JP2024062284A - Freshness-preserving packaging containers - Google Patents

Abstract

[Problem] The object is to provide a packaging container that is particularly suitable for preserving the freshness of fruits and vegetables, and has good anti-fogging properties and easy-opening properties. [Solution] A packaging container for preserving the freshness of food, comprising a heat-sealable lid material and a container whose main component is polyethylene terephthalate, wherein the heat-sealable lid material is composed of two or more layers including a base layer whose main component is polyethylene terephthalate and a heat-seal layer containing an anti-fogging agent, the heat-seal strength between the heat-sealable lid material and the container is 1.0 N/15 mm or more and 15 N/15 mm or less, the oxygen transmission rate of the heat-sealable lid material measured in an environment at a temperature of 23°C and a relative humidity of 65% RH is 200 [cc/(m2·d·atm)] or less, and the water vapor transmission rate of the heat-sealable lid material measured in an environment at a temperature of 40°C and a relative humidity of 90% RH is 100 [g/(m2·d)] or less. [Selected Figure] None

Description

The present invention relates to a food packaging container.

Aromatic polyesters, such as polyethylene terephthalate (PET), are widely used as food and beverage containers. For example, containers with a fitting lid on a molded container are used to package soups, salads, raw vegetables, etc., taking advantage of their excellent transparency and airtightness. In particular, when packaging foods such as salads and raw vegetables, moisture from the vegetables can cause the inside of the container to fog up, so anti-fogging properties are required.

Patent Document 1 discloses a packaging container that is manufactured by thermoforming a container body and a lid body coated with an anti-fogging agent using vacuum and compressed air forming, and then bonding the container body and the lid body by heat sealing.

In recent years, there has been a trend towards top-sealed lids in order to make them easier to open, standardize lid materials (mono-materials), and reduce waste by reducing thickness. Top-sealed lids are required to be transparent, airtight, anti-fogging, and easy to open, just like snap-on lids.

JP 2021-14295 A

The present invention aims to provide a packaging container that is particularly suitable for preserving the freshness of fruits and vegetables, and that not only extends the shelf life (best before date) but also enhances the product value (appearance), has good anti-fogging properties, and is easy to open.

As a result of intensive research, the inventors have discovered that a food packaging container composed of a heat-sealable lid material including a base layer mainly composed of polyethylene terephthalate and a heat-seal layer containing an anti-fogging agent, and a container mainly composed of polyethylene terephthalate, has an excellent effect in preserving the freshness of fruits and vegetables, etc., and have completed the following representative inventions.

(Item 1)
A packaging container for preserving the freshness of food, comprising a heat-sealable lid material and a container mainly composed of polyethylene terephthalate,
The heat-sealable lid material is composed of two or more layers including a base layer mainly composed of polyethylene terephthalate and a heat-seal layer containing an anti-fogging agent,
The heat seal strength between the heat sealable lid material and the container is 1.0 N/15 mm or more and 15 N/15 mm or less,
The heat-sealable lid material has an oxygen permeability of 200 cc/(m2·d·atm) or less, as measured in an environment of a temperature of 23° C. and a relative humidity of 65% RH;
The heat-sealable lid material has a water vapor permeability of 100 g/(m2·d) or less in an environment of a temperature of 40° C. and a relative humidity of 90% RH.
(Item 2)
Item 2. The freshness-preserving packaging container according to item 1, wherein the heat seal layer of the heat sealable lid material contains a polyester resin and an antifogging agent, and the mass ratio of the polyester resin to the antifogging agent in the heat seal layer is 99:1 to 80:20.
(Item 3)
Item 3. The freshness-preserving packaging container according to item 1 or 2, wherein the heat-sealable lid material has a breaking strength in either the longitudinal direction or the width direction of 180 MPa or more and 260 MPa or less.
(Item 4)
Item 4. The freshness-preserving packaging container according to any one of items 1 to 3, wherein the heat-sealable lid material has a breaking elongation in either the longitudinal direction or the width direction of 80% or more and 170% or less.
(Item 5)
Item 5. The freshness-preserving packaging container according to any one of items 1 to 4, wherein the heat-sealable lid material has a heat-seal layer, a base layer, and a printing layer in this order.

The heat-sealable lid material of the packaging container of the present invention has low oxygen permeability and a water vapor permeability within a specified range, and the anti-fogging heat seal layer of the heat-sealable lid material contains an anti-fogging agent. By setting the heat seal strength between the container body and the heat-sealable lid material to 1 N/mm or more and 15 N/mm or less, it is possible to extend the shelf life of the contents such as fruits and vegetables and improve their commercial value, and to obtain a packaging container with good anti-fogging properties and easy-opening properties.

FIG. 2 is a schematic diagram of the shape of an A-PET container used in the examples.

The present invention is described in detail below.

(Heat-sealable lid material)
The base layer in the heat-sealable lid material of the present invention is mainly composed of a polyethylene terephthalate resin, uses terephthalic acid as a dicarboxylic acid component, and uses ethylene glycol as a diol component. Here, "mainly composed" means that when the total amount of all components is 100 mol%, it contains 50 mol% or more, preferably 80% or more, and more preferably 90% or more.

Other dicarboxylic acid components and diol components may be copolymerized as long as the object of the present invention is not hindered. The upper limit of the copolymerization amount of the other dicarboxylic acid components and diol components is preferably 15 mol% or less, more preferably 10 mol% or less, and particularly preferably 5 mol% or less, based on the total dicarboxylic acid components or diol components. By setting the copolymerization amount of the dicarboxylic acid components and diol components to 15 mol% or less, thickness unevenness is improved and a decrease in anti-fogging property due to offset of the anti-fogging agent when stored in a roll form can be suppressed.

The other dicarboxylic acid components include aromatic dicarboxylic acids such as isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, 4,4'-dicarboxybiphenyl, and 5-sodium sulfoisophthalic acid, alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexadicarboxylic acid, 2,5-norbornenedicarboxylic acid, and tetrahydrophthalic acid, and aliphatic dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, octadecanedioic acid, fumaric acid, maleic acid, itaconic acid, mesaconic acid, citraconic acid, and dimer acid.

The other diol components mentioned above include aliphatic diols such as 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,3-propanediol, 2-amino-2-ethyl-1,3-propanediol, 2-amino-2-methyl-1,3-propanediol, 1,10-decanediol, dimethyloltricyclodecane, and triethylene glycol; alicyclic diols such as bisphenol A, bisphenol S, bisphenol C, bisphenol Z, bisphenol AP, ethylene oxide adducts or propylene oxide adducts of 4,4'-biphenol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, and 1,4-cyclohexanedimethanol.

The substrate layer in the heat-sealable lid material of the present invention is preferably biaxially stretched in order to reduce thickness unevenness. Biaxial stretching can be performed by a conventional method. For example, an unstretched resin sheet extruded onto a cooling drum is subsequently heated by roll heating, infrared heating, or the like, and stretched in the longitudinal direction to form a longitudinally stretched film. This stretching is preferably performed using the peripheral speed difference between two or more rolls. The longitudinal stretching is usually performed in a temperature range of 50 to 120°C. In addition, the longitudinal stretching ratio is preferably 3.0 to 4.0 times. By setting the stretching ratio at 3.0 times or more, the thickness unevenness of the biaxially oriented film obtained is improved, and not only can the anti-fogging property decrease due to the offset of the anti-fogging agent when stored in a roll form be suppressed, but the mechanical strength is also sufficiently strong.

The longitudinally stretched film is then subjected to the sequential processes of transverse stretching, heat setting, and heat relaxation to become a biaxially oriented film. Transverse stretching is usually performed at a temperature range of 60 to 130°C. The transverse stretching ratio is preferably 3.0 to 5.0 times. By setting the stretching ratio at 3.0 times or more, the thickness unevenness of the biaxially oriented film obtained is improved, and not only can the anti-fogging property decrease due to the anti-fogging agent being offset when stored in a rolled form be suppressed, but the mechanical strength is also sufficiently strong. In addition, by setting the stretching ratio at 5.0 times or less, breakage during film formation can be suppressed. After the transverse stretching, a heat setting treatment is performed, and the preferred temperature range is 170°C to 240°C. The heat setting time is also preferably 1 to 60 seconds. For applications requiring a reduction in the thermal shrinkage rate, a relaxation treatment may be performed as necessary.

The lower limit of the thickness of the base layer in the heat-sealable lid material of the present invention is preferably 5 μm, more preferably 10 μm, and particularly preferably 15 μm. By making it 5 μm or more, it is possible to maintain the impact strength and tear strength. The upper limit of the thickness of the base layer is preferably 100 μm, more preferably 80 μm, and particularly preferably 50 μm. By making it 100 μm or less, it can be suitably used as a lid material.

The upper limit of the thickness unevenness of the base layer in the heat-sealable lid material of the present invention is preferably 15%, more preferably 10%, and particularly preferably 5%. By keeping it at 15% or less, it is possible to prevent localized winding stress from being applied to areas with poor thickness unevenness when stored in roll form, and as a result, it is possible to suppress a decrease in anti-fogging properties due to the anti-fogging agent being transferred to the non-anti-fogging surface.

A printing layer may be laminated on the substrate layer in the heat-sealable lid material of the present invention. As the printing ink for forming the printing layer, water-based and solvent-based resin-containing printing inks can be preferably used. Examples of the resin used in the printing ink include acrylic resins, urethane resins, polyester resins, vinyl chloride resins, vinyl acetate copolymer resins, and mixtures thereof. The printing ink may contain known additives such as antistatic agents, light blocking agents, ultraviolet absorbers, plasticizers, lubricants, fillers, colorants, stabilizers, lubricants, defoamers, crosslinking agents, anti-blocking agents, and antioxidants.

The printing method for providing the printing layer is not particularly limited, and known printing methods such as offset printing, gravure printing, and screen printing can be used. To dry the solvent after printing, known drying methods such as hot air drying, heated roll drying, and ultraviolet drying can be used.

The base layer in the heat-sealable lid material of the present invention may be provided with a gas barrier layer such as an inorganic thin film layer or a metal layer, as long as this does not impair the object of the present invention.

When an inorganic thin film layer is used as the gas barrier layer, the inorganic thin film layer is a thin film made of a metal or an inorganic oxide. There are no particular limitations on the material that forms the inorganic thin film layer as long as it can be made into a thin film, but from the viewpoint of gas barrier properties, inorganic oxides such as aluminum, silicon oxide (silica), aluminum oxide (alumina), and mixtures of silicon oxide and aluminum oxide are preferred. In particular, a composite oxide of silicon oxide and aluminum oxide is preferred from the viewpoint of achieving both flexibility and density of the thin film layer.

In this composite oxide, the mixing ratio of silicon oxide and aluminum oxide is preferably in the range of 20 to 70% Al by mass ratio of the metal content. On the other hand, if it is 70% or less, the inorganic thin film layer can be softened, and the thin film can be prevented from being destroyed during secondary processing such as printing and lamination, resulting in a decrease in gas barrier properties. Note that silicon oxide here refers to various silicon oxides such as SiO and _SiO2, or mixtures thereof, and aluminum oxide refers to various aluminum oxides such as AlO and _{Al2O3 ,} or mixtures thereof.

The thickness of the inorganic thin film layer is usually 1 to 100 nm, preferably 5 to 50 nm. If the thickness of the inorganic thin film layer is 1 nm or less, more satisfactory gas barrier properties are likely to be obtained. On the other hand, if the thickness is 100 nm or less, it is advantageous in terms of bending resistance and manufacturing costs.

The method for forming the inorganic thin film layer is not particularly limited, and may be any known deposition method such as a physical deposition method (PVD method) such as a vacuum deposition method, a sputtering method, or an ion plating method, or a chemical deposition method (CVD method). A typical method for forming the inorganic thin film layer will be described below using a silicon oxide/aluminum oxide thin film as an example. For example, when a vacuum deposition method is adopted, a mixture of SiO ₂ and Al ₂ O ₃ , or a mixture of SiO ₂ and Al, is preferably used as the deposition raw material. Particles are usually used as these deposition raw materials, and in this case, it is desirable that the size of each particle is such that the pressure during deposition does not change, and the preferred particle diameter is 1 to 5 mm. Heating can be performed using methods such as resistance heating, high-frequency induction heating, electron beam heating, and laser heating. It is also possible to introduce oxygen, nitrogen, hydrogen, argon, carbon dioxide gas, water vapor, etc. as a reactive gas, or to adopt reactive deposition using means such as ozone addition or ion assist. Furthermore, the film formation conditions can be arbitrarily changed, such as applying a bias to the deposition target (laminated film to be subjected to deposition), or heating or cooling the deposition target. The deposition material, reactive gas, bias, heating/cooling of the deposition target, etc. can be changed in the same manner when the sputtering method or CVD method is adopted. Furthermore, a printing layer may be laminated on the inorganic thin film layer.

(Heat seal layer containing anti-fog agent)
The heat seal layer containing the antifogging agent in the heat sealable lid material of the present invention preferably contains as its constituent a polyester resin having a chemical structure obtained by polycondensation of a carboxylic acid component consisting of a divalent or higher polycarboxylic acid compound and an alcohol component consisting of a divalent or higher polyalcohol compound. The heat seal layer containing the antifogging agent is required to exhibit antifogging properties when used as a lid material, and is therefore desirably located as the outermost layer of the heat sealable lid material.

As the polyvalent carboxylic acid component, aromatic dicarboxylic acid or aliphatic carboxylic acid is preferred, and aromatic dicarboxylic acid is more preferred. The lower limit of the copolymerization amount of the dicarboxylic acid component is preferably 40 mol% or more, more preferably 45 mol% or more, and particularly preferably 50 mol% or more, based on the total amount of the carboxylic acid component. By making it 40 mol% or more, sufficient heat seal strength with the polyester container can be obtained. The upper limit of the copolymerization amount of the dicarboxylic acid component is preferably 80 mol% or more, more preferably 75 mol% or more, and particularly preferably 70 mol% or more, based on the total amount of the carboxylic acid component. By making it 80 mol% or less, it is possible to prevent the heat seal strength with the polyester container from being too strong, resulting in poor easy-opening properties.

Dicarboxylic acid components include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, and nalthane dicarboxylic acid, aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, and dodecane dicarboxylic acid, p-oxybenzoic acid, p-(hydroxyethoxy)benzoic acid, fumaric acid, maleic acid, itaconic acid, hexahydrophthalic acid, tetrahydrophthalic acid, 1,4-cyclohexanedicarboxylic acid, trimellitic acid, trimesic acid, and pyromellitic acid, etc.

As the polyhydric alcohol component, an aliphatic diol is preferred. The lower limit of the copolymerization amount of the diol component is preferably 70 mol%, more preferably 75 mol%, and particularly preferably 80 mol%. By making it 70 mol% or more, sufficient heat seal strength with a polyester container can be obtained.

Diol components include, but are not limited to, ethylene glycol, propylene glycol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 2-methyl-1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, etc.

The heat seal layer containing the antifogging agent in the heat sealable lid material of the present invention may contain at least two different polyester resins (A) and (B). In particular, by using at least two polyester resins with different glass transition temperatures, cohesive failure can be selectively caused in the heat seal layer, making it possible to achieve easy opening over a wide temperature range.

The glass transition temperature of the polyester resin (A) is preferably in the range of 0°C to 40°C, more preferably in the range of 5°C to 35°C, and particularly preferably in the range of 10°C to 30°C. By being within the above range, it is possible to obtain easy opening while maintaining the heat seal strength necessary to retain the contents.

The lower limit of the reduced viscosity (ηsp/c) of the polyester resin (A) is preferably 0.2 dl/g, more preferably 0.4 dl/g, and particularly preferably 0.6 dl/g. By making it 0.2 dl/g or more, the resin cohesive force is expressed, and the heat seal strength is expressed.

The lower limit of the number average molecular weight (Mn) of the polyester resin (A) is preferably 5,000, more preferably 10,000, and particularly preferably 15,000. By making it 5,000 or more, the resin cohesive force is exerted and the heat seal strength is exerted.

The glass transition temperature of the polyester resin (B) is preferably in the range of 41°C to 80°C, more preferably in the range of 46°C to 75°C, and particularly preferably in the range of 51°C to 60°C. By being within the above range, it is possible to obtain easy opening while maintaining the heat seal strength necessary to retain the contents.

The lower limit of the reduced viscosity (ηsp/c) of the polyester resin (B) is preferably 0.1 dl/g, more preferably 0.2 dl/g, and particularly preferably 0.3 dl/g. By making it 0.1 dl/g or more, the resin cohesive force is exerted and the heat seal strength is exerted.

The lower limit of the number average molecular weight (Mn) of the polyester resin (B) is preferably 2000, more preferably 5000, and particularly preferably 10000. By making it 2000 or more, the resin cohesive force is exerted and the heat seal strength is exerted.

The lower limit of the mass ratio of polyester resin (A) to polyester resin (B) is preferably polyester resin (A): polyester resin (B) = 50: 50 mass%, more preferably 45: 55 mass%, particularly preferably 60: 40 mass%. By making it 50: 50 mass% or more, the anti-fogging layer can be made brittle, the heat seal strength can be within the range described in the claims over a wide temperature range, and easy opening can be obtained.
The upper limit of the mass ratio of polyester resin (A) to polyester resin (B) is preferably polyester resin (A): polyester resin (B) = 90: 10 mass%, more preferably 85: 15 mass%, and particularly preferably 80: 20 mass%. By making it 90: 10 mass% or less, the seal strength of the seal surface can be increased, the heat seal strength can be within the range described in the claims over a wide temperature range, and easy opening can be obtained.

The heat seal layer containing the antifogging agent of the heat sealable lid material of the present invention preferably contains an antifogging agent. The antifogging agent is not particularly limited as long as it provides antifogging properties, and for example, anionic surfactants, nonionic surfactants, cationic surfactants, amphoteric surfactants, etc. can be used. Of these, it is preferable to use nonionic surfactants. Here, antifogging properties refer to the ability to prevent fogging of the inner surface of the packaging material due to the evaporated moisture when water is poured into a packaging container and sealed with a lid material. In particular, the lid material of packaging containers for fresh foods such as vegetables and fruits may require excellent antifogging properties because the evaporation of moisture can make it difficult to see the contents, reducing the product value.

For example, anionic surfactants include sulfate salts of higher alcohols, higher alkyl sulfonates, higher carboxylates, alkylbenzene sulfonates, polyoxyethylene alkyl sulfate salts, polyoxyethylene alkyl phenyl ether sulfate salts, and vinyl sulfosuccinate salts. For nonionic surfactants, compounds having a polyoxyethylene structure, such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyethylene glycol fatty acid esters, ethylene oxide propylene oxide block copolymers, polyoxyethylene fatty acid amides, and ethylene oxide-propylene oxide copolymers, and sorbitan derivatives can be mentioned. For cationic surfactants, alkylamine salts, dialkylamine salts, trialkylamine salts, alkyltrimethylammonium chloride, dialkyldimethylammonium chloride, and alkylbenzalkonium chloride can be mentioned. For amphoteric surfactants, lauryl betaine and lauryl dimethylamine oxide can be mentioned.

Specific examples of nonionic surfactants include sorbitan-based surfactants such as sorbitan monostearate, sorbitan distearate, sorbitan monopalmitate, sorbitan dipalmitate, sorbitan monobehenate, sorbitan dibehenate, sorbitan monolaurate, and sorbitan dilaurate; glycerin-based surfactants such as glycerin monolaurate, glycerin dilaurate, diglycerin monopalmitate, diglycerin dipalmitate, glycerin monostearate, glycerin distearate, diglycerin monostearate, diglycerin distearate, diglycerin monolaurate, and diglycerin dilaurate; polyethylene glycol monostearate, polyethylene glycol Examples of such surfactants include polyethylene glycol surfactants such as lycol monopalmitate, trimethylolpropane surfactants such as trimethylolpropane monostearate, diethanolalkylamine and diethanolalkylamide surfactants such as lauryl diethanolamine, oleyl diethanolamine, stearyl diethanolamine, lauryl diethanolamide, oleyl diethanolamide, and stearyl diethanolamide, pentaerythritol surfactants such as pentaerythritol monopalmitate, and mono- and distearates of sorbitan-diglycerin condensates. These surfactants can be used alone or in combination of two or more.

Specific examples of cationic surfactants include amine salts such as laurylamine acetate, triethanolamine monoformate, and stearamidoethyl diethylamine acetate, and quaternary ammonium salts such as lauryltrimethylammonium chloride, stearyltrimethylammonium chloride, dilauryldimethylammonium chloride, distearyldimethylammonium chloride, lauryldimethylbenzylammonium chloride, and stearyldimethylbenzylammonium chloride. These can be used alone or in combination of two or more.

The lower limit of the content of the antifogging agent in the heat seal layer containing the antifogging agent in the heat sealable lid material of the present invention is preferably 99:1, more preferably 97:3, and particularly preferably 94:6, in terms of the mass ratio of the polyester resin (A or A+B) to the antifogging agent. Antifogging properties are exhibited by making it 99:1 or more. The upper limit of the content of the antifogging agent is preferably 80:20, more preferably 83:17, and particularly preferably 86:14, in terms of the mass ratio of the polyester resin (A or A+B) to the antifogging agent. By making it 80:20 or more, sufficient heat seal strength with the polyester container can be obtained.

The heat seal layer containing the antifogging agent in the heat sealable lid material may contain an antiblocking agent for the purpose of imparting blocking resistance. Examples of antiblocking agents include inorganic particles, organic particles, waxes, etc., and can be contained to the extent that the heat seal strength is not reduced. These antiblocking agents can be used alone or in combination of two or more types. The content of the antiblocking agent is preferably 0.1 mass% or more, more preferably 0.3 mass%, and particularly preferably 0.5 mass%, in terms of the solid content concentration of the antifogging layer. If it is 0.1 mass% or more, blocking resistance is expressed. The content of the antiblocking agent is preferably 5.0 mass% or less, more preferably 4.5 mass%, and particularly preferably 4.0 mass%, in terms of the solid content concentration of the antifogging layer. If it is 5.0 mass% or less, the heat seal strength is not inhibited.

The lower limit of the thickness of the heat seal layer containing the antifogging agent in the heat sealable covering material of the present invention is preferably 0.1 μm, more preferably 0.2 μm, and particularly preferably 0.4 μm. By making it 0.1 μm or more, antifogging properties and heat sealing properties are exhibited. The upper limit of the thickness of the heat seal layer is preferably 5 μm, more preferably 3 μm, and particularly preferably 1 μm. By making it 5 μm or less, it is possible to prevent the thickness unevenness of the laminate from worsening, and as a result, it is possible to suppress the deterioration of antifogging properties due to the offset of the antifogging agent.

The upper limit of thickness unevenness of the heat-sealable lid material of the present invention is preferably 15%, more preferably 10%, and particularly preferably 5%. By keeping it at 15% or less, it is possible to prevent localized winding stress from being applied to areas with poor thickness unevenness when stored in roll form, and as a result, it is possible to suppress a decrease in anti-fogging properties due to the anti-fogging agent being transferred to the non-anti-fogging surface.

(Physical properties of heat sealable lid material)
The heat-sealable lid material of the present invention preferably has a haze of 1% or more and 15% or less. If the haze exceeds 10%, the transparency of the film is poor, and when the film is used as a package, the visibility of the contents is poor. The upper limit of the haze is more preferably 13% or less, and particularly preferably 11% or less. The lower the haze, the higher the transparency, which is preferable, but the lower limit at the current technical level is 1%, and even if it is 2% or more, it can be said that it is sufficiently preferable for practical use.

The lower limit of the breaking strength in the longitudinal or transverse direction of the heat-sealable lid material of the present invention is preferably 180 MPa, more preferably 185 MPa, and particularly preferably 190 MPa. If it is 180 MPa or more, the mechanical strength will be sufficient when used as a lid material for a container. The upper limit of the breaking strength is not particularly limited, but is preferably 260 MPa, more preferably 255 MPa, and particularly preferably 250 MPa. If it is 260 MPa or less, breaking caused by foreign matter during the stretching process can be substantially suppressed, and film formability will be good.

The lower limit of the longitudinal or transverse breaking elongation of the heat-sealable lid material of the present invention is preferably 80%, more preferably 90%, and particularly preferably 100%. If it is 80% or more, it is possible to substantially suppress breakage caused by foreign matter during the stretching process, and the film-forming properties are good. The upper limit of the breaking elongation is not particularly limited, but is preferably 170%, more preferably 160%, and particularly preferably 150%. If it is 170% or less, the mechanical strength when used as a lid material for a container is sufficient.

The heat-sealable lid material of the present invention has an oxygen permeability of 200 [cc/( ^m2 ·d·atm)] or less under an environment of 23°C and 65% RH. This is because the oxygen concentration in the container changes when fruits and vegetables are placed in the container. In the container, oxygen is consumed by the respiration of the fruits and vegetables, generating carbon dioxide. By setting the oxygen permeability of the film within the above range, the exchange of oxygen with the outside is reduced, making it easier to create a low-oxygen state that is preferable for MA packaging, and as a result, the respiration of the fruits and vegetables is suppressed, leading to an extension of the shelf life. If the oxygen permeability exceeds 200 [cc/( ^m2 ·d·atm)], the respiration rate of the fruits and vegetables increases, which is likely to cause discoloration, etc., and is therefore undesirable. The oxygen permeability is preferably 150 [cc/( ^m2 ·d·atm)] or less, and more preferably 100 [cc/( ^m2 ·d·atm)] or less.

The heat-sealable lid material of the present invention has a water vapor permeability of 100 [g/( ^m2 ·d)] or less under an environment of a temperature of 40° C. and a relative humidity of 90% RH. If the water vapor permeability exceeds 100 [g/( ^m2 ·d)], when fruits and vegetables are placed in the package, the moisture in the package is easily released to the outside, which is undesirable because it increases the amount of moisture evaporating from the fruits and vegetables. The water vapor permeability is preferably 80 [g/( ^m2 ·d)] or less, and more preferably 60 [g/( ^m2 ·d)] or less.

(container)
The packaging container of the present invention includes a molded container made of a polyester resin containing polyethylene terephthalate (PET) as a main component. Here, "containing PET as a main component" means that the total amount of components is 100 mol% and contains 50 mol% or more, preferably 80% or more, and more preferably 90% or more. By making the container from a polyester resin, it is possible to achieve mono-materialization in which the lid material and the container are made of the same resin, and an effect of contributing to the environment is obtained. The layer structure of the container of the present invention is not particularly limited, and it may be a single-layer structure or a structure in which two, three, four or more layers are laminated.

The polyester resin may be in either a crystalline or amorphous state. From the viewpoint of moldability, an amorphous polyester resin is preferred, and an amorphous PET is more preferred. Note that the amorphous state includes PET in an uncrystallized state such as A-PET and PET in a completely uncrystallized state such as PET-G.

(Contents)
The contents to be put into the container of the present invention include broccoli, komatsuna, napana, tomyo, green onions, tomatoes, snow peas, beans, spinach, komatsuna, chrysanthemums, Chinese leek, chestnuts, Chinese chives, green onions, parsley, mizuna, bell peppers, cucumbers, bitter melon, eggplants, green asparagus, white asparagus, myoga, bean sprouts, radish sprouts, Chinese cabbage, cabbage, carrots, pumpkins, lettuce, paprika, onions, Chinese yam, radish, turnips, corn, potatoes and other beans, grains, all kinds of vegetables including herbs, shiitake mushrooms, mushrooms, nameko mushrooms, shimeji mushrooms, enoki mushrooms, king oyster mushrooms, Examples of such ingredients include mushrooms such as maitake mushrooms, matsutake mushrooms, and edamame beans; fruits such as sudachi, kabosu, grapes, melons, cherries, strawberries, persimmons, kiwi fruit, cherries, green plums, pears, apples, bananas, peaches, loquats, blueberries, and watermelons; flowers such as roses, carnations, chrysanthemums, calla lilies, cherry blossoms, peaches, plums, freesias, almeria, sweet peas, hydrangeas, saffron, and dahlias; fish meat such as salmon, flounder, turbot, saury, sardines, mackerel, octopus, squid, sea bream, alphonsinodon, tuna, thin fish, hairtail, flying fish, conger eel, and eel; shellfish such as ark shells, clams, littleneck clams, clams, scallops, and aoyagi shellfish; and meat such as pork, beef, chicken, and lamb. Among these contents, it is preferably used for vegetables and fruits, and more preferably for packages of broccoli, lettuce, green onions, cabbage, shiitake mushrooms, and shimeji mushrooms. These contents may be placed in the package with inedible parts such as skin and branches still attached, or may be placed in the package with the inedible parts removed and the edible parts cut, or with the inedible parts still attached and the edible parts cut.

(Example of synthesis of polyester for lid material)
When the temperature of the esterification reactor reached 200°C, a slurry consisting of terephthalic acid [86.4 parts by mass] and ethylene glycol [64.4 parts by mass] was charged, and antimony trioxide [0.017 parts by mass] and triethylamine [0.16 parts by mass] were added as catalysts while stirring. The mixture was then heated to increase the temperature, and a pressurized esterification reaction was carried out under conditions of a gauge pressure of 0.34 MPa and 240°C.

Then, the pressure inside the esterification reaction vessel was returned to normal pressure, and magnesium acetate tetrahydrate [0.071 parts by mass] was added, followed by trimethyl phosphate [0.014 parts by mass]. The temperature was then raised to 260°C over 15 minutes, after which trimethyl phosphate [0.012 parts by mass] was added, followed by sodium acetate [0.0036 parts by mass]. After 15 minutes, the mixture was dispersed using a high-pressure disperser, and 0.10 parts by mass of silica particles with an average particle size of 2.5 μm was added relative to the entire composition. After 15 minutes, the resulting esterification reaction product was transferred to a polycondensation reaction vessel, and polycondensation reaction was carried out under reduced pressure at 280°C to obtain a polyester for a lid material.

(Synthesis Example of Polyester for Heat Seal Layer Containing Antifog Agent)
(Polyester A-1)
In an ester reaction vessel, terephthalic acid [445 parts by mass], isophthalic acid [74 parts by mass], sebacic acid [270 parts by mass], ethylene glycol [277 parts by mass], 2,2-dimethyl-1,3-propanediol [465 parts by mass] and tetrabutyl titanate [0.5 parts by mass] were charged, and the ester exchange reaction was carried out over 4 hours while heating to 230°C. After the ester exchange reaction was completed, the system was heated to 250°C while reducing the pressure to 10 torr over 60 minutes, and a polycondensation reaction was carried out at 250°C for 60 minutes. Thereafter, nitrogen was flowed into the system, and the polycondensation reaction was terminated by breaking the vacuum. After the reaction was completed, the polyester resin was taken out and cooled to obtain polyester A-1. The glass transition temperature was 7°C.

(Polyester B-1)
Polyester B-1 was obtained in the same manner as Polyester A-1, except that the raw materials were changed to dimethyl terephthalate [455 parts by mass], dimethyl isophthalate [455 parts by mass], ethylene glycol [291 parts by mass], 2,2-dimethyl-1,3-propanediol [488 parts by mass], and tetrabutyl titanate [0.5 parts by mass]. The glass transition temperature was 67°C.

(Container manufacturing example)
The polyester used for the container body was prepared in the same manner as the polyester for the lid material, except that no silica particles were added, to obtain a polyester for containers. The A-PET sheet prepared using the polyester for containers was softened at 130°C and subjected to vacuum and pressure forming using a mold to produce an A-PET container having the shape shown in Figure 1. The bottom was 100 mm x 100 mm, the height was 80 mm, the lid material seal part was 8 mm wide, and the thickness was about 300 μm.

The physical properties were measured using the following methods.

[Oxygen permeability]
The oxygen permeability was measured according to JIS K7126-2. The lid material was measured for oxygen permeability by passing oxygen through it in an environment of temperature 23°C and humidity 65% RH using an oxygen transmission amount measuring device (OX-TRAN 2/20 manufactured by MOCON). Before the measurement, the sample was left in an environment of temperature 23°C and humidity 65% RH for 4 hours to condition the moisture.

[Water vapor permeability]
The water vapor permeability was measured according to JIS K7126 B method. The water vapor permeability of the lid material was measured by passing a humidity-control gas through it in an environment of temperature 40°C and humidity 90% RH using a water vapor permeability measuring device (MOCON, PERMATRAN-W3/33MG). Before the measurement, the sample was left for 4 hours in an environment of temperature 23°C and humidity 65% RH to control the humidity.

[Hayes]
The measurement was performed in accordance with JIS K7136. The haze of the covering material was measured using a haze meter (NDH8000, manufactured by Nippon Denshoku Industries Co., Ltd.). The measurement was performed twice, and the average value was calculated.

[Breaking strength]
Measurement was performed in accordance with JIS K7113. For the lid material, a strip-shaped film sample having a measurement direction (film longitudinal direction, width direction) of 140 mm and a direction perpendicular to the measurement direction of 20 mm was cut out. Using a tensile tester (Shimadzu Corporation, Autograph AG-Xplus), both ends of the sample were held with chucks by 20 mm on each side (chuck distance 100 mm), and a tensile test was performed under conditions of an atmospheric temperature of 23 ° C. and a tensile speed of 200 mm / min, and the strength (MPa) at break was measured twice, and the average value was calculated.

[Elongation at break]
Measurement was performed in accordance with JIS K7113. For the lid material, a strip-shaped film sample with a measurement direction (film longitudinal direction, width direction) of 140 mm and a direction perpendicular to the measurement direction of 20 mm was cut out. Using a tensile tester (Shimadzu Corporation, Autograph AG-Xplus), both ends of the sample were held with chucks at 20 mm on each side (distance between chucks: 100 mm), and a tensile test was performed under conditions of an atmospheric temperature of 23°C and a tensile speed of 200 mm/min, and the elongation (%) at break was measured. The measurement was performed twice, and the average value was calculated.

[Heat seal strength]
The PET container piece and the heat seal layer side containing the antifogging agent of the lid material were overlapped. This sample was bonded with a heat sealer. The heat seal conditions were upper bar temperature 170°C, lower bar 30°C, pressure 0.2 MPa, and time 2 seconds. The heat seal sample was cut out so that the seal width was 15 mm. The heat seal strength was measured at a tensile speed of 200 mm/min using a tensile tester "AGS-KNX" (manufactured by Shimadzu Corporation). The heat seal strength is expressed as the strength per 15 mm (N/15 mm).

[Easy to open]
The lid seal part of the PET container was overlapped with the heat seal layer side containing the antifogging agent of the lid. The lid was heat sealed from above. The heat sealing conditions were a temperature of 120°C, a pressure of 0.2 MPa, and a time of 2 seconds. After that, the ease of peeling when the lid was peeled off by hand was evaluated by the following tactile sensation.
Evaluation: Good: Adhered well and easily peeled off by hand Evaluation: △: Adherence was insufficient and peeled off without much force Evaluation: ×: Adherence was too strong and unable to peel off by hand or lid was damaged

[Freshness retention performance]
Broccoli was placed in a sealed packaging container with a container and a lid, and left for 3 days in an environment of 30°C and 85% RH, after which the freshness-keeping performance was evaluated based on the change in color value and odor. The detailed method is shown below.

First, a bunch of flower buds (approximately 5 cm) was cut from a commercially available broccoli, and the L ^* and b ^* values of the top part of the flower bud were measured using a color meter (Konica Minolta, Color Reader CR-20). The color L ^* and b ^* values were measured three times, and the average values of each were used.

Next, the broccoli whose L ^* and b ^* color values had been measured was placed in a container, and the heat-sealed layer of the lid containing the antifogging agent was placed over the sealed portion of the lid of the container. The lid was then sealed from above with a cup sealer (EPK-Hand Sealer NO, manufactured by Eishin Pack Kogyo Co., Ltd.).

The container in which the broccoli was sealed in the above manner was placed in a thermohygrostat (Espec Corp., LHU124) set at a temperature of 30°C and a humidity of 85% RH, and left for 3 days. The broccoli was then removed from the container, and the color value was measured. The method for measuring the color value was the same as that used before leaving the broccoli. The color value changes after leaving were calculated for the color L ^* value and b ^* value, respectively, using the following formula.
Color value change = (color value after leaving) - (color value before leaving)

The color value change was evaluated according to the following criteria:
◯: The change in both the color L ^* value and the color b ^* value is 5 or less. △: The change in either the color L ^* value or the color b ^* value is 5 or less. ×: The change in both the color L ^* value and the color b ^* value is more than 5.

[Anti-fog evaluation]
After the broccoli was left to stand, the water droplets on the inner surface of the package (the surface in contact with the fruit or vegetable) were visually evaluated according to the following criteria.
○: No water droplets on the inner surface of the lid of the packaging container (less than 1/5 of the area of the lid of the packaging container)
×: Water droplets are present on the inner surface of the lid of the packaging container (1/5 or more of the area of the lid of the packaging container)

[Odor evaluation]
The odor of the broccoli when it was left to stand and then removed from the packaging container was evaluated according to the following criteria.
○: No odor when the packaging container is opened ×: An odor (pickle odor) is detected when the packaging container is opened

[Example 1]
The polyester for the lid material was put into an extruder. After the resin was melted at 280° C. in the extruder, it was cast from a T-die at 280° C. and adhered to a cooling roll at 10° C. by electrostatic adhesion to obtain an unstretched sheet.
The unstretched sheet was then stretched 3.2 times in the longitudinal direction at a temperature of 115°C, and then passed through a tenter to stretch 4.0 times in the transverse direction at 110°C, followed by heat setting treatment at 220°C for 3 seconds and relaxation treatment of 5% for 1 second to obtain a biaxially oriented polyester film (substrate layer) having a thickness of 25 μm.

Polyester A-1 [75.8% by mass], polyester B-2 [19.3% by mass], and anti-fogging agent (Rikemal L-71-D, nonionic surfactant, HLB 7.3, manufactured by Riken Vitamin Co., Ltd.) [4.9% by mass] were heated and stirred in an ethyl acetate solution (solid content concentration 10%) to obtain coating agent A. Coating agent A was applied to this biaxially oriented polyester film by wire bar coating method, and dried at 90°C for 20 seconds to laminate an anti-fogging layer, thereby obtaining a heat-sealable lid material. The thickness of the coated layer was 1.5 μm.

[Example 2]
A heat-sealable lid material was obtained in the same manner as in Example 1, except that the base layer was changed to a biaxially stretched inorganic binary vapor deposition barrier film (manufactured by Toyobo Co., Ltd., Ecosiale (registered trademark) VE100-12 μm).

[Comparative Example 1]
Polyester A-1 [79.7% by mass] and polyester B-2 [20.3% by mass] were heated and stirred in an ethyl acetate solution (solid content concentration 10%) to obtain coating agent B. This coating agent B was applied to the biaxially oriented polyester film produced in Example 1 by wire bar coating method, and dried at 90°C for 20 seconds to obtain a heat-sealable lid material. The thickness of the coated layer was 1.5 μm.

[Comparative Example 2]
A non-oriented linear low-density polyethylene film (Rix (registered trademark) L4102-40 μm, manufactured by Toyobo Co., Ltd.) was used as the base layer to prepare a heat-sealable lid material.

The physical properties of the heat-sealable lid materials and the evaluation results of the packaging containers for Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Table 1.

All of the packages in Examples 1 and 2 were excellent in each characteristic, and good evaluation results were obtained. The package container in Comparative Example 1 had good freshness retention performance, with little change in color value and no odor. However, the anti-fogging layer of the laminated film did not exhibit anti-fogging properties, and was therefore poor. The package container in Comparative Example 2 had high oxygen permeability and low water vapor permeability, and was unable to allow moisture emitted from the vegetables to escape, resulting in poor freshness retention performance. In addition, anti-fogging properties were not exhibited, and was therefore poor. Furthermore, the adhesive strength between the sealing layer of the base material layer and the PET container was too weak, resulting in poor ease of opening.

Claims (5)

A packaging container for preserving the freshness of food, comprising a heat-sealable lid material and a container mainly composed of polyethylene terephthalate,
The heat-sealable lid material is composed of two or more layers including a base layer mainly composed of polyethylene terephthalate and a heat-seal layer containing an anti-fogging agent,
The heat seal strength between the heat sealable lid material and the container is 1.0 N/15 mm or more and 15 N/15 mm or less,
The heat-sealable lid material has an oxygen permeability of 200 cc/( ^m2 ·d·atm) or less, as measured in an environment of a temperature of 23° C. and a relative humidity of 65% RH;
The heat-sealable lid material has a water vapor permeability of 100 g/( ^m2 ·d) or less in an environment of a temperature of 40° C. and a relative humidity of 90% RH. The freshness-preserving packaging container according to claim 1, wherein the heat seal layer of the heat sealable lid material contains a polyester resin and an anti-fogging agent, and the mass ratio of the polyester resin to the anti-fogging agent in the heat seal layer is 99:1 to 80:20. The freshness-preserving packaging container according to claim 1, wherein the heat-sealable lid material has a breaking strength in either the longitudinal or transverse direction of 180 MPa or more and 260 MPa or less. The freshness-preserving packaging container according to claim 1, wherein the heat-sealable lid material has a breaking elongation in either the longitudinal or transverse direction of 80% or more and 170% or less. The freshness-preserving packaging container according to claim 1, wherein the heat-sealable lid material has a heat-seal layer, a base layer, and a print layer in this order.