JP7775911B2 - Laminate and packaging bag - Google Patents

Description

This disclosure relates to laminates and packaging bags.

A laminate is known that includes a biaxially oriented PET (polyethylene terephthalate) film, which has excellent heat resistance and toughness, as the base film, and a polyolefin film such as polyethylene or polypropylene as the sealant layer (see, for example, Patent Document 1).

Japanese Patent Application Laid-Open No. 2017-178357

In recent years, growing environmental awareness stemming from issues such as marine plastic waste has led to calls for even more efficient sorting and recycling of plastic materials. This means that even in packaging laminates, where high performance has traditionally been achieved by combining a variety of different materials, there is now a demand for mono-materialization.

To achieve a mono-material laminate, the constituent films must be made of the same material. However, if a laminate is made using polyolefin-based films, there is a risk that the resulting packaging bag will not be able to withstand high-temperature retort processing.

The present invention was made in consideration of the above circumstances, and aims to provide a laminate that, when made into a packaging bag, can undergo high-temperature retort treatment, even when primarily composed of a polyolefin-based film. Another aim of the present invention is to provide a packaging bag using this laminate.

One aspect of the present invention provides a laminate comprising a first substrate layer, a second substrate layer, and a sealant layer in this order, wherein the first substrate layer, the second substrate layer, and the sealant layer all comprise a polyolefin film, and the polyolefin film of the first substrate layer or the second substrate layer has an inorganic oxide layer on at least one surface, and the heat shrinkage rate in the running direction (MD direction) of each of the first substrate layer, the second substrate layer, and the sealant layer after heating at 120°C for 15 minutes satisfies the following formula:
Heat shrinkage rate of the second base layer ≦5% (Equation 1)
Heat shrinkage rate of second base material layer≧Heat shrinkage rate of first base material layer (Equation 2)
Heat shrinkage rate of second base material layer≧Heat shrinkage rate of sealant layer (Equation 3)
(Here, heat shrinkage rate (%) = (length in the running direction before heating - length in the running direction after heating) / length in the running direction before heating x 100)

In one embodiment, the heat shrinkage rate may satisfy the following formula:
Heat shrinkage rate of the second base material layer−Heat shrinkage rate of the first base material layer≧0.3% (Equation 4)
Heat shrinkage rate of second base layer−Heat shrinkage rate of sealant layer≧0.5% (Equation 5)

In one embodiment, the heat shrinkage rate may satisfy the following formula:
Heat shrinkage rate of sealant layer≦2% (Equation 6)

In one aspect, the polyolefin film of the first substrate layer or the second substrate layer has the inorganic oxide layer on at least one surface and a gas barrier coating layer on the inorganic oxide layer, and the gas barrier coating layer may be formed using a gas barrier coating layer-forming composition containing at least one selected from the group consisting of a hydroxyl group-containing polymer compound, a metal alkoxide, a silane coupling agent, and hydrolysates thereof.

In one embodiment, the inorganic oxide layer may include silicon oxide.

In one embodiment, the laminate may be for use in a retort pouch.

Another aspect of the present invention provides a packaging bag made from the above-described laminate.

The present invention provides a laminate that can be subjected to high-temperature retort treatment when made into a packaging bag, even when the laminate is primarily composed of a polyolefin-based film. Furthermore, the present invention also provides a packaging bag that uses the laminate.

FIG. 1 is a schematic cross-sectional view showing a laminate according to one embodiment.

Preferred embodiments of the present disclosure will be described in detail below, with reference to the drawings where appropriate. Note that identical or equivalent parts in the drawings will be designated by the same reference numerals, and duplicate explanations will be omitted. Furthermore, the dimensional proportions in the drawings are not limited to those shown.

<Laminate>
FIG. 1 is a schematic cross-sectional view showing a laminate according to one embodiment. The laminate 100 shown in FIG. 1 includes a first substrate layer 11, a second substrate layer 12, and a sealant layer 13, in this order. The first substrate layer 11 and the second substrate layer 12, and the second substrate layer 12 and the sealant layer 13 may each be bonded with an adhesive layer S. The first substrate layer, the second substrate layer, and the sealant layer all include a polyolefin film. The polyolefin film of the first substrate layer or the second substrate layer includes an inorganic oxide layer on at least one surface to improve gas barrier properties against, for example, water vapor and oxygen. Since a polyolefin film including an inorganic oxide layer has gas barrier properties, such a first substrate layer or second substrate layer can be referred to as a gas barrier substrate layer. Each layer will be described below.

[First base layer 11]
The first substrate layer is a film that serves as one of the supports and includes a polyolefin film. The first substrate layer may be made of a polyolefin film. Examples of polyolefin films include polyethylene film (PE), polypropylene film (PP), and polybutene film (PB). Examples of polyolefin films include acid-modified polyolefin films obtained by graft-modifying polyolefins with unsaturated carboxylic acids, acid anhydrides of unsaturated carboxylic acids, esters of unsaturated carboxylic acids, and the like.

The polyolefin film constituting the first base layer may be a stretched film or a non-stretched film. However, from the standpoint of impact resistance, heat resistance, water resistance, dimensional stability, etc., the polyolefin film may be a stretched film. This makes the laminate more suitable for use in applications that require retort processing or boiling. The stretching method is not particularly limited, and any method that can provide a dimensionally stable film, such as inflation stretching, uniaxial stretching, or biaxial stretching, may be used.

There are no particular limitations on the thickness of the polyolefin film. Depending on the application, the thickness can be 6 to 200 μm, but from the perspective of achieving excellent impact resistance and excellent gas barrier properties, it may be 9 to 50 μm or 12 to 38 μm.

The polyolefin film may be subjected to various pretreatments such as corona treatment, plasma treatment, and flame treatment on its lamination surface, as long as the barrier performance is not impaired, or a coating layer such as an easy-adhesion layer may be applied.

(adhesion layer)
When the polyolefin film has an inorganic oxide layer, an adhesive layer (anchor coat layer) may be provided on the surface of the polyolefin film on which the inorganic oxide layer is to be laminated. The adhesive layer is provided on the polyolefin film, and can achieve two effects: improving the adhesion performance between the polyolefin film and the inorganic oxide layer and improving the smoothness of the polyolefin film surface. The improved smoothness makes it easier to form the inorganic oxide layer uniformly without defects, and thus makes it easier to exhibit high barrier properties. The adhesive layer can be formed using an anchor coat agent.

Anchor coating agents include, for example, polyester-based polyurethane resins and polyether-based polyurethane resins. From the standpoints of heat resistance and interlayer adhesive strength, polyester-based polyurethane resins are preferred as anchor coating agents.

The thickness of the adhesive layer is not particularly limited, but is preferably in the range of 0.01 to 5 μm, more preferably 0.03 to 3 μm, and particularly preferably 0.05 to 2 μm. If the thickness of the adhesive layer is equal to or greater than the lower limit above, more sufficient interlayer adhesive strength tends to be obtained, while if the thickness is equal to or less than the upper limit above, the desired gas barrier properties tend to be more easily achieved.

The method for applying the adhesive layer to the polyolefin film can be any known application method, without particular limitation. Examples include immersion (dipping) methods, and methods using a spray, coater, printer, or brush. Furthermore, the types of coaters and printers used in these methods and their application methods include gravure coaters such as direct gravure, reverse gravure, kiss reverse gravure, and offset gravure, reverse roll coaters, microgravure coaters, chamber doctor combined coaters, air knife coaters, dip coaters, bar coaters, comma coaters, and die coaters.

The coating amount of the adhesion layer is preferably 0.01 to 5 g/ ^m2 , more preferably 0.03 to 3 g/ ^m2 , in terms of the mass per ^m2 after the anchor coating agent has been applied and dried. If the mass per ^m2 after the anchor coating agent has been applied and dried is equal to or greater than the above lower limit, sufficient film formation tends to occur, whereas if it is equal to or less than the above upper limit, sufficient drying tends to occur and the solvent is less likely to remain.

The method for drying the adhesive layer is not particularly limited, but examples include natural drying, drying in an oven set at a predetermined temperature, and using a dryer attached to the coater, such as an arch dryer, floating dryer, drum dryer, or infrared dryer. Furthermore, drying conditions can be appropriately selected depending on the drying method. For example, when drying in an oven, it is preferable to dry at a temperature of 60 to 100°C for about 1 second to 2 minutes.

Instead of the polyurethane resin, a polyvinyl alcohol resin can be used for the adhesion layer. The polyvinyl alcohol resin may be any resin containing vinyl alcohol units formed by saponifying vinyl ester units, such as polyvinyl alcohol (PVA) and ethylene-vinyl alcohol copolymer (EVOH).

Examples of PVA include resins obtained by homopolymerizing vinyl esters such as vinyl acetate, vinyl formate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl pivalate, and vinyl versatate, followed by saponification. PVA may also be modified PVA, which has been copolymerized or post-modified. Modified PVA can be obtained, for example, by copolymerizing a vinyl ester with an unsaturated monomer copolymerizable with the vinyl ester, followed by saponification. Examples of unsaturated monomers copolymerizable with vinyl esters include olefins such as ethylene, propylene, isobutylene, α-octene, α-dodecene, and α-octadecene; hydroxy group-containing α-olefins such as 3-buten-1-ol, 4-pentyn-1-ol, and 5-hexen-1-ol; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, and undecylenic acid; nitriles such as acrylonitrile and methacrylonitrile; diacetone acrylamide, acrylic acid, and the like. amides such as acrylamides and methacrylamides; olefin sulfonic acids such as ethylene sulfonic acid, allyl sulfonic acid, and methallylsulfonic acid; vinyl compounds such as alkyl vinyl ethers, dimethylallyl vinyl ketone, N-vinylpyrrolidone, vinyl chloride, vinyl ethylene carbonate, 2,2-dialkyl-4-vinyl-1,3-dioxane, glycerin monoallyl ether, and 3,4-diacetoxy-1-butene; vinylidene chloride, 1,4-diacetoxy-2-butene, and vinylene carbonate.

The degree of polymerization of PVA is preferably 300 to 3000. If the degree of polymerization is less than 300, the barrier properties tend to decrease, while if it exceeds 3000, the viscosity becomes too high and the coating suitability tends to decrease. The degree of saponification of PVA is preferably 90 mol% or more, more preferably 95 mol% or more, and even more preferably 99 mol% or more. The degree of saponification of PVA may be 100 mol% or less, or 99.9 mol% or less. The degree of polymerization and degree of saponification of PVA can be measured in accordance with the method described in JIS K 6726 (1994).

EVOH is generally obtained by saponifying a copolymer of ethylene and a vinyl acid ester such as vinyl acetate, vinyl formate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl pivalate, or vinyl versatate.

The degree of polymerization of EVOH is preferably 300 to 3000. If the degree of polymerization is less than 300, the barrier properties tend to decrease, while if it exceeds 3000, the viscosity becomes too high and the coating suitability tends to decrease. The degree of saponification of the vinyl ester component of EVOH is preferably 90 mol% or more, more preferably 95 mol% or more, and even more preferably 99 mol% or more. The degree of saponification of EVOH may be 100 mol% or less, or 99.9 mol% or less. The degree of saponification of EVOH is determined by nuclear magnetic resonance (1H-NMR) measurement, from the peak area of the hydrogen atoms contained in the vinyl ester structure and the peak area of the hydrogen atoms contained in the vinyl alcohol structure.

The ethylene unit content of EVOH is 10 mol% or more, more preferably 15 mol% or more, even more preferably 20 mol% or more, and particularly preferably 25 mol% or more. Furthermore, the ethylene unit content of EVOH is preferably 65 mol% or less, more preferably 55 mol% or less, and even more preferably 50 mol% or less. An ethylene unit content of 10 mol% or more can maintain good gas barrier properties or dimensional stability under high humidity conditions. On the other hand, an ethylene unit content of 65 mol% or less can improve gas barrier properties. The ethylene unit content of EVOH can be determined by NMR.

When a polyvinyl alcohol resin is used as the adhesion layer, methods for forming the adhesion layer include coating using a polyvinyl alcohol resin solution and multi-layer extrusion.

(Inorganic oxide layer)
When the polyolefin film has an inorganic oxide layer, examples of the inorganic oxide contained in the inorganic oxide layer include aluminum oxide, silicon oxide, magnesium oxide, and tin oxide. From the viewpoint of transparency and barrier properties, the inorganic oxide may be selected from the group consisting of aluminum oxide, silicon oxide, and magnesium oxide. Furthermore, from the viewpoint of excellent tensile stretchability during processing, it is preferable that the inorganic oxide layer be a layer using silicon oxide. By using an inorganic oxide layer, high barrier properties can be obtained with a very thin layer that does not affect the recyclability of the laminate.

The O/Si ratio of the inorganic oxide layer is preferably 1.7 or higher. An O/Si ratio of 1.7 or higher suppresses the content of metallic Si, making it easier to achieve good transparency. Furthermore, the O/Si ratio is preferably 2.0 or lower. An O/Si ratio of 2.0 or lower increases the crystallinity of SiO, preventing the inorganic oxide layer from becoming too hard and providing good tensile strength. This prevents cracks from occurring in the inorganic oxide layer when laminating a gas barrier coating layer. Even after forming into a packaging bag, polyolefin films can shrink due to heat during boiling or retort treatment. However, an O/Si ratio of 2.0 or lower allows the inorganic oxide layer to easily adapt to this shrinkage, preventing a decrease in barrier properties. To more fully achieve these effects, the O/Si ratio of the inorganic oxide layer is preferably 1.75 to 1.9, and more preferably 1.8 to 1.85.

The O/Si ratio of the inorganic oxide layer can be determined by X-ray photoelectron spectroscopy (XPS). For example, the measurement device is an X-ray photoelectron spectrometer (manufactured by JEOL Ltd., product name: JPS-90MXV), and the X-ray source is non-monochromated MgKα (1253.6 eV), with an X-ray output of 100 W (10 kV-10 mA). Quantitative analysis to determine the O/Si ratio can use relative sensitivity factors of 2.28 for O1s and 0.9 for Si2p, respectively.

The thickness of the inorganic oxide layer is preferably 10 nm or more and 50 nm or less. A thickness of 10 nm or more can achieve sufficient water vapor barrier properties. Furthermore, a thickness of 50 nm or less can prevent cracks from occurring due to deformation caused by internal stress in the thin film, and can prevent a decrease in water vapor barrier properties. Note that a thickness exceeding 50 nm is undesirable from an economic standpoint, as it increases costs due to increased material usage and longer film formation times. From the same perspective as above, it is more preferable that the thickness of the inorganic oxide layer be 20 nm or more and 40 nm or less.

The inorganic oxide layer can be formed, for example, by vacuum deposition. Vacuum deposition can be performed using physical vapor deposition or chemical vapor deposition. Physical vapor deposition methods include, but are not limited to, vacuum evaporation, sputtering, and ion plating. Chemical vapor deposition methods include, but are not limited to, thermal CVD, plasma CVD, and photo-CVD.

The most preferred vacuum deposition methods are resistance heating vacuum deposition, EB (Electron Beam) heating vacuum deposition, induction heating vacuum deposition, sputtering, reactive sputtering, dual magnetron sputtering, and plasma enhanced chemical vapor deposition (PECVD). However, considering productivity, vacuum deposition is currently the most superior method. The heating method for vacuum deposition is preferably electron beam heating, resistance heating, or induction heating.

(Gas barrier coating layer)
The polyolefin film may have a gas barrier coating layer on the inorganic oxide layer, the gas barrier coating layer being a layer formed using a gas barrier coating layer-forming composition containing at least one selected from the group consisting of a hydroxyl group-containing polymer compound, a metal alkoxide, a silane coupling agent, and a hydrolyzate thereof.

The gas barrier coating layer is a coating layer with gas barrier properties and is formed using a gas barrier coating layer-forming composition (hereinafter also referred to as a coating agent) whose main component is an aqueous solution or water/alcohol mixed solution containing at least one selected from the group consisting of hydroxyl-containing polymer compounds, metal alkoxides, silane coupling agents, and their hydrolysates. To maintain gas barrier properties after hot water treatment such as retort treatment, the coating agent preferably contains at least a silane coupling agent or its hydrolysate. It more preferably contains at least one selected from the group consisting of hydroxyl-containing polymer compounds, metal alkoxides, and their hydrolysates, and a silane coupling agent or its hydrolysate. It is even more preferable to contain a hydroxyl-containing polymer compound or its hydrolysate, a metal alkoxide or its hydrolysate, and a silane coupling agent or its hydrolysate. The coating agent can be prepared, for example, by mixing a metal alkoxide and a silane coupling agent directly, or after prior hydrolysis, with a solution of a hydroxyl-containing polymer compound, which is a water-soluble polymer, dissolved in an aqueous solvent (water or a water/alcohol mixture).

The components contained in the coating agent used to form the gas barrier coating layer are described in detail below. Hydroxyl-containing polymer compounds used in coating agents include polyvinyl alcohol, polyvinylpyrrolidone, starch, methyl cellulose, carboxymethyl cellulose, and sodium alginate. Among these, polyvinyl alcohol (PVA) is preferred when used as the coating agent for the gas barrier coating layer, as it provides particularly excellent gas barrier properties.

From the viewpoint of obtaining excellent gas barrier properties, the gas barrier coating layer is preferably formed from a composition containing at least one selected from the group consisting of metal alkoxides represented by the following general formula (I) and hydrolysates thereof:
M(OR ¹ ) _m (R ² ) _nm ...(I)
In the general formula (I), ^R1 and ^R2 are each independently a monovalent organic group having 1 to 8 carbon atoms, and are preferably an alkyl group such as a methyl group or an ethyl group. M represents an n-valent metal atom such as Si, Ti, Al, or Zr. m is an integer from 1 to n. When there are multiple ^R1s or ^R2s , the ^R1s or ^R2s may be the same or different.

Specific examples of metal alkoxides include tetraethoxysilane [Si(OC ₂ H ₅ ) ₄ ], triisopropoxyaluminum [Al(O-2′-C ₃ H ₇ ) ₃ ], etc. Tetraethoxysilane and triisopropoxyaluminum are preferred because they are relatively stable in aqueous solvents after hydrolysis.

Examples of the silane coupling agent include compounds represented by the following general formula (II).
Si(OR ¹¹ ) _p (R ¹² ) _3-p R ¹³ ...(II)
In the general formula (II), ^R11 represents an alkyl group such as a methyl group or an ethyl group, ^R12 represents a monovalent organic group such as an alkyl group, an aralkyl group, an aryl group, an alkenyl group, an alkyl group substituted with an acryloxy group, or an alkyl group substituted with a methacryloxy group, ^R13 represents a monovalent organic functional group, and p represents an integer of 1 to 3. When a plurality of ^R11 or ^R12 are present, the ^R11s or ^R12s may be the same or different. Examples of the monovalent organic functional group represented by ^R13 include a monovalent organic functional group containing a glycidyloxy group, an epoxy group, a mercapto group, a hydroxyl group, an amino group, an alkyl group substituted with a halogen atom, or an isocyanate group.

Specific examples of silane coupling agents include vinyltrimethoxysilane, γ-chloropropylmethyldimethoxysilane, γ-chloropropyltrimethoxysilane, glycidoxypropyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, and γ-methacryloxypropylmethyldimethoxysilane.

The silane coupling agent may also be a polymer formed by polymerization of the compound represented by the general formula (II) above. A trimer is preferred as the polymer, and 1,3,5-tris(3-trialkoxysilylalkyl)isocyanurate is more preferred. This is a condensation polymer of 3-isocyanatoalkylalkoxysilane. While the isocyanate moiety of 1,3,5-tris(3-trialkoxysilylalkyl)isocyanurate is chemically non-reactive, the polarity of the nurate moiety ensures reactivity. It is generally added to adhesives, similar to 3-isocyanatoalkylalkoxysilane, and is known as an adhesion improver. Therefore, adding 1,3,5-tris(3-trialkoxysilylalkyl)isocyanurate to a hydroxyl group-containing polymer compound can improve the water resistance of the gas barrier coating layer through hydrogen bonding. While 3-isocyanate alkyl alkoxysilanes are highly reactive and have low liquid stability, 1,3,5-tris(3-trialkoxysilylalkyl) isocyanurates are not water-soluble due to the polarity of the nurate moiety, but are easily dispersed in aqueous solutions and can maintain a stable liquid viscosity. Furthermore, the water resistance of 3-isocyanate alkyl alkoxysilanes and 1,3,5-tris(3-trialkoxysilylalkyl) isocyanurates is equivalent.

Some 1,3,5-tris(3-trialkoxysilylalkyl)isocyanurates are produced by thermal condensation of 3-isocyanatepropylalkoxysilane, and may contain the raw material 3-isocyanatepropylalkoxysilane, but this does not pose any particular problems. 1,3,5-tris(3-trialkoxysilylpropyl)isocyanurate is more preferred, and 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate is even more preferred. The methoxy group hydrolyzes quickly, and those containing propyl groups are relatively inexpensive, making 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate advantageous in practical use.

In addition, known additives such as isocyanate compounds, dispersants, stabilizers, viscosity modifiers, and colorants can be added to the coating agent as needed, as long as the gas barrier properties are not impaired.

The thickness of the gas barrier coating layer is preferably 50 to 1,000 nm, and more preferably 100 to 500 nm. A gas barrier coating layer thickness of 50 nm or more tends to provide more sufficient gas barrier properties, while a thickness of 1,000 nm or less tends to maintain sufficient flexibility.

The coating liquid for forming the gas barrier coating layer can be applied by, for example, dipping, roll coating, gravure coating, reverse gravure coating, air knife coating, comma coating, die coating, screen printing, spray coating, gravure offset, etc. The coating film obtained by applying this coating liquid can be dried by, for example, hot air drying, hot roll drying, high-frequency irradiation, infrared irradiation, UV irradiation, or a combination thereof.

The temperature at which the coating film is dried can be, for example, 50 to 150°C, and preferably 70 to 100°C. By keeping the drying temperature within this range, the occurrence of cracks in the inorganic oxide layer and gas barrier coating layer can be further suppressed, allowing for the development of excellent barrier properties.

The gas barrier coating layer may be formed using a coating agent containing a polyvinyl alcohol resin and a silane compound. If necessary, an acid catalyst, an alkali catalyst, a photopolymerization initiator, etc. may be added to the coating agent.

The polyvinyl alcohol resin is as described above. Examples of silane compounds include silane coupling agents, polysilazanes, and siloxanes. Specific examples include tetramethoxysilane, tetraethoxysilane, glycidoxypropyltrimethoxysilane, acryloxypropyltrimethoxysilane, and hexamethyldisilazane.

(Printing layer)
A printing layer can be provided on the second substrate layer side of the first substrate layer. The printing layer is provided in a position visible from the outside of the laminate for the purpose of displaying information about the contents, identifying the contents, or improving the design of the packaging bag. The printing method and printing ink are not particularly limited, and are appropriately selected from known printing methods and printing inks taking into consideration printability on the film, design such as color tone, adhesion, safety as a food container, etc. Examples of printing methods that can be used include gravure printing, offset printing, gravure offset printing, flexographic printing, and inkjet printing. Among these, gravure printing is preferably used from the viewpoints of productivity and high resolution of the image.

To improve adhesion of the printed layer, the surface of the first substrate layer facing the second substrate layer may be subjected to various pretreatments such as corona treatment, plasma treatment, or flame treatment, or a coating layer such as an easy-adhesion layer may be provided. Examples of the surface of the first substrate layer facing the second substrate layer include the surface of a polyolefin film (when the first substrate layer is not a gas barrier substrate layer) and the surface of a gas barrier coating layer (when the first substrate layer is a gas barrier substrate layer).

[Second base layer 12]
For the configuration of the second substrate layer, the above description of the configuration of the first substrate layer can be referred to as appropriate. When the first substrate layer is a gas barrier substrate layer, the second substrate layer does not need to be a gas barrier substrate layer, and conversely, when the second substrate layer is a gas barrier substrate layer, the first substrate layer does not need to be a gas barrier substrate layer.

(Adhesive layer S)
The first base material layer and the second base material layer can be laminated via an adhesive layer. Examples of adhesive materials that can be used include polyester-isocyanate resins, urethane resins, and polyether resins. When using the packaging bag for retort applications, a two-component curing urethane adhesive that is retort-resistant can be preferably used.

[Sealant layer 13]
The sealant layer is a layer that provides heat-sealing properties to the laminate, and includes a polyolefin film. The sealant layer may be made of a polyolefin film.

Polyolefin-based thermoplastic resins are commonly used as materials for the sealant layer. Specific examples include ethylene-based resins such as low-density polyethylene resin (LDPE), medium-density polyethylene resin (MDPE), linear low-density polyethylene resin (LLDPE), ethylene-vinyl acetate copolymer (EVA), ethylene-alpha-olefin copolymer, and ethylene-(meth)acrylic acid copolymer; blends of polyethylene and polybutene; and polypropylene-based resins such as homopolypropylene resin (PP), propylene-ethylene random copolymer, propylene-ethylene block copolymer, and propylene-alpha-olefin copolymer. These thermoplastic resins can be selected appropriately depending on the intended use and temperature conditions, such as boiling and retort processing.

The polyolefin film that makes up the sealant layer may contain various additives such as flame retardants, slip agents, antiblocking agents, antioxidants, light stabilizers, and tackifiers.

The thickness of the sealant layer is determined by factors such as the weight of the contents and the shape of the packaging bag, but a thickness of approximately 30 to 150 μm is preferred.

The sealant layer can be formed by any of the known lamination methods, including dry lamination, in which a film-like sealant layer made from the above-mentioned thermoplastic resin is bonded with an adhesive such as a one-component or two-component curing urethane adhesive; non-solvent dry lamination, in which a film-like sealant layer is bonded with a solvent-free adhesive; and extrusion lamination, in which the above-mentioned thermoplastic resin is heated and melted, extruded into a curtain shape, and bonded together.

Of the above forming methods, the dry lamination method is preferred because it has high resistance to retort treatment, especially high-temperature hot water treatment at 120°C or higher. However, if the packaging bag is to be used for applications where it will be treated at temperatures of 85°C or lower, there are no particular restrictions on the lamination method.

[Thermal shrinkage rate of each layer]
In the laminate, the heat shrinkage rate in the machine direction (MD direction) of each of the first base material layer, the second base material layer, and the sealant layer after heating at 120°C for 15 minutes satisfies the following formula.
Heat shrinkage rate of the second base layer ≦5% (Equation 1)
Heat shrinkage rate of second base material layer≧Heat shrinkage rate of first base material layer (Equation 2)
Heat shrinkage rate of second base material layer≧Heat shrinkage rate of sealant layer (Equation 3)

Here, the heat shrinkage rate (%) is a value calculated by the following formula.
(Length in the running direction before heating - Length in the running direction after heating) / Length in the running direction before heating x 100
The procedure for measuring the heat shrinkage rate is as follows.
(1) The layer to be measured is cut into a 20 cm x 20 cm sample to be used as a measurement sample.
(2) A 10 cm line is drawn in the running direction of the measurement sample (the running direction length before heating).
(3) The measurement sample is heated at 120°C for 15 minutes.
(4) The length of the written line in the running direction is measured (length in the running direction after heating).
(5) The thermal shrinkage rate is calculated using the above formula.

The heat shrinkage rate of the second base layer is 5% or less. This makes it less likely for cracks to occur in the gas barrier coating layer during barrier processing, resulting in good barrier properties. From this perspective, the heat shrinkage rate can be 4% or less, and may be 3% or less. However, producing a film with an extremely small heat shrinkage rate requires a long time for heat setting (thermal fixation), which significantly reduces productivity. From this perspective, the lower limit of the heat shrinkage rate of the second base layer can be 1.5% or more.

The heat shrinkage rate of the second base layer is equal to or greater than the heat shrinkage rate of the first base layer and equal to or greater than the heat shrinkage rate of the sealant layer. By sandwiching and laminating the second base layer between a first base layer and a sealant layer, each of which has a heat shrinkage rate lower than that of the second base layer, the heat shrinkage rate of the laminate can be reduced. This suppresses the reduction in gas barrier function during retort processing, thereby maintaining barrier properties after retort processing. From this perspective, the heat shrinkage rate of the second base layer minus the heat shrinkage rate of the first base layer can be 0.3% or greater, and may be 0.5% or greater. If the heat shrinkage rate of the second base layer is greater than that of the first base layer by a certain amount, it becomes difficult to achieve the effect of reducing the shrinkage rate of the laminate. As a result, it becomes difficult to suppress the reduction in gas barrier function, so the upper limit of the heat shrinkage rate of the second base layer minus the heat shrinkage rate of the first base layer can be 1.5% or less. Furthermore, the heat shrinkage rate of the second base layer minus the heat shrinkage rate of the sealant layer can be 0.5% or greater, and may be 1.0% or greater. If the heat shrinkage rate of the second base layer is greater than that of the sealant layer by a certain amount, it becomes difficult to achieve the desired shrinkage reduction effect for the laminate. This makes it difficult to prevent a decrease in gas barrier function, so the upper limit for the heat shrinkage rate of the second base layer minus the heat shrinkage rate of the sealant layer can be set to 3.0% or less.

The heat shrinkage rate of the sealant layer can be 2% or less. This makes it easier to suppress the thermal shrinkage of the laminate. From this perspective, the heat shrinkage rate may be 1% or less.

As mentioned above, all of the films that make up the laminate can be polyolefin films. Such a laminate can be said to be a mono-material packaging material that is highly recyclable. From this perspective, the total mass of components other than the polyolefin component (e.g., adhesive and ink components) can be 10% by mass or less, and may be 7.5% by mass or less, relative to the total mass of the laminate.

Furthermore, when made into a packaging bag, the above laminate can be subjected to high-temperature retort treatment, making it suitable for use in retort pouches.

<Packaging bag>
The packaging bag is obtained by forming the above-described laminate. The packaging bag may be formed into a bag shape by folding one laminate in half so that the sealant layers face each other and then heat-sealing three sides, or may be formed into a bag shape by stacking two laminates so that the sealant layers face each other and then heat-sealing four sides. The packaging bag can contain contents such as food, medicine, etc., and can be subjected to heat sterilization treatment such as retort treatment or boiling treatment.

Retort processing is a method of sterilizing microorganisms such as mold, yeast, and bacteria under pressure, typically to preserve food, medicine, etc. Typically, food packaging bags are sterilized at 105-140°C and 0.15-0.30 MPa for 10-120 minutes. Retort processing equipment comes in two types: steam type, which uses heated steam, and hot water type, which uses pressurized heated water. The appropriate type is used depending on the sterilization conditions of the food contents. Boiling is a moist heat sterilization method for preserving food, medicine, etc. Typically, food packaging bags are sterilized at 60-100°C and atmospheric pressure for 10-120 minutes, depending on the contents. Boiling is usually performed in a hot water bath at 100°C or below. Methods include a batch method, in which the food is immersed in a hot water bath at a constant temperature for a set period of time and then removed, and a continuous method, in which the food is passed through a tunnel-type hot water bath for processing.

The above-mentioned packaging bag is particularly suitable for use in applications requiring retort treatment at temperatures of 120°C or higher. Packaging bags using the above-mentioned laminate can maintain excellent barrier properties even when subjected to retort treatment.

The present disclosure will be further described in detail with reference to the following examples, but the present disclosure is not limited to these examples.

<Preparation of First Substrate Layer and Second Substrate Layer>
The following stretched polypropylene films were prepared as the first base layer and the second base layer. Note that the "shrinkage rate" hereinafter refers to the heat shrinkage rate in the running direction (MD direction) after heating at 120°C for 15 minutes.
[First base layer]
OPP (shrinkage rate 2.0%): thickness 20 μm
OPP (shrinkage rate 3.1%): thickness 20 μm
OPP (shrinkage rate 4.1%): thickness 20 μm
EVOH-OPP (shrinkage rate 2.0%): Thickness 18 μm. An EVOH layer (adhesion layer) is provided on an OPP film.
[Second base layer]
OPP (shrinkage rate 2.6%): thickness 20 μm
OPP (shrinkage rate 2.6%): thickness 18 μm
OPP (shrinkage rate 3.8%): thickness 20 μm
EVOH-OPP (shrinkage rate 2.6%): Thickness 18 μm. An EVOH layer (adhesion layer) is provided on an OPP film.

The gas barrier substrate layer is manufactured as follows:

[Preparation of Adhesion Layer-Forming Composition]
Acrylic polyol and tolylene diisocyanate were mixed such that the number of NCO groups in tolylene diisocyanate was equal to the number of OH groups in the acrylic polyol, and the mixture was diluted with ethyl acetate to a total solids content (total amount of acrylic polyol and tolylene diisocyanate) of 5 mass%. β-(3,4-epoxycyclohexyl)trimethoxysilane was further added to the diluted mixture in an amount of 5 parts by mass per 100 parts by mass of the total amount of acrylic polyol and tolylene diisocyanate, and these were mixed to prepare a composition for forming an adhesion layer (anchor coating agent).

[Preparation of composition for forming gas barrier coating layer]
A composition for forming a gas barrier coating layer was prepared by mixing the following liquids A, B, and C in a mass ratio of 65/25/10, respectively.
Solution A: A hydrolysis solution with a solid content of 5% by mass ( _{SiO2 equivalent} ) obtained by adding 72.1 g of 0.1 N hydrochloric acid to 17.9 g of tetraethoxysilane (Si( _OC2H5 ) ₄ ) and 10 g of methanol and stirring for 30 minutes to hydrolyze the mixture.
Solution B: a 5% by mass water/methanol solution of polyvinyl alcohol (water:methanol mass ratio 95:5).
Liquid C: A hydrolysis solution obtained by diluting 1,3,5-tris(3-trialkoxysilylpropyl)isocyanurate with a mixed liquid of water and isopropyl alcohol (water:isopropyl alcohol mass ratio 1:1) to a solids content of 5 mass %.

[Production of gas barrier OPP]
The adhesion layer-forming composition was applied to the corona-treated surface of a stretched polypropylene film by gravure roll coating, followed by drying and curing at 60°C to form an adhesion layer composed of a polyester-based polyurethane resin with a coating weight of 0.1 g/ ^m² . Next, a 30 nm-thick transparent inorganic oxide layer (silica deposition layer) composed of silicon oxide was formed using a vacuum deposition apparatus employing an electron beam heating system. The deposition material type was adjusted to form a deposition layer with an O/Si ratio of 1.8. The O/Si ratio was measured using an X-ray photoelectron spectrometer (manufactured by JEOL Ltd., product name: JPS-90MXV) using a non-monochromated MgKα (1253.6 eV) X-ray source at an X-ray output of 100 W (10 kV-10 mA). Quantitative analysis to determine the O/Si ratio was performed using relative sensitivity factors of 2.28 for O1s and 0.9 for Si2p, respectively.

When forming an alumina vapor deposition layer as an inorganic oxide layer, the following procedure was used. Specifically, aluminum was evaporated using electron beam vacuum deposition while introducing oxygen, forming an AlOx vapor deposition film with a thickness of 10 nm.

Next, the composition for forming the gas barrier coating layer was applied onto the inorganic oxide layer using a gravure roll coating method, and then heated and dried in an oven under conditions of a tension of 20 N/m and a drying temperature of 120°C to form a gas barrier coating layer with a thickness of 0.3 μm. This resulted in a gas barrier film having a laminated structure of film substrate/adhesion layer/inorganic oxide layer/gas barrier coating layer.

[Production of gas barrier EVOH-OPP]
An inorganic oxide layer (silica vapor deposition layer) was formed on the EVOH layer (adhesion layer) of an EVOH-OPP film in the same manner as described above. Next, a gas barrier coating layer was formed on the inorganic oxide layer using the gas barrier coating layer-forming composition in the same manner as described above. This resulted in a gas barrier film having a laminated structure of film substrate/EVOH layer (adhesion layer)/inorganic oxide layer/gas barrier coating layer.

<Preparation of sealant layer>
The following polypropylene films were prepared as sealant layers.
CPP (shrinkage rate 0.2%): thickness 70 μm
CPP (shrinkage rate 1.8%): thickness 70 μm

<Measurement of shrinkage rate of each layer>
The heat shrinkage rate (%) of each layer was measured according to the following procedure.
(1) The layer to be measured was cut into a 20 cm x 20 cm sample.
(2) A 10 cm line was written in the running direction of the measurement sample (the running direction length before heating).
(3) The measurement sample was heated at 120°C for 15 minutes.
(4) The length of the written line in the running direction was measured (length in the running direction after heating).
(5) The thermal shrinkage rate was calculated using the following formula.
(Length in the running direction before heating - Length in the running direction after heating) / Length in the running direction before heating x 100

<Production of Laminate>
Laminates of the examples (however, examples 1 to 7 and 10 are reference examples) and comparative examples were produced based on the combinations of layers shown in Table 1. The method for producing the laminates is as follows.

<Manufacturing of packaging bags>
The laminate obtained in each example was cut into a size of 15 cm x 10 cm, and two of the cut packaging films were stacked so that the sealant layers faced each other. After impulse sealing on three sides to form a pouch, 100 ml of tap water was added as the contents, and the remaining side was impulse sealed. This produced a pouch (packaging bag) sealed on four sides.

<Gas barrier property evaluation>
The pouches obtained in each example were subjected to retort treatment in a retort apparatus at 0.2 MPa and 121°C for 30 minutes. After retort treatment, the tap water in the pouches was discarded, and the pouches were thoroughly dried before evaluation of gas barrier properties. Specifically, the oxygen transmission rate (OTR) and water vapor transmission rate (WTR) were measured as follows. The results are shown in Table 2.
Oxygen permeability:
Oxygen permeability measuring device (manufactured by MOCON, product name: OX-TRAN2/20)
Measured at a temperature of 30°C and a relative humidity of 70% (JIS K-7126, Method B)
Measurement values are expressed in units of [cc/ ^m2 ·day·MPa].
Water vapor permeability:
Water vapor permeability measuring device (manufactured by MOCON, product name: PERMATRAN-W 3/33)
Measured at a temperature of 40°C and a relative humidity of 90% (JIS K-7126, Method B)
The measured values are expressed in units of [g/ ^m2 ·day].

The laminate of the present invention can be used to create packaging bags that can be retorted, and its constituent films can be substantially all polyolefin films. Such a laminate can be said to be a packaging material made from a single material (mono-material), and is expected to have excellent recyclability.

11...first substrate layer, 12...second substrate layer, 13...sealant layer, S...adhesive layer, 100...laminated body.

Claims (6)

a first substrate layer, a second substrate layer, and a sealant layer in this order;
the first substrate layer, the second substrate layer, and the sealant layer all comprise a polyolefin film;
the polyolefin film of the first base material layer or the second base material layer has an adhesive layer and an inorganic oxide layer in this order on at least one surface thereof;
the adhesion layer contains a polyvinyl alcohol-based resin,
A laminate in which the thermal shrinkage rate in the machine direction (MD direction) after heating at 120°C for 15 minutes in each of the first base material layer, the second base material layer, and the sealant layer satisfies the following formula:
1.5%≦heat shrinkage rate of second base layer≦5% (Equation 1)
Heat shrinkage rate of second base material layer>Heat shrinkage rate of first base material layer (Equation 2)
Heat shrinkage rate of second base material layer>Heat shrinkage rate of sealant layer (Equation 3)
(Here, heat shrinkage rate (%) = (length in the running direction before heating - length in the running direction after heating) / length in the running direction before heating x 100) The laminate according to claim 1 , wherein the heat shrinkage rate satisfies the following formula:
1.5%≧heat shrinkage rate of second base material layer−heat shrinkage rate of first base material layer≧0.3% (Equation 4)
3.0%≧(heat shrinkage rate of second base layer)−(heat shrinkage rate of sealant layer)≧0.5% (Equation 5) The laminate according to claim 1 or 2, wherein the heat shrinkage rate satisfies the following formula:
Heat shrinkage rate of sealant layer≦2% (Equation 6) The laminate according to any one of claims 1 to 3, wherein the polyvinyl alcohol-based resin is polyvinyl alcohol (PVA) or ethylene-vinyl alcohol copolymer (EVOH). The laminate according to any one of claims 1 to 4, which is for use in a retort pouch. A packaging bag made from the laminate described in any one of claims 1 to 5.