Document M9eX2zMLRqg78k62e4jn5Noj

FILE NAME: Dow Chemical (DOW) DATE: 1978 June 6 DOC#: DOW003 DOCUMENT DESCRIPTION: US Patent - Bonded Asbestos Diaphragms [Fwd: FW: dow asbestos research program] Prev. List Hit List Next List ... -- ..... View Cart .................. i Previous Add to Cart Images Next Bottom (402 of465) United States Patent Beaver, et al. 4,093,533 June 6,1978 Bonded asbestos diaphragms Abstract Improved asbestos diaphragms for use in electrolytic chlor-alkali cells are prepared by using polymeric fluorocarbons as binders for mixtures o f chyrsotile asbestos and crocidolite asbestos. Inventors: Beaver; Richard N. (Angleton, TX); Becker; Charles W. (Angleton, TX) Assignee: The Dow Chemical Company (Midland, MI) Appi. No.: 640119 Filed: December 12,1975 Current U.S. Class: Intern'l Class: Field of Search: 204/296; 204/295; 205/519 C25B 013/02; C25B 013/06; C25B 013/08 204/295,296 162/155,169 3505200 3723264 3853721 3928166 3945910 3980613 4000057 References Cited IReferenced Bvl Apr., 1970 Mar., 1973 Dec., 1974 Dec., 1975 Mar., 1976 Sep., 1976 Dec., 1976 U.S. Patent Documents Grotheer Leduc et al. Darlington et al. O'Leary et al. DeCeuster et al. Bachot et al. Mrazek et al. 204/295. 204/80. 204/98. 204/282. 204/296. 264/45. 204/296. Primary Examiner: Edmundson; F.C. Attorney, Agent or Firm: Lee; Walter J. Claims 10 o f 29 4/10/2003 10:27 PM [Fwd: FW: dow asbestos research program] We claim: 1. An improved diaphragm for use in an electrolytic cell wherein brine is electrolyzed to produce chlorine, caustic, and hydrogen and wherein a polymeric fluorocarbon-bonded asbestos diaphragm is positioned between the electrodes, the improvement which comprises the use o f a mixture of crocidolite and chrysotile as the asbestos, said mixtures o f crocidolite/chrysotile being o f a weight ratio in the range o f about 33/67 to about 75/25. 2. The improved diaphragm o f claim 1 wherein the weight ratio o f crocidolite/chrysolite is about 50/50. 3. The improved diaphragm o f claim 1 wherein the polymeric fluorocarbon employed in the polymeric fluorocarbon-bonded diaphragm is selected from the group consisting o f polymers and copolymers of tetrafluoroethylene, trifluoroethylene, monochlorotrifluoroethylene, dichlorodifluoroethylene, vinyl fluoride, vinylidene fluoride, and fluorinated olefin polymers. 4. The improved diaphragm o f claim 1 wherein the polymeric fluorocarbon is polyvinylidene fluoride. 5. The improved diaphragm o f claim 1 wherein the polymeric fluorocarbon is polytetrafluoroethylene. 6. The improved diaphragm o f claim 1 wherein the polymeric fluorocarbon is fluorinated ethylene/propylene copolymer. Description BACKGROUND OF THE INVENTION The use o f asbestos as diaphragm material in electrolytic chlor-alkali cells is well known. Ordinarily the diaphragms are prepared by vacuum-drawing a slurry of chrysotile asbestos fibers onto a porous cathode, thereby depositing a matte o f asbestos on the cathode. It has been previously taught that polymeric fluorocarbons may be used as binders for asbestos diaphragms. The most relevant technique taught is, in general, to mix a slurry o f particulate binder material with the asbestos fibers, then draw or deposit the materials in the form o f a matte on the porous cathode, then heat-sinter to effect bonding. Patents which teach the use of binders o f polymers for use in asbestos diaphragms are, for example as follows: U.s. pat. No. 1,942,183 --teaches use o f organic glutinous material as binder for asbestos diaphragms. U.s. pat. No. 2,731,068 --teaches impregnation o f asbestos fabric with dispersion of polytetrafluoroethylene, followed by heat-sintering. U.s. pat. No. 2,840,881 --teaches non-woven asbestos batt having superposed thereon a non-woven batt o f polytetrafluoroethylene. 11 o f 29 4/10/2003 10:28 PM [Fwd: FW: dow asbestos research program] U.s. pat. No. 2,944,956 - teaches use o f the polytetrafluoroethylene (and polymonochiorotrifluoroethylene) screen along with asbestos diaphragm. U.s. pat. No. 2,945,831 -- teaches mixing o f fluorocarbon dispersions (and other polymers) with dispersion o f asbestos, then forming a crack-free coalesced film on a substrate. U.s. pat. No. 3,017,338 --teaches polymer-bonded asbestos diaphragms and membranes. U.s. pat. No. 3,097,990 - teaches, among other things, use o f polytetrafluoroethylene dispersions to certain pre-treated asbestos sheets. U.s. pat. No. 3,153,610 --teaches preparation o f asbestos paper from an aqueous blend of asbestos particles and polymer particles, the polymer being of "ethylenically unsaturated compounds". U.s. pat. No. 3,551,205 --teaches addition o f polytetrafluoroethylene aqueous emulsion to an asbestos slurry to prepare bonded web for use as a "paper" electrode structure in a voltaic cell. Other patents which teach the use o f fluorocarbon polymers as binders for asbestos in preparing diaphragms or membranes for use in electrolytic cells are, e.g.: U.S. Pat. No. 3,583,891; U.S. Pat. No. 3,694,281; U.S. Pat. No. 3,704,221; and U.S. Pat. No. 3,723,264. These four patents teach mixing of fluorocarbon polymer dispersions with asbestos fibers prior to forming the diaphragm. SUMMARY OF THE INVENTION It has now been found that asbestos diaphragms for use in electrolytic cells, especially chlor-alkali cells, which are prepared by using a fluorocarbon polymer as a binder, are improved by employing certain mixtures o f chrysotile asbestos and crocidolite asbestos. DETAILED DESCRIPTION OF THE INVENTION In the present invention, fibers o f chrysotile asbestos and crocidolite asbestos are combined in aqueous slurry with particulate fluorocarbon polymers and the resulting slurry is deposited on a porous cathodic substrate. The chrysotile fibers and the crocidolite fibers are preferably about 1/4 inch or more in length and the fiber bundles, as normally mined, have been refined to open up the bundles. Commercially available refined asbestos is suitable for use in the present invention. The fluorocarbon polymers may be solid, particulate polymers or copolymers o f tetrafluoroethylene, trifluoroethylene, vinylidene fluoride, vinyl fluoride, monochlorotrifluoroethylene, or dichlorodifluoroethylene or may be fluorinated ethylene/propylene copolymer commonly known as FEP. Also, a copolymer o f ethylene/chlorotrifluoroethylene known as Halar.RTM. may be used. Preferably the fluorocarbon polymer is polyvinylidene fluoride, fluorinated ethylene/propylene copolymer, or polytetrafluoroethylene. Most preferably, the fluorocarbon polymer is polyvinylidene fluoride. The ratio o f chrysotile/crocidolite is in the range o f about 90/10 to 20/80, preferably in the range of about 75/25 to about 40/60, most preferably the ratio is about 50/50. The asbestos slurry may also contain minor amounts o f processing aids such as surfactants, wetting 12 o f 29 4/10/2003 10:29 PM [Fwd: FW: dow asbestos research program] agents, or dispersing agents, or modifiers, such as pH-adjusters, inorganic metal compounds, e.g. TiO.sub.2, CaCO.sub.3, MgCO.sub.3, MgO, CaO, etc. Such processing aids or modifiers may be employed in order to help disperse the fluorocarbon polymer and the asbestos fibers uniformly in the aqueous medium and to impart certain porosity features to the diaphragm. The fluorocarbon polymer aqueous slurries or dispersions may be commercially available and generally contain such processing aids or modifiers as stabilizers, surfactants, dispersing agents, etc. Such polymer dispersions may also be prepared for use in the present invention by dispersing fine particle polymer in an aqueous medium by using wetting agents, surfactants, dispersing agents, or stabilizers which help to disperse the fluorocarbon polymers and/or stabilize such dispersions. The asbestos and fluorocarbon polymer slurry is preferably deposited on the desired porous cathode structure by being vacuum-drawn. By vacuum-drawn it is meant that a slurry o f the diaphragm ingredients {asbestos, polymer, modifiers, etc.) is contacted with one side o f a porous cathode and "vacuum" (reduced pressure) is applied to the other side to pull the solids tightly into place against the cathode while pulling the liquid on through. Other methods o f depositing the diaphragm onto the cathode include the use o f gravity flow or positive pressure to force the dispersion against a porous surface, thereby depositing the solids in the form of a matte or web while the liquid flows on through the porous surface. The matte or web of diaphragm material may be prepared on a surface other than the cathode surface (such as by using a Fourdrinier process) and then transferred to the cathode surface. It is generally recognized in the art that chlorine cell diaphragms made o f chrysotile asbestos have relatively poor resistance to low anolyte pH. Chrysotile asbestos fibers are relatively easily bonded together with polymeric fluorocarbons. Crocidolite asbestos fibers alone have good resistance to highly acid (i.e. low pH) anolyte but are not readily bonded together with polymeric fluorocarbons to form a strong diaphragm. Thus, attempts to completely substitute acid-resistant crocidolite in place of chrysotile in polymer-bonded diaphragms have not been successful. According to the present invention, the acid-resistance o f crocidolite and the bondability o f chrysotile are made available in a diaphragm which employs both forms o f asbestos. A blended composite of crocidolite and chrysotile asbestos, bonded with polymeric fluorocarbon, is found to be extremely stable in anolytes having a pH as low as about 0.5. By being able to operate at a low anolyte pH o f about 0.5 to about 1.5, the life o f graphite anodes is extended and graphite consumption per ton o f chlorine produced is substantially decreased. Furthermore, the lower anolyte pH also increases chlorine purity from the cells as the production of other electrolytic products such as oxygen, carbon dioxide, and carbon monoxide is substantially inhibited. Chlorine producers are aware that an anolyte pH lower than about 1.5 will acidize the normally-used chrysotile asbestos and result in its early destruction, therefore it has been common practice to operate at an anolyte pH o f not lower than 1.5 in order to obtain appreciable life o f the diaphragm, even though some sacrifice of the graphite anode life is encountered. The following procedures and examples are illustrative o f the present invention, except for those identified as being "comparative". Other embodiments o f the present invention will become apparent to practitioners o f the art and the present invention is limited only by the claims attached hereto. In general, the preferred method o f preparing the present diaphragms for use in an electrolytic process wherein an aqueous NaCl solution is electrolyzed to produce chlorine, hydrogen, and sodium hydroxide is as follows: 13 o f 29 4/10/2003 10:30 PM [Fwd: FW: dow asbestos research program] 1. The crocidolite fibers, chrysotile fibers, and fine particle size polymeric fluorocarbon are intimately admixed and slumed in an aqueous media. The aqueous slurry also contains any modifiers, surfactants, etc. which are desired. The amount o f fluorocarbon polymer employed may be from about 5 parts to about 100 parts per hundred parts o f total asbestos; the preferred amount is about 10 to 50 parts with about 15-40 parts being most preferred. 2. The slum ed ingredients are deposited on the foraminous cathode to the desired weight generally about 0.2 gms. to about 2.0 gms. per in..sup.2, and dried. Preferably, the weight is about 0.6 to about 0.8 gms./in.sup.2. 3. The so-coated cathode is subjected to a sufficient amount o f heat to cause sintering o f the polymer particles in the mixture; pressure may be applied, if desired, either by placing a positive force against the diaphragm or by using a vacuum (reduced pressure) on the other side o f the foraminous cathode which will draw the diaphragm tightly against the cathode during the sintering operation. The amount o f heat will depend, to a large extent, on which polymeric fluorocarbon is being used; the sintering temperature (or softening tem perature) o f the desired polymer is easily determined experimentally or is available in the publications. 4. The diaphragm-covered cathode is placed into position in the electrolytic cell and, in some cases, is "pre-wetted" by being soaked with a water-soluble wetting agent, such as, detergent, surfactant, methanol, or acetone to make the diaphragm less hydrophobic. Then it is generally flushed with water, anolyte, or brine after which the cell is filled with brine and is ready for the electrolytic process to begin. The "pre-wetting" is done for those polymeric fluorocarbons which exhibit a high degree of hydrophobicity or resistance to wetting, such as polytetrafluroroethylene. In those cases in which relatively low bonding temperatures may be used, wetting agents present in the pregnant slurry may survive the bonding without appreciable degradation and may therefore aid in the initial "wetting-out" o f the diaphragm when put into service in a chlor-alkali cell. When relatively high bonding temperatures are needed, such as with polytetrafluoroethylene, surfactants in the pregnant slurry may be thermally degraded and it may be advisable to employ a wetting agent or a "wetting-out" step for the diaphragm at the outset o f its service in a chlor-alkali cell. The electrolytic cell is the diaphragm type commonly used for electrolysis of brine to produce chlorine, caustic, and hydrogen. Historically, the diaphragm has been made o f asbestos, the anode has been made o f graphite, and the cathode has been made o f iron or steel. The diaphragm is positioned between the cathode and the anode and electric current flows through the electrolyte (brine). The porosity o f the diaphragm is important in that there must be some water-permeability without having so much permeability that the caustic in the catholyte flows freely into the anolyte. It is within the skill of practitioners o f the chlorine cell art to adjust the porosity o f the asbestos diaphragms to obtain optimum results for their particular operation. EXAMPLE 1 A diaphragm was prepared for use in a test cell as follows: A dispersion o f polyvinylidene (Kynar.RTM.) powder was prepared by mixing, in a Waring blender, 80 gms. o f Kynar.RTM., 250 ml. o f H.sub.2 O, and 8 ml. o f a non-ionic surfactant (alkyl aryl polyether alcohol + about 20% isopropanol). 14 o f 29 4/10/2003 10:30 PM [Fwd: FW: dow asbestos research program] Crocidolite asbestos (Type 713 from the North American Asbestos Company) was mixed at 0.5 pounds per gallon o f water and vigorously agitated for about 5 minutes in a Cowles Dissolver. Chrysotile asbestos (Plastibest from Johns Manville) was mixed at 0.5 pounds per gallon o f water and vigorously agitated for about 5 minutes in a Cowles Dissolver. Equal portions o f the asbestos dispersions were mixed together and diluted with water to give a slurry containing 10 gms. per liter o f asbestos with equal parts o f chrysotile and crocidolite. A volume o f the slurry, sufficient to give 21 gms. o f asbestos, was thoroughly blended with a portion of the Kynar slurry, sufficient to give 3.15 gms. o f Kynar. The resulting slurry was substantially uniformally deposited onto a 13-gauge, 33 in..sup.2, perforated steel plate cathode by vacuum-filtration. Thus, the deposited materials was in an amount o f about 0.73 gms./in..sup.2 The so-coated cathode was placed in a 180.degree. C oven for 3 hours to effect bonding. After being cooled, the diaphragm was subjected to a stream o f water and it was found that the fibers remained adhered in place and none washed off. Non-bonded diaphragms are easily washed o ff by a stream o f water. The diaphragm-covered cathode was installed in a small laboratory test chlorine cell used to evaluate diaphragm integrity and operability. After 4 days o f operation at a pH in the range o f 1.0-1.5 the cell was found to be performing excellently. For comparison purposes, another diaphragm was prepared the same way except that no binder was used; this non-bonded diaphragm had to be removed from service after 18 hours o f operation because o f disintegration which caused some fibers to wash off the cathode, stop circulation, and cause hot spots which resulted in the cell boiling and a high voltage drop across the cell. EXAMPLE 2 In a manner substantially as shown in Example 1 above, polytetrafluoroethylene (Teflon.RTM.) powder is employed as a bonding agent for a 50/50 mixture o f crocidolite and chrysotile. There are commercially available Teflon.RTM. dispersions which are suitable for use directly in this process. In this example, micron size Teflon.RTM. available in a spray can is employed. Also in this example, TiO.sub.2 is also mixed into the asbestos to aid in the wetting (since Teflon.RTM. resists wetting) and to impart greater permeability. The bonding is effected by placing the vacuum-deposited diaphragm in a 300.degree.-400.degree. C oven for about 4-8 minutes under a nitrogen atmosphere. The diaphragm-covered cathode is placed in the chlor-alkali test cell and pre-wetted with methanol, then flushed with water, thereby decreasing the hydrophobicity o f the diaphragm. The test cell is operated at a pH in the range o f about 1.0-1.5 pH and the diaphragm is found to resist disintegration at this low pH and has substantially longer life and greater operability than non-bonded asbestos diaphragms. EXAMPLES 3-21 Following the procedure, generally, as set forth above, various mixtures o f chrysotile and crocidolite and various amounts o f polymeric fluorocarbon binders are employed in preparing diaphragms on steel cathodes and tested in a chlor-alkali cell where aqueous NaCl is electrolyzed to produce chlorine, 15 o f 29 4/10/2003 10:31 PM [Fwd: FW: dow asbestos research program] caustic, and hydrogen. Table I summarizes the data and results. All samples are tested at a pH o f less than 1.5 to determine resistance to degradation in the acid environment o f the anolyte. TABLE I Parts of polymeric fluoro- Asbestos used Bonded carbon polymer used per Conditions Run 100 parts total asbestos .degree. C min. Resistance to Operability No. Crocidolite Chrysotile Parts Identity temp. time Acid Degradation Rating and 3 50 4 50 5 50 6 50 7 50 8 50 9 50 10 50 11* 100 12* 0 13 67 14 50 15 50 16 50 17* 0 18* 0 19 75 Remarks 50 5 Kynar.RTM,.-Grade 451 180 180 50 7 .5 Good Better than non-bonded 1 If II Good Better than non-bonded 50 10 II If II Very Good Better than 5% bonded 50 15 II II II Excellent Good wet-out, very good 50 20 II operation If If Excellent ii 50 30 H II II Excellent 50 40 If II II Excellent Fair wet-out, very good operation 50 75 If II If Excellent Poor wet-out, fair operation 0 15 If If II Excellent Poor bonding, short life 100 15 II II II Very Poor Degrades rapidly 33 50 FEP 316 19 Excellent Excellent operation, difficult to wet 50 5 FEP 316 18 Excellent Very good operation, good wet-out 50 25 FEP 320 13 Excellent Excellent operation, good wet-out 50 72 Kynar.RTM. -Grade 451 182 15 Excellent Difficult to wet, but once wetted out, has very good operation 100 0 -- -- -- Very Poor Degrades rapidly 100 18 Kynar.RTM.-Grade 451 182 15 Poor 25 18 Kynar.RTM.-Grade 451 Degrades rapidly, but lasts longer than with no binder 16 o f 29 4/10/2003 10:32 PM [Fwd: FW: dow asbestos research program] 20 25 21 33 75 20 67 20 182 15 Excellent Wets easily, very good operation .-Grade 451 182 15 Fair Wets easily, good operation .-Grade 451 ,,,,,,j 182 15 Very Good Wets easily^ 9 _______________________ _________ " ` operation -Comparative examples, not examples of the invention. ***** P m v . 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