JPH03189146A - Heat-shrinkable laminated film - Google Patents

Abstract

PURPOSE:To obtain the low temperature shrinkability, and at the same time, prevent thermal fusing from happening at the time of shrink packaging by a method wherein a 3-layer co-extrusion laminated sheet of which central layer is a straight-chain low density polyethylene, and of which both outer layers are a specific high density polyethylene is stretched. CONSTITUTION:For this laminated film, a 3-layer co-extrusion laminated sheet of which central layer is a straight-chain low density polyethylene with density of 0.915 - 0.935g/cm<2>, melting point of 120 - 128 deg.C, and of which both outer layers are 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> is stretched at least 2 times at an orienting temperature of not more than 100 deg.C to form the laminated film, and a thickness of the central layer is made to be 35 - 70% of the total thickness. By this method, a heat-shrinkable film, which has excellent transparency and low temperature shrinkability, does not give thermal fusing to an object to be packaged or between films, gives a beautiful packaged 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.

Although a stretched film of high density polyethylene (HDPE) does not undergo heat sealing, its 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.

[Problems to be Solved by the Invention] The object of the present invention is to provide excellent transparency, strength, and low-temperature shrinkability, as well as to prevent heat fusion with the packaged object and between films. An object of the present invention is to provide a heat-shrinkable film that gives a beautiful packaging appearance, is heat resistant, and has excellent properties such as not breaking in a shrink tunnel.

As a result of intensive research to overcome the problems of the prior art described above, the present inventors have developed a uniaxial three-layer coextruded laminate sheet with a center layer of linear low-density polyethylene and an amphoteric layer of a specific high-density polyethylene. The laminated film obtained by stretching has low-temperature shrinkability, and since the amphoteric layer is made of high-density polyethylene with a high melting point, it has excellent performance such as preventing heat fusion during shrink wrapping. The present invention was completed based on this finding.

[Means for Solving the Problems] Thus, according to the present invention, the central layer has a density of 0.915 to
0.935g/cd, straight-cut low-density polyethylene with a melting point of 120-128°C, and the amphoteric layer has a melt index of 0.
3-0.6, melting point 133-136℃, density 0.935-
A laminate film obtained by uniaxially stretching a three-layer coextruded laminate sheet made of high-density polyethylene of 0.945 g/c rd to at least twice the thickness at a stretching temperature of 100°C or less, the thickness of the center layer being A heat-shrinkable laminated film is provided that is characterized by having a total thickness of 35 to 70%.

The present invention will be explained in detail below.

In the present invention, the linear low density polyethylene (L-LDPE) serving as the center layer has a density of 0.915 to 0.935 g.
/cnf, melting point 120-128°C (differential scanning calorimeter DS
C).

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 wrinkles easily and breaks easily when the shrink tunnel temperature is high or the passage time is long, resulting in a beautiful finished appearance. The conditions for obtaining it are extremely narrow.

In the present invention, the amphoteric layer has a melt index of 0.3 to 0.6 and a melting point of 133 to 136°C (DSC
(measured at ) and high density polyethylene (HDPE) with a density of 0.935 to 0.945 g/cm2.

This HDPE has a haze of 7% or less and a tensile modulus of 5% and 3
, OOO~4, 500 kg/c rrr
This is preferable because transparency and strength are improved.

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

The laminated structure is that the center layer is L-LDPE and the amphoteric layer is HDP.
E, that is, a three-layer laminated structure of HDPE/L-LDPE/HDPH. The amphoteric layer has a high melting point (133-136
℃) By using HDPE, it is possible to effectively prevent heat fusion during shrink wrapping.

The method for producing a heat-shrinkable film of the present invention includes HDPE/L
- A 3N laminated sheet of LDPE/HDPE is melt-extruded from a lamination die by a co-extrusion method, cooled and solidified, and then reheated to a stretching temperature of 100°C or less, preferably within the range of 75 to 90°C, and stretched vertically. It is characterized in that it is uniaxially stretched in the 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 extrusion rocks 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 helix 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 limited physical properties, and HDPE other than the specific HDPE
When PE is used, whitening of the film and reduction in transparency occur.

Furthermore, when the center layer is made of low density polyethylene (LDPE) or ethylene/vinyl acetate copolymer (EVA), the tensile strength decreases and the thickness cannot be sufficiently reduced.

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 35-70% 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%, the low-temperature shrinkability will be poor; if it exceeds 70%, the low-temperature shrinkability will be good, but the heat resistance and strength will decrease.

[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 linear low-density polyethylene resin (Mitsui Oil: Urtozex 30).
21F), a three-layer unstretched sheet was prepared using high-density polyethylene resin (Toso: Niboron 5-FWO5) for the amphoteric layer. The thickness of the unstretched film at that time was 20 μm.
The thickness was 780 μm and 720 μm (center layer thickness 67%).

Next, the film was stretched 3 times in the longitudinal direction using a -axial stretching machine at a stretching temperature of 85° 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 all! ! : Place the film in a constant temperature bath at 80℃ and heat it for 5 minutes.
After a minute had elapsed, the sample was taken out and the dimensional shrinkage rate in the longitudinal direction (stretching direction) was measured.

? ljl[: Heat shrinkage conditions (shrinkage tunnel temperature, tunnel passage time) were changed as shown in Table 1, and paper container stacks with straws were shrink-wrapped with heat-shrinkable film, and the resulting packaged products were Observations were made for the presence or absence of heat fusion with the packaged item and between the films, whether the packaging had a tight and beautiful appearance, and whether the film was torn or not.

The results are shown in Table 1.

[Comparative Example 1] Both outer layers were made of high-density polyethylene resin (Mitsubishi Yuka: Yucalon EX40, melt index 1.1,
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 15 μm/90 μm/15
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 of the center layer was 75%, and the film was evaluated in the same manner.

[Comparative Example 3] The thickness of each layer of the unstretched sheet was 45 μm/730 μm/45
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 of the center layer was 25%, and the film was evaluated in the same manner.

[Comparative Example 4] 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.

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, there is no heat fusion with the packaged object or between films, and a beautiful packaging appearance is provided. Provided is a heat-shrinkable film that has excellent heat resistance and does not break in the shrink tunnel.

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

Furthermore, by adopting the laminated structure of the present invention, a general-purpose shrink wrapping machine can be applied, and the film will not break even if the temperature of the heat shrink tunnel is roughly controlled (even if the temperature varies).

Claims (1)

[Claims] (1) The center layer has a density of 0.915 to 0.935 g/cm^
2. Linear low-density polyethylene with a melting point of 120-128°C, both outer layers have a melt index of 0.3-0.6, a melting point of 133-136°C, and a density of 0.935-0.945 g/c.
A laminate film obtained by uniaxially stretching a three-layer coextruded laminate sheet made of high-density polyethylene of m^2 to at least twice the thickness at a stretching temperature of 100°C or lower, and in which 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 70%.