Substrate film for packaging film

Abstract

Provided is a substrate film (F) for forming a packaging film by being laminated with a sealant film. The substrate film (F) is composed of a stretched film comprising at least one base layer (LP1, LP2) containing, as a main component, a high-density polyethylene having a density of at least 0.95 g / cm3, and has a thermal shrinkage rate of less than 5% in the stretching direction at 120 °C and a piercing strength of at least 2.0 N.

Description

Base film for packaging films

[0001] The present invention relates to a base film used to form various packaging films.

[0002] Films used to form packaging bags such as pouches and gusset bags used to package daily necessities, food, etc. (so-called packaging films) are made more functional by laminating films made of different materials such as the following: - Base film: This film improves strength and serves as a printing base material, and is made of nylon, polyester, polypropylene, etc. - Barrier film: This film blocks oxygen, water vapor, light, etc., and is made of barrier resins such as EVOH (ethylene-vinyl alcohol copolymer), or synthetic resin films vapor-deposited with metals such as aluminum or inorganic materials such as its oxides. - Sealant film: This film serves to heat-seal (thermally fuse) films together, and is made of low-density polyethylene, linear low-density polyethylene, etc.

[0003] In recent years, there has been a demand for reducing the environmental impact of plastic products in general, and recycling of packaging films is also being sought. However, laminated films of different materials as described above have the problem that they are difficult to separate and recycle. Therefore, as a means of improving recyclability, a method of using a single resin raw material has been proposed. In view of the fact that polyethylene is the most widely used packaging material, a packaging film with excellent recyclability has been developed, in which both the sealant film and the base film are made of polyethylene, as in Patent Document 1.

[0004] However, the packaging film in which both the sealant film and the base film of Patent Document 1 are made of polyethylene has a drawback in that it is prone to misalignment during printing (so-called printing misalignment) due to insufficient mechanical strength. Therefore, in order to prevent such printing misalignment, a method has been proposed in which a uniaxially stretched film made of polyethylene is used as the base film, as in Patent Document 2.

[0005] JP 2022-053859 A JP 2020-055162 A

[0006] However, the packaging film of Patent Document 2 uses medium-density polyethylene as the raw material for the base film, and therefore has bag-breaking resistance, but the difference between the heat-resistant temperature and the melting temperature of the sealant film is small, so heat resistance is insufficient, and wrinkles occur during the heat-sealing process during bag production. In addition, although a uniaxially oriented film is used as the base film, its rigidity is not sufficient, and positional deviation (i.e., printing deviation) may occur due to elongation caused by stress during the printing process.

[0007] An object of the present invention is to provide a base film for packaging films that not only overcomes the problems of conventional base films for packaging films such as those described in Patent Document 2, but also has excellent recyclability, is free from printing misalignment due to stretching during the printing process, has heat resistance that prevents wrinkles from forming when heat-sealed during bag production, and is resistant to bag rupture without the formation of pinholes.

[0008] The present invention relates to a base film for forming a packaging film by laminating it with a sealant film, and the base film has a density of 0.95 g / cm ３ The film is made of a stretched film consisting of at least one base layer mainly composed of the above high-density polyethylene, and is characterized by a heat shrinkage rate of less than 5% in the stretching direction at 120°C and a puncture strength of 2.0 N or more. ３ The above high-density polyethylene is hereinafter referred to as specific high-density polyethylene.

[0009] The invention described in claim 2 is characterized in that, in the invention described in claim 1, the molecular weight distribution of the high-density polyethylene is 3.0 (Mw / Mn) or less, and the content ratio of the specific high-density polyethylene in the base layer is 90 mass% or more.

[0010] The above-mentioned "Mw / Mn" refers to the ratio (molecular weight distribution) of the weight average molecular weight (Mw) to the number average molecular weight (Mn), and the Mw and Mn of polyethylene refer to polystyrene-equivalent values ​​obtained by gel permeation chromatography (GPC) measurement.

[0011] The invention described in claim 3 is a barrier substrate film characterized by comprising two base layers, one on the front side and one on the back side, each made of the specific high-density polyethylene having a molecular weight distribution of 3.0 (Mw / Mn) or less described in claim 1 or claim 2, and a barrier layer made of a barrier resin between the two base layers.

[0012] The invention described in claim 4 is characterized in that, in the invention described in claim 3, the barrier resin contains an ethylene-vinyl alcohol copolymer as a main component.

[0013] The invention described in claim 5 is characterized in that a vapor deposition layer is provided on one side of the substrate film described in claim 1 or claim 2.

[0014] The sixth aspect of the present invention is characterized in that a coating layer is provided on one side of the substrate film according to the first or second aspect.

[0015] The seventh aspect of the present invention is characterized in that a sealant film is laminated on one of the base layers of the barrier substrate film according to the third aspect.

[0016] The base film for packaging film (hereinafter simply referred to as the base film) described in claim 1 is made of polyethylene, and therefore does not require separation when discarding packaging film laminated with a sealant film made of polyethylene, making it highly recyclable. Furthermore, the base film described in claim 1 has excellent mechanical strength and rigidity, so printing misalignment due to stretching during the printing process does not occur, and it has excellent heat resistance, making it possible to prevent wrinkles from occurring due to heat sealing during bag production. In addition, the base film described in claim 1 has high puncture strength and excellent bag tear resistance, making it possible to prevent the occurrence of pinholes.

[0017] The substrate film described in claim 2 has a high ratio of specific high-density polyethylene with a very narrow molecular weight distribution in the base layer, and therefore has excellent recyclability and can be melted and reused.In addition, it can very effectively prevent printing misalignment due to stretching during the printing process, wrinkles due to heat sealing, and pinholes.

[0018] The barrier substrate film described in claim 3 has a barrier layer made of a barrier resin between two base layers, one on the front and one on the back, made of specific high-density polyethylene with an extremely narrow molecular weight distribution, and therefore can impart high barrier properties (properties that inhibit the permeation of gases such as oxygen and water vapor) to the packaging bag after bag production.

[0019] The barrier substrate film according to claim 4 contains a barrier resin mainly composed of an ethylene-vinyl alcohol copolymer, and therefore can impart very high barrier properties to the packaging bag after bag formation without affecting good recyclability.

[0020] The barrier substrate film according to claim 5 has a vapor-deposited layer on one side of the substrate film, and therefore can impart extremely high barrier properties to the packaging bag after it is made without affecting its good recyclability.

[0021] The barrier substrate film according to claim 6 has various coating layers on one side of the substrate film, and therefore can impart extremely high barrier properties to the packaging bag after it is made without affecting its good recyclability.

[0022] The packaging film according to the seventh aspect of the present invention has a sealant film laminated on one of the base layers of the barrier substrate film, so that a packaging bag can be easily formed.

[0023] FIG. 2 is an explanatory diagram (vertical cross-sectional view) showing the layer structure of a barrier substrate film.

[0024] The substrate film according to the present invention is composed of a stretched film made of a synthetic resin containing a specific high-density polyethylene as a main component, and has a heat shrinkage rate and puncture strength adjusted to fall within predetermined ranges at 120°C. That is, the inventors of the present invention discovered that print misalignment due to elongation in the printing process, the occurrence of wrinkles due to heat sealing during bag making, and the occurrence of pinholes are related to the density of the polyethylene raw material and the heat shrinkage rate and puncture strength of the substrate film, and discovered that specifying the physical property values ​​of the density of the polyethylene raw material and the heat shrinkage rate and puncture strength of the substrate film within predetermined ranges can eliminate the occurrence of print misalignment and wrinkles and prevent the occurrence of pinholes, leading to the invention of the present invention.

[0025] The substrate film according to the present invention is made of a specific high-density polyethylene (0.95 g / cm 3 ) having a very narrow molecular weight distribution (i.e., a molecular weight distribution of 3.0 (Mw / Mn) or less). ３ It is also possible to construct the film from a stretched film having a base layer primarily composed of high-density polyethylene (high-density polyethylene having a density of 100% or higher). That is, the inventors of the present invention have discovered that the problems of conventional polyethylene-based mono-material substrate films, such as print misalignment due to stretching during the printing process and the occurrence of wrinkles due to heat sealing during bag making, are related to the density of the polyethylene raw material, and that the occurrence of pinholes is related to the molecular weight distribution of the polyethylene raw material, and have found that by specifying the properties of the polyethylene raw material to fall within predetermined numerical ranges, it is possible to eliminate the occurrence of print misalignment and wrinkles and prevent the occurrence of pinholes.

[0026] That is, the base film according to the present invention is made of a specific high-density polyethylene (i.e., 0.95 g / cm ３ It is preferable that the base layer is made of a high-density polyethylene having a density of 0.95 g / cm or higher. ３ If the density of the polyethylene is 0.95 g / cm or more, the crystallinity of the polyethylene is improved, so that the film can be stretched at a high stretch ratio, the mechanical strength (tensile modulus) is improved, and printing misalignment due to elongation in the printing process can be prevented.３ If the density of the specific high-density polyethylene in the base layer is less than 50 mass %, or if the density of the high-density polyethylene in the base layer is less than 0.95 g / cm 3 , the stretching at a high temperature is possible, thereby improving the heat resistance and suppressing the occurrence of wrinkles due to heat sealing during bag making. ３ If the stretching ratio or stretching temperature is less than 100%, the mechanical strength and heat resistance of the substrate film will be insufficient, which is undesirable. The content of the specific high-density polyethylene in the base layer is preferably 70% by mass or more, more preferably 90% by mass or more, and particularly preferably 95% by mass or more.

[0027] Also, specific high density polyethylene (i.e., 0.95 g / cm ３ Examples of methods for obtaining the above high-density polyethylene include polymerization methods (constant pressure methods) using a Ziegler-Natta catalyst, a metallocene catalyst, or the like. Polymerization using a metallocene catalyst makes the active site structure uniform, making it possible to polymerize a polymer with a small amount of low-molecular-weight components and a narrow molecular weight distribution. The density of the specific high-density polyethylene is 0.95 g / cm ３ There is no particular limitation as long as it is equal to or greater than 0.97 g / cm ３ This means that the density is preferably 0.97 g / cm or less. ３ If the density exceeds 0.955 g / cm, the film becomes hard and easily torn, which may make it difficult to produce a stretched film. ３ More preferably, it is 0.962 g / cm or more. ３ It is particularly preferable that the above value is satisfied.

[0028] When the puncture strength is increased by adjusting the properties of the polyethylene raw material, it is preferable to adjust the molecular weight distribution of the specific high-density polyethylene in the raw resin to 3.0 (Mw / Mn) or less. If the molecular weight distribution of the specific high-density polyethylene in the raw resin exceeds 3.0 (Mw / Mn), the puncture strength of the base film becomes insufficient, which is undesirable. From the viewpoint of improving puncture strength, the molecular weight distribution of the specific high-density polyethylene in the raw resin is more preferably 2.7 (Mw / Mn) or less, and particularly preferably 2.5 (Mw / Mn) or less. Note that, although a smaller molecular weight distribution of the specific high-density polyethylene in the raw resin is preferable, considering the production method of high-density polyethylene, it is considered that a minimum value of about 2.0 (Mw / Mn) is appropriate.

[0029] The melting point of the specific high-density polyethylene is preferably in the range of 130 to 140°C, and more preferably 132 to 140°C. If the melting point is less than 130°C, stretching at a temperature equal to or higher than the stretching temperature (120°C) described below may not be possible, and the heat resistance of the stretched film may decrease. The "melting point" refers to the value measured in accordance with JIS K 7121:1987, and is determined by measuring the temperature at which a main endothermic peak appears using a differential scanning calorimeter (DSC).

[0030] On the other hand, the melt mass flow rate (MFR) of the specific 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. Furthermore, from the viewpoint of improving puncture strength due to an increase in molecular weight and increased entanglement of molecular chains, a MFR of 1.0 g / 10 min or less is particularly preferred. A melt mass flow rate (MFR) of 0.01 g / 10 min or more allows molding using a general-purpose extruder without the need for special equipment, while a melt mass flow rate of 3.00 g / 10 min or less ensures sufficient film strength. The above melt mass flow rate can be obtained by measurement in accordance with the provisions of JIS K7210:1999.

[0031] Furthermore, in the present invention, as described above, it is necessary to specify the heat shrinkage rate and puncture strength at 120°C of a substrate film made of specific high-density polyethylene within a predetermined numerical range. To achieve this, it is necessary to stretch an unstretched film (raw sheet) made of specific high-density polyethylene, and a method of stretching at a higher stretching temperature than usual can be preferably used. Furthermore, the stretching direction is not particularly limited, and may be the longitudinal direction (MD) or the transverse direction (TD). (Biaxial stretching in MD and TD is also possible.) The stretching ratio is also not particularly limited, but in the case of uniaxial stretching, stretching is preferably approximately more than 4 to 10 times in the longitudinal direction or transverse direction, and in the case of biaxial stretching, stretching is preferably approximately more than 4 to 10 times in each of the longitudinal direction and transverse direction. Stretching at the above stretching ratio improves the mechanical strength (tensile modulus) of the substrate film and makes it possible to prevent printing misalignment due to elongation during the printing process. In the case of uniaxial stretching, if the stretching ratio is 3 times or less, non-uniform stretching may occur, and stretching spots may occur. Furthermore, if the stretching ratio exceeds 10 times, breakage may occur. In the case of uniaxial stretching, the stretching ratio is more preferably 6 times or more from the viewpoint of improving mechanical strength (tensile modulus). Furthermore, in the case of uniaxial stretching, in order to stretch at a stretching ratio of more than 4 times to 10 times, the density is preferably 0.95 g / cm ３ It is preferable to use a specific high density polyethylene having a density of 0.95 g / cm or more. ３ If polyethylene of less than this value is used, it may be difficult to stretch the film at a draw ratio of more than 4. As a method for stretching in the longitudinal direction, a method in which the rotation speeds of take-up rolls are made different can be suitably used, and as a method for stretching in the width direction, a method in which the left and right sides of the unstretched film are gripped in a tenter and expanded in the width direction can be suitably used.

[0032] Furthermore, the stretching temperature when obtaining the above-mentioned stretched film is not particularly limited, but from the viewpoint of improving dimensional stability by heat treatment and obtaining a film with excellent heat resistance, it is preferably 120°C or higher and lower than 130°C, which is a temperature range higher than the stretching temperature of ordinary polyethylene films. If the stretching temperature is lower than 120°C, the dimensional stability by heat treatment decreases and heat resistance decreases, which is undesirable because wrinkles tend to occur when the base film is heat-sealed to form a bag, and conversely, if the stretching temperature is 130°C or higher, melt fracture occurs during film formation, which is undesirable because it reduces the yield. The stretching temperature is more preferably 125°C or higher and lower than 130°C, and particularly preferably 127°C or higher and lower than 130°C. When the stretching temperature is 125°C or higher, the density is 0.95 g / cm ３ It is preferable to use a specific high density polyethylene having a density of 0.95 g / cm or more. ３ If the temperature is less than this, it may be difficult to stretch the film at 125°C or higher.

[0033] Furthermore, as described above, stretching improves mechanical strength (tensile modulus), but stretching the film also increases the likelihood of thermal shrinkage, making it more susceptible to wrinkles due to heat sealing during bag production. Therefore, when specifying the heat shrinkage rate and puncture strength in the stretching direction at 120°C of a substrate film (stretched film) made of specific high-density polyethylene within a predetermined range to suppress printing misalignment and wrinkles and pinholes, it is necessary for the heat shrinkage rate of the substrate film in the stretching direction at 120°C to be less than 5%. If the heat shrinkage rate of the substrate film in the stretching direction at 120°C is 5% or more, it is undesirable because it is prone to wrinkles due to heat sealing during bag production. The lower the heat shrinkage rate in the stretching direction at 120°C, the more preferable it is; specifically, it is more preferably 3% or less, even more preferably less than 2%, and particularly preferably less than 1%. A value below 0% (i.e., stretching at 120°C) is undesirable because it is more prone to wrinkles due to heat sealing.

[0034] Furthermore, as described above, stretching improves mechanical strength (tensile modulus) but reduces bag rupture resistance (puncture strength), and mechanical strength (tensile modulus) and bag rupture resistance (puncture strength) are in a trade-off relationship. Therefore, the puncture strength of the base film needs to be 2.0 N or more. If the puncture strength of the base film is less than 2.0 N, the bag rupture resistance tends to be insufficient, which is undesirable. The puncture strength of the base film is more preferably 2.5 N or more, and particularly preferably 3.0 N or more. Note that, although a higher puncture strength of the base film is preferable, a maximum value of about 5.0 N is considered to be the maximum value due to the characteristics of the specific high-density polyethylene, which is the main raw material. Furthermore, in order to achieve a puncture strength of 2.0 N or more for a base film made of specific high-density polyethylene, it is preferable to set the molecular weight distribution of the specific high-density polyethylene to 3.0 (Mw / Mn) or less. Uniaxial stretching results in significant anisotropy, resulting in a decrease in puncture strength. However, by setting the molecular weight distribution to 3.0 (Mw / Mn) or less, the entanglement of molecular chains increases, thereby improving puncture strength.

[0035] Furthermore, as mentioned above, stretching improves mechanical strength (tensile modulus), but stretching the film also makes it more susceptible to thermal shrinkage, making it more susceptible to wrinkles due to heat sealing during bag production. Therefore, when the thermal shrinkage rate and tensile modulus in the stretching direction at 120°C of a substrate film made of specific high-density polyethylene are specified within a predetermined range to simultaneously suppress printing misalignment and wrinkles, the tensile modulus of the substrate film is preferably 2,000 MPa or more, more preferably 2,500 MPa or more, and particularly preferably 3,000 MPa or more. A substrate film with a tensile modulus of less than 2,000 MPa is undesirable because it is more likely to cause printing misalignment during the printing process. The higher the tensile modulus of the substrate film, the more preferable it is, but considering the characteristics of the specific high-density polyethylene, which is the main raw material, a maximum value of about 5,000 MPa is considered appropriate.

[0036] The storage modulus of the base film at 100°C is preferably 400 MPa or more, more preferably 500 MPa or more, more preferably 600 MPa or more, and particularly preferably 700 MPa or more. If the storage modulus of the base film at 100°C is less than 400 MPa, printing misalignment is likely to occur during the printing process, which is undesirable. The higher the storage modulus of the base film at 100°C, the better, but considering the characteristics of the specific high-density polyethylene that is the main raw material, the maximum value is thought to be around 1500 MPa.

[0037] The substrate film according to the present invention has a base layer of a single layer structure made of the above-mentioned specific high-density polyethylene. Meanwhile, a film in which a barrier resin layer (i.e., a layer made of a resin with barrier properties) is interposed between base layers made of the above-mentioned specific high-density polyethylene, or a film in which a barrier resin layer is laminated on one or both sides of a base layer made of the above-mentioned specific high-density polyethylene, is called a barrier substrate film. The barrier resin in the present invention refers to a resin that has the function of inhibiting the permeation of gases such as oxygen and water vapor. Suitable examples of such barrier resins include ethylene-vinyl alcohol copolymer (EVOH), polyvinyl alcohol, and derivatives thereof.

[0038] Furthermore, when a barrier resin layer is interposed between base layers made of specific high-density polyethylene, or when a barrier resin layer is laminated on one or both sides of a base layer made of specific high-density polyethylene, an adhesive layer can be provided between the barrier resin layer and the base layer made of specific high-density polyethylene. By providing such an adhesive layer, it becomes possible to more effectively prevent delamination between the barrier resin layer and the base layer made of specific high-density polyethylene. Furthermore, acid-modified polyethylene, acid-modified polypropylene, etc. can be suitably used as such an adhesive, but acid-modified polyethylene is preferred from the viewpoint of monomateriality. In the substrate film according to the present invention, even when other layers such as barrier resin layers are interposed between base layers made of specific high-density polyethylene as described above, or when other layers such as barrier resin layers are laminated on both sides of a base layer made of specific high-density polyethylene, from the viewpoint of improving recyclability, it is preferable that 70 mass % or more, and more preferably 74 mass % or more, of the raw materials of the entire barrier substrate film be specific high-density polyethylene.

[0039] In addition, as a method for manufacturing the base film, raw materials containing the above-mentioned specific high-density polyethylene are mixed in a predetermined blending ratio, and the mixture is formed into a film by melt extrusion using an extruder equipped with a T-die, thereby obtaining a raw film before stretching the base film.

[0040] In addition, in a method for producing a barrier substrate film having a barrier resin layer between two front and back base layers each made of specific high-density polyethylene, the resins to be used for the base layer, the barrier resin layer, and the adhesive layer are each prepared, and the resins for each layer are co-extruded using a multi-layer extruder equipped with a T-die to obtain a raw film of the barrier substrate film before stretching.

[0041] The thus-produced base film or raw film before stretching of the barrier base film is uniaxially stretched under the stretching conditions as described above to obtain a stretched base film or barrier base film.

[0042] Furthermore, the thickness of the base film before stretching is preferably 50 to 300 μm, more preferably 80 to 250 μm, and particularly preferably 100 to 200 μm. If the thickness of the base film is 50 μm or more, it is possible to obtain strength that can withstand the stress during stretching. If the thickness of the base film is 300 μm or less, it is possible to obtain sufficient transparency after stretching.

[0043] The thickness of the substrate film after stretching treatment and the base layer of the barrier substrate film is preferably from 10 to 40 μm, more preferably from 15 to 35 μm, and particularly preferably from 20 to 30 μm.

[0044] On the other hand, the thickness of the barrier resin layer of the barrier substrate film after stretching is preferably 1 to 8 μm, more preferably 2 to 6 μm, and particularly preferably 2.5 to 5.0 μm. If the thickness of the barrier resin layer after stretching is 1 μm or more, sufficient oxygen and water vapor barrier properties as a barrier film can be obtained. Furthermore, if the thickness of the barrier resin layer after stretching is 8 μm or less, a film with excellent recyclability can be obtained.

[0045] The thickness of the adhesive layer of the barrier substrate film after stretching is not particularly limited, but is preferably 0.3 to 2 μm, more preferably 0.5 to 1.5 μm.

[0046] Furthermore, the base film or barrier base film according to the present invention is used in a packaging film (i.e., a film for forming a packaging bag) by laminating a sealant film on one side thereof, and then laminating the packaging film so that the sealant film surfaces are joined together, cutting the film into a predetermined shape, and heat-sealing the periphery to form a pouch, gusset bag, two-sided bag, three-sided bag, etc. As the sealant film to be attached to the base film according to the present invention, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene, etc. can be suitably used, but from the viewpoint of improving heat-sealability, low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE), which has a melting point lower than that of the base film, is preferred in order to provide a difference in melting point from the base film.

[0047] Furthermore, the substrate film according to the present invention can be used as a packaging film by laminating, if necessary, a barrier resin layer, a printed layer, a vapor-deposited layer made of a metal such as aluminum oxide, a metal oxide such as silicon dioxide, or an inorganic compound, a substrate layer made of ordinary polyethylene (low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene, high-density polyethylene, etc.) on the surface opposite to the adhesive surface of the sealant film. In addition, when a vapor-deposited layer made of a metal such as aluminum oxide, a metal oxide such as silicon dioxide, or an inorganic compound is laminated on one surface (or both surfaces) of the substrate film according to the present invention, it is preferable to provide an anchor coat layer made of a mixture of a urethane resin, a curing agent, and a silane coupling agent, etc., between the vapor-deposited layer and the substrate film in order to increase the adhesion of the vapor-deposited layer.

[0048] Furthermore, the substrate film according to the present invention can be used as a packaging film by providing a coating layer between the sealant film and the substrate film, if necessary. The coating layer is made of a barrier resin, which means a resin that has the function of suppressing the transmission of gases such as oxygen and water vapor. Suitable examples of such barrier resins include ethylene-vinyl alcohol copolymer (EVOH), polyvinyl alcohol, and derivatives thereof.

[0049] Furthermore, the melting point of the base film (after stretching) made of specific high-density polyethylene is preferably 135°C or higher. When the melting point of the base film is 135°C or higher, it is possible to increase the difference in melting point between the base film and the sealant film, and thermal shrinkage of the base film during heat sealing can be suppressed.

[0050] The substrate film according to the present invention will be described in more detail below based on examples, but the present invention is not limited to the embodiments of these examples and can be modified as appropriate within the scope of the present invention. The properties in the examples and comparative examples were evaluated as follows.

[0051] <Heat Shrinkage at 120°C> A sample of a predetermined size (12 cm x 12 cm) was cut from the stretched film produced in the Examples and Comparative Examples, and a 10 cm long, perpendicular benchmark line parallel to each side of the sample was drawn 1 cm inward from each side. The sample was placed in an oven at 120°C and heated for 10 minutes, then removed and cooled to room temperature (about 25°C). The distance between the benchmark lines in the stretching direction (i.e., MD) of the heat-treated sample was measured, and the heat shrinkage (%) was calculated from the change in the benchmark line distance in the stretching direction before and after heating using the following formula (1).

[0052] [Equation 1] Heat shrinkage rate in the stretching direction (%)=[(gauge line distance before heating−gauge line distance after heating) / gauge line distance before heating]×100 (1)

[0053] <Puncture Strength> In accordance with "7.4 Puncture Strength Test" of JIS Z 1707 "General Rules for Plastic Films for Food Packaging," a needle with a diameter of 1.0 mm and a radius of 0.5 mm was pierced at a puncture speed of 50 mm / min into the stretched films produced in the Examples and Comparative Examples, and the strength at which the needle penetrated the stretched film was measured. The strength (N) when pierced from the surface side was measured five times (N=5), and the average value was calculated.

[0054] <Bag Breaking Resistance> The bag breaking resistance was evaluated based on the above puncture strength.

[0055] ◎: 3N or more ○: 2N or more but less than 3N ×: Less than 2N

[0056] <Tensile Modulus> Test specimens were prepared by cutting the stretched films obtained in the Examples and Comparative Examples into strips measuring 200 mm in the machine direction (MD) and 10 mm in the transverse direction (TD). According to JIS-K-7127, the test specimens were stretched in the machine direction (MD) using a tensile tester (Shimadzu Corporation, Autograph®, model AG-5000A) under conditions of 25°C x 65% RH (i.e., room temperature and humidity), a chuck distance of 80 mm, and a pulling speed of 10 mm / min. The ratio of the tensile stress corresponding to two points of 0 to 1% strain to the corresponding strain was calculated as the tensile modulus (MPa).

[0057] <Appearance after printing> Packaging films were produced by laminating a 100 μm-thick linear low-density polyethylene (LLDPE) sealant film onto one side of the substrate films produced in the Examples and Comparative Examples. A commercially available gravure printing ink was then printed on the side of the packaging film opposite the sealant film using a gravure printing machine equipped with a gravure roll and doctor blade under specified conditions (printing speed = 75 m / min, drying temperature = 70°C, contact pressure of the nip roll on the substrate film = 0.15 MPa). The printed surface was then visually evaluated using the following three-point scale.

[0058] ○: No printing misalignment is observed. △: Very slight printing misalignment is observed. ×: Printing misalignment is clearly observed.

[0059] <Appearance after heat sealing> Packaging films were produced by attaching (laminating) a 100 μm thick linear low-density polyethylene (LLDPE) sealant film to one side of the base film produced in the Examples and Comparative Examples. The resulting packaging films were then overlapped so that the sealant films were bonded together, and heat-sealed at a temperature of approximately 120°C using a heat sealer. The heat-sealed surface was then visually evaluated using the following four-point scale.

[0060] ◎: No wrinkles are observed. ○: Almost no wrinkles are observed. △: Slight wrinkles are observed, but there is no problem. ×: Wrinkles are clearly observed.

[0061] The resin raw materials used in the production of the stretched films in the examples and comparative examples are as follows: HDPE1: High density polyethylene (density: 0.954 g / cm ３ , molecular weight distribution (Mw / Mn) = 2.6, melting point: 134°C, MFR (melt flow rate measured according to a method in accordance with JIS K7210, the same applies hereinafter): 1.0 g / 10 min, Neozex 5510F manufactured by Prime Polymer Co., Ltd.) HDPE2: high-density polyethylene (density: 0.960 g / cm ３, molecular weight distribution (Mw / Mn) = 8.7, melting point: 135 ° C, MFR: 1.0 g / 10 min, Novatec HY540 manufactured by Japan Polyethylene Co., Ltd.) MDPE: medium density polyethylene (density: 0.943 g / cm ３ , molecular weight distribution (Mw / Mn) = 22.4, melting point: 126°C, MFR: 0.24g / 10min, Prime Polymer Co., Ltd., Hi-Zex 5100E)

[0062] [Example 1] <Preparation of substrate film> The above-mentioned high-density polyethylene HDPE1 (i.e., molecular weight distribution (Mw / Mn) = 2.6 and density = 0.954 g / cm ３ A specific high-density polyethylene (specified high-density polyethylene) was continuously formed into a film by melt-extrusion at 200°C using an extruder (manufactured by LABTECH) equipped with a T-die, and the long film was cooled and wound up on a take-up roll to obtain an unstretched film (raw film) having a thickness of approximately 152 μm. The unstretched film was then stretched 6.0 times in the longitudinal direction (MD) while heated to 127°C, to produce a uniaxially stretched polyethylene film (i.e., substrate film) having a thickness of approximately 25 μm.

[0063] The produced base film was then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C using the methods described above. The produced base film was also used to evaluate the appearance after printing, the appearance after heat sealing, and bag rupture resistance using the methods described above. The measurement results and evaluation results for the base film of Example 1 are shown in Table 1, along with the properties of the base film.

[0064] [Examples 2, 3, 4] Uniaxially stretched polyethylene films (i.e., substrate films) of Examples 2, 3, and 4 were produced in the same manner as in Example 1, except that the stretching temperature when stretching the unstretched film (raw sheet) in the longitudinal direction (MD) was changed to 125°C, 120°C, and 128°C, respectively. The produced substrate films were then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C using the methods described above, and the appearance after printing, the appearance after heat sealing, and bag rupture resistance were evaluated. The measurement and evaluation results for the substrate films of Examples 2, 3, and 4 are shown in Table 1, along with the properties of the substrate films.

[0065] [Examples 5 and 6] Uniaxially stretched polyethylene films (i.e., substrate films) of Examples 5 and 6 were produced in the same manner as Example 1, except that the stretching ratio when stretching the unstretched film in the longitudinal direction (MD) was changed to 5 times and 10 times, respectively, and the stretching temperature for Example 5 was changed to 125°C. The produced substrate films were then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C using the methods described above, and the appearance after printing, the appearance after heat sealing, and bag rupture resistance were evaluated. The measurement and evaluation results for the substrate films of Examples 5 and 6 are shown in Table 1, along with the properties of the substrate films.

[0066] Example 7 Using a multilayer extruder (manufactured by LABTECH) equipped with a T-die, the following resins a to f were melt-extruded (extrusion temperature: 200°C) in layers to be continuously molded into a film, and the long film was taken up while being cooled on a take-up roll to obtain an unstretched film (laminated film) having a thickness of about 157 μm and a five-layer structure as shown in FIG. 1 . The unstretched film was then stretched 6.0 times in the longitudinal direction (MD) while heated to 127°C to produce the uniaxially stretched polyethylene film (i.e., barrier substrate film) of Example 7 having a total thickness of about 25 μm. Note that the first base layer (L) consisting of HDPE1 in the obtained barrier substrate film Ｐ１ ), a first adhesive layer (L ａ１ ), a barrier resin layer (LＢ ), a second adhesive layer (L ａ２ ), a second base layer (L Ｐ２ The thicknesses of the barrier substrate films were 9.3 μm, 1.5 μm, 3.3 μm, 1.5 μm, and 9.3 μm, respectively (the proportion of the specific high-density polyethylene in the entire barrier substrate film was 74.7 mass %). a. HDPE1: High-density polyethylene (density: 0.954 g / cm ３ , molecular weight distribution (Mw / Mn) = 2.6, melting point: 134°C, MFR: 1.0 g / 10 min, Prime Polymer Co., Ltd., Neozex 5510F) b. Acid-modified polyethylene: (density: 0.91 g / cm ３ c. Ethylene-vinyl alcohol copolymer: (density: 1.19 g / cm 3, melting point: 120°C, MFR: 2.3 g / 10 min, manufactured by Mitsui Chemicals, Inc., Admer NF587) ３ , melting point: 183°C, MFR: 1.6 g / 10 min, ethylene content: 32 mol%, Kuraray Co., Ltd. EVAL F171B) d. Acid-modified polyethylene: (density: 0.91 g / cm ３ , melting point: 120 ° C., MFR: 2.3 g / 10 min, Admer NF587 manufactured by Mitsui Chemicals, Inc.) e. HDPE1: high-density polyethylene (density: 0.954 g / cm ３ , molecular weight distribution (Mw / Mn) = 2.6, melting point: 1343°C, MFR: 1.0 g / 10 min, Neozex 5510F manufactured by Prime Polymer Co., Ltd.

[0067] The produced barrier substrate film was then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C by the methods described above, and the appearance after printing, the appearance after heat sealing, and the bag rupture resistance were evaluated. The measurement results and evaluation results for the substrate film of Example 7 are shown in Table 1, along with the properties of the substrate film.

[0068] [Comparative Example 1] The resin raw material was HDPE2 (i.e., molecular weight distribution (Mw / Mn) = 8.7 and density = 0.960 g / cm ３A uniaxially oriented polyethylene film (substrate film) for Comparative Example 1 was produced in the same manner as in Example 1, except that the polyethylene terephthalate (PE) was changed to a uniaxially oriented polyethylene (high-density polyethylene). The produced substrate film was then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C using the methods described above, and the appearance after printing, the appearance after heat sealing, and the bag rupture resistance were evaluated. The measurement results and evaluation results for the substrate film of Comparative Example 1 are shown in Table 1, along with the properties of the substrate film.

[0069] [Comparative Example 2] The resin raw material was MDPE (i.e., molecular weight distribution (Mw / Mn) = 22.4 and density = 0.943 g / cm ３ A uniaxially oriented polyethylene film (substrate film) of Comparative Example 2 was produced in the same manner as in Example 1, except that the uniaxially oriented polyethylene (medium-density polyethylene) was changed and the stretching temperature and stretching ratio when stretching the unstretched film (raw sheet) in the longitudinal direction (MD) were changed to 120°C and 4 times, respectively. The produced substrate film was then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C by the methods described above, and the appearance after printing, the appearance after heat sealing, and the bag rupture resistance were evaluated. The measurement and evaluation results of the substrate film of Comparative Example 2 are shown in Table 1, along with the properties of the substrate film.

[0070] [Comparative Example 3] A uniaxially stretched polyethylene film (substrate film) of Comparative Example 3 was produced in the same manner as in Example 1, except that the stretching temperature when stretching the unstretched film (raw sheet) in the machine direction (MD) was changed to 115°C. The produced substrate film was then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C using the methods described above, and the appearance after printing, the appearance after heat sealing, and bag rupture resistance were evaluated. The measurement results and evaluation results of the substrate film of Comparative Example 3 are shown in Table 1, along with the properties of the substrate film.

[0071] Comparative Example 4 An attempt was made to produce a uniaxially oriented polyethylene film of Comparative Example 4 in the same manner as in Example 1, except that the stretching temperature when stretching the unstretched film (raw sheet) in the longitudinal direction (MD) was changed to 130°C. However, because the stretching temperature was 130°C, many breaks occurred during stretching due to melting of the resin, and a uniaxially oriented polyethylene film that could be evaluated could not be obtained.

[0072] Comparative Example 5 An attempt was made to produce a uniaxially stretched polyethylene film of Comparative Example 5 in the same manner as in Example 1, except that the stretching ratio when stretching the unstretched film (raw film) in the longitudinal direction (MD) was changed to 11 times. However, because the stretching temperature ratio was 11 times, many breaks occurred during stretching, and a uniaxially stretched polyethylene film that could be evaluated could not be obtained.

[0073] Comparative Example 6 A uniaxially stretched polyethylene film of Comparative Example 6 was produced in the same manner as in Example 1, except that the stretching ratio when stretching the unstretched film (raw film) in the longitudinal direction (MD) was changed to 3 times. However, since the stretching temperature ratio was 3 times, the obtained uniaxially stretched polyethylene film had a significant degree of stretching unevenness, and it was not possible to measure various physical properties, and to evaluate the appearance after printing, the appearance after heat sealing, and the bag rupture resistance.

[0074] [Comparative Example 7] An unstretched film (raw sheet) was obtained in the same manner as in Example 1, except that the thickness was adjusted to about 26 μm. The obtained unstretched film was then used to measure the heat shrinkage, tensile modulus, and puncture strength at 120°C using the methods described above, and the appearance after printing, the appearance after heat sealing, and bag rupture resistance were evaluated. The measurement results and evaluation results of the base film of Comparative Example 7 (i.e., the unstretched film) are shown in Table 1, along with the properties of the base film.

[0075]

[0076] From Table 1, it can be seen that substrate films and barrier substrate films (stretched films) made primarily of specific high-density polyethylene and having a heat shrinkage rate and puncture strength that meet the requirements of the present invention at 120°C do not cause print misalignment when laminated with a sealant film, have good appearance after heat sealing, and have good bag rupture resistance (Examples 1 to 7). In contrast, it can be seen that a substrate film (stretched film) made primarily of specific high-density polyethylene but whose puncture strength does not meet the requirements of the present invention has poor bag rupture resistance (Comparative Example 1). Furthermore, a substrate film made primarily of medium-density polyethylene has a density of 0.950 g / cm ３ Because the stretching ratio is less than 4 times, it cannot be stretched, resulting in a tensile modulus of 2,000 MPa or less, and when a sealant film is laminated, printing misalignment is likely to occur. Furthermore, since the heat shrinkage rate at 120°C does not meet the requirements, it is easy for wrinkles to occur during heat sealing (Comparative Example 2). Furthermore, even when specific high-density polyethylene is used as the main raw material, a substrate film (stretched film) whose heat shrinkage rate at 120°C does not meet the requirements of the present invention is easy for wrinkles to occur during heat sealing (Comparative Example 3). Furthermore, it is easy for unstretched substrate films to have a tensile modulus of 2,000 MPa or less, which is easy for printing misalignment to occur (Comparative Example 7).

[0077] The substrate film according to the present invention has the excellent effects as described above, and therefore can be suitably used as a material for packaging films for forming various packaging bags.

[0078] F Barrier base film L Ｐ１ First base layer L made of specific high density polyethylene ａ１ First adhesive layer L Ｂ Barrier resin layer L ａ２ Second adhesive layer L Ｐ２ A second base layer made of specific high density polyethylene

Claims

1. A base film for laminating with a sealant film to form a packaging film, the base film having a density of 0.95 g / cm ３ A base film for packaging films, which is composed of a stretched film consisting of at least one base layer whose main component is the above-mentioned high-density polyethylene, and which has a heat shrinkage rate in the stretching direction at 120°C of less than 5% and a puncture strength of 2.0 N or more.

2. The substrate film for packaging films according to claim 1, characterized in that the molecular weight distribution of the high-density polyethylene is 3.0 (Mw / Mn) or less, and the base layer has a high-density polyethylene content of 90 mass% or more.

3. A barrier substrate film for packaging films, comprising a barrier layer made of a barrier resin between the two front and back base layers as described in claim 1 or 2.

4. The barrier substrate film for packaging films according to claim 3, characterized in that the barrier resin is comprised primarily of an ethylene-vinyl alcohol copolymer.

5. A barrier substrate film for packaging films, comprising the substrate film according to claim 1 or 2, and a vapor deposition layer provided on one side thereof.

6. A barrier substrate film for packaging films, comprising the substrate film according to claim 1 or 2, and a coating layer provided on one side thereof.

7. A packaging film comprising the barrier substrate film according to claim 3 and a sealant film laminated on one of the base layers.

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