Biaxially stretched polyamide film

Abstract

[Problem] To provide a biaxially stretched polyamide film for forming a packaging bag that can be cut easily in a straight line and that has excellent tear resistance. [Solution] This biaxially stretched polyamide film includes 70-95 parts by mass of aliphatic polyamide, 5-30 parts by mass of polymethaxylylene adipamide, and 0.01-0.20 parts by mass of an oxide or a hydroxide of a group 2 element in the periodic table. The shrinkage percentages after steam treatment at 120℃ of an end-portion film piece and a center film piece, which are cut out from a mill roll of the film, in the longitudinal direction (MD direction) and the width direction (TD direction) fall in a range of 2.0-7.0%, and the absolute value of the difference in the shrinkage percentages after steam treatment at 120℃ of the end-portion film piece and the center film piece in directions at 45° and 135° with respect to the width direction (TD direction) is at most 1.0. The end-portion film piece is a square-shaped film piece which has a side length of 200 mm and which is cut out from a film mill roll having a width of 2000 mm such that a position 300 mm inward from the right edge or the left edge in the width direction becomes the center of the square shape. The center film is a square-shaped film piece which has a side length of 200 mm and which is cut out from the mill roll such that the center position of the mill roll in the width direction becomes the center of the square shape.

Description

Biaxially oriented polyamide film

[0001] The present invention relates to a biaxially oriented polyamide film, and more particularly to a biaxially oriented polyamide film that is formed from a raw material composition containing an aliphatic polyamide and polymetaxylylene adipamide, and that has excellent linear cuttability and is resistant to tearing.

[0002] Because biaxially oriented polyamide films have excellent puncture resistance and impact strength, they are used for packaging food, medicines, industrial products, etc. For example, packaging bags made by laminating a biaxially oriented polyamide film with a heat-sealable unoriented film are used.

[0003] Packaging bags using biaxially oriented polyamide film are particularly required to have pinhole resistance and linear cut properties, but because biaxially oriented polyamide film is a tough film, in order to impart linear cut properties to the biaxially oriented polyamide film, it has been necessary to process the biaxially oriented polyamide film by perforating or slitting it, or by attaching tear tape, etc. However, because the linear cut properties imparted by such processing methods are not a property of the biaxially oriented polyamide film itself, there have been cases where the film cannot be torn linearly all the way to the end, or the film gets caught midway, causing problems such as the contents scattering.

[0004] To solve the above problems, a mixed polyamide composition containing aliphatic polyamide and polymetaxylylene adipamide (hereinafter also referred to as "MXD6"), containing aliphatic polyamide / MXD6 = 40 to 85 parts by weight / 15 to 60 parts by weight, is melt-extruded, and stretched to 2.8 times or more in both the longitudinal (MD) direction and the transverse (TD) direction using an inflation method, forming a laminate film in which a multi-layered, easily tearable film is formed as a single layer (Patent Document 1, Patent Document 2). In addition, a mixed polyamide composition consisting of 60 to 95 parts by weight of aliphatic polyamide, 5 to 40 parts by weight of polymetaxylylene adipamide, and 0.01 to 0.50 parts by weight of an alkaline earth metal oxide or hydroxide is melt-extruded, and stretched to 2 times or more using a tubular method has been proposed (Patent Document 3).

[0005] JP 5-220837, JP 5-200958, JP 2017-193616

[0006] A biaxially oriented polyamide film with excellent linear cut properties is laminated with a sealant film such as polyethylene (PE) or polypropylene (PP), folded in half parallel to the flow direction with the polyamide film facing outward, and three sides are heat-sealed and cut out to produce a three-sided sealed bag with one side open.The side to be given linear cut properties is aligned with the flow direction, and the bag is filled with contents, sealed, and put on the market.

[0007] However, bags made of laminated films including the films described in Patent Documents 1 to 3 have linear tearing properties when torn in the MD direction, but if torn unintentionally off-axis, the front and back films do not tear straight in the same direction, resulting in tearing and making the bag difficult to open. This problem is particularly likely to occur when bags are made from slit-rolled film near the end of a mill roll. This is because biaxially oriented film is prone to a bowing phenomenon, in which the molecular orientation of the film is bent in a bow shape. This bowing phenomenon is thought to make tearing more likely to occur when attempting to tear slit-rolled film, especially when tearing slit-rolled film near the end of a mill roll. Here, a mill roll refers to a film roll of the full width of a film-making device that is wound after trimming both edge portions during the film production process, and a slit roll refers to a film roll that has been slit to narrow the width for processing such as printing or lamination.

[0008] The present invention has been made in light of the above-mentioned problems, and aims to provide a laminate film that can be used for packaging bags, which has excellent linear cutting properties and tear resistance with little tearing even when torn unintentionally from the MD direction, a biaxially oriented polyamide film that constitutes the laminate film, and a method for producing the same.

[0009] As a result of intensive research conducted by the present inventors to solve the above-mentioned problems, they discovered that a biaxially oriented polyamide film containing 70 to 95 parts by mass of an aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of an element of Group 2 of the periodic table, and having a specific shrinkage rate after steam treatment at 120°C, can be used as a laminate film for packaging bags that have excellent straight-line cut properties and tear resistance, and thus completed the present invention.

[0010] Specifically, the present invention relates to [1] to [8]. [1] A biaxially stretched polyamide film containing 70 to 95 parts by mass of an aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of an element of Group 2 of the periodic table, A biaxially oriented polyamide film in which the shrinkage rates of end film pieces and central film pieces cut from the film mill roll after 120°C steam treatment in the longitudinal direction (MD direction) and width direction (TD direction) are both 2.0% to 7.0%, and the absolute value of the difference in shrinkage rates after 120°C steam treatment in directions at 45° and 135° to the width direction (TD direction) of the end film pieces and central film pieces is 1.0 or less, the end film pieces are square film pieces with sides of 200 mm that have been cut so that the center of the square is located 300 mm inside the right end or left end in the width direction of a 2000 mm wide film mill roll, and the central film is a square film piece with sides of 200 mm that has been cut so that the center of the square is located at the center of the width direction of the mill roll. [2] The biaxially oriented polyamide film according to [1], wherein the stretching ratios in the MD and TD directions are 2.7 times or more and the difference between the stretching ratios in the MD and TD directions is 0.2 times or less. [3] The biaxially oriented polyamide film according to [1], wherein the oxide or hydroxide of a Group 2 element of the periodic table is magnesium hydroxide or magnesium oxide. [4] The biaxially oriented polyamide film according to [2], wherein the oxide or hydroxide of a Group 2 element of the periodic table is magnesium hydroxide or magnesium oxide. [5] A laminate film having excellent linear cutability and consisting of multiple layers, wherein at least one layer of the multiple layers is the biaxially oriented polyamide film according to any one of [1] to [4].[6] A method for producing a biaxially oriented polyamide film according to any one of [1] to [4], comprising the steps of (A) melt-kneading a raw material composition containing 70 to 95 parts by weight of an aliphatic polyamide, 5 to 30 parts by weight of polymetaxylylene adipamide, and 0.01 to 0.20 parts by weight of an oxide or hydroxide of a Group 2 element of the periodic table, followed by cooling to form a film, thereby obtaining an unstretched film, and (B) simultaneously biaxially tubular stretching the unstretched film in both the longitudinal and transverse directions at a stretch ratio of 2.7 or more. [7] A method for producing a biaxially oriented polyamide film according to [6], further comprising a heat treatment step (C) of heat-treating the film after the simultaneous biaxial tubular stretching step (B). [8] A method for producing a biaxially oriented polyamide film according to [7], wherein the heat treatment step (C) is a heat treatment step using a heated roll, or a heat treatment step using a tenter oven after the heated roll.

[0011] According to the present invention, it is possible to provide a laminate film that can be used for packaging bags that have excellent linear cuttability and tear resistance with little tearing even if the bag is accidentally torn off-axis in the MD direction, a biaxially oriented polyamide film that constitutes the laminate film, and a method for producing the same.

[0012] Furthermore, the present invention can provide a biaxially oriented polyamide film for use in producing packaging bags that has excellent linear cut properties and that do not tear again even if the bag is accidentally torn off-axis in the MD direction, preventing the bag from becoming difficult to open, and a method for producing the same.

[0013] Figure 1 is a schematic diagram of a tubular method simultaneous biaxial stretching apparatus. Figure 2 is a cross-sectional view showing the structure of a laminate film according to one embodiment of the present invention. [(a) Cross-sectional view showing the structure of a laminate film made of a two-layer laminate. (b) Cross-sectional view showing the structure of a laminate film made of a three-layer laminate. (c) Cross-sectional view showing the structure of a laminate film made of a four-layer laminate.] Figure 3 is a diagram showing the shape of a test piece used in evaluating linear cuttability. Figure 4 is a diagram showing a test piece after linear cuttability evaluation. [(a) A diagram showing a test piece with good linear cuttability. (b) A diagram showing a test piece with poor linear cuttability.] Figure 5 is a diagram showing the shape of a test piece used in evaluating splitting resistance. Figure 6 is a diagram showing a test piece after splitting resistance evaluation. [(a) A diagram showing a test piece with good splitting resistance. (b) A diagram showing a test piece with poor splitting resistance.]

[0014] [Biaxially oriented polyamide film] The present invention provides a biaxially oriented polyamide film containing 70 to 95 parts by mass of an aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of an element of Group 2 of the periodic table, wherein the shrinkage rates of an end film piece and a central film piece of a mill roll after 120°C steam treatment in the longitudinal direction (MD) and width direction (TD) are both 2.0% to 7.0%, and the absolute value of the difference in shrinkage rates after 120°C steam treatment in directions at 45° and 135° to the width direction (TD) of the end film piece and the central film piece (hereinafter also referred to as 120°C shrinkage strain difference) is 1.0 or less, The end film pieces are each square film pieces with sides of 200 mm, cut so that the center of the square is located 300 mm inside the right or left end in the width direction of a 2000 mm wide film mill roll, and the central film is a square film piece with sides of 200 mm, cut so that the center of the square is located at the center position in the width direction of the mill roll.This is a biaxially stretched polyamide film.

[0015] The aliphatic polyamide used in the present invention is not particularly limited, and examples thereof include polyamide 6 (nylon 6), polyamide 66 (nylon 66), polyamide 46 (nylon 46), polyamide 610 (nylon 610), and polyamide 12 (nylon 12). These may be used alone or in combination of two or more. Aliphatic polyamides such as polyamide 6 (nylon 6) may have a number-average molecular weight of 10,000 to 30,000, or 22,000 to 24,000. By using an aliphatic polyamide with a number-average molecular weight of 10,000 or more, the impact strength and tensile strength of the biaxially oriented polyamide film can be ensured. Furthermore, aliphatic polyamides with a number-average molecular weight of 30,000 or less have adequate molecular chain entanglement, which reduces excessive strain during stretching and prevents breakage or punctures during stretching, leading to stable production of biaxially oriented polyamide films.

[0016] The polymetaxylylene adipamide (MXD6) used in the present invention is obtained by a polycondensation reaction between metaxylylene diamine and adipic acid. MXD6 may also be obtained by a polycondensation reaction between metaxylylene diamine, adipic acid, and a component polymerizable therewith. Polymetaxylylene adipamides may be used singly or in combination of two or more.

[0017] The resin component in the biaxially stretched polyamide film of the present invention is composed of 70 to 95 parts by mass of an aliphatic polyamide and 5 to 30 parts by mass of polymetaxylylene adipamide. The content is not particularly limited as long as it is within the above-mentioned range, but the lower limit of the aliphatic polyamide content is, for example, 70 parts by mass or more, for example, 75 parts by mass or more, for example, 80 parts by mass or more, for example, 85 parts by mass or more, for example, 90 parts by mass or more, or for example, 95 parts by mass, and the upper limit is, for example, 95 parts by mass or less, for example, 90 parts by mass or less, for example, 85 parts by mass or less, for example, 80 parts by mass or less, for example, 75 parts by mass or less, or for example, 70 parts by mass. The lower limit of the polymetaxylylene adipamide content is, for example, 5 parts by mass or more, for example, 10 parts by mass or more, for example, 15 parts by mass or more, for example, 20 parts by mass or more, for example, 25 parts by mass or more, or for example, 30 parts by mass, and the upper limit is, for example, 30 parts by mass or less, for example, 25 parts by mass or less, for example, 20 parts by mass or less, for example, 15 parts by mass or less, for example, 10 parts by mass or less, or for example, 5 parts by mass. This content allows the biaxially oriented polyamide film of the present invention to have excellent impact resistance, linear cut properties, and tear resistance. For example, if the polymetaxylylene adipamide content is less than 5 parts by mass, the resulting biaxially oriented polyamide film may lack linear cut properties and tear resistance.

[0018] In the present invention, an oxide or hydroxide of a Group 2 element of the periodic table is used to impart sufficient linear cut and tear resistance to the biaxially oriented polyamide film. The oxide or hydroxide of the Group 2 element of the periodic table is not particularly limited, and examples thereof include beryllium oxide or hydroxide, magnesium oxide or hydroxide, calcium oxide or hydroxide, strontium oxide or hydroxide, and barium oxide or hydroxide. Specific examples include beryllium oxide, beryllium hydroxide, magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, barium oxide, and barium hydroxide. Magnesium oxide and magnesium hydroxide are particularly preferred, with magnesium hydroxide being more preferred. These oxides or hydroxides of Group 2 elements of the periodic table can be used alone or in combination of two or more. Two or more types of particles of the same type but different average particle sizes and specific surface areas can be used in combination.

[0019] The content of the oxide or hydroxide of the Group 2 element of the periodic table is preferably 0.01 to 0.20 parts by mass per 100 parts by mass of the total of the aliphatic polyamide and polymetaxylylene adipamide contained in the biaxially oriented polyamide film, and the lower limit is, for example, 0.01 part by mass or more, for example, 0.03 part by mass or more, for example, 0.06 part by mass or more, for example, 0.10 part by mass or more, for example, 0.12 part by mass or more, for example, 0.15 part by mass or more, for example, 0.18 part by mass or more, or for example, 0.20 part by mass or more, and the upper limit is, for example, 0.20 part by mass or less, for example, 0.18 part by mass or less, for example, 0.15 part by mass or less, for example, 0.12 part by mass or less, for example, 0.10 part by mass or less, for example, 0.06 part by mass or less, or 0.03 part by mass or less.

[0020] The specific surface area of ​​the oxide or hydroxide of the Group 2 element of the periodic table is not particularly limited, but it is preferably 10 to 500 μm with respect to the specific surface area determined by the BET method. ２ / g is preferred, and 20 to 300 μm ２ / g is more preferable. ２By using an oxide or hydroxide of a Group 2 element of the periodic table having a moisture content of 0.1 wt % or less, the oxide or hydroxide of the Group 2 element of the periodic table is prevented from absorbing moisture rapidly, making it easier to handle the oxide or hydroxide of the Group 2 element of the periodic table during film production.

[0021] The average particle size of the oxide or hydroxide of Group 2 element of the periodic table is not particularly limited, but is preferably 5 μm or less, more preferably 1 μm or less. The average particle size means the particle size (D50) at which the cumulative number of particles in the particle size distribution determined by a laser diffraction / scattering method becomes 50%.

[0022] Although the oxide or hydroxide of a Group 2 element of the periodic table may be surface-untreated, it is preferable to use a surface-treated oxide or hydroxide in order to improve the transparency of the biaxially stretched polyamide film. The surface treatment method is not particularly limited, and examples include a wet method in which a surface treatment agent is heated and dissolved in a solvent such as water or alcohol, and oxide or hydroxide particles of a Group 2 element are added thereto and mixed and stirred, followed by removal of the solvent, or a dry method in which the oxide or hydroxide particles of a Group 2 element of the periodic table and the surface treatment agent are mixed and stirred. The surface treatment agent is also not particularly limited, and suitable examples include silane coupling agents, titanium coupling agents, aluminum-based coupling agents, higher fatty acids, higher fatty acid esters, and higher fatty acid metal salts.

[0023] The thickness of the biaxially stretched polyamide film of the present invention can be adjusted appropriately depending on the intended use, and is, for example, 8 μm to 50 μm, preferably 10 μm to 30 μm.

[0024] The biaxially stretched polyamide film of the present invention may contain conventional additives and modifiers, such as heat stabilizers, ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, tackifiers, sealability improvers, anti-fogging agents, nucleating agents, release agents, plasticizers, crosslinking agents, flame retardants, and colorants (pigments, dyes, etc.), provided that the effects of the present invention are not impaired.

[0025] The biaxially stretched polyamide film of the present invention has a shrinkage rate after 120°C steam treatment in both the longitudinal direction (MD) and the width direction (TD) of the end film pieces and the central film piece of the mill roll of the film of 2.0% or more, preferably 2.5% or more, more preferably 3.0% or more, and both 7.0% or less, preferably 6.5% or less. If the shrinkage rate after 120°C steam treatment in the longitudinal direction (MD) and the width direction (TD) is less than 2.0%, the linear cutability of packaging bags made from the biaxially stretched polyamide film is reduced and they are more likely to tear when torn. Furthermore, if the shrinkage rate after 120°C steam treatment is more than 7.0%, shrinkage occurs due to heat during the printing and laminating processes, and pitch misalignment is likely to occur during the printing and laminating processes, which is problematic in practical use.

[0026] Here, the term "end film piece" refers to a square piece of film with a side length of 200 mm that is cut so that the center of the square is located 300 mm inside the right or left end of a 2000 mm wide film mill roll, and the term "central film" refers to a square piece of film with a side length of 200 mm that is cut so that the center of the square is located at the center of the width of a 2000 mm wide film mill roll. Note that the end film piece and central film piece used are square film pieces cut so that a pair of opposing sides are parallel to the TD or MD direction of the film mill roll, respectively.

[0027] The shrinkage rate after 120°C steam treatment is a value obtained by measuring the length before treatment of a film piece cut into a 200 mm square in the TD and MD directions, allowing it to stand at 23°C and 50% RH for 2 hours to condition the moisture, and then treating the film piece in water vapor at 120°C under 0.1 MPa for 30 minutes, and then allowing it to stand at 23°C and 50% RH for 2 hours to condition the moisture, and measuring the length after the measurement, and is calculated from the dimensional change, according to the following formula: shrinkage rate after 120°C steam treatment (%) = (length before treatment - length after treatment) / length before treatment × 100

[0028] Furthermore, the biaxially stretched polyamide film of the present invention has a 120°C shrinkage strain difference in the 45° and 135° directions (i.e., diagonals of the square film pieces) relative to the width direction (TD direction) of the end film pieces and central film piece of the mill roll of the film of 1.0 or less, preferably 0.9 or less, more preferably 0.8 or less, and even more preferably 0.5 or less. If the 120°C shrinkage strain difference exceeds 1.0, packaging bags using the biaxially stretched polyamide film are likely to tear again when torn.

[0029] The shrinkage rates after 120°C steam treatment in the 45° direction and the 135° direction are calculated using the above formula, and the shrinkage rate after 120°C steam treatment in the 45° direction is compared with the shrinkage rate after 120°C steam treatment in the 135° direction to calculate the 120°C shrinkage strain difference: 120°C shrinkage strain difference = |(shrinkage rate after steam treatment in the 45° direction) - (shrinkage rate after steam treatment in the 135° direction)|

[0030] It is believed that a biaxially oriented polyamide film containing an aliphatic polyamide, polymetaxylylene adipamide, and an oxide or hydroxide of a Group 2 element of the periodic table in the above-mentioned formulation has the polymetaxylylene adipamide stably dispersed in the aliphatic polyamide in the form of long islands. When the end film pieces and the central film pieces have a shrinkage rate after 120°C steam treatment in the MD and TD directions of 2.0% to 7.0% and a difference in 120°C shrinkage strain in the 45° and 135° directions of 1.0 or less, the long islands of polymetaxylylene adipamide are present along the MD direction in the end film pieces and the central film pieces of the mill roll, and therefore a packaging bag using such a biaxially oriented polyamide film is believed to have linear cuttability and tear resistance in the MD direction.

[0031] In this specification, "excellent linear cut resistance" means that when a notch is made in the edge of the film by hand or with a blade and the film is torn from the notch in the longitudinal direction of the film, the tear-propagating edge reaches the edge opposite the notched edge, and for example, in the linear cut resistance evaluation test described in paragraph

[0048] , the evaluation value is 8 or more. In addition, in this specification, "excellent resistance to tearing" means that tearing hardly occurs, or even if tearing occurs, it is unlikely to occur to a degree that causes practical problems, and for example, in the tear resistance evaluation test described in paragraph

[0050] , the maximum width of the deviation is 5 mm or less.

[0032] [Laminate Film] The present invention also covers a multi-layer laminate film obtained by laminating the biaxially oriented polyamide film of the present invention with another film. In the multi-layer laminate film of the present invention, at least one of the layers may be the biaxially oriented polyamide film.

[0033] Examples of the laminate film of the present invention are shown in the drawings and explained below, but are not limited thereto. Examples include a laminate film 201 consisting of a two-layer laminate of a biaxially oriented polyamide film 21 and a sealant film 22 as shown in Fig. 2(a), a laminate film 202 consisting of a three-layer laminate of a biaxially oriented polyamide film 21, a gas barrier layer 23, and a sealant film 22 as shown in Fig. 2(b), and a laminate film 203 consisting of a four-layer laminate of a biaxially oriented polyester film 24, a gas barrier layer 23, a biaxially oriented polyamide film 21, and a sealant film 22 as shown in Fig. 2(c).

[0034] Examples of sealant films include films made of unstretched polyethylene, unstretched polypropylene, unstretched polyvinyl chloride, ethylene-vinyl acetate copolymer, and ionomer resin. The thickness of the sealant film can be set within a range that does not impair the effects of the present invention, for example, 20 μm to 100 μm, preferably 30 μm to 80 μm. Examples of gas barrier layers include metal foil layers such as aluminum, and oxygen-barrier biaxially oriented polybutylene terephthalate (OPBT) films such as metal vapor deposition layers (e.g., alumina-vaporized OPBT films and silica-vaporized OPBT films). The thickness of the gas barrier layer can be set within a range that does not impair the effects of the present invention, for example, 5 μm to 30 μm, preferably 7 μm to 25 μm. Examples of biaxially oriented polyester films that can be used include biaxially oriented polyethylene terephthalate (PET) films, biaxially oriented polyethylene naphthalate (PEN) films, and biaxially oriented polybutylene terephthalate (PBT) films. The thickness of the biaxially stretched polyester film can be set within a range that does not impair the effects of the present invention, for example, 10 μm to 30 μm, preferably 12 μm to 25 μm. Known adhesives can be used as dry lamination adhesives for laminating each layer, such as two-component curing polyurethane. The thickness of the laminate film can be set within a range that does not impair the effects of the present invention, for example, 35 μm to 160 μm, preferably 50 μm to 130 μm.

[0035] The laminate film of the present invention having the biaxially oriented polyamide film layer has excellent linear cutability and tear resistance in the longitudinal (MD) direction. Therefore, when the laminate film of the present invention is made into a bag so that the MD direction of the biaxially oriented polyamide film is the tear direction, a bag having linear cutability and tear resistance is obtained, and the laminate film of the present invention is useful as a laminate film for packaging bags for food products such as soup, jam, and peeled pouches, as well as medicines, daily necessities, toiletries, etc.

[0036] [Method for Producing Biaxially Stretched Polyamide Film] The present invention also covers a method for producing the biaxially stretched polyamide film of the present invention. The biaxially stretched polyamide film can be produced, for example, by a method comprising the steps of: (A) melt-kneading a raw material composition containing 70 to 95 parts by mass of an aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of a Group 2 element of the periodic table, followed by cooling to form a film, thereby obtaining an unstretched film; and (B) biaxially stretching the unstretched film in both the longitudinal and transverse directions at a draw ratio of 2.7 or more.

[0037] In the present invention, examples of a method for melt-kneading the raw material composition containing 70 parts by mass to 95 parts by mass of the aliphatic polyamide, 5 parts by mass to 30 parts by mass of polymetaxylylene adipamide, and 0.01 part by mass to 0.20 part by mass of an oxide or hydroxide of a Group 2 element of the periodic table include methods using a known melt-kneader such as a single-screw extruder or a twin-screw extruder. When the conventional additives and modifiers are added to the raw material composition, the additives and modifiers may be added before melt-kneading the raw material composition, or the conventional additives and modifiers may be added after melt-kneading the raw material composition, followed by further melt-kneading. The melt-kneading temperature is appropriately selected depending on the aliphatic polyamide and polymetaxylylene azivamide, but is usually equal to or higher than the melting temperature of the aliphatic polyamide and polymetaxylylene azivamide, and is preferably in a temperature range in which coloration due to thermal decomposition of the aliphatic polyamide and polymetaxylylene azivamide does not occur, for example, 240°C to 300°C, preferably 245°C to 280°C, and more preferably 250°C to 270°C.

[0038] The method for forming a film by cooling can be a known method, such as a T-die method or a ring die method.

[0039] Examples of methods for biaxially stretching an unstretched film include known longitudinal and transverse biaxial stretching methods such as simultaneous biaxial stretching by a tubular method, simultaneous biaxial stretching by a tenter method, and sequential biaxial stretching. Simultaneous biaxial stretching is preferred, and tubular simultaneous biaxial stretching is more preferred, as it is easy to obtain uniform longitudinal and transverse stretching and the film obtained by biaxial stretching has excellent isotropy.

[0040] The stretching ratios in both the MD and TD directions of the biaxially stretched polyamide film of the present invention are 2.7 times or more, and more preferably 2.9 times or more. By setting the stretching ratio at 2.7 times or more, a film having the desired strength properties and good linear cutability and tear resistance can be obtained. By setting the stretching ratio at 6.0 times or less, it is possible to ensure the stability of stretching and prevent problems such as film breakage.

[0041] Furthermore, in the method for producing a biaxial polyamide film of the present invention, the biaxially stretched film obtained in the biaxial stretching step can be heat-treated to improve dimensional stability. Known methods can be used to heat-treat the biaxially stretched film, preferably roll heat treatment using a heated roll. However, roll heat treatment and tenter heat treatment can also be combined. Typically, tenter heat treatment tends to cause variations in physical properties across the film width due to the influence of the bowing phenomenon at the ends and center of the film mill roll. In roll heat treatment, the film is heat-treated by contacting it with a heated roll, resulting in uniform heat treatment. From the perspective of production stability, the film is relaxed before heat treatment. The heat treatment temperature is, for example, 180°C to 220°C, more preferably 190°C to 215°C. The relaxation rate during heat treatment is, for example, 0% to 10%, more preferably 0% to 8%, and even more preferably 0% to 5%.

[0042] The method for producing a biaxial polyamide film of the present invention will be described in more detail with reference to Figure 1. However, the present invention is not limited to the following. The raw material composition is melt-kneaded in an extruder, extruded into a tubular shape through a ring-shaped die, and cooled to obtain a tubular unstretched film. This tubular film is inserted between a pair of nip rolls 2, air is blown into the tubular film to expand it, heated by a heater 3, and cooled by air blown through a cooling ring 4. The film is folded by a guide roll 5 and taken up by a pair of nip rolls 6 to obtain a biaxially stretched polyamide film by the continuous simultaneous biaxial stretching tubular method.

[0043] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0044] In the examples, the abbreviations have the following meanings: OPET: biaxially stretched polyethylene terephthalate film (manufactured by Toyobo Co., Ltd., E5102, thickness 12 μm) AL: aluminum foil (manufactured by Toyo Aluminum Co., Ltd., 1N30, thickness 9 μm) CPP: unstretched polypropylene film (manufactured by Toyobo Co., Ltd., P1146, thickness 50 μm)

[0045] Example 1 (Preparation of Biaxially Stretched Polyamide Film) 70 parts by mass of aliphatic polyamide [nylon 6: manufactured by Ube Industries, Ltd.] (PA6) with a relative viscosity of 3.5 and 30 parts by mass of polymetaxylylene adipamide [aromatic polyamide: manufactured by Mitsubishi Gas Chemical Co., Ltd.] (MXD6) with a relative viscosity of 2.7 were mixed, and 0.03 parts by mass of magnesium hydroxide [Kisuma 5B: manufactured by Kyowa Chemical Industry Co., Ltd.] was added to prepare a raw material composition. The prepared raw material composition was melt-extruded using a ring die and cooled with internal and external water-cooled mandrels to obtain a 150 μm thick tubular unstretched film (radius 750 mm). In a tubular simultaneous biaxial stretching apparatus having the structure shown in FIG. 1, the tubular unstretched film was simultaneously biaxially stretched in the MD direction and TD direction by the speed difference between the low-speed nip roll and the high-speed nip roll and the air pressure present therebetween. The tubular film was then folded and cut open at both ends into two pieces, which were then subjected to a hot roll treatment at a maximum temperature of 210°C for several seconds to obtain a biaxially oriented polyamide film 2400 mm wide and 15 μm thick. Both edges of this film were cut off to form a flat film, which was then wound around two rolls to produce a film mill roll 2000 mm wide. The raw material composition and production conditions are shown in Table 1.

[0046] (120°C Steam Treatment Shrinkage Test) A film was pulled out from a film mill roll, and square pieces of film with sides of 200 mm each were cut out so that a pair of opposing sides were parallel to the TD direction of the film, with the center of the square being the center of the film and a position 300 mm inward from the right end toward the film mill roll. These were designated (A) center film piece and (B) edge film piece. The (A) center film piece and (B) edge film piece were conditioned for 2 hours in an environment of 23°C and 50% RH. For the conditioned (A) center film piece and (B) edge film piece, the lengths of the film piece in the TD (0°), 45°, MD (90°), and 135° directions were measured to two decimal places so as to pass through the center point, with the TD direction being set to 0°. These were designated as the pre-treatment lengths. After the measurements, the (A) central film piece and the (B) edge film piece were treated in steam at 120°C under 0.1 MPa for 30 minutes using an autoclave (HVE-50, manufactured by Hirayama Manufacturing Co., Ltd.), and then the (A) central film piece and the (B) edge film piece were removed and conditioned for 2 hours in an environment of 23°C and 50% RH. For the conditioned (A) central film piece and (B) edge film piece, the TD direction was set to 0°, and the lengths of the film pieces in the TD (0°), 45°, MD (90°), and 135° directions were measured to two decimal places so as to pass through the center point, and these were defined as the post-treatment lengths. The shrinkage rates after 120°C steam treatment in the TD (0°), 45°, MD (90°), and 135° directions of the (A) central film piece and (B) edge film piece, and the 120°C shrinkage strain difference of the (A) central film piece and (B) edge film were calculated using the following formulas. In the 120°C steam treatment shrinkage rate test, two pieces of film were prepared for each of the (A) central film piece and (B) edge film piece, and measurements were taken, and the average values ​​are shown in Table 1. Shrinkage rate after 120°C steam treatment (%) = (length before treatment - length after treatment) / length before treatment x 100 120°C shrinkage strain difference = |(shrinkage rate after steam treatment in the 45° direction) - (shrinkage rate after steam treatment in the 135° direction)| A shrinkage rate of 2.0% to 7.0% after steam treatment in the TD and MD directions was judged to be good, and a rate of more than 7.0% was judged to be poor. Furthermore, when the difference in 120° C. shrinkage strain between the 45° direction and the 135° direction was 1.0 or less, it was judged as good, and when it exceeded 1.0, it was judged as poor.

[0047] (Preparation of Laminate Film) A dry laminating adhesive (manufactured by DIC Graphics Corporation: LX-703VL and KR90 (solvent: ethyl acetate)) was applied in a solid content of 3.5 g / m to a laminate of 12 μm OPET, 9 μm AL, 15 μm of the biaxially oriented polyamide film obtained by preparing the biaxially oriented polyamide film described above, and 50 μm CPP in this order. ２ The biaxially oriented polyamide film used in the production of the laminate film was extracted from a film mill roll, and rectangular pieces (A') 400 mm in the MD direction and (B') 300 mm in the TD direction were cut out so that a pair of opposing sides was parallel to the TD direction of the film, with the film being drawn out from the film mill roll and the center of the film being located 300 mm inward from the right end of the film mill roll.

[0048] (Straight Cut Property Evaluation Test) A rectangular film piece measuring 205 mm in the MD and 20 mm in the TD was cut from the laminate film containing the prepared (A') central film piece or (B') edge film piece, and a 5 mm long incision was made in the center of one of the TD sides of this film piece to prepare 10 samples (see Figure 3). Next, the film piece was manually torn in the MD direction from the incision, and the number of samples in which the tear propagation edge reached the short side opposite the incised side, as shown in Figure 4(a), was recorded as an evaluation value (evaluation value: 0 to 10). The evaluation values ​​are shown in Table 1. In Table 1, the laminate film containing the (A') central film piece in the straight cut property evaluation test is abbreviated as (A') central film piece, and the laminate film containing the (B') central film piece is abbreviated as (B') central film piece.

[0049] (Preparation of laminated packaging bag) Two rectangular film pieces measuring 130 mm in the MD direction and 160 mm in the TD direction were cut out from the laminate film containing (A') the central film piece or (B') the end film piece obtained by preparing the above laminate film, and the two film pieces were overlapped with the CPP layer on the inside, and both TD edges and one MD edge were bonded with a seal width of 5 mm to prepare a three-sided bag.

[0050] (Rip-tear Evaluation Test) Three 5 mm long slits were made on one TD edge of a three-sided bag at intervals that divided the TD edge into four equal parts (see Figure 5). The bag was then manually torn along the slits at an angle of 30° relative to the MD direction (0°), with the tear propagating edge reaching the TD edge opposite the slit. As shown in Figures 6(a) and 6(b), the maximum width of the gaps (rips) 31 and 32 between the two laminated films at the tear location was used as the evaluation value. The evaluation values ​​are shown in Table 1. In Table 1, the three-sided bag made of laminated film containing the (A') central film piece in therip-tear evaluation test is abbreviated as (A') central film piece, and the three-sided bag made of laminated film containing the (B') central film piece is abbreviated as (B') central film piece.

[0051] (Examples 2 to 10) and (Comparative Examples 1 to 13) The same procedures as in Example 1 were carried out except that the parts by mass of each component contained in the raw material composition, the stretch ratio, the heat setting method, and the maximum heat treatment temperature were changed as shown in Table 1. Biaxially stretched polyamide films were produced and subjected to a shrinkage test after steam treatment at 120°C, laminate films were produced and subjected to a linear cut evaluation test, and laminate three-sided bags were produced and subjected to a tear evaluation test. The test results are shown in Table 1. However, in Comparative Examples 1 to 13, the production of laminate three-sided bags including the (A') central film and the tear evaluation test were not carried out.

[0052] (Comparative Examples 14 and 15) Except for changing the biaxially stretched polyamide film of Example 1 to a commercially available film having linear cuttability ([Unitika Co., Ltd.: product name NCBC] or [Idemitsu Unitech Co., Ltd.: product name TB1010]), the same operations as in Example 1 were carried out to carry out a shrinkage test after 120°C steam treatment, a laminate film preparation and linear cuttability evaluation test, and a laminate three-sided bag preparation and tear resistance evaluation test, and the test results are shown in Table 1.

[0053]

[0054] The results shown in Table 1 indicate that the biaxially oriented polyamide films of Examples 1 to 10, which contain 70 to 95 parts by weight of aliphatic polyamide, 5 to 30 parts by weight of polymetaxylylene adipamide, and 0.01 to 0.20 parts by weight of an oxide or hydroxide of a Group 2 element of the periodic table, have a shrinkage rate of 2.0% to 7.0% after steam treatment at 120°C in the MD and TD directions for the center and edge film pieces, and a 120°C shrinkage strain difference of 1.0 or less in the 45° and 135° directions, were used to obtain laminate films with excellent linear cutability, even when using the (A') center film or (B') edge film. Furthermore, using these laminate films, even when torn at 30° from the MD direction, the difference between the front and back films was 2 mm or less, allowing for straight tearing in the same direction, and a laminate three-sided bag was obtained that prevented tearing problems. On the other hand, when the biaxially oriented polyamide films of Comparative Examples 1 to 6 and 10 to 13 were used, which contained 70 to 95 parts by mass of aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of a Group 2 element of the periodic table, and in which the shrinkage rates of the central film piece and edge film pieces in the MD and TD directions after 120°C steam treatment were 2.0% to 7.0%, and the 120°C shrinkage strain difference in the 45° and 135° directions exceeded 1.0, a laminate film with excellent linear cutability could be obtained even when using the (A') central film or the (B') edge film.However, it was confirmed that in a three-sided bag of the laminate film containing the (B') edge film, when torn in the 30° direction from the MD direction, the misalignment between the front and back films was 5 mm or more, and tearing was likely to occur. Furthermore, it was confirmed that laminated three-sided bags made using a commercially available tenter heat-treated film (NCBC) in which the difference in 120°C shrinkage strain between the 45° and 135° directions exceeds 1.0, or a tenter heat-treated film (TB1010) in which the shrinkage rate after 120°C steam treatment in the MD exceeds 7.0%, show a misalignment of 5 mm or more between the front and back films when torn in the 30° direction from the MD, and are prone to tearing.In addition, when the biaxially oriented polyamide films of Comparative Examples 7 to 9 were used, which contained 70 to 95 parts by mass of aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of a Group 2 element of the periodic table, and in which the difference in 120°C shrinkage strain in the 45° and 135° directions was 1.0 or less, and one or both of the shrinkage rates after 120°C steam treatment in the MD and TD directions of the central film piece and the end film pieces were outside the range of 2.0% to 7.0%, it was not possible to obtain a laminate film with excellent linear cutability even when using the (A') central film or the (B') end film.Furthermore, it was confirmed that in a three-sided bag of the laminate film containing the (B') end film, when torn in the 30° direction from the MD direction, the misalignment between the front and back films was 5 mm or more, and tearing was likely to occur.

[0055] REFERENCE SIGNS LIST 1 Unstretched film 2 Nip roll 3 Heater 4 Cooling ring 5 Guide roll 6 Nip roll 7 Biaxially stretched film 21 Biaxially stretched polyamide film 22 Sealant film 23 Gas barrier layer 24 Biaxially stretched polyester film 201 Laminate film 202 Laminate film 203 Laminate film 31, 32 Also, deviation in tearing evaluation test

Claims

1. A biaxially stretched polyamide film containing 70 to 95 parts by mass of an aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of an element of Group 2 of the periodic table, wherein the shrinkage rates of end film pieces and central film pieces cut from a mill roll of the film after 120°C steam treatment in the longitudinal direction (MD) and width direction (TD) are both 2.0% to 7.0%, and the absolute value of the difference in shrinkage rates after 120°C steam treatment in directions at 45° and 135° to the width direction (TD) of the end film pieces and central film pieces is 1.0 or less, and the end film pieces are square film pieces with sides of 200 mm that have been cut so that the center of the square is located 300 mm inward from the right or left end in the width direction of a 2000 mm wide film mill roll, The biaxially oriented polyamide film is a square film piece having sides of 200 mm, the central film being cut out so that the center of the square is located at the center position in the width direction of the mill roll.

2. A biaxially oriented polyamide film according to claim 1, which is stretched in both the MD and TD directions by 2.7 times or more, and the difference in stretching ratio between the MD and TD directions is 0.2 times or less.

3. The biaxially oriented polyamide film according to claim 1, wherein the oxide or hydroxide of a Group 2 element of the periodic table is magnesium hydroxide or magnesium oxide.

4. The biaxially oriented polyamide film according to claim 2, wherein the oxide or hydroxide of a Group 2 element of the periodic table is magnesium hydroxide or magnesium oxide.

5. A laminate film consisting of multiple layers, at least one of which is made of the biaxially oriented polyamide film according to any one of claims 1 to 4.

6. A method for producing the biaxially oriented polyamide film according to any one of claims 1 to 4, comprising: (A) melt-kneading a raw material composition containing 70 to 95 parts by mass of an aliphatic polyamide, 5 to 30 parts by mass of polymetaxylylene adipamide, and 0.01 to 0.20 parts by mass of an oxide or hydroxide of an element of Group 2 of the periodic table, followed by cooling and forming into a film to obtain an unstretched film; and (B) simultaneously biaxially stretching the unstretched film in both the longitudinal and transverse directions at a stretch ratio of 2.7 or more.

7. The method for producing a biaxially oriented polyamide film according to claim 6, further comprising a heat treatment step (C) of heat treating the film after the tubular simultaneous biaxial stretching step (B).

8. The method for producing a biaxially oriented polyamide film according to claim 7, wherein the heat treatment step (C) is a heat treatment step using a heated roll, or a heat treatment step using a heated roll followed by a tenter oven.