Abstract:
Described are novel systems for electrodeposition of paint on counter-electrodes, and membrane electrode assemblies for incorporation into such systems. In certain embodiments of the invention, electrode enclosures such as C-shaped or box-shaped enclosures include membranes sealed to secondary structural members by bonding. In other embodiments, membrane electrode cells having bumpers are mounted within paint baths in a manner wherein the bumpers are forcibly biased against the exterior wall of the bath, thereby stabilizing the position of the cells. In still further embodiments, tubular electrode cells are provided with internal valves allowing the release of liquid trapped within the tubular electrodes into the membrane shell. In this manner, removal of the electrodes for maintenance, replacement or other purposes is facilitated.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application is a divisional patent application of U.S. patent application Ser. No. 10/443,303 filed May 22, 200303 now U.S. Pat. No. 7,422,673 entitled MEMBRANE ELECTRODE ASSEMBLIES AND ELECTROPAINT SYSTEMS INCORPORATING SAME which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to membrane electrode cells and systems used in electrodeposition coating. More particularly, in certain embodiments, the present invention relates to advantageous membrane electrode cell assemblies having electrode enclosures wherein the membrane seal of the enclosure is provided by a compression-independent, bonded arrangement. 
     As further background, electrodeposition coating, or “electrocoating”, is broadly classified into two categories. Anionic electrodeposition uses anionic paints; cationic electrodeposition uses cationic paints. Both of these processes are in current commercial use. 
     Membrane electrode cells are commonly used in electrodeposition systems. Such cells act as opposing electrodes in the electrodeposition process, with the object being painted serving as the counter-electrode. Membrane electrode cells also serve in many systems to remove ions from the paint bath to maintain proper paint bath chemistry. 
     The membrane electrode cell can have many shapes, and often is shaped as a flat rectangle, arcuate or semi-circle, tube or cylinder. Electrodeposition processes employing such membrane electrode cells are disclosed for example in U.S. Pat. Nos. 4,851,102, 4,711,709 and 4,834,861. 
     The membrane used in a membrane electrode cell can be either ion-exchange or neutral. The membrane is arranged in such a fashion as to separate the electrocoating paint bath from the electrode. An electrolyte fluid flows between the inside of the membrane and the outside of the electrode. This electrolyte fluid, which is often comprised mostly of deionized water and a small amount of acid or amine (depending on the type of electrocoating employed), is responsible for flushing the ions that pass through the membrane into the membrane electrode cell from the paint bath. The conductivity of this electrolyte fluid usually is maintained in the range of 500 to 2,000 microSiemens/cm (microMho/cm). 
     To separate the electrode from the paint bath, it is a common practice to provide a seal between the membrane and other structural member(s) of the electrode cell. These seals have in the past commonly been achieved by mechanical elements such as bolted flanges, which form a pressure-dependent seal between the flange, membrane, and an outer rim or periphery of an electrode housing. These seals can be difficult to maintain, and present complications in repair and replacement operations. 
     In light of this and other background in the field, there remain needs for membrane electrode assemblies of simplified design and which are more readily repaired, replaced and/or maintained. The present invention is addressed to these needs. 
     SUMMARY OF THE INVENTION 
     Accordingly, one aspect of the present invention provides a system for electrodeposition of paint on a counter-electrode. The inventive system includes an electrodeposition chamber containing an electroconductive liquid medium including paint. At least one membrane electrode assembly is in the chamber in contact with the liquid medium. The electrode assembly includes an electrode and an enclosure separating the electrode from the liquid medium. The enclosure has a first structural member and a membrane bonded to the first structural member to provide a seal therebetween. A counter-electrode upon which paint is to be electrodeposited is in the chamber in contact with the liquid medium. Passage of electrical current between the counter-electrode and the electrode in the membrane electrode assembly through the liquid medium causes electrodeposition of the paint on the counter-electrode. In preferred embodiments, the electrode within the membrane electrode assembly is a flat or arcuate-shaped electrode, and the membrane can be bonded to the first structural member by welding, chemical bonding agents, polymers, and the like. 
     In another embodiment, the present invention provides a membrane electrode assembly for use in a paint electrodeposition chamber containing a liquid medium. The assembly includes an electrode, and an enclosure for separating the electrode from the liquid medium. The enclosure includes a first structural member, and a membrane bonded to the first structural member to provide a seal therebetween. In certain embodiments, the electrode is a flat or arcuate shaped electrode, and the membrane is bonded to a multiple-sided periphery of the structural member. 
     In another embodiment, the present invention provides a membrane electrode assembly for use in a paint electrodeposition chamber, wherein the assembly includes an electrode, and an enclosure for separating the electrode from a liquid medium within the chamber. The enclosure includes a non-electrically-conductive coating covering a first surface portion of the electrode. The enclosure further includes a membrane for separating a second surface portion of the electrode from the liquid medium. 
     In still another embodiment, the invention provides a membrane electrode assembly for use in a paint electrodeposition chamber, wherein the chamber has an electrode mount adjacent to the top of the chamber. The membrane electrode assembly of the invention includes an electrode and an enclosure for separating the electrode from the liquid medium in the chamber. The enclosure also includes a membrane, and a frame holding the membrane, the frame having a top end and a bottom end. An attachment member is located adjacent the top of the frame, for attaching the membrane electrode assembly to the electrode mount. The membrane electrode assembly includes a bumper element for contacting a wall of the electrodeposition chamber, the bumper element attached to the frame and spaced from the top end of the frame. In the inventive arrangement, the attachment member, frame and bumper element are arranged to force the bumper element against the wall when the attachment member is attached to the electrode mount. 
     In still another embodiment, the present invention provides a membrane electrode assembly for use in a paint electrodeposition chamber containing a liquid medium. The membrane electrode assembly includes a tubular electrode, and a tubular enclosure including a membrane for separating the electrode from the liquid medium. The tubular electrode received within said tubular enclosure wherein liquid can be trapped within the interior of the tubular electrode. A valve is provided in fluid communication with the interior of the tubular electrode and adapted to selectively permit fluid flow from the interior of the tubular electrode into the tubular enclosure. Electrodeposition systems and methods using such assemblies also form a part of the present invention. 
     Additional features, advantages and embodiments of the invention will be apparent to those of ordinary skill in the art from the descriptions herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  provides a perspective view of a preferred membrane electrode cell assembly of the invention. 
         FIG. 2  provides a partial cut-away view of the cell assembly of  FIG. 1 . 
         FIG. 3  provides a cross-sectional view of the cell assembly of  FIG. 1 , taken along line  3 - 3  and viewed in the direction of the arrows. 
         FIG. 4  provides a perspective view of another preferred membrane electrode cell assembly of the invention. 
         FIG. 5  provides a partial cut-away view of the cell assembly of  FIG. 4 . 
         FIG. 6  provides a side view of another preferred membrane electrode cell assembly of the invention. 
         FIG. 7  provides a cut-away cross-sectional view of the lower portions of cell assembly of  FIG. 6  taken along line  7 - 7  and viewed in the direction of the arrows. 
         FIG. 8  provides a top view of the cell assembly of  FIG. 6 . 
         FIG. 9  provides a side view of a C-shaped membrane electrode cell assembly of the present invention. 
         FIG. 10  provides a front view of the electrode cell assembly of  FIG. 9 . 
         FIG. 11  provides a rear view of the electrode cell assembly of  FIG. 9 . 
         FIG. 12  provides a cross-sectional view taken along line  12 - 12  of  FIG. 9  and viewed in the direction of the arrows. 
         FIG. 13  provides a cross-sectional view taken along line  13 - 13  of  FIG. 9  and viewed in the direction of the arrows. 
         FIG. 14  provides a cross-sectional view taken along line  14 - 14  of  FIG. 13  and viewed in the direction of the arrows. 
         FIG. 15  provides a cross-sectional view of a tubular membrane electrode assembly of the invention. 
         FIGS. 15A and 15B  provide front and rear views, respectively, of a flap valve for use in the assembly of  FIG. 15 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purpose of promoting an understanding of the principles of the invention, reference will now be made to certain preferred embodiments thereof and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations, further modifications and applications of the principles of the invention as described herein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     As disclosed above, the present invention provides membrane electrode assemblies, electodeposition systems, and related methods for electrodepositing paint on a counter-electrode. 
     In general, membrane electrode assemblies of the invention, and used in systems and methods of the invention, will include an electrode, and an enclosure for separating the electrode from liquid medium in the electrodeposition chamber. The enclosure will typically include at least one membrane, such as a selectively permeable membrane, e.g. an ion-exchange membrane. The enclosure will include at least one other element, typically a frame member, and a seal between the membrane and the frame member to complete the enclosure. In accordance with the preferred embodiments of the invention, this seal is provided by a bonding of the membrane to the frame member. This bonding may be achieved, for example, by welding, chemical bonding agents, and the like. Such bonded seals are in general compression-independent, meaning that they do not require compression of the membrane against the frame member in order to achieve a seal, as is the case in many current systems in which a flange is tightened over the membrane in order to seal it against a frame member. 
     With reference now to  FIG. 1 , disclosed is a first membrane electrode assembly  11  of the present invention. Membrane electrode assembly  11  includes an internal electrode  12 , and an enclosure enclosing the electrode. Assembly  11  further includes a membrane  13 , and at least one additional structural element in the enclosure. In the illustrated assembly  11 , the enclosure also includes four strip elements  14  arranged around the membrane  13  and electrode  12 . Strip elements  14  in general wrap around the membrane  13  and the electrode  12 , and a bonding agent can be employed at the contact locations in order to provide a seal. 
     With reference now to  FIGS. 1 and 2  together, additional components of the assembly  11  will be described.  FIG. 2  in particular shows a partial cut-away of the assembly  11  at various locations in order to reveal layers within. Membrane  13  provides the outermost layer, and is separated from electrode  12  by a separating layer  15  formed of a suitable non-conductive material such as a plastic mesh. Separating layer  15  insures that membrane  13  does not contact electrode  12  which may result in undesired degradation of membrane  13 . Located behind electrode  12  is insulative layer  16  made from a non-electrically-conductive material. Insulative layer  16  may, for example, be provided by a non-conductive substance painted or applied to the rear surface of the electrode  12 . Layer  16  may also be provided by any other suitable non-conductive layer or material that can be used to electrically isolate electrode  12 . Strip elements  14  encompass and wrap around membrane  13 , separating layer  15 , electrode  12 , and insulating layer  16 . In this fashion, electrode  12  is provided within an enclosure separated from the paint or other liquid medium in the electrodeposition bath by membrane  13 . 
     With reference now to  FIGS. 1-3  together, membrane electrode assembly  11  is also provided with a system for circulating fluid across the inner surface  13   a  of the membrane  13  in order to prevent undesirable accumulation of materials at the surface  13   a .  FIG. 3 , in particular, provides a cross-sectional view taken along line  3 - 3  of  FIG. 1  and viewed in the direction of the arrows. Assembly  11  includes a fluid inlet tube  17  and a fluid outlet tube  18  for circulating fluid within the enclosure of the membrane electrode assembly  11 . In the illustrated embodiment, fluid inlet tube  17  circulates fluid to the bottom of the assembly  11 , and has an outlet manifold  19  with multiple outlets  20  for causing the fluid to pass upwardly in the assembly  11 . Outlets  20  are positioned to cause fluid flow between the membrane  13  and the separating layer  15 . In this fashion, fluid is circulated against the inner surface  13   a  of the membrane  13  to facilitate removal of any materials becoming deposited thereon. 
     With reference now to  FIGS. 4-5 , shown is another membrane electrode cell  30  of the present invention. Membrane electrode cell  30  includes an electrode  31  preferably made of a suitable metal, enclosed by a membrane  32 , preferably an ion exchange membrane. With reference in particular to  FIG. 5 , shown is a partial cut-away view showing the various components and layers of the electrode  30 . Electrode  30  includes a first membrane layer  32  and a porous spacer element  33  that resides immediately adjacent membrane layer  32  and separates layer  32  from the electrode  31 . Electrode cell  30  also includes a second spacer element  34  which separates electrode  31  from another layer  32 A of membrane material similar to that in membrane layer  32 . Membrane layers  32  and  32 A are sealed together about their peripheries, for example by heat welding or the like. Sealed membrane layers  32  and  32 A thus enclose the electrode  31  and the spacer elements  33  and  34 . Electrode cell  30  also includes an electrolyte circulation system including an inlet tube  35  which extends from the top of the cell to a location adjacent the bottom, and an electrolyte exit tube  36 . Inlet tube  35  is connected to a laterally-extending baffle tube  38  which in turn has several fluid delivery tubes  39  extending therefrom and opening into the interior of the cell. Thus, electrolyte passed through inlet tube  35  and baffle tube  38  exits from the various outlet tubes  39  and creates an upward flow of electrolyte within the cell  30 , ultimately exiting via outlet tube  36 . Electrode cell  30  also includes a hanging bracket  37  from which the cell  30  can be suspended in a paint bath. As discussed above, in some embodiments, the electrode cell  30  is completely sealed apart from the inlet and outlet openings  35  and  36 . In other embodiments, the upper end of the electrode cell  30  may be left unsealed, and the electrode cell  30  suspended in the paint bath such that any unsealed portions are above the level of the bath. 
     With reference to  FIGS. 6-8 , shown is another membrane electrode cell  40  of the present invention. Membrane electrode cell  40  is similar in design to membrane electrode cell  11  illustrated in  FIGS. 1-3 . However, membrane electrode cell  40  includes a frame  41  constructed from tubular elements, for example, polymer tubing, including polyvinylchloride (PVC) tubing. Frame  41  as depicted includes two vertically extending tubular elements  42  and  43  and a horizontally extending element  44  all interconnected by curved or elbow sections  45  and  46 . It will be understood that frame  41  could also be made of fewer pieces or a single piece of tubing configured to an appropriate shape. Visible in  FIG. 6  are electrode cell frame  41 , outermost membrane guard  47 , and hanging bracket  48 . Also shown in phantom is the electrolyte distribution system including an inlet tube  49 , a lateral tube  50  providing a manifold, and several outlet tubes  51 . 
     With reference now particularly to  FIG. 7 , shown is a partial cut-away cross-sectional view taken along line  7 - 7  of  FIG. 6  and viewed in the direction of the arrows. Shown is lateral tube  44  of cell frame  41 . Housed within tube  44  is the lateral electrolyte distribution tube  50  with electrolyte outlet tube  51  extending therefrom. Electrolyte tube  51  extends through an aperture  52  provided through the electrode  53 . Immediately adjacent to electrode  53  is a spacer element  54  separating an ion exchange membrane  55  from the electrode  53 . A nonconductive or otherwise insulating layer  56  is provided on the back side of electrode  53  to insulate the same from the paint bath. As shown in  FIG. 7 , the electrode  53  along with the spacer  54 , membrane  55  and guard  47  all penetrate a location in the tube  44 . A similar arrangement is found in the remainder of the tubular frame  41 . In this fashion, the tubular frame  41  serves to support these structures. The penetration of the tube  44  and in other locations of the frame  41  is sealed fluid tight. In operation of cell  40 , the cell  40  is suspended into the paint bath using bracket  48 , with the upper edge  57  of the electrode  53  and associated guard, membrane and spacer remaining above the surface of the paint bath. Electrolyte is circulated through tube  49  thus exiting outlets  51  and flowing through apertures  52  in the anode thereby reaching the front surface of the anode and circulating upwardly in the cell in between the anode  53  and the membrane  55 . Thus, membrane electrode cell  40  provides a convenient, inexpensive electrode for use in paint baths and other similar applications. 
     With reference now to  FIGS. 9-11 , shown are side, front and back views of a C-shaped membrane electrode cell  60  of the present invention. Membrane electrode cell  60  generally includes a pair of elongate rails  61  and  62  interconnected to one another by a plurality of cross-brace members  63 . The rails  61  and  62  terminate in a bottom end cap  64 . A pair of support brackets  65  are attached to uppermost cross-brace member  63  and are used to suspend the membrane electrode cell  60  from a rail or other supporting device located adjacent to the edge of a paint bath. Bumper elements  79  are provided. Cell  60  also includes an electrode and associated support, spacer, membrane and guard elements all having an arcuate shape, preferably in the shape of a section of a circle, such as a semi-circle. 
     With reference now to  FIG. 12 , shown is an enlarged cross-sectional view taken along line  12 - 12  of  FIG. 9  and viewed in the direction of the arrows. Shown are the rails  61  and  62  which each have inner grooves  67  and outer groves  68 . Received within inner grove  67  are the ends of a support panel  69  extending the length of the cell  60  and having a generally arcuate cross-section. Ends of support panel  69  can be secured into grooves  67  with any suitable welding, bonding or other attachment technique. Immediately adjacent to support panel  69  is an electrode  70 , such as an anode, which is received upon support member  69  but not received within grooves  67 . Thus, in one mode of construction, electrode  70  in the completed cell  60  can be removed from the cell  60  and replaced, repaired, etc., if desired. Also shown in  FIG. 12  is membrane spacer  71  and ion exchange membrane  72 , with the membrane spacer  71  protecting against contact of the membrane  72  and the electrode  70 . Also included is membrane guard  73  which protects membrane  72  against damage resulting from contact with articles during shipment or use. 
     Referring now to  FIG. 13 , shown is an enlarged cross-sectional view taken along line  13 - 13  of  FIG. 9  and viewed in the direction of the arrows. Evident is a cross-section of the bottom end cap  64  along with an adjacent cross-brace member  63 . Cap  64  is generally arcuate in shape and includes a plurality of arcuate channels therein. A first channel  74  receives a combination of the spacer  71 , membrane  72  and guard  73 . These layers can be bonded within channel  74  using a suitable epoxy or other bonding agent. Cap  64  also includes generally arcuate channel  75  for receiving support panel  69  which likewise is bonded within channel  75 . Cap  64  further includes a channel  76  which is configured to deliver fluid in between the anode  70  and the membrane  72 , such as an electrolyte fluid. With reference now to  FIG. 14  as well, provided is a side view of the section taken along line  14 - 14  of  FIG. 13  and viewed in the direction of the arrows. Fluid is provided to channel  76  via bore  77 . For these purposes, channel  76  extends more deeply into the cap  64  than channels  67  and  68 , and bore  77  is located at a level below channels  67  and  68  so as not to intersect them and terminate into the lowermost portion of channel  76 . In this manner, electrolyte liquid can be delivered through bore  77  and exit into channel  76 , thereafter traveling upwardly in the cell in between anode  70  and membrane  72 , traversing its way through the membrane guard  71  which is configured so as to allow its passage. In use, membrane cell  60  is immersed in the paint bath, and a potential difference is applied between the electrode (anode)  70  and the counter-electrode article to be coated. The upper end of cell  60  can be left open and kept at a level above the level of the paint bath. Alternatively, the upper end of the cell  60  could be closed and means provided for the escape of any gases generated during operation. For purposes of electrolyte circulation, cell  60  can also be equipped with an outlet opening  78  such that electrolyte flowing through cell  60  exits from the outlet opening and can be routed conventionally to a treatment or disposal tank or the like. 
     With reference to  FIG. 15 , shown is a tubular membrane electrode cell according to the present invention. Except as otherwise noted herein, membrane electrode cell  80  may generally have components as described in U.S. Pat. No. 5,591,316 issued Jan. 7, 1997, which is hereby incorporated by reference in its entirety. Membrane electrode cell  80  generally includes a membrane shell including a neck  81  having attached thereto an electrolyte exit tube  82 . Neck  81  is glued into collar  83 . It will be understood in this regard that neck  81  and collar  83  may also be provided as a single, integral piece. Cell  80  also includes a cap  84  at the bottom end of the cell  80 , and a membrane guard  85  fashioned in such a manner that its inside diameter fits snugly over collar  83  and cap  84 , with guard  85  attached to collar and cap by epoxy or other suitable adhesives, or by welding, for example. The membrane shell also includes a membrane  86 , e.g. an ion-exchange membrane, encased within cloth layers  87 A and  87 B, and an inner porous guard tube  88  adjacent thereto. Membrane  86  and its adjacent cloth layers  87 A and  87 B, and inner porous tube  88  are affixed within grooves defined in collar  83  and cap  84  using epoxy or any other suitable means. 
     Membrane electrode cell  80  also includes a tubular electrode  89  disposed within the membrane shell. Electrode  89  will serve as an anode, or cathode, depending upon the type of electradeposition painting to be undertaken, as would be understood by those skilled in the art. Electrode  89  will typically be made of metal or another suitable conductive material. A preferred material is stainless steel. An electrolyte inlet tube  90  extends into the membrane shell and through the interior of the tubular electrode  89 , ultimately terminating in an internal electrode cap  91 . In use, electrolyte fluid is passed through tube  90  and exits the bottom of cap  91  thereafter flowing upwardly in the membrane shell between the outer surface of electrode  89  and the inner surface of membrane  86  and its accompanying cloth layers. In accordance with one aspect of the invention, a valve  94 , especially a one-way valve, is provided in fluid association with the electrolyte supply tube  90 , and adapted to allow fluid flow into supply tube  90 , and to resist fluid flow out of electrolyte tube  90 . Illustratively, this may be provided by the inclusion of a three-way or “T” connector  92  situated within the electrolyte supply tube  90 , having affixed within its horizontally-extending leg a section of PVC or other tubing  93 . One-way valve  94  is trapped at the shoulder of the connection of tube  93  and “T” connector  92 . In this regard,  FIGS. 15A and 15B  illustrate one suitable one-way valve for use in the invention. Shown is flat valve  94  including a flexible membrane  95  having a slit pattern  96  cut therein. With particular reference to  FIG. 15B , shown is a rear view of the valve depicted in  FIG. 15A , revealing backing screen  97  associated with one side of the valve  94 . Such valves are conventional in the art and these and other suitable one-way or check valves of a variety of designs, including for example flap valves or ball-cage valves, will be suitable for use in the present invention. 
     With continued reference to  FIGS. 15 ,  15 A and  15 B, the cell  80  is highly advantageous in use. In particular, it is sometimes necessary to remove the anode  89  and associated supply tube  90  and internal cap  91 , for replacement, repair, or otherwise. Valve  94  facilitates this operation by allowing fluid trapped within the interior of electrode  89  to drain into the membrane shell as electrode  89  is being lifted out of the membrane shell, with valve  94  opening in response to increased liquid pressure generated at the exterior of the valve. This avoids the need of lifting large amounts of water when removing electrode  89  from the cell  80 . This feature is especially advantageous in electrode cells  80  having electrodes  89  having relatively large internal diameters, for example greater than about 4 inches, and typically in the range of about 4 or 5 inches up to about 8 inches ID. It will also be understood that valve  94  could be incorporated in other positions allowing fluid communication between the interior of the electrode  89  and the membrane shell. For example, valve  94  could be positioned in internal cap  91 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.