Polyethylene composition and biaxially stretched polyethylene film

Abstract

To provide a polyethylene composition useful for biaxially stretched polyethylene films having high heat resistance and excellent machine-direction and transverse-direction mechanical strength.SOLUTION: A polyethylene composition (C) comprising polyethylene (A) having a weight-average molecular weight (Mw) measured by gel permeation chromatography of 50,000 to 200,000, a content of components each having a molecular weight of 10,000 or less of 8 mass% or less, and a density of 945 to 980 kg / m3 measured in accordance with JIS K6922-1 (1997), and polyethylene (B) having a density of 940 to 965 kg / m3 measured in accordance with JIS K6922-1 (1997) and a weight-average molecular weight (Mw) measured by gel permeation chromatography of 220,000 to 5,000,000.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a polyethylene composition and a biaxially oriented polyethylene film. [Background technology]

[0002] Polyethylene films are used as packaging materials because they have moderate flexibility, excellent transparency, moisture resistance, chemical resistance, etc., and are inexpensive.

[0003] However, polyethylene film has low rigidity, impact resistance, heat resistance, etc., and there are some applications in which it cannot be used alone. To solve these problems, laminate films obtained by laminating polyethylene film with other resin films (for example, polypropylene film, polyester film, polyamide film) are widely used as packaging materials (for example, see Patent Document 1).

[0004] Meanwhile, in recent years, social issues such as waste plastics have been attracting attention, and along with the growing demand for building a recycling-oriented society, there is a demand for improved recyclability of packaging materials. Packaging materials made from a combination of films of different materials, such as those described above, have the problem of being difficult to recycle through material recycling, chemical recycling, and other methods. In response to this, packaging materials made from the same resin materials, formed by laminating oriented polyethylene film and unoriented polyethylene film, have been proposed (see, for example, Patent Documents 2 and 3). The oriented polyethylene film is used to complement the mechanical properties of the unoriented polyethylene film and to prevent resin adhesion to the seal bar during heat sealing. However, even such oriented polyethylene film does not have the same mechanical properties as biaxially oriented polyamide film or biaxially oriented polyester film, and shrinkage is particularly severe in the heat-sealed areas. This necessitates a lower heat-sealing temperature, which may result in insufficient heat-sealing strength. As a polyethylene film having excellent heat resistance, a uniaxially stretched polyethylene film has been proposed (see, for example, Patent Document 4). However, the uniaxially stretched film has a problem in that it has poor mechanical properties in the direction perpendicular to the stretching direction and is prone to tearing. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-104525 [Patent Document 2] Japanese Patent Application Publication No. 2019-171860 [Patent Document 3] Japanese Patent Application Publication No. 2019-529165 [Patent Document 4] Japanese Patent Publication No. 2022-142174 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a biaxially oriented polyethylene film that has high heat resistance and excellent mechanical strength in both the longitudinal and transverse directions, and a polyethylene composition that is a raw material for the film. [Means for solving the problem]

[0007] As a result of extensive research aimed at solving the above problems, the present inventors have found that a specific biaxially oriented polyethylene film exhibits excellent heat resistance and mechanical properties, leading to the completion of the present invention.

[0008] That is, the respective aspects of the present invention are the following [1] to [4]. [1] The weight average molecular weight (Mw) measured by gel permeation chromatography is 50,000 to 200,000, and the proportion of molecular weights of 10,000 or less is 8% by mass or less, and the density measured by JIS K6922-1 (1997) is 945 to 980 kg / m 330 to 90 parts by mass of polyethylene (A) and a density of 940 to 965 kg / m as measured according to JIS K6922-1 (1997) 3 and 10 to 70 parts by mass of polyethylene (B) having a weight average molecular weight (Mw) of 220,000 to 5,000,000 as measured by gel permeation chromatography (the total of (A) and (B) is 100 parts by mass). [2] A biaxially oriented polyethylene film comprising the polyethylene composition according to [1], wherein the endothermic curve measured with a differential scanning calorimeter has at least one peak at 135°C or higher. [3] The biaxially oriented polyethylene film according to [2], having a breaking strength of 200 MPa or more in both the MD and TD directions. [4] A method for producing a biaxially oriented polyethylene film, comprising stretching a film or sheet made of the polyethylene composition according to [1] at a stretching temperature of 100 to 135°C, at a stretching ratio of 1.5 to 10 in the MD direction and at a stretching ratio of 1.5 to 10 in the TD direction. [Effects of the Invention]

[0009] The polyethylene composition of the present invention is useful as a raw material for biaxially oriented polyethylene films, and the resulting biaxially oriented polyethylene films have excellent heat resistance and mechanical properties and are useful as substrates for laminate films for packaging foods, beverages, pharmaceuticals, etc. Furthermore, since the resulting laminate films are composed mostly of polyethylene-based materials, they have excellent recyclability and can reduce the environmental load. DETAILED DESCRIPTION OF THE INVENTION

[0010] The polyethylene composition and biaxially oriented polyethylene film, which are one embodiment of the present invention, will be described in detail below.

[0011] The polyethylene composition (C) according to one embodiment of the present invention has a weight average molecular weight (Mw) of 50,000 to 200,000, with the proportion of molecular weights of 10,000 or less being 8% by mass or less, as measured by gel permeation chromatography, and a density of 945 to 980 kg / m, as measured in accordance with JIS K6922-1 (1997).3 Polyethylene (A) and a density measured according to JIS K6922-1 (1997) of 940 to 965 kg / m 3 and contains polyethylene (B) having a weight average molecular weight (Mw) of 220,000 to 5,000,000 as measured by gel permeation chromatography.

[0012] The polyethylene (A) has a weight average molecular weight (Mw) measured by gel permeation chromatography of 50,000 to 200,000, preferably 100,000 to 200,000, and more preferably 100,000 to 150,000. If the Mw is less than 50,000, the film will break during stretching, which is not preferred. If the Mw is more than 200,000, the extrusion load during melt extrusion will increase, which is not preferred.

[0013] The proportion of polyethylene (A) having a molecular weight of 10,000 or less as measured by gel permeation chromatography is 8% by mass or less, preferably 6% by mass or less, and more preferably 4% by mass or less. If this proportion exceeds 8% by mass, the heat resistance of the stretched film deteriorates, and the heat-sealed appearance deteriorates, which is undesirable.

[0014] Polyethylene (A) has a density of 945 to 980 kg / m as measured by JIS K6922-1 (1997). 3 and preferably 945 to 960 kg / m 3 The density is 945 kg / m 3 If the density is less than 980 kg / m, the heat resistance of the stretched film will be deteriorated, which is not preferable. 3 Polyethylene exceeding this limit is difficult to produce industrially.

[0015] Such polyethylene (A) can be obtained by homopolymerizing ethylene or copolymerizing ethylene with a small amount of an α-olefin. For polymerization, a Ziegler catalyst consisting of a solid catalyst component containing magnesium and titanium and an organoaluminum compound, a metallocene catalyst consisting of an organic transition metal compound containing a cyclopentadienyl derivative and a compound and / or an organic metal compound that reacts with the organic transition metal compound to form an ionic complex, or a vanadium-based catalyst can be used. Metallocene catalysts and vanadium-based catalysts are preferred because they make it easy to control the proportion of molecular weights of 10,000 or less. For example, polyethylene (A) can be produced by a production method such as a slurry method, a solution method, or a gas-phase method. The method for producing polyethylene (A) is not particularly limited, but a slurry method or a solution method is preferred because it makes it easy to control the proportion of molecular weights of 10,000 or less.

[0016] Polyethylene (B) has a density of 940 to 965 kg / m as measured by JIS K6922-1 (1997). 3 The density is 940 kg / m 3 If it is less than 965 kg / m, the heat resistance of the oriented polyethylene film for lamination will deteriorate, which is not preferable. 3 Polyethylene with a density above this is difficult to produce industrially.

[0017] The polyethylene (B) has a weight average molecular weight (Mw) measured by gel permeation chromatography of 220,000 to 5,000,000, preferably 220,000 to 3,000,000, more preferably 220,000 to 700,000, and even more preferably 220,000 to 300,000. If the Mw is less than 220,000, the film will break during stretching, which is not preferred. If the Mw exceeds 5,000,000, the dispersibility with the polyethylene (A) will deteriorate, which is not preferred.

[0018] The proportion of polyethylene (B) having a molecular weight of 10,000 or less as measured by gel permeation chromatography is preferably 20% by mass or less, more preferably 17% by mass or less, even more preferably 10% by mass or less, and most preferably 7% by mass or less, from the viewpoints of the heat resistance and heat seal appearance of the stretched film.

[0019] Such polyethylene (B) can be obtained by homopolymerizing ethylene or copolymerizing ethylene with a small amount of an α-olefin. For the polymerization, a Ziegler catalyst consisting of a solid catalyst component containing magnesium and titanium and an organoaluminum compound, a metallocene catalyst consisting of an organotransition metal compound containing a cyclopentadienyl derivative and a compound and / or organometallic compound that reacts with the organotransition metal compound to form an ionic complex, a vanadium-based catalyst, or the like can be used.

[0020] In the polyethylene composition (C), the mixing ratio of the polyethylene (A) to the polyethylene (B) is 30 to 90 parts by mass of the polyethylene (A) and 10 to 70 parts by mass of the polyethylene (B), preferably 90 to 50 parts by mass of the polyethylene (A) and 10 to 50 parts by mass of the polyethylene (B), and more preferably 90 to 70 parts by mass of the polyethylene (A) and 10 to 30 parts by mass of the polyethylene (B). Here, the total of (A) and (B) is 100 parts by mass. By including 30 to 90 parts by mass of the polyethylene (A), when a biaxially oriented polyethylene film is formed, film breakage during stretching can be suppressed while maintaining heat resistance.

[0021] The polyethylenes (A), (B) and the polyethylene composition (C) may contain additives that are typically used in polyolefins, such as antioxidants, lubricants, neutralizing agents, antiblocking agents, surfactants and slip agents, as needed.

[0022] The biaxially oriented polyethylene film of one embodiment of the present invention comprises the polyethylene composition (C) and exhibits at least one peak in the range of 135°C or higher in an endothermic curve measured by differential scanning calorimetry. This allows the biaxially oriented polyethylene film to exhibit high heat resistance. If the endothermic curve peak of the biaxially oriented polyethylene film is less than 135°C, the heat resistance of the film will be insufficient, and the appearance of the film will be deteriorated due to heat shrinkage caused by heat sealing, which is not preferred.

[0023] The biaxially stretched polyethylene film preferably has a breaking strength of 200 MPa or more in both the MD and TD directions.

[0024] Biaxially stretched films can be obtained by biaxially stretching in the machine direction (MD) and transverse direction (TD) within the above ranges using various known methods, such as a tubular method or a tenter method. In the case of the tenter method, the method for producing a raw film is not particularly limited, and examples include a method in which a film to be stretched is obtained by extrusion molding such as known inflation molding or T-die casting, and then stretched. Alternatively, a sheet obtained by press molding can be used as the raw film. The biaxial stretching may be simultaneous biaxial stretching or sequential biaxial stretching. Among these methods, biaxially stretched polyethylene films obtained by the tenter method are preferred.

[0025] The stretching ratio in the MD direction is generally 1.5 to 10 times, preferably 1.5 to 8 times, and the stretching ratio in the TD direction is generally 1.5 to 10 times, preferably 1.5 to 8 times.

[0026] The stretching temperature is 100 to 135°C, preferably 110 to 130°C, from the viewpoint of stretchability and heat resistance.

[0027] After biaxial stretching, the film may be heat set at a temperature in the range of 80 to 140°C depending on the application. [Example]

[0028] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Parts and percentages are based on mass unless otherwise specified. Evaluation methods in the examples and comparative examples are as follows.

[0029] <Differential scanning calorimetry> Using a differential scanning calorimeter (DSC) (DSC6220, manufactured by SII Nanotechnology Inc.), the temperature was raised from 0°C to 230°C at a rate of 10°C / min (first scan), and the endothermic peak of the first scan was measured. The sample weight of the biaxially oriented polyethylene film was 3 mg.

[0030] <Melt mass flow rate> Measurement was carried out using a melt indexer (manufactured by Takara Kogyo Co., Ltd.) based on JIS K6924-1 (under conditions of 190°C and a load of 2160 g).

[0031] <density> Measurements were made in accordance with JIS K6922-1 (1997).

[0032] <Molecular weight> The molecular weight was measured using a GPC apparatus (HLC (registered trademark)-8121GPC / HT manufactured by Tosoh Corporation) and a column (TSKgel (registered trademark) GMHhr-H(20)HT manufactured by Tosoh Corporation) at a column temperature of 140°C using 1,2,4-trichlorobenzene as an eluent. The measurement sample was prepared at a concentration of 1.0 mg / ml, and 0.3 ml was injected for measurement. The molecular weight calibration curve was calibrated using a polystyrene sample with a known molecular weight (manufactured by Tosoh Corporation; the molecular weight was converted into a PE molecular weight using a Q factor). From the chromatogram obtained as a result of the measurement, the weight-average molecular weight Mw and the proportion of components with a molecular weight of 10,000 or less were calculated.

[0033] <Melt extrusion properties> Melt-kneading was carried out at 200°C and 60 rpm using a single-screw extruder (manufactured by Toyo Seiki Seisakusho, trade name: Labo Plastomill (registered trademark) single-screw extruder 2D25S). When the extrusion load became too high and melt-kneading was not possible under the above conditions, the melt-extrudability was judged to be poor and a rating of x was given, and when melt-kneading was possible, a rating of o was given.

[0034] <Stretchability> The appearance of the biaxially stretched films obtained in the examples was evaluated, and a rating of x was given when the stretchability was poor and there were breaks in the film, and a rating of o was given when the stretchability was excellent and there were no breaks in the film.

[0035] <Breaking strength> The breaking strength of the dumbbell pieces (ATMS-1822) was measured at 25°C and 300 mm / min using an RTE-1210 (trade name, manufactured by Orientec Co., Ltd.).

[0036] <Heat seal appearance> The biaxially stretched films obtained in the examples were heat-sealed using a heat-sealing tester TP-701B (manufactured by Tester Sangyo Co., Ltd.) at a set temperature of 130°C, double-sided heating, an air pressure of 0.2 MPa, and a sealing time of 1 second, and then air-cooled, and the appearance of the biaxially stretched polyethylene film was evaluated. A rating of × was given when the laminate film had significant shrinkage and poor appearance, and a rating of ◯ was given when the shrinkage was small and the appearance was good.

[0037] <Heat shrinkage rate> The biaxially stretched films obtained in the examples were heat-sealed using a heat-sealing tester TP-701B (manufactured by Tester Sangyo Co., Ltd.) at a set temperature of 130°C, double-sided heating, an air pressure of 0.2 MPa, and a sealing time of 1 second, followed by air cooling. The heat shrinkage was calculated from the film length before and after heat sealing using the following formula.

[0038] Heat shrinkage rate = (film length before heat sealing - film length after heat sealing) / (film length before heat sealing) [Example 1] [Preparation of organically modified clay] In a 1-liter flask, 300 ml of industrial alcohol (manufactured by Japan Alcohol Sales Co., Ltd., (trade name) Equinene (registered trademark) F-3) and 300 ml of distilled water were placed, and 15.0 g of concentrated hydrochloric acid and dioleylmethylamine ((C 18 H 35)2(CH3)N (Lion Specialty Chemicals Co., Ltd., trade name Lipomin® MO) 63.7 g (120 mmol) was added and heated to 45 °C. 100 g of synthetic hectorite (BYK, trade name Laponite RD) was dispersed in the mixture, which was then heated to 60 °C and stirred for 1 hour while maintaining the temperature. The resulting slurry was filtered, washed twice with 600 ml of water at 60 °C, and dried in a dryer at 85 °C for 12 hours to obtain 130 g of organically modified clay. This organically modified clay was then jet milled to a median diameter of 15 μm.

[0039] [Preparation of polymerization catalyst] After replacing the air in a 300 mL flask equipped with a thermometer and reflux condenser with nitrogen, 25.0 g of the organically modified clay obtained in [Preparation of organically modified clay] and 108 mL of hexane were added, followed by the addition of 0.392 g (1 mmol) of bis(indenyl)zirconium dichloride and 142 mL of 20% triisobutylaluminum, and stirring for 3 hours at 60°C. After cooling to room temperature, the supernatant was removed and washed twice with 220 mL of hexane, and then 220 mL of hexane was added to obtain a catalyst suspension (solid mass content: 12.0 mass%).

[0040] [Production of polyethylene (A1) powder] A 2-liter autoclave was charged with 1.2 liters of hexane, 1.0 mL of 20% triisobutylaluminum, and 200 mg (equivalent to 24 mg of solids) of the catalyst suspension obtained in [Preparation of polymerization catalyst], and after heating to 85°C, an ethylene / hydrogen mixed gas was continuously fed so that the partial pressure became 0.90 MPa (hydrogen concentration in the ethylene / hydrogen mixed gas: 450 ppm). After 90 minutes, the pressure was released, and the slurry was filtered and dried to obtain polyethylene (A1) powder.

[0041] The obtained polyethylene (A1) powder had a weight-average molecular weight of 111,000, a proportion of components with a molecular weight of 10,000 or less of 3.4% by mass, and a density of 950 kg / m 3 It was.

[0042] The obtained polyethylene (A1) powder was melt-kneaded using a twin-screw extruder (manufactured by Technovel, trade name ULTnano25TW) with a screw diameter of 25 mm at a resin temperature of 160°C and a screw rotation speed of 300 rpm to obtain polyethylene (A1) pellets.

[0043] As the polyethylene (B), a commercially available polyethylene resin (B1) (Nipolon Hard (registered trademark) 8D01A, manufactured by Tosoh Corporation) was used.

[0044] [Preparation of polyethylene composition (C1)] 280 g of polyethylene (A1) pellets and 120 g of polyethylene (B1) were dry-blended and then melt-kneaded at 200°C using a single-screw extruder (manufactured by Toyo Seiki Seisakusho, trade name: Labo Plastomill (registered trademark) single-screw extruder 2D25S) to obtain polyethylene composition (C1). The MFR of composition (C1) was 0.44 g / 10 min. The screw rotation speed was set to 60 rpm, and melt extrudability was evaluated. The results are shown in Table 1.

[0045] Using the polyethylene composition (C1), a compression molding machine AWFA.50 (manufactured by Shinto Metal Industry Co., Ltd.) and a mold of 150 mm × 150 mm × 0.1 mm was used to perform compression molding under the conditions of a heating temperature of 230°C, a cooling temperature of 25°C, a primary pressure of 0.1 MPa × 4 minutes, a secondary pressure of 20 MPa × 4 minutes, and a cooling pressure of 20 MPa × 4 minutes, to produce a sheet with a thickness of 0.1 mm.

[0046] The prepared sheet was simultaneously biaxially stretched using a biaxial stretching device (manufactured by Toyo Seiki Seisakusho, Ltd., product name: EX10-B) at a stretching temperature of 120°C, an MD stretch ratio of 2, a TD stretch ratio of 2, and a stretching speed of 500 mm / min, and then air-cooled after stretching to obtain a biaxially stretched film. Differential scanning calorimetry was performed using the obtained biaxially stretched film. Furthermore, the heat seal appearance, heat shrinkage rate, and breaking strength were evaluated. The evaluation results are shown in Table 1.

[0047] [Example 2] A biaxially stretched film was obtained in the same manner as in Example 1, except that a commercially available polyethylene resin (B2) (Nipolon Hard (registered trademark) 8900, manufactured by Tosoh Corporation) was used as the polyethylene (B). The evaluation results are shown in Table 1.

[0048] [Example 3] The hydrogen concentration in the polyethylene powder manufacturing process was adjusted to achieve a weight average molecular weight of 131,000, a molecular weight of 10,000 or less of 1.5%, and a density of 951 kg / m 3 A biaxially stretched film was obtained in the same manner as in Example 2, except that polyethylene powder (A2) was used as polyethylene (A). The evaluation results are shown in Table 1.

[0049] [Example 4] A biaxially stretched film was obtained in the same manner as in Example 1, except that a commercially available polyethylene resin (B3) (Nipolon Hard (registered trademark) 7300A, manufactured by Tosoh Corporation) was used as the polyethylene (B). The evaluation results are shown in Table 1.

[0050] [Example 5] Polyethylene (B) was produced by two-stage polymerization in which component (X1) was polymerized followed by component (Y1) as follows: a polyethylene having a weight-average molecular weight of 270,000, a proportion of components with a molecular weight of 10,000 or less of 6%, and a density of 946 kg / m 3 Except for using the polyethylene (B4), a biaxially stretched film was obtained in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0051] [Production of polyethylene (B4)] Manufacturing of component (X1) A 10-liter autoclave was charged with 6 liters of hexane, 5.5 ml of a hexane solution of 20 wt % triisobutylaluminum, and 2.50 g (corresponding to a solid content of 310 mg) of the suspension of the production catalyst obtained in Preparation Example 1, and the temperature was raised to 60°C. Ethylene was then continuously supplied so that the ethylene partial pressure could be maintained at 0.80 MPa, and slurry polymerization was carried out for 2 hours.

[0052] Production of component (Y1) and ethylene-based resin (B4) After polymerizing the component (X1), ethylene was continuously supplied so as to maintain the ethylene partial pressure at 0.50 MPa at 60°C, and hydrogen was intermittently added so that the hydrogen concentration in the gas phase of the autoclave became 4500 ppm. Slurry polymerization was carried out for 8 hours to polymerize the component (Y1), thereby producing an ethylene-based resin (B4) powder.

[0053] The obtained polyethylene (B4) powder was melt-kneaded using a twin-screw extruder (manufactured by Technovel, trade name ULTnano25TW) with a screw diameter of 25 mm at a resin temperature of 160°C and a screw rotation speed of 300 rpm to obtain polyethylene (B4) pellets.

[0054] [Example 6] Polyethylene (B) was produced by two-stage polymerization in which component (X2) was polymerized followed by component (Y2) as follows: a polyethylene having a weight-average molecular weight of 680,000, a proportion of components with a molecular weight of 10,000 or less of 7% by mass, and a density of 945 kg / m 3 Except for using the polyethylene (B5), a biaxially stretched film was obtained in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0055] [Production of polyethylene (B5)] Component (X2) production A 10-liter autoclave was charged with 6 liters of hexane, 5.5 ml of a hexane solution of 20 wt % triisobutylaluminum, and 2.50 g (corresponding to a solid content of 310 mg) of the suspension of the production catalyst obtained in Preparation Example 1, and the temperature was raised to 60°C. Ethylene was then continuously supplied so that the ethylene partial pressure could be maintained at 0.80 MPa, and slurry polymerization was carried out for 1 hour.

[0056] Production of component (Y2) and ethylene-based resin (B5) After polymerizing the component (X2), ethylene was continuously supplied so as to maintain the ethylene partial pressure at 0.87 MPa at 60°C, and hydrogen was intermittently added so that the hydrogen concentration in the gas phase of the autoclave became 4500 ppm. Slurry polymerization was carried out for 4 hours to polymerize the component (Y2), thereby producing an ethylene-based resin (B5).

[0057] Polyethylene (B5) pellets were obtained in the same manner as in Example 5, except that polyethylene powder (B5) was used instead of polyethylene powder (B4). [Example 7] A biaxially stretched film was obtained in the same manner as in Example 2, except that the polyethylene (A1) pellets were 90% by mass and the polyethylene (B2) was 10% by mass. The evaluation results are shown in Table 1. [Example 8] A biaxially stretched film was obtained in the same manner as in Example 2, except that the polyethylene (A1) pellets were 50% by mass and the polyethylene (B2) was 50% by mass. The evaluation results are shown in Table 1.

[0058] [Comparative Example 1] A biaxially stretched polyethylene film was obtained in the same manner as in Example 1, except that polyethylene (A1) pellets were used instead of the polyethylene composition (C1). The evaluation results are shown in Table 1, but the stretched polyethylene film broke during stretching and could not be produced.

[0059] Comparative Example 2 A biaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that polyethylene (B1) was used instead of polyethylene composition (C1). The evaluation results are shown in Table 1, and the melt extrudability was poor.

[0060] Comparative Example 3 A biaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that a commercially available polyethylene resin (A3) (Nipolonhard (registered trademark) 5700, manufactured by Tosoh Corporation) was used as the polyethylene (A). The evaluation results are shown in Table 1, and the heat resistance of the film was poor.

[0061] Comparative Example 4 A biaxially stretched polyethylene film was obtained in the same manner as in Example 1, except that a commercially available polyethylene resin (B6) (Nipolonhard (registered trademark) 6000, manufactured by Tosoh Corporation) was used as the polyethylene (B). The evaluation results are shown in Table 1, but the stretched polyethylene film broke during stretching and could not be formed into a film.

[0062] [Table 1]

Claims

1. The weight average molecular weight (Mw) measured by gel permeation chromatography is 50,000 to 200,000, and the proportion of molecular weights of 10,000 or less is 8% by mass or less, and the density measured according to JIS K6922-1 (1997) is 945 to 980 kg / m ３ and a density measured according to JIS K6922-1 (1997) of 940 to 965 kg / m ３ and 10 to 70 parts by mass (the total of (A) and (B) is 100 parts by mass) of polyethylene (B) having a weight average molecular weight (Mw) of 220,000 to 5,000,000 as measured by gel permeation chromatography.

2. The polyethylene composition (C) according to claim 1, wherein the proportion of polyethylene (B) having a molecular weight of 10,000 or less as measured by gel permeation chromatography is 20 mass% or less.

3. 3. A biaxially oriented polyethylene film comprising the polyethylene composition according to claim 1 or 2, which has at least one peak at 135° C. or higher in an endothermic curve measured with a differential scanning calorimeter.

4. 4. The biaxially oriented polyethylene film according to claim 3, wherein the breaking strength in each of the MD direction and the TD direction is 200 MPa or more.

5. A method for producing a biaxially oriented polyethylene film, comprising stretching a film or sheet made of the polyethylene composition according to claim 1 or 2 at a stretching temperature of 100 to 135°C at a stretching ratio of 1.5 to 10 times in the MD direction and at a stretching ratio of 1.5 to 10 times in the TD direction.

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