CA1092761A

CA1092761A - Flexible, plastic battery separator and process for making same

Info

Publication number: CA1092761A
Application number: CA255,758A
Authority: CA
Inventors: Bruce S. Goldberg
Original assignee: Amerace Corp
Current assignee: Amerace Corp
Priority date: 1975-06-27
Filing date: 1976-06-25
Publication date: 1981-01-06
Also published as: AT360099B; NL7607035A; SE7607200L; ES449241A1; IT1065607B; LU75243A1; PT65257B; AU510614B2; AU1511076A; CH619321A5; JPS525426A; DE2629236A1; BE843328A; PT65257A; GB1553302A; FR2317777A1; ATA467276A

Abstract

ABSTRACT OF THE DISCLOSURE
A composition processable into a usable article such as, for example, a flexible, plastic-based, microporous battery separator, is formed by admixing a polymeric resin-ous binder, a plasticizer, an inorganic filler, a solvent and a nonsolvent. The composition is then extruded and calendered into a relatively thin sheet, passed through an extraction water bath, and dried.

Description

lW;~761 This invention relates generally to flexibl-, microporous plastic sheets and, moro particularly, to ~lexi-ble, microporous plastic sheets usoful as s-parators between the plates of electrical storage batteries S As is well approciated in the art, a battery separator must be porous to allow the passage of ions between the plates as well as free diffusion of acid In addition, the battery separator must bo resistant to attack by acids and electrochemical oxidation as well as being b0th strong and durable More specifically, it is highly desirable that the separator be flexible onough to resist cracking during assembly since even minute cracks, if allowed to propagate auxing the service life of the battery, could result in pr~mature batt-ry failure Moroover, the s-parator should b n~turally hydrophilic since such battery separators do not r-gulre the addition of a wetting agent, As disclosed for example, in the U S Patents to ~r Witt (2,772,322) and Selsor et al ~3,696,061), it is well known in th art to fabric~te battery soparators from compo-sitions comprising a mixturo of a plastic resin, an inorganic filler, and a solvent, in such a manner as to produce a microporous, semi-rigid shoet Attempts have been made to modi~y such prior art plastic ~as-d separators to increa~e their flexibility and 2S thus avoid cracking during assem~ly and handling, by adding plasticizers to the composition being processed It was found, however, that the mere addition of pla~ticize~s to these compositions r-sulted in separatoxs .. . . . . ~ -- - , . . ........................... .. .
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10~;~761 possessing significantly inferior physical properties and particularly inferior electrochemical properties when compar-ed with non-plasticized separators. Specifically, lt was observod that when plasticizers were added to such composi-tions the electrical resistance of the battery separatorincreased although the acid resistanee of the resulting separator decreased.
Against the foregoing background, it i8 a primary objective of the present invention to provide a composition processable into a flexible, plastic-based, microporous battery separator.
It i8 another object of the present invention to provide a composition processable into a flexible, plastic-based, microporous battery separator re6istant to cracking.
It is an additional object of the present invention to provide a composition processable into a flexible, plastie- s based, microporous battery separator with good electrical resistanee properties and whieh is re6istant to attaek by acid and to oxidation by eleetrochemieals, and, moreover, one which is normally hydrophilic and wetsoasily without requiring the addition of a wetting agent.
It is still another object of the pres-nt invention to provide a composition proeessable into a flexible, plastie-based, mieroporous battery separator in whieh the plastieizer is n-lther leached out duri~g proeessing nor during use.
It i8 yet still another objeet of the present invention to provide a eomposition processable into a flexi-ble, plastic-based, microporous battery separator whieh is less subjeet to premature failure than battery separators heretofore employed.
Aeeording to the present invention there is pro-vided a proeess for produeing a flexible mieroporous battery separator from a composition eomprising a thermoplastie resin binder in an amount between 10% and 16% by weight of tho total eomposit~on, a plastieizer in an amount between 4~ and 10% by weight of the total eomposition, an inorganie filler material in an amount between 19% and 23% by weight ..

-10S';~7~1 of the total composition, a solvent for ~aid thermoplastic resin binder in an amount between 26% and 32% by weight of the total composition, and a nonsolvent in an amount between 28% and 34~ by weight of the total composition, said process including the steps of mixing said thermoplastic resin blnder together with said plasticizer to form a master batch, add-ing to said master batch said inorganic filler material, said solvent, and said nonsolvent and mixing the composition until the constituents are uniformly dispersed to form an extrudable mixture, extruding and calendering said extrudable mixture to form a sheet-like article from said composition, passing said sheet-like article through an extraction bath to remove said solvent therefrom, and drying said sheet-like article to remove said extraction medium and ~aid non-solvent therefrom.
According to a further aspect of the inventionthere is provided a flexible, microporous, sheet-like article comprising a plasticized resin matrix and an inorganic fil-ler material, said article including a plurality of pores formed within said plasticized resin matrix between said particles of inorganic filler material and said matrix, between neighboring particles of said inorganic filler materi-al and within said matrix itself, said sheet having an electrical resistance less than 0.070 ohm/in2 and elongation greater than 40%.
The composition which is herein disclosed and which may be processed into a flexible, plastic-based, microporous battery separator, is composed of the following essential ingredient~:
a) a thermoplastic resin binder;
b) a solvent which serves to solubilize the thermo-plastic resin binder;
c) an inorganic filler such as silica;
d) a nonsolvent such as water, and e) a plasticizer.
The thermoplastic resin binder employed ~hould pre-ferably be a vinyl chloride resin binder of the "EP" or . - - , . - , ~ .
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109;~7~;1 "easing processing" type which are porou~ and highly ab~or-bent. This thermopla~tic resin binder may be a non-pla~ti-cized gamma vinyl chloride homopolymer resin or a copolymer of vinyl chloride admixed with a small amount (less tha~
about 15~) of a monoethylenic monomer such as, for example, vinyl acetate, vinylidene chloride, propylene or sthylene.
A particularly preferred thermoplastic resin binder i5 the gamma vinyl chloride homopolymer marke~-~d by Continental Oil Company under the tradomark Conoco 5385 although good results have been obta~ned with the gamma vinyl chloride homopolymers marketed by the B. F, Goodrich Company under the trademark Geon 103EP and by Solvay & Cie S.A. of Belgium under the mark Solvic 229. Additionally, other thermoplastic re~in binders may be used which, when admixed with a solvent, are converted into a doughy~ mass for oasy processing and which, upon removal of the solvent, bocome fixed shape.
Moreover, the thermoplastic resin binder selected should be rho~lly and physically stable under the conditions under which it is to be used.
The thermoplastic resin binder should comprise between about 10% and about 16% by weight of the total com-position with a range between about 11% and about 15% being preferred. Particularly good results are obtained when the amount of thermoplastic resin constitutes between about 11%
and 12~ by weight of tho total composition and, as such, thi~ rango i~ most preferred.
The inoxganic filler material should desirably be an inorganic ~olid capable of holding at least 30 parts of water or other volatile matter per lOO parts of filler material and should be able to release the volatile matter upon heating to a temperature below the decomposition point of the thermoplastic resin. While any filler material capablo of mo-ting these requirement6 may be employed, silica hydrogel or precipitated hydrated silica are preferred.
Precipitated hydrated silica is particularly preferred and may bo obtained, for example, from PPG Corporation under the ~rademark Hi Sil 233 or from Chemische Fabrik Hoosch of .
-109;~761 Germany under the trademark KS-300.
The inorganic filler material should constitute between, about 19% and about 23% by weight of the total composition with 2 range of between about 20% and about 23% by weight of the total composition being preferred.
When the range of inorganic filler material i8 between about 22% and about 23~, an article with particularly good physical and electrochemical properties is produced and, therefore, this range is most preferred.
The solvent employed, preferably an organic 801-vent, should have a solvating action on the thermoplastic resinous binder and should be capable of being absorbed by the filler material. While organic solvents such as, for example, acetone, ether, dimethyl formamide, orthochloro-15 benzene, tetrahydrofuran and certain ketones may be employed, cyclohexanone is preferred since it solubilizes polyvinyl chloride and is only slightly soluble in water, The solvent used, preferably cyclohexanone, is employed in ranges between about 26% and about 32% by weight 20 of the total composition with a range of between about 26%
a~d about 30% being preferred. Pa~ticularly good results are obtained when the amount of sol~ent constitutes between about 27% and about 28% by weight of the total composition.
Tho nonsolvent, preferably water, generally con-2S stitutes between about 28% and about 34% by weight of the r total composition with a range of between about 29% and about 33% being preferred. A nonsolvent in an amount rang-ing betweon about 31~ and about 33% of the total weight o the composition haæ been found to produce particularly good 30 physical properties and therefore is especially preferred.
The plasticizer selected should be system-compati-ble and, thus, when admixed with the above mentioned com-positions, be capable of improving the elongation properties of the resultant article while not adversely affecting such 35 physical propsrties as electrical resistance and resi~nce to attack by aci-d or resistance to oxidation by electrochemicals. In this regard, any monomeric or polymeric plasticizer which - ~ ........................................................ .
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10~ ~7 ~1 accomplishes theqe goals would be acceptable. ~hi~ would include both monomeric plasticizers such as, for example, dioctyl sebacate and polymeric plastiaizers such as, for example, elastomeric chlorinated polethylene. The use of a monomeric plasticizer, and particulàrly dioctyl phthalate and dioctyl adipate i8 preferred. It has been found that articles fabricated from composition6 containing either of these two monomeric plastici~ers po~sess ex¢ellent physical and electrochemical properties.
The plasticizer should constitute between about 4% and about 104 by weight of the totàl composition with a range of between about 4% and about 8~ being preferred.
Particularly good physical properties are observed in the resultant article when dioctyl phthalate is employed in amounts between about 6% and about 7~ by weight of the total composition and, a~ such, when dioctyl phthlate i~ the plasti-cizer, this range is most preferred. When, however, dioctyl adipate is the Plasticizer, a range of between 4.5~ and about 5.5~ is most preferred.
In addition to the foregoing ~ngredients, it will be apparent to those skilled in the art that a variety of other ingredients may be employed which do not affect the essential nature of the resultant product. Indeed, many ~uch ingredlent~ may be provided for the purpose of improv-ing other propertie~ thereof or for the purpose of improving industrial acceptance. Typical ingredients include, but are not limited to modifying or stabilizing ingredients such as, for example, oarbon black and lead stearate.
An important feature of the present invention i8 the manner in whioh the ing~edients are combined to form the composition capable of fabricating an article such as, for oxample, a flexible, plastic-based micropoxou~ battery separator.
In combining the aforementioned ingredients in the amounts and ra~ges specified, it has been found that the procedure~ re¢ited in the afor~mentioned U,S. Patent No. 3,696,061 to Selsor et al. are applicable, with certain , .

modifications. U.S. Patent No. 3,696,061, which i8 a~signed to the same assignee as the present invention, is also re-ferred to.
A master batch of the thermoplastic resin binder S and the plasticizer iB flrst prepared by preferably dry blending at room temperature the constituent parts in a low shear solids blender such as, for example, a Patterson-Kelley high intensity "liquids solids" blender for about twenty (20) minutes. This is contrasted with the processes heretofore employed wherein a high shear mixer ~uch as, for example, a Henschel mixer is used. Additionally, in the processes heretofore employed, the thermoplastic resin binder i8 admixed with the plasticizer in the presence of heat to effect better absorption of the plasticizer into the thermoplastic resin binder. The Patterson-Kelley high intensity, "liquid~ solids" blender has a v-shaped cross-section which is preferred, although it is recognized that other mixing devices may be employed for this purpose. An ali~uot of this master batch which is in damp powder form is then placed in a clean blender and the inorganic filler, in the prescribed amount, is added, The entire blend, which then resombles a dry powder, is then admixed until all in-grodlents are uniformly dispersed.
It is preferred that the prescribed amount of organlc solvent be then added. The rate at which the sol-vent i6 added is, however, of some importance 6ince the maximum absorption rate of the solvent by the inorganic filler should not be exceeded lest some of the thermoplastic resin be insolubilized.
The prescribed amount of nonsolvent ~e.g. water) is then added at a rate less than the maximum absorption rata of the solvent ladened silica. The resultant compo-sition is then in the form of a stable, damp, free-flowing powder.
In order to process this composition into a use-ful article such as, for example, a flexible, plastic-based, microporous battery separator, the composition is introduced -: ` :
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-` 109;~761 into an extruder, preferably of a vertical screw construction, wherein the free-flowing powder i8 converted into a doughy mix, shaped by a sheeting die and calendered into a continu-ous sheet.
Extrusion temperatures may range from about 80F.
to about 160F. with a narrower range of between about 115F.
and about 130F. being preferred. The temperature of the die may range between about 80F. and about 160F. The back-pressure of the extruder may range from between about 200 psig and about 500 psig with a pressure range of between about 200 psig and about 300 psig being preferred, Other extruders can, of course, be substituted.
The doughy mass formed within the extruder then passes through a screen pack of about 40 mesh and a sheet-ing pipe die before entering a geared down calender which is maintained at a temperature between about 40F. and about 60F., and preferably between about 40F. and about 45F.
The die is locked i~to the calender to avoid evaporation.
The calender employed i8 a 32" oalender preferably containing separator patterned rolls.
The composition, now in sheet form iB then sup-ported on a transport screen and pa~sed through a watçr ex-traction bath, the temperature of the water being maintained at between about 120F. and about 200F. although a narrow-er range of between about 160F. and about 180F. is pre-ferred. The sheet remains in the bath until the solvent is removed a~d is then dried by conventional means. The drying temperature should not exceed about 275F. and a tem-perature of about 225F. is preferred.
It will be appxeciated that the resultant article, after the solvent and the non-solvent constituents have been removed during prooessing, comprises a suitably plasticized thermoplastic resin binder forming a matrix, within and throughout which are dispersed particles of inorganic filler material. A network of microvoids or micropores is present in the article, being formed between neighboring or adjacent particles of dispersed filler material, between individual , - --particles of filler material and the matrix as well as withln the matrix it~elf. These microvoids or micorpores are non-uniform in size, typically ranging between about 0.01 microns and about 100 microns, and have a mean pore S diameter typically of about 1 micron a6 determined poro-simetrically by the well-known Mercury Intrusion Method.
The porosity of the resultant article when mea~ured in alcohol, ranges between about 50% and about 75%.
The resultant article possesses physical properties making it ideally suited for use as a battery ~eparator.
In particular, the resultant articlo is not only highly porous, having a total porosity of at least about 50% when measured in alcohol, but is both strong and flexible, as evi-denced by its tensile strength which is generally greater than about 200 psi and its elongation which is generally greater than about 40%. Moreover, its electrical resistance, a characteristic of significant importance with respect to battery separators, is generally no greater than about 0.070 ohms/in2 and normally less than about 0.~40 ohms/in2.
Whon the thickness of the resultant article is reduced to, for examplo, about Q.025 inchos, olectrical resistance has been further decreased to about 0.020 ohms/in2.
The following examples serve to illustrate certain preferred embodiments of the present composition and process and are not to b0 oo~shuYd as limiting the present invention.
Example I
In order to illustrate the preparation of a com-position prooessable into a useable arti¢le such as, for example, a flexible, plastic-based microporous battery se-parator in accordance with the principles of this invention,a composition was prepared which included a plasticizer as one of its constit~ent parts in a ratio of 60 parts of plasticizer to 100 parts of thermo-plastic resin binder. This composition compri~ed the fol-lowing ingredients with its respective amounts being speci-fied as a percentage weight of the total weight of the composition.

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Ingredients Percentage by Weight Conoco 5385 (a trademark for a gamma vinyl chlor-ide homopolymer) 11.17%
5 Dioctyl phthalate 6.67 Hi Sil 233 precipitated hydrated silica filler 22.20%
Cyclohexanone 27.76%
Water 32.20%
10 Carbon black .001~
The composition was prepared by first dry blend-ing in a Patter60n-Kelley high inten6ity "liquids solids"
blender the Conoco 5385 with the dioctyl phthalate to form a master batch and then adding the Hi Sil 233 followed by the cyclohexanone, water and carbon blac~.
The resultant composition was then introduced into a vertical Aragon extruder with a stainle6s steel scrow having a compres6ion ratio of about 1.4/1. The com-position was extruded at a temperature of about 120F. and at a pressure of about 250 psig. A 32" calender with a calender top, pattern separator roll was employed to pro-duce a separator patterned 6heet approximately 0.100 inches thick. The resultant sheet was then pa6sed through a water extraction tank in which the water temperature was maintained at about 160F. and then dried in an air dryer at an air temperature of about 225F.
The resultant fabricated article, in this instance, a flexible, microporous, battery 6eparator, had the follow-ing physical propertie6:
Tensile Strength 210 p8i Elongation 65%
Mullen Strength 64 psi Electrical Re6istance 0.054 Q~in2 Total Porosity (Nercury , Intru6ion Technique) 1.04 cc/g Mean Pore Diameter 0.085 ~ of Pore6 >20~ 2~5%

109;~761 Of the tests performod, the tensile strength, elongation, and Mullen ~trength data indicated that the resultant article was sufficiently strong and flexible to with~tand any damage inflicted during as~embly and evontual S use.
The electrical resistance and porosity data indi-cated a product with a fine pore size which pos~e~sod excel-lent electrochemical properties. For u~e as a battery separator, electrical resi~tanc~ ~hould be as low as po~ible, preferably less than about .070~in2. The re~ultant product's electrical resistance of .054Qin2 was, therefore, more than adequate. Additionally, the data with respect to pore ~ize and total porosity indicated that while the pore size and distribution was in the microporous range, the resultant article was highly porous. A mean pore diameter of one micron or less is preferred. Further, this data indicated that virtually no plasticizer leached out during processing.
The resultant product was hyarophilic and displayed 9OOa acid resistance.
Example II
In order to demQnstrato the effects of practicing the instant invention with a lower ratio of plasticizer to thermoplastic resin, tho same procedure as set forth in Example I was repeated with tho following ingredient~ in tho following percentage~ whorein the ratio of plasticizer to thermoplastic resin was 50 parts of plasticizer to 100 part~ resin:
Ingreaients Porcentage by Weight Conoco 5385 ~a trademark for a gamma vinyl chloride homo-polymer) lQ.98%
Dioctyl phthalate 5.49%
Hi Sil 233 precipitated hydrated silica filler 21.97%
35 Cyclohexanone 28.61%
Water 32.95%
Carbon black .001%

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lQ9;~7~i1 The resultant composition was fabricated into a useful article such as, for example, a flexible, micro-porou~ battery separator, in the ~ame manner as in Example I and the resultant article had the following phy~lcal pro-5 pertles Tensile Strength 219 psi Elongation 54%
Mullen Strength 53 psi Electrical Resistance 0 041~/in Total Porosity (Mercury Intru~ion Te~t) 0 99 cc/g Mean Pore Diameter 0 095 % of Pores >20~ 2 5%
The results of those physioal tests indicated a product with properties essontially ~imilar to the composi-tion in Example I although the elongation figure~ reflected the fact that the resultant article was somewhat less flexi-ble than the article in Example I and had a ~lightly higher electrical resi~tance The re~ultant article, however, was a commercially accoptable, flexible, pla~tic-based micro-porous battery separator Example III
In order to illustrate the preparation of a compo-sition according to the present invontion wh-rein different typ-~ of thermoplastic resin and inorganic filler materials woro mployod although with the ~amo ratio of resin to plasticizer a~ in Example II, the same procedure as, set forth in Example I was xepeated with the following ingredi-ent~ in the following percentages 30 In~redi-nts Peroentage by Weight Solvic 229 vinyl chloride homopolymer 14 23~
Dioctyl pbthalate 7 11%
KS 300 precipitated hy-35 drated sillca filler 19 96%
Cyclohexanone 28 22%
Wator 30 48%
Carbon black 0 001%

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109;~761 The resultant composition was fabricated into a useful article such as, for example, a flexible microporous battery eeparator in the same manner as in Example I and the resultant article had the following phyeical properties:
Tensile Strength 290 p8i Elongation 57%
Mullen Strength 68 p8i Electrical Resistance 0.045~/in Total Porosity (Mercury Intrusion Test) 0.87 cc/g Mean Pore Diameter 0.11 % of Pores >201~ 3 5%
In comparing the physical data of this article to tho article produced in Example I, the tensile strength of the article was higher than the article of Example I, al-though its elongation was less, indicating a stronger, but less flexible product. The electrical resistance of the article was lower than the electrical resistance of the article of Example I. Further, the porosity of the article was loes than ~he porosity of the article of Example I al-though the pore size was generally greater.
Example IV
In order to illuetrate the preparation of a compo-sition according to the present invention wherein the type of plasticizer was changed, the same procedure as set forth in Example I was r-p-at-d wher-in dioctyl adipate was chosen -as the plasticizer and used in a ratio of 50 parts of plasticizer and used in a ratio of 50 parts of plasticiz-r to 100 parts of resin. Additionally, a diff-rent thermo~
plastic resin was employed. The following percentages were employ-d:
Ingredients Percentage by Weight Geon 103EPF 10 thermo-plastic reein 11.01%
35 Dioctyl adipate 4.96%
~i Sil 233 precipitated hydrated silica filler 22.03~
Cyclohexanone 29.52%

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1()~;~761 Ingredients Percentage by Weight Water 32.48~
Carbon blsck 0.001%
The resultant composition was fabricated into a useful article such as, for example, a flexible, micro-porous battery separator, in the same manner as in Example I and the resultant article had the following physical properties:
Tensile Strength 187 p~i Elongation 62~
Mullen Strength 60 psi Electrical Resistance .031~/in Total Porosity (Mercury Intrusion Test) 0.07 cc/g % of Pores >20~ 1.3S
The physical data illustrate that the use of di-octyl adipate as a plasticizer produced an article with both physical and electrochemical propertiQs quite similar to the article of Example I wherein dioctyl phthalate was the plasticizer employed.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a flexible microporous bat-tery separator from a composition comprising a thermoplastic resin binder in an amount between 10% and 16% by weight of the total composition, said thermoplastic resin binder being of a type which when admixed with a solvent is converted into a doughy mass and upon removal of the solvent becomes fixed in shape, a plasticizer in an amount between 4% and 10% by weight of the total composition, an inorganic filler material in an amount between 19% and 23% by weight of the total composition, a solvent for said thermoplastic resin binder in an amount be-tween 26% and 32% by weight of the total composition, said sol-vent being capable of being absorbed by said filler, and a non-solvent in an amount between 28% and 34% by weight of the total composition, said process including the steps of mixing said thermoplastic resin binder together with said plasticizer under conditions of low shear and without the addition of heat to form a master batch, adding to said master batch said inorganic filler material, said solvent, and said nonsolvent and mixing the composition until the constituents are uniformly dispersed to form an extrudable mixture, extruding and calendering said ex-trudable mixture to form a sheet-like article from said composi-tion, passing said sheet-like article through an extraction me-dium to remove said solvent therefrom, and drying said sheet-like article to remove said extraction medium and said nonsolvent therefrom.

2. A process as claimed in claim 1, wherein the inorganic filler material is a solid capable of holding at least 30 parts of any volatile matter per 100 parts of said filler material.

3. A process as claimed in claim 2, wherein the ingredi-ents of said extrudable mixture are mixed together under condi-tions of low shear without the addition of heat.

4. A process as claimed in claim 3, wherein the extru-date is calendered to form said sheet-like article.

5. A process as claimed in claim 4, wherein said low shear mixing steps are carried out at a temperature between 65°F and 75°F, wherein said extruding step is carried out at a temperature of between 80°F and 160°F at a backpressure within the range of 200 psig and 300 psig, and wherein said extraction medium comprises a water bath at a temperature in the range of 120°F to 200°F.

6. A process as claimed in claim 1, wherein said plasti-cizer is dioctyl phthalate or dioctyl adipate.

7. A process as claimed in claim 6, wherein said thermo-plastic resin binder is polyvinyl chloride resin, said inorganic filler is silica, said solvent is cyclohexanone, and said non-solvent is water.

8. A process as claimed in claim 7, wherein said poly-vinyl chloride resin is a vinyl chloride homopolymer, present in an amount between 11% and 12% by weight of the total composi-tion, wherein said plasticizer is dioctyl phthalate present in an amount between 6% and 7% by weight of the total composition, wherein said silica filler material is present in an amount be-tween 22% and 23% by weight of the total composition, wherein said cyclohexanone is present in an amount between 27% and 28%
by weight of the total composition, and wherein said water is present in an amount between 31% and 33% by weight of the total composition.

9. A process as claimed in claim 7, wherein said poly-vinyl chloride resin is a vinyl chloride homopolymer present in an amount between 11% and 12% by weight of the total composi-tion, wherein said plasticizer is dioctyl adipate present in an amount between 4.5% and 5.5% by weight of the total composi-tion, wherein said silica filler is present in an amount be-tween 22% and 23% by weight of the total composition, wherein said cyclohexanone is present in an amount between 26% and 30% by weight of the total composition, and wherein said water is present in an amount between 31% and 33% by weight of the total composition.

10. A flexible, microporous, sheet-like article comprising a plasticized thermosplastic resin matrix and an inorganic filler material, said article including a plurality of pores formed within said plasticized resin matrix between particles of said inorganic filler material and said matrix, between neighboring particles of said inorganic filler material and within said matrix itself, said sheet having an electrical resistance less than 0.070 ohm/in2 and elongation greater than 40%.

11. An article as claimed in claim 10, wherein the mean pore diameter of said pores is approximately 1 micron.

12. An article as claimed in claim 10, wherein the tensile strength of said article is greater than 150 psi.

13. An article as claimed in claim 10, wherein said article is a battery separator.