JP7580493B2 - Laminated Film - Google Patents

Description

The present invention relates to a laminated film, and more specifically to a laminated film that combines high levels of recyclability with favorable film properties such as mechanical strength, and a method for producing the same.

Olefin-based polymer films such as ethylene-based polymer films are excellent in flexibility, light weight, processability, gas and liquid barrier properties, cost, etc., and are therefore widely used in various applications such as containers, packaging, substrates, and base materials.
In recent years, the plastic materials used in these films are required to be recyclable from the viewpoint of reducing the environmental load, etc. In terms of recycling, it is preferable that the plastic material is a so-called monomaterial, which is composed of a single type of polymer.
On the other hand, from the viewpoint of film strength, thinness, etc., stretching of olefin-based polymer films is widely practiced. However, films composed only of ethylene-based polymers do not necessarily have excellent stretchability, and a solution to this problem has been studied. For example, in Patent Document 1, the crosslinking degree of a polyethylene resin sheet is changed in the thickness direction, thereby improving the stretchability, especially at low temperatures. However, the production of a film with a crosslinking degree changed in the thickness direction complicates the process and is disadvantageous in terms of cost, and crosslinking is also undesirable from the viewpoint of recyclability.
Thus, there has been a demand for an olefin polymer film which has both high levels of recyclability and properties favorable for a film, such as mechanical strength and stretchability, and which can be produced relatively easily and at low cost.

Japanese Unexamined Patent Publication No. 61-74819

In view of the above technical background, the object of the present invention is to provide an olefin-based polymer film that combines high levels of recyclability with favorable film properties such as mechanical strength and stretchability, and that can be produced relatively easily and at low cost.

Means for Solving the Problems The present inventors have, as a result of intensive investigations, found that the above-mentioned object can be achieved by a laminate film having an intermediate layer (A) containing an ethylene-based polymer and a skin layer (B) containing a propylene-based polymer formed on one or both sides of the intermediate layer (A), the laminate film having a specific DSC absorption/heat generation pattern, and have thus completed the present invention.
That is, the present invention provides:
[1]
A laminated film having an intermediate layer (A) containing an ethylene-based polymer, and a skin layer (B) containing a propylene-based polymer formed on one or both sides of the intermediate layer (A), wherein a DSC curve obtained by repeating heating and cooling twice at 10°C/min has a half-width of a crystallization peak observed at 110°C or more and 125°C or less in a first heating step that is greater than 3.0°C, and has a melting point _Tm1 of 135°C or more and 165°C or less and a melting point _Tm2 of 125°C or more and less than 135°C in a second heating step;
Regarding.

Below, [2] to [7] are each a preferred aspect or embodiment of the present invention.
[2]
The laminate film according to [1], wherein the ethylene-based polymer has a crystal fusion heat ΔH in the first temperature-lowering stroke of a DSC curve of 180 to 240 J/g.
[3]
The laminate film according to [1] or [2], having a skin layer (B) formed on one side of the intermediate layer (A) and a surface layer (C) containing an ethylene-based polymer provided on the opposite side of the skin layer (B).
[4]
The laminate film according to any one of [1] to [3], wherein the thickness of the skin layer (B) (when the skin layer (B) is present on both sides of the intermediate layer (A), the sum of the thicknesses of both skin layers (B)) accounts for 5 to 60% of the total thickness of the film.
[5]
The laminate film according to [3] or [4], wherein, before stretching, the distance from the center of the intermediate layer (A) or the center of the intermediate layer (A) and the surface layer (C) to the interface with the skin layer (B) is 0.1 to 1.0 mm.
[6]
The laminate film according to any one of [1] to [5], which is a stretched laminate film.
[7]
The laminated film according to [6], wherein the stretching ratio is 2x2x or more.

The laminated film of the present invention combines high levels of recyclability with favorable film properties such as mechanical strength and stretchability, and can be produced relatively easily and at low cost. It can be suitably used in a variety of applications where conventional olefin polymer films are used, such as packaging films, while reducing the environmental impact.

The present invention relates to a laminate film having an intermediate layer (A) containing an ethylene-based polymer, and a skin layer (B) containing a propylene-based polymer formed on one or both sides of the intermediate layer (A), in which a half-width of a crystallization peak observed at 110° C. or more and 125° C. or less in a first heating stroke in a DSC curve obtained by repeating heating and cooling twice at 10° C./min is greater than 3.0° C., and the laminate film has a melting point Tm ₁ of 135° C. or more and 165° C. or less, and a melting point Tm ₂ of 125° C. or more and less than 135° C. in a second heating stroke.
That is, the easy-open film of the present invention has an intermediate layer (A) containing an ethylene-based polymer, and a skin layer (B) containing a propylene-based polymer.

Intermediate layer (A)
The intermediate layer (A) constituting the laminated film of the present invention contains an ethylene-based polymer.
The intermediate layer (A) only needs to contain an ethylene-based polymer, and therefore may contain components other than ethylene-based polymers, or may contain no components other than ethylene-based polymers and be entirely composed of ethylene-based polymers.
The intermediate layer (A) may contain only one type of ethylene-based polymer or a combination of two or more types of ethylene-based polymers.

Ethylene-Based Polymers Preferred examples of the ethylene-based polymers include ethylene homopolymers, copolymers of ethylene as a main monomer with at least one α-olefin having 3 or more carbon atoms, preferably 3 to 8 carbon atoms, ethylene-vinyl acetate copolymers, saponified products thereof, and ionomers thereof. Specific examples include copolymers of ethylene as a main monomer with at least one α-olefin having 3 to 8 carbon atoms, such as polyethylene, ethylene-propylene copolymer, ethylene-1-butene copolymer, ethylene-1-pentene copolymer, ethylene-1-hexene copolymer, ethylene-4-methyl-1-pentene copolymer, and ethylene-1-octene copolymer. The proportion of the α-olefin in these copolymers is preferably 1 to 15 mol %.
In the ethylene-based polymer, the proportion of structural units derived from ethylene exceeds 50 mol %, which distinguishes it from the propylene-based polymer described below.

The density of the ethylene polymer is preferably 0.910 to 0.970 g/ ^cm3 , and more preferably 0.940 to 0.965 g/ ^cm3 . When the density is 0.910 g/cm3 or ^more , the heat sealability is improved. When the density is 0.970 g/cm3 ^or less, the processability, toughness, and transparency are improved.

Among these ethylene-based polymers, from the viewpoint of the balance between the stretchability and heat resistance of the resulting laminated film, those having a melting point measured by a differential scanning calorimeter (DSC) in the range of 125 to 135°C, particularly 128 to 133°C, are preferred.

Specific examples of the ethylene polymer include ethylene polymers manufactured and sold under the name of polyethylene. Specifically, high-pressure low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE) are preferred, linear low-density polyethylene and high-density polyethylene are more preferred, and high-density polyethylene is particularly preferred.

The high density polyethylene (HDPE) preferably used as the ethylene-based polymer may be an ethylene homopolymer or a copolymer of ethylene and an α-olefin.

The high-density polyethylene preferably has a melt flow rate (hereinafter referred to as MFR) of 0.1 to 15 g/10 min, more preferably 0.5 to 10.0 g/10 min, and even more preferably 1.0 to 5.0 g/10 min, as measured in accordance with JIS K6922-1 at 190° C. under a load of 21.18 N.
It is preferable that the MFR is in the above range, since the load on the extruder during molding is reduced and molding stability is improved.

The high density polyethylene preferably used in the present embodiment has a density according to JIS K6922-1 of preferably 940 to 970 kg/m ³ , more preferably 945 to 970 kg/m ³ , and even more preferably 950 to 965 kg/m ³ .
It is preferable for the density to be within the above range since the heat resistance is increased, i.e., the film is not deformed by heat treatment, and the decrease in transparency is small.

The high-density polyethylene is preferably substantially linear, and for example, preferably has 0.14 or less long-chain branches per 1,000 carbon atoms in the main chain in the fraction having an Mn of 100,000 or more when fractionated by molecular weight.

The high density polyethylene (B) preferably has an Mw/Mn ratio in the range of 3.0 to 40.0, and more preferably in the range of 5.0 to 30.0.
It is preferable for the molecular weight distribution to be within the above range, since this not only provides good moldability but also improves transparency.
Moreover, when Mn is 25,000 or more, transparency is improved, which is preferable.

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

In addition, the high-density polyethylene preferably used in this embodiment can be produced by, for example, a slurry method, a solution method, a gas phase method, or the like. When producing the high-density polyethylene, a Ziegler catalyst consisting of a solid catalyst component containing magnesium and titanium and an organoaluminum compound, a metallocene catalyst consisting of an organic transition metal compound containing a cyclopentadienyl derivative and a compound and/or an organic metal compound that reacts with the cyclopentadienyl derivative to form an ionic complex, a vanadium catalyst, or the like can be used, and the high-density polyethylene can be produced by homopolymerizing ethylene or copolymerizing ethylene and an α-olefin using the catalyst. The α-olefin may be one generally called an α-olefin, and is preferably an α-olefin having 3 to 12 carbon atoms, such as propylene, butene-1, hexene-1, octene-1, or 4-methyl-1-pentene. Examples of copolymers of ethylene and α-olefin include ethylene-hexene-1 copolymer, ethylene-butene-1 copolymer, and ethylene-octene-1 copolymer.

The linear low-density polyethylene is typically a copolymer of ethylene and an α-olefin, and may be synthesized by a known manufacturing method.

The α-olefin may be a compound having 3 to 20 carbon atoms, such as propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-dodecene, 4-methyl-1-pentene, 4-methyl-1-hexene, or a mixture thereof. The α-olefin is preferably a compound having 4, 6, or 8 carbon atoms, or a mixture thereof, such as 1-butene, 1-hexene, 1-octene, or a mixture thereof.
However, ethylene can also be polymerized in the polymerization step to produce α-olefins, and in this case, ethylene can be used substantially alone as the raw material for production.

The linear low density polyethylene may be a commercially available product, for example, 2040F (C6-LLDPE, MFR; 4.0, density; 0.918 g/cm ³ ) manufactured by Ube Maruzen Polyethylene Co., Ltd., or Evolue SP2040 (product name) manufactured by Prime Polymer Co., Ltd. can be used.

The linear low density polyethylene preferably has a density of 0.905 to 0.935 g/cm ³ , more preferably 0.915 to 0.930 g/cm ³ , and an MFR of preferably 0.5 to 6.0 g/10 minutes, more preferably 2.0 to 4.0 g/10 minutes.

The molecular weight distribution of linear low-density polyethylene (expressed as the ratio of weight average molecular weight: Mw to number average molecular weight: Mn: Mw/Mn) is preferably in the range of 1.5 to 4.0, and more preferably 1.8 to 3.5. This Mw/Mn can be measured by gel permeation chromatography (GPC).

Petroleum-derived linear low-density polyethylene can be produced by a conventional production method using a conventional catalyst, including a multi-site catalyst such as a Ziegler catalyst, or a single-site catalyst such as a metallocene catalyst. From the viewpoint of obtaining linear low-density polyethylene that has a narrow molecular weight distribution and can be formed into a high-strength film, it is preferable to use a single-site catalyst.

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 weight 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 catalyst components of a transition metal compound of Group IV of the periodic table containing 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, etc. 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, etc. 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., with zirconium and hafnium being particularly preferred. The transition metal compound usually has two ligands having a cyclopentadienyl skeleton, and it is preferred that the ligands having each cyclopentadienyl skeleton are bonded to each other via 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. The substituted silylene group is preferred.

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

The above-mentioned transition metal compounds of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton can be used as a catalyst component either alone or as a mixture of two or more kinds.

The co-catalyst is one that can effectively use the above-mentioned transition metal compounds of Group IV of the periodic table as polymerization catalysts or 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 supported on an inorganic or organic carrier, preferably an inorganic or organic porous oxide, specifically ion-exchangeable layered silicate such as montmorillonite, _SiO2 , _Al2O3 , MgO, _{ZrO2 , TiO2, B2O3, CaO, ZnO, BaO, ThO2 , or mixtures thereof} .

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

The intermediate layer (A) may contain components other than the ethylene-based polymer. For example, polymers other than ethylene-based polymers, oligomers, heat stabilizers (antioxidants), weather stabilizers, ultraviolet absorbers, lubricants, slipping agents, nucleating agents, antiblocking agents, antistatic agents, antifogging agents, pigments, dyes, and various fillers such as talc, silica, and diatomaceous earth can be blended as necessary or as long as they do not contradict the object of the present invention.
These additive components may be blended in advance with the ethylene polymer, or may be added when the intermediate layer (A) is formed from the ethylene polymer.

The thickness of the intermediate layer (A) is not particularly limited, but from the viewpoint of film strength etc., it is preferably 10 μm or more, more preferably 13 μm or more, and particularly preferably 15 μm or more.
On the other hand, from the viewpoints of flexibility, economy, and the like, the thickness of the intermediate layer (A) is preferably 500 μm or less, more preferably 300 μm or less, and particularly preferably 100 μm or less.
When stretching is carried out in the production of the laminated film of the present invention, the thickness of the layer corresponding to the intermediate layer (A) before stretching is preferably from 0.2 to 1.94 mm, particularly preferably from 0.4 to 1.9 mm.
The thickness of the intermediate layer (A) can be appropriately adjusted by adjusting the stretching conditions such as the stretch ratio, the layer thickness before stretching, the lip distance of the die on the stand on which the layer is formed before stretching, and the like.

Furthermore, when stretching is performed in the production of the laminated film of the present invention, the distance from the center of the intermediate layer (A) or the center of the intermediate layer (A) and the surface layer (C) to the interface with the skin layer (B) before stretching is preferably 0.1 to 1.0 mm, more preferably 0.1 to 0.97 mm, and particularly preferably 0.25 to 0.95 mm.
The distance from the center of the intermediate layer (A) or the center of the intermediate layer (A) and the surface layer (C) to the interface with the skin layer (B) can be appropriately adjusted by adjusting the thickness of each layer before stretching, the lip spacing of the die on the stand on which the layer before stretching is formed, etc.

Skin layer (B)
The intermediate layer (B) constituting the laminated film of the present invention contains a propylene-based polymer.
The skin layer (B) may contain a propylene-based polymer, and may contain components other than the propylene-based polymer, or may contain no components other than the propylene-based polymer and be entirely composed of a propylene-based polymer.
The skin layer (B) may contain only one type of propylene-based polymer or a combination of two or more types of propylene-based polymers.

Propylene-Based Polymer As the propylene-based polymer, a resin generally manufactured and sold under the name of polypropylene can be used. Usually, a propylene homopolymer having a density of about 890 to 930 kg/ ^m3 or a propylene copolymer, that is, a copolymer comprising propylene and at least one comonomer selected from other α-olefins in a small amount, can be used.
In the case of a copolymer, it may be a random copolymer or a block copolymer. Examples of other α-olefins in this propylene copolymer include ethylene and α-olefins having about 4 to 20 carbon atoms, such as ethylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, and 4-methyl-1-pentene. Such other α-olefins may be copolymerized alone or in combination with two or more α-olefins. In addition, the presence of comonomers other than the α-olefin is not excluded.

Propylene-based polymers are distinguished from ethylene-based polymers by the proportion of propylene-derived structural units being 50 mol % or more, preferably 80 mol % or more, and particularly preferably 90 mol % or more.
Since the proportion of the structural units derived from propylene is 50 mol% or more, the proportion of the structural units derived from the comonomer is less than 50 mol%. In ordinary polypropylene, the proportion of the structural units derived from the comonomer is often 25 mol% or less. In the case of a random copolymer, it is preferably 10 mol% or less, and particularly preferably 7 mol% or less. In the case of a block copolymer, it is preferably 20 mol% or less, and particularly preferably 15 mol% or less.

Among these propylene-based polymers, from the viewpoint of the balance between the stretchability and heat resistance of the resulting laminated film, propylene-based polymers having a melting point based on a differential scanning calorimeter (DSC) in the range of 135 to 165°C, particularly 137 to 163°C, are preferred, with homopolypropylene or propylene-α-olefin random copolymers being particularly preferred.

The melt flow rate (MFR) (ASTM D1238, 230°C, 2160 g load) of the propylene-based polymer used in the skin layer (B) is not particularly limited, but is usually in the range of 0.01 to 100 g/10 min, preferably 0.1 to 70 g/10 min, in terms of stretchability, etc.

The propylene polymer (a) can be produced by various known production methods, specifically, for example, using an olefin polymerization catalyst such as a Ziegler-Natta catalyst or a single-site catalyst. In particular, it can be produced using a single-site catalyst. The single-site catalyst is a catalyst with a uniform (single-site) active site, such as a metallocene catalyst (so-called Kaminsky catalyst) or a Brookhart catalyst. The metallocene catalyst is a catalyst consisting of a metallocene transition metal compound and at least one compound selected from the group consisting of an organoaluminum compound and a compound that reacts with the metallocene transition metal compound to form an ion pair, and may be supported on an inorganic material.

Surface layer (C)
The laminate film of the present invention may be any laminate film having an intermediate layer (A) containing an ethylene-based polymer, and a skin layer (B) containing a propylene-based polymer formed on one or both sides of the intermediate layer (A), and may or may not have other layers. However, particularly when a skin layer (B) is formed on only one side of the intermediate layer (A), it is preferable for the film to have a surface layer (C) containing an ethylene-based polymer provided on the side opposite to the skin layer (B).
The surface layer (C) is preferably provided since it can impart functionality such as improved laminate strength.

The thickness of the surface layer (C) is not particularly limited, but is preferably 0.1 to 10 μm, and particularly preferably 1 to 5 μm.
When the thickness of the intermediate layer (A) is used as a reference, the thickness of the surface layer (C) is preferably 1 to 30%, particularly preferably 5 to 20%, of the thickness of the intermediate layer (A).

The surface layer (C) is not particularly limited as long as it contains an ethylene-based polymer. Therefore, the surface layer (C) may be made of the same material as the intermediate layer (A), but when there are two or more layers containing an ethylene-based polymer, the layer located outside the intermediate layer (A) and constituting the surface corresponds to the surface layer (C).
The details of the type, physical properties, etc. of the ethylene polymer in the surface layer (C) are the same as those explained above in relation to the intermediate layer (A).
Laminated Film

The laminated film of the present invention is a film having the above-mentioned intermediate layer (A) and skin layer (B). In the laminated film of the present invention, the intermediate layer (A) and the skin layer (B) are preferably directly laminated to each other, but other layers may be present therebetween.
Examples of other layers include an adhesive layer and a gas barrier layer, but are not limited to these.

In the laminate film of the present invention, the thickness of the skin layer (B) (when the skin layer (B) is present on both sides of the intermediate layer (A), the sum of the thicknesses of both skin layers (B)) is preferably 5 to 60% of the total thickness of the film.
When the thickness of the skin layer (B) accounts for 5% or more of the total thickness of the film, the stretching processability is improved and stable stretching at a high stretching ratio is possible. From this viewpoint, the thickness of the skin layer (B) (when the skin layer (B) is present on both sides of the intermediate layer (A), the sum of the thicknesses of both skin layers (B)) is preferably 5% or more, particularly preferably 10% or more, of the total thickness of the film.
When the thickness of the skin layer (B) is 60% or less of the total film thickness, the film of the present invention has excellent recyclability. From this viewpoint, the thickness of the skin layer (B) (when the skin layer (B) is present on both sides of the intermediate layer (A), the sum of the thicknesses of both skin layers (B)) is preferably 30% or less, particularly preferably 10% or less of the total film thickness.

When the laminated film of the present invention is stretched, the proportion of the thickness of the skin layer (B) to the total thickness of the film is approximately the same before and after stretching, but if there is a difference between the proportions before and after stretching, it is preferable that the proportion after stretching be within the above range.
The proportion of the thickness of the skin layer (B) to the total thickness of the film can be appropriately adjusted by adjusting the thickness of each layer before stretching, and can be appropriately adjusted by adjusting the die lip spacing when producing each layer before stretching.

The laminated film of the present invention can be produced by various known film forming methods, such as a method in which films to be the intermediate layer (A) and the skin layer (B) (or two layers, if two layers are present) are formed in advance and then the films are bonded together to form a laminated film, a method in which a multilayer film consisting of an intermediate layer (A) and a skin layer (B) is obtained using a multilayer die, and then another skin layer (B) is extruded onto the surface of the intermediate layer (A) to form a laminated film, or a method in which a laminated film consisting of the skin layer (B), the intermediate layer (A), and the skin layer (B) is obtained by co-extrusion using a multilayer die.

In addition, various known film molding methods, specifically, T-die cast film molding method, inflation film molding method, etc., can be used as the film molding method.

Since the laminated film of the present invention has excellent stretching processability, it is preferable to perform stretching for the purpose of producing a thin film, improving mechanical strength, improving transparency, etc. It is particularly preferable to perform biaxial stretching. There is no particular restriction on the stretching ratio, but in the case of biaxial stretching, it is preferable that the stretching ratio is 2 times x 2 times or more.
As the biaxial stretching, a method such as sequential biaxial stretching, simultaneous biaxial stretching, or multi-stage stretching is appropriately adopted.
The conditions for biaxial stretching may be those for producing a known biaxially stretched film. For example, in the case of a sequential biaxial stretching method, the longitudinal stretching temperature is set to 100° C. to 145° C., the stretching ratio is set to a range of 3 to 7 times, the transverse stretching temperature is set to 120° C. to 180° C., and the stretching ratio is set to a range of 3 to 11 times.

The total thickness of the laminated film of the present invention is not particularly limited, but from the viewpoint of ensuring practical strength, etc., when stretched, the total thickness after stretching is usually 15 μm or more, preferably 18 μm or more, and more preferably 20 μm or more, while from the viewpoint of having sufficient flexibility in relation to the application, etc., the total thickness is usually 500 μm or less, preferably 300 μm or less, and more preferably 100 μm or less.
When the laminated film of the present invention is stretched, the total thickness before stretching is preferably from 0.3 to 2.5 mm, particularly preferably from 0.5 to 2.0 mm.

The laminated film of the present invention is heated from -50°C to 200°C at a heating rate of 10°C/min under conditions of a sample weight of about 5.0 mg and a nitrogen gas inflow rate of 50 ml/min in accordance with JIS K7121, and then held at 200°C for 10 minutes, and then cooled and heated once each under the same conditions. The DSC curve obtained has a specific absorption/exothermic pattern. More specifically, the DSC curve obtained under the above conditions is as follows:
In the first temperature-lowering step, the half-width of the crystallization peak observed at 110°C or higher and 125°C or lower is greater than 3.0°C, and in the second temperature-highering step, the melting point Tm1 _{is greater} than 135°C or higher and 165°C or lower, and the melting point _Tm2 is greater than 125°C or higher and lower than 135°C.

In the first cooling step, the half-width of the crystallization peak observed at 110°C or higher and 125°C or lower is greater than 3.0°C, which is preferable because it allows crystallization during stretching to be appropriately suppressed and improves stretch processability.
In the first temperature-lowering step, the half-width of the crystallization peak observed at 110° C. or higher and 125° C. or lower is preferably 3.0° C. or higher, and more preferably 3.5° C. or higher.
In the first cooling step, the half width of the crystallization peak observed at 110° C. or more and 125° C. or less does not have any particular upper limit, but it is usually 10.0° C. or less, and more preferably 5.0° C. or less.
The half-value width of the crystallization peak in the first temperature-lowering step can be appropriately adjusted by changing the types of ethylene-based polymer and propylene-based polymer used, or the ratio of the thickness of the skin layer made of a propylene-based polymer to the total thickness of the film, or the like.

The laminated film of the present invention has a melting point _Tm1 of 135°C or higher and 165°C or lower and a melting point _Tm2 of 125°C or higher and lower than 135°C in the second heating step.
Since the laminated film of the present invention has the above melting points Tm ₁ and Tm ₂ , it is suitable for heat sealing processing.
In a laminate film made only of an ethylene-based polymer, the difference in melting point between the outermost layer and the seal layer of the film is small, so that a problem has been pointed out in the past that the outermost layer melts during heat sealing and becomes fused to the heat seal bar.
The laminated film of the present invention has the melting points Tm ₁ and Tm ₂ , particularly the higher Tm ₁ , and therefore can suppress heat fusion of the outermost layer (skin layer (B)) during heat sealing. For example, when used for food packaging bags, it is preferable because it can realize food packaging bags with excellent bag manufacturing suitability.
The melting point _Tm1 is preferably from 135 to 165°C, and more preferably from 137 to 160°C.
The melting point _Tm1 can be appropriately adjusted by adjusting the type, physical properties, content, etc. of the propylene-based polymer contained in the skin layer (B).
The melting point _Tm2 is preferably from 120 to 135°C, more preferably from 125 to 133°C.
The melting point _Tm2 can be appropriately adjusted by adjusting the type, physical properties, content, etc. of the ethylene polymer contained in the intermediate layer (A).

Furthermore, in the DSC curve, the crystalline melting heat ΔH (based on 100% ethylene polymer ratio) of the ethylene polymer contained in the intermediate layer (A) in the first heating step is preferably 180 to 240 J/g.
In the above DSC curve, the crystalline heat of fusion ΔH (J/g) of the entire laminate film is observed, and the crystalline heat of fusion of the ethylene-based polymer (calculated as 100% ethylene-based polymer proportion) is calculated by dividing ΔH of the melting peak of the ethylene-based polymer by the content ratio of the ethylene-based polymer (PE monomer ratio).
It is preferable that the crystalline melting heat ΔH of the ethylene polymer contained in the intermediate layer (A) in the first heating step is within the above range, since the polyethylene polymer can be stretched efficiently.
The crystalline melting heat ΔH of the ethylene polymer contained in the intermediate layer (A) in the first heating step is more preferably 180 to 240 J/g, particularly preferably 190 to 230 J/g.
The heat of crystal fusion ΔH of the ethylene-based polymer in the first temperature-lowering step can be appropriately adjusted by adjusting the type of the ethylene-based polymer contained in the intermediate layer (A) and physical properties such as the crystallinity.

The laminated film of the present invention has excellent stretch processability as described above, and this property can be utilized to realize a high elastic modulus.
When the laminated film of the present invention is stretched, after stretching, when the elastic modulus in the MD direction (machine direction) is T1 and the elastic modulus in the TD direction (transverse direction) is T2, the value of T1 + T2 is desirably 1500 (MPa) or more, more preferably 1600 (MPa) or more, even more preferably 1800 (MPa) or more, and particularly preferably 2000 (MPa) or more.
There is no particular upper limit to the value of T1+T2, but as long as the material and manufacturing method are available at reasonable cost, the upper limit is usually 4500 MPa or less, and in many cases 3500 MPa or less.
The elastic modulus of the laminated film can be measured by a method conventionally known in the art, more specifically, by performing a tensile test on a rectangular sample cut out from the laminated film, for example, by the method described in the examples of the present specification.

The laminated film of this embodiment has a high elastic modulus, and is therefore suitable for use in applications such as packaging bags. Packaging bags using a laminated film with such a high elastic modulus have a so-called firm feel, making it possible to realize packaging bags that look good when displayed.

The shape of the packaging bag in this embodiment is not particularly limited and may be any conventionally known packaging bag, with preferred examples being three-sided bags, four-sided bags, pillow bags, gusset bags, standing pouches, etc. Among these, the packaging bag is particularly suitable for use in gusset bags and standing pouches that are required to be self-supporting.
In addition, the high elastic modulus of this embodiment is preferable because it contributes to excellent processability in the lamination process, printing process, and the like.

The laminated film of the present invention preferably has a fusion temperature of 140°C or higher, more preferably 150°C or higher, and particularly preferably 160°C or higher, when the fusion temperature is defined as the temperature at which the heat seal strength is 1.0 (N/15 mm) or higher.
The heat seal strength and heat fusion temperature of the laminated film can be measured by a method conventionally known in the art, and more specifically, can be measured by performing a peel test on a 15 mm wide sample cut out from a laminate obtained by heat sealing to an adherend film at a predetermined heat seal temperature. For example, they can be measured by the method described in the examples of the present specification.

The laminated film of the present invention uses ethylene-based polymers and propylene-based polymers with excellent transparency, and the transparency can be further improved by stretching processing, so that high transparency can be achieved relatively easily and the film can be suitably used for applications such as food packaging bags. The food packaging bag of the present embodiment has high practical value, such as good appearance of printing and contents due to its high transparency.

From the standpoint of print appearance, the contents of the food packaging bag of this embodiment are not particularly limited, but from the standpoint of the appearance of the contents, the food packaging bag of this embodiment can be particularly suitably used when storing contents that should be visible to consumers, such as rice crackers, bread, cut vegetables, cut fruit, and sweets.
On the other hand, since the bag is easily shattered by impact during transportation, it is not necessarily suitable for use as a packaging bag for food containing contents that should not be shown, such as snack foods or dried small fish. However, since printing is often performed on the packaging bag in such cases, the food packaging bag of this embodiment, which has excellent print appearance, can still be used publicly.

The transparency of the laminated film of the present invention can be evaluated by its haze, which is preferably 10% or less, more preferably 8% or less, and further preferably 5% or less per sheet.
The haze of the laminated film can be measured by a conventionally known method, more specifically, by the method described in the Examples of the present specification.

When the laminated film of the present invention is used for a bag for packaging food, it is preferable to use a laminated film having high tear-openability. Examples of the form of the packaging bag include a three-sided bag, a four-sided bag, a pillow bag, a gusset bag, and a standing pouch.
More specifically, in the laminated film of this embodiment, when the tear strength in the MD direction (machine direction) is T1 (mN) and the tear strength in the TD direction is T2 (mN), the value of T1 + T2 is preferably 1000 (mN) or less, more preferably 400 (mN) or less, and particularly preferably 200 (nN) or less.
The tear strength of the laminated film can be measured by a method conventionally known in the art, more specifically, by using a light-load tear tester, for example, by the method described in the examples of the present application.
There is no particular lower limit to the tear strength of the laminated film, but from the viewpoint of avoiding unintended tearing, the value of T1+T2 is preferably 10 (mN) or more, and more preferably 20 (mN) or more.

The laminated film of the present invention has a high level of recyclability and desirable film properties such as mechanical strength and stretchability, and can be suitably used in various applications where olefin polymer films have traditionally been used. For example, it can be suitably used as a packaging material for packaging fresh foods, processed foods, daily necessities, sanitary products, pharmaceuticals, etc., as well as electrical and electronic materials and as a surface protection material for various components, and is particularly suitable for use as a packaging material.

When the laminated film of the present invention is used as a packaging material, the laminated film itself may be folded and sealed on three sides, or two sheets of the laminated film may be sealed on all four sides to form a package. Also, the laminated film or a lid material formed by bonding the laminated film to a substrate or the like may be heat-sealed to various container bodies such as cups to form a package.
A suitable example of such a package is a packaging container comprising the above-mentioned lid material and a container body containing at least one of polypropylene, polyethylene terephthalate, and polybutylene terephthalate.
There are no particular limitations on the items that can be stored in the packaging container, but the container can be preferably used to package foods, medicines, medical instruments, daily necessities, miscellaneous goods, and the like.

The present invention will be specifically described below with reference to examples and comparative examples. Note that the present invention is not limited in any way by the following examples.

The physical properties and characteristics in the examples and comparative examples were evaluated by the following methods.
(1) Maximum Stretching Ratio A stretched raw film having a thickness of 1 mm and having a layer structure shown in Table 1, in which an intermediate layer (A) and a skin layer (B) were laminated, was produced.
Using a batch-type biaxial stretching machine, the obtained stretched raw film was stretched at the temperatures shown in Table 1 (122°C to 166°C, 4°C intervals) from 2x2 to 9x9 in both length and width directions at equal stretching ratios in 0.5x intervals, and the maximum stretching ratio at which the film could be stretched without clip coming off or breaking was recorded as the maximum stretching ratio at that stretching temperature.
(2) Haze For the stretched films obtained at the stretching temperatures and stretch ratios shown in Table 2, the single-sheet haze and four-sheet haze were measured in accordance with JIS K7136 using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.). The measured values are the average values of five measurements.
(3) Elastic modulus From the stretched films obtained at the stretching temperatures and stretch ratios shown in Table 2, rectangular film pieces (length: 150 mm, width: 15 mm) were cut out in the machine direction (MD) and transverse direction (TD) as test pieces, and a tensile test was carried out using a tensile tester (manufactured by A&D Co., Ltd., RTG1210) under conditions of chuck distance: 100 mm and crosshead speed: 5 mm/min to determine the elastic modulus (MPa). The measured value is the average value of 5 runs.
(4) Tear Strength Using a light-load tear tester manufactured by Toyo Seiki Seisakusho, the tear strength in the MD direction and the TD direction of the stretched films obtained at the stretching temperatures and stretch ratios shown in Table 2 were measured under the conditions of a measurement temperature of 23±3° C. and a measurement humidity of 50±5% RH.
(5) Heat seal strength Stretched films obtained at the stretching temperatures and stretch ratios shown in Table 2 were heat sealed at temperatures ranging from 120°C to 190°C, sealed for 1 second at a pressure of 0.2 MPa using a 10 mm wide seal bar, and then cooled to prepare a measurement sample. A 15 mm wide test piece was cut from the sample, and the heat sealed portion was peeled off at a crosshead speed of 300 mm/min, and the strength was recorded as the heat seal strength (N/15 mm) at that heat seal temperature.
The temperature at which the heat seal strength became 1.0 N or more was taken as the fusion temperature of the stretched film.
(6) DSC curve Using a differential scanning calorimeter (DSC) Q100 manufactured by TA Instruments, about 5 mg of a sample cut out from the stretched film obtained at the stretching temperature and stretch ratio shown in Table 2 was precisely weighed, and in accordance with JIS K7121, the sample was heated from -50°C to 200°C at a heating rate of 10°C/min under conditions of nitrogen gas inflow of 50 ml/min, and then held at 200°C for 10 minutes, and then cooled and heated once each under the same conditions to obtain a DSC curve, from which the melting point (°C), heat of crystalline fusion ΔH (J/g), half-width of crystallization peak (°C), etc. were determined.

Details of each of the constituent components such as resins used in the examples and comparative examples are as follows.
・HDPE (high density polyethylene)
Density: 950kg/ ^m3
MFR: 1.1 g/10 min Melting point: 131° C.
・h-PP (homopolypropylene)
Density: 900kg/ ^m3
MFR: 3.0 g/10 min Melting point: 161° C.
r-PP1 (ternary random polypropylene 1)
Density: 900kg/ ^m3
MFR: 7g/10min Melting point: 139°C
・r-PP2 (ternary random polypropylene 2)
Density: 900kg/ ^m3
MFR: 5.0 g/10 min Melting point: 128° C.
・r-PP3 (metallocene binary random polypropylene)
Density: 900kg/ ^m3
MFR: 7.0 g/10 min Melting point: 125° C.

Example 1
Homopolypropylene (h-PP) as the material constituting the skin layer (B), and high density polyethylene (HDPE) as the material constituting the intermediate layer (A) were each fed into separate extruders, and a three-layer co-extruded film with a total thickness of 1.0 mm and a thickness ratio of skin layer (B)/intermediate layer (A)/skin layer (B) of 30.0:40.0:30.0 was molded using the T-die method to produce a stretched raw film.
The maximum stretching ratio of the obtained stretched raw film was evaluated according to the above-mentioned method. The results are shown in Table 1.
Next, the raw stretched film was stretched 7×7 times at 158° C. to obtain a stretched film, which was evaluated for haze, elastic modulus, tear strength, and HS strength, and the DSC curve was measured according to the above-mentioned methods. The results are shown in Table 2.

(Examples 2 to 5)
Except for changing the thickness ratio of skin layer (B)/intermediate layer (A)/skin layer (B) as shown in Table 1, a stretched raw film was prepared in the same manner as in Example 1, and the maximum stretch ratio was evaluated. The results are shown in Table 1.
Next, a stretched film was produced from the stretched raw film in the same manner as in Example 1, and the haze, elastic modulus, tear strength, and HS strength were evaluated, and the DSC curve was measured. The results are shown in Table 2.

Example 6
Homopolypropylene (h-PP) as the material constituting the skin layer (B), high density polyethylene (HDPE) as the material constituting the intermediate layer (A), and high density polyethylene (HDPE) as the material constituting the surface layer (C) were each fed into separate extruders, and a three-layer co-extruded film with a total thickness of 1.0 mm and a thickness ratio of skin layer (B)/intermediate layer (A)/surface layer (C) of 5.0:90.0:5.0 was molded by the T-die method to produce a stretched raw film.
The obtained stretched raw film was used to evaluate the maximum stretch ratio according to the above-mentioned method. The results are shown in Table 1.
Next, the stretched raw film was stretched 6x6 times at 126°C to obtain a stretched film, which was evaluated for haze, elastic modulus, tear strength, and HS strength, and the DSC curve was measured according to the above-mentioned methods. For heat sealing, the homopolypropylene of the skin layer (B) was overlapped and sealed. The results are shown in Table 2.

(Example 7)
A stretched raw film was prepared in the same manner as in Example 4, except that ternary random polypropylene (r-PP1) was used as the material constituting the skin layer (B), and the maximum stretch ratio was evaluated. The results are shown in Table 1.
Next, a stretched film was produced from the stretched raw film in the same manner as in Example 1, and the haze, elastic modulus, tear strength, and HS strength were evaluated, and the DSC curve was measured. The results are shown in Table 2.
Next, the raw stretched film was stretched 7x7 times at 130°C to obtain a stretched film, which was evaluated for haze, elastic modulus, tear strength, and HS strength, and the DSC curve was measured according to the above-mentioned methods. The results are shown in Table 2. The half-width in the first cooling step shown in Table 2 is that of the two peaks at 117.3°C.

(Example 8)
A stretched raw film was prepared in the same manner as in Example 6, except that the positions of the skin layer (B) and the surface layer (C) were interchanged and a ternary random polypropylene (r-PP1) was used as the material constituting the skin layer (B), and the maximum stretch ratio was evaluated. The results are shown in Table 1.

(Examples 9 and 10)
Except for using ternary random polypropylene (r-PP2) or metallocene binary random polypropylene (r-PP3) as the material constituting the skin layer (B), a stretched raw film was prepared in the same manner as in Example 7, and the maximum stretch ratio was evaluated. The results are shown in Table 1.

(Comparative Example 1)
High density polyethylene (HDPE) was fed into separate extruders as the material for the surface layer (C) and high density polyethylene (HDPE) was fed into separate extruders as the material for the intermediate layer (A). A three-layer co-extruded film having a total thickness of 1.0 mm and a surface layer (C)/intermediate layer (A)/surface layer (C) thickness ratio of 5.0:90.0:5.0 was formed by the T-die method, and a stretched raw film was produced.
The maximum stretching ratio of the obtained stretched raw film was evaluated according to the above-mentioned method. The results are shown in Table 1.
The stretched raw film has poor stretchability and cannot be made into a stretched film. Therefore, high density polyethylene is fed into an extruder, and a non-stretched film having a layer thickness of about 20 μm is molded by T-die method, in which the thickness ratio of surface layer (C)/middle layer (A)/surface layer (C) is 5.0:90.0:5.0. According to the above method, the haze, elastic modulus, tear strength, and HS strength are evaluated, and the DSC curve is measured. The results are shown in Table 2.

The laminated film of the present invention achieves a high level of recyclability and favorable film properties such as mechanical strength and stretchability, and can be produced relatively easily and at low cost. Therefore, the laminated film of the present invention can be suitably used in various applications in which conventional olefin polymer films are used, such as packaging films, while reducing the environmental load, and has high applicability in various industrial fields such as the electrical and electronics industry, the pharmaceutical industry, agriculture, food processing, distribution, and restaurant industry.

Claims (6)

A laminated film having an intermediate layer (A) made of an ethylene-based polymer, and a skin layer (B) made of a propylene-based polymer formed on one side of the intermediate layer (A), wherein a DSC curve obtained by repeatedly heating and cooling the laminated film twice at a rate of 10°C/min has a half-width of a crystallization peak observed at 110°C or more and 125°C or less in a first heating stroke that is greater than 3.0°C, and has a melting point _Tm1 of 135°C or more and 165°C or less and a melting point _Tm2 of 125°C or more and less than 135°C in a second heating stroke,
Further comprising a surface layer (C) made of an ethylene-based polymer,
the intermediate layer (A), the skin layer (B) and the surface layer (C) are laminated in the order of skin layer (B)/intermediate layer (A)/surface layer (C);
A laminated film, wherein the ethylene-based polymer constituting the intermediate layer (A) contains high-density polyethylene. 2. The laminate film according to claim 1, wherein the ethylene polymer constituting the intermediate layer (A) has a crystal fusion heat ΔH in a first temperature-lowering stroke of a DSC curve of 180 to 240 J/g. The laminated film according to claim 1 or 2, wherein the thickness of the skin layer (B) accounts for 5 to 60% of the total thickness of the film. The laminated film according to any one of claims 1 to 3, wherein the distance from the center of the intermediate layer (A) or the center of the intermediate layer (A) and the surface layer (C) to the interface with the skin layer (B) before stretching is 0.1 to 1.0 mm. The laminate film according to any one of claims 1 to 4, which is a stretched laminate film. A method for producing the laminated film according to claim 5, comprising a step of biaxially stretching the film at a stretching ratio of 2x2 or more.