Biaxially oriented polypropylene film

Abstract

Provided is a biaxially oriented polypropylene film that has superior rigidity and superior heat sealing strength at low temperatures.　This biaxially oriented polypropylene film has a substrate layer A that comprises a polypropylene-based resin composition, and a sealing layer C, wherein the biaxially oriented polypropylene film satisfies the following statements (1) to (3). (1) The sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23°C is 150 MPa or more. (2) When sealing layers C have been overlapped facing each other and heat-sealed at 115°C, the heat sealing strength is 1.0 N / 15 mm or more. (3) The thickness is 10 μm to 50 μm.

Description

Biaxially oriented polypropylene film

[0001] The present invention relates to a biaxially oriented polypropylene film that has excellent rigidity and heat roll strength at low temperatures. More specifically, the present invention relates to a biaxially oriented polypropylene film that easily maintains the shape of packaging bags, is resistant to deformation during processing such as printing, and is also suitable for automatic packaging.

[0002] Conventionally, as heat-sealable films for packaging, laminated polypropylene-based resin films obtained by laminating a non-stretched polyethylene-based resin film or a non-stretched polypropylene-based resin film with a stretched polypropylene-based resin film, or laminated polypropylene-based resin films obtained by laminating a low-melting-point polyolefin-based resin layer onto a layer made of a high-melting-point polypropylene-based resin and stretching the resulting film, have been widely used. Although non-stretched polyethylene-based resin films or laminated polypropylene-based resin films obtained by laminating a non-stretched polypropylene-based resin film with a stretched polypropylene-based resin film have sufficient seal strength, they require a lamination process using organic solvents, etc., which is undesirable both economically and from the standpoint of their impact on the global environment.

[0003] On the other hand, in a laminated polypropylene-based resin film obtained by co-extrusion laminating a high-melting-point polypropylene-based resin layer with a low-melting-point polyolefin-based resin layer and stretching the resulting film, heat seal strength has been improved by laminating a base layer made of a polypropylene-based resin and a seal layer made of a low-melting-point polyolefin-based resin (see, for example, Patent Documents 1 to 4, etc.).

[0004] International Publication No. WO 2022 / 118680 JP 2022-165979 A International Publication No. WO 2019 / 244708 International Publication No. WO 2021 / 5893

[0005] However, in recent years, while it has become essential to use polyolefin resins with low melting points, greater rigidity is required from the viewpoint of reducing the volume of packaging materials due to environmental considerations, and at the same time, improvement in bag-making processability is also required, so further improvement was necessary. An object of the present invention is to provide a biaxially oriented polypropylene film that is excellent in rigidity and heat-seal strength at low temperatures.

[0006] As a result of extensive research aimed at achieving this object, the present invention has been able to solve the above-mentioned problems by controlling the polypropylene resin composition and film-forming conditions of each layer of a biaxially oriented polypropylene film laminated with at least a base layer A and a seal layer C each made of a polypropylene resin composition. Specifically, the present invention has the following configuration: [1] A biaxially oriented polypropylene film having a base layer A and a seal layer C each made of a polypropylene resin composition, the biaxially oriented polypropylene film satisfying the following (1) to (3): (1) The sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the transverse direction of the biaxially oriented polypropylene film at 23°C is 150 MPa or more; (2) When the seal layers C are stacked face to face and heat-sealed at 115°C, the heat-seal strength is 1.0 N / 15 mm or more; and (3) The thickness is 10 μm or more and 50 μm or less. [2] The biaxially oriented polypropylene film according to [1], wherein the shrinkage rate of the biaxially oriented polypropylene film after heat treatment for 5 minutes at 120°C is 0% or more and 2.5% or less in the longitudinal direction and -1.0% or more and 1.3% or less in the width direction. [3] The biaxially oriented polypropylene film according to [1] or [2], wherein the relationship between the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the biaxially oriented polypropylene film at 23°C and the heat seal strength of the seal layer C at 115°C satisfies the following (Equation 1): Heat seal strength of seal layer C at 115°C [N / 15mm] ≥ -0.037 × (sum of stress at 5% elongation in the longitudinal direction and stress at 5% elongation in the width direction of the biaxially oriented polypropylene film at 23°C) [MPa] + 7.0 ... (Equation 1) [4] The biaxially oriented polypropylene film according to any one of [1] to [3], wherein the heat seal strength of the seal layer C at 130°C is 3.0 N / 15mm or more and 10.0 N / 15mm or less. [5] The biaxially oriented polypropylene film according to any one of [1] to [4], wherein an intermediate layer B made of a polypropylene-based resin composition and having a thickness of 1.0 μm or more and 5.0 μm or less is disposed between the base layer A and the seal layer C. [6] The biaxially oriented polypropylene film according to any one of [1] to [5], wherein the polypropylene-based resin composition constituting the base layer A has a mesopentad fraction of 97% or more.[7] The biaxially oriented polypropylene film according to any one of [1] to [6], wherein the polypropylene resin composition constituting the base layer A has a melt flow rate of 7.0 g / 10 min or more and 11 g / 10 min or less, measured at 230°C and 2.16 kgf. [8] The biaxially oriented polypropylene film according to any one of [1] to [7], wherein the polypropylene resin composition constituting the base layer A contains 10% by mass or more and 30% by mass or less of a polypropylene resin having a ratio (Mw / Mn) of mass average molecular weight (Mw) to number average molecular weight (Mn) of 9.0 or more and 11.0 or less. [9] The biaxially oriented polypropylene film according to any one of [1] to [8], wherein, after placing 300 cc of 50°C warm water in a 500 cc open-top container, placing the biaxially oriented polypropylene film at the opening of the container so that the surface of the seal layer C faces the inside of the container, and sealing the container, and leaving the container at 5°C for 30 minutes, the area of ​​dew on the surface of the seal layer C is one-fourth or less of the area of ​​the biaxially oriented polypropylene film placed at the opening of the container.

[10] The biaxially oriented polypropylene film according to any one of [1] to [9], wherein a functional layer D made of a polypropylene-based resin composition is laminated on the surface of the base layer A opposite to the seal layer C.

[11] The biaxially oriented polypropylene film according to

[10] , wherein the heat seal strength of the functional layer D at 130°C is 1.0 N / 15 mm or more and is equal to or less than the heat seal strength of the seal layer C at 130°C.

[12] The biaxially oriented polypropylene film according to any one of [1] to

[11] , wherein the polypropylene resin composition constituting the sealing layer C contains 30% by mass or more and 50% by mass or less of a polypropylene resin having a melting point of 60°C or more and 90°C or less.

[0007] The present invention provides a biaxially oriented polypropylene film that has high rigidity and excellent heat-sealing strength at low temperatures. Because the biaxially oriented polypropylene film of the present invention has high rigidity, it easily maintains the shape of a packaging bag when made into a bag, and even if the film is thin, it has good workability in processing. Furthermore, because it has excellent heat-sealing strength at low temperatures, it is also suitable for automatic packaging.

[0008] The biaxially oriented polypropylene film of the present invention will be described in more detail below. [Layer structure of biaxially oriented polypropylene film] The biaxially oriented polypropylene film of the present invention is a biaxially oriented polypropylene film having at least a base layer A and a seal layer C made of a polypropylene resin composition. In addition to a two-layer structure of base layer A / seal layer C, the biaxially oriented polypropylene film of the present invention may have, for example, a three-layer structure of base layer A / intermediate layer B / seal layer C, or seal layer C / base layer A / seal layer C, a four-layer structure such as functional layer D / base layer A / intermediate layer B / seal layer C, or functional surface layer E / base layer A / intermediate layer B / seal layer C, or a five-layer structure such as seal layer C / intermediate layer B / base layer A / intermediate layer B / seal layer C, or functional layer D / intermediate layer B / base layer A / intermediate layer B / seal layer C. Below, the base layer A, intermediate layer B, seal layer C, and functional layer D will each be described in detail.

[0009] [Substrate Layer A] The substrate layer A of the biaxially oriented polypropylene film of the present invention is composed of a polypropylene-based resin composition containing the following polypropylene polymer as a main component. The term "main component" means that the proportion of the polypropylene polymer in the polypropylene-based resin composition is 70% by mass or more, preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more. This proportion is preferably 100% by mass or less, and may be 99% by mass or less.

[0010] (Mesopentad Fraction of Polypropylene Resin Composition) The mesopentad fraction ([mmmm]%), which is an index of the stereoregularity of the polypropylene resin composition constituting the base layer A, is preferably in the range of 97.0% or more and 99.9% or less, more preferably in the range of 97.5% or more and 99.7% or less, even more preferably in the range of 98.0% or more and 99.5% or less, and particularly preferably in the range of 98.5% or more and 99.3% or less. When the mesopentad fraction ([mmmm]%) is 97.0% or more, the crystallinity of the polypropylene resin composition is increased, and the melting point, crystallinity, and crystalline orientation of the crystals in the film are improved, making it easier to obtain rigidity and heat resistance at high temperatures. When it is 99.9% or less, it is easier to reduce costs in terms of polypropylene production and makes the film less likely to break during film formation. The mesopentad fraction is measured by nuclear magnetic resonance (NMR).

[0011] (Melt flow rate (MFR) of polypropylene resin composition) The lower limit of the MFR (230°C, 2.16 kgf) of the polypropylene resin composition constituting the base layer A is preferably 7.0 g / 10 min, more preferably 7.5 g / 10 min, even more preferably 8.0 g / 10 min, particularly preferably 8.3 g / 10 min, and most preferably 8.5 g / 10 min. When the MFR of the polypropylene resin composition constituting the base layer A is 7.0 g / 10 min or more, the amount of low-molecular-weight components in the polypropylene resin composition constituting the film increases. Therefore, by employing a width direction stretching step in the film formation process described below, the oriented crystallization of the polypropylene resin composition is further promoted and the crystallinity of the film is more likely to be increased. In addition, entanglement of polypropylene molecular chains in the amorphous portion is reduced, making it easier to improve heat resistance. The upper limit of the MFR is preferably 11.0 g / 10 min, more preferably 10.5 g / 10 min, even more preferably 10.0 g / 10 min, and particularly preferably 9.5 g / 10 min. When the MFR of the polypropylene resin is 11.0 g / 10 min or less, the film formability is easily maintained. In order to keep the MFR of the polypropylene resin composition within the above range, it is preferable to adopt a method of controlling the average molecular weight or molecular weight distribution of the polypropylene polymer. The melt flow rate (MFR) can be measured in accordance with JIS K7210 at a temperature of 230°C and a load of 2.16 kgf (unit: g / 10 min).

[0012] (Molecular weight distribution of polypropylene resin composition) The polypropylene resin composition constituting the base layer A has a lower limit of 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 4.0, even more preferably 4.5, and particularly preferably 4.8. The upper limit of Mw / Mn is preferably 30, more preferably 25, even more preferably 23, particularly preferably 21, and most preferably 20. Mw / Mn can be obtained using gel permeation chromatography (GPC). When Mw / Mn is in the above range, it is easy to increase the amount of components having a molecular weight of 100,000 or less.

[0013] (Melting point of polypropylene resin composition) The melting point (sometimes abbreviated as Tm) of the polypropylene resin composition constituting the base layer A is preferably 160°C or higher and 180°C or lower. The lower limit of the melting point of the polypropylene resin composition constituting the base layer A is 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 likely to be obtained. The upper limit of Tm is more preferably 175°C, even more preferably 170°C, and particularly preferably 167°C. When Tm is 180°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 point was measured by a differential scanning calorimeter (DSC), and is the main peak temperature of the endothermic peak accompanying melting observed when a 5 mg sample was packed and set in an aluminum pan, 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.

[0014] (Polypropylene Polymer) The polypropylene polymer used in the base layer A is preferably a polypropylene homopolymer substantially free of ethylene and / or an α-olefin having 4 or more carbon atoms, and / or a polypropylene-α-olefin copolymer having an ethylene and / or α-olefin having 4 or more carbon atoms content of 0.3 mol% or less. The α-olefin content is more preferably 0.2 mol% or less, and even more preferably 0.1 mol% or less. Crystallinity within the above range is likely to be improved. Examples of α-olefin components 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. Two or more different polypropylene homopolymers can be blended together to form the polypropylene homopolymer. The polypropylene-α-olefin copolymer having an ethylene and / or α-olefin having 4 or more carbon atoms content of 0.3 mol% or less can be used by blending two or more different polypropylene-α-olefin copolymers each having an ethylene and / or α-olefin having 4 or more carbon atoms content of 0.3 mol% or less.

[0015] When a polypropylene-α-olefin copolymer containing more than 0.3 mol% of ethylene and / or an α-olefin component having 4 or more carbon atoms is used in the polypropylene resin composition constituting the base layer A, the content thereof is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass, based on the total polypropylene resin used in the base layer A.

[0016] (Stereoregularity of Polypropylene Polymer) The mesopentad fraction ([mmmm]%), which is an index of the stereoregularity of the polypropylene polymer used in the base layer A, is preferably in the range of 97.0% or more and 99.9% or less, more preferably in the range of 97.5% or more and 99.7% or less, even more preferably in the range of 98.0% or more and 99.5% or less, and particularly preferably in the range of 98.5% or more and 99.3% or less. When the mesopentad fraction ([mmmm]%) is 97.0% or more, the crystallinity of the polypropylene polymer is increased, and the melting point, crystallinity, and crystalline orientation of the crystals in the film are improved, making it easier to obtain rigidity and heat resistance at high temperatures. When it is 99.9% or less, it is easier to reduce costs in terms of polypropylene production and makes it less likely to break during film formation. The mesopentad fraction is measured by nuclear magnetic resonance (NMR). Specifically, the mesopentad fraction was determined by dissolving 200 mg of a sample in a 8:2 mixture of o-dichlorobenzene and heavy benzene at 135°C and １３ In order to set the mesopentad fraction of a polypropylene homopolymer and a polypropylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 or more carbon atoms of 0.3 mol % or less within the above-mentioned range, a method of washing the obtained polypropylene polymer 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 a polypropylene resin composition, and the like are preferably employed.

[0017] (Melting point of polypropylene polymer) The lower limit of the melting point (Tm) of the polypropylene polymer used in the base layer A 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 180°C, more preferably 175°C, even more preferably 170°C, still more preferably 167°C, and particularly preferably 165°C. When Tm is 180°C or lower, it is easy to suppress cost increases in terms of polypropylene production, and breakage during film formation is also less likely. The melting point can also be further increased by blending a crystal nucleating agent with the above-mentioned polypropylene resin. The melting point is measured by a differential scanning calorimeter (DSC) and is the main endothermic peak temperature associated with melting, which is observed when a 5 mg sample is packed into an aluminum pan, set, 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 at a scanning rate of 10°C / min. When the melting point (Tm) is 160°C or higher, the crystallinity of the polypropylene polymer is increased, the melting point and crystallinity of the crystals in the film are improved, and heat resistance is easily obtained.

[0018] (Melt flow rate (MFR) of polypropylene polymer) The lower limit of the MFR (230°C, 2.16 kgf) of the polypropylene polymer used in the base layer A is preferably 7.0 g / 10 min, more preferably 7.5 g / 10 min, even more preferably 8.0 g / 10 min, particularly preferably 8.3 g / 10 min, and most preferably 8.5 g / 10 min. When the MFR of the polypropylene polymer used in the base layer A is 7.0 g / 10 min or more, the amount of low molecular weight components in the polypropylene polymer constituting the film increases. Therefore, by employing a width direction stretching step in the film formation process described below, the oriented crystallization of the polypropylene resin is further promoted and the crystallinity of 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. The upper limit of the MFR is preferably 11.0 g / 10 min, more preferably 10.5 g / 10 min, even more preferably 10.0 g / 10 min, and particularly preferably 9.5 g / 10 min. When the MFR of the polypropylene resin is 11.0 g / 10 min or less, the film formability is easily maintained. In order to set the MFR of the polypropylene polymer within the above range, it is preferable to employ a method of controlling the average molecular weight or molecular weight distribution of the polypropylene polymer.

[0019] (Molecular Weight Distribution of Polypropylene Polymer) The polypropylene polymer used in the base layer A 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 4.0, even more preferably 4.5, particularly preferably 4.8, and most preferably 5.0. The upper limit of Mw / Mn is preferably 30, more preferably 25, even more preferably 23, particularly preferably 21, and most preferably 20. Mw / Mn can be obtained using gel permeation chromatography (GPC). When Mw / Mn is in the above range, it is easy to increase the amount of components having a molecular weight of 100,000 or less.

[0020] The molecular weight distribution of a polypropylene polymer can be adjusted by polymerizing components of different molecular weights in a series of plants in multiple stages, blending components of different molecular weights offline in a kneader, polymerizing by blending catalysts with different performances, or using a catalyst that can achieve a 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 (log M) of molecular weight (M) on the horizontal axis and the differential distribution value (weight fraction per log M) on the vertical axis, or may be a molecular weight distribution with multiple peaks or shoulders.

[0021] The polypropylene resin composition constituting the base layer A preferably contains a polypropylene polymer having a large Mw / Mn value in order to maintain film-formability. Specifically, the lower limit of Mw / Mn is preferably 9.0, more preferably 9.2, even more preferably 9.4, and particularly preferably 9.6. When the lower limit of Mw / Mn is 9.0 or more, film-formability can be maintained. Furthermore, the upper limit of Mw / Mn is preferably 11.0, more preferably 10.8, even more preferably 10.6, particularly preferably 10.4, and most preferably 10.2. The lower limit of the content of the polypropylene polymer having a large Mw / Mn value is preferably 10.0 mass%, more preferably 10.5 mass%, even more preferably 11.0 mass%, and particularly preferably 11.5 mass%. When the content of the polypropylene resin having a large Mw / Mn value is 10.0 mass% or more, film-formability can be maintained. The upper limit of the content of the polypropylene resin having a large Mw / Mn value is preferably 30.0 mass%, more preferably 29.5 mass%, even more preferably 28.0 mass%, and particularly preferably 27.5 mass%. When the content of the polypropylene resin having a large Mw / Mn value is 30.0 mass% or less, the crystallinity of the polypropylene resin composition can be increased, and rigidity and heat resistance can be obtained.

[0022] 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 polymer used in the base layer A is preferably 35% by mass, more preferably 38% by mass, even more preferably 40% by mass, particularly preferably 41% by mass, and most 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 a high-molecular-weight component or a long-chain branched component having a long relaxation time is contained, the amount of components having a molecular weight of 100,000 or less contained in the polypropylene polymer can be easily adjusted without significantly changing the overall viscosity, which makes it easy to improve film formability without significantly affecting rigidity or thermal shrinkage.

[0023] (Polypropylene polymer with a large Mw / Mn value) The polypropylene polymer with a large Mw / Mn value used in the base layer A is preferably a polypropylene homopolymer substantially free of ethylene and / or an α-olefin having 4 or more carbon atoms, and / or a polypropylene-α-olefin copolymer in which the amount of ethylene and / or an α-olefin having 4 or more carbon atoms is 0.3 mol% or less. The amount of the α-olefin component is more preferably 0.2 mol% or less, and even more preferably 0.1 mol% or less. When the amount is within the above range, the crystallinity is likely to be improved. Examples of the α-olefin component having 4 or more carbon atoms that constitutes such a copolymer include 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene. As the polypropylene homopolymer having a large Mw / Mn value, a blend of two or more different polypropylene homopolymers can be used. As the polypropylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 or more carbon atoms of 0.3 mol% or less, a blend of two or more different polypropylene-α-olefin copolymers having an ethylene and / or α-olefin component content of 4 or more carbon atoms of 0.3 mol% or less can be used.

[0024] When a polypropylene-α-olefin copolymer containing more than 0.3 mol% of ethylene and / or an α-olefin component having 4 or more carbon atoms is used in the polypropylene resin composition having a large Mw / Mn value constituting the base layer A, the content of the ethylene and / or α-olefin component relative to the total polypropylene resin used in the base layer A is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less, and particularly preferably 0% by mass.

[0025] (Stereoregularity of polypropylene polymer with large Mw / Mn value) The mesopentad fraction ([mmmm]%), which is an index of stereoregularity of the polypropylene polymer with large Mw / Mn value used in the base layer A, 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 ([mmmm]%) is 97.0% or more, the crystallinity of the polypropylene polymer is increased, and the melting point, crystallinity, and crystalline orientation of the crystals in the film are improved, making it easier to obtain rigidity and heat resistance at high temperatures. When it is 99.9% or less, it is easier to reduce costs in terms of polypropylene production and makes it less likely to break during film formation. The mesopentad fraction is measured by nuclear magnetic resonance (NMR). In order to set the mesopentad fraction of a polypropylene homopolymer and a polypropylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 or more carbon atoms of 0.3 mol % or less within the above-mentioned range, a method of washing the obtained polypropylene polymer 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 a polypropylene resin composition, and the like are preferably employed.

[0026] (Melting point of polypropylene polymer with large Mw / Mn value) The lower limit of the melting point (Tm) of the polypropylene polymer with large Mw / Mn value used in the base layer A 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 180°C, more preferably 175°C, even more preferably 170°C, and particularly preferably 167°C. When Tm is 180°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 point can also be further increased by blending a crystal nucleating agent with the above-mentioned polypropylene resin. The melting point is measured by a differential scanning calorimeter (DSC) and is the main endothermic peak temperature associated with melting, which is observed when a 5 mg sample is packed into an aluminum pan, set, 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 at a scanning rate of 10°C / min. When the melting point (Tm) is 160°C or higher, the crystallinity of the polypropylene polymer is increased, the melting point and crystallinity of the crystals in the film are improved, and heat resistance is easily obtained.

[0027] (Melt flow rate (MFR) of polypropylene polymer with large Mw / Mn value) The lower limit of the MFR (230°C, 2.16 kgf) of the polypropylene polymer with large Mw / Mn value used in the base layer A is preferably 7.0 g / 10 min, more preferably 7.5 g / 10 min, even more preferably 8.0 g / 10 min, particularly preferably 8.3 g / 10 min, and most preferably 8.5 g / 10 min. When the MFR of the polypropylene polymer with large Mw / Mn value used in the base layer A is 7.0 g / 10 min or more, the amount of low molecular weight components in the polypropylene polymer constituting the film increases. Therefore, by employing a width direction stretching step in the film formation process described below, the oriented crystallization of the polypropylene resin is further promoted and the crystallinity of 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 improve heat resistance. The upper limit of the MFR is preferably 11.0 g / 10 min, more preferably 10.5 g / 10 min, even more preferably 10.0 g / 10 min, and particularly preferably 9.5 g / 10 min. When the MFR of the polypropylene resin is 11.0 g / 10 min or less, the film formability is easily maintained. In order to set the MFR of the polypropylene polymer within the above range, it is preferable to employ a method of controlling the average molecular weight or molecular weight distribution of the polypropylene polymer.

[0028] The melting point of the polypropylene polymer used in the polypropylene-based resin composition constituting the base layer A is preferably 160°C or higher, and the polypropylene polymer having a melting point of 160°C or higher is preferably contained in an amount of 70% by mass or higher, more preferably 80% by mass or higher, even more preferably 90% by mass or higher, and particularly preferably 95% by mass or higher, based on the entire polypropylene resin composition constituting the base layer A.

[0029] 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 polymer having a large Mw / Mn value used in the base layer A is preferably 35% by mass, more preferably 38% by mass, even more preferably 40% by mass, particularly preferably 41% by mass, and most 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 polymer contains high-molecular-weight components or long-chain branched components having a long relaxation time, the amount of components having a molecular weight of 100,000 or less contained in the polypropylene polymer can be easily adjusted without significantly changing the overall viscosity, which makes it easy to improve film formability without significantly affecting rigidity or thermal shrinkage.

[0030] (Anti-Fog Agent) The polypropylene resin composition constituting the base layer A may contain an anti-fogging agent. Typical examples of the anti-fogging agent include fatty acid esters of polyhydric alcohols, amines of higher fatty acids, amides of higher fatty acids, and ethylene oxide adducts of amines or amides of higher fatty acids. The amount of the anti-fogging agent present in the base layer A is preferably 0.3% by mass or more and 2.0% by mass or less, more preferably 0.4% by mass or more and 1.9% by mass or less, and even more preferably 0.5% by mass or more and 1.8% by mass or less.

[0031] (Other Additives) The polypropylene resin composition constituting the base layer A can also contain known heat stabilizers, antioxidants, ultraviolet absorbers, and the like, as long as the effects of the present invention are not impaired. Anti-fogging agents and other additives can be added directly to the extruder during film production. However, a preferred method allows for uniform additive incorporation by first preparing a masterbatch containing a high concentration of additives based on a polypropylene polymer as the base resin and then adding this masterbatch to the extruder during film production. In addition to the polypropylene polymers described above, polypropylene polymers with melting points below 160°C, such as low stereoregular polypropylene polymers and polypropylene-α-olefin copolymers, may also be used as the base resin for the masterbatch, as long as the polypropylene resin composition constituting the base layer A can be within the aforementioned range. However, their content is preferably 5% by mass or less, and more preferably 3% by mass or less, relative to the total polypropylene resin composition constituting the base layer A. This content is preferably 0% by mass. In other words, it is preferable not to use a polypropylene polymer with a melting point below 160°C as the base resin for the masterbatch.

[0032] [Intermediate Layer B] In the present invention, the provision of intermediate layer B can improve the heat seal strength. Intermediate layer B is preferably made of a polypropylene-based resin composition containing 70% by mass or more of a propylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 mol% or less having 4 or more carbon atoms. The melting point of the polypropylene-based resin composition constituting intermediate layer B is preferably 150°C or higher and 160°C or lower. The lower limit of the melting point (sometimes abbreviated as Tm) of the polypropylene-based resin composition constituting intermediate layer B is more preferably 152°C, even more preferably 154°C, and even more preferably 156°C. When Tm is 150°C or higher and 160°C or lower, crystallization of seal layer C due to the influence of base layer A can be suppressed, thereby suppressing a decrease in the heat seal strength of the film. The melting point was measured by a differential scanning calorimeter (DSC), and is the main peak temperature of the endothermic peak accompanying melting observed when a 5 mg sample was packed and set in an aluminum pan, 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.

[0033] Examples of the propylene-α-olefin copolymer having 4 mol% or less of ethylene and / or α-olefins having 4 or more carbon atoms include at least one propylene-based copolymer selected from the group consisting of propylene-butene copolymer, propylene-ethylene-butene copolymer, and propylene-ethylene copolymer. The propylene-α-olefin copolymer having 4 mol% or less of ethylene and / or α-olefins having 4 or more carbon atoms is preferably contained in the polypropylene-based resin composition constituting the intermediate layer B at 80% by mass or more, more preferably 90% by mass or more, and particularly preferably 95% by mass or more. The content of the propylene-α-olefin copolymer having 4 mol% or less of ethylene and / or α-olefins having 4 or more carbon atoms in the polypropylene-based resin composition constituting the intermediate layer B is preferably 100% by mass or less, and may be 99% by mass or less. By containing 80% by mass or more of the propylene-α-olefin copolymer having 4 mol% or less of ethylene and / or α-olefins having 4 or more carbon atoms, the interlayer strength between the seal layer C and the intermediate layer B can be increased. Furthermore, the crystallization of the base layer A can be prevented from spreading to the seal layer C, which makes it easier to increase the heat seal strength of the biaxially oriented polypropylene film.

[0034] The propylene-α-olefin copolymer containing 4 mol % or less of ethylene and / or α-olefins having 4 or more carbon atoms contained in the polypropylene-based resin composition constituting the intermediate layer B is preferably at least one polypropylene-based copolymer selected from the group consisting of a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer. Preferred embodiments are shown below.

[0035] (Propylene-Ethylene-Butene Copolymer) The ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 20 mol% or less, more preferably 5 mol% or less. When the ethylene and / or butene content is 20 mol% or less, improved rigidity and heat resistance are obtained due to crystallization. There is no particular limitation on the lower limit of the ethylene and / or butene content, but if the ethylene and / or butene content is too low, the crystallization propagation caused by the base layer A cannot be suppressed sufficiently, and crystallization in the seal layer C progresses, resulting in a decrease in the heat seal strength of the film. Therefore, the ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 0.1 mol% or more. The butene content in the propylene-ethylene-butene copolymer is preferably 20 mol% or less, more preferably 5 mol% or less. When the butene content is 20 mol% or less, improved rigidity and heat resistance are obtained due to crystallization. Although there is no particular lower limit for the butene content, if the butene content is too low, the propagation of crystallization caused by the base layer A cannot be suppressed sufficiently, and crystallization in the seal layer C progresses, resulting in a decrease in the heat seal strength of the film. Therefore, the butene content in the propylene-ethylene-butene copolymer is preferably 0.1 mol % or more.

[0036] (Propylene-butene copolymer) The butene content in the propylene-butene copolymer is preferably 20 mol% or less, more preferably 5 mol% or less. When the butene content is 20 mol% or less, improvements in rigidity and heat resistance are obtained due to crystallization. There is no particular restriction on the lower limit of the butene content, but if the butene content is too low, the crystallization propagation caused by the base layer A cannot be suppressed completely, and crystallization in the seal layer C progresses, resulting in a decrease in the heat seal strength of the film. Therefore, the butene content in the propylene-butene copolymer is preferably 0.1 mol% or more.

[0037] (Propylene-ethylene copolymer) The ethylene content in the propylene-ethylene copolymer is preferably 20 mol% or less, more preferably 7 mol% or less, and even more preferably 4 mol% or less. When the ethylene content is 20 mol% or less, improvements in rigidity and heat resistance are obtained due to crystallization. There is no particular restriction on the lower limit of the ethylene content, but if the ethylene content is too low, the crystallization propagation caused by the base layer A cannot be suppressed completely, and crystallization in the seal layer C progresses, resulting in a decrease in the heat seal strength of the film. Therefore, the ethylene content in the propylene-ethylene copolymer is preferably 0.2 mol% or more.

[0038] (Melt Flow Rate (MFR) of Propylene-α-olefin Copolymer) The lower limit of the melt flow rate (MFR) (230°C, 2.16 kgf) of the propylene-α-olefin copolymer containing 4 mol% or less ethylene and / or α-olefins having 4 or more carbon atoms in the polypropylene resin composition constituting the intermediate layer B is preferably 3.0 g / 10 min, more preferably 3.5 g / 10 min, even more preferably 4.0 g / 10 min, and particularly preferably 4.3 g / 10 min. The upper limit is preferably 9.0 g / 10 min, more preferably 8.5 g / 10 min, even more preferably 8.0 g / 10 min, and most preferably 7.8 g / 10 min.

[0039] (Anti-Fog Agent) The polypropylene resin composition constituting the intermediate layer B may contain an anti-fogging agent. Typical examples of the anti-fogging agent include fatty acid esters of polyhydric alcohols, amines of higher fatty acids, amides of higher fatty acids, and ethylene oxide adducts of amines or amides of higher fatty acids. The amount of the anti-fogging agent present in the intermediate layer B is preferably 0.3% by mass or more and 2.0% by mass or less, but may also be an anti-fogging agent that has migrated from the base layer A.

[0040] (Other Additives) Known heat stabilizers, antioxidants, ultraviolet absorbers, etc. may also be added to the polypropylene resin composition constituting the intermediate layer B, as long as the effects of the present invention are not impaired.

[0041] [Sealing Layer C] The sealing layer C is composed of a polypropylene-based resin composition mainly composed of a propylene-α-olefin copolymer having a copolymerization component amount of ethylene and / or an α-olefin having 4 or more carbon atoms of 4 mol% or more. The term "main component" means that the proportion of the propylene-α-olefin copolymer in the polypropylene-based resin composition is 70% by mass or more, preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more. This proportion is preferably 100% by mass or less, and may be 99% by mass or less.

[0042] The heat of fusion of the sealing layer C at 70°C to 140°C in DSC measurement is preferably 10 J / g or more and 40 J / g or less, more preferably 10.5 J / g or more, even more preferably 11 J / g or more, and particularly preferably 11.5 J / g or more. When the heat of fusion is 10 J / g or more, the amount of resin melted in the low melting point region is sufficient, and low-temperature sealing properties are obtained. The upper limit of the heat of fusion is preferably 40 J / g, more preferably 35 J / g, even more preferably 30 J / g, and particularly preferably 25 J / g. When the heat of fusion is 40 J / g or less, the rigidity of the sealing layer C is prevented from decreasing too much, and as a result, the rigidity of the entire film is maintained. The heat of fusion of the seal layer C from 70 ° C to 140 ° C was measured using a differential scanning calorimeter (DSC). The surface of the seal layer C of the biaxially oriented polypropylene film was scraped with a razor blade to prepare a measurement sample. 5.0 ± 0.2 mg of the sample was placed in an aluminum pan and scanned from -30 ° C to 250 ° C at a heating rate of 20 ° C / min under a nitrogen atmosphere. The heat of fusion from 70 ° C to 140 ° C was calculated from the DSC curve. Low-temperature sealability refers to the ability to easily obtain sufficient heat seal strength even at relatively low seal bar temperatures during heat sealing. Specifically, if the heat seal strength of the seal layer C at 115 ° C obtained by the measurement method described below is 1.0 N / 15 mm or more, the low-temperature sealability is good, and even at relatively low seal bar temperatures during heat sealing, sufficient heat seal strength of 3.0 N / 15 mm or more is easily obtained. Therefore, automatic packaging can be performed at high speeds. In addition, because heat sealing can be performed at lower temperatures, the entire film is less likely to shrink and wrinkles are less likely to form in the sealed area.

[0043] The melting point of the polypropylene resin composition constituting the seal layer C is preferably 110°C or higher and 135°C or lower. A melting point of 135°C or lower facilitates low-temperature sealing. A melting point of 110°C or higher prevents the rigidity of the seal layer C from decreasing too much, and as a result, the rigidity of the entire film is easily maintained. The melting point is measured using a differential scanning calorimeter (DSC) and is the main endothermic peak temperature associated with melting observed when a 1-10 mg sample is packed into an aluminum pan, set, 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 at a scanning rate of 10°C / min.

[0044] As the propylene-α-olefin copolymer containing 4 mol% or more of ethylene and / or α-olefins having 4 or more carbon atoms contained in the polypropylene resin composition constituting the seal layer C, at least one propylene-α-olefin copolymer selected from the group consisting of a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer is preferred. A preferred embodiment is shown below. (Propylene-Butene Copolymer) The butene content in the propylene-butene copolymer is preferably 3 mol% or more, more preferably 20 mol% or more. A content of 3 mol% or more facilitates increased heat seal strength and improved sealability. While there is no particular upper limit for the butene content, a high butene content can cause the film surface to become sticky and reduce slipperiness and blocking resistance. Therefore, it may be determined appropriately within a range that does not cause such defects. The butene content in the propylene-butene copolymer is preferably 30 mol% or less. Examples of the propylene-butene copolymer having a high butene content include "SPX78J1" manufactured by Sumitomo Chemical Co., Ltd. and "XR110H" manufactured by Mitsui Chemicals, Inc.

[0045] (Propylene-Ethylene-Butene Copolymer) The ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 3 mol% or more, more preferably 5 mol% or more. While there are no particular limitations on the upper limit of the ethylene and / or butene content, if the ethylene and / or butene content is too high, the film surface may become sticky, and slipperiness and blocking resistance may be reduced. Therefore, the upper limit may be appropriately determined within a range that does not cause such defects. The ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 20 mol% or less. An example of a propylene-ethylene-butene copolymer with a high ethylene and / or butene content is "FSX66E8" manufactured by Sumitomo Chemical Co., Ltd. The butene content in the propylene-ethylene-butene copolymer is preferably 3 mol% or more, more preferably 5 mol% or more. The upper limit of the ethylene and / or butene content is not particularly limited, but if the ethylene and / or butene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may decrease, so it may be determined appropriately within a range that does not cause such defects. The butene content in the propylene-ethylene-butene copolymer is preferably 20 mol% or less. An example of the propylene-ethylene-butene copolymer with a high butene content is "FSX66E8" manufactured by Sumitomo Chemical Co., Ltd.

[0046] (Propylene-ethylene copolymer) The ethylene content in the propylene-ethylene copolymer is preferably 3 mol% or more, more preferably 4 mol% or more. The upper limit of the ethylene content is not particularly limited, but if the ethylene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may decrease, so it may be appropriately determined within a range that does not cause such defects. The ethylene content in the propylene-ethylene copolymer is preferably 30 mol% or less. Examples of the propylene-ethylene copolymer with a high ethylene content include "PC540R" manufactured by SunAllomer Co., Ltd. and "VM3588FL" manufactured by Mitsui Chemicals, Inc.

[0047] (Melt flow rate (MFR) of propylene-α-olefin copolymer) The lower limit of the melt flow rate (MFR) (230°C, 2.16 kgf) of the propylene-α-olefin copolymer containing 4 mol% or more ethylene and / or α-olefins having 4 or more carbon atoms contained in the polypropylene resin composition constituting the seal layer C is preferably 5.0 g / 10 min, more preferably 5.5 g / 10 min, even more preferably 6.0 g / 10 min, and particularly preferably 6.3 g / 10 min. The upper limit is preferably 8.0 g / 10 min, more preferably 7.5 g / 10 min, and even more preferably 7.0 g / 10 min. By containing 70 mass% or more of a propylene-α-olefin copolymer containing 4 mol% or more ethylene and / or α-olefins having 4 or more carbon atoms in the polypropylene resin composition constituting the seal layer C, the maximum heat seal strength can be increased, and the sealability of the bag after bag formation can be easily improved.

[0048] (Mid-melting point polypropylene copolymer) The polypropylene resin composition constituting the sealing layer C preferably contains 50% by mass or more and 90% by mass or less of a mid-melting point polypropylene resin, preferably a polypropylene copolymer, having a melting point of 115°C or more and 145°C or less. The lower limit of the melting point of the mid-melting point polypropylene copolymer is preferably 120°C, more preferably 125°C. By setting the melting point to 115°C or more, problems due to sticking during the film-forming process can be suppressed. For example, sticking to metal rolls during the longitudinal stretching process can be mentioned. The upper limit of the melting point of the mid-melting point polypropylene copolymer is preferably 135°C, more preferably 130°C. By setting the melting point to 135°C or less, it is possible to increase the melting enthalpy in the low-temperature region of the sealing layer C, thereby exhibiting low-temperature sealing properties.

[0049] The lower limit of the proportion of the mid-melting point polypropylene copolymer is preferably 53% by mass, more preferably 56% by mass. By making it 50% by mass or more, it is possible to increase the heat of fusion in the low-temperature region of the seal layer C, thereby enabling sealability at low temperatures to be achieved. The upper limit of the proportion of the mid-melting point polypropylene copolymer is preferably 80% by mass, more preferably 70% by mass. By making it 90% by mass or less, it is possible to maintain a high 5% elongation stress at 23°C, thereby achieving rigidity. The mid-melting point polypropylene copolymer is preferably a propylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 or more carbon atoms greater than 6 mol%, and for example, at least one polypropylene copolymer selected from the group consisting of a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer is preferred. Suitable embodiments are the same as those described above, but by increasing the content of the copolymerization component, a low-melting point polypropylene copolymer can be obtained.

[0050] (Low Melting Point Polypropylene-Based Copolymer) The polypropylene-based resin composition constituting the sealing layer C preferably contains 30% by mass or more and 50% by mass or less of a low-melting point polypropylene-based resin, preferably a polypropylene-based copolymer, having a melting point of 60 ° C. or more and 90 ° C. or less. The lower limit of the melting point of the low-melting point polypropylene-based copolymer is preferably 60 ° C., more preferably 64 ° C., and particularly preferably 68 ° C. By setting the melting point to 60 ° C. or more, problems due to sticking during the film-forming process can be suppressed. For example, sticking to a metal roll during the longitudinal stretching process can be mentioned. The upper limit of the melting point of the low-melting point polypropylene-based copolymer is preferably 90 ° C., more preferably 86 ° C., and particularly preferably 82 ° C. By setting the melting point to 90 ° C. or less, it is possible to increase the melting enthalpy in the low-temperature region of the sealing layer C, thereby exhibiting low-temperature sealing properties.

[0051] The lower limit of the proportion of the low-melting point polypropylene copolymer is preferably 30% by mass, more preferably 33% by mass, and particularly preferably 36% by mass. By making it 30% by mass or more, it is possible to increase the heat of fusion in the low-temperature region of the seal layer C, thereby enabling sealability at low temperatures to be achieved. The upper limit of the proportion of the low-melting point polypropylene copolymer is preferably 50% by mass, more preferably 47% by mass, and particularly preferably 44% by mass. By making it 50% by mass or less, it is possible to maintain a high 5% elongation stress at 23°C, thereby achieving rigidity. The low-melting point polypropylene copolymer is preferably a propylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 or more carbon atoms greater than 6 mol%, and for example, at least one polypropylene copolymer selected from the group consisting of a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer is preferred. Preferred embodiments are the same as those described above, but by increasing the content of the copolymerization component, a low-melting point polypropylene copolymer can be obtained.

[0052] (Anti-Fog Agent) The polypropylene resin composition constituting the sealing layer C may contain an anti-fogging agent. Typical examples of anti-fogging agents include fatty acid esters of polyhydric alcohols, amines of higher fatty acids, amides of higher fatty acids, and ethylene oxide adducts of amines or amides of higher fatty acids. The amount of the anti-fogging agent present in the sealing layer C is preferably 0.1% by mass or more and 1.0% by mass or less, but may also be an anti-fogging agent that has migrated from the substrate layer A and the intermediate layer B.

[0053] (Other Additives) As long as the effects of the present invention are not impaired, the polypropylene resin composition constituting the sealing layer C can also be blended with various additives for improving qualities such as slipperiness and antistatic properties, such as lubricants such as wax and metal soap, plasticizers, processing aids, and known heat stabilizers, antioxidants, antistatic agents, and ultraviolet absorbers that are commonly added to polypropylene films.

[0054] [Functional Layer D] The functional layer D is a layer made of a polypropylene resin composition laminated on the surface of the base layer A opposite to the seal layer C. The functional layer D is a layer for imparting a function not present in the base layer A. For example, the functional layer D is a functional layer that imparts easy slippage between films or between a film and a processing tool, a functional layer that imparts antistatic functionality, or a functional layer that imparts easy adhesion to a coating layer, an adhesive layer, a vapor deposition layer, or the like.

[0055] The functional layer D can also be a functional layer imparted with heat sealability. For example, in order to maintain the packaging shape, such as gusset packaging, by folding the heat-sealed portion after bag making to bond the outer packaging parts of the package together, so that the bag can be easily opened by hand, the functional layer D can be a sealing layer having a heat-sealing strength that is weaker than that of the sealing layer C. In this case, the heat-sealing strength of the functional layer D at 130°C is preferably 1.0 N / 15 mm or more. If it is 1.0 N / 15 mm or more, the heat-sealing strength of the functional layer D at 130°C can be folded to bond the outer packaging parts of the package together, so that the packaging shape can be maintained, such as gusset packaging. The upper limit of the heat-sealing strength of the functional layer D at 130°C is preferably equal to or lower than the heat-sealing strength of the sealing layer C at 130°C, for example, preferably 2.5 N / 15 mm or less. If it is 2.5 N / 15 mm or less, the bag can be easily opened by hand at the portion where the outer packaging parts of the package are bonded together. The heat seal strength of the functional layer D was measured by stacking the functional layers D facing each other and applying a heat seal pressure of 1 kg / cm at each temperature. ２ The measurement can be performed with a heat sealing time of 1 second and a pulling speed of 200 mm / min.

[0056] 1) When the polypropylene-based resin composition constituting the functional layer D is to impart slipperiness while maintaining high rigidity to the biaxially oriented polypropylene film, it is preferable to use a polypropylene-based resin composition having a melting point of 160°C or higher and 180°C or lower, similar to that of the base layer A, as described below.

[0057] (Polypropylene Polymer) The polypropylene polymer used in the functional layer D is preferably a polypropylene homopolymer substantially free of ethylene and / or an α-olefin having 4 or more carbon atoms, and / or a polypropylene-α-olefin copolymer having an ethylene and / or α-olefin having 4 or more carbon atoms content of 0.3 mol% or less. The α-olefin content is more preferably 0.2 mol% or less, and even more preferably 0.1 mol% or less. Crystallinity in the above range is likely to be improved. Examples of α-olefin components having 4 or more carbon atoms constituting 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. Two or more different polypropylene homopolymers can be blended and used as the polypropylene homopolymer. The polypropylene-α-olefin copolymer having an ethylene and / or α-olefin having 4 or more carbon atoms content of 0.3 mol% or less can be used by blending two or more different polypropylene-α-olefin copolymers each having an ethylene and / or α-olefin having 4 or more carbon atoms content of 0.3 mol% or less.

[0058] (Stereoregularity of Polypropylene Polymer) The mesopentad fraction ([mmmm]%), which is an index of the stereoregularity of the polypropylene polymer used in the functional layer D, is preferably in the range of 97.0% or more and 99.9% or less, more preferably in the range of 97.5% or more and 99.7% or less, even more preferably in the range of 98.0% or more and 99.5% or less, and particularly preferably in the range of 98.5% or more and 99.3% or less. When the mesopentad fraction ([mmmm]%) is 97.0% or more, the crystallinity of the polypropylene polymer is increased, and the melting point, crystallinity, and crystalline orientation of the crystals in the film are improved, making it easier to obtain rigidity and heat resistance at high temperatures. When it is 99.9% or less, it is easier to reduce costs in terms of polypropylene production and makes it less likely to break during film formation. The mesopentad fraction is measured by nuclear magnetic resonance (NMR). In order to set the mesopentad fraction of a polypropylene homopolymer and a polypropylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 or more carbon atoms of 0.3 mol % or less within the above-mentioned range, a method of washing the obtained polypropylene polymer 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 a polypropylene resin composition, and the like are preferably employed.

[0059] (Melting point of polypropylene polymer) The lower limit of the melting point (Tm) of the polypropylene polymer used in the functional layer D 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 180 ° C, more preferably 175 ° C, even more preferably 170 ° C, and particularly preferably 167 ° C. When Tm is 180 ° 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 point can also be further increased by blending a nucleating agent with the above-mentioned polypropylene resin. The melting point is measured by a differential scanning calorimeter (DSC) and is the main endothermic peak temperature associated with melting, which is observed when a 5 mg sample is packed into an aluminum pan, set, 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 at a scanning rate of 10°C / min. When the melting point (Tm) is 160°C or higher, the crystallinity of the polypropylene polymer is increased, the melting point and crystallinity of the crystals in the film are improved, and heat resistance is easily obtained.

[0060] (Melt flow rate (MFR) of polypropylene polymer) The lower limit of the MFR (230°C, 2.16 kgf) of the polypropylene polymer used in the functional layer D is preferably 7.0 g / 10 min, more preferably 7.5 g / 10 min, even more preferably 8.0 g / 10 min, particularly preferably 8.3 g / 10 min, and most preferably 8.5 g / 10 min. When the MFR of the polypropylene polymer used in the functional layer D is 7.0 g / 10 min or more, the amount of low molecular weight components in the polypropylene polymer constituting the film increases. Therefore, by employing a width direction stretching step in the film formation process described below, the oriented crystallization of the polypropylene resin is further promoted and the crystallinity of 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 improve heat resistance. The upper limit of the MFR is preferably 11.0 g / 10 min, more preferably 10.5 g / 10 min, even more preferably 10.0 g / 10 min, and particularly preferably 9.5 g / 10 min. When the MFR of the polypropylene resin is 11.0 g / 10 min or less, the film formability is easily maintained. In order to set the MFR of the polypropylene polymer within the above range, it is preferable to employ a method of controlling the average molecular weight or molecular weight distribution of the polypropylene polymer.

[0061] (Molecular Weight Distribution of Polypropylene Polymer) The polypropylene polymer used in the functional layer D 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 4.0, even more preferably 4.5, and particularly preferably 5.0. The upper limit of Mw / Mn is preferably 30, more preferably 25, even more preferably 23, particularly preferably 21, and most preferably 20. Mw / Mn can be obtained using gel permeation chromatography (GPC). When Mw / Mn is in the above range, it is easy to increase the amount of components having a molecular weight of 100,000 or less.

[0062] When a polypropylene-α-olefin copolymer containing more than 0.3 mol% of ethylene and / or α-olefin components having 4 or more carbon atoms is used in the polypropylene resin composition constituting the functional layer D, the content of the ethylene and / or α-olefin components is preferably 10 mass% or less, more preferably 5 mass% or less, even more preferably 1 mass% or less, and particularly preferably 0 mass% relative to the total polypropylene resin used in the base layer A.

[0063] 2) When imparting easy adhesion to a coating layer, adhesive layer, vapor deposition layer, etc. to the functional layer D, it is preferable to use a polypropylene-based resin composition having a melting point of 150°C or higher and 160°C or lower, similar to that of the intermediate layer B, as described below.

[0064] The polypropylene polymer used in functional layer D is preferably a propylene-α-olefin copolymer having an ethylene and / or α-olefin content of 4 mol% or less, such as at least one propylene-based copolymer selected from the group consisting of a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer. The propylene-α-olefin copolymer having an ethylene and / or α-olefin content of 4 mol% or less is preferably contained in the polypropylene-based resin composition constituting functional layer D at 80% by mass or more, more preferably 90% by mass or more, and particularly preferably 95% by mass or more. By containing 80% by mass or more of the propylene-α-olefin copolymer having an ethylene and / or α-olefin content of 4 mol% or less, the interlayer strength between the seal layer C and the intermediate layer B can be increased, while a content of 95% by mass or less facilitates the enhancement of the interlayer strength between the base layer A and the intermediate layer B. Furthermore, crystallization of the base layer A can be prevented from propagating to the functional layer D, which facilitates the enhancement of the heat seal strength of the biaxially oriented polypropylene film.

[0065] The propylene-α-olefin copolymer containing 4 mol % or less of ethylene and / or α-olefins having 4 or more carbon atoms contained in the polypropylene-based resin composition constituting the functional layer D is preferably at least one polypropylene-based copolymer selected from the group consisting of a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer. Preferred embodiments are shown below.

[0066] (Propylene-Ethylene-Butene Copolymer) The ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 7 mol% or less, more preferably 5 mol% or less. When the ethylene and / or butene content is 7 mol% or less, improved rigidity and heat resistance are obtained through crystallization. Although there is no particular limitation on the lower limit of the ethylene and / or butene content, if the ethylene and / or butene content is too low, it is not possible to prevent the crystallization of the base layer A from spreading to the functional layer D, and it becomes impossible to impart easy adhesion to a coating layer, an adhesive layer, a vapor deposition layer, etc. to the surface of the functional layer D. Therefore, the ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 1 mol% or more. The butene content in the propylene-ethylene-butene copolymer is preferably 7 mol% or less, more preferably 5 mol% or less. When the butene content is 7 mol% or less, improved rigidity and heat resistance are obtained through crystallization. Although there is no particular restriction on the lower limit of the butene content, if the butene content is too low, it will be impossible to prevent the crystallization of the base layer A from propagating to the functional layer D, and it will be impossible to impart easy adhesion to a coating layer, adhesive layer, vapor deposition layer, etc. to the surface of the functional layer D. Therefore, the butene content in the propylene-ethylene-butene copolymer is preferably 1 mol % or more.

[0067] (Propylene-butene copolymer) The butene content in the propylene-butene copolymer is preferably 20 mol% or less, more preferably 10 mol% or less. When the butene content is 20 mol% or less, improvements in rigidity and heat resistance are obtained due to crystallization. There is no particular limitation on the lower limit of the butene content, but if the butene content is too low, it is not possible to prevent the crystallization of the base layer A from spreading to the functional layer D, and it becomes impossible to impart easy adhesion to a coating layer, adhesive layer, vapor deposition layer, etc. to the surface of the functional layer D. Therefore, it is preferable that the butene content in the propylene-butene copolymer is 1 mol% or more.

[0068] (Propylene-ethylene copolymer) The ethylene content in the propylene-ethylene copolymer is preferably 10 mol% or less, more preferably 4 mol% or less. When the ethylene content is 10 mol% or less, improvements in rigidity and heat resistance are obtained due to crystallization. There is no particular restriction on the lower limit of the ethylene content, but if the ethylene content is too low, it is not possible to prevent the crystallization of the base layer A from spreading to the functional layer D, and it becomes impossible to impart easy adhesion to a coating layer, adhesive layer, vapor deposition layer, etc. to the surface of the functional layer D. Therefore, it is preferable that the ethylene content in the propylene-ethylene copolymer is 1 mol% or more.

[0069] (Melt Flow Rate (MFR) of Propylene-α-olefin Copolymer) The lower limit of the melt flow rate (MFR) (230°C, 2.16 kgf) of the propylene-α-olefin copolymer containing 4 mol% or less of ethylene and / or α-olefins having 4 or more carbon atoms contained in the polypropylene resin composition constituting the functional layer D is preferably 3.0 g / 10 min, more preferably 3.5 g / 10 min, even more preferably 4.0 g / 10 min, and particularly preferably 4.3 g / 10 min. The upper limit is preferably 9.0 g / 10 min, more preferably 8.5 g / 10 min, even more preferably 8.0 g / 10 min, and most preferably 7.8 g / 10 min.

[0070] 3) When heat sealing properties are to be imparted to the functional layer D, it can be composed of a polypropylene-based resin composition whose main component is a polypropylene-based copolymer having a copolymerization component content of 4 mol% or more of ethylene and / or an α-olefin having 4 or more carbon atoms, similar to the seal layer C, as described below.

[0071] The melting point of the polypropylene resin composition constituting the functional layer D is preferably 110°C or higher and 140°C or lower. A melting point of 140°C or lower facilitates low-temperature sealing. A melting point of 110°C or higher prevents the rigidity of the functional layer D from decreasing too much, and as a result, the rigidity of the entire film is easily maintained. The melting point is measured using a differential scanning calorimeter (DSC) and is the main endothermic peak temperature associated with melting observed when a 1-10 mg sample is packed into an aluminum pan, set, 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.

[0072] As the propylene-α-olefin copolymer containing 4 mol% or more of ethylene and / or α-olefins having 4 or more carbon atoms contained in the polypropylene-based resin composition constituting the functional layer D, at least one propylene-α-olefin copolymer selected from the group consisting of, for example, a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer is preferred.

[0073] The propylene-α-olefin copolymer containing 4 mol % or more of ethylene and / or α-olefins having 4 or more carbon atoms is preferably at least one propylene-α-olefin copolymer selected from the group consisting of a propylene-butene copolymer, a propylene-ethylene-butene copolymer, and a propylene-ethylene copolymer. Preferred embodiments are shown below.

[0074] (Propylene-Butene Copolymer) The butene content in the propylene-butene copolymer is preferably 3 mol% or more, more preferably 20 mol% or more. By making it 3 mol% or more, it is easy to increase the heat seal strength and improve the sealing property. There is no particular upper limit for the butene content, but if the butene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may decrease, so it may be appropriately determined within a range that does not cause such defects. The butene content in the propylene-butene copolymer is preferably 30 mol% or less. Examples of the propylene-butene copolymer with a high butene content include "SPX78J1" manufactured by Sumitomo Chemical Co., Ltd. and "XR110H" manufactured by Mitsui Chemicals, Inc.

[0075] (Propylene-Ethylene-Butene Copolymer) The ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 3 mol% or more, more preferably 5 mol% or more. While there are no particular limitations on the upper limit of the ethylene and / or butene content, if the ethylene and / or butene content is too high, the film surface may become sticky, and slipperiness and blocking resistance may be reduced. Therefore, the upper limit may be appropriately determined within a range that does not cause such defects. The ethylene and / or butene content in the propylene-ethylene-butene copolymer is preferably 20 mol% or less. An example of a propylene-ethylene-butene copolymer with a high ethylene and / or butene content is "FSX66E8" manufactured by Sumitomo Chemical Co., Ltd. The butene content in the propylene-ethylene-butene copolymer is preferably 3 mol% or more, more preferably 5 mol% or more. The upper limit of the butene content is not particularly limited, but if the butene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may decrease, so it may be determined appropriately within a range that does not cause such defects. The butene content in the propylene-ethylene-butene copolymer is preferably 20 mol% or less. An example of the propylene-ethylene-butene copolymer with a high butene content is "FSX66E8" manufactured by Sumitomo Chemical Co., Ltd.

[0076] (Propylene-ethylene copolymer) The ethylene content in the propylene-ethylene copolymer is preferably 3 mol% or more, more preferably 4 mol% or more. The upper limit of the ethylene content is not particularly limited, but if the ethylene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may decrease, so it may be appropriately determined within a range that does not cause such defects. The ethylene content in the propylene-ethylene copolymer is preferably 30 mol% or less. Examples of the propylene-ethylene copolymer with a high ethylene content include "PC540R" manufactured by SunAllomer Co., Ltd. and "VM3588FL" manufactured by Mitsui Chemicals, Inc.

[0077] (Melt flow rate (MFR) of propylene-α-olefin copolymer) The lower limit of the melt flow rate (MFR) (230°C, 2.16 kgf) of the propylene-α-olefin copolymer containing 4 mol% or more of ethylene and / or α-olefins having 4 or more carbon atoms contained in the polypropylene-based resin composition constituting the functional layer D is preferably 5.0 g / 10 min, more preferably 5.5 g / 10 min, even more preferably 6.0 g / 10 min, and particularly preferably 6.3 g / 10 min. The upper limit is preferably 8.0 g / 10 min, more preferably 7.5 g / 10 min, even more preferably 7.0 g / 10 min, and most preferably 6.8 g / 10 min. By containing 70 mass% or more of a propylene-α-olefin copolymer containing 4 mol% or more of ethylene and / or α-olefins having 4 or more carbon atoms in the polypropylene-based resin composition constituting the functional layer D, the maximum heat seal strength attained can be increased, and the sealability of the bag after bag formation can be easily improved.

[0078] (Low Melting Point Polypropylene-Based Copolymer) The polypropylene-based resin composition constituting the functional layer D may contain 30% by mass or more and 50% by mass or less of a low-melting point polypropylene-based resin, preferably a polypropylene-based copolymer, having a melting point of 60°C or more and 90°C or less. The lower limit of the melting point of the low-melting point polypropylene-based copolymer is preferably 60°C, more preferably 64°C, and particularly preferably 68°C. By setting the melting point to 60°C or more, defects due to sticking during the film-forming process can be suppressed. For example, sticking to a metal roll during the longitudinal stretching process can be prevented. The upper limit of the melting point of the low-melting point polypropylene-based copolymer is preferably 90°C, more preferably 86°C, and particularly preferably 82°C. By setting the melting point to 90°C or less, it is possible to increase the melting enthalpy in the low-temperature region of the functional layer D, thereby exhibiting low-temperature sealing properties.

[0079] The lower limit of the proportion of the low-melting-point polypropylene-based copolymer is preferably 30% by mass, more preferably 33% by mass, and particularly preferably 36% by mass. By making the proportion 30% by mass or more, it is possible to increase the heat of fusion in the low-temperature region of the functional layer D, thereby enabling sealability at low temperatures to be achieved. The upper limit of the proportion of the low-melting-point polypropylene-based copolymer is preferably 50% by mass, more preferably 47% by mass, and particularly preferably 44% by mass. By making the proportion 50% by mass or less, a high 5% elongation stress at 23°C can be maintained, thereby achieving rigidity. The low-melting-point polypropylene-based copolymer is preferably a propylene-α-olefin copolymer having an ethylene and / or α-olefin component content of 4 or more carbon atoms greater than 6 mol%, and for example, at least one polypropylene-based copolymer selected from the group consisting of propylene-butene copolymer, propylene-ethylene-butene copolymer, and propylene-ethylene copolymer is preferred. Preferred embodiments are the same as those described above, but by increasing the content of the copolymerization component, a low-melting-point polypropylene-based copolymer can be obtained. When functional layer D is made into a sealing layer having a heat seal reaching strength weaker than that of sealing layer C, this can be controlled by using a polypropylene-based copolymer having a higher melting point than sealing layer C as the main component, by reducing the amount of the low-melting point polypropylene-based copolymer, or by making the thickness of functional layer D thinner than that of sealing layer C.

[0080] (Anti-Fog Agent) The polypropylene resin composition constituting the functional layer D may contain an anti-fogging agent. Typical examples of the anti-fogging agent include fatty acid esters of polyhydric alcohols, amines of higher fatty acids, amides of higher fatty acids, and ethylene oxide adducts of amines or amides of higher fatty acids. The amount of the anti-fogging agent present in the functional layer D is preferably 0.1% by mass or more and 1.0% by mass or less, but may also be an anti-fogging agent that has migrated from the base layer A.

[0081] (Other Additives) Known heat stabilizers, antioxidants, ultraviolet absorbers, etc. may also be added to the polypropylene resin composition constituting the functional layer D, as long as the effects of the present invention are not impaired.

[0082] [Amount of Anti-Fog Agent in Each Layer of Biaxially Oriented Polypropylene Film] The amount of anti-fogging agent in the biaxially oriented polypropylene film of the present invention is preferably 0.1% by mass or more and 10% by mass or less, particularly 0.2% by mass or more and 5% by mass or less. To achieve this, by adding an anti-fogging agent only to the base layer A during production, the anti-fogging agent gradually migrates to the seal layer C during film production and storage after film formation, and the anti-fogging agent becomes present on the surface of the seal layer C, thereby providing anti-fogging properties. Fruit and vegetables are characterized by maintaining their physiological activity even after harvest, which is effective in preventing fogging that occurs after packaging. Furthermore, in order to maintain excellent anti-fogging properties over the long term during distribution, it is preferable to store the packaged product at room temperature rather than in a frozen state. Therefore, it is preferable to select an anti-fogging agent that continuously exhibits anti-fogging properties during repeated temperature changes between 5°C and 30°C, taking into account temperature changes during storage and distribution.

[0083] [Method for producing biaxially oriented polypropylene film] Examples of a method for producing the biaxially oriented polypropylene film of the present invention include a method in which melt lamination is performed by a T-die method, an inflation method, or the like using an extruder suitable for the number of layers, and then cooling is performed by a cooling roll method, a water cooling method, or an air cooling method to obtain an unstretched laminated film, and then stretching the obtained laminated film by a sequential biaxial stretching method, a simultaneous biaxial stretching method, a tube stretching method, or the like.

[0084] Here, the conditions for producing a biaxially oriented polypropylene film having a structure of functional layer D / substrate layer A / intermediate layer B / sealing layer C by the sequential biaxial stretching method are exemplified. The polypropylene resin compositions constituting each of the substrate layer A, intermediate layer B, sealing layer C, and functional layer D are preferably as described above. The amount of antifogging agent added to the substrate layer A, intermediate layer B, and sealing layer C is preferably adjusted taking into consideration the antifogging agent evaporating into the atmosphere when exposed to high temperatures during the film formation process.

[0085] First, a molten polypropylene resin composition multilayer sheet having a structure of functional layer D / substrate layer A / intermediate layer B / sealing layer C is extruded through a T-die. For example, a method can be used in which four extruders are used to extrude each polypropylene resin composition molten at a temperature of 200°C to 260°C from a different flow path, and the polypropylene resin compositions are co-extruded while being laminated in multiple layers using a multilayer feed block, static mixer, multilayer multi-manifold die, or the like. Alternatively, it is possible to use only one extruder and install the above-mentioned multilayering device in the melt line from the extruder to the T-die. From the viewpoint of stabilizing back pressure and suppressing thickness fluctuations, a method in which a gear pump is installed in the polymer flow path is preferred.

[0086] The molten sheet co-extruded from the T-die is cooled and solidified by being placed on a metal cooling roll. To accelerate solidification, it is preferable to further cool the sheet cooled by the cooling roll by immersing it in a water bath. The cooling roll temperature and water bath temperature are preferably set between 15°C and 40°C to suppress crystallization of the polypropylene resin composition and improve transparency.

[0087] Next, the raw sheet is heated to a temperature suitable for stretching, and the sheet is stretched in the machine direction by utilizing the speed difference between the stretching rolls. 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 process will be described in detail later. After the width direction stretching process is completed, the biaxially stretched film is heat-treated at a specific temperature to obtain a biaxially oriented film. In the heat treatment process, the film may be relaxed in the width direction. If necessary, the biaxially oriented polypropylene film thus obtained may be subjected to a corona discharge treatment on at least one side, and then wound up on a winder to obtain a film roll.

[0088] Each step is described in detail below. (Extrusion Step) The polypropylene resin compositions constituting the functional layer D, substrate layer A, intermediate layer B, and seal layer C are respectively extruded from different flow paths using four extruders. Each polypropylene resin composition is melted at a temperature of 200°C to 260°C and laminated in multiple layers using a multi-manifold die, while a molten polypropylene resin composition multilayer sheet consisting of functional layer D / substrate layer A / intermediate layer B / seal layer C is extruded from a T-die. The molten sheet co-extruded from the T-die is cooled and solidified by being grounded on a metal cooling roll. At this time, it is preferable that the sealing layer C side be grounded on the cooling roll. Alternatively, if a functional layer D is provided, the functional layer D side may be grounded on the cooling roll. The temperature of the cooling roll, or the cooling roll and water bath, is preferably in the range of 10°C to Tc. If increased transparency of the film is desired, cooling and solidification using a cooling roll at a temperature in the range of 10 to 50°C is preferred. A cooling temperature of 50°C or less tends to increase the transparency of the unstretched sheet, and is preferably 40°C or less, 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, as described above, when using a propylene homopolymer with a mesopentad fraction of 97.0% or more, a cooling temperature of 40°C or less is preferred in order to facilitate the subsequent stretching step and reduce thickness unevenness, and a temperature of 30°C or less is more preferred. 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 film thickness 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.

[0089] (Longitudinal Stretching Step) In order to make the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction at 23°C 150 MPa or more, the 5% elongation stress in the longitudinal direction at 23°C is preferably 35 MPa or more. Furthermore, in consideration of stable production without unevenness in stretching, it is preferable to set the longitudinal stretching ratio to 4.0 times or more and 6.0 times or less. The lower limit of the longitudinal stretching ratio is more preferably 4.2 times. The upper limit of the longitudinal stretching ratio is more preferably 5.5 times from the viewpoint of ease of stretching in the width direction. The lower limit of the longitudinal stretching temperature is preferably 120°C, more preferably 125°C, and even more preferably 135°C or more. Within the above range, not only can deterioration of film formability due to orientation be suppressed, but also the 5% elongation stress in the longitudinal direction at 23°C can be maintained high. The upper limit of the longitudinal stretching temperature is preferably 160°C, more preferably 155°C, and even more preferably 145°C or less. Within the above range, the heat shrinkage rate is easily reduced, and the film is less likely to adhere to the stretching rolls, making stretching difficult, or to deteriorate in quality due to increased surface roughness. The longitudinal stretching may be performed in two or more stages using three or more pairs of stretching rolls.

[0090] (Preheating step before width direction stretching) It is preferable to provide a preheating step before the width direction stretching step, in which the uniaxially stretched film after longitudinal stretching is heated to soften the polypropylene-based resin composition. Softening facilitates width direction stretching. The upper limit of the temperature in the preheating step is preferably 180°C, more preferably 178°C, even more preferably 176°C, and particularly preferably 172°C or less. Setting the upper limit of the temperature in the preheating step to 180°C not only prevents the film from melting and breaking, but also suppresses whitening caused by surface roughness due to excessive heating, and maintains a high 5% elongation stress in the width direction at 23°C. The lower limit of the temperature in the preheating step is preferably 160°C, more preferably 162°C, and particularly preferably 164°C. Setting the lower limit of the temperature in the preheating step to 160°C not only sufficiently softens the film to enable stable width direction stretching, but also makes it easy to reduce the thermal shrinkage.

[0091] (Width Direction Stretching Step) A width direction stretching step is required after the preheating step. The upper limit of the temperature in the width direction stretching step is preferably 170°C, more preferably 168°C, and particularly preferably 166°C. By setting the upper limit of the temperature in the width direction stretching step to 170°C, not only is it possible to prevent the film from melting and breaking, but also to prevent whitening caused by surface roughness due to excessive heating, and to maintain a high 5% elongation stress in the width direction at 23°C. The lower limit of the temperature in the width direction stretching step is preferably 155°C, more preferably 157°C, and particularly preferably 159°C. By setting the lower limit of the temperature in the width direction stretching step to 155°C, the film is sufficiently softened and can be stably stretched in the width direction, and the thermal shrinkage rate is also easily reduced.

[0092] In order to make the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction at 23°C 150 MPa or more, it is preferable to make the 5% elongation stress in the width direction at 23°C 110 MPa or more. Furthermore, considering stable production without unevenness in stretching, the stretch ratio in the width direction is preferably 11.0 times or more and 14.0 times or less. The lower limit of the stretch ratio in the width direction is more preferably 11.5 times, even more preferably 12.0 times, and particularly preferably 12.5 times. By making it 11.0 times or more, orientation in the width direction is strengthened, and the stress at 5% elongation in the width direction at 23°C can be increased, resulting in high rigidity. The upper limit of the stretch ratio in the width direction is preferably 14.0 times, more preferably 13.5 times, and particularly preferably 13.0 times. By making it 14.0 times or less, not only can breakage during film formation be suppressed, but also a decrease in heat seal strength can be suppressed. At a high draw ratio, the crystallinity of the base layer A becomes very high. At this time, the crystallization occurring in the base layer A propagates to and crystallizes the seal layer C, shifting the melting peak of the seal layer C to a higher temperature, reducing the heat of fusion at 70°C to 140°C, and ultimately causing a decrease in low-temperature sealability. Therefore, a draw ratio of 14.0 times or less is preferred.

[0093] (Heat Treatment Step) In order to suppress the 120°C heat shrinkage rate and simultaneously increase the tensile elongation at break, it is preferable to perform a heat treatment step in which the biaxially stretched film stretched in the longitudinal direction is heated while both edge portions are held with tenter clips. The upper temperature limit of the heat treatment step is preferably 180°C, more preferably 178°C, and particularly preferably 176°C. By setting the upper temperature limit of the heat treatment step to 180°C, not only is it possible to prevent the film from melting and breaking, but it is also possible to suppress whitening caused by surface roughness due to excessive heating, and to maintain a high 5% elongation stress in the width direction at 23°C. The lower temperature limit of the heat treatment step is preferably 160°C, more preferably 162°C, and particularly preferably 164°C. Setting the lower temperature limit of the heat treatment step to 160°C makes it easy to reduce the heat shrinkage rate. Simultaneously with the heat treatment, it is preferable to relax the film in the width direction by 4% or more while both edge portions are held with tenter clips in order to reduce the heat shrinkage rate. More preferably, it is 5% or more. The upper limit is 10% or less, and this range is preferable because it allows the film to maintain a 5% elongation stress at 23°C in both the longitudinal and transverse directions.

[0094] Thus, by using a highly crystalline polypropylene polymer with high stereoregularity and a high melting point and employing the width direction stretching process described above, the molecules of the polypropylene resin in the base layer A are highly aligned in the main orientation direction (corresponding to the width direction in the width direction stretching process described above), resulting in a very strong crystalline orientation in the resulting biaxially oriented film, which is likely to produce crystals with a high melting point. Furthermore, the orientation of the amorphous portions between the crystals is also enhanced in the main orientation direction (corresponding to the width direction in the width direction stretching process described above). Because many high-melting-point crystals exist around the amorphous portions, the elongated polypropylene molecules in the amorphous portions are less likely to relax at temperatures below the melting point of the crystals, and are more likely to maintain their tensed state. Therefore, the entire biaxially oriented film can maintain high rigidity even at high temperatures. It is noteworthy that employing such a process also tends to reduce the heat shrinkage rate at high temperatures such as 120°C. This is because, in the base layer A, many high-melting-point crystals exist around the amorphous portions, so the elongated polypropylene molecules in the amorphous portions are less likely to relax at temperatures below the melting point of the crystals, and there is less entanglement between the molecules. On the other hand, since the orientation of the seal layer C can be weakened, the heat seal strength at low temperatures is likely to be increased. Furthermore, the heat shrinkage rate at a high temperature of 120°C is likely to be reduced, and conversely, the tensile elongation at break is likely to be improved. Furthermore, when an anti-fogging agent is present, the anti-fogging properties of the final biaxially oriented film are likely to be improved.

[0095] Next, it is preferable to perform a corona discharge treatment on the surface of the sealing layer C using a corona discharge treatment machine to increase the surface tension of the sealing layer C. This can improve anti-fogging properties. In the biaxially oriented polypropylene film of the present invention, it is preferable to surface treat the base layer A or the functional layer D to improve printability, lamination properties, etc. Examples of surface treatment methods include corona discharge treatment, plasma treatment, flame treatment, acid treatment, etc., and there are no particular restrictions. It is preferable to perform corona discharge treatment, plasma treatment, or flame treatment, which can be performed continuously and can be easily performed before the winding step in the production process of this film.

[0096] [Layer structure and thickness structure of biaxially oriented polypropylene film] The biaxially oriented polypropylene film of the present invention has a base layer A and a seal layer C. It is preferable to provide an intermediate layer B between the base layer A and the seal layer C. The layer structure of the biaxially oriented polypropylene film of the present invention is not particularly limited as long as it has the seal layer C on the outermost surface, and another layer may be further provided between the base layer A and the intermediate layer B, or the intermediate layer B may be laminated directly on the base layer A. Furthermore, another layer may be provided between the intermediate layer B and the seal layer C, or the seal layer C may be laminated directly on the intermediate layer B. For example, in addition to the two-layer structure of base material layer A / seal layer C, there are also mentioned a three-layer structure of seal layer C / base material layer A / seal layer C, a three-layer structure of base material layer A / intermediate layer B / seal layer C, a four-layer structure of seal layer C1 / base material layer A / intermediate layer B / seal layer C2, a five-layer structure of seal layer C1 / intermediate layer B1 / base material layer A / intermediate layer B2 / seal layer C2, and a six-layer structure of seal layer C1 / base material layer A1 / intermediate layer B1 / base material layer A2 / intermediate layer B2 / seal layer C2. In this case, the base material layer A1 and the base material layer A2 may be made of different polypropylene-based resin compositions or may be the same, the intermediate layers B1 and B2 may be made of different polypropylene-based resin compositions or may be the same, and the seal layer C1 and the seal layer C2 may be made of different polypropylene-based resin compositions or may be the same.

[0097] When the biaxially oriented polypropylene film of the present invention has a functional layer D, the functional layer D may be laminated directly on the surface of the base layer A, or an intermediate layer B may be interposed between the base layer A and the functional layer D. The functional layer D may also be disposed between the base layer A and the intermediate layer B, or between the intermediate layer B and the sealing layer C. When the functional layer D is present, there are no particular limitations as long as the base layer A, intermediate layer B, and sealing layer C are disposed in this order, with sealing layer C being the outermost surface. Examples of such a structure include a four-layer structure of functional layer D / base layer A / intermediate layer B / sealing layer C, in which the base layer A / intermediate layer B / sealing layer C structure further has a functional layer D, and a five-layer structure of functional layer D / intermediate layer B / base layer A / intermediate layer B / sealing layer C, in which the intermediate layer B / base layer A / intermediate layer B / sealing layer C structure further has a functional layer D.

[0098] The overall thickness of the biaxially oriented polypropylene film of the present invention varies depending on the application and method of use, but is 10 μm or more and 50 μm or less from the viewpoint of film strength, airtightness, or water vapor barrier property. When used as a freshness-preserving packaging material, the thickness is preferably 33 μm or less, more preferably 28 μm or less, even more preferably 23 μm or less, and particularly preferably 18 μm or less.

[0099] The thickness of the base layer A varies depending on the application and method of use, but is preferably 5 to 48 μm, more preferably 10 to 40 μm, and even more preferably 13 to 30 μm. A thickness of 5 μm or more can improve film strength, sealing performance, and water vapor barrier properties. Furthermore, a thickness of 48 μm or less can reduce the environmental impact by reducing the volume.

[0100] The thickness of the intermediate layer B varies depending on the application and method of use, but is preferably 0.5 to 5 μm, more preferably 1.0 to 5.0 μm, and even more preferably 1 to 3 μm. By making the thickness 0.5 μm or more, the adhesion between the base layer A and the seal layer C can be increased, thereby increasing the heat seal strength. Furthermore, by making the thickness 5 μm or less, the environmental load can be reduced by reducing the volume.

[0101] The thickness of the sealing layer C varies depending on the application and method of use, but is preferably 0.5 to 5 μm, and more preferably 0.5 to 1.5 μm. A thickness of 0.5 μm or more can increase the heat seal strength. Furthermore, a thickness of 5 μm or less can reduce the environmental impact by reducing the volume.

[0102] The thickness of the functional layer D varies depending on the application and method of use, but is preferably 0.3 to 2 μm, and more preferably 0.5 to 1.5 μm. A thickness of 0.3 μm or more can increase heat seal strength. Furthermore, a thickness of 2 μm or less can reduce the environmental impact by reducing the volume.

[0103] [Film Properties of Biaxially Oriented Polypropylene Film] The biaxially oriented polypropylene film of the present invention is characterized by the following properties. Here, the "longitudinal direction" of the biaxially oriented polypropylene film of the present invention refers to the direction corresponding to the machine direction in the film production process and may be abbreviated as MD. The "transverse direction" refers to the direction perpendicular to the machine direction in the film production process and may be abbreviated as TD.

[0104] (Stress at 5% elongation at 23°C: F5 value) The sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the biaxially oriented polypropylene film of the present invention at 23°C is 150 MPa or more. When the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23°C is 150 MPa or more, the film has high rigidity, so that the bag shape when made into a packaging bag can be easily maintained even if the film thickness is reduced, and the film is less likely to deform during processing such as printing, making it easy to process. Furthermore, by increasing the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23°C, the anti-fogging properties are also improved when an anti-fogging agent is contained. The reason for this is presumed to be that the increased crystallinity of the base layer A promotes the migration of the anti-fogging agent to the surface layer C. The lower limit of the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23° C. is more preferably 160 MPa, and even more preferably 170 MPa, when making the film thinner. The upper limit of the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23° C. is preferably 200 MPa, and more preferably 190 MPa, from the viewpoint of ease of production. Methods for adjusting the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23° C. within the above range include adjusting the raw material composition of each layer (particularly the raw material composition of the base layer A), the stretch ratio during film formation, the relaxation rate, and the temperature in each film formation step.

[0105] The lower limit of the stress of the biaxially oriented polypropylene film of the present invention at 5% elongation in the longitudinal direction at 23° C. is preferably 35 MPa, more preferably 40 MPa, and particularly preferably 45 MPa, for the same reasons as the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction at 23° C. The upper limit is preferably 70 MPa, more preferably 65 MPa, and particularly preferably 60 MPa.

[0106] The stress at 5% elongation in the width direction of the biaxially oriented polypropylene film of the present invention at 23°C is preferably 110 MPa, more preferably 115 MPa, and particularly preferably 120 MPa, for the same reasons as the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction at 23°C. The upper limit is preferably 200 MPa, more preferably 180 MPa, and particularly preferably 160 MPa. The stress at 5% elongation in the longitudinal and width directions (F5) can be kept within the range by adjusting the raw material composition of each layer (particularly the raw material composition of the base layer A), the stretch ratio during film formation, the relaxation rate, and the temperature in each film formation step. The stress at 5% elongation at 23°C (F5) can be obtained by measuring the tensile strength of the film in the longitudinal and width directions at 23°C in accordance with JIS K7127. In this case, the measurement can be performed with a sample size of 15 mm x 200 mm, a chuck width of 100 mm, and a pulling speed of 200 mm / min.

[0107] (Heat seal strength of seal layer C at 115°C) The heat seal strength of seal layer C of the biaxially oriented polypropylene film of the present invention at 115°C is preferably 1.0 N / 15 mm or more and 5.0 N / 15 mm or less. The lower limit of the heat seal strength at 115°C obtained by the measurement method described below is preferably 1.5 N / 15 mm, more preferably 2.0 N / 15 mm. When the heat seal strength of seal layer C at 115°C is 1.0 N / 15 mm or more, low-temperature sealing properties are good. In other words, even if the temperature of the seal bar during heat sealing is relatively low, a sufficient heat seal strength of 3.0 N / 15 mm or more is easily obtained. This allows for high-speed operation during automatic packaging. In addition, since heat sealing can be performed at a lower temperature, the entire film is less likely to shrink and wrinkles are less likely to form in the sealed portion. The upper limit of the heat seal strength of seal layer C at 115°C is approximately 5.0 N / 15 mm. There is little need for a strength greater than this, and if it is too high, it may be more likely to stick to the seal bar. The heat seal strength of the seal layer C was measured by stacking the seal layers C facing each other and applying a heat seal pressure of 1 kg / cm at each temperature. ２ The measurement can be performed with a heat sealing time of 1 second and a pulling speed of 200 mm / min.

[0108] It is more preferable that the relationship between the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23°C and the heat seal strength of the seal layer C at 115°C satisfies the following formula (1): When the relationship is within the range of formula (1), the balance between rigidity and heat seal strength at low temperatures is excellent, and the bag-making processability, bag-making process speed, and quality of the bags are further improved. Heat seal strength of seal layer C at 115°C [N / 15mm] ≥ -0.037 x (sum of stress at 5% elongation in the longitudinal direction and stress at 5% elongation in the width direction of the biaxially oriented polypropylene film at 23°C) [MPa] + 7.0 (Equation 1) It is even more preferable that the relationship between the sum of the stress at 5% elongation in the longitudinal direction and stress at 5% elongation in the width direction of the film at 23°C and the heat seal strength at 115°C satisfies the following (Equation 2): Heat seal strength of seal layer C at 115°C [N / 15mm] ≥ -0.037 x (sum of stress at 5% elongation in the longitudinal direction and stress at 5% elongation in the width direction of the biaxially oriented polypropylene film at 23°C) [MPa] + 7.2 (Equation 2) It is particularly preferable that the relationship between the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23°C and the heat seal strength at 115°C satisfies the following (Equation 3): Heat seal strength of seal layer C at 115°C [N / 15 mm] ≥ -0.037 × (sum of stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the biaxially oriented polypropylene film at 23°C) [MPa] + 7.3 (Equation 3) When the relationship between the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the width direction of the film at 23°C and the heat seal strength at 115°C is within the range of the following (Equation 4), the bag-making processability, bag-making process speed, and quality of the bags made are sufficient. Heat seal strength of seal layer C at 115°C [N / 15 mm] ≦ -0.037 × (sum of stress at 5% elongation in the longitudinal direction and stress at 5% elongation in the width direction of the biaxially oriented polypropylene film at 23°C) [MPa] + 8.00 (Equation 4)

[0109] (Heat shrinkage at 120°C) The upper limit of the longitudinal heat shrinkage at 120°C of the biaxially oriented polypropylene film of the present invention is preferably 2.5%, more preferably 2.0%, even more preferably 1.5%, and particularly preferably 1.0%. When it is 2.5% or less, printing pitch deviation is less likely to occur when transferring printing ink. The lower limit of the longitudinal heat shrinkage at 120°C of the biaxially oriented polypropylene film of the present invention may be 0%. The upper limit of the widthwise heat shrinkage at 120°C is 1.3%, preferably 1.1%, more preferably 0.9%, even more preferably 0.7%, and particularly preferably 0.5%. When it is 1.3% or less, wrinkles are less likely to occur during heat sealing. When the longitudinal heat shrinkage at 120°C is smaller than the widthwise heat shrinkage at 120°C, printing pitch deviation is less likely to occur when transferring printing ink. The lower limit of the heat shrinkage rate in the width direction of the biaxially oriented polypropylene film of the present invention at 120°C may be -1.0%. The heat shrinkage rates in the longitudinal direction and width direction at 120°C can be kept within the range by adjusting the raw material composition of each layer (particularly the raw material composition of the base layer A), the stretch ratio during film formation, the relaxation rate, and the temperature in each film formation step. The heat shrinkage rate can be measured in accordance with JIS Z1712.

[0110] The heat seal strength of the seal layer C at 115 ° C depends on the characteristics of each layer. As mentioned above, in addition to the method of lowering the melting point of the seal layer C so that the heat of fusion of the seal layer C is increased, the method of adding a low-melting point polypropylene copolymer is effective. Furthermore, in order to prevent the crystal propagation of the base layer A from reaching the seal layer C, it is also effective to lower the melting point of the intermediate layer B or increase the layer thickness. In addition, in order to suppress the crystal propagation of the base layer A to the seal layer C, it is also effective to keep the stretch ratio low to prevent excessive promotion of crystallization, but the F5 value decreases, so it needs to be adjusted appropriately. In addition to the characteristics of each layer described above, the range can be maintained by adjusting the relaxation rate during film formation and the temperature in each film formation process.

[0111] (Heat of fusion of sealing layer C) The heat of fusion of the sealing layer C at 70°C to 140°C in DSC measurement is preferably 10 J / g or more and 40 J / g or less. The lower limit of the heat of fusion is more preferably 11 J / g, and even more preferably 12 J / g. When the heat of fusion is 10 J / g or more, the amount of resin melted in the low melting point region is sufficient, and low-temperature sealing properties are obtained. The upper limit of the heat of fusion is preferably 40 J / g, and more preferably 35 J / g. When the heat of fusion is 40 J / g or less, the rigidity of the sealing layer C is prevented from decreasing too much, and as a result, the rigidity of the entire film is maintained. The heat of fusion of the seal layer C is the heat of fusion (endotherm) from 70°C to 140°C in the DSC curve obtained by scraping the surface of the seal layer C of a biaxially oriented polypropylene film with a razor blade as a measurement sample and scanning it from -30°C to 250°C at a heating rate of 20°C / min using a differential scanning calorimeter. It is not the heat of fusion measured when measuring the melting point of the raw resin composition of the seal layer C. The heat of fusion of the seal layer C varies depending on the crystallinity, such as the melting point and crystallization temperature, of the polypropylene resin composition constituting the seal layer C, but also varies depending on the orientation and crystallization states of the polypropylene resin compositions of the adjacent base layer A and intermediate layer B. That is, it also varies depending on the stretching conditions and heat treatment conditions of the biaxially oriented polypropylene film. Methods for adjusting the heat of fusion of the seal layer C within the above range include setting the properties of the polypropylene resin compositions of the seal layer C, intermediate layer B, and base layer A, and the stretching conditions and heat treatment conditions of the biaxially oriented polypropylene film as described above.

[0112] (Heat seal strength of seal layer C at 130°C) The heat seal strength of seal layer C of the biaxially oriented polypropylene film of the present invention at 130°C is preferably 3.0 N / 15 mm or more and 10.0 N / 15 mm or less. The lower limit of the heat seal strength at 130°C obtained by the measurement method described below is more preferably 3.5 N / 15 mm, even more preferably 4.0 N / 15 mm, and even more preferably 4.5 N / 15 mm, in order to prevent the contents from falling out. The upper limit of the heat seal strength of seal layer C at 130°C is about 10.0 N / 15 mm. There is little need for a strength greater than this, and if it is too great, the bag may become difficult to open. The heat seal strength of seal layer C at 130°C can be kept within the range by adjusting the raw material composition of each of the above-mentioned layers (which depends particularly on the raw material composition of seal layer C, but may also depend on the raw materials of base layer A and intermediate layer B), the stretch ratio during film formation, the relaxation rate, and the temperature in each film formation process.

[0113] (Anti-Fog Property) The surface of the seal layer C of the biaxially oriented polypropylene film of the present invention preferably has an anti-fogging property evaluation of rank 3 or less, obtained by the measurement method described below. Rank 2 or less is more preferable, and rank 1 is even more preferable. That is, after pouring 300 cc of 50°C warm water into a 500 cc open-top container, placing the biaxially oriented polypropylene film at the opening of the container so that the surface of the seal layer C faces the inside of the container and sealing the container, and leaving it at 5°C for 30 minutes, it is preferable that the area of ​​dew adhering to the surface of the seal layer C is one-fourth or less of the area of ​​the biaxially oriented polypropylene film placed at the opening of the container. More preferably, this area is zero, i.e., no dew is adhering to the surface of the seal layer C.

[0114] (Haze) The upper limit of the haze of the biaxially oriented polypropylene film of the present invention is preferably 7.0%, more preferably 5.0%, even more preferably 4.0%, particularly preferably 3.5%, and most preferably 3.0%. A haze of 7.0% or less is easy to use in applications requiring transparency. The lower limit of the haze is preferably 0%, with 0.1% being a practical value. The haze can be kept within the range by adjusting the cooling roll temperature, the longitudinal stretching temperature, the tenter preheating temperature before widthwise stretching, the widthwise stretching temperature, or the heat setting temperature, or the amount of components in the polypropylene polymer having a molecular weight of 100,000 or less. The haze may increase depending on the addition of an antiblocking agent or the composition of the sealing layer C. The haze can be measured at 23°C in accordance with JIS K7105.

[0115] The biaxially oriented polypropylene film of the present invention or the laminate described above can be used to prepare three-side seal type, pillow type and gusset type packaging bags, which have good heat seal strength and good appearance of the sealed portion.

[0116] [Automatic Packaging] The biaxially oriented polypropylene film of the present invention is laminated with a sealing layer C, so it can be made into a packaging bag as is or after printing on the film surface. It is particularly suitable for bag production processing using an automatic packaging machine. Pillow-type packaging, an example of automatic packaging, is described below. A horizontal pillow packaging machine for packaging foods and the like is equipped with a center sealer and an end sealer. The end sealer, located downstream, seals and cuts the end seal portions, located upstream and downstream of the packaged item, of a tubular film conveyed from the center sealer in the flow direction. The center sealer is equipped with a pair of conveying rollers and a pair of heating rollers. The tubular film is conveyed by sandwiching the center seal portion, which is formed by joining both ends in the downstream width direction. The pair of heating rollers located downstream sandwich and heat the center seal portion of the conveyed tubular film. The center seal portion of the tubular film is heated and pressurized by the pair of heating rollers, thereby thermally bonding it together. The time required for heating and pressurizing the center seal portion using the pair of heating rollers is significantly shorter than the time required for heating and pressurizing the end seal portion by the end sealer, which seals the end seal portion using a box motion mechanism or the like. In addition, there is a demand for faster operation of horizontal pillow packaging machines, but if the operating speed of a horizontal pillow packaging machine is increased, the rotation speed of both heating rollers will also increase, further shortening the heating and pressurizing time.Alternatively, even if the heating temperature of the pair of heating rollers is set high to increase the seal strength of the center seal, there is a sufficient difference in melting point from the base film, so there is no slight melting of the film surface due to heating, and the molten film does not adhere to the surface of the heating roller, reducing the frequency of cleaning the heating roller.

[0117] [Laminates using biaxially oriented polypropylene film] Since the biaxially oriented polypropylene film of the present invention is laminated with the seal layer C, it can be made into a packaging bag as is or after printing on the film surface. However, depending on the application, it can also be made into the following laminates (1) to (9) and used as a packaging material. (1) Biaxially oriented PP film layer / printed layer / adhesive layer / linear low-density PE film sealant layer. (2) Biaxially oriented PP film layer / printed layer / adhesive layer / unstretched PP film sealant layer. (3) Biaxially oriented PET film layer / printed layer / adhesive layer / biaxially oriented PP film layer / adhesive layer / unstretched PP film sealant layer. (4) Biaxially oriented PET film layer / printed layer / adhesive layer / biaxially oriented PP film layer / adhesive layer / linear low-density PE film sealant layer. (5) Biaxially oriented PP film layer / anchor coat layer / inorganic thin film layer / inorganic thin film protective layer / printed layer / adhesive layer / linear low-density PE film sealant layer. (6) Linear low-density PE film sealant layer / adhesive layer / biaxially oriented PP film layer / anchor coat layer / inorganic thin film layer / adhesive layer / linear low-density PE film sealant layer. (7) Linear low-density PE film layer / adhesive layer / biaxially oriented PP film layer / anchor coat layer / inorganic thin film layer / adhesive layer / linear low-density PE film layer / low-density PE / paper / low-density PE / linear low-density PE film sealant layer. (8) Biaxially oriented PP film layer / anchor coat layer / inorganic thin film layer / inorganic thin film protective layer / printed layer / adhesive layer / unstretched PP film sealant layer. (9) Biaxially oriented PET film layer / inorganic thin film layer / inorganic thin film protective layer / printed layer / PU adhesive layer / biaxially oriented PP film layer / adhesive layer / easy peel type unoriented PP film sealant layer.

[0118] For example, to prepare the laminate (1), the biaxially oriented PP film of the present invention is printed by letterpress, lithographic, intaglio, stencil, or transfer printing, depending on the application. An adhesive solution is applied thereto, and an unstretched sheet, uniaxially oriented film, or biaxially oriented film made of low-density polyethylene, linear low-density polyethylene, ethylene-vinyl acetate copolymer, polypropylene, or polyester is laminated as a sealant film and dried (dry lamination method). Alternatively, the sealant and film are laminated together while extruding a molten adhesive resin (hot melt lamination method).

[0119] Furthermore, if gas barrier properties and heat resistance are to be improved, an unstretched sheet, uniaxially oriented film, or biaxially oriented film made of aluminum foil, polyvinylidene chloride, nylon, ethylene-vinyl alcohol copolymer, or polyvinyl alcohol can be provided as an intermediate layer between the biaxially oriented polypropylene film and the sealant film. To improve gas barrier properties, aluminum or inorganic oxides can be vapor-deposited onto the biaxially oriented polypropylene film, intermediate layer film, or sealant film. Vacuum vapor deposition, sputtering, and ion plating can be used as the vapor deposition method, but vacuum vapor deposition of silica, alumina, or a mixture of these is particularly preferred.

[0120] This application claims the benefit of priority based on Japanese Patent Application No. 2023-101750, filed on June 21, 2023. The entire contents of the specification of Japanese Patent Application No. 2023-101750, filed on June 21, 2023, are incorporated herein by reference.

[0121] The present invention will be described in more detail below with reference to examples. The properties were measured and evaluated by the following methods. (1) Melt flow rate of polypropylene polymer 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 (unit: g / 10 min).

[0122] (2) Mesopentad fraction of polypropylene polymer The mesopentad fraction ([mmmm]%) of a polypropylene resin is measured by １３The mesopentad fraction was calculated according to the method described in Zambelli et al., Macromolecules, Vol. 6, p. 925 (1973). １３ C-NMR measurement was performed using an AVANCE 500 manufactured by BRUKER, by dissolving 200 mg of a sample in a 8:2 mixture of o-dichlorobenzene and deuterated benzene at 135°C and at 110°C.

[0123] (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 polymers. Gel permeation chromatography (GPC) was used to determine the molecular weight in terms of PP using monodisperse polystyrene as the standard. When the baseline was unclear, the baseline was set 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 were as follows: Apparatus: HLC-8321PC / HT (Tosoh Corporation) Detector: RI Solvent: 1,2,4-trichlorobenzene + dibutylhydroxytoluene (0.05%) Column: TSK gelguard column HHR (30) HT (7.5 mm I.D. × 7.5 cm) × 1 + TSK gel GMHHR-H (20) HT (7.8 mm I.D. × 30 cm) × 3 Flow rate: 1.0 mL / min Injection volume: 0.3 mL Measurement temperature: 140°C The number average molecular weight (Mn) and mass average molecular weight (Mw) were determined by the molecular weight (M) at each elution position of the GPC curve obtained via a molecular weight calibration curve. ｉ ) number of molecules (N ｉ ) is defined by the following formula: 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. The proportions of components with a molecular weight of 10,000 or less and components with a molecular weight of 100,000 or less were determined from the integral curve of the molecular weight distribution obtained by GPC.

[0124] (4) Melting Point (Tm) Thermal measurements were performed under a nitrogen atmosphere using a TA Instruments Q1000 differential scanning calorimeter. Approximately 5 mg of the polypropylene resin composition pellets were cut out and sealed in an aluminum pan for measurement. The temperature was raised to 230°C and held for 5 minutes, then cooled to 30°C at a rate of -10°C / min, held at 30°C for 5 minutes, and then raised to 230°C at a rate of 10°C / min. The main endothermic peak temperature was taken as the melting point (Tm) (unit: °C). The melting points of the polypropylene resin compositions constituting the functional layer (D), substrate layer (A), intermediate layer (B), and seal layer (C) were measured by melt-mixing the raw material compositions of each layer in the composition ratios shown and pelletizing them.

[0125] (5) Film Thickness The total thickness of the film was measured using a Millitron 1202D manufactured by Seiko EM Co., Ltd. The thickness of each layer was calculated from the ratio of the total thickness measured above to the extrusion rate of each layer (unit: μm).

[0126] (6) Haze: Measured at 23° C. in accordance with JIS K7105 using an NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd. (unit: %).

[0127] (7) Stress at 5% Elongation at 23°C (F5 Value) The tensile strength of the film in the longitudinal and transverse directions was measured at 23°C in accordance with JIS K7127. A sample of 15 mm x 200 mm was cut out from the film and set in a tensile tester (Instron 5965, a dual-column tabletop tester manufactured by Instron Japan Co., Ltd.) with a chuck width of 100 mm. A tensile test was carried out at a tensile speed of 200 mm / min. The stress at 5% elongation at 23°C (F5 value) was read from the obtained strain-stress curve.

[0128] (8) Heat shrinkage was measured according to JIS Z1712 by the following method. The film was cut into pieces of 20 mm width and 200 mm length in both the longitudinal and transverse directions of the film, and then hung in a hot air oven at 120°C for 5 minutes. The length after heating was measured, and the heat shrinkage was calculated as the ratio of the shrunk length to the original length (unit: %).

[0129] (9) Heat of fusion The surface of the seal layer C of a biaxially oriented polypropylene film was scraped with a razor blade to prepare a measurement sample. The sample was weighed to 5.0±0.2 mg, and scanned under a nitrogen atmosphere from −30°C to 250°C at a heating rate of 20°C / min using a differential scanning calorimeter DSC60 (Shimadzu Corporation). The heat of fusion of the obtained DSC curve from 70°C to 140°C was calculated.

[0130] (10) Heat-sealing strength A biaxially oriented polypropylene film was cut into a length of 20 cm in the longitudinal direction and 5 cm in the width direction. The seal layers C of the two cut-out films were placed face to face, and the five films were simultaneously heat-sealed using a thermal gradient tester (manufactured by Toyo Seiki Co., Ltd.) using five heat-sealing bars with a sealing surface of 3 cm in the longitudinal direction and 1 cm in the transverse direction, with a longitudinal spacing of 1 cm between each seal bar. The temperatures of the four heat-sealing bars were set at 110°C, 115°C, 120°C, and 130°C. The heat-sealing pressure was 1 kg / cm. ２ The time was 1 second. The long axis of the heat seal bar was parallel to the longitudinal direction of the film and positioned at the center of the film's width. The distance between the end of the short axis of the sample and the seal bar was 0.5 cm. Each sealed portion (3 cm x 1 cm) was cut in the width direction, 15 mm from the center of the longitudinal direction, and attached to the upper and lower chucks of a tensile tester (Instron 5965 dual column benchtop tester). The heat seal strength of each was measured when pulled at a pulling speed of 200 mm / min (unit: N / 15 mm). The functional layers D were also stacked facing each other in the same manner, and the heat seal strength was determined using a thermal gradient tester.

[0131] (11) Anti-Fogging Property Anti-fogging property was evaluated by the following procedure. 1) 300 cc of 50°C hot water was poured into a 500 cc open-top container. 2) The obtained biaxially oriented polypropylene film was placed at the opening of the container so that the surface of the sealing layer C faced the inside of the container, and the container was sealed. 3) The container was left in a 5°C cold room for 30 minutes. 4) After leaving the container in a 5°C cold room for 30 minutes, the degree of dew adhesion to the surface of the sealing layer C was evaluated on a 5-point scale. Grade 1 (Rank 1): No dew on the entire surface (0 area of ​​dew) Grade 2 (Rank 2): Some dew adhesion (the area of ​​dew adhesion is ¼ or less of the area of ​​the biaxially oriented polypropylene film placed on the container opening) Grade 3 (Rank 3): Dew adhesion on about ½ (the area of ​​dew adhesion is more than ¼ and less than 2 / 4 of the area of ​​the biaxially oriented polypropylene film placed on the container opening) Grade 4 (Rank 4): Almost no dew adhesion (the area of ​​dew adhesion is more than ¼ and less than ¾ of the area of ​​the biaxially oriented polypropylene film placed on the container opening) Grade 5 (Rank 5): Dew adhesion on the entire surface (the area of ​​dew adhesion is more than ¾ of the area of ​​the biaxially oriented polypropylene film placed on the container opening)

[0132] (12) Strength of the film The strength of the film obtained in the examples was classified into three stages by sensory evaluation. The method was to evaluate the strength of the film when it was cut to A4 size, placed on the palm of the hand, and lightly gripped. ◎: Very strong, excellent. ○: Sufficiently strong, good. ×: Insufficient, poor.

[0133] (13) Suitability for Automatic Packaging Using a compact pillow packaging machine FW3301 (manufactured by Fujikikai Co., Ltd.), horizontal pillow packages were produced with the seal layer C of the biaxially oriented polypropylene film facing inward. Back sealing temperature: 110°C Top and bottom sealing temperature: 110°C Number of shots: 50 bags / min Bag shape: 27 cm vertically, 16 cm horizontally, with the width direction of the film as the horizontal direction. The suitability for automatic packaging was evaluated based on the sealing ability of the package as follows: ◯: Sufficient sealing, good. ×: Insufficient sealing, easy to peel, poor.

[0134] (Raw Materials Used) The polypropylene resins constituting each layer used in the following Examples and Comparative Examples are as follows: PP-1: A mixture obtained by mixing a propylene homopolymer (manufactured by Sumitomo Chemical Co., Ltd., mesopentad fraction: 98.9%, melting point: 162.5°C, MFR: 7.5 g / 10 min, Mw / Mn: 3.7, amount of components having a molecular weight of 10,000 or less: 4.0 mass%, amount of components having a molecular weight of 100,000 or less: 40.5 mass%) with stearyl diethanolamine monostearate, stearyl diethanolamine distearate, and stearyl diethanolamine, all manufactured by Toho Chemical Industry Co., Ltd., as antifogging agents, containing a total of 2.0 mass% of the antifogging agent. PP-2: Propylene homopolymer (manufactured by Sumitomo Chemical Co., Ltd., mesopentad fraction: 98.9%, melting point: 163 ° C., MFR: 7.5 g / 10 min, Mw / Mn: 3.6, amount of components with a molecular weight of 10,000 or less: 4.0 mass%, amount of components with a molecular weight of 100,000 or less: 40.5 mass%) PP-3: Propylene homopolymer with a large Mw / Mn value (manufactured by Sumitomo Chemical Co., Ltd., mesopentad fraction: 98.8%, melting point: 162 ° C., MFR: 11 g / 10 min, Mw / Mn: 9.6, amount of components with a molecular weight of 10,000 or less: 6.9 mass%, amount of components with a molecular weight of 100,000 or less: 53.1 mass%) PP-4: A mixture obtained by mixing a propylene polymer (manufactured by Sumitomo Chemical Co., Ltd., copolymerized ethylene component: 0.6 mol%, melting point: 158°C, MFR: 2.5g / 10min) with glycerin monostearate (manufactured by Toho Chemical Industry Co., Ltd.) as an anti-fogging agent, thereby containing 6.0 mass% of the anti-fogging agent in total. PP-5: A mixture obtained by mixing a propylene polymer (manufactured by Sumitomo Chemical Co., Ltd., copolymerized ethylene component: 0.6 mol%, melting point: 158°C, MFR: 7.5g / 10min) with stearyl diethanolamine monostearate, stearyl diethanolamine distearate, and stearyl diethanolamine (manufactured by Toho Chemical Industry Co., Ltd.) as anti-fogging agents, thereby containing 2.0 mass% of the anti-fogging agent in total. PP-6: A mixture containing 0.5% by mass of anti-fogging agent in total, obtained by mixing propylene-ethylene-butene copolymer (SP7843, manufactured by Sumitomo Chemical Co., Ltd., melting point: 128°C, MFR: 6.5 g / 10 min) with glycerin monostearate manufactured by Toho Chemical Industry Co., Ltd. as an anti-fogging agent.PP-7: Propylene-butene copolymer (manufactured by Mitsui Chemicals, Inc., Tafmer XM7070, melting point: 75°C, MFR: 7.0 g / 10 min) PP-8: Propylene-butene copolymer (manufactured by Sumitomo Chemical Co., Ltd., FSX66M4, melting point: 138°C, MFR: 4.5 g / 10 min) mixed with glycerin monostearate manufactured by Toho Chemical Industry Co., Ltd. as an anti-fogging agent, resulting in a mixture containing a total of 0.45 mass% of anti-fogging agent.

[0135] [Example 1] (Base Layer A) A polypropylene-based resin composition containing 59% by mass of a mixture of an anti-fog agent and a propylene homopolymer (PP-1), 18.5% by mass of a propylene homopolymer (PP-2), 20.0% by mass of a propylene homopolymer (PP-3) with a high Mw / Mn value, and 2.5% by mass of a mixture of an anti-fog agent and a propylene homopolymer (PP-4) was used. (Intermediate Layer B) A polypropylene-based resin composition containing a mixture of an anti-fog agent and a propylene polymer (PP-5) copolymerized with 0.6 mol% ethylene was used. (Sealing Layer C) A polypropylene-based resin composition containing 70% by mass of a propylene-ethylene-butene copolymer (PP-6) and 30% by mass of a propylene-butene copolymer (PP-7) was used. (Functional Layer D) A polypropylene-based resin composition containing 100% by mass of a propylene-ethylene-butene copolymer (PP-8) was used.

[0136] (Film production) First, the polypropylene-based resin compositions constituting each of the functional layer D / base layer A / intermediate layer B / sealing layer C were heated and melted in an extruder using a multi-layer feed block at 220°C, 250°C, 240°C, and 210°C, respectively, and the molten polypropylene-based resin compositions were then laminated from a T-die at 250°C to form a layer structure of functional layer D / base layer A / intermediate layer B / sealing layer C, and co-extruded so that the thickness ratio of each was 1:16:2:1.

[0137] The functional layer D side of the molten sheet was brought into contact with a cooling roll at 20 ° C. and then placed in a water bath at 20 ° C. Subsequently, the sheet was stretched 4.5 times in the longitudinal direction between metal rolls heated to 135 ° C. using the difference in peripheral speed, and then introduced into a tenter stretching machine, where it was stretched 11.2 times in the width direction at a preheating section temperature of 168 ° C. and a stretching section temperature of 163 ° C. Then, it was heat-treated at 165 ° C. while relaxing by 6.0% in the width direction. The film surface on the seal layer C side of the obtained biaxially oriented polypropylene film was subjected to corona treatment using a corona treater manufactured by Softal Corona & Plasma GmbH under conditions of an applied current of 0.75 A and an applied voltage of 1.8 kW, and then wound up on a winder to obtain the biaxially oriented polypropylene film of the present invention. The thickness of the obtained film was 20 μm. The thicknesses of the functional layer D, substrate layer A, intermediate layer B, and seal layer C of the film thus obtained were 1 μm, 16 μm, 2 μm, and 1 μm, respectively. The raw material composition of each layer, the film-forming conditions, and the film properties are shown in Table 1. The film obtained was a film with high rigidity, yet low heat shrinkage, and excellent anti-fogging properties with good heat-sealing strength at low temperatures.

[0138] [Example 2] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material for the seal layer C was a polypropylene-based resin composition containing 80 mass% of propylene-ethylene-butene copolymer PP-6 and 20 mass% of propylene-butene copolymer PP-7. As shown in Table 1, the obtained film had high rigidity, a low heat shrinkage rate, and good heat seal strength at low temperatures, and was an excellent film with anti-fogging properties.

[0139] [Example 3] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material for the seal layer C was a polypropylene-based resin composition containing 50% by mass of propylene-ethylene-butene copolymer PP-6 and 50% by mass of propylene-butene copolymer PP-7. As shown in Table 1, the obtained film had high rigidity, a low heat shrinkage rate, and good heat seal strength at low temperatures, as well as excellent anti-fogging properties.

[0140] [Example 4] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was changed to 13. As shown in Table 1, the obtained film had high rigidity, a low heat shrinkage rate, and good heat seal strength at low temperatures, and was an excellent film with anti-fogging properties.

[0141] [Example 5] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the seal layer C was changed from 1.0 μm to 0.5 μm and the thickness of the base layer A was changed from 16 μm to 16.5 μm. As shown in Table 1, the obtained film had high rigidity, a low heat shrinkage rate, and good heat seal strength at low temperatures, and was an excellent film with anti-fogging properties.

[0142] [Example 6] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the seal layer C was changed from 1.0 μm to 2.0 μm and the thickness of the base layer A was changed from 16 μm to 15 μm. As shown in Table 1, the obtained film had high rigidity, a low heat shrinkage rate, and good heat seal strength at low temperatures, and was an excellent film with anti-fogging properties.

[0143] [Example 7] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the intermediate layer B was changed from 2.0 μm to 4.0 μm, the thickness of the seal layer C was changed from 1.0 μm to 0.5 μm, and the thickness of the base layer A was changed from 16 μm to 14.5 μm. As shown in Table 1, the obtained film had high rigidity, a low heat shrinkage rate, and good heat seal strength at low temperatures, and was an excellent film with anti-fogging properties.

[0144] [Example 8] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the intermediate layer B was changed from 2.0 μm to 3.0 μm and the thickness of the seal base layer A was changed from 16 μm to 15 μm. As shown in Table 1, the obtained film had high rigidity, a low heat shrinkage rate, and good heat seal strength at low temperatures, and was an excellent film with anti-fogging properties.

[0145] Example 9 Biaxially oriented polypropylene films were obtained in the same manner as in Example 1, except that the thickness of the intermediate layer B was changed from 2.0 μm to 1.0 μm and the thickness of the base layer A was changed from 16 μm to 17 μm. As shown in Table 1, the obtained films had high rigidity, low heat shrinkage, and excellent anti-fogging properties, with good heat-sealing strength at low temperatures. The biaxially oriented polypropylene films obtained in Examples 1 to 9 were excellent in stiffness and suitability for automatic packaging, and furthermore, they easily maintained the shape of packaging bags when made into them and were less likely to deform during processing such as printing. In addition, because the heat shrinkage was low, there was little deviation in printing pitch and little wrinkling in the sealed area during heat sealing.

[0146] [Comparative Example 1] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material for the seal layer C was a polypropylene-based resin composition containing 90% by mass of propylene-ethylene-butene copolymer PP-6 and 10% by mass of propylene-butene copolymer PP-7. As shown in Table 2, the obtained film had high rigidity and a low heat shrinkage rate, but had low heat seal strength at low temperatures and was poorly suited for automatic packaging.

[0147] Comparative Example 2 A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material for the seal layer C was a polypropylene-based resin composition containing 100% by mass of propylene-ethylene-butene copolymer PP-6 and not containing the low-melting point propylene-butene copolymer PP-7. As shown in Table 2, the obtained film had high rigidity and a low heat shrinkage, but had low heat seal strength at low temperatures and was poorly suited for automatic packaging.

[0148] Comparative Example 3 A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the stretching ratio in the width direction was changed to 10.0 times. As shown in Table 2, the F5 values ​​of the obtained film in both the width direction (TD) and the longitudinal direction (MD) at 23°C were low, and the film had a weak stiffness. It was prone to deformation during processing such as printing, making processing difficult. Furthermore, when made into a packaging bag, it was difficult to maintain the bag shape.

[0149] [Comparative Example 4] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the seal layer C was changed from 1.0 μm to 0.3 μm and the thickness of the base layer A was changed from 16 μm to 16.7 μm. As shown in Table 2, the obtained film had high rigidity and a low heat shrinkage rate, but had low heat seal strength at low temperatures and was poorly suited to automatic packaging.

[0150] [Comparative Example 5] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the seal layer C was changed from 1.0 μm to 3.0 μm and the thickness of the base layer A was changed from 16 μm to 14 μm. As shown in Table 2, the F5 values ​​of the obtained film in the width direction (TD) and the longitudinal direction (MD) at 23°C were low, the film was weak, and it was prone to deformation during processing such as printing, making processing difficult. Furthermore, it was difficult to maintain the bag shape when made into a packaging bag.

[0151] [Comparative Example 6] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the thickness of the intermediate layer B was changed from 2.0 μm to 0.4 μm and the thickness of the base layer A was changed from 16 μm to 17.6 μm. As shown in Table 2, the obtained film had high rigidity and a low heat shrinkage rate, but had low heat seal strength at low temperatures and poor suitability for automatic packaging.

[0152] [Comparative Example 7] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the intermediate layer B was not provided (the thickness was changed from 2.0 μm to 0 μm) and the thickness of the base layer A was changed from 16 μm to 18 μm. As shown in Table 2, the obtained film had high rigidity and a low heat shrinkage rate, but had low heat seal strength at low temperatures and poor suitability for automatic packaging.

[0153] Comparative Example 8 In the raw material composition of the base layer A, the propylene homopolymer PP-3 with a large Mw / Mn value was not blended (20 mass% changed to 0 mass%), and instead the amount of propylene homopolymer PP-1 was increased (changed from 59 mass% to 79 mass%), and the raw material of the seal layer C was a polypropylene resin composition containing 100 mass% of propylene-ethylene-butene copolymer PP-6. An attempt was made to obtain a biaxially oriented polypropylene film in the same manner as in Example 1, but breakage occurred in the width direction stretching step, and a biaxially oriented polypropylene film could not be obtained.

[0154] [Comparative Example 9] An attempt was made to obtain a biaxially oriented polypropylene film in the same manner as in Comparative Example 8, except that the widthwise stretching ratio was changed to 10.0 times and the raw material for the seal layer C was a polypropylene-based resin composition containing 70 mass% of propylene-ethylene-butene copolymer PP-6 and 30 mass% of propylene-butene copolymer PP-7. However, breakage occurred in the widthwise stretching step, and a biaxially oriented polypropylene film could not be obtained.

[0155] [Comparative Example 10] A biaxially oriented polypropylene film was obtained in the same manner as in Comparative Example 8, except that the widthwise stretching ratio was changed to 8.0 times. As shown in Table 2, the obtained film had a low F5 value in the widthwise direction at 23°C, was weak in stiffness, and was prone to deformation during processing such as printing, making processing difficult. Furthermore, when made into a packaging bag, it was difficult to maintain the bag shape.

[0156] [Comparative Example 11] In the raw material composition of the base layer A, 50% by mass of polypropylene polymer PP-5 copolymerized with 0.6 mol% ethylene component having a melting point of 158 ° C, 47.5% by mass of highly stereoregular propylene homopolymer PP-1 having a mesopentad fraction of 98.9% and a melting point of 162.5 ° C, and 2.5% by mass of propylene homopolymer PP-4 containing an anti-fogging agent were blended, and the widthwise stretch ratio was changed to 10.0 times, and the widthwise relaxation was changed from 6.0% to 0%, to obtain a biaxially oriented polypropylene film in the same manner as in Example 1. As shown in Table 2, the obtained film had a low widthwise F5 value at 23 ° C, was a film with weak stiffness, and was prone to deformation during processing such as printing, making processing difficult. Furthermore, it was difficult to maintain the bag shape when made into a packaging bag. In addition, printing pitch deviation was likely to occur.

[0157] Comparative Example 12 A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material for the intermediate layer B was a polypropylene resin composition containing 100% by mass of propylene-ethylene-butene copolymer PP-8 having a melting point of 138°C. As shown in Table 2, the F5 values ​​of the obtained film in the width direction (TD) and longitudinal direction (MD) at 23°C were low, the film was weak, and it was prone to deformation during processing such as printing, making processing difficult. Furthermore, when made into a packaging bag, it was difficult to maintain the bag shape.

[0158] [Comparative Example 13] A biaxially oriented polypropylene film was obtained in the same manner as in Example 1, except that the raw material for the intermediate layer B was the same polypropylene-based resin composition as that for the base layer A, which had a melting point of 162°C. As shown in Table 2, the obtained film had high rigidity and a low heat shrinkage rate, but had low heat seal strength at low temperatures and poor suitability for automatic packaging.

[0159]

[0160]

[0161] The biaxially oriented polypropylene film of the present invention has high rigidity and excellent heat seal strength at low temperatures. Therefore, it is excellent in bag-making processability, easily retains the shape of the bag when made into a packaging bag, and is also excellent in suitability for automatic packaging, making it suitable for use as a packaging bag. By imparting anti-fogging properties, it is particularly suitable for packaging fruits and vegetables. Furthermore, because of its high rigidity, it is possible to make the film thinner than conventional films, or it can be used without laminating a sealant film, thereby reducing the burden on the environment.

Claims

1. A biaxially oriented polypropylene film having a base layer A made of a polypropylene resin composition and a seal layer C, which satisfies the following (1) to (3): (1) The sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the transverse direction of the biaxially oriented polypropylene film at 23°C is 150 MPa or more. (2) The heat seal strength when the seal layers C are stacked facing each other and heat sealed at 115°C is 1.0 N / 15 mm or more. (3) The thickness is 10 μm or more and 50 μm or less.

2. The biaxially oriented polypropylene film according to claim 1, wherein the shrinkage rate of the biaxially oriented polypropylene film after heat treatment at 120°C for 5 minutes is 0% or more and 2.5% or less in the longitudinal direction and -1.0% or more and 1.3% or less in the transverse direction.

3. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the relationship between the sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the transverse direction of the biaxially oriented polypropylene film at 23°C and the heat seal strength of the seal layer C at 115°C satisfies the following formula 1: Heat seal strength of seal layer C at 115°C [N / 15mm] ≧-0.037 x (sum of the stress at 5% elongation in the longitudinal direction and the stress at 5% elongation in the transverse direction of the biaxially oriented polypropylene film at 23°C) [MPa] + 7.0 ... (formula 1) 4. A biaxially oriented polypropylene film according to claim 1 or 2, wherein the heat seal strength of said seal layer C at 130°C is 3.0 N / 15 mm or more and 10.0 N / 15 mm or less.

5. A biaxially oriented polypropylene film as described in claim 1 or 2, having an intermediate layer B between the base layer A and the sealing layer C, the intermediate layer B being made of a polypropylene-based resin composition and having a thickness of 1.0 μm or more and 5.0 μm or less.

6. A biaxially oriented polypropylene film according to claim 1 or 2, wherein the polypropylene resin composition constituting the base layer A has a mesopentad fraction of 97% or more.

7. A biaxially oriented polypropylene film as described in claim 1 or 2, in which the melt flow rate of the polypropylene resin composition constituting the base layer A, measured at 230°C and 2.16 kgf, is 7.0 g / 10 min or more and 11 g / 10 min or less.

8. A biaxially oriented polypropylene film as described in claim 1 or 2, wherein the polypropylene resin composition constituting the base layer A contains 10% by mass or more and 30% by mass or less of polypropylene resin having a ratio (Mw / Mn) of mass average molecular weight (Mw) to number average molecular weight (Mn) of 9.0 or more and 11.0 or less.

9. The biaxially oriented polypropylene film according to claim 1 or 2, wherein 300 cc of 50°C warm water is poured into a 500 cc open-top container, the biaxially oriented polypropylene film is placed at the opening of the container so that the surface of the sealing layer C faces the inside of the container, the container is sealed, and the container is left to stand at 5°C for 30 minutes, after which the area of ​​dew on the surface of the sealing layer C is less than one-fourth of the area of ​​the biaxially oriented polypropylene film placed at the opening of the container.

10. A biaxially oriented polypropylene film as described in claim 1 or 2, in which a functional layer D made of a polypropylene-based resin composition is laminated on the surface of the base layer A opposite to the sealing layer C.

11. The biaxially oriented polypropylene film described in claim 10, wherein the heat seal strength of the functional layer D at 130°C is 1.0 N / 15 mm or more and is equal to or less than the heat seal strength of the seal layer C at 130°C.

12. A biaxially oriented polypropylene film as described in claim 1 or 2, wherein the polypropylene resin composition constituting the sealing layer C contains 30% by mass or more and 50% by mass or less of a polypropylene resin having a melting point of 60°C or more and 90°C or less.