JP7591199B2 - Polyethylene laminate and packaging material using same - Google Patents

Description

The present invention relates to a polyethylene laminate, and more specifically to a polyethylene laminate made of a single material, which comprises a polyethylene base layer to which heat resistance has been imparted and a polyethylene layer having heat sealability, and to a packaging material using the same.

Films made of polyethylene have moderate flexibility and are excellent in transparency, moisture resistance, and chemical resistance, as well as being inexpensive, so they are used in a variety of packaging materials. In particular, polyethylene has a melting point that varies slightly depending on the type, but is generally around 100 to 140°C, so it is commonly used as a sealant film in the packaging material field.

On the other hand, as mentioned above, polyethylene has poor heat resistance, and if used as a packaging material, it will deform or even melt during heat sheet processing. Therefore, a laminate of a base film with excellent heat resistance, such as a polyester film or a nylon film, and a polyethylene film is generally used as a packaging material. For example, a base film such as a polyester film is laminated with a polyethylene film, and bags are made by heat sealing with the polyethylene film side on the inside of the packaging bag (for example, Patent Document 1, etc.).

In recent years, with the growing demand for a recycling-oriented society, attempts have been made to recycle and reuse packaging materials. However, there is an issue with laminates made by bonding together different types of resin films as described above, in that it is difficult to separate the resins by type, making them unsuitable for recycling.

In response to the above problem, the applicant has conducted research to realize a packaging material made of a single polyethylene film without using a base film with excellent heat resistance such as a polyester film, and has proposed that a packaging material made of a polyethylene film can be realized by imparting heat resistance to one side of the polyethylene film that has conventionally been used as a heat seal layer by irradiating one side of the polyethylene film with electrons (Patent Document 2).

JP 2005-104525 A JP 2017-031233 A

As described above, the polyethylene film proposed in Patent Document 2 has one side that has been given heat resistance by electron beam irradiation and therefore functions as a substrate, and the other side has heat sealability, so that it has both heat resistance and heat sealability despite being a single film and can be suitably used as a packaging material. Furthermore, since it is made of a single polyethylene material, it is also highly recyclable. However, after further investigation by the present inventors, it was found that the surface irradiated with electron beams (i.e., the surface that has been given heat resistance) of the above polyethylene film can shrink when heat sealing is performed during bag production, and there is room for further improvement in terms of heat resistance.

SUMMARY OF THE PRESENT EMBODIMENTS Accordingly, an object of the present invention is to provide a polyethylene laminate which has high heat resistance while maintaining heat sealability, and which also has excellent recyclability.
Another object of the present invention is to provide a packaging material using the polyethylene laminate.

The inventors have found that by forming a polyethylene laminate using two types of polyethylene with different densities on the side to be irradiated with electron beams and the side not to be irradiated with electron beams, it is possible to significantly improve heat resistance while maintaining recyclability. The present invention is based on this finding.

The polyethylene laminate according to the present invention comprises:
A polyethylene laminate having a base layer and a heat seal layer,
The substrate layer comprises polyethylene having a density of 0.930 g/cm3 or ^more ,
The heat seal layer comprises polyethylene having a density of less than 0.930 g/ ^cm3 ;
the base layer and the heat seal layer are formed by co-extrusion molding of a polyethylene having a density of 0.930 g/ ^cm3 or more and a polyethylene having a density of less than 0.930 g/ ^cm3 ;
The substrate layer is characterized in that it has been subjected to an electron beam irradiation treatment.

The polyethylene laminate according to the present invention may have an intermediate layer between the base layer and the heat seal layer.

In addition, in the polyethylene laminate according to the present invention,
The intermediate layer comprises polyethylene having a density of 0.942 g/cm ³ or more,
The base layer, the intermediate layer and the heat seal layer may be formed by co-extrusion molding of a polyethylene having a density of 0.930 g/ ^cm3 or ^more , a polyethylene having a density of 0.942 g/cm3 or more, and a polyethylene having a density of less than 0.930 g/ ^cm3 .

The polyethylene laminate according to the present invention may further include a printed layer on the surface of the base layer.

The polyethylene laminate according to the present invention may further include a barrier layer on the surface of the base layer.

In another aspect of the present invention, the packaging material is made of a polyethylene laminate.

According to the present invention, in a laminate including a base layer and a heat seal layer, the base layer is made of polyethylene having a density of 0.930 g/ ^cm3 or more, the heat seal layer is made of polyethylene having a density of less than 0.930 g/ ^cm3 , and the base layer side of the laminate of the co-extruded base layer and heat seal layer is subjected to an electron beam irradiation treatment, thereby further improving the heat resistance of the base layer. As a result, a polyethylene laminate having high heat resistance and excellent recyclability while maintaining heat sealability can be realized.

1 is a cross-sectional schematic diagram of a polyethylene laminate according to one embodiment of the present invention. 1 is a cross-sectional schematic diagram of a polyethylene laminate according to one embodiment of the present invention. FIG. 1 is a perspective view showing one embodiment of a packaging material made using the polyethylene laminate of the present invention. FIG. 1 is a perspective view showing one embodiment of a packaging material made using the polyethylene laminate of the present invention.

[Polyethylene laminate]
The polyethylene laminate according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of a polyethylene laminate 10 according to one embodiment of the present invention. The polyethylene laminate 10 according to the first embodiment includes a base layer 1 and a heat seal layer 2. As described later, the polyethylene laminate of the present invention can be suitably used as a packaging material, and when the polyethylene laminate is used to make a packaging material, the base layer 1 side is used to be located on the outer surface of the packaging material, and the heat seal layer 2 is used to be located on the inner surface of the packaging material. In addition, the polyethylene laminate 10 according to another embodiment of the present invention may include an intermediate layer 3 between the base layer 1 and the heat seal layer 2. As described later, by including the intermediate layer 3, it is possible to further impart stiffness and strength to the polyethylene laminate. In addition to the above-mentioned layers, the polyethylene laminate of the present invention may also include a printed layer or a barrier layer (not shown) as appropriate so as to be suitable for various uses of the packaging material. Hereinafter, each layer constituting the polyethylene laminate of the present invention will be described.

<Base layer>
In the present invention, the base layer contains polyethylene having a density of 0.930 g/cm ³ or more. As described above, in Patent Document 2, the applicant proposed that one side of a monolayer film made of polyethylene having a density of 0.91 g/cm ³ or less is irradiated with an electron beam to improve the heat resistance of the electron beam irradiated side, while maintaining the heat sealability of the other side not irradiated with an electron beam, so that even a single polyethylene film can be used as a packaging material. This is thought to be because by irradiating polyethylene having a density of 0.91 g/cm ³ or less with an electron beam, the polyethylene on the surface irradiated with the electron beam is partially crosslinked, improving the heat resistance. However, polyethylene having a density of 0.91 g/cm ³ or less has poor processability such as stickiness, and the processability is not improved even by electron beam irradiation. In addition, since the crosslink density of polyethylene is different between the side irradiated with an electron beam and the side not irradiated with an electron beam, wrinkles may occur in the film due to the difference in heat shrinkage rate between the front and back sides of the film. In the present invention, as described above, polyethylene having a density of 0.930 g/cm3 ^or more is used as the base layer placed on the surface to be irradiated with electron beams, and the heat seal layer is formed of polyethylene having a density of less than 0.930 g/ ^cm3. This improves the heat resistance of the base layer (i.e., the occurrence of stickiness and wrinkles), and realizes a polyethylene laminate having excellent recyclability while maintaining heat sealability.

As the polyethylene having a density of 0.930 g/cm ³ or more, medium density polyethylene (MDPE) or high density polyethylene (HDPE) can be suitably used. The medium density polyethylene refers to a polyethylene having a density of 0.930 g/cm ³ or more and less than 0.942 g/cm ³ , and the high density polyethylene refers to a polyethylene having a density of 0.942 g/cm ³ or more. The density of the polyethylene constituting the base layer is preferably 0.930 g/cm ³ or more and 0.950 g/cm ³ or less, and more preferably 0.935 g/cm ³ or more and 0.945 g/cm ³ or less. The polyethylene having such a density can be prepared by mixing the above-mentioned medium density polyethylene and high density polyethylene at an appropriate compounding ratio. In addition, not limited to medium density polyethylene and high density polyethylene, high density polyethylene may be mixed with low density polyethylene (LDPE) or linear low density polyethylene (LLDPE) at an appropriate compounding ratio so as to be within the above density range. The low density polyethylene refers to polyethylene having a density of less than 0.930 g/cm ³ .

The polyethylenes with different densities and branches as described above can be obtained by appropriately selecting the polymerization method. For example, it is preferable to use a multi-site catalyst such as a Ziegler-Natta catalyst or a single-site catalyst such as a metallocene catalyst as the polymerization catalyst, and to carry out the polymerization in one stage or in two or more stages by any of the methods of gas phase polymerization, slurry polymerization, solution polymerization, and high pressure ionic polymerization.

The single-site catalyst is a catalyst capable of forming a uniform active species, and is usually prepared by contacting a metallocene transition metal compound or a nonmetallocene transition metal compound with an activating cocatalyst. Compared to multi-site catalysts, single-site catalysts have a uniform active site structure, and are therefore preferred because they can polymerize polymers with high molecular weights and highly uniform structures. As a single-site catalyst, it is particularly preferred to use a metallocene catalyst. A metallocene catalyst is a catalyst that contains each of the catalytic components of a transition metal compound of Group IV of the periodic table that contains a ligand having a cyclopentadienyl skeleton, a cocatalyst, and if necessary, an organometallic compound and a carrier.

In the above-mentioned transition metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton, the cyclopentadienyl skeleton is a cyclopentadienyl group, a substituted cyclopentadienyl group, or the like. The substituted cyclopentadienyl group has at least one substituent selected from a hydrocarbon group having 1 to 30 carbon atoms, a silyl group, a silyl-substituted alkyl group, a silyl-substituted aryl group, a cyano group, a cyanoalkyl group, a cyanoaryl group, a halogen group, a haloalkyl group, a halosilyl group, or the like. The substituted cyclopentadienyl group may have two or more substituents, and the substituents may be bonded to each other to form a ring, such as an indenyl ring, a fluorenyl ring, an azulenyl ring, or a hydrogenated product thereof. The rings formed by bonding the substituents to each other may further have substituents.

In the transition metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton, the transition metal may be zirconium, titanium, hafnium, etc., and zirconium and hafnium are particularly preferred. The transition metal compound usually has two ligands having a cyclopentadienyl skeleton, and it is preferred that the ligands having the cyclopentadienyl skeleton are bonded to each other by a bridging group. Examples of the bridging group include alkylene groups having 1 to 4 carbon atoms, silylene groups, substituted silylene groups such as dialkylsilylene groups and diarylsilylene groups, and substituted germylene groups such as dialkylgermylene groups and diarylgermylene groups. Substituted silylene groups are preferred. The above transition metal compounds of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton may be used as a catalyst component, either alone or in a mixture of two or more types.

The co-catalyst refers to a catalyst that can effectively use the above-mentioned transition metal compound of Group IV of the periodic table as a polymerization catalyst, or can balance the ionic charge in a catalytically activated state. Examples of the co-catalyst include benzene-soluble aluminoxanes of organoaluminum oxy compounds and benzene-insoluble organoaluminum oxy compounds, ion-exchangeable layered silicates, boron compounds, ionic compounds consisting of cations with or without active hydrogen groups and non-coordinating anions, lanthanoid salts such as lanthanum oxide, tin oxide, and phenoxy compounds containing fluoro groups.

The transition metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton may be used by being supported on an inorganic or organic carrier. The carrier is preferably an inorganic or organic porous oxide, and specifically includes ion-exchange layered silicate such as montmorillonite, SiO ₂ , Al ₂ O ₃ , MgO, ZrO ₂ , TiO ₂ , B ₂ O ₃ , CaO, ZnO, BaO, ThO ₂ , etc., or a mixture thereof. Furthermore, examples of the organometallic compound that is used as necessary include organoaluminum compounds, organomagnesium compounds, and organozinc compounds. Of these, organoaluminum compounds are preferably used.

Copolymers of ethylene and other monomers can also be used. Examples of ethylene copolymers include copolymers of ethylene and α-olefins having 3 to 20 carbon atoms. Examples of α-olefins having 3 to 20 carbon atoms include propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, 3-methyl-1-butene, 4-methyl-1-pentene, and 6-methyl-1-heptene. Copolymers with vinyl acetate, acrylic esters, and the like may also be used as long as they do not impair the objectives of the present invention.

In addition, in the present invention, polyethylene made from biomass-derived ethylene may be used as the raw material instead of ethylene obtained from fossil fuels. Since such biomass-derived polyethylene is a carbon-neutral material, it can be used as a packaging material with even less environmental impact. Such biomass-derived polyethylene can be produced, for example, by a method such as that described in JP 2013-177531 A. In addition, commercially available biomass-derived polyethylene (for example, Green PE available from Braskem) may be used.

The substrate layer may contain a light stabilizer. As described later, the polyethylene laminate of the present invention is one in which the polyethylene of the substrate layer has been irradiated with an electron beam. Therefore, by containing a light stabilizer in the substrate layer, the deterioration of the polyethylene constituting the substrate layer over time can be prevented. When the polyethylene is irradiated with an electron beam, the carbon-hydrogen bonds in the polyethylene are broken, and radicals are generated at the ends of the broken bonds. The generated radicals come into contact with other polyethylene molecular chains due to the molecular motion of the molecular chains, and remove hydrogen atoms to bond with carbon atoms in the polyethylene molecular chains. As a result, a crosslinked structure is formed in the polyethylene, but the molecular chains are gradually broken by the remaining radicals, which is thought to lead to the deterioration of the polyethylene. The light stabilizer has the function of trapping these remaining radicals.

Examples of light stabilizers include antioxidants such as phenol-based antioxidants, amine-based antioxidants, phosphoric acid-based antioxidants, sulfur-based antioxidants, hindered amine-based antioxidants, and hydroxylamine-based antioxidants, as well as ultraviolet absorbers such as benzotriazole-based ultraviolet absorbers, triazine-based ultraviolet absorbers, and benzophenone-based ultraviolet absorbers.

It is preferable to use an antioxidant, as it is less likely to inhibit the crosslinking reaction that occurs when the polyethylene film substrate is irradiated with an electron beam.

As for the antioxidant, it is preferable to use a primary antioxidant that captures the generated radicals in combination with a secondary antioxidant that decomposes the hydroperoxides generated from the radicals, and an antioxidant that has both the functions of a primary antioxidant and a secondary antioxidant may also be used.

Primary antioxidants include phenol-based antioxidants, amine-based antioxidants, and hindered amine-based antioxidants, while secondary antioxidants include phosphorus-based antioxidants and sulfur-based antioxidants, and antioxidants that function as both primary and secondary antioxidants include hydroxylamine-based antioxidants.

Hydroxylamine-based antioxidants and phosphorus-based antioxidants are also preferred because they can prevent discoloration of the polyethylene film substrate.

The content of the light stabilizer in the substrate layer is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 10% by mass or less, and even more preferably 0.1% by mass or more and 8% by mass or less.

The substrate layer may also contain a crosslinking agent. As described below, the substrate layer is subjected to electron beam irradiation treatment, and by containing a crosslinking agent, the degree of crosslinking of the polyethylene can be further improved. As a result, the heat resistance of the substrate layer constituting the polyethylene laminate can be further improved.

Examples of crosslinking agents include styrene-based elastomers such as styrene-polyisoprene elastomer, styrene-polybutadiene elastomer, and styrene-polyisoprene-butadiene random copolymer; ethylene-acrylate copolymers such as ethylene-methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, and ethylene-butyl acrylate copolymer; and ethylene-acrylic acid ester-glycidyl methacrylate.

The content of the crosslinking agent in the base layer is preferably 1% by mass or more and 49% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and even more preferably 15% by mass or more and 35% by mass or less.

The base layer may contain additives within the range that does not impair the characteristics of the present invention, such as fillers, reinforcing agents, antistatic agents, pigments, modifying resins, etc.

The thickness of the substrate layer can be any thickness depending on the application, but is usually about 5 μm or more and 200 μm or less, and preferably about 5 μm or more and 100 μm or less. As described below, the thickness can be appropriately adjusted by the screw rotation speed of the melt extruder and the rotation speed of the cooling roll when co-extrusion molding to form a laminate.

<Heat seal layer>
The polyethylene laminate of the present invention is provided with a heat seal layer. When the polyethylene laminate of the present invention is used to prepare a packaging material, the heat seal layer is used so as to be positioned on the inner surface of the packaging material.

The heat seal layer contains polyethylene having a density of less than 0.930 g/cm ^3. By using the same material (polyethylene) as the base layer but using polyethylene having a lower density than the polyethylene used for the base layer as the heat seal layer, it is possible to obtain a polyethylene laminate having recyclability while maintaining heat sealability.

The heat seal layer may be made of low density polyethylene or linear low density polyethylene alone, or may be made of a suitable mixture of the two. A mixture of low density polyethylene and medium density polyethylene may be used as long as the density is within the above-mentioned density range, but from the viewpoint of heat sealability, the density is preferably less than 0.925 g/ ^cm3 , and more preferably less than 0.920 g/ ^cm3 .

The polyethylene film layer may contain the above additives to the extent that they do not impair the properties of the present invention.

The thickness of the heat seal layer is arbitrary depending on the application, but is usually preferably 15 μm or more and 200 μm or less, more preferably 20 μm or more and 200 μm or less, and even more preferably 25 μm or more and 160 μm or less. This allows the heat sealability of the polyethylene laminate of the present invention to be maintained while improving its processability. The thickness of the heat seal layer can be appropriately adjusted by the screw rotation speed of the melt extruder and the rotation speed of the cooling roll when co-extrusion molding to form a laminate, as described below.

The thickness ratio of the base layer to the heat seal layer is preferably 4:1 to 1:4, and more preferably 2:1 to 1:2. The thickness ratio can be appropriately adjusted by the screw rotation speed of the co-extrusion film-forming machine, as described below.

<Middle class>
The polyethylene laminate of the present invention may have an intermediate layer between the above-mentioned base layer and the heat seal layer. By providing the intermediate layer, the polyethylene laminate can be made stiffer and have improved shape stability. In addition, by providing the intermediate layer, the polyethylene laminate can be prevented from melting and becoming thin during the heat seal process for bag making.

From the viewpoint of recyclability, the intermediate layer is preferably made of polyethylene, which is the same material as the material constituting the base layer and the heat seal layer. In this case, the intermediate layer preferably contains polyethylene having a density of 0.942 g/cm ³ or more, more preferably a polyethylene having a density of 0.950 g/cm ³ or more and 0.970 g/cm ³ or less. High density polyethylene can be preferably used as such polyethylene, but a mixture of medium density polyethylene and high density polyethylene at an appropriate blending ratio may also be used as long as the density is within the above range.

The thickness of the intermediate layer can vary depending on the application, but is usually about 5 μm or more and 100 μm or less, preferably about 10 μm or more and 80 μm or less, and more preferably about 10 μm or more and 60 μm or less.

In addition to the polyethylene described above, the intermediate layer may contain an ethylene-vinyl alcohol copolymer (EVOH). By containing an ethylene-vinyl alcohol copolymer, the intermediate layer functions as a barrier layer, thereby imparting barrier properties to the polyethylene laminate.

The thickness of the intermediate layer may be adjusted as appropriate within a range that does not impair the functionality of the intermediate layer. For example, the thickness ratio of the base layer to the intermediate layer may be in the range of 2:1 to 1:5, with a preferred range being 1:1 to 1:3. The thickness ratio of the intermediate layer to the heat seal layer may be in the range of 1:2 to 5:1, with a preferred range being 3:1 to 1:1. The thickness ratio of each layer may be adjusted as appropriate by the screw rotation speed of the co-extrusion film-forming machine when forming the polyethylene laminate, as described below.

<Other demographics>
The polyethylene laminate according to the present invention may be provided with layers other than the above-mentioned layers as long as the recyclability is not impaired. For example, a printed layer or a barrier layer may be provided on the surface of the base layer. Since the surface of the base layer of the polyethylene laminate according to the present invention is treated by electron beam irradiation, the adhesion between the printed layer or barrier layer and the base layer is excellent even if the base layer is not subjected to a particular surface treatment, but the surface may be subjected to a corona treatment, an ozone treatment, a low-temperature plasma treatment using oxygen gas or nitrogen gas, or the like, a glow discharge treatment, an oxidation treatment using chemicals, or the like, or the like.

The printing layer is a layer on which any desired printed pattern such as letters, numbers, pictures, figures, symbols, designs, etc. is formed for decoration, display of contents, display of expiration date, display of manufacturer, seller, etc., and for other display and aesthetic purposes. The printing layer may be provided on the entire surface of the base layer or the barrier layer described below, or may be provided on only a part of it. The printing layer can be formed using conventionally known pigments and dyes.

The method for forming the printing layer is not particularly limited, and can be, for example, an inkjet method, a gravure printing method, an offset printing method, a flexographic printing method, a thermal transfer method, a transfer method, or a hot stamp (foil stamping) printing method.

The printing layer preferably has a thickness of 0.1 μm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less, and even more preferably 1 μm or more and 3 μm or less.

Metal foil or a vapor-deposited layer of a metal or inorganic oxide can be suitably used as the barrier layer.

As the metal foil, any conventionally known metal foil can be used. Aluminum foil is preferred from the standpoint of gas barrier properties that prevent the transmission of oxygen gas, water vapor, etc., and light blocking properties that prevent the transmission of visible light, ultraviolet light, etc. In addition, it is possible to impart a metallic luster to the packaging bag, thereby improving the design. The thickness of the metal foil is, for example, 5 μm or more and 15 μm or less.

The metal or inorganic oxide vapor deposition layer is a layer consisting of a vapor deposition film that can be formed by a conventionally known method. By providing a vapor deposition layer, it is possible to impart or improve gas barrier properties that prevent the permeation of oxygen gas, water vapor, and the like. The barrier layer may have two or more vapor deposition layers. When two or more vapor deposition layers are provided, each layer may have the same composition or different compositions.

Metal deposition films that can be used include, for example, aluminum (Al), magnesium (Mg), tin (Sn), sodium (Na), titanium (Ti), lead (Pb), zirconium (Zr), yttrium (Y), gold (Au), and chromium (Cr). In particular, for packaging bags, it is preferable to have an aluminum deposition film.

The thickness of the metal vapor deposition film varies depending on the type of metal used, but it is desirable to select any thickness within the range of, for example, 50 Å to 2000 Å, and preferably 100 Å to 1000 Å. More specifically, in the case of an aluminum vapor deposition film, the thickness is 50 Å to 600 Å, and more preferably 100 Å to 450 Å.

As inorganic oxide vapor deposition films, for example, vapor deposition films of oxides such as silicon (Si), aluminum (Al), magnesium (Mg), calcium (Ca), potassium (K), tin (Sn), sodium (Na), boron (B), titanium (Ti), lead (Pb), zirconium (Zr), and yttrium (Y) can be used. In particular, for packaging bags, it is preferable to have a vapor deposition film of aluminum oxide or silicon oxide.

Inorganic oxides are expressed as MOX (where M represents an inorganic element and the value of X varies depending on the inorganic element), such as SiOX and AlOX. The range of X is as follows: silicon (Si) is 0-2, aluminum (Al) is 0-1.5, magnesium (Mg) is 0-1, calcium (Ca) is 0-1, potassium (K) is 0-0.5, tin (Sn) is 0-2, sodium (Na) is 0-0.5, boron (B) is 0-1.5, titanium (Ti) is 0-2, lead (Pb) is 0-2, zirconium (Zr) is 0-2, and yttrium (Y) is 0-1.5. In the above, when X=0, it is a completely inorganic element (pure substance) and is not transparent, and the upper limit of the range of X is the value when it is completely oxidized. Silicon (Si) and aluminum (Al) are preferably used for packaging materials, with silicon (Si) ranging from 1.0 to 2.0 and aluminum (Al) ranging from 0.5 to 1.5.

The thickness of the inorganic oxide vapor deposition film varies depending on the type of inorganic oxide used, but it is desirable to select any thickness within the range of, for example, 50 Å to 2000 Å, and preferably 100 Å to 1000 Å. For example, in the case of a vapor deposition film of aluminum oxide or silicon oxide, the thickness is desirably 50 Å to 500 Å, and more preferably 100 Å to 300 Å.

The vapor deposition film can be formed on the base layer, etc., using the following formation methods. Examples of the method for forming the vapor deposition film include physical vapor deposition methods (PVD methods) such as vacuum deposition, sputtering, and ion plating, or chemical vapor deposition methods (CVD methods) such as plasma chemical vapor deposition, thermal chemical vapor deposition, and photochemical vapor deposition.

If necessary, a gas barrier coating film may be provided on the vapor deposition layer. The gas barrier coating film is a coating film that functions as a layer that suppresses the permeation of oxygen gas, water vapor, and the like. The gas barrier coating film is obtained from a gas barrier composition that contains at least one alkoxide represented by the general formula R ¹ nM(OR ² )m (wherein R ¹ and R ² represent an organic group having 1 to 8 carbon atoms, M represents a metal atom, n represents an integer of 0 or more, m represents an integer of 1 or more, and n+m represents the atomic valence of M) and a polyvinyl alcohol resin and/or an ethylene-vinyl alcohol copolymer, and is further polycondensed by a sol-gel method in the presence of a sol-gel catalyst, an acid, water, and an organic solvent.

As the alkoxide represented by the general formula R ¹ nM(OR ² )m, at least one of a partial hydrolyzate of an alkoxide and a condensate of hydrolysis of an alkoxide can be used. In addition, as the partial hydrolyzate of an alkoxide, it is not necessary that all of the alkoxy groups are hydrolyzed, and one in which one or more are hydrolyzed, or a mixture thereof may be used. As the condensate of hydrolysis of an alkoxide, a dimer or higher of a partially hydrolyzed alkoxide, specifically a dimer to hexamer, is used.

In the alkoxide represented by the above general formula R ¹ nM(OR ² ) m , silicon, zirconium, titanium, aluminum, etc. can be used as the metal atom represented by M. In this embodiment, preferred metals include, for example, silicon and titanium. In the present invention, the alkoxide can be used alone or in the form of a mixture of two or more different metal atoms in the same solution.

In the alkoxides represented by the above general formula R ¹ nM(OR ² )m, specific examples of the organic group represented by R ¹ include alkyl groups such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl, t-butyl, n-hexyl, n-octyl, etc. In the alkoxides represented by the above general formula R ¹ nM(OR ² )m, specific examples of the organic group represented by R ² include methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl, etc. These alkyl groups may be the same or different in the same molecule.

When preparing the gas barrier composition, for example, a silane coupling agent may be added. As the silane coupling agent, a known organoalkoxysilane containing an organic reactive group may be used. In this embodiment, in particular, an organoalkoxysilane having an epoxy group is preferably used, and specifically, for example, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, etc. may be used. The above-mentioned silane coupling agents may be used alone or in combination of two or more.

The polyethylene laminate according to the present invention is one in which the above-mentioned layers are formed by co-extrusion. That is, it can be obtained by co-extruding polyethylene having a density of 0.930 g/ ^cm3 or more constituting the base layer, polyethylene having a density of less than 0.930 g/ ^cm3 constituting the heat seal layer, and, if desired, polyethylene having a density of 0.942 g/ ^cm3 or more constituting the intermediate layer, etc., using a co-extrusion inflation film forming machine or the like to form a film. In this way, after forming a polyethylene laminate having a base layer, a heat seal layer, and, if desired, an intermediate layer, the above-mentioned printed layer and barrier layer are formed on the surface of the base layer, whereby the polyethylene laminate of the present invention can be produced.

Next, the electron beam irradiation treatment will be described. In the polyethylene laminate according to the present invention, the base layer is irradiated with an electron beam. It is believed that the electron beam irradiation treatment of the base layer crosslinks a part of the polyethylene constituting the base layer, improving the heat resistance. The presence or absence of crosslinking (i.e., whether or not the base layer has been irradiated with an electron beam) can also be examined by immersing the polyethylene laminate in an organic solvent such as methyl ethyl ketone, drying the insoluble part that remains undissolved, measuring the mass, and calculating the gel fraction from the thickness (mass) of the base layer before dissolution and the mass of the insoluble part after drying, taking advantage of the fact that the crosslinked part does not dissolve in a solvent. The gel fraction of the polyethylene laminate is preferably 20% or more and 80% or less, calculated by the mass of the base layer.

It is preferable to change the irradiation energy of the electron beam as appropriate depending on the intended use of the package. Generally, the higher the irradiation energy of the electron beam, the greater the amount of radicals generated, making it easier to form a crosslinked structure. However, if the irradiation energy is too high, the molecular chains of the polyethylene are cut too much, tending to reduce the physical properties of the film, such as strength.

The dose of the electron beam is preferably in the range of 10 to 2000 kGy, more preferably 20 to 1000 kGy, and the acceleration voltage of the electron beam is preferably in the range of 30 to 300 kV, more preferably 50 to 300 kV, and particularly preferably 50 to 250 kV. The irradiation energy of the electron beam is preferably 20 to 750 keV, more preferably 25 to 400 keV, even more preferably 30 to 300 keV, and particularly preferably 20 to 200 keV.

As the electron beam irradiation device, a conventionally known device can be used, for example, a curtain type electron irradiation device (LB1023, manufactured by I-Electron Beam Co., Ltd.) or a line irradiation type low energy electron beam irradiation device (EB-ENGINE, manufactured by Hamamatsu Photonics Co., Ltd.) can be preferably used. In particular, a line irradiation type low energy electron beam irradiation device (EB-ENGINE, manufactured by Hamamatsu Photonics Co., Ltd.) can be preferably used.

The oxygen concentration in the electron beam irradiation device is preferably 500 ppm or less, and more preferably 100 ppm or less. By performing electron beam irradiation under such conditions, it is possible to suppress the generation of ozone and to suppress the deactivation of radicals generated by electron beam irradiation by oxygen in the atmosphere. Such conditions can be achieved, for example, by creating an inert gas (nitrogen, argon, etc.) atmosphere inside the device.

Since polyethylene film is prone to thermal shrinkage, it is preferable to irradiate it with electron beams while simultaneously cooling it using a cooling drum or the like.

<Packaging materials>
The polyethylene laminate described above can be suitably used as a packaging material. In one embodiment of the present invention, a packaging material 20 using the polyethylene laminate of the present invention has a bag-like shape as shown in FIG.

In another embodiment, the packaging material 30 using the polyethylene laminate of the present invention has a stand-up pouch shape with a body 31 and a bottom 32, as shown in FIG. 4. In this embodiment, only the body may be formed from the polyethylene co-extruded film, only the bottom may be formed from the polyethylene co-extruded film, or both the body and bottom may be formed from the polyethylene co-extruded film.

The packaging material can be produced by folding the above-mentioned polyethylene laminate in half, overlapping it so that the base layer is on the outside and the heat seal layer is on the inside, and heat sealing the ends, etc. Alternatively, it can be produced by overlapping two polyethylene laminates so that the base layer sides face each other, and heat sealing the ends, etc.

For example, various types of packaging materials can be manufactured by heat sealing using a heat seal pattern such as a side seal pattern, a two-sided seal pattern, a three-sided seal pattern, a four-sided seal pattern, an envelope seal pattern, a grooving seal pattern (pillow seal pattern), a pleated seal pattern, a flat bottom seal pattern, a square bottom seal pattern, a gusset seal pattern, and the like. In addition, for example, a self-supporting packaging bag (standing pouch) can also be manufactured. Heat sealing can be performed using known methods such as bar sealing, rotary roll sealing, belt sealing, impulse sealing, high frequency sealing, and ultrasonic sealing.

The packaging material made from the polyethylene laminate of the present invention is made of a single material (i.e., polyethylene), and therefore has better recyclability than conventional packaging materials made of a combination of different materials. In addition, the base layer of the polyethylene laminate contains polyethylene of a specific density that has been irradiated with electron beams, and therefore can satisfy the physical properties required for the outer layer of the packaging material, such as strength, dimensional stability, and printing properties. Furthermore, the heat seal layer of the polyethylene laminate contains polyethylene with a density of less than 0.930 g/ ^cm3 , and therefore has heat sealability, and can be suitably used as a film for packaging materials.

The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1
medium density polyethylene (density: 0.937 g/cm ³ , MFR: 1.8 g/10 min, manufactured by Prime Polymer Co., Ltd., product name: SP4020);
High density polyethylene (density: 0.958 g/cm ³ , MFR: 1.0 g/10 min, manufactured by Prime Polymer Co., Ltd., product name: Hizex3600F),
A polyethylene laminate was obtained by coextrusion molding in a ratio of 1:3:1 with a linear low density polyethylene (density: 0.916 g/ ^cm3 , MFR: 2.3 g/10 min, manufactured by Prime Polymer Co., Ltd., product name: SP2020). The thickness of the obtained coextruded polyethylene laminate was 60 μm. In the obtained coextruded polyethylene laminate, the layer side of the medium density polyethylene having a density of 0.937 g/ ^cm3 is referred to as the base layer, and the layer side of the linear low density polyethylene having a density of 0.916 g/ ^cm3 is referred to as the heat seal layer.

The co-extruded polyethylene laminate obtained as described above was subjected to a corona treatment from the base layer side, and further irradiated with an electron beam under the following conditions using an electron beam irradiation device (line irradiation type irradiation device EZ-CURE, manufactured by Iwasaki Electric Co., Ltd.), to obtain a co-extruded polyethylene laminate in which the base layer was irradiated with an electron beam.
(Electron beam irradiation conditions)
Voltage: 100 kV
Irradiation dose: 280kGy
Oxygen concentration in the device: 100 ppm or less Line speed: 25 m/min

Immediately after production, an image was formed on the substrate layer side of the co-extruded polyethylene laminate obtained as described above by gravure printing using a solvent-based gravure ink (manufactured by DIC Graphics Corporation, product name: XOX-901).

Example 2
A coextruded polyethylene laminate was obtained in the same manner as in Example 1, except that the electron beam irradiation conditions were as follows:
(Electron beam irradiation conditions)
Voltage: 110kV
Irradiation dose: 280kGy
Oxygen concentration in the device: 100 ppm or less Line speed: 25 m/min

Example 3
A coextruded polyethylene laminate was obtained in the same manner as in Example 1, except that the thickness of the coextruded polyethylene laminate was 100 μm.

Example 4
A coextruded polyethylene laminate was obtained in the same manner as in Example 2, except that the thickness of the coextruded polyethylene laminate was 100 μm.

Example 5
A coextruded polyethylene laminate was obtained in the same manner as in Example 1, except that the printing method after irradiating the substrate layer surface with electron beams was changed as follows.
(Printing method)
First, OP varnish (Showa Ink Industrial Co., Ltd., product name: ALEX Hakuri OP varnish) was applied to the untreated surface of a PET film (Toyobo Co., Ltd., product name: E5100, 12 μm), and an image was formed by gravure printing using a solvent-based gravure ink (Showa Ink Industrial Co., Ltd., product name: ALEX (NT) Pattern Ink). The obtained PET film with the image formed thereon and a co-extruded polyethylene laminate after electron beam irradiation obtained in the same manner as in Example 1 were dry laminated via a two-component curing urethane adhesive (Rock Paint Co., Ltd., product name: RU-40/curing agent H-4) so that the printed surface of the PET film and the electron beam irradiated surface (substrate layer surface) faced each other, to obtain a co-extruded polyethylene laminate.
Thereafter, the PET film was peeled off, and the printed layer was transferred onto the surface of the base layer side of the coextruded polyethylene laminate to form an image.

Example 6
A coextruded polyethylene laminate was obtained in the same manner as in Example 3, except that the printing method after irradiating the substrate layer surface with electron beams was changed as follows.
(Printing method)
First, OP varnish (Showa Ink Industrial Co., Ltd., product name: ALEX Hakuri OP varnish) was applied to the untreated surface of a PET film (Toyobo Co., Ltd., product name: E5100, 12 μm), and an image was formed by gravure printing using a solvent-based gravure ink (Showa Ink Industrial Co., Ltd., product name: ALEX (NT) pattern ink). The obtained PET film with the image formed thereon and a co-extruded polyethylene laminate after electron beam irradiation obtained in the same manner as in Example 3 were dry laminated via a two-component curing urethane adhesive (Rock Paint Co., Ltd., product name: RU-40/curing agent H-4) so that the printed surface of the PET film and the electron beam irradiated surface (substrate layer surface) faced each other, to obtain a co-extruded polyethylene laminate.
Thereafter, the PET film was peeled off, and the printed layer was transferred onto the surface of the base layer side of the coextruded polyethylene laminate to form an image.

<Comparative Example 1>
A coextruded polyethylene laminate was obtained in the same manner as in Example 1, except that the electron beam was not irradiated.

<Comparative Example 2>
A coextruded polyethylene laminate was obtained in the same manner as in Example 3, except that the electron beam was not irradiated.

<Heat resistance evaluation>
The coextruded polyethylene laminates obtained in the above examples and comparative examples were cut into 5 cm x 5 cm samples, each in triplicate. Each sample was heated for 5 seconds in a glycerin solution heated to 140°C. The size of the sample pieces after heating was measured in the machine direction (MD) during inflation molding and in the transverse direction (TD) perpendicular thereto. The average shrinkage rate from the time of cutting was as shown in Table 1 below.

<Seal strength evaluation>
The coextruded polyethylene laminates obtained in the above Examples and Comparative Examples immediately after production were cut into 10 cm x 10 cm samples, each in triplicate. Each sample was folded in half so that the side not irradiated with the electron beam, i.e., the polyethylene layer with a density of 0.916 g/ ^cm3 , was on the inside, and a 1 cm x 10 cm area was heat-sealed using a heat seal tester at a temperature of 180°C, a pressure of 1 kgf/cm2, and for 1 second.

After heat sealing, the sample pieces were cut into strips 15 mm wide, and both ends that were not heat sealed were clamped in a tensile tester, and the peel strength (N/15 mm) was measured at a speed of 300 mm/min and a load range of 50 N. Note that the co-extruded polyethylene laminates obtained in Comparative Examples 1 and 2 adhered to the heat seal bar, making it impossible to measure the peel strength, and were therefore marked as "-". The measurement results are shown in Table 1 below.

<Printability test>
The formed images were visually observed, and the printability of the coextruded polyethylene laminates obtained in the above Examples and Comparative Examples was evaluated based on the following evaluation criteria.
A: The dimensional stability during printing was good, and a good image was formed without rubbing, bleeding, or the like.
×: The film expanded and contracted during printing, causing rubbing and bleeding in the formed image.
The evaluation results are shown in Table 1.

<Evaluation of liquid pouch production>
Using a liquid pouch filling machine, liquid pouches measuring 100 mm in length and 80 mm in width were produced from the coextruded polyethylene films obtained in the above Examples and Comparative Examples.
The bag-making method was as follows: first, two co-extruded polyethylene laminates were stacked with their heat-sealable layers facing each other, and three sides were heat-sealed with a heat-seal bar at 140° C. Next, the heat-sealed co-extruded polyethylene laminate was cut into a size of 100 mm length × 80 mm width to prepare a liquid pouch.
The suitability of the liquid pouches for bag making was evaluated based on the following evaluation criteria.
◯: The film was not fused to the heat seal bar, and liquid pouches could be produced using the liquid pouch filling machine.
×: The film was fused to the heat seal bar, and a clean liquid pouch could not be produced using the liquid pouch filling machine.
The evaluation results are shown in Table 1.

<Stand-up pouch bag making evaluation>
Using a bag making machine, stand-up pouches measuring 110 mm in length and 150 mm in width were made from the coextruded polyethylene laminates obtained in the above Examples and Comparative Examples.
As a bag making method, first, one test piece of 110 mm length x 60 mm width was prepared from the coextruded polyethylene laminate obtained in the examples and comparative examples, and folded in a V shape (110 mm length x 30 mm width) so that the heat seal layer was on the outside. Next, two coextruded polyethylene laminates were overlapped with the heat seal layers facing each other, and the V-folded test piece obtained above was sandwiched at one end and heat sealed with a heat seal bar at 140°C to form a bottom. Next, two sides adjacent to the bottom were heat sealed in the same manner to form a cylindrical body, and the body was cut to a size of 110 mm length x 150 mm width to prepare a stand pouch.
The suitability of the stand-up pouches for bag making was evaluated based on the following evaluation criteria.
◯: The film was not fused to the heat seal bar, and a stand-up pouch could be produced using the bag making machine.
×: The film was fused to the heat seal bar, and a clean stand-up pouch could not be produced using the bag making machine.
The evaluation results are shown in Table 1.

Claims (5)

A polyethylene laminate comprising a base layer, a printing layer provided on a surface of the base layer, and a heat seal layer (excluding a heat shrinkable multilayer film comprising a base layer and at least one heat seal layer laminated on the base layer, the heat shrinkable multilayer film having a heat shrinkage rate of 10% or less at 110°C and 30% or more at 140°C in accordance with ASTM D2732 in the machine direction and in a direction perpendicular to the machine direction of the heat shrinkable multilayer film) ,
The base layer is made of polyethylene having a density of 0.930 g/cm3 or ^more ,
the substrate layer comprises high density polyethylene and/or medium density polyethylene, but does not comprise low density polyethylene;
The heat seal layer is made of polyethylene having a density of less than 0.930 g/ ^cm3 ;
the base layer and the heat seal layer are formed by co-extrusion molding of a polyethylene having a density of 0.930 g/ ^cm3 or more and a polyethylene having a density of less than 0.930 g/ ^cm3 ;
13. A polyethylene laminate, wherein the substrate layer has been subjected to an electron beam irradiation treatment. The polyethylene laminate of claim 1, comprising an intermediate layer between the base layer and the heat seal layer. The intermediate layer is made of polyethylene having a density of 0.942 g/cm3 or ^more ,
3. The polyethylene laminate according to claim 2, wherein the base layer, the intermediate layer and the heat seal layer are formed by co-extrusion molding of a polyethylene having a density of 0.930 g/ ^cm3 or more, a polyethylene having a density of 0.942 g/ ^cm3 or more, and a polyethylene having a density of less than 0.930 g/ ^cm3 . The polyethylene laminate according to any one of claims 1 to 3, further comprising a barrier layer on the surface of the base layer. A packaging material comprising the polyethylene laminate according to any one of claims 1 to 4.