Laminated film, release laminated film and release film

Abstract

To provide a laminate film that has excellent rigidity and heat resistance and is easy to peel even if the film thickness is thinner than that of a conventional product, does not easily wrinkle, and does not curl easily.SOLUTION: Provided are a biaxially oriented polypropylene film that satisfies the following (1) and (2), and a laminate film that includes release coating layer. (1) The ratio of (III) when separated into crystalline component (I), constrained amorphous component (II), and unconstrained amorphous component (III) as determined by pulse NMR according to the solid echo method is 7% or less. (2) The relationship between the loop stiffness stress (S) in the width direction and the thickness (t) satisfies the following formula. S[mN]≥0.0010×Thickness(μm)3.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polypropylene release film having excellent rigidity and heat resistance. More specifically, the present invention relates to a biaxially oriented polypropylene release film having excellent flatness during release processing and small thickness unevenness, even when the film thickness is thinner than conventional products, and is suitable for use as a process release film. [Background technology]

[0002] Biaxially oriented polypropylene films are moisture-proof and have the necessary rigidity and heat resistance, making them suitable for industrial applications such as packaging, release films, and adhesive tapes. In recent years, environmental considerations have led to demands for recyclability and volume reduction through thinner film thickness for release films, making it essential to significantly improve the rigidity of polypropylene films. Methods for improving rigidity include improving the crystallinity and melting point of polypropylene resins by improving catalysts and process technology during polymerization, and increasing the stretch ratio during the film-forming process to enhance the film's orientation. However, increasing rigidity has come with the problem of reduced heat resistance, and to date, no biaxially oriented polypropylene film has possessed sufficient rigidity and heat resistance.

[0003] In the manufacturing process of biaxially oriented polypropylene film, a method has 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 equal to or lower than 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). Another method has been proposed in which, after width direction stretching, further longitudinal stretching is performed (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 and curling 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, the orientation in the width direction is low and the rigidity is insufficient. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. WO2016 / 182003 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-177645 [Patent Document 3] Japanese Patent Application Laid-Open No. 2001-40111 Summary of the Invention [Problem to be solved by the invention]

[0005] 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 a process release film with little thickness unevenness, without wrinkling or curling during release processing, and with good flatness, even when the film thickness is thinner than conventional products to reduce film waste after use and reduce environmental impact. [Means for solving the problem]

[0006] As a result of extensive research into achieving this object, the present inventors have arrived at the following inventions [1] to

[11] . [1] A laminated film comprising a biaxially oriented polypropylene film that satisfies the following (1) and (2), and a release coating layer. (1) When separated into crystalline component (I), constrained amorphous component (II), and unconstrained amorphous component (III) as determined by pulsed NMR using the solid echo method, the ratio of (III) is 7% or less. (2) The relationship between the loop stiffness stress (S) in the width direction and the thickness (t) satisfies the following formula. S [mN] ≥ 0.0010 × thickness (μm) 3 [2] The laminated film according to [1], wherein the heat shrinkage rate of the biaxially oriented polypropylene film for release in the width direction at 150°C is 10% or less in the longitudinal direction and 30% or less in the width direction. [3] The laminated film according to [1] or [2], wherein the refractive index Ny in the width direction of the biaxially oriented polypropylene film for release is 1.5250 or more, and ΔNy is 0.0240 or more. [4] The laminated film according to any one of [1] to [3], wherein the biaxially oriented polypropylene release film has a haze of 5.0% or less. [5] [6] The laminated film according to any one of [1] to [4], wherein the polypropylene resin constituting the biaxially oriented polypropylene film for release has a mesopentad fraction of 97.0% or more. The laminated film according to any one of [1] to [5], wherein 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. [7] The laminated film according to any one of [1] to [6], wherein the polypropylene resin constituting the biaxially oriented polypropylene release film has a melt flow rate of 4.0 g / 10 min or more. [8] The laminated film according to any one of [1] to [7], wherein the content of components having a molecular weight of 100,000 or less in the polypropylene resin constituting the biaxially oriented polypropylene film for release is 35% by mass or more. [9] The laminated film according to any one of [1] to [8], wherein the release coating layer is made of a silicone resin.

[10] The laminated film according to any one of [1] to [9], which is for use as a release film.

[11] A release film comprising the laminated film according to any one of [1] to [9]. [Effects of the Invention]

[0007] The present invention relates to a biaxially oriented polypropylene release film having excellent rigidity and heat resistance. More specifically, the biaxially oriented polypropylene release film has good flatness and small thickness unevenness during release processing, even when the film thickness is thinner than conventional products, and is therefore suitable for use as a process release film. [Brief explanation of the drawings]

[0008] [Figure 1] Schematic diagram of component separation of the spin-spin relaxation time decay curve observed by 1H-pulse NMR. DETAILED DESCRIPTION OF THE INVENTION

[0009] 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.

[0010] (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. Even if ethylene and / or an α-olefin having 4 or more carbon atoms is contained, the amount of the ethylene and / or α-olefin having 4 or more carbon atoms 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. Crystallinity within the above range tends to be improved. Examples of α-olefins having 4 or more carbon atoms that constitute such copolymers 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 types of polypropylene, a copolymer with ethylene and / or an α-olefin having 4 or more carbon atoms, or a mixture thereof.

[0011] (stereoregularity) The mesopentad fraction (hereinafter sometimes 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 increases, improving the melting point, crystallinity, and crystalline orientation of the crystals in the film, making it easier to achieve 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) method. It is more preferably 99.5% or less. The mesopentad fraction is measured by nuclear magnetic resonance (NMR) spectroscopy. 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 co-catalyst, and a method of appropriately selecting the components of the polypropylene resin composition are preferably employed.

[0012] (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 crystal nucleating agent with the polypropylene resin. Tm is the main endothermic peak temperature associated with melting, 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 under 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.

[0013] (crystallization temperature) The lower limit of the crystallization temperature (Tc) of the polypropylene resin constituting the biaxially oriented polypropylene film for release use of the present invention, as measured by DSC, is 105°C, preferably 108°C, and more preferably 110°C. If 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, still more preferably 130°C, particularly preferably 128°C, and most preferably 127°C. If 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 under a nitrogen atmosphere, and cooled to 30°C at a scanning rate of -10°C / min.

[0014] (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, it is easy to obtain a biaxially oriented polypropylene film for release use with low thermal shrinkage. Furthermore, when the melt flow rate (MFR) of the polypropylene resin is 30 g / 10 min or less, the film formability is easily maintained.

[0015] 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 is large. 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, and the degree of crystallization in the film is more likely to be increased. In addition, the entanglement of polypropylene molecular chains in the amorphous portion is reduced, making it easier to increase 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 of controlling the average molecular weight or molecular weight distribution of the polypropylene resin.

[0016] 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% by mass, preferably 38% by mass, more preferably 40% by mass, even more preferably 41% by mass, and particularly preferably 42% by 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 high-molecular-weight components with long relaxation times or long-chain branched components, it becomes easier to adjust the amount of components with a molecular weight of 100,000 or less contained in the polypropylene resin without significantly changing the overall viscosity, making it easier to improve film-forming properties without significantly affecting rigidity or thermal shrinkage.

[0017] (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), 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 within the above range, it is easy to increase the amount of components with a molecular weight of 100,000 or less.

[0018] The molecular weight distribution of polypropylene resins 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 in 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.

[0019] (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.

[0020] 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 each side may be one, two, or three or more, 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.

[0021] 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.

[0022] Next, the sheet is stretched in the longitudinal direction between two pairs of heated stretching rolls by increasing the rotation speed of the rear stretching roll, to obtain a uniaxially stretched film.

[0023] Subsequently, the uniaxially stretched film is preheated and then stretched in the width direction at a specific temperature while holding the film edges in a tenter-type stretching machine to obtain a biaxially stretched film. This width direction stretching step will be described in detail later.

[0024] After the width direction stretching step is completed, the biaxially stretched film is heat-treated at a specific temperature to obtain a biaxially oriented film for release. In the heat treatment step, the film may be relaxed in the width direction.

[0025] The biaxially oriented polypropylene film for release use thus obtained can be subjected to a corona discharge treatment on at least one side as required, and then wound up on a winder to obtain a film roll.

[0026] 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 is extruded into a sheet form from a T-die and brought into contact with a metal cooling roll to be cooled and solidified. The obtained unstretched sheet is then preferably 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. 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. A cooling temperature of 50°C or less tends to increase the transparency of the unstretched sheet, and 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 3500 μm or less in terms of cooling efficiency, and more preferably 3000 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.

[0027] (Longitudinal stretching process) The lower limit of the longitudinal stretching ratio is preferably 3.5, more preferably 3.8, and particularly preferably 4.2. 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, and particularly preferably 7. When the ratio is within the above 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 the stretching rolls 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.

[0028] (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. The temperature is more preferably Tm+2 to Tm+20°C, and particularly preferably Tm+3 to Tm+15°C. Here, the highest temperature in the preheating step is defined as the preheating temperature.

[0029] (Width direction stretching process) In the width direction stretching step after the preheating step, a preferred method is as follows.

[0030] 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.

[0031] 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 small and the film is less likely to break during stretching. When a later section is added, the total stretching ratio is adjusted to be within the above range. In this case, the lower limit of the stretching 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 stretching ratio at the end of the earlier section is preferably 15 times, more preferably 14 times, and even more preferably 13 times.

[0032] It is preferable to cool the film immediately after the end of width direction stretching, i.e., when the final width direction stretch ratio is reached. The cooling temperature is preferably equal to or lower than the width direction stretching temperature and is preferably Tm-80°C or higher but not higher than Tm-15°C, more preferably Tm-80°C or higher but not higher than Tm-20°C, even more preferably Tm-80°C or higher but not higher than Tm-30°C, and particularly preferably Tm-70°C or higher but not higher than Tm-40°C. The addition of the cooling step causes crystallization and fixes the crystalline orientation, and the orientation history is maintained even when the temperature is subsequently raised to a temperature higher than the melting point, 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.

[0033] 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 stretched again in the width direction at a high temperature after cooling, the crystalline orientation of the film is easily increased, and therefore the rigidity is easily increased. The lower limit of the stretching temperature when re-stretching 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. At 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 likely to be 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 may become uneven, or the film may break.

[0034] 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 stretching can easily eliminate the effects of molecular chain entanglement. As a result, the molecular chains are less likely to be constrained, which makes it difficult for disordered molecular orientation to occur and allows sufficient crystallization to proceed. 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.

[0035] 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.

[0036] In the typical 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 temperatures 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 rate. This relaxation eliminates the difficulty of crystallization due to the constraints on molecular chains, contributing to a reduction in the heat shrinkage rate. However, on the other hand, the reduction in the transverse orientation of the molecular chains generated during the transverse stretching process also reduces rigidity, making it difficult to achieve both low heat shrinkage and high rigidity. Furthermore, there is also the problem of whitening of the film when the temperature is too high. In the normal film-making process (extrusion - longitudinal stretching - width stretching - heat treatment), increasing the temperature in the width stretching step increases the mobility of the molecular chains, and stretching is performed so as to leave no distortion. This promotes the melting of the crystals formed by the longitudinal stretching, resulting in a decrease in crystalline orientation. In the method of the present invention, the film is stretched once in the width direction to achieve sufficient width orientation, and then the film, which has sufficient tension even when the fixed crystal orientation is melted by cooling, is 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 should be at least 1.05 times, so long as it is sufficient to disentangle and align the molecular chains. A stretching ratio of 2 times or less is less likely to cause thickness unevenness.

[0037] 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), and therefore, the resulting biaxially oriented film is more likely to have a strong crystalline orientation and produce more crystals with a high melting point.

[0038] 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.

[0039] (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 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. If the rigidity is to be further increased, heat treatment may not be performed.

[0040] (cooling process) After stretching in the width direction, the film is preferably cooled immediately after being stretched again in the width direction at Tm-5°C or higher, or immediately after the heat treatment step. The cooling temperature 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.

[0041] (film thickness) The thickness of the biaxially oriented polypropylene film for release purposes 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, film rigidity 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. It is then 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.

[0042] (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, making it suitable for use in applications requiring precision. The measurement method was as follows: A test piece 40 mm in width was cut out from the 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

[0043] (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-crystal 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."

[0044] (Crystalline component (I), constrained amorphous component (II), unconstrained amorphous component (III) determined by pulsed NMR) 1 It is known that the decay time constant of the FID (Free Induction Decay) of the spin-spin relaxation time T2 observed in H-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 as the sum of three components: the crystalline component, the mesophase component, and the amorphous component, using the solid echo method of pulsed NMR. 1The spin-spin relaxation time T2 observed by H-pulse NMR decreases in the following order: crystalline, mesophase, and amorphous. The mesophase has a faster T2 than the amorphous phase, and is considered to be amorphous with restricted mobility. When stretching is performed while disentangling the molecular chains, a highly oriented crystalline component (I) is formed, and an amorphous chain component (II) with restricted mobility (corresponding to the mesophase) is formed near the crystalline component. On the other hand, when entanglement is strong 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 form. The unrestricted amorphous component (III) has high mobility and is more 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 restricted amorphous chain (II) is less likely to shrink at high temperatures because its movement is suppressed compared to the amorphous component (III).

[0045] The upper limit of the unconstrained amorphous component (III) in 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, and furthermore, the film is less likely to curl after release processing. 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, when measuring the gel permeation chromatography (GPC) integrated curve of the polypropylene resin constituting the film, to 35% by mass. Here, the fact that the amount of unconstrained amorphous component (III) determined by pulse NMR is 7% or less means that there are few molecular chains with large distortions in molecular orientation constrained by entanglement points, and that the film is less likely to shrink even when crystal melting begins, is less likely to wrinkle even when the release coating film is dried at high temperatures, and is less likely to curl after release processing.

[0046] When the amount 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 the molecular orientation, so that shrinkage occurs as soon as the crystals start to melt, and the film is prone to wrinkles after release processing at high temperatures, and the film is prone to curling after the release coating film has dried. Although there is no particular lower limit for the unconstrained amorphous component (III), a practical limit is 0.1% or more. If the unconstrained amorphous component (III) is reduced to less than 0.1%, the film must be stretched again in the width direction at a higher temperature after the sequential biaxial stretching. This reduces the tension during stretching due to melting, which may result in breakage. Furthermore, the crystalline orientation in the film may be weakened, resulting in reduced rigidity.

[0047] (Heat shrinkage rate at 150℃) 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% or less. 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% or less. A longitudinal heat shrinkage rate of 10% or less and a transverse heat shrinkage rate of 30% or less reduces wrinkles during drying of the release coating and also reduces curling of the film after release processing. In particular, a longitudinal heat shrinkage rate of 8.0% or less and a transverse heat shrinkage rate of 150°C or less at 15°C broadens the controllable range of the roll tension during release processing, thereby reducing wrinkles during drying of the release coating and reducing curling of the film after release processing. A longitudinal heat shrinkage rate of 10% or more and a transverse heat shrinkage rate of 30% or more reduces wrinkles during drying of the release coating and further reduces handleability due to curling of the release-coated film. 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, as measured by gel permeation chromatography (GPC) integration curve of the polypropylene resin constituting the film, to 35% by mass.

[0048] (Tensile modulus at 23°C) The lower limit of the longitudinal tensile modulus of the biaxially oriented polypropylene film for release films 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 rigidity is high, so that the release coating film is less likely to wrinkle when dried, and the film is less likely to curl after release processing. 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 easier and the balance of longitudinal and transverse properties is more likely to be improved. The lower limit of the tensile modulus in the width direction at 23°C of the biaxially oriented polypropylene film for release purposes of the present invention is preferably 5.5 GPa, more preferably 5.6 GPa, more preferably 5.7 GPa, even more preferably 5.8 GPa, particularly preferably 5.9 GPa, and most preferably 6.0 GPa. At 5.5 GPa or higher, the rigidity is high, so that the release coating film is less likely to wrinkle when dried, and the film is less likely to curl after release processing. 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 properties in the longitudinal and width directions 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.

[0049] (Tensile 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, the film is less likely to curl after release processing, and the film is less likely to break 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, the film is less likely to break. The lower limit of the widthwise tensile breaking strength at 23°C of the biaxially oriented polypropylene film for release purposes 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, the film is less likely to curl after release processing, and the film is less likely to break during release processing. The upper limit of the widthwise tensile breaking strength is preferably 650 MPa, more preferably 600 MPa, and even more preferably 550 MPa as a practical value. If the pressure is below 650 MPa, the film is less likely to break. The tensile breaking strength can be kept within the range by adjusting the stretching ratio, stretching temperature, and heat setting temperature.

[0050] (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% or more. 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% as a practical value, more preferably 280%.

[0051] The lower limit of the tensile elongation at break in the width direction of the biaxially oriented polypropylene film for release purposes of the present invention at 23°C 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 elongation at break 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.

[0052] (refractive index) The lower limit of the refractive index (Nx) in the longitudinal direction of the biaxially oriented polypropylene film for release molds 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 lower, the film is likely to have an excellent balance of properties in the longitudinal direction and the width direction.

[0053] The lower limit of the refractive index (Ny) in the width direction of the biaxially oriented polypropylene film for release molds of the present invention is 1.5250, preferably 1.5253, more preferably 1.5255, even more preferably 1.5260, and particularly preferably 1.5265. If it is 1.5250 or higher, the rigidity of the film is likely to be increased. 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. If it is 1.5280 or less, the film is likely to have an excellent balance of properties in the longitudinal direction and the width direction.

[0054] The lower limit of the refractive index (Nz) in the thickness direction of the biaxially oriented polypropylene film for release molds 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.

[0055] (△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. It means the degree of orientation in the width direction of the entire orientation in the longitudinal, width, and thickness directions of the film. △Ny=Ny-[(Nx+Nz) / 2]

[0056] (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.

[0057] (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 the refractive index is 1.5080 or higher, wrinkles are less likely to occur during drying of the release coating film, and the film is less likely to curl after release processing. The average refractive index can be kept within this 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 along the longitudinal, transverse, and thickness directions of the film, respectively. Average refractive index = (Nx + Ny + Nz) / 3

[0058] (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. A practical lower limit for the haze is preferably 0.1%, more preferably 0.2%, even more preferably 0.3%, and particularly preferably 0.4%. A haze of 0.1% or more facilitates production. 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 with a molecular weight of 100,000 or less. However, the haze may increase by adding an antiblocking agent or providing a sealing layer.

[0059] (FWHM of diffraction peaks originating 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° or less, more preferably 24° or less, particularly preferably 23° or less, and most preferably 22.0° or less. When the half-width (Wh) is 26° or less, the rigidity of the film is easily increased. The lower limit of Wh is preferably 15°, more preferably 16°, and even more preferably 17°.

[0060] (X-ray orientation degree) The lower limit of the degree of X-ray orientation of the biaxially oriented polypropylene film for release use of the present invention, calculated from Wh 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 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 it to 0.917 or less, film formation tends to be stable.

[0061] (Loop stiffness stress) The lower limit of the longitudinal loop stiffness stress S (mN) of the biaxially oriented polypropylene film of the present invention at 23°C is 0.00020 × t (μm), where t is the thickness of the biaxially oriented polypropylene film. 3 and preferably 0.00025×t 3 and more preferably 0.00030×t 3 and more preferably 0.00035×t 3 If it is 0.00020×t3 or more, the peeling force of the release film tends to be small. In addition, since the film rigidity is high, wrinkles are less likely to occur when the release coating film dries, and furthermore, curling of the film after release processing is less likely to occur. The upper limit of the longitudinal loop stiffness stress S (mN) at 23°C is preferably 0.00080×t 3 and more preferably 0.00075×t 3 and more preferably 0.00072×t 3 and particularly preferably 0.00070×t 3 0.00080×t 3 If it is less than this, it is easy to manufacture in practice. The lower limit of the loop stiffness stress S (mN) in the width direction of the biaxially oriented polypropylene film of the present invention at 23°C is 0.0010 × t 3 and preferably 0.0011×t 3 and more preferably 0.0012×t 3 and more preferably 0.0013×t 3 0.0010×t 3If the value is equal to or greater than this, the peeling force of the release film tends to be small. In addition, since the film rigidity is high, wrinkles are less likely to occur when the release coating film dries, and furthermore, curling of the film after release processing is less likely to occur. The upper limit of the loop stiffness stress S (mN) in the width direction at 23°C is preferably 0.0020 x t 3 and more preferably 0.0019×t 3 and more preferably 0.0018×t 3 and particularly preferably 0.0017×t 3 0.0020×t 3 If it is less than this, it is easy to manufacture in practice.

[0062] Loop stiffness stress is an index of the stiffness of a film, but it also depends on the film thickness. The measurement method is as follows: Two 110 mm x 25.4 mm strips were cut from each film, with the long axis (loop direction) aligned with the longitudinal direction of the film or the long axis (loop direction) aligned with the width direction of the film. These were then clamped between clips to create measurement loops, one with one side of the film facing the inner surface of the loop and the other with the other side facing the inner surface of the loop, with the long axis of the strip aligned with the longitudinal and width directions of the film. The measurement loops, with the long axis of the strip aligned with the longitudinal direction of the film, were placed in the chuck of a Toyo Seiki Co., Ltd. loop stiffness tester DA with the width direction perpendicular. The clips were then removed, and the loop stiffness stress was measured with a chuck spacing of 50 mm, a compression depth of 15 mm, and a compression speed of 3.3 mm / s. The measurements were made five times with one side of the film facing the inside of the loop, measuring the loop stiffness stress and thickness, and then five times with the other side facing the inside of the loop. Using the data from these 10 measurements, the cube of the thickness of each test piece was plotted on the horizontal axis and the loop stiffness stress on the vertical axis, and the curve was approximated with a straight line with an intercept of 0 to determine the slope a. The slope a value represents a characteristic value specific to the film that determines stiffness and is independent of thickness. Measurement loops in which the long axis of the strip was in the width direction of the film were also measured in the same way.

[0063] (Biaxially oriented polypropylene film with release coating) 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 biaxially oriented polypropylene film of the present invention preferably has a surface wetting tension of 38 mN / m or more. A wetting tension of 38 mN / m or more improves adhesion to a release coating layer. To achieve a wetting tension of 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 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.

[0064] The silicone resin used as the release agent in the present invention can be any silicone resin commonly used in release agents, and can be selected from silicone resins commonly used in the relevant field, such as those described in "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, and examples of ionizing radiation-curable silicone resins include ultraviolet- or electron beam-curable silicone resins. These are applied to a film substrate and then dried or cured to form a release layer.

[0065] The curable silicone resin preferably has a degree of polymerization after curing of about 500,000 to 200,000, and particularly about 1,000 to 100,000. Specific examples of such resins include the following: 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; Dow Corning DK-Q3-202, DK-Q3-203, DK-Q3-204, DK-Q3-205, DK-Q3-210, DK-Q3-240, DK-Q3-3 003, 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 can be used alone or in combination of two or more.

[0066] The silicone resin of the addition reaction system of the curable silicone resin is, for example, a silicone resin that is cured by reacting polydimethylsiloxane with a vinyl group at the end or side chain and hydrogen siloxane using a platinum catalyst. In this case, it is more preferable to use a resin that can be cured within 30 seconds at 120°C, as this allows 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 Toray Dow Corning Co., Ltd., and solvent addition + UV-cure types (X62-5040, X62-5065, X62-5072T, KS5508, etc.) and dual-cure types (X62-283, X62-2834, X62-1980, etc.) manufactured by Shin-Etsu Chemical Co., Ltd.

[0067] An example of the condensation reaction silicone resin of the curable silicone resin is one in which a three-dimensional crosslinked structure is formed by condensing a polydimethylsiloxane having an OH group at its terminal with a polydimethylsiloxane having an H group at its terminal using an organotin catalyst.

[0068] Examples of the UV-curable or electron beam-curable silicone resins among the above-mentioned curable silicone resins include, for example, the most basic types: resins that crosslink and cure by a radical reaction similar to normal silicone rubber crosslinking, resins that photocure by the introduction of acrylic groups, resin compositions in which an onium salt is decomposed by UV light to generate a strong acid that then cleaves the epoxy ring to crosslink, and resin compositions that crosslink by an addition reaction of a thiol to a vinyl siloxane. Because electron beams have more energy than UV rays, a radical-based crosslinking reaction occurs without the need for an initiator as in the case of UV curing.

[0069] The release agent used in the release coating layer of the present invention may be used alone or in combination of two or more. In order to adjust the release force, it is also possible to mix an additive such as a light release additive or a heavy release additive.

[0070] The release coating layer of the present invention 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 that form protrusions. The release coating layer in the present invention 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.

[0071] The thickness of the release coating layer in the present invention may be set depending on the 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 release performance is maintained. Furthermore, a release coating layer thickness of 2 μm or less is preferable because the curing time is not too long and there is no risk of thickness unevenness due to a decrease in the flatness of the release film.

[0072] 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 a biaxially oriented polypropylene film for release use as a substrate, and the solvent is removed by drying, followed by heat drying, heat curing, or UV curing. In this case, the drying temperature during solvent drying and heat curing is preferably 100 to 170°C. A drying temperature higher than 170°C may cause heat-induced wrinkles in the film. On the other hand, a low drying temperature may result in insufficient heat curing of the release coating layer, resulting in insufficient release properties. Biaxially oriented polypropylene films have lower thermal dimensional stability at high temperatures than biaxially oriented polyester films, and thus are prone to heat-induced wrinkles during drying of the coating film. However, the biaxially oriented polypropylene film for release use of the present invention can suppress wrinkles. The heat drying time is typically 10 to 60 seconds.

[0073] As a method for applying the release agent coating liquid, any known coating method can be applied, and for example, conventionally known methods such as roll coating methods such as gravure coating and reverse coating, bar coating methods such as wire bar coating, die coating, spray coating, and air knife coating can be used. [Example]

[0074] 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.

[0075] (2) Mesopentad fraction The mesopentad fraction ([mmmm]%) of polypropylene resin is measured as follows: 13 The mesopentad fraction was calculated according to the method described in Zambelli et al., Macromolecules, Vol. 6, p. 925 (1973). 13C-NMR measurement was performed using AVANCE500 manufactured by BRUKER at 110°C after dissolving 200 mg of a sample in a 8:2 mixture of o-dichlorobenzene and deuterated benzene at 135°C.

[0076] (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 Using gel permeation chromatography (GPC), the molecular weight was determined as a PP equivalent using monodisperse polystyrene as a standard. When the baseline was unclear, the baseline was set to 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: Device: HLC-8321PC / HT (Tosoh Corporation) Detector: RI Solvent: 1,2,4-trichlorobenzene + dibutylhydroxytoluene (0.05%) Column: TSKgelguard column HHR(30)HT (7.5 mm I.D. x 7.5 cm) x 1 + TSKgelGMHHR-H(20)HT (7.8 mm I.D. x 30 cm) x 3 Flow rate: 1.0mL / min Injection volume: 0.3mL Measurement temperature: 140℃ The number average molecular weight (Mn) and the mass average molecular weight (Mw) are the molecular weights (M i ) number of molecules (N i ) is defined by the following equation: Number average molecular weight: Mn=Σ(N i M i ) / ΣN i Mass average molecular weight: Mw=Σ(N i M i 2 ) / Σ(N i M i ) Here, the molecular weight distribution can be obtained by Mw / Mn. Furthermore, the proportion of components with a molecular weight of 100,000 or less was determined from the integral curve of the molecular weight distribution obtained by GPC.

[0077] (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 of polypropylene resin pellets were cut and sealed in an aluminum pan for measurement. The sample was heated to 230°C, held for 5 minutes, and then cooled to 30°C at a rate of -10°C / min. The exothermic peak temperature was taken as the crystallization temperature (Tc). The heat of crystallization (ΔHc) was calculated by setting a baseline that smoothly connected the exothermic peak area from the start to the end of the peak. The sample was then held at 30°C for 5 minutes, then heated to 230°C at a rate of 10°C / min. The main endothermic peak temperature was taken as the melting temperature (Tm).

[0078] (5) Film thickness

[0079] The thickness of the film was measured using a Seiko EM Millitron 1202D. (6) Hayes Measurement was carried out at 23°C in accordance with JIS K7105 using an NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd.

[0080] (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 were cut out from the film to a size of 15 mm x 200 mm and set in a tensile testing machine (Instron 5965, a dual-column tabletop testing machine manufactured by Instron Japan Co., Ltd.) with a chuck width of 100 mm. The tensile test was carried out at a pulling rate of 200 mm / min. From the obtained strain-stress curve, 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.

[0081] (8) Heat shrinkage rate Measurement was performed in accordance with JIS Z 1712 using the following method: The film was cut into pieces 20 mm wide and 200 mm long in both the longitudinal and transverse directions, 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 rate was calculated as the ratio of the shrunken length to the original length.

[0082] (9) Refractive index, △Ny, plane orientation coefficient, average refractive index Measurements were made 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 transverse 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.

[0083] (10) X-ray half-width, X-ray orientation Measurements were performed using the transmission method with an X-ray diffractometer (Rigaku Corporation, SmartLab, with αβγ attachment). X-rays with a wavelength of 1.5418 Å were used, with an X-ray output of 45 kV and 200 mA. The detector used was a hybrid multidimensional pixel detector, Hypix-3000, in 0-dimensional mode. The parallel beam method used a 2.5° Soller slit, a 10 mm long limiting slit, and a 1 mm entrance slit width as the entrance slit. A 0.228° parallel slit analyzer was used as the receiving slit. The camera length was 300 mm and the detector integration 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 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. Measurements were taken 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 resulting from oriented crystals in the width direction of the film was calculated. In addition, the degree of X-ray orientation was calculated using Wh according to the following formula. X-ray orientation degree=(180-Wh) / 180

[0084] (11) The ratio of unbound amorphous component (III) determined by pulsed NMR The film was cut and stuffed into a glass tube with an outer diameter of 10 mm to a height of 1 cm. The film was measured using the following measuring equipment and conditions to measure the strength of the biaxially oriented polypropylene film. 1 The spin-spin relaxation time T2 of H nuclei was measured and the decay curve of the magnetization intensity was obtained. Equipment: BRUKER Minispec mq20 Temperature: 40℃ Observation frequency: 20MHz 90° pulse width: 2.74 μs Pulse repetition time: 2.0s Pulse mode: Solid Echo method Number of times accumulated: 128 times Recycle Delay: 4s Acquisition Scale: 0.1 ms Measurements were initiated after placing a glass tube filled with biaxially oriented polypropylene film into the instrument and incubating for 15 minutes. To ensure that the obtained magnetization intensity decay curve matched the fitting curve, 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 Lorentzian function. The shortest component corresponds to the crystalline component (I), while 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 instrument. 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 +MII +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)

[0085] (12) Wetting tension (mN / m) In accordance with JIS K 6768: 1999, the film was left at 23°C and 50% relative humidity for 24 hours. After aging, 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) Place the test piece on the substrate of the hand coater (4.1), drop a few drops of the test mixture onto the test piece, and immediately pull the wire bar to spread it out. If a cotton swab or brush is used to spread the test mixture, the liquid should be at least 6 cm 2 Spread the liquid quickly over the above area. The amount of liquid should be enough to form a thin layer without creating any puddles. Wetting tension is determined by observing the liquid film of the test mixture in a bright place and checking 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, and conversely, if the liquid film breaks in 3 seconds or less, proceed to the mixture with the next lowest surface tension. Repeat this process to select a mixture that can accurately wet the surface of the test piece in 3 seconds.

[0086] (13) Evaluation of releasability A biaxially oriented polypropylene film (release film) coated with the release agent described in the Examples was cut from a roll to a width of 30 mm and a length of 80 mm, with the long side in the width direction, to serve as a sample for peel force measurement. After static elimination using a static eliminator (Keyence Corporation, SJ-F020), the film was peeled using a peel tester (Kyowa Interface Science Co., Ltd., VPA-3) at a peel angle of 30°, a peel temperature of 25°C, and a peel speed of 10 m / min. For the peel direction, double-sided adhesive tape (Nitto Denko Corporation, No. 535A) was attached to a SUS plate attached to the peel tester, and the release film was fixed on the double-sided tape on the side opposite the release layer. The film was then peeled by pulling the release layer side. The average peel force was calculated from the measured values ​​over peel distances of 20 mm to 70 mm, and this value was used as the peel force. Measurements were performed five times, and the average peel force was used for evaluation. The peel strength was evaluated based on the following criteria. ○: 60mN / mm or less ×: Greater than 60 mN / mm The peel angle in this evaluation method refers to the angle of the direction in which the release film is pulled relative to the axis of the evaluation sample fixed to the peel tester.

[0087] (14) Release film curl A release film was cut into a size of 10 cm x 10 cm from a roll of biaxially oriented polypropylene film (release film) coated with the release agent described in the examples, and the release film sample was placed on a glass plate with the release side facing up, and the height of the part floating above the glass plate was measured. The curl that occurred in the release film was evaluated by the following method. The height of the part that was most raised from the glass plate was taken as the measured value. The curling was evaluated according to the following criteria. ○: Curl is 5 mm or less ×: Curl is greater than 5 mm

[0088] (15) Wrinkles in release film roll The biaxially oriented polypropylene film coated with the 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. Specifically, a 60 cm wide release film was hung vertically in the longitudinal direction in a room at a temperature of 25°C and a humidity of 65%, and left to stand for 30 minutes with a load of 10 N / m applied. A fluorescent light 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 to be 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. ○: Number of wrinkles is 10 or less per meter ×: Number of wrinkles is 11 or more per meter.

[0089] Example 1 (Method of manufacturing biaxially oriented polypropylene film for release) The polypropylene resin used was a blend of 80 parts by weight of propylene homopolymer PP-1 (Sumitomo Chemical Co., Ltd., Sumitomo Noblen FLX80E4) with MFR = 7.5 g / 10 min, [mmmm] = 98.9%, Tc = 116.2 °C, and Tm = 162.5 °C, and 20 parts by weight of propylene homopolymer PP-2 (Sumitomo Chemical Co., Ltd., EL80F5) with MFR = 11 g / 10 min, [mmmm] = 98.8%, Tc = 116.5 °C, and Tm = 161.5 °C. The extruded sheet was extruded through a T-die at 250°C, contacted with a 20°C cooling roll, and then placed directly in a 20°C water bath. It was then stretched 4.5 times longitudinally between two pairs of rolls at 142°C. It was then clamped at both ends and introduced into a hot-air oven. After preheating to 170°C, it was stretched 10 times transversely at 162°C (first stage). Immediately after stretching in the transverse direction, it was cooled at 120°C while still held by the clips. It was then re-stretched 1.2 times transversely at 175°C, cooled to room temperature, and 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. The resulting film had a thickness of 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 were excellent in rigidity, and a film with low heat shrinkage at high temperatures was obtained.

[0090] (Preparation of biaxially oriented polypropylene film coated with release agent) A coating solution of a thermosetting silicone compound (KS-774, manufactured by Shin-Etsu Silicone Co., Ltd.) in toluene / methyl ethyl ketone = 50 / 50 (solid content concentration 1 wt%) was applied to the corona-treated surface of a roll of biaxially oriented polypropylene film for release, with a Pt catalyst (PL-50T, manufactured by Shin-Etsu Silicone Co., Ltd.) added at a rate of 1 part by weight per 100 parts by weight of the solid content of KS-774, at a coating amount (wet) of 3 g / m 2 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 upper and lower airflow outlets, so that the weight of the release layer after drying and hardening was 0.03 g / m 2 A release film of the formula (I) was obtained. After drying, the film was cooled at a rate of 20°C / sec using a cooling roll at 50°C and then taken up into a roll to obtain a release film roll. Table 1 shows the structure of the polypropylene resin, and Table 2 shows the film formation conditions. As shown in Table 3, the physical properties of the film were excellent in rigidity and had a low thermal shrinkage rate at high temperatures. The release film coated with the release agent had excellent releasability and was a smooth film without wrinkles or curls.

[0091] 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 film without wrinkles or curls.

[0092] 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, and immediately after stretching in the width direction, cooled to 120°C while still held by the clips, 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 thermal shrinkage at high temperatures. The release film coated with a release agent had excellent releasability and was a smooth film without wrinkles or curls.

[0093] Example 4 The same procedure as in Example 3 was repeated except that the film was re-stretched to 1.1 times its width 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, its physical properties were high in rigidity and low in thermal shrinkage at high temperatures, and the release film coated with a release agent had excellent releasability and was a smooth film without wrinkles or curls.

[0094] (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 a constant width. The thickness of the obtained film was 20.8 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 high in rigidity, but the thermal shrinkage rate at high temperatures was poor, and the release film coated with a release agent developed wrinkles and curls, resulting in poor processing suitability as a release film.

[0095] (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, and the release film coated with a release agent showed wrinkles and curls, resulting in poor processability as a release film.

[0096] (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 stretching in the width direction, it was cooled to 100°C while still held by the clips, and then heat-set at 170°C while maintaining a constant width. The thickness of the resulting 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 and curls occurred in the release film coated with the release agent, resulting in poor processability as a release film.

[0097] Comparative Example 4 The polypropylene resin used was PP-3 (manufactured by Japan Polypropylene Corporation, FL203D) with MFR = 3 g / 10 min, [mmmm] = 94.8%, Tc = 117.2 ° C, and Tm = 160.6 ° C. 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 directly into a 20 ° C water bath. It was then stretched 4.5 times in the longitudinal direction at 130 ° C., and in the width direction stretching in a tenter, the preheating temperature was set to 168 ° C., and the first stretching step was performed at 155 ° C., whereupon it was stretched 8.2 times. Immediately after width direction stretching, it was cooled at 120 ° C. while still held by the clips, and then re-stretched 1.2 times in the width direction at 170 ° C. Finally, it was cooled to room temperature. The resulting film had a thickness of 18.8 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 high in heat shrinkage at high temperatures and poor in rigidity. Wrinkles and curls occurred in the release film coated with the release agent, resulting in poor processing suitability as a release film.

[0098] (Comparative Example 5) As in Example 1, a blend of PP-1 and PP-2 was used as the polypropylene resin, 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. 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 low in heat shrinkage at high temperatures, but the rigidity was poor, and the release film coated with a release agent developed wrinkles and curls, resulting in poor processing suitability as a release film.

[0099] (Comparative Example 6) A film was obtained using PP-4 (SA4L, manufactured by Japan Polypropylene Corporation) with MFR = 5 g / 10 min, [mmmm] = 97.3%, Tc = 116.8 °C, and Tm = 161.6 °C as the polypropylene resin, and heat treatment at 168 °C was performed without re-stretching in the width direction under the film-forming conditions shown in Table 2. The thickness of the obtained film was 20.0 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 high in heat shrinkage at high temperatures and poor in rigidity. Wrinkles and curls occurred in the release film coated with the release agent, resulting in poor processing suitability as a release film.

[0100] [Table 1]

[0101] [Table 2]

[0102] [Table 3] [Industrial Applicability]

[0103] The biaxially oriented polypropylene film for release purposes of the present invention has high rigidity and can be made thin. Even when the film is thin, it is easy to peel off, wrinkles are unlikely to occur when the release agent dries, and curling of the film after coating is minimal, making it suitable for use as a release film.

Claims

1. A laminated film comprising a biaxially oriented polypropylene film satisfying the following (1) and (2), and a release coating layer: (1) 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 relationship between the loop stiffness stress (S) in the width direction and the thickness (t) satisfies the following formula: S [mN] ≧ 0.0010 × thickness (μm) ３

2. 2. The laminated film according to claim 1, wherein the heat shrinkage rate of the biaxially oriented polypropylene film in the width direction at 150°C is 10% or less in the longitudinal direction and 30% or less in the width direction.

3. 3. The laminated film according to claim 1, wherein the refractive index Ny in the width direction of the biaxially oriented polypropylene film is 1.5250 or more and ΔNy is 0.0240 or more.

4. The laminated film according to any one of claims 1 to 3, wherein the haze of the biaxially oriented polypropylene film is 5.0% or less.

5. 5. The laminated film according to claim 1, wherein the polypropylene resin constituting the biaxially oriented polypropylene film has a mesopentad fraction of 97.0% or more.

6. The laminated film according to any one of claims 1 to 5, wherein the polypropylene resin constituting the biaxially oriented polypropylene film has a crystallization temperature of 105°C or higher and a melting point of 160°C or higher.

7. 7. The laminated film according to claim 1, wherein the polypropylene resin constituting the biaxially oriented polypropylene film has a melt flow rate of 4.0 g / 10 min or more.

8. A laminated film according to any one of claims 1 to 7, wherein the amount of components having a molecular weight of 100,000 or less in the polypropylene resin constituting the biaxially oriented polypropylene film is 35% by mass or more.

9. The laminated film according to any one of claims 1 to 8, wherein the release coating layer is made of a silicone resin.

10. The laminated film according to any one of claims 1 to 9, which is for use as a release film.

11. A release film comprising the laminate film according to any one of claims 1 to 9.

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