Stretched film

Abstract

Provided is a stretched film that is laminated with a sealant film (2) to form a laminate 1. The stretched film has at least a base material layer (3) mainly composed of a high-density polyethylene having a density of 0.950 g / cm3 or more, has a tensile modulus of elasticity, in the stretching direction, of 2,000 MPa or more and less than 5,000 MPa, and has a thermal shrinkage rate inferior to 5%, in the stretching direction, when heated at 130℃for 10 minutes.

Description

stretched film

[0001] This invention relates to stretched films used in packaging films and the like.

[0002] Conventionally, packaging films used for pouches and the like have employed a packaging laminate (hereinafter sometimes simply referred to as "laminated") which consists of a base film made of a resin material and a sealant film made of a material different from the resin material that makes up the base film.

[0003] While there is a demand for reducing the environmental impact of plastics in general, and recyclability is also required for packaging films, the problem with laminates made of different materials is that separating the materials is difficult, making recycling challenging.

[0004] Therefore, in recent years, there has been a growing movement towards monomaterialization, where packaging films are made from a single material. Examples of resins used in monomaterial packaging films include polyethylene, polypropylene, and polyethylene terephthalate. Of these, polyethylene has the highest usage rate in existing packaging films and is a material for which monomaterialization is particularly in demand.

[0005] As a packaging film using polyethylene, for example, a laminate has been proposed comprising a base film and a sealant film, wherein the base film and sealant film are made of polyethylene, and the base film is subjected to a stretching treatment. It has been stated that such a configuration can provide a laminate with high recyclability, printability, and strength, as well as improved transparency of the base film (see, for example, Patent Document 1).

[0006] Japanese Patent Publication No. 2019-171860

[0007] In packaging films, various functions are required, but it is known that thermal shrinkage occurs when the film is stretched. In the laminate described in Patent Document 1 above, the density is 0.950 g / cm³. ３Because it is less than [amount missing] and mainly composed of polyethylene with a low melting point, it has a high thermal shrinkage rate, and during heat sealing performed in bag making, the base film shrinks, causing wrinkles and resulting in a poor appearance.

[0008] Therefore, the present invention has been made in view of the above problems, and aims to provide a stretched film with excellent heat resistance and printability.

[0009] To achieve the above objective, the stretched film of the present invention is a stretched film that forms a laminate when laminated with a sealant film, wherein the stretched film has a density of 0.950 g / cm³. ３ The material has at least one base layer mainly composed of high-density polyethylene, a tensile modulus in the stretching direction of 2000 MPa or more and less than 5000 MPa, and a thermal shrinkage rate of less than 5% when heated at 130°C for 10 minutes in the stretching direction.

[0010] According to the present invention, it is possible to provide a stretched film with excellent heat resistance and printability.

[0011] This is a cross-sectional view illustrating a packaging laminate using the stretched film of the present invention. This is a plan view illustrating a packaging laminate using the stretched film of the present invention. This is a cross-sectional view illustrating a modified example of the packaging laminate using the stretched film of the present invention. This is a cross-sectional view illustrating a modified example of the packaging laminate using the stretched film of the present invention. This is a cross-sectional view illustrating a modified example of the packaging laminate using the stretched film of the present invention.

[0012] The stretched film of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments, and can be modified and applied as appropriate without altering the essence of the invention.

[0013] Figure 1 is a cross-sectional view showing a packaging laminate using the stretched film of the present invention.

[0014] The packaging laminate 1 comprises a base film substrate layer (stretched film) 3 and a sealant film 2 laminated on the substrate layer 3.

[0015] <Sealant Film> From the viewpoint of monomaterials, polyethylene resins are preferred as the resin constituting the sealant film 2 of the present invention. More specifically, low-density polyethylene (LDPE, density: 0.910 g / cm³) is preferred. ３ 0.929g / cm or more ３ Below), linear low-density polyethylene (LLDPE, density: 0.910 g / cm³) ３ 0.925g / cm or more ３ The following), and medium-density polyethylene (MDPE, density: 0.930 g / cm³). ３ 0.949g / cm or more ３ Examples include the following:

[0016] Of these, linear low-density polyethylene (LLDPE) is preferable from the viewpoint of having excellent heat-sealing properties, impact resistance, and transparency.

[0017] Furthermore, the melting point of the polyethylene resin is preferably in the range of 116°C or higher, and more preferably 120°C or higher. When the melting point is 116°C or higher, the rigidity of the sealant film 2 is improved, making it possible to use it as a sealant film in packaging films that are self-supporting, such as packaging films used for packaging high-temperature contents or stand-up pouches.

[0018] The "melting point" mentioned above refers to the melting point measured in accordance with JIS K 7121, which is determined by measuring the temperature at which the main endothermic peak appears using a differential scanning calorimeter (DSC).

[0019] Furthermore, in order to improve heat-sealability, a polyethylene-based resin with a lower melting point than the base film is used to create a difference in melting points with the base film.

[0020] The polyethylene content in the sealant film 2 is preferably 70% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass.

[0021] Also, the thickness of the sealant film 2 is preferably 50 μm to 150 μm, more preferably 80 μm to 150 μm. This is because when the thickness is less than 50 μm, the rigidity of the sealant film 2 decreases, making it difficult to apply it to a self-standing packaging film that is made of only the above-mentioned packaging film, stand pouch, etc. When the thickness is 150 μm or more, the cost increases and long-time heating is required during bag making, which may lead to a decrease in productivity.

[0022] Further, the sealant film 2 may contain other components other than the above-mentioned polyethylene-based resin within a range that does not impair the properties of the sealant film 2.

[0023] Examples of other components include olefin-based resins, amide-based antiblocking agents (such as stearic acid amide), plasticizers, ultraviolet absorbers, antioxidants, weather stabilizers, antistatic agents, colorants, antifogging agents, metal soaps, waxes, fungicides, antibacterial agents, nucleating agents, flame retardants, lubricants, etc.

[0024] <Base material layer> The base material layer 3 of the present invention is a stretched film composed mainly of high-density polyethylene (HDPE). In the present invention, the density of high-density polyethylene is 0.950 g / cm ３ or more. This is because when the density is less than 0.950 g / cm ３ the melting point of polyethylene becomes low, so the heat shrinkage rate increases and the heat resistance decreases, and at the same time, the film strength decreases. Also, from the viewpoint of improving heat resistance and film strength, the density of high-density polyethylene is preferably 0.960 g / cm ３ or more.

[0025] Also, the density of high-density polyethylene is preferably 0.971 g / cm ３ or less. This is because when the density is greater than 0.971 g / cm ３ it becomes hard and prone to cracking, which may make it difficult to produce the stretched film.

[0026] In other words, in the present invention, the density of the high-density polyethylene constituting the base layer 3 is 0.950 g / cm³. ３ As described above, the melting point of polyethylene is increased, which makes uniaxial stretching at the stretching temperature described later possible and suppresses the increase in thermal shrinkage rate (i.e., the thermal shrinkage rate of the stretched film when heated at 130°C for 10 minutes in the stretching direction is less than 5%), thereby improving heat resistance and film strength.

[0027] Furthermore, from the viewpoint of further improving heat resistance, the content of high-density polyethylene relative to the total (100% by mass) of the base layer 2 is preferably 50% by mass or more (i.e., it is the main component of the base layer 3), and from the viewpoint of improving recyclability, it is more preferably 70% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more.

[0028] Furthermore, it is preferable to use high-density polyethylene (m-HDPE) polymerized using a metallocene catalyst as the high-density polyethylene constituting the base layer 3. Polymerization using a metallocene catalyst results in a uniform active site structure, making it possible to perform polymerization with fewer low molecular weight components and a smaller molecular weight distribution. Consequently, the entanglement of molecular chains increases, making it possible to improve the impact strength and puncture strength of the base layer 3.

[0029] Furthermore, the melting point of high-density polyethylene is preferably 130°C or higher and less than 140°C, more preferably 132°C or higher and less than 140°C, and even more preferably 135°C or higher and less than 140°C. This is because if the melting point is below 130°C, stretching to the stretching temperature (125°C) or higher, as described later, may not be possible, and the heat resistance of the stretched film may decrease.

[0030] Furthermore, it is preferable that the molecular weight distribution (Mw / Mn), which is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the high-density polyethylene, be 3.0 or less. With this configuration, the molecular weight distribution (Mw / Mn) becomes smaller, which increases the entanglement of molecular chains and makes it possible to improve the impact strength and puncture strength of the base layer 3.

[0031] Note that the above weight-average molecular weight (Mw) and number-average molecular weight (Mn) refer to values in terms of polystyrene obtained in gel permeation chromatography (GPC) measurement.

[0032] Also, the melt mass flow rate (MFR) of high-density polyethylene is preferably 0.01 to 3.00 g / 10 min, more preferably 0.02 to 2.50 g / 10 min, and even more preferably 0.1 to 2.00 g / 10 min. This is because when the melt mass flow rate (MFR) is 0.01 g / 10 min or more, it can be molded with a general-purpose extruder without using special equipment, and when it is 3.00 g / 10 min or less, sufficient film strength can be provided.

[0033] Note that the above melt mass flow rate is obtained by measuring in accordance with the provisions of JIS K7210:1999.

[0034] Also, the base material layer 3 may contain the above other components as long as the heat resistance of the base material layer 3 is not impaired.

[0035] <Manufacturing method of packaging laminate> Next, an example of the manufacturing method of a packaging laminate using the stretched film of the present invention will be described in detail.

[0036] First, a raw material containing the above high-density polyethylene is formed into a film shape using an extruder to produce the base material layer 3.

[0037] More specifically, high-density polyethylene and, if necessary, the above other components are mixed at a predetermined blending ratio, and melt-extruded into a film shape using an extruder equipped with a T-die to obtain an original film before stretching.

[0038] Note that, similar to the content of high-density polyethylene in the entire base material layer 3 described above, the content of high-density polyethylene in the entire original film is 50% by mass or more out of 100% by mass of the original film.

[0039] Then, by subjecting the raw film to a uniaxial stretching process, a stretched film that will become the base layer 3, as shown in Figures 1 and 2, is manufactured. The stretching method is not particularly limited and examples include roll stretching and tenter stretching.

[0040] The uniaxial stretching process described above is a stretching process performed in either the direction of the machine axis (longitudinal) of the film (hereinafter referred to as "MD") or the direction perpendicular to the MD (hereinafter referred to as "TD"), as shown in Figure 2. By performing uniaxial stretching, the high-density polyethylene is oriented and the elastic modulus of the film is improved, making it possible to prevent printing misalignment caused by elongation during the printing process.

[0041] In uniaxial stretching, the stretching temperature is 125°C or higher and less than 130°C, preferably 127°C or higher and less than 128°C, when the stretched film is composed solely of high-density polyethylene. This is because if the stretching temperature is below 125°C, the thermal shrinkage rate increases, which can reduce heat resistance, and if the stretching temperature is above 130°C, the film may melt and break.

[0042] In other words, if the stretching temperature is within the above range, the effect of heat fixation becomes greater, thus improving heat resistance.

[0043] Furthermore, the stretching ratio in the uniaxial stretching process is between 4 and 10 times. This is because if the stretching ratio is 4 times or less, the film stretches unevenly, making it difficult to suppress printing misalignment caused by film stretching during the printing process. Also, if the stretching ratio is greater than 10 times, the film may break. Moreover, from the viewpoint of suppressing printing misalignment and preventing film breakage, the stretching ratio is preferably between 5 and 8 times, more preferably between 5 and 7 times, and even more preferably between 5 and 6 times.

[0044] The stretched film produced by the stretching process described above has a density of 0.950 g / cm³. ３The main component is high-density polyethylene, and because high-density polyethylene has a high melting point, it is possible to suppress the increase in the thermal shrinkage rate. As shown in the examples below, the thermal shrinkage rate when heated at 120°C for 10 minutes, 130°C for 10 minutes, and 135°C for 10 minutes in the stretching direction of the film is less than 5%. Therefore, the dimensional stability due to heat treatment is increased, and the occurrence of wrinkles due to thermal shrinkage during the heat sealing process in bag making can be prevented, making it possible to obtain excellent heat resistance.

[0045] Furthermore, from the viewpoint of improving heat resistance, the above-mentioned heat shrinkage rates in the stretched film are preferably less than 3%, more preferably less than 2%, and particularly preferably less than 1%.

[0046] Furthermore, the aforementioned "thermal shrinkage rate" can be determined by the method described in the examples below.

[0047] Furthermore, in stretched films, the tensile modulus of elasticity in the stretching direction of the film is 2000 MPa or more and less than 5000 MPa. If the tensile modulus of elasticity is less than 2000 MPa, it may be difficult to suppress the occurrence of printing misalignment due to the stretching of the film when transporting the film during the printing process. Also, if the tensile modulus of elasticity is 5000 MPa or more, the flexibility of the film decreases, which may cause cracks or fissures.

[0048] In other words, in the present invention, since the tensile modulus of the stretched film is 2000 MPa or more and less than 5000 MPa, a decrease in flexibility can be prevented and printability can be improved.

[0049] Furthermore, the tensile modulus is preferably 2500 MPa or higher, and more preferably 3000 MPa or higher.

[0050] Furthermore, the aforementioned "tensile modulus" can be obtained by measuring it in accordance with JIS K 7127.

[0051] Furthermore, in the stretched film of the present invention, it is preferable that the storage modulus at 100°C in the stretching direction of the film is 400 MPa or more. This is because if the storage modulus is less than 400 MPa, it may be difficult to suppress the occurrence of printing misalignment due to the elongation of the film when transporting the film during the printing process.

[0052] Furthermore, the storage modulus is more preferably 500 MPa or higher, even more preferably 600 MPa or higher, and particularly preferably 700 MPa or higher.

[0053] Furthermore, the "storage modulus" mentioned above can be determined by the method described in the examples below.

[0054] Furthermore, it is preferable that the puncture strength of the stretched film be 1.5 N or higher. If the puncture strength is less than 1.5 N, the film may rub against the packaged item or the transport container (cardboard, etc.) and tear during transport.

[0055] Furthermore, a puncture strength of 2.0 N or higher is more preferable, 2.5 N or higher is even more preferable, and 3.0 N or higher is particularly preferable.

[0056] Furthermore, the above-mentioned "puncture strength" is obtained by measuring it in accordance with JIS Z 1707 ("General Rules for Plastic Films for Food Packaging," "7.4 Puncture Strength Test").

[0057] The thickness of the raw film before stretching is preferably 100 μm to 300 μm, and more preferably 100 μm to 250 μm. If the thickness of the raw film is 100 μm or more, sufficient strength to withstand the stress during stretching can be obtained. Furthermore, if the thickness of the raw film is 300 μm or less, sufficient transparency can be obtained after stretching.

[0058] Furthermore, the thickness of the stretched film after stretching is preferably 10 μm to 40 μm, more preferably 15 μm to 35 μm, and even more preferably 20 μm to 30 μm. If the thickness of the stretched film after stretching is 10 μm or more, sufficient strength can be obtained as a base film. Also, if the thickness of the stretched film after stretching is 40 μm or less, sufficient transparency can be obtained, costs can be reduced, and especially when used for flexible packaging, the amount of plastic used can be reduced, thus providing an environmentally friendly packaging film.

[0059] The stretched film used as the base film may be a single layer or a multi-layered film of two or more layers. If the stretched film is multi-layered, the composition and thickness of each layer may be the same or different.

[0060] Next, a raw material containing polyethylene-based resins such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and medium-density polyethylene (MDPE) is prepared, and a sealant film 2 is produced by forming it into a film using the extruder described above.

[0061] Then, by laminating the stretched film (base layer 3) and the sealant film 2, the packaging laminate (packaging film) 1 shown in Figure 1 is manufactured.

[0062] By the above method, the present invention makes it possible to obtain a packaging laminate with excellent heat resistance and printability.

[0063] <Other Embodiments> In the embodiments described above, a packaging laminate 1 comprising a base layer (stretched film) 3 and a sealant film 2 laminated on the base layer 3 was used as an example. However, the packaging laminate of the present invention only needs to have the base layer 3 and sealant film 2 described above. For example, as shown in Figure 3, a coating layer 4 made of polyvinyl alcohol or the like is provided on at least one surface of the base layer 3 (in Figure 3, the surface of the base layer 3 on the sealant film 2 side), and the packaging laminate 20 may have a three-layer structure in which the sealant film 2 / coating layer 4 / base layer 3 are laminated in that order.

[0064] Furthermore, by providing such a coating layer 4, it becomes possible to impart high gas barrier properties (characteristics that suppress the permeation of gases such as oxygen and water vapor).

[0065] Furthermore, as shown in Figure 4, a packaging laminate 30 may also have a five-layer structure in which a vapor-deposited layer 6 made of silica, aluminum, alumina, etc. is provided on at least one side of the base layer (stretched film) 3 (in Figure 4, the side of the base layer 3 opposite to the sealant film 2 side) via an anchor coat layer 5, and a top coat layer 7 is provided on the surface of the vapor-deposited layer 6, and the layers are stacked in the order of sealant film 2 / base layer 3 / anchor coat layer 5 / vapor-deposited layer 6 / top coat layer 7.

[0066] Furthermore, by providing such a vapor-deposited layer 6, it becomes possible to impart high gas barrier properties (characteristics that suppress the permeation of gases such as oxygen and water vapor).

[0067] Furthermore, although the above-described embodiment described an example of a packaging laminate 1 comprising a stretched film having a single-layer structure consisting only of the base material layer 3, the stretched film of the present invention only needs to have at least a base material layer. For example, as shown in Figure 5, the packaging laminate 40 may comprise a stretched film 12 having a five-layer structure in which a base material layer (first base material layer) 3 / first adhesive layer 8 / barrier layer 9 / second adhesive layer 10 / second base material layer 11 are laminated in that order (i.e., a base material layer is provided on at least one side of the barrier layer), and a sealant film 2 laminated on the stretched film 12.

[0068] The barrier layer 9 is made of a barrier resin that has the function of suppressing the permeation of gases such as oxygen and water vapor, and at least one of ethylene-vinyl alcohol copolymer (EVOH) and butenediol-vinyl alcohol copolymer (BVOH) can be used as the barrier resin.

[0069] In this case, first, resin materials for forming each layer are prepared, and then, using a co-extruder equipped with a T-die for five types and five layers, the resin materials for forming each layer are extruded at a predetermined temperature to form a five-layer film in which the first base layer, the first adhesive layer, the barrier layer, the second adhesive layer, and the second base layer are laminated in this order, thereby obtaining a raw film before stretching. Then, the raw film is stretched by performing uniaxial stretching in MD or TD under predetermined stretching temperature and stretching ratio conditions, thereby producing a stretched film having the five-layer structure shown in Figure 5.

[0070] The present invention will be described below based on examples. However, the present invention is not limited to these examples, and these examples can be modified and altered in accordance with the spirit of the invention; such modifications do not exclude them from the scope of the invention.

[0071] The materials used to produce the stretched film are as follows: (1) HDPE1: High-density polyethylene (density: 0.951 g / cm³) ３ (1) HDPE2: High-density polyethylene (density: 0.960 g / cm³) ３ (3) MDPE: Medium-density polyethylene (density: 0.943 g / cm³) ３(4) EVOH: Ethylene-vinyl alcohol copolymer (density: 1.19 g / cm³) ３ (5) Acid-modified polyethylene: (Density: 0.91 g / cm³) ３ Melting point (measured according to JIS K 7121): 120°C, MFR: 2.3 g / 10 min, manufactured by Mitsui Chemicals, Inc. (Admer NF587)

[0072] (Example 1) <Preparation of stretched film> First, high-density polyethylene as shown in Table 1 was prepared. Next, the prepared high-density polyethylene was formed into a film by melt extrusion (extrusion temperature: 200°C) using an extruder equipped with a T-die (LABTECH Co., Ltd., product name: LCR-350), and the film was wound onto a winding roll to obtain a raw film roll before stretching with the thickness shown in Table 1.

[0073] Then, using a stretching machine (LABTECH, product name: LMDO-350), the raw film was subjected to uniaxial stretching in the medium-density direction (MD) under the stretching temperature and stretching ratio conditions shown in Table 1, thereby stretching the raw film and producing a single-layer stretched film made of high-density polyethylene with the thickness shown in Table 1.

[0074] <Calculation of Heat Shrinkage Rate at 120°C> A sample of a predetermined size (12 cm x 12 cm) was cut from the prepared stretched film. Orthogonal markings, each 10 cm long and parallel to the edge, were drawn 1 cm inward from each edge of the sample. The sample was placed in an oven at 120°C and heated for 10 minutes. After removal, it was cooled to room temperature (approximately 25°C). The distance between the markings in the stretching direction (i.e., MD) was measured in the heat-treated sample. The heat shrinkage rate [%] was calculated from the change in the distance between the markings before and after heating in the stretching direction using the following formula (1), and this was used as an indicator of heat resistance. The results are shown in Table 1.

[0075] Thermal shrinkage rate in the stretching direction [%] = [(gauge distance before heating - gauge distance after heating) / gauge distance before heating] × 100 (1)

[0076] <Calculation of Heat Shrinkage Rate at 130°C> A sample of a predetermined size (12 cm x 12 cm) was cut from the prepared stretched film. Orthogonal markings, each 10 cm long and parallel to the edge, were drawn 1 cm inward from each edge of the sample. The sample was placed in an oven at 130°C and heated for 10 minutes. After removal, it was cooled to room temperature (approximately 25°C). The distance between the markings in the stretching direction (i.e., MD) was measured in the sample after heat treatment. The heat shrinkage rate [%] was calculated from the change in the distance between the markings before and after heating in the stretching direction using the above formula (1), and this was used as an indicator of heat resistance. The results are shown in Table 1.

[0077] <Calculation of Heat Shrinkage Rate at 135°C> A sample of a predetermined size (12 cm x 12 cm) was cut from the prepared stretched film. Orthogonal markings, each 10 cm long and parallel to the edge, were drawn 1 cm inward from each edge of the sample. The sample was placed in an oven at 135°C and heated for 10 minutes. After removal, it was cooled to room temperature (approximately 25°C). The distance between the markings in the stretching direction (i.e., MD) was measured in the sample after heat treatment. The heat shrinkage rate [%] was calculated from the change in the distance between the markings before and after heating in the stretching direction using the above formula (1), and this was used as an indicator of heat resistance. The results are shown in Table 1.

[0078] <Measurement of Melting Point> The melting point of the fabricated stretched film was measured by differential calorimetry (DSC measurement). More specifically, a 5 mg sample was prepared from the fabricated stretched film, and in accordance with JIS K 7121, the sample was sealed in a differential calorimetry meter (Hitachi High-Tech Science Corporation, product name: DSC7000X). Nitrogen was then flowed as a carrier gas at a rate of 30 ml / min, and the temperature was increased at a rate of 10 °C / min in the temperature range of 30 °C to 200 °C, and the melting point was measured. The results are shown in Table 1.

[0079] <Measurement of Storage Modulus at 100°C> A sample of a predetermined size (50 mm long x 4 mm wide) was cut from the prepared stretched film, and this sample was attached to the sample holder of a viscoelasticity measuring device (TA Instruments "DMA-Q800"). The measurement mode was set to tensile mode, the frequency to 1 Hz, and the load to 0.2 N. The storage modulus [MPa] at 100°C was measured in the temperature range of room temperature (approximately 25°C) to 135°C, under the condition of a heating rate of 3°C / min. The results are shown in Table 1.

[0080] <Measurement of Tensile Modulus> The tensile modulus [MPa] of the prepared stretched film was measured in accordance with JIS K 7127. More specifically, strip-shaped test pieces measuring 200 mm in the MD direction and 10 mm in the TD direction were prepared from the prepared stretched film. Using a tensile testing machine (Shimadzu Corporation, product name: Autograph AG-5000A), the test pieces were pulled in the stretching direction (MD) under the conditions of a temperature of 25°C and a humidity of 65% RH, a chuck distance of 80 mm, and a tensile speed of 10 mm / min. The ratio of the tensile stress corresponding to the strain between two points of strain from 0 to 1% and the corresponding strain was calculated, and the calculated value was defined as the tensile modulus [MPa]. The results are shown in Table 1.

[0081] <Measurement of Puncture Strength> The puncture strength [N] of the prepared stretched film was measured in accordance with JIS Z 1707 ("General Rules for Plastic Films for Food Packaging," "7.4 Puncture Strength Test"). More specifically, using a puncture tester (manufactured by IMADA Corporation, product name: TKS-250N / EMX-1000N), a needle with a diameter of φ1.0 mm × 0.5 mmR was punctured into the prepared stretched film at a puncture speed of 50 mm / min, and the strength [N] at which the needle penetrated the stretched film was measured.

[0082] Furthermore, the puncture strength was measured five times (N=5) when the stretched film was pierced from the surface side, and the average value was defined as the puncture strength [N]. The results are shown in Table 1.

[0083] (Examples 2-6, Comparative Examples 1-7) Stretched films were produced by stretching raw film rolls having the thicknesses shown in Tables 1-2, in the same manner as in Example 1 described above, except that the composition of the stretched film (i.e., the high-density polyethylene used) or the conditions for uniaxial stretching were changed to those shown in Tables 1-2.

[0084] Then, in the same manner as in Example 1 described above, the thermal shrinkage rate at 120°C, 130°C, and 135°C was calculated, the melting point was measured, the storage modulus at 100°C was measured, the tensile modulus was measured, and the puncture strength was measured. The results are shown in Tables 1 and 2.

[0085] In Comparative Example 1, the stretched film was composed solely of medium-density polyethylene, and because the density of polyethylene was low (the melting point of the stretched film was low), the thermal shrinkage rate of the stretched film at 120°C was large (i.e., the thermal shrinkage rate at 120°C was 5% or more), indicating poor heat resistance. Furthermore, the stretched film melted during the calculation of the thermal shrinkage rate at 130°C and 135°C. Therefore, in Comparative Example 1, it was not possible to calculate the thermal shrinkage rate at 130°C and 135°C.

[0086] Furthermore, in Comparative Example 2, the raw film before stretching was composed solely of medium-density polyethylene, and because polyethylene has a low density (low melting point), the stretched film melted at the stretching temperature (125°C) during film formation in the uniaxial stretching process. Consequently, in Comparative Example 2, it was not possible to calculate the thermal shrinkage rate at 120°C, 130°C, 135°C, measure the melting point, measure the storage modulus at 100°C, measure the tensile modulus, and measure the puncture strength.

[0087] Furthermore, in Comparative Example 3, the stretched film is composed solely of high-density polyethylene, and the stretching temperature during film formation in the uniaxial stretching process is low (115°C). As a result, the thermal shrinkage rates of the stretched film at 120°C, 130°C, and 135°C are high (i.e., each of the aforementioned thermal shrinkage rates is 5% or higher), indicating poor heat resistance.

[0088] Furthermore, in Comparative Example 4, the raw film before stretching was composed solely of high-density polyethylene, and the stretching temperature during film formation in the uniaxial stretching process was 130°C, causing the stretched film to melt. Consequently, in Comparative Example 4, it was not possible to calculate the thermal shrinkage rate at 120°C, 130°C, 135°C, measure the melting point, measure the storage modulus at 100°C, measure the tensile modulus, or measure the puncture strength.

[0089] Furthermore, in Comparative Example 5, the raw film before stretching was composed solely of high-density polyethylene, and the stretching ratio during film formation in the uniaxial stretching process was high (11 times), resulting in the stretched film breaking. Consequently, in Comparative Example 5, it was not possible to calculate the thermal shrinkage rate at 120°C, 130°C, 135°C, measure the melting point, measure the storage modulus at 100°C, measure the tensile modulus, or measure the puncture strength.

[0090] Furthermore, in Comparative Example 6, the raw film before stretching was composed solely of high-density polyethylene, and the stretching ratio during film formation in the uniaxial stretching process was low (3 times), resulting in uneven elongation of the film. Consequently, in Comparative Example 6, it was not possible to calculate the thermal shrinkage rate at 120°C, 130°C, 135°C, measure the melting point, measure the storage modulus at 100°C, measure the tensile modulus, and measure the puncture strength.

[0091] (Example 7) First, high-density polyethylene, ethylene-vinyl alcohol copolymer, and acid-modified polyethylene were prepared as shown in Table 3. Next, using a multilayer extruder equipped with a T-die (manufactured by LABTECH), the prepared high-density polyethylene, ethylene-vinyl alcohol copolymer, and acid-modified polyethylene were co-extruded in layers at an extrusion temperature of 200°C to form a five-layer film in which a first base layer, a first adhesive layer, a barrier layer, a second adhesive layer, and a second base layer were laminated in this order. The film was then wound onto a winding roll to obtain a raw film roll before stretching with the thickness shown in Table 3.

[0092] The thickness of the first substrate layer in the raw film was 58.5 μm, the thickness of the first adhesive layer was 9.4 μm, the thickness of the barrier layer was 20.8 μm, the thickness of the second adhesive layer was 9.4 μm, and the thickness of the second substrate layer was 58.5 μm, resulting in a total thickness of 156.6 μm for the raw film.

[0093] Then, the raw film was stretched by uniaxial stretching in the medium-density (MD) direction under the stretching temperature and stretching ratio conditions shown in Table 3, thereby producing a stretched film having the thickness shown in Table 3 and the five-layer structure shown in Figure 5.

[0094] In the stretched film, the thickness of the first substrate layer was 9.3 μm, the thickness of the first adhesive layer was 1.5 μm, the thickness of the barrier layer was 3.3 μm, the thickness of the second adhesive layer was 1.5 μm, and the thickness of the second substrate layer was 9.3 μm, resulting in a total thickness of 24.9 μm for the stretched film.

[0095] Then, in the same manner as in Example 1 described above, the thermal shrinkage rate at 120°C, 130°C, and 135°C was calculated, the melting point was measured, the storage modulus at 100°C was measured, the tensile modulus was measured, and the puncture strength was measured. The results are shown in Table 3.

[0096]

[0097]

[0098]

[0099] As shown in Tables 1 and 3, the stretched films of Examples 1 to 7 have a density of 0.950 g / cm³. ３ The above-mentioned high-density polyethylene is the main component, and the heat shrinkage rate of the stretched film when heated at 130°C for 10 minutes in the stretching direction is less than 5%, indicating high dimensional stability after heat treatment and excellent heat resistance. Furthermore, in the stretched films of Examples 1 to 7, the tensile modulus of elasticity in the stretching direction is 2000 MPa or more and less than 5000 MPa, which prevents a decrease in flexibility, and also suppresses the occurrence of printing misalignment due to film elongation when the film is transported during the printing process, thereby improving printability.

[0100] On the other hand, the film of Comparative Example 7 is a raw film (unstretched film) made of high-density polyethylene, so it has a low tensile modulus, making it difficult to suppress the occurrence of printing misalignment when transporting the film during the printing process, and thus it is clear that it has poor printability.

[0101] As described above, the present invention is suitable for stretched films used, for example, in packaging films.

[0102] 1. Packaging laminate 2. Sealant film 3. Substrate layer (stretched film) 4. Coating layer 6. Vapor deposition layer 9. Barrier layer

Claims

1. A stretched film that forms a laminate when laminated with a sealant film, wherein the stretched film has a density of 0.950 g / cm³. ３ A stretched film having at least a base layer mainly composed of high-density polyethylene, having a tensile modulus in the stretching direction of 2000 MPa or more and less than 5000 MPa, and having a thermal shrinkage rate of less than 5% when heated at 130°C for 10 minutes in the stretching direction.

2. The stretched film according to claim 1, characterized in that the molecular weight distribution (Mw / Mn), which is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the high-density polyethylene obtained by gel permeation chromatography (GPC) measurement, is 3.0 or less.

3. The stretched film according to claim 1 or 2, characterized in that the stretched film has a barrier layer formed of a barrier resin, and the base layer is provided on at least one surface of the barrier layer.

4. The stretched film according to claim 3, characterized in that the barrier resin is formed of at least one of an ethylene-vinyl alcohol copolymer and a butenediol-vinyl alcohol copolymer.

5. A laminate characterized in that a vapor-deposited layer is provided on at least one surface of the stretched film described in claim 1 or claim 2.

6. A laminate characterized in that a coating layer is provided on at least one surface of the stretched film described in claim 1 or claim 2.

7. A laminate comprising the stretched film described in claim 3 and the sealant film laminated on the stretched film.

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