Oriented polyethylene film, packaging material, and food packaging

Abstract

An oriented polyethylene film (100) comprising a high-density polyethylene layer 1 (101), a medium-density polyethylene layer (102), and a high-density polyethylene layer 2 (103) in this order, wherein the oriented polyethylene film has a crystal thickness of 16 nm or less in the MD direction as determined by small-angle X-ray scattering (SAXS) measurement.

Description

Stretched polyethylene film, packaging material and food packaging body

[0001] The present invention relates to a stretched polyethylene film, a packaging material, and a food package.

[0002] In the field of packaging films, attempts to improve various performances by devising materials used, layer structures, etc. are known.

[0003] Patent Document 1 discloses a laminated film that includes a base layer and a heat seal layer, has a stiffness of 70 mN or more in the TD direction measured under conditions of a loop length of 50 mm and a push-in length of 10 mm, and has a polyethylene content of 90 mass% or more, and describes that this laminated film can be recycled and can fully ensure the self-supporting properties of a standing pouch.

[0004] Patent Document 2 discloses a polyethylene laminate for use in packaging materials, which comprises at least an oriented polyethylene film, an adhesive layer, and a heat-sealable polyethylene layer, wherein the adhesive layer contains a solvent-free adhesive, and the oriented polyethylene film contains at least one of high-density polyethylene (HDPE) and medium-density polyethylene (MDPE), and describes that this polyethylene laminate for use in packaging materials can significantly reduce the burden on the environment, and has high printability and strength.

[0005] Patent Document 3 discloses a polyethylene co-extruded film comprising a polyethylene film substrate and a polyethylene film layer, wherein the polyethylene film substrate is an electron beam irradiated layer containing polyethylene, a light stabilizer, and a crosslinking agent, and the polyethylene film layer contains polyethylene, and the surface opposite to the surface on which the polyethylene film substrate is provided has heat-sealability. It is described that this polyethylene co-extruded film can suppress deterioration over time and further has improved heat resistance and strength.

[0006] Patent Document 4 describes a multilayer film in which a gas barrier layer formed by applying a dispersion containing an inorganic layered compound and a water-soluble polymer onto at least one surface of a base layer made of a thermoplastic resin, an overcoat layer containing a cationic resin and a resin having a hydroxyl group, an adhesive layer, and a sealant layer are laminated in this order, and it is described that this multilayer film has excellent heat sealability and gas barrier properties.

[0007] JP 2022-053864 A JP 2022-079510 A JP 2018-008455 A JP 2009-241359 A

[0008] With the recent rise in environmental awareness, particularly the focus on the problem of marine plastic pollution, packaging films have come under scrutiny from society. This has led to a greater demand than ever before for the promotion of packaging film recycling. In other words, there is a growing demand for packaging films to be designed and manufactured with "ease of recycling" in mind.

[0009] Many conventional packaging films have achieved desired effects (strength, gas barrier properties, etc.) by laminating multiple materials. For example, the multilayer film described in Patent Document 4 has at least four layers: a gas barrier layer, an overcoat layer, an adhesive layer, and a sealant layer. However, laminating multiple materials has led to difficulties in recycling. From the perspective of facilitating the recycling of packaging films, for example, it is conceivable to make the packaging film have as simple a layer structure as possible.

[0010] In terms of simplifying the layer structure, the ultimate goal is to make the packaging film "single-layer." Furthermore, there are examples such as Patent Documents 1 to 3, in which a multi-layer structure is used in which a single material is blended at a high content overall, taking into consideration ease of recycling, even if the film is not single-layered. The present inventors conducted a preliminary study on various properties that may be required of packaging films using polyethylene film, a relatively low-cost and general-purpose packaging material. As a result of the study, it was found that a "single-layer" polyethylene film has poor lamination strength, which refers to the force required when laminating and peeling the film.

[0011] The present invention provides a stretched polyethylene film having improved lamination strength and a packaging material made from such a stretched polyethylene film.

[0012] [1] A stretched polyethylene film comprising, in this order, a high-density polyethylene layer 1, a medium-density polyethylene layer, and a high-density polyethylene layer 2, wherein the stretched polyethylene film has a crystal thickness in the machine direction of 16 nm or less as determined by small-angle X-ray scattering (SAXS) measurement. [2] The stretched polyethylene film according to [1], wherein, when the entire stretched polyethylene film is taken as 100% by mass, the total amount of the high-density polyethylene layer 1 and the high-density polyethylene layer 2 in the stretched polyethylene film is 10% by mass or more and 85% by mass or less. [3] The density of the high-density polyethylene layer 1 and the high-density polyethylene layer 2, measured in accordance with JIS K 7112:1999, is 940 kg / m or less, respectively. ３ More than 970kg / m ３ [4] The stretched polyethylene film according to [1] or [2], wherein the density of the medium-density polyethylene layer measured in accordance with JIS K 7112:1999 is 910 kg / m or less. ３ More than 940kg / m ３[5] The stretched polyethylene film according to any one of [1] to [4], wherein the full width at half maximum (FWHM) of a peak in the machine direction at a diffraction angle 2θ in the range of 0.2 to 0.4°, as determined by small-angle X-ray scattering (SAXS) measurement, is 0.20° or less. [6] The stretched polyethylene film according to any one of [1] to [5], wherein the haze per sheet of the stretched polyethylene film, as measured in accordance with JIS K 7136:2000, is 8.0% or less. [7] The stretched polyethylene film according to any one of [1] to [6], wherein the haze per four sheets of the stretched polyethylene film, as measured in accordance with JIS K 7136:2000, is 33.0% or less. [8] The tensile modulus T of the stretched polyethylene film in the MD direction, measured using a tensile tester in accordance with JIS K7127:1999 under conditions of a measurement temperature of 23±2°C, 50±5% RH, and a tensile speed of 5 mm / min. １ and the tensile modulus in the TD direction T ２The stretched polyethylene film according to any one of [1] to [7], wherein the total value of (a) and (b) is 1000 MPa or more and 3300 MPa or less. [9] The stretched polyethylene film according to any one of [1] to [8], wherein the stretched polyethylene film has a heat shrinkage rate in the MD direction of 3.0% or less when heat-treated at 100°C for 15 minutes in accordance with JIS C2151:2019.

[10] The stretched polyethylene film according to any one of [1] to [9], wherein the stretched polyethylene film has a heat shrinkage rate in the MD direction of 15.0% or less when heat-treated at 120°C for 15 minutes in accordance with JIS C2151:2019.

[11] The stretched polyethylene film according to any one of [1] to

[10] , wherein at least one surface of the stretched polyethylene film is a corona-treated surface.

[12] The stretched polyethylene film according to

[11] , wherein the static friction coefficient (tan θ) of the surface of the corona-treated side of the stretched polyethylene film is 0.40 or more and 0.75 or less, as measured using a slip tester by Method 1 (inclined method) described below. (Method 1) Two sheets of the stretched polyethylene film cut to a size of 50 mm x 75 mm (hereinafter referred to as stretched polyethylene films 1 and 2) are prepared, and one of the sheets, stretched polyethylene film 1, is fixed to an inclined plate with the corona-treated side facing up. Next, a friction body with a brass bottom (41 mm x 26 mm) is fixed to the center of the surface of the other stretched polyethylene film, stretched polyethylene film 2, opposite the corona-treated side, and a weight is attached on top of the friction body so that the mass applied by the friction body to stretched polyethylene film 2 is 150 g. Next, the corona-treated sides of the two stretched polyethylene films 1 and 2 are overlapped together. Next, the inclined plate is tilted at a speed of 1° / sec, and the value of tan θ is calculated from the angle θ when the upper stretched polyethylene film 2 starts to slide.

[13] Using a Gelbo flex tester, the number of pinholes generated in the stretched polyethylene film is measured at a bending angle of 440°, a bending speed of 40 times / min, and an atmosphere of −30° C., and the number of pinholes generated in the stretched polyethylene film is 6000 / m. ２

[14] The stretched polyethylene film according to any one of [1] to

[13] , wherein the stretched polyethylene film has a tear strength in the MD direction of 50 mN or more and 1,500 mN or less, as measured using a light-load tear tester under the following conditions: test piece size: MD direction: 63.5 mm, TD direction: 50.0 mm, pendulum weight: 96.09 g, tear length: 12.7 mm, and pendulum lift angle: 90°.

[15] The stretched polyethylene film according to any one of [1] to

[14] , wherein the stretched polyethylene film has a TD tear strength of 200 mN or more and 800 mN or less, as measured using a light-load tear tester under the following conditions: test piece size: MD: 50.0 mm, TD: 63.5 mm, pendulum weight mass: 96.09 g, tear length: 12.7 mm, and pendulum lift angle: 90°.

[16] The stretched polyethylene film according to any one of [1] to

[15] , wherein at least one surface of the stretched polyethylene film is non-corona-treated, and wherein the stretched polyethylene film has a heat-fusion strength of 10.0 N / 15 mm or less when the non-corona-treated surfaces of the stretched polyethylene film are bonded together and heat-sealed at 140°C.

[17] The stretched polyethylene film according to any one of [1] to

[16] , wherein the stretched polyethylene film has a lamination strength of 0.92 N / 15 mm or more, as measured by Method 2 below. (Method 2) A test piece measuring 297 cm x 210 cm was cut out from the stretched polyethylene film, and the corona-treated side of the test piece was bonded to the corona-treated side of a 50 μm-thick cast LLDPE film, one side of which had been corona-treated, using an ester-based adhesive, and aged at 40°C for 3 days to obtain a sample. The sample was then cut into a width of 15 mm and peeled in the MD direction using a tensile tester in accordance with JIS Z 0238:1998 at a peel angle of 90°, a chuck distance of 100 mm, and a crosshead speed of 300 mm / min, to determine the peel strength, which was used as the laminate strength.

[18] The stretched polyethylene film according to any of [1] to

[17] , wherein the total thickness of the stretched polyethylene film is 10 μm or more and 100 μm or less.

[19] The stretched polyethylene film according to any one of [1] to

[18] , which is a food packaging film.

[20] A packaging material using the stretched polyethylene film according to any one of [1] to

[19] .

[21] A food package comprising the packaging material according to

[20] and food placed in the packaging material.

[0013] According to the present invention, it is possible to provide a stretched polyethylene film having improved lamination strength.

[0014] FIG. 1 is a cross-sectional view schematically illustrating an example of the structure of a stretched polyethylene film according to an embodiment of the present invention.

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In this specification, unless otherwise specified, "A to B" indicating a range of values ​​represents A or more and B or less.

[0016] <Stretched polyethylene film> The stretched polyethylene film (100) according to this embodiment comprises a high-density polyethylene layer 1 (101), a medium-density polyethylene layer (102), and a high-density polyethylene layer 2 (103) in this order, and the stretched polyethylene film has a crystal thickness in the MD direction of 16 nm or less as determined by small-angle X-ray scattering (SAXS) measurement.

[0017] High-density polyethylene is sometimes blended into packaging films to improve heat resistance and rigidity. However, when a film containing a high content of high-density polyethylene is stretched in one direction, oriented crystallization is promoted, and molecular chains are strongly oriented in the stretching direction. This can make the film more susceptible to tearing when stress is applied in the stretching direction, resulting in a significant decrease in laminate strength in the stretching direction. In contrast, the stretched polyethylene film according to this embodiment can improve laminate strength. The reason for this effect is presumably that the three-layer structure in which the medium-density polyethylene layer is sandwiched between high-density polyethylene layer 1 and high-density polyethylene layer 2 can suppress oriented crystallization in the surface layer of the stretched polyethylene film, i.e., can prevent the polyethylene molecular chains from being aligned excessively in one direction.

[0018] In the stretched polyethylene film according to this embodiment, the crystal thickness in the MD direction, as determined by small-angle X-ray scattering (SAXS) measurement, is 16 nm or less, preferably 15 nm or less, from the viewpoint of improving laminate strength, and is preferably 3 nm or more, more preferably 5 nm or more, even more preferably 7 nm or more, even more preferably 10 nm or more, and even more preferably 13 nm or more, from the viewpoint of further improving heat resistance and rigidity. The crystal thickness of the stretched polyethylene film can be adjusted, for example, by adjusting the types and contents of the high-density polyethylene layer 1, the high-density polyethylene layer 2, and the medium-density polyethylene layer contained in the stretched polyethylene film, the thickness of the stretched polyethylene film, the stretching temperature, the stretching ratio, etc.

[0019] The materials constituting the stretched polyethylene film will be described below.

[0020] In the stretched polyethylene film according to the present embodiment, the total amount of the high-density polyethylene layer 1 and the high-density polyethylene layer 2, relative to the entire stretched polyethylene film, is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, even more preferably 25% by mass or more, even more preferably 28% by mass or more, even more preferably 30% by mass or more, and even more preferably 35% by mass or more, from the viewpoint of further improving the transparency, rigidity, heat resistance, and slip property; and from the viewpoint of incorporating an appropriate amount of the medium-density polyethylene layer and further improving the laminate strength, the total amount of the high-density polyethylene layer 1 and the high-density polyethylene layer 2 is preferably 85% by mass or less, more preferably 83% by mass or less, even more preferably 80% by mass or less, even more preferably 75% by mass or less, even more preferably 65% ​​by mass or less, even more preferably 55% by mass or less, and even more preferably 45% by mass or less. Furthermore, from the viewpoint of further improving the balance between transparency, rigidity, heat resistance, heat fusion strength, and laminate strength, the total amount of high-density polyethylene layer 1 and high-density polyethylene layer 2 is more preferably 38% by mass or more, and even more preferably 65% ​​by mass or less, and even more preferably 55% by mass or less, of the entire stretched polyethylene film.

[0021] In the stretched polyethylene film of the present embodiment, the high-density polyethylene layer 1 and the high-density polyethylene layer 2 may be formed using the same material or different materials. However, when different materials are used, it is preferable that the high-density polyethylene layer 1 and the high-density polyethylene layer 2 satisfy the following configuration.

[0022] From the viewpoint of further improving transparency, rigidity, heat resistance, and slip property, the content of high-density polyethylene in high-density polyethylene layer 1 and high-density polyethylene layer 2 is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and still more preferably 98% by mass or more, based on the total content of high-density polyethylene layer 1 and high-density polyethylene layer 2. There is no upper limit to the content of high-density polyethylene in high-density polyethylene layer 1 and high-density polyethylene layer 2, but it is, for example, 100% by mass or less.

[0023] The densities of the high-density polyethylene layer 1 and the high-density polyethylene layer 2, as measured in accordance with JIS K 7112:1999, are preferably 940 kg / m, respectively, from the viewpoint of achieving a better balance of various properties such as heat resistance, transparency, mechanical properties, and rigidity. ３ More preferably, 943 kg / m ３ More preferably, 945 kg / m ３ From the viewpoint of achieving a better balance between lamination strength and film formability, it is preferably 970 kg / m ３ or less, more preferably 968 kg / m ３ More preferably, 965 kg / m or less ３ More preferably, 960 kg / m or less ３ More preferably, 955 kg / m or less ３ More preferably, 950 kg / m or less ３ The following is the result.

[0024] The density of the high-density polyethylene constituting the high-density polyethylene layer 1 and the high-density polyethylene layer 2, as measured in accordance with JIS K 7112:1999, is preferably 940 kg / m, from the viewpoint of achieving a better balance of various properties such as heat resistance, transparency, mechanical properties, and rigidity. ３ More preferably, 943 kg / m ３ More preferably, 945 kg / m ３ From the viewpoint of achieving a better balance between lamination strength and film formability, it is preferably 970 kg / m ３ or less, more preferably 968 kg / m ３ More preferably, 965 kg / m or less ３ More preferably, 960 kg / m or less ３ More preferably, 955 kg / m or less ３ More preferably, 950 kg / m or less ３ The following is the result.

[0025] The melt flow rate (MFR) of the high-density polyethylene layer 1 and the high-density polyethylene layer 2, measured in accordance with ASTM D1238 under conditions of 190°C and a load of 2.16 kg, is preferably 0.01 g / 10 min or more, more preferably 0.1 g / 10 min or more, even more preferably 0.5 g / 10 min or more, and still more preferably 1.0 g / 10 min or more, from the viewpoint of further improving fluidity and moldability; and is preferably 20 g / 10 min or less, more preferably 10 g / 10 min or less, even more preferably 5 g / 10 min or less, even more preferably 3 g / 10 min or less, even more preferably 2 g / 10 min or less, and still more preferably 1.5 g / 10 min or less, from the viewpoint of improving the stiffness of the stretched polyethylene film while maintaining the tearability of the stretched polyethylene film. The melting points of the high-density polyethylene layer 1 and the high-density polyethylene layer 2, as measured by a differential scanning calorimeter (DSC), are each preferably 120°C or higher, more preferably 125°C or higher, and preferably 135°C or lower, from the viewpoint of further improving the balance of thermal dimensional stability, heat resistance, mechanical properties, rigidity, bag-formability, flowability, moldability, and the like.

[0026] The melt flow rate (MFR) of the high-density polyethylene constituting the high-density polyethylene layer 1 and the high-density polyethylene layer 2, measured in accordance with ASTM D1238 under conditions of 190°C and a load of 2.16 kg, is preferably 0.01 g / 10 min or more, more preferably 0.1 g / 10 min or more, even more preferably 0.5 g / 10 min or more, and still more preferably 1.0 g / 10 min or more, from the viewpoint of further improving fluidity and moldability; and from the viewpoint of improving the stiffness of the stretched polyethylene film while maintaining the tearability of the stretched polyethylene film, is preferably 20 g / 10 min or less, more preferably 10 g / 10 min or less, even more preferably 5 g / 10 min or less, even more preferably 3 g / 10 min or less, even more preferably 2 g / 10 min or less, and still more preferably 1.5 g / 10 min or less. The melting points of the high-density polyethylenes constituting the high-density polyethylene layer 1 and the high-density polyethylene layer 2, as measured by a differential scanning calorimeter (DSC), are preferably 120°C or higher, more preferably 125°C or higher, and preferably 135°C or lower, from the viewpoint of further improving the balance of thermal dimensional stability, heat resistance, mechanical properties, rigidity, bag-formability, flowability, moldability, and the like.

[0027] When two or more types of polyethylene are used as the polyethylene constituting the high-density polyethylene layer, the density, MFR, and melting point of the high-density polyethylene layer can be measured values ​​for a mixture obtained by melt-blending two or more types of polyethylene by a known method. The melting point of the high-density polyethylene layer can be the peak temperature of the maximum melting peak.

[0028] From the viewpoint of suppressing oriented crystallization of the surface layer of the stretched polyethylene film and further improving the laminate strength, the content of medium-density polyethylene in the medium-density polyethylene layer is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and still more preferably 98% by mass or more, based on the entire medium-density polyethylene layer. There is no upper limit to the content of medium-density polyethylene in the medium-density polyethylene layer, but it is, for example, 100% by mass or less.

[0029] The density of the medium-density polyethylene layer, measured in accordance with JIS K 7112:1999, is preferably 910 kg / m from the viewpoint of improving mechanical properties, rigidity, and flexibility. ３ More preferably, 915 kg / m ３ More preferably, 920 kg / m ３ More preferably, 925 kg / m ３ From the viewpoint of achieving a better balance between laminate strength and flexibility, it is preferably 940 kg / m ３ less than 933 kg / m ３ More preferably, 930 kg / m or less ３ The following is the result.

[0030] The density of the medium-density polyethylene constituting the medium-density polyethylene layer, measured in accordance with JIS K 7112:1999, is preferably 910 kg / m from the viewpoint of improving mechanical properties, rigidity, and flexibility. ３ More preferably, 915 kg / m ３ More preferably, 920 kg / m ３ More preferably, 925 kg / m ３ From the viewpoint of achieving a better balance between laminate strength and flexibility, it is preferably 940 kg / m ３ less than 933 kg / m ３ More preferably, 930 kg / m or less ３ The following is the result.

[0031] The melt flow rate (MFR) of the medium-density polyethylene layer, measured in accordance with ASTM D1238 under conditions of 190°C and a load of 2.16 kg, is preferably 0.01 g / 10 min or more, more preferably 0.1 g / 10 min or more, even more preferably 0.5 g / 10 min or more, and still more preferably 1.0 g / 10 min or more, from the viewpoint of further improving processability; and from the viewpoint of improving the stiffness of the stretched polyethylene film while maintaining the tearability of the stretched polyethylene film, is preferably 20 g / 10 min or less, more preferably 10 g / 10 min or less, even more preferably 5 g / 10 min or less, even more preferably 3 g / 10 min or less, and still more preferably 2 g / 10 min or less. The melting point of the medium-density polyethylene layer, as measured by a differential scanning calorimeter (DSC), is preferably 120°C or higher, more preferably 125°C or higher, from the viewpoint of improving heat resistance and stiffness while maintaining processability and adhesiveness, and is preferably 135°C or lower, more preferably 130°C or lower, from the viewpoint of improving adhesiveness while maintaining heat resistance.

[0032] The melt flow rate (MFR) of the medium-density polyethylene constituting the medium-density polyethylene layer, measured in accordance with ASTM D1238 under conditions of 190°C and a load of 2.16 kg, is preferably 0.01 g / 10 min or more, more preferably 0.1 g / 10 min or more, even more preferably 0.5 g / 10 min or more, and still more preferably 1.0 g / 10 min or more, from the viewpoint of further improving processability; and from the viewpoint of improving the stiffness of the stretched polyethylene film while maintaining the tearability of the stretched polyethylene film, is preferably 20 g / 10 min or less, more preferably 10 g / 10 min or less, even more preferably 5 g / 10 min or less, even more preferably 3 g / 10 min or less, and still more preferably 2 g / 10 min or less. The melting point of the medium-density polyethylene constituting the medium-density polyethylene layer, as measured by a differential scanning calorimeter (DSC), is preferably 120°C or higher, more preferably 125°C or higher, from the viewpoint of improving heat resistance and stiffness while maintaining processability and adhesiveness, and is preferably 135°C or lower, more preferably 130°C or lower, from the viewpoint of improving adhesiveness while maintaining heat resistance.

[0033] When two or more types of polyethylene are used as the polyethylene constituting the medium-density polyethylene layer, the density, MFR, and melting point of the medium-density polyethylene layer can be measured values ​​for a mixture obtained by melt-blending two or more types of polyethylene by a known method. The melting point of the medium-density polyethylene layer can be the peak temperature of the maximum melting peak.

[0034] At least one surface of the stretched polyethylene film of this embodiment is preferably a corona-treated surface (a surface modified by corona discharge irradiation). By making at least one surface of the stretched polyethylene film of this embodiment a corona-treated surface, the printing properties, coating properties, and lamination properties with other films of the stretched polyethylene film of this embodiment can be further improved.

[0035] The high-density polyethylene layer 1, the high-density polyethylene layer 2, and the medium-density polyethylene layer may contain various additives within the scope of the present invention, such as heat stabilizers, weather stabilizers, antioxidants, ultraviolet absorbers, lubricants, slip agents, nucleating agents, antiblocking agents, antistatic agents, antifogging agents, pigments, dyes, inorganic or organic fillers, etc.

[0036] The thicknesses of high-density polyethylene layer 1 and high-density polyethylene layer 2 are each preferably 1 μm or more, more preferably 2 μm or more, from the viewpoint of achieving a more appropriate range of orientation crystallinity, and preferably 17 μm or less, more preferably 15 μm or less, even more preferably 13 μm or less, even more preferably 10 μm or less, even more preferably 9 μm or less, and even more preferably 7 μm or less. The thicknesses of high-density polyethylene layer 1 and high-density polyethylene layer 2 may be the same or different, but are preferably the same from the viewpoint of achieving a uniform amount of strain on the front and back sides of the stretched polyethylene film on the high-density polyethylene layer 1 side and the high-density polyethylene layer 2 side. The thickness of the medium-density polyethylene layer is preferably 2 μm or more, more preferably 3 μm or more, even more preferably 5 μm or more, even more preferably 10 μm or more, from the viewpoint of achieving a more appropriate range of orientation crystallinity, and is preferably 30 μm or less, more preferably 25 μm or less, even more preferably 20 μm or less, and even more preferably 15 μm or less. The thickness of the entire stretched polyethylene film is preferably 10 μm or more, more preferably 13 μm or more, and even more preferably 15 μm or more, from the viewpoint of maintaining the tearability and further improving the mechanical strength of the stretched polyethylene film, and is preferably 100 μm or less, more preferably 70 μm or less, even more preferably 50 μm or less, even more preferably 40 μm or less, and even more preferably 30 μm or less, from the viewpoint of further improving the tearability, handleability, formability, bag-making suitability, lightness, etc. of the stretched polyethylene film.

[0037] Next, the physical properties of the stretched polyethylene film will be described.

[0038] In the stretched polyethylene film of this embodiment, the full width at half maximum (FWHM) of the peak at a diffraction angle 2θ in the MD direction in the range of 0.2 to 0.4°, as determined by small-angle X-ray scattering (SAXS) measurement, is preferably 0.20° or less, more preferably 0.19° or less, from the viewpoint of suppressing oriented crystallization in the surface layer of the stretched polyethylene film and further improving the balance of tear strength, lamination strength, transparency, thermal dimensional stability, and slip properties. The full width at half maximum (FWHM) of the peak at a diffraction angle 2θ in the MD direction in the range of 0.2 to 0.4° of the stretched polyethylene film is preferably 0.05° or more, more preferably 0.10° or more, even more preferably 0.13° or more, even more preferably 0.14° or more, and even more preferably 0.15° or more, from the viewpoint of further improving film formability, bag formability, lamination strength, and thermal dimensional stability. The full width at half maximum (FWHM) of the peak in the MD diffraction angle 2θ range of 0.2 to 0.4°, which is determined from such small-angle X-ray scattering (SAXS) measurement, can be adjusted by, for example, adjusting the types and contents of the high-density polyethylene layer 1, the high-density polyethylene layer 2, and the medium-density polyethylene layer contained in the stretched polyethylene film, the thickness of the stretched polyethylene film, the stretch ratio, etc.

[0039] In the stretched polyethylene film of this embodiment, the haze per sheet of stretched polyethylene film measured in accordance with JIS K 7136: 2000 is, from the viewpoint of further improving the transparency of the stretched polyethylene film, preferably 8.0% or less, more preferably 7.5% or less, even more preferably 7.0% or less, even more preferably 6.5% or less, even more preferably 6.0% or less, even more preferably 5.5% or less, even more preferably 5.0% or less, and even more preferably 4.5% or less. There is no lower limit for the haze per sheet of stretched polyethylene film, but it may be, for example, 0.1% or more, or alternatively 1.0% or more, or 3.0% or more.

[0040] In the stretched polyethylene film of this embodiment, the haze per four sheets of stretched polyethylene film measured in accordance with JIS K 7136:2000 is preferably 33.0% or less, more preferably 30.0% or less, even more preferably 28.0% or less, even more preferably 25.0% or less, even more preferably 23.0% or less, even more preferably 20.0% or less, and even more preferably 18.0% or less, from the viewpoint of further improving the transparency of the stretched polyethylene film. There is no lower limit for the haze per four sheets of stretched polyethylene film, but it may be, for example, 1.0% or more, 5.0% or more, 10.0% or more, or 15.0% or more. Such a haze can be adjusted, for example, by adjusting the types and contents of the high-density polyethylene layer 1, the high-density polyethylene layer 2, and the medium-density polyethylene layer contained in the stretched polyethylene film, the thickness of the stretched polyethylene film, the stretch ratio, etc.

[0041] In the stretched polyethylene film of this embodiment, the tensile modulus T of the stretched polyethylene film in the MD direction is measured using a tensile tester in accordance with JIS K7127:1999 under the conditions of a measurement temperature of 23±2°C, 50±5% RH, and a tension speed of 5 mm / min. １ and the tensile modulus in the TD direction T ２The total value of (a) and (b) is preferably 1000 MPa or more, more preferably 1300 MPa or more, even more preferably 1500 MPa or more, even more preferably 1550 MPa or more, even more preferably 1600 MPa or more, and even more preferably 1800 MPa or more, from the viewpoints of achieving a better balance between the transparency and thermal dimensional stability of the stretched polyethylene film and further improving the stiffness of the stretched polyethylene film; and from the viewpoints of making troubles such as breakage less likely to occur during molding of the stretched polyethylene film, facilitating continuous stretch molding of the film, and further improving industrial continuous productivity, the total value is preferably 3300 MPa or less, more preferably 3100 MPa or less, even more preferably 3000 MPa or less, and even more preferably 2950 MPa or less. Furthermore, from the viewpoints of further improving the balance between the transparency, rigidity, heat resistance, and the heat-fusion strength and lamination strength, the tensile modulus T １ and the tensile modulus in the TD direction T ２ The total value of (a) and (b) is more preferably 1830 MPa or more, even more preferably 1840 MPa or more, and even more preferably 2800 MPa or less, even more preferably 2600 MPa or less, even more preferably 2400 MPa or less, and even more preferably 2200 MPa or less. Such a tensile modulus is a substitute value for quantitatively measuring the stiffness of a film, and can be adjusted, for example, by adjusting the types and contents of the high-density polyethylene layer 1, the high-density polyethylene layer 2, and the medium-density polyethylene layer contained in the stretched polyethylene film, the thickness of the stretched polyethylene film, the stretch ratio, etc.

[0042] Tensile modulus T of stretched polyethylene film in the MD direction １is preferably 500 MPa or more, more preferably 600 MPa or more, even more preferably 630 MPa or more, even more preferably 650 MPa or more, even more preferably 670 MPa or more, and even more preferably 700 MPa or more, from the viewpoint of further improving the balance of the thermal dimensional stability, bag formability, handleability, and packaging suitability of the stretched polyethylene film, and is preferably 1500 MPa or less, more preferably 1480 MPa or less, even more preferably 1450 MPa or less, even more preferably 1430 MPa or less, and even more preferably 1400 MPa or less, from the viewpoint of further improving the balance of the thermal dimensional stability, bag formability, and packaging suitability of the stretched polyethylene film. ２ is preferably 600 MPa or more, more preferably 700 MPa or more, even more preferably 730 MPa or more, even more preferably 750 MPa or more, even more preferably 770 MPa or more, even more preferably 800 MPa or more, and even more preferably 830 MPa or more, from the viewpoint of further improving the balance of the formability, mechanical properties, transparency, bag-formability, handleability, and packaging suitability of the stretched polyethylene film; and is preferably 1800 MPa or less, more preferably 1750 MPa or less, even more preferably 1700 MPa or less, even more preferably 1650 MPa or less, even more preferably 1600 MPa or less, and even more preferably 1550 MPa or less, from the viewpoint of further improving the balance of the bag-formability and packaging suitability of the stretched polyethylene film.

[0043] In the stretched polyethylene film of this embodiment, the heat shrinkage rate in the MD direction of the stretched polyethylene film when heated at 100°C for 15 minutes is preferably 3.0% or less, more preferably 2.8% or less, and even more preferably 2.5% or less, from the viewpoint of further improving thermal dimensional stability and bag formability, and may be 0.1% or more, 0.5% or more, 1.0% or more, 1.5% or more, 1.6% or more, or 1.7% or more. Furthermore, from the viewpoint of further improving the balance between transparency, rigidity, heat resistance, and heat-fusion strength and lamination strength, the heat shrinkage rate in the MD direction of the stretched polyethylene film when heated at 100°C for 15 minutes is more preferably 2.4% or less, and may be 2.0% or more. Furthermore, the heat shrinkage rate of the stretched polyethylene film can be measured in accordance with JIS C2151:2019.

[0044] In the stretched polyethylene film of this embodiment, the heat shrinkage rate in the MD direction of the stretched polyethylene film when heated at 120°C for 15 minutes is preferably 15.0% or less, more preferably 13.0% or less, and even more preferably 11.0% or less, from the viewpoint of further improving thermal dimensional stability and bag formability, and may be 1.0% or more, 3.0% or more, 5.0% or more, or even 8.0% or more. Furthermore, from the viewpoint of further improving the balance between transparency, rigidity, heat resistance, and heat-fusion strength and lamination strength, the heat shrinkage rate in the MD direction of the stretched polyethylene film when heated at 120°C for 15 minutes is more preferably 10.5% or less, even more preferably 9.0% or more, and even more preferably 9.5% or more.

[0045] When either the high-density polyethylene layer 1 or the high-density polyethylene layer 2 of the stretched polyethylene film is corona-treated, the static friction coefficient (tan θ) of the stretched polyethylene film surface may be measured between treated surfaces, between untreated surfaces, or between treated and untreated surfaces. In particular, from the viewpoint of facilitating the formation of a thin and uniform layer, it is preferable that the static friction coefficient (tan θ) of the surface on the corona-treated side be within the above-mentioned range. The static friction coefficient (tan θ) of the corona-treated side of the stretched polyethylene film is preferably 0.40 or more, more preferably 0.42 or more, even more preferably 0.45 or more, and is preferably 0.75 or less, more preferably 0.70 or less, even more preferably 0.65 or less, even more preferably 0.60 or less, even more preferably 0.56 or less, and even more preferably 0.54 or less. Furthermore, from the viewpoint of further improving the handleability of the stretched polyethylene film, it is preferable that the static friction coefficient (tan θ) of the surface on the corona-untreated side be within the above-mentioned range. The static friction coefficient (tan θ) of the surface of the non-corona treated side of the stretched polyethylene film is preferably 0.40 or more, more preferably 0.42 or more, even more preferably 0.45 or more, and is preferably 0.75 or less, more preferably 0.70 or less, even more preferably 0.65 or less, even more preferably 0.60 or less, even more preferably 0.56 or less, even more preferably 0.54 or less.

[0046] The static friction coefficient (tan θ) of the corona-treated surface of a stretched polyethylene film is measured using the following method. Two pieces of stretched polyethylene film (hereinafter referred to as stretched polyethylene films 1 and 2) cut to a size of 50 mm x 75 mm are prepared. One of the stretched polyethylene films, stretched polyethylene film 1, is fixed to an inclined plate with the corona-treated surface facing up. Next, a friction body with a brass bottom (41 mm x 26 mm) is fixed to the center of the surface of the other stretched polyethylene film, stretched polyethylene film 2, opposite the corona-treated surface. A weight is attached to the friction body so that the mass applied to the stretched polyethylene film 2 from the friction body is 150 g. Next, the corona-treated surfaces of the two stretched polyethylene films 1 and 2 are placed face-to-face. The inclined plate is then tilted at a rate of 1° / sec, and the value of tan θ is determined from the angle θ at which the upper stretched polyethylene film 2 begins to slide. When measuring the static friction coefficient (tan θ) of the corona-untreated surface, the static friction coefficient (tan θ) of the corona-treated surface is measured in the same manner as in the above method for measuring the static friction coefficient (tan θ) of the corona-treated surface, except that the corona-treated surface is replaced with the corona-untreated surface.

[0047] In the stretched polyethylene film of this embodiment, the number of pinholes generated in the stretched polyethylene film is preferably 6000 / m, as measured by a bending test using a Gelbo flex tester at a bending angle of 440 degrees, a bending speed of 40 times / min, and an atmosphere of −30° C., 3000 times. ２ Less than or equal to 5000 particles / m ２ More preferably, 4000 pieces / m or less ２ More preferably, 3000 pieces / m or less ２ More preferably, 2800 pieces / m or less ２ More preferably, 2500 pieces / m or less ２ There is no lower limit to the number of pinholes that occur in the stretched polyethylene film, but for example, it is set to 100 pinholes / m ２ or more, and ２ It may be 500 pieces / m or more. ２ It may be 1000 pieces / m or more. ２or more. The number of pinholes generated represents an index of the flex resistance of the stretched polyethylene film of this embodiment, and the fewer the number of pinholes generated, the better the flex resistance. When the number of pinholes generated in the stretched polyethylene film is within the above range, the generation of pinholes due to flexing during low-temperature filling or low-temperature transportation can be further suppressed. The Gelbo Flex Tester that can be used is, for example, one manufactured by Tester Sangyo Co., Ltd.

[0048] The tear strength in the MD direction of the stretched polyethylene film, measured using a light-load tear tester under the conditions of test piece size: MD direction: 63.5 mm, TD direction: 50.0 mm, pendulum weight mass: 96.09 g, tear length: 12.7 mm, and pendulum lift angle: 90°, is preferably 50 mN or more, more preferably 55 mN or more, even more preferably 60 mN or more, still more preferably 65 mN or more, and still more preferably 70 mN or more, from the viewpoint of improving film-forming properties and bag-making processability; and is preferably 1500 mN or less, more preferably 1200 mN or less, even more preferably 1000 mN or less, still more preferably 800 mN or less, still more preferably 600 mN or less, still more preferably 400 mN or less, and still more preferably 200 mN or less, from the viewpoint of improving tearability while maintaining the heat-sealability and stiffness of the stretched polyethylene film.

[0049] The tear strength in the TD direction of the stretched polyethylene film, measured using a light-load tear tester under the conditions of test piece size: MD direction: 50.0 mm, TD direction: 63.5 mm, pendulum weight mass: 96.09 g, tear length: 12.7 mm, and pendulum lift angle: 90°, is preferably 200 mN or more, more preferably 250 mN or more, even more preferably 300 mN or more, still more preferably 310 mN or more, still more preferably 320 mN or more, and still more preferably 330 mN or more, from the viewpoint of improving film-forming properties and bag-making processability; and is preferably 800 mN or less, more preferably 750 mN or less, even more preferably 700 mN or less, preferably 650 mN or less, even more preferably 630 mN or less, still more preferably 600 mN or less, and still more preferably 500 mN or less, from the viewpoint of improving tearability while maintaining the heat-sealability and stiffness of the stretched polyethylene film. The light-load tear tester may be, for example, a Model D manufactured by Toyo Seiki Seisakusho, Ltd. In order to achieve such tear strength, the density, thickness, etc. of each of the high-density polyethylene layer 1, the high-density polyethylene layer 2, and the medium-density polyethylene layer contained in the stretched polyethylene film may be appropriately adjusted.

[0050] When at least one of the stretched polyethylene films is non-corona-treated, the non-corona-treated surfaces of the stretched polyethylene films are bonded together and heat-sealed at 140°C. From the viewpoint of improving heat resistance, the heat-seal strength is preferably 10.0 N / 15 mm or less, more preferably 9.0 N / 15 mm or less, even more preferably 7.0 N / 15 mm or less, even more preferably 6.8 N / 15 mm or less, even more preferably 6.5 N / 15 mm or less, even more preferably 6.3 N / 15 mm or less, and even more preferably 6.0 N / 15 mm or less. There is no lower limit to the heat-seal strength, but it is, for example, 0.5 N / 15 mm or more. The heat-seal strength is measured as follows. A laminated film is obtained by heat-sealing the non-corona-treated surfaces of two stretched polyethylene films cut to a width of 15 mm together under conditions of 140°C, a pressure of 2.0 kgf, and a sealing time of 1.0 second. Next, using the laminated film cut to a width of 15 mm, two stretched polyethylene films are peeled off under the conditions of 90° peeling, a peeling speed of 300 mm / min, and pulling in the MD direction, and the peel strength at this time is taken as the heat fusion strength (N / 15 mm).

[0051] The laminate strength of the stretched polyethylene film, measured in accordance with JIS Z 0238:1998, is preferably 0.92 N / 15 mm or more, more preferably 0.93 N / 15 mm or more, even more preferably 0.94 N / 15 mm or more, even more preferably 0.95 N / 15 mm or more, even more preferably 1.00 N / 15 mm or more, and still more preferably 1.05 N / 15 mm or more; from the viewpoint of achieving a better balance between adhesiveness and easy-opening properties, it is preferably 1.20 N / 15 mm or less, more preferably 1.18 N / 15 mm or less, preferably 1.15 N / 15 mm or less, preferably 1.13 N / 15 mm or less, and preferably 1.10 N / 15 mm or less. The laminate strength is measured as follows: A 297 cm x 210 cm test piece is cut out from a stretched polyethylene film, and the corona-treated side of the test piece is bonded to the corona-treated side of a 50 μm-thick cast LLDPE film, one side of which has been corona-treated, with an ester adhesive. The resulting mixture is aged at 40°C for 3 days to obtain a sample. The sample is then cut to a width of 15 mm and peeled in the machine direction using a tensile tester in accordance with JIS Z 0238:1998 at a peel angle of 90°, a chuck distance of 100 mm, and a crosshead speed of 300 mm / min. The peel strength is then determined and used as the laminate strength.

[0052] <Identification of High-Density Polyethylene Layer and Medium-Density Polyethylene Layer> For example, it can be determined that the stretched polyethylene film of this embodiment is composed of high-density polyethylene layer 1, medium-density polyethylene layer, and high-density polyethylene layer 2 by cutting a cross section of the stretched polyethylene film and measuring the melting point of each layer. The melting points of high-density polyethylene layer 1 and high-density polyethylene layer 2 are, for example, in the range of 130°C or higher and 140°C or lower, and the melting point of the medium-density polyethylene layer is, for example, in the range of 110°C or higher and 129°C or lower.

[0053] <Method of manufacturing stretched polyethylene film> The stretched polyethylene film of the present embodiment is stretched uniaxially or biaxially from the viewpoint of improving the orientation crystallinity and increasing the mechanical strength, and is preferably stretched uniaxially from the viewpoint of improving productivity. Furthermore, by stretching the film uniaxially in the MD direction, the tensile modulus T １ This can further improve the properties. The stretched polyethylene film can be obtained by, for example, co-extrusion molding a high-density polyethylene resin for forming the high-density polyethylene layer 1, a medium-density polyethylene resin for forming the medium-density polyethylene layer, and a high-density polyethylene resin for forming the high-density polyethylene layer 2 in this order into a film, and then stretching the resulting film using a known stretched film production method such as a uniaxial stretching method, a simultaneous biaxial stretching method, or a sequential biaxial stretching method. The molding apparatus and molding conditions are not particularly limited, and conventionally known molding apparatuses and molding conditions can be used. Examples of molding apparatus that can be used include a T-die extruder, a multilayer T-die extruder, an inflation molding machine, and a multilayer inflation molding machine. Conditions for the uniaxial stretching and biaxial stretching methods can be, for example, those used for producing known stretched polyethylene films. More specifically, in the case of a uniaxial stretching method, the longitudinal stretching temperature may be 100°C to 145°C, and the longitudinal stretching ratio may be 4.5 to 6 times. In the sequential biaxial stretching method, for example, the longitudinal stretching temperature may be 100°C to 145°C, the longitudinal stretching ratio may be in the range of 4.5 to 6 times, the transverse stretching temperature may be 110°C to 160°C, and the transverse stretching ratio may be in the range of 9 to 11 times.

[0054] <Uses of Stretched Polyethylene Film / Packaging Material / Food Package> The stretched polyethylene film of this embodiment can be suitably used, specifically, as a food packaging film. The stretched polyethylene film of this embodiment can also be suitably used as a packaging material. When used as a packaging material, the stretched polyethylene film of this embodiment can be used alone, or other layers can be laminated to form the packaging material. Examples of such other layers include a substrate layer, a coating layer, an adhesive layer, and a heat-sealing layer. From the perspective of ease of recycling, it is preferable that these layers be formed from a polyethylene-based resin. The packaging material of this embodiment can also be suitably used for food packaging. The food packaging is used, for example, for the purpose of packaging food, and specifically includes the packaging material of this embodiment and the food contained therein. Depending on the application, only a portion of the food packaging may be made of the packaging material of this embodiment, or substantially the entire food packaging may be made of the packaging material of this embodiment.

[0055] There are no particular limitations on the method for producing a food package from the stretched polyethylene film or packaging material, and any method known in the field of packaging materials / packages, such as heat sealing or fusing, can be used.

[0056] The stretched polyethylene film according to this embodiment is preferably used for food packaging that requires good laminate strength. The food packaging may be in the form of, for example, a two-sided bag or a standing pouch (pouch packaging). These forms are preferred because they can provide good laminate strength.

[0057] Furthermore, when a food package (such as a packaging bag) is constructed using the stretched polyethylene film or packaging material of this embodiment, it is preferable that the corona-treated surface is the inner surface and the non-corona-treated surface is the outer surface. Furthermore, as described above, when another layer is laminated on the stretched polyethylene film, it is preferable that the layer be laminated on the corona-treated surface. That is, when a laminate using the stretched polyethylene film of this embodiment is used for a food package (such as a packaging bag), it is preferable that the stretched polyethylene film of this embodiment be the outermost layer of the food package.

[0058] There are no limitations on the foods that can be packaged in the food packaging, but examples include baked goods, rice crackers, snacks, sprinkles, grain powders, and the like.

[0059] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various other configurations may be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of achieving the object of the present invention are included in the present invention.

[0060] <Raw Materials> The raw materials used in the Examples and Comparative Examples are listed below. The density was measured in accordance with JIS K 7112:1999. The MFR was measured in accordance with ASTM D1238 under conditions of 190°C and a load of 2.16 kg. The melting point was measured using a differential scanning calorimeter (DSC).

[0061] (High density polyethylene) High density polyethylene (HDPE1): Density: 949 kg / m ３ , MFR: 1.1 g / 10 min, melting point: 130°C (medium density polyethylene) Medium density polyethylene (MDPE1): Density: 928 kg / m ３ , MFR: 1.7 g / 10 min, melting point: 127°C

[0062] <Production of Stretched Polyethylene Films> High-density polyethylene (HDPE1), medium-density polyethylene (MDPE1), and high-density polyethylene (HDPE1) from the Examples and Comparative Examples, in the formulations shown in Table 1, were T-die extruded in this order to form a film, yielding a cast sheet, which was then uniaxially stretched. The high-density polyethylene layer 2 side of the uniaxially stretched cast sheet was then subjected to a corona treatment to produce the stretched polyethylene film of each Example. The extrusion and uniaxial stretching conditions are shown below. Multilayer extruder: 75 mmφ multilayer T-die extruder (L / D = 29, manufactured by Mitsubishi Heavy Industries, Ltd.) Extrusion set temperature: 230°C, Processing speed: 70 m / min, Longitudinal stretching temperature: 110 to 130°C, Longitudinal stretching ratio: 5x. The densities of the high-density polyethylene layer 1, high-density polyethylene layer 2, and medium-density polyethylene layer were measured for the stretched polyethylene films obtained in each Example. The following evaluations were also performed. The results are shown in Table 1.

[0063] <Small-angle X-ray scattering (SAXS) measurement of stretched polyethylene film> For the stretched polyethylene film of each example, the MD direction of the film was set to the top and bottom, the TD direction to the left and right, and the measurement film was set in the following device so that the angle between the X-ray source direction and the film surface was perpendicular. Small-angle X-ray scattering (SAXS) measurement was carried out using the following device and conditions. Device: Manufactured by RIGAKU Corporation, product name: Ultima IV (small-angle scattering attachment system) X-ray incidence direction: normal to the film X-ray wavelength: 0.15418 nm Optical unit specifications: 1. Optical system selection slit: for small-angle scattering 0.03 mm (=1 ｓｔ . Slit) 2. DS; Anti-scattering slit 1.00 mm (= 2 ｎｄ . Slit) 3. Soller slit on the entrance side: 5° flexible optical system used 4.1 ｓｔ .~2 ｎｄ Distance between slits: 70 mm 5.2 ｎｄ. Distance between sample and sample: 98 mm 6. Vacuum path length: 100 mm (installed on a dedicated stand in front of the receiving slit box) 7. RS, SS: Scattering slit 0.20 mm, receiving slit 0.10 mm 8. Camera length: 285 mm 9. Receiving side Soller slit: 5° using a flexible optical system 10. Monochromatization: None (monochromatization on the incident side using a multilayer mirror) 11. Detector: RIGAKU scintillation detector (HV: 762 V) (one-dimensional) X-ray irradiation conditions: A. Scan axis: 2 theta B. Measurement method: continuous C. Scan start angle: 0.1° D. Scan end angle: 1.0° E. Sampling width: 0.02° F. Scan speed: 0.5° / min G. Voltage and current: 40 kV-40 mA H. Number of sample stacks: To obtain sufficient scattering intensity, the samples were aligned and stacked to a distance of approximately 0.5 mm. The X-ray scattering pattern obtained under the above measurement conditions was subjected to air scattering correction for the detector to obtain the SAXS profile I(q). The diffraction angle θ of the scattering vector magnitude of the peak derived from the crystalline long period of the SAXS profile I(q) was calculated using equation (1), and this was then substituted into Bragg's equation (2) to calculate the crystalline long period (d). q = 4π sin θ / λ ... (1) θ: diffraction angle q: scattering vector magnitude λ: X-ray wavelength 2d sin θ = λ ... (2) d: crystalline long period θ: diffraction angle λ: X-ray wavelength Furthermore, referring to the structural analysis of crystalline polymer materials by scattering method in Nichias Technical Review (2014) No. 2, No. 365, the electron density correlation function γ(r) was calculated by Fourier transforming the SAXS profile I(q) using the following equation (3): γ(r) has a special property that can be used directly for structural characterization, and the amorphous thickness (da) of each stretched polyethylene film obtained as structural information was calculated. r represents distance (nm).

[0064]

[0065] The crystal thickness (dc) was calculated by subtracting the amorphous thickness (da) from the crystalline long period (d). The full width at half maximum (FWHM) of the peak in the MD direction diffraction angle 2θ range of 0.2 to 0.4° was calculated using the X-ray analysis software PDXL-2 (manufactured by RIGAKU Corporation). Specifically, air scattering was removed from the SAXS profile I(q) obtained above. Using the obtained values, the software separated the crystalline scattering and amorphous scattering, and the full width at half maximum (FWHM) of the peak in the MD direction diffraction angle 2θ range of 0.2 to 0.4° was calculated from the peak fitting results of the crystalline scattering according to the following (analysis conditions). (Analysis conditions) Fitted peak shape: Split pseudo-Voigt function Crystallite size distribution type: Lorentz model

[0066] <Haze> In accordance with JIS K7136:2000, the haze of a single stretched polyethylene film and a stack of four stretched polyethylene films of each example was measured using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.) Here, the four stretched polyethylene films were stacked with the orientations of the stretched polyethylene films in the MD and TD directions aligned, and the measurement was performed.

[0067] <Tensile modulus> A test piece of 15 mm x 15 cm was cut out from the stretched polyethylene film of each example. Then, the tensile modulus T of the test piece in the MD direction was measured using a tensile tester manufactured by Orientec Co., Ltd., in accordance with JIS K7127:1999 under the conditions of a measurement temperature of 23±2°C, 50±5% RH, and a pulling speed of 5 mm / min. １ and tensile modulus in the TD direction T ２ From the obtained values, T １ and T ２ The total value was calculated.

[0068] <Heat shrinkage in MD direction of stretched polyethylene film at 100°C> The heat shrinkage in MD direction of stretched polyethylene film at 100°C was measured in accordance with JIS C2151:2019. A 10 cm x 10 cm test piece was cut out from the stretched polyethylene film of each example. At this time, the test piece was heated in a hot air circulation type thermostatic chamber (manufactured by ADVANTEC, product name: DRM620DE) while hanging without applying force. Next, the above test piece was heat-treated at 100°C for 15 minutes. The length in the MD direction of the test piece after heat treatment was measured as the MD １００ [cm], and the heat shrinkage rate [%] in the MD direction is calculated as 100 × (10 - MD １００ The above measurement was carried out three times, and the average value of the obtained measurements was used as the heat shrinkage rate of the stretched polyethylene film at 100°C.

[0069] <Heat shrinkage in MD direction of stretched polyethylene film at 120°C> The heat shrinkage in MD direction of stretched polyethylene film at 120°C was measured in accordance with JIS C2151:2019. A 10 cm x 10 cm test piece was cut out from the stretched polyethylene film of each example. At this time, the test piece was heated by hanging it without applying force in a hot air circulation type thermostatic oven (manufactured by ADVANTEC, product name: DRM620DE). Next, the above test piece was heat-treated at 120°C for 15 minutes. The length in the MD direction of the test piece after heat treatment was measured as the MD １２０ [cm], and the heat shrinkage rate [%] in the MD direction is calculated as 100 × (10 - MD １２０ The above measurement was carried out three times, and the average value of the obtained measurements was used as the heat shrinkage rate of the stretched polyethylene film at 120°C.

[0070] <Static Friction Coefficient (Corona-Treated Surface / Corona-Treated Surface)> The static friction coefficient of the corona-treated surface was measured as follows. Two sheets of stretched polyethylene film of each example (hereinafter referred to as stretched polyethylene films 1 and 2) were cut to a size of 50 mm x 75 mm and prepared. One of the stretched polyethylene films, stretched polyethylene film 1, was fixed to an inclined plate with the corona-treated surface facing up. Next, a friction body with a brass bottom (41 mm x 26 mm) was fixed to the center of the surface of the other stretched polyethylene film, stretched polyethylene film 2, opposite the corona-treated surface. A weight was attached to the friction body so that the mass applied to the stretched polyethylene film 2 from the friction body was 150 g. Next, the corona-treated surfaces of the two stretched polyethylene films 1 and 2 were placed face-to-face. Next, the inclined plate was tilted at a rate of 1° / sec, and the value of tan θ was calculated from the angle θ when the upper stretched polyethylene film 2 began to slide.

[0071] <Tear Strength> A test piece measuring 63.5 mm in the MD direction and 50.0 mm in the TD direction was cut out from the stretched polyethylene film of each example. For each test piece, the tear strength (mN) in the MD direction was measured using a light-load tear tester (manufactured by Toyo Seiki Seisakusho, Ltd., Model-D) under the following conditions: pendulum weight: 96.09 g, tear length: 12.7 mm, pendulum lift angle: 90°. Furthermore, a test piece measuring 50.0 mm in the MD direction and 63.5 mm in the TD direction was cut out from the stretched polyethylene film of each example. The tear strength (mN) in the TD direction of each test piece was measured in the same manner as for the measurement of the tear strength (mN) in the MD direction.

[0072] <Flexibility> A test piece measuring 297 cm x 210 cm was cut out from the stretched polyethylene film of each example, and subjected to a flex test 3,000 times using a Gelbo Flex Tester (manufactured by Tester Sangyo Co., Ltd.) at a flex angle of 440° and a flex speed of 40 times / minute in an atmosphere of -30°C. After the flex test, a bag was made from the test piece, and the number of pinholes generated was measured using Ageless Seal Check Liquid (manufactured by Mitsubishi Gas Chemical Company).

[0073] <Heat-sealing strength> Two sheets of the stretched polyethylene film of each example, cut to a width of 15 mm, were heat-sealed together with their non-corona-treated surfaces at 140°C, a pressure of 2.0 kgf, and a sealing time of 1.0 second to obtain a laminated film. The two stretched polyethylene films were then peeled together under the conditions of a width of 15 mm, a 90° peel angle, a peel rate of 300 mm / min, and tension in the MD direction, and the peel strength at this time was recorded as the heat-sealing strength (N / 15 mm).

[0074] <Laminate Strength> A 297 cm x 210 cm test piece was cut from each stretched polyethylene film, and the corona-treated side of the test piece was bonded to the corona-treated side of a 50 μm thick cast LLDPE film (TUX FCS #50 manufactured by Mitsui Chemicals Tohcello) with one side corona-treated using an ester-based adhesive (Mitsui Chemicals Takelac A310 / Takenate A3 / ethyl acetate = 12 / 1 / 7). The resulting mixture was aged at 40 °C for 3 days to obtain a sample. The sample was cut to a width of 15 mm, and the peel strength was measured in accordance with JIS Z 0238:1998 using a tensile tester (Orientec Co., Ltd. Tensilon Universal Testing Machine RTC-1225) at a peel angle of 90°, a chuck distance of 100 mm, and a crosshead speed of 300 mm / min. This peel strength was used to measure the laminate strength.

[0075]

[0076] When the stretched polyethylene film of the example was used, a food packaging film with improved lamination strength was obtained. It was also found that when the stretched polyethylene film of the example was used, bending resistance was also improved.

[0077] This application claims priority based on Japanese Patent Application No. 2022-157673, filed September 30, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0078] 100 Stretched polyethylene film 101 High density polyethylene layer 1 102 Medium density polyethylene layer 103 High density polyethylene layer 2

Claims

1. A high density polyethylene layer 1; A medium density polyethylene layer; A high density polyethylene layer 2; A stretched polyethylene film comprising, in this order: The stretched polyethylene film has a crystal thickness in the MD direction of 16 nm or less, as determined by small angle X-ray scattering (SAXS) measurement.

2. 2. The stretched polyethylene film according to claim 1, wherein a total amount of the high density polyethylene layer 1 and the high density polyethylene layer 2 in the stretched polyethylene film is 10 mass% or more and 85 mass% or less, when the entire stretched polyethylene film is taken as 100 mass%.

3. The density of the high density polyethylene layer 1 and the high density polyethylene layer 2 measured in accordance with JIS K 7112:1999 is 940 kg / m ３ More than 970kg / m ３ 3. The stretched polyethylene film according to claim 1 , wherein:

4. The medium density polyethylene layer has a density of 910 kg / m as measured in accordance with JIS K 7112:1999. ３ More than 940kg / m ３ 3. The stretched polyethylene film according to claim 1 or 2, wherein the stretched polyethylene film has a viscosity of less than 1000 s.p.m.

5. 3. The stretched polyethylene film according to claim 1 or 2, wherein the stretched polyethylene film has a full width at half maximum (FWHM) of a peak in the MD direction at a diffraction angle 2θ in the range of 0.2 to 0.4°, as determined by small angle X-ray scattering (SAXS) measurement, of 0.20° or less.

6. 3. The stretched polyethylene film according to claim 1, wherein the haze of the stretched polyethylene film per sheet measured in accordance with JIS K 7136:2000 is 8.0% or less.

7. 3. The stretched polyethylene film according to claim 1 or 2, wherein the haze per four sheets of the stretched polyethylene film measured in accordance with JIS K 7136:2000 is 33.0% or less.

8. The tensile modulus T of the stretched polyethylene film in the MD direction is measured using a tensile tester at a measurement temperature of 23±2° C., a relative humidity of 50±5%, and a tensile speed of 5 mm / min in accordance with JIS K7127:1999. １ and the tensile modulus in the TD direction T ２ The stretched polyethylene film according to claim 1 or 2, wherein the total value of is 1000 MPa or more and 3300 MPa or less.

9. The stretched polyethylene film according to claim 1 or 2, wherein the stretched polyethylene film has a heat shrinkage rate in the MD direction of 3.0% or less when heat-treated at 100 ° C. for 15 minutes in accordance with JIS C2151:2019.

10. The stretched polyethylene film according to claim 1 or 2, wherein the stretched polyethylene film has a heat shrinkage rate in the MD direction of 15.0% or less when heat-treated at 120 ° C. for 15 minutes in accordance with JIS C2151:2019.

11. 3. The stretched polyethylene film according to claim 1, wherein at least one surface of the stretched polyethylene film is a corona-treated surface.

12. The stretched polyethylene film according to claim 11, wherein the static friction coefficient (tan θ) of the surface of the corona-treated surface of the stretched polyethylene film, as measured using a slip tester by the following method 1 (incline method), is 0.40 or more and 0.75 or less. (Method 1) Two pieces of the stretched polyethylene film cut to a size of 50 mm x 75 mm (hereinafter referred to as stretched polyethylene films 1 and 2) are prepared, and one of the stretched polyethylene films, the stretched polyethylene film 1, is fixed to an inclined plate so that the corona-treated surface side is on top. Next, a friction body with a bottom surface (size 41 mm x 26 mm) made of brass is fixed to the center of the surface opposite to the corona-treated surface side of the other stretched polyethylene film 2, and a weight is attached on top of the friction body so that the mass applied from the friction body to the stretched polyethylene film 2 is 150 g. Next, the corona-treated surfaces of the two stretched polyethylene films 1 and 2 are overlapped. Next, the inclined plate is inclined at a speed of 1° / sec, and the value of tan θ is calculated from the angle θ when the upper stretched polyethylene film 2 starts to slide.

13. The number of pinholes generated in the stretched polyethylene film is 6,000 / m, as measured by a Gelbo flex tester at a bending angle of 440 degrees, a bending speed of 40 times / min, and an atmosphere of -30°C, and 3,000 times bending. ２ 3. The stretched polyethylene film according to claim 1 , wherein:

14. 3. The stretched polyethylene film according to claim 1 or 2, wherein the tear strength in the MD direction of the stretched polyethylene film is measured using a light-load tear tester under the conditions of test piece size: MD direction: 63.5 mm, TD direction: 50.0 mm, pendulum weight: 96.09 g, tear length: 12.7 mm, and pendulum lift angle: 90°, of 50 mN or more and 1500 mN or less.

15. 3. The stretched polyethylene film according to claim 1 or 2, wherein the tear strength in the TD direction of the stretched polyethylene film is measured using a light-load tear tester under the conditions of test piece size: MD direction: 50.0 mm, TD direction: 63.5 mm, pendulum weight mass: 96.09 g, tear length: 12.7 mm, and pendulum lift angle: 90°, of 200 mN or more and 800 mN or less.

16. At least one surface of the stretched polyethylene film is a non-corona treated surface, 3. The stretched polyethylene film according to claim 1, wherein the heat fusion strength when the non-corona treated surface of the stretched polyethylene film is bonded to a substrate and heat sealed at 140°C is 10.0 N / 15 mm or less.

17. 3. The stretched polyethylene film according to claim 1, wherein the stretched polyethylene film has a lamination strength of 0.92 N / 15 mm or more as measured by the following method 2. (Method 2) A test piece measuring 297 cm x 210 cm was cut out from the stretched polyethylene film, and the corona-treated surface of the test piece was bonded to the corona-treated surface of a 50 μm-thick cast LLDPE film, one side of which had been corona-treated, with an ester-based adhesive, and aged at 40°C for 3 days to obtain a sample. The sample was then cut to a width of 15 mm, and peeled in the MD direction using a tensile tester in accordance with JIS Z 0238:1998 at a peel angle of 90°, a chuck distance of 100 mm, and a crosshead speed of 300 mm / min, to determine the peel strength, which is taken as the laminate strength.

18. 3. The stretched polyethylene film according to claim 1, wherein the total thickness of the stretched polyethylene film is from 10 μm to 100 μm.

19. 3. The stretched polyethylene film according to claim 1 or 2, which is a food packaging film.

20. A packaging material using the stretched polyethylene film according to claim 1 or 2.

21. A packaging material according to claim 20; and a food product within the packaging material.