WO2025164065A1 - Biaxially oriented polypropylene film - Google Patents

Abstract

The purpose of the present invention is to provide a biaxially oriented polypropylene film having a great clarity, a high storage modulus at high temperature, and a low heat shrinkage rate at high temperature. A biaxially oriented polypropylene film according to the present invention is characterized by: having a base material layer A made of a polypropylene-based resin composition and a seal layer B made of a polypropylene-based resin composition; and satisfying the following (1)-(4).　(1) The seal layer B is provided on at least one of the outermost surfaces.　(2) The clarity is 90-100%.　(3) The storage modulus at 140°C in the width direction is 0.7-5.0 GPa.　(4) The 150°C heat shrinkage rate in the width direction is at most 10%.

Description

Biaxially oriented polypropylene film

The present invention relates to a biaxially oriented polypropylene film.

Conventionally, polypropylene-based resin films have been widely used as heat-sealable films for packaging. One example of a polypropylene-based resin film with heat-sealability is a laminated polypropylene-based resin film made by laminating an unstretched polyethylene-based resin film or an unstretched polypropylene-based resin film with an oriented polypropylene-based resin film. However, while the above-mentioned laminated polypropylene-based resin film has sufficient sealing strength, it requires a lamination process that uses organic solvents, making it undesirable from both a cost perspective and an environmental impact perspective.

Another example is a laminated polypropylene resin film obtained by stretching a sheet in which a layer of high-melting-point polypropylene resin and a layer of low-melting-point polyolefin resin are co-extruded together. For example, Patent Documents 1 and 2 disclose biaxially oriented polypropylene resin films that have excellent rigidity, transparency, and heat sealability.

International Publication No. 2021/5893 Pamphlet International Publication No. 2019/244708

However, the films in Patent Documents 1 and 2 had the problem of poor processability. There was also a demand for improved transparency.

The object of the present invention is to provide a biaxially oriented polypropylene film that has high clarity, a high storage modulus at high temperatures, and a low heat shrinkage rate at high temperatures.

As a result of extensive research to achieve the above object, the inventors have developed a biaxially oriented polypropylene film with high clarity, high storage modulus at high temperatures, and low heat shrinkage at high temperatures, thereby completing the present invention. That is, the present invention includes the following inventions.
[1] A biaxially oriented polypropylene film having a base layer A made of a polypropylene-based resin composition and a seal layer B made of a polypropylene-based resin composition, and satisfying the following (1) to (4):
(1) The seal layer B is provided on at least one outermost surface.
(2) Clarity is between 90% and 100%.
(3) The storage modulus in the width direction at 140° C. is 0.7 GPa or more and 5.0 GPa or less.
(4) The thermal shrinkage rate at 150°C in the width direction is 10% or less.
[2] The biaxially oriented polypropylene film according to [1 _] , wherein, in a thermomechanical analysis, when heated from 30°C to 160°C at a heating rate of 10°C/min, the temperature at which the widthwise length is 0.9950 x _X0 or less relative to the widthwise length X0 at 30°C is 129°C or higher, and the storage modulus in the longitudinal direction at 23°C is 2.0 GPa or higher and the storage modulus in the widthwise direction at 23°C is 7.0 GPa or higher.
[3] The biaxially oriented polypropylene film according to [1] or [2], wherein the storage modulus in the longitudinal direction at 120°C is 0.5 GPa or more and the storage modulus in the width direction at 120°C is 1.5 GPa or more.
[4] The biaxially oriented polypropylene film according to any one of [1] to [3], wherein the heat shrinkage rate in the longitudinal direction at 120 ° C is 2.5% or less and the heat shrinkage rate in the width direction at 120 ° C is 1.1% or less.
[5] The biaxially oriented polypropylene film according to any one of [1] to [4], wherein the stress at 5% elongation in the width direction at 23°C is 120 MPa or more.
[6] The biaxially oriented polypropylene film according to any one of [1] to [5], having a haze of 7.0% or less.
[7] The biaxially oriented polypropylene film according to any one of [1] to [6], containing an anti-fogging agent in an amount of 0.2% by mass or more and 2.0% by mass or less.
[8] The biaxially oriented polypropylene film according to any one of [1] to [7], wherein the base layer A contains 90% by mass or more of a polypropylene resin having a mesopentad fraction of 97.0% or more.
[9] The biaxially oriented polypropylene film according to any one of [1] to [8], wherein the seal layer B contains 70% by mass or more of a polypropylene copolymer, and the polypropylene copolymer contains 4% by mol or more of an α-olefin other than propylene.
[10] The biaxially oriented polypropylene film according to any one of [1] to [9], which has an intermediate layer C made of a polypropylene-based resin composition between the base layer A and the seal layer B, and the melting point of the polypropylene-based resin composition constituting the intermediate layer C is higher than the melting point of the polypropylene-based resin composition constituting the seal layer B.
[11] The biaxially oriented polypropylene film according to any one of [1] to [10], having a thickness of 10 μm or more and 100 μm or less.

The polypropylene film of the present invention is a heat-sealable film with excellent rigidity and heat resistance, so it easily maintains its bag shape when made into a packaging bag. It can also be used suitably in applications requiring high rigidity without laminating a sealant film, and can maintain its strength even when the film is thin. Furthermore, because it has excellent heat resistance, there are fewer wrinkles in the sealed area when heat-sealed, resulting in an excellent appearance when made into a bag.

FIG. 1 is a diagram showing the relationship between temperature and the length in the width direction of the film in Example 1 according to the present invention, Comparative Example 1, and Comparative Example 4. FIG. 2 is a diagram showing the relationship between temperature and loss modulus in Example 1 according to the present invention and Comparative Example 1. FIG. 3 is a diagram showing the relationship between temperature and storage modulus in Example 1 according to the present invention and Comparative Example 1.

The biaxially oriented polypropylene film of the present invention will be described below.

Layer structure of biaxially oriented polypropylene film The biaxially oriented polypropylene film of the present invention has a base layer A made of a polypropylene-based resin composition and a seal layer B made of a polypropylene-based resin composition. The biaxially oriented polypropylene film of the present invention may have an intermediate layer C made of a polypropylene-based resin composition between the base layer A and the seal layer B. Furthermore, the biaxially oriented polypropylene film of the present invention may have a functional layer D.
The substrate layer A, the seal layer B, the intermediate layer C, and the functional layer D will be described in detail below.

1. Base material layer A
The base layer A made of a polypropylene-based resin composition is preferably made of a polypropylene-based resin composition containing a polypropylene homopolymer as a main component. The term "main component" means that 80% by mass or more of the entire base layer A is polypropylene homopolymer, more preferably 90% by mass or more of the entire base layer A is polypropylene homopolymer, even more preferably 95% by mass or more of the entire base layer A is polypropylene homopolymer, particularly preferably 97% by mass or more of the entire base layer A is polypropylene homopolymer, and most preferably 99% by mass or more of the entire base layer A is polypropylene homopolymer. The upper limit of this ratio is not particularly limited, but 100% by mass or less of the entire base layer A may be polypropylene homopolymer.

1-1. Polypropylene Homopolymer The polypropylene homopolymer used in the base layer A is a polypropylene polymer that is substantially free of α-olefin components other than propylene. Specifically, it is a 100 mol% propylene homopolymer or a polypropylene copolymer having structural units of more than 0 mol% and 1 mol% or less of α-olefin components other than propylene and 99 mol% or more but less than 100 mol% of propylene (total 100 mol%). α-olefin components other than propylene refer to ethylene and α-olefins having 4 or more carbon atoms. Even when α-olefin components other than propylene are contained, the content of α-olefin components other than propylene is 1 mol% or less, as described above, preferably 0.3 mol% or less, more preferably 0.2 mol% or less, and even more preferably 0.1 mol% or less. Within the above range, crystallinity is likely to be improved.

Examples of α-olefin components having 4 or more carbon atoms include 1-butene, 1-pentene, 3-methyl-1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 5-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene.

Two or more different types of polypropylene homopolymers can also be used, in which case it is preferable that the total content is within the above range. The polypropylene homopolymer used in base layer A includes not only polypropylene homopolymers containing no α-olefin components other than propylene, but also polypropylene copolymers whose constituent units are more than 0 mol% and not more than 1 mol% α-olefin components other than propylene and 99 mol% or more but less than 100 mol% propylene.

The following describes various suitable physical properties of polypropylene homopolymer. When two or more different types of polypropylene homopolymer are used, the physical property values are the mass average values of the physical properties of each polypropylene homopolymer.

The melting point Tm _a of the polypropylene homopolymer used in the base layer A is preferably 160°C or higher and 170°C or lower. If Tm _a is 160°C or higher, rigidity and heat resistance at high temperatures are likely to be obtained. If Tm _a is 170°C or lower, it is easy to suppress an increase in the cost of polypropylene production and the film is less likely to break during film formation. _{Tm a} is more preferably 161°C or higher, even more preferably 162°C or higher, more preferably 169°C or lower, even more preferably 168°C or lower, particularly preferably 167°C or lower, and most preferably 166°C or lower. Tm _a can also be further increased by blending a crystal nucleating agent with the above-mentioned polypropylene resin.
The melting point of the resin is the main peak temperature of the endothermic peak accompanying melting, which is observed when 5 mg of polypropylene homopolymer is packed into an aluminum pan, set in a differential scanning calorimeter (DSC), heated from 30°C to 230°C at a heating rate of 20°C/min in a nitrogen atmosphere, and held at 230°C for 5 minutes to melt the polypropylene homopolymer, then cooled to 30°C at a heating rate of -10°C/min, held at 30°C for 5 minutes, and then heated at a heating rate of 10°C/min.

The polypropylene homopolymer used in the base layer A preferably has a mesopentad fraction ([mmmm]%), which is an index of stereoregularity, of 97.0% or more and 99.9% or less. A mesopentad fraction of 97.0% or more enhances the crystallinity of the polypropylene resin, improving the melting point Tm _a of the crystals in the base layer A, the degree of crystallinity, and the degree of crystal orientation, making it easier to obtain rigidity and heat resistance at high temperatures. A mesopentad fraction of 99.9% or less makes it easier to reduce the cost of polypropylene production and makes the film less susceptible to breakage during film formation. The mesopentad fraction is more preferably 97.5% or more, even more preferably 98.0% or more, and more preferably 99.7% or less, and even more preferably 99.5% or less. The mesopentad fraction is measured by nuclear magnetic resonance (NMR) spectroscopy. In order to control the mesopentad fraction of the polypropylene homopolymer within the above 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 a co-catalyst, and a method of appropriately selecting components of the polypropylene resin composition are preferably employed.

The melt flow rate (MFR) of the polypropylene homopolymer used in the base layer A is preferably 5.0 g/10 min or more and 30 g/10 min or less when measured at a temperature of 230°C and a load of 2.16 kgf in accordance with condition M of JIS K 7210 (1995).
When the polypropylene resin has an MFR of 5.0 g/10 min or more, the polypropylene resin constituting the base layer A contains a large amount of low-molecular-weight components, and therefore, by employing a width direction stretching step in the film formation process described below, oriented crystallization of the polypropylene resin is further promoted, the crystallinity of the base layer A is more likely to be increased, and entanglement of polypropylene molecular chains in the amorphous portion is reduced, making it easier to improve heat resistance. Furthermore, when the polypropylene resin has an MFR of 30 g/10 min or less, the film formability of the film is easily maintained.
The MFR is more preferably 5.5 g/10 min or more, even more preferably 6.0 g/10 min or more, particularly preferably 6.3 g/10 min or more, and most preferably 6.5 g/10 min or more. It is more preferably 25 g/10 min or less, even more preferably 22 g/10 min or less, particularly preferably 20 g/10 min or less, and most preferably 10 g/10 min or less.
In order to set the MFR of the polypropylene homopolymer within the above range, it is preferable to employ a method for controlling the molecular weight or molecular weight distribution of the polypropylene homopolymer.

The polypropylene homopolymer used in the base layer A has a lower limit of _Mw / _Mn , which is an index of 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.
When _Mw / _Mn is within the above range, it is easy to increase the amount of components having a molecular weight of 100,000 or less. _Mw / _Mn can be obtained using gel permeation chromatography (GPC).

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

The amount of components with a molecular weight of 100,000 or less in the GPC cumulative curve of the propylene-based resin composition constituting the base layer A is preferably 38% by mass or more, and more preferably 38% by mass or more and 65% by mass or less. By ensuring that the amount of components with a molecular weight of 100,000 or less is 38% by mass or more, heat resistance is easily improved. If the amount of components with a molecular weight of 100,000 or less is 65% by mass or less, film strength is less likely to decrease. In this case, if high-molecular-weight components with long relaxation times or long-chain branched components are included, it becomes easier to adjust the amount of components with a molecular weight of 100,000 or less contained in the polypropylene resin without significantly changing the overall viscosity, making it easier to improve film formability without significantly affecting rigidity or heat resistance. The above amount is even more preferably 40% by mass or more, particularly preferably 41% by mass or more, most preferably 42% by mass or more, even more preferably 60% by mass or less, particularly preferably 55% by mass or less, and most preferably 50% by mass or less.

1-2. Anti-Fog Agent It is preferable to blend an anti-fogging agent into the polypropylene resin composition constituting the base layer A. The biaxially oriented polypropylene film of the present invention can be processed into packaging bags, and when fruits and vegetables are placed in the packaging bags, the addition of an anti-fogging agent can prevent fogging of the packaging bags, since the physiological functions of the fruits and vegetables continue even after harvest.

Examples of the antifogging agent that can be used include known antifogging agents such as ethylene oxide adducts of aliphatic amines, ethylene oxide adducts of aliphatic amides, esters of ethylene oxide adducts of aliphatic amines and fatty acids, fatty acid esters of polyhydric alcohols, fatty acid amines, fatty acid amides, etc. Among these, it is preferable to include at least one selected from the group consisting of ethylene oxide adducts of aliphatic amines and esters of ethylene oxide adducts of aliphatic amines and fatty acids, and it is more preferable to include ethylene oxide adducts of aliphatic amines and esters of ethylene oxide adducts of aliphatic amines and fatty acids.
The ester of an ethylene oxide adduct of an aliphatic amine and a fatty acid preferably comprises at least one selected from the group consisting of stearyl diethanolamine monoester and stearyl diethanolamine diester. Packaging bags are often stored at room temperature rather than frozen, and in order to maintain excellent anti-fogging properties over the long term during distribution, it is preferable to use an anti-fogging agent that continuously exhibits anti-fogging properties over repeated temperature changes between 5°C and 30°C, taking into account temperature changes during storage or distribution. However, the anti-fogging agent is not limited to the above preferred embodiment and may be selected appropriately depending on the application. One type of anti-fogging agent may be used alone, or two or more types may be used in combination.

Examples of stearyl diethanolamine monoesters include stearyl diethanolamine monolaurate, stearyl diethanolamine monomyristate, stearyl diethanolamine monopalmitate, stearyl diethanolamine monostearate, and stearyl diethanolamine monooleate, with stearyl diethanolamine monostearate being preferred.
Examples of stearyl diethanolamine diesters include stearyl diethanolamine dilaurate, stearyl diethanolamine dimyristate, stearyl diethanolamine dipalmitate, stearyl diethanolamine distearate, and stearyl diethanolamine dioleate, with stearyl diethanolamine distearate being preferred.
Examples of the ethylene oxide adducts of aliphatic amines include lauryldiethanolamine, myristyldiethanolamine, palmityldiethanolamine, and stearyldiethanolamine, with stearyldiethanolamine being preferred. The ethylene oxide adducts of aliphatic amines may be used alone or in combination of two or more.

The amount of antifogging agent in the propylene-based resin composition constituting base layer A is preferably 0.2% by mass or more and 2.0% by mass or less. This amount is more preferably 0.3% by mass or more, even more preferably 0.35% by mass or more, more preferably 1.8% by mass or less, and even more preferably 1.6% by mass or less. However, as described below, the antifogging agent may migrate from base layer A to other layers such as intermediate layer C during the film-forming process.

1-3. Others The polypropylene resin composition constituting the base layer A may contain resins other than polypropylene homopolymers, as well as known additives such as heat stabilizers, antioxidants, ultraviolet absorbers, nucleating agents, adhesives, flame retardants, and inorganic or organic fillers, as long as the effects of the present invention are not impaired.
However, it is preferable that these be added in small amounts, and in the polypropylene resin composition constituting the base layer A, the amount of resins other than polypropylene homopolymers is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, particularly preferably 2% by mass or less, and most preferably 1% by mass or less. Furthermore, in the polypropylene resin composition constituting the base layer A, the amount of additives other than resins is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 2% by mass or less, and particularly preferably 1% by mass or less. These resins other than polypropylene homopolymers and/or additives other than resins do not need to be blended into the base layer A. That is, the amount of resins other than polypropylene homopolymers and/or additives other than resins in the base layer A is 0% by mass or more.
Examples of resins other than polypropylene homopolymers include polyolefin resins other than the polypropylene homopolymer used in the base layer A, various elastomers, etc. These may be used by sequential polymerization using a multi-stage reactor, blending with polypropylene resin in a Henschel mixer, diluting master pellets prepared in advance using a melt kneader with polypropylene to a predetermined concentration, or melt-kneading the entire amount in advance. If the surface resistance of the polypropylene homopolymer used in the base layer A is too high, a surfactant may be added to reduce the surface resistance.

2. Sealing layer B
2-1. Polypropylene Resin Composition The seal layer B made of a polypropylene resin composition preferably contains a polypropylene copolymer containing an α-olefin other than propylene. That is, the polypropylene resin composition constituting the seal layer B preferably contains a polypropylene copolymer containing an α-olefin other than propylene. In the description of the seal layer B, even if the term "polypropylene copolymer (contained in seal layer B)" is simply used, it refers to a polypropylene copolymer containing propylene and an α-olefin other than propylene. The seal layer B preferably contains 70% by mass or more of the polypropylene copolymer. By containing 70% by mass or more of the polypropylene copolymer, it is easy to improve the interlayer adhesion between the seal layer B and the intermediate layer C, thereby further increasing the heat seal strength of the biaxially oriented polypropylene film. The proportion is more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more. The polypropylene copolymer contained in the seal layer B can also be two or more different polypropylene copolymers, and the total content is preferably within the above range. There is no particular upper limit to this proportion, but the polypropylene copolymer may account for up to 100% by mass of the entire seal layer B.
The content of α-olefin components other than propylene in the polypropylene copolymer contained in the seal layer B is preferably 4.0 mol% or more. In this case, the propylene content in the polypropylene copolymer is 96 mol% or less, and the sum of the propylene content and the content of α-olefin components other than propylene is 100 mol%. The content of α-olefin components is the total amount of ethylene and α-olefins having 4 or more carbon atoms. The content of α-olefin components other than propylene is more preferably 5.0 mol% or more, even more preferably 6.0 mol% or more, particularly preferably 7.0 mol% or more, and more preferably 15 mol% or less, even more preferably 12 mol% or less, and particularly preferably 10 mol% or less.
Examples of the α-olefin component having 4 or more carbon atoms include 1-butene, 1-pentene, 3-methyl-1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 5-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene.
The α-olefin component other than propylene is preferably an α-olefin component other than propylene having 2 or more and 20 or less carbon atoms, more preferably an α-olefin component other than propylene having 2 or more and 10 or less carbon atoms, even more preferably an α-olefin component other than propylene having 2 or more and 6 or less carbon atoms, and particularly preferably an α-olefin component other than propylene having 2 or more and 4 or less carbon atoms.

The melting point _Tmb of the polypropylene resin composition constituting the seal layer B is preferably 150°C or lower, more preferably 145°C or lower, even more preferably 140°C or lower, particularly preferably 135°C or lower, and most preferably 130°C or lower. By setting _Tmb within the above range, it is easy to lower the heat seal start temperature and also to increase the heat seal strength. There is no particular restriction on the lower limit of the melting point _Tmb of the polypropylene resin composition constituting the seal layer B, and it is, for example, 110°C or higher. Furthermore, from the viewpoint of increasing the heat seal strength, it is preferable that the melting point Tmb of the polypropylene resin composition constituting the seal layer _B is lower than the melting point _Tmc of the polypropylene resin composition constituting the intermediate layer C.

The melt flow rate (MFR) of the polypropylene resin composition constituting seal layer B, measured at a temperature of 230°C and a load of 2.16 kgf, is preferably 5.0 g/10 min or more and 8.0 g/10 min or less. The MFR is more preferably 5.5 g/10 min or more, even more preferably 6.0 g/10 min or more, and particularly preferably 6.3 g/10 min or more, and more preferably 7.5 g/10 min or less, even more preferably 7.0 g/10 min or less, and particularly preferably 6.8 g/10 min or less. Furthermore, from the perspective of increasing heat seal strength, the melt flow rate of the polypropylene resin composition constituting seal layer B is preferably higher than the melt flow rate of the polypropylene resin composition constituting intermediate layer C.

The polypropylene copolymer contained in sealing layer B preferably includes at least one selected from the group consisting of propylene-butene copolymer, propylene-ethylene-butene copolymer, and propylene-ethylene copolymer, and more preferably includes propylene-butene copolymer.

2-2. Propylene-Ethylene-Butene Copolymer The content of α-olefin components other than propylene in the propylene-ethylene-butene copolymer is preferably 4 mol% or more. When the content of α-olefin components other than propylene is 4 mol% or more, the interlayer adhesion between the intermediate layer C and the seal layer B is likely to be improved, and as a result, the heat seal strength and hermetic sealability are likely to be improved. The content of α-olefin components other than propylene is more preferably 5 mol% or more, and even more preferably 6 mol% or more. There is no particular upper limit for the content of α-olefin components other than propylene, and it is, for example, 25 mol% or less. When there are two or more α-olefin components other than propylene, the total amount is taken as the content of the α-olefin components other than propylene.
The ethylene content is preferably 1 mol% or more, more preferably 2 mol% or more. Although there is no particular upper limit to the ethylene content, if the ethylene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may decrease, so the upper limit is, for example, 12 mol% or less.
The butene content is preferably 1 mol% or more, more preferably 2 mol% or more. Although there is no particular upper limit to the butene content, if the butene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may be reduced, so the upper limit is, for example, 16 mol% or less.
As the propylene-ethylene-butene copolymer having a total content of α-olefin components other than propylene of 4 mol % or more, a commercially available product may be used, for example, FSX66E8 manufactured by Sumitomo Chemical Co., Ltd.

2-3. Propylene-Butene Copolymer The butene content in the propylene-butene copolymer is preferably 4 mol% or more. A butene content of 4 mol% or more tends to improve the interlayer adhesion between the intermediate layer C and the seal layer B, and as a result, tends to improve heat seal strength and hermetic sealability. The butene content is more preferably 5 mol% or more, and even more preferably 6 mol% or more. 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 for example, it is 16 mol% or less, preferably 12 mol% or less. As a propylene-butene copolymer having a butene content of 4 mol% or more, commercially available products may be used, such as SP7843 manufactured by Sumitomo Chemical Co., Ltd., SPX78J1 manufactured by Sumitomo Chemical Co., Ltd., and XR110H manufactured by Mitsui Chemicals, Inc.

2-4. Propylene-Ethylene Copolymer The ethylene content of the propylene-ethylene copolymer is preferably 4 mol% or more. When the ethylene content is 4 mol% or more, the interlayer adhesion between the intermediate layer C and the seal layer B is likely to be improved, and as a result, the heat seal strength and hermetic sealability are likely to be improved. The ethylene content is more preferably 5 mol% or more, and even more preferably 6 mol% or more. There is no particular upper limit for the ethylene content, but if the ethylene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may be reduced, so for example, it is 12 mol% or less. As a propylene-ethylene copolymer having an ethylene content of 4 mol% or more, commercially available products may be used, such as PC540R manufactured by SunAllomer Co., Ltd. and VM3588FL manufactured by Mitsui Chemicals, Inc.

2-5. Anti-Fog Agent The polypropylene resin composition constituting the seal layer B may or may not contain an anti-fogging agent. The anti-fogging agents described in the description of the substrate layer A can be used as the anti-fogging agent. Even if the polypropylene resin composition constituting the seal layer B does not contain an anti-fogging agent, the anti-fogging agent may migrate from the substrate layer A to the seal layer B during the film formation process, and the obtained biaxially oriented polypropylene film may contain the anti-fogging agent in the seal layer B.
Furthermore, as long as the effects of the present invention are not impaired, the polypropylene-based resin composition constituting the sealing layer B may contain additives such as resins other than polypropylene copolymers, the above-mentioned antifogging agents, known heat stabilizers, antioxidants, UV absorbers, nucleating agents, adhesives, flame retardants, inorganic or organic fillers, etc. Examples of resins other than polypropylene copolymers include polyolefin resins other than the polypropylene copolymers used in the sealing layer B, and various elastomers. However, it is preferable that these be added in small amounts, and the amount of resins other than polypropylene copolymers in the polypropylene-based resin composition constituting the sealing layer B is preferably 30% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, particularly preferably 2% by mass or less, and may even be 0% by mass or more. Furthermore, the amount of additives other than resins in the polypropylene-based resin composition constituting the sealing layer B is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 2% by mass or less, particularly preferably 1% by mass or less, and may even be 0% by mass or more.

3. middle class C
3-1. Polypropylene Resin Composition The intermediate layer C made of a polypropylene resin composition preferably contains a polypropylene copolymer containing an α-olefin other than propylene. That is, the polypropylene resin composition constituting the intermediate layer C preferably contains a polypropylene copolymer containing an α-olefin other than propylene. The provision of the intermediate layer C can improve heat seal strength. In the description of the intermediate layer C, even when the term "polypropylene copolymer" is simply used, it refers to a polypropylene copolymer containing an α-olefin other than propylene. The intermediate layer C preferably contains 70% by mass or more of the polypropylene copolymer. By containing 70% by mass or more of the polypropylene copolymer, it is easy to improve the interlayer adhesion between the intermediate layer C and the seal layer B and between the base layer A and the intermediate layer C, thereby further increasing the heat seal strength of the biaxially oriented polypropylene film. The proportion of the polypropylene copolymer in the intermediate layer C is more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and may even be 100% by mass or more. In addition, two or more different polypropylene copolymers can be used as the polypropylene copolymer contained in the intermediate layer C, and it is preferable that the total content is within the above range.
The content of α-olefin components other than propylene in the polypropylene copolymer contained in the intermediate layer C, i.e., the total amount of ethylene and α-olefins having 4 or more carbon atoms, is preferably 4.0 mol% or more. In this case, the propylene content in the polypropylene copolymer is 96 mol% or less, and the sum of the propylene content and the content of α-olefin components other than propylene is 100 mol%. The amount is more preferably 4.0 mol% or more, even more preferably 5.0 mol% or more, particularly preferably 6.0 mol% or more, more preferably 12 mol% or less, even more preferably 11 mol% or less, and particularly preferably 10 mol% or less.
Examples of the α-olefin component having 4 or more carbon atoms include 1-butene, 1-pentene, 3-methyl-1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 5-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene.
The α-olefin component other than propylene is preferably an α-olefin component other than propylene having 2 or more and 20 or less carbon atoms, more preferably an α-olefin component other than propylene having 2 or more and 10 or less carbon atoms, even more preferably an α-olefin component other than propylene having 2 or more and 6 or less carbon atoms, and particularly preferably an α-olefin component other than propylene having 2 or more and 4 or less carbon atoms.

The following describes various suitable physical properties of the polypropylene resin composition. When two or more different polypropylene copolymers are used, the physical property values are the mass average values of the physical properties of each polypropylene copolymer. The same applies to the sealing layer B described above and the functional layer D described below.

The melting point _Tmc of the polypropylene resin composition constituting the intermediate layer C is preferably 150°C or lower, more preferably 145°C or lower, and even more preferably 140°C or lower. By setting _Tmc within the above range, the heat seal strength can be increased. The lower limit of the melting point _Tmc of the polypropylene resin composition constituting the intermediate layer C is not particularly limited, and is, for example, 120°C or higher. Furthermore, from the viewpoint of increasing the heat seal strength, it is preferable that the melting point _Tmc of the polypropylene resin composition constituting the intermediate layer C is higher than the melting point Tmb of the polypropylene resin composition constituting the seal layer _B.

The melt flow rate (MFR) of the polypropylene resin composition constituting intermediate layer C, measured at a temperature of 230°C and a load of 2.16 kgf, is preferably 3.0 g/10 min or more and 6.0 g/10 min or less. The melt flow rate value is more preferably 3.5 g/10 min or more, even more preferably 4.0 g/10 min or more, particularly preferably 4.3 g/10 min or more, more preferably 5.5 g/10 min or less, even more preferably 5.0 g/10 min or less, and particularly preferably 4.8 g/10 min or less. Furthermore, from the perspective of increasing heat seal strength, the melt flow rate of the polypropylene resin composition constituting intermediate layer C is preferably lower than the melt flow rate of the polypropylene resin composition constituting seal layer B.

The polypropylene copolymer contained in the intermediate layer C is preferably at least one selected from the group consisting of propylene-butene copolymer, propylene-ethylene-butene copolymer, and propylene-ethylene copolymer, with propylene-ethylene-butene copolymer being more preferred.

3-2. Propylene-Ethylene-Butene Copolymer The content of the α-olefin component other than propylene in the propylene-ethylene-butene copolymer in the intermediate layer C is preferably 4 mol% or more. When the content of the α-olefin component other than propylene is 4 mol% or more, the interlayer adhesion between the intermediate layer C and the seal layer B is likely to be improved, and as a result, the heat seal strength and hermetic sealability are likely to be improved. The content of the α-olefin component other than propylene is more preferably 5 mol% or more, and even more preferably 6 mol% or more. There is no particular upper limit for the content of the α-olefin component other than propylene, and it is, for example, 25 mol% or less. When there are two or more types of α-olefin components other than propylene, the total amount is taken as the content of the α-olefin components other than propylene.
The ethylene content is preferably 1 mol% or more, more preferably 2 mol% or more. Although there is no particular upper limit for the ethylene content, if the ethylene content is too high, crystallization is too suppressed, resulting in lower crystallinity compared to propylene homopolymer, which may result in a decrease in the stiffness of the film. Therefore, for example, the upper limit is 10 mol% or less.
The butene content is preferably 1 mol% or more, more preferably 2 mol% or more. Although there is no particular upper limit for the butene content, if the butene content is too high, crystallization is too suppressed, resulting in lower crystallinity compared to propylene homopolymer, which may result in a decrease in the stiffness of the film. Therefore, for example, the upper limit is 16 mol% or less.
As the propylene-ethylene-butene copolymer having a total content of α-olefin components other than propylene of 4 mol % or more, a commercially available product may be used, for example, FSX66E8 manufactured by Sumitomo Chemical Co., Ltd.

3-3. Propylene-Butene Copolymer The butene content in the propylene-butene copolymer is preferably 4 mol% or more. A butene content of 4 mol% or more tends to improve the interlayer adhesion between the intermediate layer C and the seal layer B, and as a result, the heat seal strength and hermetic sealability are likely to be improved. The butene content is preferably 5 mol% or more, and even more preferably 6 mol% or more. There is no particular upper limit for the butene content, but if the butene content is too high, crystallization is too suppressed, resulting in lower crystallinity compared to propylene homopolymer, which may result in a decrease in the stiffness of the film. Therefore, for example, the upper limit is 16 mol% or less, and preferably 12 mol% or less. As a propylene-butene copolymer having a butene content of 4 mol% or more, commercially available products may be used, such as SP7843 manufactured by Sumitomo Chemical Co., Ltd., SPX78J1 manufactured by Sumitomo Chemical Co., Ltd., and XR110H manufactured by Mitsui Chemicals, Inc.

3-4. Propylene-Ethylene Copolymer The ethylene content of the propylene-ethylene copolymer is preferably 4 mol% or more. An ethylene content of 4 mol% or more tends to improve the interlayer adhesion between the intermediate layer C and the seal layer B, and as a result, tends to improve heat seal strength and hermetic sealability. The ethylene content is more preferably 5 mol% or more, and even more preferably 6 mol% or more. There is no particular upper limit for the ethylene content, but if the ethylene content is too high, crystallization is too suppressed, resulting in lower crystallinity compared to propylene homopolymer, which may result in a decrease in the stiffness of the film. Therefore, for example, the upper limit is 12 mol% or less. As a propylene-ethylene copolymer having an ethylene content of 4 mol% or more, commercially available products may be used, such as PC540R manufactured by SunAllomer Co., Ltd. and VM3588FL manufactured by Mitsui Chemicals, Inc.

3-5. Anti-Fog Agent The polypropylene resin composition constituting the intermediate layer C may or may not contain an anti-fogging agent. As the anti-fogging agent, the anti-fogging agents described in the description of the base layer A can be used. Even if the polypropylene resin composition constituting the intermediate layer C does not contain an anti-fogging agent, the anti-fogging agent may migrate from the base layer A to the intermediate layer C during the film formation process, and the obtained biaxially oriented polypropylene film may contain the anti-fogging agent in the intermediate layer C.
Furthermore, as long as the effects of the present invention are not impaired, the polypropylene-based resin composition constituting the intermediate layer C may contain additives such as resins other than polypropylene copolymers, the above-mentioned antifogging agents, known heat stabilizers, antioxidants, UV absorbers, nucleating agents, adhesives, flame retardants, inorganic or organic fillers, etc. Examples of resins other than polypropylene copolymers include polyolefin resins other than the polypropylene copolymers used in the intermediate layer B, and various elastomers. However, the amount of these additives is preferably small, and the amount of resins other than polypropylene copolymers in the polypropylene-based resin composition constituting the intermediate layer C is preferably 30% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, particularly preferably 2% by mass or less, and may even be 0% by mass or more. Furthermore, the amount of additives other than resins in the polypropylene-based resin composition constituting the intermediate layer C is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 2% by mass or less, particularly preferably 1% by mass or less, and may even be 0% by mass or more.

4. Functional layer D
The functional layer D is preferably provided on the surface of the base layer A on which the seal layer B is not provided. The functional layer D is not particularly limited and may be a layer of the same composition as the intermediate layer C. In order to impart functions such as easy lubricity between films or between the film and a processing tool, or antistatic properties to the functional layer D, an antiblocking agent, a lubricant such as wax or metal soap, a plasticizer, a processing aid, an antistatic agent, etc. may be blended into the polypropylene resin composition constituting the functional layer D.

4-1. Polypropylene Resin Composition The functional layer D is preferably a layer made of a polypropylene resin composition, and more preferably contains a polypropylene copolymer containing an α-olefin other than propylene. That is, the polypropylene resin composition constituting the functional layer D more preferably contains a polypropylene copolymer containing an α-olefin other than propylene. In the description of the functional layer D, even when the term "polypropylene copolymer" is simply used, it refers to a polypropylene copolymer containing an α-olefin other than propylene. The functional layer D preferably contains 70% by mass or more of the polypropylene copolymer, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more. This proportion may be 100% by mass or less. The polypropylene copolymer contained in the functional layer D can also be two or more different polypropylene copolymers, with the total content preferably being within the above range. By containing the polypropylene copolymer in an amount of 70% by mass or more and 100% by mass or less, it is easy to improve the interlayer adhesion between the functional layer D and the layer adjacent to the functional layer D, thereby further increasing the heat seal strength of the biaxially oriented polypropylene film.
The content of α-olefin components other than propylene in the polypropylene copolymer contained in the functional layer D, i.e., the total amount of ethylene and α-olefins having 4 or more carbon atoms, is preferably 4.0 mol% or more. In this case, the propylene content in the polypropylene copolymer is 96 mol% or less, and the sum of the propylene content and the content of α-olefin components other than propylene is 100 mol%. The amount is more preferably 4.0 mol% or more, even more preferably 5.0 mol% or more, particularly preferably 6.0 mol% or more, more preferably 12 mol% or less, even more preferably 11 mol% or less, and particularly preferably 10 mol% or less.
Examples of the α-olefin component having 4 or more carbon atoms include 1-butene, 1-pentene, 3-methyl-1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 5-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, 1-octadecene, and 1-eicosene.
The α-olefin component other than propylene is preferably an α-olefin component other than propylene having 2 or more and 20 or less carbon atoms, more preferably an α-olefin component other than propylene having 2 or more and 10 or less carbon atoms, even more preferably an α-olefin component other than propylene having 2 or more and 6 or less carbon atoms, and particularly preferably an α-olefin component other than propylene having 2 or more and 4 or less carbon atoms.

The melting point _Tmd of the polypropylene resin composition constituting the functional layer D is preferably 150°C or lower, more preferably 145°C or lower, and even more preferably 140°C or lower. By setting _Tmd within the above range, the heat seal strength can be increased. The lower limit of the melting point _Tmd of the polypropylene resin composition constituting the functional layer D is not particularly limited, and is, for example, 120°C or higher.

The melt flow rate (MFR) of the polypropylene resin composition constituting functional layer D, measured at a temperature of 230°C and a load of 2.16 kgf, is preferably 3.0 g/10 min or more and 6.0 g/10 min or less. The melt flow rate is more preferably 3.5 g/10 min or more, even more preferably 4.0 g/10 min or more, and particularly preferably 4.3 g/10 min or more. It is more preferably 5.5 g/10 min or less, even more preferably 5.0 g/10 min or less, and particularly preferably 4.8 g/10 min or less.

The polypropylene copolymer contained in functional layer D preferably contains at least one selected from the group consisting of propylene-butene copolymer, propylene-ethylene-butene copolymer, and propylene-ethylene copolymer, and more preferably contains propylene-ethylene-butene copolymer.

4-2. Propylene-Ethylene-Butene Copolymer The content of α-olefin components other than propylene in the propylene-ethylene-butene copolymer is preferably 4 mol% or more. When the content of α-olefin components other than propylene is 4 mol% or more, the interlayer adhesion between the functional layer D and the layer adjacent to the functional layer D is likely to be improved, and as a result, the heat seal strength and hermetic sealability are likely to be improved. The content of α-olefin components other than propylene is more preferably 5 mol% or more, and even more preferably 6 mol% or more. There is no particular upper limit for the content of α-olefin components other than propylene, and it is, for example, 25 mol% or less. When there are two or more types of α-olefin components other than propylene, the total amount is taken as the content of the α-olefin components other than propylene.
The ethylene content is preferably 1 mol% or more, more preferably 2 mol% or more. Although there is no particular upper limit to the ethylene content, if the ethylene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may decrease, so the upper limit is, for example, 10 mol% or less.
The butene content is preferably 1 mol% or more, more preferably 2 mol% or more. Although there is no particular upper limit to the butene content, if the butene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may be reduced, so the upper limit is, for example, 16 mol% or less.
As the propylene-ethylene-butene copolymer having a total content of α-olefin components other than propylene of 4 mol % or more, a commercially available product may be used, for example, FSX66E8 manufactured by Sumitomo Chemical Co., Ltd.

4-3. Propylene-Butene Copolymer The butene content in the propylene-butene copolymer is preferably 4 mol% or more. A butene content of 4 mol% or more tends to improve the interlayer adhesion between the functional layer D and the layer adjacent to the functional layer D, and as a result, tends to improve heat seal strength and hermetic sealability. The butene content is more preferably 5 mol% or more, and even more preferably 6 mol% or more. 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. Therefore, for example, it is 16 mol% or less, preferably 12 mol% or less. As a propylene-butene copolymer having a butene content of 4 mol% or more, commercially available products may be used, such as SP7843 manufactured by Sumitomo Chemical Co., Ltd., SPX78J1 manufactured by Sumitomo Chemical Co., Ltd., and XR110H manufactured by Mitsui Chemicals, Inc.

4-4. Propylene-Ethylene Copolymer The ethylene content of the propylene-ethylene copolymer is preferably 4 mol% or more. When the ethylene content is 4 mol% or more, the interlayer adhesion between the functional layer D and the layer adjacent to the functional layer D is likely to be improved, and as a result, the heat seal strength and hermetic sealability are likely to be improved. The ethylene content is more preferably 5 mol% or more, and even more preferably 6 mol% or more. There is no particular upper limit for the ethylene content, but if the ethylene content is too high, the film surface may become sticky and the slipperiness and blocking resistance may be reduced, so for example, it is 12 mol% or less. As a propylene-ethylene copolymer having an ethylene content of 4 mol% or more, commercially available products may be used, such as PC540R manufactured by SunAllomer Co., Ltd. and VM3588FL manufactured by Mitsui Chemicals, Inc.

4-5. Anti-Fog Agent The polypropylene resin composition constituting the functional layer D may or may not contain an anti-fogging agent. As the anti-fogging agent, the anti-fogging agents described in the description of the base layer A can be used. Even if the polypropylene resin composition constituting the functional layer D does not contain an anti-fogging agent, the anti-fogging agent may migrate from the base layer A to the functional layer D during the film formation process, and the functional layer D may contain the anti-fogging agent in the obtained biaxially oriented polypropylene film.
Furthermore, as long as the effects of the present invention are not impaired, the polypropylene-based resin composition constituting the functional layer D may contain additives such as resins other than polypropylene copolymers, the above-mentioned anti-fogging agents, known heat stabilizers, antioxidants, ultraviolet absorbers, nucleating agents, adhesives, flame retardants, inorganic or organic fillers, etc.
Examples of resins other than polypropylene copolymers include polyolefin resins other than the polypropylene copolymers used in the functional layer D, various elastomers, and the like. However, it is preferable that these be added in small amounts, and the amount of resins other than polypropylene copolymers in the polypropylene-based resin composition constituting the functional layer D is preferably 30% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, particularly preferably 2% by mass or less, and can be 0% by mass or more. Furthermore, the amount of additives other than resins in the polypropylene-based resin composition constituting the functional layer D is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 2% by mass or less, particularly preferably 1% by mass or less, and can be 0% by mass or more.

5. Content of Anti-Fog Agent in Film The content of the 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. The content is more preferably 0.15% by mass or more, even more preferably 0.2% by mass or more, particularly preferably 0.25% by mass or more, most preferably 0.3% by mass or more, more preferably 5.0% by mass or less, even more preferably 3.0% by mass or less, particularly preferably 2.0% by mass or less, and most preferably 1.5% by mass or less. Even if the anti-fogging agent is added only to the polypropylene resin composition constituting the base layer A, the anti-fogging agent may migrate from the base layer A to other layers during film formation and storage after film formation, and the anti-fogging agent may migrate to the outermost seal layer B, causing the anti-fogging agent to be present on the surface of the seal layer B, thereby providing anti-fogging properties.

6. 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 B. An intermediate layer C may be present between the base layer A and the seal layer B. The layer structure of the biaxially oriented polypropylene film of the present invention has the base layer A and the seal layer B, and as long as the seal layer B is present on at least one outermost surface, the intermediate layer C may be present between the base layer A and the seal layer B, or another layer may be present between the base layer A and the intermediate layer C, or the intermediate layer C may be laminated directly on the base layer A. Furthermore, another layer may be present between the intermediate layer C and the seal layer B, or the seal layer B may be laminated directly on the intermediate layer C. Examples of the biaxially oriented polypropylene film of the present invention include a two-layer structure of base layer A/seal layer B, a three-layer structure of base layer A/intermediate layer C/seal layer B, a three-layer structure of seal layer B1/base layer A/seal layer B2, a four-layer structure of seal layer B1/base layer A/intermediate layer C/seal layer B2, a five-layer structure of seal layer B1/intermediate layer C1/base layer A/intermediate layer C2/seal layer B2, and a six-layer structure of seal layer B1/base layer A1/intermediate layer C1/base layer A2/intermediate layer C2/seal layer B2. In this case, the base layer A1 and the base layer A2 may be made of different polypropylene-based resin compositions or may be the same, the intermediate layers C1 and C2 may be made of different polypropylene-based resin compositions or may be the same, and the seal layer B1 and the seal layer B2 may be made of different polypropylene-based resin compositions or may be the same.

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 substrate layer A, or an intermediate layer C may be interposed between the substrate layer A and the functional layer D. The functional layer D may also be located between the substrate layer A and the intermediate layer C, or between the intermediate layer C and the sealing layer B. When the functional layer D is present, there are no particular limitations as long as the substrate layer A, intermediate layer C, and sealing layer B are present in this order, with sealing layer B on at least one outermost surface. Examples of such a structure include a four-layer structure of functional layer D/substrate layer A/intermediate layer C/sealing layer B, in which the substrate layer A/intermediate layer C/sealing layer B structure further has a functional layer D, and a five-layer structure of functional layer D/intermediate layer C/substrate layer A/intermediate layer C/sealing layer B, in which the intermediate layer C/substrate layer A/intermediate layer C/sealing layer B structure further has a functional layer D.

The overall thickness of the biaxially oriented polypropylene film of the present invention will vary depending on the application and method of use, but from the viewpoints of film strength, airtightness, and water vapor barrier properties, it is preferably 5 μm or more and 100 μm or less. The thickness is more preferably 10 μm or more, even more preferably 18 μm or more, more preferably 80 μm or less, and even more preferably 50 μm or less. When the biaxially oriented polypropylene film of the present invention is 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.

The thickness of the base layer A varies depending on the application and method of use, but is preferably 5 μm or more and 90 μm or less. A thickness of 5 μm or more can improve film strength, sealing properties, and water vapor barrier properties. Furthermore, a thickness of 90 μm or less can reduce the environmental impact by reducing the volume. The thickness of the base layer A is more preferably 10 μm or more, even more preferably 15 μm or more, more preferably 50 μm or less, and even more preferably 30 μm or less.

The thickness of intermediate layer C varies depending on its application and method of use, but is preferably 0.5 μm or more and 4 μm or less. A thickness of 0.5 μm or more can improve adhesion between base layer A and seal layer B, thereby increasing seal strength. Furthermore, a thickness of 4 μm or less can reduce the environmental impact by reducing volume. The thickness of intermediate layer C is more preferably 1 μm or more and 3 μm or less.

The thickness of sealing layer B varies depending on its application and method of use, but is preferably 0.3 μm or more and 2 μm or less. 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. The thickness of sealing layer B is more preferably 0.5 μm or more and 1.5 μm or less.

The thickness of functional layer D varies depending on its application and method of use, but is preferably 0.3 μm or more and 2 μm or less. 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. The thickness of functional layer D is more preferably 0.5 μm or more and 1.5 μm or less.

7. Method for Producing Biaxially Oriented Polypropylene Film The method for producing the biaxially oriented polypropylene film of the present invention is not particularly limited, but examples thereof include a method in which melt lamination is performed by a T-die method, inflation method, or the like using an extruder suitable for the number of layers, followed by cooling by a cooling roll method, water cooling method, or air cooling method to obtain an unstretched laminated film, and then stretching the obtained film by a sequential biaxial stretching method, simultaneous biaxial stretching method, tube stretching method, or the like.

Below is an example of a method for producing a biaxially oriented polypropylene film consisting of functional layer D/substrate layer A/intermediate layer C/sealing layer B using the sequential biaxial stretching method. Note that the following example can be modified as appropriate depending on the layer configuration, etc. The polypropylene resin compositions that make up each of the substrate layer A, intermediate layer C, sealing layer B, and functional layer D are as described above. Furthermore, it is preferable to adjust the amount of anti-fogging agent added to the substrate layer A, intermediate layer C, and sealing layer B, taking into account that the anti-fogging agent will evaporate into the atmosphere when exposed to high temperatures during the film manufacturing process.

The polypropylene resin composition constituting each of the functional layer D, substrate layer A, intermediate layer C, and seal layer B is melt-extruded using one or more extruders, and the extruded multilayer sheet is cooled on a cooling roll to form an unstretched sheet. The resulting unstretched sheet is then stretched in the longitudinal direction (MD). The stretched sheet is then preheated, stretched in the transverse direction (TD), and finally heat-set to obtain the biaxially oriented polypropylene film of the present invention. If necessary, at least one side of the biaxially oriented polypropylene film can be surface-treated, and then wound up on a winder to obtain a film roll.

7-1. Extrusion Process The polypropylene resin compositions constituting the functional layer D, base layer A, intermediate layer C, and seal layer B are each melted at, for example, 200°C or higher and 260°C or lower, and the molten polypropylene resin compositions are delivered from different flow paths using four extruders. The delivered polypropylene resin compositions are laminated in multiple layers using a multi-layer feed block, static mixer, multi-layer multi-manifold die, or the like, and a multi-layer sheet laminated in the order of functional layer D/base layer A/intermediate layer C/seal layer B is extruded from a T-die. It is also possible to obtain a multi-layer sheet using only one extruder by introducing a multi-layering device into the melt line from the extruder to the T-die. Furthermore, from the viewpoints of stabilizing back pressure and suppressing thickness fluctuations, it is preferable to install a gear pump in the polymer flow path.

The thickness of the unstretched multilayer sheet is preferably 3500 μm or less from the perspective of improving cooling efficiency, and more preferably 3000 μm or less, but this can be adjusted appropriately depending on the film thickness after sequential biaxial stretching. The thickness of the unstretched multilayer sheet can be adjusted by the extrusion speed of the polypropylene resin composition and the lip width of the T-die, etc. There is no particular lower limit to the thickness of the unstretched multilayer sheet, but it can be set to 500 μm, for example.

7-2. Cooling step The unstretched multilayer sheet co-extruded from the T-die into a sheet form is brought into contact with a metal cooling roll and cooled and solidified. At this time, it is preferable that the seal layer B side is grounded on the cooling roll. In order to promote solidification, it is also preferable to further cool the unstretched multilayer sheet cooled by the cooling roll by, for example, immersing it in a water bath.
The temperature of the chill roll is preferably 10°C or higher and lower than the crystallization temperature of the polypropylene resin composition. When increasing the transparency of the film, it is preferable to cool and solidify using a chill roll at 10°C or higher and 50°C or lower. Setting the chill roll temperature to 50°C or lower tends to increase the transparency of the unstretched multilayer sheet, and it is more preferably 40°C or lower, and even more preferably 30°C or lower. In order to increase the degree of crystalline orientation after sequential biaxial stretching, a cooling temperature of 40°C or higher may be preferable. However, when using a propylene homopolymer with a mesopentad fraction of 97.0% or higher as described above, the chill roll temperature is preferably 40°C or lower, more preferably 30°C or lower, from the viewpoint of facilitating the subsequent stretching step and reducing thickness unevenness. When a water bath is used, the temperature of the water bath is also preferably 10°C or higher and 50°C or lower, more preferably 40°C or lower, and even more preferably 30°C or lower, for the same reasons as above.

7-3. Longitudinal Stretching Process The longitudinal stretching temperature is preferably Tm-30°C or higher and Tm-7°C or lower. If the temperature is Tm-30°C or higher, the subsequent widthwise stretching becomes easier and film thickness unevenness tends to be reduced. If the temperature is Tm-7°C or lower, the thermal shrinkage rate is easily reduced, and there is little risk of the film becoming difficult to stretch when applied to the stretching rolls or of the film becoming rough on the surface, resulting in a decrease in quality. The temperature is more preferably Tm-27°C or higher, even more preferably Tm-25°C or higher, more preferably Tm-12°C or lower, and even more preferably Tm-10°C or lower. Here, Tm refers to the Tm of the resin forming the thickest layer.
The stretching ratio in the longitudinal direction is preferably 3.5 times or more and 8.0 times or less. When the stretching ratio is 3.5 times or more, it is easy to increase the strength and reduce thickness unevenness. Furthermore, when the stretching ratio is 8.0 times or less, it is easy to perform width direction stretching in the width direction stretching step, and it is easy to improve productivity. The stretching ratio is more preferably 3.8 times or more, even more preferably 4.2 times or more, and more preferably 7.0 times or less, and even more preferably 6.0 times or less.
The longitudinal stretching may be performed in two or more stages using three or more pairs of stretching rolls, but it is preferable to perform stretching in one stage using two pairs of stretching rolls. When performing stretching in multiple stages, it is preferable that the highest stretching temperature is within the above range.

7-4. Preheating Step It is preferable to heat the uniaxially stretched film after longitudinal stretching in a preheating step to sufficiently soften the polypropylene resin composition before the widthwise stretching step. The heating temperature in the preheating step is preferably Tm or higher and Tm + 25°C or lower. By setting the heating temperature in the preheating step to the melting point or higher, softening proceeds, facilitating widthwise stretching. Furthermore, by setting the heating temperature in the preheating step to Tm + 25°C or lower, orientation proceeds during widthwise stretching, making it easier to develop rigidity. The heating temperature is more preferably Tm + 2°C or higher, even more preferably Tm + 3°C or higher, more preferably Tm + 20°C or lower, and even more preferably Tm + 15°C or lower. Note that if the preheating step consists of multiple zones, the temperature of the hottest zone among them is taken as the preheating temperature. Note that Tm here refers to the Tm of the resin forming the thickest layer.

7-5. Width Direction Stretching Process The width direction stretching temperature is preferably Tm-10°C or higher and the heating temperature in the preheating process or lower. If the temperature is Tm-10°C or higher, the rigidity of the resulting film is easily improved, and if the temperature is lower than the heating temperature in the preheating process, stretching unevenness is less likely to occur. The temperature is more preferably Tm-9°C or higher, even more preferably Tm-7°C or higher, particularly preferably Tm-5°C or higher, and more preferably Tm+10°C or lower, even more preferably Tm+7°C or lower, and particularly preferably Tm+5°C or lower. Note that Tm here refers to the Tm of the resin forming the thickest layer.
In the width direction stretching step, it is preferable to add a later stretching step in which the film is stretched at a lower temperature following the width direction stretching step in the above temperature range (hereinafter sometimes referred to as the "early stretching step"). By providing the later stretching step, the rigidity of the film can be easily increased.
The stretching ratio in the width direction is preferably 10 times or more and 20 times or less. When the stretching ratio is 10 times or more, rigidity is easily increased and thickness unevenness is easily reduced. Furthermore, when the stretching ratio is 20 times or less, the heat shrinkage rate is easily reduced and the film is less likely to break during stretching. The stretching ratio is more preferably 11 times or more, even more preferably 12 times or more, particularly preferably 12.5 times or more, and more preferably 17 times or less, and even more preferably 15 times or less.
When a later stretching step is added, it is preferable that the total stretching ratio is set within the above range.

7-6. Heat Treatment Step It is preferable to perform heat treatment after the width direction stretching step. Specific means for heat treatment include providing a zone with a higher temperature than the stretching zone after the width direction stretching is completed, or increasing the zone temperature in the latter half of stretching and passing the film through a zone at the same temperature after the stretching is completed to increase the film temperature. Examples of heating methods include blowing hot air or heating with an infrared heater, but there are no particular limitations as long as the method increases the film temperature from the temperature at the end of the width direction stretching step.

The heat treatment step is preferably carried out immediately after the widthwise stretching step is completed, i.e., immediately after the widthwise stretching has reached the final stretch ratio. The temperature in the heat treatment step is preferably higher than that at the end of the widthwise stretching step, and specifically, is preferably at least 1°C higher than the widthwise stretching temperature. It is also preferable to carry out the heat treatment step in two stages, with the early heat treatment step being performed at a temperature higher than that at the end of the widthwise stretching step, and the later heat treatment step being performed at a temperature lower than that in the early heat treatment step. The early and later heat treatment steps are explained below.

7-6-1. Preliminary Heat Treatment Step The heating temperature in the preliminary heat treatment step is preferably Tm or higher and Tm + 20°C or lower. Heating at a temperature above Tm promotes relaxation, reducing tension in the molecular chains and allowing crystallization to proceed more reliably. On the other hand, heating at a temperature below Tm + 20°C suppresses melting while suppressing relaxation of the oriented molecular chains, thereby more reliably preventing a decrease in rigidity. The heating temperature is more preferably Tm + 3°C or higher, even more preferably Tm + 4°C or higher, particularly preferably Tm + 5°C or higher, more preferably Tm + 18°C or lower, even more preferably Tm + 14°C or lower, and particularly preferably Tm + 10°C or lower. Heating in the heat treatment step after the stretching step relaxes the molecular chain orientation formed during stretching, making crystallization more likely to occur in the final heat treatment step. The temperature can be gradually increased from the temperature at the end of width direction stretching to the temperature during heating, or it can be increased in stages or in a single step. It is preferable to raise the temperature stepwise or in one step, since this makes it easier to control the orientation of molecular chains in the film. Note that Tm here refers to the Tm of the resin that forms the thickest layer.
In the heat treatment step, the film may or may not be relaxed in the width direction. Specifically, the relaxation rate is preferably 0% or more and 3% or less. If the relaxation rate is within this range, the rigidity is less likely to decrease and the film thickness fluctuation is likely to be small. The relaxation rate is more preferably 0% or more and 1% or less, and even more preferably 0%, i.e., no relaxation. If it is desired to further increase the rigidity, relaxation is not necessary. Furthermore, the film may be slightly expanded to suppress sagging, etc., as long as it does not impair the effects of the present invention.

7-6-2. Later Heat Treatment Step The heating temperature in the later heat treatment step is preferably Tm-70°C or higher and Tm or lower. If the heating temperature is Tm-70°C or higher, lamellar thickening proceeds and the melting point of the film is likely to increase. In other words, heat resistance at high temperatures can be obtained. On the other hand, if the heating temperature is Tm or lower, crystallization proceeds and the heat shrinkage rate is likely to decrease. The heating temperature is more preferably Tm-50°C or higher, even more preferably Tm-40°C or higher, particularly preferably Tm-30°C or higher, more preferably Tm-1°C or lower, even more preferably Tm-2°C or lower, and particularly preferably Tm-3°C or lower. Note that Tm here refers to the Tm of the resin forming the thickest layer.
In the latter heat treatment step, the film may be relaxed in the width direction in order to adjust the thermal shrinkage rate. When the film is relaxed, the relaxation rate is preferably 1% or more and 8% or less. When the relaxation rate is within this range, the rigidity is less likely to decrease and the film thickness fluctuation tends to be small. The relaxation rate is more preferably 2% or more, even more preferably 3% or more, and more preferably 6% or less, and even more preferably 5% or less. However, if it is desired to further increase the rigidity, relaxation is not necessary.

During the widthwise stretching process, molecular chains are oriented by stretching, but remain strongly entangled, resulting in an excessively constrained state of the molecular chains. If a heat treatment process is performed in this state, the excessively constrained molecular chains due to entanglement make it difficult to increase the crystallinity, and the lamellae in the crystalline region do not increase in thickness. This leads to the formation of crystalline regions that melt at lower temperatures, resulting in insufficient heat resistance at high temperatures. Therefore, in conventional film-forming processes, in order to eliminate the entanglement of molecular chains after widthwise stretching, the film is relaxed by several percent to several tens of percent during the heat treatment process to promote crystallization. However, relaxation reduces the molecular chain orientation generated during the widthwise stretching process, resulting in a decrease in the rigidity of the film. Therefore, it is difficult to achieve both heat resistance and rigidity in conventional film-forming processes. Furthermore, heat treatment at high temperatures can cause excessive melting, resulting in the whitening of the film.
To solve this problem, it is preferable to perform heat treatment immediately after the end of the width direction stretching step at a temperature higher than that used for width direction stretching, for example, at a relaxation rate of 1% to 5%, preferably 3% or less, to relieve the constraint of the molecular chains due to excessive entanglement while maintaining the molecular chain orientation. By performing this heat treatment step, the presence of constrained molecular chains due to entanglement is reduced, which increases the crystallinity and makes it easier to increase the thickness of the lamellae in the crystalline portion, thereby enabling the film to exhibit sufficient heat resistance even at high temperatures.

Furthermore, increasing the amount of low-molecular-weight polypropylene components in the polypropylene resin composition that makes up the biaxially oriented polypropylene film can reduce entanglement of molecular chains, thereby weakening the heat shrinkage stress in areas other than the lamellae of the crystalline portion and further reducing the heat shrinkage rate, which is preferable.

7-7. Cooling Step It is preferable to cool the film immediately after the heat treatment step. The cooling temperature is preferably 10°C or higher and 140°C or lower. The cooling temperature is more preferably 15°C or higher, even more preferably 20°C or higher, more preferably 135°C or lower, even more preferably 130°C or lower, particularly preferably 80°C or lower, and most preferably 50°C or lower. By providing a cooling step, the state of molecular orientation within the film can be fixed.

7-8. Surface Treatment Step In order to improve printability and lamination properties, the biaxially oriented polypropylene film of the present invention is preferably surface-treated on at least one of the seal layer B and the surface layer opposite to the seal layer B, and it is more preferable to surface-treat the seal layer B from the viewpoint of increasing the surface tension of the seal layer B. Examples of the surface treatment method include corona discharge treatment, plasma treatment, flame treatment, and acid treatment, and there are no particular restrictions on the method. However, from the viewpoints that continuous treatment is possible and that it can be easily carried out before the winding step in the film production process, it is preferable to carry out corona discharge treatment, plasma treatment, or flame treatment, and from the viewpoint of improving anti-fogging properties, it is more preferable to carry out corona discharge treatment.

8. Various Properties of the Biaxially Oriented Polypropylene Film of the Present Invention The biaxially oriented polypropylene film of the present invention preferably has the following properties. Here, the "longitudinal direction (MD direction)" of the biaxially oriented polypropylene film of the present invention refers to the direction corresponding to the flow direction in the film production process, and the "transverse direction (TD direction)" refers to the direction perpendicular to the flow direction in the film production process, and the same applies hereinafter.
For polypropylene films whose flow direction in the film manufacturing process is unknown, wide-angle X-rays are incident perpendicularly to the film surface, and the scattering peaks resulting from the (110) plane of the α-type crystals are scanned in the circumferential direction. The direction with the greatest diffraction intensity in the obtained diffraction intensity distribution is designated the "longitudinal direction," and the direction perpendicular to that is designated the "width direction."

8-1. Width Direction Length In thermomechanical analysis, when the temperature of the biaxially oriented polypropylene film of the present invention is increased from 30°C to 130°C at a rate of 10°C/min, where _X0 is the width direction length at 30°C, _X1 is the maximum width direction length during the temperature increase, and _X2 is the minimum width direction length during the temperature increase, the percentage of ( _X1 - _X0 )/ _X0 is preferably 0.50% or less. When this percentage is 0.50% or less, film deformation can be suppressed even after heating at high temperatures, such as during heat processing with a roll, printing, or heat sealing. This reduces the deterioration of film flatness and improves film processability. Furthermore, high-temperature printing ink is transferred during printing, which reduces the occurrence of printing pitch deviation. The percentage is more preferably 0.45% or less, even more preferably 0.40% or less, particularly preferably 0.35% or less, and most preferably 0.30% or less. The percentage of (X ₁ -X ₀ )/X ₀ is preferably small, and although there is no particular lower limit, it is, for example, 0.01% or more, preferably 0.02% or more, in view of technical difficulties.
The percentage of (X ₂ -X ₀ )/X ₀ of the biaxially oriented polypropylene film of the present invention is preferably -0.50% or more. When this percentage is -0.50% or more, deformation of the film can be suppressed even after heating at high temperatures, such as during heat processing with a roll, printing, or heat sealing. This prevents deterioration of the film's flatness and improves the film's processability. The percentage is more preferably -0.45% or more, even more preferably -0.40% or more, particularly preferably -0.35% or more, and most preferably -0.30% or more. A larger percentage of (X ₂ -X ₀ )/X ₀ is preferable. While there is no particular upper limit, it is, for example, 0.01% or less, preferably 0.00% or less, due to technical difficulties.

In thermomechanical analysis, when the biaxially oriented polypropylene film of the present invention is heated from 30°C to 160°C at a heating rate of 10°C/min, the temperature at which the width direction length of the film is 0.9950 x _X0 or less, i.e., the temperature at which the film shrinks by 0.5%, is preferably 129°C or higher, more preferably 130°C or higher, even more preferably 131°C or higher, particularly preferably 132°C or higher, and most preferably 133°C or higher. The "temperature at which the width direction length is 0.9950 x X0 _or less" refers to the lowest temperature at which the width direction length is 0.9950 x X0 _or less when the film is heated from 30°C to 160°C at a heating rate of 10°C/min. When the temperature at which the width direction length is 0.9950 x _X0 or less is 129°C or higher, deformation of the film can be suppressed even after heating at high temperatures, such as during heat processing with a roll, printing, or heat sealing. This makes it less likely that the flatness of the film will deteriorate and improves the processability of the film. The temperature at which 0.9950 × X ₀ or less is preferably higher, and the upper limit is not particularly limited, but is, for example, 160° C. or less, preferably 156° C. or less. When the temperature is 160° C. or less, practical production is easy and transparency is easily maintained.

8-2. Loss Modulus The loss modulus is determined by dynamic viscoelasticity measurement. Specifically, the temperature is raised from -60°C to 160°C at a rate of 5°C/min under a nitrogen atmosphere with a measurement load of 10 g and a frequency of 10 Hz, and the loss modulus is measured at each temperature during the temperature rise. The inventors have found that while increasing the rigidity and heat resistance alone may not be enough to maintain the flatness of the film after heating, controlling the loss modulus in a predetermined temperature range to fall within a predetermined range makes it possible to maintain the flatness of the film even after heating.

Below, we will explain five parameters related to the loss modulus: E"(A), the maximum value of the loss modulus between -25°C and 25°C; E"(B), the minimum value of the loss modulus between 25°C and 75°C; E"(C), the maximum value of the loss modulus between 100°C and 160°C; E"(C)/E"(A); and E"(B)/E"(C). Note that in this specification, even when simply referring to "loss modulus," it always refers to the loss modulus in the width direction.

8-2-1. Maximum loss modulus E" (A) from -25°C to 25°C
Takayanagi Motoo, "Temperature Dispersion of Crystalline Polymers," Polymer, Society of Polymer Science, 1961, Vol. 10, No. 3, pp. 289-295, describes that relaxation (main dispersion) due to micro-Brownian motion of the main chain occurs in polypropylene films at temperatures between -25°C and 25°C, and also describes that the loss modulus between -25°C and 25°C increases as the degree of orientation by stretching increases. The present inventors have found that by using highly stereoregular polypropylene and employing the above-mentioned width direction stretching process, it is possible to increase the orientation in the film, i.e., to increase the value of the loss modulus between -25°C and 25°C.

E"(A) of the biaxially oriented polypropylene film of the present invention is preferably 0.40 GPa or more. When E"(A) is 0.40 GPa or more, rigidity tends to be high. E"(A) is more preferably 0.42 GPa or more, even more preferably 0.44 GPa or more, particularly preferably 0.46 GPa or more, and most preferably 0.48 GPa or more. There is no particular upper limit for E"(A), but a realistic value is, for example, 0.70 GPa or less, and preferably 0.60 GPa or less.

8-2-2. Minimum loss modulus E" (B) from 25°C to 75°C
Takayanagi Motoo, "Temperature Dispersion of Crystalline Polymers," Polymer, Society of Polymer Science, 1961, Vol. 10, No. 3, pp. 289-295, describes that increasing the crystallinity in a film, which contributes greatly to heat resistance, and reducing the amount of crystals that melt in a relatively low temperature range above the glass transition temperature of the polypropylene resin (hereinafter referred to as the low temperature range) makes it difficult for relaxation to occur due to melting, which in turn suppresses relaxation of the amorphous portion, i.e., even when treated at high temperatures, the mobility of the amorphous portion is reduced, resulting in good heat resistance. The present inventors have found that by using a highly stereoregular polypropylene and employing the width direction stretching process described above, it is possible to reduce the amount of crystals that melt in the low temperature range, thereby reducing the change in loss modulus from the glass transition temperature to 75°C.

E"(B) of the biaxially oriented polypropylene film of the present invention is preferably 0.16 GPa or more. If E"(B) is 0.16 GPa or more, the heat shrinkage rate is likely to decrease. Furthermore, if E"(B) is 0.16 GPa or more, fewer crystals will melt in the above-mentioned low temperature range, thereby improving flatness. E"(B) is more preferably 0.17 GPa or more, even more preferably 0.18 GPa or more, particularly preferably 0.19 GPa or more, and most preferably 0.20 GPa or more. There is no particular upper limit for E"(B), but a realistic value is, for example, 0.60 GPa or less, and preferably 0.50 GPa or less.

8-2-3. Maximum loss modulus E" (C) from 100°C to 160°C
Takayanagi Motoo, "Temperature Dispersion of Crystalline Polymers," Polymer, Society of Polymer Science, 1961, Vol. 10, No. 3, pp. 289-295, describes that when a stretched polypropylene film is heated and the loss modulus is measured at each temperature, a peak due to crystalline dispersion appears above 100°C. This crystalline dispersion peak is believed to be due to an increase in frictional viscosity between the planes of the crystalline structure, and increases when the film is stretched at an optimal temperature during film formation. Increased frictional viscosity indicates strong stress transmission within the crystalline phase and is thought to correlate with increased rigidity. Based on the above, the present inventors have discovered that the maximum value of the loss modulus above 100°C is increased by using highly stereoregular polypropylene and employing the above-described width direction stretching process.

The E"(C) of the biaxially oriented polypropylene film of the present invention is preferably 0.28 GPa or more and 0.80 GPa or less. When E"(C) is 0.28 GPa or more, the high rigidity makes it easier to maintain the shape of the bag when made into a packaging bag, and the film is less likely to deform during processing such as printing. Furthermore, when E"(C) is 0.80 GPa or less, practical manufacturing is easier and the film is less likely to tear in the width direction. E"(C) is more preferably 0.29 GPa or more, even more preferably 0.30 GPa or more, particularly preferably 0.31 GPa or more, and most preferably 0.32 GPa or more. It is more preferably 0.75 GPa or less, even more preferably 0.70 GPa or less, particularly preferably 0.65 GPa or less, and most preferably 0.60 GPa or less.

The E"(C)/E"(A) ratio of the biaxially oriented polypropylene film of the present invention is preferably 0.55 or greater, and more preferably 0.55 or greater and 1.30 or less. When E"(C)/E"(A) is 0.55 or greater, the film has high rigidity, making it easier to maintain the shape of the packaging bag when made into it, and the film is less likely to deform during processing such as printing. When E"(C)/E"(A) is 1.30 or less, practical manufacturing is easier and the film is less likely to tear in the width direction. E"(C)/E"(A) is more preferably 0.60 or greater or 0.62 or greater, particularly preferably 0.64 or greater, and most preferably 0.66 or greater. It is even more preferably 1.20 or less or 1.10 or less, particularly preferably 1.00 or less, and most preferably 0.90 or less.

The E"(B)/E"(C) ratio of the biaxially oriented polypropylene film of the present invention is preferably 0.55 or greater, and more preferably 0.55 or greater and 1.30 or less. When E"(B)/E"(C) is 0.55 or greater, fewer crystals melt in the low-temperature range, making relaxation associated with melting less likely to occur. This in turn suppresses relaxation of the amorphous portion. As a result, the mobility of the amorphous portion is reduced even when processed at high temperatures, improving flatness and heat resistance. When E"(B)/E"(C) is 1.30 or less, rigidity is less likely to decrease and film thickness fluctuations tend to be small. E"(B)/E"(C) is more preferably 0.60 or greater or 0.61 or greater, particularly preferably 0.62 or greater, and most preferably 0.63 or greater. It is even more preferably 1.25 or less or 1.20 or less, particularly preferably 1.15 or less, and most preferably 1.10 or less.

The storage modulus is determined by dynamic viscoelasticity measurement. Specifically, the temperature is increased from -60°C to 160°C at a rate of 5°C/min under a nitrogen atmosphere with a measurement load of 10 g and a frequency of 10 Hz, and the storage modulus is measured at each temperature during the temperature increase.

The storage modulus in the longitudinal direction of the biaxially oriented polypropylene film of the present invention at 23° C. is preferably 2.0 GPa or more and 5.0 GPa or less. The storage modulus is more preferably 2.3 GPa or more, even more preferably 2.5 GPa or more, particularly preferably 2.9 GPa or more, most preferably 3.0 GPa or more, and more preferably 4.5 GPa or less, even more preferably 4.3 GPa or less, particularly preferably 4.2 GPa or less, and most preferably 4.0 GPa or less.
The storage modulus in the width direction of the biaxially oriented polypropylene film of the present invention at 23° C. is preferably 7.0 GPa or more and 15.0 GPa or less. The storage modulus is more preferably 7.3 GPa or more, even more preferably 7.6 GPa or more, and particularly preferably 8.0 GPa or more, and more preferably 14.0 GPa or less, even more preferably 13.5 GPa or less, and particularly preferably 13.0 GPa or less.
When the storage modulus at 23°C in the longitudinal and transverse directions is within the above range, the strength of the biaxially oriented polypropylene film is significantly increased, and even if the film is thin, it can maintain its stiffness and strength, which greatly contributes to reducing the volume of the film.

The storage modulus in the longitudinal direction of the biaxially oriented polypropylene film of the present invention at 120°C is preferably 0.5 GPa or more and 2.5 GPa or less. The storage modulus is more preferably 0.6 GPa or more, even more preferably 0.7 GPa or more, and particularly preferably 0.8 GPa or more, and is more preferably 2.3 GPa or less, even more preferably 2.1 GPa or less, and particularly preferably 2.0 GPa or less. When the storage modulus in the longitudinal direction at 120°C is within the above range, strength at high temperatures tends to be high, and printing pitch deviation is less likely to occur when hot printing ink is transferred during printing.

The storage modulus in the width direction of the biaxially oriented polypropylene film of the present invention at 120°C is preferably 1.5 GPa or more and 8.0 GPa or less. The storage modulus is more preferably 1.8 GPa or more, even more preferably 2.0 GPa or more, and particularly preferably 2.1 GPa or more, and is more preferably 7.8 GPa or less, even more preferably 7.6 GPa or less, and particularly preferably 7.4 GPa or less. When the storage modulus in the width direction at 120°C is within the above range, strength at high temperatures tends to be increased, and printing pitch deviation is less likely to occur when hot printing ink is transferred during printing. Furthermore, the flatness of the film is less likely to deteriorate, and the processability of the film can be improved.

The storage modulus in the longitudinal direction of the biaxially oriented polypropylene film of the present invention at 140° C. is preferably 0.3 GPa or more and 1.5 GPa or less, more preferably 0.35 GPa or more, even more preferably 0.4 GPa or more, particularly preferably 0.45 GPa or more, and more preferably 1.45 GPa or less, even more preferably 1.4 GPa or less, particularly preferably 1.35 GPa or less.
The storage modulus in the width direction of the biaxially oriented polypropylene film of the present invention at 140° C. is preferably 0.9 GPa or more and 5.0 GPa or less, more preferably 1.0 GPa or more, even more preferably 1.2 GPa or more, and particularly preferably 1.3 GPa or more, and more preferably 4.5 GPa or less, even more preferably 4.0 GPa or less, and particularly preferably 3.5 GPa or less.
When the storage modulus at 140°C in the longitudinal and transverse directions is within the above range, the strength at high temperatures tends to be high, and printing pitch deviation is less likely to occur when transferring high-temperature printing ink during printing. In addition, the flatness of the film is less likely to deteriorate and the processability of the film can be improved.

The storage modulus in the longitudinal direction of the biaxially oriented polypropylene film of the present invention at 150° C. is preferably 0.1 GPa or more and 1.0 GPa or less, more preferably 0.15 GPa or more, even more preferably 0.2 GPa or more, particularly preferably 0.25 GPa or more, and more preferably 0.9 GPa or less, even more preferably 0.8 GPa or less, particularly preferably 0.7 GPa or less.
The storage modulus in the width direction of the biaxially oriented polypropylene film of the present invention at 150° C. is preferably 0.85 GPa or more and 2.5 GPa or less, more preferably 0.9 GPa or more, even more preferably 0.95 GPa or more, and particularly preferably 1.0 GPa or more, and more preferably 2.4 GPa or less, even more preferably 2.3 GPa or less, and particularly preferably 2.2 GPa or less.
When the storage modulus at 150°C in the longitudinal and transverse directions is within the above range, the strength at high temperatures tends to be high, and printing pitch deviation is less likely to occur when transferring high-temperature printing ink during printing. In addition, the flatness of the film is less likely to deteriorate and the processability of the film can be improved.

The sum of the longitudinal storage modulus at 23° C. and the longitudinal storage modulus at 140° C. of the biaxially oriented polypropylene film of the present invention is preferably 2.8 GPa or more and 8.0 GPa or less. The sum of these storage moduli is more preferably 3.0 GPa or more, even more preferably 3.1 GPa or more, and particularly preferably 3.2 GPa or more, and more preferably 7.5 GPa or less, even more preferably 7.0 GPa or less, and particularly preferably 6.5 GPa or less.
The sum of the storage modulus in the width direction at 23° C. and the storage modulus in the width direction at 140° C. of the biaxially oriented polypropylene film of the present invention is preferably 8.5 GPa or more and 19.0 GPa or less. The sum of these storage moduli is more preferably 8.8 GPa or more, even more preferably 9.1 GPa or more, and particularly preferably 9.5 GPa or more, and is more preferably 18.0 GPa or less, even more preferably 17.0 GPa or less, and particularly preferably 16.0 GPa or less.
When the sums are within the above ranges, the strength at high temperatures is likely to be high, and printing pitch deviation is unlikely to occur when transferring high-temperature printing ink during printing. In addition, the flatness of the film is unlikely to be deteriorated and the processability of the film can be improved.

8-4. Physical properties other than loss modulus and storage modulus 8-4-1.5% elongation stress (F5)
The stress at 5% elongation in the longitudinal direction (F5) of the biaxially oriented polypropylene film of the present invention at 23°C is preferably 40 MPa or more and 70 MPa or less. If the F5 is 40 MPa or more, the film has high rigidity, making it easier to maintain the shape of the bag when made into a packaging bag, and the film is less likely to deform during processing such as printing. If the F5 is 70 MPa or less, practical production becomes easier and the balance between the longitudinal direction and the width direction tends to be improved. The F5 is more preferably 42 MPa or more, even more preferably 46 MPa or more, particularly preferably 48 MPa or more, and more preferably 65 MPa or less, even more preferably 62 MPa or less, and particularly preferably 60 MPa or less. The F5 in the longitudinal direction can be adjusted within the above range by adjusting the stretch ratio or relaxation rate or adjusting the temperature during film formation.

The stress at 5% elongation in the width direction (F5) of the biaxially oriented polypropylene film of the present invention at 23°C is preferably 90 MPa or more and 280 MPa or less. If the F5 is 90 MPa or more, the film has high rigidity, making it easier to maintain the shape of the bag when made into a packaging bag, and the film is less likely to deform during processing such as printing. Furthermore, if the F5 is 90 MPa or more, flatness can be improved. If the F5 is 280 MPa or less, practical manufacturing is easier and the film is less likely to tear in the width direction. The F5 is preferably 110 MPa or more or 120 MPa or more, more preferably 130 MPa or more, particularly preferably 150 MPa or more, most preferably 156 MPa or more, more preferably 250 MPa or less, even more preferably 230 MPa or less, particularly preferably 210 MPa or less, and most preferably 200 MPa or less. The F5 in the width direction can be adjusted within the above range by adjusting the stretch ratio and relaxation rate, or by adjusting the temperature during film formation.

8-4-2. Heat Shrinkage at 120°C The heat shrinkage in the longitudinal direction of the biaxially oriented polypropylene film of the present invention at 120°C is preferably 2.5% or less. If the heat shrinkage is 2.5% or less, printing pitch deviation is less likely to occur when transferring printing ink. The heat shrinkage is more preferably 2.0% or less, even more preferably 1.7% or less, and particularly preferably 1.5% or less. The lower the heat shrinkage in the longitudinal direction at 120°C, the more preferable it is. Although there is no particular lower limit, in view of technical difficulties, it is, for example, 0.1% or more, preferably 0.3% or more. The heat shrinkage in the longitudinal direction at 120°C can be adjusted to fall within the above range by adjusting the stretch ratio, stretching temperature, and heat treatment temperature.

The heat shrinkage rate in the width direction of the biaxially oriented polypropylene film of the present invention at 120°C is preferably 1.1% or less. If the heat shrinkage rate is 1.1% or less, wrinkles are less likely to occur during heat sealing. The heat shrinkage rate is more preferably 1.0% or less, even more preferably 0.7% or less, particularly preferably 0.5% or less, and most preferably 0.3% or less. There is no particular restriction on the lower limit of the heat shrinkage rate in the width direction at 120°C, but it is, for example, -0.2%. The heat shrinkage rate in the width direction at 120°C can be kept within the above range by adjusting the stretch ratio, stretching temperature, and heat treatment temperature.

The sum of the thermal shrinkage rates in the longitudinal and transverse directions of the biaxially oriented polypropylene film of the present invention at 120°C is preferably 3.5% or less. When this sum is 3.5% or less, flatness is easily improved and printing pitch deviation when transferring printing ink is less likely to occur. This sum is more preferably 3.0% or less, even more preferably 2.5% or less, particularly preferably 2.0% or less, and most preferably 1.7% or less. A lower sum is preferable, and although there is no particular lower limit, considering technical difficulties, it is, for example, 1.0% or more, preferably 1.3% or more. The sum of the thermal shrinkage rates in the longitudinal and transverse directions at 120°C can be kept within the above range by adjusting the stretch ratio, stretching temperature, and heat treatment temperature.

8-4-3. Heat Shrinkage at 150°C The heat shrinkage in the longitudinal direction of the biaxially oriented polypropylene film of the present invention at 150°C is preferably 10% or less. If the heat shrinkage is 10% or less, printing pitch deviation is less likely to occur when transferring printing ink. The heat shrinkage is more preferably 7.0% or less, even more preferably 6.0% or less, particularly preferably 5.0% or less, and most preferably 4.0% or less. The lower the heat shrinkage in the longitudinal direction at 150°C, the more preferable it is. Although there is no particular lower limit, considering technical difficulties, it is, for example, 0.1% or more, preferably 0.5% or more. The heat shrinkage in the longitudinal direction at 150°C can be adjusted to fall within the above range by adjusting the stretch ratio, stretching temperature, and heat treatment temperature.

The heat shrinkage rate in the width direction of the biaxially oriented polypropylene film of the present invention at 150°C is preferably 20% or less. If the heat shrinkage rate is 20% or less, wrinkles are less likely to occur during heat sealing. The heat shrinkage rate is more preferably 15% or less, even more preferably 12% or less, and particularly preferably 10% or less. There is no particular lower limit for the heat shrinkage rate in the width direction at 150°C, but it is, for example, 0% or more, and preferably 1% or more. The heat shrinkage rate in the width direction at 150°C can be kept within the above range by adjusting the stretch ratio, stretching temperature, and heat treatment temperature.

If the heat shrinkage rate in the longitudinal direction at 150°C is 10% or less and the heat shrinkage rate in the width direction is 20% or less, wrinkles are less likely to occur during heat sealing. In particular, if the heat shrinkage rate in the longitudinal direction at 150°C is 8.0% or less and the heat shrinkage rate in the width direction at 150°C is 15% or less, distortion when the zipper part is fused to the opening is small, which is preferable. To reduce the heat shrinkage rate at 150°C, it is effective to set the amount of components with a molecular weight of 100,000 or less to 35% by mass or more when measuring the gel permeation chromatography (GPC) cumulative curve of the polypropylene resin composition that makes up the film.

8-4-4. Thickness Uniformity The lower limit of the thickness uniformity of the biaxially oriented polypropylene film of the present invention is preferably 0%, more preferably 0.1%, even more preferably 0.5%, and particularly preferably 1%. The upper limit of the thickness uniformity is preferably 20%, more preferably 17%, even more preferably 15%, particularly preferably 12%, and most preferably 10%. Within the above range, defects are unlikely to occur during post-processing such as coating or printing, making the film suitable for use in applications requiring precision.
The thickness uniformity was measured as follows: A test piece 40 mm in width was cut out from a steady region where the film properties were stable in the length direction of the film, and the film thickness was measured continuously over 20,000 mm using a film feeder manufactured by Micron Measurement Instruments (product number: A90172) and a continuous film thickness measuring instrument manufactured by Anritsu Corporation (product name: K-313A wide-range high-sensitivity electronic micrometer), and the thickness uniformity was calculated using the following formula.
Thickness uniformity (%) = [(maximum thickness - minimum thickness) / average thickness] x 100

8-4-5. Haze The upper limit of the haze of the biaxially oriented polypropylene film of the present invention is preferably 7.0%. A haze of 7.0% or less makes it easy to use in applications requiring transparency. The haze is more preferably 5.0% or less, even more preferably 4.0% or less, particularly preferably 3.5% or less, and most preferably 3.0% or less. The lower limit of the haze is preferably 0%, with 0.1% being a practical value. The haze can be adjusted within the above range by adjusting the cooling roll temperature, the longitudinal stretching temperature, the tenter preheating temperature before widthwise stretching, the widthwise stretching temperature, the heat setting temperature, or the amount of polypropylene polymer components with 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 B.

8-4-6. Image clarity The lower limit of the image clarity of the biaxially oriented polypropylene film of the present invention is preferably 55%. An image clarity of 55% or higher is easy to use in applications requiring transparency. The image clarity is more preferably 57% or higher, even more preferably 59% or higher, particularly preferably 61% or higher, and most preferably 65% or higher. The upper limit of the image clarity is preferably 100%, with 95% being a realistic value. The image clarity can be kept within the range by adjusting the chill roll temperature, longitudinal stretching temperature, tenter preheating temperature before widthwise stretching, widthwise stretching temperature, heat setting temperature, or the amount of polypropylene polymer components with a molecular weight of 100,000 or less. The image clarity may be increased by adding an antiblocking agent or by adjusting the composition of the sealing layer B.

8-4-7. Clarity The lower limit of the clarity of the biaxially oriented polypropylene film of the present invention is preferably 90%. A clarity of 90% or higher makes it easy to use in applications requiring transparency. The clarity is more preferably 92% or higher, even more preferably 93% or higher, particularly preferably 94% or higher, and most preferably 95% or higher. The upper limit of the clarity is preferably 100%, with 99% being a realistic value. The clarity can be adjusted within the above range by adjusting the cooling roll temperature, longitudinal stretching temperature, tenter preheating temperature before widthwise stretching, widthwise stretching temperature, heat setting temperature, or the amount of polypropylene polymer components with a molecular weight of 100,000 or less. The clarity may be increased by adding an antiblocking agent or by adjusting the composition of the sealing layer B.

9. Applications The biaxially oriented polypropylene film of the present invention exhibits little dimensional change over the entire temperature range from room temperature to 130°C. Therefore, deformation of the film can be suppressed even after heating at high temperatures, such as during heat processing with a roll, printing, or heat sealing. This makes it difficult for the film's flatness to deteriorate and improves its processability. Furthermore, it allows for thinner films, which contributes to reducing the volume of packaging materials.
From the above, when the biaxially oriented polypropylene film of the present invention is used to make a packaging bag, the bag shape is easily maintained, the film is less likely to deform during processing such as heat sealing at high temperatures, and printing pitch deviation is less likely to occur during printing, making it suitable for packaging. In addition, the film is less likely to lose flatness even after being coated with a silicone release agent and heated and dried, making it suitable as a release film for optical applications and other applications where a high degree of flatness is required.

To form a bag for packaging food, the contents are filled into a pre-made bag, and the film is heated to melt and fuse, sealing it. This process is often repeated when making bags while filling them with food. Typically, a sealant film made of polyethylene resin, polypropylene resin, or the like is laminated onto the biaxially oriented polypropylene film of the present invention as the base film, and the sealant film surfaces are fused together. The heating method involves applying pressure from a heating plate from the base film side to hold down the film and seal it, with a seal width of approximately 10 mm being common. The base film is also heated during this process, and the resulting expansion and contraction causes wrinkles. Fewer wrinkles are preferable for bag durability and to increase consumer appetite. While the sealing temperature can be around 120°C, higher temperatures are required to increase the bag-making processing speed, and even in this case, small expansion and contraction are preferable. If a zipper is to be fused to the opening of the bag, sealing at even higher temperatures is required.

10. Heat Seal Strength In order to prevent the contents from falling out, the biaxially oriented polypropylene film of the present invention preferably has a heat seal strength at 130°C on the seal layer B side, measured by the measurement method described below, of 4.8 N/15 mm or more, more preferably 5.0 N/15 mm or more, even more preferably 5.5 N/15 mm or more, and particularly preferably 6.0 N/15 mm or more. The upper limit is about 8.0 N/15 mm. There is little need for a strength greater than this, and if it is too great, the bag may be difficult to open.
Furthermore, when one surface of the biaxially oriented polypropylene film of the present invention is a sealing layer B and the other surface is a functional layer D, in order to maintain the packaging form, for example, as in Z-packaging, the heat-sealed portion after bag-making processing is folded to bond the functional layers D together.However, so that the bag can be easily opened by hand, it is preferable that the heat seal strength at 130°C on the functional layer D side is lower than the heat seal strength at 130°C on the sealing layer B side, for example, 3.5 N/15 mm or less, preferably 2.0 N/15 mm or less, and more preferably 1.0 N/15 mm or less.

11. Heat-sealing start temperature The heat-sealing start temperature of the seal layer B of the biaxially oriented polypropylene film of the present invention is preferably 90°C or higher and 130°C or lower. When the heat-sealing start temperature of the seal layer B is 130°C or lower, high heat-sealing strength can be achieved at a relatively low temperature of around 130°C, allowing the temperature of the processing equipment during heat-sealing to be relatively low, enabling high-speed operation during automatic packaging. Furthermore, since heat-sealing processing can be performed at a relatively low temperature, the entire film is less likely to shrink and wrinkles are less likely to form in the sealed area. By setting the heat-sealing start temperature of the seal layer B to 90°C or higher, the film can be made less likely to fuse to the film-forming equipment. The above temperature is more preferably 100°C or higher, even more preferably 110°C or higher, and more preferably 125°C or lower, and even more preferably 120°C or lower.
The heat seal rise temperature can be set within the above range by adjusting the raw material composition of each layer, particularly the raw material composition of the base layer A, the stretching ratio during film formation, the relaxation rate, the temperature of each film formation step, etc.

12. Coefficient of Dynamic Friction The coefficient of dynamic friction of the biaxially oriented polypropylene film of the present invention is preferably 0.50 or less on both sides, i.e., on both the seal layer B and the surface layer opposite to the seal layer B. If the coefficient of dynamic friction is 0.50 or less on both sides, the film can be smoothly unwound from the roll film, facilitating printing processing. The coefficient of dynamic friction is more preferably 0.48 or less, and even more preferably 0.45 or less. There is no particular restriction on the lower limit of the coefficient of dynamic friction, but it is, for example, 0.10 or more.

13. Wet Tension The wet tension of the surface of the seal layer B of the biaxially oriented polypropylene film of the present invention is preferably 35 mN/m or more. A wet tension of 35 mN/m or more can improve adhesion to the surface layer opposite the seal layer B. Furthermore, a wet tension of 35 mN/m or more can improve the anti-fogging properties of the film. To achieve a wet tension of 35 mN/m or more, it is preferable to perform a physicochemical surface treatment such as corona treatment or flame treatment. In corona treatment, it is preferable to use a preheating roll and a treatment roll and perform discharge in the air. The wet tension is more preferably 37 mN/m or more. If the wet tension is too high, the effect will saturate, so it is preferably 43 mN/m or less.

14. Anti-Fog Property The anti-fogging property of the surface of the seal layer B of the biaxially oriented polypropylene film of the present invention is preferably grade 1 to 3, more preferably grade 1 or 2, and even more preferably grade 1, as determined by the evaluation method described below.

15. Packaging Materials When the biaxially oriented polypropylene film of the present invention is used as a packaging material, it may be used alone or with a printed layer provided thereon.
The biaxially oriented polypropylene film of the present invention can be used as a packaging material to produce three-side seal type, pillow type and gusset type packaging bags having good seal strength and good appearance of the sealed portion.
The biaxially oriented polypropylene film of the present invention can be subjected to relief printing, lithographic printing, intaglio printing, stencil printing and transfer printing depending on the application.

This application claims the benefit of priority from Japanese Patent Application No. 2024-14405, filed February 1, 2024. The entire contents of the specification of Japanese Patent Application No. 2024-14405, filed February 1, 2024, are incorporated herein by reference.

The present invention will be described below with reference to examples, but the present invention is not limited to these examples.
The evaluation methods used in each of the examples and comparative examples are as follows.
Furthermore, the "surface layer opposite to the sealing layer B" refers to the base material layer A in Examples 3 and 4, and to the functional layer D in Examples 1 and 2 and Comparative Examples 1 to 4.

(1) Melt Flow Rate of Polypropylene Resin The melt flow rate (MFR) was measured in accordance with JIS K7210 at a temperature of 230°C and a load of 2.16 kgf.

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

(3) Amount of components with a molecular weight of 100,000 or less: Using gel permeation chromatography (GPC), an integral curve of the polypropylene-equivalent molecular weight was obtained using monodisperse polystyrene as a standard. When the baseline was unclear, the baseline was set in the range from the lowest point of the high-molecular-weight side base of the elution peak closest to the elution peak of the standard substance.
The GPC measurement conditions are as follows.
・Device: "HLC-8321PC/HT" manufactured by Tosoh Corporation ・Detector: RI
Solvent: 1,2,4-trichlorobenzene + dibutylhydroxytoluene (0.05%)
Column: TSK gelguard column HHR (30) HT (7.5 mm I.D. x 7.5 cm) x 1 + TSK gel GMHHR-H (20) HT (7.8 mm I.D. x 30 cm) x 3 Flow rate: 1.0 mL/min Injection volume: 0.3 mL
・Measurement temperature: 140℃
The mass proportion of components having a molecular weight of 100,000 or less was determined from the integral curve of the molecular weight obtained by GPC.

(4) Melting point and crystallization temperature Thermal measurements were performed using a differential scanning calorimeter ("Q1000" manufactured by TA Instruments). Approximately 5 mg was cut out from the pellet, placed in an aluminum pan for measurement, and set in a differential scanning calorimeter. Under a nitrogen atmosphere, the resin was melted by heating from 30 ° C. to 230 ° C. at a heating rate of 20 ° C./min and holding at 230 ° C. for 5 minutes, and then cooled to 30 ° C. at a heating rate of -10 ° C./min and held at 30 ° C. for 5 minutes. The temperature was then increased to 230 ° C. at a heating rate of 10 ° C./min. The melting point was the main peak temperature of the endothermic peak associated with melting observed during the second heating. The crystallization temperature was the main peak temperature of the exothermic peak observed when the temperature was lowered from 230 ° C. to 30 ° C.

(5) Film Thickness The thickness of the film was measured using a stylus film thickness meter ("Militron 1202D" manufactured by Seiko EM Corporation).

(6) Thermomechanical analysis (TMA) measurement (measurement of film width direction length from 30°C to 130°C)
A film was cut out so that the width direction of the film was 40 mm and the length direction of the film was 4 mm, and the film was set in a thermomechanical analyzer ("TMA-60", manufactured by Shimadzu Corporation) so that the chuck width was 10 mm. The temperature was raised from 30°C to 130°C at a temperature increase rate of 10°C/min with a measurement load of 0.5 g, and the length X of the sample in the width direction during the temperature increase was continuously measured. The maximum value of the length between the chucks during the temperature increase was defined as _X1 (mm), and the minimum value of the length between the chucks during the temperature increase was defined as _X2 (mm), and the percentages of (X1 - ₁₀ )/10 and (X2 _- 10)/10 were calculated.

(7) Thermomechanical analysis (TMA) measurement (temperature measurement at 0.5% shrinkage)
A film was cut out so that the width direction of the film was 40 mm and the length direction of the film was 4 mm, and the film was set in a thermomechanical analyzer ("TMA-60", manufactured by Shimadzu Corporation) so that the chuck width was 10 mm. The temperature was raised from 30°C to 160°C at a heating rate of 10°C/min with a measurement load of 0.5 g, and the width direction length of the sample was continuously measured during the temperature rise. From the measurement results, the lowest temperature at which the width direction length of the sample was 9.95 mm or less was determined to be the temperature at which 0.5% shrinkage occurred.

(8) Loss modulus by dynamic mechanical analysis (DMA) A film was cut to a width of 40 mm and a length of 4 mm, and set in a solid viscoelastic analyzer (RSA-G2, manufactured by TA Instruments Japan) with a chuck width of 10 mm. The film was heated from -60°C to 160°C at a heating rate of 5°C/min under a nitrogen atmosphere with a measuring load of 10 g and a frequency of 10 Hz, and the loss modulus of the film was measured during the heating in the width direction. A graph was drawn with temperature on the horizontal axis and loss modulus on the vertical axis, and the maximum loss modulus E"(A) from -25°C to 25°C, the minimum loss modulus E"(B) from 25°C to 75°C, and the maximum loss modulus E"(C) at 100°C or higher were calculated. The values of E"(C)/E"(A) and E"(B)/E"(C) were also calculated.

(9) Storage modulus by dynamic mechanical analysis (DMA) The storage modulus in the longitudinal and transverse directions of the film was measured by the following method. A film was cut so that the measurement direction was 40 mm and the direction perpendicular thereto was 4 mm, and the film was set in a solid viscoelasticity analyzer ("RSA-G2" manufactured by TA Instruments Japan) so that the chuck width was 10 mm. The temperature was raised from -60 ° C to 160 ° C at a heating rate of 5 ° C / min under a nitrogen atmosphere with a measurement load of 10 g and a frequency of 10 Hz, and the storage moduli at 23 ° C, 120 ° C, 140 ° C, and 150 ° C were determined. Furthermore, the sum of the storage moduli at 23 ° C and 140 ° C in the longitudinal direction and the sum of the storage moduli at 23 ° C and 140 ° C in the transverse direction were calculated from the obtained values.

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

(11) Image clarity Using an image clarity measuring instrument ("ICM-1T" manufactured by Suga Test Instruments Co., Ltd.), image clarity was measured in accordance with the transmission method of JIS K7374: 2007, with the slit width of the optical comb set to 0.5 mm. During measurement, the sample was set so that the width direction was perpendicular to the optical comb of the measuring instrument.

(12) Clarity Clarity was measured using a transparency measuring device ("Hazeguard i" manufactured by BYK).
The light transmitted through the film during measurement includes straight light that travels straight along the optical axis of the incident parallel light and narrow-angle scattered light that has an angle of ±2.5° or less with respect to the optical axis of the parallel light.
When the amount of light of straight light is I _c and the amount of light of narrow-angle scattered light within ±2.5° is I _s , the clarity is calculated by the following formula.
Clarity (%) = (I _c - _{I s} ) / (I _c + I _s ) × 100

(13) Stress at 5% Elongation (F5), Tensile Modulus, Tensile Breaking Strength, and Tensile Breaking Elongation Various physical properties were measured at 23°C during tensile testing of the film in the longitudinal and transverse directions in accordance with JIS K7127. The film was cut so that the measurement direction was 200 mm and the direction perpendicular thereto was 15 mm, and the chuck width was 100 mm. The film was set in a tensile testing machine ("Dual Column Tabletop Tester Instron 5965" manufactured by Instron Japan Co., Ltd.). The tensile test was performed at a pulling rate of 200 mm/min. From the obtained strain-stress curve, the tensile modulus was calculated from the slope of the linear portion at the beginning of elongation, and the stress at 5% elongation was designated as F5. The tensile breaking strength and tensile breaking elongation were defined as the strength and elongation, respectively, at the time the sample broke.

(14) Heat Shrinkage Ratio The heat shrinkage ratios of the film in the longitudinal and transverse directions were measured according to JIS Z1712 by the following method. A film was cut so that the measurement direction was 200 mm and the direction perpendicular to this was 20 mm, and the film was hung in a hot air oven at 120°C and heated for 5 minutes. The length after heating was measured, and the heat shrinkage ratio was calculated as the ratio of the shrunken length to the original length.

(15) Dynamic friction coefficient A sample measuring 400 mm in length and 100 mm in width was cut out from the film. This was aged for 12 hours in an atmosphere of 23°C and 65% RH, and then divided into a sample for a test table measuring 300 mm in length and 100 mm in width and a sample for a sliding piece measuring 100 mm in length and 100 mm in width.
The test table sample was set on the test table, and the slider sample was attached to the bottom surface (square with an area of 39.7 mm ² ) of a metal slider with a load of 1.5 kg so that the sealing layers B were in contact with each other.
In accordance with JIS K-7125, a tensile tester ("Tensilon RTG-1210" manufactured by A&D Co., Ltd.) was used to measure the dynamic friction coefficient of the seal layer surface B under conditions of a sliding speed of the test piece of 200 mm/min, 23°C, and 65% RH, and the average of three measurements was used.
The coefficient of dynamic friction of the surface opposite to the sealing layer B was also determined in the same manner as above, except that the surfaces opposite to the sealing layer B were attached so as to be in contact with each other.

(16) Wetting tension (mN/m)
The film was cut into a size of 297 mm in the longitudinal direction and 210 mm in the width direction, and after aging for 24 hours at a temperature of 23°C and a relative humidity of 50%, the wet tension of the surface on the seal layer B side of the sample was measured by the following procedure in accordance with JIS K 6768. The wet tension was measured in a laboratory atmosphere at a temperature of 23°C and a relative humidity of 50%, in accordance with JIS K7100.
The sample was placed on a flat substrate, and several drops of a wetting tension standard solution (a test mixture according to JIS K 6768) were placed on the sealing layer B of the sample. The wetting tension standard solution was then spread over the surface of the sealing layer B with a cotton swab to an area of 6 ^cm2 or more. The state of the liquid film was visually observed in a bright place 3 seconds after application. If the liquid film did not break 3 seconds after formation and remained in the same state as when it was applied, this meant that the liquid film was wet. Therefore, a liquid film was formed using a wetting tension standard solution with a surface tension one level higher than the wetting tension standard solution used to form the liquid film. On the other hand, if the liquid film broke within 3 seconds, a liquid film was formed using a wetting tension standard solution with a surface tension one level lower than the wetting tension standard solution used to form the liquid film. A new cotton swab was used for each liquid film formation.
The above liquid film formation was repeated, and the value of the maximum wetting tension standard solution among the wetting tension standard solutions that could wet the surface of the sealing layer B in 3 seconds was taken as the wetting tension. Note that a liquid film was formed three times using the wetting tension standard solution with the maximum value, and it was confirmed that the state at the time of application was maintained after 3 seconds had elapsed.
The wetting tension of the surface layer on the opposite side to the sealing layer B was also measured using the same measuring and calculation methods as above.

(17) Anti-Fogging Properties Using a film, the anti-fogging properties of the film surface on the seal layer B side and the film surface on the opposite side to the seal layer B were evaluated according to the following procedure.
1) 300 mL of hot water at 50°C was placed in a 500 mL open-top container.
2) The opening of the container was sealed with the film, with the surface of the film on the side where the anti-fogging properties of the film were to be measured facing inward.
3) The film was left in a cold room at 5° C. for 30 minutes, and then the state of dew on the film surface on the side where the anti-fogging property was to be measured was evaluated on the following 5-point scale.
Grade 1: No dew on the entire surface (0 adhesion area)
Grade 2: Dew adheres to a small area of the surface (adhesion area is greater than 0 and less than 1/4)
Grade 3: Dew on just under half of the surface (adhesion area: over 1/4 and less than 2/4)
Grade 4: Dew on most of the surface (adhesion area: over 2/4 and less than 3/4)
Grade 5: Dew on almost the entire surface (more than 3/4 of the surface area)

(18) Heat-sealing Rise Temperature Two samples measuring 20 cm in the longitudinal direction and 5 cm in the width direction were cut out from the film. The two cut-out samples were stacked with the seal layers B facing each other, and then heat-sealed using a thermal gradient tester (manufactured by Toyo Seiki Co., Ltd.). Five rectangular heat-sealed surfaces were placed in a row in the center of the width direction of the sample, parallel to the longitudinal direction of the sample. The heat-sealed surfaces were placed 1 cm in the longitudinal direction of the sample and 3 cm in the width direction of the sample, with a 1.5 cm gap between adjacent heat-sealed surfaces. The heat-sealing temperatures were different for each heat-sealed surface, and were 80°C, 85°C, 90°C, 95°C, and 100°C. The heat-sealing pressure was 1 kg/ ^cm2 , and the heat-sealing time was 1 second.
The sample was then cut to a length of 19 cm in the longitudinal direction and a central 1.5 cm in the width direction so that five heat-sealed surfaces were included. The sample was cut in the longitudinal direction so that the longitudinal direction of the sample and the longitudinal direction of the cut sample were parallel, and the heat-sealed surface heat-sealed at 90°C was located in the longitudinal center. The cut sample was attached to the upper and lower chucks of a tensile tester ("5965 Dual Column Tabletop Tester" manufactured by Instron), and the heat-seal strength was measured for each heat-sealed surface when pulled at a pulling speed of 200 mm/min (unit: N/15 mm).
In addition, two new cut-out samples were prepared and heat-sealed at 105°C, 110°C, 115°C, 120°C, and 125°C in the same manner as above, except that five measurement samples were prepared and the heat-seal strength was measured.
A linear graph was drawn with the horizontal axis representing temperature and the vertical axis representing heat seal strength, and the temperature at which the heat seal strength reached 1 N/15 mm was taken as the heat seal initiation temperature. Another sample was prepared and the heat seal strength was measured two more times at 80 to 125°C to determine the heat seal initiation temperature, and the average of the three calculated values was taken as the heat seal initiation temperature of the film.
For the surface layer on the opposite side to seal layer B, the heat seal rise temperature of the film was determined using the same measurement and calculation methods as above, except that the surface layers on the opposite side to seal layer B were stacked facing each other.

(19) Heat Seal Strength Two samples measuring 29.7 cm in the longitudinal direction and 21.0 cm in the width direction were cut out from the film. The seal layers B of the two cut out samples were placed face to face, and then heat sealed at 130 ° C. using a thermal gradient tester (manufactured by Toyo Seiki Co., Ltd.). A rectangular heat seal surface was placed in the center of the width direction of the sample so that it was parallel to the longitudinal direction of the sample. The heat seal surface was placed so that it was 1.5 cm in the longitudinal direction of the sample and 3 cm in the width direction of the sample. Then, the sample was cut to a length of 9.5 cm in the longitudinal direction and 1.5 cm in the central part of the width direction. In the longitudinal direction, the sample was cut so that the longitudinal direction of the sample and the longitudinal direction of the cut sample were parallel, and the heat seal surface was located in the center of the longitudinal direction. The cut sample was attached to the upper and lower chucks of a tensile tester (Instron's "5965 Dual Column Tabletop Tester") and pulled at a pulling speed of 200 mm/min to measure the heat seal strength (unit: N/15 mm). Two other samples were prepared and their heat seal strengths were measured, and the average of the three calculated values was used as the heat seal strength of the film.
The heat seal strength of the film was determined using the same measuring and calculating methods as above, except that the surface layers of the two cut-out samples on the side opposite to the seal layer B were placed face to face.
Furthermore, the heat seal strength of the seal layer B and the heat seal strength of the surface layer opposite to the seal layer B were determined using the same measuring and calculation methods as above, except that the heat seal temperature was set to 140°C.

(20) Appearance Evaluation of Heat-Sealed Portions The obtained film was heat-sealed to form a 130 mm x 180 mm three-sided sealed bag using a heat sealer, with the sealant films of the laminate heat-sealed to each other. The pressure was 0.2 MPa for 1 second, the seal bar width was 10 mm, and the heat-sealing temperature was 150°C. The appearance of wrinkles in the heat-sealed portions was visually evaluated.
A: No wrinkles can be seen in the heat-sealed area in either the width or length direction of the film. B: Wrinkles can be seen in the heat-sealed area in only one of the width or length directions of the film. C: Wrinkles can be seen in the heat-sealed area in both the width and length directions of the film.

(21) Flatness Evaluation (Flatness after 130°C Treatment)
A sample was cut out from the film so that both the width and length directions were 200 mm, and used as an evaluation sample. The sample was hung in a hot air oven at 130°C and heated for 5 minutes. After cooling to room temperature, the film was placed on a black mount, and the film surface was observed at a 45-degree angle under a fluorescent lamp.
A: No heat wrinkles or large swells of 5 mm or more exist. B: No heat wrinkles are visible, but large swells of 5 mm or more can be seen. C: Heat wrinkles can be seen.

Raw Materials Used The polypropylene resins constituting the layers of the films in the following Examples and Comparative Examples are as follows.
PP-1: Propylene homopolymer ("FLX80H5" manufactured by Sumitomo Chemical Co., Ltd., mesopentad fraction: 98.9%, melting point: 163°C, MFR: 7.5g/10min, amount of components with molecular weight of 10,000 or less: 4.0% by mass, amount of components with molecular weight of 100,000 or less: 40.5% by mass)
PP-2: Propylene homopolymer ("EL80F5" manufactured by Sumitomo Chemical Co., Ltd., mesopentad fraction: 98.8%, melting point: 162°C, MFR: 11 g/10 min, amount of components with a molecular weight of 10,000 or less: 6.9% by mass, amount of components with a molecular weight of 100,000 or less: 53.1% by mass)
PP-3: A composition containing PP-1 and, as an anti-fogging agent, stearyl diethanolamine monostearate, stearyl diethanolamine distearate, and stearyl diethanolamine in a total amount of 1.7% by mass, and glycerin monostearate in an amount of 0.25% by mass. PP-4: Propylene homopolymer ("FY6H", manufactured by Japan Polypropylene Corporation, MFR: 1.9 g/10 min, melting point: 163°C, mesopentad fraction: 98.9%)
PP-5: A composition containing propylene homopolymer ("FL203D" manufactured by Japan Polypropylene Corporation, mesopentad fraction: 94.8%, melting point: 161°C, MFR: 3g/10 min, amount of components with a molecular weight of 10,000 or less: 3.0% by mass, amount of components with a molecular weight of 100,000 or less: 37.1% by mass), 1.7% by mass of stearyl diethanolamine monostearate as an anti-fogging agent, and 0.25% by mass of glycerin monostearate. PP-6: A composition containing propylene-ethylene-butene copolymer ("FSX66M4" manufactured by Sumitomo Chemical Co., Ltd., melting point: 138°C, MFR: 4.5g/10 min, ethylene content: 3.3 mol%, butene content: 2.9 mol%, glycerin monostearate content: 0.45% by mass).
PP-7: Propylene-butene copolymer ("SP7843" manufactured by Sumitomo Chemical Co., Ltd., melting point: 128°C, MFR: 6.5 g/10 min, butene content: 8.2 mol%)

Example 1
(1) Preparation of raw materials for base layer A A polypropylene-based resin composition containing 20% by mass of propylene homopolymer PP-1, 20% by mass of propylene homopolymer PP-2, and 60% by mass of propylene homopolymer PP-3 was used as the raw material. When the physical properties of each polypropylene homopolymer were averaged by mass, the polypropylene-based resin composition constituting base layer A had a mesopentad fraction of 98.88%, a melting point of 162.8°C, an MFR of 8.2 g/10 min, a content of components with a molecular weight of 10,000 or less of 4.58% by mass, and a content of components with a molecular weight of 100,000 or less of 43.02% by mass.
(2) Raw Materials for Sealing Layer B Propylene-butene copolymer PP-7 was used as the raw material.
(3) Raw Materials for Functional Layer D Propylene-ethylene-butene copolymer PP-6 was used as the raw material.
(4) Film Preparation First, the polypropylene resin compositions constituting each of the functional layer D, base layer A, and seal layer B were heated and melted in an extruder using a multi-layer feed block at 250°C, 250°C, and 210°C, respectively, and the molten polypropylene resin compositions were then laminated from a T-die at 250°C to produce a 1.8 mm thick laminated molten sheet, with the thickness ratio of the functional layer D, base layer A, and seal layer B being 1/14/1.
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. The sheet was then stretched 4.5 times in the longitudinal direction using two pairs of rolls at 142 ° C., then clamped at both ends with clips and introduced into a hot air oven, preheated to 172 ° C., and then stretched 12.7 times in the width direction at 162 ° C. Immediately after stretching in the width direction, the sheet was heat-treated at 170 ° C. while still held by the clips without relaxation, and then heat-treated at 140 ° C. to achieve a relaxation rate of 3% in the width direction. Finally, the sheet was cooled to room temperature. The surface of the seal layer B side of the obtained biaxially oriented polypropylene film was subjected to corona treatment using a corona treatment machine (manufactured by Softal Corona & Plasma GmbH) at 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 16 μm. In the obtained film, the thicknesses of the functional layer D, the base layer A and the seal layer B were 1 μm, 14 μm and 1 μm, respectively.
The raw material composition of each layer and the film-forming conditions are shown in Table 1, and various physical properties of the film are shown in Table 2. In the table, "%" indicates % by mass.
Despite its high rigidity, the biaxially oriented polypropylene film had low heat shrinkage at high temperatures, high seal strength, and excellent anti-fogging properties. It also had excellent flatness after treatment at 130°C. The three-side sealed bag produced using the film of Example 1 had a good heat-sealed appearance and excellent handleability due to the excellent stiffness of the bag.

Examples 2 to 4
In Examples 2 to 4, films were produced using the same production method as in Example 1, except that the film production conditions were changed to those shown in Table 1. Various physical properties of the films are shown in Table 2. The biaxially oriented polypropylene films of Examples 2 to 4 had high rigidity like that of Example 1, but also had low heat shrinkage at high temperatures, high seal strength, and excellent anti-fogging properties.

Examples 5 and 6
The same materials as those used in Examples 1 to 4 were used as the raw materials for the base layer A and the seal layer B, and the propylene-ethylene-butene copolymer PP-6 was used as the raw material for the intermediate layer C.
First, the polypropylene resin compositions constituting each of the base layer A, intermediate layer C, and seal layer B were heated and melted in an extruder using a multi-layer feed block at 250°C, 250°C, and 210°C, respectively, and the molten polypropylene resin compositions were laminated from a T-die at 250°C to produce a 1.8 mm thick laminated molten sheet, with the thickness ratio of the base layer A, intermediate layer C, and seal layer B being 13/2/1.
A molten sheet was obtained in the same manner as in Example 1 and stretched in the longitudinal and width directions. Immediately after stretching in the width direction, the sheet was heat-treated at 170°C while held by the clips without relaxation, then heat-treated at 140°C to relax the sheet in the width direction by 3%, and finally cooled to room temperature. The surface of the resulting biaxially oriented polypropylene film on the side of the seal layer B was corona-treated, and the resulting film was wound up on a winder to form a biaxially oriented polypropylene film of the present invention. The thickness of the resulting film was 16 μm. In the resulting film, the thicknesses of the base layer A, intermediate layer C, and seal layer B were 13 μm, 2 μm, and 1 μm.
The raw material composition of each layer and the film-forming conditions are shown in Table 1, and various physical properties of the film are shown in Table 2.
The biaxially oriented polypropylene films of Examples 5 and 6 had high rigidity, but also had low heat shrinkage at high temperatures, high seal strength, and excellent anti-fogging properties.

Example 7
The same materials as those used in Examples 1 to 4 were used as the raw materials for the base layer A, the sealing layer B, and the functional layer D, and the propylene-ethylene-butene copolymer PP-6 was used as the raw material for the intermediate layer C.
First, the polypropylene resin compositions constituting each of the functional layer D, base layer A, intermediate layer C, and seal layer B were heated and melted in an extruder using a multi-layer feed block at 250°C, 250°C, 250°C, and 210°C, respectively, and the molten polypropylene resin compositions were laminated from a T-die at 250°C to produce a 1.8 mm thick laminated molten sheet, with the thickness ratio of the functional layer D, base layer A, intermediate layer C, and seal layer B being 1/16/2/1.
A molten sheet was obtained in the same manner as in Example 1 and stretched in the longitudinal and width directions. Immediately after stretching in the width direction, the sheet was heat-treated at 170°C while held by the clips without relaxation, then heat-treated at 140°C to relax the sheet in the width direction by 3%, and finally cooled to room temperature. The surface of the resulting biaxially oriented polypropylene film on the side of the seal layer B was corona-treated, and the resulting film was wound up on a winder to form a biaxially oriented polypropylene film of the present invention. The thickness of the resulting film was 20 μm. In the resulting film, the thicknesses of the functional layer D, base layer A, intermediate layer C, and seal layer B were 1 μm, 16 μm, 2 μm, and 1 μm.
The raw material composition of each layer and the film-forming conditions are shown in Table 1, and various physical properties of the film are shown in Table 2.
The biaxially oriented polypropylene film of Example 7 had high rigidity, but also had low heat shrinkage at high temperatures, high seal strength, and excellent anti-fogging properties.

Comparative Examples 1 to 6
In Comparative Examples 1 to 6, the same raw materials as in Example 7 were used to form a layer structure of functional layer D/substrate layer A/intermediate layer C/sealing layer B, and except for Comparative Example 3, the thickness structure was also the same as in Example 7, with a total thickness of 20 μm, that is, 1 μm/16 μm/2 μm/1 μm. In Comparative Example 3, the thicknesses of the layers were 1 μm/31 μm/2 μm/1 μm, and the total thickness was 35 μm.
The film-forming conditions for Comparative Examples 1 to 6 are shown in Table 3, and various physical properties of the films are shown in Table 4.

Comparative Example 7
In Comparative Example 7, the layer structure was the same as in Example 7, that is, functional layer D/substrate layer A/intermediate layer C/sealing layer B, and the thickness structure was also the same as in Example 7, that is, 1 μm/16 μm/2 μm/1 μm, for a total thickness of 20 μm. The base layer A was made from a polypropylene resin composition containing 43% by mass of PP-6 ("FY6H" manufactured by Japan Polypropylene Corporation, MFR: 1.9 g/10 min, melting point: 163°C, mesopentad fraction: 98.9%), a propylene homopolymer, and 57% by mass of PP-7 (a composition obtained by adding 0.16% by mass of glycerin monostearate (TB-123 manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.), 0.2% by mass of polyoxyethylene (2) stearylamine (TB-12 manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.), and 0.6% by mass of polyoxyethylene (2) stearylamine monostearate (Elex 334 manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.) to 100% by mass of the above PP-1). The film formation conditions are shown in Table 3, and the physical properties of the film are shown in Table 4.

As shown in Table 4, in Comparative Examples 1 to 7, at least one of the percentages ( _X1 - _X0 )/ _X0 or ( _X2 - _X0 )/ _X0 ( _X0 : 10 mm, _X1 : maximum value of chuck gap during temperature increase (mm), _X2 : minimum value of chuck gap during temperature increase (mm)) was outside the specified range, and therefore the films of Comparative Examples 1 to 7 developed wrinkles in the sealed portion heat-sealed at 150°C. In addition, the flatness of the films after treatment at 130°C was poor.
On the other hand, as shown in Table 3, when the percentages of (X ₁ −X ₀ )/X ₀ and (X ₂ −X ₀ )/X ₀ were predetermined values, the change in the length of the film, particularly the length in the width direction, was small even during and after the temperature increase, the occurrence of wrinkles was suppressed, and the flatness was excellent.

Fig. 1 is a diagram showing the relationship between temperature and the length in the width direction of the film in Example 1, Comparative Example 1, and Comparative Example 4, and more precisely, the relationship between temperature and the percentage of (X ₁ - X ₀ )/X _0. Fig. 2 is a diagram showing the relationship between temperature and loss modulus in Example 1 and Comparative Example 1, and Fig. 3 is a diagram showing the relationship between temperature and storage modulus in Example 1 and Comparative Example 1.

The biaxially oriented polypropylene film of the present invention has excellent rigidity and heat resistance, making it easy to process into bags and easily retaining the shape of the bags when made into packaging bags. Therefore, the biaxially oriented polypropylene film of the present invention can be used for packaging bags, and packaging bags to which an anti-fog agent has been added are particularly suitable for packaging fresh produce. Furthermore, the biaxially oriented polypropylene film of the present invention can be suitably used in applications requiring high rigidity even without laminating a sealant film, and since it can maintain its strength even when the film is thin, it can reduce the burden on the environment.

Claims (11)

A biaxially oriented polypropylene film having a substrate layer A made of a polypropylene-based resin composition and a seal layer B made of a polypropylene-based resin composition, and satisfying the following (1) to (4):
(1) The seal layer B is provided on at least one outermost surface.
(2) Clarity is between 90% and 100%.
(3) The storage modulus in the width direction at 140° C. is 0.7 GPa or more and 5.0 GPa or less.
(4) The thermal shrinkage rate at 150°C in the width direction is 10% or less. 2. The biaxially oriented polypropylene film according to claim _1, wherein, in a thermomechanical analysis, when the temperature is raised from 30°C to 160°C at a heating rate of 10°C/min, the temperature at which the width direction length is 0.9950 x _X0 or less relative to the width direction length X0 at 30°C is 129°C or higher, the storage modulus in the longitudinal direction at 23°C is 2.0 GPa or higher, and the storage modulus in the width direction at 23°C is 7.0 GPa or higher. The biaxially oriented polypropylene film according to claim 1 or 2, having a longitudinal storage modulus of 0.5 GPa or more at 120°C and a transverse storage modulus of 1.5 GPa or more at 120°C. The biaxially oriented polypropylene film according to claim 1 or 2, having a longitudinal heat shrinkage rate of 2.5% or less at 120°C and a transverse heat shrinkage rate of 1.1% or less at 120°C. The biaxially oriented polypropylene film according to claim 1 or 2, having a stress of 120 MPa or more at 5% elongation in the width direction at 23°C. The biaxially oriented polypropylene film according to claim 1 or 2, having a haze of 7.0% or less. The biaxially oriented polypropylene film according to claim 1 or 2, containing 0.2% by mass or more and 2.0% by mass or less of an anti-fogging agent. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the base layer A contains 90% by mass or more of a polypropylene resin having a mesopentad fraction of 97.0% or more. The biaxially oriented polypropylene film according to claim 1 or 2, wherein the sealing layer B contains 70% by mass or more of a polypropylene copolymer, and the polypropylene copolymer contains 4% by mole or more of an α-olefin other than propylene. The biaxially oriented polypropylene film according to claim 1 or 2, which has an intermediate layer C made of a polypropylene-based resin composition between the base layer A and the sealing layer B, and the melting point of the polypropylene-based resin composition constituting the intermediate layer C is higher than the melting point of the polypropylene-based resin composition constituting the sealing layer B. The biaxially oriented polypropylene film according to claim 1 or 2, having a thickness of 10 μm or more and 100 μm or less.