Note: Descriptions are shown in the official language in which they were submitted.
1- t3~8~L62
TITLE OF THE INVENTION:
LINEAR POLYETHYLENE FILM AND PROCESS
FOR PRODUCING THE SAME
BACKGROUND OF THE INVENTION:
. . _ _ _
The present invention relates to a film
having high tear and impact strength,
stif~ness and tensile strength in the longitudinal direction
(stretched direction) and a process for producing the film.
More particularly the invention provides a film for packaging
bags which comprises principally a linear polyethylene,
i5 capable of greater reduction of thickness than possible
with the conventional films and is suitable for packaging
the relatively heavy articles such as rice, grain,
fertilizers, etc.
High-pressure low-densi~y polyethylenes produced
by radical polymerization under the high-temperature and
high-prassure conditions have been popularly used as the
material of bags or sacks for packaging heavy articles.
In recent years, however, such high-pressure low-de~sity
polyethylenes have been rapidly superseded by linear
polyethylenes, especially lineax low-density polyethylenes.
The lineax low-density polyethylenes with few
branches produced from copolymerization of ethylene and
a-olefins have many excellent properties, especially
strength properties such as tensile strength, impact
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- 2 - ~32~2
strength and stiffness, environmental stress cracking
resistance (ESCR), heat resistance, heat-sealing charac-
teristics, etc., in comparison with the high-pressure low-
density polyethylenes and are used not only as a material
for films for packaging bags but also in various
other fields.
The non-stretched films or sheets produced from linear low-
density polyethylenes according to a T-die or blown film forming
(inflation forming) are incapable of reduct~on of thickness
to an extreme extent due to restrictions on forming opera-
tions. Further, such non-stretched films or sheets are
poor in strenyth. To eliminate such problems, it has been
proposed to subject such films or sheets to a stretching
treatment.
A preferred mode of such treatment is biaxial
stretching of the non-stretched films or sheets. This
treatment, however, necessarily leads to an increased
equipment cost. Also, very strict control of operations
is required because of the narrow range of stretching
conditions. For these reasons, such biaxial stretching
has ac~ually been practiced for production of the films or
sheet to be used for certain specific purposes.
The known techniques of longitudinal monoaxial
stretching require no high equipment cost and are also
easy in operational control, but the involve the problem
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_ 3 _ ~3~ 2
in anisotropy of the stretched film properties, especially
tear strength and surface strength in the longitudinal
direction (stretched direction) and were unable to produce
the stretched films which could safely be put to practical
use.
Generally, the packaging bags made by
using linear polyethylenes can be made smaller in thickness
and have higher strength than those made by using high-
pressure low-d~nsity polyethylenes, as for instance dis-
closed in Japanese Patent ~pplication Laid-Open (KOKAI)
No. 61-18313~ (1986). But the bags made of
linear polyethylenes lack the strength and tensile strength
at break of the body portion for realizing a further
reduction of thickness. Japanese Patent Application
Laid-Open (KOKaI) No. 62-32134 proposes two-layer laminating
of the stretched films to impxove tensile strength at
break of-the body portion. In a bag produced
from such films, however, when the bag is dropped
with the .~ealed portion down, the bo y portion of the bag
could be elongated in the circumerential direction
and deformed, so that such bag is not suitable for
practical use. Also, such packaging bag because
of its poor stiffness, had the problem of improper filling
due to adhesion of the inner aces thereof or buckling of
the body portion at the time of automatic filling.
_ 4 _ 1 328~ 62
In view of the above, the present inventors have
pursued their studies for eliminating said problems of
the prior art and producing a thin (stretched) film with
excellent strength properties by using linear polyethylenes
as the material and, as a result, found that a film having
a specific range of heat shrinkage obtained by molding a
specific linear polyethylene into a non-stretched film or
sheet and stretching it under the specific conditions has
excellent tear strength, impact strength, stiffness and
tensile strength even when reduced in thickness, and that
use of such film for making a packaging bag can
improve the stiffness of the packaging bag and
its deformation properties under dropping while maintaining
the strength of the body portion. The present invention
have been accomplished on the basis o this finding.
SUMMAR~ OF THE INVENTION:
In a first aspect of the present invention, there
is provided a mono- or biaxially stretched film having a
heat shrinkage of 20% or more in one of the longi~udinal
and transverse directions and 60% or more in the other
direction, comprising 100 to 50 parts by weight of a linear
polyethylene having a density of 0.910 to 0O965 g/cm3, a
melt index of 2 g/10 min or less and a fluidity ratio of
50 or below;
.. . . . . . .
. . . .
~32~1~2
~ 5 -
0 to 50 parts by weight of a branched low-density
polyethylene having a melt index of 2 g/10 min or less,
a fluidity ratio of 70 or below and a density of 0.930 g/cm3
or below; and 0.0001 to 0.1 part by weight of a radical
initiator as an op~ional component.
In a second aspect of the present-invention, there
is provided a process for producing a mono- or biaxially
stretched film having a heat shrinkage of 20% or more in
one of the longitudinal and transverse directions and 60%
or more in the other direction produced by monoaxially stretching
in the longi-tudinal or transverse.direction.or biaxially stretching. .
which comprises a composition of 100 to 50 parts by weight
of a linear polyethylene having a density of 0.910 to
0.965 g/cm3, a melt index of 2 g/10 min or less and a
fluidity ratio of 50 or below; o to 50 parts by weight
of a branched low-density polyethylene having a melt index
of 2 g/10 min or less, a fluidity ratio of 70 or below and
a density of 0.930 g/cm3 or below; and 0.0001 to 0.1 part
by weight of a radical initiator as an optional component.
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-- 6 --
In a third aspect of the present invention, there
is provided a longitudinally monoaxially stretched film
having a heat shrinkage of 20% or more in one of the
longitudinal and transverse directions and 60% or more in
the other direction, comprising 100 to 50 parts by weight
of a linear polyethylene having a density of 0.91 to 0.965
g/cm3, a melt index of 2 g/10 min or less and a fluidity
ratio of higher than 50 and not higher than 120,
and 0 to 50 parts by weight of a branched
low-density polyethylene having a density of 0.930
g/cm3 or. below, a melt index of 2 g/10 min or less and a
fluidity ratio of 70.or below.
In a fourth aspect of the present invention,
there is provided a proces~ for producing a film
which comprises subjecting a composition comprising
100 to 50.parts by weight of a linear polyethylene having
a density of 0.91 to 0.965 g/cm3, a mel~-index of 2 g/10 min
or less and a 1~idity ratio of higher.than-50 .
and not higher than 120, and 0 to 50 parts
by weight of a branched low-density polyethylene having
a density of 0.930 g/cm3 or below, a melt index of 2 g/10 min
or...less and a fluidity ratio of 70 or below, to blown
film forming under the conditions o~ blow-up
ratio of 2 to 8 and frost line height of 2 to 50 times the
die diameter, to obtain a non-stretched film or sheet, and
monoaxially stretching said non-stretched film or sheet by
l.S to 8 times the original length in the longitudinal
direction at a temperature of Tm - 70C to Tm - 20C
.: . ~. . .
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- 7 _ ~32~2
wherein Tm is the melting point of said composition.
DETAILED DESCRIPTION OF THE INVENTION:
The present invention relates to a film having a
heat shrinkage of 20~ or more in one of the longitudinal
and transverse directions and 60% or more in the other
direction, which is obtained by forming a non-stretched
film or sheet from a composition comprising a linear
polyethylene and a branched low-density
polyethylene, and monoaxially or biaxially stretching the
non-stretched film or sheet under sp~cific conditions.
The present invention also relates to a process
for producing the film.
The present invention will be described more in
detail below.
. A linear low-density polyethylene having a density
of 0~91 to 0.95 g/cm3 and a high-density polyethylene having
a density of 0.965 g/cm3 or below are used as the linear
polyethylene used in the present invention.
The linear low-density polyethylene is a copolymer
of ethylene and other a-olefin and different from the
conventional branched low-density polyethylenes produced by
the high-pressure process. Such linear low-density poly
ethylene is produced, for example, by copolymerizing ethylene
with 4 to 17~ by weight, preferably 5 to 15% by weight of
- t~2~1~2
other a-olefin such as butene, hexene, octene, decene t 4~
methyl~l-pentene, etc., in the presen~e of a Ziegler or
Phillips catalyst which is generally used in the production
of moderate- to low-pressure high-density polyethylenes.
The linear low-density polyethylene is lowered in its
density to the order of 0.91 to 0.95 g/cm3 due to a
molecular structure having short branchings of the copoly-
merizing components. Thus, this polyethylene has a
linearlity of the chains higher than the conventional
branched low-density polyethylenes, and is of a structure
having a greater number of branches than high-density
polyethylenes.
The high-density polyethylene used as another
component of the linear polyethylene is an ethylene homo-
polymer obtained by polymerizing ethylene alone by using
a Ziegler or Phillips catalyst and having a density of
0.965 g/cm3 or below.
As the linear polyethylene in
this invention t the linear low-densit~ polyethylene and
high-density polyethylene are used alone or as a mixture
thereof. When they are used as a mixture, the mixing ratio
is not strictly limited.
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Preferably said linear polyethylene has a mel~
index of not higher than 2 g/10 min, preferably not higher
than 1 g/10 min, more preferably in the range of 0.001 to
1 g/10 min.
When the linear polyethylene has a melt index
higher than 2 g/10 min, the surface strength tends to be
lowered.
- It is further preferred that the linear
polyethylene has a density in the range of 0.910 to 0.965
g/cm3, preferably 0.910 to 0.950 g/cm3, more preferably
O.915 to 0.940 g/cm3. When the linear polyethylene has a
density above 0.965 g/cm3~ the impact resistance is greatly
deteriorated, and when the linear polyethylene is of a
density below 0.910 g/~m3, both stiffness and tensile
strength are lowered.
In the present lnvention, the values of melt index
are those determined according to the formula 4 in Table 1
of JIS K 7210 which is the standard referred to in
JIS K 6760. Fluidity ratio represents the ratio o~
extrusion rates (g/10 min) under shearing force of
106 dyn/cm2 (load of 11,131 g-) and 105 dyn/cm2 (load of
1,113 g) and is calculated from:
:
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-- 10 --
extrusion rate under load of 11,131 g
Fluidity ratio = (for 10 min)
extruslon rate under load of 1,113 g
(for 10 min)
Density was measured according to JIS K 6760.
Fluidity ratio is discussed as a possible index of molecular
weight distribution of the resin used. That i5, a small value
of fluidity ratio represents a sharp molecular weight
distribution and a large value of fluidity ratio represents
a broad molecular weight distribution.
In the present invention, the linear polyethylene
alone may be used, but it is preferred to blend a specified
amount of a branched low-density polyethylene in the
linear polyethylene used as base, because in the latter
case the film-forming properties and stretchability are
improved.
The branched low-density polyethylenes that can
be blended in the linear polyethylene in the present ~
invention include ethylene homopolymers and copolymers of
ethylene and other ~opolymerizable materials.
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11 - ~ 32~1~2
Examples of the copolymerizable materials are
vinyl compounds such as vinyl acetate, ethyl acrylate,
methyl acrylate, etc., and olefins having 3 or more carbon
atoms such as hexene, propylene, octene, 4-methyl-1-pentene,
etc. The amount of such copolymerizable material used
in the copolymerization is in the range of 0.5 to 18%
by weight, preferably ~ to 10% by weight. These branched
low-density polyethylenes are preferably the ones obtained
from radical polymerization using a radical initiator
such as oxygen, organic peroxides, etc., according to a
known high-pressure (1,000 - 3,000 kg/cm2) process.
The branched low-density polyethylene used in
the present invention is the one having a melt index of
not exceeding 2 g/10 min, preferably in the range of 0.1 to
1 g/10 min, and a fluidity ratio o not greater than 70,
preferably in the range of 30 to 70. When the melt index
of the branched low-density polyethylene is outside
the range, the strength of the produced film is lowered
and when such film is mads into a packaging bag, the
strength of its body portion proves low. The same holds true
when fluidity ratio of the branched low-density polyethylene
is outside the range. It is further preferred that the
branched low-density polyethylene has a density of not
exceeding 0.930 g/cm3,~preferably in the range of 0.915 to
0.925 g/cm3, for attaining an improvement of strength
of the film as well as an improvement of body strength and
_ 12 _ 1 3 2 81 62
heat-seal strength of the bag made from such film.
The blending ratio of the branched low-density
polyethylene to the linear polyethylene is 0 - 50 parts by
weight, preferably 10 - 30 parts by weight of branched low-
density polyethylene to 100 - 50 part~ by weight, preferably
90 - 70 parts by weight of linear polyethylene.
In the present invention; a radical initiator
may be added to the linear polyethylene or a blend of the
linear polyethylene and the branched low-density polyethylene.
Addition of such radical initiator is preferable as it
improves film-forming properties and the other properties,
especially strength, of the produced film.
~ he radical initi~tor used in the present
invention is preferably of the type whose decomposition
temperature at which the half-life period is one minute i~
in the range of 130 - 300C, the examples of such radical
forming agent being dicumyl peroxide, 2,5 dimethyl-2,5-
di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-di(t-butylperoxy~-
hexyne-3, ~,a'-bis~t-butylpexoxyisopropyl)benzene, dibenzoyl
peroxide, di-t-butyl peroxide and the like.
The amount of such radical initiator blended
is not more than ~.1 part by weight, preferably 0.0001 to
0.1 part by weight, more preferably 0.001 to 0.1 part by
weight based on the amount of the linear polyethylene or
the total amount of the linear polyethylene and branched
low-density polyethylene. Use of the radical initiator
,
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_13 -
in excess of the amount re~ults in a too low melt index,
which tends to cause film break or melt ~racture in the
course of blown film forming.
In the present invention, any suitable method
may be employed for blending the radical initiator in
the linear polyethylene and branched low-density polyethylene,
and decomposing and reacting such radical initiator with
the polyethylenes. For instance, the following methods may
be~employed.
(1) The linear polyethylene, branched low~density
polyethylene and radical initiator are fed simultaneously
or successively and melt extruded at the time of inflation
molding.
(2) By using an extruder and/or a kneader such as
Banbury mixer, the linear polyethylene, branched low-density
polyethylene and radical initiator are mixed and reacted,
then pelletized and subjected to inflation molding.
(3) There is first prepared a pelletized master
batch by blending an excess amount of radical initiator
~usually 5,000 - 10,000 ppm) in a polyethylene such as linear
low-density polyethylene~ branched low-density polyethylene
and high-density polyethylene and mixing and melting them
at a temperature above the melting point of the
polyethylene and below a temperature at which no substantial
decomposition of the radical initia~or is caused,
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and this master batch is blended with the linear polyethylene
or a mixture of the linear polyethylene and branched low-
density polyethylene and subjected to blown film forming.
The radical initiator may be used either in
the ~orm as it is or in the form of a solution in a solvent.
The reaction of the linear polyethylene and
branched low-density polyethylene with the radical initiator
causes intermolecular coupling of the polyethylenes
to~increase the high-molecular weight component and make it
possible to obtain a modified polyethylene with a reduced
melt index. Such modified polyethylene is more liable to
be oriented in the transverse direction at the time of blown
film forming than the non-modified linear polyethylene
or the blend of the non-modified linear polyethylene and
non-modified branched low-density polyethylene, and the
film produced therefrom, when subjected to a stretching
treatment, is markedly improved in longitudinal tear
strength and impact strength~
The polyethylene resin comprising the linear
polyethylene or a blend of the linear polyethylene and
branched low-density polyethylene or a modified version
thereof may contain according to necessity a known additive
or additives such as antioxidant, ultraviolet absorber,
antistatic agen~, slip agent, etc., which are usually
used in the preparation of polyethylene products.
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In the present invention, the composition prepared
by adding a radical initiator to the linear poly-
ethylene or a mixture of the linear polyethylene and
branched low-density polyethylene is formed into a non-
stretched film by the inflation method and this non-stretched
film is then stretched in the longitudinal direction (film
take-up direction) to form a stretched film.
Blown film forming of the non-stretched film is
carried out under the conditions wh re the blow-up ratio
is 2 - 8, preferably 3 - 8, and the frost line height
(height from die surface to frost line) is 2 - 50 times,
preferably 5 - 50 times the die diameter. When the blow~up
ratio is below the range, both longitudinal tear strength
and impact strength of the film are lowered, while when the
blow-up ratio is higher than the range, t~e bubble forming
stability is deteriorated. Also, when the frost line height
is below the range, the longitudinal ~ear strength of the
film is reduced, while when the frost line height is above
the range, the bubble forming stability is deteriorated.
The non-stretched film is then monoaxially
stretched in the longitudinal direction at a temperature
of Tm - 70 to Tm - 20C (Tm being the melting point of the
polyethylene composition comprising the non-stretch~d film
at a stretch ratio of 1.5 to 8.
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The stretching temperature is in the range from
Tm - 70 to Tm - 20C, preferably Tm - 60 to Tm - 30C.
At a temperature below the range, there may take place
non-uniform stretch of the film. At a temperature above
the range, the produced film is greatly lowered in impact
strength.
The film is stretched at a stretch ratio of 1.5 to
8, preferably 2 to 5. When the stretch ratio is less than
1.5, no desired effect of stretching is provided, resulting
in an unsatisfactory stiffness and tensile strength of the
film. When the stretch ratio exceeds 8, the stretched
film has excessive molecular orientation in the longitudinal
direction, resulting in a reduction of longitudinal tear
strength of the film.
In the above process for producing the monoaxially
stretched film, a linear polyethylene having a melt index
of 2 g/10 min or below, pxeferably 1 g/10 min or below, more
preferably in the range of 0.001 to 1 g/10 min, and a fluidity
ratio of 50 or below, preferably in the range of 10 to 50 is
preferably used. When the fluidity ratio is above 50, the
surface strength o~ the produced film may be undesirably
lowered.
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- 17 -
As the branched low-density polyethylene, one having a melt
index of 2 g/10 min or below, preferably in the range of 0.1
to 1 g/10 min, and a fluidity ratio of 70 or below, pre~erably
in the range of 30 to 70 is used. When the fluidity ratio
is above 70, the surface strength of the produced film is
lowered and when such film is made into a packaging bag, the
strength of its body portion proves low.
According to the same process as the above-described
process except for stretching at a temperature of Tm - 70
to Tm - 20C, preferably Tm - 60 to Tm - 30C (Tm being
the melting point of the composition comprising a linear
low-density polyethylene and a branched low-density
polyethylene), it is possible to obtain a longitudinally
monoaxially stretched film having a heat shrinkage of 20%
or more in one of the longitudinal and transverse directions
and 60% or more in the other direction by using a composition
comprising 100 to 50 parts by weight of a linear
polye hylene having a density of 0.91 to 0.965 g/cm3, a melt
index of 2 g/10 min or l~ss and a fluidity ratio of
larger than 50 and not larger than 120,
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and 0 to 50 parts by weight of a branched low-density
polyethylene having a density of 0.930 g/cm3 or below, a melt
index of 2 g/10 min or less and a fluidity ratio of 70 or
below.
The linear polyethylene used in the
above process is the one having a density in the range of
0.91 to 0.965 g~cm3, preferably 0.91 to 0.95 g/cm3,
more preferably 0.915 to 0.940 g/cm3, a melt
index of 2 g/10 min or less, preferably 0.1 to 1 g/10 min
or less, and a fluidity ratio in the range of higher than
50 and not higher than 120, preferably 60 to ~0. -
When the density of the linear low-density poly-
ethylene used is below the range, there can not be obtained
the desired stiffness and tensile strength of the ilm, and
when the density is above the range, the produced film proves
remarkedly poor in impact strength. When-the melt index of
the linear polyethylene is abo~e 2 g/10 min,
the film is not provided with desired strength.
Either when the fluidity ratio o the linear
polyethylene is above or below the range, there results a
deterioration of processability and strength.
In the process described above, it is possible
to use linear polyethylenes having a relatively broad
molecular weight distribution with the fluidity ratio
of higher than 50 and not higher than 120.
Any known method can be used for the prepara-
tion of linear polyethylene having a broad
` ~ 19 - ~3~62
molecular weiyht distribution. For example, there can be
favorably employed a method in which a blend of polymers
with different molecular weights is produced by using two
or more polymerization vessel~ The linear
polyethylene alone may be used, but
it is preferred to blend a specified amount of a branched
low-density polyethylene in the linear low-density poly-
ethylene as such blend improves the film processability and
tensile properties. The "branched low-density poly-
ethylene" refers to the same ethylene homopolymers and
copolymers of ethylene and other copolymeriæable materials
as mentioned above, but in the composition described above~
there are used those having a melt index not exceeding
2 g/lO min, preferably in the range of 0.1 tQ lg/10 min,
and a fluidity ratio not higher than 70, preferably in the
range of 30 to 70. When the melt index is above 2 g~lO min,
the produced film proves poor in strength. A
deterioration of strength of the produced film is
also seen when the fluidity ratio of the linear
polyethylene is higher than the range. It is also preferable
for the improvement of strength of the film that the
branched low-density polyethylene has a density in the range
of 0.91 to 0.930 g/cm3, preferably 0.915 to 0.925 g/cm3.
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~32~62
- 20 -
The amount of said branched low-density polyethylene
to be blended in the linear polyethylene is in the
range of 0 to 50 parts by weight, preferably lO to 30 parts by
weight, to lO0 to 50 parts by weight, preferably 90 to 70 parts
by weight of the linear polyethy~ene.
In the case of using the composition, mentioned above,
which comprises 100 to 50 parts by weight of a l~near polyethylene
.... . .. .
having a density of O.9lO to 0.965 g/cm3, a melt index of
2 g/10 min or less and a fluidity ratio of 50 or below;
0 to 50 parts by weight of a branched low-density polyethylene
having a density of 0.930 g/cm3 or below, a melt index of
2 g/lO min or less and a fluidity ratio of 70 or below; and
O.OOOl to O.l part by weight of a radicai initiator as
an optional component, the foilowing
132~162
- 21 -
method may be also employed to form the film according to
the present invention.
By employing an ordinary film or shee~ forming
equipment and method, for example, blown film forming method
using a circular die or T-die method, the above
composition i~ formed at a resin temperature of 150 - 250C
and a draft ratio of 1 to 50 to obtain a non-stretched film
or sheet. Then this non-stretched film or sheet is stretched
in at least one of the longitudinal and transverse directions
so that the surface area of said film or sheet is stretched
1.2 to g times the original area to obtain a stretched film.
Stretching of the non-stretched film or sheet is
performed either by monoaxially stretching it in the
transvexse direction (the direction orthogonal to the film
take-up direction of the film forming machine) or by
biaxially stretching the film or sheet in both longitudinal
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direction (film take-up direction) and transverse direction.
Monoaxial stretching in the transverse direction
can be accomplished, ~or example, in the ~ollowing way.
The non-stretched film obtained by the T-die method or blown film
forming method is ~lit to a desired width and heated,
and then stretched in the transverse direction under heating
with the ends of the ~ilm fixed.
In the ~ase o biaxial stretching, the non-stretched
film obtained by the T-die method or blown film forming method
is slit to a desired width and stretched in both longi-
tudinal and transverse directions either successively
or simultaneously. In the case of successi~e stretching,
the film is ~irst stretched in the longitudinal direction
and then stretched in the transverse direction, or vice
versa. In the case o~ simultaneous biaxial stretching,
the time allocation for longitudinal and trans~erse
stretching is optional, for example, longitudinal stretching
may be continued gradually until transverse stretching
is completed, or stretching in both longitudinal and
transverse directions may be started simultaneously,
or stretching in the longitudinal direction may be completed
first.
Tentering method, successive biaxial stretching
method, tubular stretching method and simultaneous biaxial
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stretching method can be favorably used for the biaxial
stretching in the present invention. The stretching
temperature is in the range of Tm - 70 to Tm - 5C,
preferably Tm - 60 to Tm - 15C (Tm being the melting point
of the composition of the linear polyethylene and the
branched low-density polyethylene, or the modified poly-
ethylene composition obtained therefrom by the reaction of
the radical initiator). At a temperature lower than
Tm - 70C, the mobility of molecular chain is so poor that
the film tends to break when stretched, and e~en if the
film could be stretched, the desired stretch ratio would
not be attained, making it unable to obtain a stretched
film with excellent properties. At a temperature higher
than Tm - 5C, the non-stretched film may be partly melted
and becomes unable to have desired orientation, so that
in this case, too, it is impossible to obtain a stxetched
film with excellent properties.
The stretching rate is in the range of 2 to 40~/sec,
preferably 10 to 20~/sec. A stretching rate lower than
2%/sec tends to impair the stretchability due to oriented
crystallization in the course of stretching, while a
stretching rate higher than 40%/sec will make the polymer
deformation unable to keep pace with the stretching rate
to cause break of the film during stretching.
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In view of the stretching workability (easiness
to st-etch) and the properties of the obtained stretched
film the stretch ratio is in the range of 1.2 to 9
times, preferably 1.2 to 5 times, more preferably 105
to 5 times in terms of areal stretch ratio (in the
case of transverse monoaxial stretching, the stretch ratio
in the longitudinal direction is supposed to be 1) in
the txansverse direction or in both longitudinal and
transverse directions. In the case of biaxial stretching,
the film is stretched 1.2 to 3 times, preferably 1.2 to
2 times in the longitudinal direction and 3 to 7.5 times,
preferably 4 to 7 times in the ~ransverse direction~
When the areal stretch ratio is less than 1.2 times,
there can not be obtained the desired effect of improY~ment
of strength properties and tensile strength at break of
the film. When the areal stretch ratio is greater than ~
times, the stxetching workahility is deteriorated to ~ake
it unabl~ to obtain a satisactory stretched film.
The monoaxially or biaxially stxetched film of
the present invention produced according to the abo~e-
mentioned processes by using the composition prefera~ly has
a thickness of 30 to 120 ~m, more preferably 40 to 10~ ~m,
and a heat shrinkage of 20% or moxe, preferably 30% ox more
in one of the longitudinal and transverse directions and 60%
~ - 25 - ~328~2
or more, preferably 70~ or more in the other direction.
The monoaxially or biaxially stretched film of this invention
can be favorably used for a packaging bag, but in case
the heat shrinkage of the film in at least one direction
is less than said values, the bag made therefrom is found
unsatisfactory in deformability and stiffness when the bag
is dropped.
The known methods can be employed for producing
a packaging bag by using the stretched film of the present
invention. For example, the following methods may be
employed.
(1) The stretched film is turned into a cylindrical
~orm by heat-sealing both edges thereof or by sealing
both edges with an adhesive (hereinafter referred to as
adhesive sealing), and then the top and bottom of this
cylindrical film are closed by heat sealing~ adhes~e
sealing or sewing to foxm a bag.
(2) Both top and bottom ends of the cylindrical
stretched film are heat-sealed or adhesi~e~sealed, and then
the side edges are joined by heat sealing, adhesi~e sealing
or sewing.
When heat sealing is employed fox joining the
edges in the production of the packaging bag, such heat
sealing is preferably conducted so that the direction
.
- 26 - ~ 3~8~
of heat sealing will coincide with the direction in which
the heat shrinkage of the stretched film is small. When
the part to be heat sealed is kept pressed for a long
time by a heating means such as heat bar or heat belt
used in heat sealing, there may take place thermal relaxation
to weaken the strength of the heat-sealed part, so that
it is preferable to employ a method in which the part
to be heat-sealed is heated quickly at a temperature of
about 230 - 280C with no pressing force applied thereto
to keep it in a free state so that shrinkage will occur
at the heat-sealed part.
It is also possible to form a double-wall bag
by using the mono- or biaxially stretched fil~ of the present
invention as the inner bag while using paper for the outer
bag. The paper used for the outer bag is not li~ited to
specific types. Any type of paper generally used for the
industrial packaging materials is usable in the present
invention. Kraft paper, extensible paper
and the like are especially pxeferred. The basis weight
(weight per unit area, an index of thickness) of such paper
is in the range of 73 to 88 g/m2. Also, such paper may be
polyethylene-laminated on ~he inside.
Known methods are employable for producing such
double-wall packaging bag.
.
- 27 - ~328~2
The outer and inner bags that constitute the
double-wall packaging bag may be simply lapped one o~er
the other or may be bonded together with an adhesive.
Sealing may be also effected by known methods,
for example, (1) the inner bag is heat-sealed or adhesi~e-
sealed, and the outer bag is sewn up, or (2) both inner
and outer bags are sewn up.
Heat sealing in the production of the inner bag
i5 preferably conducted in the same way as described above.
The double-wall bag obtained in the manner described
above is free of the problems accompanying the con~entional
bags of this type, such as multi-wall kraft bag using
polyethylene or nylon films, cross-laminated kraft bag
made by laminating woven fabrics made of stretched cords
of synthetic resin on kraft paper, and bags made of synthetic
resin films alone, which p~oblems include wetting of the
contained material and break o bag due to wetting with
water, high production cos~ due to complication of the
production process, break of bag by a protuberance or
othex object during transport, phenomenon that when the
filled bag is carried with hands, the finger tips holding
the bag penetrate into the bag, and other troubles resulting
from the reduced tensile strength at break of the body
portion of the bag due to reduction of film thickness of
.
`" `` - 28 - ~3~ 2
the bag. FurthPr, the bag made by using the stretched film
according to the present invention is excellent in water
resistance, mechanical strength and anti-slip properties
and suited for packaging and transporting heavy materials.
The present invention will hereinafter be described
more in detail referring to the following non-limitative
examples.
Example 1: ~:
~13 Production o~ stretched fil~
Eighty parts by weight of a lin~ar low-density
polyethylene (MI: 0.5 g/10 min, fluidity ratio: 20, density:
0.921 g/cm3, copolymerized material: butene-l, amount thereof
copolymérized: 10% by weight, melting point: 118C) and 20
parts by weight of a high-pressure branched low-density
polyethylene ~MI: 0.4 g/10 min, fluidity ratio: 45, density:
0.922 g/cm3) were dry blended ~melting point of the mixture:
118C~, and this blend was further mixed with 0.03 part by
weight of 2,5-dimethyl-2,5-di(t-butyl-peroxy)hexyne-3 to
prepare the starting material. This starting material was
formed into a blown film under the conditions of
extrusion rate of 80 kg/hr, blow-up ratio (suR) of 3 and
frost line hight/die diameter (FLH/D) of 8 by using a blown film
extrusion equipment ~a Modern Machinary's ~xtruder Delser~
, ~
~, :
.. . ._. - : : ~
:' ' " ~ ~ ;
- 29 - ~ 328~
65~) adapted with an inflation die having a circular slit
with a diameter of 250 mm and a width of 4 mm and a cooling
air ring. This base film was slit in the film take-up
direction and stretched by using a roll stretcher at a
stretching temperature of 80C and a stretch ratio of
3 (in the longitudinal direction) to obtain a longitudinally
monoaxially stretched film having a thickness of 80 ~mO
Ev~luation methods
(a) Film strength
Elemendorf tear strength was measured according
to JIS P8116, and dart drop impact ~DDI) test was conducted
according to ASTM D1709.
(b) Strength against finger penetration
For examining tensile strength of the film, the
test for strength against finger penetration was conducted
in the following way.
The longitudinally streched film obtained ~nd
described in (1) above was cut to a length of 760 mm in
the stretched direction and then cut to a length of 1,000 mm
in the transverse direction (widthwise direction of the
film). This piece of film was rounded up and joined
edgewise so that the lapped portion would measure ~O mm in
width. A hot melt adhesive (GEade HX-960 made by Nitta
Gelatin Co., Ltd.) was applied to said lapped portion and
_ 30 _ 132~
then this portion was heated and bonded by a hot gun to
form a cylindrical body. Either top or bottom of this
cylindrical body was heat sealed by using a heat sealer
(Model HS 22B-Z made by New Long Co., Ltd.) to form a bag. This
bag was packed with 20 kg of fertilizer and the opening
of the bag was heat sealed in the same way as described
above to obtain a packaging bag for testing. This bag
containing 20 kg of fertilizer was lifted up with the
hands so that the heat sealed portion would be parallel
to the floor surface, and it was obser~ed whethex the
finger tips would bite into the fil~ surface of the bag.
Evaluation
A: The finger tips didn't bite into the bag film at
all. No problem at all.
B: The finger tips bit slightly into the bag film,
but no serious problem was posed.
C: The finger tips bit largely into the bag film,
posing the serious problem.
~c) Determination of heat shrinkage
A circular test piece of 6 mm in diameter was
cut out from a suitable position of the film. This test
piece was placed on a hot plate having a surface tempera~ure
of 200C for 20 seconds and the changes in length in the
longitudinal direction (film take-up direction) and in
, ' ~
- 31 - ~32~2
the transverse direction (film widthwise direction)were
measured and shown by percent to the original lengths.
The results are shown in Table l.
Example 2:
A stretched film was obtained and a packaging
bag was made therefrom in the same way as Example l except
that a mixture of lO0 parts by weight of the same linear
low-density polyethylene as used in Example l and 0.0~
part by weight of 2,5-dimethyl-2,5-di(t-butylpexoxy)hexyne-3
was used as the starting material, and that FLH/D wa~
adjusted to 15. The results are shown in Table l.
Comparative Example 1:
Th~ procedure of Example 1 was followed except
that the blow-up ratio (BUR) was changed to 1.5.
Compara~ive Examples 2 6:
.
The procedure of Example 1 was followed except
that the amounts of linear polyethylene and radical initiator,
the forming conditions and the stretching conditions
were changed as shown in Table l.
Example 3:
Two types of linear polyethylene ha~ing a butene-l
content of 4.5% by weight and MI of 200 and 0.055, respec-
tively, were prepared by using a solid catalyst obtained
' . . . ' ~ ' ' '
,
- 32 - ~ 3~62
by reacting 5 g of magnesium ethylate and 50 cc of titanium
tetrachloride at 130C. Ten kg each of said two types of
linear polyethylene were mixed with 0.05 part by weight of
2,6-ditertialybutyl-paracresol (BHT) and 0.05 part by weight
of calcium stearate, and mixed well by Banbury mixer.
The thus obtained modified linear polyethylene
had an MI of 0.3 g/10 min, a fluidity ratio of 70 and a
density of 0.93 g/cm3.
This modified linear polyethylene was formed into
a 200 ~m thick blown film under the conditions of
extrusion rate of 80 kg/hr, BUR of 3 and FLH/D of 8 by using a similar
blown film extrusion equipment (Modern Machinary's Delser
65~ extruder) to that used in Example 1. This base film
was slit in the film take-up direction and stxetched under
the conditions of stretching temperature of 80C and stretch
ratio of 3 (in the longitudinal direction~ by using a roll
stre~cher to obtain an 80 ~m thick monoaxially (longitudinally)
stretched film. The results are shown in Table 2.
Example 4:
The procedure o~ Example 3 was followed except
for use of a blend of 80 parts by weight of modified linear
polyethylene of Example 3 and 20 parts by weight of a
branched low-density polyethylene having an MI of 0.4 g/10
min, a fluidity ratio of 40 and a density of 0.922 g/cm3.
;
. ., :
- . .
:
- 33 ~ 1 3 2~ 1 ~2
The results are shown in Table 2.
Example 5:
To 100 parts by weight of a linear polyethylene
having an MI of 0.5 g/10 min, a fluidity ratio of 20, a
density of 0.921 g/cm3 and a butene-1 content of 8% by
weight, was added 0.03 part by weight of 2,5-dimethyl-2,5-
di(t-butylperoxy)hexyne-3, and the mixture was treated at
250C by using a Modern Machinary's 50 mm~ extruder to
obtain a modified linear polyethylene ha~ing an MI o~ ~.12
and a fluidity ratio of 75.
By using a blend of 80 parts by weight of said
modified linear polyethylene and 20 parts by weight of the
branched low-density polyethylene used in Example 4, a
monoaxially stretched film was produced by following the
procedure of Example 3.
The results are shown in Table 2.
Comparative Examples ? - 12
The procedure of Example 3 was follo~ed except
for the changes of the compositions of the resins used
and the forming conditions as shown in Table 2.
The results are shown in Table 2.
Example 6:
~1) Preparation of stretched film
Used as the starting material was a dry blend
. , ' ~ .:..: .
." , .
:-:
.
~ 34 ~ 1 3~ 81 ~2
(melting point: 118C~ of 90 parts by weight of a linear
low-density polyethylene (MI: 0.5 g/10 min, fluidity xatio:
20, density: 0.921 g/cm3, copolymerized material: butene-l,
amount thereof: 10~ by weight) and 10 parts by weight of
a high-pressure branched low-density polyethylene (MI: 0.4
g/10 min, fluidity ratio: 20, density: 0.922 g/cm3). This
starting material was formed into a 300 ~m thick blown
film under the conditions of extrusion rate of 50 kg/hr,
BUR of 2 and draft ratio of 6.7 by using the same blown film extrusion
equipment as used in Example 3. This base film was slit in
the film take-up direction and stretched by using a tenter
type successive biaxial stretching machine under the
conditions of stretching temperature of 105C, stretching
rate of 10~/sec, and stretch ratio of 1.2 in the longitudinal
direction and 5 in the transverse direction to produce a
biaxially stretched film h~ving a thickness of 50 ~m.
The thus obtained stretched film was rounded up
and joined edgewise so that the lapped portion would measure
100 mm in width. Then said lapped portion, after applying
thereto a hot melt adhesive (Grade HX-960 a~ailable from
Nitta Gelatin Co., Ltd.), was heated and bonded by usin~
a hot gun to form a cylindrical body designed to serve as
an inner bag having an internal ~olume of 25 kg). Then an
outer bag made of extensive ~aper having a basis weight of
~ 3 ~ 2
- 35 -
83 g/m2 was fitted around said cylindrical inner bag to
form a double-wall cylindrical body, and the bottom end
of this double~wall cylindrical body was sewn up at a
pitch of 7 m/m by using New Long's DS-5 sewing machine to
made a double-wall bag.
This double-wall bag was packed with 25 kg of
polyethylene chips and then the top end of the bag was
sewn up in the same way as described above.
Performance tests of packagin~ bag
(1) Drop test
The double-wall bag was dropped onto a concrete
surface with the bag placed parallel to the concrete
surface from a height of 1.5 meters. The bag was dropped
10 times in all, 5 times with the same side down and 5
times with the opposite side down. The drop test was
made on 20 bags, and the number of times of drop conducted
till the bag was broken was counted. The a~erage of the
20 bags was calculated.
(2) Transport test
1) There were prepared 500 bags each being packed
with 25 kg of chips~ These bags were loaded in a pallet
and transported by a freight car over a distance of 400 km.
Upon arrival at the destination, the number of the bags
which were broken during the transport was counted.
.~ : ., , :
- 36 - ~ 3 ~ 8 1 ~ 2
2) Of the bags loaded in the pallet in 1) above
the number of crumbled bags during transport was counted
after arrival at the destination.
Example 7:
The procedure of Example 6 was followed except
that a mixture obtained by mixing 0.03 part by weight of
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 to a dry blend
of 90 parts by weight of a linear low-density poly-
ethylene and 10 parts by weight of a high-pressure branched
low-density polyethylene was used as starting material,
and that said starting material was formed under the
conditions shown in Table 3. The results are shown in
Table 3.
~omparative Example _3
The procedure of Example 6 was followed except
that a doulbe-wall kraft pa~kaging bag was made without
using any polyethylene resin but by using paper (extensive paper)
having a basis weight of 83 g/m2)-alone. The results are
shown in Table 3~
Comparative Example 14
.
The procedure of Example 6 was followed e~cept
that no paper was used and the forming conditions were
changed as shown in Table 3 to make a single-wall poly-
ethylene film bag. The results are shown in Table 3.
~ 3~8~ ~
- - 37 -
Comparative_Example 15:
The procedure of Example 6 was followed except
that the forming conditions were changed as shown in Table 3.
The results are shown in Table 3.
Example 8:
tl) Preparation of stretched film
Used as starting material was a dry blend of 90
parts by weight of a linear low-density polyethylene
(MI: 0.5 gtlO min, fluidity ratio: 20, density: 0.921 g/cm3,
copolymerized material: butene-l, amount thereof: 10~ by
weight) and 10 parts by weight of a high-pressure branched
low-density polyethylene (MI: 0.4 g/10 min, fluidity ratio:
20, density: 0.922 ~/cm3). This starting material was
formed into a 450 ~m thick blown film under the conditions
of extrusion rate of 50 kg/hr, BUR of 2 and draft xatio of 6.7 by
using the same blown film extrusion equipment as used in Example 3.
This base film was slit in the film take-up direction and
stretched by using a tenter type successive biaxial
stretcher under the conditions of stretching temperature of
105C, stretching rate of 10~/sec, and stretch ratio of 1.5
in the longitudinal direction and 3 in the trans~erse
direction to produce a biaxially stretched film having a
thickness of 100 ~m.
`'` ' : '- ' ' - -, ~ `:
- 38 - ~ 32~62
For decidiny the heat sealing direction in
producing a packaging bag from said biaxially stretched
film, the heat shrinkage of the film was determined
according to the method in Example 1. The results are
shown in Table 4.
Based on the obtained results of determination
of heat shrinkage, the direction of smaller heat shrinkage
was decided as the heat sealed direction (corresponding
to the top and bottom openings of the bag) and the direction
of greater heat shrinkage was decided as the adhesi~e-
sealed direction (corresponding to the body portion of
the bag).
~2) Production of packaging bag
The biaxially stretched film obtained according
to ~1~ above was cut to a length of 890 mm in the direction
of smaller heat shrinkage, viz. the longitudinal direction
(film take-up direction) and to a length of 670 mm in the
direction of greater heat shrinkage, viz. the transverse
direction (film widthwise direction). This piece of film
was rounded and joined edgewise so that the lapped portion
would measure 100 mm in width. Then the lapped portion,
after applying thereto a hot melt adhesive (Grade HX-960
made by Nitta Gelatin Co., Ltd.), was heated and bonded
by using a hot gun to form a cylindrical body. One slde
.: .
.. -.,.
~3~8~ ~
- 39
of one of the openings of said cylindrical film was heat
sealed at a posltion of 1.5 cm from the end by using a
New Long's heat sealer Model HS 22B-2 (length of heating
section: 150 mm, clearance of heating section: 0.3 mm,
length of cooling section: 150 mm, clearance of cooling
section: 1 mm) under the conditions of heat sealing
temperature (heating section surface temperature) of 250C,
cooling section temperature of 30C and film feed rate of
15 m/sec. The heat sealed portion was shrunk in the film
take-up direction (longitudinal direction) and had a
thickne~s greater than the original film thickness.
The obtained bag was packed with 20 kg of ferti-
lizer and then the opening was heat sealed under the same
conditions as described above to obtain a packaging bag
for drop test.
(3) Performance tests of packaging bag
(A) Drop test
The packaging bags obtained according to ~) abo~e
were subjected to a sidewise drop test and a leng~hwise
drop test in the manners described below.
The sidewise drop test was conducted by keeping
the test room temperature at -10C and the lengthwise
drop test at -5C. Each bag was dropped 5 times from a
height of 1.5 meters. The percent of the number o~ broken
.,:
_ 40 _ 1 328 ~ ~2
bags to the total number of bags tested was determined
and shown as broken bag ratio. The results are shown in
Table 4.
(a) Sidewise drop test
Each packed bag was dropped in such a way that
the body portion of the bag would remain parallel to the
floor surface while the heat sealed portion substantially
vertical,,thereto. The test was conducted by dropping 20
bags in said way~ and the broken bag ratio was calculated.
This sidewise drop test was made for determining the
strength of the heat sealed portion.
(b) Lengthwise drop test
20 bags were dropped in such a way that the heat
sealed portion of the bag would remain parallel to the
floor surface while the body portion substantially vertical
thereto, and the broken bag ratio was calculated. This
lengthwise drop test was made for determining the strength
of the body portion.
(B) Deformation test
The bags obtained according to ~2) described above
were subjected to the lengthwise drop test described in
(A)(b) above~ and the circumferential length of the body
portion of the bag before drop and that after drop were
measured. The degree of deformation (hereinafter referred
- 41 - ~ 328~2
to as deformation strength ratio) was determined from the
following formula:
Deformation strength ratio = circumferential length of body
clrcumferential length of body
ortion of bag after dro
portion of bag before drop
A high deformation strength ratio signifies a
high deforming aptitude of the bag.
(C~ Film stiffness
This was determined in the following way by using
a film sti~fness tester made by Toyo Fine Machinary Co., Ltd.
An 80 x 100 mm sample (film used for said packaging bag) was
placed flat on a sample holder and both ends were clamped
to a chuck. Since a marginal area of clamping clearance
at each end was 10 mm, the effective area of the sample
tested was 8~ x 80 mm.
The sample bending handle was turned to narrow
the clamping distance of the chuck to thereby bend the
sample. Then the top o the bend was depressed by an
indenter and the load was measured electrically.
D) Automatic fillability test
If the stiffness of the bag is weak, the bag may
bend and can not be opened when subjected to automatic
filling operation in which the opening end of the bag is
- ., , . . .
. ;' ' ' - " '' :' :, "
' ' ~ "
- . . . . ~-
- 42 ~ 2
spread open by a sucker. So, 100 bags obtained according
to (2) described above were subjected to an automatic filling
test and the de~ective filling rate was determined.
Example 9:
The procedure of Example 8 was followed except
that a mixture prepared by mixing 0.03 part by weight of.
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 with a dry
blend of 90 parts by weight of a linear low-density
polyethylene and 10 parts by weight of a high-pressure
branched low-density polyethylene was used as starting
material, and that the forming conditions shown in Table 4
were used. The results are shown in Table 4.
Example 10:
The procedure of Example 8 was followed except
for the use of the forming conditions shown in Table 4.
The results are shown in Ta~le 4.
Comparative Example 16-
.
The procedure of Example 8 was followed except
that a mixture prepared by mixing 0.005 part by weight of
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 with a dry blend
of 90 parts by weight of a linaar low-density polyethylene
and 10 parts by weight of a high-pressure branched low-
density polyethylene was used as starting material, and
~32~62
. - 43 -
that this starting material was treated under the conditions
shown in Table 4. The results are shown in Table 4.
Comparative Examples 17 - 19:
The procedure of Example 8 was followed except
for use of the conditions shown in Table 4. The results
are shown in Table 4.
Comparative Examples 20 and 21:
~ The procedure of Example 8 was followed except
that two pieces of stretched film were placed one over the
other to form a double-wall bag, and that the conditions
shown in Table 4 were used. The results are shown in Table ~.
.. . .
: .
~, . .
. . .
.
- 44 - 1 32 ~ ~2
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