U.S. patent application number 10/107781 was filed with the patent office on 2002-09-26 for multilayer battery separators.
Invention is credited to Call, Ronald W., Simmons, Donald K., Yu, Ta-Hua.
Application Number | 20020136945 10/107781 |
Document ID | / |
Family ID | 27804373 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020136945 |
Kind Code |
A1 |
Call, Ronald W. ; et
al. |
September 26, 2002 |
Multilayer battery separators
Abstract
A microporous battery separator is provided having a first
co-extruded multilayered portion and a second co-extruded
multilayered portion. The two portions are bonded together. In a
preferred embodiment, the battery separator has two substantially
identical multilayered portions bonded together face-to-face. Each
of the two multilayered portions has at least one strength layer
and at least one shutdown layer. Methods for making the battery
separators are also provided. Preferably, a tubular multilayered
film is extruded, and collapsed onto itself to form a multilayered
battery separator precursor. The precursor is then bonded and
annealed before it is stretched to form a microporous multilayer
battery separator.
Inventors: |
Call, Ronald W.; (Rock Hill,
SC) ; Simmons, Donald K.; (Charlotte, NC) ;
Yu, Ta-Hua; (Nanuet, NY) |
Correspondence
Address: |
ROBERT H. HAMMER III, P.C.
13777 BALLANTYNE COPORATE PLACE
SUITE 250
CHARLOTTE
NC
28277
US
|
Family ID: |
27804373 |
Appl. No.: |
10/107781 |
Filed: |
March 27, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10107781 |
Mar 27, 2002 |
|
|
|
09484184 |
Jan 18, 2000 |
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Current U.S.
Class: |
429/144 ;
428/516; 429/145 |
Current CPC
Class: |
B29C 66/83413 20130101;
B32B 37/153 20130101; B29C 66/91935 20130101; H01M 50/457 20210101;
B29C 48/10 20190201; B29C 48/305 20190201; B29K 2023/06 20130101;
H01M 50/403 20210101; B29C 66/919 20130101; H01M 50/449 20210101;
B29K 2105/256 20130101; H01M 50/406 20210101; B29K 2023/065
20130101; B29C 66/91421 20130101; B29C 48/0018 20190201; B32B 37/28
20130101; B29C 66/727 20130101; B29C 66/91645 20130101; B29C 66/929
20130101; H01M 50/417 20210101; H01M 10/4235 20130101; Y02E 60/10
20130101; Y10T 428/31913 20150401; B29C 48/185 20190201; B29K
2023/0625 20130101; B32B 2038/0028 20130101; B29K 2105/04 20130101;
H01M 50/463 20210101; B29K 2023/0633 20130101; B32B 27/32 20130101;
B29C 48/21 20190201; B29C 55/005 20130101; B29K 2023/12 20130101;
B29C 65/18 20130101; B29C 66/9141 20130101; H01M 50/489 20210101;
B29C 66/723 20130101; B29K 2105/16 20130101; B29C 48/0012 20190201;
B29L 2009/00 20130101; H01M 50/411 20210101; B32B 38/0032
20130101 |
Class at
Publication: |
429/144 ;
429/145; 428/516 |
International
Class: |
H01M 002/16; H01M
002/18; B32B 027/08 |
Claims
What is claimed is:
1. A battery separator comprising: a microporous film having a
first co-extruded multilayered portion and a second co-extruded
multilayered portion, said first portion and said second portion
being substantially identical, being bonded face-to-face.
2. The battery separator of claim 1 wherein said film being formed
by collapsing a tubular film onto itself.
3. The battery separator of claim 1 wherein said co-extruded
multilayered portion further comprising at least one strength layer
and at least one shutdown layer.
4. The battery separator of claim 1 wherein a uniaxial orientation
of said first portion being at an angle relative to a uniaxial
orientation of said second portion.
5. The battery separator of claim 1 wherein said film further
comprising four layers, the first and fourth layers consisting
essentially of polypropylene, the second and third layers being
bonded together and consisting essentially of polyethylene.
6. The battery separator of claim 1 wherein said film further
comprising six layers, the second and fifth layers consisting
essentially of polyethylene, the first, third, fourth and sixth
layers consisting essentially of polypropylene, said third and
fourth layers being bonded together.
7. The battery separator of claim 1 wherein said film having a
thickness of from about 0.5 mil to about 1.5 mils.
8. The battery separator of claim 1 wherein said film having a peel
strength of at least about 5 grams/inch.
9. The battery separator of claim 1 wherein said battery separator
has a thickness of from about 0.5 mil to about 1.5 mils and a
puncture strength of at least about 500 grams.
10. The battery separator of claim 3 wherein each said multilayered
portion having one said strength layer and one said shutdown layer,
wherein said multilayered portions being bonded together such that
the uniaxial orientation of each portion is at an angle relative to
the other and that said film having two strength layers sandwiching
two shutdown layers bonded together, said film having a thickness
of from about 0.5 mil to about 1.5 mils and a puncture strength of
at least about 500 grams.
11. The battery separator of claim 3 wherein each said multilayered
portion having two said strength layers sandwiching one said
shutdown layer, said strength layers and said shutdown layer,
wherein the two multilayered portions being bonded together such
that the uniaxial orientation of each portion is at an angle
relative to the other and that said film having a six-layer
construction, the first, third, fourth and sixth layers being
strength layers and the second and fifth layers being shutdown
layers, said film having a thickness of from about 0.5 mil to about
1.5 mils and a puncture strength of at least about 500 grams.
12. A method for making a multilayer battery separator comprising
the steps of: extruding a tubular multilayered film; collapsing the
tubular multilayered film onto itself to form a multilayered
battery separator precursor having two plies of the multilayered
film; bonding and annealing the precursor; and stretching the
bonded and annealed precursor to form a microporous multilayer
battery separator.
13. The method of claim 12 wherein said precursor is bonded by
thermocompression bonding.
14. The method of claim 12 wherein said multilayered tubular film
has two layers including a shutdown layer and a strength layer.
15. The method of claim 14 wherein said strength layer consists
essentially of polypropylene, and said shutdown layer consists
essentially of polyethylene.
16. The method of claim 12 wherein said multilayered tubular film
has three layers including two strength layers sandwiching a
shutdown layer.
17. The method of claim 16 wherein said strength layer consists
essentially of polypropylene, and said shutdown layer consists
essentially of polyethylene.
18. A method for making a multilayer battery separator, comprising
the steps of: co-extruding a first multilayered flat sheet;
co-extruding a second multilayered flat sheet; laminating said
first and second flat sheets to form a battery separator precursor;
bonding and annealing said precursor; and stretching the bonded and
annealed precursor to form a multilayer battery separator.
19. The method of claim 18 further comprising extracting an
extractable material from said precursor.
Description
RELATED APPLICATION
[0001] This application is a continuation of co-pending application
Ser. No. 09/484,184 filed Jan. 18, 2000.
FIELD OF THE INVENTION
[0002] This invention relates to battery separators, in particular
to battery separators with improved strength properties and methods
for making same.
BACKGROUND OF THE INVENTION
[0003] Microporous film battery separators are used in various
batteries, particularly rechargeable batteries, such as lithium
batteries. Such battery separators allow electrolytes to cross
through the battery separators while preventing any contact between
electrodes of opposite polarity. Typically, the microporous film
comprises one or more layers of microporous membranes.
[0004] In lithium batteries, particularly secondary lithium
batteries (rechargeable lithium batteries), overheating problems
can occur and cause thermal runaway in the battery. Thus, shutdown
separators were developed to prevent thermal runaway. See e.g.,
U.S. Pat. Nos. 4,650,730 and 4,731,304. A shutdown battery
separator has a microporous membrane that closes its pores at a
temperature substantially lower than the temperature that could
cause thermal runaway in the lithium battery.
[0005] Some multilayer shutdown separators are known in the art.
For example, U.S. Pat. No. 4,650,730 discloses a bilayer battery
separator having an unfilled microporous sheet and a filled
microporous sheet. Each sheet is formed separately by an extraction
process using a suitable solvent. The two microporous sheets are
then laminated together to form the shutdown separator.
[0006] The Celgard.RTM. battery separators, which have been
commercially available for a number of years, are typically formed
by a stretch method. For example, a non-porous tubular
polypropylene film is first formed by blown film extrusion. The
tubular film is collapsed onto itself to form a non-porous flat
sheet having two polypropylene plies. Optionally, the die assembly
is rotated slowly twisting the tubular film somewhat to prevent and
remove wrinkles and uneven distribution so that the surface of the
film is substantially smooth. The flat sheet is then annealed and
stretched to impart microporosity therein. The two microporous flat
sheets are then de-plied into two layers of microporous battery
separator. Normally, the adhesion force between the two plies in
the flat sheet must be sufficiently low such that the two plies can
be separated without damaging the plies. However, when a separator
having two layers of the microporous polypropylene film is desired,
the adhesion force can be higher, e.g., 5 grams/inch to about 35
grams/inch, which can be caused by, e.g., bonding the plies after
collapsing the tubular film.
[0007] U.S. Pat. No. 5,691,077 discloses a trilayer battery
separator. In a preferred embodiment disclosed therein, the
separator has a polypropylene-polyethylene-polypropylene
construction, and is made by laminating and bonding microporous
layers. Each microporous layer is formed by a Celgard.RTM. process
described above involving a de-plying step.
[0008] U.S. Pat. No. 5,691,047 also discloses a microporous
trilayer battery separator having a
polypropylene-polyethylene-polypropylene construction. A plurality
of non-porous single-layered precursors are first extruded by cast
extrusion. The non-porous single layers are laminated and bonded
together into a precursor of a
polypropylene-polyethylene-polypropylene structure. The precursor
is then annealed and stretched to form a microporous trilayer
battery separator.
[0009] A number of co-extrusion processes for making multilayer
battery separators have also been proposed. For example, UK Patent
Publication No. GB 2,298,817 describes a microporous trilayer
battery separator made by co-extruding a trilayer film precursor
having a non-porous polypropylene-polyethylene-polypropylene
construction using a T-die, annealing the trilayer precursor, and
then stretching the annealed trilayer precursor to form the porous
trilayer battery separator.
[0010] A porous trilayer separator is also proposed in Japanese
Patent Application No. 56320/1995 (JP8-250097A) filed by Kureha
Chemical Industry Co. Ltd. The Kureha separator is prepared by a
process that includes the steps of co-extruding a trilayer
precursor containing a solvent extractable material as pore forming
aid, and forming pores in the precursor by solvent extraction of
the extractable material in the precursor.
[0011] U.S. Pat. No. 6,346,350 discloses making a multilayer
battery separator by co-extrusion in a blown film process. The
co-extruded molten film is rapidly quenched such that it is in a
substantially solidified state. The co-extruded film is then
annealed and stretched to impart microporosity therein.
[0012] A multilayer microporous shutdown separator should be as
thin as possible in order to minimize the space it occupies in a
battery and also to reduce electrical resistance (ER). Nevertheless
the shutdown separator must also have sufficient strength to resist
puncture. Punctured battery separators are ineffective in
preventing the contact between the electrodes of opposite polarity.
Under overheating conditions, a punctured battery separator cannot
shut down effectively to prevent the electrolytes from crossing the
battery separator, and thus is ineffective in preventing thermal
runaway. Battery separators with low puncture strength are
difficult to handle especially in the battery separator
manufacturing processes. Once punctured, battery separators are
prone to splitting, i.e., tearing.
[0013] Thus, it is an objective in the art to further develop
efficient methods for making relatively thin battery separators
with improved puncture strength.
SUMMARY OF THE INVENTION
[0014] A battery separator is a microporous film. The film has a
first co-extruded multilayered portion and a second co-extruded
multilayered portion. The first portion and the second portion are
substantially identical. The first portion and the second portion
are bonded face-to-face. The film may be formed by collapsing a
tubular film onto itself.
[0015] The separator is preferably made by first extruding a
tubular multilayered film. The tubular film is collapsed onto
itself to form a multilayered battery separator precursor having
two plies of the multilayered film. The precursor is bonded and
annealed. Then, the precursor is stretched to form a microporous
multilayered battery separator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For the purpose of illustrating the invention, there is
shown in the drawings a form that is presently preferred; it being
understood, however, that this invention is not limited to the
precise arrangements and instrumentalities shown.
[0017] FIG. 1 is a schematic diagram showing the cross sectional
view of a battery separator of this invention having four
layers;
[0018] FIG. 2 illustrates the cross section of another embodiment
of the battery separator of this invention having four layers;
[0019] FIG. 3 is a schematic diagram demonstrating an embodiment of
the battery separator of this invention having six layers;
[0020] FIG. 4 shows the cross sectional view of the different
construction of a battery separator of this invention having six
layers;
[0021] FIG. 5 illustrates the construction of another battery
separator of this invention having six layers;
[0022] FIG. 6 is a cross sectional view of a battery separator of
this invention having four outer polypropylene layers and two inner
polyethylene layers;
[0023] FIG. 7 is a cross sectional view of a battery separator of
this invention having four inner polyethylene layers and two outer
polypropylene layers; and
[0024] FIG. 8 illustrates a cross section of a battery separator of
this invention having four outer polyethylene layers and two inner
polypropylene layers.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The present invention provides a microporous battery
separator having two portions bonded together. Each portion
contains two or more co-extruded membrane layers. To obtain greater
puncture strength over the prior art, i.e., a single multilayered
separator of a given thickness, the instant invention bonds at
least two multilayered precursors that are sized, when combined, to
have the same thickness of the prior art separator.
[0026] The battery separator can have a wide range of thickness.
Preferably the battery separator has a thickness of less than 5
mils, normally no greater than 2 mils, and most preferably no
greater than 1.5 mils. The puncture strength of the battery
separator is at least about 400 grams, preferably at least about
450 grams. Normally, the puncture strength is at least about 500
grams, and advantageously is greater than about 550 grams.
[0027] In accordance with this invention, the membrane layers can
be extruded from any compositions of film-forming polymers suitable
for making battery separators, preferably polyolefins.
Homopolymers, heteropolymers, such as block polymers, random
copolymers and terpolymers, can all be used. Polymers should be
chosen such that the battery separator made therefrom exhibits a
low degree of electrical resistance and is stable in the battery
environment. The polymer resin compositions may also include
additives such as antioxidants, stabilizers, surfactants, and other
processing aids as known in the art.
[0028] Preferably, polymers are chosen such that the battery
separator exhibits shutdown properties. That is, the separator
closes its pores at a temperature typically below the temperature
that could cause thermal runaway in a lithium battery. Preferably,
polyolefins are used including, but not limited to polyethylene,
polypropylene, polybutene, ethylene-butene copolymers,
ethylene-hexene copolymers, ethylene-methacrylate copolymers, and a
blend thereof. Polyethylenes such as low density polyethylene
(LDPE), linear low density polyethylene (LLDPE), and high density
polyethylene (HDPE) can all be used. Typically, suitable
polyolefins can have an average molecular weight of from about
100,000 to about 5,000,000. Normally, the battery separator of this
invention exhibits shutdown properties, i.e., the micropores of the
separators close at a temperature typically below the temperature
that could cause thermal runaway in a lithium battery. Normally,
the battery separator of this invention has a shutdown temperature,
i.e., the temperature at which the battery separator closes its
micropores, of from about 80.degree. C. to about 140.degree. C.,
preferably from about 100.degree. C. to about 135.degree. C.
[0029] As used herein, the term "microporous" means that the
battery separator of this invention has micropores normally with an
average pore size of about 0.005 to about 10 microns, preferably
from about 0.01 to about 5 microns. Advantageously, the average
pore size is from about 0.05 to about 2 microns. The battery
separator of this invention typically has a Gurley value of from
about 5 seconds to about 100 seconds, preferably from about 10
seconds to about 60 seconds, as measured by the method of
ASTM-D726(B).
[0030] Each of the two portions of the battery separator of this
invention has two or more layers that are co-extruded during the
process of making the battery separator. As known in the art,
co-extrusion entails simultaneously extruding two or more polymer
compositions through a single die, e.g., a multimanifold internal
combining die, joining the two or more layers together. Typically,
the two or more polymer compositions enter the die separately and
join just before they pass through the final die orifice. Upon
extrusion, the multiple polymer compositions form multiple discrete
layers laminated and in intimate contact with each other. Normally,
the co-extruded layers become inseparable from each other without
being subject to a bonding step. That is, it is normally infeasible
to peel one layer away from an adjacent layer while maintaining the
integrity of the layers.
[0031] Any conventional co-extrusion techniques can be used for the
purpose of this invention. For example, co-extrusion can be done by
melt extrusion using a T-die as is known in the art. Alternatively,
co-extrusion is conducted by a blown film process, also known as
blown film extrusion. In "blown film extrusion," polymer
compositions are extruded from two or more extruders with an
annular die to form a tubular film (or parison) having two or more
layers. The tubular film or parison is then pulled away from the
die and collapsed or flattened using a collapsing frame, nip rolls
or the like. Normally, as the tubular film is formed, a fluid such
as air is continuously blown onto the tubular film from within the
parison. Thus, a bubble of air is trapped within the tubular film
and between the die and the collapsing device. In addition, as the
tubular film is pulled away from the die, air is also blown around
the film outside surface to stabilize and quench the tubular film
from the exterior. As a result, the extruded film is cooled to a
substantially crystalline solid state before it is collapsed. Many
conventional blown film processes are known in the art and can all
be used in this invention.
[0032] Preferably, bonding takes place after each of the two
portions is extruded and before they are made microporous.
Typically, two multilayered non-porous flat sheets (e.g., the
collapsed bubble) are bonded to form a battery separator precursor.
The precursor can be bonded, and then made microporous by a
conventional method known in the art to form a multilayer battery
separator.
[0033] Bonding is used to make the two multilayered portions stick
together (due to adhesion) so that the two portions are not readily
separable and cannot be readily de-plied. Thus, the battery
separator of this invention must have a minimum adhesion, which can
be measured as peel strength. As used herein, "peel strength" is
measured using a tension and compression tester to determine the
force in grams required to separate two one-inch wide sections of
bonded membrane at a peel rate of 10 inches per minute. The peel
strength should be at least about 5 grams/inch, preferably at least
about 8 grams/inch, and advantageously should be at least about 10
grams/inch. Methods for laminating and bonding two membranes are
generally known in the art and are disclosed in, e.g., U.S. Pat.
No. 5,565,281, which is incorporated herein by reference. Suitable
bonding methods for use in this invention include calendaring,
adhering with adhesives, and welding. The application of adhesives
may include air atomizing, gravure/screen printing, hydraulic
spraying, and ultrasonic spraying. The choice of adhesive and the
rate of adhesive application must be chosen so that the
characteristics of the battery separator formed thereof is not
adversely affected. The welding technique includes thermowelding
and ultrasonic welding. The amount of energy for either welding
procedure and the pattern of weld should be chosen so that the
separator's porosity is not adversely affected. Preferably, bonding
in this invention is performed by thermocompression bonding. For
example, bonding can be achieved by calendaring, with nip rolls
closed, at a temperature of at least 1.degree. C. below the melting
point of the polymer in the multilayered potions, preferably at a
temperature of at least 5.degree. C. below the melting point of the
multilayered portions. Normally, for precursors made of
polypropylene and polyethylene, the bonding temperature ranges from
about 100.degree. C. to about 150.degree. C., preferably from about
125.degree. C. to about 135.degree. C. The residence time at the
bonding temperature can be up to about 30 minutes. The bonding
conditions for precursors made from other polymers will be apparent
to a skilled artisan apprised of the present disclosure.
[0034] The multilayered portions, separate or bonded together, can
be made microporous by conventional methods known in the art for
making microporous membranes, including but not limited to a
stretch method, an extraction (or phase inversion) method, and a
particle stretch method.
[0035] Briefly, in a phase inversion method, a microporous membrane
can be formed from a composition comprising a polymer and an
extractable material. The extractable material is selected such
that it is miscible with the polymer at least at the melting point
of the polymer. Thus, in this process, the composition is heated to
the melting temperature of the polymer to form a homogenous phase.
A membrane is then extruded from the homogenous liquid composition.
Phase separation occurs between the polymer and the extractable
material as the membrane is extruded and while the temperature is
lowered down. The extractable material may be extracted from the
membrane by a suitable solvent that dissolves the extractable
material but not the polymer thus forming a microporous structure
in the membrane. optionally, either before or after the removal of
the extractable material, the extruded membrane in the phase
inversion method may be oriented or stretched beyond its elastic
limit so as to impart a permanent structure of a network of
interconnected micropores. Any method of stretching known in the
art may be suitable for this invention. Stretching can be in the
uniaxial or transverse direction. See, e.g., U.S. Pat. No.
5,281,491 and Japanese Patent Application No. 56320/1995, filed by
Kureha Chemical Industry Co. Ltd. on Mar. 15, 1995, both of which
are incorporated herein by reference.
[0036] By "particle stretch method" is intended a method of forming
a microporous film by stretching a precursor film made from a
polymer matrix filled with solid fillers dispersed therein. The
stretching results in pore formation due to stress concentration,
whereby the film is rendered microporous. Any particle stretch
methods known in the art may be used for making this invention.
Examples of such methods can be found in, for example, U.S. Pat.
Nos. 3,870,593; 4,350,655; 4,698,372; and 4,777,073, all of which
are incorporated herein by reference.
[0037] Preferably, the microporosity in the battery separator of
this invention are imparted by a stretch method, which involves
subjecting pre-formed non-porous flat sheets to uniaxial or biaxial
stretching to make them microporous. Broadly speaking, the
preferred stretch method includes annealing the multilayered
non-porous flat sheet(s) (separate or bonded together), and
subsequently stretching the annealed flat sheet(s). By way of
non-limiting example, suitable methods for this purpose are
disclosed in U.S. Pat. Nos. 5,565,281, 5,691,047, 5,691,077, and
5,824,430, each of which is incorporated herein by reference.
[0038] Preferably, in a stretch method, after the extrusion, the
non-porous flat sheet(s) are annealed before further stretching. As
generally known in the art, annealing is a heating process which
improves the crystalline structure in the precursor and facilitates
the micropore formation during the stretching step. Annealing can
be conducted by any conventional methods. For example, the film
precursor can be contacted with a heated roll or a heated metal
plate, or can be heated in air or an inert gas. Alternatively, the
film precursor can be wound around a core and heated in a roll form
in a gaseous phase. A release sheet such as polyethylene
terephthalate films, fluorine resin films, and paper or plastic
films coated with, e.g., silicone resin, may be used to prevent the
blocking of the film in the roll form. Typically, annealing can be
performed at a temperature of from about 90.degree. C. to about
150.degree. C., for a time period of from about 5 minutes to about
30 minutes.
[0039] The annealed film precursor can then be stretched (or
"oriented") to cause the formation of micropores in the structure
of the film precursor. Typically, the annealed film precursor is
uniaxially stretched in the machine direction, and optionally in
the transverse direction as well. Stretching can include several
steps, e.g., a cold drawing step, a hot drawing step, and a relax
or heat-treating step. The relax or heat-treating step is to reduce
internal stress within the separator and may be accomplished with
either negative draw ratio or substantially no draw tension at
various heat profiles. Stretching can be a continuous process
performed in ovens containing a draw frame. The temperatures and
draw ratios can be set by a skilled artisan without undue
experimentation.
[0040] In a preferred embodiment, each co-extruded multilayered
portion in the battery separator of this invention has at least one
strength layer and at least one shutdown layer. As used herein, the
term "shutdown layer" refers to a microporous membrane layer that
closes its pores at the shutdown temperature of the battery
separator, i.e., a relatively low temperature of from about
80.degree. C. to about 135.degree. C. In contrast, a "strength
layer" is a microporous membrane layer that has a substantially
higher melting point, e.g., above 145.degree. C., preferably about
160.degree. C. The strength layer typically is capable of
maintaining the melt integrity of the battery separator at a
relatively high temperature. Preferably, the shutdown layers in the
battery separator of this invention are made of polyethylene,
ethylene-butene copolymers, ethylene-propylene copolymers,
ethylene-hexene copolymers, or a blend thereof. Preferably high
density polyethylene (HDPE) is used. More preferred, the HDPE has a
density ranging from 0.959-0.964 g/min (ASTM D792) and a MFI
ranging from 0.42-0.33 dg/min @ 190.degree. C./2.16 Kg (ASTM
D1238). The strength layers in the battery separator of this
invention typically are made of polypropylene or a polypropylene
copolymer. Preferably, a polypropylene homopolymer having a density
of about 0.905 g/cc (ASTM D1505) and a MFI of about 1.5 g/10 min @
230.degree. C./2.16 Kg.
[0041] In a preferred embodiment of this invention, the two
multilayered portions of the battery separator are substantially
identical and are bonded together face-to-face. By "substantially
identical", it is intended to mean that the two bonded multilayered
portions are similar to each other with respect to their physical
structures and chemical compositions. As used herein, the term
"face-to-face" means that the two substantially identical portions
are bonded together such that the two portions are substantially
symmetrically arranged in the resultant structure as shown below in
reference to FIGS. 1-8.
[0042] FIGS. 1-8 are schematic diagrams illustrating the cross
sections of some of the preferred embodiments of the battery
separators of this invention. Each embodiment has two laminated
portions that have two or more co-extruded layers. As shown in FIG.
1, battery separator 10 has two substantially identical portions 12
and 12'. The portion 12 has a co-extruded bilayer structure
including a polypropylene layer 14 and a polyethylene layer 16. The
portion 12' has a co-extruded bilayer structure including a
polypropylene layer 14' and a polyethylene layer 16'. The portions
12 and 12' are bonded together in a face-to-face manner such that
the two polyethylene layers 16 and 16' are in contact with each
other.
[0043] In FIG. 2, the battery separator 20 has two portions 22 and
22', which are same as the two portions 12 and 12' in FIG. 1.
However, the two portions in battery separator 20 are bonded
face-to-face with the two polypropylene layers in contact with each
other.
[0044] Referring now to FIG. 3, each of the two portions 32 and 32'
of separator 30 has a trilayer structure with two polypropylene
layers 34 (34') and 38 (38') sandwiching a polyethylene layer 36
(36'). The two portions are bonded face-to-face such that the
polypropylene layers 38 and 38' are in contact with each other.
[0045] FIG. 4 is another exemplary embodiment of the battery
separator of this invention. Battery separator 40 includes two
identical portions 42 and 42' bonded together. Each portion has a
co-extruded trilayer construction including a polypropylene layer
44 or 44' sandwiched between two polyethylene layers 46 and 48, or
46' and 48'. The two portions are bonded face-to-face such that
polyethylene layers 48 and 48' are in contact with each other.
[0046] Referring now to FIG. 5, the battery separator 50 has two
portions 52 and 52'. Each portion has a polyethylene layer (54 or
54') and two adjacent polypropylene layers (56 and 58 or 56' and
58'). The two portions are bonded together face-to-face by bonding
the two polypropylene layers 58 and 58'.
[0047] The two portions 62 and 62' in the battery separator 60
shown in FIG. 6 are identical to the two portions in the battery
separator 50 shown in FIG. 5. However, the two portions 62 and 62'
are bonded together by bonding the two polyethylene layers 64 and
64' together. Thus, in battery separator 60, the first, second,
fifth and sixth layers are polypropylene layers and the third and
fourth layers are polyethylene layers.
[0048] FIG. 7 shows battery separator 70 which has a hexa-layer
structure having two portions 72 and 72' bonded together. Each
portion has two polyethylene layers 74 and 76 or 74' and 76' and a
polypropylene layer 78 or 78'. The two portions are bonded together
by bonding the two polyethylene layers 76 and 76' together.
[0049] In FIG. 8, the two portions 82 and 82' of the battery
separator 80 are same as the two portions 72 and 72' in FIG. 7,
except that the two portions 82 and 82' are bonded face-to-face by
bonding the two polypropylene layers 88 and 88' together. Thus, the
battery separator 80 has a hexa-layer structure with two inner
polypropylene layers sandwiched between four polyethylene
layers.
[0050] Preferably, the two multilayered portions in the battery
separator of this invention are arranged in a cross-plied manner,
i.e., the uniaxial orientation of one portion is at an angle
relative to the uniaxial orientation of the other portion. The
angle can be within the range of from about 0.degree. to about
90.degree.. Cross-ply can be achieved by cross-ply laminating the
two multilayered portions at the time of bonding the two
multilayered portions.
[0051] The battery separator precursor is then bonded and annealed.
Bonding is for joining together the two laminated portions of the
battery separator precursor. Bonding may be performed by passing
the precursor between the heated nip rolls under a pressure exerted
by the two closed nip rolls. Preferably, bonding is performed at a
temperature ranging from about 100.degree. C. to about 150.degree.
C., at about 125.degree. C. to about 135.degree. C. The bonding
temperature is chosen so that the two layers are united and stick
together to achieve in the finished battery separator a sufficient
adhesion without affecting the molecular orientation in the
precursor. The pressure exerted by the nip rolls can be greater
than 1 pound per linear inch (pli), or from about 1 to about 3 pli,
or preferably, about 1.2 to about 2.5 pli. The target adhesion is
at least about 5 grams/inch, preferably at least about 10
grams/inch. Annealing of the battery separator precursor 110 can be
performed at temperatures ranging from 105-150.degree. C.,
preferably from about 110 to 130.degree. C. if
polyethylene-polypropylene are employed, other annealing
temperatures should be used for other polymer employed.
[0052] As a result of the twisting of the tubular film, when the
tubular film is collapsed onto itself to form a battery precursor,
the uniaxial orientation of one portion is angularly biased against
the uniaxial orientation of the other portion, and thus cross-ply
lamination is achieved. A cross-ply battery separator can be
obtained after the subsequent steps of bonding, annealing, and
stretching as described above.
[0053] This invention is further illustrated with reference to the
examples set forth below. In the following examples, Gurley is
measured by the ASTM D-726 (B) method. As used herein, Gurley is
the resistance to air flow measured by the Gurley Densometer (e.g.
Model 4120). The Gurley values set forth herein are expressed as
the time in seconds required to pass 10 cc of air through one
square inch of product under a pressure 12.2 inches of water.
[0054] Peel strength (adhesion) measured using a tension and
compression tester to determine the force in grams required to
separate two one-inch wide sections of bonded membrane. The peel
rate is 10 inches/minute. Three measurements are taken across the
web and averaged.
[0055] The thickness of the battery separator is measured by the
T411om-83 method developed under the auspices of the Technical
Association of the Pulp and Paper Industry. Thickness is determined
using a precision micrometer with a 1/2 inch diameter, circular
shoe contacting the sample at seven (7) psi. Up to 30 individual
micrometer readings taken across the width of the sample are
averaged.
[0056] Puncture strength is measured as follows: up to 30
measurements are made across the width of the stretched product and
averaged. A Mitech Stevens LFRA Texture Analyzer is used. The
needle is 1.65 mm in diameter with 0.5 mm radius. The rate of
descent is 2 mm/sec and the amount of deflection is 6 mm. The film
is held tight in the clamping device with a central hole of 11.3
mm. The displacement (in mm) of the film that was pierced by the
needle was recorded against the resistance force (in gram force)
developed by the tested film. The maximum resistance force is the
puncture strength.
EXAMPLES
[0057] CE1 and CE2 are comparative examples. CE1 is a conventional
trilayer battery separator having a
polyethylene-polypropylene-polyethyle- ne (PE/PP/PE) construction
and prepared in a conventional manner by a blown film extrusion
process. CE2 is a conventional trilayer battery separator having a
PP/PE/PP construction and prepared in a conventional manner by a
blown film extrusion process. CE1 and CE2 are not made the
inventive technique.
[0058] E1, E1A, E2, E3, and E3A are examples of instant invention.
E1 and E1A start with three layer parison, PE/PP/PE, and yields a
hexa-layer product, PE/PP/PE/PE/PP/PE. E2 starts with a three layer
parison, PP/PE/PP, and yields a hexa-layer product,
PP/PE/PP/PP/PE/PP. E3 and E3A start with a two layer parison,
PP/PE, and yields a four layer product PP/PE/PE/PP.
[0059] All examples were made in a conventional manner with
conventional materials: PE-HDPE, density=0.959 g/cc (ASTM D792),
MFI=0.42 dg/min @ 190.degree. C./2.16 Kg (ASTM D 1238); and
PP-isotactic PP, density=0.905 g/cc (ASTM D 1505), MFI=1.5 g/10 min
@ 230.degree. C./2.16 Kg. Product results are reported in Table
1.
1 TABLE 1 CE1 E1 E1A CE2 E2 E3 E3A Thickness 1 1.2 1.1 0.83-1.05
1.1-1.2 1.1 1.1 (mil) Gurley 26 32 27-28 25-33 34-44 30-31 35-36
(Sec) Puncture 354 445-455 400-495 400-495 580-620 544-566 561-562
Strength (g) Adhesion* NA NR 27 NA 10-18 32-40 NR (g/in) NA-not
applicable; NR-not reported; *adhesion is between the bonded plies,
individual co-extruded layers are not separable.
[0060] Comparison of CE1 and E1, E1A shows that using the inventive
technique yields a separator with significantly greater strength at
equivalent thicknesses and Gurleys. Likewise, comparison of CE2 and
E2, E3, E3A shows significantly greater strength at equivalent
thicknesses and Gurleys.
[0061] The present invention may be embodied in other forms without
departing from the spirit and the essential attributes thereof,
and, accordingly, reference should be made to the appended claims,
rather than to the foregoing specification, as indicated the scope
of the invention.
* * * * *