JP7581884B2 - Multilayer film, packaging material and packaging body - Google Patents

Description

The present invention relates to a multilayer film, a packaging material, and a package. More specifically, the present invention relates to a polypropylene-based multilayer film that has excellent heat resistance and transparency and can be suitably used as a sealant film for packaging materials, even in harsh treatments such as boiling water treatment and retort treatment, and to a packaging material and a package obtained using the polypropylene-based multilayer film.

Polypropylene films are sometimes used as sealant films in various packaging materials, such as food packaging, because they have excellent rigidity and heat resistance and are inexpensive.

Patent Document 1 proposes a polypropylene-based composite film that is composed of three layers, in which the middle layer is made of a propylene-ethylene block copolymer and both surface layers are made of a propylene-based random copolymer.

Patent Document 2 proposes a multilayer film consisting of surface layers containing an ethylene-α-olefin copolymer and a propylene-based polymer, and an intermediate layer containing a propylene polymer and an ethylene-propylene copolymer.

Patent Document 3 proposes a multilayer film consisting of two surface layers made of polypropylene or an ethylene-propylene random copolymer, and an inner layer made of an ethylene-propylene block copolymer or a blend of an ethylene-propylene block copolymer and a polyolefin polymer that is well compatible with the rubber component in the block copolymer.

JP 2017-132186 A JP 2017-105174 A Japanese Unexamined Patent Publication No. 62-3950

Polypropylene-based multilayer films are now required to have heat resistance that can withstand retort treatment, which involves pressurizing at high temperatures of, for example, 120 to 135°C to perform sterilization and disinfection, and transparency that allows the contents to be easily seen. However, with conventional polypropylene-based films, it is currently difficult to maintain excellent heat resistance, transparency, and cold impact resistance at the same time.

The present invention has been made in consideration of the above circumstances, and aims to provide a polypropylene-based multilayer film that has excellent heat resistance, transparency, and cold impact resistance. The present invention also aims to provide a packaging material and a package obtained by using the multilayer film.

As a result of intensive research to solve the above problems, the inventors discovered that it is important to mix different propylene-based polymers into the outer layer of a polypropylene-based multilayer film, and then add a specified amount of a nucleating agent to adjust the crystallinity, which led to the completion of the present invention.

The multilayer film according to one aspect of the present invention is a multilayer film comprising, in this order, a first outer layer which is a heat seal layer containing a propylene homopolymer (A), a propylene-ethylene random copolymer (B) and a nucleating agent, an inner layer containing a propylene-ethylene block copolymer (C) and an ethylene-propylene copolymer elastomer (D), and a second outer layer containing a propylene homopolymer (A) and a propylene-ethylene random copolymer (B), and the content of the nucleating agent is 1 to 5 parts by mass per 100 parts by mass of the total amount of the propylene homopolymer (A) and the propylene-ethylene random copolymer (B).

In the multilayer film, the outer layer contains a propylene homopolymer (A), a propylene-ethylene random copolymer (B), and a predetermined amount of a nucleating agent. This makes it possible to maintain heat resistance (high level of heat resistance that makes it difficult for the film to deform or fuse on the inner surface when used as a package), while suppressing surface irregularities that reduce the transparency of the film, maintaining the flexibility of the film, and ensuring excellent cold impact resistance. Such effects cannot be obtained when a propylene-based random copolymer is simply used in the outer layer (for example, Patent Document 1 above), when an ethylene-α-olefin copolymer and a propylene-based polymer are used (for example, Patent Document 2 above), or when either polypropylene or an ethylene-propylene random copolymer is used (for example, Patent Document 3 above), and are particularly suitable for retort processing (including high retort processing) applications.

In one embodiment, the first outer layer and the second outer layer may contain 70 to 30 parts by mass of a propylene homopolymer (A) and 30 to 70 parts by mass of a propylene-ethylene random copolymer (B). This makes it easier to achieve both heat resistance and transparency.

In one embodiment, the propylene homopolymer (A) may satisfy the following requirement (a1), and the propylene-ethylene random copolymer (B) may satisfy the following requirement (b1), which makes it easier to achieve both heat resistance and transparency.
Requirement (a1): The melting onset temperature is 150° C. or higher and the melting peak temperature is 155° C. or higher, as measured by differential scanning calorimetry (JIS K 7121).
Requirement (b1): The ethylene content is 5% by mass or less.

In one embodiment, the inner layer may contain 90 to 50% by mass of a propylene-ethylene block copolymer (C) and 10 to 50% by mass of an ethylene-propylene copolymer elastomer (D). This maintains the flexibility of the film and makes it easier to obtain excellent cold impact resistance.

A packaging material according to one aspect of the present invention comprises the above-mentioned multilayer film and a substrate.

The packaging body according to one aspect of the present invention is made from the above-mentioned packaging material.

According to the present invention, it is possible to provide a polypropylene-based multilayer film having excellent heat resistance, transparency, and cold impact resistance. The present invention also provides a packaging material and a package obtained by using the multilayer film.

FIG. 1 is a cross-sectional view of a multilayer film according to one embodiment of the present invention. FIG. 2 is a cross-sectional view of a packaging material according to one embodiment of the present invention.

<Polypropylene-based multi-layer film>
Fig. 1 is a cross-sectional view of a multilayer film according to one embodiment of the present invention. The multilayer film 10 includes a first outer layer 1a, an inner layer 2, and a second outer layer 1b in this order. The multilayer film can be used as a polypropylene-based non-stretch sealant film.

[First outer layer and second outer layer]
The first outer layer contains a propylene homopolymer (A), a propylene-ethylene random copolymer (B), and a nucleating agent. The first outer layer can be said to be a layer formed from a resin composition containing a propylene homopolymer (A), a propylene-ethylene random copolymer (B), and a nucleating agent. When used as a packaging material, the first outer layer plays a role as a heat seal layer (has heat sealability) and is arranged so as to contact the contents. Therefore, the first outer layer has a heat resistance to such an extent that the first outer layers are not fused together during retort treatment.

The second outer layer contains a propylene homopolymer (A) and a propylene-ethylene random copolymer (B). The second outer layer can be said to be a layer formed from a resin composition containing a propylene homopolymer (A) and a propylene-ethylene random copolymer (B).

The first outer layer and the second outer layer have the above-mentioned configuration, which can impart heat resistance and transparency to the multilayer film. The first outer layer and the second outer layer may be collectively referred to simply as the outer layers.

(Propylene homopolymer (A))
The method for producing the propylene homopolymer (A) is not particularly limited, and it can be obtained, for example, by a method for homopolymerizing propylene using a Ziegler-Natta catalyst, a metallocene catalyst, or a half metallocene catalyst. By containing the propylene homopolymer (A) in the outer layer, excellent heat resistance can be imparted to the outer layer.

As the propylene homopolymer (A), one having a melting onset temperature of 150°C or higher and a melting peak temperature of 155°C or higher when measured by differential scanning calorimetry (JIS K 7121) can be used. One having both a melting onset temperature and a melting peak temperature within this range has excellent heat resistance, and is less likely to fuse on the inner surface of the package after, for example, retort treatment at high temperatures.

As the propylene homopolymer (A), one having a melt flow rate (MFR: ISO 1133) (temperature 230°C, load 2.16 kg) in the range of 2.0 to 7.0 g/10 min can be used. The melt flow rate is a parameter that indicates the fluidity of a polymeric material when melted, and is also a parameter that indicates the molecular weight. Therefore, if the melt flow rate is too high, the impact resistance of the polymeric material is likely to decrease, and if it is too low, the load on the extruder during molding processing increases, the processing speed decreases, and the productivity is likely to decrease. From these viewpoints, the melt flow rate can be 2.0 to 7.0 g/10 min, and may be 2.0 to 5.0 g/10 min.

(Propylene-Ethylene Random Copolymer (B))
The method for producing the propylene-ethylene random copolymer (B) is not particularly limited, but it can be obtained, for example, by copolymerizing ethylene as a comonomer into the main monomer consisting of propylene using a Ziegler-Natta catalyst, a metallocene catalyst, or a half metallocene catalyst. By containing the propylene-ethylene random copolymer (B) in the outer layer, a multilayer film having excellent transparency can be obtained.

The propylene-ethylene random copolymer (B) may be one that has a melting onset temperature of 140°C or higher and a peak melting temperature of 145°C or higher when measured by differential scanning calorimetry (JIS K 7121). One that has both a melting onset temperature and a peak melting temperature within this range has excellent heat resistance, and is unlikely to fuse on the inner surface of the package after, for example, a severe retort treatment at 135°C for 40 minutes.

The propylene-ethylene random copolymer (B) may be one in which the ratio ΔHh/ΔHl of the heat of fusion ΔHh on the higher side than a measurement temperature of 135°C to the heat of fusion ΔHl on the lower side is in the range of 1.5 to 2.5 when differential scanning calorimetry (JIS K 7121) is performed. By keeping the above ratio below the upper limit, the flexibility of the film is maintained, edge tearing of the heat-sealed portion after retort processing can be suppressed, and the heat-seal strength is less likely to decrease. The lower limit of the above ratio can be set to 1.5 from the viewpoint of preventing fusion from occurring on the inner surface of the package after retort processing.

The ethylene content of the propylene-ethylene random copolymer (B) can be 5% by mass or less. By having an ethylene content of the upper limit or less, the heat resistance is not excessively decreased while maintaining transparency, and it becomes easier to suppress fusion on the inner surface of the package after retort treatment. From this viewpoint, the ethylene content may be 4.5% by mass or less, or may be 4% by mass or less. The lower limit of the ethylene content is not particularly limited, but it can be 2% by mass from the viewpoints of maintaining the flexibility of the film, suppressing edge tearing at the heat seal portion after retort treatment, and making it difficult for the heat seal strength to decrease.

The ethylene content of the propylene-ethylene random copolymer (B) can be measured according to the ethylene content determination method (IR method) described on pages 412-413 of the Polymer Analysis Handbook (May 10, 2013, 3rd printing) edited by the Polymer Analysis Roundtable of the Japan Analytical Society.

The outer layer may contain 70 to 30 parts by mass of propylene homopolymer (A) and 30 to 70 parts by mass of propylene-ethylene random copolymer (B). When the content of propylene homopolymer (A) is 30 parts by mass or more, excellent heat resistance is easily maintained. Furthermore, when the content of propylene homopolymer (A) is 70 parts by mass or less, that is, when the content of propylene-ethylene random copolymer (B) is at least 30 parts by mass or more, excellent transparency and heat sealability are easily achieved. From these viewpoints, the outer layer may contain 60 to 40 parts by mass of propylene homopolymer (A) and 40 to 60 parts by mass of propylene-ethylene random copolymer (B). From a similar viewpoint, the second outer layer may contain 70 to 30% by mass of propylene homopolymer (A) and 30 to 70% by mass of propylene-ethylene random copolymer (B), or may contain 60 to 40% by mass of propylene homopolymer (A) and 40 to 60% by mass of propylene-ethylene random copolymer (B).

(Nucleating Agent)
Nucleating agents (or nucleating agents, crystal nucleating agents) are additives for promoting the crystallization of polymer compounds, and are agents for adjusting the physical properties of polymer compounds by adding a small amount to the polymer compounds. Nucleating agents are not particularly limited as long as they exhibit such effects, and examples thereof include α crystal nucleating agents such as sorbitol compounds, organic phosphate metal salt compounds, organic carboxylate metal salt compounds, and rosin compounds, β crystal nucleating agents such as amide compounds and quinanacridone compounds, benzoic acid metal salts, alkyl fatty acid salts, and talc.

It is believed that by incorporating a nucleating agent into the first outer layer, which serves as the heat seal layer, the crystalline regions in the first outer layer increase, improving heat resistance. This makes it possible to suppress fusion on the inner surface of the package after retort processing. It is also believed that the nucleating agent refines the crystals in the first outer layer. This makes the crystals smaller and more uniform than the wavelength of visible light, suppressing diffuse reflection of visible light, and providing excellent heat resistance while maintaining transparency.

The content of the nucleating agent can be 1 to 5 parts by mass per 100 parts by mass of the total amount of the propylene homopolymer (A) and the propylene-ethylene random copolymer (B). When the content is equal to or greater than the lower limit, heat resistance is improved. On the other hand, when the content is equal to or less than the upper limit, the flexibility of the film is maintained, edge tearing of the heat-sealed portion after retort treatment can be suppressed, and excellent heat sealability can be obtained. In addition, costs can be suppressed. From this viewpoint, the content of the nucleating agent may be 1.5 to 4.5 parts by mass.

[Inner layer]
The inner layer contains a propylene-ethylene block copolymer (C) and an ethylene-propylene copolymer elastomer (D). It is desirable for the inner layer to have flexibility in order to obtain impact resistance that does not cause the bag to break in a low-temperature environment.

(Propylene-Ethylene Block Copolymer (C))
The propylene-ethylene block copolymer (C) is a copolymer that can be obtained by producing a propylene polymer (C1) in a first step, and then producing an ethylene-propylene copolymer (C2) by gas phase polymerization in a second step. The propylene-ethylene block copolymer (C) is not a block copolymer in which a propylene polymer end and an ethylene-propylene copolymer end are bonded, but is a type of blend copolymer. By containing the propylene-ethylene block copolymer (C) in the inner layer, the flexibility of the film is maintained, and excellent cold impact resistance is easily obtained.

As the propylene-ethylene block copolymer (C), one having a melt flow rate (MFR: ISO 1133) (temperature 230°C, load 2.16 kg) in the range of 0.5 to 2.5 g/10 min can be used. If the melt flow rate is too high, the impact resistance of the film is likely to decrease, and if it is too low, the load on the extruder during molding processing increases, the processing speed decreases, and productivity is likely to decrease. From these viewpoints, the melt flow rate can be 0.5 to 2.5 g/10 min, and may be 1.0 to 2.0 g/10 min.

The propylene-ethylene block copolymer (C) may contain 90 to 60% by mass of the above propylene polymer (C1) and 10 to 40% by mass of the ethylene-propylene copolymer (C2). By having each component in this range, it is easy to obtain excellent cold impact resistance.

The ethylene content of the ethylene-propylene copolymer (C2) contained in the propylene-ethylene block copolymer (C) is not particularly limited, but can be 20 to 40% by mass. By having the ethylene content be equal to or less than the upper limit, the tackiness of the product can be suppressed, and contamination due to the tackiness of the product during production is unlikely to occur, making it easier to maintain excellent productivity. By having the ethylene content be equal to or more than the lower limit, the flexibility of the film is maintained, making it easier to obtain excellent cold impact resistance.

(Ethylene-propylene copolymer elastomer (D))
The ethylene-propylene copolymer elastomer (D) can be obtained by, for example, a slurry polymerization method carried out in the presence of an inert hydrocarbon such as hexane, heptane, kerosene, or a liquefied α-olefin solvent such as propylene, or a gas phase polymerization method in the absence of a solvent. Specifically, the ethylene-propylene copolymer elastomer (D) can be obtained by a known multi-stage polymerization method. That is, it is a polymerized polypropylene-based resin with a high rubber content, which can be obtained by polymerizing propylene and/or propylene-α-olefin polymer in a first-stage reaction and then copolymerizing propylene with an α-olefin in a second-stage reaction. By containing the ethylene-propylene copolymer elastomer (D) in the inner layer, flexibility can be easily imparted to the film, and excellent cold impact resistance can be easily obtained.

The ethylene-propylene copolymer elastomer (D) may have a melt flow rate (MFR: ISO 1133) (temperature 230°C, load 2.16 kg) in the range of 0.5 to 3.5 g/10 min. When the melt flow rate is equal to or higher than the lower limit, the load on the extruder during molding is small, the processing speed is unlikely to decrease, and excellent productivity can be easily maintained. When the melt flow rate is equal to or lower than the upper limit, the compatibility between the propylene-ethylene block copolymer (C) and the ethylene-propylene copolymer elastomer (D) is good, and transparency is unlikely to decrease.

The ethylene-propylene copolymer elastomer (D) may have a ratio of propylene content to ethylene content (propylene content/ethylene content) in the range of 1.5 to 4. When the ratio is equal to or greater than the lower limit, the flexibility of the film is maintained and excellent cold impact resistance is easily obtained. When the ratio is equal to or less than the upper limit, the compatibility between the propylene-ethylene block copolymer (C) and the ethylene-propylene copolymer elastomer (D) is good and transparency is less likely to decrease.

The inner layer may contain 90 to 50% by mass of propylene-ethylene block copolymer (C) and 10 to 50% by mass of ethylene-propylene copolymer elastomer (D). When the content of propylene-ethylene block copolymer (C) is 50% by mass or more, excellent cold impact resistance is easily maintained. Furthermore, when the content of propylene-ethylene block copolymer (C) is 90% by mass or less, that is, when the content of ethylene-propylene copolymer elastomer (D) is at least 10% by mass or more, even better heat sealability can be achieved. From these viewpoints, the inner layer may contain 80 to 60% by mass of propylene-ethylene block copolymer (C) and 20 to 40% by mass of ethylene-propylene copolymer elastomer (D).

There are no particular limitations on the thickness of the multilayer film, so long as it is within a range that allows it to be used as a film for packaging materials, for example, but if the film is too thick, it will have a cost disadvantage. For this reason, the thickness of the multilayer film can be 100 μm or less, and may be 50 to 70 μm.

The thickness of the outer layer (i.e., the total thickness of the first outer layer and the second outer layer) may be 16 to 42% or 25 to 42% based on the thickness of the multilayer film. When the ratio of the thickness of the outer layer is equal to or greater than the lower limit, excellent transparency is easily obtained, and when the ratio is equal to or less than the upper limit, deterioration of the impact resistance and heat sealability of the film can be suppressed, making it easier to obtain practical use.

The thickness of the inner layer can be 20 μm or more, and may be 30 μm or more. This maintains the flexibility of the film, makes the film less likely to break after retort processing, and reduces the heat seal strength.

<Method of manufacturing multi-layer film>
The method for producing the multilayer film is not particularly limited, and known methods can be used. For example, examples of the thermoforming process include a melt-kneading method using a general mixer such as a single-screw extruder, a twin-screw extruder, or a multi-screw extruder, and a method in which the components are dissolved or dispersed and mixed, and then the solvent is removed by heating. In consideration of workability, a single-screw extruder or a twin-screw extruder can be used. When a single-screw extruder is used, the screw can be a full-flight screw, a screw with a mixing element, a barrier flight screw, a fluted screw, or the like, which can be used without any particular restrictions. As the twin-screw kneading device, a co-rotating twin-screw extruder, a counter-rotating twin-screw extruder, or the like can be used, and the screw shape can be a full-flight screw, a kneading disk type, or the like, which can be used without any particular restrictions.

In the above method, it is possible to use a method in which the multilayer film is melted using a single-screw extruder or twin-screw extruder, etc., and then formed into a film using a T-die via a feed block or multi-manifold.

If necessary, the obtained multilayer film may be subjected to a surface modification treatment to improve suitability for subsequent processes. For example, to improve printability when used as a single film or lamination suitability when used in layers, a surface modification treatment may be performed on the printing surface or the surface that comes into contact with the substrate. Examples of surface modification treatments include treatments that generate functional groups by oxidizing the film surface, such as corona discharge treatment, plasma treatment, and frame treatment, and modification treatments using a wet process that forms an easy-adhesion layer by coating.

<Packaging materials>
The multilayer film may be used as a single film or may be laminated with a substrate, and the method of use as a packaging material is not particularly limited.

When the multilayer film is laminated with a substrate, the packaging material can include the multilayer film and the substrate. Specifically, such packaging material can be obtained by laminating at least one layer of a substrate such as a biaxially oriented polyamide film (ONy), a biaxially oriented polyester film (PET), a printed paper, a metal foil (AL foil), or a transparent vapor deposition film to the multilayer film 10 to form a laminate. FIG. 2 is a cross-sectional view of a packaging material according to one embodiment of the present invention. The packaging material 100 shown in the figure includes a multilayer film 10, an adhesive layer 3, a substrate film 4, an adhesive layer 5, and a transparent vapor deposition film 6 in this order. The laminate can be manufactured by a conventional dry lamination method in which the films constituting the laminate are bonded together using an adhesive, but if necessary, a method in which the multilayer film is directly extrusion laminated onto the substrate can also be employed.

The laminate structure of the laminate can be adjusted as appropriate according to the required properties of the packaging material, such as barrier properties that meet the shelf life of the packaged food, size and impact resistance that can accommodate the weight of the contents, visibility of the contents, etc.

<Packaging>
The packaging body may be made from the above packaging material, and there is no particular restriction on the form of the bag. For example, the above packaging material (laminate) can be used for a flat bag, a three-sided bag, a two-sided bag, a gusseted bag, a standing pouch, a pouch with a spout, a pouch with a beak, etc., using a multilayer film as a sealing material.

The present invention will be described in detail below using examples, but the present invention is not limited to the following examples.

<Preparing various materials>
The following propylene homopolymer (A), propylene-ethylene random copolymer (B), propylene-ethylene block copolymer (C), ethylene-propylene copolymer elastomer (D), and nucleating agent were prepared.

(Propylene homopolymer (A))
A propylene homopolymer having a melting onset temperature of 153°C, a melting peak temperature of 159°C, and a melt flow rate (MFR: ISO 1133) (temperature 230°C, load 2.16 kg) of 3.0 g/10 min when measured by differential scanning calorimetry (JIS K 7121).

(Propylene-Ethylene Random Copolymer (B))
A propylene-ethylene random copolymer having a melting onset temperature of 142° C., a melting peak temperature of 147° C., a ΔH _h /ΔH ₁ ratio of 1.84, and an ethylene content of 3.4 mass % when measured by differential scanning calorimetry (JIS K 7121).

The ethylene content was measured according to the ethylene content determination method (IR method) described on pages 412-413 of Polymer Analysis Handbook (May 10, 2013, 3rd printing), edited by the Polymer Analysis Roundtable, Japan Analytical Society.
ΔH _h /ΔH _l is the ratio of the heat of fusion ΔH _h on the higher side than a measurement temperature of 135° C. to the heat of fusion ΔH _l on the lower side when differential scanning calorimetry (JIS K 7121) is performed.

(Propylene-Ethylene Block Copolymer (C))
A propylene-ethylene block copolymer having a melt flow rate (MFR: ISO 1133) (temperature 230°C, load 2.16 kg) of 2.0 g/10 min, containing 77.1 mass % of a propylene polymer and 22.9 mass % of an ethylene-propylene copolymer, and the ethylene content in the ethylene-propylene copolymer is 28.7 wt %.

(Ethylene-propylene copolymer elastomer (D))
An ethylene-propylene copolymer elastomer having a melt flow rate (MFR: ISO 1133) (temperature 230° C., load 2.16 kg) of 0.6 g/10 min and a propylene content/ethylene content ratio of 2.7.

(Nucleating Agent)
Clearmaster H8020 (sorbitol-based crystal nucleating agent) manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.
Clearmaster R8020 (sorbitol-based crystal nucleating agent) manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

<Preparation of Laminated Film>
Example 1
For the formation of the first outer layer, a resin mixture was prepared by mixing 50 parts by mass of propylene homopolymer (A), 50 parts by mass of propylene-ethylene random copolymer (B) and CLEARMASTER H8020 as a nucleating agent in a pellet state. The amount of the nucleating agent added was 2.0 parts by mass relative to a total of 100 parts by mass of the propylene homopolymer (A) and the propylene-ethylene random copolymer (B).
For forming the second outer layer, a resin mixture was prepared by mixing 50 parts by mass of a propylene homopolymer (A) and 50 parts by mass of a propylene-ethylene random copolymer (B) in the form of pellets.
For forming the inner layer, a resin mixture was prepared by mixing 67.8 parts by mass of the propylene-ethylene block copolymer (C) and 32.2 parts by mass of the ethylene-propylene copolymer (D) in a pellet state.
Each resin mixture was fed to an extruder whose temperature was controlled at 250°C, kneaded in a molten state, and laminated in a T-die extruder equipped with a feed block so that the first outer layer and second outer layer were each 10 μm thick, and the inner layer was 40 μm thick, to produce the film of Example 1.

Example 2
A film of Example 2 was produced in the same manner as in Example 1, except that the blending ratio of the nucleating agent was changed as shown in Table 1.

Example 3
A film of Example 3 was prepared in the same manner as in Example 1, except that Clear Master R8020 was used instead of Clear Master H8020.

Example 4
A film of Example 4 was produced in the same manner as in Example 3, except that the blending ratio of the nucleating agent was changed as shown in Table 1.

(Comparative Example 1)
A film of Comparative Example 1 was produced in the same manner as in Example 1, except that only the propylene homopolymer (A) was used to form the outer layer.

(Comparative Example 2)
A film of Comparative Example 2 was prepared in the same manner as in Example 1, except that only the propylene-ethylene random copolymer (B) was used to form the outer layer.

(Comparative Example 3)
A film of Comparative Example 3 was prepared in the same manner as in Example 1, except that no nucleating agent was used.

(Comparative Example 4)
A resin mixture obtained by mixing 67.8 parts by mass of the propylene-ethylene block copolymer (C) and 32.2 parts by mass of the ethylene-propylene copolymer (D) in a pellet state was used to prepare a film of Comparative Example 4 having a thickness of 60 μm.

(Comparative Example 5)
A film of Comparative Example 5 was produced in the same manner as in Example 1, except that the blending ratio of the nucleating agent was changed as shown in Table 1.

(Comparative Example 6)
A film of Comparative Example 6 was produced in the same manner as in Example 1, except that the blending ratio of the nucleating agent was changed as shown in Table 1.

<Various evaluations>
The films obtained in each example were subjected to the following evaluations, and the results are shown in Table 1.

[Haze measurement]
A bag of 297 mm x 420 mm was prepared using the film obtained in each example, 800 g of water was placed inside, and retort treatment was performed for 40 minutes at 135° C. A film sample of 50 mm x 50 mm was cut out from the bag after retort treatment, and evaluation was performed using a haze meter (model number COH7700) manufactured by Nippon Denshoku Industries Co., Ltd. in accordance with the haze measurement method described in JIS K7136.

[Cold impact resistance evaluation]
Using a film impact tester manufactured by Toyo Seiki Co., Ltd., the cold impact resistance of the films obtained in each example was evaluated under conditions of a temperature of -5°C, a weight of 1.5 J, and a bullet size of 1/2 inch.

[Welding strength measurement]
The films obtained in each example were heat-sealed together using a heat sealer manufactured by Tester Sangyo Co., Ltd. under the conditions of a sealing pressure of 0.03 MPa, a sealing time of 30 seconds, a sealing width of 10 mm, and a sealing temperature of 135° C. The heat-sealed film was then cut into a piece of 15 mm width×80 mm, and a tensile tester manufactured by Shimadzu Corporation was used to perform T-peel at a tensile speed of 300 mm/min to measure the fusion strength of the heat-sealed portion.

The polypropylene-based multilayer film of the present invention has high levels of heat resistance, transparency, and cold impact resistance, making it suitable for use as a sealant film for retort packaging.

10...multilayer film, 100...packaging material, 1a...first outer layer, 1b...second outer layer, 2...inner layer, 3...adhesive layer, 4...base film, 5...adhesive layer, 6...transparent vapor deposition film.

Claims (6)

a first outer layer which is a heat seal layer containing a propylene homopolymer (A), a propylene-ethylene random copolymer (B) and a nucleating agent;
an inner layer containing a propylene-ethylene block copolymer (C) and an ethylene-propylene copolymer elastomer (D);
a second outer layer containing a propylene homopolymer (A) and a propylene-ethylene random copolymer (B) in this order , and not containing a nucleating agent ;
a content of the nucleating agent in the first outer layer is 1 to 5 parts by mass relative to 100 parts by mass of the total amount of the propylene homopolymer (A) and the propylene-ethylene random copolymer (B). The multilayer film according to claim 1, wherein the first outer layer and the second outer layer contain 70 to 30 parts by mass of the propylene homopolymer (A) and 30 to 70 parts by mass of the propylene-ethylene random copolymer (B). The multilayer film according to claim 1 or 2, wherein the propylene homopolymer (A) satisfies the following requirement (a1), and the propylene-ethylene random copolymer (B) satisfies the following requirement (b1):
Requirement (a1): The melting onset temperature is 150° C. or higher and the melting peak temperature is 155° C. or higher, as measured by differential scanning calorimetry (JIS K 7121).
Requirement (b1): The ethylene content is 5% by mass or less. The multilayer film according to any one of claims 1 to 3, wherein the inner layer contains 90 to 50% by mass of the propylene-ethylene block copolymer (C) and 10 to 50% by mass of the ethylene-propylene copolymer elastomer (D). A packaging material comprising the multilayer film according to any one of claims 1 to 4 and a substrate. A package made from the packaging material according to claim 5.

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