U.S. patent application number 13/129670 was filed with the patent office on 2011-09-29 for microporous membranes and methods for producing and using such membranes.
This patent application is currently assigned to TORAY TONEN SPECIALTY SEPARATOR GODO KAISHA. Invention is credited to Yoichi Matsuda, Sadakatsu Suzuki, Junko Takita, Kotaro Takita.
Application Number | 20110236764 13/129670 |
Document ID | / |
Family ID | 42169947 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110236764 |
Kind Code |
A1 |
Takita; Kotaro ; et
al. |
September 29, 2011 |
MICROPOROUS MEMBRANES AND METHODS FOR PRODUCING AND USING SUCH
MEMBRANES
Abstract
The invention relates to microporous polymeric membranes
suitable for use as battery separator film. The invention also
relates to a method for producing such membranes, batteries
containing such membranes as battery separators, methods for making
such batteries, and methods for using such batteries.
Inventors: |
Takita; Kotaro;
(Nasushiobara, JP) ; Takita; Junko; (Nasushiobara,
JP) ; Matsuda; Yoichi; (Nasushiobara, JP) ;
Suzuki; Sadakatsu; (Nasushiobara, JP) |
Assignee: |
TORAY TONEN SPECIALTY SEPARATOR
GODO KAISHA
Nasushiobara-shi, Tochigi
JP
|
Family ID: |
42169947 |
Appl. No.: |
13/129670 |
Filed: |
October 30, 2009 |
PCT Filed: |
October 30, 2009 |
PCT NO: |
PCT/JP2009/069016 |
371 Date: |
May 17, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61115410 |
Nov 17, 2008 |
|
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61115405 |
Nov 17, 2008 |
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Current U.S.
Class: |
429/249 ;
521/134 |
Current CPC
Class: |
B01D 67/0027 20130101;
B29K 2023/12 20130101; B29L 2031/3468 20130101; B01D 2323/10
20130101; H01M 50/411 20210101; B29L 2031/755 20130101; B29L
2009/00 20130101; B29K 2105/256 20130101; H01M 50/449 20210101;
H01M 50/403 20210101; H01M 10/052 20130101; B29K 2023/06 20130101;
B29C 55/023 20130101; Y02E 60/10 20130101; B29K 2995/0097 20130101;
B01D 71/26 20130101 |
Class at
Publication: |
429/249 ;
521/134 |
International
Class: |
H01M 2/16 20060101
H01M002/16; C08J 9/00 20060101 C08J009/00; C08F 10/02 20060101
C08F010/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2009 |
EP |
09151318.4 |
Jan 26, 2009 |
EP |
09151320.0 |
Claims
1. A monolayer microporous membrane comprising a polyolefin having
an Mw>1.0.times.10.sup.6, the membrane having a normalized air
permeability .ltoreq.4.0.times.10.sup.2 seconds/100 cm.sup.3/20
.mu.m, and a heat shrinkage at 130.degree. C. of .ltoreq.15% in at
least one planar direction.
2. The microporous membrane of claim 1, wherein the planar
direction is TD and the membrane has heat shrinkage at 105.degree.
C. in at least one planar direction of .ltoreq.2.5%.
3. The membrane of claim 1, wherein the membrane has a maximum
shrinkage in the molten state in at least one planar direction of
.ltoreq.10.0%.
4. The microporous membrane of claim 1, wherein the polyolefin
comprises (a) a first polyethylene, and further comprises at least
one of (b) polypropylene having an Mw>1.0.times.10.sup.6 or (c)
a second polyethylene having an Mw>1.0.times.10.sup.6.
5. The microporous membrane of claim 4, wherein the first
polyethylene has an Mw in the range of 1.0.times.10.sup.5 to
9.0.times.10.sup.5 and an MWD in the range of from 3.0 to 15, the
second polyethylene has an Mw in the range of 1.1.times.10.sup.6 to
5.0.times.10.sup.6 and an MWD in the range of 4.0 to 20.0, and the
polypropylene has an Mw in the range of from 1.05.times.10.sup.6 to
about 2.0.times.10.sup.6, an MWD in the range of 2.0 to 6.0, and a
.DELTA.Hm.gtoreq.100.0 J/g.
6. The microporous membrane of claim 5, wherein the polyolefin
comprises from 60.0 wt. % to 99.0 wt. % of the first polyethylene
and from 1.0 wt. % to 40.0 wt. % of the second polyethylene.
7. The microporous membrane of claim 6, wherein the membrane has
one or more of (1) a thickness in the range of 1.0 .mu.m to 50.0
.mu.m, (2) a porosity in the range of from 25.0% to 80.0%, (3) a
normalized pin puncture strength .gtoreq.3.0.times.10.sup.3 mN/20
.mu.m, (4) a tensile strength .gtoreq.4.0.times.10.sup.4 kPa, (5) a
TD tensile elongation .gtoreq.100%, (6) a meltdown temperature
.gtoreq.145.degree. C., (7) a shutdown temperature
.ltoreq.140.0.degree. C., (8) a thickness variation after heat
compression .ltoreq.20%, (9) an air permeability after heat
compression .ltoreq.7.0.times.10.sup.2 sec/100 cm.sup.3, or (10) TD
heat shrinkage at 105.degree. C. in the range of 0.25% to 1.5%.
8. The microporous membrane of claim 4, wherein the polyolefin
comprises (a) from 1.0 wt. % to 50.0 wt. % of the polypropylene,
(b) from 25.0 wt. % to 99.0 wt. % of the first polyethylene, and
(c) from 0.0 wt. % to 50.0 wt. % of the second polyethylene.
9. The microporous membrane of claim 8, wherein the polypropylene
is an isotactic polypropylene having an Mw in the range of
1.1.times.10.sup.6 to 1.5.times.10.sup.6, and a .DELTA.Hm in the
range of 110 J/g to 120 J/g.
10. The microporous membrane of claim 9, wherein the membrane has
one or more of (1) a thickness in the range of 1.0 .mu.m to 50.0
.mu.m, (2) a porosity in the range of from 25% to 80.0%, (3) a
normalized pin puncture strength .gtoreq.3.5.times.10.sup.3 mN/20
.mu.m, (4) a tensile strength .gtoreq.4.0.times.10.sup.4 kPa, (5) a
tensile elongation .gtoreq.100%, (6) a meltdown temperature
.gtoreq.170.0.degree. C., (7) a shutdown temperature
.ltoreq.140.0.degree. C., (8) a thickness variation after heat
compression .ltoreq.20%, (9) an air permeability after heat
compression .ltoreq.7.0.times.10.sup.2 sec/100 cm.sup.3, or (10) a
TD heat shrinkage at 105.degree. C. in the range of 1.0% to
2.3%.
11. A method for manufacturing a microporous membrane, comprising:
(a) stretching an extrudate in at least one of MD or TD, the
extrudate comprising diluent and a polyolefin having an
Mw>1.0.times.10.sup.6, and then removing at least a portion of
the diluent from stretched extrudate to form a membrane having a
first length along MD and a first width along TD; (b) stretching
the membrane in MD from the first length to a second length larger
than the first length by a first magnification factor in the range
of from about 1.1 to about 1.5 and stretching the membrane in TD
from the first width to a second width that is larger than the
first width by a second magnification factor in the range of from
about 1.1 to about 1.3; and then (c) reducing the second width to a
third width, the third width being in the range of from the first
width to about 1.1 times larger than the first width.
12. The method of claim 11, wherein the MD and TD stretching of
step (a) are each conducted to a magnification factor in the range
of 3 fold to 9 fold while the extrudate is exposed to a temperature
during stretching in the range of Tcd to Tm.
13. The method of claim 11, further comprising heat setting the
membrane following steps (b) and/or (c).
14. The method of claim 11, wherein during step (b) the MD
stretching is conducted before the TD stretching, wherein the first
magnification factor is >the second magnification factor, and
wherein (i) the MD stretching is conducted while the membrane is
exposed to a first temperature in the range of Tcd-30.degree. C. to
about Tm-10.degree. C. and (ii) the TD stretching is conducted
while the membrane is exposed to a second temperature that is
higher than the first temperature but lower than Tm; and wherein
the reducing of step (c) is conducted while the membrane is exposed
to a temperature .gtoreq.the second temperature.
15. The method of claim 11, wherein the polyolefin comprises (a) a
first polyethylene, and further comprises at least one of (b)
polypropylene having an Mw>1.0.times.10.sup.6 or (c) a second
polyethylene having an Mw>1.0.times.10.sup.6.
16. The method of claim 15, wherein the second polyethylene has an
Mw in the range of 1.1.times.10.sup.6 to about 5.0.times.10.sup.6
and an MWD in the range of 4.0 to 20.0, the first polyethylene has
an Mw in the range of 1.0.times.10.sup.5 to 9.0.times.10.sup.5 and
an MWD in the range of from 3.0 to 15.0, and the polypropylene has
an Mw in the range of from 1.05.times.10.sup.6 to about
2.0.times.10.sup.6, an MWD in the range of 2.0 to 6.0, and a
.DELTA.Hm.gtoreq.100.0 J/g
17. The method of claim 16, wherein the polyolefin comprises from
60.0 wt. % to 99.0 wt. % of the first polyethylene and from 1.0 wt.
% to 40.0 wt. % of the second polyethylene.
18. The method of claim 15, wherein the polyolefin comprises (a)
from 1.0 wt. % to 50.0 wt. % of the polypropylene, (b) from 25.0
wt. % to 99.0 wt. % of the first polyethylene, and (c) from 0.0 wt.
% to 50.0 wt. % of the second polyethylene.
19. The method of claim 18, wherein the polypropylene is an
isotactic polypropylene having an Mw in the range of
1.1.times.10.sup.6 to 1.5.times.10.sup.6, and a .DELTA.Hm in the
range of 110 J/g to 120 J/g.
20. (canceled)
21. A battery comprising an anode, a cathode, an electrolyte, and a
monolayer microporous membrane comprising polypropylene having an
Mw>1.0.times.10.sup.6, the membrane having a normalized air
permeability .ltoreq.4.0.times.10.sup.2 seconds/100 cm.sup.3/20
.mu.m, and a heat shrinkage at 105.degree. C. in at least one
planar direction .ltoreq.2.5%; wherein the microporous membrane
separates at least the anode from the cathode.
22-23. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Prov. App. Ser.
No. 61/115,405, filed 17 Nov. 2008, U.S. Prov. App. Ser. No.
61/115,410, filed 17 Nov. 2008, EP09151320.0 filed 26 Jan. 2009,
and EP09151318.4 filed 26 Jan. 2009, the contents of each of which
are incorporated by reference in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to microporous polymeric membranes
suitable for use as battery separator film. The invention also
relates to a method for producing such membranes, batteries
containing such membranes as battery separators, methods for making
such batteries, and methods for using such batteries.
BACKGROUND OF THE INVENTION
[0003] Microporous membranes can be used as battery separators in,
e.g., primary and secondary lithium batteries, lithium polymer
batteries, nickel-hydrogen batteries, nickel-cadmium batteries,
nickel-zinc batteries, silver-zinc secondary batteries; etc. When
microporous polyolefin membranes are used for battery separators,
particularly lithium ion battery separators, the membranes'
characteristics significantly affect the properties, productivity
and performance of the batteries. Accordingly, it is desirable for
the microporous membrane to have resistance to thermal shrinkage,
particularly at elevated temperature. Resistance to heat shrinkage
can improve the battery's protection against internal short
circuiting that might otherwise occur as the separator shrinks away
from the edges of the battery's electrodes at elevated
temperature.
[0004] European Patent Application Publication No. EP 1 905 586
(published Feb. 2, 2008) discloses multi-layer polymeric membranes
useful as battery separator film. One of the membranes exemplified
has a transverse direction heat shrinkage of 2% at 105.degree.
C.
[0005] Japanese patent document JP2000198866 (published Jul. 18,
2000) discloses multi-layer battery separator films having heat
shrinkage values of 10%. The membrane comprises layers containing
alpha-olefin/carbon monoxide copolymers and an inorganic species
(cross-linked silicone powders).
[0006] PCT publication WO2007-049568 (published May 3, 2007) also
discloses multi-layer battery separator films having a machine
direction heat-shrinkage value of 4% and a transverse direction
heat shrinkage value of 3%. The films of this reference comprise a
core layer containing heat-resistant polymers or an inorganic
filler.
[0007] U.S. Patent Publication 2007/0218271 discloses monolayer
microporous films having machine and transverse direction heat
shrinkage values of 4% or less. The films of this reference are
produced from high density polyethylene having a weight-average
molecular weight of 2.times.10.sup.5 to 4.times.10.sup.5,
containing not more than 5 wt. % of molecules with a molecular
weight of 1.times.10.sup.4 or less and not more than 5 wt. % of
molecules having a molecular weight of 1.times.10.sup.6 or
more.
[0008] Japanese Patent Application Laid Open. No. JP2001-192467
discloses monolayer microporous membranes having transverse
direction heat shrinkage values as low as 1.8%, but at a relatively
low permeability (Gurley value of 684 seconds). Similarly, Japanese
Patent Application Laid Open No. JP2001-172420 discloses monolayer
microporous membranes having transverse direction heat shrinkage
values as low as 1.1%, but at a Gurley value above 800.
[0009] While improvements have been made, there is still a need for
battery separator film having increased resistance to heat
shrinkage.
SUMMARY OF THE INVENTION
[0010] In an embodiment, the invention relates to a method for
manufacturing a microporous membrane, comprising:
(a) stretching an extrudate in at least one of MD or TD, the
extrudate comprising diluent and polyolefin having an
Mw>1.0.times.10.sup.6, and then removing at least a portion of
the diluent from stretched extrudate to form a membrane having a
first length along MD and a first width along TD; (b) stretching
the membrane in MD from the first length to a second length larger
than the first length by a magnification factor in the range of
from about 1.1 to about 1.5 and stretching the membrane in TD from
the first width to a second width that is larger than the first
width by a magnification factor in the range of from about 1.1 to
about 1.3; and then (c) reducing the second width to a third width,
the third width being in the range of from the first width to about
1.1 times larger than the first width.
[0011] In another embodiment, the invention relates to monolayer
microporous membrane comprising a polyolefin having an
Mw>1.0.times.10.sup.6, the membrane having a normalized air
permeability .ltoreq.4.0.times.10.sup.2 seconds/100 cm.sup.3/20
.mu.m, and a heat shrinkage at 130.degree. C. of .ltoreq.15% in at
least one planar direction.
[0012] In yet another embodiment, the invention relates to a
battery comprising an anode, a cathode, an electrolyte, and
monolayer microporous membrane comprising a polyolefin having an
Mw>1.0.times.10.sup.6, the membrane having a normalized air
permeability .ltoreq.4.0.times.10.sup.2 seconds/100 cm.sup.3/20
.mu.m, and a heat shrinkage at 130.degree. C. of .ltoreq.15% in at
least one planar direction, wherein the microporous membrane
separates at least the anode from the cathode. The battery can be,
e.g., a lithium ion primary or secondary battery. The battery can
be used as a source or sink of electric charge, e.g., as a power
source for an electric vehicle or hybrid electric vehicle.
[0013] In another embodiment, the invention relates to monolayer
microporous membrane comprising a polyolefin, the polyolefin having
an Mw>1.0.times.10.sup.6, wherein the membrane has been
subjected to orientation (a) in a first planar direction from a
first size to a second size, the second size being in the range of
from about (1.1the first size) to about (1.5the first size), (b) in
a second planar direction from a third size to a fourth size, the
first and second planar directions defining a planar angle the
range of 60.degree. to 120.degree. and the fourth size being in the
range of (1.1the third size) to (1.3the third size); and (c) in the
second direction from the fourth size to a fifth size, the fifth
size being (i) <the fourth size and (ii) in the range of from
the third size to (1.1the third size). Optionally, the membrane is
an extruded membrane, wherein the first direction is the machine
direction and the second direction is the transverse direction.
Optionally, the oriented membrane has a normalized air permeability
.ltoreq.4.0.times.10.sup.2 seconds/100 cm.sup.3/20 .mu.m, and a
heat shrinkage at 130.degree. C. of .ltoreq.15% in at least one
planar direction. Optionally, the orientation is conducted while
the membrane is exposed to a temperature .ltoreq.the polyolefin's
lowest melting peak, e.g., in the range of from (a) 30.degree. C.
less than the polyolefin's lowest crystal dispersion temperature to
(b) the polyolefin's lowest melting peak; such as in the range of
70.0.degree. C. to about 135.degree. C., for example from about
80.0.degree. C. to about 132.degree. C. when the membrane comprises
polyethylene or a mixture of polyethylene and polypropylene.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross-sectioned, perspective view showing one
example of cylindrical type lithium ion secondary battery
comprising an electrode assembly of the present invention.
[0015] FIG. 2 is a cross-sectioned view showing the battery in FIG.
1.
[0016] FIG. 3 is an enlarged cross-sectioned view showing a portion
A in FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
[0017] In an embodiment, the invention relates to a microporous
film having improved resistance to heat shrinkage at elevated
temperature. In another embodiment, the invention relates to a
microporous membrane having a good balance of important properties
including resistance to heat shrinkage at elevated temperature,
high porosity, with suitable mechanical strength, permeability, and
compression resistance. The membrane is permeable to liquids (e.g.,
water or polar electrolyte) at atmospheric pressure, and,
consequently, the membrane can be used as battery separator
film.
[0018] One battery failure mode involves the high temperature
softening of membranes used as battery separator film, resulting in
a loss of dimensional stability especially near the membrane's
edges. Should the membrane's width decrease at a temperature above
the membrane's shutdown temperature (generally much higher than
105.degree. C.), the close spacing between anode, cathode, and
separator can lead to an internal short circuit in the battery.
This is particularly the case in prismatic and cylindrical
batteries, where even a small change in membrane width can result
in anode-cathode contact at or near the battery's edges.
[0019] The invention relates to the discovery of microporous
membranes having better dimensional stability at elevated
temperature, e.g., improved heat shrinkage properties. The
improvement in heat shrinkage properties is observed not only at
relatively low temperatures (e.g., below about 110.degree. C.,
which is within the operating temperature range of conventional
lithium ion batteries), but also at relatively high temperatures
(e.g., above 125.degree. C., or above 135.degree. C., e.g., above
the shutdown temperature of conventional battery separator film for
lithium ion batteries).
[0020] Since the battery separator film might not be softened
sufficiently at 105.degree. C. to exhibit poor heat shrinkage, the
film's heat shrinkage performance at 105.degree. C. is not always a
reliable indicator of the potential for internal battery short
circuiting. In contrast, the film's maximum TD heat shrinkage in
the molten state is measured at a temperature that is above the
membrane's shutdown temperature, and thus can be a better indicator
for this type of internal short circuiting. TD heat shrinkage in
the molten state is generally not predictable solely from the
membrane's heat shrinkage performance at 105.degree. C.
[1] Composition and Structure of the Microporous Membrane
[0021] In an embodiment, the microporous membrane comprises a
polyolefin having a weight average molecular weight
("Mw")>1.0.times.10.sup.6. The polyolefin can comprise, e.g.,
(a) a first polyethylene having a weight average molecular weight
("Mw").ltoreq.1.0.times.10.sup.6 (referred to as the "first
polyethylene") and at least one of (b) a polypropylene having an
Mw>1.0.times.10.sup.6 or (c) a second polyethylene having an
Mw>1.0.times.10.sup.6. In an embodiment, the microporous
membrane is a monolayer membrane, i.e., it is not laminated or
coextruded with additional layers. The membrane produced from the
extrudate can consist essentially of or even consist of a single
layer comprising polyethylene or polyethylene and
polypropylene.
[0022] For the purpose of this description and the appended claims,
the term "polymer" means a composition including a plurality of
macromolecules, the macromolecules containing recurring units
derived from one or more monomers. The macromolecules can have
different size, molecular architecture, atomic content, etc. The
term "polymer" includes macromolecules such as copolymer,
terpolymer, etc., and encompasses individual polymer components
and/or reactor blends. "Polypropylene" means polyolefin containing
recurring propylene-derived units, e.g., polypropylene homopolymer
and/or polypropylene copolymer wherein at least 85% (by number) of
the recurring units are propylene units. "Polyethylene" means
polyolefin containing recurring ethylene-derived units, e.g.,
polyethylene homopolymer and/or polyethylene copolymer wherein at
least 85% (by number) of the recurring units are ethylene
units.
[0023] Selected embodiments will now be described in more detail,
but this description is not meant to foreclose other embodiments
within the broader scope of the invention.
[0024] In an embodiment, the microporous membrane comprises the
polypropylene in an amount .ltoreq.50.0 wt. %, the first
polyethylene in an amount .ltoreq.99.0 wt. %, and the second
polyethylene in an amount .ltoreq.50.0 wt. %, the weight percents
being based on the weight of the microporous membrane, provided the
microporous membrane contains at least one of the polypropylene or
the second polyethylene. For example, in one embodiment the
microporous membrane comprises (a) from about 1.0 wt. % to about
50.0 wt. %, e.g., from about 2.0 wt. % to about 40.0 wt. %, such as
from about 5.0 wt. % to about 30.0 wt. %, of the polypropylene; (b)
from about 25.0 wt. % to about 99.0 wt. %, e.g., from about 50.0
wt. % to about 90.0 wt. %, such as from about 60.0 wt. % to about
80.0 wt. % of the first polyethylene; and (c) from about 0.0 wt. %
to about 50.0 wt. %, e.g., from about 5.0 wt. % to about 30.0 wt.
%, such as from about 10.0 wt. % to about 20.0 wt. % of the second
polyethylene.
[0025] In another embodiment, the microporous membrane comprises
the first and second polyethylenes, the first polyethylene being
present in an amount .gtoreq.60.0 wt. %, and the second
polyethylene being present in an amount .ltoreq.40.0 wt. %, the
weight percents being based on the weight of the microporous
membrane. The membrane can be a polyethylene membrane that does not
contain a significant amount of polypropylene, e.g., <about 1.0
wt. %, such as 0.0 wt. % to about 0.1 wt. % polypropylene based on
the weight of the membrane. For example, in one embodiment the
microporous membrane is a polyethylene membrane which comprises
from about 1.0 wt. % to about 40.0 wt. %, e.g., from about 10.0 wt.
% to about 30.0 wt. %, of the second polyethylene and from about
60.0 wt. % to about 99.0 wt. %, e.g., from about 70.0 wt. % to
about 90.0 wt. %, of the first polyethylene.
[0026] In an embodiment, the invention relates to a method for
producing a mono-layer microporous membrane. In the production
method, an initial method step involves combining polymer resins,
such as polyethylene resins, with diluent and then extruding the
diluent to make an extrudate. The process conditions in this
initial step can be the same as those described in PCT Publication
No. WO 2008/016174, for example, which is incorporated by reference
herein in its entirety.
[0027] The polypropylene, the first and second polyethylenes, and
the diluent used to produce the extrudate and the microporous
membrane will now be described in more detail.
[2] Materials Used to Produce the Microporous Membrane
[0028] In an embodiment, the extrudate is produced from, at least
one diluent and polyolefin having an Mw>1.0.times.10.sup.6,
e.g., the first polyethylene and at least one of the polypropylene
or the second polyethylene. Optionally, inorganic species (such as
species containing silicon and/or aluminum atoms, e.g., TiO.sub.2),
and/or heat-resistant polymers such as those described in PCT
Publications WO 2007/132942 and WO 2008/016174 (both of which are
incorporated by reference herein in their entirety) can be used to
produce the extrudate. In an embodiment, these optional species are
not used.
A. The First Polyethylene
[0029] The first polyethylene has an Mw.ltoreq.1.0.times.10.sup.6,
e.g., in the range of from about 1.0.times.10.sup.5 to about
9.0.times.10.sup.5, for example from about 4.0.times.10.sup.5 to
about 8.0.times.10.sup.5. Optionally, the polyethylene has a
molecular weight distribution ("MWD").ltoreq.50.0, e.g., in the
range of from about 1.2 to about 25, such as from about 3.0 to
about 15. For example, the first polyethylene can be one or more of
a high density polyethylene ("HPDE"), a medium density
polyethylene, a branched low density polyethylene, or a linear low
density polyethylene.
[0030] In an embodiment, the first polyethylene has an amount of
terminal unsaturation .gtoreq.0.2 per 1.0.times.10.sup.5 carbon
atoms, e.g., .gtoreq.5 per 1.0.times.10.sup.5 carbon atoms, such as
.gtoreq.10 per 1.0.times.10.sup.5 carbon atoms. The amount of
terminal unsaturation can be measured in accordance with the
procedures described in PCT Publication WO97/23554, for
example.
[0031] In an embodiment, the first polyethylene is at least one of
(i) an ethylene homopolymer or (ii) a copolymer of ethylene and
.ltoreq.10.0 mole % of a comonomer such as .alpha.-olefins, based
on 100% by mole of the copolymer. Such a polymer or copolymer can
be produced using a single-site catalyst. The comonomer can be, for
example, one or more of propylene, butene-1, pentene-1, hexene-1,
4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, or
styrene.
[0032] The Mw and molecular weight distribution ("MWD", defined as
Mw divided by the number average molecular weight, "Mn") of the
first polyethylene are determined using a High Temperature Size
Exclusion Chromatograph, or "SEC", (GPC PL 220, available from
Polymer Laboratories), equipped with a differential refractive
index detector (DRI). Three PLgel Mixed-B columns (available from
Polymer Laboratories) are used. The nominal flow rate is 0.5
cm.sup.3/min, and the nominal injection volume was 300 .mu.L.
Transfer lines, columns, and the DRI detector are contained in an
oven maintained at 145.degree. C. The measurement is made in
accordance with the procedure disclosed in "Macromolecules, Vol.
34, No. 19, pp. 6812-6820 (2001)".
[0033] The GPC solvent used is filtered Aldrich reagent grade
1,2,4-Trichlorobenzene (TCB) containing approximately 1000 ppm of
butylated hydroxy toluene (BHT). The TCB is degassed with an online
degasser prior to introduction into the SEC. Polymer solutions are
prepared by placing dry polymer in a glass container, adding the
desired amount of above TCB solvent, then heating the mixture at
160.degree. C. with continuous agitation for about 2 hours. The
concentration of UHMWPE solution is 0.25 to 0.75 mg/ml. The sample
solution is filtered offline before injecting into the GPC with 2
.mu.m filter using a model SP260 Sample Prep Station (available
from Polymer Laboratories).
[0034] The separation efficiency of the column set is calibrated
with a calibration curve generated using seventeen individual
polystyrene standards ranging in Mp ("Mp" being defined as the peak
in Mw) from about 580 to about 10,000,000, which is used to
generate the calibration curve. The polystyrene standards are
obtained from Polymer Laboratories (Amherst, Mass.). A calibration
curve (log Mp 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.
B. The Second Polyethylene
[0035] The second polyethylene has an Mw>1.0.times.10.sup.6,
e.g., in the range of 1.1.times.10.sup.6 to about
5.0.times.10.sup.6, for example from about 1.2.times.10.sup.6 to
about 3.0.times.10.sup.6, such as about 2.0.times.10.sup.6.
Optionally, the second polyethylene has an MWD.ltoreq.50.0, e.g.,
from about 1.5 to about 25, such as from about 4.0 to about 20.0 or
about 4.5 to 10.0. For example, the second polyethylene can be an
ultra-high molecular weight polyethylene ("UHMWPE"). In an
embodiment, the second polyethylene is at least one of (i) an
ethylene homopolymer or (ii) a copolymer of ethylene and
.ltoreq.10.0 mole % of a comonomer, such as .alpha.-olefin, based
on 100% by mole of the copolymer. The comonomer can be, for
example, one or more of propylene, butene-1, pentene-1, hexene-1,
4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate, or
styrene. Such a polymer or copolymer can be produced using a
single-site catalyst.
C. The Polypropylene
[0036] The polypropylene has an Mw>1.0.times.10.sup.6, for
example from about 1.05.times.10.sup.6 to about 2.0.times.10.sup.6,
such as from about 1.1.times.10.sup.6 to about 1.5.times.10.sup.6.
Optionally, the polypropylene has an MWD.ltoreq.50.0, e.g., from
about 1.2 to about 25, or about 2.0 to about 6.0; and/or a heat of
fusion (".DELTA.Hm").gtoreq.100.0 J/g, e.g., 110 J/g to 120 J/g,
such as from about 113 J/g to 119 J/g or from 114 J/g to about 116
J/g. The polypropylene can be, for example, one or more of (i) a
propylene homopolymer or (ii) a copolymer of propylene and
.ltoreq.10.0 mole % of such a comonomer, such as .alpha.-olefin,
based on 100% by mole of the entire copolymer. The copolymer can be
a random or block copolymer. The comonomer can be, for example, one
or more of .alpha.-olefins 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. Optionally, the
polypropylene has one or more of the following properties: (i) the
polypropylene is isotactic; (ii) the polypropylene has an
elongational viscosity of at least about 50,000 Pa sec at a
temperature of 230.degree. C. and a strain rate of 25 sec.sup.-1;
(iii) the polypropylene has a melting peak (second melt) of at
least about 160.0.degree. C.; and/or (iv) the polypropylene has a
Trouton's ratio of at least about 15 when measured at a temperature
of about 230.degree. C. and a strain rate of 25 sec.sup.-1.
[0037] The polypropylene's .DELTA.Hm, Mw, and MWD are determined by
the methods disclosed in PCT Patent Publication No. WO2007/132942,
which is incorporated by reference herein in its entirety.
[0038] The diluent is generally compatible with the polymers used
to produce the extrudate. For example, the diluent can be any
species capable of forming a single phase in conjunction with the
resin at the extrusion temperature. Examples of the diluent include
aliphatic or cyclic hydrocarbon such as noname, decane, decalin and
paraffin oil, and phthalic acid ester such as dibutyl phthalate and
dioctyl phthalate. Among them, preferable is paraffin oil, which is
harmless to human body, has high boiling point, and contains small
amount of volatile components. Paraffin oil with kinetic viscosity
of 20-200 cSt at 40.degree. C. can be used. The diluent can be the
same as those described in U.S. Patent Publication Nos.
2008/0057388 and 2008/0057389, both of which are incorporated by
reference in their entirety.
[0039] In an embodiment, the polyolefin in the extrudate comprises
polypropylene present in an amount of from about 1.0 wt. % to about
50.0 wt. %, e.g., from about 2.5 wt. % to about 40.0 wt. %, such as
from about 5.0 wt. % to about 30.0 wt. %. The amount of first
polyethylene used to produce the extrudate can be in the range of
from about 25 wt. % to about 99.0 wt. %, e.g., from about 50.0 wt.
% to about 90.0 wt. %, such as 60.0 wt. % to about 80.0 wt. %. The
amount of second polyethylene used to produce the extrudate can be,
e.g., in the range of from 0.0 wt. % to about 50.0 wt. %, e.g.,
from about 5.0 wt. % to about 30.0 wt. %, such as about 10.0 wt. %
to about 20.0 wt. %. The weight percents of the polypropylene and
first and second polyethylenes are based on the weight of the
polymer used to produce the extrudate. When the membrane comprises
polypropylene in amount >2.0 wt. %, and particularly greater
than 2.5 wt. %, the membrane generally has a meltdown temperature
that is higher than that of a membrane which does not contain a
significant amount of polypropylene.
[0040] In another embodiment, the membrane does not contain a
significant amount of polypropylene. In this embodiment, the
polyolefin used to produce the extrudate comprises less than 0.1
wt. % polypropylene, such as when the polyolefin consists of or
consists essentially of polyethylene. In this embodiment, the
amount of second polyethylene used to produce the extrudate can be,
e.g., in the range of from about 1.0 wt. % to about 40.0 wt. %,
such as from about 10.0 wt. % to about 50.0 wt. %; and the amount
of first polyethylene used to produce the extrudate can be, e.g.,
in the range of from about 60.0 wt. % to about 99.0 wt. %, such as
from about 70.0 wt. % to about 90.0 wt. %. The weight percents of
the first and second polyethylenes are based on the weight of the
polymer used to produce the extrudate.
[0041] The extrudate is produced by combining polymer and at least
one diluent. The amount of diluent used to produce the extrudate
can be in the range, e.g., of from about 25.0 wt. % to about 99.0
wt. % based on the weight of the extrudate, with the balance of the
weight of the extrudate being the polymer used to produce the
extrudate, e.g., the combined first polyethylene and second
polyethylene.
[0042] While the extrudate and the microporous membrane can contain
copolymers, inorganic species (such as species containing silicon
and/or aluminum atoms, e.g., SiO.sub.2, TiO.sub.2, Al.sub.2O.sub.3,
etc.), and/or heat-resistant polymers such as those described in
PCT Publication WO 2008/016174, these are not required. In an
embodiment, the extrudate and membrane is substantially free of
such materials. Substantially free in this context means the amount
of such materials in the microporous membrane is less than 1.0 wt.
%, or less than 0.1 wt. %, or less than 0.01 wt. %, based on the
total weight of the polymer used to produce the extrudate.
[0043] The microporous membrane generally comprises the polyolefin
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 1.0 wt. % based on the weight of the
microporous 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 Mw of the polymers in the membrane to differ
from the Mw of the polymers used to produce the membrane by no more
than about 50.0%, or no more than about 1.0%, or no more than about
0.1%.
[0044] In one embodiment, the invention relates to a microporous
membrane comprising (a) from about 1.0 wt. % to about 50.0 wt. %,
e.g., from about 2.0 wt. % to about 40.0 wt. %, such as from about
5.0 wt. % to about 30.0 wt. %, of the polypropylene; (b) from about
25.0 wt. % to about 99.0 wt. %, e.g., from about 50.0 wt. % to
about 90.0 wt. %, such as 60.0 wt. % to about 80.0 wt. % of a first
polyethylene; and (c) from about 0.0 wt. % to about 50.0 wt. %,
e.g., from about 5.0 wt. % to about 30.0 wt. %, such as about 10.0
wt. % to about 20.0 wt. % of a second polyethylene; the membrane
having a TD heat shrinkage at 105.0.degree. C..ltoreq.about 2.5%,
e.g., in the range of about 1.0% to about 2.3%, a TD heat shrinkage
at 130.0.degree. C..ltoreq.about 15.0%; e.g., in the range of about
5.0% to about 14.0%, and a maximum TD heat shrinkage in the molten
state .ltoreq.10.0%, e.g., in the range of about 1.0% to about
9.0%; the first polyethylene having an
Mw.ltoreq.1.0.times.10.sup.6, e.g., in the range of from about
1.0.times.10.sup.5 to about 9.0.times.10.sup.5, such as from about
4.0.times.10.sup.5 to about 8.0.times.10.sup.5, and an
MWD.ltoreq.50.0, e.g., in the range of from about 1.2 to about 25,
such as from about 3.0 to about 15; the second polyethylene having
an Mw>1.0.times.10.sup.6, e.g., in the range of about
1.1.times.10.sup.6 to about 5.0.times.10.sup.6, such as from about
1.2.times.10.sup.6 to about 3.0.times.10.sup.6, and an
MWD.ltoreq.50.0, e.g., from about 1.5 to about 25, such as from
about 4.0 to about 20.0; and the polypropylene having an
Mw>1.0.times.10.sup.6, e.g., from about 1.05.times.10.sup.6 to
about 2.0.times.10.sup.6, such as about 1.1.times.10.sup.6 to about
1.5.times.10.sup.6, an MWD.ltoreq.50.0, e.g., from about 1.2 to
about 25, such as about 2 to about 6, and a .DELTA.Hm.gtoreq.100.0
J/g, e.g., about 110 J/g to about 120 J/g, such as about 114 J/g to
about 116 J/g.
[0045] In another embodiment, the microporous membrane contains
polypropylene in an amount <0.1 wt. %, based on the weight of
the microporous membrane. Such a membrane can comprise, for
example, (a) from about 1.0 wt. % to about 40.0 wt. %, e.g., from
about 10.0 wt. % to about 30.0 wt. %, of a first polyethylene; and
(b) from about 60.0 wt. % to about 99.0 wt. %, e.g., from about
70.0 wt. % to about 90.0 wt. %, of a second polyethylene; the
membrane having a normalized air permeability
.ltoreq.4.0.times.10.sup.2 seconds/100 cm.sup.3/20 .mu.m, e.g., in
the range of about 20.0 seconds/100 cm.sup.3/20 .mu.m to about
400.0 seconds/100 cm.sup.3/20 .mu.m, a TD heat shrinkage at
105.degree. C.<1.9%, e.g., in the range of about 0.25% to about
1.5%, a TD heat shrinkage at 130.degree. C..ltoreq.15%, e.g., in
the range of about 5.0% to 15%, and a maximum TD heat shrinkage in
the molten state .ltoreq.10.0%, e.g., in the range of about 1.0% to
about 7.0%; the first polyethylene having an
Mw.ltoreq.1.0.times.10.sup.6, e.g., in the range of from about
1.0.times.10.sup.5 to about 9.0.times.10.sup.5, such as from about
4.0.times.10.sup.5 to about 8.0.times.10.sup.5, and an
MWD.ltoreq.50.0, e.g., in the range of from about 1.2 to about 25,
such as from about 3.0 to about 15; and the second polyethylene
having an Mw>1.0.times.10.sup.6, e.g., in the range of
1.1.times.10.sup.6 to about 5.0.times.10.sup.6, such as from about
1.2.times.10.sup.6 to about 3.0.times.10.sup.6, and an
MWD.ltoreq.50.0, e.g., from about 1.2 to about 25, such as from
about 4.0 to about 20.0.
[0046] In an embodiment, the fraction of polyolefin in the membrane
having a molecular weight >1.0.times.10.sup.6 is at least 1.0
wt. %, based on the weight of the polyolefin in the membrane, e.g.,
at least 2.5 wt. %, such as in the range of about 2.5 wt. % to 50.0
wt. %.
[0047] Selected embodiments for producing the microporous membrane
will now be described in more detail, but this description is not
meant to foreclose other embodiments within the broader scope of
the invention.
[3] Method of Producing the Microporous Membrane
[0048] In an embodiment, the microporous membrane is a monolayer
(i.e., single-layer) membrane produced from polymer and
diluent.
[0049] For example, the microporous membrane can be produced by a
process comprising: producing a polymeric article, e.g., by
combining polymer and diluent and extruding the combined polymer
and diluent through a die to form an extrudate; optionally cooling
the extrudate to form a cooled extrudate, e.g., a gel-like sheet;
stretching the cooled extrudate in at least one planar direction;
removing at least a portion of the diluent from the extrudate or
cooled extrudate to form a membrane and optionally removing any
remaining volatile species from the dried membrane. The dried
membrane is subjected to orientation, e.g., stretching, (a) in a
first planar direction from a first size to a second size, the
second size being in the range of from about (1.1the first size) to
about (1.5the first size), (b) in a second planar direction from a
third size to a fourth size, the first and second planar directions
defining a planar angle the range of 60.degree. to 120.degree. and
the fourth size being in the range of (1.1the third size) to
(1.3the third size); and (c) in the second direction from the
fourth size to a fifth size, the fifth size being (i) <the
fourth size and (ii) in the range of from the third size to (1.1the
third size).
[0050] An optional hot solvent treatment step, an optional heat
setting step, an optional cross-linking step with ionizing
radiation, and an optional hydrophilic treatment step, etc., as
described in PCT Publication WO2008/016174 can be conducted if
desired. Neither the number nor order of the optional steps is
critical.
Combining Polymer and Diluent
[0051] The polymers as described above can be combined, e.g., by
dry mixing or melt blending, and then the combined polymers can be
combined with at least one diluent (e.g., a membrane-forming
solvent) to produce a mixture of polymer and diluent, e.g., a
polymeric solution. Alternatively, the polymer(s) and diluent can
be combined in a single step. The polymer-diluent mixture can
contain additives such as one or more antioxidant. In an
embodiment, the amount of such additives does not exceed 1 wt. %
based on the weight of the polymeric solution.
[0052] The amount of diluent used to produce the extrudate is not
critical, and can be in the range, e.g., of from about 25 wt. % to
about 99 wt. % based on the weight of the combined diluent and
polymer, with the balance being polymer, e.g., the combined first
and second polyethylene.
Extruding
[0053] In an embodiment, the combined polymer and diluent are
conducted from an extruder to a die.
[0054] The extrudate or cooled extrudate should have an appropriate
thickness to produce, after the stretching steps, a final membrane
having the desired thickness (generally 3 .mu.m or more). For
example, the extrudate can have a thickness in the range of about
0.1 mm to about 10.0 mm, or about 0.5 mm to 5.0 mm. Extrusion is
generally conducted with the mixture of polymer and diluent in the
molten state. When a sheet-forming die is used, the die lip is
generally heated to an elevated temperature, e.g., in the range of
140.degree. C. to 250.degree. C. Suitable process conditions for
accomplishing the extrusion are disclosed in PCT Publications WO
2007/132942 and WO 2008/016174. The machine direction ("MD") is
defined as the direction in which the extrudate is produced from
the die. The transverse direction ("TD") is defined as the
direction perpendicular to both MD and the thickness direction of
the extrudate. The extrudate can be produced continuously from a
die, or it can be produced from the die in portions (as is the case
in batch processing) for example. The definitions of TD and MD are
the same in both batch and continuous processing.
Formation of a Cooled Extrudate
[0055] The extrudate can be exposed to a temperature in the range
of 15.0.degree. C. to 25.0.degree. C. to form a cooled extrudate.
Cooling rate is not particularly critical. For example, the
extrudate can be cooled at a cooling rate of at least about
30.0.degree. C./minute until the temperature of the extrudate (the
cooled temperature) is approximately equal to the extrudate's
gelation temperature (or lower). Process conditions for cooling can
be the same as those disclosed in PCT Publications No. WO
2008/016174 and WO 2007/132942, for example.
Stretching the Extrudate
[0056] The extrudate or cooled extrudate is stretched in at least
one direction. The extrudate can be stretched by, for example, a
tenter method, a roll method, an inflation method or a combination
thereof, as described in PCT Publication No. WO 2008/016174, for
example. The stretching may be conducted monoaxially or biaxially,
though the biaxial stretching is preferable. In the case of biaxial
stretching, any of simultaneous biaxial stretching, sequential
stretching or multi-stage stretching (for instance, a combination
of the simultaneous biaxial stretching and the sequential
stretching) can be used, though simultaneous biaxial stretching is
preferable. When biaxial stretching is used, the amount of
magnification need not be the same in each stretching
direction.
[0057] The stretching magnification can be, for example, 2 fold or
more, preferably 3 to 30 fold in the case of monoaxial stretching.
In the case of biaxial stretching, the stretching magnification can
be, for example, 3 fold or more in any direction, namely 9 fold or
more, such as 16 fold or more, e.g. 25 fold or more, in area
magnification. An example for this stretching step would include
stretching from about 9 fold to about 49 fold in area
magnification. Again, the amount of stretch in either direction
need not be the same. The magnification factor operates
multiplicatively on film size. For example, a film having an
initial width (TD) of 2.0 cm that is stretched in TD to a
magnification of 4 fold will have a final width of 8.0 cm.
[0058] While not required, the stretching can be conducted while
exposing the extrudate to a temperature in the range of from about
the Tcd temperature Tm.
[0059] Tcd and Tm are defined as the crystal dispersion temperature
and melting point of the polyethylene having the lowest melting
point among the polyethylenes used to produce the extrudate (i.e.,
the first and second polyethylene). The crystal dispersion
temperature is determined by measuring the temperature
characteristics of dynamic viscoelasticity according to ASTM D
4065. In an embodiment where Tcd is in the range of about
90.0.degree. C. to 100.0.degree. C., the stretching temperature can
be from about 90.0.degree. C. to 125.0.degree. C.; e.g., from about
100.0.degree. C. to 125.0.degree. C., such as from 105.0.degree. C.
to 125.0.degree. C.
[0060] In an embodiment, the stretched extrudate undergoes an
optional thermal treatment before diluent removal. In the thermal
treatment, the stretched extrudate is exposed to a temperature that
is higher (warmer) than the temperature to which the extrudate is
exposed during stretching. The planar dimensions of the stretched
extrudate (length in MD and width in TD) can be held constant while
the stretched extrudate is exposed to the higher temperature. Since
the extrudate contains polymer and diluent, its length and width
are referred to as the "wet" length and "wet" width. In an
embodiment, the stretched extrudate is exposed to a temperature in
the range of 120.0.degree. C. to 125.degree. C. for a time
sufficient to thermally treat the extrudate, e.g., a time in the
range of 1.0 second to 1.0.times.10.sup.2 seconds while the wet
length and wet width are held constant, e.g., by using tenter clips
to hold the stretched extrudate along its perimeter. In other
words, during the thermal treatment, there is no magnification or
demagnification (i.e., no dimensional change) of the stretched
extrudate in MD or TD.
[0061] In this step and in other steps such as dry orientation and
heat setting where the sample (e.g., the extrudate, dried
extrudate, membrane, etc.) is exposed to an elevated temperature,
this exposure can be accomplished by heating air and then conveying
the heated air into proximity with the sample. The temperature of
the heated air, which is generally controlled at a set point equal
to the desired temperature, is then conducted toward the sample
through a plenum for example. Other methods for exposing the sample
to an elevated temperature, including conventional methods such as
exposing the sample to a heated surface, infrared heating in an
oven, etc. can be used with or instead heated air.
Removal of the Diluent
[0062] In an embodiment, at least a portion of the diluent is
removed (or displaced) from the stretched extrudate to form a dried
membrane. A displacing (or "washing") solvent can be used to remove
(wash away, or displace) the diluent, as described in PCT
Publication No. WO 2008/016174, for example.
[0063] In an embodiment, at least a portion of any remaining
volatile species (e.g., washing solvent) is removed from the dried
membrane after diluent removal. Any method capable of removing the
washing solvent can be used, including conventional methods such as
heat-drying, wind-drying (moving air), etc. Process conditions for
removing volatile species such as washing solvent can be the same
as those disclosed in PCT Publication No. WO 2008/016174, for
example.
Stretching the Membrane (Dry Orientation)
[0064] The dried membrane is stretched (called "dry stretching"
since at least a portion of the diluent has been removed or
displaced) in at least MD. A dried membrane that has been dry
stretched is called an "oriented" membrane. Before dry stretching,
the dried membrane has an initial size in MD (a first dry length)
and an initial size in TD (a first dry width). As used herein, the
term "first dry width" refers to the size of the dried membrane in
the transverse direction prior to the start of dry orientation. The
term "first dry length" refers to the size of the dried membrane in
the machine direction prior to the start of dry orientation. Tenter
stretching equipment of the kind described in WO 2008/016174 can be
used, for example.
[0065] The dried membrane can be stretched in MD from the first dry
length to a second dry length that is larger than the first dry
length by a first magnification factor (the "MD dry stretching
magnification factor") in the range of from about 1.1 to about 1.5.
When TD dry stretching is used, the dried membrane can be stretched
in TD from the first dry width to a second dry width that is larger
than the first dry width by a second magnification factor (the "TD
dry stretching magnification factor"). Optionally, the TD dry
stretching magnification factor is .ltoreq.the MD dry stretching
magnification factor, e.g., the first magnification factor is
>the second magnification factor. The TD dry stretching
magnification factor can be in the range of from about 1.1 to about
1.3. The dry stretching (also called re-stretching since the
diluent-containing extrudate has already been stretched) can be
sequential or simultaneous in MD and TD. Since TD heat shrinkage
generally has a greater effect on battery properties than does MD
heat shrinkage, the amount of TD magnification generally does not
exceed the amount of MD magnification. When TD dry stretching is
used, the dry stretching can be simultaneous in MD and TD or
sequential. When the dry stretching is sequential, generally MD
stretching is conducted first followed by TD stretching.
[0066] The dry stretching can be conducted while exposing the dried
membrane to a temperature .ltoreq.Tm, e.g., in the range of from
about Tcd-30.degree. C. to Tm. In an embodiment, the dry stretching
is conducted with the membrane exposed to a temperature in the
range of from about 70.0.degree. C. to about 135.0.degree. C., for
example from about 80.0.degree. C. to about 132.0.degree. C. In an
embodiment, the MD stretching is conducted before TD stretching,
and [0067] (i) the MD stretching is conducted while the membrane is
exposed to a first temperature in the range of Tcd-30.0.degree. C.
to about Tm-10.0.degree. C., for example 70.0.degree. C. to about
125.0.degree. C., or about 80.0.degree. C. to about 120.0.degree.
C. and [0068] (ii) the TD stretching is conducted while the
membrane is exposed to a second temperature that is higher than the
first temperature but lower than Tm, for example about 70.0.degree.
C. to about 135.0.degree. C., or about 127.0.degree. C. to about
132.0.degree. C., or about 129.0.degree. C. to about 131.0.degree.
C.
[0069] In an embodiment, the MD stretching magnification is in the
range of from about 1.1 to about 1.5, such as 1.2 to 1.4; the TD
dry stretching magnification is in the range of from about 1.1 to
about 1.3, such as 1.15 to 1.25; the MD dry stretching is conducted
before the TD dry stretching, the MD dry stretching is conducted
while the membrane is exposed to a temperature in the range of
80.0.degree. C. to about 120.0.degree. C., and the TD dry
stretching is conducted while the membrane is exposed to a
temperature in the range of 129.0.degree. C. to about 131.0.degree.
C.
[0070] The stretching rate is preferably 3%/second or more in the
stretching direction (MD or TD), and the rate can be independently
selected for MD and TD stretching. The stretching rate is
preferably 5%/second or more, more preferably 10%/second or more,
e.g., in, the range of 5%/second to 25%/second. Though not
particularly critical, the upper limit of the stretching rate is
preferably 50%/second to prevent rupture of the membrane.
Controlled Reduction of the Membrane's Width
[0071] Following the dry stretching, the dried membrane is
subjected to a controlled reduction in width from the second dry
width to a third width, the third dry width being in the range of
from the first dry width to about 1.1 times larger than the first
dry width. The width reduction generally conducted while the
membrane is exposed to a temperature .gtoreq.Tcd-30.0.degree. C.,
but no greater than Tm. For example, during width reduction the
membrane can be exposed to a temperature in the range of from about
70.0.degree. C. to about 135.degree. C., such as from about
127.degree. C. to about 132.degree. C., e.g., from about
129.degree. C. to about 131.degree. C. In an embodiment, the
decreasing of the membrane's width is conducted while the membrane
is exposed to a temperature that is lower than Tm. In an
embodiment, the third dry width is in the range of from 1.0 times
larger than the first dry width to about 1.1 times larger than the
first dry width.
[0072] It is believed that exposing the membrane to a temperature
during the controlled width reduction that is .gtoreq.the
temperature to which the membrane was exposed during the TD
stretching leads to greater resistance to heat shrinkage in the
finished membrane.
Optional Heat Set
[0073] Optionally, the membrane is thermally treated (heat-set) at
least once following diluent removal, e.g., after dry stretching,
the controlled width reduction, or both. It is believed that
heat-setting stabilizes crystals and makes uniform lamellas in the
membrane. In an embodiment, the heat setting is conducted while
exposing the membrane to a temperature in the range Tcd to Tm,
e.g., a temperature e.g., in the range of from about 100.degree. C.
to about 135.degree. C., such as from about 127.degree. C. to about
132.degree. C., or from about 129.degree. C. to about 131.degree.
C. Generally, the heat setting is conducted for a time sufficient
to form uniform lamellas in the membrane, e.g., a time in the range
of 1 to 100 seconds. In an embodiment, the heat setting is operated
under conventional heat-set "thermal fixation" conditions. The term
"thermal fixation" refers to heat-setting carried out while
maintaining the length and width of the membrane substantially
constant, e.g., by holding the membrane's perimeter with tenter
clips during the heat setting.
[0074] Optionally, an annealing treatment can be conducted after
the heat-set step. The annealing is a heat treatment with no load
applied to the membrane, and can be conducted by using, e.g., a
heating chamber with a belt conveyer or an air-floating-type
heating chamber. The annealing may also be conducted continuously
after the heat-setting with the tenter slackened. During annealing
the membrane can be exposed to a temperature in the range of Tm or
lower, e.g., in the range from about 60.0.degree. C. to about
Tm-5.degree. C. Annealing is believed to provide the microporous
membrane with improved permeability and strength.
[0075] Optional heated roller, hot solvent, cross linking,
hydrophilizing, and coating treatments can be conducted if desired,
e.g., as described in PCT Publication No. WO 2008/016174.
[4] Structure, Properties, and Composition of Microporous
Membrane
[0076] In an embodiment, the membrane is a monolayer microporous
membrane. The thickness of the monolayer membrane is generally in
the range of from about 1.0 .mu.m to about 1.0.times.10.sup.2
.mu.m, e.g., from about 5.0 .mu.m to about 30.0 .mu.m. The
thickness of the microporous membrane can be measured by a contact
thickness meter at 1.0 cm longitudinal intervals over the width of
20.0 cm, and then averaged to yield the membrane thickness.
Thickness meters such as the Litematic available from Mitsutoyo
Corporation are suitable. This method is also suitable for
measuring thickness variation after heat compression, as described
below. Non-contact thickness measurements are also suitable, e.g.,
optical thickness measurement methods.
[0077] Optionally, the microporous membrane has one or more of the
following properties.
(a) Normalized Air Permeability .ltoreq.4.0.times.10.sup.2 sec/100
cm.sup.3/20 .mu.m
[0078] In an embodiment, the membrane's normalized air permeability
(Gurley value, normalized to an equivalent membrane thickness of 20
.mu.m) is .ltoreq.400.0 seconds/100 cm.sup.3/20 .mu.m, e.g., in the
range of about 20.0 seconds/100 cm.sup.3/20 .mu.m to about 400.0
seconds/100 cm.sup.3/20 .mu.m. Since the air permeability value is
normalized to a film thickness of 20 .mu.m, the air permeability
value is expressed in units of "seconds/100 cm.sup.3/20 .mu.m". In
an embodiment, the normalized air permeability is in the range of
100.0 seconds/100 cm.sup.3/20 .mu.m to about 375 seconds/100
cm.sup.3/20 .mu.m. In an embodiment, the membrane comprises <0.1
wt. % polypropylene, based on the weight of the membrane, and the
membrane's normalized air permeability is in the range of 100.0
seconds/100 cm.sup.3/20 .mu.m to about 275 seconds/100 cm.sup.3/20
.mu.m. 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 a thickness of
20 .mu.m.
(b) Porosity in the Range of from about 25.0% to about 80.0%
[0079] In an embodiment, the membrane has a porosity .gtoreq.25.0%,
e.g., in the range of about 25.0% to about 80.0%, or 30.0% to
60.0%. The membrane's porosity is measured conventionally by
comparing the membrane's actual weight to the weight of an
equivalent non-porous membrane of the same composition (equivalent
in the sense of having the same length, width, and thickness).
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.
(c) Normalized Pin Puncture Strength .gtoreq.3.0.times.10.sup.3
mN/20 .mu.m
[0080] In an embodiment, the membrane has a normalized pin puncture
strength .gtoreq.3.0.times.10.sup.3 mN/20 .mu.m, e.g., in the range
of 3.5.times.10.sup.3 mN/20 .mu.m to 1.0.times.10.sup.4 mN/20
.mu.m, such as 3,750 mN/20 .mu.m to 5,500 mN/20 .mu.m. In one
embodiment, the membrane comprises polypropylene having an
Mw>1.0.times.10.sup.6, in an amount .gtoreq.about 0.1 wt. %
(e.g., .gtoreq.1.0 wt. %, such as .gtoreq.about 2.5 wt. %) based on
the weight of the membrane. In this embodiment, the membrane can
have a normalized pin puncture strength of, e.g., .gtoreq.3500
mN/20 .mu.m. Pin puncture strength is defined as the maximum load
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 normalized to a value at a
membrane thickness of 20 .mu.m using the equation S.sub.2=20
.mu.m*(S.sub.1)/T.sub.1, where S.sub.1 is the measured pin puncture
strength, S.sub.2 is the normalized pin puncture strength, and
T.sub.1 is the average thickness of the membrane.
(d) Tensile Strength .gtoreq.4.0.times.10.sup.4 kPa
[0081] In an embodiment, the membrane has an MD tensile strength
.gtoreq.9.5.times.10.sup.4 kPa, e.g., in the range of 95,000 to
110,000 kPa, and a TD tensile strength .gtoreq.9.0.times.10.sup.4
kPa, e.g., in the range of 9.0.times.10.sup.4 kPa to
1.1.times.10.sup.5 kPa. Tensile strength is measured in MD and TD
according to ASTM D-882A.
(e) Tensile Elongation of 100% or More
[0082] Tensile elongation is measured according to ASTM D-882A. In
an embodiment, the membrane's MD and TD tensile elongation are each
.gtoreq.100.0%, e.g., in the range of 125.0% to 350.0%. In another
embodiment, the membrane's MD tensile elongation is in the range
of, e.g., 125.0% to 250.0% and TD tensile elongation is in the
range of, e.g., 140.0% to 300.0%.
(f) Thickness Variation Ratio after Heat Compression
.ltoreq.20%
[0083] In an embodiment, the membrane's thickness variation ratio
after heat compression is .ltoreq.20% of the thickness of the
membrane before the heat compression, e.g., in the range of 5% to
10%. Thickness variation after heat compression is measured by
subjecting the membrane to a compression of 2.2 MPa (22
kgf/cm.sup.2) in the thickness direction for five minutes while the
membrane is exposed to a temperature of 90.degree. C. The
membrane's thickness variation ratio is defined as the absolute
value of (average thickness after compression-average thickness
before compression)/(average thickness before
compression).times.100.
(g) Air Permeability after Heat Compression
.ltoreq.7.0.times.10.sup.2 sec/100 cm.sup.3
[0084] In an embodiment, the membrane's air permeability after heat
compression is .ltoreq.7.0.times.10.sup.2 seconds/100 cm.sup.3,
e.g., 100.0 seconds/100 cm.sup.3 to 675 seconds/100 cm.sup.3. Air
permeability after heat compression is measured according to JIS
P8117 after the membrane is subjected to a compression of 2.2 MPa
(22 kgf/cm.sup.2) in the thickness direction for five minutes while
the membrane is exposed to a temperature of 90.degree. C. In an
embodiment, the membrane comprises <0.1 wt. % polypropylene,
based on the weight of the membrane, and the membrane's air
permeability after heat compression is in the range of
1.0.times.10.sup.2 seconds/100 cm.sup.3 to about 5.0.times.10.sup.2
seconds/100 cm.sup.3/20 .mu.m.
(h) Shutdown Temperature .ltoreq.140.0.degree. C.
[0085] In an embodiment, the membrane has a shutdown temperature
.ltoreq.140.0.degree. C., e.g., 132.degree. C. to 138.degree. C.
The shutdown temperature of the microporous membrane is measured by
a thermomechanical analyzer (TMA/SS6000 available from Seiko
Instruments, Inc.) as follows: 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 the transverse direction of
the microporous membrane and the short axis is aligned with the
machine direction. The sample is set in the thermomechanical
analyzer at a chuck distance of 10.0 mm, i.e., the distance from
the upper chuck to the lower chuck is 10.0 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. Starting at 30.degree. C., the temperature inside the tube
is elevated 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 temperature is
increased to 200.degree. C. "Shutdown temperature" is defined as
the temperature of the inflection point observed at approximately
the melting point of the polymer having the lowest melting point
among the polymers used to produce the membrane.
(i) Electrolytic Solution Absorption Speed .gtoreq.2.5
[0086] In an embodiment; the membrane has an electrolytic solution
absorption speed .gtoreq.2.5, e.g., in the range of 3.0 to 5.0.
Using a dynamic surface tension measuring apparatus (DCAT21 with
high-precision electronic balance, available from Eko Instruments
Co., Ltd.), a microporous membrane sample is immersed in an
electrolytic solution for 6.0.times.10.sup.2 seconds (electrolyte:
1 mol/L of LiPF.sub.6, solvent: ethylene carbonate/dimethyl
carbonate at a volume ratio of 3/7) kept at 18.degree. C., to
determine an electrolytic solution absorption speed by the formula
of [weight (in grams) of microporous membrane after
immersion/weight (in grams) of microporous membrane before
immersion]. The electrolytic solution absorption speed is expressed
by a relative value, assuming that the electrolytic solution
absorption rate in the microporous membrane of Comparative Example
1 is 1.0. Battery separator film having a relatively high
electrolytic solution absorption speed (e.g., .gtoreq.2.5) are
desirable since less time is required for the separator to uptake
the electrolyte during battery manufacturing, which in turn
increases the rate at which the batteries can be produced.
(j) TD Heat Shrinkage at 105.degree. C..ltoreq.2.5%
[0087] In an embodiment, the membrane has a TD heat shrinkage at
105.degree. C..ltoreq.2.5%, for example from 1.0% to 2.3%. In
another embodiment, the membrane comprises <0.1 wt. %
polypropylene, based on the weight of the membrane, and the
membrane has a TD heat shrinkage at 105.degree. C. of, e.g.,
<1.9%, for example from 0.25% to 1.5%. The membrane's heat
shrinkage in orthogonal planar directions (e.g., MD or TD) at
105.degree. C. is measured as follows:
(i) Measure the size of a test piece of microporous membrane at
ambient temperature in both MD and TD, (ii) expose the test piece
to a temperature of 105.degree. C. for 8 hours with no applied
load, and then (iii) measure the size of the membrane in both MD
and TD. The heat (or "thermal") shrinkage in either the MD or TD
can be obtained by dividing the result of measurement (i) by the
result of measurement (ii) and expressing the resulting quotient as
a percent.
[0088] In an embodiment, the membrane has an MD heat shrinkage at
105.degree. C..ltoreq.10%, for example from 1% to 8%.
(k) TD Heat Shrinkage at 130.degree. C..ltoreq.15%
[0089] The membrane can also be characterized by a heat shrinkage
value measured at 130.degree. C. The measurement is slightly
different from the measurement of heat shrinkage at 105.degree. C.,
reflecting the fact that the edges of the membrane parallel to the
transverse direction are generally fixed within the battery, with a
limited degree of freedom allowed for expansion or contraction
(shrinkage) in the transverse direction, particularly near the
center of the edges parallel to the machine direction. Accordingly,
a square sample of microporous film measuring 50 mm along TD and 50
mm along MD is mounted in a frame, with the edges parallel to TD
fixed to the frame (e.g., by tape) leaving a clear aperture of 35
mm in MD and 50 mm in TD. The frame with sample attached is then
exposed to a temperature of 130.degree. C. for thirty minutes, and
then cooled. TD heat shrinkage generally causes the edges of the
film parallel to MD to bow slightly inward (toward the center of
the frame's aperture). The shrinkage in TD (expressed as a percent)
is equal to the length of the sample in TD before heating divided
by the narrowest length (within the frame) of the sample in TD
after heating times 100 percent.
[0090] In an embodiment, the membrane has a TD heat shrinkage at
130.degree. C..ltoreq.15%, for example from about 3.0% to about
15%. In another embodiment, the membrane has a TD heat shrinkage at
130.degree. C. in the range of 5.0% to 13%.
(l) Maximum TD Shrinkage in Molten State .ltoreq.10.0%
[0091] Maximum shrinkage in the molten state in a planar direction
of the membrane is measured by the following procedure:
[0092] Using the TMA procedure described for the measurement of
meltdown temperature, the sample length measured in the temperature
range of from 135.degree. C. to 145.degree. C. are recorded. The
membrane shrinks, and the distance between the chucks decreases as
the membrane shrinks. The maximum shrinkage in the molten state is
defined as the sample length between the chucks measured at
23.degree. C. (L1 equal to 10 mm) minus the minimum length measured
generally in the range of about 135.degree. C. to about 145.degree.
C. (equal to L2) divided by L1, i.e., [L1-L2]/L1*100%. When TD
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 the transverse
direction of the microporous membrane as it is produced in the
process and the short axis is aligned with the machine direction.
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 the machine
direction of the microporous membrane as it is produced in the
process and the short axis is aligned with the transverse
direction.
[0093] In one embodiment, the membrane comprises polypropylene
having an Mw>1.0.times.10.sup.6, in an amount .gtoreq.about 0.1
wt. % (e.g., .gtoreq.1.0 wt. %, such as .gtoreq.about 2.5 wt. %)
based on the weight of the membrane. In this embodiment, the
membrane's maximum MD heat shrinkage in the molten state can be,
e.g., .ltoreq.25%, or .ltoreq.20.0%, such as in the range of 1.0%
to 25%, or 2.0% to 20.0%. In this embodiment, the membrane's
maximum TD shrinkage in the molten state can be, e.g.,
.ltoreq.10.0%, such as in the range of from about 1.0 to about
9.0%.
[0094] In another embodiment, the membrane comprises <0.1 wt. %
polypropylene, based on the weight of the membrane. In this
embodiment, the membrane's maximum MD heat shrinkage in the molten
state can be, e.g., .ltoreq.35%, or .ltoreq.30.0%, such as in the
range of 1.0% to 30.0%, or 2.0% to 25%. In this embodiment, the
membrane's maximum TD shrinkage in the molten state can be, e.g.,
.ltoreq.10.0%, such as in the range of from about 1.0% to about
7.0%.
(m) Meltdown Temperature .gtoreq.145.degree. C.
[0095] Meltdown temperature is measured by the following procedure:
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 the transverse direction of the microporous membrane
as it is produced in the process and the short axis is aligned with
the machine direction. The sample is set in a thermomechanical
analyzer (TMA/SS6000 available from Seiko Instruments, Inc.) at a
chuck distance of 10.0 mm, i.e., the distance from the upper chuck
to the lower chuck is 10.0 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. Starting at
30.degree. C., the temperature inside the tube is elevated 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 temperature is increased to
200.degree. C. The meltdown temperature of the sample is defined as
the temperature at which the sample breaks, generally at a
temperature in the range of about 170.degree. C. to about
200.degree. C.
[0096] In one embodiment, the membrane comprises polypropylene
having an Mw>1.0.times.10.sup.6, in an amount .ltoreq.about 0.1
wt. % (e.g., .ltoreq.1.0 wt. %, such as .ltoreq.about 2.5 wt. %)
based on the weight of the membrane. In this embodiment, the
membrane's meltdown temperature can be, .gtoreq.170.0.degree. C.,
e.g., in the range of from 170.0.degree. C. to 180.0.degree. C.
[0097] In another embodiment, the membrane contains <0.1 wt. %
polypropylene, based on the weight of the membrane. In this
embodiment, the meltdown temperature can be, e.g., in the range of
from 145.degree. C. to 155.degree. C., such as 147.degree. C. to
152.degree. C.
Microporous Membrane Composition
Polymer
[0098] The microporous membrane generally comprises the same
polymers used to produce the polymeric composition, in generally
the same relative amounts. Washing solvent and/or process solvent
(diluent) can also be present, generally in amounts less than 1 wt.
% based on the weight of the microporous membrane. A small amount
of polymer molecular weight degradation might occur during
processing, but this is acceptable. In an embodiment where the
polymer is polyolefin and the membrane is produced in a wet
process, molecular weight degradation during processing, if any,
causes the value of Mw of the polyolefin in the membrane to differ
from the Mw of the polymer used to produce the membrane by no more
than about 50%, or no more than about 1%, or no more than about
0.1%.
[5] Battery
[0099] The microporous membranes of the invention are useful as
battery separators in e.g., lithium ion primary and secondary
batteries. Such batteries are described in PCT publication, WO
2008/016174 which is incorporated by reference herein in its
entirety. The membrane generally has a thickness in the range of
about 3.0 .mu.m to about 200.0 .mu.m, or about 5.0 .mu.m to about
50.0 .mu.m. Depending, e.g., on the choice of electrolyte,
separator swelling might increase the final thickness to a value
larger than 200 .mu.m.
[0100] FIG. 1 shows an example of a cylindrical-type lithium ion
secondary battery comprising two battery separators. The
microporous membranes of the invention are suitable for use as
battery separators in this type of battery. The battery has a
toroidal-type electrode assembly 1 comprising a first separator 10,
a second separator 11, a cathode sheet 13, and an anode sheet 12.
The separators' thicknesses are not to scale, and are greatly
magnified for the purpose of illustration. The toroidal-type
electrode assembly 1 can be wound, e.g., such that the second
separator 11 is arranged on an outer side of the cathode sheet 13,
while the first separator 10 is arranged on the inner side of the
cathode sheet. In this example, the second separator 11 is arranged
on inside surface of the toroidal-type electrode assembly 1, as
shown in FIG. 2.
[0101] In this example, an anodic active material layer 12b is
formed on both sides of the current collector 12a, and a cathodic
active material layer 13b is formed on both sides of the current
collector 13a, as shown in FIG. 3. As shown in FIG. 2, an anode
lead 20 is attached to an end portion of the anode sheet 12, and a
cathode lead 21 is attached to an end portion of the cathode sheet
13. The anode lead 20 is connected with battery lid 27, and the
cathode lead 21 is connected with the battery can 23.
[0102] While a battery of cylindrical form is illustrated, the
invention is not limited thereto, and the separators of the
invention are suitable for use in e.g., prismatic batteries such as
those containing electrodes in the form of stacked plates of
anode(s) 12 and a cathode (3) 13 alternately connected in parallel
with the separators situated between the stacked anodes and
cathodes.
[0103] When the battery is assembled, the anode sheet 12, the
cathode sheet 13, and the first and second separators 10, 11 are
impregnated with the electrolytic solution, so that the separator
10, 11 (microporous membranes) are provided with ion permeability.
The impregnation treatment is can be conducted, e.g., by immersing
electrode assembly 1 in the electrolytic solution at room
temperature. A cylindrical type lithium ion secondary battery can
be produced by inserting the toroidal-type electrode assembly 1
(see FIG. 1) into a battery can 23 having a insulation plate 22 at
the bottom, injecting the electrolytic solution into the battery
can 23, covering the electrode assembly 1 with a insulation plate
22, caulking a battery lid (24, 25, 26, and 27) to the battery can
23 via a gasket 28. The battery lid functions as an anode
terminal.
[0104] FIG. 3 (oriented so that the battery lid, i.e., the anode
terminal of FIG. 1, is toward the right) illustrates the advantage
of using a separator having diminished tendency to TD heat
shrinkage as the battery temperature increases. One role of the
separator is to prevent contact of the anodic active material layer
and the cathodic active material layer. In the event of a
significant amount of TD heat shrinkage, the thin edges of the
separators 10 and 11 move away from the battery lid (move leftward
in FIG. 3), thereby allowing contact between the anodic active
material layer and the cathodic active material layer, resulting in
a short circuit. Since the separators can be quite thin, usually
less than 200 .mu.m, the anodic active material layer and the
cathodic active material layer can be quite close. Consequently,
even a small decrease in the amount of separator TD shrinkage at
elevated battery temperature can make a significant improvement in
the battery's resistance to internal short circuiting.
[0105] 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 moving an electric vehicle or hybrid electric
vehicle, for example. In one embodiment, the battery is
electrically connected to an electric motor and/or an electric
generator for powering an electric vehicle or hybrid electric
vehicle.
[0106] The present invention will be explained in more detail
referring to Examples below without intention of restricting the
scope of the present invention.
Example 1
[0107] A polyolefin composition is made by dry mixing (a) 70 wt. %
of a first polyethylene resin having an Mw of 5.6.times.10.sup.5
and an MWD of 4.05, (b) 30 wt. % polypropylene resin having an Mw
of 1.1.times.10.sup.6, an MWD of 5, and a heat of fusion of 114
J/g. The first polyethylene has a Tm of 135.degree. C. and a Tcd of
100.degree. C.
[0108] A polyolefin solution is produced as follows: 30 wt. % of
the combined polyethylene and polypropylene (the polyolefin
composition) is charged into a strong-blending double-screw
extruder having an inner diameter of 58 mm and L/D of 42, and 70
wt. % of liquid paraffin diluent (50 cst at 40.degree. C.) is
supplied to the double-screw extruder via a side feeder, the weight
percents being based on the weight of the polyolefin solution.
Melt-blending is conducted at 210.degree. C. and 200 rpm. The
polyolefin solution is extruded from a T-die mounted to the
double-screw extruder. The extrudate is cooled while passing
through cooling rolls having, a surface temperature of 40.degree.
C., to form a cooled extrudate, i.e. gel-like sheet.
[0109] Using a tenter-stretching machine, the extrudate (gel-like
sheet) is biaxially stretched (simultaneously in MD and TD) to a
magnification of 5 fold in each of MD and TD while exposing the
extrudate to a temperature of 117.degree. C. The stretched gel-like
sheet is fixed to an aluminum frame of 20 cm.times.20 cm, immersed
in a bath of methylene chloride controlled at 25.degree. C. to
remove the liquid paraffin with vibration of 100 rpm for 3 minutes,
and dried by an air flow at room temperature. At the start of dry
orientation, the membrane has an initial size in TD (the first dry
width) and an initial size in MD (the first dry length). The dried
membrane is first stretched by a batch-stretching machine to an MD
magnification of 1.4 fold (to a second dry length) while exposing
the membrane to a temperature of 110.degree. C. and holding ht
membrane's width constant. The dried membrane is then stretched by
a batch-stretching machine to a TD magnification of 1.2 fold (to a
second dry width) while exposing the membrane to a temperature of
130.degree. C. and holding the membrane's length constant at the
second dry length. The membrane is then subjected to a controlled
reduction in width from the second dry width to a third dry width
that is equal to the first dry width, i.e., to a final
magnification of 1.0 fold, while exposing the membrane to a
temperature of 130.degree. C. and maintaining the membrane's length
constant at the second dry length. In other words, the membrane's
width is reduced to the membrane's initial size in TD at the start
of dry orientation while holding the membrane's length in MD
constant at the second dry length. After the membrane's width is
reduced to the initial width, it is then heat-set by exposing the
membrane to a temperature of 129.degree. C. for 10 minutes.
Example 2
[0110] Example 1 is repeated except that the polyolefin composition
is produced from 70 wt. % of the first polyethylene, 10 wt. % of a
second polyethylene having an Mw of 1.9.times.10.sup.6 and an MWD
of 5.09, and 20 wt. % of the polypropylene; the membrane is exposed
to a temperature of 90.degree. C. during the MD dry orientation;
the MD dry orientation magnification is 1.3 fold; and the heat
setting temperature is 130.degree. C.
Example 3
[0111] Example 2 is repeated except that the polyolefin composition
is produced from 60 wt. % of the first polyethylene, 20 wt. % of
the second polyethylene, and 20 wt. % of the polypropylene; the
membrane is exposed to a temperature of 115.degree. C. during the
MD dry orientation; and the MD dry orientation magnification is 1.2
fold.
Example 4
[0112] Example 2 is repeated except that the polyolefin composition
is produced from 60 wt. % of the first polyethylene, 30 wt. % of
the second polyethylene, and 10 wt. % of the polypropylene; and the
membrane is exposed to a temperature of 115.degree. C. during the
MD dry orientation.
Example 5
[0113] Example 1 is repeated except the polyolefin composition
comprises 70 wt. % of the first polyethylene resin and 30% of the
second polyethylene resin, the extrudate is exposed to a
temperature of 120.degree. C. during biaxial orientation; and the
membrane is exposed to a temperature of 130.degree. C. during heat
setting.
Example 6
[0114] Example 5 is repeated except the polyolefin composition
comprises 80% of the first polyethylene resin and 20% of the second
polyethylene resin; 25 wt. % of the polyolefin composition is
charged into the double-screw extruder; the simultaneous biaxial
stretching is conducted while exposing the extrudate to a
temperature of 116.degree. C.; the magnification in the MD dry
stretching is 1.3; the membrane is exposed to a temperature of
129.degree. C. during TD dry stretching and the width reduction;
and the membrane is exposed to a temperature of 129.degree. C.
during heat setting.
Example 7
[0115] Example 6 is repeated except 28.5 wt. % of the polyolefin
composition is charged into the double-screw extruder; the
extrudate is exposed to a temperature of 117.degree. C. during
simultaneous biaxial stretching; the magnification factor in the MD
dry stretching is 1.2; the MD dry stretching is conducted while the
membrane is exposed to a temperature of 120.degree. C.; the
membrane is exposed to a temperature of 128.degree. C. during the
TD stretching and the width reduction; and the membrane is exposed
to a temperature of 128.degree. C. during the heat setting.
Example 8
[0116] Example 6 is repeated except; the membrane is exposed to a
temperature of 90.degree. C. during the MD dry stretching; the
membrane is exposed to a temperature of 130.degree. C. during the
TD dry stretching and width reduction; the membrane's width is
reduced to a magnification of 1.1 fold; and the membrane is exposed
to a temperature of 130.degree. C. during the heat setting.
Comparative Example 1
[0117] Example 2 is repeated that the polyolefin composition is
produced from 70 wt. % of the first polyethylene and 30 wt. % of
the second polyethylene (no polypropylene); the extrudate is
exposed to a temperature of 117.degree. C. during the simultaneous
biaxial orientation; the membrane was not subjected to dry
orientation; and the heat setting temperature is 127.degree. C.
Comparative Example 2
[0118] Example 2 is repeated that the polyolefin composition is
produced from 60 wt. % of the first polyethylene, 10 wt. % of the
second polyethylene, and 30 wt. % of the polypropylene; the
extrudate is exposed to a temperature of 118.degree. C. during the
simultaneous biaxial orientation; the membrane was not subjected to
MD dry orientation; TD dry orientation was conducted to a
magnification of 1.3 while exposing the membrane to a temperature
of 125.degree. C., with no reduction in width after TD dry
orientation, and (v) and the heat setting temperature is
125.degree. C.
Comparative Example 3
[0119] Comparative Example 2 is repeated that the membrane is
subjected to MD dry orientation, with the membrane exposed to a
temperature of 115.degree. C. during the MD dry orientation; the MD
dry orientation magnification is 1.2 fold; the membrane is exposed
to a temperature of 130.degree. C. during the TD dry orientation;
and the heat setting temperature is 130.degree. C.
Comparative Example 4
[0120] Comparative Example 3 is repeated except that the
polypropylene's Mw is 1.5.times.10.sup.6, MWD is 3.2, and heat of
fusion is 78 J/g; (ii) the membrane is subjected to TD dry
orientation at a magnification of 1.2 fold and then a controlled
reduction in width to a final magnification of 1.0 fold while
exposing the membrane to a temperature of 130.degree. C.
Comparative Example 5
[0121] Example 3 is repeated except that the polypropylene has an
Mw of 7.times.10.sup.5, an MWD of 11, and a heat of fusion of 103
J/g; the extrudate was exposed to a temperature of 113.5.degree. C.
during biaxial extrudate stretching, the membrane is exposed to a
temperature of 115.degree. C. during the MD dry orientation; the MD
dry orientation magnification is 1.3 fold; and the membrane is
exposed to a temperature of 127.degree. C. during the TD dry
stretching, the controlled reduction in width, and the heat
setting.
Comparative Example 6
[0122] Comparative Example 1 is repeated except the polyolefin
composition comprises 95 wt. % of the first polyethylene resin, and
5 wt. % of the second polyethylene resin; 40 wt. % of the
polyolefin composition is charged into the double-screw extruder;
the extrudate is exposed to a temperature of 119.degree. C. during
the simultaneous biaxial stretching; there is no MD dry stretching;
the membrane is exposed to a temperature of 119.degree. C. during
the TD dry stretching; the TD dry stretching is conducted to a
magnification of 1.4 fold; and the membrane is exposed to a
temperature of 130.degree. C. during the heat-setting. There is no
width reduction step.
Comparative Example 7
[0123] Comparative Example 1 is repeated except the polyolefin
composition comprises 80 wt. % of the first polyethylene resin and
20 wt. % of the second polyethylene resin; there is no MD dry
stretching; the membrane is exposed to a temperature of 115.degree.
C. during TD dry stretching; the TD dry stretching is conducted to
a magnification of 1.4 fold; width reduction is conducted to a
magnification of 1.0 fold at 126.degree. C.; and the membrane is
exposed to a temperature of 126.degree. C. during the
heat-setting.
Properties
[0124] The properties of the microporous membranes obtained in the
Examples and Comparative Examples are measured by the methods
described above. The results are shown in the following tables.
TABLE-US-00001 TABLE 1 PROPERTIES Example 1 Example 2 Example 3
Example 4 Example 5 Example 6 Example 7 Example 8 Thickness .mu.m
21.0 19.5 20.4 20.9 20.2 16.3 16.0 16.0 Normalized Air Perm. 320
275 315 250 251 185 150 145 (sec/100 cm.sup.3/2 .mu.m) Porosity %
42 37 36 37 37 37 42 43 Normalized Punct. 4067 3802 3822 4018 4410
3724 4214 3920 Strength (mN) Tensile Strength 107800 102900 98000
107800 156800 133770 132300 132300 MD//TD (kPa) 99960 95060 97020
102900 137200 99960 107800 99960 Tensile Elongation 135 150 160 155
125 140 140 140 MD//TD (%) 155 300 290 260 180 160 180 180 Heat
Shrinkage 105.degree. C. 7.5 6.0 6.0 6.5 6.5 4.5 2.0 2.7 MD/TD (%)
2.3 1.5 1.6 2.0 1.0 0.8 0.5 1.0 Heat Shrinkage 130.degree. C. TD
13.1 9.4 11.2 10.3 15.0 9.5 5.2 10.0 (%) Elec. Soln. Sorpt. Speed
3.1 3.2 3.1 3.0 3.5 3.9 3.7 3.8 Thick. Var. Aft. Heat 8.0 9.0 9.0
8.0 7.0 9.0 9.0 9.0 Comp. (%) Air Perm. Aft. Heat Comp. 650 600 640
590 480 430 268 258 (seconds/100 cm.sup.3) Shutdown Temp. .degree.
C. 134 134 134 134 132 132 132 133 Meltdown Temp. .degree. C. 174
173 173 171 149 147 149 148 Max. MD Shrinkage in 24 22 19 22 20.0
30.5 12.5 12.5 Molten State (%) Max. TD Shrinkage in 8.0 2.0 4.0
3.0 5.9 4.4 2.0 5.1 molten state (%)
TABLE-US-00002 TABLE 2 Comparative Comparative Comparative
Comparative Comparative Comparative Comparative PROPERTIES Example
1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7
Thickness .mu.m 20.1 21.2 19.6 19.9 21.8 19.5 20.1 Normalized Air
Perm. 550 215 197 420 248 250 255 (sec/100 cm.sup.3/2 .mu.m)
Porosity % 39 46 38 41 40 40 40 Normalized Punct. Strength (mN)
5390 4459 4018 3234 3528 4704 3577 Tensile Strength 181300 102900
106820 82320 90160 117600 107800 MD//TD (kPa) 147000 139160 112700
73500 81340 151900 93100 Tensile Elongation 140 150 160 110 140 150
180 MD//TD (%) 240 145 190 180 200 110 270 Heat Shrinkage
105.degree. C. 6.5 6.5 5.0 5.5 5.0 2.0 6.0 MD/TD (%) 6.0 9.9 6.0
2.7 2.4 3.0 0.0 Heat Shrinkage 130.degree. C. TD (%) 35 38 29 19
17.2 15.7 20.4 Elec. Soln. Sorpt. Speed 1.0 3.3 3.6 2.3 3.4 4.0 1.0
Thick. Var. Aft. Heat Comp. (%) 7.0 12 10.0 14 13 7.0 11 Air Perm.
Aft. Heat Comp. 1049 610 490 980 580 550 1445 (seconds/100
cm.sup.3) Shutdown Temp. .degree. C. 134 134 134 134 134 133 132
Meltdown Temp. .degree. C. 153 174 174 162 170 144 150 Max. MD
Shrinkage 38 25 18 25 19 17.5 30.2 In Molten State (%) Max. TD
Shrinkage 35 39 34 10.0 10.0 38.0 10.0 In Molten State (%)
[0125] It is noted from Table 1 that the microporous membrane of
the present invention exhibits a good balance of important
properties such as a TD heat shrinkage at 105.degree. C. of 2.5% or
less, a TD heat shrinkage at 130.degree. C. of 15% or less, and a
maximum TD shrinkage in the molten state of 10.0% or less, with
good mechanical strength and compression resistance. The
microporous membranes of the invention also have suitable air
permeability, pin puncture strength, tensile rupture strength and
tensile rupture elongation, with little variation of thickness and
air permeability after heat compression. On the other hand, the
microporous membrane products of the Comparative Examples exhibit
generally higher air permeability Gurley values, higher air
permeability after heat compression Gurley values, and higher
maximum TD shrinkage in the molten state.
[0126] Battery separators formed by the microporous polyolefin
membranes of the present invention provide batteries with suitable
safety, heat resistance, storage properties and productivity.
[0127] 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.
[0128] 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.
[0129] When numerical lower limits and numerical upper limits are
listed herein, ranges from any lower limit to any upper limit are
contemplated.
* * * * *