JP7779141B2 - Biaxially oriented polypropylene release film and release film - Google Patents

Description

The present invention relates to a biaxially oriented polypropylene release film that has excellent rigidity and heat resistance. More specifically, it relates to a biaxially oriented polypropylene release film that has good flatness during release processing even when made thinner than conventional products, and is suitable as a process release film with minimal thickness unevenness.

Biaxially oriented polypropylene film is moisture-proof and has the necessary rigidity and heat resistance, making it suitable for industrial applications such as packaging, release films, and adhesive tape. In recent years, environmental considerations have led to demands for release films to be more recyclable and thinner for reduced volume, making it essential to significantly improve the rigidity of polypropylene film as well. Methods for improving rigidity include improving the crystallinity and melting point of polypropylene resin by improving the catalyst or process technology used during polymerization of the polypropylene resin, and increasing the stretch ratio during the film-making process to increase the degree of orientation of the film. However, increasing rigidity also comes with the problem of reduced heat resistance, and to date, no biaxially oriented polypropylene film has possessed sufficient rigidity and heat resistance.

In the manufacturing process of biaxially oriented polypropylene film, methods have been proposed in which, after stretching in the width direction, the film is subjected to a first-stage heat treatment while relaxing at a temperature below that used for width direction stretching, and then a second-stage heat treatment is performed at a temperature between the first stage and the width direction stretching temperature (e.g., Patent Document 1), while methods have been proposed in which further stretching in the longitudinal direction is performed after width direction stretching (e.g., Patent Documents 2 and 3). However, the film described in Patent Document 1 has low orientation and insufficient rigidity. The film described in Patent Document 2 has excellent rigidity but poor heat resistance, and suffers from the problem of wrinkling during release processing at temperatures above 120°C. Furthermore, the film described in Patent Document 3 is sequentially biaxially stretched, oriented in the width direction, and then re-stretched in the longitudinal direction, resulting in insufficient alignment of molecular chains in the longitudinal direction and low rigidity in the longitudinal direction. Furthermore, because the film is relaxed in the width direction, its orientation in the width direction is low and its rigidity is insufficient.

International Publication No. WO2016/182003 JP 2013-177645 A Japanese Patent Application Laid-Open No. 2001-40111

The present invention aims to solve the above-mentioned problems. Specifically, it relates to a biaxially oriented polypropylene release film that combines film rigidity with heat resistance at temperatures as high as 150°C. The object of the present invention is to provide a polypropylene release film that is suitable for use as an industrial release film, as it eliminates the above-mentioned conventional problems, has good flatness and does not develop wrinkles during release processing, and has little thickness unevenness, even when the film thickness is thinner than conventional products to reduce film waste after use and reduce environmental impact.

As a result of intensive research into achieving the above object, the present inventors have arrived at the following inventions [1] to [10].
[1] A biaxially oriented polypropylene film for release purposes that satisfies the following (1) and (2):
(1) In the azimuth angle dependence of the (110) plane of polypropylene α-crystals obtained by wide-angle X-ray diffraction measurement, the half-value width of the peak derived from oriented crystals in the width direction is 26° or less.
(2) When separated into a crystalline component (I), a constrained amorphous component (II), and an unconstrained amorphous component (III) as determined by pulsed NMR using the solid echo method, the ratio of (III) is 7% or less.
[2] The biaxially oriented polypropylene film for release purposes according to [1] , which satisfies the following (3), (4) and (5):
(3) The thermal shrinkage at 150°C is 10% or less in the longitudinal direction and 30% or less in the transverse direction.
(4) The stress at 5% elongation in the width direction at 23°C is 140 MPa or more.
(5) The heat shrinkage rate (%) in the width direction at 150°C and the stress (MPa) at 5% elongation in the width direction at 23°C satisfy the following formulas:
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 140
[3] The biaxially oriented polypropylene film for release purposes according to [1 ] or [2], which satisfies the following (6) and (7):
(6) The heat shrinkage rate at 120°C is 2.0% or less in the longitudinal direction and 10.0% or less in the transverse direction.
(7) The heat shrinkage rate (%) in the width direction at 120°C and the tensile modulus (GPa) in the width direction at 23°C satisfy the following formulas.
Tensile modulus (GPa) in the width direction at 23°C ≥ Heat shrinkage (%) in the width direction at 120°C × 0.3 + 7.0
[4] The biaxially oriented polypropylene film for release has a refractive index Ny of 1.5250 or more in the width direction and a ΔNy of 0.0240 or more. [1] to [3].
[5] The biaxially oriented polypropylene film for release purposes according to any one of [1] to [4], wherein the haze of the biaxially oriented polypropylene film for release purposes is 5.0% or less.
[6] The biaxially oriented polypropylene film for release molds according to any one of [1] to [5], wherein the mesopentad fraction of the polypropylene resin constituting the biaxially oriented polypropylene film for release molds is 97.0% or more.
[7] The polypropylene resin constituting the biaxially oriented polypropylene film for release has a crystallization temperature of 105°C or higher and a melting point of 160°C or higher. [1] to [6].
[8] The polypropylene resin constituting the biaxially oriented polypropylene film for release purposes has a melt flow rate of 4.0 g/10 min or more. [1] to [7].
[9] The amount of components of the polypropylene resin constituting the biaxially oriented polypropylene film for release use of 100,000 or less in molecular weight is 35% by mass or more. [1] to [8]. A biaxially oriented polypropylene film for release use according to any one of [1] to [8].
[10] A release film in which a release coating layer is laminated on at least one surface of the biaxially oriented polypropylene film for release use according to any one of [1] to [9].
[11] A laminated film comprising a biaxially oriented polypropylene film satisfying the following (1) and (2) and a release coating layer:
(1) In the azimuth angle dependence of the (110) plane of polypropylene α-crystals obtained by wide-angle X-ray diffraction measurement, the half-value width of the peak derived from oriented crystals in the width direction is 26° or less.
(2) When separated into a crystalline component (I), a constrained amorphous component (II), and an unconstrained amorphous component (III) as determined by pulsed NMR using the solid echo method, the ratio of (III) is 7% or less.

The present invention relates to a biaxially oriented polypropylene release film that has excellent rigidity and heat resistance. More specifically, even when the film thickness is thinner than conventional products, it has good flatness during release processing and little thickness unevenness, making it suitable for use as a process release film.

Schematic diagram of component separation of the decay curve of spin-spin relaxation time observed by ¹ H-pulse NMR. Graph comparing Examples and Comparative Examples with respect to F5 and 150°C heat shrinkage in the width direction

The biaxially oriented polypropylene release film of the present invention will be described in more detail below.
The biaxially oriented polypropylene release film of the present invention is made of a polypropylene resin composition containing a polypropylene resin as a main component. Here, "main component" means that the proportion of the polypropylene resin in the polypropylene resin composition is 90% by mass or more, more preferably 93% by mass or more, even more preferably 95% by mass or more, and particularly preferably 97% by mass or more.

(Polypropylene resin)
The polypropylene resin used in the present invention may be a polypropylene homopolymer or a copolymer with ethylene and/or an α-olefin having 4 or more carbon atoms. A propylene homopolymer that is substantially free of ethylene and/or an α-olefin having 4 or more carbon atoms is preferred, and even when it contains ethylene and/or an α-olefin having 4 or more carbon atoms, the amount of the ethylene and/or α-olefin having 4 or more carbon atoms component is preferably 1 mol % or less, more preferably 0.5 mol % or less, even more preferably 0.3 mol % or less, and particularly preferably 0.1 mol % or less. Within the above range, crystallinity is likely to be improved. Examples of the α-olefin component having 4 or more carbon atoms that constitutes such a copolymer include 1-butene, 1-pentene, 3-methylpentene-1 , 3-methylbutene-1 , 1-hexene , 4-methylpentene-1, 5-ethylhexene-1 , 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene.
The polypropylene resin may be a homopolymer of two or more different polypropylenes, a copolymer with ethylene and/or an α-olefin having 4 or more carbon atoms, or a mixture thereof.

(stereoregularity)
The mesopentad fraction (hereinafter may be abbreviated as [mmmm]%), which is an index of the stereoregularity of the polypropylene resin used in the present invention, is preferably in the range of 97.0 to 99.9%, more preferably in the range of 97.5 to 99.7%, even more preferably in the range of 98.0 to 99.5%, and particularly preferably in the range of 98.5 to 99.3%.
When the mesopentad fraction is 97.0% or more, the crystallinity of the polypropylene resin is increased, and the melting point, crystallinity, and crystalline orientation of the crystals in the film are improved, making it easier to obtain rigidity and heat resistance at high temperatures. When the mesopentad fraction is 99.9% or less, it is easier to reduce costs in terms of polypropylene production and the film is less likely to break during production. The mesopentad fraction is measured by nuclear magnetic resonance (NMR) spectroscopy.
The mesopentad fraction is more preferably 99.5% or less. The mesopentad fraction is measured by nuclear magnetic resonance spectroscopy (the so-called NMR method).
In order to set the mesopentad fraction of the polypropylene resin within the above range, a method of washing the obtained polypropylene resin powder with a solvent such as n-heptane, a method of appropriately selecting a catalyst and/or a co-catalyst, and a method of appropriately selecting the components of the polypropylene resin composition are preferably employed.

(Melting temperature)
The lower limit of the melting temperature (Tm) measured by DSC of the polypropylene resin constituting the biaxially oriented polypropylene film for release use of the present invention is preferably 160°C, more preferably 161°C, even more preferably 162°C, still more preferably 163°C, and even more preferably 164°C. When Tm is 160°C or higher, rigidity and heat resistance at high temperatures are easily obtained. The upper limit of Tm is preferably 170°C, more preferably 169°C, even more preferably 168°C, still more preferably 167°C, and particularly preferably 166°C. When Tm is 170°C or lower, it is easy to suppress cost increases in terms of polypropylene production and the film is less likely to break during film formation. The melting temperature can also be further increased by blending a nucleating agent with the polypropylene resin.
Tm is the main endothermic peak temperature associated with melting, which is observed when 1 to 10 mg of a sample is packed into an aluminum pan and set in a differential scanning calorimeter (DSC), melted at 230°C for 5 minutes in a nitrogen atmosphere, cooled to 30°C at a scanning rate of -10°C/min, held for 5 minutes, and then heated again at a scanning rate of 10°C/min.

(crystallization temperature)
The lower limit of the crystallization temperature (Tc) measured by DSC of the polypropylene resin constituting the biaxially oriented polypropylene film for release use of the present invention is 105°C, preferably 108°C, and more preferably 110°C. If the Tc is 105°C or higher, crystallization is likely to proceed during width direction stretching and the subsequent cooling process, making it easier to obtain rigidity and heat resistance at high temperatures. The upper limit of Tc is preferably 135°C, more preferably 133°C, even more preferably 132°C, even more preferably 130°C, particularly preferably 128°C, and most preferably 127°C. If the Tc is 135°C or lower, it is difficult to increase the cost of polypropylene production and the film is less likely to break during film formation. The crystallization temperature can also be further increased by blending a crystal nucleating agent with the polypropylene resin.
Tc is the main peak temperature of the exothermic peak observed when 1 to 10 mg of a sample is packed into an aluminum pan, set in a DSC, melted at 230°C for 5 minutes in a nitrogen atmosphere, and cooled to 30°C at a scanning rate of -10°C/min.

(Melt Flow Rate)
The melt flow rate (MFR) of the polypropylene resin constituting the biaxially oriented polypropylene film for release use of the present invention, when measured in accordance with condition M (230°C, 2.16 kgf) of JIS K 7210 (1995), is preferably 4.0 to 30 g/10 min, more preferably 4.5 to 25 g/10 min, even more preferably 4.8 to 22 g/10 min, particularly preferably 5.0 to 20 g/10 min, and most preferably 6.0 to 20 g/10 min.
When the melt flow rate (MFR) of the polypropylene resin is 4.0 g/10 min or more, a biaxially oriented polypropylene film for release purposes having low thermal shrinkage is likely to be obtained.
Furthermore, when the melt flow rate (MFR) of the polypropylene resin is 30 g/10 min or less, the film formability is easily maintained.

From the viewpoint of film properties, the lower limit of the melt flow rate (MFR) (230°C, 2.16 kgf) of the polypropylene resin constituting the film is preferably 5.0 g/10 min, more preferably 5.5 g/10 min, even more preferably 6.0 g/10 min, particularly preferably 6.3 g/10 min, and most preferably 6.5 g/10 min.
When the melt flow rate (MFR) of the polypropylene resin is 5.0 g/10 min or more, the amount of low-molecular-weight components in the polypropylene resin constituting the film increases. Therefore, by employing a width direction stretching step in the film production process described below, the oriented crystallization of the polypropylene resin is further promoted, the degree of crystallization in the film is more likely to increase, and in addition, the entanglement of polypropylene molecular chains in the amorphous portion is reduced, making it easier to improve heat resistance.
In order to set the melt flow rate (MFR) of the polypropylene resin within the above range, it is preferable to employ a method for controlling the average molecular weight or molecular weight distribution of the polypropylene resin.

That is, the lower limit of the amount of components having a molecular weight of 100,000 or less in the GPC cumulative curve of the polypropylene resin constituting the film of the present invention is 35 mass%, preferably 38 mass%, more preferably 40 mass%, even more preferably 41 mass%, and particularly preferably 42 mass%.
The upper limit of the amount of components having a molecular weight of 100,000 or less in the GPC cumulative curve is preferably 65% by mass, more preferably 60% by mass, and even more preferably 58% by mass. When the amount of components having a molecular weight of 100,000 or less in the GPC cumulative curve is 65% by mass or less, the film strength is less likely to decrease.
In this case, if the polypropylene resin contains a high-molecular-weight component or a long-chain branched component having a long relaxation time, the amount of components having a molecular weight of 100,000 or less contained in the polypropylene resin can be easily adjusted without significantly changing the overall viscosity, which makes it easier to improve film formability without significantly affecting rigidity or thermal shrinkage.

(molecular weight distribution)
The polypropylene resin used in the present invention has a lower limit of the mass average molecular weight (Mw)/number average molecular weight (Mn) ratio, which is an index of the breadth of the molecular weight distribution, of preferably 3.5, more preferably 4, even more preferably 4.5, and particularly preferably 5. The upper limit of Mw/Mn is preferably 30, more preferably 25, even more preferably 23, particularly preferably 21, and most preferably 20.
Mw/Mn can be obtained using gel permeation chromatography (GPC). When Mw/Mn is in the above range, it is easy to increase the amount of components having a molecular weight of 100,000 or less.

The molecular weight distribution of polypropylene resin can be adjusted by polymerizing components of different molecular weights in multiple stages in a series of plants, blending components of different molecular weights offline in a kneader, polymerizing by blending catalysts with different performance, or using a catalyst that can achieve the desired molecular weight distribution. The shape of the molecular weight distribution obtained by GPC may be a gentle molecular weight distribution with a single peak on a GPC chart with the logarithm (logM) of molecular weight (M) on the horizontal axis and the differential distribution value (weight fraction per logM) on the vertical axis, or a molecular weight distribution with multiple peaks or shoulders.

(Method for producing biaxially oriented polypropylene film for release)
The biaxially oriented polypropylene film for release use of the present invention is preferably obtained by preparing an unstretched sheet made of a polypropylene resin composition containing the above-mentioned polypropylene resin as a main component, and biaxially stretching the sheet. The biaxial stretching method may be any of simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, and sequential tenter biaxial stretching, but from the viewpoint of film formation stability and thickness uniformity, sequential tenter biaxial stretching is preferred. It is particularly preferred to stretch the film in the longitudinal direction and then in the width direction, but a method in which stretching in the width direction and then in the longitudinal direction may also be used.

Next, a method for producing the biaxially oriented polypropylene release film of the present invention will be described below, but the method is not necessarily limited thereto. The biaxially oriented polypropylene release film of the present invention may have a layer having another function laminated on at least one side. Lamination may be on one or both sides. In this case, the resin composition of the other layer and the central layer may be the polypropylene resin composition described above. It may also be a polypropylene resin composition different from the above-mentioned one.
The number of layers to be laminated on one side may be one, two, or three or more layers, but from the viewpoint of manufacturing, one or two layers are preferred. As a lamination method, for example, coextrusion using a feed block method or a multi-manifold method is preferred. In particular, for the purpose of improving the processability of the biaxially oriented polypropylene film for release, a resin layer having heat sealability can be laminated to the extent that the properties are not deteriorated. Furthermore, to impart printability, one or both sides can be subjected to corona treatment.

In the following, an example of a single layer will be described in which a tenter sequential biaxial stretching method is employed.
First, a resin composition containing a polypropylene resin is heated and melted in a single-screw or twin-screw extruder, extruded into a sheet from a T-die, and cooled and solidified by being placed on a cooling roll. In order to promote solidification, it is preferable to further cool the sheet cooled by the cooling roll by immersing it in a water tank, for example.

The sheet is then stretched longitudinally between two pairs of heated stretching rolls by increasing the rotation speed of the rear stretching roll, resulting in a uniaxially stretched film.

The uniaxially stretched film is then preheated and stretched in the width direction at a specific temperature while holding the film edges in a tenter-type stretching machine, yielding a biaxially stretched film. This width direction stretching process will be described in detail later.

After the widthwise stretching process is completed, the biaxially stretched film is heat-treated at a specific temperature to obtain a biaxially oriented film for release. During the heat-treatment process, the film may be relaxed in the widthwise direction.

If necessary, the biaxially oriented polypropylene film for release purposes obtained in this manner can be subjected to a corona discharge treatment on at least one side, and then wound up on a winder to obtain a film roll.

Each step will be described in detail below.
(Extrusion process)
First, a polypropylene resin composition containing a polypropylene resin as a main component is heated and melted in a range of 200 to 300°C in a single-screw or twin-screw extruder, and the molten polypropylene resin composition in a sheet form is extruded from a T-die and brought into contact with a metal cooling roll to be cooled and solidified. The obtained unstretched sheet is preferably further placed in a water tank.
The temperature of the cooling roll, or the cooling roll and water bath, is preferably in the range of 10°C to Tc, and when it is desired to increase the transparency of the film, it is preferable to cool and solidify the film using a cooling roll at a temperature in the range of 10 to 50°C. If the cooling temperature is 50°C or less, the transparency of the unstretched sheet tends to increase, and it is preferably 40°C or less, and more preferably 30°C or less. In order to increase the degree of crystalline orientation after sequential biaxial stretching, a cooling temperature of 40°C or more may be preferable. However, when using a propylene homopolymer with a mesopent fraction of 97.0% or more as described above, a cooling temperature of 40°C or less is preferable in order to facilitate the subsequent stretching step and reduce thickness unevenness, and a cooling temperature of 30°C or less is more preferable.
The thickness of the unstretched sheet is preferably 3,500 μm or less in terms of cooling efficiency, and more preferably 3,000 μm or less, and can be appropriately adjusted depending on the thickness of the film after sequential biaxial stretching. The thickness of the unstretched sheet can be controlled by the extrusion speed of the polypropylene resin composition, the lip width of the T-die, etc.

(Longitudinal stretching step)
The lower limit of the longitudinal stretching ratio is preferably 3.5 times, more preferably 3.8 times, and particularly preferably 4.2 times. Within this range, strength can be easily increased and thickness unevenness can be reduced.
The upper limit of the longitudinal stretching ratio is preferably 7.0, more preferably 6.0. When the stretching ratio is within this range, the width direction stretching is facilitated in the width direction stretching step, and productivity is improved.
The lower limit of the longitudinal stretching temperature is preferably Tm - 30°C, more preferably Tm - 27°C, and even more preferably Tm - 25°C. Within this range, subsequent width direction stretching becomes easier and thickness unevenness becomes less. The upper limit of the longitudinal stretching temperature is preferably Tm - 7°C, more preferably Tm - 10°C, and even more preferably Tm - 12°C. Within this range, the heat shrinkage rate is easily reduced, and there is little risk of difficulty in stretching by applying it to a stretching roll or of deterioration in quality due to increased surface roughness.
The longitudinal stretching may be carried out in two or more stages using three or more pairs of stretching rolls.

(Preheating process)
Prior to the width direction stretching step, the uniaxially stretched film after longitudinal stretching must be heated in the range of Tm to Tm + 25°C to soften the polypropylene resin composition. By setting the temperature at or above Tm, softening proceeds, making width direction stretching easier. By setting the temperature at or below Tm + 25°C, orientation proceeds during transverse stretching, making it easier to develop rigidity. A temperature of Tm + 2 to Tm + 20°C is more preferred, and a temperature of Tm + 3 to Tm + 15°C is particularly preferred. Here, the highest temperature in the preheating step is defined as the preheating temperature.

(Width direction stretching process)
In the width direction stretching step after the preheating step, a preferred method is as follows.

In the width direction stretching step, stretching is preferably performed at a temperature of Tm-10°C or higher and the preheating temperature or lower. In this case, the width direction stretching may be started when the preheating temperature is reached, or when the temperature is lowered after reaching the preheating temperature to a temperature lower than the preheating temperature.
The lower limit of the temperature in the width direction stretching step is more preferably Tm - 9° C., even more preferably Tm - 7° C., and particularly preferably Tm - 5° C. When the width direction stretching temperature is within this range, the rigidity of the obtained biaxially oriented film is easily improved.
The upper limit of the temperature in the width direction stretching step is preferably Tm+10° C., more preferably Tm+7° C., and particularly preferably Tm+5° C. When the width direction stretching temperature is within this range, stretching unevenness is unlikely to occur.
In the width direction stretching step, a later stretching step of stretching at a lower temperature may be added subsequent to the width direction stretching within the above temperature range.
That is, a section (early section) in which the film is stretched at a temperature of Tm-10°C or more and Tm+10°C or less may be followed by a section (late section) in which the film is stretched at a temperature lower than the temperature in the early section and at Tm-70°C or more and Tm-5°C or less. Providing an early section and a late section makes it easier to increase rigidity.
The lower limit of the stretching temperature in the latter section is preferably Tm-65° C., more preferably Tm-60° C., and even more preferably Tm-55° C. When the stretching temperature in the latter section is within this range, film formation tends to be stable.

The lower limit of the final width direction stretching ratio in the width direction stretching step is preferably 10 times or more, more preferably 11 times or more, and even more preferably 11.5 times or more. If it is 10 times or more, the rigidity of the film tends to be increased and thickness unevenness tends to be reduced. The upper limit of the width direction stretching ratio is preferably 20 times, more preferably 17 times, and even more preferably 15 times. If it is 20 times or less, the heat shrinkage rate tends to be reduced and the film is less likely to break during stretching.
When a later section is added, the total draw ratio is adjusted to be within the above range. In this case, the lower limit of the draw ratio in the earlier stretching step is preferably 4 times, more preferably 5 times, even more preferably 6 times, and particularly preferably 6.5 times. The upper limit of the draw ratio at the end of the earlier section is preferably 15 times, more preferably 14 times, and even more preferably 13 times.

It is preferable to cool the film immediately after the end of width direction stretching, i.e., when the final width direction stretching ratio is reached. The cooling temperature at this time is preferably equal to or lower than the width direction stretching temperature and is preferably at least Tm-80°C and not more than Tm-15°C, more preferably at least Tm-80°C and not more than Tm-20°C, even more preferably at least Tm-80°C and not more than Tm-30°C, and particularly preferably at least Tm-70°C and not more than Tm-40°C. By adding a cooling step, crystallization occurs and the crystalline orientation is fixed. Thereafter, even when the temperature is raised to a temperature higher than the melting temperature, the orientation history is maintained, resulting in increased crystalline orientation in the film.
The temperature can be gradually lowered from the temperature at the end of width direction stretching to the temperature at the time of cooling, or can be lowered in stages or in a single step. Lowering the temperature in stages or in a single step is preferred because it facilitates greater crystal orientation in the film.

After cooling the film, it is preferable to stretch it again in the width direction at a high temperature (hereinafter also referred to as width direction re-stretching). When the film is cooled and then stretched again in the width direction at a high temperature, the crystalline orientation of the film is easily increased, and therefore the rigidity is easily increased. The lower limit of the stretching temperature when stretching again in the width direction is Tm - 5°C, preferably Tm°C, more preferably Tm + 5°C, even more preferably Tm + 7°C, and particularly preferably Tm + 9°C. When the temperature is Tm - 5°C or higher, the rigidity is easily increased and the heat shrinkage rate is easily reduced.
The upper limit of the width direction re-stretching temperature is preferably Tm+20° C., more preferably Tm+18° C., and even more preferably Tm+16° C. When the temperature is Tm+20° C. or lower, the rigidity is easily increased.
The lower limit of the widthwise re-stretching ratio at high temperature is preferably 1.05 times, more preferably 1.1 times, and even more preferably 1.15 times.
The upper limit of the widthwise re-stretching ratio at high temperature is preferably 2, more preferably 1.7, and even more preferably 1.5. If the re-stretching ratio is too high, the heat shrinkage may become too large, thickness unevenness may occur, or the film may break.

In other words, it has been found that, instead of immediately relaxing the film at a high temperature after widthwise stretching as in the conventional method, it is possible to further improve the rigidity while reducing the heat shrinkage rate by again stretching the film in the widthwise direction at a temperature sufficiently higher than the melting point.
That is, in the step after the first width direction stretching, it is preferable to perform stretching at Tm + 5° C. or higher. At Tm + 5° C. or higher, the mobility of molecular chains is sufficiently high, and the influence of entanglement of molecular chains is easily resolved by stretching. As a result, the molecular chains are less likely to be constrained, and therefore, disordered molecular orientation is less likely to occur, and crystallization also progresses sufficiently.
By cooling the film below a temperature at which crystallization is possible after stretching it in the width direction at a high temperature, the crystal orientation is fixed, and a high-melting film with a high degree of crystallization and thick crystalline lamellae can be obtained.

Furthermore, even in areas other than the crystalline lamellae, there are few molecular chains with large strains in molecular orientation constrained by entanglement points, so the film is less likely to shrink even when the crystals begin to melt. Furthermore, as the degree of crystallization improves and the crystalline lamellae become thicker, the melting point tends to increase, making it difficult for melting to occur below the melting point. This tends to reduce the thermal shrinkage rate.
As a result, it has become possible to further improve rigidity while reducing the thermal shrinkage rate.

In the conventional film-making process (extrusion - longitudinal stretching - transverse stretching - heat treatment), to eliminate distortion caused by transverse stretching at temperatures lower than the melting point, the film is exposed to a high temperature above the melting point during the heat treatment process, and relaxation is performed by several to several tens of percent, thereby reducing the heat shrinkage. Relaxation eliminates the difficulty of crystallization due to the constraints on molecular chains, contributing to a reduction in the heat shrinkage. However, conversely, the reduction in the transverse orientation of molecular chains generated during the transverse stretching process also reduces rigidity, making it difficult to achieve both low heat shrinkage and high rigidity. Furthermore, excessively high temperatures can cause the film to whiten.
In a typical film-forming process (extrusion - longitudinal stretching - width stretching - heat treatment), increasing the temperature in the width stretching step increases the mobility of molecular chains, and stretching is performed so as not to leave any residual strain. This promotes melting of the crystals formed by the longitudinal stretching, resulting in a decrease in crystalline orientation. In the method of the present invention, after the width stretching is performed once to achieve sufficient width orientation, the film has sufficient tension even if the fixed crystalline orientation melts upon cooling, and is then stretched again at a high temperature of Tm + 5°C or higher. Therefore, even when stretched again, the film has sufficient tension, and there is little concern about thickness unevenness or film breakage.
The stretching ratio at high temperatures may be 1.05 or more, as long as it is sufficient to disentangle and align the molecular chains. A stretching ratio of 2 or less is less likely to cause thickness unevenness.

In this way, by using a highly crystalline polypropylene resin with high stereoregularity and a high melting point and employing the above-mentioned longitudinal stretching process, width direction stretching process, cooling process, and high-temperature stretching process, the molecules of the polypropylene resin are aligned to a significantly high degree in the main orientation direction (this corresponds to the width direction in the above-mentioned width direction stretching process), which makes it easier to produce a greater number of crystals with strong crystalline orientation and a high melting point in the resulting biaxially oriented film.

In addition, by increasing the low molecular weight component of the polypropylene resin, the entanglement of molecular chains is reduced, which makes it easier to increase the crystallinity of the film and reduce the area other than the crystalline lamellae.Furthermore, by weakening the heat shrinkage stress of the area other than the crystalline lamellae, it is easier to further reduce the heat shrinkage rate.
In conventional technology, it has been difficult to achieve both strong crystalline orientation and a reduction in amorphous components not bound by the crystals. In other words, if either rigidity or thermal shrinkage is improved, the other property tends to deteriorate. Considering these factors, the present invention can be said to have a revolutionary effect.

(Heat treatment process)
The biaxially stretched film can be heat-treated as needed to further reduce the heat shrinkage. The upper limit of the heat treatment temperature is preferably the high-temperature re-stretching temperature described above, more preferably the high-temperature re-stretching temperature -2°C, and even more preferably the high-temperature re-stretching temperature -3°C. By setting the temperature at or below the high-temperature re-stretching temperature, the rigidity is less likely to decrease, the film surface roughness does not become too large, and the film is less likely to whiten. The lower limit of the heat treatment temperature is preferably Tm -3°C, more preferably Tm -2°C, and particularly preferably Tm.
For the purpose of adjusting the thermal shrinkage rate, the film may be relaxed (relaxed) in the width direction during heat treatment, but the upper limit of the relaxation rate is preferably 5%, more preferably 3%, and even more preferably 1%. Within the above range, the rigidity is less likely to decrease and the film thickness fluctuation tends to be small. When the rigidity is to be further increased, heat treatment may not be performed.

(cooling process)
After stretching in the width direction, it is preferable to cool the film immediately after stretching again in the width direction at Tm-5°C or higher, or immediately after the heat treatment step. The cooling temperature at this time is preferably 10°C or higher and 140°C or lower, more preferably 20°C or higher and 120°C or lower, even more preferably 80°C or lower, and particularly preferably 50°C or lower. By providing a cooling step, the state of the film can be fixed.
The biaxially oriented polypropylene film thus obtained is preferably subjected to corona discharge, plasma treatment, flame treatment, etc., as required.

(film thickness)
The thickness of the biaxially oriented polypropylene film for release use of the present invention is set according to each application, but in order to obtain film strength, the lower limit of the film thickness is preferably 2 μm, more preferably 3 μm, even more preferably 4 μm, particularly preferably 8 μm, and most preferably 10 μm. When the film thickness is 2 μm or more, the rigidity of the film is easily obtained. The upper limit of the film thickness is preferably 100 μm, more preferably 80 μm, even more preferably 60 μm, particularly preferably 50 μm, and most preferably 40 μm. When the film thickness is 100 μm or less, the cooling rate of the unstretched sheet during the extrusion process is less likely to decrease.
The biaxially oriented polypropylene release film of the present invention is usually produced as a roll having a width of about 2000 to 12000 mm and a length of about 1000 to 50000 m, and wound into a film roll. Furthermore, it is slit according to the intended use and supplied as a slit roll having a width of about 300 to 2000 mm and a length of about 500 to 5000 m. The biaxially oriented polypropylene release film of the present invention can be used to obtain a longer film roll.

(Thickness uniformity)
The lower limit of the thickness uniformity of the biaxially oriented polypropylene film for release use of the present invention is preferably 0%, more preferably 0.1%, even more preferably 0.5%, and particularly preferably 1%. The upper limit of the thickness uniformity is preferably 20%, more preferably 17%, even more preferably 15%, particularly preferably 12%, and most preferably 10%. Within the above range, defects are less likely to occur during post-processing such as coating or printing, and the film is easily usable in applications requiring precision.
The measurement method was as follows: A test piece 40 mm in width was cut out from a steady region where the film properties were stable in the length direction of the film, and the film thickness was measured continuously over 20,000 mm using a film feeder manufactured by Micron Measurement Instruments Co., Ltd. (product number: A90172) and a continuous film thickness measuring instrument manufactured by Anritsu Corporation (product name: K-313A wide-range high-sensitivity electronic micrometer), and the thickness uniformity was calculated using the following formula.
Thickness uniformity (%) = [(maximum thickness - minimum thickness) / average thickness] x 100

(Film characteristics)
The biaxially oriented polypropylene film for release use of the present invention is characterized by the following properties. Here, the "longitudinal direction" of the biaxially oriented polypropylene film for release use of the present invention is the direction corresponding to the flow direction in the film production process, and the "width direction" is the direction perpendicular to the flow direction in the film production process. For polypropylene films whose flow direction in the film production process is unknown, wide-angle X-rays are incident in a direction perpendicular to the film surface, and the scattering peak derived from the (110) plane of the α-type crystals is scanned in the circumferential direction, and the direction with the greatest diffraction intensity in the obtained diffraction intensity distribution is defined as the "longitudinal direction," and the direction perpendicular to that is defined as the "width direction."

The heat shrinkage rate (%) in the width direction at 150°C and the stress (MPa) at 5% elongation in the width direction at 23°C of the biaxially oriented polypropylene film for release mold of the present invention satisfy the following formulas.
By satisfying the following formula, the rigidity is higher and the thermal shrinkage rate at high temperatures is smaller, so that when used as a release film, it is easier to maintain its shape, it is less likely to deform during processing of ceramic capacitors, printed circuit board materials, etc., and wrinkles are suppressed, resulting in improved appearance quality. It also becomes possible to make the film thinner, which contributes to reducing the volume of the release film.
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 140
Furthermore, it is preferable that the heat shrinkage rate (%) in the width direction at 150°C and the stress (MPa) at 5% elongation in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention satisfy the following formulas.
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 150
Furthermore, it is more preferable that the heat shrinkage rate (%) in the width direction at 150°C and the stress (MPa) at 5% elongation in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention satisfy the following formulas.
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 160

(Crystalline component (I), constrained amorphous component (II), unconstrained amorphous component (III) determined by pulsed NMR)
It is known that the decay time constant of the FID (Free Induction Decay) of the spin-spin relaxation time T2 observed by ^1H -pulse NMR is observed as the sum of two or more decay time constants. For example, according to Polymer Journal, Vol. 3, No. 4, pp. 448-462 (1972), the decay time constant of the relaxation time of a crystalline polymer is analyzed by the solid echo method of pulse NMR as the sum of the three components: the crystalline component, the mesophase component, and the amorphous component.
The spin-spin relaxation time T2 observed by ^1H -pulse NMR becomes slower in the order of crystalline, mesophase, and amorphous. The mesophase has a faster T2 than the amorphous phase, and is considered to be an amorphous phase with restricted mobility. When stretching is performed while disentangling the molecular chains, a crystalline component (I) with strong orientation is generated, and an amorphous chain component (II) with restricted mobility (corresponding to the mesophase) is generated near the crystal. On the other hand, when entanglement is high and a disordered orientation is achieved during stretching, an amorphous component (III) (corresponding to the amorphous component) that is not restricted by the crystal is likely to be generated. The unrestricted amorphous component (III) has high mobility and is likely to move to relieve strain at high temperatures, which is thought to be the cause of shrinkage at high temperatures. On the other hand, the movement of the restricted amorphous chain (II) is suppressed even at high temperatures compared to the amorphous component (III), and therefore it is thought to be less likely to shrink at high temperatures.

The upper limit of the unconstrained amorphous component (III) of the biaxially oriented polypropylene release film of the present invention determined by pulse NMR is 7%, preferably 6%, more preferably 5%.
When the amorphous component (III) is 7% or less, the release coating film is less likely to wrinkle when dried. In addition, when formed into a release film, it is easier to maintain its shape, and deformation is less likely to occur during processing of ceramic capacitors, printed circuit board materials, etc., and wrinkles are suppressed, thereby further improving the appearance quality.
In order to reduce the amount of the unconstrained amorphous component (III), it is particularly effective to increase the area ratio during film formation and, after sequential biaxial stretching, to stretch again in the width direction at a high temperature.
It is also effective to use a polypropylene raw material with a high mesopentad fraction.
Furthermore, it is effective to set the lower limit of the amount of components having a molecular weight of 100,000 or less, as measured by gel permeation chromatography (GPC) integration curve of the polypropylene resin constituting the film, to 35% by mass.
Here, the fact that the content of the unconstrained amorphous component (III) determined by pulse NMR is 7% or less means that there are few molecular chains with large distortion in molecular orientation constrained by entanglement points, and that the film is less likely to shrink even when crystal melting begins, and is less likely to wrinkle even when the release coating film is dried at high temperatures.

When the content of the unconstrained amorphous component (III) determined by pulse NMR exceeds 7%, there are many molecular chains that are constrained by entanglement points and have large distortions in molecular orientation, so that shrinkage occurs simultaneously with the start of crystal melting, resulting in a film that is prone to wrinkles when the release coating film dries.
Although the lower limit of the unconstrained amorphous component (III) is not particularly limited, a practical limit is 0.1% or more. If the unconstrained amorphous component (III) is set to less than 0.1%, the film must be stretched again in the width direction at a higher temperature after the sequential biaxial stretching, which may reduce the tension during stretching due to melting, resulting in breakage. Furthermore, the crystal orientation in the film may be weakened, resulting in reduced rigidity.

(Heat shrinkage rate at 150°C)
The upper limit of the heat shrinkage rate in the longitudinal direction of the biaxially oriented polypropylene film for release purposes of the present invention at 150°C is 10%, preferably 7.0%, more preferably 6.0%, even more preferably 5.0%, and particularly preferably 4.0 % . The upper limit of the heat shrinkage rate in the width direction at 150°C is 30%, preferably 20%, more preferably 16%, and particularly preferably 15 % .
When the heat shrinkage rate in the longitudinal direction is 10% or less and the heat shrinkage rate in the width direction is 30% or less, wrinkles are less likely to occur during the release process. In particular, when the heat shrinkage rate in the longitudinal direction at 150°C is 8.0% or less and the heat shrinkage rate in the width direction at 150°C is 15% or less, the control range of the roll tension during the release process is wider, resulting in less wrinkles. When the heat shrinkage rate in the longitudinal direction is 10% or more and the heat shrinkage rate in the width direction is 30% or more, shrinkage of the film increases during drying of the release coating film, causing unevenness due to non-uniform drying, and releasability is likely to deteriorate. To reduce the heat shrinkage rate at 150°C, it is effective to set the lower limit of the amount of components with a molecular weight of 100,000 or less to 35% by mass when measuring the gel permeation chromatography (GPC) integrated curve of the polypropylene resin constituting the film.

(F5: stress at 5% elongation at 23°C)
The lower limit of F5 in the width direction of the biaxially oriented polypropylene release film of the present invention at 23° C. is 140 MPa, preferably 160 MPa, more preferably 180 MPa, and even more preferably 190 MPa. When the stiffness is 140 MPa or more, the film is highly rigid, so that it is easy to maintain its shape when made into a release film, and deformation of the film is unlikely to occur during processing into ceramic capacitors, printed circuit board materials, etc.
The upper limit of F5 in the width direction at 23° C. is preferably 300 MPa, more preferably 290 MPa, and even more preferably 280 MPa . If it is 300 MPa or less, practical production is easy and the longitudinal-width balance is likely to be improved.
The lower limit of F5 in the longitudinal direction of the biaxially oriented polypropylene release film of the present invention at 23° C. is preferably 40 MPa , more preferably 42 MPa , even more preferably 46 MPa , and particularly preferably 48 MPa . When the strength is 40 MPa or more, the rigidity is high, so that the shape is easily maintained when made into a release film, and deformation of the film is less likely to occur during processing into ceramic capacitors, printed circuit board materials, etc.
The upper limit of F5 in the longitudinal direction at 23° C. is preferably 70 MPa, more preferably 65 MPa, even more preferably 62 MPa, and particularly preferably 60 MPa . At 70 MPa or less, practical production becomes easy and the longitudinal-width balance tends to improve.
F5 can be kept within the range by adjusting the stretching ratio and relaxation rate, or by adjusting the temperature during film formation.

The biaxially oriented polypropylene film for release of the present invention preferably has the following properties and structure.
(Heat shrinkage rate at 120°C)
The upper limit of the longitudinal heat shrinkage rate of the biaxially oriented polypropylene film for release use of the present invention at 120°C is preferably 2.0%, more preferably 1.5%, even more preferably 1.2%, and particularly preferably 1.0%. If it is 2.0% or less, appearance defects such as vertical streaks are less likely to occur during release processing. The upper limit of the widthwise heat shrinkage rate at 120°C is 10.0 % , preferably 5.0%, more preferably 3.5%, and particularly preferably 2.5%. If it is 10.0% or less, wrinkles are less likely to occur in the film after release processing.
The heat shrinkage rate at 120°C can be kept within the range by adjusting the stretching ratio, stretching temperature and heat setting temperature.

It is preferable that the heat shrinkage rate (%) in the width direction at 120°C and the tensile modulus (GPa) in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention satisfy the following formulas.
By satisfying the following formula, the rigidity is higher and the thermal shrinkage rate at high temperatures is smaller, so that the film is less likely to wrinkle after release processing when it is made into packaging bags.
Tensile modulus (GPa) in the width direction at 23°C ≥ Heat shrinkage (%) in the width direction at 120°C × 0.3 + 7.0
It is more preferable that the heat shrinkage rate (%) in the width direction at 120°C and the tensile modulus (GPa) in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention satisfy the following formulas.
Tensile modulus (GPa) in the width direction at 23°C ≥ Heat shrinkage (%) in the width direction at 120°C × 0.3 + 8.0
Furthermore, it is more preferable that the heat shrinkage rate (%) in the width direction at 120°C and the tensile modulus (GPa) in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention satisfy the following formulas.
Tensile modulus (GPa) in the width direction at 23°C ≥ Heat shrinkage (%) in the width direction at 120°C × 0.3 + 9.0

The heat shrinkage rate (%) in the width direction at 150°C and the stress (MPa) at 5% elongation in the width direction at 23°C of the biaxially oriented polypropylene film for release mold of the present invention satisfy the following formulas.
By satisfying the following formula, the rigidity is higher and the thermal shrinkage rate at high temperatures is smaller, which helps prevent the film from wrinkling due to heat during the drying process of the release coating. Wrinkles are caused by film shrinkage due to heat and film deformation due to tension during roll transport, and by optimizing the balance between these two, wrinkling can be prevented. In addition, the film is more likely to maintain its shape when used as a release film, and is less likely to deform during processing of ceramic capacitors and printed circuit board materials, improving the appearance quality, such as preventing wrinkles. Furthermore, it allows for thinner films, which also contributes to reducing the volume of the release film.
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 140
Furthermore, it is preferable that the heat shrinkage rate (%) in the width direction at 150°C and the stress (MPa) at 5% elongation in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention satisfy the following formulas.
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 150
Furthermore, it is more preferable that the heat shrinkage rate (%) in the width direction at 150°C and the stress (MPa) at 5% elongation in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention satisfy the following formulas.
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 160

(Tensile modulus at 23°C)
The lower limit of the longitudinal tensile modulus of the biaxially oriented polypropylene film for release use of the present invention at 23°C is 2.0 GPa, preferably 2.1 GPa, more preferably 2.2 GPa, even more preferably 2.3 GPa, particularly preferably 2.4 GPa, and most preferably 2.6 GPa. At 2.0 GPa or higher, the film has high rigidity, making it easy to maintain its shape when used as a release film, and is less likely to deform during processing of ceramic capacitors, printed circuit board materials, etc. The upper limit of the longitudinal tensile modulus is preferably 4.0 GPa, more preferably 3.8 GPa, even more preferably 3.7 GPa, particularly preferably 3.6 GPa, and most preferably 3.5 GPa. At 4.0 GPa or lower, practical production is easy, and the balance of longitudinal and transverse properties is easily improved.
The lower limit of the tensile modulus in the width direction at 23°C of the biaxially oriented polypropylene film for release use of the present invention is preferably 6.0 GPa, more preferably 6.5 GPa, more preferably 6.7 GPa, even more preferably 7.0 GPa, particularly preferably 8.0 GPa, and most preferably 8.5 GPa. At 6.0 GPa or higher, the film has high rigidity, making it easy to maintain its shape when used as a release film, and the film is less likely to deform during processing into ceramic capacitors, printed circuit board materials, etc. The upper limit of the tensile modulus in the width direction is preferably 15 GPa, more preferably 13 GPa, and even more preferably 12 GPa. At 15 GPa or lower, practical production is easier and the balance of longitudinal and width direction properties is more likely to be improved.
The tensile modulus can be kept within the range by adjusting the stretching ratio and relaxation rate, or by adjusting the temperature during film formation.

(Tensile breaking strength at 23°C)
The lower limit of the longitudinal tensile breaking strength at 23°C of the biaxially oriented polypropylene film for release use of the present invention is preferably 90 MPa, more preferably 100 MPa, even more preferably 110 MPa, and particularly preferably 115 MPa. If it is 90 MPa or higher, wrinkles are less likely to occur during release film processing, and film breakage is less likely to occur during release processing. The upper limit of the longitudinal tensile breaking strength is preferably 200 MPa as a practical value, more preferably 180 MPa, and even more preferably 160 MPa. If it is 200 MPa or lower, film breakage is less likely.
The lower limit of the widthwise tensile breaking strength at 23°C of the biaxially oriented polypropylene film for release films of the present invention is preferably 400 MPa, more preferably 420 MPa, even more preferably 440 MPa, and particularly preferably 450 MPa. If it is 400 MPa or higher, wrinkles are less likely to occur during release film processing, and film breakage is less likely to occur during release processing. The upper limit of the widthwise tensile breaking strength is preferably 650 MPa as a practical value, more preferably 600 MPa, and even more preferably 550 MPa. If it is 650 MPa or lower, film breakage is less likely.
The tensile breaking strength can be kept within the range by adjusting the stretching ratio, stretching temperature, and heat setting temperature.

(Tensile elongation at break at 23°C)
The lower limit of the longitudinal tensile elongation at break of the biaxially oriented polypropylene film for release purposes of the present invention at 23°C is preferably 180%, more preferably 190%, more preferably 200%, and particularly preferably 210 % . If it is 180% or more, the film tends to break less. The upper limit of the longitudinal tensile elongation at break at 23°C is preferably 300%, more preferably 280%, as a practical value.

The lower limit of the tensile breaking elongation in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention is preferably 15%, more preferably 20%, and more preferably 30%. If it is 15% or more, the film is less likely to break. The upper limit of the tensile breaking elongation in the width direction at 23°C is preferably 60%, more preferably 55%, and even more preferably 50%. If it is 60% or less, the film is less likely to break during release processing.
The tensile elongation at break can be kept within the range by adjusting the draw ratio, draw temperature, and heat setting temperature.

(Refractive Index)
The lower limit of the refractive index (Nx) in the longitudinal direction of the biaxially oriented polypropylene film for release use of the present invention is preferably 1.4950, more preferably 1.4970, even more preferably 1.4980, particularly preferably 1.4990, and most preferably 1.5000. If it is 1.4950 or higher, the rigidity of the film is likely to be increased. The upper limit of the refractive index (Nx) in the longitudinal direction is preferably 1.5100, more preferably 15070, and even more preferably 1.5050. If it is 1.5100 or less, the film is likely to have an excellent balance of properties in the longitudinal direction and the width direction.

The lower limit of the refractive index (Ny) in the width direction of the biaxially oriented polypropylene film for release purposes of the present invention is 1.5250, preferably 1.5253, more preferably 1.5255, even more preferably 1.5260, and particularly preferably 1.5265. A refractive index of 1.5250 or higher tends to increase the rigidity of the film. The upper limit of the refractive index (Ny) in the width direction is preferably 1.5280, more preferably 1.5275, and even more preferably 1.5270. A refractive index of 1.5280 or lower tends to achieve an excellent balance of properties in the longitudinal and width directions of the film.

The lower limit of the refractive index (Nz) in the thickness direction of the biaxially oriented polypropylene film for release use of the present invention is preferably 1.4960, more preferably 14965, even more preferably 1.4970, particularly preferably 1.4980, and most preferably 1.4990. If it is 1.4960 or higher, the rigidity of the film is likely to be increased. The upper limit of the refractive index (Nz) in the thickness direction is preferably 1.5020, more preferably 1.5015, and even more preferably 1.5010. If it is 1.5020 or lower, the heat resistance of the film is likely to be increased.
The refractive index can be adjusted within the range by adjusting the stretching ratio, stretching temperature, and heat setting temperature.

(△Ny)
The lower limit of ΔNy of the biaxially oriented polypropylene film for release use of the present invention is 0.0240, preferably 0.0245, more preferably 0.0247, even more preferably 0.0250, particularly preferably 0.0255, and most preferably 0.0260. If it is 0.0240 or higher, the rigidity of the film tends to be high. The upper limit of ΔNy is preferably 0.0280 as a practical value, more preferably 0.0277, even more preferably 0.0273, and particularly preferably 0.0270. If it is 0.0280 or less, thickness unevenness tends to be improved. ΔNy can be kept within the range by adjusting the stretch ratio, stretching temperature, and heat setting temperature of the film.
ΔNy is calculated using the following formula, where Nx, Ny, and Nz are the refractive indices along the longitudinal, width, and thickness directions of the film, respectively, and means the degree of orientation in the width direction of the entire orientation of the film in the longitudinal, width, and thickness directions.
△Ny=Ny-[(Nx+Nz)/2]

(Plane Orientation Coefficient)
The lower limit of the planar orientation coefficient (ΔP) of the biaxially oriented polypropylene film for release use of the present invention is preferably 0.0135, more preferably 0.0138, and even more preferably 0.0140. If it is 0.0135 or more, the balance in the plane direction of the film is good and thickness unevenness is also good. The upper limit of the planar orientation coefficient (ΔP) is preferably 0.0155 as a practical value, more preferably 0.0152, and even more preferably 0.0150. If it is 0.0155 or less, it tends to have excellent heat resistance at high temperatures. The planar orientation coefficient (ΔP) can be kept within the range by adjusting the stretch ratio, stretching temperature, and heat setting temperature.
The plane orientation coefficient (ΔP) was calculated using the formula [(Nx+Ny)/2]-Nz.

(average refractive index)
The lower limit of the average refractive index of the biaxially oriented polypropylene film for release purposes of the present invention is preferably 1.5080, more preferably 1.5081, even more preferably 1.5082, particularly preferably 1.5083, and most preferably 1.5090. The upper limit of the average refractive index is preferably 1.5150 as a practical value, more preferably 1.5140, even more preferably 1.5135, and particularly preferably 1.5130. If it is 1.5080 or higher, the film is less likely to wrinkle when the release coating film is dried at high temperatures. The average refractive index can be kept within the range by adjusting the film stretch ratio, stretching temperature, and heat setting temperature.
The average refractive index is calculated by the following formula, where Nx, Ny, and Nz are the refractive indices in the longitudinal, transverse, and thickness directions of the film, respectively.
Average refractive index=(Nx+Ny+Nz)/3

(Hayes)
When transparency is required for the biaxially oriented polypropylene film for release use of the present invention, the upper limit of the film haze is preferably 5.0%, more preferably 4.5%, even more preferably 4.0%, particularly preferably 3.5%, and most preferably 3.0%. A haze of 5.0% or less is easy to use in applications requiring transparency. The lower limit of the haze is preferably 0.1%, more preferably 0.2%, even more preferably 0.3%, and particularly preferably 0.4% as a practical value. A haze of 0.1% or more is easy to manufacture. The haze can be kept within the range by adjusting the chill roll (CR) temperature, width direction stretching temperature, tenter preheating temperature before width direction stretching, width direction stretching temperature, or heat setting temperature, or the amount of components of the polypropylene resin having a molecular weight of 100,000 or less. However, the haze may increase by adding an antiblocking agent or providing a sealing layer.

(Half-width of diffraction peak derived from oriented crystals)
In the biaxially oriented polypropylene release film of the present invention, in the azimuth angle dependence of the scattering peak of the (110) plane of the polypropylene α-crystals obtained by wide-angle X-ray measurement with incident X-rays perpendicular to the film surface, the upper limit of the half-width (Wh) of the diffraction peak derived from oriented crystals in the width direction of the film is 26°, preferably 25° , more preferably 24 ° , particularly preferably 23 ° , and most preferably 22.0 ° . When the half-width (Wh) is 26° or less, the rigidity of the film is likely to be increased. The lower limit of Wh is preferably 15°, more preferably 16°, and even more preferably 17°.

(X-ray orientation degree)
The lower limit of the degree of X-ray orientation calculated from Wh of the biaxially oriented polypropylene film for release use of the present invention by the following formula is preferably 0.856, more preferably 0.861, even more preferably 0.867, particularly preferably 0.872, and most preferably 0.878. By making it 0.856 or more, it is easy to increase the rigidity.
X-ray orientation degree = (180-Wh)/180
The upper limit of the degree of X-ray orientation is preferably 0.917, more preferably 0.911, and even more preferably 0.906. By setting the degree to 0.917 or less, film formation tends to be stable.

(Biaxially oriented polypropylene film for release with release coating layer)
The biaxially oriented polypropylene film for release purposes of the present invention may be used as a release film without any special release treatment, but may also be used with a release agent such as a silicone resin applied to its surface.
The surface wetting tension of the biaxially oriented polypropylene film for release of the present invention is preferably 38 mN/m or more. When the wetting tension is 38 mN/m or more, the adhesion to the release coating layer is improved. In order to make the wetting tension 38 mN/m or more, it is preferable to perform a physicochemical surface treatment such as corona treatment, flame treatment, or plasma treatment.
For example, in the corona treatment, it is preferable to use a preheating roll and a treatment roll and perform discharge in the air. The wetting tension is preferably 44 mN/m or less, more preferably 43 mN/m or less, and even more preferably 42 mN/m or less.
The release agent to be applied to the biaxially oriented polypropylene film for release use of the present invention is not particularly limited, and silicone resins, fluororesins, alkyd resins, various waxes, aliphatic olefins, etc. can be used, and each resin can be used alone or in combination of two or more types.

The silicone resin used as the release agent in this invention can be any silicone resin commonly used in release agents, and can be selected from those commonly used in the relevant field, as described in, for example, "Silicone Materials Handbook" (edited by Toray Dow Corning, August 1993). Generally, heat-curable or ionizing radiation-curable silicone resins (which encompass both resins and resin compositions) are used. Examples of heat-curable silicone resins include condensation reaction and addition reaction silicone resins, while examples of ionizing radiation-curable silicone resins include ultraviolet or electron beam-curable silicone resins. These are applied to the substrate film and then dried or cured to form a release layer.

The above-mentioned curable silicone resin preferably has a degree of polymerization after curing of approximately 500,000 to 200,000, and particularly approximately 1,000 to 100,000. Specific examples of these include the following resins: KS-718, KS-774, KS-775, KS-778, KS-779H, KS-830, KS-835, KS-837, KS-838, KS-839, KS-841, KS-843, and KS-848 manufactured by Shin-Etsu Chemical Co., Ltd. -847, KS-847H, X-62-2418, X-62-2422, X-62-2125, X-62-2492, X-62-2494, X-62-5048, X-62-470, X-62-2366, X-62-630, X-92-140, X-92-128, KS-723A/B, KS-705F, KS-708A, KS-883, KS-709, KS-719; manufactured by Toshiba Silicon Corporation TPR-6701, TPR-6702, TPR-6703, TPR-3704, TPR-6705, TPR-6721, TPR-6722, TPR-6700, XSR-7029, YSR-3022, YR-3286; DK-Q3-202, DK-Q3-203, DK-Q3-204, DK-Q3-205, DK-Q3-210, DK-Q3-240, DK-Q3-3 manufactured by Dow Corning Corporation; 003, DK-Q3-3057, SFXF-2560; SD-7226, SD-7229, SD-7320, BY-24-900, BY-24-171, BY-24-312, BY-24-374, SRX-375, SYL-OFF23, SRX-244, SEX-290 manufactured by Dow Corning Toray Silicone Co., Ltd.; and SILCOLEASE 425 manufactured by ICI Japan Co., Ltd. Furthermore, silicone resins described in JP-A-47-34447 and JP-B-52-40918 can also be used. These curable silicone resins may be used alone or in combination of two or more.

Examples of the above-mentioned addition reaction silicone resin include those that cure by reacting polydimethylsiloxane, which has vinyl groups introduced into the terminals or side chains, with hydrogen siloxane using a platinum catalyst. In this case, it is more preferable to use a resin that can cure within 30 seconds at 120°C, as this allows for processing at low temperatures. Examples include low-temperature addition cure types (LTC1006L, LTC1056L, LTC300B, LTC303E, LTC310, LTC314, LTC350G, LTC450A, LTC371G, LTC750A, LTC755, LTC760A, etc.) and thermal UV cure types (LTC851, BY24-510, BY24-561, BY24-562, etc.) manufactured by Dow Corning Toray Co., Ltd., and solvent addition + UV cure types (X62-5040, X62-5065, X62-5072T, KS5508, etc.) and dual cure types (X62-2835, X62-2834, X62-1980, etc.) manufactured by Shin-Etsu Chemical Co., Ltd.

An example of the condensation reaction silicone resin is one in which a three-dimensional crosslinked structure is formed by condensing polydimethylsiloxane having terminal OH groups with polydimethylsiloxane having terminal H groups using an organotin catalyst.

Examples of the UV-curable or electron beam-curable silicone resins mentioned above include, for example, the most basic types: resins that crosslink and cure through a radical reaction, similar to normal silicone rubber crosslinking; resins that photocure through the introduction of acrylic groups; resin compositions in which UV decomposes onium salts to generate strong acids that cleave epoxy rings and crosslink; and resin compositions that crosslink through an addition reaction of thiols to vinyl siloxanes. Because electron beams have more energy than UV rays, a radical-based crosslinking reaction occurs without the need for an initiator, as is the case with UV curing.

When using a release agent, you can use one type or a mixture of two or more types. It is also possible to add additives such as light release additives and heavy release additives to adjust the release force. The resulting biaxially oriented polypropylene film with a release coating layer has particularly excellent release properties.

The release coating layer may contain particles having a particle size of 1 μm or less, but from the viewpoint of preventing pinholes, it is preferable that the release coating layer does not substantially contain particles or other particles that form protrusions.
The release coating layer may contain additives such as an adhesion improver and an antistatic agent. In order to improve adhesion to the substrate, it is also preferable to subject the surface of the biaxially oriented polypropylene film for release to a pretreatment such as anchor coating, corona treatment, plasma treatment, or atmospheric pressure plasma treatment before providing the release coating layer.

The thickness of the release coating layer can be set depending on its intended use and is not particularly limited, but is preferably in the range of 0.005 to 2 μm after curing. A release coating layer thickness of 0.005 μm or more is preferable because it maintains release performance. Furthermore, a release coating layer thickness of 2 μm or less is preferable because it does not require too long a curing time and there is no risk of thickness unevenness due to a decrease in the flatness of the release film. Another layer may be provided between the biaxially oriented polypropylene release film and the release coating layer.

In the present invention, the method for forming the release coating layer is not particularly limited. A coating liquid containing a release resin dissolved or dispersed therein is applied to one side of the biaxially oriented polypropylene release film substrate by coating or the like, and the solvent is then removed by drying, followed by heat drying, heat curing, or ultraviolet curing. In this case, the drying temperature during solvent drying and heat curing is preferably 100 to 170°C. Drying temperatures higher than 170°C may cause the film to wrinkle due to heat. On the other hand, drying temperatures lower than this may result in insufficient heat curing of the release coating layer, making it difficult to achieve release properties. Biaxially oriented polypropylene film has lower thermal dimensional stability at high temperatures than biaxially oriented polyester film, making it prone to heat-induced wrinkles during drying of the coating film. However, the biaxially oriented polypropylene release film of the present invention can prevent wrinkles from forming. The heat drying time is typically 10 to 60 seconds.

The coating liquid can be applied by any known coating method, including roll coating methods such as gravure coating and reverse coating, bar coating methods such as wire bars, die coating, spray coating, and air knife coating.

The present invention will be described in detail below with reference to examples, in which the properties were measured and evaluated by the following methods.
(1) Melt Flow Rate The melt flow rate (MFR) was measured in accordance with JIS K7210 at a temperature of 230°C and a load of 2.16 kgf.

(2) Mesopentad Fraction The mesopentad fraction ([mmmm]%) of the polypropylene resin was measured using ¹³ C-NMR. The mesopentad fraction was calculated according to the method described in Zambelli et al., Macromolecules, Vol. 6, p. 925 (1973). ¹³ C-NMR measurement was performed using an AVANCE 500 manufactured by BRUKER, by dissolving 200 mg of a sample in an 8:2 mixture of o-dichlorobenzene and deuterated benzene at 135°C, and then at 110°C.

(3) Number average molecular weight, weight average molecular weight, amount of components with molecular weights of 100,000 or less, and molecular weight distribution of polypropylene resin. Gel permeation chromatography (GPC) was used to determine the molecular weight in terms of PP using monodisperse polystyrene as the standard. When the baseline was unclear, the baseline was set within the range of the lowest point on the high molecular weight side of the elution peak closest to the elution peak of the standard substance.
The GPC measurement conditions are as follows.

Apparatus: HLC-8321PC/HT (manufactured by Tosoh Corporation)
Detector: RI
Solvent: 1,2,4-trichlorobenzene + dibutylhydroxytoluene (0.05%)
Column: TSK gelguard column HHR (30) HT (7.5 mm I.D. x 7.5 cm) x 1 + TSK gel GMHHR-H (20) HT (7.8 mm I.D. x 30 cm) x 3 Flow rate: 1.0 mL / min
Injection volume: 0.3mL
Measurement temperature: 140℃

The number average molecular weight (Mn) and the weight average molecular weight (Mw) are each defined by the following formula using the number of molecules (N _i ) of the molecular weight (M _i ) at each elution position of the GPC curve obtained via a molecular weight calibration curve.
Number average molecular weight: Mn=Σ(N _i・M _i )/ΣN _i
Mass average molecular weight: Mw = Σ( _Ni · _Mi ² )/Σ( _Ni · _Mi )
Here, the molecular weight distribution can be obtained by Mw/Mn.
Furthermore, the proportion of components having a molecular weight of 100,000 or less was determined from the integral curve of the molecular weight distribution obtained by GPC.

(4) Crystallization temperature (Tc), melting temperature (Tm)
Thermal measurements were performed under a nitrogen atmosphere using a TA Instruments Q1000 differential scanning calorimeter. Approximately 5 mg was cut out from the polypropylene resin pellets and sealed in an aluminum pan for measurement. The temperature was raised to 230 ° C and held for 5 minutes, then cooled to 30 ° C at a rate of -10 ° C / min, and the exothermic peak temperature was taken as the crystallization temperature (Tc). The heat of crystallization (ΔHc) was determined by setting a baseline so that the exothermic peak area smoothly connected from the start of the peak to the end of the peak. The temperature was then held at 30 ° C for 5 minutes, and then raised to 230 ° C at 10 ° C / min, and the main endothermic peak temperature was taken as the melting temperature (Tm).

(5) Film Thickness The film thickness was measured using a Millitron 1202D manufactured by Seiko EM Corporation.

(6) Haze: Measured at 23° C. in accordance with JIS K7105 using an NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd.

(7) Tensile Test The tensile strength of the film in the longitudinal and transverse directions was measured at 23°C in accordance with JIS K7127. Samples of 15 mm x 200 mm were cut out of the film and set in a tensile tester (Instron 5965, a dual-column tabletop tester manufactured by Instron Japan Co., Ltd.) with a chuck width of 100 mm. The tensile test was carried out at a tensile speed of 200 mm/min. From the obtained strain-stress curve, the tensile modulus was determined from the slope of the linear portion at the beginning of elongation, and the stress at 5% elongation was determined as F5.
The tensile breaking strength and tensile breaking elongation were the strength and elongation, respectively, at the time when the sample broke.

(8) Heat shrinkage was measured according to JIS Z1712 by the following method. The film was cut into pieces of 20 mm width and 200 mm length in both the longitudinal and transverse directions of the film, and then hung in a hot air oven at 120°C or 150°C for 5 minutes. The length after heating was measured, and the heat shrinkage was calculated as the ratio of the shrunk length to the original length.

(9) Refractive index, ΔNy, plane orientation coefficient, average refractive index Measured using an Abbe refractometer manufactured by Atago Co., Ltd. at a wavelength of 589.3 nm and a temperature of 23°C. The refractive indices along the longitudinal and width directions of the film were designated Nx and Ny, respectively, and the refractive index along the thickness direction was designated Nz. ΔNy was calculated using Nx, Ny, and Nz using the formula Ny-[(Nx+Nz)/2]. The plane orientation coefficient (ΔP) was calculated using the formula [(Nx+Ny)/2]-Nz. The average refractive index was calculated using the formula (Nx+Ny+Nz)/3.

(10) X-ray half-width and degree of X-ray orientation: These were measured by the transmission method using an X-ray diffractometer (SmartLab manufactured by Rigaku Corporation, with an αβγ attachment). X-rays with a wavelength of 1.5418 Å were used, and the X-ray output was 45 kV and 200 mA. A hybrid multidimensional pixel detector Hypix-3000 was used in zero-dimensional mode as the detector. In the parallel beam method, a solar slit of 2.5°, a length limiting slit of 10 mm, and an entrance slit width of 1 mm were used as the entrance slit. A parallel slit analyzer of 0.228° was used as the receiving slit.
The camera length was 300 mm, and the detector integral width was 2 mm.
A sample was prepared by stacking films to a thickness of 400 μm. A detector was placed at the diffraction peak position (diffraction angle 2θ = 14.1°) of the (110) plane of the α-crystal of the polypropylene resin, and the sample was rotated 360° around the axis in the thickness direction of the film to obtain the azimuth angle dependence of the diffraction intensity of the (110) plane. The measurement was performed at a step interval of 0.5° and a measurement speed of 60°/min. From this azimuth angle dependence, the half-width Wh of the diffraction peak derived from the oriented crystals in the width direction of the film was calculated.
Further, the degree of X-ray orientation was calculated using Wh according to the following formula.
X-ray orientation degree = (180-Wh)/180

(11) Ratio of Unconstrained Amorphous Component (III) Determined by Pulse NMR The film was cut and packed into a glass tube having an outer diameter of 10 mm to a height of 1 cm. The spin-spin relaxation time T2 of ^1H nuclei of the biaxially oriented polypropylene film was measured using the following measuring device and measuring conditions to obtain a decay curve of the magnetization intensity.
Device: BRUKER Minispec mq20
Temperature: 40℃
Observation frequency: 20MHz
90° pulse width: 2.74 μs
Pulse repetition time: 2.0 s
Pulse mode: Solid Echo method Number of integrations: 128 times Recycle Delay: 4 seconds
Acquisition Scale: 0.1 ms
Measurements were initiated after placing a glass tube filled with biaxially oriented polypropylene film into the apparatus and incubating for 15 minutes. To ensure that the obtained magnetization intensity decay curve and fitting curve matched, the component with the shortest relaxation time was separated by the least squares method using a Gaussian function, and the second and third shortest components were separated using a Lorentz function, and their respective proportions were obtained. The shortest component corresponds to the crystalline component (I), the second and third shortest components correspond to the constrained amorphous component (II) and the unconstrained amorphous component (III), respectively. Fitting and analysis were performed using the software (TD-NMR Analyzer) provided with the measurement apparatus.
The ratio of the unconstrained amorphous component (III) is the ratio (%) of the amorphous component (III) to the total of the crystalline component (I), the constrained amorphous component (II), and the unconstrained amorphous component (III) obtained by the above method.
was calculated using the following formula (1).
Ratio of unconstrained amorphous component (III) = M _III / (M _I + M _II + M _III ) (1)
M _I : Amount of crystalline component (I) M _II : Amount of constrained amorphous component (II) M _III : Amount of unconstrained amorphous component (III)

(12) Wetting tension (mN/m)
In accordance with JIS K 6768:1999, the film was aged at 23°C and a relative humidity of 50% for 24 hours, and then the corona-treated surface of the film was measured according to the following procedure.
Step 1)
Measurements are carried out in a standard laboratory atmosphere (see JIS K 7100) at a temperature of 23°C and a relative humidity of 50%.
Step 2)
The test piece is placed on the substrate of the hand coater (4.1), a few drops of the test mixture are placed on the test piece, and immediately spread by pulling the wire bar.
If using a cotton swab or brush to spread the test mixture, spread the liquid quickly over an area of at least 6 ^cm² , using enough liquid to form a thin layer without pooling.
The wetting tension is judged by observing the liquid film of the test mixture in a bright place and determining the state of the liquid film after 3 seconds. If the liquid film does not break and remains in the same state as when it was applied for 3 seconds or more, it is considered to be wet. If the wetness remains for 3 seconds or more, proceed to the mixture with the next highest surface tension. Conversely, if the liquid film breaks in 3 seconds or less, proceed to the mixture with the next lowest surface tension.
This procedure is repeated to select a mixture that can accurately wet the surface of the test piece in 3 seconds.

(13) Releasability (tape peeling force)
A laminate was produced by laminating the adhesive layer surface of a 50 mm wide acrylic adhesive tape No. 31B (manufactured by Nitto Denko Corporation) using a hand roller to the release agent layer surface of a biaxially oriented polypropylene film coated with the release agent described in the Examples. After storing this laminate at 23°C for 24 hours, it was pulled at a speed of 300 mm/min using a T-peel method with the release film facing downward and the adhesive tape facing upward in a tensile tester, and the peel force measured in an atmosphere of 23°C was evaluated as follows.
○: Peeling strength is less than 1000 mN/50 mm ×: Peeling strength is 1000 mN/50 mm or more

(14) Wrinkles in Release Film Roll The biaxially oriented polypropylene film coated with a release agent described in the Examples was unwound from the roll, and wrinkles that occurred in the release film were evaluated by the following method.
That is, a 60 cm wide release film was hung in a room at a temperature of 25°C and a humidity of 65% so that the film's longitudinal direction was vertical, and a load of 10 N/m was applied and left to stand for 30 minutes. A fluorescent lamp was projected onto the film surface from 45° above and 1 m away from the surface on which the number of continuous corrugated wrinkles in the longitudinal direction was to be counted, and the number of wrinkles was visually counted and evaluated from 45° below and 0.5 m away from the surface on which the wrinkles were counted. A wrinkle that was convex in the longitudinal direction of the film relative to the surface being observed was counted as one wrinkle, and the number of wrinkles in the width direction of the film was counted.
◯: The number of wrinkles was 10 or less per meter. ×: The number of wrinkles was 11 or more per meter.

Example 1
(Method for producing biaxially oriented polypropylene film for release)
As the polypropylene resin, 80 parts by mass of propylene homopolymer PP-1 (manufactured by Sumitomo Chemical Co., Ltd., Sumitomo Noblen FLX80E4) having MFR = 7.5 g / 10 min, [mmmm] = 98.9%, Tc = 116.2 ° C., Tm = 162.5 ° C., and 20 parts by mass of propylene homopolymer PP-2 (manufactured by Sumitomo Chemical Co., Ltd., EL80F5) having MFR = 11 g / 10 min, [mmmm] = 98.8 % , Tc = 116.5 ° C., Tm = 161.5 ° C., were blended and used.
The film was extruded into a sheet form through a T-die at 250 ° C, brought into contact with a 20 ° C cooling roll, and then placed in a water bath at 20 ° C. It was then stretched 4.5 times in the longitudinal direction with two pairs of rolls at 142 ° C., then clamped at both ends with clips and introduced into a hot air oven. After preheating at 170 ° C., it was stretched 10 times in the width direction at 162 ° C. as the first stage. Immediately after stretching in the width direction, it was cooled at 120 ° C. while still held by the clips, and then re-stretched 1.2 times in the width direction at 175 ° C., after which it was cooled to room temperature. One side of the film was corona-treated using a corona treater manufactured by Softal Corona & Plasma GmbH at an applied current of 0.75 A, and then wound up on a winder. The thickness of the film thus obtained was 18.6 μm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the film obtained were very excellent in rigidity, and the film had a low thermal shrinkage rate at high temperatures.

(Preparation of biaxially stretched polypropylene film coated with release agent)
On the corona-treated surface side of a roll of biaxially oriented polypropylene film for release, a coating liquid was added to a toluene/methyl ethyl ketone ( 50/50 ) solution (solids concentration 1% by mass ) of a thermosetting silicone compound (KS-774, manufactured by Shin-Etsu Silicones Co., Ltd.) with 1 part by mass of Pt catalyst (PL-50T, manufactured by Shin-Etsu Silicones Co., Ltd.) per 100 parts by mass of the solids content of KS-774 at a coating amount (wet) of 3 g/m ² using a coater, and the release layer was dried for 20 seconds at a conveying tension of 2000 kPa and a drying temperature of 150 ° C. using an air floating conveying dryer with a distance of 38 cm between the lower and upper air flow outlets, to obtain a release film with a mass of 0.03 g/m ² after drying. After drying, the film was cooled at a rate of 20 ° C./sec using a 50 ° C. cooling roll and then wound up into a roll to obtain a release film roll. The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the film were excellent in rigidity, had a low thermal shrinkage rate at high temperatures, and the release film coated with a release agent had excellent releasability and was a smooth, wrinkle-free film.

Example 2
The same procedure as in Example 1 was carried out, except that the film was re-stretched to 1.2 times its width at 165°C. The thickness of the obtained film was 18.4 µm. Table 1 shows the structure of the polypropylene resin, and Table 2 shows the film-forming conditions. As shown in Table 3, the physical properties of the film were excellent in rigidity, had a low thermal shrinkage rate at high temperatures, and the release film coated with a release agent had excellent releasability and was a smooth, wrinkle-free film.

Example 3
The procedure was the same as in Example 1, except that the film was stretched in the longitudinal direction at 147°C, then stretched 10 times in the width direction at 165°C as the first stage, cooled to 120°C while still held by the clips immediately after the width direction stretching, and then re-stretched 1.2 times in the width direction at 177°C. The thickness of the resulting film was 18.9 µm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the film were high rigidity and low heat shrinkage at high temperatures, and the release film coated with the release agent had excellent releasability and was a smooth, wrinkle-free film.

Example 4
The same procedure as in Example 3 was repeated except that the film was re-stretched to 1.1 times the width direction at 177° C. The thickness of the obtained film was 20.6 μm. Table 1 shows the structure of the polypropylene resin, and Table 2 shows the film-forming conditions.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the film were high in rigidity and low in heat shrinkage at high temperatures, and the release film coated with the release agent had excellent releasability and was a smooth, wrinkle-free film.

(Comparative Example 1)
The same procedures as in Example 1 were carried out, except that the film was stretched 12 times in the width direction at 162°C as the first stage, and immediately after the width direction stretching, it was cooled to 100°C while still held by the clips, and then heat-set at 170°C while maintaining the width constant. The thickness of the obtained film was 20.8 µm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the film were high in rigidity, but poor in thermal shrinkage at high temperatures, and wrinkles occurred in the release film coated with a release agent, resulting in poor processability as a release film.

(Comparative Example 2)
The same procedure as in Example 1 was repeated, except that the film was stretched 12 times in the width direction at 162°C as the first stage, and immediately after the width direction stretching, it was not cooled while still held by the clips, but was heat-set at 172°C while maintaining a constant width. The thickness of the obtained film was 23.1 µm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the film were poor in rigidity, and the release film coated with the release agent developed wrinkles, resulting in poor processability as a release film.

(Comparative Example 3)
The same procedures as in Example 1 were carried out except that the film was stretched 12 times in the width direction at 168°C as the first stage, and immediately after the width direction stretching, it was cooled to 100°C while still held by the clips, and then heat-set at 170°C while maintaining the width constant. The thickness of the obtained film was 18.7 µm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the film were high in heat shrinkage at high temperatures, poor in rigidity, and wrinkles occurred in the release film coated with the release agent, resulting in poor processability as a release film.

(Comparative Example 4)
As the polypropylene resin, PP-3 (manufactured by Japan Polypropylene Corporation, FL203D) having MFR = 3 g / 10 min, [mmmm] = 94.8%, Tc = 117.2 ° C, Tm = 160.6 ° C was used. It was extruded into a sheet from a T-die at 250 ° C, brought into contact with a 20 ° C cooling roll, and then placed in a water bath at 20 ° C. Thereafter, it was stretched 4.5 times in the longitudinal direction at 130 ° C., and in the width direction stretching with a tenter, the preheating temperature was set to 168 ° C., and 8.2 times stretching was performed at 155 ° C. as the first stage of stretching. Immediately after width direction stretching, it was cooled at 120 ° C. while held by the clip, and then re-stretched 1.2 times in the width direction at 170 ° C. Finally, it was cooled at room temperature. The thickness of the obtained film was 18.8 μm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As shown in Table 3, the physical properties of the polypropylene resin were high in heat shrinkage at high temperatures and poor in rigidity, and the release film coated with a release agent had poor releasability and wrinkles occurred, resulting in poor processability as a release film.

(Comparative Example 5)
A film was obtained using a blend of PP-1 and PP-2 as the polypropylene resin in the same manner as in Example 1, without re-stretching in the width direction, and heat-treated at 168°C under the film-forming conditions shown in Table 2. The thickness of the obtained film was 20.0 µm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As for the physical properties, as shown in Table 3, the heat shrinkage rate at high temperatures was low, but the rigidity was poor, and the release film coated with the release agent developed wrinkles, resulting in poor processability as a release film.

(Comparative Example 6)
A polypropylene resin, PP-4 (SA4L, manufactured by Japan Polypropylene Corporation) having an MFR of 5 g/10 min, [mmmm] of 97.3%, Tc of 116.8°C, and Tm of 161.6°C, was used, and a film was obtained under the film-forming conditions shown in Table 2, in which the film was heat-treated at 168°C without being re-stretched in the width direction. The thickness of the obtained film was 20.0 μm.
The structure of the polypropylene resin is shown in Table 1, and the film-forming conditions are shown in Table 2. As for the physical properties, as shown in Table 3, the heat shrinkage rate at high temperatures was low, but the rigidity was poor, and the release film coated with the release agent developed wrinkles, resulting in poor processability as a release film.

The biaxially oriented polypropylene film for release of the present invention has high rigidity and can be made thin. Even when the film is thin, it is easy to peel off and there is little wrinkling during the coating process of a release agent, so it can be suitably used as a release film.

Claims (11)

A biaxially oriented polypropylene film for release purposes that satisfies the following (1) and (2):
(1) In the azimuth angle dependence of the (110) plane of polypropylene α-crystals obtained by wide-angle X-ray diffraction measurement, the half-value width of the peak resulting from oriented crystals in the width direction is 26° or less.
(2) When separated into a crystalline component (I), a constrained amorphous component (II), and an unconstrained amorphous component (III) as determined by pulsed NMR using the solid echo method, the ratio of (III) is 7% or less. 2. The biaxially oriented polypropylene film for release purposes according to claim 1, which satisfies the following (3), (4) and (5):
(3) The thermal shrinkage at 150°C is 10% or less in the longitudinal direction and 30% or less in the transverse direction.
(4) The stress at 5% elongation in the width direction at 23°C is 140 MPa or more.
(5) Heat shrinkage rate (%) in the width direction at 150°C and stress (MPa) at 5% elongation in the width direction at 23°C
satisfies the following formula:
Stress (MPa) at 5% elongation in the width direction at 23°C ≥ Heat shrinkage rate (%) in the width direction at 150°C × 4.0 + 140 The biaxially oriented polypropylene film for release molds according to claim 1 or 2, wherein the biaxially oriented polypropylene film for release molds satisfies the following (6) and (7):
(6) The heat shrinkage rate at 120°C is 2.0% or less in the longitudinal direction and 10.0% or less in the transverse direction.
(7) The heat shrinkage rate (%) in the width direction at 120°C and the tensile modulus (GPa) in the width direction at 23°C satisfy the following formulas.
Tensile modulus (GPa) in the width direction at 23°C ≥ Heat shrinkage (%) in the width direction at 120°C × 0.3 + 7.0 The biaxially oriented polypropylene film for release purposes according to any one of claims 1 to 3, wherein the refractive index Ny in the width direction of the biaxially oriented polypropylene film for release purposes is 1.5250 or more and ΔNy is 0.0240 or more. The biaxially oriented polypropylene film for release purposes according to any one of claims 1 to 4, wherein the haze of the biaxially oriented polypropylene film for release purposes is 5.0% or less. The biaxially oriented polypropylene release film according to any one of claims 1 to 5, wherein the polypropylene resin constituting the biaxially oriented polypropylene release film has a mesopentad fraction of 97.0% or more. The biaxially oriented polypropylene film for release molds according to any one of claims 1 to 6, wherein the polypropylene resin constituting the biaxially oriented polypropylene film for release molds has a crystallization temperature of 105°C or higher and a melting point of 160°C or higher. The biaxially oriented polypropylene release film according to any one of claims 1 to 7, wherein the polypropylene resin constituting the biaxially oriented polypropylene release film has a melt flow rate of 4.0 g/10 min or more. The biaxially oriented polypropylene film for release molds according to any one of claims 1 to 8, wherein the amount of components with a molecular weight of 100,000 or less in the polypropylene resin constituting the biaxially oriented polypropylene film for release molds is 35% by mass or more. A release film in which a release coating layer is laminated on at least one surface of the biaxially oriented polypropylene film for release use described in any one of claims 1 to 9. A laminated film comprising a biaxially oriented polypropylene film satisfying the following (1) and (2), and a release coating layer:
(1) In the azimuth angle dependence of the (110) plane of polypropylene α-crystals obtained by wide-angle X-ray diffraction measurement, the half-value width of the peak resulting from oriented crystals in the width direction is 26° or less.
(2) When separated into a crystalline component (I), a constrained amorphous component (II), and an unconstrained amorphous component (III) as determined by pulsed NMR using the solid echo method, the ratio of (III) is 7% or less.