CA1076994A - Molded, form retaining and electrolyte resistant filled polymeric plastic electrolytic cell frame - Google Patents

Abstract

MOLDED, FORM-RETAINING AND ELECTROLYTE RESISTANT
FILLED POLYMERIC PLASTIC ELECTROLYTIC CELL FRAME

ABSTRACT OF THE DISCLOSURE

A molded plastic frame for housing electrodes and a membrane of an electrolytic cell, which possesses good dimensional stability, is machinable, facilitates good sealings, resists attacks by electrolyte and electrolytic products and is capable of being made in various complex designs, is of a mixture of polypropylene (which may include copolymer) and a filler, such as calcium silicate, asbestos and mica or a mixture of such fillers, and includes a plurality of inlets and outlets for electro-lytic process fluids and mounting means for positioning cell components, such as the electrodes and the membrane. Passages and headers, molded into the cells, convey water and brine feeds to the cathode and anode compart-ments, respectively, preferably through metering orifices, and overflows from these compartments automatically maintain desired electrolyte levels.
Some metering orifices may be located and operated so that the discharges are discontinuous and into gas phases, to minimize current leakages (and to provide accessibility for cleaning) and the overflows, which may be such as to produce increased current paths through the liquid products, along which paths the overflow streams may be broken up, at least in part, also help to minimize current leakages.

Description

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This invention relates to a cell frame for use in making electrolytic cells and assemblies thereof. More particularly, it relates to a molded synthetic organic polymeric electrolytic cell frame having molded-in parts and passageways for holding cell elements, feeding and removing cell fluids and aligning and fastening the cell frames together in assemblies.
For the manufacture of chlorine, sodium hydroxide and hydrogen from aqueous sodium chloride solutions electrolytic cells employed have in the past usually been of either the-diaphragm or mercury cell types. In recent years, due to increases in the cost of mercury ; and pollution problems attending its use, diaphragm cells have been of increasing importance. Most such cells have in the past been made of concrete but the use of liners of various polymeric materials, including polyvinyl chloride and polypropylene, has been suggested.
With the advent of dimensionally stable anodes and effective permselective membranes it has now become possible to increase the productivity of membrane cell assemblies. Because of the possibility of using flat electrodes with a thin separating membrane between them the manufacture of thin cell assemblies has become feasible. Accordingly, massive cell wall constructions are no longer necessary or desirable and efforts have been made to discover structural materials which can be made into satisfactory electrolytic cells which are thin, readily manufactured and capable of withstanding electrolysis condit10ns, reactants and products. Efforts have been made to machine cells and cell frames ~5 from synthetic organic polymeric materials but this has often been expensiveand the product made has not been resistant to electrolytic conditions, possibly because of strains developed in the plastic during machining or because of incipient weaknesses created thereby. Machining of filled polymers exposes surfaces of the filler and this may be undesirable for long term electrolytic applications, especially in the presence of electrolyte . :

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`~ ' ~.0'7~i,9~4 ; and cell products at elevated temperatures or when the temperature varies over a range which is sufficiently wide to cause expansions and contractions which can develop weaknesses in the filled polymer surface where the filler is exposed.
Polypropylene has been utilized as an acid-resistant and caustic resistant material of construction and has been filled with various high temperature resistant materials, such as talc, mica and asbestos but has not previously been employed for the manufacture of molded frames of the present type for joinder together to form electrolytic cell assemblies.
Such prod~cts are comparatively simple to make, assemble and use and are of long life, with the frames being highly resistant to checking, warping, expanding, contracting, creeping and other distortions during the electrolysis of aqueous sodium chloride solutions.
In accordance with the invention an electrolytic cell frame is a synthetic organic polymeric frame for housing an anode and a cathode and for supporting a membrane, which frame, in coniunction with a matching contiguous frame containing an electrode of opposite sign to that nearer to it in the present frame, forms an electrolytic cell having an anode, a cathode and a membrane between them, said frame including inlets and outlets for electrolytic process fluids to anolyte and catholyte compartments so that the cell formed from two contiguous frames has at least one inlet to the anolyte compartment, at least one outlet from the anolyte compartment, at least one inlet to the catholyte compartment and at least one outlet from the catholyte compartment, each of which inlets and outlets is communicated with respective headers or manifolds integral with the cell frame. In preferred embodiments of the invention the synthetic organic polymer is polypropylene containing a filler such as asbestos, mica, calcium silicate, talc or mixtures thereof, the frame is injection molded, includes integral external alignment and fastening means for holding '~ .

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~C~7~9~4 a plurality of frames together in a cell assembly and includes molded-in mounting means for holding various cell parts in desired positions. Also, within the invention are methods of electrolyzing, utilizing electrolytic cells incorporatdng the present framesi methods for making the frames; and intermediate mounting means or frames for electrodes to hold them in desired positions in the present frames.
The invention will be readily understood from the description thereof herein, taken in conjunction with the drawing in which:
FIG. 1 is a perspective view of a cell assembly incorporating a plurality of frames of this invention;
FIG. 2 is a partial cutaway vertical sectional view of a pair of frames of this invention and parts of other frames adjacent to them, with two complete frames being combined to make an electrolytic cell;
FIG. 3 is a partially cutaway sectional elevational view along plane 3-3 of FIG. 2; and FIG. 4 is a partial vertical sectional view of the T-shaped rectangular "picture frame" intermediate mount for joining an electrode to the polymeric plastic frame of this invention.
In FIG. 1 electrolytic cell assembly 11 comprises a base 13, individual cell frames 15 mounted together with gaskets (not shown) between them for sealing purposes, compression front and rear end plate members 17 and 19, respectively, horizontal side supporting bars 21 and 23, held to base 13 and supported above it by vertical members 25, 27, 29 and 31, and rods 33, 35, 37, 39 and 41, each of which is threaded at both ends thereof and each of which has tightening nuts thereon for pressing the end plates together and thereby holding the frames together in fluid-tight relation-ship, usually with the aid of the gaskets between them. Threaded ends ; 43 and 45 and tightening nuts 47 and 49 on rod 41 are illustrated and it " 10~7699~

is evident that the other rods contain similar threaded ends and tightening nuts. A roller which functions as a bearing to facilitate sliding movement of plate 17 is identified by numeral 51. Plates 17 and 19 contain passages therein or connected to them for holding the tightening rods in position and for transmitting tightening forces to the plates. Cell frames 15, the structures of which will be further illustrated in FIG'S. 2 and 3, contain external alignment means 53, which comprises external members integral with the frames and molded (at 54) to fit and rest on support bar 23 to facilitate assembly together of the cell frames, which will be described later, in conjunction with FIG'S.

2 and 3.
In FIG. 2 wherein the combinations of cell frames to form electrolytic cells are shown without the external parts thereof, except for electrical connectors, and from which, for the sake of clarity, upper inlet and outlet passages and manifolds are not illustrated (they are shown in FIG. 3), cell frames 15 are held together, with gasket 61 or a plurality of gaskets between them holding them in fluid-tight contact, thereby forming an , electrolytic cell when electrodes and an interposed membrane are in place.
'~ Frames 15 have walls 63 which bound the cell created by joinder of the frames and prevent leakage into the next adjoining cells, 65 and 67. Cell ; 69, formed by joinder of frames 15, includes anode 71, cathode 73 and membrane 75, shown herein held to the inner, inactive face of anode 71.
Anode 71 is electrically and mechanically (by welding) connected to conductor 77 and cathode 73 is similarly connected to conductor 79. Both such conductors are positioned substantially vertically and have upper ends thereof passing through open~ngs 81 and 83, respectively, in the frames and into contacts with connectors 85 and 87, through which electricity is fed to cell 69 from the cathode conductor of cell 67 and is transmitted from cell 69 to the anode conductor of cell 65. Sealing means 89 and 91 are provided to make a fluid-tight seal to prevent gas or liquid loss.

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As is shown for the anode conductor exit from cell 65, the seal comprises a plurality of resilient neoprene washers or 0-rings 93 which are compressed together against a ledge 95 by tightening of a cap 97 through threaded collar 99 against force transmitting rigid washer 101.
Thus~ the washers or 0-rings or fasteners of other suitable types, such as wedges, are compressed against the anode conductor and hold it tightly in place, while preventing fluid loss from the cell.
Conductors 77 and 79, and through them electrodes 71 and 73, are supported in place by supporting brackets 103 and 105, which are fastened by screws 107 and 109 to molded-in bosses 111 and 113. The conductors may be welded or otherwise positively held to the brackets for additional support.
As illustrated, cathode 73 fits into a prepared opening in the frame at the cathode ends and is additionally supported by the pressure of the gasket against it when the cell frames are tightened together. To improve the rigidity of the anode, it is mounted on an angle frame or series of angle brackets 115, which frame is held in place with respect to body frame 15, by suitable means, including screws, cements, pressure holding by gasket 61, etc. The use of the angle 115 or a T-shaped "picture frame" type of inter~ediate mounting means, such as is illustrated in FIG. 4 and which is interchangeable with the present angle, serves to provide additional strength and prevents undesirable flexing of the anode material.
Membrane 75, in the present embodiment shown held against anode 71, is maintained in position by being sealed in place between angle 115, frame portions 117, 119 and 121 and also in suitable channels or grooves such as those at 123 and 125 holding members 127 and 129, held in place on frame 15 by screws. It will be noted that in the illustration given the membrane is not perfectly vertical at all locations but is pressed ., .

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against by the anode to help tighten it in place. It is important that the distance between the electrodes be maintained constant for best electrolytic effects and it is also important for the membrane to be held tightly against the anode to prevent chlorine gas generated at the active side of the anode, away from the membrane, from collecting between the anode and the membrane. Similarly, the membrane-cathode distance should be maintained constant to prevent interference with the removal of hydrogen gas generated and allow free flow of electrolyte.
It will be evident that in other embodiments of the invention, wherein the membrane is positioned between the two electrodes with clearances between the membrane and the electrode, it may be desirable to have the active side of the anode facing the membrane or to have both sides of the anode active or when the membrane is to be placed against the cathode the structure employed may be modified accordingly.
An important feature of the present invention is in the molding-in of the various means for adding and removing fluid from the electrolytic cells produced, so that it is unnecessary to drill or bore such passageways in the frame. In FIG. 2 the only such passageways illustrated are for feeds to the anolyte and catholyte compartments and the anolyte feed header or manifold. Other feed headers and discharge passageways and headers are shown in FIG. 3. Anolyte header 131, molded into frame 15, communicates through passageway 133 with anolyte compartment l35 of cell 69. At the end of passageway 133 nearer to the anolyte compartment an orifice plug 137 is screwed into place in previously molded-in threads in frame 15 or is otherwise fastened therein so as to restrict the passageway to the orifice shown at 139. In a similar manner, a catholyte feed manifold 134 (FIG. 3) communicates through passage 141 with catholyte compartment 143 and is stopped down by orifice 145.

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Use of the orifices, which are interchangeable with others of different sizes, allows regulation of flow rates into the cell.
In FIG. 3 cell frame 15 and cell 69 include an additional optional feed manifold 147, chlorine outlet manifold 151, hydrogen outlet manifold 149, caustic soda solution take-off header 153 and depleted anolyte take- ~
off header 155. It will be noted that overflow passageways 157 and 159 -carry the respective catholyte and anolyte effluents to the headers from anolyte 161 and catholyte 163 (behind the membrane). Frame lS also contains optional anolyte feed header 167 and optional catholyte feed header 165, both of which may be connected by channels or passages, not shown, to the cell interior. Such passages may be drilled in the frame if it is desired to use feeds from the top of the cell or may be molded in and plugged when not in use. Similarlyj if it is desired not to use bottom feeds, passages 133 and 141 may be plugged. Manifold 147 is useful when one wants to employ a plurality of membrane, forming a buffer compartment, to which feed is to be added. As was mentioned with respect to the other optional headers, a passageway may be opened up or plugged, according to the desired use.
Although not specifically illustrated, it is within the invention to employ "tortuous passage" feed and outlet streams so as to increase the resistance of any stream through which current leakage might occur.
Thus, with respect to feeds of liquids through orifices, because of the smaller streams the resistances thereof will be greater and current leakages will be diminished. This will be especially effective with respect to the upper feed locations when orifices are employed which are small enough to produce droplets of feed liquids which are discontinuous ,' as they fall through the chlorine and hydrogen gas phases above the electrolyte in the anode and cathode compartment, respectively. Such j discontinuous streams prov1de very high resistance against current leakage.
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Similarly, such interrupted streams may be produced wherein the overflow is withdrawn, with the cell liquor and depleted anolyte dropping from the cell electrolyte levels to lower levels, in which dropping discontinuous "streams" are created. Of course, the electrolytes may also be broken-up when removed from the cell bank to prevent leakage through them. In this respect, the polymeric plastic material of construction for the cell frames is useful because it is non-conductive.
In FIG. 4 is shown a part of a T-shaped "picture frame" intermediate mounting means for holding an electrode, such as expanded mesh anode 71 of FIG's. 2 and 3 to frame 15. In FIG. 2 the framework 115 is shown as an angle and in FIG. 4 it is a T 169, having a horizontal leg portion 171, and upper vertical "top" part 173 and a lower vertical "top" part ' 175. The upper part of the T top 173 is held to a body portion of frame 15 and the lower portion has the expanded mesh anode 177 held to it. As shown, a clearance is created between anode 177 and membrane 179 which is held between the upper part of the T and the gasket 181. Of course, by changing proportions and locations, it can be arranged that this clearance is greater, lesser or non-existent.
In modifications of the structure illustrated, a monopolar type of cell is readily produced by merely changing the electrical connectors so that ealch cell is independently charged with an electrolyzing voltage.
Similarly, electrical connections between bipolar cells may be made internally, rather than externally of the cell. Instead of having the electrical connections at the top of the cell they may be at the side thereof. The major separators in the cell, shown as vertical central structural members of the frames, may be replaced by pluralities of such members, both being thinner, to conserve plastic. They may be moved to either side of the cell frame but it is preferred to have them substantially centrally located, as indicated, for dividing ~he cell into g ~769~4 approximately equal compa~tments and for greater strength and dimensional stability of the molded plastic frame. Instead of using the neoprene washers, which are squeezed tightly against the electrode conductors by compression of a packing nut, other forms of seals may be employed, including single 0-rings, cylinders, hollow cone wedges, etc. Such materials may be of neoprene or other suitable polymers, e.g., poly-tetrafluoroethylene and other fluorinated polymers. Instead of using positively held means for fastening the membrane and/or electrodes in - place, such means may be elastic, such as neoprene bands, pressing the gasket or other parts against the frame during assembly, possibly fitting into hollows in the frame. Such temporary means can be employed until the various frames are tightly pressed together during construction of the cell assembly. Alternatively, instead of employing such means, a cement, such as neoprene cement, which holds the gasketsl electrodes and membranes in place temporarily, can be used. Such cement should be sufficiently strong to hold the various parts together in correct relationship until "permanently" fastened by pressing together of the various frames. Upon disassembly the cement would not so tightly bind the various parts as to make them unremovable without permanent damage. In some cases, it may be possible to dispense with the use of gaskets and employ mating parts of the polypropylene frames which are so tightly held together by the assembly compressing means as to maintain the various interposed parts in desired "permanent" relationship without the need for softer gasketing materials to prevent leakages. Where one dispenses with the neoprene gaskets the resilience of the polypropylene frame may be increased by including rubber or other elastomeric materials in the molding compositions, e.g., 5 to 25% of ethylene propylene elastomer.
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The cells illustrated in FIG'S 2 and 3 are those of a centralportion of the cell assembly. It is evident that the terminal frames are of a different design, being essentially half cell frames with the exterior sides blanked off. It is not thought to be necessary to il-lustrate such structures here because they are considered to be self-evident.
: The only material of construction of the present cell frames which has been found to be satisfactory in use and can be injection molded readily is polypropylene, more particularly, polypropylene filled with a suitable "filler" such as one selected from the group consisting of asbestos, mica, calcium silicate, talc and mixtures thereof. The mix-tures of fillers mentioned may be of two, three or four components but it is considered especially beneficial to include both the calcium silicate and mica in such mixtures. In some cases art-recognized equivalent fillers may also be used. I~ only one of the fillers is to be employed it is preferred that it is calcium silicate, of which the , fibrous form is considered to be best. The polypropylene referred to may be a normal resin intended for injection molding, such as the un-modified copolymer, inert filled or impact (rubber modified) polypro-pylenes that result in products of the properties given in 1973-1974 Modern Plastics Encyclopedia, at page 552. Thus, such resins, when injection molded according to the methods described at pages 338-410 of that publication, produce useful electrolytic cell frames suitable for employment in "filter press" assemblies of from about 10 to 60 of such frames, for the electrolysis of aqueous sodium chloride solutions.
The frames described resist the electrolyte and products of electro-'r,( lysis and are satisfactory dimensionally stable during electrolysis, even ` over temperature variations from 40 to 95C. and pH variations from 3 to 14. Although it is possible to utilize polypropylene resin containing _ 11 , ~,~

' 1~'7~9~9~

no copolymer and no rubber, providing that the desired content of "filler" sufficiently improves the dimensional stability, heat resistance, etc., to make the material satisfactorily operative in commercial chlorine cells, usually one will employ a mixture of homo-polymer and copolymer. In such mixture the proportion thereof will normally be from 10% to 40% of copolymer, such as that sold by Shell Chemical Company as Shell 7525 and 10 to 40% of homopolymer, with the total content of such components being 40 to 90% of the final product. Instead of the homopolymer, copolymers of it with other materials may be utilized, e.g., propylene-acrylic acid copolymers such as Exxon D-561. The proportion of rubber impact modifier may be from 0 to 20% but it is preferably present and will normally be from 3 to 15 of the product. When the term "polypropylene" is used herein in a general sense it is inclusive of copolymer and copolymers of homopolymer and linking agents such as unsaturated lower organic acids, e.g., acrylic acid, methacrylic acid and other monoalkenoic acids of 3 to 6 carbon atoms and equivalents, as well as homopolymers.
The "filler" while it may talc, asbestos, calcium silicate or mica or a mixture thereof, preferably includes calcium silicate fibers, such as Wollastonite fibers or synthetic calcium silicate fibers and may preferably include additionally, mica flakes or platelets. The calcium silicate, either natural or synthetic, asbestos and talc fibers may be of a variety of diameters commercially available, for the -180 Angstroms for chrysotile asbestos to as much as one millimeter, although usually the diameters will be less than 0.1 mm., e.g., 0.001 to 0.05 mm.
Fiber lengths will normally be in the range of 10 to one million times the diameter, preferably 20 to 1,000 times, and will normally be in the range of 1 mm. to 2 cm. Similarly, the mica employed will usually be finely enough divided to pass through a 140 mesh screen, preferably through a "' ~~ io ~99,~

.
200 mesh screen, (United States Standard Sieve Series). Although f these sizes are mentioned as guides, it will be evident that in somecases it may be desirable to utilize different sizes of materials for special effects.
The proportion of the described inorganic filler material in the present injection molded frames is from 10 to 60%, preferably 15 to 50% and more preferably 20 to 40%. Preferably from 50 to 100%
thereof is calcium silicate fiber. However, in some embodiments of the invention from 10 to 30% of asbestos will be employed with 90 to 70% of polypropylene homopolymer or 5 to 20% of mica flake, 10 to 30% asbestos fiber, 15 to 50% of polypropylene homopolymer and 20 to 60% of polypropylene copolymer may be utilized and will produce satisfactory products. Instead of some of the calcium silicate there may be substituted ~! an e~ual proportion, from 10 to 50% thereof, of talc powder. Also, the calcium silicate and/or talc may be treated with silanes or silicones to modify the silanol and siloxane groups thereon and to vary properties of the molded polypropylene. Various rubber impact modifiers of known types may be employed for their obvious purpose. The rubbers utilized may be any of those normally acceptable for this purpose in the polymer art ! 20 and mixtures of these may also be employed but elastomers based on poly-ethylene or polypropylene are preferred, as are those based on both such polymers.
.., Various other additives may be present in small proportions, usually to the extent of no more than 10% and preferably no more than 5%, in the present molded products, e.g., colorants, mold released agents and fire retardant chemicals. Descriptions of such materials and of the polymers, fillers and rubbers employed in making the injection moldable polypropylene resin composition utilized in this invention are described in more detail ., .

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in the 1973-1974 Modern Plastics Encyclopedia and in the 1972-1973 eclition of this work and accordingly, are not described further herein.
The injection molding of the frames is in accordance with known p~ocedures for molding large items (the frame size is about 1.1 meter by 1.1 meter by 9 cm. and the distance between anode and cathode in the cell formed by fitting together of two frames is normally about 1 cm.).
A description of such molding is found in the article entitled Giant, Thick-Sectioned Plastic Parts Achieved by New Method, appearing at page 42 of the May 1972 issue of Product Engineering and in the article en-10 titled Unusual Technique Makes 'Impossibl--e' Parts, at page 53 of the -September 1971 issue of Modern Plastics. Such molding techniques are those developed and practiced by Eimco Envirotech. Details of suitable molding methods are given in the article.
The molded frame made, although produced from thermoplastic mate-rials, does not soften at the operating temperatures of the presentelectrolytic cells, which can go as high as 95C. The cells operate continuously for lengthy periods of time, e.g., six months, without ; warping, cracking, failing, sagging or otherwise showing objectionableevidence of lack of dimensional stability. Without the reinforcing in-organic filler materials results are not as satisfactory but the molded frames are still useful. It is considered that the molding operation, which tends to cover all fibrous material near the surfaces of the molded item, thereby prevents any exposure of such reinforcing material to the contents of the electrolytic cell, thereby aiding in stabilizing the in-25 jection molded frame. Thus, such products are considered to be superior `
to similar ones wherein a cell frame is machined from stock material of reinforced polypropylene.
The materials of construction of the various components of thepresent cells are chosen for resistance to the particular environments encountered. The anode, which preferably of expanded titanium mesh, al-though other _ 14 ' .

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valve metals are also useful, e.g., tantalum, and which may be coated on an active surface thereof with a noble metal or noble metal oxide, e.g., ruthenium oxide, is resistant to the chlorine and acidic brine of the anolyte compartment. The conductor rods are utilized in such compartment are preferably of copper for good conductivity and clad with titanium for resistance to the electrolyte. The openwork portion of the anode has openings of 2 to 700 sq./mm., preferably 100 to 600 sq./mm. and the proportion of openings to the nominal single major face area of the anode is in the range of about 25 to 70%, preferably 40 to 70%. The mesh is normally from 0.2 to 2.5 mm. thick, preferably 1 to 2 mm. and the strand or section width thereof is from 0.7 to 2.5 mm., preferably 1 to 2 mm. Preferably the anode is activated with ruthenium oxide on the back surface thereof, away from the membrane.
The cathodes utilized may be of any electrically conductive material that resists the attack of cell liquor, which is comparatively high in sodium hydroxide content. Suitable cathodes are made of steel mesh and they are joired to a copper conductor but other cathode materials and conductor materials may also be employed, among which are iron, graphite, lead dioxide on graphite, lead dioxide on titanium and noble metals, such as platimum, iridium, ruthenium, and rhodium. The noble metals may be deposited as surfaces on conductive substrates, such as those of copper, silver, aluminum, steel and iron. The cathodes used preferably will be of screen or expended metal mesh and, like the anodes, will be flat or of other conforming shapes so that the inter-electrode distances will be approximately the same throughout. The openings in the cathode screen or mesh will normally be at least 25% of the surface area of a face the cathode, preferably 30 to 80% thereof and most preferably about 45 to 65% thereof. The areas of openings are usually 0.5 to 1,000 sq. mm., 10769~4 preferably 2 to 100 sq. mm. When a wire screen is employed the strands thereof will preferably be about 0.5 to 3 mm. in diameter. Both the electrodes will normally be maintained in perfectly vertical or sub-stantially vertical position, usually not being more than 10 from the vertical and preferably not more than 5 therefrom.
The presently preferred cation permselective membrane is of a hydrolyzed copolymer of perfluorinated hydrocarbon and a fluorosulfo-nated perfluorovinyl ether. The perfluorinated hydrocarbon is prefer-ably tetrafluoroethylene, although other perfluorinated and saturated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized, of which the monolefinic hydrocarbons are preferred, especially those of 2 to 4 carbon atoms and most especially those of 2 to 3 carbon atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated per-fluorovinyl ether which is most useful is that of the formula FS02CF2CF20CF(CF3)CF20CF=CF2. Such a material, named as perfluoro[2-(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to henceforth as PSEPVE, may be modified to equivalent monomers, as by modifying the ii internal perfluorosulfonylethoxy component to the corresponding propoxy component and by altering the propyl to ethyl or butyl, plus rearranging positions of substitution of the sulfonyl thereon and utilizing isomers of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.
The method of manufacture of the hydrolyzed copolymer is described . in Example XVII of U.S. patent 3,282,865 and an alternative method is mentioned in Canadian patent 849,670, which also discloses the use of the finished membrane in fuel cells, characterized therein as electroehemical cells. In short, the copolymer may be made by reacting PSEPVE or equi-valent with tetrafluoroethylene or equivalent in desired proportions in .

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water at elevated temperature and pressure for over an hour, after which time the mix is cooled. It separates into a lower perfluoro-ether layer and an upper layer of aqueous medium with dispersed desired polymer. The molecular weight is indeterminate but the equivalent weight is about 900 to 1,600 preferably 1,100 to 1,400 and the percentage of PSEPVE or corresponding compound is about 10 to 30%, preferably 15 to 20% and most preferably about 17%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or membranes, which may vary in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze pendant -S02F groups to -S03H groups, as by treating with 10% sulfuric acid or by the methods of the patents previously mentioned. The presence of the -S03H groups may be verified by titration, as described in the Canadian patent. Additional details of various processing steps are described in Canadian Patent 752,427 and U.S. Patent 3,041,317.
Improved versions of the above-described copolymers may be made by chemical treatment of surfaces thereof, as by treatments to modify the -S03H group thereon. For example, the sulfonic group may be altered on the membrane to produce a concentration gradient or may be replaced in part with a phosphoric or phosphonic moiety. Such changes may be made in the manufacturing process or after production of the membrane. When effected as subsequent surface treatment of a membrane the depth of treatment will usually be from 0.001 to 0.01 mm. In some instances it may be desirable to convert the sulfonyl or sulfonic acid group of the membrane on one side (such as the anode side) to a sulfoamide, which is more hydro-philic, which may be effected in the manner described in U.S. Patent 10'7699~

3,784,399. Also, the membrane may be in laminated form, which is now most preferred, with the laminae being of a thickness in the range of 0.07 to 0.17 mm. on the anode side and 0.01 to 0.07 mm.
on the cathode side, which laminae are respectively, of equivalent weights in the ranges of 1,000 to 1,200 and 1,350 to 1,600. A
preferred thickness for the anode side lamina is in the range of 0.07 to 0.12 mm. thick and most preferably this is about 0.1 mm., .,. , - .
with the preferred thickness of the lamina on the cathode side being O.Q2 to 0.07 mm., most preferably about 0.05 mm. The pre-ferred and most preferred equivalent weights are 1,050 to 1,150 and 1,100 and 1,450 to 1,550 and 1,500, respectively. The higher the equivalent weight of the individual lamina the lesser the thickness preferred to be used, within the ranges given.
In addition to the copolymers previously discussed, including modifications thereof, it has been found that another type of mem-brane material is also superior to prior art films for applications in the present processes. Although it appears that tetrafluoro-ethylene (TFE) polymers which are sequentially styrenated and sulfonated are not useful for making satisfactory cation-active permselective membranes for use in the present electrolytic processes it has been established that per~luorinated ethylene propylene polymer (FEP) which is styrenated and sulfonated makes a useful membrane. The sulfostyrenated FEP's are surprisingly resistant to hardening and otherwise failing in use under the present process conditions.
Examples of useful membranes made by the described process are products of RAI Research Corporation, Hauppauge, New York, identified as 18ST12S and 16ST13S, the former being 18% styrenated and having 2/3 of the phenyl groups monosulfonated and the latter being 16% styrenated . .

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and having 13/16 of the phenyl groups monosulfonated. To obtain 18%
styrenation a solution of 17-1/2% of styrene in methylene chloride is utilized and to obtain the 16% styrenation a solution of 16% of styrene in methylene chloride is employed.
The membrane walls will normally be from 0.02 to 0.5 mm. thick, preferably from 0.07 to 0.4 mm. and most preferably 0.1 to 0.2 mm.
Ranges of thicknesses for the portions of the laminated membranes previously described have already been given. When mounted on a polytetrafluoro-r ethylene asbestos, titanium or other suitable network, for support, the network filaments or fibers will usually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15 mm., corresponding to up to the thickness of the membrane. Often it will be preferable for the fibers to be less than half the film thickness but filament thicknesses greater than that of ~ the film may also be successfully employed, e.g., 1.1 to 5 times the film ; 15 thickness. The networks, screens or cloths have an area percentage of openings therein from about 8 to 80%, preferably 10 to 70% and most preferably 30 to 70%. Generally the cross-sections of the filaments will be circular but other shapes, such as ellipses, squares and rectangles, are also useful. The supporting network is preferably a screen or cloth and although it may be cemented to the membrane it is preferred that it be fused to it by high temperature, high pressure compression before hydrolysis of the copolymer. Then, the membrane-network composite can be clamped or otherwise fastened in place in a holder or support. To maintain the desired spacings between membrane and one, the other or both electrodes, gaskets and/or vertical wires or strips of suitable plastic material, e.g., polytetrafluoroethylene, are utilized. Generally the cell width will be from 0.3 to 1 cm. and the distance from the anode to the membrane will be from 1.5 to 6 mm. but may be O mm., too. Similar measurements also apply to cathode-membrane distances.

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After the injection molding of the cell frame at elevated temperature and pressure the frames are assembled into cells and into a cell assembly, as shown in FIG'S. 1-4. When the T or right angle picture frame for holding the electrode (and possibly also the membrane) in place is utilized it will be of a material which withstands the electrolyte with which it is brought into contact. Thus, when the frame is on the anode side it will often be of titanium or titanium clad metal and when on the cathode side will usually be of soft steel. Similar rules apply to various molding devices, supporting brackets, etc.
After assembly of the frames into cells and tightening of these into a cell assembly, as shown in FIG. 1, the cells are charged with electrolyte and electrolysis is begun.
The anode compartment is filled with a nearly saturated salt solution or brine of sodium chloride content of about 25% and the cathode compartment 15 is filled with water, initially containing a small quantity of salt or brine to improve its conductivity. Current is turned on and chlorine and hydrogen produced in the cells are taken off continuously, in some embodiments of the invention with the chlorine and anolyte being separated after removal from the cell rather than before. Usually sodium hydroxide 20 iS removed continuously during electrolysis but it may be removed at the completion of an electrolytic cycle. Depleted anolyte is passed through ` a resaturator wherein sodium chloride content is increased and it is then returned to the anode compartment. Generally, the sodium chloride content of the withdrawn anolyte is about 22% by weight and that of the returned 25 anolyte from the resaturator is about 25%. The anolyte may be acidified and preferably is of a pH in the range of 1 to 6, preferably 1 to 5 ' and most preferably about 3 to 4.5, e.g., 3.9 to 4.3. Of course, the catholyte pH is about 14, due to the high content of sodium hydroxide.

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The temperature of the electrolyte will be maintained at less than 105C., preferably being 20 to 95O and more preferably being about 80 to 95C. Electrolyte temperatures may be controlled by recirculation of portions thereof, by regulation of proportions of feed for the various zones and by changing the temperatures of the feeds. Refrigeration and other cooling means may also be employed.
The voltage impressed per cell will usually be from 2~3 to 5 or 6 volts and is preferably in the range of 3.3 to 4.3 volts. Sometimes in a preferred method it may be as high as 4.5 volts. Most preferably the voltage will be from 3.3 to 4.1 volts. The current density will generally - be from 0.2 to 0.5 ampere/sq. centimeter, preferably about 0.3 ampere/sq.
cm. The take-off of caustic from the catholyte compartment is at such a rate that the caustic produced is at a concentration of 5 to 45%, preferably 5 to 25% and most preferably about 8 to 12%.
The following examples illustrate but do not limit the invention.
Unless otherwise mentioned, all parts are by weight and all temperatures are in C.

2~ A frame of the type illustrated in FIG'S. 2 and 3 is injection molded by the Eimco Envirotech method for injection molding large plastic pieces, at normal elevated injection molding temperatures and pressures.
The mold is so constructed that passageways, bosses, channels, ledges, ribs, manifolds, alignment and tightening means are molded into the frame produced. Where feasible, threaded or partly threaded openings are also molded so as to be available for and receptive to threaded orifices or other parts of the cell frame. The frames are molded of a mixture of ' 25% of calcium silicate fiber, 10% EP rubber impact modifier, 37.5%
- Exxon D-561 propylene-acrylic acid polymer and 27.5% Shell 75~5 poly-' ., .

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propylene copolymer and the products made are examined and tested. It is found that the surfaces of the frames are richer in resin than the interiors and few or no fibers of inorganic material are at the surfaces, uncovered by resin. Accordingly, when tested in representative aqueous electrolyte suitable for use in an electrolytic cell for the production of chlorine and caustic from sodium chloride solution, the frame shows no significant weakening, possibly because there are no incipient cracks or points at which the calcium silicate is exposed. Also, when tested for dimensional stability at temperatures over the range of 40 to ~5C., which temperatures may be used in normal electrolysis with this type of cell, the frames are stable and do not warp, crack, check, craze or otherwise distort.
~The frames made are assembled into cells, using neoprene gaskets, ; titanium "picture frame" intermediate retainers for the anodes and similar steel picture frame retainers for the cathode, with nylon screws and poly-propylene rectangular "ring" members for holding the membrane in the frame and tightly against the anode. The anode employed is an expanded titanium mesh anode and the cathode is a steel screen. The conductor to the anode is titanium clad copper and that to the cathode is copper. The anode expanded mesh is of a thickness of about 2.0 mm. and the strand width is about 2 mm. The mesh is of diamond configuration with a long axis being horizontal and includes about 50% open area at a surface thereof. Distances across the diamonds are 0.75 cm. and 1.25 cm. The anode is coated on the side away from the membrane with an active coating of ruthenium oxide 0.07 mm. thick, which is applied according to standard methods known for making dimensionally stable anodes. The cathode is of mild steel wire mesh, about 2.0 mm. in equivalent diameter, with about f 50% open area.

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The membrane is of the PSEPVE type (hydrolyzed) described in the foregoing specification and is a laminate 0.17 mm. thick, of which thickness 2/3 is of an equivalent weight of about 1,100 and 1/3 is of an equivalent weight of about 1,500. The high molecular weight side of the membrane faces the cathode and the flatter, thicker, lower molecular weight side, backed with a polytetrafluoroethylene supporting network of filaments or threads of diameters of about 0.3 mm., woven into a screen or cloth which has an area percentage of openings therein of about 22%, is pressed tightly against the anode.
The frames and cell walls are about 1.1 meter x 1.1 meter and the cells are about 11 cm. thick. The cell walls and other plastic parts thereof in the cell, such as the bosses, walls defining the headers and passageways, and ledges and channels are usually from 1 to 3 cm. thick.
Between the cathode and the membrane is a series of vertical polytetrafluoroethylene flexible separating cylinders or lines, each 2.5 mm. in diameter, which are employed as vertical spacers every 15 cm.
across the membrane-cathode gap and together with the gasket, which is of a thickness such as to produce about a 2.5 mm. gap between the cathode and the membrane, these maintain the membrane at a regular distance from the cathode and hold it tightly against the anode.
In the assembly of the cell the various frames are positioned on the temporary aligning means or bar, the electrodes are applied and the membranes are held in place by the temporary cement (Velcro or other suitable temporary and permanent seals can also be used) and later by the sealing gaskets. Before sealing the various frames together the membrane is normally conditioned in position on the frame by application of a suita61e solvent, e.g., glycerol, which prevents drying of the membrane during cell assembly and is especially useful, as in the present case, when a plurality of cells (35) is being joined in a cell assembly.

.
, .

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7~994 After assembly of the cells together in desired position, in filter press type arrangement, they are drawn tight so as to pre~ent flu-id leakages. See FIG. l for a view of the assembled cell stack or bank. They are then connected to sources of feeds and electricity and to discharge piping.
Next, the anode compartments are filled with a nearly saturated acidified salt solution of about 25% concentration and of a p~ of about

4.1 and the cathode compartment is filled with water, initially containing a small quantity of sodium hydroxide to improve its conductivity. The current is turned on and a current density of 0.3 ampere/sq. cm. results.
The voltage drop across each cell is noted during equilibrium production of aqueous sodium hydroxide solution, chlorine and hydrogen. Under such conditions the cell voltage drop is about 4 volts, the load is about 3 kiloamperes and the total voltage is about 120 volts D.C. At an operating 15 temperature of about 90C and at other temperatures in the 85 to 95C range the stack or cell assembly produces about 3 tons per day of chlorine at a chlorine efficiency of about 96%, with acid addition, and a caustic efficiency of about 90% at a caustic concentration of 80 to lO0 g./l.
Chlorine efficiency without acid addition is on the order of about 90%.
After about six months operation under the conditions described above the cell stack is opened and the electrodes, membrane, frame, gaskets and ; fastening means are inspected. The electrodes and the membrane are still ; operative and the frame shows no signs of significant weakening or dis-tortion, despite the fact that during the normal operation of the cells, including some shutdowns, the temperature varies over the range of 40 to 95~C. The neoprene cement used is still readily removable from the membrane and frame wall, so that the membranes can be employed again. In a modification of the experiment, when Velcro fasteners are employed to hold the membranes in position, they are readily separable and the membranes are removable without damaging them.
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In variations of this experiment, the filled polypropylene resin employed is one made from a resin mix of 30% of the described calcium silicate fiber, 10% EP rubber impact modifier, 45% Exxon D-561 propylene-acrylic acid copolymer mix and 15% of Shell 7525 polypropylene copolymer.
The results obtained are almost as good as those of the formula previously given except that it is noted that the frames are somewhat more susceptible to cracking under extreme conditions. In a further variation, there is employed 20% of asbestos and 80% of polypropylene homopolymer. Although this frame is a useful one, it is not as good as that initially described.
However, an ;mprovement on it, made and tested the same way as described above, is one in which the resin mix is of 20% of asbestos, 10~ mica, 40% of homopolymer and 30% of copolymer.

The procedure of Example 1 is repeated, utilizing the 30% calcium silicate fiber - 10% EP rubber impact modifier - 45% Exxon D-561 propylene-acrylic acid copolymer resin - 15% Shell 7525 polypropylene copolymer formula but employing as the membrane the unlaminated PSEPVE hydrolyzed copolymer having a thickness of about 0.2 mm. with the same type of rein-forcing polytetrafluoroethylene screen. In a similar manner, there are substituted for the membrane RAI Research Corporation membranes identified as 18ST12s and 16ST13s, of equivalent thicknesses. Utilizing 0.3 ampere/sq.
cm. current densities in all such cases the cells operate satisfactorily without damage to the frame material from either the acidic anolyte or ; alkaline catholyte. In modifications of the cells the ruthenium oxide on titanium anode is changed so as to be active on all surfaces thereof and the cathode-membrane distance is lowered to 2 mm. At such conditions efficient electrolysis is effected without damage to the framing material.

. ~ . . . . . .

Also, when the electrodes are held by T-shaped intermediate holding members they are less apt to sag than when merely gasket held, aithough -angle frames are also satisfactorily rigid.
The cells of this example also work satisfactorily when the membranes S are held against the cathodes or are positioned halfway between the anodesand cathodes, held in desired positions by Teflon line spacers l.S mm. in diameter.

Frames are made by the method described in Example 1 from 35% of polypropylene homopolymer, 35% of polypropylene copolymer and the remainder (no impact modifiers present) of mica flakes (200 mesh), calcium silicate fibers (Wollastonite), calcium silicate fibers (synthetic), talc or asbestos (chroysotile). Unmodified homopolymer, copolymer and 50:50 homopolymer-copolymer mixtures are also used. Frames are molded of each of these. When tested by practical use test, in electrolytes under electrolyzing conditions, it is found that the use of the calcium silicate fibers in a homopolymer-copolymer mix is best and that the filled polymers are superior in physical properties, especially dimensional stability and heat resistance, to the unfilled materials. However, while the differences are important it is possible to utillze all of the built polypropylene frames in commercial application and the unbuilt frames, while inferior, are still operative.
Frames made of other polymers, such as polyvinyl chloride, polytetrafluoro-ethylene and polyethylene, while not normally commercially acceptable - for long term use, are operative for short term use in electrolytic cells and are advantageous when they have molded into them by ordinary injection molding techniques, when applicable, the various headers, passageways, orifices, alignment, mounting and fastening means of the apparatuses of this invention.

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Still, the combination of polypropylene frame and cation-active permselective membrane of the DuPont Nafion XR type, especially when the membrane is held directly to the frame, without intervening gaskets, is best, with respect to dimensional stability, chemical resistance and long life in use and is preferred.

The procedures of Example 1 are repeated but the temperatures, voltages and pH's of the various processes described are varied over the ranges given, from 20 to 95C, from 3.3 to 4.3 volts, from 3 to 4.5 anolyte pH and from 0.2 to 0.4 ampere/sq. cm. Also, the electrodes are varied over the open area size and proportion ranges and inter-electrode distances given in the preceding specification. In such cases, adequate electrolysis results and the frames are sufficiently stable to be commercially acceptable.
No significant problems are encountered in the operations of these cells ~, 15 and the cost of manufacture of the frames is diminished to an important extent because of the ease of making the frames by injection molding.
Furthermore, with the pre-molded bosses, ledges, channels, etc. in the frames, assembly of parts is facilitated and is accomplished in shorter periods of time, decreasing the expense of celi assembly. Accordingly, the present cells represent an important advance in the art of electrolysis of aqueous hallde solutions.
The invention has been described with respect to illustrations and specific working examples thereof but is not to be llmited to these because it is evident that one of skill in the art with the present specification before him will be able to utilize substitutes and equivalents without departing from the invention.
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Claims

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

A synthetic organic polymeric electrolytic cell frame for housing an anode and a cathode and for supporting a membrane, which frame, in conjunction with a matching contiguous frame containing an electrode of opposite sign to that nearer to it in the present frame, forms an electrolytic cell having an anode, a cathode and membrane between them, said frame including inlets and outlets for electrolytic process fluids to anolyte and catholyte compartments so that the cell formed from two contiguous frames has at least one inlet to the anolyte compartment, at least one outlet from the anolyte compartment, at least one inlet to the catholyte compartment and at least one outlet from the catholyte compartment, each of which inlets and outlets is communicated with respective headers or manifolds integral with the cell frame, and in which the synthetic organic polymer is a blend of polypropylene copolymer, propylene-acrylic acid polymer, rubber modifier and a filler selected from the group consisting of asbestos, mica, calcium silicate, talc and mixtures thereof.

An electrolytic cell frame according to Claim 1 which is of molded construction so that the surfaces thereof are richer in polymer than the interiors thereof, to improve resistance of the frame to the electrolye in contact with it.

An electrolytic cell frame according to Claim 2 comprising about 15 to 50% of calcium silicate fibers, mica flakes or a mixture thereof and being resistant to distortion during use in an elect-rolytic cell for the production of chlorine and sodium hydroxide from aqueous sodium hydroxide solution.

An electrolytic cell frame according to Claim 2 which includes molded-in mounting means for holding electrical conductors for electrodes in desired positions and which includes mounting means for holding in position an intermediate mount for an electrode.

An electrolytic cell frame according to Claim 4 wherein the intermediate mount is of a rectangular "picture frame" T-shape with the leg of the top T being horizontal and the top thereof being vertical, one portion of the top being adapted to be fastened to the frame and the other being adapted to be fastened to the electrode, whereby the electrode is rigidified by the T-frame and thereby held in desired position.

A method of making a frame for an electrolytic cell for the electrolysis of aqueous solutions of sodium chloride which comprises injection molding such frame from a mixture of polypropylene and filler therefor, which filler is selected from the group consisting of asbestos, mica, calcium silicate, talc and mixtures thereof, in the form of an integral member of parts of two different cells, each of which is joinable to another such frame to form a complete cell, which frames and formed cells have molded-in feed and discharge passageways and mounting means for holding cell parts.