CA1172310A

CA1172310A - Battery separator material

Info

Publication number: CA1172310A
Application number: CA000379738A
Authority: CA
Inventors: Stanley J. Strzempko; John F. Waterhouse
Original assignee: Texon Inc
Current assignee: Texon Inc
Priority date: 1980-06-30
Filing date: 1981-06-15
Publication date: 1984-08-07
Also published as: DE3153271C2; IT1171347B; DE3125751A1; DE3153271A1; IT8148790A1; JPS5744969A; GB2078769A; DE3125751C2; FR2485815A1; GB2078769B; FR2485815B1; IT8148790A0

Abstract

ABSTRACT OF THE DISCLOSURE

A method of forming a battery separator material which comprises forming a slurry of discrete polyolefin pulp fibers and a particulate filler in water. The slurry is deposited to form a wet web which is saturated with a polymeric organic binder. The web is compressed to form a coherent sheet which is then dried to form a battery separator material at a temperature below the fusion temperature of the polyolefin pulp fibers.

Description

! 1~2310 sATTERY SEPARATOR MATERIA~
BACKGROUND OF TE~E INVENTION
Field of_the Invention The invention relates to a me-thod of forming a battery separator material to fully envelope the elec-trolytic plates of the battery.
~escription of t:he Prior Art Electrolytic cells (i.e. batteries) formed by spaced apart metal plates connected in series for storage of elec-tric energy are useful for a variety of purposes. The plates of the electrolytic cells tend to form an active substance on the surface of the electrodes. When the active substance drops off, battery capacity drops, and also the degradation of the metal electrodes forms products including metal salts and other conductive substances which tend to bridge between two electrodes, thereby short circuit-ing the cell. These salts and degraded metal products are primarily due to the highly acidic environment in storage batteries.
In order to protect the electrode plates, separators have been used between the plates which are porous, thereby allowing ionic exchange through the separator while providing adequate separation between the cells to prevent short circuiting. Typically, these separators are constructed of cellulose fiber or fused polyolefin sheets. Exemplary of battery separators are those disclosed in U.S. Patents Nos.

2,973,398, 3,890,184, 4,024,323, 4,055,711, 4,113,927,

3,753,7~4, 3,694,265, 3,773,590, 3,351,495, 3,340,100;
3,055,966, 3,205,098 and 2,978,529.
Battery separators having improved properties have been required with the introduction of what is known as the "maintenance free battery". The maintenance free battery is one which is a sealed unit and does not require the intermittent addition of water thereto. Typically, the battery separator in a maintenance free battery is pre-Eerably an envelope sealed on at least three sides in order to prevent bridging with metal salts between electrodes. A

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primary requirement of these battery separators is that they must have sufficient porosity in order for the ionic exchange to occur while the pores of the separator must be sufficiently small to prevent the migration of heavy metal ions and, consequently, bridging which short circuit the cell.
Further requirements of a battery separator for a lead-acid battery are resistance to acid and oxidative and reduc-tive chemical reactions because of the highly acidic environ-ment within the battery. Also ! separators should have as low an electrical resistance as possible to provide good cold discharge performance.
In accordance with the invention, a battery separator material is provided which has excellent filtering, electrical, chemical and physical properties which is readily fabricated and may be folded and heat sealed along the edges in the form of an envelope to act as a battery separator, particularly in a maintenance free battery.
In accordance with a broad aspect of the present inven-tion there is provided a method of forming a battery separator ; 20 material which comprises forming a slurry of discrete poly-olefin pulp fibers and a particulate filler in water. The slurry is deposited to form a wet web which is sa-turated with a polymeric organic binder. The web is compressed to form a coherent sheet which is then dried to form a battery separator material at a temperature below the fusion temperature of the polyolefin pulp fibers.
According to a further broad aspect of the present invention there is provided a battery separator material comprised of discrete polyolefin pulp fibers in a sufficient amount to enable the battery separator material to be heat sealed to itself along its edges. The fibers are bound by an organic polymeric binder to form a sheet material.
BRIEF ~ESCRIPTION OF THE INVENTION
A battery separator material which is folded to form an envelope is heat sealed along at least two opposing edges thereof to form a battery envelope. The separator material is comprised of discrete polyolefin pulp fibers in a sufficient amount to heat seal the separator along its edges. The fibers are bound by an organic polymeric binder.
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- 3a -DETAILED DESCRIPTION OF THE I~VENTION
The fibers useful in the practice of the invention are those fibers capable of withstanding strong acid, such as is present in electrolytic cells.
A major portion of the fibrous content of the battery separator material of the invention is polyolefin pulp fibers which are synthesized from the polymerization of ethylene and/or propylene or mixtures thereof, such as to produce polyethylene, polypropylene or poly(ethylene-propylene~
copolymers and have a fiber diameter of up to 100 microns.
Preferably, these polyolefin pulp fibers have a fiber diameter of 0.01 to 20 microns, have a softening point below 340F, and are up to 0!5 inch in length. The polyolefin pulp fibers are present in a weight range from 30 to 100 percent by weight, and preferably 70 to 90 percent by weight based upon the nonbinder constituents of the separator material~
The polyolefin fibers most useful in the practice of the invention are those which are characterized as synthetic wood pulps. These polyolefin fibers have a surface treatment which imparts wettability and ease of dispersion in water to the fibers. Typically, the surface tension of the polyolefin fibers is about 70 mN/m. Typical properties of the poly-ethylene and polypropylene fibers are those which have- (1) a specific gravity of less than one and more preferably between about 0.900 and 0.965 kg/dm3, (2) a melting point of between about 250 and 340Ft (3) a yield stress of greater than 300 daN/cm2, (4) a tensile strength at break of greater than 200 daN/cm2, (5) a modulus of elasticity in tension of 7,000 to 20,000 daN/cm2, and (6) a dielectric constant of 2 to 4, a dielectric strength of 2 to 5 x 102 kv/cm and a transverse resistivity of 1015 to 1018_/ L cm.
The polyolefin pulp fibers comprise a major portion of the battery sep-arator material for several reasons. Among the reasons are that they are substantially inert to acidic conditions such as are present in electrolytic cells and, secondly, they have the desired softening point of below 340li so that the fabricated battery separator can be heat sealed along at least two opposing edges thereof to form an envelope. They have low ohmic resistance due to their diameters. Further, the polyolefin pulp fibers have sufficient flexibility so that the final battery separator material can be folded and worked while providing good envelope integrity and ease of pro-cessing on papermalcing equipment.
In addition to the polyolefin pulp fibers, staple glass fibers may be in-corporated into the battery separator material in order to impart rigidity and tensile strength while maintaining the inert chemical characteristics and low ohmic resistance of the battery separator. Preferably, the glass fibers useful in the practice of the invention have fiber diameters less than 20 microns as the mean diameter. The fibrous component as glass consti-tutes up to 60 percent by weight based on the nonbinder content of the separator material and preferably 5 to 15 percent by weight. Exemplary of the glass fibers useful in the practice of the invention are the glass micro-fibers, i.e. those having fiber diameters of 0.20 to 4.0 microns. These glass fibers may be of preferred compositions known as soda-lime borosilicate or C glass with excellent chemical durability sold by Johns Mansville Corpora-2 5 tion.
In addition to the polyolefin pulp fibers and the glass fibers, polyester and/or polyolefin staple fibers and/or cellulose pulp fibers may be incorpora-ted into the battery separator material. These polyester and/or polyolefin staple fibers preferably have a denier of 0.5 to 1.5 and are incorporated at a level of up to 30 percent and preferably 5 to 15 percent by weight based on the nonbinder constituents of the separator material.
In addition, fillers may be used to reduce the pore size of the battery separator material. The fillers are used at a level of up to 60 percent and preferably 40 to 60 percent by weight based upon the nonbinder content of the separator material. The fillers useful in the practice of the invention are those which are particulate fillers having an average particle size of ~ `I '723~,~

0.02 to 20 microns, such as Icaolinite, halloysite, montrnorittonite, tinite and illite which are all clays, and other fillers such as silica, quartz, cal-site, luminite, gypsum, muscavite, diatomaceous earth and the like. In addition to the inorganic fillers, organic fillers having a particle size of 0.2to 50 microns may also be used for the same purpose as the inorganic fillers. These organic fillers are typically inert thermoplastic organic poly-mers such as hydrocarbon polymer powders. Typical polymers are polysty-rene and polyolefin polymers and eopolymers. The fillers reduce the ohmic resistance and pore size along with the cost of the battery separator material.
The fibers and filler are bound together by an organic b;nder preferably supplied as a latex or aqueous dispersion. Preferably, the binder is the poly-mer of monoethylenically unsaturated monomers. "Monoethylenically unsat-urated", as used herein, is characterized by the monomer haYing a ~ C = CH2 group. These monoethylenically unsaturated monomers are, but not limited to, the acrylic monomers such as methacrylic acid, acrylic acid, acrylonitrile, methacrylonitrile, methylacrylate, methylmethacrylate, ethylacrylate, ethylmethacrylate, acrylamide and the like; olefinic hydro-carbons such as ethylene, butylene, propylene, styrene, alpha-methylstyrene and the like; and other functional unsaturated monomers SUCIl as vinyl pyridine, vinyl pyrollidone and the like. Typically, these polymers are acrylic polymers dispersed in water at a level of 3 0 to 50 percent by weight and are in the form of a latex. Additionally, the polymer should be film-forming.
Although the polymers useful in the practice of the invention may be provided with sufficient functional groups to self-crosslink, i.e. crosslink without the addition of other materials, crosslinking agents may be added to provide the required crosslinking characteristics. Preferably, the polymers will crosslink at a temperature below 200F and in a preferred range of 150 to 190F.
Crosslinking agents suitable in the practice of the invention include aldehydes such as formaldehyde, glyoxal, acrolien and the like; synthetic resin precondensates obtained by the reaction of an aldehyde generally with compounds containing nitrogen, like dimethylol urea, dimethylolethylene urea, di- and trimethylol triazon dimethyluron, di- and trimethylol melamine and other cyclic or noncyclic, water soluble or non-water soluble precondensates 1 117231~

of urea and melamine formaldehyde. The r eactive methylol groups may be blocked or partially blocked by alcohols having l to a~ carbon atoms. Apart from the above, other known crosslinking agents too may be used, such as diepoxides and epichlorin derivatives thereof, dichlorophenols, beta substituted5 diethyle sulfones, sulfonium salts, N-methylolacrylamide and methylacrylamide and derivatives thereof, diisocyanates and the like. Up to ~ percent of the crosslinking agent may be incorporated into the binder composition. Too much crosslinking agent may render the sheet material too brittle or hard and not provide the required flexibility for forming the battery separator.
The use of an organic binder for binding the polyolefin pulp fibers to form a battery separator material is a substantial improvement over the prior art. While the prior art teaches that polyolefin fibers of continuous or long staple characteristics can be heat fused to form a separator material such heat fusion requires complicated and expensive processing. To the con-15 trary binding of the polyolefin pulp fibers with an organic binder avoids these complicated and expensive processing steps by providing a battery separator material which can be fabricated on standard papermaking equipment. In addition, the battery separator material in accordance with the invention shows increased strength and elongation over prior art pulp fiber separator 20 material, th~ls providing a material with improved integrity and improved handling and fabrication properties.
The battery separator material which is formulated in accordance with the invention has a preferred porosity between 50 and 70 percent, as calcu-lated by the following equation:
% E = (1 ~ a ) l O0 f where E is porosity,~3a is apparent density of the battery separator material and J~f is the columbic density of the material.
Further, the battery separator material has an ohmic resistance of 30 between .001 ohms in2 to 0.025 ohms in2 and more preferably below 0.020 ohms in .
The battery separator material of the present invention may be formed on standard papermaking apparatus such as a fourdrinier, an inclined four-drinier, a cylinder machine, a rotoformer or the like. A typical process 35 involves charging the fibrous constituents along with the filler and a large quantity of water to a pulper. The pulper disperses the fiber and filler in i ~723~) the water. Various polyelcctrolytes and surface active agents may be added to provide the appropriate chemical and physical properties in accord-ance with standard papermaking techniques. The fiber and filler slurry is then charged to the machine chest of a fourdrinier papermaking machine.
5 From the machine chest the slurry is transferred onto the moving wire of the fourdrinier and water is allowed to drain therefrom. After the draining of tlIe water by gravity, suction is applied to the wet web to further remove water from the web. The web is then pressed between felt rolls. After wet pressing, the wet web passes through a saturator, where the binder is 10 applied. Finally, the sheet is dried on drying cans. The drying temperature r~quired is between 230 to 260F. During the drying cycle, the binder may be crosslinked if it has suff~cient functionality to do so, or there is a cross-linking agent in the furnish or in the binder. It is preferred that the drying be conducted at a temperature below the fusion temperature of the polyole-15 fin fibers so as to maintain them as discrete fibers rather than fused fibers.The sheet material is then calendered to a uniform thickness and may be embossed on one side thereof and wound on rolls. In the alternative, the binder can be applied to the dry web and redried. Preferably, the battery separator material has a thickness of 0.005 inch to 0.040 inch. These 20 materials may be used for enveloping or may be cut to the appropriate size and used as a leaf separator per se.
Preferably, the spearator material is embossed on one side thereof and a rib is formed of extruded polyolefin plastic on the opposing side of the embossment. The rib is applied by extrusion of molten polyolefin resin onto 25 the separator in a generally cross-sectionally triangular configuration through a die.
In forming the rib, low density polyolefin pellets are fed into the hopper of an extruder which is in communication with a single piece die head. A
series of polyolefin strands of circular cross-section are extruded through the 30 die lip attached to the front of the die head. The strands are aligned and directed onto the cooled female portion of the die roll pair just prior to the nip formed between the profiled female and the male die. At the nip the polyethylene strand is contacted with the separator web and the profile of the rib is estabilshed, together with the emboss on the back web of the 35 separator material. Contact is maintained with the cooled female die roll to insure solidification and release of the profiled rib. The polyolefin strand g ~7~3~

is at a sufficient temperature to soften the p~lIp fibers in the web, thereby fusing the rib and the web into a unitary construction.
The triangular configuration of the rib provides necessary spacing between the electrolytic plate and the separator material to allow for cir-culation of the electrolytic solution about the plate. Further, the top of the rib aligns with the embossment on the opposing side so that there is a guide for true rolling of the battery separator material during winding sub-sequent to production and unwinding for fabrication into battery separators.
Preferably, the height of the triangular cross-section is about 0.010 to 0.100 inch with a base correspondingly configured to generally form an equilateral triangle.
In producing the battery separator from the battery separator material which is wound on reels, the reel is unwound and fed into an enveloping machine which folds the battery separator material around the electrolytic plate and heat seals it along opposing edges thereof. The electrolytic plate may also be inserted into a preformed envelope and the eveloped electroly-tic plate is mounted withhl a battery casing.
The invention is illustrated by reference to the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a top view of the battery separator of the invention;
Figure 2 is an end cross-sectional view of the battery separator of Figure 1;
Figure 3 is a cross-section taken through one of the ribs shown in Figures 1 and 2;
Figure 4 is a front view partially in section o~ the battery separator material of Figure 1 on a roll; and Figure 5 is an exploded view of a portion of the roll of Figure 4.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to Figures 1 through 5, wherein lil<e reference numerals refer to like parts, there is shown a battery separator material generally designated by the numeral 1, which is composed of a continuous web formed as previously described with a plurality of polyole rin ribs 3 of generally tri-angular cross-sectional configuration on one side 7 of the web 2. On the opposing side 4 of the web 2 there is provided a plurality of embossed grooves 5 which are adapted to receive the ribs 3 when the battery sep-arator material is wound on a roll 6, as is shown in Figure 4. The emboss-'~7~

ment 5 is con~igured so as to receive the apex ~ of the rib 3 so that when tlle material is rolled the apex 8 of the rib 3 and the embossment 5 mate to form an interlocking arrangement for true rolling. More specifically, Figure 5 shows the interengagement of the embossment 5 and the rib 3 5 when the material is wound on a roll.
The invention will be further illustrated by the following Examples.
E~AMPLE 1 A furnish composed of the following ingredients was prepared:
IngredientParts by Weight Pulpex EA 80 (by Hercules synthetic wood pulp - avg. fiber diameter 4.9 microns) Microglass 112 *15 (glass microfibers sold by Johns Mansville) Dacron*l/4 inch, 5 1.5 denier The above furnish was charged to a pulper and 24 parts water was 20 added thereto The material was agitated in the pulper until a uniform slurry was prepared. The slurry was pumped to the machine chest of a four--- drinier papermaking machine.
The slurry was pumped to the head box of the fourdrinier. The slurry was fed onto the moving screen of the fourdrinier, water was allowed to 25 drain by gravity, and then suction was applied. The wet web was saturated in a saturator with 11 parts by weight of a self-crosslinking film-forming acrylic latex sold under the trademark Polymerics 144[) having a pll 5.45 and a weight average molecular weight of about 10~ and 0.11 parts by weight of Aerosol O'~*(sodium dioctyl sulfosuccinate) an ionic surfactant by 30 American Cyanamide. The wet web was then dried by passing it over heated rolls to impart a temperature of 260F to the web.
The sheet material from the fourdrinier was calendered and embossed to form linear indentations longitudinally oriented on the sheet materiaL A
triangularly cross-sectioned polyolefin rib of low density polyolefin having a 35 melt index of 5 g/10 min. at 300F (manufactured by Rexene Polyolefins * Reg . T . M.

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Co.) having a base of 0.036 inch and a height of 0.034 inch was extruded on the material opposing the inclentations.
The material was wound as a roll with the apex of the triangular rib nested in opposing indentations on the material.
I`he material was unwound and fed into a Dale enveloping machine which cut the material to a predetermined size, folded each piece in half and heat sealed opposing open edges of the envelope.
The battery separator had the following properties:
Property Value 10 Backweb tb ins 00.013 Overall to ins 00.038 Weight/area g/m2 89.000 Air porosity 1/4"
orifice, secs. 93.100 l5 Ohmic Resistance 20 minsfL in2 00.019 Ohmic Resistance 24 hours~ in2 00.015 Apparent density 1 00.377 20 ,~ Porosity1.2 67.200 Pore diarneter avg.
in microns 08.700 % Volume greater than 20 microns 11.000 25 % Columbic density 01.100 Tensile strength lbs. 04.600 % Elongation 06.500 Measured on an ~minco Mercury Porosimeter under liquid mercury pressure 30 2 % Porosity = 1 - apparent density x 100 columbic density The battery separator fabricated in accordance w;th Example 1 was used in a lead~acid storage battery and tested for performance. The battery tests were run according to the Battery Counsel Industry (BCI) recom mended 35 specifications for vehicular, ignition lighting and starting types. The results are as follows.

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Discharge Performance Characteristics Reserve Capacity 25 amps at 80F
minutes to 10.5 volts 109 minutes Cold Crank 450 amps at 0F
voltage at 30 sec. 7.25 volts EXAM P_~ 2 A furnish composed of the following ingredients was prepared:
Ingredient Parts by Weight Pulpex EA 50 Hi Sil 233 (silica) 40 Dacron 0.75 denier 10 The above furnish in hand sheet form was processed and the binder 15 applied similar to the production machine, Example 1, except that the ionic surfactant was eliminated. The battery separator was formed in accordance with Example 1 and had the following properties:
Property Value Backweb tb ins 0.011 Weight/area g/m2 92~8 Porosity 1/4" secs. 176 Ohmic Resistance 20 minQ in2 0 009 Ohmic Resistance 24 hours~L in2 0.008 Apparent density 0.337 % Porosity4 63.4 (calculated max.
73.5%) Pore diameter avg.
diameter microns 13~9 Tensile strength lbs. 6.1 % Elongation 15.9

4 Limited by porosimeter range As is demonstrated by Example 2, a battery separator material having 35 a high filler loading has excellent physical and electrical properties.

As can be seen from the foregoing description of the invention, a battery separator material is provided which can be easily fabricated into a battery separator having excellent chemical, physical and electrical properties.

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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 method of forming a battery separator material comprising:
a) forming a slurry of discrete polyolefin pulp fibers and a particulate filler in water, b) depositing said slurry to form a wet web, c) saturating the wet web with a polymeric organic binder for said web, d) compressing the web to form a coherent sheet, and e) drying the sheet to form a battery separator material at a temperature below the fusion temperature of said polyolefin pulp fibers.

2. The method of claim 1 wherein said polyolefin pulp fibers are present at a level of 30 to 100 percent by weight based on the nonbinder content of said material.

3. The method of claim 1 including glass fibers slurried with said polyolefin pulp fibers.

4. The method of claim 3 wherein said glass fibers are present at a level of up to 60 percent by weight based on the nonbinder content of said material.

5. A battery separator material comprised of discrete polyolefin pulp fibers in a sufficient amount to enable the battery separator material to be heat sealed to itself along its edges, said fibers being bound by an organic polymeric binder to form a sheet material.

6. The material of claim 5 wherein said polyolefin pulp fibers are present at a level of 30 to 100 percent by weight based on the nonbinder content of said material.

7. The material of claim 6 wherein said polyolefin pulp fibers are present at a level of 70 to 90 percent.

8. The material of claim 5 wherein said polyolefin pulp fibers have a length of up to 0.5 ins.

9. The material of claim 5 wherein said polyolefin pulp fibers have a diameter of up to 20 microns.

10. The material of claim 5 including glass fibers.

11. The material of claim 10 wherein said glass fibers are present at a level of up to 60 percent by weight based on the nonbinder content of said material.

12. The material of claim 11 wherein said glass fibers are present at a level of 5 to 15 percent by weight.

13. The material of claim 5 wherein said organic polymeric binder is an acrylic polymer.

14. The material of claim 13 wherein said acrylic polymer is self-crosslinked.

15. The material of claim 5 wherein said polymeric binder is present at a level of 5 to 30 percent add-on based upon the nonbinder content of said material.

16. The material of claim 5 including a particulate filler.

17. The material of claim 16 wherein said particulate filler has a particle size of 0.02 to 20 micrometers.

18. The material of claim 16 wherein said particulate filler is inorganic.

19. The material of claim 18 wherein said filler is silica.

20. The material of claim 5 includes staple fibers selected from the group consisting of polyester and polyolefin staple fibers.

21. The material of claim 20 wherein said polyester or polyolefin staple fibers are present at a level of 5 to 15 percent based upon the nonbinder content of the material.