Method for producing biaxially oriented polyamide film

Abstract

To provide a biaxially oriented polyamide film that is excellent in puncture strength, impact resistance, and abrasion pinhole resistance, and that can reduce environmental load by using polyamide 6 chemically recycled from waste polyamide products as a raw material. [Solution] A method for producing a single-layer biaxially oriented polyamide film, comprising a polyamide resin composition containing 70% by mass or more of polyamide 6, of which 4 to 90% by mass is chemically recycled polyamide 6, and 5 to 60% by mass is mechanically recycled polyamide 6, and containing a lubricant.

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polyamide film that has excellent puncture strength, impact resistance, and abrasion pinhole resistance, and that reduces the environmental impact by using polyamide 6 chemically recycled from waste polyamide products as a raw material. The biaxially oriented polyamide film of the present invention is suitable for use as a food packaging film, etc. [Background technology]

[0002] In recent years, as calls for the creation of a recycling-oriented society have grown, there has been a desire to move away from fossil fuels in the materials field, just as there is in the energy field. In recent years, marine plastic pollution has become a major problem. When plastic marine litter flows into the ocean, it breaks down into tiny particles (microplastics) due to ultraviolet rays and physical wear. When marine organisms ingest these particles, they may be exposed to chemicals contained in or adsorbed to them, and this may have an impact on top predators through the food chain, making this a global problem. Much of the marine plastic waste mentioned above has washed up from land, and while most of it is disposable plastic packaging, it also includes fishing line and nets. Given this background, recycling and effectively utilizing plastic waste is an effective way to reduce marine plastic waste.

[0003] On the other hand, biaxially oriented films made of aliphatic polyamides, such as polyamide 6, have been widely used as various packaging films due to their excellent impact resistance and resistance to pinholes caused by bending. However, these polyamide films used for packaging also contribute to the aforementioned plastic waste, and therefore there is a demand for the use of recycled materials.

[0004] There are three ways to recycle nylon 6 (also called polyamide 6): thermal recycling, in which it is incinerated and the heat energy is recovered; material recycling, in which it is melted and remolded for reuse; and chemical recycling, in which it is chemically depolymerized to return it to the raw materials for nylon and reused in nylon production, etc. Nylon is also called polyamide.

[0005] Of these, chemical recycling is an industrially useful recycling method because it breaks down nylon 6 into its raw material, caprolactam, which can then be recovered and reused as a raw material for nylon 6.

[0006] For example, Patent Document 1 discloses a recycling method in which used nylon clothing products are collected, depolymerized to recover ε-caprolactam, which is then purified, polymerized, and melt-spun or molded into nylon fibers or molded nylon products. This technology makes it possible to recycle collected clothing products by returning them to their raw materials and reusing them. Furthermore, by breaking down and refining collected clothing products, high-purity, high-quality raw materials (raw monomers) can be obtained, which can then be recycled to produce high-quality nylon 6 products and enable repeated recycling. Furthermore, this technology significantly reduces the work of collecting and sorting collected clothing products.

[0007] Nylon resin recycled by the above-mentioned chemical recycling method has been used mainly as a raw material for fibers and molded products, but has not been put to practical use as food packaging film. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 7-310204 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention was devised in view of the above-mentioned conventional techniques. An object of the present invention is to provide a biaxially oriented polyamide film that has excellent puncture strength, impact resistance, and abrasion pinhole resistance, and that can reduce the environmental impact by using polyamide 6 chemically recycled from waste polyamide products as the raw material. [Means for solving the problem]

[0010] That is, the present invention comprises the following: [1] A biaxially oriented polyamide film comprising a polyamide resin composition containing 70% by mass or more of polyamide 6 and 4 to 90% by mass of chemically recycled polyamide 6. [2] The biaxially oriented polyamide film according to [1], characterized in that the biaxially oriented polyamide film contains 5 to 60 mass % of mechanically recycled polyamide 6. [3] A biaxially oriented polyamide film having a surface layer (B layer) laminated on at least one side of a base layer (A layer), wherein the A layer is the biaxially oriented polyamide film described in [1] or [2], and the B layer is made of a polyamide resin composition containing 70% by mass or more of polyamide 6. [4] The biaxially oriented polyamide film according to [3], wherein the A layer contains 5 to 80% by mass of mechanically recycled polyamide 6, and the B layer contains 0 to 30% by mass of mechanically recycled polyamide 6. [5] The biaxially oriented polyamide film according to any one of the above items [1] to [4], which satisfies the following (a) and (b): (a) Piercing strength of 0.65 N / μm or more; (b) Impact strength is 0.9 J / 15 μm or more. [6] The biaxially stretched polyamide film according to any one of [1] to [5], which satisfies the following (c): (c) In the friction pinhole resistance test, the distance until pinholes appear is 2900 cm or more. [7] The biaxially oriented polyamide film according to any one of the above items [1] to [6], which satisfies the following (d) and (e): (d) haze of 2.6% or less; (e) The coefficient of kinetic friction is 1.0 or less. [8] The biaxially oriented polyamide film according to any one of [1] to [7], characterized in that the lamination strength after being attached to a polyethylene sealant film is 4.0 N / 15 mm or more. [9] A laminated film obtained by laminating a sealant film on the biaxially stretched polyamide film according to any one of [1] to [8].

[10] [9] A packaging bag using the laminated film described in. [Effects of the Invention]

[0011] The biaxially oriented polyamide film of the present invention is made primarily of polyamide 6, and by blending polyamide 6 chemically recycled from waste polyamide products and employing specific film-forming conditions, a biaxially oriented polyamide film that is excellent in puncture strength, impact resistance, flex pinhole resistance, and abrasion pinhole resistance and that can reduce the environmental impact can be obtained. Furthermore, by blending mechanically recycled polyamide 6 into the raw material, a biaxially oriented polyamide film that can further reduce the environmental impact can be obtained. [Brief explanation of the drawings]

[0012] [Figure 1] Schematic diagram of friction pinhole resistance evaluation device [Explanation of symbols]

[0013] 1: Head of fastness tester 2: Cardboard 3: Mount for holding samples 4: Film sample folded in four 5: Rubbing amplitude direction DETAILED DESCRIPTION OF THE INVENTION

[0014] The biaxially oriented polyamide film of the present invention will be described in detail below. The biaxially oriented polyamide film of the present invention is a biaxially oriented polyamide film comprising: a biaxially oriented polyamide film (layer A) made of a polyamide resin composition containing 70% by mass or more of polyamide 6, wherein 4 to 90% by mass of the polyamide 6 is chemically recycled polyamide 6; and a surface layer (layer B) made of a polyamide resin composition containing 70% by mass or more of polyamide 6 laminated on at least one side of the base layer (layer A).

[0015] [Layer A (biaxially oriented polyamide film or substrate layer)] Layer A in the present invention contains 70% by mass or more of polyamide 6, and thereby obtains the excellent mechanical strength such as impact strength and gas barrier properties such as oxygen that are inherent to biaxially oriented polyamide films made of polyamide 6. Layer A in the present invention is a layer made of a polyamide resin composition containing at least 70% by mass or more of polyamide 6, of which 4 to 90% by mass is polyamide 6 chemically recycled from waste polyamide 6 products such as waste plastic products, waste tire rubber, fibers, and fishing nets. Layer A in the present invention contains 4 to 90% by mass of polyamide 6 chemically recycled from waste polyamide 6 products such as waste plastic products, waste tire rubber, fibers, and fishing nets, making it possible to provide a biaxially oriented polyamide film that uses raw materials recycled from polyamide products that would previously have been discarded as garbage, thereby reducing the environmental impact. Furthermore, by selecting a specific stretching method, it is possible to obtain a biaxially oriented polyamide film that is simultaneously excellent in puncture resistance, impact resistance, and abrasion pinhole resistance.

[0016] [Polyamide 6] The polyamide 6 used in the present invention is usually produced by ring-opening polymerization of ε-caprolactam. The polyamide 6 obtained by ring-opening polymerization is usually subjected to removal of lactam monomer with hot water, followed by drying and melt-extrusion in an extruder. The relative viscosity of the polyamide 6 used in the present invention is preferably 1.8 to 4.5, and more preferably 2.6 to 3.2. If the relative viscosity is less than 1.8, the impact strength of the film will be insufficient. If the relative viscosity is more than 4.5, the load on the extruder will be large, making it difficult to obtain an unstretched film before stretching.

[0017] [Chemically recycled polyamide 6] The polyamide 6 used in Layer A is not only that polymerized from commonly used fossil fuel-derived monomers, but also polyamide 6 chemically recycled from waste polyamide 6 products such as waste plastic products, waste tire rubber, textiles, and fishing nets.

[0018] A method for obtaining chemically recycled polyamide 6 from waste polyamide 6 products can be, for example, the method disclosed in the above-mentioned Patent Document 1. That is, a method can be used in which used nylon (polyamide) products are collected, depolymerized to recover ε-caprolactam, which is then purified and polymerized.

[0019] <Depolymerization conditions> In the depolymerization carried out to produce the chemically recycled polyamide 6 used in the A layer, the polyamide 6 fiber is usually depolymerized by heating. The depolymerization may or may not use a catalyst. The depolymerization may be carried out in the absence (dry process) or in the presence (wet process) of water.

[0020] The pressure for depolymerization when producing the chemically recycled polyamide 6 used in layer A may be reduced pressure, normal pressure, or increased pressure. The depolymerization temperature is usually 100°C to 400°C, preferably 200°C to 350°C, and more preferably 220°C to 300°C. If the temperature is low, the polyamide 6 product does not melt, resulting in a slow depolymerization rate. If the temperature is high, decomposition of unnecessary polyamide 6 monomers (i.e., caprolactam) may occur, potentially reducing the purity of the recovered caprolactam.

[0021] When a catalyst is used in the depolymerization carried out in producing the chemically recycled polyamide 6 used in Layer A, an acid catalyst or a base catalyst is typically used. Examples of acid catalysts include phosphoric acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids, solid acids, and their salts. Examples of base catalysts include alkali hydroxides, alkali salts, alkaline earth hydroxides, alkaline earth salts, organic bases, and solid bases. Examples of preferred catalysts include phosphoric acid, boric acid, organic acids, alkali hydroxides, and alkali salts. Examples of more preferred catalysts include phosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate.

[0022] The amount of the acid catalyst used in the depolymerization is usually preferably 0.01 to 50% by mass relative to the polyamide 6 fiber component, more preferably 0.01 to 20% by mass, and even more preferably 0.5 to 10% by mass. If the amount of catalyst used is small, the reaction rate will be slow, while if it is large, side reactions will increase and the catalyst cost will increase, which is economically disadvantageous.

[0023] The depolymerization can be carried out either in the absence of water (dry process) or in the presence of water (wet process). In the case of wet depolymerization, the amount of water used is preferably 0.1 to 50 times by mass relative to the polyamide 6 product components such as fibers. More preferably, it is 0.5 to 20 times by mass, and even more preferably, it is 1 to 10 times by mass. If the amount of water used is small, the reaction rate will be slow, and if it is large, the concentration of the recovered caprolactam aqueous solution will be low, which is disadvantageous in obtaining caprolactam.

[0024] The method for recovering the caprolactam recovered by the above method is not particularly limited. For example, when dry depolymerization is performed, the produced caprolactam is distilled from the reactor by vacuum distillation to obtain recovered caprolactam. After the depolymerization reaction is completed, caprolactam may be extracted by vacuum distillation. Alternatively, caprolactam may be extracted continuously as the reaction proceeds. When wet depolymerization is performed, the produced caprolactam is distilled from the reactor together with water to obtain a recovered aqueous caprolactam solution. After the depolymerization reaction is completed, caprolactam may be extracted by vacuum distillation. Alternatively, caprolactam may be extracted continuously as the reaction proceeds. To obtain caprolactam of even higher purity, the recovered caprolactam can be combined with other purification methods, such as precision distillation of the recovered caprolactam, distillation under reduced pressure with the addition of a small amount of sodium hydroxide, treatment with activated carbon, ion exchange treatment, and recrystallization.

[0025] [Mechanically recycled polyamide 6] Layer A can also contain polyamide 6, which is mechanically recycled waste material generated during the manufacturing and processing of biaxially oriented polyamide film.

[0026] The mechanically recycled polyamide 6 referred to above is a raw material made by recovering scrap material generated during the production of biaxially oriented polyamide film, such as non-standard, unshippable film and off-cuts (edge ​​trims), and pelletizing them through melt extrusion or compression molding.

[0027] The lower limit of the amount of mechanically recycled polyamide 6 added to Layer A is preferably 10% by mass, more preferably 15% by mass, and even more preferably 20% by mass. If the amount of mechanically recycled polyamide 6 added is less than the above range, the recycled content in the film will be low. The upper limit of the amount of mechanically recycled polyamide 6 added to Layer A is preferably 50% by mass, more preferably 40% by mass, and even more preferably 30% by mass. If the amount of mechanically recycled polyamide added exceeds this range, the film may become more colored or have a high haze value, which may impair the appearance of the film. Alternatively, the amount of degraded material may increase during film production, which may worsen film formability.

[0028] [Subsidiary materials, additives] The biaxially oriented polyamide film or substrate layer (layer A) in the present invention may contain various additives such as other thermoplastic resins, lubricants, heat stabilizers, antioxidants, antistatic agents, anti-fogging agents, ultraviolet absorbers, dyes, pigments, etc., as needed.

[0029] <Other thermoplastic resins> The biaxially stretched polyamide film or substrate layer (layer A) of the present invention may contain a thermoplastic resin in addition to the polyamide 6 and the polyamide resin at least partly derived from biomass, provided that the object of the present invention is not impaired. Examples of such polyamide resins include polyamide 12, polyamide 66, polyamide 6·12 copolymer, polyamide 6·66 copolymer, and polyamide MXD6. If necessary, thermoplastic resins other than polyamides, for example, polyester polymers such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, and polyolefin polymers such as polyethylene and polypropylene, may be contained. If the raw materials for these thermoplastic resins are derived from biomass, they will not affect the increase or decrease in carbon dioxide on the ground, and therefore are preferable because they can reduce the environmental load.

[0030] <Lubricant> In the present invention, the biaxially oriented polyamide film or the substrate layer (layer A) preferably contains fine particles or an organic lubricant such as a fatty acid amide as a lubricant to improve the slipperiness and make it easier to handle. The biaxially oriented polyamide film of the present invention has an effect of reducing breakage of packaging bags due to friction by improving the slipperiness.

[0031] The fine particles can be appropriately selected from inorganic fine particles such as silica, kaolin, zeolite, etc., and polymeric organic fine particles such as acrylic and polystyrene fine particles, etc. From the viewpoints of transparency and lubricity, it is preferable to use silica fine particles. The average particle size of the fine particles is preferably 0.5 to 5.0 μm, more preferably 1.0 to 3.0 μm. If the average particle size is less than 0.5 μm, a large amount of addition is required to obtain good slip properties. On the other hand, if the average particle size exceeds 5.0 μm, the surface roughness of the film tends to become too large, resulting in poor appearance.

[0032] When the silica fine particles are used, the pore volume of the silica is preferably in the range of 0.5 to 2.0 ml / g, more preferably 0.8 to 1.6 ml / g. If the pore volume is less than 0.5 ml / g, voids are likely to occur, resulting in poor film transparency, while if the pore volume exceeds 2.0 ml / g, the fine particles tend to be less likely to form protrusions on the surface.

[0033] The biaxially stretched polyamide film or the base layer (layer A) in the present invention may contain a fatty acid amide and / or a fatty acid bisamide to improve slipperiness. Examples of the fatty acid amide and / or fatty acid bisamide include erucic acid amide, stearic acid amide, ethylene bisstearic acid amide, ethylene bisbehenic acid amide, and ethylene bisoleic acid amide. The content of fatty acid amide and / or fatty acid bisamide in the biaxially stretched polyamide film of the present invention is preferably 0.01 to 0.40% by mass, more preferably 0.05 to 0.30% by mass. If the content of fatty acid amide and / or fatty acid bisamide is less than the above range, the slip properties tend to be poor. On the other hand, if the content exceeds the above range, the wettability tends to be poor.

[0034] In the present invention, a polyamide resin such as polyamide MXD6, polyamide 12, polyamide 66, polyamide 6·12 copolymer, polyamide 6·66 copolymer, etc. can be added to the biaxially stretched polyamide film or base layer (layer A) to improve slip properties. Polyamide MXD6 is particularly preferred, and it is preferable to add it in an amount of 1 to 10 mass %.

[0035] <Antioxidants> The biaxially oriented polyamide film or the substrate layer (layer A) in the present invention may contain an antioxidant. The antioxidant is preferably a phenolic antioxidant. The phenolic antioxidant is preferably a fully hindered phenolic compound or a partially hindered phenolic compound. Examples of such antioxidants include tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane, stearyl-β-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, and 3,9-bis[1,1-dimethyl-2-[β-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]2,4,8,10-tetraoxaspiro[5,5]undecane. The inclusion of the phenolic antioxidant improves the film-forming operability of biaxially oriented polyamide films. In particular, when recycled films are used as raw materials, thermal degradation of the resin is likely to occur, which can lead to operational problems in film-forming and increased production costs. In contrast, the inclusion of an antioxidant suppresses thermal degradation of the resin and improves operability.

[0036] [B layer (surface layer)] The layer B in the present invention is a layer containing polyamide 6 in an amount of 70% by mass or more. Layer B in the present invention contains 70% by mass or more of polyamide 6, so that a biaxially oriented polyamide film having excellent mechanical strength such as impact strength and gas barrier properties such as oxygen can be obtained. As the polyamide 6, polyamide 6 polymerized from new raw materials, chemically recycled polyamide 6, and mechanically recycled polyamide 6 can be used, similar to the polyamide 6 used in the above-mentioned layer A. Layer B in the present invention may contain various additives such as other thermoplastic resins, lubricants, heat stabilizers, antioxidants, antistatic agents, antifogging agents, ultraviolet absorbers, dyes, pigments, etc., depending on the functions to be imparted to the surface of layer B. When Layer B is used on the outside of a packaging bag, it needs to be resistant to pinholes caused by friction, so it is undesirable to include soft resins such as polyamide elastomers or polyolefin elastomers or substances that generate a large amount of voids. Also, if good pinhole resistance caused by friction is desired, the content of mechanically recycled polyamide 6 should be less than 30% by mass, and more preferably 15% by mass or less.

[0037] Layer B in the present invention may contain a thermoplastic resin other than the above-mentioned polyamide 6, provided that the object of the present invention is not impaired. Examples of such polyamide resins include polyamide MXD6, polyamide 11, polyamide 12, polyamide 66, polyamide 6-12 copolymer, and polyamide 6-66 copolymer. If necessary, thermoplastic resins other than polyamides, for example, polyester polymers such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, and polyolefin polymers such as polyethylene and polypropylene, may be contained.

[0038] In the present invention, the layer B preferably contains fine particles or an organic lubricant as a lubricant in order to improve the film slip properties. By improving the slipperiness, the handling of the film is improved and the breakage of the packaging bag due to friction is reduced.

[0039] The fine particles can be appropriately selected from inorganic fine particles such as silica, kaolin, zeolite, etc., and polymeric organic fine particles such as acrylic and polystyrene fine particles, etc. From the viewpoints of transparency and slipperiness, it is preferable to use silica fine particles.

[0040] The average particle size of the fine particles is preferably 0.5 to 5.0 μm, more preferably 1.0 to 3.0 μm. If the average particle size is less than 0.5 μm, a large amount of addition is required to obtain good slip properties. On the other hand, if the average particle size exceeds 5.0 μm, the surface roughness of the film tends to become too large, resulting in poor appearance.

[0041] When using the above silica fine particles, the pore volume of the silica is preferably in the range of 0.5 to 2.0 ml / g, more preferably 0.8 to 1.6 ml / g. If the pore volume is less than 0.5 ml / g, voids are likely to occur, resulting in poor film transparency. If the pore volume exceeds 2.0 ml / g, the fine particles tend to be less likely to form protrusions on the surface.

[0042] The organic lubricant may contain a fatty acid amide and / or a fatty acid bisamide, such as erucic acid amide, stearic acid amide, ethylene bisstearic acid amide, ethylene bisbehenic acid amide, or ethylene bisoleic acid amide. The content of fatty acid amide and / or fatty acid bisamide added to Layer B is preferably 0.01 to 0.40% by mass, more preferably 0.05 to 0.30% by mass. If the content of fatty acid amide and / or fatty acid bisamide is less than the above range, slip properties tend to be poor. On the other hand, if it exceeds the above range, wettability tends to be poor.

[0043] To improve the slipperiness of the film, polyamide resins other than polyamide 6, such as polyamide MXD6, polyamide 11, polyamide 12, polyamide 66, polyamide 6-12 copolymer, and polyamide 6-66 copolymer, can be added to Layer B in the present invention. Polyamide MXD6 is particularly preferred, and it is preferable to add it in an amount of 1 to 10% by mass. If it is less than 1% by mass, the effect of improving the slipperiness of the film is small. If it is more than 10% by mass, the effect of improving the slipperiness of the film becomes saturated. Polyamide MXD6 resin is produced by polycondensation of metaxylylenediamine and adipic acid. The relative viscosity of the polyamide MXD6 is preferably 1.8 to 4.5, and more preferably 2.0 to 3.2. If the relative viscosity is less than 1.8 or more than 4.5, it may be difficult to knead it with the polyamide resin in an extruder.

[0044] To improve adhesion, layer B can also contain polyamide resins other than polyamide 6. In this case, copolymer polyamide resins such as polyamide 6·12 copolymer and polyamide 6·66 copolymer are preferred.

[0045] Subsidiary materials and additives such as lubricants and antioxidants can be added to Layer A and / or Layer B of the biaxially oriented polyamide film of the present invention by adding them during resin polymerization or melt extrusion in an extruder. Alternatively, a high-concentration masterbatch can be prepared and added to the polyamide resin during film production. Such known methods can be used.

[0046] [Thickness composition of biaxially oriented polyamide film] The thickness of the biaxially stretched polyamide film in the present invention is not particularly limited, but when used as a packaging material, it is usually 100 μm or less, and a thickness of 5 to 50 μm is generally used, and a thickness of 8 to 30 μm is particularly used.

[0047] In the thickness configuration of each layer of the biaxially stretched polyamide film of the present invention, when slipping property and friction pinhole resistance are to be imparted to Layer B, the thickness of Layer B is preferably 0.5 to 8 μm in order to exhibit these functions. In addition, in order to increase the recycling rate, the thickness of Layer A is preferably 50 to 93%, particularly 70 to 93%, of the total thickness of Layer A and Layer B.

[0048] [Method of manufacturing biaxially oriented polyamide film] The biaxially stretched polyamide film of the present invention can be produced by the following production methods. For example, sequential biaxial stretching and simultaneous biaxial stretching are mentioned. The sequential biaxial stretching method is preferred because it can increase the film production speed and is advantageous in terms of production costs. The method for producing the biaxially stretched polyamide film of the present invention will now be described in further detail. First, the raw material resin is melt-extruded using an extruder, extruded into a film form from a T-die, and cast onto a cooling roll to cool, thereby obtaining an unstretched film. The melting temperature of the resin is preferably 200 to 300° C. If it is lower than this, unmelted matter may occur, resulting in defects and other poor appearance, while if it exceeds this, deterioration of the resin may be observed, resulting in a decrease in molecular weight and a deterioration in appearance. When a surface layer (layer B) is laminated on a base layer (layer A), it is preferable to obtain an unstretched film by co-extrusion using a feed block or a multi-manifold.

[0049] The cooling roll temperature is preferably from -30 to 80°C, and more preferably from 0 to 50°C. To obtain an unstretched film by casting the film-like melt extruded from a T-die onto a rotating cooling drum and cooling it, methods such as an air knife method and an electrostatic adhesion method in which a static charge is applied are preferably used, with the latter being particularly preferred.

[0050] It is also preferable to cool the surface of the cast unstretched film opposite the cooling roll. For example, it is preferable to use a method in which a cooling liquid in a tank is brought into contact with the surface of the unstretched film opposite the cooling roll, a method in which a vaporizing liquid is applied with a spray nozzle, or a method in which a high-velocity fluid is sprayed onto the surface to cool the film. The unstretched film obtained in this manner is stretched biaxially to obtain a biaxially stretched polyamide film.

[0051] As a method for stretching in the MD direction, multi-stage stretching such as one-stage stretching or two-stage stretching can be used. As will be described later, multi-stage MD stretching such as two-stage stretching is preferred over single-stage stretching in terms of physical properties and uniformity of physical properties in the MD and TD directions (isotropy). In the sequential biaxial stretching method, stretching in the MD direction is preferably performed by roll stretching.

[0052] The lower limit of the MD stretching temperature is preferably 50° C., more preferably 55° C., and even more preferably 60° C. If the temperature is less than 50° C., the resin does not soften, making stretching difficult. The upper limit of the MD stretching temperature is preferably 120° C., more preferably 115° C., and even more preferably 110° C. If the temperature exceeds 120° C., the resin becomes too soft and stable stretching may not be possible.

[0053] The lower limit of the stretching ratio in the MD direction (when stretching is performed in multiple stages, the total stretching ratio obtained by multiplying each stretching ratio) is preferably 2.2, more preferably 2.5, and even more preferably 2.8. If it is less than 2.2, the thickness accuracy in the MD direction decreases, and the crystallinity becomes too low, which may reduce the impact strength.

[0054] The upper limit of the stretching ratio in the MD direction is preferably 5.0, more preferably 4.5, and most preferably 4.0. If the stretching ratio exceeds 5.0, subsequent stretching may become difficult.

[0055] When stretching in the MD direction is performed in multiple stages, the above-mentioned stretching is possible in each stretching, but the stretching ratios must be adjusted so that the product of all MD stretching ratios is 5.0 or less. For example, in the case of two-stage stretching, the first stage stretching is preferably 1.5 to 2.1 times, and the second stage stretching is preferably 1.5 to 1.8 times.

[0056] The film stretched in the MD direction is stretched in the TD direction using a tenter, heat-set, and then subjected to a relaxation treatment (also called a relaxation treatment). The lower limit of the TD stretching temperature is preferably 50° C., more preferably 55° C., and even more preferably 60° C. If the temperature is less than 50° C., the resin does not soften, making stretching difficult. The upper limit of the TD stretching temperature is preferably 190° C., more preferably 185° C., and even more preferably 180° C. If the temperature exceeds 190° C., crystallization may occur, making stretching difficult.

[0057] The lower limit of the stretching ratio in the TD direction (when stretching is performed in multiple stages, the total stretching ratio obtained by multiplying each stretching ratio) is preferably 2.8, more preferably 3.2, even more preferably 3.5, and particularly preferably 3.8. If it is less than 2.8, the thickness accuracy in the TD direction decreases, and the crystallinity becomes too low, which may reduce the impact strength. The upper limit of the stretching ratio in the TD direction is preferably 5.5 times, more preferably 5.0 times, even more preferably 4.7 times, particularly preferably 4.5 times, and most preferably 4.3 times. If the stretching ratio exceeds 5.5 times, productivity may decrease significantly.

[0058] The selection of the heat setting temperature is an important factor in the present invention. As the heat setting temperature increases, the crystallization and orientation relaxation of the film progress, improving the impact strength and reducing the heat shrinkage rate. On the other hand, if the heat setting temperature is low, the crystallization and orientation relaxation are insufficient, making it impossible to sufficiently reduce the heat shrinkage rate. Moreover, if the heat setting temperature is too high, the resin deteriorates and the film rapidly loses its toughness, such as its impact strength.

[0059] The lower limit of the heat setting temperature is preferably 180° C., more preferably 200° C. If the heat setting temperature is too low, the heat shrinkage rate becomes too large, which tends to deteriorate the appearance after lamination and reduce the laminate strength. The upper limit of the heat setting temperature is preferably 230° C., more preferably 220° C. If the heat setting temperature is too high, the impact strength tends to decrease.

[0060] The heat setting time is preferably 0.5 to 20 seconds, and more preferably 1 to 15 seconds. The heat setting time can be adjusted appropriately by balancing the heat setting temperature and the air speed in the heat setting zone. If the heat setting conditions are too weak, crystallization and orientation relaxation will be insufficient, causing the above problems. If the heat setting conditions are too strong, the film toughness will decrease.

[0061] Relaxation treatment after heat setting is effective in controlling the heat shrinkage rate. The temperature for relaxation treatment can be selected within the range from the heat setting temperature to the glass transition temperature (Tg) of the resin, but is preferably from the heat setting temperature -10°C to Tg + 10°C. If the relaxation temperature is too high, the shrinkage rate will be too fast, which is undesirable and can cause distortion. Conversely, if the relaxation temperature is too low, relaxation treatment will not occur and the film will simply become loose, which will not reduce the heat shrinkage rate and will result in poor dimensional stability.

[0062] The lower limit of the relaxation rate in the relaxation treatment is preferably 0.5%, more preferably 1%. If it is less than 0.5%, the heat shrinkage rate may not be reduced sufficiently. The upper limit of the relaxation rate is preferably 20%, more preferably 15%, and even more preferably 10%. If it exceeds 20%, sagging occurs in the tenter, which may make production difficult.

[0063] In order to increase the adhesive strength with the sealant film or the printed layer, the surface of the laminated stretched polyamide film may be subjected to corona treatment, flame treatment, or the like. The biaxially oriented polyamide film of the present invention thus obtained can prevent bag breakage caused by friction with transport packaging such as cardboard during transportation of the bag-made product, which can be caused by the film being scraped by the friction. It can also prevent bag breakage due to flex fatigue caused by contact between bags. Furthermore, the high water-resistant adhesive strength between the polyamide film and the sealant film provides high bag breakage prevention.

[0064] [Characteristics of biaxially oriented polyamide film] The biaxially stretched polyamide film of the present invention preferably has fewer than 20 pinhole defects when subjected to a twist and flex test at a temperature of 1°C 1,000 times using a Gelbo flex tester according to the measurement method described in the Examples. The number of pinhole defects is more preferably fewer than 10. The fewer the number of pinhole defects after the flex test, the better the pinhole resistance to flexing. If the number of pinholes is 10 or less, a packaging bag that is less likely to develop pinholes even when a load is applied to the packaging bag during transportation, etc., can be obtained.

[0065] Furthermore, the biaxially stretched polyamide film of the present invention preferably has a distance until pinholes appear in an abrasion pinhole resistance test of 2000 cm or more, more preferably 2900 cm or more, and even more preferably 3000 cm or more. The longer the distance until pinholes appear, the better the abrasion pinhole resistance. If the distance until pinholes appear is 2900 cm or more, a packaging bag that is less likely to develop pinholes can be obtained, even if the packaging bag rubs against a cardboard box or the like during transportation. In the present invention, a biaxially oriented polyamide film having excellent properties in both flexural pinhole resistance and friction pinhole resistance can be obtained by optimizing the raw material compositions of Layer A and Layer B. The biaxially oriented polyamide film of the present invention having these properties is highly useful as a packaging film because it is less likely to develop pinholes during transportation.

[0066] The biaxially stretched polyamide film of the present invention preferably has a heat shrinkage rate of 0.6 to 5.0%, more preferably 0.6 to 3.0%, in both the machine direction (hereinafter abbreviated as MD) and the width direction (hereinafter abbreviated as TD) at 160°C for 10 minutes. If the heat shrinkage rate exceeds 5.0%, curling or shrinkage may occur when heat is applied in subsequent processes such as lamination or printing. Furthermore, the lamination strength with the sealant film may be weakened. Although it is possible to reduce the heat shrinkage rate to less than 0.6%, this may result in mechanical embrittlement. Furthermore, such a rate is undesirable because it reduces productivity.

[0067] Since excellent impact resistance is a feature of biaxially oriented polyamide films, the impact strength of the highly adhesive polyamide film of the present invention is preferably 0.7 J / 15 μm or more, and more preferably 0.9 J / 15 μm or more. Although a higher impact strength is preferable, it is difficult to increase it above 1.5 J / 15 μm. The puncture strength of the highly adhesive polyamide film of the present invention is preferably 0.65 N / μm or more. The puncture strength is more preferably 0.70 N / μm or more. Although a higher puncture strength is preferable, it is difficult to increase the puncture strength beyond 1.0 N / μm. The planar orientation coefficient of the highly adhesive polyamide film of the present invention is preferably 0.045 or more, and more preferably 0.050 or more. A larger planar orientation coefficient is preferable because it increases the impact strength and puncture strength, but achieving a coefficient greater than 0.080 requires a higher stretch ratio, which is difficult because the film is prone to breakage during the stretching process.

[0068] The haze value of the biaxially stretched polyamide film of the present invention is preferably 10% or less, more preferably 5% or less, and even more preferably 2.6% or less. A low haze value means good transparency and gloss, so when used for packaging bags, beautiful printing is possible, increasing the product value. Adding fine particles to improve the slipperiness of the film increases the haze value, so it is preferable to add fine particles only to the surface layer, layer B, or to include a larger amount of fine particles therein, and to reduce the amount in layer A, as this will result in a film with good slipperiness and a small haze value.

[0069] The biaxially oriented polyamide film of the present invention preferably has a laminate strength of 4.0 N / 15 mm or more after being bonded to the polyethylene sealant described in the examples. The biaxially oriented polyamide film of the present invention is usually laminated with a sealant film and then processed into a packaging bag. If the laminate strength is 4.0 N / 15 mm or more, when packaging bags are produced using the biaxially oriented polyamide film of the present invention in various lamination configurations, sufficient strength can be obtained in the seal portion, resulting in a strong packaging bag that is resistant to tearing. In order to increase the lamination strength to 4.0 N / 15 mm or more, the biaxially oriented polyamide film of the present invention can be subjected to corona treatment, coating treatment, flame treatment, or the like.

[0070] Furthermore, the biaxially stretched polyamide film of the present invention can be subjected to heat treatment or humidity conditioning treatment to improve dimensional stability depending on the application. In addition, to improve the adhesion of the film surface, corona treatment, coating treatment, flame treatment, etc., can be performed, and printing and vapor deposition of metals, inorganic oxides, etc. are also possible. Vapor deposition films formed by vapor deposition are preferably aluminum vapor deposition films, and vapor deposition films of silicon oxide or aluminum oxide alone or in combination. Furthermore, by coating these vapor deposition films with a protective layer, etc., oxygen and hydrogen barrier properties can be improved.

[0071] The biaxially stretched polyamide film of the present invention is laminated with a sealant film or the like to form a laminate film, which is then processed into packaging bags such as bottom-sealed bags, side-sealed bags, three-side-sealed bags, pillow bags, standing pouches, gusseted bags, and square-bottom bags. Examples of the sealant film include unstretched linear low-density polyethylene film, unstretched polypropylene film, and ethylene-vinyl alcohol copolymer resin film. The layer structure of the laminated film using the biaxially stretched polyamide film of the present invention is not particularly limited as long as it contains the highly adhesive polyamide film according to the embodiment of the present invention in the laminated film. The film used in the laminated film may be made from either petroleum-derived or biomass-derived raw materials, but polylactic acid, polyethylene terephthalate, polybutylene succinate, polyethylene, polyethylene furanoate, etc., polymerized using biomass-derived raw materials are preferred in terms of reducing environmental impact.

[0072] Examples of layer structures of the laminated film of the present invention, where the boundary of the layers is represented by / , include ONY / bond / LLDPE, ONY / bond / CPP, ONY / bond / Al / bond / CPP, ONY / bond / Al / bond / LLDPE, ONY / PE / Al / bond / LLDPE, ONY / bond / Al / PE / LLDPE, PET / bond / ONY / bond / LLDPE, PET / bond / ONY / PE / LLDPE, PET / bond / ONY / Al / bond / LLDP E, PET / Contact / Al / Contact / ONY / Contact / LLDPE, PET / Contact / Al / Contact / ONY / PE / LLDPE, PET / PE / Al / PE / ONY / PE / LLDPE, PET / Contact / ONY / Contact / CPP, P ET / Contact / ONY / Contact / Al / Contact / CPP, PET / Contact / Al / Contact / ONY / Contact / CPP, ONY / Contact / PET / Contact / LLDPE, ONY / Contact / PET / PE / LLDPE, ONY / Contact / PET / Contact / CPP , ONY / / Al / / PET / / LLDPE, ONY / bonded / Al / bonded / PET / PE / LLDPE, ONY / PE / LLDPE, ONY / PE / CPP, ONY / PE / Al / PE, ONY / PE / Al / PE / LLDPE, OPP / bonded / ONY / bonded / LLDPE, ONY / bonded / EVOH / bonded / LLDPE, ONY / bonded / EVOH / bonded / CPP, ONY / bonded / aluminum or inorganic oxide vapor-deposited PET / bonded / LLDPE, Examples include ONY / coated / aluminum-vaporized PET / coated / ONY / coated / LLDPE, ONY / coated / aluminum-vaporized PET / PE / LLDPE, ONY / PE / aluminum-vaporized PET / PE / LLDPE, ONY / coated / aluminum-vaporized PET / coated / CPP, PET / coated / aluminum-vaporized PET / coated / ONY / coated / LLDPE, CPP / coated / ONY / coated / LLDPE, ONY / coated / aluminum-vaporized LLDPE, and ONY / coated / aluminum-vaporized CPP. The abbreviations used in the above layer structure are as follows: ONY: biaxially oriented polyamide film of the present invention, PET: oriented polyethylene terephthalate film, LLDPE: unoriented linear low-density polyethylene film, CPP: unoriented polypropylene film, OPP: oriented polypropylene film, PE: extrusion laminated or unoriented low-density polyethylene film, Al: aluminum foil, EVOH: ethylene-vinyl alcohol copolymer resin, Adhesion: adhesive layer that bonds films together, aluminum or inorganic oxide vapor deposition indicates that aluminum or inorganic oxide is vapor-deposited. [Example]

[0073] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples. The films were evaluated using the following measurement methods. Unless otherwise specified, measurements were carried out in a measurement room at 23°C and a relative humidity of 65%.

[0074] (1) Film recycling rate The recycling rate of biaxially oriented polyamide film is calculated as the proportion of chemically recycled raw materials and mechanically recycled raw materials relative to the total raw materials of the film, and is expressed as a percentage. (2) Film thickness The film was divided into 10 equal parts in the TD direction (for narrow films, the division was made to ensure a width that would allow thickness measurement), and 10 100mm films were cut out in the MD direction, and conditioned for at least 2 hours in an environment at a temperature of 23°C and a relative humidity of 65%. The thickness of the center of each sample was measured using a thickness measuring device made by Tester Sangyo, and the average value was taken as the thickness.

[0075] (3) Haze value of film Measurement was carried out using a direct reading haze meter manufactured by Toyo Seiki Seisakusho Co., Ltd. in accordance with JIS-K-7105. (4) Dynamic friction coefficient of film The coefficient of dynamic friction between the outer surfaces of the film rolls was evaluated under the following conditions in accordance with JIS-C2151: The test specimen was 130 mm wide and 250 mm long, and the test speed was 150 mm / min.

[0076] (5) Impact strength of the film Measurements were made using a film impact tester manufactured by Toyo Seiki Seisakusho, Ltd. The measured values ​​were converted to values ​​per 15 μm thickness and expressed as J (joules) / 15 μm. (6) Planar orientation of film The refractive index of the sample in the longitudinal direction (Nx), width direction (Ny), and thickness direction (Nz) of the film was measured using an Abbe refractometer with sodium D line as a light source according to JIS K 7142-1996 A method, and the plane orientation coefficient was calculated using formula (1). Planar orientation coefficient (ΔP) = (Nx + Ny) / 2 - Nz (1) (7) Film puncture strength The obtained polyester film was sampled into a 5 cm square, and the puncture strength of the film was measured in accordance with JIS Z1707 using a digital force gauge "ZTS-500N," an electric test stand "MX2-500N," and a puncture jig "TKS-250N" manufactured by Imada Co., Ltd. The unit of measurement was N / μm.

[0077] (8) Pinhole resistance of film when bent The number of pinholes due to bending fatigue was measured using a Gelbo Flex Tester manufactured by Rigaku Kogyosha Co., Ltd., according to the following method. A polyester adhesive was applied to the film prepared in the examples, followed by dry lamination with a 40 μm-thick linear low-density polyethylene film (L-LDPE film: Toyobo Co., Ltd., L4102). The resulting laminate film was then aged for 3 days at 40°C to produce a laminate film. The resulting laminate film was cut into a 12-inch x 8-inch piece and formed into a 3.5-inch diameter cylinder. One end of the cylindrical film was fixed to the fixed head of a Gelbo Flex Tester, and the other end was fixed to the movable head, with an initial gripping distance of 7 inches. A 440° twist was applied over the first 3.5 inches of the stroke, followed by a linear horizontal movement for the next 2.5 inches. Flex fatigue was performed 1,000 times at a speed of 40 times / min, and the number of pinholes that appeared in the laminate film was counted. Measurements were performed at 1°C. The test film was placed on filter paper (Advantec, No. 50) with the L-LDPE film side facing down, and the four corners were secured with Scotch tape (registered trademark). Ink (Pilot ink (product number INK-350-Blue) diluted 5 times with pure water) was applied to the test film and spread over the entire surface using a rubber roller. After wiping off any excess ink, the test film was removed and the number of ink dots on the filter paper was counted.

[0078] (9) Film abrasion and pinhole resistance A friction test was carried out using a fastness tester (manufactured by Toyo Seiki Seisakusho) according to the following method, and the distance at which pinholes occurred was measured. A test sample was made by folding the same laminate film as that produced in the pinhole resistance evaluation above into quarters to sharpen the corners, and then rubbing it against the inside surface of cardboard using a fastness tester with an amplitude of 25 cm, an amplitude speed of 30 times / min, and a weight of 100 g. The cardboard used was K280 x P180 x K210 (AF) = (surface liner x core material x backing liner (type of flute)). The pinhole occurrence distance was calculated according to the following procedure: The longer the pinhole occurrence distance, the better the abrasion pinhole resistance. First, a friction test was conducted with an amplitude of 100 times and a distance of 2500 cm. If no pinholes were found, the friction test was conducted with an amplitude of 20 times and a distance increased by 500 cm. If no pinholes were found, the friction test was conducted with an amplitude of 20 times and a distance increased by 500 cm. This process was repeated, and the distance at which a pinhole appeared was marked with an "X" and assigned a level of 1. If a pinhole appeared at an amplitude of 100 times and a distance of 2500 cm, the friction test was conducted with an amplitude of 20 times and a distance reduced by 500 cm. If a pinhole appeared, the friction test was conducted with an amplitude of 20 times and a distance reduced by 500 cm. This process was repeated, and the distance at which no pinholes appeared was marked with a "O" and assigned a level of 1. Next, for level 2, if the final result in level 1 was ○, the number of amplitudes was increased by 20 and a friction test was conducted, and if no pinholes were formed, a ○ was marked, and if a pinhole was formed, an X was marked.If the final result in level 1 was ×, the number of amplitudes was decreased by 20 and a friction test was conducted, and if no pinholes were formed, a ○ was marked, and if a pinhole was formed, an X was marked. Further, for levels 3 to 20, if the previous level was marked with an O, increase the number of amplitudes by 20 and conduct a friction test; if no pinholes appear, mark with an O; if pinholes appear, mark with an X. If the previous level was marked with an X, decrease the number of amplitudes by 20 and conduct a friction test; if no pinholes appear, mark with an O; if pinholes appear, mark with an X. Repeat this process and mark levels 3 to 20 with an O or X. For example, the results obtained are shown in Table 1. Using Table 1 as an example, we will explain how to determine the pinhole occurrence distance. Count the number of trials that are correct and incorrect for each distance. The distance with the most tests was used as the median, and the coefficient was set to 0. For distances longer than that, the coefficient was set to +1, +2, or +3 for every 500 cm, and for distances shorter than that, the coefficient was set to -1, -2, or -3 for every 500 cm. For all tests from levels 1 to 20, the number of tests in which no holes were formed was compared with the number of tests in which holes were formed, and the friction pinhole occurrence distance was calculated using the following formula for cases A and B. A: In all tests, the number of tests without holes is equal to or greater than the number of tests with holes. Distance at which friction pinholes occur = median + 500 x (Σ (coefficient x number of tests where no holes occurred) / number of tests where no holes occurred) + 1 / 2) B: In all tests, the number of tests without holes is less than the number of tests with holes. Distance at which friction pinholes occur = Median + 500 x (Σ (Coefficient x Number of tests with holes) / Number of tests with holes) - 1 / 2)

[0079] [Table 1]

[0080] (10) Heat shrinkage rate of film The heat shrinkage was measured according to the following formula in accordance with the dimensional change test method described in JIS C2318, except that the test temperature was 160°C and the heating time was 10 minutes. Heat shrinkage rate = [(length before treatment - length after treatment) / length before treatment] x 100 (%)

[0081] (11) Lamination strength with polyethylene sealant A laminate film prepared in the same manner as described in the evaluation of pinhole resistance was cut into strips measuring 15 mm wide x 200 mm long. One end of the laminate film was peeled off at the interface between the biaxially oriented polyamide film and the linear low-density polyethylene film. Using an autograph (manufactured by Shimadzu Corporation), the laminate strength was measured three times in each of the MD and TD directions under the conditions of a temperature of 23°C, a relative humidity of 50%, a pulling speed of 200 mm / min, and a peel angle of 90°, and the average value was used to evaluate the laminate strength.

[0082] (12) Relative viscosity of raw polyamide 0.25 g of polyamide was dissolved in 96% sulfuric acid in a 25 ml measuring flask to a concentration of 1.0 g / dl, and the relative viscosity of the resulting polyamide solution was measured at 20°C. (13) Melting point of raw material polyad Measurement was carried out in accordance with JIS K7121 using a differential scanning calorimeter, model SSC5200, manufactured by Seiko Instruments Inc., in a nitrogen atmosphere with a sample weight of 10 mg, a temperature rise starting temperature of 30°C, and a temperature rise rate of 20°C / min, and the endothermic peak temperature (Tmp) was determined as the melting point.

[0083] [Polyamide 6(a-1) newly polymerized from petrochemical-derived raw materials] As the polyamide 6 (a-1) newly polymerized from petrochemical-derived raw materials, polyamide 6 manufactured by Toyobo Co., Ltd., having a relative viscosity of 2.8 and a melting point of 220°C, was used. [Production of chemically recycled polyamide 6 (a-2)] Polyamide 6 fibers recovered from waste materials and a 75% by mass aqueous solution of phosphoric acid, a depolymerization catalyst, were placed in a depolymerization reactor and heated to 260°C under a nitrogen atmosphere. The reaction was initiated by blowing superheated steam into the depolymerization reactor, and the ε-caprolactam and water vapor continuously distilled from the depolymerization reactor were cooled to recover the ε-caprolactam distillate. The recovered distillate was concentrated in an evaporator, and the resulting ε-caprolactam was repolymerized to obtain chemically recycled polyamide 6. The relative viscosity of polyamide 6 (a-2) was 2.7, and the melting point was 221°C.

[0084] [Production of mechanically recycled polyamide 6 (a-3)] Off-spec film and scraps (edge ​​trimmings) generated from stretched film obtained by the method described in Example 1 below were collected and crushed, kneaded in an extruder at a cylinder temperature of 270°C, pelletized, and dried at 100°C under reduced pressure to obtain mechanically recycled polyamide 6. The relative viscosity of polyamide 6 (a-3) was 2.6 and the melting point was 221°C.

[0085] Example 1 Using an apparatus consisting of one extruder and a 380 mm wide single-layer T-die, a molten resin of the following polyamide resin composition was extruded from the T-die into a film, which was then cast onto a cooling roll controlled to a temperature of 20°C and electrostatically adhered to obtain an unstretched film having a thickness of 200 μm. The polyamide resin composition is a polyamide resin composition consisting of 95 parts by mass of polyamide 6(a-1), 5.0 parts by mass of polyamide 6(a-2), 0.45 parts by mass of porous silica microparticles (manufactured by Fuji Silysia Chemical Ltd., average particle size 2.0 μm, pore volume 1.6 ml / g), and 0.15 parts by mass of fatty acid bisamide (ethylene bisstearamide, manufactured by Kyoeisha Chemical Co., Ltd.). The extrusion rate of the extruder was adjusted so that the total thickness of the biaxially stretched polyamide film would be 15 μm. The resulting unstretched film was fed into a roll-type stretching machine and stretched 1.73 times in the MD direction at 80°C using the differential speed of the rolls, followed by a further stretch of 1.85 times at 70°C. This uniaxially stretched film was then continuously fed into a tenter-type stretching machine, preheated at 110°C, and stretched 1.2 times in the TD direction at 120°C, 1.7 times at 130°C, and 2.0 times at 160°C. It was then heat-set at 218°C and relaxed 7% at 218°C. The surface to be dry-laminated with a linear low-density polyethylene film was then corona-discharge-treated to obtain a biaxially stretched polyamide film. The evaluation results of the resulting biaxially stretched polyamide film are shown in Table 2.

[0086] (Examples 2 to 6 and Reference Example 1) A biaxially stretched film was obtained in the same manner as in Example 1, except that the film-forming conditions, such as the polyamide resin composition, stretch ratio, and heat setting temperature, were changed as shown in Table 1. The evaluation results of the obtained biaxially stretched film are shown in Table 2.

[0087] Example 7 To simultaneously biaxially stretch an unstretched sheet of the polyamide resin composition shown in Table 1, the sheet was placed in a hot water bath adjusted to 50°C and immersed in water for 2 minutes to adjust the moisture content to about 4%, after which the sheet was held by the clips of a tenter-type simultaneous biaxial stretching machine and subjected to corona discharge treatment under the film-forming conditions, such as the stretch ratio and heat setting temperature, shown in Table 1 to obtain a biaxially stretched film. The evaluation results of the resulting biaxially stretched film are shown in Table 2.

[0088] [Table 2]

[0089] As shown in Table 2, the biaxially oriented polyamide films using chemically recycled polyamide 6 (a-2) shown in the Examples or the biaxially oriented polyamide films using chemically recycled polyamide 6 (a-2) and mechanically recycled polyamide 6 (a-3) showed good properties in terms of impact resistance, puncture strength, and flex pinhole resistance. Compared to the biaxially oriented polyamide film of Reference Example 1, which used only polyamide 6 (a-1) newly polymerized from non-recycled petrochemical-derived raw materials, the Examples produced biaxially oriented polyamide films with similar properties. Furthermore, the films had low haze, good transparency, and high lamination strength with sealant films, making them excellent packaging films.

[0090] Example 8 Using an apparatus consisting of two extruders and a 380 mm wide co-extrusion T-die, the layers were laminated in a B layer / A layer / B layer configuration using the feed block method, and the molten resin was extruded from the T-die into a film, which was then cast onto a cooling roll controlled to 20°C and electrostatically adhered to obtain an unstretched film with a thickness of 200 μm. The resin compositions of the A layer and the B layer are as follows. Resin composition constituting layer A: a polyamide resin composition consisting of 95 parts by mass of polyamide 6 (a-1) and 5 parts by mass of polyamide resin (a-2). Resin composition constituting layer B: a polyamide resin composition consisting of 95 parts by mass of polyamide 6(a-1), 5.0 parts by mass of polyamide 6(a-2), 0.45 parts by mass of porous silica fine particles (manufactured by Fuji Silysia Chemical Ltd., average particle size 2.0 μm, pore volume 1.6 ml / g), and 0.15 parts by mass of fatty acid bisamide (ethylene bisstearamide, manufactured by Kyoeisha Chemical Co., Ltd.). The feed block configuration and extrusion rate of the extruder were adjusted so that the total thickness of the biaxially oriented polyamide film was 15 μm, with the thickness of the base layer (layer A) being 12 μm and the thickness of the front and back surface layers (layer B) being 1.5 μm each. The resulting unstretched film was introduced into a roll-type stretching machine and stretched 1.73 times in the MD direction at 80°C using the differential speed of the rolls, followed by a further stretch of 1.85 times at 70°C. This uniaxially stretched film was then continuously introduced into a tenter-type stretching machine, preheated at 110°C, and stretched 1.2 times in the TD direction at 120°C, 1.7 times at 130°C, and 2.0 times at 160°C. It was then heat-set at 218°C and relaxed 7% at 218°C. The surface to be dry-laminated with a linear low-density polyethylene film was then corona-discharge-treated to obtain a biaxially stretched polyamide film. The evaluation results of the resulting biaxially stretched polyamide film are shown in Table 3.

[0091] Examples 9 to 14 A biaxially stretched polyamide film was obtained in the same manner as in Example 8, except that the film-forming conditions, such as the polyamide resin composition, stretch ratio, and heat setting temperature, were changed as shown in Table 3. The evaluation results of the obtained biaxially stretched polyamide film are shown in Table 3.

[0092] Example 15 An unstretched film was prepared in the same manner as in Example 8, except that the film-forming conditions, such as the polyamide resin composition, stretch ratio, and heat setting temperature, were changed as shown in Table 3, and simultaneous biaxial stretching was carried out in the same manner as in Example 7 to obtain a biaxially stretched polyamide film. The evaluation results of the obtained biaxially stretched polyamide film are shown in Table 3.

[0093] [Table 3]

[0094] As shown in Table 3, the biaxially oriented polyamide films using chemically recycled (a-2) polyamide 6 shown in the Examples, or the biaxially oriented polyamide films using chemically recycled polyamide 6 (a-2) and mechanically recycled polyamide 6 (a-3), all had good properties in terms of impact resistance, puncture strength, and abrasion pinhole resistance. Compared to the biaxially oriented polyamide film of Reference Example 2, which uses only polyamide 6 (a-1) newly polymerized from non-recycled petrochemical-derived raw materials as raw material, biaxially oriented polyamide films with similar properties were obtained in the Examples. Furthermore, the films had low haze, good transparency, and high lamination strength with sealant films, making them excellent packaging films.

[0095] In Reference Examples 3 and 4, a large amount of mechanically recycled polyamide 6 was used in the surface layer (layer B). In these cases, the transparency (haze) and abrasion pinhole resistance were inferior to those of the films of Examples 8 to 15, in which the content of mechanically recycled polyamide 6 in the surface layer was 10 mass % or less.

[0096] [Example 15] The biaxially stretched polyamide film produced in Example 11 was used to produce laminates having the following configurations (1) to (9), and the laminates (1) to (9) were used to produce three-side seal type and pillow type packaging bags. Packaging bags with good appearance and resistance to tearing in a drop impact test were produced. (1) Biaxially oriented polyamide film layer / printing layer / polyurethane adhesive layer / linear low-density polyethylene film sealant layer. (2) Biaxially oriented polyamide film layer / printing layer / polyurethane adhesive layer / unoriented polypropylene film sealant layer. (3) Biaxially oriented PET film layer / printed layer / polyurethane adhesive layer / biaxially oriented polyamide film layer / polyurethane adhesive layer / unstretched polypropylene film sealant layer. (4) Biaxially oriented PET film layer / printed layer / polyurethane adhesive layer / biaxially oriented polyamide film layer / polyurethane adhesive layer / linear low-density polyethylene film sealant layer. (5) Biaxially oriented polyamide film layer / anchor coat layer / inorganic thin film layer / inorganic thin film protective layer / printing layer / polyurethane adhesive layer / linear low-density polyethylene film sealant layer. (6) Linear low-density polyethylene film sealant layer / polyurethane adhesive layer / biaxially oriented polyamide film layer / anchor coat layer / inorganic thin film layer / polyurethane adhesive layer / linear low-density polyethylene film sealant layer. (7) Linear low-density polyethylene film layer / polyurethane adhesive layer / biaxially oriented polyamide film layer / anchor coat layer / inorganic thin film layer / polyurethane adhesive layer / linear low-density polyethylene film layer / low-density polyethylene / paper / low-density polyethylene / linear low-density polyethylene film sealant layer (8) Biaxially oriented polyamide film layer / anchor coat layer / inorganic thin film layer / inorganic thin film protective layer / printing layer / polyurethane adhesive layer / unstretched polypropylene film sealant layer. (9) Biaxially oriented PET film layer / inorganic thin film layer / inorganic thin film protective layer / printed layer / polyurethane adhesive layer / biaxially oriented polyamide film layer / polyurethane adhesive layer / easy-peel type unstretched polypropylene film sealant layer. [Industrial Applicability]

[0097] The biaxially oriented polyamide film of the present invention has excellent puncture strength, impact resistance, flex pinhole resistance, and abrasion pinhole resistance, making it suitable for use as a packaging material for food packaging and the like. Furthermore, the use of polyamide 6 chemically recycled from waste polyamide products can contribute to reducing the environmental impact. Furthermore, blending mechanically recycled polyamide 6 into the raw material allows for the production of a biaxially oriented polyamide film that can further reduce the environmental impact.

Claims

1. A method for producing a single-layer biaxially oriented polyamide film, characterized in that the biaxially oriented polyamide film is made from a polyamide resin composition containing 70% by mass or more of polyamide 6, of which 4 to 90% by mass is chemically recycled polyamide 6 and 5 to 60% by mass is mechanically recycled polyamide 6, and further contains a lubricant.

2. A method for producing a biaxially oriented polyamide film as described in claim 1, characterized in that the relative viscosity of the polyamide 6 is 1.8 to 4.

5.

3. A method for producing a biaxially oriented polyamide film, characterized in that the biaxially oriented polyamide film obtained by the method for producing a biaxially oriented polyamide film described in claim 1 or 2 satisfies the following (a) and (b): (a) a puncture strength of 0.65 N / μm or more; (b) Impact strength of 0.9 J / 15 μm or more.

4. 4. The method for producing a biaxially oriented polyamide film according to claim 1, wherein the laminate strength after being bonded to a polyethylene sealant film is 4.0 N / 15 mm or more.

5. A method for producing a laminated film, comprising laminating a sealant film on the biaxially oriented polyamide film obtained by the method for producing a biaxially oriented polyamide film according to any one of claims 1 to 4.

6. A method for producing a packaging bag using a laminated film obtained by the method for producing a laminated film according to claim 5.

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