JP7464034B2 - 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 bags, 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 three-layer polypropylene composite film, in which the middle layer is made of a propylene-ethylene block copolymer and both surface layers are made of a propylene random copolymer.

Patent Document 2 proposes a multilayer film consisting of two 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.

JP 2017-132186 A JP 2017-105174 A

Polypropylene-based multilayer films are now required to have heat resistance that can withstand retort treatment, which involves pressurizing at high temperatures of 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 achieve both excellent heat resistance and transparency.

The present invention has been made in consideration of the above circumstances, and aims to provide a polypropylene-based multilayer film that is capable of achieving both excellent heat resistance and transparency. Another aim of the present invention is to provide a packaging material and a package obtained using the multilayer film.

As a result of intensive research aimed at solving the above problems, the inventors discovered that it is important to incorporate a predetermined amount of a mixture of different propylene-based polymers into the outer layer of a polypropylene-based multilayer film, and thus completed the present invention.

The multilayer film according to one aspect of the present invention comprises, in this order, a first outer layer containing 70 to 30% by mass of a propylene homopolymer (A) and 30 to 70% by mass of a propylene-ethylene random copolymer (B) having an ethylene content of 5% by mass or less, an inner layer containing a propylene-ethylene block copolymer (C) and an ethylene-propylene copolymer elastomer (D), and a second outer layer containing 70 to 30% by mass of a propylene homopolymer (A) and 30 to 70% by mass of a propylene-ethylene random copolymer (B) having an ethylene content of 5% by mass or less.

In the above multilayer film, the outer layer contains a propylene homopolymer (A) and a propylene-ethylene random copolymer (B) having a specified ethylene content in a specific ratio. This makes it possible to maintain heat resistance while suppressing surface irregularities, which are a factor in reducing the transparency of the film. This effect cannot be obtained when a propylene-based random copolymer is used in the outer layer (for example, Patent Document 1 above) or when an ethylene-α-olefin copolymer and a propylene-based polymer are used (for example, Patent Document 2 above), and is particularly suitable for retort treatment applications.

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, suppresses edge tearing of the heat-sealed portion after retort processing, and makes it easier to obtain excellent heat sealability.

In one embodiment, the total thickness of the first outer layer and the second outer layer may be 16 to 42% of the thickness of the multilayer film. This makes it easier to achieve both transparency and heat sealability.

In one embodiment, the thickness of the inner layer may be 20 μm or more. This makes it easier to maintain the flexibility of the film.

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 that is capable of achieving both excellent heat resistance and transparency. In addition, according to the present invention, it is possible to provide 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 diagram showing the total heat of fusion of the propylene-ethylene random copolymer (B) used in Example 1 and the results of dividing the heat of fusion at 135° C. FIG. 3 is a diagram showing the total heat of fusion of the propylene-ethylene random copolymer (E) used in Comparative Example 3 and the results of dividing the heat of fusion at 135° C.

<Multilayer 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 and the second outer layer contain a propylene homopolymer (A) and a propylene-ethylene random copolymer (B). The first outer layer and the second outer layer may be collectively referred to simply as the outer layer. The first outer layer and the second outer layer may have the same composition or different compositions.

(Propylene homopolymer (A))
The propylene homopolymer (A) can be obtained by homopolymerizing propylene using, for example, a Ziegler-Natta catalyst or a metallocene catalyst. By including the propylene homopolymer (A) in the outer layer, excellent heat resistance can be imparted to the outer layer.

The propylene homopolymer (A) may be one that has 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). One that has 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 a packaging bag, for example, after 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. 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 less likely to decrease, and excellent productivity can be easily maintained. In addition, when the melt flow rate is equal to or lower than the upper limit, the outer layer is likely to have excellent impact resistance.

(Propylene-Ethylene Random Copolymer (B))
The propylene-ethylene random copolymer (B) can be obtained by copolymerizing ethylene as a comonomer into the main monomer consisting of propylene using, for example, a Ziegler-Natta catalyst or a 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 less likely to fuse on the inner surface of a packaging bag, for example, after retort treatment at high temperatures.

The propylene-ethylene random copolymer (B) may have a ratio ΔH h /ΔH _l of the heat of fusion ΔH _h on the higher side than 135° C. to the heat of fusion ΔH _l on the lower side in differential scanning calorimetry (JIS _K 7121) of 1.5 to 2.5. When the ratio 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 the heat-seal strength is less likely to decrease. The lower limit of the ratio can be set to 1.5 from the viewpoint of preventing fusion on the inner surface of the packaging bag after retort treatment.

The ethylene content of the propylene-ethylene random copolymer (B) is 5% by mass or less. By having an ethylene content of less than the upper limit, heat resistance is not excessively decreased while maintaining transparency, and fusion on the inner surface of the packaging bag after retort treatment can be suppressed. 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 may be 2% by mass from the viewpoints of maintaining the flexibility of the film, suppressing edge tearing at the heat-sealed 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 contains 70 to 30% by mass of propylene homopolymer (A) and 30 to 70% by mass of propylene-ethylene random copolymer (B) having an ethylene content of 5% by mass or less. When the content of propylene homopolymer (A) is 30% by mass or more, excellent heat resistance can be maintained. From this viewpoint, the content may be 35% by mass or more, or 40% by mass or more. When the content of propylene homopolymer (A) is 70% by mass or less, that is, when the content of propylene-ethylene random copolymer (B) having an ethylene content of 5% by mass or less is at least 30% by mass or more, excellent transparency and heat sealability can be exhibited. From this viewpoint, the content of propylene homopolymer (A) may be 65% by mass or less, or 60% by mass or less. From the above viewpoint, the content of propylene-ethylene random copolymer (B) having an ethylene content of 5% by mass or less may be 35 to 65% by mass, or 40 to 60% by mass.

[Inner layer]
The inner layer contains a propylene-ethylene block copolymer (C) and an ethylene-propylene copolymer elastomer (D).

(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, edge tearing of the heat-sealed portion after retort treatment can be suppressed, and excellent heat sealability can be 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. 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 less likely to decrease, and excellent productivity can be easily maintained. When the melt flow rate is equal to or lower than the upper limit, the inner layer is likely to have excellent flexibility, edge cutting in the heat-sealed portion can be suppressed, and excellent heat sealability can be easily obtained.

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 heat sealability.

The ethylene content of the ethylene-propylene copolymer (C2) contained in the propylene-ethylene block copolymer (C) is not particularly limited, but can be in the range of 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 less likely to occur, making it easier to maintain excellent productivity. By having the ethylene content be equal to or greater than the lower limit, the flexibility of the film can be maintained, edge tearing of the heat-sealed portion after retort processing can be suppressed, making it easier to obtain excellent heat-sealability.

(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 type high rubber content polypropylene-based resin that can be obtained by polymerizing propylene and/or propylene-α-olefin polymer in a first-stage reactor and then copolymerizing propylene and 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, edge cutting of the heat-sealed portion can be suppressed, and excellent heat sealability 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, edge tearing of the heat-sealed portion after retort processing can be suppressed, and excellent heat sealability can be 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, it is easy to maintain excellent heat sealability. From this viewpoint, the content may be 60% by mass or more, or 70% by mass or more. 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, it is possible to develop even more excellent heat sealability. From this viewpoint, the content of propylene-ethylene block copolymer (C) may be 87.5% by mass or less, or 85% by mass or less. From the above viewpoint, the content of ethylene-propylene copolymer elastomer (D) may be 12.5 to 40% by mass, or 15 to 30% by mass.

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% based on the thickness of the multilayer film. When the thickness ratio of the outer layer is equal to or greater than the lower limit, excellent transparency is easily obtained, and when the thickness ratio is equal to or less than the upper limit, deterioration of the heat sealability of the film can be suppressed, making it easier to obtain practical use. From this perspective, the thickness ratio of the outer layer may be 20 to 35%.

The thickness of the inner layer may be 20 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. From this perspective, the thickness of the inner layer may be 25 μm or more, or may be 30 μm or more. There is no particular upper limit to the thickness of the inner layer, but it can be set to 50 μm because it would be cost disadvantageous.

<Characteristics of multi-layer film>
The surface roughness of the multilayer film, i.e., the arithmetic mean roughness Ra (JIS B 0601-2001) of the outer layer constituting the multilayer film, may be 0.05 to 0.15 μm. When Ra is equal to or greater than the lower limit, the multilayer film tends to have good sliding properties with respect to each other, and excellent processability is easily maintained. When Ra is equal to or less than the upper limit, diffuse reflection due to unevenness on the film surface is unlikely to occur, and excellent transparency is easily obtained.

The haze value of a 60 μm-thick multilayer film measured in accordance with JIS K 7136 may be 15% or less. A haze value of 15% or less means that visibility of the contents is easily maintained.

In a fusion test conducted in accordance with the following, the fusion strength of the multilayer film may be 2.0 N/15 mm or less. A fusion strength of 2.0 N/15 mm or less means that the films are unlikely to fuse together when subjected to retort treatment at a high temperature of 135°C.

[Fusion test]
The multilayer films are 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 multilayer film is then cut into a piece of 15 mm width×80 mm, and a tensile tester manufactured by Shimadzu Corporation is used to perform T-peel at a tensile speed of 300 mm/min to measure the fusion strength of the heat-sealed portion.

<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 to form a laminate. The laminate can be preferably manufactured by a normal 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 palm-shaped 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.

<Preparation of Laminated Film>
Example 1
The following propylene homopolymer (A), propylene-ethylene random copolymer (B), propylene-ethylene block copolymer (C), and ethylene-propylene copolymer elastomer (D) 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 of the measurement temperature than 135° C. to the heat of fusion ΔH _l on the lower side when differential scanning calorimetry (JIS K 7121) is performed. Fig. 2 is a diagram showing the total heat of fusion of the propylene-ethylene random copolymer (B) and the results of dividing the heat of fusion at 135° C.

(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.

For the formation of the outer layer, a resin mixture in which 70% by mass of propylene homopolymer (A) and 30% by mass of propylene-ethylene random copolymer (B) were mixed in a pellet state was used, and for the formation of the inner layer, a resin mixture in which 83% by mass of propylene-ethylene block copolymer (C) and 17% by mass of ethylene-propylene copolymer elastomer (D) were mixed in a pellet state was used. Each raw material was fed to an extruder whose temperature was adjusted to 250°C, kneaded in a molten state, and laminated in a T-die extruder with a feed block so that the first outer layer and the 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 mixing ratio of the propylene homopolymer (A) and the propylene-ethylene random copolymer (B) was changed as shown in Table 1.

Example 3
The film of Example 3 was produced in the same manner as in Example 1, except that the mixing ratio of the propylene homopolymer (A) and the propylene-ethylene random copolymer (B) was changed as shown in Table 1.

Example 4
The film of Example 4 was produced in the same manner as in Example 2, except that the thicknesses of the outer layer and the inner layer were changed as shown in Table 1.

Example 5
The film of Example 5 was produced in the same manner as in Example 2, except that the mixing ratio of the propylene-ethylene block copolymer (C) and the ethylene-propylene copolymer elastomer (D) was changed as shown in Table 1.

Example 6
The film of Example 6 was produced in the same manner as in Example 2, except that the mixing ratio of the propylene-ethylene block copolymer (C) and the ethylene-propylene copolymer elastomer (D) 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 the outer layer was formed using only the propylene-ethylene random copolymer (B).

(Comparative Example 2)
The film of Comparative Example 2 was produced in the same manner as in Example 1, except that the propylene-ethylene random copolymer (B) was replaced with the following propylene-ethylene random copolymer (E), and the mixing ratio of the propylene homopolymer (A) and the propylene-ethylene random copolymer (E) was changed as shown in Table 1.
(Propylene-Ethylene Random Copolymer (E))
A propylene-ethylene random copolymer having a melting onset temperature of 122° C., a melting peak temperature of 133° C., a ΔH _h /ΔH ₁ ratio of 0.275, and an ethylene content of 5.8% by mass, as measured by differential scanning calorimetry (JIS K 7121).

Figure 3 shows the total heat of fusion of propylene-ethylene random copolymer (E) and the results of dividing the heat of fusion at 135°C.

(Comparative Example 3)
A film of Comparative Example 3 was produced in the same manner as in Example 1, except that the propylene-ethylene random copolymer (B) was replaced with the following elastomer resin, and the mixing ratio of the propylene homopolymer (A) and the elastomer resin was changed as shown in Table 1.
(Elastomer resin)
An olefin-based elastomer resin using a metallocene catalyst, with ethylene as the main monomer and butene-1 as the comonomer, and having a melt flow rate (MFR: ISO 1133) (temperature 190°C, load 2.16 kg) of 3.6 g/10 min.

(Comparative Example 4)
A resin mixture obtained by mixing 83% by mass of the propylene-ethylene block copolymer (C) and 17% by mass of the ethylene-propylene copolymer elastomer (D) in a pellet state was used to produce a film of Comparative Example 4 having a thickness of 60 μm.

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

[Surface roughness measurement]
The surface roughness of the film obtained in each example (surface roughness of the outer layer) was measured. A small surface roughness tester, Surf Test, manufactured by Mitutoyo Corporation, was used as the measuring device. The arithmetic mean roughness Ra (JIS B 0601-2001) was measured with a cutoff value λc of 0.25.

[Haze measurement]
According to the haze measurement method described in JIS K7136, the evaluation was carried out using a haze meter (model number HM-150) manufactured by Murakami Color Research Laboratory.

[Heat sealability evaluation]
A biaxially oriented polyester film (PET) having a thickness of 12 μm, a biaxially oriented polyamide film (ONy) having a thickness of 15 μm, an AL foil having a thickness of 9 μm, and the film obtained in each example (polypropylene-based film) were bonded together by a normal dry lamination method using a urethane-based adhesive to form a laminate having the following configuration.
Laminate structure: PET/adhesive/ONy/adhesive/AL foil/adhesive/polypropylene film The polypropylene films of this laminate were placed facing each other and heat sealed using a heat sealer manufactured by Tester Sangyo Co., Ltd. under the conditions of a sealing pressure of 0.2 MPa, a sealing time of 1 second, a sealing width of 5 mm, and a sealing temperature of 200° C. Then, a retort treatment was performed for 40 minutes at 135° C. The film that had been subjected to the retort treatment was cut into a size of 15 mm wide x 80 mm, and a T-shaped peel was performed at a tensile speed of 300 mm/min using a tensile tester manufactured by Shimadzu Corporation to measure the heat seal strength.

[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 combines high levels of heat resistance and transparency, making it suitable for use as a sealant film for retort packaging.

10...polypropylene-based multilayer film, 1a...first outer layer, 1b...second outer layer, 2...inner layer.

Claims (6)

a first outer layer containing a propylene homopolymer (A ) and a propylene-ethylene random copolymer (B ) having an ethylene content of 5% by mass or less;
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 ) having an ethylene content of 5% by mass or less, in this order;
In the propylene-ethylene random copolymer (B), when differential scanning calorimetry (JIS K 7121) is performed, the ratio ΔH h /ΔH l of the heat of fusion ΔH h on the higher side than 135° C. _to the _heat of fusion _{ΔH l} on the lower side is in the range of 1.5 to 2.5.
Multilayer film. The multilayer film according to claim 1, 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). The multilayer film according to claim 1 or 2, wherein the total thickness of the first outer layer and the second outer layer is 16 to 42% of the thickness of the multilayer film. The multilayer film according to any one of claims 1 to 3, wherein the thickness of the inner layer is 20 μm or more. 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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