Method for manufacturing biaxially oriented polyamide film

Abstract

[Problem] To provide a biaxially oriented polyamide film capable of reducing environmental load by using, as a raw material, polyamide 6 that has exceptional puncture strength, impact resistance, and friction pinhole resistance and is chemically recycled from waste polyamide products. [Solution] A biaxially oriented polyamide film (layer A) characterized by being formed from a polyamide resin composition containing 70 mass% or more of polyamide 6 and 4-90 mass% of chemically recycled polyamide 6; and a biaxially oriented polyamide film characterized by having a layer B (surface layer) formed from a polyamide resin composition containing 70 mass% or more of polyamide 6 laminated on at least one surface of the layer A (base layer).

Description

Technical Field

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[0003]

[0001] The present invention relates to a biaxially stretched polyamide film that is excellent in puncture strength, impact resistance, and friction-resistant pinhole resistance, and can reduce environmental impact by using polyamide 6 obtained by chemical recycling from waste polyamide products as a raw material. The biaxially stretched polyamide film of the present invention is suitably used for food packaging films and the like.

Background Art

[0002] In recent years, with the increasing demand for building a recycling-based society, the materials field, like the energy field, is expected to move away from fossil fuels. In recent years, marine plastic pollution has become a major problem. When plastic in marine waste flows into the sea, it breaks into pieces due to ultraviolet rays and physical abrasion, becoming tiny plastic particles (microplastics). When marine organisms ingest such particles, they may be exposed to chemical substances contained in or adsorbed on the particles, and further, through the food chain, it may also affect top predators, which has become a global problem. Most of the above-mentioned marine plastic waste has flowed in from land, and most of it is plastic container packaging assuming single use, while fishing lines and fishing nets are also included. Under such circumstances, to reduce plastic waste, recycling and effectively utilizing these plastic wastes are effective for reducing marine plastic waste.

[0003] On the other hand, conventionally, biaxially stretched films made of aliphatic polyamides typified by polyamide 6 are excellent in impact resistance and flexure-resistant pinhole resistance, and are widely used as various packaging material films. Even in these polyamide films used for packaging, due to being one of the causes of the above-mentioned plastic waste, the use of recycled materials is required.

[0004] Methods for recycling nylon 6 (also known as polyamide 6) include thermal recycling, which involves incineration to recover thermal energy; material recycling, which involves melting and then reshaping the material for reuse; and chemical recycling, which involves chemically depolymerizing the material to return it to its raw material form for reuse in nylon manufacturing, etc. Note that nylon is also known as polyamide.

[0005] Of these methods, the chemical recycling method is considered an industrially useful recycling method because it breaks down nylon 6 into its raw material, caprolactam, which is then 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, purified, polymerized, and then melt-spinned or molded to produce nylon fibers or nylon molded products. This technology makes it possible to recycle collected clothing products by returning them to their raw material state for reuse. Furthermore, by disassembling and refining collected clothing products, high-purity, high-quality raw material (raw material monomer) can be obtained, resulting in high-quality nylon 6 products that can be recycled repeatedly. In addition, the collection and sorting work of collected clothing products is significantly reduced.

[0007] The nylon resin recycled by the aforementioned chemical recycling method has so far been mainly used as a raw material for fibers and molded products, but it had not been put into practical use as a film for food packaging. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 7-310204 [Overview of the project] [Problems that the invention aims to solve]

[0009] This invention was conceived in view of the prior art. The object of this invention is to provide a biaxially oriented polyamide film that is excellent in puncture strength, impact resistance and abrasion pinhole resistance, and that can reduce environmental impact by using polyamide 6 chemically recycled from waste polyamide products as a raw material. [Means for solving the problem]

[0010] In other words, the present invention consists of the following configuration. [1] A biaxially oriented polyamide film characterized by 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% by 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], characterized in that layer A contains 5 to 80% by mass of mechanically recycled polyamide 6 and layer B contains 0 to 30% by mass of mechanically recycled polyamide 6. [5] The biaxially oriented polyamide film according to any one of the above [1] to [4], characterized in that the biaxially oriented polyamide film satisfies (a) and (b) below. (a) Puncture strength of 0.65 N / μm or more, (b) Impact strength of 0.9 J / 15 μm or more. [6] A biaxially oriented polyamide film according to any one of [1] to [5], characterized in that the biaxially oriented polyamide film satisfies (c) below. (c) In the abrasion resistance pinhole test, the distance until a pinhole occurs is 2900 cm or more. [7] The biaxially oriented polyamide film according to any one of [1] to [6] above, characterized in that it satisfies the following (d) and (e). (d) The haze is 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] above, characterized in that the laminate strength after being laminated with a polyethylene-based sealant film is 4.0 N / 15 mm or more. [9] A laminated film obtained by laminating a sealant film on the biaxially oriented polyamide film according to any one of [1] to [8].

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

Advantages of the Invention

[0011] The biaxially oriented polyamide film of the present invention is mainly composed of polyamide 6, and by blending polyamide 6 obtained by chemical recycling from waste polyamide products and adopting specific film-forming conditions, a biaxially oriented polyamide film excellent in puncture strength, impact resistance, flexing pinhole resistance, and friction pinhole resistance and capable of reducing environmental load can be obtained. Furthermore, by blending mechanically recycled polyamide 6 as a raw material, a biaxially oriented polyamide film capable of further reducing environmental load can be obtained.

Brief Description of the Drawings

[0012] [Figure 1] Schematic diagram of a friction pinhole resistance evaluation apparatus

Explanation of Reference Numerals

[0013] 1: Head part of a durability tester 2: Corrugated cardboard 3: Mounting board for sample holding 4: Four-folded film sample 5: Rubbing amplitude direction

Embodiments for Carrying Out the Invention

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

[0015] [A layer (biaxially stretched polyamide film or base material layer)] By containing 70% by mass or more of polyamide 6 in the A layer in the present invention, excellent mechanical strength such as impact strength and gas barrier properties such as oxygen inherent in the biaxially stretched polyamide film made of polyamide 6 can be obtained. The A layer in the present invention is a layer composed of at least a polyamide resin composition containing 70% by mass or more of polyamide 6, and 4 to 90% by mass of the polyamide 6 is composed of polyamide 6 chemically recycled from waste polyamide 6 products such as waste plastic products, waste tire rubber, fibers, and fishing nets. By containing 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 in the A layer in the present invention, a biaxially stretched polyamide film with reduced environmental load using recycled raw materials from polyamide products that have been discarded as waste can be provided. And by selecting a specific stretching method, a biaxially stretched polyamide film having excellent puncture resistance, impact resistance, and friction pinhole resistance at the same time can be obtained.

[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 dried after removing the lactam monomer with hot water and then melt-extruded by an extruder. The relative viscosity of the polyamide 6 used in this 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 it is greater than 4.5, the load on the extruder will increase, making it difficult to obtain the unstretched film before stretching.

[0017] [Chemically recycled polyamide 6] As for the polyamide 6 used in layer A, in addition to polyamide 6 polymerized from monomers derived from fossil fuels that are commonly used, polyamide 6 chemically recycled from waste polyamide 6 products such as waste plastic products, waste tire rubber, fibers, and fishing nets is also used.

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

[0019] <Depolymerization conditions> In the depolymerization process used to produce the chemically recycled polyamide 6 for layer A, the polyamide 6 fibers are typically depolymerized by heating. Depolymerization may or may not involve the use of a catalyst. Furthermore, depolymerization can be carried out in the absence of water (dry process) or in the presence of water (wet process).

[0020] The depolymerization pressure used when producing the chemically recycled polyamide 6 for use in layer A may be reduced pressure, atmospheric pressure, or increased pressure. The depolymerization temperature is usually between 100°C and 400°C, preferably between 200°C and 350°C, and more preferably between 220°C and 300°C. If the temperature is too low, the polyamide 6 product will not melt, resulting in a slow depolymerization rate. If the temperature is too 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 process for producing the chemically recycled polyamide 6 used in layer A, an acid catalyst or a base catalyst is usually used. Examples of acid catalysts include phosphoric acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids, solid acids, and their salts, while examples of base catalysts include alkali hydroxides, alkali salts, alkaline earth hydroxides, alkaline earth salts, organic bases, and solid bases. Preferably, phosphoric acid, boric acid, organic acids, alkali hydroxides, and alkali salts are used. More preferably, phosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate are used.

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

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

[0024] The method for recovering the caprolactam obtained by the above method is not particularly limited. For example, when dry depolymerization is performed, the generated caprolactam is distilled off the reactor by vacuum distillation to obtain the recovered caprolactam. Alternatively, the caprolactam may be removed by vacuum distillation after the depolymerization reaction is complete, or it may be removed continuously as the reaction progresses. When wet depolymerization is performed, the generated caprolactam is distilled off the reactor along with water to obtain a recovered aqueous caprolactam solution. After the depolymerization reaction is complete, the caprolactam may be extracted by vacuum distillation. Alternatively, it may be extracted continuously as the reaction progresses. Furthermore, methods to obtain even higher purity caprolactam include combining the recovered caprolactam with other purification methods such as precision distillation, vacuum distillation with the addition of a small amount of sodium hydroxide, activated carbon treatment, ion exchange treatment, and recrystallization.

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

[0026] The mechanically recycled polyamide 6 mentioned above is a raw material obtained by recovering waste materials such as non-standard films that cannot be shipped or cut-off trims generated during the production of biaxially oriented polyamide films, and then 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, the recycling ratio in the film will be low. The upper limit for 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 the above, it may impair the appearance of the film, such as causing strong coloration or a high haze value. Alternatively, it may increase the amount of degraded material during film manufacturing, worsening the film-forming properties.

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

[0029] <Other thermoplastic resins> The biaxially oriented polyamide film or substrate layer (Layer A) in the present invention may contain a thermoplastic resin in addition to the polyamide 6 and a polyamide resin whose raw materials are at least partially derived from biomass, to the extent that the objectives of the present invention are not impaired. Examples of 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, such as polyester polymers like polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, or polyolefin polymers like polyethylene and polypropylene, may be included. If the raw materials for these thermoplastic resins are derived from biomass, they do not affect the increase or decrease of carbon dioxide on Earth, thus reducing the environmental burden, which is preferable.

[0030] <Lubricant> In the present invention, it is preferable to include organic lubricants such as fine particles or fatty acid amides as lubricants in the biaxially oriented polyamide film or substrate layer (Layer A) to improve slipperiness and ease of handling. The biaxially oriented polyamide film in this invention also has the effect of reducing the tearing of packaging bags due to friction by improving their slipperiness.

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

[0032] When using the aforementioned silica fine particles, the pore volume of the silica is preferably in the range of 0.5 to 2.0 ml / g, and more preferably in the range of 0.8 to 1.6 ml / g. If the pore volume is less than 0.5 ml / g, voids are more likely to occur, and the transparency of the film deteriorates. If the pore volume exceeds 2.0 ml / g, surface protrusions caused by the fine particles tend to be less likely to form.

[0033] In the present invention, the biaxially oriented polyamide film or substrate layer (Layer A) may contain fatty acid amides and / or fatty acid bisamides for the purpose of improving slipperiness. Examples of fatty acid amides and / or fatty acid bisamides include erucic acid amide, stearic acid amide, ethylenebisstearate amide, ethylenebisbehenate amide, and ethylenebisoleate amide. The content of fatty acid amides and / or fatty acid bisamides in the biaxially oriented polyamide film of the present invention is preferably 0.01 to 0.40% by mass, and more preferably 0.05 to 0.30% by mass. If the content of fatty acid amides and / or fatty acid bisamides falls below the above range, the slipperiness tends to deteriorate. On the other hand, if it exceeds the above range, the wettability tends to deteriorate.

[0034] In the present invention, polyamide resins such as polyamide MXD6, polyamide 12, polyamide 66, polyamide 6-12 copolymer, and polyamide 6-66 copolymer may be added to the biaxially oriented polyamide film or substrate layer (layer A) to improve its slipperiness. Polyamide MXD6 is particularly preferred, and it is preferable to add it in an amount of 1 to 10% by mass.

[0035] <Antioxidant> The biaxially oriented polyamide film or substrate layer (layer A) in the present invention may contain an antioxidant. As antioxidants, phenolic antioxidants are preferred. Of the phenolic antioxidants, fully hindered phenolic compounds or partially hindered phenolic compounds are preferred. Examples 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. By incorporating the above-mentioned phenolic antioxidant, the operational efficiency of biaxially oriented polyamide film production is improved. In particular, when recycled film is used as raw material, thermal degradation of the resin is likely to occur, leading to poor film production operations and a tendency to increase production costs. In contrast, by incorporating the antioxidant, thermal degradation of the resin is suppressed, and operational efficiency is improved.

[0036] [B layer (surface layer)] In this invention, layer B is a layer containing 70% by mass or more of polyamide 6. In the present invention, layer B contains 70% by mass or more of polyamide 6 to obtain a biaxially oriented polyamide film with excellent mechanical strength such as impact strength and gas barrier properties such as oxygen. As the polyamide 6 mentioned above, polyamide 6 polymerized from new raw materials, chemically recycled polyamide 6, or mechanically recycled polyamide 6 can be used, similar to the polyamide 6 used in layer A. In the present invention, layer B may contain various additives such as other thermoplastic resins, lubricants, heat stabilizers, antioxidants, antistatic agents, antifogging agents, ultraviolet absorbers, dyes, and pigments, depending on the function required for the surface of layer B. When layer B is used on the outside of a packaging bag, abrasion resistance to pinholes is required, so it is undesirable to include soft resins such as polyamide elastomers or polyolefin elastomers, or substances that generate a large amount of voids. Furthermore, if abrasion resistance to pinholes is to be improved, it is better to reduce the content of mechanically recycled polyamide 6 to less than 30% by mass, and more preferably 15% by mass or less.

[0037] In the present invention, layer B may contain a thermoplastic resin in addition to the polyamide 6 described above, as long as it does not impair the objectives of the present invention. Examples include polyamide resins such as polyamide MXD6, polyamide 11, polyamide 12, polyamide 66, polyamide 6-12 copolymer, and polyamide 6-66 copolymer. If necessary, thermoplastic resins other than polyamides, such as polyester polymers like polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, or polyolefin polymers like polyethylene and polypropylene, may be included.

[0038] In the present invention, it is preferable to include fine particles or an organic lubricant as a lubricant in layer B in order to improve the film's slipperiness. Improving the slipperiness enhances the handling of the film and reduces the risk of packaging bags tearing due to friction.

[0039] The above-mentioned fine particles can be appropriately selected and used from inorganic fine particles such as silica, kaolin, and zeolite, and polymeric organic fine particles such as acrylic and polystyrene. However, silica fine particles are preferable in terms of transparency and slipperiness.

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

[0041] When using the above-mentioned silica microparticles, the pore volume range of the silica is preferably 0.5 to 2.0 ml / g, and more preferably 0.8 to 1.6 ml / g. If the pore volume is less than 0.5 ml / g, voids are more likely to occur, and the transparency of the film deteriorates. If the pore volume exceeds 2.0 ml / g, surface protrusions caused by the microparticles tend to be less likely to form.

[0042] The above-mentioned organic lubricant may contain fatty acid amides and / or fatty acid bisamides. Examples of fatty acid amides and / or fatty acid bisamides include erucic acid amide, stearic acid amide, ethylenebisstearate amide, ethylenebisbehenate amide, and ethylenebisoleate amide. The content of fatty acid amides and / or fatty acid bisamides added to layer B is preferably 0.01 to 0.40% by mass, and more preferably 0.05 to 0.30% by mass. If the content of fatty acid amides and / or fatty acid bisamides is below the above range, the slipperiness tends to deteriorate. On the other hand, if it exceeds the above range, the wettability tends to deteriorate.

[0043] In the present invention, layer B may contain polyamide resins other than polyamide 6, such as polyamide MXD6, polyamide 11, polyamide 12, polyamide 66, polyamide 6-12 copolymer, or polyamide 6-66 copolymer, in order to improve the slipperiness of the film. Polyamide MXD6 is particularly preferred, and it is preferable to add 1 to 10% by mass. Below 1% by mass, the effect of improving the slipperiness of the film is small. Above 10% by mass, the effect of improving the slipperiness of the film saturates. Polyamide MXD6 resin is produced by polycondensation of metaxylylenediamine and adipic acid. The relative viscosity of the polyamide MXD6 described above 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 greater than 4.5, it may be difficult to knead it with the polyamide resin in an extruder.

[0044] Furthermore, polyamide resins other than polyamide 6 can be added to layer B to improve adhesion. In this case, copolymerized polyamide resins such as polyamide 6-12 copolymer and polyamide 6-66 copolymer are preferred.

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

[0046] [Thickness composition of biaxially oriented polyamide film] The thickness of the biaxially oriented 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 generally films with a thickness of 5 to 50 μm are used, with films with a thickness of 8 to 30 μm being particularly common.

[0047] In the thickness configuration of each layer of the biaxially oriented polyamide film of the present invention, when providing slipperiness or friction pinhole resistance to layer B, the thickness of layer B is preferably 0.5 to 8 μm in order to exhibit these functions. Furthermore, 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 layers A and B.

[0048] [Method for producing biaxially oriented polyamide film] The biaxially oriented polyamide film of the present invention can be manufactured by the following methods. For example, sequential biaxial stretching and simultaneous biaxial stretching methods can be used. Sequential biaxial stretching is preferred because it allows for a higher film formation rate, which is advantageous in terms of manufacturing costs. The method for producing the biaxially oriented polyamide film according to the present invention will be further explained. First, the raw resin is melt-extruded using an extruder, extruded into a film shape from a T-die, cast onto a cooling roll to cool, and an unstretched film is obtained. The melting temperature of the resin is preferably 200 to 300°C. If it is below this temperature, unmelted material may be generated, resulting in defects and other appearance problems. If it exceeds this temperature, deterioration of the resin may be observed, leading to a decrease in molecular weight and a deterioration in appearance. When laminating a surface layer (layer B) onto a base layer (layer A), it is preferable to obtain an unstretched film by co-extrusion using a feed block or multi-manifold.

[0049] The cooling roll temperature is preferably -30 to 80°C, and more preferably 0 to 50°C. To obtain an unstretched film by casting the film-like molten material extruded from a T-die onto a rotating cooling drum and cooling it, methods such as using an air knife or an electrostatic adhesion method that applies an electrostatic charge can be preferably applied. The latter is particularly preferred.

[0050] Furthermore, it is preferable to cool the opposite side of the cooling roll of the cast unstretched film. For example, it is preferable to use a combination of methods such as bringing the cooling liquid in the tank into contact with the opposite side of the cooling roll of the unstretched film, applying a liquid that evaporates with a spray nozzle, or cooling by spraying a high-speed fluid. The unstretched film obtained in this way is stretched in two directions to obtain a biaxially oriented polyamide film.

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

[0052] The lower limit of the stretching temperature in the MD direction is preferably 50°C, more preferably 55°C, and even more preferably 60°C. Below 50°C, the resin may not soften, making stretching difficult. The upper limit of the stretching temperature in the MD direction is preferably 120°C, more preferably 115°C, and even more preferably 110°C. If the temperature exceeds 120°C, the resin may become too soft, making stable stretching impossible.

[0053] The lower limit of the stretching ratio in the MD direction (or the total stretching ratio obtained by multiplying each ratio in the case of multi-stage stretching) is preferably 2.2 times, more preferably 2.5 times, and even more preferably 2.8 times. If it is less than 2.2 times, the thickness accuracy in the MD direction will decrease, and the degree of crystallinity may become too low, which may reduce the impact strength. The upper limit of the stretching ratio in the MD direction is preferably 5.0 times, more preferably 4.5 times, and most preferably 4.0 times. If it exceeds 5.0 times, subsequent stretching may become difficult.

[0054] Furthermore, when stretching in the MD direction is performed in multiple stages, the stretching described above is possible for each stage, but the stretching ratio must be adjusted so that the product of the stretching ratios in all MD directions is 5.0 or less. For example, in the case of two-stage stretching, it is preferable to stretch the first stage by 1.5 to 2.1 times and the second stage by 1.5 to 1.8 times.

[0055] The film, stretched in the MD direction, is then stretched in the TD direction using a tenter, heat-set, and subjected to a relaxation treatment (also called a easing treatment). The lower limit of the stretching temperature in the TD direction is preferably 50°C, more preferably 55°C, and even more preferably 60°C. If the temperature is below 50°C, the resin may not soften, making stretching difficult. The upper limit of the stretching temperature in the TD direction is preferably 190°C, more preferably 185°C, and even more preferably 180°C. Exceeding 190°C may cause crystallization, making stretching difficult.

[0056] The lower limit of the stretching ratio in the TD direction (or the total stretching ratio obtained by multiplying each ratio in the case of multi-stage stretching) 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 will decrease, and the degree of crystallinity may become too low, resulting in a decrease in 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. Exceeding 5.5 times may significantly reduce productivity.

[0057] The selection of the heat-setting temperature is an important element in this invention. As the heat-setting temperature increases, crystallization and orientation relaxation of the film progress, improving impact strength and reducing thermal shrinkage. On the other hand, if the heat-setting temperature is low, crystallization and orientation relaxation are insufficient, and the thermal shrinkage cannot be sufficiently reduced. Furthermore, if the heat-setting temperature is too high, the resin deteriorates, and the toughness of the film, such as impact strength, is rapidly lost.

[0058] The lower limit of the heat-setting temperature is preferably 180°C, and more preferably 200°C. If the heat-setting temperature is too low, the thermal shrinkage rate becomes too large, which tends to degrade the appearance after lamination and reduce the laminate strength. The upper limit of the heat-fixing temperature is preferably 230°C, and more preferably 220°C. If the heat-fixing temperature is too high, the impact strength tends to decrease.

[0059] The heat setting time is preferably 0.5 to 20 seconds, and more preferably 1 to 15 seconds. The appropriate heat setting time can be determined by balancing the heat setting temperature and the airflow speed in the heat setting zone. If the heat setting conditions are too weak, crystallization and orientation relaxation will be insufficient, leading to the problems mentioned above. If the heat setting conditions are too strong, the film toughness will decrease.

[0060] Performing a relaxation treatment after heat setting treatment is effective in controlling the thermal shrinkage rate. The relaxation treatment temperature can be selected within the range from the heat setting treatment temperature to the glass transition temperature (Tg) of the resin, but preferably from the heat setting treatment temperature -10°C to Tg +10°C. If the relaxation temperature is too high, the shrinkage rate will be too fast, which can cause distortion and other problems, so this is undesirable. Conversely, if the relaxation temperature is too low, the relaxation treatment will not occur, and the material will simply sag, the thermal shrinkage rate will not decrease, and dimensional stability will be poor.

[0061] The lower limit of the relaxation rate for the relaxation treatment is preferably 0.5%, and more preferably 1%. If it is less than 0.5%, the thermal shrinkage rate may not decrease sufficiently. The upper limit of the relaxation rate is preferably 20%, more preferably 15%, and even more preferably 10%. If it exceeds 20%, sagging may occur within the tenter, making production difficult.

[0062] To increase the adhesive strength with sealant films and printed layers, the surface of the laminated stretched polyamide film may be subjected to corona treatment, flame treatment, or the like. The biaxially oriented polyamide film obtained in this way can suppress bag tearing even when friction occurs with transport packaging such as corrugated cardboard during the transportation of the bagged product, as this friction causes the film to abrade. Furthermore, it can suppress bag tearing due to bending fatigue caused by contact between bags. In addition, the high water-resistant adhesive strength between the polyamide film and the sealant film provides high bag tear prevention performance.

[0063] [Properties of biaxially oriented polyamide film] The biaxially oriented polyamide film of the present invention preferably has fewer than 20 pinhole defects when subjected to a torsional bending test using a Gelboflex tester according to the measurement method described in the examples, 1000 times at a temperature of 1°C. More preferably, it has fewer than 10 pinhole defects. The fewer the number of pinhole defects after the bending test, the better the bending pinhole resistance. If the number of pinholes is 10 or less, a packaging bag can be obtained that is less likely to develop pinholes even when the packaging bag is subjected to load during transportation, etc.

[0064] Furthermore, the biaxially oriented polyamide film of the present invention preferably has a distance of 2000 cm or more before a pinhole occurs in the abrasion resistance pinhole test. More preferably, it is 2900 cm or more, and even more preferably 3000 cm or more. The longer the distance at which a pinhole occurs, the better the abrasion resistance to pinholes. If the distance at which a pinhole occurs is 2900 cm or more, a packaging bag can be obtained that is less prone to pinholes even when the packaging bag rubs against cardboard boxes or the like during transportation. In this invention, by optimizing the raw material compositions of layers A and B, a biaxially oriented polyamide film with excellent properties in both flexural pinhole resistance and abrasion pinhole resistance can be obtained. The biaxially oriented polyamide film of this invention, possessing these properties, is extremely useful as a packaging film because it is less prone to pinhole formation during transportation.

[0065] The biaxially oriented polyamide film of the present invention preferably has a heat shrinkage rate of 0.6 to 5.0% in both the flow direction (hereinafter abbreviated as MD direction) and the width direction (hereinafter abbreviated as TD direction) at 160°C for 10 minutes, and more preferably 0.6 to 3.0%. 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. In addition, the lamination strength with sealant film may be weakened. Although it is possible to set the heat shrinkage rate to less than 0.6%, it may become mechanically brittle. Furthermore, it is undesirable because it reduces productivity.

[0066] Since excellent impact resistance is a characteristic of biaxially oriented polyamide films, the impact strength of the easily adhering polyamide film of the present invention is preferably 0.7 J / 15 μm or higher. A more preferable impact strength is 0.9 J / 15 μm or higher. While a higher impact strength is preferable, it is difficult to achieve a strength greater than 1.5 J / 15 μm. The puncture strength of the easily adhering polyamide film of the present invention is preferably 0.65 N / μm or higher. A more preferable puncture resistance is 0.70 N / μm or higher. While a higher puncture strength is preferable, it is difficult to achieve a strength greater than 1.0 N / μm. The surface orientation coefficient of the easily adhering polyamide film of the present invention is preferably 0.045 or higher. A more preferable surface orientation coefficient is 0.050 or higher. A larger surface orientation coefficient is preferable because it increases impact strength and puncture strength, but increasing it above 0.080 requires a higher stretching ratio, which makes the film more prone to breakage during the stretching process and is therefore difficult.

[0067] The haze value of the biaxially oriented 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 results in good transparency and gloss, allowing for clean printing and enhancing the product's value when used for packaging bags. Adding fine particles to improve the slipperiness of the film increases the haze value. Therefore, it is preferable to add or increase the amount of fine particles only in the surface layer B, and reduce the amount in layer A, in order to obtain a film with good slipperiness and a low haze value.

[0068] The biaxially oriented polyamide film of the present invention preferably has a laminate strength of 4.0 N / 15 mm or higher after being bonded with the polyethylene-based sealant described in the examples. The biaxially oriented polyamide film of the present invention is typically laminated with a sealant film before being processed into a packaging bag. If the lamination strength is 4.0 N / 15 mm or higher, when packaging bags are made using the biaxially oriented polyamide film of the present invention in various lamination configurations, sufficient strength of the seal portion is obtained, resulting in a strong, tear-resistant packaging bag. To achieve a laminate strength of 4.0 N / 15 mm or higher, the biaxially oriented polyamide film of the present invention can be subjected to corona treatment, coating treatment, flame treatment, etc.

[0069] Furthermore, the biaxially oriented polyamide film of the present invention can be subjected to heat treatment or humidity control treatment to improve dimensional stability depending on the application. In addition, corona treatment, coating treatment, flame treatment, etc. can be applied to improve the adhesion of the film surface, and printing, vapor deposition of metals or inorganic oxides can also be performed. As the vapor deposition film formed by vapor deposition, aluminum vapor deposition films, and vapor deposition films of silicon oxide or aluminum oxide as a single substance or mixture are preferably used. Furthermore, by coating these vapor deposition films with a protective layer, the oxygen and hydrogen barrier properties can be improved.

[0070] The biaxially oriented polyamide film of the present invention is laminated by adding a sealant film or the like, and then processed into packaging bags such as bottom-seal bags, side-seal bags, three-sided-seal bags, pillow bags, standing pouches, gusset bags, and square-bottom bags. Examples of sealant films 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 oriented polyamide film of the present invention is not particularly limited as long as it contains the easily adhering polyamide film according to the embodiment of the present invention within the laminated film. Furthermore, the film used in the laminated film may be derived from petrochemical raw materials or biomass raw materials, but polylactic acid, polyethylene terephthalate, polybutylene succinate, polyethylene, polyethylene furanoate, etc., polymerized using biomass-derived raw materials are preferable in terms of reducing environmental impact.

[0071] As an example of the layer structure of the laminated film of the present invention, if we represent the layer boundaries with / , for example, ONY / contact / LLDPE, ONY / contact / CPP, ONY / contact / Al / contact / CPP, ONY / contact / Al / contact / LLDPE, ONY / PE / Al / contact / LLDPE, ONY / contact / Al / PE / LLDPE, PET / contact / ONY / contact / LLDPE, PET / contact / ONY / PE / LLDPE, PET / contact / ONY / contact / Al / contact / 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 / coated / Al / coated / PET / PE / LLDPE, ONY / PE / LLDPE, ONY / PE / CPP, ONY / PE / Al / PE, ONY / PE / Al / PE / LLDPE, OPP / coated / ONY / coated / LLDPE, ONY / coated / EVOH / coated / LLDPE, ONY / coated / EVOH / coated / CPP, ONY / coated / aluminum or inorganic oxide vapor-deposited PET / coated / LLDPE, Examples include ONY / coated / aluminum-coated PET / coated / ONY / coated / LLDPE, ONY / coated / aluminum-coated PET / PE / LLDPE, ONY / PE / aluminum-coated PET / PE / LLDPE, ONY / coated / aluminum-coated PET / coated / CPP, PET / coated / aluminum-coated PET / coated / ONY / coated / LLDPE, CPP / coated / ONY / coated / LLDPE, ONY / coated / aluminum-coated LLDPE, and ONY / coated / aluminum-coated CPP. The abbreviations used in the above layer structure are as follows: ONY: Biaxially oriented polyamide film of the present invention, PET: Stretched polyethylene terephthalate film, LLDPE: Unstretched linear low-density polyethylene film, CPP: Unstretched polypropylene film, OPP: Stretched polypropylene film, PE: Extruded laminate or unstretched low-density polyethylene film, Al: Aluminum foil, EVOH: Ethylene-vinyl alcohol copolymer resin, Adhesive: Adhesive layer for bonding films together, Aluminum or inorganic oxide deposition indicates that aluminum or inorganic oxide has been deposited. [Examples]

[0072] 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 by the following measurement method. Unless otherwise specified, measurements were taken in a measurement room with an environment of 23°C and 65% relative humidity.

[0073] (1) Film recycling rate The recycling rate for biaxially oriented polyamide film is calculated as a percentage of the total raw materials used in the film, representing the ratio of chemically recycled materials to mechanically recycled materials. (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 sufficient for thickness measurement), and 10 sheets of 100mm film were cut out in the MD direction, stacked together. These were then conditioned for at least 2 hours 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 instrument manufactured by Tester Industries, and the average value was used as the thickness.

[0074] (3) Film haze value Measurements were taken using a direct-reading haze meter manufactured by Toyo Seiki Seisakusho Co., Ltd., in accordance with JIS-K-7105. (4) Coefficient of dynamic friction of the film In accordance with JIS-C2151, the coefficient of dynamic friction between the outer surfaces of the film rolls was evaluated under the following conditions. The test specimens were 130 mm wide and 250 mm long, and the test speed was 150 mm / min.

[0075] (5) Impact strength of the film Measurements were taken using a film impact tester manufactured by Toyo Seiki Seisakusho Co., Ltd. The measured values ​​were converted to J (joules) / 15μm and expressed as J / 15μm. (6) Degree of surface orientation of the film For the sample, the refractive index in the longitudinal direction of the film (Nx), the refractive index in the width direction (Ny), and the refractive index in the thickness direction (Nz) were measured using an Abbe refractometer with sodium D line as the light source, according to JIS K 7142-1996 Method A, and the surface orientation coefficient was calculated using the formula in equation (1). Surface orientation coefficient (ΔP) = (Nx + Ny) / 2 - Nz (1) (7) Puncture strength of the film The obtained polyester film was sampled in 5cm squares, and its puncture strength was measured in accordance with JIS Z1707 using an IMADA Corporation digital force gauge "ZTS-500N", an electric measuring stand "MX2-500N", and a puncture jig "TKS-250N". The unit is expressed in N / μm.

[0076] (8) Film's resistance to bending pinholes The number of bending fatigue pinholes was measured using a Gelboflex tester manufactured by Rigaku Kogyo Co., Ltd., according to the following method. After applying a polyester adhesive to the film prepared in the example, a 40 μm thick linear low-density polyethylene film (L-LDPE film: manufactured by Toyobo Co., Ltd., L4102) was dry-laminated onto it, and the film was aged for 3 days at 40°C to obtain a laminated film. The obtained laminated film was cut to 12 inches x 8 inches to form a cylindrical shape with a diameter of 3.5 inches. One end of the cylindrical film was fixed to the fixed head side of the Gelboflex tester, and the other end to the movable head side, with an initial gripping distance of 7 inches. A 440-degree twist was applied to the first 3.5 inches of the stroke, and then the entire stroke was completed with a linear horizontal movement for the remaining 2.5 inches. This bending fatigue was performed 1000 times at a speed of 40 times / minute, and the number of pinholes generated in the laminated film was counted. The measurement was 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 cellophane 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 evenly 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 measured.

[0077] (9) Film's resistance to abrasion and pinholes Using a durability testing machine (Toyo Seiki Seisakusho), friction tests were conducted using the following method, and the distance to pinhole occurrence was measured. A test sample was prepared by folding a laminate film similar to the one used in the above bending resistance pinhole evaluation into quarters and sharpening the corners. This sample was then rubbed against the inner surface of corrugated cardboard using a durability tester with an amplitude of 25 cm, an amplitude speed of 30 times / minute, and a load of 100 g. The corrugated cardboard used was K280×P180×K210(AF) = (outer liner × core material × inner liner (flute type)). The pinhole generation distance was calculated according to the following procedure. A longer pinhole generation distance indicates better resistance to friction-induced pinholes. First, a friction test was performed with an amplitude of 100 times at a distance of 2500 cm. If no pinhole appeared, the friction test was repeated with an increased amplitude of 20 times at a distance of 500 cm. If no pinhole appeared again, the friction test was repeated with an additional 20 times at a distance of 500 cm. This process was repeated, and the distance at which a pinhole appeared was marked with an "X" and designated as level 1. If a pinhole appeared at an amplitude of 100 times at a distance of 2500 cm, the friction test was repeated with an additional 20 times at a distance of 500 cm. If a pinhole appeared again, the friction test was repeated with an additional 20 times at a distance of 500 cm. This process was repeated, and the distance at which no pinhole appeared was marked with a "○" and designated as level 1. Next, for Level 2, if the final result in Level 1 was ○, the number of amplitude cycles was increased by 20 and a friction test was performed. If no pinhole appeared, it was marked ○; if a pinhole appeared, it was marked ×. If the final result in Level 1 was ×, the number of amplitude cycles was decreased by 20 and a friction test was performed. If no pinhole appeared, it was marked ○; if a pinhole appeared, it was marked ×. Next, for levels 3 through 20, if the previous level was marked with a circle (○), increase the number of amplitude cycles by 20 and perform a friction test. If no pinhole appears, mark it with a circle (○); if a pinhole appears, mark it with an X (×). If the previous level was marked with an X (×), decrease the number of amplitude cycles by 20 and perform a friction test. If no pinhole appears, mark it with a circle (○); if a pinhole appears, mark it with an X (×). Repeat this process and mark levels 3 through 20 with a circle (○) or an X (×). For example, the results shown in Table 1 were obtained. We will explain how to determine the pinhole occurrence distance using Table 1 as an example. Count the number of tests with a "○" and a "×" for each distance. The distance with the most tests was used as the median, and its coefficient was set to zero. For distances longer than this, the coefficient was increased by +1, +2, +3, etc., for every 500cm increment; for distances shorter than this, the coefficient was decreased by -1, -2, -3, etc., for every 500cm increment. For all tests from level 1 to 20, the number of tests in which no holes occurred was compared to the number of tests in which holes occurred, and the friction pinhole generation distance was calculated using the following formulas for cases A and B. A; If the number of tests that did not develop holes is greater than or equal to the number of tests that did develop holes. Friction pinhole occurrence distance = median + 500 × (Σ(coefficient × number of tests where no holes occurred) / number of tests where no holes occurred) + 1 / 2) B: In all tests, the number of tests that did not develop holes is less than the number of tests that developed holes. Friction pinhole occurrence distance = median + 500 × (Σ(coefficient × number of tests with holes) / number of tests with holes) - 1 / 2)

[0078] [Table 1]

[0079] (10) Thermal shrinkage rate of the film Except for a test temperature of 160°C and a heating time of 10 minutes, the thermal shrinkage rate was measured according to the dimensional change test method described in JIS C2318 using the following formula. Thermal shrinkage rate = [(Length before treatment - Length after treatment) / Length before treatment] × 100 (%)

[0080] (11) Lamination strength with polyethylene sealant A laminate film prepared in the same manner as described in the explanation of the evaluation of flexural pinhole resistance was cut into strips measuring 15 mm in width and 200 mm in length. 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. The laminate strength was measured three times each in the MD direction and TD direction using an Autograph (manufactured by Shimadzu Corporation) under the conditions of a temperature of 23°C, relative humidity of 50%, tensile speed of 200 mm / min, and peel angle of 90°, and the average value was used for evaluation.

[0081] (12) Relative viscosity of raw material polyamide The relative viscosity of a polyamide solution prepared by dissolving 0.25 g of polyamide in 96% sulfuric acid in a 25 ml volumetric flask to a concentration of 1.0 g / dl was measured at 20°C. (13) Melting point of the raw material polyad In accordance with JIS K7121, measurements were taken using a Seiko Instruments SSC5200 differential scanning calorimetry analyzer in a nitrogen atmosphere with a sample weight of 10 mg, a heating start temperature of 30°C, and a heating rate of 20°C / min. The endothermic peak temperature (Tmp) was determined as the melting point.

[0082] [Polyamide 6(a-1) newly polymerized from petrochemical-derived raw materials] For the newly polymerized polyamide 6(a-1) derived from petrochemical raw materials, we used polyamide 6 manufactured by Toyobo Co., Ltd., with a relative viscosity of 2.8 and a melting point of 220°C. [Manufacturing of chemically recycled polyamide 6(a-2)] Polyamide 6 fibers recovered from waste materials and a 75% by mass aqueous phosphoric acid solution, which serves as a depolymerization catalyst, were placed in a depolymerization apparatus and heated to 260°C under a nitrogen atmosphere. The reaction was started by blowing superheated steam into the depolymerization apparatus, and the ε-caprolactam-water vapor continuously distilled from the depolymerization apparatus was 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.

[0083] [Manufacturing of mechanically recycled polyamide 6(a-3)] The waste materials generated from the stretched film obtained by the method described in Example 1 below, including off-spec film and cut-off scraps (edge ​​trim), were collected and crushed, kneaded in an extruder at a cylinder temperature of 270°C, pelletized, and then 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.

[0084] (Example 1) Using an apparatus consisting of one extruder and a 380 mm wide single-layer T-die, molten resin of the polyamide resin composition described below was extruded from the T-die into a film, cast onto a cooling roll heated to 20°C, and electrostatically adhered to obtain an unstretched film with a thickness of 200 μm. The polyamide resin composition consists 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 Silicia Chemical Co., Ltd., average particle size 2.0 μm, pore volume 1.6 ml / g), and 0.15 parts by mass of fatty acid bisamide (manufactured by Kyoeisha Chemical Co., Ltd., ethienbisstearate amide). The extruder's discharge rate was adjusted so that the total thickness of the biaxially oriented polyamide film was 15 μm. The obtained unstretched film was guided to a roll-type stretcher, and using the difference in peripheral speed of the rolls, it was stretched 1.73 times in the MD direction at 80°C, and then further stretched 1.85 times at 70°C. Subsequently, this uniaxially oriented film was continuously guided to a tenter-type stretcher, preheated at 110°C, and then stretched 1.2 times in the TD direction at 120°C, 1.7 times at 130°C, and 2.0 times at 160°C. After heat setting at 218°C, a 7% relaxation treatment was performed at 218°C, and then the surface on the side to be dry laminated with a linear low-density polyethylene film was treated with corona discharge to obtain a biaxially oriented polyamide film. The evaluation results of the obtained biaxially oriented polyamide film are shown in Table 2.

[0085] (Examples 2-6 and Reference Example 1) A biaxially oriented film was obtained using the same method as in Example 1, except that the film-forming conditions, such as the polyamide resin composition, stretching ratio, and heat-fixing temperature, were changed as shown in Table 1. The evaluation results of the obtained biaxially oriented film are shown in Table 2.

[0086] (Example 7) To simultaneously biaxially stretch the unstretched polyamide resin compositions shown in Table 1, they were sent to a hot water bath adjusted to 50°C and subjected to a 2-minute immersion treatment to adjust the moisture content to approximately 4%. The sheets were then held in clips on a tenter-type simultaneous biaxial stretching machine and subjected to corona discharge treatment under the film-forming conditions, such as the stretching ratio and heat-fixing temperature, shown in Table 1, to obtain biaxially oriented films. The evaluation results of the obtained biaxially oriented films are shown in Table 2.

[0087] [Table 2]

[0088] As shown in Table 2, the biaxially oriented polyamide films using chemically recycled polyamide 6(a-2) as shown in the examples, or the biaxially oriented polyamide films using chemically recycled polyamide 6(a-2) and mechanically recycled polyamide 6(a-3), exhibited good properties in impact resistance, puncture strength, and bending pinhole resistance. Compared to the biaxially oriented polyamide film using only polyamide 6(a-1) newly polymerized from non-recycled petrochemical raw materials as a raw material in Reference Example 1, the examples obtained biaxially oriented polyamide films with equivalent properties. Furthermore, they exhibited low haze, good transparency, and high lamination strength with sealant films, making them excellent as packaging films.

[0089] (Example 8) Using an apparatus consisting of two extruders and a 380 mm wide co-extrusion T-die, the molten resin was laminated in a B layer / A layer / B layer configuration using the feed block method, extruded from the T-die into a film, cast onto a cooling roll temperature-controlled to 20°C, and electrostatically adhered to obtain an unstretched film with a thickness of 200 μm. The resin compositions of layers A and B 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 Silicia Chemical Co., Ltd., average particle size 2.0 μm, pore volume 1.6 ml / g), and 0.15 parts by mass of fatty acid bisamide (ethienbisstearate amide, manufactured by Kyoeisha Chemical Co., Ltd.). The feed block configuration and extruder discharge rate were adjusted so that the total thickness of the biaxially oriented polyamide film was 15 μm, with the substrate layer (Layer A) being 12 μm thick and the front and back surface layers (Layer B) each being 1.5 μm thick. The obtained unstretched film was guided to a roll-type stretcher, and using the difference in peripheral speed of the rolls, it was stretched 1.73 times in the MD direction at 80°C, and then further stretched 1.85 times at 70°C. Subsequently, this uniaxially oriented film was continuously guided to a tenter-type stretcher, preheated at 110°C, and then stretched 1.2 times in the TD direction at 120°C, 1.7 times at 130°C, and 2.0 times at 160°C. After heat setting at 218°C, a 7% relaxation treatment was performed at 218°C, and then the surface to be dry-laminated with a linear low-density polyethylene film was treated with corona discharge to obtain a biaxially oriented polyamide film. The evaluation results of the obtained biaxially oriented polyamide film are shown in Table 3.

[0090] (Examples 9-14) A biaxially oriented polyamide film was obtained in the same manner as in Example 8, except that the film-forming conditions, such as the polyamide resin composition, stretching ratio, and heat-fixing temperature, were changed as shown in Table 3. The evaluation results of the obtained biaxially oriented polyamide film are shown in Table 3.

[0091] (Example 15) Except for changing the film-forming conditions such as the polyamide resin composition, stretching ratio, and heat-fixing temperature as shown in Table 3, an unstretched film was prepared in the same manner as in Example 8, and simultaneous biaxial stretching was performed in the same manner as in Example 7 to obtain a biaxially oriented polyamide film. The evaluation results of the obtained biaxially oriented polyamide film are shown in Table 3.

[0092] [Table 3]

[0093] As shown in Table 3, the biaxially oriented polyamide films using chemically recycled (a-2) polyamide 6 as shown in the examples, or the biaxially oriented polyamide films using chemically recycled polyamide 6 (a-2) and mechanically recycled polyamide 6 (a-3), exhibited good properties in impact resistance, puncture strength, and abrasion pinhole resistance. Compared to the biaxially oriented polyamide film using only polyamide 6 (a-1) newly polymerized from non-recycled petrochemical raw materials as a raw material in Reference Example 2, the examples obtained biaxially oriented polyamide films with equivalent properties. Furthermore, they exhibited low haze, good transparency, and high lamination strength with sealant films, making them excellent as packaging films.

[0094] In Reference Examples 3 and 4, a large amount of mechanically recycled polyamide 6 was used in the surface layer (layer B). In this case, the transparency (haze) and abrasion pinhole resistance were inferior to the films in Examples 8-15, where the content of mechanically recycled polyamide 6 in the surface layer was 10% by mass or less.

[0095] [Example 15] Using the biaxially oriented polyamide film prepared in Example 11, laminates with the following configurations (1) to (9) were fabricated, and three-side seal type and pillow-type packaging bags were made using these laminates. Packaging bags with a good appearance and resistance to tearing in drop impact tests were successfully produced. (1) Biaxially oriented polyamide film layer / Printed layer / Polyurethane adhesive layer / Linear low-density polyethylene film sealant layer. (2) Biaxially oriented polyamide film layer / Printed 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 / Unoriented 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 / printed 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 / printed layer / polyurethane adhesive layer / unoriented 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 unoriented polypropylene film sealant layer. [Industrial applicability]

[0096] The biaxially oriented polyamide film of the present invention exhibits excellent puncture strength, impact resistance, bending pinhole resistance, and abrasion pinhole resistance, making it suitable for use as a packaging material for food packaging and other applications. Furthermore, by using polyamide 6 chemically recycled from waste polyamide products, it is possible to contribute to reducing the environmental burden. Moreover, by blending mechanically recycled polyamide 6 into the raw material, a biaxially oriented polyamide film with even greater environmental impact can be obtained.

Claims

1. A method for producing a biaxially oriented polyamide film for food packaging, wherein a surface layer (B layer) is laminated on both sides of a base layer (A layer), characterized in that the A layer is made of a polyamide resin composition containing 70% by mass or more of polyamide 6, and of the polyamide 6, 4 to 90% by mass of polyamide 6 recovered from used polyamide products and chemically recycled, and 5 to 60% by mass of mechanically recycled polyamide 6, the B layer is made of a polyamide resin composition containing 70% by mass or more of polyamide 6, and does not contain polyamide elastomers or polyolefin elastomers, and the biaxially oriented polyamide film satisfies the following (a) and (b). (However, the chemically recycled polyamide 6 excludes, in the form of polyamide resin, resin waste generated during polymerization, waste from switching between brands during polymerization, and waste from switching until the film product is commercialized, as well as waste such as trimming waste and slitting waste generated during film manufacturing, and films that were not commercialized as defective products, and in the form of oligomers, the residue after filtration, which includes highly water-soluble oligomers and less water-soluble 2-8-mers recovered from the scouring water generated during the scouring of polyamide resin, and is chemically recycled using these as raw materials for depolymerization.) (a) Puncture strength of 0.65 N / μm or more, (b) Impact strength of 0.9 J / 15 μm or more.

2. A method for producing a biaxially oriented polyamide film according to claim 1, characterized in that the polyamide resin composition of the base layer (layer A) contains 5 to 60% by mass of mechanically recycled polyamide 6, and the polyamide resin composition of the surface layer (layer B) contains 0 to 30% by mass of mechanically recycled polyamide 6.

3. The method for producing a biaxially oriented polyamide film according to claim 1 or 2, characterized in that the relative viscosity of the polyamide 6 is 1.8 to 4.

5.

4. A method for producing a biaxially oriented polyamide film according to any one of claims 1 to 3, characterized in that the laminate strength after bonding with a polyethylene-based sealant film is 4.0 N / 15 mm or more.

5. A method for producing a laminated film for food packaging, comprising laminating a sealant film onto a 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 manufacturing a food packaging bag using a laminated film for food packaging obtained by the method for manufacturing a laminated film for food packaging described in claim 5.

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