JP2007152749A - Multiple layer heat-shrinkable film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a film have heat-shrinkability at a low temperature and a proper shrinking stress, and also, to maintain the rigidity of the film, and to provide an excellent packaging machine adaptability. <P>SOLUTION: This multiple layer heat-shrinkable film having at least 2 layers which satisfy the following (1) to (4) conditions has at least one internal layer (X) and first and second surface layers made of an ethylene-based polymer. In this case, the internal layer (X) is made of a mixture of a block copolymer of a conjugated diene and a vinyl aromatic compound, and/or its hydrogenation substance (A) and a polystyrene-based resin (B). (1) The heat shrinkability at 100°C is 30 to 80%. (2) The maximum heat shrinking stress at 40°C to 140°C is 0.9 to 4.0 MPa. (3) The elasticity is 200 to 900 MPa. (4) The haze after 30% shrinkage on the film area is 6% or lower. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a multilayer heat shrinkable film having good packaging machine suitability and excellent shrinkage. In particular, it can be packaged without applying excessive heat to the package that is inferior in heat resistance, so there is no deformation due to heat of the package, it can be tightly packed, and the packages are packed together when packed in a box etc. The present invention relates to a multilayer heat shrinkable film that easily slides and packs, a method for producing the film, and a package using the film.

Conventionally, shrink wrapping (synonymous with shrink wrapping) can quickly and tightly package multiple products at the same time, regardless of the shape and size of the package, and the resulting package has a beautiful appearance. In addition, it is used for packaging food, sundries, etc., because the contents are kept hygienic and visual quality control is easy. Examples of the method of covering an object to be packaged with a packaging film include pillow shrink packaging and L-shaped packaging. As an example, a method of continuous packaging in L-shaped packaging will be described here.
First, the package body is placed between the half-folded films, and the package body is covered with a margin of 5 to 200% with respect to the vertical and horizontal lengths of the package body. The body flow direction and the half-folded film opening are sealed with an L-type sealer and cut simultaneously to obtain one package. Subsequently, the obtained package passes through a shrink tunnel whose temperature is adjusted in advance to the shrinkage temperature of the film, so that the film is thermally contracted to obtain a tightly finished package.

  As a sealing method in heat shrink packaging, there are 1) a heat sealing method such as a bar sealing method and a heat roller method, 2) an impulse sealing method, and 3) a fusing sealing method, and these sealing methods are used in an appropriate combination. ing. The above 1) is basically a face seal, and a so-called seal-and-cut method is adopted in which it is cut by a cutter almost immediately at the same time as the seal. In 2) and 3), as described above, a separate cutter is not required, and the film is melted and sealed with a heated metal wire or metal blade (sealing method). As a simple sealing method, it is widely used for various films for packaging.

  Moreover, in the heat shrinking process of the packaging film, a small hole for venting air is opened in advance in the packaging film using any one of a needle, a heat needle, a laser, etc., and the air in the packaging film bag is vented during the heat shrinking. It is known that a shrink package finished in a tight manner can be obtained. Hot air, steam or the like can be used as a heating method at the time of heat shrinkage, but hot air is usually used. As described above, shrink wrap is generally a method in which the film is heat-shrinked by hot air in a shrink tunnel after first wrapping the contents by heat seal, fusing seal, etc. with a little allowance on the film, A tight and beautiful finish is obtained. The packaging speed of the packaging machine has been increased by automation of the packaging machine, and when it becomes a high-speed automatic continuous packaging machine, it is possible to package about 40 pieces per minute, and it tends to increase further. Therefore, the packaging film is strongly required to have suitability for the packaging speed, such as slipperiness and heat shrinkage characteristics.

  The characteristics required for a heat-shrinkable packaging film are as follows: 1) shrinkage characteristics, 2) optical characteristics, 3) suitability for packaging machines, 4) good slipperiness of the package, and 1) in particular. The film is highly shrinkable to finish tightly after shrinking, and it is low-temperature shrinkable to wrap the contents without applying excessive heat. Is transparent and the contents can be confirmed. For 3), as the packaging speed increases, the film has rigidity (so-called waist), and the sliding property between the film and the mechanical metal part is good. And having strength against various external loads (including tear strength, puncture strength, etc.), including transportation and storage after packaging, 4) Packaging that is packaged with a film when the packaged item is packed in a box or the like Same thing Slip, it tends to load filling (the film each other slippery) is obtained.

Practically, the required characteristics described above vary depending on the type of package, the distribution process, how it is handled during storage, and various environmental conditions, and heat shrink wrapping films must meet this requirement. On the other hand, on the other hand, as a result of considering environmental needs such as cost, resource saving, and garbage reduction, several types of film thickness are usually required. As a heat-shrinkable packaging film used for these, a multilayer film using a polyolefin resin having excellent transparency is conventionally known.
Patent Document 1 discloses a multilayer film made of an ethylene-based polymer resin, and describes that it has heat shrinkability and the objects to be packaged have good slipperiness. Patent Document 2 discloses a multilayer heat-shrinkable film having a surface layer, a core layer made of a specific polypropylene resin, and an inner layer made of an ethylene α-olefin copolymer, and has excellent slipperiness. It is described that the packaging machine is good.

Patent Document 3 discloses a multilayer heat-shrinkable film containing a hydrogenated copolymer comprising a conjugated diene and a vinyl aromatic compound, and has mechanical strength, transparency, stretchability, binding property, and elastic recovery. It is described as having excellent properties. Patent Document 4 discloses a multilayer heat-shrinkable film containing an α-olefin vinyl aromatic random copolymer, which is described as having a high elastic modulus and low shrinkage stress.
However, in the technique disclosed in Patent Document 1, the obtained film has a high heat shrinkage temperature, and it is necessary to set the hot air temperature in the shrink tunnel high, and the packaged body is deformed by heat. The problem remains. In the technique disclosed in Patent Document 2, there remains a problem that the merchantability of the packaged body is deteriorated due to the lack of the film heat shrinkage rate.

The film containing a conjugated diene and a vinyl aromatic copolymer disclosed in Patent Document 3 has a problem that the rigidity (so-called waist) of the film is low and the suitability for packaging machinery is poor. In the multilayer film containing the α-olefin / vinyl aromatic linear random copolymer disclosed in Patent Document 4, the rigidity of the film is improved by increasing the vinyl aromatic ratio, and the suitability of the packaging machine is increased. The shrinkage stress is reduced, and particularly at the bottom surface of the packaged body, the thermal shrinkage of the film is hindered by the weight of the packaged body, shrinkage spots occur, and a tight packaging finish cannot be obtained. .
JP 2002-36358 A JP-A-11-70625 Japanese Patent Laid-Open No. 2003-246021 JP 2004-526593 A

  An object of the present invention is to provide a multilayer heat-shrinkable film that achieves a tight packaging finish while maintaining transparency and has both excellent shrinkage characteristics and appropriate film rigidity. In particular, by having heat shrinkability at low temperature, it is possible to wrap a package body with low heat resistance, and by having an appropriate shrinkage stress that does not deform the package body when the film is thermally contracted, a beautiful packaging finish can be obtained. It is to provide a film excellent in suitability for packaging machinery by having the rigidity of the film.

As a result of intensive studies to achieve the above-mentioned problems, the present inventor has completed the present invention.
That is, the present invention is as follows.
1. At least one inner layer (X) comprising a mixture of a conjugated diene and vinyl aromatic copolymer and / or a hydrogenated product thereof (A) and a polystyrene resin (B), and a first comprising an ethylene polymer, And a second surface layer and satisfying the following 1) to 4):
1) 30-80% thermal shrinkage at 100 ° C
2) Maximum heat shrinkage stress at 40 ° C. to 140 ° C. is 0.9 to 4.0 MPa.
3) Elastic modulus is 200 to 900 MPa
4) The haze after 30% shrinkage in the film area is 6.0% or less. The copolymer of conjugated diene and vinyl aromatic and / or the hydrogenated product thereof (A) contains at least one polymer block composed of vinyl aromatic monomer units. A multilayer heat shrinkable film as described in 1.
3. The polystyrene resin (B) is a dispersed phase comprising a copolymer of styrene units and methacrylate units, a copolymer of styrene units and alkyl acrylate units, and a polybutadiene or a copolymer of styrene units and butadiene units. 1 or a mixture of two or more selected from copolymers having a continuous phase, the main component of which is styrene / methacrylic acid ester. A multilayer heat shrinkable film as described in 1.
4). The mixing weight ratio of the conjugated diene and the vinyl aromatic copolymer and / or the hydrogenated product (A) thereof and the polystyrene resin (B) in the inner layer (X) is (A) :( B) = 90: 10 It is 10:90. A multilayer heat shrinkable film as described in 1.
5. The thickness ratio of at least one inner layer (X) comprising a copolymer of a conjugated diene and vinyl aromatic copolymer and / or a hydrogenated product thereof (A) and a polystyrene resin (B) is 1. It is characterized by being 10 to 80%. A multilayer heat shrinkable film as described in 1.
6). The first and second surface layers made of an ethylene polymer resin contain 0.05 to 3.0% by weight of an antiblocking agent and 0.05 to 3.0% by weight of a slip agent. 1. A multilayer heat shrinkable film as described in 1.
7). Between the inner layer (X) and the first and second surface layers, an ethylene vinyl acetate copolymer, an ethylene / methyl methacrylic acid copolymer, a metal partial neutralized product of an ethylene / methacrylic acid copolymer, ethylene Acrylic acid copolymer, ethylene / ethyl acrylic acid copolymer, ethylene / maleic anhydride copolymer, ethylene / ethylene acrylate / maleic anhydride copolymer, ethylene / maleic acid modified resin, and Vicat softening temperature of 50 to 50 1. It contains at least one layer made of one kind or a mixture of two or more kinds selected from ethylene / α-olefin copolymer at 68 ° C. A multilayer heat shrinkable film as described in 1.
8). 1. Gel fraction is 0.05 to 20% by weight The multilayer heat-shrink film as described in any one of these.
9. The raw materials used for the inner layer, the first and second surface layers are melted from the extrusion machine, the resin of each layer is coextruded from the annular die, and the obtained tube parison is cooled. 5 to 120 kGy of cross-linking treatment is performed from both sides, and then the tube-like parison is guided into the stretching machine, and air is injected between nip rolls with a speed difference while heating the stretching start point at 80 to 140 ° C. Stretching is performed at a magnification of 2 to 10 times in the (MD) direction and the width (TD) direction, respectively. ~ 8. The manufacturing method of the multilayer heat shrink film of any one of these.
10.1. ~ 8. A package for a recording medium, which is packaged using the multilayer heat-shrinkable film described in 1.

  By using the film of the present invention, a tight packaging finish can be obtained while maintaining transparency. In addition, the film of the present invention has good packaging machine suitability due to its rigidity. And since the film of this invention is excellent in the heat shrink property in low temperature compared with the conventional heat shrink packaging film, it becomes possible to package the to-be-packaged body with low heat resistance. In addition, when a heavy object is packaged, a tight packaging finish can be obtained because it has an appropriate shrinkage stress.

The present invention will be described in detail below with a focus on preferred embodiments.
The multilayer heat-shrinkable film of the present invention has at least one inner layer (X) made of a mixture of a conjugated diene and vinyl aromatic copolymer and / or a hydrogenated product (A) thereof and a polystyrene resin (B). By using a copolymer of a conjugated diene and a vinyl aromatic and / or its hydrogenated product (A), heat shrinkability at a low temperature can be obtained, and by using a polystyrene resin (B), rigidity of a film can be obtained. .
The ratio of the vinyl aromatic compound in the copolymer comprising the conjugated diene and the vinyl aromatic compound and / or the hydrogenated product (A) thereof used for the inner layer (X) of the present invention is 55 to 85% by weight. Preferably, it is 60 to 80% by weight. When the ratio of the vinyl aromatic compound is 55% by weight or more, it is preferable in terms of obtaining an appropriate rigidity by improving the elastic modulus of the film, and when it is 80% by weight or less, the film has heat shrinkability at low temperature. It is preferable at the point obtained.

Examples of the conjugated diene include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3 butadiene, 1,3-pentadiene, 2-methyl 1,3-pentadiene, 1, 3-hexadiene etc. are mentioned. You may use these 1 type or in mixture of 2 or more types.
Examples of vinyl aromatic compounds include styrene, o-methyl styrene, p-methyl styrene, m-methyl styrene, α-methyl styrene, ethyl styrene, α-methyl-p-methyl styrene, 2,4-dimethyl styrene, monochloro. Examples include styrene, p-methylstyrene, p-tert-butylstyrene, α-ethylstyrene, and the like. You may use these 1 type or in mixture of 2 or more types.

The weight average molecular weight of the copolymer comprising a conjugated diene and a vinyl aromatic compound used in the present invention is preferably from 20,000 to 600,000, more preferably from 20,000 to 400,000, still more preferably from the viewpoint of heat shrinkability of the film. Is 20,000 to 300,000. The molecular weight distribution (Mw / Mn) is preferably 10.0 or less, more preferably 1.1 to 5.0, and still more preferably 1.1 to 4.0, from the viewpoint of film transparency. All of the above characteristics are measured by a gel permeation chromatography apparatus (GPC).
As the at least one inner layer (X) used in the present invention, a copolymer obtained by hydrogenating an unsaturated portion of a conjugated diene portion of a copolymer composed of a conjugated diene and a vinyl aromatic compound may be used. Examples thereof include hydrogenated styrene / isoprene block copolymer (SIS), hydrogenated styrene / ethylene / butadiene block copolymer (SEBS), hydrogenated styrene / isoprene / butadiene block copolymer (SIBS), and the like.

The polystyrene-based resin (B) used in the present invention includes a styrene-based compound homopolymer, a copolymer of two or more styrene-based compounds, and a rubbery heavy polymer in a matrix composed of the styrene-based polymer. Examples thereof include rubber-modified styrene resins in which the coalesced particles are dispersed, and a mixture of two or more of these can also be suitably used.
Specific examples of the styrene compound include styrene, o-methylstyrene, p-methylstyrene, m-methylstyrene, α-methylstyrene, ethylstyrene, α-methyl-p-methylstyrene, 2,4-dimethyl. Examples thereof include styrene, monochlorostyrene, p-methylstyrene, p-tert-butylstyrene, α-ethylstyrene and the like. These may be used alone or in combination of two or more.

A copolymer of styrene units and methacrylate units, a copolymer of styrene units and alkyl acrylate units, or a copolymer of polybutadiene or styrene units and butadiene units is the dispersed phase. A copolymer having a continuous phase as a main component is preferable in that the conjugated diene and the vinyl aromatic copolymer and / or the hydrogenated product thereof have good compatibility and do not impair transparency.
The range of the melt flow rate of the preferred polystyrene resin is preferably 0.5 to 20 g / 10 min, more preferably 0.5 to 15 g / 10 min, and still more preferably 0.5 to 10 g / 10 min, from the viewpoint of film rigidity. It is.

The method for producing the polystyrene resin used in the present invention is not limited, and it can be obtained by polymerization by any of bulk polymerization, solution polymerization, emulsion polymerization and suspension polymerization. Examples of the three-dimensional structure of the styrene resin molecule include atactic, isotactic, syndiotactic, and the like, and any of them can be suitably used.
The mixed weight ratio of the conjugated diene and vinyl aromatic copolymer and / or its hydrogenated product (A) and the polystyrene resin (B) used in at least one inner layer of the present invention is high in the film. From the viewpoint of rigidity, it is preferable that (A) :( B) = 10: 90 to 90:10, and more preferably (A) :( B) = 30: 70 to 70:30.

The at least one inner layer (X) composed of a copolymer of a conjugated diene and a vinyl aromatic copolymer and / or a hydrogenated product thereof (A) and a polystyrene resin (B) used in the present invention is based on the total layer. The thickness ratio is preferably 10 to 80% from the viewpoint of the high rigidity of the film, more preferably 20 to 70%, and still more preferably 25 to 65%.
The multilayer heat-shrinkable film of the present invention has first and second surface layers made of an ethylene polymer. By having the first and second surface layers made of an ethylene polymer, the transparency of the film can be obtained.
In the present invention, examples of the ethylene polymer resin used in the first and second surface layers include an ethylene-α-olefin copolymer, a low density polyethylene, and an ethylene vinyl acetate copolymer. These resins may be used alone or in combination of two or more. An ethylene-α-olefin copolymer is preferable from the viewpoint of transparency and resistance to tearing when the package is transported.

As the ethylene-α-olefin copolymer, a random copolymer of ethylene and at least one monomer selected from α-olefins having 3 to 18 carbon atoms is preferable. As the α-olefin, propylene- 1, butene-1, pentene-1, 4-methyl-pentene-1, hexene-1, octene-1, decene-1, dodecene-1, and the like.
The resin may be polymerized with either a multi-site catalyst or a single-site catalyst, but when packaging a content requiring transparency, it is preferable to use a polymer polymerized with a single-site catalyst. In that case, when the molecular weight distribution (Mw / Mn) measured by GPC is 3.5 or less, the transparency is particularly good. The density of the ethylene -α- olefin copolymer, the rigidity of the film is preferably 0.870~0.930g / cm ³ from the viewpoint of optical properties, preferably density of 0.880~0.920g / cm ³ is there.

In the multilayer heat shrinkable film of the present invention, the ethylene polymer resin used for the first and second surface layers has an anti-blocking agent of 0.05 to 3.0% by weight and a slip agent of 0.05 to 3 It is preferable to contain 0.0% by weight. Examples of the anti-blocking agent include silica, calcium carbonate, talc and the like, and examples of the slip agent include polyethylene wax, higher fatty acid amide, higher fatty acid ester and the like.
As long as the properties of the multilayer heat-shrinkable film of the present invention are not impaired, plasticizers, antioxidants, surfactants, ultraviolet absorbers, inorganic fillers, antifogging agents, antistatic agents, crystal nucleating agents, and low molecular weight petroleum resins. A representative adhesive, colorant, or the like may be included in any layer of the multilayer heat shrinkable film. As a method for adding the additive to the resin, the additive may be directly kneaded and added to the target resin layer, or in some cases, a master batch may be prepared in advance and diluted.

  The multilayer heat-shrinkable film of the present invention may be provided with at least one layer between the inner layer (X) and the first and second surface layers, and the resin used in this layer is an ethylene vinyl acetate copolymer. , Ethylene / methyl methacrylic acid copolymer, ethylene / methacrylic acid copolymer metal partial neutralized product, ethylene / acrylic acid copolymer, ethylene / ethyl acrylic acid copolymer, ethylene / maleic anhydride copolymer, Use one or a mixture of two or more selected from ethylene / ethylene acrylate / maleic anhydride copolymer, ethylene / maleic acid modified resin, and ethylene / α-olefin copolymer having a Vicat softening temperature of 50 to 68 ° C. Is preferred. When a resin selected from these is used, it is possible to prevent delamination of the multilayer film in the packaging process and the transport process, and to reduce the heat shrinkage stress.

From the viewpoint of high rigidity of the film, the melt flow rate of other resins used for the inner layer (X) is 0.5 to 5.0 g / 10 min. Is more preferable, and more preferably 0.7 to 3.5 g / 10 min. It is.
The gel fraction of the multilayer heat shrinkable film of the present invention is preferably 0.05 to 20% by weight, more preferably 0.07 to 15% by weight from the viewpoint of packaging machine suitability. By cross-linking the multilayer heat-shrinkable film of the present invention before the stretching process, it is easy to adjust the stretching temperature and magnification, and it is possible to produce a film with excellent shrinkability while maintaining film formation stability, making it ideal for shrink wrapping. Heat shrinkage rate and heat shrinkage stress can be obtained. Moreover, even if it raises shrinkage | contraction temperature beyond the melting | fusing point of resin which comprises a film, shrink packaging can be performed without melting, and it is also possible to provide heat resistance to a packaging film.

  The heat shrinkage rate of the multilayer heat shrinkable film of the present invention is measured according to ASTM D-2732. The measurement temperature is evaluated at 100 ° C. assuming a packaged body with poor heat resistance. The heat shrinkage rate at 100 ° C. of the film of the present invention is 30 to 80% in order to improve high-speed packaging suitability and to wrap without excessive heat on a package having poor heat resistance, Preferably, it is 35 to 80%. When the heat shrinkage rate at 100 ° C. is 30% or more, a tight packaging finish is obtained, and even a packaged body having low heat resistance does not deform due to heat. Further, when the heat shrinkage rate at 100 ° C. is 80% or less, the packaging body is less likely to be deformed when the flexible packaging body is packaged.

The heat shrinkage stress of the multilayer heat shrinkable film of the present invention is measured between 40 and 140 ° C., which is a temperature at which shrink wrap is actually performed according to ASTM D-2838. The heat shrinkage stress of the packaging film of the present invention is the maximum measured between 40 to 160 ° C. in order to obtain a tight packaging finish even in heavy weight packaging without deforming the packaged body during film heat shrinkage. The shrinkage stress value is 0.9 to 4.0 MPa. When the maximum shrinkage stress value between 40 and 140 ° C. is 0.9 MPa or more, the shrinkage finish is good, and when it is 4.0 MPa or less, the package body is hardly deformed by the shrinkage stress.
The elastic modulus of the multilayer heat shrinkable film of the present invention is measured according to ASTM D-882. That is, the film is stretched to add a strain of 2% to the film and obtained from the stress at that time. The elastic modulus of the film of the present invention is 200 to 900 MPa, preferably 220 MPa to 850 MPa, from the viewpoint of high rigidity (so-called waist) of the film. When the elastic modulus is 200 MPa or more, the packaging machine suitability is good, and when it is 900 MPa or less, the rigidity of the film is excellent.

The haze of the multilayer heat shrinkable film of the present invention is determined by measuring the film after heat shrinking the film area to 30% at a hot air temperature of 140 ° C. according to ASTM D-1003. The haze after shrinking the film of the present invention by 30% in terms of film area is 6.0% or less, preferably 5.5% or less, more preferably 5.3% or less. If the haze when shrinking 30% in film area is 6.0% or less, for example, when purchasing a product, the product recognition bar code for product management and price confirmation at the cash register is easy to read and posted on the outer box of the product It is easy to check the content display.
The dynamic friction coefficient of the multilayer heat shrinkable film of the present invention is measured using a 500 g satin metal rider in accordance with ASTM D-1894. The coefficient of dynamic friction between the film and the metal of the film of the present invention is preferably 0.30 or less, more preferably 0.28 or less, and still more preferably 0.25 or less. When the coefficient of dynamic friction between the film and the metal is 0.30 or less, there is no resistance when the film travels at high speed on the packaging machine, and excessive tension is not applied to the film, so that the film does not break. In addition, when the coefficient of dynamic friction between the film after heat shrinkage is 0.30 or less, a bag breakage occurs due to rubbing between the packaged bodies when the individually packaged packages are packed in cardboard or the like. Absent.

The manufacturing method of the multilayer heat-shrink film of this invention is demonstrated.
The raw materials used in each layer are melted from several types of extrusion machines, and the respective resins are coextruded from an annular die to obtain a tubular unstretched parison. The tube-shaped parison uses water as the cooling medium from the outside, and the surface from which water is passed through the inside of the tube-shaped parison from the outside as the cooling medium is blasted, along the rough cooling mandrel, There is a case where the tube-like parison is cooled and solidified from both the inside and outside, and this is rapidly cooled and solidified, and any method may be used.
Next, the cooled and solidified tubular parison is guided into a stretching machine, and air is injected between nip rolls provided with a speed difference while heating the stretching start point between 80 to 140 ° C. From the viewpoint of stretching stability. In the flow (MD) direction and the width (TD) direction, stretching is preferably performed at a magnification of 2 to 10 times, more preferably 3 to 7 times. The stretching start point refers to a position at which expansion starts in the TD direction due to the internal pressure of the bubble.

Examples of the film stretching method of the present invention include a single bubble inflation method, a double bubble inflation method, a triple bubble inflation method, and a tenter method. From the viewpoint of shrinkage, the single bubble inflation method, the double bubble inflation method, the triple bubble inflation method, and the like. The method is preferred. Among these, the double bubble inflation method is particularly preferable.
From the viewpoint of film formation stability and heat resistance, a crosslinking treatment with an electron beam may be performed. The electron beam cross-linking treatment may be performed either before stretching or after stretching, but it is preferable to perform it before stretching because a relatively small cross-linking apparatus can be used. Electron beam irradiation may be performed from one side or both sides of the film. The amount of electron beam irradiation is preferably 5 to 120 kGy, more preferably 10 to 100 kGy from the viewpoints of film formation stability and heat resistance.

When the surface treatment such as corona treatment, ozone treatment, or flame treatment is performed after stretching, a film suitable for printing applications can be obtained. The obtained film is slit to a predetermined size.
The thickness of the multilayer heat shrinkable film of the present invention is preferably 5 to 60 μm, more preferably 6 to 50 μm, still more preferably 7 to 40 μm. When the thickness of the film is 5 μm or more, the stiffness of the film is improved and workability at the time of packaging is good. When the thickness is 60 μm or less, the amount of heat required for heat shrinkage of the film can be reduced, and heat shrinkage can be performed in a shorter time or at a lower temperature.
Since the packaging body using the multilayer heat-shrinkable film of the present invention does not require excessive heat and shrink packaging is performed, the packaging body is packaged without thermal deformation. Moreover, it is suitable when the packaged body is a recording medium such as a CD, a DVD, and a video, and packaging is performed without causing deterioration of recording due to heat during shrink packaging. In addition, since the multilayer heat shrinkable film of the present invention has an appropriate shrinkage stress, it can be packaged without causing deformation or breakage such as warpage of the packaged body due to the shrinkage force of the film.

Next, the present invention will be specifically described with reference to examples and comparative examples.
The evaluation method used in the present invention is as follows.
<Elastic modulus (film rigidity)>
Measured in accordance with ASTM D-882, the film is sampled to a width of 10 mm x a length of 100 mm, stretched in the length direction, and obtained from the stress when 2% strain is applied. Three points are measured in each of the MD direction and TD direction of the film, and the average value is obtained.
<Heat shrinkage>
Measurement is performed according to ASTM D-2732, the film is sampled so as to be 10 cm × 10 cm in the MD direction and TD direction, respectively, and is shrunk for 30 minutes at a temperature of 100 ° C. using a hot air dryer. Five films are shrunk, a dimensional change is calculated | required, and the average value of MD and TD direction is calculated | required.

<Heat shrinkage stress>
It measures according to ASTM D-2838, and evaluates by the maximum heat shrinkage stress value between 40-140 degreeC using an oil bath. The film is sampled to a width of 10 mm and a length of 50 mm, and measurements are taken three times at each temperature in the MD direction and in the TD direction to obtain an average value.
<Gel fraction>
A sample placed in a 150-mesh wire mesh in boiling p-xylene is immersed for 12 hours, and the ratio of the insoluble portion is expressed by the following formula, which is 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
<Vicat softening temperature>
The Vicat softening temperature is determined according to JIS-K7206.

<Melt flow rate of resin used>
1) Polystyrene resin: determined according to JIS-K6871.
2) Polyethylene resin: Determined according to JIS-K-7210.
3) Ethylene vinyl acetate copolymer: Determined according to JIS-K-6922-2.
<Weight average molecular weight and molecular weight distribution (Mw / Mn)>
1) Weight average molecular weight and molecular weight distribution in a copolymer comprising a conjugated diene and a vinyl compound: G
Using a PC (manufactured by Tosoh Corporation, GPC apparatus HLC-8020 type), the column was measured using Tosoh TSK series, the solvent was tetrahydrofuran, column temperature 40 ° C., flow rate 1 ml / min, concentration 3 mg / ml. A weight average molecular weight and a molecular weight distribution (Mw / Mn) are determined from the weight average molecular weight (Mw) and the number average molecular weight (Mn) converted from the composition curve of standard polystyrene.

<Density of polyethylene resin>
Calculate according to JIS-K-7112.
<Haze>
Measured according to ASTM D-1003. In addition, it measures using the film heat-shrinked at the temperature of 140 degreeC to 30% in a film area.
<Slip property between film and metal>
Measured in accordance with ASTM D-1894, and evaluated by a dynamic friction coefficient measured using a 500 g rider (made of satin metal).

<Slippery between films>
Measured in accordance with ASTM D-1894, a 200 g rider (made of satin metal bonded with a 2 mm thick foam sheet) was stretched with a film thermally contracted to 140% at 140 ° C. and measured. Evaluation is based on the dynamic friction coefficient.
<Packaging suitability evaluation>
The packaging suitability is evaluated using an L-type automatic packaging machine HP-10 (trade name) manufactured by Hanagata Corporation. A packaging speed of 20 packs / minute is used, and a paper box (mass of 300 g) of 200 mm × 300 mm × 60 mm is used as an object to be packaged. Evaluate as ◯ if the product was able to be packaged without any problem, and x (describe the reason) if there was any problem (shrinkage failure, seal failure, etc.).
<High-speed packaging suitability evaluation>
Evaluation is performed in the same manner as in <Package suitability evaluation> except that the packaging speed is changed to 50 packs / minute. Evaluate as ◯ if the product was able to be packaged without any problem, and x (describe the reason) if there was any problem (shrinkage failure, seal failure, etc.).

<Resin used in Examples and Comparative Examples>
・ SB1:
A copolymer comprising a conjugated diene and a vinyl aromatic compound. Copolymer comprising butadiene as the conjugated diene and styrene as the vinyl aromatic compound and having a weight average molecular weight of 150,000 and Mw / Mn = 1.3. SB2:
A hydrogenated product of a copolymer comprising a conjugated diene and a vinyl aromatic compound. Copolymer comprising butadiene as conjugated diene and styrene as vinyl aromatic compound, having a weight average molecular weight of 210,000 and Mw / Mn = 2.2. PS1:
Styrene-alkyl acrylate copolymer. MFR = 6.5 g / 10 min.
・ PE1:
Ethylene / 1-octene copolymer. Density = 0.905 g / cm3, MFR = 0.8 g / 10 min.
・ PE2:
Low density polyethylene. Density = 0.920 g / cm3, MFR = 0.4 g / 10 min.PE3:
Ethylene / 1-octene copolymer. Density = 0.908g / cm3, MFR = 4.0g / 10min. PE4:
Ethylene / 1-butene copolymer. Density = 0.900 g / cm3, MFR = 0.8
・ EVA1:
Ethylene-vinyl acetate copolymer. Vinyl acetate content = 15 wt%, MFR = 1.0 g / 10 min.
Examples 1 to 6 are shown in Table 1, and Comparative Examples 1 and 2 are shown in Table 2.

[Example 1]
Three extruders were used to obtain a film by the double bubble inflation method. The resin mixture of SB1 and PS1 was put into an extruder for the inner layer. The mixing weight ratio of SB1 and PS1 was SB1: PS1 = 70: 30. The first and second surface layer extruders were charged with a resin mixture of PE1 and PE2. The mixing weight ratio of PE1 and PE2 was set to PE1: PE2 = 70: 30. Further, 0.5% by weight of silica and 1.0% by weight of erucic acid amide were added to the PE1 and PE2 mixture.
Extruded two-type three-layer parison with a thickness ratio of 35/30/35% using an in-die bonded annular die, and the inside of the tube-shaped parison is water-filled with water as the cooling medium from the outside. The resulting surface was blasted and roughened, followed by a cooling mandrel, and cooled and solidified from both the inside and outside of the tube to produce a tubular parison with a width of 180 mm and a thickness of 450 μm.
This tubular parison was guided to an electron beam irradiation device, irradiated with an electron beam, and subjected to crosslinking treatment at a dose of 50 kGy from both sides of the tubular parison. 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 120 ° C., and the MD direction is set to 5. Each film was stretched 5 times and 5.5 times in the TD direction to obtain a film having a thickness of about 15 μm.
While cutting both ends of the obtained tube-like film, it was cut into a size of 400 mm in width to form two films, each of which was made into one film having a width of 450 mm, an inner diameter of 76.2 mm, and a thickness of 10 mm. The film was wound with a length of 200 m with a tension that does not cause wrinkles in the cigarette to obtain a multilayer film for evaluation.

[Example 2]
A film having a thickness of about 15 μm was obtained by performing the same operation as in Example 1 except that a resin mixture of SB2 and PS1 was used in the extruder for the inner layer and the mixing weight ratio was SB2: PS1 = 70: 30.

[Example 3]
A film having a thickness of about 15 μm was obtained by performing the same operation as in Example 1 except that a resin mixture of SB2 and PS1 was used in the extruder for the inner layer and the mixing weight ratio was SB2: PS1 = 50: 50.

[Example 4]
A resin mixture of SB1 and PS1 is used for the extruder for the inner layer, the mixing weight ratio is SB1: PS1 = 50: 50, and a mixture of PE2, PE3, and PE4 is used for the first and second surface layers, and the mixing weight ratio Was changed to PE2: PE3: PE4 = 30: 30: 40, and the same operation as in Example 1 was performed to obtain a film having a thickness of about 15 μm.

[Example 5]
The same SB1 and PS1 resin mixture as in Example 4 is used for the inner layer, and the same PE2, PE3 and PE4 resin mixture as in Example 4 is used for the first and second surface layers. As an intermediate layer, a mixture of EVA1 and PE4 is used as an intermediate layer, the mixing weight ratio is EVA1: PE4 = 50: 50, and the thickness ratio is 15/20/30/20/15%. And the same operation as in Example 1 was performed to obtain a film having a thickness of about 15 μm.

[Example 6]
A film having a thickness of about 15 μm was obtained by performing the same operation as in Example 3 except that the thickness ratio of the surface layer to the inner layer was 20/60/20.

[Comparative Example 1]
A film having a thickness of about 15 μm was obtained by performing the same operation as in Example 1 except that SB1 alone was used for the inner layer extruder. The resulting film had a low elastic modulus, the film lacked rigidity, the film-metal slip property and the film-to-film slip property were lowered, and the packaging suitability was poor.

[Comparative Example 2]
A film having a thickness of about 15 μm was obtained in the same manner as in Example 1 except that the same mixture of PE1 and PE2 as the first and second surface layers was used in the extruder for the inner layer. The obtained film had a low elastic modulus, the film had insufficient rigidity, and its packaging ability was poor.

  The multilayer heat shrinkable film of the present invention can be suitably used as a heat shrinkable packaging film. It is particularly suitable for packaging recording media such as CDs and DVDs with low heat resistance.

Claims (10)

At least one inner layer (X) composed of a copolymer of a conjugated diene and vinyl aromatic copolymer and / or a hydrogenated product thereof (A) and a polystyrene resin (B), and a first and a second composed of an ethylene polymer. A multilayer heat-shrinkable film having at least three layers, characterized by having a second surface layer and satisfying the following 1) to 4).
1) 30-80% thermal shrinkage at 100 ° C
2) Maximum heat shrinkage stress at 40 ° C. to 140 ° C. is 0.9 to 4.0 MPa.
3) Elastic modulus is 200 to 900 MPa
4) The haze after shrinkage of 30% in the film area is 6.0% or less.   The copolymer of conjugated diene and vinyl aromatic and / or the hydrogenated product thereof (A) contains at least one polymer block composed of vinyl aromatic monomer units. Multilayer heat shrink film.   The polystyrene resin (B) is a dispersed phase comprising a copolymer of styrene units and methacrylate units, a copolymer of styrene units and alkyl acrylate units, and a polybutadiene or a copolymer of styrene units and butadiene units. The multilayer heat-shrinkable film according to claim 1, wherein the multilayer heat-shrinkable film is one or a mixture of two or more selected from copolymers having a continuous phase mainly composed of styrene / methacrylic acid ester.   The mixing weight ratio of the conjugated diene and vinyl aromatic copolymer and / or its hydrogenated product (A) and the polystyrene resin (B) in the inner layer (X) is (A) :( B) = 90: It is 10-10: 90, The multilayer heat-shrink film of Claim 1 characterized by the above-mentioned.   The thickness ratio of at least one inner layer (X) comprising a copolymer of a conjugated diene and vinyl aromatic copolymer and / or a hydrogenated product thereof (A) and a polystyrene resin (B) is 10 with respect to the total layer. The multilayer heat shrinkable film according to claim 1, which is ˜80%.   The first and second surface layers made of an ethylene polymer resin contain 0.05 to 3.0% by weight of an antiblocking agent and 0.05 to 3.0% by weight of a slip agent. The multilayer heat shrinkable film according to claim 1.   Between the inner layer (X) and the first and second surface layers, an ethylene vinyl acetate copolymer, an ethylene / methyl methacrylic acid copolymer, a metal partial neutralized product of an ethylene / methacrylic acid copolymer, ethylene / acrylic Acid copolymer, ethylene / ethyl acrylic acid copolymer, ethylene / maleic anhydride copolymer, ethylene / ethylene acrylate / maleic anhydride copolymer, ethylene / maleic acid modified resin, and Vicat softening temperature of 50 to 68 2. The multilayer heat shrinkable film according to claim 1, comprising at least one layer composed of one or a mixture of two or more selected from ethylene / α-olefin copolymers at 0 ° C. 3.   The multilayer heat-shrinkable film according to claim 1, wherein the gel fraction is 0.05 to 20% by weight.   The raw materials used for the inner layer, the first and second surface layers are melted from the extrusion machine, the resin of each layer is coextruded from the annular die, and the resulting tube-shaped parison is cooled. 5 to 120 kGy of cross-linking treatment is performed from both sides, and then the tubular parison is guided into the stretching machine, and air is injected between nip rolls with a speed difference while heating the stretching start point at 80 to 140 ° C. The method for producing a multilayer heat-shrinkable film according to any one of claims 1 to 8, wherein stretching is performed at a magnification of 2 to 10 times in each of the (MD) direction and the width (TD) direction.   The package for recording media packaged using the multilayer heat-shrink film of any one of 1-8.