JPH0337497B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention comprises an olefin resin as the outermost layer, a vinylidene chloride copolymer as the core layer, a polyamide as the innermost layer, and an adhesive layer between the outermost layer and the core layer and between the innermost layer and the core layer. A polymer obtained by modifying a copolymer of ethylene and vinyl ester or acrylic ester with an ethylenically unsaturated carboxylic acid or its acid anhydride, or a polymer obtained by modifying the acid-modified polymer with a metal compound, into a tubular form. The polyamide is melted and laminated, and while being cooled with hot water having a temperature equal to or higher than the secondary transition temperature of the polyamide and not higher than it by 20°C, it is pulled along a conical internal mandrel attached below the center of the annular die and in the winding direction. By simultaneous biaxial stretching while tightening, and then enclosing air in the tubular body, the diameter of the tubular body in the transverse direction perpendicular to the winding direction is equal to or larger than the maximum diameter of the conical internal mandrel. The present invention relates to a method for producing a heat-shrinkable composite film, which comprises expanding and simultaneously biaxially stretching. Conventionally, pasty meats such as hams and sausages have been filled and packaged in thermoplastic casing films, and the packaged products have been heat-treated in a temperature range of approximately 70 to 95° C. for several minutes to several hours before being provided to the market. Appropriate packaging materials for packaging foods that undergo such heat treatment must satisfy the following requirements. (1) The casing film must have sufficient oxygen gas shielding properties to prevent the contents from spoiling during the appropriate distribution period. (2) In order to prevent jelly such as meat juice from depositing on the interface between the casing film and the contents, especially when the contents are meat such as ham or sausage, the casing film must be firmly attached to the contents. . (3) The casing film should not be wrinkled, as a wrinkled or folded appearance on the packaged product gives consumers the impression that the product is old. (4) The pressure when the casing film is filled (generally
0.2 to 0.5 Kg/cm ² G) and the cylindrical shape of the packaged product should not be broken or deformed by the pressure caused by the expansion of the contents during heat treatment. In other words, the casing film is particularly required to have high temperature creep resistance.
It is required not to deform under stress of 0.2Kg/mm ² to 0.3Kg/mm ² . (5) The casing film must have sufficient flexibility for operation, and no pinholes will form during filling. (6) Heat the packaged product in a temperature range of approximately 70 to 95℃ for several minutes.
No melt holes should occur in the casing film after heat treatment for several hours. (7) The casing film must have excellent strength (Young's modulus, cold resistance strength). Regarding the requirements for such packaging films, a two-layer coextruded casing film with an inner layer of homopolyamide (nylon 11 and nylon 12) and an outer layer of nylon 6, which is known, is easily deformed by the filling pressure, and is not a non-stretched film. Therefore, after heat treatment, the casing wrinkles significantly during cooling. On the other hand, JP-A-55-74744 discloses a heat-shrinkable casing film made of polyamide or a mixture of these polyamides and an olefinic resin. Furthermore, casing films made of thermoplastic polyester (for example polyethylene terephthalate) are also known for heat shrink packaging. However, all of the above-mentioned casing films have a common drawback in that they do not have sufficient oxygen gas shielding properties and water vapor shielding properties, so that the edible storage period must be shortened. Furthermore, the casing film made of polyester has the disadvantage that it does not adhere firmly to the contents of ham, sausage, etc. On the other hand, casing films made of vinylidene chloride copolymer resins that have oxygen gas shielding properties and water vapor shielding properties are already widely used as food packaging materials. Not only is the degree of adhesion insufficient, but when a large amount of contents are packaged, the cylindrical shape tends to deform due to the weight of the contents during heat treatment, and it has been pointed out that the high temperature creep resistance is insufficient. Furthermore, a casing film made of a vinylidene chloride copolymer has the disadvantage that pinholes are likely to occur during filling. As an example of a laminate film having a vinylidene chloride copolymer, the core layer is a vinylidene chloride copolymer, the outermost layer is an olefin resin, the innermost layer is an ionomer, and the core layer, outermost layer, and innermost layer are ethylene. - A heat-shrinkable 5-layer laminate film bonded with an adhesive layer such as an acetic acid copolymer can be prepared by immersing a packaged product containing a large amount of contents in boiling water (90-98°C) for 10 minutes. If the above-mentioned material is applied and subjected to heating and sterilization treatment, it has the disadvantage of producing melt holes and creep deformation. Furthermore, the innermost layer of the five-layer laminate has the disadvantage that it is difficult to adhere to the contents. The purpose of the present invention is to overcome these drawbacks of the prior art, to provide a product that has excellent oxygen gas shielding properties, water vapor shielding properties, high temperature creep resistance, pinhole resistance, content adhesion to meat, and hygienic properties, and is free from folds and wrinkles. It is an object of the present invention to provide a method for producing a heat-shrinkable composite film for food packaging, which does not sag and has excellent strength, particularly strength against forces in the lateral direction perpendicular to the winding direction. The vinylidene chloride copolymer used as the core layer in the present invention is a copolymer mainly composed of vinylidene chloride, and contains 65 to 95% by weight of vinylidene chloride and at least one monomer copolymerizable therewith. It is a copolymer consisting of 35 to 5% by weight. Examples of copolymerizable monomers include vinyl chloride, acrylonitrile, acrylic acid alkyl esters (alkyl group having 1 to 18 carbon atoms), methacrylic acid alkyl esters (alkyl group having 1 to 18 carbon atoms), and acrylic acid. ,
It is selected from methacrylic acid and the like. In vinylidene chloride copolymer, vinylidene chloride is 65
If the amount is less than % by weight, the vinylidene chloride copolymer becomes rubbery at room temperature, and a molded article with stable shape cannot be obtained. Furthermore, if the amount of vinylidene chloride is more than 95% by weight, the melting point will be too high and it will be easily thermally decomposed, making stable melt extrusion processing difficult. The vinylidene chloride copolymer may contain small amounts of plasticizers, stabilizers, and other additives, if necessary. These additives are known to those skilled in the art, and typical plasticizers used in vinylidene chloride copolymers include dioctyl sebacate, dibutyl sebacate, acetyl triptyl citrate, etc., and stabilizers include Examples include epoxidized soybean oil. The vinylidene chloride copolymer layer of the heat-shrinkable composite film in the present invention has a thickness of 3 to 30 microns.
If the thickness is less than 3μ, it will be difficult to maintain the desired oxygen gas and water vapor shielding properties, and
If the thickness exceeds 30μ, it becomes difficult to prevent the occurrence of cracks or pinholes due to low temperature brittleness even if protected by the outermost polyolefin resin layer and the innermost polyamide layer. The olefin resin that forms the outermost layer is
High-pressure polyethylene, medium-low pressure polyethylene, ethylene-vinyl acetate copolymer, ethylene-acrylic acid ester copolymer, polypropylene, ethylene-propylene copolymer, ethylene-α-olefin copolymer, so-called low-density linear polyethylene,
Ionomers are used. The olefin resin layer is effective in imparting mechanical strength, particularly cold resistance strength and flexibility, to the heat-shrinkable composite film. Although there are no particular restrictions, the thickness of the olefin resin layer of the heat-shrinkable composite film is preferably 5 to 50 microns in order not to impede the stretchability of the heat-shrinkable composite film. Examples of the polyamide forming the innermost layer include nylon-6 (polycapramide), nylon 6-6 (polyhexamethylene adipamide), and nylon 6-66 (ε
- a copolymer of caprolactam and hexamethylene adipamide), nylon 6-10 (polyhexamethylene sepacamide), and nylon 12 (a ring-opened polymer of laurin lactam). These polyamides have a melt viscosity in the range of 5 x 10 ³ to 50 x 10 ³ poise at 230°C and a shear rate of 100 sec ^-1 using an enhanced flow tester from the viewpoint of extrusion processability and operability in the drawing process. is preferable, and most preferably 10 to 20×10 ³ poise. On the other hand, since a large stretching force is required when simultaneously biaxially stretching a laminated tubular body, the laminated film itself must be able to withstand stress during the stretching. Furthermore, the heat-shrinkable composite film must have properties that can withstand pressure during filling and high-temperature creep resistance during heat treatment (several minutes to several hours at 70 to 95°C). In order to meet this requirement, the thickness of the polyamide resin layer of the heat-shrinkable composite film is preferably 5 to 50 microns, preferably 20 to 40 microns. In addition, the polyamide resin layer has oil resistance that prevents it from being swollen by the fat contained in the food, and it also has the property of adhering tightly to the contents of ham and sausage, so it is placed as the innermost layer. It is preferable. In the present invention, an adhesive layer is used to firmly adhere the vinylidene chloride copolymer of the core layer and the olefin resin or polyamide of the outer layer. The adhesive used is a mixture of ethylene-vinyl acetate copolymer and ethylene-acrylic acid copolymer (Japanese Unexamined Patent Publication No. 1973
-41476), ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer (JP-A No. 51
-19075) is known. However, as in the present invention, each layer is bonded in a die and then coextruded, or extruded and then laminated, and the resulting tubular body is made into two
When producing a heat-shrinkable film by axial stretching, an adhesive with extremely high adhesive strength is required to prevent delamination during stretching. However, with the above-mentioned conventional adhesives, during the production of the heat-shrinkable composite film, which is the object of the present invention, delamination occurs during stretching, so that sufficiently satisfactory results have not yet been obtained. In the present invention, a polymer obtained by modifying a copolymer of ethylene and vinyl ester or acrylic ester with an ethylenically unsaturated carboxylic acid or an acid anhydride, or a polymer obtained by modifying the acid-modified polymer with a metal compound is used as an adhesive. For the first time, it became possible to achieve that purpose by using it in layers. As the vinyl ester copolymerized with ethylene in the adhesive of the present invention, vinyl acetate and vinyl propionate are used, and as the acrylic ester, an acrylic ester having an alkyl group having 1 to 8 carbon atoms is preferred. The polymers used in the adhesive of the present invention include these ethylene-vinyl esters, ethylene-acrylic ester copolymers, and unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and fumaric acid, or their acid anhydrides. For example, a polymer modified with maleic anhydride is used. The term "modification" as used herein refers to introducing an unsaturated carboxylic acid or its acid anhydride into a copolymer through a graft polymerization reaction or the like. Furthermore, as the adhesive of the present invention, polymers obtained by modifying these acid-modified polymers with metal compounds, preferably alkali metal salts, alkali metal oxides, etc., or alkaline earth metal salts, alkaline earth oxides, etc. can also be used. . Modification here refers to introducing a metal into a polymer through a neutralization reaction or the like. The amount of unsaturated carboxylic acid in the acid-modified polymer is preferably from 0.01 to
10% by weight, and the amount of metal in the metal-modified polymer is preferably in the range of 1/50 to 10% by mole. A typical adhesive is the product name N-
Examples include polymers (products of Nippon Petrochemicals Co., Ltd.). The thickness of the adhesive layer of heat-shrinkable composite film is 1μ
or more, less than 5μ, preferably 1.5 to 4.0μ.
If it is thinner than 1μ, suitable adhesive force cannot be exhibited. The laminated film of the present invention is extruded into a cylindrical shape using an annular die using four extruders, so that the inside of the die is laminated outside the die. Next, the tubular bodies melted and laminated at a temperature higher than the melting point of polyamide, which is a high melting point resin, are quickly cooled from the molten state to the drawing temperature, and at the same time, sharp drawing is started, thereby forming spherulites in each of the constituent resins. It is possible to obtain a heat-shrinkable composite film which has excellent transparency and has been given a stretching orientation effect. An inflation method in which air is enclosed can be used as a method for stretching the tubular film, but when stretching the composite five-layer film containing the polyamide layer of the present invention, the inflation pneumatic pressure is required because the required stretching tension is large. must be made larger.
As a result, stress concentration occurs on slight unevenness in the thickness of the film, resulting in uneven thickness of the film.
Continuous stretching and winding is difficult with the inflation method in which rupture occurs and air is enclosed. The melt-laminated tubular body of the present invention is rapidly cooled from the molten state to the drawing temperature, and at the same time, the tubular body is placed along the conical internal mandrel attached below the center of the annular die and taken off. 2 at the same time while being pulled in the winding direction by the roll.
axial stretching, and then immediately enclosing air in the tubular body and expanding it so that the diameter of the tubular body in the transverse direction perpendicular to the winding direction is larger than the maximum diameter of the conical internal mandrel, and simultaneously biaxially stretching the tubular body. By stretching, a heat-shrinkable composite that has sufficiently excellent high-temperature creep resistance even under harsher conditions than before, and that does not cause thickness unevenness in the film after stretching and does not rupture during stretching. It became possible to stretch and wind the film continuously. In order to obtain satisfactory high temperature creep resistance even under severe conditions, it is necessary that the stretch orientation ratio in the transverse direction perpendicular to the winding direction of the composite film is higher than the stretch orientation ratio in the winding direction. . That is,
When the packaging product is assumed to be cylindrical, and the pressure applied to the contents is P ₀ , the thickness of the composite film is t, and the radius of the contents is R, then the stress applied in the winding direction L of the composite film (σ _L ): σ _L = R×P ₀ /2t Stress applied in the transverse direction T perpendicular to the winding direction of the composite film (σ _T ): σ _T = R×P ₀ /t, which is perpendicular to the winding direction of the composite film The stress in the lateral direction (σ _T ) is twice the stress in the winding direction (σ _L ), and the composite film is subjected to a larger deformation force in the lateral direction perpendicular to the winding direction. Therefore, it is preferable that the composite film is stretched and oriented more highly in the transverse direction perpendicular to the winding direction than in the winding axis direction. Hereinafter, the method for manufacturing a composite film of the present invention will be explained with reference to the accompanying drawings. In Figs. 1 and 2, extruder 7 (actually 4
The cylindrical five-layer tubular body 14 extruded through the annular die 1 by a stand (not shown in the figure) is kept at a temperature such that its tubular shape can be easily maintained by the air ring 2 installed immediately after the annular die lip. , preferably to a temperature 5 to 10°C higher than the crystallization temperature of the innermost polyamide layer in order to maintain an essentially amorphous state, and then shearing 3 to cool the innermost polyamide layer to a secondary state. The 5-layer tubular body is stretched at a temperature ( The polyamide is cooled to a temperature between the crystallization temperature and the second order transition temperature). Next, the cooled five-layer tubular body is placed along the conical inner mandrel 4 which is vertically attached via a support rod 13 downward from the center of the annular die, and the composite film 15 is pulled by the take-off roll 9. Perform simultaneous biaxial stretching. At this time, if necessary, the temperature of the internal mandrel is preferably set to 30 to 80 by using the heat medium circulation temperature controller 8.
The temperature is adjusted to ℃ and the tubular body is also cooled from the inside. However, as described above, in the method of simply stretching the transverse direction orthogonal to the winding direction using the conical internal mandrel 4 to a predetermined stretching ratio (2 to 5 times), which is the same as the winding direction, the 5 The layered tubular body 15 tends to be re-stretched in the direction of the winding axis due to the tension applied by the winding roll 9, and tends to become a five-layer tubular heat-shrinkable film in which the stretching orientation is dominant in the direction of the winding axis. It is difficult to obtain a stretched and oriented film. Therefore, in the present invention, in order to improve this point, after stretching with an internal mandrel, the air pressure regulating valve 1 is
By enclosing air whose pressure has been appropriately adjusted by rollers 7, the five-layer tubular body, which has been simultaneously biaxially stretched, is expanded to a diameter greater than the outer diameter of the internal mandrel, and the five-layer tubular body, which has been simultaneously biaxially stretched, is expanded by a take-up roller 9. Simultaneous biaxial stretching is carried out by taking off the layered tubular body. At this time, the hot water sprayed from the shower ring 3 is removed by the presser ring 5. In this case, the ratio of the die lip diameter of the annular die to the diameter of the maximum diameter part of the conical inner mandrel is 1.5 to 2.5.
Double is preferred. Further, by adjusting the ratio between the diameter of the die lip and the diameter of the maximum diameter portion of the bubble portion expanded by air pressure, and the ratio between the axial speed of the tubular body 14 immediately before stretching and the speed of the take-up roller 9, the winding shaft can be adjusted. The direction and the stretching ratio in the transverse direction perpendicular to the direction are determined. Generally, a magnification of 2 to 5 times is preferred in both the winding axis direction and the lateral direction. As described above, the conical inner mandrel and the film 15 immediately after being stretched by air pressure do not need to be further stretched, so that the film 15, especially the innermost layer, is sprayed with cooling water of 5 to 30°C by the cooling ring 6. cooled to below the second-order transition temperature of the polyamide,
Stretching of the composite film 15 in the direction of the winding axis is prevented. Since the composite film 15 is sufficiently cooled at this time, the innermost layers will not stick together even if pinched, so there is no need to use a pile agent. Therefore, when this tubular heat-shrinkable composite film is applied to casings for sausages, etc., it is extremely convenient as there is no substance that would inhibit the adhesion between the contents such as ham, sausage, etc. and the innermost polyamide resin layer. . The stretched composite film 15 passes through a take-up roller 9, and a pair of pinch rollers 10 and 11 (the pinch roller 11 is at the same speed as the pinch roller 10) is used to relax the excessively stretched composite film and to remove wrinkles. (which is driven at a slightly slower speed), the composite film is relaxed between
Preferably, it is subjected to a relaxation treatment by enclosing air, and if necessary, it is heated in a far-infrared oven 12 in an atmosphere controlled at 40 to 100°C, and then rolled up with a winding roll 16 to make it transparent. A 5-layer tubular heat-shrinkable film with good glossiness can be obtained. When the heat-shrinkable composite film of the present invention obtained by the manufacturing method of the present invention as described above is heated in a relaxed state at 90°C for 3 seconds, the heat shrinkage rate is 15 on average in the winding axis direction L and the transverse direction T, respectively. % or more. Furthermore, when the biaxially stretched heat-shrinkable composite film obtained by the method of the present invention is used as a film for food packaging, appropriate shrinkage occurs during heating after filling the contents, and the contents It adheres closely to food paste products, etc., and does not appear wrinkled or wrinkled. Examples are shown below. Example 1 Polymer A described below: Vinylidene chloride-vinyl chloride copolymer (83/17% by weight) 100 parts by weight dibutyl separate 1 Epoxidized soybean oil 2 B: Ethylene-ethyl acrylate copolymer (ethyl A polymer obtained by graft-polymerizing acrylate content (15% by weight) with maleic anhydride is further
Polymer modified with _MgCO3 . Melt Index 6g/10min. Maleic anhydride in the graft polymer is 0.5% by weight. Mg in Polymer B was 0.84 mol%. C; Low density polyethylene Melt Index 1.22
g/10 minutes, density 0.92 D; Polyamide nylon 6-66, manufactured by Toyo Rayon Co., Ltd., trade name CM-6041, melting point 200°C, crystallization temperature 150°C, secondary transition point 60°C, η ^* = 1.7× 10 ⁴
Poise (220℃) is extruded separately using four extruders, and the molten polymer is introduced into an annular die, which allows it to flow from the inside to the outside.
DBABC was melted and bonded in the order of 5 layers in a die and coextruded. The resin temperature of the tubular body 14 at the die exit is
It was 220℃. The tubular body has a diameter of 0.2 mm at the die lip.
It was cooled by an air ring 2 blowing air at an air pressure of Kg/cm ² G, and further cooled to the drawing temperature by spraying hot water at 60° C. by a shower ring 3. Next, initial stretching is carried out along a conical internal mandrel 4 whose lower maximum diameter is twice the diameter of the die lip, and immediately after that, the air pressure control valve 17 is used to draw 0.3 kg/
It was stretched to three times the diameter of the die lip using the enclosed air adjusted to cm ² G. At this time, the mandrel has 60
℃ medium was circulated, the speed of the take-up roller 9 at this time was 20 m/min, the speed in the direction of the winding axis of the tubular body immediately before stretching was 6 m/min, and the magnification in the direction of the winding axis was approximately 3.3.
It was twice as hot. The presser ring 5 removes hot water and at the same time compressed air sealed inside the bubble is released into the tubular body 1.
The film that is being stretched is held against the conical internal mandrel 4 to prevent it from leaking to the film. 15 by the valve cooling ring 6 to further cool the stretched film.
Cooling water at ℃ was sprayed on the fabric, and the wrinkles were smoothed out in the next relaxation zone 18, and then the fabric was rolled up. The folding width of the biaxially stretched film is
The thickness was 230 mm, and the thickness structure was: D layer = 30 μm, B layer = 3 μm, A layer = 10 μm, C layer = 10 μm, and the total thickness was 56 μm. The physical properties of the film and clip package obtained in Example 1 were measured using the methods shown in Table 1. The results are shown in Table 2. The stretched film has a higher heat shrinkage rate in the transverse direction than in the winding axis direction. The resulting heat-shrinkable composite film was filled with approximately 6 kg of sausage using a conventional method, and then heat-treated (90°C 4
In both the packaged products that had been stored for 30 minutes and those that had been stored in a refrigerator at 5° C. for one day, there were no wrinkles in the composite film and no change in the outer diameter of the packaged products.

【table】

[Table] Example 2 Same as Example 1 except that Polymer E: Low density linear polyethylene (manufactured by Mitsui Chemicals, Urtozex 2021L) was used instead of Polymer C: Low density polyethylene of Example 1. The same polymer was extruded separately using four extruders, and the molten polymers were introduced into an annular die, where they were melt-bonded in the order of DBABE from the inside and coextruded into five layers within the die. Subsequently, a composite film was produced using the same method as described in Example 1. The thickness of each layer of the composite film is: D layer = 30μ, B layer = 3μ, A layer = 10μ.
and E layer = 10μ, and the total layer thickness was 56μ. The physical properties of the film and clip package obtained in Example 2 were measured using the methods shown in Table 1. The results are shown in Table 2.

[Table] The packaged product prepared by filling the heat-shrinkable composite film produced in Example 2 with approximately 6 kg of sausage by a normal method and then heat-treating it by suspending it at 90°C for 4 hours was also heat-treated in a refrigerator at 5°C for 24 hours. Things that have been stored for a while,
There were no wrinkles in the composite film and no change in the outer diameter of the packaged product was observed. Furthermore, the physical properties of the heat-shrinkable composite films produced in Examples 1 and 2 were measured using the methods shown in Table 3. The results are shown in Table 4.

【table】

【table】

[Table] From Table 4, the biaxially stretched composite films of Examples 1 and 2 have a higher heat shrinkage rate in the transverse direction perpendicular to the winding axis direction than in the winding axis direction. Further, from the values in Table 4 regarding tensile strength, tensile elongation, heat shrinkage stress, and hot water creep, it is clear that the stretching orientation in the transverse direction perpendicular to the winding axis direction is higher than that in the winding axis direction.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a layout for manufacturing the composite film of the present invention, and FIG.
FIG. 2 is an enlarged view of the die and conical inner mandrel portion of the figure; 1... Annular die, 2... Air ring, 3... Shear ring, 4... Conical internal mandrel, 5...
... Pressing ring, 6 ... Cooling ring, 9 ... Take-up roller, 10, 11 ... Pinch roller, 14 ... Tubular body, 15 ... Composite film.

Claims (1)

[Scope of Claims] 1. Olefin resin as the outermost layer, vinylidene chloride copolymer as the core layer, polyamide as the innermost layer, and between the outermost layer and the core layer and between the innermost layer and the core layer. For the adhesive layer, a polymer obtained by modifying a copolymer of ethylene and vinyl ester or acrylic ester with an ethylenically unsaturated carboxylic acid or its acid anhydride, or a polymer obtained by modifying the acid-modified polymer with a metal compound. A conical internal mandrel is attached to the lower center of the annular die while melt-laminating the resulting tubular body in the form of a tubular body and cooling the resulting tubular body with hot water at a temperature not lower than the second-order transition temperature of the polyamide and not higher than it by more than 20°C. At the same time, the inner diameter of the tubular body in the transverse direction perpendicular to the winding direction is expanded by biaxial stretching while being pulled along the winding direction and in the winding direction. A method for producing a heat-shrinkable composite film, which comprises expanding the film to a maximum diameter of a mandrel or more and simultaneously biaxially stretching the film. 2 The thickness of both outer layers of the heat-shrinkable composite film is 5~
50μ, the thickness of the core layer is 3 to 30μ, and the thickness of the adhesive layer is 1μ or more and less than 5μ. 3. The heat treatment according to claim 1, characterized in that simultaneous biaxial stretching is carried out so that the stretching magnification in the winding direction of the tubular body and in the transverse direction orthogonal to the winding direction is 2 to 5 times, respectively. A method for producing a shrinkable composite film.