JP2015093885A - Polyethylene-based crosslinked shrink film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyethylene-based crosslinked shrink film having sufficiently practical rigidity and heat resistance even with a thin thickness in which one surface is excellent in heat sealability and the other surface is excellent in wear resistance and heat resistance.SOLUTION: There is provided a polyethylene-based crosslinked shrink film which is composed of at least one layer and has one outermost surface A and the other outermost surface B, where when the surface roughness of the surface A observed by an atomic force microscope is defined as RaA and the surface roughness RaB of the surface B is defined as RaB, RaA is 4.0 to 10.0 and RaB is 3.0 to 5.0 and the relation between RaA and RaB satisfies the following expression (1): 1.1≤RaA/RaB≤3.0 (1).

Description

  The present invention relates to a polyethylene-based crosslinked shrink film.

  The shrink film for packaging (shrink film) can quickly and tightly package a plurality of products at the same time, regardless of the shape and size of the package. In addition, the resulting packaging has a beautiful appearance, exhibits a display effect, enhances the value of the product, keeps the contents hygienic, and makes it easy to visually check the quality. ing.

  As a packaging method using such a shrinkable film for packaging, a method is generally used in which the film is heat-shrinked with hot air after the contents are primarily packaged with some allowance for the film. This is a representative example. In this method, generally, a food or other packaged object stored in a container or tray is covered with a film in a cylindrical shape, and then a sealing wire is formed on the back surface of the packaged object by a center seal device such as a rotary roller type. Gaze heat-sealing so that it comes. Subsequently, both open ends of the tubular film are heat-sealed to form a bag, and heated by hot air in a box called a shrink tunnel, and this is heated while degassing the air from the holes provided in advance. Shrink. In addition to the above, this pillow shrink packaging includes a three-sided seal and a method of heating a four-side sealed bag-like film.

  Main examples of packages to be subjected to such pillow shrink wrapping are heat-resistant containers such as made of polystyrene or filled polypropylene (PP) with lids containing lunch boxes or side dishes, without lids containing meat or fish Examples include trays made of expanded polystyrene, PP, paper, etc. In any case, containers and trays are loosely wrapped with sufficient margins, and then hot air is blown to shrink to reduce the remaining corners. A beautiful package is obtained.

In recent years, it has been desired to reduce the heating temperature during shrink wrapping in order to reduce carbon dioxide emissions and reduce costs by increasing the speed of the packaging line. Shrink film shrinkage characteristics are as low as possible. High shrinkage is preferable.
However, in the packaging of a packaged object such as the above-mentioned lunch box or side dish, reheating in a microwave oven is often performed after packaging, and depending on the temperature, the container may become soft due to heat. Therefore, when a package packaged using a heat-shrinkable film is reheated in a microwave oven, the container may be deformed due to the shrinkage of the film.
In addition, since containers and trays are becoming thinner from the viewpoint of resource saving, the containers are easily deformed by reheating. Therefore, from the viewpoint of preventing container deformation, the shrinkage characteristic of the packaging film is preferably low shrinkage at 100 ° C. or less in consideration of the temperature during reheating.

On the other hand, it is also desired to reduce the thickness of the film itself, and the packaging machine suitability (sealability and pinhole resistance) and transport suitability (scratch resistance) equivalent to those of conventional thickness films are required.
When packaging lunch boxes, side dishes, etc. in pillow packaging, a generally used pillow wrapping machine pulls both ends of the film about 10% in the width direction during primary packaging of the body to be wrapped. The end of the film is guided and heated continuously by being sandwiched between a pair of heat rollers to seal the palms together (sealing between the inner surfaces of the package). For this reason, it is desirable that the film is cross-linked in order to prevent the film from being melted and wound around the heat roller.

  For example, Patent Documents 1 and 2 disclose a crosslinked multilayer shrink film using an ethylene-α-olefin copolymer.

Japanese Patent No. 5025412 JP 2007-118576 A

By the way, in the case of packaging with a thin film (for example, the film thickness is less than 9 μm), the thinner the film is, the more the outer surface of the film (the surface that becomes the outside of the packaging body) when guided to the bottom of the packaging body in the primary packaging. In many cases, fine scratches are caused by rubbing against metal parts, and in the subsequent shrinking process, a melt hole is generated starting from the scratches, and a phenomenon of pinhole formation is likely to occur.
Therefore, when pillow packaging is performed using a thin film, it is necessary to solve the difficult problem that the inner surface side of the package requires stable heat sealability and the outer surface side requires wear resistance and heat resistance. Is required.

In order to obtain wear resistance and heat resistance, for example, a crosslinking treatment may be applied to a polyethylene resin. On the other hand, when cross-linking occurs between molecules, the fluidity of the molecules decreases at the time of sealing, which may reduce the heat sealability.
Here, in the film of Patent Document 1, the acceleration voltage at the time of electron beam irradiation is high, the degree of cross-linking of the surface on the side that becomes the heat seal surface, and the cross-linking of the other surface that requires wear resistance and heat resistance. The degree is expected to be the same. Therefore, for example, when a thin film having a thickness of less than 9 μm is used, it is difficult to achieve both the sealability of the heat seal surface and the wear resistance and heat resistance of the opposite surface. On the other hand, in the relationship between the film thickness of the film and the acceleration voltage disclosed in Patent Document 2, the acceleration voltage is low, the surface on the heat seal surface side is almost non-crosslinked, and due to the orientation difference in the degree of crosslinking between both surfaces. It is thought that the film curls and causes a conveyance failure during packaging.

  An object of the present invention is to provide a polyethylene-based crosslinked shrink film in which one surface is heat-sealable and the other surface is excellent in wear resistance and heat resistance, and has a sufficiently practical rigidity and heat resistance even in a thin wall. .

  As a result of intensive studies to achieve the above-mentioned problems, the present inventors have found that, in a polyethylene-based crosslinked shrink film, if one film surface is low-crosslinked, the surface roughness observed by an atomic force microscope is large and heat It was found that if the other surface is highly cross-linked with excellent sealing properties, the surface roughness is small, a smooth surface state is obtained, and the wear resistance is excellent, leading to the present invention.

That is, the present invention is as follows.
[1] A polyethylene-based cross-linked shrink film comprising at least one layer and having a surface A as one outermost surface and a surface B as the other outermost surface, the surface A being observed by an atomic force microscope When surface roughness is RaA and surface B is RaB, RaA is 4.0 to 10.0, RaB is 3.0 to 5.0, and the relationship between RaA and RaB is as follows. A polyethylene-based crosslinked shrink film satisfying the formula (1).
1.1 ≦ RaA / RaB ≦ 3.0 (1)
[2] The gel fraction of the surface layer SA having at least three layers and having the surface A is 0.5 to 5.0% by mass, and the gel fraction of the surface layer SB having the surface B is 5.0 to 15.0. The polyethylene-based crosslinked shrink film of [1], which is% by mass.
[3] The polyethylene-based crosslinked shrink film of [1] or [2], wherein the gel fraction of the entire film is 10 to 35% by mass.
[4] A method for producing a polyethylene-based crosslinked shrink film according to any one of [1] to [3], wherein an electron beam is applied at an accelerating voltage that satisfies the following formulas (2) and (3) with respect to the original fabric before stretching: And a step of performing a crosslinking treatment, and a step of stretching the original fabric at a melting peak temperature or higher of the resin or resin composition constituting the original fabric.
T × D = α (2)
α + 10 ≦ V ≦ α + 70 (3)
[In the above formulas (2) and (3), T represents the thickness of the original fabric (μm), D represents the average density of the original fabric (g / cm ³ ), and V represents the acceleration voltage (kV)]
[5] The polyethylene-based crosslinked shrink film of any one of [1] to [3] is folded back so that the surface A is on the inner side, and the surfaces A facing each other are sealed in at least three directions to shrink the contents. Wrapped package.

  According to the polyethylene-based crosslinked shrink film of the present invention, one surface is excellent in heat sealability, and the other surface is excellent in wear resistance and heat resistance. A shrink film can be provided. Further, according to the present invention, since the film can be thinned, waste can be reduced, it is excellent in environmental suitability, and is effective for global warming countermeasures.

It is a schematic cross section of the polyethylene type crosslinked shrink film (one layer) of this embodiment. It is a schematic cross section of the polyethylene type crosslinked shrink film (three layers) of this embodiment.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as the present embodiment) will be described. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

The polyethylene-based crosslinked shrink film according to this embodiment is a polyethylene-based crosslinked shrink film comprising at least one layer and having a surface A that is one outermost surface and a surface B that is the other outermost surface, When the surface roughness of surface A observed by a force microscope is RaA, and the surface roughness of surface B is RaB, RaA is 4.0 to 10.0, RaB is 3.0 to 5.0, and , RaA and RaB are films satisfying the following formula (1).
1.1 ≦ RaA / RaB ≦ 3.0 (1)

  FIG. 1 is a schematic cross-sectional view of a polyethylene-based crosslinked shrink film of this embodiment when the film is a single layer. As shown in FIG. 1, the polyethylene type crosslinked shrink film 1 has a surface A (2a) which is one outermost surface and a surface B (3a) which is the other outermost surface.

  The polyethylene-based crosslinked shrink film 1 of the present embodiment is low-crosslinked because the surface A side is used as a seal surface. Moreover, since the surface B side used as the back surface when the surface A side is made into the surface is used so that it may become an outer side when the polyethylene-type crosslinked shrink film 1 is used as a package, it is highly bridge | crosslinked than the surface A side.

The atomic force microscope can evaluate the uneven shape of the sample surface by scanning the surface of the sample with a probe attached to the tip of the cantilever and measuring the vertical displacement of the cantilever. When the surface of the crosslinked and stretched film is observed with an atomic force microscope, a network-like molecular orientation-like shape is observed, but the higher the cross-linking, the denser the molecular orientation-like network. The lower the cross-linking, the less dense the network is observed.
At this time, the information in the height direction can be expressed as the surface roughness parameter (Ra). However, the higher the bridge, the smoother the network becomes flat by stretching, and the Ra value becomes smaller. Moreover, since the lower the cross-linking, the less affected by stretching, the surface is less likely to be leveled, and the Ra value increases. That is, the Ra value can be used as an index of the degree of crosslinking on the surface of the polyethylene-based crosslinked shrink film.

  RaA which is the surface roughness of the surface A observed by an atomic force microscope is 4.0 or more from the viewpoint of sealing properties and 10.0 or less from the viewpoint of transparency. RaA becomes like this. Preferably it is 4.5-9.5, More preferably, it is 4.5-9.0, More preferably, it is 5.0-8.5.

  The RaB, which is the surface roughness of the surface B observed with an atomic force microscope, is 3.0 or more from the viewpoint of stretchability and 5.0 or less from the viewpoint of suppressing rubbing and tearing. RaB becomes like this. Preferably it is 3.3-4.8, More preferably, it is 3.5-4.7, More preferably, it is 3.7-4.6.

In addition, when the difference in the degree of cross-linking between the front and back of the film is too large, the orientation balance of the film is deteriorated, so that the film curls with respect to the weakly oriented side (low cross-linked side). From the viewpoint of preventing curling of the film, the upper limit of RaA / RaB, which is the ratio of RaA to RaB, is 3.0 or less. Further, from the viewpoint of the sealing performance of the surface A and the scuff resistance of the surface B, the lower limit value of RaA / RaB is 1.1 or more. RaA / RaB is preferably 1.15 to 2.9, more preferably 1.2 to 2.8, and still more preferably 1.25 to 2.7.

The polyethylene-based crosslinked shrink film 1 of the present embodiment is preferably a resin or resin composition containing a polyethylene-based resin having a density of 0.900 to 0.930 g / cm ³ and formed into a film shape.

Among polyethylene resins, an ethylene-α-olefin copolymer is preferable because of its excellent mechanical strength. If the density of the ethylene-α-olefin copolymer is 0.900 g / cm ³ or more, it is preferable because the film tends to rise and the running property in a packaging machine tends to improve, and 0.930 g / cm. If it is ³ or less, it is preferable because the shrinkage rate near 110 ° C. or more tends to be improved. The density of the ethylene-α-olefin copolymer is more preferably 0.907 to 0.928 g / cm ³ , further preferably 0.910 to 0.926 g / cm ³ , and most preferably 0.910 to 0. 920 g / cm ³ .

  Here, the ethylene-α-olefin copolymer refers to a copolymer obtained by copolymerizing at least one monomer selected from ethylene and an α-olefin having 3 to 18 carbon atoms.

  Examples of the α-olefin used as the monomer of the ethylene-α-olefin copolymer include propylene, butene-1, pentene-1, 4-methyl-pentene-1, hexene-1, octene-1, and decene-1. , Dodecene-1 and the like.

  The polymerization catalyst used for producing the ethylene-α-olefin copolymer is not particularly limited, and examples thereof include a multisite catalyst and a single site catalyst.

  As long as the ethylene-α-olefin copolymer is within the above-specified density range, one or two or more types having different densities or comonomer types may be mixed and used depending on the purpose. Since ethylene-α-olefin copolymers generally have low melt tension, it is considered difficult to use alone as a layer constituent material from the viewpoint of extrusion stability of unstretched tubes or sheets. It is practiced to adjust the melt tension of the layer constituent material by mixing with a high-pressure low-density polyethylene or ethylene-vinyl acetate copolymer having a high tension. However, since the high pressure method low density polyethylene and the ethylene-vinyl acetate copolymer have lower tear strength than the ethylene-α-olefin copolymer, the higher the compounding ratio of the high pressure method low density polyethylene, The tear strength tends to decrease.

  In this embodiment, it is preferable that the resin or resin composition constituting the polyethylene-based crosslinked shrink film 1 contains 50 to 95% by weight of an ethylene-α-olefin copolymer. The lower limit of the content ratio of the ethylene-α-olefin copolymer is preferably 60% by weight, more preferably 70% by weight. By using in such a content ratio, extrudability, extrusion moldability, and stretching stability can be obtained, and tear strength is improved, and troubles such as tear propagation during packaging tend to be reduced.

  Moreover, if it is the range which is 0 to 30 weight% of resin or resin composition which comprises the polyethylene-type crosslinked shrink film 1, unless the transparency of a film is impaired, polypropylene-type resin, styrene resin, polybutene-type resin, ethylene Any other resin such as a resin may be included.

  The polyethylene-based crosslinked shrink film according to this embodiment may be a multilayer film composed of at least three layers. FIG. 2 is a schematic cross-sectional view of the polyethylene-based crosslinked shrink film of the present embodiment when the film has three layers. 2 includes a surface layer SA (4) having a surface A (2b), a surface layer SB (5) having a surface B (3b), a surface layer SA, and a surface layer SB. And an inner layer 6.

  In the present embodiment, the gel fraction of the surface layer SA is preferably 0.5 to 5.0% by mass, and the gel fraction of the surface layer SB having the surface B is preferably 5.0 to 15.0% by mass.

The gel fraction is used as a measure of the degree of cross-linking, and is, for example, the ratio of the portion that remains undissolved after the sample is immersed in boiling paraxylene for 12 hours, and is represented by the following equation.
Gel fraction (% by weight) = (sample weight after extraction / sample weight before extraction) × 100

  Since the surface layer SA side having the surface A is a low-crosslinking side, it is used as a heat seal surface on the inner surface in the case of a package. In the multilayer polyethylene-based crosslinked shrink film 10, it is possible to cut out only the surface layer SA and measure the gel fraction. The gel fraction of the surface layer SA is more preferably 1 to 4% by weight, still more preferably 1.5 to 3.9% by weight.

  Since the surface layer SB side having the surface B is a highly cross-linked side, improvement in wear resistance and heat resistance can be expected when used on the outer surface in the case of a package. The lower limit of the gel fraction of the surface layer SB in the multilayer polyethylene-based crosslinked shrink film 10 is more preferably 6.0% by mass or more, and still more preferably 7.0% by weight or more. Further, when the gel fraction is too high, the fusing cutability at the time of packaging is lowered, so the upper limit of the gel fraction is more preferably 13.0% by mass or less, and further preferably 14.0% by weight or less. It is.

  Moreover, in the polyethylene type crosslinked shrink film in the present embodiment, the gel fraction of at least one layer or at least three layers (film total layer) is 10 to 35% by mass from the viewpoint of stretchability and heat resistance of the film. Is preferable, and it is more preferable that it is the range of 15-30 mass%.

For the surface layer SA and / or the surface layer SB, it is preferable to use an ethylene-α-olefin copolymer in order to impart heat seal strength. The density of the ethylene-α-olefin copolymer is 0.900 g / cm ^{3 to} 0.930 g / cm ³ , more preferably 0.907 / cm ^{3 to} 0.928 g / cm ³ , still more preferably 0.910 to 0 926 g / cm ³ , most preferably 0.910 to 0.920 g / cm ³ .

  The ethylene-α-olefin copolymer forming the surface layer SA and / or the surface layer SB may be adjusted in density by blending two or more types having different densities and melt indexes.

  Even if the resin or resin composition mainly composed of the ethylene-α-olefin copolymer forming the surface layer SA and / or the surface layer SB is composed of the ethylene-α-olefin copolymer alone, It may be a mixture with coalescence. In the case of a mixture with another polymer, the content of the other polymer is preferably 50 parts by weight or less, and 30 parts by weight or less with respect to 100 parts by weight of the ethylene-α-olefin copolymer. It is more preferable that it is 20 parts by weight or less.

As the other polymer, a high-pressure low-density polyethylene having a density of 0.930 g / cm ³ or less is preferably 1 to 25 parts by weight, more preferably 5 to 20 parts per 100 parts by weight of the ethylene-α-olefin copolymer. Addition of parts by weight is preferable because the hot tack sealability and transparency are improved.

For the inner layer 6, an ethylene-α-olefin copolymer may be used as in the case of the surface layer SA and / or the surface layer SB, and other polymers may be added. For example, the density is 0.930 g / High pressure process low density polyethylene of cm ³ or less may be added to the ethylene-α-olefin copolymer.

  Moreover, a recycled raw material can be used for the inner layer 6 in this embodiment. The recycled material is obtained by melt-extrusion of a pulverized film or the like remaining when slitting into a predetermined width and pelletizing it. Compared with non-recycled raw materials, these recycled materials may have increased molecular weight due to cross-linking treatment, etc., and may have a higher melt viscosity. The surface may be rough and the transparency may be reduced. However, when used in the inner layer, surface roughness is unlikely to occur and the transparency of the film can be maintained, which is preferable from the viewpoint of resource saving and the like.

  The method for producing a polyethylene-based crosslinked shrink film of the present embodiment includes a step of irradiating an electron beam with an accelerating voltage under a specific condition to a raw material before stretching, a crosslinking treatment, and a resin or a resin composition constituting the raw material And a step of stretching the original fabric at a temperature equal to or higher than the melting peak temperature of the product. Here, there are no particular limitations on the method for obtaining the original fabric before stretching. When a multilayer film is to be obtained, it is preferably produced by coextrusion by a melt extrusion method. For example, the raw material of the polyethylene type | system | group crosslinked shrink film is obtained by melt | dissolving resin or resin composition which comprises each layer with each extruder, and coextruding with a multilayer circular die etc.

  In the method for producing a polyethylene-based crosslinked shrink film of the present embodiment, in order to impart the respective properties to both surfaces of the film, the raw material before stretching is subjected to crosslinking treatment with an electron beam.

As a suitable condition for the crosslinking treatment with an electron beam, from the raw fabric thickness T (unit: μm) and the average density D (unit: g / cm ³ ) of the polyethylene-based crosslinked shrink film, the following formula (2), The acceleration voltage V (unit: kV) that satisfies (3) is set, and the irradiation process is performed. When the acceleration voltage is lower than the lower limit (α + 10) of the formula (3), the degree of crosslinking on the heat seal surface side becomes too low, and the film tends to curl and become difficult to handle. Moreover, when exceeding the upper limit ((alpha) +70) of Formula (3), there exists a tendency for the crosslink degree by the side of a heat seal surface to become high, and to inhibit heat seal property.
T × D = α (2)
α + 10 ≦ V ≦ α + 70 (3)
[T: thickness of original fabric (μm), D: average density of original fabric (g / cm ³ ), V: acceleration voltage (kV)]

  In the present embodiment, when the acceleration voltage V is set to 300 to 450 kV, the intended crosslinking treatment tends to be effectively performed.

  By irradiating an electron beam with the above acceleration voltage, for example, from the side where high cross-linking is desired (for example, the surface B side), a gradient of the degree of cross-linking occurs in the thickness direction of the original fabric, and one surface (for example, the surface A side) is low cross-linked. The other surface (for example, the surface B side) can be highly crosslinked.

  The range of preferable irradiation dose is 40 to 200 kGy, and 60 to 150 kGy is more preferable from the viewpoints of heat sealability and stretching stability.

  In the present embodiment, the polyethylene-based crosslinked shrink film is subjected to crosslinking treatment on an unstretched (before stretching) raw material, and the stretching start temperature is 10 ° C. or more higher than the melting peak temperature of the resin composition to be formed. It is preferable to perform sequential biaxial stretching or simultaneous biaxial stretching of 5 times or more in each of MD and width direction (TD). When the final thickness of the film is set to 9 μm or less, the draw ratio is preferably 5 to 9 times in the flow direction. When it is 5 times or more, stretching is stable, and when it is 9 times or less, film breakage is difficult to occur during stretching. More preferably, it is 5.5 to 8.5 times. The stretching ratio in the width direction is preferably in the range of 5 to 8 times from the viewpoint of film thickness control and stretching stability.

  In order to perform the stretching treatment, it is particularly preferable to use a double bubble inflation method. Since stretching is performed in a temperature range where no crystals above the melting peak temperature exist, the effect of cross-linking stretching is maximized, and more advanced molecular orientation is observed with an atomic force microscope, improving wear resistance and heat resistance. It is thought to let you.

  On the other hand, when stretched in the temperature range below the melting peak temperature of the constituent resin composition, it becomes stretched while leaving the crystal, so in the observation with an atomic force microscope, it is a medium level derived from the part that seems to be a crystal. Since the state of orientation is observed, it is considered that the wear resistance and heat resistance are not sufficiently improved.

  In the production of the polyethylene-based crosslinked shrink film in the present embodiment, specifically, in the case of a multilayer film using an extruder, the resin composition constituting each layer is melt-extruded and sequentially layered one by one in an annular die. It is preferable to obtain a multi-layered tube-shaped unstretched raw fabric by merging or merging at once in an annular die. At this time, one extruder may be used per layer, or it may be divided into two or more before the resin composition flows into the annular die from one extruder, and may be a plurality of layers. . This is cooled and solidified, guided into a stretching machine, and the stretching start point is heated between the melting peak temperature of the resin composition and the melting peak temperature + 40 ° C. It is preferable to perform the injection.

  Thus, Ra (surface roughness) on both surfaces of the polyethylene-based crosslinked shrink film according to the present embodiment is specified in the present embodiment by combining the crosslinking treatment condition satisfying the above formula (3) and the stretching method. (RaA is 4.0 to 10.0, RaB is 3.0 to 5.0).

  The thickness of the polyethylene-based crosslinked shrink film of this embodiment is preferably 5 to 9 μm from the viewpoints of resource saving and film practicality. More preferably, it is 6-8.5 micrometers, More preferably, it is 7-8 micrometers.

  When the polyethylene-based crosslinked shrink film of the present embodiment is a multilayer film, the thickness ratio of the surface layer SA and the surface layer SB to the entire thickness is determined from the viewpoints of extrudability, sealability, transparency, and the like. On the other hand, it is preferably 5 to 50%, more preferably 8 to 30%, still more preferably 10 to 20%.

  The shrinkage rate of the polyethylene-based crosslinked shrink film of this embodiment is generally 0% or more and less than 30% at 100 ° C., and 15% or more and 95% or less, more preferably 35% or more and 95% at 110 ° C. It is as follows. Further, at 120 ° C., it is 50% or more and 95% or less, more preferably 60% or more and 95% or less.

In the present embodiment, the shrinkage rate refers to a value represented by the following equation, and the film shrinkage rate refers to the average value of the shrinkage rate in the flow direction and the shrinkage rate in the width direction of the film.
Shrinkage rate (%) = {(length before shrinkage−length after shrinkage) / length before shrinkage} × 100

  It is preferable that the shrinkage rate of the polyethylene-based crosslinked shrink film of the present embodiment is in the above range because it is possible to suppress the deformation of the container at the time of reheating by microwave heating or the like and achieve low-temperature packaging. Low temperature packaging here includes lowering the set temperature of the shrink tunnel, shortening the transit time in the tunnel, suppressing shrinkage wrinkles, lowering the position of the seal line before and after the package, and the like.

  In addition, when pillow-wrapping, if the polyethylene-based cross-linked shrink film is easy to tear in the longitudinal direction (flow direction), contact with the package body when temporarily packaging the package body with sharp corners with a heat-shrinkable film. The film may tear from the portion, and the tear may propagate to the vicinity of the feeding portion of the heat-shrinkable film. In such a case, it is necessary to start the film through from the beginning, which may cause a large loss.

The obtained polyethylene-based crosslinked shrink film can be slit into a predetermined size and used for packaging. The heat seal bar is set to 110 to 120 ° C. depending on the packaging machine, and is set to a temperature at which a practically satisfactory seal strength can be obtained within a range in which no melt hole or seal breakage occurs in the heat seal portion of the film.
Further, the polyethylene-based cross-linked shrink film can be folded back so that the surface A is on the inner side, and the surfaces A facing each other can be sealed in at least three directions to obtain a package formed by shrink-wrapping the contents. . Note that the three directions refer to, for example, the bottom, back, and top (opening before sealing) of the bag.

  The package using the polyethylene-based crosslinked shrink film in the present embodiment is excellent in the visibility of the contents because the transparency of the film is high even after shrinking. As a measure of visibility, the transparency (haze value) after shrinking is preferably less than 2.5%. When the content is 2.5% or more, the visibility of the film is lowered due to the cloudiness of the film.

  The gloss (gross value) of the package using the polyethylene-based crosslinked shrink film in the present embodiment is preferably 140% or more from the viewpoint of display properties.

  In the polyethylene-based crosslinked shrink film in the present embodiment, a surfactant or an antifogging agent may be contained in any of the constituent layers. For example, a resin composition constituting each layer with at least one additive such as an antifogging agent selected from glycerin fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, ethylene oxide adduct, and liquid paraffin as a plasticizer When blended in an amount of 0.1 to 10.0% by weight based on the total amount of the material, it is preferable because processability and film runnability during packaging are improved. In particular, it is more preferable to add 0.5 to 10% by weight of polyglycerin fatty acid ester or the like from the viewpoint of transparency, and it is further preferable to add 0.8 to 6% by weight from the viewpoint of antistatic properties and slipperiness. preferable.

  The polyethylene-based crosslinked shrink film in the present embodiment has a natural silica or synthetic silica, a saturated fatty acid amide, an unsaturated fatty acid amide, talc, etc. as a lubricant in any layer that does not impair the original characteristics and transparency. May be blended.

  Further, the polyethylene-based crosslinked shrink film in the present embodiment has a tackifier resin or petroleum-based resin such as Alcon (trademark), Clearon (registered trademark), Imabe (registered trademark) as a plasticizer in any of the layers constituting the polyethylene-based crosslinked shrink film. ) And the like. If the content is 0.1 to 10% by weight with respect to the total amount of the resin composition constituting each layer, shrinkage and transparency tend to be improved.

  The polyethylene-based crosslinked shrink film in the present embodiment can be made into a film suitable for printing applications by performing surface treatment such as corona treatment, ozone treatment, and flame treatment.

  When performing the printing process, 0.5 to 5.0% by weight of glycerin fatty acid ester or the like is added to the material of the surface layer to be the printing surface, and after the film formation, the printing surface is subjected to a corona treatment, It is preferable to perform a printing process. From the viewpoint of antistatic properties and prevention of ink peeling, the addition amount of glycerin fatty acid ester and the like is more preferably 0.8 to 3.0% by weight with respect to the resin composition constituting the surface layer.

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

[1] Measurement of surface roughness Using an atomic force microscope “Cypher” (manufactured by Asylum Research), measurement was performed in the following procedure. Films were cut into 5 × 5 mm squares at arbitrary locations, and their front and back surfaces (the other surface) were fixed to a sample fixing disk. These samples were subjected to surface morphological observation using an atomic force microscope (Cypher, manufactured by Asylum Research) using AC mode imaging (cantilever: Olympus AC200TS, observation field: 2 × 2 μm ² , resolution: 512 × 512 pixels). An observation image was obtained. For each sample, the measurement of arbitrary three visual fields was performed. Next, the form of the obtained observation image was compared with the phase image obtained at the same time, and the difference in structure was confirmed. Moreover, Ra (average of the absolute value of the difference from the average value) generally used for surface roughness evaluation was calculated, and the surface roughness of each sample was evaluated. When moisture adhered to the film surface, it was easily affected by the measurement. Therefore, the film was sandwiched between neutral paper having a thickness of 85 μm, stored for 24 hours in a desiccator with a relative humidity of 30% or less, and conditioned.

[2] Evaluation of curling property The film is cut into a size of 5 × 5 cm, and talc is coated on both sides of the film to cover the film surface to prevent the films from adhering to each other, and left on a glass plate for 5 minutes. The curling property was evaluated according to the standard.
◯: The film center or edge lift is less than 5 mm.
X: The lift of the center or edge of the film is 5 mm or more.

[3] Measurement of gel fraction The film was heated in a hot air drier set at 140 ° C. and sampled to a thickness of 200 μm or more was sampled at a weight of 100 mg, extracted in boiling paraxylene for 12 hours, A gel fraction was obtained by expressing the ratio of the insoluble matter by the following formula, and used as a measure of the degree of crosslinking of the film.
Gel fraction (% by weight) = (sample weight after extraction / sample weight before extraction) × 100

[4] Measurement of shrinkage rate A 100 × 100 mm square film is placed in an air oven type high-temperature tank set at a predetermined temperature (100, 110, 120 ° C.), heat-treated for 1 minute, and the film flow direction at each temperature ( MD), the amount of shrinkage in the width direction (TD) is measured, and the percentage ratio of the value before shrinkage, that is, 100 mm, is taken as the shrinkage rate in the flow direction and width direction, respectively, and the average value of these is the film The shrinkage rate of

[5] Evaluation of suitability for packaging machine The film was slit to a width of 500 mm, and “FW-3451A-αV (trade name)” manufactured by Fujikikai Co., Ltd. was used, and “ES-Xinjiang (middle) manufactured by FP Corporation Co., Ltd. (Product name) "About 30g of 20 ° C cooked rice is pillow-wrapped, and the tunnel temperature and transit time are set so that each film is finished best, and heat treatment is performed. The packaging machine rubbing property, sealing property, and transparency of the packaged product after shrinkage were evaluated according to the following criteria.
5-1. Packaging machine rubbing evaluation: The bottom of the container after packaging was observed.
○: No tear or pinhole.
Δ: Pinholes with a diameter of less than 1 mm and film tears.
X: A pinhole having a diameter of 1 mm or more or a film torn.
5-2. Sealability evaluation: The seal bar temperature was changed to 120 ° C. for the bottom and 140 ° C., 160 ° C. and 180 ° C. for the top, and the seal properties were evaluated.
○: No seal puncture (the seal part peeled off at the film interface) after passing through the shrink tunnel.
×: Seal punctured after passing through the shrink tunnel.
5-3. Transparency evaluation after shrinking: The transparency of the film on the upper surface of the package after shrinking was evaluated.
○: High transparency and good visibility of contents.
Δ: The film is cloudy, and the contents appear cloudy when the viewing angle is changed.
X: The film surface is rough and the contents are difficult to see.

[6] Transparency (haze) and gloss (gloss) after shrinkage
Haze: The upper part of the package obtained in (Packaging machine suitability evaluation) was cut out, and the haze after shrinkage was determined according to ASTM D-1003.
Gross: The upper part of the package obtained in (Evaluation of suitability for packaging machine) was cut out and made into a gloss after shrinkage according to ASTM D-2457.

  The resin and film manufacturing methods used in Examples and Comparative Examples are as follows.

The following resins were used as materials for the surface layer and the inner layer.
LL1: ethylene-α-olefin copolymer (polymerized with a single-site catalyst, comonomer = hexene, density = 0.913 g / cm ³ , MI = 2.0 g / 10 min, melting main peak temperature = 113 ℃)
LL2: ethylene-α-olefin copolymer (polymerized with multisite catalyst, comonomer = octene, density = 0.926 g / cm ³ , MI = 2.0 g / 10 min, melting main peak temperature = 121 ℃)
LL3: ethylene-α-olefin copolymer (polymerized with a single site catalyst, comonomer = hexene, density = 0.916 g / cm ³ , MI = 2.3 g / 10 min, melting main peak temperature = 114 ℃)
LL4: ethylene-α-olefin copolymer (polymerized with a single-site catalyst, comonomer = hexene, density = 0.910 g / cm ³ , MI = 2.0 g / 10 min, melting main peak temperature = 103 ℃)
LL5: ethylene-α-olefin copolymer (polymerized with a single-site catalyst, comonomer = hexene, density = 0.926 g / cm ³ , MI = 2.5 g / 10 min, melting main peak temperature = 121 ℃)
LD1: High-pressure method low-density polyethylene (density = 0.922 g / cm ³ , MI = 0.2 g / 10 minutes, melting main peak temperature = 110 ° C.)

[Example 1]
After extruding a resin composition having the composition shown in Table 1 with 2.0 mass% of a 1: 1 mixture of diglycerin oleate and glycerin monooleate as a multi-layer (three layers) raw fabric from a cyclic die, It was cooled and solidified with cold water to prepare a tube-shaped multilayer original fabric having a folding width of 120 mm and a thickness of 400 μm. This was induced into an electron beam irradiation apparatus (Curetron (trade name), manufactured by NHV Corporation), and the crosslinking treatment was performed by setting the acceleration voltage and irradiation dose shown in Table 1. The irradiation dose at this time is a total value of irradiation in which the tube-shaped original fabric is folded in half, irradiated from one side and then irradiated from the opposite side, and this is repeated once more, for a total of four times. This is induced in a stretching machine and reheated, passed between two pairs of differential nip rolls, bubbles are formed by air injection, the heating temperature at the stretching start point is set to 140 ° C., and 7. Each film was stretched at a magnification of 8 times and 6.4 times in the width direction, an average thickness of 8 μm, and a thickness ratio (%) of the surface layer / inner layer / surface layer was 15/70/15, respectively. Obtained. For the film thus obtained, measurement of surface roughness with an atomic force microscope, curl evaluation, gel fraction, shrinkage, packaging machine suitability evaluation (packaging machine runnability evaluation (packing machine rubbing property), seal Evaluation, transparency evaluation after shrinking), transparency (haze), and gloss (gloss).

[Examples 2-6, Comparative Examples 1-6]
A shrink film having an average thickness of 8 μm was obtained according to the procedure of Example 1 except that the composition / layer structure of the film, the thickness of the original fabric, and the crosslinking conditions were changed to those shown in Tables 1 and 3. About the film thus obtained, surface roughness measurement by atomic force microscope, curl evaluation, gel fraction, shrinkage, packaging machine suitability evaluation (packaging machine runnability evaluation, sealability evaluation, after shrinking, Evaluation of transparency), transparency (haze), and gloss (gloss) were evaluated.

[Examples 7 to 9, Comparative Example 7]
After extruding a resin composition having the composition shown in Table 2 with a mixture of diglycerin oleate and glycerin monooleate (1: 1% by mass) as a single-layer raw fabric from a circular die, it is cooled with cold water. After solidification, a tube-shaped multi-layer original fabric having a folding width of 120 mm and a thickness of 400 μm was prepared. This was guided to an electron beam irradiation apparatus (Curetron (trade name), manufactured by NHV Corporation) and subjected to crosslinking treatment under the conditions of Tables 2 and 3. This is guided into a stretching machine and reheated, passed between two pairs of differential nip rolls, bubbles are formed by air injection, the heating temperature at the stretching start point is set to 140 ° C., and 8 times in the flow direction The film was stretched at a magnification of 7 times in the width direction to obtain a single-layer shrink film having an average thickness of 8 μm. About the film thus obtained, surface roughness measurement by atomic force microscope, curl evaluation, gel fraction, shrinkage, packaging machine suitability evaluation (packaging machine runnability evaluation, sealability evaluation, after shrinking, Evaluation of transparency), transparency (haze), and gloss (gloss) were evaluated.

  As shown in Tables 1 and 2, Examples 1 to 9 are within the specified ranges of the surface roughness, gel fraction and shrinkage rate according to the present invention, do not curl, and run the packaging machine in the packaging machine suitability evaluation. It was excellent in property evaluation, sealing property evaluation, packaging finish, transparency (haze) and gloss (gloss) after packaging.

  On the other hand, as shown in Table 3, in Comparative Example 1, since the acceleration voltage in electron beam irradiation was too high with respect to the original thickness, Ra (RaA) of the surface A on the seal surface side was small, and the gel fraction was Since it was also expensive, it was a high bridge. Therefore, it is considered that seal puncture is likely to occur during packaging.

  In Comparative Example 2, since the acceleration voltage in electron beam irradiation is too low relative to the thickness of the original fabric, the Ra (RaA) of the surface A which is the seal surface side is large, and the gel fraction is too low. It is considered that the orientation difference in the degree of crosslinking has increased. Moreover, since the surface roughness was great, transparency and gloss were lowered.

  In Comparative Example 3, since the stretching temperature is equal to or lower than the melting peak temperature, sufficient stretching orientation cannot be applied, and since the acceleration voltage and irradiation dose during irradiation are low, the RaB of the surface B is increased. It is thought that rubbing and tearing in the packaging machine was likely to occur.

  In Comparative Example 4, since the acceleration voltage in electron beam irradiation is low with respect to the thickness of the original fabric, Ra (RaA) of the surface A on the seal surface side is large, and the gel fraction is too low, so curling is likely to occur. Transparency and gloss decreased.

  In Comparative Example 5, RaA / RaB was larger than 3.0, and thus curled easily.

  In Comparative Example 6, Ra (RaA and RaB) on surfaces A and B was less than 3 and the degree of crosslinking was high, resulting in poor sealing properties.

  Since Comparative Example 7 has a high Ra (RaB) on the surface B, the rubbing property of the packaging machine is not sufficient, and since the Ra (RaA) on the surface A is high, the film surface is too rough, resulting in poor transparency. became.

  The polyethylene-based crosslinked shrink film of the present invention can be suitably used as a thin packaging film. In particular, it is suitable for packaging containers reheated in a microwave oven, and can be suitably used for packaging lunch boxes, side dishes, and the like.

DESCRIPTION OF SYMBOLS 1 ... Polyethylene type crosslinked shrink film, 2a, 2b ... Surface A, 3a, 3b ... Surface B, 4 ... Surface layer SA, 5 ... Surface layer SB, 6 ... Inner layer, 10 ... Multilayer polyethylene type crosslinked shrink film.

Claims (5)

A polyethylene-based crosslinked shrink film comprising at least one layer and having a surface A which is one outermost surface and a surface B which is the other outermost surface,
When the surface roughness of the surface A observed by an atomic force microscope is RaA and the surface roughness of the surface B is RaB,
RaA is 4.0 to 10.0, RaB is 3.0 to 5.0,
And the polyethylene type crosslinked shrink film whose relationship of RaA and RaB satisfy | fills following formula (1).
1.1 ≦ RaA / RaB ≦ 3.0 (1)   The gel fraction of the surface layer SA having at least three layers and having the surface A is 0.5 to 5.0 mass%, and the gel fraction of the surface layer SB having the surface B is 5.0 to 15.0 mass%. %, The polyethylene-based crosslinked shrink film according to claim 1.   The polyethylene type crosslinked shrink film according to claim 1 or 2, wherein the gel fraction of the whole film is 10 to 35% by mass. It is a manufacturing method of the polyethylene system crosslinked shrink film according to any one of claims 1 to 3,
A step of irradiating an electron beam with an accelerating voltage satisfying the following formulas (2) and (3) to the raw material before stretching, and crosslinking treatment:
And a step of stretching the original fabric at a temperature equal to or higher than the melting peak temperature of the resin or resin composition constituting the original fabric.
T × D = α (2)
α + 10 ≦ V ≦ α + 70 (3)
[In the above formulas (2) and (3), T represents the thickness of the original fabric (μm), D represents the average density of the original fabric (g / cm ³ ), and V represents the acceleration voltage (kV)] The polyethylene-based crosslinked shrink film according to any one of claims 1 to 3 is folded back so that the surface A is on the inside, and the surfaces A facing each other are sealed at least in three directions, and the contents are sealed. A package made by shrink wrapping.

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