JP7471830B2 - Polyamide Film - Google Patents

Description

The present invention relates to a polyamide film.

Films containing nylon resin are widely used in various fields as films with gas barrier properties, toughness, etc., and are particularly widely used for product packaging.

This film is preferably used as a packaging film for food products and other items distributed on the market. However, pinholes can occur during transportation and handling, and various improvements are being made from the perspective of protecting the contents.

The applicant has developed a polyamide-based film having at least a polyamide layer containing 86% to 98% by weight of polyamide and 2% to 14% by weight of a flex-resistant agent (Patent Document 1). This polyamide-based multilayer film has excellent pinhole resistance, with softness to withstand flexing and hardness to withstand wear due to repeated contact. This polyamide-based film has excellent pinhole resistance due to flexing and pinhole resistance due to repeated contact, and also exhibits excellent puncture resistance.

JP 2017-2114 A

Compared to food packaging bags transported at room temperature, food packaging bags transported at low temperatures, especially frozen, tend to have a higher incidence of pinholes. When the contents contain moisture, pinholes that develop during frozen transport can cause problems such as the contents spilling out when thawed.

The present invention aims to provide a polyamide-based film that has excellent pinhole resistance and puncture resistance for use in packaging food that is transported at low temperatures, particularly in a frozen state.

As a result of intensive research into solving the above problems, the inventors discovered that a polyamide film having at least a polyamide layer containing an aliphatic polyamide and a polyester-based elastomer can solve the above problems, and thus completed the present invention.

That is, the present invention provides the following polyamide-based film:

Item 1.
A polyamide-based film having a layer mainly composed of an aliphatic polyamide, the layer mainly composed of an aliphatic polyamide further comprising a first layer containing a polyester-based elastomer and a second layer not containing a polyester-based elastomer,
The first layer and the second layer are adjacent to each other.
Polyamide film.

Item 2.
2. The polyamide film according to claim 1, wherein the first layer contains 80% by weight to 99.5% by weight of an aliphatic polyamide and 0.5% by weight to 20% by weight of a polyester elastomer.

Item 3.
3. The polyamide film according to item 1 or 2, wherein the aliphatic polyamide has a relative viscosity of 3.0 to 5.5 as measured in accordance with JIS K6920-2:2009 under conditions of a sample concentration of 1.0% by weight in 96% _H2SO4 solvent and _a temperature of 25°C.

Item 4.
A method for suppressing the occurrence of pinholes using a film used in food packaging bags, comprising:
The film is a polyamide-based film having at least a first layer containing an aliphatic polyamide and a polyester-based elastomer,
packaging a food product with the film;
After freezing and thawing the packaged food,
A method for suppressing the occurrence of pinholes in the packaging bag.

The polyamide film of the present invention has excellent pinhole resistance and puncture resistance when used for packaging food that is transported at low temperatures, particularly in a frozen state.

The present invention will be described in detail below.

[1] Polyamide film The polyamide film of the present invention includes the following aspects.

The polyamide film of the present invention is
The laminate has a layer mainly composed of an aliphatic polyamide,
The layer mainly composed of an aliphatic polyamide further includes a first layer including a polyester-based elastomer and a second layer not including a polyester-based elastomer,
The first layer and the second layer are adjacent to each other.

In the polyamide-based film of the present invention, the first layer containing the aliphatic polyamide and polyester-based elastomer preferably contains 80% by weight to 99.5% by weight of aliphatic polyamide and 20% by weight to 0.5% by weight of polyester-based elastomer.

In the polyamide film of the present invention, the aliphatic polyamide preferably has a relative viscosity of 3.0 to _5.5 , measured according to a measurement method in accordance with JIS K6920-2:2009 under conditions of a sample concentration of 1.0 wt.% and a temperature of 25°C using 96% _H2SO4 as a solvent.

When the food to be packaged contains relatively sharp contents (food), the packaging bag must be able to withstand punctures by the contents during transportation and handling, and must also be able to prevent pinholes from forming. In particular, when the contents are frozen foods, the packaging bag must have excellent puncture resistance.

In addition, when packaging frozen foods that are liquid at room temperature, the weight of the food can be heavy, so the packaging bag must have excellent impact resistance.

The polyamide-based film of the present invention is a film that, when used as a food packaging film, particularly for packaging frozen filled products and frozen distribution products, has excellent puncture resistance and impact resistance as well as excellent flex resistance under frozen conditions. The polyamide-based film of the present invention is a film that shows excellent toughness and is less likely to develop pinholes when packaging food and transporting, conveying, etc., not only in normal temperature environments but also in frozen environments. Improvements in freezing technology have made it possible to freeze a variety of foods, enabling the transport of foods over long distances. The polyamide-based film of the present invention is useful when transporting foods over long distances in a frozen state.

In the present invention, "freezing" refers to cooling food or raw materials to below -5°C, preferably below -15°C, and then freezing and storing them.

(1) Aliphatic polyamides Aliphatic polyamides include aliphatic nylons and their copolymers. Specific examples include polycapramide (nylon-6), poly-ω-aminoheptanoic acid (nylon-7), poly-ω-aminononanoic acid (nylon-9), polyundecaneamide (nylon-11), polylauryl lactam (nylon-12), polyethylenediamineadipamide (nylon-2,6), polytetramethyleneadipamide (nylon-4,6), polyhexamethyleneadipamide (nylon-6,6), polyhexamethylenesebacamide (nylon-6,10), polyhexamethylenedodecamide (nylon-6,12), polyoctamethyleneadipamide (nylon-8,6), polydecamethyleneadipamide (nylon-10,8), caprolactam/lauryl lactam copolymer (nylon-6/12), caprolactam, Examples of the copolymer include caprolactam/ω-aminononanoic acid copolymer (nylon-6/9), caprolactam/hexamethylenediammonium adipate copolymer (nylon-6/6,6), lauryl lactam/hexamethylenediammonium adipate copolymer (nylon-12/6,6), ethylenediamine adipamide/hexamethylenediammonium adipate copolymer (nylon-2,6/6,6), caprolactam/hexamethylenediammonium adipate/hexamethylenediammonium sebacate copolymer (nylon-6,6/6,10), and ethyleneammonium adipate/hexamethylenediammonium adipate/hexamethylenediammonium sebacate copolymer (nylon-6/6,6/6,10).

These aliphatic polyamides may be used alone or in combination of two or more.

Preferred aliphatic polyamides include nylon-6, nylon-6,6, and nylon-6/6,6 (a copolymer of nylon 6 and nylon 6,6), more preferably nylon-6 and nylon-6/6,6, and particularly preferably nylon-6. As a combination of two or more aliphatic polyamides, a combination of nylon-6 and nylon-6/6,6 is preferred.

The weight ratio of nylon-6:nylon-6/6,6 in the above combination is preferably about 50:50 to 95:5.

In the polyamide-based film of the present invention, the aliphatic polyamide preferably has a relative viscosity of 3.0 to 5.5 as measured by a measurement method conforming to JIS K6920-2:2009 under the conditions of a sample concentration of 1.0% by weight and a temperature of 25° C. using 96% H ₂ SO ₄ as a solvent. The relative viscosity is more preferably 3.2 to 4.2.

The polyamide-based film of the present invention can further improve the puncture resistance by using a polyamide having the above relative viscosity as the polyamide.

When two or more polyamides are mixed together in the polyamide-based film of the present invention, the relative viscosity of the polyamide is determined by measuring the relative viscosity of each polyamide to be mixed, and taking the weighted average of the measured values.

In addition, in the polyamide-based film of the present invention, the relative viscosities of the aliphatic polyamides used in the front layer, back layer, and intermediate layer may be the same or different.

The polyamide film of the present invention may contain polyamides other than aliphatic polyamides.

Aromatic polyamide As the polyamide, the above aliphatic polyamide may contain an aromatic polyamide.

Examples of aromatic polyamides include crystalline aromatic polyamides obtained by polycondensation reaction of aromatic diamines such as metaxylylenediamine and paraxylylenediamine with dicarboxylic acids or derivatives thereof such as adipic acid, suberic acid, sebacic acid, cyclohexanedicarboxylic acid, terephthalic acid, and isophthalic acid. The aromatic polyamide is preferably a crystalline aromatic polyamide such as polymetaxylyleneadipamide (MXD-nylon). Specific examples of aromatic polyamides include S-6007 and S-6011 (both manufactured by Mitsubishi Gas Chemical Company, Inc.).

Examples of aromatic polyamides include amorphous aromatic polyamides (amorphous nylons) obtained by polycondensation reaction of aliphatic diamines such as hexamethylenediamine with dicarboxylic acids such as terephthalic acid and isophthalic acid or their derivatives. A preferred example of an aromatic polyamide is a copolymer of hexamethylenediamine-terephthalic acid-hexamethylenediamine-isophthalic acid. A specific example of the aromatic polyamide is Sealer PA (manufactured by DuPont-Mitsui Polychemicals Co., Ltd.).

Combinations of Polyamides Preferred combinations of aliphatic polyamides and aromatic polyamides include a combination of nylon-6 and MXD-nylon, and a combination of nylon-6 and non-crystalline aromatic polyamide (amorphous nylon).

When aliphatic polyamide and aromatic polyamide are mixed as the polyamide, the content of aliphatic polyamide in the polyamide is preferably 80% to 100% by weight, and more preferably 90% to 100% by weight, assuming the amount of polyamide to be 100% by weight. The content of aliphatic polyamide in the polyamide is further preferably 91% to 99% by weight, particularly preferably 92% to 98% by weight, and most preferably 93% to 97% by weight.

When the polyamide contains an aliphatic polyamide and an aromatic polyamide (such as an amorphous aromatic polyamide), the content of the aromatic polyamide in the polyamide is preferably 0% to 20% by weight, and more preferably 0% to 10% by weight, with the amount of polyamide being 100% by weight. The content of the aromatic polyamide in the polyamide is further preferably 1% to 9% by weight, particularly preferably 2% to 8% by weight, and most preferably 3% to 7% by weight.

The polyamide-based film of the present invention can exhibit better stretching film-forming properties when it contains an aliphatic polyamide and an aromatic polyamide as the polyamide. By setting the content of the aliphatic polyamide in the polyamide within the above range, the pinhole resistance and puncture resistance of the polyamide-based film due to repeated contact can be further improved.

(2) Polyester-based elastomers Examples of polyester-based elastomers include modified polyester-based elastomers, which are obtained by modifying a saturated polyester-based thermoplastic elastomer containing a polyalkylene ether glycol segment with an unsaturated carboxylic acid or a derivative thereof.

Specifically, it is a modified polyester elastomer obtained by modifying a saturated polyester thermoplastic elastomer with a polyalkylene ether glycol segment content of 58% to 73% by weight with an unsaturated carboxylic acid or its derivative in the presence of a radical generator. Reactive groups are introduced by the graft reaction and terminal addition reaction of the unsaturated carboxylic acid or its derivative, improving chemical and hydrogen bonding properties with various resins.

Saturated polyester-based thermoplastic elastomers are block copolymers consisting of soft segments containing polyalkylene ether glycol segments and hard segments containing polyester, and the content of polyalkylene ether glycol segments in the polyester-based elastomer is approximately 58% to 73% by weight.

Examples of polyalkylene ether glycols constituting the soft segment include polyethylene glycol, poly(1,2 and 1,3-propylene ether) glycol, poly(tetramethylene ether) glycol, and poly(hexamethylene ether) glycol. The number average molecular weight of the polyalkylene ether glycol is preferably about 400 to 6,000.

Examples of unsaturated carboxylic acids or derivatives thereof include unsaturated carboxylic acids such as acrylic acid, maleic acid, and fumaric acid, as well as their acid anhydrides, esters, and metal salts.

Radical generators include peroxides such as t-butyl hydroperoxide and cumene hydroperoxide, and azo compounds such as 2,2'-azobisisobutyronitrile (AIBN).

The blending ratio of each of the above components is preferably 0.01 to 30 parts by weight of unsaturated carboxylic acid or its derivative and 0.001 to 3 parts by weight of radical generator per 100 parts by weight of saturated polyester thermoplastic elastomer.

The preparation method of the modified polyester elastomer is not particularly limited, but it can be prepared, for example, by the method described in JP 2002-155135 A. The melt flow rate (MFR) of the resulting modified polyester elastomer is preferably 40 g/10 min to 300 g/10 min. The melt flow rate (MFR) is a value measured under conditions of 230°C and 2.16 kg using a measurement method conforming to JIS K7210.

A specific example of a commercially available modified polyester elastomer is Tefablock (manufactured by Mitsubishi Chemical Corporation).

The polyamide film of the present invention has a layer whose main component is the aliphatic polyamide. In the present invention, the "main component" refers to a component that is contained in an amount of 70% by weight or more, preferably 80% by weight or more, and more preferably 90% by weight or more, assuming the total weight of the resin components that make up the layer to be 100% by weight.

In the polyamide-based film of the present invention, the content of the aliphatic polyamide and polyester-based elastomer in the first layer is preferably 80% by weight to 99.5% by weight of aliphatic polyamide and 20% by weight to 0.5% by weight of polyester-based elastomer, with the weight of the entire layer being 100% by weight.

The content of the aliphatic polyamide in the first layer is more preferably 99.5% to 85% by weight, even more preferably 99.5% to 90% by weight, particularly preferably 99.5% to 93% by weight, and most preferably 99.5% to 98.1% by weight. The content of the polyester-based elastomer in the layer is more preferably 0.5% to 15% by weight, more preferably 0.5% to 10% by weight, particularly preferably 0.5% to 7% by weight, and most preferably 0.5% to 1.9% by weight, based on 100% by weight of the entire polyamide layer.

By setting the polyamide content in the first layer within the above range, the pinhole resistance and puncture resistance due to bending of the polyamide-based film can be further improved.

By setting the content of the polyester-based elastomer in the first layer within the above range, the pinhole resistance of the polyamide-based film can be improved.

In the polyamide film of the present invention, the polyester elastomer contained in the polyamide layer functions as a flex resistance agent. The polyester elastomer is effective in improving the pinhole resistance of the polyamide film due to flexing, particularly in a freezing environment.

In the polyamide-based film of the present invention, the second layer is mainly composed of the aliphatic polyamide and does not contain a polyester-based elastomer.

Since the second layer does not contain a polyester-based elastomer, the puncture resistance of the polyamide-based film can be improved.

(3) Bending Resistance Agent In the polyamide-based film of the present invention, the first layer may contain a thermoplastic elastomer as a bending resistance agent in addition to the polyester-based elastomer.

Thermoplastic elastomers are thermoplastic materials that have rubber-like elasticity, and examples of such materials include polyamide-based elastomers, polyolefin-based elastomers, polystyrene-based elastomers, polyurethane-based elastomers, polyvinyl chloride-based elastomers, and ionomer polymers.

Among these, polyamide-based elastomers and polyolefin-based elastomers are preferred, with polyamide-based elastomers being more preferred, since they can further improve the pinhole resistance of polyamide-based films caused by bending.

These thermoplastic elastomers may be used alone or in combination of two or more.

The thermoplastic elastomer may be modified to the extent that the pinhole resistance due to bending of the polyamide film is not reduced. For example, it may be a modified product of the thermoplastic elastomer. Modification of the thermoplastic elastomer may be, for example, modification by copolymerization or graft modification, modification by adding a polar group, etc. Addition of a polar group may be performed by graft modification. Examples of such polar groups include epoxy groups, carboxyl groups, acid anhydride groups, hydroxyl groups, amino groups, oxo groups, etc. Polar groups can be added by one type or a combination of multiple types. Therefore, modified products to which polar groups are added include, for example, epoxy modified products, carboxy modified products, acid anhydride modified products, hydroxy modified products, amino modified products, etc. of thermoplastic elastomers.

Examples of polyamide elastomers include polyamide block copolymers that are made up of hard segments formed from polyamide components and soft segments formed from polyoxyalkylene glycol components.

The polyamide component constituting the hard segment of the polyamide block copolymer is selected from the group consisting of (1) lactams, (2) ω-aminoaliphatic carboxylic acids, (3) aliphatic diamines and aliphatic dicarboxylic acids, or (4) aliphatic diamines and aromatic dicarboxylic acids. Specific examples include lactams such as ε-caprolactam, aliphatic diamines such as aminoheptanoic acid, aliphatic dicarboxylic acids such as adipic acid, and aromatic dicarboxylic acids such as terephthalic acid.

Examples of the polyoxyalkylene glycol component that constitutes the soft segment of the polyamide block copolymer include polyoxytetramethylene glycol, polyoxyethylene glycol, and polyoxy-1,2-propylene glycol.

The melting point of polyamide block copolymers is determined by the type and ratio of hard segments made up of polyamide components and soft segments made up of polyoxyalkylene glycol components, but those in the range of 120°C to 180°C are usually used.

By using a polyamide block copolymer as a component of a laminated biaxially oriented polyamide film, it is effective in improving the flex fatigue resistance of the laminated biaxially oriented polyamide film, especially in low temperature environments.

Examples of polyolefin-based elastomers include resins obtained by graft-modifying or copolymerizing polyolefin resins with unsaturated carboxylic acids or their anhydrides.

Polyolefin resins that can be used as the base polymer for polyolefin-based elastomers include polymers selected from homopolymers of α-olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and 4-methyl-1-pentene, random copolymers of two or more of these, block copolymers of two or more of these, copolymers of ethylene and vinyl acetate, and copolymers of ethylene and ethyl (meth)acrylate, and other ethylene-polar monomer copolymers.

Specific examples of polyolefin resins that can be used as the base polymer include high-density polyethylene, medium-density polyethylene, high-pressure low-density polyethylene, linear low-density polyethylene (copolymer of ethylene and α-olefin having 3 or more carbon atoms), polypropylene (homopolymer, random polymer, block copolymer, etc.), poly-1-butene, poly-4-methyl-1-pentene, ethylene-vinyl acetate copolymer, etc. These may be produced using any catalyst system; for example, in the case of the linear low-density polyethylene, those produced using a metallocene catalyst or a multi-site catalyst can be used.

The polyolefin elastomer can be obtained by grafting an unsaturated carboxylic acid onto the polyolefin resin, which is the base polymer, or by copolymerizing an olefin with a small amount of unsaturated carboxylic acid. Examples of unsaturated carboxylic acids or their anhydrides used for grafting or copolymerizing include acrylic acid, methacrylic acid, fumaric acid, maleic anhydride, itaconic anhydride, and norbornene-2,3-dicarboxylic anhydride, with acid anhydrides being particularly preferred, and maleic anhydride being particularly preferred.

The polyolefin elastomer is preferably a maleic anhydride-modified ethylene copolymer. Examples of maleic anhydride-modified ethylene copolymers include maleic anhydride-grafted ethylene copolymers and maleic anhydride-ethylene copolymers, and a partially saponified ethylene-vinyl acetate copolymer modified with maleic anhydride can be preferably used.

In the polyamide film of the present invention, the polyamide layer containing the aliphatic polyamide and polyester elastomer preferably contains 80% to 99% by weight of the aliphatic polyamide, with the weight of the entire polyamide layer being 100% by weight, and the bending resistance agent, together with the polyester elastomer, preferably contains 20% to 0.5% by weight.

In the polyamide film of the present invention, the content of the other flex resistance agents in the polyamide layer, combined with the polyester elastomer, is 20% by weight to 1% by weight, which further improves pinhole resistance due to flexing and pinhole resistance due to repeated contact.

(4) Other Components The layer mainly composed of an aliphatic polyamide may contain other additives as necessary within the range that does not impair the effects of the present invention. Examples of other additives include antiblocking agents, lubricants, nucleating agents, antioxidants, etc.

(5) Layer structure The polyamide film of the present invention includes at least a first layer and a second layer adjacent to each other. As described above, the first layer and the second layer are layers mainly composed of the aliphatic polyamide, the first layer is a layer containing a polyester elastomer, and the second layer is a layer not containing a polyester elastomer.

Specific examples of the polyamide film of the present invention include a three-layer structure of first layer/second layer/first layer, or second layer/first layer/second layer, or a five-layer structure of first layer/second layer/first layer/second layer/first layer, or second layer/first layer/second layer/first layer/second layer.

The total thickness of the polyamide film of the present invention is preferably about 10 μm to 50 μm, more preferably 10 μm to 30 μm, and even more preferably 15 μm to 25 μm.

The thickness of the first layer is preferably 0.5 μm to 30.0 μm, more preferably 1.0 μm to 20.0 μm. The thickness of the second layer is preferably 0.5 μm to 30.0 μm, more preferably 1.0 μm to 20.0 μm.

The polyamide film of the present invention has a multilayer structure having at least the first layer and the second layer. In the multilayer structure, the outermost layers are referred to as the front layer and the back layer.

The polyamide film of the present invention is excellent in pinhole resistance due to bending and excellent resistance to puncture when used for packaging frozen foods that are transported at low temperatures. The polyamide film of the present invention has excellent stretchability and the film hardly breaks due to stretching. The polyamide film of the present invention is suitable for packaging food, portioning, packaging food that contains moisture, etc., and can be suitably used for packaging frozen foods that are transported at low temperatures.

[2] Method for Producing Polyamide Film The method for producing the polyamide film of the present invention is not particularly limited, and may be any of the conventionally known methods for forming a laminate.

One example of a manufacturing method for the present invention is to co-extrude the resin compositions of the layers of the polyamide-based film from a T-die onto a chill roll through which cooling water circulates, for example, in the order of second layer (surface layer)/first layer/second layer (back layer), to prepare a flat, multi-layered polyamide-based film.

The resulting polyamide film may be uniaxially or biaxially stretched (simultaneous biaxial stretching or sequential biaxial stretching). The stretching ratio is, for example, 2.5 to 4.5 times in the longitudinal direction (MD) and 2.5 to 5.0 times in the transverse direction (TD).

For example, in the case of sequential biaxial stretching, the film is stretched longitudinally 2.5 to 4.5 times using a roll stretching machine at 50°C to 80°C, then stretched transversely 2.5 to 5.0 times using a tenter stretching machine in an atmosphere of 80°C to 140°C, and then heat-treated using the same tenter in an atmosphere of 180°C to 220°C to obtain the desired film.

The polyamide film of the present invention may be uniaxially or biaxially stretched (simultaneous biaxial stretching or sequential biaxial stretching), and the resulting polyamide film may be subjected to a corona discharge treatment on both surfaces or on one surface, if necessary.

[3] Method for suppressing the occurrence of pinholes in a packaging bag The present invention includes the following method for suppressing the occurrence of pinholes in a packaging bag.

The method for suppressing the occurrence of pinholes in a packaging bag of the present invention includes the steps of:
A method for suppressing the occurrence of pinholes using a film used in food packaging bags, comprising:
The film is a polyamide-based film having at least a polyamide layer (first layer) containing an aliphatic polyamide and a polyester-based elastomer,
packaging a food product with the film;
After freezing and thawing the packaged food,
This is a method for suppressing the occurrence of pinholes in the packaging bag.

As described above, the first layer is a layer whose main component is an aliphatic polyamide, and further contains a polyester-based elastomer.

In the method of the present invention for suppressing the occurrence of pinholes in a packaging bag, it is preferable that the polyamide layer contains 80% to 99.5% by weight of an aliphatic polyamide and 20% to 0.5% by weight of a polyester-based elastomer.

In the method of the present invention for suppressing the occurrence of pinholes in a packaging bag, it is preferable that the aliphatic polyamide has a relative viscosity of 3.0 to _5.5 measured using a measurement method in accordance with JIS K6920-2:2009 under conditions of a sample concentration of 1.0 weight% and a temperature of 25°C using 96% _H2SO4 as a solvent.

In the method of the present invention for preventing pinholes from occurring in packaging bags, the film described above in [1] Polyamide-based film can be used as the film used in food packaging bags.

Manufacturing Method of Packaging Bags In a method for preventing the occurrence of pinholes using the film for food packaging bags of the present invention, the polyamide film of the present invention is processed into a bag shape to manufacture the packaging bag.

In the method of the present invention for suppressing the occurrence of pinholes in a packaging bag, a film is used for the food packaging bag, and the film is a polyamide-based film having at least a polyamide layer (first layer) containing an aliphatic polyamide and a polyester-based elastomer.

The packaging bag can be formed by an automatic filling and packaging machine or the like using a laminate film in which a sealant film is laminated on one side of the polyamide film of the present invention. The device for processing the polyamide film into a bag shape and automatically packaging food includes, but is not limited to, a horizontal pillow type packaging machine, a vertical pillow type packaging machine, a three-side seal packaging machine, a four-side seal packaging machine, a stick packaging machine, and the like.

Details of the polyamide film are as described above.

The method of the present invention for preventing pinholes from occurring in packaging bags can effectively prevent pinholes from occurring due to bending in packaging bags for frozen foods that are transported at low temperatures.

[4] Effects of polyamide films
(1) Puncture resistance (puncture strength)
The puncture resistance of the polyamide film of the present invention is a puncture strength, and is evaluated by a measurement method based on JIS Z-1707 (1997) as described in the Examples below. The puncture strength of the polyamide film of the present invention is preferably 8.0 N or more at 23° C. The puncture strength of the polyamide film of the present invention is preferably 13.0 N or more at −25° C.

(2) Impact resistance (impact strength)
The impact resistance of the polyamide film of the present invention is impact strength, and is evaluated using an impact tester as described in the Examples below. The impact strength of the polyamide film of the present invention is preferably 1.0 J or more under a condition of 23° C. The impact strength of the polyamide film of the present invention is preferably 0.5 J or more under a condition of −25° C.

The polyamide-based film of the present invention has excellent pinhole resistance due to bending and pinhole resistance due to repeated contact, and also exhibits excellent puncture resistance, even when used for packaging frozen foods that are transported at low temperatures. The polyamide-based film of the present invention has excellent stretchability and the film is hardly broken by stretching. The polyamide-based film of the present invention is suitable for packaging heavy items, serving food, rice cakes, sausages, and other foods, and can be suitably used for packaging frozen foods that are transported at low temperatures.

(3) Bending resistance (resistance to pinholes caused by bending)
The flex resistance of the polyamide film of the present invention is pinhole resistance due to flexing, and is evaluated using a Gelbo flex tester as described in the Examples below. In the polyamide film of the present invention, the number of pinholes generated after 1,000 flexes at 5°C using a ³⁰⁰ cm2 sample is preferably 3 or less. In the polyamide film of the present invention, the number of pinholes generated is preferably 7 or less under -5°C conditions. In the polyamide film of the present invention, the number of pinholes generated is preferably 25 or less under -25°C conditions.

The present invention will be described in more detail below with reference to examples and comparative examples.
The present invention is not limited to these examples.

(1) Raw material for polyamide film: Aliphatic polyamide 1: Nylon 6, relative viscosity 3.4
Aliphatic polyamide 2: Nylon 6, relative viscosity 4.1
Aliphatic polyamide 3: Nylon 6, relative viscosity 4.0
Polyester elastomer: Maleic anhydride modified polyester elastomer
(Mitsubishi Chemical's Tifa Block)

(2) Production of polyamide film Using the above-mentioned raw materials, resin compositions for forming each polyamide layer were prepared according to the formulations shown in Table 1. In addition to the formulations shown in Table 1, these resin compositions contained 900 ppm of silica with an average particle size of 3 μm as an antiblocking agent and 300 ppm of ethylene bisstearic acid amide as a lubricant in the outermost layers (front and back layers).

The resin compositions constituting each polyamide layer were fed into an extruder at 250°C and co-extruded from a T-die at 250°C onto a chill roll through which cooling water was circulating in the order of surface layer/middle layer/back layer to obtain a flat polyamide film with three polyamide layers.

In the layer structure of surface layer/middle layer/back layer, as in Examples 1 and 2, when the surface layer and back layer are the first layers, the middle layer is the second layer.

In the layer structure of surface layer/middle layer/back layer, as in Examples 3 and 4, when the surface layer and back layer are the second layers, the middle layer is the first layer.

This film was stretched longitudinally 3.0 times using a roll stretching machine at 65°C, then stretched transversely 4.0 times using a tenter stretching machine in an atmosphere of 110°C, and further heat-treated in an atmosphere of 210°C using the same tenter to obtain a polyamide-based film with three polyamide layers and a thickness of 15 μm.

The following evaluations were carried out using the polyamide film prepared above.

(3) Evaluation of polyamide films
(3-1) Puncture Resistance The puncture strength of the polyamide film was measured by a method in accordance with JIS Z-1707 (1997).

The test specimen was fixed and pierced with a semicircular needle with a diameter of 1.0 mm and a tip radius of 0.5 mm at a speed of 50 ± 5 mm per minute, and the maximum stress (piercing strength (N)) until the needle penetrated was measured. At least five test specimens were used, and the average was calculated to obtain the measurement result.

Puncture strength (at 23°C): A strength of 8.0N or more is considered good.

Puncture strength (at -25°C): A strength of 13.0N or more is considered good.

(3-2) Impact Resistance The impact strength (J) of the polyamide film was measured using an impact tester (manufactured by Toyo Seiki Seisakusho Co., Ltd.).

Impact strength (at 23°C): Strength is good if it is 0.9 J or more, and even better if it is 1.0 J or more.

Impact strength (at -25°C): Strength is good if it is 0.3J or more, and even better if it is 0.5J or more.

(3-3) Flex Resistance The pinhole resistance of the polyamide film due to bending was measured using a Gelbo Flex Tester manufactured by Rikagaku Kogyo Co., Ltd.

A polyamide film bag made into a cylindrical shape with a folded diameter of 150 mm and a length of 300 mm was attached to a Gelbo Flex Tester, and a 440° twist was applied at the first 88.9 mm, followed by a linear horizontal motion of 63.5 mm. The film was then subjected to 1,000 repetitions of linear bending motion at a test speed of 40 times/min under conditions of 5°C, -5°C and -25°C, and the number of pinholes was then counted using a penetrant. The number of pinholes was measured at a 300 ^cm2 location in the center of the sample. The number of pinholes was measured for three samples, and the average was taken as the measurement result.

Flex resistance (at 5°C): 5 or less pinholes is good, and 3 or less is even better.

Flex resistance (at -5°C): 7 or less pinholes is good, and 5 or less is even better.

Flex resistance (at -25°C): A pinhole count of 27 or less is good, and 25 or less is even better.

(3-4) Evaluation results

The polyamide film of the present invention (Example) has excellent puncture strength and impact strength compared to the conventional technology (Comparative Example), while also exhibiting significantly superior bending resistance in a freezing environment.

The polyamide-based film of the present invention exhibited excellent pinhole resistance due to bending and pinhole resistance due to repeated contact, as well as excellent puncture resistance, even when used for packaging frozen foods that are transported at low temperatures. The polyamide-based film of the present invention is suitable for packaging food, portioning, packaging foods that contain moisture, etc., and it has been evaluated that it can be suitably used for packaging frozen foods that are transported at low temperatures.

Claims (2)

A polyamide-based film having a layer mainly composed of an aliphatic polyamide and used for packaging bags for frozen foods ,
The layer mainly composed of an aliphatic polyamide further includes a first layer including a polyester-based elastomer and a second layer not including a polyester-based elastomer,
the second layer is disposed on either side of the first layer;
The first layer contains 78% to 99.5% by weight of an aliphatic polyamide and 0.5% to 22% by weight of a polyester-based elastomer;
the second layer contains 70% by weight or more of an aliphatic polyamide when the total weight of the resin components constituting the second layer is 100% by weight,
the first layer comprises an aliphatic polyamide of a type not included in the second layer;
the relative viscosity of the aliphatic polyamide in the first layer is lower than the relative viscosity of the aliphatic polyamide in the second layer;
The first layer and the second layer are adjacent to each other.
Polyamide film. A method for suppressing the occurrence of pinholes using a film used in packaging bags for frozen foods, comprising the steps of:
The film is a polyamide-based film having at least a first layer containing an aliphatic polyamide and a polyester-based elastomer, and a second layer containing an aliphatic polyamide but not a polyester-based elastomer,
the second layer is disposed on either side of the first layer;
The first layer contains 78% to 99.5% by weight of an aliphatic polyamide and 0.5% to 22% by weight of a polyester-based elastomer;
the second layer contains 70% by weight or more of an aliphatic polyamide when the total weight of the resin components constituting the second layer is 100% by weight,
the first layer comprises an aliphatic polyamide of a type not included in the second layer;
the relative viscosity of the aliphatic polyamide in the first layer is lower than the relative viscosity of the aliphatic polyamide in the second layer;
packaging a food product with the film;
After freezing and thawing the packaged food,
A method for suppressing the occurrence of pinholes in the packaging bag.