Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Background of the Invention
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This invention relates to a dry-sintering process
(i.e., without solvents, plasticizers, etc.) such as
described in Bahler et al U.S.P. 3,551,210. More spe~ifically,
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 elec-
trically isolate the positive and negative plates from
each other. They prevent direct contact and suppress
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"treeing' or interelectrode dendrite growth which causes
shorting of the respective plates. ~n 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, ho~ever, is thwarted by practical manu-
facturing limitations. Processes are sought which will
yield maximum total porosity and thinness (i.e., for
achieving low electrical resistance), and minimu~ pore
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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 ~he
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Gurley porisimeter method and is referred to herein as the
' separator's "porosity profile."
Heretofore, high-speed, dry-sintering processes
- ol the Bahler et al-type have been able to produce separa-
tors 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
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- 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 comm~rcially practical rates. Moreover, separators that
have been made have proven unacceptable ~or applications
such as Pb-Ca maintenance-free batteries which have a hi~her
"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
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electrical xesistance (i.e., at ~0 ~ 'nich does not
exceed about 0.0012 Ohrns/inch2 for each 0.001 inch of web
thickness, and have a tree resisting, porosity profile
yielding an air permeability of not less than 2~ secs for
passing 300 ccs of air in~a i~lodel 4100 Gurley Densometer
ith a 0~025 inch2 orifice and a 5 oz. ~7eignt (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 comrnercially prac~ical dry-sinteriny ~ethod for making
PVC separators which are less than 0.012 inch thick yet have
a porosity pro~ile resulting in high "treeing" resistance
and low electrical resistance. This and other objects of
this invention will become more apparent frorn the detailed
description which follows.
The Invention
The Invention comprehends: sintering PVC powder
mlxed with about 3% to about 15% b~ volume of leachable,
pore-forming particles which are less than about 10 microns in
diameter (average); warm deforming or calendarinc~ o~ the
sintered mix within fixed temperature lirni~s to reduce the
as-sintered pore siæe without collapsing them, and to stàbilize
the strip agalnst in-servlce yro;~th; and then leaching out the
particles leaving only the smaller pores. ~arlier attempts
to reduce the pore size by sinply calendarinc~ the sintered
sheet but ~ithout the pore-~orrners or wit'~ the ~ore-formers
but at too high a tem~eratuYe only increased the electrical
resistance to an unaccep,able level. Moreover, calen~arlng
at too low a temperature following sinterin~ would not fix the
separator's thic~ness against in-service expansion as will
be poinLed out hereinafter.
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The pore-forming particles used in co~bination
with the warm deformation step pre-ferably 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 ofL
about 200/o of its weight as C02 at 410~ 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.
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In carrying out the process of this invention, the
mix is spread onto a moving metal belt as a ~ayer less than
0.014 inch thick (usually about 0.010 inch). The particle
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layer is heated (i.e., about 415 F.) as it passes through
an elongated oven to sinter the PVC particles into a concinu--
ous strip. Following sintering, the strip is cooled to a
temperature of about 250 F. to about 300 F. (pre~ferabl~
about 275 F.), and at this temperature compressed between
caiendar rolls to a thickness which is no greater than about
one-half (preferably about one-third) its as-sin~ered thick-
ness. This warm compressioll de~orms the warm PVC particles,
-improves their bond strength to each other ana shrinks the
pores bett~een them In this step, the leachable particles
serve to prevent collapse of the parts and, like a co.e in
molding, to some extent generally defines the pores t'nemselves.
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Following compression, the still warm strip
elastically recovers much, but not all, of its lost
thickness and hence remains somewhat permanently de-
formed. More specifically, it recovers about 75% to
about gO% (i.e., preferably about 90%) of its as-sintered
thickness. The precise a~ount o~ recovery in each instance
will vary with the degree or compression and the compres-
sion temperature used. In this regard, it has generally
been observed that greater compression is required at
the lower compression temperatures (i.e., nearer 250~
; F.) to achieve the desired pore size and recovery than
is needed at the higher compression temperatures (i.e.,
nearer 300 F.).
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
remo~od as ~y 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 desree 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
individua separators or separator-envelopes according
to 'he many techniques known to those ski]lea in the art.
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Conventional spacer ribs may be formed on the separator
at the time the powder layer is spread onto the belt
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during calendaring or at any other time as is well
known to those skilled in the art.
'~ Warm compression in the 250 F. to 300 F.
; range 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 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 particular-
ly noticed in automobile SLI batteries located in engine
compartments which see as much as 230~ F. temperatures.
On the other hand, strips compressed at temperatures
above about 300 do not recover as much after compres-
sion and tend to yield separators with unnecessarily highelectrical 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
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warm compression step. Most preferably, the gasifiable
pore-forming particles are sodium bicarbonate in the 1
; - to 7 micron particle range which evolve harmless-CO2 and
leave sodium carbonate as the residue Jhich does not
upset the battery chemistry ~hen removed by the electro-
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lyte in the com letely assembled battery.
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- The Figures generally ill~strate, in side
elevation, apparatus for carrying out the process of
the present invention. Figure 2 is an enlargement of
portions of Figure 1.
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~/O 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 separa-
tors. 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 merçury porosi-
meter 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 preferxed PVC
particles which vary for the most part from about 15 microns
to about '18 microns and have an averaye 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 Co~lter Electronics Counter ~iodel PAll~
The pore-forming particles have an average particle
size which is no greater than the 10 micron pore si,.e sought
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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
sodiu~ 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 electri-
cal 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 weaX and fragile to sustain the normal
handling in the plant.
The gasifiable, pore-forming particles which
leave leachable residues (i.e., NaHCO3) 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 loft-
ing affect on the PVC which lowers the density of the as-
sintered strip prior to compressing. It is also noted that the
pore-orming particles themse~ves 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
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yielded an 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.P. 3,551,210.
Generally speaking though, the Figures of this application depict
apparatus like that of Figure 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. Figure
1 depicts a conditioning and mixing means 2 for providing
the PVC-pore-forming mix to a feed hopper 4 ~see Figure 2
for enlargement). The hopper 4 dispenses the mix onto a
continuous stainless steel belt 6 (i.e., about 0.032 inch
; 30 thick) behind a comblike scraper blade 8 which is pro-
filed to form conventional spacing ribs on the strip
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while spreading the powders. In this regard, the spacerribs 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
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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 o 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
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^ thick powder layer 11 downstream thereof. The height o-E 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
~; 20 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~
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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 F.) to just
- below its sintering temperature, and then more slowly heated
to sintering of the PVC at about 410 F.-415 F. In the
particular embodiment shown, the initial rapid heatup of
the particles to the 375 F. presintering temperature is
accomplished in the first two stages of four-stage oven 20
having gas burners 22 heating the separators through the
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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 F. The second
oven stage is about 28 feet long and is maintained at an
. oven temperature of about 400 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
tempexatures of about 610 F. and 475 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.
. 20 After sintering, the strip is cooled to a tempera-
ture of about 2S0 F. to about 300 F. as determined by a
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temperature probe 13 (see Figure 2 enlargement) contacting
the underside of the belt 6 just before the compression
means. ~Ihile forced cooling ~Jould 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 F. to 300 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
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~ flat or may have annular grooves for accommodating the
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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 2~, the strip recovers
to about 8~/o~95% ti.e. D depending on the temperature of
the PVC and degree of compression) of its as-sintered
thickness before compression, which is the design thicknes~
of the separa~or. Air 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 F.) to about one-third their as-sintered thickness
using the preferred 5% NaHC03 mix have demonstrated resis-
tances o about 0.010 ohms/inch2 and Gurley air permeabilities
of about 30 secs. ~ith the same material, 0.012 inch thick
PVC separators made this way have demonstrated 0.013 ohms/
inch2 and Gurley air permeabilities of about 42 secs. With
the same material, 0.008 inch thic~ separators made this
way have demonstrated 0.009 ohms/inch resistance and Gurley
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air permeabilities of about 33 secs. These resistance
measurements were determined in a typical battery separator
test cell at 80 F. using 1.280 specific gravity H2S0~. --
While this invention has been described in terms
of certain embodiments thereof, it is not intended to be
- 30 restricted thereto, but rather only to the extent deined
hereafter in the claims which follow.
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SUPPLEMENTARY DISCLOSURE
The invention described in the principal specification
involves 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 within fixed temperature limits
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.
In carrying out the process according to the principal
disclosure 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 (i.e., about 415F.) as it passes through
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an elongated oven to sinter the PVC particles into a continuous
: strip. Following sintering, the strip is cooled to a temperature
of about 250F. to about 300F. (preferably about 275F.), and
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at this temperature compressed 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.
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Further experiments have revealsed that the useful
- temperature range for compression of the strip following sintering
is broader than earlier predicted. In this regard, 300F is no
longer considered the upper temperature limit for compression.
By using higher belt speeds and water cooled compression rollers,
strip temperatures as high as 450F have been used.
Accordingly, the modified process is carried out in
the manner described in the principal disclosure with the particle
layer being heated as it passes through an elongated oven to
sinter the PVC particles into a continuous strip. Following
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sintering, the strip is compressed at temperatures in the range
of about 250F to 450F to the thickness described previously.
The principal disclosure contains a description of
typical belt speeds and oven temperatures. 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 F.-200 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 450F. -
600F. and the second, third and fourth stages were held to about
490F., 610F. and 640F. respectively ~ 40F. per stage. Under
these conditions, the strip exits the oven and enters the nip
of the rollers at temperatures as high as about 450F. To effect
satisfactory compression at these temperatures, the rollers were
water cooled to a surface temperature of about 170F., 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
65F. - 80 F. water.
It is theorized that at the higher belt speeds,
only the surfaces of the PVC particles achieve the higher
temperatures (i.e., 450F.) observed while the core of the
particles remain at a lower temperature. This theory is re-
inforced 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.
Since the process and the resulting product are
otherwise basically the same as described in the principal
disclosure, there is no need for them to be further described here.
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