IL28563A - Microporous plastic member such as a battery separator and process for making same - Google Patents

Description

/ / MICR02P0R0US PLASTIC MEMBER SUCH AS A BATTER! , SEPARATOR AND PROCESS FOR MAKING SAME ,ms»^ ΠΒ*®·» "US » T)BB 71:0 D'»B'i Di)ip,» o'apj ya »t»¾a MICROPOROUS PLASTIC MEMBER SUCH AS A BATTERY SEPARATOR AND PROCESS FOR MAKING SAME ABSTRACT OF DISCLOSURE Microporous plastic members such as sheets useful, for example, as separators for electric storage batteries are made by (a) mixing (i) a- thermoplastic resin (for instance, polyvinyl chloride)., (ii) an organic solvent (for instance, cyclohexanone) to plasticize the resin and (iii) a pore forming filler such as powdered silica hydrogel or o.ther material containing a vaporizable constituent and capable of shrinkage by irreversibly releasing it at the ^proper time so that the material will not thereafter swell substantially; (b) forming a member of the desired configuration from the resulting plastic dough; (c) introducing the doughy member promptly and with a minimum loss of its volatile components into a bath of an extractant liquid (for instance, water) ; (d) extracting the plasticizing organic solvent from the member in the bath and thereby restoring rigidity to the resin while minimizing evaporation of the volatile components in the member; and (e) thereafter creating porosity in the deplasticized or rigid member by drying it and driving the volatile components therefrom.

The porous members made according to this invention may also be useful as filters, breathable plastics, etc.

BACKGROUND OF INVENTION Numerous attempts have been made to produce plastic separators both reinforced and unreinforced and many such attempts have been successful to the extent that the separators are commercially available and used within the industry concerned. These separators include blanks of cellulose fibers; for example , paper impregnated with resins insoluble in sulfuric acid or they comprise woven fabrics of polyvinylidene chloride and other plastic fibers. They also include glass fibers alone or in combination with other .materials such as diatomaceous earth.

These separators have many disadvantages and imperfections which include porosity that is too low leading to high internal resistance in the battery which is undesirable and pore size that is sufficiently large to permit shorting between the negative and positive plates or material transfer, such as antimony, which reduces the capacity of the battery.

Other separators in existence consist' of unreinforced plastic material. These have preferably been prepared by mixing powdered thermoplastic material with a porous or pore producing material and a solvent to make the plastic processable. After a dough has been made this is formed by extrusion or calendering into a sheet of suitable thickness and the solvent evaporated. For instance, see U.S. Patent 2,707,201. If the pore forming material is porous as in the case of diatomite, the sheets need only to be cut to the desired size and used as separators. In this case, stresses are set up in the plastic structure which causes shrinkage and reduces the effective porosity of the product.

If, on the other hand, the pore forming material consists of a water soluble material such as an inorganic salt or organic material such as starch or sugar, this must be leached from the extruded or calendered sheet and the result is a porous sheet which is then dried. U.S. Patent 3,055,966 describes one such process in which the pore making material consists of an inorgani salt and- starch. However, separators whose porosity is caused by pore' formers that have been, leached or washed out tend to have . relatively low residual porosity due to shrinkage during such processing. Also, these separators have poor wetting characteristics which tend to retard -passage of electrolyte and thereby cause high internal resistance when assembled in a battery.

The main object of the present invention is to produce a microporous member useful, for example, as a separator for electric storage batteries, with high porosity, good and permanent wetting characteristics, which separator is sufficientl rigid and impact resistant to be inserted even by machinery between the battery plates.

Further, the separator has sufficient dimensional stability, mechanical strength, and insolubility that it maintain its dimensions and characteristics during service in an electric, storage battery.

This and other objects of the invention as well as the nature, operation and use of the invention will become more fully apparent from the subsequent description and appended claims.

All amounts and proportions of materials are expressed herein on a weight basis unless indicated otherwise.

GENERAL DESCRIPTION In accordance with the present invention, poromeric or microporous plastic sheets are made from a mixture compounded fro the following essential kinds of ingredients: (1) a thermoplastic resin, for example, resinous polyvinyl chloride; (2) a solvent which serves to plasticize the resin, e. g. , cyclohexanone ; and (3) a pore former such, as silica hydrogel (which, for i-nstance, m easily hold from 20 up to about 80% water that can be released therefrom by heating and which then irreversibly shrinks in size) or any other carrier or substrate for volatile matter which serves .to import a controlled amount of volatile matter into the resinous matrix whence the volatile matter is later removed under controll conditions leaving behind 'the desired system of micropores.

Instead of polyvinyl chloride, other similar resilient thermoplastic vinyl resins or polymers of ethylenically unsaturate monomers can be used, e.g., copolymers of about 85 to 97% vinyl chloride and correspondingly 15 to 3% vinyl acetate/ polyvinylide chloride, polystyrene or styrene copolymers, polyethylene, polypropylene and the like. The thermoplastic resin must of course be a material (1) which can be converted to a doughy, plasticized state with the aid of a solvent, without or with the aid of moderate heat so as to be readily capable of shaping by extrusion or calendering while in this plasticized state; and which then upon removal of solvent and consequently deplasticization retains the resulting shape at whatever temperatures it is ultimately intended to function; and (2) which is chemically stable under the conditions of intended use, that is, if it is intended to serve as a battery separator it should resist attack by battery acid or the reaction products which can occur in a battery cell nor should it thermally decompose or become embrittled at temperatures at which batteries are expected to be used and function. To facilitate mixing or compounding, the resin is preferably used in the form of a powder or fine granules. Commercial vinyl chlori polymer, such as "QYNA" made by Union Carbide Corporation, represents a suitable species of resi . If desired, a coloring agent or filler, such as carbon black, may be included in the composition in conventional proportions, e.g., about 1/2 to 2 parts per part of thermoplastic resin, the appropriate amount being somewhat dependent on the specific character and proportion of the other ingredients of the mix as well as on the desired working characteristics of the dough and the performance characteristics of the final product.

Instead of silica hydrogel or hydrated silica, in making a sheet of about 0.6 mm thickness, the pore former can be any other relatively little soluble inorganic or organic solid capable of holding at least 30 parts water or other volatile matter per 100 parts non-volatile material and capable of irreversibly shrinking in volume when such volatile matter is driven off or released upon heating to an appropriate temperature below the decomposition of the thermoplastic resin used in the process. In this manner, dehydration or devolatilization and shrinkage of the filler material in the rigid, deplasticized sheet brings about the . formation of the desired system of micropores within the sheet. Among the more readily available pore forming materials of this kind are aluminum hydroxide, ferro hydroxide, hydrated adsorbent clays .or diatomaceous earths, borax and acetyl salicylic acid.

Of' course, the material should be one which is not readily extracted from the extruded or calendered plastic sheet in the solvent removal or deplasticizing bath. Furthermore, when the poromeric product is intended for use as a battery separator, the pore former as well as all the other materials used in its production should be essentially free from components or impuriti such as iron, manganese, arsenic, etc., which have an adverse effect on the battery performance. .1 to 25 or more parts by weight, preferably 5 to 12 parts, of the pore forming filler can be used per part of thermoplastic resin. Silica hydrogel containing 70 to 75% water is a particularly convenient material to use. When the releasable volatile content of the filler and consequently its shrinkage capability are low it is often advantageous to use more than 12 parts of filler per part of resin. _ The selection and, optimum amount of solvent depends on the resin used. For instance, in the case of vinyl chloride polymers it. is convenient to use ketones as solvents, e.g., eyelohe anone, methyl ethyl ketone, tetrahydrofuran or phorone (diisopropylidene acetone) . When using polystyrene, the solvent can be an aromatic hydrocarbon such as benzene or xylene, a chlorinated hydrocarbon such as methylchloroform or an ester such as ethyl acetate. Polyolefin resin can be dissolved or solvated in solvents such as ethylene dichloride. There is nothing particularly critical about the kind or amount of solvent used except that upon mixing with the resin and' the pore former and whatever other ingredients may be present it should yield a dough of a consistency suitable for molding or extrusion in the form of thin sheets and should not attack the pore former. Moreover, sinc the solvent must ultimately be extracted from the formed sheet while minimizing its loss by evaporation, it is desirable that it be relatively non-volatile, e.g., preferably have a boiling point of. at least 120°C. , and relatively easily extractable with water' or some other relatively inexpensive and relatively volatil secondary solvent which does not appreciably solvate the resin or the porous filler used.

Though generally not required when a pore former such as silica hydrogel is used, which in itself facilitates wetting, in some instances the extraction of the plasticizing solvent from the plastic sheet may be facilitated by including a small amount, e.g., 0.001 to 0.1 part of a conventional wetting agent per part of thermoplastic resin in the dough forming mixture.

Cationic or anionic emulsifiers can be used but non-ionics such as sulfonated red oil (a sulfated oleic acid) are particularly effective in decreasing extraction time. When the primary solvent is a ketone its extraction can also be facilitated by adding a minor proportion of a - alkanol such as methanol, ethanol or isopropyl alcohol to the water used as. the extraction medium. When the primary solvent is an aromatic hydrocarbon or an ester, ethanol is a convenient extraction medium.

It is important that when the dough is extruded in the desired shape such as a sheet that it be conveyed into the extraction medium under conditions such that a significant amount of the primary solvent or of the volatile content of the pore former does not evaporate from the sheet prior to. its immersion in the extraction medium. The extraction medium in the extraction step" is desirably maintained at a temperature which is at least nearly the same -and is preferably higher than the maximum temperature used in the preceding dough forming and shaping steps. For instance, if the temperature of the mixture in the dough forming step is raised from room temperature to 50°C. or 70°C . and the dough is extruded or otherwise shaped at about this temperature to form a sheet, the extraction liquid is desirably maintained at a temperature about 5° to 30°C. higher than the shaping temperature but substantially below the boiling point of the extraction liquid and of the volatile content of the pore former, e.g., at 75°C. or 80°C. For -instance, when water forms the volatile content of the pore former, bath temperatures approaching 100°C. should be avoided when operating at atmospheric pressure as at such excessive temperatures water vapor is liberated in the plastic sheet and tends to "blow" out therefrom causing the formation of macropores of undesirably large diameters instead of the desired micropore system. On the other hand, of.-course, the use of a suitable elevated bath temperature has the advantage of accelerating the extraction of the plasticizing solvent. If this temperature approaches the softening point of the.resin used, it may be desirable to support the plastic sheet in the extraction bath on an appropriate support such as a conveyor belt to avoid unwanted deformation.

By maintaining the extraction bath at such an elevated temperature, strain's which may have previously developed in the doughy sheet are relaxed and the primary solvent is replaced therein with the secondary solvent without intorducing any new stresses, thereby minimizing ' shrinkage and maximizing ultimate porosity. It should be understood that, depending on the amount of solvent, the dry volume of the thermoplastic resin used to form the sheet may swell greatly when it is solvated or transformed into a gel upon mixing with the plasticizing solvent and that stresses may develop in the sheet when it is mechanically formed from the resulting gel and particularly if the solvent is then evaporated from the shaped dough mass. These stresses then tend to cause unwanted shrinkage of the sheet and thereby diminish its porosity. In the present invention shrinkage is minimized and the desired microporosity is produced and maintained. This is. achieved here because as the primary solvent is extracted from the plastic sheet it is largely replaced in the sheet by the extraction liquid. For instance, though the organic solvent in a typical embodiment may constitute 20 to 30% by volume of the dough, upon its removal by extraction of the extruded plastic sheet in accordance with this invention the total volume of the sheet will shrink relatively little, usually about 5% or less. Thus, unlike in the case of solvent removal by evaporation, the. removal of the primary solvent according to the present invention does not leave behind any . substantial vacant volume for the deplast'icized resinous mass to shrink into. Instead, it produces .a dimensionally stable, deplastieized . resinous matrix which contains a substantial proportion of non-solvating liquid and in which the desired system of micropores can thereafter be developed by drying out its liquid content and .by driving, off the volatile content of the pore forming substance which is dispersed in or held together by the resin.

In preparing the doughy mass it is most convenient to blend the dry pulverulent resin and particles of pore former in a dry mixer. Thus, for instance, 1 part of polyvinyl chloride can be mixed with 10 to 12 parts, of silica hydrogel particles containing 50 to 75% water. After a homogeneous pulverulent mixtur is obtained a solvent for the resin, e.g., cyclohexanone when polyvinyl chloride is used, is added in an amount sufficient to form a kneadable, coherent dough that is capable of being formed into a sheet of desired thickness, e.g., 0.001 to 1.5 mm, preferably about 0.6 to 0.7 mm, by extrusion, calendering, or other suitable method. The resulting sheet is then directly introduced into an extraction liquid which is capable of extracting the primary solvent from the formed sheet and which replaces the primary solvent in the sheet without appreciably, dissolving or swelling the resin in the sheet. When the primary solvent has been · substantially completely extracted from the sheet, and the latter has thus been deplasticized and made dimensionally quite stable, the extraction liquid is removed ■from the sheet by evaporative drying. Ultimately the desired microporosity is created, in the sheet by heating it at a temperature- at which the pore former shrinks in volume due to release of its volatile content or other partial or total decomposition. For instance, when silica hydrogel is used as the pore former, its desired dehydration and shrinkage within the plastic sheet and the consequent formation of micropores _ can be achieved by passing the deplasticized sheet including the pore former through a dry air oven maintained at a temperature between about 65° and 150°C. Quite high air temperatures may be maintained if the plastic sheet which is being dried or in which i the micropores are being created is passed through the oven at j ! a rapid rate, particularly when evaporation of substantial ' j quantities of water or other liquid from the sheet prevents the ; temperature of the latter from exceeding the boiling point of the liquid being evaporated. However, the heating conditions should preferably be such as to avoid raising the temperature of the sheet itself substantially above the softening point of the resin of which the sheet is essentially composed, i.e., in the case of a sheet made from a typical PVC resin the sheet temperature should desirably be kept at not more' than about 100°C, and preferably below about 95°C. When the desired micropore structure has been created, the sheet can be cut to 10 the desired size or configuration, preferably after it is cooled to room temperature.

EXAMPLE 1 One part powdered polyvinyl chloride resin is mixed dry .with 12 parts powdered silica hydrogel (73% water content) until 15 II the mixture is homogeneous, whereupon 4 parts cyclohexanone is added to the mixed powdered solids. As those skilled in the art will' readily understand, the optimum proportion- of organic solvent used in making up a dough of the proper consistency- is somewhat dependent on the specific characteristics of the resin 2011 and solvent used as well as on water which may be present in the mixture and which may tend to weaken the solvent power or plasticizing effect of the organic solvent. Thus, for instance, in this specific example the solvent requirement was l'part cyclohexanone per part of polyvinyl chloride plus 0.25 part cyclohexanone per part of the silica hydrogel which contained 73% water. Less cyclohexanone is needed if a silica gel of lower water content is used but in this event less filler shrinkage and hence less porosity is ultimately developed in the plastic sheet.

In making up the dry mixture the resin powder is preferably sifted to remove large particles. The sifting is done for example through a 100 mesh screen, i.e., through a screen having openings about 0.15 mm wide, directly into the mixer and the silica gel is thereafter sifted through a screen of the same mesh directly into the mixer where a dry homogeneous mixture is formed. Thereafter 4 parts of cyclohexanone is added and the mixing is continued at room temperature for a period of j about 3 to 10 minutes or until homogeneity is obtained. Thereupon the temperature is gradually raised under continued mixing to about 70°C. and after this temperature has been reached the mixing continues for ah additional 5 minutes until a substantially homogeneous dough is obtained.

. Next the warm dough is extruded in an otherwise conventional manner to form a flat band or sheet approximately 0.6 to 0.7 mm thick and as wide as desired, e.g., 25 cm wide.

Immediately after its formation the band is then ' introduced into a water .bath maintained at about 80°C. and serving as the extracting liquid, it being important that no significant amount of solvent evaporates from the band in the interval between its formation and its introduction into the extraction bath. While different extraction bath temperatures may be used, it is preferred to use temperatures above about 50°C. in order to accelerate the extraction process. On the other hand, it is important to keep the extraction bath temperature substantially below the point at which vapors or gases are generated or released within the plastic sheet either from the filler or from the liquid solvent composition. Accordingly, when- using a water-containing pore former such as silica hydrogel and when operating at atmospheric pressure, it is desirable to maintain the extraction bath at a temperature below 100°C .· and' prefer bly below about 90°C. Sudden release of any. substantial volume of vapor or gas in the plastic sheet is undesirable in that it tends to "explode" the sheet "and create large channels or macropores therein in place of the desired micropores .

The organic solvent content of the extraction bath is preferably kept 'as low as possible by known methods, for example by periodic distillation of the extraction bath or by continuous removal of a portion of the extraction bath to a distillation column and return of the water back to the bath after the organic solvent has been stripped therefrom. With a band thickness of 0.6 to 0.7 mm substantially complete extraction of the organic solvent can be achieved in water in about 35 to 40 minutes at a bath temperature of 80°C. However, a minor proportion of ethanol in the water bath can be used to shorten the extraction time of cyclohexanone , the inclusion of about 20% ethanol in the water bath serving to shorten the extraction time by about 25% as compared to the use of a plain water bath.

As the extraction of the organic solvent from the plastic sheet progresses, the organic solvent becomes largely replaced therein by water and the plastic thus becomes deplasticized and rigid. For instance, whereas cyclohexanone originally constituted about 25% by volume of the' extruded sheet, its essentially complete removal from the sheet in . . . o accordance with the present invention results in the sheet shrinking only about 5% or less in volume.

When the plastic band or sheet has become practically free from solvent it sets up into a stiff or dimensionally stable product which is then dried and heated to remove water or other extraction solvent and ultimately cause the desired pore system j to form in the product by dehydration or decomposition of the pore former dispersed therein. For instance, in the specific embodiment described in this example, the extracted sheet may be passed through an air oven wherein the air temperature may gradually decrease in the oven from well above 100°C. , e.g., 150°C, at the sheet entrance to under 100°C. , e.g., "about 90° - 95°Ce , in the later drying stages. Relatively high temperatures can be tolerated in the early stages of the drying process because evaporation of water from the sheet automatically keeps the sheet temperature from exceeding 100°C. However, in the later stages when most of the water is gone from the sheet it is advantageous to reduce the air temperature below the softening point of the polyvinyl chloride or other resin used in making the sheet.

In these later stages the silica -hydrogel or equivalent pore former is dehydrated and shrunk such that its remaining solid residue separates from the resinous matrix and thus forms the desired fissures or pores therein. Excessive softening of the resin at this stage is undesirable in that it may cause the resin to flow and thus close or reduce the pores which have been formed therein.

' ■ ' ·' EXAMPLE 2 One part powdered , polyvinyl chloride resi is mixed wit 5 parts silica hydrogel containing 70% water and with 2.3 parts cyclohexanone to form a dough and the latter extruded through a suitable die to form a thin band, all substantially as described in Example 1. The band is extruded directly into a' tank containing 'water at room temperature. At room temperature cyclohexanone is soluble in water in the proportion of about 8 to 10 parts per hundred.

The extruded band is then transferred to another water bath wherein the temperature is raised to about 100°C. , or just under the boiling point of water. Since at this temperature the solubility of cyclohexanone in water is less than at room temperature it is exchanged in the plastic band or sheet for more water while substantially maintaining the volume or porosity of the plastic composition. Internal stresses in the processed material tend to -be released in the hot bath. Thereafter, the extracted and consequently rigid plastic band is dried and micropores formed therein by controlled heating as in Example Finally, the band is cut into sheets of the desired size for use as battery separators.

The invention is particularly pointed out in the appended claims.

Claims (1)

1. '3. Process for producing microporous plastic sheets which comprises dry mixing 1 part of a powdered thermoplastic vinyl resin with 1 to 25 parts of powdered hydrated silica, thereafter adding an organic solvent for the plastic material under continued mixing until a homogeneous coherent dough is obtained; mechanically forming the dough' into a sheet about 0.3 to 1.5 mm thick; introducing the solvent-containing sheet before any substantial evaporation of solvent occurs into a bath of an extracting liquid in which the solvent is extracted from, the sheet without _ substantial dissolution of the vinyl resin and the inorganic solid; and drying and heating the resulting extracted sheet at a temperature below the softening point of the resin until the hydrated silica dispersed therein becomes substantially dehydrated , thereby forming micropores in said sheet. 4. Process as in claim 3 , wherein the powdered resin is polyethylene. 5. Process as in claim 3 , wherein the powdered resin is polypropylene. 6. Process as in claim 3 , wherein the powdered resin is polystyrene. 7. Process for producing microporous battery separator sheets which comprises dry mixing 1 part powdered polyvinyl chloride resin with 5 to 12 parts of powdered silica hydrogel and thereafter adding a water-miscibie ketone which is a solvent for the resin under continued mixing- until ' a homogeneous dough is obtained; .· · ° o ■ o ' • heating the dough to between about 50 and 70 C. and mechanically forming the resulting warm dough' into a sheet about 0.3 to 1.5. mm thick; immediately introducing the formed sheet into an aqueous bath to extract the ketone solvent from the sheet; removing said sheet from the bath when the ketone solvent, has been extracted from the sheet; and heating said extracted sheet to a temperature above about 50°G. but below the boiling point of water until said silica hydrogel becomes substantially dehydrated. 8. Process as in claim 1 , wherein the aqueous bath is maintained at a temperature between about 50°C. and 90°C. 19. Method according to claim 1 , wherein the resin is polyvinyl chloride and the porous if i initially mixed therewith is silica hyd agel containing about 70 to 75$ absorbed water." FOR AND OR BEHALF