U.S. patent application number 13/259172 was filed with the patent office on 2012-02-09 for microporous membranes, methods for making such membranes, and the use of such membranes as battery separator film.
This patent application is currently assigned to TORAY TONEN SPECIALTY SEPARATOR GODO KAISHA. Invention is credited to Patrick Brant, Donna J. Crowther, Takeshi Ishihara, Koichi Kono, Satoshi Miyaoka.
Application Number | 20120034518 13/259172 |
Document ID | / |
Family ID | 42828625 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120034518 |
Kind Code |
A1 |
Ishihara; Takeshi ; et
al. |
February 9, 2012 |
MICROPOROUS MEMBRANES, METHODS FOR MAKING SUCH MEMBRANES, AND THE
USE OF SUCH MEMBRANES AS BATTERY SEPARATOR FILM
Abstract
A microporous membrane comprising polyolefin copolymer, the
membrane having a shutdown temperature.ltoreq.130.5.degree. C. and
a shutdown activation energy E2.gtoreq.3.5.times.10.sup.3
J/mol.
Inventors: |
Ishihara; Takeshi;
(Nasushiobara, JP) ; Miyaoka; Satoshi;
(Nasushiobara, JP) ; Kono; Koichi; (Nasushiobara,
JP) ; Crowther; Donna J.; (Seabrook, TX) ;
Brant; Patrick; (Seabrook, TX) |
Assignee: |
TORAY TONEN SPECIALTY SEPARATOR
GODO KAISHA
Nasushiobara, Tochigi
JP
|
Family ID: |
42828625 |
Appl. No.: |
13/259172 |
Filed: |
March 5, 2010 |
PCT Filed: |
March 5, 2010 |
PCT NO: |
PCT/US2010/026426 |
371 Date: |
September 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61164827 |
Mar 30, 2009 |
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61164833 |
Mar 30, 2009 |
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61164817 |
Mar 30, 2009 |
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61164824 |
Mar 30, 2009 |
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61177060 |
May 11, 2009 |
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61220094 |
Jun 24, 2009 |
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Current U.S.
Class: |
429/207 ;
264/210.1; 264/210.7; 429/233; 429/253; 521/139; 521/143 |
Current CPC
Class: |
H01M 50/409 20210101;
H01M 50/40 20210101; Y02E 60/10 20130101; B29C 48/00 20190201; H01M
50/411 20210101; B01D 71/26 20130101; B32B 27/32 20130101; B01D
71/76 20130101 |
Class at
Publication: |
429/207 ;
429/253; 429/233; 521/143; 521/139; 264/210.1; 264/210.7 |
International
Class: |
H01M 2/16 20060101
H01M002/16; H01M 10/056 20100101 H01M010/056; B29C 47/08 20060101
B29C047/08; C08L 23/04 20060101 C08L023/04; B29C 45/00 20060101
B29C045/00; H01M 4/64 20060101 H01M004/64; C08F 110/02 20060101
C08F110/02 |
Foreign Application Data
Date |
Code |
Application Number |
May 25, 2009 |
EP |
09160964.4 |
May 25, 2009 |
EP |
09160965.1 |
May 25, 2009 |
EP |
09160966.9 |
May 25, 2009 |
EP |
09160967.7 |
Jun 25, 2009 |
EP |
09163698.5 |
Aug 19, 2009 |
EP |
09168194.0 |
Claims
1. A microporous membrane comprising polyolefin copolymer, the
membrane having a shutdown temperature.ltoreq.130.5.degree. C. and
a shutdown activation energy E2.gtoreq.3.5.times.10.sup.3
J/mol.
2. The microporous membrane of claim 1, wherein the polyolefin
copolymer is an ethylene-alpha olefin copolymer having a
Tm.ltoreq.130.0.degree. C., an Mw.ltoreq.9.0.times.10.sup.5, an
MWD.ltoreq.10.0, and a comonomer Content in the range of from 1.0
to 5.0 mol. %.
3. The microporous membrane according to claim 1, wherein the
copolymer is produced by a process using a single site
catalyst.
4. The microporous membrane according to claim 1, wherein the
membrane comprises polyolefin copolymer in an amount in the range
of 8.0 to 13.0 wt. %, based on the weight of the membrane.
5. The microporous membrane according to claim 1, wherein the
membrane further comprises a second polyethylene having a
Tm.gtoreq.131.0.degree. C. and an Mw in the range of
1.0.times.10.sup.5 to 9.0.times.10.sup.5 and a third polyethylene
having an Mw>1.0.times.10.sup.6; and the membrane's shutdown
activation energy E2 is in the range of 3.60.times.10.sup.3 J/mol
to 5.50.times.10.sup.3 J/mol.
6. The microporous membrane according to claim 1, wherein the
polyolefin copolymer has a Tm in the range of 122.0.degree. C. to
126.0.degree. C.
7. The microporous membrane, according to claim 1, wherein the
polyolefin copolymer has an Mw in the range of from
1.0.times.10.sup.4 to 1.0.times.10.sup.5.
8. The microporous membrane according to claim 5, wherein the
polyolefin copolymer is a copolymer of ethylene and a second
alpha-olefin, the second alpha olefin being propylene, butene-1,
pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate,
methylmethacrylate, styrene, other comonomer, or combinations
thereof.
9. The microporous membrane of claim 8, wherein the copolymer has
an MWD in the range of 2.5 to 4.5.
10. A battery separator film comprising the microporous membrane of
claim 1.
11. A method for producing a microporous membrane, comprising: (1)
extruding a mixture of diluent and polymer, the polymer comprising
polyolefin copolymer having a Tm.gtoreq.130.0.degree. C., an
Mw.ltoreq.9.0.times.10.sup.5 an MWD.ltoreq.10.0, and a comonomer
content in the range from 1.0 mol. % to 5.0 mol. %, (2) stretching
the extrudate in at least one planar direction, and (3) removing at
least a portion of the diluent from the stretched extrudate to form
the microporous membrane.
12. The method of claim 11, wherein the polymer further comprises
(ii) a second polyolefin having a Tm.gtoreq.131.0.degree. C. and an
Mw.ltoreq.1.0.times.10.sup.6 and/or (iii) a third polyolefin having
an Mw>1.0.times.10.sup.6.
13. The method of claim 12, wherein the first, second, and third
polyolefins are polyethylenes.
14. The method of claim 12, wherein the first polyolefin's Tm is in
the range of 122.0.degree. C. to 126.0.degree. C., the first
polyolefin's Mw is in the range of 1.0.times.10.sup.4 to
1.0.times.10.sup.5, and the first polyolefin's MWD is in the range
of 2.5 to 4.5.
15. The method of claim 11, further comprising stretching the
membrane in at least one planar direction following step (3).
16. The method of claim 11 further comprising subjecting the
membrane to a thermal treatment following step (3).
17. The method of claim 11, further comprising cooling the
extrudate before step (2).
18. The method of claim 11, wherein the stretching of step (2) is
conducted biaxially to a magnification factor in the range of from
9-fold to 49-fold in area, while exposing the extrudate to
temperature (Tm -10.degree. C.).
19. The method of claim 11, further comprising removing any
remaining volatile species from the membrane after step (3).
20. The method of claim 13, wherein the polyolefin copolymer is
present in an amount in the range of 1.0 wt. % to 20.0 wt. %, based
on the weight of the polymer in the polymer-diluent mixture, the
second polyethylene is present in an amount in the range of 25.0
wt. % to 99.0 wt. %, and the third polyethylene is present in an
amount in the range of 0 wt. % to 74.0 wt. %.
21. The membrane product of claim 11.
22. A battery comprising an electrolyte, an anode, a cathode, and a
separator situated between the anode and the cathode, wherein the
separator comprises a microporous membrane having a shutdown
temperature.ltoreq.130.5.degree. C. and a shutdown activation
energy E2.gtoreq.3500 J/mol.
23. The battery of claim 21, wherein the battery is a lithium ion
secondary battery, a lithium-polymer secondary battery, a
nickel-hydrogen secondary battery, a nickel-cadmium secondary
battery, a nickel-zinc secondary battery, or a silver-zinc
secondary battery.
24. The battery of claim 22, wherein the cathode comprises a
current collector, and a cathodic active material layer on the
current collector capable of absorbing and discharging lithium
ions.
25. The battery of claim 23, wherein the electrolyte comprises
lithium salts in an organic solvent.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application Ser. No. 61/177,060 filed May 11, 2009, and EP
09163698.5 filed Jun. 25, 2009; U.S. Provisional Application Ser.
No. 61/164,824 filed Mar. 30, 2009, and EP 09160964.4 filed May 25,
2009; U.S. Provisional Application Ser. No. 61/164,817 filed Mar.
30, 2009, and EP 09160965.1 filed May 25, 2009; U.S. Provisional
Application Ser. No. 61/164,833 filed Mar. 30, 2009, and EP
09160966.9 filed May 25, 2009; U.S. Provisional Application Ser.
No. 61/164,827 filed Mar. 30, 2009, and EP 09160967.7 filed May 25,
2009; U.S. Provisional Application Ser. No. 61/220,094 filed Jun.
24, 2009, and EP 09168194.0 filed Aug. 19, 2009, the contents of
each of which are incorporated by reference in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to microporous membranes comprising
polyolefin and having a good balance of air permeability, shutdown
temperature and shutdown speed. The invention also relates to
methods for making such membranes, and the use of such membranes as
battery separator film in, e.g., lithium ion secondary
batteries.
BACKGROUND OF THE INVENTION
[0003] Microporous membranes can be used as battery separators in,
e.g., primary and secondary lithium batteries, lithium polymer
batteries, nickel-hydrogen batteries, nickel-cadmium batteries,
nickel-zinc batteries, silver-zinc secondary batteries, etc. When
microporous membranes are used for battery separators, particularly
lithium ion battery separators, the membranes' characteristics
significantly affect the properties, productivity and performance
of the batteries. Accordingly, the microporous membrane should have
appropriate permeability, mechanical properties, heat resistance,
dimensional stability, shut down properties, melt down properties,
etc. It is desirable for such batteries to have a relatively low
shutdown temperature, rapid shutdown speed and a relatively high
meltdown temperature for improved battery safety properties,
particularly for batteries exposed to high temperatures under
operating conditions.
[0004] Microporous membranes having a relatively low shutdown
temperature have been produced using polyethylene having a low
molecular weight and/or low melting temperature. While the
membrane's shutdown properties are improved, the use of such
polyethylenes can lead to decreased air permeability and increased
heat shrinkage.
[0005] For example, Japanese Patent Application Laid Open No.
JP2002-128943 discloses microporous membranes having shutdown
temperatures of 130.degree. C. and heat shrinkage
values.gtoreq.8.0%. Other references disclose the microporous
membrane having relatively rapid shutdown speed. For example,
WO96-027633 and Japanese Patent Application Laid Open No.
JP11-269289 discloses a method for using the relationship between
temperature and impedance (or permeability) to evaluate the
membrane's shutdown speed.
[0006] While improvements have been made, microporous membranes
having relatively low shutdown temperature and relatively rapid
shutdown speed are desired.
SUMMARY OF INVENTION
[0007] In an embodiment, the invention relates to microporous
membrane comprising, polyolefin copolymer, the membrane having a
shutdown temperature.ltoreq.130.5.degree. C. and a shutdown
activation energy E2 (as defined below).gtoreq.3.50.times.10.sup.3
J/mol.
[0008] In another embodiment, the invention relates to a
microporous membrane comprising polyolefin copolymer (such as an
ethylene/.alpha.-olefin copolymer) having a melting peak
("Tm").ltoreq.130.0.degree. C., a weight average molecular weight
("Mw").ltoreq.9.0.times.10.sup.5 (e.g.,
Mw.ltoreq.5.0.times.10.sup.5), a molecular weight distribution (MWD
defined as Mw/Mn).ltoreq.10.0, and a comonomer content in the range
of from about 1.0 to 5.0 mol. %.
[0009] In another embodiment, this invention relates to a method
for producing a microporous membrane, comprising: [0010] (1)
extruding a mixture of diluent and polymer, the polymer comprising
polyolefin copolymer having a Tm.ltoreq.130.0.degree. C., an
Mw.ltoreq.9.0.times.10.sup.5, an MWD.ltoreq.10.0, and a comonomer
content in the range from 1.0 mol. % to 5.0 mol. %, [0011] (2)
stretching the extrudate in at least one planar direction, and
[0012] (3) removing at least a portion of the diluent from the
stretched extrudate to form the microporous membrane.
[0013] In another embodiment, the invention relates to a
microporous membrane produced by the preceding process.
[0014] In yet another embodiment, the invention relates to a
battery comprising an anode, a cathode, an electrolyte, and at
least one battery separator located between the anode and the
cathode, the battery separator comprising the microporous membrane
of any of the preceding embodiments. The battery can be, e.g., a
lithium ion primary or secondary battery. The battery can be used
as a power source for, e.g., a power tool such as a
battery-operated saw or drill, an electric vehicle, or a hybrid
electric vehicle, etc.
BRIEF DESCRIPTION OF THE FIGURES
[0015] FIG. 1 is a plot of air permeability as a function of
temperature for a microporous membrane comprising polyethylene, the
membrane having a shutdown temperature of 130.67.degree. C. and a
meltdown temperature of 148.89.degree. C.
[0016] FIG. 2A is a plot of the natural logarithm of the change in
air permeability (Gurley Value) as a function of reciprocal
temperature (in inverse Kelvin) for two microporous membranes
comprising polyolefin copolymer (solid rectangle points and solid
diamond points).
[0017] FIG. 2B is an enlargement of FIG. 2A containing a fit of the
first seven data points for each membrane. The data points are
fitted to the equation
Ln .DELTA. G = Ao E 2 .cndot. A R ( T - 1 ) ##EQU00001##
to obtain the values for E2.
[0018] FIG. 3 shows that the value of E2 is approximately constant
for microporous membranes having a polyolefin copolymer content in
the range of 5 wt. % to 20 wt. %.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The invention is based on the discovery that when a
microporous membrane comprises polyolefin copolymer having a Tm, an
Mw, and a comonomer content in desired ranges, the shutdown
characteristics of the membrane (shutdown temperature and shutdown
speed) is improved. The membrane's shutdown speed is modeled as a
reaction where at least a portion of the polyolefin in the
microporous membrane migrates into the membrane's pores, thereby
reducing membrane permeability. The shutdown reaction can be
described thermodynamically in terms of the Arrhenius equation
k=Ae.sup.E2/RT where the rate constant k is the change in membrane
permeability, E2 is the shutdown activation energy, R is the gas
constant, T is absolute temperature, and A is a pre-exponential
factor. See FIGS. 1 and 2.
[0020] It has been discovered that membrane's shutdown
characteristics are improved when the membrane comprises polyolefin
copolymer having a Tm.ltoreq.130.0.degree. C., an
Mw.ltoreq.9.times.10.sup.5, an MWD.ltoreq.10, and a comonomer
content in the range of from about 1.0 to 5.0 mol. %. FIG. 3 shows
that for amounts of such a polyolefin copolymer greater than a
minimum value of about 3 wt. % based on the weight of the membrane,
the value of E2 depends on the selected polyolefin copolymer
species and is relatively independent of the amount used (as is
expected for reactions satisfying an Arrhenius-type relationship).
Moreover, FIGS. 2 and 3 show that the relationship between the
amount of selected polyolefin copolymer and value of the E2/RT
slope is specific to the selected polyolefin copolymer and is
relatively independent of the amount present in the membrane.
[0021] While not wishing to be bound by any theory or model, it is
believed that the increase in membrane shutdown speed results at
least in part from copolymer swelling that occurs as the copolymer
becomes molten. For copolymer having a Tm.ltoreq.130.0.degree. C.,
the largest change in copolymer density (greatest amount of
swelling) is observed when the comonomer content is .ltoreq.5.0
mol. %.
[1] COMPOSITION AND STRUCTURE OF THE MICROPOROUS MEMBRANE
[0022] In an embodiment, the invention relates to microporous
membranes, especially monolayer membranes, having a low shutdown
temperature, improved shutdown speed, and good air permeability;
and an improved balance of these properties. In another embodiment,
the invention relates to a method for producing such membranes. In
the production method, an initial method step involves combining
polymer resins, e.g., polyolefin resins such as polyethylene
resins, with a diluent, and then extruding the polymer and diluent
to make an extrudate. The process conditions in this initial step
can be the same as those described in PCT Publications WO
2007/132942 and WO 2008/016174, for example, which are incorporated
by reference herein in their entirety.
[0023] In an embodiment, the microporous membrane comprises the
polymer used to produce the extrudate, wherein the polymer resin
can comprise, e.g., (a) a first polyolefin comprising polyolefin
copolymer having an Mw.ltoreq.9.0.times.10.sup.5, an MWD.ltoreq.10,
a Tm.ltoreq.130.0.degree. C., and comonomer content in the range
from 1.0 wt. % to 3.0 mol. %, (b) a second polyolefin having a
Tm.gtoreq.130.degree. C. and an Mw.ltoreq.1.times.10.sup.6 and (c)
a third polyolefin having an Mw>1.times.10.sup.6. In an
embodiment, the microporous membrane is a monolayer membrane, i.e.,
it is not laminated or coextruded with additional layers. The
membrane produced from the extrudate can consist essentially of or
even consist of a single layer comprising polyethylene.
[0024] In an embodiment, the microporous membrane comprises the
polyolefin copolymer in an amount.gtoreq.1.0 wt. %, e.g., in the
range of from about 1.0 wt. % to about 20.0 wt. %; the second
polyolefin in amount.ltoreq.99.0 wt. %; and the third polyolefin in
an amount.ltoreq.99.0 wt. %, the weight percents being based on the
weight of the microporous membrane. For example, in one embodiment
the microporous membrane comprises (a) from about 1.0 wt. % to
about 20.0 wt. %, e.g., from about 4.0 wt. % to about 17.0 wt. %,
such as from about 8.0 wt. % to about 13.0 wt. %, of the first
polyolefin (i.e., the polyolefin copolymer); (b) from about 25.0
wt. % to about 99.0 wt. %, e.g., from about 50.0 wt. % to about
95.0 wt. %, such as from about 60.0 wt. % to about 85.0 wt. % of
the second polyolefin; and (c) from about 0 wt. % to about 74.0 wt.
%, e.g., from about 1.0 wt. % to about 46.0 wt. %, such as from
about 7.0 wt. % to about 32.0 wt. % of the third polyolefin.
[0025] In an embodiment, the invention relates to a method for
producing a mono-layer microporous membrane. In an embodiment, the
microporous membrane further comprises a second membrane. The
second membrane can be, e.g., a microporous membrane; and can be,
e.g., in the form of a layer in the first microporous membrane.
[2] MATERIALS USED TO PRODUCE THE MICROPOROUS MEMBRANE
[0026] In an embodiment the microporous membrane is made by
extruding a mixture of polymer and diluent. The diluent can be a
solvent for the polymer. When the polymer is soluble in or miscible
with the diluent, the polymer-diluent mixture can be called a
polymeric solution. When the polymer is polyolefin and the diluent
is liquid paraffin, the mixture can be called a polyolefin
solution. When the polymer is a mixture of polymers, e.g., a
combination of polyolefins, it can be called a polymeric
composition, e.g., a polyolefin composition. The polymer can be a
mixture of individual polymer components or a reactor blend, for
example. In an embodiment, the membrane is produced from diluent
and a mixture of polyolefin, where the diluent is a solvent for the
polyolefin mixture such as liquid paraffin. Examples of polyolefin
useful in this embodiment will now be described in more detail.
[0027] In an embodiment, the microporous membrane comprises a
combination (e.g., a mixture, reactor blend, etc.) of polyolefins,
such as a mixture of polyethylenes. The polyethylenes can comprise
polyolefin (homopolymer or copolymer) containing recurring ethylene
units. Optionally, the polyethylene comprises polyethylene
homopolymer or copolymer wherein at least 85% of the recurring
units are ethylene units.
[0028] While the invention is described in terms of these
embodiments, it is not limited thereto, and the description of
these embodiments is not meant to foreclose other embodiments
within the broader scope of the invention.
(1) The First Polyolefin
[0029] In an embodiment, the first polyolefin comprises
polyethylene copolymer, the polyethylene copolymer having an
Mw.ltoreq.9.0.times.10.sup.5, e.g., in the range of from about
1.times.10.sup.4 to about 5.times.10.sup.5, for example from about
3.0.times.10.sup.3 to about 3.0.times.10.sup.5, such as from
1.0.times.10.sup.4 to 1.0.times.10.sup.5. Optionally, the first
polyethylene copolymer has an MWD.ltoreq.10.0, e.g., from about 2.0
to about 10.0, such as from about 2.5 to about 4.5. The
polyethylene is a copolymer of ethylene and a comonomer such as
.alpha.-olefin. The .alpha.-olefin can be, e.g., propylene,
butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl
acetate, methyl methacrylate, styrene, other comonomer, or
combinations thereof. In an embodiment, the .alpha.-olefin is
propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1,
octene-1, and combinations thereof. In another embodiment, the
comonomer is hexene-1 and/or octene-1. The amount of comonomer in
the comonomer is .ltoreq.5.0 mol. %, e.g., in the range of 1.0 mol.
% to 5.0 mol. %, such as from 1.25 mol. % to 4.50 mol. %.
[0030] The polymer can be produced by any convenient process, such
as those using a Ziegler-Natta or single-site polymerization
catalyst. Optionally, the first polyethylene is one or more of a
low density polyethylene ("LDPE"), a medium density polyethylene, a
branched low density polyethylene, or a linear low density
polyethylene, such as a polyethylene produced by metallocene
catalyst. For example, the polymer can be produced according to the
methods disclosed in U.S. Pat. No. 5,084,534 (such as the methods
disclosed therein in examples 27 and 41), which is incorporated by
reference herein in its entirety.
[0031] Tm is measured in accordance with JIS K7122 as follows. A
sample of the first polyethylene is prepared as a 0.5-mm-thick
molding that is melt-pressed at 210.degree. C. and then stored for
about 24 hours while exposed to a temperature of about 25.degree.
C. The sample is then placed in a sample holder of a differential
scanning calorimeter (Pyris Diamond DSC available from Perkin
Elmer, Inc.) and exposed to a temperature of 25.degree. C. in a
nitrogen atmosphere. The sample is then exposed to an increasing
temperature (the first heating cycle) at a rate of 10.degree.
C./minute until a temperature of 230.degree. C. is reached. The
sample is exposed to the 230.degree. C. temperature for 1 minute
and then exposed to a decreasing temperature at a rate of
10.degree. C./minute until a temperature of 30.degree. C. is
reached. The sample is exposed to the 30.degree. C. temperature for
1 minute, and is then exposed to an increasing temperature at a
rate of 10.degree. C./minute (the second heating cycle) until a
temperature of 230.degree. C. is reached. The DSC records the
amount of heat flowing to the sample during the second heating
cycle. Tm is the temperature of the maximum heat flow to the sample
as recorded by the DSC in the temperature range of 30.degree. C. to
200.degree. C. Polyethylene may show secondary melting peaks
adjacent to the principal peak, and/or the end-of-melt transition,
but for purposes herein, such secondary melting peaks are
considered together as a single melting point, with the highest of
these peaks being considered the Tm.
[0032] In an embodiment, the first polyethylene has
Tm.ltoreq.130.0.degree. C., e.g., in the range of from
120.0.degree. C. to 128.0.degree. C., such as 120.0.degree. C. to
126.0.degree. C., or 121.0.degree. C. to 124.0.degree. C. In
another embodiment, the first polyethylene has Tm in the range of
from 122.0.degree. C. to 126.0.degree. C.
(2) The Second Polyolefin
[0033] In an embodiment, the second polyolefin comprises
ethylene-based homopolymer or copolymer (the second polyethylene)
having a Tm>130.0.degree. C., such as Tm.gtoreq.131.degree. C.,
and an Mw.ltoreq.1.0.times.10.sup.6, e.g., in the range of from
1.0.times.10.sup.5 to 9.0.times.10.sup.5, for example from about
4.times.10.sup.5 to about 8.times.10.sup.5. Optionally, the second
polyethylene has a molecular weight distribution
("MWD").ltoreq.100, e.g., in the range of from about 1 to about
100, such as from about 3.0 to 20.0. For example, the second
polyethylene can be one or more of a high density polyethylene
("HPDE"), a medium density polyethylene, a branched low density
polyethylene, or a linear low density polyethylene. Optionally, the
second polyethylene is HDPE. Optionally, the second polyethylene
has terminal unsaturation. For example, the second polyethylene can
have an amount of terminal unsaturation.gtoreq.0.20 per 10,000
carbon atoms, e.g., .gtoreq.5 per 10,000 carbon atoms, such as
.gtoreq.10 per 10,000 carbon atoms. The amount of terminal
unsaturation can be measured in accordance with the procedures
described in PCT Publication WO1997/23554, for example. In another
embodiment, the second polyethylene has an amount of terminal
unsaturation<0.20 per 10,000 carbon atoms. Optionally, the
amount of second polyethylene is in the range of from 25.0 wt. % to
99.0 wt. %, or 50.0 wt. % to 95.0 wt. %, based on the total amount
of polymer used to produce the membrane.
[0034] In an embodiment, the second polyethylene is at least one of
(i) an ethylene homopolymer or (ii) a copolymer of ethylene and
.ltoreq.10.0 mol. % of a comonomer such as propylene, butene-1,
pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate,
methyl methacrylate, styrene, other comonomer, and combinations
thereof.
[0035] The second polyethylene can be produced by any suitable
polymerization process, such as those using a Ziegler-Natta or
single-site polymerization catalyst.
(3) The Third Polyolefin
[0036] In an embodiment, the third polyolefin comprises
ethylene-based homopolymer or copolymer (the third polyethylene)
having an Mw>1.0.times.10.sup.6, e.g., in the range of
1.1.times.10.sup.6 to about 5.times.10.sup.6, for example from
about 1.2.times.10.sup.6 to about 3.times.10.sup.6, such as about
2.times.10.sup.6. Optionally, the third polyethylene has an
MWD.ltoreq.100, e.g., from about 2 to about 100, such as from about
4 to about 20 or 4.5 to 10.0. For example, the third polyethylene
can be an ultra-high molecular weight polyethylene ("UHMWPE").
Optionally, the amount of the third polyethylene is in the range of
from 0 wt. % to 74.0 wt. %, 1.0 wt. % to 46.0 wt. %, or 7.0 wt. %
to 32.0 wt. %, based on the total weight of polymer used to produce
the membrane. In an embodiment, the third polyethylene is at least
one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene
and .ltoreq.10.0 mol. % of a comonomer such as propylene, butene-1,
pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate,
methyl methacrylate, styrene, other comonomer, and combinations
thereof.
(4) Characterization of Polyethylene Resin
[0037] The Mw of the first, second and third polyethylene is
determined using a High Temperature Size Exclusion Chromatograph,
or "SEC", (GPC PL 220, available from Polymer Laboratories),
equipped with a differential refractive index detector (DRI). Three
PLgel Mixed-B columns (available from Polymer Laboratories) are
used. The nominal flow rate is 0.5 cm.sup.3/min, and the nominal
injection volume was 300 .mu.L. Transfer lines, columns, and the
DRI detector are contained in an oven maintained at 145.degree. C.
The measurement is made in accordance with the procedure disclosed
in "Macromolecules, Vol. 34, No. 19, pp. 6812-6820 (2001)".
[0038] The GPC solvent used is filtered Aldrich reagent grade
1,2,4-Trichlorobenzene (TCB) containing approximately 1000 ppm of
butylated hydroxy toluene (BHT). The TCB is degassed with an online
degasser prior to introduction into the SEC. Polymer solutions are
prepared by placing dry polymer in a glass container, adding the
desired amount of above TCB solvent, then heating the mixture at
160.degree. C. with continuous agitation for about 2 hours. The
concentration of polymer in the polymer solution is 0.25 to 0.75
mg/ml. The sample solution is filtered off-line before injecting
into the GPC with 2 .mu.m filter using a model SP260 Sample Prep
Station (available from Polymer Laboratories).
[0039] The separation efficiency of the column set is calibrated
with a calibration curve generated using seventeen individual
polystyrene standards ranging in Mp ("Mp" being defined as the peak
in Mw) from about 580 to about 10,000,000, which is used to
generate the calibration curve. The polystyrene standards are
obtained from Polymer Laboratories (Amherst, Mass.). A calibration
curve (logMp vs. retention volume) is generated by recording the
retention volume at the peak in the DRI signal for each PS
standard, and fitting this data set to a 2nd-order polynomial.
Samples are analyzed using IGOR Pro, available from Wave Metrics,
Inc.
[0040] The content of comonomer in the first polyethylene is
determined by measurement of methyl group content in the copolymer.
The instrument used is a 400 MHz Varian pulsed Fourier transform
NMR spectrometer equipped with a variable temperature proton
detection probe operating at 120.degree. C. The polymer sample is
dissolved in 1,1,2,2-tetrachloroethane-d.sub.2 and transferred into
a 5 mm glass NMR tube. The acquisition parameters are SW=10 KHz,
pulse width=30.degree., acquisition time=2 s, acquisition delay=5 s
and number of scans=120. The methyl region from 0.85 to 1.05 ppm
(MRA) is integrated separately from the aliphatic region integrated
from 0 to 2.1 ppm (IA). The number of methyl groups/1000 C's is
determined from the formula:
(MRA.times.1000/3)/(IA/2.times.MIS/IS). MIS is the methyl integral
scale between 0.85 and 1.05 ppm and IS is the aliphatic integral
scale between 0 and 2.1 ppm.
Additional Polymer
[0041] In addition to the polyethylene resin(s), the polyolefin
mixture can optionally contain additional polymers such as a fourth
polyolefin. The fourth polyolefin can be one or more homopolymer or
copolymer of, e.g., polypropylene, polybutene-1, polypentene-1,
poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl
acetate, polymethyl methacrylate, polystyrene, etc. Optionally, the
fourth polyolefin has an Mw in the range of about 1.times.10.sup.4
to about 4.times.10.sup.6. When used, the amount of the fourth
polyolefin is generally in the range of <20.0 wt. %, based on
the weight of the polymer used to produce the microporous membrane,
such as in the range of 0.5 wt. % to 10.0 wt. %. The polyolefin
composition can also contain a polyethylene wax, e.g., one having
an Mw of about 1.times.10.sup.3 to about 1.times.10.sup.4. When
used, the amount of polyethylene wax is generally <about 20.0%
wt. % of the combined weight of the first second and third polymers
and the polyethylene wax used to produce the microporous membrane.
In an embodiment, the amount of polyethylene wax is <10.0 wt. %,
such as in the range of 0.5 wt. % to 10 wt. %. When used, the
amount of fourth polymer and/or polyethylene wax is not critical
provided they are not used in an amount that would cause
significant deterioration of the properties of the microporous
polyolefin membrane. In an embodiment, the fourth polymer is
polypropylene having an Mw.gtoreq.1.0.times.10.sup.6 and a heat of
fusion (second melt).gtoreq.90 J/g. Suitable polypropylenes are
described in PCT Patent Publication No. WO2007/132942, which is
incorporated by reference herein in its entirety.
[3] METHOD FOR PRODUCING THE MICROPOROUS MEMBRANE
[0042] In an embodiment, the microporous membrane is a monolayer
(i.e., single-layer) membrane produced from the extrudate. The
extrudate can be produced from polyolefin and diluent as
follows.
[0043] In an embodiment, the microporous membrane is produced by a
process comprising: (1) combining diluent (e.g., a membrane-forming
solvent) and polyolefin, (2) extruding the combined diluent and
polyolefin through a die to form an extrudate; (3) optionally
cooling the extrudate to form a cooled extrudate, e.g., a gel-like
sheet; (4) stretching the cooled extrudate in at least one planar
direction, e.g., stretching in the transverse direction (TD), the
machine direction (MD), or both; and (5) removing at least a
portion of the diluent from the extrudate or cooled extrudate to
form a membrane.
[0044] Optionally, the process further comprises (6) removing at
least a portion of any remaining volatile species from the membrane
at any time after step (5).
[0045] Optionally, the process further comprises (7) subjecting the
membrane to a thermal treatment (such as heat setting or annealing)
at any time after step (5).
[0046] Optionally, the process further comprises stretching the
membrane in at least one planar direction at any time after step
(5), e.g., between steps (6) and (7). For example, the process can
further comprise (8) stretching the dried membrane of step (6) in
the MD from the first dry length to a second dry length larger than
the first dry length by a magnification factor in the range of from
about 1.1 to about 1.5 and stretching the membrane in the TD from a
first dry width to a second width that is larger than the first dry
width by a magnification factor in the range of from about 1.1 to
about 1.3; and then (9) decreasing the second dry width to a third
dry width, the third dry width being in the range of from the first
dry width to about 1.1 times larger than the first dry width.
[0047] An optional hot solvent treatment step, an optional heat
setting step, an optional cross-linking step with ionizing
radiation, and an optional hydrophilic treatment step, etc., as
described in PCT Publication WO2008/016174 can be conducted if
desired. Neither the number nor order of the optional steps is
critical.
(1) Combining Polyolefin and Diluent
[0048] The polyolefin mixture as described above can be combined,
e.g., by dry mixing or melt blending, and then the polyolefin
mixture can be combined with at least one diluent to produce a
polyolefin-diluent mixture, e.g., a polyolefin solution.
Alternatively, the polyolefin mixture and diluent can be combined
in a single step. The resins and solvents can be added
sequentially, in parallel, or in a combination thereof.
Alternatively, a polyolefin mixture can produced by first combining
at least a portion of the resins to make a polyolefin composition,
and then combining the polyolefin composition with at least one
membrane-forming solvent (and optionally additional portions of the
resins and/or additional resins) to produce a polyolefin solution.
Optionally, the polyolefin solution contains additives such as one
or more of antioxidant, fine silicate powder (pore-forming
material), etc. The amount of such additives is not critical,
provided they are not present in amounts large enough to adversely
affect the membrane's properties. Generally, the amount of such
additives in aggregate does not exceed 1 wt. %, based on the weight
of the polyolefin solution.
[0049] The use of a diluent comprising liquid membrane-forming
solvent can make it less difficult to conduct stretching at
relatively high magnifications. The liquid solvents can be, for
example, aliphatic, alicyclic or aromatic hydrocarbons such as
nonane, decane, decalin, p-xylene, undecane, dodecene; liquid
paraffin; mineral oil distillates having boiling points comparable
to those of the above hydrocarbons; and phthalates liquid at room
temperature, such as dibutyl phthalate, dioctyl phthalate, etc. The
use of a non-volatile solvent such as liquid paraffin can make it
easier to obtain a gel-like molding (or gel-like sheet) having a
stable solvent content. In an embodiment, one or more solid
solvents, which are miscible with the polyolefin solution or
polyolefin composition during melt-blending but solid at room
temperature, may be added to the liquid solvent. Such solid
solvents can be, e.g., stearyl alcohol, ceryl alcohol, paraffin
waxes, etc. Solid solvent can be used without liquid solvent, but
in this case it can be more difficult to evenly stretch the
gel-like sheet during step (4).
[0050] In an embodiment, the viscosity of the liquid solvent ranges
from about 30 cSt to about 500 cSt, or from about 30 cSt to about
200 cSt, when measured at a temperature of 25.degree. C. Although
the choice of viscosity is not particularly critical, when the
viscosity at 25.degree. C. is less than about 30 cSt, the
polyolefin solution might foam, resulting in difficulty in
blending. On the other hand, when the viscosity is more than about
500 cSt, it can be more difficult to remove the solvent during step
(5). The polyolefin solution can contain additives such as one or
more antioxidant. In an embodiment, the amount of such additives
does not exceed 1 wt. % based on the weight of the polyolefin
solution.
[0051] The amount of membrane-forming solvent used to produce the
extrudate is not critical, and can be in the range, e.g., of from
about 25 wt. % to about 99 wt. % based on the weight of the
combined membrane-forming solvent and polyolefin composition with
the balance being polymer, e.g., the combined first, second, and
third polyethylene.
(2) Extruding
[0052] In an embodiment, the combined polyolefin composition and
the diluent (a membrane-forming solvent in this case) are conducted
from an extruder to a die.
[0053] The extrudate or cooled extrudate should have an appropriate
thickness to produce, after the stretching steps, a final membrane
having the desired thickness (generally 3 .mu.m or more). For
example, the extrudate can have a thickness in the range of about
0.1 mm to about 10 mm, or about 0.5 mm to 5 mm. Extrusion is
generally conducted with the mixture of polyolefin composition and
membrane-forming solvent in the molten state. When a sheet-forming
die is used, the die lip is generally heated to an elevated
temperature, e.g., in the range of 140.degree. C. to 250.degree. C.
Suitable process conditions for accomplishing the extrusion are
disclosed in PCT Publications WO 2007/132942 and WO 2008/016174.
The machine direction ("MD") is defined as the direction in which
the extrudate is produced from the die. The transverse direction
("TD") is defined as the direction perpendicular to both MD and the
thickness direction of the extrudate. The extrudate can be produced
continuously from a die, or it can be produced from the die in
portions (as is the case in batch processing) for example. The
definitions of TD and MD are the same in both batch and continuous
processing.
(3) Optional Extrudate Cooling
[0054] Optionally the extrudate can be exposed to a temperature in
the range of 5.degree. C. to 40.degree. C. to form a cooled
extrudate. Cooling rate is not particularly critical. For example,
the extrudate can be cooled at a cooling rate of at least about
30.degree. C./minute until the temperature of the extrudate (the
cooled temperature) is approximately equal to the extrudate's
gelation temperature (or lower). Process conditions for cooling can
be the same as those disclosed in PCT Publications No. WO
2008/016174 and WO 2007/132942, for example.
(4) Stretching the Extrudate
[0055] The extrudate or cooled extrudate is stretched in at least
one direction. The extrudate can be stretched by, for example, a
tenter method, a roll method, an inflation method or a combination
thereof, as described in PCT Publication No. WO 2008/016174, for
example. The stretching may be conducted monoaxially or biaxially,
though the biaxial stretching is preferable. In the case of biaxial
stretching, any of simultaneous biaxial stretching, sequential
stretching or multi-stage stretching (for instance, a combination
of the simultaneous biaxial stretching and the sequential
stretching) can be used, though simultaneous biaxial stretching is
preferable. When biaxial stretching is used, the amount of
magnification need not be the same in each stretching
direction.
[0056] The stretching magnification factor can be, for example, 2
fold or more, preferably 3 to 30 fold in the case of monoaxial
stretching. In the case of biaxial stretching, the stretching
magnification factor can be, for example, 3 fold or more in any
direction (e.g., in the range of 3 fold to 30 fold), such as 16
fold or more, e.g., 25 fold or more, in area magnification. An
example of this stretching step includes stretching from about 9
fold to about 49 fold in area magnification. Again, the amount of
stretch in each direction need not be the same. The magnification
factor operates multiplicatively on film size. For example, a film
having an initial width (TD) of 2.0 cm that is stretched in TD to a
magnification factor of 4 fold will have a final width of 8.0 cm.
The machine direction ("MD") is a direction in the plane of the
film (the extrudate in this instance) which is oriented
approximately along the direction of travel as the film is formed,
i.e., the longest axis of the film during production. The
transverse direction ("TD") also lies in the plane of the film and
is approximately perpendicular to both the machine direction and a
third axis approximately parallel to the thickness of the film.
[0057] While not required, the stretching can be conducted while
exposing the extrudate to a temperature (the stretching
temperature) in the range of from about Tcd to Tm, where in this
instance Tcd and Tm are the crystal dispersion temperature and
melting peak of the polyethylene having the lowest melting peak
among the polyethylenes used to produce the extrudate (usually the
first polyethylene). The crystal dispersion temperature is
determined by measuring the temperature characteristics of dynamic
viscoelasticity according to ASTM D 4065. In an embodiment where
Tcd is in the range of about 90.degree. C. to 100.degree. C., the
stretching temperature can be from 90.0.degree. C. to 125.0.degree.
C.; e.g., from about 100.degree. C. to 125.0.degree. C., such as
from 105.degree. C. to 125.0.degree. C. Optionally, the stretching
temperature is .ltoreq.(Tm -10.0.degree. C.).
[0058] In an embodiment, the stretched extrudate undergoes an
optional thermal treatment before diluent removal. In the thermal
treatment, the stretched extrudate is exposed to a temperature that
is higher (warmer) than the temperature to which the extrudate is
exposed during stretching. The planar dimensions of the stretched
extrudate (length in MD and width in TD) can be held constant while
the stretched extrudate is exposed to the higher temperature. Since
the extrudate contains polyolefin and diluent, its length and width
are referred to as the "wet" length and "wet" width. In an
embodiment, the stretched extrudate is exposed to a temperature in
the range of 120.0.degree. C. to 125.0.degree. C. for a time
sufficient to thermally treat the extrudate, e.g., a time in the
range of 1 second to 100 seconds while the wet length and wet width
are held constant, e.g., by using tenter clips to hold the
stretched extrudate along its perimeter. In other words, during the
thermal treatment, there is no magnification or demagnification
(i.e., no dimensional change) of the stretched extrudate in MD or
TD.
[0059] In this step and in other steps such as dry orientation and
heat setting where the sample (e.g., the extrudate, dried
extrudate, membrane, etc.) is exposed to an elevated temperature,
this exposure can be accomplished by heating air and then conveying
the heated air into proximity with the sample. The temperature of
the heated air, which is generally controlled at a set point equal
to the desired temperature, is then conducted toward the sample
through a plenum for example. Other methods for exposing the sample
to an elevated temperature, including conventional methods such as
exposing the sample to a heated surface, infra-red heating in an
oven, etc. can be used with or instead heated air.
(5) Diluent Removal
[0060] In an embodiment, at least a portion of the diluent is
removed (or displaced) from the stretched extrudate to form a dried
membrane. A displacing (or "washing") solvent can be used to remove
(wash away, or displace) the diluent, as described in PCT
Publication No. WO 2008/016174, for example.
(6) Drying the Membrane
[0061] In an embodiment, at least a portion of any remaining
volatile species (e.g., washing solvent) is removed from the dried
membrane after diluent removal. Any method capable of removing the
washing solvent can be used, including conventional methods such as
heat-drying, wind-drying (moving air), etc. Process conditions for
removing volatile species such as washing solvent can be the same
as those disclosed in PCT Publication No. WO 2008/016174, for
example.
(7) Thermal Treatment
[0062] In am embodiment, the membrane is subjected to a thermal
treatment such as heat setting. During heat-setting, the membrane
is, e.g., exposed to a temperature in the range of from about Tcd
to about Tm, for example from 90.0.degree. C. to 130.0.degree. C.,
from about 100.degree. C. to 128.degree. C. or 105.degree. C. to
125.degree. C. In this instance Tm is the melting peak of the
polymer having the lowest melting peak among the polymers used to
produce the membrane, e.g., the first polyethylene.
(8) Stretching the Membrane (Dry Orientation)
[0063] Optionally, the dried membrane of the step (6) can be
stretched (called "dry stretching" since at least a portion of the
diluent has been removed or displaced) in at least one direction
between the step (6) and (7). A dried membrane that has been dry
stretched is called an "oriented" membrane. Before dry stretching,
the dried membrane has an initial size in MD (a first dry length)
and an initial size in TD (a first dry width). As used herein, the
term "first dry width" refers to the size of the dried membrane in
the TD prior to the start of dry orientation. The term "first dry
length" refers to the size of the dried membrane in the MD prior to
the start of dry orientation. Tenter stretching equipment of the
kind described in WO 2008/016174 can be used, for example.
[0064] The dried membrane can be stretched in MD from the first dry
length to a second dry length that is larger than the first dry
length by a magnification factor (the "MD dry stretching
magnification factor") in the range of from about 1.1 to about 1.5.
When TD dry stretching is used, the dried membrane can be stretched
in TD from the first dry width to a second dry width that is larger
than the first dry width by a magnification factor (the "TD dry
stretching magnification factor"). Optionally, the TD dry
stretching magnification factor is .ltoreq.the MD dry stretching
magnification factor. The TD dry stretching magnification factor
can be in the range of from about 1.1 to about 1.3. The dry
stretching (also called re-stretching since the membrane-forming
solvent-containing extrudate has already been stretched) can be
sequential or simultaneous in MD and TD. Since TD heat shrinkage
generally has a greater effect on battery properties than does MD
heat shrinkage, the amount of TD magnification generally does not
exceed the amount of MD magnification. When TD dry stretching is
used, the dry stretching can be simultaneous in MD and TD or
sequential. When the dry stretching is sequential, generally MD
stretching is conducted first followed by TD stretching.
[0065] The dry stretching can be conducted while exposing the dried
membrane to a temperature.ltoreq.Tm, e.g., in the range of from
about Tcd-30.degree. C. to Tm. In this instance Tm is the melting
peak of the polymer having the lowest melting peak among the
polymers used to produce the membrane, e.g., the first
polyethylene. In an embodiment, the stretching temperature is
conducted with the membrane exposed to a temperature in the range
of from about 70.0 to about 130.0.degree. C., for example from
about 80.degree. C. to about 129.0.degree. C. In an embodiment, the
MD stretching is conducted before TD stretching, and [0066] (i) the
MD stretching is conducted while the membrane is exposed to a first
temperature in the range of Tcd -30.degree. C. to about Tm
-10.degree. C., for example 70.0.degree. C. to 129.0.degree. C., or
about 80.degree. C. to about 125.degree. C., and [0067] (ii) the TD
stretching is conducted while the membrane is exposed to a second
temperature that is higher than the first temperature but lower
than Tm, for example 70.0.degree. C. to 129.0.degree. C., or about
105.degree. C. to about 125.degree. C., or about 110.degree. C. to
about 120.degree. C.
[0068] In an embodiment, the total MD dry stretching magnification
factor is in the range of from about 1.1 to about 1.5, such as 1.2
to 1.4; the total TD dry stretching magnification factor is in the
range of from about 1.1 to about 1.3, such as 1.15 to 1.25; the MD
dry stretching is conducted before the TD dry stretching, the MD
dry stretching is conducted while the membrane is exposed to a
temperature in the range of 80.0.degree. C. to about 120.0.degree.
C., and the TD dry stretching is conducted while the membrane is
exposed to a temperature in the range of 115.0.degree. C. to about
130.0.degree. C., but less than Tm.
[0069] The stretching rate is preferably 3%/second or more in the
stretching direction (MD or TD), and the rate can be independently
selected for MD and TD stretching. The stretching rate is
preferably 5%/second or more, more preferably 10%/second or more,
e.g., in the range of 5%/second to 25%/second. Though not
particularly critical, the upper limit of the stretching rate is
preferably 50%/second to prevent rupture of the membrane.
(9) Controlled Reduction of the Membrane's Width (Heat-Relaxing of
the Membrane)
[0070] Following the dry stretching, the dried membrane is
optionally subjected to a controlled reduction in width from the
second dry width to a third dry width, the third dry width being in
the range of from the first dry width to about 1.1 times larger
than the first dry width. The width reduction generally conducted
while the membrane is exposed to a temperature.ltoreq.Tcd
-30.degree. C., but no greater than Tm of the first polyethylene.
For example, during width reduction the membrane can be exposed to
a temperature in the range of from 70.0.degree. C. to about
130.0.degree. C., such as from about 115.degree. C. to about
130.0.degree. C., e.g., from about 120.degree. C. to about
128.degree. C. In an embodiment, the decreasing of the membrane's
width is conducted while the membrane is exposed to a temperature
that is lower than Tm of the first polyethylene. In an embodiment,
the third dry width is in the range of from 1.0 times larger than
the first dry width to about 1.1 times larger than the first dry
width.
[0071] It is believed that exposing the membrane to a temperature
during the controlled width reduction that is .gtoreq.the
temperature to which the membrane was exposed during the TD
stretching leads to greater resistance to heat shrinkage in the
finished membrane.
[4] THE PROPERTIES OF THE MICROPOROUS MEMBRANE
[0072] The final microporous membrane generally comprises the
polymer used to produce the extrudate. A small amount of diluent or
other species introduced during processing can also be present,
generally in amounts less than 1 wt. % based on the weight of the
microporous polyolefin membrane. A small amount of polymer
molecular weight degradation might occur during processing, but
this is acceptable. In an embodiment, molecular weight degradation
during processing, if any, causes the value of MWD of the polymer
in the membrane to differ from the MWD of the first or second
polyethylene before extrusion by no more than, e.g., about 10%, or
no more than about 1%, or no more than about 0.1%.
[0073] While the extrudate and the microporous membrane can contain
copolymers, inorganic species (such as species containing silicon
and/or aluminum atoms), and/or heat-resistant polymers such as
those described in PCT Publications WO 2007/132942 and WO
2008/016174, these are not required. In an embodiment, the
extrudate and membrane are substantially free of such materials.
Substantially free in this context means the amount of such
materials in the microporous membrane is less than 1 wt. %, based
on the total weight of the polymer used to produce the
extrudate.
(1) Structure, Properties and Composition
[0074] The following embodiments further exemplify the invention.
These embodiments are descriptions of aspects of the invention and
are not meant to foreclose other embodiments within the broader
scope of the invention.
[0075] In one embodiment, the microporous membrane comprises from
about 4.0 wt. % to about 17.0 wt. % of polyolefin copolymer
(comprising, e.g., ethylene-hexene copolymer and/or ethylene-octene
copolymer), based on the weight of the membrane, the polyolefin
copolymer having an Mw in the range of 3.0.times.10.sup.3 to
3.0.times.10.sup.5, an MWD in the range of 2 to 10, and a comonomer
content in the range of 1.0 mol. % to 5.0 mol. %. Optionally, the
polyolefin copolymer has a Tm in the range of 120.0.degree. C. to
128.0.degree. C. Optionally, the membrane further comprises from
50.0 wt. % to 95.0 wt. % of a second polyolefin (comprising, e.g.,
polyethylene homopolymer or copolymer) having an
Mw.ltoreq.1.0.times.10.sup.6 and a Tm.gtoreq.131.0.degree. C. and
from 1.0 wt. % to 46.0 wt. % of a third polyolefin (comprising,
e.g., polyethylene homopolymer or copolymer) having an
Mw>1.0.times.10.sup.6. The weight percents are based on the
weight of the membrane.
[0076] In another embodiment, the microporous membrane comprises
from about 8.0 wt. % to about 13.0 wt. % of a polyethylene-hexene
and/or polyethylene-octene copolymer, the copolymer having an Mw in
the range of 1.0.times.10.sup.4 to 1.0.times.10.sup.5, an MWD in
the range of from 2.5 to 4.5, and a comonomer amount in the range
of 1.25 mol. % to 4.50 mol. %. Optionally, the copolymer has a Tm
in the range of from 122.0.degree. C. to 126.0.degree. C., such as
from about 123.0.degree. C. to 125.0.degree. C. Optionally, the
membrane further comprises from 60.0 wt. % to 85.0 wt. % of a
second polyethylene comprising polyethylene homopolymer or
copolymer, the second polyethylene having an Mw in the range of
from 1.0.times.10.sup.5 to 9.0.times.10.sup.5 and a Tm
131.0.degree. C. and from about 7.0 wt. % to about 32.0 wt. % of a
third polyethylene comprising polyethylene homopolymer or copolymer
having an Mw in the range of 1.1.times.10.sup.6 to
5.0.times.10.sup.6. The weight percents are based on the weight of
the membrane.
[0077] The membranes provide a good balance of membrane
permeability, shutdown temperature, and shutdown speed. When
comonomer content is <1.0 mol. %, it is more difficult to
produce a membrane having a suitable permeability. When comonomer
content is >5.0 mol. %, it is more difficult to produce a
microporous membrane having low shutdown temperature and rapid
shutdown speed.
[0078] In an embodiment, the membrane's thickness is generally in
the range of from about 1 .mu.m to about 100 .mu.m, e.g., from
about 5 .mu.m to about 30 .mu.m. The thickness of the microporous
membrane can be measured by a contact thickness meter at 1 cm
longitudinal intervals over the width of 20 cm, and then averaged
to yield the membrane thickness. Thickness meters such as the
Litematic available from Mitsutoyo Corporation are suitable. This
method is also suitable for measuring thickness variation after
heat compression, as described below. Non-contact thickness
measurements are also suitable, e.g., optical thickness measurement
methods.
[0079] Optionally, the microporous membrane has one or more of the
following properties.
A. Porosity of about 25% to about 80%
[0080] The membrane's porosity is measured conventionally by
comparing the membrane's actual weight to the weight of an
equivalent non-porous membrane of 100% polyethylene (equivalent in
the sense of having the same length, width, and thickness).
Porosity is then determined using the formula: Porosity
%=100.times.(w2-w1)/w2, wherein "w1" is the actual weight of the
microporous membrane and "w2" is the weight of an equivalent
non-porous membrane of 100% polyethylene having the same size and
thickness.
[0081] In an embodiment, the membrane has a porosity.gtoreq.25%,
e.g., in the range of about 25% to about 80%, or 30% to 60%. The
membrane's porosity is measured conventionally by comparing the
membrane's actual weight to the weight of an equivalent non-porous
membrane of the same composition (equivalent in the sense of having
the same length, width, and thickness).
B. Normalized Air Permeability.ltoreq.8.0.times.10.sup.2
Seconds/100 cm.sup.3/20 .mu.m
[0082] The membrane's air permeability value is normalized to the
value for an equivalent membrane having a film thickness of 20
.mu.m. The membrane's air permeability value is thus expressed in
units of "seconds/100 cm.sup.3/20 .mu.m". Normalized air
permeability is measured according to JIS P8117, and the results
are normalized to the permeability value of an equivalent membrane
having a thickness of 20 .mu.m using the equation A=20
.mu.m*(X)/T.sub.1, where X is the measured air permeability of a
membrane having an actual thickness T.sub.1 and A is the normalized
air permeability of an equivalent membrane having a thickness of 20
.mu.m.
[0083] In an embodiment, the normalized air permeability is
.ltoreq.8.0.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m, e.g., in the
range of 1.0.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m to
8.0.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m. In another
embodiment, the normalized air permeability is in the range of
about in the range of 1.0.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m
to 7.0.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m, or
1.0.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m to about
6.5.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m, or about
1.5.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m to about
5.0.times.10.sup.2 sec/100 cm.sup.3/20 .mu.m.
C. Normalized Pin Puncture Strength of about 3,000 mN/20 .mu.m or
More
[0084] Pin puncture strength is defined as the maximum load
measured (in grams Force or "gF") when a microporous membrane
having a thickness of T.sub.1 is pricked with a needle of 1 mm in
diameter with a spherical end surface (radius R of curvature: 0.5
mm) at a speed of 2 mm/second. The pin puncture strength is
normalized to a value at a membrane thickness of 1.0 .mu.m using
the equation L.sub.2=(L.sub.1)/T.sub.1, where L.sub.1 is the
measured pin puncture strength, L.sub.2 is the normalized pin
puncture strength, and T.sub.1 is the average thickness of the
membrane.
[0085] In an embodiment, the pin puncture strength of the membrane
(normalized to a thickness of 20 .mu.m) is 3.0.times.10.sup.3 mN/20
.mu.m or more, or 3.5.times.10.sup.3 mN/20 .mu.m or more, or in the
range of 3.0.times.10.sup.3 mN/20 .mu.m to 5.0.times.10.sup.3 mN/20
.mu.m.
D. 105.degree. C. Heat Shrinkage Ratio.ltoreq.10% in at Least One
Planar Direction
[0086] The shrinkage ratio of the microporous membrane orthogonal
planar directions (e.g., machine direction or transverse direction)
is measured as follows:
[0087] (i) Measure the size of a test piece of microporous membrane
at ambient temperature in both the machine direction and transverse
direction, (ii) equilibrate the test piece of the microporous
membrane at a temperature of 105.degree. C. for 8 hours with no
applied load, and then (iii) measure the size of the membrane in
both MD and TD. The 105.degree. C. heat shrinkage ratio in either
MD or TD can be obtained by dividing the result of measurement (i)
by the result of measurement (ii) and expressing the resulting
quotient as a percent.
[0088] In an embodiment, the heat shrinkage ratio measured at
105.degree. C. in at least one planar direction is .ltoreq.10%, or
alternatively.ltoreq.9.0%, or alternatively.ltoreq.8.5%, or
alternatively.ltoreq.8%. In another embodiment, the 105.degree. C.
heat shrinkage ratio in the membrane's TD is .ltoreq.10%, or
alternatively.ltoreq.9.0%, or alternatively.ltoreq.8.5%, or
alternatively.ltoreq.8.0%, or alternatively in the range of 2.0% to
8.0%. In yet another embodiment, the membrane's MD heat shrinkage
ratio is .ltoreq.10% and the membrane's transverse direction heat
shrinkage ratio is .ltoreq.10%, or .ltoreq.9.0.
E. Shutdown Temperature
[0089] The microporous membrane's shutdown temperature is measured
by the method disclosed in PCT publication WO2007/052663, which is
incorporated by reference herein in its entirety. According to this
method, the microporous membrane is exposed to an increasing
temperature (5.degree. C./minute beginning at 30.degree. C.) while
measuring the membrane's air permeability. The microporous
membrane's shutdown temperature is defined as the temperature at
which the microporous membrane's air permeability (Gurley Value)
first exceeds 100,000 seconds/100 cm.sup.3. The microporous
membrane's air permeability is measured according to JIS P8117
using an air permeability meter (EGO-1T available from Asahi Seiko
Co., Ltd.).
[0090] In an embodiment, the membrane's shutdown temperature is in
the range of 125.degree. C. to 130.degree. C., e.g., in the range
of 126.degree. C. to 129.degree. C.
F. Rupture Temperature.gtoreq.150.degree. C.
[0091] Rupture temperature is measured as follows. A microporous
membrane of 5 cm.times.5 cm is sandwiched by blocks each having a
circular opening of 12 mm in diameter, and a tungsten carbide ball
of 10 mm in diameter was placed on the microporous membrane in the
circular opening. The membrane is then exposed to an increasing
temperature at a rate of 5.degree. C./minute. The membrane's
rupture temperature is defined as the temperature at which the ball
first breaks through the membrane. The membrane's meltdown
temperature is defined as the temperature at which the ball
completely penetrates the sample, i.e., the temperature at which
the sample breaks, generally at a temperature in the range of about
140.degree. C. to about 200.degree. C.
[0092] In an embodiment, the meltdown temperature is in the range
of from 150.0.degree. C. to 200.0.degree. C.
G Shutdown Activation Energy E2.gtoreq.3,500
[0093] The membrane's shutdown activation energy E2 is directly
related to the speed at which membrane permeability decreases near
the membrane's shutdown temperature (the shutdown speed). Shutdown
activation energy determined from the rate of change in membrane
normalized air permeability (Gurley value measured in units of
seconds/100 cm.sup.3/20 .mu.m) as a function of the reciprocal of
absolute temperature in the range of 1/T<0.00248 (T in
Kelvin).
[0094] Specifically, E2 is determined from the formula:
Ln .DELTA.G=A.sub.0-E2/RT, [0095] where [0096] .DELTA.G: change in
normalized air permeability (Gurley value in units of sec/100
cm.sup.3/20 .mu.m [0097] T: absolute temperature (K) [0098] E2:
shutdown activation energy (J/mol) [0099] A.sub.0=additive
constant, and [0100] R=8.31 (J/mol K).
[0101] FIG. 1 shows the relationship between membrane temperature
and air permeability. The temperature-permeability curve exhibits
two distinct regions: a first region showing the change in
permeability at the start of shutdown and a second region showing
the change in permeability as shutdown comes to completion as a
result of polymer migration into the membrane's micropores thereby
closing the micropores. When the change in the natural logarithm of
permeability is plotted as a function of the reciprocal of absolute
temperature, the slope of the resulting curve shows the rate of
permeability change, corresponding to shutdown speed. See FIG. 2.
The value of E2 is calculated from slope E2/RT using at least five
data point in the permeability range of 9.0.times.10.sup.3
seconds/100 cm.sup.3/20 .mu.m to 10.0.times.10.sup.3 seconds/100
cm.sup.3/20 .mu.m.
[0102] In an embodiment, the membrane's shutdown activation energy
is .gtoreq.3.50.times.10.sup.3 J/mol, e.g.,
.gtoreq.4.00.times.10.sup.3 J/mol. Optionally, the membrane's
shutdown activation energy is in the range of 3.60.times.10.sup.3
J/mol to 5.50.times.10.sup.3 J/mol.
Battery Separator and Battery
[0103] The microporous membrane of this invention has well-balanced
properties of a shutdown temperature, air permeability, and pin
puncture strength. The microporous membrane is permeable liquids
(aqueous and non-aqueous) at atmospheric pressure. Thus, the
microporous membrane is useful as a battery separator, filtration
membrane, and so on. The microporous membrane can be particularly
applied to a secondary battery separator, such as a nickel-hydrogen
battery, nickel-cadmium battery, nickel-zinc battery, silver-zinc
battery, lithium-ion battery, lithium-ion polymer battery
separator. In an embodiment, the membrane is used as a battery
separator film in lithium-ion secondary batteries.
[0104] Such batteries are described in PCT publication WO
2008/016174 which is incorporated by reference herein in its
entirety.
[0105] This invention will be described in more detail with
reference to Examples below without intention of restricting the
scope of this invention.
[7] EXAMPLES
Example 1
(1) Preparation of First Polyolefin Solution
[0106] A polyolefin blend is prepared by combining (a) 8.2 wt. % of
first polyethylene (a polyethylene copolymer) having a weight
average molecular weight ("Mw") of 3.8.times.10.sup.4, a molecular
weight distribution ("MDW") of 3.0 with (b) 73.8 wt. % of second
polyethylene having an Mw of 5.6.times.10.sup.5, an MWD of 4.1, (c)
18 wt. % of third polyethylene having an Mw of 2.0.times.10.sup.6
and a MWD of 5 (the first polyethylene).
[0107] Twenty-five parts by weight of the resultant first
polyolefin blend is charged into a strong-blending double-screw
extruder having an inner diameter of 58 mm and L/D of 42, and 65
parts by mass of liquid paraffin (50 cst at 40.degree. C.) is
supplied to the double-screw extruder via a side feeder to produce
a polyolefin solution.
(2) Production of Membrane
[0108] The polyolefin solution is supplied from the double-screw
extruders to a single-layer-extruding T-die, and extruded therefrom
to form an extrudate. The extrudate is cooled while passing through
cooling rollers controlled at 20.degree. C., to form a gel-like
sheet, which is simultaneously biaxially stretched at 115.0.degree.
C. to a magnification factor of 5 fold in both MD and TD by a
tenter-stretching machine. The stretched gel-like sheet is fixed to
an aluminum frame of 20 cm.times.20 cm, immersed in a bath of
methylene chloride controlled at 25.degree. C. to remove liquid
paraffin with vibration of 100 rpm for 3 minutes, and dried by air
flow at room temperature. The dried membrane is re-stretched by a
batch-stretching machine to a magnification factor of 1.4 fold in
the transverse direction (TD) while exposed to a temperature of
115.0.degree. C., and then relaxed (tenter clips re-adjusted to a
narrower width) to a TD magnification factor of 1.2 fold at the
same temperature, the magnification factors being based on the
width of the membrane (TD) before dry stretching. The re-stretched
membrane, which remains fixed to the batch-stretching machine, is
heat-set at 115.0.degree. C. for 10 minutes to produce a mono-layer
microporous membrane. The properties of the membrane are shown in
Table 1.
Example 2
[0109] Example 1 is repeated except that the amount of the first
polyethylene is 12.3 wt. %. The properties of the membrane are
shown in Table 1.
Example 3
[0110] Example 1 is repeated except that the amount of the first
polyethylene is 16.4 wt. %. The properties of the membrane are
shown in Table 1.
Example 4
[0111] Example 1 is repeated except that the first polyethylene is
copolymer of ethylene with hexene. The copolymer has Mw of
8.5.times.10.sup.4 and comonomer content of 1.9 mol. %. The
properties of the membrane are shown in Table 1.
Example 5
[0112] Example 4 is repeated except that the amount of the first
polyethylene is 16.4 wt. %. The properties of the membrane are
shown in Table 1.
Example 6
[0113] Example 3 is repeated except that the comonomer content of
first polyethylene is 1.4 mol. % and Mw is 1.2.times.10.sup.5. The
properties of the membrane are shown in Table 1.
Example 7
[0114] Example 4 is repeated except that the comonomer content of
first polyethylene is 1.4 mol. % and Mw is 1.7.times.10.sup.5. The
properties of the membrane are shown in Table 1.
Example 8
[0115] Example 7 is repeated except that the amount of the first
polyethylene is 16.4 wt. %. The properties of the membrane are
shown in Table 1.
Example 9
[0116] Example 5 is repeated except that the comonomer content of
first polyethylene is 1.3 mol. % and Mw is 2.6.times.10.sup.5. The
properties of the membrane are shown in Table 1.
Example 10
[0117] Example 3 is repeated except that the first polyethylene is
a copolymer of ethylene with propylene, having comonomer content of
2.7 mol % and Mw of 7.5.times.10.sup.5. The properties of the
membrane are shown in Table 1.
Comparative Example 1
[0118] Example 3 is repeated except that the first polyethylene is
ethylene homopolymer having Mw of 1.7.times.10.sup.5. The
properties of the membrane are shown in Table 1.
Properties
[0119] The properties of the multi-layer microporous membranes of
Examples 1-8 and Comparative Examples 1 are measured by the
procedures described in Section 4. The results are shown in Table
1.
[0120] The membranes of Examples 1-10 exhibit improved shutdown
temperature and/or improved shutdown speed over the membrane of
Comparative Example 1. Examples 1-3 show the microporous membrane
having desirable low shutdown temperature and fast shutdown speed
can be produced by using a polyolefin copolymer having relatively
low Mw, narrow MWD, and relatively low comonomer content. Low
shutdown temperature and high shutdown speed is achieved even using
relatively large amounts of the copolymer. A comparison between
Examples 1-3 and Examples 4-6 shows that a relatively fast shutdown
speed can be achieved even at a relatively broader MWD provided a
lower comonomer content is needed is used. Examples 7-10 show
desirable results, however, shutdown speed is less rapid than is
the case for membranes containing copolymer having a lower Mw. It
is believed that at relatively high copolymer Mw
(Mw.gtoreq.1.times.10.sup.5) there is a slight increase in the
probability of the occurrence of a critical segment length in the
polymer chain sufficient to reduce chain mobility.
[0121] The invention is further described by the following
embodiments. The invention is not limited to these embodiments.
1. A microporous membrane comprising 1.0 wt. % to 20.0 wt. % of a
polyolefin copolymer based on the weight of the membrane, the
copolymer having an Mw.ltoreq.9.0.times.10.sup.5 and a comonomer
content in the range of from 1.0 mol. % to 5.0 mol. %. 2. The
microporous membrane of embodiment 1, wherein the copolymer
comprises ethylene-hexene copolymer and/or ethylene-octene
copolymer having an MWD in the range of 2.0 to 10.0 and a Tm in the
range of 120.0.degree. C. to 128.0.degree. C. 3. The microporous
membrane of embodiment 1 or 2, wherein the copolymer has an Mw in
the range of from 1.0 to 10.sup.4 to 1.0.times.10.sup.5, an MWD in
the range of 2.5 to 4.5, a comonomer content in the range of 1.25
mol. % to 4.50 mol. %, and a Tm in the range of 123.0.degree. C. to
125.0.degree. C. 4. A battery separator film comprising the
microporous membrane of any of embodiments 1-3.
[0122] All patents, test procedures, and other documents cited
herein, including priority documents, are fully incorporated by
reference to the extent such disclosure is not inconsistent and for
all jurisdictions in which such incorporation is permitted.
[0123] While the illustrative forms disclosed herein have been
described with particularity, it will be understood that various
other modifications will be apparent to and can be readily made by
those skilled in the art without departing from the spirit and
scope of the disclosure. Accordingly, it is not intended that the
scope of the claims appended hereto be limited to the examples and
descriptions set forth herein but rather that the claims be
construed as encompassing all the features of patentable novelty
which reside herein, including all features which would be treated
as equivalents thereof by those skilled in the art to which this
disclosure pertains.
[0124] When numerical lower limits and numerical upper limits are
listed herein, ranges from any lower limit to any upper limit are
contemplated.
TABLE-US-00001 TABLE 1 No. Example 1 Example 2 Example 3 Example 4
Example 5 Example 6 Resin Composition First Polyethylene Mw 3.8
.times. 10.sup.4 3.8 .times. 10.sup.4 3.8 .times. 10.sup.4 8.5
.times. 10.sup.4 8.5 x 10.sup.4 1.2 .times. 10.sup.5 Mw/Mn 3.0 3.0
3.0 4.0 4.0 2.5 % by mass 8.2 12.3 16.4 8.2 16.4 16.4 comonomer
Octene Octene Octene Hexene Hexene Octene % by mol 2.4 2.4 2.4 1.9
1.9 1.4 Tm (.degree. C.) 125.8 125.8 125.8 124.1 124.1 123.5 Second
Polyethylene Mw 5.6 .times. 10.sup.5 5.6 .times. 10.sup.5 5.6
.times. 10.sup.5 5.6 .times. 10.sup.5 5.6 .times. 10.sup.5 5.6
.times. 10.sup.5 Mw/Mn 4.1 4.1 4.1 4.1 4.1 4.1 % by mass 73.8 69.7
65.6 73.8 65.6 65.6 Third Polyethylene Mw 2.0 .times. 10.sup.6 2.0
.times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0
.times. 10.sup.6 2.0 .times. 10.sup.6 Mw/Mn 5.1 5.1 5.1 5.1 5.1 5.1
% by mass 18 18 18 18 18 18 Production Condition Polymer
concentration 25 25 25 25 25 25 Stretching of Gel-Like sheet
Temperature (.degree. C.) 115 115 115 115 115 115 Magnification (MD
.times. TD).sup.(3) 5 .times. 5 5 .times. 5 5 .times. 5 5 .times. 5
5 .times. 5 5 .times. 5 Stretching of dried membrane
Temperature(.degree. C.) 115 115 115 115 115 115 Magnification (TD)
1.4 1.4 1.4 1.4 1.4 1.4 Relaxation of re-stretched membrane
Temperature(.degree. C.) 115 115 115 115 115 115 Final
magnification (TD) 1.2 1.2 1.2 1.2 1.2 1.2 Heat setting treatment
Temperature(.degree. C.) 115 115 115 115 115 115 Time (min) 10 10
10 10 10 10 Properties Average thickness (.mu.m) 16.0 19.1 22.9
22.2 13.4 25.9 Normalized Air Permeability 337 220 250 323 1649 773
(sec/100 cm.sup.3/20 .mu.m) Porosity % 44.0 48.5 49.8 42.4 31.2
41.5 Normalized Puncture Strength (mN/20 .mu.m) 3400 1800 1700 1900
2800 1300 Heat shrinkage MD/TD (%) 7.5/7.5 7.2/7.6 8.0/7.7 7.4/5.7
4.8/4.9 2.9/2.3 Shut Down Temp. .degree. C. 129.8 126.3 127.0 128.3
122.8 125.9 Shut Down Activation Energy 4200 4200 4200 4000 4000
4200 Rupture Temp. .degree. C. 145.7 145.2 146.0 145.9 144.0 144.2
No. Comparative Example 7 Example 8 Example 9 Example 10 Example 1
Resin Composition First Polyethylene Mw 1.7 .times. 10.sup.5 1.7
.times. 10.sup.5 2.6 .times. 10.sup.5 7.5 .times. 10.sup.5 1.7
.times. 10.sup.5 Mw/Mn 2.8 2.8 2.5 3.6 3.0 % by mass 8.2 16.4 16.4
16.4 16.4 comonomer Hexene Hexene Hexene Propylene -- % by mol 1.4
1.4 1.3 2.7 0 Tm (.degree. C.) 123.0 123.0 122.5 120.0 130.4 Second
Polyethylene Mw 5.6 .times. 10.sup.5 5.6 .times. 10.sup.5 5.6
.times. 10.sup.5 5.6 .times. 10.sup.5 5.6 .times. 10.sup.5 Mw/Mn
4.1 4.1 4.1 4.1 4.1 % by mass 73.8 65.6 65.6 65.6 65.6 Third
Polyethylene Mw 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0
.times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 Mw/Mn
5.1 5.1 5.1 5.1 5.1 % by mass 18 18 18 18 18 Production Condition
Polymer concentration 25 25 25 25 25 Stretching of Gel-Like sheet
Temperature (.degree. C.) 115 115 115 115 115 Magnification (MD
.times. TD).sup.(3) 5 .times. 5 5 .times. 5 5 .times. 5 5 .times. 5
5 .times. 5 Stretching of dried membrane Temperature(.degree. C.)
115 115 115 115 115 Magnification (TD) 1.4 1.4 1.4 1.4 1.4
Relaxation of re-stretched membrane Temperature(.degree. C.) 115
115 115 115 115 Final magnification (TD) 1.2 1.2 1.2 1.2 1.2 Heat
setting treatment Temperature(.degree. C.) 115 115 115 115 115 Time
(min) 10 10 10 10 10 Properties Average thickness (.mu.m) 22.3 19.4
14.3 10.0 12.0 Normalized Air Permeability 329 721 993 795 388
(sec/100 cm.sup.3/20 .mu.m) Porosity % 46.6 43.8 37.1 37.0 38.0
Normalized Puncture Strength (mN/20 .mu.m) 1800 1600 1800 2500 2200
Heat shrinkage MD/TD (%) 6.4/5.8 4.7/4.7 8.8/7.2 5.7/5.8 1.7/1.8
Shut Down Temp. .degree. C. 129.1 126.0 126.2 129.2 131.6 Shut Down
Activation Energy 3500 3500 4000 3800 4000 Rupture Temp. .degree.
C. 145.3 144.4 144.2 144.4 144.5
* * * * *