Polyethylene film

Abstract

A polyethylene film in which, when the direction in which tensile strength is greatest is defined as the main orientation direction, and the direction orthogonal to the main orientation direction in the plane of the film is defined as the main orientation orthogonal direction, the sum of the thermal shrinkage rate in the main orientation direction and the thermal shrinkage rate in the main orientation orthogonal direction when heated for eight hours at 100°C is -5.0% to 10.0%, the tensile strength in the main orientation orthogonal direction is 200 MPa to 5000 MPa, and the internal haze is 0% to 80%.　Provided is polyethylene film having excellent heat resistance, mechanical strength, quality, and transparency.

Description

Polyethylene film

[0001] The present invention relates to a polyethylene film that is excellent in heat resistance, mechanical strength, quality, and transparency and can be suitably used as an industrial material film.

[0002] Polyethylene films are widely used in various packaging materials, medical films, and other applications due to their excellent physical properties, including light weight, moisture resistance, and chemical resistance. Ultra-high molecular weight polyethylene films, in particular, have superior abrasion resistance and tensile strength compared to general-purpose polyethylene films, and are therefore used as sliding materials and substrates for adhesive films. However, because ultra-high molecular weight polyethylene resins have extremely high melt viscosity, they are difficult to form into films by conventional extrusion or injection molding. Films are typically formed by cutting out compression-molded materials, but this method results in insufficient molecular chain orientation, resulting in low mechanical strength, and difficulty in thinning the film, resulting in poor transparency.

[0003] To address these problems, for example, Patent Document 1 describes an example in which a gel-like sheet obtained by dissolving an ultra-high molecular weight polyethylene resin in a solvent is biaxially stretched, and then pressure treatment is performed after removing the solvent to obtain a thin film. Patent Document 2 also describes an example in which a specific hydrocarbon-based plasticizer is mixed with an ultra-high molecular weight polyethylene resin and extrusion-molded to obtain a biaxially stretched film. Furthermore, Patent Document 3 describes an example in which a sheet obtained by compression-molding an ultra-high molecular weight polyethylene resin is uniaxially stretched at a high ratio to obtain a high-strength film.

[0004] JP-A No. 60-228122 JP-A No. 6-262679 JP-A No. 2014-111384

[0005] However, the polyethylene film described in Patent Document 1 has high strength due to high-magnification stretching, but the molecular chains of the high-molecular-weight components are not sufficiently relaxed, resulting in heat shrinkage during use at high temperatures and other issues with heat resistance. The polyethylene film described in Patent Document 2 has insufficient mechanical strength due to the addition of a plasticizer, and also has the problem of the plasticizer bleeding out over long-term use, resulting in a deterioration in quality. Furthermore, the polyethylene film described in Patent Document 3 has high strength due to high-magnification stretching, but is uniaxially stretched, resulting in issues with mechanical strength in the width direction. Therefore, the object of the present invention is to solve the above-mentioned problems. That is, to provide a polyethylene film that has excellent heat resistance, mechanical strength, quality, long-term storage stability, and transparency.

[0006] In order to solve the above-mentioned problems, the polyethylene film of the present invention has the following configuration: The polyethylene film of the present invention is a polyethylene film in which, when the direction in which the tensile strength is greatest is defined as the main orientation direction and the direction in the film plane perpendicular to the main orientation direction is defined as the orthogonal direction to the main orientation direction, the sum of the heat shrinkage percentages in the main orientation direction and the orthogonal direction to the main orientation direction when heated at 100°C for 8 hours is from −5.0% to 10.0%, the tensile strength in the orthogonal direction to the main orientation is from 200 MPa to 5000 MPa, and the internal haze is from 0% to 80%.

[0007] The method for producing a polyethylene film of the present invention may also be configured as follows: That is, the method for producing a polyethylene film of the present invention is a method for producing a polyethylene film having an internal haze of 0% or more and 80% or less, which is characterized by comprising a heat treatment step of biaxially stretching a sheet containing polyethylene having a weight-average molecular weight Mw of 500,000 or more and 5,000,000 or less and a plasticizer, extracting the plasticizer, and then heat-treating the sheet.

[0008] The present invention provides a polyethylene film excellent in heat resistance, mechanical strength, quality, long-term storage stability, and transparency. Because the polyolefin film of the present invention has the above-mentioned excellent properties, it can be suitably used in a wide range of applications, such as industrial material films, surface protection films, process films, release films, heat dissipation films, low-temperature films, adhesive film substrates, sliding films, medical films, and capacitor films.

[0009] The polyethylene film of the present invention is a polyethylene film in which, when heated at 100°C for 8 hours, the sum of the heat shrinkage percentages in the main orientation direction and the direction orthogonal to the main orientation direction is from -5.0% to 10.0%, the tensile strength in the direction orthogonal to the main orientation is from 200 MPa to 5000 MPa, and the internal haze is from 0% to 80%.

[0010] A polyethylene film is a film containing more than 50% by mass but not more than 100% by mass of polyethylene resin, where the total mass of all components constituting the film is 100% by mass. The content of polyethylene resin in a polyethylene film is preferably 70% by mass or more but not more than 100% by mass, more preferably 90% by mass or more but not more than 100% by mass, even more preferably 95% by mass or more but not more than 100% by mass, particularly preferably 96% by mass or more but not more than 100% by mass, and most preferably 97% by mass or more but not more than 100% by mass. When a film contains multiple components that qualify as polyethylene resins, the film is considered to qualify as a polyethylene film if the combined total of these components is more than 50% by mass but not more than 100% by mass. A polyethylene resin is a resin in which ethylene units account for more than 50 mol% but not more than 100 mol% of all structural units constituting the resin.

[0011] From the viewpoint of improving heat resistance, it is important that the polyethylene film of the present invention, when heated at 100°C for 8 hours, has a sum of the heat shrinkage in the main orientation direction and the heat shrinkage in the direction perpendicular to the main orientation direction, of -5.0% to 10.0%, where the direction with the greatest tensile strength is the main orientation direction and the direction perpendicular to the main orientation direction in the film plane is the main orientation direction. The upper limit of the sum of the heat shrinkage in the main orientation direction and the heat shrinkage in the direction perpendicular to the main orientation direction when heated at 100°C for 8 hours is preferably 8.0%, more preferably 6.0%, and even more preferably 4.0%. The lower limit of the sum of the heat shrinkage in the main orientation direction and the heat shrinkage in the direction perpendicular to the main orientation direction when heated at 100°C for 8 hours is preferably -2.0%, more preferably -1.0%, and even more preferably 0.0%, since the film may expand. Here, a heat shrinkage of 0.0% means that the film neither shrinks nor expands, and a negative value means that the film expands rather than shrinks. When the film expands in both the main orientation direction and the direction perpendicular to the main orientation, or when the film expands in one of the main orientation direction and the direction perpendicular to the main orientation and the degree of expansion is greater than the degree of shrinkage in the other direction, the "sum of the thermal shrinkage in the main orientation direction and the thermal shrinkage in the direction perpendicular to the main orientation" becomes a negative value.

[0012] Here, the "main orientation direction" in the present invention refers to the direction showing the largest value when tensile strength is measured in each direction forming an angle of 0° to 175° in 5° increments relative to an arbitrary direction in the plane of the film, with the arbitrary direction being set as 0°, and the "direction perpendicular to the main orientation" refers to the direction perpendicular to the main orientation direction in the plane of the film. The tensile strength can be measured using a tensile tester in accordance with the method specified in JIS K7161 (2014), and details of the measurement method are shown in the Examples.

[0013] When the sample width is less than 50 mm and the tensile strength cannot be determined using a tensile tester, the crystal orientation of the (110) plane of the polyethylene film is measured using wide-angle X-rays as follows, and the main orientation direction is determined based on the following criteria. That is, X-rays (CuKα rays) are incident perpendicular to the film surface, and the crystal peak at 2θ = approximately 22° (the (110) plane) is scanned circumferentially. The direction with the highest diffraction intensity in the obtained diffraction intensity distribution is determined as the main orientation direction, and the direction perpendicular to this is determined as the direction perpendicular to the main orientation direction. In the present invention, the direction parallel to the film-forming direction of the polyethylene film is referred to as the film-forming direction, longitudinal direction, or MD direction, and the direction perpendicular to the film-forming direction within the film plane is referred to as the width direction or TD direction.

[0014] By setting the sum of the heat shrinkage rate in the main orientation direction and the heat shrinkage rate in the direction perpendicular to the main orientation when heated at 100°C for 8 hours to be −5.0% or more and 10.0% or less, or within the above preferred range, the dimensional stability of the polyethylene film is improved, and deterioration in quality, such as wrinkles in the roll due to shrinkage and expansion of the polyethylene film while the polyethylene film is wound into a roll and stored, can be reduced.

[0015] To achieve a sum of the heat shrinkage in the main orientation direction and the heat shrinkage in the direction perpendicular to the main orientation when heated at 100°C for 8 hours of -5.0% to 10.0%, for example, a method can be used in which the composition of the polyethylene film is within the range described below and the film-forming conditions are within the range described below. In particular, it is effective to set the weight average molecular weight of the film measured using high-temperature GPC (hereinafter sometimes referred to as "weight average molecular weight" or "weight average molecular weight of the film") within the range described below and to relax the shrinkage of the film at high temperatures in the heat treatment step.

[0016] From the viewpoint of enhancing mechanical strength, the polyethylene film of the present invention has a tensile strength in the direction orthogonal to the main orientation of 200 MPa or more and 5000 MPa or less. From the above viewpoint, the lower limit of the tensile strength in the direction orthogonal to the main orientation is preferably 300 MPa, more preferably 400 MPa, even more preferably 450 MPa, and particularly preferably 500 MPa. Furthermore, from the viewpoint of feasibility, the upper limit of the tensile strength in the direction orthogonal to the main orientation is preferably 2000 MPa, more preferably 1000 MPa. By setting the tensile strength in the direction orthogonal to the main orientation to 200 MPa or more and 5000 MPa or less, the film is less likely to break even when made thinner or when used under high tension, and can be suitably used as a process film.

[0017] To achieve a tensile strength of 200 MPa or more and 5000 MPa or less in the direction perpendicular to the main orientation, for example, a method can be used in which the composition of the polyethylene film is within the range described below and the film-forming conditions are within the range described below. In particular, it is effective to set the molecular weight of the film within the range described below and stretch it at a high stretch ratio.

[0018] In order to increase the tensile strength, it is common to stretch a film at a higher draw ratio, but increasing the draw ratio can sometimes increase the heat shrinkage, and it has traditionally been difficult to achieve both a low heat shrinkage and a high tensile strength in a polyethylene film. However, for example, by using a method in which the composition of the polyethylene film is within the range described below and the film-forming conditions are within the range described below, it is possible to achieve both a low heat shrinkage and a high tensile strength. In particular, it is effective to set the molecular weight of the film within the range described below and to relax the shrinkage of the film at a high temperature in the heat treatment step.

[0019] The polyethylene film of the present invention has an internal haze of 0% or more and 80% or less from the viewpoint of enhancing transparency. The upper limit of the internal haze is preferably 70%, more preferably 60%, even more preferably 50%, and particularly preferably 40%. A low internal haze means high transparency. By setting the internal haze to 0% or more and 80% or less, visibility after film attachment can be improved when the polyethylene film is used as an adhesive film. Furthermore, light transmittance can be improved when the polyethylene film is used as an optical film. The internal haze can be measured using a haze meter, and details of the measurement method are shown in the examples.

[0020] To set the internal haze to 0% or more and 80% or less, for example, a method can be used in which the composition of the polyethylene film is set within the range described below and the film-forming conditions are set within the range described below. In particular, it is effective to close voids in the film by heat treatment / re-stretching at a high temperature.

[0021] The polyethylene film of the present invention preferably has a ratio T1 / T2 of the tensile elongation T1 in the main orientation direction to the tensile elongation T2 in the direction perpendicular to the main orientation direction of 0.10 or more and 10 or less (hereinafter, the ratio of the tensile elongation T1 in the main orientation direction to the tensile elongation T2 in the direction perpendicular to the main orientation direction may be simply referred to as T1 / T2). The upper limit of T1 / T2 is more preferably 5.0, even more preferably 2.0, and particularly preferably 1.1, and the lower limit of T1 / T2 is more preferably 0.20, even more preferably 0.50, and particularly preferably 0.60. By setting T1 / T2 to 0.10 or more and 10 or less, the mechanical properties of the film become isotropic, and the film is less likely to tear even when made thinner or used under high tension, making it suitable for use as a process film.

[0022] To set T1 / T2 within the above preferred range, a method of setting the composition of the polyethylene film and film-forming conditions within the ranges described below can be used. In particular, it is effective to set the stretch ratios in the MD and TD directions within the ranges described below.

[0023] The polyethylene film of the present invention preferably has a sum of the tensile elongation in the main orientation direction and the direction orthogonal to the main orientation direction of 160% or more and 500% or less. The lower limit of the sum of the tensile elongation in the main orientation direction and the direction orthogonal to the main orientation direction is more preferably 170%, even more preferably 180%, and particularly preferably 190%, and the upper limit is more preferably 450%, even more preferably 400%, and particularly preferably 350%. By setting the sum of the tensile elongation in the main orientation direction and the direction orthogonal to the main orientation direction to 160% or more and 500% or less, the film is less likely to break even when used under high tension, and can be suitably used as a process film. The tensile elongation can be measured using a tensile tester in accordance with the method specified in JIS K7161 (2014); details of the measurement method are shown in the examples.

[0024] In order to make the sum of the tensile elongations in the main orientation direction and the direction perpendicular to the main orientation 160% to 500%, for example, a method can be used in which the composition of the polyethylene film is within the range described below and the film-forming conditions are within the range described below. In particular, it is effective to make the stretch ratios in the MD and TD directions within the range described below.

[0025] The thickness of the polyethylene film of the present invention is preferably 25 μm or less. The upper limit of the thickness is more preferably 20 μm, even more preferably 15 μm, and particularly preferably 10 μm. The lower limit is not particularly limited, but from the viewpoint of film production feasibility, it is substantially about 0.1 μm. By making the thickness of the polyethylene film 25 μm or less, it is possible to improve the conformability to the adherend when used as a release film or a protective film. Furthermore, it is possible to reduce the volume when used as a packaging film.

[0026] To achieve a polyethylene film thickness of 25 μm or less, a method can be used in which the film-forming conditions for the polyethylene film are set within the ranges described below. It is particularly effective to set the stretch ratios in the MD and TD directions within the ranges described below. Furthermore, the thickness can be adjusted by the screw rotation speed of the extruder, the width of the unstretched sheet, the film-forming speed, the stretch ratio, and the like, within a range that does not deteriorate other physical properties. The thickness of the polyethylene film can be measured using a known micro thickness meter, and details of the measurement method are shown in the Examples.

[0027] From the viewpoint of improving heat resistance and mechanical strength, the polyethylene film of the present invention preferably has a temperature distribution curve of the heat of crystalline fusion measured by differential scanning calorimetry, in which the proportion of the heat of crystalline fusion at 140°C or higher to the total heat of crystalline fusion is 30% to 90%. The lower limit of the proportion of the heat of crystalline fusion at 140°C or higher to the total heat of crystalline fusion is more preferably 40%, even more preferably 50%, and particularly preferably 60%. The proportion of the heat of crystalline fusion at 140°C or higher to the total heat of crystalline fusion corresponds to the proportion of a structure consisting of highly oriented molecular chains in the film. By setting this value to 30% to 90%, the heat resistance and mechanical strength of the film are improved, making it suitable for use as a processing film. The proportion of the heat of crystalline fusion at 140°C or higher to the total heat of crystalline fusion can be measured by differential scanning calorimetry (DSC) according to JIS K7121 (2012), and details of the measurement method are shown in the examples.

[0028] In order to make the proportion of the heat of crystalline fusion at 140°C or higher to the total heat of crystalline fusion 30% to 90% by weight, for example, a method can be used in which the composition of the polyethylene film is within the range described below and the film-forming conditions are within the range described below. In particular, it is effective to make the molecular weight of the film within the range described below and stretch it at a high stretch ratio.

[0029] From the viewpoint of enhancing mechanical strength and transparency, the polyethylene film of the present invention has a Gurley value of 1×10 measured by an Oken air resistance meter. ４ seconds / 100cm ３ The Gurley value is preferably 5×10 or more. ４ seconds / 100cm ３ More preferably, 7 × 10 ４ seconds / 100cm ３ More preferably, 1×10 ５ seconds / 100cm ３ The Gurley value measured by the Oken air resistance meter may be simply referred to as the "Gurley value" below.

[0030] Gurley value is 1 x 10 ４ seconds / 100cm ３By making the value equal to or greater than this, it is possible to suppress the generation of voids penetrating the film in the thickness direction, and to improve the mechanical strength and transparency. ６ seconds / 100cm ３ The Gurley value can be measured using an Oken air resistance meter in accordance with JIS P-8117 (2009), and details of the measurement method will be shown in the examples.

[0031] Gurley value is 1 x 10 ４ seconds / 100cm ３ To achieve the above, for example, a method can be used in which the composition of the polyethylene film is set within the range described below and the film-forming conditions are set within the range described below. In particular, it is effective to close voids in the film by heat treatment / re-stretching at a high temperature.

[0032] From the viewpoint of improving heat resistance and mechanical strength, the polyethylene film of the present invention preferably has a weight average molecular weight (sometimes referred to as "weight average molecular weight" or "film weight average molecular weight") measured using high-temperature GPC of 500,000 or more and 1,900,000 or less. The upper limit of the film's weight average molecular weight is more preferably 1,700,000, even more preferably 1,500,000, and the lower limit is more preferably 700,000, even more preferably 900,000. By setting the film's weight average molecular weight to 1,900,000 or less, high molecular weight components that are difficult to relax can be suppressed, improving the film's heat resistance. Furthermore, by setting the film's weight average molecular weight to 500,000 or more, the film's mechanical strength can be improved. The film's weight average molecular weight can be measured by high-temperature GPC, and details of the measurement method are shown in the examples.

[0033] In order to set the weight-average molecular weight of the film to 500,000 or more and 1,900,000 or less, a method can be used in which the composition of the polyethylene film is set within the range described below and the film-forming conditions are set within the range described below. In particular, it is effective to set the weight-average molecular weight of the polyethylene resin within the range described below.

[0034] From the viewpoint of enhancing mechanical strength, the polyethylene film of the present invention preferably has a thermal conductivity in the main orientation direction of 0.7 W / m / K or more, more preferably 1.0 W / m / K or more, even more preferably 2.0 W / m / K or more, particularly preferably 3.0 W / m / K or more, and most preferably 5.0 W / m / K or more.

[0035] By setting the thermal conductivity in the main orientation direction to 0.7 W / m / K or more, the orientation of molecular chains in the main orientation direction of the film can be improved, thereby increasing mechanical strength. Furthermore, when used as a heat dissipation film, the diffusion of heat generated from a heat source can be improved. The upper limit of the thermal conductivity in the main orientation direction is not particularly limited, but is substantially about 5000 W / m / K, preferably about 100 W / m / K, and more preferably about 30 W / m / K. The thermal conductivity in the main orientation direction can be measured by an optical alternating current method, and details of the measurement method are shown in the examples.

[0036] To achieve a thermal conductivity of 0.7 W / m / K or more in the main orientation direction, for example, a method can be used in which the composition of the polyethylene film is within the range described below and the film-forming conditions are within the range described below. In particular, it is effective to set the molecular weight of the film within the range described below and stretch it at a high draw ratio after extracting the plasticizer.

[0037] Hereinafter, the ultra-high molecular weight polyethylene resin (sometimes referred to as polyethylene resin A) that is suitable as the component contained in the largest amount in the polyethylene film of the present invention will be described. Here, polyethylene resin A (ultra-high molecular weight polyethylene) is polyethylene having a weight-average molecular weight Mw of 500,000 or more, and examples that can be used include "Hi-Zex Million" (registered trademark) manufactured by Mitsui Chemicals, Inc. and "Sunfine" (registered trademark) manufactured by Asahi Kasei Corporation. Note that when multiple types of polyethylene resins that satisfy the above requirements are contained, if the total content of these components exceeds 50 mass% of the entire film, polyethylene resin A can be considered to be the one that is contained in the largest amount.

[0038] The weight-average molecular weight Mw of polyethylene resin A is preferably 500,000 or more and 5,000,000 or less from the viewpoint of achieving both heat resistance and mechanical strength of the film. The upper limit of Mw of polyethylene resin A is more preferably 2,000,000, even more preferably 1,800,000, and particularly preferably 1,500,000, and the lower limit is more preferably 700,000, even more preferably 900,000, and particularly preferably 1,100,000. The weight-average molecular weight Mw of polyethylene resin A can be measured by high-temperature GPC, and details of the measurement method are shown in the examples.

[0039] The melting point of polyethylene resin A is preferably 120°C or higher and 150°C or lower, from the viewpoint of achieving both heat resistance and film formability of the film. The upper limit of the melting point of polyethylene resin A is more preferably 145°C, even more preferably 140°C, and the lower limit is more preferably 125°C, even more preferably 130°C. The melting point of polyethylene resin A can be measured by differential scanning calorimetry (DSC) based on JIS K7121 (2012), and details of the measurement method are shown in the examples. The same applies to the melting point of polyethylene resin B described below.

[0040] Polyethylene resin A may contain copolymer components such as copolymers of other unsaturated hydrocarbons within the scope of the present invention. Examples of monomer components constituting such copolymer components include propylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, 5-methyl-2-norbornene, vinyl acetate, methyl methacrylate, and styrene.

[0041] From the viewpoint of dimensional stability when the polyethylene film is formed, the copolymerization amount is preferably less than 10 mol%, and more preferably 5 mol% or less, when all the structural units constituting the polyethylene resin A are taken as 100 mol%. Here, the copolymerization amount of the polyethylene resin A is calculated based on the entire polyethylene resin A contained in the film. That is, when all the structural units constituting the polyethylene resin A are taken as 100 mol%, the copolymerization amount can be considered to be less than 10 mol%, not only when the film is made only of polyethylene resin A containing less than 10 mol% of a copolymerization component, but also when the film contains polyethylene resin A containing 10 mol% or more of a copolymerization component but the copolymerization component is less than 10 mol% as a whole. The same applies to the copolymerization amount of polyethylene resin B described below.

[0042] The polyethylene film of the present invention may contain, in addition to polyethylene resin A, a polyethylene resin other than ultra-high molecular weight polyethylene (polyethylene resin B). When the polyethylene film contains polyethylene resin B, void formation in the film can be suppressed and transparency can be improved. As polyethylene resin B, high-density polyethylene, low-density polyethylene, very-low-density polyethylene, linear low-density polyethylene, low-molecular-weight polyethylene, etc. can be used.

[0043] Here, high density polyethylene has a density of 0.930 g / cm ３ Examples of the polyethylene that can be used include "Hi-Zex" (registered trademark) manufactured by Prime Polymer Co., Ltd., "Evolue" (registered trademark) H manufactured by Prime Polymer Co., Ltd., "Suntech" (registered trademark) HD ​​manufactured by Asahi Kasei Corporation, and "Novatec" (registered trademark) HD ​​manufactured by Japan Polyethylene Co., Ltd.

[0044] Low density polyethylene has a density of 0.910 g / cm ３ 0.930g / cm or more ３ For example, "Suntech" (registered trademark) LD manufactured by Asahi Kasei Corporation, "Novatec" (registered trademark) LD manufactured by Japan Polyethylene Corporation, etc. can be used.

[0045] Ultra-low density polyethylene has a density of 0.910 g / cm ３ For example, "LUMITAC" (registered trademark) manufactured by Tosoh Corporation can be used.

[0046] The linear low-density polyethylene is a low-density polyethylene produced by catalytic polymerization, and examples of such polyethylene include "Evolue" (registered trademark) manufactured by Prime Polymer Co., Ltd. and "Novatec" (registered trademark) LL manufactured by Japan Polyethylene Co., Ltd.

[0047] The low-molecular-weight polyethylene is polyethylene having a weight-average molecular weight of less than 100,000, and examples thereof include "Hiwax" (registered trademark) manufactured by Mitsui Chemicals, Inc. and "Sunwax" manufactured by Sanyo Chemical Industries, Ltd.

[0048] Polyethylene is classified according to density, manufacturing method, molecular weight, etc., and therefore may fall into two or more types. In the present invention, polyethylene resin A is one having a weight-average molecular weight of 500,000 or more, and polyethylene resin B is one having a weight-average molecular weight of less than 500,000.

[0049] From the viewpoint of enhancing the transparency of the film, the weight average molecular weight Mw of polyethylene resin B is preferably 1,000 or more and less than 500,000. From the above viewpoint, the upper limit of Mw of polyethylene resin B is more preferably 400,000, even more preferably 300,000, and particularly preferably 200,000, and the lower limit is more preferably 5,000, even more preferably 10,000, and particularly preferably 20,000.

[0050] From the viewpoint of enhancing the transparency of the film, the melting point of polyethylene resin B is preferably 90° C. or higher and 140° C. or lower. From the viewpoints mentioned above, the upper limit of the melting point of polyethylene resin B is more preferably 135° C., even more preferably 130° C., and particularly preferably 125° C., and the lower limit is more preferably 95° C., even more preferably 100° C., and particularly preferably 110° C.

[0051] Polyethylene resin B may contain copolymerization components such as copolymerization components based on other unsaturated hydrocarbons, provided that the objectives of the present invention are not impaired. Examples of monomer components constituting such copolymerization components include propylene, 1-butene, 1-pentene, 3-methylpentene-1, 3-methylbutene-1, 1-hexene, 4-methylpentene-1, 5-ethylhexene-1, 1-octene, 1-decene, 1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, 5-methyl-2-norbornene, vinyl acetate, methyl methacrylate, and styrene. From the viewpoint of dimensional stability when formed into a polyethylene film, the copolymerization amount is preferably less than 10 mol%, and more preferably 5 mol% or less, when all structural units constituting the low-molecular-weight polyethylene resin are taken as 100 mol%.

[0052] The polyethylene film of the present invention may contain a resin other than polyethylene as long as the object of the present invention is not impaired. Examples of such resins other than polyethylene include polypropylene, polymethylpentene, polybutene, olefin-based thermoplastic elastomers, polystyrene, polyvinylidene fluoride, polyethylene oxide, polyester, etc. When a resin other than polyethylene is contained, the content thereof is preferably less than 20% by mass, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, based on 100% by mass of the total amount of resin components, from the viewpoint of mechanical strength when formed into a polyethylene film.

[0053] The polyethylene film of the present invention may contain various additives, such as nucleating agents, antioxidants, heat stabilizers, slipping agents, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as the additives do not impair the object of the present invention. Among these, the selection of the type and amount of antioxidant is important from the viewpoint of suppressing oxidative degradation of the polyethylene resin due to thermal history. That is, such antioxidants are preferably phenolic antioxidants having steric hindrance, and at least one of them is preferably a high-molecular-weight type having a molecular weight of 500 or more. Specific examples thereof include various compounds, and it is preferable to use, for example, one or more compounds selected from 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4) together with 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (for example, "Irganox" (registered trademark) 1330 manufactured by BASF: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (for example, "Irganox" (registered trademark) 1010 manufactured by BASF: molecular weight 1177.7).

[0054] The total content of these antioxidants is preferably in the range of 0.01 part by mass or more and 1.0 part by mass or less, relative to 100 parts by mass of the total polyethylene resin. By using an antioxidant in an amount of 0.01 part by mass or more, it is possible to suppress discoloration of the film due to polymer degradation during the extrusion process and improve long-term heat resistance. Furthermore, by using an antioxidant in an amount of 1.0 part by mass or less, it is possible to suppress bleed-out of the antioxidant and improve the transparency of the polyethylene film. From the above viewpoints, the lower limit of the antioxidant content is more preferably 0.05 part by mass, even more preferably 0.1 part by mass, relative to 100 parts by mass of the total polyethylene resin, and the upper limit is more preferably 0.9 part by mass, even more preferably 0.8 part by mass.

[0055] The polyethylene film of the present invention preferably does not contain inorganic particles. The polyethylene resin that can be preferably used as the main component of the polyethylene film of the present invention has low affinity with inorganic particles, and therefore, the inorganic particles may fall off from the film during the manufacturing process, contaminating the manufacturing line or the product. Furthermore, if large protrusions are formed by inorganic particles with high hardness, when the film is used as a protective film or processing film for optical components, the irregularities may be transferred to the resin layer of the optical component. Therefore, when the film is used as a protective film or a manufacturing base film for products that require high quality, such as display components, this may cause a decrease in quality. From the above perspective, the polyethylene film of the present invention preferably does not contain a lubricant such as organic particles.

[0056] In the polyethylene film of the present invention, the proportions of polyethylene resin A and polyethylene resin B in 100% by mass of the total resin components are preferably as follows: The proportion of polyethylene resin A is preferably more than 50% by mass and not more than 100% by mass, from the viewpoint of the heat resistance and mechanical strength of the polyethylene film. From the above viewpoints, the lower limit of the proportion of polyethylene resin A is more preferably 60% by mass, and even more preferably 70% by mass. The proportion of polyethylene resin B is preferably 0% by mass or more and 40% by mass or less of the entire film, and its upper limit is more preferably 30% by mass, and even more preferably 10%. Here, "0% by mass" means that the component is not included, and when polyethylene resin A accounts for 100% by mass, the proportion of polyethylene resin B is 0% by mass.

[0057] The layer structure of the polyethylene film of the present invention is not particularly limited, and it may have either a single layer or a laminate structure.

[0058] The polyethylene film of the present invention may include only one polyethylene resin layer, or may include two or more polyethylene resin layers. A polyethylene resin layer refers to a layer containing more than 50% by mass and not more than 100% by mass of polyethylene resin, assuming that all components constituting the layer are 100% by mass. In this case, when a layer contains two or more components corresponding to polyethylene resin, the layer is considered to be a "layer containing polyethylene resin as the main component" if the total content of these components is more than 50% by mass and not more than 100% by mass.

[0059] The content of the polyethylene resin in the "layer containing polyethylene resin as a main component" is more preferably 90% by mass or more and 100% by mass or less, even more preferably 95% by mass or more and 100% by mass or less, still more preferably 96% by mass or more and 100% by mass or less, particularly preferably 97% by mass or more and 100% by mass or less, and most preferably 98% by mass or more and 100% by mass or less, when all components constituting the layer are taken as 100% by mass. When the polyethylene film of the present invention has a single layer configuration, the main component of the polyethylene film itself is polyethylene resin.

[0060] The polyethylene film of the present invention is preferably biaxially stretched using the above-mentioned resin. The biaxial stretching method may be any of simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, and sequential biaxial stretching using a roll stretching machine and a tenter stretching machine. Among these, however, tenter simultaneous biaxial stretching or sequential biaxial stretching is preferred in terms of film formation stability, thickness uniformity, and controlling the high rigidity and dimensional stability of the resulting polyethylene film, and it is preferable to employ all of the following steps (a) to (e). It is particularly important to include step (e) of heat treatment / re-stretching after extracting the plasticizer. Below, an example of a method for producing a polyethylene film using the above-mentioned raw materials will be described, but the method for producing the polyethylene film of the present invention is not necessarily limited thereto. (a) preparing a polyethylene resin solution by kneading and dissolving polymer materials including polyethylene alone, a polyethylene mixture, a polyethylene solvent (plasticizer) mixture, additives, and a polyethylene mixture; (b) extruding the melt, forming it into a sheet, and cooling and solidifying it; (c) subjecting the obtained sheet to sequential or simultaneous biaxial stretching using a roll stretching machine and / or a tenter stretching machine; (d) subsequently extracting the plasticizer from the obtained stretched film and drying the film; and (e) subsequently performing heat treatment / re-stretching. Each step is described below.

[0061] (a) Preparation of Polyethylene Resin Solution A polyethylene resin solution is prepared by heating and dissolving polyethylene resin in a plasticizer. In this case, polyethylene with a weight-average molecular weight Mw of 500,000 to 5,000,000 is preferred. The plasticizer is not particularly limited as long as it can sufficiently dissolve the polyethylene resin, but a plasticizer that is liquid at room temperature is preferred to enable relatively high-magnification stretching. Examples of plasticizers include aliphatic, cycloaliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, as well as mineral oil fractions with corresponding boiling points, and phthalate esters that are liquid at room temperature, such as dibutyl phthalate and dioctyl phthalate. Among these, nonvolatile plasticizers such as liquid paraffin are preferred to obtain a gel-like sheet with a stable plasticizer content. Here, a gel-like sheet refers to a sheet-like molded product containing a plasticizer that is liquid at room temperature. A plasticizer that is miscible with polyethylene in a melt-kneaded state but solid at room temperature may be mixed with a plasticizer that is liquid at room temperature. Examples of plasticizers that are solid at room temperature include stearyl alcohol, ceryl alcohol, paraffin wax, etc. However, if only such plasticizers are used, unevenness in stretching may occur.

[0062] The blending ratio of the polyethylene resin and the plasticizer is 100% by mass, and the content of the polyethylene resin may be appropriately selected within a range that does not impair moldability, but is preferably 5% by mass or more and 90% by mass or less. The upper limit of the polyethylene resin content is more preferably 70% by mass, even more preferably 50% by mass, and particularly preferably 30% by mass, and the lower limit is more preferably 7% by mass, even more preferably 10% by mass, and particularly preferably 15% by mass. By making the polyethylene resin 5% by mass or more (and the plasticizer 95% by mass or less), swelling and necking at the outlet of the die can be suppressed when molding into a sheet, thereby improving sheet moldability and film formability. On the other hand, by making the polyethylene resin 90% by mass or less (and the plasticizer 10% by mass or more), shrinkage in the thickness direction can be suppressed, improving moldability.

[0063] The viscosity of the plasticizer that is liquid at room temperature is preferably 20 cSt or more and 200 cSt or less at 40°C. If the viscosity at 40°C is 20 cSt or more, the sheet extruded from the polyethylene resin solution through the die is less likely to be non-uniform. On the other hand, if the viscosity is 200 cSt or less, the plasticizer can be easily removed. The viscosity of the plasticizer that is liquid at room temperature is the viscosity measured at 40°C using an Ubbelohde viscometer.

[0064] (b) Formation of extrudate and sheet formation The method for uniformly melt-kneading the polyethylene resin solution is not particularly limited, but for example, when a high-concentration polyethylene resin solution is to be prepared, it is preferable to carry out the melt-kneading in a twin-screw extruder. If necessary, various additives such as antioxidants may be added within a range that does not impair the effects of the present invention. In particular, it is preferable to add an antioxidant to prevent oxidation of the polyethylene resin.

[0065] In the extruder, the polyethylene resin solution is uniformly mixed at a temperature at which the polyethylene resin is completely melted. The melt-kneading temperature varies depending on the polyethylene resin used, but is preferably (the melting point of the polyethylene resin + 10°C) or higher (the melting point of the polyethylene resin + 120°C) or lower. Specifically, the melt-kneading temperature is preferably 140°C or higher and 260°C or lower, with the upper limit being more preferably 230°C, and even more preferably 210°C. The lower limit of the melt-kneading temperature is more preferably 150°C, and even more preferably 160°C.

[0066] From the viewpoint of suppressing resin degradation, a lower melt-kneading temperature is preferred. By setting the melt-kneading temperature to 260°C or lower, thermal decomposition of polyethylene can be suppressed, and the mechanical strength of the resulting film can be improved. In addition, precipitation of decomposition products on chill rolls and rolls in the stretching process can be suppressed, and deterioration of the appearance of the film can be suppressed. On the other hand, by setting the melt-kneading temperature to 140°C or higher, unmelted material in the extrudate extruded from the die can be suppressed, and film breakage in the subsequent stretching process can be prevented. After kneading within the above temperature range, it is preferable to remove foreign matter and denatured polymers using a filter.

[0067] Next, the resulting extrudate is cooled to obtain a sheet containing polyethylene and a plasticizer, and the gel structure of the polyethylene resin containing the plasticizer can be fixed by cooling. The cooling temperature is preferably 10° C. or higher and 50° C. or lower. By setting the cooling temperature within the above preferred range, the gel structure becomes finer, making it easier to perform uniform stretching in the subsequent stretching step.

[0068] The cooling method may be a method of directly contacting the material with cold air, cooling water or other cooling medium, a method of contacting the material with a roll cooled with a cooling medium, or a method using a casting drum or the like.

[0069] (c) Stretching Step Next, the obtained sheet is stretched. Examples of stretching methods include MD uniaxial stretching using a roll stretching machine, TD uniaxial stretching using a tenter stretching machine, sequential biaxial stretching using a combination of a roll stretching machine and a tenter stretching machine, or a combination of a tenter stretching machine and a tenter stretching machine, and simultaneous biaxial stretching using a simultaneous biaxial tenter stretching machine. However, biaxial stretching is preferred from the viewpoints of film formation stability, thickness uniformity, and controlling the high rigidity and dimensional stability of the resulting polyethylene film. The stretching ratio varies depending on the thickness of the sheet, but from the viewpoint of film thickness uniformity, it is preferably 5.0 times or more in either direction. The area ratio is preferably 25.0 times or more, more preferably 49.0 times or more, and even more preferably 64.0 times or more. An area ratio of 25.0 times or more not only achieves sufficient film uniformity, but also improves the mechanical strength of the film because unstretched portions are less likely to remain. The area magnification is preferably 150.0 times or less, more preferably 120.0 times or less, and even more preferably 100.0 times or less. When the area magnification is 150.0 times or less, breakage during film production can be reduced.

[0070] The stretching temperature in each direction is preferably (melting point of the sheet + 10°C) or lower, specifically preferably 90°C or higher and 130°C or lower. The upper limit of the stretching temperature is more preferably 125°C, even more preferably 120°C, and the lower limit is more preferably 95°C, even more preferably 100°C. Stretching at 90°C or higher can suppress stretching unevenness and improve film thickness uniformity.

[0071] (d) Plasticizer Extraction (Washing) and Drying Step: Next, the plasticizer remaining in the stretched sheet is removed using a washing solvent. Since the polyethylene resin phase and the plasticizer phase are separated, a polyethylene film is obtained by removing the plasticizer. Examples of washing solvents include saturated hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; ethers such as diethyl ether and dioxane; ketones such as methyl ethyl ketone; and chain fluorocarbons such as trifluoroethane. These washing solvents can be selected appropriately depending on the plasticizer and can be used alone or in combination.

[0072] The washing method can be a method of immersing the stretched sheet in a washing solvent, a method of showering the stretched sheet with the washing solvent, or a combination of these methods. The washing temperature is preferably 15° C. or higher and 30° C. or lower.

[0073] Thereafter, the washing solvent in the polyethylene film is removed by drying in a drying step. The drying method is not particularly limited, and a method using a metal heating roll or a method using hot air can be selected.

[0074] (e) Heat Treatment / Re-Stretching Step It is important to relax and heat-treat the dried polyethylene film in the width direction while tensely gripping both widthwise ends with clips. From the viewpoint of the heat resistance of the film, the relaxation rate is preferably 5.0% or more and 25% or less. The upper limit of the relaxation rate is more preferably 20%, and even more preferably 18%. The lower limit is more preferably 8.0%, and even more preferably 10%, and considering the appearance during long-term storage, 11% is particularly preferable. Furthermore, the heat treatment temperature is preferably 130°C or more from the viewpoint of closing voids in the film and increasing the transparency of the film. From the above viewpoints, the upper limit of the heat treatment temperature is preferably 160°C, more preferably 155°C, and even more preferably 150°C. The lower limit is more preferably 135°C, and even more preferably 140°C. By setting the relaxation rate and the heat treatment temperature within the above ranges, residual stress in the film can be alleviated and the thermal shrinkage rate can be reduced. After the heat treatment, the film is preferably cooled to 50°C or higher but lower than 130°C while the clips continue to tensely hold both widthwise ends, and then led to the outside of the tenter, where the clips on both widthwise ends are released. The upper limit of the temperature in the cooling step is more preferably 120°C, and even more preferably 110°C. Next, in a winding step, the film edges are slit, and the polyethylene film is wound into a roll. The heat treatment may be performed using a method such as a roll press or a belt press, which applies heat and pressure uniformly in the thickness direction of the film.

[0075] If necessary, it is preferable to stretch (re-stretch) the film at least uniaxially after the plasticizer extraction (washing) and drying steps. When re-stretching is performed, heat treatment is performed after the re-stretching. The re-stretching can be performed using a tenter stretching machine or the like while heating the polyethylene film, similar to the above-mentioned stretching. The re-stretching may be uniaxial or biaxial. In the case of multi-stage stretching, sequential stretching and / or simultaneous stretching are combined.

[0076] The re-stretching temperature is preferably 70° C. or higher and 160° C. or lower. The upper limit is more preferably 150° C., and even more preferably 140° C., from the viewpoint of improving the mechanical strength of the film. The lower limit is more preferably 80° C., and even more preferably 90° C., from the viewpoint of improving the transparency of the film. By re-stretching at a high temperature, voids in the film can be closed, and the transparency of the film can be increased.

[0077] In the case of uniaxial stretching, the re-stretching ratio is preferably more than 1.00 times and not more than 20 times, and particularly preferably more than 1.00 times and not more than 10 times in the TD direction. The lower limit of the re-stretching ratio in the TD direction is more preferably 1.20 times, and even more preferably 1.50 times. In the case of biaxial stretching, it is preferable to stretch at more than 1.00 times and not more than 5.00 times in both the MD and TD directions, and this may be different in the MD and TD directions. The re-stretching ratio varies depending on the stretching ratio in the stretching step described above, but from the viewpoint of preventing film breakage, it is preferable to adjust the re-stretching ratio so that the final stretching ratio (the product of the stretching ratio in the stretching step and the re-stretching ratio in the re-stretching step) is 500.0 times or less in area ratio. The final stretching ratio is more preferably 300.0 times or less, even more preferably 200.0 times or less, and particularly preferably 160.0 times or less. By setting the temperature and ratio of the re-stretching within the above ranges, the crystal orientation is promoted, and the mechanical strength of the film can be improved.

[0078] (f) Other Steps Furthermore, depending on other applications, the polyethylene film may be subjected to a hydrophilization treatment. The hydrophilization treatment may be performed by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer grafting is preferably performed after the crosslinking treatment. The polyethylene film is preferably crosslinked by irradiation with ionizing radiation such as α-rays, β-rays, γ-rays, or electron beams. In the case of electron beam irradiation, the electron beam dose is preferably 0.1 Mrad to 100 Mrad, and the acceleration voltage is preferably 100 kV to 300 kV.

[0079] In the surfactant treatment, any of nonionic, cationic, anionic, and amphoteric surfactants can be used, but nonionic surfactants are preferred. The polyethylene film is immersed in a solution prepared by dissolving the surfactant in water or a lower alcohol such as methanol, ethanol, or isopropyl alcohol, or the solution is applied to the polyethylene film by the doctor blade method.

[0080] In the case of corona discharge, it is preferable to carry out the corona discharge treatment in air, nitrogen, carbon dioxide gas, or a mixture of these gases.

[0081] A metal film can also be applied to at least one side of the polyethylene film. When applying a metal film to a polyethylene film, it is preferable to perform a hydrophilization treatment by corona discharge to improve the adhesion of the vapor-deposited metal. In the present invention, the method for applying the metal film is not particularly limited, but it may be performed using a continuous or batch-type vacuum deposition machine, electrothermal heating, sputtering, ion plating, ion beam, or the like. For example, a method in which aluminum or an alloy of aluminum and zinc is vapor-deposited on at least one side of the polyethylene film to form a metal film is preferably used. In this case, other metal components such as nickel, copper, gold, silver, and chromium can also be vapor-deposited simultaneously with or successively with the aluminum. The thickness of the metal layer is not particularly limited, but is preferably 10 nm to 250 nm.

[0082] The polyethylene film of the present invention obtained as described above can be used for various industrial applications such as packaging films, surface protection films, process films, release films, heat dissipation films, low-temperature films, sliding films, adhesive film substrates, sanitary products, agricultural products, construction products, medical products, and capacitor films, but because it has particularly excellent heat resistance, mechanical strength, quality, and transparency, it can be preferably used as packaging films, surface protection films, process films, release films, heat dissipation films, low-temperature films, and adhesive film substrates. Furthermore, a metal film-laminated film obtained by providing a metal film on at least one side of the polyethylene film of the present invention can be preferably used as a radiant heat reflection film, packaging film, or capacitor film.

[0083] Here, a surface protection film refers to a film that is attached to an object such as a molded body or film and has the function of protecting it from scratches and contamination that occur during processing or transportation. A process film refers to a film that is attached to an object such as a molded body or film to protect it from scratches and contamination that occur during manufacturing or processing, and is discarded when used as a final product. A release film refers to a film that has high releasability and functions as a film that is attached to an object such as a molded body or film to protect it from scratches and contamination that occur during processing or transportation, and can be easily peeled off and discarded when used as a final product. A packaging film refers to a film used for packaging food and various products. A heat dissipation film refers to a film used to diffuse heat generated from heat sources such as electronic components. A low-temperature film refers to a film used at low temperatures below room temperature, such as in frozen packaging, or at extremely low temperatures such as in a liquid nitrogen environment. A pressure-sensitive adhesive film refers to a film made of a base film with an adhesive layer on one or both sides, and is used by being attached to an adherend. Radiant heat reflective film refers to a film used for heat shielding by reflecting radiant heat, and capacitor film refers to a film that is rolled up and used in film capacitors.

[0084] The present invention will be described in detail below with reference to examples. The properties were measured and evaluated by the following methods.

[0085] (1) Film Thickness: Measured using a micro thickness meter (manufactured by Anritsu Corporation). The film was sampled into a 10 cm square, and the thickness was measured at five arbitrarily selected points. The average value of the measured values ​​was taken as the film thickness (μm).

[0086] (2) Tensile Strength and Tensile Elongation A rectangular sample measuring 150 mm in length (measurement direction) x 10 mm in width was cut out from a polyethylene film. The sample was set in a tensile tester (Orientec "Tensilon" (registered trademark) UCT-100) with an initial chuck distance of 50 mm, and a tensile test of the film was performed at room temperature at a tensile speed of 300 mm / min, and the tensile strength and tensile elongation were calculated according to the method specified in JIS K7161 (2014). Measurements were performed five times for each sample, and the average values ​​were used as the tensile strength and tensile elongation of the sample.

[0087] (3) Main orientation direction, direction perpendicular to main orientation When the tensile strength was measured in the film plane, an arbitrary direction was set as 0°, and each direction forming an angle of 0° to 175° in 5° increments with respect to the arbitrary direction was set as the measurement direction, the direction showing the largest value was set as the main orientation direction, and the direction perpendicular to the main orientation direction in the film plane was set as the direction perpendicular to the main orientation direction. The tensile strength was measured by the method described in (2).

[0088] (4) Heat shrinkage when heated at 100°C for 8 hours The heat shrinkage of a film was measured by cutting a 10 cm square film with its sides in the main orientation direction and the direction perpendicular to the main orientation, sandwiching it between 0.09 mm thick paper sheets, and heating it in an oven heated to 100°C for 8 hours. The dimensional change rate of the film in the main orientation direction and the direction perpendicular to the main orientation direction before and after heating was measured. The measurement point for the dimensions was the length of a line connecting the center positions of each of the opposing sides of a 10 cm square film. The above measurement was performed five times at different locations on the same polyethylene film, and the average values ​​were used as the heat shrinkage rate in the main orientation direction and the direction perpendicular to the main orientation direction.

[0089] (5) Internal haze: A haze meter (HGM-2DP) manufactured by Suga Test Instruments Co., Ltd. was used. A sample was cut into a size of 6.0 cm x 3.0 cm, inserted into a quartz cell filled with purified water and having an optical path length of 1 cm, and measured by irradiating light perpendicularly to the sample surface to obtain the internal haze value. The measurement was performed five times, and the average value was taken as the internal haze.

[0090] (6) Melting Point of Polyethylene Film and Polyethylene Resin The melting points of the polyethylene film and polyethylene resin were measured by differential scanning calorimetry (DSC) according to JIS K7121 (2012). A 3.0 mg sample was sealed in an aluminum pan and heated from 25°C to 250°C at a rate of 20°C / min under a nitrogen atmosphere using a differential scanning calorimeter (EXSTAR DSC6220 manufactured by Seiko Instruments, Inc.). The peak temperature of the resulting melting endothermic curve was taken as the melting point of the polyethylene film and polyethylene resin.

[0091] (7) Proportion of the heat of crystalline fusion at 140 ° C or higher to the total heat of crystalline fusion The proportion of the heat of crystalline fusion at 140 ° C or higher of polyethylene film was calculated based on the results of measurement by differential scanning calorimetry (DSC) in accordance with JIS K7121 (2012). 3.0 mg of sample was sealed in an aluminum pan, and the temperature was raised from 25 ° C to 250 ° C at 20 ° C / min under a nitrogen atmosphere using a differential scanning calorimeter (Seiko Instruments EXSTAR DSC6220), to obtain a melting endothermic curve. For the obtained melting endothermic curve during melting, a linear baseline was set in the range of 60 ° C to 200 ° C, and the heat was calculated from the area enclosed by the linear baseline and the endothermic melting curve, and this was converted to a value per sample mass to obtain the total heat of fusion S ａｌｌ In addition, the heat of fusion S at 140°C or higher was calculated from the area enclosed by the linear baseline and the endothermic melting curve, and this was converted to the heat of fusion S at 140°C or higher per sample mass. ≧１４０℃ The total heat of fusion S ａｌｌ and 140℃ or more fusion heat S ≧１４０℃ was substituted into the following formula to determine the proportion S of the heat of crystalline fusion of the polyethylene film at 140°C or higher. The measurement was carried out three times for each sample, and the average value was taken as the proportion of the heat of crystalline fusion of 140°C or higher to the total heat of crystalline fusion of the sample.

[0092] S (%) = S ≧１４０℃ ×100 / S ａｌｌ .

[0093] (8) Gurley value According to JIS P-8117 (2009), the air resistance (sec / 100 cm ) of a polyethylene film was measured using an Oken type air resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). ３ The above measurement was carried out at five different points on the same polyethylene film, and the average value was taken as the Gurley value of the film.

[0094] (9) Weight-average molecular weight of polyethylene film and polyethylene resin The weight-average molecular weight of the polyethylene film and polyethylene raw material was determined by gel permeation chromatography (GPC) under the following conditions: Measuring device: GPC-150C manufactured by Waters Corporation Column: Shodex UT806M manufactured by Showa Denko K.K. Column temperature: 160°C Solvent (mobile phase): 1,2,4-trichlorochlorobenzene Solvent flow rate: 1.0 ml / min Sample concentration: 0.1 wt% (dissolution conditions: 135°C / 1 h) Injection amount: 500 μl Detector: Differential refractometer (RI detector) manufactured by Waters Corporation Calibration curve: Created using a polyethylene conversion factor (0.46) from a calibration curve obtained using a monodisperse polystyrene standard sample.

[0095] (10) Density of Polyethylene Film The polyethylene film was sampled to a size of 10 cm square, and the mass (kg) was measured. Then, the volume (m ３ ) was calculated, and the density of the polyethylene film (kg / m ３ The density (kg / m ３ ) = mass (kg) / volume (m ３ ).

[0096] (11) Specific Heat of Polyethylene Film The specific heat of the polyethylene film was calculated based on the results of measurement by differential scanning calorimetry (DSC) under the following conditions in accordance with JIS K7123 (1987). The measurement was carried out three times for each sample, and the average value was used as the specific heat of the sample. Measuring device: Differential scanning calorimeter DSC8500 manufactured by Perkin-Elmer Heating rate: 10°C / min Standard sample: sapphire (α-Al２ O ３ ) Atmosphere: dry nitrogen gas flow Measurement temperature: 25°C Sample container: aluminum container.

[0097] (12) Thermal Conductivity in the Main Orientation Direction The thermal conductivity in the main orientation direction was measured by an optical alternating current method. A rectangular sample measuring 30 mm in length (main orientation direction) x 5 mm in width was cut out from the polyethylene film, colored with black paint, and used for measurement. The sample was set in a thermal diffusivity measurement device (LaserPIT manufactured by ULVAC-RIKO), and cyclic heating was performed by irradiating the sample with a semiconductor laser in a vacuum. The thermal diffusivity was calculated from the attenuation constant of the temperature wave from the heating position in the main orientation direction. Using the density and specific heat of the polyethylene film measured by the methods described in (10) and (11), the thermal conductivity of the polyethylene film in the main orientation direction was calculated using the following formula. Similar measurements were performed three times, and the average value was used as the thermal conductivity of the sample in the main orientation direction. Thermal conductivity (W / m / K) = thermal diffusivity (m ２ / s) x density (kg / m ３ ) x specific heat (J / kg / K).

[0098] (13) Appearance Evaluation After Long-Term Storage A 500 mm wide polyethylene film was wound into a roll with a length of 200 m to prepare a film roll. The obtained film roll was stored at 50°C for 200 hours. The appearance of the film roll after storage and the flatness when 1 m of the film roll was unwound under free tension (a state in which the film hung vertically due to its own weight) or when a tension of 1 kg / m was applied uniformly and without unevenness across the entire width of the film were visually confirmed and evaluated according to the following criteria.

[0099] S: There were no areas of poor appearance such as wrinkles or slack on the exterior of the film roll. A: Areas of poor appearance were found on the exterior of the film roll, but areas of poor flatness such as wrinkles, slack, and dents disappeared under free tension. B: Areas of poor flatness were found under free tension, but disappeared under a tension of 1 kg / m. C: Areas of poor flatness were found even under a tension of 1 kg / m.

[0100] (Polyethylene Resin, etc.) In producing the polyethylene films of the Examples and Comparative Examples, polyethylene resins having the weight-average molecular weight Mw and melting point Tm shown in Table 1 below were used. These values ​​were evaluated in the form of resin pellets. Four types of polyethylene resin A and five types of polyethylene resin B were used.

[0101] <Polyethylene Resin A> Polyethylene resin A1 (PE A1): ultra-high molecular weight polyethylene, manufactured by Mitsui Chemicals, Inc., "Hi-Zex Million" (registered trademark) 145M Polyethylene resin A2 (PE A2): ultra-high molecular weight polyethylene, manufactured by Asahi Kasei Corporation, "Sunfine" (registered trademark) UH650 Polyethylene resin A3 (PE A3): ultra-high molecular weight polyethylene, manufactured by Mitsui Chemicals, Inc., "Hi-Zex Million" (registered trademark) 240M Polyethylene resin A4 (PE A4): ultra-high molecular weight polyethylene, manufactured by Mitsui Chemicals, Inc., "Hi-Zex Million" (registered trademark) 630M.

[0102] <Polyethylene Resin B> Polyethylene resin B1 (PE B1): high-density polyethylene, "Evolue" (registered trademark) H SP50800P manufactured by Prime Polymer Co., Ltd. Polyethylene resin B2 (PE B2): low-density polyethylene, "Novatec" (registered trademark) LD LF128 manufactured by Japan Polyethylene Co., Ltd. Polyethylene resin B3 (PE B3): high-density polyethylene, "Novatec" (registered trademark) HD ​​HF111K manufactured by Japan Polyethylene Co., Ltd. Polyethylene resin B4 (PE B4): high-density polyethylene, "Sunfine" (registered trademark) SH800 manufactured by Asahi Kasei Corporation Polyethylene resin B5 (PE B5): low-molecular-weight polyethylene, Paraffin Wax-155 manufactured by Nippon Seiro Co., Ltd.

[0103]

[0104] Example 1 20 parts by mass of polyethylene resin A1 was blended with 0.04 parts by mass of BASF's "Irganox" (registered trademark) 1010 as an antioxidant, and the blend was fed into a twin-screw extruder. 80 parts by mass of liquid paraffin (35 cSt (40°C)) as a plasticizer was fed from the side feeder of the twin-screw extruder, and the mixture was melt-kneaded at 180°C to prepare a polyethylene resin solution. The polyethylene resin solution was extruded from the twin-screw extruder, filtered to remove foreign matter, and then fed to a T-die. The sheet-like extrudate was cooled and solidified while being taken up by a cooling roll controlled at 30°C, to obtain a gel-like sheet. The take-up speed was 5 m / min. The obtained gel-like sheet was stretched 9.6 times in the MD direction at 120°C using a roll stretcher, and then stretched 10 times in the TD direction at 120°C using a tenter stretcher. The stretched film was immersed in a methylene chloride bath adjusted to 25°C in a washing tank to remove the liquid paraffin and then air-dried at room temperature. The dried film was then re-stretched in the TD direction by 1.56 times at 120°C using a tenter stretching machine, and then heat-treated at 146°C while providing a 12% relaxation in the TD direction. The film was then cooled to 100°C, and then guided outside the tenter stretching machine. The clips on both widthwise ends were released, and the film edges were slit in a winding process and wound around a core to obtain a polyethylene film with a thickness of 5 μm. The physical properties and evaluation results of the resulting film are shown in Table 2.

[0105] (Examples 2 to 9, Comparative Examples 1 to 3, 5 to 6) Polyethylene films were obtained in the same manner as in Example 1, except that the compositions and film-forming conditions were as shown in Table 2. In this case, the thickness was adjusted by adjusting the discharge rate during extrusion and the speed of the casting drum (the same applies to the other Examples and Comparative Examples below). The physical properties and evaluation results of the obtained films are shown in Table 2.

[0106] Comparative Example 4 A polyethylene film was obtained in the same manner as in Example 1, except that the composition and film-forming conditions were as shown in Table 2 and the film was stretched again without being immersed in a methylene chloride bath after the first stretching.

[0107] (Example 10) A polyethylene film produced under the film-forming conditions described in Example 1 was heated at 25 W·min / m２ The film was then placed in a vacuum deposition apparatus equipped with a film transport device and subjected to a corona discharge treatment at a treatment intensity of 1.00 × 10 －２ After the pressure was reduced to a high pressure of 100 Pa, the film was passed through a cooled metal drum at 20°C, and aluminum metal was heated and evaporated to form a vapor-deposited thin film layer on the film surface (the side in contact with the cooling roll). The vapor-deposited film was controlled to a thickness of approximately 100 nm. After vapor deposition, the pressure inside the vacuum vapor deposition apparatus was returned to normal pressure, yielding a metal film laminated film with a metal film on one side. The metal film laminated film obtained in this manner was wrinkle-free and allowed for uniform vapor deposition.

[0108] (Example 11) A polyethylene film produced under the film-forming conditions described in Example 1 was heated at 25 W·min / m ２ A corona discharge treatment was performed at a treatment strength of 100 μm. Thereafter, an acrylic adhesive (manufactured by Soken Chemical & Engineering Co., Ltd., "SK Dyne" (registered trademark) 1310) was diluted with ethyl acetate, toluene, and methyl ethyl ketone (MEK), and a curing agent (manufactured by Nippon Polyurethane Industry Co., Ltd., "Coronate" (registered trademark) D-90) 2.0 parts by mass was mixed with the adhesive layer coating agent for 100 parts by mass of the adhesive solids content. The coating agent was applied to the film surface (the cooling roll contact surface side) using a gravure coater. Subsequently, the film was introduced into a drying oven at 80 ° C. and conveyed for 30 seconds, and the solvent in the coating agent was removed to obtain an adhesive film with an adhesive layer thickness of 0.7 μm. The adhesive film obtained in this way was wrinkle-free and the adhesive layer could be uniformly coated.

[0109]

[0110] The polyethylene film of the present invention can be used for various industrial applications such as packaging films, surface protection films, process films, heat dissipation films, low-temperature films, sliding films, adhesive film substrates, sanitary products, agricultural products, construction products, medical products, and capacitor films. However, since the polyethylene film of the present invention is particularly excellent in heat resistance, mechanical strength, quality, and transparency, it can be preferably used as surface protection films, process films, release films, packaging films, heat dissipation films, low-temperature films, and adhesive film substrates.

Claims

1. A polyethylene film in which, when heated at 100°C for 8 hours, the sum of the heat shrinkage rates in the main orientation direction and the direction orthogonal to the main orientation direction is -5.0% or more and 10.0% or less, where the direction in which the tensile strength is greatest is the main orientation direction and the direction orthogonal to the main orientation direction is the direction orthogonal to the main orientation direction, the tensile strength in the direction orthogonal to the main orientation is 200 MPa or more and 5000 MPa or less, the internal haze is 0% or more and 80% or less, and the Gurley value measured with an Oken air resistance meter is 1 x 10 4 seconds / 100 cm 3 or more.

2. 2. The polyethylene film according to claim 1, wherein a ratio T1 / T2 of a tensile elongation T1 in the main orientation direction to a tensile elongation T2 in a direction perpendicular to the main orientation direction is 0.10 or more and 10 or less.

3. 3. The polyethylene film according to claim 1, wherein the sum of the tensile elongation in the main orientation direction and the direction perpendicular to the main orientation is 160% or more and 500% or less.

4. 3. The polyethylene film according to claim 1, wherein the proportion of the heat of crystalline fusion at 140°C or higher to the total heat of crystalline fusion in a temperature distribution curve of the heat of crystalline fusion measured by differential scanning calorimetry is 30% or more and 90% or less.

5. 3. The polyethylene film according to claim 1, wherein the weight average molecular weight measured by high-temperature GPC is 500,000 or more and 1,900,000 or less.

6. 3. The polyethylene film according to claim 1, which is mainly composed of polyethylene having a weight-average molecular weight of 500,000 or more and 5,000,000 or less.

7. 3. The polyethylene film according to claim 1, wherein the thermal conductivity in the main orientation direction is 0.7 W / m / K or more.

8. 3. The polyethylene film according to claim 1, having a thickness of 25 μm or less.

9. 3. An adhesive film comprising the polyethylene film according to claim 1 or 2, and an adhesive layer provided on one or both sides thereof.

10. A metal film laminated film having a metal film on at least one surface of the polyethylene film according to claim 1 or 2.

11. A release film comprising the polyethylene film according to claim 1 or 2.

12. A heat dissipation film comprising the polyethylene film according to claim 1 or 2.

13. A low-temperature film comprising the polyethylene film of claim 1 or 2.

14. A polyethylene film roll obtained by winding the polyethylene film according to claim 1 or 2 around a core.

15. A method for producing a polyethylene film as described in claim 1 or 2, characterized in that it includes a heat treatment step of biaxially stretching a sheet containing polyethylene having a weight average molecular weight Mw of 500,000 or more and 5,000,000 or less and a plasticizer, extracting the plasticizer, and then heat treating the sheet.

16. The method for producing a polyethylene film according to claim 15, wherein the temperature in the heat treatment step is 130°C or higher.

17. The method for producing a polyethylene film according to claim 15, further comprising a step of stretching the film at least uniaxially after extracting the plasticizer.