Heat-shrinkable polyethylene film

Abstract

To provide a heat-shrinkable polyethylene film with excellent low-temperature shrinkage properties and high impact strength. [Solution] A heat-shrinkable polyethylene film containing polyethylene resin, wherein the birefringence Δnyx, calculated from the refractive index in the width direction (Ny) and the refractive index in the longitudinal direction (Nx) measured according to JIS K 7142:2014 using the formula: Δnyx=Ny-Nx, is 0.0250 or higher, and the degree of crystallinity is 40% or higher.

Description

Technical Field

[0001] The present invention relates to a heat-shrinkable polyethylene film and the like.

Background Art

[0002] PET bottles are mainly widely distributed as containers for beverages, and due to the large volume of their distribution, the importance of recycling is high. Many PET bottles are labeled with a label (shrink label) in a form that adheres along the bottle shape by heating a heat-shrinkable film at a relatively low temperature. Therefore, when recycling, a separation operation of the shrink label is required. The separation operation of the shrink label can be efficiently performed by using floatation separation that utilizes the difference in specific gravity between the shrink label material and the PET bottle material. However, polystyrene and polyethylene terephthalate, which are generally used as shrink label materials, have no or a small difference in specific gravity from the PET bottle material, so it is difficult to use them for the above floatation separation.

[0003] On the other hand, polyethylene has a relatively small specific gravity, so it can be used for the above floatation separation. However, polyethylene films have low low-temperature shrinkability, and their use for the above shrink labels has not advanced.

[0004] In Patent Document 1, a method of adding a petroleum resin and / or a terpene resin and a copolymer of ethylene and an α-olefin having 4 or more carbon atoms to an ethylene-propylene random copolymer has been proposed. Also, in Patent Document 2, a method of adding a specific alicyclic hydrocarbon resin to a specific propylene-α-olefin random copolymer for the purpose of improving low-temperature shrinkability and specific gravity has been proposed. Although these all improve low-temperature shrinkability, the amount of low-molecular-weight components increases, resulting in a decrease in impact resistance, and thereby troubles such as perforation and breakage occur in the processing step. Also, the raw material production cost is high.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Application Publication No. 63-304032 [Patent Document 2] Japanese Patent Publication No. 2002-060566 [Overview of the project] [Problems that the invention aims to solve]

[0006] The object of this invention is to provide a heat-shrinkable polyethylene film having excellent low-temperature shrinkage properties and high impact strength. [Means for solving the problem]

[0007] In view of the above problems, the inventors diligently conducted research and found that a heat-shrinkable polyethylene film containing polyethylene resin has a birefringence Δnyx of 0.0250 or higher, calculated from the refractive index in the width direction (Ny) and the refractive index in the longitudinal direction (Nx) measured according to JIS K 7142:2014 using the formula: Δnyx = Ny - Nx, and a crystallinity of 40% or higher, which solves the above problems. Based on this finding, the inventors furthered their research and completed the present invention. That is, the present invention encompasses the following aspects.

[0008] Item 1. A heat-shrinkable polyethylene film containing polyethylene resin, wherein the birefringence Δnyx, calculated from the refractive index in the width direction (Ny) and the refractive index in the longitudinal direction (Nx) according to JIS K 7142:2014 using the formula: Δnyx = Ny - Nx, is 0.0250 or higher, and the degree of crystallinity is 40% or higher.

[0009] Item 2. The heat-shrinkable polyethylene film according to Item 1, wherein the birefringence Δnyx is 0.0250 or more and 0.0550 or less.

[0010] Item 3. The heat-shrinkable polyethylene film according to item 1 or 2, wherein the degree of crystallinity is 40% or more and 65% or less.

[0011] Item 4. A heat-shrinkable polyethylene film according to any one of items 1 to 3, wherein the heat shrinkage rate in the width direction at 110°C is 16% or more, and the heat shrinkage rate in the longitudinal direction at 110°C is 5% or less.

[0012] Item 5. A stretched film, which is a heat-shrinkable polyethylene film as described in any of items 1 to 4.

[0013] Item 6. A heat-shrinkable polyethylene film according to any of items 1 to 5, which is a uniaxially oriented film.

[0014] Item 7. A heat-shrinkable polyethylene film according to any of items 1 to 6, having a thickness of 10 μm or more and 50 μm or less.

[0015] Item 8. A heat-shrinkable polyethylene film according to any one of items 1 to 7, which is a label or packaging film.

[0016] Item 9. A laminate comprising a heat-shrinkable polyethylene film and other layers as described in any of Items 1 to 8. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a heat-shrinkable polyethylene film having excellent low-temperature shrinkage properties and high impact strength. [Modes for carrying out the invention]

[0018] In this specification, the terms “contains” and “includes” include the concepts of “contains,” “includes,” “substantially consist of,” and “consist solely of.”

[0019] In this specification, the "~" in numerical ranges means "greater than or equal to" and "less than or equal to." That is, the notation α~β means α or greater and β or less, or β or greater and α or less, and the range includes α and β.

[0020] In this specification, when the upper limit value and the lower limit value are described separately, the ranges formed by arbitrarily combining the described upper limit value and the lower limit value are also disclosed in this specification.

[0021] In identifying the inventions included in the present disclosure, each configuration (properties, structure, functions, etc.) described in each embodiment of the present disclosure may be combined in any manner. That is, the present disclosure includes all the subjects consisting of any combinations of the combinable configurations described in this specification.

[0022] 1. Heat-shrinkable polyethylene film In one aspect, the present invention relates to a heat-shrinkable polyethylene film containing a polyethylene resin, having a birefringence Δnyx calculated by the formula: Δnyx = Ny - Nx from the refractive index (Ny) in the width direction and the refractive index (Nx) in the longitudinal direction measured based on JIS K 7142:2014 of 0.0250 or more, and a crystallinity of 40% or more. (In this specification, it may also be referred to as "the polyethylene film of the present invention".) This will be described below.

[0023] The polyethylene film of the present invention exhibits excellent shrinkability at a relatively low temperature (for example, about 110°C) and has high impact resistance.

[0024] The present inventor has found that by adjusting the birefringence Δnyx calculated by the formula: Δnyx = Ny - Nx and the crystallinity from the refractive index (Ny) in the width direction and the refractive index (Nx) in the longitudinal direction, the above two properties can be adjusted. By adjusting the birefringence Δnyx and the crystallinity within the above predetermined ranges, the above two properties can be made compatible at a high level.

[0025] The birefringence Δnyx is preferably 0.0255 or higher, more preferably 0.0260 or higher, even more preferably 0.0265 or higher, and even more preferably 0.0270 or higher, from the viewpoint of low-temperature shrinkage and impact strength (particularly from the viewpoint of low-temperature shrinkage). In one embodiment, the birefringence Δnyx is preferably even higher from the above viewpoint, for example, 0.0280 or higher, 0.0300 or higher, 0.0320 or higher, 0.0340 or higher, 0.0360 or higher, 0.0380 or higher, 0.0400 or higher, 0.0420 or higher, 0.0440 or higher, or 0.0460 or higher, 0.0480 or higher. The upper limit of the birefringence Δnyx is not particularly limited, but for example, it is 0.0570 or lower, preferably 0.0550 or lower, more preferably 0.0530 or lower, even more preferably 0.0510 or lower, and even more preferably 0.0500 or lower. By setting the upper limit of the birefringence Δnyx to this extent, in addition to achieving the effects described above, it is possible to suppress film breakage during the process of slitting the product roll to a predetermined width, thereby improving yield.

[0026] The birefringence Δnyx is a value measured according to the method of (4-1) in the example described below.

[0027] From the viewpoint of low-temperature shrinkage and impact strength (particularly from the viewpoint of impact strength), the degree of crystallinity is preferably 40.2% or higher, more preferably 40.4% or higher, even more preferably 40.6% or higher, and even more preferably 40.8% or higher. In one embodiment, the degree of crystallinity is even more preferably higher from the above viewpoint, for example, 41% or higher, 42% or higher, 43% or higher, 44% or higher, 45% or higher, 46% or higher, 47% or higher, 48% or higher, 49% or higher, 50% or higher, or 51% or higher. The upper limit of the degree of crystallinity is not particularly limited, but is preferably 65% ​​or lower, more preferably 60% or lower, even more preferably 55% or lower, and even more preferably 52% or lower. By setting the upper limit of the degree of crystallinity to this extent, in addition to the effects described above, it is possible to improve the stretchability in the tenter during stretching and improve the yield.

[0028] The degree of crystallinity is a value measured according to the method of (4-2) in the example described below.

[0029] The heat-shrinkable polyethylene film of the present invention has excellent low-temperature shrinkage, preferably excellent low-temperature shrinkage in the width-to-width direction. The heat shrinkage rate at 110°C in the width-to-width direction (TD) is preferably 16% or more, more preferably higher, for example, 17% or more, 18% or more, 19% or more, 20% or more, 21% or more, 22% or more, 23% or more, 24% or more, 25% or more, 26% or more, 27% or more, 28% or more, 29% or more, or 30% or more. The upper limit of the heat shrinkage rate is not particularly limited, for example, 55% or less, 50% or less, 45% or less, or 40% or less.

[0030] When the heat-shrinkable polyethylene film of the present invention has excellent low-temperature shrinkage in the width direction, it is preferable that the low-temperature shrinkage in the longitudinal direction is low. The heat shrinkage rate at 110°C in the longitudinal direction (MD) is preferably 5% or less, more preferably 4% or less, even more preferably 3% or less, even more preferably 2.5%, particularly preferably 2% or less, and particularly more preferably 1.5% or less. The lower limit of the heat shrinkage rate is not particularly limited and may be, for example, 0% or more, 0.1% or more, 0.2% or more, or 0.5% or more.

[0031] The thermal shrinkage rate is a value measured according to the method of (5-1) in the example described below.

[0032] The heat-shrinkable polyethylene film of the present invention has excellent impact strength. The impact strength is preferably 5 kgf·cm or more, and more preferably higher, for example, 6 kgf·cm or more, 7 kgf·cm or more, 8 kgf·cm or more, 9 kgf·cm or more, 10 kgf·cm or more, 11 kgf·cm or more, 12 kgf·cm or more, 13 kgf·cm or more, 14 kgf·cm or more, 15 kgf·cm or more, 16 kgf·cm or more, 17 kgf·cm or more, 18 kgf·cm or more, 19 kgf·cm or more, or 20 kgf·cm or more. The upper limit of the impact strength is not particularly limited, for example, 40 kgf·cm or less, 30 kgf·cm or less, or 25 kgf·cm or less.

[0033] The impact strength is a value measured according to the method of (5-2) in the example described below.

[0034] The polyethylene film of the present invention contains one or more types of polyethylene resin. The polyethylene film of the present invention contains polyethylene resin as a main component. In this specification, "containing polyethylene resin as a main component" means that the polyethylene film contains 50% by mass or more of polyethylene resin relative to the entire polyethylene film (when the entire polyethylene film is considered to be 100% by mass). The polyethylene resin content relative to the entire polyethylene film of the present invention is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and especially preferably 99% by mass or more. The upper limit of the polyethylene resin content is, for example, 100% by mass and 99.9% by mass relative to the entire polyethylene film of the present invention.

[0035] Polyethylene resin is not particularly limited in its origin; for example, it can be a resin made from petroleum-derived raw materials, or it can be a resin made from plant-derived raw materials (so-called biomass plastic).

[0036] Among polyethylene resins, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), or high-density polyethylene (HDPE) are preferred, more preferably LLDPE or HDPE, and particularly preferably LLDPE.

[0037] The polyethylene resin is preferably a combination of LLDPE and HDPE. This makes it easier to adjust the birefringence Δnyx and degree of crystallinity to a suitable range, and allows for a higher level of both low-temperature shrinkage and impact strength. In this case, the HDPE content is, for example, 1% to 99% by mass, preferably 1% to 90% by mass, more preferably 1% to 70% by mass, even more preferably 1% to 50% by mass, even more preferably 2% to 30% by mass, particularly preferably 5% to 20% by mass, and particularly more preferably 7% to 15% by mass, based on 100% by mass of the total of LLDPE and HDPE.

[0038] The weight-average molecular weight (Mw) of polyethylene resin is preferably 200,000 to 400,000, and more preferably 210,000 to 300,000, from the viewpoint of thickness uniformity, mechanical properties, and thermal-mechanical properties. In a preferred embodiment of the present invention, the weight-average molecular weight (Mw) is 210,000 to 280,000 (more preferably 210,000 to 250,000).

[0039] The number-average molecular weight (Mn) of the polyethylene resin is preferably 80,000 or less, and more preferably 10,000 to 70,000, from the viewpoint of suppressing the elastic modulus after stretching and obtaining a flexible film. In one preferred embodiment of the present invention, the number-average molecular weight (Mn) is 15,000 to 70,000 (more preferably 50,000 to 70,000).

[0040] The molecular weight distribution (Mw / Mn) of polyethylene resin, calculated as the ratio of weight-average molecular weight (Mw) to number-average molecular weight (Mn), is preferably 3 to 13, and more preferably 3.3 to 12, from the viewpoint of obtaining appropriate resin fluidity during biaxial stretching and efficiently obtaining a film without breakage. In a preferred embodiment of the present invention, the molecular weight distribution (Mw / Mn) is 3.3 to 11.5 (more preferably 3.3 to 5).

[0041] The melt flow rate (MFR) of polyethylene resin at 190°C and a load of 2.16 kg is not particularly limited, but from the viewpoint of reducing the mechanical load in the film-forming process, it is preferably 5 g / 10 min or less, and from the viewpoint of making the thickness of the polyethylene film of the present invention uniform, it is more preferably 0.2 g / 10 min or more and 4 g / 10 min or less. Furthermore, from the viewpoint of suppressing curling due to heat, the MFR is preferably 0.8 g / 10 min or more, more preferably 1.2 g / 10 min or more, even more preferably 1.25 g / 10 min or more, and even more preferably 1.5 g / 10 min or more.

[0042] The average molecular weight (Mz) of polyethylene resin is, for example, between 500,000 and 1,800,000. In a preferred embodiment of the present invention, the average molecular weight (Mz) is between 500,000 and 1,700,000 (more preferably between 500,000 and 900,000).

[0043] The average molecular weight and molecular weight distribution of the polyethylene resin are values ​​measured according to the method of (2-1) in the example described below. Furthermore, the MFR of the polyethylene resin is a value measured according to the method of (2-2) in the example described below.

[0044] The polyethylene film of the present invention may contain other components besides the polyethylene resin, as long as the effects of the present invention are not hindered. Examples of other components include additives contained in known resin films, such as antioxidants, heat stabilizers, light stabilizers, ultraviolet absorbers, plasticizers, lubricants, crosslinking agents, flame retardants, antistatic agents, heat resistance improvers, antiblocking agents, inorganic particles, resin particles, chlorine scavenging agents, antifogging agents, hydrolysis inhibitors, and the like. Each of these components may be used individually or in combination as needed. When the polyethylene film of the present invention contains the other components, their content is 10% by mass or less, preferably 5% by mass or less, more preferably 1% by mass or less, and particularly preferably 0.5% by mass or less, based on the total mass of the polyethylene film.

[0045] The polyethylene film of the present invention is preferably a stretched film. It is particularly preferable that the polyethylene film of the present invention is a uniaxially oriented film stretched in a uniaxial direction. If the polyethylene film of the present invention is a uniaxially oriented film, it is preferably a uniaxially oriented film stretched in the width direction (TD).

[0046] The polyethylene film of the present invention can have a single-layer structure or a multi-layer structure. A single-layer structure is preferred for the polyethylene film of the present invention. If the polyethylene film of the present invention has a multi-layer structure, each layer contains the aforementioned polyethylene resin. In this case, the polyethylene resins contained in each layer may be the same, or at least one or all of them may be different.

[0047] The thickness of the polyethylene film of the present invention is not particularly limited and can be set to any desired thickness depending on the intended application. From the viewpoint of avoiding film breakage and obtaining a stable and uniform thickness, the lower limit of the thickness is preferably 2 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, even more preferably 15 μm or more, and particularly preferably 20 μm or more. On the other hand, from the viewpoint of suppressing production costs and reducing the mechanical load of the film-forming process, the upper limit of the thickness is preferably 100 μm or less, more preferably 90 μm or less, even more preferably 80 μm or less, even more preferably 70 μm or less, particularly preferably 50 μm, and especially preferably 30 μm or less. Note that if the polyethylene film of the present invention has the multilayer structure described above, the thickness of the polyethylene film of the present invention refers to the sum of the thicknesses of each layer.

[0048] 2. Manufacturing method The method for manufacturing the polyethylene film of the present invention is not particularly limited, and a wide range of methods similar to those used for known films can be employed. Specifically, for example, the polyethylene film of the present invention can be manufactured by a manufacturing method that includes the step of obtaining a cast sheet containing polyethylene resin and stretching the cast sheet in the width direction (TD). An example of such a method will be described in detail below.

[0049] Cast sheets, which are the stretching precursors, can be obtained using known methods. For example, polyethylene resin pellets, dry-mixed polyethylene resin pellets, or mixed polyethylene resin pellets prepared by pre-melting and kneading can be supplied to an extruder, heated and melted, foreign matter and modified polymers removed through a filter, then extruded into a sheet from a T-die, and cooled and solidified in at least one cooling drum to obtain a cast sheet.

[0050] Inside the extruder, polyethylene resin undergoes some degree of degradation due to thermal and oxidative stress. From the viewpoint of suppressing such polymer degradation, the resin temperature during melt extrusion should be between 170°C and 320°C, preferably between 200°C and 300°C. Furthermore, degradation can be suppressed by adjusting the nitrogen purging inside the extruder, the screw shape, the internal shape of the T-die during casting, and the amount of antioxidant added.

[0051] The temperature of the cooling drum is preferably between 20°C and 90°C, and more preferably maintained between 40°C and 80°C. Any method can be used to bring the sheet resin into contact with the casting drum, such as the air knife method, touch roll method, electrostatic application method, or water-cooled casting method, but the air knife method is preferred because it allows for easy adjustment when bringing the sheet resin into contact with the cooling drum and is easy to handle. When using an air knife, the temperature of the blown air (AK air temperature) is preferably between 10°C and 90°C, more preferably between 15°C and 50°C, and even more preferably between 20°C and 35°C.

[0052] The polyethylene film of the present invention can be obtained by stretching the cast sheet in the width direction (TD).

[0053] The cast sheet is guided into a tenter and stretched in the width direction (TD stretching). The temperature for stretching in the width direction (TD temperature) is 80°C to 160°C, preferably 85°C to 155°C, and more preferably 90°C to 145°C. The stretching ratio in the width direction (TD ratio) is 4 to 13 times, preferably 4.5 to 12 times, and more preferably 5 to 11 times. The TD temperature should be as low as possible within the range where molding is possible (preferably 135°C or lower, more preferably 120°C or lower, even more preferably 110°C or lower, even more preferably 100°C or lower, and particularly preferably 95°C or lower) from the viewpoint of improving low-temperature shrinkage. The TD ratio should be as high as possible (preferably 6 times or more, more preferably 7 times or more, even more preferably 8 times or more, and particularly preferably 8.5 times or more) from the viewpoint of improving low-temperature shrinkage and impact strength. By setting the TD temperature and TD magnification within the aforementioned range, the molecular chains are highly oriented (i.e., the birefringence Δnyx is improved), and amorphous chains are incorporated into the crystalline layer, improving the degree of crystallinity.

[0054] In one embodiment of the present invention, after the TD stretching (first stage) described above, the material can be further stretched in the width direction (TD stretching, second stage). This can further improve the birefringence Δnyx. The temperature for the second stage TD stretching is the same as the temperature for the first stage TD stretching described above. The stretching ratio for the second stage TD stretching is, for example, 1.01 times or more and 2 times or less, preferably 1.02 times or more and 1.7 times or less, more preferably 1.05 times or more and 1.5 times or less, and even more preferably 1.07 times or more and 1.3 times or less.

[0055] Finally, the uniaxially oriented film is relaxed in the width direction to obtain the polyethylene film of the present invention. The relaxation rate in the width direction (TD relaxation rate) is 5% or more and less than 23%, preferably 8% or more and less than 22%, and more preferably 9% or more and less than 21%.

[0056] The film fed from the tenter is wound into a roll by a winding machine to obtain the polyethylene film of the present invention. At this time, the ratio of the tenter's transport speed to the winding roll speed of the winding machine (TU draw ratio: winding speed / tenter transport speed × 100) is 98% or more and less than 104%, preferably 98.5% or more and less than 103.5%, and more preferably 99% or more and less than 103%. By setting the TU draw ratio within the above range, the tension state of the molecular chains after leaving the tenter can be maintained. This suppresses subsequent relaxation of the molecular chains, and as a result, the molecular chains are highly oriented (i.e., the birefringence Δnyx is improved) and the degree of crystallinity is improved.

[0057] Furthermore, the biaxially oriented polyethylene film of the present invention can be surface-treated according to its application, as long as its properties are not impaired. Examples of surface treatments include corona discharge treatment, plasma treatment, and flame treatment.

[0058] 3.Applications The polyethylene film of the present invention can be applied to a variety of uses. In particular, the polyethylene film of the present invention is suitable as a label or packaging film, and especially as a film for labeling or packaging (shrink label or shrink packaging film) that adheres closely to the shape of an object by heating at a relatively low temperature.

[0059] Since the polyethylene film of the present invention is mainly composed of polyethylene resin, it is preferable to use it for labels or packaging of objects (e.g., PET bottles) made of materials with a specific gravity difference from polyethylene (e.g., polyethylene terephthalate). In this case, the object and the label or packaging can be efficiently separated by buoyancy separation utilizing the difference in specific gravity.

[0060] The polyethylene film of the present invention can also be used as a laminate for various applications by arranging other layers (such as a coating layer like a gas barrier layer) on one or both of its surfaces as needed. [Examples]

[0061] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0062] (1) Preparation of polyethylene resin The polyethylene resins used in the examples and comparative examples are as follows: • PE1: Dow Chemical's "TF80" (LLDPE) • PE2: Prime Polymer's "SP3010" (LLDPE) • PE3: SABIC "BX202" (LLDPE) • PE4: LG Chem Co., Ltd. "LO4904P" (HDPE) The physical properties of these polyethylene resins are shown below. The measurement method is as follows.

[0063] [Table 1]

[0064] (2) Measurement of the physical properties of polyethylene resin (2-1) Measurement of various average molecular weights and molecular weight distributions of polyethylene resins Using SEC (size exclusion chromatography), the average molecular weight and molecular weight distribution of various materials were measured under the following conditions. Equipment: HLC-8321GPC / HT (Detector: Differential Refractometer (RI)) (Manufactured by Tosoh Corporation) Column: TSKgel guardcolumnH HR (30)HT(7.5mmI.D.×7.5cm)×1 + TSKgel GMH HR -H(20)HT (7.8mm I.D. x 30cm) x 3 pieces (manufactured by Tosoh Corporation) Eluent: 1,2,4-Trichlorobenzene (for GPC, manufactured by Fujifilm Wako Pure Chemical Industries) + Dibutylhydroxytoluene (0.05%) Flow rate: 1.0mL / min Detection condition: polarization=(-) Injection volume: 300μL Column temperature: 140℃ Temperature: 40°C Sample concentration: 1 mg / mL Pretreatment: The sample was weighed, and dissolved in a solvent (1,2,4-trichlorobenzene with 0.1% dibutylhydroxytoluene) by shaking at 140°C for 1 hour. The solution was then filtered by heating through a 0.5 μm sintered filter. No insoluble material was observed in any of the sample solutions during visual inspection. Calibration Curve: A calibration curve for a quintic approximation curve was created using standard polystyrene manufactured by Tosoh Corporation. Therefore, the obtained values ​​represent the molecular weight on a polystyrene basis.

[0065] From the obtained calibration curve and SEC chromatogram, the number-average molecular weight (Mn), weight-average molecular weight (Mw), and Z-average molecular weight (Mz) were obtained using analysis software for the measurement device. The molecular weight distribution (Mw / Mn) was then obtained using these Mw and Mn values.

[0066] (2-2) Measurement of Melt Flow Rate (MFR) For each resin, the melt flow rate (MFR) in the form of raw resin pellets was measured in accordance with JIS K 7210:1999 using a melt indexer from Toyo Seiki Co., Ltd. Specifically, first, a weighed 4g sample was inserted into a cylinder heated to the test temperature (190°C) and preheated for 3.5 minutes under a load of 2.16kg. Then, the weight of the sample extruded from the bottom hole over 30 seconds was measured to determine the MFR (g / 10min). The above measurement was repeated three times, and the average value was used as the measured MFR.

[0067] (3) Preparation of polyethylene film (Example 1) PE1 was supplied to an extruder and melted at a resin temperature of 260°C. After removing foreign matter and modified polymer using a filter installed in the middle of the polymer tube, it was extruded using a T-die and wrapped around a casting drum with a surface temperature maintained at 50°C to solidify and produce a cast sheet. An air knife was used to ensure tight contact with the casting drum, and the temperature of the blown air was set to 25°C.

[0068] The cast sheet was guided to a tenter, held at both ends with clips, and stretched 6.5 times in the width-to-length direction (TD) at 110°C, after which it was relaxed by 10% in the same direction. Subsequently, after the film temperature was cooled after uniaxial stretching, the clips on the tenter were released, and the film was wound into a roll using a winding machine to obtain a transversely uniaxially oriented polyethylene film with a thickness of 25 μm. The ratio of the tenter's transport speed to the winding roll speed of the winding machine (TU draw ratio: winding speed / tenter transport speed × 100) (TU draw ratio) was set to 101%.

[0069] The film thickness was measured using a micrometer (JIS-B7502:2016) in accordance with JIS-C2330:2014.

[0070] (Example 2) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that it was stretched five times in the width direction.

[0071] (Example 3) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that it was stretched nine times in the width direction.

[0072] (Example 4) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the stretching temperature in the width direction was set to 100°C and the TU draw ratio was set to 100%.

[0073] (Example 5) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the stretching temperature in the width direction was set to 130°C, the film was stretched nine times in the width direction, and the TU draw ratio was set to 100%.

[0074] (Example 6) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the stretching temperature in the width direction was set to 90°C, the film was stretched nine times in the width direction, and the TU draw ratio was set to 103%.

[0075] (Example 7) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the resin supplied to the extruder was changed to PE3, the stretching temperature in the width direction was set to 100°C, the stretching was relaxed by 12% in the width direction, and the TU draw ratio was set to 99%.

[0076] (Example 8) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the resin supplied to the extruder was changed to PE3, the stretching temperature in the width direction was set to 145°C, and the film was stretched five times in the width direction.

[0077] (Example 9) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that it was stretched 6.5 times in the width direction at 110°C (first stage), followed by a subsequent stretching 1.1 times in the width direction (second stage).

[0078] (Example 10) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that it was stretched 6.5 times in the width direction at 110°C (first stage), followed by a subsequent stretching 1.3 times in the width direction (second stage).

[0079] (Example 11) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the resin supplied to the extruder was changed to a dry blend resin of PE1 and PE4 (by mass ratio, PE1:PE4 = 90:10), the stretching temperature in the width direction was set to 130°C, the film was stretched nine times in the width direction, and the TU draw ratio was set to 103%.

[0080] (Example 12) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the resin supplied to the extruder was changed to a dry blend resin of PE1 and PE4 (by mass ratio, PE1:PE4 = 60:40), the stretching temperature in the width direction was set to 130°C, the film was stretched nine times in the width direction, and the TU draw ratio was set to 103%.

[0081] (Example 13) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the resin supplied to the extruder was changed to PE4, the stretching temperature in the width direction was set to 130°C, the film was stretched nine times in the width direction, and the TU draw ratio was set to 103%.

[0082] (Comparative Example 1) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that the stretching temperature in the width direction was set to 155°C, the film was stretched 4.5 times in the width direction, and the TU draw ratio was set to 98%.

[0083] (Comparative Example 2) A transversely uniaxially oriented polyethylene film was obtained in the same manner as in Example 1, except that it was stretched nine times in the width direction and the TU draw ratio was set to 97%.

[0084] (Comparative Example 3) PE2 was melt-kneaded and a film was formed by the inflation method to obtain a polyethylene film with a thickness of 30 μm.

[0085] (Comparative Example 4) PE1 was supplied to an extruder and melted at a resin temperature of 260°C. After removing foreign matter and modified polymers using a filter installed in the polymer tube, the melted PE1 was extruded using a T-die and solidified on a casting drum maintained at a surface temperature of 50°C to produce a cast sheet. An air knife was used to ensure tight contact with the casting drum, with the blown air temperature set to 25°C. The resulting cast sheet was preheated to 100°C and stretched five times in the longitudinal direction (MD). It was then wound into a roll using a winding machine to obtain a 25 μm thick uniaxially oriented polyethylene film.

[0086] (Comparative Example 5) PE1 was supplied to an extruder and melted at a resin temperature of 260°C. After removing foreign matter and modified polymers using a filter installed in the polymer tube, the melted PE1 was extruded using a T-die and solidified on a casting drum maintained at a surface temperature of 50°C to produce a cast sheet. An air knife was used to ensure tight contact with the casting drum, with the blown air temperature set to 25°C. The resulting cast sheet was preheated to 100°C and stretched five times in the longitudinal direction (MD). The cast sheet was then guided to a tenter, where both ends were held with clips and stretched eight times in the widthwise direction (TD) at 125°C, followed by a 10% relaxation in the same direction. Subsequently, after cooling the biaxially stretched film, the tenter clips were released, and the film was wound into a roll using a winding machine to obtain a biaxially oriented polyethylene film with a thickness of 25 μm. The ratio of the tenter's conveying speed to the winding machine's winding speed was set to 100%.

[0087] (4) Measurement of the physical properties of polyethylene film (4-1) Measurement of physical properties of birefringence Δnyx For the polyethylene films of the examples and comparative examples, the refractive index in the width-to-length direction (TD) (Ny) and the refractive index in the length-to-length direction (MD) (Nx) were measured using an Abbe refractometer (DR-A1-Plus, manufactured by Atago Corporation) in accordance with JIS K 7142:2014. Then, the birefringence Δnyx was calculated from these two refractive indices using the formula: Δnyx = Ny - Nx.

[0088] (4-2) Measurement of the physical properties of crystallinity The degree of crystallinity of the polyethylene films in the examples and comparative examples was measured using a differential scanning calorimeter (Diamond DSC, PerkinElmer). Specifically, using a 5 mg sample, measurements were taken in the range from 25°C to 170°C at a heating rate of 10°C / min. The degree of crystallinity (%) of the polyethylene film was calculated by dividing the amount of endothermic heat of the melting peak observed during heating by the theoretical heat of fusion of perfect polyethylene crystals (293 J / g).

[0089] (5) Performance evaluation of polyethylene film (5-1) Measurement of thermal shrinkage rate The heat shrinkage rate at 110°C was measured for the polyethylene films of the examples and comparative examples. First, to measure the heat shrinkage rate in the longitudinal direction (MD), a polyethylene film was cut into a rectangle measuring 20 mm x 150 mm, and a 100 mm mark was made using a ruler to obtain a sample. In this sample, the 150 mm side extends in the longitudinal direction. The top end (short side end) of this sample was clipped and suspended in a dryer, where it was heat-treated at 110°C for 15 minutes. The sample was removed from the dryer, the distance between the markings was measured with a ruler, and the heat shrinkage rate was calculated using the formula: Heat shrinkage rate = (distance between markings before heat treatment - distance between markings after heat treatment) / distance between markings before heat treatment × 100.

[0090] Next, a sample was prepared to measure the thermal shrinkage rate in the width-to-length direction (TD). This sample was identical to the sample used for longitudinal measurement, except that the 150mm side extended in the width-to-length direction. The same heat treatment was performed on this sample as described above, and the thermal shrinkage rate was calculated.

[0091] (5-2) Measurement of impact strength The impact strength of the films in the examples and comparative examples was measured using a Yasuda Seiki film impact tester, in accordance with ASTM-D3420. A 15 kgf·cm weight was used for the measurement, with a 1 / 4-inch metal ball as the tip. Three impact strength measurements were performed per sample, and the average value was taken as the impact strength.

[0092] (6) Results Table 2 shows the resin composition, manufacturing conditions, physical property measurement results, and performance evaluation results.

[0093] [Table 2]

Claims

1. A heat-shrinkable polyethylene film containing polyethylene resin, wherein the birefringence Δnyx, calculated from the refractive index in the width direction (Ny) and the refractive index in the longitudinal direction (Nx) according to JIS K 7142:2014 using the formula: Δnyx = Ny - Nx, is 0.0250 or higher, and the degree of crystallinity is 40% or higher.

2. The heat-shrinkable polyethylene film according to claim 1, wherein the birefringence Δnyx is 0.0250 or more and 0.0550 or less.

3. The heat-shrinkable polyethylene film according to claim 1, wherein the degree of crystallinity is 40% or more and 65% or less.

4. A heat-shrinkable polyethylene film according to claim 1, wherein the heat shrinkage rate in the width direction at 110°C is 16% or more, and the heat shrinkage rate in the length direction at 110°C is 5% or less.

5. A stretched film, the heat-shrinkable polyethylene film according to claim 1.

6. The heat-shrinkable polyethylene film according to claim 1, which is a uniaxially oriented film.

7. A heat-shrinkable polyethylene film according to claim 1, wherein the thickness is 10 μm or more and 50 μm or less.

8. A heat-shrinkable polyethylene film according to any one of claims 1 to 7, which is a film for labels or packaging.

9. A laminate comprising a heat-shrinkable polyethylene film and other layers according to any one of claims 1 to 7.

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