METHOD OF AND APPARATUS FOR INTRODUCING AN IMPREGNATING FLUID INTO A POROUS SUBSTRATE REGION

Abstract: A method for making an impregnated fuel cell edge seal utilizing specialized tooling and modifying the exemplary carbon/graphite impregnant dispersion, which might include, for example, 66 % carbon or graphite by weight, by adding to it a minor amount (e.g., 7 %) of a concentrated solution of ammonium hydroxide (e.g., about 58 % by weight of the NH4OH in water, equivalent to about 28-30 % by weight of NH3 gas added to water to make the solution) to the dispersion, increasing the pH of the impregnant up to a level of about of 10.5 to 11.5, improving the flow characteristics of the dispersion and decreasing the tendency of such a dispersion to solidify during the impregnation process. The modified impregnant is forced into the edge area of the electrode substrate (E.S.) of the fuel cell, producing an integral edge seal, using a specialized tool (10 or 10') having a rigid die (12) or a resiliently yieldable die (12') with a cooperating substantially rigid plunger (11 or 11'), which define between them an elongated chamber or channel (13 or 13') for the impregnant (30; see figs. 1-3 or 5). Such a tool (10 or 10') overcomes the effects of thixotropy, as the volume of the impregnant (30) is constrained by the volume of the chamber (13 or 13'), producing a seal which is uniform in density and dimensionally controlled by the shape of the channel (13), eliminating the varying shear forces of the prior art. The specialized tooling (10 or 10') can also be used to impregnate wetproofing material, particularly a fluorocarbon, into an electrode substrate to form corrosion resistant seals.

Description: Description Method of and Apparatus for Introducing an Impregnating Fluid into a Porous Substrate Region Technical Field The present invention relates to the introduction of impregnating substances into porous substrate regions in general, and more particularly to a method of forming an improved impregnated seal, especially an edge seal or a corrosion resistant coating or seal, in a porous electrode substrate of a fuel cell, using dispersions, for example, aqueous, highly thixotropic, non-Newtonian dispersions of graphite and carbon for the edge seals, or, for further example, aqueous dispersions of wetproofing materials such as, for example, fluorocarbons, for the corrosion resistant seal, using specialized tooling. The invention also relates to specialized tooling used to form, for example, such impregnated edge seals and corrosion resistant coatings or seals in porous electrode substrates to uniformly force the impregnant into the substrate with, for example, consistent shear forces in the impregnant throughout the seal area, overcoming the effects of thixotropy. Additionally, the present invention relates to the modification of the dispersions of graphite and carbon for the edge seals to maintain such disper¬ sions in an enhanced.liquid state. Background Art Fuel cell powerplants produce electric power by electrochemically combining a fuel and an oxidant in one or more electrochemical cells. The oxidant may be pure oxygen or a mixture of gases containing oxygen, such as air. The fuel may be hydrogen. Each fuel cell generally has electrodes to which the respective gases are supplied, including an anode electrode for the gaseous fuel and a cathode electrode for the gaseous oxidant, with the electrodes being provided in the form of porous substrates to be permeable to such gases. The cathode electrode is spaced from the anode electrode, and a matrix saturated with electrolyte (acid or alkaline) typically is disposed between the electrodes. A fuel cell electrolyte retention section is included to retain the electrolyte within the cell. Thus, in a fuel cell a matrix layer filled with electrolyte typically is sandwiched between a pair of electrodes, a cathode and an anode making up each pair. Each electrode comprises a substrate with a thin layer of catalyst disposed on the surface thereof facing the electrolyte. Each electrode substrate is constructed to permit a reactant gas (generally either air or hydrogen) to pass therethrough and contact the catalyst. This is the gas diffusion type of electrode. For further background information on such fuel cells, note, for example, assignee's U.S. Patents 4,738,872 of Messrs. Lee & Emanuelson issued April 19, 1988 and 4,938,942 of Messrs. Gorman, Breault, Donahue & Bose issued July 3, 1990, both entitled "Carbon-Graphite Component for an Electrochemical Cell and Method for Making the Component," the disclosures of which are incorporated herein by reference. A common characteristic of all fuel cells is the necessity for preventing leakage and inadvertent mixing of the reactant gases both within and externally of the cell. Since the electrode substrates (and certain other components of the fuel cell stack) are gas permeable or porous, means must be provided for preventing "in-plane" gas leakage through the edge regions of these substrates. Thus, in such fuel cells, it is necessary to form, for example, edge seals in the electrode substrates, either by densification or impregnation. For examples of some prior patents having to do with edge sealing by densification or impregnation of fuel cell components, note, for example, U.S. Patents 4,269,642 of Messrs. DeCasperis, Roethlein & Breault issued May 26, 1981 entitled "Method of Forming Densified Edge Seals for Fuel Cell Components," and 4,786,568 of Messrs. El ore & Roethlein issued Nov. 22, 1988 entitled "Electrode Substrate with Integral Edge Seal and Method of Forming the Same", respectively, with both patents being owned by the assignee hereof, the disclosures of which patents are incorporated herein by reference. Note also U.S. Patent 4,233,369 of Messrs. Breault, Roethlein & Congdon issued Nov. 11, 1980 entitled "Fuel Cell Cooler Assembly and Edge Seal Means Therefor" of a related company and its disclosure with respect to edge seal means. As disclosed therein, exemplary impregnants to form edge seals include impregnant dispersions made up of, for example, aqueous, highly thixotropic, non-Newtonian dispersions of graphite and carbon. Such edge seal areas, when properly impregnated and heat treated or otherwise dried, effectively contain the acid electrolyte of the fuel cell(s) within the confines of the fuel cell electrolyte retention section. Typically, in the prior art, the dispersions of graphite and carbon, which can contain up to, for example, sixty-six percent (66%) of carbon or graphite by weight, were forced into the edge regions of the porous electrode substrates using hydraulic pressure existing in the dispersion as was is being supplied to such regions. However, when the prior art method was used, it has been relatively difficult to dimensionally control the extent of the affected edge regions and such prior art method has not produced completely consistent results in other respects either. In particular, the prior art included the method disclosed in a commonly assigned U.S. patent No. 4,855,840 to Messrs. Elmore and Roethlein titled "Electrode Substrate with Integral Edge Seal and Method of Forming the Same" which method involved, as may be seen particularly in Figure 4 of this patent, hydraulically pumping the dispersion into a cavity or channel having the requisite shape of the desired seal and that cavity was held against the substrate in the area to be impregnated. Increasing the hydraulic system pressure forced the impregnant into the region to be sealed. Unfortunately, the viscosity of the impregnant also varies according to the shear forces applied to it. That is, at the entrance port into the cavity or channel, turbulence was high, while the viscosity could be as low as, for example, 2000 cP (centipoise) . At a channel location away from the entry port, the turbulence was minimal, while viscosity was measured at, for example, 12000 cP. These viscosity variations can produce erratic penetration and improper dimensions of the seal areas. Moreover, in such fuel cells, it is also often necessary to form corrosion resistant coatings or seals in the cathode substrates in the areas where air is introduced into the fuel cells. Were it not for these corrosion coatings or seals, it would be possible for air and electrolyte to mix in the presence of an electrical potential or voltage, causing corrosion to occur. Attempts have been made to form such corrosion resistant coatings using aqueous dispersions of wetproofing materials, such as, for example, fluorocarbons. Such corrosion resistant areas, when properly coated and dried, were able to effectively prevent corrosion from occurring at the coated areas. However, the aqueous dispersions of the fluorocarbons where merely dripped onto the external surfaces of the electrode substrate in the areas where the corrosion resistant coatings were desired and allowed to soak into the substrate. However, this takes a relatively long time and has produced non-uniform coatings or seals. The present invention in general is thus par¬ ticularly applicable to properly impregnating selected areas or regions of porous substrates or articles, especially of components of fuel cells, particularly electrode substrates, to either form edge seals using impregnant dispersions made up of, for example, aqueous or non-aqueous, highly thixotropic, non-Newtonian dispersions of graphite and carbon, or to form corrosion resistant seals using impregnant dispersions made up of aqueous or non-aqueous dispersions of wetproofing materials, particularly fluorocarbons. In contrast to the prior art, the method of the present invention using specialized tooling and related methodology, as explained more fully below, results in a more uniform seal, whether for an edge seal or for a corrosion resistant seal, with improved dimensional control. It also is more amenable to continuous production techniques, takes less time and using relatively simple tooling. Additionally, with respect to edge sealing, it has been found that, although the particle size of the dispersion was supposed to be below, for example, one micron, in fact due to agglomerations of the particles being formed, the effective "particle" size became much greater, increasing the tendency of the impregnant to solidify, diminishing the sealing effectiveness of the impregnating process. Another methodological aspect of the present invention, which uses a chemical additive for the impregnant, solves this prior art problem. Disclosure of the Invention Accordingly, it is an object of the present invention to provide a method of forming fuel cell edge seals or fuel cell corrosion resistant seals, which method is improved and achieves improved results relative to those obtainable in accordance with the heretofore used approaches. A further object of the present invention is to enhance the flow characteristics and reduce the solidification tendencies of the impregnant to be used to form the edge seals. It is a further object of the present invention to design specialized tooling for forming such seals, which tooling is suited for enhancing the ease and uniformity of the penetration of the impregnant into and its distribution throughout the affected regions of the substrate. It is still a further object to produce a more uniform impregnated fuel cell seal either both of the edge seal type or of the corrosion resistant type, with improved dimensional control, using methodology and tooling which is more amenable to continuous production techniques than prior art approaches. A concomitant object of the present invention is to develop both mechanical and chemical means capable of to accomplish the overall desired results contemplated by the invention, insofar as the invention relates to edge seals, including maintaining the aqueous dispersion in a liquid state using a pH increasing chemical additive, as well as to overcoming the effects of thixotropy using specialized but simple tooling. In keeping with these objects and others which will become apparent hereafter, one feature of the present invention resides in a method of impregnating a selected region of a porous substrate, this method comprising the steps of: (a) preparing an impregnant; (b) introducing a predetermined amount of the impregnant into a cavity of an impregnant injection die having an opening that opens onto a major surface of the die and is to be juxtaposed with the selected region of the substrate, said cavity being partially delimited by a bottom surface that is movable relative to said die to vary the volume of said cavity; (c) positioning said die into sealing contact with said substrate, with the opening of the cavity in juxtaposition with the selected region to be sealed; and (d) causing said movable bottom surface to move into said cavity to expel the impregnant under pressure out of said cavity and into said selected region of the substrate with the impregnant flowing into the interstices of the porous substrate region to form a seal therein. According to another aspect of the invention, there is provided a system for making an area seal in a porous substrate using an impregnant which is to be forced into the porous substrate in the region to be sealed to seal such region area, this system comprising: a platen; a die associated with said platen, said die having an opening in it and a movable surface contained within said opening, said opening and said movable surface defining an impregnant cavity into which the impregnant is placed prior to its being forced into the porous substrate; a work area located between said platen and said die of a size in which the substrate to have the region on it sealed can be positioned, said opening in said die extending out to the exterior surface of said die facing said work area forming an open top in the die; sealing means associated with said die extending at least around said open top for sealing the portion of said die about said open top to the substrate in the substrate area to be impregnated; and pressure means associated with said movable surface for applying pressure in the direction of said platen to said movable surface to move it through said cavity forcing out any impregnant in said cavity through said open top under pressure to impregnate the area of the substrate to be sealed with the impregnant. In further accord with the invention, the properties of the impregnant are improved by adding thereto a minor percentage of a concentrated solution of an alkaline or caustic substance, such as, for example, ammonium hydroxide. Experimental observations conducted as part of the inventior indicate that the tendency of, for example, aqueous carbon/graphite dispersions to solidify as the solids content exceeds, for example, sixty-six (66%) percent can be mitigated by adding a minor amount of a concentrated solution of an alkaline or caustic material, such as preferably ammonium hydroxide, to the dispersion to increase its pH level. Sufficient ammonium hydroxide should be added to increase the pH level, preferably up to about a range of 10.5 to 11.5. An optimized solution has been found to be up to about fifty-eight percent of ammonium hydroxide (e.g., about 58% by weight of the NH.OH in water, which is equivalent to about 28-30% by weight of the NH- gas added to water to make the aqueous solution) , with the concentrated solution being added being up to about seven percent (7%) by weight of the total mixed impregnant. The maintenance of the dispersion in a liquid state is very important if satisfactory substrate penetration and sealing impregnation is to occur, and the present invention with its modified impregnant maintains the dispersion in such an enhanced liquid state. The special tooling used in this invention to overcome the effect of thixotropy comprises, as an example, an extended, rigid die chamber or channel having a movable, rigid, internal surface, such as, for example, a bottom surface or plunger. After filling the rigid channel with the impregnant and placing it in sealing engagement with the substrate in the area where impregnant or densifying sealing is desired, the bottom of the channel is forced upward, uniformly forcing the impregnant into the substrate. As a result of the tooling aspects of the invention the shear forces on the impregnant in the die channel are substantially consistent and uniform throughout the seal area. In the invention the volume of the impregnant is constrained by the volume of the chamber, and consequently a seal is produced which is uniform in density and dimensionally controlled by the shape of the channel, rather than being distorted by varying shear forces, as occurred in the hydraulic pumping system of the prior art. Additionally, using a rigid die channel with a rigid, internal moving surface, such as a plunger, similar to that used to make edge seals, to impregnate the desired corrosion resistant sealing areas of the cathode substrate also produces a superior corrosion resistant seal, with the corrosion resistant seal being made in less time and with greater simplicity than the coatings of the prior art. Brief Description of the Drawings The foregoing and other features and advantages of the present invention will become more apparent from the following further description and its related drawings in which: Figure 1 is a simplified, partial, close-up, side, cross-sectional view of an exemplary embodiment of the specialized tooling used in the preferred embodiment of the present invention; Figure 2 is a similar side view of the specialized tooling of Figure 1, but in a subsequent stage of impregnation, with the plunger of the tooling having been pushed in against compression springs to begin the initial step of impregnation; Figure 3 is a similar side view of the specialized tooling of Figures 1 and 2, but in the final stage of impregnation, with the plunger of the tooling having been pushed completely up against the compression springs to perform the final step of impregnation; Figure 4 is a simplified, plan view of an exemplary cathode substrate with the areas of the edge seals and the corrosion resistant seals produced using the preferred methodology and specialized tooling of the present invention indicated in phantom line; and Figure 5 is a view similar to that of Figure 1 but showing a modified construction of the specialized tooling. Best Mode of Carrying out the Invention As can be seen in Figure 1, one exemplary embodiment of a specialized tooling constructed in accordance with the present invention includes a lower tooling assembly 10 underlying a stationary, combined vacuum chuck and platen 20, which carries on its underside the electrode substrate "E.S." The electrode substrate E.S. is maintained in place on the vacuum chuck 20 by means of vacuum pressure. Alternatively, the vacuum chuck aspect of the stationary platen 20 could be eliminated, in which case the electrode substrate E.S. could be placed and carried on the upper side of the lower tooling 10. The lower, impregnating tooling 10 includes a movable plunger 11 riding in an opening in a rigid die structure 12 with a set of compression springs 14 between them, holding or biasing them apart, but allowing for reciprocal, "up" and "down" movement of the plunger stem 11A within the die 12. The plunger 11 is rigid, with its stem 11A made of, for example, "Teflon", and its head 11B made of a suitable metal, such as, for example, aluminum. When the tool assembly 10 is in its initial disposition, i.e. with the plunger 11 fully withdrawn as shown in Figure 1, an elongated cavity or channel trough 13 of a certain volume is formed above the stem 11A of the plunger 11 within the opening in the die 12 into which the plunger stem 11A moves and into which channel 13 an impregnant 30 is eventually placed (see Fig. 2) . The channel 13 has an elongated, rectangularly configured top opening through which the impregnant 30 will ultimately be forced into the substrate region to be impregnated, and it in essence forms a moat across the upper side of the die 12. The elongated channel 13 has four, fixed sides and a movable bottom surface formed by the stem ending of the plunger 11. The compression springs 14 can be regular springs as illustrated or be in other forms, such as, for example, elastomeric springs, or a combination of the two. Additionally, by including threaded bolts extending through the body of the die 12 and threaded into the head 11A of the plunger 11, the plunger 11 can be preloaded to a desired initial pressure by screwing down the bolts, thereby lessening the volume of the impregnant chamber 13 and increasing the preloaded pressure on the plunger 11 tending to overcome some of the biasing pressure of the spring members 14. The spring members 14 and the threaded, preloading bolts are provided between the die 12 and the plunger 11 in a spaced series positioned along the length of the elongated plunger 11. A series of "O" type, surrounding rings 15 is included around the stem 11A of the plunger 11 interfacing with the sides 12A of the die opening to provide a seal, preventing loss of the impregnant down about the plunger stem 11A. An additional series of "O" type rings 16 is provided on the upper side of the die 12 extending peripherally about the die opening for sealing the interface with the underside of the substrate E.S. when the die 12 is pressed up against the substrate E.S. , working against the fixed platen 20. This second set of sealing rings 16 prevents the sideward escape of the impregnant 30 when the die 12 is pushed up against the underside of the substrate E.S. and the plunger 11 is forced upward, i.e. when the plunger 11 and the die 12 are in the dispositions of Figures 2 and 3. The electrode substrate E. . is typically rectangular in plan view (note Fig. 4) and the plunger 11 and die 12 are longitudinally extended along and parallel to the side edges of the substrate E.S. to be sealed, defining the extended channel or trough 13 for the impregnant 30. In the case of a typical cathode substrate C.S. illustrated in Figure 4, which substrate C.S. will need four regions 1 to 4 to be edge sealed, there usually will be four orthogonal, abutting plunger/die sections 11/12, forming in combination a rectangular outline following the dimensions of the four edge areas 1 to 4 to be sealed. The upper, open parts of the cL .inels 13 are positioned sufficiently close together in the abutted plunger/die sections 11/12 that the edge seals formed are continuous, being interconnected about all four sides of the substrate C.S. in the regions 1 to 4. The dimensions of the open top of the channel 13 are comparable to the region(s) 1 to 4 to be sealed but typically a little less than them, because some side flow of the impregnant will occur during the forced impregnation steps. Typically, the edge seal regions 1 and 2 that are parallel to ribs 5 of the cathode substrate C.S. are wider than the two edge seal regions 3 and 4 that are perpendicular to the ribs 5. For a typical cathode electrode C.S., two corrosion resistant seal regions 6 and 7 are included. In the case of a typical anode substrate (not illustrated) , usually only two opposed, parallel side edge regions are to be edge sealed (comparable to regions 1 and 2 of the cathode substrate C.S.), in which case just two longitudinally elongated, separate, spaced, parallel plunger/die sections 11/12 are used. Typically, no corrosion resistant seals are needed for the anode substrate. Exemplary dimensions for the width of the impregnant channel or trough 13 to produce edge seals in a cathode substrate C.S. for the wider regions 1 and 2 and for the narrower regions 3 and 4 are about one and three-tenths inches to about one and eighth-tenths inches (1.3" - 1.8"), and about a half inch to about seven-tenths of an inch (0.5" - 0.7"), respectively. Exemplary dimensions for the width of the impregnant channel or trough 13 to produce edge seals in an anode substrate for the regions comparable to the cathode regions l and 2 is about one and one-tenths inches minimum to about one and seven-tenths inches maximum (1.1" min. to 1.7" max.) . When the tooling 10 is used to form the corrosion resistant area coatings or seals in the areas 6 and 7 on the cathode substrate C.S. of Figure 4, a typical width of the impregnant channel 13 would be about one and a quarter inches (1.25") . Of course, these width dimensions are merely exemplary and, of course, are subject to much variation. The extended lengths of the plunger 11, the die 12 and hence the defined channel trough 13 will depend on the size of the fuel cell electrode substrates E.S. and in particular the lengths of the regions to be sealed by the tool assembly 10. However, an exemplary size of a cathode die, made up of four, orthogonal die sections 12, is approximately forty-five inches by approximately forty-five inches (45"x45") in plan view. It should also be noted that the drawings are generalized, simplified ones, not necessarily repre¬ senting the actual relative sizes of the components of the specialized tooling. Thus, for example, the impregnant chamber 13 may be many times longer in its height or depth (viewed in the perspective of Figs. 1-3) than it is wide. In non-mechanized operation, an operator initially fills up the impregnant channels 13 defined by the various plunger/die sections 11/12 by pouring in the desired impregnant, preferably using a metered container. Alternatively, mechanized, traveling flow lines with metered or timed pumps could be used. When the combined vacuum chuck/platen 20 of Figures 1-3 is being used, the lower assembly tool 10 and/or the platen 20 are moved relative to one another until the impregnating tool 10 is appropriately positioned under the substrate E.S. If the vacuum chuck aspect is not being used, the substrate E.S. is appropriately positioned on top of the die 12. In essence, a work area between the die 12 the platen 20 exists having a size to accommodate the fuel cell component, typically an electrode substrate E.S. , having one or more regions to be sealed. In speaking of "regions" to be sealed, it should be understood that a three dimensional volume is actually being considered, namely the volume underlying the surface areas of the substrate E.S. through which the impregnant is to be forced, and the seal will typically extend from the surface area on the side directly exposed to the open top of the die 12 all the way through to the opposite side. As illustrated in Figure 2, pressure is applied to the head 11B of the plunger 11, driving it upwards and forcing the impregnant 30 out of the channel 13 and into the porous substrate E.S, with the quantity of the impregnant thusly introduced into the substrate E.S. being indicated at 31. The driving pressure can be applied to the head 11B of the plunger 11 by suitable mechanical, hydraulic and/or pneumatic means (note lower directional arrows in Figure 2 representing the upwardly directed, initial forces on the head 11B of the plunger 11) . In Figure 3 additional force has been applied to the plunger 11, forcing it further upward against the force of the compression springs 14 (note lower directional arrows in Figure 3 representing the upwardly directed, now greater, forces on the head 11B of the plunger 11) until it has completed its stroke. This further movement in turn forces the rest of the liquid impregnant 30 previously contained in the channel 13, or at least most of it, into the body of the substrate E.S, to form the impregnant quantity 31. For example, an air inflatable bag could be used to apply a final pressure (Fig. 3) of, for example, thirty pounds per square inch (30 psi) , with the plunger 11 having been initially pre-loaded to a pressure of, for example, twenty (20) psi. After the impregnant has been added to the substrate E.S., the substrate E.S. is dried, preferably with heating, to remove the liquid components of the impregnant quantity 31 from the substrate E.S., leaving behind a dry edge seal forming an effective edge seal in the substrate E.S. Turning now to Figure 5 of the drawing, it may be seen that it depicts an alternative construction of the tooling of the present invention that is similar to that discussed above in conjunction with Figures 1 to 3 of the drawing that the same reference numerals, merely supplemented with primes where needed, have been used to identify corresponding parts therein. The assembly 10', consisting of the die 12' and the plunger 11', is shown therein in a situation occurring basically between those depicted in Figures 1 and 2, that is, after the channel 13' has been filled to the desired extent with the metered amount of the impregnant 30 but before the assembly 10* has been brought toward and into contact with the electrode substrate E.S. In this instance, the material of the die 12' , rather than being rigid, is elastically yieldable or elastomeric so that it can be compressed to occupy only a fraction of its original volume that is illustrated in Figure 5. Advantageously, this die material is of the foamed type, be it foamed rubber, foamed latex, or a similar foamed elastomeric material. Preferably, this material is of the so-called closed-cell variety, i.e. it includes a great number of relatively minuscule "air bubbles" or voids which do not communicate with one another. In this case, at least some of the externally or internally exposed zones of the die 12' (i.e. those facing the substrate E.S., various portions of the plunger 11', or the channel 13', as the case may be) need not necessarily be provided with smooth, fluid-impermeable surfaces or "skins", since the lack of communication between the "closed cells" or voids will prevent penetration of the impregnant 30 to any appreciable extent into the die 12' . However, it is advantageous for at least that external surface that faces the electrode substrate E.S. (and advantageously also that which faces the plunger head 11'A) to be provided with such a "skin" to improve contact at and thus minimize leakage through the respective interfaces. However, it is also possible to use an "open-cell" type of a foamed elastomeric material for the die 12*; in this case, it is mandatory to provide such "skins" at least on the zones facing the channel 13' and preferably all around to prevent penetration of the impregnant 30 into the die 12• . The plunger head 11*B in this instance is constructed as a rigid, preferably metallic (aluminum, for example) plate that constitutes a rigid support and force transmission and distribution medium for the elastically yieldable or compressible die 12* . In view of the inherent elasticity of the die 12' , the previously mentioned springs 14 are omitted in the construction of Figure 4. Also, inasmuch as the die material is elastomeric, it will seal the respective interfaces on its own, so that the O-ring seals 15 and 16 are not needed in, and thus are absent from, the construction of Figure 5. The plunger head or plate 11'B is shown to be provided with a recess that partially accommodates the die 12' (or separate portions thereof). The die 12' (or its portions) can be held in such a recess by friction and/or gravity alone, but if a greater degree of retention security is needed, it or they could be additionally held in the recess or in the respective associated recesses by an adhesive or the like. In the alternative, the recess or recesses could be omitted from the plunger plate 11'B and the die 12' (or its portions) could be held on the plate 11'B by adhesion alone. It may also be seen in Figure 5 that the plunger stem 11'A includes, in addition to a metal portion thereof that is constituted by a projection of the plunger head or plate 11*B, a contact portion 11'C which covers the metal portion in its entirety and thus separates the latter from the channel 13 ' , thus presenting a bottom surface bounding the channel 13• . The contact portion 11•C is preferably of a material exhibiting at least a small degree of elasticity, such as of hard rubber. Because of its compliancy, this material will not damage or exert undue pressure on the electrode substrate should it come into contact therewith. The contact portion 11'C is connected, for instance adhesively, to the metallic portion of the plunger stem 11'A so as to share in its movement. Of course, this expedient can also be used in the construction depicted in Figures 1 to 3 of the drawing, if so desired. The assembly 10* is to be used in a manner similar to that of the assembly or tooling 10, namely, the channel 13' is to be filled with a metered amount of the impregnant 30 first, resulting in the situation illustrated in Figure 5. Thereafter, the assembly 10' is brought into contact with the electrode substrate E.S. in the same manner as discussed above in conjunction with Figures 1 and 2 of the drawing, and pressure is exerted against the plunger head or plate 11*B, like it is in Figures 2 and 3. However, this time it is the die 12' (rather than any now absent springs) that resiliently yields or is compressed to enable the plunger stem 11*A to move toward the electrode substrate E.S. and thus to reduce the vertical dimension of the channel 13 * , forcing the impregnant 30 to enter and penetrate the affected region of the electrode substrate E.S. Of course, after the completion of such operation and termination of the action of the upwardly oriented pressure forces on the plunger head 11'B, and especially in the course of movement of the plunger head 11'B away from the substrate E.S., the resiliency of the material of the die 12' will cause the latter to change its configuration back to that illustrated in Figure 5, making the assembly ready for the next impregnation operation. A particular advantage of this latter approach, as compared to that of the first-discussed one, is that of an improved sealing effect at the various interfaces. This is especially true with respect to the interface between the die 12* and the electrode substrate E.S. where, because of its resiliency, the material of the die 12' will "go around the corner" at the outer edge of the substrate E.S., thus increasing the sealing effect at this area. However, Figure 5 also shows, in broken lines, a modified approach that is particularly useful when the electrode substrate E.S. is relatively thick. In this instance, a strip or component 17' of, for instance, hard rubber is connected, preferably adhesively, to the top surface of the die 12' in such a manner that its inward lateral surface, that that which is closer to the channel 13' , is in substantial registry with the outwardly facing lateral surface of the electrode substrate E.S. in the position illustrated in Figure 5. This registry is useful not only because it facilitates proper positioning of the electrode substrate E.S. and the tooling 10' relative to one another by acting as an abutment or registration aid, but also because the aforementioned inward and outwardly facing surfaces are in close proximity to one another during the actual impregnating operation, leaving substantially no gap therebetween, so that there is no void into which the material of the die 12* could penetrate on compression, which penetration would be detrimental in the case of relatively thick substrates. As mentioned before, the electrode substrate E.S. typically is made of a porous carbon material and is to be edge sealed using for the impregnant an aqueous, highly thixotropic, non-Newtonian di version of graphite and carbon. As noted, typically, dispersions of this kind can contain up to sixty-six percent (66%) of carbon or graphite by weight. However, in such high concentrations, such a dispersion has a tendency toward solidification. An exemplary impregnant 30, prior to modification by the addition of an alkaline or caustic solution to increase its pH in accordance with an aspect of the invention, would be an aqueous dispersion of less than one micron size particles of carbon black, graphite, silicon carbide, or other inorganic solids compatible with phosphoric acid at, for example, temperatures of 400 F, or mixtures of such solids. The final solids content of the impregnant generally can be in the range of, for example, about fifty percent (50%) to about seventy-four percent (74%) by weight, although about a sixty-six percent (66%) by weight is considered the most practical from a manufacturing standpoint. Additionally, solids content below about fifty-seven percent (57%) is considered outside the preferred range. A suitable binder, for example, a fluorocarbon, can be added if desired. Additionally, a stearic type of thickener may be added to optimize the impregnant rheology, in particular raising its viscosity, shear sensitivity and its pseudo-plasticity. Also, a dispersant of a suitable concentration may be necessary to minimize particle settling over long storage periods. However, in the invention the dispersion is modified by adding a minor amount of a concentrated solution of an alkaline or caustic material, such as, for example, ammonium hydroxide or sodium hydroxide, to the dispersion before it is impregnated into the electrode substrate E.S. , thereby enhancing the flow characteristics of the modified dispersion and reducing the solidification tendencies of the dispersion. An ammonium hydroxide or sodium hydroxide solution as an alkaline or caustic material increases the pH of the mixed dispersion, and, it is believed, causes the particles to become charged, repelling one another and thereby chemically breaking up any agglomerations of particles. Sufficient alkaline or caustic material, preferably ammonium hydroxide, should be added to increase the pH level up to about a range of 10.5 to 11.5. Additionally, whichever alkaline or caustic material is used, it should not leave any fuel cell "poisonous" residue after the impregnated substrate is dried. Ammonium hydroxide decomposes and is driven off in the drying stage, leaving no fuel cell "poisonous" residue, and is preferred. Testing has shown that up to about a seven percent (7%) by weight of about a fifty-eight percent (58%) by weight concentrated aqueous solution of ammonium hydroxide can be used and is preferred. However, it should be understood that such "58%" is based on the weight of the NH OH in water, which is equivalent to about twenty-eight to thirty percent (28% - 30%) by weight of NH 3 gas added to water to make the aqueous ammonium hydroxide solution, with the concentrated solution added being up to about seven percent (7%) by weight of the total mixed impregnant. It is noted that, to some degree, the maximum amount of ammonium hydroxide available is desired as needed to increase the pH level to about a range of 10.5 to 11.5. However, when an ammonium hydroxide solution is substantially greater than "58%", it can be difficult to handle and use. Also, adding substantially more than a "7%" solution to the impregnant can decrease the solids contents of the modified impregnant to too great an extent, decreasing its effectiveness in the sealing process. Thus, the methodology of the invention insofar as it applies to making edge seals preferably includes the preliminary step of adding the aforementioned minor amount of such a concentrated solution to the aqueous dispersion, thereby modifying it to have an increased pH level up to about a range of 10.5 to 11.5. The then so modified dispersion 30 is poured or pumped in metered fashion into each of the rigid channels 13 above the plunger stem 11A of each of the plunger/die sections 11/12 in the construction of Figure 1, or into the channel 13• of the construction of Figure 5. The rigid die 12, or the elastomeric die 12 ' , after the channel 13 or 13 • has been filled with the impregnant 30 and with its plunger 11 preferably unpressurized, is placed or driven up against the underside of the edge area of the substrate E.S. for the rigid die 12 to be sealed by the ring seals 16 to the substrate E.S. and for the elastomeric die 12 ' to seal the affected interface by itself due to its compliancy, or, as the case may be, the substrate E.S. itself is placed on top of the die 12 or 12' in sealed engagement therewith. In the following text, the operation of the specialized tooling will be discussed only in conjunction with the construction of Figures 1 to 3 to avoid unnecessary repetition; however, this discussion is equally applicable to that of Figure 5, with appropriate modifications. The head 11B of the plunger 11 of each section is pressurized or otherwise driven upward, causing the preferably modified impregnant 30 to be forced into the porous substrate E.S. , as shown in Figure 2. Because of the shape of the channel 13 and the use of an integral moving wall portion, namely the plunger head 11A, the lower tool assembly 10 uniformly forces the impregnant 30 into the substrate E.S. The head 11B of each section is further pressurized, or driven up until the desired amount of impregnation is reached, which typically is designed to occur when the plungers 11 have fully forced all of the impregnant 30, which previously had occupied the channels 13, into the porous substrate E.S. (note Fig. 3) . This causes the edge areas of the electrode substrate E.S. to become sealed all of the way through the depth of the substrate E.S. The die 12 and its plunger 11 are then drawn back down to the disposition of Figure 1 for recharging of each of the channels 13 with impregnant (after the impregnated substrate E.S. has been removed from the die 12 if it had been carried up on the die 12) and subsequent re-positioning of the die and plunger 12 and 11 for use in edge sealing another electrode substrate E.S. The edge impregnated substrate E.S. is then removed from the work station for drying out the impregnated areas (e.g., areas 1-4), typically by heating it, removing the liquid carrier materials and leaving the solids in the interstices of the impregnated areas of the substrate E.S. I.iuε , using the specialized tooling 10 of the invention, such a modified impregnant forced into the desired side areas of the porous electrode substrate E.S., namely, for example, side areas 1 to 4 of the cathode suι.-.--trate C.S. of Figure 4, uniformly and with proper dimensional control fill up the porous interstices of the substrate E.S. , or at least sufficiently reduce the size of the capillaries to prevent the seeping or in-plane transference of any significant acid electrolyte or gas across the seal area, thereby making an effective side edge seal. As an alternative to the carbon and graphite dispersion for making the substrate edge seal, it is also contemplated to use a wetproofing material, including particularly fluorocarbons, as the impregnant 30 to form corrosion resistant seals after being pressure injected by the specialized tooling of Figure 1 or 5, using the above described operation described particularly in connection with Figures 2 and 3. Like the edge seals, the corrosion seals preferably will extend all of the way through the substrate body E.S. Suitable fluorocarbon materials in aqueous dis¬ persions are Dupont's "Teflon 30" and "FEP 120". To these aqueous dispersions of fluorocarbon material is added a thickening agent, for example, B.F. Goodrich's "Carbopol" (grade 941) , which is an acrylic acid polymer having the following chemical structure: In contrast, the fluorocarbon dispersions used to make corrosion resistant coatings using the methodology of the prior art were diluted down, rather than thickened, to create a dispersion having a concentration compatible with the amount of "Teflon" desired for the final product. A suitable exemplary impregnant 30 for making a corrosion resistant seal using the specialized tooling of Figure 1 has been found to be: FEP 22.9% by weight H 2 0 76.2% by weight "Carbopol" 0.4% by weight ammonium hydroxide 0.5% by weight, which forms a slightly alkaline dispersion, not to be confused with the highly alkaline dispersion formed in the preferred impregnant for making edge seals using ammonium hydroxide, as described above. Although this invention has been shown and described with respect to detailed, exemplary embodiments thereof, it should be understood by those skilled in the art that various changes in form, detail, methodology and/or approach may be made without departing from the spirit and scope of this invention. So, for instance, it is also contemplated in accordance with the present invention to use the specialized tooling 10 or 10' described above for impregnating porous substrates other than electrode substrates, and to use the tooling 10 or 10' to force impregnating substances other than those specifically disclosed above into any of the porous substrate types mentioned before. Having thus described exemplary embodiments of the invention, that which is new and desired to be secured by Letters Patent is claimed below.

Claims: Claims 1. A method of impregnating a selected region of a porous substrate, comprising the steps of: (a) preparing an impregnant; (b) introducing a predetermined amount of the impregnant into a cavity of an impregnant injection die having an opening that opens onto a major surface of the die and is to be juxtaposed with the selected region of the substrate, said cavity being partially delimited by a bottom surface that is movable relative to said die to vary the volume of said cavity; (c) positioning said die into sealing contact with said substrate, with the opening of the cavity in juxtaposition with the selected region to be sealed; and (d) causing said movable bottom surface to move into said cavity to expel the impregnant under pressure out of said cavity and into said selected region of the substrate with the impregnant flowing into the interstices of the porous substrate region to form a seal therein. 2. The method of claim 1, wherein the seal to be formed is an edge seal, and wherein there is included the steps of: preparing the impregnant from a non-Newtonian aqueous carbon/graphite dispersion; and forcing said impregnant into the substrate under pressure with substantially uniform shear forces throughout the opening of the cavity. 3. The method of claim 2 , wherein in step "a" there is included the step of: adding a minor amount of a concentrated solution of an alkaline or caustic material to the dispersion to increase its pH level. 4. The method of claim 2, wherein in step "a" there is included the step of: adding a minor amount of a concentrated solution of an alkaline or caustic material to the dispersion to increase its pH level up to about a range of 10. 5 to 11.5. 5. The method of claim 2, wherein in step "a" there is included the step of: adding a minor amount of a concentrated solution of about fifty-eight percent of ammonium hydroxide, with the concentrated solution being added being up to about seven (7%) percent by weight of the total mixed impregnant. 6. The method of claim 1, wherein the seal to be formed is a corrosion resistant seal and wherein there is included in step "a" the step of: preparing the impregnant from a wetproofing, fluorocarbon dispersion. 7. The method of claim 6, wherein in step "a" there is included the step of: thickening the fluorocarbon dispersion. 8. The method of claim 6, wherein in step "a" there is included the step of: thickening the fluorocarbon dispersion with an acrylic acid polymer. 9. The method of claim 6, wherein in step "a" there is included the step of: mixing the fluorocarbon dispersion with addi¬ tional materials to produce a slightly alkaline dispersion. 10. The method of claim 1, wherein in step "b" there is included the step of: providing said moving surface in the form of an elongated plunger riding in the die opening for reciprocal movement therein, with biasing spring means biasing the plunger and the die apart, with the head of the plunger and the die walls defining the opening at least in the area defining said cavity being rigid. 11. The method of claim 10, wherein in step "d" there is included the step of: placing pressure on the head of said plunger with a maximum pressure of about thirty (30) psi. 12. The method of claim 10, wherein in step "d" there is included the step of: placing pressure on the head of said plunger with a maximum pressure of about thirty (30) psi in expelling the impregnant out of the cavity into the substrate. 13. A system for making an area seal in a porous substrate using an impregnant which is to be forced into the porous substrate in the region to be sealed to seal such region area, comprising: a platen; a die associated with said platen, said die having an opening in it and a movable surface contained within said opening, said opening and said movable surface defining an impregnant cavity into which the impregnant is placed prior to its being forced into the porous substrate; a work area located between said platen and said die of a size in which the substrate to have the region on it sealed can be positioned, said opening in said die extending out to the exterior surface of said die facing said work area forming an open top in the die; sealing means associated with said die extending at least around said open top for sealing the portion of said die about said open top to the substrate in the substrate area to be impregnated; and pressure means associated with said movable surface for applying pressure in the direction of said platen to said movable surface to move it through said cavity forcing out any impregnant in said cavity through said open top under pressure to impregnate the area of the substrate to be sealed with the impregnant. 14. The system of claim 13, wherein: said movable surface is a plunger having a stem tightly fitting within said opening and a head extending out of said die against which the pressure of said pressure means is applied. 15. The system of claim 14, wherein said open top forms an extended rectangular configuration; and wherein said sealing means comprises: at least one "O" ring type seal extending around and surrounding said open top, with the outer surface of said seal used to contact in sealing engagement the substrate to be impregnated. 16. The system of claim 14, wherein there is further included: a series of spring members positioned along the length of said elongated plunger biasing the head of said plunger away from said die and away from said work area. 17. The system of claim 16, wherein there is further included: a series of bolts positioned along the length of said elongated plunger extending between said plunger and said die, preloading the pressure on said plunger head toward said die against the biasing action of said spring members. 18. The system of claim 14, wherein said die is of an elastomeric and compliant material; and wherein said plunger includes a head that is in contact with said die over substantially the entire surface thereof that faces away from said work area to act on said die and compress the same between itself and said substrate with attendant movement of said plunger into said cavity.

Inventors: DAYTON JAMES E, MCGUIRE JAMES L, STEWART ROBERT C, DAYTON, JAMES, E., MCGUIRE, JAMES, L., STEWART, ROBERT, C.
Applicants: INT FUEL CELLS CORP, INTERNATIONAL FUEL CELLS CORPORATION
Application year: 1991
Main classification: B29C67
IPC codes: B29C70, H01M8