Heat-shrinkable multilayer film and method for producing the same

Abstract

The present invention provides a multilayer film configured from 80 mass% or more of a polyolefin resin, the multilayer film having exceptional mechanical characteristics and thermal shrinkage properties.　A thermally shrinkable multilayer film according to the present invention comprising at least three layers, including an outer surface layer (a) containing a polyolefin resin, an intermediate layer (b) containing a gas-barrier resin, and an inner surface layer (c) containing a polyolefin resin, wherein: at least one layer in the multilayer film contains a cyclic olefin copolymer; and the mass ratio of the polyolefin resin to the multilayer film is 80 mass% or greater, and the mass ratio of the cyclic olefin copolymer to the multilayer film is 15 mass% or greater, where 100 mass% represents the total mass of the multilayer film.

Description

Technical Field

[0001] The present invention relates to a heat-shrinkable multilayer film containing a polyolefin resin and a method for producing the same.

Background Art

[0002] Heat-shrinkable films for packaging ham, sausage, and other foods have multiple required properties such as sealability for packaging the contents, mechanical properties for protecting the contents, gas barrier properties for maintaining freshness, and shrinkability for maintaining a good appearance. In order to satisfy these properties, laminated films of multiple resins are widely used (see, for example, Patent Documents 1 to 3).

[0003] As described in Patent Document 1, such heat-shrinkable multilayer films often contain a polyolefin resin (hereinafter sometimes abbreviated as "PO") excellent in sealability and extrusion characteristics, and a polyamide resin (hereinafter sometimes abbreviated as "PA") excellent in mechanical properties and stretchability as main layers.

[0004] In recent years, the demand for recycling of packaging materials has been increasing. For example, CEFLEX, a European consortium, has presented guidelines for making packaging materials with 90% by mass or more of PO and 5% by mass or less of each resin other than PO such as PA material recyclable. Here, material recycling refers to the act of reusing plastic waste after performing treatments such as crushing and melting.

[0005] The heat-shrinkable multilayer film laminated with PO and PA as described above often does not meet the CEFLEX guidelines. Since the resins of each layer are incompatible, even if they are melted, mixed, and remolded after use, the mechanical properties and appearance are significantly impaired due to layer separation, and practical performance as a material recycled product cannot be obtained.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] International Publication No. 2007 / 094144 [Patent Document 2] Japanese Patent Publication No. 2015-512801 [Patent Document 3] International Publication No. 2013 / 121874 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] Therefore, in order to obtain a material composition suitable for material recycling, it is desirable to adopt a configuration with the highest possible ratio of PO. However, when the ratio of PO is increased and the usage ratio of PA is decreased, the stretchability and mechanical properties required for forming a heat-shrinkable multilayer film are impaired.

[0008] Here, the mechanical properties mainly refer to the strength against puncturing with a needle, and are required to suppress pinholes that occur when packaging contents having protrusions such as meat with bones. Also, the stretchability refers to, for example, the stable film-forming property of the inflation bubble (the second bubble) in the inflation (triple bubble process) method described in Patent Document 2. Heat-shrinkable multilayer films for meat packaging applications are often required to shrink at temperatures of 100°C or lower, and are formed by the inflation (triple bubble process) method that can obtain a highly oriented film at low cost to meet this requirement.

[0009] Patent Document 2 discloses a heat-shrinkable multilayer film using a cyclic olefin copolymer (hereinafter sometimes abbreviated as COC). However, the film in this document was subjected to a piercing test at a speed of 1 mm / min using a rod having a spherical ball with a tip diameter of φ2.5 and has a fracture resistance of 2 to 10 J / mm (≈1 to 150 N / mm), but it is difficult to say that it has sufficient strength to package contents having protrusions such as meat with bones. Further, the film in this document is implemented in a form containing PA and is hardly a film suitable for recycling. Furthermore, this document does not mention the material recyclability, stretchability, elastic modulus, use in a sealant layer, and performance (seal strength and blocking property) at that time of a film containing COC.

[0010] Also, in film formation by the inflation (triple bubble process) method, when only PO is stretched without using PA, since bubbles cannot be formed unless PO is preheated to near the crystal melting point, the orientation of molecular chains is weak and it is difficult to obtain sufficient heat shrinkability and mechanical properties.

[0011] As a film formation technique for a conventional heat-shrinkable multilayer film that does not use PA, as described in Patent Document 3, there are examples such as imparting a crosslinked structure by electron beam irradiation to PO and using an ionomer. However, crosslinked PO gels during melting, which inhibits material recyclability, and ionomers are expensive, which increases costs. Further, compared with the case of using PA, the mechanical properties are inferior and the elastic modulus of the film is low, so the firmness is lost and the suitability for packaging machines is impaired.

[0012] The present invention has been made in view of the above problems, and an object thereof is to provide a multilayer film composed of 80% by mass or more of a polyolefin-based resin and having excellent mechanical properties and heat shrinkability.

Means for Solving the Problems

[0013] The inventors of the present invention have found that the above problems can be solved by a heat-shrinkable multilayer film containing a cyclic olefin copolymer in at least one layer of the multilayer film and having a mass ratio of the cyclic olefin copolymer in the multilayer film of 15% by mass or more, and have thus completed the present invention.

[0014] The heat-shrinkable multilayer film according to the present invention is a multilayer film composed of at least three layers including an outer surface layer (a) containing a polyolefin resin, an intermediate layer (b) containing a gas barrier resin, and an inner surface layer (c) containing a polyolefin resin. At least one layer of the multilayer film contains a cyclic olefin copolymer. When the total mass of the multilayer film is 100% by mass, the mass ratio of the polyolefin resin in the multilayer film is 80% by mass or more, and the mass ratio of the cyclic olefin copolymer in the multilayer film is 15% by mass or more.

[0015] The polyolefin resin in the outer surface layer (a) and / or the inner surface layer (c) preferably contains at least one selected from the group consisting of ultra-low density polyethylene and linear low density polyethylene, and a cyclic olefin copolymer.

[0016] The cyclic olefin copolymer preferably contains either or both of a cyclic olefin copolymer having a glass transition temperature of 50 to 68°C and a cyclic olefin copolymer having a glass transition temperature of -10 to 20°C.

[0017] The gas barrier resin of the intermediate layer (b) preferably contains at least one selected from the group consisting of a partially saponified ethylene-vinyl acetate copolymer and a glycolic acid (co)polymer resin.

[0018] In the heat-shrinkable multilayer film, the tensile modulus in the transverse direction (TD) is preferably 200 to 600 MPa.

[0019] The manufacturing method of the heat-shrinkable multilayer film according to the present invention is such that the surface temperature of the resin at the position immediately before the TD-direction stretching (position α in FIG. 1) is 75 to 90 °C, and the surface temperature of the resin at the lower shoulder forming portion of the inflation bubble (position β in FIG. 1) is in the range of 5 to 30 °C lower than the surface temperature of the resin at the position immediately before the TD-direction stretching.

Effect of the Invention

[0020] The heat-shrinkable multilayer film of the present invention is composed of 80% by mass or more of a polyolefin resin and has excellent mechanical properties and heat shrinkability. Therefore, a heat-shrinkable multilayer film that can be used as a packaging material having pinhole resistance even against contents having protrusions while being an environmentally friendly packaging film can be obtained.

Brief Description of the Drawings

[0021]

Figure 1

Embodiments for Carrying Out the Invention

[0022] The heat-shrinkable multilayer film according to the present invention is a multilayer film composed of at least three layers including an outer surface layer (a) containing a polyolefin resin, an intermediate layer (b) containing a gas barrier resin, and an inner surface layer (c) containing a polyolefin resin. At least one layer of the multilayer film contains a cyclic olefin copolymer. When the total mass of the multilayer film is 100% by mass, the proportion of the mass of the polyolefin resin in the multilayer film is 80% by mass or more, and the proportion of the mass of the cyclic olefin copolymer in the multilayer film is 15% by mass or more.

[0023] The olefin referred to in the present invention is C n H 2nIt refers to a hydrocarbon having a double bond, represented by the following formula. Further, a cyclic olefin refers to a hydrocarbon having a cyclic structure formed of carbon atoms and having a carbon-carbon double bond in the cyclic structure, and the number of the carbon-carbon double bonds may be one or more (however, it does not include an aromatic ring). And in this specification, a homopolymer or copolymer containing 50 mol% or more of a structural unit derived from an olefin and / or a cyclic olefin is defined as a polyolefin-based resin.

[0024] The heat-shrinkable multilayer film according to the present invention contains a cyclic olefin copolymer in at least one layer of the multilayer film. Among them, it is preferable to contain a cyclic olefin copolymer in the outer surface layer (a) and / or the inner surface layer (c), and it is more preferable to contain a cyclic olefin copolymer in the inner surface layer (c).

[0025] In the heat-shrinkable multilayer film of the present invention, when the total mass of the multilayer film is 100% by mass, the proportion of the mass of the polyolefin-based resin in the multilayer film is 80% by mass or more, preferably 90% by mass or more, and more preferably 95% by mass or more from the viewpoint of material recyclability. If the proportion of the mass of the polyolefin-based resin is less than the above range, the material recyclability is impaired due to the incompatibility with components other than the polyolefin-based resin. The proportion of the mass of PO (polyolefin-based resin) can be calculated from the following calculation formula. Proportion of the mass of polyolefin-based resin (mass%) = 100 × (total of "density × thickness" of the layers containing polyolefin-based resin) / (total of "density × thickness" of all layers) In Table 2 described later, the proportion of the mass of PO (mass%) is described as "PO mass ratio (wt%)". However, the "density" of a layer in which two or more resins are blended is the sum of the densities obtained by multiplying the density of each resin by the mixing ratio of the resins.

[0026] The heat-shrinkable multilayer film of the present invention has a proportion of the mass of the cyclic olefin copolymer in the multilayer film of 15% by mass or more, preferably 15 to 80% by mass, more preferably 15 to 50% by mass, and most preferably 16 to 29% by mass when the total mass of the multilayer film is 100% by mass. When the proportion of the mass of the cyclic olefin copolymer is less than 15% by mass, the tensile strength during stretching is poor and it is difficult to stably form a film. When it is more than 80% by mass, the flexibility of the film tends to be impaired. The proportion of the mass of the COC (cyclic olefin copolymer) can be calculated from the following formula. Proportion of the mass of the cyclic olefin copolymer (%) = 100 × (total of "density × thickness × COC mixing ratio" of the layer containing COC) / (total of "density × thickness" of all layers) In Table 2 described later, the proportion of the mass of the COC (mass %) is described as "COC mass ratio (wt%)". However, the "density" of the layer in which two or more resins are blended is the sum of the densities obtained by multiplying the density of the resin by the mixing ratio of the resin. In the present invention, "stretching" refers to the biaxial stretching process in the film formation of a heat-shrinkable film. Representative biaxial stretching methods include the tenter method and the inflation method. The film formation process of the heat-shrinkable film is not limited to any of these methods, but from the perspective of cost, the inflation method is particularly preferred. Further, the inflation method is divided into a direct method (direct inflation) in which a fluid is directly introduced into the molten resin extruded into a tubular shape, and a tubular stretching method (tubular biaxial stretching) including a process of once cooling and solidifying the molten resin extruded into a tubular shape to form a tubular body as described in Patent Document 2, then heating this tubular body to an appropriate temperature and stretching it in the longitudinal direction (MD) and the transverse direction (TD) while introducing a fluid into the tubular body. However, as the film formation method of the heat-shrinkable film, the tubular stretching method is particularly preferred. By using the tubular stretching method as the film formation process of the heat-shrinkable film, compared with the tenter method and the direct method, the stress during stretching tends to be high, so a heat-shrinkable film with an excellent shrinkage rate tends to be formed. Tubular biaxial stretching may be referred to as double bubble inflation (double bubble process) and triple bubble inflation (triple bubble process) depending on the number of bubbles formed in the film formation process. The difference between the two lies in the difference in the heat treatment process after biaxial stretching. Either film formation method can be used, but the triple bubble inflation is more preferred because the dimensional stability of the heat-shrinkable film tends to be good.

[0027] The outer surface layer (a) contains a polyolefin resin as a thermoplastic resin to provide excellent shrinkage characteristics, barrier properties against water vapor, and material recyclability.

[0028] As the PO (polyolefin resin) in the outer surface layer (a), for example, ethylene homopolymer; propylene homopolymer; copolymers of α-olefins having 2 to 8 carbon atoms such as VLDPE (very low density polyethylene), LLDPE (linear low density polyethylene); polyolefin copolymers such as propylene-ethylene copolymer, propylene-ethylene-butene-1 copolymer, EVA (ethylene-vinyl acetate copolymer), EAA (ethylene-acrylic acid copolymer), EMAA (ethylene-methacrylic acid copolymer), EMA (ethylene-methyl acrylate copolymer), EEA (ethylene-ethyl acrylate copolymer), EBA (ethylene-butyl acrylate copolymer), etc. are included, and cyclic olefin copolymers are also included. These polyolefin resins may be used alone or in combination of two or more. Among these, LLDPE or VLDPE is particularly preferred, and these include resins produced by Ziegler-Natta type reaction catalysts and resins produced by metallocene or single-site reaction catalysts.

[0029] As the polyolefin resin in the outer surface layer (a), a mixture of at least one selected from VLDPE and LLDPE and COC can be used.

[0030] The density of LLDPE or VLDPE is preferably 0.880 to 0.920 g / cm 3 , more preferably 0.890 to 0.915 g / cm 3 is. Also, the melt flow index (MI) of LLDPE or VLDPE is preferably 0.5 to 7.0 g / 10 min, more preferably 1.0 to 5.0 g / 10 min, and even more preferably 3.2 to 4.0 g / 10 min. When the MI is 3.2 to 4.0 g / 10 min, while maintaining the puncture strength of the film, the fluidity during melting is also excellent, and the variation in the film thickness tends to be reduced. An example is a product sold under the name "Evolue" by Prime Polymer Co., Ltd., and in this product, the α-olefin other than ethylene is 1-hexene. In addition, MI is measured at 190 °C in accordance with JIS K 7210.

[0031] The COC in the outer surface layer (a) is a resin having a structural unit derived from a cyclic olefin and a structural unit derived from an olefin such as ethylene other than the cyclic olefin. Examples of the cyclic olefin include norbornene and its derivatives (e.g., 2-norbornene, 5-methyl-2-norbornene); cyclopentadiene and its derivatives (e.g., dicyclopentadiene, 2,3-dihydrocyclopentadiene), etc., but are not limited thereto. Examples of the olefin other than the cyclic olefin include α-olefins such as ethylene, propylene, and butylene, etc., but are not limited thereto.

[0032] In addition, as the COC in the outer surface layer (a), for example, those containing 60 to 90 mol%, preferably 65 to 85 mol% of the structural unit derived from ethylene and 5 to 45 mol%, preferably 10 to 40 mol% of the structural unit derived from the cyclic olefin can be used.

[0033] Also, the glass transition temperature (Tg) of the COC is measured from the peak temperature of the loss modulus E” of the data obtained under the conditions of a heating rate of 5°C / min, a frequency of 62.8 rad / sec, and a strain of 0.1% by the tensile mode of dynamic viscoelasticity measurement (DMA), and the value is preferably 50 to 68°C. When the glass transition temperature is within the above range, in the film stretching process, the surface temperature of the resin at the part immediately before stretching in the TD direction (the part immediately before forming the lower shoulder of the inflation bubble: the position of α in FIG. 1) is 75 to 90°C, and the surface temperature of the resin at the lower shoulder forming part of the inflation bubble (the position of β in FIG. 1) is 5 to 30°C lower than the surface temperature of the resin at the part immediately before stretching in the TD direction (the position of α in FIG. 1). By setting such conditions, sufficient tensile stress (tensile strength) can be developed against the internal pressure of the fluid blown into the tubular body during stretching, and there is a tendency that a film can be stably formed by the triple bubble inflation method. When the glass transition temperature is lower than 50°C, the tensile strength during stretching is poor, and it tends to be difficult to stably form a film. In addition, the elastic modulus of the film becomes small, and the mechanical suitability during content filling tends to be inferior. Also, when the glass transition temperature is higher than 68°C, it is necessary to increase the resin temperature during stretching, and the orientation becomes small, so the hot water shrinkage rate tends to be small. In addition, the elongation at break during the puncture test tends to be small.

[0034] Also, the melt flow index (MI) of the COC is preferably 0.5 to 5.0 g / 10 min, more preferably 0.8 to 4.0 g / 10 min, and still more preferably 1.0 to 3.0 g / 10 min. When MI is lower than 0.5 g / 10 min, the fluidity during melting deteriorates, and the film thickness unevenness and productivity tend to be impaired. When MI is greater than 5.0 g / 10 min, the elongation at break may deteriorate when the film is punctured with a needle-like protrusion. COC is sold, for example, by Polyplastics Co., Ltd. under the name “TOPAS”. Note that MI was measured at 190°C in accordance with JIS K 7210.

[0035] The polyolefin resin in the outer surface layer (a) preferably has a melting point of 80 to 170 °C, more preferably 95 to 160 °C, as obtained by DSC (differential scanning calorimeter) at a heating rate of 10 °C / min in accordance with JIS K 7121. If the melting point is lower than 80 °C, the heat resistance of the multilayer film is insufficient, and problems such as melting during hot water shrinkage and sticking to the sealing substrate during heat sealing are likely to occur. On the other hand, if it is higher than 170 °C, when the heat-shrinkable multilayer film after use is recycled by melt-kneading, the temperature during melt-kneading tends to be high, so the recycled product tends to be yellowed due to heat deterioration.

[0036] The gas barrier property referred to in the present invention indicates the difficulty of oxygen gas permeation. The gas barrier resin in the present invention means that the oxygen permeability per 25 μm of the resin film at 23 °C and 0% RH is 0.01 to 300 cm 3 / m 2 ·day·atm, preferably 0.05 to 200 cm 3 / m 2 ·day·atm, most preferably 0.1 to 100 cm 3 / m 2 ·day·atm. The gas barrier resin in the intermediate layer (b) can be selected from ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as EVOH), a partially saponified ethylene-vinyl acetate copolymer, glycolic acid (co)polymer resin (hereinafter sometimes abbreviated as PGA), vinylidene chloride copolymer (hereinafter sometimes abbreviated as PVDC), polyvinyl alcohol (PVA), crystalline polyamide (MXD6) obtained from the polycondensation reaction of metaxylenediamine and adipic acid, aliphatic polyamide (PA) resin (for example, nylon 6, etc.), amorphous aromatic polyamide resin (for example, nylon 6I-6T, etc.), polyacrylonitrile copolymer (PAN), polyethylene terephthalate (PET), etc. The intermediate layer (b) is preferably a resin that provides excellent barrier properties against oxygen to prevent oxidative deterioration of the packaged product.

[0037] A preferred gas barrier resin for the intermediate layer (b) is EVOH. In particular, EVOH having an ethylene content of 20 to 60 mol%, preferably 30 to 50 mol%, and a saponification degree of 95% or more is preferred. When the ethylene content of EVOH is lower than the above range, the drawability tends to be inhibited, and when the ethylene content is more than 60 mol%, the oxygen gas barrier property tends to deteriorate. Further, when the saponification degree of EVOH is less than 95%, the oxygen gas barrier property tends to deteriorate. EVOH is sold, for example, by Kuraray Co., Ltd. under the name "EVAL".

[0038] As a preferred gas barrier resin for the intermediate layer (b), there is a glycolic acid (co)polymer resin. Examples of the glycolic acid (co)polymer resin include a homopolymer of polyglycolic acid and a copolymer of polyglycolic acid. In the present invention, it is preferable that the glycolic acid (co)polymer resin is formed from polyglycolic acid containing a repeating unit represented by the following general formula at a ratio of 60% by mass or more, more preferably formed from polyglycolic acid containing the repeating unit at a ratio of 70% by mass or more, and particularly preferably formed from polyglycolic acid containing the repeating unit at a ratio of 80% by mass or more. When the repeating unit represented by the following general formula in the polyglycolic acid is less than the above lower limit, the crystallinity inherent in the polyglycolic acid is impaired, and the gas barrier property and heat resistance of the resulting heat-shrinkable multilayer film tend to decrease. [Chemical formula] Since the glycolic acid (co)polymer resin has a lower oxygen gas permeability per unit thickness compared to other gas barrier resins such as EVOH and PVDC, even if the thickness of the intermediate layer (b) is made thinner, a practical gas barrier property can be exhibited. As a result, it leads to an increase in the volume ratio of the polyolefin layer, and an improvement in recyclability can be expected. The glycolic acid (co)polymer resin is a resin having hydrolyzability. When the molecular chain is randomly cleaved by hydrolysis and the molecular weight is reduced, the resin disintegrates when the molecular weight is reduced to such an extent that the strength cannot be maintained, and it is known to dissolve in water when the molecular weight reduction further progresses. By recycling the heat-shrinkable multilayer film of the present invention after the intermediate layer (b) is hydrolyzed, it is possible to recycle at a higher polyolefin ratio, and an improvement in recyclability can be expected.

[0039] The inner surface layer (c) contains a polyolefin resin as a heat-sealable thermoplastic resin. The inner surface layer (c) is preferably a resin having excellent heat-sealability among the properties required for the heat-shrinkable multilayer film. In addition to that, it is more preferable that the inner surface layer (c) has mechanical properties, shrinkage properties, barrier properties against water vapor, and material recyclability.

[0040] The resin used for the inner surface layer (c) can be selected from the same polyolefin resins as those listed for the outer surface layer (a). In particular, considering the heat-sealability, a resin having a melting point obtained by a DSC differential scanning calorimeter at a heating rate of 10 °C / min in accordance with JIS K 7121 is preferably 80 to 150 °C, more preferably 95 to 130 °C. If the melting point of the film is lower than 80 °C, sticking (blocking) between the inner surface layers (c) is likely to occur. If it is higher than 150 °C, the sealable temperature becomes high, and the stretchability in triple bubble inflation tends to be inhibited. In addition, additives such as lubricants can be added within a range that does not inhibit the transparency of the film.

[0041] As the polyolefin resin in the inner surface layer (c), in order to suppress the adhesion (blocking) between the inner surface layers (c), a mixture of at least one selected from VLDPE and LLDPE and COC can be used. The mass ratio (mixing ratio) of COC in the mixture is preferably 1 to 70% by mass, more preferably 20 to 60% by mass, and still more preferably 40 to 50% by mass. When the mass of COC is less than 1% by mass, the adhesion suppression effect tends to be poor, and when it is more than 70% by mass, the heat sealability tends to decrease.

[0042] The COC used in the inner surface layer (c) can be selected from the same COCs as those mentioned for the outer surface layer (a). For example, a COC with a glass transition temperature of 50 to 68°C can be used. Also, in order to improve the heat sealability at low temperatures of the inner surface layer (c), a COC with a glass transition point of 0 to 10°C can be further blended. That is, either or both of a COC with a glass transition temperature of 50 to 68°C and a COC with a glass transition temperature of -10 to 20°C can be used. The mass ratio (mixing ratio) of the COC with a glass transition point of 0 to 10°C is preferably 5 to 30% by mass, more preferably 10 to 20% by mass when the resin constituting the inner surface layer (c) is 100% by mass. When the COC with a glass transition point of 0 to 10°C is less than 5% by mass, the effect of improving the low-temperature heat sealability tends to be poor, and when it is more than 30% by mass, the effect of suppressing blocking tends to decrease.

[0043] The heat-shrinkable multilayer film of the present invention includes, as essential constituent layers, an outer surface layer (a) containing the above-mentioned polyolefin resin, an intermediate layer (b) containing a gas barrier resin, and an inner surface layer (c) containing a polyolefin resin. In addition, for the purpose of improving the functionality or processability of the multilayer film, a plurality of other layers (z) can be included as needed in the intermediate layers other than the intermediate layer (b) containing the gas barrier resin. Examples of such other layers (z) preferably include the layer containing the polyolefin resin. Also, when the adhesive strength between each layer is not sufficient, an adhesive resin layer can be provided.

[0044] The adhesive resin layer can be provided between each layer as needed. As the adhesive resin, EVA, EEA, acid-modified polyolefin (such as the reaction product of an olefin homopolymer or copolymer and an unsaturated carboxylic acid such as maleic acid or fumaric acid, acid anhydride, ester or metal salt, for example, acid-modified VLDPE, acid-modified LLDPE, acid-modified EVA), etc. can be used. Preferred examples include olefin resins modified with acids such as maleic acid or their anhydrides.

[0045] The melting point of the thermoplastic resin selected for the other layer (z) is preferably 40 to 170 °C, more preferably 50 to 160 °C. If the melting point is lower than 40 °C, the heat resistance of the multilayer film tends to deteriorate. If it is higher than 170 °C, when recycling the heat-shrinkable multilayer film after use by melt-kneading, the temperature during melt-kneading tends to be high, so the recycled product tends to turn yellow due to thermal degradation.

[0046] Also, in the above layer structure, an organic lubricant, an inorganic lubricant (anti-blocking agent) and / or an antistatic agent can be added to any of the layers.

[0047] The organic and / or inorganic lubricant is preferably included in the inner surface layer (c) or the outer surface layer (a) in particular to improve the slipperiness of the film during film production, the bag-making property during secondary processing, and the suitability of the packaging machine in filling the contents. Furthermore, powdering such as corn starch can be performed on the inner and outer surfaces of the film as needed.

[0048] Examples of the organic lubricant include hydrocarbon-based lubricants, fatty acid-based lubricants, fatty acid amide-based lubricants, ester-based lubricants, metal soaps, etc. The organic lubricant may be liquid or solid. Among these lubricants, fatty acid amide-based lubricants and metal soaps are preferably used because of their excellent compatibility with polyolefin resins. The organic lubricant is preferably used in a proportion of 0.1 to 0.2% by mass in the desired layer.

[0049] As the inorganic lubricant (anti-blocking agent), known inorganic fillers added to the resin for the purpose of suppressing the adhesion between films, such as talc, diatomaceous earth, silica, zeolite, calcium carbonate, aluminosilicate, etc. can be used. For example, silica, aluminosilicate, zeolite, etc. are preferably used from the viewpoints of refractive index and dispersibility.

[0050] Also, the median volume average particle diameter D50 of the inorganic lubricant measured by the Coulter counter method is preferably 0.5 to 10 μm, more preferably 1 to 7 μm. For the inorganic lubricant having the above average particle diameter, it is more preferable to use the one with the portion exceeding 10 μm in particle diameter cut off. The addition amount of the inorganic lubricant is, for example, 0.05 to 2% by mass in the desired layer, and particularly preferably 0.1 to 1% by mass.

[0051] Both organic and inorganic lubricants are preferably added to the desired resin layer as a masterbatch containing the lubricant in a resin constituting the desired layer or a resin having an affinity with it at a concentration of about 1 to 10% by mass.

[0052] As the antistatic agent, a surfactant is preferably used. As the surfactant, anionic surfactants, cationic surfactants, nonionic surfactants, amphoteric surfactants and mixtures thereof can be used. The antistatic agent can be added at a ratio of 0.05 to 2% by mass with respect to the resin of the layer to be added as necessary.

[0053] The heat-shrinkable multilayer film of the present invention is preferably formed into a film having a final thickness in the range of 10 to 150 μm, particularly 30 to 120 μm, by laminating and stretching each layer.

[0054] More specifically, the outer surface layer (a) is, for example, 0.5 to 50 μm, preferably 0.5 to 30 μm, more preferably 0.5 to 20 μm. Further, the intermediate layer (b) preferably has a thickness of 0.5 to 30 μm, particularly preferably 1 to 10 μm. When the thickness of the intermediate layer (b) made of a gas barrier resin is less than 0.5 μm, the effect of improving the oxygen barrier tends to be poor. When it is more than 30 μm, it becomes difficult to stretch the multilayer film, and the material recyclability tends to be inhibited. In addition, the inner surface layer (c) preferably has a thickness of 0.5 to 50 μm, more preferably 1 to 30 μm, in order to impart sufficient seal strength to the multilayer film. In addition, a plurality of adhesive resin layers can be provided, and the thickness of each layer is preferably 0.5 to 15 μm. When a glycolic acid (co)polymer resin is used for the intermediate layer (b), since it has excellent oxygen gas barrier properties, the intermediate layer (b) may have a thickness of 0.1 to 30 μm, preferably 0.2 to 10 μm.

[0055] The oxygen gas permeability (O2TR) of the heat-shrinkable multilayer film used in the present invention was measured under the conditions of a temperature of 23°C and a relative humidity of 80% on both sides using an oxygen permeability tester (manufactured by MOCON, "OX-TRAN 2 / 20") in accordance with JIS K 7126. For example, when packaging fresh meat, the oxygen gas permeability of the heat-shrinkable multilayer film is preferably 1 to 100 cm 3 / m 2 ·day·atm, more preferably 1 to 80 cm 3 / m 2 ·day·atm, particularly preferably 1 to 60 cm 3 / m 2 ·day·atm. When the oxygen gas permeability exceeds 100 cm 3 / m 2 ·day·atm, the preservability deteriorates due to oxidative degradation, and when packaging fresh meat, it tends to become impossible to store it for 40 days under the condition of 5°C or lower.

[0056] The heat-shrinkable multilayer film of the present invention, when pierced from the inner surface layer of a sample fixed at a speed of 50 mm / min using a piercing pin having a hemispherical tip with a curvature radius of 0.5 mm in an atmosphere of 23°C and 50% RH, the value obtained by dividing the measured value (N) at the maximum point until breakage by the thickness (μm) (the piercing strength per unit thickness) is preferably 0.20 to 0.50 N / μm, more preferably 0.22 to 0.45 N / μm, and most preferably 0.30 to 0.40 N / μm. When the piercing strength is less than 0.20 N / μm, especially when distributing a package containing protrusions such as meat with bones or hard foods, a larger thickness is required to suppress the occurrence of bag breakage or pinholes due to impacts such as dropping, which is not preferable from the viewpoints of cost and film formability. When the piercing strength is greater than 0.50 N / μm, since the rigidity of the film increases, the bag-making property during secondary processing using the film and the openability of the package during filling of the contents tend to be inferior, which is not preferable from the viewpoint of such workability.

[0057] The heat-shrinkable multilayer film of the present invention preferably has a hot water shrinkage rate of 40 to 60%, preferably 43 to 55%, in either the MD (longitudinal direction) / TD (transverse direction) after being immersed in hot water at 80°C for 10 seconds. When the hot water shrinkage rate is lower than 40%, depending on the size and shape of the non-packaged body, it may not be possible to tightly fit the contents, and there is a tendency that a beautiful package cannot be made. Also, when it is greater than 60%, the contents are overly tightened, and for example, when packaging meat, drips (blood) tend to come out easily.

[0058] The heat-shrinkable multilayer film of the present invention preferably has a tensile elastic modulus in the TD direction of 200 to 600 MPa, more preferably 300 to 550 MPa. When the elastic modulus is lower than 200 MPa, the mechanical suitability during content filling tends to be inferior, and when it is greater than 600 MPa, it tends to be hard and inferior in followability to the contents. In this specification, the tensile elastic modulus in the TD direction is a value measured by the method described in the examples below.

[0059] When the heat-shrinkable multilayer film of the present invention contains a polyamide resin, due to its incompatibility with polyolefin resins, its material recyclability becomes poor. For the above reasons, when a polyamide resin is included, when the total thickness of all layers is taken as 100%, the ratio of the thickness of the layer containing polyamide in all layers is preferably less than 10%, more preferably less than 5%, and even more preferably less than 2%. Since polyamide inhibits hot water shrinkage, when it is 10% or more, it tends to be difficult to make the hot water shrinkage rate in the MD (longitudinal direction) or TD (transverse direction) of the film after immersion in hot water at 80 °C for 10 seconds 40% or more.

[0060] As the polyamide resin, an aliphatic polyamide resin and an amorphous aromatic polyamide resin are preferably used. As the aliphatic polyamide resin, a polyamide 6 (nylon 6) polymer, a polyamide 6-66 (nylon 6-66) copolymer, a polyamide 6-69 (nylon 6-69) copolymer, a polyamide 6-12 (nylon 6-12) copolymer, and a polyamide 6-66-12 (nylon 6-66-12) copolymer are preferably used. As the amorphous aromatic polyamide resin, for example, a polycondensate of an aliphatic diamine having isophthalic acid and terephthalic acid as main acid components is used. As the acid component, a mixture containing 40 mol% or more and 98 mol% or less of the isophthalic acid component and 2 mol% or more and 60 mol% or less of the terephthalic acid component is preferably used. Particularly from the viewpoint of versatility, an amorphous nylon copolymer commonly known as nylon 6I-6T (Ny6I-6T) in which the aliphatic diamine consists of hexamethylene alone is preferably used.

[0061] When the heat-shrinkable multilayer film of the present invention contains a polyester resin, due to its incompatibility with polyolefin resins, its material recyclability deteriorates. For the above reasons, when the polyester resin is included, when the total thickness of all layers is 100%, the proportion of the thickness of the layer containing polyester in all layers is preferably less than 10%, more preferably less than 5%, and even more preferably less than 2%. When the polyester resin is 10% or more, when the heat-shrinkable multilayer film is melt-kneaded for material recycling, due to its incompatibility with polyolefin, the haze value and internal haze value of the recycled sheet tend to increase.

[0062] As the polyester resin, for example, an aliphatic polyester resin and an aromatic polyester resin are used. As the dicarboxylic acid component used in this polyester resin, any component that can obtain a polyester by a normal production method may be used. For example, terephthalic acid, isophthalic acid, dimer acid composed of dimers of unsaturated fatty acids, adipic acid, oxalic acid, malonic acid, succinic acid, azelaic acid, sebacic acid, phthalic acid, 5-t-butylisophthalic acid, naphthalenedicarboxylic acid, diphenyl ether dicarboxylic acid, cyclohexanedicarboxylic acid, etc. may be mentioned, and two or more kinds may be used. Further, as the diol component used in the polyester resin, any component that can obtain a polyester by a normal production method may be used. For example, ethylene glycol, propylene glycol, tetramethylene glycol, neopentyl glycol, hexamethylene glycol, diethylene glycol, polyalkylene glycol, 1,4-cyclohexanedimethanol, 2-alkyl-1,3-propanediol, 1,4-butanediol, etc. may be mentioned, and two or more kinds may be used.

[0063] The heat-shrinkable multilayer film of the present invention preferably does not contain an ionomer and an electron beam crosslinked layer. The ionomer is not preferable from the viewpoint of cost, and also tends to have a low puncture strength and elastic modulus per thickness. The electron beam crosslinked layer tends to inhibit material recyclability because it gels during melting.

[0064] When the heat-shrinkable multilayer film of the present invention contains an ionomer resin, from the viewpoints of cost and puncture strength per unit thickness, when the total thickness of all layers is taken as 100%, the ratio of the thickness of the layer containing the ionomer in all layers is preferably less than 20%, more preferably less than 10%, and most preferably less than 5%. When the ionomer resin is 20% or more, the puncture strength per unit thickness becomes small. Therefore, when distributing a package containing a protrusion or a hard food, a larger thickness tends to be required to suppress the occurrence of bag breakage or pinholes due to impacts such as dropping, which is not preferable from the viewpoints of cost and film-forming property.

[0065] Examples of the ionomer resin include resins obtained by using, as a base polymer, an ethylene-unsaturated carboxylic acid copolymer or an ethylene-ethylenically unsaturated carboxylic acid-ethylenically unsaturated carboxylic acid ester terpolymer (preferably an ethylene-ethylenically unsaturated carboxylic acid-ethylenically unsaturated carboxylic acid ester terpolymer) and neutralizing the carboxyl groups in these copolymers with cations. As the unsaturated carboxylic acid, methacrylic acid and acrylic acid are preferable, and as the unsaturated carboxylic acid ester, an alkyl ester having 1 to 6 carbon atoms of methacrylic acid or acrylic acid is preferable. Further, as the terpolymer, ethylene-methacrylic acid (or acrylic acid)-alkyl methacrylate (or alkyl acrylate) such as ethylene-methacrylic acid-isobutyl acrylate is preferable.

[0066] Examples of the cation include Na + , K + , Li + , Cs + , Ag + , Hg + , Cu + , Mg 2+ , Zn 2+ , Be 2+ , Ca 2+ , Ba 2+ , Cu 2+ , Cd 2+ , Hg 2+ , Sn 2+ , Pb 2+ , Fe 2+, Co 2+ , Ni 2+ , Al 3+ , Sc 3+ , Fe 3+ , Y 3+ Metal ions such as, and organic amines etc. may be mentioned. Among these cations, Na + , K + , Ca 2+ , Zn 2+ are preferred.

[0067] When the heat-shrinkable multilayer film of the present invention is melt-molded for reuse of its recovered product, deterioration of the resin (decomposition / gelation) and deterioration of appearance due to layer separation are suppressed, and it is characterized by excellent material recyclability.

[0068] The haze value (haze value) of the sheet formed to a thickness of 70 to 120 μm at a pressure of 150 kgf / cm 2 after kneading the film as it is in a melt kneader and then sheet-forming using a press machine is preferably 1 to 85% as the value (HA) per 100 μm of sheet thickness, more preferably 1 to 80%. In the present invention, the smaller the haze value (HA), the better. When it is larger than 85%, the appearance of the material recycled product is impaired. Furthermore, the internal haze value of the sheet formed to a thickness of 70 to 120 μm at a pressure of 150 kgf / cm 2 after kneading the film as it is in a melt kneader and then sheet-forming using a press machine is preferably 1 to 80% as the value (HI) per 100 μm of sheet thickness, more preferably 1 to 75%. When the internal haze value (HI) in the present invention is larger than 80%, the compatibility of the resin contained in the sheet may be extremely poor, and there is a tendency to be inferior in mechanical properties. Furthermore, the yellowness (b 2 of the sheet formed to a thickness of 70 to 120 μm at a pressure of 150 kgf / cm *The value is desirably 1 to 10, more preferably 1 to 8, and most preferably 1 to 6 as the value (BY) per 100 μm of the sheet thickness. b in the present invention * The smaller the value (BY), the better. The larger the numerical value, the more likely the resin is to decompose. When it is larger than 10, the appearance of the material recycled product is impaired. In addition, after kneading the film as it is in a melt kneader, sheet molding is performed, and the haze value (haze value), internal haze value, and yellowness (b * ) The method for measuring the value is the method described in the examples below. In addition, the haze value (HA) per 100 μm of the sheet thickness, the internal haze value (HI), b * The value (BY) can be calculated from the following calculation formula. Haze value per 100 μm (HA) = measured value of haze value × measured sheet thickness (μm) / 100 Internal haze value per 100 μm (HI) = measured value of internal haze value × measured sheet thickness (μm) / 100 b per 100 μm * Value (BY) = b * Measured value of the value × measured sheet thickness (μm) / 100

[0069] Examples of preferred embodiments of the layer structure of the heat-shrinkable multilayer film of the present invention are shown below. However, these are merely examples, and the present invention is not limited to these only. Note that COC2 means a type of COC different from COC. (1) LLDPE + COC / adhesive resin / EVOH / adhesive resin / LLDPE (2) LLDPE / adhesive resin / EVOH / adhesive resin / LLDPE + COC (3) LLDPE / adhesive resin / EVOH / adhesive resin / LLDPE + COC + COC2 (4) LLDPE + COC / adhesive resin / LLDPE / adhesive resin / EVOH / adhesive resin / LLDPE (5) LLDPE / adhesive resin / LLDPE + COC / adhesive resin / EVOH / adhesive resin / LLDPE (6) LLDPE / Adhesive Resin / LLDPE / Adhesive Resin / EVOH / Adhesive Resin / LLDPE + COC (7) LLDPE / Adhesive Resin / LLDPE / Adhesive Resin / EVOH / Adhesive Resin / LLDPE + COC + COC2 (8) LLDPE + COC / Adhesive Resin / LLDPE + COC / Adhesive Resin / EVOH / Adhesive Resin / LLDPE (9) VLDPE + COC / Adhesive Resin / EVOH / Adhesive Resin / VLDPE (10) VLDPE / Adhesive Resin / EVOH / Adhesive Resin / VLDPE + COC (11) VLDPE / Adhesive Resin / EVOH / Adhesive Resin / VLDPE + COC + COC2 (12) VLDPE + COC / Adhesive Resin / VLDPE / Adhesive Resin / EVOH / Adhesive Resin / VLDPE (13) VLDPE / Adhesive Resin / VLDPE + COC / Adhesive Resin / EVOH / Adhesive Resin / VLDPE (14) VLDPE / Adhesive Resin / VLDPE / Adhesive Resin / EVOH / Adhesive Resin / VLDPE + COC (15) VLDPE / Adhesive Resin / VLDPE / Adhesive Resin / EVOH / Adhesive Resin / VLDPE + COC + COC2 (16) VLDPE + COC / Adhesive Resin / VLDPE + COC / Adhesive Resin / EVOH / Adhesive Resin / VLDPE (17) LLDPE + COC / Adhesive Resin / EVA / Adhesive Resin / EVOH / Adhesive Resin / LLDPE (18) LLDPE / Adhesive Resin / EVA / Adhesive Resin / EVOH / Adhesive Resin / LLDPE + COC (19) LLDPE / Adhesive Resin / EVA / Adhesive Resin / EVOH / Adhesive Resin / LLDPE + COC + COC2 (20) VLDPE + COC / Adhesive Resin / EVA / Adhesive Resin / EVOH / Adhesive Resin / VLDPE (21) VLDPE / Adhesive Resin / EVA / Adhesive Resin / EVOH / Adhesive Resin / VLDPE + COC (22) VLDPE / Adhesive Resin / EVA / Adhesive Resin / EVOH / Adhesive Resin / VLDPE + COC + COC2 (23) LLDPE / Adhesive Resin / LLDPE + COC / Adhesive Resin / PGA / Adhesive Resin / LLDPE + COC (24) LLDPE + COC / Adhesive Resin / PGA / Adhesive Resin / LLDPE (25) LLDPE / Adhesive Resin / PGA / Adhesive Resin / LLDPE + COC (26) LLDPE / Adhesive Resin / PGA / Adhesive Resin / LLDPE + COC + COC2 (27) LLDPE + COC / Adhesive Resin / LLDPE / Adhesive Resin / PGA / Adhesive Resin / LLDPE (28) LLDPE / Adhesive Resin / LLDPE + COC / Adhesive Resin / PGA / Adhesive Resin / LLDPE (29) LLDPE / Adhesive Resin / LLDPE / Adhesive Resin / PGA / Adhesive Resin / LLDPE + COC (30) LLDPE / Adhesive Resin / LLDPE / Adhesive Resin / PGA / Adhesive Resin / LLDPE + COC + COC2 (31) LLDPE + COC / Adhesive Resin / LLDPE + COC / Adhesive Resin / PGA / Adhesive Resin / LLDPE (32) VLDPE + COC / Adhesive Resin / PGA / Adhesive Resin / VLDPE (33) VLDPE / Adhesive Resin / PGA / Adhesive Resin / VLDPE + COC (34) VLDPE / Adhesive Resin / PGA / Adhesive Resin / VLDPE + COC + COC2 (35) VLDPE + COC / Adhesive Resin / VLDPE / Adhesive Resin / PGA / Adhesive Resin / VLDPE (36) VLDPE / Adhesive Resin / VLDPE + COC / Adhesive Resin / PGA / Adhesive Resin / VLDPE (37) VLDPE / Adhesive Resin / VLDPE / Adhesive Resin / PGA / Adhesive Resin / VLDPE + COC (38) VLDPE / Adhesive Resin / VLDPE / Adhesive Resin / PGA / Adhesive Resin / VLDPE + COC + COC2 (39)VLDPE + COC / Adhesive resin / VLDPE + COC / Adhesive resin / PGA / Adhesive resin / VLDPE (40)LLDPE + COC / Adhesive resin / EVA / Adhesive resin / PGA / Adhesive resin / LLDPE (41)LLDPE / Adhesive resin / EVA / Adhesive resin / PGA / Adhesive resin / LLDPE + COC (42)LLDPE / Adhesive resin / EVA / Adhesive resin / PGA / Adhesive resin / LLDPE + COC + COC2 (43)VLDPE + COC / Adhesive resin / EVA / Adhesive resin / PGA / Adhesive resin / VLDPE (44)VLDPE / Adhesive resin / EVA / Adhesive resin / PGA / Adhesive resin / VLDPE + COC (45)VLDPE / Adhesive resin / EVA / Adhesive resin / PGA / Adhesive resin / VLDPE + COC + COC2

[0070] The heat - shrinkable multilayer film according to the present invention is, for example, a step of co - extruding the molten resin in a tubular shape to form a tubular body composed of at least three layers including an outer surface layer (a) containing a polyolefin - based resin, an intermediate layer (b) containing a gas - barrier resin, and an inner surface layer (c) containing a polyolefin - based resin; a step of cooling the tubular body to a temperature below the melting point of the resin, then reheating the tubular body to a temperature below the melting point of the resin, and stretching the tubular body in the longitudinal direction (MD) and the transverse direction (TD) while introducing a fluid into the tubular body to form a biaxially stretched film; Next, it can be manufactured by a method including a step of folding the biaxially stretched film, and then performing a heat treatment from the outer surface side of the biaxially stretched film while introducing a fluid into the biaxially stretched film, and performing a relaxation (shrinkage) treatment in the longitudinal direction and the transverse direction simultaneously with the heat treatment. In the step of forming the biaxially stretched film, the surface temperature of the resin of the tubular body at the position immediately before stretching in the TD direction (position α in FIG. 1) is 75 - 90°C, and it is preferable that the surface temperature of the resin at the lower shoulder forming part of the inflation bubble (position β in FIG. 1) is reduced in the range of 5 - 30°C compared to the surface temperature of the resin at the position immediately before stretching in the TD direction.

[0071] More specifically, the heat-shrinkable multilayer film of the present invention can be manufactured using, for example, the apparatus shown in FIG. 1. In the apparatus shown in FIG. 1, for example, an extruder 1 (only one is shown in the figure) corresponding to the number of types of laminated resins constituting the multilayer film passes through an annular die 2, and while enclosing an unsealing agent typified by soybean oil, fatty acid esters of glycerin, propylene glycol, etc. as necessary, a tubular body (parison) 3 having an outer surface layer, an intermediate layer, and an inner surface layer is co-extruded. The molten tubular body 3 immediately after co-extrusion is flattened by a pinch roller 5 while being cooled by a water bath 4 to a temperature below the melting point of the main resin occupying each layer, preferably 20°C or lower, more preferably 15°C or lower, and then taken out. Next, while enclosing an unsealing agent typified by soybean oil, fatty acid esters of glycerin, propylene glycol, etc. as necessary, the drawn flat body 3a (multilayer film) is introduced into a warm water bath 6 at, for example, 75 to 95°C below the melting point of the main resin occupying each layer, and the heated flat body 3b is pulled out upward. Then, a bubble-shaped tubular body 3c is formed from the flat body 3b by the fluid air introduced between a pair of pinch rollers 7 and 8, and while cooling with a cooling medium blown out from an air ring device at 10 to 30°C and 5 to 20 m / s, it is simultaneously biaxially stretched in the machine direction (longitudinal direction, MD) and the direction perpendicular to the machine direction (transverse direction, TD). At this time, the surface temperature of the tubular body 3c at the position immediately before stretching in the TD direction (position α in FIG. 1) is preferably 75 to 90°C, more preferably 80 to 88°C, in a measurement with an infrared thermograph (Teledyne FLIR LLC "FLIR E4") with the emissivity set to 0.83. When the surface temperature at the position immediately before stretching in the TD direction is lower than 75°C, the yield point stress of the tubular body 3c is large, so the internal pressure of the enclosed fluid air becomes high and stretching tends to be difficult. When it is higher than 90°C, it tends to be difficult to stop stretching in the TD direction. Further, the surface temperature of the lower shoulder forming portion (position of β in FIG. 1) of the tubular body 3c is preferably cooled in the range of 5 to 30 ° C, more preferably in the range of 10 to 20 ° C, from the immediately preceding portion extending in the TD direction (position of α in FIG. 1). When it is less than 5 ° C, the tubular body 3c tends to rupture, and when it is greater than 30 ° C, the width of the tubular body 3c tends to become smaller. The draw ratio is calculated based on the following formula, and is preferably 2.0 to 4.0 times, more preferably 2.5 to 3.5 times, and particularly preferably 2.8 to 3.5 times in both directions. Formula 1: Longitudinal direction (MD) draw ratio = take-up speed of pinch roller 8 / take-up speed of pinch roller 7 Formula 2: Transverse direction (TD) draw ratio = width of flat body 3d / width of flat body 3b

[0072] Next, the drawn flat body 3d is pulled downward and a bubble-shaped tubular body 3e is formed again from the flat body 3d by the fluid air introduced between the pair of pinch rollers 10 and 11, and is held in the heat treatment cylinder 12. Then, steam is sprayed alone or together with air from the blowout port 13 of this heat treatment cylinder 12, and the tubular body 3e during heat treatment is preferably at 50 ° C or higher and 100 ° C or lower, more preferably at 60 ° C or higher and 95 ° C or lower, and is preferably heat-treated for 1 second or more and 20 seconds or less, more preferably about 1.5 seconds or more and 10 seconds or less. Then, the tubular body 3e during heat treatment is relaxed (contracted) so that the relaxation rate in the longitudinal direction (MD) and the transverse direction (TD) is preferably 2% or more and 40% or less, more preferably 5% or more and 30% or less. The relaxation rate in both directions is calculated based on the following formula. Formula 3: Longitudinal direction (MD) relaxation rate = (1 - (take-up speed of roller 11 / take-up speed of roller 10)) × 100 (%) Formula 4: Transverse direction (TD) relaxation rate = (1 - (width of flat body 3f / width of flat body 3d)) × 100 (%)

[0073] The flat body 3f after such relaxation heat treatment corresponds to the heat-shrinkable multilayer film of the present invention. and is wound around the take-up roll 14.

Example

[0074] Hereinafter, the present invention will be described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[0075] The resins used in the Examples and Comparative Examples are summarized in Table 1 below together with their abbreviations. Note that ○ in the Remarks column means that it is a polyolefin resin in this specification.

[0076] [Table 1]

[0077] (Example 1) Using an inflation device, each resin was extruded by a plurality of extruders so that the layer structure was LLDPE1 / MA-g-PE / EVA1 / MA-g-PE / EVOH / MA-g-PE / LLDPE1+COC1 (50% by mass + 50% by mass) from the outside to the inside, and the melted resins were introduced into an annular die, where they were melt-bonded to form the above layer structure and co-extruded. The molten tubular body flowing out of the die outlet was rapidly cooled to 10 - 25°C in a water bath to form a flat body with a flat width of 157 mm and a thickness of 270 μm. Then, after passing the flat body through a warm water bath controlled at 90°C ± 5°C, it was formed into a tubular film in the shape of a bubble and simultaneously biaxially stretched in the longitudinal direction (MD) by a factor of 2.4 and in the transverse direction (TD) by a factor of 2.8 by the inflation method while cooling with a cooling medium blown out from an air ring at 20°C and 15 m / s. At this time, the surface temperature of the tubular body at the position immediately before stretching in the TD direction (position α in Fig. 1) was 80°C, and the temperature of the lower shoulder of the tubular body (position β in Fig. 1) was 65°C, which was 15°C lower than the surface temperature at the position immediately before stretching in the TD direction. Next, the biaxially stretched film was heat-treated while being relaxed (shrunk) by 3% in the longitudinal direction and 6% in the transverse direction by an IR heat treatment heater at 360°C to produce a biaxially stretched film (heat-shrinkable multilayer film). The layer structure, the thickness (μm) of the whole and each layer, the mass ratio of the polyolefin-based resin and the cyclic olefin copolymer in the whole, and the possibility of stretching by the inflation method of the obtained biaxially stretched film are shown together with those of other examples and comparative examples in Table 2. Note that "※" in Comparative Example 3 means an electron beam cross-linked layer, and "〇" and "×" in "Possibility of stretching" mean stretchable and non-stretchable, respectively. Here, the arithmetic calculation method of "PO mass ratio (wt%)" and "COC mass ratio (wt%)" of Example 1 described in Table 2 is exemplified. "PO mass ratio (wt%)": 95≒(100×(0.903×1.4 + 0.9×1.0 + 0.94×20.0 + 0.9×1.0 + 0.9×1.0 + (0.903×0.5 + 1.01×0.5)×12.0)) / (0.903×1.4 + 0.9×1.0 + 0.94×20.0 + 0.9×1.0 + 1.12×1.6 + 0.9×1.0 + (0.903×0.5 + 1.01×0.5)×12.0)) "COC mass ratio (wt%)": 16 ≒ (100×(0.903×0.5 + 1.01×0.5)×12.0×0.5)) / ((0.903×1.4 + 0.9×1.0 + 0.94×20.0 + 0.9×1.0 + 1.12×1.6 + 0.9×1.0 + (0.903×0.5 + 1.01×0.5)×12.0))

[0078] (Examples 2 to 5, Comparative Example 1) Except for changing the layer structure as described in Table 2, various biaxially stretched films (heat-shrinkable multilayer films) were obtained in the same manner as in Example 1. Note that for Comparative Example 1, a biaxially stretched film (heat-shrinkable multilayer film) could not be obtained because it could not be biaxially stretched by the inflation method.

[0079] (Comparative Example 2) Using an inflation apparatus, each resin was extruded with a plurality of extruders so that the layer structure was Co-PET / MA-g-PE / PA6+A-PA (85 mass% + 15 mass%) / EVOH / MA-g-PE / VLDPE from the outside to the inside, and the melted resins were introduced into an annular die, where they were melt-bonded to form the above layer structure and co-extruded. The molten tubular body flowing out from the die outlet was rapidly cooled to 20°C in a water bath to form a film-like flat body. Then, the flat body was passed through a warm water bath at 87°C and then formed into a tubular film in a bubble shape, and simultaneously biaxially stretched by the inflation method at a draw ratio of 3.0 times in the longitudinal direction (MD) and 3.1 times in the transverse direction (TD) while cooling with an air ring at 22°C. Next, the biaxially stretched film was introduced into a heat treatment tower having a barrel length of about 2 m, formed into a tubular film in a bubble shape, heated to 70°C by steam ejected from the wiping outlet, and heat-treated while relaxing (shrinking) 6% in the longitudinal direction and 6% in the transverse direction to produce a biaxially stretched film (heat-shrinkable multilayer film).

[0080] (Comparative Example 3) Using an inflation device, each resin was extruded by a plurality of extruders so that the layer structure was VLDPE / EVA2 / EMA / PVDC / EMA / EVA2 / EVA3 from the outside to the inside, and the melted resin was introduced into an annular die, where it was melt-bonded to form the above layer structure and co-extruded. The molten tubular body flowing out from the die outlet was rapidly cooled to 12°C in a water bath to form a flat body. Next, the flat body was irradiated with an electron beam from the outside of the flat body in an electron beam irradiation device with an acceleration voltage of 300 keV to give a radiation dose of 80 kilograys. Next, after passing through a hot water bath at 82°C, it was formed into a tubular film in a bubble shape, and simultaneously biaxially stretched at a draw ratio of 3.1 times in the longitudinal direction (MD) and 3.0 times in the transverse direction (TD) by the inflation method while cooling with an air ring at 15°C or higher and 20°C or lower to produce a biaxially stretched film (heat-shrinkable multilayer film).

[0081] <Evaluation of Heat-Shrinkable Multilayer Film> By the following method, the puncture strength, hot water shrinkage rate, tensile elastic modulus, and material recyclability of the heat-shrinkable multilayer films obtained in the examples and comparative examples were measured. The puncture strength was measured from the inner surface layer side of the film. The numerical values obtained by the measurement are shown in Table 2.

[0082] (1) Puncture Strength In accordance with JIS Z 1707, using a tensilon universal material testing machine (manufactured by Orientec, model "RTC-1210") equipped with a puncture pin having a hemispherical tip with a curvature radius of 0.5 mm, the puncture pin was punctured from the inner surface layer of the fixed sample at a speed of 50 mm / min, and the value obtained by dividing the measured value (N) of the maximum point until breakage by the thickness (μm) was defined as the puncture strength from the inner surface layer side.

[0083] (2) Hot Water Shrinkage Rate A film sample with marks made at a distance of 10 cm in the machine direction (longitudinal direction, MD) and in the direction perpendicular to the machine direction (transverse direction, TD) of the film was immersed in hot water adjusted to 80 °C for 10 seconds, then taken out and immediately cooled with water at room temperature (25 ± 5 °C). After that, the marked distance was measured, and the ratio of the decrease value from 10 cm to the original length of 10 cm was expressed as a percentage for both the longitudinal and transverse directions. Five tests were conducted on one sample, and the average values for both the longitudinal direction (MD) and the transverse direction (TD) were obtained and taken as the hot water shrinkage rate.

[0084] (3) Tensile modulus In accordance with JIS K 7127, in an atmosphere of 23 °C and 50% RH, a strip-shaped film sample with a width of 20 mm and a length of 130 mm was attached to a tensilon universal material testing machine (model "RTC - 1210" manufactured by Orientec) so that the distance between the chucks was 100 mm, and it was stretched by 5 mm at a tensile speed of 10 mm / min in the machine direction (longitudinal direction, MD) and in the direction perpendicular to the machine direction (transverse direction, TD) of the film. At that time, the strain and load were measured. From the obtained strain and load, the tensile modulus was calculated in accordance with JIS K 7161.

[0085] (4) Material recyclability The heat - shrinkable multilayer films of Examples 1 - 5, Comparative Examples 2 and 3 were put into a kneading and extrusion molding evaluation test apparatus, Laboplastmill (model "4C150" manufactured by Toyo Seiki Seisakusho Co., Ltd.), and melt - kneaded at a mixer rotation speed of 50 rpm for 3 minutes. After the kneading was completed, 5 g of the molten resin was recovered with a spatula, etc., and stacked in order from the bottom to form a press sample with a 400 - μm - thick SUS plate, a 250 - μm - thick PTFE - impregnated glass cloth sheet, 5 g of the above - mentioned melt - kneaded material, a 250 - μm - thick PTFE - impregnated glass cloth sheet, and a 400 - μm - thick SUS plate. This sample was compression (press) molded by a compression (press) molding machine (model "AYSR.5" manufactured by Kinteng Metal Industry Co., Ltd.) with a preheating time of 1 minute, a pressurizing time of 1 minute, and a pressure of 150 kgf / cm 2Press molding was performed, the sheets were formed into films, and the thickness of each sheet was measured. As a result, Example 1 was 97 μm, Example 2 was 110 μm, Example 3 was 104 μm, Example 4 was 100 μm, Example 5 was 83 μm, Comparative Example 2 was 84 μm, and Comparative Example 3 was 115 μm. The temperatures during the lab plastomill and compression molding were 190 °C for Examples 1 to 4 and Comparative Example 3, and 240 °C for Example 5 and Comparative Example 2. This temperature setting is preferably based on the melting point of the resin with the highest melting point contained in the multilayer film, and is preferably from the reference temperature +10 °C to 60 °C, more preferably from +15 °C to 50 °C, and most preferably from +20 °C to 35 °C. When the set temperature is lower than the reference temperature +10 °C, unmolten resin tends to remain as foreign matter in the sheet, and when it is higher than the reference temperature +60 °C, the formed sheet is likely to be colored due to resin decomposition. In Examples 1 to 4, the resin with the highest melting point was EVOH (157 °C), in Example 5 it was PGA (220 °C), in Comparative Example 2 it was Co-PET (220 °C), and in Comparative Example 3 it was PVDC (160 °C). The melting point was measured by DSC (differential scanning calorimeter) in accordance with JIS K 7121 under the condition of a heating rate of 10 °C / min. The haze degree (haze value) of the obtained sheets was measured using a HazeMeter (manufactured by Nippon Denshoku Industries Co., Ltd., "NDH7000") with a light source D65 in accordance with JIS K 7136. Also, using the same HazeMeter, silicon was applied to the surface of the film to measure the internal haze value. Furthermore, the b * value of the obtained sheets was measured by the reflection method using a Spectrophotometer (manufactured by Nippon Denshoku Industries Co., Ltd., "SE7700") with a light source D65 in accordance with JIS K 7373 with a white plate placed on the sheet. The b * value was used as an index of the yellowness of the sheet.

[0086]

Table 2

Claims

1. A multilayer film comprising at least three layers: an outer surface layer (a) containing a polyolefin resin, an intermediate layer (b) containing a gas barrier resin, and an inner surface layer (c) containing a polyolefin resin, wherein the polyolefin resin in the outer surface layer (a) and / or the inner surface layer (c) contains at least one selected from the group consisting of an ethylene homopolymer, a propylene homopolymer, an ultra-low density polyethylene, a linear low density polyethylene, a propylene-ethylene copolymer, a propylene-ethylene-butene-1 copolymer, an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, an ethylene-methacrylic acid copolymer, an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, and an ethylene-butyl acrylate copolymer, and a cyclic olefin copolymer, wherein at least one layer of the multilayer film contains a cyclic olefin copolymer, when the total mass of the multilayer film is 100% by mass, the proportion of the mass of the polyolefin resin in the multilayer film is 80% by mass or more, and a heat-shrinkable multilayer film in which the proportion of the mass of the cyclic olefin copolymer in the multilayer film is 15% by mass or more.

2. A multilayer film comprising at least three layers: an outer surface layer (a) containing a polyolefin resin, an intermediate layer (b) containing a gas barrier resin, and an inner surface layer (c) containing a polyolefin resin, wherein the polyolefin resin in the outer surface layer (a) contains at least one selected from the group consisting of an ethylene homopolymer, a propylene homopolymer, an ultra-low density polyethylene, a linear low density polyethylene, a propylene-ethylene copolymer, a propylene-ethylene-butene-1 copolymer, an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, an ethylene-methacrylic acid copolymer, an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, and an ethylene-butyl acrylate copolymer, and a cyclic olefin copolymer, The polyolefin resin in the inner surface layer (c) is a resin having a melting point of 80 to 150°C obtained by a DSC differential scanning calorimeter in accordance with JIS K 7121 at a heating rate of 10°C / min. At least one layer of the multilayer film contains a cyclic olefin copolymer. When the total mass of the multilayer film is 100% by mass, The proportion of the mass of the polyolefin resin in the multilayer film is 80% by mass or more, and A heat-shrinkable multilayer film in which the proportion of the mass of the cyclic olefin copolymer in the multilayer film is 15% by mass or more.

3. A multilayer film comprising at least three layers of an outer surface layer (a) containing a polyolefin resin, an intermediate layer (b) containing a gas barrier resin, and an inner surface layer (c) containing a polyolefin resin, The polyolefin resin in the outer surface layer (a) and / or the inner surface layer (c) contains at least one selected from the group consisting of ultra-low density polyethylene and linear low density polyethylene, and a cyclic olefin copolymer. At least one layer of the multilayer film contains a cyclic olefin copolymer. When the total mass of the multilayer film is 100% by mass, The proportion of the mass of the polyolefin resin in the multilayer film is 80% by mass or more, and A heat-shrinkable multilayer film in which the proportion of the mass of the cyclic olefin copolymer in the multilayer film is 15% by mass or more.

4. The heat-shrinkable multilayer film according to claim 3, wherein the cyclic olefin copolymer contains either or both of a cyclic olefin copolymer having a glass transition temperature of 50 to 68°C and a cyclic olefin copolymer having a glass transition temperature of -10 to 20°C.

5. The heat-shrinkable multilayer film according to claim 1, wherein the gas barrier resin of the intermediate layer (b) contains at least one selected from the group consisting of a partially saponified ethylene-vinyl acetate copolymer and a glycolic acid (co)polymer resin.

6. The heat-shrinkable multilayer film according to claim 1, having a tensile modulus in the transverse direction (TD) of 200 to 600 MPa.

7. A method for producing the heat-shrinkable multilayer film according to any one of claims 1 to 6, wherein the surface temperature of the resin immediately before stretching in the TD direction is 75 to 90 °C, and the surface temperature of the resin in the lower shoulder forming portion of the inflation bubble is decreased in the range of 5 to 30 °C from the surface temperature of the resin immediately before stretching in the TD direction.

Images