Wettable battery separator for alkaline batteries

A wettable battery separator for alkaline batteries is formed of a microporous plastic sheet selected from porous films, fabrics and papers. The separator is impregnated with a resin which has one or more carboxyl groups which have been neutralized by a base so as to form a salt. The resin is present in an amount from about 0.0001 to 3% by weight of the separator. Alkali resistant fillers may also be included.

The present invention relates to a separator for alkaline batteries. More 
particularly, it relates to wettable separators for alkaline batteries. 
BACKGROUND OF THE INVENTION 
Alkaline batteries have become increasingly more popular because of their 
high energy density. As such, these batteries are increasingly used in 
applications normally reserved for the traditional lead-acid battery 
systems. 
In order to achieve extended battery life and efficiency in alkaline 
systems, the use of battery separators is required and the use of battery 
interseparators, as well, is preferred. The battery separators are located 
between the positive and negative plates so as to provide, (1) a 
separation between the electrodes of opposite charge, (2) an electrolyte 
reservoir, (3) a uniform electrolyte distribution across the electrode 
surface so as to permit uniform current density and (4) a space for 
electrode expansion. 
Battery separators and interseparators used in alkaline batteries at 
present are commonly formed of a polyolefin, preferably polypropylene, 
polyamide or nylon non-woven sheet. 
Other separators in use in alkaline systems today are shown in U.S. Pat. 
Nos. 4,264,691 and 4,330,602 in which the separator/interseparator is 
formed of synthetic pulp, alkali resistant inorganic filler and a long 
fiber of polyester, polyacrylic, polyamide or polyolefin materials. 
Another type of separator is a microporous plastic film such as is 
described in U.S. Pat. No. 3,351,495. 
Until now, these separators have been suitable for existing technology. 
Battery makers now require a separator with better performance 
characteristics than is currently available with the current separators, 
especially those based upon nylon or treated polypropylene. Preferably, 
they desire greater and faster wettability of the separator by the 
alkaline electrolyte. 
The present invention overcomes the problems inherent in the currently used 
separators and provides a wettable sheet material with the desired tensile 
strength, chemical inertness, electrolyte absorption and electrical 
resistance properties which is usable in alkaline batteries. 
OBJECT OF THE INVENTION 
It is an object of the present invention to provide a wettable sheet 
material useful as a separator or interseparator in alkaline batteries. 
It is another object of the present invention to provide a sheet material 
which is wettable by electrolyte and has good electrolyte absorption and 
chemical inertness in an alkaline battery system. 
Another object of the present invention is to provide a wettable separator 
suitable for use in alkaline battery systems comprised of one or more 
fibers, and a carboxyl group containing resin binder/wetting agent. 
An additional object of this invention is to provide a battery separator or 
interseparator comprised of synthetic pulp, one or more short fibers, one 
or more long fibers and a carboxyl group resin binder/wetting agent. The 
separator has excellent tensile strength, wicking properties, wettability 
and alkali resistance. 
It is a further object of the present invention to provide a battery 
separator for alkaline batteries formed of synthetic pulp, one or more 
short fibers, one or more long fibers, wherein at least some of the long 
fibers are water swellable and an acrylic resin binder/wetting agent. 
An additional object of the present invention is to provide a separator for 
alkaline batteries comprised of a microporous, plastic sheet material and 
a carboxyl containing wetting agent coated thereon. 
An even further embodiment of the present invention is to provide improved 
wettability to existing alkaline separators by the incorporation of a 
carboxyl group containing wetting agents, wherein the carboxyl groups have 
been neutralized by treatment with a base material, preferably so as to 
form a salt. 
SUMMARY OF THE INVENTION 
A sheet material suitable for use in alkaline battery systems as a battery 
separator or interseparator is described which is formed of a microporous 
sheet and from about 0.0001 to 3 percent of a carboxyl group containing a 
wetting agent. Wherein the carboxyl group is neutralized with a base 
material. The sheet may be formed of a porous film or it may be formed of 
a fibrous material such as a woven or nonwoven fabric or synthetic paper.

DETAILED DESCRIPTION OF THE INVENTION 
All measurements stated in percent in this application, unless otherwise 
stated, are measured by weight based upon 100% of the finished product 
weight. Thus, for example 30% represents 30 weight parts out of every 100 
weight parts of the finished product. 
The present invention relates to a battery separator formed of a 
microporous plastic sheet coated or impregnated with a carboxyl group 
containing resin. The sheet may be a microporous plastic film, fabric 
(woven or nonwoven) or paper. Such materials are well known and 
commercially available as described herein. 
The carboxyl group containing resin acts as a wetting agent and in some 
cases, such as the paper, additionally as a binder to hold the fibers 
together. The resin may be any polymer which contains one or more carboxyl 
groups which are capable of reacting with a base material. Preferred 
resins have more than one, preferably more than two carboxyl groups. It is 
preferred that the groups be attached along the backbone of the polymer as 
this allows for a large number of sites for each polymer chain. A 
simplified diagram of such a polymer is shown below: 
______________________________________ 
COOH COOH 
C C C 
COOH COOH 
______________________________________ 
It is preferred to use a homopolymer type of resin rather than a copolymer 
or terpolymer. 
The carboxyl group is preferably a carboxylic acid such as acrylic acid, 
methacrylic acid, the aconic acid and fumaric acid. More preferably the 
group is acrylic acid. 
The carboxyl groups must be arranged on the polymer so as to be capable of 
reacting with a base, preferably a metal hydroxide or other alkaline 
material. Preferably, the base is potassium hydroxide. The alkaline 
material neutralizes the carboxyl group forming in most instances a salt. 
The resin which is used to impregnate or saturate the microporous sheet is 
preferably an acrylic resin which has one or more pendant carboxylic 
groups. One preferred acrylic resin is known as CARBOPOL and is sold by 
B.F. Goodrich. It is an acrylic acid homopolymer, typically of high 
molecular weight. The preferred acrylic acid homopolymer used in the 
present invention should have a molecular weight (average) of from about 
200,000 to about 2,000,000. Preferably, the resin has one or more 
carboxylic acids attached to it, either at the ends of the polymer or 
along the length of the polymer chain. 
The selected resin should be dissolvable in a solvent, preferably water. To 
prepare the resin, such as the CARBOPOL resin, for use in the invention, 
one dilutes or dissolves the resin in the solvent and raises the pH of the 
solution to above 6. Preferably, the pH is raised by the addition of a 
metal hydroxide such as potassium hydroxide, or other alkaline materials. 
The carboxylic groups of the polymer react with the base to form salts 
such as potassium salts when potassium hydroxide is used. These salts 
provide the desired wettability characteristics. 
The amount of resin which is present in the sheet, after impregnation and 
drying, varies from about 0.0001 to about 3% by weight of the sheet. This 
small amount has been shown to provide good binding properties for the 
fibrous plastic papers allowing the sheet to be flexible and formstable. 
Moreover, the addition of a small amount of resin binder improved the 
wetting ability of the sheet whether it was a film, a fabric or paper. 
(e.g. ability to quickly and completely absorb electrolyte). 
In a first embodiment of the present invention, the separator is a fibrous 
sheet material, more preferably a paper comprising a mixture of polyolefin 
synthetic pulp, of one or more fibers, and from about 0.0001 to 3 percent 
by weight of the sheet of resin binder. Preferably some of the fibers are 
long fibers. Additionally, it is preferred that one of the fibers be water 
swellable. 
The synthetic pulp useful in the present invention is a polyolefin of short 
fibers having a fiber size and shape similar to that of cellulose pulp. 
Such synthetic pulps are described in U.S. Pat. Nos. 4,264,691 and 
4,330,602, the teachings of which are incorporated herein by reference. 
The length of the synthetic pulp is from about 0.8 to 5 millimeters, 
preferably from about 1 to 4 millimeters. The preferred pulp is a 
polyethylene based pulp. Other pulps are equally useful and may be made of 
other polyolefins such as polypropylene. Pulp fibers with a high degree of 
branching or fibrillation are most preferred in the present invention. 
The one or more fibers used in the present invention are formed of 
synthetic polymers. Preferably at some or all of which are water 
swellable. More preferably, the invention contains at least two types of 
fibers; one water swellable, and at least one other being nonwater 
swellable. 
The water swellable fiber, if used, is formed of a material such as a vinyl 
alcohol polymer (PVA) or a copolymer of a polyvinyl chloride (PVC) and a 
polyvinyl alcohol or a grafted copolymer comprising a vinyl chloride 
backbone grafted with a vinyl alcohol polymer. Such fibers are 
commercially available. A commercially available example of a vinyl 
alcohol polymer water swellable fiber is MEWLON SML by Unitika Kasei, Ltd. 
An example of a commercially available polyvinyl chloride/polymer alcohol 
copolymer is sold by Kohjin Co. Preferably, this fiber is a "long" fiber 
having a length of from about 3 to 38 millimeters (0.10 to 1.5 inches). 
The non swellable fiber(s) can be a nonwater swellable PVA, such as MEWLON 
F, by Unitika Kasei, Ltd., or other nonwater swellable fibers such as 
polyester, nylon, polyacrylic, polyamide, polyolefin, polyvinyl chloride 
bicomponent fibers or mixtures thereof. Such fibers are well known and 
commercially available. 
Some or all of the fibers may be along fiber (as described above), 
especially where no water swellable fibers are used. 
Preferably, the fiber component is a blend of several different fibers. 
Typically, a blend of polypropylene, polyethylene, nonwater swellable PVA, 
a bicomponent fiber and a water swellable PVA fiber are used. 
By "bicomponent" it is meant a fiber which has core formed of one polymer 
and an outer sheath, jacket or coating formed of a different polymer. 
Typically, the outer layer is formed of a polymer having a lower melting 
point than the polymer of the core. 
A preferred bicomponent fiber is based upon a polypropylene core covered a 
polyethylene jacket. Such fibers are sold by DIAWABO Co. Ltd. of Japan and 
are known as the DIAWABO NBF fibers. Other suppliers are also available 
and known to those skilled in the art. 
Most, if not all of the fibers should have a denier of from about 1 to 
about 12 and a length of 3 to 10 millimeters. The total amount of these 
fiber should constitute from 80 to 90% of the sheet material. The 
remainder of the fibers being "long" fibers, having a length of from about 
3 to about 38 millimeters, and as described above, may be water swellable, 
nor nonwater swellable or a blend of both and can be selected from the 
synthetic fibers described above. 
Optionally, an inorganic alkali resistant filler may be used in the present 
invention. The alkali resistant inorganic filler can be any particulate 
material which is chemically inert to alkaline electrolytes. Alkali 
resistant inorganic fillers suitable for use in the present invention 
include for example, titanium dioxide, alumina, calcium oxide, calcium 
hydroxide, calcium titanate, potassium titanate, magnesium hydroxide, 
magnesium oxide calcium hydroxide or zirconium hydroxide and admixtures 
thereof. Of course, any other alkali resistant filler, which is compatible 
with the other ingredients used in the sheet of the present invention and 
known to one skilled in the art can be used as well. A preferred filler is 
potassium titanate. Another preferred filler is titanium dioxide. The 
filler preferably has a particle size of from about 0.001 to about 1 
microns, a surface area of from about 5 to about 200 square meters per 
gram and a pore volume of from about 0.01 to about 1 cc per gram when 
present, the filler should be in an amount from 5 to 100 weight percent of 
the fibers in the separator. 
Sheet material formed in accordance with the present invention are porous 
materials having a median pore diameter of less than 40 microns and a 
maximum pore diameter of no more than about 140 microns. Additionally, 
wherein the form of a paper it may have a ream weight of from about 15 to 
100 pounds, porosity of from about 50 to 90%, an electrolyte retention 
capability of from 200 to 700% and an electrical resistance of less than 5 
ohms/cm. 
The process by which the synthetic paper sheet of the present invention is 
formed is similar to that described in U.S. Pat. Nos. 4,264,691 and 
4,330,602 which are incorporated by reference herein. The process is 
carried out on normal paper-making machinery, such as a rotoformer or 
Fourdinier paper machine. A slurry is formed in a conventional 
paper-making pulper first by charging the synthetic pulp with water and 
pulping the material until it is thoroughly dispersed. Various dispersants 
may be added if necessary or desired, or more preferably the pulp may 
optionally contain a dispersant. The pulper content is then discharged 
into the chest of a rotoformer or a Fourdinier paper machine. A water 
swellable long fiber if used, is added to the chest and mixed for a 
sufficient amount of time so as to allow the fiber to swell. Preferably, 
the slurry is warmed to room temperature or greater to aid in the swelling 
of the fiber. Next one or more nonwater swellable fibers are mixed into 
the chest. After thoroughly mixing the components in the chest, ground 
alum, (aluminum sulphate) preferably iron free, is added. The aqueous 
slurry is then transferred from the chest to a dilution box upstream of a 
web forming headbox. There, the mixture is diluted with water until the 
solids concentration is less than 5 percent preferably less than 1 
percent. The mixture is then transferred to the headbox and a web is 
formed on the rotoformer or Fourdinier machine. A lump breaker operating 
at 20 to 80 psi. smooths the upper surface of the web. The web is 
transferred from the rotoformer or Fourdinier machine to an oven and/or 
one or more drying cans where the web is dried. During the drying or 
subsequent thereto, the web may optionally be heated to a temperature of 
from about 125.degree. C. to 150.degree. C. to allow for the partial 
fusing of the synthetic pulp fibers. 
The thickness of the resultant web is directly related to the rate at which 
the slurry is deposited onto the web forming apparatus, the solids 
concentration at that moment and the speed of the web forming apparatus. 
The sheet material should have a thickness of less than 20 mils. 
Preferably the resultant sheet material should be less than 12 mils, 
requiring therefore that the deposition of the slurry be at a grammage of 
less than about 120 grams per square meter (gm/m.sup.2), preferably 40 to 
120 gm/m.sup.2. Of course, should a thicker sheet material be desired, one 
could easily increase the deposition grammage until a sheet of desired 
thickness is achieved. 
The dried web may be calendered at sufficient pressure and temperature to 
form a sheet having a thickness of less than 12 mils, preferably 5 to 10 
mils. 
The use of a retention aid is not required in the present invention 
especially when a water swellable fiber is used as it has been found that 
the water swellable fiber attracts and retains any filler, if used, to its 
surface so that the amount of filler lost during drying is substantially 
reduced. One could use retention aids in the process to reduce the loss of 
filler during drying especially if no water-swellable fiberic used. If 
used, preferably the slurry is treated with an ionic retention aid such as 
a cationic polyacrylamide and then an anionic polyacrylamide retention 
aid. An example of a cationic acrylamide useful in this invention is RETEN 
210, a product of Hercules, Inc. A suitable anionic retention aid is RETEN 
421 or 521, an anionic acrylamide copolymer of Hercules, Inc. Typical 
concentrations are 0.04% in water at 600 to 800 ml/minute for the cationic 
retention aid and 0.025% in water at 600 to 800 mil/minute for the anionic 
retention aid. 
Other aids, such as surfactants to improve the electrical resistance of the 
sheet material, and wetting agents are also usable in the present 
invention, so long as they do not have an adverse effect on battery 
performance or life. 
A second embodiment of the present invention is the microporous film type 
sheet as disclosed in U.S. Pat. No. 3,351,495 which is incorporated 
herein by reference in its entirety. The film is formed by extruding one 
or more plastics preferably polyethylene or polypropylene with a pore 
forming agent (such as mineral oil) through a sheet die and/or calender 
roll and then extracting some or all of the pore forming agent to create 
the microporous structure. Optionally and preferably, an inorganic alkali 
resistant filler, as described above with the first embodiment, is used. 
Such materials are commercially available, such as 3A or DARAMIC.RTM. 
separators from W. R. Grace & Co.-Conn. 
A third embodiment of the present invention is the fabric type of sheet 
which can be a woven fabric such as rip stop nylon or a nonwoven such as a 
spun bonded polyester fabric. The materials are commercially available 
such as REEMAY or TYVEK nonwoven fabrics. 
After formation of the sheet, regardless of whether it is a paper, film or 
fabric, it is saturated, coated, or impregnated with the wetting agent and 
then dried in an oven. 
Alternatively, and especially with the synthetic paper embodiment the resin 
can be applied after the sheet formation, but prior to drying by a sizing 
step. "Sizing" refers to the addition of a chemical to the sheet during 
processing. The resin is simply sprayed or otherwise applied to sheet, 
preferably before calendaring in an amount sufficient to saturate the 
entire sheet. 
The following examples are provided to explain and more fully teach the 
present invention. They are not intended to restrict or limit the scope of 
the claims of the present invention. 
EXAMPLE 1 
A sheet according to the present invention was formed of the following 
components: 
15% synthetic polyethylene pulp; 
30% nonwater swellable polyvinyl alcohol fiber (1 dernier by 3 
millimeters); 
10% water swellable polyvinyl alcohol long fiber; 
25% polypropylene fiber (average length 6 millimeters); 
20% bicomponent fiber formed of a polypropylene core and polyethylene 
sheath, (2 dernier by 3 to 5 millimeter length). (all percentages by 
weight of the sheet). 
The components were added to a wet laid paper making machine and formed 
into a sheet about 15 mils mm in thickness. The sheet was dried and then 
impregnated with an aqueous solution of CARBOPOL acrylic resin and dried. 
The amount of resin in the dried sheet was about 0.01% of the sheet 
weight. 
The sheet was used as a separator in a nickel/metal hydride battery and was 
tested against a nickel/metal hydride battery using a microporous nylon 
separator. Both batteries were charged to capacity and then stored for 30 
days after which their energy capacities were determined. 
The control battery with the nylon separator retained 50% of its energy. 
The battery using the separator of the present invention retained better 
than 60% of its energy. 
The present invention was shown to provide excellent separator properties 
and provided the battery with improved storage capabilities over the 
existing separators. 
EXAMPLE 2 
A separator based upon a synthetic paper of polyethylene pulp and 
polypropylene fiber was treated with 0.005% by weight solution of a 
carboxyl group containing acrylic resin [CARBOPOL resin], reacted with 
potassium hydroxide, rinsed and then dried. A similar untreated separator 
was tested along with the treated separator by placing a same size sheet 
of each separator on top of a bowl of alkaline electrolyte (potassium 
hydroxide [KOH]). The time until each separator was completely wetted was 
recorded. The treated separator was found to wet out (i.e, become 
completely absorbed in the electrolyte) in 3 minutes as compared to the 
untreated separator which after 60 minutes had still not wet out. 
While this invention has been described with reference to its preferred 
embodiments, other embodiments can achieve the same result. Variations and 
modifications of the present invention will be obvious to those skilled in 
the art and it is intended to cover in the appended claims all such 
modifications and equivalents as fall within the true spirit and scope of 
this invention.