US 1929923 A
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Patented Ooi. 1o, 1933 UNITED STATES PEBMEABLE -DIAPHRAGM Henry C. Howard and Ervin S. Kern, Akron, Ohio, assign'ors to American Anodc, Inc., Akron, hio, a corporation of Delaware Application September 16,1929 Serial No. 393,133
A5 Claims. (Cl. ZIM-fzs) This invention relates to the arts of electroplating and electrodeposition and has as its object to provide an improved porous diaphragm permeable to the electric current, and particular- 5 1y a diaphragm having an unusually high capacity for electro-endosmosis.
Permeable diaphragms are employed in many electrolytic processes'for the purpose of separating the different uids employed. For example,
they may be employed .in the cells used for the electrolytic decomposition of water into oxygen and hydrogen, in orderto preventv the readrnixture of these two gases. They are also frequently employed in galvanic cells to separate the dif Aferent solutions which are placed in contact with the anode andcathode. Of particular importance, however, are the diaphragms which are frequently employed in the cells used for the electrophoretic deposition of suspended particles from aqueous dispersions.
In the production` of coatings by electrophoresis it is of the utmost importance that no gas bubbles be formed or entrapped within the' 25. deposited coating to render it porous.` Gas evolution at the anode-upon which such deposits are ordinarily formedis 'readily prevented by employing an anode of a metal such as zinc or cadmium which readily goes into solution under the influence oi the current. Elimination ofthe gas at the cathode, however, is more diilicult and is ordinarily eilected by interposing a permeable diaphragm between the cathode and the disper-` (chiey water) from the dispersion at the same time that the dispersed particles are-'removed by electrodepositionfand coagulation, and at such a rate that the concentration ofthe 'dispersed material remaining in the liquidv is mainiotained substantiall` unaltered.. When a porous diaphragm is disposed adjacent the cathode, the
electro-endomotic flow of the aqueous liquid is'v normallydirected toward' the cathode, so that by choosing -a diaphragm through which the sa" new iosumcienuy great, the withdrawn or the dispersion medium through the cathode diaphragm can be caused'to correspond in quantity to the deposition of the dispersed substance upon the-anode, leaving the dispersion reduced in quantity but unchanged in concentration.
We have discovered that sheets of coagulated, but undried, rubber are particularly useful diaphragrns for any of the above purposes. Their permeability to the .electric current is high,
while the pore sizel is suiilciently small that susl Y pended particles cannot penetrate them. On the other hand, the pores are `of such a uniform size, and the character of the material is such that an unusually high electro-endosmotic flow may be attained. y 70 It is well known that if the current is maintained at a constant value, the volume of liquid transmitted through a single pore or capillary by electro-endosmosis is independent of the di'- ameter and length of the capillaries, therefore, the amount transmitted per unit are of diaphragm if the current over the unit area.l re'- mains constant is independent off the number or sizeof the capillaries and. depends Vonly on the viscosity of the liquid, on its dielectric con.- stant, and on the potential at the electrical double layer at the interface between the liquid and the wall of the capillary., That is to say that'the electro-endosmotic ow through a dia-i pliragm canbe modified by changing the char- 85 acter'istics of either the liquid or the material of the diaphragm: Since itis ordinarily undes irable to modify thev liquid to any great extent, the' characteristics ofv the diaphragm will ordinarilyl be modified to meet different condtions, the maximum flow.) being attained by employing diaphragms made .by the coagulation of rubber latex without the addition of foreign substances, and variations therefrom being obtainable by suitable pigmentation or sub` 95 stitution of artificial aqueous dispesions.
Because of 'thefa'bove noted characteristics, the diaphragms employed in the process of this invention may be made from natural or artiiicial dispersio'ns of rubber or rubber-like sub- 100v stances either with or without the addition of other suspended or dispersed materials, but are preferably made from the substantially unadulterated natural latex of rubber, such as the ammomia-preserved latex of commerce, derived '105A from the Hevea brasiliensis. Articial dispersions of rubber tend to give rise to soft dia-` phragms of little mechanical strength, which are readily` injured when not carefully handled. The addition of considerable proportions of llers likewise reduces the strength of the diaphragms, and also impairs their electro-endosmotic efficiency. Nevertheless, for some purposes such modifications of the materials from which the diaphragms are made may be very desirable.
The diaphragms are readily prepared by the deposition of the above-mentioned rubber dispersions on forns of the desired size and shape. The deposition is effected by ionic diifusion, either alone or assisted by the electric current. In either case the deposition may take place either on a smooth-surfaced form or on a foraminous supporting webV which may be retained within thefinished diaphragm.
In the accompanying drawing Fig. 1 represents a plain diaphragm and Fig. 2 a diaphragm -reenforced with a wire fabric, which is shown.
. inch thickness is formed. About 15 minutes will be required. The deposit is; then carefully removed from the mold.
The electrodeposition is effected in the well known manner upon a metallic anode or upon a porous form impregnated with a suitable electrolyte such as alkaline ammonium chloride. The electric current simultaneously causes an electrophoretic migration of the rubber particles toward the anode and a movement of coagulating ions from the anode toward the deposit. As a result the deposit formed is somewhat more firm and compact than that produced by the simple ionic diffusion described above. The concen'tration,` alkalinity; and conductivity of the latex or equivalent dispersion should be so regulated that the deposit formed is smooth, firm, and coherent, without being so dense that it is impermeable. Ammonia-preserved latex which has been aerated until the greater part of the ammonia has been eliminated and the alkalinity reduced'to a value corresponding to a pH of 9 to 9.5 is quite suitable. The current density at the anode may conveniently be from 0.05 to 0.2 amperes per square inch, and the deposition may be continued until the deposit has attained the desired thickness.
The deposited diaphragm is preferably washed thoroughly to eliminate coagulating substances from its pores, and is impregnated with a neutral or slightly alkaline electrolyte. Since coagulated rubber shrinks upon drying, the walls of the pores fusing together upon the elimination of the water contained therein, and the whole structure becoming permanently impermeable, it is necessary to maintain the diaphragms continuously moist until they are used. Preferably they are preserved byimmersion in a slightly alkaline solution containing a preservative such as phenol which prevents the decomposition or putrefaction of the sugars or proteins naturally present in rubber.
The freshly coagulated deposits are quite :liexible and extensible, hence they must ordinarily be supported by some rigid or inextensible means when they are employed as diaphragms. They diaphragm being 9/64 inches, was found to have within the deposit.
may, for example, be confined between perforated sheets of a non-conducting material such as hard rubber or between `rubber-insulated zwoven wire fabric or even between ba e metal supports, but are preferably supported ternally.
For example, a particularly useful diaphragm is produced )by depositing rubber upon an open weave textile fabric or a fine mesh woven wire screen until the rubber has completely fllled all the openings and the diaphragms has attained a substantially constant thickness.
The rubber diaphragm prepared as describe above have pores large enough to permit the flow of the electric current without undue resistance-'yet small enough that differences in the liquid level on the two sides of the diaphragm do not cause any appreciable ow. A diaphragm deposited by electrophoresis on a 1/8 inch mesh wire screen, the total thickness of the an electro-endosmotic flow of 1.2 cubic centimeters per ampere-minute 'when impregnated with a slightly alkaline water solution. This is considerably greater than the flow through porcelain or asbestos diaphragms, which varies from about 0.5 to 1 cubic c'entermeters per ampere-minute. The electrical resistance, however, is practically the same.
The particular mode of employing the above rubber diaphragms in any electrolytic or electrophoretic cell will be obvious, the rubber diaphragm simply being substituted for that previously employed. The sole precaution necessary is that the diaphragms must be continuously maintained moist, as drying out will impair their usefulness. v
It is to be understood that the term rubber, unless otherwise modified, is employed in the appended claims in a generic sense to include natural or synthetic rubber, reclaimed rubber, gutta-percha and balata, rubber-isomers, and like substances. The term undried is used to indicate that the material of the diaphragm has been maintained continuously in a moist condition from the time when the diaphragmwas originally formed.
Inasmuch as numerous embodiments of this invention'may be made without exceeding the scope thereof, it is not intended to limit this invention to the specific embodiment hereinabove described except as indicated in the appended claims.
l. Permeable diaphragms comprising undried unvulcanizedcoagulated rubber occupying the foramina of a foraminous support. 1
2. Permeable diaphragms comprising undried unvulcanized coagulated rubber surrounding and supported by a woven wire screen, the mesh of the screen being completely filled with the l deposited rubber.
3. Permeable diaphragms comprising undried unvulcanized coagulated substantially pure rubber supported by a foraminous web contained, 4
4. Permeable diaphragms comprising undried unvulcanized electrodeposited rubber occupying the foramina of a foraminous support.
5. Permeable diaphragms comprising undried unvulcanized electrodeposited rubber surrounding and supported by a woven wire screen, the mesh of the screen being completely filled with the deposited rubber.
HENRY C. HOWARD.
ERVIN S. KERN.