Battery separator

A battery separator, and a method of forming the same, which exhibits good electrical conductivity and a high degree of inhibition to dendrite formation, is in the form of a thin sheet formed from a substantially uniform mixture of a thermoplastic rubber and a filler in a volume ratio of from about 1:0.15 to 1:0.6. The thermoplastic rubber is preferably a styrene/elastomer/styrene block copolymer.

BACKGROUND OF THE INVENTION 
The subject invention is directed to battery separators which exhibit a 
high degree of conductivity and inhibition to dendrite formation and which 
are capable of being formed in an economically improved manner. 
Storage batteries, in general, utilize either acid or alkaline electrolyte 
with compatible electrode systems. The term "acid battery system" or 
"alkaline battery system," used in the present application, refers to 
battery systems which utilize, respectively, an acidic or an alkaline 
solution as the electrolyte. An example of an acid battery system is lead 
acid batteries which are in common use, while examples of alkaline battery 
systems are those which use silver-cadmium or nickel-zinc electrodes in an 
alkaline solution such as an aqueous solution of potassium hydroxide. 
Because of their high energy density, alkaline batteries, such as 
nickel-zinc secondary alkaline battery system, have great potential for 
replacing the more conventional lead acid battery system in a number of 
terrestrial applications. However, extending the cyclic life of the 
battery beyond that presently attainable and reducing the cost of the cell 
components are required criterias which must be met to make the alkaline 
battery system an effective energy source. 
Battery separators are recognized as a key component in attaining an 
extended battery life and efficiency. Separators are located between 
plates of opposite polarity to prevent contact between the plates while 
freely permitting electrolytic conduction. Contact between plates of 
opposite polarity may be due to imperfections in the plate structure, such 
as warping or wrinkling of the plate. Such macro deformations are readily 
inhibited by any type of a sheet material which is coextensive with the 
plates and is capable of permitting suitable eletrolyte passage. Contact 
may also occur by formation of dendrites or localized needle like growths 
on an electrode, such as zinc dendrites formed on the zinc electrode in an 
alkaline nickel-zinc battery system. Separators which are commonly used 
today are in the form of sheet structures which during formation normally 
have pores and imperfections of sufficient size to readily permit 
dendrites to bridge the gap between electrodes of opposite polarity and, 
thereby, short out the battery system and reduce the battery life. 
Various non-elastomeric polymers have been used for forming separators. The 
term "elastomeric" or "elastomer," or "rubber," as used in the present 
application, refers to polymer materials which are capable of exhibiting a 
high degree of elongation and recovery. Elastomeric materials are 
distinguished from other polymeric materials, such as polyethylene, 
polypropylene, polystyrene and the like which are not capable of 
exhibiting such stress/strain recovery properties. 
U.S. Pat. No. 3,351,495 teaches that certain non-elastomeric polyolefins, 
such as polyethylene and polypropylene, can be compounded with filler and 
plasticizer to form a sheet material which, after extraction of some or 
all plasticizer, forms a microporous matrix suitable as a battery 
separator. The required use of a high amount of plasticizer and the needed 
extraction step to form a suitable separator material is costly and, in 
certain instances, produces irregular results. Separators formed from 
polyolefins, such as polyethylene, have been irradiated in attempts to 
increase the structural integrity of the formed sheet material. The 
resultant crosslinked material, when used as a separator, have been 
observed to exhibit high electrical resistance and, therefore, generally 
detract from the formation of effective and efficient battery system. 
More recently, certain rubber materials, including synthetic rubbers, have 
been used in the preparation of battery separators. These rubber materials 
are solvent cast onto a highly porous substrate support sheet which is 
normally formed from cellulose or asbestos material. The process of 
forming such composite separators is both complex and costly and requires 
removal of the casting solvent and extraction of plasticizer contained in 
the rubber to form a microporous membrane. Further, these separators are 
normally of a thickness which reduces the efficiency of the formed battery 
system. 
A battery separator which is capable of increasing the efficiency of a 
battery system is highly desired. It is generally agreed such a separator 
should be in the form of a thin, microporous sheet which is resistant to 
degradation by electrolyte solution, exhibits a high degree of inhibition 
to dendrite formation and dendrite growth, and has a high degree of 
electrical conductivity. Further, the battery separator should be of a 
composition which is capable of being processsed and formed into thin 
microporous sheet material in an efficient and cost effective means. 
SUMMARY OF THE INVENTION 
The present invention is directed to battery separators which have high 
electrical conductivity, high inhibition to dendrite formation and growth, 
are resistant to degradation by common electrolyte solutions, and is 
readily formed in a cost effective, simple manner. The present separator 
is formed from a substantially uniform mixture of a thermoplastic rubber 
material, preferably a styrene/elastomer/styrene triblock copolymer and a 
filler which is essentially chemically inert to the electrolyte of the 
battery system. The volume ratio of the thermoplastic rubber to filler is 
from 1:0.15 to 1:0.6. The mixture is capable of being formed into sheets 
of less than 10 mil thickness of a structure which permits good electrical 
conductivity and high inhibition to dendrite formation and growth. 
DETAILED DESCRIPTION OF THE INVENTION 
The subject invention is directed to the utilization of a combination of 
components which unexpectedly permits the formation of a battery separator 
having highly desired properties in a simple, cost effective manner. 
Further, the presently described invention is directed to a process which 
forms an integral sheet product in a simple, cost effective manner. 
The required components to be used in forming the subject separator are a 
thermoplastic rubber material in combination with an inert filler such 
that the volume ratio of thermoplastic rubber to filler is from 1:0.15 to 
1:0.6 and, preferably, from 1:0.25 to 1:0.4. It has been unexpectedly 
found that when one utilizes a thermoplastic rubber material, as more 
fully described herein below, in a combination with an inert filler in the 
particular volume ratio described herein, one is capable of forming a 
homogeneous mixture which can be processed by standard techniques and 
equipment to form the desired sheet material suitable as a battery 
separator. The composition of the present invention does not require the 
utilization of a plasticizer, oil or other such additives to aid in the 
processing of the subject composition. 
The polymer matrix required for use in forming the subject separators are 
of the class of materials known as thermoplastic rubbers. The polymers are 
required to have an elastomeric segment which forms the major amount of 
the polymer chain. The elastomeric material generally has a glass 
transition temperature below room temperature and is substantially 
amorphous in morphology. The polymer chain further comprises a 
thermoplastic segment which has a glass transition temperature above 
75.degree. C. and, preferably, above 100.degree. C., and which forms a 
minor portion of the polymeric chain. The thermoplastic segments normally 
form end blocks of the polymer chain. The material can, therefore, be 
viewed as a block copolymer having a structure of 
thermoplastic/elastomer/thermoplastic polymer chains. Examples of such 
polymer materials formed of three block segments which an elastomeric 
block in the center of the polymeric chain and a thermoplastic block on 
each end are, for example, styrene/elastomer/styrene block copolymer. Such 
thermoplastic rubbers are commercially available, including materials sold 
under the tradename of Kraton by Shell Chemical Company. 
The thermoplastic rubbers found useful in the present invention consist of 
triblock copolymers in which the mid-block of the molecule is an elastomer 
such as polybutadiene, isoprene, poly(ethylene-butylene) and the like. The 
mid-block segment should be present as the major volume percent of the 
triblock copolymer and have an average molecular weight which is 
sufficient to impart elastomeric rubber properties to the resultant 
copolymer. Such molecular weights are normally at least about 40,000. 
Elastomers of an average molecular weight of from about 40,000 to 100,000 
are most suitable for the intended purpose. 
The thermoplastic blocks form segments at each end of the elastomeric block 
to complete the triblock configuration of the subject thermoplastic 
rubber. The preferred thermoplastic rubbers have end blocks formed from 
styrene. In lieu of styrene, the end blocks can be formed from other vinyl 
monomers which are capable of forming polymer segments which are 
thermodynamically incompatible with the elastomer segment while capable of 
imparting thermoplastic properties to the resultant rubber. Other monomers 
suitable for forming the end blocks include substituted styrene, such as 
alpha-methyl styrene, paraphenyl styrene, as well as acrylonitrile and the 
like. The thermoplastic polymeric end groups should have a glass 
transition temperature of above about 75.degree. C. and preferably above 
about 100.degree. C. Each of the end blocks should have an average 
molecular weight between a lower limit set by the minimum chain size 
required for phase separation, while the upper limit is governed by the 
effect of viscosity on the efficiency of phase separation. In most 
instances, the average molecular weight of such end blocks are from about 
5,000 to 25,000 and, preferably, from 10,000 to 20,000. In the case of 
polystyrene end blocks, it is preferred that the styrene content be up to 
about 40 volume percent of the triblock copolymer, as described herein. 
The preferred thermoplastic rubbers to be used in the present invention are 
those of a block copolymer formed from styrene/elastomer/styrene wherein 
the elastomer is butadiene, isoprene or ethylene/butylene copolymers. The 
polystyrene concentration may range from about 15% to about 40% by weight 
such as to give the resultant polymer thermoplastic properties while 
allowing the polymer to retain the elastomeric properties attributable to 
the mid-block. 
The thermoplastic rubber found useful in the subject invention have a melt 
index, as determined by the standard procedures indicated in ASTM D-1238 
(Condition G), of less than about 10 gms/10 min, and have tensile 
strengths of from about 3,000 to 5,000 psi, 300% modulus of elasticity of 
from about 10 to 900 psi, and elongation at break of from about 500 to 
1,300 percent, as measured in accordance with ASTM method D-412. 
The thermoplastic rubbers, as illustrated by styrene/elastomer/styrene, are 
generally prepared by anionic polymerization, preferably anionic solution 
polymerization using solvents, initiators, temperatures, and techniques 
which are well known to those skilled in the art. Four methods used for 
preparing block copolymers by anionic polymerization are applicable to 
preparing the thermoplastic elastomers used in this invention and are 
summarized as follows: 
1. Sequential Polymerization 
Styrene is first polymerized in preferably non-polar solvents, such as 
heptane or cyclohexane, using suitable alkyl lithium initiators, such as 
sec-butyl lithium or isopropyl lithium. After all the styrene has been 
polymerized, the elastomeric block may be initiated from the end of 
styrene block by addition of a suitable elastomer forming monomer, such as 
butadiene. In order to initiate the third (polystyrene) block after the 
elastomer has been consumed, a polar solvent must be added along with 
styrene. This method can be used in the preparation of triblock copolymer 
free of any homopolymer or diblock, provided rigorous exclusion of 
impurities is observed. 
2. Difunctional Initiation 
Polymerization of the desired elastomer, such as butadiene, is initiated by 
means of a dilithium initiator. Styrene monomer is added after formation 
of polystyrene blocks at each end of the elastomer. 
3. Diblock Synthesis 
A polystyrene-diene diblock copolymer is prepared in a manner similar to 
that described in Method 1 above. The active chain ends are then coupled 
using a coupling agent (e.g., dichlorodimethyl silane) to yield triblock 
material. 
4. Two-Stage Process 
Polystyrene is formed and followed by addition of a styrene-diene mixture 
whereby the elastomeric block is formed preferentially, followed by the 
polystyrene block. 
Although not to be a limitation on the subject invention, it is believed 
that the thermoplastic rubbers of the subject invention, as illustrated by 
styrene/elastomer/styrene block copolymers, are capable of exhibiting both 
thermoplastic and elastomeric properties due to the thermodynamic 
incompatability between the polystyrene thermoplastic moieties and the 
elastomer blocks contained in the rubber molecule and matrix. Because of 
this incompatibility, the polystyrene end blocks, being in a minor portion 
of the rubber matrix, unite to form submicroscopic regions or domains 
(about 300 to 400 Angstroms) which are substantially uniformly distributed 
throughout the matrix. These domains create a crosslinking network of a 
physical nature. The continuous phase between and around the domains is 
occupied by the elastomeric moieties and imparts the rubber properties to 
the polymer material. The domains may be disrupted and, therefore, the 
physical crosslinking may be readily broken through the application of 
stress or elevated temperature or a combination of the same, and will 
depend upon the exact glass transition temperature of the resultant 
thermoplastic polymer. The subject thermoplastic rubbers have or exhibit 
two glass transition temperatures, one associated predominantly with the 
elastomeric moiety, and a second associated with the thermoplastic end 
blocks. For comparison, random copolymers of styrene and butadiene 
normally exhibit a single glass transition temperature. 
The subject rubbers may be further enhanced by incorporating therewith an 
antioxidant, an antiozonant, as well as other conventional additives or 
combinations thereof in conventional amounts and methods as is well known 
to those skilled in the art. Some antioxidants which have been found 
useful with respect to thermoplastic rubbers are, for example, zinc 
dibutyl dithiocarbamate, thiodipropionate, triphenyl phosphite and the 
like. Antiozonants, which are commercially available, include, for 
example, nickel dibutyldithiocarbamate, dibutylthiourea and the like. 
It has been found that the above described polymer matrix when utilized in 
combination with filler materials, as described herein below, forms a 
unique composition which is capable of forming a sheet material suitable 
for use as a battery separator. The filler materials should be 
substantially chemically inert with respect to the specific electrolyte 
solution with which it is contemplated the separator will come in contact. 
Generally, the size of the filler particle can range from an average of 
about 0.01 micron to about 10 microns in diameter and, preferably, from 
about 0.01 to 0.25 microns. The surface area of the filler can range from 
about 10 to 950 square meters per gram, as determined by standard 
techniques. 
The fillers may be any ingredient which is substantially chemically inert 
with respect to the electrolyte to which it is to come in contact. For 
example, the filler can be carbon black, coal dust, or graphite; it may 
also be a metal oxide or hydroxide such as those of silicon, aluminum,, 
calcium, magnesium, barium, titanium, iron, zinc, and tin; it may also be 
a metal carbonates such as those of calcium, magnesium or the like; 
synthetic and natural zeolites; Portland cement; precipitated metal 
silicates, such as calcium silicate, and aluminum polysilicate; alumina 
and silica gels, or mixtures of said fillers. 
When the separator material is contemplated for use as a component of an 
alkaline battery system, it is preferred that the filler material utilized 
by aluminum oxide, magnesium oxide, titanium dioxide, carbon, or 
combinations thereof. If the battery separator is contemplated for use as 
a component of an acid battery system, the filler material may include 
silicon compounds, such as silicon oxide, silica gels, polysilicates, and 
the like. The volume ratio of polymer to filler should be between 1:0.15 
and 1:06 and, preferably, between 1:0.25 to 1:04. It has been found that 
by utilizing a polymer to filler volume ratio, as described herein above, 
the components will form a uniform mixture which is capable of being 
processed to a battery separator sheet without the aid of additional 
components, such as plasticizers, oil extenders, and the like at levels 
which require extraction. Therefore, the present composition is capable of 
forming a battery separator without the conventional process step of 
extracting or removing the plasticizers or oils subsequent to forming of 
the sheet product. 
The above described thermoplastic rubber-filler composition may be mixed 
with minor amounts of such organic additives which further improve its 
thermoplastic or viscoelastic properties, or a combination thereof. The 
additional material should be present in amounts of not greater than 10 
percent by weight, based on the total weight of the resultant composition. 
In order to further enhance the viscoelastic properties of the subject 
thermoplastic rubber, one may utilize an additional material which will 
associate with the elastomeric phase of the matrix. The thermoplastic or 
high temperature performance of the rubber may be modified by ingredients 
which will associate with the thermoplastic phase or domains of the 
matrix. For example elastomer associated materials are organic compounds 
and low molecular weight polymers having a softening point of less than 
about 75.degree. C., and which include low molecular weight 
polyisobutylene, polybutadiene, polypropylene, ethylene-propylene 
copolymers, poly(vinyl acetate), ethylene-vinyl acetate copolymers, 
polyterpene, as well as esters, polyesters, and the like. The 
thermoplastic properties of the subject polymer can be modified by the 
addition of compounds which will associate with the thermoplastic moiety, 
such as polystyrene, poly(alpha-methylstyrene), and the like. 
The components of the instant composition can be mixed by any conventional 
manner which will produce a substantially uniform distribution of the 
filler throughout the polymer matrix. Mixing temperatures of up to about 
175.degree. C. and preferably, from about 70.degree. to 150.degree. C. are 
suitable for processing the compounds to be utilized in the instant 
invention. At such elevated temperatures, the polymeric matrix is softened 
sufficiently to be handled as a thermoplastic material. To produce a 
particularly uniform mixture, the components can be premixed at room 
temperature in a blender or the like, and then fed to a heated mixer, such 
as an internal mixer of the Banbury type or the like which are suitable 
compounding processors. 
After being substantially uniformly mixed, the resultant composition can be 
molded or shaped by any conventional manner, such as, for example, by 
utilizing extrusion, injection molding, or compression molding apparatus 
to form the final sheet product. The shaping can be readily done by 
utilizing elevated temperature, pressure or shear force or a combination 
thereof. The exact temperature or force will depend on the particular 
composition used, as well as the particular shaping process used, and can 
be determined by standard techniques. Temperatures, as described above 
with respect to mixing, have been found most suitable in shaping the 
subject material. It has been found that the composition described herein 
readily forms a sheet material suitable for use as a battery separator 
merely by mixing the components together under suitable temperature 
conditons for a time sufficient to produce a substantially uniform 
mixture, and then shaping the resultant mixture into the desired sheet 
material. 
The term "sheet" is intended in the subject application to define a planar 
material which may or may not contain, in addition, rib or web members 
embossing or patterns. The sheet material should be of a film which is 
generally less than about 10 mils thick and, preferably, less than 7 mils 
in thickness. The ribs or webs, as part of the configuration of acid 
battery system separators, may add to this thickness. 
It has been unexpectedly found that the sheet material formed in accordance 
with the present invention can be readily superimposed upon an additional 
sheet material of like kind so as to form a composite sheet material of 
less than about 10 mils thick. Due to the nature of subject composition, 
the resultant films may adhere to one another solely due to the adhesive 
nature of the materials. Alternatively, composite sheet material can be 
formed by subjecting the sheets to sufficient elevated temperatures to 
enhance tackiness of the surface and to pass the combined sheets through a 
pair of nip rollers or the like. The composite sheet material can be 
formed from sheets of the same or different thermoplatic rubber matrix. In 
the former preferred case, the resultant composite sheet will be of a 
substantially uniform composition throughout the formed separator. By 
utilizing a multiplicity of the subject sheet materials, one attains a 
composite sheet which alleviates the effect of pinholes that may occur 
during the formation of the separator sheet. Such pinholes normally permit 
ready growth of dendrites and the shorting out of the battery system 
resulting therefrom. 
The final composition of the separator will be substantially the same as 
and defined by the polymer and filler used to form the initial 
composition. The resultant sheets formed from such compositions have been 
found to be substantially absent of pores. Even though the sheet material, 
as formed, lacks pores, as observed by electron microscopy of the surface 
and cross-sectioned areas as well as by other conventional techniques, it 
has been unexpectedly found that they exhibit a high degree of electrical 
conductivity when in association with an acid or alkaline battery system. 
Battery separators formed by the above described compositions are believed 
to inhibit growth of dendrites and the resulting shorting out of the 
system while permitting a high degree of electrical conductivity by 
certain physical interaction between the electrolyte and the composition. 
This interaction, though unresolved, may be in the form of swelling, 
shrinking, wicking, etc. which causes only minute tortuous pores to form 
while permitting passage of electrolyte ions. 
The thickness of the battery separators will vary, depending upon the type 
of battery in which they are used. In general, the thickness of web sheet 
material can range for 1 to 50 mils, as is useful in lead acid batteries 
systems. For alkaline batteries, the preferred thickness is generally less 
than about 10 mils thick. The subject composition can be utilized to form 
both lead acid batteries and alkaline battery separator materials. The 
lead acid batteries can be made in a combination of a web sheet material 
to be used alone or in combination with a thin, from about 1 to 10 mils 
thick, sheet material which aids in forming the tortuous part of the 
resultant battery separator. For the alkaline battery systems, either a 
single sheet of material or a plurality of sheets of material can be 
utilized to form a unitary sheet capable of inhibiting the formation and 
growth of dendrites between electrodes of opposite plurity. 
In order to be commercially acceptable, a battery separator must meet 
minimum electrical resistant requirements. Generally, the acceptable value 
is less than about 100 ohm-cm and, preferably, less than 20 ohm-cm, as 
measured by standard techniques. 
Further, the battery separator should possess certain chemical properties 
such as resistance to oxidation and resistance to attack by acid or 
alkaline material, as is appropriate for the particular battery separator 
system. It has been found that the presently formed battery materials are 
capable of retaining their configuration and are not destroyed when placed 
in contact with alkali or acid materials over an extended period of time.

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 1 
A battery separator was formed by introducing into a Brabender mixer 44 
parts of a commercially available styrene/butadiene/styrene triblock 
copolymer crumb material which has a 30:70 styrene:butadiene ratio, 
contains no plasticizer or oil, has a melt index of 1 gm/10 min, Condition 
G, a Brookfield viscosity (25 wt. % in Toluene) of 4000 at 25.degree. C., 
and a density of 0.94 gm/cc, with 54 parts a commercial TiO.sub.2 powder 
(DeGusso P-25) having a surface area of 65 m.sup.2 /gm and a density of 
4.3 gm/cc, a rubber modifier of 2 parts of a polyester condensation 
product of azelaic acid and propanediol (Emergy Industries P-9720), and 
0.5 parts of tetra-bis 
methylene-3-(3,5-ditertiary-butyl-4-hydroxyphenyl)propionate methane. The 
materials were mixed at 160.degree. C. at 50 RPM in two passes until a 
substantially homogeneous mixture was obtained. 
Films of the material were formed by pressing small amounts of the 
resultant formulation in a Walash press at approximately 1,600 psi with 
both plates of the press being maintained at 145.degree. to 150.degree. C. 
The resultant film has a thickness of approximately 10 mils and is free of 
pinholes and imperfections, as observed. Random sections of the surface 
and cross-sectional area of the resultant film were observed using 
standard technique of electron microscopy and showed the film to be 
substantially free of pores. 
The films were utilized as separators in nickel-zinc alkaline secondary 
battery system (electrolyte:KOH). The electrical resistance of the films 
was determined by the direct current method, as described by J. J. Lander 
and R. D. Weaver in Characteristics of Separators for Alkaline Silver 
Oxide-Zinc Secondary Batteries: Screening Methods, ed. by J. E. Cooper and 
A. Fleischer, Chapter 6. Resistance to dendrite penetration was measured 
according to the method described by G. A. Dalen and F. Solomon, Chapter 
12, ibid. Prior to any testing, the films were conditioned by subjection 
to 45 weight percent KOH solution at 100.degree. C. for 90 minutes. 
Physical properties of modulus of elasticity (psi.times.10.sup.-3) and 
elongation at break (%) of the resultant sheets were tested in accordance 
with ASTM-638 procedure at an elongation rate of 0.2 in/min and initial 
jaw separation of 0.4 in. prior to contact with KOH solution and 
subsequent to soaking in 45 weight percent KOH solution at 80.degree. C. 
for 96 hours. 
In accordance with the above tests procedures, the electrical resistance of 
the resultant sheet material was determined to be 32 ohm-cm; zinc dendrite 
resistance was 37.7 min/mil thickness; the modulus of elasticity of blank 
and KOH treated material was 30 and 22, respectively; and the elongation 
at break was 790 and 320, respectively. 
EXAMPLE II 
A battery separator was formed and tested in accordance with the 
description given in Example I above except that the triblock copolymer 
used therein was substituted with a commercially available lower molecular 
weight polymer of styrene/butadiene/styrene crumb material having a 28/72 
styrene to butadiene ratio, a melt index (Condition G) of 6, a density of 
0.94 gm/cc, a Brookfield viscosity (25 wt. % polymer in Toluene) of 1,200 
cps at 25.degree. C., and containing no plasticizer or oil. 
The resultant sheet material was approximately 6 mils thick and exhibited 
electrical resistance of 9.7 ohm-cm and zinc dendrite resistance of 9.1 
min/mil. 
EXAMPLE III 
Separator sheet materials were formed and tested in accordance with the 
procedures indicated in Example I above. The thermoplastic rubber used was 
a commercially available styrene/butadiene/styrene triblock copolymer 
crumb material having a 28/72 styrene to butadiene ratio, a melt index 
(Condition G) of 6, a density of 0.94 gm/cc and contained no plasticizer 
or oil. The filler used was a commercially available TiO.sub.2 powder (De 
Gussa P-25) having a surface area of 65 in.sup.2 /gm and a density of 4.3 
gm/cc. Certain of the samples were formulated with a small amount of 
rubber modifier as indicated in Table I below. 
TABLE I 
__________________________________________________________________________ 
Dendrite 
Modulus Elongation 
Thermoplastic 
Filler 
Modifier 
El. Resist. 
Resist. 
(psi .times. 10.sup.-3) 
(%) 
Sample 
Rubber (Parts) 
(Parts) 
(Parts) 
(ohm-cm) 
(min/mil) 
Blank 
KOH Blank 
KOH 
__________________________________________________________________________ 
A 65 35 -- 39 20 3.3 8.4 950 540 
B 36 64 -- 12 14 -- -- -- -- 
C 36 64.sup.1 
-- 18 -- 16 56 530 -- 
D 29 63.sup.1 
10.sup.2 
56 -- 44 18 210 20 
E 40 45 10.sup.3 
9 24 4.6 14 780 80 
F 30 60 10.sup.4 
49 39 9.5 18 560 430 
G 44 54 2.sup.5 
85 27 27 29 100 60 
__________________________________________________________________________ 
.sup.1 Two parts are carbon black surface area (N.sub.2 SA) 230 m.sup.2 
/gm, part. size (E.M.) 30 millimicron. 
.sup.2 Ethylenepropylene rubber (Vistalon 404). 
.sup.3 HIgh density polyethylene, M.I. =.sub.2 O. 
.sup.4 Styrene/butadiene/styrene copolymer (Soloprene 416). 
.sup.5 Ethylenevinyl acetate copolymer (25% VA). 
EXAMPLE IV 
A separator sheet product was formed and tested using the procedure 
described in Example I above except that the sheet composition was formed 
from the thermoplastic rubber described in Example II with Al.sub.2 
O.sub.3 powder filler having a surface area of 94 m.sup.2 /gm and a BET 
Nitrogen pore volume of 0.8 cc/gm. No additional modifiers were used. The 
sheet product exhibited electrical resistance of 43 ohm-cm, a modulus of 
elasticity (psi.times.10.sup.-3) of 14 before subjection to KOH solution 
and 18 after, and elongation at break of 240 both before and after 
subjection to KOH solution. 
EXAMPLE V 
Separators were formed using the procedures and test method described in 
Example I above, except that the thermoplastic rubber used therein was 
substituted by other commercially available rubbers described herein 
below. The thermoplastic rubbers used were a 
styrene/ethylene-butylene/styrene (SE-BS) block copolymer having a density 
of 0.91 gm/cc, styrene to rubber ratio of 28 to 72, and a Brookfield 
viscosity (20 wt % in Toluene) of 1,500; and a lower molecular weight 
SE-BS polymer (SE-BS-1) having a styrene-rubber ratio of 29/71, a density 
of 0.91, and a Brookfield viscosity (20 wt % in Toluene) of 550. The 
resultant sheets were all less than 10 mils thick and exhibited the 
results summarized in Table II. 
TABLE II 
__________________________________________________________________________ 
Dendrite 
Modulus Elongation 
Thermoplastic 
Filler El. Resist. 
Resist. 
(psi .times. 10.sup.-3) 
(%) 
Rubber TiO.sub.2 
Modifier.sup.1 
(ohm-cm) 
(min/mil) 
Blank 
KOH Blank 
KOH 
__________________________________________________________________________ 
35 (SE-BS) 
55 10 47 22 6 33 590 30 
35 (SE-BS-1) 
55 10 26 19 -- -- -- -- 
40 (SE-BS-1) 
50 10 36 36 4 60 375 22 
__________________________________________________________________________ 
.sup.1 Contains 0.5 percent Irganox 1010 antioxidant. 
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.