JPS625060B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat-shrinkable packaging film with excellent gas barrier properties and a method for producing the same, and more specifically relates to a low-temperature heat-shrinkable film with an intermediate layer made of a saponified ethylene-vinyl acetate copolymer resin with excellent gas barrier properties. The present invention relates to a multilayer heat-shrinkable film for food packaging that has good properties such as flexibility, transparency, toughness, and hygiene, and a method for producing the same. Conventionally, vinylidene chloride resin films have been used for packaging processed meat products such as hams and sausages, and processed fish meat products such as kamaboko. Vinylidene chloride resin has excellent gas barrier properties, but residual vinylidene chloride monomer poses food hygiene concerns, and some packaging applications require even higher oxygen gas barrier properties.
In place of vinylidene chloride resin, saponified ethylene-vinyl acetate copolymer resin has recently attracted attention. However, while the saponified ethylene-vinyl acetate copolymer resin has extremely excellent gas barrier properties, it has the drawback that its gas barrier properties decrease due to moisture absorption, and its mechanical performance also decreases significantly. Furthermore, it is extremely difficult to biaxially stretch a single-layer extruded tube or extruded sheet of saponified ethylene-vinyl acetate copolymer resin at a stretching temperature of 100°C or less in a sufficiently dry state. It has been difficult to obtain a low-temperature heat-shrinkable biaxially oriented film that can be used for vacuum shrink packaging such as the following. The present inventors have developed a saponified ethylene-vinyl acetate copolymer resin with excellent gas barrier properties, which has stable and high oxygen gas barrier properties over a long period of time.
In addition, we conducted research to obtain a packaging film that exhibits high heat shrinkability in hot water, and developed a coextrusion laminate consisting of three layers: an ethylene-vinyl acetate copolymer saponified resin layer, an adhesive resin layer, and an ionomer resin layer. The inventors discovered that the sheet can be biaxially stretched at a temperature of 100° C. or lower, and after further various studies, they completed the present invention. The first object of the present invention is to provide shrink packaging that has stable oxygen gas barrier properties and moisture proof properties over a long period of time, and has excellent low-temperature heat shrinkability, transparency, pinhole resistance, and hygiene. It is an object of the present invention to provide a suitable heat-shrinkable film and a method of manufacturing such a film. The present invention provides an ionomer resin layer/modified polyolefin adhesive resin layer or copolymerized polyamide resin layer/ethylene-vinyl acetate copolymer saponified resin layer/modified polyolefin adhesive resin layer or copolymerized polyamide resin layer, and an ionomer Resin layer/
Modified polyolefin adhesive resin layer or copolymerized polyamide resin layer / ethylene-vinyl acetate copolymer saponified resin layer / modified polyolefin adhesive resin layer or copolymerized polyamide resin layer / ionomer resin layer or ethylene-vinyl acetate copolymer The composite is a multilayer plastic film with a laminated structure of adjacent layers in this order, and the ethylene-vinyl acetate copolymer saponified resin layer has an ethylene content of 25 to 75 mol% and a saponification degree of vinyl acetate in the copolymer. but
The film is composed of 90% or more resin, has a total thickness of 5μ to 150μ, and is characterized by a dimensional shrinkage rate of over 10% in both length and width after 10 seconds of being placed in an atmosphere at a temperature of 100°C. Ethylene-vinyl acetate copolymer saponified resin has a softening temperature (flow tester method) of 150 to 175.
℃, the ethylene content is 25 to 75 mol%, the degree of saponification of vinyl acetate in the copolymer is 90% or more, and it has extremely high gas barrier properties in a dry state. For saponified ethylene-vinyl acetate copolymer resins, the higher the concentration of saponified vinyl acetate in the copolymer, the higher the softening temperature and improved gas barrier properties, but on the other hand, the higher the concentration of saponified vinyl acetate in the copolymer, the higher the gas barrier properties. This results in disadvantages such as poor performance. Therefore, in the heat-shrinkable film of the present invention, a saponified ethylene-vinyl acetate copolymer resin having a softening temperature of 165°C to 175°C is preferred. Ionomer resins are ionic copolymers made by reacting metal ions with ethylene-methacrylic acid copolymers, ethylene-acrylic acid copolymers, ethylene-itaconic acid copolymers, ethylene-methyl methacrylate copolymers, etc. It is a polymer, especially ethylene-methacrylic acid copolymer containing Na ions and Mg.
A resin treated with ions or Zn ions is preferable. Ionomer resins not only have excellent water vapor barrier properties, but also have the advantage of excellent low-temperature heat sealing properties when heat sealing between films in secondary processing. Adhesive resins include modified polyethylene adhesive resins whose molecular chain ends have been treated with reactive functional groups, modified polypropylene adhesive resins, modified ethylene-vinyl acetate copolymer adhesive resins, copolymerized polyamide resins, etc. Among these, modified ethylene-vinyl acetate copolymer adhesive resins and copolyamide resins are particularly preferred. The total thickness of the film of the present invention is 5μ to 150μ
Films with a thickness within this range are suitable for shrink-wrapping foods. The thickness of the saponified ethylene-vinyl acetate copolymer resin layer is preferably about 20% to 5% of the ionomer resin layer, and the thickness of the adhesive resin layer is about 50% of the ionomer resin layer.
% to about 5% is preferable. The method for producing a heat-shrinkable film of the present invention is based on an ethylene copolymer having an ethylene content of 25 to 75 mol% and a degree of saponification of vinyl acetate in the copolymer of 90% or more.
A multilayer plastic sheet including a laminated structure in which a saponified vinyl acetate copolymer resin layer, an adhesive resin layer, and an ionomer resin layer are adjacent to each other in this order,
A multilayer sheet in which the saponified ethylene-vinyl acetate copolymer resin layer is not exposed on the surface is melt-extruded from a laminated die using a coextrusion method, solidified by cooling, and then stretched at a temperature within the range of 45°C to 100°C. It is characterized by being reheated and biaxially stretched by 1.5 times or more in the machine direction and 1.5 times or more in the transverse direction, and then cooled. The coextrusion method for multilayer sheets requires a plurality of extruders, and is preferably extruded into a tube shape using a circular rider or extruded into a flat shape using a T-die. When melt extruding a multilayer sheet into a tube shape from a circular die,
Either cool and solidify the tube by immersing it in a water tank, or
Preferably, cooling air is blown into the tube to cool and solidify it. Next, this tube is reheated to a stretching temperature within the range of 45°C to 100°C, stretched by more than 1.5 times in the longitudinal direction using a take-up roll, and in the transverse direction, air is introduced into the tube to increase the pressure inside the tube. Inflate the tube and stretch it more than 1.5 times. In this case, a film with good appearance can be produced over a wide range of stretching ratios by simultaneously stretching in the longitudinal and lateral directions, or by first stretching in the transverse direction and then in the longitudinal direction. When melt-extruding a multilayer sheet in a flat form from a T-die, the sheet is cooled by contacting the surfaces of two or three rolls, cooling air is blown onto the sheet surface using an air knife, or the sheet is It is preferable to cool it by placing it in a water tank. Next, this flat sheet is reheated to a stretching temperature within the range of 45°C to 100°C, and stretched by more than 1.5 times in the longitudinal direction using a combination of stretching rolls, and in the transverse direction, the edges of the sheet are fixed with chucks. A tenter method of stretching in which the film is stretched continuously in the transverse direction is preferred. Even in the case of a flat sheet, a film with good appearance can be produced over a wide range of stretching ratios by simultaneous biaxial stretching or sequential biaxial stretching in which the film is first stretched in the transverse direction and then in the longitudinal direction. Regardless of the stretching method, the stretching temperature is 45℃.
As long as the temperature is within the range of 100° C., it is not necessarily necessary to stretch at the same temperature in both length and width. In the case of the multilayer laminated sheet of the present invention, when first stretched in the longitudinal direction and then stretched in the transverse direction, the first longitudinal stretching emphasizes the longitudinal lines of the original multilayer sheet, and the second These lines may not disappear even after transverse stretching, resulting in poor appearance of the biaxially stretched film. Therefore, by adopting simultaneous biaxial stretching or a sequential biaxial stretching method in which horizontal stretching is first performed and then longitudinal stretching, it is possible to eliminate the vertical lines of the original sheet and create a two-dimensional structure with a good appearance. An axially stretched film can be produced. Since the heat-shrinkable film of the present invention is intended for shrink-wrapping foods, raw materials such as processed meat products are filled into a bag made of the film and the opening is heat-sealed for 70 minutes.
Shrink-wrap or sterilize by boiling in boiling water at a temperature between 95°C and 95°C. For this purpose, the heat-shrinkable film must have a dimensional shrinkage rate of more than 10% in both length and width after 10 seconds of being placed in an atmosphere at a temperature of at least 100°C. The film of the present invention satisfies these requirements and has stable high oxygen gas barrier properties over a long period of time. In order to manufacture the heat-shrinkable film of the present invention,
Heating the original multilayer sheet to 45℃ to 100℃,
Biaxial stretching is necessary, and biaxial stretching at such a low temperature has become possible for the first time with the multilayer sheet containing the laminated structure of the present invention. A single-layer sheet of saponified ethylene-vinyl acetate copolymer resin can be stretched by heating it to 140°C to 180°C, near its softening temperature, but a film biaxially stretched at such a high temperature cannot be heated to 1000°C. However, it shows almost no shrinkage. 100 pieces of this multilayer sheet
Even if you try to stretch it by heating it to a temperature below ℃, the resin is hard and will cause the so-called netting phenomenon.
The sheet breaks during stretching. Therefore, it is extremely difficult to produce a low-temperature heat-shrinkable film that shrinks at temperatures below 100° C. by stretching a single-layer sheet of this resin. On the other hand, a single-layer sheet of ionomer resin can be stretched over a very wide temperature range. For example, in the case of an ethylene-methacrylic acid copolymer ionomer resin with a melting point of 99°C and a Vikato softening point of 80°C, it can be stretched within a range of 60°C to 95°C. Biaxial stretching can be easily performed even at temperatures below the melting point, and a heat-shrinkable film with a good appearance can be obtained without causing netting. The main effects of the present invention are that the ionomer resin has good biaxial stretching properties in a wide temperature range below the crystal melting point and in the temperature range below 100°C, and the ionomer resin has excellent biaxial stretching properties at temperatures below 100°C. The saponified ethylene-vinyl acetate copolymer resin is laminated with the saponified ethylene-vinyl acetate copolymer resin through an adhesive resin, and this three-layer structure allows the biaxial stability of the saponified ethylene-vinyl acetate copolymer resin layer at temperatures below 100°C. The reason is that it makes stretching possible. In the case of a multilayer sheet in which different resin layers are laminated, each resin generally has a different stretchable temperature range, so the stretchable temperature range tends to become narrower. In addition, in the case of multilayer sheets, the sheet with the largest thickness ratio or the highest elastic modulus at the stretching temperature tends to dominate the stretchable temperature range, and the ethylene-vinyl acetate copolymer saponified resin layer with a high softening temperature tends to dominate the stretchable temperature range. It is not easy to find a resin that allows biaxial stretching at temperatures below 100°C. Ionomer resin has a crystalline melting point around 100°C, and can be biaxially stretched without necking in a temperature range where it has a relatively high elastic modulus below the crystallographic melting point. By laminating an ethylene-vinyl acetate copolymer saponified resin layer through a resin layer, it is possible to achieve good low-temperature biaxial stretching, which was difficult with other combinations, and as a result, impart low-temperature heat shrinkability. is now possible. The present invention is based on the above-mentioned three-layer laminated structure, but also includes a surface layer to prevent this layer from being exposed to the surface in order to prevent the saponified ethylene-vinyl acetate copolymer resin from absorbing moisture. is preferable,
Further layers can be added depending on the application. In the case of a laminate sheet with a five-layer structure of ionomer resin/adhesive resin/saponified ethylene-vinyl acetate copolymer resin/adhesive resin/ionomer resin,
By biaxially stretching in a temperature range of 50°C to 80°C, a heat-shrinkable film that exhibits high dimensional shrinkage in an atmosphere of 100°C or lower can be obtained. This film has a symmetrical structure on the front and back, so the curl of the film is small, and since the ionomer resin layer is on the surface, it has good heat sealability, making it suitable for applications such as vacuum shrink packaging. In this case, the ionomer resin is preferably a resin prepared by reacting Na ions or Zn ions with an ethylene-methacrylic acid copolymer having a melting point of 85° C. to 100° C. and a Vicat softening point of 60° C. to 80° C. The adhesive resin is modified ethylene.
Vinyl acetate copolymer adhesive resins are preferred. In the case of a sheet with a five-layer structure of ionomer resin/adhesive resin/saponified ethylene-vinyl acetate copolymer resin/adhesive resin/ethylene-vinyl acetate copolymer resin, biaxial stability occurs in the temperature range of 60°C to 80°C. By stretching, it is possible to obtain a heat-shrinkable film that exhibits a dimensional shrinkage of 10% or more at temperatures below 100°C. This film has excellent flexibility, pinhole resistance, and heat sealability, and is suitable for shrink-wrapping meat, etc. In this case as well, the adhesive resin layer is preferably a modified ethylene-vinyl acetate copolymer adhesive resin. Ionomer resin/Copolymer polyamide resin/
In the case of a sheet with a four-layer structure of ethylene-vinyl acetate copolymer saponified resin/copolymerized polyamide resin,
The copolyamide resin in the middle layer acts as an adhesive resin. The copolymerized polyamide resin is preferably a copolymer of ε-caprolactam and hexamethylene diammonium adipate. In the case of this sheet, by biaxially stretching it at a temperature range of 60°C to 80°C, a heat-shrinkable film that exhibits high dimensional shrinkage in an atmosphere of 100°C or lower can be obtained. In the case of this configuration, in order to obtain a good biaxially stretched film, it is preferable that the ionomer resin layer is sufficiently thicker than the copolyamide resin layer. This film has excellent transparency and heat resistance, and can withstand long-term heat treatment in boiling water, such as boiling sterilization. The stretching ratio of the biaxial stretching is 1.5 times or more in both length and width, preferably 2 times to 4 times. The dimensional shrinkage rate after 10 seconds after being placed in a 100℃ atmosphere is
To exceed 10%, it is necessary to stretch 1.5 times or more. The dimensional shrinkage rate (α) is the length of the original film.
1o, put in a constant temperature atmosphere, and let the length of the film after a certain time be 1, then α = 100 (1o -
1)/1o (%). The heat shrinkage rate of the present invention is 10% when placed in an atmosphere of 70℃ to 90℃.
It is preferable that the dimensional shrinkage after seconds is 20% to 50%. In order to obtain such a film, it is preferable to carry out biaxial stretching of 2 to 4 times both length and width at a temperature of 45°C to 100°C. Next, examples of the present invention will be described. Examples Table 1 shows examples and comparative examples of the present invention.

[Table] The raw sheets of films No. 1 to No. 10 shown in Table 1 were all manufactured by the T-die extrusion method.
160μ thick sheets were produced by melt extrusion from a single layer die or a multilayer coextrusion die. Next, the sheet was reheated using a sequential biaxial stretching device, and then stretched twice in both length and width to form a film with a thickness of about 40 μm, and its stretchability and shrinkability were evaluated. The optimum stretching temperature range is a temperature range in which a film with a relatively good appearance can be stretched without causing film breakage during stretching. Films with good transparency and smoothness and no stretching unevenness were evaluated as good, and films with stretching unevenness due to netting or poor smoothness and transparency were evaluated as poor. Heat shrinkability was determined by placing the film in boiling water, taking it out after 10 seconds, and cooling it. Those exhibiting a dimensional shrinkage rate of more than 10% both in length and width before and after boiling were considered good, and those less than that were considered poor. In Table 1, ionomer resin has a melting point of 99
It is a resin (Himilan manufactured by Mitsui Polychemicals Co., Ltd.) made by acting on Zn ions on an ethylene-methacrylic acid copolymer with a Vikatsuto softening point of 80% °C.A modified ethylene-vinyl acetate copolymer adhesive resin is The saponified ethylene-vinyl acetate copolymer resin is Eval manufactured by Kuraray Co., Ltd., which has a softening temperature (flow tester method) of 170°C, and the copolyamide polyamide has a melting point of 190°C. Ube Industries
The polypropylene copolymer is Sumitomo Noblen manufactured by Sumitomo Chemical Co., Ltd. with a melting point of 150℃, and the low density polyethylene is Sumitomo Chemical Co., Ltd. with a melting point of 105℃.
The ethylene-vinyl acetate copolymer resin is Evatate, manufactured by Sumitomo Chemical Co., Ltd., and the vinyl acetate content in the copolymer is 7 mol%. Table 2 shows the thickness of each layer of films No. 1 to No. 10. (The abbreviations for each layer are the same as in Table 1.)

[Table] The order of the film structure and the order of the thickness structure are shown in correspondence from left to right. As is clear from Examples No. 1 to No. 5, the heat-shrinkable film of the present invention has extremely good film appearance and low-temperature heat-shrinkability. As is clear from Comparative Examples No. 6 to No. 10, films that do not contain an ionomer resin layer have poor low-temperature heat shrinkability and an unsatisfactory appearance, making them unsuitable for shrink-wrapping processed meat products and the like.

Claims (1)

[Claims] 1. Ionomer resin layer/modified polyolefin adhesive resin layer or copolymerized polyamide resin layer/saponified ethylene-vinyl acetate copolymer resin layer/modified polyolefin adhesive resin layer or copolymerized polyamide resin layer However, it is a multilayer plastic film with a laminated structure of adjacent layers in this order. The film is made of 90% or more resin, has a total thickness of 5μ to 150μ, and has a dimensional shrinkage rate of more than 10% in both length and width after 10 seconds of being placed in an atmosphere at a temperature of 100°C. Shrinkable film. 2. The heat-shrinkable film according to claim 1, wherein the ionomer resin is a resin in which metal ions are applied to an ethylene-methacrylic acid copolymer. 3. The heat-shrinkable film according to claim 1 or 2, wherein the modified polyolefin adhesive resin is a modified ethylene-vinyl acetate copolymer. 4 Ionomer resin layer / modified polyolefin adhesive resin layer or copolymerized polyamide resin layer / ethylene-vinyl acetate copolymer saponified resin layer / modified polyolefin adhesive resin layer or copolymerized polyamide resin layer / ionomer resin layer or ethylene ―
It is a multilayer plastic film with a laminated structure in which vinyl acetate copolymer is adjacent to each other in this order, and the ethylene-vinyl acetate copolymer saponified resin layer has an ethylene content of 25 to 75 mol% and acetic acid in the copolymer. The film must be made of a resin with a saponification degree of 90% or more relative to vinyl, the total thickness of the film is 5μ to 150μ, and the dimensional shrinkage rate in both length and width after 10 seconds after being placed in an atmosphere at a temperature of 100℃ exceeds 10%. A heat-shrinkable film featuring: 5. The heat-shrinkable film according to claim 4, wherein the ionomer resin is a resin in which metal ions are applied to an ethylene-methacrylic acid copolymer. 6. The heat-shrinkable film according to claim 4 or 5, wherein the modified polyolefin adhesive resin is a modified ethylene-vinyl acetate copolymer.