JPS6140184B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a multilayer plastic film having a Japanese paper-like texture and a method for producing the same. More specifically, the present invention relates to a method for producing a highly heat-shrinkable, tubular thin multilayer plastic film having a Japanese paper-like texture due to a fine fiber structure on the inner surface by a coextrusion inflation method.

In recent years, with the development of manufacturing technology for plastic biaxially stretched films, heat-shrinkable films have become widely used for integrated shrink packaging and individual shrink packaging. In Shrink packaging, tube-shaped inflation films made of polyethylene, polypropylene, etc. are widely used, and the packaged items are placed inside these tube-shaped films and passed through an infrared heating tunnel to tightly fit the packaged items. Can be shrink wrapped. Conventional Shrink packaging films have the advantages of being strong and transparent, but have the disadvantages of lacking cushioning properties and a calm texture.

Styrofoam sheets as cushioning packaging materials,
Foamed polyethylene sheets and the like have been developed, and are widely used for packaging products that require impact resistance by allowing the foam to maintain appropriate cushioning properties. However, since these foam sheets are thick and opaque, they have disadvantages such as bulky packaging, almost invisible packaging, and non-shrinkable properties that prevent them from being used for shrink wrapping. be.

In addition, non-woven fabrics have recently been developed and widely used as packaging materials with a texture similar to Japanese paper.
It has drawbacks such as insufficient sealing properties and lack of heat shrinkability.

In view of the current situation, the present inventors developed a heat-shrinkable material that has a Japanese paper-like texture, is heat-shrinkable, has appropriate airtightness or air permeability, and is transparent. As a result of various studies to obtain film,
A resin composition containing a mixture of low-density polyethylene resin, polypropylene resin, and a foaming agent is used as the main component of the foamable resin layer, and a resin composition containing low-density polyethylene as the main component is used as the non-foamable resin layer. The inventors discovered that a heat-shrinkable film having a texture very similar to that of Japanese paper could be produced by coextrusion inflation, and conducted further detailed research to complete the present invention.

A first object of the present invention is to provide a heat-shrinkable plastic film with a Japanese paper-like feel suitable for heat-shrink packaging and excellent transparency.

A second object of the present invention is to provide a multilayer plastic film that is light, strong, and has a Japanese paper-like texture with good tearability.

A third object of the present invention is to provide a microporous film having air permeability intermediate between that of paper and conventional plastic film.

The multilayer plastic film of the present invention is a multilayer plastic film having a thickness of 20μ to 200μ and consisting of at least two layers, a foamed resin layer and a non-foamed resin layer, and the foamed resin layer is made of polypropylene resin based on 100 parts by weight of polyethylene. It consists of a resin composition containing 30 to 300 parts by weight and 0.3 to 10 parts by weight of a blowing agent, the non-foamed resin layer consists of a resin composition whose main component is a thermoplastic resin, and the foamed resin layer is the outermost layer. It is characterized by a Japanese paper-like texture due to its fine fiber structure.

The method for producing a multilayer plastic film of the present invention comprises a foamed resin layer mainly composed of a thermoplastic resin to which a blowing agent is added and a non-foamed resin layer mainly composed of a thermoplastic resin to which no blowing agent is added. In the film manufacturing method, a foamed resin composition in which 30 to 300 parts by weight of a polypropylene resin and 0.3 to 10 parts by weight of a blowing agent are added to 100 parts by weight of a polyethylene resin, and a non-foamed resin composition whose main components are a thermoplastic resin. After kneading and melting the materials in separate extruders, the foamed resin layer and non-foamed resin layer are melted and bonded inside a cylindrical coextrusion die, and then from the exit of the extrusion die, at least the inner surface is the foamed resin layer. The tube is then extruded into a laminated tube shape so that the tube is expanded in the width direction due to the difference in air pressure between the inside and outside of the tube, and is stretched in the take-up direction. Cooling fluid is then sprayed onto the surface of the tube to cool the tube from the outside. At the same time, the tube is expanded until the blow ratio finally reaches 3 or more, and at the same time, the tube is stretched in the drawing direction until the take-up ratio reaches 2 or more. The foamed resin layer is compressed to form a Japanese paper-like texture due to the fine fiber structure of the foamed resin layer.

In the present invention, the foamed resin layer contains 30 parts by weight of polypropylene resin per 100 parts by weight of polyethylene resin.
The main component is a resin composition to which 300 parts by weight and 0.3 to 10 parts by weight of a blowing agent are added. When the foamed resin layer is made of low-density polyethylene or polypropylene alone, the film has no special texture, but when the foamed resin layer is made of a mixture of low-density polyethylene and polypropylene, it has a characteristic texture similar to Japanese paper. It was found that there is a possibility that the As a result of manufacturing films by changing the mixing ratio of low density polyethylene and polypropylene in the foamed resin composition, it was found that polypropylene was mixed with 100 parts by weight of low density polyethylene.
Films containing 80 to 150 parts by weight of polypropylene have a texture most similar to that of Japanese paper;
It was found that a formulation in which 300 parts by weight was added was within a preferable range.

As the polypropylene resin added to the foamed resin layer, a propylene-based random copolymer resin is most preferable, and a mixture of a propylene homopolymer and a low-density polyethylene results in a film that somewhat lacks a soft feel. High-density polyethylene may be used instead of low-density polyethylene, but the texture of the film will change slightly. Therefore, a mixture of low density polyethylene and polypropylene is most preferred.

The reason why a mixture of low-density polyethylene and polypropylene is preferable for creating a Japanese paper-like texture is not clear, but perhaps the poor compatibility between low-density polyethylene and polypropylene contributes to improving the texture of the film. It is thought that the When a foaming agent is added to a mixture of low-density polyethylene and polypropylene, which have poor compatibility, a fine fiber structure is created during the foaming and stretching process of the film, resulting in a unique Japanese paper-like texture. It is estimated that

The blowing agent is added in an amount of 0.3 to 10 parts by weight per 100 parts by weight of polyethylene. The amount of the blowing agent added depends on the type of blowing agent, but does not exceed the range of usual amounts. The type of blowing agent is not particularly limited, but includes sodium bicarbonate, azodicarbonamide, p-
Toluenesulfonyl semicarbazide, trihydrazinotriazine and the like are preferred.

The non-foamed resin layer is a thermoplastic resin, and the resin in the layer adjacent to the foamed resin layer is a polyolefin resin, particularly low-density polyethylene, high-density polyethylene, polypropylene, ethylene-vinyl acetate copolymer resin, 1.2 polybutadiene resin, Ionomer resins and the like are preferred. For applications requiring a high thermal shrinkage rate, low density polyethylene or ionomer resin is preferred as the non-foamed resin layer. Even if the type of resin in the non-foamed resin layer changes, the effect on the Japanese paper-like texture of the film is small, but polyolefin resin is suitable as a resin that has good adhesiveness with the foamed resin layer.

After heating, kneading, and melting the foamed resin composition and the non-foamed resin composition in separate extruders, the foamed resin layer and the non-foamed resin layer are melted and bonded inside a cylindrical coextrusion die, and the extrusion die extrude from the outlet into the laminated tube. In-die adhesion between the foamed resin layer and the non-foamed resin layer is important for producing a Japanese paper-like texture in the film. By melting and adhering the foamed resin layer and the non-foamed resin layer sufficiently firmly in the die, the foamed resin layer and the non-foamed resin layer are stretched together with the non-foamed resin layer during blow stretching of the laminated tube, creating a fine fiber structure. is thought to be formed.

It is necessary that the inner surface of the laminated tube be made of a foamed resin layer. Comparing the microstructure of the film when the foamed resin layer is on the inner surface of the tube and when it is on the outer surface, it is found that when the foamed resin layer is on the outer surface, network-like unevenness is more likely to be formed on the film surface, and the film has a Japanese paper-like appearance. However, if the foamed resin layer is on the inner surface, a fine fiber structure is formed on the film surface, resulting in a film with a Japanese paper-like texture.

Due to the difference in air pressure between the inside and outside, the laminated tube expands in the width direction and stretches in the take-up direction, and cools the tube from the outside by spraying cooling fluid onto the surface of the tube, eventually reaching a blow ratio of 3 or more. The tube is inflated until the tube is expanded and simultaneously stretched in the take-off direction until the take-off ratio reaches 2 or more. The blowing ratio is the ratio between the diameter of the expanded tubular film and the diameter of the exit, or lip, of the cylindrical die, and the takeoff ratio is the ratio between the passing speed of the produced film through the gap between the takeoff rolls and the diameter of the cylindrical die. It is defined as the ratio to the passage speed of the tube-shaped molten resin passing through the outlet. In order to form a Japanese paper-like texture on the film and impart high heat shrinkability to the film, it is preferable that the final blow ratio be 4 or more and the take-up ratio be 3 or more. By significantly increasing the blow ratio and take-up ratio in this way, a structure is created in which fine fibers are piled up and thinly scattered and attached to the surface of the thin film of the non-foamed resin layer. A texture similar to that of Japanese paper is formed by appropriately distributing areas with poor transparency to which fine fibers are densely adhered and areas with good transparency to which fine fibers are sparsely adhered.

The thickness of the film of the present invention is 20μ to 200μ. The thickness of the film was measured using a micrometer with a diameter of 6 mm, and the measurement surfaces of the anvil and spindle were both circular. Although the thickness of the film varies locally depending on the density distribution of the foamed resin layer, the variation in the thickness of the film measured by this method is small. If the thickness of the film is less than 20 μm, the strength will be significantly reduced, making it impractical, and if the thickness of the film is more than 200 μm, the Japanese paper-like texture will deteriorate.

The thin film of the present invention having a thickness of about 50 μm is light and strong, and has extremely good tearability, and can be easily torn straight in the longitudinal and width directions of the film. This is due to the layered combination of the fine fiber structure of the foamed resin layer and the thin non-foamed resin layer, and the tearability of this film is similar to that of paper.

In order to form a Japanese paper-like texture on the film, it is necessary to make the foamed resin layer the outermost layer. However, the film of the present invention is not limited to two layers, a foamed resin layer and a non-foamed resin layer, but by having foamed resin layers on both the front and back sides and a non-foamed resin layer in the middle, it is possible to improve the wind resistance of the film. The texture can be made even closer to that of Japanese paper. In order to impart gas barrier properties to the film, a multilayer film having a structure in which a foamed resin layer/low density polyethylene layer/adhesive polyolefin layer/saponified ethylene-vinyl acetate copolymer resin layer is arranged in this order is also preferred. This film has a texture similar to Japanese paper, and has excellent gas barrier properties and heat shrinkability, making it suitable as a packaging film.

Although the film of the present invention has lower air permeability than paper, it has higher air permeability than the generally widely used low density polyethylene film, and also has the features of a microporous film.

Examples are shown below.

Example: 100 parts by weight of low-density polyethylene (Sumikasen, manufactured by Sumitomo Chemical Co., Ltd.), 100 parts by weight of propylene random copolymer (Sumitomo Noblen, MI = 7, manufactured by Sumitomo Chemical Co., Ltd.) and 2 parts by weight of chemical blowing agent azodicarbonamide. After kneading and melting a foamed resin composition mixed with a non-foamed resin composition made of an ionomer resin (Himilan manufactured by Mitsui Polychemical Co., Ltd.) using two 40 mmφ extruders,
A laminated tube with a foamed resin inner layer and a non-foamed resin outer layer is extruded from the die lip's cylindrical two-layer coextrusion die with a diameter of 50 mm at a discharge rate of 10 kg/hr, and the air-cooled inflation method is used to extrude the laminated tube at a rate of approximately 20 m/min. A film with a fold diameter of 380 mm and a thickness of 60 μm was produced at this speed.
The blow ratio of this film was set at 4.8, and the take-off ratio was set at 3.1. The produced film has a Japanese paper-like feel due to its fine fiber structure, is light and strong, and has excellent tearability in the width direction and the pulling direction.

Put this film in a glycerin bath at 120℃ 2
The heat shrinkability after seconds was 45% in the vertical direction and 55% in the horizontal direction (width direction).

This film has excellent heat sealability and is suitable for shrink packaging such as cosmetic boxes.

Claims (1)

[Scope of Claims] 1. A multilayer plastic film with a thickness of 20 to 200 μ, consisting of at least two layers, a foamed resin layer and a non-foamed resin layer, in which the foamed resin layer has a thickness of 30 to 200 μm to 100 parts by weight of polyethylene. The main component is a resin composition containing 300 parts by weight of polypropylene and 0.3 to 10 parts by weight of a foaming agent, the non-foamed resin layer is made of a resin composition containing a thermoplastic resin as the main component, and the foamed resin layer is the outermost layer. A multilayer plastic film characterized by forming a Japanese paper-like texture due to its fine fiber structure. 2. The multilayer plastic film according to claim 1, wherein the polyethylene of the foamed resin layer is low density polyethylene. 3. The multilayer plastic film according to claim 1 or 2, wherein the thermoplastic resin of the non-foamed resin layer is low density polyethylene. 4. In a method for producing a multilayer plastic film consisting of a foamed resin layer mainly composed of a thermoplastic resin to which a blowing agent has been added and a non-foamed resin layer mainly composed of a thermoplastic resin to which no blowing agent is added, 100 parts by weight of A foamed resin composition prepared by adding 30 to 300 parts by weight of polypropylene and 0.3 to 10 parts by weight of a blowing agent to polyethylene and a non-foamed resin composition whose main component is a thermoplastic resin are kneaded in separate extruders. After melting, the foamed resin layer and the non-foamed resin layer are melted and bonded inside a cylindrical coextrusion die, and then extruded from the exit of the extrusion die into a laminated tube shape so that at least the inner surface becomes the foamed resin layer. The tube is expanded in the width direction and stretched in the take-up direction due to the difference in air pressure inside and outside the tube, and cooling fluid is sprayed onto the surface of the tube to cool the tube from the outside until the blow ratio reaches 3. At the same time, the tube is expanded in the drawing direction until the drawing ratio reaches 2 or more, and then the tube is compressed in the gap between at least one set of drawing rolls to form a foamed resin layer. A method for producing a multilayer plastic film characterized by forming a Japanese paper-like texture due to its fine fiber structure. 5. The method for producing a multilayer plastic film according to claim 4, wherein the polypropylene is a propylene random copolymer resin.