JPH06115026A - Multi-layer stretch shrink film - Google Patents

Abstract

PURPOSE:To obtain a multi-layer stretch shrink film for forming a package superior in not only transparency and glossiness but also elastic recovery power and binding power without occurrence of odor of an acetate in stretch/shrink packaging. CONSTITUTION:A core layer is made of an ethylene-propylene copolymer, an ethylene-butene-propylene copolymer, or the mixture thereof. Each outer layer is made of a resin composition obtained by mixing a straight-chain low-density polyethylene having a density of 0.910 to 0.925g/cm<3> and a melt index of 0.5-10g/10min with a straight-chain low-density polyethylene having a density of 0.890 to 0.907g/cm<3> and a melt index of 0.5-10g/10min in a mixing ratio of 11:5 to 5:1. In addition, the total thickness of both the outer layers is 10-90% of the total film thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stretch shrink film which is stretched to some extent during wrapping and then heat-shrinks the film to generate tension. Note that such a packaging method is hereinafter referred to as "stretch shrink packaging".

[0002]

2. Description of the Related Art A method of wrapping a film while stretching it, that is, a stretch wrapping method has been well known. For example, it is described in "Packaging Technology Handbook" published by Japan Packaging Technology Association. Moreover, as a film used for the packaging, not only a single-layer film but also a multi-layered film as described in Japanese Patent Publication No. 2-12187 is known recently. However, it is difficult to obtain a package having sufficient tenacity because it is difficult to package the whole by uniformly stretching it in the packaging method. Therefore, recently, the stretch shrink wrapping method has been used. As a stretch shrink film used for such packaging, a multilayer film in which a core layer is made of polypropylene resin and both outer layers are made of ethylene-vinyl acetate copolymer is used.
However, the stretch shrink wrapping method is not suitable for packaging foods, etc., because acetic acid odor is generated in the film due to being heated in the heat shrinking tunnel and the odor is transferred to the object to be packaged.

Therefore, the inventors of the present invention have previously used a stretch shrink wrapping film which does not emit an acetic acid odor even after passing through a heat shrinking tunnel and has a high heat shrinkage stress and a high elastic recovery rate. These problems have been solved by using a film-type resin in which both outer layers are made of a linear low-density polyethylene resin. (Patent application 3
However, although the reason for this method is not clear, stretch-shrink packaging with good surface roughness and poor smoothness when producing unstretched original fabric by the coextrusion method, and having good transparency and gloss. Film was not obtained.

Japanese Patent Publication No. 61-3264 discloses a multilayer thermoplastic film in which at least one side of an oriented polypropylene base layer is coated with a low density linear copolymer of ethylene and an α-olefin having 3 to 10 carbon atoms. Have been described. However, in the invention, although it is specified that the film is excellent in heat sealability, there is no description that it can be used for stretch shrink packaging,
Moreover, there is no description suggesting such a thing.

[0005]

DISCLOSURE OF THE INVENTION The present invention does not generate an acetic acid odor even after passing through a heat shrinking tunnel, has a strong shrinking stress, and has a high elastic recovery. An object of the present invention is to provide a multilayer stretch shrink film having excellent optical properties.

[0006]

The present invention has taken the following means in order to solve these problems. That is, the core layer is composed of ethylene-propylene copolymer or ethylene-butene-propylene copolymer or a mixture thereof,
Both outer layers have a density of 0.910 to 0.925 g / cm ³ ,
Linear low density polyethylene (A) having a melt index (hereinafter referred to as MI) of 0.5 to 10 g / 10 min.
And density 0.890 to 0.907 g / cm ³ , MI
A mixture of linear low-density polyethylene (B) of 0.5 to 10 g / 10 min with a mixing ratio of 1: 5 to 5:
The resin composition of No. 1 was used, and the sum of the thicknesses of the outer layers was 10 to 90% of the total thickness of the film.

The core layer is not a homopolymer of propylene but an ethylene-propylene copolymer, an ethylene-butene-propylene copolymer or a mixture thereof in view of stretchability, heat shrinkability and elastic recovery rate. It is necessary to use a polypropylene resin composed of The MI of the resin is preferably 1.2 to 6.0 g / 10 min.
Further, the copolymerization ratio of the resin is such that in the case of ethylene-propylene copolymer, the ethylene content is 3 to 5 wt%.
In the case of ethylene-butene-propylene copolymer, the total content of ethylene and butene is 4 to 13
wt% is preferred. By using the resin of these copolymers, it becomes possible to easily perform the laminated stretching with the linear low-density polyethylene layers of both outer layers, and moreover, since the low temperature stretching becomes possible, the heat shrinkability at low temperature is improved. Get better

Linear low-density polyethylene is used for both outer layers from the viewpoint of avoiding the odor of acetic acid and high heat shrinkage stress. Moreover,
In order to obtain a multi-layer stretch shrink film having excellent film forming properties, stretch processability, and optical characteristics, the density is 0.9
10 to 0.925 g / cm ³ , MI 0.5 to 10 g
/ 10 min linear low-density polyethylene (A) and a density of 0.890 to 0.907 g / cm ³ and MI 0.5 to 10 g / 10 min linear low-density polyethylene (B) in a mixture ratio. Uses a resin composition of 1: 5 to 5: 1.

Linear low density polyethylene (A) and (B)
It is necessary that the MI of both be in the range of 0.5 to 10 g / 10 min. If MI exceeds 10 g / 10 min, a strong heat shrinkage stress cannot be obtained. Also, MI is 0.5
If it is less than 10 g / 10 min or more than 10 g / 10 min, film-forming property and stretchability by co-extrusion with the polypropylene resin layer of the core layer are deteriorated. It is more preferable to use linear low-density polyethylene, which has similar melting characteristics to the polypropylene resin used for the core layer, from the viewpoint of film formability and stretchability. Further, the mixing ratio of the linear low-density polyethylene (A) and (B) needs to be within the range of 1: 5 to 5: 1. When the mixing ratio of the linear low-density polyethylene (A) exceeds this range, the elastic recovery rate of the obtained film is poor, and the linear low-density polyethylene (B) is also low.
If the mixing ratio exceeds the above range, the optical properties of the obtained film will be poor.

When both outer surface layers are linear low-density polyethylene (A) only, the optical properties are good, but the elastic recovery rate of the obtained film is poor, and it is not preferable as a stretch shrink film. When both outer surface layers are linear low-density polyethylene (B) only, the elastic recovery rate of the obtained film is good, but the surface is rough when co-extruding the unstretched raw film. The smoothness is inferior, and when it is stretched, the optical properties such as transparency and gloss are inferior.

Further, the sum of the thicknesses of the two outer layers must be 10 to 90% of the total thickness of the film in terms of stretchability and elastic recovery rate. If the sum of the thicknesses of both outer layers is less than 10%, the elastic recovery rate is poor, and if it exceeds 90%, the heat shrinkage stress becomes weak. In addition, it is preferable that the outer layers have substantially the same thickness on the left and right sides in terms of eliminating curl of the obtained film.

The multilayer stretch shrink film of the present invention is required to have heat shrinkability by coextruding into the above-mentioned film constitution and stretching the unstretched original fabric. Moreover, it is preferable that the film causes heat shrinkage at a low temperature. In that respect, since the above film structure can be easily stretched at a low temperature, it can be heat-shrinked at a low temperature. The stretching method is not particularly limited, and a tenter method or an inflation method that has been generally used in the past can be adopted. And, it is desirable that it is stretched in the biaxial direction from the viewpoint of the finished packaging.

[0013]

[Working] The multilayer stretch shrink film of the present invention does not emit an acetic acid odor because it does not contain any ethylene-vinyl acetate copolymer. In addition, by using a mixture of linear low-density polyethylene (A) and (B) having heat shrinkability in both outer layers, the heat shrinkage stress characteristic of the polypropylene resin used in the core layer can be greatly reduced. It is possible to exert a strong heat-shrinkage stress without lowering to a low level, and to generate a good elastic recovery rate. Furthermore, by using the mixture, the surface of the co-extruded unstretched raw material becomes smooth, and the optical properties of the obtained film are improved.

[0014]

[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
It goes without saying that the matters described in the examples are not limiting. In the present invention, the elastic recovery rate was measured by the following method. First, width 10m
50 mm between the marked lines at the center of the test piece of m and length of 200 mm
Mark the line. The test piece is mounted on a tensile tester so that the chuck gap is 100 mm, and is pulled at a speed of 200 mm / min until the marked gap is 100 mm. Then, it is left as it is for 1 minute. Next, the test piece is removed from the chuck by loosening it at a speed of 200 mm / min until the tensile stress becomes zero, and left for 60 minutes.
Then, the distance L (mm) between the marked lines was measured and determined by the following formula.

Elastic recovery rate (%) = {(100−L) ÷ 50} × 100

The heat shrinkage ratio at 100 ° C. was measured by the following method. That is, a test piece cut into a square having a side of 100 mm is immersed in a glycerin bath at 100 ° C. The amount of heat shrinkage generated at that time was determined as a 100-percentage ratio of the length of the base test piece. Further, the heat shrinkage stress was measured by the following method. That is, width 10 mm, length 50 mm
The test piece of No. 1 is mounted on a heat shrinkage stress measuring machine so that the distance between chucks is 30 mm, and immersed in a glycerin bath at 100 ° C. At that time, the stress generated between the chucks was determined as a heat shrinkage stress in the unit of “kg / cm ² ”.

[0017]

Example 1 The core layer was an ethylene-propylene copolymer having an ethylene content of 4.7 wt%, and both outer layers had a density of 0.920.
g / cm ^3, linear low density polyethylene MI0.8g / 10min and (A) Density 0.900g / cm ^3, MI
A multilayer tube-shaped unstretched raw fabric composed of a 1: 1 mixture of 2.0 g / 10 min of linear low-density polyethylene (B) was obtained by a co-extrusion method. The surface of the obtained tube-shaped unstretched original fabric was excellent in smoothness. The raw fabric was biaxially stretched by an inflation method to obtain a multilayer stretch shrink film. The obtained film had a total thickness of about 15 μm, and each outer layer had a thickness of about 3.5.
μm (the sum of the thicknesses of both outer layers to the thickness of the entire film was about 47%). The obtained multilayer stretch shrink film was used to carry out stretch shrink packaging of cucumbers piled on a tray. As a result, even after passing through the heat-shrinking tunnel, there was no odor of acetic acid in the package, the transparency and gloss were good, and the display effect was excellent. In addition, it was a stretch-shrink package that was excellent in cohesion and tension, and was highly elastic in recovery. The heat shrinkage of the film at 100 ° C. was 45%, the heat shrinkage stress was 16 kg / cm ² , and the elastic recovery was 90%. Also, Haze, G
The loss was 3.0% and 155%, respectively, and the optical characteristics were excellent.

[0018]

[Comparative Example 1] A vinyl acetate content of 1.5 wt.
%, MI 3.0 g / 10 min, except that the ethylene-vinyl acetate copolymer was replaced by the same method as in Example 1 to obtain a multilayer stretch shrink film. The film was used to carry out stretch shrink wrapping of cucumbers piled on a tray in the same manner as in Example 1. However, the odor of acetic acid drifts inside the package when it is opened, which greatly reduces the commercial value. Moreover, the binding strength was also poor.
The heat shrinkage stress of the film was 8.0 kg / cm ² ,
The elastic recovery rate was 40%. Also, Haze, Glos
s was 1.0% and 140%, respectively.

[0019]

Comparative Example 2 A multilayer stretch shrink film was obtained in the same manner as in Example 1 except that the ethylene-propylene copolymer of the core layer in Example 1 was changed to a polypropylene homopolymer. Using the film, Example 1
In the same manner as above, stretch-wrap shrink wrapping of cucumbers on a tray was performed. However, the film was inferior in heat shrinkability and elastic recovery rate, and a good stretch shrink package was not obtained. The heat shrinkage rate at 100 ° C. of the film was 20%, and the elastic recovery rate was 35%, which were both inferior. Also, Haze and Gloss are 2.5% and 1 respectively.
It was 25%.

[0020]

[Comparative Examples 3 to 4] A mixture of linear low-density polyethylene for both outermost layers in Example 1 was used to obtain a density of 0.920 g / cm 3.
³ , MI 1.2 g / 10 min linear low density polyethylene simple substance (Comparative Example 3), and density 0.900 g / cm
³ , a multilayer stretch shrink film was obtained by the same method as in Example 1 except that the linear low-density polyethylene simple substance with MI 1.2 g / 10 min (Comparative Example 4) was used.
The film was used to carry out stretch shrink wrapping of cucumbers piled on a tray in the same manner as in Example 1. However, the film of Comparative Example 3 was inferior in elastic recovery rate, and the film of Comparative Example 4 was inferior in optical characteristics.
It significantly reduced the product value. The elastic recovery rate of the film of Comparative Example 3 was 55%, and the Haze and Glass of the film of Comparative Example 4 were 2.0% and 1.
Both were inferior at 25%.

[0021]

[Comparative Examples 5 to 6] A mixture of linear low-density polyethylene in both outermost layers in Example 1 was used to obtain a density of 0.935 g / cm 3.
³ , linear low-density polyethylene with MI 1.0 g / 10 min, density 0.900 g / cm ³ , MI 2.0 g / 1
0 min linear low density polyethylene 1: 1 mixture (Comparative Example 5), and density 0.920 g / cm ³ , MI0.
Example 1 except that a mixture of 8 g / 10 min linear low-density polyethylene and linear low-density polyethylene 1: 1 having a density of 0.910 g / cm ³ and MI 2.0 g / 10 min (Comparative Example 6) was used. A multilayer stretch shrink film was obtained by the same method. The film was used to carry out stretch shrink wrapping of cucumbers piled on a tray in the same manner as in Example 1. However, the films of Comparative Examples 5 and 6 were inferior in elastic recovery rate, and a good stretch shrink package was not obtained. The elastic recovery rate of the film of Comparative Example 5 was 50%, and the elastic recovery rate of the film of Comparative Example 6 was 55%, both inferior. Also, Haze
And Glass were 2.0% and 120%.

[0022]

Example 2 and Comparative Examples 7 to 12 As the polypropylene resin for the core layer, an ethylene-butene-propylene copolymer having an ethylene content of 2.8 wt% and a butene content of 3.6 wt% was used, and both the outer layers were directly laminated. As the chain low density polyethylene (A) (B), the resins shown in Table 1 are used,
A multilayer stretch shrink film was obtained by the same method as in Example 1. The total thickness of the film is 15
μm, both outermost layers have the same thickness, and the total thickness is shown in Table 1.
As shown in. The results are shown in Table 1.

[0023]

[Table 1]

As is clear from Table 1, both outer layers are linear low density polyethylene (A) having a density of 0.920 g / cm ³ and MI 0.8 g / 10 min, and a density of 0.900 g / cm ³ .
cm ³ , using a resin composition in which the mixing ratio of the linear low density polyethylene (B) of MI 2.0 g / 10 min is 1: 1,
The film of Example 2 in which the sum of the thicknesses of both outer layers is 10.5 μm (the sum of the thicknesses of both outer layers with respect to the thickness of the entire film is 70%) has a strong shrinkage stress, an excellent elastic recovery rate, and
It had excellent optical properties and was suitable for stretch shrink wrapping.

On the other hand, the film of Comparative Example 7 in which the mixing ratio of the linear low-density polyethylene (A) is more than 5: 1 is inferior in elastic recovery, and the mixing ratio of the linear low-density polyethylene (B) is low. The film of Comparative Example 8 having a ratio of more than 1: 5 was inferior in optical characteristics. Further, the sum of the thicknesses of the linear low-density polyethylene mixture layers of both outer layers is 1.3 μm (the sum of the thicknesses of both outer layers with respect to the entire film thickness is 10% or less).
The film of Comparative Example 9 was inferior in elastic recovery rate and was 13.8 μm.
m (the sum of the thicknesses of both outer layers with respect to the entire film thickness is 90
% Or more), the heat shrinkage stress of the film of Comparative Example 10 is weak,
Both were unsuitable for stretch shrink wrapping. Comparative Example 1 in which the MI of the linear low-density polyethylene resin composition for both outer layers was less than 0.5 g / 10 min, respectively
The film No. 1 and the film of Comparative Example 12 having an MI of more than 10.0 g / 10 min produced melt fracture during co-extrusion processing with the polypropylene resin of the core layer, were inferior in optical characteristics, and had poor drawability. It was inferior. Further, the obtained stretched film had poor thickness accuracy.

[0026]

[Effect] The package using the multilayer stretch shrink film of the present invention is suitable for packaging foods and the like because it does not generate an acetic acid odor even when it passes through a heat shrinking tunnel.
Since it has good optical properties such as transparency and gloss, it has excellent display effects on products. In addition, since the packaging body using the multilayer stretch shrink film of the present invention has a high recovery elastic modulus, dents and the like formed by being pressed by a finger or the like disappear with the passage of time. Furthermore, since the multilayer stretch shrink film of the present invention has a strong shrinkage stress, an integrated package having a strong binding force can be obtained.

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display area B29L 9:00 4F

Claims (1)

[Claims] 1. The core layer comprises an ethylene-propylene copolymer or an ethylene-butene-propylene copolymer or a mixture thereof, and both outer layers have a density of 0.910 to 0.
925 g / cm ³ , melt index 0.5 to 10
g / 10 min linear low density polyethylene (A),
A mixture of linear low-density polyethylene (B) having a density of 0.890 to 0.907 g / cm ³ and a melt index of 0.5 to 10 g / 10 min and a mixing ratio of 1: 5.
To 5: 1, and the sum of the thicknesses of both outer layers is 10 to 90% of the total thickness of the film.
A multi-layer stretch shrink film in which a coextruded unstretched raw material is stretched.