Classifications

• • D—TEXTILES; PAPER
  • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
  • D01D—MECHANICAL METHODS OR APPARATUS IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS
  • D01D5/00—Formation of filaments, threads, or the like
  • D01D5/40—Formation of filaments, threads, or the like by applying a shearing force to a dispersion or solution of filament formable polymers, e.g. by stirring
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B13/00—Diaphragms; Spacing elements
  • C25B13/04—Diaphragms; Spacing elements characterised by the material

Definitions

• Diaphragms for electrolytic cells used to produce chlorine, and sodium hydroxide or potassium hydroxide from brine are conventionally asbestos fiber mat structures supported directly by the cathode of the chlor-alkali cell.
• Such asbestos diaphragms suffer the serious disadvantage of swelling under load, sometimes, for example, swelling up to 800 percent. Such swelling can result in filling the anode diaphragm gap, thereby increasing cell voltage and subjecting the diaphragm itself to attrition by gas released at the anode surface proximate to the swollen diaphragm.
• Another proposal for overcoming the disadvantages of asbestos diaphragms involves a process which includes depositing a diaphragm from a slurry of asbestos fibers and polytetrafluorethylene fibers on a foraminous cathode and heat treating the deposit to physically bind it and to strengthen the diaphragm.
• the cost of these die-drawn polytetrafluoroethylene fibers of relatively large diameter are excessive and exceed the cost of fibrids as described in the instant invention.
• the amount of such fibers required in the diaphragm for a given level of operation exceeds the amounts required when fibrids are utilized.
• the invention is directed to new and improved electrodes coated with fiber-like polytetrafluoroethylene (PTFE) modified asbestos diaphragms, to the production of these diaphragms, and to use of these diaphragms in chlor-alkali cells.
• PTFE polytetrafluoroethylene
• the new and improved diaphragms of the present invention include a foraminous substrate which is electrically conductive which is coated with a random mixture of asbestos fibers and polytetrafluoroethylene fibrids (described below) and which is subsequently subjected to temperatures effective to cause the fibrous PTFE component in the coating to shrink and form an interlocking matrix.
• the new and improved diaphragm of the present invention is dimensionally stable and exhibits substantially less swelling during use than prior diaphragms. Power efficiencies of cells incorporating the new and improved diaphragm of the invention are accordingly superior to power efficiencies of conventional asbestos diaphragms when used in chlor-alkali cells. Significantly, new and improved diaphragms of the present invention exhibit substantially increased lifetimes compared to conventional diaphragms used in chlor-alkali cells.
• the present invention provides for a process for making an electrolytic cell diaphragm which comprises depositing a slurry of asbestos fibres and polytetrafluoroethylene upon a foraminous electrically-conductive substrate so as to form a diaphragm mat and heating the deposited slurry to dry the mat and cause the polytetrafluoroethylene to fuse, characterised in that the polytetrafluoroethylene comprises polytetrafluoroethylene fibrids and that the heating is effected so as to fuse and shrink the polytetrafluoroethylene fibrids in the dried mat.
• the new and improved diaphragms of the present invention are produced by depositing a random mixture of asbestos fibers and polytetrafluoroethylene fibrids (i.e. fibrous material of various lengths and about 0.2 micrometers to about 100 micrometers in diameter, which are distinct and well separated and which are produced by a shearing action on fibrillatable polytetrafluoroethylene as differentiated from a fiber made by die-drawing) onto an electrically foraminous substrate, and heating the deposit to temperatures sufficient to fuse the deposit and to shrink the deposit.
• polytetrafluoroethylene fibrids i.e. fibrous material of various lengths and about 0.2 micrometers to about 100 micrometers in diameter, which are distinct and well separated and which are produced by a shearing action on fibrillatable polytetrafluoroethylene as differentiated from a fiber made by die-drawing
• a product of the present invention resulting from this process, can accordingly be described as a foraminous electrode coated on its electrically active surface with a porous, fused, coherent, adherent, dimensionally stable deposit of a random mixture of asbestos fibers and polytetrafluoroethylene fibrids.
• the fused deposit contains polytetrafluoroethylene fibrids in an amount of at least 5 percent by weight and up to about 25 percent by weight, based on the weight of the fused deposit.
• the fused deposit may contain other fibers and fibrids, in addition to those specified; for example, the fused deposit may also contain conventional polytetrafluoroethylene fibers, made by die-drawing.
• the random mixture of asbestos fibers and polytetrafluoroethylene fibrids can be prepared by first forming the polytetrafluoroethylene fibrids and then admixing the fibrids with the asbestos fibers.
• Polytetrafluoroethylene fibrid formation involves subjecting particulate polytetrafluoroethylene to shear conditions.
• the particle diameters of the particulate polytetrafluoroethylene may range from about 0.01 ⁇ m, or less, to about 50 ⁇ m, preferably between about 0.1 pm, to about 0.5 ⁇ m.
• the particulate polytetrafluoroethylene can be either in the form of a wet (water) dispersion or dry powder.
• the concentration of polytetrafluoroethylene in a water dispersion should be a concentration sufficiently high to facilitate fibrid formation as polytetrafluoroethylene fibrid initiation becomes difficult at extremely low concentrations; but the concentration must be Jow enough to obviate large clump formation.
• PTFE fibrid formation has been induced in aqueous dispersions containing as low as 1 percent by weight particulate polytetrafluoroethylene to concentrations of about 30 percent by weight.
• Certain commercially available products contain particulate polytetrafluoroethylene particles having diameters ranging up to about 0.5 ⁇ m, required for the production of fibrids.
• Fluon CD1@ sold by Imperial Chemical Industries Ltd.
• Teflon 30B® solid by E. I. duPont deNemours & Co.
• These dispersions are hydrophilic, negatively charged colloid dispersions, containing particles having diameters preferably of about 0.05 pm to about 0.5 ⁇ m, suspended in water.
• Teflon type T-6 also sold by DuPont
• Teflon type T-6 can also be used to form the fibrids of the invention; it is a powder agglomerate produced from Teflon 30B.
• the primary diameters of particles of polytetrafluoroethylene in the DuPont Teflon 30B and T-6 dispersions average from about 0.2 ⁇ m to about 0.5 ⁇ m, while powder agglomerates of the T-6 powder average about 500 micrometers.
• Formation of the polytetrafluoroethylene fibrids is effected by suspending a particulate fibrid inducing substrate in a mass or dispersion of particulate polytetrafluoroethylene and subjecting the polytetrafluoroethylene particles to shearing forces to form fibrids of polytetrafluoroethylene.
• the fibrid inducing substrate comprises coarse particles of suitable materials.
• the materials used as the fibrid inducing substrate are substantially physically and chemically inert to the particulate polytetrafluoroethylene. By physically and chemically inert, it is meant that the substrate material will not absorb the polytetrafluoroethylene dispersion and will not chemically react with the polytetrafluoroethylene.
• the materials used as the fibrid inducing substrate include any solid granular inert material which is easily separated from the fibrids.
• Suitable fibrid inducing substrate materials include alumina, limestone, salt, sugar, sand, and graphite.
• salt that is pulverized sodium chloride is used.
• Coarse particles of suitable materials may be illustrated by noting that the diameters of particles of particulate alumina, when used as the fibrid inducing substrate, usually range from about 1 ⁇ m to about 800 ⁇ m, and preferably from about 100 ⁇ m to about 200 ⁇ m.
• Asbestos fibers are admixed with the polytetrafluoroethylene fibrids after fibrid formation.
• These asbestos fibers may be any product used to form conventional asbestos mat diaphragms.
• asbestos fibers of standard length combinations are employed, based on the Quebec Standard for length.
• a standard length combination which can be used in accordance with the invention comprises two parts short asbestos fibers to one part long asbestos fibers.
• a mixture of VAG #2@ short fibers having lengths ranging from 1/32 inch (0.079 cm) to 1 inch (2.54 cm) with an average length of 1/4 inch (0.635 cm) and of VAG #10 long fibers having an average length of 1/2 inch (1.27 cm) may be employed.
• Asbestos fibers are not generally used as the fibrid inducing substrate.
• Shearing conditions which affect fibrid formation include the time, the temperature and the shearing force applied to the mixture of particulate polytetrafluoroethylene and substrate.
• the temperature of the shearing step is a temperature sufficient to render the polytetrafluoroethylene sufficiently plastic to form fibrids.
• the time duration of the shearing action is temperature dependent, and thus the polytetrafluoroethylene will be maintained at the temperature of the shearing step for time sufficient to allow substantial fibrid formation.
• the temperature during the shearing step may range from about 20°C up to about 250°C, preferably from about 60°C to about 200°C. Most preferably, the polytetrafluoroethylene is heated to a temperature of from about 75°C to about 100°C during the shearing step.
• the shearing action used to form the polytetrafluoroethylene fibrids is generally a compressive shearing action obtained by mulling or stirring.
• Various means may be employed to effect a compressive shearing action, including a spatula and beaker, a mortar and pestle, ribbon blade, a small ball mill, a double screw blender and a Banbury mixer or a Hobart mixer.
• the result of the shearing action is the production of fibrids which may be highly branched or singular fibers or a mixture of both.
• These fibrids are composed of polytetrafluoroethylene particles having diameters of from about 0.1 ⁇ m, or less, up to about 100 ⁇ m.
• the lengths of the fibrids is not critical; the fibrids of experiments reported below are generally less than about one-half inch (1.27 cm).
• a random mixture of the polytetrafluoroethylene fibrids and the asbestos fibers is deposited on the foraminous electrically conductive substrate.
• the polytetrafluoroethylene fibrid content of the slurry can be quite variable ranging from about 1 to about 10 grams per liter of slurry volume.
• the slurry is applied to the foraminous substrate by gravity feed and/or by vacuum applied downstream from the site of deposit.
• the foraminous electrically conductive substrate can be disposed in a vacuum filtration funnel; vacuum facilitates removal of water from the deposit and matting of the deposit. Thereafter the mat is dried.
• the electrically conductive foraminous substrate is a metal mesh or a metal alloy mesh.
• the substrate is a mesh electrode.
• the mesh sizes of the substrate are not critical.
• a 6- mesh electrode or perforated screen, specifically a mesh cathode is described in the examples.
• chemically stable metallic mesh electrodes having in excess of 8 mesh to the linear inch (2.54 cm) and width openings of less than 0.06 inch (0.15 cm) have been used in chlor-alkali cells.
• 6 mesh is the equivalent of an opening size of 3.35 mm, an open area of 0.112 cm 2 , and 4.8 openings/cm 2 .
• the foraminous substrate may be any cathode currently used in chlor-alkali cells.
• mesh cathodes, wire cathodes, or Ryerson cathodes perforated steel plate
• the PTFE fibrid-asbestos fiber deposit After drying the PTFE fibrid-asbestos fiber deposit at a temperature of about 100°C, it is heated to temperatures sufficient to fuse the polytetrafluoroethylene of the deposit. Fusion occurs at temperatures around the melting point of polytetrafluoroethylene (327 ⁇ 1 ⁇ °C). Preferably, however, the temperature of fusion is at least about 340°C. Generally, fusion is undertaken at temperatures ranging from about 340°C to about 370°C for about one-quarter hour to two hours. Although temperatures as high as 400°C can be utilized with appropriate shortening of the time, temperatures above 380°C should be avoided as degradation of the polytetrafluoroethylene starts at about that temperature and interferes to a degree with the effectiveness of the process.
• the polytetrafluoroethylene fibrids form a reticulate or matrix configuration and shrinks.
• the network or matrix acts to hold in or enclose asbestos fibers for improved dimensional stability.
• the diaphragm is made more porous. The increased porosity of the diaphragm so produced reduces the electrical resistance in an operating cell and results in consequent power savings.
• CaC0 3 powder (Fisher 2-20 microns) was used in the following manner: To 98 parts by weight of the CaC 03 was added 3.3. parts by weight of a 60% solids PTFE dispersion (Teflon 30B). Shearing was applied by mulling in a mortar and pestle at 80°C for 10 min. The CaC0 3 was then removed from the mixture by leaching with dilute HCI, and the resulting fibrid residue was washed and then dried at 100°C for 1 hour to yield about 2.0 parts of Teflon fibrids.
• Teflon 30B 60% solids PTFE dispersion
• the slurry used for deposition of the diaphragm consisted of the following components:
• the diaphragm is constructed by taking an aliquot portion, approximately 360 milliliters, and passing it by gravity over a 6 mesh cathode (0.093" steel wire calendared to a thickness of 0.155", which is the equivalent of an opening size of 3.35 mm, an open area of 0.112 cm2, and 4.8 openings/cm 2 ) centered in a 450 ml filtration funnel.
• a vacuum is applied to the suction flask ranging from 0-2.5" (0 cm to 6.35 cm) of mercury for about 5 minutes and gradually increased over a five minute interval to 17" (43.18 cm) Hg vacuum and then holding for drying the deposited diaphragm for a period of 10 minutes.
• the diaphragm was next heated to 100°C for one hour for additional drying, and fused at 350°C for one hour.
• the resulting mat had a density of 1.25 g/sq inch (0.194 g/cm 2 ) and contained 20% by weight of PTFE fibrids.
• a power muller was used.
• a 2 wheel Cincinnati brand muller (a grinding device with compressive shear motion) was used, with 1-1/2" (3.81 cm) wide 8" (20.3 cm) diameter wheels in a 12" (30.5 cm) pan.
• Fibrids were made with a granular salt substrate at 21 °C for 40 minutes, with a 1 kilogram mass on the wheels, using about a 1 kilogram charge.
• the fibrids were recovered in the same manner as described above and a diaphragm made in exactly the same manner as above, including the drying and fusing.
• the resulting diaphragm had a density of 1.22 g/sq inch (0.19 g/cm 2 ) and contained 15% fibrids by weight.
• the diaphragm performance was comparable to Examples 1 and 2, as set forth in Table 1.
• Example 3 Exactly the same procedure as in Example 3 was repeated, except that the mass was dried before mulling. This yielded a diaphragm with very similar parameters as above with a mat density of 1.17 and contained 15% by weight fibrids.
• the diaphragms of the following examples were used using fibrids made from DuPont Teflon T-6 (the solid agglomerate particles made by evaporation of Teflon 30B dispersion) and DuPontTeflon 30B (dispersion of PTFE particles).
• the performance results of these 5 diaphragms are set forth in Table 2.
• Fibrids were made by using 2% Teflon solids from Teflon K-20 (6.6 g of a 30% Teflon solids dispersion) with 98% granular salt, heating to 130°C for one-half hour and then mulling the wet mix with a spatula in the beaker for about 3 minutes to induce fibrid formation. Fibrids were recovered by leaching out the salt with water, washing and drying. The slurry mix was made by the formula of Example 1, sheared by dispersion action for 3 minutes and a diaphragm was deposited as before. The diaphragm was dried and fused as above. The resulting diaphragm had a density of 1.00 g/sq inch (0.155 g/cm 2 ) and contained 15% fibrids. Performance parameters were similar to those described above.
• Teflon type T-6 particulate PTFE powder was used. Twenty-five parts of Teflon type T-6 and 75 parts of granular salt were added to a mortar. The mix was mulled with a pestle for 60 minutes at 21 °C. The fibrids were recovered by leaching out the salt; the fibrids were then washed and dried.
• This experiment further demonstrates the use of a power muller, such as the automated mortar and pestle to make fibrids from Teflon type T-6.
• a power muller such as the automated mortar and pestle to make fibrids from Teflon type T-6.
• Two parts T-6 and 98 parts of granular salt were placed in a power mortar and pestle. This mixture was mulled at room temperature for 40 minutes and then at 85°C for 1 minute.
• the resulting fibrids were made into a diaphragm with asbestos fibers in the manner described above and had a density of 1.15 g/sq inch (0.178 g/cm 2 ) and contained 15% fibrids.
• the superior operating parameters obtained are set forth in Table 2.
• Example 9 The procedure of Example 9 was repeated except that the density was increased a little at 1.22 g/sq inch (0.189 g/cm 2 ).
• the superior operating parameters obtained are set forth in Table 2.
• Teflon 30B was premixed with 100 mesh salt, as a 4% blend, in a 1 cu ft. (0.03 cm3) ribbon blender at 80-90°C for 45 minutes. It was compression sheared in a Banbury mixer for 13 minutes as described in Example 11. The fibrids salt mixture was dissolved in water, and the major portion of the salt water was removed to yield a saline slurry containing about 8.8 grams/liter of fibrids.

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• Mechanical Engineering (AREA)
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• Compositions Of Macromolecular Compounds (AREA)
• Chemical Or Physical Treatment Of Fibers (AREA)

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• 1981
  • 1981-08-12 CA CA000383721A patent/CA1207705A/en not_active Expired
  • 1981-09-17 BR BR8105955A patent/BR8105955A/pt not_active IP Right Cessation
  • 1981-09-21 DE DE8181304327T patent/DE3169066D1/de not_active Expired
  • 1981-09-21 JP JP14929581A patent/JPS5785985A/ja active Pending
  • 1981-09-21 MX MX18924581A patent/MX167581B/es unknown
  • 1981-09-21 EP EP19810304327 patent/EP0048617B1/en not_active Expired

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