JP7722544B2 - Manufacturing method for biaxially oriented polyamide film for food packaging - Google Patents

Description

The present invention relates to a biaxially oriented polyamide film that exhibits excellent impact resistance, flex pinhole resistance, and abrasion pinhole resistance, and is made from raw materials derived from biomass (organic resources derived from living organisms such as plants) and polyamide 6 chemically recycled from waste polyamide products. The biaxially oriented polyamide film of the present invention is suitable for use as a food packaging film, etc.

Biaxially oriented films made from aliphatic polyamides, such as polyamide 6, have traditionally been widely used as packaging films for various purposes due to their excellent impact resistance and resistance to pinholes caused by bending.

A film made by mixing a polyamide-based elastomer with an aliphatic polyamide is known as a means of improving the above-mentioned pinhole resistance under bending (see, for example, Patent Document 1). This film has good pinhole resistance and impact resistance under bending in low-temperature environments, and is less likely to develop pinholes due to bending fatigue, even under low-temperature conditions. However, it has been found that the polyamide-based elastomer added during film production is subject to thermal degradation, easily forming a degraded substance known as "pitting resin" at the die lip exit, which can cause deterioration in the accuracy of film thickness. Furthermore, the degraded substance itself falls, resulting in defective products and reducing production efficiency during continuous film production.

Pinholes can occur not only due to bending, but also due to friction (rubbing). The methods for improving pinholes caused by bending and pinholes caused by friction are often contradictory. For example, increasing the flexibility of a film makes it less likely for pinholes to form due to bending, but the softer the film, the more likely it is for pinholes to form due to friction. In response to this, a packaging laminate has been proposed that has excellent bending resistance and friction pinhole resistance by applying a surface coating agent to the outer surface of a biaxially oriented polyamide film (see, for example, Patent Document 2). However, this method is not very effective in preventing friction pinholes. It also requires a coating process.

In recent years, the use of biomass as an alternative to fossil fuels has been attracting attention in the materials sector in order to build a recycling-oriented society. Biomass is an organic compound formed by photosynthesis from carbon dioxide and water, and when used, it can be converted back into carbon dioxide and water, making it a so-called carbon-neutral raw material (the amount of carbon dioxide emitted and absorbed in the environment is the same, thereby suppressing the increase of carbon dioxide, a greenhouse gas). Recently, the practical application of biomass plastics made from these biomass raw materials has progressed rapidly, and attempts are also being made to produce polyester, a general-purpose polymer material, from these biomass raw materials.

In addition, there is a demand for the use of recycled materials to reduce the amount of plastic waste generated. Methods for recycling polyamide 6 include thermal recycling, in which it is incinerated and the heat energy 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 its polyamide raw materials, which can then be reused in polyamide production, etc.

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

For example, Patent Document 3 discloses a recycling method in which used polyamide clothing products are collected, depolymerized to recover ε-caprolactam, which is then purified, polymerized, and melt-spun or molded into polyamide fibers or molded polyamide products. This technology makes it possible to recycle collected clothing products by returning them to their raw materials and reusing them. Furthermore, by decomposing and refining collected clothing products, high-purity, high-quality raw materials (raw monomers) can be obtained, which allows for recycling to produce high-quality polyamide 6 products and enables repeated recycling. Furthermore, the work of collecting and sorting collected clothing products is significantly reduced.

Polyamide resin recycled using the chemical recycling method described above has primarily been used as a raw material for fibers and molded products, but has not yet been put to practical use as food packaging film.

Japanese Patent Application Publication No. 11-254615 Japanese Patent Application Laid-Open No. 2001-205761 Japanese Patent Application Publication No. 7-310204

The present invention was devised in light of these problems with the prior art. The object of the present invention is to provide a biaxially oriented polyamide film that has excellent impact resistance, flex pinhole resistance, and abrasion pinhole resistance, while also reducing the environmental impact.

That is, the present invention comprises the following:
[1] The polyamide resin contains 70 to 99% by mass of polyamide 6 and 1 to 30% by mass of polyamide whose raw materials are at least partly derived from biomass;
The biaxially stretched polyamide film, wherein the polyamide 6 contains 5 to 100 parts by mass of polyamide 6 obtained by chemical recycling, based on 100 parts by mass of polyamide 6.
[2] The biaxially stretched polyamide film according to [1], wherein the polyamide 6 contains, per 100 parts by mass of polyamide 6, polyamide 6 obtained by chemical recycling and 5 to 50 parts by mass of polyamide 6 obtained by mechanical recycling.
[3] The biaxially oriented polyamide film according to [1] or [2], characterized in that the content of biomass-derived carbon as determined by radiocarbon (C ¹⁴ ) measurement is 1 to 15% of the total carbon in the biaxially oriented polyamide film.
[4] The biaxially stretched polyamide film according to any one of [1] to [3], wherein the polyamide resin at least partly derived from biomass is at least one polyamide resin selected from the group consisting of polyamide 11, polyamide 410, polyamide 610, and polyamide 1010.
[5] A biaxially oriented polyamide film according to any one of [1] to [4], wherein one or more layers are laminated on at least one side of the biaxially oriented polyamide film, the laminated layer containing 70 to 100 mass% of polyamide 6, and the polyamide 6 contains 5 to 100 mass parts of polyamide 6 obtained by chemical recycling, based on 100 mass parts of polyamide 6.
[6] The biaxially stretched polyamide film according to [5], wherein the polyamide 6 contains, per 100 parts by mass of polyamide 6, polyamide 6 obtained by chemical recycling and 5 to 50 parts by mass of polyamide 6 obtained by mechanical recycling.
[7] The biaxially oriented polyamide film according to [5] or [6], wherein the thickness of the laminated layer is 7 to 50% of the total thickness of the film.
[8] The biaxially stretched polyamide film according to any one of [1] to [7], which satisfies the following (a) and (b):
(a) The number of pinhole defects is 10 or less when a twisting and bending test using a Gelbo flex tester is performed 1,000 times at a temperature of 1°C,
(b) The distance until pinholes appear in a friction pinhole resistance test is 2900 cm or more. [9] The biaxially oriented polyamide film according to any one of [1] to [8], characterized in that the haze is 10% or less and the dynamic friction coefficient is 1.0 or less.
[10] The biaxially oriented polyamide film according to any one of [1] to [8], characterized in that the lamination strength after being attached to a polyethylene sealant film is 4.0 N/15 mm or more.
[11] A laminated film in which a sealant film is laminated on at least one side of the biaxially stretched polyamide film according to any one of [1] to [10].
[12] A packaging bag using the laminated film described in [11].

The biaxially oriented polyamide film of the present invention is primarily composed of polyamide 6 resin. By blending it with a polyamide resin polymerized from specific biomass-derived raw materials and employing specific film-forming conditions, the film exhibits impact resistance, flex pinhole resistance, abrasion pinhole resistance, and adhesion to sealant films, while also producing a carbon-neutral polyamide film.

The biaxially oriented polyamide film of the present invention can also be produced using or blending polyamide 6 chemically recycled from waste polyamide products, thereby reducing the environmental impact.

Furthermore, according to the present invention, since the biomass-derived polyamide is effective in improving pinhole resistance, there is no need to add a polyamide-based elastomer. This prevents the adhesion of degradation products to the inner surface of the die caused by the polyamide-based elastomer and the adhesion of resin to the die lip outlet. As a result, it is possible to prevent the thickness unevenness of the film from worsening and enable long-term continuous production.

Schematic diagram of friction pinhole resistance evaluation device

[Biaxially stretched polyamide film]
The biaxially stretched polyamide film of the present invention contains, as polyamide resins, 70 to 99% by mass of polyamide 6 and 1 to 30% by mass of a polyamide at least partially derived from biomass. The polyamide 6 further contains 5 to 100 parts by mass of polyamide 6 obtained by chemical recycling, per 100 parts by mass of polyamide 6. By containing 70% by mass or more of polyamide 6, the film can achieve the excellent mechanical strength, such as impact strength, and gas barrier properties, such as oxygen barrier properties, inherent to biaxially stretched polyamide films made of polyamide 6.

The biaxially oriented polyamide film of the present invention has improved flex pinhole resistance and abrasion pinhole resistance due to the inclusion of 1 to 30 mass % of a polyamide resin, at least a portion of which is derived from biomass. Conventionally used flex pinhole resistance improvers, such as polyamide elastomers and polyolefin elastomers, improve flex pinhole resistance but degrade abrasion pinhole resistance. By including a polyamide resin, at least a portion of which is derived from biomass, a biaxially oriented polyamide film with excellent flex pinhole resistance and abrasion pinhole resistance can be obtained. Furthermore, the resulting film is carbon-neutral and has little effect on the increase or decrease of terrestrial carbon dioxide.

The polyamide 6 used in the biaxially oriented polyamide film of the present invention preferably comprises 5 to 100 parts by mass of chemically recycled polyamide 6, based on 100 parts by mass of the total polyamide 6. Examples of raw materials for chemical recycling include, but are not limited to, discarded polyamide 6 products such as discarded plastic products, discarded tire rubber, fibers, and fishing nets. By using chemically recycled polyamide 6, a polyamide film can be obtained that reduces the environmental impact.

[Polyamide 6]
The polyamide 6 resin used in the present invention is usually produced by ring-opening polymerization of ε-caprolactam. The polyamide 6 resin 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 polyamide 6 resin 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 film will have insufficient impact strength. If it is more than 4.5, the load on the extruder will increase, making it difficult to obtain unstretched film before stretching.

The polyamide 6 used in the biaxially oriented polyamide film of the present invention can be polyamide 6 polymerized from commonly used fossil fuel-derived monomers, as well as polyamide 6 chemically recycled from waste polyamide 6 products such as waste plastic products, waste tire rubber, fibers, and fishing nets. One method for obtaining chemically recycled polyamide 6 from waste polyamide 6 products is to collect used polyamide products, depolymerize them to obtain ε-caprolactam, purify this, and then polymerize polyamide 6.

<Depolymerization conditions>
In the depolymerization carried out when producing the chemically recycled polyamide 6 used in the biaxially oriented polyamide film of the present invention, the polyamide 6 product is usually depolymerized by heating. The depolymerization may be performed with or without the use of a catalyst.

The depolymerization pressure may be reduced, normal, or increased. The depolymerization temperature is typically 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 will not melt, slowing down the depolymerization rate. If the temperature is high, decomposition of unnecessary polyamide 6 monomer (i.e., caprolactam) may occur, potentially reducing the purity of the recovered caprolactam.

When a catalyst is used in the above depolymerization, 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. Phosphoric acid, boric acid, organic acids, alkali hydroxides, and alkali salts are preferred. More preferred examples include phosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate.

The amount of acid catalyst used in the above depolymerization is typically 0.01 to 50% by mass relative to the polyamide 6 component. It is preferably 0.01 to 20% by mass, and more preferably 0.5 to 10% by mass. Using a small amount of catalyst slows the reaction rate, while using a large amount increases the number of side reactions and increases catalyst costs, making it economically disadvantageous.

The above depolymerization can be carried out in the absence (dry) or presence (wet) of water. In wet depolymerization, the amount of water used is 0.1 to 50 times by weight relative to the polyamide 6 product components, such as fibers. Preferably, it is 0.5 to 20 times by weight, and more preferably, it is 1 to 10 times by weight. Using a small amount of water slows the reaction rate, while using a large amount of water reduces the concentration of the recovered caprolactam aqueous solution, which is disadvantageous in terms of obtaining caprolactam. When wet depolymerization is performed, the produced caprolactam is distilled from the reactor together with water to obtain the recovered caprolactam aqueous solution. After the depolymerization reaction is complete, caprolactam can be extracted by vacuum distillation. Alternatively, it can be extracted continuously as the reaction progresses. When dry depolymerization is performed, the produced caprolactam is distilled from the reactor by vacuum distillation to obtain the recovered caprolactam. After the depolymerization reaction is complete, caprolactam can be extracted by vacuum distillation. Alternatively, it can be removed continuously as the reaction progresses.

To obtain even higher purity caprolactam, the recovered caprolactam can be combined 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.

The biaxially oriented polyamide film of the present invention can also be used in combination with polyamide 6, which is mechanically recycled waste material generated during the biaxially oriented polyamide film manufacturing process.

The above-mentioned mechanically recycled polyamide 6 is a raw material made by recovering scrap material generated during the production of biaxially oriented polyamide film, such as off-spec, unshippable film and offcuts (edge trim), and pelletizing it through melt extrusion or compression molding.

There is no particular lower limit to the proportion of mechanically recycled polyamide 6 added to the biaxially stretched polyamide film of the present invention. The upper limit is preferably 50 parts by mass, more preferably 40 parts by mass, and even more preferably 30 parts by mass, based on 100 parts by mass of the total polyamide 6. If the proportion of mechanically recycled polyamide added exceeds the above range, the film may become more discolored 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.

[Polyamide resin at least part of which is derived from biomass]
Examples of polyamide resins used in the present invention, at least a part of which is derived from biomass, include polyamide 11, polyamide 410, polyamide 610, polyamide 1010, polyamide MXD10 resin, and polyamide 11-6T copolymer resin.

Polyamide 11 is a polyamide resin with a structure in which monomers containing 11 carbon atoms are bonded via amide bonds. Polyamide 11 is typically obtained using aminoundecanoic acid or undecane lactam as the monomer. Aminoundecanoic acid, in particular, is a monomer derived from castor oil, making it desirable from a carbon-neutral perspective. The structural units derived from these monomers containing 11 carbon atoms preferably account for 50 mol% or more of the total structural units in polyamide 11, more preferably 80 mol% or more, and even 100 mol%. Polyamide 11 is typically produced by the ring-opening polymerization of undecane lactam, as described above. Polyamide 11 obtained by ring-opening polymerization is typically subjected to removal of the lactam monomer with hot water, followed by drying and melt extrusion in an extruder. The relative viscosity of polyamide 11 is preferably 1.8 to 4.5, more preferably 2.4 to 3.2. A relative viscosity of less than 1.8 results in insufficient film impact strength. If it is greater than 4.5, the load on the extruder will increase, making it difficult to obtain unstretched film before stretching.

Polyamide 610 is a polyamide resin having a structure formed by polymerizing a diamine having six carbon atoms and a dicarboxylic acid having ten carbon atoms. Hexamethylenediamine and sebacic acid are typically used. Of these, sebacic acid is a monomer obtained from castor oil, making it desirable from a carbon-neutral perspective. The combined total of the structural units derived from these monomers having six carbon atoms and the structural units derived from monomers having ten carbon atoms in PA610 preferably accounts for 50 mol% or more of all structural units, more preferably 80 mol% or more, and may even be 100 mol%.

Polyamide 1010 is a polyamide resin with a structure formed by polymerizing a diamine having 10 carbon atoms and a dicarboxylic acid having 10 carbon atoms. Polyamide 1010 typically uses 1,10-decanediamine (decamethylenediamine) and sebacic acid. Decamethylenediamine and sebacic acid are monomers obtained from castor oil, making them desirable from a carbon-neutral perspective. The combined total of the structural units derived from these diamines having 10 carbon atoms and the structural units derived from dicarboxylic acids having 10 carbon atoms in PA1010 preferably accounts for 50 mol% or more of all structural units, more preferably 80 mol% or more, and may even be 100 mol%.

Polyamide 410 is a polyamide resin with a copolymer structure of a monomer with four carbon atoms and a diamine with ten carbon atoms. Polyamide 410 is typically made from sebacic acid and tetramethylenediamine. From an environmental perspective, it is preferable to use sebacic acid derived from castor oil, a vegetable oil. From an environmental protection perspective (particularly from the perspective of carbon neutrality), it is desirable to use sebacic acid derived from castor oil.

In the biaxially stretched polyamide film of the present invention, the lower limit of the content of polyamide resin at least partly derived from biomass raw materials is not particularly limited, but is preferably 1% by mass, and more preferably 3% by mass or more. The upper limit of the content is 30% by mass, and more preferably 20% by mass. If the content of polyamide resin at least partly derived from biomass raw materials exceeds 30% by mass, the molten film may become unstable when cast, making it difficult to obtain a homogeneous unstretched film.

[Subsidiary materials, additives]
The biaxially stretched polyamide film of 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.

<Other thermoplastic resins>
The biaxially stretched polyamide film of the present invention may contain a thermoplastic resin in addition to the polyamide 6 and a polyamide resin at least partially derived from biomass, as long as the objectives of the present invention are not impaired. Examples include polyamide-based resins such as polyamide 12 resin, polyamide 66 resin, polyamide 6-12 copolymer resin, polyamide 6-66 copolymer resin, and polyamide MXD6 resin. If necessary, thermoplastic resins other than polyamide-based resins, such as polyester polymers such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, and polyolefin polymers such as polyethylene and polypropylene, may also be included. It is preferable that the raw materials for these thermoplastic resins are biomass-derived, as this does not affect the increase or decrease in terrestrial carbon dioxide and therefore reduces the environmental burden.

<Lubricant>
The biaxially oriented polyamide film of the present invention 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. By improving the slipperiness of the biaxially oriented polyamide film of the present invention, it is also effective in reducing breakage of packaging bags due to friction.

The fine particles can be appropriately selected from inorganic fine particles such as silica, kaolin, and zeolite, and polymeric organic fine particles such as acrylic and polystyrene-based fine particles. From the perspectives of transparency and slipperiness, it is preferable to use silica fine particles. The average particle size of the fine particles is preferably 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 must be added to achieve good slipperiness. On the other hand, if the average particle size exceeds 5.0 μm, the surface roughness of the film tends to become too great, resulting in a poor appearance.

When using the silica microparticles, the pore volume of the silica is preferably in the range of 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, reducing the transparency of the film. If the pore volume exceeds 2.0 ml/g, the microparticles tend to be less likely to form protrusions on the surface.

The biaxially stretched polyamide film of 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 erucamide, stearamide, ethylene bisstearamide, ethylene bisbehenamide, and ethylene bisoleamide.
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.

To improve slip properties, polyamide resins such as polyamide MXD6 resin, polyamide 12 resin, polyamide 66 resin, polyamide 6-12 copolymer resin, and polyamide 6-66 copolymer resin can be added to the biaxially stretched polyamide film of the present invention. Polyamide MXD6 resin is particularly preferred, and it is preferable to add it in an amount of 1 to 10 mass %.

<Antioxidants>
The biaxially stretched polyamide film of the present invention may contain an antioxidant. Phenol-based antioxidants are preferred. The phenol-based antioxidant is preferably a fully hindered phenolic compound or a partially hindered phenolic compound. Examples of the phenol-based antioxidant 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 a phenol-based antioxidant improves the film-forming operability of the biaxially stretched polyamide film. In particular, when recycled films are used as raw materials, the resin is prone to thermal degradation, which can lead to operational problems in film production and increased production costs. In contrast, the inclusion of an antioxidant suppresses thermal degradation of the resin and improves operational efficiency.

[Surface layer]
In the present invention, one or more surface layers made of the same or different resin compositions can be laminated on one or both sides of the biaxially stretched polyamide film. By laminating at least one surface layer, the surface properties can be improved.

The surface layer is preferably a layer made of a resin composition containing 70 to 100% by mass of polyamide 6. Preferably, the polyamide 6 content is 80% by mass or more, and more preferably 90% by mass or more. The upper limit is 100% by mass, preferably 99% by mass, and more preferably 97% by mass. By containing 70% by mass or more of polyamide 6, a biaxially oriented polyamide film with excellent mechanical strength, such as impact strength, and gas barrier properties, such as oxygen, can be obtained.

Furthermore, it is preferable that the polyamide 6 obtained by chemical recycling be contained in an amount of 5 to 100 parts by mass per 100 parts by mass of polyamide 6. In addition to the polyamide 6 obtained by chemical recycling, polyamide 6 obtained by mechanical recycling can also be used in combination. The proportion of polyamide 6 obtained by mechanical recycling is preferably 0 to 50 parts by mass per 100 parts by mass of polyamide 6. If the proportion of mechanically recycled polyamide used exceeds the above range, the film may become more discolored or have a high haze value, which may impair the appearance of the film. Alternatively, degradation products may increase during film production, which may deteriorate film formability.

The surface layer can contain various additives, such as other thermoplastic resins, lubricants, heat stabilizers, antioxidants, antistatic agents, anti-fogging agents, UV absorbers, dyes, and pigments, depending on the function to be imparted to the surface of the surface layer. When the surface layer is used on the outside of a packaging bag, it must be resistant to abrasion and pinholes, so it is undesirable to include soft resins such as polyamide elastomers or polyolefin elastomers, or substances that generate a large number of voids.

The surface layer may contain thermoplastic resins other than the polyamide 6 described above, provided that the objectives of the present invention are not impaired. Examples include polyamide-based resins such as polyamide MXD6 resin, polyamide 11 resin, polyamide 12 resin, polyamide 66 resin, polyamide 6-12 copolymer resin, and polyamide 6-66 copolymer resin. If necessary, the surface layer may also contain thermoplastic resins other than polyamide-based resins, such as polyester-based polymers such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, and polyolefin-based polymers such as polyethylene and polypropylene.

The surface layer preferably contains fine particles or an organic lubricant as a lubricant to improve the film's slipperiness. Improving the slipperiness improves the film's handling properties and reduces the risk of packaging bags breaking due to friction.

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

The preferred average particle size of the microparticles 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 must be added to achieve good slip properties. On the other hand, if it exceeds 5.0 μm, the film surface roughness tends to become too great, resulting in a poor appearance.

When using the above-mentioned silica microparticles, the pore volume of the silica is preferably in the range of 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, resulting in a decrease in film transparency. If the pore volume exceeds 2.0 ml/g, the microparticles tend to be less likely to form surface protrusions.

The 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, ethylene bisstearic acid amide, ethylene bisbehenic acid amide, and ethylene bisoleic acid amide. The content of fatty acid amides and/or fatty acid bisamides added to the surface layer 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 less than the above range, slip properties tend to deteriorate. On the other hand, if the content exceeds the above range, wettability tends to deteriorate.

To improve the film's slipperiness, polyamide resins other than polyamide 6 can be added to the surface layer, such as polyamide MXD6 resin, polyamide 11, polyamide 12 resin, polyamide 66 resin, polyamide 6-12 copolymer resin, and polyamide 6-66 copolymer resin. Polyamide MXD6 resin is particularly preferred, and it is preferable to add it at 1 to 10% by mass. At less than 1% by mass, the film's slipperiness improvement effect is minimal. At more than 10% by mass, the film's slipperiness improvement effect saturates. Polyamide MXD6 resin is produced by polycondensation of metaxylylenediamine and adipic acid. The relative viscosity of 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.

When adding fine particles, organic lubricants, or polyamide resins such as polyamide MXD6 resin to the surface layer to improve the film's slipperiness, it is preferable to reduce the amount of these added to the biaxially oriented polyamide film base, as this will result in a film with excellent transparency and slipperiness.

In addition, polyamide resins other than polyamide 6 can be added to the surface layer to improve adhesion. In this case, copolymer polyamide resins such as polyamide 6-12 copolymer resin and polyamide 6-66 copolymer resin are preferred.

Addition of secondary materials and additives such as lubricants and antioxidants to the biaxially oriented polyamide film and surface layer that serve as the base material of the present invention can be achieved by adding them during resin polymerization or melt extrusion in an extruder. A high-concentration masterbatch can also be prepared and added to the polyamide resin during film production. These well-known methods can be used.

[Biaxially stretched polyamide film]
The thickness of the biaxially oriented polyamide film of 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.

When one or more surface layers are laminated on at least one side of the biaxially oriented polyamide film of the present invention to form a laminate film, if the thickness of the surface layer accounts for a large proportion of the total film thickness, the bending pinhole resistance will decrease. The thickness of the laminated layer is preferably 7 to 50% of the total film thickness, and more preferably 7 to 30%.

The biaxially stretched polyamide film of the present invention has 10 or fewer pinhole defects when subjected to a twist-bending test at a temperature of 1°C 1,000 times using a Gelbo flex tester according to the measurement method described in the Examples. More preferably, it has 5 or fewer 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 fewer, the resulting packaging bag is less likely to develop pinholes even when stress is applied to the packaging bag during transportation, etc.

Furthermore, in the biaxially stretched polyamide film of the present invention, in the friction pinhole resistance test described in the examples, the distance until pinholes appear is 2900 cm or more. More preferably, it is 3100 cm or more, and even more preferably, it is 3300 cm or more. The longer the distance until pinholes appear, the better the friction pinhole resistance is. 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, etc.

The biaxially oriented polyamide film of the present invention is characterized by its excellent properties of both the flexural pinhole resistance and abrasion pinhole resistance described above. The biaxially oriented polyamide film of the present invention, which has these properties, is highly effective as a packaging film because it is less likely to develop pinholes during transportation.

The film of the present invention has a heat shrinkage rate of 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, and preferably 0.6 to 2.5%. If the heat shrinkage rate exceeds 3.0%, 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. While it is possible to reduce the heat shrinkage rate to less than 0.6%, this may result in mechanical embrittlement. Furthermore, this is undesirable because it reduces productivity.

Since excellent impact resistance is a feature of biaxially oriented polyamide film, the impact strength of the biaxially oriented polyamide film of the present invention is preferably 0.7 J/15 μm or more. More preferably, the impact strength is 0.9 J/15 μm or more.

The haze value of the biaxially oriented polyamide film of the present invention is preferably 10% or less. More preferably, it is 7% or less, and even more preferably, it is 5% or less. A low haze value means good transparency and gloss, so when used for packaging bags, beautiful printing is possible and commercial value is increased. Since the addition of fine particles to improve the film's slipperiness increases the haze value, when the film has two or more layers, adding fine particles only to the surface layer reduces the haze value.

The dynamic friction coefficient of the biaxially oriented polyamide film of the present invention is preferably 1.0 or less. More preferably, it is 0.7 or less, and even more preferably, it is 0.5 or less. A film with a low dynamic friction coefficient has good slipperiness and is easier to handle. If the dynamic friction coefficient of the film is too small, it will be too slippery and difficult to handle, so the dynamic friction coefficient of the biaxially oriented polyamide film of the present invention is preferably 0.15 or more.

The biaxially oriented polyamide film of the present invention preferably has a biomass-derived carbon content (also referred to as biomass ratio) of 1 to 15% of the total carbon in the polyamide film, as determined by radiocarbon ( ^C14 ) measurement according to ASTM D6866-16. Atmospheric carbon dioxide contains a certain proportion of ^C14 (105.5 pMC), and it is known that the ^C14 content in plants that grow by absorbing atmospheric carbon dioxide, such as corn, is also approximately 105.5 pMC. It is also known that fossil fuels contain very little ^C14 . Therefore, the proportion of biomass-derived carbon can be calculated by measuring the proportion of ^C14 in the total carbon atoms in the polyester.

The biaxially oriented polyamide film of the present invention has a laminate strength of 4.0 N/15 mm or more after being bonded to the polyethylene sealant described in the examples. Biaxially oriented polyamide films are typically laminated with a sealant film before being processed into packaging bags. When the above-mentioned 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 layer configurations, sufficient strength is obtained in the seal portion, resulting in a strong, tear-resistant packaging bag. To achieve a laminate strength of 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, etc.

[Film production method]
The biaxially stretched polyamide film of the present invention can be produced by a known production method.
For example, sequential biaxial stretching and simultaneous biaxial stretching are mentioned. The sequential biaxial stretching method is preferable 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 be described below.
First, a raw material resin is melt-extruded using an extruder, extruded from a T-die into a film, and cast onto a cooling roll to cool, thereby obtaining an unstretched film.

When producing a biaxially oriented polyamide film in which a surface layer is laminated on at least one side of a base film, coextrusion using a feed block or multi-manifold is preferred to obtain a laminated unstretched film. In addition to coextrusion, dry lamination and extrusion lamination can also be used. When laminating using coextrusion, it is desirable to minimize the difference in melt viscosity between the polyamide resin compositions used for each layer.

The resin melting temperature is preferably 220-350°C. If it is lower than this, unmelted material may occur, resulting in defects and other poor appearance. If it is higher than this, deterioration of the resin may be observed, resulting in a decrease in molecular weight and a deterioration in appearance. The die temperature is preferably 250-350°C.

The cooling roll temperature is preferably from -30 to 80°C, more preferably from 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 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.

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 combination of methods such as bringing a cooling liquid in a tank into contact with the surface of the unstretched film opposite the cooling roll, applying a vaporizing liquid with a spray nozzle, or spraying a high-velocity fluid onto the surface to cool it. The unstretched film obtained in this manner is stretched biaxially to obtain the biaxially stretched polyamide film of the present invention.

The stretching method may be either simultaneous biaxial stretching or sequential biaxial stretching. In either case, multi-stage stretching such as one-stage stretching or two-stage stretching can be used as the MD stretching method. 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). Roll stretching is preferred for MD stretching in sequential biaxial stretching.

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 below 50°C, the resin will not soften and stretching may be 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 may become too soft and stable stretching may not be possible.

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 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 crystallinity may become too low, resulting in reduced 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.

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

The film stretched in the MD direction is then stretched in the TD direction using a tenter, heat-set, and relaxed (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 below 50°C, the resin will not soften and stretching may be 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.

The lower limit of the stretch ratio in the TD direction (when stretching is performed in multiple stages, the total stretch ratio obtained by multiplying each stretch 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 will decrease, and the crystallinity may become too low, resulting in reduced impact strength. The upper limit of the stretch ratio in the TD direction is preferably 5.5, more preferably 5.0, even more preferably 4.7, particularly preferably 4.5, and most preferably 4.3. If it exceeds 5.5, productivity may decrease significantly.

The selection of the heat setting temperature is an important factor in the present invention. As the heat setting temperature increases, the film's crystallization and orientation relaxation progress, improving impact strength and reducing heat shrinkage. On the other hand, if the heat setting temperature is low, crystallization and orientation relaxation are insufficient, making it impossible to sufficiently reduce heat shrinkage. Furthermore, if the heat setting temperature is too high, the resin will deteriorate and the film's toughness, including impact strength, will be rapidly lost.

The lower limit of the heat setting temperature is preferably 210°C, more preferably 212°C. If the heat setting temperature is too low, the thermal shrinkage rate will be too large, which will result in a poor appearance after lamination and a decrease in laminate strength. The upper limit of the heat setting temperature is preferably 220°C, more preferably 218°C. If the heat setting temperature is too high, the impact strength will tend to decrease.

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, resulting in the above problems. If the heat setting conditions are too strong, the film's toughness will decrease.

Performing a relaxation treatment after heat setting is effective in controlling the thermal shrinkage rate. The temperature for the relaxation treatment can be selected from the range of the heat setting temperature to the glass transition temperature (Tg) of the resin, but a temperature of -10°C to Tg + 10°C is preferable. If the relaxation temperature is too high, the shrinkage rate will be too fast, which can cause distortion and other problems, so it is not desirable. Conversely, if the relaxation temperature is too low, the relaxation treatment will not occur and the material will simply become loose, which will not reduce the thermal shrinkage rate and will result in poor dimensional stability.

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 thermal 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 may occur in the tenter, making production difficult.

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. Additionally, to improve the adhesion of the film surface, it can be subjected to corona treatment, coating treatment, flame treatment, etc., or printing or vapor deposition of metals or inorganic oxides, etc. Suitable vapor deposition films formed by vapor deposition include aluminum vapor deposition films and vapor deposition films of silicon oxide or aluminum oxide, either alone or as a mixture. Furthermore, by coating these vapor deposition films with a protective layer, etc., oxygen barrier properties can be improved.

[Laminated film and bags]
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, square-bottom bags, etc. Examples of sealant films include unstretched linear low-density polyethylene films, unstretched polypropylene films, and ethylene-vinyl alcohol copolymer resin films.

The layer structure of the laminate film of the present invention using the biaxially oriented polyamide film of the present invention is not particularly limited, as long as the biaxially oriented polyamide film of the present invention is contained within the laminate film. Furthermore, the film used in the laminate film may be made from either petroleum-derived or biomass-derived raw materials, but polylactic acid, polyethylene terephthalate, polybutylene succinate, polyethylene, polyethylene furanoate, and the like, which are polymerized using biomass-derived raw materials, are preferred in terms of reducing the environmental impact.

Examples of layer configurations of the laminated film of the present invention include ONY/adhesive/LLDPE, ONY/adhesive/CPP, ONY/adhesive/Al/adhesive/CPP, ONY/adhesive/Al/adhesive/LLDPE, ONY/PE/Al/adhesive/LLDPE, ONY/adhesive/Al/PE/LLDPE, PET/adhesive/ONY/adhesive/LLDPE, PET/adhesive/ONY/PE/LLDPE, PET/adhesive/ONY/adhesive/Al/adhesive/LLDPE, and PET/adhesive. Adhesive/Al/Adhesive/ONY/Adhesive/LLDPE, PET/Adhesive/Al/Adhesive/ONY/PE/LLDPE, PET/PE/Al/PE/ONY/PE/LLDPE, PET/Adhesive/ONY/Adhesive/CPP, PET/Adhesive/ONY/Adhesive/Al/Adhesive/CPP, PET/Adhesive/Al/Adhesive/ONY/Adhesive/CPP, ONY/Adhesive/PET/Adhesive/LLDPE, ONY/Adhesive/PET/PE/LLDPE, ONY/Adhesive/PET/Adhesive/ CPP, ONY/adhesive/Al/adhesive/PET/adhesive/LLDPE, ONY/adhesive/Al/v/PET/PE/LLDPE, ONY/PE/LLDPE, ONY/PE/CPP, ONY/PE/Al/PE, ONY/PE/Al/PE/LLDPE, OPP/adhesive/ONY/adhesive/LLDPE, ONY/adhesive/EVOH/adhesive/LLDPE, ONY/adhesive/EVOH/adhesive/CPP, ONY/adhesive/aluminum-deposited PET/adhesive/LLDPE, ON Examples include Y/adhesive/aluminum-vapor-deposited PET/adhesive/ONY/adhesive/LLDPE, ONY/adhesive/aluminum-vapor-deposited PET/PE/LLDPE, ONY/PE/aluminum-vapor-deposited PET/PE/LLDPE, ONY/adhesive/aluminum-vapor-deposited PET/adhesive/CPP, PET/adhesive/aluminum-vapor-deposited PET/adhesive/ONY/adhesive/LLDPE, CPP/adhesive/ONY/adhesive/LLDPE, ONY/adhesive/aluminum-vapor-deposited LLDPE, and ONY/adhesive/aluminum-vapor-deposited CPP.
The abbreviations used in the above layer structure are as follows:
/ : indicates the boundary of layers ONY: biaxially oriented polyamide film 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 Adhesive: adhesive layer that bonds films together Aluminum vapor deposition: indicates that aluminum is vapor-deposited

Next, the present invention will be described in more detail using examples, but the present invention is not limited to the following examples. Film evaluations were performed using the following measurement methods. Unless otherwise specified, measurements were performed in a measurement room at 23°C and a relative humidity of 65%.

(1) Haze Value of Film: Measured using a direct reading haze meter manufactured by Toyo Seiki Seisakusho Co., Ltd. in accordance with JIS K7105.
(2) Film Thickness The film was divided into 10 equal parts in the TD direction (narrow films were divided so that the width was wide enough to measure the thickness), and 10 100 mm 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 instrument manufactured by Tester Sangyo, and the average value was taken as the thickness.

(3) Biomass Degree of Film The biomass degree of the obtained film was measured by radiocarbon (C ¹⁴ ) measurement according to ASTM D6866-16 Method B (AMS).
(4) Heat Shrinkage 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 (%)
(5) Impact Strength of Film: Measured using a film impact tester manufactured by Toyo Seiki Seisakusho, Ltd. The measured value was converted to a value per 15 μm thickness and expressed as J (joule)/15 μm.
(6) Dynamic Friction Coefficient of Film The dynamic friction coefficient between the outer surfaces of the film rolls was evaluated under the following conditions in accordance with JIS C2151. The test specimen had a width of 130 mm and a length of 250 mm, and the test speed was 150 mm/min.

(7) Bending Pinhole Resistance of Film Using a Gelbo Flex Tester manufactured by Rigaku Kogyosha, the number of pinholes caused by bending fatigue was measured by 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 three 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 cylindrical shape with a diameter of 3.5 inches. 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 to complete the entire stroke. Flex fatigue was performed 1,000 times at a speed of 40 times/min, and the number of pinholes that developed 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 downwards, and the four corners were secured with Scotch tape (registered trademark). Ink (Pilot ink (product number INK-350-Blue) diluted five 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.

(8) Resistance to Pinholes Caused by Friction of Film Using a fastness tester (manufactured by Toyo Seiki Seisakusho), a friction test was carried out by the following method, and the distance at which pinholes occurred was measured.
A test sample was prepared by folding a laminate film similar to that prepared for the pinhole resistance evaluation in the above, and sharpening the corners. The test sample was rubbed against the inner surface of a cardboard box 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 formed, the friction test was conducted with an amplitude of 20 times and a distance increased by 500 cm. If no pinholes were formed, 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 was formed was marked with an "X" and assigned a level of 1. If a pinhole was formed with 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 was formed, 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 a pinhole was not formed 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.
Furthermore, for levels 3 to 20, if the previous level was rated ○, increase the number of amplitudes by 20 and conduct a friction test; if no pinholes appear, mark it with an ○; if pinholes appear, mark it with an X. If the previous level was rated ×, decrease the number of amplitudes by 20 and conduct a friction test; if no pinholes appear, mark it with an ○; if pinholes appear, mark it with an X. Repeat this process and mark levels 3 to 20 with an ○ or ×.
For example, the results obtained are shown in Table 1. Using Table 1 as an example, a method for determining the pinhole occurrence distance will be explained.
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 longer distances, the coefficient was set to +1, +2, +3, etc. for every 500 cm, and for shorter distances, the coefficient was set to -1, -2, -3, etc. for every 500 cm.
In 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 for each of the following cases A and B using the respective formulas.
A: In all tests, the number of tests in which no holes were formed is equal to or greater than the number of tests in which holes were formed. Distance at which pinholes were formed = median + 500 x (Σ (coefficient x number of tests in which no holes were formed) / number of tests in which no holes were formed) + 1/2)
B: In all tests, the number of tests in which no holes were formed is less than the number of tests in which holes were formed. Distance at which pinholes were formed by friction = median + 500 x (Σ (coefficient x number of tests in which holes were formed) / number of tests in which holes were formed) - 1/2)

(9) Laminate strength with polyethylene sealant A laminate film prepared in the same manner as described in the evaluation of pinhole resistance for bending was cut into strips of 15 mm width x 200 mm length, and 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 for evaluation.

(10) Film-forming stability during casting The process of extruding a molten resin into a film form from a T-die, casting it onto a cooling roll and cooling it to obtain an unstretched film was visually observed, and the film-forming stability was evaluated as follows.
A: Film formation was stable and a homogeneous unstretched film was obtained.
B: Film formation was a little unstable, and fluctuations were observed in the width of the unstretched film, but biaxial stretching was possible.
C: Film formation was unstable and the unstretched film was inhomogeneous, so a biaxially stretched film could not be obtained.
A rating of B or higher indicates practicality.
(11) Generation Cycle of Thermally Degraded Products Formed at the Die Lip Exit After cleaning the die lip, film production was started, and the time until thermally degraded products formed on the die lip was observed.

(12) Relative Viscosity of Raw Material 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 polyamide solution was measured at 20°C.
(13) Melting Point of Raw Material Polyamide Measurement was carried out in accordance with JIS K7121 using a differential scanning calorimeter, SSC5200, manufactured by Seiko Instruments Inc., in a nitrogen atmosphere at 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.

[Polyamide 6]
The polyamide 6 used in the examples and comparative examples is as follows.
Polyamide 6 (a-1)
Relative viscosity 2.8, melting point 220°C, manufactured by Toyobo Co., Ltd.

Polyamide 6 (a-2) obtained by chemical recycling
Relative viscosity 2.7, melting point 221°C
Polyamide 6 fibers recovered from waste materials and a 75% by mass aqueous solution of phosphoric acid as a depolymerization catalyst were charged into a depolymerization apparatus and heated to 260°C under a nitrogen atmosphere. The reaction was initiated by blowing superheated steam into the depolymerization apparatus, and the ε-caprolactam and water vapor continuously distilled from the depolymerization apparatus were cooled to recover an ε-caprolactam distillate. The recovered distillate was concentrated using an evaporator, and the resulting ε-caprolactam was repolymerized to obtain a chemically recycled polyamide resin.

Polyamide 6 (a-3) obtained by mechanical recycling
Relative viscosity: 2.6, melting point: 221°C
The non-standard film and scrap material generated as cut off pieces (edge trims) from the stretched film produced in Example 1 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.

[Example 1-1]
Using an apparatus consisting of an extruder and a 380 mm wide T-die, the following molten resin composition was extruded from the T-die into a film, which was then cast onto a cooling roll whose temperature was controlled to 20°C and electrostatically adhered to obtain an unstretched film having a thickness of 200 μm.
(Resin composition)
Polyamide 6 (a-1): 92 parts by mass Polyamide 6 (a-2): 5 parts by mass Polyamide 11 (manufactured by Arkema, relative viscosity 2.5, melting point 186 ° C.): 3 parts by mass Porous silica fine particles (manufactured by Fuji Silysia Chemical Ltd., average particle diameter 2.0 μm, pore volume 1.6 ml / g): 0.45% by mass
Fatty acid bisamide (ethylene bisstearic acid amide manufactured by Kyoeisha Chemical Co., Ltd.): 0.15% by mass

The resulting unstretched film was fed into a roll-type stretching machine and, utilizing the difference in peripheral speed between the rolls, stretched 1.73 times in the MD direction at 80°C, and then further stretched 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 200°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 film are shown in Table 2.

[Examples 1-2 to 1-12]
Biaxially stretched films were obtained in the same manner as in Example 1-1, except that the raw material resin composition and film-forming conditions such as the heat setting temperature were changed as shown in Table 2. The evaluation results of the obtained biaxially stretched films are also shown in Table 2. In the examples and comparative examples, the following polyamide resins at least part of which are derived from biomass were used as raw materials.
Polyamide 410: (manufactured by DSM, ECOPaXX Q150-E, melting point 250°C)
Polyamide 610: (Arkema, RilsanS SMNO, melting point 222°C)
Polyamide 1010: (Arkema, RilsanT TMNO, melting point 202°C)

As shown in Table 2, the films obtained in the examples exhibited both good resistance to pinholes due to bending and good resistance to pinholes due to friction. They also had low haze, good transparency, high impact strength, and high lamination strength with sealant films, making them excellent for use as packaging films.

[Comparative Examples 1-1 to 1-5]
Biaxially stretched films were obtained in the same manner as in Example 1-1, except that the raw material resin composition and film-forming conditions such as the heat setting temperature were changed as shown in Table 3. The evaluation results of the obtained biaxially stretched films are shown in Table 3. However, in Comparative Example 1-4, the molten resin could not be stably extruded from the T-die into a film shape, and a homogeneous unstretched film could not be obtained, so biaxial stretching was not possible.

As shown in Table 3, the biaxially oriented polyamide films of Comparative Examples 1-1 and 1-2, which did not contain a material that modified pinhole resistance, and the biaxially oriented polyamide film of Comparative Example 1-3, which contained too little polyamide 11, had poor pinhole resistance. In Comparative Example 1-4, the polyamide 11 content was too high, so the molten resin could not be stably extruded into a film from the T-die, and a homogeneous unstretched film was not obtained, resulting in a biaxially oriented polyamide film. In Comparative Example 1-5, a polyamide elastomer conventionally used as a material that modified pinhole resistance for bending was used. Although the film had good pinhole resistance for bending, the film had poor pinhole resistance due to friction. Furthermore, there was a drawback in that degraded materials were easily deposited on the die during long-term production, making long-term continuous production impossible.

[Example 2-1]
Using an apparatus consisting of two extruders and a 380 mm wide coextrusion T-die, layers were laminated in a B layer/A layer/B layer configuration by 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 at 20° C. and electrostatically adhered to obtain an unstretched film with a thickness of 200 μm. The feed block configuration and the extruder output were adjusted so that the thickness of the biaxially stretched polyamide film was 15 μm in total, with the thickness of the base layer (A layer) being 12 μm and the thicknesses of the front and back surface layers (B layers) being 1.5 μm each.

The resin compositions of the A layer and the B layer used in Example 2 were as follows.
(Resin composition constituting layer A)
Polyamide 6 (a-1): 92 parts by mass Chemically recycled polyamide 6 (a-2): 5 parts by mass Polyamide 11 (manufactured by Shusei Co., Ltd., relative viscosity 2.5, melting point 186°C): 3 parts by mass (resin composition constituting layer B)
Polyamide 6 (a-1): 90 parts by mass Chemically recycled polyamide 6 (a-2): 5 parts by mass Polyamide MXD6 (manufactured by Mitsubishi Gas Chemical Co., Ltd., relative viscosity 2.1, melting point 237 ° C.): 5 parts by mass Porous silica fine particles (manufactured by Fuji Silysia Chemical Ltd., average particle diameter 2.0 μm, pore volume 1.6 ml / g): 0.54 mass%
Fatty acid bisamide (ethylene bisstearic acid amide manufactured by Kyoeisha Chemical Co., Ltd.): 0.15% by mass

The resulting unstretched film was fed into a roll-type stretching machine and, utilizing the difference in peripheral speed between the rolls, stretched 1.73 times in the MD direction at 80°C, and then further stretched 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 film are shown in Table 4.

[Examples 2-2 to 2-12]
A biaxially stretched film was obtained in the same manner as in Example 2-1, except that the resin compositions of Layer A and Layer B, the heat setting temperature, and other film-forming conditions were changed as shown in Table 4. The evaluation results of the obtained biaxially stretched film are also shown in Table 4.

As shown in Table 4, the films obtained in the examples exhibited both good resistance to pinholes due to bending and good resistance to pinholes due to friction. They also had low haze, good transparency, high impact strength, and high lamination strength with sealant films, making them excellent packaging films.

[Comparative Examples 2-1 to 2-7]
A biaxially stretched film was obtained in the same manner as in Example 2-1, except that the film-forming conditions, such as the resin compositions of Layer A and Layer B and the heat setting temperature, were changed as shown in Table 5. The evaluation results of the obtained biaxially stretched film are also shown in Table 5. However, in Comparative Example 2-4, the molten resin could not be stably extruded from the T-die into a film shape, and a homogeneous unstretched film could not be obtained, so biaxial stretching was not possible.

The biaxially oriented polyamide films of Comparative Examples 2-1 and 2-2, which did not contain a material for improving pinhole resistance, and the biaxially oriented polyamide film of Comparative Example 2-3, which contained too little polyamide 11, exhibited poor pinhole resistance. In Comparative Example 2-4, the polyamide 11 content was too high, so the molten resin could not be stably extruded into a film from the T-die, resulting in a homogeneous unstretched film and no biaxially oriented polyamide film. In Comparative Example 2-5, the thickness and thickness ratio of Layer A were small, resulting in poor pinhole resistance. In Comparative Example 2-6, the amount of polyamide MXD6 in Layer B was high and the amount of polyamide 6 resin was low, resulting in poor pinhole resistance and abrasion resistance. In Comparative Example 2-7, a polyamide elastomer conventionally used as a material for improving pinhole resistance was used, and the film exhibited good pinhole resistance but poor abrasion resistance. Furthermore, there was a drawback in that deterioration materials tended to adhere to the die during long-term production, making it impossible to carry out long-term continuous production.

[Examples 3 and 4]
The biaxially stretched polyamide films prepared in Examples 1-2 and 2-2 were used to prepare laminate films having the following configurations (1) to (9). The laminate films (1) to (9) were then used to prepare three-side seal type and pillow type packaging bags. Packaging bags with good appearance and resistance to tearing in a drop impact test were successfully prepared.
(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 / unstretched polypropylene film sealant layer (3) Biaxially oriented PET film layer / printing layer / polyurethane adhesive layer / biaxially oriented polyamide film layer / polyurethane adhesive layer / unstretched polypropylene film sealant layer (4) Biaxially oriented PET film layer / printing 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 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/non-oriented 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 non-oriented polypropylene film sealant layer

The biaxially oriented polyamide film of the present invention has excellent impact resistance, flex pinhole resistance, and abrasion pinhole resistance, making it suitable for use as a packaging material, such as food packaging. Furthermore, because it uses a resin polymerized from raw materials derived from biomass that is originally found on land, it is a carbon-neutral film and has little impact on the increase or decrease of carbon dioxide on land, thereby reducing the environmental burden. Furthermore, because it contains polyamide 6 chemically recycled from waste polyamide products, it allows for the reuse of plastic products that would previously have been discarded, thereby reducing the environmental burden and contributing to a reduction in the amount of plastic waste generated.

1: Head of fastness tester 2: Cardboard board 3: Mounting board for holding sample 4: Film sample folded in four 5: Direction of vibration of rub

Claims (9)

The resin contains only a polyamide resin, and the polyamide resin contains only 70 to 99 mass% of polyamide 6 and 1 to 30 mass% of polyamide 11, at least a part of which is derived from biomass,
A method for producing a biaxially oriented polyamide film for food packaging, wherein the polyamide 6 contains 5 to 95 parts by mass of polyamide 6 obtained by chemical recycling, and more than 0 part by mass but not more than 50 parts by mass of polyamide 6 obtained by mechanical recycling, based on 100 parts by mass of polyamide 6. The method for producing a biaxially oriented polyamide film for food packaging according to claim 1, characterized in that the content of biomass-derived carbon determined by radiocarbon (C ¹⁴ ) measurement is 1 to 15% of the total carbon in the biaxially oriented polyamide film for food packaging. 3. A method for producing a biaxially oriented polyamide film for food packaging, in which one or more layers are laminated on both sides of a biaxially oriented polyamide film for food packaging obtained by the method for producing a biaxially oriented polyamide film for food packaging according to claim 1 or 2, wherein the laminated layers contain 70 to 100 mass% of polyamide 6, and the polyamide 6 contains 5 to 95 mass parts of polyamide 6 obtained by chemical recycling, and more than 0 mass parts but not more than 50 mass parts of polyamide 6 obtained by mechanical recycling, based on 100 mass parts of polyamide 6. The method for producing a biaxially oriented polyamide film for food packaging according to claim 3, wherein the thickness of the laminated layer is 7 to 50% of the total thickness of the film. A method for producing a biaxially oriented polyamide film for food packaging, characterized in that the biaxially oriented polyamide film for food packaging obtained by the method for producing a biaxially oriented polyamide film for food packaging according to any one of claims 1 to 4 satisfies the following (a) and (b):
(a) The number of pinhole defects is 10 or less when a twisting and bending test using a Gelbo flex tester is performed 1,000 times at a temperature of 1°C,
(b) The distance until pinholes appear in the friction pinhole resistance test is 2900 cm or more. A method for producing a biaxially oriented polyamide film for food packaging, characterized in that the biaxially oriented polyamide film for food packaging obtained by the method for producing a biaxially oriented polyamide film for food packaging described in any one of claims 1 to 5 has a haze of 10% or less and a dynamic friction coefficient of 1.0 or less. A method for producing a biaxially oriented polyamide film for food packaging, characterized in that the biaxially oriented polyamide film for food packaging obtained by the method for producing a biaxially oriented polyamide film for food packaging described in any one of claims 1 to 5 has a laminate strength of 4.0 N/15 mm or more after being laminated with a polyethylene sealant film. A method for producing a laminated film for food packaging, in which a sealant film is laminated on at least one side of a biaxially oriented polyamide film for food packaging obtained by the method for producing a biaxially oriented polyamide film for food packaging described in any one of claims 1 to 7. A method for producing a food packaging bag using a food packaging laminate film obtained by the food packaging laminate film manufacturing method described in claim 8.