JPH0639974A - Heat-shrinkable laminated film for sleeve packaging - Google Patents

Abstract

PURPOSE:To provide a heat-shrinkable laminated film for sleeve packaging capable of automatically sleeve-packaging at a high speed since a self-cutting property at a perforation part is good without any whitening phenomenon in shrinkage by heating. CONSTITUTION:Both surface layers consist of 20-90% gel fraction polyethylene resin, and a core layer consists of polypropylene resin. A propylene unit content of the core layer is 35-85wt.% of all constitutional resin of the film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-shrinkable laminated film for sleeve packaging, which is used for sleeve packaging which is self-cut by a perforation portion for separation during automatic packaging. In particular, it is preferably used for sleeve packaging in integrated packaging such as cans, bottles and dry batteries.

[0002]

2. Description of the Related Art Conventionally, a heat-shrinkable film provided with a perforation for separation that is intermittent in the width direction at regular intervals in the length direction is formed into a cylindrical body continuous in the length direction, and each perforation part is formed. After inserting the packaged object in between, heat-shrink the tubular heat-shrinkable film in which the packaged object is inserted as it is in the heat-shrink tunnel to heat the perforation part between each packaged object and the film. A sleeve wrapping method is known, in which the film is self-cut by its own heat shrinkage stress, and the film is tightly adhered to the periphery of the packaged object, and further, the film cut end is also heat-shrinked so as to be closely adhered to the packaged object. (For example, Japanese Patent Publication No. 56-125
33, etc.).

As a film used in the sleeve packaging method, a vinyl chloride resin film having a larger heat shrinkability in the lateral direction than in the longitudinal direction is preferably used. However, the film is expensive and has a problem that toxic gas or the like is generated when it is incinerated. Therefore, it has been desired to use a single-layer or multi-layer heat-shrinkable film made of a polyolefin resin such as polyethylene or polypropylene that is inexpensive and does not generate a toxic gas when incinerated. However, the film is inferior in heat resistance and is liable to cause a whitening phenomenon in the heat shrink tunnel, and the most characteristic feature of the sleeve packaging method is heating in the heat shrink tunnel and perforation due to heat shrink stress of the film itself. There was a problem that the automatic cutting property at the portion was insufficient and, moreover, the resin was continuous in a thread form between the cutting ends.

[0004]

SUMMARY OF THE INVENTION The present invention, in a heat-shrinkable film used for sleeve packaging that is self-determined by a separating perforation section during automatic packaging, suppresses the whitening phenomenon in a heat-shrinking tunnel, and further, perforation. It is an object of the present invention to provide a heat-shrinkable laminated film for sleeve packaging made of a polyolefin-based resin, which has a good automatic cutting property at a portion and does not have continuous cut ends connected by a thread-like resin.

[0005]

According to the present invention, both surface layers are made of polyethylene resin, and the core layer is made of polyethylene resin cross-linked with a gel fraction of 20 to 90%. -Based resin, the propylene unit content of the core layer is 35
A heat-shrinkable laminated film for sleeve packaging is provided, which is made of polypropylene resin in an amount of 85 to 85 wt%.

That is, both surface layers made of polyethylene resin are cross-linked within a specific range, and by specifying the propylene content of the polypropylene resin used for the core layer, heat resistance and self-cutting in the perforation part are achieved. The inventors have found that all of the properties are good and have completed the present invention.

The polyethylene resin in the present invention includes low-density polyethylene, ethylene-vinyl acetate copolymer, or ethylene-α-olefin copolymer such as linear low-density polyethylene or linear ultra-low-density polyethylene. Examples thereof include ethylene homopolymers such as polymers, and resin compositions prepared by mixing one or more selected from ethylene copolymers having an ethylene unit as a main component. Among them, ethylene-α-olefin copolymers in which the α-olefin has 4 to 6 carbon atoms are particularly preferable.

In the heat-shrinkable laminated film for sleeve packaging of the present invention, the degree of cross-linking between both surface layers made of the polyethylene resin must be such that the gel fraction is in the range of 20 to 90%. is there. However, the cross-linking degree of both surface layers does not have to be the same, and may be different in both surface layers. When the gel fraction is less than 20 wt%, whitening phenomenon is likely to occur during heat shrinkage in the heat shrink tunnel, so it is necessary to lower the heat shrink tunnel temperature, which not only deteriorates the packaging finish but also causes the heat shrink stress of the film. It becomes weak and the automatic cutability of the perforation part deteriorates. Moreover,
When the perforation part is separated, stringing is likely to occur at the cutting part, and the cutting ends are likely to be connected by the thread-like resin. Further, when the gel fraction exceeds 90 wt%, the heat resistance is further improved, the heat shrinkage stress is improved, and the packaging finish is good, but the film strength becomes too strong accordingly, so that the perforation part becomes too strong. The self-cutting property of is reduced. Further, the heat-sealing property at the time of molding the tubular body becomes poor, and the perforation formability becomes difficult.

The gel fraction in the present invention represents the degree of crosslinking, and is measured by the following method. That is, about 0.3 g of the sample was placed in a wire mesh container of 80 mesh, extracted with toluene under the condition of 100 ° C. for 1 hour, and the residual amount was measured.

The method for cross-linking the polyethylene resin of both surface layers is not particularly limited, but in general, the method of cross-linking by electron beam irradiation is simple and the gel fraction can be easily specified. This is a useful method because it can be done.

The sum of the thicknesses of both surface layers made of polyethylene resin is preferably in the range of 25 to 70% with respect to the total thickness, and both surface layers are effective in preventing curling of the obtained film. It is preferable that the thickness is almost the same from the viewpoint. If necessary, the surface layer may be subjected to surface activation treatment such as corona discharge treatment, and may be subjected to processing such as printing or metal vapor deposition.

Examples of the polypropylene resin include a simple substance of a propylene-α-olefin copolymer having a propylene unit as a main component, or a mixture of the copolymer or polypropylene and a polyethylene resin. The propylene-α-olefin copolymer may, for example, be a propylene-ethylene copolymer, a propylene-butene copolymer or a propylene-ethylene-butene terpolymer. As the polyethylene resin, the above-mentioned polyethylene resin is used.

It is necessary to select a resin having a propylene content such that the propylene content of the core layer falls within the range of 35 to 85 wt% with respect to the resin of the entire constitution. . When a polypropylene resin whose propylene content is less than 35 wt% with respect to the resin of all components is used, the heat shrinkage stress becomes weak,
The self-cutting property of the perforation part becomes difficult. or,
The stiffness of the film is weakened, and the suitability for packaging machines and the formation of perforations are poor. On the other hand, when a polypropylene resin whose propylene content exceeds 85 wt% with respect to the resin of the entire composition is used, the heat shrinkage stress of the film becomes too strong rather than the strength reduction of the perforation part due to heating, and the heat of the film The object to be packaged is also drawn together with the contraction, the space between the objects to be packaged is narrowed, and the self-cutting property of the perforation portion becomes poor. In addition, the proper temperature range for heat shrink packaging is biased toward the high temperature side and is narrowed, resulting in poor packaging finish.

The heat-shrinkable laminated film for sleeve packaging according to the present invention is, of course, a film having a three-layer structure composed of both surface layers made of a crosslinked polyethylene resin and a core layer made of a polypropylene resin. In addition to the above three layers, a film having a multilayer structure of four or more layers including another resin layer between the surface layer and the core layer is also included.

The method for producing the heat-shrinkable laminated film for sleeve packaging of the present invention is not particularly limited, but it is preferably produced by the following method. That is, a predetermined resin composition is laminated by a coextrusion method so as to have a predetermined thickness constitution to obtain an unstretched laminated original sheet. The obtained raw sheet is irradiated with an electron beam and crosslinked so that both surface layers have a predetermined gel fraction. And
The unstretched laminated original sheet subjected to the cross-linking treatment is reheated and stretched. As a stretching method, a generally used method such as a sequential biaxial stretching method in which a roll stretching method and a tenter stretching method are combined, or a simultaneous biaxial stretching method such as an inflation method or a tenter method is used. Incidentally, the heat-shrinkable laminated film for sleeve packaging of the present invention preferably has a higher heat shrinkage ratio in the transverse direction than in the longitudinal direction. Therefore, after being stretched in the longitudinal direction by a roll stretching method, it is stretched in the transverse direction by a tenter method. It is preferably stretched. Further, in the case of stretching by the inflation method, the unstretched raw sheet needs to be in a tubular shape, so that it is necessary to use a multilayer annular die as the multilayer die for coextrusion.

A sleeve packaging method using the heat-shrinkable laminated film for sleeve packaging of the present invention is carried out by the method described in the above-mentioned Japanese Patent Publication No. 56-12533. That is, first, a perforation portion for separation that is intermittent in the width direction is provided at regular intervals in the length direction of the film. Next, the film is formed into a cylindrical body that is continuous in the length direction, and the body to be packaged is inserted between the perforations. Then, the heat-shrinkable film of the tubular body in which the packaged object is inserted is directly introduced into the heat-shrinkable tunnel, and the film is heat-shrinked to heat the perforation part between the packaged objects and the heat-shrinkable stress of the film. Self-cut by. Moreover, the film is heat-shrinked so as to be in close contact with the periphery of the packaged object.

[0017]

In the heat-shrinkable laminated film for sleeve packaging of the present invention, since the core layer is made of polypropylene resin, heat shrinkage stress is strong, and both surface layers are made of cross-linked polyethylene resin. Is good. Therefore, since the perforation portion can be heated to a high temperature, a strong tearing stress can be applied to the perforation portion, and as a result, the perforation portion can be easily cut. Further, since the polyethylene-based resin has been subjected to a cross-linking treatment, it does not have stringing at the cut portion and exhibits good cuttability.

[0018]

[Examples and Comparative Examples] Examples and comparative examples will be shown below.
The contents of the present invention will be described more specifically. The present invention is
Of course, it is not limited only by the matters described in the examples. The physical property evaluation and various packaging suitability evaluations in the present invention were evaluated by the following methods. Tensile strength, elongation Measured according to ASTM D882. Thermal contraction stress A sample with a width of 10 mm in each length and width, with a chuck distance of 30 mm
Was attached to the chuck with no load and without slack, and immersed in a glycerin bath at 120 ° C. to measure the initial maximum shrinkage stress generated between the chucks. Formability of Perforation Formability of the perforation by the rotary blade for perforation molding was evaluated. ◯: Perforations are formed well. X: The film is too hard or too soft, and it is difficult to form perforations. Self-cutting property of perforation part The self-cutting property of the perforation part when passing through a heat-shrinking tunnel during automatic packaging of four AA batteries was evaluated. ◯: Good cuttability. Thread: The cut part is continuous with thread-shaped resin. X: The object to be packaged is attracted and the cutting is defective. Heat resistance It was visually judged whether the film was whitened due to melting during passage through a heat shrink tunnel during automatic packaging. ◯: No whitening. ×: There is whitening. Heat-sealing property Heat-sealing is performed under the optimum conditions with a fusing sealing machine.
The heat seal strength was measured. ◯: 800 g / 10 mm or more x: Less than 800 g / 10 mm

Examples 1 and 2 and Comparative Examples 1 and 2 As an ethylene resin for both surface layers, an ethylene-butene copolymer (butene content: 15.0 wt%, density: 0.9
05G / cm ³ a), the propylene as propylene resin core layer - ethylene copolymer (propylene content: 95
wt%) and an ethylene-butene copolymer (butene content: 15.0 wt%, density: 0.905 g / cm ³ ) were mixed at a ratio of 8: 2, and the thickness composition ratio was 1 A laminated unstretched raw sheet of: 3: 1 was obtained by a coextrusion method. Both sides of the obtained sheet were irradiated with an electron beam to have gel fractions of 85 wt% (Example 1), 76 wt% (Example 2), 95 wt% (Comparative Example 1), and 10 wt% (Comparative Example 2). Thus, the cross-linking treatment was performed. The obtained various sheets were stretched 3.0 times in the longitudinal direction by a roll stretching method, and then stretched 7.0 times in the transverse direction by a tenter method, and had a thickness of 25 μm and were heat-shrinkable for sleeve packaging. A laminated film was obtained. In addition, in each film, the propylene content of the core layer was 57 w relative to the resin of the entire film constitution.
It was t%. AA dry cell by a sleeve packaging method in which the obtained film is subjected to a perforation process for cutting such that it is intermittently cut in the width direction at regular intervals in the length direction, and is self-cut at the cutting perforation part during the automatic packaging described above. Four sleeve packages were made.
The results are shown in Table 1 together with the physical properties of each film.

[0020]

[Table 1]

The sleeve packages using the films of Examples 1 and 2 in which the gel fraction of the polyethylene-based resin of both surface layers is within the range of the present invention have good heat resistance and are free from whitening phenomenon in the tunnel. It did not occur, and the self-cutting property in the perforation part was good, and the stringing phenomenon did not occur. The sleeve package using the film of Comparative Example 1 in which the gel fraction of the polyethylene-based resin in both surface layers exceeds the range of the present invention, the film strength is too strong, so the self-cutting property of the perforation part is poor, and the perforation by the rotary blade is poor. Was also poor in moldability. Also, the heat sealability during automatic packaging was poor. The heat-shrinkable laminated film for sleeve packaging of Comparative Example 2 in which the gel fraction of the polyethylene-based resin in both surface layers was less than the range of the present invention had weak heat-shrinkage stress, and the self-cutting property of the perforation part was poor. In addition, the film had a weak rigidity and was inferior in packaging machine suitability and perforation forming property.

Comparative Examples 3 to 4 The same ethylene-butene copolymer as in Example 1 was used as the ethylene resin for both surface layers, and the propylene-butene copolymer was used as the propylene resin for the core layer (propylene content: 76
wt%) and ethylene-butene copolymer (butene content:
15.0 wt%, density: 0.905 g / cm ³ ) 2:
The resin composition mixed in a ratio of 8 (Comparative Example 3) or the propylene-butene copolymer simple substance (Comparative Example 4) was used, and various thickness composition ratios were 1: 1: 1 (Comparative Example 3), Alternatively, a 1: 6: 1 (Comparative Example 4) laminated unstretched raw sheet was obtained by a coextrusion method. The propylene content of each film was 26 wt% (Comparative Example 3) and 90% with respect to all the constituent resins.
It was wt% (Comparative Example 4). Each of the obtained sheets was irradiated with an electron beam to perform a cross-linking treatment in which the gel fraction of both surface layers was 85 wt%, and was stretched 3.0 times in the machine direction by a roll stretching method as in Examples 1 and 2. Then, the film was stretched 7.0 times in the transverse direction by a tenter method to obtain a thickness of 2
A 5 μm heat-shrinkable laminated film for sleeve packaging was obtained.
Using each of the obtained films, four AA batteries were sleeve-packaged in the same manner as in Example 1 and the like. The results are shown in Table 2 together with the physical properties of each film.

[0023]

[Table 2] PB: Propylene-butene copolymer EB: Ethylene-butene copolymer

The sleeve package using the film of Comparative Example 3 having a propylene content of less than 35 wt% with respect to all the constituent resins of the film had weak heat shrinkage stress and poor self-cutting property in the perforation part. Further, the sleeve package using the film of Comparative Example 4 in which the propylene content of all the constituent resins of the film exceeds 85 wt%, the heat shrinkage stress is strong, and the AA dry battery which is the packaged object is attracted and the spacing between the perforations is increased. Was narrowed, and the self-cutting property of the perforation part was poor.

[0025]

EFFECT OF THE INVENTION The heat-shrinkable laminated film for sleeve packaging of the present invention has no whitening phenomenon in the heat-shrinkable tunnel, has good self-cutting property in the perforation part, and has cut ends connected with a thread-like resin. It is possible to do automatic sleeve packaging at high speed because there is no problem. Moreover, since the packaging finish is good, it can be suitably used for packaging integrated sleeves such as cans, bottles and dry batteries.

Claims (1)

[Claims] 1. Both surface layers are made of polyethylene resin, and are made of polyethylene resin cross-linked with a gel fraction of 20 to 90%, and the core layer is made of polypropylene resin. A heat-shrinkable laminated film for sleeve packaging, comprising a polypropylene-based resin having a propylene unit content of 35 to 85 wt% of the total resin constituting the film.