Process for manufacturing ultra-thin sintered PVC battery separators

Dry mixed and sintered PVC battery separators having predetermined thicknesses less than 0.012 inch, porosities greater than about 50%, and pores sufficiently small to substantially prevent interplate "treeing" (i.e., less than about 10 microns average) are produced by: mixing about 3% to about 15% by volume of leachable, pore-forming particles with the PVC particles; scraping the particle mix into a layer less than about 0.012 inch thick; sintering the PVC into a continuous strip; compressing the strip in the temperature range of about 250.degree. F.-450.degree. F. to a thickness no greater than about 50% of its as-sintered thickness; thereafter immediately allowing the strip to recover much, but not all, of its as-sintered thickness; and cooling the strip to substantially fix the separator thickness at the recovered thickness. The leachable, pore-forming particles are preferably formed in situ by the thermal degradation of a gasifiable filler such as sodium bicarbonate.

This invention relates to a dry-sintering process (i.e., without solvents, 
plasticizers, etc.) such as described in Bahler et al U.S. Pat. No. 
3,551,210, filed in the U.S. Feb. 3, 1969 and assigned to the assignee of 
the present invention. More specifically, the invention relates to 
dry-sintering PVC battery separators in thinner (i.e., less than 0.012 
inch) strips than it was practically possible to do heretofore without 
materially reducing the separator's strength, its "treeing" resistance and 
its electrical conductivity in the battery. 
Battery separators function essentially to electrically isolate the 
positive and negative plates from each other. They prevent direct contact 
and suppress "treeing" or interelectrode dendrite growth which causes 
shorting of the respective plates. An ideal separator would isolate the 
plates without inhibiting electrolyte mobility, and without increasing the 
battery's internal resistance. The separator manufacturer's ability to 
achieve the ideal, however, is thwarted by practical manufacturing 
limitations. Processes are sought which will yield maximum total porosity 
and thinness (i.e., for achieving low electrical resistance), and minimum 
pore size (i.e., for achieving maximum "treeing" resistance). The 
relationship that exists between the total porosity and size of the pores 
defining that porosity for a separator of given thickness can be 
quantitatively characterized in terms of the separator's air permeability 
according to the Gurley porisimeter method and is referred to herein as 
the separator's "porosity profile." 
Heretofore, high-speed, dry-sintering processes of the Bahler et al-type 
have been able to produce separators having thicknesses as low as 0.014 
inch and total porosities of about 50%, but with average pore sizes no 
less than about 14 microns. With pores this large, the 0.014 inch 
thickness is necessary to provide adequate strength and "treeing" 
resistance. Prior to the present invention, Bahler et al-type processes 
have not been able to produce acceptable separators less than 0.012 inch 
thick at commercially practical rates. Moreover, separators that have been 
made have proven unacceptable for applications such as Pb-Ca 
maintenance-free batteries which have a higher "treeing" resistance 
requirement not met by the larger pores in the thinner separators. In this 
regard then, acceptable separators are herein intended to mean those which 
have an electrical resistance (i.e., at 80.degree. F.) which does not 
exceed about 0.0012 Ohms/inch.sup.2 for each 0.001 inch of web thickness, 
and have a tree resisting, porosity profile yielding an air permeability 
of not less than 24 secs for passing 300 ccs of air in a Model 4100 Gurley 
Densometer with a 0.025 inch.sup.2 orifice and a 5 oz. weight (i.e., 24 
Gurley). Separators with Gurleys above about 60 secs, on the other hand, 
tend to have too high a resistance for most applications. 
It is an object of the present invention to provide a commercially 
practical dry-sintering method for making PVC separators which are less 
than 0.012 inch thick yet have a porosity profile resulting in high 
"treeing" resistance and low electrical resistance. This and other objects 
of this invention will become more apparent from the detailed description 
which follows. 
THE INVENTION 
The Invention comprehends: sintering PVC powder mixed with about 3% to 
about 15% by volume of leachable, pore-forming particles which are less 
than about 10 microns in diameter (average); warm deforming or calendaring 
of the sintered mix at a temperature above 250.degree. F. to reduce the 
as-sintered pore size without collapsing them, and to stabilize the strip 
against in-service growth; and then leaching out the particles leaving 
only the smaller pores. Earlier attempts to reduce the pore size by simply 
calendaring the sintered sheet but without the pore-formers or with the 
pore-formers but at too high a temperature only increased the electrical 
resistance to an unacceptable level. Moreover, calendaring at too low a 
temperature following sintering would not fix the separator's thickness 
against in-service expansion as will be pointed out hereinafter. 
The pore-forming particles used in combination with the warm deformation 
step preferably range from about 1 to about 7 microns in diameter and have 
the average particle size of less than about 4 microns. Preferably, the 
pore-forming particles are comprised of materials which are gasifiable 
under PVC sintering conditions (i.e., evolve a gas in the sintering oven) 
yet leave soluble (i.e., in acid or water) residue amongst the PVC. One 
such preferred gasifiable material is sodium bicarbonate which gives off 
about 20% of its weight as CO.sub.2 at 410.degree. F. and leaves somewhat 
smaller (i.e., about 10%) sodium carbonate particles in their stead. A 
particular advantage of sodium bicarbonate over other gasifiable 
pore-formers is that its bulk density (i.e., ca 0.47 g/cc) is near that of 
the PVC (i.e., ca 0.53 g/cc) which greatly simplifies mixing and 
fluidization of the PVC-bicarb mixes. 
In carrying out the process of this invention, the mix is spread onto a 
moving metal belt as a layer less than 0.014 inch thick (usually about 
0.010 inch). The particle layer is heated as it passes through an 
elongated oven to sinter the PVC particles into a continuous strip. 
Following sintering, the strip is compressed at temperatures in the range 
of about 250.degree. F.-450.degree. F. between calendar rolls to a 
thickness which is no greater than about one-half (preferably about 
one-third) its as-sintered thickness. This warm compression deforms the 
warm PVC particles, improves their bond strength to each other and shrinks 
the pores between them. In this step, the leachable particles serve to 
prevent collapse of the pores and, like a core in molding, to some extent 
generally defines the pores themselves. 
Following compression, the still warm strip elastically recovers much, but 
not all, of its lost thickness and hence remains somewhat permanently 
deformed. More specifically, it recovers about 75% to about 90% (i.e., 
preferably about 90%) of its as-sintered thickness. The precise amount of 
recovery in each instance will vary with the degree of compression and the 
compression temperature used. In this regard, it has generally been 
observed that greater compression is required at the lower compression 
temperature (i.e., nearer 250.degree. F.) to achieve the desired pore size 
and recovery than is needed at the higher compression temperatures. 
Following recovery, the strip is cooled to fix the post-compression 
thickness achieved at the exit of the calendaring rolls. The leachable 
pore-forming particles remain with the PVC throughout the foregoing, but 
are ultimately removed by the time the battery is in service. In this 
regard, they may be immediately removed as by a distinct leaching step, 
but preferably are left in situ and are ultimately removed in the battery 
by the action of the acid therein. The particular combination of process 
parameters (e.g., composition, layer thickness, sintering 
time/temperature, and degree and temperature of compression, etc.) is 
chosen to achieve a particular design thickness after the calendaring 
rolls. Following cooling and fixing of the separator's thickness, the 
separator strip is ready for cutting and forming into individual 
separators or separator-envelopes according to the many techniques known 
to those skilled in the art. Conventional spacer ribs may be formed on the 
separator at the time the powder layer is spread onto the belt during 
calendaring or at any other time as is well known to those skilled in the 
art. 
Warm compression at temperatures in excess of 250.degree. F. following 
sintering has been found essential to fix the post-compression thickness 
against further growth during the service life of the battery. In this 
regard, it has been observed that when the PVC is compressed at 
temperatures less than about 250.degree. F., an initial partial elastic 
recovery occurs immediately after compression, but that this thickness is 
not permanent and a secondary elastic recovery later occurs in the battery 
in service which unduly internally stresses its elements. This problem has 
been particularly noticed in automobile SLI batteries located in engine 
compartments which see as much as 230.degree. F. temperatures. On the 
other hand, strips compressed at temperatures above about 450.degree. F. 
do not recover as much after compression and tend to yield separators with 
unnecessarily high electrical resistance. 
As indicated, the pore-forming particles preferably gasify in the sintering 
oven and yield a pore-forming residue which is then leached out after the 
warm compression step. Most preferably, the gasifiable pore-forming 
particles are sodium bicarbonate in the 1 to 7 micron particle range which 
evolve harmless CO.sub.2 and leave sodium carbonate as the residue which 
does not upset the battery chemistry when removed by the electrolyte in 
the completely assembled battery.

Fixed thickness PVC battery separators can be made by the process of this 
invention which are less than about 0.010 inch thick, have greater than 
50% porosity, have pores which are, for the most part, less than about 10 
microns in diameter and have Gurley air permeabilities greater than 24 
secs. The high porosity helps to keep the electrical resistance low by 
insuring adequate electrolyte volume and mobility within the cell while 
the small pore size inhibits the "treeing" through of these thin 
separators. Separators have been made by this invention as low as 0.008 
inch thick and with an average pore size of about 7.5 microns (as 
determined by a mercury porosimeter Aminico Model 7-7118). 
Separator-grade PVC particles useful with this invention comprise for the 
most part particle mixes in which the particles range in diameter from 
about 13 microns to about 67 microns with an average particle size of less 
than 36 microns. Thinner separators are made with preferred PVC particles 
which vary for the most part from about 15 microns to about 48 microns and 
have an average particle diameter of less than 30 microns. Particle sizes 
and distributions herein for both the PVC and pore-forming agents are as 
determined by a Coulter Electronics Counter Model 1. 
The pore-forming particles have an average particle size which is no 
greater than the 10 micron pore size sought to be obtained in the finished 
separator. Particular success has been obtained with sodium bicarbonate 
particles ranging from about 1 micron to about 7 microns in diameter and 
an average particle size of about 3.2 microns. The sodium bicarbonate 
content of the PVC-bicarbonate mix can vary from as low as about 3% to as 
high as about 15% by volume, but about 5% to about 10% yields consistently 
acceptable results. The 5% sodium bicarbonate-PVC mixes seem to achieve 
about the best tradeoff between acceptable electrical resistance, 
"treeing" resistance and handling strength. Otherwise, when the 
bicarbonate content falls below about 3%, the resistance of the compressed 
separator is unacceptably high. On the other hand, when the bicarbonate 
content exceeds about 15%, the separator has a lower resistance to 
"treeing" and is generally too weak and fragile to sustain the normal 
handling in the plant. 
The gasifiable, pore-forming particles which leave leachable residues 
(i.e., NaHCO.sub.3) are preferred over particles which are leachable but 
do not gas in the oven. In this regard, the gasifiable pore-formers yield 
as-sintered strips whose porosity (i.e., before compression) is higher 
than that predictable based solely on the volume of pore-former alone. 
Just why this is so is not clearly understood though it is believed that 
the gassing in the oven has a lofting affect on the PVC which lowers the 
density of the as-sintered strip prior to compressing. It is also noted 
that the pore-forming particles themselves grow somewhat smaller during 
gassing which contributes to the small pore formation achieved during the 
warm compression step of the process. In one example of this apparent 
lofting phenomena, a control sample of PVC powder (i.e., without a 
pore-former) was sintered and yielded as uncompressed separator with a 
porosity of about 50%. The same PVC powder, but with 5% by volume sodium 
bicarbonate added, had an uncompressed porosity of about 62% (i.e., with 
the carbonate residue still present). When the residue was leached out, 
the uncompressed porosity of the separator rose to about 65%. It has 
further been observed that 50% porous PVC control samples (i.e., without 
gassing pore-formers) have a porosity approaching only about 40% after the 
warm compression step whereas those containing soda, as above, are about 
48% porous after warm compression (i.e., before removal of the salt), and 
in excess of 50% (i.e., 51%-52%) after the carbonate is leached out. 
Separator strip material made in accordance with this invention may be 
processed in substantially the same manner as described in Bahler et al 
U.S. Pat. No. 3,551,210 and accordingly, for much of the detail thereof, 
Bahler et al is intended to be incorporated herein by reference. Generally 
speaking though, the Figures of this application depict apparatus like 
that of FIG. 2 of Bahler et al but with the addition of means for the warm 
compression of the separator strip following sintering. In carrying out 
the present process, the PVC particles are conditioned as necessary for 
moisture and agglomeration control followed by homogeneous mixing with the 
pore-forming particles. The specific means for accomplishing this is not 
part of the present invention but both conditioning and mixing may be 
conveniently achieved by known fluidization techniques. FIG. 1 depicts a 
conditioning and mixing means 2 for providing the PVC-pore-forming mix to 
a feed hopper 4 (see FIG. 2 for enlargement). The hopper 4 dispenses the 
mix onto a continuous stainless steel belt 6 (i.e., about 0.032 inch 
thick) behind a comblike scraper blade 8 which is profiled to form 
conventional spacing ribs on the strip while spreading the powders. In 
this regard, the spacer ribs are preferably combed into the powder layer 
while it is being spread onto the belt as in Bahler et al, and the 
compression means merely compresses the webs between the ribs without 
appreciably acting on the ribs themselves. It is recognized, however, that 
the powder may be spread flat and the ribs put thereon after compression 
and recovery as by hot melt beading, corrugating, embossing or the like as 
is well known in the art. 
The belt 6 moves at a rate of about 200 ft./min. under the feeding hopper 4 
and thereunder receives a layer of mix having a thickness equal to about 
the height of the dam 10 above the belt 6. The dam 10 is positioned about 
0.025 inch above the belt 6 and the comb 8 adjusted (i.e., to about 0.02 
inch above the belt) to produce a 0.012 inch thick powder layer 11 
downstream thereof. The height of the dam 10 and comb 8 can be varied by 
appropriate dam and comb adjusting means 12 and 14, respectively. Excess 
powders mound up behind the comb 8 which mound 16 is kept in a constant 
rolling or eddy-like motion by means of a vacuum skimming device 18 which 
is so located as to prevent excess powders upstream of the comb 8 from 
raising the head of the mound 16 to the point that it becomes stagnant. 
The powder layer 11 flowing from under the comb 8 is then heated and 
sintered in a long oven 20. Preferably it is rapidly preheated (i.e., to 
about 375.degree. F.) to just below its sintering temperature, and then 
more slowly heated to sintering of the PVC at about 410.degree. 
F.-415.degree. F. In the particular embodiment shown, the initial rapid 
heatup of the particles to the 375.degree. F. presintering temperature is 
accomplished in the first two stages of four-stage oven 20 having gas 
burners 22 heating the separators through the stainless steel belt 6 which 
tends to form a thin skin on the bottom of the strip where the PVC is 
hottest. This skin has a somewhat higher density than the rest of the 
separator, but even here the pore-forming particles serve to keep the skin 
from completely sealing off that surface of the separator. The first two 
burners are located approximately 2 inches below the stainless steel belt 
6. The first oven stage is approximately 48 feet long and the oven 
temperature is maintained at about 600.degree. F. The second oven stage is 
about 28 feet long and is maintained at an oven temperature of about 
400.degree. F. The third and fourth oven stages finish the heating and 
sintering and are 28 feet and 32 feet long, respectively, and maintained 
at oven temperatures of about 610.degree. F. and 475.degree. F., 
respectively. It is to be appreciated that these temperature readings will 
vary depending on the location of the temperature sensor in each oven, but 
they do serve to indicate the nature of the preheating and sintering steps 
used to manufacture separators by the process of this invention. 
After sintering, the strip is cooled to a temperature of about 250.degree. 
F. to about 300.degree. F. as determined by a temperature probe 13 (see 
FIG. 2 enlargement) contacting the underside of the belt 6 just before the 
compression means. While forced cooling would be acceptable, it appears 
that merely extending the length of the line between the oven exit and the 
compression rollers (discussed hereafter) is sufficient to permit adequate 
cooling before compression. At the aforesaid 250.degree. F. to 300.degree. 
F. temperature, the sintered strip enters the nip of compression rollers 
24 which compress the strip between the upper roller and the belt 6. As 
indicated above, the compression rollers may have flat surfaces if the 
strip is flat or may have annular grooves for accommodating the ribs if 
they are already formed on the strip. In this latter case, only the 
portions of the rollers that are between the annular recesses compress the 
web portions (i.e., between the ribs) of the separator strip. Upon exiting 
the compressing rollers 24, the strip recovers to about 80%-95% (i.e., 
depending on the temperature of the PVC and degree of compression) of its 
as-sintered thickness before compression, which is the design thickness of 
the separator. Cooling to room temperature after the warm compression 
fixes or permanizes the thickness of the strip against further elastic 
recovery and swelling while in service. Finally, the strip is peeled from 
the belt 6 as by a stripper means 26 and cut into desired lengths as by 
blade 28. 
0.010 inch thick PVC separators compressed (i.e., at about 275.degree. F.) 
to about one-third their as-sintered thickness using the preferred 5% 
NaHCO.sub.3 mix have demonstrated resistances of about 0.010 
ohms/inch.sup.2 and Gurley air permeabilities of about 30 secs. With the 
same material, 0.012 inch thick PVC separators made this way have 
demonstrated 0.013 ohms/inch.sup.2 Gurley air permeabilities of about 42 
secs. With the same material, 0.008 inch thick separators made this way 
have demonstrated 0.009 ohms/inch.sup.2 resistance and Gurley air 
permeabilities of about 33 secs. These resistance measurements were 
determined in a typical battery separator test cell at 80.degree. F. using 
1.280 specific gravity H.sub.2 SO.sub.4. Higher belt speeds (i.e., up to 
about 300 ft/min) may be used if the oven temperatures are increased and 
the compression rollers are cooled (i.e., about 100.degree. F.-200.degree. 
F. surface temperatures). For example, acceptable separators have been 
made at the rate of 240 ft/min under conditions where the first oven stage 
varied from 450.degree. F.-600.degree. F. and the second, third and fourth 
stages were held to about 490.degree. F., 610.degree. F. and 640.degree. 
F. respectively .+-.40.degree. F. per stage. Under these conditions, the 
strip exits the oven and enters the nip of the rollers at temperatures as 
high as about 450.degree. F. To effect satisfactory compression at these 
temperatures, the rollers were water cooled to a surface temperature of 
about 170.degree. F., and the strip compressed to about 30% of its 
as-sintered thickness (i.e., 70% thickness reduction). Following 
compression, the strip is immediately cooled by spraying the underside of 
the belt with 65.degree. F.-80.degree. F. water. 
It is theorized that at the higher belt speeds, only the surfaces of the 
PVC particles achieve the higher temperatures (i.e., 450.degree. F.) 
observed while the core of the particles remain at a lower temperature. 
This theory is reinforced by the observation that any delay in cooling the 
strip after exiting the rollers causes greater initial coalescence of the 
particles on the belt and eventual complete charring of the strip. 
While this invention has been described in terms of certain embodiments 
thereof, it is not intended to be restricted thereto, but rather only to 
the extent defined hereafter in the claims which follow.