Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the application of electrochemically
active thixotropic paste onto suitable substrates for the preparation of
battery plates for electrochemical cells and batteries.
Canadian Patent application 225,095 and Canadian Patent 995,744
describe method and apparatus for which a thixotropic paste of cement-like
character is discharged at a predetermined volumetric flow rate onto a soft
porous grid, of expanded pure lead, for instance. In those applications, as
is characteristic elsewhere in the prior art, the paste is applied directly
to a grid. While this method has proved quite satisfactory, it does suffer
from the objection that it is extremely difficult to control certain paste
variables, such as paste density or thickness, principally because the grid
which is made of lead intefers with the operation of detection devices, such
as radiation sources. The previous methods wherein the paste was applied
directly to the grid substrate also required careful control to prevent loss
of paste by gravity drop through the interstices of the grid or by sticking
to a feed roll over which the grid was passed prior to introduction into the
compressing means.
It is a primary object of the subject invention to provide
method and apparstus by which the paste variables such as width, thickness
and density are carefully monitored and controlled prior to bringing the
paste into intimate contact with the grid substrate. It is a separate object
to provide a transport medium, other than the grid itself, for delivering the
paste in the form of a ribbon of controlled dimensions and characteristics
prior to compression into the grid substrate. It is a further object to trans-
fer the ribbon of paste to the substrate and thereafter strip the web of
material from the pasted substrate so that the paste is transferred essentially
completely to the substrate without leaving residual paste on the stripped
web.
Briefly described, the method for applying a thixotropic electro-
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chemically active paste onto a continuous length of grid substrate having
interstitial pores therein, includes the steps of (a) positioning a pair of
juxtaposed compressing means, such as rolls, a predetermined distance apart;
(b) passing a web of flexible material which has been rendered non-hygroscopic
through the nip of the compressing means; (c) discharging a ribbon of the
paste at a predetermined volumetric flow rate onto one side of the web from
a discharge means positioned in close proximity to one side of the web and
located upstream and remote from the nip of the compressing means; (d) pass-
ing a continuous length of the grid substrate through the nip of the
compressing means in a manner such that the ribbon of paste carried by the
web comes into intimate contact with the grid substrate and is compressed
into its interstitial pores; and (e) stripping the web from the pasted
substrate without leaving substantial residue of paste adhered to the
stripped web.
The apparatus for performing the method of the invention includes
the pair of juxtaposed compressing means discussed above for reception of
the continuous web of flexible material together with the continuous length
of grid substrate, the means for discharging the thixotropic paste, and
the means for delivering the pasted web through the nip of the compressing
means. In the preferred embodiments, the apparatus further includes means
situated intermediate the discharge means and the compressing means for
detecting a paste characteristic and generating a signal responsive thereto,
together with means for controlling said paste characteristic in response to
said signal.
The invention will be more particularly set forth in alternate
embodiments by reference to the accompanying drawings, wherein like numeral
designate like parts, and in which:
Figure 1 depicts in perspective the pasting system including means
for detecting paste variables;
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Figure 2 is a modification of Figure 1 leaving off many of
the details of Figure 1 and focusing on the positioning of the discharge
means relative to the rolls, and without the use of a detection device; and
Figure 3 is a block flow diagram illustrating the detection
and control feedback system of Figure 1.
While the invention will be particularly described with
reference to manufacture of electrode plates for lead-acid electrochemical
cells, it is to be understood that the invention applies to the preparation
of other types of electrode plates utilizing thixotropic pastes.
Referring to Figure 1, there is shown an extruding device
comprising discharge nozzle 10 having a substantially circular inlet 12 con-
nected to the output of a positive tisplacement metering pump 14, which in
turn preferably forms a portion of a paste circulation loop (not shown) more
particularly described in Applicants' commonly assigned United States patent
number 3,814,628. Briefly, as disclosed in that patent, the paste circulation
loop is substantially closed to preclude air degradation of the paste and to
allow the circulating paste to be agitated and thoroughly mixed prior to
discharge through the nozzle 10. Because of the particular consistency of
the paste, it is preferred to employ a positive displacement pump 14, such
as of the Archimedes' screw type, which initiates and diverts from the closed
loop a predetermined volumetric flow rate of paste to the discharge means 10.
To enhance the consistency of the paste discharged from the nozzle, it is
preferred to the amount of paste diverted from the loop to the discharge
nozzle 10 through metering pump 14 be only a small proportion of the a unt
of paste allowed to recirculate in the loop, e.g., from about l/15th to about
1/50th. The positive displacement pump 14 is preferably driven by a variable
speed motor 33 as also shown in the referenced patent.
The paste employed is highly viscous, thixotropic, resembling a
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slurry-like suspension of discrete crystalline finely divided particles, such
as lead-oxide suspended in aqueous vehicle. Such paste may rheologically
resemble "quick clays", red mortar, cement or the like. The finely divided
particles of active material are generally macro-molecular in size, rather
than colloidal. The paste materials, by virtue of their thixotropic character,
are essentially non-plastic and behave ]ike non-Newtonian fluids insomuch as
the viscosity of the paste varies with its rate of flow in the closed loop
network. Because of this particular nature of the paste~ the processing
parameters, particularly residence time, are critical to avoid "setting up"
of the paste somewhere along the system. Specific examples of paste com-
positions which have the above characteristics include negative and positive
lead-acid battery pastes which comprise a major proportion of oxides of
lead, e.g., litharge, suspended in a minor proportion of a vehicle, e.g.,
water. Oftentimes additional paste constituents, such as expanders,
elemental lead and Pb304 may be present. It is preferred that the battery
paste contain no entrained fluids which may have the effect of degrading
the paste by oxidation or other mechanism.
The discharge nozzle 10 delivers the thixotropic paste to a
transport medium (other than the grid) consisting of a flexible web of
material 16. The web material may be composed of one or more layers of
paper, such as of cellulosic base, delivered from feed roll 18. The flexible
web is rendered non-hygroscopic to prevent subsequent preferential adherence
of the paste to the flexible web once the substrate has been pasted. The
web may be rendered non-hygroscopic by various means, such as by passing it
over one or more wetting rolls 20 which are rotatable in trough 22 supplied
with a source of fluid compatible with the paste, and which renders the paper
or web non-hygroscopic. Such a fluid is preferably aqueous, such as distilled
water.
The nozzle 10 delivers the paste in a ribbon-like configuration 23
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from a substantially rectangular slot 24 onto the flexible web 16. Prefer-
ably the dischsrge nozzle has a flow cross-section and internal surface con-
figuration to permit the viscous thixotropic paste to be discharged at a
predetermined volumetric flow rate without substantial "channeling" e.g.,
partial plugging, of the nozzle. A flared surface with increase in cross-
section toward the discharge slit has been found suitable. The outlet face
of the nozzle is in close proximity to the moving web of material, meaning
that it is either in contact with the web or spaced slightly above it. If
the outlet face of the nozzle is positioned essentially parallel to the top
surface of the moving web, then the slot 24 functions in a manner similar
to a doctor blade, to deliver the ribbon of paste at the desired width and
thickness.
It is preferred that a support 30 be provided beneath the
flexible web and nozzle to insure that the thin ribbon of paste which is
metered onto the flexible web is of uniform dimensions. As shown in Figure
2, this support member may be provided directly by one of the compressing
mesns or rolls 26. In this embodiment, the surface of the nozzle in closest
relation to the support is curved at 28 to correspond to the curvature of
the support roll.
As shown in the preferred embodiment of Figure 1, the dis-
charge means 10 is spaced a considerable distance upstream from the compress-
ing rolls 26 and 27 and intermediate therebetween is positioned means for
detecting a paste characteristic and generating a signal responsive thereto,
which signal may be used to control a paste variable. In this respect, a
radiation source 30 talso provided as the mechanical support in this case)
is employed on one side of the pasted web, and on the other side opposite
the radiation source there is positioned a detector 32 which may be the
customary ionization chamber. The source or emitter 30 and detector 32 may
comprise a conventional radioactive transmission system such as the trans-
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mission beta gauge using a Strontium-90 or Krypton-85 source. Beta radiation
emitted by the source is attenuated by the measured layer of thixotropic
paste, and depending upon a characteristic of the paste such as thickness or
density, a certain portion of the beta radiation completely penetrates the
paste and produces ions in the ionization chamber of the detector 32. Thus,
as the weight of the measured paste is increased, the number of beta
particles reaching the ionization chamber will be decreased. Conversely,
decreasing the weight of the measured paste will result in a greater number
of beta particles reaching the ionization chamber.
Since the paste composition is generally homogeneous, and further
since the paper material does not interfere appreciably with transmission
of the radioactive particles, the output signal from the detector device
can be used to monitor either thickness or density of the paste. Beta
transmission gauges are well known in the art, and more details in this
respect may be found from a number of sources, including Bulletin 6005-TAl
of the LFE Corporation, Waltham, Massachusetts 02154. Of particular interest
in that bulletin is the absorption curve of Strontium-90, plotting the percent
of total detected radiation versus the weight per unit area of the measured
material.
As aforementioned, the monitoring signal produced at the detector
32 may be used to control a paste variable. Referring to Figure 3, the
signal is employed in a paste thickness control system. Based on the
volumetric flow rate of paste which is desired for producing a pasted
substrate of the desired density and geometry, a reference paste thickness
tl is selected. Detector 32 will "see" an actual paste thickness t2 passing
by the detector. A control panel (not shown) of conventional design can be
used to compare the reference paste thickness tl with the actual paste
thickness t2, and on this basis provide a correction signal to the metering
pump motor 33 to either slow up or speed up the discharge rate of the
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metering pump 14. mus, the paste flow rate will be brought into conformance
with the desired volumetric flow rate. The controller sub-system in known
manner may generally employ solid state electronics with built in control to
compensate for any delay in response time and backlash compensation. In
this manner the controller can maintain a certain tolerance level on the
output of the paste from nozzle 10. Control systems are commercially
available which are suitable for this purpose, and include systems offered
by Industrial Nucleonics Corporation of Columbus, Ohio 43202, and particular-
ly their Accuray System (a registered trademark of Industrial Nucleonics).
The monitoring and control system above described employs
transmission of radioactive particles through the paste to be measured in
which the particles are absorbed on the reverse side of the paste. It is
contemplated that various electromagnetic radiation sources other than the
transmission type could be employed. For instance, one could employ a
single combined source and detector operating on the principle of
reflectivity. In the latterinstance, the emitter could bounce waves similar
to sonar or radar off of the traversing paste, and based on the absorption of
the reflected waves monitor a paste characteristic from which a control
signal indicative thereof is produced. If different paste characteristics
other than thickness and density are desired to be measured, other
radiation sources would be effective. For instance, infrared rays could be
used to measure the moisture content of the paste. Those skilled in the art
will appreciate the various types of devices which could be utilized to
measure different paste variables and produce a signal responsive to that
measurement for control, as desired.
In the event a characteristic of lead or other high shield paste
is being measured, depending on the thickness of paste ribbon alpha or beta
particle radiation may not possesssufficient energy to penetrate the ribbon
and reach a detection device. In this event electromagnetic radiation of
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high energy form, such as X or gamma rays, may be employed. Alternatively
in such cases where the paste is of a character and thickness to act as
a radiation shield, the physical dimensions of the paste (width and thickness)
can be monitored and from this the density of the paste can be determined,
assuming knowledge of the paste constituents together with its homogeneity.
Edge control can be employed in this regard using, for instance, electric
eyes positioned respectively on the underside of the pasted web and
opposite the edge of the ribbon from which a shadow graph is produced and
detected. In this manner any impermissible deviations in width or thickness
of the paste will be detected.
While automatic control has been discussed, the emitted signal
could be used to power a visual readout device, from which an operator could
manually adjust appropriate hardware in the system, such as the motor driven
output flow rate of the metering pump, or the speed of a variable speed motor
driving rolls 26 and 27, or the gap between such rolls, etc.
After the ribbon of paste has been discharged onto the flexible web
and either moved past the monitoring devices (Figure 1) or not (Figure 2),
the thus pasted web is introduced into the nip of a compressing means, shown
as vertically spaced compressing rolls 26 and 27 driven by meshed gears 31,
35. Simultaneous to the introduction of the pasted web to the nip of the
rolls is the cocurrent introduction of the grid substrate 34, which may be
supplied from a roll (not shown) over a supply roller or guide 36 and a
sandwiching lower flexible web 44, supplied from roll 46 over water roller
48. The grid substrate 34 is preferably made of a soft, flexible and malle-
able material which contains interstitial pores, which may be foraminous
cavities or voids which extend completely through the thickness of the grid,
for receipt of the paste therein. An expanded mesh grid has been found
particularly suitable for this purpose. It is preferred that the presence
in the substrate of any hydrogen or oxygen overvoltage reducing impurities,
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such as antimony, be minimized to preclude passivation of the plate during
cycling of an electrochemical cell containing such plates. It i9 preferred
that relatively pure lead grids of at least 99.9 percent and more preferably
of at least 99.99 percent purity of lead be utilized. However, impurities
(e.g., calcium or tin) aggregating greater than 0.1 percent which do not
have the effect of substantially reducing the hydrogen overvoltage of the
resultant plate may be utilized, even though the plate in this instance
loses flexibility.
As the grid substrate is passed through the nip of the compressing
means it intimately encounters the ribbon of paste carried by the flexible
web. The roll gap is chosen so that the ribbon of paste is necessarily
forced into the interstitial pores of the substrate. In a preferred
embodiment, the roll gap is chosen sufficiently low so as to place the pasted
substrate in tension and actually drive the pasted substrate and web through
the nip and toward a pair of paper stripping rolls 38, 40.
The paste has greater affinity for the substrate than for the
treated paper web, thereby permitting stripping of the paper from the
pasted substrate without leaving substantial amounts of residual paste
adhered to the stripped web. In this respect, the non-hygroscopic character
of the paper is essential in preventing adherence of the paste to the
stripped paper web.
~ ecause the paste is accurately metered onto the flexible web at
a predetermined volumetric flow rate, and because the speed of the grid
substrate and pasted web are coordinated, the correct amount of paste will
be forced into the grid interstices to produce the finished pasted grid at
42. The width and thickness of the plates, as well as the resultant density
of the paste is thus accurately de*ermined. The continuous length of pasted
grid may then be subjected to subsequent operations of a conventional nature,
including slitting, trimming, cutting and sizing for fabrication of the final
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electrode structure. The resultant plate will be suitable for use in
parallel stacked plate configurations or spirally wound configurations, for
instance.
It should be understood that the invention is capable of a variety
of modifications and variations which will become apparent to those skilled
in the art upon a reading of the specification. Such modifications are
intended to be encompassed within the scope and sphere of the invention
as defined by the appended claims. For instance, the illustrated embodiments
have shown the application of paste to the top flexible web and then deliver-
ing this pasted web about the surface of a compressing roll. The pasted
web is then made to travel in an opposite direction from which it was
originally pasted. Clearly, the paste could be applied to the top surface
of the wetted bottom paper layer 44 prior to introduction into the nip of
the rolls 26 and 27. Also, while two paper layers have been shown to
sandwich the grid substrate and assist in positioning the paste within the
grid and to prevent adherence to the rolls, one of the paper layers (the
one which does not receive the ribbon of paste) could be omitted by
providing that the associated roll is rendered non-adherent with respect to
the paste. This can be accomplished by providing a porous surface for the
roll and internally pressurizing it with water so that the surface of such
roll is rendered non-hygroscopic.
As an additional alternative, paste can be applied to both webs
and compressed simultaneously into the grid from its top and bottom sides
as the pasted webs engage the grid and pass through the compressing means.
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