CA1078564A - Method for improving selectivity of membranes used in chlor-alkali cells - Google Patents
Method for improving selectivity of membranes used in chlor-alkali cellsInfo
- Publication number
- CA1078564A CA1078564A CA262,439A CA262439A CA1078564A CA 1078564 A CA1078564 A CA 1078564A CA 262439 A CA262439 A CA 262439A CA 1078564 A CA1078564 A CA 1078564A
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- centigrade
- temperature
- minutes
- thermally treating
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Abstract
Abstract of the Disclosure Membranes for use in chlor-alkali cells, made of a copolymer of tetrafluoroethylene and sulfonylfluoride per-fluorovinyl ether, have their selectivity improved, with resulting substantial decrease in consumption of electric power per mole of sodium hydroxide produced, by being heat-treated at 100 to 275° Centigrade for several hours to four minutes. Desirably, the membranes are also subjected to pressure, such as up to 9.76 kilograms per square centimeter (10 tons per square foot). The current efficiency is sub-stantially increased, and the power consumption, per unit of sodium hydroxide produced, is usually decreased by about 10 percent or more.
Description
0~ 8 S ~ ~
Background o~ the Invention 1. Field of the Invention:--This invention relates to the art of providing membranes for use in chlor-alkali elec~
trolysis membrane cells~ and in particular, to a method o~
pre-treating said membranes, before their insertion into a cell, t~o improve the quality of said membranes
Background o~ the Invention 1. Field of the Invention:--This invention relates to the art of providing membranes for use in chlor-alkali elec~
trolysis membrane cells~ and in particular, to a method o~
pre-treating said membranes, before their insertion into a cell, t~o improve the quality of said membranes
2. Description of the Prior Art:--The use of membrane-type - electrolysis cells for the electrolysis of brine, producing chlorine, hydrogen, and sodium hydroxide, is well known, as for example, from U S Patent No 2J967J807. I~ is well known that such membrane-type cells can be made by using a sheet or film, approximately 0 1 to 0.25 or 0.5 millimeters (about 4 to 10 or 20 mils) thick, of a copolymer of tetra-fluoroethylene and sulfonylfluoride perfluorovinyl ether.
Suitable copolymer material is disclosed in U S. Patent No. 3,282,875. Such cells offer an attractive alternative to the customary diaphragm-type cells, using a diaphragm made of asbestos or the like, because of the health hazards posed by the manufacture and use of asbestos It is known that such membranes have a tendency, when put into service, to swell, thereby crea~ing water domains through which hydroxide ions are transported much more readily than sodium ions, owing to the Grotthus mechanism. I am not aware that anyone has hitherto proposed any method or S~4 practiceJ by means of which such swelling of the membrane may be reduced, with corresponding favorable e~fects upon the effective selectivity of the membranes and upon the current efficiency o~ the chlor-alkali membrane cells ln which they are used.
Summary of the Invention By subjecting them to a heat treatment at 100 to 275 Centigrade for a time of several hours to four minutes, membranes for use in chlor-alkali cells are given improved properties: they contain less water, have improved selec-tivity, exhibit higher current efficiency and lower power consumption per unlt of product, and afford a product having a lower salt content. It is also desirable, during ~he heat i treatment, to apply a pressure of up to 9.76 kilograms per square centimeter (10 tons per square foot).
Description of the Preferred Embodiments The present invention is practiced upon membranes for use in membrane-type chlor-alkali cells for the elec-trolysis of brine to produce alkali-metal hydroxide, chlorine3 and hydrogen. In particular, it is practiced on membranes that are made of a copolymer of tetrafluoro-ethylene and sulfonated perfluorovinyl ether, such as a copolymer of tetrafluoroethylene and sulfonylfluoride per-fluorovlnyl ethers. Such material is commercially available 7 ~ 56 ~
for use in such cells in the form of membranes having a thickness ordinarily on the order of 0.10 to 0.25 milli-meters ~4 to 10 mils) and having an equivalent weight number on the order of 1000 to 1500. To improve ~he strength of the membrane, some of the membranes are provided with reinforcement of polytetrafluoroethylPne or the like;
others are not. Such membranes are useful ln their un-treated condition~ but by the practice of the present in-vention, their performance can be considerably improved.
In the practice of the invention, a membrane to be treated is preferably placed between a pair of sligh~ly larger thin sheets of polytetrafluoroethylene, to insure against having the membrane adhere to anything with which it is in contact during the thermal treatment. A conven-ient way of practicing the invention is to insert the sand-wich thus prepared into a hydraulic press having a pair of electrically heated flat plates, and while exerting some ; pressure upon the sandwich, to bring it to a desired tem-perature and hold it at such temperature for an appropriate period of time. Satisfactory results have been ob~ained without the exertion of any pressure, but in most instances it is desirable to use a small pressure, such as o.976 to 4.88 kilograms per square centimeter (l to 5 ton8 per Y8~;~;4 square foot). Pressuresas grea~ as 9.76 kilograms per square centimeter (10 tons per square foot) can be used.
The duration of the heat treatment depends upon the temperature. At a high temperature, such as 275 Centi-grade, a short time such as four to five minutes is suf-ficient, whereas at a low temperature such as 100 Centi-grade, a time of several hours may be required. Preferably, the temperature used is between 175 and 225 Centigrade, and at that temperature, a time of five to twelve minutes is satisfactory. Pre~erred results are obtained with the use of a temperature of 200 Centigrade for seven minutes.
After the thermal treatment, the membrane i5 -.
allowed to cool to room temperature. Rapid cooling (one min-or less) is acceptable, but a slower cooling rate (about fifteen minutes) i~ preferred.
The trea~ed membrane is then inserted into a chlor-alkali cell and used in the same manner as an un-treated membrane~
The invention described above is illustrated by the following specific examples.
Example 1 A 0.125-millimeter (five-mil) thick piece of polytetrafluoroethylene-reinforced membrane materialJ made ~5~
.. , .. ~ , . ...
" ~ 7 8 ~ ~
of a copolymer o~ tetrafluoroethylene wlth sulfona~ed per-fluorovinyl ether and having an equivalent weight number o about 1100, was boiled briefly in a 1 Normal aqueous solu-tion of hydrochloric acid and then removed. In this state, it could have been inserted directly into a chlor-alkali cell. The piece was wiped dry, sandwiched between two sheets of polytetrafluoroethylene, and placed into a hy-draulic press that had been pre-heated to 225 Centigrade.
A pressure of 6.83 kilograms per square centimeter (7 tons per square foot) was then applied for a period o five minutes, and the membrane was then allowed to cool in the press after the pressure had been released. This took about 15 minutes. The membrane was removed from between the sheets of polytetrafluoroethylene and inserted into a chlor-alkali cell having dimensionally stable anodes and steel cathodes. The cell was then operated at a cell current of 25 amperes. Saturated brine having a pH of 4 was fed to the anode compartment at a rate of about 200 milliliters per hour7 and 80 milliliters per hour of water were fe~ to the cathode compartment, which produced an 18 weight percent aqueous solution of sodium hydroxide. The cell operated at ~.85 volts and with a current efficiency of 78 percent.
The energy consumption was 1~2 wat~-hours per mole of sodium hydroxlde.
.
.
~7~ 5~ ~
For comparison, a similar membrane was inser~ed into a similar chlor-alkali cell, immediately after having been boiled brie~ly in hydrochloric acid. This chlor-alkali cell was operated under substantially th~ same conditions, exhibiting a cell voltage of 3.~5 volts, a curren~ effi-ciency of 59 percent, and an energy consumption o~ 152 watt-hours per mole of sodium hydroxide. The thermal treat-ment according to the invention increased the current efi-ciency from 59 percent to 78 percent, and it lowered the energy consumption from 152 to 132 watt-hours per mole.
Example 2 Example 1 was repeated, except that (1) the mem- -brane was 0.178 millimeters (7 mils) thick and had an equivalent weight number of 1200, and (2) the temperature used in the thermal treatment was 250 Centigrade. Again, for comparison, the results with an identical but untreated membrane were observed. The ~reated membrane gave current efficiency of 82 percent, a cell voltage of ~.7 volts, and an energy consumption of 121 watt-hours per mole. When the treated membrane was used for a period of ~ive months3 the current efficiency remained at about 80 percent. The un-treated membrane gave a current eficiency of 69 percent, a cell voltage of ~.85 volts, and an energy consumption of . ~ . . . ............................ . .
, ~ 85~4 149 watt-hours per mole.
Example 3 An unreinforced membrane of 1200 equivalent weight number and having a thickness of 0.254 millimeters (10 mils) was thermally treated at 250 Centigrade for five minutes~ It was then inserted into a cell, as in Example 1, and used to produce an aqueous solution containing 18 weight percent of sodium hydroxide. The current efficiency was 82 percent, the cell voltage was 3.3 volts, and the energy consumption was 108 watt-hours per mole. The sodium chloride content of the product from the cell containing the treated membrane was 200 milligrams per liter.
In comparison, when a substantially identical but ! untreated membrane was used, the current efficiency was 64 percent~ the cell voltage was 3.1 volts, and the energy consumption was 130 watt-hours per mole. Moreover, the hydroxide product contained 1.5 grams per liter of sodium chloride.
- Example 4 Copolymerized tetrafluoroethylene and sulfonated - perfluorinated vinyl ether of equivalent weight number 1350 was used to prepare a reinforced membrane 0.1016 millimeters (~ mils) thick. The membrane was thermally treated a~ 225 .. ..
-8- ~
..
. . .. ~ . : .
.
~ ~ 7~ S ~ ~
Cen~igrade for five minutes. When lnserted lnto a cell, the thermally treated membrane gave a cell voltage of 4.0, a current efficiency of 90 percent, and an energy con-sumption o~ 120 watt-hours per mole. In comparison, an untreated membrane gave a cell voltage of 3.3, a current efficiency of 68 percent, and an energy consumption of 130 watt-hours per mole.
! Example 5 Example ~ was repeated, except that the thermal treatment was conduc~ed at 200 Centigrade for four minutes and at a pressure of 2.928 kilograms per square cen~imeter ;
(three tons per square foot). When tested in a chlor-alkali cell, the resulting membrane gave a current efficiency of 82 percent, the same as in Example 3, in which the un~reated - membrane had a current efficiency of 64 percent.
Example 6 Example ~ was repeated, except that the thermal ; treatment was conducted in an oven at 200 Centigrade for ~hirty minutes without any pressure. When tested in a chlor-alkali cell, the mem~rane so treated gave a current efficiency of 78 p~rcent.
" ~Z .
Example 3 was repeated, except that the thermal .
.
_9 .
.
56~
treatment was conducted in an oven at 110 Centigrade or a period of ~our hours. When tes~ed in a chlor~alkali cell, the membrane gave a current efficiency of 77 percent.
In addition, tests were conducted wi~h respect to membranes of 1100 and 1200 equivalent weight num~er to demonstrate that a thermal treatment in accordance with the present invention yields a membrane which has a decreased tendency to absorb water (in comparison with an untreated membrane). Treated and corresponding untreated ~embranes were brought to equilibrium in an atmosphere having 50 percent xelative humidity, and the water contents of the membranes were then determined by the base catalyzed, extrapolated Karl Fischqr me~hod. For membranes of 1100 equivalent weight number, the wa~er contents were 8.72 weigh~ percent for the untreated and 7.50 weight percent for the treated, a difference of 14 percent. For membranes of 1200 equivalent weight number, the water contents were 7.45 weight percent for the untreated and 6.55 weight percent for the treated, a difference of 12 percent.
While there have been shown and described herein certain embodiments of the invention, it is intended that - - there be covered as well any change or modification therein which may be made withou~ departing from the spiri~ and ;
scope of the invention. -~
Membrane~ treated as herein taught may find other uses in which greater selectivity is wanted.
-10- , . ... .
.
. . . .
. . . . ..
~ 1~785ti~
SUPPLEMENTARY DISC~OSURE
_ . _ _ . _ . .
In the application as originally iled, a method ~or obtaining a selective membrane for use in chlor-alkali cells has already been disclosed. Said membrane prior to use is subjected to a particular thermal treatment.
Now, it has been found that the thermal treatment may be conducted at a temperature of 175 to 225 Centigrade, for a time of three hours to one half hour. Preferred results are obtained with the use of a temperature of 200Centigrade for two hours.
Further, after the thermal treatment, the membrane is allowed to cool to room temperature. Rapid cooling (one minute or less) is acceptable, but a slower cooling ra-te ~at least 15 minutes, and up to several hours, preferably about 2 or 3 hours) is preferred.
As a result of such thermal treatment, the membranes exhibit improved properties: they have improved selectivity, give higher current efficiencies and lower power consumption per unit of product obtained, and afford a product having a lower salt content.
The underlying reason for these changes is that as a result of the heat treatment, there occurs a morphological transition in the membrane material. This can be seen clearly from X-ray diffraction data upon membranes in their untreated and treated states. Untreated, the membrane is characterized by two lacttice constants, one at 5.7 Angstrom units and one at 34 Angstrom units. The former is attributable to the lateral spacing of the polymer chains. The latter is related to the spa-cing of the sulfonic acid groups. Treated, the membrane exhibitslattice constants of 5.7, 27, and 140 Angstrom units. The X-ray diffraction data demonstrate that the spacing between sulfonic acid ~ 'r groups has heen diminished~ as is evidenced b~ the decrease in lattice constant from 34 to 27 Anys-trom units. Those skilled in the art of ion-exchange membranes know ~hat closer spacing o sulfonic acid groups means better membrane selectivity.
Moreover, the appearance of an overstructure with a spacing of 140 Angstrom units indicates that, af~er treatment, there is a more regular ordering of the resin. Those skilled in the art will again appreciate that the more regular ordering can be expected to improve the selectivity of the membrane and its other mechanical and transport properties. Indeed, the treated membrane, as compared to one untreated, was 25% higher in tensile strength and 50% lower in permeability for gases.
The new reacting conditions of preparing the membrane will now be further understood by means of the following non-res-trictive example.
Example 8 Example 6 of the original application was repeated, except that the treatment at 200 Centigrade was or two hours.
The curr~nt efficiency was 81%.
. . - . .
Suitable copolymer material is disclosed in U S. Patent No. 3,282,875. Such cells offer an attractive alternative to the customary diaphragm-type cells, using a diaphragm made of asbestos or the like, because of the health hazards posed by the manufacture and use of asbestos It is known that such membranes have a tendency, when put into service, to swell, thereby crea~ing water domains through which hydroxide ions are transported much more readily than sodium ions, owing to the Grotthus mechanism. I am not aware that anyone has hitherto proposed any method or S~4 practiceJ by means of which such swelling of the membrane may be reduced, with corresponding favorable e~fects upon the effective selectivity of the membranes and upon the current efficiency o~ the chlor-alkali membrane cells ln which they are used.
Summary of the Invention By subjecting them to a heat treatment at 100 to 275 Centigrade for a time of several hours to four minutes, membranes for use in chlor-alkali cells are given improved properties: they contain less water, have improved selec-tivity, exhibit higher current efficiency and lower power consumption per unlt of product, and afford a product having a lower salt content. It is also desirable, during ~he heat i treatment, to apply a pressure of up to 9.76 kilograms per square centimeter (10 tons per square foot).
Description of the Preferred Embodiments The present invention is practiced upon membranes for use in membrane-type chlor-alkali cells for the elec-trolysis of brine to produce alkali-metal hydroxide, chlorine3 and hydrogen. In particular, it is practiced on membranes that are made of a copolymer of tetrafluoro-ethylene and sulfonated perfluorovinyl ether, such as a copolymer of tetrafluoroethylene and sulfonylfluoride per-fluorovlnyl ethers. Such material is commercially available 7 ~ 56 ~
for use in such cells in the form of membranes having a thickness ordinarily on the order of 0.10 to 0.25 milli-meters ~4 to 10 mils) and having an equivalent weight number on the order of 1000 to 1500. To improve ~he strength of the membrane, some of the membranes are provided with reinforcement of polytetrafluoroethylPne or the like;
others are not. Such membranes are useful ln their un-treated condition~ but by the practice of the present in-vention, their performance can be considerably improved.
In the practice of the invention, a membrane to be treated is preferably placed between a pair of sligh~ly larger thin sheets of polytetrafluoroethylene, to insure against having the membrane adhere to anything with which it is in contact during the thermal treatment. A conven-ient way of practicing the invention is to insert the sand-wich thus prepared into a hydraulic press having a pair of electrically heated flat plates, and while exerting some ; pressure upon the sandwich, to bring it to a desired tem-perature and hold it at such temperature for an appropriate period of time. Satisfactory results have been ob~ained without the exertion of any pressure, but in most instances it is desirable to use a small pressure, such as o.976 to 4.88 kilograms per square centimeter (l to 5 ton8 per Y8~;~;4 square foot). Pressuresas grea~ as 9.76 kilograms per square centimeter (10 tons per square foot) can be used.
The duration of the heat treatment depends upon the temperature. At a high temperature, such as 275 Centi-grade, a short time such as four to five minutes is suf-ficient, whereas at a low temperature such as 100 Centi-grade, a time of several hours may be required. Preferably, the temperature used is between 175 and 225 Centigrade, and at that temperature, a time of five to twelve minutes is satisfactory. Pre~erred results are obtained with the use of a temperature of 200 Centigrade for seven minutes.
After the thermal treatment, the membrane i5 -.
allowed to cool to room temperature. Rapid cooling (one min-or less) is acceptable, but a slower cooling rate (about fifteen minutes) i~ preferred.
The trea~ed membrane is then inserted into a chlor-alkali cell and used in the same manner as an un-treated membrane~
The invention described above is illustrated by the following specific examples.
Example 1 A 0.125-millimeter (five-mil) thick piece of polytetrafluoroethylene-reinforced membrane materialJ made ~5~
.. , .. ~ , . ...
" ~ 7 8 ~ ~
of a copolymer o~ tetrafluoroethylene wlth sulfona~ed per-fluorovinyl ether and having an equivalent weight number o about 1100, was boiled briefly in a 1 Normal aqueous solu-tion of hydrochloric acid and then removed. In this state, it could have been inserted directly into a chlor-alkali cell. The piece was wiped dry, sandwiched between two sheets of polytetrafluoroethylene, and placed into a hy-draulic press that had been pre-heated to 225 Centigrade.
A pressure of 6.83 kilograms per square centimeter (7 tons per square foot) was then applied for a period o five minutes, and the membrane was then allowed to cool in the press after the pressure had been released. This took about 15 minutes. The membrane was removed from between the sheets of polytetrafluoroethylene and inserted into a chlor-alkali cell having dimensionally stable anodes and steel cathodes. The cell was then operated at a cell current of 25 amperes. Saturated brine having a pH of 4 was fed to the anode compartment at a rate of about 200 milliliters per hour7 and 80 milliliters per hour of water were fe~ to the cathode compartment, which produced an 18 weight percent aqueous solution of sodium hydroxide. The cell operated at ~.85 volts and with a current efficiency of 78 percent.
The energy consumption was 1~2 wat~-hours per mole of sodium hydroxlde.
.
.
~7~ 5~ ~
For comparison, a similar membrane was inser~ed into a similar chlor-alkali cell, immediately after having been boiled brie~ly in hydrochloric acid. This chlor-alkali cell was operated under substantially th~ same conditions, exhibiting a cell voltage of 3.~5 volts, a curren~ effi-ciency of 59 percent, and an energy consumption o~ 152 watt-hours per mole of sodium hydroxide. The thermal treat-ment according to the invention increased the current efi-ciency from 59 percent to 78 percent, and it lowered the energy consumption from 152 to 132 watt-hours per mole.
Example 2 Example 1 was repeated, except that (1) the mem- -brane was 0.178 millimeters (7 mils) thick and had an equivalent weight number of 1200, and (2) the temperature used in the thermal treatment was 250 Centigrade. Again, for comparison, the results with an identical but untreated membrane were observed. The ~reated membrane gave current efficiency of 82 percent, a cell voltage of ~.7 volts, and an energy consumption of 121 watt-hours per mole. When the treated membrane was used for a period of ~ive months3 the current efficiency remained at about 80 percent. The un-treated membrane gave a current eficiency of 69 percent, a cell voltage of ~.85 volts, and an energy consumption of . ~ . . . ............................ . .
, ~ 85~4 149 watt-hours per mole.
Example 3 An unreinforced membrane of 1200 equivalent weight number and having a thickness of 0.254 millimeters (10 mils) was thermally treated at 250 Centigrade for five minutes~ It was then inserted into a cell, as in Example 1, and used to produce an aqueous solution containing 18 weight percent of sodium hydroxide. The current efficiency was 82 percent, the cell voltage was 3.3 volts, and the energy consumption was 108 watt-hours per mole. The sodium chloride content of the product from the cell containing the treated membrane was 200 milligrams per liter.
In comparison, when a substantially identical but ! untreated membrane was used, the current efficiency was 64 percent~ the cell voltage was 3.1 volts, and the energy consumption was 130 watt-hours per mole. Moreover, the hydroxide product contained 1.5 grams per liter of sodium chloride.
- Example 4 Copolymerized tetrafluoroethylene and sulfonated - perfluorinated vinyl ether of equivalent weight number 1350 was used to prepare a reinforced membrane 0.1016 millimeters (~ mils) thick. The membrane was thermally treated a~ 225 .. ..
-8- ~
..
. . .. ~ . : .
.
~ ~ 7~ S ~ ~
Cen~igrade for five minutes. When lnserted lnto a cell, the thermally treated membrane gave a cell voltage of 4.0, a current efficiency of 90 percent, and an energy con-sumption o~ 120 watt-hours per mole. In comparison, an untreated membrane gave a cell voltage of 3.3, a current efficiency of 68 percent, and an energy consumption of 130 watt-hours per mole.
! Example 5 Example ~ was repeated, except that the thermal treatment was conduc~ed at 200 Centigrade for four minutes and at a pressure of 2.928 kilograms per square cen~imeter ;
(three tons per square foot). When tested in a chlor-alkali cell, the resulting membrane gave a current efficiency of 82 percent, the same as in Example 3, in which the un~reated - membrane had a current efficiency of 64 percent.
Example 6 Example ~ was repeated, except that the thermal ; treatment was conducted in an oven at 200 Centigrade for ~hirty minutes without any pressure. When tested in a chlor-alkali cell, the mem~rane so treated gave a current efficiency of 78 p~rcent.
" ~Z .
Example 3 was repeated, except that the thermal .
.
_9 .
.
56~
treatment was conducted in an oven at 110 Centigrade or a period of ~our hours. When tes~ed in a chlor~alkali cell, the membrane gave a current efficiency of 77 percent.
In addition, tests were conducted wi~h respect to membranes of 1100 and 1200 equivalent weight num~er to demonstrate that a thermal treatment in accordance with the present invention yields a membrane which has a decreased tendency to absorb water (in comparison with an untreated membrane). Treated and corresponding untreated ~embranes were brought to equilibrium in an atmosphere having 50 percent xelative humidity, and the water contents of the membranes were then determined by the base catalyzed, extrapolated Karl Fischqr me~hod. For membranes of 1100 equivalent weight number, the wa~er contents were 8.72 weigh~ percent for the untreated and 7.50 weight percent for the treated, a difference of 14 percent. For membranes of 1200 equivalent weight number, the water contents were 7.45 weight percent for the untreated and 6.55 weight percent for the treated, a difference of 12 percent.
While there have been shown and described herein certain embodiments of the invention, it is intended that - - there be covered as well any change or modification therein which may be made withou~ departing from the spiri~ and ;
scope of the invention. -~
Membrane~ treated as herein taught may find other uses in which greater selectivity is wanted.
-10- , . ... .
.
. . . .
. . . . ..
~ 1~785ti~
SUPPLEMENTARY DISC~OSURE
_ . _ _ . _ . .
In the application as originally iled, a method ~or obtaining a selective membrane for use in chlor-alkali cells has already been disclosed. Said membrane prior to use is subjected to a particular thermal treatment.
Now, it has been found that the thermal treatment may be conducted at a temperature of 175 to 225 Centigrade, for a time of three hours to one half hour. Preferred results are obtained with the use of a temperature of 200Centigrade for two hours.
Further, after the thermal treatment, the membrane is allowed to cool to room temperature. Rapid cooling (one minute or less) is acceptable, but a slower cooling ra-te ~at least 15 minutes, and up to several hours, preferably about 2 or 3 hours) is preferred.
As a result of such thermal treatment, the membranes exhibit improved properties: they have improved selectivity, give higher current efficiencies and lower power consumption per unit of product obtained, and afford a product having a lower salt content.
The underlying reason for these changes is that as a result of the heat treatment, there occurs a morphological transition in the membrane material. This can be seen clearly from X-ray diffraction data upon membranes in their untreated and treated states. Untreated, the membrane is characterized by two lacttice constants, one at 5.7 Angstrom units and one at 34 Angstrom units. The former is attributable to the lateral spacing of the polymer chains. The latter is related to the spa-cing of the sulfonic acid groups. Treated, the membrane exhibitslattice constants of 5.7, 27, and 140 Angstrom units. The X-ray diffraction data demonstrate that the spacing between sulfonic acid ~ 'r groups has heen diminished~ as is evidenced b~ the decrease in lattice constant from 34 to 27 Anys-trom units. Those skilled in the art of ion-exchange membranes know ~hat closer spacing o sulfonic acid groups means better membrane selectivity.
Moreover, the appearance of an overstructure with a spacing of 140 Angstrom units indicates that, af~er treatment, there is a more regular ordering of the resin. Those skilled in the art will again appreciate that the more regular ordering can be expected to improve the selectivity of the membrane and its other mechanical and transport properties. Indeed, the treated membrane, as compared to one untreated, was 25% higher in tensile strength and 50% lower in permeability for gases.
The new reacting conditions of preparing the membrane will now be further understood by means of the following non-res-trictive example.
Example 8 Example 6 of the original application was repeated, except that the treatment at 200 Centigrade was or two hours.
The curr~nt efficiency was 81%.
. . - . .
Claims (12)
1. A method for improving the properties of a membrane of copolymerized polytetrafluoroethylene and sulfonated perfluorinated polyvinyl ether having an equivalent weight number of approximately 1000 to 1500 and a thickness of approximately 0.1 to 0.5 millimeters, said method comprising the step of thermally treating said membrane by subjecting it to a temperature of 100 to 275° Centigrade for a period of several hours to four minutes.
2. A method as defined in claim 1, wherein during said step said membrane is subjected to a substantial pressure of up to 9.76 kilograms per square centimeter.
3. A method as defined in claim 2, wherein said step of thermally treating said membrane is conducted at a temperature of 175 to 225° Centigrade for a time of five to 12 minutes.
4. A method as defined in claim 1, wherein said step of thermally treating said membrane is conducted at a temperature of 175 to 225° Centigrade for a time of five to 12 minutes.
5. In a method of electrolyzing an alkali-metal halide by subjecting an aqueous solution of said halide to electrolysis in a cell having anode and cathode compartments separated by a membrane of copolymerized tetrafluoro-ethylene and sulfonated perfluorinated vinyl ether having an equivalent weight number of approximately 1000 to 1500 and a thickness of approximately 0.1 to 0.25 millimeters, the improvement which consists in the step of thermally treating said membrane prior to use in said cell at a temperature of 100 to 275° Centigrade for a period of several hours to four minutes.
6. An improvement as defined in claim 5, wherein during said step said membrane is subjected to a pressure of up to 9.76 kilograms per square centimeter.
7. An improvement as defined in claim 6, wherein said step of thermally treating said membrane is conducted at a temperature of 175 to 225° Centigrade for a time of five to 12 minutes.
8. An improvement as defined in claim 5, wherein said step of thermally treating said membrane is conducted at a temperature of 175 to 225° Centigrade for a time of five to 12 minutes.
9. A method as defined in claim 1, wherein said membrane has a thickness of 0.1 to 0.25 millimeters.
10. A membrane made by the method of claim 1.
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE
11. A method as defined in claim 1, wherein said step of thermally treating said membrane is conducted at a temperature of 175 to 225° Centigrade for a time of three hours to one half hour.
12. A method as defined in claim 5, wherein said step of thermally treating said membrane is conducted at a temperature of 175 to 225° Centigrade for a time of three hours to one half hour.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US61960675A | 1975-10-06 | 1975-10-06 | |
| US05/729,201 US4089759A (en) | 1975-10-06 | 1976-10-04 | Method for improving selectivity of membranes used in chlor-alkali cells |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1078564A true CA1078564A (en) | 1980-06-03 |
Family
ID=27088554
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA262,439A Expired CA1078564A (en) | 1975-10-06 | 1976-09-30 | Method for improving selectivity of membranes used in chlor-alkali cells |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1078564A (en) |
-
1976
- 1976-09-30 CA CA262,439A patent/CA1078564A/en not_active Expired
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