Abstract:
An electrode assembly including a shell made of electro-conductive polymeric material in which is embedded a metallic insert such as a thin plate or wire mesh. The insert has a number of surface irregularities such as holes, indentations, or protuberances. The polymeric material shrinks after the insert is encapsulated within the shell resulting in contact pressure being exerted by the material on the irregularities and surrounding surface areas. One method of making the assembly includes coating the insert with a slurry including heat-curable polymeric material, which shrinks when cured. Another method includes encapsulating the insert within molten or liquidized polymeric material, which shrinks when cooled. Contact pressure is permanent because the high temperatures reached during thermosetting or overmolding are never reached while the assembly is being used in applications such as heating or sterilizing water, electrolysis, or oil well drilling.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electrode assembly having low bulk resistance, low contact resistance at its connection to an electrical source, high structural integrity, and excellent resistance to corrosion when immersed in an electrically conductive liquid. More particularly, the invention relates to an electrode assembly having an electro-conductive polymeric shell in which a metallic insert, strongly bonded to the polymer, is embedded. 
         [0003]    2. Related Art 
         [0004]    Electrodes used for heating, purifying, or electrolyzing water commonly are made of metal. Electrodes used in boilers typically use iron rods, sometimes coated with rhodium or another corrosion-resistant material. Electrodes in purifiers and electrolyzers typically are made of stainless steel or titanium. Although the choice of metal and/or coating can extend life expectancy, due to electrolytic corrosion, any metallic electrode will eventually shrink in size thereby decreasing its efficiency. Consequently, even if a unit is not discarded, there will be recurring part and labor costs in replacing the electrodes. 
         [0005]    Electrodes made of pure carbon have been used for heating water and in vaporizers and humidifiers. Aside from their high cost, pure carbon electrodes, being soft, are vulnerable to cracking under pressure making good electrical contact difficult to achieve. 
         [0006]    Electrical devices, which include an electro-conductive polymer physically and electrically, connected to at least one electrode suitable for attachment to a source of electrical power are well known. Among the types of electrodes that have been used are solid and stranded wires, metal foils, perforated and expanded metal sheets, porous electrodes, and conductive inks and paints. 
         [0007]    When the conductive polymer is in the form of a sheet or a laminar element, metal foil electrodes that are directly attached to opposed surfaces of the conductive polymer, sandwiching the element, are particularly preferred. It is known that metal foils having micro rough surfaces can provide improved adhesion, resulting in improved physical and electrical stability, when used as electrodes in contact with conductive polymers. For example, U.S. Pat. No. 4,689,475 to Matthiesen discloses metal foils having microscopic surface irregularities protruding from the surface by 0.1 to 100 microns and having at least one dimension parallel to the surface, which are at most 100 microns. The primary mechanism for forming a good bond between a polymer and micro rough foil is mechanical interlocking achieved by embedding the rough surface of the foil into the polymer by heating the polymer above its melting point during the electroding process. U.S. Pat. No. 6,987,440 to Becker et al. discloses that improved electroding can be accomplished using foil having a combination of surface features making up the surface roughness of even smaller dimensions. 
         [0008]    Overmolding a metallic contact pin that extends far into an electro-conductive polymer can increase electrical efficiency by providing a low resistance path to the far end of the electrode. However, difficulties occur when there is poor bonding between the polymer and metal. The polymeric material tends to delaminate due to a difference between the respective coefficients of thermal expansion. The poor electrical bond causes high-resistance contact arcing between the polymeric material and pin, which further increases resistance, resulting in internal burning of the material 
         [0009]    U.S. Pat. No. 6,188,308 to Kojima et al. is directed to a positive temperature coefficient (PTC) thermostat and a manufacturing method thereof. A conductive polymer sheet is sandwiched from the top and bottom by metal foils and integrated by heat pressing to form a laminated body. The body is then sandwiched from the top and bottom by other conductive polymer sheets, and the laminated body and conductive polymer sheets are sandwiched from the top and bottom by the metal foils. 
         [0010]    The assembly is then integrated by heat pressing. A side electrode having multiple layers is disposed at the center of the side of the laminated body so as to be electrically coupled to the inner and outer electrodes. U.S. Pat. No. 6,597,551 to Heaney is directed to a polymer current limiting device (PCL) wherein a fluoropolymer heated to about 350° C. is sandwiched between thin metal foil electrodes and the assembly is bonded at a pressure of 40,000 psi. 
         [0011]    U.S. Pat. No. 5,955,936 to Shaw, Jr. et al. discloses a polymer PTC electrical circuit protection device wherein an electro-conductive polymer is sandwiched between a pair of electrodes, each having a three-dimensional, initially open cellular structure such as nickel foam. Under high temperature and pressure, the cells fill with polymeric material resulting in a laminate having lower resistance and improved mechanical adhesion. 
         [0012]    U.S. Pat. No. 6,051,778 to Ichinose et al. discloses an electrode structure formed by superposing a bar-shaped or linear metal member on an electro-conductive polymeric layer. In one embodiment the layer has a thickness larger than the diameter or thickness of the metal member so as to fully embed the metal member and connect the member to a busbar. The structure exhibits low resistance, high adhesion and high reliability. 
       SUMMARY OF THE INVENTION 
       [0013]    In one aspect an electrode assembly according to the invention provides a shell made of electro-conductive polymeric material, which has at least one outer surface. A metallic insert having at least one outer surface with a plurality of irregularities at least one surface is embedded within the shell. The polymeric material encapsulating the insert undergoes shrinkage such that The material on the irregularities as well as on contiguous surface areas exerts contact pressure. 
         [0014]    In a second aspect an electrode assembly includes a rectangular-shaped shell and a thin metallic plate embedded within the shell. The plate has a plurality of holes, each of which has a bore surface. 
         [0015]    The plate terminates at one end in a metallic wire connector tab, which protrudes through a shell edge. The polymeric material encapsulating the plate undergoes shrinkage such that contact pressure is exerted by the material on the bore surfaces as well as on plate surface areas contiguous to the holes. 
         [0016]    In a third aspect an electrode assembly includes a rectangular-shaped shell and a thin metallic wire mesh embedded within the shell. The mesh has a plurality of holes formed by crisscrossed wire segments, and terminates in an end protruding through a shell edge. The polymeric material encapsulating the mesh undergoes shrinkage such that the material on the wire segments, which determine the holes, exerts contact pressure. 
         [0017]    In a fourth aspect the invention provides a method for making an electrode assembly, including the steps of: (a) forming a slurry consisting of a heat-curable electro-conductive polymeric material and a solvent; (b) coating the slurry over a metallic insert having at least one outer surface with a plurality of irregularities at least one surface; (b) evaporating the solvent, and (c) heat curing the polymer material, thereby shrinking the material so as to exert contact pressure on the irregularities and on contiguous surface areas. 
         [0018]    In a fifth aspect the invention provides a method for making an electrode assembly, including the steps of: (a) heating to its melting point an electro-conductive polymeric material which shrinks when cooled; (b) encapsulating within the molten material a metallic insert having at least one outer surface with a plurality of irregularities at least one surface; and (c) cooling the material, thereby shrinking the material so as to exert contact pressure on the irregularities and contiguous surface areas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a perspective sectional view of an electrode assembly according to a first embodiment of the invention, including an electro-conductive polymeric shell and a metallic plate embedded within the shell. 
           [0020]      FIG. 2  shows the  FIG. 1  plate having a plurality of holes therethrough, and attached at one end to a wire connector tab. 
           [0021]      FIG. 3  is a perspective view of the  FIG. 1  electrode assembly with the shell encapsulating the plate and the  FIG. 2  tab extending through a shell edge. 
           [0022]      FIGS. 4 and 5  are cross-sectional views taken, respectively, along lines  4 - 4  and  5 - 5  of  FIG. 3 . 
           [0023]      FIG. 6  is a perspective sectional view of an electrode assembly according to a second embodiment of the invention, including an electro-conductive polymeric shell and a wire mesh screen embedded within the shell, with an end portion of the screen protruding through the shell. 
           [0024]      FIG. 7  shows the  FIG. 6  screen having a plurality of holes therethrough. 
           [0025]      FIG. 8  is a perspective view of the  FIG. 6  electrode assembly with the shell encapsulating the plate, and the screen end portion protruding through a shell edge to serve as a wire connector. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    While the present invention is open to various modifications and alternative constructions, the two preferred embodiments shown in the drawings are described herein in detail. It is to be understood, however, there is no intention to limit the invention to the particular forms disclosed. On the contrary, it is intended that the invention cover all modifications, equivalences and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims. 
         [0027]    As used herein, the term “metallic insert” means a metal structure, which is totally embedded within an electro-conductive polymeric shell. The insert may be planar-shaped such as a plate or mesh, or be symmetric about a longitudinal axis such as a cylinder, or have any other shape compatible with shell dimensions. As used herein, the term “plurality of irregularities” means a number of discontinuities at least one outer surface of the insert, each of which may be a hole through the insert, an indentation terminating within the insert body, or a protuberance extending from the outer surface. As illustrated in  FIGS. 2 and 7 , this number may be in a range from tens to hundreds. As used herein, the term “shell” means a body made of electro-conductive polymeric material, which can be of any shape and dimensions compatible with the intended use of the electrode assembly. 
         [0028]    Referring to  FIG. 1 , a first embodiment of an electrode assembly  20  according to the invention includes an electro-conductive polymeric shell  22  having opposed, generally planar outer surfaces  24 A,  24 B, opposed, generally planar edges  26 A,  26 B, and opposed, generally planar edges  28 A,  28 B generally orthogonal to edges  26 A,  26 B and surfaces  24 A,  248 . An insert in the shape of a thin, generally planar metallic plate  30  is embedded within the shell  22 . Referring to  FIG. 2 , the plate  30  has a plurality of holes  32  therethrough, each having a bore surface  32 B, and terminates at an end  34  attached to a metallic wire connector tab  36 . Referring to  FIG. 3 , shell  22  encapsulates plate  30 , and tab  36  protrudes through shell edge  28 A. Tab  36  is an electrical contact lug or quick-connect type connector, which is potted to seal it from ambient liquid. 
         [0029]    Electrode assembly  20  may be formed by dipping the plate  30  into an electro-conductive slurry so as to coat the plate with slurry, which is then allowed to dry. A slurry composed of a vinyl chloride polymer (PVC) and a solvent is preferred when dipping or manual application is used. When a phenolic such as RESOL™ or NOVOLAC™ is used, heat curing in a range of 60 to 80° C., the molding of which is well known causes shrinkage of the thermoset material. 
         [0030]    Alternatively, electro-conductive polymer material may be overmolded onto plate  30  using injection molding, transfer molding, or compression molding. For injection molding the plate is placed within a mold and polymer pellets added. As the pellets are heated to the melt temperature they expand and melt, while the insert expands far less. The molten polymer, under pressure, encapsulates the insert. The assembly is then cooled prior to being removed from the mold. Depending on the polymeric material used, its shrink rate is in a range of about 0.2 percent to about 1.9 percent. Once the molten material has cooled, a mechanical pressure equal to the tensile strength of the polymer at its respective shrink factor provides an excellent electrical contact pressure that will endure for the life of the assembly. 
         [0031]    When the slurry technique is used, liquid polymeric material covers all plate surfaces and fills the holes  32 . When the material is heated to curing temperature, the material shrinks thereby exerting contact pressure against the bore surfaces  328  and the plate surfaces. 
         [0032]    When overmolding is used, the polymeric material is heated to a molten state and undergoes thermal expansion before being introduced to encapsulate the plate. As the assembly cools, polymeric material  38  filling the holes  32  shrinks thereby exerting contact pressure against bore surfaces  328  and plate surfaces. 
         [0033]      FIG. 4  shows exemplary holes  32  filled with the material  38 .  FIG. 5  shows the direction, after cooling, of the stress vector against each bore surface  32 B for the 3×11 array of holes shown in  FIG. 2 . 
         [0034]    Shrinkage provides contact pressure between the bore surfaces and material which is not relieved thereafter because the high temperatures reached during thermosetting or molding are never reached while the assembly is being used in applications such as heating or sterilizing water, electrolysis, and oil well drilling. Such applications would never exceed the polymer&#39;s glass transition temperature, so that the contact pressure would last indefinitely. 
         [0035]    In normal use the difference in the coefficient of thermal expansion of the metal, relative to that of the polymer at the molding temperature is approximately 13-fold. Approximately the same shrink factor applies for a PVC slurry. 
         [0036]    The permanent mechanical stress of the polymer against the metallic surfaces provides excellent and multiple electrical surfaces dispersed throughout the bulk of the assembly, enabling optimum electric conductivity within the assembly. 
         [0037]    Polymeric material used in the invention can be of any polymeric compound such as polypropylene, polyethylene, polyphenylene sulfide, ethylene vinyl acetate, polycarbonate, nylon, phenolic, or any other polymer having rigid or semi-flexible properties. The material is compounded with conductive particles such as carbon black, exfoliated graphite, lampblack, carbon fibers, nanotubes or any other conductive particles including metallic particles, to form an electro-conductive polymer. 
         [0038]    The thickness of shell  22  must not be so large as to cause overheating of the polymer. Acceptable thicknesses are in a range from ¼-inch to a thin layer just coating the plate  30  to prevent electrolysis of the metal. Preferably, the thickness is ⅛-inch; most preferably, it is 1/32 inch. Such a thin layer decreases the power density throughout the bulk of the polymeric material, thereby reducing internal heating. 
         [0039]    The number density and size of the holes  32  are important. The spacing between nearest neighbor holes must be sufficiently small and the bore surface of each hole sufficiently large to prevent delamination, particularly at high current. The metal face spacing between holes can be 3/16 of an inch, preferably ⅛ inch and most preferably 1/32 of an inch. The metal plates can be any metal such as aluminum or brass, preferably mild steel and most preferably stainless steel such as a 316L. Electrode size is dependent on its use. No particular size electrode fits all applications, however there are limitations in fabrication. For electrolyzers, water heaters, and other small devices, an opposed face area between electrode pairs can be 5 square inches for small electrodes to as much as 120 square inches for tubular electrodes. A preferred opposed face area for electrolyzers can be approximately 36 square inches. 
         [0040]    For down hole electrodes used in the oil-drilling field, tubular electrodes can have 800 to 1,000 square inches. For this application, only a single electrode is used to descend into the well as the pipe is charged with the opposite polarity. 
         [0041]    An electrode assembly according to the invention provides several advantages. No bonding or cross-linking agents are used during fabrication. 
         [0042]    No chemical bonding agents, coupling agents, conductive electrical coupling epoxies or chemicals are required to ensure excellent electrical contact between the polymer and metallic surfaces. 
         [0043]    Electro-conductive polymeric material is expensive and many applications require several electrodes. A metallic insert reduces the amount of material by as much as 30 percent while also providing structural strength to the material, which may be fragile or brittle. Also, as required by Underwriters&#39; Laboratory, a metal-to-metal electrical connection can be made directly to tab  36 , thus eliminating the need to make a high resistance connection directly to shell  22 . Referring to  FIG. 6 , a second embodiment of an electrode assembly  50  according to the invention includes an electro-conductive polymeric shell  52  having opposed, generally planar outer surfaces  54 A,  54 B, opposed, generally planar edges  56 A,  56 B, and opposed, generally planar edges  58 A,  58 B generally orthogonal to edges  56 A,  56 B and surfaces  54 A,  54 B. An insert in the shape of a thin, generally planar metallic wire mesh  60  is embedded within the shell  52 . Referring to  FIG. 7 , the mesh  60  has a plurality of holes  62  formed by crisscrossed wire segments  62 W, and terminates in an end portion  64 . Referring to  FIGS. 6 and 8 , shell  52  encapsulates mesh  60 , and end portion  64  protrudes through shell edge  58 A to serve as an electrical connector. 
         [0044]    The electrode assembly  50  may be formed by dipping the mesh  60  into an electro-conductive slurry or painting the slurry onto the mesh, such as for the first embodiment. The coated mesh is then allowed to dry before the assembly is heated to curing temperature. Alternatively, as for the first embodiment, electro-conductive polymer material may be overmolded onto mesh  60  using injection molding, transfer molding, or compression molding. 
         [0045]    As in the first embodiment, the shrink rate of the polymeric material is in a range of about 0.2 percent to about 1.9 percent. As polymer material filling the holes  62  and coating the wire segments  62 W shrinks, the material exerts contact pressure against the segments.