JPH03189148A - Heat-shrinkable laminated film - Google Patents

Abstract

PURPOSE:To obtain a laminated film which has good transparency and strength and also has low temperature shrinkability by a method wherein a 3-layer co-extrusion laminated sheet, of which central layer is a specific high density polyethylene and of which both outer layers are straight-chain low density polyethylene, is stretched. CONSTITUTION:A 3-layer co-extrusion laminated sheet, of which central layer is a high density polyethylene with melt index of 0.3 - 0.6, melting point of 133 - 136 deg.C and density of 0.935 - 0.945g/cm<2>, and of which both outer layers are straight-chain low density polyethylene with density of 0.915 - 0.935g/cm<2> and melting point of 120 - 128 deg.C, is stretched at least 2 times at an orienting temperature of not more than 100 deg.C, to form a laminated film, and thickness of the central layer is made to be 35 - 80% of the total thickness. By doing this, a heat-shrinkable film, which has excellent transparency and strength, by which thermal fusing with an object to be packaged is reduced, and which gives a beautiful packaging appearance and has heat resistance and does not rupture in a shrink tunnel, can be obtained.

Description

[Detailed description of the invention] [Industrial application field] The present invention has low-temperature shrinkability and heat resistance, and also has transparency and
This invention relates to a heat-shrinkable laminated film with excellent strength.

The heat-shrinkable laminated film of the present invention is particularly suitable as a film for shrink-wrapping food containers and the like.

[Conventional technology]

In general, shrink wrapping films are required to have appropriate shrinkage rates, film strength, transparency, etc.

In addition, in shrink packaging, after being fused and sealed, the shrinkage is completed by passing through a shrink tunnel using infrared heating, etc., but in order to avoid the negative effects of heating on the contents, the shrink film must have low-temperature shrinkability. Ru. Furthermore, heat resistance and no heat fusion to the packaged items are also important performance requirements.

However, conventional shrink films are still insufficient in various performances in view of recent required standards.

Conventionally, shrink wrapping films made of low density polyethylene (LDPE) have been widely used for the integrated packaging of various cartons including beverage containers and glass bottles. However, L
Shrink packaging films made of DPE do not have a beautiful finish because the films adhere to each other during heat shrinkage, and in the case of integrated packaging for beverage paper containers, the paper containers themselves or attached straws and the film may adhere to each other. There was a flaw in that it ended up happening.

Furthermore, when shrink wrapping films made of LDPE are used for building materials such as aluminum sash and decorative plywood, they have the disadvantage of being thermally fused to the masking film on the surface of the building material during heat shrinkage.

There are products that have increased low-temperature shrinkability by stretching low-density polyethylene, but although heat fusion can be prevented by lowering the shrinkage temperature, they lack cohesive strength due to low-temperature shrinkage. Therefore, if the shrinkage temperature is slightly raised, the shrinkage rate increases and the film breaks within the shrinkage tunnel, resulting in a drawback that the shrinkage temperature range is extremely narrow.

A stretched film of linear low-density polyethylene (L-LDPE) has improved performance over a stretched film of low-density polyethylene, but it is not sufficient.

A stretched film of ethylene/vinyl acetate copolymer (EVA) also has a narrow shrinkage temperature range, similar to a stretched film of low density polyethylene.

Stretched films of high density polyethylene (HDPE) are not heat-sealable, but their shrinkage performance is insufficient, and a beautiful finish cannot be obtained due to insufficient shrinkage.

Low-density polyethylene film stretched by irradiation with electron beams does not undergo heat sealing and produces a beautiful packaging finish, but because it is cross-linked, general-purpose packaging machines cannot be used, and special machines are required for sealing. There were disadvantages, such as the necessity and the high cost of the film.

[Problem to be Solved by the Invention J] The purpose of the present invention is to provide excellent transparency, strength and low-temperature shrinkability,
To provide a heat-shrinkable film which has excellent properties such as alleviating heat-sealing phenomenon, giving a tight and beautiful packaging finish appearance, being heat resistant, and not breaking in a shrink tunnel. There is a particular thing.

Another object of the present invention is to provide an inexpensive shrink film that can be applied to general-purpose shrink wrapping machines.

As a result of intensive research to overcome the problems of the prior art described above, the present inventors developed a three-layer co-extruded draft layer sheet whose central layer is a specific high-density polyethylene and whose outer layer is a linear low-density polyethylene. It was discovered that a laminated film obtained by uniaxial stretching has excellent properties such as good transparency, strength, and low-temperature shrinkability, and based on this knowledge, the present invention was completed.

[Means for Solving the Problems] Thus, according to the present invention, the center layer has a melt index of 0.3 to 0.6, a melting point of 133 to 136°C, and a density of 0.9.
35-0.945 g/crrr high-density polyethylene, the outer wound layer has a density of 0.915-0.935 g/crrr,
A laminated film obtained by uniaxially stretching a three-layer coextruded layer sheet made of linear low-density polyethylene with a melting point of 120 to 128° C. at a stretching temperature of 100° C. or lower to at least twice the amount, wherein the center layer A heat-shrinkable laminated film characterized in that the thickness is 35 to 80% of the total thickness is provided.

The present invention will be explained in detail below.

In the present invention, the linear low density polyethylene (L-LDPE) serving as the outer layer has a density of 0.915 to 0.935 g/
It is a polymer with a melting point of 120 to 128°C (measured by differential scanning calorimeter DSC).

This L-LDPE is a copolymer of ethylene and α-olefin such as butene, and has superior tensile strength, puncture strength, tear strength, etc. compared to LDPH, and can be used as a single film for processed foods. It is used for packaging such as.

Stretched heat-shrinkable film made of L-LDPE alone has a high shrinkage rate and shrinks rapidly, so it is more prone to wrinkles (and easily breaks when the shrink tunnel temperature is high or the passage time is long, resulting in a beautiful finished appearance) The conditions for obtaining this are extremely narrow.

In the present invention, the core layer has a melt index of 0.3 to 0.6 and a melting point of 133 to 136°C (DSC
It is high-density polyethylene (HDPE) with a density of 0.935 to 0.945 g/c.

This HDPE has a haze of 7% or less and a tensile modulus of 5% and 3
,000 to 4,500 kg/crtr is preferable because transparency and strength are good.

Good low-temperature shrinkability cannot be obtained with a heat-shrinkable film made of HDPE alone.

The laminated structure consists of a specific HDPE in the center layer and an L-layer in the outer layer.
LDPE, i.e. L-LDPE/HDPE/L-LD
It has a 3-layer laminated structure of PH.

By forming the outer layer L-LDPE, (1) after stretching,
(2) It is heat resistant compared to LDPE and EVA shrink films, and can be heat-fused to the contents during shrink wrapping. (3) The film has high tensile strength. Even thin films can be put to practical use.

By placing a specific HDPE in the center layer, (1)
Heat-shrinkable film made of L-LDPE alone has a large shrinkage rate and is prone to rapid shrinkage and breakage, but HDPE
(2) Improves heat resistance and makes it difficult to break in the shrink tunnel. Therefore, the shrink temperature range is expanded,
Improves work efficiency.

The method for producing a heat-shrinkable film of the present invention includes L-LDPE
A three-layer laminated sheet of /HDPE/L-LDPE is melt-extruded from a laminated goo using a co-extrusion method, cooled and solidified,
Next, the film is characterized by being reheated to a stretching temperature of 100° C. or lower, preferably within a range of 75 to 90° C., uniaxially stretched in the machine direction at a stretching ratio of 2 times or more, and then cooled.

For coextrusion of the laminated sheet, it is preferable to use a plurality of extruders and extrude it into a tube shape using a circulider (inflation method) or extrude it into a flat shape using a T die (T die method).

When melt extruding into a tube shape, it is immersed in a water bath or cooled and solidified by blowing cooling air, and then heated to 100°C.
The film is reheated to the following stretching temperature and stretched to more than twice the length using a take-up roll in the machine direction. When melt-extruding a laminated sheet into a flat shape using a T-die, it is brought into contact with a cooling roll or cooling air to rapidly cool and solidify, and then
The film is reheated to a stretching temperature of 0.degree.

If the stretching temperature exceeds 100°C, low-temperature heat shrinkability cannot be obtained.The heat-shrinkable laminated film of the present invention has a dimensional shrinkage rate of 20% in the longitudinal direction after 5 minutes of being placed in an atmosphere at a temperature of 80°C. The above results indicate good low-temperature shrinkability.

Further, the haze of the uniaxially stretched laminated film of the present invention is 1
0% or less, the transparency is good, but this can only be achieved by using HDPE with specific and limited physical properties, and if a general-purpose HDPE other than the specific HDPE is used, Whitening of the film and reduction in transparency occur.

Furthermore, in the laminated structure of EVA/HDPE/L-LDPH, the film whitens when stretched at a low temperature of 100° C. or lower.

As mentioned above, L-LDPE/HDPE/L-LDP
The laminated structure E also lacks transparency.

The heat-shrinkable laminated film of the present invention usually has a total thickness of 25
~50 μm, and the thickness of the center layer should be 35-80% of the total thickness.

Good shrinkage performance can be obtained within the range of the above thickness structure. If the thickness of the center layer is less than 35%, heat resistance will decrease, the heat shrinkage temperature range will become narrow, and workability will decrease.
On the other hand, if it exceeds 80%, the low-temperature shrinkability is reduced, a good finished appearance cannot be obtained, and the transparency is significantly reduced.

[Example] The present invention will be specifically described below with reference to Examples and Comparative Examples.

[Example 1] In a coextruded three-layer inflate, the center layer was made of high-density polyethylene resin (Toso: Niboron 5-FWO5), and the outer layer was made of linear low-density polyethylene resin (Mitsui Oil: Urtozex 3021F). ), a three-layer unstretched sheet was produced. The thickness of the unstretched film at that time was 30 gm/
It was 60 gm/30 μm (center layer thickness 50%).

Next, the film was stretched 3 times in the longitudinal direction using a -axis stretching machine at a stretching temperature of 90° C., and then cooled to obtain a heat-shrinkable film with a total thickness of 40 μm.

Regarding the obtained heat-shrinkable laminated film, the packaging appearance was observed while changing the transparency (haze), the longitudinal heat-shrinkage rate (80° C. x 5 minutes), and the heat-shrinkage conditions.

The method for measuring physical properties is as follows.

Haze: JIS K-7105 LILIL! ! : The film was placed in a constant temperature bath at 80° C. and taken out after 5 minutes, and the dimensional shrinkage rate in the longitudinal direction (stretching direction) was measured.

1. Outside I: The heat shrinkage conditions (shrinkage tunnel temperature, tunnel passage time) were changed as shown in Table 1, and the straw
Shrink-wrapping paper container collections with heat-shrinkable film, and checking whether the resulting packaged products are thermally fused to the packaged items or between the films, and whether the packaging has a tight and beautiful appearance. The film was also observed to see if it was torn or not.

The results are shown in Table 1.

[Comparative Example 1] As the center layer, a high-density polyethylene resin (Mitsubishi Yuka: Yucalon EX40, melt index 1.l,
A heat-shrinkable laminate film having a three-layer laminate structure was prepared in the same manner as in Example 1, except that a film with a density of 0.960) was used, and evaluated in the same manner.

[Comparative Example 2] The thickness of each layer of the unstretched sheet was 40 μm/40 μm/40
A heat-shrinkable laminate film having a three-layer laminate structure was prepared in the same manner as in Example 1 except that the thickness was 33% in the center layer, and the film was evaluated in the same manner.

[Comparative Example 3] A heat-shrinkable laminate film having a three-layer laminate structure was prepared in the same manner as in Example 1 except that the stretching temperature was changed to 110° C., and evaluated in the same manner.

[Comparative Example 4] A commercially available low-density polyethylene axially stretched type heat-shrinkable film (40 μm) was similarly evaluated.

The results are summarized in Table 1.

(The following is a blank space) [Effects of the Invention] According to the present invention, it has excellent transparency and low-temperature shrinkability, reduces heat fusion with the packaged object, provides a beautiful packaging appearance, and
Provided is a heat-shrinkable film that has excellent heat resistance and does not break within the shrink tunnel.

Furthermore, since the film has high strength and can be made thinner, costs can be reduced. The laminated structure is symmetrical,
There is no curling of the original material, and it is easy to handle.

Furthermore, by employing the laminated structure of the present invention, a general-purpose shrink wrapping machine can be applied, and the film is difficult to break even if the temperature range of the heat shrink tunnel is wide, improving workability.

Claims (1)

[Claims] (1) The center layer has a melt index of 0.3 to 0.6, a melting point of 133 to 136°C, and a density of 0.935 to 0.945 g/
cm^2 high-density polyethylene, both outer layers have a density of 0.9
15~0.935g/cm^2, melting point 120~128℃
A laminate film obtained by uniaxially stretching a three-layer coextruded laminate sheet made of linear low-density polyethylene at a stretching temperature of 100° C. or lower to at least twice the thickness, wherein the thickness of the center layer is the same as the total thickness. A heat-shrinkable laminated film characterized in that it has a content of 35 to 80%.