Alkaline battery with separator of high surface area

The instant invention is directed to battery separator membranes useful in alkaline battery systems formed from a homogeneous admixture of a polyolefin, a plasticizer, and a filler selected from particulate material having a surface area of from 100 to 385 m.sup.2 /cc and a pore volume of at least 0.075 cc/gm.

BACKGROUND OF THE INVENTION 
The instant invention is directed to an improved battery separator membrane 
suitable for use in alkaline battery systems, such as nickel-zinc systems, 
which is capable of exhibiting an unexpected high degree of dendrite 
inhibition, electrical conductivity, stability to alkali, and ability to 
form very thin membranes. 
Alkaline battery systems, because of their high energy density, have great 
potential for replacing the more conventional lead-acid battery system in 
a number of terrestrial applications. However, extending the cycle life of 
such batteries beyond that presently attainable and reducing the cost of 
all the components are required criterias which must be met to make the 
alkaline battery an effective energy source. 
One of the recognized key components in extending the life and efficiency 
of the battery is its separator. The separator is a membrane located 
between the plates of opposite polarity to prevent contact between the 
plates while freely permitting electrolytic conduction. Contact between 
plates may be due to imperfections in the plate structure or due to 
warping or wrinkling of the plate during use. Such macro deformations are 
readily inhibited by any type of sheet material which is coextensive with 
that of the plates. Contact may also occur due to the formation of 
dendrites or localized needle like growths on an electrode, such as zinc 
dendrites formed on a zinc electrode in an alkaline nickel-zinc battery 
system. These dendrites bridge the gap between electrodes of opposite 
polarity either by puncturing the separator membrane located in the gap, 
or by passing through the pores of the separator. The high degree of 
solubility of zinc oxide in alkaline electrolytes normally permits 
extensive loss of active material from the negative electrode through 
deposition of the zinc oxide in the separator pores and onto the positive 
electrode. These factors cause shorting out of the battery system and 
significantly reduce its effective life. The ability to produce a 
separator membrane which can effectively act as a dendristatic diaphragm 
is a required criteria for forming an effective battery system. 
Further, a separator suitable for use in forming a highly effective 
alkaline battery system must be capable of exhibiting a high degree of 
electrical conductivity. Stated another way, an effective separator 
membrane must exhibit a low electrical resistance and good wetting 
properties. 
U.S. Pat. No. 3,351,495 discloses battery separators for use in both acid 
and alkaline battery systems formed from very high molecular weight 
polyolefin compounded with a plasticizer and an inert filler. The 
reference further teaches that, in alkaline battery separators, filler 
having relatively low surface areas, e.e., one square meter per gram or 
less, are satisfactorily employed. Battery separators formed in accordance 
with the general procedure and materials disclosed in this patent exhibit 
a high degree of electrical resistance and poor wetting properties. These 
separators, therefore, do little to enhance the efficiency and 
effectiveness of the resultant battery systems. 
U.S. Pat. No. 4,024,323 is directed to a variation of the 3,351,495 battery 
separator which aids in processability. The resultant separator has 
similar defects. 
A battery separator which is capable of increasing the efficiency of a 
battery system and cause it to have a high energy density is highly 
desired, especially with respect to alkaline battery systems. It is 
generally agreed that such separators should be (a) resistant to 
degradation by the alkaline electrolyte and by oxidation due to nascent 
oxygen, (b) be very thin, (c) exhibit a high degree of inhibition to 
dendrite formation and growth, and (d) exhibit a high degree of electrical 
conductivity. The first two elements and the last two elements are each 
thought to be counter productive with respect to each other. For example, 
very thin sheets have a high surface area to volume ratio and are, 
therefore, more susceptible to attack by the strong alkaline electrolyte 
solution and to oxidation. With respect to the latter two criterias, it is 
known that separator membranes which are nonporous normally exhibit a high 
degree of inhibition to dendrite formation, but have low electrical 
conductivity. Microporous separators, that is those which have discrete 
pores usually in the form of a tortuous network, have a high degree of 
electrolyte permeability but they lack the ability to inhibit dendritic 
shorting. 
SUMMARY OF THE INVENTION 
The present invention is directed to an improved separator membrane which 
is resistant to degradation by common alkaline electrolyte solutions and 
oxidation, can be formed into thin sheets, exhibits a high degree of 
inhibition to dendrite formation and growth while also exhibiting a high 
degree of electrical conductivity (a low degree of resistivity). The 
present separator which unexpectedly exhibits this combination of 
properties is sheet formed from a homogeneous admixture of a polyolefin, a 
plasticizer for said polyolefin, and a particulate filler material 
selected from titania, alumina, magnesium or calcium hydroxide or mixtures 
thereof having a surface area in the range of from 100 to 385 m.sup.2 /cc 
and a pore volume of 0.075 cc/gm. 
DETAILED DESCRIPTION OF THE INVENTION 
The alkaline battery separator of the present invention is in the form of a 
very thin sheet which is required to be formed from a homogeneous 
admixture of a polyolefin, a plasticizer for the polyolefin and a 
particulate filler, each described herein below. The components of the 
admixture are present in from 5 to 20 weight percent of the polyolefin, 
from 10 to 60 weight percent of the platicizer and from 30 to 75 weight 
percent of the filler. 
The present invention requires the utilization of a polyolefin, preferably 
polyethylene or polypropylene of high density. The polyolefin must have an 
average molecular weight of at least 100,000, and can be selected from 
polyolefins having average molecular weights of from 100,000 to about 
2,000,000. The polyolefin can be selected from homopolymers, such as 
polyethylene or polypropylene or from copolymers formed from a mixture of 
hydrocarbon olefinic monomers, such as ethylene, propylene, butene and the 
like, or from a mixture of at least 90 percent by weight of hydrocarbon 
olefinic monomer with other olefinic monomer, such as acrylic acids and 
esters. 
The polyolefin can be comprised of a mixture of a high molecular weight 
polyolefin and a low molecular weight polyolefin. Representative of 
polyolefins of high and low molecular weight which are operable in the 
instant invention are polyethylene, polypropylene, polybutene, 
ethylene-propylene copolymers, ethylene-butene copolymers, 
propylene-butene copolymers, ethylene-acrylic acid copolymers and the 
like. The mixture can be formed from about 5 to 95 weight percent high 
molecular weight polymer with the corresponding about 95 to 5 weight 
percent of low molecular weight polymer. It is preferred that the low 
molecular weight polymer be the major component of the polyolefin mixture. 
The term "high molecular weight polyolefin," as used herein, is intended to 
refer to a polyolefin having an average molecular weight of at least 
500,000. The term "low molecular weight polyolefin," as used herein, 
refers to polyolefins having an average molecular weight of from 100,000 
to 500,000. 
When only one polyolefin is used in forming the subject separator, the 
average molecular weight should preferably be greater than 150,000 and, 
preferably, greater than 200,000. 
The polyolefin must be substantially insoluble in the solvents used and at 
the temperatures used to extract the plasticizer from the 
polyolefin-filler-plasticizer composition. Such insolubility or inertness 
to the action of solvents is imparted to the polyolefin by its 
crystallinity content or by the judicious choice of solvent used in the 
extraction procedure. The partially crystalline polyolefin, such as 
polyethylene and isotactic polypropylene are ideally suited to such an 
application because they are substantially insoluble in common 
hydrocarbons and other organic and aqueous solvents at low temperatures. 
Conventional stabilizers or antioxidants are employed in the compositions 
of the present invention to prevent thermal and oxidative degradation of 
the polyolefin component. Representative of the stabilizers are 4,4 
thiobis (6-tert-butyl-m-cresol) ("Santonox"), and 
2,6-di-tert-butyl-4-methylphenol ("Ionol"). 
The plasticizer of the instant composition improves the processability of 
the composition, i.e., lower the melt viscosity, or reduces the amount of 
power input which is required to compound and to fabricate the composition 
and aids in inducing porosity, as discussed herein below. 
The plasticizer can be soluble or insoluble in water. Representative of the 
water-insoluble plasticizers are organic esters, such as the sebacates, 
phthalates, stearates, adipates, and citrates; epoxy compounds such as 
epoxidized vegetable oil; phosphate esters such as tricresyl phosphate; 
hydrocarbon materials such as petroleum oil including lubricating oils and 
fuel oils, hydrocarbon resin and asphalt and pure compounds such as 
eicosane; low molecular weight polymers such as polyisobutylene, 
polybutadiene, polystyrene, atactic polypropylene, ethylene-propylene 
rubber; ethylene-vinyl acetate copolymer, oxidized polyethylene, 
coumarone-indene resins and terpene resins; tall oil and linseed oil. 
Illustrative of the water-soluble plasticizers are ethylene glycol, 
polyethylene glycol, polypropylene glycol, glycerol, and ethers and esters 
thereof; alkyl phosphates such as triethyl phosphate; polyvinyl alcohol, 
polyacrylic acid and polyvinyl pyrolidone. 
When a plasticizer is used which is not totally removed from the 
composition during the extraction step but forms part of the battery 
separator, it imparts flexibility, high elongation and resistance to the 
battery separator. 
There are a number of water-soluble, normally solid plasticizers which are 
sufficiently inert to form a part of the battery separator. Typical 
examples of these plasticizers are polyisobutylene, polybutadiene, 
polystyrene, atactic polypropylene, ethylene-propylene rubber, and 
ethylene vinyl acetate copolymer. Generally, when this type of plasticizer 
is used, it can be included in the battery separator in an amount as high 
as 40 percent by volume of the battery separator composition. 
The fillers required for formation of the improved alkaline separator are 
titania, alumina, magnesium or calcium hydroxide or mixtures thereof which 
have the specifically required properties of surface area of from 100 to 
385 m.sup.2 /cc and pore volume (BET) of at least 0.075 cc/gm and, 
preferably, from 0.08 to 0.8 cc/gm. The most desirable separator is formed 
from the above-described polyolefin and plasticizer with titania, having 
surface area of from 180 to 325 m.sup.2 /cc and more preferably from 200 
to 300 m.sup.2 /cc, and a pore volume of at least 0.075 cc/gm, and more 
preferred from 0.1 to 0.4 cc/gm. The surface area of the presently 
required fillers is determined in surface area per unit volume. This takes 
into account variations in densities of the respective fillers. The 
density of the desired fillers are about 3.9 to 4.2 for alumina; about 3.8 
to 4.3 for titania; about 2.4 for magnesium hydroxide; and about 3.3-3.4 
for calcium hydroxide. 
It has been presently found that when the above-described particulate 
filler material is used, the electrical conductivity and the dendristatic 
ability of the separator is greatly enhanced over that obtained by 
conventional separator membranes. It is believed that the presently 
required filler material has certain properties which allow the filler to 
interact with the electrolyte in a manner which causes exceptionally good 
conduction of the electrolyte through the separator membrane. The enhanced 
properties attained by the subject fillers is due to a combination of 
effects. It is presently believed that the porosity is sufficient to 
permit conduction of electrolyte therethrough while being sufficiently low 
to aid in inhibition of dendrite growth and formation. The required 
surface area of the filler is believed to aid in permitting wetting of the 
filler particles and, therefore, the separator with electrolyte and to be 
sufficient to also aid in holding at least a part of plasticizer during 
processing. Finally, it is believed that the chemical nature of the 
required fillers permits absorption of the electrolyte by the filler and 
thereby acts as a micro wick which acts as a ready means of passage of 
electrolyte through the separator membrane, and to permit more even 
wetting of the membrane by the electrolyte. 
The filler particles can be of a size ranging from an average of about 0.01 
micron to about 10 micron in diameter. 
It should be understood that any of the commercially available wetting 
agents known to the art, such as sodium alkyl benzene sulfonate, sodium 
lauryl sulfate, dioctyl sodium sulfosuccinate, and isooctyl phenyl 
polyethoxy ethanol, can be used to enhance the wettability of the filler 
prior to its inclusion in the composition and to thereby cause a more 
uniform distribution of the filler in the admixture. 
A preferred embodiment of the subject invention further requires the 
inclusion of carbon black in less than 10 weight percent, based on the 
total weight of the admixture. The carbon black should be of the type 
conventionally known as conductive carbon black, which exhibit low 
hydrogen overvoltage. They must also have a surface area of at least 100 
m.sup.2 /cc and more preferably from 250 to 2000 m.sup.2 /cc. The particle 
size of the carbon blacks are from about 1 to 75 millimicrons (BET). The 
preferred and most effective amount is from 0.25 to 5 weight percent. The 
carbon blacks meeting these requirements are commercially available. It 
has been unexpectedly found that when one includes the above-described 
conductive carbon black, one effectively reduces the shorting between 
electrodes of opposite polarity, due to dendrite growth. 
According to the invention, the battery separator is produced by a process 
which comprises blending a composition of from 5 to 25 weight percent of 
polyolefin, 30 to 75 weight percent of filler material, and from 20 to 60 
weight percent plasticizer, forming said composition into sheet form and, 
subsequently, extracting from said sheet by means of a suitable solvent at 
least a portion of the plasticizer. The preferred weight percent of the 
above components of the admixture are, respectively, 5 to 20; 35-70; and 
15 to 50. 
The composition of the resultant separator will depend upon the degree of 
extraction of the plasticizer. The plasticizer can be substantially 
completely removed, leaving a highly filled polymeric sheet product or, 
alternatively, can have 60 percent and, preferably, 75 percent of the 
plasticizer of the admixture removed. These products normally show good 
retention of physical properties, as well as good electrical stability and 
dendrite inhibition properties. The extracted separator membrane normally 
has from about 7 to 30 percent polyolefin, about 50 to 93 percent filler, 
and from about 0 to 15 percent plasticizer. The more preferred separators 
comprise a mixture of from 10 to 25 percent polyolefin, 60 to 90 percent 
filler, and from 2 to 8 percent plasticizer. 
In a preferred embodiment, 8 weight percent polyolefin, 70 weight percent 
filler, and 22 weight percent plasticizer are blended together, extruded 
to provide a flat sheet and then sufficient plasticizer is extracted to 
provide a finished separator composed of 10 weight percent polyolefin, 
87.5 weight percent filler, and 2.5 weight percent plasticizer. 
A particularly preferred composition consists essentially of polyethylene 
having at least 50 percent by weight crystallinity, finely-divided 
titania, conductive carbon black, and petroleum oil. 
The components of the instant composition can be mixed by any conventional 
manner which will produce a substantially uniform mixture. To produce a 
particularly uniform mixture, the components can be premixed at room 
temperature in a blender. The polyolefin-filler-plasticizer dry blends are 
then fluxed in a conventional mixer, such as a Banbury mixer or melt 
homogenized in a conventional two roll mill. 
After being suitably mixed, the composition is molded or shaped in any 
conventional manner. Specifically, it can be fed to an extrusion, 
calendering, injection molding, or compression molding machine to be 
processed into its final form. 
The term "sheet" or "membrane" as used in the subject application is 
intended to define a planar material which is formed from the initially 
composed admixture both prior and subsequent to extraction of plasticizer 
therefrom. The sheet material should be preferably, a film which is less 
than about 10 mils thick and, more preferably, less than 7 mils thick with 
from about 2 to 7 being most perferred. It has been found that the highly 
filled polymeric composition of the present invention can be readily 
formed into such thin sheet material by conventional techniques. 
The process of forming the subject separator comprises blending the 
components described herein above to form a substantially uniform 
admixture thereof, forming the admixture into a sheet product and, 
subsequently, extracting from said sheet at least a portion of the 
plasticizer contained therein. 
The procedure for extraction of the plasticizer from a sheet product is 
well known and is not meant to form a part of the present invention, per 
se. A single stage extraction can be used. The solvent or extraction 
conditions should be chosen so that the polyolefin and filler are 
essentially insoluble. For example, when petroleum oil is to be extracted 
from the molded composition, the following solvents are suitable; 
chlorinated hydrocarbons, such as trichloroethylene, tetrachloethylene, 
carbon tetrachloride, methylene chloride, tetrachloroethane, etc; 
hydrocarbon solvents such as hexane, benzene, petroleum ether, toluene, 
cyclohexane, gasoline, etc. If polyethylene glycol is to be extracted, the 
extraction medium can be water, ethanol, methanol, acetone, aqueous or 
alcoholic sodium hydroxide, potassium hydroxide, and the like. Generally, 
acids such as hydrochloric acid should not be used as these would attack 
the required filler component. 
The extraction temperature can range anywhere from room temperature up to 
the melting point of the polyolefin as long as the polyolefin does not 
dissolve. 
The time of the extraction will vary depending upon the temperature used 
and the nature of the plasticizer or filler being extracted. For example, 
when a higher temperature is used, the extraction time for an oil of low 
viscosity can be only a few minutes, whereas if the extraction is 
performed at room temperature, the time requirement for a polymeric 
plasticizer can be in order of several hours. 
The final composition of the separator will depend upon the original 
composition and the degree of extraction of the plasticizer from the sheet 
product. 
The surface area, pore sizes and the pore volumes of the filler of the 
instant invention were measured using the nitrogen absorption method 
described by S. Brunauer, P. J. Emett, and E. Teller in the Journal of 
American Chemical Society, Vol. 6, page 308 (1938), and commonly known as 
the BET method. 
The average pore sizes of the resultant separator membrane were measured by 
water permeability method in accordance with the procedure described in 
"Characteristics of Separator for Alkaline Silver Oxide-Zinc Secondary 
Batteries-Screening Methods" by J. E. Cooper and A. Fleischer. 
The electrical resistances of the resultant separators were determined by 
the direct current method described by J. E. Cooper and A. Fleischer, 
supra. 
The porosity volume percents or void volume percent were calculated for the 
resultant separator from wet weight minus dry weight divided by the 
separator sample geometric wet volume. 
Chemical and oxidation stability of the resultant separator was measured by 
subjecting duplicate samples of separator to KOH solution (45%) at 0.4 
amp/cm.sup.2 overcharage for 96 hours at 80.degree. C. and compared weight 
loss and/or physical property changes with that of untreated sample. 
Tensile tests were conducted on a Scott Tester or Instron Tensile Testor 
(model TM) using a sample width of 0.25 inch and a 0.4 inch jaw 
separation, and a cross head speed of 0.2"/min. 
The following examples are given for illustrative purposes only and are not 
meant to be a limitation on the subject invention, except as made in the 
claims appended hereto. All parts and percentages are by weight unless 
otherwise indicated.

EXAMPLE I 
A separator membrane of the present invention was formed by initially 
introducing into a B-Banbury internal mixer 10 parts of commercially 
available high density polyethylene (MW of 250,000), 5 parts of 
commercially available high density polyethylene (MW of about 2,000,000) 
and 38 parts of low aromatic, saturated hydrocarbon petroleum oil 
(Shellflex 411; 547 SSU at 110.degree. F.) and 0.1 part of Santonox 
followed by 47 parts of titanium dioxide (P-25; 5% rutile, 95% anatase, 
density 4.3, BET surface area 280 m.sup.2 /cc, BET pore volume 0.35 cc/g, 
average pore diameter 212A). After complete addition of the components, 
they were compounded in the mixer at 400.degree. F. for about 8 minutes. 
The resultant composition was removed from the mixer, cooled, and ground 
to a coarse powder in a Wiley mill. The powder was fed into a one inch 
single screw extruder, operated at 400.degree. F., and extruded as pellets 
which were passed through two 40 mesh (U.S. Standard) screens to remove 
any large agglomerates. The pellets were subjected to a second extruder 
similar to the first, except that it was equipped with an eight inch 
sheeting die capable of forming sheet material of approximately 9-10 mil 
thick at the rate of 3 ft/min. The sheet was passed over 2 annealing rolls 
maintained at 200.degree. F. and then 3 cooling rolls. The sheet product 
was immersed in 1,1,1-trichloroethane for 30 minutes, air dried, and then 
immersed in a second fresh bath of trichloroethane for 30 minutes. Samples 
of the extracted sheet separator were analyzed and showed that greater 
than 90 percent of the plasticizer was removed. 
The material was tested by standard techniques to determine dendrite 
penetration, electrical resistance, and stability. Results are given in 
Table I below. 
EXAMPLE II 
A battery separator was formed in accordance with the preferred embodiment 
of the present invention in the same manner as described in Example I 
above, except that 2 parts of the titanium dioxide filler were substituted 
by 2 parts of a conductive carbon black capable of exhibiting low hydrogen 
overvoltage (Vulcan XC-72; 414 m.sup.2 /cc BET surface area, 1.8 density, 
BET particle size of m.mu.). 
The extracted separator membrane had greater than 90 percent of plasticizer 
removed. The separator was tested for dendrite penetration, electrical 
resistance, and stability. Results are given in Table I below. To readily 
compare samples, a figure of merit is calculated by dividing the dendrite 
resistance by the electrical resistance. Since a high dendrite resistance 
and a low electrical resistance are desired features, it is readily seen 
that the higher the figure of merit value, the more desirable the 
separator overall properties. 
TABLE I 
______________________________________ 
Thickness ER Dendrite Resistance 
Figure 
Sample 
(Mils) ohm-cm Min Min/Mil 
of Merit 
______________________________________ 
I 9.9 9.8 164 16.6 1.7 
II 10 13.3 490 49.0 3.7 
______________________________________ 
EXAMPLE III 
Battery separators were formed using titanium dioxide fillers having 
different surface areas and pore volumes. In this manner, filler materials 
meeting the physical property criterias of the present invention are 
compared with fillers outside of the present invention. The titanium 
dioxide fillers had the following properties: 
______________________________________ 
TiO.sub.2 Fillers 
BET 
Den- Surface 
N.sub.2 Pore 
Average 
Rutile Anatase sity Area Volume Pore 
Sample 
% % g/cc (m.sup.2 /g) 
(cc/g) Diameter 
______________________________________ 
A 99 3.9 9.17 0.0042 166 
B 5 95 4.3 65 0.3450 212 
C 97 4.2 10.11 0.0225 89 
D 82 3.8 54 0.1100 82 
______________________________________ 
Separator samples were formed by introducing into a Brabender internal 
mixer 13 parts of a commercially available high density polyethylene 
(MW=250,000), 45 parts of petroleum oil (Shellflex 411) and 0.1 part of 
Santonox. The components were mixed for less than 5 minutes and than 40 
parts of TiO.sub.2 particulate material (range from 0.01 to 7 microns), 
and 2 parts conductive carbon black (Vulcan XC-72) were added. The 
materials were mixed at 50 rpm for 10 minutes at a head temperature of 
160.degree. C. The material was removed from the mixer and then reinserted 
to insure thorough mixing. The second mixing period was done at 50 rpm, 
160.degree. C. for 10 minutes. The material was cooled and ground to a 
coarse powder. Two parts of each of the resultant materials were placed 
between sheets of Mylar and pressed at 1,200 psi for 5 minutes at 
140.degree. C. The Mylar sandwiched samples were placed between one-half 
inch thick aluminum slabs to cause rapid cooling of the samples. 
Each sample was subjected to extraction by immersing the sample in a bath 
of hexane for one-half hour at room temperature and then in a fresh bath 
for an additional half hour. Analysis of each sample established that 
greater than 97 percent of the plasticizer (Shellflex) content was 
removed. The percent pore volume of the samples were each about 65%. 
The samples were tested for electrical resistance, dendrite penetration 
resistance and stability to alkali and oxidation. The results are given in 
Table II below: 
TABLE II 
______________________________________ 
Dendrite 
Thick- ER Resis- Stability Per- 
Sam- ness ohm tance cent Elongation 
% 
ple TiO.sub.2 
in mils cm in min. 
Before 
After Change 
______________________________________ 
A A 4.7 24.6 68 640 239 62.7 
B B 8.3 11.8 67 307 -- -- 
C C 6.2 21.7 29 400 147 63.3 
D D 5.0 13.2 42 868 590 31.6 
______________________________________ 
The above data comparatively shows that separator membranes formed from 
titania having the physical properties required by the present invention 
(Samples B and D) have superior electrical resistance, in general, than 
Samples A and C. 
EXAMPLE IV 
Battery separator membranes were formed in the manner described in Example 
III above using the components described therein with titanium dioxde 
samples A, C, and D, Sample D representing the filler required by the 
present invention. The amount of each component was 8 parts polyolefin, 68 
parts TiO.sub.2, 22 parts plasticizer, 2 parts conductive carbon black, 
and 0.1 part stabilizer. The membranes, after extraction with hexane had a 
composition of 10 parts polyolefin, 87 parts TiO.sub.2, 0.7 parts 
plasticizer, 0.1 part stabilizer, and 2.2 parts carbon black. The percent 
pore volume of each sample was about 48 percent. The separator samples 
were tested for electrical conductivity, dendrite resistance, and physical 
properties before and after subjection to alkali and oxidation stability. 
TABLE III 
______________________________________ 
Thick- Dendrite 
ness ER Resistance 
% Elongation 
% 
TiO.sub.2 
in mils ohm-cm in min. Before 
After Change 
______________________________________ 
A 7.4 13.1 41 547 14 97.5 
C 6.1 12.6 85 694 141 79.7 
D 7.1 6.6 67 172 143 16.7 
______________________________________ 
The overall performance of the separator containing titania "D" was 
superior than that of the other samples. 
EXAMPLE V 
Separator membranes were formed in the same manner as described in Example 
IV above, except that different commercially available conductive carbon 
blacks were used for the Vulcan XC-72. The various conductive carbon 
blacks were: 
Vulcan XC-72 with surface area of 414 m.sup.2 /cc, density of 1.8 gm/cc and 
particle size of 15 m.mu.. 
Elftex 132 (Cabot) with surface area of 133 m.sup.2 /cc, density of 1.8 
gm/cc and particle size of 45 m.mu.. 
Vulcan C (Cabot) with surface area of 225 m.sup.2 /cc, density of 1.8 gm/cc 
and particle size of 27 m.mu.. 
Black EC (Ketjen) with surface area of 1800 m.sup.2 /cc, density of 1.8 
gm/cc and particle size of 3.5 m.mu.. 
Each sample showed an excellent combination of properties, as indicated in 
Table IV below: 
TABLE IV 
______________________________________ 
Sample by 
Carbon Thick- ER Dendrite % 
Black ness ohm- Resistance 
Stress Psi 
Elongation 
By Name in mils cm in min B A* B A 
______________________________________ 
XC-72 7.1 6.6 67 485 505 172 143 
Elftex 6.1 7.1 57 562 424 33 123 
Vulcan C 
7.6 8.9 72 462 441 185 418 
Black EC 
8.4 8.6 133 552 429 89 192 
______________________________________ 
*Before (B) and after (A), subjection to KOH (45%) at 80.degree. C. for 9 
hours or rapid oxidation test at 0.4 amp/cm.sup.2 overcharge for 96 hours 
at 80.degree. C. 
EXAMPLE VI 
A separator was formed in accordance with the present invention in 
accordance with the procedure of Example I above, except that the feed 
components were 15 parts high density polyethylene (MW=250,000), 45 parts 
TiO.sub.2 (P-25), 38 parts petroleum oil (Shellflex 411), and 2 parts 
conductive carbon black (XC-72). The resultant separator had a composition 
of 22 parts polyolefin, 65 parts filler, 3 parts carbon black, and 10 
parts plasticizer. The membrane was 9 mils thick and had electrical 
resistance of 20 ohm-cm, dendrite resistance of 45 min/mil or 405 min, and 
calculated porosity of 48.5 volume percent. 
EXAMPLE VII 
A separator was formed in the same manner as described in Example III 
above, except that alumina having a density of 3.97, a surface area of 373 
m.sup.2 /cc, and a N.sub.2 pore volume of 0.8 cc/gm was used. The 
components were 30 parts of the alumina, 26 parts high density 
polyethylene (MW=250,000), and 44 parts petroleum oil. The resultant 
membrane had a thickness of 10 mils, electrical resistance of 20 ohm-cm, 
and a dendrite penetration of 43 min. The material showed excellent 
stability to alkali (KOH) and oxidation. 
In comparison, a separator which is formed from low surface area alumina 
exhibits a poor combination of properties. 
EXAMPLE VIII 
A separator was formed in the same manner as described in Example III 
above, except that calcium hydroxide having a density of 3.4 gm/cc, a 
surface area 110 m.sup.2 /cc, and a N.sub.2 pore volume of 0.11 cc/gm 
instead of the titania. The components of the admixture forming the 
separator were in the amounts of 12 parts polyolefin, 7 parts conductive 
carbon black, 17 parts petroleum oil, and 63 parts calcium hydroxide. The 
resultant separator had a thickness of 7.8 mils, an electrical resistance 
of 44 ohm-cm, and a dendrite resistance of 300 min. 
A similar sample is formed with a filler of lower surface area and exhibits 
poorer properties. 
While the invention has been described in connection with certain preferred 
embodiments, it is not intended to limit the invention to the particular 
form set forth, but on the contrary, it is intended to cover such 
alternatives, modifications, and equivalents as defined by the appended 
claims.