JPH0815774B2 - Heat shrinkable film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The heat-shrinkable film of the present invention wraps a product roughly and heat-shrinks it with a heating device (shrink tunnel).
It is used for heat-shrink packaging in which tight packaging is performed by closely adhering to the shape of the product, and particularly, it exhibits an excellent effect for integrated packaging of products.

(Prior Art) A conventional heat-shrinkable film made of low-density polyethylene does not have a sufficient orientation effect, is inferior in heat-shrinkage characteristics such as heat-shrinkage rate and heat-shrinkage stress at a low temperature, and has a low temperature. Heat shrink wrapping was not possible at the shrink tunnel temperature, and a good heat shrink wrap could not be obtained unless the temperature was close to the melting point.
Also, the transparency was inferior to that of the heat-shrinkable film made of other resin.

Then, when linear low density polyethylene became commercially available, various attempts were made to solve these problems by using the resin, and some such heat shrinkable films were commercially available. It came to be done. For example, a method of adding an ethylene-vinyl acetate copolymer or a copolymer of ethylene and α-olefin to a linear low density polyethylene in order to improve heat shrinkage characteristics and transparency at low temperature is disclosed in JP-A-2000-242. It is disclosed in JP-A-61-123516 and JP-A-61-112627. Also, Japanese Patent Publication No. 61-60770 discloses a method of laminating a low-density polyethylene layer and a linear low-density polyethylene layer in order to enhance the cohesive strength. Furthermore, in order to improve the stretchability of linear low-density polyethylene, a method of adding modified polyethylene is disclosed in JP-A-59-21.
It is disclosed in Japanese Patent No. 5828.

However, no sufficient solution has been found yet, the drawability is poor, the heat shrinkage property at low temperature is insufficient, and the temperature range suitable for heat shrinkable packaging is very narrow. At present, it is difficult to obtain a shrink package.

A heat-shrinkable film having improved heat-shrinkability by cross-linking low-density polyethylene is widely used. However, this method has problems in sealing property and recycling.

(Problems to be Solved by the Invention) The present invention provides a heat-shrinkable film using a linear low-density polyethylene, which can be easily stretched and has a heat-shrinkable property at low temperature. It is the one we are trying to provide. That is, the present invention can easily perform simultaneous biaxial stretching by a tenter system or an inflation system which is generally used in the past, and the heat-shrinkable film obtained is required for heat-shrinkable packaging from low temperature to high temperature. An object of the present invention is to provide a heat-shrinkable film which exhibits a heat-shrinkage rate and a heat-shrinkage stress and exhibits a wide temperature range suitable for heat-shrinkable packaging.

(Means for Solving Problems) The present invention provides a polyethylene-based heat-shrinkable film, which has a good stretch processability and is easily provided with a good heat-shrinkable package. The film structure is as follows. That is, the heat-shrinkable film of the present invention, both outer layers have a density of 0.915 to 0.935 g / cm ³ ,
Made of linear low-density polyethylene with a melting point of 120-128 ° C,
The middle layer has a density of 0.890 to 0.910 g / cm ³ , and a Vicat softening point of 75 ° C.
The film is composed of the following linear low-density polyethylene, and the thickness of the intermediate layer is 30 to 80% of the total thickness.

This will be described in detail below. First, as the linear low-density polyethylene used for both outer layers, ethylene is propylene, butene-1, pentene-1, hexene-1, octene-1, 4-methylpentene-1. By copolymerizing α-olefins such as, by introducing an appropriate number of short chain branches into the linear main chain, the density is 0.915 ~ 0.935 g / cm ³ , the melting point of 120 ~ 128 ℃ resin Adopted as a suitable one.

Further, as the linear low-density polyethylene used for the intermediate layer, a resin obtained by copolymerizing ethylene and α-olefin and introducing a short-chain branch into the linear main chain is preferable as in the case of the both outer layers. Adopted as a thing. In this case, as the number of short chain branches of the resin, a larger number than the resin used for both the outer layers was introduced, and as a result, the density was 0.890 to
It is kept as low as 0.910 g / cm ³ , and the Vicat softening point is set to 75 ° C or lower.

If the density of the linear density polyethylene used for both outer layers exceeds the above range and exceeds 0.935 g / cm ³ , the stretchability will be poor and the transparency will be poor. Also, the density is 0.
If it is less than 915 g / cm ³ , the heat shrinkage stress is weakened, the heat shrinkable package obtained is inferior in tightness and looks bad, and the binding force as a heat shrinkable package is inferior. Further, if the melting point of the resin exceeds 128 ° C., not only the stretchability deteriorates, but also stretching processing at low temperature becomes impossible. Therefore, since the film is stretched at a high temperature, the low temperature heat shrinkage property is deteriorated and the heat shrinkable packaging suitability is deteriorated. Further, if the melting point is less than 120 ° C., the heat resistance is poor, and the effect of widening the temperature range suitable for heat-shrinkable packaging due to the multilayer structure is lost.

Then, the density of the linear low density polyethylene used for the intermediate layer stretchability is deteriorated and exceeds 0,910g / cm ³ deviates from the above range, also weakly heat shrinkage stress is less than 0.890 g / cm ³ As a result, not only the heat-shrinkable package obtained is inferior in tightness and appearance, but also the binding force as a heat-shrinkable package is inferior. Furthermore, if the Vicat softening point of the resin exceeds 75 ° C., the stretching processability at low temperatures is poor, so that stretching is performed at the high temperature side, so heat shrinkage properties at low temperatures are poor, and heat shrinkable packaging suitability becomes poor. .

That is, when the heat-shrinkable film has the film constitution of the present invention, the stretching process at a low temperature can be easily performed, and further, the low-temperature stretching becomes possible, so that the heat-shrinkability and the heat-shrinkability at a low temperature can be achieved. The stress is improved. It is considered that the reason is that the linear low-density polyethylene used for the intermediate layer is inferior in crystallinity and has a low Vicat softening point, so that the drawability at low temperature is improved. Incidentally, the fact that the linear low-density polyethylene of the intermediate layer has poor crystallinity is expected due to its low density.

Furthermore, the heat-shrinkable film of the present invention makes it possible to obtain a heat-shrinkable package having a good heat-shrinkable tunnel temperature in a wide temperature range from a low temperature to a high temperature when heat-shrink wrapping.
The reason for this is that, since the low-temperature stretching described above was first possible, the heat-shrinkable packaging at low temperature was possible. Next, both outer layers use linear low-density polyethylene, which has better heat resistance than the linear low-density polyethylene used for the intermediate layer, so it protects the intermediate layer, which has poor heat resistance. Therefore, it became possible to perform heat shrink packaging at high temperature. Moreover, since the melting point of the linear low-density polyethylene used for both outer layers is higher than that of the low-density polyethylene used for general heat-shrinkable films, it has excellent heat resistance. .

Regarding the thickness constitution of each layer, it is necessary that the thickness ratio of the intermediate layer is within the range of 30 to 80% with respect to the total thickness.
If the thickness ratio of the intermediate layer deviates from the above range and is less than 30%, the drawability is poor and the drawability is particularly difficult at low temperatures. Even if the film can be stretched, the obtained heat-shrinkable film is inferior in heat shrinkage suitability at a low temperature and is difficult to heat shrink-wrap. On the other hand, if it exceeds 80%, not only the binding strength as a heat-shrinkable package is poor, but also it becomes difficult to expand the temperature range suitable for heat-shrinkable packaging.

The overall thickness of the heat-shrinkable packaging film of the present invention,
Considering heat shrink packaging, it is preferably in the range of 12 to 35 μ.

It should be noted that mixing of other resins, additives, or the like, or provision of a new layer in each layer is not hindered within the scope of the present invention.

As a method for producing the heat-shrinkable film of the present invention,
Although not particularly limited, it is preferably manufactured by the following method. That is, the laminated unstretched sheet is coextruded from the laminated die using a plurality of extruders. And
After the laminated unstretched sheet is cooled and solidified, it is reheated to a temperature at which it can be stretched to obtain at least 2.5 in both the longitudinal and transverse directions.
After stretching twice or more, it is cooled. The stretching method may be either a tenter method or an inflation method, but it is preferable to use the inflation method because it is easy to make the heat shrinkage characteristics in the machine direction and the transverse direction similar to each other. Therefore, the laminated unstretched sheet needs to be obtained as a multilayer tubular sheet by using a multilayer circular die. The resulting heat-shrinkable film, if left to stand naturally, will have its deformation and surface condition deteriorated by the natural shrinkage of the film itself, so in order to prevent this, the heat-shrinkable property is not significantly deteriorated. It is more preferable to reduce the amount of natural shrinkage by heat treatment.

As a heat-shrinkable packaging method using the heat-shrinkable film of the present invention, a heat-shrinkable packaging line which has been conventionally used can be used as it is. As compared with the conventional heat-shrinkable film made of low-density polyethylene, a heat-shrinkable package having excellent tightness even when the shrinking tunnel temperature is low and having a strong binding force can be obtained. In addition, since heat shrink wrapping is possible in a wide temperature range, a good heat shrink wrap can be easily obtained without setting the temperature in the shrink tunnel on the civer. Further, since it has excellent heat resistance, it does not cause melt whitening even when it is heat-shrink packaged at high temperature.

(Effects of the Invention) As is apparent from the above description, the heat-shrinkable film of the present invention is excellent in stretchability and therefore has good productivity. Next, since the temperature range suitable for heat shrink wrapping is wide, a good heat shrink wrap can be easily obtained. Moreover, the obtained heat-shrinkable package has excellent effects such as excellent tightness, good appearance, and excellent binding force.

(Example) Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

Example 1 Linear low-density polyethylene having a density of 0.920 g / cm ³ and a melting point of 123 ° C. was used as both outer layers, and the density was 0.900 g / cm ³ and the Vicat softening point was 67.
A tubular coextruded laminated unstretched raw fabric sheet having a linear low-density polyethylene at 0 ° C. as an intermediate layer was obtained by three extruders and a three-layer coextrusion circular die. The total thickness of the obtained laminated unstretched original fabric sheet was about 145 μ, and the thickness ratio of each layer was 1: 2: 1 from one outer layer. In the extrusion molding, immediately after the coextrusion, the water was rapidly cooled by the conventional water cooling method.

This laminated unstretched raw sheet was stretched 3.5 times in the machine direction and the transverse direction by the conventional inflation method, but it can be easily stretched at a lower temperature than the conventional linear low density polyethylene. An axially stretched film could be obtained. Then, this laminated biaxially stretched film was heat-set while being relaxed to obtain a heat-shrinkable film having an overall thickness of about 15 μm.

Using the obtained heat-shrinkable film, four toilet papers were integrated and packaged. As a result, even if the shrinking tunnel temperature set at 140 ° C is changed up and down by about 15 ° C, it does not affect the finish of the heat-shrinkable package such as the tightness and the binding force, and the heat shrinkage. At times, there was no occurrence of melt whitening.

Example 2 Linear low-density polyethylene having a density of 0.930 g / cm ³ and a melting point of 125 ° C. was used as both outer layers, and the density was 0.900 g / cm ³ and the Vicat softening point was 70.
A laminated unstretched raw fabric sheet having a straight-chain low-density polyethylene at 0 ° C. as an intermediate layer was coextruded in the same manner as in Example 1. The total thickness was about 190μ, and the thickness ratio of each layer was 1: 4: 1 from one outer layer.

The obtained laminated unstretched original sheet was treated in the same manner as in Example 1.
The film was biaxially stretched by a conventional inflation method and then heat-fixed to easily obtain a heat-shrinkable film having an overall thickness of about 20μ.

Using the obtained heat-shrinkable film, four boxes of tissue paper were integrated and packaged. As a result, even if the shrinking tunnel temperature set at 145 ° C is changed up and down by about 15 ° C,
There was no influence on the finish of the heat-shrinkable package such as tightness and binding force, and there was no occurrence of melt whitening or the like during heat shrinkage.

Comparative Example 1 The linear low-density polyethylene used in both outer layers of Example 1 and the linear low-density polyethylene used in the intermediate layer were 1: 1.
To obtain a tubular single-layer unstretched raw sheet having the same thickness as in Example 1.

The obtained unstretched raw sheet was biaxially stretched by the conventional inflation method as in Example 1.
However, the stretching stability was inferior, the swaying of the bubble was large, and the puncture was likely to occur. Further, the obtained biaxially stretched film was inferior in uneven thickness accuracy. Furthermore, Example 1
When heat-shrinkable packaging is performed using a heat-shrinkable film obtained by heat setting in the same manner as in Example 1, heat resistance is inferior to that of the heat-shrinkable film of Example 1, and heat-shrinkable packaging at low temperature is also performed. The suitability was not sufficient, and the temperature range suitable for heat shrink wrapping was narrow, and heat shrink wrapping was difficult.

Comparative Example 2 A laminated unstretched raw fabric sheet having linear low-density polyethylene used for both outer layers of Example 1 as both outer layers and low-density polyethylene having a density of 0.920 g / cm ³ and a melting point of 109 ° C. as an intermediate layer, By the same method as in Example 1, the same overall thickness and the same thickness constitution of each layer were coextruded.

The obtained laminated unstretched original sheet was treated in the same manner as in Example 1.
Biaxial stretching was attempted by the conventional inflation method. However, it was inferior in drawability and it was very difficult to perform biaxial draw at any draw temperature.

Comparative Examples 3 to 4 Thickness ratio of each layer of the laminated unstretched sheet used in Example 1 1:
A laminated unstretched raw sheet having a ratio of 2: 1 to 0.5: 9: 0.5 in Comparative Example 3 and 2: 1: 2 in Comparative Example 4 was coextruded by the same method as in Example 1. The total thickness was the same as in Example 1.

Using these obtained laminated unstretched raw fabric sheets, a heat-shrinkable film was trial-produced by the same method as in Example 1.

As a result, in Comparative Example 3, although low-temperature stretching was possible, the bubbles swayed greatly and punctures were likely to occur.
Further, the obtained biaxially stretched film was inferior in the uneven thickness accuracy. In Comparative Example 4, the stretchability was poor and it was difficult to perform biaxial stretching. In particular, the stretching process at a low temperature was impossible.

When the heat-shrinkable film of Comparative Example 3 was used for heat-shrinkable packaging, the binding strength was poor and the temperature range suitable for heat-shrinkable packaging was narrow, making it difficult to obtain a good heat-shrinkable package.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location B29K 105: 02 B29L 9:00

Claims (1)

[Claims] 1. Both outer layers have a density of 0.915 to 0.935 g / cm ³ and a melting point of 120.
〜128 ℃ linear low density polyethylene, the intermediate layer has a density of 0.890 to 0.910 g / cm ³ , Vicat softening point of 75 ℃ or less linear low density polyethylene, and the total thickness of the intermediate layer A heat-shrinkable film that is 30-80% of the thickness.