Composite Fiber Reinforced Plastic Electrode Frame

The present invention relates generally to electrode frames used in electrolytic cells. More specifically, the present invention relates to an improved composite fiber reinforced plastic frame that may be employed in monopolar filter press membrane type of electrolytic cells, especially those used to produce chlorine and caustic.

Chlorine and caustic, products of the electrolytic process, are base chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type of electrolytic cell or the flowing mercury cathode-type of cell.

The development of a hydraulically impermeable membrane has promoted the advent of commercial filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product can obviate the need for caustic purification and concentration processes. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, advances continue to be made in the development of monopolar filter press membrane cells which have caused increasing attention to be focused on the development of improved and more economical electrodes and electrode frames.

Early filter press membrane cells were constructed of heavy plastic frames. Typically, these cells were bipolar and utilized a solid sheet or backplate which was a divider between the cells and was fabricated integrally with the frame. Bipolar cells of this type followed well developed filter press fabrication principles. The integral frame-backplate construction provided excellent stiffening of the frame structure. The backplate frequently was covered with a resin or rubber coating that was not readily attacked by the chlorinated brine. The frames for these cells were molded from hard rubber, filled polypropylene, polyester fiberglass, polyester or any other material that was chemically resistant. Frequently, the anode frame was formed of these materials while the cathode frame continued to be formed from steel.

The filter press membrane cell frames tend to be limited in size for several reasons. These include the high cost for very large molds and the warping that tends to occur in the heavy plastic frames when the frames are subjected to operating temperatures during actual cell use. Additionally, the plastic parts employed in these cells tend to have a high coefficient of expansion compared to the metal parts. This results in a disparity in expansion between the cell parts during operation that tends to cause distortion. Also, the filled plastic frames are susceptible to corrosion by the chlorine, especially in the filler material. Lastly, the presence of calcium and magnesium in these plastic frames has been found to be detrimental when membranes are used because of the adverse affect of these elements on membrane life.

Thus, because of these aforementioned deficiencies, monopolar filter press membrane cells, as well as bipolar filter press membrane cells, normally employ metal frames. Typically, these metal frames use titanium in the anodic electrode and nickel in the cathodic electrode. This metal frame construction offers the advantages of high strength, small cross section of structural members, corrosion resistance, resistance to warping, large size and compatibility with metal electrode surfaces. However, the single most notable disadvantage of metal frames is their very high fabrication cost. Metals such as titanium and nickel, and the fabricating facilities necessary to produce the electrode frames, are particularly susceptible to the soaring costs associated with high technology.

Therefore, attempts are continuing to employ plastic frames in filter press membrane cells that will give the advantages that metal frames offer without the high costs. Pultruded members of fiberglass polyester resin offer the advantages of low cost, a low coefficient of thermal expansion similar to that of metal, and high strength. However, this type of plastic frame construction is deficient because of inadequate corrosion resistance, the variability of the thickness and straightness of the pultruded sections, and the difficulty of obtaining strong, leak-free, corrosion-resistant corner joints.

The foregoing problems are solved in the design of the composite fiber reinforced plastic frames and the method of making these composite fiber reinforced plastic frames of the present invention.

It is an object of the present invention to provide a frame for use in a filter press membrane type of electrolytic cell that incorporates a corrosion resistant liner with an inexpensive, but high strength structural core.

It is another object of the present invention to provide a frame for use in a filter press membrane type of electrolytic cell that is made of a composite fiber reinforced plastic which has a coefficient of thermal expansion compatible with metal electrode surfaces.

It is another object of the present invention to provide a method of making the composite fiber reinforced plastic frame for use in a monopolar filter press membrane type of electrolytic cell.

It is a feature of the present invention that a corrosion resistant liner is utilized to form a shell frame of selected thermoplastic material that is open-topped and generally U-shaped.

It is another feature of the present invention that glass fiber impregnated with a catalyzed polyester resin is wrapped continuously in a roving fashion within a shell frame formed of corrosion resistant liner material.

It is a feature of an alternative embodiment of the present invention that the glass fiber roving is alternated with pultruded strips of fiber reinforced plastic or pultruded blocks of fiber reinforced plastic.

It is a feature of the method of making the composite plastic frame of the present invention that the shell frame is mounted on a rotatable jig during the continuous roving of the glass fiber within the shell frame.

It is an advantage of the present invention that the thermoplastic corrosion resistant liner material employed is sufficiently flexible to permit the frame members to expand to the shape of the mold formed by the jig.

It is another advantage of the present invention that the thermoplastic corrosion resistant liner material provides a smooth gasket face for use in assembled filter press membrane type of electrolytic cells.

It is another advantage of the present invention that the resin impregnated glass fiber roving bonds the entire frame structure into a high strength, durable unit.

It is still another advantage of the present invention that the core material of resin impregnated glass fiber roving is sufficiently resistant to corrosion to withstand leaks through the liner material.

It is yet another advantage of the present invention that special adhesive material may be applied to the inner surface of the thermoplastic corrosion resistant liner to enhance the bonding of the resin impregnated glass fiber material thereto.

It is another advantage of the present invention that in one embodiment a polyester mat or cloth is hot pressed into the interior surface of the polyvinyl difluoride thermoplastic corrosion resistant liner to provide a surface that is conducive to bonding with the core material.

It is still another advantage that monopolar electrodes of the design of the present invention may be assembled in direct contact, since the frames are electrically nonconductive, obviating the need for insulating material between the adjacent electrodes.

These and other objects, features, and advantages, are obtained in a composite fiber reinforced plastic frame and the method of making such a frame by providing a thin corrosion resistant liner material having a highly reinforced structural core that is at least partially formed by wrapped roving layers of glass fiber impregnated with a catalyzed thermosetting resin which possesses high strength and a low coefficient of thermal expansion.

The advantages of this invention will become apparent upon consideration of the following disclosure of the invention, especially when it is taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a side elevational view of a monopolar electrode adaptable for use in a filter press membrane type of electrolytic cell having the electrode surfaces broken away;

FIG. 2 is a sectional view taken along the lines 2--2 of FIG. 1 showing one embodiment of the frame member structure;

FIG. 3 is a sectional view showing an alternative embodiment of the structure of the frame member shown in FIG. 2;

FIG. 4 is a sectional view showing another alternative embodiment of the structure of the frame member shown in FIG. 2; and

FIG. 5 is a sectional view taken along the lines 5--5 of FIG. 1 showing the lining material that is inserted in an opening in the frame with the conductor rod removed.

Looking at FIG. 1, an electrode indicated generally by the numeral 10 is shown. The electrode may be either an anode or a cathode, depending upon the materials employed at specific locations. It is intended that the term electrode encompass all of the elements and components normally associated with an electrode unit that is assembled in a filter press membrane type of electrolytic cell, including opposing electrode surfaces, conductor rods, process inlet and process outlet means.

Electrode 10 is shown to include a generally rectangular outer frame, indicated generally by the numeral 11. Frame 11 is comprised of four separate members, specifically a top member 12, a bottom member 14, a first generally vertically oriented side member 15 and opposing second generally vertical side member 16. Opposing electrode surfaces 18, only partially shown, are mounted to the electrode frame by being appropriately fastened, such as by welding, to the electrode rods 19. Electrode rods 9 pass through predrilled holes or openings in an appropriate side of the frame 11 and extend generally horizontally thereacross.

Conductor rods 19 may be flanged to make internal seals on the inner faces 28 of side members 15 and 16 or may be grooved on one end and collared on the opposing end to permit the use of O-rings to seal the frames 11. The ends of the conductor rods 19 extending through the side member 16 are the current leads to the cell. These are connectable to cell terminals (not shown) in an assembled filter press membrane electrolytic cell. At the opposite ends of the conductor rods 19, capscrews 20 are inserted through side member 15 and are screwed into female threaded end portions (not shown) of the conductor rods 19. The conductor rods 19 likewise hold and seal side members 16 by means of externally threaded nuts that are screwed onto the outer ends of conductor rods 19.

A process inlet means 21 is shown entering the frame 11 through the bottom member 14. A process outlet means 22 is similarly shown in the top member 12. The process inlet means 21 and the process outlet means 22 serve to carry the electrolyte and entrained product gas through the electrode compartment (not shown) that is generally defined in an assembled cell by the pair of membranes that are placed adjacent, but exteriorly of each electrode's two opposing surfaces 18. The inlet process means 21 may connect to an infeed manifold or header (not shown) through which fresh brine, in the anode, and deionized water and caustic, in the cathode, are fed. The process outlet means 22 leads to a gas-liquid disengager (not shown) into which chlorine gas and anolyte, in the case of the anode, and hydrogen gas and caustic, in the case of the cathode, are released.

A corrosion resistant liner material 24 is seen surrounding the outer portions of the frame 11 in FIG. 1. FIGS. 2 through 4 show in more detailed fashion the placement of the liner material 24 on the individual members of the frame 11. In FIG. 2, the liner material 24 is shown having a first side face 25 and an opposing second side face 26 interconnected by a third side face 28. This same structural element is utilized in each of the alternate embodiments in FIGS. 3 and 4. Within the open-topped channel that is formed by the side faces 25, 26, and 28, a core material 29 is emplaced. The core material 29 is of a predetermined width or thickness that is equal to the width of the channel formed by the side faces 25, 26, and 28. Preferably, the core material 29 is formed from resin impregnated glass fiber roving that is applied continuously in layers wrapped around all four sides of the frame 11. The resin is a catalyzed thermosetting resin, such as a polyether or polyester resin. An epoxy resin could also be employed.

Alternate configurations of filler may be used in the channel formed by the side faces 25, 26, and 28 to reduce the amount of heat that builds up in the frame 11 during the curing of the resin. High heat build up can cause the liner material 24 to warp and can be reduced by the use of a filler material interspersed or inserted between the layers of resin impregnated glass fiber. Suitable filler material is pultruded fiber reinforced plastic. FIG. 3 shows an initial core material 29 on top of which is placed a solid pultruded block of fiber reinforced plastic 30. A top layer of core material 29 is then placed atop the fiber reinforced plastic block 30 to form a multi-layered laminate between the side faces 25, 26, and 28. Similarly, FIG. 4 shows another embodiment of a multi-layered laminate. The core material 29 has placed in alternating fashion between its layers strips of fiber reinforced plastic 31. While the pultruded fiber reinforced plastic block 30 of FIG. 3 is shown extending across the full width of the channel, the fiber reinforced plastic strips 31 of FIG. 4 are shown being a distance less than the width of the channel. Either of the configurations are interchangeable, so that the pultruded fiber reinforced plastic block 30 could extend some distance less than the width of the channel or the pultruded fiber reinforced plastic strips 31 could extend along the full width of the channel. The fiber reinforced plastic block 30 and the fiber reinforced plastic strips 31 are generally pultruded and extend along substantially the entire length of each individual member of the frame 11.

The core material 29, as has been previously described, is preferably formed from glass fiber roving and is impregnated with a catalyzed thermosetting resin. This is formed by passing glass fiber through a resin bath prior to its being positioned within the shell frame 11. The impregnated glass fiber is then wound about the top member 12, the first side member 15, the bottom member 14, and the second side member 16, and in layered roving fashion. Where the embodiments of either FIG. 3 or FIG. 4 are employed, a predetermined amount of the core material 29 is wound into the shell frame 11. Then either the pultruded fiber reinforced plastic block 30 or one of the pultruded fiber reinforced plastic strips 31 are positioned on top of the base of core material 29. Additional rovings of resin impregnated glass fiber are then wound about the members of the frame 11. If the embodiment of FIG. 4 is employed, after a predetermined amount of the glass fiber rovings have been wound in layers, an additional strip of the pultruded fiber reinforced plastic is inserted within the frame 11. This procedure is continued until the desired number of strips 31 are employed. For both the embodiments of FIG. 3 and FIG. 4, a final layer of core material 29, such as the resin impregnated glass fiber, is then wound about the frame 11 until the core material 29 is at least flush with the open-topped channel between the side faces 25 and 26. After the resin is cured, excess core material 29 may be removed by appropriate trimming.

FIG. 5 shows a lining material 36 inserted within each hole that is drilled through any of the members of the frame 11. As can be seen, the lining material 36 extends from the interconnecting third side face 28 outwardly to the exterior or periphery of the core material 29. The material for the lining material 36 is the same material as used in the liner material 24 which will be discussed in greater detail. As can be seen, this is applied to locations where holes are drilled through the second side member 16 for the passage of each of the conductor rods 19, as well as for the holes passed through the top member 12 and the bottom member 14 for the process outlet means 22 and the process outlet means 21, respectively. Appropriate adhesives may be employed to bond the lining material 36 to the core material 29 and liner material 24, which is clad thereto. The lining material 36 within each hole is heat welded to the liner material 24 to make a liquid-tight seal.

Similarly, the individual members 12, 14, 15 and 16 of the frame 11 may employ an adhesive to promote the cladding of the liner material 24 to the core material 29 and any pultruded fiber reinforcement plastic blocks 30 or strips 31. As best seen in FIG. 2, the first side face inner surface 32, the second side face inner surface 34, and the third side face inner surface 35 provide surface area on which the appropriate adhesive may be placed prior to the roving and winding of the core material 29 into the empty channel of the shell frame 11. An appropriate adhesive material has been found to be commercially available from Ashland Chemical Company and sold under the name CRYSTIC 392. This adhesive is especially useful with liner material made from chlorinated polyvinyl chloride.

The liner material 24 will vary depending upon whether the particular electrode is to be used as an anode or a cathode. Where electrodes will be employed as an anode, it has been found that the preferred liner material 24 is made from polyvinyl difluoride (PVDF). Alternate materials may include chlorinated polyvinyl chloride (CPVC) or polyvinyl dichloride (PVDC). The PVDF has been found to be more resistant to chlorine and therefore is especially desirable for use in chloralkali cell applications. Where the electrode is used as the cathode, the preferred liner material 24 has been found to be CPVC since it is resistant to caustic. The distinguishing characteristics must be the use of a material which is resistant to chlorine or caustic, as appropriate, and can withstand operating temperatures of approximately 90.degree. C. Other thermoplastic materials resistant to caustic or chlorine, as appropriate, may also be employed as a suitable liner material 24, such as polypropylene or products sold under the trademarks of Teflon.RTM. PFA, Teflon.RTM. ETFE, Aflon.RTM. COP or Uniroyal Rovel.RTM. thermoplastics. However, in all cases, presence of calcium, magnesium, and iron should be minimized to avoid detrimental effect to the membranes utilized in the fully assembled filter press membrane type of electrolytic cell.

Certain liner materials 24, such as PVDF, are very difficult to bond with adhesives. Hence, special grades of material are used in which bonding layers of polyester mat or glass fiber mat are pressed into one surface during manufacture. The incorporation of such a mat into the liner material 24 of the frame 11 permits a strong bond to be obtained between the core material 29 and the PVDF liner material 24.

The electrodes 10 can be of varying sizes, with side and top member lengths varying in length from about 20 inches to about 200 inches. The more commonly employed member lengths vary from about 40 inches to about 80 inches, while the preferred lengths are from about 42 inches to about 45 inches. The electrodes 10 can be either rectangular or square in shape.

The side and top members 15, 16, 12 and 14, respectively, may have cross section side dimensions that vary from about 1/2 inch to about 6 inches. The more common cross section dimensions vary from about 1 inch to about 3 inches. The preferred size has been about 2 inches by about 2 inches in section for each member.

The electrode surfaces 18 are appropriately connected to the conductor rods 19, such as by welding at selected locations. Once connected the entire electrode surface-conductor rod assembly is removably inserted into the generally rectangular or square hollow center section formed by the frame 11. The electrode surfaces 18 are those standardly employed in the industry for use in anodic or cathodic conditions.

The conductor rods 19 are about 1/2 inch to about 3 inches in diameter. The preferred size is about 1 inch in diameter. The conductor rods 19 are copper with titanium or nickel coatings for use in an anode or a cathode, as appropriate.

A gasket (not shown) may be employed between adjacent electrodes in an assembled cell by machining a suitably depthed groove, such as approximately 1/16 inch deep and approximately 1/2 inch wide, in the liner material 24 of the cathode and placing an O-ring gasket of approximately 1/8 inch diameter therein. The liner material 24 is approximately 1/8 inch in thickness. Alternate assemblies may use no gaskets or gaskets with no grooves in the cathode liner material 24. Where no gaskets are employed the liner material 24 possesses sufficient resiliency to obviate the need for gaskets by compressing tightly enough between adjacent frames to effect liquid-tight seals in an assembled cell. Where gaskets with no grooves in the cathode liner material 24 are employed in an assembled cell, the gaskets are placed flush against the appropriate side faces of the frames 11.

The frames 11 of the instant invention are made by selecting a thin, heated-weldable, corrosion-resistant liner material 24 to cover the two opposing side faces, 25 and 26, and the interconnecting interior side face 28. The shell frame formed by the heat welding of the mitred corners of the liner material 24 is then mounted on a rotatable jig so that the open-topped frame 11 has the open-topped portion facing outwardly about its entire periphery. The jig may be made from appropriate materials, such as two pieces of plywood or metal plates of appropriate thickness with accurately machined inner surfaces bolted together with spacers to give the desired frame member thickness. An axle member projects through the center of the opposing plates to permit the jig to rotate. The liner material 24 is sufficiently flexible to permit the frame members with its core material 29 to expand outwardly to conform to the shape of the mold formed by the jig. Glass fibers pass from rolls of tow through a tray filled with resin. The resin impregnated glass fiber is then would in rovings into the channel formed by the open-topped U-shaped shell frame 11 as the frame is rotated with the jig. The glass fiber impregnated with the catalyzed thermosetting resin is wound in roving fashion into the frame 11 until a predetermined thickness of the resin impregnated glass fiber is built up.

Where a solid pultruded fiber reinforced plastic block 30 or a plurality of pultruded fiber reinforced plastic strips 31 are employed, the pultrusions are positioned appropriately within the channel formed by the shell frame 11. Additional rovings of resin impregnated glass fiber are then placed on top of the pultruded fiber reinforced plastic block 30 until the level of core material 29 is flush with the tops of the first side face 25 and the opposing second side face 26. Where a plurality of pultruded FRP strips 21 are employed, additional strips 31 are dispersed between the layers of resin impregnated glass fiber rovings until the depth of core material 29 is generally flush with the tops of the aforementioned first side face 25 and opposing second side face 26.

When the desired depth of core material 29 has been achieved, plates are clamped in place against the open-topped side to exert a predetermined amount of pressure on the core material 29 during its curing process. A layer of cellophane or polyethylene sheet between the plate and the resin impregnated glass fiber may be employed to prevent adherence of the resin to the pressure plate. Preferably, the core material 29 is permitted to cure at ambient temperature. The core material 29 generally hardens within about 1 hour, but is preferably allowed to cure over a 24 hour period. Alternately, a different resin formulation may be employed that requires heating in an oven so that annealing of the liner material 24 and curing of the core material 29 may be simultaneously accomplished. After the core material 29 is cured, the frame 11 is removed from the jig. Frame 11 with the core material 29 is then trimmed of any excess to provide a smooth exterior or periphery. The thermoplastic side faces 25 and 26 of the frame 11 may be lightly machined to prepare a gasket face or groove as discussed above.

It should also be noted that the high ratio of glass fiber to resin in the core material 29 provides a frame 11 that is high strength and yet possesses a low coefficient of thermal expansion that is compatible with the expansion that occurs in the metal components of the electrode 10 during operation. The percentage by volume of glass fiber can be as high as approximately 80% or as low as approximately 45%. The preferred percentage is from approximately 60% to approximately 70%.

While the preferred structure in which the principles of the present invention have been incorporated is shown and described above, it is to be understood that the invention is not to be limited to the particular details thus presented, but in fact, widely different means may be employed in the practice of the broader aspects of this invention. The scope of the appended claims is intended to encompass all obvious changes in the details, materials, and arrangement of parts which will occur to one of skill in the art upon reading the disclosure.