U.S. patent application number 12/741187 was filed with the patent office on 2010-11-25 for microporous polymeric membrane, battery separator, and battery.
Invention is credited to Takeshi Ishihara, Kohtaro Kimishima.
Application Number | 20100297491 12/741187 |
Document ID | / |
Family ID | 40560262 |
Filed Date | 2010-11-25 |
United States Patent
Application |
20100297491 |
Kind Code |
A1 |
Ishihara; Takeshi ; et
al. |
November 25, 2010 |
Microporous Polymeric Membrane, Battery Separator, and Battery
Abstract
The invention relates to microporous polymeric membrane having a
good balance of rupture temperature and air permeability. The
invention also relates to a battery separator formed by such a
microporous membrane, and a battery comprising such a separator.
Another aspect of the invention relates to a method for making the
microporous polymeric membrane, a method for making a battery using
such a membrane as a separator, and a method for using such a
battery.
Inventors: |
Ishihara; Takeshi;
(Saitama-ken, JP) ; Kimishima; Kohtaro;
(Kanagawa-ken, JP) |
Correspondence
Address: |
EXXONMOBIL CHEMICAL COMPANY
5200 BAYWAY DRIVE, P.O. BOX 2149
BAYTOWN
TX
77522-2149
US
|
Family ID: |
40560262 |
Appl. No.: |
12/741187 |
Filed: |
November 17, 2008 |
PCT Filed: |
November 17, 2008 |
PCT NO: |
PCT/JP2008/071182 |
371 Date: |
July 29, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60991384 |
Nov 30, 2007 |
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61052438 |
May 12, 2008 |
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Current U.S.
Class: |
429/145 ;
210/500.36; 264/45.9; 428/315.9 |
Current CPC
Class: |
Y10T 428/24998 20150401;
B32B 27/08 20130101; B32B 2307/518 20130101; B32B 2307/724
20130101; B32B 2307/516 20130101; H01M 10/052 20130101; B32B 3/26
20130101; B32B 27/32 20130101; B32B 2250/24 20130101; B01D 71/26
20130101; B29C 55/005 20130101; B32B 2270/00 20130101; Y02E 60/10
20130101; H01M 50/449 20210101; B32B 1/08 20130101; Y02T 10/70
20130101; H01M 50/411 20210101; B01D 69/125 20130101; Y10T
428/249978 20150401; B29K 2105/04 20130101; B32B 2307/736 20130101;
B32B 5/32 20130101; B32B 2457/10 20130101 |
Class at
Publication: |
429/145 ;
210/500.36; 428/315.9; 264/45.9 |
International
Class: |
H01M 2/16 20060101
H01M002/16; B32B 5/32 20060101 B32B005/32; B32B 27/32 20060101
B32B027/32; B01D 67/00 20060101 B01D067/00; B01D 71/26 20060101
B01D071/26 |
Claims
1. A multi-layer microporous membrane, comprising: a first layer
material comprising polyethylene and a second layer material
comprising polypropylene, the polypropylene having (1) a
weight-average molecular weight .gtoreq.6.times.10.sup.5, (2) a
heat of fusion .gtoreq.90 J/g, and (3) an MWD in the range of 2 to
6, wherein (a) the multi-layer microporous membrane has at least a
first microporous layer containing the first layer material, a
third microporous layer containing the first layer material, and a
second microporous layer containing the second layer material, the
second microporous layer being located between the first and third
microporous layers, and (b) the total amount of polypropylene in
the multi-layer microporous membrane is at least 2.0 wt. % based on
the total weight of the multi-layer microporous membrane.
2. The multi-layer microporous membrane of claim 1, wherein (a) the
polyethylene of the first layer material is a first polyethylene,
the first polyethylene being present in the first layer material in
an amount in the range of from 80 wt. % to 100 wt. % based on the
weight of the first layer material, (b) the polypropylene of the
second layer material is present in the second layer material in an
amount in the range of from 1 wt. % to 100 wt. %, based on the
weight of the second layer material, and (c) the second layer
material further comprises a second polyethylene, the second
polyethylene being present in the second layer material in an
amount in the range of from 0 wt. % to 99 wt. %, based on the
weight of the second layer material.
3. The multi-layer microporous membrane of claim 1, wherein the
first microporous layer material consists essentially of
polyethylene.
4. The multi-layer microporous membrane of claim 1, wherein the
multi-layer microporous membrane is a three-layer membrane and
wherein the amount of polypropylene in the second layer material is
in the range of 20 wt. % to 50 wt. %.
5. The multi-layer microporous membrane of claim 1, wherein the
first layer further comprises polypropylene that is optionally the
same as the polypropylene of the second layer material, the
polypropylene of the first layer material being present in an
amount in the range of from 0.5 wt. % to 10 wt. % based on the
weight of the first layer material.
6. The multi-layer microporous membrane of claim 1, wherein (a) the
first and/or second polyethylene comprises PE1, PE2, or both PE1
and PE2, and wherein (1) the first polyethylene has a Mw in the
range of 1.times.10.sup.5 to 5.times.10.sup.6; (2) the second
polyethylene has an Mw ranging from 1.times.10.sup.5 to
5.times.10.sup.6; (3) PE1 is one or more of a high-density
polyethylene, a medium-density polyethylene, a branched low-density
polyethylene, or a linear low-density polyethylene; (4) PE1 is at
least one of (i) an ethylene homopolymer or (ii) a copolymer of
ethylene and a comonomer selected from propylene, butene-1,
hexene-1; (5) PE2 has an Mw of at least 1.times.10.sup.6; (6) PE2
is at least one of (i) an ethylene homopolymer or (ii) a copolymer
of ethylene and a comonomer selected from propylene, butene-1,
hexene-1; (7) the amount of the PE1 in the first microporous layer
material is in the range of from 50 wt. % to 100 wt. %, based on
the weight of the first microporous layer material; (8) the amount
of the PE2 in the first microporous layer material is in the range
of from 0 wt. % to 50 wt. %, based on the weight of the first
microporous layer material; (9) the amount of the PE1 in the second
microporous layer material is in the range of from 40 wt. % to 60
wt. %, based on the weight of the second microporous layer
material; (10) the amount of the PE2 in the second microporous
layer material is in the range of from 0 wt. % to 50 wt. %, based
on the weight of the second microporous layer material; (11) the
first and/or second polyethylene has Mw/Mn in the range of 5 to
300; and (b) the second polypropylene has at least one of: (1) an
Mw/Mn in the range of 2 to 6; (2) an MW in the range of
9.times.10.sup.5 to 2.times.10.sup.6; and (3) a heat of fusion is
100 J/g or more.
7. The multi-layer microporous membrane of claim 1, wherein the
second microporous layer has a thickness in the range of about 4.6%
to about 50% of the total thickness of the multi-layer microporous
membrane.
8. The multi-layer microporous membrane of claim 6, wherein the
first microporous layer material further comprises
polypropylene.
9. The multi-layer microporous membrane of claim 6, wherein PE1 is
high-density polyethylene and the PE2 is ultra-high molecular
weight polyethylene.
10. The multi-layer membrane of claim 9, wherein the first and/or
second polyethylene comprises 8 wt. % or less of PE2 and 92 wt. %
or more of PE1.
11. A method for producing a microporous membrane, comprising, (1)
combining a polyethylene resin and a first diluent, (2) combining a
polypropylene resin, and a second diluent; the polypropylene resin
having the polypropylene having (a) a weight-average molecular
weight of 6.times.10.sup.5 or more, (b) a heat of fusion of 90 J/g
or more, and (c) an MWD in the range of 2 to 6; (3) extruding at
least a portion of the combined first polyethylene resin and first
diluent, and extruding at least a portion of the combined first
polypropylene resin and second diluent to produce a multi-layer
extrudate which comprises a first layer comprising the combined
polyethylene and first diluent, a second layer comprising the
polypropylene and the second diluent, and a third layer comprising
the combined polyethylene and first diluent, wherein the second
layer is located between the first and third layers and wherein the
polypropylene is present in the extrudate in an amount .gtoreq.2.0
wt. % based on the weight of polymer in the extrudate; and then (4)
cooling the multi-layer extrudate to form a multi-layer sheet, (5)
removing at least a portion of the first and second diluents from
the multi-layer sheet to form a diluent-removed sheet, and (6)
removing at least a portion of any volatile species from the sheet
to form the microporous membrane.
12. The method of claim 11, wherein the combined polyethylene and
first diluent is a first polyolefin solution, wherein the combined
polypropylene and second diluent is a second polyolefin solution,
and wherein the polyethylene resin is present in the first
polyolefin solution in an amount in the range of from about 0.5 wt.
% to about 75 wt. % based on the total weight of polyolefin in the
first polyolefin solution, the first polyolefin solution comprises
polypropylene resin, the polypropylene resin being present in the
first polyolefin solution in an amount in the range of from about 0
wt. % to about 10 wt. % based on the total weight of polyolefin in
the first polyolefin solution, the second polyolefin solution
comprises polyethylene resin, the polyethylene resin being present
in the second polyolefin solution in an amount in the range of from
about 40 wt. % to about 90 wt. % based on the total weight of
polyolefin in the second polyolefin solution, the polypropylene
resin is present in the second polyolefin solution in an amount in
the range of from about 10 wt. % to about 60 wt. % based on the
total weight of polyolefin in the second polyolefin solution; the
first diluent is present in the first polyolefin solution in an
amount in the range of from about 25 wt. % to about 99 wt. % based
on the weight of the first polyolefin solution; and the second
diluent is present in the second polyolefin solution in an amount
in the range of from about 25 wt. % to about 99 wt. % based on the
weight of the second polyolefin solution.
13. The method of claim 12 wherein (a) the polyethylene of the
first, second, and third layer independently comprise a PE1, a PE2,
or both PE1 and PE2, wherein (1) the first polyethylene has a Mw in
the range of 1.times.10.sup.5 to 5.times.10.sup.6; (2) the second
polyethylene has an Mw ranging from 1.times.10.sup.5 to
5.times.10.sup.6; (3) PE1 is one or more of a high-density
polyethylene, a medium-density polyethylene, a branched low-density
polyethylene, or a linear low-density polyethylene; (4) PE1 is at
least one of (i) an ethylene homopolymer or (ii) a copolymer of
ethylene and a comonomer selected from propylene, butene-1,
hexene-1; (5) PE2 has an Mw of at least 1.times.10.sup.6; (6) PE2
is at least one of (i) an ethylene homopolymer or (ii) a copolymer
of ethylene and a comonomer selected from propylene, butene-1,
hexene-1; (7) the amount of the PE1 in the first microporous layer
material is in the range of from 50 wt. % to 100 wt. %, based on
the weight of the first microporous layer material; (8) the amount
of the PE2 in the first microporous layer material is in the range
of from 0 wt. % to 50 wt. %, based on the weight of the first
microporous layer material; (9) the amount of the PE1 in the second
microporous layer material is in the range of from 40 wt. % to 60
wt. %, based on the weight of the second microporous layer
material; (10) the amount of the PE2 in the second microporous
layer material is in the range of from 0 wt. % to 50 wt. %, based
on the weight of the second microporous layer material; (11) the
first and/or second polyethylene has Mw/Mn in the range of 5 to
300; and (b) the second polypropylene has at least one of: (1) an
Mw/Mn in the range of 2 to 6; (2) an MW in the range of
9.times.10.sup.5 to 2.times.10.sup.6; and (3) a heat of fusion is
100 J/g or more. (b) the polypropylene has at least one
characteristic selected from: (1) a weight-average molecular weight
of 9.times.10.sup.5 to 2.times.10.sup.6, (2) a heat of fusion of
100 J/g or more, and (3) a molecular weight distribution (Mw/Mn) in
the range of 2 to 6.
14. A battery comprising an anode, a cathode, an electrolyte, and
the multi-layer microporous membrane of claim 1, wherein the
multi-layer microporous membrane separates at least the anode from
the cathode.
15. The battery of claim 14, wherein the electrolyte contains
lithium ions and the battery is a secondary battery.
16. The battery of claim 14 used as a source or sink of electric
charge.
17. A microporous membrane having Rupture temperature 180.degree.
C. or higher and comprising polypropylene wherein the total amount
polypropylene in the membrane is .gtoreq.2 wt. %, based on the
total weight of the membrane.
18. The microporous membrane of claim 17, wherein the total amount
polypropylene in the membrane having (a) an
Mw.gtoreq.6.times.10.sup.5, (b) an MWD in the range of 2 to 6, and
(c) a heat of fusion of 90 J/g or higher is .gtoreq.2 wt. %, based
on the total weight of the membrane.
19. The microporous membrane of claim 17 wherein the membrane has
Normalized Air Permeability satisfying the relationship
A.ltoreq.(M*P)-I where A is the microporous membrane's Normalized
Air Permeability thickness, P is the microporous membrane's
Normalized Pin Puncture Strength, M is a slope in the range of
about 0.09 to about 0.1, or about 0.95 to about 0.99, and I is
.gtoreq.100.
20. A battery comprising the separator of claim 19.
21. A battery separator made by the process of claim 11.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a microporous membrane having
suitable permeability, pin puncture strength, shutdown temperature,
and rupture temperature. The invention also relates to a battery
separator formed by such multi-layer, microporous membrane, and a
battery comprising such a separator. Another aspect of the
invention relates to a method for making the multi-layer,
microporous polyolefin membrane, a method for making a battery
using such a membrane as a separator, and a method for using such a
battery.
BACKGROUND OF THE INVENTION
[0002] 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'
performance significantly affects the properties, productivity and
safety of the batteries. Accordingly, the microporous polyolefin
membrane should have suitable mechanical properties, heat
resistance, permeability, dimensional stability, shutdown
properties, meltdown properties, etc. As is known, it is desirable
for the batteries to have a relatively low shutdown temperature and
a relatively high meltdown (or rupture) temperature for improved
battery-safety properties, particularly for batteries that are
exposed to high temperatures during manufacturing, charging,
re-charging, use, and/or storage. Improving separator permeability
generally leads to an improvement in the battery's storage
capacity. High shutdown speed is desired for improved battery
safety, particularly when the battery is operated under overcharge
conditions. Improving pin puncture strength is desired because
roughness of the battery's electrode can puncture the separator
during manufacturing leading to a short circuit. Improved thickness
uniformity is desired because thickness variations lead to
manufacturing difficulties when winding the film on a core.
[0003] In general, microporous membranes containing polyethylene
only (i.e., the membrane consists of, or consists essentially of,
polyethylene) have low meltdown temperatures, while microporous
membranes containing polypropylene only have high shutdown
temperatures. Accordingly, microporous membranes comprising
polyethylene and polypropylene as main components have been
proposed as improved battery separators. It is therefore desired to
provide microporous membranes formed from polyethylene resin and
polypropylene resin, and multi-layer, microporous membranes
comprising polyethylene and polypropylene.
[0004] JP7-216118A, for example, discloses a battery separator
comprising a multi-layer, porous film having two microporous
layers. Both layers can contain polyethylene and polypropylene, but
in different relative amounts. For example, the percentage of the
polyethylene is 0 wt. % to 20 wt. % in the first microporous layer,
and 21 wt. % to 60 wt. % in the second microporous layer, based on
the combined weight of the polyethylene and polypropylene. The
total amount of polyethylene in the film (i.e., both microporous
layers) is 2 wt. % to 40 wt. %, based on the weight of the
multi-layer microporous film.
[0005] JP10-195215A discloses a relatively thin battery separator
having conventional shutdown and pin-pulling characteristics. The
term "pin pulling" refers to the relative ease of pulling a metal
pin from a laminate of a separator, a cathode sheet and an anode
sheet, which is wound around the pin, to form a toroidal laminate.
The multi-layer, porous film contains polyethylene and
polypropylene, but in different relative amounts. The percentage of
polyethylene is 0 wt. % to 20 wt. % in the inner layer and 61 wt. %
to 100 wt. % in the outer layer, based on the total weight of the
polyethylene and polypropylene.
[0006] JP10-279718A discloses a separator designed to prevent
unacceptably large temperature increases in a lithium battery when
the battery is overcharged. The separator is formed from a
multi-layer, porous film made of polyethylene and polypropylene,
with different relative amounts of polyethylene and polypropylene
in each layer. The film has a polyethylene-poor layer whose
polyethylene content is 0 wt. % to 20 wt. %, based on the weight of
the polyethylene-poor layer. The second layer is a
polyethylene-rich layer which contains 0.5 wt. % or more of
polyethylene having a melt index of 3 or more and has a
polyethylene content of 61 wt. % to 100 wt. %, based on the weight
of the polyethylene-rich layer.
[0007] It would be desirable to further improve the permeability
and pin puncture strength of microporous membranes.
SUMMARY OF THE INVENTION
[0008] In an embodiment, the invention relates to a multi-layer
microporous membrane, comprising:
[0009] a first layer material comprising polyethylene and a second
layer material comprising polypropylene, the polypropylene having
(1) a weight-average molecular weight of 6.times.10.sup.5 or more,
(2) a heat of fusion of 90 J/g or more, and (3) a molecular weight
distribution ("MWD" defined as Mw/Mn) in the range of about 2 to
about 6, wherein
[0010] (a) the multi-layer microporous membrane has at least a
first microporous layer containing the first layer material, a
third microporous layer containing the first layer material, and a
second microporous layer containing the second layer material, the
second microporous layer being located between the first and third
microporous layers, and
[0011] (b) the total amount of polypropylene in the multi-layer
microporous membrane is at least 2.0 wt. % based on the total
weight of the microporous membrane.
[0012] In another embodiment, the invention relates to a method for
producing a microporous membrane, comprising, [0013] (1) combining
a polyethylene resin and a first diluent, [0014] (2) combining a
polypropylene resin, and a second diluent; the polypropylene resin
having the polypropylene having (a) a weight-average molecular
weight of 6.times.10.sup.5 or more, (b) a heat of fusion of 90 J/g
or more, and (c) an MWD in the range of 2 to 6; [0015] (3)
extruding at least a portion of the combined first polyethylene
resin and first diluent, and extruding at least a portion of the
combined first polypropylene resin and second diluent to produce a
multi-layer extrudate which comprises [0016] a first layer
comprising the combined polyethylene and first diluent, a second
layer comprising the polypropylene and the second diluent, and a
third layer comprising the combined polyethylene and first diluent,
wherein the second layer is located between the first and third
layers and wherein the polypropylene is present in the extrudate in
an amount .gtoreq.2.0 wt. % based on the weight of polymer in the
extrudate; and then [0017] (4) cooling the multi-layer extrudate to
form a multi-layer sheet, [0018] (5) removing at least a portion of
the first and second diluents from the multi-layer sheet to form a
diluent-removed sheet, and [0019] (6) removing at least a portion
of any volatile species from the sheet to form the microporous
membrane.
[0020] In yet another embodiment, the invention relates to a
microporous polymeric membrane having a rupture temperature of
180.degree. C. or higher and an air permeability satisfying the
relationship A.ltoreq.0.097 P-I where A is the microporous
membrane's Normalized Air Permeability expressed in the units of
sec/100 cm.sup.3/25 .mu.m, P is the microporous membrane's Pin
Puncture Strength expressed in the units of mN/25 .mu.m, and I is
in the range of from 100 to about 250, or about 110 to about 230.
In an embodiment, I is about 110, in which case the relationship
can be expressed as A.ltoreq.0.097 P-110.
[0021] In yet another embodiment, the invention relates to a
microporous membrane having Rupture temperature 180.degree. C. or
higher and comprising polypropylene wherein the total amount
polypropylene in the membrane is .gtoreq.2 wt. %, based on the
total weight of the membrane. The invention is not limited to
microporous membranes. For example, separators comprising such
membranes, batteries comprising such separators, and the use of
such batteries are all within the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a graph showing Air permeability (Y axis) and Pin
Puncture Strength (X axis) for various microporous membranes. The
membranes described in the Examples are represented by diamonds and
the membranes described in the Comparative Examples are represented
by triangles. Membranes of the invention but not further
exemplified are represented by rectangles. Points represented by
circles are membranes which have (i) a Rupture temperature lower
(cooler) than 180.degree. C. and/or (ii) an Air Permeability that
is greater than 0.097P-I, where I is in the range of about 100 to
about 250. I represents the Y intercept of the line plotted on FIG.
1, which has a slope of about 0.097. Air Permeability is expressed
in the units of sec/100 cm.sup.3/25 .mu.m and P is the microporous
membrane's Pin Puncture Strength expressed in the units of mN/25
.mu.m (where "/25 .mu.m" means normalized to the value for a
membrane of 25 .mu.m thickness).
[0023] FIG. 2 is a graph showing the Rupture temperature of
microporous polymeric membranes as a function of the membrane's
total polypropylene content, e.g., the total amount as measured by
wt. % of first and second polypropylene in the membrane, based on
the total weight of the membrane. The membranes described in the
Examples are represented by diamonds and the membranes described in
the Comparative Examples are represented by triangles. Membranes
not further exemplified are represented by asterisks.
DETAILED DESCRIPTION OF THE INVENTION
[1] Composition and Structure of the Microporous Membrane
[0024] In an embodiment, the multi-layer, microporous membrane
comprises three layers, wherein the outer layers (also called the
"surface" or "skin" layers) comprise the first microporous layer
material and at least one intermediate layer (or "core" layer)
comprises the second microporous layer material. In a related
embodiment, the multi-layer, microporous membrane can comprise
additional layers, i.e., in addition to the two skin layers and the
core layer. In a related embodiment where the multi-layer,
microporous membrane comprises three or more layers, the outer
layers consist essentially of (or consists of) the first
microporous layer material and at least one intermediate layer
consists essentially of (or consists of) the second microporous
layer material. While it is not required, the core layer can be in
planar contact with one or more of the skin layers in a stacked
arrangement such as A/B/A with face-to-face stacking of the layers.
The membrane can be referred to as a "polyolefin membrane" when the
membrane contains polyolefin. While the membrane can contain
polyolefin only, this is not required, and it is within the scope
of the invention for the membrane to contain polyolefin and
materials that are not polyolefin.
[0025] When the multi-layer, microporous membrane has three or more
layers, the multi-layer, microporous polyolefin membrane has at
least one layer comprising the first microporous layer material and
at least one layer comprising the second microporous layer
material.
[0026] In an embodiment, the microporous membrane is a three layer
membrane wherein the thickness of the core layer is in the range of
about 4.6% to about 50%, or from about 5% to about 30%, or from 5%
to about 15% of the total thickness of the multi-layer microporous
membrane.
[0027] In an embodiment, the first microporous layer material
comprises a first polyethylene and optionally a first
polypropylene. The second microporous layer material comprises a
second polypropylene and optionally a second polyethylene. The
total amount of polyethylene in the multi-layer, microporous
polyolefin membrane is in the range of from about 70 wt. % to about
98 wt. %, or from about 90 wt. % to about 97.95 wt. %, or from
about 95 wt. % to about 97.9 wt. %, based on the weight of the
multi-layer, microporous membrane. The total amount of
polypropylene in the multi-layer, microporous membrane is generally
greater than 2.0 wt. % based on the total weight of the membrane.
When the membrane contains material in addition to polyolefin, the
wt. % is based on the weight of the membrane's total polyolefin
content. For example, the total amount of polypropylene in the
multi-layer, microporous membrane can be in the range of from about
2.0 wt. % to about 30 wt. %, e.g., about 2.05 wt. % to about 10 wt.
%, or from about 2.1 wt. % to about 5 wt. %, based on the weight of
the multi-layer, microporous membrane. In an embodiment, the first
polyethylene is present in the first microporous layer material in
a first polyethylene amount in the range of from about 80 wt. % to
about 100 wt. % based on the weight of the first microporous layer
material; the first polypropylene is present in the first
microporous layer material in a first polypropylene amount in the
range of from about 0 wt. % to about 10 wt. % (e.g., from about 0.5
wt. % to about 10 wt. %) based on the weight of the first
microporous layer material; the second polyethylene is present in
the second microporous layer material in a second polyethylene
amount in the range of from about 0 wt. % to about 99 wt. %, e.g.,
from about 40 wt. % to about 90 wt. %, such as from about 50 wt. %
to about 80 wt. % based on the weight of the second microporous
layer material; and the second polypropylene is present in the
second microporous layer material in a second polypropylene amount
in the range of from about 1 wt. % to about 100 wt%, e.g., from
about 10 wt. % to about 60 wt. %, such as from about 20 wt. % to
about 50 wt. % based on the weight of the second microporous layer
material.
[0028] The first and second polyethylene and the first and second
polypropylene will now be described in more detail.
A. The First Polyethylene
[0029] In an embodiment, the first polyethylene is a polyethylene
having an Mw in the range of about 1.times.10.sup.4 to about
1.times.10.sup.7, or about 1.times.10.sup.5 to about
5.times.10.sup.6, or about 1.times.10.sup.5 to about
9.times.10.sup.5. Although it is not critical, the first
polyethylene can have terminal unsaturation of, e.g., two or more
per 10,000 carbon atoms in the polyethylene. Terminal unsaturation
can be measured by, e.g., conventional infrared spectroscopic
methods. The first polyethylene can be one or more varieties of
polyethylene, e.g., PE1, PE2, etc. In one embodiment, the first
polyethylene comprises PE1. PE1 comprises polyethylene having an Mw
ranging from about 4.times.10.sup.5 to about 8.times.10.sup.5.
Optionally, the PE1 can be one or more of an high density
polyethylene ("HDPE"), a medium-density polyethylene, a branched
low-density polyethylene, or a linear low-density polyethylene. In
an embodiment, PE1 has an Mn/Mw of .ltoreq.100, e.g., in the range
of about 3 to about 20. In an embodiment, PE1 is at least one of
(i) an ethylene homopolymer or (ii) a copolymer of ethylene and a
comonomer such as propylene, butene-1, hexene-1, etc, typically in
a relatively small amount compared to the amount of ethylene, e.g.,
10 mol % or less based on 100% by mol of the copolymer. Such a
copolymer can be produced using a single-site catalyst.
[0030] In an embodiment, the first polyethylene comprises PE2. PE2
comprises polyethylene having an Mw of at least about
1.times.10.sup.6. For example, PE2 can be an ultra-high molecular
weight polyethylene ("UHMWPE"). In an embodiment, PE2 is at least
one of (i) an ethylene homopolymer or (ii) a copolymer of ethylene
and a comonomer which is typically present in a relatively small
amount compared to the amount of ethylene, e.g., 10 mol % or less
based on 100% by mol 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. Optionally, the Mw of PE2 can be in the range, e.g., of
from about 1.times.10.sup.6 to about 15.times.10.sup.6, or from
about 1.times.10.sup.6 to about 5.times.10.sup.6, or from
1.times.10.sup.6 to about 3.times.10.sup.6. Optionally, the PE2 has
an Mw/Mn in the range of about 4.5 to about 10.
[0031] In an embodiment, the first polyethylene comprises both PE1
and PE2. In this case, the amount of PE2 in the first polyethylene
can be, e.g., in the range of about 0 wt % to about 50 wt %, or
about 1 wt. % to about 50 wt. %, based on the weight of the first
polyethylene. In one embodiment, the first polyethylene comprises
PE1 in an amount .gtoreq.92 wt. % and PE2 in an amount .ltoreq.8
wt. %, based on the weight of the first polyethylene.
[0032] In one embodiment, the first polyethylene has one or more of
the following independently-selected features: [0033] (1) The first
polyethylene comprises PE1. [0034] (2) The first polyethylene
consists essentially of, or consists of, PE1. [0035] (3) The PE1 is
one or more of a high-density polyethylene, a medium-density
polyethylene, a branched low-density polyethylene, or a linear
low-density polyethylene. [0036] (4) PE1 is one or more of a
high-density polyethylene having an Mw ranging from about
1.times.10.sup.5 to about 5.times.10.sup.6, e.g., from about
2.times.10.sup.5 to about 9.times.10.sup.5, such as from about
4.times.10.sup.5 to about 8.times.10.sup.5. [0037] (5) PE1 is at
least one of (i) an ethylene homopolymer or (ii) a copolymer of
ethylene and a third .alpha.-olefin selected from the group of
propylene, butene-1, hexene-1. [0038] (6) The first polyethylene
comprises both PE1 and PE2. [0039] (7) PE2 has an Mw ranging from
about 1.times.10.sup.6 to about 15.times.10.sup.6, e.g., from about
1.times.10.sup.6 to about 5.times.10.sup.6, such as from about
1.times.10.sup.6 to about 3.times.10.sup.6, and an Mw/Mn in the
range of about 4.5 to about 10. [0040] (8) PE2 is
ultra-high-molecular-weight polyethylene. [0041] (9) PE2 is at
least one of (i) an ethylene homopolymer or (ii) a copolymer of
ethylene and a fourth .alpha.-olefin selected from the group of
propylene, butene-1, hexene-1. [0042] (10) The first polyethylene
has a molecular weight distribution ("Mw/Mn") of about 5 to about
300, or about 5 to about 100, or optionally from about 5 to about
30.
[0043] In an embodiment, the microporous membrane is characterized
by one or more of: [0044] (1) the amount of the PE1 in the first
microporous layer material is in the range of from about 50 wt. %
to about 100 wt. %, based on the weight of the first microporous
layer material; [0045] (2) the amount of the PE2 in the first
microporous layer material is in the range of from about 0 wt. % to
about 50 wt. %, based on the weight of the first microporous layer
material; [0046] (3) the amount of the PE1 in the second
microporous layer material is in the range of from about 40 wt. %
to about 60 wt. %, based on the weight of the second microporous
layer material; or [0047] (4) the amount of the PE2 in the second
microporous layer material is in the range of from about 0 wt. % to
about 50 wt. %, based on the weight of the second microporous layer
material.
B. The Second Polyethylene
[0048] The second polyethylene can be selected from among the same
polyethylenes as the first polyethylene. For example, the second
polyethylene comprise PE1, PE2, or both PE1 and PE2. When the
second polyethylene comprises PE1 and PE2, the amount of PE2 in the
second polyethylene can be in the range of 0 wt % to about 50 wt.
%, or about 1 wt. % to about 50 wt. %, based on the weight of the
second polyethylene. Optionally, the second polyethylene is
substantially the same as the first polyethylene.
[0049] Mw and MWD of the polyethylene and polypropylene are
determined using a High Temperature Size Exclusion Chromatograph,
or "SEC", (GPC PL 220, Polymer Laboratories), equipped with a
differential refractive index detector (DRI). The measurement is
made in accordance with the procedure disclosed in "Macromolecules,
Vol. 34, No. 19, pp. 6812-6820 (2001)". Three PLgel Mixed-B columns
available from (available from Polymer Laboratories) are used for
the Mw and MWD determination. For polyethylene, the nominal flow
rate is 0.5 cm.sup.3/min; the nominal injection volume is 300
.mu.L; and the transfer lines, columns, and the DRI detector are
contained in an oven maintained at 145.degree. C. For
polypropylene, the nominal flow rate is 1.0 cm.sup.3/min; the
nominal injection volume is 300 .mu.L; and the transfer lines,
columns, and the DRI detector are contained in an oven maintained
at 160.degree. C.
[0050] The GPC solvent used is filtered Aldrich reagent grade
1,2,4-Trichlorobenzene (TCB) containing approximately 1000 ppm of
butylated hydroxy toluene (BHT). The TCB was degassed with an
online degasser prior to introduction into the SEC. The same
solvent is used as the SEC. Polymer solutions were prepared by
placing dry polymer in a glass container, adding the desired amount
of the TCB solvent, and then heating the mixture at 160.degree. C.
with continuous agitation for about 2 hours. The concentration of
UHMWPE solution was 0.25 to 0.75 mg/ml. Sample solution are
filtered off-line before injecting to GPC with 2 .mu.m filter using
a model SP260 Sample Prep Station (available from Polymer
Laboratories).
[0051] The separation efficiency of the column set is calibrated
with a calibration curve generated using a seventeen individual
polystyrene standards ranging in Mp ("Mp" being defined as the peak
in Mw) from about 580 to about 10,000,000. The polystyrene
standards are obtained from Polymer Laboratories (Amherst, Mass.).
A calibration curve (logMp vs. retention volume) is generated by
recording the retention volume at the peak in the DRI signal for
each PS standard and fitting this data set to a 2nd-order
polynomial. Samples are analyzed using IGOR Pro, available from
Wave Metrics, Inc.
C. The First Polypropylene
[0052] Besides polyethylene, the first layer materials can
optionally comprise polypropylene. The polypropylene can be, for
example, one or more of (i) a propylene homopolymer or (ii) a
copolymer of propylene and a comonomer. The copolymer can be a
random or block copolymer. The comonomer can be, e.g., 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. The amount of the fifth olefin
in the copolymer is preferably in a range that does not adversely
affect properties of the multi-layer microporous membrane such as
heat resistance, compression resistance, heat shrinkage resistance,
etc. For example, the amount of the comonomer is generally >10%
by mol based on 100% by mol of the entire copolymer. Optionally,
the polypropylene has one or more of the following properties: (i)
the polypropylene has an Mw ranging from about 1.times.10.sup.4 to
about 4.times.10.sup.6, or about 3.times.10.sup.5 to about
3.times.10.sup.6; (ii) the polypropylene has an Mw/Mn ranging from
about 1.01 to about 100, or about 1.1 to about 50; (iii) the
polypropylene's tacticity is isotactic; (iv) the polypropylene has
a heat of fusion of at least about 90 Joules/gram; (v)
polypropylene has a melting peak (second melt) of at least about
160.degree. C., (vi) 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; and/or (vii) 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. Optionally, the first polypropylene is
substantially the same as the second polypropylene.
D. The Second Polypropylene
[0053] In an embodiment, the second polypropylene has one or more
of the following characteristics. The second polypropylene
preferably has a weight-average molecular weight of
6.times.10.sup.5 or more (e.g., in the range of about
9.times.10.sup.5 to about 2.times.10.sup.6) and a heat of fusion
.DELTA.Hm (measured by a differential scanning calorimeter (DSC)
according to JIS K7122) of 90 J/g or more, and an Mw/Mn.ltoreq.100,
e.g., in the range of about 2 to about 6. Because polypropylene
having a weight-average molecular weight of less than
6.times.10.sup.5 has low dispersibility in the polyethylene resin,
its use makes stretching difficult, providing large micro-roughness
to a surface of the second porous layer and large thickness
variation to the multi-layer, microporous membrane. When the
polypropylene has a heat of fusion .DELTA.Hm of less than 90 J/g,
the resultant multi-layer, microporous membrane may have low
meltdown properties and permeability.
[0054] The weight-average molecular weight of the second
polypropylene can be, e.g., 8.times.10.sup.5 or more, or in the
range 8.times.10.sup.5 to 2.0.times.10.sup.6. The heat of fusion
.DELTA.Hm of the polypropylene is 95 J/g or more, or 100 J/g or
more, or 110 J/g or more, or 115 J/g or more, e.g., in the range of
100 J/g to 120 J/g. The molecular weight distribution, Mw/Mn, is
.gtoreq.2, e.g., in the range of 2 to 6. In an embodiment,
Mw/Mn.gtoreq.3, e.g, in the range of 3 to 10.
[0055] The polypropylene content of the second layer material can
be, e.g., in the range of from about 1 wt. % to about 100 wt. %,
e.g., from about 20 wt. % to about 80 wt. %, such as from about 20
wt. % to about 50 wt. %, based on the weight of the second layer
material.
[0056] As long as the above conditions of the weight-average
molecular weight, and the heat of fusion are met, the type of the
polypropylene is not particularly critical, but may be a propylene
homopolymer, a copolymer of propylene and a comonomer, or a mixture
thereof, the homopolymer being preferable. The copolymer may be a
random or block copolymer. The comonomer can be, for example,
ethylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene,
vinyl acetate, methyl methacrylate, styrene, and combinations
thereof The amount of comonomer is, e.g., less than 10 mol % based
on 100 mol % of the copolymer. Optionally, the second polypropylene
has one or more of the following properties: (i) the polypropylene
has an Mw in the range of from about 1.times.10.sup.4 to about
4.times.10.sup.6, or about 6.times.10.sup.5 to about
3.times.10.sup.6; (ii) the polypropylene has an Mw/Mn in the range
of from about 1.01 to about 100, or about 2 to about 6; (iii) the
polypropylene's tacticity is isotactic; (iv) the polypropylene has
a heat of fusion of at least about 95 Joules/gram; (v) the
polypropylene has a melting peak "Tm" (second melt) of at least
about 160.degree. C., (vi) 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; and/or (vii) 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. In an embodiment, the polypropylene has a
crystallinity .gtoreq.50%, e.g., in the range of 55% to about 75%,
such as in the range of 65% to 70%.
[0057] The .DELTA.Hm, Mw, and Mn of polypropylene are determined by
the methods disclosed in PCT Patent Publication No. WO2007/132942,
which is incorporated by reference herein in its entirety.
[2] Materials Used to Produce the Multi-Layer, Microporous
Polyolefin Membrane
[0058] A. Polymer Resins Used to make the First Microporous Layer
Material
[0059] In an embodiment, the first microporous layer material is
made by combining a first polyolefin composition and a first
diluent to form, e.g., a first polyolefin solution. Since the
diluent produces a multi-layer microporous membrane, the diluent
can also be called a process solvent or a membrane-forming solvent.
The resins used to make the first polyolefin composition will now
be described in more detail.
(1) The First Polyethylene Resin
[0060] In an embodiment, the first polyethylene resin comprises the
first polyethylene, where the first polyethylene is as described
above in section [1]. For example, the first polyethylene resin can
be a mixture of a polyethylene resin having a lower Mw than UHMWPE
(such as HDPE) and UHMWPE resin.
[0061] Multi-stage polymerization can be used to obtain the desired
Mw/Mn ratio in the first polyethylene resin. For example, a
two-stage polymerization method can be used, forming a relatively
high-molecular-weight polymer component in the first stage, and
forming a relatively low-molecular-weight polymer component in the
second stage. While not required, this method can be used, for
example, when the first polyethylene resin comprises PE1. When the
first polyethylene resin comprises the PE1 and PE2, the desired
Mw/Mn ratio of the polyethylene resin can be selected by adjusting
the relative molecular weights and relative amounts of the first
and second polyethylene.
(2) The First Polypropylene Resin
[0062] Besides the first polyethylene resin, the first polyolefin
composition can optionally further comprise a first polypropylene
resin. In an embodiment, the first polypropylene resin comprises
the first polypropylene, where the first polypropylene is as
described above in section [1].
(3) Formulation
[0063] The amount of process solvent in the first polyolefin
solution can be in the range. e.g., of from about 25 wt. % to about
99 wt. % based on the weight of the first polyolefin solution. In
an embodiment, the amount of the first polyethylene resin in the
first polyolefin composition can be in the range. e.g., of from
about 50 wt. % to about 99 wt. % based on the weight of the first
polyolefin composition. The balance of the first polyolefin
composition can be the first polypropylene.
B. Polymer Resins Used to Produce the Second Microporous Layer
Material
[0064] In an embodiment, the second microporous layer material is
made from a second polyolefin solution that is generally selected
independently of the first polyolefin solution. The second
polyolefin solution comprises a second polyolefin composition and a
second diluent which can be the same as the first diluent. As is
the case in the first polyolefin solution, the second diluent can
be referred to as a second membrane-forming solvent or a process
solvent. In an embodiment, the second polyolefin composition
comprises a second polyethylene resin and a second polypropylene
resin. The second polyethylene resin comprises the second
polyethylene as described above in section [1]. The second
polypropylene resin comprises the second polypropylene as described
above in section [1].
[0065] The amount of process solvent in the second polyolefin
solution can be in the range. e.g., of from about 25 wt. % to about
99 wt. % based on the weight of the second polyolefin solution. In
an embodiment, the amount of the second polyethylene resin in the
second polyolefin composition can be in the range. e.g., of from
about 5 wt. % to about 95 wt. % based on the weight of the second
polyolefin composition. The balance of the second polyolefin
composition can be the second polypropylene.
C. Third Polyolefin
[0066] Although it is not required, each of the first and second
polyolefin compositions can further comprise a third polyolefin
selected from the group consisting of polybutene-1, polypentene-1,
poly-4-methylpentene-1, polyhexene-1, polyoctene-1, polyvinyl
acetate, polymethyl methacrylate, polystyrene and an ethylene
.alpha.-olefin copolymer (except for an ethylene-propylene
copolymer). In an embodiment where a third polyolefin is used, the
third polyolefin can, for example, have an Mw in the range of about
1.times.10.sup.4 to about 4.times.10.sup.6. In addition to or
besides the third polyolefin, the first and/or second polyolefin
composition can further comprise a polyethylene wax, e.g., one
having an Mw in the range of about 1.times.10.sup.3 to about
1.times.10.sup.4. When used, these species should be present in
amounts less than an amount that would cause deterioration in the
desired properties (e.g., meltdown, shutdown, etc.) of the
multi-layer, microporous membrane. When the third polyolefin is one
or more of polybutene-1, polypentene-1, poly-4-methylpentene-1,
polyhexene-1, polyoctene-1, polyvinyl acetate, polymethyl
methacrylate, and polystyrene, the third polyolefin need not be a
homopolymer, but may be a copolymer containing other
.alpha.-olefins.
[0067] The multi-layer microporous membrane generally comprises the
polyolefin used to form the polyolefin solution. A small amount of
washing solvent and/or process solvent can also be present,
generally in amounts less than 1 wt % based on the weight of the
microporous polyolefin membrane. A small amount of polyolefin
molecular weight degradation might occur during processing, but
this is acceptable. In an embodiment, molecular weight degradation
during processing, if any, causes the value of Mw/Mn of the
polyolefin in the membrane to differ from the Mw/Mn of the first or
second polyolefin solution by no more than about 50%, or no more
than about 1%, or no more than about 0.1%.
[3] Production Method of Multi-Layer, Microporous Polyolefin
Membrane
[0068] In an embodiment, the microporous polyolefin membrane is a
two-layer membrane. In another embodiment, the microporous
polyolefin membrane has at least three layers. For the sake of
brevity, the production of the microporous polyolefin membrane will
be mainly described in terms of two-layer and three-layer
membranes, although those skilled in the art will recognize that
the same techniques can be applied to the production of membranes
or membranes having at least four layers.
[0069] In an embodiment, the three-layer microporous polyolefin
membrane comprises first and third microporous layers constituting
the outer layers of the microporous polyolefin membrane and a
second layer situated between (and optionally in face-to-face
contact with) the first and third layers. In an embodiment, the
first and third layers are produced from a first mixture of polymer
and diluent, e.g., a first polyolefin solution and the second (or
inner) layer is produced from the second mixture of polymer and
diluent, e.g., a second polyolefin solution. In another embodiment,
the first and third layers are produced from the second polyolefin
solution and the second layer is produced from the first polyolefin
solution. While the invention is described in terms of extruding
polyolefin solutions, it is not limited thereto, and any extrudable
mixture of polymer and diluent can be used.
A. First Production Method
[0070] The first method for producing a multi-layer membrane
comprises the steps of (1) combining (e.g., by melt-blending) a
first polyolefin composition and a first diluent to prepare a first
polyolefin solution, (2) combining a second polyolefin composition
and a second diluent to prepare a second polyolefin solution, (3)
extruding (preferably simultaneously) the first and second
polyolefin solutions through at least one die to form an extrudate,
(4) cooling the extrudate to form a cooled extrudate, e.g., a
multi-layer, gel-like sheet, (5) removing the membrane-forming
solvent from the multi-layer, sheet to form a solvent-removed
sheet, and (6) drying the solvent-removed gel-like sheet to remove
volatile species, if any, in order to form the multi-layer,
microporous polyolefin membrane. An optional stretching step (7),
and an optional hot solvent treatment step (8), etc. can be
conducted between steps (4) and (5), if desired. After step (6), an
optional step (9) of stretching a multi-layer, microporous
membrane, an optional heat treatment step (10), an optional
cross-linking step with ionizing radiation (11), and an optional
hydrophilic treatment step (12), etc., can be conducted if desired.
The order of the optional steps is not critical.
(1) Preparation of First Polyolefin Solution
[0071] The first polyolefin composition comprises polyolefin resins
as described above that can be combined, e.g., by dry mixing or
melt blending with an appropriate membrane-forming solvent to
produce the first polyolefin solution. Optionally, the first
polyolefin solution can contain various additives such as one or
more antioxidant, fine silicate powder (pore-forming material),
etc., provided these are used in a concentration range that does
not significantly degrade the desired properties of the
multi-layer, microporous polyolefin membrane.
[0072] The first process solvent is optionally a solvent that is
liquid at room temperature. While not wishing to be bound by any
theory or model, it is believed that the use of a liquid solvent to
form the first polyolefin solution makes it possible to conduct
stretching of the gel-like sheet at a relatively high stretching
magnification. In an embodiment, the first membrane-forming solvent
can be at least one of aliphatic, alicyclic or aromatic
hydrocarbons such as nonane, decane, decalin, p-xylene, undecane,
dodecane, liquid paraffin, etc.; mineral oil distillates having
boiling points comparable to those of the above hydrocarbons; and
phthalates liquid at room temperature such as dibutyl phthalate,
dioctyl phthalate, etc. In an embodiment where it is desired to
obtain a multi-layer, gel-like sheet having a stable liquid solvent
content, non-volatile liquid solvents such as liquid paraffin can
be used, either alone or in combination with other solvents.
Optionally, a solvent which is miscible with polyethylene in a melt
blended state but solid at room temperature can be used, either
alone or in combination with a liquid solvent. Such solid solvent
can include, e.g., stearyl alcohol, ceryl alcohol, paraffin waxes,
etc. Although it is not critical, it can be more difficult to
evenly stretch the gel-like sheet or resulting membrane when the
solution contains no liquid solvent.
[0073] The viscosity of the liquid solvent is not a critical
parameter. For example, the viscosity of the liquid solvent can
range from about 30 cSt to about 500 cSt, or from about 30 cSt to
about 200 cSt, at 25.degree. C. Although it is not a critical
parameter, when the viscosity at 25.degree. C. is less than about
30 cSt, it can be more difficult to prevent foaming the polyolefin
solution, which can lead to difficulty in blending. On the other
hand, when the viscosity is greater than about 500 cSt, it can be
more difficult to remove the liquid solvent from the multi-layer
microporous polyolefin membrane.
[0074] In an embodiment, the resins, etc., used to produce to the
first polyolefin composition are dry mixed or melt-blended in,
e.g., a double screw extruder or mixer. For example, a conventional
extruder (or mixer or mixer-extruder) such as a double-screw
extruder can be used to combine the resins, etc., to form the first
polyolefin composition. The membrane-forming solvent can be added
to the polyolefin composition (or alternatively to the resins used
to produce the polyolefin composition) at any convenient point in
the process. For example, in an embodiment where the first
polyolefin composition and the first membrane-forming solvent are
melt-blended, the solvent can be added to the polyolefin
composition (or its components) at any of (i) before starting
melt-blending, (ii) during melt blending of the first polyolefin
composition, or (iii) after melt-blending, e.g., by supplying the
first membrane-forming solvent to the melt-blended or partially
melt-blended polyolefin composition in a second extruder or
extruder zone located downstream of the extruder zone used to
melt-blend the polyolefin composition.
[0075] When melt-blending is used, the melt-blending temperature is
not critical. For example, the melt-blending temperature of the
first polyolefin solution can range from about 10.degree. C. higher
than the melting point Tm.sub.1 of the first polyethylene resin to
about 120.degree. C. higher than Tm.sub.1. For brevity, such a
range can be represented as Tm.sub.1+10.degree. C. to
Tm.sub.1+120.degree. C. In an embodiment where the first
polyethylene resin has a melting point of about 130.degree. C. to
about 140.degree. C., the melt-blending temperature can be in the
range of from about 140.degree. C. to about 250.degree. C., or from
about 170.degree. C. to about 240.degree. C.
[0076] When an extruder such as a double-screw extruder is used for
melt-blending, the screw parameters are not critical. For example,
the screw can be characterized by a ratio L/D of the screw length L
to the screw diameter D in the double-screw extruder, which can
range, for example, from about 20 to about 100, or from about 35 to
about 70. Although this parameter is not critical, when L/D is less
than about 20, melt-blending can be more difficult, and when L/D is
more than about 100, faster extruder speeds might be needed to
prevent excessive residence time of the polyolefin solution in the
double-screw extruder (which can lead to undesirable molecular
weight degradation). Although it is not a critical parameter, the
cylinder (or bore) of the double-screw extruder can have an inner
diameter of in the range of about 40 mm to about 100 mm, for
example.
[0077] The amount of the first polyolefin composition in the first
polyolefin solution is not critical. In an embodiment, the amount
of first polyolefin composition in the first polyolefin solution
can range from about 1 wt. % to about 75 wt. %, based on the weight
of the polyolefin solution, for example from about 20 wt. % to
about 70 wt. %. Although the amount of first polyolefin composition
in the first polyolefin solution is not critical, when the amount
is less than about 1 wt. %, it can be more difficult to produce the
multi-layer microporous polyolefin membrane at an acceptably
efficient rate. Moreover, when the amount is less than 1 wt. %, it
can be more difficult to prevent swelling or neck-in at the die
exit during extrusion, which can make it more difficult to form and
support the multi-layer, gel-like sheet, which is a precursor of
the membrane formed during the manufacturing process. On the other
hand, when the amount of first polyolefin composition solution is
greater than about 75 wt. %, it can be more difficult to form the
multi-layer, gel-like sheet. The amount of polymer (e.g., the first
polyethylene resin) in the first polyolefin solution is preferably
in the range of 1 wt. % to 50 wt. %, e.g., 20 wt. % to 40 wt. %,
based on the weight of the first polyolefin solution. When the
amount of polymer is less than 1 wt. %, swelling or neck-in may
occur at the die exit during the extrusion of the first polyolefin
solution to form a gel-like molding, resulting in decrease in the
formability and self-support of the gel-like molding. On the other
hand, when the amount of polymer is more than 50 wt. %, the
formability of the gel-like molding is more difficult.
(2) Preparation of Second Polyolefin Solution
[0078] The second polyolefin solution can be prepared by the same
methods used to prepare the first polyolefin solution. For example,
the second polyolefin solution can be prepared by melt-blending a
second polyolefin composition with a second membrane-forming
solvent. The second membrane-forming solvent can be selected from
among the same solvents as the first membrane-forming solvent. And
while the second membrane-forming solvent can be (and generally is)
selected independently of the first membrane-forming solvent, the
second membrane-forming solvent can be the same as the first
membrane-forming solvent, and can be used in the same relative
concentration as the first membrane-forming solvent is used in the
first polyolefin solution.
[0079] The second polyolefin composition is generally selected
independently of the first polyolefin composition. The second
polyolefin composition comprises the second polyethylene resin and
the second polypropylene resin.
[0080] In an embodiment, the method for preparing the second
polyolefin solution differs from the method for preparing the first
polyolefin solution, only in that the mixing temperature is
preferably in a range from the melting point (Tm2) of the second
polypropylene to Tm2+90.degree. C.
[0081] In an embodiment, the first polyethylene resin is present in
the first polyolefin solution in an amount in the range of from
about 0.5 wt. % to about 75 wt. % based on the total weight of
polyolefin in the first polyolefin solution, and the first
polyolefin solution optionally comprises a first polypropylene
resin, the polypropylene resin being present in the first
polyolefin solution in an amount in the range of from about 0 wt. %
to about 10 wt. % based on the total weight of polyolefin in the
first polyolefin solution.
[0082] In an embodiment, the second polypropylene resin is present
in the second polyolefin solution in an amount in the range of from
about 10 wt. % to about 60 wt. % based on the total weight of
polyolefin in the second polyolefin solution, and the second
polyolefin solution optionally comprises a second polyethylene
resin, the second polyethylene resin being present in the second
polyolefin solution in an amount in the range of from about 40 wt.
% to about 90 wt. % based on the total weight of polyolefin in the
second polyolefin solution.
(3) Extrusion
[0083] In an embodiment, the first polyolefin solution is conducted
from a first extruder to a first die and the second polyolefin
solution is conducted from a second extruder to a second die. A
layered extrudate in sheet form (i.e., a body significantly larger
in the planar directions than in the thickness direction) can be
extruded from the first and second die. Optionally, the first and
second polyolefin solutions are co-extruded from the first and
second die with a planar surface of a first extrudate layer formed
from the first polyolefin solution in contact with a planar surface
of a second extrudate layer formed from the second polyolefin
solution. A planar surface of the extrudate can be defined by a
first vector in the machine direction of the extrudate and a second
vector in the transverse direction of the extrudate.
[0084] In an embodiment, a die assembly is used where the die
assembly comprises the first and second die, as for example when
the first die and the second die share a common partition between a
region in the die assembly containing the first polyolefin solution
and a second region in the die assembly containing the second
polyolefin solution.
[0085] In another embodiment, a plurality of dies is used, with
each die connected to an extruder for conducting either the first
or second polyolefin solution to the die. For example, in one
embodiment, the first extruder containing the first polyolefin
solution is connected to a first die and a third die, and a second
extruder containing the second polyolefin solution is connected to
a second die. As is the case in the preceding embodiment, the
resulting layered extrudate can be co-extruded from the first,
second, and third die (e.g., simultaneously) to form a three-layer
extrudate comprising a first and a third layer constituting surface
layers (e.g., top and bottom layers) produced from the first
polyolefin solution; and a second layer constituting a middle or
intermediate layer of the extrudate situated between and in planar
contact with both surface layers, where the second layer is
produced from the second polyolefin solution.
[0086] In yet another embodiment, the same die assembly is used but
with the polyolefin solutions reversed, i.e., the second extruder
containing the second polyolefin solution is connected to the first
die and the third die, and the first extruder containing the first
polyolefin solution is connected to the second die.
[0087] In any of the preceding embodiments, die extrusion can be
conducted using conventional die extrusion equipment. For example,
extrusion can be conducted by a flat die or an inflation die. In
one embodiment useful for co-extrusion of multi-layer gel-like
sheets, multi-manifold extrusion can be used, in which the first
and second polyolefin solutions are conducted to separate manifolds
in a multi-layer extrusion die and laminated at a die lip inlet. In
another such embodiment, block extrusion can be used, in which the
first and second polyolefin solutions are first combined into a
laminar flow (i.e., in advance), with the laminar flow then
connected to a die. Because multi-manifold and block processes are
known to those skilled in the art of processing polyolefin films
(e.g., as disclosed in JP06-122142 A, JP06-106599A), they are
deemed conventional, therefore, their operation will be not
described in detail.
[0088] Die selection is not critical, and, e.g., a conventional
multi-layer-sheet-forming, flat or inflation die can be used. Die
gap is not critical. For example, the multi-layer-sheet-forming
flat die can have a die gap of about 0.1 mm to about 5 mm. Die
temperature and extruding speed are also non-critical parameters.
For example, the die can be heated to a die temperature ranging
from about 140.degree. C. to about 250.degree. C. during extrusion.
The extruding speed can range, for example, from about 0.2 m/minute
to about 15 m/minute. The thickness of the layers of the layered
extrudate can be independently selected. For example, the gel-like
sheet can have relatively thick surface layers (or "skin" layers)
compared to the thickness of an intermediate layer of the layered
extrudate.
[0089] While the extrusion has been described in terms of
embodiments producing two and three-layer extrudates, the extrusion
step is not limited thereto. For example, a plurality of dies
and/or die assemblies can be used to produce multi-layer extrudates
having four or more layers using the extrusion methods of the
preceding embodiments. In such a layered extrudate, each surface or
intermediate layer can be produced using either the first
polyolefin solution and/or the second polyolefin solution.
(4) Formation of a Multi-Layer, Gel-Like Sheet
[0090] The multi-layer extrudate can be formed into a multi-layer,
gel-like sheet by cooling, for example. Cooling rate and cooling
temperature are not particularly critical. For example, the
multi-layer, gel-like sheet can be cooled at a cooling rate of at
least about 50.degree. C./minute until the temperature of the
multi-layer, gel-like sheet (the cooling temperature) is
approximately equal to the multi-layer, gel-like sheet's gelation
temperature (or lower). In an embodiment, the extrudate is cooled
by exposing the extrudate to a temperature of about 25.degree. C.
or lower in order to form the multi-layer gel-like sheet. While not
wishing to be bound by any theory or model, it is believed that
cooling the layered extrudate sets the polyolefin micro-phases of
the first and second polyolefin solutions for separation by the
membrane-forming solvent or solvents. It has been observed that in
general a slower cooling rate (e.g., less than 50.degree.
C./minute) provides the multi-layer, gel-like sheet with larger
pseudo-cell units, resulting in a coarser higher-order structure.
On the other hand, a relatively faster cooling rate (e.g.,
80.degree. C./minute) results in denser cell units. Although it is
not a critical parameter, when the cooling rate of the extrudate is
less than 50.degree. C./minute, increased polyolefin crystallinity
in the layer can result, which can make it more difficult to
process the multi-layer, gel-like sheet in subsequent stretching
steps. The choice of cooling method is not critical. For example
conventional sheet cooling methods can be used. In an embodiment,
the cooling method comprises contacting the layered extrudate with
a cooling medium such as cooling air, cooling water, etc.
Alternatively, the extrudate can be cooled via contact with rollers
cooled by a cooling medium, etc.
(5) Removal of the First and Second Membrane-Forming Solvents
[0091] In an embodiment, at least a portion of the first and second
membrane-forming solvents are removed (or displaced) from the
multi-layer gel-like sheet in order to form a solvent-removed
gel-like sheet. A displacing (or "washing") solvent can be used to
remove (wash away, or displace) the first and second
membrane-forming solvents. While not wishing to be bound by any
theory or model, it is believed that because the polyolefin phases
in the multi-layer gel-like sheet produced from the first
polyolefin solution and the second polyolefin solution (i.e., the
first polyolefin and the second polyolefin) are separated from the
membrane-forming solvent phase, the removal of the membrane-forming
solvent provides a porous membrane constituted by fibrils forming a
fine three-dimensional network structure and having pores
communicating three-dimensionally and irregularly. The choice of
washing solvent is not critical provided it is capable of
dissolving or displacing at least a portion of the first and/or
second membrane-forming solvent. Suitable washing solvents include,
for instance, one or more of volatile solvents such as saturated
hydrocarbons such as pentane, hexane, heptane, etc.; chlorinated
hydrocarbons such as methylene chloride, carbon tetrachloride,
etc.; ethers such as diethyl ether, dioxane, etc.; ketones such as
methyl ethyl ketone, etc.; linear fluorocarbons such as
trifluoroethane, C.sub.6F.sub.14, C.sub.7F.sub.16, etc.; cyclic
hydrofluorocarbons such as C.sub.5H.sub.3F.sub.7, etc.;
hydrofluoroethers such as C.sub.4F.sub.9OCH.sub.3,
C.sub.4F.sub.9OC.sub.2H.sub.5, etc.; and perfluoroethers such as
C.sub.4F.sub.9OCF.sub.3, C.sub.4F.sub.9OC.sub.2F.sub.5, etc.
[0092] The method for removing the membrane-forming solvent is not
critical, and any method capable of removing a significant amount
of solvent can be used, including conventional solvent-removal
methods. For example, the multi-layer, gel-like sheet can be washed
by immersing the sheet in the washing solvent and/or showering the
sheet with the washing solvent. The amount of washing solvent used
is not critical, and will generally depend on the method selected
for removal of the membrane-forming solvent. For example, the
amount of washing solvent used can range from about 300 to about
30,000 parts by mass, based on the mass of the gel-like sheet.
While the amount of membrane-forming solvent removed is not
particularly critical, generally a higher quality (more porous)
membrane will result when at least a major amount of first and
second membrane-forming solvent is removed from the gel-like sheet.
In an embodiment, the membrane-forming solvent is removed from the
gel-like sheet (e.g., by washing) until the amount of the remaining
membrane-forming solvent in the multi-layer gel-like sheet becomes
less than 1 wt. %, based on the weight of the gel-like sheet.
(6) Drying of the Solvent-Removed Gel-Like Sheet
[0093] In an embodiment, the solvent-removed multi-layer, gel-like
sheet obtained by removing at least a portion of the
membrane-forming solvent is dried in order to remove the washing
solvent. Any method capable of removing the washing solvent can be
used, including conventional methods such as heat-drying,
wind-drying (moving air), etc. The temperature to which the
gel-like sheet is exposed during drying (i.e., drying temperature)
is not critical. For example, the drying temperature can be equal
to or lower than the crystal dispersion temperature Tcd. Tcd is the
lower of the crystal dispersion temperature Tcd.sub.1 of the first
polyethylene resin and the crystal dispersion temperature Tcd.sub.2
of the second polyethylene resin (when used). For example, the
drying temperature can be at least 5.degree. C. below the crystal
dispersion temperature Tcd. The crystal dispersion temperature of
the first and second polyethylene resin can be determined by
measuring the temperature characteristics of the kinetic
viscoelasticity of the polyethylene resin according to ASTM D 4065.
In an embodiment, at least one of the first or second polyethylene
resins have a crystal dispersion temperature in the range of about
90.degree. C. to about 100.degree. C.
[0094] Although it is not critical, drying can be conducted until
the amount of remaining washing solvent is about 5 wt. % or less on
a dry basis, i.e., based on the weight of the dry multi-layer,
microporous polyolefin membrane. In another embodiment, drying is
conducted until the amount of remaining washing solvent is about 3
wt. % or less on a dry basis. Insufficient drying can be recognized
because it generally leads to an undesirable decrease in the
porosity of the multi-layer, microporous membrane. If this is
observed, an increased drying temperature and/or drying time should
be used. Removal of the washing solvent, e.g., by drying or
otherwise, results in the formation of the multi-layer, microporous
polyolefin membrane.
(7) Stretching
[0095] Prior to the step for removing the membrane-forming solvents
(namely prior to step 5), the multi-layer, gel-like sheet can be
stretched in order to obtain a stretched, multi-layer, gel-like
sheet. It is believed that the presence of the first and second
membrane-forming solvents in the multi-layer, gel-like sheet
results in a relatively uniform stretching magnification. Heating
the multi-layer, gel-like sheet, especially at the start of
stretching or in a relatively early stage of stretching (e.g.,
before 50% of the stretching has been completed) is also believed
to aid the uniformity of stretching.
[0096] Neither the choice of stretching method nor the degree of
stretching magnification is particularly critical. For example, any
method capable of stretching the multi-layer, gel-like sheet to a
predetermined magnification (including any optional heating) can be
used. In an embodiment, the stretching can be accomplished by one
or more of tenter-stretching, roller-stretching, or inflation
stretching (e.g., with air). Although the choice is not critical,
the stretching can be conducted monoaxially (i.e., in either the
machine or transverse direction) or biaxially (both the machine or
transverse direction). In an embodiment, biaxial stretching is
used. In the case of biaxial stretching (also called biaxial
orientation), the stretching can be simultaneous biaxial
stretching, sequential stretching along one planar axis and then
the other (e.g., first in the transverse direction and then in the
machine direction), or multi-stage stretching (for instance, a
combination of the simultaneous biaxial stretching and the
sequential stretching). In an embodiment, simultaneous biaxial
stretching is used.
[0097] The stretching magnification is not critical. In an
embodiment where monoaxial stretching is used, the linear
stretching magnification can be, e.g., about 2 fold or more, or
about 3 to about 30 fold. In an embodiment where biaxial stretching
is used, the linear stretching magnification can be, e.g., about 3
fold or more in any planar direction. In another embodiment, the
area magnification resulting from stretching is at least about 9
fold, or at least about 16 fold, or at least about 25 fold.
Although it is not a critical parameter, when the stretching
results in an area magnification of at least about 9 fold, the
multi-layer microporous polyolefin membrane has a relatively higher
pin puncture strength. When attempting an area magnification of
more than about 400 fold, it can be more difficult to operate the
stretching apparatus.
[0098] The temperature to which the multi-layer, gel-like sheet is
exposed during stretching (namely the stretching temperature) is
not critical. In an embodiment, the temperature of the gel-like
sheet during stretching can be about (Tm+10.degree. C.) or lower,
or optionally in a range that is higher than Tcd but lower than Tm,
wherein Tm is the lesser of the melting point Tm.sub.1 of the first
polyethylene and the melting point Tm.sub.2 of the second
polyethylene (when used). Although this parameter is not critical,
when the stretching temperature is higher than approximately the
melting point Tm+10.degree. C., at least one of the first or second
polyethylene can be in the molten state, which can make it more
difficult to orient the molecular chains of the polyolefin in the
multi-layer gel-like sheet during stretching. And when the
stretching temperature is lower than approximately Tcd, at least
one of the first or second polyethylene can be so insufficiently
softened that it is difficult to stretch the multi-layer, gel-like
sheet without breakage or tears, which can result in a failure to
achieve the desired stretching magnification. In an embodiment, the
stretching temperature ranges from about 90.degree. C. to about
140.degree. C., or from about 100.degree. C. to about 130.degree.
C.
[0099] While not wishing to be bound by any theory or model, it is
believed that such stretching causes cleavage between polyethylene
lamellas, making the polyethylene phases finer and forming large
numbers of fibrils. The fibrils form a three-dimensional network
structure (three-dimensionally irregularly connected network
structure). Consequently, the stretching when used generally makes
it easier to produce a relatively high-mechanical strength
multi-layer, microporous polyolefin membrane with a relatively
large pore size. Such multi-layer, microporous membranes are
believed to be particularly suitable for use as battery
separators.
[0100] Optionally, stretching can be conducted in the presence of a
temperature gradient in a thickness direction (i.e., a direction
approximately perpendicular to the planar surface of the
multi-layer, microporous polyolefin membrane). In this case, it can
be easier to produce a multi-layer, microporous polyolefin membrane
with improved mechanical strength. The details of this method are
described in Japanese Patent 3347854.
(8) Hot Solvent Treatment Step
[0101] Although it is not required, the multi-layer, gel-like sheet
can be treated with a hot solvent between steps (4) and (5). When
used, it is believed that the hot solvent treatment provides the
fibrils (such as those formed by stretching the multi-layer
gel-like sheet) with a relatively thick leaf-vein-like structure.
Such a structure, it is believed, makes it less difficult to
produce a multi-layer, microporous membrane having large pores with
relatively high strength and permeability. The term
"leaf-vein-like" means that the fibrils have thick trunks and thin
fibers extending therefrom in a network structure. The details of
this method are described in WO 2000/20493.
(9) Stretching of Multi-Layer, Microporous Membrane ("Dry
Stretching")
[0102] In an embodiment, the dried multi-layer, microporous
membrane of step (6) can be stretched, at least monoaxially. The
stretching method selected is not critical, and conventional
stretching methods can be used such as by a tenter method, etc.
While it is not critical, the membrane can be heated during
stretching. While the choice is not critical, the stretching can be
monoaxial or biaxial. When biaxial stretching is used, the
stretching can be conducted simultaneously in both axial
directions, or, alternatively, the multi-layer, microporous
polyolefin membrane can be stretched sequentially, e.g., first in
the machine direction and then in the transverse direction. In an
embodiment, simultaneous biaxial stretching is used. When the
multi-layer gel-like sheet has been stretched as described in step
(7) the stretching of the dry multi-layer, microporous polyolefin
membrane in step (9) can be called dry-stretching, re-stretching,
or dry-orientation.
[0103] The temperature to which the dry multi-layer, microporous
membrane is exposed during stretching (the "dry stretching
temperature") is not critical. In an embodiment, the dry stretching
temperature is approximately equal to the melting point Tm or
lower, for example in the range of from about the crystal
dispersion temperature Tcd to the about the melting point Tm. When
the dry stretching temperature is higher than Tm, it can be more
difficult to produce a multi-layer, microporous polyolefin membrane
having a relatively high compression resistance with relatively
uniform air permeability characteristics, particularly in the
transverse direction when the dry multi-layer, microporous
polyolefin membrane is stretched transversely. When the stretching
temperature is lower than Tcd, it can be more difficult to
sufficiently soften the first and second polyolefins, which can
lead to tearing during stretching, and a lack of uniform
stretching. In an embodiment, the dry stretching temperature ranges
from about 90.degree. C. to about 135.degree. C., or from about
95.degree. C. to about 130.degree. C.
[0104] When dry-stretching is used, the stretching magnification is
not critical. For example, the stretching magnification of the
multi-layer, microporous membrane can range from about 1.1 fold to
about 1.8 fold in at least one planar (e.g., lateral) direction.
Thus, in the case of monoaxial stretching, the stretching
magnification can range from about 1.1 fold to about 1.8 fold in
the longitudinal direction (i.e., the "machine direction") or the
transverse direction, depending on whether the membrane is
stretched longitudinally or transversely. Monoaxial stretching can
also be accomplished along a planar axis between the longitudinal
and transverse directions.
[0105] In an embodiment, biaxial stretching is used (i.e.,
stretching along two planar axis) with a stretching magnification
of about 1.1 fold to about 1.8 fold along both stretching axes,
e.g., along both the longitudinal and transverse directions. The
stretching magnification in the longitudinal direction need not be
the same as the stretching magnification in the transverse
direction. In other words, in biaxial stretching, the stretching
magnifications can be selected independently. In an embodiment, the
dry-stretching magnification is the same in both stretching
directions. If desired, the membrane can be stretched to a
magnification that is larger than 1.8 fold, particularly when
during subsequent processing (e.g., heat treatment) the membrane
relaxes (or shrinks) in the direction(s) of stretching to a achieve
a final magnification of about 1.1 to about 1.8 fold compared to
the size of the film at the start of the dry orientation step.
(10) Heat Treatment
[0106] In an embodiment, the dried multi-layer, microporous
membrane can be heat-treated following step (6). It is believed
that heat-treating stabilizes the polyolefin crystals in the dried
multi-layer, microporous polyolefin membrane to form uniform
lamellas. In an embodiment, the heat treatment comprises
heat-setting and/or annealing. When heat-setting is used, it can be
conducted using conventional methods such as tenter methods and/or
roller methods. Although it is not critical, the temperature to
which the dried multi-layer, microporous polyolefin membrane is
exposed during heat-setting (i.e., the "heat-setting temperature")
can range from the Tcd to about the Tm. In an embodiment, the
heat-setting temperature ranges from about the dry stretching
temperature of the multi-layer, microporous polyolefin membrane
.+-.5.degree. C., or about the dry stretching temperature of the
multi-layer, microporous polyolefin membrane .+-.3.degree. C.
[0107] Annealing differs from heat-setting in that it is a heat
treatment with no load applied to the multi-layer, microporous
polyolefin membrane. The choice of annealing method is not
critical, and it can be conducted, for example, by using a heating
chamber with a belt conveyer or an air-floating-type heating
chamber. Alternatively, the annealing can be conducted after the
heat-setting with the tenter clips slackened. The temperature of
the multi-layer, microporous polyolefin membrane during annealing
(i.e., the annealing temperature) is not critical. In an
embodiment, the annealing temperature ranges from about the melting
point Tm or lower, or in a range from about 60.degree. C. to
(Tm-10.degree. C.). It is believed that annealing makes it less
difficult to produce a multi-layer, microporous polyolefin membrane
having relatively high permeability and strength.
(11) Cross-Linking
[0108] In an embodiment, the multi-layer, microporous polyolefin
membrane can be cross-linked (e.g., by ionizing radiation rays such
as .alpha.-rays, .beta.-rays, .gamma.-rays, electron beams, etc.)
after step (6). For example, when irradiating electron beams are
used for cross-linking, the amount of electron beam radiation can
be about 0.1 Mrad to about 100 Mrad, using an accelerating voltage
in the range of about 100 kV to about 300 kV. It is believed that
the cross-linking treatment makes it less difficult to produce a
multi-layer, microporous polyolefin membrane with relatively high
meltdown temperature.
(12) Hydrophilizing Treatment
[0109] In an embodiment, the multi-layer, microporous polyolefin
membrane can be subjected to a hydrophilic treatment (i.e., a
treatment which makes the multi-layer, microporous polyolefin
membrane more hydrophilic). The hydrophilic treatment can be, for
example, a monomer-grafting treatment, a surfactant treatment, a
corona-discharging treatment, etc. In an embodiment, the
monomer-grafting treatment is used after the cross-linking
treatment.
[0110] When a surfactant treatment is used, any of nonionic
surfactants, cationic surfactants, anionic surfactants and
amphoteric surfactants can be used, for example, either alone or in
combination. In an embodiment, a nonionic surfactant is used. The
choice of surfactant is not critical. For example, the multi-layer,
microporous polyolefin membrane can be dipped in a solution of the
surfactant and water or a lower alcohol such as methanol, ethanol,
isopropyl alcohol, etc., or coated with the solution, e.g., by a
doctor blade method.
B. Second Production Method
[0111] The second method for producing the multi-layer, microporous
polyolefin membrane comprises the steps of (1) combining (e.g., by
melt-blending) a first polyolefin composition and a first
membrane-forming solvent to prepare a first polyolefin solution,
(2) combining a second polyolefin composition and a second
membrane-forming solvent to prepare a second polyolefin solution,
(3) extruding the first polyolefin solution through a first die and
the second solution through a second die and then laminating the
extruded first and second polyolefin solutions to form a
multi-layer extrudate, (4) cooling the multi-layer extrudate to
form a multi-layer, gel-like sheet, (5) removing at least a portion
of the membrane-forming solvent from the multi-layer, gel-like
sheet to form a solvent-removed gel-like sheet, and (6) drying the
solvent-removed gel-like sheet in order to form the multi-layer,
microporous membrane. An optional stretching step (7), and an
optional hot solvent treatment step (8), etc., can be conducted
between steps (4) and (5), if desired. After step (6), an optional
step (9) of stretching a multi-layer, microporous membrane, an
optional heat treatment step (10), an optional cross-linking step
with ionizing radiations (11), and an optional hydrophilic
treatment step (12), etc., can be conducted.
[0112] The process steps and conditions of the second production
method are generally the same as those of the analogous steps
described in connection with the first production method, except
for step (3). Consequently, step (3) will be explained in more
detail.
[0113] The type of die used is not critical provided the die is
capable of forming an extrudate that can be laminated. In one
embodiment, sheet dies (which can be adjacent or connected) are
used to form the extrudates. The first and second sheet dies are
connected to first and second extruders, respectively, where the
first extruder contains the first polyolefin solution and the
second extruder contains the second polyolefin solution. While not
critical, lamination is generally easier to accomplish when the
extruded first and second polyolefin solution are still at
approximately the extrusion temperature. The other conditions may
be the same as in the first method.
[0114] In another embodiment, the first, second, and third sheet
dies are connected to first, second and third extruders, where the
first and third sheet dies contain the first polyolefin solutions,
and the second sheet die contains the second polyolefin solution.
In this embodiment, a laminated extrudate is formed constituting
outer layers comprising the extruded first polyolefin solution and
one intermediate comprising the extruded second polyolefin
solution.
[0115] In yet another embodiment, the first, second, and third
sheet dies are connected to first, second, and third extruders,
where the second sheet die contains the first polyolefin solution,
and the first and third sheet dies contain the second polyolefin
solution. In this embodiment, a laminated extrudate is formed
constituting outer layers comprising the extruded second polyolefin
solution and one intermediate comprising extruded first polyolefin
solution.
C. Third Production Method
[0116] The third method for producing the multi-layer, microporous
polyolefin membrane comprises the steps of (1) combining (e.g., by
melt-blending) a first polyolefin composition and a
membrane-forming solvent to prepare a first polyolefin solution,
(2) combining a second polyolefin composition and a second
membrane-forming solvent to prepare a second polyolefin solution,
(3) extruding the first polyolefin solution through at least one
first die to form at least one first extrudate, (4) extruding the
second polyolefin solution through at least one second die to form
at least one second extrudate, (5) cooling first and second
extrudates to form at least one first gel-like sheet and at least
one second gel-like sheet, (6) laminating the first and second
gel-like sheet to form a multi-layer, gel-like sheet, (7) removing
at least a portion of the membrane-forming solvent from the
resultant multi-layer, gel-like sheet to form a solvent-removed
gel-like sheet, and (8) drying the solvent-removed gel-like sheet
in order to form the multi-layer, microporous membrane. An optional
stretching step (9), and an optional hot solvent treatment step
(10), etc., can be conducted between steps (5) and (6) or between
steps (6) and (7), if desired. After step (8), an optional step
(11) of stretching a multi-layer, microporous membrane, an optional
heat treatment step (12), an optional cross-linking step with
ionizing radiations (13), and an optional hydrophilic treatment
step (14), etc., can be conducted.
[0117] The main difference between the third production method and
the second production method is in the order of the steps for
laminating and cooling.
[0118] In the second production method, laminating the first and
second polyolefin solutions is conducted before the cooling step.
In the third production method, the first and second polyolefin
solutions are cooled before the laminating step.
[0119] The steps of (1), (2), (7) and (8) in the third production
method can be the same as the steps of (1), (2), (5) and (6) in the
first production method as described above. For the extrusion of
the first polyolefin solution through the first die, the conditions
of step (3) of the second production method can be used for step
(3) of the third production method. For the extrusion of the second
solution through the second die, the conditions of step (4) in the
third production method can be the same as the conditions of step
(3) in the second production method. In one embodiment, either the
first or second polyolefin solution is extruded through a third
die. In this way, a multi-layer laminate can be formed having two
layers produced from the first polyolefin solution and a single
layer produced from the second polyolefin solution, or vice
versa.
[0120] Step (5) of the third production method can be the same as
step (4) in the first production method except that in the third
production method the first and second gel-like sheets are formed
separately.
[0121] The step (6) of laminating the first and second gel-like
sheets will now be explained in more detail. The choice of
lamination method is not particularly critical, and conventional
lamination methods such as heat-induced lamination can be used to
laminate the multi-layer gel-like sheet. Other suitable lamination
methods include, for example, heat-sealing, impulse-sealing,
ultrasonic-bonding, etc., either alone or in combination.
Heat-sealing can be conducted using, e.g., one or more pair of
heated rollers where the gel-like sheets are conducted through at
least one pair of the heated rollers. Although the heat-sealing
temperature and pressure are not particularly critical, sufficient
heating and pressure should be applied for a sufficient time to
ensure that the gel-like sheets are appropriately bonded to provide
a multi-layer, microporous membrane with relatively uniform
properties and little tendency toward delamination. In an
embodiment, the heat-sealing temperature can be, for instance,
about 90.degree. C. to about 135.degree. C., or from about
90.degree. C. to about 115.degree. C. In an embodiment, the
heat-sealing pressure can be from about 0.01 MPa to about -50
MPa.
[0122] As is the case in the first and second production method,
the thickness of the layers formed from the first and second
polyolefin solution (i.e., the layers comprising the first and
second microporous layer materials) can be controlled by adjusting
the thickness of the first and second gel-like sheets and by the
amount of stretching (stretching magnification and dry stretching
magnification), when one or more stretching steps are used.
Optionally, the lamination step can be combined with a stretching
step by passing the gel-like sheets through multi-stages of heated
rollers.
[0123] In an embodiment, the third production method forms a
multi-layer, polyolefin gel-like sheet having at least three
layers. For example, after cooling two extruded first polyolefin
solutions and one extruded second polyolefin solution to form the
gel-like sheets, the multi-layer gel-like sheet can be laminated
with outer layers comprising the extruded first polyolefin solution
and an intermediate layer comprising the extruded second polyolefin
solution. In another embodiment, after cooling two extruded second
polyolefin solutions and one extruded first polyolefin solution to
form the gel-like sheets, the multi-layer gel-like sheet can be
laminated with outer layers comprising the extruded second
polyolefin solution and an intermediate layer comprising the
extruded first polyolefin solution.
[0124] The stretching step (9) and the hot solvent treatment step
(10) can be the same as the stretching step (7) and the hot solvent
treatment step (8) as described for the first production method,
except stretching step (9) and hot solvent treatment step (10) are
conducted on the first and/or second gel-like sheets. The
stretching temperatures of the first and second gel-like sheets are
not critical. For example, the stretching temperatures of the first
gel-like sheet can be, e.g., Tm.sub.1+10.degree. C. or lower, or
optionally about Tcd.sub.1 or higher but lower than about Tm.sub.1.
The stretching temperature of the second gel-like sheet can be,
e.g., Tm.sub.2+10.degree. C. or lower, or optionally about
Tcd.sub.2 or higher but lower than about Tm.sub.2.
D. Fourth Production Method
[0125] The fourth method for producing the multi-layer, microporous
polyolefin membrane comprises the steps of (1) combining (e.g., by
melt-blending) a first polyolefin composition and a
membrane-forming solvent to prepare a first polyolefin solution,
(2) combining a second polyolefin composition and a second
membrane-forming solvent to prepare a second polyolefin solution,
(3) extruding the first polyolefin solution through at least one
first die to form at least one first extrudate, (4) extruding the
second polyolefin solution through at least one second die to form
at least one second extrudate, (5) cooling first and second
extrudates to form at least one first gel-like sheet and at least
one second gel-like sheet, (6) removing at least a portion of the
first and second membrane-forming solvents from the first and
second gel-like sheets to form solvent-removed first and second
gel-like sheets, (7) drying the solvent-removed first and second
gel-like sheets to form at least one first polyolefin membrane and
at least one second polyolefin membrane, and (8) laminating the
first and second microporous polyolefin membranes in order to form
the multi-layer, microporous polyolefin membrane.
[0126] A stretching step (9), a hot solvent treatment step (10),
etc., can be conducted between steps (5) and (6), if desired. A
stretching step (11), a heat treatment step (12), etc., can be
conducted between steps (7) and (8), if desired. After step (8), a
step (13) of stretching a multi-layer, microporous membrane, a heat
treatment step (14), a cross-linking step with ionizing radiations
(15), a hydrophilic treatment step (16), etc., can be conducted if
desired.
[0127] Steps (1) and (2) in the fourth production method can be
conducted under the same conditions as steps of (1) and (2) in the
first production method. Steps (3), (4), and (5) in the fourth
production method can be conducted under the same conditions as
steps (3), (4), and (5) in the third method. Step (6) in the fourth
production method can be conducted under the same conditions as
step (5) in the first production method except for removing the
membrane-forming solvent from the first and second gel-like sheets.
Step (7) in the fourth production method can be conducted under the
same conditions as step (6) in the first production method except
that in the fourth production method the first and second
solvent-removed gel-like sheets are dried separately. Step (8) in
the fourth production method can be conducted under the same
conditions as the step (6) in the third production method except
for laminating the first and second polyolefin microporous
membranes. The stretching step (9) and the hot solvent treatment
step (10) in the fourth production method can be conducted under
the same conditions as step (9) and (10) in the third production
method. The stretching step (11) and the heat treatment step (12)
in the fourth production method can be conducted under the same
conditions as steps (9) and (10) in the first production method
except that in the fourth production method the first and second
polyolefin microporous membranes are stretched and/or heat
treated.
[0128] In an embodiment, in the stretching step (11) in the fourth
production method, the stretching temperature of the first
polyolefin microporous membranes can be about Tm.sub.1 or lower, or
optionally about Tcd.sub.1 to about Tm.sub.1, and the stretching
temperature of the second polyolefin microporous membrane can be
about Tm.sub.2 or lower, or optionally about Tcd.sub.2 to about
Tm.sub.2.
[0129] In an embodiment, the heat treatment step (12) in the fourth
production method can be HS and/or annealing. For example, in the
heat treatment step (12) in the fourth production method, the
heat-setting temperature of the first polyolefin microporous
membranes can be about Tcd.sub.1 to about Tm.sub.1, or optionally
about the dry stretching temperature .+-.5.degree. C., or
optionally about the dry stretching temperature .+-.3.degree. C. In
an embodiment, in the heat treatment step (12) in the fourth
production method, the heat-setting temperature of the second
microporous membrane can be about Tcd.sub.2 to about Tm.sub.2, or
optionally the dry stretching temperature .+-.5.degree. C., or
optionally the dry stretching temperature .+-.3.degree. C. When the
HS is used, it can be conducted by, e.g., a tenter method or a
roller method.
[0130] In an embodiment, in the heat treatment step (12) in the
fourth production method, the annealing temperature of the first
microporous membrane can be about Tm.sub.1 or lower, or optionally
about 60.degree. C. to about (Tm.sub.1-10.degree. C.). In an
embodiment, in the heat treatment step (12) in the fourth
production method, the annealing temperature of the second
microporous membranes can be about Tm.sub.2 or lower, or optionally
about 60.degree. C. to about (Tm.sub.2-10.degree. C.).
[0131] The conditions in step (13), stretching a multi-layer,
microporous membrane, a heat treatment step (14), a cross-linking
step with ionizing radiations (15), and a hydrophilic treatment
step (16) in the fourth production method can be the same as those
for steps (9), (10), (11) and (12) in the first production
method.
[4] The Properties of a Multi-Layer, Microporous Polyolefin
Membrane
[0132] In an embodiment, the multi-layer, microporous polyolefin
membrane has a thickness ranging from about 3 .mu.m to about 200
.mu.m, or about 5 .mu.m to about 50 .mu.m. Optionally, when the
membrane is a multi-layer, microporous polyolefin membrane, it has
one or more of the following characteristics.
A. Porosity of about 25% to about 80%
[0133] When the porosity is less than 25%, the multi-layer,
microporous polyolefin membrane generally does not exhibit the
desired air permeability for use as a battery separator. When the
porosity exceeds 80%, it is more difficult to produce a battery
separator of the desired strength, which can increase the
likelihood of internal electrode short-circuiting. In an
embodiment, the membrane is characterized by or has a least one
layer characterized by a hybrid structure, i.e., a relatively broad
distribution of differential pore volume as a function of pore
diameter. For example, the differential pore volume as a function
of pore diameter can have two or more peaks or modes.
B. Weight Per Unit Area of about 5 g/m.sup.2 to 19 g/m.sup.2 at
25-.mu.m Thickness
[0134] When the basis weight of the multi-layer, microporous
polyolefin membrane ranges from about 5 g/m.sup.2 to 19 g/m.sup.2
at 25-.mu.m thickness, the membrane has appropriate porosity.
C. Air Permeability of about 20 seconds/100 cm.sup.3 to about 700
Seconds/100 cm.sup.3 (Normalized to Value at 25-.mu.m
Thickness)
[0135] In an embodiment, the membrane's normalized air permeability
is .ltoreq.350 seconds/100 cm.sup.3, e.g. in the range of 100
seconds/100 cm.sup.3 to 340 seconds/100 cm.sup.3. When the
normalized air permeability of the membrane (as measured according
to JIS P8117) ranges from about 20 seconds/100 cm.sup.3 to about
700 seconds/100 cm.sup.3, it is less difficult to form batteries
having the desired charge storage capacity and desired cyclability.
When the air permeability is less than about 20 seconds/100
cm.sup.3, it is more difficult to produce a battery having the
desired shutdown characteristics, particularly when the
temperatures inside the batteries are elevated. Air permeability
P.sub.1 measured on a multi-layer, microporous membrane having a
thickness T.sub.1 (in .mu.m) according to JIS P8117 can be
normalized to air permeability AP.sub.2 at a thickness of 25 .mu.m
by the equation of AP.sub.2=(AP.sub.1.times.25)/T.sub.1.
[0136] In an embodiment, the membrane has Normalized Air
Permeability satisfying the relationship A.ltoreq.(M*P)-I where A
is the microporous membrane's Normalized Air Permeability expressed
in units of sec/100 cm.sup.3 and normalized to a 25 .mu.m membrane
thickness and P is the microporous membrane's Normalized Pin
Puncture Strength expressed in units of mN and normalized to a 25
.mu.m membrane thickness. M is a slope (using the axes and units of
FIG. 1) in the range of about 0.09 to about 0.1, or about 0.95 to
about 0.99. In an embodiment, M is equal to 0.097. "I" is an
intercept on the Y axis (using the axes and units of FIG. 1) that
is .gtoreq.100, e.g., .gtoreq.110, such as .gtoreq. or 150, or
.gtoreq.200, or .gtoreq.250; or, e.g., in the range of about 100 to
about 250, such as from about 110 to about 240.
[0137] In another embodiment, the membrane has a Normalized Air
Permeability and Normalized Pin Puncture Strength that fall on or
within the boundary of the ellipse shown in FIG. 1. In yet another
embodiment, the membrane has an Air Permeability satisfying the
relationship
(M.sub.2*P)-I.sub.2.ltoreq.A.ltoreq.(M.sub.1*P)-I.sub.1.
[0138] M.sub.1 and M.sub.2 are independently selected and can each
be, e.g., in the range of about 0.09 to about 0.1, or about 0.95 to
about 0.99. In an embodiment, M.sub.1 and M.sub.2 are equal. For
example, M.sub.1 and M.sub.2 can be 0.097; and "I.sub.1" can be,
e.g., in the range of about 100 to about 240, such as from about
110 to about 230. In an embodiment, I.sub.1 is 110. "I.sub.2" can
be, e.g., .gtoreq.260. For example, I.sub.2 can be in the range of
about 260 to about 450. Units and axes are the same as in FIG.
1.
D. Pin Puncture Strength of about 3,000 mN/25 .mu.m or More
[0139] The pin puncture strength (normalized to the value at a
25-.mu.m membrane thickness) is the maximum load measured when the
multi-layer, microporous polyolefin membrane is pricked with a
needle 1 mm in diameter with a spherical end surface (radius R of
curvature: 0.5 mm) at a speed of 2 mm/second. When the pin puncture
strength of the multi-layer, microporous polyolefin membrane is
less than 3,000 mN/25 .mu.m, it is more difficult to produce a
battery having the desired mechanical integrity, durability, and
toughness. In an embodiment, the membrane's Normalized Pin Puncture
strength is in the range of 4500 mN/25.mu. to 600 mN/25 g.
E. Heat Shrinkage Ratio of 10% or Less
[0140] When the heat shrinkage ratio measured after holding the
multi-layer, microporous membrane at a temperature of about
105.degree. C. for 8 hours exceeds 10% in both longitudinal and
transverse directions, it is more difficult to produce a battery
that will not exhibit internal short-circuiting when the heat
generated in the battery results in the shrinkage of the
separators. In an embodiment, the membrane's heat shrinkage at
105.degree. C. is in the range of 1% to 5% (MD) and 1% to 5% (TD),
such as 1% to 3% (TD).
F. Shutdown Temperature of about 140.degree. C. or Lower
[0141] When the shutdown temperature of the multi-layer,
microporous polyolefin membrane exceeds 140.degree. C., it is more
difficult to produce a battery separator with the desired shutdown
response when the battery is overheated. One way to determine
shutdown temperature involves determining the temperature at a
point of inflection observed near the melting point of the
multi-layer, microporous polyolefin membrane, under the condition
that a test piece of 3 mm in the longitudinal direction and 10 mm
in the transverse direction is heated from room temperature at a
speed of 5.degree. C./minute while drawing the test piece in the
longitudinal direction under a load of 2 g. In an embodiment, the
shutdown temperature is in the range of about 120-140.degree.
C.
G. Rupture Temperature of at least about 180.degree. C.
[0142] The microporous membrane should have a rupture temperature
of about 180.degree. C. or higher, or about 185.degree. C. or
higher, or about 190.degree. C. or higher. In an embodiment, the
rupture temperature is in the range of about 180.degree. C. to
about 195.degree. C., or about 185.degree. C. to about 190.degree.
C. Rupture temperature can be measured as follows. A microporous
membrane of 5 cm.times.5 cm is sandwiched by blocks each having a
circular opening of 12 mm in diameter, and a tungsten carbide ball
of 10 mm in diameter was placed on the microporous membrane in the
circular opening. While heating at a temperature-elevating speed of
5.degree. C./minute, the temperature at which the microporous
polyolefin membrane is ruptured by melting is measured and recorded
as the Rupture temperature. FIG. 2 shows that when the total amount
polypropylene in the membrane is 2 wt. % or higher, based on the
total weight of the membrane, the membrane's Rupture temperature
180.degree. C. or higher. In an embodiment, the total amount
polypropylene in the membrane having [0143] (a) an Mw of 600,000 or
higher, [0144] (b) a molecular weight distribution in the range of
3 to 10, and [0145] (c) a heat of fusion of 90 J/g or higher [0146]
is 2 wt. % or higher, or 2.05 wt. % or higher, or 2.1 wt. % or
higher, based on the total weight of the membrane.
H. Maximum Shrinkage in Molten State of 30% or Less
[0147] The multi-layer microporous membrane should exhibit a
maximum shrinkage in the molten state (about 140.degree. C.) of
about 30% or less, preferably about 20% or less as measured by a
thermomechanical analyzer, ("TMA").
[5] Battery Separator
[0148] In and embodiment, the battery separator formed by the above
multi-layer, microporous polyolefin membrane has a thickness in the
range of about 3 .mu.m to about 200 .mu.m, or about 5 .mu.m to
about 50 .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.
[6] Battery
[0149] In an embodiment, the multi-layer, microporous polyolefin
membrane can be used as a separator for primary and secondary
batteries such as lithium ion batteries, lithium-polymer secondary
batteries, nickel-hydrogen secondary batteries, nickel-cadmium
secondary batteries, nickel-zinc secondary batteries, silver-zinc
secondary batteries, and particularly for lithium ion secondary
batteries. Explanations will be made below on the lithium ion
secondary batteries.
[0150] The lithium secondary battery comprises a cathode, an anode,
and a separator located between the anode and the cathode. The
separator generally contains an electrolytic solution
(electrolyte). The electrode structure is not critical, and
conventional electrode structures can be used. The electrode
structure may be, for instance, a coin type in which a disc-shaped
cathode and anode are opposing, a laminate type in which a planar
cathode and anode are alternately laminated with at least one
separator situated between the anode and the cathode, a toroidal
type in which ribbon-shaped cathode and anode are wound, etc.
[0151] The cathode generally comprises a current collector, and a
cathodic-active material layer capable of absorbing and discharging
lithium ions, which is formed on the current collector. The
cathodic-active materials can be, e.g., inorganic compounds such as
transition metal oxides, composite oxides of lithium and transition
metals (lithium composite oxides), transition metal sulfides, etc.
The transition metals can be, e.g., V, Mn, Fe, Co, Ni, etc. In an
embodiment, the lithium composite oxides are lithium nickelate,
lithium cobaltate, lithium manganate, laminar lithium composite
oxides based on .alpha.-NaFeO.sub.2, etc. The anode generally
comprises a current collector, and a negative-electrode active
material layer formed on the current collector. The
negative-electrode active materials can be, e.g., carbonaceous
materials such as natural graphite, artificial graphite, cokes,
carbon black, etc.
[0152] The electrolytic solutions can be obtained by dissolving
lithium salts in organic solvents. The choice of solvent and/or
lithium salt is not critical, and conventional solvents and salts
can be used. The lithium salts can be, e.g., LiClO.sub.4,
LiPF.sub.6, LiAsF.sub.6, LiSbF.sub.6, LiBF.sub.4,
LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiC(CF.sub.3SO.sub.2).sub.3, Li.sub.2B.sub.10Cl.sub.10,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiPF.sub.4(CF.sub.3).sub.2,
LiPF.sub.3(C.sub.2F.sub.5).sub.3, lower aliphatic carboxylates of
lithium, LiAlCl.sub.4, etc. The lithium salts may be used alone or
in combination. The organic solvents can be organic solvents having
relatively high boiling points (compared to the battery's shut-down
temperature) and high dielectric constants. Suitable organic
solvents include ethylene carbonate, propylene carbonate,
ethylmethyl carbonate, .gamma.-butyrolactone, etc.; organic
solvents having low boiling points and low viscosity such as
tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane,
dioxolane, dimethyl carbonate, diethyl carbonate, and the like,
including mixtures thereof. Because the organic solvents generally
having high dielectric constants generally also have a high
viscosity, and vice versa, mixtures of high- and low-viscosity
solvents can be used.
[0153] When the battery is assembled, the separator is generally
impregnated with the electrolytic solution, so that the separator
(multi-layer, microporous membrane) is provided with ion
permeability. The choice of impregnation method is not critical,
and conventional impregnation methods can be used. For example, the
impregnation treatment can be conducted by immersing the
multi-layer, microporous membrane in an electrolytic solution at
room temperature.
[0154] The method selected for assembling the battery is not
critical, and conventional battery-assembly methods can be used.
For example, when a cylindrical battery is assembled, a cathode
sheet, a separator formed by the multi-layer, microporous membrane
and an anode sheet are laminated in this order, and the resultant
laminate is wound to a toroidal-type electrode assembly. A second
separator might be needed to prevent short-circuiting of the
toroidal windings. The resultant electrode assembly can be
deposited into a battery can and then impregnated with the above
electrolytic solution, and a battery lid acting as a cathode
terminal provided with a safety valve can be caulked to the battery
can via a gasket to produce a battery.
[7] Examples
[0155] The present invention will be explained in more detail
referring to the following non-limiting examples.
Example 1
(1) Preparation of First Polyolefin Solution
[0156] A first polyolefin composition comprising (a) 80% of PE1
having a weight average molecular weight of 5.6.times.10.sup.5 and
a molecular weight distribution of 4.05, (b) 20% of PE2 having a
weight average molecular weight of 1.9.times.10.sup.6 and a
molecular weight distribution of 5.09, is prepared by dry-blending.
The polyethylene composition has a melting point of 135.degree. C.
and a crystal dispersion temperature of 100.degree. C.
[0157] Twenty-five parts by weight of the resultant first
polyolefin composition is charged into a strong-blending
double-screw extruder having an inner diameter of 58 mm and L/D of
42, and 65 parts by mass of liquid paraffin (50 cst at 40.degree.
C.) is supplied to the double-screw extruder via a side feeder.
Melt-blending is conducted at 210.degree. C. and 200 rpm to prepare
a first polyolefin solution.
(2) Preparation of Second Polyolefin Solution
[0158] A second polyolefin solution is prepared in the same manner
as above except as follows. A second polyolefin composition
comprising (a) 69% of PE1 having a weight average molecular weight
of 5.6.times.10.sup.5 and a molecular weight distribution of 4.05,
and (b) 1% of PE2 having a weight average molecular weight of
1.9.times.10.sup.6 and a molecular weight distribution of 5.09, and
(c) 30% of polypropylene resin having a weight average molecular
weight of 1.6.times.10.sup.6, a molecular weight distribution of
5.21 and a heat of fusion of 114.0 J/g, by weight of the second
polyolefin composition, is prepared by dry-blending. The polyolefin
composition has a melting point of 135.degree. C. and a crystal
dispersion temperature of 100.degree. C. Thirty-five parts by
weight of the resultant second 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 parts by mass of liquid
paraffin (50 cst at 40.degree. C.) is supplied to the double-screw
extruder via a side feeder. Melt-blending is conducted at
210.degree. C. and 200 rpm to prepare a second polyolefin
solution.
(3) Production of Membrane
[0159] The first and second polyolefin solutions are supplied from
their respective double-screw extruders to a three-layer-extruding
T-die, and extruded therefrom to form an extrudate (also called a
laminate) of first polyolefin solution layer/second polyolefin
solution layer/first polyolefin solution layer at a layer thickness
ratio of 46/8/46. The extrudate is cooled while passing through
cooling rollers controlled at 20.degree. C., to form a three-layer
gel-like sheet, which is simultaneously biaxially stretched at
115.degree. C. to a magnification of 5 fold in both machine
(longitudinal) and transverse directions by a tenter-stretching
machine. The stretched three-layer gel-like sheet is fixed to an
aluminum frame of 20 cm.times.20 cm, immersed in a bath of
methylene chloride controlled at 25.degree. C. to remove liquid
paraffin with vibration of 100 rpm for 3 minutes, and dried by air
flow at room temperature. The dried membrane is re-stretched by a
batch-stretching machine to a magnification of 1.4 fold in a
transverse direction at 127.degree. C. The re-stretched membrane,
which remains fixed to the batch-stretching machine, is heat-set at
127.degree. C. for 10 minutes to produce a three-layer microporous
membrane.
Example 2
[0160] Example 1 is repeated except the dried membrane is
re-stretched to a magnification of 1.6 fold in a transverse
direction at 127.degree. C. and contracted to a magnification of
1.4 fold in the direction at 127.degree. C. compared with original
size.
Example 3
[0161] Example 1 is repeated except the first polyolefin content in
the first polyolefin solution is 25%, the second polyolefin
composition comprises (a) 64% of PE1, (b) 1% of PE2 and (c) 35% of
the second polypropylene, a layer thickness ratio of first
microporous membrane/second microporous membrane/first microporous
membrane is 47/6/47, the gel-like sheet is stretched at 119.degree.
C. and the dried membrane is re-stretched to a magnification of 1.5
fold in a transverse direction at 127.degree. C. and shrank to a
magnification of 1.3 fold in the direction at 127.degree. C.
Example 4
[0162] Example 3 is repeated except the second polyolefin
composition comprises (a) 49% of PE1, (b) 1% of PE2 and (c) 50% of
the second polypropylene, a layer thickness ratio of first
microporous membrane/second microporous membrane/first microporous
membrane is 46/8/46, the gel-like sheet is stretched at 115.degree.
C.
Example 5
[0163] Example 4 is repeated except the second polyolefin comprises
(a) 79% of PE1, (b) 1% of PE2, and (c) 20% of second polypropylene
and a layer thickness ratio of first microporous membrane/second
microporous membrane/first microporous membrane is 35/30/35.
Example 6
[0164] Example 4 is repeated except a layer thickness ratio of
first microporous membrane/second microporous membrane/first
microporous membrane is 40/20/40.
Example 7
[0165] Example 2 is repeated except the first polyolefin content in
the first polyolefin solution is 25% and the second polypropylene
resin in the second polyolefin composition has a weight average
molecular weight of 0.90.times.10.sup.6, a molecular weight
distribution of 4.5 and a heat of fusion of 106.0 J/g.
Example 8
[0166] Example 7 is repeated except the first polyolefin
composition comprises (a) 80% of PE1, (b) 12% of PE2 and (c) 8% of
the first polypropylene having a weight average molecular weight of
6.6.times.10.sup.5, a molecular weight distribution of 1 and a heat
of fusion of 103.3 J/g.
Comparative Example 1
[0167] Example 1 is repeated except there is no second polyolefin
solution. In other words, the membrane is a monolayer membrane
produced from the first polyolefin solution. This comparative
example also differs from Example 1 in that the dried membrane is
not re-stretched.
Comparative Example 2
[0168] Example 2 is repeated except there is no first polyolefin
solution. In other words, the membrane is a monolayer membrane
produced from the second polyolefin solution.
Comparative Example 3
[0169] Example 2 is repeated except the first polyolefin content in
the first polyolefin solution is 25% and a layer thickness ratio of
first microporous membrane/second microporous membrane/first
microporous membrane is 47/6/47.
Comparative Example 4
[0170] Example 2 is repeated except the second polypropylene resin
in the second polyolefin composition has a weight average molecular
weight of 0.54.times.10.sup.6, a molecular weight distribution of
4.7 and a heat of fusion of 94.0 J/g.
Comparative Example 5
[0171] Example 2 is repeated except the second polypropylene resin
in the second polyolefin composition has a weight average molecular
weight of 1.56.times.10.sup.6, a molecular weight distribution of
3.2 and a heat of fusion of 78.40 J/g.
Comparative Example 6
[0172] Example 2 is repeated except the second polypropylene resin
in the second polyolefin composition has a weight average molecular
weight of 2.67.times.10.sup.6, a molecular weight distribution of
2.6 and a heat of fusion of 99.4.
Properties
[0173] The properties of the multi-layer microporous membranes of
Examples 1-8 and Comparative Examples 1-6 are measured by the
following methods. The results are shown in Tables 1 and 2.
(1) Average Thickness (.mu.m)
[0174] The thickness of each microporous membrane is measured by a
contact thickness meter at 10 mm intervals in the area of 10
cm.times.10 cm of the membrane, and averaged. The thickness meter
used is a Litematic made by Mitsutoyo Corporation.
(2) Porosity (%)
[0175] Measured by a weight method using the formula: Porosity
%=100.times.(w2-w1)/w2, wherein "w1" is the actual weight of film
and "w2" is the assumed weight of 100% polyethylene.
(3) Weight Per Unit Area (g/m.sup.2)
[0176] A weight per unit area of the membrane is the weight at 1
square meter and calculated based on the weight of above square
membrane.
(4) Air Permeability (sec/100 cm.sup.3/25 .mu.m)
[0177] Air permeability P.sub.1 measured on each microporous
membrane having a thickness T.sub.1 according to JIS P8117 is
converted to air permeability P.sub.2 at a thickness of 25 .mu.m by
the equation of P.sub.2=(P.sub.1.times.25)/T.sub.1.
(5) Pin Puncture Strength (mN/25 .mu.m)
[0178] The maximum load is measured when each 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 measured maximum load L.sub.1 is
converted to the maximum load L.sub.2 at a thickness of 25 .mu.m by
the equation of L.sub.2=(L.sub.1.times.25)/T.sub.1, and is used as
pin puncture strength.
(6) Heat Shrinkage Ratio (%)
[0179] The shrinkage ratios of each microporous membrane in both
longitudinal and transverse directions are measured three times
when exposed to 105.degree. C. for 8 hours, and averaged to
determine the heat shrinkage ratio.
(7) Shut Down Temperature (.degree. C.)
[0180] The shut down temperature is measured as follows: A
rectangular sample of 3 mm.times.50 mm is cut out of the
microporous membrane such that the longitudinal direction of the
sample is aligned with the transverse direction of the microporous
membrane, and set in a thermomechanical analyzer (TMA/SS6000
available from Seiko Instruments, Inc.) at a chuck distance of 10
mm. With a load of 19.6 mN applied to a lower end of the sample,
the temperature is elevated at a rate of 5.degree. C./minute to
measure its size change. A temperature at a point of inflection
observed near the melting point is defined as the shutdown
temperature.
(8) Rupture Temperature (.degree. C.)
[0181] A microporous membrane of 5 cm.times.5 cm is sandwiched by
blocks each having a circular opening of 12 mm in diameter, and a
tungsten carbide ball of 10 mm in diameter was placed on the
microporous membrane in the circular opening. While heating at a
temperature-elevating speed of 5.degree. C./minute, the temperature
at which the microporous polyolefin membrane is ruptured by melting
is measured and recorded as the Rupture temperature.
(9) Maximum Shrinkage in Molten State
[0182] The maximum shrinkage in the molten state is measured as
follows: A rectangular sample of 3 mm.times.50 mm is cut out of the
microporous membrane such that the longitudinal direction of the
sample is aligned with the transverse direction of the microporous
membrane, and set in a thermomechanical analyzer (TMA/SS6000
available from Seiko Instruments, Inc.) at a chuck distance of 10
mm. With a load of 19.6 mN applied to a lower end of the sample,
the temperature is elevated at a rate of 5.degree. C./minute to
measure the membrane sample's size change. A size change ratio is
calculated relative to the size at 23.degree. C., to obtain a
temperature-size change ratio curve. The maximum shrinkage ratio in
the molten state is observed in a temperature range of from
135.degree. C. to 145.degree. C.
TABLE-US-00001 TABLE 1 No. Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Ex 7 Ex 8
Resin composition First Polyolefin PE1 Mw 3.0 .times. 10.sup.5 3.0
.times. 10.sup.5 3.0 .times. 10.sup.5 3.0 .times. 10.sup.5 3.0
.times. 10.sup.5 3.0 .times. 10.sup.5 3.0 .times. 10.sup.5 3.0
.times. 10.sup.5 Mw/Mn 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 % by mass 80
80 80 80 80 80 80 80 PE2 Mw 2.0 .times. 10.sup.6 2.0 .times.
10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times.
10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times.
10.sup.6 Mw/Mn 8 8 8 8 8 8 8 8 % by mass 20 20 20 20 20 20 20 12
1.sup.st PP Mw -- -- -- -- -- -- -- 6.6 .times. 10.sup.5 Mw/Mn --
-- -- -- -- -- -- 11 Heat of fusion(J/g) -- -- -- -- -- -- -- 103.3
% by mass -- -- -- -- -- -- -- 8 Conc. of PO Comp. % by mass 35 35
25 25 25 25 25 25 Second Polyolefin PE1 Mw 3.0 .times. 10.sup.5 3.0
.times. 10.sup.5 3.0 .times. 10.sup.5 3.0 .times. 10.sup.5 3.0
.times. 10.sup.5 3.0 .times. 10.sup.5 3.0 .times. 10.sup.5 3.0
.times. 10.sup.5 Mw/Mn 8.6 8.6 8.6 8.6 8.6 8.6 8.6 8.6 % by mass 69
69 64 49 79 49 69 69 PE2 Mw 2.0 .times. 10.sup.6 2.0 .times.
10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times.
10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times.
10.sup.6 Mw/Mn 8 8 8 8 8 8 8 8 % by mass 1 1 1 1 1 1 1 1 2.sup.nd
PP Mw 1.10 .times. 10.sup.6 1.10 .times. 10.sup.6 1.10 .times.
10.sup.6 1.10 .times. 10.sup.6 1.10 .times. 10.sup.6 1.10 .times.
10.sup.6 0.90 .times. 10.sup.6 1.10 .times. 10.sup.6 Mw/Mn 5.0 5.0
5.0 5.0 5.0 5.0 4.5 5.0 Heat of fusion(J/g) 114.0 114.0 114.0 114.0
114.0 114.0 106 114.0 % by mass 30 30 35 50 20 50 30 30 Conc. of PO
Comp. % by mass 30 30 30 30 30 30 30 30 Production condition
Extrudate Layer structure (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I)
(I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I)
Layer thickness ratio 46/8/46 46/8/46 47/6/47 46/8/46 35/30/35
40/20/40 46/8/46 46/8/46 Total PP content % by mass 2.08 2.08 2.49
5.43 6.79 11.5 3.31 11.9 Stretching of Gel-Like sheet Temperature
(.degree. C.) 115 115 119 115 115 115 115 115 Magnification (MD
.times. TD) 5 .times. 5 5 .times. 5 5 .times. 5 5 .times. 5 5
.times. 5 5 .times. 5 5 .times. 5 5 .times. 5 Stretching of dried
membrane Temperature (.degree. C.) 127 127 127 125 125 125 127 127
Magnification (TD) 1.4 1.6 -> 1.4 1.5 -> 1.3 1.6 -> 1.4
1.6 -> 1.4 1.6 -> 1.4 1.6 -> 1.4 1.6 -> 1.4 Heat
setting treatment Temperature(.degree. C.) 127 127 127 125 125 125
127 127 Time (min) 10 10 10 10 10 10 10 10 Properties Average
thickness (.mu.m) 25.9 25.6 24.9 25.1 24.7 25.0 25.5 24.7
Normalized Air Permeability 270 262 230 236 211 326 240 289
(sec/100 cm.sup.3/25 .mu.m) Porosity % 47.0 46.5 48.7 47.2 49.1
43.3 48.2 47.3 Weight per unit area 13.6 13.6 12.6 13.3 12.3 14.2
13.1 12.9 Normalized Puncture Strength 5586 5468 3826 5292 5133
4732 5194 4508 (mN/25 .mu.m) Heat shrinkage MD/TD (%) 3.5/4.9
3.4/1.3 3.5/1.5 3.3/1.2 3.7/1.3 3.6/1.1 3.3/1.6 3.2/1.2 Shut Down
Temp. .degree. C. 132 132 132 133 132 132 132 133 Rupture Temp.
.degree. C. 187 189 184 191 188 190 185 191 Maximum shrinkage (TMA)
% 29 16 12 15 16 14 15 15 Is A .ltoreq. 0.097P-110? Yes yes yes yes
yes yes yes yes
TABLE-US-00002 TABLE 2 Comp. Ex Comp. Ex Comp. Ex Comp. Ex Comp. Ex
Comp. Ex No. 1 2 3 4 5 6 Resin composition First Polyolefin PE1 Mw
3.0 .times. 10.sup.5 -- 3.0 .times. 10.sup.5 3.0 .times. 10.sup.5
3.0 .times. 10.sup.5 3.0 .times. 10.sup.5 Mw/Mn 8.6 -- 8.6 8.6 8.6
8.6 % by mass 80 -- 80 80 80 80 PE2 Mw 2.0 .times. 10.sup.6 -- 2.0
.times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0
.times. 10.sup.6 Mw/Mn 8 -- 8 8 8 8 % by mass 20 -- 20 20 20 20
1.sup.st PP Mw -- -- -- -- -- -- Mw/Mn -- -- -- -- -- -- Heat of
fusion(J/g) -- -- -- -- -- -- % by mass -- -- -- -- -- -- Conc. of
PO Comp. % by mass 30 -- 25 35 35 25 Second Polyolefin PE1 Mw --
3.0 .times. 10.sup.5 3.0 .times. 10.sup.5 3.0 .times. 10.sup.5 3.0
.times. 10.sup.5 3.0 .times. 10.sup.5 Mw/Mn -- 8.6 8.6 8.6 8.6 8.6
% by mass -- 69 69 69 69 49 PE2 Mw -- 2.0 .times. 10.sup.6 2.0
.times. 10.sup.6 2.0 .times. 10.sup.6 2.0 .times. 10.sup.6 2.0
.times. 10.sup.6 Mw/Mn -- 8 8 8 8 8 % by mass -- 1 1 1 1 1 2.sup.nd
PP Mw -- 1.10 .times. 10.sup.6 1.10 .times. 10.sup.6 0.54 .times.
10.sup.6 1.56 .times. 10.sup.6 2.67 .times. 10.sup.6 Mw/Mn -- 5.0
5.0 4.7 3.2 2.6 Heat of fusion(J/g) -- 114.0 114.0 94.0 78.4 99.0 %
by mass -- 30 30 30 30 50 Cone, of PO Comp. % by mass -- 30 30 30
30 35 Production condition Extrudate Layer structure (I) (II)
(I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) (I)/(II)/(I) Layer thickness
ratio 100 100 47/6/47 46/8/46 46/8/46 40/20/40 Total PP content %
by mass 0 30 1.89 2.08 2.08 13.0 Stretching of Gel-Like sheet
Temperature (.degree. C.) 115 115 115 115 115 115 Magnification (MD
.times. TD) 5 .times. 5 5 .times. 5 5 .times. 5 5 .times. 5 5
.times. 5 5 .times. 5 Stretching of dried membrane
Temperature(.degree. C.) -- 127 127 127 127 127 Magnification (TD)
-- 1.6 -> 1.4 1.6 -> 1.4 1.6 -> 1.4 1.6 -> 1.4 1.6
-> 1.4 Heat setting treatment Temperature(.degree. C.) 127 127
127 127 127 127 Time (min) 10 10 10 10 10 10 Properties Average
thickness (.mu.m) 25.5 24.9 25.1 25.4 25.1 24.6 Normalized Air
Permeability 550 446 256 223 290 460 (sec/100 cm.sup.3/25 .mu.m)
Porosity % 38.4 46.1 47.0 47.8 46.7 42.6 Weight per unit area 15.6
13.3 13.2 13.1 13.2 14.0 Normalized Puncture Strength 5488 4939
5322 5107 5603 5341 (mN/25 .mu.m) Heat shrinkage MD/TD (%) 5.5/5.0
4.3/1.8 3.4/1.4 3.2/1.2 3.6/1.5 4.2/2.1 Shut Down Temp. .degree. C.
132 136 132 132 132 132 Rupture Temp. .degree. C. 150 181 156 165
164 190 Maximum shrinkage (TMA) % 32 13 15 14 18 21 Is A .ltoreq.
0.097P-110? No no yes yes yes no
[0183] It is noted from Table 1 that the microporous membrane of
the present invention has well-balanced properties, including air
permeability, pin puncture strength, shut down temperature and
rupture down temperature, as well as low maximum shrinkage in the
molten state. Lithium ion secondary batteries comprising the
microporous membranes of the present invention have high capacity
and high safety performance. As discussed in the Background, the
selection of polymer type and content for microporous polymeric
membranes represents a trade-off. Polymeric materials and membrane
structures that conventionally provide a relatively high porosity
(or high Air Permeability as characterized by a shorter time to
pass a volume of air through the pores of the membrane) generally
lead to a lower membrane Pin Puncture Strength, particularly when a
relatively high Rupture temperature is desired, e.g., 180.degree.
C. or higher. The invention is based in part on the discovery that
three-layer membranes as described above exemplified in the
Examples 1 through 8 achieve an optimization of Air Permeability,
Pin Puncture Strength, and Rupture temperature.
[0184] On the other hand, the microporous membranes of the
Comparative Examples exhibit a poorer balance of these properties.
For example, even though Comparative Examples 3, 4, and 5 have an
Air Permeability satisfying the relationship A.ltoreq.0.097P-110,
they do not have a meltdown temperature in the range of 180.degree.
C. or higher.
[0185] The microporous membranes of the present invention have
well-balanced properties and the of such microporous membrane as a
battery separator provides batteries having excellent safety, heat
resistance and productivity.
[0186] 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.
[0187] 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.
[0188] When numerical lower limits and numerical upper limits are
listed herein, ranges from any lower limit to any upper limit are
contemplated.
* * * * *