JP2024066486A - Biaxially-oriented polypropylene film - Google Patents

Abstract

To provide a biaxially-oriented polypropylene film that features superior heat shrinkability and planarity and is suitable as release films such as cover films, protective films, and process films for precision components.SOLUTION: A biaxially-oriented polypropylene film is provided. When considering the main shrinkage direction of the film as the X direction, the thermal shrinkage stress value at 150°C in the X direction is 0.40 N/2 mm or more and 0.80 N/2 mm or less, and the initiation temperature of thermal shrinkage stress in the X direction is 80°C or more and 90°C or less.SELECTED DRAWING: None

Description

The present invention relates to a biaxially oriented polypropylene film that has excellent heat shrinkability and flatness in high-temperature environments and can be suitably used as a release film for precision components, such as a cover film, protective film, or process film.

Heat-shrinkable films are used in a variety of applications, including food packaging, label packaging for adding design and protecting the contents, processing substrates for functional layer coatings for controlling the orientation of the functional layer, and cover and protective films for electronic components. Among these, heat-shrinkable films made from polypropylene are particularly preferred for these applications due to their excellent strength and releasability.

Furthermore, in recent years, as electronic devices become smaller and more precise, there is an increasing demand for surface smoothness in cover films, protective films, and processing films. In light of the above, heat-shrinkable films using α-olefin copolymers (e.g., Patent Documents 1, 2, and 3) and heat-shrinkable films using polypropylene resins with low crystallinity (e.g., Patent Documents 4 and 5) have been reported.

International Publication No. 2020/080483 JP 2021-178974 A JP 2019-007006 A JP 2022-122963 A International Publication No. 2014/148547

However, the heat shrinkable films of Patent Documents 1 to 5 have the problem that when they are heated and bonded to a mating member, wrinkles and bubbles are generated, causing surface defects on the mating member, and there is also room for improvement in flatness. Therefore, the object of the present invention is to solve the above problems and provide a biaxially oriented polypropylene film that has excellent heat shrinkability and flatness and can be suitably used as a release film for cover films, protective films, process films, etc. for precision members.

In order to solve the above-mentioned problems, the biaxially oriented polypropylene film of the present invention has the following configuration. That is, when the main shrinkage direction of the film is the X direction, the heat shrinkage stress value in the X direction at 150°C is 0.40 N/2 mm or more and 0.80 N/2 mm or less, and the onset temperature of the heat shrinkage stress in the X direction is 80°C or more and 90°C or less.

In order to solve the above problems, the biaxially oriented polypropylene film of the present invention may have the following configuration, and may be wound around a core to form a film roll.
(1) A biaxially oriented polypropylene film, in which, when the main shrinkage direction of the film is the X direction, a heat shrinkage stress value in the X direction at 150°C is 0.40 N/2 mm or more and 0.80 N/2 mm or less, and a heat shrinkage stress onset temperature in the X direction is 80°C or more and 90°C or less.
(2) The biaxially oriented polypropylene film according to _{(1), which satisfies Formula 1 when the direction perpendicular to the X direction in the film plane is the Y direction, the heat shrinkage in the X direction at 140°C is Δ 140X %} , the heat shrinkage in the Y direction at 140°C is Δ _140Y %, the heat shrinkage in the X direction at 160°C is Δ 160X %, and the heat shrinkage in the Y direction at 160°C is Δ _160Y %,.
Formula 1: 0.10<(Δ _140X + Δ _140Y )/(Δ _160X + Δ _160Y )<0.40
(3) The biaxially oriented polypropylene film according to (2), wherein the Δ _140X is 6.0 or more and 12.0 or less, and the Δ _160X is 30.0 or more and 50.0 or less.
(4) The biaxially oriented polypropylene film according to any one of (1) to (3), having a film surface roughness SRa of 10 nm or more and 100 nm or less on at least one surface.
(5) The biaxially oriented polypropylene film according to any one of (1) to (4), which has a laminated structure of at least two layers and a thickness of 40 μm or more and 80 μm or less.
(6) A film roll comprising the biaxially oriented polypropylene film according to any one of (1) to (5) wound around a core.

The present invention provides a biaxially oriented polypropylene film that has good flatness despite having high heat shrinkability in a specific direction in the film plane in a high temperature environment. Since the polypropylene film of the present invention has the above-mentioned properties, it can be suitably used as a release film for cover films, protective films, process films, etc. for precision components.

The biaxially oriented polypropylene film of the present invention is characterized in that, when the main shrinkage direction of the film is the X direction, the heat shrinkage stress value in the X direction at 150°C is 0.40 N/2 mm or more and 0.80 N/2 mm or less, and the heat shrinkage stress rise temperature in the X direction is 80°C or more and 90°C or less. The biaxially oriented polypropylene film of the present invention will be specifically described below.

The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film obtained by stretching a cast sheet (unstretched sheet) mainly composed of polypropylene resin in two directions perpendicular to each other in its plane. That is, biaxial orientation here means stretching in two directions perpendicular to each other in its plane (for example, the longitudinal direction and the width direction). By making the film a biaxially oriented film, the film has excellent surface smoothness and excellent heat shrinkage properties, which will be described later. The longitudinal direction (MD) refers to the direction in which the film runs during film formation (the winding direction in the case of a film roll), and the width direction (TD) refers to the direction perpendicular to the longitudinal direction in the film plane. The main component refers to a component that is contained in the film at more than 50% by mass and not more than 100% by mass when the total components constituting the film are taken as 100% by mass (the main component can be interpreted in the same way hereinafter), and the polypropylene film refers to a film mainly composed of polypropylene resin. The polypropylene resin refers to a resin that contains propylene units at more than 50% by mass and not more than 100% by mass when the total constituent units constituting the resin are taken as 100% by mass.

In the present invention, the main shrinkage direction (X direction) refers to the direction in the film plane that shows the highest value when the heat shrinkage is measured after treatment at 160°C for 15 minutes in the MD and in each direction that forms an angle of 15°, 30°, 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, and 165° with the MD, with the MD being the 0° direction. If the MD cannot be identified from the appearance of the film, the heat shrinkage is measured in the same manner with an arbitrary direction set to 0°, and the direction with the highest heat shrinkage is regarded as the main shrinkage direction (X direction). The method for measuring the heat shrinkage is described below.

When the biaxially oriented polypropylene film of the present invention is used by laminating it with a substrate that shrinks when heated at a temperature exceeding 140°C, such as a substrate used in a wiring board or the like (hereinafter, such a substrate may be referred to as a heat-shrinkable substrate), it is important from the viewpoint of realizing favorable heat shrinkage characteristics that the heat shrinkage stress value in the X direction at 150°C is 0.40 N/2 mm or more and 0.80 N/2 mm or less when the main shrinkage direction of the film is the X direction. From the above viewpoint, the heat shrinkage stress value in the X direction at 150°C is preferably 0.50 N/2 mm or more and 0.70 N/2 mm or less, more preferably 0.53 N/2 mm or more and 0.57 N/2 mm or less.

Examples of heat-shrinkable substrates include substrates whose main component is a thermosetting resin such as phenolic resin, polycarbonate resin, melamine resin, glass epoxy resin, and epoxy resin, and the process of curing these often involves heating at temperatures exceeding 140°C. Therefore, the process film used by laminating the substrate before curing in the curing process of such a heat-shrinkable substrate is required to have heat shrinkability at the heating temperature during curing to a degree that follows the shrinkage of the heat-shrinkable substrate associated with curing, from the viewpoints of adhesion to the heat-shrinkable substrate and prevention of wrinkles in the heat-shrinkable substrate after curing.

By having a heat shrinkage stress value of 0.4 N/2 mm or more in the X direction at 150°C, when the biaxially oriented polypropylene film is superimposed on the heat-shrinkable substrate before curing and heated, an appropriate heat shrinkage stress is applied, resulting in high adhesion and suppressing a decrease in yield and the occurrence of wrinkles. On the other hand, by having a heat shrinkage stress value of 0.80 N/2 mm or less in the main shrinkage direction at 150°C, room temperature shrinkage of the biaxially oriented polypropylene film is suppressed, thereby reducing the deterioration of the quality of the product roll. In addition, if the heat shrinkage stress value in the main shrinkage direction at 150°C is 0.80 N/2 mm or less, deformation and damage caused by sudden stress applied to the heat-shrinkable substrate before curing during heating are also reduced, thereby suppressing a decrease in yield.

As a method for making the heat shrinkage stress value at 150°C in the main shrinkage direction 0.40 N/2 mm or more and 0.80 N/2 mm or less, or the above-mentioned preferred range, for example, a method using a polypropylene resin having an MFR of 0.1 g/10 min or more and 2.0 g/10 min or less for a biaxially oriented polypropylene film can be mentioned. In addition, in regard to the film formation conditions using such a polypropylene resin, it is effective to set the stretching temperature in the transverse stretching (stretching in the width direction) process to 145°C to 180°C, preferably 151°C to 175°C, the relaxation rate in the heat treatment process to 1.0% to 20%, preferably 1.0 to 14%, and the temperature in the cooling process to 130°C to 145°C. By setting the transverse stretching, relaxation, and cooling under the above conditions, stretching and heat setting can be performed in a state in which the stress inside the film is strengthened, so that the obtained biaxially oriented polypropylene film has a high heat shrinkage stress. Note that these conditions can be combined as appropriate.

In order to realize favorable heat shrinkage properties when used in combination with a heat shrinkable substrate before curing, it is important that the heat shrinkage stress rise temperature in the X direction is 80°C or more and 90°C or less. The heat shrinkage stress rise temperature in the X direction can be measured by the TMA (Thermo Mechanical Analysis) method described later. If the heat shrinkage stress rise temperature in the X direction is less than 80°C, the film will not follow the heat shrinkable substrate and cause a decrease in adhesion and wrinkles when used in combination with a heat shrinkable substrate, since the film will start shrinking at a temperature lower than the shrinkage start temperature of the heat shrinkable substrate. On the other hand, if the heat shrinkage stress rise temperature in the X direction is 90°C or more, the film will not follow the heat shrinkable substrate and cause a decrease in adhesion and wrinkles when used in combination with a heat shrinkable substrate. From the above viewpoint, the heat shrinkage stress rise temperature in the X direction is preferably 83°C or more and 90°C or less, and more preferably 83°C or more and 87°C or less.

A method for setting the onset temperature of the heat shrinkage stress in the X direction to 80°C or more and 90°C or less can be mentioned, for example, by using a polypropylene resin having a viscosity of 0.1 g/10 min or more and 2.0 g/10 min or less, setting the temperature in the longitudinal stretching (stretching in the longitudinal direction) to 140°C or more and 160°C or less, and setting the stretching ratio to 3.0 times or more and less than 5.0 times. Furthermore, it is also effective to set the relaxation rate in the heat treatment process to 1.0 to 14% and the temperature in the cooling process to 130°C to 145°C. These methods can also be used in combination as appropriate. By setting the relaxation rate in the heat treatment process to the above conditions, the orientation state of the polymer chains inside the film can be suitably controlled, so that a biaxially oriented polypropylene film with high heat shrinkage stress and excellent thermal dimensional stability can be obtained. In addition, by setting the temperature in the cooling process to the above conditions, the temperature difference with the outside of the tenter can be controlled to increase the crystallinity inside the film, so that the obtained biaxially oriented polypropylene film has high heat shrinkage stress and excellent thermal dimensional stability.

From the viewpoint of achieving both adhesion to a heat-shrinkable substrate and flatness, it is preferable that the biaxially oriented polypropylene film of the present invention satisfies Formula 1 when the direction perpendicular to the X direction in the film plane is the Y direction, the heat shrinkage in the X direction at 140°C is Δ _140X %, the heat shrinkage in the Y direction at 140°C is Δ _140Y %, the heat shrinkage in the X direction at 160°C is Δ _160X %, and the heat shrinkage in the Y direction at 160°C is Δ _160Y %.
Equation 1: 0.10<(Δ _140X +Δ _140Y )/(Δ _160X +Δ _160Y )<0.40.

Here, the Y direction refers to the direction perpendicular to the X direction determined by the above-mentioned method in the film plane. Δ _140X refers to the shrinkage rate in the X direction when heated at 140°C for 15 minutes, and Δ _140Y , Δ _160X , and Δ _160Y can be interpreted in the same way except that the measurement direction and measurement temperature are different. The details of the measurement method of Δ _140X , Δ _140Y , Δ _160X , and Δ _160Y will be described later.

From the above viewpoint, the value of (Δ _140X + Δ _140Y ) / (Δ _160X + Δ _160Y ) is more preferably 0.15 to 0.35, even more preferably 0.20 to 0.30, and particularly preferably 0.20 to 0.26, from the viewpoint of both adhesion to the heat-shrinkable substrate and flatness. The biaxially oriented polypropylene film of the present invention is required to have a low shrinkage rate at 140°C, which is the stage before the heat-shrinkable substrate shrinks, from the viewpoint of conformity to the heat-shrinkable substrate in the curing process. On the other hand, from the viewpoint of adhesion to the heat-shrinkable substrate, a high shrinkage rate is required at 160°C, where the substrate shrinks, within a range that does not impair flatness due to the occurrence of wrinkles, etc.

In the biaxially oriented polypropylene film of the present invention, by making the value of (Δ _140X + Δ _140Y ) / (Δ _160X + Δ _160Y ) less than 0.40, when the film is laminated with a heat-shrinkable base material and heated, the shrinkage rate in the X direction and Y direction is kept low until the temperature at which the base material shrinks is reached, so that the deterioration of the winding shape and the deterioration of flatness due to the occurrence of wrinkles, etc., when the film is made into a product roll are reduced. In addition, if the value of (Δ _140X + Δ _140Y ) / (Δ _160X + Δ _160Y ) is less than 0.40, the biaxially oriented polypropylene film also shrinks following the heat-shrinkable base material under temperature conditions at which the heat-shrinkable base material shrinks (temperatures exceeding 140 ° C.), so that the adhesion to the heat-shrinkable base material is improved. From the above viewpoint, the smaller the value of (Δ _140X + Δ _140Y ) / (Δ _160X + Δ _160Y ) is, the more preferable it is, but from the viewpoint of feasibility, it exceeds 0.10.

As a method for satisfying the formula 1 (or making (Δ _140X +Δ _140Y )/(Δ _160X +Δ _160Y ) the above preferred range), for example, there is a method of using a polypropylene resin having a viscosity of 0.1 g/10 min or more and 2.0 g/10 min or less, and setting the preheating temperature in the longitudinal stretching step to 125° C. to 175° C. and the stretching temperature to be higher than 140° C. and lower than 160° C. Increasing the preheating temperature in the longitudinal stretching and stretching the film in a state in which the internal stress of the film is relaxed can relax the thermal shrinkage in the longitudinal direction. In addition, there is also a method of setting the preheating temperature in the transverse stretching step to 160° C. to 185° C., the stretching temperature to be 145° C. to 180° C. (preferably 151° C. to 180° C.), the heat setting temperature to be higher than 140° C. and lower than 160° C., the cooling step temperature to be 130° C. to 145° C., and the relaxation rate to be 1.0% to 20%. By setting the transverse stretching temperature to a temperature higher than 140° C., preferably 151° C. or higher, it is possible to maintain a high internal stress at 160° C. while alleviating the internal stress at 140° C. Furthermore, by setting the heat setting temperature to a temperature higher than 140° C. and 160° C. or lower, it is possible to reduce shrinkage at 140° C. Note that these methods can be used in appropriate combination.

The biaxially oriented polypropylene film of the present invention preferably has a Δ _140X of 6.0 or more and 12.0 or less, and a Δ _160X of 30.0 or more and 50.0 or less. When the Δ _140X is 6.0 or more, for example, when the biaxially oriented polypropylene film of the present invention is laminated with a heat-shrinkable base material before curing and heated, the biaxially oriented polypropylene film of the present invention shrinks to a degree that follows the shrinkage of the heat-shrinkable base material, so that the adhesion between the two is increased and the heat-shrinkable base material can be sufficiently protected. On the other hand, when the Δ _140X is 12.0% or less, the shrinkage at 140 ° C. is too large, so that wrinkles may occur when the film is laminated with the heat-shrinkable base material and heated, or the flatness may deteriorate, resulting in poor appearance. From the above viewpoint, the upper limit of Δ _140X is preferably 12.0, more preferably 11.0, even more preferably 10.8, and particularly preferably 10.0, and the lower limit is preferably 6.0, more preferably 7.0, even more preferably 8.0, and particularly preferably 9.6.

In addition, when Δ _160X is 30.0 or more, the biaxially oriented polypropylene film also shrinks following the heat-shrinkable base material at 160 ° C., where the heat-shrinkable base material shrinks, and the adhesion between the biaxially oriented polypropylene film and the heat-shrinkable base material is maintained. On the other hand, when Δ _160X is 50.0 or less, in an environment of 160 ° C., where the heat-shrinkable base material shrinks, the biaxially oriented polypropylene film does not shrink excessively compared to the shrinkage of the base material, and deterioration of flatness due to the occurrence of wrinkles, etc. is reduced. From the above viewpoint, the upper limit of Δ _160X is preferably 50.0, more preferably 46.0, and even more preferably 43.5, and the lower limit is preferably 30.0, more preferably 39.5, and even more preferably 40.5.

As a method for controlling _Δ140X and _Δ160X to 6.0 or more and 12.0 or less and 30.0 or more and 50.0 or less, respectively, a method similar to the method for setting the preferable range of the above formula 1 can be used. However, the preheating temperature in the transverse stretching step is more preferably 166° C. or more, the transverse stretching temperature is more preferably 156° C. to 164° C., and the relaxation rate in the relaxation treatment is preferably 1.0% to 14%.

In the biaxially oriented polypropylene film of the present invention and the film roll obtained by winding it, it is preferable that the longitudinal direction is the Y direction and the width direction is the X direction. By adopting such an embodiment, in a continuous process such as a roll-to-roll process, the biaxially oriented polypropylene film shrinks appropriately when it is laminated with a heat-shrinkable substrate before curing and heated, so that high adhesion to the heat-shrinkable substrate can be obtained. In addition, since the longitudinal direction is the Y direction and the width direction is the X direction, the shrinkage of the biaxially oriented polypropylene film in the film advance direction when heated is relatively small, so that the occurrence of wrinkles, air bubbles, etc. due to interference between the force conveying the film and the shrinkage in the roll-to-roll process is reduced, and as a result, adhesion to the heat-shrinkable substrate can be easily maintained.

As a method for obtaining a biaxially oriented polypropylene film in which the longitudinal direction is the Y direction and the width direction is the X direction, and a film roll obtained by winding this film, for example, a method in which the stretching ratio in the width direction is greater than the stretching ratio in the longitudinal direction can be mentioned, and further, a method similar to the method in which the above formula 1 is in the preferred range can also be used. Note that these methods may be used in combination as appropriate.

The biaxially oriented polypropylene film of the present invention preferably has a film surface roughness SRa of 10 nm to 100 nm on at least one side, more preferably 25 nm to 50 nm. When the film surface roughness SRa is 10 nm to 100 nm on at least one side of the biaxially oriented polypropylene film, the roughness of the film surface is appropriate, so that the film can be bonded well to the heat-shrinkable substrate, and the shape change and defects of the heat-shrinkable substrate can be reduced. In addition, by making the film surface roughness SRa of the biaxially oriented polypropylene film 100 nm or less on at least one side, when the laminate with the heat-shrinkable substrate is wound into a roll, dents and defects due to protrusions are unlikely to occur on the substrate, and the decrease in yield due to these can be reduced. On the other hand, by making the surface roughness SRa 10 nm or more, it is possible to reduce winding misalignment due to excessively high smoothness. From the above viewpoint, it is even more preferable that the film surface roughness SRa on both sides is 10 nm to 100 nm or less or in the above preferred range.

As a method for making the film surface roughness SRa on at least one side 10 nm or more and 100 nm or less or the above-mentioned preferred range, for example, a method of controlling the surface temperature of the cast drum to 20°C or more and 80°C or less during the cooling process during casting can be used. By making the surface temperature of the cast drum within the above range, the crystal form formed on the film surface can be controlled and the surface roughness SRa can be reduced. In addition, the most effective method for adjusting SRa is to adjust the surface temperature of the cast drum, but SRa is also affected by the preheating temperature before stretching, and can be increased, for example, by increasing the preheating temperature before transverse stretching within the preferred range described below.

From the viewpoint of being used by laminating with a heat-shrinkable substrate, the biaxially oriented polypropylene film of the present invention preferably has a thickness of 40 μm or more and 80 μm or less, more preferably 50 μm or more and 70 μm or less, and even more preferably 55 μm or more and 65 μm or less. When the thickness of the biaxially oriented polypropylene film is 40 μm or more and 80 μm or less, the biaxially oriented polypropylene film is stiff, and handling is improved when laminating with a shrinkable substrate before curing. In addition, by having a thickness of 40 μm or more, wrinkles are unlikely to occur even if the biaxially oriented polypropylene film shrinks by heating, and defects in the heat-shrinkable substrate caused by this can be reduced. In order to make the thickness of the biaxially oriented polypropylene film within the above range, it is effective to adjust the amount of resin discharged when forming a cast sheet, the rotation speed of the casting drum, the width of the slit of the die, and the stretching ratio.

The film roll of the present invention is formed by winding the biaxially oriented polypropylene film of the present invention around a core. The material of the core is preferably plastic, fiber-reinforced plastic, or metal, which are less likely to deform, and fiber-reinforced plastic is more preferably used from the viewpoint of strength. Examples of fiber-reinforced plastic cores include resin-impregnated cores formed by winding carbon fiber or glass fiber into a cylindrical shape, impregnating this with a thermoplastic resin such as unsaturated polyester resin, and hardening the core.

In the film roll of the present invention, it is preferable to control the amount of one-sided elongation within an appropriate range from the viewpoint of the running property of the biaxially oriented polypropylene film in the process of unwinding and processing the biaxially oriented polypropylene film from the film roll in the longitudinal direction. More specifically, it is preferable to set the amount of one-sided elongation to 10.0 mm/10 m length or less, more preferably 7.0 mm/10 m length or less, and even more preferably 5.0 mm/10 m length or less.

One-sided elongation refers to the phenomenon in which the biaxially oriented polypropylene film curves in an arc shape when it is unwound from the film roll in the longitudinal direction. The amount of one-sided elongation referred to in the present invention can be measured by the following procedure. First, the biaxially oriented polypropylene film is unwound from the film roll in the longitudinal direction for a length of 10 m and cut to obtain a rectangular measurement sample with a size of 10 m (longitudinal direction) x film roll width (width direction). A thread is attached parallel to the longitudinal direction so as to connect the vertices of the obtained sample (fixed at the vertices, two threads are also attached as there are two opposing sides), and the distance between the thread and the biaxially oriented polypropylene film is measured at the midpoint of the thread (5 m from each vertex). The same measurement is performed for two threads, and the average value is taken as the amount of one-sided elongation (unit: mm/10 m length).

A large amount of one-sided stretch means that the biaxially oriented polypropylene film unwound from the film roll is curved in an arc shape. By setting the amount of one-sided stretch within the above range, not only running properties but also winding properties are improved when performing lamination processing in a roll-to-roll process. Furthermore, by setting the amount of one-sided stretch to 10.0 mm/10 m length or less, meandering of the biaxially oriented polypropylene film during unwinding and transport can be reduced. Therefore, by adjusting the running state using an EPC (edge position control device) or a cross guider, it is possible to reduce the occurrence of defects such as uneven processing, wrinkles, and uneven edge alignment during winding, and further improve productivity. In order to set the amount of one-sided stretch in the longitudinal direction within the above range, it is effective to adjust the tension of the transport film immediately after the clip is separated by adjusting the speed of the transport roll based on the transport film running speed at the time of clip separation in the transverse stretching process as described below.

In order to achieve both windability and flatness, the film roll of the present invention preferably has an air entrapment rate of 5.0% or less, and more preferably 3.0% or less. By having an air entrapment rate of 5.0% or less in the film roll, the occurrence of wrinkles in the biaxially oriented polypropylene film due to air accumulation can be reduced. The lower limit of the air entrapment rate in the film roll is 1.0% in order to reduce blocking due to room temperature shrinkage of the wound film roll.

The air entrapment rate in the film roll can be calculated from the length and thickness of the biaxially oriented polypropylene film wound on the roll, and the diameter of the film roll and core, and will be described in detail later. One method for keeping the air entrapment rate in the film roll at 5.0% or less or within the above-mentioned preferred range is to control the winding conditions, such as the winding tension and winding surface pressure, within a preferred range in the slitting process described later.

Next, the layer structure and raw materials of the biaxially oriented polypropylene film of the present invention will be described, but are not necessarily limited thereto. The biaxially oriented polypropylene film of the present invention may be a single layer structure or a laminated structure of two or more layers, but it is preferable that it has a laminated structure of two or more layers from the viewpoint of imparting various effects. When the biaxially oriented polypropylene film of the present invention has a laminated structure of two or more layers, it is preferable that it has a laminated structure of a base layer (hereinafter sometimes referred to as A layer) mainly composed of polypropylene resin A and a surface layer (hereinafter sometimes referred to as B layer) mainly composed of polypropylene resin B and located on at least one surface, and it is more preferable that it has B layer on both sides of A layer. Here, the main component refers to a component that is contained in more than 50 mass% to 100 mass% or less when the total components constituting the layer are 100 mass%. In addition, polypropylene resins A and B are distinguished from each other by the smaller MFR measured under the conditions described below as polypropylene resin A and the larger MFR measured under the conditions described below as polypropylene resin B.

In addition, lamination methods that can be used when the biaxially oriented polypropylene film of the present invention is laminated include, for example, coextrusion methods using the feed block method or the multi-manifold method, and methods in which films are bonded together by lamination.

As long as the object of the present invention is not impaired, polypropylene resin A and polypropylene resin B may be homopolymers of propylene or copolymers containing copolymerization components with other unsaturated hydrocarbons. Examples of 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, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-eicosene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene. In addition, resins other than polypropylene resins may be blended into layers A and B, and examples of resins that can be blended include resins whose main constituent units are structural units derived from the above monomers (which may contain propylene as a copolymerization unit).

However, from the viewpoint of transparency and heat resistance, the content of polypropylene units in the structural units constituting all the resin components constituting the biaxially oriented polypropylene film of the present invention is preferably 95% by mass or more and 100% by mass or less. More preferably, it is 96% by mass or more and 100% by mass or less, even more preferably, it is 97% by mass or more and 100% by mass or less, and particularly preferably, it is 98% by mass or more and 100% by mass or less.

When polypropylene resin A or polypropylene resin B contains ethylene units, the content of the ethylene units contained is preferably 10% by mass or less of all constituent units. More preferably, it is 5% by mass or less, and even more preferably, it is 3% by mass or less. The more ethylene units there are, the lower the crystallinity of the polypropylene resin is, and the easier it is to improve transparency. On the other hand, by keeping the amount of ethylene units to 10% by mass or less, it is possible to reduce deterioration of the heat shrinkage rate due to a decrease in the strength and heat resistance of the biaxially oriented polypropylene film.

In addition, when polypropylene resin A or B is blended with polyethylene resin, the surface may become rougher and the resin may deteriorate during the extrusion process. As a result, flatness may deteriorate due to an increase in fish eyes caused by the polyethylene resin and an increase in foreign matter due to surface scraping. From the viewpoint of reducing the deterioration of flatness due to such mechanisms, when polypropylene resin A or B is blended with polyethylene resin, it is preferable to keep the amount of polypropylene resin A or B to 10% by mass or less when the total of the polypropylene resin and the polyethylene resin is 100% by mass.

The biaxially oriented polypropylene film of the present invention may also contain various additives, such as crystal nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as components other than the above resins, provided that the object of the present invention is not impaired. Among these, the selection of the type and amount of antioxidant to be added is important from the viewpoint of reducing bleed-out of the antioxidant. 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 of antioxidants that can be used in the biaxially oriented polypropylene film of the present invention include various ones, but for example, it is preferable to use 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) in combination with 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox" (registered trademark) 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox" (registered trademark) 1010: molecular weight 1177.7). The total content of these antioxidants is preferably in the range of 0.03% to 1.0% by mass when the total components constituting the biaxially oriented polypropylene film are taken as 100% by mass. If the amount of antioxidant is too small, the polymer may deteriorate during the extrusion process, causing the film to become discolored, or the long-term heat resistance may be poor. On the other hand, if the amount of antioxidant is too large, the transparency may decrease due to the antioxidant bleeding out. From the above viewpoint, the content of the antioxidant is more preferably 0.05% by mass to 0.90% by mass, and even more preferably 0.10% by mass to 0.80% by mass, when the total components constituting the biaxially oriented polypropylene film are taken as 100% by mass.

The MFR of polypropylene resin A constituting the biaxially oriented polypropylene film of the present invention, when measured in accordance with condition M (230°C, 2.16 kg) of JIS K 7210-2:2014, is preferably 0.1 g/10 min to 2.0 g/10 min, more preferably 0.5 g/10 min to 2.0 g/10 min, and even more preferably 1.0 g/10 min to 2.0 g/10 min. The MFR of polypropylene resin B, measured in the same manner, is greater than the MFR of polypropylene resin A and is preferably 1.0 g/10 min to 10 g/10 min, more preferably 2.0 g/10 min to 8.0 g/10 min, and even more preferably 3.0 g/10 min to 5.0 g/10 min.

In general, when the MFR of polypropylene resin A is 0.1 g/10 min or more, the film-forming property is stable and thickness unevenness is suppressed, so that when the polypropylene resin is laminated with a heat-shrinkable base material before curing and heated, the adhesion between the two is improved. In order to set the MFR of polypropylene resins A and B within the above range, it is preferable to select a polypropylene polymerization catalyst, add a polymerization accelerator, or control the average molecular weight and molecular weight distribution by the amount of propylene monomer added. Polypropylene resins A and B can be appropriately selected from polypropylene resins that satisfy the above MFR, and examples of polypropylene resins that can be suitably used as polypropylene resin A include, for example, "Noblen" (registered trademark) S131 manufactured by Sumitomo Chemical. Examples of polypropylene resins that can be suitably used as polypropylene resin B include, for example, "Novatec" (registered trademark) (FL203D, etc.) manufactured by Nippon Polypropylene and "Noblen" (registered trademark) (AH561, etc.) manufactured by Sumitomo Chemical.

Next, the method for producing the biaxially oriented polypropylene film of the present invention will be specifically described using one embodiment as an example, but the biaxially oriented polypropylene film of the present invention is not necessarily limited to that obtained by the following method.

The biaxially oriented polypropylene film of the present invention is preferably biaxially stretched using the above-mentioned raw materials. As the biaxial stretching method, any of the inflation simultaneous biaxial stretching method, the stenter simultaneous biaxial stretching method, and the stenter sequential biaxial stretching method can be used. Among these, the stenter sequential biaxial stretching method is preferably used in terms of film formation stability, thickness uniformity, and control of high film rigidity and dimensional stability.

First, polypropylene resin A is fed to a single-screw extruder for the base layer (A layer), and polypropylene resin B is fed to a single-screw extruder for the surface layer (B layer), and melt extrusion is performed at 200 to 280°C, more preferably 220 to 280°C, and even more preferably 240 to 270°C. Then, after removing foreign matter and modified polymers with a filter installed in the middle of the polymer tube, the molten resin is laminated to a two-type three-layer structure of B layer/A layer/B layer using a multi-manifold type composite T-die with a temperature adjusted to 240°C to 270°C, and then discharged onto a cast drum and cooled and solidified to obtain a laminated unstretched sheet having a layer structure of B layer/A layer/B layer. At this time, the lamination thickness ratio of B layer/A layer/B layer is preferably in the range of 1/38/1 to 1/78/1. By setting the lamination thickness ratio in the above range, stiffness is created in the obtained biaxially oriented polypropylene film, and handling is improved when it is laminated to a heat-shrinkable substrate.

The surface temperature of the cast drum is preferably 20°C to 80°C, more preferably 25°C to 60°C, and even more preferably 30°C to 50°C, from the viewpoint of controlling the surface roughness SRa of the biaxially oriented polypropylene film within an appropriate range. As a method for adhering the molten resin to the cast drum, any of the following methods may be used: electrostatic application method, adhesion method using the surface tension of water, air knife method, press roll method, underwater casting method, etc., but the air knife method is preferred from the viewpoint of good flatness of the obtained biaxially oriented polypropylene film and easy control of the surface roughness SRa. At this time, the air temperature of the air knife is preferably 20°C to 80°C, and the blowing air pressure is preferably 0.01 MPa to 0.1 MPa. In addition, it is also preferable to appropriately adjust the position of the air knife so that air flows downstream of the film production in order to prevent vibration of the film.

The unstretched sheet thus obtained is introduced into the longitudinal stretching process. In the longitudinal stretching process, the unstretched sheet is first heated by passing it through an oven maintained at 125°C to 175°C, preferably 135°C to 165°C, more preferably 150°C to 160°C, and then stretched in the longitudinal direction between rolls with a difference in peripheral speed, and then cooled to room temperature. From the viewpoint of appropriately controlling the heat shrinkage characteristics, the stretching temperature is more than 140°C to 160°C, preferably more than 140°C to 155°C, more preferably more than 140°C to 150°C. In addition, from the viewpoint of increasing the strength while suppressing breakage of the film during stretching, the stretching ratio is preferably 3.0 times or more and less than 5.0 times, more preferably 3.0 times or more and less than 5.0 times, and even more preferably 4.0 times or more and less than 5.0 times.

In the preheating process for longitudinal stretching (stretching in the lengthwise direction), it is important to heat the entire sheet evenly to suppress temperature unevenness in the unstretched sheet from the viewpoint of smoothing the surface of the resulting biaxially oriented polypropylene film. Furthermore, if the preheating temperature and the stretching temperature are significantly different, the film may shrink unevenly in the width direction when it touches the high-temperature stretching roll, which may result in wrinkles. As a countermeasure to this, a ceramic roll can be used for the stretching roll. The film slides easily on the ceramic roll, and the film shrinks more uniformly, making it possible to stretch the film while suppressing the occurrence of wrinkles caused by the above mechanism.

Next, the uniaxially oriented film obtained in the longitudinal stretching process is held at both ends in the width direction by clips, introduced into a tenter for preheating, and then transversely stretched 5.0 to 12.0 times in the width direction. By adjusting the stretching temperature during transverse stretching, the heat setting temperature, and the relaxation rate during heat setting, it is possible to control the heat shrinkage stress and heat shrinkage rate. From the above viewpoint, the preheating temperature is preferably 160°C to 185°C, more preferably 165°C to 180°C, and even more preferably 166°C to 175°C. From the same viewpoint, the stretching temperature is preferably 145°C to 180°C, more preferably 150°C to 175°C, even more preferably 151°C to 175°C, particularly preferably 155°C to 170°C, and most preferably 156°C to 164°C. When the preheating and stretching temperature approaches the melting point of polypropylene, the polypropylene crystals transition to amorphous, improving the mobility of the polymer, and the heat shrinkage rate and heat shrinkage stress decrease. For this reason, from the above viewpoint, a low preheating/stretching temperature is preferable, but if the preheating/stretching temperature is too low, the uniaxially oriented film will be stretched with poor polymer mobility, causing breakage and thickness unevenness during stretching, so the above preheating/stretching temperature range is preferable.

In the subsequent heat treatment step, the film is heated at a temperature of 140°C to 160°C, preferably 145°C to 160°C, more preferably 150°C to 160°C, while being relaxed at a relaxation rate of 1.0% to 20% in the width direction while being held taut at both ends in the width direction with clips. Since the film is less likely to shrink at a temperature equal to or higher than the heat treatment temperature by applying a heat treatment temperature while keeping the film width fixed in the heat treatment step, it is preferable to heat treat the film at a temperature exceeding 140°C from the viewpoint of suitably carrying out processing by laminating the film with a heat shrinkable base material. In addition, since the orientation of the amorphous molecular chains is high and the thermal dimensional ratio after winding the film into a roll is poor, it is preferable to relax the orientation by applying a relaxation rate to the film. However, as the relaxation rate increases, the film after transverse stretching becomes looser and is more likely to come into contact with the inside of the tenter, causing film tearing, so the upper limit of the relaxation rate is preferably 20%, more preferably 14%, and even more preferably 5.0%. On the other hand, from the viewpoint of controlling the heat shrinkage stress onset temperature and the value of (Δ _140X + Δ _140Y )/(Δ _160X + Δ _160Y ) within a suitable range, the lower limit of the relaxation rate is more preferably 2.0%. Thereafter, while being tension-held in the width direction with clips, the film is passed through a cooling process at 130°C to 145°C and guided to the outside of the tenter, the clips at both ends in the width direction are released, and the film edges are slit in a winder process to obtain a biaxially oriented polypropylene film.

Finally, the biaxially oriented polypropylene film thus obtained is slit to a predetermined width and length in a slitting process, and wound around a core as a film roll. In the present invention, from the viewpoint of productivity, the film roll width (the length in the width direction of the biaxially oriented polypropylene film) is preferably 1000 mm or more and 2000 mm or less, and more preferably 1300 mm or more and 1700 mm or less. The film length is preferably 1000 m or more and 3000 m or less, and even more preferably 1500 m or more and 3000 m or less. Taking into consideration the productivity and the difficulty of the winding technique, it is even more preferably 2000 m or more and 3000 m or less.

The slitting speed in the slitting process is preferably 100 m/min to 200 m/min, more preferably 100 m/min to 190 m/min, from the viewpoint of optimal control of the air entrapment rate and surface hardness of the film roll, and from the viewpoint of productivity. By setting the slitting speed at 200 m/min or less, the accompanying air current generated by the traveling biaxially oriented polypropylene film (hereinafter sometimes referred to as the conveying film) is reduced, so the occurrence of wrinkles during conveying is suppressed. Therefore, it is possible to reduce the occurrence of wrinkles when forming the film roll, and the deterioration of flatness due to the wrinkles being folded.

The unwinding force in the slitting process is preferably 600N to 800N in terms of suppressing wrinkles during transport. If the unwinding force is less than 600N, the transport film will slacken due to insufficient tension, making it more likely to wrinkle. On the other hand, if the unwinding force exceeds 800N, if there is unevenness in the thickness of the transport film, the transport film will be pulled strongly, causing a large stress difference in the width direction, making it more likely to wrinkle. In addition, if the unwinding force exceeds 800N, the transport film will be pulled strongly and wound up in a deformed state, making it more likely to wrinkle as the deformation returns to normal within the film roll.

In terms of air entrapment rate and winding hardness, the initial winding tension in the slitting process is preferably 60 N/m to 100 N/m, and more preferably 70 N/m to 90 N/m. If the initial winding tension is less than 60 N/m, the conveying film will slacken due to insufficient tension, making it easier for wrinkles to occur during winding, and therefore easier for winding misalignment to occur. On the other hand, if the initial winding tension exceeds 100 N/m, if the conveying film has uneven thickness, the conveying film will be pulled strongly, causing a large stress difference in the width direction, making it easier for wrinkles and winding misalignment to occur. In addition, if the initial winding tension exceeds 100 N/m, the conveying film will be pulled strongly and wound up while deformed, so wrinkles will also easily occur due to the deformation returning in the film roll. The initial winding tension refers to the winding tension at the initial timing when the biaxially oriented polypropylene film slit in the slitting process is wound around the core.

From the viewpoints of reducing the occurrence of wrinkles in the biaxially oriented polypropylene film or film roll during transport, controlling the air entrapment rate, and reducing winding misalignment, it is preferable that the winding tension taper (winding tension when the film roll is wound up/initial winding tension x 100) in the slitting process is 70% to 100%. By setting the winding tension taper to 70% or more, the hardness of the surface layer of the film roll is maintained, and buckling and winding misalignment can be reduced. In addition, wrinkles in the biaxially oriented polypropylene film during transport that occur due to a sudden change in winding tension can also be reduced. On the other hand, by setting the winding tension taper to 100% or less, the occurrence of wrinkles and unevenness can be reduced, especially near the surface layer of the film roll.

From the viewpoint of controlling the surface hardness of the film roll, the initial winding surface pressure in the slitting process is preferably 200 N/m to 400 N/m, and more preferably 210 N/m to 340 N/m. By setting the initial winding surface pressure at 200 N/m or more, the surface hardness of the film roll is maintained, and buckling and winding misalignment can be reduced. On the other hand, by setting the initial winding surface pressure at 400 N/m or less, an excessive increase in the surface hardness of the film roll can be suppressed, and blocking between film layers, the occurrence of wrinkles in the center of the film roll due to bending of the contact roll, and the associated winding misalignment can be reduced. The initial winding surface pressure refers to the surface pressure at the initial timing when the biaxially oriented polypropylene film slit in the slitting process is wound around the core.

The winding surface pressure taper in the slitting process (winding surface pressure when the film roll is wound up/initial winding surface pressure x 100) is preferably 80% to 120%, more preferably 90% to 120%, from the viewpoint of controlling the surface hardness. By setting the winding surface pressure taper to 80% or more, the surface hardness of the film roll is maintained and buckling and winding misalignment can be reduced. On the other hand, by setting the winding surface pressure taper to 120% or less, an excessive increase in the surface hardness of the film roll is suppressed, and blocking between film layers and the occurrence of wrinkles in the center of the film roll due to bending of the contact roll can be reduced.

In the slitting process, if thickness unevenness exists in the biaxially oriented polypropylene film, the wound film roll may easily become uneven due to the thickness unevenness. To solve this problem, it is preferable to perform so-called oscillation, in which the film roll on the unwinding side or the film roll on the winding side is repeatedly moved in the width direction to disperse the effects of thickness unevenness. The oscillation width in the slitting process is preferably 5 mm to 50 mm, and more preferably 10 mm to 30 mm, from the viewpoint of dispersing thickness unevenness and reducing winding misalignment. The oscillation speed is preferably 5 mm/min to 30 mm/min.

The biaxially oriented polypropylene film of the present invention obtained in the above manner has excellent high-temperature heat shrinkability and flatness at high temperatures, and can be suitably used as a release film for precision components, such as a cover film, a protective film, or a processing film.

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

(1) Melt flow rate (MFR, g/10 min)
Measurement was performed in accordance with JIS K 7210-2:2014, condition M (230°C, 2.16 kg).

(2) Thickness of Biaxially Oriented Polypropylene Film The thickness was measured at five arbitrarily selected points using a micro thickness meter (manufactured by Anritsu Corporation), and the average value was regarded as the thickness of the biaxially oriented polypropylene film.

(3) Identification of heat shrinkage rate and main shrinkage direction at 140°C and 160°C Five pieces of biaxially oriented polypropylene film with a size of 200 mm (measurement direction) x 10 mm (total of 10 pieces) were cut out so that the longitudinal direction and the width direction were the measurement directions, and used as samples. Marks were made at positions 50 mm from both ends of the obtained sample to set the test length l ₀ to 100 mm. Next, the sample was hung in an oven kept at 140°C and heated for 15 minutes, then removed, cooled at room temperature, and the dimension (l ₁ ) was measured to determine the heat shrinkage rate according to the following formula 2. The average value of the five measured values was calculated for each of the longitudinal direction and the width direction, and the obtained value was taken as the heat shrinkage rate (%) of the biaxially oriented polypropylene film at 140°C. The heat shrinkage rate at 160°C was also measured in the same manner, except that the oven temperature was 160°C. In each example and each comparative example, since the stretching direction was the longitudinal direction and the width direction, the direction with the higher shrinkage rate at 160°C among these directions was taken as the main shrinkage direction.
Equation 2: Shrinkage rate (%)={(l ₀ −l ₁ )/l ₀ }×100.

(4) TMA Measurement The rate of change of the sample at each temperature was measured by the TMA (Thermo Mechanical Analysis) method, and a curve (TMA curve) was plotted with the horizontal axis representing temperature and the vertical axis representing the amount of change in the length of the sample. The temperature at which the thermal shrinkage stress in the X direction started was read from the TMA curve, and the thermal shrinkage stress at each temperature was measured. The measuring device and measuring 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: Placed in a temperature increasing apparatus at a constant rate of 10° C./min. Atmosphere: Air at room temperature Sample: Rectangle of 15 mm×2 mm (main shrinkage direction (X direction) which is the measurement direction is 15 mm)

(5) Arithmetic mean height (SRa)
Measurements were made using a non-contact surface/layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka Systems Co., Ltd. The contact surface with the cast drum was used as the measurement surface, and three locations randomly selected in the longitudinal direction from the center position in the width direction of the film roll after slitting were set as measurement locations, and the average value of the measurements at each measurement location was taken as the arithmetic mean height (SRa) of the biaxially oriented polypropylene film in the film roll. Detailed conditions for one measurement were as follows. Note that one visual field (visual field area 1,252 μm × 939 μm = 1,175,628 μm ² ) was observed for one measurement.

(6) Air entrapment rate The air entrapment rate was calculated from the thickness of the biaxially oriented polypropylene film, the length of the biaxially oriented polypropylene film wound on the film roll, the diameter of the film roll, and the diameter of the core according to the following formula 3. The diameter of the film roll was measured and calculated according to the following procedure. First, the outer periphery of the film roll was measured at a point 5 mm from one of the widthwise ends selected arbitrarily using a tape measure with a dimensional accuracy of 10 μm, and the diameter of the film roll was calculated from the outer periphery. Next, the measurement point was shifted 50 mm toward the center in the width direction and the same measurement was performed, and this was repeated over the entire width.
Formula 3: α={1−t1L/((d1 ² −d2 ² )π/4)}×100
Here, α is the air entrapment rate (%), t1 is the film thickness (μm, the value measured by the method of (2) is used), L is the length (m) of the biaxially oriented polypropylene film wound around the film roll, d1 is the diameter (mm) of the film roll, and d2 is the diameter (mm) of the core.

(7) One-sided elongation First, the biaxially oriented polypropylene film was unwound from the film roll in the longitudinal direction by 10 m and cut to obtain a rectangular measurement sample with a size of 10 m (longitudinal direction) x film roll width (width direction). Next, a thread was attached parallel to the longitudinal direction so as to connect the vertices of the obtained sample (fixed at the vertices, two threads were also attached because there were two sides facing each other), and the distance between the thread and the biaxially oriented polypropylene film was measured at the midpoint of the thread (5 m from each vertex). The same measurement was performed for two threads, and the average value was taken as the one-sided elongation of the biaxially oriented polypropylene film (unit: mm/10 m).

(8) Flatness Evaluation The appearance of the film roll wound in the slitting process was checked to confirm the state of wrinkles. Next, 1 m of the biaxially oriented polypropylene film was pulled out from the film roll, both ends were supported with fingers, and the state of wrinkles when pulled with a force of 10 N was confirmed. The evaluation was made in the order of excellent to poor, with poor being unsuitable for practical use.
A: No wrinkles were observed on the appearance of the film roll.
Δ: Wrinkles were observed on the appearance of the film roll, but when the film was pulled out and pulled with a force of 10 N, the wrinkles disappeared.
x: Wrinkles were observed in the appearance of the film roll, and the wrinkles did not disappear even when the film was pulled out and pulled with a force of 10 N.

(9) Evaluation of runnability The biaxially oriented polypropylene film taken up in the slitting process was unwound to 1000 m, and the maximum deviation between the center of the transport roll and the center of the film in the width direction was measured and evaluated according to the following criteria. The evaluation was made in the order of excellent to excellent, with × being unsuitable for practical use.
◯: The maximum deviation between the center of the transport roll and the center of the film in the width direction was less than 5 mm.
Δ: The maximum deviation between the center of the transport roll and the center of the film in the width direction was 5 mm or more and less than 10 mm.
x: The maximum deviation between the center of the transport roll and the center of the film in the width direction was 10 mm or more.

(10) Overall evaluation of processability When unwinding a biaxially oriented polypropylene film from a film roll and processing it such as laminating, it is necessary to have both "flatness" and "running property." If either one is lacking, problems such as wrinkles will occur in the film during processing. Therefore, based on the evaluation results of (8) and (9), an overall evaluation of processability was performed using the following criteria. Since flatness has a greater effect on processability than running property, flatness was given a greater weight in the evaluation. In the evaluation, ◎, ○, and △ were considered to be acceptable.
⊚: Both the flatness evaluation and the running performance evaluation were ◯.
◯: The flatness was evaluated as ◯, and the running performance was evaluated as Δ.
Δ: The flatness was evaluated as Δ, and the running performance was evaluated as ◯ or Δ.
×: At least one of the flatness evaluation and the running property evaluation was ×.

(11) Evaluation of Adhesion to Heat-Shrinkable Substrate The biaxially oriented polypropylene film was unwound from the film roll, and superimposed on an epoxy resin plate (G-10 manufactured by PCB Materials, thickness: 2.0 μm), and treated with a metal mirror roll heated to 160 ° C. under conditions of a linear pressure of 60 kg / cm and a speed of 40 m / min to obtain a sample of a laminate of a biaxially oriented polypropylene film and an epoxy resin plate. The obtained sample was aged for 3 hours in an atmosphere of a temperature of 25 ° C. and 50% RH. Then, the two were peeled off with a tensile tester under conditions of a load of 400 g / 15 mm and a speed of 300 mm / min. The adhesion was evaluated according to the following criteria based on wrinkles and twists when the two were bonded together and the ease of peeling.
⊚: Both the biaxially oriented polypropylene film and the substrate were free of wrinkles, twists, and air bubbles, and the biaxially oriented polypropylene film and the substrate could not be peeled off under the above tensile test conditions.
◯: There were no wrinkles, twists, or bubbles in either the biaxially oriented polypropylene film or the substrate, but the biaxially oriented polypropylene film and the substrate could be peeled off under the above tensile test conditions.
Δ: The biaxially oriented polypropylene film and the substrate could not be peeled off under the above tensile test conditions, but wrinkles, twists, or bubbles were observed in at least one of the biaxially oriented polypropylene film and the substrate.
×: Wrinkles, kinks, or bubbles were observed in at least one of the biaxially oriented polypropylene film and the substrate, and the biaxially oriented polypropylene film and the substrate could be peeled off under the above tensile test conditions.

(12) Winding Misalignment and End Face Evaluation After cutting, the state in which the film protrudes from the end face (the face perpendicular to the central axis) of the film roll was considered as winding misalignment. The presence or absence of such misalignment was confirmed, and the length was measured with a vernier caliper to determine the amount of winding misalignment. In addition, the end face of the film roll was observed, and the presence or absence of scratches, spoked wrinkles, flower patterns, and wheel-shaped wrinkles was visually confirmed. The evaluation was performed based on the obtained results according to the following criteria, with the evaluation results being in the order of ◎, ◯, △, and ×, with × being judged to be unsuitable for practical use. In addition, the end faces were observed on both sides, and if the evaluation results differed, the worse evaluation result was used.
⊚: The amount of winding misalignment was less than 5 mm (including cases where there was no winding misalignment), and no scratches or wrinkles were observed in visual inspection of the end surface of the film roll.
◯: The amount of winding misalignment was 5 mm or more and less than 10 mm, and neither scratches nor wrinkles were observed in visual inspection of the end surface of the film roll.
Δ: The amount of winding misalignment was 10 mm or more, but no scratches or wrinkles were observed in visual inspection of the end surface of the film roll.
x: At least one of scratches and wrinkles was observed in visual inspection of the end surface of the film roll.

[Example 1]
A homopolypropylene resin (MFR=1.3g/10min. hereinafter, sometimes referred to as resin A) was melt-extruded in a first extruder, and an ethylene-propylene random copolymer (MFR=4.0g/10min. hereinafter, sometimes referred to as resin B) was melt-extruded in a second extruder, and resin B/resin A/resin B were co-extruded in a die in the order of resin B/resin A/resin B in a sheet shape at 265°C using a T-die method, and then cooled with a cooling roll at 35°C to obtain an unstretched film (hereinafter, the layer made of resin A may be referred to as layer A, and the layer made of resin B may be referred to as layer B). The lamination thickness ratio at this time was resin B/resin A/resin B=1/58/1. The obtained unstretched film was then preheated at 150°C and stretched at 145°C in the longitudinal direction at a magnification of 4.6 times to obtain a uniaxially oriented film. Then, the uniaxially stretched film was guided to a tenter by holding both ends in the width direction with clips, preheated at 170°C, stretched in the width direction at 160°C at a ratio of 7.7 times, and further heat-treated at 150°C while applying 2.7% relaxation in the width direction. Then, the biaxially oriented polypropylene film after the heat treatment was guided outside the tenter through a cooling process at 145°C, and the clips were released. Then, both sides of the obtained biaxially oriented polypropylene film were subjected to corona treatment, and the film edge portion was slit with a winder to obtain a biaxially oriented polypropylene film. Finally, the film was cut to a width of 1500 mm with a slit under the following conditions to obtain a wound film roll. The evaluation results are shown in Table 1.
<Winding conditions at the slit>
Slitting speed: 180 m/min
Unwinding force: 650N
Initial winding tension: 80 N/m
Winding tension taper: 90%
Initial winding surface pressure: 300 N / m
Winding surface pressure taper: 100%.

[Examples 2 to 14, Comparative Examples 1 to 8]
A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the resin A (MFR), the stretching and heat treatment conditions in the machine direction and width direction, and the slitting conditions were as shown in Table 1. The evaluation results are shown in Table 1.

According to the present invention, a biaxially oriented polypropylene film having good flatness despite having high heat shrinkability in a specific direction in the film plane under high temperature environment can be provided. Since the polypropylene film of the present invention has the above-mentioned properties, it can be suitably used as a release film for precision components such as a cover film, a protective film, and a processing film.

Claims (6)

A biaxially oriented polypropylene film in which, when the main shrinkage direction of the film is the X direction, the heat shrinkage stress value in the X direction at 150°C is 0.40 N/2 mm or more and 0.80 N/2 mm or less, and the onset temperature of the heat shrinkage stress in the X direction is 80°C or more and 90°C or less. 2. The biaxially oriented polypropylene film according to claim 1, which satisfies formula 1 when the direction perpendicular to the X direction in the film plane is the Y direction, the heat shrinkage in the X direction at 140°C is Δ _140X %, the heat shrinkage in the Y direction at 140°C is Δ _140Y %, the heat shrinkage in the X direction at 160°C is Δ _160X %, and the heat shrinkage in the Y direction at 160°C is Δ _160Y %, respectively.
Formula 1: 0.10<(Δ _140X + Δ _140Y )/(Δ _160X + Δ _160Y )<0.40 The biaxially oriented polypropylene film according to claim 2, wherein the Δ _140X is 6.0 or more and 12.0 or less, and the Δ _160X is 30.0 or more and 50.0 or less. The biaxially oriented polypropylene film according to any one of claims 1 to 3, in which the film surface roughness SRa is 10 nm or more and 100 nm or less on at least one side. The biaxially oriented polypropylene film according to any one of claims 1 to 3, having a laminated structure of at least two layers and a thickness of 40 μm or more and 80 μm or less. A film roll comprising the biaxially oriented polypropylene film according to any one of claims 1 to 3 wound around a core.