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

[Technical Field]

[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. [Background technology]

[0002] Polyethylene film is widely used in various packaging materials and medical films due to its excellent physical properties such as lightness, moisture resistance, and chemical resistance. In particular, ultra-high molecular weight polyethylene film has superior abrasion resistance and tensile strength compared to general-purpose polyethylene film, and is used as a sliding material and a base material for adhesive films. However, because ultra-high molecular weight polyethylene resin has an extremely high melt viscosity, it is difficult to produce films by conventional extrusion molding or injection molding, and is mainly produced by cutting compression-molded material. However, this method results in insufficient orientation of molecular chains, leading to low mechanical strength, and also has the problem of poor transparency due to the difficulty in forming thin films.

[0003] To address these problems, for example, Patent Document 1 describes an example in which a gel-like sheet obtained by dissolving ultra-high molecular weight polyethylene resin in a solvent is biaxially stretched, and then subjected to pressure treatment after solvent removal to obtain a thin film. Patent Document 2 also describes an example in which a biaxially oriented film is obtained by mixing a specific hydrocarbon-based plasticizer with ultra-high molecular weight polyethylene resin and extruding it. Furthermore, Patent Document 3 describes an example in which a sheet obtained by compression molding ultra-high molecular weight polyethylene resin is stretched at a high magnification in the uniaxial direction to obtain a high-strength film. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 60-228122 [Patent Document 2] Japanese Patent Application Publication No. 6-262679 [Patent Document 3] Japanese Patent Publication No. 2014-111384 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, the polyethylene film described in Patent Document 1, while having high strength due to high-magnification stretching, suffers from insufficient relaxation of the molecular chains of high molecular weight components, resulting in problems with heat resistance, such as thermal shrinkage when used at high temperatures. The polyethylene film described in Patent Document 2 has insufficient mechanical strength due to the addition of plasticizers, and also suffers from a decrease in quality due to plasticizer bleed-out during long-term use. Furthermore, the polyethylene film described in Patent Document 3, while having high strength due to high-magnification stretching, is uniaxially stretched, resulting in problems 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 with excellent heat resistance, mechanical strength, quality, long-term storage properties, and transparency. [Means for solving the problem]

[0006] To solve the above-mentioned problems, the polyethylene film of the present invention has the following configuration. Specifically, the polyethylene film of the present invention is a polyethylene film in which, when the direction with the greatest tensile strength is defined as the principal orientation direction and the direction perpendicular to the principal orientation direction within the film plane is defined as the orthogonal direction to the principal orientation, the sum of the thermal shrinkage rate in the principal orientation direction and the thermal shrinkage rate in the orthogonal direction to the principal orientation when heated at 100°C for 8 hours is -5.0% or more and 10.0% or less, the tensile strength in the orthogonal direction to the principal orientation is 200 MPa or more and 5000 MPa or less, and the internal haze is 0% or more and 80% or less.

[0007] Furthermore, the following manufacturing method is provided as a method for producing polyethylene film according to the present invention. Specifically, the method for producing polyethylene film according to the present invention is a method for producing polyethylene film having an internal haze of 0% or more and 80% or less, and is characterized by including a heat treatment step of biaxially stretching a sheet containing polyethylene with a weight-average molecular weight Mw of 500,000 or more and a plasticizer, and then heat-treating the sheet after extracting the plasticizer. [Effects of the Invention]

[0008] The present invention provides a polyethylene film with excellent heat resistance, mechanical strength, quality, long-term storage properties, and transparency. Due to its superior properties, the polyolefin film of the present invention can be widely and suitably used as a film for industrial materials, a surface protection film, a process film, a release film, a heat dissipation film, a low-temperature film, a base material for adhesive films, a sliding film, a medical film, and a film for capacitors. [Modes for carrying out the invention]

[0009] The polyethylene film of the present invention will now be described. The polyethylene film of the present invention is a polyethylene film in which, when the direction with the greatest tensile strength is defined as the principal orientation direction and the direction perpendicular to the principal orientation direction within the film plane is defined as the orthogonal direction to the principal orientation, the sum of the thermal shrinkage rate in the principal orientation direction and the thermal shrinkage rate in the orthogonal direction to the principal orientation when heated at 100°C for 8 hours is -5.0% or more and 10.0% or less, the tensile strength in the orthogonal direction to the principal orientation is 200 MPa or more and 5000 MPa or less, and the internal haze is 0% or more and 80% or less.

[0010] A polyethylene film refers to a film that contains polyethylene resin in an amount exceeding 50% by mass and not exceeding 100% by mass when the total components constituting the film are 100% by mass. The content of the polyethylene resin in the polyethylene film is preferably 70% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less, still more preferably 95% by mass or more and 100% by mass or less, particularly preferably 96% by mass or more and 100% by mass or less, and most preferably 97% by mass or more and 100% by mass or less when the total components constituting the film are 100% by mass. When there are a plurality of components corresponding to the polyethylene resin, if the total of these components exceeds 50% by mass and does not exceed 100% by mass, it is considered to correspond to a polyethylene film. The polyethylene resin refers to a resin in which the ethylene unit occupies more than 50 mol% and not more than 100 mol% of all the constituent units constituting the resin.

[0011] From the viewpoint of improving heat resistance, for the polyethylene film of the present invention, when the direction in which the tensile strength is the largest is defined as the main orientation direction and the direction orthogonal to the main orientation direction within the film plane is defined as the main orientation orthogonal direction, it is important that the sum of the heat shrinkage rate in the main orientation direction and the heat shrinkage rate in the main orientation orthogonal direction when heated at 100°C for 8 hours is -5.0% or more and 10.0% or less. The upper limit of the sum of the heat shrinkage rate in the main orientation direction and the heat shrinkage rate in the main orientation orthogonal direction when heated at 100°C for 8 hours is preferably 8.0%, more preferably 6.0%, and still more preferably 4.0%. The lower limit of the sum of the heat shrinkage rate in the main orientation direction and the heat shrinkage rate in the main orientation orthogonal direction when heated at 100°C for 8 hours may be such that the film expands, and is preferably -2.0%, more preferably -1.0%, and still more preferably 0.0%. Here, a heat shrinkage rate 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 the main orientation direction and the main orientation orthogonal direction, or expands in one of the main orientation direction and the main orientation orthogonal direction and the degree of expansion is larger than the degree of shrinkage in the other direction, the "sum of the heat shrinkage rate in the main orientation direction and the heat shrinkage rate in the main orientation orthogonal direction" becomes a negative value.

[0012] Here, the main orientation direction in the present invention means that, in the film plane, with an arbitrary direction taken as 0°, when the tensile strength is measured in each direction forming an angle of 0° to 175° in 5° increments with respect to the arbitrary direction, the direction showing the largest value is referred to. The main orientation orthogonal direction means the direction orthogonal to the main orientation direction in the film plane. The tensile strength can be measured by a tensile testing machine in accordance with the method specified in JIS K7161 (2014), and the details of the measurement method are shown in the examples.

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

[0014] By setting 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 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 during storage by winding such a polyethylene film into a roll, it is possible to reduce the deterioration in quality, such as shrinkage and expansion of the polyethylene film and the formation of wrinkles on the roll.

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

[0016] The polyethylene film of the present invention has a tensile strength of 200 MPa or more and 5000 MPa or less in the direction orthogonal to the main orientation, from the viewpoint of increasing mechanical strength. 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, and 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 thinned or 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 orthogonal to the principal 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 stretching ratio.

[0018] While it is common to increase tensile strength by stretching at a higher ratio, increasing the stretch ratio can lead to increased thermal shrinkage, making it conventionally difficult to achieve both low thermal shrinkage and high tensile strength in polyethylene films. However, by using a method that sets the composition of the polyethylene film within the range described below, and the film-forming conditions within the range described below, it is possible to achieve both low thermal shrinkage and high tensile strength. In particular, to achieve both low thermal shrinkage and high tensile strength, it is effective to set the molecular weight of the film within the range described below and to mitigate film shrinkage at high temperatures during the heat treatment process.

[0019] The polyethylene film of the present invention has an internal haze of 0% to 80% 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% to 80%, the visibility after film application can be improved when the polyethylene film is used as an adhesive film. Furthermore, the 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 achieve an internal haze of 0% to 80%, for example, a method can be used in which the composition of the polyethylene film is within the range described later, and the film-forming conditions are within the range described later. In particular, closing voids in the film by performing heat treatment / re-stretching at high temperatures is effective.

[0021] The polyethylene film of the present invention preferably has a ratio T1 / T2 of tensile elongation T1 in the principal orientation direction to tensile elongation T2 in the direction perpendicular to the principal orientation direction of 0.10 or more and 10 or less (hereinafter, the ratio of tensile elongation T1 in the principal orientation direction to tensile elongation T2 in the direction perpendicular to the principal orientation direction may simply be 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, making it less prone to tearing even when the film is thinned or used under high tension, and thus suitable for use as a process film.

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

[0023] The polyethylene film of the present invention preferably has a sum of tensile elongation in the principal orientation direction and the direction perpendicular to the principal orientation of 160% or more and 500% or less. The lower limit of the sum of tensile elongation in the principal orientation direction and the direction perpendicular to the principal orientation 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 tensile elongation in the principal orientation direction and the direction perpendicular to the principal orientation 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 testing machine in accordance with the method specified in JIS K7161 (2014), and details of the measurement method are shown in the examples.

[0024] To achieve a sum of tensile elongation in the principal orientation direction and the direction perpendicular to the principal orientation of 160% or more and 500% or less, for example, one can use a method in which the composition of the polyethylene film is within the range described later, and the film formation conditions are within the range described later. In particular, it is effective to set the stretch ratios in the MD direction and TD direction within the range described later.

[0025] The polyethylene film of the present invention preferably has a thickness of 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-forming ability, it is substantially about 0.1 μm. By making the polyethylene film thickness 25 μm or less, the conformability to the adherend can be improved when used as a release film or protective film. Furthermore, volume reduction can be achieved when used as a packaging film.

[0026] To achieve a polyethylene film thickness of 25 μm or less, the polyethylene film formation conditions can be adjusted to the range described below. In particular, it is effective to set the stretching ratios in the MD and TD directions to the range described below. Furthermore, the screw rotation speed of the extruder, the width of the unstretched sheet, the film formation speed, and the stretching ratio can be adjusted within a range that does not degrade other physical properties. The thickness of the polyethylene film can be measured with a known micro-thickness gauge, 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 ratio of the amount of heat of fusion at 140°C or higher to the total amount of heat of fusion, as measured by differential scanning calorimetry and shown in the temperature distribution curve of the amount of heat of fusion at 140°C or higher, which is 30% to 90%. The lower limit of the ratio of the amount of heat of fusion at 140°C or higher to the total amount of heat of fusion is more preferably 40%, even more preferably 50%, and particularly preferably 60%. The ratio of the amount of heat of fusion at 140°C or higher to the total amount of heat of fusion corresponds to the ratio of structures 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, and it can be suitably used as a process film. The ratio of the amount of heat of fusion at 140°C or higher to the total amount of heat of fusion can be measured by differential scanning calorimetry (DSC) based on JIS K7121 (2012), and details of the measurement method are shown in the examples.

[0028] In order to make the ratio of the crystal melting heat quantity at 140°C or higher to the total crystal melting heat quantity be 30% or more and 90% or less, for example, a method can be used in which the composition of the polyethylene film is within the range described later and the film-forming conditions are within the range described later. In particular, it is effective to set the molecular weight of the film within the range described later and stretch it at a high draw ratio.

[0029] From the viewpoint of enhancing mechanical strength and transparency, the Gurley value measured by the Ogawa-type air permeability meter of the polyethylene film of the present invention is 1×10 4 seconds / 100 cm 3 or more, preferably. The Gurley value is more preferably 5×10 4 seconds / 100 cm 3 or more, still more preferably 7×10 4 seconds / 100 cm 3 or more, particularly preferably 1×10 5 seconds / 100 cm 3 or more. Hereinafter, the Gurley value measured by the Ogawa-type air permeability meter may be simply referred to as the "Gurley value".

[0030] By setting the Gurley value to 1×10 4 seconds / 100 cm 3 or more, the generation of voids penetrating in the thickness direction of the film can be suppressed, and the mechanical strength and transparency can be enhanced. The upper limit of the Gurley value is not particularly limited, but it is about 1×10 6 seconds / 100 cm 3 in terms of measurement. The Gurley value can be measured by an Ogawa-type air permeability meter in accordance with JIS P-8117(2009), and the details of the measurement method are shown in the examples.

[0031] In order to make the Gurley value 1×10 4 seconds / 100 cm 3 or more, for example, a method can be used in which the composition of the polyethylene film is within the range described later and the film-forming conditions are within the range described later. In particular, it is effective to block the voids in the film by performing heat treatment / re-drawing 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 "weight-average molecular weight of the film") of 500,000 to 1,900,000, measured using high-temperature GPC. The upper limit of the weight-average molecular weight of the film 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 weight-average molecular weight of the film to 1,900,000 or less, high molecular weight components that are difficult to relax can be suppressed, and the heat resistance of the film can be improved. Furthermore, by setting the weight-average molecular weight of the film to 500,000 or more, the mechanical strength of the film can be improved. The weight-average molecular weight of the film can be measured by high-temperature GPC, and details of the measurement method are shown in the examples.

[0033] To achieve a film weight-average molecular weight of 500,000 to 1,900,000, 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 polyethylene resin within the range described below.

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

[0035] By setting the thermal conductivity in the main orientation direction to 0.7 W / m / K or higher, the orientation of the molecular chains in the main orientation direction of the film can be enhanced, thereby increasing its mechanical strength. Furthermore, when used as a heat dissipation film, the diffusion of heat generated from a heat source can be improved. There is no particular upper limit to the thermal conductivity in the main orientation direction, but it is practically around 5000 W / m / K, preferably around 100 W / m / K, and more preferably around 30 W / m / K. The thermal conductivity in the main orientation direction can be measured by the optical AC 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 higher 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 later, and the film-forming conditions are within the range described later. In particular, it is effective to set the molecular weight of the film within the range described later, and to stretch it at a high stretching ratio after extracting the plasticizer.

[0037] The following describes the ultra-high molecular weight polyethylene resin (sometimes referred to as polyethylene resin A) that is suitable as the most abundant component in the polyethylene film of the present invention. Here, polyethylene resin A (ultra-high molecular weight polyethylene) is polyethylene with a weight-average molecular weight Mw of 500,000 or more, and examples include "Hyzex Million" (registered trademark) manufactured by Mitsui Chemicals, Inc. and "Sunfine" (registered trademark) manufactured by Asahi Kasei Corporation. If multiple types of polyethylene resins that meet the above requirements are included, polyethylene resin A can be considered to be the most abundant component if the total content of these components exceeds 50% by mass of the entire film.

[0038] The weight-average molecular weight Mw of polyethylene resin A is preferably between 500,000 and 5,000,000, from the viewpoint of achieving both heat resistance and mechanical strength of the film. The upper limit of Mw for polyethylene resin A is more preferably 2,000,000, even more preferably 1,800,000, and particularly preferably 1,500,000, while 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 to 150°C, from the viewpoint of achieving both heat resistance and film-forming properties 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, which will be described later.

[0040] Polyethylene resin A may contain copolymer components with other unsaturated hydrocarbons, etc., to the extent that it does not impair the objectives of the present invention. Examples of monomer components that constitute 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 it becomes a polyethylene film, the copolymerization amount is preferably less than 10 mol%, and more preferably 5 mol% or less, when the total constituent units of polyethylene resin A are set to 100 mol%. Here, the copolymerization amount of polyethylene resin A is calculated for the entire polyethylene resin A contained in the film. That is, not only when the film consists only of polyethylene resin A containing less than 10 mol% copolymerization components, but also when the film contains polyethylene resin A containing 10 mol% or more copolymerization components, if the copolymerization amount of the film as a whole is less than 10 mol%, the copolymerization amount can be considered to be less than 10 mol% when the total constituent units of polyethylene resin A are set to 100 mol%. The copolymerization amount of polyethylene resin B, which will be described later, is the same.

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

[0043] Here, high-density polyethylene refers to a polyethylene with a density of 0.930 g / cm³. 3 The polyethylenes mentioned above include, for example, "Hyzex" (registered trademark) manufactured by Prime Polymer Co., Ltd., "Evolu" (registered trademark) H manufactured by Prime Polymer Co., Ltd., "Suntec" (registered trademark) HD ​​manufactured by Asahi Kasei Corporation, and "Novatec" (registered trademark) HD ​​manufactured by Nippon Polyethylene Co., Ltd.

[0044] Low-density polyethylene has a density of 0.910 g / cm³. 3 More than 0.930g / cm 3 Polyethylene less than 100 lb is used, such as Asahi Kasei Corporation's "Suntec" (registered trademark) LD, Nippon Polyethylene Co., Ltd.'s "Novatec" (registered trademark) LD, etc.

[0045] Ultra-low density polyethylene has a density of 0.910 g / cm³. 3 It is polyethylene of a certain weight or less, and for example, "LUMITAC" (registered trademark) manufactured by Tosoh Corporation can be used.

[0046] Linear low-density polyethylene refers to polyethylene produced by catalytic polymerization, and examples include "Evolu" (registered trademark) manufactured by Prime Polymer Co., Ltd. and "Novatec" (registered trademark) LL manufactured by Nippon Polyethylene Co., Ltd.

[0047] Low molecular weight polyethylene refers to polyethylene with a weight-average molecular weight of less than 100,000, and products such as "Highwax" (registered trademark) manufactured by Mitsui Chemicals, Inc. and "Sunwax" manufactured by Sanyo Chemical Industries, Ltd. can be used.

[0048] Polyethylene is classified according to its density, manufacturing method, molecular weight, etc., and may fall into two or more categories. However, in this invention, polyethylene resin A is defined as having a weight-average molecular weight of 500,000 or more, and polyethylene resin B is defined as having a weight-average molecular weight of less than 500,000.

[0049] The weight-average molecular weight Mw of polyethylene resin B is preferably 1,000 or more and less than 500,000, from the viewpoint of improving the transparency of the film. 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 improving the transparency of the film, the melting point of polyethylene resin B is preferably 90°C to 140°C. From the above viewpoint, 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 copolymer components of other unsaturated hydrocarbons, etc., to the extent that it does not impair the objectives 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. 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 the total constituent units constituting the low molecular weight polyethylene resin are set to 100 mol%.

[0052] The polyethylene film of the present invention may contain resins other than polyethylene, to the extent that it does not impair the objectives of the present invention. Examples of such resins other than polyethylene include polypropylene, polymethylpentene, polybutene, olefin-based thermoplastic elastomers, polystyrene, polyvinylidene fluoride, polyethylene oxide, and polyester. When resins other than polyethylene are included, their content 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, from the viewpoint of mechanical strength when it is made into a polyethylene film.

[0053] The polyethylene film of the present invention may also contain various additives, such as nucleating agents, antioxidants, heat stabilizers, lubricants, antistatic agents, antiblocking agents, fillers, viscosity modifiers, and color inhibitors, as long as they do not impair the objectives 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 its thermal history. Specifically, such antioxidants are preferably phenolic types with steric hindrance, and at least one of them is preferably a high molecular weight type with a molecular weight of 500 or more. Various specific examples can be given, but it is preferable to use one or more selected from 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene (e.g., BASF's "Irganox"® 1330: molecular weight 775.2) or tetrakis[methylene-3(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (e.g., BASF's "Irganox"® 1010: molecular weight 1177.7) along with 2,6-di-t-butyl-p-cresol (BHT: molecular weight 220.4).

[0054] The total content of these antioxidants is preferably in the range of 0.01 parts by mass to 1.0 part by mass per 100 parts by mass of the total polyethylene resin. By using 0.01 parts by mass or more of antioxidants, discoloration of the film due to polymer degradation during the extrusion process can be suppressed and long-term heat resistance can be improved. Furthermore, by using 1.0 part by mass or less of antioxidants, bleed-out of the antioxidants can be suppressed and the transparency of the polyethylene film can be improved. From the above viewpoint, the lower limit of the antioxidant content is more preferably 0.05 parts by mass, even more preferably 0.1 parts by mass per 100 parts by mass of the total polyethylene resin, and the upper limit is more preferably 0.9 parts by mass, and even more preferably 0.8 parts 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 for inorganic particles, which can cause inorganic particles to detach from the film during the manufacturing process, contaminating the production line and products. Furthermore, if coarse protrusions are formed by highly rigid inorganic particles, these may transfer unevenness to the resin layer of optical components when used as a protective film or process film. Therefore, this can lead to a decrease in quality when used as a protective film or manufacturing base film for products requiring high quality, such as display components. From this perspective, it is preferable that the polyethylene film of the present invention also does not contain lubricants such as organic particles.

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

[0057] The polyethylene film of the present invention is not particularly limited in its layer structure and can take the form of either a single layer or a laminated structure.

[0058] The polyethylene film of the present invention may contain only one polyethylene resin layer, or it may contain two or more polyethylene resin layers. A polyethylene resin layer is defined as a layer in which polyethylene resin is present in an amount greater than 50% by mass and less than or equal to 100% by mass, when the total components constituting the layer are considered to be 100% by mass. In this case, if the layer contains two or more components equivalent to polyethylene resin, the layer shall be considered a "layer mainly composed of polyethylene resin" if the sum of these components is greater than 50% by mass and less than or equal to 100% by mass.

[0059] The polyethylene resin content in the "layer mainly composed of polyethylene resin" is more preferably 90% to 100% by mass, even more preferably 95% to 100% by mass, even more preferably 96% to 100% by mass, particularly preferably 97% to 100% by mass, and most preferably 98% to 100% by mass, when the total components constituting the layer are considered as 100% by mass. Note that if the polyethylene film of the present invention has a single-layer structure, the main component of the polyethylene film itself is polyethylene resin.

[0060] The polyethylene film of the present invention is preferably produced by biaxial stretching using the resin described above. The biaxial stretching method may be any of the following: simultaneous inflation biaxial stretching, simultaneous tenter biaxial stretching, or sequential biaxial stretching using a roll stretcher and a tenter stretcher. However, among these, simultaneous tenter biaxial stretching or sequential biaxial stretching is preferred in terms of controlling film formation stability, thickness uniformity, and the high rigidity and dimensional stability of the resulting polyethylene film, and it is preferable to have all of the following steps (a) to (e). It is particularly important to include step (e), in which heat treatment / re-stretching is performed after the extraction of the plasticizer. An example of a method for producing polyethylene film using the above raw materials is described below, but the method for producing polyethylene film of the present invention is not necessarily limited thereto. (a) A polyethylene resin solution is prepared by kneading and dissolving polymer materials containing polyethylene alone, polyethylene mixtures, polyethylene solvent (plasticizer) mixtures, additives, and polyethylene kneaded products. (b) Extrude the molten material, mold it into a sheet, and cool and solidify it. (c) The obtained sheet is subjected to sequential biaxial stretching or simultaneous biaxial stretching using a roll stretcher and / or a tenter stretcher. (d) After that, the plasticizer is extracted from the stretched film and the film is dried. (e) Next, heat treatment / re-stretching is performed. The following describes each step.

[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 as the polyethylene resin. The plasticizer is not particularly limited as long as it can sufficiently dissolve the polyethylene resin, but it is preferable that it be liquid at room temperature in order to enable relatively high-magnification stretching. Examples of plasticizers include aliphatic, cyclic aliphatic, 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, it is preferable to use a non-volatile plasticizer such as liquid paraffin in order to obtain a gel-like sheet with a stable plasticizer content. Here, a gel-like sheet refers to a sheet-like molded article containing a plasticizer that is liquid at room temperature. Note that a plasticizer that is miscible with polyethylene in a molten-kneaded state but is solid at room temperature may be mixed with a plasticizer that is liquid at room temperature. Examples of solid plasticizers at room temperature include stearyl alcohol, ceryl alcohol, and paraffin wax. However, using only such plasticizers may result in uneven stretching.

[0062] The blending ratio of polyethylene resin to plasticizer is set with the total of polyethylene resin and plasticizer at 100% by mass. The polyethylene resin content can be appropriately selected within a range that does not impair moldability, but 5% to 90% by mass is preferred. 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. The lower limit is more preferably 7% by mass, even more preferably 10% by mass, and particularly preferably 15% by mass. By using 5% by mass or more of polyethylene resin (and 95% by mass or less of plasticizer), swell and neck-in at the die exit can be suppressed when forming into a sheet, improving the moldability of the sheet and enhancing film-forming properties. On the other hand, by using 90% by mass or less of polyethylene resin (and 10% by mass or more of plasticizer), shrinkage in the thickness direction can be suppressed, improving moldability.

[0063] The viscosity of the liquid plasticizer at room temperature is preferably between 20 cSt and 200 cSt at 40°C. A viscosity of 20 cSt or higher at 40°C reduces the likelihood of unevenness in the polyethylene resin solution extruded from the nozzle. On the other hand, a viscosity of 200 cSt or lower facilitates the removal of the plasticizer. The viscosity of the liquid plasticizer at room temperature is measured at 40°C using an Ubbelohde viscometer.

[0064] (b) Formation of extruded products and sheet formation The method for uniformly melting and kneading the polyethylene resin solution is not particularly limited, but for example, if a high-concentration polyethylene resin solution is to be prepared, it is preferable to do so in a twin-screw extruder. If necessary, various additives such as antioxidants may be added, as long as they do not impair the effects of the present invention. It is particularly preferable to add antioxidants 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 completely melts. The melt-mixing temperature varies depending on the polyethylene resin used, but it is preferably between (melting point of polyethylene resin + 10°C) and (melting point of polyethylene resin + 120°C). Specifically, the melt-mixing temperature is preferably between 140°C and 260°C, with the upper limit being more preferably 230°C and even more preferably 210°C. The lower limit of the melt-mixing 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 preferable. 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, the deposition of decomposition products on chill rolls and rolls in the stretching process can be suppressed, thereby preventing deterioration of the film's appearance. On the other hand, by setting the melt-kneading temperature to 140°C or higher, unmelted material in the extruded product from the die can be suppressed, preventing film breakage in the subsequent stretching process. After kneading within the above temperature range, it is preferable to remove foreign matter and modified polymer using a filter.

[0067] Next, by cooling the obtained extruded material, a sheet containing polyethylene and a plasticizer is obtained. Cooling can fix the gel structure of the polyethylene resin containing the plasticizer. The cooling temperature is preferably between 10°C and 50°C. By setting the cooling temperature within the above preferred range, the gel structure is refined, making it easier to perform uniform stretching in the subsequent stretching process.

[0068] Cooling methods include direct contact with cold air, cooling water, or other cooling media; contact with a roll cooled by a refrigerant; and the use of a casting drum.

[0069] (c) Stretching process Next, the obtained sheet is stretched. Examples of stretching methods include MD uniaxial stretching using a roll stretcher, TD uniaxial stretching using a tenter stretcher, sequential biaxial stretching using a combination of a roll stretcher and a tenter stretcher, or two tenter stretchers, and simultaneous biaxial stretching using a simultaneous biaxial tenter stretcher. However, biaxial stretching is preferred in terms of controlling film formation stability, thickness uniformity, and the high rigidity and dimensional stability of the resulting polyethylene film. The stretching ratio varies depending on the sheet thickness, but from the viewpoint of film thickness uniformity, it is preferable to set it at 5.0 times or more in any direction. In terms of area ratio, 25.0 times or more is preferable, 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 ensures sufficient film uniformity but also improves the mechanical strength of the film because less unstretched portion remains. Furthermore, the area magnification ratio is preferably 150.0 times or less, more preferably 120.0 times or less, and even more preferably 100.0 times or less. Setting the area magnification ratio to 150.0 times or less reduces film breakage during manufacturing.

[0070] The stretching temperature in each direction is preferably (melting point of the sheet + 10°C) or lower, and specifically, 90°C to 130°C is preferred. 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 the uniformity of the film thickness.

[0071] (d) Plasticizer extraction (washing) and drying process Next, any 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-like fluorocarbons such as trifluorinated ethane. These washing solvents can be appropriately selected depending on the plasticizer and can be used individually or in mixtures.

[0072] The cleaning method can be carried out by immersing the stretched sheet in a cleaning solvent, showering the stretched sheet with the cleaning solvent, or a combination of these methods. The cleaning temperature is preferably between 15°C and 30°C.

[0073] Subsequently, the cleaning solvent in the polyethylene film is dried and removed during the drying process. There are no particular limitations on the drying method; methods such as using a metal heating roll or using hot air can be selected.

[0074] (e) Heat treatment / re-stretching process It is important to heat-treat a dried polyethylene film by tensing both ends in the width direction with clips and then relaxing it in the width direction. From the viewpoint of the heat resistance of the film, a relaxation rate of 5.0% to 25% is preferable. The upper limit of the relaxation rate is more preferably 20%, and even more preferably 18%. The lower limit is more preferably 8.0%, even more preferably 10%, and especially preferably 11%, considering the appearance during long-term storage. Furthermore, the heat treatment temperature is preferably 130°C or higher from the viewpoint of closing voids in the film and improving the transparency of the film. From the above viewpoint, 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 keeping the relaxation rate and heat treatment temperature within the above ranges, residual stress in the film can be relieved and the thermal shrinkage rate can be reduced. After heat treatment, it is preferable to continue holding both ends in the width direction with clips, and then guide the film to the outside of the tenter and release the clips at both ends in the width direction, after a cooling process at 50°C to less than 130°C. The upper limit of the temperature in the cooling process is more preferably 120°C, and even more preferably 110°C. Next, the film edge is slit in the winding process and the polyethylene film is wound into a roll. The heat treatment may be performed using a method that applies heat and pressure uniformly in the thickness direction of the film, such as a roll press or a belt press.

[0075] If necessary, it is preferable to stretch (re-stretch) the film at least uniaxially after the plasticizer extraction (washing) and drying process. If re-stretching is performed, heat treatment should be carried out after re-stretching. Re-stretching can be performed using a tenter stretcher or the like while heating the polyethylene film, similar to the stretching described above. Re-stretching may be uniaxial or biaxial. In the case of multi-stage stretching, it should be performed by combining sequential stretching and / or simultaneous stretching methods.

[0076] The re-stretching temperature is preferably between 70°C and 160°C. 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. Re-stretching at a high temperature can close voids in the film, thereby increasing its transparency.

[0077] For uniaxial stretching, the re-stretching ratio is preferably greater than 1.00 and less than or equal to 20, and particularly preferably greater than 1.00 and less than or equal to 10 in the TD direction. The lower limit of the re-stretching ratio in the TD direction is more preferably 1.20, and even more preferably 1.50. For biaxial stretching, it is preferable to stretch in the MD direction and the TD direction, respectively, with a ratio greater than 1.00 and less than or equal to 5.00, although these ratios may differ between the MD and TD directions. The re-stretching ratio varies depending on the stretching ratio in the stretching process described above, but from the viewpoint of suppressing film breakage, it is preferable to adjust the re-stretching ratio so that the final stretching ratio (product of the stretching ratio in the stretching process and the re-stretching ratio in the re-stretching process) is 500.0 or less in terms of area ratio. The final stretching ratio is more preferably 300.0 or less, even more preferably 200.0 or less, and particularly preferably 160.0 or less. By keeping the re-stretching temperature and magnification within the above-mentioned range, crystal orientation can be promoted, improving the mechanical strength of the film.

[0078] (f) Other processes Furthermore, depending on the application, the polyethylene film can be subjected to hydrophilic treatment. Hydrophilic treatment can be carried out by monomer grafting, surfactant treatment, corona discharge, etc. It is preferable to perform monomer grafting after crosslinking treatment. It is preferable to perform crosslinking treatment on the polyethylene film by irradiation with ionizing radiation such as alpha rays, beta rays, gamma rays, or electron beams. In the case of electron beam irradiation, an electron dose of 0.1 Mrad or more and 100 Mrad or less is preferable, and an acceleration voltage of 100 kV or more and 300 kV or less is preferable.

[0079] In the case of surfactant treatment, nonionic surfactants, cationic surfactants, anionic surfactants, or amphoteric surfactants can be used, but nonionic surfactants are preferred. The polyethylene film is immersed in a solution obtained 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 perform the corona discharge treatment in air, nitrogen, carbon dioxide, or a mixture of these gases.

[0081] Furthermore, a metal film can 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 hydrophilic treatment by corona discharge to improve the adhesion of the deposited metal. In the present invention, the method of applying the metal film is not particularly limited, but it may be done by electric heating, sputtering, ion plating, ion beam, etc., using a continuous or batch type vacuum deposition machine. For example, a preferred method is to deposit aluminum or an alloy of aluminum and zinc onto at least one side of the polyethylene film to provide a metal film. At this time, other metal components such as nickel, copper, gold, silver, and chromium can be deposited simultaneously or sequentially with the aluminum. The thickness of the metal layer is not particularly limited, but it is preferably 10 nm to 250 nm.

[0082] The polyethylene film of the present invention obtained as described above can be used in a variety of 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, building materials, medical products, and capacitor films. In particular, it is excellent in heat resistance, mechanical strength, quality, and transparency, and is therefore preferably used as a packaging film, surface protection film, process film, release film, heat dissipation film, low-temperature film, and adhesive film substrate. Furthermore, a metal film laminated film obtained by applying a metal film to at least one side of the polyethylene film of the present invention can be preferably used as a radiant heat reflective film, packaging film, and capacitor film.

[0083] Here, surface protection film refers to a film that is applied to an object such as a molded body or film to prevent scratches and contamination that occur during processing or transportation. Process film refers to a film that is applied to an object such as a molded body or film to prevent scratches and contamination that occur during manufacturing or processing, and is discarded when the final product is used. Release film refers to a film that has high release properties, is applied to an object such as a molded body or film to prevent scratches and contamination that occur during processing or transportation, and can be easily peeled off and discarded when the final product is used. Packaging film refers to a film used to package food and various other products. Heat dissipation film refers to a film used to dissipate heat generated from heat sources such as electronic components. 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. Adhesive film refers to a film in which an adhesive layer is provided on one or both sides of a base film, and is used by being applied to an object. Radiant heat reflective film refers to a film used for heat shielding purposes by reflecting radiant heat. Capacitor film, on the other hand, refers to a film that is wound and used in film capacitors. [Examples]

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

[0085] (1) Film thickness The thickness was measured using a micro-thickness gauge (manufactured by Anritsu Corporation). The film was sampled in 10cm square sections, and the thickness was measured at five arbitrarily selected points. The average value of the obtained measurements was defined as the film thickness (μm).

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

[0087] (3) Main orientation direction, direction perpendicular to the main orientation Within the film surface, the tensile strength was measured in each direction that forms an angle from 0° to 175° in 5° increments relative to any given direction, with the arbitrary direction set as 0°. The direction showing the largest value was defined as the principal orientation direction, and the direction perpendicular to the principal orientation direction within the film surface was defined as the orthogonal direction to the principal orientation. The tensile strength was measured using the method described in (2).

[0088] (4) Heat shrinkage rate when heated at 100°C for 8 hours The thermal shrinkage rate of the film was measured by cutting a 10cm square piece of film, with its sides aligned with the film's principal orientation direction and the direction perpendicular to the principal orientation, sandwiching it between 0.09mm thick paper, and heating it in an oven preheated to 100°C for 8 hours. The percentage change in the dimensions of the film in the principal orientation direction and the direction perpendicular to the principal orientation before and after heating was measured. For the dimensions, the measurement point was the length of the line connecting the centers of opposite sides of the 10cm square film. The above measurement was performed five times at different locations within the same polyethylene film, and the average value was taken as the thermal shrinkage rate in the principal orientation direction and the direction perpendicular to the principal orientation.

[0089] (5) Internal haze A haze meter (HGM-2DP) manufactured by Suga Test Instruments Co., Ltd. was used. A sample measuring 6.0 cm × 3.0 cm was cut and inserted into a quartz cell with a path length of 1 cm filled with purified water. Light was incident perpendicularly to the sample surface to measure the internal haze value. Five measurements were taken, and the average value was taken as the internal haze value.

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

[0091] (7) The proportion of the heat of fusion of crystals at temperatures above 140°C to the total heat of fusion of crystals The percentage of the heat of fusion of polyethylene film at temperatures above 140°C was calculated based on the results obtained by differential scanning calorimetry (DSC) analysis according to JIS K7121 (2012). A 3.0 mg sample was sealed in an aluminum pan, and the temperature was increased from 25°C to 250°C at a rate of 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, a linear baseline was set in the range of 60°C to 200°C, and the heat amount was calculated from the area enclosed by the linear baseline and the endothermic melting curve. This was then converted to the total melting heat S per sample mass. all The following was calculated. In addition, the amount of heat was calculated from the area enclosed by the linear baseline and the endothermic melting curve above 140°C, and this was converted to the amount of heat of fusion S above 140°C per sample mass. ≧140℃ The total heat of fusion S was calculated. all and heat of fusion S at temperatures above 140℃ ≧140℃ The following formula was used to determine the percentage S of the heat of fusion (CFL) at temperatures above 140°C for polyethylene film. Each sample was measured three times, and the average value was taken as the percentage of the heat of fusion at temperatures above 140°C relative to the total heat of fusion for that sample.

[0092] S(%)=S ≧140℃ ×100 / S all .

[0093] (8) Gurley value In accordance with JIS P-8117 (2009), the air permeability resistance (sec / 100cm) of polyethylene film was measured using a Wangyan-type air permeability resistance meter (manufactured by Asahi Seiko Co., Ltd., EGO-1T). 3 The following measurements were taken: The above measurements were performed at five different locations within 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 polyethylene film and polyethylene raw materials was determined by gel permeation chromatography (GPC) under the following conditions. • Measuring device: Waters Corporation GPC-150C • Column: Showa Denko Corporation Shodex UT806M Column temperature: 160℃ • Solvent (mobile phase): 1,2,4-trichlorochlorobenzene • Solvent flow rate: 1.0 ml / min • Sample concentration: 0.1 wt% (Dissolution conditions: 135°C / 1h) Injection volume: 500 μl • Detector: Differential refractometer (RI detector) manufactured by Waters Corporation • Calibration curve: Created from a calibration curve obtained using monodisperse polystyrene standard samples, with a polyethylene conversion factor (0.46).

[0095] (10) Density of polyethylene film A 10cm square sample of polyethylene film was taken and its mass (kg) was measured. Subsequently, the volume (m³) was calculated using the thickness measured by the method described in (1). 3 The density of the polyethylene film (kg / m³) is calculated using the following formula. 3 ) was calculated. Density (kg / m 3 ) = Mass (kg) / Volume (m³) 3 ).

[0096] (11) Specific heat of polyethylene film The specific heat of polyethylene film was calculated based on differential scanning calorimetry (DSC) measurements performed under the following conditions, in accordance with JIS K7123 (1987). Each sample was measured three times, and the average value was used as the specific heat of that sample. • Measuring device: Perkin-Elmer DSC8500 differential scanning calorimeter • Heating rate: 10°C / min • Standard sample: Sapphire (α-Al2O3) • Atmosphere: Dry nitrogen stream ·Measurement temperature: 25℃ • Sample container: Aluminum container.

[0097] (12) Thermal conductivity in the principal orientation direction The thermal conductivity in the main orientation direction was measured by the optical AC method. A rectangular sample measuring 30 mm in length (main orientation direction) x 5 mm in width was cut from a polyethylene film, colored with black paint, and used for measurement. The sample was placed in a thermal diffusivity measuring device (ULVAC Riko LaserPIT), and the sample was periodically heated by irradiating it with a semiconductor laser in a vacuum. The thermal diffusivity was determined from the attenuation constant of the temperature wave in the main orientation direction from the heating position. Using the density and specific heat of the polyethylene film measured by the methods described in (10) and (11), the thermal conductivity in the main orientation direction of the polyethylene film was calculated using the following formula. The same measurement was performed three times, and the average value was taken as the thermal conductivity in the main orientation direction of the sample. Thermal conductivity (W / m / K) = Thermal diffusivity (m 2 / s) x density (kg / m 3 ) × specific heat (J / kg / K).

[0098] (13) Appearance evaluation during long-term storage A 500mm wide polyethylene film was wound into a 200m long roll to form a film roll. The resulting film roll was stored at 50°C for 200 hours. After storage, the appearance of the film roll and its flatness were visually inspected when 1m was unwound from the film roll and left free-tensioned (hanging vertically due to the film's own weight), or when a uniform tension of 1kg / m was applied evenly across the entire width of the film. The flatness was then evaluated according to the following criteria.

[0099] S: The film roll had no cosmetic defects such as wrinkles or sagging. A: Although there were some cosmetic defects on the film roll, wrinkles, sagging, dents, and other flatness defects disappeared when the film was free-tensioned. B: Areas with poor flatness were observed under free tension, but these areas with poor flatness disappeared under a tension of 1 kg / m. Even with a tension of 1 kg / m, areas of poor flatness were observed.

[0100] (Polyethylene resin, etc.) For the production of the polyethylene films in the examples and comparative examples, the polyethylene resins with 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., "Hyzex 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., "Hyzex Million" (registered trademark) 240M Polyethylene resin A4 (PE A4): Ultra-high molecular weight polyethylene, manufactured by Mitsui Chemicals, Inc., "Hyzex Million" (registered trademark) 630M.

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

[0103] [Table 1]

[0104] (Example 1) A polyethylene resin solution was prepared by mixing 20 parts by mass of polyethylene resin A1 with 0.04 parts by mass of BASF's "Irganox" (registered trademark) 1010 as an antioxidant and supplying it to a twin-screw extruder. 80 parts by mass of liquid paraffin (35 cSt (40°C)) was supplied as a plasticizer from the side feeder of the twin-screw extruder, and the mixture was melt-kneaded at 180°C. The polyethylene resin solution was extruded from the twin-screw extruder, foreign matter was removed by filtering, and it was supplied to a T-die. The sheet-like extruded material was cooled and solidified while being taken up by a cooling roll temperature-controlled to 30°C to obtain a gel-like sheet. The take-up speed at this time 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 heated to 25°C in a washing tank to remove the liquid paraffin, and then air-dried at room temperature. Next, the dried film was re-stretched to 1.56 times its original length in the TD direction at 120°C using a tenter stretcher, and then heat-treated at 146°C while applying a relaxation rate of 12% in the TD direction. Furthermore, after a cooling process at 100°C, the film was guided to the outside of the tenter stretcher, the clips at both ends in the width direction were released, the film edges were slit in a winder process, and the film was wound onto a core to obtain a polyethylene film with a thickness of 5 μm. The physical properties and evaluation results of the obtained film are shown in Table 2.

[0105] (Examples 2-9, Comparative Examples 1-3, 5-6) 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. In this case, the thickness was adjusted by adjusting the discharge amount during extrusion and the speed of the casting drum (the same applies to other examples and comparative examples below). The physical properties and evaluation results of the obtained film 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 re-stretched after the first stretching without immersion in a methylene chloride bath.

[0107] (Example 10) The polyethylene film prepared under the film-forming conditions described in Example 1 had a film surface (cooling roll contact side) with a load of 25 W·min / m². 2 Corona discharge treatment was performed at a processing intensity of 1.00 × 10⁻¹⁰ -2 After creating a high-pressure environment of Pa, the film was run through a 20°C cooling metal drum to heat and evaporate the aluminum metal, forming a vapor-deposited thin film layer on the film surface (the side in contact with the cooling roll). The vapor deposition thickness was controlled to approximately 100 nm. After deposition, the vacuum deposition apparatus was returned to atmospheric pressure to obtain a metal film laminate with a metal film on one side. The resulting metal film laminate was wrinkle-free and allowed for uniform deposition.

[0108] (Example 11) The polyethylene film prepared under the film-forming conditions described in Example 1 had a film surface (cooling roll contact side) with a load of 25 W·min / m². 2 Corona discharge treatment was performed at the specified treatment intensity. Subsequently, an acrylic adhesive (manufactured by Soken Chemical Co., Ltd., "SK Dyne" (registered trademark) 1310) was diluted with ethyl acetate, toluene, and methyl ethyl ketone (MEK). A coating agent for the adhesive layer was prepared by mixing 2.0 parts by mass of a curing agent (manufactured by Nippon Polyurethane Industry Co., Ltd., "Coronate" (registered trademark) D-90) with 100 parts by mass of solid content of the adhesive, and this coating agent was applied to the film surface (cooling roll contact side) using a gravure coater. Next, the film was guided to an 80°C drying oven and transported for 30 seconds to remove the solvent in the coating agent, obtaining an adhesive film with an adhesive layer thickness of 0.7 μm. The adhesive film obtained in this way was wrinkle-free and allowed for a uniform coating of the adhesive layer.

[0109] [Table 2] [Industrial applicability]

[0110] The polyethylene film of the present invention can be used in a variety of industrial applications, including packaging films, surface protection films, process films, heat dissipation films, low-temperature films, sliding films, adhesive film substrates, sanitary products, agricultural products, building materials, medical products, and capacitor films. In particular, it is preferable to use it as a surface protection film, process film, release film, packaging film, heat dissipation film, low-temperature film, and adhesive film substrate because of its excellent heat resistance, mechanical strength, quality, and transparency.

Claims

1. A polyethylene film in which, when the direction with the greatest tensile strength is defined as the principal orientation direction and the direction perpendicular to the principal orientation direction within the film plane is defined as the orthogonal direction to the principal orientation, the sum of the thermal shrinkage rate in the principal orientation direction and the thermal shrinkage rate in the orthogonal direction to the principal orientation is -5.0% or more and 10.0% or less when heated at 100°C for 8 hours, the tensile strength in the orthogonal direction to the principal 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 by a Wang Ren type air permeability resistance meter is 1 × 10⁴ seconds / 100 cm³ or more.

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

3. The polyethylene film according to claim 1 or 2, wherein the sum of the tensile elongation in the principal orientation direction and the direction perpendicular to the principal orientation is 160% or more and 500% or less.

4. The polyethylene film according to claim 1 or 2, wherein the proportion of the heat of fusion of crystals at temperatures above 140°C to the total heat of fusion of crystals, as measured by differential scanning calorimetry, is 30% or more and 90% or less.

5. The polyethylene film according to claim 1 or 2, wherein the weight-average molecular weight measured using high-temperature GPC is 500,000 or more and 1,900,000 or less.

6. A polyethylene film according to claim 1 or 2, wherein polyethylene having a weight-average molecular weight of 500,000 or more and 5,000,000 or less is the main component.

7. The polyethylene film according to claim 1 or 2, wherein the thermal conductivity in the main orientation direction is 0.7 W / m / K or more.

8. A polyethylene film according to claim 1 or 2, wherein the thickness is 25 μm or less.

9. An adhesive film comprising an adhesive layer provided on one or both sides of a polyethylene film according to claim 1 or 2.

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

11. A release film having the polyethylene film described in claim 1 or 2.

12. A heat dissipation film having the polyethylene film described in claim 1 or 2.

13. A low-temperature film having the polyethylene film described in claim 1 or 2.

14. A polyethylene film roll comprising a polyethylene film according to claim 1 or 2 wound around a core.

15. A method for producing a polyethylene film according to 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 a plasticizer, and then heat-treating the sheet after extracting the plasticizer.

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

17. A method for producing a polyethylene film according to claim 15, characterized by including a step of stretching the film in at least one axial direction after extracting the plasticizer.

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