Recombinant battery separator

A recombinant battery separator pad is made from a mat of meltblown ultrafine polymer fibers, with the fibers being treated with an agent to render them permanently wettable. The fibers include at least ten percent of less than one micron, with the majority less than five microns. The mat has a liquid porosity of at least 90% and a surface area of at least 1.0 m.sup.2 /g.

BACKGROUND OF THE INVENTION 
This invention relates to porous separators which are disposed between the 
electrode plates of a battery. 
Storage batteries include a plurality of electrode plates which are 
arranged to provide alternating positive and negative electrodes. The 
separators are made from an insulating porous material and hold battery 
electrolyte, such as acid, and allow passage of ionic current between the 
plates. 
Battery separators in general must possess certain properties. The 
separator medium must be resistant to degradation and instability in the 
environment of the battery, such as degradation by strong acid solutions 
at ambient and elevated temperatures and strong oxidative attacks. The 
separator should also be capable of allowing a high degree of ionic 
movement or should have a low electrical resistance. The separator should 
also be capable of inhibiting the formation of conductive paths between 
plates and consequent shorting. This latter problem can arise during 
battery operation when parts of the battery electrode become dispersed in 
the electrolyte and precipitate or become deposited in the separator. 
Flooded cell lead acid batteries have been in general use for many years. 
In such batteries, the separators employed typically have a fixed 
thickness. These type of separators are not highly porous and do not 
absorb significant amounts of acid. They serve primarily to prevent 
migration of particles and typically have ribs to physically separate or 
space the electrodes in the cell. 
A recently developed electrochemical cell is commonly referred to as a 
sealed or valve regulated recombinant design. In certain types of 
recombinant batteries, the reservoir of electrolyte is completely 
contained or absorbed by the separator media, and the separator is in full 
contact with adjacent electrodes and fills the entire space between the 
electrodes. 
Battery separators of the recombinant type must have a degree of empty void 
volume to permit transport of oxygen gas generated at the positive 
electrode, during charging or overcharging, to the negative electrode 
where such gas is reduced. In lead-acid batteries, generated oxygen must 
pass from the positive electrode through the separator to the surface of 
the negative electrode, which is damp with sulfuric acid. The oxygen then 
combines with the lead to form lead oxide, which is in turn converted to 
lead sulfate and free water. 
To achieve the above properties, it is known to employ a mat or felt of 
borosilicate glass microfibers as the separator media. These separators 
generally comprise a blend of glass fibers of varying length and diameter. 
GB patent no. 1,364,283 describes a separator medium made up of fine glass 
fibers. The fiber mat has a small pore size and provides a very high 
volume retentivity of electrolyte per unit volume of separator. The 
capillarity of the mat retains the electrolyte stably within the 
separator. The mat is designed to be saturated with liquid electrolyte to 
about 85-95 percent of the available void volume, with the remaining void 
volume being open to allow gas transfer. 
Separators containing submicron glass fibers have several disadvantages 
which have not been adequately resolved. Health concerns have been 
expressed about extremely fine fibers of this nature. Glass fiber mats are 
difficult to process on high speed production equipment due to poor 
mechanical properties, and they tend to release airborne particles. 
Several early proposals were made to use meltblown fibers to make battery 
separators for conventional flooded cell acid batteries. U.S. Pat. Nos. 
3,847,676, 3,972,759 and 4,165,351 all disclose the formation of battery 
separators from fine meltblown fibers. The fibers are rendered wettable by 
the addition of internal or external surfactants. In all cases, the mat is 
permanently compressed, usually by use of heat and pressure, in order to 
make the pad rigid and to reduce pore size to an acceptable level. 
Up to the present time, however, the only material available for use in 
lead acid batteries of the recombinant type have been mats made of the 
aforementioned fine glass fibers. Mats made from the meltblown polymers 
described in the above references are not suitable because of their low 
porosity, large pore size even when compressed, and inability to 
completely absorb the acid electrolyte while retaining an empty void 
volume capable of transmitting gas between electrodes. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a battery separator pad uniquely 
suitable for batteries of the recombinant or sealed type is made from 
extremely fine meltblown fibers self-bonded in a cohesive, uncompressed 
mass. At least 10% of the fibers have a diameter of less than one micron, 
and a majority of the fibers have a diameter of less than five microns. In 
order to obtain a mat of polymeric fibers suitable for use in recombinant 
batteries, the surface area of the fibers in the mat exceeds 1.0 m.sup.2 
/g. Also, the mat, which is not permanently compressed, has a porosity of 
greater than 90% and a mean pore size of from about five to about fifteen 
microns. 
The fiber mat is treated in order to render it wettable by battery acid. 
This may be accomplished by addition of a suitable surface active agent to 
the polymer prior to extrusion, or by covalently bonding hydrophilic 
groups to the surface of the fibers after formation. 
Unlike meltblown battery separators of the prior art, the separator of the 
present invention completely wicks and absorbs the acid electrolyte over 
its entire dimensions and completely fills the space between electrodes. 
DETAILED DESCRIPTION 
The substrate of the recombinant battery separator of the present invention 
is formed using a conventional melt blowing apparatus. Such an apparatus 
typically includes pressurized, heated die through which a plurality of 
filaments of molten thermoplastic polymer are extruded. The die also uses 
heated and pressurized air flowing in the direction of extrusion to 
attenuate the molten polymer upon exit from the orifices. The fibers are 
continuously deposited on a moving conveyor to form a consolidated flat 
web of desired thickness, which may be cut into the desired shape. 
The construction and operation of a melt blowing apparatus for forming a 
coherent mat are considered conventional, and the design and operation are 
well within the ability of those skilled in the art. Suitable apparatus 
and methods are described in U.S. Pat. No. 3,849,241 and U.S. Pat. No. 
3,972,759, incorporated herein by reference. 
The polymers used to make the substrate include thermoplastic polymers 
capable of being melt extruded into a submicron size diameter, and 
resistant to strong acids. Potential candidates include polystyrene, 
polyamides, polyesters and polyolefins, but polypropylene is preferred. 
Several approaches are available in the selection of a suitable resin. 
So-called metallocene polypropylene resins, produced by single-site 
catalysis, have a narrow distribution of molecular weight. A conventional 
polypropylene resin may be treated with known viscosity reducing agents 
such as peroxides. Also, untreated resins having melt flow rates greater 
than 1000 and preferably greater than 1200 may be employed. 
In order to achieve submicron diameters and high surface area, the 
processing conditions must be optimized for the particular resin employed. 
For conventional polypropylene resins having a high MFR, the temperature 
of the attenuating air must be greater than the temperature of the polymer 
melt, and preferably at least 15.degree. C. higher. The rate of flow of 
the attenuating air may be increased from normal levels until ultrafine 
fibers are produced. Also, the thruput of the resin may be reduced from 
normal, with the normal rate usually being one gram/hole/minute. 
From the above considerations, a person skilled in the art will be able to 
prepare a meltblown web, or mat of uniform thickness, with a distribution 
of fiber sizes which are necessary for a recombinant battery separator. 
The web or mat of fibers must contain at a minimum at least about 10% 
fibers having diameters of less than one micron and preferably 5% less 
than 0.5 micron. Most preferably, the web will contain more than 15% 
fibers having diameters of less than one micron. Also, the average fiber 
diameter of fibers in the web will be less than 5 microns, and more than 
60% of the web will have fibers with diameters less than 5 microns. 
The thickness and the basis weight of the meltblown web as produced will 
depend on the particular design of the battery. The thickness may vary 
widely, for example, from 5 to 200 mils, with a basis weight in the order 
of 16 to 660 grams per square meter. 
The fiber size distribution and the essential properties of the meltblown 
mat are determined by standard test procedures. In order to be suitable 
for use in a recombinant battery the mat will have a liquid porosity of 
greater than 87% and preferably greater than 90%. Despite the high 
porosity, the mat will have a mean pore diameter in the order of 5 to 15 
microns and preferably in the order of 8 to 12 microns. Due to the 
relatively high proportion of ultrafine fibers, the surface area of the 
fibers in the mat is greater than 1.0 m.sup.2 /g, allowing for effective 
wicking of the electrolyte, whereby the electrolyte is substantially 
uniformly distributed throughout the volume of the separator. 
The mat is employed in uncompressed form and the thickness is not altered 
by any procedure such as heating under compression. It may be desirable to 
provide a mat which is slightly thicker than the space between electrodes 
in order to assure good contact with the electrode surfaces, but no 
permanent pre-compression is involved. 
While the meltblown web may be simply cut into flat pieces and used as 
such, additional forms are envisioned. For example, the web may be 
reinforced with one or more thin layers of spunbond fabric. Also, pieces 
of the fabric can be thermally bonded together around three edges to form 
a pocket which is then applied over an electrode to cover both sides. 
The meltblown fabric is treated to render it wettable by battery acid. One 
suitable method is to incorporate an internal additive into the molten 
polymer before it is extruded into fibers. These additives are resistant 
to strong acids and may be added at levels of from about 0.5 to 5 percent 
by weight. Some suitable additives which have been identified include 
polytetrahydrafuran, mono and diglycerides from fatty acids, and 
dimethylsilicone oxyalkylene copolymers. The additives are preferably not 
added to the polymer directly but are preferably preformed into pellets 
with the polymer. For example, 25% of the additive may be mixed with 75% 
polypropylene and extruded into filaments. The filaments are allowed to 
cool and chopped into micropellets. Then, about 5 to 20% of the 
micropellets are added to pure polypropylene pellets and fed to an 
extruder and through the melt blowing apparatus. The surface active agent 
tends to migrate toward and coat the surface of the fiber, rendering it 
wettable by acid. 
Another method is to alter the surface of the fiber to render it wettable. 
As an example, a hydrophilic polymer may be chemically bonded to the 
surface of the fiber. This may be accomplished by graft polymerizing of 
the substrate with a hydrophilic monomer, such as an acrylic or 
methacrylic monomer having alcohol functional groups, with the energy for 
the reaction being furnished by radiation. 
In accordance with one preferred embodiment, a hydrophilic compound such as 
polyvinyl pyrrolidone or polyacrylamide is immobilized on the surface of 
the fibers. The hydrophilic agent is either photoactivatible itself or is 
combined with a photoactivatible cross linking agent. The agent is coated 
onto the substrate and irradiated. Various compounds of this nature are 
available from BSI Corporation, Eden Prairie, Minn. See, for example, U.S. 
Pat. No. 5,414,075, incorporated by reference. In this embodiment, the 
hydrophilic polymer is covalently bonded to the meltblown substrate and is 
hydrolytically stable. 
Other treatments to surfaces of polyolefin articles and fibers have been 
suggested to render them hydrophilic or wettable. These include techniques 
to render the surface rough or porous such as by treatment with a plasma 
or corona discharge. 
It has been found that topical application of surfactants to the separator 
pads of the present invention is not acceptable. In evaluating this 
approach, it has been found that topical surfactants, even if they are not 
washed away or chemically degraded, usually cause formation of bubbles in 
the empty void space and may prevent transmission of gas. 
The following are examples of treatments of meltblown polypropylene fabrics 
to render them permanently wettable.

EXAMPLE 1 
Meltblown polypropylene (PP) mat was made wettable using the 
photoactivatible crosslinker, PR03 (provided by BSI Corporation) to 
immobilize polyvinyl pyrrolidone (PVP, BASF K30). PR03 at 0.35g/l and PVP 
at 2.0 g/l was dissolved 0.8% v/v hexanol in water. Meltblown PP was 
saturated with this solution and processed once on each side through on a 
conveyor belt at 30.5 cm per minute, under a Fusion Systems light source 
which was 8.9 cm from the mat. The Fusion System light source P-300, with 
300 watt/inch d bulb. The treated mat was then dried in a convection oven 
until it reached 100 C. This treatment resulted in mat that completely 
saturated with water and wicked 2.5 cm above vertical dip grade in 6.8 
seconds. 
EXAMPLE 2 
Meltblown PP mat was made wettable using the photoactivatible PVP (PV03 
which was provided by BSI Corporation) reagent. PV03 was dissolved at 1.0 
g/l in 75% water and 25% Isopropanol (IPA). The mat was saturated with 
said reagent and illuminated for 60 seconds using two Dymax lamps (PC-2, 
400 watt metal halide/mercury vapor bulbs) 15 cm from the mat on each 
side. The treated mat was allowed to air dry. This treatment resulted in a 
mat that completely saturated with water and wicked 5 cm above vertical 
dip grade in 28 seconds. 
EXAMPLE 3 
Meltblown PP mat was made wettable using the photoactivatible PVP (PV05 
which was provided by BSI Corporation) reagent. PV05 was dissolved at 1.0 
g/l in 0.8% v/v hexanol in water. Meltblown PP mat was saturated with this 
solution and processed once on each side through a conveyor belt at 61 cm 
per minute, under a Fusion Systems light source which was 8.9 cm from the 
mat. The Fusion System light source P-300, with 300 watt/inch H bulb. An 
alternative light source is a pulsed UV Xenon bulb. The treated mat was 
then dried in a convection oven until it reached 100 C. This treatment 
resulted in a mat that completely saturated with water and wicked 5 cm in 
51 seconds. 
EXAMPLE 4 
Meltblown PP mat was made wettable in the same manner as example 1 except 
by changing the parameters listed in the following table. The resultant 
wetting characteristics are listed per each set of parameters: 
______________________________________ 
PVP 4 PR03 0.50 Oven Temp 
100 
(g/l) 2 (g/l) 0.38 (degree C.) 
100 
2 0.25 100 
5 0.75 125 
2 0.75 100 
5 0.75 100 
2 0.25 100 
3 0.50 100 
2 0.38 100 
2 0.50 100 
______________________________________ 
Wick Time Wick Time 
to 2.5 cm to 5 cm 
(seconds) (seconds) 
______________________________________ 
10 35 
9 37 
10 35 
9 30 
6.5 27.5 
6.5 26.5 
7.0 28.5 
8.0 31.0 
7.8 31.0 
6.5 27.5 
______________________________________ 
All samples listed in the aforementioned table resulted in a mat that 
completely saturated with water. 
EXAMPLE 5 
Meltblown PP mat was made wettable using the photoactivatible PVP (PV03 
which was provided by BSI Corporation) reagent. PV03 was dissolved at 1.0 
g/l in water. The mat was pretreated with an oxygen plasma at 100 watts 
for 3 minutes on each side and then saturated with said reagent and 
illuminated for 4 minutes using two Dymax lamps (PC-2, 400 watt metal 
halide/mercury vapor bulbs) 15.2 cm from the mat on each side. The treated 
mat was allowed to air dry. This treatment resulted in a mat that 
completely saturated with water. 
EXAMPLE 6 
Meltblown PP mat was made wettable as described in example 5, except that 
photoactivatible Polyacrylamide (4 which was provided by BSI 
Corporation) was used as a reagent in 1.0 g/l in water. 
EXAMPLE 7 
Meltblown polypropylene (PP) mat was made wettable using the 
photoactivatible crosslinker, PR03 (provided by BSI Corporation) to 
immobilize polyvinyl pyrrolidone (PVP, BASF K90). PR03 at 0.25 g/l and PVP 
at 2.0 g/l was dissolved 25% IPA in water. Meltblown PP was saturated with 
this solution and processed once on each side through on a conveyor belt 
at 152 cm per minute, under a Fusion Systems light source which was 3.5 
inches from the mat. The Fusion System light source P-300, with 300 
watt/inch H bulb. The treated mat was then allowed to air dry. This 
treatment resulted in mat that completely saturated with water and wicked 
5 cm in 66 seconds. 
EXAMPLE 8 
Poly(tetra hydrafuran) BASF polyTHF 2000! 25% by weight, was mixed with 
high melt flow poly(propylene). The mixture was fed into a single screw 
extruder and filaments were drawn of the mixture. The filaments were 
allowed to cool in the air and then were chopped into micropellets. The 
polyTHF concentrate, 20% by weight, was then mixed into more high melt 
flow poly(propylene) and feed into another extruder. This extruder was 
equipped with a meltblown die and polypropylene mat was formed. The 
resultant mat had fine denier and completely saturated with water and 
acid. 
EXAMPLE 9 
Humko Chemical of American Ingredience, Atmul 124 mono and diglycerides 
from fatty acids! were added, 25% by weight, to polypropylene. The 
material was mixed and extruded into filaments using a single screw 
extruder. After cooling in air the polymer filaments were chopped into 
micropellets. The Atmul 124 concentrate pellets, 5-10% by weight, were 
then mixed with high melt flow poly(propylene) powder and meltblown mat 
was formed. The resultant mat had fine denier and completely saturated 
with water. 
EXAMPLE 10 
PPG's Masil SF 19.RTM. dimethylsilane onyalkylene copolymer! additive was 
added, 25% by weight, to polypropylene. The material was mixed and 
extruded into filaments using a single screw extruder. After cooling in 
air the polymer filaments were chopped into micropellets. The Masil SF 
19.RTM. concentrate, 5-10% by weight, was then added to poly(propylene) 
and meltblown mat was formed. The mat had fine denier and completely 
saturated with water. 
When wetted with water or acid, the liquid wicks through the entire 
structure and the liquid is entirely absorbed by the separator. In 
practice, sufficient acid is added so that 85 to 95% of the void volume is 
saturated. The remaining space is in the form of an empty interconnected 
porous structure, allowing transfer of gas between electrodes.