CA1056905A - Method and apparatus for pasting battery plates using compression - Google Patents

Abstract

Abstract of the Disclosure Thixotropic battery paste is discharged at a substantially predetermined volumetric flow rate onto a flexible web transport medium and thereafter introduced into the nip of a compressing means, e.g., a pair of rolls, where the paste is transferred by compression into interstitial pores of a grid substrate. Means are provided intermediate the paste discharge point and the nip of the rolls for monitoring and controlling paste variables such as density, thickness and volumetric flow rate. The flexible web is stripped from the thus pasted grid substrate without leaving substantial amounts of residual paste adhered to the stripped web.

Description

1056;905 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-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;

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 ~, ," , 105~905 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 - - -lOS~;90S

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-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 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 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, - - .

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 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.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for applying a thixotropic electrochemically active paste onto a continuous length of grid substrate having interstitial pores, comprising: positioning a pair of juxtaposed compressing means a predetermin-ed distance apart; passing a web of flexible material which has been rendered non-hygroscopic through the nip of said compressing means; discharging a uniform ribbon of paste at a substantially predetermined volumetric flow rate onto one side of said web from a discharge means positioned in close proximity to said one side of the web and located upstream and remote from the nip of the compressing means; passing 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 to form a pasted substrate; and stripping the web from the pasted substrate so that the paste is transferred to the substrate without leaving substantial amounts of residual paste adhered to the stripped web.

2. The method of Claim 1 wherein a second web of flexible material is passed through the nip of said compressing means on the opposite side of the grid substrate from said web of flexible material upon which the ribbon of paste has been discharged.

3. The method of Claim 1 including the following additional step:
positioning detector means intermediate the point where said ribbon of paste is discharged onto said web and the nip of said compressing means, said detector sensing a paste characteristic and generating a signal responsive thereto.

4. The method of Claim 3 wherein said signal is fed to a control mechanism which controls the paste characteristic within a given pre-determined tolerance range.

5. The method of Claim 1 wherein said web of flexible material prior to receiving the ribbon of paste thereon is passed over at least one wetting roll so as to imbibe water on the surface of said web.

6. Apparatus for the production of continuous lengths of substrates having interstitial pores pasted with thixotropic pastes for use in electrochemical devices, comprising: a pair of juxtaposed compressing means spaced a predetermined distance apart and defining a nip therebetween for reception of a continuous length of the substrate, and also for reception of at least one continuous web of flexible material which is or has been rendered non-hygroscopic and which is supplied to the nip interposed between the compressing means and the substrate; means positioned upstream of the compressing means for discharging thixotropic paste onto the web of flexible material at a predetermined volumetric flow rate in the from of a ribbon deposit; and means for delivering the thus pasted web through the nip of the compressing means whereby the paste is transferred from the web to the interstitial pores of the substrate.

7. The apparatus of Claim 6 including means for stripping the web from the pasted substrate substantially without adherence of residual paste to the stripped web.

8. The apparatus of Claim 6 including means situated intermediate said discharge means and said compressing means for detecting a paste characteristic and generating a signal responsive thereto.

9. The apparatus of Claim 8 further including means operable by said signal to control said paste characteristic within a given predetermined tolerance range.

10. A battery pasting system, in combination including the apparatus of Claim 6 together with: a supply of said substrate having interstitial pores and made of lead having no substantial amounts of impurities which will have the effect of substantially reducing the hydrogen overvoltage of the pasted plate produced.

11. The battery pasting system of Claim 10 further including said continuous web of flexible material which has been rendered non-hygroscopic.

12. The battery pasting system of Claim 11 wherein said continuous web of flexible material is a thin sheet of flexible paper.

13. The battery pasting system of Claim 12 wherein said paper is cellulosic based and has been wetted with aqueous solution.