U.S. patent application number 13/260621 was filed with the patent office on 2012-02-16 for microporous membranes and methods for making and using such membranes.
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 | 20120040232 13/260621 |
Document ID | / |
Family ID | 42828625 |
Filed Date | 2012-02-16 |
United States Patent
Application |
20120040232 |
Kind Code |
A1 |
Ishihara; Takeshi ; et
al. |
February 16, 2012 |
MICROPOROUS MEMBRANES AND METHODS FOR MAKING AND USING SUCH
MEMBRANES
Abstract
This invention relates to microporous membranes comprising
polyolefin, the use of such membranes as battery separators, and
methods for producing such microporous membranes. In particular,
the invention relates to microporous membranes having a shutdown
temperature in the range of 120.0.degree. C. to 130.0.degree. C.
and a maximum solid state heat shrinkage .ltoreq.30.0%.
Inventors: |
Ishihara; Takeshi; (Kawagoe,
JP) ; Miyaoka; Satoshi; (Nasushiobara, JP) ;
Kono; Koichi; (Aseka, JP) ; Crowther; Donna J.;
(Seabrook, TX) ; Brant; Patrick; (Seabrook,
TX) |
Assignee: |
Toray Tonen Specialty Separator
Godo Kaisha
Nasushiobara
JP
|
Family ID: |
42828625 |
Appl. No.: |
13/260621 |
Filed: |
March 5, 2010 |
PCT Filed: |
March 5, 2010 |
PCT NO: |
PCT/US10/26425 |
371 Date: |
September 27, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61164817 |
Mar 30, 2009 |
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61164824 |
Mar 30, 2009 |
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61164833 |
Mar 30, 2009 |
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61164827 |
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: |
429/145 ;
264/210.5; 429/249; 521/134; 521/143; 521/144 |
Current CPC
Class: |
B32B 27/32 20130101;
B01D 71/26 20130101; B29C 48/00 20190201; Y02E 60/10 20130101; H01M
50/40 20210101; H01M 50/409 20210101; H01M 50/411 20210101; B01D
71/76 20130101 |
Class at
Publication: |
429/145 ;
521/143; 521/144; 429/249; 264/210.5; 521/134 |
International
Class: |
H01M 2/16 20060101
H01M002/16; C08L 1/00 20060101 C08L001/00; C08L 23/06 20060101
C08L023/06; 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, and having a
shutdown temperature .ltoreq.130.0.degree. C. and a maximum solid
state heat shrinkage .ltoreq.30.0%.
2. The microporous membrane of claim 1, wherein the membrane has a
maximum solid state heat shrinkage .ltoreq.25.0%, a shut-down speed
.gtoreq.10000.0 sec/100 cm.sup.3/.degree. C., and a pin puncture
strength .gtoreq.5.0 N/25 .mu.m.
3. The microporous membrane of claim 1, wherein the membrane has an
air permeability in the range of 50.0 sec/100 cm.sup.3/20 .mu.m to
500.0 sec/100 cm.sup.3/20 .mu.m.
4. The microporous membrane according to claim 1, wherein the
polyolefin comprises a first polyethylene having an
Mw.gtoreq.1.0.times.10.sup.6; a second polyethylene having an Mw in
the range of 1.0.times.10.sup.5 to 0.95.times.10.sup.6 and a
Tm>127.0.degree. C.; and a third polyethylene being an
ethylene-alpha-olefin copolymer having an Mw in the range of
1.0.times.10.sup.4 to 5.0.times.10.sup.5 and having a Tm in the
range of 115.0.degree. C. to 127.0.degree. C.
5. The microporous membrane according to claim 4, wherein the
second polyethylene has a terminal vinyl group content .ltoreq.0.20
per 10,000 carbons.
6. The microporous membrane according to claim 4, wherein the
membrane contains the first polyethylene in an amount in the range
of from 5 wt. % to 40 wt. %, based on the weight of the
membrane.
7. The microporous membrane according to claim 4, wherein the
membrane contains the third polyethylene in an amount in the range
of from 5 wt. % to 30 wt. %, based on the weight of the
membrane.
8. The microporous membrane according to claim 4, wherein the
membrane contains the second polyethylene in an amount in the range
of from 5 wt. % to 90 wt. %, based on the weight of the
membrane.
9. A battery separator film comprising the microporous membrane of
claim 1.
10. A battery comprising an anode, a cathode, an electrolyte, and a
separator situated between the anode and the cathode, the separator
comprising the microporous membrane of claim 1.
11. A method for producing a microporous membrane, comprising: (a)
extruding a mixture of diluent and polyolefin copolymer having an
Mw in the range of 1.0.times.10.sup.4 to 5.0.times.10.sup.5 and
having a melting point in the range of 115.0.degree. C. to
127.0.degree. C., (b) stretching the extrudate, (c) removing at
least a portion of the diluent from the stretched extrudate, (d)
stretching the diluent-removed extrudate, and then (e) exposing the
stretched, diluent-removed extrudate to an elevated
temperature.
12. The method of claim 11, wherein the copolymer comprises
ethylene-alpha-olefin copolymer.
13. The method of claim 11, wherein the mixture further comprises a
first polyethylene having an Mw.gtoreq.1.0.times.10.sup.6 and a
second polyethylene having an Mw.ltoreq.1.0.times.10.sup.6 and a
Tm>127.0.degree. C.
14. The method of claim 11, wherein the copolymer has an MWD in the
range of 1.5 to 5.
15. The method of claim 11, further comprising combining the
diluent with a fourth polymer selected from the group of
polypropylene, polybutene-1, polypentene-1,
poly(4-methyl-pentene-1), polyhexene-1, polyoctene-1,
poly(vinylacetate), polymethyl methacrylate, polyester, polyamide,
polyarylenesulfide, and mixtures thereof.
16. The method of claim 11, wherein the stretching of step (d) is
conducted while exposing the membrane to a temperature in the range
of 116.0.degree. C. to 125.0.degree. C. and achieves magnification
factor in the range of 1.5 fold to 2.5 fold in at least one planar
direction.
17. The method of claim 11, wherein during step (e) the membrane is
exposed to a temperature in the range of 116.0.degree. C. to
125.0.degree. C. and subjected to a thermal relaxation to achieve a
magnification factor in the range of 1.2 fold to 1.5 fold in at
least one planar direction, based on the membrane's size at the
start of step (d).
18. The method of claim 11, wherein the stretching of step (b) is
conducted while exposing the sheet to a temperature in the range of
110.0.degree. C. to 125.0.degree. C. to achieve an area
magnification factor in the range of 20 fold to 60 fold.
19. The method of claim 11, wherein the stretching of step (b) is
conducted in at least one planar direction while exposing the sheet
to a temperature in the range of 20.0.degree. C. to 90.degree. C.,
and further comprising second stretching the stretched sheet in at
least one direction while exposing the membrane to a temperature in
the range of 110.0.degree. C. to 125.0.degree. C., the first and
second stretching achieving an area magnification in the range of
from 20 fold to 60 fold.
20. The method of claim 11, further comprising heat setting the
membrane.
21. A membrane made by the process of claim 11.
22. A battery separator film comprising a membrane made by the
process of claim 11.
23. A microporous membrane comprising polyolefin copolymer having
an Mw in the range of 1.0.times.10.sup.4 to 5.0.times.10.sup.5 and
having a Tm in the range of 115.0.degree. C. to 127.0.degree.
C.
24. The microporous membrane of claim 23, wherein the membrane has
a shutdown temperature .ltoreq.130.0.degree. C. and a maximum solid
state heat shrinkage .ltoreq.30.0%.
25. The microporous membrane of claim 23 and a second membrane.
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 thermally-stable microporous
membranes comprising polyolefin, the use of such membranes as
battery separators, and methods for producing such microporous
membranes.
BACKGROUND OF THE INVENTION
[0003] Microporous membranes can be used as separation membranes,
such as battery separators and electrolytic capacitor separators,
etc. A lithium-ion battery contains an anode, a cathode, an aprotic
polar organic solvent as an electrolyte solvent, a lithium salt as
an electrolyte, and a battery separator located between the anode
and the cathode. One common battery separator is a battery
separator film comprising polyolefin, such as a microporous
membrane or a non-woven film. Battery separator film comprising
polyolefin, e.g., polyethylene and/or polypropylene, is desirable
because the polyolefin is insoluble in the organic solvent, and
reacts with neither the electrolyte nor the electrodes.
[0004] Recently, a microporous membrane having a high strength and
high modulus by using an ultra-high-molecular-weight polyolefin has
been developed. For example, JP60-242035A discloses a membrane
produced by molding a gel-like sheet, which is extruded from a
solution containing solvent and polyolefin having an average
molecular weight .ltoreq.7.times.10.sup.5. The gel-like sheet is
then stretched and the solvent is removed to produce the membrane.
JP03-064334A discloses a microporous membrane having a specific
polyolefin molecular weight distribution in order to produce the
microporous membrane from a highly concentrated polyolefin
solution.
[0005] For microporous membranes used as battery separator film, it
is desirable for the pores of the microporous membrane to close
automatically at elevated temperatures as might be encountered
under overcharge or short-circuiting conditions. This property,
called "shutdown", results from molten polymer closing the
membrane's micropores. It is desired that shutdown occur at a
temperature lower than that at which the membrane loses structural
integrity (the "meltdown" temperature). Increasing both the
difference between the shutdown and meltdown temperatures and the
speed at which the membrane's pores close at temperatures above the
shutdown temperature (the shutdown speed) improves the battery's
safety margin. Shutdown temperatures .ltoreq.130.degree. C. are
desired. Using branched low density polyethylene (LDPE) (and/or) a
linear low density polyethylene (LLDPE) to lower the membrane's
shutdown temperature has been disclosed in JP60-023954A,
JP03-201360A and JP05-025305A.
[0006] JP11-269289A discloses a polyethylene microporous membrane
having improved (lower) shutdown temperature. The membrane
comprises 20 to 98 wt. % of polyethylene having a weight average
weight molecular weight .gtoreq.5.times.10.sup.5 and 2 to 80 wt. %
of an ethylene-alpha-olefin copolymer having a melting point of
95.degree. C. to 125.degree. C. and a substantially linear
structure. JP2002-338730A discloses a polyethylene microporous
membrane comprising a high density polyethylene (HDPE) having a
viscosity average molecular weight in the range of 1.times.10.sup.5
to 4.times.10.sup.6 and a polyethylene (PE) having a melting point
in the range of from 125.degree. C. to 132.degree. C. The PE can be
an ethylene-alpha-olefin copolymer, the alpha-olefin being an
olefin having 4 carbons or more.
[0007] WO2007/060990 and WO2007/060991 disclose microporous
membranes containing polyethylene produced using a single-site
catalyst. The references disclose membranes having improved heat
shrinkage, i.e., a reduced tendency for the membrane to shrink in a
planar direction at elevated temperature. Since electrode spacing
in lithium ion batteries is very small (a fraction of a
millimeter), separator heat shrinkage can result in direct
electrode contact (short circuit). The references also disclose
membranes having improved shutdown temperature, but neither
discloses membranes having a shutdown temperature
.ltoreq.130.degree. C.
[0008] While improvements have been made in battery separator film
shutdown performance, further improvements are desired. It is
particularly desired to produce a battery separator film having a
shutdown temperature .ltoreq.130.degree. C. with low heat
shrinkage.
SUMMARY OF THE INVENTION
[0009] In an embodiment, this invention relates to a microporous
membrane comprising polymer and having a shutdown temperature
.ltoreq.130.0.degree. C. and a maximum solid state heat shrinkage
.ltoreq.30.0%.
[0010] In another embodiment, this invention relates to a
microporous membrane, prepared by [0011] (a) extruding a mixture of
polymer and diluent to form a sheet, [0012] (b) stretching the
sheet, [0013] (c) removing at least a portion of the diluent from
the stretched sheet to form a microporous sheet, [0014] (d)
stretching the microporous sheet, and then [0015] (e) exposing the
stretched microporous sheet to an elevated temperature to produce a
microporous membrane having a shutdown temperature
.ltoreq.130.0.degree. C. and a maximum solid state heat shrinkage
.ltoreq.30.0%.
[0016] In another embodiment, this invention relates to a
microporous membrane comprising polyolefin, the membrane having a
shutdown temperature in the range of from 120.0.degree. C. to
130.0.degree. C., a maximum solid state heat shrinkage
.ltoreq.30.0%, a shut-down speed .gtoreq.10000.0 sec/100
cm.sup.3/.degree. C., and a pin puncture strength .gtoreq.5N/25
.mu.m. Hereinafter, this embodiment is referred to as the "first
embodiment."
[0017] In another embodiment, this invention relates to a
microporous membrane comprising polyolefin, the membrane having a
shutdown temperature in the range of from 120.0.degree. C. to
130.0.degree. C., a maximum solid state heat shrinkage
.ltoreq.30.0%, an air permeability in the range of 50.0 sec/100
cm.sup.3/20 .mu.m to 500.0 sec/100 cm.sup.3/20 .mu.m, and a pin
puncture strength .gtoreq.5.0 N/25 .mu.m. Hereinafter, this
embodiment is referred to as the "second embodiment."
[0018] In another embodiment, this invention relates to a method
for producing a microporous membrane, comprising: [0019] (a)
extruding a mixture of polymer and diluent, [0020] (b) stretching
the extrudate, [0021] (c) removing at least a portion of the
diluent from the stretched extrudate, [0022] (d) stretching the
diluent-removed extrudate, and [0023] (e) exposing the stretched,
diluent-removed extrudate to an elevated temperature. In an
embodiment, the microporous membrane produced by the process has a
shutdown temperature .ltoreq.130.0.degree. C. and a maximum solid
state heat shrinkage .ltoreq.30.0%.
[0024] In another embodiment, this invention relates to a method
for producing a microporous membrane, comprising: [0025] (1)
extruding a mixture of diluent and (A) a first polyethylene, (B) a
second polyethylene and (C) a third polyethylene, [0026] (2)
stretching the extrudate by a first magnification factor .gtoreq.4
fold in a first planar direction and by a second magnification
factor .gtoreq.4 fold in a second planar direction substantially
perpendicular to the first planar direction, the product of the
first and second magnification factors being in the range of 20
fold to 60 fold in area, [0027] (3) removing at least a portion of
the diluent from the stretched extrudate, [0028] (4) stretching the
diluent-removed extrudate by a third magnification factor in the
range of 1.5 fold to 2.5 fold in at least one direction while
exposing the diluent-removed extrudate to a temperature in the
range of 116.degree. C. to 125.degree. C., and [0029] (5) reducing
the size of the stretched, diluent-removed extrudate in the
stretching direction of step (4) to a fourth magnification factor
in the range 1.2 fold to 1.5 fold while exposing the stretched,
diluent-removed extrudate to a temperature in the range of
116.degree. C. to 125.degree. C. In an embodiment, the microporous
membrane produced by the process has a shutdown temperature in the
range of from 120.degree. C. to 130.degree. C., a maximum solid
state heat shrinkage .ltoreq.30.0%, a shut-down speed
.gtoreq.10000.0 sec/100 cm.sup.3/.degree. C., and a pin puncture
strength .gtoreq.5.0 N/25 .mu.m.
[0030] In another embodiment, this invention relates to a method
for producing a microporous membrane, comprising: [0031] (1)
extruding a mixture of diluent and (A) a first polyethylene, (B) a
second polyethylene and (C) a third polyethylene and cooling the
extrudate, [0032] (2) stretching the extrudate in at least one
direction while exposing the extrudate to a temperature in the
range of 20.degree. C. to 90.degree. C., [0033] (3) stretching the
stretched extrudate in at least one direction while exposed to a
temperature in the range of 110.degree. C. to 125.degree. C., the
stretching of steps (2) and (3) resulting in an area magnification
factor in the range of from 20 fold to 60 fold, [0034] (4) removing
at least a portion of the diluent from the stretched extrudate,
[0035] (5) stretching the diluent-removed extrudate in at least one
direction by a first magnification factor in the range of 1.5 fold
to 2.5 fold while exposing the diluent-removed extrudate to a
temperature in the range of 116.degree. C. to 125.degree. C., and
then either [0036] (6a) reducing the size of the stretched,
diluent-removed extrudate in the stretching direction of step (5)
to a second magnification factor in the range 1.2 fold to 1.5 fold
while exposing the stretched, diluent-removed extrudate to a
temperature in the range of from 116.degree. C. to 125.degree. C.
or [0037] (6b) exposing the stretched, diluent-removed extrudate to
a temperature in the range of 116.degree. C. to 125.degree. C. with
no substantial change in the first magnification factor. In an
embodiment, the microporous membrane produced by the process has a
shutdown temperature in the range of 120.0.degree. C. to
130.0.degree. C., a maximum solid state heat shrinkage
.ltoreq.30.0%, a shutdown speed .gtoreq.10000.0 sec/100
cm.sup.3/.degree. C. and a pin puncture strength .gtoreq.5.0 N/25
.mu.m.
[0038] In another embodiment, this invention relates to a battery
separator comprising a microporous membrane, comprising polymer and
having a shutdown temperature in the range of 120.0.degree. C. to
130.0.degree. C. and a maximum solid state heat shrinkage
.ltoreq.30.0%.
[0039] In another embodiment, this invention relates to a battery
comprising an anode, a cathode, an electrolyte, and a microporous
membrane situated between the anode and the cathode, the
microporous membrane having a shutdown temperature in the range of
120.0.degree. C. to 130.0.degree. C. and a maximum solid state heat
shrinkage .ltoreq.30.0%.
[0040] In yet another embodiment, the invention relates to a
microporous membrane comprising ethylene-.alpha.-olefin copolymer
having a melting peak in the range of 115.0.degree. C. to
127.0.degree. C. and a weight-average molecular weight in the range
of 1.0.times.10.sup.4 to 5.0.times.10.sup.5.
DETAILED DESCRIPTION OF THE INVENTION
[0041] The invention is based in part on the discovery that battery
separator film comprising a lower-crystallinity polymer and a
higher-crystallinity polymer such as a high density polyethylene or
an ultra-high molecular weight polyethylene exhibits a relatively
low shutdown temperature but with relatively poor heat shrinkage.
It has been found that this difficulty can be overcome when the
lower-crystallinity polymer comprises polyethylene having a weight
average molecular weight ("Mw") in the range of 1.0.times.10.sup.4
to 5.0.times.10.sup.5 and a melting peak ("Tm") in the range of
115.0.degree. C. to 127.0.degree. C. Microporous membranes produced
using such a lower-crystallinity polymer have a shutdown
temperature in the range of 120.0.degree. C. to 130.0.degree. C.
and a maximum solid state heat shrinkage .ltoreq.30.0%. The first
and second embodiments will now be described in more detail. While
the invention is described in terms of the first and second
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.
First Embodiment
Polymer Resin Starting Materials
[0042] In this embodiment, the microporous membrane is produced
from first, second, and third polyethylenes. The first, second, and
third polyethylenes can be combined from component polyethylenes
or, alternatively, can be a reactor blend.
The First Polyethylene
[0043] The first polyethylene has an Mw.gtoreq.1.0.times.10.sup.6,
e.g., in the range of 1.0.times.10.sup.6 to 15.times.10.sup.6, such
as 1.1.times.10.sup.6 to 5.times.10.sup.6, or 1.1.times.10.sup.6 to
3.times.10.sup.6. While not wishing to be bound by any theory or
model, it is believed that using the first polyethylene results in
a microporous membrane of greater mechanical strength. In an
embodiment, the microporous membrane contains the first
polyethylene in an amount in the range of from 5 wt. % to 70 wt. %,
e.g., 5 wt. % to 40 wt. %, such as 5 wt. % to 30 wt. %, based on
the weight of the microporous membrane. When the amount of the
first polyethylene in the microporous membrane is .gtoreq.70 wt. %,
it can be more difficult to achieve both improved shutdown
characteristics (high shutdown speed and low shutdown temperature)
and low heat shrinkage. The first polyethylene can be an
ultra-high-molecular-weight polyethylene ("UHMWPE"). The first
polyethylene can be (i) a homopolymer or (ii) a copolymer of
ethylene and a comonomer selected from propylene, butene-1,
pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinylacetate,
methylmethacrylate, styrene, other monomers, or combinations
thereof. When used, the comonomer content in the copolymer is
.ltoreq.10 mol. %. The first polyethylene can be a blend (e.g., a
mixture of polyethylenes including mixtures of polyethylene(s) and
other polymers such as polyolefin) or a single component. The first
polyethylene can be prepared by a single stage polymerization or
multi-stage polymerization. Optionally, the first polyethylene has
a molecular weight distribution ("MWD" defined as the ratio of
weight average molecular weight to number average molecular weight)
in the range of 3 to 10.
The Second Polyethylene
[0044] The second polyethylene has an Mw<1.0.times.10.sup.6,
e.g., in the range of 1.times.10.sup.4 to 0.95.times.10.sup.6, and
has a Tm>127.0.degree. C. Although it is not critical, the
second polyethylene can have an Mw in the range of 1.times.10.sup.4
to 1.times.10.sup.6, such as 1.times.10.sup.5 to 8.times.10.sup.5,
or 1.5.times.10.sup.5 to 7.5.times.10.sup.5. Using the second
polyethylene as a starting material results in a polymeric sheet
that is easier to mold. It is also believed that the second
polyethylene is useful for improving the membrane's shutdown
properties. 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. In an embodiment, the Tm of
the second polyethylene is >Tm of the third polyethylene. In one
embodiment, the second polyethylene has a linear structure. It is
believed that when the second polymer has a linear structure the
microporous membrane's mechanical strength is improved. In one
embodiment, the second polyethylene has a terminal vinyl group
content <0.20 per 10,000 carbon atoms, e.g., .ltoreq.0.19 per
10,000 carbon atoms. The amount of terminal vinyl group can be
measured in accordance with the procedures described in PCT
Publication WO97/23554, for example. It is believed that when the
second polyethylene's terminal vinyl group content is .ltoreq.0.20
per 10,000 carbons, it is less difficult to produce a microporous
membrane having improved shutdown speed. In an embodiment, the
amount of the second polyethylene in the microporous membrane is in
the range of from 5 wt. % to 90 wt. %, such as 30 wt. % to 90 wt.
%, or 40 wt. % to 90 wt. %, based on the weight of the membrane.
The second polyethylene can selected from high density polyethylene
(HDPE), medium density polyethylene (MDPE), a branched low density
polyethylene (LDPE), linear low density polyethylene (LLDPE) or a
combination thereof. In one embodiment, the second polyethylene is
HDPE. The second polyethylene can be (i) a homopolymer or (ii) a
copolymer of ethylene and a comonomer selected from propylene,
butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1,
vinylacetate, methylmethacrylate, styrene, other monomers or
combinations thereof. When used, the comonomer content in the
copolymer should be .ltoreq.10 mol. %. The second polyethylene can
be a blend (e.g., a mixture of polyethylenes including mixtures of
polyethylene(s) and other polymers such as polyolefin) or a single
component. In an embodiment, the second polyethylene has an MWD in
the range of 3 to 15.
The Third Polyethylene
[0045] The third polyethylene has an Mw in the range of
1.0.times.10.sup.4 to 5.0.times.10.sup.5 and a Tm.ltoreq.127.0,
such as in the range of 115.0.degree. C. to 127.0.degree. C. It is
believed that using the third polyethylene improves the membrane's
shutdown temperature.
[0046] Although it is not critical, the third polyethylene can be
produced by using a single site catalyst, such as a metallocene
catalyst. The third polyethylene can be prepared by methods
disclosed in JP58-019309A, JP59-095292A, JP60-035005A,
JP60-035006A, JP60-035007A, JP60-035008A, JP60-035009A,
JP61-130314A, JP03-163088A, EP0420436A1, U.S. Pat. No. 5,055,438A,
and WO91/004257A, for example.
[0047] Optionally, the third polyethylene has an Mw in the range of
1.times.10.sup.4 to 2.times.10.sup.5, e.g., 2.times.10.sup.4 to
1.times.10.sup.5. When the third polyethylene has an Mw in the
range of 1.times.10.sup.4 to 5.times.10.sup.5, it is easier to
produce a microporous membrane having a relatively rapid shutdown
speed. In an embodiment, the third polyethylene is a copolymer of
ethylene and a comonomer selected from butene-1, hexene-1,
octene-1, or combinations thereof. The copolymer's comonomer
content can be, e.g., in the range of 1.2 mol. % to 20.0 mol. %,
such as 1.2 mol. % to 10 mol. %, or 1.5 mol. % to 8 mol. %. When
the comonomer content is <1.2 mol. %, it can be more difficult
to produce a microporous membrane having improved shutdown
temperature and improved shutdown speed. When the comonomer content
is >20.0 mol. %, it can be more difficult to produce the
microporous membrane having good pin puncture strength. Optionally,
the amount of third polyethylene in the microporous membrane is in
the range of 5 to 30 wt. %, e.g., 5 to 20 wt. %, such as 10 to 20
wt. %, based in the total amount of the first, second and third
polyethylene in the membrane. When the amount of the third
polyethylene in the membrane is less <5wt. %, it can be more
difficult to improve the membrane's shutdown temperature. When the
content of the third polyethylene is >30 wt. %, it is less
difficult to reduce membrane shutdown temperature, but more
difficult to maintain membrane pin puncture strength. Optionally,
the third polyethylene has a Tm in the range of 116.degree. C. to
125.degree. C. When the Tm of the third polyethylene is
>127.0.degree. C., it can be more difficult to improve shutdown
temperature. When the Tm of the third polyethylene is
<115.0.degree. C., it can be more difficult to produce a
membrane having sufficiently low heat shrinkage at a higher
temperature (e.g., 120.degree. C.). Optionally, the third
polyethylene has an MWD in the range of 1.5 to 5, or 1.7 to 3.
While not wishing to be bound by any theory or model, it is
believed that when the third polyethylene's MWD is >5, it can be
more difficult to improve the membrane's shutdown speed.
A Fourth Polymer
[0048] In one embodiment, the membrane comprises the third
polyethylene and a fourth polymer. Optionally, the membrane further
comprises the second and third polyethylene. The fourth polymer can
be polyolefin, such as a polypropylene (PP), polybutene-1 (PB-1),
polypentene-1, poly(4-methyl-pentene-1) (PMP), polyhexene-1,
polyoctene-1, poly(vinylacetate), polymethyl methacrylate;
heat-resistant polymer such as polyester, polyamide,
polyarylenesulfide; and mixtures thereof. In an embodiment, the
fourth polymer is polyolefin, such as polypropylene. When used, the
polypropylene can be a homopolymer or a copolymer of propylene and
.ltoreq.10 mol. % of a comonomer selected from .alpha.-olefin, such
as ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1,
octene-1, vinyl acetate, methyl methacrylate, and styrene, etc.,
and diolefins such as butadiene, 1,5-hexadiene, 1,7-octadiene,
1,9-decadiene, etc.
[0049] Mw and MWD of the first, second, and third polyethylenes are
determined using a High Temperature Size Exclusion Chromatograph,
or "SEC", (GPC PL 220, Polymer Laboratories), equipped with a
differential refractive index detector (DRI). The measurement is
made in accordance with the procedure disclosed in "Macromolecules,
Vol. 34, No. 19, pp. 6812-6820 (2001)". Three PLgel Mixed-B columns
available from (available from Polymer Laboratories) are used for
the Mw and MWD determination. The nominal flow rate is 0.5
cm.sup.3/min; the nominal injection volume is 300 .mu.L; and the
transfer lines, columns, and the DRI detector are contained in an
oven maintained at 145.degree. C.
[0050] 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. The same solvent is
used as the SEC eluent. Polymer solutions were prepared by placing
dry polymer in a glass container, adding the desired amount of the
TCB solvent, and then heating the mixture at 160.degree. C. with
continuous agitation for about 2 hours. The concentration of
polymer solution is 0.25 to 0.75 mg/ml. Sample solution are
filtered off-line before injecting to GPC with 2 .mu.m filter using
a model SP260 Sample Prep Station (available from Polymer
Laboratories).
[0051] The separation efficiency of the column set is calibrated
with a calibration curve generated using a seventeen individual
polystyrene standards ranging in Mp ("Mp" being defined as the peak
in Mw) from about 580 to about 10,000,000. 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.
Method for Producing a Microporous Membrane
[0052] The microporous membrane of the first embodiment can be
prepared by: [0053] (1) combining diluent (e.g., a solvent) and the
first, second and third polyethylenes to form polymeric mixture
(e.g., a polyethylene solution); [0054] (2) extruding the mixture
from a die, and cooling the extrudate to form a gel-like sheet;
[0055] (3) stretching the gel-like sheet by a first magnification
factor .gtoreq.4 fold in the machine direction ("MD") and a second
magnification factor .gtoreq.4 fold in the transverse direction
("TD"), to achieve an area magnification factor in the range of 20
fold to 60 fold; [0056] (4) removing at least a portion of the
diluent from the stretched sheet to form a microporous sheet;
[0057] (5) stretching the microporous sheet in at least one
direction from a first size to a second size greater than the first
size by a third magnification factor in the range of 1.5 fold to
2.5 fold while exposing the microporous sheet to a temperature in
the range of 116.degree. C. to 125.degree. C.; and [0058] (6)
reducing the second size to a third size that is less than the
second size but greater than the first size by a magnification
factor in the range of 1.2 fold to 1.5 fold while exposing the
microporous sheet to a the temperature in the range of 116.degree.
C. to 125.degree. C.
[0059] One or more optional steps such as a heat treatment step
(7), an cross-linking step with ionizing radiation (8), a
hydrophilic treatment step (9), and a surface coating step (10)
etc., can be conducted if desired. Such optional steps are
disclosed in PCT Publications WO2007/052663A and WO2007/117042A,
for example.
[0060] The process will now be described in more detail.
Combining the Polymer and Diluent
[0061] In an embodiment, diluent (such as a solvent) is combined
with the first, second, and third polyethylenes and, optionally,
the fourth polymer, to form a mixture. When the mixture contains
polyethylene and a solvent for the polyethylene the mixture can be
called a polyethylene solution. As long as the polymer and diluent
can be combined for extrusion, diluent selection is not critical.
Melt-blending can be used to combine at least some of the polymer
before diluent is added, but this is not required. Optionally, the
mixture of polymer and diluent contains various additives such as
one or more antioxidant, inorganic materials (such as pore-forming
material), etc., provided these are used in a concentration range
that does not significantly degrade the desired properties of the
microporous membrane. Suitable additives are described in PCT
publication WO2008/140835 A1, which is incorporated by reference
herein in its entirety.
[0062] The diluent is preferably a solvent that is liquid at room
temperature. In an embodiment, the solvent can be at least one of
aliphatic, alicyclic or aromatic hydrocarbons such as nonane,
decane, decalin, p-xylene, undecane, dodecane, liquid paraffin,
etc., 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.
Suitable diluents are described in WO2007/132942 A, which is
incorporated by reference herein in its entirety.
[0063] When melt-blending is used for combining the polymer, the
melt-blending temperature is not critical. For example, the
temperature of the polymeric solution during melt-blending (the
melt-blending temperature) can range, e.g., from about 10.degree.
C. higher than the Tm of the polyethylene having the lowest Tm in
the mixture to about 120.degree. C. higher than Tm. Such a range
can be represented as Tm+10.degree. C. to Tm+120.degree. C. In an
embodiment where Tm is in the range of about 130.degree. C. to
about 140.degree. C., the melt-blending temperature can range from
about 140.degree. C. to about 250.degree. C., or from about
170.degree. C. to about 240.degree. C. The diluent can be provided
before, after, or during the melt-blending of the polyethylene.
[0064] The amount of polymer in the polymer-diluent mixture is not
critical. In an embodiment, the amount of polymer is in the range
of from about 1 wt. % to about 75 wt. %, based on the weight of the
polymer-diluent mixture, for example from about 20 wt. % to about
70 wt. %.
Extruding the Polymer-Diluent Mixture
[0065] In an embodiment, the polymer-diluent mixture is extruded
from a die to form an extrudate which is then cooled to form a
sheet. When the cooled sheet has the appearance of a gel, it can be
called a gel-like sheet. The polymer-diluent mixture can be
extruded and then conducted directly from a first extruder to the
die. The polymer-diluent mixture is generally in the molten state
during extrusion, and can be exposed to a temperature in the range
of about 140.degree. C. to about 250.degree. C. during extrusion.
The extrudate is typically cooled until the extrudate reaches a
temperature.ltoreq.the extrudate's gelation temperature (i.e., the
temperature at which the extrudate sheet begins to gel) at a
cooling rate of e.g., .gtoreq.about 50.degree. C./minute. Suitable
process conditions for accomplishing the extrusion are disclosed in
PCT Publication WO 2007/132942.
Stretching the Sheet Comprising Polymer and Diluent
[0066] In an embodiment, the sheet (i.e., the cooled extrudate) is
stretched in at least one direction to form a stretched sheet.
Since the sheet comprises polymer and diluent, this can be called
"wet stretching" to distinguish it from the stretching conducted
later in the process after a portion of the diluent has been
removed ("dry stretching"). The choice of stretching method is not
critical. In an embodiment, the stretching is accomplished by one
or more of tenter stretching, roller stretching or inflation
stretching (e.g., with air). Biaxial stretching involves stretching
the cooled extrudate in two planar directions, e.g., both the
machine direction and transverse direction. In this context, the
machine direction ("MD") is a direction in the plane of the film
(the cooled 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. In
the case of biaxial stretching (also called biaxial orientation),
the stretching can be simultaneous biaxial stretching, sequential
stretching along one axis and then the other or multi-stage
stretching (for instance, a combination of the simultaneous biaxial
stretching and the sequential stretching). In one embodiment,
simultaneous biaxial stretching is used. It is believed that
biaxially stretching the cooled extrudate improves the finished
membrane's pin puncture strength.
[0067] In an embodiment, the cooled extrudate is exposed to a
temperature in the range of about 90.degree. C. to 125.degree. C.
during wet stretching. It is believed that when the wet stretching
temperature is >125.degree. C., it can be more difficult to
orient the molecular chains of the polyethylene in the cooled
extrudate. And when the wet stretching temperature is
<90.degree. C., it can be more difficult to stretch the cooled
extrudate without breakage or tears, which can result in a failure
to achieve the desired wet stretching magnification.
[0068] When biaxial stretching is used, the linear stretching
magnification can be, e.g., at least about 4 fold in at least two
approximately perpendicular planar directions, e.g., the MD and TD,
to achieve a magnification factor in area in the range of about 20
fold to about 60 fold. It is believed that using a linear
stretching magnification factor of .gtoreq.4 fold and an area
magnification factor .gtoreq.about 16 fold produces a microporous
membrane having a relatively high pin puncture strength. It is
believed that stretching to achieve an area magnification factor of
.ltoreq.about 60 fold produces a microporous membrane having a
relatively low heat shrinkage. The magnification factor operates
multiplicatively on the 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.
[0069] Optionally, stretching can be conducted in the presence of a
temperature gradient in the extrudate's thickness direction (i.e.,
a direction approximately perpendicular to the planar surface of
the microporous polyolefin membrane). In this case, it can be
easier to produce a microporous polyolefin membrane with improved
mechanical strength. The details of this method are described in
Japanese Patent No. 3347854.
Diluent Removal Step
[0070] In an embodiment, at least a portion of the diluent is
removed (or displaced) from the stretched sheet in order to form a
microporous sheet. When a second diluent is used for this purpose,
it is called a second diluent to distinguish it from the diluent
used to produce the polymer-diluent mixture (the first diluent).
The second diluent should be soluble in or miscible with the first
diluent. See, e.g., PCT Publication No. WO 2008/016174, for
example.
[0071] In an embodiment, at least a portion of any remaining
volatile species (e.g., a portion of the second diluent) is removed
from the stretched sheet after diluent removal. Any method capable
of removing the volatile species can be used, including
conventional methods such as heat-drying, wind-drying (moving air),
etc. Process conditions for removing volatile species such as the
second diluent (which can be called a "washing solvent" because it
washes away the first diluent) can be the same as those disclosed
in PCT Publication No. WO 2008/016174, for example.
Stretching the Microporous Sheet
[0072] After at least a portion of the first diluent has been
removed, the microporous sheet is subjected to stretching during at
least 2 stages of heat treatment, i.e., a second stretching
treatment followed by a thermal relaxation treatment. The second
stretching can be called "dry" stretching or dry orientation since
at least a portion of the first diluent is removed before the
second stretching begins.
[0073] During dry stretching, the microporous sheet is stretched in
at least one direction. It is believed that dry stretching results
in an improvement in both membrane permeability and pin puncture
strength. The dry stretching can be conducted while the microporous
sheet is exposed to a temperature in the range of 116.0.degree. C.
to 125.0.degree. C.
[0074] The temperature to which the microporous sheet is exposed
during dry stretching affects the properties of the finished
membrane. When the membrane is exposed to a temperature
<116.0.degree. C. during dry stretching, the resulting membrane
can have an undesirably large heat shrinkage. Exposing the membrane
to a temperature >125.0.degree. C. during dry stretching,
generally results in a loss of membrane air permeability.
[0075] In an embodiment, the microporous sheet is stretched in at
least one direction from a first size to a second size that is
larger than the first size by a linear magnification factor in the
range of 1.5 fold to 2.5 fold. The magnification factor achieved
during dry stretching affects the properties of the finished
membrane. For example, when the linear magnification factor is
<1.5 fold, it is more difficult to produce a microporous
membrane having improved air permeability and pin puncture
strength. When the linear magnification is >2.5 fold, it can be
more difficult to produce a microporous membrane having improved
heat shrinkage (although pin puncture strength does not appear to
be adversely affected). Optionally, the dry stretching is conducted
to achieve an area magnification in the range of 1.5 fold to 3
fold. Conventional stretching methods can be used for the dry
stretching, e.g., one or more of tenter stretching, roller
stretching or inflation stretching (e.g., with air). The methods
described in Publications No. WO 2008/016174 and WO 2007/132942 can
be used, for example.
Thermal Relaxation of the Microporous Sheet
[0076] In an embodiment, the size of the microporous sheet along
the stretching direction of step (5) is reduced from the second
size to a third size that is less than the second size but greater
than the first size by a magnification factor in the range of 1.2
fold to 1.5 fold while exposing the microporous sheet to a
temperature in the range of 116.0.degree. C. to 125.0.degree. C.
This step can be called ("thermal relaxation"). The temperature to
which the microporous sheet is exposed during the size reduction
can affect the properties of the finished membrane. For example,
when the microporous sheet is exposed to a temperature
<116.0.degree. C. during size reduction, strain concentration in
the sheet can lead to worsening heat shrinkage characteristics.
When the microporous sheet is exposed to a temperature
>125.0.degree. C., it can be more difficult to produce a
membrane having the desired air permeability.
[0077] Optionally, thermal relaxation is conducted for a time in
the range of 10 seconds to 5 hours, for example. The choice of the
thermal relaxation method is not critical. In an embodiment, the
thermal relaxation is conducted using a tenter machine, where the
distance between opposed tenter clips is gradually reduced in TD.
Thermal relaxation produces a finished microporous membrane having
low shutdown temperature, fast shutdown speed, high pin puncture
strength and low heat shrinkage.
Second Embodiment
[0078] The microporous membrane of the second embodiment will now
be described.
Polymers Used to Produce the Microporous Membrane of the Second
Embodiment
[0079] The microporous membrane of the second embodiment can be
produced from first, second and third polyethylenes and optionally
a fourth polymer. The first and third polyethylenes and the fourth
polymer can be the same as those used in the first embodiment. The
second polyethylene is similar to the second polyethylene used to
produce the microporous membrane of the first embodiment, except
that the second polyethylene of the second embodiment has an Mw in
the range of 1.times.10.sup.4 to 1.times.10.sup.6 and the terminal
vinyl group content is not relevant.
Method for Producing a Microporous Membrane
[0080] The microporous membrane in the second embodiment can be
produced by a method similar to that used to produce the
microporous membranes of the first embodiment. The combining step
(1) and the extruding step (2) are the same as in the first
embodiment. The wet stretching step (3) differs from that of the
first embodiment. In the second embodiment, the sheet (the cooled
extrudate) is stretched in at least one direction while exposing
the sheet to a temperature in the range of 20.0.degree. C. to
90.0.degree. C. (primary wet stretching). Following primary wet
stretching, the stretched sheet is stretched in at least one
direction (the same direction or a different direction from the
direction of primary stretching) while exposing the sheet to a
temperature in the range of 110.0.degree. C. to 125.0.degree. C.,
(secondary wet stretching) in order to achieve an area
magnification factor in the range of 20 fold to 60 fold, based on
the area of the sheet at the start of primary stretching. The
diluent removal, dry stretching, thermal relaxation, and the
optional steps are the same as in the first embodiment. A thermal
treatment step (heat-setting), where the membrane is exposed to a
temperature in the range of 116.0.degree. C. to 125.0.degree. C.
while maintaining the microporous sheet's size substantially
constant, can be substituted for the thermal relaxation step as
described below.
[0081] The primary wet stretching step will now be described in
more detail.
Primary Wet Stretching Step
[0082] The primary wet stretching step can be accomplished by one
or more of tenter stretching, roller stretching or inflation
stretching (e.g., with air). Conventional stretching methods can be
used, such as those described in the previously cited PCT
Publications. As in the first embodiment, the stretching can be
along one direction (e.g., a planar direction such as MD or TD) or
two directions (e.g., substantially perpendicular planar directions
such as MD and TD). When the stretching is biaxial, the stretching
can be sequential (first stretching in one direction and then
another) or simultaneous. In one embodiment, simultaneous biaxial
stretching is used.
[0083] During primary stretching, the cooled extrudate is exposed
to a temperature in the range of 20.0.degree. C. to 90.0.degree. C.
Temperatures higher than 90.0.degree. C. can result in cracking of
the gel-like sheet, which can lead to undesirable permeability and
pore size in the finished membrane. Temperatures lower than
20.0.degree. C. can result in increased sheet tension during
stretching, making it more difficult to achieve the desired
magnification factor.
[0084] In an embodiment, simultaneous biaxial stretching is used
for primary stretching. The cooled extrudate is stretched in
substantially perpendicular planar directions, e.g., MD and TD, to
a magnification factor of in the range of 1.01 to 5 fold, such as
1.05 to 4 fold in each planar direction. A magnification factor
<1.01 fold can result in a finished membrane having an
undesirable pore diameter and permeability. A magnification factor
>5 fold can result in a finished membrane having an undesirably
large heat shrinkage. Optionally, the area magnification factor
resulting from primary biaxial stretching is in the range of 1.1 to
20 fold, or 1.2 to 16 fold, based on the area of the membrane at
the start of primary stretching. Area magnification factors of
<1.1 can result in a finished microporous membrane having
undesirable pore diameter and permeability. Area magnification
factors >20 can result in a finished membrane having undesirably
large heat shrinkage. Secondary wet stretching is conducted after
primary wet stretching, optionally with no intervening
processing.
[0085] Secondary wet stretching results in an increase in film size
to a total area magnification factor in the range of 20 fold to 60
fold. An area magnification factor <20 fold can result in a
finished membrane having undesirable pin puncture strength. An area
magnification factor >60 fold can result in a finished membrane
having undesirably large heat shrinkage. The stretching method can
be the same as that used in primary stretching, e.g., one or more
of tenter stretching, roller stretching, etc.
[0086] The membrane is exposed to a temperature in the range of
110.0.degree. C. to 125.0.degree. C. during secondary wet
stretching. When the temperature is >125.0.degree. C. sheet
melting can make it more difficult to achieve the desired
magnification factor. When the temperature is <110.0.degree. C.,
it can be difficult to it can be more difficult to stretch the
cooled extrudate evenly.
[0087] Optionally, secondary wet stretching is conducted by
simultaneously stretching the cooled extrudate in two substantially
perpendicular planar directions, each to a magnification factor in
the range of 2.0 to 10.0 fold, such as 2.5 to 8 fold. A linear
stretching magnification factor <2.0 fold can result in a
finished microporous membrane having an undesirable pin puncture
strength. A linear stretching magnification >10.0 fold can
result in a finished microporous membrane having an undesirable
heat shrinkage ratio. Optionally, secondary stretching is conducted
to achieve an area magnification factor in the range of 2.0 fold to
50.0 fold, such as 2.5 fold to 45 fold.
Thermal Relaxation and/or Heat-Setting
[0088] Following diluent removal, the microporous sheet is
subjected to thermal relaxation, heat setting, or both. When
thermal relaxation is used, it can be conducted under the same
conditions as in the first embodiment. Suitable heat setting
conditions include those described in the previously cited PCT
publications. Optionally, the microporous sheet is exposed to a
temperature in the range of 116.0.degree. C. to 125.0.degree. C.
during heat setting. Temperatures <116.0.degree. C. can result
in undesirable heat shrinkage values in the finished membrane,
resulting it is believed from internal strain concentration.
Temperatures >125.0.degree. C. can result in a loss of membrane
permeability.
The Microporous Membrane
[0089] The finished microporous membranes have a shutdown
temperature in the range of 120.0.degree. C. to 130.0.degree. C.
and a maximum solid state heat shrinkage .ltoreq.30.0%. 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. Non-contact
thickness measurements are also suitable, e.g., optical thickness
measurement methods.
[0090] The microporous membrane can be a monolayer membrane. In an
embodiment, the microporous membrane further comprises a second
membrane. The second membrane can be, e.g., a microporous
layer.
Shutdown Temperature
[0091] 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 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.).
[0092] In an embodiment, the membrane has a shutdown temperature in
the range of 120.0.degree. C. to 130.0.degree. C., e.g., in the
range of 124.degree. C. to 129.degree. C.
Maximum Solid State Shrinkage
[0093] Microporous membrane's maximum solid state heat shrinkage is
measured using a thermomechanical analyzer, (TMA/S S6000 available
from Seiko Instruments, Inc.). When TD maximum shrinkage is
measured, a rectangular sample of 3 mm.times.50 mm is cut out of
the microporous membrane such that the long axis of the sample is
aligned with TD and the short axis is aligned with MD. When MD
maximum shrinkage is measured, the rectangular sample of 3
mm.times.50 mm used is cut out of the microporous membrane such
that the long axis of the sample is aligned with MD and the short
axis is aligned with TD.
[0094] The sample is set in the thermomechanical analyzer with the
long axis of the sample between the analyzer's chucks. The chuck
distance is set to 10 mm, i.e., the distance from the upper chuck
to the lower chuck is 10 mm. The lower chuck is fixed and a load of
19.6 mN applied to the sample at the upper chuck. The chucks and
sample are enclosed in a tube which can be heated to expose the
microporous membrane sample to elevated temperature. Starting at
30.degree. C., the temperature inside the tube is increased at a
rate of 5.degree. C./minute, and sample length change under the
19.6 mN load is measured at intervals of 0.5 second and recorded as
temperature is increased. The maximum solid state heat shrinkage is
a ratio defined as the maximum sample length between the chucks
measured at 23.degree. C. (L1 equal to 10 mm) minus the minimum
length measured in the range of about 125.degree. C. to about
135.degree. C. (equal to L2) divided by L1, i.e.,
[L1-L2]/L1*100%.
[0095] In an embodiment, the microporous membrane's maximum TD and
MD solid state heat shrinkages are each .ltoreq.30.0%, such as
.ltoreq.25%. Optionally, the TD maximum solid state heat shrinkage
is the range of 5.0% to 30.0%, e.g., 10% to 25%. Optionally, the MD
maximum solid state heat shrinkage is in the range of 5.0% to
30.0%, e.g., 10% to 25%.
[0096] 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 polymer used to
produce the membrane by no more than, e.g., about 10%, or no more
than about 1%, or no more than about 0.1%.
[0097] While the extrudate and the microporous membrane can contain
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.
[0098] Optionally, the microporous membrane can have one or more of
the following:
Shutdown Speed
[0099] Optionally, the microporous membrane has a shutdown speed
.gtoreq.10000 sec/100 cm.sup.3/.degree. C., for example
.gtoreq.12000 sec/100 cm.sup.3/.degree. C., such as .gtoreq.15000
sec/100 cm.sup.3/.degree. C. In an embodiment, the microporous
membrane has a shutdown speed in the range of 10000 sec/100
cm.sup.3/.degree. C. to 50000 sec/100 cm.sup.3/.degree. C. The
shutdown speed is measured as follows. 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 speed is defined as the
gradient of air permeability (Gurley value) as a function of
temperature at a Gurley value of 10000 sec/100 cm.sup.3 when
measured during heating.
Pin Puncture Strength
[0100] The microporous membrane's pin puncture strength is
expressed as the equivalent pin puncture strength for a membrane
having a thickness of 25 .mu.m. In other words, the measured pin
puncture strength of the microporous membrane is normalized to the
equivalent value at a membrane thickness of 25 .mu.m, and expressed
in units of [force/25 .mu.m].
[0101] Optionally, the microporous membrane's pin puncture strength
is .gtoreq.5 N/25 .mu.m, e.g., .gtoreq.6 N/25 .mu.m, such as
.gtoreq.6.2 N/25 .mu.m, or .gtoreq.6.5 N/25 .mu.m. In an
embodiment, the microporous membrane's pin puncture strength is in
the range of 5.0 N/25 .mu.m to 10.0 N/25 .mu.m. Pin puncture
strength is measured as follows. The maximum load is measured 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 ("S") is defined using the equation S.sub.2=25
.mu.m*(S.sub.1)/T.sub.1, where S.sub.1 is the measured pin puncture
strength, S.sub.2 is the pin puncture strength normalized to that
of an equivalent membrane thickness of 25 .mu.m, and T.sub.1 is the
average thickness of the membrane.
Air Permeability
[0102] Optionally, the microporous membrane has an air permeability
.gtoreq.50.0 sec/100 cm.sup.3/20 .mu.m (Gurley value, normalized to
that of an equivalent membrane thickness of 20 .mu.m). Since the
air permeability value is normalized to that of an equivalent
membrane having a film thickness of 20 .mu.m, the air permeability
value is expressed in units of [seconds/100 cm.sup.3/20.mu.]. In an
embodiment, the membrane has an air permeability in the range of
50.0 sec/100 cm.sup.3/20 .mu.m to 500.0 sec/100 cm.sup.3/20 .mu.m.
When the microporous membrane's air permeability is <than 50.0
sec/100 cm.sup.3/20 .mu.m, it is more difficult to prevent the
formation of lithium dendrites within the microporous membrane when
the membrane is used as battery separator film.
[0103] Normalized air permeability is measured according to JIS
P8117, and the results are normalized to a value at 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 at an equivalent
membrane thickness of 20 .mu.m.
Porosity
[0104] The microporous membrane optionally has a porosity of
.gtoreq.30.0%, such as in the range of 30% to 95%. 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 the equivalent non-porous membrane having the
same size and thickness.
A Meltdown Temperature
[0105] The microporous membrane optionally has a meltdown
temperature .gtoreq.145.0.degree. C., such as .gtoreq.150.degree.
C. Meltdown temperature can be measured as follows. A sample of the
microporous membrane measuring 5 cm.times.5 cm is fastened along
its perimeter by sandwiching the sample between metallic blocks
each having a circular opening of 12 mm in diameter. The blocks are
then positioned so the plane of the membrane is horizontal. A
tungsten carbide ball of 10 mm in diameter is placed on the
microporous membrane in the circular opening of the upper block.
Starting at 30.degree. C., the membrane is then exposed to an
increasing temperature at rate of 5.degree. C./minute. The
temperature at which the microporous membrane is ruptured by the
ball is defined as the membrane's meltdown temperature.
[0106] In an embodiment, the membrane has a meltdown temperature in
the range of 145.degree. C. to 200.degree. C.
Battery Separator Film
[0107] The microporous membranes of the invention are useful as
battery separator film in e.g., lithium ion primary and secondary
batteries. Such batteries are described in PCT publication WO
2008/016174.
[0108] The battery is useful as a power source for one or more
electrical or electronic components. Such components include
passive components such as resistors, capacitors, inductors,
including, e.g., transformers; electromotive devices such as
electric motors and electric generators, and electronic devices
such as diodes, transistors, and integrated circuits. The
components can be connected to the battery in series and/or
parallel electrical circuits to form a battery system. The circuits
can be connected to the battery directly or indirectly. For
example, electricity flowing from the battery can be converted
electrochemically (e.g., by a second battery or fuel cell) and/or
electromechanically (e.g., by an electric motor operating an
electric generator) before the electricity is dissipated or stored
in a one or more of the components. The battery system can be used
as a power source for powering relatively high power devices such
as electric motors in power tools.
[0109] The invention is not limited to the use of the microporous
membrane as battery separator film in lithium ion secondary
batteries. Because the microporous membrane of this invention has
well-balanced properties such as relatively low shutdown
temperature, relatively high shutdown speed, relatively high pin
puncture strength and relatively low heat shrinkage, the
microporous membrane is useful as battery separator film in primary
and secondary batteries such as nickel-hydrogen batteries,
nickel-cadmium batteries, nickel-zinc batteries, silver-zinc
batteries, lithium-ion batteries, lithium-ion polymer batteries.
The membrane is also useful for filtration and separation.
EXAMPLES
[0110] The following examples further describe embodiments of the
invention. The third polyethylene used in the Examples and
Comparative Examples below was produced by the method described in
PCT publication WO2000/068279A, which is incorporated by reference
herein in its entirety.
[0111] At first, we will describe examples of the first
embodiment.
Example 1
[0112] 100 parts by mass of a polyethylene composition is prepared.
The polyethylene composition comprises the first, second, and third
polyethylene as described above in the following amounts: 30% by
mass of an UHMWPE having an Mw of 1.95.times.10.sup.6 (the first
polyethylene-"component A"), 57% by mass of an HDPE having an Mw of
5.6.times.10.sup.5, a Tm of 135.0.degree. C., and a terminal vinyl
group content of 0.15 per 10000 carbons (the second
polyethylene-"component B"), and 13% by mass of an ethylene-octene
copolymer (m-PE) having an Mw of 2.0.times.10.sup.4 (prepared by a
single cite catalyst), containing 4.0 mol. % of an octene
comonomer, and having a Tm of 120.0.degree. C. and an MWD of 2.0
(the third polyethylene-component C). The polyethylene composition
is dry-blended with 0.375 parts by mass of
tetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate]me-
thane as an antioxidant.
[0113] 30 parts by mass of the antioxidant polyethylene composition
mixture is charged into a strong-blending double-screw extruder
having an inner diameter of 58 mm and L/D of 42, and 70 parts by
mass of liquid paraffin (50 cst at 40.degree. C.) is supplied to
the double-screw extruder via a side feeder. Melt-blending is
conducted at 210.degree. C. and 200 rpm to prepare a polyethylene
solution.
[0114] The polyethylene solution is extruded from a T-die mounted
to a tip end of the double-screw extruder, and drawn and cooled by
cooling rolls controlled at 50.degree. C. while reeling up, to form
a gel-like sheet having a thickness of 1.0 mm. Using a
tenter-stretching machine, the gel-like sheet is simultaneously
biaxially stretched while exposing the sheet to a temperature of
116.degree. C. (a wet stretching), such that the stretching
magnification factor is 5 fold in both MD and TD. The stretched
gel-like sheet is fixed to an aluminum frame of 20 cm.times.20 cm
and immersed in a washing bath of methylene chloride controlled at
25.degree. C., and washed with the vibration of 100 rpm for 3
minutes to remove the liquid paraffin.
[0115] The washed gel-like sheet is dried by air at room
temperature to form a microporous membrane, and simultaneously
biaxially re-stretched to 1.5 fold in both MD and TD at 118.degree.
C. by a batch-stretching machine (dry stretching). Then, thermal
relaxation is conducted to the dry-stretched microporous membrane
to reduce the membrane's size in both MD and TD to a magnification
factor of 1.4 fold based on the membrane's size in MD and TD at the
start of dry orientation. In other words, the amount of thermal
relaxation is 6.7%. Selected properties of the membrane are shown
in Table 1.
Example 2
[0116] A microporous polyethylene membrane is produced in the same
manner as in Example 1, except for using 30% by mass of the UHMWPE
as a component (A), 55% by mass of the HDPE as a component (B) and
15% by mass of the ethylene-octene copolymer as a component
(C).
Example 3
[0117] A microporous polyethylene membrane is produced in the same
manner as in Example 1, except for using an ethylene-hexene
copolymer, containing an octene comonomer content of 3.7 mol. % and
having an Mw of 3.0.times.10.sup.4, a melting point of
121.4.degree. C. and an MWD of 2.5 as a component (C).
Example 4
[0118] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that the gel-like sheet is
simultaneously biaxially stretched at 115.degree. C. (primary
stretching) such that the stretching magnification is 6 fold in
both MD and TD directions by using a tenter stretching machine,
that the microporous membrane is simultaneously biaxially
dry-stretched to 1.8 fold in both MD and TD at 119.degree. C. by a
batch-stretching machine and that the thermal relaxation is
conducted to the to achieve a final magnification factor in MD and
TD (based on the membrane's size at the start of dry stretching) of
1.6 fold while exposing the membrane to a temperature of
119.degree. C. In other words, the thermal relaxation results in a
reduction in the membrane's Td and MD size of 11.1%.
Example 5
[0119] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that the gel-like sheet is
simultaneously biaxially stretched while exposing the membrane to a
temperature of 115.degree. C. (a primary stretching) such that the
stretching magnification is 7 fold in MD and 5 fold in TD, that the
microporous membrane is simultaneously biaxially dry-stretched to
an MD magnification factor of 1.5 fold and 2.1 fold in TD while
exposing the membrane to a temperature of 119.degree. C. and that
thermal relaxation is conducted to the dry-stretched microporous
membrane to a achieve a final magnification factor in MD of 1.4
fold (based on the MD size of the membrane at the start of dry
stretching) and a final TD magnification factor of 1.8 fold (based
on the TD size of the membrane at the start of dry orientation)
while exposing the membrane to a temperature of 119.degree. C. In
other words, the thermal relaxation results in an MD size decrease
of 6.7% and a TD size decrease of 14.3%.
Example 6
[0120] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except for using an HDPE having an Mw of
7.5.times.10.sup.5 and a terminal vinyl group of 0.85 per 10000
carbons as component (B).
Example 7
[0121] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except without using the UHMWPE as a
component (A) and using 85% by mass of an HDPE having a weight
average molecular weight of 5.6.times.10.sup.5 and a terminal vinyl
group of 0.15 per 10000 carbons as component (B).
Example 8
[0122] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that using 10% by mass of an
ethylene-octene copolymer having an Mw of 2.0.times.10.sup.4, an
MWD of 5.0 a melting point of 120.degree. C., and containing an
octene comonomer content of 8.0 mol. %.
Example 9
[0123] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that the microporous membrane is
simultaneously biaxially dry-stretched to a magnification factor of
1.3 fold in both MD and TD and the thermal relaxation is conducted
to the dry-stretched microporous membrane to achieve a final
magnification factor (based on the size of the membrane at the
start of dry orientation) in MD and TD of 1.2 fold. In other words,
the thermal relaxation results in an MD and TD size decrease of
7.7%.
Example 10
[0124] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that the thermal relaxation is
conducted to the dry-stretched microporous membrane to achieve a
final magnification factor in MD and TD of 1.1 fold based on the
size of the membrane at the start of dry stretching. In other
words, the thermal relaxation results in an MD and TD size decrease
of 26.7%.
Comparative Example 1
[0125] 100 parts by mass of a polyethylene composition comprising
80% by mass of an HDPE having Mw of 5.6.times.10.sup.5, a melting
point of 135.0.degree. C. and a terminal vinyl group of 0.15 per
10000 carbons as component (B) and 20% by mass of an
ethylene-hexene copolymer (m-PE) having Mw of 7.0.times.10.sup.4
(prepared by a single cite catalyst), containing an octene
comonomer amount of 1.0 mol. % and having a melting point of
127.0.degree. C. and an MWD of 4.2 as component (C) were
dry-blended with 0.3 parts by mass of
tetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate]me-
thane as an antioxidant.
[0126] 45 parts by mass of the resultant mixture is charged into a
strong-blending double-screw extruder having an inner diameter of
58 mm and L/D of 42, and 55 parts by mass of liquid paraffin (50
cst at 40.degree. C.) is supplied to the double-screw extruder via
a side feeder. Melt-blending is conducted at 240.degree. C. and 200
rpm to prepare a polyethylene solution. The polyethylene solution
is extruded from a T-die mounted to a tip end of the double-screw
extruder, and drawn and cooled by cooling rolls controlled at
50.degree. C. while reeling up, to form a gel-like sheet having a
thickness of 1.0 mm. Using a tenter-stretching machine, the
gel-like sheet is simultaneously biaxially stretched at 116.degree.
C. (wet stretching), such that the stretching magnification factor
is 7 fold in both MD and TD. The stretched gel-like sheet is fixed
to an aluminum frame of 20 cm.times.20 cm and immersed in a washing
bath of methyl ethyl ketone (MEK) controlled at 25.degree. C., and
washed with the vibration of 100 rpm for 3 minutes to remove the
liquid paraffin.
[0127] The washed gel-like sheet is dried by air at room
temperature to form a microporous membrane, and simultaneously
biaxially dry-stretched to a magnification factor of 1.3 fold in MD
and TD while exposing the membrane to a temperature of 118.degree.
C. using a batch-stretching machine Then, heat-setting is conducted
by exposing the dry-stretched microporous membrane to a temperature
118.degree. C. while maintaining the membrane's MD and TD size
substantially constant. This Comparative Example is conducted
according to the method disclosed in JP2002-338730A.
Comparative Example 2
[0128] A microporous polyethylene membrane is produced in the same
manner as in Example 4 except that thermal relaxation is not
conducted.
Comparative Example 3
[0129] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that the wet stretching is conducted
while exposing the sheet to a temperature of 110.degree. C.
Comparative Example 4
[0130] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that the wet stretching is conducted
at a temperature of 127.degree. C.
Comparative Example 5
[0131] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that the microporous membrane is
simultaneously biaxially dry-stretched to 3 fold in both MD while
exposing the membrane to a temperature of 118.degree. C.
Comparative Example 6
[0132] A microporous polyethylene membrane is produced in the same
manner as in Example 1 except that an ethylene-octene copolymer
(prepared by a single cite catalyst), containing an octene
comonomer content of 15.0 mol. % and having a melting point of
110.degree. C. and a MWD of 2.0 is used as a component (C).
[0133] The properties of the polyethylene microporous membrane
produced in Example 1 to 10 and Comparative Example 1 to 5 were
measured as described above. Selected membrane properties are shown
in Table 1 and 2. Membrane thickness was measured using a contact
thickness meter at a selected MD position. The measurement was
conducted at points along the membrane's TD (width) over a distance
of 30 cm at 5-mm intervals. The arithmetic mean of the measured
values is the sample thickness. Other membrane properties are
measured as described above.
TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Example 4
Example 5 PO composition First PE Mw 1.95 .times. 10.sup.6 1.95
.times. 10.sup.6 1.95 .times. 10.sup.6 1.95 .times. 10.sup.6 1.95
.times. 10.sup.6 % by mass 30 30 30 25 30 Second PE 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 % by mass 57 55 57 65 57 Terminal
vinyl content (per 10,000 carbons) 0.15 0.15 0.15 0.15 0.15 Melting
point (.degree. C.) 135.0 135.0 135.0 135.0 135.0 Third PE Mw 2.0
.times. 10.sup.4 2.0 .times. 10.sup.4 3.0 .times. 10.sup.4 2.0
.times. 10.sup.4 2.0 .times. 10.sup.4 Mw/Mn 2.0 2.0 2.5 2.0 2.0 %
by mass 13 15 13 10 13 Kind of comonomer octene octene hexene
octene octene Comonomer content (mol. %) 4.0 4.0 3.7 4.0 4.0
Melting point (.degree. C.) 120.0 120.0 121.4 120.0 120.0
Production Conditions PE concentration (% by mass) 30 30 30 30 30
Wet stretching Temperature (.degree. C.) 116 116 116 115 115
Magnification (MD .times. TD) 5 .times. 5 5 .times. 5 5 .times. 5 6
.times. 6 7 .times. 7 Re-stretching Temperature (.degree. C.) 118
118 118 119 119 Magnification (MD .times. TD) 1.5 .times. 1.5 1.5
.times. 1.5 1.5 .times. 1.5 1.8 .times. 1.8 1.5 .times. 1.5
Heat-relaxing (annealing) Temperature (.degree. C.) 118 118 118 119
119 Magnification before re-stretching (MD .times. TD) 1.4 .times.
1.4 1.4 .times. 1.4 1.4 .times. 1.4 1.6 .times. 1.6 1.4 .times. 1.4
Heat-setting Temperature (.degree. C.) NA NA NA NA NA Physical
property Average Thickness (.mu.m) 24 24 23 25 24 Air Permeability
(sec/100 cm.sup.3/20 .mu.m) 720 740 745 650 510 Porosity (%) 38 37
37 42 42 Pin Puncture Strength (N/25 .mu.m) 6.1 6.0 6.3 7.0 7.7
Max. Shrinkage Ratio %) (MD/TD) (in a solid state) 18.1/19.0
17.8/18.5 18.4/19.2 20.1/21.4 24.1/23.1 Shutdown Temperature
(.degree. C.) 122.0 123.0 122.5 124.5 124.0 Shutdown Speed (sec/100
cm.sup.3/.degree. C.) 11,000 11,540 11,100 10,300 11,500 Example 6
Example 7 Example 8 Example 9 Example 10 PO composition First PE Mw
1.95 .times. 10.sup.6 NA 1.95 .times. 10.sup.6 1.95 .times.
10.sup.6 1.95 .times. 10.sup.6 % by mass 30 NA 30 30 30 Second PE
Mw 7.5 .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 % by mass 57 85 57 57 57
Terminal vinyl content (per 10,000 carbons) 0.85 0.15 0.15 0.15
0.15 Melting point (.degree. C.) 135.0 135.0 135.0 135.0 135.0
Third PE Mw 2.0 .times. 10.sup.4 2.0 .times. 10.sup.4 2.0 .times.
10.sup.4 2.0 .times. 10.sup.4 2.0 .times. 10.sup.4 Mw/Mn 2.0 2.0
5.0 2.0 2.0 % by mass 13 15 13 13 13 Kind of comonomer octene
octene octene octene octene Comonomer content (mol. %) 4.0 4.0 4.0
4.0 4.0 Melting point (.degree. C.) 120.0 120.0 120.0 120.0 120.0
Production Conditions PE concentration (% by mass) 30 30 30 30 30
Wet stretching Temperature (.degree. C.) 116 116 115 116 116
Magnification (MD .times. TD) 5 .times. 5 5 .times. 5 7 .times. 7 5
.times. 5 5 .times. 5 Re-stretching Temperature (.degree. C.) 118
118 119 118 118 Magnification (MD .times. TD) 1.5 .times. 1.5 1.5
.times. 1.5 1.5 .times. 1.5 1.3 .times. 1.3 1.5 .times. 1.5
Heat-relaxing (annealing) Temperature (.degree. C.) 118 118 119 118
118 Magnification before re-stretching (MD .times. TD) 1.4 .times.
1.4 1.4 .times. 1.4 1.4 .times. 1.4 1.2 .times. 1.2 1.1 .times. 1.1
Heat-setting Temperature (.degree. C.) NA NA NA NA NA Physical
property Average Thickness (.mu.m) 24 24 24 23 25 Air Permeability
(sec/100 cm.sup.3/20 .mu.m) 890 450 630 470 400 Porosity (%) 36 47
39 42 44 Pin Puncture Strength (N/25 .mu.m) 6.3 4.6 6.0 4.5 5.0
Max. Shrinkage Ratio %) (MD/TD) (in a solid state) 19.0/20.1
23.1/24.5 25.0/26.1 11.4/13.0 15.2/16.4 Shutdown Temperature
(.degree. C.) 122.0 121.2 121.3 122.0 120.4 Shutdown Speed (sec/100
cm.sup.3/.degree. C.) 9,700 12,500 9,200 11,000 11,400
TABLE-US-00002 TABLE 2 Comparative Comparative Comparative
Comparative Comparative Comparative Example 1 Example 2 Example 3
Example 4 Example 5 Example 6 PO composition First PE Mw NA 1.95
.times. 10.sup.6 1.95 .times. 10.sup.6 1.95 .times. 10.sup.6 1.95
.times. 10.sup.6 1.95 .times. 10.sup.6 % by mass NA 30 30 30 30 30
Second PE 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 % by mass 80 57 57 57 57 57 Terminal vinyl content (per
10,000 0.15 0.15 0.15 0.15 0.15 0.15 carbons) Melting point
(.degree. C.) 135.0 135.0 135.0 135.0 135.0 135.0 Third PE Mw 7.0
.times. 10.sup.4 2.0 .times. 10.sup.4 3.0 .times. 10.sup.4 2.0
.times. 10.sup.4 2.0 .times. 10.sup.4 2.0 .times. 10.sup.4 Mw/Mn
4.2 2.0 2.5 2.0 2.0 2.0 % by mass 13 13 13 13 13 13 Kind of
comonomer Octene Octene Octene Octene Octene Octene Comonomer
content (mol. %) 1.0 4.0 4.0 4.0 4.0 4.0 Melting point (.degree.
C.) 127.0 120.0 121.4 120.0 120.0 120.0 Production Conditions PE
concentration (% by mass) 45 30 30 30 30 30 Wet stretching
Temperature (.degree. C.) 116 116 110 127 115 115 Magnification (MD
.times. TD) 7 .times. 7 5 .times. 5 5 .times. 5 5 .times. 5 7
.times. 7 7 .times. 7 Re-stretching Temperature (.degree. C.) 118
118 118 118 118 118 Magnification (MD .times. TD) 1.3 .times. 1.3
1.8 .times. 1.8 1.5 .times. 1.5 1.5 .times. 1.5 1.4 .times. 1.4 1.4
.times. 1.4 Heat-relaxing (annealing) Temperature (.degree. C.) NA
NA 118 118 119 118 Magnification before re-stretching (MD .times.
TD) NA NA 1.4 .times. 1.4 1.4 .times. 1.4 1.4 .times. 1.4 1.4
.times. 1.4 Heat-setting Temperature (.degree. C.) 118 NA NA NA NA
NA Physical property Average Thickness (.mu.m) 25 25 24 24 24 24
Air Permeability (sec/100 cm.sup.3/20 .mu.m) 350 420 660 1050 470
820 Porosity (%) 40 43 39 30 40 38 Pin Puncture Strength (N/25
.mu.m) 4.9 6.4 7.0 3.5 7.5 5.2 Max. Shrinkage Ratio (%) (MD/TD)
38.7/32.5 30.2/31.4 31.4/32.5 9.7/11.2 35.3/36.2 32.4/33.3 (in a
solid state) Shutdown Temperature (.degree. C.) 129.5 126.3 123.5
130.5 131.0 113.0 Shutdown Speed (sec/100 cm.sup.3/.degree. C.)
11,400 12,200 11,400 12,700 12,500 9,600
[0134] Table 1 shows that the microporous membrane of Examples 1 to
10 has both a relatively low heat shrinkage ratio and a relatively
low shutdown temperature in the range of 120.degree. C. to
130.degree. C. Furthermore, Table 1 shows that the polyethylene
microporous membrane of Examples 1 to 5 has a relatively high
shutdown speed and a relatively high pin puncture strength. The
microporous membrane of Examples 1 to 5 corresponds to the first
embodiment, as described above. Table 2 shows the polyethylene
microporous membrane of Comparative Examples of 1 to 6. Comparative
Example 1 has a relatively high heat shrinkage ratio, because, it
is believed, the third polyethylene has a higher melting point and
a relatively broad MWD. Furthermore, Comparative Example 1 has a
lower pin puncture strength, because, it is believed, the membrane
does not contain UHMWPE. Comparative Example 2 had a high heat
shrinkage ratio, because a heat-relaxing step (annealing) is
omitted. It is believed that Comparative Example 3 exhibits a
relatively high heat shrinkage ratio because the wet stretching
temperature is too low. It is believed that Comparative Example 4
exhibits a relatively high shutdown temperature and a relatively
low permeability because the wet stretching temperature is too
high. It is believed that Comparative Example 5 exhibits a
relatively high shutdown temperature and a relatively high heat
shrinkage ratio because the dry-stretching magnification factor is
too large. It is believed that Comparative Example 6 exhibits a
relatively large high heat shrinkage ratio because the third
polyethylene has too low a low melting point.
[0135] The following examples pertain to the second embodiment.
Example 11
[0136] 100 parts by mass of a polyethylene (PE) composition
comprising 30% by mass of an UHMWPE having an Mw of
1.95.times.10.sup.6 as a component (A), 57% by mass of an HDPE
having an Mw of 5.6.times.10.sup.5, a melting point of
135.0.degree. C., and a terminal vinyl group content of 0.15 per
10000 carbons as a component (B); and 13% by mass of an
ethylene-octene copolymer (m-PE) prepared by a single cite
catalyst, containing an octene comonomer amount of 8.0 mol. %,
having Mw of 2.0.times.10.sup.4, and having a melting point of
120.degree. C., and an MWD of 2.0 as a component (C) were
dry-blended with 0.375 parts by mass of
tetrakis[methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)-propionate]me-
thane as an antioxidant.
[0137] 30 parts by mass of the resultant mixture is charged into a
strong-blending double-screw extruder having an inner diameter of
58 mm and L/D of 42, and 70 parts by mass of liquid paraffin (50
cst at 40.degree. C.) is supplied to the double-screw extruder via
a side feeder. Melt-blending is conducted at 210.degree. C. and 200
rpm to prepare a polyethylene solution.
[0138] The polyethylene solution is extruded from a T-die mounted
to a tip end of the double-screw extruder, and drawn and cooled by
cooling rolls controlled at 50.degree. C. while reeling up, to form
a gel-like sheet having a thickness of 1.0 mm. Using a
tenter-stretching machine, the gel-like sheet is simultaneously
biaxially stretched while exposing the sheet to a temperature of
80.degree. C. (a primary wet stretching), to achieve a primary wet
stretching magnification factor of 2 fold in both MD and TD. The
sheet is then simultaneously biaxially stretched while exposing the
sheet to a temperature of 116.degree. C. (secondary wet
stretching), to achieve a secondary wet stretching magnification
factor of 3.5 fold in both MD and TD. The total magnification
factor at the end of primary and secondary wet stretching is 49
fold. The stretched gel-like sheet is fixed to an aluminum frame of
20 cm.times.20 cm and immersed in a washing bath of methylene
chloride controlled at 25.degree. C., and washed with a vibration
of 100 rpm for 3 minutes to remove the liquid paraffin.
[0139] The washed gel-like sheet was dried by air at room
temperature to form a microporous membrane, and simultaneously
biaxially dry-stretched to a magnification factor of 2.0 fold in
both MD and TD while exposing the membrane to a temperature of
120.degree.. Then, thermal relaxation is conducted to the
dry-stretched microporous membrane to achieve a final TD and MD
magnification factor of 1.4 fold (based on the size of the membrane
at the start of dry stretching) while exposing the membrane to a
temperature of 122.degree. C. In other words, the thermal
relaxation results in an MD and TD size decrease of 30% in both MD
and TD.
Example 12
[0140] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except for using HDPE having Mw of
7.46.times.10.sup.5 and a terminal vinyl group content of 0.88 per
10000 carbons as a component (B).
Example 13
[0141] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except that the gel-like sheet is
simultaneously biaxially stretched while exposed to a temperature
of 85.degree. C. during primary wet stretching, the primary wet
stretching magnification factor is 3.5 fold in both MD and TD, and
then secondary wet stretching is conducted while exposing the sheet
to a temperature of 114.degree. C. to achieve a secondary wet
stretching magnification factor of 2 fold in both MD and TD.
Example 14
[0142] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except that the microporous membrane is
biaxially dry-stretched to a magnification factor of 1.7 fold in
both MD and TD while exposing the membrane to a temperature of
118.degree. C. by a batch-stretching machine (dry-stretching) and
then conducting thermal relaxation of the dry-stretched microporous
membrane to achieve a final magnification factor of 1.4 fold in MD
and TD while exposing the membrane to a temperature of 122.degree.
C. In other words, the thermal relaxation results in an MD and TD
size decrease of 17.6%.
Example 15
[0143] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except that the gel-like sheet is
simultaneously biaxially stretched while exposing the sheet to a
temperature of 80.degree. C. (primary wet stretching), such that
the stretching magnification is 2.5 fold in both MD and TD, and
then simultaneously biaxially stretching the sheet (secondary wet
stretching) while exposing the sheet to a temperature of
116.degree. C. to achieve a secondary stretching magnification
factor of 2 fold in both MD and TD. The membrane is simultaneously
biaxially dry-stretched to a magnification factor of 1.5 fold in
both MD and TD while exposing the membrane to a temperature of
120.degree. C. by a batch-stretching, and then the membrane is
exposed to a temperature of 121.degree. C. while maintaining the MD
and TD size of the membrane substantially constant to heat set the
membrane. No thermal relaxation was used.
Example 16
[0144] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except that the primary wet stretching is
not conducted.
Example 17
[0145] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except that component A (UHMWPE) was
omitted and using 87% by mass of an HDPE having an Mw of
3.0.times.10.sup.5 and a terminal vinyl group content of 0.15 per
10000 carbons as a component (B).
Example 18
[0146] A microporous polyethylene membrane is produced in the same
manner as in Example 11 except that the microporous membrane is
simultaneously biaxially dry-stretched to 1.3 fold in both MD and
TD by using a tenter stretching machine (dry-stretching) and then
subjecting the membrane to thermal relaxation to achieve a final MD
and TD magnification factor of 1.2 fold, based on the size of the
membrane at the start of dry stretching. In other words, the
thermal relaxation results in an MD and TD size decrease of
7.7%.
Comparative Example 7
[0147] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except that the gel-like sheet is
simultaneously biaxially stretched at 80.degree. C. (primary wet
stretching) to achieve a primary wet stretching magnification of 2
fold in both MD and TD, and then simultaneously biaxially stretched
while exposing the sheet to a temperature of 116.degree. C. (wet
secondary stretching) to achieve a secondary wet stretching
magnification of 4 fold in both MD and TD.
Comparative Example 8
[0148] A microporous polyethylene membrane is produced in the same
manner as in Example 11, except component (B) (HDPE) is omitted and
for using 30% by mass of UHMWPE as a component (A) and 70% by mass
of m-PE as a component (C).
Comparative Example 9
[0149] A microporous polyethylene membrane is produced in the same
manner as in Example 16 except that the microporous membrane is
simultaneously biaxially dry-stretched to a magnification factor of
3 fold in both MD and TD while exposing the membrane to a
temperature of 118.degree. C.
Comparative Example 10
[0150] A microporous polyethylene membrane is produced in the same
manner as in Example 16 except that neither thermal relaxation nor
heat-setting is conducted.
[0151] The properties of the polyethylene microporous membrane
produced in Example 11 to 18 and Comparative Example 7 to 10 were
measured by the above-described methods. Selected properties of the
polyethylene microporous membrane are shown in Table 3 and 4.
TABLE-US-00003 TABLE 3 Example Example Example Example Example
Example Example Example 11 12 13 14 15 16 17 18 PO composition
First PE Mw 1.95 .times. 10.sup.6 1.95 .times. 10.sup.6 1.95
.times. 10.sup.6 1.95 .times. 10.sup.6 1.95 .times. 10.sup.6 1.95
.times. 10.sup.6 NA 1.95 .times. 10.sup.6 % by mass 30 30 30 30 30
30 NA 30 Second PE Mw 5.6 .times. 10.sup.5 7.5 .times. 10.sup.5 5.6
.times. 10.sup.5 5.6 .times. 10.sup.5 5.6 .times. 10.sup.5 7.5
.times. 10.sup.5 5.6 .times. 10.sup.5 5.6 .times. 10.sup.5 % by
mass 57 57 57 57 57 57 87 57 Terminal vinyl content 0.15 0.88 0.15
0.15 0.15 0.85 0.15 0.15 (per 10,000 carbons) Melting point
(.degree. C.) 135.0 135.0 135.0 135.0 135.0 135.0 135.0 135.0 Third
PE Mw 2.0 .times. 10.sup.4 2.0 .times. 10.sup.4 2.0 .times.
10.sup.4 2.0 .times. 10.sup.4 2.0 .times. 10.sup.4 2.0 .times.
10.sup.4 2.0 .times. 10.sup.4 2.0 .times. 10.sup.4 Mw/Mn 2.0 2.0
2.0 2.5 2.0 2.0 2.0 5.0 % by mass 13 15 13 13 13 13 13 13 Kind of
comonomer octene octene octene octene octene octene octene octene
Comonomer content 4.0 4.0 4.0 4.0 8.0 8.0 8.0 8.0 (mol. %) Melting
point (.degree. C.) 120.0 120.0 120.0 120.0 120.0 120.0 120.0 120.0
Production Conditions PE concentration (% by mass) 30 30 30 30 30
30 30 30 Primary Wet stretching Temperature (.degree. C.) 80 80 85
80 80 NA 80 80 Magnification (MD .times. TD) 2 .times. 2 2 .times.
2 3.5 .times. 3.5 2 .times. 2 2 .times. 2 NA 2 .times. 2 2 .times.
2 Secondary Wet stretching Temperature (.degree. C.) 116 116 114
116 116 116 116 116 Magnification (MD .times. TD) 3.5 .times. 3.5
3.5 .times. 3.5 2 .times. 2 3.5 .times. 3.5 3.5 .times. 3.5 3.5
.times. 3.5 3.5 .times. 3.5 3.5 .times. 3.5 Re-stretching
Temperature (.degree. C.) 120 120 120 118 120 120 120 120
Magnification (MD .times. TD) 2.0 .times. 2.0 2.0 .times. 2.0 2.0
.times. 2.0 1.7 .times. 1.7 1.5 .times. 1.5 2.0 .times. 2.0 2.0
.times. 2.0 1.3 .times. 1.3 Heat-relaxing (annealing) Temperature
(.degree. C.) 122 122 122 122 NA 122 122 122 Magnification before
re-stretching 1.4 .times. 1.4 1.4 .times. 1.4 1.4 .times. 1.4 1.4
.times. 1.4 NA 1.4 .times. 1.4 1.4 .times. 1.4 1.2 .times. 1.2 (MD
.times. TD) Heat-setting Temperature (.degree. C.) NA NA NA NA 121
NA NA NA Physical property Average Thickness (.mu.m) 24 24 23 25 24
25 24 24 Air Permeability 210 280 185 310 200 870 190 620 (sec/100
cm.sup.3/20 .mu.m) Porosity (%) 48 46 49 46 49 32 49 42 Pin
Puncture Strength (N/25 .mu.m) 6.7 6.6 6.2 6.7 6.9 4.5 4.9 6.2
Maximum Shrinkage Ratio (%) 20.0/20.9 21.1/22.0 28.2/29.1 22.4/23.2
25.4/26.3 19.8/20.1 18.0/19.2 14.2/15.5 (MD/TD)(in a solid state)
Shutdown Temperature (.degree. C.) 123.0 122.4 124.0 123.5 123.5
120.0 121.0 121.5
TABLE-US-00004 TABLE 4 Comparative Comparative Comparative
Comparative Example 7 Example 8 Example 9 Example 10 PO composition
First PE Mw 1.95 .times. 10.sup.6 1.95 .times. 10.sup.6 1.95
.times. 106 1.95 .times. 10.sup.6 % by mass 30 30 30 30 Second PE
Mw 5.6 .times. 10.sup.5 NA 5.6 .times. 10.sup.5 5.6 .times.
10.sup.5 % by mass 57 NA 57 57 Terminal vinyl content 0.15 NA 0.15
0.15 (per 10,000 carbons) Melting point (.degree. C.) 135.0 NA
135.0 135.0 Third PE Mw 2.0 .times. 10.sup.4 2.0 .times. 10.sup.4
2.0 .times. 10.sup.4 2.0 .times. 10.sup.4 Mw/Mn 2.0 2.0 2.0 2.0 %
by mass 13 70 13 13 Kind of comonomer octene octene octene octene
Comonomer content (mol.%) 4.0 4.0 4.0 4.0 Melting point (.degree.
C.) 120.0 120.0 120.0 120.0 Production Conditions PE concentration
(% by mass) 30 30 30 30 Primary Wet stretching Temperature
(.degree. C.) 80 80 80 80 Magnification (MD .times. TD) 2 .times. 2
2 .times. 2 2 .times. 2 2 .times. 2 Secondary Wet stretching
Temperature (.degree. C.) 116 116 116 116 Magnification (MD .times.
TD) 4 .times. 4 3.5 .times. 3.5 3.5 .times. 3.5 3.5 .times. 3.5
Re-stretching Temperature (.degree. C.) 120 120 118 120
Magnification (MD .times. TD) 2.0 .times. 2.0 2.0 .times. 2.0 3.0
.times. 3.0 2.0 .times. 2.0 Heat-relaxing (annealing) Temperature
(.degree. C.) 122 122 122 NA Magnification before re-stretching (MD
.times. TD) 1.4 .times. 1.4 1.4 .times. 1.4 1.4 .times. 1.4 NA
Heat-setting Temperature (.degree. C.) NA NA NA NA Physical
property Average Thickness (.mu.m) 24 24 24 22 Air Permeability
(sec/100 cm.sup.3/20 .mu.m) 190 1100 180 200 Porosity (%) 48 29 48
47 Pin Puncture Strength (N/25 .mu.m) 7.7 3.9 7.1 6.3 Maximum
Shrinkage Ratio (%)(MD/TD) 35.1/36.2 30.2/31.1 40.0/41.2 35.0/36.1
(in a solid state) Shutdown Temperature (.degree. C.) 131.0 120.2
129.0 124.0
[0152] Table 3 shows that the polyethylene microporous membrane of
Examples 11 to 18 has a relatively low heat shrinkage ratio and a
shutdown temperature in the range of 120.degree. C. to130.degree.
C. Furthermore, Table 3 shows that the polyethylene microporous
membrane of Examples 11 to 15 have a relatively high pin puncture
strength, improved air permeability and relatively low heat
shrinkage.
[0153] Table 4 shows the microporous membrane of Comparative
Examples of 7 to 10. It is believed that the microporous membrane
of Comparative Example 7 has a relatively high shrinkage ratio and
relatively high shutdown temperature because the total wet
stretching magnification (64 fold) is >60 fold. It is believed
that the microporous membrane of Comparative Example 8 has a
relatively high heat shrinkage because it does not contain HDPE,
and because the content of the third polyethylene is >30% by
mass. It is believed that the microporous membrane of Comparative
Example 9 has a relatively high shrinkage ratio because the
magnification of re-stretching is too large. It is believed that
the microporous membrane of Comparative Example 10 has a relatively
large heat shrinkage ratio because neither thermal relaxation nor
heat-setting step is conducted.
[0154] The invention is further exemplified in the following
embodiments. [0155] 1. A polymer microporous membrane comprising
polyolefin, and having a shutdown temperature of 120.degree. C. or
more but less than 130.degree. C. and a maximum heat shrinkage
.ltoreq.30% in a solid state. [0156] 2. A polymer microporous
membrane, prepared by the following steps of: [0157] (a) combining
a polymer comprising a polyolefin and a diluent to prepare a
polymer solution, [0158] (b) extruding the polymer solution to form
a gel-like sheet, [0159] (c) stretching the gel-like sheet, [0160]
(d) removing the diluent from the stretched sheet, [0161] (e)
heat-stretching the diluent-removed sheet, and [0162] (f)
heat-relaxing the heat-stretched-diluent-removed sheet to form the
membrane comprising polyolefin, and having a shutdown temperature
of 120.degree. C. or more but less than 130.degree. C. and a
maximum heat shrinkage .ltoreq.30% in a solid state. [0163] 3. The
polymer microporous membrane according to embodiments 1 or 2,
further having a shutdown speed .gtoreq.10000 sec/100 cm3/.degree.
C. and a pin puncture strength .gtoreq.5 N/25 .mu.m. [0164] 4. The
polymer microporous membrane according to embodiments 1 or 2,
further having an air permeability of 50 sec/100cm.sup.3/20 .mu.m
to 500 sec/100 cm.sup.3/20 .mu.m and a pin puncture strength
.gtoreq.5 N/25 .mu.m. [0165] 5. A battery separator comprising a
polymer microporous membrane, comprising a polyolefin, and having a
shutdown temperature of 120.degree. C. or more but less than
130.degree. C. and a maximum heat shrinkage .ltoreq.30% in a solid
state. [0166] 6. The battery separator comprising a polymer
microporous membrane according to embodiment 5, further having a
shut-down speed .gtoreq.10000 sec/100 cm.sup.3/.degree. C. and a
pin puncture strength .gtoreq.5N/25 .mu.m. [0167] 7. The battery
separator comprising a polymer microporous membrane according to
embodiment 5, further having an air permeability of 50 sec/100
cm.sup.3/20 .mu.m to 500 sec/100 cm.sup.3/20 .mu.m and a pin
puncture strength .gtoreq.5N/25 .mu.m. [0168] 8. A battery using
the microporous membrane comprising a polyolefin as a battery
separator, wherein the membrane has a shutdown temperature of
120.degree. C. or more but less than 130.degree. C. and a maximum
heat shrinkage .ltoreq.30% in a solid state. [0169] 9. The battery
using the microporous membrane comprising a polyolefin as a battery
separator according to embodiment 8, wherein the membrane further
has a shutdown speed .gtoreq.10000 sec/100 cm3/.degree. C. and a
pin puncture strength .gtoreq.5N/25 .mu.m. [0170] 10. The battery
using the microporous membrane comprising a polyolefin as a battery
separator according to embodiment 8, wherein the membrane further
has an air permeability of 50 sec/100 cm.sup.3/20 .mu.m to 500
sec/100 cm.sup.3/20 .mu.m and a pin puncture strength .gtoreq.5N/25
.mu.m.
[0171] 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.
[0172] 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.
[0173] When numerical lower limits and numerical upper limits are
listed herein, ranges from any lower limit to any upper limit are
contemplated.
* * * * *