Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
HEAT SHRINKABLE POLYETHYLENE LAMINATE FILM
TECHMCAL FIELD
The present invention relates to a heat
shrinkable polyethylene laminate film, and more
particularly to a heat shrinkable laminate film made of
specific ethylene copolymers, having an excellent
applicability to packaging machines.
BACKGROUND ART
Stretched films of polyvinyl chloride,
polypropylene, polyethylene and the like are hitherto
known as heat shrinkable films. Among them, heat
shrinkable polyethylene films have been put to practical
use from the viewpoints of possession of heat sealability
and low cost. Particularly, in recent years, a heat
shrinkable polyethylene film using a linear low density
copolymer of ethylene and an a -olefin (hereinafter
referred to as " linear low density polyethylene" )
attracts attention in that the impact resistance and the
heat seal strength are excellent, and its utilization in
various field is expected.
The present inventors previously proposed a heat
shrinkable film composed mainly of a specific ethylene/
a -olefin copolymer (Japanese Patent Publication Kokai No.
62-201229). By practicing this proposed method, it became
possible to obtain a film having a less unevenness in
thickness and having a better low temperature
shrinkability than films obtained by inflation. However,
in case of applicating to automatic packaging machines
(such as pillow type packaging machine and automatic
packaging machine using centerfold films), the proposed
film encounters a problem that a heat seal failure
(non-sealing in part) which has not been generated, is
generated because the packaging speed of packaging
machines is markedly increased in recent years. When
~~~.~~~~z
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packaging is made by automatic packaging machines,it is
general to conduct the heat sealing by bead sealingusing
a heat knife. The sealing failure phenomenon means
that a
sealed portion is peeled off in a shrinking step, film
a
is not well cut off with causing stringiness
at sealed
portion when a large tension is applied to the (for
film
example, when bulky goods are packaged), pinholes are
generated at sealed portion, or in an extreme casefilm
a
is not sealed at all.
The sealing failure as mentioned above also
occurs when a molten resin adheres to the edge heat
of a
knife or to a heat knife pedestal.
DISCLOSURE OF THE INVENTION
As a result of making further investigation
thoroughly in order to eliminate the above-mentioned
sealing failure, the present inventors have confirmed the
following matters.
That is to say, time until a film is shrinked in
a shrinking tunnel is shortened with increase of packaging
speed, and when solidification of a molten resin does not
proceed within such a short time, a part of the molten
resin is drawn apart to produce pinholes, or in an extreme
case to cause a complete peeling of the sealed portion.
Also, in case that a film lacks firmness (in case
that the modulus of tensile elasticity is small), the film
is easy to crease at the time of running and, therefore,
folding of the film increases at the portion to be sealed
by bead sealing, resulting in increase of pinhole
formation.
Also, in case that running property of a film in
a packaging machine is insufficient because the film is
poor in slipping property, or in case that articles to be
packaged is bulky, the bead seal portion is subject to a
larger tension, and a part of the seal portion is drawn
apart prior to solidification of the molten resin, thus
increasing formation of pinholes.
Further, in case that the viscosity of the molten
a ~~;~~7
~~, J4.,ti;.,
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resin is low, the molten resin is easy to adhere to the
edge of a heat knife and a pedestal for the heat knife,
thus pinholes are formed at sealed portion, or the film is
not cut, or in an extreme case the film is not sealed at
all.
In order to eliminate the above-mentioned
problems and to provide a heat shrinkable film having a
good applicability to packaging machines, the present
inventors have made a study about a lamination
constitution of films and various kinds of raw material
resins. As a result, the present inventors have found
that a heat shrinkable film having a good packaging
machine applicability is obtained by using a resin having
a low melt index and a high total heat of fusion and
having a proportion of an endothermic area above the
melting point which is not less than a certain range, in
an intermediate layer in order to accelerate the cooling
solidification speed at the time of bead sealing, and by
using a resin having a higher melt index than the
intermediate layer and similar characteristics to those of
the intermediate layer in the innermost and outermost
layers so as not to impair the transparency. Thus, the
present inventors have arrived at the present invention.
That is to say, the present invention relates to
a heat shrinkable polyethylene laminate film biaxially
stretched, having an excellent packaging machine
applicability, characterized by having a modulus of
tensile elasticity of at least 3, 000 kg/cm2 and an area
shrinkage of at least 20 % at 90 C and including as an
intermediate layer at least one layer made of a
composition comprising, as a main component, (A) a linear
low density polyethylene having a density of 0.910 to
0.930 g/cm3 and a melt index of 0.1 to 0.8 g/10 minutes
and showing a fusion curve by DSC wherein the total heat
of fusion is at least 135 mJ/mg in a fusion curve obtained
when the temperature is kept at 190 C for 30 minutes,
dropped down to 20°C at a temperature dropping rate of
10 C /minute and then raised at a temperature rising rate
CA 02118002 2003-O1-08
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of 10C /minute,
and the endothermic
area in the
range of
not lower than the main peak temperature (melting
temperature) is at least 12 % of the total endothermic
area, and including, as an innermost layer and an
outermost
layer, layers
made of a composition
comprising,
as a main component, (B) a linear low density polyethylene
having a melt
index of 0.8
to 5.0 g/10
minutes wherein
the
total heat of fusion in the fusion curve is from 135 to
160 mJ/mg and the endothermic area in the range of not
lower than the main peak temperature is at least 12 % of
the total endothermic area, the thickness of said
intermediate layer being at least 60 % of all layers.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a sectional view illustrating a tubular
biaxial stretching apparatus used in Examples.
Fig. 2 is a schematic view illustrating a stress-
strain curve for calculating a modulus of tensile
elasticity in the Examples.
BEST MODE FOR CARRING OUT THE INVENTION
CA 02118002 2003-O1-08
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The linear low density polyethylene (A) used in
the present invention as a main component of at least one
15 layer, an intermediate layer, is those having a density of
0.910 to 0.930 g/cm3 and a melt index of 0.1 to 0.8 g/10
minutes, preferably those having a density of 0.915 to
0.925 g/cm3 and a melt index of 0.2 to 0.7 g/10 minutes.
When the density is less than 0.910 g/cm3, the modulus of
20 tensile elasticity is lowered, and when the density is
more than 0. 9 3 0 g/cm3, the low ' temperature shrinkability
is insufficient. Those having a melt index of less than
0.1 g/10 minutes are not preferable in that the load on a
motor at the time of melt extrusion is increased, so the
2 5 processability deteriorates. When the melt index is . more
than 0.8 g/10 minutes, the sealability by bead sealing
deteriorates.
Also, the above-mentioned linear low density
polyethylene (A) is required to be that in the fusion
30 curve in DSC measurement the total heat of fusion is not
less than 135 mJ/mg and the endothermic area in the range
of not lower than the main peak temperature is not less
than 12 °~ of the total endothermic area. Those not
satisfying this requirement do not provide a good
35 sealability by bead sealing, since the cooling
solidification speed of molten resin is slow.
It is necessary that the thickness of the
intermediate layer is not less than 60 % of the total
~i_.~oJ~~,..,
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thickness of all layers. When the thickness of the
intermediate layer is less than 60 %, no excellent bead
sealing sealability is exhibited.
The linear low density polyethylene (B) used as
the main component of each of the innermost layer and the
outermost layer is those having a melt index of 0.8 to 5.0
g/10 minutes and having a total heat of fusion in the
fusion curve within the range of 135 to 160 mJ/mg and an
endothermic area in the range of not lower than the main
peak temperature of at least 12 % based on the total
endothermic area.
When the melt index is less than 0.8 g/10
minutes, the transparency is lowered owing to roughening
of the film surface, and when the melt index is more than
5.0 g/10 minutes, heat seal strength is lowered and it
also exerts a bad influence on stretching processability.
Also, when the total heat of fusion is less than 135
mJ/mg, no good bead sealing sealability is obtained, and
when it is more than 160 mJ/mg, lowering of the
transparency is observed.
The above-mentioned linear low density
polyethylenes (A) and (B) are linear copolymers of
ethylene and an a -olefin. The a -olefin to be
copolyrnerized with ethylene is not particularly limited.
For instance, there are mentioned a -olefins 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 and the like. Alpha-olefins having
4 to 8 carbon atoms are more preferably used. These
linear copolymers (A) and (B) of ethylene and a -olefin
can be easily obtained by a low pressure or medium
pressure method using a Ziegler-Natta catalyst. The
preparation of these copolymers can be made according to a
technique disclosed, for instance, in Japanese Patent
Publication Kokoku No. 50-32270 and Japanese Patent
Publication Kokai No. 49-35345, No. 55-78004, No. 55-86804
and No. 54-154488.
Also, the resin compositions used in each of the
above-mentioned layers may be used alone or in admixture
thereof, and further they can be used with polyolefin
resins such as high pressure polyethylene, ethylene-vinyl
acetate copolymer, ionomer, ethylene-propylene copolymer
and the like so long as the objects of the present
invention are not hindered.
In addition to the intermediate layer, the
innermost layer and the outermost layer, the laminate film
of the present invention may include one or more
intermediate layers made of a polyolefin resin other than
the above-mentioned linear low density polyethylene resins
(A) and (B), so long as the above-mentioned requirement
for the thickness of the respective layers is satisfied.
Examples of the polyolefin resin used in such intermediate
layers are linear low density polyethylene resins other
than the above-mentioned linear low density polyethylene
resins (A) and (B), high pressure polyethylene, ethylene
propylene copolymer and others. They can be suitably
selected and used alone or in admixture thereof so long as
the objects of the present invention are not hindered.
In addition, additives such as a lubricant, an
anti-blocking agent, an antistatic agent and an anti-
fog agent can be suitably used for the purpose of
imparting their effective actions, as occasion demands.
They are particularly effective for use in the innermost
layer and the outermost layer.
The preparation of non-stretched films used in
the present invention and the stretching can be conducted
by known methods. The detailed explanation is given below
taking the case of preparation and stretching of a three-
layered tubular laminate film.
Firstly, the linear low density polyethylene (A)
of ethylene and a -olefin and the linear low density
polyethylene (B) of ethylene and a -olefin are melt-
kneaded by three extruders, co-extruded in a tubular form
through a three-layered circular die so that the linear
low density polyethylene (A) forms an intermediate layer
and the linear low density polyethylene (B) forms an
CA 02118002 2003-O1-08
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innermost layer and an outermost layer, and once
solidified by rapid ~ cooling without stretching to give a
tubular non-stretched film.
The obtained tubular non-stretched film 1 is fed to .
a tubular stretching apparatus A, for instance, as shown in
Fig. 1, and is subjected to a simultaneous biaxial
orientation by inflation stretching within a temperature
range capable of a high degree of orientation, for
example, at a temperature lower than the melting point of
the intermediate layer resin by at least 10°C , preferably
at least 15°C , applying a gas pressure to the inside of
the tube. The stretching ratios are not always required
to be the same in the longitudinal and transverse
directions, but it is preferable for obtaining excellent
physical properties such as strength and rate of shrinkage
to stretch the film at least 2 times, preferably at least
2.5 times, more preferably at least 3 times, in both the
longitudinal and transverse directions. In Fig. 1, numeral 2 is low
speed nip rolls, and numeral 3 is high speed nip rolls. Numeral 4 is a
pre-heater, numeral 5 is a main heater, numeral 6 is a cooling air ring,
and numeral 7 is collapser rolls.
The film taken out of the stretching apparatus A can be
annealed if desired, and spontaneous shrinkage during the storage can
be inhibited by this annealing.
The present invention is more specifically
explained by means of the following Examples, but is not
limited to the Examples.
The measurements shown in the Examples were made
35 by the following methods.
1) Total heat of fusion
After weighing out 8 to IO mg of a sample, it was
sealed in an aluminum pan. In a differential scanning
~~~ ~~'~:~
-
calorimeter (model DSC-100 made by Seiko Denshi Kabushiki
Kaisha), the sample was heated in a nitrogen stream of 30
m.~ /minute from room temperature to 190 C , kept at that
temperature for 30 minutes and then cooled to room
temperature at a rate of 10 C /minute. Thereafter, a
fusion curve was obtained at a rate of temperature rise of
C /minute and the heat of fusion was calculated from the
area of endothermic peak by using the fusion curve.
2) Proportion of endothermic area
10 Proportion of the area in the range of not lower
than the main peak temperature (melting point) based on
the total endothermic area in the above-mentioned fusion
curve was represented by percentage (%).
3 ) Thickness of each layer
The thickness of each layer of built-up layers
was measured by observing the cross section of a film with
a microscope.
4 ) Haz a
An integrating sphere type light transmittance
measuring device according to JIS K 6714 was used, and
proportion of the transmittance of scattered rays based on
the transmittance of parallel rays was represented by
percentage (°/).
5) Area shrinkage
A film cut in a square of 10 cm x 10 cm was
dipped in a glycerol bath at a predetermined temperature
for 10 seconds, and the rate of area shrinkage was
calculated according to the following equation.
Rate of area shrinkage = 100 - A X B
wherein A and B are lengths (cm) in the machine and
transverse directions after dipping.
6) Modulus of tensile elasticity
A specimen was cut from a film sample in 15 mm
width in MD (machine direction) and 300 mm length in TD
(transverse direction), and its thickness was measured.
The specimen was then gripped and attached at 50
i. ~.i
9 _
mm spacing to a universal tensile tester made by Kabushiki
Kaisha Orientech, and was measured under conditions of
rate of tension 40 mm/minute, rate of recording paper 500
mm/minute and full scale 2 kg. The calculation was made
according to the following equation.
Modulus of tensile elasticity =
0 L/L
P/S
wherein P is a strength (kg) of full scale, S is a
sectional area (cm2) of a film, p L is a distance (mm)
between L1 and L2 in the stress-strain curve shown in Fig.
2, and L is a grip spacing of a specimen.
7) Sealability by bead sealing
A film folded in half having a width of 400 mm
was fed to an automatic packaging machine of the type
using centerfold films (model AT-500) made by Kyowa Denki
Kabushiki Kaisha, and 100 pieces of a lunch box (200 g)
having a length of 23.5 cm, a width of 15.5 cm and a
height of 5.6 cm were continuously wrapped at a rate of 25
pieces/minute to measure the percent non-defective.
Percent non-defective 100 % in the sealing temperature
range of 220° to 250 C was estimated as Q , percent
non-defective between less than 100 % and 80 % was
estimated as p , and percent non-defective of less than 80
% was estimated as X .
With respect to the standard of the percent non
defective, a case where no stringiness and no pinhole
having a size of at least 1 mm are observed in a sealed
portion after shrink packaging, is regarded as non
defective.
Also, the percent margine in pre-packaging was
set 13 % in each of length and width.
Example 1
A linear low density polyethylene resin having
characteristics as shown in Table 1 which was a copolymer
of ethylene and 4-methylpentene-1 comonomer, for an
intermediate layer and a linear low density polyethylene
~'~~ :~i9~~'~~,
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having characteristics as shown in Table 1 which was a
copolymer of ethylene and octene-1 comonomer, for inner
and outer layers were melt-kneaded at a temperature of
170° to 240 C respectively in three extruders (for the
intermediate layer, for the innermost layer and for the
outermost layer). The extrusion outputs from the
respective extruders were set under the estimation of the
thickness ratio shown in Table 1, and they were co
extruded downward from a three-layer circular die kept at
240 C .
The formed three-layer tube was cooled by sliding
the inner side on the outer surface of a cylindrical
cooling mandrel wherein a cooling water was circulated and
by passing the outer side through a water bath, and drawn
to give a non-stretched film having a diameter of about 75
mm and a thickness of 320 ,u m. The adjustment of
thickness of each layer was conducted by adjusting the
number of revolutions of a screw of an extruder and a
drawing speed. This tubular non-stretched film was led to
a tubular biaxial stretching apparatus shown in Fig. 1 and
stretched 4 times in both machine and transverse
directions at a temperature of 95° to 105 C to give a
biaxially stretched laminate film. The stretched film was
then treated with a hot air of 75°C for 10 seconds in a
tube annealing apparatus, cooled to room temperature,
collapsed and taken up.
The stability during the stretching was good,
vertical motion of the stretching point and swaying of the
stretched tube did not occur, and also nonuniform
stretching state such as necking was not observed. The
obtained stretched film had a thickness constitution as
shown in Table 1, and had excellent transparency and low
temperature shrinkability and a high modulus of tensile
elasticity. Also, in estimation by actual packaging of
lunch boxes using an automatic packaging machine of the
type using centerfold films, there was no failure of
sealed portion, thus the film had a good packaging machine
applicability in a wide temperature range:
N ,~ ,~. J J ~'~
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Example
2
A heat shrinkable laminate film was
prepared
in
the same manner as in Example 1 using the resin
constitution shown Table 1. The obtainedstretched
in
film had excellent transparencyand low temperature
shrinkability and a high modulusof tensileelasticity.
Also, like Example the film an excellentpackaging
1, had
machine applicability.
Example 3
A heat shrinkable laminate film was prepared
in
the same manner as in Example 1 using the resin
constitution shown in Table 1. the inner and outer
To
layers was added 5, ppm of stearic
000 acid monoglyceride
as an anti-fog agent.The film excellent transparency
had
and low temperature shrinkability,a high modulus of
tensile elasticity and an excellent
packaging
machine
applicability.
Comparative Example 1
Using a linear low density polyethylene having a
melt index of 2.0 g/10 minutes in the intermediate layer
and using the same linear low density polyethylene resin
as that used for the inner and outer layers in Example 1
in the inner and outer layers, they were extruded under
the same conditions as in Example 1, cooled and drawn to
give a non-stretched laminate film having a diameter of
about 75 mm and a thickness of 320 ,u m. The adjustment of
thickness of each layer was conducted by adjusting the
number of rotations of a screw of an extruder and the
drawing speed.
In the same manner as in Example 1, this tubular
non-stretched film was led to the tubular biaxial
stretching apparatus shown in Fig. 1 and stretched 4 times
in both machine and transverse directions at a temperature
of 95° to 105 C to give a biaxially stretched laminate
film. This stretched film was then treated with a hot air
of 75°C for 10 seconds in a tube annealing apparatus,
w~?.~~r~
- 12 -
cooled to room temperature, collapsed and taken up.
There was no problem in the stability during
stretching and the obtained film had excellent
transparency and low temperature shrinkability, but the
modulus of tensile elasticity was small. Estimation of
the applicability of this film to packaging machine showed
that adhesion of a resin to a bead sealing bar and
stringiness were somewhat improved, but pinholes were easy
to be formed, so the sealability was insufficient.
Comparative Examples 2 and 3
Heat shrinkable laminate films were obtained in
the same manner as in Example 1 by using in the
intermediate layer for Comparative Example 2 a linear low
density polyethylene whose total heat of fusion in the
fusion curve was 132.0 mJ/mg which was not more than 135
mJ/mg, and by using in the intermediate layer for
Comparative Example 3 a linear low density polyethylene
whose ratio of endothermic area in the range of not less
than melting point in the fusion curve was 11.0 % which
was not more than 12 % and by using in the inner and outer
layers for both Comparative Examples 2 and 3 the same
linear low density polyethylene as in Example 1.
Both heat shrinkable films obtained in
Comparative Examples 2 and 3 had excellent transparency
and low temperature heat shrinkability, but their modulus
of tensile elasticity was small. In actual packaging test
using a packaging machine, both films were easy to form
pinholes in sealed portion and were insufficient in
sealability.
Comparative
Example 4
A heat shrinkablelaminate filmwas obtainedin
the same manner as in Example 1 using in the
by
intermediatelayer the same linear low density
polyethylene as in Example and by using in the innerand
1
outer layers density polyethylene a
a linear having
low
melt index of 0.6 g/10 not within the
minutes which
was
~~~ 3~v~
- 13 -
range of 1.0 to 5.0 g/10 minutes. The obtained film had
an excellent low temperature shrinkability and a large
modulus of tensile elasticity and showed a good
sealability by bead sealing in actual packaging test using
a packaging machine, but it was poor in transparency.
Comparative Example 5
A heat shrinkable laminate film was obtained in
the same manner as in Example 1 by using in the
intermediate layer the same linear low density
polyethylene as in Example 2 and by using in the inner and
outer layers a linear low density polyethylene whose total
heat of fusion in the fusion curve was 121 mJ/mg which was
not within the range of 135 to 160 mJ/mg. The obtained
film had excellent transparency and low temperature
shrinkability, but it was somewhat inferior in modulus of
tensile elasticity, and adhesion of a resin to a heat
knife and sticking of the film to a heat knife pedestal
were observed in the actual packaging test using a
packaging machine, and a hole having a length of about 3
mm was sometimes formed in sealed portion, so the
sealability was very unstable.
Comparative Example 6
A heat shrinkable laminate film was obtained in
the same manner as in Example 1 by using in the
intermediate layer the same linear low density
polyethylene as in Example 1 and by using in the inner and
outer layers a linear low density polyethylene whose total
heat of fusion in the fusion curve was 162.4 mJ/mg which
was more than 160 mJ/mg. The stability in the stretching
step was bad, and the obtained film was inferior in
transparency and low temperature shrinkability. In the
actual packaging test using a packaging machine, the
sealed portion was easy to be wrinkled and some pinholes
were observed.
- 14 -
Comparative Example 7
A heat shrinkable laminate film was obtained in
the same manner as in Example 1 by using the same linear
low density polyethylenes as Example 1 in the intermediate
layer and the inner and outer layers except that the
proportion of the thickness of the intermediate layer was
made 50 % of the entire thickness. The obtained film were
excellent in transparency and low temperature
shrinkability, but some pinholes were observed in sealed
portion, so the sealability was insufficient.
INDUSTRIAL APPLICABILITY
The heat shrinkable laminate film of the present
invention provides a shrinkable film which has excellent
transparency and low temperature shrinkability and a high
rate of cooling solidification of a molten resin at the
time of bead sealing, because of being formed using raw
materials satisfying specific conditions as those for
respective layers, and which has an excellent packaging
machine applicability because of using raw materials which
give a high modulus of tensile elasticity.
rat
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