JP2023135593A - Biaxially-oriented laminated polypropylene film and release film - Google Patents

Abstract

To provide a biaxially-oriented laminated polypropylene film with excellent releasability and windability.SOLUTION: A biaxially-oriented laminated polypropylene film has at least one face with a peeling force of 1.0 N or more and 2.2 N or less; at least one face with a minimum self-correlation length (Sal) of 41 μm or more and 100 μm or less; and a Sal difference between both faces of 5 μm or more and 40 μm or less.SELECTED DRAWING: None

Description

The present invention relates to a biaxially oriented laminated polypropylene film with excellent mold releasability and winding performance, and a mold release film using the same.

Polypropylene films such as biaxially oriented laminated polypropylene films have excellent properties such as transparency, mechanical properties, and electrical properties, so they are used for packaging, mold release, tape, electrical applications such as cable wrapping and capacitors, etc. It is used for various purposes. In particular, since it has excellent surface release properties and mechanical properties, it is suitably used as a release film or process film for various members such as plastic products, building materials, and optical members.

In recent years, polypropylene films are sometimes used as cover films for adhesive resin layers such as photosensitive resin layers. When using a polypropylene film as such a cover film, if the mold releasability is poor, the polypropylene film cannot be peeled off cleanly from the adherend, and the shape of the resin layer on the adherend may change or the adherend may Peeling marks may remain on the resin layer of the body. In order to solve these problems, a method is used in which the surface of the film is roughened to reduce the contact area with the resin layer, thereby improving the mold releasability. On the other hand, if the surface roughness of the cover film is increased, coarse protrusions are more likely to be formed, and the surface irregularities of the cover film caused by the coarse protrusions may be transferred to the resin layer of the adherend, affecting the visibility of the product. There is.

From the above, in order to use polypropylene film as a release film in the field of optical components, etc., which have high requirements for release properties and surface uniformity, it is necessary to have a uniform and finely roughened surface with few coarse protrusions. surface. For example, the following method is known as a means for obtaining a polypropylene film having such a dense and rough surface.

Patent Document 1 describes an example in which low melting point polypropylene is added to the surface of a polypropylene film and stretched at a temperature higher than the melting point to partially melt and roughen the surface, improving mold releasability and quality. There is. Patent Documents 2 to 5 describe examples in which a high-melting point polypropylene resin is used in the main layer of a polypropylene film to form a dense and rough surface, thereby improving surface smoothness and mold releasability. Further, Patent Document 6 describes an example in which a β-crystal nucleating agent is added to the surface layer to roughen the surface of the film more than the examples in Patent Documents 2 to 5, thereby improving mold releasability. In addition, Patent Documents 1 to 6 have in common that the size of craters on the film surface is controlled by stretching while adjusting the degree of formation of β-crystalline spherulites, which is one of the crystal forms of polypropylene. Examples of improved performance, quality, and process transportability are described.

International Publication No. 2020/071291 International Publication No. 2018/147334 International Publication No. 2017/169952 Japanese Patent Application Publication No. 2020-132877 JP 2021-20472 Publication International Publication No. 2016/006578

However, in the methods described in the above-mentioned Patent Documents 1 to 5, the surface roughness of the polypropylene film was insufficient, and there were problems in mold releasability, transportability, and winding performance. More specifically, in the method described in Patent Document 1, when used as a release film, the smooth surface adhered too strongly to the adherend, resulting in insufficient release properties. Furthermore, due to the flexible surface formed of a low melting point polypropylene resin or the like, the laminated films do not slide against each other when wound up into a roll, resulting in wrinkles. In the methods described in Patent Documents 2 to 6, the surface on which sharp protrusions formed from highly stereoregular polypropylene resin etc. are densely arranged comes into contact with the adherend, and the protrusions bite into the adherend, thereby forming a polypropylene film. The adherend adhered strongly and the mold releasability was insufficient.

Therefore, an object of the present invention is to solve the above-mentioned problems. That is, an object of the present invention is to provide a biaxially oriented laminated polypropylene film with excellent mold releasability and winding properties.

In order to solve the above-mentioned problems, the biaxially oriented laminated polypropylene film of the present invention has the following configuration. That is, the biaxially oriented laminated polypropylene film of the present invention has a peel force of 1.0 N or more and 2.2 N or less on at least one surface, and a minimum autocorrelation length (Sal) of at least one surface of 41 μm or more and 100 μm or less. This is a biaxially oriented laminated polypropylene film, characterized in that the difference in Sal on both sides is 5 μm or more and 40 μm or less.

In addition, the biaxially oriented laminated polypropylene film of the present invention can also be made into the following embodiments, and can also be suitably used as a release film.
(1) The peeling force on at least one side is 1.0N or more and 2.2N or less, the minimum autocorrelation length (Sal) on at least one side is 41μm or more and 100μm or less, and the difference in Sal on both sides is 5μm. A biaxially oriented laminated polypropylene film, characterized in that the film has a diameter of 40 μm or more.
(2) The biaxially oriented laminated polypropylene film according to (1), wherein the arithmetic mean height (Sa) of at least one surface is 80 nm or more and 700 nm or less, and the difference in Sa between both surfaces is 50 nm or more and 400 nm or less.
(3) The biaxially oriented laminated polypropylene film according to (1) or (2), wherein at least one surface has an aspect ratio (Str) of 0.40 or more and 0.90 or less.
(4) It has a surface layer (A layer) and a base layer (B layer), the A layer, the B layer, and the A layer are located in this order, and the A layer contains branched polypropylene in an amount of 10% by mass or more. The biaxially oriented laminated polypropylene film according to any one of (1) to (3), containing 30% by mass or less.
(5) Static friction coefficient (μs) and dynamic friction coefficient (μd) measured by contacting one surface with the opposite surface are both 0.40 or more and 0.55 or less, (1) to (4) The biaxially oriented laminated polypropylene film according to any one of the above.
(6) A release film comprising the biaxially oriented laminated polypropylene film according to any one of (1) to (5).

According to the present invention, it is possible to provide a biaxially oriented laminated polypropylene film which has excellent mold releasability and winding property and can be suitably used as a mold release film.

The biaxially oriented laminated polypropylene film of the present invention has a peel force of 1.0 N or more and 2.2 N or less on at least one surface, and a minimum autocorrelation length (Sal) of at least one surface of 41 μm or more and 100 μm or less. , the difference in Sal between both surfaces is 5 μm or more and 40 μm or less. Hereinafter, the biaxially oriented laminated polypropylene film of the present invention will be specifically explained.

Polypropylene film refers to a sheet-shaped molded product whose main component is polypropylene resin. The principal component refers to a component that is present in an amount greater than 50% by mass and less than or equal to 100% by mass when the total constituent components of the object (here, the film) are 100% by mass.Hereinafter, principal components will be interpreted in the same manner. be able to. In addition, when a plurality of components corresponding to polypropylene resin are included, if the total amount is more than 50% by mass, it is considered that polypropylene resin is the main component. Polypropylene resin refers to a resin whose main structural unit is a propylene unit, and the main structural unit refers to a structural unit that is contained in more than 50 mol% and less than 100 mol% when the total structural units are 100 mol%. The definition can be similarly interpreted for other olefin resins, except that the propylene units are replaced with other olefin units.) Here, biaxial orientation refers to molecular orientation in two orthogonal directions, and can be achieved by stretching an unstretched sheet in two orthogonal directions (usually the longitudinal direction and the width direction). Lamination refers to two or more types of layers having different compositions overlapping in the thickness direction.

From the viewpoint of mold releasability, the biaxially oriented laminated polypropylene film of the present invention has a peeling force of 1.0 N or more and 2.2 N or less on at least one surface, and a minimum autocorrelation length (Sal) of at least one surface. is 41 μm or more and 100 μm or less, and it is important that the difference in Sal on both surfaces is 5 μm or more and 40 μm or less. Note that "peel force is 1.0 N or more and 2.2 N or less" and "minimum autocorrelation length (Sal) is 41 μm or more and 100 μm or less" only need to be satisfied on at least one side, and not necessarily on both sides. Although it is not essential to satisfy the conditions at the same time, from the viewpoint of mold releasability, it is preferable to satisfy the conditions at the same time on the same side, and it is more preferable to satisfy conditions at the same time on both sides.

Peeling force is defined as when one side of a biaxially oriented laminated polypropylene film is laminated on a polyester adhesive tape with a tape width of 19 mm, and the polyester adhesive tape is peeled off at a peeling speed of 300 mm/min and a peeling angle of 180°. Represents the force (N) at a time. Here, the polyester adhesive tape is polyester adhesive tape No. manufactured by Nitto Denko Corporation. 31B or a polyester adhesive tape having adhesive strength equivalent to that.

The minimum autocorrelation length (Sal) is one of the three-dimensional parameters of the surface texture, and is a parameter that is an index of the period of the surface texture in the in-plane direction. To explain more specifically, the autocorrelation length (Sal) is the direction in which the autocorrelation function defined by ISO25178-2 (2012) attenuates fastest to a specific value s (s=0.2 in the present invention). is the horizontal distance of A small value of the minimum autocorrelation length (Sal) indicates that the surface has a finer (shorter period) unevenness in the in-plane direction, while a larger value of the minimum autocorrelation length (Sal) indicates that the surface has a rougher (longer period) uneven shape in the in-plane direction. Note that hereinafter, the "minimum autocorrelation length Sal" and the "difference between Sal on both sides" may be simply referred to as "Sal" and "Sal difference," respectively. By adopting an embodiment in which the Sal of at least one surface is 41 μm or more and 100 μm or less, when the surface of the biaxially oriented laminated polypropylene film is bonded to an adherend, excellent mold releasability and rollability can be achieved. Easy handling can be achieved.

Sal, the arithmetic mean height Sa, and the surface texture aspect ratio Str, which will be described later, can be measured using a known layer cross-sectional shape measuring device. A layer cross-sectional shape measuring system “VertScan” (registered trademark) 2.0 or the like can be used. Note that specific methods for measuring these parameters using the same device will be described later.

The biaxially oriented laminated polypropylene film of the present invention has a peeling force on at least one side of 1.0 N or more and 2.2 N or less, preferably 1.2 N or more and 2.0 N, from the viewpoint of mold releasability and winding property. It is as follows. Similarly, Sal is 41 μm or more and 100 μm or less, preferably 43 μm or more and 90 μm or less, and more preferably 45 μm or more and 85 μm or less on at least one surface. Note that it is preferable that the peeling force and Sal are simultaneously within the above ranges on one surface. The Sal difference is 5 μm or more and 40 μm or less, preferably 6 μm or more and 35 μm or less.

When a biaxially oriented laminated polypropylene film having no surface in which each of the above parameters is within the above range is used as a release film, various problems occur in terms of release properties as described below. If the peeling force of the surface to be bonded to the adherend (hereinafter sometimes referred to as the When laminating a biaxially oriented laminated polypropylene film with a polypropylene film, the adhesion may be insufficient and the film may peel off during the conveyance process. On the other hand, if the peeling force in the X plane exceeds 2.2N, the adherend and the biaxially oriented laminated polypropylene film will adhere too strongly. If the adhesion between the two becomes too strong, when the biaxially oriented laminated polypropylene film is peeled off from the adherend, the biaxially oriented laminated polypropylene film may pull a part of the surface of the adherend and cause it to peel off, or This may cause the adherend to break.

Further, if the Sal of both the X-plane and the opposite side is less than 41 μm, a problem occurs due to the protrusion size on the surface of the biaxially oriented laminated polypropylene film becoming excessively small. Specifically, when the Sal of the X-plane is less than 41 μm, the protrusions tend to bite into the adherend, resulting in excessively strong adhesion between the X-plane and the adherend, and the biaxially oriented laminated polypropylene film is removed from the adherend. When peeling off the adherend, part of the adherend may be peeled off, dents from the protrusions may remain on the surface of the adherend, or the adherend may be broken. Furthermore, if the Sal on the opposite side is less than 41 μm, the protrusions on the surfaces that come into contact when the biaxially oriented laminated polypropylene film is wound up into a roll will not interfere with each other, and the biaxial The oriented laminated polypropylene film becomes more slippery than necessary, which causes winding misalignment in the film roll.

On the other hand, when the Sal of both the X-plane and the opposite side exceeds 100 μm, problems arise due to excessively large protrusions on the surface of the biaxially oriented laminated polypropylene film. Specifically, when the Sal of the X-plane exceeds 100 μm, the protrusion size becomes excessively large and the surface becomes smooth, resulting in strong adhesion between the X-plane and the adherend, which deteriorates the mold releasability. There is. Furthermore, if the Sal on the opposite side exceeds 100 μm, the protrusions on the surfaces that come into contact when the biaxially oriented laminated polypropylene film is wound up into a roll will interfere with each other excessively, resulting in The slipperiness of the film deteriorates, causing wrinkles in the film roll.

In addition, if the difference in Sal on both sides of the biaxially oriented laminated polypropylene film is less than 5 μm, there will be no difference in protrusion size between the front and back sides, resulting in poor slipperiness and wrinkles when wound into a roll. There is. On the other hand, when the difference in Sal on both sides of a biaxially oriented laminated polypropylene film exceeds 40 μm, the disparity in protrusion size between the front and back sides becomes maximum, resulting in high slipperiness, causing the films to slip against each other when wound into a roll. A deviation in the direction may occur.

That is, the biaxially oriented laminated polypropylene film of the present invention has a peel force of 1.0 N or more and 2.2 N or less on at least one surface, a Sal of 41 μm or more and 100 μm or less on at least one surface, and a Sal of 41 μm or more and 100 μm or less on both surfaces. When the difference is 5 μm or more and 40 μm or less, damage to the adherend is reduced and the behavior of the film is stabilized during roll-up. Therefore, it has excellent mold releasability and winding property, and can be suitably used as a mold release film.

In order to make the difference between the peel force and Sal on at least one side and the Sal on both sides within the above range, it is effective to set the raw material composition and film forming conditions of the biaxially oriented laminated polypropylene film to the ranges described below. be. More specifically, a resin composition containing a branched polypropylene resin in a preferred amount as 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 single layer film), and casting. By setting the surface temperature of the drum within the preferred range described below and setting the film temperature (preheating temperature) during longitudinal stretching within the range described below, the peeling force and Sal and Sal difference are controlled within the preferred range. be able to. The term "longitudinal direction" as used herein refers to the direction in which the film travels in the film manufacturing process, and in the case of a film roll, refers to the direction in which the film is wound. Further, the direction perpendicular to the longitudinal direction within the film plane is referred to as the width direction.

The biaxially oriented laminated polypropylene film of the present invention has a peel force of 1.0 N or more and 2.2 N or less on at least one surface, a Sal of 41 μm or more and 100 μm or less of at least one surface, and a difference in Sal of both surfaces. There is no particular limitation as long as it is 5 μm or more and 40 μm or less, but from the viewpoint of mold releasability and winding property, it is more preferable to have a surface with both peeling force and Sal within the above range, and furthermore, it is preferable to have a surface with a peeling force and Sal in the above range. In view of improving the conveyance and winding properties of the biaxially oriented laminated polypropylene film itself, it is more preferable that both the peeling force and Sal of both surfaces are within the above ranges.

The biaxially oriented laminated polypropylene film of the present invention has an arithmetic mean height (Sa) of 80 nm or more on at least one surface from the viewpoint of reducing damage to adherends and improving film transportability and winding performance. It is preferably 700 nm or less, more preferably 150 nm or more and 650 nm or less, and still more preferably 250 nm or more and 600 nm or less. Further, from the same viewpoint, the difference in Sa between both surfaces is preferably 50 nm or more and 400 nm or more, more preferably 55 nm or more and 350 nm or less. Note that hereinafter, "arithmetic mean height Sa" and "difference between Sa on both sides" may be simply referred to as "Sa" and "Sa difference."

Arithmetic mean height (Sa) is one of the three-dimensional parameters of surface texture defined in ISO25178-2 (2012), and is the absolute value of the difference in height of each point with respect to the average height of the surface. This is a parameter that represents the average.

By setting Sa of at least one, preferably both, of the X-plane and the opposite side to 80 nm or more, the adhesion between the adherend and the biaxially oriented laminated polypropylene film does not become excessively large, thereby improving mold releasability. This can prevent the biaxially oriented laminated polypropylene film from pulling a part of the surface of the adherend and causing it to peel off, and from causing dents on the surface of the adherend. In addition, the roughened surface that comes into contact with the film when it is wound into a roll allows it to slip properly, and wrinkles that occur during the transport process and between the roll layers are naturally alleviated by the firmness of the film, making it easier to wind. will improve.

On the other hand, if the Sa of at least one, preferably both, of the X-plane and the opposite side is 700 nm or less, the adhesion force is insufficient when bonding the adherend and the biaxially oriented laminated polypropylene film, resulting in a transfer process. In addition, when the film is rolled up into a roll, the contact surface does not become excessively rough, and the film that is being laminated into a roll is properly fixed, preventing roll misalignment. can be suppressed.

Furthermore, when the Sa difference is 50 nm or more, the films can appropriately slide against each other when wound up into a roll, thereby suppressing the generation of wrinkles. On the other hand, when the Sa difference is 400 nm or less, it is possible to suppress deviation in the width direction due to excessive slipping of the films when wound into a roll.

In order to make the Sa of at least one surface in the range of 80 nm or more and 700 nm or less, it is effective to set the raw material composition of the film in the range described below, and to set the film forming conditions in the range described below. More specifically, a method similar to the method for controlling the peeling force, Sal, and Sal difference within a preferable range can be used. In addition, from the viewpoint of suppressing the transfer of dents on the back side and transporting and winding properties of the film, it is preferable that Sa on both sides of the biaxially oriented laminated polypropylene film is 80 nm or more and 700 nm or less, and 150 nm or more and 650 nm or less. is more preferable, and even more preferably 250 nm or more and 600 nm or less. Further, from the same viewpoint, the Sa difference is preferably 55 nm or more and 350 nm or less, and more preferably 60 nm or more and 300 nm or less.

The biaxially oriented laminated polypropylene film of the present invention has a surface texture with an aspect ratio Str of 0.40 or more and 0.90 or less on at least one surface from the viewpoint of improving mold releasability, transportability, and winding property. It is preferably 0.45 or more and 0.85 or less. Note that hereinafter, the "surface texture aspect ratio Str" may be simply referred to as "Str."

Str is one of the three-dimensional parameters of the surface texture, and is used as an index of isotropy of the surface texture. To explain more specifically, Str is the horizontal distance (as described above) in the direction in which the autocorrelation function specified by ISO25178-2 (2012) attenuates fastest to a specific value s (s = 0.2 in the present invention). (equivalent to Sal) divided by the horizontal distance in the direction of slowest decay to the value s. In other words, the Str of a biaxially oriented laminated polypropylene film theoretically has a value between 0 and 1, and the fact that Str is close to 0 means that the biaxially oriented laminated polypropylene film has a surface texture with a regular pattern with high anisotropy. On the other hand, when Str is close to 1, it means that the biaxially oriented laminated polypropylene film has a highly isotropic random pattern surface texture.

In the X plane of the biaxially oriented laminated polypropylene film, when Str is 0.40 or more, the surface texture of the plane becomes a random pattern with appropriate anisotropy. Therefore, when the adherend and the biaxially oriented laminated polypropylene film are laminated on that surface, unevenness in adhesion is suppressed, and mold releasability can be improved. It is possible to prevent part of the surface of the adherend from being pulled and peeled off, and to prevent dents on the surface of the adherend. Furthermore, by setting Str to 0.40 or more on at least one surface of the biaxially oriented laminated polypropylene film, air trapped when the biaxially oriented laminated polypropylene film is wound into a roll can be appropriately eliminated. Therefore, the stiffness of the film itself has the effect of naturally alleviating wrinkles generated during the conveyance process or between the roll layers, and the winding properties are improved. On the other hand, since Str is 0.90 or less on at least one surface of the biaxially oriented laminated polypropylene film, uneven adhesion is moderate when the adherend and the biaxially oriented laminated polypropylene film are bonded to the adherend. Since the film remains on the surface, peeling between the two during the conveyance process can be reduced, and mold releasability is improved. From the above viewpoint, it is preferably 0.40 or more and 0.90 or less or the above preferable range on the X plane, and more preferably 0.40 or more and 0.90 or less or the above preferable range on both sides.

In order to set the Str of at least one side within the above range, it is effective to set the raw material composition and film forming conditions of the biaxially oriented laminated polypropylene film to the ranges described below. More specifically, a method similar to the method for controlling the peeling force, Sal, and Sal difference within a preferable range can be used.

The biaxially oriented laminated polypropylene film of the present invention has a surface layer (layer A) and a base layer (layer B) from the viewpoint of achieving both heat resistance and mold release properties, and the layer A, layer B, and layer A are arranged in this order. It is preferable that the A layer contains 10% by mass or more and 30% by mass or less of branched polypropylene. "Layer A contains branched polypropylene in an amount of 10% by mass or more and 30% by mass or less" means that the content of branched polypropylene in layer A is 10% by mass when all components constituting layer A are 100% by mass. It means that it is not less than 30% by mass and not more than 30% by mass. From the above viewpoint, the amount of branched polypropylene in layer A is more preferably 13% by mass or more and 27% by mass or less, and even more preferably 16% by mass or more and 24% by mass or less. "Layer A contains branched polypropylene in an amount of 10% by mass or more and 30% by mass or less" means that at least one of the A layers contains branched polypropylene in an amount of 10% by mass or more and 30% by mass or less. It is more preferable that layer A contains branched polypropylene in an amount of 10% by mass or more and 30% by mass or less.

In addition, in the biaxially oriented laminated polypropylene film of the present invention, from the viewpoint of surface roughness controllability, the B layer also contains branched polypropylene within a range below the content (mass%) of branched polypropylene in the A layer. Polypropylene may also be included. When layer B contains branched polypropylene, it is preferably 15% by mass or less, more preferably 10% by mass or less, and 5.0% by mass or less, more preferably 10% by mass or less when the total components constituting layer B are 100% by mass. It is more preferably at most 2.5% by mass, particularly preferably at most 2.5% by mass.

The branched polypropylene referred to here is a polypropylene resin having internal tri-substituted propylene at 5 or less locations in 10,000 carbon atoms in the molecular chain, and the presence of this internal tri-substituted propylene is reflected in ^{the 1} H-NMR spectrum. This can be confirmed by the proton ratio.

By adopting such an aspect, the layer (B layer) that plays the role of ensuring thermal dimensional stability as a biaxially oriented laminated polypropylene film and excellent mold releasability when bonded to an adherend are realized. Since a layer (layer A) that plays the role of oxidation can be provided, it is easy to produce a biaxially oriented laminated polypropylene film that has both heat resistance and mold releasability.

In addition, by containing 10% by mass or more of branched polypropylene in layer A, the size of spherulites generated in the cooling process of the melt-extruded polypropylene resin can be increased, resulting in excellent mold release properties. A biaxially oriented laminated polypropylene film can be obtained. On the other hand, since the amount of branched polypropylene contained in layer A is 30% by mass or less, the size of spherulites generated in the cooling process of the melt-extruded polypropylene resin does not become excessively large. Thereby, the height of the protrusion does not become excessively high, and the adherend can be peeled off from the surface of the A layer without damaging the adherend on the surface of the A layer (corresponding to the above-mentioned X plane).

Furthermore, by containing branched polypropylene in layer B, more protrusions can be formed from the inside as well. Thereby, the surface of the biaxially oriented laminated polypropylene film can be made into a suitably dense rough surface, and the surface roughness can be controlled more broadly. On the other hand, if the amount of branched polypropylene in the B layer is excessive, excessive protrusion formation occurs, so that the surface of the biaxially oriented laminated polypropylene film becomes smooth beyond the moderately dense and rough surface, and the mold releasability deteriorates. By setting the content of branched polypropylene in layer B to 15% by mass or less, smoothing due to the above mechanism is suppressed, so it is easy to obtain a biaxially oriented laminated polypropylene film with excellent mold release properties. Moreover, it is possible to reduce winding misalignment due to deterioration of slipperiness.

In addition, the biaxially oriented laminated polypropylene film of the present invention has one surface in contact with the opposite surface in order to reduce problems such as meandering and wrinkles during transport, winding, and unwinding of the film. The measured static friction coefficient (μs) and dynamic friction coefficient (μd) are preferably both 0.40 or more and 0.55 or less, more preferably both 0.42 or more and 0.53 or less. The coefficient of static friction and the coefficient of dynamic friction referred to here mean that under an environment where the temperature is 23 ± 3°C and the humidity is 65 ± 5%, films are stacked so that different surfaces are in contact with each other, and a load of 200 g is applied from above. When the upper film is slid, it is a value calculated from the maximum resistance value at the moment it starts sliding and the resistance value while it is sliding, using the following formula. Note that hereinafter, the "static friction coefficient (μs)" and the "dynamic friction coefficient (μd)" may be collectively referred to as a friction coefficient, and both can be calculated from the following equation.
Friction coefficient = resistance value (g) / load (g) Formula (1).

Since the coefficient of friction of the biaxially oriented laminated polypropylene film is 0.40 or more, a certain degree of slipperiness is ensured, so it is possible to reduce problems such as wrinkles occurring in the film during transportation and winding. This leads to stabilization of the winding shape. On the other hand, when the coefficient of friction is 0.55 or less, the slipperiness is not excessive, and meandering and winding deviation that occur when the biaxially oriented laminated polypropylene film is wound up as a roll can be reduced.

The friction coefficient can be adjusted, for example, by adjusting Sa, and more specifically, the friction coefficient can be increased by lowering Sa. Note that the method for adjusting Sa is as described above.

Next, the layer structure and raw materials of the biaxially oriented laminated polypropylene film of the present invention will be explained, but the layer structure and raw materials of the biaxially oriented laminated polypropylene film of the present invention are not necessarily limited thereto.

In the biaxially oriented laminated polypropylene film of the present invention, the content of polypropylene resin is preferably 95% by mass or more and 100% by mass or less, more preferably 96% by mass or less, when all components constituting the film are 100% by mass. The content is preferably 97% by mass or more and 100% by mass or less, particularly preferably 98% by mass or more and 100% by mass or less.

The film thickness of the biaxially oriented laminated polypropylene film of the present invention is not particularly limited, but from the viewpoint of handling properties, it is preferably 1 μm or more and 100 μm or less, more preferably 5 μm or more and 50 μm or less, and 10 μm or more and 30 μm or less. It is even more preferable that there be. The film thickness can be measured with a known electronic micrometer (details will be described later), and can be adjusted by adjusting the screw rotation speed of the extruder, the width of the unstretched sheet, the film forming speed, the stretching ratio, etc.

The main component of the base layer (layer B) of the biaxially oriented laminated polypropylene film of the present invention is preferably a polypropylene resin. Hereinafter, the polypropylene resin that is the main component of the base layer (layer B) of the biaxially oriented laminated polypropylene film may be referred to as polypropylene resin I. The polypropylene resin I of the base layer (B layer) is more preferably 90% by mass or more and 100% by mass or less, still more preferably 95% by mass or more and 100% by mass or less, even more preferably 96% by mass or more and 100% by mass or less, particularly preferably is 97% by mass or more and 100% by mass or less, most preferably 98% by mass or more and 100% by mass or less.

The polypropylene resin I in the biaxially oriented laminated polypropylene film of the present invention is preferably homopolypropylene from the viewpoints of strength and heat resistance. Homopolypropylene refers to a polypropylene resin in which 99 mol% or more and 100 mol% or less are propylene units, when the total constituent units constituting the resin are 100 mol%.

The polypropylene resin I preferably has a melting point of 155°C or higher, more preferably 160°C or higher. When the melting point is 155° C. or higher, the heat resistance of the biaxially oriented laminated polypropylene film is improved. Therefore, when a biaxially oriented laminated polypropylene film is used, for example, as a release film, the biaxially oriented laminated polypropylene film may soften during the process of applying heat after being bonded to an adherend, and the tension may be applied in the direction of the softening. This suppresses the elongation of the adherend and reduces the deformation of the adherend. Further, the melting point is preferably 200°C or lower, more preferably 190°C or lower, and still more preferably 180°C or lower. Since the melting point of the polypropylene resin I is 200° C. or lower, restrictions on equipment during melt extrusion are unlikely to occur.

The mesopentad fraction of the polypropylene resin I is preferably 0.90 or more, more preferably 0.93 or more, still more preferably 0.94 or more. The mesopentad fraction is an index indicating the stereoregularity of the crystalline phase of polypropylene measured by nuclear magnetic resonance (NMR), and generally, the higher the value, the higher the crystallinity and melting point. Therefore, by using polypropylene with a high mesopentad fraction as the polypropylene resin I, the dimensional stability at high temperatures when formed into a film becomes high. Although the upper limit of the mesopentad fraction is not particularly specified, it is set to 0.99 from the viewpoint of use in film formation.

In order to obtain polypropylene with a high mesopentad fraction, there are several methods, such as washing the obtained resin powder with a solvent such as n-heptane, selecting the catalyst and/or co-catalyst, and selecting the composition as appropriate. Preferably adopted. Generally, a polymerization catalyst containing a metallocene compound having a cyclopentadienyl skeleton in the molecule is preferably used as the catalyst.

In addition, from the viewpoint of film formability and strength when formed into a film, the polypropylene resin I preferably has a melt flow rate (MFR) of 1 g/10 minutes or more and 10 g/10 minutes or less, more preferably 1 g/10 minutes. It is 8 g/10 minutes or more, more preferably 2 g/10 minutes or more and 5 g/10 minutes or less. The MFR referred to here refers to the MFR measured under the conditions of a temperature of 230° C. and a load of 21.18 N, and the same applies to MFR hereinafter. In order to bring the melt flow rate (MFR) of the polypropylene resin to the above value, a method of controlling the average molecular weight and molecular weight distribution is adopted. More specifically, the MFR can be increased by decreasing the number average or weight average molecular weight or molecular weight distribution.

The polypropylene resin I may contain copolymerization components of unsaturated hydrocarbons other than the main structural units, as long as the object of the present invention is not impaired. Monomers constituting such a copolymerization component include, for example, 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, 5-methyl-2-norbornene and the like. The amount of copolymerization is preferably less than 1 mol% from the viewpoint of dimensional stability when formed into a film.

Further, the base layer (layer B) may contain a polyolefin resin other than the polypropylene resin I, which is the main component. Here, the polyolefin resin other than polypropylene resin I refers to a polyolefin resin whose main structural unit is different from that of polypropylene resin I. The main constituent units of polyolefin resins other than polypropylene resin I include ethylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethyl Examples include structural units derived from hexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, 5-methyl-2-norbornene, and the like. The content of polyolefin resins other than polypropylene resin I is preferably 15% by mass or less, and preferably 10% by mass or less, when all components constituting the base layer (B layer) are 100% by mass. More preferred. Further, the base layer (layer B) can also contain a branched polypropylene resin as a polyolefin resin other than polypropylene resin I from the viewpoint of controllability of surface roughness, and the preferable content thereof is as described above.

When the polypropylene resin I is a polypropylene resin and contains polyethylene as a polyolefin resin other than the polypropylene resin I, the polyethylene resin contained in the base layer (layer B) is preferably 10% by mass or less, more preferably 5% by mass. % or less, more preferably 3% by mass or less. The higher the polyethylene resin content, the lower the crystallinity when formed into a film, making it easier to improve transparency. On the other hand, by setting the content of polyethylene resin to 10% by mass or less, it is possible to reduce the decrease in strength and heat resistance when made into a film, and also to reduce the deterioration of the resin during the extrusion process. The occurrence of fish eyes can also be suppressed.

Next, the polypropylene resin used for the surface layer (layer A) of the biaxially oriented laminated polypropylene film of the present invention will be explained. For the surface layer (layer A) of the present invention, from the viewpoint of achieving both heat resistance and mold releasability, it is preferable to use a resin composition in which polypropylene resin I contains a branched polypropylene resin. The branched polypropylene resin in the surface layer (layer A) is preferably 10% by mass or more and 30% by mass or less, more preferably 13% by mass or more and 27% by mass or less, and 16% by mass or more and 24% by mass, based on all the components constituting the layer. % or less is more preferable.

As the branched polypropylene resin, it is preferable to use a polypropylene resin having a branched structure in its molecular chain. In addition, "polypropylene resin having a branched structure in the molecular chain" is a polypropylene resin having internal trisubstituted propylene at 5 or less locations in 10,000 carbon atoms in the molecular chain, and the presence of internal trisubstituted propylene can be confirmed by the proton ratio in the ¹ H-NMR spectrum. Further, the MFR of the branched polypropylene resin is preferably 1.0 g/10 minutes or more and 10.0 g/10 minutes or less from the viewpoint of film forming stability. Note that the same branched polypropylene resin used for layer B is also suitable.

Examples of branched polypropylene resins that can be suitably used for the surface layer (layer A) and base layer (layer B) of the biaxially oriented laminated polypropylene film of the present invention include "Profax" (registered trademark) manufactured by Lyondell Basell Co., Ltd. PF-814, etc.), "Daploy" (trademark) manufactured by Borealis (WB130HMS, WB135HMS, etc.), "WAYMAX" (registered trademark) manufactured by Nippon Polypro Co., Ltd. (MFX3, MFX6, MFX8, EX6000, EX8000, etc.).

Although the branched polypropylene resin itself has an α-crystal or β-crystal nucleating effect, other types of α-crystal nucleating agents (for example, dibenzylidene sorbitol, (sodium benzoate, etc.), β-crystal nucleating agents (e.g., potassium 1,2-hydroxystearate, magnesium benzoate, amide compounds such as N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide, etc.), quinacridone compounds, etc. ) etc. may be added. In addition, these crystal nucleating agents may be added to either the base layer (B layer) or the surface layer (A layer), or may be added to both.

However, excessive addition of the above-mentioned different types of nucleating agents may cause a decrease in transparency and strength due to decreases in film stretchability and void formation, etc., so the amount added should be 100% by mass of all components constituting the film. It is usually 0.5% by mass or less, preferably 0.1% by mass or less, and more preferably 0.05% by mass or less.

The biaxially oriented laminated polypropylene film of the present invention may contain various additives such as antioxidants, heat stabilizers, slip agents, antistatic agents, antiblocking agents, fillers, and viscosity additives within the range that does not impair the purpose of the present invention. It is also possible to contain a regulating agent, a coloring preventive agent, and the like. Among these, selection of the type and amount of antioxidant is important from the viewpoint of antioxidant bleed-out. As such an antioxidant, a phenol type antioxidant having steric hindrance properties is preferable, and when multiple types of antioxidants are used together, at least one type is preferably a high molecular weight type with a molecular weight of 500 or more. There are various specific examples thereof, including 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) and 1,3,5-trimethyl-2,4,6 -tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (for example "Irganox" 1330 manufactured by BASF, molecular weight 775.2) or tetrakis[methylene-3(3,5-di -t-butyl-4-hydroxyphenyl)propionate] methane (for example, "Irganox" (registered trademark) 1010 manufactured by BASF, molecular weight 1177.7) is preferably used in combination.

The total content of these antioxidants was set to 100% by mass of the entire raw material for obtaining the biaxially oriented laminated polypropylene film, from the viewpoint of reducing coloration due to polymer deterioration and decrease in transparency due to antioxidant bleed-out. Sometimes, a range of 0.03% or more and 1.0% by mass or less is preferable. When the antioxidant content is 0.03% by mass or more, it is possible to reduce coloration of the film and decrease in long-term heat resistance due to deterioration of the polymer during the extrusion process. On the other hand, when the antioxidant content is 1.0% by mass or less, a decrease in transparency due to antioxidant bleed-out can be suppressed. From the above viewpoint, 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. In addition, these antioxidants may be added to either the base layer (B layer) or the surface layer (A layer), or may be added to both.

Further, the biaxially oriented laminated polypropylene film of the present invention preferably does not contain organic particles or inorganic particles. For example, the polypropylene resin that can be suitably used in the biaxially oriented laminated polypropylene film of the present invention has a low affinity with organic particles and inorganic particles, so these particles fall off during the manufacturing process and contaminate the manufacturing line and products. In some cases, coarse protrusions formed by highly hard particles may be transferred to the resin layer of the adherend as irregularities. Therefore, when used as a protective film for optical components used in products that require high quality, such as display components, or as a manufacturing base film, it is important that biaxially oriented laminated polypropylene films do not contain lubricants such as organic particles or inorganic particles. preferable.

As the method of biaxial stretching to obtain the biaxially oriented laminated polypropylene film of the present invention, any of the simultaneous inflation biaxial stretching method, the simultaneous biaxial stretching method with a stenter, and the sequential biaxial stretching method with a stenter may be adopted. Among these, it is preferable to employ the stenter sequential biaxial stretching method in terms of controlling film formation stability, thickness uniformity, 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 the width direction are performed in separate steps, and at least the stretching in the width direction is performed using a stenter.

Next, one embodiment of the method for producing a biaxially oriented laminated polypropylene film of the present invention will be described, but the method is not necessarily limited thereto. First, polypropylene resin I or a mixture of polypropylene resin I and branched polypropylene resin is supplied to a single screw extruder for the base layer (layer B), and the mixture of polypropylene resin I and branched polypropylene resin is supplied to the surface layer (layer A). ), then melt extrusion is performed at a temperature of 200°C or higher and 280°C or lower, more preferably 220°C or higher and 280°C or lower, and still more preferably 240°C or higher and 270°C or lower. After removing foreign substances and modified polymers using a filter installed in the middle of the polymer tube, the layers are stacked using a multi-manifold type composite T-die to form layer A/layer B/layer A on a casting drum. An unstretched sheet is obtained by discharging the mixture onto the top and cooling and solidifying it.

At this time, casting is performed in two stages, and the surface temperature of both casting drums is preferably 40°C or more and 100°C or less, more preferably 50°C or more and 100°C or less, and even more preferably 60°C or more and 100°C or less. . By setting the surface temperature of the two casting drums within the above range, spherulites of appropriate size can be provided, and a biaxially oriented laminated polypropylene film with excellent mold release properties can be obtained. Furthermore, it is preferable that the two casting drums have different temperatures. In this case, the difference in surface temperature between the two casting drums (surface temperature of the first casting drum - surface temperature of the second casting drum) is preferably 5°C or more and 60°C or less, more preferably 10°C or more and 50°C or less. , more preferably 15°C or more and 40°C or less. By using two-stage casting drums with different temperatures, it is possible to create a difference in the size of the spherulites on both sides of the film, improving the winding performance.

Any method may be used to adhere the film to the casting drum, such as an electrostatic application method, an adhesion method using the surface tension of water, an air knife method, a press roll method, or an underwater casting method, but depending on the flatness of the film, The air knife method is preferred from the viewpoint of improving the surface roughness and making it easier to control the surface roughness. The air temperature of the air knife is preferably 40° C. or more and 80° C. or less, and the blowing air speed is preferably 130 m/s or more and 150 m/s or less. Further, in order to prevent vibration of the sheet, it is also preferable to adjust the position of the air knife as appropriate so that air flows downstream of film formation.

Next, the obtained unstretched sheet is biaxially stretched and biaxially oriented. Specific stretching and preheating conditions are as follows. As the preheating method, a method using a temperature-controlled rotating roll, a method using a hot air oven, etc. can be used alone or in an appropriate combination. The preheating temperature for both sides of the unstretched film is preferably 130° C. or higher and 160° C. or lower, more preferably 135° C. or higher and 160° C. or lower, from the viewpoint of controlling protrusions on the film surface within a preferable range by high-temperature stretching. More preferably, the temperature is 140°C or higher and 154°C or lower. At this time, it is preferable that the preheating temperatures for both sides of the unstretched film are different, and the preheating temperature difference for both sides of the unstretched film is within the above-mentioned preferred range of 5°C or more and 30°C or less, more preferably 5°C or more and 20°C or less, and Preferably it is 5°C or more and 13°C or less. By providing a difference in preheating temperature to both sides of the unstretched film, it is possible to provide a difference in the height of the protrusions on both sides, and the winding property is improved.

After preheating under the above conditions, from the viewpoint of controlling the protrusions on the film surface within a preferable range by high-temperature stretching, the stretching temperature when stretching in the longitudinal direction is preferably 140°C or higher and 160°C or lower, more preferably 144°C. The temperature is preferably 148°C or more and 155°C or less, more preferably 148°C or more and 155°C or less. Further, from the same viewpoint, it is preferable that the stretching temperature in the longitudinal direction is equal to or higher than the preheating temperature of the surface having a relatively higher preheating temperature.

The stretching ratio is preferably 3.0 times or more and 6.0 times or less, more preferably 4.0 times or more and 5.7 times or less, in order to control the mechanical properties and thermal properties within a preferable range. Preferably it is 4.5 times or more and 5.5 times or less. When the stretching ratio is 3.0 times or more, more uniform stretching can be achieved and thickness unevenness can be suppressed. On the other hand, when the stretching ratio is 6.0 times or less, film breakage in the longitudinal stretching process and the next horizontal stretching process is reduced.

Next, the longitudinally uniaxially stretched film obtained by stretching in the longitudinal direction is once cooled to 10° C. or more and 70° C. or less. When the cooling temperature is 10° C. or higher, curling of the film can be suppressed. On the other hand, when the cooling temperature is 70° C. or lower, promotion of thermal crystallization can be suppressed. Thereafter, the sheet is guided into a tenter, its widthwise ends are gripped with clips, preheated, and then laterally stretched by 7.0 times or more and 13 times or less in the width direction. Considering the viewpoint of controlling the protrusions on the film surface within a preferable range by high-temperature stretching, the preheating and stretching temperature is 156°C or higher and 166°C or lower, more preferably 159°C or higher and 165°C or lower, and even more preferably 160°C or higher and 164°C or higher. below ℃.

Furthermore, heat treatment may be performed in a tenter as it is, and at this time, in order to control the heat shrinkage rate in the width direction, the heat treatment temperature is preferably 110°C or more and less than 170°C, and preferably 150°C or more and less than 165°C. and more preferable. Further, the heat treatment may be performed while relaxing the film in the width direction, and in particular, the relaxation rate in the width direction is set to 2.0% or more and 20.0% or less, more preferably 7.0% or more and 15.0% or less. By doing so, the heat shrinkage rate in the width direction can be set within an appropriate range, and the balance of dimensional stability can be optimized.

The biaxially oriented laminated polypropylene film obtained as described above can be used in various applications such as packaging films, surface protection films, process films, sanitary products, agricultural products, construction products, and medical products, but especially Since it has excellent surface smoothness, it can be preferably used as a surface protection film, a process film, and a release film.

Hereinafter, the biaxially oriented laminated polypropylene film of the present invention will be explained in more detail with reference to Examples, but the biaxially oriented laminated polypropylene film of the present invention is not limited to such embodiments. In addition, the characteristics were measured and evaluated by the following method.

[Evaluation method of each characteristic]
(1) Film Thickness Five biaxially oriented laminated polypropylene films were stacked to form a measurement sample, and the thickness of the measurement sample was measured using an electronic micrometer (TT80 manufactured by TESA). Next, the obtained value was divided by 5 to calculate the thickness per biaxially oriented laminated polypropylene film. Similar measurements were performed using a measurement point further shifted by 50 mm in the width direction, and the same measurements were repeated five times in total. From the obtained measurement results, the maximum value, minimum value, and average value of the thickness per biaxially oriented laminated polypropylene film were determined, and the average value was taken as the film thickness (μm).

(2) Minimum autocorrelation length (Sal), arithmetic mean height (Sa), aspect ratio of surface texture (Str)
Measurement was performed using a non-contact surface/layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 (model: R3300GL-Lite-AC) manufactured by Ryoka System Co., Ltd. under the following procedure and conditions. First, a biaxially oriented laminated polypropylene film is unwound from a film roll, and measurement samples are taken at 10 randomly determined points on a straight line passing through the center in the width direction and parallel to the longitudinal direction. Sal, Sa, and Str were measured at the 10 locations. The average value of each of the obtained measured values was calculated and used as Sal and Sa of the biaxially oriented laminated polypropylene film. In addition, in one measurement, one visual field (field area: 939 μm vertically×1,252 μm horizontally = 1,175,628 μm ² ) was measured.

A. Measurement conditions CCD camera: SONY HR-57 1/2
Objective lens: 10X
Lens barrel: 0.5X BODY
Wavelength filter: 530 white
Measurement mode: Wave
Field of view size: 640 x 480
Scan range: (start) 5 μm, (stop) -5 μm.

B. Method for fixing the measurement sample A dedicated sample holder was used to fix the measurement sample. The sample holder consists of two removable metal plates with a circular hole in the center, and the measurement sample was sandwiched and fixed between them without wrinkles, and the measurement sample located in the circular hole was measured.

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

(3) Peeling force/mold release property evaluation Nitto Denko Corporation polyester adhesive tape NO. 31B was applied with a roller and cut into a 19 mm width to prepare a sample. The sample was peeled using a tensile testing machine (model: VPA-2) manufactured by Kyowa Interface Science Co., Ltd. at a peeling rate of 300 mm/min and a peeling angle of 180°, and the value output by the tensile testing machine was used as the peeling force. (N). The releasability was evaluated according to the following criteria (◯ and △, the film could be used as a release film without any problems and was considered to be a pass). Note that the peeling force was evaluated on both sides. After evaluating the mold releasability on both sides, the side with the better evaluation when applied to the following three criteria was adopted.
Good: No delamination occurred between the surface layer and the base layer, and peeling was possible at a constant speed.
Δ: No interlayer peeling occurred between the surface layer and the base layer, but peeling resistance was somewhat high and peeling was unstable.
×: None of the requirements of 〇 or △ were satisfied, and the peeling behavior was accompanied by defects exceeding △.

(4) Coefficient of friction A test piece (measurement standard length: width 75 mm x length 100 mm) whose temperature and humidity were controlled under the specified environment (temperature: 23 ± 3°C, humidity: 65 ± 5%) was They were stacked so that they were touching each other and set in a measuring device (slip tester). A load of 200 g was placed on the two stacked test pieces, and the resistance value was measured by sliding the upper test piece with a measuring device, and the friction coefficient was calculated using the following formula (1). Note that both the static friction coefficient (μs) and the dynamic friction coefficient (μd) were calculated using the following formula (1), but for the resistance value (g), the maximum resistance value at the moment when the slide started was calculated for the former, and the latter The resistance value during sliding was used for calculation.
Friction coefficient = resistance value (g)/load (g). Formula (1).

(5) Product roll winding appearance After winding the product roll, wrinkles and winding misalignment in the product roll were visually confirmed and evaluated according to the following criteria. In addition, cases where the evaluation result was ○ or △ were considered to be passed.
○: No wrinkles or misalignment occurred in the product roll.
△: Wrinkles and winding misalignment occurred in the product roll, but the level was slight enough to cause no actual damage during processing.
×: Wrinkles and winding misalignment occurred in the product roll, which was at an excessive level that caused actual damage in processing.

[material]
The following raw materials were used to produce the biaxially oriented laminated polypropylene films of Examples and Comparative Examples. Note that the following antioxidants were already mixed in the polypropylene resin I and the branched polypropylene resin, so fine adjustment of the content was not performed in the Examples and Comparative Examples. In addition, the amount of antioxidant contained in each resin is very small (total amount less than 1.0% by mass), and considering the volatile content when formed into a film, the amount of antioxidant will be even smaller. The manufacturing methods and Table 1 in Examples and Comparative Examples are not described.

(1) Resin polypropylene resin I (homopolypropylene resin): manufactured by Prime Polymer Co., Ltd., a highly stereoregular homopolymer with an MFR of 2.9 g/10 min, a melting point of 164°C, and a mesopentad fraction of 0.94. Polypropylene resin.
Branched polypropylene resin: "WAYMAX" (registered trademark) (MFX3) manufactured by Nippon Polypro Co., Ltd. A branched polypropylene resin having an MFR of 9.0 g/10 minutes.
Polymethylpentene (PMP) resin: “TPX” (registered trademark) MX004 manufactured by Mitsui Chemicals.

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

(Example 1)
As a raw material for the base layer (layer B), a mixture of 98 parts by mass of polypropylene resin I and 2 parts by mass of branched polypropylene resin was supplied to a single screw extruder, and as a raw material for the surface layer (layer A), 80 parts by mass of a mixture of polypropylene resin I and 2 parts by mass of branched polypropylene resin A mixture of parts by mass of polypropylene resin I and 20 parts by mass of branched polypropylene resin was supplied to another single-screw extruder, and melt extrusion was performed at 260°C. Next, after removing foreign matter with a 20 μm cut sintered filter, each raw material was stacked using a feed block type composite T-die so that the thickness ratio of A layer/B layer/A layer was 1/8/1. This was formed into a sheet and discharged, and was brought into close contact with a casting drum using an air knife, cooled and solidified to obtain an unstretched sheet. At this time, two types of casting drums are used; first, the surface side that will become the inner surface of the film roll is brought into close contact with the casting drum whose surface temperature has been adjusted to 90°C, and then the surface temperature is adjusted to 60°C. The surface side that would become the outer surface of the film when it was finally formed into a film roll was brought into close contact with a casting drum set at .degree. Further, on the first casting drum, the sheet was cooled by injecting compressed air at a temperature of 65° C. at a blowing air speed of 140 m/s onto the non-drum contact surface side. Note that no air knife was used on the second casting drum. Subsequently, the unstretched sheet was preheated to 142° C. on the inner surface and 151° C. on the outer surface using conveyor rolls, and then the unstretched sheet was preheated in the longitudinal direction between rolls at a temperature of 155° C. with a difference in circumferential speed. The film was stretched 0 times and cooled with a roll at 40°C to obtain a uniaxially stretched film. Next, the uniaxially stretched film was held at both ends in the width direction with clips and introduced into a tenter-type stretching machine, and after preheating at 162°C for 3 seconds, it was stretched 8.8 times at 162°C to 10.5% in the width direction. The heat treatment was performed at 163° C. while providing relaxation. After that, the film is guided to the outside of a tenter-type stretching machine through a cooling process at 135°C, the clips at the ends in the width direction are released, and both ends in the width direction (the part held by the clips) are cut off with a slitter. The film was wound up to obtain a biaxially oriented laminated polypropylene film having a thickness of 15 μm. Table 1 shows the physical properties and evaluation results of the obtained biaxially oriented laminated polypropylene film. Note that the inner surface of the winding means 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 winding means the surface opposite to the inner surface of the winding.

(Examples 2 to 7, Comparative Examples 1 to 7)
In Example 1, a biaxially oriented laminated 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. Table 1 shows the physical properties and evaluation results of the obtained biaxially oriented laminated polypropylene film. The film thickness was adjusted by adjusting the screw rotation speed of the extruder, the width of the unstretched sheet, the film forming speed, the stretching ratio, etc.

In the table, since the amount of antioxidant in the film composition is extremely small relative to the resin, it is not taken into account when calculating the composition.

As mentioned above, the biaxially oriented laminated polypropylene film of the present invention has excellent mold releasability and winding property, so it can be suitably used as a mold release film or a process film, and can be used as a cover film for an adhesive resin layer, etc. It is particularly suitable as a mold release film. Furthermore, since the biaxially oriented laminated polypropylene film of the present invention has excellent surface smoothness, it can be preferably used as a release film or a process film for applications requiring surface smoothness of products. Furthermore, the biaxially oriented laminated polypropylene film of the present invention having the above characteristics can be used in various applications such as packaging films, process films, sanitary products, agricultural products, construction products, and medical products.

Claims (6)

The peeling force on at least one side is 1.0 N or more and 2.2 N or less, the minimum autocorrelation length (Sal) on at least one side is 41 μm or more and 100 μm or less, and the difference in Sal on both sides is 5 μm or more and 40 μm or less. A biaxially oriented laminated polypropylene film characterized by: The biaxially oriented laminated polypropylene film according to claim 1, wherein the arithmetic mean height (Sa) of at least one side is 80 nm or more and 700 nm or less, and the difference in Sa of both sides is 50 nm or more and 400 nm or less. The biaxially oriented laminated polypropylene film according to claim 1 or 2, wherein at least one surface has an aspect ratio (Str) of 0.40 or more and 0.90 or less. It has a surface layer (A layer) and a base layer (B layer), the A layer, the B layer, and the A layer are located in this order, and the A layer contains 10% by mass or more and 30% by mass of branched polypropylene. The biaxially oriented laminated polypropylene film according to claim 1 or 2, comprising: The biaxial shaft according to claim 1 or 2, wherein the static friction coefficient (μs) and the dynamic friction coefficient (μd) measured by bringing one surface into contact with the opposite surface are both 0.40 or more and 0.55 or less. Oriented laminated polypropylene film. A release film comprising the biaxially oriented laminated polypropylene film according to claim 1 or 2.