Battery separator

A battery separator in the form of a microporous membrane composed of a substantially uniform composition of a polymer mixture of ultra high molecular weight polyolefin, polyethylene terpolymer and polyvinyl chloride, in combination with at least one plasticizer for the polymer mixture and an inert filler.

BACKGROUND OF THE INVENTION 
The instant invention is directed to an improved battery separator which 
exhibits very high degree of stability with respect to the conditions 
encountered in a battery system. In particular, the instant separator 
exhibits prolonged resistance to oxidative degradation. The present 
separator aids in providing a battery system capable of exhibiting 
extended activity. 
Storage batteries are generally composed of at least one pair of electrodes 
of opposite polarity and an electrolyte. The battery may employ an acid or 
an alkaline electrolyte. Conventional acid batteries are exemplified by 
the lead-acid (sulfuric acid) batteries used in automobiles and the like. 
Alkaline batteries include secondary batteries such as nickel-zinc, 
nickel-cadmium, mercury-zinc and the like. 
In addition to the electrodes and electrolyte, one of the recognized key 
components in a battery is the separator. Separators are elements located 
between electrodes to prevent direct contact between plates of opposite 
polarity while freely permitting electrolytic conduction. The separator 
must be porous to permit the electrolyte to be the sole internal 
conducting path between electrodes. Thus, although the separator itself 
must be substantially non-conducting, it must be capable of exhibiting low 
resistivity when in a battery system. 
The separator is preferably in the form of a diaphragm or envelope (in 
which an electrode of at least one polarity is encased) which is 
microporous and has a high void volume. Such configurations permit the 
necessary free flow of electrolytic conductivity (low resistivity) while 
inhibiting active materials from passing through the separator causing 
unwanted bridging of plates of opposite polarity. Such contact may be due 
to imperfections in the plate structure or due to dendrite growth on the 
electrode during use or the like. 
In addition to the above requirements the separator must be capable of 
being formed into a very thin sheet which is substantially inert to the 
environment established by the battery system. For example, it is well 
known that the battery forms an oxidative environment which causes 
degradation of materials used in forming conventional separators which, in 
turn, causes disintegration of the thin separator membrane. The ability to 
produce a separator membrane which is capable of withstanding the 
oxidative forces of the battery environment while fulfilling the other 
criteria of being microporous, having a high void volume, having a very 
thin cross-section, being inert to the battery components, exhibiting low 
electrical resistivity, and exhibiting high resistivity to passage of 
active material is highly desired. 
U.S. Pat. No. 3,351,495 discloses a battery separator having a relatively 
low pore size and satisfactory electrical resistance characteristics made 
from a high molecular weight polyolefin having an average molecular weight 
of at least 300,000, a standard load melt index of substantially zero, and 
a reduced viscosity of not less than 4. The separator is manufactured by 
extruding the high molecular weight polyolefin in admixture with an inert 
filler and a plasticizer and then extracting the plasticizer by the use of 
a suitable solvent. 
U.S. Pat. No. 4,024,323 discloses a variation to the '495 battery 
separator. The composition contains small amounts of a copolymer formed 
from an olefin or mixture of olefins (e.g., ethylene, propylene)and an 
ethylenically unsaturated carboxylic acid (e.g., acrylic or methacrylic 
acid) to aid in processability. 
U.S. Pat. No. 4,287,276 is directed to the formation of a battery separator 
specifically useful in alkaline battery systems. The polyolefin matrix is 
highly filled with a particular class of fillers to form a microporous 
sheet having enhanced resistance to dendrite formation. 
While the above separators have shown good stability, they do not exhibit 
the desired prolonged stability against oxidative forces encountered in 
the modern day, extended life batteries. Thus, these separators are known 
to degrade and permit shorting over a period of time. Extending the life 
of the battery can be achieved by using the presently described separator. 
SUMMARY OF THE INVENTION 
The present invention is directed to an improved separator which exhibits a 
high degree of stability with respect to a battery environment, can be 
formed into thin sheets, exhibit a high degree of inhibition to passage of 
active material and exhibits a high degree of electrical conductivity. 
The present separator is a microporous membrane composed of a substantially 
homogeneous composition of (a) a polymer mixture formed from a polyolefin 
having a weight average molecular weight of at least 3 million, a 
partially crosslinked ethylene terpolymer and a vinyl or vinylidene halide 
polymer; (b) an inert filler and (c) a plasticizer for at least one of the 
polymer components. 
DESCRIPTION OF THE INVENTION 
The battery separator of the present invention is in the form of a very 
thin membrane or sheet which is formed from a substantially homogeneous 
mixture of a polymer blend, an inert filler and a plasticizer. Each of the 
components is fully described hereinbelow. 
A mixture of polymers is required to form the subject separator sheet 
product. The mixture must be formed using a polyolefin of ultra high 
molecular weight. The term "ultra-high molecular weight," as used herein 
and in the appended claims refers to polymers having a weight average 
molecular weight of at least about three million, preferably at least 
about four million, as determined according to ASTM D-4020 or DIN-53493. 
The polyolefin can be homopolymers of ethylene or of propylene or a 
mixture of the formed homopolymers or a copolymer of ethylene and 
propylene. These polymers can be formed from very pure monomer stock using 
a Natta type catalyst in manners known to those skilled in the art. 
A second component of the polymer mixture is composed of a polymer blend 
formed from a polyethylene terpolymer and a vinyl or vinylidene halide 
polymer as fully described hereinbelow. The polyethylene terpolymer is 
formed from (1) ethylene with, (2) an ethylenically unsaturated organic 
monomer other than an unsaturated carboxylic acid such as acrylic acid, 
maleic acid, and the like, and (3) an ethylenically unsaturated carboxylic 
acid such as acrylic acid, maleic acid, and the like, carbon monoxide, or 
sulfur dioxide. Exemplary of the organic monomers (2) are those selected 
from the group consisting of esters of said unsaturated mono- or 
dicarboxylic acids, vinyl esters of saturated carboxylic acids where the 
acid group has 2-18 carbon atoms, vinyl alkyl ethers wherein the alkyl 
group has 1-18 carbon atoms, vinyl or vinylidene halides, acrylonitrile, 
methacrylonitrile, norbornene, alpha-olefins of 3-12 carbon atoms, and 
vinyl aromatic compounds. Preferred organic monomers include methyl 
acrylate, butyl acrylate and vinyl acetate. The melt index range for these 
terpolymers is from 0.1 to 1000 (ASTM D-1238), preferably from about 1 to 
100. The preferred terpolymers are formed from ethylene, an ester and 
carbon monoxide or a carboxylic acid. 
The ethylene terpolymers used to form the present composition should have 
sufficient comonomer copolymerized therein to aid in providing 
compatability with the vinyl and vinylidene halide polymers described 
below. Generally speaking, the ethylene content in these terpolymers 
should be 40 to 85 percent, the content of the second monomer should be 
from 1 to 60 percent and the third monomer content should be from 1 to 30 
percent, all based on the terpolymer weight. The lack of compatability can 
be tested by forming a blend of the ethylene terpolymer and the vinyl or 
vinylidene polymer described below and observing if the blend is opaque, 
show stress whitening when stretched and lack of required recovery to be 
considered elastomeric. A more detailed discussion of the conpatability of 
these ethylene copolymers with vinyl and vinylidene halide polymers, as 
well as a discussion of the preparation of the copolymers can be found in 
Polymer-Polymer Miscibility, O. Olabisi, L. M. Robeson and M. T. Shaw, 
Academic Press, N.Y., N.Y., 1979, U.S. Pat. No. 3,684,778 and U.S. Pat. 
No. 3,780,140, all herein incorporated by reference. 
The polymer blend further comprises a vinyl or vinylidene halide polymer 
including copolymers resulting from copolymerization with a comonomer 
selected from the group consisting of vinyl esters, acrylonitrile, acrylic 
esters, vinylidene chloride, vinyl chloride, esters of unsaturated 
carboxylic acids and vinyl ethers. The weight ratio of ethylene terpolymer 
to vinyl or vinylidene halide polymer should be 19:1 to 1:3 and, 
preferably, 4:1 to 2:3. The preferred polymers are chlorides such as 
homopolymers of polyvinyl chloride or polyvinylidene chloride. For 
example, polyvinyl chloride having an inherent viscosity of 0.30 to 1.4 
(ASTM D-1243) is generally useful in the practice of the subject 
invention. 
The blending of the ethylene terpolymer with the vinyl or vinylidene halide 
polymer is accomplished by any one of a number of conventional techniques, 
for example, in a Banbury mixer, two-roll mill or extruder. The blending 
can be done prior to mixing with the polyolefin or concurrently with the 
formation of the polymer mixture. Preferably, the blending of the 
terpolymer and vinyl or vinylidene halide polymer is done prior to forming 
the mixture. This blending is done at a temperature high enough to soften 
the polymers for adequate blending, but not so high as to degrade the 
vinyl or vinylidene halide polymer. Generally speaking, this blending 
temperature ranges from 140.degree. to 200.degree. C., and blending is 
carried out for a time sufficient to homogeneously blend the components. 
The ethylene terpolymer in the compatible blend is partially crosslinked. 
This can be carried out using any one or more of the well known 
crosslinking techniques including electron beam irradiation, gamma 
irradiation and free radical curatives such as peroxides. The crosslinking 
of the ethylene copolymer according to this invention can be carried out 
before or concurrently with blending with the vinyl or vinylidene halide 
polymers, or after such blending when using radiation techniques to 
effectuate the crosslinking. If the ethylene terpolymer in the blend 
contains carbon monoxide, diamines such as methylene dianiline or 
p-phenylene diamine can be used to effectuate the desired crosslinking. If 
the ethylene terpolymer is ethylene/vinyl acetate/carbon monoxide, sulfur 
vulcanizing agents can be used as detailed in U.S. Pat. No. 4,172,939. For 
crosslinking ethylene terpolymers containing carboxylic acid 
functionalities, the formation of ionic crosslinks is suitable in the 
practice of the subject invention, and is achieved with various metal 
oxides or hydroxides such as ZnO and NaOH, or with organometallics such as 
chromium acetylacetone, as detailed in U.S. Pat. No. 4,304,887. 
The term "partially crosslinked" refers to a degree of crosslinking of the 
polymers, in particular, the ethylene terpolymer of the blend. The term 
"partially crosslinked" of the polymer blend refers to blends which have a 
gel content of 5 to 90 percent, preferably 10 to 70 percent based on total 
polymer blend. To quantify the degree of crosslinking, the amount of 
insoluble, and hence crosslinked, polymer can be determined by soaking a 
sample of the polymer blend, after crosslinking, in tetrahydrofuran at 
23.degree. C. for 16 hours, isolating the insoluble portion and weighing 
and dried residue. If other components are present, such as polyolefin or 
filler, one must make suitable corrections based upon knowledge of the 
composition. For example, the weight of components soluble in 
tetrahydrofuran such as plasticizers are subtracted from the initial 
weight; and components insoluble in tetrahydrofuran, such as fillers, are 
subtracted from both the initial and final weight. The insoluble polymer 
recovered is reported as percent gel content. This procedure is based on a 
conventional procedure for quantifying degree of crosslinking as is more 
fully detailed in U.S. Pat. No. 3,203,937. The conditions under which this 
crosslinking is carried out, i.e., type and quantity of crosslinking 
agent, crosslinking time and temperature to arrive at a composition having 
a gel content within this operable range, can be determined empirically. 
When chemical crosslinking agents are utilized, it is preferable that they 
be substantially totally consumed during the crosslinking step. Further 
description of polymer blends suitable for use in forming the subject 
separator is made in U.S. Pat. No. 4,613,533, the teachings of which are 
incorporated herein by reference. 
The mixture of polymers shall be composed of from at least 40 to 95 
(preferably 60 to 95) weight percent polyolefin and from 60 to 5 
(preferably 40 to 5) weight percent of the above described polymer blend. 
In another embodiment of the subject invention, the polymer mixture can be 
composed of the above described ultra-high molecular weight polyolefin and 
the above described polyethylene terpolymer wherein the monomer (3) is 
selected from carbon monoxide or sulfur dioxide. The polymer mixture can 
be formed from at least 40 to 95 (preferably 60-95) weight percent 
polyolefin and from 60 to 5 (40-5) weight percent polyethylene terpolymer. 
The plasticizer of the instant composition further 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 
hereinbelow. The microporous separator of the present invention is formed 
from an initial composition having a very high content of plasticizer 
therein, such as, at least about 30 vol. percent and preferably at least 
50 vol. percent based on the initial composition. 
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, glycerol, and ethers and esters thereof; alkyl 
phosphates such as triethyl phosphate; polyvinyl alcohol, polyacrylic acid 
and polyvinyl pyrrolidone. The preferred plasticizers are selected from 
organic esters, including oligimers and hydrocarbon materials including 
petroleum oils. 
There are a number of water-insoluble, normally solid plasticizers which 
are sufficiently inert to form a part of the battery separator. Typical 
examples of these plasticizers are low molecular weight 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 resultant battery separator 
composition. 
The term "plasticizer" as used herein and in the appended claims refers to 
a material or materials capable of interacting with at least one of the 
polymers, preferably at least the polyolefin, forming the polymer blend 
under conditions used to blend the components and to form the initial 
composition into a sheet material. The interaction of the plasticizer and 
polymer is such that the filled polymeric composition exhibits reduced 
viscosity to aid in forming a substantially uniform sheet product. 
The term "sheet" or "membrane" as used in the subject application is 
intended to define a substantially 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 20 mils thick and more preferably, less than 10 
mils thick with from about 2 to 7 being most preferred. 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. Separators formed from the sheet can be used in their planar 
form between electrodes of opposite polarity or can be formed into other 
configurations, such as pocket or envelopes, of suitable size to encase an 
electrode and provide separation between electrode pairs. 
Filler material is the major component of the subject separator. The filler 
is particulate material having a small particle size ranging from about 
0.01 to about 100 microns in average diameter. Preferred filler average 
diameter is from about 0.01 to about 20. The filler can have a surface 
area of from about 30 to 950 m.sup.2 /gm. It is preferred that the surface 
area of the filler be high such as at least 100 m.sup.2 /g. In the 
application relating to alkaline separators, the preferred filler has a 
surface area of from about 100 to about 400 m.sup.2 /cc U.S. Pat. No. 
4,287,276. 
The filler component can be chosen from a wide variety of materials 
provided the filler is inert with respect to the battery components, such 
as the electrolyte composition, the electrodes and the like of the battery 
system in which the separator is contemplated for use. The filler 
component must also be substantially inert with respect to the other 
components of the subject separator including the polymers forming the 
polymer mixture, the plasticizer and the like. Finally, the filler 
component should not be electrically conductive nor electrochemically 
active with respect to the battery system. Fillers which meet the above 
criteria will depend on the type of battery system in which it will be 
employed (acid or alkaline), the particular components of the battery, 
etc., and can be readily ascertained by those skilled in this art. For 
example, the term "inert" would require that battery separators with 
alkali insoluble fillers should be used only in alkaline batteries, and 
acid insoluble fillers should be used only in acid batteries. If so used, 
the filler is not extracted by the battery electrolyte. Neutral fillers, 
or fillers that do not react with either acid or alkaline electrolytes, 
can of course be used with either acid or alkaline batteries. 
Examples of material which are suitable as fillers in appropriate 
application include materials which are soluble or insoluble in water. 
Representative of the fillers which are substantially water insoluble and 
operable in the instant invention are carbon black, coal dust and 
graphite; metal oxides and hydroxides such as those of silicon, aluminum, 
calcium, magnesium, barium, titanium, iron, zinc, and tin; metal 
carbonates such as those of calcium and magnesium; minerals such as mica, 
montmorillonite, kaolinite, attapulgite, asbestos, talc, diatomaceous 
earth and vermiculite; synthetic and natural zeolites; portland cement; 
precipitated metal silicates such as calcium silicate and aluminum 
polysilicate; alumina silica gels; wood flour, wood fibers and bark 
products; glass particles including microbeads, hollow microspheres, 
flakes and fibers. 
Illustrative of the water-soluble fillers operable in the present invention 
are inorganic salts such as the chlorides of sodium, potassium, and 
calcium, acetates such as those of sodium, potassium, calcium, copper and 
barium; sulfates such as those of sodium, potassium and calcium; 
phosphates such as those of sodium and potassium; nitrates such as those 
of sodium and potassium; carbonates such as those of sodium and potassium 
and sugar. 
The preferred filler is silica when the formed separator is to be used in 
an acid battery system. When the separator is to be used in an alkaline 
battery, the preferred fillers are titania, alumina or calcium or 
magnesium hydroxide. Neutral fillers, that is those that do not react with 
either acid or alkaline electrolytes can be used in either system and 
include carbon black, graphite, coal dust and the like. 
Although highly filled polymeric compositions have been previously used to 
form separators for battery systems, the resultant products have, in 
certain instances, not met acceptable minimum electrical resistance, have 
not provided sufficient current flow to occur via the electrolyte, and/or 
have not possessed acceptable tensile properties. In addition, known 
highly filled polymeric separators do not exhibit a high degree of 
stability, especially resistance to oxidative degradation. The presently 
formed separator unexpectedly achieves all of the above criteria and thus 
provides a separator which enhances the battery life. 
In addition to the above required components the separator can contain 
effective amounts of wetting agents, stabilizers, processing aids and the 
like. For example, commercially available wetting agents include sodium 
alkyl benzene sulfonate, sodium lauryl sulfate dioctyl sodium 
sulfosuccinate, isooctyl phenyl polyethoxyethanol and the like; 
stabilizers include 4,4-thiobis(6-tert-butyl-m-cresol), 
2,6-di(tert-butyl)-4-methylphenol and the like; processing aids include 
sodium stearate and the like. 
The battery separator is formed by a process which comprises blending a 
composition comprising from about 5 to about 25 (preferably 5-20) weight 
percent of the polymer mixture, from about 25 to about 75 (preferably 
35-70) weight percent filler components and from about 15 to about 80 
(preferably 40-75) weight percent plasticizer, forming the composition 
into sheet form and then extracting from the sheet by means of a suitable 
solvent at least a portion of the plasticizer. 
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. 
The extracted separator membrane normally has from about 7 to 50 percent 
of the polymer mixture about 50 to 93 percent filler, and from about 0 to 
20 percent plasticizer. The more preferred separators comprise a mixture 
of from 10 to 25 percent polymer mixture, 60 to 90 percent filler, and 
from 2 to 15 percent plasticizer. 
The process of forming the subject separator comprises blending the 
components described hereinabove 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. In view of the amount of filler and ultra high molecular weight 
pololefin, it has been suprisingly found that the subject composition is 
capable of being blended into a uniform admixture using relatively low 
energy. 
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 polymers and filler components 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, tetrachloroethane, 
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 when alkali 
resistant fillers are used. Generally, acids such as hydrochloric acid can 
be used when the filler, such as silica, is acid resistant. 
The extraction temperature can range anywhere from room temperature up to a 
temperature below (preferably at least 10.degree. C. below) the melting or 
degradation temperature of the polymers used. 
The time of the extraction will vary depending upon the temperature used 
and the nature of the plasticizer being extracted. For example, when a 
higher temperature is used, the extraction time for an oil of low 
viscosity may 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. 
When the separator is to be provided with rib members (such as for use in 
an acid battery) these members can be formed from the same composition or 
from other polymeric compositions which are compatible with the present 
composition. For example, other polymer compositions can be filled, 
unfilled or foamed polyolefins, polyvinyl chloride and the like. 
Alternately, the separator sheet can be grooved or embossed to provide the 
channel members to permit egress of gaseous products from the battery 
system. Other similar modifications can be made in known manners. The 
instant process and composition produce microporous battery separators 
which exhibit low electrical resistance, readily permits electrical 
conductivity via the electrolyte, and possess excellent tensile properties 
to accommodate the various physical forces encountered in the battery 
during operation. In addition, the present separator exhibits a high 
degree of stability and lack of degradation to the various chemical and 
electrochemical forces encountered in the battery.

The following examples are given for illustrative purposes only and are not 
meant to be a limitation on the claims appended hereto. All parts and 
percentages are by weight unless otherwise indicated. 
EXAMPLE I 
9.9 parts of a commercially available polyethylene having a weight average 
molecular weight of 5,000,000 and a density of 0.96 was mixed with 4.3 
parts of a previously made blend of partially crosslinked polyethylene 
terpolymer (ethylene/vinyl acetate/CO) and polyvinyl chloride (Alcryn--80A 
of Dupont having a Shore A Hardness of 78 and a density of 1.25; 27.5 
parts of finely divided silica (HiSil 233) having an average particle 
diameter of 0.06 micron, a surface area of 165 m.sup.2 /gm and a density 
of 2.2; and 66.3 parts of hydrocarbon oil (Sunthene 255; density of 0.89 
g/cc, 54 suu at 210.degree. F., flash point of 390.degree. F.) were 
blended in a dry atmosphere using a high shear mixer with twin roller 
style mixing rotors. The blending was continued for thirty minutes while 
maintaining the mix at 175.degree. C. The torque required was measured to 
be 750 gram.multidot.meters. 
The compounded composition was pressed into sheets of approximately 10 mils 
thickness using a hydraulic press with heated plates (150.degree. C.) at a 
pressure of 3000 psi. The formed sheets were observed to be a cohesive 
flexible material which was suitable for further handling and processing. 
The sheets were immersed in a bath of hexane maintained at ambient 
temperature for 15 minutes to extract out the petroleum oil and then 
dried. The extracted materials were microporous sheets having 
cross-sectional thickness of 10 mils and a void volume of about 73 volume 
percent. The highly filled sheets were composed of 34.2 percent polymer 
and 65.8 percent silica. 
The sheets were analyzed for electrical resistivity according to the 
procedure described by J. J. Lander and R. D. Weaver in Chapter 6a 
entitled "Electrical Resistance Direct Current Method" Page 53-68 
contained in "Characteristics of Separators, Screening Methods" ed. by J. 
E. Cooper and A. Fleischer except that the electrolyte was a 35 weight 
percent H.sub.2 SO.sub.4 aqueous solution. Further, the samples were 
analyzed to determine their capacity to resist oxidative degradation as 
would occur in a battery environment by an accelerated oxidative 
degradation test performed by immersing the samples for a short period in 
an aqueous 30% isopropyl alcohol solution to insure wettability of the 
samples and then immersing the samples in a strong lead(IV) oxidant 
solution formed by dissolving 50 parts by wt. of lead dioxide in 660 parts 
of 1 molar hydrochloric acid. Preformed tensile samples were immersed in 
the lead oxidant solution and removed after three, seven and ten day 
periods and tested for modification to its physical properties (tensile 
tests done according to ASTM D-412). Degradation is observed by the degree 
of decrease in elongation failure and increase in modulus. The results of 
the tests are given in Table I 
TABLE I 
______________________________________ 
Percent 
Electrical Tensile Elongation 
Retention 
Number of 
Resistance Modulus Percent at 
of Orig. 
Days (m ohm in.sup.2) 
(lb/in.sup.2) 
Failure Elongation 
______________________________________ 
0 8.27 6,560 1,080 -- 
3 -- 6,840 882 82 
7 -- 8,290 833 77 
10 -- 9,640 444 41 
______________________________________ 
The sample exhibited low electrical resistance (thus showing excellent 
ability to provide free conductivity by the electrolyte) and good physical 
properties over the entire test period (thus showing that such material 
would exhibit a high degree of stability over extended periods while 
subjected to a conventional battery environ. 
EXAMPLE II 
A separator membrane was formed in the same manner as described in Example 
I above except that 6.6 parts (6.9 vol. percent) of polyethylene of 
5,000,000 weight average molecular weight and 8.6 parts (6.9 vol. percent) 
of the polymer blend used in Example I above (Alcryn--80A) were used to 
form the composition. The torque required for mixing the composition was 
measured to be 100 gram .multidot. meters. The extracted sheet had a 
thickness of 10 mils. The sample was tested in the same manner as 
described in Example 1. The results are shown in Table II below. 
TABLE II 
______________________________________ 
Percent 
Electrical Tensile Elongation 
Retention 
Number of 
Resistance Modulus Percent at 
of Orig. 
Days (m ohm .multidot. in.sup.2) 
(lb/in.sup.2) 
Failure Elongation 
______________________________________ 
0 7.27 3,300 789 -- 
3 -- 3,350 600 76 
7 -- 4,030 512 64 
10 -- 4,960 122 15 
______________________________________ 
Again the sample showed that it has good electrical resistance and 
maintained good physical properties after subjection to accelerated 
oxidation. 
EXAMPLE III 
For comparative purposes, samples were formed in a similar manner to that 
described in Example I using the same materials except that the 
compositions were formed from A) only polyethylene homopolymer of 
5,000,000 molecular weight and B) only polyethylene homopolymer of 
2,000,000 molecular weight (density of 0.96), as described in U.S. Pat. 
No. 3,351,495. 
The samples were formed using 13.2 parts of polyethylene, 27.5 parts of 
silica, and 66.3 parts of hydrocarbon oil (Sunthene 255). The torque 
required to mix the composition was measured to be 1,500 gram .multidot. 
meters. Thus, it can be observed that the present composition, as 
illustrated in Examples I and II, require much less energy to form the 
desired homogeneous blend. 
The samples were tested in the same manner as described above in Example I. 
The results are given in Table III below. 
TABLE III 
______________________________________ 
Percent 
Electrical Tensile Elongation 
Retention 
Number of 
Resistance Modulus Percent at 
of Orig. 
Days (m .OMEGA. .multidot. in.sup.2) 
(lb/in.sup.2) 
Failure Elongation 
______________________________________ 
SAMPLE A 
0 7.09 4,810 225 -- 
3 -- 5,670 147 65 
7 -- 19,000 12.3 5 
10 -- 23,800 4.80 2 
SAMPLE B 
0 9.45 5,480 1,180 -- 
3 -- 16,700 483 41 
7 -- 18,600 24.4 2 
10 -- 24,100 9.60 0.8 
______________________________________ 
The above results show that although the samples are microporous and 
exhibit the desired low electrical resistance, they each severely 
deteriorate due to oxidation. In each case the material became brittle, 
would be easily damaged due to the forces exerted on a separator during 
use and would thereby permit shortening of the battery life. 
EXAMPLE IV 
For comparative purposes, samples were formed in the same manner as 
described in Example I except the polymer content was substituted by 6.6 
parts (6.9 vol. percent of composition) of polyethylene of 2,000,000 
molecular weight and 8.3 parts (6.9 vol. percent of composition) of 
polyvinyl chloride (Geon 121). The torque required to form the mixture was 
measured to be 1500 gram .multidot. meter. The sheet material was tested 
as described above in Example I. The results are given in Table IV below. 
TABLE IV 
______________________________________ 
Percent 
Electrical Tensile Elongation 
Retention 
Number of 
Resistance Modulus Percent at 
of Orig. 
Days (m .multidot. in.sup.2) 
(lb/in.sup.2) 
Failure Elongation 
______________________________________ 
0 32.7 5,640 377 -- 
3 -- 6,820 171 45 
7 -- 10,300 1.90 0.5 
10 -- 15,700 2.40 0.6 
______________________________________ 
The above results shows that the formed microporous sheet exhibits high 
electrical resistance and poor oxidation stability.