JP7619530B1 - Biaxially oriented polyamide film - Google Patents

Abstract

The object of the present invention is to provide a biaxially oriented polyamide film that is excellent in bending pinhole resistance, puncture strength, dimensional stability, and ethanol permeability. The present invention relates to a biaxially oriented polyamide film that contains polyamide 6 as a main component, and has an ethanol permeability of 6000 to 10000 g μm/ ^m2 24 hr (50% RH/40°C), a thickness of 15 μm or less, heat shrinkage rates of 2.0% or less in both the machine direction and width direction of the film, a surface crystallization parameter in the range of 1.0 to 1.5, and a puncture strength of 6.0 N or more.

Description

The present invention relates to a biaxially oriented polyamide film that is suitable for use as a food packaging film and a packaging material for alcohol-evaporating agents, etc.

Biaxially stretched films made of aliphatic polyamides such as polyamide 6 have excellent impact resistance and pinhole resistance due to bending, and are widely used as various packaging film materials. In Patent Documents 1 and 2, polyamide films are generally highly permeable to alcohol, and therefore are also used as base films for packaging bags for alcohol evaporative agents and freshness-preserving packaging bags, which require specific gas permeability. Patent Document 3 describes how the desired gas permeability can be obtained by reducing the thickness of the biaxially stretched polyamide film.

JP 2003-211604 A JP 2002-204652 A JP 2010-167652 A

Patent Document 3 shows that reducing the thickness of the nylon film is effective in improving ethanol permeability, but the inventors have found that reducing the film thickness leads to problems with reduced pinhole resistance and puncture strength. Patent Document 3 also found that there is room for improvement in the dimensional stability of the film.

The present invention aims to provide a biaxially oriented polyamide film that has excellent bending pinhole resistance, puncture strength, dimensional stability, and excellent ethanol permeability.

As a result of intensive research into achieving the above object, the inventors have discovered that by dispersing a resin other than polyamide 6 having a maximum dispersion diameter of 1 μm or more in a polyamide 6 resin in a biaxially oriented polyamide film and imparting a specific degree of planar orientation and a specific degree of crystallinity, a biaxially oriented polyamide film having good bending pinhole resistance, puncture strength, dimensional stability, and ethanol permeability can be obtained. The inventors have continued to conduct further research and improvements, and have completed the following representative inventions.

[1] A biaxially oriented polyamide film containing polyamide 6 as a main component, the biaxially oriented polyamide film having an ethanol permeability of 6,000 to 10,000 g μm/ ^m2 24 hr (50% RH/40°C), a thickness of 15 μm or less, a thermal shrinkage rate of 2.0% or less in both the machine direction and the width direction of the film when heat-treated at 160°C for 10 minutes, a surface crystallization parameter of 1.0 to 1.5, and a puncture strength of 6.0 N or more.
[2] The biaxially stretched polyamide film according to [1], having at least one layer containing 60% by mass or more and 99% by mass or less of a polyamide 6 resin (a) and 2% by mass or more and 40% by mass or less of a resin other than polyamide 6 (b).
[3] The biaxially stretched polyamide film according to [2], wherein the resin (b) other than polyamide 6 is at least one selected from the group consisting of thermoplastic elastomers such as polyolefins, ionomer polymers, polyester-based elastomers, polyamide-based elastomers, polyolefin-based elastomers, polystyrene-based elastomers, polyurethane-based elastomers, and polyvinyl chloride-based elastomers, aliphatic polyester resins, aliphatic-aromatic polyester resins, aliphatic polyamide resins, and aliphatic-aromatic polyamide resins.
[4] The biaxially stretched polyamide film according to either [2] or [3], wherein the aliphatic aromatic polyester resin is a polybutylene adipate terephthalate resin.
[5] The biaxially stretched polyamide film according to [2], wherein the resin (b) other than polyamide 6 is derived from biomass and is at least one polyamide selected from the group consisting of polyamide 11, polyamide 410, polyamide 610, and polyamide 1010.
[6] The biaxially stretched polyamide film according to any one of [2] to [5], wherein the maximum dispersion diameter of the resin (b) other than polyamide 6 is 1 μm or more.
[7] The biaxially stretched polyamide film according to any one of [1] to [6], wherein the stress at 5% elongation in both the machine direction and the width direction of the film is 70.0 MPa or less.
[8] The biaxially stretched polyamide film according to any one of [1] to [7], having a planar orientation degree (ΔP) of 0.050 or more.
[9] The biaxially oriented polyamide film according to any one of [1] to [8], wherein a laminate film of the biaxially oriented polyamide film and a polyethylene film having a thickness of 40 μm is prepared, and the film is subjected to a bending treatment 1000 times in an atmosphere at a temperature of 1° C. using a Gelbo flex tester. When the number of pinholes generated is 20 or less.
[10] A food packaging material comprising the biaxially oriented polyamide film according to any one of [1] to [9] and a sealant film.
[11] A packaging material for an ethanol evaporative agent, comprising a laminate including at least two layers of the biaxially oriented polyamide film according to any one of [1] to [9] and a breathable resin film or nonwoven fabric.

The present invention provides a biaxially oriented polyamide film having excellent pinhole resistance, puncture strength, dimensional stability, and ethanol permeability, as well as a food packaging material and a packaging material for ethanol evaporative agents using the biaxially oriented polyamide film.

1 is a transmission electron microscope photograph showing the dispersion state of polyamide 6 resin (a) and a resin other than polyamide 6 (b) contained in a film.

The biaxially stretched polyamide film of the present invention is described in detail below. In this specification, a numerical range expressed using "~" means a range including the numerical values written before and after "~" as the lower and upper limits.

The biaxially stretched polyamide film of the present invention preferably contains, as a main component, polyamide 6. Here, "containing as a main component" means that the content of the component in the polyester resin composition constituting the film is 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more, based on 100% by mass of all the components of the polyester resin.
The biaxially stretched polyamide film of the present invention is preferably a biaxially stretched polyamide film having a functional layer laminated on at least one side of a substrate layer. The biaxially stretched polyamide film of the present invention is preferably a film having at least two layers, a substrate layer and a functional layer, and may have a two-layer structure of substrate layer/functional layer or a three-layer structure of functional layer/substrate layer/functional layer.
Each layer will be described in detail below.

[Base layer A]
The first resin composition forming the base layer contains at least polyamide 6 resin (a). The first resin composition preferably contains 60% by mass or more of polyamide 6, more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on the first resin composition being 100% by mass. If the content of polyamide 6 is less than 60%, the impact resistance may decrease and the gas permeability may increase significantly. By containing 80% by mass or more of polyamide 6, a polyamide film having mechanical strength such as impact strength and gas barrier properties can be obtained. The upper limit of the content of polyamide 6 is preferably 98% by mass or less, more preferably 97% by mass or less, based on the first resin composition being 100% by mass. That is, polyamide 6 is preferably 60 to 98% by mass, more preferably 80 to 98% by mass, and even more preferably 85 to 97% by mass. In this specification, the upper and lower limits of the numerical range may be appropriately combined to form a suitable numerical range (hereinafter the same applies to the numerical range).

The first resin composition forming the base layer contains at least one type of resin (b) other than polyamide 6. Resin (b) is a substance that has the effect of imparting flexibility to the base layer, and is not particularly limited as long as it is incompatible with polyamide 6, but is preferably at least one type selected from the group consisting of thermoplastic elastomers such as polyolefins, ionomer polymers, polyester-based elastomers, polyamide-based elastomers, polyolefin-based elastomers, polystyrene-based elastomers, polyurethane-based elastomers, and polyvinyl chloride-based elastomers, aliphatic polyester resins, aliphatic aromatic polyester resins, aliphatic polyamide resins, and aliphatic aromatic polyamide resins.

Examples of polyolefins include homopolymers of olefins, copolymers of olefins with other monomers, and mixtures thereof, and polyolefins grafted with unsaturated carboxylic acids. Examples of homopolymers of olefins include polyethylene (high density, low density) and polypropylene. Examples of copolymers of olefins with other monomers include ethylene-vinyl acetate copolymer, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene-acrylic acid copolymer, and ethylene-methacrylic acid copolymer. Examples of unsaturated carboxylic acids used in grafting polyolefins with unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, and other carboxylic acids, and acid anhydrides such as maleic anhydride, citraconic anhydride, and itaconic anhydride. Unsaturated carboxylic acids may be used alone or in combination.

An ionomer polymer is a polymer composed of a polyolefin and an α,β-ethylenically unsaturated monomer having a carboxyl moiety, the carboxyl moiety being neutralized with a metal ion having a valence of 1 to 3. Examples of polyolefins include copolymers of polyethylene or ethylene with at least one α-olefin having usually 3 to 8 carbon atoms and a diolefin, such as 1,4-hexadiene. Examples of α,β-ethylenically unsaturated monomers having a carboxyl moiety include methacrylic acid, acrylic acid, maleic acid, maleic anhydride, and fumaric acid. Examples of metal ions include zinc ions, sodium ions, lithium ions, potassium ions, and magnesium ions. Ionomer polymers are produced by a direct synthesis method in which an α-olefin and an olefin monomer having a carboxyl moiety are polymerized, or by a grafting method in which a monomer having a carboxyl moiety is added to the olefin skeleton.

Examples of thermoplastic elastomers include polyolefins, ionomer polymers, polyester elastomers, polyamide elastomers, polyolefin elastomers, polystyrene elastomers, polyurethane elastomers, polyvinyl chloride elastomers, etc., aliphatic polyester resins, aliphatic aromatic polyester resins, aliphatic polyamide resins, and aliphatic aromatic polyamide resins. For example, polyamide elastomers, polyester elastomers, polyurethane elastomers, acrylic thermoplastic elastomers, polystyrene elastomers, and hydrogenated products thereof may be used.

Examples of polyamide-based elastomers include polyetheramides, polyetheresteramides, and polyesteramides. In addition, polyamide-based elastomers may be copolymerized with dicarboxylic acids such as dodecanedicarboxylic acid, adipic acid, and terephthalic acid as optional components.

A polyester-based elastomer refers to a block copolymer composed of a crystalline hard segment and a flexible soft segment. Among them, a block copolymer having a hard segment made of a cyclic polyester and a soft segment made of a polyalkylene ether, and a block copolymer having a hard segment made of a cyclic polyester and a soft segment made of a linear aliphatic polyester are preferred, and a cyclic polyester-polyalkylene ether block copolymer is more preferred. In the present invention, "cyclic polyester" refers to a raw material dicarboxylic acid or its alkyl ester that contains a dicarboxylic acid or its alkyl ester having a cyclic structure.

From the viewpoint of moldability, examples of polyurethane-based thermoplastic elastomers include those having a hard segment consisting of a diisocyanate compound and a glycol having a molecular weight of about 50 to 500, and a soft segment consisting of a diisocyanate compound and a long-chain glycol. The long-chain glycol may be a polyether-based one such as a polyalkylene glycol having a molecular weight of about 500 to 10,000, or a polyester-based one such as polyalkylene adipate, polycaprolactone, or polycarbonate. Examples of diisocyanate compounds include phenylene diisocyanate, trigen diisocyanate, xylylene diisocyanate, 4,4-diphenylmethane diisocyanate, and hexamethylene diisocyanate. The diisocyanate compounds in the soft segment and the hard segment may be the same or different.

Examples of acrylic thermoplastic elastomers include ethylene-acrylic ester copolymer elastomers, ethylene-methacrylic ester copolymer elastomers, and acrylic ABA triblock copolymers consisting of acrylic esters and methacrylic esters.

Examples of polystyrene-based elastomers include styrene-butadiene copolymer rubber and styrene-isoprene copolymer rubber.

As the aliphatic polyester resin, polybutylene succinate and polybutylene succinate adipate are preferably used. As the aliphatic aromatic polyester resin, an aliphatic aromatic polyester resin containing an adipic acid component is preferred, and polybutylene adipate terephthalate is particularly preferred. More preferred are aliphatic or aliphatic aromatic polyester resins with a glass transition temperature (Tg) of -30°C or less. By using a polyester copolymer with a glass transition temperature of -30°C or less, excellent pinhole resistance can be achieved even in a freezing environment.

As the aliphatic polyamide resin, polyamide 11, polyamide 610, polyamide 1010 and polyamide 410 are preferably used. Furthermore, the polyamide resin may be at least partly derived from biomass. By containing a polyamide resin containing a biomass-derived raw material, the bending pinhole resistance can be further improved.
Examples of the aliphatic aromatic polyamide resin include polymetaxylylene adipamide (MXD6), polyamide 6T, polyamide 6I, polyamide 6T6I, and polyamide 9T.
Among these, MXD6 is suitable from the viewpoint of improving stretchability and adhesiveness.

The content of the resin (b) other than polyamide 6 contained in the first resin composition forming the base layer is preferably 2% by mass to 40% by mass, or 20% by mass or less, and more preferably 4% by mass to 15% by mass, based on 100% by mass of the first resin composition. By containing 2% by mass or more of the resin (b) other than polyamide 6, it is possible to obtain the effect of bending pinhole resistance. By containing 40% by mass or less, preferably 20% by mass or less of the resin (b) other than polyamide 6, it is possible to prevent the film from becoming soft and the puncture strength and impact strength from decreasing. In addition, the film becomes more stretchable, which can prevent pitch deviation during processing such as printing.

The first resin composition forming the base layer may contain a thermoplastic resin other than polyamide 6 resin, provided that the object of the present invention is not impaired. Examples of the polyamide resin include polyamide 12 resin, polyamide 66 resin, polyamide 6-12 copolymer resin, polyamide 6-66 copolymer resin, polyamide MXD6 resin, polyamide MXD10 resin, and polyamide 11-6T copolymer resin. If necessary, a thermoplastic resin other than polyamide may be contained, for example, a polyester polymer such as polyethylene terephthalate, polybutylene terephthalate, or polyethylene-2,6-naphthalate, or a polyolefin polymer such as polyethylene or polypropylene.

The substrate layer may contain various additives as needed, such as other thermoplastic resins, lubricants, heat stabilizers, antioxidants, antistatic agents, anti-fogging agents, UV absorbers, dyes, pigments, etc.

[Functional Layer B]
The second resin composition forming the functional layer preferably contains at least polyamide 6. The second resin composition preferably contains 70% by mass or more of polyamide 6, more preferably 80% by mass or more, and particularly preferably 90% by mass or more, based on 100% by mass of the second resin composition. By containing 70% by mass or more of polyamide 6, a polyamide film having mechanical strength such as impact strength and gas barrier properties can be obtained. The upper limit of the content of polyamide 6 is preferably 99% by mass or less, more preferably 96% by mass or less, based on 100% by mass of the first resin composition. That is, the content of polyamide 6 is preferably 70 to 99% by mass, more preferably 80 to 96% by mass, and even more preferably 90 to 96% by mass. The polyamide 6 can be the same polyamide 6 as that used in the first resin composition.

The second resin composition forming the functional layer may contain a polyamide resin other than polyamide 6. Examples of polyamide resins other than polyamide 6 include polyamide MXD6 resin, polyamide 11, polyamide 12, and polyamide 66. When the second resin composition contains a polyamide resin other than polyamide 6, the content is preferably 1% by mass or more and 30% by mass or less, more preferably 3% by mass or more and 20% by mass or less, and particularly preferably 5% by mass or more and 10% by mass or less, based on 100% by mass of the second resin composition. By containing 1% by mass or more, it is possible to impart slipperiness to the film. On the other hand, if it is more than 30% by mass, the effect of improving the slipperiness of the film may become saturated. As a polyamide resin other than polyamide 6, polyamide MXD6 resin is preferably used.

The second resin composition that forms the functional layer can contain microparticles or organic lubricants as lubricants to improve the film's slipperiness. By improving the slipperiness, the film can be made easier to handle and the risk of packaging bags breaking due to friction can be reduced.

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 standpoint of transparency and slipperiness, it is preferable to use silica fine particles.

The preferred average particle size of the fine particles is 0.5 μm or more and 5.0 μm or less, and more preferably 1.0 μm or more and 3.0 μm or less. By having an average particle size of 0.5 μm or more, good slip properties can be expected, and by having an average particle size of 5.0 μm or less, it is expected to prevent poor appearance due to increased surface roughness of the film.

When silica microparticles are used as the microparticles, the range of the pore volume of the silica is preferably 0.5 ml/g or more and 2.0 ml/g or less, and more preferably 0.8 ml/g or more and 1.6 ml/g or less.

As the organic lubricant, fatty acid amide and/or fatty acid bisamide can be used. Examples of fatty acid amide and/or fatty acid bisamide include erucic acid amide, stearic acid amide, ethylene bisstearic acid amide, ethylene bisbehenic acid amide, and ethylene bisoleic acid amide. The content of fatty acid amide and/or fatty acid bisamide contained in the second resin composition forming the functional layer is preferably 0.01% by mass or more and 0.40% by mass or less, and more preferably 0.05% by mass or more and 0.30% by mass or less, based on 100% by mass of the second resin composition. By being 0.01% by mass or more, a slip effect can be expected. By being 0.40% by mass or less, the wettability of the printing ink when a printing layer is provided on the functional layer side can be ensured.

The functional layer may contain various additives such as other thermoplastic resins, lubricants, heat stabilizers, antioxidants, antistatic agents, anti-fogging agents, UV absorbers, dyes, pigments, etc.

[Biaxially oriented polyamide film]
The biaxially stretched polyamide film of the present invention is preferably a film having at least two layers, a substrate layer and a functional layer, and may have a two-layer structure of substrate layer/functional layer or a three-layer structure of functional layer/substrate layer/functional layer.

The ethanol permeability of the biaxially stretched polyamide film of the present invention is preferably 6000 g μm/m ² 24 hr (50% RH/40° C.) or more. When the biaxially stretched polyamide film of the present invention is used as a packaging material for an ethanol transpiration agent, the ethanol impregnated into the nonwoven fabric as an ethanol transpiration agent slowly transpires, and a sufficient ethanol gas concentration for maintaining the freshness of the food in the food packaging can be obtained. If the ethanol permeability is less than 6000 g μm/m ² 24 hr (50% RH/40° C.), the permeation of the alcohol gas transpired from the alcohol transpiration agent is inhibited, and it is undesirable that a sufficient alcohol gas concentration for maintaining the freshness of the food in the food packaging cannot be obtained. The ethanol permeability is preferably 10000 g μm/m ² 24 hr (50% RH/40° C.) or less. By keeping the resistance to 10,000 g·μm/m ² ·24 hr (50% RH/40° C.) or less, it is possible to prevent condensation of ethanol inside the package that would occur due to a large amount of ethanol permeating through the package.

The thickness of the biaxially oriented polyamide film of the present invention is preferably 15 μm or less, and more preferably 5 to 12 μm. A thickness of 15 μm or less provides excellent alcohol permeability and contributes to resource conservation and waste reduction. A thickness of 5 μm or more is preferable because it provides sufficient mechanical strength required for secondary processing such as lamination, coating, and slitting.

The thickness of the substrate layer is preferably 50% to 90%, particularly preferably 60% to 80%, of the total thickness of the substrate layer and the functional layer (if there are multiple substrate layers and functional layers, the total thickness of each), taken as 100%. By making the thickness of the substrate layer 50% or more, bending pinhole resistance can be imparted. By making the thickness of the substrate layer 90% or less, friction pinhole resistance in the functional layer can be imparted.

The biaxially stretched polyamide film of the present invention preferably has a stress at 5% elongation obtained by a tensile test in the machine direction and width direction of the film, i.e., F5 value, of 70 MPa or less. More preferably, it is 60 MPa or less, and even more preferably, it is 50 MPa or less. By making the F5 value 70 MPa or less, the film becomes flexible and is less likely to break due to bending fatigue. The lower limit of the F5 value is preferably 20 MPa or more. Making the F5 value 20 MPa or more is preferable in terms of preventing printing pitch deviation due to film elongation during secondary processing. That is, the F5 value is preferably 20 to 70 MPa, more preferably 20 to 60 MPa, and even more preferably 20 to 50 MPa. Means for making the F5 value 20 MPa or more and 70 MPa or less include low-fold stretching, high-temperature stretching, and addition of resins other than nylon 6, but addition of resins other than nylon 6 is preferable from the viewpoint of productivity.

The biaxially stretched polyamide film of the present invention has excellent pinhole resistance when subjected to a twist bending test using a Gelbo flex tester 1000 times at a temperature of 1°C, and the number of pinhole defects is 1520 or less, more preferably 15 or less, and even more preferably 10 or less. The fewer the number of pinhole defects after the bending test, the better the pinhole resistance when bending. If the number of pinholes is preferably 1520 or less, more preferably 15 or less, a packaging bag that is less likely to develop pinholes even when a load is applied to the packaging bag during transportation, etc. can be obtained.

The biaxially stretched polyamide film of the present invention preferably has a crystallization parameter (1200 cm ^-1 /1370 cm ^-1 ) of 1.0 or more, which is calculated from the intensity ratio of the peaks around 1200 cm ^-1 and 1370 cm ^-1 in the spectrum obtained by ATR measurement of FT-IR. Here, the peak at 1200 cm ^-1 is an absorption derived from the α-crystal of nylon 6, and the peak at 1370 cm ^-1 is an absorption unrelated to crystals. By setting this crystallization parameter in the range of 1.0 to 1.5, dimensional stability can be obtained. By setting the crystallization parameter to 1.0 or more, dimensional stability is improved. By setting the crystallization parameter to 1.5 or less, mechanical strength can be obtained.

The biaxially stretched polyamide film of the present invention preferably has a heat shrinkage rate at 160°C for 10 minutes in the range of 0.6% to 3.0% in both the machine direction (hereinafter sometimes abbreviated as MD direction) and the width direction (hereinafter sometimes abbreviated as TD direction), more preferably 0.5% to 2.0%. A heat shrinkage rate of 3.0% or less can suppress the occurrence of curling or shrinkage when heat is applied in the next process, such as lamination or printing. A heat shrinkage rate of 0.6% or more is preferable in terms of excellent impact resistance.

The planar orientation (ΔP) of the biaxially stretched polyamide film in the present invention is preferably 0.050 to 0.070. The planar orientation is determined by measuring the birefringence with a refractometer and calculating it from the following formula, where Nx is the refractive index in the longitudinal direction, Ny is the refractive index in the width direction, and Nz is the refractive index in the thickness direction.
ΔP=(Nx+Ny)/2-Nz
The plane orientation can be increased by increasing the biaxial stretch ratio, particularly the stretch ratio in the TD direction, and a plane orientation of 0.05 or more can provide the film with mechanical strength such as puncture strength, while a plane orientation of 0.07 or less can suppress a decrease in productivity.

The impact strength of the biaxially oriented polyamide film of the present invention is preferably 0.6 J or more, and more preferably 0.7 J or more.

The puncture strength of the film of the present invention is preferably 6.0 N or more. By making the puncture strength 6.0 N or more, even when filled with solid contents, it is possible to prevent the contents from puncturing the bag and causing holes in the bag, or to prevent the bag from being caused by external factors during transportation. There is no particular upper limit to the puncture strength, but it may be 100 N or less, or 50 N or less.

The maximum dispersion diameter of the resin (b) other than polyamide 6 contained in the base layer is preferably 1.0 μm or more. Here, the maximum dispersion diameter refers to the longest length of the resin (b) dispersed in the base layer when the base layer is observed with a transmission electron microscope (TEM): unit μm. An example is shown in FIG. 1. FIG. 1 is a photograph of a cross section of the film in the machine direction observed with a transmission electron microscope at a magnification of 2000 times, and the darkly dyed domain is the resin (b). By having the dispersion diameter of the resin (b) be 1.0 μm or more, good bending pinhole resistance and ethanol permeability can be obtained.
The reason why good pinhole resistance is obtained is believed to be that the flexibility of the entire film is improved and destruction due to bending fatigue is less likely to occur due to the dispersion of the flexible resin (b), which is incompatible with polyamide 6. The reason why good ethanol permeability is obtained is believed to be that the dispersion of the incompatible resin (b) in polyamide 6 forms an interface with weak adhesion between polyamide 6 and resin (b), and ethanol permeates the film through this interface.
When the maximum dispersion diameter of the resin (b) is less than 1.0 μm, compatibility is promoted and the characteristics of the base material, polyamide 6, become apparent, resulting in a decrease in pinhole resistance and ethanol permeability.

[Method for producing biaxially stretched polyamide film]
The method for producing the biaxially oriented polyamide film of the present invention will now be described.
First, the raw resin is melt-extruded using an extruder, extruded from a T-die into a film, cast onto a cooling roll and cooled to obtain an unstretched film in which at least a substrate layer and a functional layer are laminated. In order to obtain an unstretched laminated film, a co-extrusion method using a feed block or a multi-manifold is preferred. In addition to the co-extrusion method, a dry lamination method, an extrusion lamination method, etc. can also be selected. When laminating by the co-extrusion method, it is desirable to reduce the difference in melt viscosity between the first resin composition forming the substrate layer and the second resin composition forming the functional layer.

The melting temperature of the resin is preferably 220°C or higher and 350°C or lower. If the temperature is lower than the above, unmelted material may occur, and defects such as poor appearance may occur. If the temperature exceeds the above, deterioration of the resin may be observed, and molecular weight may decrease and appearance may deteriorate. The die temperature is preferably 250°C or higher and 350°C or lower. The cooling roll temperature is preferably -30°C or higher and 80°C or lower, and more preferably 0°C or higher and 50°C or lower. To obtain an unstretched film by casting the film-like molten material extruded from the T-die onto a rotating cooling drum and cooling it, for example, a method using an air knife or an electrostatic adhesion method in which a static charge is applied can be preferably applied. In addition, 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 method of contacting the opposite side of the cooling roll of the unstretched film with a cooling liquid in a tank, a method of applying a liquid that evaporates with a spray nozzle, a method of spraying a high-speed fluid, or the like in combination. The unstretched film obtained in this way is stretched in a biaxial direction to obtain the biaxially stretched polyamide film of the present invention.

The stretching method may be either a simultaneous biaxial stretching method or a sequential biaxial stretching method. The sequential biaxial stretching method is preferable because it is advantageous in that it increases the degree of surface orientation and makes it easier to obtain puncture strength, that it is easier to adjust the crystallization parameters to obtain the desired dimensional stability, and that it is easier to reduce production costs by increasing the film production speed. 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 preferable in terms of physical properties and uniformity of physical properties in the MD and TD directions (isotropy) rather than one-stage stretching. Roll stretching is preferable for MD stretching in the sequential biaxial stretching method.

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. If it is less than 50°C, the resin does not soften and stretching may be 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 it exceeds 120°C, the resin may become too soft and stable stretching may not be possible. Stretching at 120°C or less is preferable in terms of preventing sticking to the roll and breakage. In other words, the stretching temperature in the MD direction is preferably 50 to 120°C, more preferably 55 to 115°C, and even more preferably 60 to 110°C.

The lower limit of the stretch ratio in the MD direction (when stretching in multiple stages, the total stretch ratio multiplied by each stretch 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 decreases, and the crystallinity may become too low, resulting in a decrease in impact strength. The upper limit of the stretch ratio in the MD direction is preferably 5.0 times, more preferably 4.5 times, and most preferably 4.0 times. A stretch ratio of 5.0 times or less is preferable from the viewpoint of achieving both productivity and film formation stability. That is, the stretch ratio in the MD direction is preferably 2.2 to 5.0 times, more preferably 2.5 to 4.5 times, and even more preferably 2.8 to 4.0 times.

In addition, when stretching in the MD direction is performed in multiple stages, the above-mentioned stretching is possible in each stretching, but it is preferable to adjust the stretching ratio 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 to set the first stage stretching to 1.5 to 2.1 times and the second stage stretching to between 1.5 to 1.8 times.

The film stretched in the MD direction is stretched in the TD direction using a tenter, heat-set, and relaxed (also called relaxation 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 less than 50°C, the resin does not soften and stretching may be 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. Setting the stretching temperature in the TD direction to 190°C or less is preferable from the viewpoint of suppressing film crystallization and preventing breakage. In other words, the stretching temperature in the TD direction is preferably 50 to 190°C, more preferably 55 to 185°C, and even more preferably 60 to 180°C.

The lower limit of the stretch ratio in the TD direction (when stretching in multiple stages, the total stretch ratio multiplied by each stretch ratio) is preferably 2.8 times, more preferably 3.2 times, even more preferably 3.5 times, and particularly preferably 3.8 times. If it is less than 2.8 times, the thickness accuracy in the TD direction decreases, and the crystallinity may become too low, resulting in a decrease in impact strength. The upper limit of the stretch ratio in the TD direction is preferably 5.5 times, more preferably 5.0 times, even more preferably 4.7 times, especially preferably 4.5 times, and most preferably 4.3 times. It is preferable to set the stretch ratio in the TD direction to 5.5 times or less from the viewpoint of preventing breakage. That is, the stretch ratio in the TD direction is preferably 2.8 to 5.5 times, more preferably 3.2 to 5.0 times, even more preferably 3.5 to 4.7 times, especially preferably 3.8 to 4.5 times, and most preferably 3.8 to 4.3 times.

The lower limit of the heat setting temperature is preferably 210°C, more preferably 212°C. If the heat setting temperature is low, the heat shrinkage rate becomes too large, which deteriorates the appearance after lamination and tends to reduce the 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 tends to decrease. The heat setting time is preferably 0.5 to 20 seconds. It is further preferably 1 to 15 seconds. The heat setting time can be set to an appropriate time in consideration of the heat setting temperature and the wind speed in the heat setting zone. It is preferable to set the heat setting temperature to 220°C or less from the viewpoint of preventing embrittlement of the film. It is preferable to set the heat setting temperature to 210°C or more from the viewpoint of improving dimensional stability and laminate strength. That is, the heat setting temperature is preferably 210 to 220°C, more preferably 212 to 218°C.

Performing a relaxation process after heat setting is effective in controlling the heat shrinkage rate. The temperature for the relaxation process can be selected in the range from the heat setting temperature to the Tg of the resin, but is preferably (heat setting temperature (℃) - 10) ℃ or higher (Tg (℃) + 10) ℃ or lower. If the relaxation temperature is too high, the shrinkage rate will be too fast, which is not preferable as it may cause distortion. Conversely, if the relaxation temperature is too low, the film will not be relaxed, but will simply slacken, and the heat shrinkage rate will not decrease, resulting in poor dimensional stability. The lower limit of the relaxation rate for the relaxation process is preferably 0.5%, more preferably 1%. If it is less than 0.5%, the heat 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 in the tenter, making production difficult. A relaxation rate of 0.5% or higher is preferable in terms of reducing the heat shrinkage rate. A relaxation rate of 20% or lower is preferable in terms of preventing film sagging. That is, the relaxation rate is preferably 0.5 to 20%, more preferably 1 to 15%, and even more preferably 1 to 10%.

Furthermore, the biaxially oriented polyamide film of the present invention can be subjected to heat treatment or humidity conditioning treatment to improve dimensional stability depending on the application. In addition, it is also possible to perform corona treatment, coating treatment, flame treatment, etc., to improve the adhesion of the film surface, printing processing, and deposition processing of metal objects, inorganic oxides, etc.

In one embodiment of the present invention, the biaxially oriented polyamide film of the present invention is processed into a laminated film by laminating a sealant film, thereby providing a packaging material. Examples of the sealant film include unstretched linear low density polyethylene (LLDPE) film, unstretched polypropylene (CPP) film, and ethylene-vinyl alcohol copolymer resin (EVOH) film. The sealant film may be laminated so as to be in direct contact with the biaxially oriented polyamide film, or may be laminated via another layer such as an adhesive layer. The packaging material may include a printing layer. The packaging material of the present invention is made into a bag and processed into a packaging bag.

Another embodiment of the present invention provides a packaging material for ethanol evaporative agents, which is composed of a laminate including at least two layers of the biaxially oriented polyamide film of the present invention and a breathable resin film or nonwoven fabric. The breathable resin film is not particularly limited as long as it is a film having breathability or ethanol permeability, and examples thereof include unstretched linear low density polyethylene (LLDPE) film, unstretched polypropylene (CPP) film, ethylene-vinyl alcohol copolymer resin (EVOH) film, and ethylene-vinyl acetate copolymer resin (EVA) film. In addition, a nonwoven fabric can be used instead of or in combination with the breathable resin film. For example, polyester, polypropylene, rayon, nylon, biodegradable fiber, pulp, cotton, etc. may be used as the nonwoven fabric substrate, and polypropylene is preferably used. As the nonwoven fabric, either a short fiber nonwoven fabric or a long fiber nonwoven fabric can be used. The nonwoven fabric may be a paper substrate.

This application claims the benefit of priority based on Japanese Patent Application No. 2023-055926, filed on March 30, 2023. The entire contents of the specification of Japanese Patent Application No. 2023-055926, filed on March 30, 2023, are incorporated by reference into this application.

The films were evaluated using the following measurement methods. Unless otherwise specified, measurements were performed in a measurement room with an environment of 23°C and a relative humidity of 65%.

(1) Film Thickness Divide the film into 10 equal parts in the TD direction (for narrow films, divide the film into 10 pieces so that the width is sufficient to measure the thickness), cut out 10 pieces of 100 mm film in the MD direction, and condition them for 2 hours or more in an environment of 23°C and 65% relative humidity. The thickness of the center of each sample was measured using a thickness measuring device manufactured by Tester Sangyo, and the average value was taken as the thickness.

(2) Heat Shrinkage of Film The heat shrinkage of the film 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 (%)

(3) F5 value of film The obtained biaxially stretched polyamide film was left to stand for 2 hours in a room adjusted to 23°C and 50% RH relative humidity, and then cut into strips of 150 mm (gauge length 100 mm) in the measurement direction of the MD and TD of the film and 15 mm in the direction perpendicular to the measurement direction to obtain samples. A tensile test was carried out at a test speed of 200 mm/min using a tensile tester (AG-1I manufactured by Shimadzu Corporation) equipped with a 1 kN load cell and a sample chuck. From the obtained stress-elongation curve, the stress when the sample was elongated by 5% was taken as the F5 value and treated as an alternative value for the elastic modulus. Measurements were carried out on three samples, and the average value of each was calculated.

(4) Impact Strength of Film: Measurement was performed using a film impact tester manufactured by Toyo Seiki Seisakusho Co., Ltd.

(5) Puncture strength of film The puncture strength was measured in accordance with "2. Test method for strength, etc." in "Standards and standards for food, additives, etc., Part 3: Apparatus and containers and packaging" (Ministry of Health and Welfare Notification No. 20, 1982) in the Food Sanitation Act. A needle with a tip diameter of 0.7 mm was pierced into the film at a piercing speed of 50 mm/min, and the strength at which the needle penetrated the film was measured and used as the puncture strength. The measurement was performed at room temperature (23°C), and the obtained value was used as the puncture strength of the film (unit: N).

(6) Crystallization parameters FT-IR ATR measurements were performed on the front and back surfaces of the sample under the following conditions.
FT-IR device: Bio Rad DIGILAB FTS-60A/896
Single reflection ATR attachment: golden gate MKII (SPECAC)
Internal reflection element: Diamond Incident angle: 45°
Resolution: 4 ^cm
The degree of crystallinity was calculated from the intensity ratio (1200 cm ^-1 /1370 cm ^-1 ) of the absorption appearing near 1200 cm ^-1 and the absorption appearing near 1370 cm ^-1 . Here, 1200 cm ^-1 is the absorption of α-crystals of nylon 6, and 11370 cm ^-1 is the absorption unrelated to crystals.

(7) Planar Orientation Degree of Film For a sample, the refractive index (nx), the refractive index (ny), and the refractive index (nz) in the longitudinal direction, width direction, and thickness direction of the film were measured using an Abbe refractometer with sodium D line as a light source according to JIS K 7142-1996, Method A, and the planar orientation coefficient was calculated according to the formula (1).
Planar orientation coefficient (ΔP) = (nx + ny) / 2 - nz

(8) Ethanol Permeability Absorbent cotton soaked in ethanol was placed in a cup, the cup was covered with a film, and the cup was left for 24 hours under conditions of a relative humidity of 50% and a temperature of 40° C., after which the ethanol permeability was measured from the weight loss.

(9) Dispersion diameter of resin The film was cut so that the cross section in any direction could be observed, and embedded in epoxy resin. Ultrathin sections were prepared from the embedded samples using a microtome equipped with an ultrasonic knife. In order to confirm the dispersion shape of resin (b), appropriate dyeing treatment was performed according to the type of resin (b). Carbon deposition was performed on the test pieces to prepare samples for TEM observation. Observation and photography were performed using a JEOL JEM2100 transmission electron microscope at an acceleration voltage of 200 kV. When taking the photographs, the magnification was adjusted so that the entire substrate layer was within the photographing range. The longest major axis length of the resin dispersed in the substrate layer in the obtained photograph was taken as the maximum dispersion diameter.

(10) Bending Pinhole Resistance of Film The number of pinholes due to bending fatigue was measured by the following method using a Gelbo Flex Tester manufactured by Tester Sangyo Co., Ltd. Specifically, a polyester-based adhesive was applied to the biaxially oriented polyamide film manufactured in the examples, and then a linear low-density polyethylene film (L-LDPE film: manufactured by Toyobo Co., Ltd., L4102) having a thickness of 40 μm was dry-laminated, and aged for 3 days in an environment of 40° C. to obtain a laminate film. The obtained laminate film was cut into 12 inches by 8 inches, and made into 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 Gelbo Flex Tester, and the other end was fixed to the movable head side, with an initial gripping distance of 7 inches. A bending fatigue test was performed 1000 times at a speed of 40 times/min, in which a 440-degree twist was given in the first 3.5 inches of the stroke, and the entire stroke was completed in a linear horizontal motion for the next 2.5 inches, and the number of pinholes generated in the laminate film was counted. The measurements were carried out in an environment of 1°C. The test film was placed on a filter paper (Advantec, No. 50) with the L-LDPE film side facing down, and the four corners were fixed with Cellotape (registered trademark). Ink (Pilot ink (product number INK-350-Blue) diluted 5 times with pure water) was applied to the test film and spread over the entire surface using a rubber roller. After wiping off unnecessary ink, the test film was removed, and the number of ink dots on the filter paper was counted.

The following resin compositions were used for the base layer and the functional layer.
Resin composition constituting base layer A:
Polyamide 6 (manufactured by Toyobo Co., Ltd., relative viscosity 2.8, melting point 220° C.)
Resin other than polyamide 6 (b)
・Polybutylene adipate terephthalate PBAT (BASF Ecoflex F-Blend C1200)
- Polyester elastomer PEE (Tefabloc TPC A1400N, manufactured by Mitsubishi Chemical Corporation)
・Polyamide elastomer PAE (Arkema PEBAX 4033 SA01)
・Polyolefin elastomer POE (MELSON H6051, manufactured by Tosoh Corporation)
Polybutylene succinate PBS (Showa Polymer Co., Ltd., Bionolle 1001)
Polyamide 11: (Arkema Rilsan, melting point 186°C)

Resin composition constituting functional layer B:
Polyamide 6 (manufactured by Toyobo Co., Ltd., relative viscosity 2.8, melting point 220° C.) 90% by mass;
Polyamide MXD6 (manufactured by Mitsubishi Gas Chemical Co., Inc., relative viscosity 2.1, melting point 237° C.) 10% by mass;
Porous silica fine particles (manufactured by Fuji Silysia Chemical Ltd., average particle size 2.0 μm, pore volume 1.6 ml/g) 0.54% by mass;
Fatty acid bisamide (ethylene bis stearic acid amide, manufactured by Kyoeisha Chemical Co., Ltd.) 0.15% by mass

(Examples 1 to 7)
The raw materials for the base layer: A layer and the functional layer: B layer were mixed in the formulation shown in Table 1. Using an apparatus consisting of two extruders and a 380 mm wide co-extrusion T-die, lamination was performed using the feed block method in a configuration of functional layer: B layer/base layer: A layer/functional layer: B layer, and the molten resin of the resin composition shown in Table 1 was extruded from the T-die into a film, which was then cast onto a cooling roll whose temperature was controlled at 20°C and electrostatically adhered to obtain an unstretched film with a thickness of 100 to 200 μm.

The unstretched film obtained was introduced into a roll-type stretching machine, 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 stretched film was introduced into a tenter-type stretching machine, preheated at 60°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, heat-set at 218°C, and then relaxed 7% at 218°C. One surface was corona discharge-treated to obtain a biaxially stretched polyamide film. The extrusion rate of the extruder was adjusted so that the thickness of the base layer was 8 μm and the thickness of the functional layer was 1 μm on each side, relative to the thickness of the biaxially stretched polyamide film of 10 μm.

(Examples 8 and 9)
A biaxially oriented polyamide film was obtained in the same manner as in Examples 1 to 7, except that the feed block configuration and the extrusion rate of the extruder were adjusted so that the total thickness of the biaxially oriented polyamide film was 15 μm, the thickness of the base layer was 12 μm, and the thickness of the functional layer was 1.5 μm on each side with the composition shown in Table 1.

(Comparative Examples 1 to 4, 6)
A biaxially stretched polyamide film was obtained in the same manner as in Example 1, except that the resin compositions of the base layer and the functional layer were changed as shown in Table 2. The evaluation results of the obtained film are shown in Table 2.

(Comparative Example 5)
Biaxially oriented polyamide films were obtained in the same manner as in Examples 1 to 7 and Comparative Examples 1 to 4 and 6, except that the rotation speed of the extruder for extruding the resin composition constituting the base layer was increased within a range that did not increase the target thickness, and the kneading was strengthened.

The evaluation results of the films obtained in the examples and comparative examples are shown in Table 1.

The films described in Examples 1 to 9 had good pinhole resistance, dimensional stability, puncture strength, and ethanol permeability, making them suitable for use as alcohol evaporative agents. The plane orientation degree was 0.5 or more, indicating high orientation, and the crystallization parameter was 1.0 or more, indicating high crystallization. However, the resin (b) was dispersed with a dispersion diameter of 1 μm or more, forming an interface with relatively low adhesion strength with the polyamide resin, and it is believed that this interface was utilized to improve ethanol permeability. Furthermore, the flexibility of the base layer A itself was improved by the dispersion of resin (b), which is believed to have improved pinhole resistance.

The film described in Comparative Example 1 had excellent dimensional stability and puncture strength, but poor bending pinhole resistance and ethanol permeability. When the plane orientation ΔP was 0.05 or more and the crystallization parameter was in the range of 1 to 1.5, the dimensional stability and puncture strength were good, but it is believed that the brittleness of the film increased due to the progress of crystallization, causing pinhole resistance and ethanol permeability to decrease.

Although the film described in Comparative Example 2 had a thickness of 15 μm, the base layer contained no resin other than polyamide 6, and therefore no dispersion structure was formed due to phase separation, and ethanol permeation did not occur using the dispersion interface, so the ethanol permeability was not satisfactory for use as an alcohol evaporant.

The film described in Comparative Example 3 was thick at 25 μm, and since the base layer contained no resin other than polyamide 6, the pinhole resistance and ethanol permeability were not satisfactory for use as an alcohol evaporant.

The film described in Comparative Example 4 had a small amount of PBAT added as resin (b), and the interface between polyamide 6 and PBAT was small, resulting in insufficient ethanol permeability.

The film described in Comparative Example 5 had a small maximum dispersion diameter of resin (b) of 0.5 μm, and compatibility with polyamide 6 progressed, so good ethanol permeability and good pinhole resistance were not obtained.

The film described in Comparative Example 6 was added with 40% PBAT, and as a result, the ethanol permeability increased to 16000 g·μm/ ^m2 ·24 hr (50% RH/40°C). As an ethanol transpiration agent, the result was that ethanol transpire quickly.

Claims (11)

A biaxially oriented polyamide film containing polyamide 6 as a main component, the biaxially oriented polyamide film having an ethanol permeability of 6000 to 10000 g μm/ ^m2 24 hr (50% RH/40°C), a thickness of 15 μm or less, heat shrinkage rates in both the machine direction and width direction of the film when heat-treated at 160°C for 10 minutes of 2.0% or less, a surface crystallization parameter in the range of 1.0 to 1.5, and a puncture strength of 6.0 N or more. The biaxially oriented polyamide film according to claim 1, having at least one layer containing 60% by mass or more and 98% by mass or less of polyamide 6 resin (a) and 2% by mass or more and 40% by mass or less of a resin other than polyamide 6 (b). The biaxially stretched polyamide film according to claim 2, wherein the resin (b) other than polyamide 6 is at least one selected from the group consisting of thermoplastic elastomers such as polyolefins, ionomer polymers, polyester-based elastomers, polyamide-based elastomers, polyolefin-based elastomers, polystyrene-based elastomers, polyurethane-based elastomers, and polyvinyl chloride-based elastomers, aliphatic polyester resins, aliphatic-aromatic polyester resins, aliphatic polyamide resins, and aliphatic-aromatic polyamide resins. The biaxially oriented polyamide film according to claim 3, wherein the aliphatic aromatic polyester resin is a polybutylene adipate terephthalate resin. The biaxially stretched polyamide film according to claim 2, wherein the resin (b) other than polyamide 6 is derived from biomass and is at least one polyamide selected from the group consisting of polyamide 11, polyamide 410, polyamide 610, and polyamide 1010. The biaxially oriented polyamide film according to claim 2, wherein the maximum dispersion diameter of the resin (b) other than polyamide 6 is 1.0 μm or more. A biaxially oriented polyamide film as described in claim 1, in which the stress at 5% elongation in both the machine direction and width direction of the film is 70.0 MPa or less. The biaxially stretched polyamide film according to claim 1, having a planar orientation degree (ΔP) of 0.050 or more. The biaxially oriented polyamide film according to claim 1, wherein a laminate film of the biaxially oriented polyamide film and a polyethylene film having a thickness of 40 μm is prepared, and when the film is bent 1,000 times using a Gelbo flex tester in an atmosphere at a temperature of 1°C, the number of pinholes that are generated is 20 or less. A food packaging material comprising a biaxially oriented polyamide film and a sealant film according to any one of claims 1 to 9. A packaging material for an ethanol transpiration agent, comprising a laminate including at least two layers of the biaxially oriented polyamide film according to any one of claims 1 to 9 and a breathable resin film or nonwoven fabric.