WO2013080546A1

WO2013080546A1 - Heat-shrinkable multilayer film and process for producing the heat-shrinkable multilayer film

Info

Publication number: WO2013080546A1
Application number: PCT/JP2012/007653
Authority: WO
Inventors: Yuta Sekiya; Tadayoshi Itoh; Takahisa Ueyama
Original assignee: Kureha Corporation
Priority date: 2011-11-30
Filing date: 2012-11-29
Publication date: 2013-06-06
Also published as: JP2014533612A

Abstract

An object of the present invention is to provide a heat-shrinkable multilayer film, which is excellent in transparency and overlap seal properties and has heat shrinkability, gas barrier characteristics, and heat-sealing characteristics, and a process for producing the heat-shrinkable multilayer film. The heat-shrinkable multilayer film according to the present invention, comprises a multilayer structure having a superficial layer including a surface layer, a gas barrier layer, and an inner layer including a seal layer, in the multilayer structure, the surface layer being disposed on one surface, and the seal layer being disposed on the other surface,wherein the surface layer contains a silicone, and the superficial layer is crosslinked.

Description

HEAT-SHRINKABLE MULTILAYER FILM AND PROCESS FOR PRODUCING THE HEAT-SHRINKABLE MULTILAYER FILM

The present invention relates to a heat-shrinkable multilayer film, which enables effective packaging of raw meat, processed meat, and the like, and a process for producing the heat-shrinkable multilayer film.

Since raw meat, processed meat, and the like have irregular shapes, they are usually shrink-packed in terms of appearance and keeping freshness. A film used in shrink packaging is constituted of multiple layers including at least a surface layer forming an outer surface, a gas barrier layer having gas barrier characteristics, and a seal layer forming an inner surface and is imparted, according to the characteristics of each layer, heat shrinkability, transparency, gas barrier characteristics, or heat-sealing characteristics as characteristics for realizing good appearance and keeping freshness.

Further, a film used for shrink packaging is required to have overlap seal properties for workability enhancement purposes. The term "overlap seal properties" means properties in which when a packaging bag is constituted of a multilayer film so that a seal layer faces inside and the packaging bags are overlapped and heat-sealed, the respective seal layers of the packaging bags are heat-sealed together, and, at the same time, the respective surface layers are not heat-sealed or are heat-sealed to the extent that it is peelable. When the film has the overlap seal properties, the packaging bags can be heat-sealed while being partially overlapped with each other, and therefore the number of the packaging bags which can be heat-sealed at once can be increased, so that the working is made more efficient. Moreover, since the packaging bags are not required to be arranged so as not to be overlapped with each other, the workability of workers can be improved.

As a heat-shrinkable multilayer film having the overlap seal properties, there has been disclosed a film imparted the overlap seal properties by crosslinking of a surface layer forming an outer surface and uncrosslinking of a seal layer forming an inner surface (for example, see Patent Literature 1). Further, there has been disclosed a film whose outer surface is formed of a resin having a relatively high melting point such as polyamide resin and polyester resin and inner surface is formed of a resin having a relatively low melting point such as polyolefin resin, and a temperature difference of the material melting point is provided between the outer surface and the inner surface, whereby the film is imparted the overlap seal properties (for example, see Patent Literatures 2 to 5).

Patent Literature 1: Japanese Patent Application Laid-Open No. 9-39179
Patent Literature 2: International Publication WO 2008/099799
Patent Literature 3: European Patent No. 1131205
Patent Literature 4: European Patent Application No. 1985443
Patent Literature 5: European Patent Application No. 2147783

In the film disclosed in the Patent Literature 1, the temperature range in which the inner surfaces can be heat-sealed without heat-sealing the outer surfaces to each other is narrow, and the overlap seal properties are insufficient. Although the films disclosed in the Patent Literatures 2 to 5 have the excellent overlap seal properties, polyamide resin or polyester resin has a lower heat shrinkability than polyolefin resin. Therefore, the shrinkage of the outer surface cannot follow the shrinkage of the inner surface, and waving occurs on the outer surface, so that optical properties such as glossiness and transparency are reduced. In a resin having a high melting point such as polyamide resin and polyester resin, the temperature of extruding is naturally required to be increased. However, when polyvinylidene chloride resin (PVDC) is used in an intermediate layer in order to impart the gas barrier characteristics, considering degradation of PVDC, a process temperature cannot be increased. Thus, a die temperature cannot be increased, and since the resin having a high melting point such as polyamide resin and polyester resin cannot be stably extruded, the flatness of the film surface is deteriorated, and poor appearance may occur.

An object of the present invention is to provide a heat-shrinkable multilayer film, which is excellent in transparency and overlap seal properties and has heat shrinkability, gas barrier characteristics, and heat-sealing characteristics, and a process for producing the heat-shrinkable multilayer film.

A heat-shrinkable multilayer film according to the present invention, comprises a multilayer structure having a superficial layer including a surface layer, a gas barrier layer, and an inner layer including a seal layer, in the multilayer structure, the surface layer being disposed on one surface, and the seal layer being disposed on the other surface, wherein the surface layer contains a silicone, and the superficial layer is crosslinked.

In the heat-shrinkable multilayer film according to the present invention, it is preferable that a gel fraction of the superficial layer is within a range of not less than 20% and not more than 80%. An overlap-sealable temperature range can be further widened.

In the heat-shrinkable multilayer film according to the present invention, it is preferable that the surface layer contains a polyolefin-based resin. The heat resistance and the glossiness can be improved.

In the heat-shrinkable multilayer film according to the present invention, it is preferable that the superficial layer contains not less than 0.05% by mass and not more than 5% by mass of the silicone. The overlap-sealable temperature range can be further widened. Moreover, the transparency can be improved.

In the heat-shrinkable multilayer film according to the present invention, it is preferable that the surface layer contains not less than 1% by mass and not more than 10% by mass of the silicone. The overlap-sealable temperature range can be further widened. Moreover, the transparency can be improved.

A process for producing a heat-shrinkable multilayer film according to the present invention, which contains a multilayer structure having a superficial layer including a surface layer, a gas barrier layer, and an inner layer including a seal layer, in the multilayer structure, the surface layer being disposed on one surface, and the seal layer being disposed on the other surface, comprising: a step A of extruding at least a silicone-containing resin composition for the surface layer formation, a resin composition for the gas barrier layer formation, and a resin composition for the seal layer formation and forming a laminate having the multilayer structure; a step B of stretching the laminate; and a step C of irradiating an electron beam to the surface layer and crosslinking the superficial layer, wherein a process flow I in which the step B and the step C are sequentially progressed after the step A or a process flow II in which the step C and the step B are sequentially progressed after the step A is performed.

In the process for producing a heat-shrinkable multilayer film according to the present invention, it is preferable that the step C is a step of crosslinking the superficial layer until a gel fraction of the superficial layer is within a range of not less than 20% and not more than 80%. The overlap-sealable temperature range can be further widened.
EFFECTS OF THE INVENTION

The present invention can provide a heat-shrinkable multilayer film, which is excellent in transparency and overlap seal properties and has heat shrinkability, gas barrier characteristics, and heat-sealing characteristics, and a process for producing the heat-shrinkable multilayer film.

Hereinafter, the present invention will be described in detail with reference to embodiments, but the present invention shall not be interpreted as being limited to these descriptions. The embodiments may be variously modified as long as the effects of the present invention are obtained.

A heat-shrinkable multilayer film according to the present embodiment, comprises a multilayer structure having a superficial layer including a surface layer, a gas barrier layer, and an inner layer including a seal layer, in the multilayer structure, the surface layer being disposed on one surface, and the seal layer being disposed on the other surface, wherein the surface layer contains a silicone, and the superficial layer is crosslinked.

(Superficial layer)
The superficial layer includes at least a surface layer. The surface layer is disposed on one surface of the multilayer structure, is a layer being an outer surface of a bag, and serves to impart heat resistance and glossiness. In the heat-shrinkable multilayer film according to the present embodiment, in terms of heat resistance and glossiness, it is preferable that the surface layer contains a polyolefin-based resin. The polyolefin-based resin includes, for example, low-density polyethylene (LDPE), medium density polyethylene (MDPE), polypropylene (PP), a copolymer of propylene and an a-olefin having a carbon number of 2 or 4 to 8, an ethylene-a-olefin copolymer, an ethylene-polar comonomer copolymer such as an ethylene-vinyl acetate copolymer (EVA), an ethylene-alkyl acrylate having a carbon number of 1 to 4, an ethylene-methacrylic acid copolymer and an ethylene-methacrylic acid-unsaturated carboxylic acid copolymer, or an ionomer. The ethylene-a-olefin copolymer includes a copolymer prepared by using a Ziegler-Natta catalyst and a copolymer prepared by using a metallocene catalyst. The a-olefin as a comonomer used in polymerization of the ethylene-a-olefin copolymer is butene-1 having a carbon number of 4, pentene-1 having a carbon number of 5, 4-methylpentene-1 or hexane-1 having a carbon number of 6, or octane-1 having a carbon number of 8, for example. Specific examples of the ethylene-a-olefin copolymer include very low density polyethylene (VLDPE) whose density is 0.900 g/cm³ to 0.909 g/cm³ and linear low-density polyethylene (LLDPE) whose density is 0.910 g/cm³ to 0.925 g/cm³. These may be used alone, or two kinds or more of them may be used in combination. Among them, particularly preferred are VLDPE and LLDPE in terms of stretchability, particularly preferred is PP in terms of heat resistance.

The density of the resin used in the surface layer is preferably not less than 0.880 g/cm³ and not more than 0.940 g/cm³, more preferably not less than 0.900 g/cm³ and not more than 0.925 g/cm³.

The melting point of the resin used in the surface layer is preferably not less than 90^oC and not more than 160^oC, more preferably not less than 110^oC and not more than 150^oC. When the melting point is less than 90^oC, the heat resistance may be insufficient. When the melting point is more than 160^oC, the extruding temperature becomes high, and therefore, the surface may have less flatness, or the stretchability may be inhibited. When PVDC is used in a gas barrier layer, PVDC may be degraded.

The surface layer contains the silicone. In the heat-shrinkable multilayer film according to the present embodiment, the surface layer preferably contains not less than 1% by mass and not more than 10% by mass of the silicone, more preferably not less than 1% by mass and not more than 5% by mass, particularly preferably not less than 1.25% by mass and not more than 2.5% by mass. When the silicone content is less than 1% by mass, the temperature range exhibiting the overlap seal properties may be narrowed. When the silicone content is more than 10% by mass, the surface layer may have less transparency. The surface layer may contain various additives such as heat stabilizer, plasticizer, antioxidant, and softener as well as polyolefin-based resin and silicone. It is preferable that, among the additives, the surface layer contains the softener in terms of capable of suppressing a film bending phenomenon at the time of shrinkage. The softener includes a polyolefin-based elastomer such as an ethylene-a-olefin copolymer and a propylene-a-olefin copolymer, an ethylene-based copolymer such as an ethylene-vinyl acetate copolymer, polyisobutylene, polybutene, polybutadiene, a butadiene-styrene copolymer, neoprene, or a natural rubber, for example.

The silicone is a compound having a main backbone of siloxane bonds and is, for example, polydimethylsiloxane, polymethyl phenyl siloxane, or polymethylvinylsiloxane. These may be used alone, or two kinds or more of them may be used in combination. Among them, more preferred is polydimethylsiloxane.

The kinematic viscosity of the silicone is correlative to the molecular weight of the silicone. Namely, as the kinematic viscosity of the silicon becomes high, the molecular weight of the silicone tends to increase. In the heat-shrinkable multilayer film according to the present embodiment, the kinematic viscosity of the silicone at 25^oC is preferably not less than 100 mm²/sec (cSt) and not more than 1,500,000 mm²/sec (cSt), more preferably not less than 500 mm²/sec and not more than 1,000,000 mm²/sec, particularly preferably not less than 1000 mm²/sec and not more than 100,000 mm²/sec. When the kinematic viscosity of the silicone at 25^oC is less than 100 mm²/sec, a friction force against a barrel or a screw is reduced in melt extrusion, and stable extrusion cannot be performed at times. When the kinematic viscosity of the silicone at 25^oC is more than 1,500,000 mm²/sec, it becomes difficult to mix with a resin, or a mixing failure may easily occur.

The thickness of the surface layer is preferably not less than 0.5 mm and not more than 40 mm, more preferably not less than 1 mm and not more than 10 mm.

The superficial layer preferably further includes an intermediate layer T1 between the surface layer and the gas barrier layer. The intermediate layer T1 serves to enhance transparency and glossiness. The intermediate layer T1 preferably contains a polyolefin-based resin. The polyolefin-based resin exemplified in the surface layer can be used, and in terms of stretchability, adhesiveness to the surface layer, and transparency, it is particularly preferable to use an ethylene-polar comonomer copolymer such as an EVA, an ethylene-alkyl acrylate having a carbon number of 1 to 4, an ethylene-methacrylic acid copolymer and an ethylene-methacrylic acid-unsaturated carboxylic acid copolymer, or an ionomer. The resin used in the intermediate layer T1 may be each independently used, or two or more kinds of the resins may be used in combination. The intermediate layer T1 may contain various additives such as heat stabilizer, plasticizer, antioxidant, and softener as well as a polyolefin-based resin. It is preferable that, among the additives, the intermediate layer T1 contains the softener in terms of capable of suppressing a film bending phenomenon at the time of shrinkage. The softener includes a polyolefin-based elastomer such as an ethylene-a-olefin copolymer and a propylene-a-olefin copolymer, an ethylene-based copolymer such as an ethylene-vinyl acetate copolymer, polyisobutylene, polybutene, polybutadiene, a butadiene-styrene copolymer, neoprene, or natural rubber, for example.

Although the melting point of the resin used in the intermediate layer T1 is not particularly limited, the melting point is preferably not less than 70^oC and not more than 120^oC, more preferably not less than 80^oC and not more than 100^oC.

The thickness of the intermediate layer T1 is preferably not less than 5 mm and not more than 50 mm, more preferably not less than 10 mm and not more than 30 mm. The intermediate layer T1 may be constituted of one or two or more layers. When the intermediate layer T1 is constituted of two or more layers, the respective layers may have the same or different compositions.

The superficial layer preferably further includes an adhesive layer S1 as a layer adjacent to the gas barrier layer. The adhesive layer S1 serves to improve adhesiveness to the gas barrier layer. The adhesive layer S1 preferably contains, for example, an adhesive resin such as an ethylene-polar comonomer copolymer and an acid-modified polyolefin. The ethylene-polar comonomer copolymer is, for example, an ethylene-vinyl acetate copolymer, an ethylene-alkyl acrylate having a carbon number of 1 to 4, an ethylene-methacrylic acid copolymer, an ethylene-methacrylic acid-unsaturated carboxylic acid copolymer, or an ethylene-acrylic acid copolymer. An acid-modified polyolefin is, for example, a reactant of olefins alone or an olefin copolymer and an unsaturated carboxylic acid such as maleic acid or fumaric acid, an acid anhydride, an ester, or a metal salt. The resin used in the adhesive layer S1 may be used alone, or two kinds or more of the resins may be used in combination. The adhesive layer S1 may contain various additives such as heat stabilizer, plasticizer, and antioxidant as well as the adhesive resin.

Although the melting point of the resin used in the adhesive layer S1 is not particularly limited, the melting point is preferably not less than 70^oC and not more than 130^oC, more preferably not less than 80^oC and not more than 120^oC.

The thickness of the adhesive layer S1 is preferably not less than 0.5 mm and not more than 10 mm, more preferably not less than 1 mm and not more than 5 mm.

The superficial layer is crosslinked. The degree of crosslinking can be typically represented by a gel fraction. The gel fraction can be obtained as follows. When the superficial layer separated from the heat-shrinkable multilayer film is immersed in a solvent such as 1, 2, 4-trichlorobenzene, a crosslinked portion remains as an insoluble substance (gel). The gel fraction is the ratio of a dry mass W2 of the insoluble substance remaining without being dissolved and a mass W1 of the superficial layer before being dissolved by a solvent expressed in percentage. Namely, the gel fraction can be obtained by the Mathematical formula 1:
(Mathematical formula 1) gel fraction [%] = (W2/W1) x 100.

In the heat-shrinkable multilayer film according to the present embodiment, the gel fraction of the superficial layer is preferably within a range of not less than 20% and not more than 80%. The gel fraction is more preferably not less than 25% and not more than 75%, particularly preferably not less than 30% and not more than 70%. When the gel fraction is less than 20%, the temperature range exhibiting the overlap seal properties may be narrowed. The present inventors have found that as the degree of crosslinking becomes high (the gel fraction becomes high), the overlap-sealable temperature range tends to be widened. When the gel fraction is more than 80%, problems may occur in stretching processability. The present invention is not limited to a crosslinking reaction and a cross-linked structure.

In the present embodiment, a layer other than the surface layer constituting the superficial layer (for example, the intermediate layer T1 or the adhesive layer S1) may contain a silicone in a range where the effects of the present invention are not impaired. When the surface layer and the layer other than the surface layer contain silicone, or when only the surface layer contains the silicone, the superficial layer preferably contains not less than 0.05% by mass and not more than 5% by mass of the silicone, more preferably not less than 0.1% by mass and not more than 2.5% by mass of the silicone, particularly preferably not less than 0.3% by mass and not more than 2.2% by mass of the silicone. When the superficial layer contains less than 0.05% by mass of the silicone, the temperature range expressing the overlap seal properties may be narrowed. When the superficial layer contains more than 5% by mass of the silicone, there may be less transparency. For example, when the surface layer and the intermediate layer T1 contain the silicone, the intermediate layer T1 is crosslinked as well as the surface layer.

(Gas barrier layer)
The gas barrier layer serves to suppress transmission of oxygen, water vapor, and so on and prevent deterioration of contents. The gas barrier layer contains, as a gas barrier resin, a polyvinylidene chloride (PVDC)-based resin or an ethylene-vinyl alcohol copolymer (EVOH), for example.

The polyvinylidene chloride-based resin is, for example, a copolymer of 60 to 98% by mass of vinylidene chloride (VDC) and 2 to 40% by mass of other copolymerizable monomer (comonomer).

The comonomer includes, for example, vinyl chloride; acrylic acid alkyl ester (the carbon number of alkyl group is 1 to 18) such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and stearyl acrylate; alkyl ester methacrylate (the carbon number of alkyl group is 1 to 18) such as methyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, and stearyl methacrylate; vinyl cyanide such as acrylonitrile and methacrylonitrile; aromatic vinyl such as styrene; vinyl esters of aliphatic carboxylic acids having a carbon number of 1 to 18 such as vinyl acetate; alkyl vinyl ether having a carbon number of 1 to 18; vinyl-polymerizable unsaturated carboxylic acid such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconic acid; alkyl ester (containing partial ester and the carbon number of alkyl group is 1 to 18) of vinyl-polymerizable unsaturated carboxylic acid such as maleic acid, fumaric acid, and itaconic acid; diene-based monomer, functional group-containing monomer, and multifunctional monomer.

The above comonomers may be each independently used, or two kinds or more of them may be used in combination. Among those comonomers, preferred are vinyl chloride, methyl acrylate, or lauryl acrylate. When the copolymerization ratio of the comonomer is too small, internal plasticization is insufficient, and the melt processability is reduced. When the copolymerization ratio of the comonomer is too large, the gas barrier characteristics are reduced. The copolymerization ratio of the comonomer is preferably 3 to 35% by mass, more preferably 5 to 30% by mass, particularly preferably 10 to 25% by mass.

In terms of processability when it is formed into a film, packaging machine aptitude, and cold resistance, the reduced viscosity (hsp/C) of the polyvinylidene chloride-based resin is preferably 0.035 to 0.070, more preferably 0.040 to 0.065, particularly preferably 0.045 to 0.063. When the reduced viscosity of the polyvinylidene chloride-based resin is too low, the processability is lowered. When the reduced viscosity of the polyvinylidene chloride-based resin is too high, discoloration tendency is exhibited, and therefore, any of these cases are not preferable. Two or more kinds of the polyvinylidene chloride-based resins with different reduced viscosities can be used in combination, whereby the processability can be improved. When two or more kinds of the polyvinylidene chloride-based resins are used in combination, the reduced viscosity of the mixed resin is preferably within the above range.

The polyvinylidene chloride-based resin can be blended with other resins as desired. The other resins include, for example, an ethylene-vinyl acetate copolymer, (meth)acrylic acid ester, and preferably (co)polymer of (meth)acrylic acid alkyl ester having a carbon number of an alkyl group of 1 to 18 [for example, methyl (meth)acrylate-butyl (meth)acrylate copolymer], and a methyl methacrylate-butadiene-styrene copolymer. Those other resins may be blended when a polyvinylidene chloride-based resin composition is prepared or may be contained in a resin composition for coloration to be blended with a polyvinylidene chloride-based resin. The other reins are usually used in an amount of not more than 20 parts by mass based on 100 parts by mass of the polyvinylidene chloride-based resin.

The ethylene-vinyl alcohol copolymer includes, for example, a saponified ethylene-vinyl acetate copolymer. It is preferable that a ethylene content of the saponified ethylene-vinyl acetate copolymer is 25 to 48 mol%, and the saponification degree is not less than 98%. When the ethylene content of the saponified ethylene-vinyl acetate copolymer is less than 25 mol%, an insoluble substance tends to be easily produced. When the ethylene content of the saponified ethylene-vinyl acetate copolymer is more than 48 mol%, oxygen gas barrier characteristics tend to be deteriorated. When the saponification degree of the saponified ethylene-vinyl acetate copolymer is less than 98%, oxygen gas barrier characteristics tend to be deteriorated.

The gas barrier resin may be used alone, or two or more kinds of the gas barrier resins may be used in combination. The gas barrier resin and the polyolefin-based resin can obtain good adhesiveness by interposing the above-mentioned adhesive layer S1 and an adhesive layer S2 to be described later between them. The gas barrier layer may contain various additives such as heat stabilizer, plasticizer, and antioxidant as well as the gas barrier resin.

The thickness of the gas barrier layer is preferably not less than 1 mm and not more than 40 mm, more preferably not less than 3 mm and not more than 30 mm, particularly preferably not less than 4 mm and not more than 10 mm. The gas barrier layer may be constituted of one or two or more layers. When the gas barrier layer is constituted of two or more layers, the respective layers may have the same or different compositions.

(Inner layer)
The inner layer contains at least a seal layer. The seal layer is disposed on a surface on the opposite side of the surface layer, becomes an inner surface of a bag, and serves to seal the bag by being heat-sealed. The seal layer preferably contains a polyolefin-based resin. The polyolefin-based resin includes, for example, low-density polyethylene (LDPE), medium density polyethylene (MDPE), an ethylene-a-olefin copolymer, an ethylene-polar comonomer copolymer such as an ethylene-vinyl acetate copolymer (EVA), an ethylene-alkyl acrylate having a carbon number of 1 to 4, an ethylene-methacrylic acid copolymer and an ethylene-methacrylic acid-unsaturated carboxylic acid copolymer, or an ionomer. The ethylene-a-olefin copolymer includes a copolymer prepared by using a Ziegler-Natta catalyst and a copolymer prepared by using a metallocene catalyst. An a-olefin as a comonomer used in polymerization of the ethylene-a-olefin copolymer is butene-1 having a carbon number of 4, pentene-1 having a carbon number of 5, 4-methylpentene-1 or hexane-1 having a carbon number of 6, or octane-1 having a carbon number of 8, for example. Specific examples of the ethylene-a-olefin copolymer include very low density polyethylene (VLDPE) whose density is not less than 0.900 g/cm³ and not more than 0.909 g/cm³ and linear low-density polyethylene (LLDPE) whose density is not less than 0.910 g/cm³ and not more than 0.925 g/cm³. These may be used alone, or two kinds or more of them may be used in combination. Among them, particularly preferred are EVA and ionomer in terms of stretchability and low-temperature seal properties. The resin used in the seal layer may be used alone, or two kinds or more of the resins may be used in combination. The seal layer may contain various additives such as heat stabilizer, plasticizer, and antioxidant as well as the polyolefin-based resin.

The density of the resin used in the seal layer is preferably not less than 0.880 g/cm³ and not more than 0.940 g/cm³, more preferably not less than 0.900 g/cm³ and not more than 0.925 g/cm³.

The melting point of the resin used in the seal layer is preferably not less than 80^oC and not more than 110^oC, more preferably not less than 85^oC and not more than 100^oC. When the melting point is less than 80^oC, blocking occurs at the time of stretching, there may be less stretching film forming properties. When the melting point is more than 110^oC, the overlap-sealable temperature range may be narrowed.

It is preferable that the melting point of the seal layer is lower than the melting point of the surface layer and the melting point difference is not less than 10^oC and not more than 60^oC. The melting point difference is more preferably not less than 15^oC and not more than 55^oC, particularly preferably not less than 20^oC and not more than 50^oC. When the melting point difference between the surface layer and the seal layer is less than 10^oC, the temperature range expressing the overlap seal properties may be narrowed. When the melting point difference between the surface layer and the seal layer is more than 60^oC, the working temperature difference is increased, and when the melting point of the seal layer is relatively high, the melting point of the surface layer is relatively high, whereby the extruding temperature becomes high, and therefore, as a result, the surface may have less flatness, or stretchability may be inhibited. Meanwhile, when the melting point of the surface layer is relatively low, the melting point of the seal layer is relatively low, whereby blocking of the seal layer may occur.

The thickness of the seal layer is preferably not less than 3 mm and not more than 50 mm, more preferably not less than 5 mm and not more than 30 mm, particularly preferably not less than 8 mm and not more than 20 mm.

It is preferable that the inner surface further includes the intermediate layer T2 between the gas barrier layer and the seal layer. The intermediate layer T2 serves to enhance transparency and stretchability. It is preferable that the intermediate layer T2 contains a polyolefin-based resin. The polyolefin-based resin exemplified in the surface layer can be used, and in terms of stretchability, adhesiveness to the surface layer, and transparency, particularly preferred are an ethylene-polar comonomer copolymer such as EVA, an ethylene-alkyl acrylate having a carbon number of 1 to 4, an ethylene-methacrylic acid copolymer and an ethylene-methacrylic acid-unsaturated carboxylic acid copolymer, or an ionomer. The resin used in the intermediate layer T2 may be used alone, two or more kinds of the resins may be used in combination. The intermediate layer T2 may contain various additives such as heat stabilizer, plasticizer, and antioxidant as well as the polyolefin-based resin.

Although the melting point of the resin used in the intermediate layer T2 is not particularly limited, the melting point is preferably not less than 70^oC and not more than 120^oC, more preferably not less than 80^oC and not more than 100^oC.

The thickness of the intermediate layer T2 is preferably not less than 3 mm and not more than 50 mm, more preferably not less than 5 mm and not more than 30 mm, particularly preferably not less than 8 mm and not more than 20 mm. The intermediate layer T2 may be constituted of one or two or more layers. When the intermediate layer T2 is constituted of two or more layers, the respective layers may have the same or different compositions.

It is preferable that the inner layer further includes the adhesive layer S2 as a layer adjacent to the gas barrier layer. The adhesive layer S2 serves to improve adhesiveness to the gas barrier layer. The adhesive resin exemplified in the adhesive layer S1 can be used as the adhesive layer S2, and the adhesive resin may be used alone, or two or more kinds of the adhesive resins may be used in combination. The adhesive layer S2 may contain various additives such as heat stabilizer, plasticizer, and antioxidant as well as the adhesive resin.

Although the melting point of the resin used in the adhesive layer S2 is not particularly limited, the melting point is preferably not less than 70^oC and not more than 130^oC, more preferably not less than 80^oC and not more than 120^oC.

The thickness of the adhesive layer S2 is preferably not less than 0.5 mm and not more than 10 mm, more preferably not less than 1 mm and not more than 5 mm.

(Heat-shrinkable multilayer film)
The heat-shrinkable multilayer film according to the present embodiment has a multilayer structure having at least the superficial layer, the gas barrier layer, and the inner layer. In the multilayer structure, the surface layer may be disposed on one surface, the seal layer may be disposed on the other surface, and various forms can be provided depending on the application. The embodiments of the multilayer structure include a three-layer structure constituted of the surface layer, the gas barrier layer, and the seal layer laminated in sequence, a five-layer structure constituted of the surface layer, the adhesive layer S1, the gas barrier layer, the adhesive layer S2, and the seal layer laminated in sequence, a six-layer structure constituted of the surface layer, the intermediate layer T1, the adhesive layer S1, the gas barrier layer, the adhesive layer S2, and the seal layer laminated in sequence, and a seven-layer structure constituted of the surface layer, the intermediate layer T1, the adhesive layer S1, the gas barrier layer, the adhesive layer S2, the intermediate layer T2, and the seal layer laminated in sequence. However, these structures are just examples, and the present invention is not limited only to this.

The thickness of the heat-shrinkable multilayer film according to the present embodiment is preferably not less than 20 mm and not more than 150 mm, more preferably not less than 30 mm and not more than 120 mm. When the thickness is less than 20 mm, mechanical strength may be insufficient. When the thickness is more than 150 mm, it takes a long time for heat sealing, and there may be less packaging aptitude. Moreover, there may be less stretching processability.

In the heat-shrinkable multilayer film according to the present embodiment, a haze measured in accordance with JIS K 7136:2000 "Plastics - Determination of haze for transparent materials" is preferably not more than 25%, more preferably not more than 20%.

In the heat-shrinkable multilayer film according to the present embodiment, a hot water shrinkage at 80^oC in at least one of a machine direction of a film (MD direction) and a direction transverse to the machine direction of a film (TD direction) is preferably not less than 30% and not more than 60%, more preferably not less than 35% and not more than 55%. When the hot water shrinkage is less than 30%, the shrinkage is insufficient, and the appearance of a package may be deteriorated. When the hot water shrinkage is more than 60%, the contents may be deformed by excessive shrinkage. Here, the hot water shrinkage is expressed by a percentage by dividing the difference between a length of a film in the MD direction or the TD direction before being immersed in the hot water (80^oC) and the length of a film in the MD direction or the TD direction after being immersed in the hot water by the length of a film in the MD direction or the TD direction before being immersed in the hot water.

The heat-shrinkable multilayer film according to the present embodiment has the overlap seal properties. Here, the term "overlap seal properties" means properties in which when a packaging bag is constituted of a multilayer film so that a seal layer faces inside and the packaging bags are overlapped and heat-sealed, the respective seal layers of the packaging bags are heat-sealed together, and, at the same time, the respective surface layers are not heat-sealed or are heat-sealed to the extent that it is peelable. At this time, the heat-seal strength (also referred to as fusion strength, bonding strength) between the seal layers is preferably not less than 5 N/15 mm, more preferably 1 not less than 10 N/15 mm, particularly preferably not less than 15 N/15 mm. When the heat-seal strength between the seal layers of the packaging bags is less than 5 N/15 mm, sealability is insufficient, and air may enter inside the bag in a packaging process or at the time of transportation. Meanwhile, the heat-seal strength between the surface layers is preferably not more than 1.5 N/15 mm, more preferably 1.0 N/15 mm. When the heat-seal strength between the surface layers is more than 1.5 N/15 mm, the resistance is large when the packaging bags are separated from each other, and it is hard to say that the heat-shrinkable multilayer film has practical overlap seal properties. The method of measuring the seal strength is as described in Examples.

Next, a process for producing a heat-shrinkable multilayer film according to the present embodiment will be described.

In the process for producing a heat-shrinkable multilayer film according to the present embodiment, which contains a multilayer structure having a superficial layer including a surface layer, a gas barrier layer, and an inner layer including a seal layer, in the multilayer structure, the surface layer being disposed on one surface, and the seal layer being disposed on the other surface, comprising; a step A of extruding at least a silicone-containing resin composition for the surface layer formation, a resin composition for the gas barrier layer formation, and a resin composition for the seal layer formation and forming a laminate having the multilayer structure; a step B of stretching the laminate; and a step C of irradiating an electron beam to the surface layer and crosslinking the superficial layer, wherein a process flow I in which the step B and the step C are sequentially progressed after the step A or a process flow II in which the step C and the step B are sequentially progressed after the step A is performed. Then, the process flow I will be described taking as an example.

In the process for producing a heat-shrinkable multilayer film according to the present embodiment, although a method of forming a multilayer structure is not particularly limited, preferred is a melt extrusion method. The melt extrusion method is an inflation method or a T-die method, for example. Among those methods, more preferred is the inflation method. Next, the production process using the inflation method will be described taking as an example.

(Step A)
In the step A, an non-stretched laminate having a multilayer structure is formed. First, resin compositions for the formation of the respective layers, including at least the silicone-containing resin composition for the surface layer formation, the resin composition for the gas barrier layer formation, and the resin composition for the seal layer formation and, according to need, including a resin composition for the intermediate layer T1 formation, a resin composition for the intermediate layer T2 formation, a resin composition for the adhesive layer S1 formation, and a resin composition for the adhesive layer S2 formation are introduced into an extruder and melted. Then, the molten resin compositions are melted and joined to a multilayer structure in which the surface layer is disposed on one surface and the seal layer is disposed on the other surface and co-extruded in a tubular form through an annular die. At this time, it is preferable to dispose the surface layer outside the primary tube and dispose the seal layer inside the primary tube. In the subsequent step C, an electron beam is efficiently irradiated to the surface layer, and the superficial layer can be more efficiently crosslinked. The primary tube is cooled by cooling water, and a flat tubular non-stretched laminate is obtained.

In the step A, silicone may be blended in the form of being contained in a master batch. In a silicone master batch, a base resin is not particularly limited as long as the effects are not impaired in the present invention and is, for example, LDPE, VLDPE, LLDPE, or EVA.

(Step B)
In the step B, the obtained flat tubular non-stretched laminate is stretched, and a stretched film is formed. First, the flat tubular non-stretched laminate is heated by passing through a hot water bath, for example. After that, air is blown into the primary tube, a bubble-shaped tubular film is formed, and the film is stretched simultaneously and biaxially in the MD direction and the TD direction while being cooled by airing with cold air. In the step B, the temperature at which the non-stretched laminate is heated is preferably 70 to 95^oC, more preferably 75 to 90^oC. The temperature of airing with cold air is preferably 5 to 25^oC. The stretching ratio is preferably 2 to 4 times in each of the directions of MD and TD. The stretching ratio in the MD direction and the stretching ratio in the TD direction may be the same as each other or different from each other.

In the present embodiment, it is preferable to perform thermal relaxation treatment after stretching in terms of dimensional stability.

(Step C)
In the step C, an electron beam (EB) is irradiated to the surface layer, and the superficial layer is crosslinked. In the process for producing a heat-shrinkable multilayer film according to the present embodiment, the step C is preferably a step of crosslinking the superficial layer until the gel fraction of the superficial layer is within a range of not less than 20% and not more than 80% in terms that the overlap-sealable temperature range can be further widened. The gel fraction is more preferably not less than 25% and not more than 75%, particularly preferably not less than 30% and not more than 70%. The degree of crosslinking can be controlled by adjusting various conditions such as absorbed dose and acceleration voltage.

In the step C, an irradiation conditions of the electron beam may be suitably set in accordance with a target degree of crosslinking, and as one example, the absorbed dose is preferably 50 to 250 kGy (kilogray) within an acceleration voltage range of 150 to 500 kV, more preferably 80 to 200 kGy. In the present embodiment, in the irradiation with the electron beam, there may be adopted an in-line method in which after the formation of a stretched film, the electron beam is irradiated not through a winding process, or there may be adopted an off-line method in which after the formation of the stretched film, the electron beam is irradiated through the winding process.

Although the process flow I in which the step B and the step C are sequentially progressed after the step A has been described, the process flow II in which the step C and the step B are sequentially progressed after the step A may be performed. Further, a step D in which the electron beam is irradiated to the non-stretched laminate before being stretched in the step B after the process A may be provided. By performing the step D, a cross-linking reaction between a resin contained in the surface layer and a resin contained in the intermediate layer is progressed, for example, and stretchability and heat resistance can be further enhanced.

Next, although the present invention will be described using Examples, the present invention is not limited to the Examples.

(Step A)
A first layer as the resin composition for the surface layer formation which is a product (hereinafter referred to as VLDPE-1 + 5 wt% MB-1) prepared by blending 5% by mass of a silicone master batch (Hekisashirikonku ML-950 manufactured by Hexa Chemical Co., Ltd., base resin: LDPE, silicon content: 50% by mass, and hereinafter referred to as MB-1) with very low density polyethylene (Moretec V0398CN manufactured by Prime Polymer Co., Ltd., density: 0.907 g/cm³, melting point (Tm): 117^oC, and hereinafter referred to as VLDPE-1), a second layer as the resin composition for the intermediate layer T1 formation layer which is a product (hereinafter referred to as EVA-1 + 5 wt% MB-1) prepared by blending 5% by mass of MB-1 with an ethylene-vinyl acetate copolymer (Evaflex V430RC, manufactured by DU PONT-MISTUI POLYCHEMICALS CO., LTD., vinyl acetate content: 19 mol%, density: 0.94 g/cm³, melting point: 84^oC, and hereinafter referred to as EVA-1), a third layer as the resin composition for the adhesive layer S1 formation which is an ethylene-methyl acrylate copolymer (Elvaloy 1218AC manufactured by DU PONT-MISTUI POLYCHEMICALS CO., LTD., density: 0.94 g/cm³, melting point: 94^oC, and hereinafter referred to as EMA-1), a fourth layer as the resin composition for the gas barrier layer formation which is a vinylidene chloride-vinyl chloride copolymer (manufactured by Kureha Corporation, density: 1.71 g/cm³, melting point: 145^oC, and hereinafter referred to as PVDC-1), a fifth layer as the resin composition for the adhesive layer S2 formation which is EMA-1, and a six layer as the resin composition for the seal layer formation which is an ionomer (Himilan 1601 manufactured by DU PONT-MISTUI POLYCHEMICALS CO., LTD., density: 0.94 g/cm³, melting point 97^oC, and hereinafter referred to as Ionomer-1 ) are extruded by a plurality of extruders individually, and a molten resin is introduced into an annular die, melted and joined so as to provide a six layer structure constituted of the surface layer, the intermediate layer T1, the adhesive layer S1, the gas barrier layer, the adhesive layer S2, and the seal layer laminated in sequence from outside to inside, and co-extruded. A molten primary tube flowed through a die exit is cooled by cold water showering of 10 to 20^oC, and a primary tube with a flat width of 160 mm is obtained.

(Step B)
The flat primary tube obtained in the step A is passed through a hot water bath of 86^oC and then is formed into a bubble-shaped tubular film, and the film is stretched simultaneously and biaxially at stretching ratios of 2.0 times in the machine direction (MD direction) and 2.8 times in the transverse direction (TD direction) by the inflation method while being cooled by airing with cold air of 15 to 20^oC. Then, the biaxially stretched film is heated at 40^oC for 1 second and thermally relaxed, and a biaxially stretched film (a heat-shrinkable multilayer film) is produced. The flat width of the obtained heat-shrinkable multilayer film is 400 mm. In the thickness of each layer, the surface layer has a thickness of 3 mm, the intermediate layer T1 has a thickness of 10 mm, the adhesive layer S1 has a thickness of 1.5 mm, the gas barrier layer has a thickness of 4 mm, the adhesive layer S2 has a thickness of 1.5 mm, and the seal layer has a thickness of 30 mm, and the total thickness of the film is 50 mm.

(Step C)
The biaxially stretched film obtained in the step B is irradiated off-line with the electron beam in an electron beam irradiation apparatus with an acceleration voltage of 275 kV. As the electron beam irradiation conditions, the electron beam with an absorbed dose of 100 kGy is irradiated.

A heat-shrinkable multilayer film is produced as in the Example 1 except that MB-1 blended with the resin composition for the surface layer formation and the resin composition for the intermediate layer T1 formation is changed to a silicone master batch (silicone concentrate BY27-002 manufactured by Dow Corning Toray Co., Ltd., base resin: LDPE, silicone content: 50% by mass, and hereinafter referred to as MB-2) in the Example 1.

A heat-shrinkable multilayer film is produced as in the Example 1 except that MB-1 is not blended with the resin composition for the intermediate layer T1 formation and the acceleration voltage in the electron beam irradiation is changed to 250 kV in the Example 1.

A heat-shrinkable multilayer film is produced as in the Example 3 except that the thickness of the surface layer is 1.5 mm and the thickness of the intermediate layer T1 is 11.5 mm in the Example 3.

A heat-shrinkable multilayer film is produced as in the Example 3 except that a product (hereinafter referred to as VLDPE-1 + 10 wt% MB-1) prepared by blending 10% by mass of MB-1 with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 5 wt% MB-1 and the acceleration voltage in the electron beam irradiation is changed to 275 kV in the Example 3.

A heat-shrinkable multilayer film is produced as in the Example 3 except that a product (hereinafter referred to as VLDPE-1 + 2.5 wt% MB-1) prepared by blending 2.5% by mass of MB-1 with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 5 wt% MB-1 and the acceleration voltage in the electron beam irradiation is changed to 275 kV in the Example 3.

A heat-shrinkable multilayer film is produced as in the Example 3 except that the exposure dose in the electron beam irradiation is changed to 80 kGy in the Example 3.

A heat-shrinkable multilayer film is produced as in the Example 3 except that the exposure dose in the electron beam irradiation is changed to 150 kGy in the Example 3.

A heat-shrinkable multilayer film is produced as in the Example 1 except that adhesive polyethylene (Admer SF730 manufactured by Mitsui Chemicals, Inc., density: 0.92 g/cm³, melting point: 119^oC, and hereinafter referred to as mod-VL) is used as the resin composition for the adhesive layer S1 formation and the resin composition for the adhesive layer S2 formation instead of EMA-1 and a saponified ethylene-vinyl acetate copolymer (EVAL EPG156B manufactured by KURARAY CO., LTD., ethylene content: 48 mol%, density: 1.11 g/cm³, melting point 160^oC, and hereinafter referred to as EVOH-1) is used as the resin composition for the gas barrier layer formation instead of PVDC-1 in the Example 1.

A heat-shrinkable multilayer film is produced as in the Example 3 except that VLDPE-1 is used as the resin composition for the intermediate layer T1 formation instead of EVA-1 in the Example 3. In the obtained heat-shrinkable multilayer film, a boundary between the surface layer and the intermediate layer T1 is unclear, and the heat-shrinkable multilayer film has a five layer structure constituted of the surface layer, the adhesive layer S1, the gas barrier layer, the adhesive layer S2, and the seal layer artificially laminated from outside to inside.

A heat-shrinkable multilayer film is produced as in the Example 3 except that a product (hereinafter referred to as LLDPE-1 + 5 wt% MB-1) prepared by blending 5% by mass of MB-1 with linear low-density polyethylene (Moretec 0238CN manufactured by Prime Polymer Co., Ltd., density: 0.916 g/cm³, melting point: 119^oC, and hereinafter referred to as LLDPE-1) is used as the resin composition for the surface layer formation instead of VLDPE-1 + 5 wt% MB-1 in the Example 3.

A heat-shrinkable multilayer film is produced as in the Example 4 except that a product (hereinafter referred to as VLDPE-1 + 28 wt% MB-3) prepared by blending 28% by mass of a silicone master batch (prototype, silicone: polydimethylsiloxane (Shinetsu Silicone KF-96H-1,000,000 cs manufactured by Shinetsu Chemical Industry Co., Ltd.), silicon content: 9% by mass, kinematic viscosity of silicone: 1,000,000 mm²/sec, base resin: VLDPE-1, and hereinafter referred to as MB-3) with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 5 wt% MB-1 in the Example 4.

A heat-shrinkable multilayer film is produced as in the Example 4 except that a product (hereinafter referred to as VLDPE-1 + 25 wt% MB-4) prepared by blending 25% by mass of a silicone master batch (prototype, silicone: polydimethylsiloxane (Shinetsu Silicone KF-96H-100,000 cs manufactured by Shinetsu Chemical Industry Co., Ltd.), silicon content: 10% by mass, kinematic viscosity of silicone: 100,000 mm²/sec, base resin: VLDPE-1, and hereinafter referred to as MB-4) with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 5 wt% MB-1 in the Example 4.

A heat-shrinkable multilayer film is produced as in the Example 4 except that a product (hereinafter referred to as VLDPE-1 + 25 wt% MB-5) prepared by blending 25% by mass of a silicone master batch (prototype, silicone: polydimethylsiloxane (Shinetsu Silicone KF-96H-10,000 cs manufactured by Shinetsu Chemical Industry Co., Ltd.), silicon content: 10% by mass, kinematic viscosity of silicone: 10,000 mm²/sec, base resin: VLDPE-1, and hereinafter referred to as MB-5) with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 5 wt% MB-1 in the Example 4.

A heat-shrinkable multilayer film is produced as in the Example 4 except that a product (hereinafter referred to as VLDPE-1 + 12.5 wt% MB-6) prepared by blending 12.5% by mass of a silicone master batch (prototype, silicone: polydimethylsiloxane (Shinetsu Silicone KF-96-1,000 cs manufactured by Shinetsu Chemical Industry Co., Ltd.), silicon content: 10% by mass, kinematic viscosity of silicone: 1,000 mm²/sec, base resin: VLDPE-1, and hereinafter referred to as MB-6) with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 5 wt% MB-1 in the Example 4.

(Step A)
A first layer as the resin composition for the surface layer formation which is VLDPE-1 + 10 wt% MB-1, a second layer as the resin composition for the intermediate layer T1 formation which is an ethylene-vinyl acetate copolymer (Evaflex V5715 manufactured by DU PONT-MISTUI POLYCHEMICALS CO., LTD., vinyl acetate content: 19 mol%, density: 0.94 g/cm³, melting point: 89^oC, and hereinafter referred to as EVA-2), a third layer as the resin composition for the adhesive layer S1 formation which is EMA-1, a fourth layer as the resin composition for the gas barrier layer formation which is PVDC-1, a fifth layer as the resin composition for the adhesive layer S2 formation which is EMA-1, a sixth layer as the resin composition for the intermediate layer T2 formation which is an ethylene-vinyl acetate copolymer (Evaflex V57141C manufactured by DU PONT-MISTUI POLYCHEMICALS CO., LTD., vinyl acetate content: 15 mol%, density: 0.94 g/cm³, melting point: 89^oC, and hereinafter referred to as EVA-3), and a seventh layer as the resin composition for the seal layer formation which is an ionomer (Himilan AM79301 manufactured by DU PONT-MISTUI POLYCHEMICALS CO., LTD., density: 0.94 g/cm³, melting point 91^oC, and hereinafter referred to as Ionomer-2) are extruded by a plurality of extruders individually, and a molten resin is introduced into an annular die, melted and joined so as to provide a seven layer structure constituted of the surface layer, the intermediate layer T1, the adhesive layer S1, the gas barrier layer, the adhesive layer S2, the intermediate layer T2, and the seal layer laminated in sequence from outside to inside, and co-extruded. A molten primary tube flowed through a die exit is cooled by cold water showering of 10 to 20^oC, and a primary tube with a flat width of 113 mm is obtained.

(Step D)
The flat primary tube obtained in the step A is irradiated in-line with the electron beam in an electron beam irradiation apparatus with an acceleration voltage of 275 kV. As the electron beam irradiation conditions, the electron beam with an absorbed dose of 100 kGy is irradiated.

(Step B)
After the step D, the flat primary tube is passed through a hot water bath of 81.5^oC and then is formed into a bubble-shaped tubular film, and the film is stretched simultaneously and biaxially at stretching ratios of 3.5 times in the machine direction (MD direction) and 3.4 times in the transverse direction (TD direction) by the inflation method while being cooled by airing with cold air of 5 to 20^oC. Then, the biaxially stretched film is heated at 42^oC for 0.5 second and thermally relaxed, and a biaxially stretched film (a heat-shrinkable multilayer film) is produced. The flat width of the obtained heat-shrinkable multilayer film is 380 mm. In the thickness of each layer, the surface layer has a thickness of 1.5 mm, the intermediate layer T1 has a thickness of 22 mm, the adhesive layer S1 has a thickness of 1.5 mm, the gas barrier layer has a thickness of 7 mm, the adhesive layer S2 has a thickness of 1.5 mm, the intermediate layer T2 has a thickness of 10 mm, and the seal layer has a thickness of 10 mm, and the total thickness of the film is 53.5 mm.

(Step C)
The biaxially stretched film obtained in the step B is irradiated off-line with the electron beam in an electron beam irradiation apparatus with an acceleration voltage of 250 kV. As the electron beam irradiation conditions, the electron beam with an absorbed dose of 100 kGy is irradiated.

A heat-shrinkable multilayer film is produced as in the Example 16 except that a product (hereinafter referred to as VLDPE-1 + 25 wt% MB-4) prepared by blending 25% by mass of MB-4 with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 10 wt% MB-1 in the Example 16.

A heat-shrinkable multilayer film is produced as in the Example 16 except that a product (hereinafter referred to as VLDPE-1 + 25 wt% MB-5) prepared by blending 25% by mass of MB-5 with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 10 wt% MB-1 in the Example 16.

A heat-shrinkable multilayer film is produced as in the Example 18 except that EVA-1 is used as the resin composition for the intermediate layer T2 formation instead of EVA-3 and EVA-3 is used as the resin composition for the seal layer formation instead of Ionomer-2.

(Step A)
A first layer as the resin composition which is a product (hereinafter referred to as VLDPE-1 + 56 wt% MB-3) for the surface layer formation prepared by blending 56 wt% of MB-3 with VLDPE-1, a second layer as the resin composition for the intermediate layer T1 formation which is EVA-1, a third layer as the resin composition for the adhesive layer S1 formation which is EMA-1, a fourth layer as the resin composition for the gas barrier layer formation which is PVDC-1, a fifth layer as the resin composition for the adhesive layer S2 formation which is EMA-1, and a sixth layer as the resin composition for the seal layer formation which is Ionomer-1 are extruded by a plurality of extruders individually, and a molten resin is introduced into an annular die, melted and joined so as to provide a six layer structure constituted of the surface layer, the intermediate layer T1, the adhesive layer S1, the gas barrier layer, the adhesive layer S2, and the seal layer laminated in sequence from outside to inside, and co-extruded. A molten primary tube flowed through the die exit is cooled by cold water showering of 10 to 20^oC, and a primary tube with a flat width of 160 mm is obtained.

(Step C)
The flat primary tube obtained in the step A is irradiated off-line with the electron beam with an acceleration voltage of 250 kV and an absorbed dose of 100 kGy.

(Step B)
The flat primary tube obtained in the step C is passed through a hot water bath of 86^oC and then is formed into a bubble-shaped tubular film, and the film is stretched simultaneously and biaxially at stretching ratios of 2.0 times in the machine direction (MD direction) and 2.8 times in the transverse direction (TD direction) by the inflation method while being cooled by airing with cold air of 15 to 20^oC. Then, the biaxially stretched film is heated at 40^oC for 1 second and thermally relaxed, and a biaxially stretched film (a heat-shrinkable multilayer film) is produced. The flat width of the obtained heat-shrinkable multilayer film is 400 mm. In the thickness of each layer, the surface layer has a thickness of 1.5 mm, the intermediate layer T1 has a thickness of 11.5 mm, the adhesive layer S1 has a thickness of 1.5 mm, the gas barrier layer has a thickness of 4 mm, the adhesive layer S2 has a thickness of 1.5 mm, and the seal layer has a thickness of 30 mm, and the total thickness of the film is 50 mm.

A heat-shrinkable multilayer film is produced as in the Example 20 except that a product (hereinafter referred to as VLDPE-1 + 25 wt% MB-5) prepared by blending 25% by mass of MB-5 with VLDPE-1 is used as the resin composition for the surface layer formation instead of VLDPE-1 + 56 wt% MB-3 in the Example 20.

<Comparative Example 1>
A heat-shrinkable multilayer film is produced as in the Example 1 except that MB-1 is not blended with the resin composition for the surface layer formation and the resin composition for the intermediate layer T1 formation in the Example 1.

<Comparative Example 2>
A heat-shrinkable multilayer film is produced as in the Example 1 except that the step C is not performed in the Example 1.

<Comparative Example 3>
A heat-shrinkable multilayer film is produced as in the Example 16 except that MB-1 is not blended with the resin composition for the surface layer formation and the step C is not performed in the Example 16.

<Comparative Example 4>
A heat-shrinkable multilayer film is produced as in the Example 20 except that MB-3 is not blended with the resin composition for the surface layer formation and the resin composition for the intermediate layer T1 formation in the Example 20.

In the obtained heat-shrinkable multilayer films in the Examples and the Comparative Examples, the kinds of the used resins are grouped in Table 1, the kinds of the silicones are grouped in Table 2, and the kinds of the master batches are grouped in Table 3. The layer structure in the Examples and the Comparative Examples are shown in Table 4, and the stretching conditions and the electron beam irradiation conditions are shown in Table 5.

For the obtained heat-shrinkable multilayer film in the Examples and the Comparative Examples, the following evaluation is performed. The results are shown in Table 6.

<Hot water shrinkage>
In accordance with ASTM D-2732, a film sample marked at a distance of 10 cm in both the machine direction of a film (longitudinal direction, MD direction) and a direction transverse to the machine direction (horizontal direction, TD direction) is immersed for 10 seconds in hot water regulated to 80^oC, then taken from the hot water, and immediately cooled by water at ordinary temperatures. After that, the marked distance is measured, and a reduction from 10 cm is displayed in percentage as the ratio to the original length 10 cm. A test is performed five times, and the respective average values in the MD direction and the TD direction are assumed as the hot water shrinkages.

<Transparency> (Haze)
In accordance with JIS K 7136:2000, a haze of a film (Haze [%]) is measured using a haze meter (NDH2000 manufactured by Nippon Denshoku Industries CO., LTD.). The Haze value means that the smaller the value, the more excellent the transparency, and the larger the value, the more worse the transparency.

<Overlap seal properties>
Two pairs of the obtained tubular films are overlapped with each other, a vacuum packaging machine (AGW manufactured by Multivac) is used, a vacuum timer is fixed at 3.0, a seal bar temperature is changed between 155^oC, 160^oC, and 165^oC, and sealing is performed so that the TD direction and a seal line are parallel to each other at each temperature. The seal bar temperature is measured by applying a thermo label (5E, manufactured by Nichiyu Giken Kogyo Co., Ltd.) to a seal bar. While the two pairs of the films are overlapped with each other, the overlap portion is cut into a width of 15 mm to form a sample piece. Hereinafter, as a matter of convenience, a seal between the seal layers in the film on the side in contact with the seal bar is referred to as an upper seal inner surface, and a seal between the seal layers in the film on the side not in contact with the seal bar is referred to as a lower seal inner surface. A seal between the surface layers in the overlap portion between the two pairs of the films is referred to as a seal outer surface. The seal strength in each portion is measured using a universal tensile tester (Tensilon RTM-100 manufactured by ORIENTEC Co., Ltd.). At this time, a distance between chucks is 20 mm, and test speed is 300 mm/min. The overlap seal properties are judged from each seal strength of the upper seal inner surface, the lower seal inner surface, and the seal outer surface as follows:
O: the seal strength of the upper seal inner surface and the lower seal inner surface is not less than 5 N/15 mm, the seal strength of the seal outer surface is not more than 1 N/15 mm, and the overlap seal properties are exhibited (practical level);
X: the seal strength of the seal outer surface is not less than 1 N/15 mm, and the overlap seal properties are not exhibited (unpractical level).

<Comprehensive evaluation of overlap seal properties>
In the overlap seal properties evaluation, the comprehensive evaluation is performed as follows:
+++: three O in the overlap seal properties evaluation (practical level);
++: two O in the overlap seal properties evaluation (practical level);
+: one O in the overlap seal properties evaluation (practical limit level); and
-: no O in the overlap seal properties evaluation (unpractical level).

<Inner surface seal strength>
In the overlap seal properties evaluation, the seal strength of the upper seal inner surface and the lower seal inner surface at a seal temperature of 155^oC is evaluated as an inner surface seal strength. The seal strength of not less than 5 N/15 mm of the upper seal inner surface and the lower seal inner surface is regarded as practical level, and the seal strength of less than 5 N/15 mm of the upper seal inner surface and the lower seal inner surface is regarded as unpractical level.

<Gel fraction>
Superficial layers (first, second, and third layers) are peeled from a base material and used as a sample, and the mass of the sample is represented by W1 [g]. Next, the sample is immersed in 10 ml of 1, 2, 4-trichlorobenzene and held at 120^oC for two hours, and then insoluble substances are collected and dried overnight. The dry mass of the insoluble substances is represented by W2 [g], and the gel fraction is calculated by the Mathematical formula 1:
(Mathematical formula 1) gel fraction [%] = (W2/W1) x 100.

As shown in table 6, the heat-shrinkable multilayer films of the Examples are excellent in transparency and hot water shrinkability and have practical overlap seal properties. In any production process, including the process flow I and the process flow II, it is confirmed that the heat-shrinkable multilayer film having good transparency, hot water shrinkability, and overlap seal properties is obtained. Among those process flows, the process flow I is particularly good.

Meanwhile, in the Comparative Example 1, since the surface layer does not contain silicone, there is no overlap seal properties even if the electron beam is irradiated. In the Comparative Example 2, although the surface layer contains silicone, the electron beam is not irradiated, and since the surface layer is not crosslinked, there is no overlap seal properties. In the Comparative Example 3, since the surface layer does not contain silicone, there is no overlap seal properties. In the Comparative Example 4, since the surface layer does not contain silicone, there is no overlap seal properties even if the electron beam is irradiated.

Claims

A heat-shrinkable multilayer film, comprising:
a multilayer structure having a superficial layer including a surface layer, a gas barrier layer, and an inner layer including a seal layer, in the multilayer structure, the surface layer being disposed on one surface, and the seal layer being disposed on the other surface,
wherein the surface layer contains silicone, and the superficial layer is crosslinked.
The heat-shrinkable multilayer film according to claim 1,
wherein a gel fraction of the superficial layer is within a range of not less than 20% and not more than 80%.
The heat-shrinkable multilayer film according to claim 1 or 2,
wherein the surface layer contains a polyolefin-based resin.
The heat-shrinkable multilayer film according to any one of claims 1 to 3,
wherein the superficial layer contains not less than 0.05% by mass and not more than 5% by mass of the silicone.
The heat-shrinkable multilayer film according to any one of claims 1 to 4,
wherein the surface layer contains not less than 1% by mass and not more than 10% by mass of the silicone.
A process for producing a heat-shrinkable multilayer film, which contains a multilayer structure having a superficial layer including a surface layer, a gas barrier layer, and an inner layer including a seal layer, in the multilayer structure, the surface layer being disposed on one surface, and the seal layer being disposed on the other surface, comprising:
a step A of extruding at least a silicone-containing resin composition for the surface layer formation, a resin composition for the gas barrier layer formation, and a resin composition for the seal layer formation and forming a laminate having the multilayer structure;
a step B of stretching the laminate; and
a step C of irradiating an electron beam to the surface layer and crosslinking the superficial layer,
wherein a process flow I in which the step B and the step C are sequentially progressed after the step A or a process flow II in which the step C and the step B are sequentially progressed after the step A is performed.
The process for producing a heat-shrinkable multilayer film according to claim 6,
wherein the step C is a step of crosslinking the superficial layer until a gel fraction of the superficial layer is within a range of not less than 20% and not more than 80%.