JP7843124B2 - Sealant film for retort food packaging containers - Google Patents

Description

This invention relates to a sealant film for retort food packaging containers, and more specifically, to a sealant film for retort food packaging containers that achieves a high level of balance between retort suitability (heat resistance at retort temperatures) and impact resistance at low temperatures.

In recent years, the demand for retort food packaging containers has been steadily increasing. There is a growing need for retort food packaging containers suitable not only for room-temperature storage and distribution, but also for low-temperature storage and distribution (frozen to chilled). Therefore, in addition to the conventionally required heat-sealing properties and high-temperature resistance, sealant films for such retort food packaging containers now require impact resistance at low temperatures.

To achieve both low-temperature impact resistance and high-temperature heat resistance, a sealant film for retort foods has been proposed, comprising a laminate in which at least a first, second, and third layer are stacked in that order, characterized in that the first layer contains a specific ethylene-α-olefin copolymer, the second layer contains a specific ethylene-α-olefin copolymer, and the third layer contains a specific ethylene-α-olefin copolymer and high-density polyethylene (see, for example, Patent Document 1).

Similarly, a laminate suitable for films and sheets for packaging cooked foods, possessing both impact resistance and heat resistance, has been proposed. This laminate is characterized by comprising at least two layers: a first layer containing linear low-density polyethylene (A) made of a copolymer of ethylene and an α-olefin having 3 to 20 carbon atoms, and high-density polyethylene (B) made of an ethylene homopolymer or a copolymer of ethylene and an α-olefin having 3 to 20 carbon atoms; and a second layer containing high-density polyethylene (C) made of an ethylene homopolymer or a copolymer of ethylene and an α-olefin having 3 to 20 carbon atoms, produced using a metallocene catalyst or a single-site catalyst (see, for example, Patent Document 2).

In recent years, the required standards for retort suitability (heat resistance at retort temperatures) have become increasingly stringent. From this perspective, there is a growing demand for sealant films for retort food packaging that offer a higher level of balance between heat resistance at retort temperatures and impact resistance at low temperatures. For example, there is a need for sealant films for retort food packaging that suppress heat fusion between the innermost layers (seal layers) of the packaging container over extended periods, even at retort processing temperatures, and that do not easily rupture even after repeated low-temperature drop tests after the packaging container has been formed.

Japanese Patent Publication No. 2019-131271 Japanese Patent Publication No. 2002-361811

In view of the above technical background, the object of the present invention is to provide a sealant film for retort food packaging containers that achieves a high level of balance between heat resistance at retort temperatures and impact resistance at low temperatures, exceeding the limitations of the prior art.

As a result of diligent research, the present inventors have discovered that in a sealant film for retort food packaging containers having (A) a seal layer, (B) a core layer, and (C) a laminate layer, each containing linear polyethylene, by setting the density of each layer within a specific range and using linear polyethylene polymerized with a metallocene catalyst in the (B) core layer and (C) laminate layer, it is possible to achieve a high level of both heat resistance at retort temperatures and impact resistance at low temperatures, exceeding the limitations of conventional technology, and thus completed the present invention.
In other words, the present invention is
[1]
A sealant film for retort food packaging containers having (A) a seal layer, (B) a core layer, and (C) a laminate layer, each containing linear polyethylene,
(A) The density of the seal layer is 915 kg/ ^m³ or more and 960 kg/ ^m³ or less.
The density of both the (B) core layer and the (C) laminate layer is 932 kg/ ^m³ or more and less than 940 kg/ ^m³ .
(B) The core layer and (C) the laminate layer are both composed of linear polyethylene polymerized using a metallocene catalyst.
Sealant film for retort food packaging containers,
That is the case.

Hereinafter, [2] to [5] are all preferred embodiments or models of the present invention.
[2]
The sealant film for retort food packaging containers according to [1], wherein the melt flow rate (190°C, 2160 g load) of both the core layer (B) and the laminate layer (C) is 6.0 g/10 min or less.
[3]
(A) A sealant film for retort food packaging containers according to [1] or [2], wherein the density of the sealing layer is 940 kg/ ^m³ or more and 953 kg/ ^m³ or less.
[4]
A retort food packaging container film comprising a sealant film for retort food packaging containers and a base film as described in any one of items [1] to [3], wherein the base film is laminated with the sealant film for retort food packaging containers in a (C) laminate layer.
[5]
A retort food packaging container, including the film for retort food packaging containers described in [4].

The sealant film for retort food packaging containers of the present invention surpasses the limitations of conventional technology, achieving a high level of both heat resistance at retort temperatures and impact resistance at low temperatures. For example, even at retort processing temperatures, heat fusion between the innermost layers (seal layers) of the packaging container is suppressed for extended periods, and the packaging container does not easily rupture even after repeated drops at low temperatures after formation. These are just some of the remarkable technical benefits achieved.

The present invention relates to a sealant film for retort food packaging containers having (A) a seal layer, (B) a core layer, and (C) a laminate layer, each containing linear polyethylene,
(A) The density of the seal layer is 915 kg/ ^m³ or more and 960 kg/ ^m³ or less.
The density of both the (B) core layer and the (C) laminate layer is 932 kg/ ^m³ or more and less than 940 kg/ ^m³ .
(B) The linear polyethylene constituting the core layer and (C) the laminate layer is polymerized using a metallocene catalyst.
This is a sealant film for retort food packaging containers.
In other words, the sealant film for retort food packaging containers of the present invention has a seal layer (A), a core layer (B), and a laminate layer (C).

Seal layer (A)
The seal layer (A) constituting the sealant film for retort food packaging containers of the present invention contains linear polyethylene. Therefore, the seal layer (A) may consist only of linear polyethylene, or it may contain other components in addition to linear polyethylene, such as other resins, or various additives such as antiblocking agents and slipping agents.
By including linear polyethylene in the sealing layer (A), the sealant film for retort food packaging containers of the present invention can achieve a high level of balance between retort suitability (heat resistance at retort temperatures) and impact resistance at low temperatures, and can also appropriately possess physical properties generally required for sealant films, such as heat sealability, flexibility, pliability, and airtightness.

The density of the sealing layer (A) is 915 kg/ ^m³ to 960 kg/ ^m³ . This density of 915 kg/ ^m³ to 960 kg/ ^m³ , combined with the other requirements of the present invention, allows the sealant film for food packaging containers of the present invention to achieve a high level of balance between retort suitability (heat resistance at retort temperatures) and low-temperature impact resistance.
The density of the sealing layer (A) is preferably 930 kg/ ^m³ or more and 955 kg/ ^m³ or less, and particularly preferably 940 kg/ ^m³ or more and 953 kg/ ^m³ or less.
The density of the sealing layer (A) can be appropriately adjusted by adjusting the type and amount of material constituting the sealing layer (A), and the manufacturing conditions of the sealant film for retort food packaging containers of the present invention, which includes the sealing layer (A).
If the seal layer (A) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additive, the density of the seal layer (A) is largely determined by the density of the linear polyethylene. In such cases, the density of the seal layer (A) can be kept within a predetermined range by using linear polyethylene with ^a density of 915 kg/ ^m³ to 960 kg/ ^m³ , preferably 930 kg/m³ to 955 kg/ ^m³ , and more preferably 940 kg/ ^m³ to 953 kg/ ^m³ .

There are no particular restrictions on the melting point of the sealing layer (A), and it can be set appropriately depending on the application and usage of the sealant film for retort food packaging containers, but it is preferably 120 to 135°C.
By having the melting point of the sealing layer (A) within the above range, the sealant film for food packaging containers of the present invention can be given even better retort suitability (heat resistance at retort temperatures).
The melting point of the sealing layer (A) is more preferably 122 to 134°C, and particularly preferably 125 to 133°C.
The melting point of the seal layer (A) can be appropriately adjusted by adjusting the type and amount of material constituting the seal layer (A), and the manufacturing conditions of the sealant film for retort food packaging containers of the present invention, which includes the seal layer (A).
If the seal layer (A) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additive, the melting point of the seal layer (A) is largely determined by the melting point of the linear polyethylene. In such cases, the melting point of the seal layer (A) can be kept within the preferred range by using linear polyethylene having a melting point of 120 to 135°C, preferably 125 to 133°C.

The sealing layer (A) preferably has a melting rate at the retort temperature, for example, a melting rate of 45% by mass or less at 121°C.
By ensuring that the melting rate of the seal layer (A) at the retort temperature is within the above range, the sealant film for retort food packaging containers of the present invention can be given even better retort suitability (heat resistance at the retort temperature).
The melting rate of the seal layer (A) at 121°C is more preferably 40% by mass or less, and particularly preferably 35% by mass or less.
The melting rate of the seal layer (A) at the retort temperature can be appropriately adjusted by adjusting the type and amount of material constituting the seal layer (A), and the manufacturing conditions of the sealant film for retort food packaging containers of the present invention, which includes the seal layer (A).

There are no particular restrictions on the MFR (melt flow rate) of the seal layer (A), and it can be set appropriately according to the manufacturing conditions of the seal layer (A) or the sealant film for retort food packaging containers containing the seal layer (A), and the physical properties required for the sealant film for retort food packaging containers. However, it is preferable that the MFR (190°C, 2160g load) is 0.1 to 15 g/10 min.
By having the MFR of the seal layer (A) within the above range, the manufacturing of the sealant film for food packaging containers of the present invention becomes even easier, and desirable effects such as the resin pressure during extrusion being within an appropriate range can be achieved.
The MFR (at 190°C and 2160g load) of the sealing layer (A) is more preferably 0.5 to 12 g/10 min, and particularly preferably 0.7 to 11 g/10 min.
The MFR of the seal layer (A) can be adjusted as appropriate by selecting and adjusting the type and amount of material that constitutes the seal layer (A).
If the seal layer (A) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additive, the MFR of the seal layer (A) is largely determined by the melting point of the linear polyethylene. In such cases, the MFR of the seal layer (A) can be brought within the above preferred range by using linear polyethylene having an MFR (190°C, 2160g load) of 0.1 to 15 g/10 min, more preferably 0.5 to 12 g/10 min.

The linear polyethylene constituting the linear polyethylene seal layer (A) may be any homopolymer of ethylene having a substantially linear main chain, or a copolymer of ethylene and another monomer; there are no other restrictions. Typical forms of linear polyethylene include high-density polyethylene and linear low-density polyethylene.
In the sealing layer (A), linear polyethylene is used that allows the density of the sealing layer (A) to be between 915 kg/ ^m³ and 960 kg/ ^m³ .
The linear polyethylene content of the sealing layer (A) is preferably 20% by mass or more, more preferably 25 to 100% by mass, and particularly preferably 30 to 100% by mass.
In the sealing layer (A), only one type of linear polyethylene may be used, or two or more types of linear polyethylene may be used in combination. When two or more types of linear polyethylene are used in combination, the linear polyethylene content described above is calculated based on the total mass of the two or more types of linear polyethylene.

High-density polyethylene (HPE) is a typical form of linear polyethylene, and as such, any HPE generally known in the art can be used as appropriate.
High-density polyethylene (HDPE) may be an ethylene homopolymer or a copolymer of ethylene and α-olefin.

The above-mentioned high-density polyethylene preferably has a melt flow rate (hereinafter referred to as MFR) of 0.1 to 15 g/10 min, more preferably 0.5 to 12.0 g/10 min, and even more preferably 0.7 to 11.0 g/10 min, measured at 190°C and a load of 2160 g in accordance with JIS K6922-1.
Having the MFR within the above range is preferable because it reduces the load on the extruder during molding and improves molding stability.

In this embodiment, the high-density polyethylene preferably has a density of 940 to 975 kg/ ^m³ , more preferably 945 to 965 kg/ ^m³ , and even more preferably 950 to 960 kg/ ^m³ , in accordance with JIS K6922-1.
Having the density within the above range makes it easy to set the density of the sealing layer (A) to between 915 kg/ ^m³ and 960 kg/ ^m³ .

The high-density polyethylene preferably used in this embodiment may be a commercially available product, such as Nipolon Hard 5700, 8500, and 8022 manufactured by Tosoh Corporation (product names), or Hyzex® 3300F manufactured by Prime Polymer Co., Ltd.

Furthermore, the high-density polyethylene preferably used in this embodiment can be manufactured by methods such as the slurry method, solution method, or gas phase method. When manufacturing the high-density polyethylene, generally, a Ziegler catalyst consisting of a solid catalyst component containing magnesium and titanium and an organoaluminum compound, a metallocene catalyst consisting of an organotransition metal compound containing a cyclopentadienyl derivative and a compound that reacts with it to form an ionic complex, and/or an organometallic compound, or a vanadium-based catalyst can be used. It can be manufactured by homopolymerizing ethylene or copolymerizing ethylene with α-olefins using these catalysts. The α-olefin can be any α-olefin generally referred to as such, and is preferably an α-olefin having 3 to 12 carbon atoms, such as propylene, butene-1, hexene-1, octen-1, or 4-methyl-1-pentene. Examples of copolymers of ethylene and α-olefins include ethylene-hexene-1 copolymer, ethylene-butene-1 copolymer, and ethylene-octen-1 copolymer.

Linear low-density polyethylene (SLIP ) is a typical form of linear low-density polyethylene, and any type of linear low-density polyethylene generally known in the art can be used as appropriate. Such linear low-density polyethylene can be a copolymer of ethylene and α-olefin, and can be synthesized using a production method with known catalysts such as Ziegler catalysts or metallocene catalysts.

As the α-olefin, compounds having 3 to 20 carbon atoms can be used, such as propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-dodecene, 4-methyl-1-pentene, 4-methyl-1-hexene, etc., and mixtures thereof may also be used. Preferably, the α-olefin is a compound having 4, 6, or 8 carbon atoms or a mixture thereof, such as 1-butene, 1-hexene, 1-octene, or a mixture thereof.
However, it is also possible to increase the amount of ethylene during the polymerization process to produce α-olefins, in which case it is possible to manufacture them using virtually only ethylene as a raw material.

The linear low-density polyethylene may be a commercially available product, for example, 2040F ( _C6 -LLDPE, MFR; 4.0, density; 0.918 g/ ^cm³ ) manufactured by Ube Maruzen Polyethylene Co., Ltd., or Evolu® SP2040 manufactured by Prime Polymer Co., Ltd. can be used.

The density of the linear low-density polyethylene is preferably 890 to 940 kg/ ^m³ , and more preferably 900 to 930 kg/ ^m³ . From the viewpoint of setting the density of the seal layer (A) to 915 kg/ ^m³ or more and 960 kg/ ^m³ or less, and/or a more preferable density range, it is particularly preferable to use a relatively high density within the above density range. Furthermore, it is preferable to use a small amount of the relatively low-density material.
The density of linear low-density polyethylene can be adjusted as needed by adjusting the comonomer content, and also by selecting and adjusting polymerization conditions such as catalysts and polymerization temperature.

From the viewpoint of setting the MFR (190°C, 2160g load) of the sealing layer (A) to the above preferred range, the MFR (190°C, 2160g load) of linear low-density polyethylene is preferably 0.1 to 15 g/10 min, more preferably 0.5 to 12 g/10 min, and particularly preferably 0.7 to 11 g/10 min.
The molecular weight-free ratio (MFR) of linear low-density polyethylene (at 190°C and 2160 g load) can be adjusted as appropriate by conventionally known methods, such as by adjusting polymerization conditions like polymerization temperature or by introducing molecular weight modifiers.

Linear low-density polyethylene can be produced by conventionally known manufacturing methods using conventionally known catalysts, including multi-site catalysts such as Ziegler catalysts and single-site catalysts such as metallocene catalysts. From the viewpoint of obtaining linear low-density polyethylene with a narrow molecular weight distribution and capable of forming high-strength films, the use of a single-site catalyst is preferable.

The single-site catalyst described above is a catalyst capable of forming a uniform active species, and is typically prepared by contacting a metallocene transition metal compound or a non-metallocene transition metal compound with an activation co-catalyst. Single-site catalysts are preferred over multi-site catalysts because their active site structure is more uniform, allowing for the polymerization of polymers with high molecular weight and high uniformity. Metallocene catalysts are particularly preferred as single-site catalysts. A metallocene catalyst is a catalyst comprising a transition metal compound of Group IV of the periodic table containing a ligand with a cyclopentadienyl skeleton, a co-catalyst, an organometallic compound as needed, and the catalyst components of a support.

In the transition metal compounds of Group IV of the periodic table containing the ligand having the cyclopentadienyl skeleton described above, the cyclopentadienyl skeleton is a cyclopentadienyl group, a substituted cyclopentadienyl group, etc. The substituted cyclopentadienyl group has at least one substituent selected from hydrocarbon groups having 1 to 30 carbon atoms, silyl groups, silyl-substituted alkyl groups, silyl-substituted aryl groups, cyano groups, cyanoalkyl groups, cyanoaryl groups, halogen groups, haloalkyl groups, halosilyl groups, etc. The substituted cyclopentadienyl group may have two or more substituents, and the substituents may bond to each other to form a ring, forming an indenyl ring, a fluorenyl ring, an azlenyl ring, or its hydrogenated form, etc. The ring formed by the bonding of substituents may further have substituents on each other.

In a transition metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton, examples of transition metals include zirconium, titanium, and hafnium, with zirconium and hafnium being particularly preferred. The transition metal compound typically has two ligands having a cyclopentadienyl skeleton, and it is preferable that each ligand is bonded to the others by a bridging group. Examples of bridging groups include alkylene groups having 1 to 4 carbon atoms, silylene groups, substituted silylene groups such as dialkylsilylene groups and diarylsilylene groups, and substituted germylene groups such as dialkylgermylene groups and diarylgermylene groups. A substituted silylene group is preferred.

In transition metal compounds of Group IV of the periodic table, typical ligands other than those containing a cyclopentadienyl skeleton include hydrogen, hydrocarbon groups having 1 to 20 carbon atoms (alkyl groups, alkenyl groups, aryl groups, alkylaryl groups, aralkyl groups, polyenyl groups, etc.), halogens, metaalkyl groups, and metaaryl groups.

The transition metal compounds of Group IV of the periodic table containing the ligand having the cyclopentadienyl skeleton described above can be used as catalyst components, either individually or as a mixture of two or more.

As co-catalysts, we refer to those that can effectively utilize the transition metal compounds of Group IV of the periodic table as polymerization catalysts, or that can balance the ionic charge in a catalytically activated state. Examples of co-catalysts include benzene-soluble aluminoxanes and benzene-insoluble organoaluminum oxy compounds, ion-exchangeable layered silicates, boron compounds, ionic compounds consisting of cations containing or not containing active hydrogen groups and non-coordinating anions, lanthanide salts such as lanthanum oxide, tin oxide, and phenoxy compounds containing fluoro groups.

Transition metal compounds of Group IV of the periodic table containing ligands having a cyclopentadienyl skeleton may be used by being supported on an inorganic or organic compound. The support is preferably a porous oxide of an inorganic or organic compound, and specifically includes _ion -exchangeable layered silicates such as _{montmorillonite} , _SiO₂ , _Al₂O₃ , MgO, _ZrO₂ , _TiO₂ , _B₂O₃ , CaO, ZnO, BaO, _ThO₂ , or mixtures thereof.

Furthermore, examples of organometallic compounds that may be used as needed include organoaluminum compounds, organomagnesium compounds, and organozinc compounds. Of these, organoaluminum compounds are preferred.

From the viewpoint of obtaining linear low-density polyethylene with a broad molecular weight distribution and excellent flexibility and moldability, it is preferable to use multi-site catalysts such as Ziegler catalysts and Phillips catalysts.
Preferred Ziegler catalysts are those commonly known as Ziegler catalysts used in the coordination polymerization of ethylene and α-olefins, such as catalysts containing titanium compounds and organoaluminum compounds. Examples include catalysts composed of titanium halide compounds and organoaluminum compounds, and catalysts composed of solid catalyst components such as titanium, magnesium, and chlorine, and organoaluminum compounds. Examples of such catalysts include catalysts comprising a catalyst component (a) obtained by reacting an alcohol pretreatment product of an anhydrous magnesium dihalide with an organometallic compound, and an organometallic compound (b); catalysts comprising a catalyst component (A) obtained by reacting magnesium metal with an organic hydroxide compound or an oxygen-containing organic compound such as magnesium, an oxygen-containing organic compound of a transition metal, and an aluminum halide, and an organometallic compound catalyst component (B); catalysts comprising (i) metallic magnesium and at least one selected from organic hydroxide compounds, oxygen-containing organic compounds of magnesium, and halogen-containing compounds; (ii) at least one selected from oxygen-containing organic compounds of transition metals and halogen-containing compounds; (iii) a reaction product obtained by reacting a silicon compound with (iv) an aluminum halide compound, and a solid catalyst component (A) and an organometallic compound catalyst component (B).

Furthermore, the Phillips catalyst can be any generally known Phillips catalyst used in the coordination polymerization of ethylene and α-olefins. For example, it may be a catalyst system containing chromium compounds such as chromium oxide. Specifically, examples include catalysts in which chromium compounds such as chromium trioxide and chromate esters are supported on solid oxides such as silica, alumina, silica-alumina, and silica-titania.

The sealing layer (A) may contain components other than the linear polyethylene described above. For example, polymers other than linear polyethylene, oligomers, antiblocking agents, slip agents, heat stabilizers (antioxidants), weather stabilizers, ultraviolet absorbers, lubricants, nucleating agents, antistatic agents, antifogging agents, pigments, dyes, and various fillers such as talc, silica, and diatomaceous earth may be added as needed or insofar as they do not contradict the purpose of the present invention.
These additive components may be incorporated into the linear polyethylene beforehand, or they may be added when forming the intermediate layer (A) from the linear polyethylene.

There are no particular restrictions on the thickness of the sealing layer (A), but from the viewpoint of good sealing characteristics, it is preferable that it be 3 μm or more, more preferably 4 μm or more, and particularly preferably 5 μm or more.
On the other hand, from the viewpoint of flexibility and economic efficiency, the thickness of the seal layer (A) is preferably 30 μm or less, more preferably 25 μm or less, and particularly preferably 20 μm or less.
When stretching is performed in the manufacture of the laminated film of the present invention, the stretching ratio is preferably 1 to 100 times in terms of area, and particularly preferably 4 to 80 times. The thickness of the layer corresponding to the seal layer (A) before stretching is preferably 0.3 to 12 mm, and particularly preferably 0.5 to 4 mm.
The thickness of the seal layer (A) can be appropriately adjusted by adjusting the stretching conditions such as the stretching ratio, the thickness of the layer before stretching, and the lip spacing of the die when forming the layer before stretching.

Core layer (B)
The core layer (B) constituting the sealant film for retort food packaging containers of the present invention contains linear polyethylene polymerized using a metallocene catalyst. Therefore, the core layer (B) may consist only of linear polyethylene polymerized using a metallocene catalyst, or it may contain other components in addition to the linear polyethylene polymerized using a metallocene catalyst, such as other resins or various additives such as antiblocking agents and slipping agents.
By including linear polyethylene polymerized using a metallocene catalyst in the core layer (B), the sealant film for retort food packaging containers of the present invention can achieve a high level of balance between retort suitability (heat resistance at retort temperatures) and impact resistance at low temperatures, and can also appropriately possess physical properties generally required for sealant films, such as heat sealability, flexibility, pliability, and airtightness.

The density of the core layer (B) is 932 kg/ ^m³ or more and less than 940 kg/ ^m³ . The density of the core layer (B) being 940 kg/ ^m³ or more and 960 kg/ ^m³ or less, combined with the other requirements of the present invention, allows the sealant film for food packaging containers of the present invention to achieve a high level of balance between retort suitability (heat resistance at retort temperatures) and impact resistance at low temperatures.
The density of the core layer (B) is preferably 934 kg/ ^m³ or more and less than 940 kg/ ^m³ , and particularly preferably 936 kg/ ^m³ or more and less than 940 kg/ ^m³ .
The density of the core layer (B) can be appropriately adjusted by adjusting the type and amount of material constituting the core layer (B), and the manufacturing conditions of the sealant film for retort food packaging containers of the present invention, which includes the core layer (B).
If the core layer (B) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additives, the density of the core layer (B) is largely determined by the density of the linear polyethylene. In such cases, the density of the core layer (B) can be kept within a predetermined range by using linear polyethylene with a density of 932 kg/ ^m³ or more and less than 940 kg/ ^m³ , preferably 934 kg/ ^m³ or more and less than 940 kg/ ^m³ .

There are no particular restrictions on the melting point of the core layer (B), and it can be set appropriately depending on the application and usage of the sealant film for retort food packaging containers, but it is preferably 120 to 135°C.
By having the melting point of the core layer (B) within the above range, the sealant film for food packaging containers of the present invention can be given even better retort suitability (heat resistance at retort temperatures).
The melting point of the core layer (B) is more preferably 122 to 134°C, and particularly preferably 125 to 133°C.
The melting point of the core layer (B) can be appropriately adjusted by adjusting the type and amount of material constituting the core layer (B), and the manufacturing conditions of the sealant film for retort food packaging containers of the present invention, which includes the core layer (B) or the core layer (B).
If the core layer (B) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additives, the melting point of the core layer (B) is largely determined by the melting point of the linear polyethylene. In such cases, the melting point of the core layer (B) can be kept within the preferred range by using linear polyethylene having a melting point of 120 to 135°C, preferably 122 to 134°C.

The core layer (B) preferably has a melting rate at the retort temperature, for example, a melting rate at 121°C of 95% by mass or less.
By ensuring that the melting rate of the core layer (B) at the retort temperature is within the above range, the sealant film for food packaging containers of the present invention can be given even better retort suitability (heat resistance at the retort temperature).
The melting rate of the core layer (B) at 121°C is more preferably 45 to 95% by mass, and particularly preferably 47 to 93% by mass.
The melting rate of the core layer (B) at the retort temperature can be appropriately adjusted by adjusting the type and amount of material constituting the core layer (B), and the manufacturing conditions of the sealant film for retort food packaging containers of the present invention, which includes the core layer (B).

There are no particular restrictions on the MFR (melt flow rate) of the core layer (B). It can be set appropriately according to the manufacturing conditions of the core layer (B) or the sealant film for retort food packaging containers containing the core layer (B), and the physical properties required for the sealant film for retort food packaging containers. However, it is preferable that the MFR (190°C, 2160g load) is 6.0g/10min or less.
By having a core layer (B) with an MFR of 6.0 g/10 min or less, the heat resistance at retort temperatures and impact resistance at low temperatures of the sealant film for food packaging containers of the present invention can be achieved at an even higher level, resulting in desirable effects such as improved impact resistance.
The MFR (at 190°C and 2160g load) of the core layer (B) is more preferably 0.1 to 15 g/10 min, and particularly preferably 0.7 to 11 g/10 min.
The MFR of the core layer (B) can be adjusted as appropriate by selecting and adjusting the type and amount of materials that make up the core layer (B).
If the core layer (B) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additives, the MFR of the core layer (B) is largely determined by the melting point of the linear polyethylene. In such cases, the MFR of the core layer (B) can be brought within the above preferred range by using linear polyethylene having an MFR (190°C, 2160g load) of 0.1 to 15 g/10 min, more preferably 0.7 to 11 g/10 min.

The linear polyethylene constituting the linear polyethylene core layer (B), polymerized using a metallocene catalyst , is basically the same type of resin as the linear polyethylene constituting the seal layer (A) described above.
However, the linear polyethylene constituting the core layer (B) is limited to that which has been polymerized using a metallocene catalyst. Therefore, the above description of the linear polyethylene constituting the seal layer (A), specifically the description of the production of linear polyethylene using catalysts other than metallocene catalysts, does not apply to the linear polyethylene constituting the core layer (B).
Furthermore, since the density of the core layer (B) is between 932 kg/ ^m³ and less than 940 kg/ ^m³ , linear polyethylene with a density such that the overall density of the layer falls within this range is used in the core layer (B).

Linear polyethylene polymerized using a metallocene catalyst exhibits excellent impact resistance. Therefore, by using linear polyethylene polymerized with a metallocene catalyst as the core layer (B), impact resistance can be further improved, achieving a higher level of balance between heat resistance and low-temperature impact resistance.

The density of the linear polyethylene constituting the core layer (B) is set such that the density of the core layer (B) is between 932 kg/ ^m³ and less than 940 kg/ ^m³ .
More specifically, the density of the linear polyethylene constituting the core layer (B) is preferably in the range of 932 to less than 940 kg/ ^m³ , more preferably in the range of 934 to less than 940 kg/ ^m³ , and particularly preferably in the range of 936 to less than 940 kg/ ^m³ .
From this perspective, linear low-density polyethylene (LLDPE) is preferred over high-density polyethylene (HDPE) as the linear polyethylene constituting the core layer (B), and it is preferable to use only linear low-density polyethylene or a combination in which linear low-density polyethylene makes up the majority.

The linear polyethylene content of the core layer (B) is preferably 70% by mass or more, more preferably 80 to 100% by mass, and particularly preferably 90 to 98% by mass.
In the core layer (B), only one type of linear polyethylene may be used, or two or more types of linear polyethylene may be used in combination. When two or more types of linear polyethylene are used in combination, the linear polyethylene content described above is calculated based on the total mass of the two or more types of linear polyethylene.

There are no particular limitations on the thickness of the core layer (B), but it is preferable that it be greater than the thickness of the seal layer (A) and the thickness of the laminate layer (C), and it is particularly preferable that it be greater than the sum of the thicknesses of the seal layer (A) and the laminate layer (C).
More specifically, the thickness of the core layer (B) is preferably 9 μm or more, and particularly preferably 15 μm or more.
On the other hand, from the viewpoint of raw material costs, the thickness of the core layer (B) is preferably 90 μm or less, and particularly preferably 60 μm or less.

Laminate layer (C)
The laminate layer (C) constituting the sealant film for retort food packaging containers of the present invention contains linear polyethylene polymerized using a metallocene catalyst. Therefore, the laminate layer (C) may consist only of linear polyethylene polymerized using a metallocene catalyst, or it may contain other components in addition to linear polyethylene polymerized using a metallocene catalyst, such as other resins or various additives such as antiblocking agents and slipping agents.
By including linear polyethylene polymerized using a metallocene catalyst in the laminate layer (C), the sealant film for retort food packaging containers of the present invention can achieve a high level of balance between retort suitability (heat resistance at retort temperatures) and impact resistance at low temperatures, and can also appropriately possess physical properties generally required for sealant films, such as heat sealability, flexibility, pliability, and airtightness.

The density of the laminate layer (C) is 932 kg/ ^m³ or more and less than 940 kg/ ^m³ . The density of the laminate layer (C) being 940 kg/ ^m³ or more and 960 kg/ ^m³ or less, combined with the other requirements of the present invention, allows the sealant film for food packaging containers of the present invention to achieve a high level of balance between retort suitability (heat resistance at retort temperatures) and impact resistance at low temperatures.
The density of the laminate layer (C) is preferably 934 kg/ ^m³ or more and less than 940 kg/ ^m³ , and particularly preferably 936 kg/ ^m³ or more and less than 940 kg/ ^m³ .
The density of the laminate layer (C) can be appropriately adjusted by adjusting the type and amount of materials constituting the laminate layer (C), and the manufacturing conditions of the laminate layer (C) or the sealant film for retort food packaging containers of the present invention that includes the laminate layer (C).
If the laminate layer (C) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additives, the density of the laminate layer (C) is largely determined by the density of the linear polyethylene. In such cases, the density of the laminate layer (C) can be kept within a predetermined range by using linear polyethylene with a density of 932 kg/ ^m³ or more and less than 940 kg/ ^m³ , preferably 934 kg/ ^m³ or more and less than 940 kg/ ^m³ .

There are no particular restrictions on the melting point of the laminate layer (C), and it can be set appropriately depending on the application and usage of the sealant film for retort food packaging containers, but it is preferably 120 to 135°C.
By having the melting point of the laminate layer (C) within the above range, the sealant film for food packaging containers of the present invention can be given even better retort suitability (heat resistance at retort temperatures).
The melting point of the laminate layer (C) is more preferably 122 to 134°C, and particularly preferably 125 to 133°C.
The melting point of the laminate layer (C) can be appropriately adjusted by adjusting the type and amount of materials constituting the laminate layer (C), and the manufacturing conditions of the laminate layer (C) or the sealant film for retort food packaging containers of the present invention that includes the laminate layer (C).
If the laminate layer (C) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additives, the melting point of the laminate layer (C) is largely determined by the melting point of the linear polyethylene. In such cases, the melting point of the laminate layer (C) can be kept within the preferred range by using linear polyethylene having a melting point of 120 to 135°C, preferably 122 to 134°C.

The laminate layer (C) preferably has a melting rate at the retort temperature, for example, a melting rate at 121°C, of 95% by mass or less.
By ensuring that the melting rate of the laminate layer (C) at the retort temperature is within the above range, the sealant film for food packaging containers of the present invention can be given even better retort suitability (heat resistance at the retort temperature).
The melting rate of the laminate layer (C) at 121°C is more preferably 45 to 95% by mass, and particularly preferably 47 to 93% by mass.
The melting rate of the laminate layer (C) at the retort temperature can be appropriately adjusted by adjusting the type and amount of materials constituting the laminate layer (C), and the manufacturing conditions of the laminate layer (C) or the sealant film for retort food packaging containers of the present invention that includes the laminate layer (C).

There are no particular restrictions on the MFR (melt flow rate) of the laminate layer (C), and it can be set appropriately according to the manufacturing conditions of the laminate layer (C) or the sealant film for retort food packaging containers containing the laminate layer (C), and the physical properties required for the sealant film for retort food packaging containers. However, it is preferable that the MFR (190°C, 2160 g load) is 6.0 g/10 min or less.
By having an MFR of 6.0 g/10 min or less in the laminate layer (C), the heat resistance at retort temperatures and impact resistance at low temperatures of the sealant film for food packaging containers of the present invention can be achieved at an even higher level, and desirable effects such as improved impact resistance can be realized.
The MFR (at 190°C and 2160g load) of the laminate layer (C) is more preferably 0.1 to 15 g/10 min, and particularly preferably 0.7 to 11 g/10 min.
The MFR of the laminate layer (C) can be adjusted as needed by selecting and adjusting the type and amount of materials that make up the laminate layer (C).
If the laminate layer (C) is composed solely of linear polyethylene, or linear polyethylene and a small amount of additives, the MFR of the laminate layer (C) is largely determined by the melting point of the linear polyethylene. In such cases, the MFR of the laminate layer (C) can be brought within the above preferred range by using linear polyethylene having an MFR (190°C, 2160 g load) of 0.1 to 15 g/10 min, more preferably 0.7 to 11 g/10 min.

The linear polyethylene constituting the linear polyethylene laminate layer (C) polymerized using a metallocene catalyst is basically the same type of resin as the linear polyethylene constituting the seal layer (A) described above.
However, the linear polyethylene constituting the laminate layer (C) is limited to that which has been polymerized using a metallocene catalyst. Therefore, the description above regarding the production of linear polyethylene using catalysts other than metallocene catalysts for the linear polyethylene constituting the seal layer (A) does not apply to the linear polyethylene constituting the laminate layer (C).
Furthermore, since the density of the laminate layer (C) is between 932 kg/ ^m³ and less than 940 kg/ ^m³ , linear polyethylene with a density such that the overall density of the layer falls within this range is used for the laminate layer (C).

Linear polyethylene polymerized using a metallocene catalyst exhibits excellent impact resistance. Therefore, by using linear polyethylene polymerized with a metallocene catalyst as the linear polyethylene constituting the laminate layer (C), impact resistance can be further improved, achieving a higher level of balance between heat resistance and low-temperature impact resistance.

The density of the linear polyethylene constituting the laminate layer (C) is set such that the density of the laminate layer (C) is between 932 kg/ ^m³ and less than 940 kg/ ^m³ .
More specifically, the density of the linear polyethylene constituting the laminate layer (C) is preferably in the range of 932 to less than 940 kg/ ^m³ , more preferably in the range of 934 to less than 940 kg/ ^m³ , and particularly preferably in the range of 936 to less than 940 kg/ ^m³ .
From this perspective, linear low-density polyethylene (LLDPE) is preferred over high-density polyethylene (HDPE) as the linear polyethylene constituting the laminate layer (C), and it is preferable to use only linear low-density polyethylene or a combination in which linear low-density polyethylene makes up the majority.

The linear polyethylene content of the laminate layer (C) is preferably 70% by mass or more, more preferably 80 to 100% by mass, and particularly preferably 90 to 98% by mass.
In the laminate layer (C), only one type of linear polyethylene may be used, or two or more types of linear polyethylene may be used in combination. When two or more types of linear polyethylene are used in combination, the linear polyethylene content described above is calculated based on the total mass of the two or more types of linear polyethylene.

The laminate layer (C) can be laminated with other layers, including the base layer (D) described later, as necessary or desired.
Therefore, it is preferable to design the laminate layer (C) taking into consideration the lamination strength between it and other layers, including the base layer (D).
The laminate layer (C) may be subjected to surface treatments such as corona treatment to improve its adhesion to other layers.

From the viewpoint of preventing blocking when storing the sealant film for retort food packaging containers of the present invention, the laminate layer (C) may contain a blocking inhibitor.
As an anti-blocking agent, powdered silica, preferably synthetic silica, etc., can be suitably used. From the viewpoint of uniformly dispersing the powdered silica in the laminate layer (C), the powdered silica may be dispersed in a resin that has excellent miscibility with the resin constituting the laminate layer (C) to form a masterbatch, and then the masterbatch may be added to the laminate layer (C).

There are no particular restrictions on the thickness of the laminate layer (C), but from the viewpoint of suppressing film curling, it is preferable that it be 3 μm or more, and particularly preferable that it be 5 μm or more.
On the other hand, from the viewpoint of raw material costs, etc., it is preferable that the particle size be 30 μm or less, and particularly preferable that it be 20 μm or less.

Any of the seal layer (A), core layer (B), and laminate layer (C) may contain various additives and fillers other than those mentioned above, as long as they do not contradict the objectives of the present invention. For example, heat stabilizers, antioxidants, light stabilizers, antistatic agents, antiblocking agents, lubricants, nucleating agents, flame retardants, pigments, dyes, calcium carbonate, barium sulfate, magnesium hydroxide, mica, talc, clay, antibacterial agents, antifogging agents, etc. Furthermore, other thermoplastic resins, thermoplastic elastomers, rubbers, etc. (other than those described above) may be blended in as long as they do not contradict the objectives of the present invention.
Various additives may be added in masterbatch form.

Sealant film (laminated film) for retort food packaging containers
The sealant film for retort food packaging containers of the present invention comprises a seal layer (A), a core layer (B), and a laminate layer (C). In the sealant film for retort food packaging containers of the present invention, the laminate layer (C) and the seal layer (A) are preferably directly laminated via the core layer (B), but other layers may be present in between.

The sealant film for retort food packaging containers of the present invention can be manufactured using various known film molding methods. For example, a method may be employed in which films to form a laminate layer (C), a core layer (B), and a seal layer (A) are pre-formed, and then the films are laminated together to form a sealant film for retort food packaging containers; a method may be employed in which a multilayer film consisting of a core layer (B) and a seal layer (A) is obtained using a multilayer die, and then the laminate layer (C) is extruded onto the surface of the core layer (B) to form a sealant film for retort food packaging containers; a method may be employed in which a multilayer film consisting of a laminate layer (C) and a core layer (B) is obtained using a multilayer die, and then the seal layer (A) is extruded onto the surface of the core layer (B) to form a sealant film for retort food packaging containers; or a method may be employed in which a sealant film consisting of a laminate layer (C), a core layer (B), and a seal layer (A) is obtained using a multilayer die.

Furthermore, various known film forming methods can be employed, specifically, T-die-cast film forming methods, inflation film forming methods, and the like.

The thickness of the sealant film for retort food packaging containers of the present invention is not particularly limited, but from the viewpoint of ensuring practical strength, it is usually 20 μm or more, preferably 30 μm or more, and more preferably 40 μm or more. On the other hand, from the viewpoint of having practical flexibility even after lamination with a base layer (D), for example, it is usually 200 μm or less, preferably 170 μm or less, and more preferably 150 μm or less.

The sealant film for retort food packaging containers of the present invention may be a stretched film or an unstretched film, but an unstretched film is preferred from the viewpoint of ease of manufacture and flexibility.
On the other hand, from the viewpoint of improving mechanical properties, it is preferable that the film be stretched, and particularly preferable that it be a biaxially oriented film.
Biaxial stretching can be performed using methods such as sequential biaxial stretching, simultaneous biaxial stretching, or multi-stage stretching, as appropriate.
As for the conditions for biaxial stretching, known manufacturing conditions for biaxially oriented films include, for example, in the sequential biaxial stretching method, setting the longitudinal stretching temperature to 100°C to 145°C and the stretching ratio to 4 to 7 times, and the transverse stretching temperature to 150 to 190°C and the stretching ratio to 8 to 11 times.

The sealant film for retort food packaging containers of the present invention has a predetermined density for each of the (A) seal layer, (B) core layer, and (C) laminate layer, and the linear polyethylene constituting the (B) core layer and (C) laminate layer is polymerized using a metallocene catalyst. This allows for a high level of both heat resistance at retort temperatures and impact resistance at low temperatures, exceeding the limitations of conventional technology.
Here, heat resistance at retort temperature means that when a retort food packaging made using a sealant film for retort food packaging containers is heat-treated at retort temperature (120°C in this application), the seal layers on the inner surface of the packaging do not heat-fuse together, and the packaging maintains its proper condition.

The heat resistance of a sealant film for retort food packaging containers at retort temperatures can be evaluated by forming a packaging container using the sealant film so that the seal layers (A) are in contact with each other, performing a retort process at a predetermined retort temperature, and then determining whether or not the seal layers (A) are fused together. More specifically, it can be evaluated by, for example, the method described in the embodiment of this application.
When evaluated by the method described in the embodiments of this application, packaging containers formed using the sealant film for retort food packaging containers of the present invention can effectively prevent fusion, such as not fusion occurring or only very slight fusion occurring, allowing for easy peeling when retorted at 121°C for 45 minutes.
In the present invention, the heat resistance at retort temperatures can be further improved by further optimizing the resin composition. For example, by setting a higher density for each layer, particularly for the sealing layer (A), the heat resistance at retort temperatures can be further improved.

Here, impact resistance at low temperatures can be evaluated by the drop-and-break resistance, more specifically by forming a bag-shaped packaging container filled with water using a sealant film for retort food packaging containers, cooling it to 5°C, and then repeatedly dropping it from a predetermined height until it breaks, and more specifically by the method described in the embodiment of this application.
When evaluated by the method described in the embodiments of this application, a packaging container formed using the sealant film for retort food packaging containers of the present invention usually requires 15 or more drops, preferably 20 or more drops, and more preferably 25 or more drops to break.
In this invention, impact resistance at low temperatures can be further improved by further optimizing the resin composition. For example, impact resistance at low temperatures can be further improved by setting a lower density for each layer.

Base material layer (D)
If desired, the sealant film for retort food packaging containers of the present invention can be laminated with a base layer (D) in its laminate layer (C).

There are no particular restrictions on the base layer (D), but a film with excellent heat resistance and gas barrier properties, commonly used for retort food packaging containers, can be suitably used.
Examples of preferred materials for the base layer (D) include plastic films made from thermoplastic resins such as polyamides like nylon 6, nylon 66, para- or metaxylylene adipamide; polyolefins such as crystalline polypropylene, crystalline propylene-ethylene copolymer, crystalline polybutene-1, crystalline poly-methylpentene-1, low-, medium-, or high-density polyethylene, ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), and ion-crosslinked olefin copolymer (ionomer); aromatic vinyl copolymers such as polystyrene and styrene-butadiene copolymer; halogenated vinyl polymers such as polyvinyl chloride and vinylidene chloride resin; nitrile polymers such as acrylonitrile-styrene copolymer and acrylonitrile-styrene-butadiene copolymer; polyesters such as polyethylene terephthalate (PET) and polytetramethylene terephthalate; various polycarbonates; and polyacetals such as polyoxymethylene. Furthermore, if the contents to be packaged are sensitive to oxygen, the above-mentioned film may be coated with a metal, metal oxide, or the like, or with an organic compound, or a layer made of ethylene vinyl alcohol copolymer (EVOH) resin may be provided.
Plastic films made from these materials may be used in an unstretched state, or they may be used after being uniaxially stretched or biaxially stretched.

These plastic films can be used as a single layer or as a laminate of two or more types as the base layer (D). Alternatively, one or more of these plastic films can be laminated with metal foil such as aluminum, paper, cellophane, etc.
Preferred base layer (D) includes, for example, a single-layer film made of stretched nylon film or stretched polyester film, a two-layer film made by laminating a polyolefin film such as low-density polyethylene or polypropylene with PET, and a three-layer film made by laminating PET/nylon/polyethylene. When manufacturing these laminated films, adhesives and anchoring agents may be interposed between each layer as needed. An ink layer for expressing the design may also be provided.

There are no particular restrictions on the method of laminating the base layer (D) onto the laminate layer (C), but for example, the base layer (D) can be directly laminated onto the laminate layer (C) by extrusion lamination or the like. Alternatively, the base layer (D) can be laminated onto the laminate layer (C) via an adhesive layer by dry lamination or the like. As the adhesive, ordinary adhesives such as urethane adhesives, acid-modified polyolefin adhesives, polyester adhesives, polyether adhesives, and polyamide adhesives can be used.
The thickness of the substrate layer (D) can be set arbitrarily, but is usually selected from the range of 5 to 1000 μm, preferably 9 to 100 μm.

The sealant film for retort food packaging containers of the present invention, and the laminated film obtained by laminating a base layer (D) onto a laminate layer (C) of the sealant film for retort food packaging containers of the present invention, are suitable films for constituting all or part of a retort food packaging container.
There are no particular restrictions on the shape of the retort food packaging container, but suitable examples include bag-shaped packaging containers and packaging containers consisting of a lid and a cup or similar body.
When using the sealant film for retort food packaging containers of the present invention in a retort food packaging container, for example, the sealant film for retort food packaging containers itself or a laminated film obtained by laminating it with a base layer (D) may be folded and sealed on three sides, or two sheets of sealant film for retort food packaging containers or the laminated film may be sealed on all four sides to form a bag-shaped packaging container. Alternatively, the sealant film for retort food packaging containers or a lid material made by laminating it with a base layer (D) may be heat-sealed to a container body such as a cup to form a packaging container. A suitable example of such a packaging container is one comprising the lid material and a container body containing at least one of polypropylene, polyethylene terephthalate, and polybutylene terephthalate.

The sealant film for retort food packaging containers of the present invention, or a lid material laminated thereto with a base layer (D), can form a heat seal by heat-sealing it to various container bodies at the seal layer (A). Examples of container body materials include propylene polymers, polystyrene, polyester, polycarbonate, and polyvinyl chloride. These container bodies can take various shapes, such as films, sheets, trays, cups, and bottles.

There are no particular restrictions on the types of food that can be stored in the retort food packaging containers; various foods that have traditionally been offered as retort foods can be stored in them.
The sealant film for retort food packaging containers of the present invention surpasses the limitations of conventional technology by achieving a high level of balance between heat resistance at retort temperatures and impact resistance at low temperatures. Therefore, retort food packaging containers using this film can be suitably used for heat and pressure sterilization treatment at 100 to 135°C, preferably 121°C or lower. Furthermore, by utilizing its impact resistance at low temperatures, it can be suitably used in low-temperature storage/distribution (frozen to chilled).

The present invention will be described in detail below with reference to examples and comparative examples. However, the present invention is not limited in any way by the following examples.

The physical properties and characteristics of the examples/comparative examples were evaluated by the following methods.
(1) Heat resistance (suitability for retort processing)
Laminated films were prepared by laminating the sealant film laminate layer (C) of each example/comparative example onto a base material (15 μm thick nylon film).
Two of the laminated films described above were placed with their sealing layer (A) sides overlapped, and the three sides were heat-sealed to create a three-sided bag.
After degassing the three-sided bag so that the insides were in contact, the remaining side of the bag was heat-sealed, and then it was retorted at 115°C or 120°C for 45 minutes.
The presence and degree of fusion inside the three-sided seal bags were evaluated by sensory testing based on the following criteria. Bags that could be easily opened were judged as pass (○), and those that were difficult to open due to fusion were judged as fail (×).

(2) Low-temperature impact resistance (drop rupture)
Laminated films were prepared by laminating the sealant film laminate layer (C) of each example/comparative example onto a base material (15 μm thick nylon film).
Two of the laminated films described above were placed with their sealing layer (A) sides overlapped, and the three sides were heat-sealed to create a three-sided bag.
300 ml of water was filled into the three-sided bag described above, and the remaining side was heat-sealed to form a sealed bag, which was then retorted at 121°C.
Next, the bags were stored in a 5°C refrigerator for 24 hours, and then repeatedly dropped in the 5°C refrigerator until they burst. The number of drops until the bag burst was recorded. A 1.0 kg weight was placed on top of the bag, and it was dropped from a height of 60 cm along with the weight.
The sample size N was set to 10, and the average number of drops when the bag broke was used as the evaluation value for the example/comparative example.

(3) Melting point The melting point was measured using a differential scanning calorimeter (DSC) (TA Instruments, "Q200", software: "TA Universal Analysis") when heated at a heating rate of 10°C/min.

Details of each component of the resins used in the examples/comparative examples are as follows.

・LLDPE-1
Metallocene LLDPE (linear low-density polyethylene)
Composition: Ethylene-C6 copolymer Melting point: 130°C
Density: 936 kg/ ^m3
MFR (2.16kg, 190℃): 1.8g/10min

LLDPE-2
Metallocene LLDPE (linear low-density polyethylene)
Composition: Ethylene-C6 copolymer Melting point: 129°C
Density: 938kg/ ^m3
MFR (2.16kg, 190℃): 3.8g/10min

・LLDPE-3
Non-metallocene LLDPE (linear low-density polyethylene)
Composition: Ethylene-α-olefin copolymer Melting point: 127°C
Density: 941kg/ ^m3
MFR (2.16 kg, 190°C): 4.0 g/10 min

LLDPE-4
Metallocene LLDPE (linear low-density polyethylene)
Composition: Ethylene-C6 copolymer Melting point: 94°C
Density: 904kg/ ^m3
MFR (2.16kg, 190℃): 1.2g/10min

LLDPE-5
Metallocene LLDPE (linear low-density polyethylene)
Composition: Ethylene-α-olefin copolymer Melting point: 130°C
Density: 946kg/ ^m3
MFR (2.16kg, 190℃): 2.8g/10min

LLDPE-6
Non-metallocene LLDPE (linear low-density polyethylene)
Composition: Ethylene-C6 copolymer Melting point: 126°C
Density: 937kg/ ^m3
MFR (2.16kg, 190℃): 2.3g/10min

・HDPE-1
HDPE (High-density polyethylene)
Composition: Ethylene copolymer Melting point: 131°C
Density: 949kg/ ^m3
MFR (2.16kg, 190℃): 1.1g/10min

• Antiblocking agent masterbatch (Product name: EAZ-30, manufactured by Prime Polymer Co., Ltd.)
A composition containing 30% by mass of synthetic zeolite as an antiblocking agent and 70% by mass of low-density polyethylene as a base resin.

- Slipping agent masterbatch (Product name: ESQ-4, manufactured by Prime Polymer Co., Ltd.)
A composition containing 4% by mass of erucic acid amide as a slipping agent and 96% by mass of low-density polyethylene as a base resin.

(Example 1)
Each layer of resin listed in Table 1 was supplied to a separate extruder, and a sealant film for retort food packaging containers was formed by the T-die method. This film consisted of a three-layer co-extruded film with a total thickness of 50 μm, comprising a seal layer (A), a core layer (B), and a laminate layer (C), with a thickness ratio of 15:65:20.
In addition to the resins of each layer listed in Table 1, 6% by mass of antiblocking agent masterbatch and 1.5% by mass of slipping agent masterbatch were added to the seal layer (A), and 1% by mass of antiblocking agent masterbatch was added to the laminate layer (C).
The obtained sealant films for retort food packaging containers were evaluated for heat resistance (suitability for retort packaging) and low-temperature impact resistance (drop rupture) according to the method described above.
The results are shown in Table 1.

(Examples 2 to 4, and Comparative Examples 1 to 3)
Except for changing the resin of each layer as shown in Table 1, sealant films for retort food packaging containers were prepared and evaluated in the same manner as in Example 1.
The results are shown in Table 1.

The sealant film for retort food packaging containers of the present invention combines heat resistance at retort temperatures and impact resistance at low temperatures at a level far exceeding the limitations of conventional technology. Therefore, it can provide retort food packaging containers that are suitable not only for room temperature storage but also for low-temperature distribution (frozen to chilled), thus offering high applicability in various industrial fields such as food processing, distribution, restaurants, tourism, and healthcare.

Claims (5)

A sealant film for retort food packaging containers having (A) a seal layer, (B) a core layer, and (C) a laminate layer, each containing linear polyethylene,
(A) The density of the seal layer is 940 kg/ ^m³ or more and 960 kg/ ^m³ or less.
The density of both the (B) core layer and the (C) laminate layer is 932 kg/ ^m³ or more and less than 940 kg/ ^m³ .
(B) The core layer and (C) the laminate layer are both composed of linear polyethylene polymerized using a metallocene catalyst.
Sealant film for retort food packaging containers. The sealant film for retort food packaging containers according to claim 1, wherein the melt flow rate (190°C, 2160 g load) of both the core layer (B) and the laminate layer (C) is 6.0 g/10 min or less. (A) A sealant film for retort food packaging containers according to claim 1 or 2, wherein the density of the sealing layer is 940 kg/ ^m³ or more and 953 kg/ ^m³ or less. A film for retort food packaging containers comprising a sealant film for retort food packaging containers and a base film according to any one of claims 1 to 3, wherein the base film is laminated with the sealant film for retort food packaging containers in a (C) laminate layer. A retort food packaging container comprising the film for retort food packaging containers described in claim 4.