Heat shrinkable laminated film

Description: TECHNICAL FIELD [Detailed description of the invention]

Industrial application field The present invention comprises an aliphatic polyamide mixed resin layer (A) in which an aliphatic polyamide having a specific crystal melting point (hereinafter abbreviated as PA resin) is mixed at a specific ratio, and an ethylene-vinyl acetate copolymer saponified product (hereinafter abbreviated as PA resin). The present invention relates to a heat-shrinkable multilayer film composed of a mixed resin layer (B) containing (abbreviated as EVOH resin) as a main component. Conventional technology Many fatty foods such as raw meat and processed meat have irregular shapes and irregular sizes. As a packaging method for foods having various forms like this, shrinkage packaging using a heat-shrinkable film has been industrially adopted. The general operating procedure for this method is to put the contents in a bag-shaped film, then remove the air inside the bag under reduced pressure, seal the opening of the bag, and then heat the film to shrink it. This is a method of obtaining a beautiful package by sticking it to the contents. The heat treatment in this method also serves as sterilization of the contents, and is usually treated at a temperature of 70 to 120 ° C. The film used for such shrinkage packaging has excellent gas barrier properties and adhesion, and has the property of being sufficiently heat-shrinked by heating with hot water or warm air at 70 to 120 ° C. It is necessary. As the film for shrink wrapping as described above, it has been considered to use an EVOH resin film having a high gas barrier property, but an EVOH resin film having an excellent gas barrier property has a drawback of being inferior in stretchability. .. Various methods have been proposed to solve this drawback. For example, a composite film in which an EVOH resin film and a PA resin film are adhered and laminated and then stretched (Japanese Patent Laid-Open No. 52-115880), a laminated film composed of a polyolefine layer, a PA resin layer and an EVOH resin layer (Japanese Patent Laid-Open No. 56-136365) are proposed. Has been done. However, even if EVOH resin alone is laminated with PA resin, the stretchability is not always satisfactory, and it is hard to say that a film having uniform heat shrinkage can be obtained. Laminating PA resin is also necessary to obtain heat resistance, but dimensional stability deteriorates, so heat fixing such as heat treatment must be performed, which causes a decrease in workability and product yield. Become. Therefore, it is desired to provide a multilayer film having an EVOH resin layer and a PA resin layer having gas barrier properties, heat resistance, and more excellent stretchability, heat shrinkage, and dimensional stability. The present inventors previously proposed in patent application No. 25237, 1987, a laminated film composed of a mixed resin layer of PA resin and EVOH resin and a resin layer mainly composed of EVOH resin. Furthermore, as a result of diligent research to improve the stretchability, heat shrinkage, and dimensional stability of the EVOH resin while maintaining the gas barrier property of the EVOH resin and the heat resistance of the PA resin, the present inventors have conducted intensive studies to improve the stretchability of the EVOH resin itself. When a mixed resin containing EVOH resin as a main component is used in order to improve the above, it is found that gas barrier property and stretchability can be compatible to some extent, and a specific melting point of the mixed resin layer containing EVOH resin as a main component is obtained. It was found that a multilayer film having extremely improved stretchability, uniform heat shrinkage, and excellent dimensional stability can be obtained by laminating a PA resin in which a PA resin having a specific ratio is mixed with a specific ratio. The present invention has been made based on the above. Means to solve the problem The multilayer film of the present invention has an aliphatic polyamide resin having a crystal melting point of 175 to 240 ° C (hereinafter abbreviated as PA resin-1) of 55 to 90% by weight and an aliphatic polyamide resin having a crystal melting point of 120 ° C or more and less than 175 ° C. A heat-shrinkable multilayer film consisting of at least two layers, a mixed resin layer (A) consisting of 45 to 10% by weight (hereinafter abbreviated as PA resin-2) and a mixed resin layer (B) containing EVOH resin as a main component. Is. Examples of PA resin-1 include nylon 6, nylon 9, nylon 69, nylon 610, nylon 612, nylon 11, nylon 12, nylon 6-66, nylon 6-69, and nylon 6-12, but from the viewpoint of heat resistance. PA resin with a crystal melting point of 195 to 240 ° C: Nylon 6, Nylon 69, Nylon 610, Nylon 612, Nylon 6-66, Nylon 6-69, Nylon 6-12 are preferable. PA resin-1 having a crystal melting point exceeding 240 ° C has high crystallinity and tends to have poor stretchability. Also, Examples of PA resin-2 include nylon 6-69, nylon 6-12, nylon 6-66-610, and nylon 6-66-610-12, which are preferably stretchable, heat-shrinkable, and dimensionally stable. PA resin with a crystal melting point of 120 to 160 ° C is selected. PA resin-2 having a crystal melting point of less than 120 ° C is inferior in stretchability and heat resistance. The EVOH resin used for the layer (B) of the present invention is a resin obtained by partially saponifying a copolymer of ethylene and vinyl acetate, and has an ethylene content of 25 to 49 mol%, preferably 35 to 49 mol%. The degree of saponification is 95% or more. If the ethylene content exceeds 49 mol%, the gas barrier property is poor, and if the ethylene content is less than 25 mol%, the stretchability is poor. In order to obtain uniform stretchability, an ethylene content of 35 mol% or more is preferable. Layer (A) is a mixed resin of PA resin-1 55 to 90% by weight and PA resin-2 45 to 10% by weight, preferably PA resin-1 55 to 75% by weight and PA resin-2 45 to 25% by weight. It consists of a mixed resin. If PA resin-1 exceeds 90% by weight, uniform heat shrinkage and dimensional stability cannot be obtained. If PA resin-1 is less than 55% by weight, heat resistance and uniform heat shrinkage cannot be obtained. When the layer (B) is made of a mixed resin containing EVOH resin as the main component, 70 to 99% by weight of EVOH resin and polyester elastomer (block copolymer type thermoplastic polyester elastomer, etc.) are used from the viewpoint of gas barrier property and stretchability. , Polyethylene elastomers (polyesteramide elastomers, polyetheramide elastomers, etc.), ethylene-vinyl carboxylic acid ester copolymers such as ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid ester copolymers, etc. A mixed resin layer composed of 1 to 30% by weight of the obtained resin is preferable. The resin to be mixed with the EVOH resin is preferably a polyester elastomer, an ethylene-vinyl carboxylic acid ester copolymer having an ethylene content of 75 to 95 mol%, and an ethylene-acrylic acid ester copolymer. These resins are relatively easy to mix with EVOH resins. If the EVOH resin is less than 70% by weight, the gas barrier property is insufficient, and if the EVOH resin alone is used, the stretchability for obtaining a uniform heat shrinkage rate is insufficient, which is not preferable. An EVOH resin in the range of 85 to 97% by weight is preferable for obtaining gas barrier property and uniform stretchability. The multilayer film of the present invention is composed of at least a layer (A) and a layer (B), and each of the layer (A) and the layer (B) may have two or more layers. When two or more layers (A) and layers (B) are provided, the resin compositions constituting each layer may be the same or different. However, from the viewpoint of workability and obtaining a uniform layer thickness, it is preferable that the layers (A) and (B) are each one layer. The thickness of the multilayer film composed of the layer (A) and the layer (B) of the present invention is preferably 6 μm or more and 45 μm or less from the viewpoint of flexibility. The thickness of the layer (A), the layer (B) and the thermoplastic resin layer is preferably 15 μ or more and 120 μ or less. The thickness of the layer (A) is preferably thicker than the thickness of the layer (B), and when there are two or more layers (A) and layers (B), the total thickness of the layers (A) is the layer (B). ) Is preferably thicker than the total thickness. If the thickness of the layer (A) is thinner than the thickness of the layer (B), the stretchability of the film may decrease. The multilayer film of the present invention requires at least two layers, a layer (A) and a layer (B), but other thermoplastic resins can be laminated in order to add various functions. As the thermoplastic resin, an olephine resin is preferable from the viewpoint of extrusion moldability, stretchability and sealing property. The olephine resin is a copolymer of ethylene and vinyl ester monomer, for example, a copolymer of ethylene and a monomer selected from EVA, resin group unsaturated carboxylic acid, and aliphatic unsaturated carboxylic acid ester, for example. Copolymer of acrylic acid, acrylic acid ester, methacrylic acid, methacrylic acid ester, etc. and ethylene, ionoma resin, linear low density polyethylene (hereinafter abbreviated as LLDPE) and mixed resin of LLDPE and EVA, density 0.91 or less Bicut softening point (measured by ASTM D-1525) Ultra-low density polyethylene (hereinafter abbreviated as VLDPE) of 90 ° C or less, preferably 80 ° C or less, a mixed resin of VLDPE and a small amount of LLDPE, crystalline propylene- A mixed resin of an ethylene copolymer and a polypropylene-based elastomer is preferable. The EVA preferably has a vinyl acetate content of 3 to 19% by weight. The proportion of EVA in the mixed resin of LLDPE and EVA is preferably at least 55% by weight from the viewpoint of stretchability. LLDPE is a copolymer of ethylene and a small amount of butene-1, penten-1, 4-methyl-pentene-1, hexene-1, octene-1 and other α-olephine having 4 to 18 carbon atoms, and has a crystal melting point. The one at 118 to 125 ° C is preferable. Some ionomer resins include copolymers of olephine such as ethylene and propylene with unsaturated carboxylic acids such as acrylic acid, methacrylic acid and maleic acid, and copolymers of the olephine and unsaturated carboxylic acid esters. The anion part of the converted polymer is Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺An ion-crosslinked product obtained by partially neutralizing a metal ion such as the above is used. Usually Na⁺And Zn²⁺Is used. The ionomer resin partially neutralized with divalent metal ions may contain a polyamide oligomer. At least one layer of the olephine resin in the multilayer film of the present invention can be laminated, and when two or more layers are laminated, the olephine resin may be the same or different. The olephine resin may be arranged between the layer (A) and the layer (B), and is preferably laminated on one side or both sides of the laminated film of the layer (A) and the layer (B). By laminating the olephine resin, extrusion moldability, dimensional stability and the like can be improved, and excellent sealing properties and the like can be imparted. Inorganic additives such as thermoplastic resins, inorganic fillers and pigments and / or organic additives may be added to the layers (A), layers (B) and olephine resin layers as long as they do not impair stretchability and gas barrier properties. You can. Since the layer (A) and the layer (B) have good adhesiveness, an adhesive layer is not particularly required, but an adhesive layer may be provided if necessary. It is preferable to provide an adhesive layer between the olephine resin layer and the layer (A) or the layer (B) in order to improve the adhesiveness. As the adhesive, polyolephine modified with a carboxylic acid such as fumaric acid or maleic acid or copolymerized polyolephine is preferable. The method for producing the multilayer film of the present invention will be described below. First, the layer (A) and the layer (B) are laminated and extruded in a tube shape using an annular die equipped with an extruder corresponding to the number of resins to be laminated. Lamination after forming the layer (A) and the layer (B) into films, for example, external bonding of dies, is not preferable because the adhesion between the films may reduce the stretchability. The extruded tube-like laminate is immediately quenched, the resulting substantially amorphous tube-like film is heated, and then the inflation method is used to increase the length and width by 1.3 to 4.0 times, preferably 1.5 to 3 times. Axial stretching is performed to form a multilayer film. The heating temperature is 60 ° C or more and less than 100 ° C, preferably 80 to 95 ° C. If the heating temperature is less than 60 ° C, the stretchability deteriorates and the dimensional change becomes large. Further, when the heating temperature becomes 100 ° C. or higher, the desired heat shrinkage rate cannot be obtained. The heat shrinkage of the multilayer film of the present invention needs to be 10% or more, preferably 20% or more in the vertical and horizontal directions when immersed in hot water at 90 ° C for 1 minute. If the heat shrinkage rate is less than 10%, wrinkles of the film will be generated on the surface of the packaged product, the adhesion between the filling material and the packaged film will be impaired, and the appearance of the product will be deteriorated. Effect of the invention The multi-layer film of the present invention is biaxially stretched by laminating a mixed resin layer mainly composed of EVOH resin having an excellent balance between gas barrier property and stretchability, and a PA resin in which a PA resin having a specific melting point is mixed at a specific ratio. By laminating these mixed resin layers, it is possible to obtain a film for food packaging having uniform heat shrinkage, excellent stretchability and dimensional stability. This is because the laminated film of the present invention has extremely good stretchability. Practical dimensional stability can be obtained without unreasonable stretching orientation, uniform heat shrinkage and heat treatment, and delamination does not occur during high temperature treatment in the sterilization heat shrinkage step. Further, the multilayer film of the present invention is also excellent in adhesion to the contents. Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples. Examples 1 to 7, Comparative Examples 1 to 5 The resins shown in Table 1 were separately extruded by a plurality of extruders, flowed into an annular die, laminated, and co-extruded in the layer structure shown in Table 2. The tubular body extruded from the die was cooled in a cooling tank at 5 to 20 ° C. to obtain a tube-shaped laminate having a folding width of 175 mm and a thickness of 80 to 265 μm. The tube-shaped laminate is heated for about 6 seconds in a heating tank adjusted to the temperature shown in Table 2, and is 2.3 times in the longitudinal direction (L) by the inflation method, and the diameter direction of the tube-shaped laminate. It was stretched 2.3 times. The folded width of the obtained biaxially stretched film was about 403 mm, and the thickness was 15 to 50 μm. Example 8 The resins shown in Table 1 were separately extruded by a plurality of extruders, flowed into an annular die, laminated, and co-extruded in the layer structure shown in Table 2. The tubular body extruded from the die was cooled in a cooling tank at 5 to 20 ° C. to obtain a tube-shaped laminate having a folding width of 262 mm and a thickness of 95 μm. The tube-shaped laminate is heated for about 6 seconds in a heating tank adjusted to the temperature shown in Table 2, and the diameter of the tube-shaped laminate is 1.5 times in the longitudinal direction (L) by the inflation method. It was stretched 1.5 times in the direction. The folded width of the obtained biaxially stretched film was about 393 mm, and the thickness was 42 μm. Example 9 The resins shown in Table 1 were separately extruded by a plurality of extruders, flowed into an annular die, laminated, and co-extruded in the layer structure shown in Table 2. The tubular body extruded from the die was cooled in a cooling tank at 5 to 20 ° C. to obtain a tube-shaped laminate having a folding width of 133 mm and a thickness of 378 μm. The tube-shaped laminate is heated for about 6 seconds in a heating tank adjusted to the temperature shown in Table 2, and the diameter of the tube-shaped laminate is tripled in the longitudinal direction (L) by the inflation method. It was stretched 3 times in the direction. The folded width of the obtained biaxially stretched film was about 399 mm, and the thickness was 42 μm. Table 1 shows the types and physical properties of the resins used in Examples and Comparative Examples, Table 2 shows the layer structure and characteristic test results of the obtained laminated film, and Table 3 shows the characteristic test method.

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As shown in Examples 1 to 9, it can be seen that the heat-shrinkable multilayer film of the present invention has excellent stretchability, uniform heat-shrinkability and practical dimensional stability. On the other hand, in Comparative Example 1, since the film is a laminated film of a PA resin layer and an EVOH resin single layer, the stretchability is unsatisfactory and a practical dimensionally stable film cannot be obtained. In Comparative Example 2, the PA resin layer is the same as that of the present invention, but since the second layer is an EVOH resin single layer, the stretchability is improved, but the dimensional stability is not practical. In Comparative Examples 3 to 5, the same EVOH resin layer as in the present invention is used, but since the fourth PA resin layer is outside the scope of the present invention, uniform stretchability and practical dimensional stability can be obtained. I can't.