CA1076065A - Method of making porous plastic diaphragms and the resulting novel diaphragms - Google Patents

Abstract

Abstract A method is disclosed for manufacturing novel electrolytic cell diaphragms which are consolidated by a fluorinated polymer resin.
The method comprises agitating a suspension of asbestos fibers in water, a sulfonic anionic surfactant, a fluorinated polymer resin latex and a solid mineral pore-former, to form a stable, homogeneous suspension, depositing and filtering the suspension on a grid or screen to produce a preform. The preform is dried, sintered at a temperature above the crystalline melting point of the fluorinated polymer resin, and the solid mineral pore-former is removed to form a uniform, porous membrane having uniform pore size. This membrane is combined with a reinforcing element by integrating the reinforcing element with the anode face of the membrane. The reinforcing element has a structure which is at least as open with respect to the passage of liquor as the porous membrane.
The novel diaphrams are particularly useful in the electrolysis of alkali metal chlorides.

Description

107606iS

Ihe present invention relates to a novel diaphragm, particularly for use in electrolysis cells, and to a novel method of m~king said diaphragm. ~he invention is an ~mprovement in the diaphragm disclosed in our Canadian patent 1,037,671, issued September 5, 1978.
The prior patent describes a method of making porous diaphragms -of asbestos which is deposited and consolidated by a fluorinated polymeric i resin, characterized in that a stable, homogeneous suspension is formed by adding a fluorinated polymeric resin latex and a pore-forming agent to ; a suspension of asbestos fibers in water, in the presence of a sulphonic anionic surfactant, followed by placing said suspension in a filtration mold, drying and fritting the resulting casting at a temperature above the crystaIline melting point of said fluorinated polymeric resin, and with the pore-forming agent being finally eleminated by decomposition or chemical action.
One of the requirements of the diaphgragm of zn electrolytic cell is that it behave as a porous medium: (1) on the one hand, it allows current to flow with a slight ohmic drop, and on the other hand, (2) it permits an even flow of electrolyte from one compartment of an electrolytic cell to the other. There is, consequently, a co~bination of mechanical, electrical and hydraulic conditions which are all the more critical now that electrolytic cells are required to function with high current density, sub~ect to tolerzting prohibitive ohmic drops. The properties required are rather contradictory. Thus, from a mechanical point of view, the diaphra m must have a well-defined, -relatively permanent geometry or shape. The diaphragm must not be subject to swelling and being permanently deformed during use. In other words, such a diaphragm must h~v~ good mechanical strength. However, it must also have good wettability properties, enabling gases to be released and electxolyte to be circulated, while preventing diffusion of hydroxide ions in the opposite direction to the liquid flow. Such diffusion is respon-- sible for the formation of chlorate which leads to a drop in yield.
To express the siiuation differently, a diaphragm for electrolysis must have low relative resistance. Relative resis-tance is understood as being the quotient of the resistance of a medium consisting of the diaphragm impregnated with electrolyte and the resistance of the same medium consisting of electrolyte.
It has been observed that the relative resistance is bound up with the porosity of the diaphragm, but also with the shape of the flow passa~es.
Attempts have therefore been made to associate a plurality of elements so as to meet all the above requirements simultaneously. Thus, U. S. patent No. 3,694,281 proposes forming a mechanical support consisting of plastic filaments and arranged on at least one face of the diaphragm, or making a laminate with threads, fibers or particles inserted in the interface between the asbestos and the substrate.
French patent No. 2,170,247 proposes a simple arrange-ment which makes use of a support consisting of a cloth, e.g., of polypropylene.
Belgian patent No. 814,510 proposes improving the rigidity of a diaphra~m by applying the diaphragm to at least one sheet of chrysotile asbestos. In a special embodiment of the diaphragm, the sheet comprising the diaphragm is sandwiched between and bonded to two sheets of chrysotile asbestos.

10760~;5 However, the main difficulty encountered in providing a preferred diaphragm is to maintain the advantages of an open structure as provided by the diaphragm produced in accordance with our prior Canadian Patent 1,037,671, while improving its mechanical properties. When such diaphragms are used in-dustrially, they are in fact found to have the disadvantage of showing struc-tural deformation, particularly localized loss of cohesion, and to result in performance which cannot be perfectly reproduced.
It has been found that this defect can be avoided if the diaphragm obtained according to Patent 1,037,671 is associated with a reinforcing element, by integrating it with the anode face of the porous membrane, the reinforcing element having a structure which is at least as open, relative to the passage of liquid during electrolysis, as the porous membrane.
In one general aspect, the present invention comprises an electro-lytic cell diaphragm in accordance with Canadian Patent 1,037,671 which has associated with it a reinforcing element integrated with the anode face of the porous electrolytic cell diaphragm. The reinforcing element shall have a structure which is at least as open or porous relative to the passage of liquid during electrolysis as the porous membrane.
It has unexpectedly been observed that if the same reinfor^ing ele-ment is placed on the cathode face rather than the anode face, the above-men-tioned disadvantages of the prior art membranes will still be experienced.
Accordingly, the invention provides a method of manufacturing por-ous electrolytic cell diaphragms which are consolidated by a fluorinated poly-mer resin, comprising agitating a suspension of asbestos fibers in water, a sul-fonic anionic surfactant, a fluorinated polymer resin latex and a solid mineral pore-former, to form a stable, homogeneous suspension, depositing and filter-ing said suspension on a grid or screen to form a preform, drying the result-ing preform, sintering it by heat at a temperature above the crystalline melt-ing point of the fluorinated polymer resin, removing said solid minerql pore-former to form a uniform, porous membrane having uniform pore size, combiningsaid resulting uniform, porous, homogeneous membrane with a reinforming ele-ment by integrating the said reinforcing element with the anode face of said ~076065 uniform, porous, homogeneous membrane, wherein said reinforcing element has a structure which is at least as open with respect to the passage of liquor as said uniform, porous, homogeneous membrane.
The reinforcing 01ement advantageously comprises a cloth or sheet of asbestos fibers.
The asbestos cloth from 0.5 to 1 mm. thick and less than 500 grams per square meter in weight may advantageously be associated with a micro-porous membrane produced in accordance with our Canadian Patent 1,037,671, in which the proportion of charge of pore-forming agent relative to the weight lQ of asbestos is at least about 100% and preferably between about 250 and 600%.
In a preferred embodiment the diaphragm according to the invention comprises as asbestos cloth from 0.5 to 1 mm., weighing less than 500 grams per square meter and incorporated in the anode face of a microporous membrane in which the proportion of charge comprising calcium carbonate or other solid mineral pore-forming agent, relative to the weight of asbestos, is from about 250 to 600~. The diaphragm has a relative resistance of less than 4, a liquid flow rate through the diaphragm of from about 0.25 to 0.02 ml./min./cm.2 under 50 g./cm of liquid charge at 20C., and a thickness of about 2.5 to 5 mm. and preferably from about 3 to 4 mm.
Such a diaphragm can be obtained by applying a method similar to that described in Canadian Patent 1,037,671.

~gJI

` 1076065 In practice, an asbestos suspension is prepared by dispersing by agitation a mixture which contains (by weight):
1 part of asbestos about 5 to 100 parts of water about 0~015 to 0.1 part of anionic surfactant.
The asbestos used is preferably made up of 0.5 to 50 millimeter fibers. The surfactant, a sulfonic anionic surfac-- tant and preferably a sodium sulphosuccinate, i6 used in pure .
form or in solution in alcohol. Vigorous agitation gives a stable, well-dispersed asbestos suspension.
Other sulfonic anioni¢ surfactants may be empLoyed such as alkyl sulfonates, sulfosuccinates and sulfosuccinamates.
The latex of a fluorinated polymeric resin and the pore-forming agent are added to ~he suspension so as to give :.
the following proportions by weight:
100 parts asbestos about 60 to 200 parts of fluorinated polymeric ~, resin, on dry basis about 100 to 1400 parts of pore-forming agent.
Agitation is then continued for about 1 to 20 minutes, - preferably 5 to 10 minutes, depending upon conditions employed, particularly of speed. The final concentration of the suspension may be adjusted by adding water, at the end of the agitation process, in proportions best adapted to the depositing condi-tions observed.
; The latex of fluorinated polymeric resin, preferably of polytetrafluoroethylene, is preferably a suspension of approxi-mately 60% polytetrafluoroethylene in water. It may be replaced by other latices of fluorinated resins (a polymerized mixture OL
tetrafluoroethylene-hexafluoropropylene, or polymerized poly-chlorotrifluoroethylene etc.)~

The pore-forming agent used is desirably a mineral material and may he calcium carbonate, colloidal alumina, metal oxides or any substances adapted to be eliminated by a solvent or by destruction at the end of the operation. It must have a well-defined particle size. It is preferable to use a calcium carbonate consisting of particles with an average diameter OL

2 to 25 microns.
When a flat diaphragm is being made, the stable, homogeneous mixture of the various constituents is poured onto the reinforcing element in a quantity such that the desired thickness is obtained. It is then filtered under vacuum then dried. Drying takes place at a temperature of over 100C., at approximately 150C. for 3 to 24 hours.
The sheet is then fritted or sintered by placing it in a furnace at a temperature above the crystalline melting point of the fluorinated polymer resin, preferably about 25 to 75C.
above it, for a period of 2 to 20 minutes and preferably about 6 to 10 minutes. The optimum temperature chosen depends some-what on the fritting time but also on the thickness and compo-sition of the diaphragm.
; When the diaphragm has cooled and employs calcium carbona*e as the pore-forming agent, it is immersed in a 10 to 20% by weight aqueous solution of a weak acid, in the presence of a wetting agen~, for a period ranging from 24 to 72 hours depending on the thickness. Acetic acid is preferred, but other weak acids may be used with the same degree of success. Where ctherpoxe-forming agents are employed, another suitahle solvent or reagent for decomposing and dissolving the pore-forming agent may be employed. Thus, depending upon the pore-former employed, any agent which will dissolve or decompose the pore-former may be employed, provided it does not dissolve or decompose the ~076065 fluorinated polymer resin. If the mineral pore-former is alumina, acid o~ alkali solutions may be employed. ~ith other metal oxides, other dissolving agents well known to the chemist may be employed.
The diaphragm obtained is then washed in water to eliminate the acid, and kept under water.

Specific Description of the Invention In order to disclose more clearly the nature of the present invention, the following examples illustrating the invention are given. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims. In the examples which follow and through-out the specification, the quantities of ma~erial are expressed in terms of parts by weight, unless otherwise specified.
:
EXAMPL~ 1 This example is a control experiment which differs from the method of the invention, in that the reinforcing element is arranged on the cathode face of the diaphragm.
In this experiment, a suspension of asbestos fibers is prepared, containing:
100 g. of asbestos fibers of an average length of 1 to 2 mm.
2500 g. of water 2.5 g. of a 75% solution of sodium dioctylsulpho-succinate in alcohol.
Dispersion is effected by agitating for 60 minutes using an agitator of the drum roller type.
The following are then added:
300 g. of polytetrafluoroethylene polymer in the form of a latex with 60% of dry polymer 560 g. of calcium carbonate, marketed under the registered trademark "BLE O~1YA".

The mixture is agitated for 5 minutes.
170 g. of the suspension is drained on an asbestos cloth measuriny 1 square decimeter, using the following vacuum program:
1 1 minute of decantation 2 minutes at a pressure of 200 mm. of Hg. (mercury) 2 minutes at 300 mm. of Hg.
10 minutes at 740 mm. of Hg.
The asbestos cloth used has a thickness of 0.76 mm.
for a weight of 255 grams per square meter. It comprises warp and weft threads of 111 tex (weight in grams of 1,000 meters of thread) and has 13 threads/cm. in the warp and 7 threads/cm. in the weft.
The form obtained, thus combined with the asbestos cloth, is dried in an oven at 150C. for 5 hours. The reinforced - form is then fritted in a furnace which is brought to 360C. for 7 minutes. The carbonate is eliminated in a 20% by weight solu-tion of acetic acid in water for 72 hours. The diaphragm thus o,btained is washed with water.
This gives a diaphragm which has the following pro-perties:

weight in grams per 170 square decimeter thickness in mm. 3,55 liquid flow rate in ml./ 0.15 min./cm.2 (wlder a charge of 50 g./cm.2) relative resistance R/Ro 2.6 The diaphragm is used as a separator in electrolysis 0 of a solution o~ sodium chloride under the ~ollowing conditions:
asbestos cloth placed at cathode side electrodes formed by a grating (platinum-plated titanium at anode side and iron at cathode side) 7 mm. apart.

current density--25 amperes per square decimeter temperature--85C.
The following properties are observed during operation of the cell:
loss of cohesion on the anode face tension of cell in equilibrium 3.44 volts composition of liquor:
soda g./1. = 125 chlorate g./l. - 1 liquid charge on diaphragm = 15 cm. water (H) ~ EXAMPLE 2 - This example is identical with the previous one, except that the same asbestos cloth is arranged at the anode side.
The liquor is found to have the same composition of soda and chlorate, but the equilibrium tension is found to have decreased to 3.17 volts; relative resistance is 2.1 and the liquid charge on the diaphragm is 12 cm. of water.

. EX~PLES 3 to 14 In all these examples the proportions of the various -constituents are as follows:
asbestos fibers 100 g.
water 2500 g.
- polytetrafluoroethylene 300 g.
resln sodium dioctylsulphosuccinate 2.5 g.
The other conditions and the results in electrolysis under the same conditions as in Examples 1 and 2 are set forth in the table which follows.

O_ -- C~ O N _ O _ l O __ Zo _~ o ~ _~ o t~ _I N
~ _ _ _ _ _ _ _ _ _ __ _ _ O _~Ul O O O U~ U~ 00 U~ O
bON N ':t N ~) N O N l O
_ _ _ _ _ _ _ _ ~d- _ ~d- c~- .
~I~ ~ d' X N ~t N Il~ b O ~ h _ _ _ _ _ _ _ _l O _ O O
oo ~ L~ O I~
E-~O ' t~ ro t-~ t~ t~ t~ t~ t~ ~
O _ _ _ _ _ _ _ _ _ _ _ _ ~: ~ a~ ~ ~D cr~ O ~ _1 O u~ 00 1~ _I N N t-~ N t ) t~ ~ ~ I~ O O~
_ _ _ _ _ _ _ _ _ _ _ N N ., q~ ~ O Ln n I~ _l 1-~ _~ t~ ';t C~ n N O _l O 1~ O O
.~, h N N O O O O O O O O O O

h _ _ _ _ _ _ _ _ _ _ _ _ ~: h ~
~0 ~. ~ o~ ~ _1 ~ Ln I~ el~ oo ~ n ~ `D
~ ~ t ~ u~ ~ t~ t~ t~ N t ~ N t~
_ _ _ _ _ _ _ _ _ _ _ _ ~0 ~ rl h ~
.,~ ~, a) O O O O O O O Q O O O O
ID ~ 1~ ~D ~ O ~ a~ N ~0 a- N ~ 01 N
--~ ~1 N --I _I N _~ _I N _~ ~1 N
_ _ _ _ _ _ _ _ _ _ _ _ 0~0 O O O O O O O O O O O O
C~ 1: N `D ~7 N N N ~t ~ ~
_ _ _ _ _ _ _ _ _ _ _ _ X ¦ t~l ~ u~ ~0 1~ oO O- O _~ N ~ ~:t ~11 , _ _ _ _ _ _ _ _1 _1 _~ _1 ~

The foregoing table shows the properties and advan-tages of the diaphragm according to the invention.
Thus, it will be noted that the charge valu~s for Examples 10 to 14 are very high or too great, making the dia-phragms virtually useless in electrolysis.
Number 9 is an example of a diaphragm which tolerates a high proportion of chlorate, thus making it unsuitable.
The examples show that a diaphragm structure which is too closed gives poor results in electrolysis.
Conversely, too open a structùre leads to fairly good results in electrolysis, but the diaphragm obtained is fragile, which means that a thicker diaphragm has to be used; this is detrimental to application in electrolysis, as can be seen from Example 3.
It will be seen from Examples 4 to 8 that there is a preferred range which gives an excellent overall result.

EX~MPLE 15 This example is carried out under the same conditions as previously and in the preferred range as defined above. It illustrates an experiment with a reinforcing element on the cathode face, different from that described in Example 1.
In this example, the charge of pore-forming agent (CaCO3) is 560 g. and the quantity of material filtered 120 g.
The thickness of the diaphragm is 3.10 mm.
Relative resistance is 2.25, tension in equilibrium is

3.30 volts, the liquid charge, H, is 2.5 cm. and the composition of the liquor:
soda = 116 g./l.
chlorate = 1.7 g./l.
These conditions are unacceptable in electrolysis;

the liquid charge is too small and the chloratè content too high.

Moreover, it will be noted, e.g., that the tension of the cell corresponds to that in Example 5, where the quantity of material filtered is 200 g., the liquid charge 24, and the quantity of chlorate 0.9.
It has been seen that the solution comprising increasing the quantity of solid in the porous membrane to obtain a lower chlorate content and an adequate liquid charge leads to a high equilibrium tension, whereas according to the invention with a larger quantity of filtered material, 140 g. instead of 120 g.
(see Example 4), the liquid charge on the diaphragm is greater, the proportion of chlorate reduced, and the tension of the cell weaker.
Even with a larger quantity of pore-forming filler, better results are obtained in electrolysis (Examples 2 and 3).
The examples thus illustrate the new and advantageous ~; effects of the invention, such as the absence of any loss of cohesion and the very marked improvement in performance in electrolysis.
It should also be pointed out that the following effects are observed with diaphragms according to the invention:
(1) the diaphragms of the invention have greater resistance, at the anode side, to abrasion by gases;
(2) the diaphragms of the invention have greater resistance to bending and punching, operations which are sometimes necessi-tated by cell technology; they are also less vulnera~le to handling, particularly during the shaping or forming of the diaphragms;
(3) in situ clearing operations are facilitated.
Above all, it should be noted tha~, in contrast with what usually happens, the mechanical, hydrodynamic and electro-chemical re~uirements of electrolysis are all unexpectedly met better by diaphragms according to the invention.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the eatures shown and described or portions thereof, but it is recognized that various modifi-cations are possible within the scope of the invention claimed.

Claims (16)

WHAT IS CLAIMED IS:

1. A method of manufacturing porous electrolytic cell diaphragms which are consolidated by a fluorinated polymer resin, comprising agitating a suspension of asbestos fibers in water, a sulfonic anionic surfactant, a fluorinated polymer resin latex and a solid mineral pore-former, to form a stable, homo-geneous suspension, depositing and filtering said suspension on a grid or screen to form a preform, drying the resulting pre-form, sintering it by heat at a temperature above the crystalline melting point of the fluorinated polymer resin, removing said solid mineral pore-former to form a uniform, porous membrane having uniform pore size, combining said resulting uniform, porous, homogeneous membrane with a reinforcing element by inte-grating the said reinforcing element with the anode face of said uniform, porous, homogeneous membrane, wherein said rein-forcing element has a structure which is at least as open with respect to the passage of liquor as said uniform, porous, homo-geneous membrane.

2. A method of manufacturing a porous diaphragm according to Claim 1, wherein said reinforcing element comprises an asbestos cloth from about 0.5 to 1 mm. thick and weighing less than 500 grams per square meter.

3. A method of manufacturing a porous diaphragm according to Claim 1, wherein said uniform, porous, homogeneous membrane, with which the said reinforcing element is combined, employs a proportion of said solid mineral pore-former relative to the weight of asbestos of over about 100%.

4. A method of manufacturing a porous diaphragm according to Claim 1, wherein said uniform, porous, homogeneous membrane, with which the said reinforcing element is combined, employs a proportion of said solid mineral pore-former relative to the weight of asbestos of between about 250 and 600%.

5. A method of manufacturing a porous diaphragm according to Claim 1, wherein said surfactant is sodium dioctyl sulphosuccinate.

6. A method of manufacturing a porous diaphragm according to Claim 1, wherein said solid mineral pore-former is calcium carbonate.

7. A method of manufacturing a porous diaphragm according to Claim 1, characterized in that the fluorinated polymer resin latex is of polytetrafluoroethylene polymer.

8. A method according to Claim 1, wherein said asbestos fibers are of 0.5 to 50 millimeters in length.

9. A method according to Claim 1, wherein said solid mineral pore-former is a member selected from the group con-sisting of calcium carbonate, colloidal alumina and metallic oxides.

10. A method according to Claim 1, wherein said sulfonic anionic surfactant is a member selected from the group consisting of an alkyl sulfonate, sulfosuccinate and sulfosucci-namate.

11. A method according to Claim 1, wherein said sintering of the preform is by heat and is effected for a period of about 2 to 20 minutes.

12. A method according to Claim 1, wherein said solid mineral pore-former is removed by decomposition.

13. A method according to Claim 1, wherein the said solid mineral pore-former is removed by extraction.

14. An integrated, porous diaphragm suitable for use in an electrolytic cell consisting of an asbestos cloth of from about 0.5 to 1 millimeter thick, weighing less than about 500 grams per square meter, integrated with the anode face of a microporous, uniform membrane of substantially uniform pore size and comprising asbestos fibers, said membrane, being consolidated by a fluorinated polymer resin, said asbestos fibers of between about 0.5 and 50 millimeters in length, said diaphragm having a relative resis-tance less than about 4, a flow rate of liquid through the diaphragm of from about 0.25 to 0.02 milliliters per minute per square centimeter under 50 grams per square centimeter of liquid charge at 20°C. and said resulting diaphragm being of total thickness of from about 2.5 to 5 millimeters.

15. An integrated, porous diaphragm according to claim 14, wherein said total thickness of said diaphragm is between about 3 and 4 millimeters.

16. An integrated, porous diaphragm according to claim 14, wherein the fluorinated polymer resin is polytetrafluoroethylene polymer.