Coating procedure for tape separator of heat sterilizable batter

Dixon, G. D.

Scala, L. C.

English

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UNCLASSIFIED
SEPARATOR DEVELOPMENT FOR A HEAT STERILIZABLE BATTERY
L. C. Scala and G. D. Dixon
July 31, 1968
Contract No. 951525
"This work was performed for the Jet Propulsion Laboratory,
California Institute of Technology, pursuant to a sub-
contract issued winder Prime Contract NAS 7-100 between the
California Institute of Technology and the United States
of America represented by the National Aeronautics and
Space Administration."
F
Westinghouse Electric Corporation
Research Laboratories
Pittsburgh, Pennsylvania 15235
Eighth Quarterly Report
Period: April 1 to June 30, 1968
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UNCLASSIFIED
SEPARATOR DEVELOPMENT FOR A FEAT STERILIZABLE BATTERY
L. C. Scala and G. D. Dixon
July 31, 1968
Contract No. 951525
"This work was performed for the Jet Propulsion Laboratory,
California Institute of Technology, pursuant to a sub-
contract issued under Prime ^nntract NAS 7-100 between the
California Institute of Tech,-.logy and the United States
of America represented by the National Aeronautics and
Space Administration."
Westinghouse Electric Corporation
Research Laboratories
Pittsburgh, Pennsylvania 15235
Eighth Quarterly Report
Period: April 1 to June 30, 1968
SEPARATOR DEVELOPMENT FOR A HEAT
STERILIZABLE BATTERY
Eighth Quarterly Report
L. C. Scala and G. D. Dixon
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ABSTRACT
It is now possible continuously to dip-coat Webril support
tape which is 12 inches wide. Long runs have been made producing
separator material having satisfactory resistivity values and hydroxyl
ion diffusion rates. Several alternative supporting tapes have been
evaluated. Flame absorption spectroscopic analysis indicated that
zincate ion diffusion was very slow. The addition of free zirconia to
the sterilizing mixture reduced the weight loss and decreased the re-
sistivity of most separators.
I. INTRODUCTION
This quarterly report covers the work done during the period
April 1 to June 30. The effort expended during this time and reported
here includes:
1. Evaluation of the industrial size coating tower for
continuous preparation of separator material.
2. Investigation of the varying parameters involved in a
continuous coating process.
3. Evaluation of new candidate substrates by sterilization
tests.
4. Investigation of the effectf of adding zirconia to sterilizing
medium.
5. Study of the effects upon separators of aging in a
40% KOH solution containing silver ions.
6. Measurements of zincate ion diffusion through the
separators.
II. PROGRESS
1. Modification of coating tower
Coating supporting tapes using a dip tank and wiper rods has
proven to be the best method tried. However, because the coating mixture
is a slurry, an efficient means of agitation is necessary to prevent the
inorganic filler from settling out. This problem has been solved by re-
designing the dipping tank so that a circulating pump could be included.
In order to reduce the amount of tape shrinkage at the tem-
peratures used to give a dry coated tape, the vertical drying oven was
doubled in length. This resulted in higher coating speeds at lower
temperatures giving dry coated tapes with little or no shrinkage.
Most of the work has been done using 3" wide supporting tape.
Several runs have been made with 12" wide tape and these showed the
need to modify the take-up system which is under way. With rollers only
12" wide, the tape continuously overlapped the rollers. Also better
roller alignment is necessary because the web-guiding system did not
work. This is controlled by a light beam and the coated tape was
transparent to this beam. A pneumatic, web-guide, or curved spreader
rollers will probably obviate this problem.
2. Coating of candidate support_ng tapes
A new Webril tape (E1403, 5 mil thick printed polypropylene,
3 inches wide, similar to H739 an4 1:751 which were previously tested)
was tower coated with a mixture consisting of 3:1 20-50 oxide: PS and
1% Alkaterage by weight of soiids. The coating mixture covered the
tape very well at speeds ranging from 3.5 to 6 feet per minute. The
coated tape was thick, ranging from 6.8 to 7.3 mils. In all cases very
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minute, regularly spaced pinholes were visible; they allowed the
electrolyte through relatively rapidly (pH 13 reached in 5 minutes);
however, the specific resistivity of these samples ranged from 10 to
20 ohms-inch. This material appears to be promising, even though it
is rather thick, because of the regularity of its structure which
presents no large voids.
Two long runs were made (C58-67-1 and C58-67-2) using Webril
type 1403, mentioned above. This material, although possibly too thick,
has already been shown to hold promise for our applications. Resin
mix C58-67-1 consisted of 3:1 oxide (20:50 mesh): polysulfone, and 1%
Alkaterge E; mix C58-67-2 was the same, except that the oxide used was
of 100-200 mesh. Both runs gave good looking, regularly coated tapes
at coating speeds of 4,5,6 and 7 feet per minute, except where the
Webril tape presented ragged edges. The build varied from 1.2 to 1.8
mils, and curling of the coated cape edges was not very pronounced,
probably due to the fact that the temperature of the oven did not exceed
60°C. Only at 7 ft/min was the coated tape slightly damp; in general, the
coatings were smooth and uniform. Extremely small pinholes showed when
a strong light was shined at the tapes; however, they may have been coated
over with a very thin film of polysulfone:oxide. The resistance to
electrolyte flow of samples from all coating speeds was low and decreased
with increasing speed of coating. Although the resistivity values were
low, they were not so low as to indicate excessively rapid KOH passage.
Again, this material seems well suited to our application.
As requested by the contract, the ability to supply one-foot
wide battery separators must be demonstrated. To this aim the coating
tower used to coat 3 inch wide tape was modified to accept 12 inch wide
tape; new, larger and improved dip and coagulating tanks were constructed,
together with longer driving and take up rollers. The first run was
made using formulation C58-70-4 (3:1 ziiconia: PS, 1% Alkaterge, 10%
extra DMAC) and 12 inch wide Webril 1403, which was about 2 mils thick
and exhibited the nonuniform; tgeak characteristics discussed in the
previous quarter. With this arrangement, the operation was performed
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at about 6 feet per minute, at a temperature ranging from 40 to 60°C in
the oven. About 100 feet of tape was run through. General observations
on this run are as follows: a. the coating mixture was too thin; b. the
heater temperature gave best results at around 40°C; c. some shrinking
was noticed due to the open weave of the tape and to the high oven heat.
A second run employing the same 12 inch wide Webril fabric
was made using a thicker mix (C58-70-5); adjustments were made on the
tower to correct misalignments. This run was considerably better than
the first one. At a coating speed of 5 feet per minute and at 35-40°C
in the oven, tine tape coated uniformly and showed no pinholes. Difficulties
were encountered in the taking up of the coated tape, which was wrinkled
but otherwise looked promising. After about 50 feet, pinholes appeared,
due to severe stretching of the coated tape over the top sheaves. This
problem was cured by over driving the pay-off rollers. In general, this
run showed the feasibility of coating one foot wide tape. Stronger and
more uniform tape would cure most of the problems mentioned above.
Work on modification of the coating tower to accept one foot
wide tape is continuing. During this work, a material balance of the
coating mixture was determined for two runs. These showed that, while
the solids content decreased only slightly as a function of coating time,
the oxide: polysulphone ratio decreased considerably, showing that the
oxide was used up at a faster rate than the polysulphone.
3. Sterilization tests on new candidate substrates and coated tapes
Coating runs C58-67-1, C58-67-2 and C58-61-2 (see monthly Report
No. 21) were distinguished by having been made with a Webril tape which
is about 5 mils thick and which exhibits a much more uniform and stronger
structure than the Webril tape used up to now. The electrical and
mechanical properties obtained using this kind of tape were satisfactory
and reproducible. As commonly found, the sterilization cycle lowered
the resistivity of the samples, rather considerably in some cases. How-
ever, it was noticed that the weight :_osses were relatively high. It is
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postulated that the increased thickness of tape and coating (to an
average value of 7 mils) may have trapped some of the solvent, which
then subverted the original weight measurements.
Some new materials were received and tested. From Union Carbide
we received very small samples of their new and very expensive yttria-
stabilized zirconia cloth, with nominal thickness.s of about 15 mil.
These samples appeared to be relatively weak mechanically, and to be
filled with some powder or dust. They were immersed in 40% KOH for
2 days; when they were taken out of the caustic for resistivity measure-
ments, it was found that their mechanical strength had diminished
further, to the point where they could 'hardly be handled. Only one
sample could be tested for resistivity; all the others had practically
disintegrated in the KOH. As it appears now, this material is too
fragile for our purposes.
Two 9 x 11 inch sheets of DALKASEP film were received from
Dewey and Almy; they intend to manufacture it in continuous lengths if
it becomes commercial. This material consists of microporous polyethylene
designed for use as separator in alkaline batteries, particularly silver-
zinc batteries.
Samples of DALKASEP underwent standard sterilization cycle
(60 hours in 40% KOH at 135°C). At the end of the test the samples had
shrunk drastically, losing about 80% cf their original area. Conversely,
their thickness had increased two fold. Their resistivity before
cycling was too low to measure on our meter; after cycling the samples
were too small to fit our resistivity cell. In addition, the weight
of the samples after sterilization had increased fifty percent, probably
indicating a chemical reaction with the caustic. These results indicate
that this material does not lend itself to use as battery zeparator
material which must be able to withstand a temperature of 135°C in 40%
KOH. Dewey and Almy later confirmed that 135% "approaches what we
consider to be the upper temperature limit for DALKASEP."
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Two polypropylene substrates, Pellon styles 2530 and FT 2140,
were tested for resistivity and electrolyte flow properties. The
thickness of the former was 6 mils on the average, while the latter
averaged 7 mils. They both looked uniform in their felt-like structure,
but were soft and compressible. They were, however, recommended in a
recent Texas Instrument Co. report for use as sterilizable battery
separators.
Tape style 2530 offered negligible resistance to KOH passage
(pH 14 in 1.5 minutes) and exhibited a specific resistivity of 10 ohm-
inch. Style FT 2140 tape had higher electrolyte flow resistance (pH
14 in 25 minutes, 13.5 in 2.5 minutes) and a specific resistivity of
23 ohm-ir ►ch. Several rolls both 3 and 12 inches wide have been ordered
from the Pellon Company.
4. Addition of free zirconia to sterilizing system
The possibility of reducing the weight loss, sustained by
battery separator material as a result of sterilization, by introducing
free zirconia in the Teflon cells used in the sterilization cycle, was
investigated. Fresh samples of runs which had already been tested were
sterilized in the presence of 0.01 g of 20-50 mesh zirconia (approximately
10% of the weight of zirconia in the sample). The results indicated that,
except in one case, the presence of frLs zirconia lowered the weight
loss induced by the sterilization step; in three cases the specific
resistivity values also declined, and in all cases the resistivity was
below the limit imposed by the contract.
This effect was further evaluated by increasing the amount of
added zirconia to 0.03 g (approximately 30% of total zirconia present).
It was shown that a larger amount of free zirconia in the cells further
reduces the sterilization weight loss. Again, except in one case, the
resistivity values also decrease as the amount of zirconia in the cell
increases. On the basis of these results it seems advisable to add
free zirconia, in the ratio of between 10 and 30% of the weight of the
battery separator composite, to the battery cell in order to reduce both
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dissolution of zirconia from the separator into the electrolyte and the
resistivity of the separator.
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	 5. Separator aging in 40% KOH containing silver ions
In order to find out whether long term aging in concentrated
-	 caustic in the presence of silver affected the electrical and physical
d
properties of battery separators, the samples of Webril-polysulfone-
zirconia separators used to determine silver ion transfer were tested
for resistivity and for physical damage. They had been aged for about
three months. The resistivity tests were carried out as follows: the
first series was run on the samples as they came out of the original KOH
solution, and the second series was run after the samples had been
vigorously washed in water (in an attempt to remove the black silver
or silver oxide precipitate) and then had been resoaked in KOH before
testing.
In 75% of the cases the long term aging in KOH with silver
present has no deleterious effect on the specific resistivity of the
untreated samples. However, the simple act of washing the samples with
water resulted in greatly increased specific resistivity values. It is
conceivable that the washing action shook loose the solid particles of
the silver or silver oxide precipitate, which then redistributed them-
selves and obstructed the pores of the separator composite. No obvious
correlations could be made between the resistivity values and the different
formulations and coating speeds of the samples. No physical changes were
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observed.
6. Measurement of zincate ion diffusion
The apparatus consisted of two Teflon cups (2" I.D., 100 cc.)
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	 each with a flat edge. A hole (1/2" diam.) was drilled in each flat
face, a square of separator to be tested placed between the holes, and
a the cups clamped together. One cup was filled with 40% KOH and the
other cup filled with 40% KOH containing 40 p.p.m. zinc ions. 1 cc
samples were taken at regular intervals and the zinc concentration
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measured by flame absorption spectroscopy. The samples tested had
previously been tested for silver ion diffusion. The results showed
that the rate of zincate ion diffusion was of the same order as silver
ion iiffusion, with a maximum of 4.2 ppm after 150 hours.
In the case of silver ion permeability, a comparison between
values obtained by the Goddard Space Flight Center on Dewey and Almy
E-5114 membrane and values obtained on our separator specimens indicates
that the D. and A. membrane's permeability is three orders of magnitude
greater than that of our membranes (83 x 10-10 vs 70 x 10-13 moles/sec/
cm2).
M. FUTURE WORK
While new and promising polypropylene substrates will continue
to be tested, the main effort will be directed toward establishing a
coating procedure for the production of 12 inch wide coated tape
exhibiting resproducible and satisfactory sterilizable battery separator
properties.
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Separator development for a heat sterilizable battery

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