Note: Descriptions are shown in the official language in which they were submitted.
33~
DESCRIPT10N OF THE INVENTION
The present invention is directed to a siliceous filler-
containing battery separator and the novel siliceous filler used to pro-
pare the battery separator. In commonly used electric storage batteries,
such as the well-known 12-volt battery employed in automobiles, sepal
rotors sure placed between battery plates of opposite polarity to prevent
the two plates from touching each other and causing an electrical short.
The separator is typically a mlcroporous article fabricated from a polyp
metric material, e.g., natural or synthetic rubber, or a polyolefin. The
separator may have a backing material of, for example, a non-woven web.
The pores of the separator should be as small as possible since this
reduces the danger of active materials being forced through or growing
through the separator, thereby causing an electrical short.
The separator should also have a low electrical resistance in
order to maximize the power output from the battery. Lower electrical
resistance can be obtained by reducing the overall thickness of the sepal
rotor; however, thinner separators are Gore subject to corrosion and
other physical factors affecting the service life of the separator.
Certain siliceous fillers have been used to prepare micro porous
battery separators. See, for example, U.S. Patent 2,302,832, which
~Z336~
describes the use of a silica hydrogen in a rubber binder, U.S. Patent
3,351,495, which describes synthetic and natural zealots, precipitated
metal silicates, such as calcium silicate, and silica gels as the nor-
genie filler and extender for separators of high molecular weight polyp
olefins, and U.S. Patents 3,696,061, 4,226,926; and 4,237,083, which
describe the use of finely divided, precipitated amorphous silica, such
as Hazel 233 siliceous pigment, in micro porous battery separators.
Hazel 233 amorphous silica is prepared by uninterrupted acidification,
e.g., with carbonic acid, of an aqueous solution of sodium silicate to
produce a finely-divided powder having a reported BET surface area of
between 140 and 160 square meters per gram. See, for example, U.S.
Patent 2,9~0,~30.
Amorphous precipitated silica is used as the vehicle for intro-
during porosity into and for reinforcing the polymeric material utilized
to fabricate the battery separator. Such precipitated silica is highly
absorbent and can absorb a substantial quantity of an aqueous or organic
liquid while remaining free flowing. In practice, the amorphous precipi-
toted silica is loaded with a liquid of choice, e.g., water or oil, and
then blended with the polymeric material. The liquid absorbed by the
silica filler is subsequently removed to impart porosity to the polymer.
It has now been discovered that certain novel amorphous precipi-
toted silica permits fabrication of battery separators having reduced
electrical resistance compared to separators prepared with conventional
amorphous precipitated silica, such as the aforementioned Hazel 233
silica. In addition, the precipitated silica of the present invention
provides reinforcement and strength to the polymeric material. Further,
the agglomerated particles of precipitated silica of the present invent
lion possess a structure that resists breakage caused by the mechanical
;~336~
stresses to which the silica is subjected during production of the
separator.
DETAILED DESCRIPTION OF THE INVENTION_
Corpus precipitated silica used to produce the reinforced
~lcropo~Qus polymeric battery separators of the present invention are
popper by a process involving a sequence of several steps in which the
process conditions are carefully controlled. In the first step of the
process, a firs aqueous solution of alkali metal silicate braving an
alkali metal oxide concentration of from about 5.6 to 7.2, e.g., 5.6 to
6.3, grams per liter and a temperature of between about 190F. (88C.)
and about 198F. (92C.) is established in a precipitation vessel
equipped with agitation means. Further alkali metal silicate in an
amount equal to from about 2 to about 5, preferably about 2 to 3, times
the amount of alkali metal silicate present in the first aqueous solution
is then added slowly to the precipitation vessel while simultaneously
adding acidifying agent to the aqueous solution in amounts sufficient to
maintain the alkali metal oxide concentration in the first aqueous soul-
lion at substantially the same level. Following addition of the further
alkali metal silicate to the precipitation vessel, additional acidifying
agent is added to the resulting slurry until the pi thereof is from about
8 to 9, e.g., about 8.5. This slightly alkaline slurry is then aged at
between about 188F. (87C.) and about 198F. (92C.) for from about 15
to about 90, preferably from about 30 to 45, minutes. Subsequent to the
aging step, additional acidifying agent is added to the aged slurry until
the pi thereof is from about 4.0 to about 4.7. The precipitated silica
in the acidified slurry is then recovered, washed and dried. If nieces-
spry, the dried product can be milled to break up large agglomerates to
36~1
obtain a finely divided white product in which the median aggregate part-
ale size is between about 6 and about 15, preferably between about 8 and
abut 12, microns, as measured by a Courter counter.
Any suitable-water soluble alkali metal silicate may serve as a
s~rce of the silica. Such alkali metal silicate may contain from 1 to 5
ooze of Sue per mule of alkali metal oxide. Sodium silicate contain-
in from 2 to 4 Poles Six per mole of Noah is the more widely avail-
ye a used material and hence is preferred. Typically, the
Asia ratio is about 1:3.3. Other alkali metal silicates, such
as lithium or potassium silicate, may also be used.
The first aqueous solution of alkali metal silicate, i.e., the
s~lutlon containing from about 5.6 to 7.2 grams per liter of alkali metal
ox is typically prepared by adding an aqueous alkali metal silicate
lotion e.g., sodium silicate having a Nash ratio of 1/3.3, to
a predetermined quantity of water heated to between about 190F. (80C.)
and about 198F., (92C.) e.g., about 195F. (91C.), in amounts suffix
Kent to establish the desired concentration. This solution is agitated
to assure efficient mixing of the alkali metal silicate added to the
water and subsequent mixing of the further alkali metal silicate and acid-
ifyi~g agent added thereto.
In the second step of the process, further alkali metal sift-
cat and acidifying agent are then added slowly and simultaneously to the
first aqueous solution. These are added in relative amounts and at rates
sufficient to maintain the alkali metal oxide concentration in the first
aqueous solution substantially constant, i.e., at substantially its in-
trial value. Thus, the amount of acidifying agent added to the vessel
containing the first aqueous alkali metal silicate solution will be
slightly less than the stoichiometric amount required for the further
I
alkali metal silicate added to the precipitation vessel so as to compel-
sate for the effect of dilution by the alkali metal silicate and acidify-
in agent added during the second step. The amount of further alkali
metal silicate added is from about 2 to 5, preferably 2 to 3, times the
amount of alkali metal silicate initially present in the first aqueous
solution.
The further alkali metal silicate is typically added over a
period of from about 60 to 150 minutes, e.g., about 90 minutes. The par-
titular addition time will, of course depend on the multiple of further
alkali metal silicate added, e.g., multiples of 2 to 5. During addition
of the further alkali metal silicate and acidifying agent, the tempera-
lure of the resulting slurry in the precipitation vessel is maintained at
about the temperature of the starting alkali metal silicate aqueous soul-
lion, e.g., between about 190F. (88C.) and 198F. (92C.). The further
alkali metal silicate added to the precipitation vessel will typically
have the same alkali metal cation as the cation of the starting alkali
metal silicate solution, e.g., sodium.
Acidifying agent used to neutralize Lye alkali metal silicate
typically is carbonic acid or an inorganic mineral acid, e.g., hydrochlo-
fig acid or sulfuric acid. Different acids may be used in the various
process steps if desired. The carbonic acid acidifying agent can be fur-
nighed by introducing carbon dioxide into the alkali metal silicate aqua-
out solution. Acidifying agent is added gradually to the precipitation
vessel and the amount required is determined by monitoring the pi of the
alkali metal silicate solution or slurry in such vessel. The pi can be
measured by any convenient commercially available pi meter.
After completing the addition of further alkali metal silicate
and acidifying agent to the precipitation vessel, additional acidifying
~2~36~
agent is added slowly, and preferably at the same rate used during the
preceding (second) process step until the pi of the siliceous slurry is
between about 8 and about 9, i.e., slightly alkaline, e.g., about 8.5.
Thereupon, the slurry is agitated and allowed to age at between about
188F. (87C.~ and 198~F. (92C.), i.e., substantially the temperature
maintained during the precipitation step, for from about 15 to about 90
minutes, usually between about 30 and about 45 minutes. Following the
aging step, additional acidifying agent is added slowly with agitation to
the slurry until the pi thereof reaches between about 4.0 and about 4.7,
e.g., between about 4.3 and about 4.7.
The precipitated silica in the slurry is recovered from the
slurry by any suitable solid-liquid separating means such as a filter
press, centrifuge etc. The resulting filter cake can be washed with
water to remove water soluble salts, e.g., sodium chloride and/or sodium
sulfate. Silica prepared by the above-described method using sulfuric
acid as the acidifying agent will typically have a sodium chloride con-
tent of less than about 0.10 weight percent, e.g., less than 0.07 weight
percent, a sodium sulfate content of less than about 2.5, preferably less
than 2.0, weight percent, and a sodium oxide content less than about 1.5
weight percent, e.g., preferably not more than about 1.0 weight percent.
The aforesaid values can be determined by X-Ray fluorescence
spectroscopy.
Following washing, the filter cake is dried by any suitable
drying means, e.g., spray drying, tray and compartment drying, or rotary
drying. The dried silica may be used as recovered from the drying step
if the particles are sufficiently finely-divided, e.g., such as product
recovered from a spray dryer. of, however, the drying step produces
large, hard agglomerates or cemented particles, the product can be sub-
1~3~
jetted to a milling or grinding step to produce a more finely-divided
product having the appropriate aggregate particle size.
After drying, the silica is a white, fluffy, pulverulent powder
that is dry to touch. Despite appearing dry, the silica normally con-
twins water, e.g., between about 2 and 8 percent "free water" by weight.
Free water is that water which is removed from the silica by heating it
at 105C. for 24 hours. The silica also contains "bound water", which
refers to that water removed by heating the silica at ignition tempera-
lure, i.e., 1000C. to 1200C. for an extended period, e.g., 24 hours.
Bound water can constitute between about 2 and 6 percent of the slice.
Chemically, the finely-divided, amorphous precipitated hydrated silica
contains at least 85, preferably at least 90 and more preferably 93 to 97
weight percent Sue on an an hydrous basis, i.e., not including free
water.
The precipitated silica prepared by the above described process
will typically have a BET surface area of between about 130 and 180,
e.g., about 150, square meters per gram (m go and an oil absorption of
between about 200 and 270, e.g., preferably between about 230 and about
260, milliliters of oil per hundred grams of silica. In addition, the
silica will typically have a water absorption of between about 160 and
about 180 milliliters per 100 grams of silica and a median agglomerate
particle size of between about 6 and about 15, preferably between about 8
and 12 microns (micrometers), as measured by a Courter counter. The bulk
density is from about 8 to 12 pounds per cubic foot (12.8-19.2 kg/m3),
e.g., 10 pounds per cubic foot (16 kg/m3).
The surface are of finely-divided silica can be determined by
the method of Browner, Emmett and Teller, J. Am. Chum. So., 60, 309
(1938). This method, which is often referred to as the BET method, meat
123;~
surges the absolute surface area of the material by measuring the amount of gas adsorbed under special conditions of low temperature and pros-
sure. The BET surface areas reported herein were obtained using nitrogen
as the gas adsorbed and liquid nitrogen temperatures (-196C.) and at a
0.2 relative pressure. Oil absorption and water absorption values are
the volumes of dibutylphthalate oil and water respectively necessary to
wet 100 grams of the silica. These values can be obtained using a method
like the method described in ASTM D2414-65.
The specific volume of the precipitated silica prepared by the
above-described process will be at least 3.5 cubic centimeters per gram
(cm3/g), e.g., 3.5-4.7 cm3/g when compacted with an applied pressure
of 17 pounds per square inch (Sue (117 spa), and will be at least 2.5
cm3/g, e.g., 2.5-2.7 cm3/g when compacted at an applied compaction
pressure of 280 psi (1~31 spa).
Compaction of precipitated silica and measurement of the spew
cilia volume thereof as a function of the applied pressure is useful for
discriminating between different silicas. For example, the specific volt
use of a silica (which is a value obtained by dividing the sample volume
at a given applied pressure by the sample weight) may be correlated to
other physical properties of the silica, such as its porosity. Different
silicas exhibit unique compaction behavior as eke applied pressure is
increased, thereby gradually eliminating the silica's porosity.
Specific volume measurements, as reported herein, may be cowlick-
fated using the loading curve generated by an Instron mechanical testing
machine, the sample weight, and dimensions of the die chamber (and hence
the sample volume) at any given applied pressure.
The amorphous precipitated silica prepared by the above-
described process is a finely-divided solid, particulate material in the
e yoke - 8 -
12;~36~1
form of reinforced floes or agglomerates of smaller particles of sift-
Swiss material. As initially precipitated, amorphous silica is composed
of ultra fine, solid spherical particles having an average diameter of
about 0.02 microns which appear as strands of beads. These strands got-
feat and intertwine to form a loose aggregate structure with open pyres-
try. In the present process, the valleys and gaps i.e., the interstices,
of the individual strands are filled in with further silica particles to
produce strands having a smoother, e.g., more rod-like, appearance.
These reinforced, intertwined strands provide a final agglomerate snug
lure which is more resistant to the mechanical forces, i.e., resists
being broken down into smaller aggregate structures, applied to it during
preparation of the battery separator than non-reinforced amorphous precip-
stated silica.
between about 10 and about 90 weight percent, basis the polyp
metric material, of the amorphous precipitated silica described herein
above is used to produce the reinforced micro porous polymeric battery
separator. More particularly, between about 20 and 75, e.g., between 30
and 60, weight percent of the silica is so used.
The polymeric material into which the silica is incorporated to
prepare the micro porous battery separator can be any of the conventional
natural and synthetic polymeric materials conventionally used to
fabricate battery separators. Among such materials, there can be
mentioned natural rubber, styrene-butadiene rubber, nitrile-butadiene
rubber, polyisoprene, high molecular weight olefins such as polyethylene,
polypropylene, polybutene, ethylene-propylene copolymers, ethylene-butene
copolymers, propylene-butene copolymers, ethylene-propylene-butene Capella-
mews, polyvinyl chloride and vinyl chloride-vinyl acetate copolymers.
Mixtures of such materials have also been used to prepare battery swooper-
ions.
~LX~3~
Other conventional materials added to the polymeric material,
such us plasticizers, antioxidant, wetting agents, carbon black and cur-
in agents e.g., sulfur, for rubbery polymeric materials may also be
added to the composition used to prepare the battery separator.
battery separators incorporating the above-described precipi-
toted silica can be prepared in accordance with known techniques for pro-
paring such articles. A typical procedure fur preparing a battery swooper-
ion utilizing a curable rubber is described in U.S. Patent 4,226,926. In
that patent, the siliceous filler is dehydrated to levels of between 65
and 75 percent by admixing the siliceous filler with water. The result-
in free flowing dehydrated silica powder is admixed with the polymeric
material, e.g., in a Danbury mixer. Thereafter, the mixture (including
any additional additives required for curing the polymeric particle) is
milled on a 2-roll mill to produce a milled sheet. The milled sheet is
soaked in hot water and then calendered for contours. Optionally a back-
in such as paper or a heat-bonded mat is added to the milled sheet. The
calendered sheet is then cut into appropriate sizes.
Another similar procedure is described in U.S. Patent
3,351,495. There, the polymeric material, e.g., a polyolefin having a
molecular weight of at least 300,000, is blended with the inert filler,
e.g., silica, and a plasticizer. The blend, which may also contain con-
ventional stabilizers or antioxidant, is molded or shaped, e.g., by
extrusion, calendering, injection molding or compression, into sheets.
Plasticizer and/or filler is removed from the sheet by soaking the sheet
in a suitable solvent, e.g., chlorinated hydrocarbons for a petroleum oil
plasticizer, and water, ethanol, acetone, etc. for a polyethylene glycol
plasticizer.
-- 10 --
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The present invention it more particularly described in the
fulled examples which are intended as illustrative only since numerous
modiflcatio~s and variations therein will be apparent to those skilled in
the art.
sample 1
5vdium silicate having an Nash ratio of about 3.3 was
added with agitation to water heated to 192F. in a precipitation vessel
twill the aye concentration in this first aqueous solution was 7.0 +
0.2 Russ per liter. While maintaining the first aqueous solution at
192~F., three limes the initial volume of sodium silicate and concern-
treated sulfuric acid were added with agitation over 90 minutes to the
precipitation vessel. The rate of acid addition was calculated to main-
lain the NATO concentration in the vessel at about 7.0 grams per
liter. When addition of the three additional volumes of sodium silicate
was completed, acid addition was continued until the pi of the resulting
slurry was about 8.5. Acid addition was then interrupted for 45 minutes
Jo permit the slurry to age. At the end of the aging period, concern-
treated sulfuric acid was added to lower the pi of the slurry to about
4.6. The precipitated silica was filtered and the filter cake washed
with water to reduce the level of sodium sulfate by-product salt in the
wake to less than 2.5 weight percent. The washed filter cake was ruffled-
iced and spray dried. The dried silica was then milled. The product was
submitted for physical analysis. Results are tabulated in Table I. The
silica was also analyzed for chloride ion, sulfate ion, and sodium ion by
X-Ray fluorescence spectroscopy using a model XRD-41Q automated X-Ray
spectrograph. The spectrograph employs a dual target X-Ray tube (lung-
stenJchromium) operating at 60 kilovolts and 50 milliamperes for excite-
lion of the minor and trace element spectra.
-- 11 --
~'~12. or
~Z33~
Example 2
The process of Example 1 was repeated except that the tempera-
lure of precipitation was 195 OF. Results of analyses of the silica there-
by produced are tabulated in Table I.
Table I
Nail, No SO , No 0,
Silica OAT- H O/A S A 3C.C.4 FOE- % 2% 4 %2
-2
Example 1 253 176 149 12 6.3.03 1.761.37
Example 2 b 240 158 133 END .06 1.3i 0.61
icily 233 200 153 150 13 6 .07 1.8 1.0
1. OX = oil absorption, ml/100 grams
2. HOWE = Water absorption, m~/100 grams
3. SPA. = BET surface area m gram
4. COCK = Courter counter aggregate particle size, micrometers
5. FEW. = Free water loss at 105~C., percent
a. Average of 5 values
b. Typical analysis
c. Average of 4 values
NOD. - jot Determined
The precipitated silica of Example 1 and a Hazel 233 precipi-
toted silica were used to prepare battery separators. The electrical
resistance of samples of such battery separators was measured in sulfuric
acid (Specific Gravity 1.223~ at about room temperature after soaking in
the acid for 24 hours to eliminate air bubbles.
The electrical resistance (milliohm - in Molly of thickness)
of the battery separator prepared with the Hazel 233 type pigment was
found to be 5.00 (average of two samples). By comparison, the electrical
resistance of the battery separators prepared with the siliceous filler
of Example 1 was 3.77 leverage of three values). This represents a reduce
lion in electrical resistance of 1.23 or about 24 percent.
- 12 -
I
Example 3
Two samples of precipitated silica sold for use in rubber rein-
forcemeDt and eye samples of precipitated silica (A-G) prepared in
accordance lath the process described in the present application were
loaded into a cylindrical steel die having an inside diameter of 1.25
inches ~3.J8 centimeters) and a height of 2.5 inches (6.35 centimeters)
ox a Instron Model TO mechanical testing machine. The die was filled
capitol with the silica and gently vibrated to ensure that the die
cell was uniformly filled. The loaded silica was continuously compacted
at a machine crosshead velocity of 0.02 inch/minute (.05 centimeter/min-
vie) in the double action mode. A dial indicator measured plunger disk
placements. The silicas were compacted to a load near 10,000 pounds
(4536 kg), the capacity of the machine, after which the compacted silica
samples were unloaded and accurately weighed. The specific volume of
each of the silicas was calculated at 17 psi (117 spa), 280 psi (1931
spa), 4500 psi (31 Ida and 8000 psi (55 Ida Results are tabulated in
Tale I.
Table II
Specific Volume
Silica Compaction Pressure (PSI) 17 280 4500 8000
issue aye 2.967 2.195 1.092 0.851
Hazel 260 b 3.361 2.504 1.362 1.131
Ultrasil~ VN-3 3.155 2.409 1.270 1.112
a Precipitated silica with physical properties like Hazel 233.
b Silica Product of Degas Corp. hiving thy following reported prop-
reties: BET surface area - 170 mug DIP Oil Absorption - 225
ml/100 g; bulk density - 15 lb/t-t .
- 13 -
4.~72 -- 1.291 1.017
B 3.5792.489 1.179 0.913
C 4.0182.518 1.152 0.187
3.72~2.608 1.325 1.102
3.7002.664 1.320 1.108
F 3.6882.611 1.315 1.103
G 3.9672.713 1.351 1.061
The data of Table II show that the precipitated silica of the
present 1n~entLon, i.e., silica samples A-G, have higher specific volumes
thaw the to commercial grades of silica when compacted at 17 psi (117
up cub precipitated silicas, therefore have higher porosity which
contributes Jo the improved performance of battery separators prepared
with such silicas. As the applied pressure us increased and the porosity
of the silica is eliminated, the specific volumes are lowered accordingly
and top d~ffPre.nces between the commercial silica samples and those
prepared by thy a~ove-described process are reduced.
hole eke invention has been described in detail with respect
to certain embodiments thereof, it is to be understood that the invention
is not intended to be limited to such details except as and insofar as
they appear in the appended claims.
