Note: Descriptions are shown in the official language in which they were submitted.
~a
BACKGROIND OF THE Ir~'VENTiON
The present invention relates to a microporous
membrane of a polyolefin composition containing an ultra-high-
molecular-weight polyolefin, and a method for efficiently
producing such a micropozous membrane.
Microporous membranes are widely used in various
applications such as battery separators, electrolytic capacitor
separators, various filters, moisture-permeable, waterproof
clothes, reverse osmosis membranes, ultrafiltration membranes,
microfiltration membranes, etc.
Microporous polyolefin membranes are
conventionally produced by various processes. One example of
such processes is an extraction process comprising the steps of
1 ~ mixing a polyolefin with a pore-forming agent such as fine
powder of different polymers in such a manner as to achieve
micro-dispersion and subsequently extracting the dispersed
pore-forming agent. Another process is a phase separation
process by which polyolefin is divided into fine phases by a
2 0 solvent, thereby forming a porous structure. Further, thexe is a
drawing process comprising the steps of forming a polyolefin
article containing solid fillers finely dispersed therein and
imparting a strain to the article by drawing to break the
interfaces between the polymer phase and the solid fillers,
2 5 thereby forming pores in the article. In these processes,
however, polyolefins having a molecular weight lower than
SOO,OOU are usually used, so that the thinning and strengthening
of membranes by drawing are limited.
-1-
~~~a~a~0
Recently, there has been developed an ultra-high-
molecular-weight polyolefin which can be formed into a high-
strength, high-modulus membrane. With this development,
there have been proposed various processes for producing a
high-strength rnicroporous membrane from it.
One of such processes is disclosed in, for example,
Japanese Patent Laid-Open No. 58-5228. According to this
process, an ultra-high-molecular-weight polyolefin is dissolved
in a nonvolatile solvent and the resulting solution is made into a
gel in the form of fiber or membrane. The solvent-containing gel
is subjected to an extraction treatment with a volatile solvent
and then stretched while heating. This process is, however,
disadvantageous in that the gel cannot be biaxially oriented at a
high draw ratio because it has a porous structure highly swollen
with a nonvolatile solvent. 'The resulting membxane has a low
strength and a large pore diameter on account of its reticulate
structure which easily expands and breaks. Another
disadvantage of this process is that the gel in the form of a
membrane is liable to warpage because of uneven evaporation of
2 0 the volatile solvent. Further, it cannot be subjected to
orientation at a high draw ratio because of the shrinkage and
cornpaction of the reticulate structure of the gel which take place
after the extraction of the nonvolatile solvent by a volatile
solvent.
2 5 Various attempts have been proposed to produce a
microporous membrane of an ultra-high-molecular-weight
polyolefin (polyethylene) by forming a gel-like sheet from a
heated solution of an ultra-high-molecular-weight polyolefin
-2-
having a weight-average molecular weight of 5 x 105 or more,
controlling a solvent amount in the gel-like sheet by a solvent-
removing treatment, and then stretching it while heating
thereby removing the remaining solvent.
Japanese Patent Laid-Open No. 60-242035 discloses a
process for producing a microporous ultra-high-molecular-
weight polyethylene membrane having a thickness of 10~,m or
less, a breaking stxength of 200 kg/cm2 or more, and a void
volume of 30% or more by dissolving ultra-high-molecular-
weight polyethylene having a weight-average molecular weight
of 5 x 105 or more in a solvent while heating, forming a gel-like
sheet from the resulting solution, removing a solvent from the
gel-like sheet until the solvent content decreases to 10-80
weight %, and then stretching the sheet while heating, thereby
removing a residual solvent.
Japanese Patent Laid-Open No. 61-195132 discloses a
method of producing a rnicroporous membrane by forming a gel-
like article from a solution of an a-olefin polymer having a
weight-average molecular weight of 5 x 10~ or more, removing
2 0 at least 10 weight % of a solvent from the gel-like article so that
the ec-olefin polymer content in the gel-like article becomes 10-
90 weight %, stretching it at a temperature equal to or lower
than a melting point of the cc-olefin polymer + 10°C, and
removing the remaining solvent from the resulting stretched
2 5 article.
Japanese Patent Laid-Open No. 61-195133 discloses a
microporous membrane made of an a-olefin polymer having a
weight-average molecular weight of 5 x 105 or more and having
-3-
~~~~0
an average pore diameter of 0.001-1 p,rn and a porosity of 30-
90%, which is stretched two times or more in one direction at an
areal stretching ratio of 20 times or more.
Japanese Patent Laid-Qpen No. 63-3602 discloses a
method of producing a microporous polyethylene membrane
having a pure water permeability o.f 100 I/m2~hraatm or more
and a 'y-globulin block ratio of 50°l0 or more, which comprises
forming a gel-like article from a solution of polyethylene having
a weight-average molecular weight of 5 x 105 or more, removing
a solvent from the gel-like article so that the solvent content in
the gel-like article becomes more than 80 weight % and 95
weight % or less, stretching it two times in one direction and 20
times or more in an areal ratio at a temperature of 120°C or
lower, and removing the remaining solvent fxom the resulting
stretched article. This microporous polyethylene membrane is
excellent in water permeability and suitable for separating
proteins, etc. because of its fine pores.
Further, Japanese Patent Laid-Open No. 63-2?3651
discloses a method of producing a microporous membrane by
2 0 pxeparing a solution of an ultra-high-molecular-weight
polyolefin having a weight-average molecular weight of 5 x 105
or more, extruding the solution through a die while rapidly
cooling the solution to a gelation temperature or lower, so that
an ultra-high-molecular-weight polyolefin content in the gel-like
2 5 article becomes 10-90 weight %, stretching it at a temperature
equal to or lower than a melting point of the ultra-high-
molecular-weight polyolefin + 10°C, and then removing the
remaining solvent. This method can provide a microporous
membrane having a thickness of 10 p,m or more, which is
suitable for applications requiring large strength and pressure
resistance.
However, in any of the above methods, since the
ultra-high-molecular-weight polyolefin is biaxially oriented, a
diluted solution of palyolefin should be prepared. Accordingly,
the ultra-high-molecular-weight polyolefin solution suffers from
large swelling and neck-in at the exit of a die, resulting in the
difficulty of sheet formation. Further, since the resulting sheet
contains an excess amount of a solvent, mere stretching fails to
provide a desired microporous membrane. Accordingly, a
solvent-removing treatment is necessary to control the solvent
content in the sheet before stxetching, meaning that the
productivity of a microporous membrane is relatively low.
OBJECT AN37 SUMMARY OF T1-IE INVENTION
Accordingly, an object of the present invention is to
provide a microporous polyolefin membrane having good
stretchability and made from a high-concentration polyolefin
2 4 composition solution.
Another object of the present invention is to provide
a method for efficiently producing a microporous membrane
from a high-concentration polyolefin composition solution
containing an ultra-high-molecular-weight polyolefin.
2 5 To achieve the above-mentioned objects, the present
inventors have carried out intense research, which has led to the
finding that it is possible to form a high-concentration solution
by using a polyolefin composition containing an ultra-high-
-5-
~~~~~a
molecular-weight polyolefin in such an amount that a weight-
average molecular weight/number-average molecular weight
ratio of the polyolefin composition is in a desired range, and that
a microporous polyolefin membrane having excellent properties
S can be efficiently produced from this solution. The present
invention has been completed on the basis of this finding.
Thus, the micropoxous polyolefin membrane
according to the present invention is made of a polyolefin
composition containing 1 weight % or more of an ultra-high-
molecular-weight polyolefin having a weight-average molecular
weight of 7 x 105 or more and having a weight-average
molecular weight/number-average molecular weight ratio of 10-
300, the microporous membrane having a thickness of 0.1-25
~tm, a porosity of 35-95%, an average pore diameter of 0.001-
0.2 ~.m and a breaking strength of 0.2 kg or mare per 15 mm
width.
The method of producing a microporous polyolefin
membrane according to the present invention comprises the
steps of:
2 0 (a) preparing a solution comprising 10-SO weight % of a
polyolefin composition containing 1 weight % or more
of an ultra-high-molecular-weight polyolefin having
a weight-average molecular weight of 7 x 105 or
more and having a weight-average molecular
2 5 weight/number-average molecular weight ratio of
10-300, and 50-90 weight % of a solvent;
(b) extruding the solution through a die;
(c) cooling the extruded solution to form a gel-like
-6-
~~~~aa~~~
article;
(d) stretching the gel-like article at a temperature equal
to or lower than a melting paint of the polyolefin
composition + 10°C; and
(e) removing the remaining solvent.
DETAILED DESCRIPTION OF TI-IE INVENTION
The microporous polyolefin membrane of the present
invention is made of a polyolefin composition containing 1
weight % or more of an ultra-high-molecular-weight polyalefin
having a weight-average molecular weight of 7 x 105 or more
and having a weight-average molecular weight/number-aver<ige
molecular weight ratio of 10-300.
A weight-average molecular weight/number-average
molecular weight ratio of the polyolefin composition is 10-300,
preferably 12-250. When it is less than 10, the polyolefin
composition has large average molecular chain length, resulting
in an excessively high density of entanglement of molecular
chains when dissolved. Accordingly, it is difficult to prepare a
2 0 high-concentration polymer solution. On the other hand, when
the ratio exceeds 300, lower molecular weight components tend
to be cut when stretched, resulting in the decrease in the overall
strength of the resulting membrane.
Incidentally, the weight-average molecular
2 5 weight/number-average molecular weight ratio is a measure of
a molecular weight distribution, and the larger this molecular
weight ratio, the wider the molecular weight distribution. In the
polyolefin composition comprising polyolefins having different
_7_
weight-average molecular weights, the larger the ratio of
molecular weights in the composition, the larger the difference
of weight-average molecular weights of polyolefins, and vice
versa.
In the present invention, the ratio of weight-average
molecular weight/number-average molecular weight of the
polyolefin composition is 10-300, larger than the weight-average
molecular weight/number-average molecular weight ratio of
ultra-high-molecular-weight polyolefin itself (usually about 6).
As a result, the molecular weight distribution of the polyolefin
composition expands toward a lower molecular weight region,
making it possible to prepare a high-concentration polyolefin
solution.
Such polyolefin composition of the present invention
can be prepared by rni.xing an ultra-high-molecular-weight
polyolefin having a weight-average molecular weight of 7 x 10~
or more with a polyolefin having a weight-average molecular
weight of less than 7 x 105 in such a proportion that a ratio of
weight-average molecular weight/nurnber-average molecular
2 0 weight of the resulting composition is within the above range.
The ultra-high-molecular-weight polyolefin that can
be used in the present invention is one which has a weight-
average molecular weight of 7 x 105 or more, preferably in the
range of 1 x I06 to 15 x 106. With a weight-average molecular
I 2 5 weight lower than 7 x 105, the composition has low maximum
stretching (draw} ratio, failing to provide a desired microporous
membrane. Although there is no upper limit in molecular
weight, polyolefins having a molecular weight in excess of
-g-
15 x 10~ are poor in formability of their gel-like articles.
Examples of such ultra-high-molecular-weight
polyolefins include crystalline homopolymers or copolymers of
ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene,
etc. Preferable among them is ultra-high-molecular-weight
polyethylene, particularly ultra-high-molecular-weight, high-
density polyethylene.
The content of the ultra-high-molf;cular-weight
polyolefin in the polyolefin composition is 1 weight % or more
1 0 per 100 weight % of the polyolefin composition. When the
content of the ultra-high-molecular-weight polyolefin is less
than 1 weight %, the entanglement of molecular chains
contributing to the improvement of stretchability does not take
place, failing to provide a high-strength microporous membrane.
On the other hand, the upper limit of the ultra-high-molecular-
weight polyolefin content is not particularly limited, but when it
exceeds 90 weight %, the desired high-concentration polyolefin
solution cannot be obtained.
Other polyolefins than the ultra-high-molecular-
2 0 weight polyolefin in the polyolefin composition are those having
weight-average rnolecuiar weights of less than 7 x 10s. As a
lower limit of the molecular weight, it is preferably 1 x 104 or
more. When a polyolefin having a weight-average molecular
weight of less than 1 x 104 is used, rupture is likely to take place
2 5 when stretched, failing to provide a desired microporous
membrane. Particularly, the polyolefin having a weight-average
molecular weight of 1 x 105 or more and less than 7 x 105 is
preferably added to the ultra-high-molecular-weight polyolefin.
_g_
CA 02025520 2000-11-02
72177-20
As such polyolefins, the same types of polyolefins as
the above ultra-high-molecular-weight polyolefin can be used,
and particularly high-density polyethylene, which is a polymer
based on ethylene, is preferable.
Incidentally, the above-mentioned ultra-nign-
molecular-weight polyolefin may be incorporated with various
additives such as antioxidants, ultraviolet absorbers, slip agents,
antiblocking agents, pigments, dyes, inorganic fillers, etc., if
required, within limits not harmful to the object of the present
invention.
The method of producing the microporous polyolefin
membrane according to the present invention will be explained
below.
In the present invention, the high-concentration
solution of the polyolefin composition is prepared by dissolving
the above-mentioned polyolefin composition in a solvent while
heating. The solvent is not specifically limited so long as it is
capable of dissolving the polyolefin composition. Examples of
the solvents include aliphatic or cyclic hydrocarbons such as
2 0 nonane, decane, undecane, dodecane, paraffin oils, etc., and
fractions of mineral oils having boiling points substantially equal
to those of these hydrocarbons. Nonvolatile solvents such as
paraffin oils are desirable to obtain the gel-like articles in which
the solvent content is stable.
2 S Dissolution of the polyolefin composition while
heating should be carried out by stirring its solution at a
temperature at which it is completely dissolved in a solvent.
The dissolving temperature varies depending on the types of
- 10 -
polymers and solvents used. It is generally in the range of 140-
250°C in the case of polyethylene composition. The
concentration of the polyolefin compositian solution is preferably
10-50 weight %, preferably 10-40 weight °lo. When the
concentration is less than 10 weight %, a large amount of a
solvent has to be used, and swelling and neck-in are likely to
take place at the exit of a die in the process of forming sheets.
Accordingly, it is difficult to produce large sheets. On the other
hand, when the concentration exceeds SO weight %, it is difficult
to prepare a uniform solution. Incidentally, it is desirable to add
an antioxidant to the solution to protect the polyolefin from
degradation by oxidation.
Next, a heated solution of this polyolefin composition
is extruded through a die. Usually used as a die is a sheet die
having a rectangular orifice, but a hollow die having a circular
orifice, an inflation die, ete. may be used. When the sheet die is
used, a die gap is usually 0.1-5 mm, and heated at 140-250°C in
the extrusion process. In this case, an extrusion speed is usually
20-30 cm/minute to 2-3 m/minute.
2 0 The solution extruded through the die is formed into
a gel-like article by cooling. The cooling is preferably conducted
to a gelation temperature or lowez at a speed of 50°C/minute or
more. When the cooling speed is too low, the crystallization
degree of the gel-like article increases, unsuitable for stretching.
2 5 As a method of cooling, direct contact With cooling air, cooling
water and other cooling media, contact with a roll cooled by a
coolant, etc. may be employed. The solution extruded through a
die may be drawn at a take-up ratio of 1-10, preferably 1-5
- 11 -
iv!
before or after cooling. When the take-up ratio is more than 10,
neck-in is likely to take place, undesirably causing the
breakdown of the sheet when stretched.
The gel-like article is then subjected to an orientation
(stretching) treatment at a predetermined draw ratio while
heating. Orientation is accomplished by an ordinary method
such as a Center method, a roll method, an inflation method, a
calendering method, or a combination thereof. Biaxial
orientation is desirable. It may be carried out by stretching the
sheet in longitudinal and transverse directions simultaneously or
sequentially, and simultaneous biaxial orientation is more
preferable.
The orientation temperature should be equal to or
lower than a temperature which is 10°C above the melting point
of the ultra-high-molecular-weight polyalefin, preferably in the
range from the crystal dispersion temperature to the crystal
melting point. In the case of polyethylene, it is 90-140°C,
preferably 100-130°C. If the orientation temperature is higher
than the melting point plus 10°C, the molecular orientation does
2 0 not take place because the resin melts. If the orientation
temperature is lower than the crystal dispersion temperature,
the membrane tends to break on account of the insufficient
softening of the resin, and the membrane cannot be oriented at a
high draw ratio.
2 5 The draw ratio varies depending on the thickness of
the original membrane. The linear draw ratio in one horizontal
(longitudinal or transverse) direction should be greater than
twofold, preferably 3- to 20-fold, and the areal draw ratio
- 12 -
should be greater than tenfold, preferably 20- to 400-fold. With
an areal draw ratio smaller than 10-fold, the resulting
microporous membrane lacks high modulus and high strength on
account of insufficient orientation. On the other hand, with an
areal draw ratio in excess of 400-fold, difficulties exist in the
orientation operation.
The thus oriented product is subjected to a solvent-
removing treatment. Solvents used for this solvent-removing
treatment may be highly volatile solvents including
hydrocarbons such as pentane, hexane, heptane, etc.; chlorinated
hydrocarbons such as methylene chloride and carbon
tetrachloride; fluorinated hydrocarbons such as trifluoroethane;
and ethers such as diethyl ether and dioxane. These volatile
solvents may be used individually or in combination, and their
1 S selection depends on the types of the nonvolatile solvents used
to dissolve the ultra-high-molecular-weight polyolefin. Washing
methods with these solvents include an extraction method with
solvents, a method of spraying solvents or a combination thereof.
The washing of the stretched article with a solvent
2 0 should be performed to such an extent that the content of the
residual solvent in the oriented product is Iess than 1 weight %.
The stretched article is finally dried to remove the washing
solvent by a heating method, an air-cooling method, etc. The
dried article is desirably heat-set in a temperature range
2 5 between the crystal dispersion temperature and the melting
point.
The microporous ultra-high-molecular-weight
polyolefin membrane produced as mentioned above has a
- 13 -
porosity of 35-95°lo and an average pore diameter of 0.001-0.2
~.m, and a breaking strength of 0.2 kg or more per 15 rnm
width. The thickness of the microporous polyolefin membrane
may vary depending upon its applications, but it is generally
0.1-25 ~.m, preferably 2-20 p.m.
Incidentally, the resulting microporous polyolefin
membrane is, if necessary, subjected to a hydrophilic treatment
by plasma irradiation, impregnation with surfactant, surface
grafting, etc.
The present invention will be explained in further
detail by the following Examples. The test methods used in
Examples are as follows:
( 1 ) Weight-average molecular weight and molecular weight
distribution: Measured by gel permeation
1 5 chromatography (GPC) method at 135°C at a flow rate of
1.0 ml/minute by a GPC apparatus manufactured by
Waters whose column is GMH-6 manufactured by Tosoh
Corporation, using o-dichlorobenzene as a solvent.
(2) Membrane thickness: Determined by measuring a cross
2 0 section of a microporous membrane by a scanning electron
microscope.
(3) Tensile breaking strength: Measured according to ASTM
D882, and expressed in term of load at a break point for a
15-mm-wide specimen.
2 5 (4) Air permeability: Measured according to JIS P8117.
(5) Water permeability: Expressed in terms of the amount of
filtrate which passed through the hydrophilicized
microporous membrane under a hydraulic pressure of 380
- 14 -
mmHg. The hydrophilicization was accomplished by
passing a 50/50 (by volume) mixture of distilled water
and ethanol through the microporous membrane set in a
flat module, followed by thorough washing with water.
(b) Average pore diameter: Expressed in terms of the
concentration of pullulan (manufactured by ~l~owa Denko
K.~.) contained in a filtrate which passed through the
microporous membrane under the differential pressure of
3$0 mmHg when a 0.05 weight °lo aqueous solution of
pullulan was circulated in the module mentioned in (5)
above. The concentration of pullulan was determined by
differential refractometry. The ratio of blocking pullulan
was calculated by the following formula:
Ratio of blocking pulluian = (1 - A/B) x 100
where A is the concentration of pullulan in the filtrate and
B is the concentration of pullulan in the original solution.
High-molecular polymer chains in the solution are
like spherically entangled yarns whose diameter d,
relative to a mean-square distance [Y2] of both ends of a
2 0 molecular chain, is approximately expressed as follows:
[d/2]2 = [y2] ...(1)
According to the Florry theory concerning a viscosity and
expansion of molecular chains in a high-molecular
polymer solution,
2 5 [r)] M=2.1 x 1021 [Y2] ~/2
is satisfied regardless of types of high-molecular
polymers. Accordingly, by the equations ( 1 ) and (2), the
diameter d of linear, high-molecular polymer can be
- 15 -
CA 02025520 2000-11-02
72177-20
calculated from the measured value of [~ ] and a molecular
weight M at which the block ratio is 50%. This d was used
as an average pore diameter of a microporous
polyethylene membrane.
Example 1
A polyethylene composition solution was prepared
from a resin material (Mw/Mn = 16.8) consisting of 2 parts by
weight of ultra-high-molecular-weight polyethylene having a
weight-average molecular weight (Mw) of 2.5 x 106 and 8 parts
by weight of polyethylene having a weight-average molecular
weight of 6.8 x 105, and 90 parts by weight of liquid paraffin (64
cSt at 40°C), and an antioxidant composed of 0.125 parts by
weight of 2,6-di-tert-butyl-p-cresol ("BHT," manufactured by
Sumitomo Chemical Industries Co., Ltd.) and 0.25 parts by
weight of tetrakis [methylene-3-(3,5-di-tert-butyl-4-
hydroxylphenyl)-propionate] methane ("Irganoxk 1010,"
manufactured by Ciba Geigy) were added to 100 parts by weight
of the resulting polyethylene composition solution. The mixture
2 0 was introduced into an autoclave equipped with a stirrer and
stirred to give a uniform solution.
The solution was fed to an extruder of 45 mm in
diameter and extruded from a T-die and taken up by a cooling
roll as a gel-like sheet.
2 5 This sheet was subjected to simultaneous biaxial
orientation by a biaxial stretching machine at 115°C, at a draw
speed of 0.5 m/min and at a draw ratio of 7 x 7. The resulting
oriented membrane was washed with methylene chloride to
*Trade-mark
- 16 -
remove residual liquid paraffin and then dried. Thus there was
obtained a microporous membrane of a polyethylene
composition having a thickness of ~. p,m. The properties of the
microporous membrane are shown in Table 1.
Example 2
A 5-p.m-thick, microporous membrane of a
polyethylene composition was prepared in the same manner as
in Example 1 except that a resin material (Mw/Mn = 16.7)
consisting of 2 parts by weight of ultra-high-molecular-weight
polyethylene having a weight-average molecular weight (Mw) of
2.5 x 106 and 13 parts by weight of polyethylene having a
weight-average molecular weight of 2.4 x lOs, and 85 parts by
weight of liquid paraffin were used to prepare a polyethylene
composition solution. The properties of the microporous
membrane are shown in Table 1.
Example 3
A 4~-p.m-thick, microporous membrane of a
polyethylene composition was prepared in the same manner as
in Example 1 except that a resin material (Mw/Mn = 190)
2 0 consisting of 2 parts by weight of ultra-high-molecular-weight
polyethylene having a weight-average molecular weight (Mw) of
2.5 x 106 and 13 parts by weight of polyethylene having a
weight-average molecular weight of ~.1 x 105, and 85 parts by
weight of liquid paraffin were used to prepare a polyethylene
2 5 composition solution. The properties of the microporous
membrane are shown in Table 1.
Example 4
- 17 -
~a°~ ~"s
.A 16-p.m-thick, microporous membrane of a
polyethylene composition was prepared in the same manner as
in Example 1 except that a resin material (Mw/Mn = 150)
consisting of 1 part by weight of ultra-high-molecular-weight
polyethylene having a weight-average molecular weight (Mw) of
2.5 x 106 and 19 parts by weight of polyethylene having a
weight-average molecular weight of 4.1 x 105, and 80 parts by
weight of liquid paraffin were used to prepare a polyethylene
composition solution. The properties of the microporous '
membrane are shown in Table 1.
Example 5
A 12-~.m-thick, microporous membrane of a
polyethylene composition was prepared in the same manner as
in Example 1 except that a resin material (Mw/Mn ; 24U)
consisting of I part by weight of ultra-high-molecular-weight
polyethylene having a weight-average molecular weight (Mw) of
2.5 x 106 and 39 parts by weight of polyethylene having a
weight-average molecular weight of 3.5 x 105, and 60 parts by
weight of liquid paraffin were used to prepare a polyethylene
2 0 composition solution. The properties of the microporous
membrane are shown in Table 1.
Comparative Example 1
A membrane of polyethylene was prepared in the
same manner as in Example 1 except that 12 parts by weight of
2 5 polyethylene having a weight-average molecular weight (Mw) of
6.8 x 105 (Mw/Mn = 8.0) and 88 parts by weight of liquid
paraffin were used to prepare a polyethylene solution. However,
_ Z8 _
orientation could not be achieved at a high draw ratio, failing to
provide a microporous polyethylene membrane.
Comparative Example 2
A microporous membrane of a polyethylene
composition was prepared in the same manner as in Example 1
except that a resin material (Mw/N%n = 350) consisting of 2 parts
by weight of ultra-high-molecular-weight polyethylene having a
weight-average molecular weight (Mw) of 2.5 x 106 and 13 parts
by weight of polyethylene having a weight-average molecular
weight (Mw) of 5.9 x 105, and 85 parts by weight of liquid
paraffin were used to prepare a polyethylene composition
solution. However, a microporous membrane thus obtained
(thickness: 10 p.m) had poor breaking strength.
omparative Example 3
12 paxts by weight of ultra-high-molecular-weight
polyethylene having a weight-average molecular weight (Mw) of
2.5 x 106 (Mw/Mn = 6.0) was added to 88 parts by weight of
liquid paraffin to prepare a polyethylene solution. Howevex, a
uniform solution could not be prepared.
2 0 Comparative Example 4
A membrane of a polyethylene composition was
prepared in the same manner as in Example 1 except that a resin
material (Mw/Mn = 120) consisting of 0.2 parts by weight of
ultra-high-molecular-weight polyethylene having a weight-
2 5 average molecular weight (Mw) of 2.5 x 106 and 24.8 parts by
weight of polyethylene having a weight-average molecular
weight of 3.5 x 10~, and 75 parts by weight of liquid paraffin
were used to prepare a polyethylene composition solution.
- 19 -
However, the resulting polyethylene composition membrane was
frequently ruptured when stretching, failing to provide a
microporous membrane.
Comparative Example 5
A resin material (Mw/Mn = 14.5) consisting of 2
parts by weight of ultra-high-molecular-weight polyethylene
having a weight-average molecular weight (Mw) of 2.5 x 106 and
58 parts by weight of polyethylene having a weight-average
molecular weight of 2.4 x 106 was added to 40 parts by weight
of liquid paraffin to prepare a polyethylene composition solution.
However, a uniform solution could not be prepared.
- 20 -
f~'r~y~
~ ~~
Table
1
Example No. 1 2 3 4 5
Polyolefin Composition
Mw/Mn 16.8 16.7 190 150 240
Ultra-High-Molecular-Weight
Polyethylene
Mw (x 106)~la 2.5 2.5 2.5 2.5 2.5
Content 20.0 13.3 13.3 5.0 2.5
Other Polyethylene
Mw (x 105)~1~ 6.8 2.4 4.1 4.1 3.5
2 Concentration in
0
Solution~~> (Wt. %) 1 0 1 5 1 S 2 0 4 0
t etching Conditions
2 Stretching Temperature
5
(C) lI5 115 115 115 115.
Stretching Ratio
MDxTD(times) 7x? 7x7 7x7 7x7 7x7
30
Properties
Thickness (~.m) 4 5 4 16 12
35
Freaking Strength
(kg/15 mm width) 0.70 0.65 0.35 0.35 0.40
Air Permeability
4 (sec/100 cc) 14 8 3 7 3 0 5 5 112
0
Water Permeability3 ~ 2 7 2 7 8 4 6 8 0 415
3 4 2
Pore Diameter (~.m) 0.02 0.03 0.04 0.05 0.04
45
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Table 1 (Continued)
Comparative
Examine No. 1 2 3 4 5
Pol.,~,olefin Composition
Mw/Mn 8.0 350 6.0 120 14.5
Ultra-High-Molecular-Weight
Polyethylene
Mw (x 106)~1> - 2.5 2.5 2.5 2.5
Content 0 13.3 100 0.8 3.3
Other Polyethylene
Mw (x 105)~1> 6.8 5.9 - 3.5 2.4
Concentration
in Solutiont2> (Wt. %) 15 12 2 5 6 0
12
Stretchin~Con itions
Stretching Temperature
(C) - 115 - 115 -
Stretching Ratio
3 MD x TD (times) - 7 x 7 - 7 x 7 -
0
Properties
3 5 Thickness (~.m) - 1 0 - - -
Breaking Strength
(kg/15 mm width) - 0.12 - - -
4 0 Air Permeability
(sec/100 cc) - 10 8 - -
Water Permeability~3> - ~ 530 - -
4 5 Pore Diameter (~,m) - 0.05 - -
- 22 -
~,~~
As described above in detail, a polyolefin
composition solution in as high a concentration as 10-50 weight
% can be prepared according to the present invention.
Accordingly, only a small amount of solvent is used, and the
solvent content needs not be controlled in the gel-:like sheet
before stretching. Therefore, the microporous polyolefin
membzane can be produced efficiently. In addition, a gel-like
sheet extruded for the production of a sheet does not
substantially suffer from swelling and neck-in.
The microporous membrane of the present invention
had a high strength and is excellent in handling and working
when laminated with an unwoven fabric. Further, since it has
goad water permeability, it can efficiently separate out fractions
having molecular weights of several tens of thousand to several
hundreds of thousand.
Such microporous polyolefin membrane is suitable
for battery separators, electrolytic capacitor separators,
ultrafiltration membranes, miczofiltration membranes, various
filters, moisture-permeable, waterproof clothes, etc.
2 0 The present invention has been described by the
above Examples, but it should be noted that any modifications
can be made unless they deviate from the scope of the present
invention defined by the claims attached hereto.
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