JP7697244B2 - Heat-resistant sealant film - Google Patents

Description

The present invention relates to a heat-resistant sealant film that has excellent tearability in the machine direction (MD) of the film, and a food packaging film using the same.

In recent years, with the spread of the individual meal culture and the increase in convenience stores, the performance requirements for food packaging bags are also changing with the times. For example, food packaging bags are required to withstand high-temperature heat sterilization to extend the life of the food, and to have strength suitability over a wide range of temperatures, such as when the food is heated in a microwave oven or retort after being distributed at low temperatures in the frozen or chilled state. Since such foods are heated after being refrigerated or transported and stored in the frozen state, the packaging bags are required to be both cold-resistant and heat-resistant. Furthermore, there is also the problem that opening the bag at a high temperature after cooking by boiling or using a microwave oven makes it difficult to open by hand, as the film tends to stretch.
Additionally, stand-up pouch packaging designs are being adopted to improve visibility at storefronts and maintain shape during cooking, requiring the constituent film to be highly rigid.
Conventionally, polypropylene-based resins with high melting points have often been used in the sealing layer of packaging bags compatible with microwave ovens and retorts. However, polypropylene does not have cold resistance, and there is a problem that the bag breaks during transportation. On the other hand, polyethylene, which is often used in sealant films, is known to have excellent cold resistance (Patent Document 1), but has a lower melting point and poorer heat resistance than polypropylene, and there is a problem that the films fuse together after retort.
Furthermore, as the population is rapidly aging and the birthrate is declining, there is a growing demand for barrier-free packaging, such as easy-to-open bags.

Patent Publication No. 2020-15804

In view of the problems with the prior art described above, the object of the present invention is to provide a heat-resistant sealant film that is heat-resistant and has excellent tearability in the machine direction (MD) of the film, and a food packaging film using the same.

As a result of extensive research into solving the above problems, the inventors discovered that the above problems could be solved by using a linear medium-density polyethylene resin with specific physical properties that was recently newly manufactured and developed, and further using a film with a specific resin layer configuration in a multilayer film having an innermost layer and other layers, and based on these findings, they have completed the present invention.

That is, according to the first aspect of the present invention, there is provided a heat-resistant sealant film which is a multilayer film having at least an innermost layer and other layers, the innermost layer containing 65 to 100% by weight of (a-1) linear medium-density polyethylene (A) having a density of 0.925 to 0.940 g/cm ³ and (a-2) MFR of 0.1 to 20 g/10 min, the linear medium-density polyethylene (A) being copolymerized with ethylene and an α-olefin having 6 or more carbon atoms, and a layer in contact with the innermost layer containing (b-1) linear low-density polyethylene (B) having a density of 0.910 to 0.950 g/cm ³ and (b-2) MFR of 0.1 to 15 g/10 min, the linear low-density polyethylene being copolymerized with ethylene and an α-olefin having 4 carbon atoms.
According to a second invention, there is provided the heat-resistant sealant film according to the first invention, characterized in that the linear medium-density polyethylene (A) is an ethylene/α-olefin copolymer further having the following physical properties (a-3) and (a-4).
(a-3) Molecular weight distribution (Mw/Mn) of 1.8 to 3.5
(a-4) According to the third invention, in which there are multiple peaks in the elution temperature-elution amount curve measured by continuous temperature rising elution fractionation (TREF), there is provided the heat-resistant sealant film according to the first or second invention, characterized in that the tear strength in the machine direction (MD) of the film is 20 N/mm or less in an Elmendorf tear test measured in accordance with JIS K7128-2.
According to a fourth invention, there is provided a heat-resistant sealant film according to any one of the first to third inventions, characterized in that in an Elmendorf tear test measured in accordance with JIS K7128-2, the ratio of tear strength (S _MD ) in the machine direction (MD) of the film to tear strength (S _TD ) in the perpendicular direction (TD) is S _MD /S _TD < 0.5.
According to a fifth aspect of the present invention, there is provided a heat-resistant sealant film according to any one of the first to fourth aspects of the present invention, characterized in that at least one base layer selected from PET, Ny, OPP, and AL is laminated on the layer opposite the innermost layer.
According to the sixth invention, there is provided a heat-resistant sealant film according to any one of the first to fifth inventions, in which the innermost layers are adhered to each other and, after heating and pressurizing at 115°C, no fusion between the innermost layers is observed.
According to a seventh aspect of the present invention, there is provided a food packaging film using the heat-resistant sealant film according to any one of the first to sixth aspects of the present invention.

The heat-resistant sealant film of the present invention has excellent tearability in the machine direction (MD) of the film, and is characterized by the fact that no fusion between films is observed after heating and pressurizing at 115°C, and that the tearability is maintained. Therefore, by using this heat-resistant sealant film, it is possible to provide a food packaging film or a medical packaging film that is easy to open even after heating and pressurizing sterilization or retort processing.

The present invention is a heat-resistant sealant film comprising an innermost layer made of linear medium density polyethylene (A) and a layer in contact with the innermost layer made of C4LLDPE (B). The components constituting each ethylene-based copolymer composition, each ethylene-based copolymer composition, their properties, and food packaging films using them are described in detail below.

(1) Linear medium density polyethylene (A)
The linear medium-density polyethylene (A) of the present invention is specifically an ethylene-α-olefin copolymer having a medium density and a linear molecular structure, which is obtained by copolymerizing ethylene with an α-olefin having 6 to 18 carbon atoms by a catalytic polymerization method.
Examples of α-olefins having 6 to 18 carbon atoms include 1-hexene, 4-methyl-1-hexene, 1-heptene, 1-octene, 1-decene, and 1-octadecene. Among these, α-olefins having 6 to 12 carbon atoms are preferred, and those having 6 to 10 carbon atoms, such as 1-hexene, 4-methyl-1-hexene, 1-heptene, 1-octene, and 1-decene, are particularly preferred.

The α-olefin content in the ethylene/α-olefin copolymer is preferably 3 to 24% by weight, more preferably 5 to 20% by weight, and even more preferably 7 to 15% by weight. If the α-olefin content is less than 3% by weight, the film tends to have poor pinhole resistance.

Furthermore, the linear medium density polyethylene of component (A) in the present invention must satisfy the following properties (a-1) and (a-2).
(a-1) Density The density of component (A) is 0.925 to 0.940 g/cm ³ , preferably 0.928 to 0.940 g/cm ³ , and more preferably more than 0.930 g/cm ³ to 0.938 g/cm ^3. Here, the density is a value measured in accordance with Method D (density gradient tube method) of JIS K7112-1999 "Method of measuring density and specific gravity of plastics-nonfoamed plastics."

(a-2) Melt flow rate (MFR)
The MFR of component (A) is 0.1 to 20 g/10 min, preferably 0.3 to 15 g/10 min, and more preferably 0.5 to 10 g/10 min. If the MFR is less than 0.1 g/10 min, the resin pressure increases during molding into a film, resulting in poor processability, whereas if the MFR exceeds 20 g/10 min, the mechanical strength required for a packaging film and processability such as bubble stability during film molding are poor.
Here, MFR is a value measured in accordance with JIS K7210-1999 "Test method for melt mass flow rate (MFR) and melt volume flow rate (MVR) of plastics - thermoplastic plastics" under test conditions of 190°C and a load of 21.18N (2.16kg).

The linear medium-density polyethylene (A) used in the present invention is preferably one copolymerized in the presence of a Kaminsky type catalyst rather than one copolymerized in the presence of a Ziegler-Natta type catalyst, a Phillips type catalyst or the like.
Ethylene/α-olefin copolymers produced using a Kaminsky type catalyst are described, for example, in JP-A-58-19309, JP-A-59-95292, JP-A-60-35005, JP-A-60-35006, JP-A-60-35007, JP-A-60-35008, JP-A-60-35009, JP-A-61-130314, and JP-A-3-163088, European Patent Publication No. 420436, and U.S. Pat. No. 505,543 The polyolefin resin can be produced by using a metallocene catalyst, particularly a metallocene-alumoxane catalyst, as described in, for example, International Publication WO 91/04257, or by using a catalyst comprising a metallocene compound and a compound which reacts with the metallocene compound to form a stable anion, as described, for example, in International Publication WO 92/07123, and by a polymerization method such as a gas phase method, a slurry method, a solution method, or a high-pressure ionic polymerization method.

Among them, the ethylene/α-olefin copolymer in the present invention is preferably one polymerized using a catalyst in which the metallocene compound is a tetravalent transition metal compound such as titanium, zirconium, nickel, palladium, hafnium, or platinum, having a mono-, di-, or tri-cyclopentadienyl ring or a substituted cyclopentadienyl ring as a ligand, and in particular, one polymerized using a catalyst in which the metallocene compound is a transition metal compound having a hafnium compound as a central metal.
More preferably, ethylene/α-olefin copolymers produced using a special catalyst, such as those described in Japanese Patent No. 3,539,801, are used.
The linear medium-density polyethylene (A) is preferably an ethylene/α-olefin copolymer having the following physical property (a-3).
(a-3) Molecular weight distribution (Mw/Mn) of 1.8 to 3.5
(a-4) The copolymer satisfies (a-3) and (a-4) above has a narrower molecular weight distribution than a polymer obtained using a normal Ziegler-Natta type catalyst, and in addition, has a plurality of peaks in the elution temperature-elution amount curve, whereas a copolymer obtained using a typical metallocene catalyst usually has one peak. The catalyst used in the production of the ethylene/α-olefin copolymer, the production method thereof, and the method for measuring the molecular weight distribution and TREF are as described in Japanese Patent No. 3,539,801, etc.

In recent years, the applicant has succeeded in using the above catalyst species to produce linear medium density polyethylene having the above physical properties in the medium density region, a region whose production control is considered difficult.
The linear medium-density polyethylene (A) used in the present invention is available, for example, as a new medium-density resin in the "Harmolex" (trade name) series by the present applicant (Japan Polyethylene Co., Ltd.) Such a copolymer is a polyethylene resin which has a narrower molecular weight distribution than polymers obtained with normal Ziegler-Natta type catalysts, and in addition has strength and high rigidity over a wide temperature range, and is optimized in terms of basic qualities required for polyethylene films, such as heat sealability, impact strength, low odor, and low fisheyes.

(2) Linear low-density polyethylene (B)
The linear low density polyethylene (B) of the present invention is specifically an ethylene/α-olefin copolymer obtained by copolymerizing ethylene with an α-olefin having 4 carbon atoms by catalytic polymerization. Hereinafter, it is also referred to as C4LLDPE.
By using, as the linear low-density polyethylene (B), an ethylene-α-olefin in which the comonomer species is limited to an α-olefin having 4 carbon atoms, it is possible to obtain a heat-resistant sealant film that is excellent in ease of tearing in the MD direction while satisfying heat resistance, which is preferable.
The content of the α-olefin having 4 carbon atoms (referred to as C4) in the linear low density polyethylene (B) is preferably 3 to 24% by weight, more preferably 5 to 20% by weight, and even more preferably 7 to 15% by weight. If the α-olefin content is less than 3% by weight, the film tends to have poor pinhole resistance.

Furthermore, the linear low-density polyethylene (B) in the present invention preferably satisfies the following properties (b-1) and (b-2).
(b-1) Density The density of component (B) is 0.910 to 0.950 g/cm ³ , preferably 0.915 to 0.940 g/cm ^3. More preferably, it is 0.915 to 0.938 g/cm ³ or less. Here, the density is a value measured in accordance with Method D (density gradient tube method) of JIS K7112-1999 "Method of measuring density and specific gravity of plastics-non-foamed plastics."

(b-2) Melt flow rate (MFR)
The MFR of the linear low density polyethylene (B) is 0.1 to 15 g/10 min, preferably 0.3 to 10 g/10 min. If the MFR is less than 0.1 g/10 min, the resin pressure increases during molding into a film, resulting in poor processability, whereas if the MFR exceeds 15 g/10 min, the mechanical strength required for a packaging film and the processability such as bubble stability during film molding are poor.
Here, MFR is a value measured in accordance with JIS K7210-1999 "Test method for melt mass flow rate (MFR) and melt volume flow rate (MVR) of plastics - thermoplastic plastics" under test conditions of 190°C and a load of 21.18N (2.16kg).

The linear low-density polyethylene (B) used in the present invention may be selected from known linear low-density polyethylenes such as those copolymerized in the presence of a Ziegler-Natta catalyst. Among them, a linear low-density polyethylene that contains a C4 component as an essential component may be selected. For example, a resin that contains a C4 component as an essential component may be selected from "Novatec LL" (trademark) manufactured by Japan Polyethylene Corporation.

(3) Other Additives The ethylene copolymer composition constituting each layer of the present invention may contain, if necessary, other resins or rubbers, as well as various additives commonly used in thermoplastic resins, such as heat stabilizers, light stabilizers, UV absorbers, nucleating agents, neutralizing agents, lubricants, antistatic agents, antiblocking agents, slip agents, antifogging agents, dispersants, flow improvers, release agents, adhesion promoters, flame retardants, colorants, fillers, etc., within the scope of not impairing the effects of the present invention. These components may be contained in each component, or may be blended during the production of the ethylene copolymer composition.

(4) Film Molding The method for producing the film is not particularly limited, and may be a common molding method such as a multi-layer inflation molding method, in which a resin molten in an extruder is joined at the tip of a multi-layer die to form a laminated structure, a co-extrusion molding method such as a multi-layer T-die molding method, or a multi-layer blow molding method.

(5) Structure of the multilayer film The heat-resistant sealant film of the present invention is composed of at least two layers, an innermost layer and a layer in contact with the innermost layer. The innermost layer is a layer located on the surface of the multilayer film, and when the film is used to form a bag or the like, the layer located on the inside is called the innermost layer.
In the multi-layer structure, any other layer having a barrier property or adhesive property may be provided between the innermost layer and the other layers, but a simple structure is preferable. The layer ratio between the innermost layer and the other layers is not particularly limited. The total thickness of the multi-layer film is 10 to 500 μm, preferably 20 to 200 μm.

(6) Innermost Layer The innermost layer of the present invention is characterized by containing 65 to 100% by weight of (a-1) linear medium-density polyethylene (A) having a density of 0.925 to 0.940 g/cm ³ and (a-2) MFR of 0.1 to 20 g/10 min, which is preferable for obtaining a film that has excellent tearability even after heat and pressure treatment. In particular, it is preferable for the innermost layer to contain 70 to 100% by weight of linear medium-density polyethylene (A). The preferable characteristics of the linear medium-density polyethylene (A) are as described above.

(7) Layer in Contact with Innermost Layer The layer in contact with the innermost layer of the present invention preferably contains 50 to 100% by weight of (b-1) C4LLDPE (B) having a density of 0.910 to 0.950 g/cm ³ and (b-2) MFR of 0.1 to 15 g/10 min.
This configuration makes it possible to impart easy tearability while maintaining the heat resistance required for a heat-resistant sealant film.

(8) Tear Properties The heat-resistant sealant film of the present invention is characterized by having a specific Elmendorf tear strength as a preferred property. That is, the Elmendorf tear strength measured in accordance with JIS K7128-2 is preferably 20 N/mm or less in the machine direction (MD) of the film. The lower limit is not particularly limited.
Furthermore, in an Elmendorf tear test measured in accordance with JIS K7128-2, it is preferable that the ratio of tear strength (S _MD ) in the machine direction (MD) of the film to tear strength (S _TD ) in the perpendicular direction (TD) of the film be S _MD /S _TD <0.5, since this indicates ease of tearing in the MD direction.

(9) Heat Resistance A preferred characteristic of the heat-resistant sealant film of the present invention is that the innermost layers do not fuse together even after heating and pressurizing at 115° C. The pressure and treatment time during this heating are not particularly limited, but for example, heating is performed at 115° C. for 30 minutes.

(10) Combination with other substrates The heat-resistant sealant film of the present invention may be used to form a pouch by disposing the sealant film on the content side of a substrate film having oxygen barrier properties. Specifically, a substrate layer selected from PET, Ny, OPP, and Al is laminated on the layer opposite to the side where the innermost layer of the heat-resistant sealant film is provided. By combining with a substrate having other functions, it becomes a suitable embodiment as a package for retort food or a package for long-life food.

The present invention will be explained in more detail below with examples and comparative examples, but the present invention is not limited to the following examples as long as it does not deviate from the gist of the invention. The test and evaluation methods and materials used in the examples are as follows.

1. Test and Evaluation Methods (1) Density: Measured in accordance with JIS K7112-1999 "Method for measuring density and specific gravity of plastics-non-foamed plastics" Method D (density gradient tube method).
(2) MFR: Measured in accordance with JIS K7210-1999 "Test method for melt mass flow rate (MFR) and melt volume flow rate (MVR) of plastics-thermoplastics" under test conditions of 190°C and a load of 21.18N (2.16kg).
(3) Elmendorf Tear Strength Measured according to JIS K7128-2 using the following device and conditions, where MD is the machine direction and TD is the transverse direction.
Equipment: Digital Elmendorf tear tester, model SA (manufactured by Toyo Seiki Seisakusho Co., Ltd.)
Measurement environment: temperature 23℃, humidity 50%
(4) Heat and pressure treatment Heat and pressure treatment was carried out using the following equipment and conditions: For the treatment, the heat-resistant sealant film was combined with a nylon film "EMBLEM (ONM)" manufactured by Unitika Ltd., and a pouch was prepared with an appropriate amount of water placed inside.
Equipment: High-temperature, high-pressure shower sterilization (disinfection) equipment YRF-40/50E (manufactured by Sakura SI Co., Ltd.)
Treatment conditions: 115° C.×30 minutes Heat resistance was evaluated according to the following criteria.
◯: No adhesion between the innermost layers is observed by visual inspection. ×: Adhesion between the innermost layers is observed by visual inspection.

2. Material (1) Component (A)
(i) A: Metallocene-based linear medium density polyethylene LL (A) having a density of 0.935 g/cm ³ , an MFR of 1.5 g/10 min, and an Mw/Mn of 3.42
(A polyethylene resin was used, which was produced using a metallocene catalyst under production conditions set so as to have a predetermined density and MFR range. This resin had multiple peaks in the elution temperature-elution amount curve measured by continuous temperature rising elution fractionation (TREF).)
(2) Component (B)
(B-1)
Linear low-density polyolefin LL having a density of 0.927 g/cm ³ and an MFR of 2.1 g/10 min ("UF641" manufactured by Japan Polyethylene Corporation)
(B-2)
Linear low-density polyolefin LL having a density of 0.938 g/cm ³ and an MFR of 2.1 g/10 min ("UF943" manufactured by Japan Polyethylene Corporation)
(3) AB Agent (Anti-Blocking Agent)
"Hermolex MBN560B" manufactured by Japan Polyethylene Corporation
(4) Slip agent: "Harmolex MBN560S" manufactured by Japan Polyethylene Corporation

Example 1
According to the formulation in Table 1, a three-layered sheet film having a total thickness of 50 μm was molded using a multi-layer T-die molding machine (die lip: 1 mm, die temperature: 220° C.). The thicknesses of the innermost layer, intermediate layer, and outermost layer were 10 μm: 30 μm: 10 μm. The Elmendorf tear strength and heat resistance of the obtained film were evaluated. The results are shown in Table 3.

Example 2
A film was prepared in the same manner as in Example 1 according to the formulation in Table 1. The results are shown in Table 3.

Example 3
A film was prepared in the same manner as in Example 1 according to the formulation in Table 1. The results are shown in Table 3.

Example 4
A film was prepared in the same manner as in Example 1 according to the formulation in Table 1. The results are shown in Table 3.

(Comparative Example 1)
A film was prepared in the same manner as in Example 1 according to the formulation in Table 2. The results are shown in Table 3.

(Comparative Example 2)
A film was prepared in the same manner as in Example 1 according to the formulation in Table 2. The results are shown in Table 3.

From Table 3, it can be seen that, compared to Comparative Examples 1 and 2, Examples 1 to 4 have heat resistance and are excellent in tearability in the machine direction (MD) of the film (the ratio S MD /S _TD (shown as TD, MD, and MD/TD in the table) of the Elmendorf tear strength S _MD in the machine direction ( _MD ) of the film to the tear strength S _TD in the transverse direction (TD) of the film is less than 0.5).
Therefore, the heat-resistant sealant film of the present invention has both heat resistance against heating and pressurizing treatment at temperatures equivalent to retort sterilization treatment and easy tearability so that it can be easily torn by hand, and it is clear that it has great technical significance.

The heat-resistant sealant film made of the multi-layer structure of the ethylene copolymer composition of the present invention does not fuse together between the innermost layers even after heating and pressurizing at 115°C, and has excellent tearability (easy opening), so it is suitable for use as a food packaging film, including retort foods that require heating and pressurizing sterilization, and as medical product packaging.

Claims (6)

The heat-resistant sealant film is a multilayer film having at least an innermost layer and other layers, the innermost layer containing 65 to 100% by weight of a ^linear medium-density polyethylene (A) copolymerized with ethylene and an α-olefin having 6 or more carbon atoms, the medium-density polyethylene (A) having (a-1) a density of 0.925 to 0.940 g/cm 3 , (a-2) an MFR of 0.1 to 20 g/10 min, (a-3) a molecular weight distribution (Mw/Mn) of 1.8 to 3.5, and (a-4) a curve showing multiple peaks in an elution temperature-elution amount curve measured by a continuous temperature rising elution fractionation method (TREF), and a layer adjacent to the innermost layer containing (b-1) ^a linear low-density polyethylene (B) copolymerized with ethylene and an α-olefin having 4 carbon atoms, the linear low-density polyethylene (B) having a density of 0.927 to 0.950 g/cm 3 and (b-2) an MFR of 0.1 to 15 g/10 min. 2. The heat-resistant sealant film according to claim 1 , characterized in that the tear strength in the machine direction (MD) of the film is 20 N/mm or less in an Elmendorf tear test measured in accordance with JIS K7128-2. 3. The heat-resistant sealant film according to claim 1, characterized in that in an Elmendorf tear test measured in accordance with JIS K7128-2, the ratio of tear strength (S _MD ) in the machine direction (MD) of the film to tear strength (S _TD ) in the perpendicular direction (TD) is S _MD /S _TD < 0.5 . 4. The heat-resistant sealant film according to claim 1 , further comprising at least one substrate layer selected from the group consisting of PET, Ny, OPP and Al laminated on the layer opposite to the innermost layer. 5. The heat-resistant sealant film according to claim 1 , wherein the innermost layers are not fused to each other after being subjected to a heat and pressure treatment at 115°C. A food packaging film using the heat-resistant sealant film according to any one of claims 1 to 5 .