JP2024092920A - Polypropylene film and release film - Google Patents

Abstract

To provide a polypropylene film with superior transfer resistance and handleability.SOLUTION: A polypropylene film has an A face on at least one side, where the A face has a sharpness Sku of surface height distribution of 1.0 or more and 50.0 or less and a maximum peak height Sp of 30 nm or more and 300 nm or less.SELECTED DRAWING: None

Description

The present invention relates to a polypropylene film that has excellent transfer resistance and handling properties, and a release film using the same.

Since polypropylene film has excellent properties such as transparency, mechanical properties, and electrical properties, it is used in a variety of applications such as packaging, release, tape, cable wrapping, and electrical applications such as capacitors. In particular, since it has excellent surface peeling properties and mechanical properties, it is ideally used as a release film or process film to protect various components such as plastic products, building materials, and optical components.

Normally, the required properties for a release film are set appropriately depending on its application, but with the recent trend toward smaller devices and higher precision, the products to be protected may also require a thin film and high quality. If the surface smoothness of a polypropylene film is poor, for example when used as a release film for optical components, the surface irregularities of the polypropylene film may be transferred to the optical components, affecting the visibility of the optical components.

For this reason, attempts have been made to reduce the surface roughness of polypropylene films. For example, Patent Document 1 discloses a method for finely roughening the surface of a polypropylene film without forming coarse protrusions. Patent Document 2 describes a polypropylene film with a surface shape that makes it possible to uniformly control the amount of air and gap distance between the films when rolled.

International Publication No. 2020/071291 International Publication No. 2020/142713

However, due to further improvements in precision and miniaturization of products to which release films are applied, there have been cases where the transfer of surface irregularities leads to reduced quality and yield, even in polypropylene films in which surface protrusions have been suppressed by the methods described in Patent Documents 1 and 2. In addition, when a film with a smooth surface design to suppress transfer is wound into a roll, there are problems with handling, such as the inability to retain air between the films and the formation of wrinkles. Therefore, the object of the present invention is to solve the above problems. In other words, the object of the present invention is to provide a polypropylene film with excellent transfer resistance and handling properties.

In order to solve the above-mentioned problems, the polypropylene film of the present invention has the following configuration. That is, the polypropylene film of the present invention is a polypropylene film characterized in that, when the surface height distribution sharpness Sku is 1.0 or more and 50.0 or less, and the maximum peak height Sp is 30 nm or more and 300 nm or less, the surface A is defined as the surface A, and at least one surface of the polypropylene film is the surface A.

The polypropylene film of the present invention can also be in the following form and can also be suitably used as a release film.
(1) A polypropylene film, characterized in that at least one side is side A, the side A having a surface height distribution sharpness Sku of 1.0 or more and 50.0 or less and a maximum peak height Sp of 30 nm or more and 300 nm or less.
(2) The polypropylene film according to (1), characterized in that both sides are the A-sides.
(3) The polypropylene film according to (1) or (2), wherein at least one of the A sides has a maximum valley depth Sv of 30 nm or more and 300 nm or less.
(4) A polypropylene film according to any one of (1) to (3), having a heat shrinkage stress value in the film width direction at 150°C of 0.05 N/2 mm or more and 0.20 N/2 mm or less, and a heat shrinkage stress onset temperature of 130°C or more and 150°C or less.
(5) The polypropylene film according to any one of (1) to (4), in which the variation Δt of the thermal shrinkage rate in the film width direction when treated at 150° C. for 15 minutes is 0.01% or more and 0.50% or less.
(6) The polypropylene film according to any one of (1) to (5), which has a laminated structure of at least two layers.
(7) A release film comprising the polypropylene film according to any one of (1) to (6).

The present invention provides a polypropylene film that has excellent transfer resistance and handling properties and can be suitably used as a release film.

The polypropylene film of the present invention is characterized in that, when the surface height distribution sharpness Sku is 1.0 or more and 50.0 or less, and the maximum peak height Sp is 30 nm or more and 300 nm or less, and the surface A is defined as the surface A, at least one of the surfaces is the surface A. The polypropylene film of the present invention will be specifically described below.

A polypropylene film is a sheet-like molded product whose main component is polypropylene resin. The main component refers to a component that is contained in an amount of more than 50% by mass and not more than 100% by mass when the total components of the object (here, the film) are taken as 100% by mass, and the main component can be interpreted in the same way below. When multiple components corresponding to polypropylene resin are contained, if the total amount is more than 50% by mass, polypropylene resin is considered to be the main component. In the polypropylene film of the present invention, the content of polypropylene resin is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 96% by mass or more, particularly preferably 97% by mass or more, and most preferably 98% by mass or more of the total components of the film.

Polypropylene resin refers to a resin whose main structural unit is a propylene unit, and a main structural unit refers to a structural unit that is contained in an amount of more than 50 mol% to 100 mol% or less when all structural units are taken as 100 mol% (this definition can be interpreted similarly for other olefin resins, except that the propylene unit is replaced by another olefin unit).

The polypropylene film of the present invention is preferably a biaxially oriented film, particularly from the viewpoint of surface smoothness, quality, and heat shrinkage characteristics described later. Biaxial orientation means having molecular orientation in two perpendicular directions, and can be achieved by stretching an unstretched sheet in two perpendicular directions (usually the longitudinal direction and the width direction). The longitudinal direction means the direction in which the polypropylene film runs during the manufacturing process (corresponding to the winding direction in the case of a film roll), and the width direction means the direction perpendicular to the longitudinal direction within the film plane.

In addition, from the viewpoint of multi-functionality, the polypropylene film of the present invention preferably has a laminated structure of at least two layers. Laminated means that two or more layers with different compositions are laminated in the thickness direction (direction perpendicular to the film surface). By adopting such an embodiment, for example, by having a laminated structure of a layer that plays a role in increasing strength and a layer that plays a role in increasing releasability, the polypropylene film can achieve both handleability and releasability.

From the viewpoints of transfer resistance and handling, it is important that at least one of the polypropylene film of the present invention is the A-side, with the A-side being the surface having a surface height distribution sharpness Sku of 1.0 or more and 50.0 or less and a maximum peak height Sp of 30 nm or more and 300 nm or less. From the above viewpoints, it is preferable that the A-side has an Sku of 1.0 or more and 40.0 or less and an Sp of 30 nm or more and 200 nm or less, and it is more preferable that the Sku of 1.0 or more and 29.0 or less and an Sp of 30 nm or more and 150 nm or less. Hereinafter, the "surface height distribution sharpness Sku" and the "maximum peak height Sp" are sometimes simply referred to as "Sku" and "Sp".

Sku is a parameter that represents the sharpness of the height distribution of the surface shape. More specifically, a larger value means that the surface has more sharp peaks and valleys, and a smaller value means that the surface is flatter. If the Sku of the surface of a polypropylene film exceeds 50.0, the surface will have more sharp peaks and valleys, and if the film is used as a release film and the adherend that will become the product comes into contact with the surface, there is a possibility that transfer marks will occur on the adherend, resulting in a significant decrease in quality. On the other hand, if the Sku of the surface of a polypropylene film falls below 1.0, the surface approaches a flat surface, making it difficult to form gaps between the polypropylene films when rolled, resulting in decreased handling and winding properties.

Sp is a parameter that represents the maximum peak height of the surface characteristics, and more specifically, the larger the Sp, the more coarse protrusions there are on the surface. If the Sp of the surface of the polypropylene film exceeds 300 nm, coarse protrusions will be present on the surface, and when the film is used as a release film and the adherend that will become the product comes into contact with the surface, transfer marks may occur on the adherend, significantly reducing the quality. On the other hand, if the Sp is below 30 nm, the surface approaches a flat surface, making it difficult to form gaps between the polypropylene films when rolled, and reducing handling and winding properties.

In other words, by having an A-side, the polypropylene film does not deteriorate the quality of the adherend due to transfer when the A-side is bonded to the adherend, and excellent handling properties can be achieved. Therefore, such polypropylene films can be suitably used as release films.

Sku and Sp can be measured using a known layer cross-sectional shape measuring device, such as the non-contact surface/layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 manufactured by Ryoka Systems Co., Ltd. The specific method for measuring Sku and Sp using this device will be described later.

To set the sharpness of the surface height distribution Sku and the maximum peak height Sp within the above ranges, it is effective to set the raw material composition of the polypropylene film and the film-forming conditions within the ranges described below. More specifically, it is effective to incorporate a high melting point resin into the surface layer (I) of the laminated structure including the surface layer (I) and the base layer (II), to set the temperature of the casting drum to 15°C or higher and 60°C or lower, and to set the contact time between the casting drum and the sheet to 10 to 20 seconds. These methods can be combined as appropriate.

In the polypropylene film of the present invention, it is preferable that both sides are A-sides from the viewpoint of reducing damage to the adherend to be the product and improving the transportability and winding properties of the film. If Sku on one side is less than 1.0 or Sp is less than 30 nm, it becomes difficult to retain air between the film layers when the polypropylene film is wound into a roll, which may cause winding wrinkles. If Sku on one side exceeds 50 or Sp exceeds 300 nm, it may cause back transfer to the other side due to surface protrusions when the film is wound into a roll and laminated. When both sides are A-sides, Sku and Sp may be the same or different values on both sides, and the same applies to the maximum valley depth Sv described below.

From the viewpoint of the quality and handling of the adherend when used as a release film, the polypropylene film of the present invention preferably has a maximum valley depth Sv of 30 nm or more and 300 nm or less on at least one A side. The maximum valley depth Sv is a value that represents the maximum valley height of the surface properties, and a larger value means that the surface of the polypropylene film has a deeper concave shape. Hereinafter, the "maximum valley depth Sv" may be simply referred to as "Sv". From the above viewpoint, the Sv of the A side is more preferably 30 nm or more and 200 nm or less, and even more preferably 30 nm or more and 150 nm or less.

When the Sv of side A is 300 nm or less, there will be no excessively deep concave shape on side A, and a significant decrease in quality due to the transfer of the surface shape to the adherend can be reduced. On the other hand, when the Sv is 30 nm or more, side A will not become excessively flat, and an appropriate gap will be formed between the films when rolled, improving winding properties. From the above viewpoint, when both sides are side A, it is preferable that the maximum valley depth Sv on both sides is 30 nm or more and 300 nm or less, or within the above preferred range.

The maximum valley depth Sv can be measured using a known layer cross-sectional shape measuring device, such as the non-contact surface/layer cross-sectional shape measuring system "VertScan" (registered trademark) 2.0 manufactured by Ryoka Systems Co., Ltd. The specific method for measuring Sv using this device will be described later.

To set the maximum valley depth Sv within the above range, it is effective to set the raw material composition of the polypropylene film and the film-forming conditions within the ranges described below, and it is also effective to control the formation of β crystals by adjusting the cast drum temperature and the contact time with the cast drum. More specifically, it is effective to incorporate a high melting point resin in the surface layer (I) of a laminate structure including a surface layer (I) and a base layer (II), to set the temperature of the cast drum to 15°C or higher and 60°C or lower, and to set the contact time between the cast drum and the sheet to 10 to 20 seconds. These methods can be combined as appropriate.

From the viewpoint of heat resistance, the polypropylene film of the present invention preferably has a heat shrinkage stress value in the film width direction at 150°C of 0.05 N/2 mm or more and 0.20 N/2 mm or less, and a heat shrinkage stress rise temperature in the film width direction of 130°C or more and 150°C or less. The heat shrinkage stress value in the film width direction at 150°C and the heat shrinkage stress rise temperature can be measured by the TMA (Thermo Mechanical Analysis) method, the details of which will be described later.

For example, when the polypropylene film of the present invention is used as a release film, if the heat shrinkage stress value in the film width direction at 150°C is 0.20 N/2 mm or less, poor flatness due to heat shrinkage of the polypropylene film is suppressed, and processing suitability is improved. On the other hand, if the heat shrinkage stress value in the film width direction at 150°C is 0.05 N/2 mm or more, slackness during processing is suppressed.

In addition, if the heat shrinkage stress rise temperature is 130°C or higher, the polypropylene film is less likely to shrink during the processing step, resulting in good processing suitability. In addition, if the heat shrinkage stress rise temperature is 150°C or lower, sagging of the polypropylene film during the processing step is suppressed, similarly resulting in good processing suitability.

As a method for making the heat shrinkage stress value in the film width direction at 150°C 0.05 N/2 mm or more and 0.20 N/2 mm or less, for example, a method of forming a laminated structure having a surface layer (I) and a base layer (II) and using a polypropylene resin having an MFR of 1.0 g/10 min or more and 5.0 g/10 min or less, preferably 2.0 g/10 min or more and 5.0 g/10 min or less, as the base layer (II) of the polypropylene film can be mentioned. In addition, in addition to using such a polypropylene resin, it is also effective to set the stretching temperature in the transverse stretching (stretching in the width direction) process to 150°C to 165°C (preferably 160°C to 165°C), the relaxation rate in the heat treatment process to 5.0% to 15.0% (preferably 7.0% to 13.0%), and the heat treatment temperature to 141°C to 155°C, in terms of the film formation conditions. Note that these methods can be appropriately combined. By setting the transverse stretching and heat treatment under the above conditions, stretching and heat setting can be performed in a state where the stress inside the polypropylene film is relaxed, so the resulting polypropylene film has reduced heat shrinkage stress. The above methods can also be used to set the heat shrinkage stress onset temperature to 130°C or higher and 150°C or lower, and can be combined appropriately in the same manner.

In addition, from the viewpoint of uniformity of flatness in the width direction, the polypropylene film of the present invention preferably has a variation Δt in the heat shrinkage rate in the width direction of the film when treated at 150°C for 15 minutes of 0.01% or more and 0.50% or less, and more preferably has a variation Δt in the heat shrinkage rate in the width direction of the film when treated at 150°C for 15 minutes of 0.01% or more and 0.35% or less. Note that hereinafter, "variation Δt in the heat shrinkage rate in the width direction of the film when treated at 150°C for 15 minutes" may be simply referred to as "Δt", and the measurement method will be described later.

By making the Δt of the polypropylene film 0.50% or less, the deterioration of the uniformity of the surface flatness due to the difference in the local shrinkage behavior and expansion behavior in the width direction is suppressed, and the deterioration of the formation of the coating layer is reduced when used as a release film. In addition, by making the Δt 0.01% or more, the polypropylene film has a certain degree of variation in the thermal shrinkage rate in the film width direction, and the occurrence of thermal wrinkles due to the tension and heat load received when processing into a release film, etc. is reduced. The mechanism of the suppression of thermal wrinkles is assumed to be that the expansion and contraction of the polypropylene main chain that occurs in the polypropylene film when subjected to the tension and heat load received during thermal processing has a certain amount of variation in the dimensional change rate in the width direction, so that the shrinking and expanding parts are uniform, thereby obtaining the effect of suppressing the occurrence of thermal wrinkles.

The method for making Δt 0.01% or more and 0.50% or less is not particularly limited, but examples include a method for reducing the variation in the thickness of the polypropylene film and a method for controlling the temperature variation in the width direction during the width direction stretching process (preheating, stretching) and the heat treatment process. These methods can be combined as appropriate.

The thickness of the polypropylene film of the present invention is adjusted appropriately depending on the application and is not particularly limited, but is preferably 5.0 μm or more and 70.0 μm or less. If the thickness is 5 μm or less, handling is good, and if it is 70.0 μm or less, the decrease in productivity due to an increase in the amount of resin is suppressed. Even if the thickness is reduced, the polypropylene film of the present invention can maintain a moderate strength (Young's modulus) and handleability. In order to make the most of such characteristics, the thickness of the polypropylene film is more preferably 5.0 μm or more and 60.0 μm or less, and even more preferably 10.0 μm or more and 40.0 μm or less. The thickness of the polypropylene film can be measured with a known electronic micrometer (details of the measurement method will be described later) and can be adjusted by the screw rotation speed of the extruder, the width of the unstretched sheet, the film-forming speed, the stretching ratio, etc.

The polypropylene film of the present invention may contain resins other than polypropylene resins as long as the effects of the present invention are not impaired. Examples of resins other than polypropylene resins that can be used in the polypropylene film of the present invention include polyolefins whose main structural units are 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, etc. (among which, polyethylene and polymethylpentene are preferred), and polyesters such as polyethylene terephthalate and polybutylene terephthalate, but are not limited to the above examples. These may be added to the polypropylene resin alone or in combination of two or more types in any ratio, and may be homopolymers or copolymers. In addition, from the standpoint of dielectric breakdown resistance and dimensional stability, the content of these resins is preferably less than 10% by mass of all components constituting the polypropylene film.

In the polypropylene film of the present invention, the melt flow rate (hereinafter referred to as MFR) of the main component polypropylene resin (sometimes referred to as polypropylene resin A) is preferably 1.0 g/10 min or more and 5.0 g/10 min or less, more preferably 2.0 g/10 min or more and 5.0 g/10 min or less, and even more preferably 2.2 g/10 min or more and 5.0 g/10 min or less, when measured in accordance with condition M (230°C, 2.16 kg) of JIS K 7210 (1999) from the viewpoint of film forming stability and thickness unevenness, from the viewpoint of film forming stability and thickness unevenness. In order to make the melt flow rate (MFR) of the polypropylene resin the above value, a method of controlling the average molecular weight or molecular weight distribution is adopted.

By having the MFR of the main component, polypropylene resin A, be 1.0 g/10 min or more, film-forming properties are stable and thickness unevenness is suppressed. On the other hand, by having the MFR of the polypropylene resin be 5.0 g/10 min or less, the thermal shrinkage rate when made into a polypropylene film can be kept low.

In addition to the resins, various additives such as crystal nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors can be added as long as they do not impair the object of the present invention. It is also possible to use a combination of multiple types of these components as long as they do not impair the effect of the present invention.

The selection of antioxidants and adjustment of the amount added are important from the viewpoint of antioxidant bleed-out. Phenol-based antioxidants with steric hindrance are preferred, and when multiple types of antioxidants are used in combination, it is preferable that at least one of them is a high molecular weight type with a molecular weight of 500 or more. Specific examples of antioxidants that can be used in the 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 entire raw material for obtaining the polypropylene film is taken as 100% by mass, from the viewpoint of reducing coloring due to degradation of the polymer and loss of transparency due to bleed-out of the antioxidant. By using 0.03% by mass or more of the antioxidant, coloring of the film caused by degradation of the polymer in the extrusion process and loss of long-term heat resistance can be reduced. On the other hand, by using 1.0% by mass or less of the antioxidant, loss of transparency due to bleed-out of the antioxidant can be suppressed. 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. In addition, when the polypropylene film has a laminated structure, these antioxidants may be added to any layer, or may be added to multiple layers.

From the viewpoint of strength, heat resistance, and film formation stability, the mesopentad fraction of polypropylene resin A is preferably 0.90 or more, and more preferably 0.93 or more. The mesopentad fraction is an index of the stereoregularity of the crystalline phase of polypropylene measured by nuclear magnetic resonance (NMR), and the higher the value, the higher the crystallinity, the higher the melting point, and the higher the dimensional stability at high temperatures, which is preferable. There is no particular upper limit for the mesopentad fraction. In order to obtain a resin with such high stereoregularity, it is preferable to use a method of washing the resin powder obtained with a solvent such as n-heptane, or a method of appropriately selecting a catalyst and/or co-catalyst and a composition.

Polypropylene resin A may contain copolymerization components of other unsaturated hydrocarbons within the scope of the present invention. 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, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, and 5-methyl-2-norbornene. From the viewpoints of dielectric breakdown resistance and dimensional stability, it is preferable that the copolymerization amount is less than 1 mol%.

The polypropylene film of the present invention preferably contains a resin having a melting point of 180°C or more (hereinafter, high melting point resin). The melting point of the high melting point resin is more preferably 180°C or more and 240°C or less. By incorporating a high melting point resin into the surface layer of the polypropylene film and forming the film under the conditions described below, it becomes easy to form a surface shape within the above-mentioned range for Sku, Sp, and Sv, and the slipperiness can be improved. In such a case, it is preferable that the surface layer (I) contains a high melting point resin after forming a laminate film consisting of at least two layers, a surface layer (I) and a base layer (II). If a high melting point resin is present in the surface layer (I), it melts and disperses in the polypropylene in the melt extrusion process described below, and it becomes possible to form the above-mentioned protrusions without deformation in the stretching process. The blending conditions of the high melting point resin and the polypropylene resin, and the melt extrusion conditions during film formation will be described later. In the present invention, the melting point of the resin is measured by a differential scanning calorimeter (DSC) in accordance with JIS K7121-1987.

In order to form the fine protrusions as described above, it is preferable to finely disperse the high melting point resin in the polypropylene during the melt extrusion process, and therefore the high melting point resin is required to have a high affinity with the polypropylene resin. From this viewpoint, the high melting point resin is preferably an olefin resin, and among olefin resins, an olefin resin (polymethylpentene) having 4-methylpentene-1 units as the main constituent unit is particularly preferable. Resins containing 4-methylpentene-1 units have a higher affinity with polypropylene resins than non-olefin resins, and therefore can improve dispersibility. Examples of olefin resins containing 4-methylpentene-1 units that can be suitably used in the polypropylene film of the present invention include "TPX" (registered trademark) DX310, "TPX" (registered trademark) DX231, and "TPX" (registered trademark) MX004, all manufactured by Mitsui Chemicals, Inc.

In the polypropylene film of the present invention, the content of the olefin resin containing 4-methylpentene-1 units in the surface layer (I) is preferably 0.1 to 4.5% by mass, more preferably 0.1 to 4.0% by mass, even more preferably 0.1 to 3.0% by mass, and even more preferably 0.1 to 2.5% by mass. When the content of the olefin resin containing 4-methylpentene-1 units in the surface layer (I) is 4.5% by mass or less, the protrusions do not become mountain-like in the longitudinal direction, and the size of the protrusions formed is suppressed. Therefore, when such a polypropylene film is used as a base film, cover film, or release film, the transfer of unevenness to the surface of the adherend is suppressed. In addition, in cases where winding is difficult, such as when a coating layer is applied and the film is used as a protective film, defects such as air entrapment can be suppressed. When the content of the olefin resin containing 4-methylpentene-1 units in the surface layer (I) is 0.1% by mass or more, the frequency of the protrusions formed is appropriately maintained, resulting in good slip properties and improved winding properties.

In order to obtain the raw material used in the surface layer (I) of the polypropylene film of the present invention, it is preferable to blend the high melting point resin and polypropylene resin A, and more preferably, to knead both in advance in a twin-screw extruder to form chips. In this case, the kneading temperature is preferably higher than the melting point of the high melting point resin from the viewpoint of uniform dispersion, and the difference between the kneading temperature and the melting point of the high melting point resin is more preferably 10°C or more, and even more preferably 20°C or more. By having the kneading temperature higher than the melting point of the high melting point resin, dispersibility is improved and the formation of coarse protrusions is suppressed. There is no particular upper limit for the kneading temperature, but 280°C is preferable from the viewpoint of suppressing thermal decomposition of polypropylene resin A.

In terms of winding property and suppression of variation in thermal shrinkage behavior in the width direction, the polypropylene film of the present invention has a thickness unevenness in the width direction of 5.0% or less, more preferably 3.0% or less. If the thickness unevenness in the width direction is more than 5.0%, the film may deform at the thickness unevenness location, air may accumulate locally and cause wrinkles, or the shrinkage behavior may vary when heat treated. In order to achieve the above-mentioned range of local thickness unevenness, the transverse stretching conditions, heat setting conditions, and relaxation conditions during film formation may be set within the ranges described below. More specifically, for example, it is effective to set the transverse stretching ratio to 8.0 times or more and 11 times or less, and the heat treatment temperature to 110°C or more and 160°C or less, and these may be combined appropriately.

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

First, the above-mentioned preferred polypropylene resin is fed to a melt extruder and melt extruded at 230°C to 260°C. Next, foreign matter and modified polymers are removed using a filter installed in the middle of the polymer tube, and the resin is discharged onto a casting drum from a T-die to obtain an unstretched sheet. In addition, in order to control the surface Sku, Sp, and Sv within appropriate ranges, the surface temperature of the casting drum is preferably 15°C to 60°C, more preferably 15°C to 45°C, and even more preferably 15°C to 30°C. Methods for adhering the sheet to the casting drum include electrostatic application, air knife, nip roll, and underwater casting, but the air knife method is preferred from the viewpoint of foreign matter-free and film cooling.

It is also preferable to cool the sheet at the above temperature for 10 to 20 seconds, preferably 13 to 20 seconds, after it has been firmly attached to the casting drum. By controlling the temperature and time of the casting drum as described above, the formation of β crystals can be suppressed, so that no coarse protrusions are formed, and smooth, broad protrusions are formed.

Next, the unstretched sheet obtained is biaxially stretched to be biaxially oriented. As specific stretching conditions, first, the temperature at which the unstretched sheet is stretched in the longitudinal direction is controlled. The temperature control method can be a method using a temperature-controlled rotating roll, a method using a hot air oven, or the like. The film temperature (preheating temperature and stretching temperature) when stretching in the longitudinal direction is preferably 140°C or more and 155°C or less, taking into consideration the control of the surface Sku, Sp, and Sv within a preferred range, stable film formation, and reduction of stain-like defects. The stretching ratio is preferably 3.5 times or more and 6.0 times or less in order to control the heat shrinkage rate and the breaking elongation within a preferred range, and more preferably 4.0 times or more and 5.0 times or less. By setting the stretching ratio at 3.5 times or more, the stretching can be performed more uniformly, thereby reducing thickness unevenness. On the other hand, by suppressing the stretching ratio to 6.0 times or less, film breakage during the longitudinal stretching process and film breakage during the subsequent transverse stretching process can be reduced.

Next, the uniaxially oriented film obtained by longitudinal stretching is cooled to 10°C or higher and 70°C or lower. If the cooling temperature is 10°C or higher, curling of the film can be suppressed. On the other hand, if the cooling temperature is 70°C or lower, promotion of thermal crystallization can be suppressed, and the thermal shrinkage stress and shrinkage stress onset temperature can be easily controlled.

Next, the uniaxially oriented film thus obtained is gripped at both widthwise ends with clips and introduced into a tenter-type stretching machine, where it is stretched in the widthwise direction. From the viewpoint of thickness unevenness and stable film formation, it is preferably heated to 145°C to 170°C, more preferably 150°C to 165°C, and even more preferably 160°C to 165°C, and stretched in the widthwise direction at a ratio of 7.0 to 12 times, more preferably 8.0 to 11 times.

Then, heat treatment is performed in the tenter as it is. At this time, in order to control the thermal shrinkage rate in the width direction, the heat treatment temperature is preferably 110°C or more and 160°C or less, more preferably 120°C or more and 155°C or less, and even more preferably 141°C or more and 155°C or less. Furthermore, the heat treatment may be performed while relaxing the film in the width direction. In particular, by setting the relaxation rate in the width direction to 5.0% or more and 15.0% or less, more preferably 7.0% or more and 13.0% or less, it is preferable from the viewpoint of keeping the thermal shrinkage rate in the width direction in an appropriate range and optimizing the balance of dimensional stability.

The biaxially oriented polypropylene film thus obtained can be used for various purposes such as packaging film, surface protection film, processing film, sanitary products, agricultural products, construction products, and medical products, but since it has particularly excellent surface smoothness and quality, it can be preferably used as a surface protection film, processing film, release film, cable wrapping, capacitors and other electrical application films, and is particularly preferably used as a release film. Here, a release film refers to a film that is peeled off from a component or product before being used as a final product, such as a substrate used to protect the surface of a component or form a resin.

The present invention will be described in detail below with reference to examples. The properties of the film obtained by the manufacturing method were measured and evaluated by the following methods.

[Evaluation method for each characteristic]
(1) Film Thickness, Thickness Unevenness Five polypropylene films were stacked to prepare a measurement sample, and the thickness of the measurement sample was measured using an electronic micrometer (TT80 manufactured by TESA). The obtained value was then divided by 5 to calculate the thickness per polypropylene film. The same measurement was then performed at a position shifted 50 mm in the width direction as the measurement point, and the same measurement was repeated a total of five times. From the obtained measurement results, the maximum, minimum, and average values of the thickness per polypropylene film were obtained, and the average value was taken as the film thickness (μm), and the thickness unevenness (%) was calculated using the following formula.
(Formula 1) Thickness unevenness (%)=((maximum thickness value−minimum thickness value)/average thickness value)×100.

(2) Sharpness of surface height distribution (Sku), maximum peak height (Sp), maximum valley depth (Sv)
The measurements were performed 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., under the following procedure and conditions. First, the polypropylene film was unwound from the film roll, and measurement samples were taken so that 10 measurement points were determined randomly on a straight line passing through the center of the width direction and parallel to the longitudinal direction, and Sku, Sp, and Sv were measured at the 10 points. The average values of the obtained measurements were calculated and were defined as Sku, Sp, and Sv of the polypropylene film, respectively. Thereafter, the measurement surface was changed and the same measurement was performed. In addition, in one measurement, one visual field (visual field area: vertical 939 μm × horizontal 1,252 μm = 1,175,628 μm ² ) was measured.

A. Measurement conditions CCD camera: SONY HR-57 1/2
Objective lens: 10X
Optical tube: 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 of fixing the measurement sample A dedicated sample holder was used to fix the measurement sample. The sample holder is two detachable metal plates with a circular hole in the center, and the measurement sample is sandwiched and fixed between them without any wrinkles, and the measurement sample located in the circular hole was measured.

C. Analysis method The data obtained by the above measurement was analyzed using the image analysis software VS-Viewer of "VertScan" (registered trademark) 2.0. First, noise was removed using a median filter (5 x 5), and waviness components were removed using a Gaussian filter with a cutoff value of 250 μm. Next, Spd, Spc, and Sa defined in ISO25178 (2012) were measured using the "ISOPara" function. In addition, in the "ISOPara" function, the S-Filter was set to 6.0 μm.

(3) Thermal shrinkage stress value in the film width direction 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 drawn by plotting the temperature on the horizontal axis and the change in the length of the sample on the vertical axis, and the start temperature of the thermal shrinkage stress in the width direction 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)

(4) Variation in thermal shrinkage rate Δt when treated at 150° C. for 15 minutes in the transverse direction (TD)
Two lines were drawn on the surface of a polypropylene film with a width of 10 mm and a measurement length of approximately 100 mm (with the measurement length in the longitudinal direction), and the distance between the two lines was accurately measured and designated as L0 at 23° C. The sample was left in a 100° C. oven for 30 minutes under a load of 1.5 g, and then the distance between the two lines was measured again in a 23° C. environment, designated as L1, and the dimensional change rate at each temperature was calculated using the following formula.
(Formula 2) Dimensional change rate (%) = [(L0 - L1) / L0] x 100.
The polypropylene film to be evaluated was similarly measured every 100 mm from one end to the other end in the width direction, and the difference between the maximum value and the minimum value was calculated as Δt.

(5) Winding appearance of product roll After winding the product roll, the product roll was visually inspected and evaluated according to the following criteria. The evaluation results of ◯ and △ were deemed to be acceptable.
(The transfer marks were judged by shining fluorescent light on the polypropylene film unwound from the roll and visually checking the reflected light.)
◯: No wrinkles, winding misalignment, or transfer marks were observed on the product roll.
Δ: At least one of wrinkles, winding misalignment, and transfer marks occurred on the product roll, but it was so minor that it did not actually harm processing.
×: At least one of wrinkles, misalignment, and transfer marks was observed on the product roll, to a level that caused actual damage to processing.

[material]
The following raw materials were used to manufacture the polypropylene films of the Examples and Comparative Examples. The antioxidants listed below were already mixed into each polypropylene resin raw material, so the content was not finely adjusted in the Examples and Comparative Examples. The antioxidants contained in each polypropylene resin raw material were in trace amounts (less than 1.0 mass% of all components), and considering the volatile content when the film was made, the amount of antioxidant would be even smaller, so they will not be listed in the manufacturing methods and Table 1 in the Examples and Comparative Examples described below.

(1) Resin Polypropylene resin raw material I: manufactured by Prime Polymer Co., Ltd., a highly stereoregular homopolypropylene resin (Homo PP1) having an MFR of 2.9 g/10 min, a melting point of 164° C., and a mesopentad fraction of 0.94.
Polypropylene resin raw material II: A mixture of homo-PP1 and 2% by mass of 4-methyl-1-pentene polymer (manufactured by Mitsui Chemicals, Inc., "TPX" (registered trademark) MX004, melting point: 230°C) was prepared by the following procedure.
Mixing Procedure
Each resin was fed from a metering hopper to a twin-screw extruder so that Homo PP1 was 90 parts by mass and "TPX" (registered trademark) MX004 was 10 parts by mass, melt-kneaded at 260°C, discharged from a die in the form of a strand, cooled and solidified in a water tank at 25°C, and then cut into chips to obtain a mixed polypropylene raw material. The obtained mixed polypropylene raw material and polypropylene resin raw material I were dry-blended in a mass ratio of 20:80 to obtain polypropylene resin raw material II.
Polypropylene resin raw material III: A homopolypropylene resin (Homo PP2) manufactured by Sumitomo Chemical Co., Ltd., having an MFR of 2.1 g/10 min and a melting point of 160° C.
Polypropylene resin raw material IV: manufactured by Prime Polymer Co., Ltd., a homopolypropylene resin (Homo PP3) having an MFR of 2.0 g/10 min and a melting point of 161° C. mixed with 0.5% by mass of silica particles having an average particle size of 2 μm.
Polypropylene resin raw material V: manufactured by Sumitomo Chemical Co., Ltd., a homopolypropylene resin (Homo PP4) having an MFR of 1.3 g/10 min and a melting point of 159° C.
Polypropylene resin raw material VI: manufactured by Sumitomo Chemical Co., Ltd., a homopolypropylene resin (Homo PP5) having an MFR of 4.0 g/10 min and a melting point of 158° C. mixed with 0.3% by mass of acrylic bead particles having an average particle size of 2 μm.
Branched polypropylene resin: "WAYMAX" (registered trademark) (MFX3) manufactured by Japan Polypropylene Corporation. A branched polypropylene resin (branched PP) having an MFR of 9.0 g/10 min.

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

Example 1
Polypropylene resin raw material II was fed to a separate single-screw extruder as a raw material for the surface layer (I), and polypropylene resin raw material I (homo PP1) was fed to a separate single-screw extruder as a raw material for the base layer (II). Then, the resin composition was melted and extruded at 240°C in each single-screw extruder, and foreign matter was removed from each molten resin with a 30 μm cut sintered filter. Next, each raw material was laminated in a feed block type composite T die so that I/III/I had a thickness ratio of 0.5/23.5/0.5, discharged onto a casting drum whose surface temperature was controlled at 20°C, and adhered to the casting drum by an air knife to obtain an unstretched sheet. At this time, the peripheral speed of the casting drum was adjusted so that the adhesion time between the casting drum and the film was 15 seconds, and the air temperature of the air knife was set to 30°C. Next, the unstretched sheet was preheated to 148 ° C. using a conveying roll, stretched 4.2 times in the longitudinal direction at a temperature of 144 ° C. between rolls with a peripheral speed difference, and further cooled with a roll at 40 ° C. Next, the obtained uniaxially stretched film was introduced into a tenter-type stretching machine with both ends in the width direction held by clips, preheated at 162 ° C. for 3 seconds, and then stretched 8.8 times in the width direction at 162 ° C. Then, in the same tenter-type stretching machine, heat treatment was performed at 150 ° C. while giving 12% relaxation in the width direction, and the clip was released by being led to the outside of the tenter-type stretching machine through a cooling process at 140 ° C. Next, the polypropylene film with both ends in the width direction removed was wound up by a winder to obtain a roll of polypropylene film having a thickness of 24.5 μm. The physical properties and evaluation results of the obtained polypropylene film are shown in Table 1.

(Examples 2 to 8, Comparative Examples 1 to 5)
In Example 1, a roll of polypropylene film having a thickness shown in Table 1 was obtained in the same manner as in Example 1, except that the film configuration and film-forming conditions were as shown in Table 1. The physical properties and evaluation results of the obtained polypropylene film are shown in Table 1. The thickness was adjusted by controlling the resin discharge amount by the rotation speed of the extruder, and by controlling the cast speed, the longitudinal stretch ratio, and the transverse stretch ratio. For the surface layer raw materials of Comparative Example 2 and Comparative Example 4, polypropylene resin raw material IV and polypropylene resin raw material VI were used, respectively, instead of the surface layer raw material of Example 1. For the surface layer raw material of Comparative Example 1, a mixture of homo PP1 and branched PP in a mass ratio of 8:2 was used, and for the base layer raw material, a mixture of homo PP1 and branched PP so that the branched PP was 2 mass% was used.

As described above, the polypropylene film of the present invention has excellent transfer resistance and handling properties, and therefore can be suitably used as a release film or a processing film, and is particularly suitable as a release film, such as a cover film for an adhesive resin layer. Furthermore, the polypropylene film of the present invention has excellent surface smoothness, and therefore can be suitably used as a release film or a processing film for members that require surface smoothness. In addition, the polypropylene film of the present invention having the above characteristics can be used for various purposes such as packaging films, sanitary products, agricultural products, building products, and medical products.

Claims (7)

A polypropylene film, characterized in that at least one of its surfaces is surface A, where the surface height distribution sharpness Sku is 1.0 or more and 50.0 or less, and the maximum peak height Sp is 30 nm or more and 300 nm or less. The polypropylene film according to claim 1, characterized in that both sides are the A side. The polypropylene film according to claim 1 or 2, wherein the maximum valley depth Sv is 30 nm or more and 300 nm or less on at least one of the A sides. The polypropylene film according to claim 1 or 2, in which the heat shrinkage stress value in the film width direction at 150°C is 0.05 N/2 mm or more and 0.20 N/2 mm or less, and the heat shrinkage stress onset temperature is 130°C or more and 150°C or less. The polypropylene film according to claim 1 or 2, in which the variation Δt in the thermal shrinkage rate in the film width direction when treated at 150°C for 15 minutes is 0.01% or more and 0.50% or less. The polypropylene film according to claim 1 or 2, having a laminated structure of at least two layers. A release film comprising the polypropylene film according to claim 1 or 2.