JP7809963B2 - Multilayer packaging film and packaging materials using the same - Google Patents

Description

The present invention relates to a packaging material for packaging food and other items, and to a multilayer film that combines high antibacterial and antifungal properties with safety, even when the food being packaged contains little moisture, thereby extending the edible period of the food, and a packaging material made from said multilayer film.

In recent years, the disposal of food that has passed its expiration date or fruit and vegetables that have lost their freshness has become a problem as food waste. Furthermore, at home, people sometimes store leftover food by resealing food packaging after opening it, but this can lead to mold growing on the food, which is then discarded. This has led to a growing demand for packaging materials that can maintain the freshness and hygiene of food and extend the edible period of food.

Films containing antibacterial agents are being considered as packaging materials to extend the edible period of food. When antibacterial agents are added to food packaging materials, they must be safe enough to come into contact with the food contained inside and the human body.
Powders made by calcining raw materials such as seashells or eggshells are known as highly safe antibacterial additives. Calcium hydroxide, the main component of these calcined powders, reacts with water to generate hydroxide ions, creating a strongly alkaline environment with a pH of 10 or higher. Viruses, bacteria, and fungi that cause mold are inactivated in a strongly alkaline environment, so antibacterial agents containing calcium hydroxide exhibit antibacterial, antiviral, and antifungal effects.

Patent Document 1 is known as a packaging material containing an antibacterial agent containing calcium hydroxide. The food packaging film in Patent Document 1 is made of a film or sheet primarily made of a plastic resin, and has an antibacterial layer containing antibacterial powder primarily made of calcium hydroxide obtained by baking and hydrating seashells at high temperatures.

Patent Document 2 is also known as a packaging material containing an antibacterial agent containing calcium hydroxide. The food packaging film in Patent Document 2 has a first resin layer containing a first resin and calcium hydroxide particles as its outermost layer, and the first resin is a food packaging film containing an ethylene-propylene-butene terpolymer, which prevents fresh foods such as vegetables from fermenting or spoiling due to the action of various bacteria.

Patent Document 3 is also known as a packaging material containing an antibacterial agent containing calcium hydroxide. The food packaging container in Patent Document 3 is an antibacterial container made by laminating a resin base layer and a functional layer, and the functional layer is formed by blending antibacterial powder into a resin layer, and the antibacterial powder contains calcium hydroxide derived from at least one of seashells and eggshells.

However, these conventional packaging materials containing antibacterial agents including calcium hydroxide were used for foods with a high moisture content, such as lettuce and salad. On the other hand, when packaging foods with a relatively low moisture content, such as bread, there is less moisture available to react with the calcium hydroxide, resulting in a decrease in antibacterial performance. In other words, conventional packaging materials containing antibacterial agents including calcium hydroxide were unable to exert sufficient antibacterial properties depending on the food being packaged.

JP 2017-030842 A Japanese Patent Application Laid-Open No. 2020-196489 Japanese Patent Application Laid-Open No. 2019-014500

In light of the above situation, the objective of the present invention is to provide a multilayer film that can exhibit antibacterial properties even when used to package foods with relatively low moisture content, and a packaging material that uses the film.

As a result of intensive research conducted by the present inventors to solve the above problems, they discovered that the above problems can be solved by a multilayer film including a sealing layer, in which the sealing layer contains an antibacterial agent containing calcium hydroxide, and in which the pH of the sealing layer side surface of the multilayer film is 8.5 or higher and the water vapor permeability of the multilayer film is 9 g/m ² /24 hours or less, and thus completed the present invention.

That is, the present invention provides a multilayer film including a sealing layer, characterized in that the sealing layer contains an antibacterial agent containing calcium hydroxide, the pH of the sealing layer side surface of the multilayer film being 8.5 or higher, and the water vapor permeability of the multilayer film being 9 g/m ² /24 hours or lower.

The present invention also provides a multilayer film further comprising a cyclic olefin resin layer.

The present invention also provides a multilayer film in which the thickness of the cyclic olefin resin layer is 2 μm or more.

The present invention also provides a multilayer film comprising a sealing layer and a cyclic olefin resin layer, wherein the sealing layer contains an antibacterial agent containing calcium hydroxide.

The present invention also provides a multilayer film in which the ratio of the thickness of the sealing layer to the particle size of the antibacterial agent is 1:8 to 8:1.

The present invention also provides packaging materials having the above-mentioned multilayer film, and packaging materials for food packaging.

The multilayer film of the present invention and packaging materials using it exhibit antibacterial and antifungal properties, even when used to package foods with relatively low moisture content, thereby extending the edible period of the food.

The following provides a detailed description of the multilayer film of the present invention and the various components that make up the packaging material made using it.

<Sealing layer>
The multilayer film of the present invention includes a sealing layer, which constitutes a surface layer on one side of the multilayer film of the present invention and is a layer that comes into direct contact with food or the like contained therein.
The sealing layer also contains an antibacterial agent containing calcium hydroxide, and has an antibacterial effect.

The sealing layer of the multilayer film of the present invention preferably contains a polyolefin resin as its main component. Examples of such polyolefin resins include polyethylene resins and polypropylene resins, and there are no particular limitations on the resin as long as it can be laminated with layers other than the sealing layer of the multilayer film.

Examples of the polyethylene resin include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE), and high-density polyethylene (HDPE), and these may be used alone or in combination. Of these, linear low-density polyethylene is preferred.

The linear low-density polyethylene is produced by copolymerizing ethylene monomer, the main component, with an α-olefin such as butene-1, hexene-1, octene-1, or 4-methylpentene as a comonomer, using a low-pressure radical polymerization method with a single-site catalyst. The comonomer content in LLDPE is preferably in the range of 0.5 to 10 mol%, and more preferably in the range of 1 to 7 mol%. Using butene-1 as a comonomer is preferred because it improves transparency, impact resistance, and tearability, and in this case, the butene monomer content is most preferably in the range of 1 to 5 mol%.

The single-site catalyst may be a metallocene catalyst system that combines a metallocene compound of a transition metal from Group IV or V of the periodic table with an organoaluminum compound and/or an ionic compound. Furthermore, because single-site catalysts have uniform active sites, the molecular weight distribution of the resulting resin is sharper than with multi-site catalysts, which have non-uniform active sites. This results in less precipitation of low-molecular-weight components when the resin is formed into a film, and is therefore preferred because it produces a resin with excellent physical properties, such as stable seal strength and excellent blocking resistance.

The polyethylene resin preferably has a melt flow rate (hereinafter sometimes referred to as MFR) (at 190°C) of 0.5 to 30.0 g/10 min, as this facilitates extrusion molding, and more preferably an MFR of 2.0 to 15.0 g/10 min. Furthermore, if the polyethylene resin has a melting point of 80 to 135°C, the film is less likely to shrink during heat sealing, improving packaging suitability. A melting point of 90 to 130°C is more preferred.

Examples of the polypropylene-based resin include propylene homopolymer, propylene-ethylene copolymer, propylene-butene-1 copolymer, propylene-ethylene-butene-1 copolymer, and metallocene-catalyzed polypropylene. These may be used alone or in combination. When such a polypropylene-based resin is used in the sealing layer of the present invention, the heat resistance of the film is improved and the heat seal strength can be increased, making it particularly suitable for use as a packaging material for heavy items.

The polypropylene resin preferably has an MFR (at 230°C) of 0.5 to 30.0 g/10 min and a melting point of 120 to 165°C, and more preferably an MFR (at 230°C) of 2.0 to 15.0 g/10 min and a melting point of 125 to 162°C. If the MFR and melting point are within these ranges, the film shrinks less during heat sealing, and the film's film-forming properties are also improved.

<Antibacterial agent containing calcium hydroxide>
The antibacterial agent used in the present invention is an antibacterial agent containing calcium hydroxide, which is obtained by calcining seashells or eggshells and then hydrating them. The calcium hydroxide contained in the antibacterial agent is preferably 40 mass% or more relative to the total amount of the antibacterial agent. When the ratio of calcium hydroxide contained in the antibacterial agent is within this range, a sufficient antibacterial effect can be exhibited.
Examples of shells used as materials include scallop shells, abalone shells, turban shells, surf clam shells, sea urchin shells, and coral shells, which may be natural or cultivated. Of these, it is preferable to use scallop shells because of their uniform shell composition and large supply.

Antibacterial agents can be produced by methods known to those skilled in the art. For example, shells can be fired at high temperatures of 850-1200°C in a special electric furnace or the like to convert the calcium carbonate contained in the shells into calcium oxide, which can then be hydrated to produce an antibacterial agent containing calcium hydroxide. Commercially available antibacterial agents can also be used. Commercially available antibacterial agents containing calcium hydroxide include, but are not limited to, Scallow Premium, Scallow Premium S, Scallow Premium HK, and Scallow Premium R (all manufactured by WM Co., Ltd.), and scallop shell fired powder (Unicera Co., Ltd.).

The amount of the antibacterial agent used in the present invention is preferably 0.1% by mass or more, more preferably 3% by mass or more, and particularly preferably 5% by mass or more, based on the total mass of the sealing layer used in the present invention. Using the antibacterial agent in this range in the sealing layer can achieve excellent antibacterial effects. The upper limit of the amount is preferably less than 20% by mass, more preferably less than 15% by mass, and particularly preferably less than 10% by mass, based on the total mass of the sealing layer. Using the antibacterial agent in this range in the sealing layer can achieve both transparency and antibacterial effects in the multilayer film of the present invention.

(Particle size of antibacterial agent)
The particle size of the antibacterial agent containing calcium hydroxide used in the present invention can be adjusted by performing a pulverization process using various pulverizers during the calcination or hydration process of the antibacterial agent. The preferred range of particle size of the antibacterial agent is preferably a lower limit of 0.1 μm or more, more preferably 0.5 μm or more, and particularly preferably 2 μm or more. If the lower limit is within the preferred range, mesh clogging is less likely to occur when the antibacterial agent is made into a masterbatch. Furthermore, the upper limit is preferably less than 1000 μm, more preferably less than 100 μm, and particularly preferably less than 20 μm. If the upper limit is below the preferred range, the antibacterial agent can be uniformly dispersed when mixed with a resin, and transparency is more easily maintained when made into a multilayer film.

The particle size of the antibacterial agent can be measured using a laser diffraction/scattering particle size distribution analyzer (LMS-2000e, manufactured by Seishin Enterprise Co., Ltd.). Specifically, the antibacterial agent particles are dispersed in an IPA solvent in a water tank, and the light diffraction/scattering intensity distribution is measured and analyzed using the aforementioned device. The particle size and volume-based particle distribution can then be calculated to obtain the D50 value, which can be used as the particle size of the antibacterial agent.

<Ratio of sealing layer thickness and antibacterial agent particle size>
The ratio of the thickness of the seal layer used in the present invention to the particle size of the antibacterial agent used in the present invention is preferably 1:8 to 8:1, and particularly preferably 1:2 to 2:1. The reason why the multilayer film of the present invention exhibits good antibacterial effects when the ratio of the seal layer thickness to the particle size of the antibacterial agent is within the preferred range is unclear, but the inventors speculate as follows: Because the particle size of the antibacterial agent is larger than the thickness of the seal layer, the antibacterial agent in the seal layer protrudes from the seal layer, and the protruding antibacterial agent is distributed on the surface of the multilayer film facing the seal layer. As a result, a high antibacterial effect can be obtained with a small amount of antibacterial agent.

In order to add the antibacterial agent used in the present invention to the film, it is preferable to prepare an antibacterial masterbatch by mixing it with a resin in advance. The resin is preferably the same as that used for the sealing layer, because this makes it easier to mix the antibacterial masterbatch with the resin used for the sealing layer.
The method for mixing the antibacterial masterbatch with the resin used in the sealing layer is not particularly limited, and can be any conventionally known method. For example, dry blending or melt blending may be used. Among these, melt blending is preferred. Specifically, for example, the antibacterial masterbatch is preferably produced by melt blending the antibacterial agent and the resin in a melt kneading device such as an extruder, followed by pelletization. Alternatively, a commercially available antibacterial masterbatch may be used. An example of a commercially available antibacterial masterbatch is Scallow Premium Antibacterial PE Masterbatch (WM Corporation).

<Cyclic olefin resin layer>
The multilayer film of the present invention preferably includes a cyclic olefin resin layer, which is mainly composed of a cyclic olefin resin, i.e., the cyclic olefin resin layer contains 60% by mass or more of the cyclic olefin resin relative to the total mass of the cyclic olefin resin layer.

Examples of the cyclic olefin resin include norbornene polymers, vinyl alicyclic hydrocarbon polymers, and cyclic conjugated diene polymers. Of these, norbornene polymers are preferred. Norbornene polymers include ring-opening polymers of norbornene monomers (hereinafter sometimes referred to as "COP") and norbornene copolymers (hereinafter sometimes referred to as "COC") obtained by copolymerizing norbornene monomers with olefins such as ethylene. Hydrogenated products of COP and COC are particularly preferred. The weight-average molecular weight of the cyclic olefin resin is preferably 5,000 to 500,000, and more preferably 7,000 to 300,000.

The norbornene monomers used as raw materials for the norbornene polymers are alicyclic monomers having a norbornene ring. Examples of such norbornene monomers include norbornene, tetracyclododecene, ethylidenenorbornene, vinylnorbornene, ethylidetetracyclododecene, dicyclopentadiene, dimethanotetrahydrofluorene, phenylnorbornene, methoxycarbonylnorbornene, and methoxycarbonyltetracyclododecene. These norbornene monomers may be used alone or in combination of two or more.

The norbornene copolymer (COC) is a copolymer of the norbornene monomer and a copolymerizable olefin. Examples of such olefins include olefins having 2 to 20 carbon atoms, such as ethylene, propylene, and 1-butene; cycloolefins, such as cyclobutene, cyclopentene, and cyclohexene; and non-conjugated dienes, such as 1,4-hexadiene. These olefins can be used alone or in combination of two or more.

The content of norbornene-based monomers in the norbornene-based copolymer (COC) is preferably 40 to 90 mol %, and more preferably 50 to 80 mol %. A content within this range improves the film's rigidity, tearability, and processing stability.

Commercially available products that can be used as the cyclic olefin resin include ring-opening polymers (COPs) of norbornene monomers such as "ZEONOR" manufactured by Zeon Corporation, and norbornene copolymers (COCs) such as "APEL" manufactured by Mitsui Chemicals, Inc. and "TOPAS" manufactured by TICONA.

The content of the cyclic olefin resin in the cyclic olefin resin layer used in the present invention is preferably 60% by mass or more, and from the viewpoint of superior tearability in the longitudinal and transverse directions, it is preferably 80% by mass or more.
The cyclic olefin resin layer may contain a resin other than the cyclic olefin resin as long as the effects of the present invention are not impaired, and the resin other than the cyclic olefin resin is not particularly limited as long as it is a resin that can be mixed with the cyclic olefin resin. For example, it may be a mixture with a low-density polyethylene resin, a mixture with a high-density polyethylene resin, or a mixture of a low-density polyethylene resin, a high-density polyethylene resin, and a cyclic polyolefin resin.

<Thickness of Cyclic Olefin Resin Layer>
The thickness of the cyclic olefin resin layer used in the present invention is preferably 2 μm or more, more preferably 3 μm or more, and particularly preferably 5 μm or more. When the thickness of the cyclic olefin resin layer is 2 μm or more, the layer exhibits a water vapor barrier function, making it difficult for water vapor emitted from the contents to escape to the outside of the package of the present invention, and it is presumed that the water vapor emitted from the contents reacts efficiently with the antibacterial agent used in the present invention containing calcium hydroxide, creating a strongly alkaline environment and exhibiting excellent antibacterial effects.
Furthermore, the multilayer film of the present invention can exhibit water vapor barrier function even when it is thinner than multilayer films using resins other than cyclic olefin resins due to the cyclic olefin resin layer, and is therefore particularly useful as a packaging film, which is required to be thinner from the standpoint of economic efficiency and environmental considerations.

(outermost layer)
The multilayer film of the present invention preferably includes, in addition to the sealing layer and the cyclic olefin resin layer, an outermost layer as the other surface layer that is not the sealing layer.
The resin used in the outermost layer is preferably a polypropylene-based resin. Examples of the polypropylene-based resin include propylene homopolymers and copolymers of propylene and other α-olefins. Examples of the propylene homopolymer include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene, with isotactic polypropylene being preferred.

Examples of the other α-olefins include ethylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-heptene, 4-methyl-pentene-1, and 4-methyl-hexene-1, and two or more of these may be copolymerized simultaneously. Either random copolymerization or block copolymerization can be used as the copolymerization method. Among these, copolymers with ethylene are preferred.
The content of the other α-olefin in the copolymer is preferably 2.0 to 23 mol %, more preferably 2.5 to 15 mol %.

The resin used in the outermost layer preferably contains 50% by mass or more of the polypropylene-based resin, but other resins may be used in combination as long as the effects of the present invention are not impaired. It is preferable that the polypropylene-based resin account for 70% by mass or more, and it is particularly preferable that the polypropylene-based resin account for 90% by mass or more.

(middle class)
The multilayer film of the present invention may include, in addition to the sealing layer and the cyclic olefin resin layer, an intermediate layer located between the sealing layer and the outermost layer. The intermediate layer preferably contains an olefin resin.

Examples of the olefin-based resin used in the intermediate layer include polyethylene-based resins and polypropylene-based resins. Examples of such polyethylene-based resins include low-density polyethylene-based resins and high-density polyethylene-based resins.

The low-density polyethylene resin may be any polyethylene resin having a density of 0.900 to 0.940 g/cm ³ , and examples thereof include polyethylene resins such as very low-density polyethylene (VLDPE), linear low-density polyethylene (LLDPE), and low-density polyethylene (LDPE); ethylene copolymers such as ethylene-vinyl acetate copolymer (EVA), ethylene-methyl methacrylate copolymer (EMMA), ethylene-ethyl acrylate copolymer (EEA), ethylene-methyl acrylate (EMA) copolymer, ethylene-ethyl acrylate-maleic anhydride copolymer (E-EA-MAH), ethylene-acrylic acid copolymer (EAA), and ethylene-methacrylic acid copolymer (EMAA); and ionomers of ethylene-acrylic acid copolymers and ionomers of ethylene-methacrylic acid copolymers. Among these, low-density polyethylene and linear low-density polyethylene are preferred because of their good balance between tearability and pinhole resistance.

The low-density polyethylene may be a branched low-density polyethylene obtained by high-pressure radical polymerization, and is preferably a branched low-density polyethylene obtained by homopolymerizing ethylene by high-pressure radical polymerization.

The linear low-density polyethylene is produced by copolymerizing ethylene monomer, the main component, with an α-olefin such as butene-1, hexene-1, octene-1, or 4-methylpentene as a comonomer, using a low-pressure radical polymerization method with a single-site catalyst. The comonomer content in LLDPE is preferably in the range of 0.5 to 10 mol%, and more preferably in the range of 1 to 7 mol%. Using butene-1 as a comonomer is preferred because it improves transparency, impact resistance, and tearability, and in this case, the butene monomer content is most preferably in the range of 1 to 5 mol%.

The single-site catalyst may be a metallocene catalyst system that combines a metallocene compound of a transition metal from Group IV or V of the periodic table with an organoaluminum compound and/or an ionic compound. Furthermore, because single-site catalysts have uniform active sites, the molecular weight distribution of the resulting resin is sharper than with multi-site catalysts, which have non-uniform active sites. This results in less precipitation of low-molecular-weight components when formed into a film, and is preferable because it produces a resin with excellent physical properties, such as excellent blocking resistance and stable seal strength when an intermediate layer and a seal layer are adjacent to each other.

As mentioned above, the density of the low-density polyethylene resin is 0.900 to 0.940 g/ ^cm3 , but is more preferably in the range of 0.905 to 0.935 g/ ^cm3 . A density within this range provides appropriate rigidity, excellent mechanical strength such as pinhole resistance, and improved film formability and extrusion suitability. The melting point is preferably in the range of 95 to 120°C, more preferably 100 to 130°C. A melting point within this range improves processing stability. The low-density polyethylene resin also preferably has an MFR (190°C, 21.18 N) of 2 to 20 g/10 min, more preferably 3 to 10 g/10 min. An MFR within this range improves film extrusion moldability.

The above-mentioned low-density polyethylene resin has weak mechanical strength, making it relatively brittle and tear-resistant compared to other polyolefin resins. It also maintains interlayer adhesive strength with layers other than the intermediate layer without the need for adhesive resins, and its flexibility results in good pinhole resistance. Furthermore, to improve pinhole resistance, it is preferable to use linear low-density polyethylene.

The high-density polyethylene resin is used to improve tearability and packaging suitability, and may be ^any polyethylene having a density of 0.950 g/cm or more, with polyethylene having a density of 0.955 g/cm ^or more being preferred.

Furthermore, the high-density polyethylene resin is preferably high-density polyethylene (HDPE), which is generally used in extrusion molding such as film formation, and has good fluidity, such as a high-density polyethylene with an MFR (190°C) of 5 to 20 g/10 min, because when melt-kneaded with the low-density polyethylene resin and extruded, it disperses relatively well and produces a film with a smooth surface and good transparency.

Examples of the polypropylene-based resin include those similar to those that can be used in the outermost layer.

As the resin component of the intermediate layer, it is preferable to use the low-density polyethylene resin and the high-density polyethylene resin in combination.
The content of the low-density polyethylene resin in the intermediate layer is preferably 25 to 65% by mass, and particularly preferably 35 to 65% by mass from the viewpoint of superior tear resistance in the longitudinal and transverse directions. The content of the high-density polyethylene resin in the intermediate layer is preferably 35 to 75% by mass, and more preferably 35 to 65% by mass, from the viewpoints of transparency, smoothness, water vapor permeability, and adhesive strength to layers other than the intermediate layer.

<Multilayer film>
The multilayer film of the present invention comprises a sealing layer, which contains an antibacterial agent containing calcium hydroxide, and the pH of the sealing layer side surface of the multilayer film is 8.5 or higher and the water vapor permeability of the multilayer film is 9 g/m ² /24 hours or lower.

<pH of the seal layer side surface>
The pH of the seal layer surface of the multilayer film of the present invention is 8.5 or higher. The pH of the seal layer surface refers to the result of measuring the pH using a flat pH meter (FPH70) manufactured by AS ONE Corporation after dropping 300 μL or more of ultrapure water onto the seal layer surface of the film.

Calcium hydroxide, the main component of the antibacterial agent used in the present invention, reacts with water to generate hydroxide ions, creating a strongly alkaline environment. Viruses, bacteria, and fungi that cause mold are inactivated in a strongly alkaline environment, so antibacterial agents containing calcium hydroxide exhibit antibacterial, antiviral, and antifungal effects.
Therefore, a pH of 8.5 or higher on the sealing layer side surface means that the antibacterial agent used in the present invention generates hydroxide ions and exerts an antibacterial effect.

When the pH is 8.5 or higher, it is above the pH at which fungi such as Penicillium diffus can survive, thereby providing an antifungal effect. Furthermore, when the pH is 9 or higher, it is above the pH at which bacteria such as E. coli and Staphylococcus aureus can survive, providing a stronger antibacterial effect. Furthermore, when the pH is 11 or higher, it is above the pH at which viruses such as norovirus and influenza virus can survive, providing an antiviral effect.

<Water vapor permeability of multilayer film>
The water vapor permeability of the multilayer film of the present invention is a result of measurement using a water vapor permeability meter (LyssyL80-5000, manufactured by Systec Illinois) in accordance with JIS K7129A under conditions of 40°C, 90% RH, and a measurement time of 24 hours. The unit in this case is "g/ ^m2 /24 hours".

The water vapor permeability of the multilayer film of the present invention is preferably 9 g/m ² /24 hours or less. A low water vapor permeability enhances the antibacterial effect. Although the reason for this is unclear, it is presumed that a low water vapor permeability makes it difficult for water vapor emitted from the contents to escape to the outside of the package of the present invention, and the water vapor emitted from the contents reacts efficiently with the antibacterial agent used in the present invention, including calcium hydroxide, to create a strongly alkaline environment.

(Layer structure of multilayer film)
The multilayer film of the present invention includes a sealing layer, and preferably includes a cyclic olefin-based resin layer. Furthermore, in the multilayer film of the present invention, in order to prevent the antibacterial agent contained in the sealing layer from migrating to an adjacent layer, the layer adjacent to the sealing layer is preferably a layer mainly composed of a resin different from that of the sealing layer. In particular, from the viewpoint of water vapor permeability, the layer adjacent to the sealing layer is preferably a cyclic olefin-based resin layer.

Examples of the layer configuration of the multilayer film include, but are not limited to, outermost layer/cyclic olefin-based resin layer/sealing layer, outermost layer/intermediate layer/cyclic olefin-based resin layer/sealing layer, outermost layer/intermediate layer/intermediate layer/cyclic olefin-based resin layer/sealing layer, outermost layer/cyclic olefin-based resin layer/intermediate layer/sealing layer, and outermost layer/intermediate layer/cyclic olefin-based resin layer/intermediate layer/sealing layer.
Also, a laminate structure of outermost layer/cyclic olefin resin layer/sealing layer is preferred, and a laminate structure of outermost layer/intermediate layer/cyclic olefin resin layer/sealing layer is also preferred.

As described above, the ratio of the thickness of the sealing layer to the particle size of the antibacterial agent containing calcium hydroxide used in the present invention is preferably 1:8 to 8:1. Since the particle size of the antibacterial agent is preferably in the range of 2 μm to 20 μm, the thickness of the sealing layer is preferably 2.5 μm to 16 μm.
As described above, the thickness of the cyclic olefin resin layer is preferably 2 μm or more, and more preferably 5 μm or more.

The thickness of the outermost layer is preferably 5 to 35%, more preferably 6 to 25%, of the total thickness of the multilayer film of the present invention, from the viewpoints of film rigidity, suitability for packaging, transparency, surface gloss, and ease of tearing in the lateral direction.
The thickness of the intermediate layer is preferably in the range of 30 to 80% of the total thickness of the multilayer film of the present invention, and more preferably 35 to 70%. When the ratio of the thickness of the intermediate layer to the total thickness of the multilayer film is in this range, the transparency, tearability, pinhole resistance, and heat sealability are improved.

Furthermore, the multilayer film of the present invention preferably has a film thickness of 15 to 90 μm, more preferably 20 to 80 μm. A film thickness within this range provides stable seal strength, suitability for packaging machines, excellent pinhole resistance, easy tearing, and other properties.

The above-mentioned layers of the multilayer film of the present invention may optionally contain components such as antifogging agents, antistatic agents, heat stabilizers, nucleating agents, antioxidants, lubricants, antiblocking agents, release agents, UV absorbers, and colorants, provided that the objects of the present invention are not impaired. In particular, to ensure processability during film formation and packaging suitability for filling machines, the coefficient of friction of the outermost layer and sealing layer, which form the surface of the multilayer film, is preferably 1.5 or less, and more preferably 1.0 or less. Therefore, it is preferable to add lubricants and antiblocking agents to the outermost layer and sealing layer as appropriate.

(Method for manufacturing multilayer film)
The method for producing the multilayer film of the present invention is not particularly limited, but examples include a coextrusion method in which each resin or resin mixture used for the outermost layer, intermediate layer, cyclic olefin resin layer, and seal layer is heated and melted in separate extruders, and then laminated in the molten state in the order of outermost layer/intermediate layer/cyclic olefin resin layer/seal layer using a method such as a coextrusion multilayer die method or a feed block method, and then molded into a film using an inflation method or a T-die chill roll method. This coextrusion method is preferred because it allows relatively free adjustment of the thickness ratio of each layer and produces a multilayer film that is excellent in hygiene and cost performance. Furthermore, the large difference in softening point (melting point) between the low-density polyethylene resin used in the intermediate layer of the present invention and the high-density polyethylene can lead to phase separation or gelation. To prevent such phase separation or gelation, a T-die chill roll method is preferred, which allows melt extrusion at a relatively high temperature.

When laminating the above-mentioned resin mixture into each layer, the dry-blended resin mixture can be directly extruded using a co-extruder to form the layers. Alternatively, the resin mixture can be melt-blended in advance using a melt-kneading device such as a single-screw extruder, twin-screw extruder, or Brabender mixer, then pelletized and extruded using a co-extruder to form the layers.

The multilayer film of the present invention is obtained as a substantially unstretched multilayer film using the above manufacturing method, making it possible to perform secondary forming such as deep drawing by vacuum forming.

Furthermore, to improve adhesion to printing ink and suitability for lamination, it is preferable to subject the outermost layer to a surface treatment. Examples of such surface treatments include surface oxidation treatments such as corona treatment, plasma treatment, chromic acid treatment, flame treatment, hot air treatment, and ozone/ultraviolet treatment, as well as surface roughening treatments such as sandblasting, with corona treatment being preferred.

<Packaging material>
Packaging materials made of the multilayer film of the present invention include packaging bags and containers used for food, pharmaceuticals, industrial parts, miscellaneous goods, magazines, etc. To maximize the effects of the present invention, the contents are preferably food, and particularly preferably food with a relatively low moisture content. Food with a relatively low moisture content refers to food with a water activity value (Aw) of 0.95 or less, including, but not limited to, rice cakes, bread, fish paste products, rice, and confectionery. The multilayer film of the present invention can exhibit suitable antibacterial properties even for food with a water activity value (Aw) of 0.95 or less.

The above-mentioned packaging bag is preferably one formed by overlapping and sealing the sealing layers of the multilayer film of the present invention, or by overlapping and sealing the outermost layer and the sealing layer. For example, two sheets of the multilayer film can be cut to the desired size of the packaging bag, overlapped, and sealed on three sides to form a bag, after which the contents can be filled in through the one unsealed side and sealed. Furthermore, a packaging bag can also be formed by sealing the ends of a roll of film into a cylindrical shape using an automatic packaging machine, and then sealing the top and bottom.

The multilayer film of the present invention can also be used to form packaging bags and containers by overlapping and sealing the sealing layer used in the present invention with another sealable film. In this case, a film with relatively low mechanical strength, such as LDPE or EVA, can be used as the other film. Also, a laminate film can be used in which a film such as LDPE or EVA is bonded to an oriented film with relatively good tearability, such as biaxially oriented polyethylene terephthalate film (OPET) or biaxially oriented polypropylene film (OPP).

(Sealing method)
The multilayer film of the present invention has heat-sealing properties and can be used to form packages by heat sealing. The heat-sealing strength of the multilayer film of the present invention can be adjusted appropriately depending on the intended use. For example, the multilayer film of the present invention is heat-sealed to an A-PET sheet (softening point 77°C, crystallization temperature 126°C) at a temperature of 170°C and a pressure of 0.2 MPa for 1.0 second, and then a 15 mm wide test piece is cut out. The maximum load when peeled in a 180° direction at a tensile speed of 300 mm/min in a thermostatic chamber at 23°C and 50% RH is preferably 5 N/15 mm or more, more preferably 6 N/15 mm or more. The upper limit of the maximum load is preferably less than 20 N/15 mm, more preferably less than 15 N/15 mm. By achieving this peel strength, the multilayer film is less likely to peel or fall off, and is particularly easy to open.
In addition to heat sealing, the multilayer film of the present invention can also be sealed by ultrasonic waves. There are no particular limitations on the method of ultrasonic sealing, and any known ultrasonic sealing method or method using a known ultrasonic sealing device can be appropriately selected depending on the purpose.

For packaging materials using the coextruded multilayer film of the present invention, it is preferable to form any tear initiation portion, such as a V notch, I notch, perforation, or micropores, in the sealed portion to reduce the initial tear strength and improve openability.

Next, the present invention will be explained in more detail using examples and comparative examples.

<Measurement of particle size of antibacterial agent>
The particle size of the antibacterial agent was measured using a laser diffraction/scattering particle size distribution analyzer (LMS-2000e, manufactured by Seishin Enterprise Co., Ltd.). Specifically, the antibacterial agent particles were dispersed in an IPA solvent in a water tank, and the light diffraction/scattering intensity distribution was measured and analyzed using the above-mentioned device. The D50 value, which can be calculated by measuring the particle size and volume-based particle distribution, was used as the particle size of the antibacterial agent. According to this measurement method, the particle size of the antibacterial agent (Scallow Premium S, manufactured by WM Corporation) was 4 μm.

Example 1
The outermost layer resin was a resin mixture of 50 parts by mass of propylene homopolymer (density 0.900 g/cm ³ , MFR 8.0 g/10 min) (hereinafter referred to as HOPP) and 50 parts by mass of propylene-ethylene random copolymer (density 0.900 g/cm ³ , MFR 7.0 g/10 min) (hereinafter referred to as COPP). The intermediate layer resin was a resin mixture of 65 parts by mass of linear low-density polyethylene (density 0.935 g/cm ³ , MFR 4.0 g/10 min) (hereinafter referred to as LLDPE) and 35 parts by mass of high-density polyethylene (density 0.960 g/cm ³ , MFR 8.0 g/10 min) (hereinafter referred to as HDPE). The resin for the cyclic olefin resin layer was 100 parts by mass of norbornene copolymer (density 1.010 g/cm ³ , MFR 7.0 g/10 min) (hereinafter referred to as COC). The resin for the seal layer was a resin mixture of 91.4 parts by mass of LLDPE and 8.6 parts by mass of Scallow Premium antibacterial PE masterbatch (manufactured by WM Co., Ltd., particle size 4 μm, antibacterial concentration 35%) (hereinafter referred to as antibacterial PEMB). These resins were fed to an extruder for the outermost layer (50 mm diameter), an extruder for the intermediate layer (50 mm diameter), an extruder for the cyclic olefin resin layer (50 mm diameter), and an extruder for the seal layer (50 mm diameter), respectively, and melted at 200-230°C. The molten resins were then fed to a co-extrusion multilayer film manufacturing apparatus using a T-die/chill roll method (feed block and T-die temperature: 250°C) equipped with a feed block, and co-melt extrusion was carried out to obtain a multilayer film having a four-layer structure of outermost layer/intermediate layer/cyclic olefin resin layer/seal layer, with thicknesses of 9.2 μm/20.0 μm/3.6 μm/7.2 μm (total 40 μm). The concentration of the antibacterial agent contained in the seal layer was 3% by mass.

Example 2
A multilayer film of Example 2 was obtained in the same manner as in Example 1, except that the resin composition of the seal layer in Example 1 was changed to a resin mixture of 81.4 parts by mass of LLDPE and 18.6 parts by mass of antibacterial MB.

Example 3
A multilayer film of Example 3 was obtained in the same manner as in Example 2, except that the thicknesses of the cyclic olefin resin layer and the seal layer in Example 2 were changed to 7.2 μm and 3.6 μm, respectively.

Example 4
A multilayer film of Example 4 was obtained in the same manner as in Example 3, except that the thicknesses of the intermediate layer and the cyclic olefin resin layer in Example 3 were changed to 12.8 μm and 14.4 μm, respectively.

Example 5
A multilayer film of Example 5 was obtained in the same manner as in Example 4, except that the resin composition of the seal layer in Example 4 was changed to a resin mixture of 77.1 parts by mass of LLDPE and 22.9 parts by mass of antibacterial MB.

Example 6
A multilayer film of Example 6 was obtained in the same manner as in Example 4, except that the thicknesses of the outermost layer/intermediate layer/cyclic olefin resin layer/sealing layer in Example 4 were changed to 6.9 μm/9.6 μm/10.8 μm/2.7 μm (total 30 μm).
Example 7
The antibacterial masterbatch for the sealing layer was prepared by melt-blending 20 parts by mass of an antibacterial agent (Scallow Premium S, manufactured by WM Co., Ltd.) and 80 parts by mass of COPP in an extruder (diameter 50 mm) to prepare compounded pellets (hereinafter referred to as antibacterial PPMB). The multilayer film of Example 7 was obtained in the same manner as in Example 3, except that the resin composition of the sealing layer in Example 3 was changed to a resin mixture of 60 parts by mass of LLDPE and 40 parts by mass of antibacterial PPMB.

(Comparative Example 1)
A multilayer film of Comparative Example 1 was obtained in the same manner as in Example 1, except that the resin composition of the seal layer in Example 1 was changed to 100 parts LLDPE (not containing antibacterial MB).

(Comparative Example 2)
In Example 1, 100 parts by mass of COPP was used as the resin for the outermost layer. Furthermore, a resin mixture of 70 parts by mass of a propylene-ethylene random copolymer (density 0.900 g/cm ³ , MFR 5.0 g/10 min) (hereinafter referred to as COPP (2)) and 30 parts by mass of a propylene-1-butene copolymer (density 0.90 g/cm ³ , MFR 4.0 g/10 min) (hereinafter referred to as BP-1) was used as the resin for the seal layer. Furthermore, a resin mixture of 80 parts by mass of HOPP and 20 parts by mass of COPP (2) was used as the resin for the intermediate layer. A multilayer film of Comparative Example 2 was obtained in the same manner as in Example 1, except for changing the resin composition of each layer.

(Comparative Example 3)
In Comparative Example 2, the resin composition of the seal layer was changed to a resin mixture of 51.4 parts by mass of COPP (2), 30 parts by mass of BP-1, and 18.6 parts by mass of antibacterial PEMB. A multilayer film of Comparative Example 3 was obtained in the same manner as in Comparative Example 2.
(Comparative Example 4)
A multilayer film of Comparative Example 4 was obtained in the same manner as in Example 7, except that the cyclic olefin resin layer in Example 7 was omitted and the thicknesses of the outermost layer/intermediate layer/sealing layer were changed to 8.4 μm/18.3 μm/3.3 μm (total 30 μm).

<pH Measurement>
The pH of the sealing layer surface was measured for the multilayer films produced in Examples 1 to 7 and Comparative Examples 1 to 4. The pH was measured by dropping 300 μL or more of ultrapure water onto the sealing layer surface using a flat pH meter (FPH70) manufactured by AS ONE Corporation.

<Measurement of water vapor permeability>
The water vapor permeability of each of the multilayer films produced in Examples 1 to 7 and Comparative Examples 1 to 4 was measured using a water vapor permeability meter (LyssyL80-5000, manufactured by Systec Illinois, Inc.) under conditions of 40°C, 90% RH, and a measurement time of 24 hours in accordance with JIS K7129 A.

(Creating packaging bags)
Each of the multilayer films produced in Examples 1 to 7 and Comparative Examples 1 to 4 was cut into an A4 size piece, folded in half, and heat-sealed at 0.2 MPa, 1 second, and 120°C along the two short sides, leaving one long side, to produce a packaging bag. Two packaging bags were produced for each type of multilayer film, and the test was conducted with N=2.
A commercially available bag of bread (6 slices) was opened and quickly removed while wearing nitrile gloves, one slice of bread was placed in each packaging bag, and the open long side was impulse sealed. The bread-containing packaging bags made from the multilayer films of Examples 1 to 7 and Comparative Examples 1 to 4 (N=2, a total of 22 bags) were placed on a tray and stored in a constant temperature and humidity room at 23°C and 55%RH.

<Calculating mold growth area>
The packaging bag containing the bread was visually inspected 12 days after sealing, and photographs of each side (front and back) were taken. Using image analysis software (GIMP), the mold area, which was the area where mold was growing, was selected, and the total number of pixels was calculated in the histogram dialog to quantify the proportion of the mold area to the total area of the bread, which had been calculated in advance.
The arithmetic mean value of N=2 was calculated, and the evaluation results were shown in Tables 1 and 2 as mold growth area.

As is clear from the above table, the multilayer films of the present invention in Examples 1 to 7 had a pH of 8.5 or higher and a water vapor permeability of 9 g/m ² /24 hours or lower, and were found to have a high mold growth inhibitory effect. Furthermore, the multilayer films of the present invention in Examples 1 to 7 contained a sealing layer containing an antibacterial agent containing calcium hydroxide and a cyclic olefin resin layer, and it was found that the thicker the cyclic olefin resin layer, the higher the mold growth inhibitory effect.
On the other hand, Comparative Example 1, which did not contain an antibacterial agent, had a low water vapor permeability but a low pH, and no mold growth inhibitory effect was observed. Comparative Example 2, which did not contain an antibacterial agent in the sealing layer and did not have a cyclic olefin-based resin layer, had a low pH and a high water vapor permeability, and no mold growth inhibitory effect was observed. Furthermore, Comparative Examples 3 and 4, which contained an antibacterial agent in the sealing layer but did not have a cyclic olefin-based resin layer, had a high pH but also a high water vapor permeability, and were unable to exhibit a mold growth inhibitory effect.

Claims (7)

A multilayer film including a sealing layer and a cyclic olefin-based resin layer, wherein the sealing layer contains an antibacterial agent containing calcium hydroxide,
The thickness of the cyclic olefin resin layer is 2 μm or more,
An antibacterial multilayer film for food packaging, wherein the pH of the surface of the multilayer film on the seal layer side is 8.5 or higher .
The antibacterial multilayer film for food packaging according to claim 1 , wherein the sealing layer and the cyclic olefin resin layer are adjacent to each other.
2. The antibacterial multilayer film for food packaging according to claim 1, wherein the thickness of the cyclic olefin resin layer is 5 μm or more, and the thickness of the multilayer film is 15 to 90 μm.
The particle size of the antibacterial agent is 2 μm or more and less than 20 μm,
The amount of the antibacterial agent is 3% by mass or more and less than 10% by mass based on the total mass of the seal layer.
The antibacterial multilayer film for food packaging according to claim 1.
5. The antibacterial multilayer film for food packaging according to claim 1, wherein the ratio of the thickness of the sealing layer to the particle size of the antibacterial agent is 1:2 to 2:1.
6. The antibacterial multilayer film for food packaging according to claim 1, wherein the multilayer film has a water vapor permeability of 9 g/m ² /24 hours or less.

A packaging material comprising the antibacterial multilayer film for food packaging according to any one of claims 1 to 6.