JP2017030242A - Polyethylene-based crosslinked shrinkable film and packaging bag for refrigerated or frozen noodle formed of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film capable of forming a packaging bag which can completely seal an object to be packaged during storage, allows a heat seal portion to be opened in such a degree as to let steam escape from the inside of a packaging bag when heated in a microwave oven, and allows the object to be packaged to be easily taken out therefrom after heating.SOLUTION: There is provided a polyethylene-based crosslinked shrinkable film having a heat seal layer containing a polyethylene-based resin. A heat seal strength when the heat seal layers are heat-sealed satisfies: (1) heat seal temperature of 120°C, and heat seal strength when heat-sealed at heat seal pressure of 0.65 MPa is 2-10 N/30 mm; and (2) heat seal temperature of 120°C, and heat seal strength when heat-sealed at heat seal pressure of 2.0 MPa is 12-20 N/30 mm.SELECTED DRAWING: None

Description

  The present invention relates to a polyethylene-based crosslinked shrink film and a refrigerated noodle or frozen noodle packaging bag made of the film.

  Conventionally, various types of plastic packaging containers have been developed and proposed for filling and packaging foods and drinks, pharmaceuticals, cosmetics, industrial members, chemical products, and other articles. In order to fill and package a plastic packaging container, it is necessary to seal the opening of the plastic packaging container with a film. And the request | requirement, such as protection of the quality of contents, such as food / beverage products, and the extension of a preservation | save period, requires that the film which seals an opening part is excellent in heat-sealability. It is also required that the film used for filling and packaging completely seals the contents.

  Examples of packaging methods for covering food products include household wrap packaging, overlap packaging, twist packaging, bag packaging, skin packaging, shrink packaging, stretch packaging, and pillow packaging. In particular, shrink wrapping, pillow wrapping, and top seal wrapping continuous wrapping machines have been in the development trend of higher speeds in recent years. Accordingly, films having various layer configurations and resin compositions have been proposed for the required characteristics of films used in continuous packaging.

  For example, Patent Document 1 uses a medium-density polyethylene and a high-pressure low-density polyethylene for the base material layer, and uses an ethylene-α-olefin copolymer for the heat seal layer. A heat-shrinkable multilayer shrink film for use in frozen noodles such as ramen and ramen and packaging bags for refrigerated noodles is disclosed.

JP 2010-094967 A

  However, in the case of a packaging bag in which frozen noodles are heat-sealed with a general-purpose packaging machine using the film disclosed in Patent Document 1, since the sealing strength is strong and difficult to open, it takes time to take out the contents. There is a case.

  In addition, some packaging bags such as frozen noodles and refrigerated noodles are cooked by heating directly in a microwave oven from the viewpoint of ease of cooking. Therefore, when heating in a microwave oven, the heat seal strength that opens the heat seal part to the extent that the steam can escape during heating in order to prevent the steam inside the packaging bag from expanding and greatly expanding and bursting It is required to have.

Accordingly, one of the problems to be solved by the present invention is that the packaged item can be completely sealed during storage, and the heat seal part opens to the extent that the vapor inside the packaging bag escapes during microwave heating, and is easy after heating. Another object of the present invention is to provide a film capable of forming a packaging bag that can be opened and taken out.
Another problem to be solved by the present invention is to provide a packaging bag made of the above film.

  As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by a polyethylene-based crosslinked shrink film having a specific range of heat seal strength, and have completed the present invention.

That is, the present invention provides the following polyethylene-based crosslinked shrink film and packaging bag.
[1] A polyethylene-based crosslinked shrink film having a heat seal layer containing a polyethylene-based resin,
A polyethylene-based crosslinked shrink film, wherein the heat seal strength when the heat seal layers are heat sealed satisfies the following (1) and (2).
(1) Heat seal strength when heat-sealed at a heat seal temperature of 120 ° C. and a heat seal pressure of 0.65 MPa is 2 to 10 N / 30 mm.
(2) The heat seal strength when heat-sealed at a heat seal temperature of 120 ° C. and a heat seal pressure of 2.0 MPa is 12 to 20 N / 30 mm.
[2] The polyethylene-based crosslinked shrink film according to [1], wherein the gel fraction of the heat seal layer is 15 to 50% by mass.
[3] A refrigerated noodle or frozen noodle packaging bag comprising the polyethylene-based crosslinked shrink film according to [1] or [2].

  According to the present invention, the object to be packaged can be completely sealed during storage, and when the microwave oven is heated, the heat seal portion is opened to the extent that the vapor inside the packaging bag is released, and after heating, the package is easily opened and taken out. A film capable of forming a packaging bag is provided. Moreover, the packaging bag which consists of the said film is provided.

A preferred embodiment of the present invention will be described below.
Hereinafter, an embodiment for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

[Polyethylene-based cross-linked shrink film]
The polyethylene-based crosslinked shrink film of the present embodiment has at least a heat seal layer, and may further have a base material layer and an intermediate layer. Moreover, the polyethylene type crosslinked shrink film of the present embodiment may be composed of a single layer of the heat seal layer or may be composed of a plurality of layers including the heat seal layer.
In the present specification, the polyethylene-based crosslinked shrink film may be simply referred to as “film”.

  It is essential that the film of this embodiment is cross-linked. Since the heat seal layer of the film is cross-linked, the flow of the resin on the heat seal surface is limited during heat sealing. For this reason, the maximum value of the heat seal strength does not become too large, and it becomes easy to adjust the heat seal strength to an appropriate value, and when the microwave oven is heated, the vapor is opened to prevent bursting of the packaging bag. Moreover, after microwave oven heating, a packaging bag can be easily opened from the opened part and the contents can be taken out easily.

(Physical properties of polyethylene-based crosslinked shrink film)
The film of this embodiment has a heat seal strength of 2 when the heat seal layers are heat sealed at a heat seal temperature of 120 ° C. and a heat seal pressure of 0.65 MPa (hereinafter referred to as “low pressure seal condition”). It is 10N / 30mm, Preferably it is 4-8N / 30mm.
If the heat seal strength under the low-pressure seal condition is less than 2 N / 30 mm, the heat seal portion is likely to open during transportation and storage, and if it exceeds 10 N / 30 mm, the heat seal portion may be difficult to open during microwave heating.
The heat seal strength under the low pressure seal condition can be adjusted by, for example, the gel fraction of the heat seal layer, the resin composition of the heat seal layer, and the like.

The film of this embodiment has a heat seal strength of 12 when the heat seal layers are heat sealed at a heat seal temperature of 120 ° C. and a heat seal pressure of 2.0 Mpa (hereinafter referred to as “high pressure seal condition”). It is -20N / 30mm, Preferably it is 13-18N / 30mm.
When the heat seal strength is less than 12 N / 30 mm, the airtightness during transportation and storage may be poor. If it exceeds 20 N / 30 mm, the film may burst during heating in a microwave oven by exceeding the pressure resistance of the film.
The heat seal strength under the high pressure seal condition can be adjusted by, for example, the gel fraction of the heat seal layer, the resin composition of the heat seal layer, and the like.

  The difference between the heat seal strength under the high pressure seal condition and the heat seal strength under the low pressure seal condition is preferably 0.5 to 2.0 MPa. Where the difference in heat seal strength between the high-pressure seal condition and the low-pressure seal condition is less than 0.5 MPa, where the opening by steam occurs when heating the microwave oven when the packaging bag is made under the high-pressure seal condition or the low-pressure seal condition It may be difficult to grasp.

  As for the polyethylene type crosslinked shrink film of this Embodiment, it is preferable that the gel fraction of the said heat seal layer is 15-50 mass%. When the gel fraction is 15 to 50% by mass, a film having excellent heat resistance is realized while maintaining an appropriate heat seal strength. The gel fraction is more preferably 20 to 45% by mass, still more preferably 25 to 45% by mass.

  The gel fractions of the layers other than the heat seal layer (base material layer, intermediate layer) of the polyethylene-based crosslinked shrink film of the present embodiment are each preferably 10 to 50% by mass, more preferably 15 to 45%. It is 20 mass%, More preferably, it is 20-40 mass%. Heat resistance at the time of microwave heating can be provided because a gel fraction is 10-50 mass%.

It is preferable that the gel fraction of the polyethylene type crosslinked shrink film of this Embodiment is 5-60 mass%, More preferably, it is 10-50 mass%, More preferably, it is 15-45 mass%. When the gel fraction is 5 to 60% by mass, the packaged item can be completely sealed during storage, and when the microwave oven is heated, the heat-sealed part is easy to open to the extent that the vapor inside the packaging bag is released, and easy after heating It is easy to obtain a heat seal strength that can be easily opened and taken out.
The gel fraction can be obtained by extracting a sample in boiling p-xylene for 12 hours and calculating the proportion of the insoluble portion by the following formula.
Gel fraction (mass%) = (mass of sample after extraction / mass of sample before extraction) × 100

(Heat seal layer)
The heat seal layer includes at least one polyethylene-based resin. Furthermore, an additive may be included.

-Polyethylene resin-
Polyethylene resins are classified by density according to JIS K 6922. In the classification, density 0.942 g / cm ³ or more polyethylene resin is high density polyethylene (HDPE), density of 0.930 g / cm ³ or more 0.942 g / cm ³ less than the polyethylene-based resin is, Medium density polyethylene (MDPE) is used, and polyethylene having a density of 0.910 g / cm ³ or more and less than 0.930 g / cm ³ is used as low density polyethylene (LDPE).
In addition, a polyethylene-type resin shows the copolymer of ethylene only, or the copolymer of ethylene and C3 or more alpha olefins. Examples of the α-olefin having 3 or more carbon atoms include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like.

  In the present embodiment, “density” means a value measured according to JIS K 6922. As a method for measuring the density, specifically, there is a method of measuring the density with a density gradient tube in accordance with JIS K 6922.

  Examples of the polyethylene resin contained in the heat seal layer include high density polyethylene (HDPE), high pressure method low density polyethylene, linear low density polyethylene, and the like. The said polyethylene resin may be used individually by 1 type, and may use 2 or more types together.

The high-density polyethylene is preferably a polyethylene resin having a density of 0.942 to 0.970 g / cm ³ from the viewpoint of having good rigidity and further preventing shrinkage during heating in the microwave oven. A 0.966 g / cm ³ polyethylene resin is more preferable, and a polyethylene resin having a density of 0.950 to 0.960 g / cm ³ is more preferable.

  The melt flow rate of the high-density polyethylene (hereinafter sometimes simply referred to as “MFR”) is preferably 0.2 to 7.0 g / 10 min, more preferably 0.5 to 6.0 g. / 10 minutes. When the melt flow rate of the high density polyethylene is 0.2 g / 10 min or more, it is preferable from the viewpoint that the strength of the film is further improved, and when it is 7.0 g / 10 min or less, further stability in the production process is obtained. This is preferable.

  In the present embodiment, the melt flow rate (MFR) is an index indicating the fluidity at the time of melting, and means a value measured according to JIS K 7210. As a method for measuring MFR, specifically, there is a method for measuring MFR with a melt indexer according to JIS K 7210.

  The high-pressure method low-density polyethylene is a low-density polyethylene produced by a high-pressure method, and is a polyethylene-based resin having a long-chain branch, in which ethylene of repeating units are randomly bonded with branches.

The density of the high-pressure process low-density polyethylene is preferably 0.910 to 0.929 g / cm ³ , more preferably 0.915 to 0.929 g / cm ³ . When the density of the high-pressure method low-density polyethylene is not less than the above lower limit, it is easy to impart rigidity to the film, and it is possible to further suppress the slackness of the film. Moreover, the cloudiness of a film can fully be maintained to a practical level as the density of a high pressure method low density polyethylene is below the said upper limit.

  As a method for producing the high-pressure method low-density polyethylene, generally known methods can be used. In general, high-pressure low-density polyethylene is produced by polymerizing ethylene and α-olefin in an autoclave or tube reactor in the presence of a free radical generator such as peroxide at a high temperature and high pressure of 100 to 300 ° C and 100 to 350 MPa. can do.

  The melt flow rate of the high-pressure low-density polyethylene is preferably from 0.1 to 5.0 g / 10 minutes, more preferably from 0.2 to 4.0 g / 10 minutes. A melt flow rate of 0.1 g / 10 min or more is preferable in that the strength of the film is further improved, and 5.0 g / 10 min or less is preferable in that further stability in the production process can be obtained.

  In the present embodiment, the linear low density polyethylene is preferably a copolymer of ethylene and an α-olefin having 3 to 18 carbon atoms. The α-olefin having 3 to 18 carbon atoms is preferably selected from propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene and the like.

The density of the linear low density polyethylene is preferably less than 0.910 g / cm ³ or more 0.930 g / cm ^3, more preferably 0.910~0.920g / cm ^3. When the density of the linear low density polyethylene is within the above range, the rigidity of the film becomes more suitable as a packaging film, and adhesion between the films can be prevented by appropriate surface roughness of the film. In addition, it is preferable that the adhesiveness between films is low, for example, in that a seal part in an automatic packaging machine is finished neatly.

  The melt flow rate of the linear low density polyethylene is preferably 0.1 to 10.0 g / 10 minutes, and more preferably 1.0 to 8.0 g / 10 minutes. A melt flow rate of 0.1 g / 10 min or more is preferable from the viewpoint of obtaining better film strength, and 10.0 g / 10 min or less is preferable from the viewpoint of obtaining stability in the production process.

  The polymerization catalyst used when producing the linear low density polyethylene is not particularly limited, and any of a multisite catalyst, a single site catalyst, and the like may be used.

  From the viewpoint that the heat seal layer is easily cross-linked, can be easily adjusted to an appropriate heat seal strength, and can be made into a film excellent in suitability for an automatic packaging machine, it is a high pressure method low density polyethylene, It is preferable to contain linear low density polyethylene and high density polyethylene.

  Specifically, the high-pressure method low-density polyethylene is 20 to 50% by mass, the high-density polyethylene is 30 to 60% by mass, and the linear low-density polyethylene is 10 to 40% by mass with respect to the total amount of the heat seal layer. It is preferable.

  When the high-pressure method low-density polyethylene is 20 to 50% by mass, the processability of the film can be improved and an appropriate heat seal strength can be imparted, and more preferably 25 to 45% by mass.

  By making high density polyethylene 30-60 mass%, it becomes possible to improve the slipperiness between films, without adding inorganic fillers, such as silica and talc, More preferably, it is 35-55 mass%. is there.

  By making the linear low density polyethylene 10 to 40% by mass, it is easy to obtain heat seal strength that opens the heat seal part to escape the steam inside the packaging bag during microwave heating, and heat that can withstand distribution and storage Seal strength can be obtained, and it is more preferably 15 to 35% by mass.

-Additives-
In the present embodiment, a glycerin fatty acid ester such as glycerin monooleate can be blended as the additive from the viewpoint of improving slipperiness and antifogging property.

  In the present embodiment, in order for the glycerin fatty acid ester to be present on the surface of the heat seal layer, it is preferable to set the temperature of the extruder to 250 ° C. or higher and knead with the polyethylene resin at a high shear rate. It is preferable to finely disperse the glycerin fatty acid ester. Bleedout is an important factor with different effects depending on the amount and manner of existence. As a way of existence, it is preferable that the glycerin fatty acid ester is present in the form of a layer, not a droplet, in a substantially continuous state on the surface of the heat seal layer.

  In this Embodiment, it is preferable to contain 0.1-5.0 mass% of glycerol fatty acid ester with respect to the heat seal layer whole quantity from a viewpoint of anti-fogging property and the slipperiness of a packaging machine and a film.

(Base material layer)
As said base material layer, the layer containing the said polyethylene-type resin etc. are mentioned, for example. Especially, the layer containing the said high density polyethylene and the said high voltage | pressure low density polyethylene is preferable.

  In the present embodiment, when the polyethylene-based crosslinked shrink film is a laminate having a multi-layer structure, it is preferable to include a heat seal layer and a base material layer, and the heat seal layer (I) and the base material layer (II As long as the heat seal layer (I) is at least one outermost layer of the laminate, the arrangement is not particularly limited. As a structure of the said laminated body, for example, in the case of two layers consisting of a heat seal layer (I) and a base material layer (II): (I) / (II), both surface layers are from the heat seal layer (I) In the case of three layers: (I) / (II) / (I) and the like.

  In this Embodiment, the thickness of a polyethylene type crosslinked shrink film becomes like this. Preferably it is 5-40 micrometers, More preferably, it is 8-30 micrometers. If the thickness of the film is in the range of 5 to 40 μm, it is difficult for tearing to occur for packages to be packaged including heavy objects and protrusions, and it is possible to produce a packaging bag stably. The thickness of the film can be adjusted to a desired value depending on the discharge amount or stretching ratio of each layer extruder during production.

  In the film of the present embodiment, when it is a laminate, the ratio of the total thickness of the heat seal layer (I) is 5 to 50% with respect to the thickness of the entire film layer in terms of film strength. Is more preferable, and 10 to 30% is more preferable.

[Method for producing polyethylene-based crosslinked shrink film]
Examples of the film production method of the present embodiment include a direct inflation method, a double bubble inflation method, a triple bubble inflation method, and a tenter method.

In the inflation method, for example, a predetermined resin composition melt-kneaded using an extruder is extruded with an annular die, and rapidly cooled with cooling water to collect an unstretched original fabric. The method of extrusion is not particularly limited, and a method using a multi-layer T die, a method using a multi-layer circular die, and the like can be adopted, and among these, a method using a multi-layer circular die is preferable.
Next, this raw fabric is subjected to a crosslinking treatment, and then the raw fabric is heated to a melting point or higher by heat transfer heating with hot air or radiation heating such as an infrastructure heater, and then the raw fabric is given a speed ratio between two nip rolls. While stretching in the flow direction (MD), air is injected into the tube to stretch in the vertical direction (TD).

The polyethylene-based crosslinked shrink film of the present embodiment needs to be crosslinked.
Generally known methods can be used for the crosslinking treatment. As a method of crosslinking treatment, for example, a method of adding a crosslinking agent and heating to a temperature equal to or higher than the decomposition temperature of the crosslinking agent to carry out crosslinking, ionizing radiation such as α rays, β rays, γ rays, neutron rays, electron beams, etc. The method of irradiating is mentioned.
By performing the crosslinking treatment, the haze and gloss after shrinkage of the film can be improved. The crosslinking treatment also has an aim of preventing tearing due to melting of the film when it is shrunk by heating above the melting point of the resin constituting the film.

  Since the polyethylene type crosslinked shrink film of the present embodiment is appropriately crosslinked, stable stretching can be performed even at a temperature equal to or higher than the melting point of the resin constituting the film. That is, by crosslinking, the stretching temperature and the stretching ratio can be easily adjusted, and a film having a low heat shrinkage stress can be produced. In addition, most of the film does not shrink in the heat seal temperature range, and a stable heat seal property can be exhibited.In the temperature range of the hot air shrink tunnel, the optimum heat shrinkage rate and heat shrink stress for shrink wrapping are achieved. It becomes possible to have.

  In this Embodiment, it is preferable to implement a crosslinking process so that the gel fraction of a heat seal layer may be 15-50 mass%. Moreover, it is more preferable to implement a crosslinking process so that the gel fraction of a heat seal layer may be 20-45 mass%, More preferably, it is 25-45 mass%.

  When the crosslinking treatment is performed by irradiation with ionizing radiation, the irradiation dose is preferably 40 to 120 kGy in consideration of mechanical unevenness. Since the film tends to have better haze and gloss after heat shrinkage, the irradiation dose is preferably 40 kGy or more, and the heat shrinkage stress tends to be more suitable for practical use, so the irradiation dose is 120 kGy or less. Is preferred. In addition, since the relationship between the degree of irradiation and the degree of crosslinking differs depending on the type of resin, it is desirable to appropriately adjust the irradiation dose and the irradiation voltage so that the above-described preferable gel fraction can be obtained.

[Packaging bag]
The packaging bag of this Embodiment is a packaging bag which consists of a polyethylene type crosslinked shrink film of this Embodiment, and it is preferable to heat seal the above-mentioned polyethylene type crosslinked shrink film to make a packaging bag. Moreover, the packaging bag of this Embodiment may package the to-be packaged object using the above-mentioned heat shrink multilayer film.

In the packaging bag of the present embodiment, examples of a method for packaging an article to be packaged include pillow packaging, stretch packaging, and the like, and it is particularly preferable that three directions are sealed by a pillow packaging machine.
Examples of the pillow packaging method include a method in which both ends of the film are sealed together to form a cylinder, and a package is put in the cylinder and sealed before and after to obtain individual packaging bags. It is done.

In the packaging bag of this embodiment, as a method of sealing the polyethylene-based crosslinked shrink film, a method of sealing the film while sandwiching it with a roller-shaped center sealer that rotates in a different direction toward the traveling direction can be mentioned. Then, it cut | disconnects, applying a pressure with the end sealer heated from the top and bottom of the film.
In general, a center seal portion (portion that seals both ends of a film together) in an automatic packaging machine is heat sealed in a state of being sandwiched between disc-shaped heat sealers that rotate in different directions. On the other hand, since the pressure is applied with a tough spring at the time of sealing at the end seal portions orthogonal to each other, the pressure tends to be higher than that at the center seal portion.
When a dish is heated for a predetermined time in a microwave oven with the center seal portion having a low heat seal strength facing the top surface, the end seal portion having a high heat seal strength is not opened, so that the contents are not easily spilled.
In order to release steam from a specific seal part during microwave heating, which is the object of the present invention, a heat seal method in which the seal strength can be adjusted by temperature and pressure is preferable.

Examples of the heat seal temperature of the center seal part include 100 to 150 ° C. Moreover, as a heat seal pressure of the said center seal part, 0.3-0.8 Mpa etc. are mentioned, for example.
As a heat seal temperature of the said end seal part, 100-150 degreeC etc. are mentioned, for example. Moreover, as a heat seal pressure of the said end seal part, 0.8-2.3MPa etc. are mentioned, for example.

  The package is not particularly limited, but frozen noodles such as ramen and udon or refrigerated noodles are suitable.

As an example of a process for obtaining a packaging bag using the polyethylene-based crosslinked shrink film of the present embodiment as a packaging film, a method for continuously packaging an article to be packaged in pillow packaging will be described.
The packaging speed of the continuous wrapping machine is such that about 20 wraps per minute, but some recent high-speed continuous wrapping machines wrap about 30 to 100 per minute. Therefore, the packaging film is strongly required to have suitability for the packaging speed, for example, slipperiness, heat sealability, and heat shrinkage characteristics. The polyethylene-based crosslinked shrink film of the present embodiment is a film excellent in slipperiness, heat sealability, and heat shrinkage characteristics as a packaging film.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment.

  Hereinafter, the present embodiment will be described more specifically with reference to examples and comparative examples. However, the present embodiment is not limited to only these examples. The evaluation method used in the present embodiment is as follows.

[Gel fraction]
In boiling p-xylene, the film or the heat seal layer taken out from the film was extracted for 12 hours, and the proportion of the insoluble portion was calculated by the following formula. Used as a measure of the degree of crosslinking of the film.
Gel fraction (mass%) = (mass of sample after extraction / mass of sample before extraction) × 100

[Heat seal strength]
A sample for heat seal strength measurement was prepared using a heat sealer TP-701 manufactured by Tester Sangyo Co., Ltd. The film was heat-sealed with the heat seal layer surfaces of the film facing each other.
(Heat seal condition)
・ Heat seal bar: width 5mm, length 300mm (uses no surface irregularities)
・ Heat seal temperature: 120 ℃
Heat seal time: 0.3 seconds Heat seal pressure: 0.65 MPa, 2.0 MPa
・ Specimen width: 30 mm
The sample prepared under the above conditions was cut into a 30 mm width, and the sample was placed so that the seal part of the test piece was at the center of 10 mm length between chucks using Autograph (trade name) AG-I manufactured by Shimadzu Corporation. The seal part was peeled off at a tensile speed of 200 mm / min, and the strongest peel strength at each seal pressure was defined as the heat seal strength.

[Microwave aptitude]
The obtained film is slit to a width of 300 mm, and the temperature of the center seal part is set to 120 ° C. and the temperature of the end seal part is set to 120 ° C. using “FSP-1561N (trade name)” manufactured by Ibaraki Seiki Co., Ltd. The frozen sealing udon (200 g) is packaged at a rate of 100 pieces / minute with the heat sealing pressure of the center sealing part lower than the heat sealing pressure of the end sealing part, and 30 pieces are used for each example and comparative example. A packaging bag was obtained.
In a microwave oven (National, NE-EH21A), install one packaging bag so that the center seal part is on the top, set the output to 500 W, and set the heating time to 3 minutes and 30 seconds. Each of the 30 packaging bags was heated, and the suitability of the microwave oven was evaluated according to the following criteria.
(Evaluation criteria)
A: The center seal part of the packaging bag was peeled off in less than 2 minutes due to the pressure of water vapor generated during heating in all the packaging bags, and the steam was released.
B: At least one packaging bag took 2 minutes or more and less than 3 minutes until the center seal part was peeled off by the pressure of water vapor generated during heating and the release was performed.
C: At least one packaging bag did not peel off the center seal part for 3 minutes or more, the packaging bag expanded, and sufficient vapor release was not performed.

[sealability]
The obtained film is slit to a width of 300 mm, and the temperature of the center seal part is set to 120 ° C. and the temperature of the end seal part is set to 120 ° C. using “FSP-1561N (trade name)” manufactured by Ibaraki Seiki Co., Ltd. The frozen sealing udon (200 g) was packaged at a rate of 100 pieces / minute with the heat sealing pressure of the center sealing part lower than the heat sealing pressure of the end sealing part. A packaging bag was obtained.
After fixing each packaging bag to a fastness tester (trade name “RT-200”, manufactured by Daiei Scientific Research Laboratories) with the center seal part facing up and repeatedly vibrating for 5 minutes at a speed of 50 reciprocations / minute The state of the seal part was confirmed and the sealing performance was evaluated.
A: All the packaging bags were not opened, and the sealing property could be maintained.
B: The seal part of one packaging bag opened.
C: The seal part of two or more packaging bags opened.

Resins and additives used in Examples and Comparative Examples are as follows.
(Polyethylene resin)
HD: high density polyethylene (density = 0.952 g / cm ³ , MFR = 0.8 g / 10 min), melting point = 133 ° C.
LD: high pressure method low density polyethylene (density = 0.922 g / cm ³ , MFR = 0.2 g / 10 min), melting point = 110 ° C.
LL: single-site linear low density polyethylene (α-olefin comonomer = 1-hexene, density = 0.913 g / cm ³ , MFR = 2.0 g / 10 min), melting point = 112 ° C.
(Additive)
Ad: Glycerol monooleate

In the examples and comparative examples, films were generally produced by the following method.
[Film Production Method]
Resin for forming a film was supplied to the extruder, and in the case of multiple layers, in each extruder, mixing and melting were performed while injecting a predetermined amount of a predetermined additive with an injection pump. Each of the mixed and melted resin compositions was supplied to an annular die, laminated with the die, and coextruded. Immediately below the annular die, the molten resin discharged from the die was quenched with cooling water while forming the first bubble. After quenching, it was pinched with a pinch roll, and an unstretched original fabric was collected.
This original fabric was adjusted to have a desired thickness and a layer ratio shown in Table 1. This original fabric was subjected to a crosslinking treatment using an electron beam irradiation apparatus having an acceleration voltage of 500 kV. At this time, adjustment was performed so that the gel fraction of each layer was within a desired value. By heating the processed raw fabric in a heating furnace maintained at an atmospheric temperature of 170 ° C., stretching in the flow direction by a speed ratio between two nip rolls, and injecting air into the tube Stretched in the direction perpendicular to the machine flow direction. After extending | stretching, it cooled by applying cold wind from the air ring to the largest diameter part of the bubble. Thereafter, it was folded to obtain a film.

Below, each Example and a comparative example are explained in full detail.
[Example 1]
A resin composition containing 30% by mass of HD, 49% by mass of LD, 20% by mass of LL, and 1.0% by mass of Ad was extruded using a cyclic die.
Thereafter, it was cooled and solidified with cooling water, and a tube-shaped stretched original fabric having a width of 130 mm and a thickness of 550 μm and a uniform thickness accuracy was collected. Then, this stretched raw fabric is guided to an electron beam irradiation apparatus of 500 kV, subjected to crosslinking treatment with an absorbed dose of 100 kGy, heated in a heating furnace maintained at an atmospheric temperature of 170 ° C., and the speed between the two sets of nip rolls. The film was stretched 6.0 times in the flow direction (MD) by the ratio and 6.2 times in the vertical direction (TD) by injecting air into the tube to obtain a film having a thickness of 15 μm.
The evaluation results of the obtained film are shown in Table 1. Using the resulting film, the frozen udon is packaged with an automatic packaging machine and heated with a microwave oven. It opens from the center seal, and the contents can be easily removed even after the heating is complete. It was a film.

[Examples 2 to 4]
A film having a thickness of 15 to 20 μm was obtained in the same manner as in Example 1 except that the resin for forming each layer, its ratio, and the thickness of the film were changed as shown in Table 1. In addition, the draw ratio was made the same as Example 1, and the final thickness of the film was adjusted suitably by changing the thickness of the tube-shaped extending original fabric before extending | stretching.
The evaluation results of the obtained film are shown in Table 1. In both cases, using the obtained film, wrapping frozen udon with an automatic packaging machine and heating it with a microwave oven, it opens from the center seal part, and the contents can be easily taken out after heating, making it practical. It was an excellent film.

[Comparative Example 1]
A film having a thickness of 15 μm was obtained in the same manner as in Example 1 except that the resin for forming each layer and the ratio thereof were changed as shown in Table 1.
The evaluation results of the obtained film are shown in Table 1. Since the obtained packaging bag had a large amount of linear low-density polyethylene used for the heat seal layer, the overall seal strength was high, and it took time to release the vapor during heating in the microwave oven.

[Comparative Example 2]
A film having a thickness of 15 μm was obtained in the same manner as in Example 1 except that the resin for forming each layer and the ratio thereof were changed as shown in Table 1 and the irradiation dose was changed to 10 kGy.

[Comparative Example 3]
A resin having a thickness of 15 μm was obtained in the same manner as in Example 1 except that the resin for forming each layer and the ratio thereof were changed as shown in Table 1 and the irradiation dose was changed to 50 kGy.
The evaluation results of the obtained film are shown in Table 1. In Comparative Examples 2 and 3, since the gel fraction of the film is low and the degree of crosslinking is too low, the fluidity of the resin in the heat seal layer is increased and the seal strength is too strong, and steam release is performed during microwave heating. Ruptured from the side of the packaging bag.

[Comparative Example 4]
A resin having a thickness of 15 μm was obtained in the same manner as in Example 1 except that the resin for forming each layer and the ratio thereof were changed as shown in Table 1 and the irradiation dose was changed to 250 kGy.
The evaluation results of the obtained film are shown in Table 1. Since the gel fraction of the film was high and the degree of crosslinking was too high, the fluidity of the resin in the heat seal layer was lowered, resulting in too weak seal strength, and steam release during microwave heating could be performed. In the test, the seal part opened.

[Comparative Example 5]
A resin having a thickness of 15 μm was obtained in the same manner as in Example 1 except that the resin for forming each layer and the ratio thereof were changed as shown in Table 1 and the irradiation dose was changed to 140 kGy.
The evaluation results of the obtained film are shown in Table 1. Since the gel fraction of the film was high and the degree of crosslinking was too high, the fluidity of the resin in the heat seal layer was lowered, resulting in too weak seal strength, and steam release during microwave heating could be performed. In the test, the seal part opened.

  The polyethylene-based crosslinked shrink film of the present invention has a heat-sealed portion that opens when heated in a microwave oven, and it is possible to moderately suppress expansion during heating by letting steam escape, so microwave cooking of frozen noodles and refrigerated noodles It can be suitably used as a packaging material that can cope with the above.

Claims (3)

A polyethylene-based crosslinked shrink film having a heat seal layer containing a polyethylene-based resin,
A polyethylene-based crosslinked shrink film, wherein the heat seal strength when the heat seal layers are heat sealed satisfies the following (1) and (2).
(1) Heat seal strength when heat-sealed at a heat seal temperature of 120 ° C. and a heat seal pressure of 0.65 MPa is 2 to 10 N / 30 mm.
(2) The heat seal strength when heat-sealed at a heat seal temperature of 120 ° C. and a heat seal pressure of 2.0 MPa is 12 to 20 N / 30 mm.   The polyethylene type crosslinked shrink film according to claim 1 whose gel fraction of said heat seal layer is 15-50 mass%.   A packaging bag for refrigerated noodles or frozen noodles comprising the polyethylene-based crosslinked shrink film according to claim 1 or 2.