JPH01301251A - Polyethylenic heat-shrinkable multilayered film - Google Patents

Abstract

PURPOSE:To obtain excellent low temp. heat sealability, by using linear copolymers each consisting of specific ethylene and alpha-olefin and setting a specific layer constitutional ratio to perform the co-extrusion of both copolymers. CONSTITUTION:Linear copolymers are melted and kneaded by two extruders so that a linear copolymer (A) of ethylene and alpha-olefin having density of 0.915 to 0.930g/cm<3> and a melt index of 0.1 to 3.0g/10min becomes an intermediate layer and a linear copolymer (B) of ethylene and alpha-olefin having density of 0.870 to 0.915g/cm<3> and a melt index of 0.2 to 3.0g/10min becomes inner and outer layers and subjected to co- extrusion from a three-layer annular die and solidified under cooling to form a tubular non-stretched film. Subsequently, this tubular non-stretched film is supplied, for example, to a tubular stretching apparatus and gas pressure is adapted to the interior of the film tube in an orientation possible temp. region to inflate and stretch said tube two times of or more, pref., 2.5 times or more in respective longitudinal and lateral directions to perform simultaneous biaxial orientation. By this method, a polyethylenic heat-shrinkable multilayered film wherein the ratio of the intermediate layer to all of the layers of the stretched film is 50% or more and the thickness of each of the inner and outer layers is 1mum more and an area shrinkage factor at 90 deg.C is 20% or more is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a shrink packaging material, and more particularly to a polyethylene heat-shrinkable multilayer film that has good suitability for packaging machines and excellent low-temperature heat sealability.

(Prior Art) Stretched films of polyvinyl chloride, polypropylene, polyethylene, and the like are conventionally known as heat-shrinkable films.

Among these, polyethylene-based heat-shrinkable films have been put into practical use because of their heat-sealability and low cost, and in recent years, linear low-density copolymers of ethylene and α-olefin (hereinafter simply referred to as linear Abbreviated as low-density polyethylene.μ polyethylene heat-shrinkable film is attracting attention for its excellent impact resistance and heat-sealing strength, and is expected to be used in many fields. .

(Problems to be Solved by the Invention) The present inventors have previously proposed a heat-shrinkable film mainly composed of a specific ethylene-α-olefin copolymer (Japanese Patent Laid-Open No. 62-201229). .

By implementing this proposed method, it became possible to obtain a film with small thickness unevenness and good low-temperature shrinkability compared to films obtained by inflation. When shrink-wrapping with a packaging machine, etc., if the film is not stiff, the film will not run smoothly and the edges of the film will be uneven, resulting in insufficient sealing (poor suitability of the packaging machine).

In addition, the film around the seal portion of the heat seal portion may shrink, and sealing may not be performed satisfactorily in some cases.

If the heat sealing temperature is lowered to prevent shrinkage of the film around the sealed portion, the problem arises that the sealing strength decreases, and this has not always been completely satisfactory.

Therefore, there has been a desire for a heat-shrinkable film that has transparency and low-temperature shrinkability, has good suitability for packaging machines, and has excellent low-temperature heat sealability.

(Means for Solving the Problems) The present inventors have conducted extensive studies in order to solve the above-mentioned conventional problems and provide a polyethylene heat-shrinkable film that has good packaging machine suitability and excellent low-temperature heat sealability. As a result, they discovered that the object of the present invention could be achieved for the first time by using a linear copolymer of a specific ethylene and α-olefin and coextruding it with a specific layer composition ratio. This invention has been achieved.

That is, in the present invention, the intermediate layer has a density of 0.915 to 0.9308.
/C113, melt index 0.1-3.0g/
Linear copolymer of ethylene and α-olefin for 10 minutes (
A), the inner and outer layers have a density of 0.870 to 0.9153
/cn13, melt index 0.2-3.0g/
It is made of a linear copolymer (B) of 10-minute ethycine and α-olefin, and the ratio of the intermediate layer to all the layers of the stretched film after stretching is 50% or more, and the thickness of the inner and outer layers is at least 1 μm each. The present invention relates to a biaxially stretched polyethylene heat-shrinkable multilayer film characterized by an area shrinkage rate of 20% or more at 90°C.

The linear copolymer (A) of ethylene and α-olefin used in the intermediate layer of the present invention has a density of 0.915 to 0.930.
8/cm3, melt index 0.1-3.0g/
A material having characteristic values of 10 minutes is used, and more preferably a material having a density of 0.915 to 0.925 g/cm3 and a melt index of 0.2 to 3.0 g/10 minutes is used.

If the density is less than 0.9153/cm3, the effect of improving the stiffness of the film will be small, and if it exceeds 0.9308/cm3, the low-temperature shrinkability will be insufficient, which is not preferable.

Those with a melt index of less than 0.1 g/10 minutes are undesirable because the motor load during melt extrusion increases and processing suitability deteriorates, and furthermore, melt index of less than 0.2
g/10 minutes or more is more preferably used. Moreover, if it exceeds 3.0 g/10 minutes, the stability in the stretching process will be insufficient, which is not preferable.

The linear copolymer (A) of ethylene and α-olefin is
It may be used alone or in a mixture of two or more. The α-olefin copolymerized with ethylene is not particularly limited, and includes those having 4 to 12 carbon atoms, such as 1-butene, 1-pentene, 1-hexene, and 1-heptene.
1, octene-1,4-methylpentene-1, decene-
1, undecene-1, dodecene-1, etc., but α-olefins having 4 to 8 carbon atoms are more preferably used.

The linear copolymer (B) of ethylene and α-olefin used for the inner and outer layers has a density of 0.870 to 0.9158/c
m3, and a melt index of 0.2 to 3.0 g/10 minutes is used.

If the density is less than 0.8703/cm3, the heat sealing strength will be low, and if the density exceeds 0.9153/cm3, the heat sealing temperature will become high, which is not preferable.

If the melt index is less than 0.2 g/10 minutes, it is unfavorable because of a decrease in processability and a decrease in transparency due to roughening of the film surface, and if it exceeds 3.0 g/10 minutes, the heat sealing strength will decrease. Undesirable.

The linear copolymer (B) of ethylene and α-olefin is 1
It may be a single species or a mixture of two or more species. The α-olefin copolymerized with ethylene is not particularly limited, and includes those having 4 to 12 carbon atoms, such as butene-1, pentene-1, hexene-1, heptene-1
, octene-1,4-methylpentene-1, decene-1
, undecene-1, dodecene-1, etc., but α-olefins having 4 to 8 carbon atoms are more preferably used.

These linear copolymers of ethylene and α-olefin (
A) and (B) can be easily obtained by a low-medium pressure method using a so-called Ziegler-Natsuta type catalyst, and their production method is described in Japanese Patent Publication No. 5-32270,
JP-A-49-35345, JP-A-55-7800
Publication No. 4, JP-A-55-86804, JP-A-54
The technology disclosed in Japanese Patent No. 154488 and the like can be used.

The ratio of the open layer to the total layer needs to be 50% or more. If the ratio of the intermediate layer is less than 50%, the stiffness improvement effect of the film will be small and the suitability for packaging machines will be insufficient. Further, it is necessary to select the thicknesses of the inner and outer layers after stretching to be at least 1 μm or more each. If the thickness of the inner and outer layers after stretching is less than 1 μm, sufficient low-temperature heat sealability cannot be exhibited.

Furthermore, additives such as lubricants, anti-blocking agents, antistatic agents, and antifogging agents may be used as appropriate to provide their respective effective effects, as long as they do not impede the purpose of the present invention. Of course.

The heat-shrinkable multilayer film of the present invention is produced by a biaxial stretching method, and a tubular one-type production method will be described in detail below as an example.

First, the linear copolymer of ethylene and monoolefin (
Aμ intermediate layer, linear copolymer of ethylene and α-olefin (Bμ inner and outer layers are melt-kneaded using two extruders, coextruded through a three-layer annular die, and cooled once without stretching) Solidify to produce a tubular unstretched film.

Next, this tubular unstretched film is
The tube is supplied to a tubular stretching device as shown in the figure, and gas pressure is applied to the inside of the tube in a temperature range that allows orientation, and the tube is expanded and stretched by at least 2 times, preferably 2.5 times or more, in both the length and width directions to produce simultaneous biaxial stretching. Orient.

The film taken out from the stretching device can be annealed if necessary. This annealing can suppress natural shrinkage during storage.

(Examples) The present invention will be specifically explained below using Examples, but the present invention is not limited to these Examples.

The various measurements shown in this example were carried out by the following methods.

(1) Transparency (Haze) Using an integrating sphere type light transmittance measuring device based on JIS-6714, the ratio of scattered light transmittance to parallel light transmittance was expressed in %.

(2) Area shrinkage rate A film cut into a square of 10 cm in length and width was immersed in a glycerin bath at a predetermined temperature for 10 seconds, and the shrinkage was calculated using the following formula.

Area shrinkage rate = 10O-AXB However, A and B are the vertical and horizontal lengths after immersion, and the unit is c
mμ is shown.

(3) Heat-seal strength: Using a heat-seal bar with a width of 10 mm, after sealing under the conditions of heat-sealing pressure 1.5 kg/cm2 and heat-sealing time 1.0 seconds at each set temperature, the width of the heat-sealed film sample was 15 mm. A rectangular test piece was cut out and tested at a tensile speed of 300 using a Tensilon tensile tester.
A tensile load was applied at nn+/min, T-shaped peeling was performed, and the heat seal strength was measured.

(4) Young's Modulus Test pieces are taken from the film sample in the MD (longitudinal direction) and TD (horizontal direction), each having a width of 15 mm and a length of 300 mm, and the thicknesses thereof are measured.

Next, the test piece was placed in a universal tensile testing machine manufactured by Toyo Saiki Co., Ltd. with a spacing of 50 mm, and the tensile speed was 40 mm/mi.
n, a recording paper speed of 500 mm/min, and a full scale of 2 kg, and was calculated using the following formula.

However, P: Full scale strength (kg) S: Cross-sectional area of the film (cm2) ΔL: Distance from the load-deformation curve shown in Fig. 2 to L2 (mm) L: Gripping interval of the test piece Example I Density 0.9203/cm3, melt index 2.
0g/10min linear low density polyethylene (product name: IJ
It, ZEX 202OL Mikata Petrochemical μ intermediate layer, density 0.890g/cm3. Melt index 1.
og/10 min linear low density polyethylene (product name:
NUC-FLX DFDA-1210 manufactured by Nippon Unicar μ 200 ~
3P melted and kneaded at 250°C and kept at 250°C! It was coextruded downward from an annular die. At this time, the ratio between the inner and outer layers and the middle layer was set as shown in Table 1. The slit diameter of the three-layer annular die is 75 mm, and the slit gap is 0.
It was 8 mm.

The coextruded molten tubular film was slid on the outer surface of a cylindrical mandrel with an outer diameter of 66 mm installed directly below the die and inside which circulated cooling water at 20°C, while the outer surface was passed through a water tank. The film was cooled with water and then cooled to room temperature to obtain a tubular unstretched film having a diameter of about 65 mm+ and a thickness of 320 μm.

This raw unstretched film was introduced into the biaxial stretching apparatus shown in FIG. 1, and stretched 4 times in length and width at 95 to 1.05°C. The stretched film was annealed with hot air at 75° C. for 10 seconds in a tubular annealing device, cooled to room temperature, folded, and rolled up.

The obtained stretched film has a thickness of 20.2μ (thickness of the inner and outer layers is 3.071 from the set layer ratio) and a haze of 2.6%.
The area shrinkage rate at 90° C. was 28.8%.

Further, the heat seal strength at 135°C was sufficiently large to cause no practical problems. The results including other data are shown in Table 1.

Using this film, a bag making, filling and packaging machine connected with a shrink tunnel was used to create a bag with a width of 80 mm, a length of 160 mm, and a height of 4 mm.
When wrapping paper containers containing 0mm rice crackers, the film ran very smoothly on the packaging machine, and there was no sealing failure at the heat sealing part, resulting in a good packaging that tightly adhered to the container. No change was observed in the packaged items.

Example 2 Film formation, stretching, and annealing were carried out in the same manner and under the same conditions as in Example 1, except that the ratio of the inner and outer layers to the intermediate layer was changed to 10/90.

The obtained stretched film had a thickness of 20.8 μm, a haze of 3.5%, and an area shrinkage rate at 90° C. of 24.8%. Furthermore, the heat sealing strength was 135°C, a large value that poses no practical problem.

The results, including other data, are shown in Table 1.

When this film was used for shrink-wrapping in the same manner as in Example 1, there were no troubles with the packaging machine, no sealing defects at the heat-sealed part, and the packaging was in a good state, tightly adhering to the container.

Example 3 Film formation, stretching, and annealing were performed in the same manner and under the same conditions as in Example 1, except that the ratio of the inner and outer layers to the intermediate layer was changed to 40,760.

The obtained stretched film had a thickness of 20.1 μm, a haze of 2.4%, and an area shrinkage rate at 90° C. of 30.2%. The heat seal strength showed a sufficient value at 135°C. The results, including other data, are shown in Table 1.

Example 4 Density 0.9233/cm3, Mel I index 0
．． 8g/10min linear low density polyethylene (product name: N
eozex 2006H Mikata Petrochemical μ intermediate layer, density 0.906g/clTI3, melt index 0.8
g/10 min linear low density polyethylene (product name:
NtJC-FLX DFDA-11'', 37 ro Hon Unicar μ inner and outer layers, the thickness of the unstretched film was 190 ll, the stretching temperature was 95 to 105°C, and the length and width were stretched 23 times each. Film formation, stretching, and annealing were performed using the same method and conditions as in Example 1.

The obtained stretched film has a thickness of 20.6μ, a haze of 3.2%, and an area shrinkage rate of 24.3% at 90°C.
Met. The heat sealing strength showed a sufficient value at 135°C. The results, including other data, are shown in Table 1.

Comparative Example 1 Film formation, stretching, and annealing were performed in the same manner and under the same conditions as in Example 1, except that the inner and outer layers of Example 1 were made of the linear low-density polyethylene used in the intermediate layer of Example 4.

j! ) The thickness of the stretched film was 21.2μ,
The haze is 4.3%, and the area shrinkage rate at 90°C is 18.
3%, but the heat sealing strength was almost negligible at 135°C, and the heat sealing start temperature was 145°C, about 10°C higher than Examples 1-4.

Table 1 includes other physical property data.

When this film was subjected to the same shrink wrapping test as in Example 1, no particular problem was observed in the running properties of the film, but under the same heat sealing conditions, the sealing properties were poor and punctures due to poor sealing properties occurred. It happened in the shrink tunnel. Therefore, a shrink wrapping test was conducted with the sealing temperature raised by about 10° C., but the film around the sealed portion shrank and the condition of the sealed portion was not fully satisfactory.

Comparative Example 2 Density 0.9123/cm3, Melt Index 1.
0g/10min linear low density polyethylene (Product name: D
awlex 4001 manufactured by Dow Chemical, the intermediate layer was used, the linear low-density polyethylene used in Example 4 was used as the inner and outer layers, and the stretching temperature was 90 to 100°C. Film, stretching, and annealing were performed.

The obtained stretched film had a thickness of 20.2 μm, a haze of 2.7%, and an area shrinkage rate at 90° C. of 28.9%. Further, the heat seal strength was a sufficient value at 135°C. Table 1 includes other physical property data.

When a shrink wrapping test similar to that in Example 1 was conducted using this film, the running properties of the film were not smooth.
Stretching and twitching were observed in some areas, and the edges of the film in the sealed area were not aligned, resulting in poor sealing.

Comparative Example 3 Same as Example 1 except that in Example 1, the density was 0.9123/cm3, the melt index was 3°3g/10 min linear low density polyethylene (trade name: Dowlex 4002 manufactured by Dow Chemical) and the μ inner and outer layers were used. Film formation, stretching, and annealing were carried out using different methods and conditions.

The thickness of the stretched film obtained was 20.7μ, and...
The size was 3.5% and the area shrinkage rate at 90°C was 25.8%, but the heat seal strength was lower than Examples 1 to 4. When a shrink wrapping test similar to Example 1 was performed, Punctures sometimes occurred in the shrink tunnel due to weak seal strength.

Table 1 includes other physical property data.

Comparative Example 4 Film formation, stretching, and annealing were performed in the same manner and under the same conditions as in Example 1, except that the ratio of the inner and outer layers to the intermediate layer was changed to 5/95.

The obtained stretched film had a thickness of 20.8 μm, a haze of 3.6%, and an area shrinkage rate at 90° C. of 22.5%. The heat sealing strength at 135°C was that of Examples 1 to 4.
It was weaker than. Table 1 includes other physical property data.

When this film was subjected to the same shrink wrapping test as in Example 1, there was no problem with the film runnability in the packaging machine, but under the same heat sealing conditions, the sealing performance was poor and punctures due to poor sealing performance occurred in the shrink tunnel. It happened in

[Hereinafter, blank space (effects of the invention) The polyethylene heat-shrinkable multilayer film of the present invention is constructed using materials that satisfy specific conditions as raw materials for each layer, so it has transparency and low-temperature shrinkability. It is possible to stably produce a heat-shrinkable multilayer film that has good packaging machine suitability and excellent low-temperature heat sealability.

[Brief explanation of the drawing]

FIG. 1 is an explanatory cross-sectional view of a biaxial stretching apparatus used in an example of the present invention. FIG. 2 is a schematic diagram for explaining a load-deformation curve for calculating Young's modulus in the example.

Claims (1)

[Claims] 1) The intermediate layer has a density of 0.915 to 0.9308/cm^3
, made of a linear copolymer (A) of ethylene and α-olefin with a melt index of 0.1 to 3.0 g/10 min, and the inner and outer layers have a density of 0.870 to 0.915 g/cm^3,
Consisting of a linear copolymer (B) of ethylene and α-olefin with a melt index of 0.2 to 3.0 g/10 min,
A biaxially stretched polyethylene heat-shrinkable product, characterized in that the ratio of the intermediate layer to all layers is 50% or more, the thickness of the inner and outer layers is at least 1μ or more, and the area shrinkage rate at 90°C is 20% or more. multilayer film. 2) Linear copolymer of ethylene and α-olefin (A)
Claim 1, wherein the α-olefin is an α-olefin selected from the group consisting of 1-butene, 1-pentene, 1-hexene, 1-octene, and 1-4-methylpentene. The polyethylene heat-shrinkable multilayer film described above. 3) Linear copolymer of ethylene and α-olefin (B)
The polyethylene heat-shrinkable multilayer film according to claim 1, wherein is an ethylene-butene-1 copolymer.