CA1207705A - Dimensionally stable asbestos-polytetrafluoroethylene diaphragms for chlor-alkali electrolytic cells - Google Patents

Dimensionally stable asbestos-polytetrafluoroethylene diaphragms for chlor-alkali electrolytic cells

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Publication number
CA1207705A
CA1207705A CA000383721A CA383721A CA1207705A CA 1207705 A CA1207705 A CA 1207705A CA 000383721 A CA000383721 A CA 000383721A CA 383721 A CA383721 A CA 383721A CA 1207705 A CA1207705 A CA 1207705A
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Canada
Prior art keywords
fibrids
polytetrafluoroethylene
diaphragm
mat
asbestos
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CA000383721A
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French (fr)
Inventor
Shan-Pu Tsai
Leo L. Benezra
Lewis R. Horvat
John P. Hazzard
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Eltech Systems Corp
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Eltech Systems Corp
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Classifications

    • DTEXTILES; PAPER
    • D01NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
    • D01DMECHANICAL METHODS OR APPARATUS IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS
    • D01D5/00Formation of filaments, threads, or the like
    • D01D5/40Formation of filaments, threads, or the like by applying a shearing force to a dispersion or solution of filament formable polymers, e.g. by stirring
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25BELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B13/00Diaphragms; Spacing elements
    • C25B13/04Diaphragms; Spacing elements characterised by the material

Abstract

ABSTRACT

The invention provides for diaphragm coated electrodes and processes for producing these diaphragm coated electrodes, as well as application of the diaphragm coated electrodes in electrolytic cells used in the electrolysis of brine to produce chlorine and caustic. Specifically is disclosed a process whereby a slurry of asbestos fibers and polytetrafluoro-ethylene fibrids are deposited on a foraminous electrically conductive substrate to form a mat for a diaphragm. The deposited slurry is heated to dry the mat and then there is a subsequent heating which may be around the transition temperature of polytetrafluoroethylene (327°C), and up to a temperature no more than 400°C in order to avoid degradation of the polytetrafluoroethylene. The polytetrafluoroethylene fibrids are produced by shearing action on fibrillatable polymer, as opposed to the die-drawn polymer of the prior art. The cost of the die-drawn polytetrafluoroethylene fibers of relatively large diameter is excessive and exceeds the cost of fibrids as described in the instant invention.

Description

~2~'~7~

IMPROVED DIMENSIONALLY STABLE
ASBESTOS-POLYTETRAFLUOROETHYLENE DIAPHRAGMS FOR
CHLOR-ALKALI ELECTROLYTIC CELLS

Diaphragms for electrolytic cells used to produce chlorine, and sodium hydroxide or potassium hydroxid from brine (hereinafter gen-erally referred to as "chlor-alkali" cells) are conventionally asbestos fiber mat structures sup-: ported directly by the cathode of the chlor-alkali cell~ Such asbestos diaphragms suf~er the serious disadvantage of swelling under load, sometimes, for ~ 10 example, swelling up to 800 percent. Such swelling ; can result in filling the anode diaphragm gap, there-: : by increasing cell voltage and subjecting the dia-; : phragm itself to at~ri~ion by gas released at the anode surface proximate ~o the swollen diaphraqm.
15~ The result of severe swelling of the asbestos dia-; : phragm and of attrition caused by gas relea~ed at the anode proximate the swollen diaghragm is to limit the lifetime of such diaphrayms, used commer-cially, to approximately six mGnths. ' Many attempts and proposals for overcoming the problems of the conventional asbes-tos diaphragms involve polymer modification of the asbestos sheets.
However, the majority of existing diaphragm-type cells are of complex geometric design; accordingly, the composite sheets are necessarily formed exterior to the cell and cannot be employed without signi-ficantly reducing the available diaphragm surface area. Moreover, such composite sheet diaphragms must be used in the filter press or "sandwich" type cell design to be useful.
$~

ane -qpecific ~ug~esti~n, ~r ~vercomin~
the problems of asbestos diaphra~s, in~olves impregnating a preformed asbestos di~phragm with monomer or polymer and subsequently polymerizing ~he monomer in situ, or curing the polymer. How-ever, such impregnation of asbestos fiber dia-phragms results in the formation of continuous polymer coating on the surface of the asbestos fibers; the continuous polymer coating eliminates the water permeability Qroperties of the asbes~os fibers. .~oreover, impregnation techni~ues which, by design or by accident, form a con~inuous skin on one surface of t~e asbestos, render the asbestos impermeable to the elec~rolv~.
Another proposal for overcoming th~ dis-advantages of asbestos dlaphra~ms invol~es a pro-cess which includes depositlng a diaphragm from a slurry of asbestos fibers and polyte~rafluorethylene fibers on a foraminous cathode and heat treating the deposit to physically blnd it and to strengthen the diaphragm. However, the cost of these die-drawn polytetrafluoroethvlene fibers of relati~relY largP
diameter i3 excessive and excee~s the cost of fibrids : as described in the ins~an~ inven~ion. Likewise, the amount of such fibers reouired in the diaphraym for a given level of o~eration exceeds the amounts reauired when fibrids are utilized.

SUMMARY OF T~E INVENTION
The invention i5 directed to new and im-proved electrodes coated with fiber~ e polytetra-fluoroe~hylene (PTF~) modified asbestos diaphragms, to the production of these diaphragms, and to use :~2~7~

these diaphragms in chlor-alkali cells.

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 polytetra1uoroethylene fibrids (described below) and which is subsequently subjected to temperatures effective to cause the fibrous P~FE 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 usa than prior diaphragms. Power efficiencies of cells incorporating the new and improved diaphragm of the invention are accordingly superior to power efficiencies o~ 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 diaphragrns used in chlor-alkali cells.

Therefore, the present invention provides for a process for making a diaphragm coated cathode for an electrolytic cell, said diaphragm containing polytetra1uoroethylene fibrid and asbestos fiber, comprising:

forming an aqueous slurry of asbestos fibers and polytetrafluoroethylene fibrids, wherein said polytetrafluoroethylene fibrids comprise from about 1 to 10 grams/liter of said slurry, and wherein said polytetrafluoroethylene fibrids before shrinking are between 0.1 microns to 100 microns in diameter, by s dissolving a solid, granular, inert material intimately admixed with polytetrafluoroethylene fibrids and mixing same with asbestos fiber, said disxolving occurring ~efore or after said mixing;

S depositing said slurry of asbesto6 fibers and polytetraEluoroethylene fibrids on a foraminous electrically conductive substrate to form a mat for a diaphragm while removing the majority of the dissolved solid, granular, inert material, and heating said deposited slurry to dry said mat, and thereafter ~eating sai~ dry mat to tempera~ures < 400 C., to ~hrink and/or fuse said polytetrafluoroeth~lene fibrids in said dried mat to form a matrix holding said asbestos fibers in a polytetrafluoroethylene fibrid and asbestos fiber containing diaphragm, said fused deposit containing polytetrafluoroethylene fibrids comprising from S to 25 percent by weight of said diaphragm.

The present invention also provides for a process for making a diaphragm coa~ed electrode for an electrolytic ~ell, said diaphragm containing a polytetrafluoroethylene fibrid and asbestos fiber, comprising:

forming said polytetrafluoroethylene fibrids by com~ining particulate polytetrafluoroethylene with a fibrid inducing su~strate, wherein said fibrid inducing substrate comprises a solid, granular, inert material which is easily separated from polytetrafluoroethylene fibrids~ and subjecting said suspension to a compressive shearing action at a temperature between 20 C. and 250 C.;

- 3B - ~

forming an aqueous slurry of asbestos fibers and polytetrafluoroethylene fibrids in said fibrid inducing substrate and dissolving said fibrid inducing substrate;

depositing said slurry of asbestos ibers and polytetrafluoroethylene fibrids on a foraminous electrically conductive substrate to form a mat for a diaphragm while rernoving most of said dissolved fibrid inducing substrate; and heating said deposited slurry to dry said mat, and thereafter heating said dry ma~ to temperatures ~ 400 C., to shrink and/or fuse said polytetrafluoroethylene fibrids in said dried mat to form a matxix holding said asbestos fibers in a polytetrafluoroethylene fibrid and asbestos fiber containing diaphragm.

DETAILED SUBSCRIPTION OF THE INVENTION

The new and improved diaphragms of thè present invention are produced by depositing a random mixture of asbestos fibers and polytetrafluoroethylene fibrids (i.e~
fibrous material of various lengths and about 0.1 microns to about 100 microns in d.iameter, 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 ~2~7'~

heating the deposit to temperatures sufficient to fuse the deposit and to shrink the deposit. 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, ad-herent, dimensionally stable deposit of a random mixture of asbestos fibers and polytetrafluoroethyl-ene 1brids. The fused deposit contains polytetra-fluoroethylene 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 poly-tetrafluoroethylene fibers, made by die-drawing.
The random mixture of asbestos fiber~ and polytetrafluoroe~hylene 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 polytetrafluoro-ethylene to shear conditions. The particle dia-meters of the particulate polytetrafluoroethylene may range from about 0.01~, or less, to about 50~, preferably between about 0.1~ to about 0.5~. The particulate polytetrafluoroethylene can be either in the form of a wet (water) dispersion or dry powder. The concentration of polytetrafluoro-ethylene in a water dis~ersion should be a concen-tration sufficiently high to facilitate fibridformation as ~olytetrafluoroethylene fibrid ini-tiation becomes difficult at extremely low, concen-trations; but the concentration must be low enough 7~

to obviate large clump formation, By way of ex-ample, it is noted that PTFE fibrid formation has been induced in aqueous dispersions containing as low as 1 percent by weight particulate polytetra-fluoroethylene to concentrations of about 30 per-cen~ by weight.
Certain commercially available products contain particulate polytetrafluoroethylene parti-cles having diamet~rs ranging up ~o about 0.5~ re-quired for the production of fibrids. For example,both Fluon~CDl, sold b~ Imoerial Chemical Indus-tries Ltd., and Teflon~30B, sold by E. I. duPont deNemours ~ Co., have been found to be quite suit-able, both contain about 60 percent solids in a water dispersion, including about 6 percent wetting agents, based on the weight of solids. These dis-persions are described in U.S. Patent No. 4,047,537, These dispersions are hydrophilic, negatively charged colloid dispersions, containing particles having dia-meters preferably of about 0.05~ to about 0.5~, sus-: pended in water Another commercially available produc~, Teflon ~ype T-6 (also sold ~y DuPont) 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 polytetrafluoro-ethylene in the DuPont Teflon 30B and T-6 dispersions average from about 0.2~ to about O.S~ , while powder agglomerates sf the T-6 powder average about 50C
microns.

~IL2~ 5 Formation of the polytetrafluoroethylene fibrids is effected by suspending a particulate fibrid inducing substrate in a mass or dispersion of particulate polytetrafluoroethy~ne and sub~
jecting the polytetrafluoroethylene particles to shearing forces to form fibrids of polytetrafluoro-ethylene. The fibrid inducing substrate comprises coarse particles of suitable materials. The mate-rials used as the fibrid inducing substrate are sub-stantially physically and chemically inert to theparticulate polytetrafluoroethylene. By physically and chemically inert, it is meant that the substrate material will not absorb the polytetrafluoroethylene dispexsion and will not chemically react with the 15~ polytetrafluoroethylene. The materials used as the fibrid inducing substrate include any solid gran-ular inert material which is easily separated from the fibrids. Suitable fibrid inducing substrate materials include alumina, limestone, salt, sugar, sand, and graphite. Preferably salt that is pul-verized 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 abou~ 1~ to abou~ 800~, and pre-ferably from about 100~ to about 200~.
AsbPstos fibers are admixed with the poly-tetrafluoroethylene fibrids after fibrid formation.
These asbestos fibers may be any product used to r~s form conventional asbestos mat diaphragms, Generally, in the e~periments repor~ed below, asbestos fibers of standard length combinations are employed, based on the Quebec Standard for length. For instance, a standard length combina-tion which can be used in accordance with theinvention comprises two parts short asbestos fibers to one part long asbestos fibers. For example, a mixture of VAG ~2 short fibers having lengths xanging from 1/32 inch to 1 inch with an average length of 1/4 inch and of VAG #l long fibers having an average length of 1/2 inch 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 par-ticulate polytetrafluoroethylene and substrate.
The temperature of the shearing step is a tempera-ture sufficient to render the polytetrafluoroethyl-ene sufficiently plastic to form fibrids. The time duration of the shearing action is temperature de-pendent, and thus the polytetrafluoroethylene will be maintained at the temperature of ~he shearing step for time sufficient to allow subs~antial fibrid formationO The temperature during the shearing step may range from about 20C up to about 250C, preferably from about 60C to about 200C. Most preferably, the polytetrafluoroethylene is heated to a temperature of from about 75C to about 100C
during the shearing step.

3~ 7~

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 kall mill, a double screw blender and a Banbury~mixer or a Hobart~mixer. The re-sult of the shearing action is the production of fibrids which may be highly branched or sing-ular fibers or a mixture of both. These fibridsare composed of poly~etrafluoroethylene particles having diameters of from about 0.1~, or less, up to about 100~. The leng~hs of the fibrids is not critical; the fibrids of experiments reDorted below are generally less than about one-half inch.
After polytetrafluoroethylene fibrid forma-tion, a random mixture of the ~olytetrafluoroe~hy~
ene fibrids and the asbestos fibers is deposited on the foraminous electrically conductive subs~rate.
This has been done by forming an aqueous slurry o the polytetrafluoroethylene fibrids and asbestos fibers in water, in cell liquor, in caustic, in a salt solution or in admixtures thereof. The poly-tetrafluoroethylene 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. For example, the foraminous electrically conductive substrate can be disposed in a vacuum fil-tration funnel; vacuum facilitates removal o~ water from the depoæit and matting of the deposit. There-after the mat is dried.

~ ~ \
~2~37~

The electrically conductive foraminous substrate is a metal mesh or a metal alloy mesh.
In other words, the substrate is a mesh electrode.
The mesh sizes of the substrate are not critical.
Below, a 6-mesh electrode or perforated screen, specifically a mesh cathode, is described in the examples. However, chemically stable metallic mesh electrodes having in excess of 8 mesh to the linear inch and width openings of less than 0.06 inch have been used in chlor-alkali cells~ In the embodiment of the invention which is directed to chlor-alkali cells equipped with diaphragms of the invention, the foraminous substrate may be any cathode currently used in chlor-alkali cells. For example, mesh cathodes, wire cathodes, or Ryerson cathodes (per-forated steel plate) may be used.
After drying the PTFE fibrid-asbestos fiber deposit at a temperature of about 100C, it is heated to temperatures sufficient to fuse the polytetrafluoroethylene of the deposit. Fusion occurs at temperatures around the melting point of polytetrafluoroethylene (327 ~ 10C). Preferably, however, the temperature of fusion is at least about 340C. Generally, fusion is undertaken at temperatures ranging from about 340C ~o about 370C for about cne-quarter hour to two hours.
Although temperatures as high as 400C can be uti-lized with appropriate shortening of the time, temper-atures above 380C should be avoided as degradation of the polytetrafluoroethylene starts at about that temperature and interferes to a degree with the effectiveness of the process.

7~S

As a result of fusion, the polytetrafluoro-ethylene fibrids form a recticulate or matrix con-figuration and shrinks. The network or matrix acts to hold in or enclose asbestos fibers for improved dimensional stability. As a result of shrinkage S during fusion, 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.
The following examples are presented to 10 illustrate the invention and specific embodiments by which the invention can be effected.

EX2~1PLE 1 To demonstrate the suitable need for fibrid inducing substrate, CaCO3 powder (Fisher ~-20 microns) 15 was used in the following manner: To 98 parts by weight of the CaCO3 was added 3.3 parts by weight of a 60~ solids PTFE dispersion (Teflon 30B). Shearing was app~ied by mulling in a mortar and pestle at 80C
for 10 min. The CaCO3 was then removed from the mix-~ tuxe by leaching wi~h dilute HCl, and the resulting fibrid residue was washed and then dried at 100C
for l hour to yield about 2.0 parts of Teflon fibrids.
The slurry used for deposition of the dia~
phragm consisted of the following components:
3.6 g fibrids made as above ~.6 g ~2 VAG Short Asbestos fibers 4.8 g #l VAG Long Asb~sto~ fibers 0.1 g of NOPCO PE 260, a no~-ionic dis-persant, all in l liter of cell liquor (11% NaOH and 16% NaCl solution).
A11 the above were then stirred vigorously with a dispersator at moderate speeds for about 5 minutes.
The diaphragrn is constructed by taking an aliquot portion, approximately 360 milliliters, and 35 passing it by gravity over a 6 mesh cathode (0.093l' :~LZ~ 7~
.

steel wire calendared to a thickness of 0.155") cen-tered in a 450 ml filtration funnel. A vacuum is applied to the suction flask ranging from 0-2.5" of mercury for about S minutes and gradually increased over a five minute interval to 17" ~g vacuum and then S holding for drying the deposited diaphragm for a period of 10 minutes. ~he diaphragm was next heated to 100C for one hour for additional drying, and fused at 350C for one hour. The r~sulting ~at had a den~ity of 1.25 g/sq inch (0.002755 lb/sq.in.) and contained 20% by weight of PTFE fibrids.
When this 3 sq inch assembly of diaphragm a~d cathode is installed in a laboratory chlor-alkali cell t it demonstrated yood dimensional stability with a very good brine head and permeability (a~ judged by the flow ratç through the diaphragm of .185 mls/min/sq inoh ~0.0065 ounce~/min./sq.in.3). Conditions o opera-tion were 93C with the electrodes }:eing separated at ~" apart .
Voltage w~s measured and ~ound to be exoellent at 2.98 v~lts at one asi.
These results should be com~ared with an asbestos dia-phragm made in a conventional way but containing nofibrids, in which the diaphragm showed instability, lasting less than 2 days at 3.23 volts (see Table 1).
Compare also with a dia~hraom made with 2$~ of Teflon fibers (6.6 de~ier) with the asbestos firbers, all other conditions being the same; stability and voltage are about the same, showing an equivalency of pexformance using Teflon f.ibers and 20~ PTFE fibrids. See Table 1 for other examples.

To demonstrate the use of another fibrid inducing substrate, 98 parts of granular NaCl (ap-prox. 50 mesh) were added to 3.3 parts of a 60%
Teflon 30B dispersion. Fibrids were formed by mull~

7~5 ing in a mortar and ~estle for 30 minutes at 21C, followed by continuous mulling at 130C for 3 minutes.
The salt substrate was removed by leaching with water, washed and dried as above. A diaphragm was formed in the same manner as above. It was dried and fused as above. The resulting diaphragm had a mat density of 1.17 g/sq inch and contained 15~ by weight of fibrids.
It had good performance parameters as set forth in Table 1.

EXAMoeLE 3 To demonstrate the use of other equipment to form fibrids, a ~ower muller was ased. A 2 wheel Cincinnati brand muller was used, with 1~1/2" wide 8" diameter wheels in a 12" pan. Fibrids were made with a granular salt substrate at 21C for 40 minutes, with a 1 kilogram mass on the wheels~ using about a 1 kilsgram 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 and containedl 15~ fibrids by weight. The diaphragm performance was comparable to Examples 1 and 2, as set forth in Table 1.

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~

, 7~

Table 1 A Comparison of Asbestos 3iaphragms Modified _y the Addition of Fi~rids in Chlor-Alkali Pexformance Brine Days Ex. % Head Volts NaO~ Current on No. Fibrids Substrate ~inches) 1 asi q/l Eff.~ Stability Line . . _ _ CaC03 3 2.98 124 92 Stable ~14
2 15 NaCl 3-1/4 3.04 128 92 Stable ~29
3 15 NaCl 5-1/2 3.03 128 93 Stable >61
4 15 NaCl 6-1/4 3.02 131 92 Stable >52 * 25 -- 2-1/2 2.98 130 93 Stable ~20 ** none -- 3-1/2 3.23 130 93 Unstable < 2 Notes: * 25% Teflon fibers, 6.6 denier, all conditions standard as per Example 1.
** No fibex or fibrid binder, made in the conventional manner.
EXAMPLES 5 through 10 The diaphrc~ s of the following examples were used using fibrids made from DuPont Teflon T-6 (the solid agglomera~e particles made by evaporation of Teflon 30B dispersion) and DuPont Teflon 30B (dis-persion of PTFE particles). The performance results of these 5 diaphragms are set foxth in Table 2.

-Fibrids were made by using 2% Teflon solids from Teflon K-20 (6.6g of a 30% Teflon solids disper-sion) with 98% granular salt, heating to 130C for one-half hour and then mulling the wet mix with a spatula in the beaker for about 3 minu~es to induce 7~

fibrid formation. ~ibrids 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 disperator action for 3 minutes and a diaphragm was deposited as before. The diaphragrn was dried and fused as above. The resulting diaphragm had a density of 1.00 g/sq inch and contained 25~
fibrids. Performance parameters were similar to those described above.

In this experiment, Teflon type T-6 particulate PTFE powder was used. Twenty-five parts of Teflon type T-6 and 7S parts of granular salt ~ere added to a mortar. The mix was mulled with a pestle for 60 minutes at 21C. The fibrids were recovered by leaching out the salt; the fibrids were then w~shed and dried.

Asbestos, the fibri~s made above, Teflon 6.6 denier fibers, and a small quantity of dispersant were added to cell liquor. The resulting diaphragm, made in the manner previously described, was dried and fused at 350C Eor one hour. The resul-ting diaphragm had 7% fibrids, and 15% Teflon fibers with a mat density of 1.15 g/sq inch~ When the diaphragm was installed in a chlor-alkali cell, the voltage, concentration and current efficiency all showed superior characteristics as set forth in Table 2.

In this experiment the use of a ball mill and mulling was demonstrated. Eigh~ parts of Teflon 30B
(a ~0% solids dispersion) and 92 parts oE granular salt, were ball-milled for 2 hours at 21C, and 7~

removed. The mixture was then preheated in an oven to }-30c and mulled for 5 minutes to foL~ the com-ple~ed fibrids. The fibrids wer~ recovered and dia-phragms were made in the manner described above.
S The resulting diaphragm had a density of 1.21 and contained 20% flbrids. The superior operating para-meters obtained as set forth in Table. 2.

This expeximent illustrates the use of both shearing equipment as wPll as the use of particulate Teflon type ~6.In an automated power mortar and pes-tle were added 5 parts of Teflon T-6 and 95 parts granular salt, and mulled for 40 minutes at 21C.
The recovered fibrids were admixed with asbestos and Teflon 6.6 denier fibPrs as described abo~e. The re-sulting diaphragm made in the manner described above had a density of 1.22 g/~q inch and contained 7%
fibrids and 15~ Teflon fibers. The superior opera-ting parameters obtained are set forth in Table 2.

.
This experiment further demonstrates the use of a power muller, such as the automated mor~ar and pestle to make fibrids from Tefloh 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 ~oo~ ~am~xatur~ for 4Q minutes and then at 85C for } rinute. The resulting fi~brids were made into a diaphragm wi~h asbestos fibers in the manner described above ~hich had a density of 1.15 g/sq inch and contained 15% fibrids. The superior operating parameters obtained are set forth in Table 2.

~77r~

The procedure of Example 9 was repeated except that the density was increased a little at 1.22 g/sq inch. The superior operating para-meters obtained are set forth in Table 2.

Table 2 Comparison of Asbestos Diaphragms Modified by the Addition of Fibrids and Teflon Fibers Made in Various Different Ways in Chlor-Alkali Cell Performance Brine Days Percent Head Volts NaO~ ~xent on E~le Fibrids Substrate (inches) 1 asi* ~ Eff.% S~ility Line NaCl 1-1/2 3.01 129 91 Stable >29 6 7%+
15~ NaCl 2-1/4 3.02 129 93 Stable jl3 f~s 7 20 NaCl 4-1/2 3.05 130 94 Stable >51 15~
fikers NaCl 3-1/2 3.06 131 93 Stable >60 9 15 NaCl 4-V2 3.09 130 94 Stable >91 ld 15 NaCl 4 3.03 131 94 Stable >100 * amperes per square inch The followinq exam~lss show th~ processin~ of fibr'ds by pilot pla~t ~ca~e equipment.

;

s 3 . 59 pounds of salt were pulverized to 100 mesh by a hammermill,added to .~ pound of a Teflon type 30B dispersion (60% Teflon solids dis-S persion) and mixed dry. This is a 4% Teflon fi-brid blend. The mixture was then compression sheared in a 4 pound ~anbury brand mixer for 13 minutes with a partial ram pressure. The result-ing fibrids were re~overed in the usual manner, washed and dried, and diaphragms prepared in the same manner as in Example 1, from an approximately 18 grams/liter slurry from cell liquor, again in the usual manner. The density of the diaphragm was 1.30 g/sq inch with a fibrid content of 15% . The superior operating parameters obtained in a chlor-alkali cell are set forth in Table 3.

This is another example of pilot scale fibrid production. A 5~ blend of Teflon 30B solids (from a 60% Teflon solids dispersion) of 100 mesh salt was processed in a Banbury brand mixer for 9 minutes as above with partial ra~. pressure. The fibrids, recovered in the usual manner, were added to the cell liquor with asbestos fiber and air/vacuum 2S agitated for 1 hour. The resulting diaphragm had a density of 1.25 g/sq inch and a fibrid content of 15%. The superior operating parameters obtained in a chlor-alkali cell are set forth in Table 3.

This experiment illustrates a va~iation in the processing of fibrids. Teflon 30s was pre-mixed with 100 mesh salt, as an 8% blend, in a ~z~q77rlls ribbon blender at80-90C for 30 minutes. No Banbury mixer was used. The diaphragm was made in the manner described above and contained 20~
of the fibrids as recovered fxom the ribbon blender.
The superior operating parameters obtained in a chlor-alkali cell are set forth in Table 3.

This experiment again is a variation in processing equipment and conditions. Teflon 30B
was premixed with 100 mesh salt, as a 4% blend, in a 1 cu ft. ribbon bl.ender at 80-90C for 45 minutes.
It was compression sheared in a Banbury ~ixer for 13 minutes as described in Example 11. The fi-brids salt mixture was dissolved in water, and the major portion of the salt water was removed to yield a saline slurry containing about-3.8 grams/liter of fibrids. About 250 mls of this slurry (2.2 g of fibrids) was added to 500 mls of!water, 250 mls of standard cell liquor, 15.8 g of asbesto~ fibers and mechanically sheared by a dispersator. A stable dia-phragm was made from this slurry in the usual mannerand had a density of 1.20 g/l with 12~ fibrids. Its performance in a chlor-alkalicell was au~ted byusing approximately half the normal gap between electrodes as well as the use of a porous nickel-coated steel cathode to yield an unusually low voltage (2.64 volts vs. a normal 3.00 volts -- 1 asi).

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Thus it ls apparent that there has been provided, in accordance with the invention, a diaphragm coated foraminous electrode that fully satisfies the objects, aims, and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident ~hat many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.

,

Claims (12)

WHAT IS CLAIMED IS:
1. A process for making a diaphragm coated cathode for an electrolytic cell, said diaphragm containing polytetra-fluoroethylene fibrid and asbestos fiber, comprising:
forming an aqueous slurry of asbestos fibers and poly-tetrafluoroethylene fibrids, wherein said polytetrafluoro-ethylene fibrids comprise from 1 to 10 grams/liter of said slurry, and wherein said polytetrafluoroethylene fibrids before shrinking are between 0.1 micron to 100 microns in diameter, by fribrillating polytetrafluoroethylene particles in the presence of a solid, granular, inert material intimately admixed therewith, and mixing the resulting polytetrafluoro-ethylene fibrids with asbestos fiber, said solid granular material being dissolved in the aqueous slurry medium before or after said mixing;
depositing said slurry of asbestos fibers and polytetra-fluoroethylene fibrids on a foraminous electrically conductive substrate to form a mat for a diaphragm while removing the greater part of the dissolved solid, granular, inert material; and heating said deposited mat to dry it, and thereafter heating said dry mat to temperatures ? 400°C., to shrink and/or fuse said polytetrafluoroethylene fibrids in said dried mat to form a matrix holding said asbestos fibers in a polytetrafluoroethylene fibrid and asbestos fiber con-taining diaphragm, said fused deposit containing polytetrafluoroethylene fibrids comprising from 5 to 25 percent by weight of said diaphragm.
2. The process of claim 1, wherein said heating of said dry mat is at temperatures between the fusion temperature of the polytetrafluoroethylene fibers (327° C.? 10° C.) and 370° C.
3. The process as stated in claim 2, wherein said heating of said dry mat is conducted at temperatures between 340° C. and 370° C.
4. The process of claim 1, 2, or 3, wherein the solid, granular, inert material is alumina, limestone, salt, sugar, sand or graphite.
5. The process of claim 1, 2, or 3, wherein the depositing of the slurry on the foraminous substrate is via gravity feed or vacuum, the heating of the dry mat is for 0.25 - 2 hours, and the granular material is CaCO3 or NaCl.
6. A process for making a diaphragm coated electrode for an electrolytic cell, said diaphragm containing polytetrafluoroethylene fibrid and asbestos fiber, comprising:
forming said polytetrafluoroethylene fibrids which, before shrinking, are between 0.1 micron to 100 microns in diameter, by combining particulate polytetrafluoroethylene and a fibrid inducing substrate comprising solid, granular, inert material which is easily separated from polytetra-fluoroethylene fibrids, and subjecting said combination to a compressive shearing action at a temperature between 20°C.
and 250°C.;
forming an aqueous slurry of asbestos fibers and polytetrafluoroethylene fibrids plus fibrid inducing substrate, and having sufficient fibrids to comprise from 5 to 25 percent by weight of said diaphragm, while dissolving said fibrid inducing substrate in aqueous slurry medium;
depositing said slurry of asbestos fibers and polytetrafluoroethylene fibrids on a foraminous electrically conductive substrate to form a mat for a diaphragm while removing most of said dissolved fibrid inducing substrate: and heating said deposited slurry to dry said mat, and thereafter heating said dry mat to temperatures ?
400° C., to shrink and/or fuse said polytetrafluoroethylene fibrids in said dried mat to form a matrix holding said asbestos fibers in a polytetrafluoroethylene fibrid and asbestos fiber containing diaphragm.
7. The process of claim 6, wherein said heating of said dry mat is at temperatures between the fusion temperature of the polytetrafluoroethylene fibers (327° C. ? 10° C.) and 370°C.
8. The process set forth in claim 7, wherein said compressive shearing action takes place at a temperature between 75° C. and 100° C.
9. The process as stated in any of claims 6, 7, or 8, wherein said heating of said dry mat is conducted at temperatures between 340° C, and 370°
C.
10. The process of claim 6, 7, or 8, wherein the fiber inducing substrate is a solid, granular, inert material selected from alumina, limestone, salt, sugar, sand, or graphite.
11. The process of claim 6, 7, or 8, wherein the fiber inducing substrate is a solid, granular, material selected from CaC03 or NaCl, the depositing of the slurry on the foraminous substrate is via gravity feed or vacuum, and the heating of the dry mat is for 0.25 - 2 hours.
12. A diaphragm coated electrode for an electrolytic cell containing polytetrafluoroethylene fibrids and asbestos fibers made by the process of claims 1, 2 or 3.
CA000383721A 1980-09-22 1981-08-12 Dimensionally stable asbestos-polytetrafluoroethylene diaphragms for chlor-alkali electrolytic cells Expired CA1207705A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US18903680A 1980-09-22 1980-09-22
US189,036 1980-09-22

Publications (1)

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CA1207705A true CA1207705A (en) 1986-07-15

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CA000383721A Expired CA1207705A (en) 1980-09-22 1981-08-12 Dimensionally stable asbestos-polytetrafluoroethylene diaphragms for chlor-alkali electrolytic cells

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EP (1) EP0048617B1 (en)
JP (1) JPS5785985A (en)
BR (1) BR8105955A (en)
CA (1) CA1207705A (en)
DE (1) DE3169066D1 (en)
MX (1) MX167581B (en)

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4563260A (en) * 1983-01-27 1986-01-07 Eltech Systems Corporation Modified liquid permeable asbestos diaphragms with improved dimensional stability
DE19746404A1 (en) 1997-10-21 1999-04-22 Basf Ag Process for the production of composite fibers and disphragmas

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4070257A (en) * 1970-08-13 1978-01-24 Electrode Corporation Electrolytic process using novel diaphragm
ZA74315B (en) * 1973-01-17 1975-03-26 Diamond Shamrock Corp Dimensionally stable asbestos diaphragms
AR206735A1 (en) * 1975-04-09 1976-08-13 Hooker Chemicals Plastics Corp A METHOD FOR PRODUCING A CATHODE COATED WITH A DIAPHRAGM FOR ELECTROLYTIC CELLS AND THE CATHODE SO PRODUCED

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EP0048617A1 (en) 1982-03-31
BR8105955A (en) 1982-06-08
DE3169066D1 (en) 1985-03-28
EP0048617B1 (en) 1985-02-20
MX167581B (en) 1993-03-30
JPS5785985A (en) 1982-05-28

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