CA1130515A

CA1130515A - Cationic membranes

Info

Publication number: CA1130515A
Application number: CA322,382A
Authority: CA
Inventors: Luigi Giuffre; Oronzio De Nora; Giovanni Modica
Original assignee: Oronzio de Nora Impianti Elettrochimici SpA
Current assignee: De Nora SpA
Priority date: 1979-01-17
Filing date: 1979-02-27
Publication date: 1982-08-31
Also published as: US4246090A; DE2967209D1; EP0013677B1; IT7919354A0; US4295952A; IT1166581B; EP0013677A1; JPS5597255A

Abstract

ABSTRACT OF THE DISCLOSURE
Cationic membranes permeable to ions but fluid imper-meable comprising a partly sufonated termpolymer of styrene, divinylbenzene and at least one member of the group consist-ing of 2-vinylpyridine, 4-vinylpyridine and acrylic acid, the degree of sulfonation varying through the membrane cross-section from a maximum on the anodic surface to a minimum at the cathodic surface, their preparation, their use in elec-trolysis cells and electrochemical processes.

Description

3~S15 STATE OF TEIE ART
Cationic membranes which are selectively permeable to cations are widely used in electrochemical systems such as in electrolysis cells, batteries, electrodialysis cells, etc.
In chlor-alkali cells, for example, they act as a barrier against the back-diffusion of hydroxyl ions produced in the - cathode compartment towards the anode compartment while allow-ing migration of alkali metal ions in the opposite direction.
Generally speaking,a cationic membrane, like any ionic mem-brane, should exhibit a good mechanical and chemi.cal resist~
ance, a low ohmic drop,.imperviousness to fluid flow and a high transport number or permselectivity.
Commercially available cationic membranes, particu-larly for use in chlor-alkali cells, are most often made of thin sheets of perfluorocarbon polymer having pendant chains ' , ~ O ~ ' , jrc ~

~ L~L3~35~
earrying negative polar groups such as sulEonic acid groups and/or carboxylic acid groups. These membranes have a good ehemieal stability due to the absence of carbon-hydrogen bonds, and a reasonable mechanical stability though they are not cross-linked. However, they present serious limitations in terms of cation selectivity whic:h arise from the fact that by increasing the density of sulfonic acid groups or of earboxylic acid groups to lower the electrical resistance of the membrane, the ion selectivity clrastically falls, especially when operating with high concentrations of caustic soda in the cathodic compartment. This lowering of the transport number is caused by counter migration of hydroxyl ions through the hydration water inside the membrane. In fact, sulfonic acid groups and, to a lesser degree, carboxylic acid groups have strong hydrophilic properties.
In practice, a compromise is sought which gives a reasonably low voltage drop across the membrane together with a reasonable current efficiency. Usually-, laminated m~nbraneshaving a thickness of about 0.13-0.5 mm have equi-valent weights of about 1500 to lO00. Nevertheless, the cur-rent efficiency rarely exceeds 80% and usually falls drasti-cally with caustic concentration in the catholyte above 5 N.
Various expedients have been tried to improve the hydroxyl ion rejection properties of cationic membranes such as producing a 0.005-0.25 mm thick layer of perfluorocarbon resin eontaining sulfonamide ~roups over the surace of the perfluorosulfonic acid membrane exposed to caustic to reduce ~ ~ .

~3rc ~

-- ~IL130S15 the hydrophilic properties of the resin and therefore the hydroxyl ion back migration.
The substitution of sulfonic acid groups with the less hydrophilic carboxylic acid groups as the cation exrhange agents in the pendant c:hains of the per1uoro-carbon polymer have also been at;tempted with the same intent of reducing the back migration of hydroxyl ions, but carboxylic acid groups have poor ion-exchange properties with respect to sulfonic acid groups. Sulfonated copolymers of divinyl-benzene and styrene which are used satisfactorily as cation exchange membranes in electrodialysis have not found appli-cation in chlor-alkali cells because of extremely po~r ion selectivity.
OBJECTS OF THE INVENTION
It is therefore a primary object of the present invention to provide an improved cationic membrane having a low electrical resistance and a high selectivity suitable for chlor-alkali electrolysis cells.
It is another object to provide a novel method for preparing said improved cationic membranes.
It is a further object of the invention to provide ~ novel electrochemical cells equipped with the improved membranes and to provide a novel electrochemical process where the anode and cathod~ compartments are separated by the said membranes.
These and other objects and advantages will become apparent from the ~ollowing description.

~rc ~

. ~ . . .. .
. ~
~; . . ~ ,, ~ .,.. ,, ;
. - . . .. .
, .
- - . . . ~ ~ . . ~.

s~
THE INVENTION
. . . .. .
The novel cationic membranes of the invention which are fluid impermeable but permeable to :ions comprise a partly sulfonated terpolymer of styrene, divinylbenzene and at least one member of the group consisting of 2-vinylpyridine, 4-vinylpyridine and acrylic acicl, the degree of sulfonation varying through the membrane cross-section from a maximum on the anodic surface to a minimum at the cathodic surface.
~ he membranes may be in t:he form of thin sheets of the terpolymer per se or may be formed by polymerizing the mixture of monomers directly on a supportiny matrix made of an inert porous material in sufficient amounts to make the same fluid impervious. The pyridine containing polymers are preferably formed on a matrix support such as asbestos paper or felts or meshes of synthetic fibers such as polyesters or polytetrafluoroethylene.
It has been ascertained that an extremely efficient cation exchange membrane is obtained when the equivalent weight which is the molecular weight o~ the polymer divided by the number of sulfonic acid groups is between 400 and 1300, preferably 600 to 1200, on the anode side of the membrane cross-section and is minimum on the cathode side of the mem-brane cross-section where the equivalent weight is between 1000 and infinity, preferably between 1400 and 2000.

~ : , .

: jrc.

.,. . - , . :

~05~S
The cation exchange sited near the cathode side of the membrane are supplied either hy the nitrogen atom of the pyridine molecule which in the strongly basic environment such as that existing in the cathode compartment of a chlor-alkali cell, assumes a negative polarity due to the presence of its electro-pair or doublets or by the carboxylic acid groups of the acrylic acid. Both of the said groups have a polarity substantially lower than the sulfonic acid group and therefore the polarity of the sulfonated copolymer may be controlled along the cross-section of the membrane by varying the degree of sulfonation.
The reduced hydrophilic properties of carboxylic acid groups and especially of the pyridine groups appear to be ; effective in reducing hydroxyl ions back migration. More-over, it has been unexpectedly ohserved that even though the presence of the highly hydrophilic sulfonic acid groups is ; substantially excluded for a depth of about 0.025 to 0.075mm from the surface of the membrane exposed to the catholyte, the electric resistance of the membrane does not increase significantly. On the other hand, if, for example, the poly-mer of styrene, divinylbenzene and 2- or 4-vinylpyridine is not sulfonated for a substantial depth from the surface ` of the membrane exposed to the anolyte, that is down to about 0.075-0.1 mm from the cathodic surface, the membrane is sub-¦~ stantially an insulator and ionic current cannot be passed ~ through the cell.
, l .. . .

_ 5 _ ~rc: ~

. . - .. . .. . .

~13~515 The novel cationic membranes of the invention have a strong polarity over the portion of kheir cross-section facing the anolyte which i5 imparted by the high degree of sulfonation of the polymer in this region and have a lower polarity over the portion of their cross-section facing the catholyte due to a lower degree of sulfonation of the polymer or to the substantial absence of sulfonic acid groups in this region.
Therefore, the high cation-exchange properties of sulfonic acid groups is associated with the lower hydrophilic prop-e~ties of pyridine or of carboxylic acid groups.
The sulfonation degree may decrease gradually from the anodic side to the cathodic side throughout the thickness of the membrane by providing a sheet of terpolymer of styrene, divinylbenzene and either 2- or 4-vinylpyridine or acrylic acid and then contacting only one side o the sheet with the appropriate sulfonating agent and controlling the tem-perature and the time of exposure to obtain a decreasing degree of sulfonation from a maximum at the exposed surface to a mimimum at the unexposed surface.
The molar ratio between styrene and 2- or ~-vinyl-pyridine in the terpolymer may vary between 1/8 and 1/1 and the molar ratio between styrene and acrylic acid in the ter polymer may vary between 1/2 and 2/1~ The molar amount of cross-linking agent or divinylben2ene, may vary, in both cases, be~ween 8% and 20%.

~ , .

~rc ~
-t 5~5 Preferably, the thickness of the membrane is between 0.13-2.5 mm and more preferably, when it is an unsupported membrane,it may be from O.S 1.25 mm thick.
According to another embodiment of the invention, the degree of sulfonation, that is of polarity, may be a stepwise change. This is achieved by superimposing on a f~.rst layer of highly sulfonated terpolymer of styrene and at least divinylbenzene with opt:ionally 2- or 4-vinylpyridine or acrylic acid as a third co-monomer, a second layer of an unsulfonated copolymer of divinylbenzene and at least one monomer belonging to the group comprising 2- or 4-vinyl-pyridine and acrylic acid, optionally wlth styrene as a third co-monomer. When using only two co-monomers for both the sulfonated layer and the unsulfonated layer, the degree of cross-linking, that is the amount of divinylbenzene contained in the formed copolymer, may be higher than 20% and may be as high as 50~ or more.
The unsulfonated layer should not be thicker than 3 to 4 mils because beyond this.maximum thickness, the voltage . 20 drop in the membrane is found to increase. Again, the total thickness of the two layered membrane may vary between 5 to 100 mil. A membrane supported on an asbestos paper, for example, will have an apparent thi.ckness similar to the thickness of the asbestos paper which may be between 30 to . 100 mil. However, the effective thickness of the ion-exchange resin impregnating the asbestos may be expected to vary between 5 and 20 mil.

"
....

_ 7 _ tr~

S~5 The method of the invention for the preparation of the novel cationic memhrane compris~s impregnating an inert, porous support such as an.asbestos paper, a felt or a mesh or PTFE, a felt or a mesh of polyester fibers with a solution or mixture of the said monomers i.n the appropriate ratio with up to 1~ molar of a polymerization initiator such as dibenzoyl-peroxide or ~ azobisisobutyronitrile in a solvent such as benzene, acetone, toluene or xylene or any other suitable solvent, evaporat:ing the solvent under vacuum at room temperature, placing the impregnated support in a closed reactor with a minimum gas space, preferably inside a Teflon ~ coated mould and heating at a temperature of about 60 to 100C, preferably 80 to 90C, for a period of 1 to 4 hours to attain complete copolymerization, cooling the material to room temperature, and preferably washing the membrane with solvent to remove low molecular weight polymers and unreacted monomers and then drying the latter. After swelling in sym-dichloroethane or other halogenated solvents, the membrane material is contacted over one side only with liquid sulfur trioxide diluted with liquid sulfur dioxide for a controlled period of time ranging from 5 min. to 60 minO
depending on the thickness and on the type of porous support used and on the temperature which may ~ary from -40C up to room temperature to sulfonate the styrene aromatic rings : down to a substantial depth from the surface exposed to contact with the sulfonating liquid mixture.

~rc:~ .

- ~3~;3SlS
The method produces a membrane with a sulfonation degree and therefore a negative polarity which is maxirnum at the surface in contact with the sulfonating mixture and which decreases in a substantially gradual manner over the cross-secticn of the membrane down to a minimum at the unexposed surface of the membrane. Preferably, the equival-ent weight referred to the sulfonic acid group is comprised between 600 and 1200 at the exposed~surface and between 1200 and 2000 at the unexposed surface. This procedure 1~ will be hereinafter referred to as "partial sulfonation".
The sulfonated film of polymer may then be contacted with an aqueous alcoholic solution of an alkali metal hydroxide for a sufficient time to convert the -SO3H groups into their more stable form of -SO3Me where Me is an alkali metal such as sodium or potassium. This treatment stabilizes the membrane which may then be heated to 70 to 100C, prefer-ably 80 to 90C under vacuum, to release the water of hydration to obtain a dehydrated or dried stable membrane in its metal salt form which may be stored for any length of time.
In a modified embodiment of the invention, a layer of unsulfonated.copolymer may be provided over one side of an uniformly sulfonated layer of the polymer to form a less hydrophilic layer having a greater ability to reject hydroxyl ions on the side of the membrane which contacts the stron~ly alkaline catholyte during operation in a chlor-alkali cell.
The layer of sulfonated polymer may be a sulfonated copolymer I ~

~, : ' .' . ' , . :

~ 3~ S

of styrene and divinyl benzene optionally containing also the third co-monomer, that is 2- or ~-vinylpyridine or acrylic acid.
~ he layer of unsulfonated copolymer may be of the same type as that of the sulfonated copolymer, that i5 a terpolymer of styrene-divinylbenzene and 2- or 4-vinylpyridine or of styrene-divinylbenzene and acrylic acid, but preferably the layer of unsulfonated copolymer is a copoly~er of divinyl-benzene and 2- or 4-vinylpyridine or of divinylbenzene and acrylic acid. Most preferably, it is a layer of copolymer of divinylbenzene and 2- or 4-vinylpyridine.

Therefore, a modified method of the invention for preparing the cation-exchange membrane of the invention com-prises a) forming a thin sheet of copolymer, preferably by copolymerizing the mixture of co-monomers on an inert support and in the absence of solvent, b) optionally treating the formed sheet with a swelling agent, preferably a halo-genated solvent such as symdichloroethane, for several hours, normally for 2 to 1~ hours at a temperature which may vary from room temperature up to 80C; c) evenly sulfonating the ; sheet by immersing it in a solution of sulfur trioxide in liquid sulfur dioxide at a temperature between -40C and +20C for a period of time of 20 min. to 90 min.; d) stabilizing the sulfonic groups by soaking the membrane in an aqueous alcoholic solution of an alkali metal hydroxide for a time sufficient to convert substantially all the -SO3~1 groups to their metal salt form, that is -SO3Me, where Me is an alkali metal such as sodium; e) dehydrating or drying the membrane trc~

.. . . . ~ , ~.:

s-at a suitable temperature such as 70-100C preferably under vacuum; f~ applying over one side of the sulfonated membrane a solution or a mixture of di~inylbenzene and 2- or 4-vinyl-pyridine, or of acrylic acid and divinylben~ene or of styrene, divinylbenzene and 2- or 4-vinylpyridine or acrylic acid;
g) evaporating any solvent and then copolymerizing said monomers to form a layer of unsulfonated copolymer over ~he side of the membrane; and h) optionally repeating steps f) and g) to form a layer of unsulfonated copolymer having the desired thickness, preferably from 0.1 to 3 mil., over the side of the sulfonated membrane.
Another modification of the process of the invention .for preparing the cation-exchange membrane comprises a) forming a thin sheet of copolymer, preferably by copolymerizing the mixture of co-monomers on an inert support and in the absence of a solvent, b) applying over one side of the formed sheet a mixture or a solution of 2- or 4-vinylpyridine and divinylbenzene, c) evaporating any solvent, and then copolymeriz-ing the two monomers to form a layer over the previously formed sheet of terpolymer of styrene, divinylbenzene and 2- or 4-vinyl- .
pyridine or acrylic acid; d) optionally repeating steps b) and c) to form a layer of copolymer having the desired thickness, preferably from 0.1 to 3 mil; e) treating the two layered sheet with.a swelling agent such as sym-dichloroethane for several hours, normally from 2 to 12 hours at a temperature between 20 and 80C and f) sulfonating the two layered sheet by immersing the sheet in a solution of sulfur trloxide in liquid sulfur dioxide at a temperature between -40 and 20C for a period of time of 20 to 90 min.

' . ~rc~
, .

. . . . .. .
. .
.
~ . ~

~3C~51S

The absence of styrene and of acrylic acid in the 0.1 to 3 mil thick copolymer layer formed on one side o~the membrane i5 hi~hly preferred because it allows a much simplified procedure of sulfonation. The formed sheet may be sul~onated without strict control of time and temperature of sulfonation because of the practical impossibility of the sulfonating agent, namely SO3, to react with the copolymer of vinyl-pyridine and divinylbenzene. These modified methods produce membrances with a high sulfonation degree ~high polarity) which remains substantially uniform for a substantial depth from the anode side of the membrane and which falls to sub-stantially nil near the cathode side of the membrane which region corresponds to the layer of unsulfonated copolymer formed over the side of the previously sulfonated copolymer sheet.
Various modifications of general methods described may be resorted to. For example, the porous support of inert material may not be used and membranes may be prepared by hot-pressing a mixture of the co-monomers, in the substantial absence of a solvent in a Teflon-lined press. Typical conditions are for 1 hour at 80C with a pressure o O.5 kg/cm to induce copolymerization and to obtain sheets of copolymer which may then be sulfonated by the above procedures.
The membranes of the invention are particularly useful in the electrolysis of alkali metal halide brines to produce the halogen and the alkali metal hydroxide such as chlorine and caustic soda but may also be used in other electrolysis reactions.
~ $~

~rc:~
, . ~ . - . . . . . .... . .

~05~5 ~ he novel electrolytic process of the invention for the preparation of a chemical product comprises providing an electrolyte containing the elements of the product to be produced in an electrolytic cell with an anode and a cathode separated by a cationic membrane permeable to ions but ~luid impermeable comprised o~ a partly sulfonated terpolymer of styrene, diviny.lbenzene and at least one member of the group consisting of 2-vinylpyridine, 4--vinylpyridine and acrylic acid, the degree of sulfonation varying through the membrane cross-section from a maximum on the anodic surface to a minimum at the cathodic surface, passing an electrolysis cur-rent through the anode, cathode and electrolyte and recover-ing said chemical product from the said cell.
The novel electrolytic cell of the invention com-prises a cell housing containing at least one anode in an anode compartment and at least one cathode in a cathode com partment forming an interelectrodic gap with a cation exchange member of the invention separating the anode and cathode compartments.
Divinylbenzene has the formula ,CH2=C~I- ~ -CH=CH2 In the following examples there are described several preferred embodiments to illustrate the invention.
However, it is to be understood that the invention is not intended to be limited to the spe_ific embodiments.

.

t` `~, , i . . .

jrc~

, .:

:

~L3g35~5 EXAMP~E 1

2 pieces of asbestos paper (0~25 L/FX 3~ of John Mansville) measuring 120 x 120 mm were soaked in a solution of 75% by volume at 20C of 4-vinylpyridine-styrene-divinyl-benæene mixture (molar composition 80~ - 10% - 10%) in 25% by volume of benzene. The solution also contained as a poly-merization/initiator l mole percent of dibenzoylperoxide (based on monomers). The papers were removed from the solution and the benzene was evaporated under reduced pressure. The papers were then placed in a closed reactor with a minimum of empty space and were held at 80C for two hours to effect poly-merization. The papers were then washed with benzene to remove unreacted monomers and low molecular weight polymers and were then dried.
The amount of polymer in each case represented 38%
by weight of the treated paper and both sheets were substantial-ly impermeable to fluid flow. One of the sheets was allowed to stand in sym-dichlorethane for 12 hours at 70C and was then placed in a reactor designed to allow contact of only one side of the sheet with a solution of liquid sulfur trioxide in liquid sulfur dioxide at -30C. The temperature of the solution was slowly raised over 30 minutes to 10C and the paper was dried to obtain a membrane A with a different polarity on each side. A sample was taken from each side of the membrane and the elemental analysis and the equivalent weights (EW) with respect to the sulfonic acid groups are reported in Tab~e I.

, ::
.... ~. .

jrc~

:. , :. : .. : .

~ ~ ~iL130S:L5 TABL~ I .

- %C %H ~N ~_ ~S EW
. . , _ Sample taken from the 76.8% 6.45% 9.65% 4.2~ 2.~% 1152 surface of the side :.
contacted with the sulfonation solution . __ _ sample taken from the 78.65~ 6.6~ 9.9~ 2.86% 1.9% 1700 surface of side not in .
contact wikh the . .
sulfonation solution _ For comparison purposes, the second sheet was similarly treated with sym-dichloroethane and thereafter sulfonated by the same method as described for membrane (A), except that in this case the contact with the sulfonating solution was made to occur on both sides of the sheet to obtain a uniformly sulfonated membrane (B). Samples taken from both sides of the sulfonated membrane were analyzed and the results of the elemental analysis were the same for both side as indicated hereinbelow:

. _ _ . __ .
~C. ~H %~ ~O %S
76,95 6~4 9.~ 4.l~ ~.77 .

The transport number was measured for both membranes;

membranes tA) showed a transport number of 0.98 and membranes (B) showed also a transport number of 0.98 so that no appreciable --- . : - . .: .

.: . . , ~ , . : .
~ ; . , ~ .. ~: -:

~ ~3~5i1S
difference in transport number was observed under the conditions of measurement.
The two membranes thus obtained were placed in two similar test ceLls for the electrolysis of sodium chloride brine. The test conditions were as follows:

. , - anode Titanium mesh activated with mixed oxides of ruthenium and titanium - cathode low carbon steel mesh - anolyte NaCl 310 g/l - catholyte Ca2 + Mg2~ 25 ppm NaOH 25%
- temperature 85C
- current density over membrane surface 1800 A/m After 1500 hours of operation, the average operating result~
for the test cell equipped with membrane (A) and membrane (B~
were respectively as reported in Table II.
; TABLE II
~.
:: .
Cell Voltage %Cathodic Cur- g/l Chloride Content ~20 V cent Efficiency in Catholyte . . .
membrane (A) 3.7 90 to 91 0.01 membrane (B~ 3.65 82 to 84 0.0l .

-.
, : - .... ' ' ' ' .:;` :`
- ~6 ~- jrc:~.

3~ S
The data of Tahle II indicates that mem~rane (A) having a sulfonation degree which varies through the cros -section of the membrane from a maximum at the anode side of the membrane to a minimum at the cathode s:ide of the membrane, shows a much higher efficiency tharl membrane (B) which is uniformly sulfonated throughout its thickness. Moreover, the cell voltage in the case of membrane (A) is only slightly higher than in the case of membrane (B).
In both cases, the cell voltage did not show any remarkable increase with time of opexation, although a relatively high amount of calcium and magnesium had been purposedly added to the brine. This appears to indicate that the cross-linked sulfonated copolymer membranes are not as strongly susceptible to aging in the presence of calcium and magnesium cations as are perfluorosulfonic acid membranes commercially used.

A 150 x 150 mm Teflon ~ felt with a thickness of 0.5 mm was soaked with a mixture of 4-vinylpyridine, styrene and divinylbenzene having the following molar composition: 70~ of

4-vinylpyridine, 20% of styrene and 10~ of divinylbenzene.
Dibenzoylperoxide was present in an amount of 1 mole percent with respect to the overall moles of monomers as copolymerization initiator.
The soaked felt was then hot-pressed in a Teflon lined press at a temperature of 80C and at a pressure of 0.5 kg/cm for one and half hours to effect copolymerization, - A compact sheet, 0.5 mm thick and impervious to fluid flow, was obtained whexein the copolymer was present in the amount ~ ~;

~-rc~
:, :. " . ' .' ' : :' :. '.

. . .' ' : ' ` . ''. "~ ' ','`` ~'i '` ' ~

~3(;~SlS

of 60% by weight. The sheet was t~en treated first in-sym-dichloroethane and then contacted on one side only with the sulfonating solution by the procedure of Example 1 for the case of membrane (A). Samples from the two sides of the membrane, on elemental analysis, were characterized by the following composition:
- TABLE III

_ .%C ~H ~N ~O ~S
_ . _ _ _ . . _ __ _ Sample from con- 72.8 6.7 7.8 7... 7 5.2 tacted side . .
... ~
sample from uncon- 78.7 7.23 8.38 3.4 2.26 tacted side ~he equivalent weights of the two samples, that is of the two sides of the membrane, with respect to the sulfonic . groups, were 610 for the contac~ed side and 1400 for the uncontacted side.
The sulfonated membrane was tested for the electrolysis of brine in the same test cell and under the same conditions as :~ indicatea in Example 1. After 500 hours of operation, the average operating results were as follows:
cell voltage - 3.9 V; cathodic curr~ent efficiency - 90% and . chloride content in the catholyte - 0.005 g/l.

A 120 x 120 mm Teflon ~ felt with a thickness of 0.6 mm was soaked with a mi~ture having the following molar composition:
: 40~ of styrene, 40% of 2-vinylpyridine, 20% of divinylbenzene ~nd : 1% ~by moles with respect to the total moles of the monomers~ of A~ .. i. ~ -; . . . : , - -. . . ; .

C)5:~5 dibenzoylperoxide. n~1 The felt was then placed in a Teflo ined press and held at 80% for 1 hour under a pressure of 0.5 kg/cm to obtain a fluid-impervious sheet with a polymeric content of 50% by weight and a ~hickness of 0.6 mm. The sheet was immersed in sym-dichloroethane for 12 hours at 80C and thereafter sulfonated at -30C with liquid sulfur trioxide in liquid sulfur dioxide while slowly raising the temperature to -10C over a period of twenty m:inutes. After the sulfonation, the membrane thus obtained was stabilized by treating it with a sodium hydroxide hydroalcoholic solution whereby the -SO3H
groups were converted to ~SO3Na groups. The membrane was then dried at 80C under vacuum. One side of the membrane was coated with a mixture of the following molar composition: 92~ of 2-vinyl pyridine, 8~ of divinylbenzene and 1~ (moles of respect to the total moles of monomers) of dibenzoylperoxide and was held for 3 hours at 80C in a reactor to effect copolymerization to obtain a copolymeric layer 0.05 mm thick.
A sample was taken from the sulfonated membrane, prior to stabilization with NaOH, and another was taken from the 2-vinylpyridine/divinylbenzene copolymerlc layer added thereon and their elemental analysis gave the following results:

.
%C %H ~N %O %S
. ~ _ . ~ -- ' '' ' . .
sulfonated copolymer 71.84 5.96 4.18 10.75 7.17 sample .
~_ _ .. ~
unsulfonated copolymer 81~1 6.75 12.0 trace trace sampl-!

. ~

S-~5 The presence of trace-amounts of oxygen and sulfur in the unsulfonated copolymer sample was due to imprecise sampling which caused contamination of the sample by material coming from the underlying sulfonated copolymer.
The equivalent weight (EW) with respect to the sulfonic groups of the two samples, that is of the two sides of the membrane, was 450 for the sulfonated sample and extremely high for the unsulfonated sample.
The two layer membrane was tested for the electrolysis of brine in the same test cell and under the same conditions as indicated in Example l. After 48 hours of operation, average operating results were as follows: cell voltage -3.5 V, cathodic current efficiency - 91.5% and chloride content in the cathode - 0.01 g/l.
EXAMPLE ~
Two identical 0.6 mm thick Teflon ~ felts were soaked with a mixture having the followin~ molar composition: 40~ of acrylic acid, 50% of styrene, 10% of divinylbenzene and 1% ;`
(with respect to the total moles of monomers) of ~ azobis-fR~
; 20 isobutyronitrile. The felts were placed in a Teflon~lined - press and held at 80C for 3 hours under a pressure of 0.5 kg/cm2.
The fluid impervious sheets thus obtained were treated with sym-dichloroethane at 70C for 12 hours, and were then sulfonated with liquid SO3 in liquid SO2, sta~ting from a temperature of -30C and slowly raising it up to 20C over a period of thirty minutes. Sulfonation occurred mainly at the aromatic ring of styrene and to a lesser extent, at the ~-carbon atom at acryli~
acid.
One of the membranes thus obtained (C) was treated with an aqueous ethanol solu ion of NaOH to convert the ~SO3H

0 ~ .

: ~ - , , . , ~ ,: ", .: i :' ~ ,: ,. .

)S~L5 .
groups to their sodium salt form. The membrane ~C) was then spread on one side w1th a mixture of the following molar com-position: 90% of 4-vinylpyridine, 10% of divinylbenzene and l~
:~by moles with respect to the total moles of monomers) dibenzoyl-peroxide and then was heated in a reactor at 80C for 3 hours to effect copolymerlzation to finally obtain an 0.05 mm thick 4-vinylpyridine/divinylbenzene copolymer layer on one side of the sulfonated membrane (C).
The two-layer membrane ~C) and the single layer membrane (D) were both tested in the same cell and under the same conditions as described in Example 1. After 100 hours of operation, the average operatiny results were reported in the following Table IV:
TAELE IV

I Cell Cathodic CurrentChloride Content : . Efficiencyln the Catholyte membrane (C) 3.8 V 90% O.005 g/l membrane (D) 3.2 V 50~ , The relative cathodic current efficiencies obtained with the two membranes show that the unsulfonated copolymer layer applied over the cathode side of the sulfonated copolymer membrane - greatly improves the OH rejection with a marked favorable ~: effect on efficiency.
EXAMPLE S
: A 120 x 120 mm asbestos paper of the 0.025" L/FX36"
~: : type manufactured by John Mansville Co. was soaked with a mixture ~ having the following molar composition: 66~ of styrene (ST), 34%

,:
: :
--:

051~
of divinylbenzene (DVB) and 1~ (molar with respect to the total moles of monomers) ~ dibenzoylperoxide. The paper was then placed in a Tefl lined press and held at 80C for 1 hour at a pressure of 0.5 kg/cm2 to produce a fluid-impervious sheet with a ST/DVB polymer load of 65~ by weight. One side thereof was then coated with a mixture of the following composition by moles: 85~ of 4-vinylpyridine (4VP), 15~ of divinylbenzene (DVB) and 1~ (molar ratio with respect to the total moles of monomers) of dibenzoylperoxide and was then held for 2.5 hours at 80C in a reactor to effect copolymer-ization to obtain a 0.03 mm-thick 4-VP/DVB polymer layer. A
double-layer membrane was thus obtained which, after swelling for 10 hours in sym-dichloroethane at 70C, was then sulfonated at -30C with liquid SO3 dissolved in liquid SO2 while raising the temperature to -10C over 20 minutes to obtain a sulfonated membrane exhibiting a different polarity on each side. Samples were taken from both sides of the membrane and the elemental analysis thereof ~ave the following results:
._ _ _ ~C %H ~N %O ~S
. . _ _ .
sulfonated ST/DVB 79.8 6.62 -- 9.05 5.36 copolymer sample . :, unsolfonated 4 VP/DVB 82.2 6.83 10.9 tace trace copolymer sample - The presence of trace amounts of oxygen and sulfur in the unsulfonatecl copolymer sample is due to imprecise sampling which caused contamination of the sample .

rc~ v-- ;, : , . : ,: , ~ 051S
by material coming from the underlying sulfonated copolymer.
Th~ equivalent weights (EW) with respect to the sulfonic groups of the two samples of the two sides of the membrane were 550 for the sulfonated sample and extremely high for the unsulfonated sample.
The two layer membrane was tested for brine electrolysis in the same test-cell and under the same conditions as indicated in Example 1. After three days oi-- operation, the average operating results were as follows: cell voltage - 3.8 V, cathodic current efficiency - 91~ and chloride content in the catholyte - 0.01 g/l. Also in this case, as in Example 4, . the unsulfonated copolymer layer of the.cathode side of the membrane appears to be highly effective in hindering hydroxyl ion back-diffusion.
Various modifications of the products and processes of the invention may be made without departing from the spirit or scope thereof and it should be understood that the invention is interded to be l'mited only as defined in the appended claims.

..' :

; .

~ .

.
, .. .
. .

~~ ~rc~

.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fluid impermeable cation exchange membrane comprising a partly sulfonated terpolymer of styrene, divinyl-benzene and at least one member of the group consisting of 2-vinylpyridine and 4-vinylpyridine, the degree of sulfonation varying through the membrane cross section from a maximum on the surface adapted to face an anode to a minimum at the surface adapted to face a cathode.

2. The cation exchange membrane of claim 1 wherein the molar ratio of styrene to the vinylpyridine is between 1/8 and 1/1 and the molar percentage of divinylbenzene in the copolymer is from 8 to 20%.

3. The cation exchange membrane of claim 1 wherein the degree of sulfonation expressed in equivalent weight referred to the sulfonic acid groups is between 400 and 1300 at the surface of the membrane facing the anode and is between 1000 and 3000 at the surface of the membrane facing the cathode.

4. A fluid impermeable cation exchange membrane according to claim 1 having a first layer of a water wettable and water insoluble sulfonated polymer of styrene and divinyl-benzene and a second layer of an unsulfonated polymer of divinylbenzene and at least one monomer selected from the group consisting of 2-vinylpyridine and 4-vinylpyridine.

5. The membrane of claim 4 wherein the first layer polymer also contains at least one member of the group consisting of 2-vinylpyridine and 4-vinylpyridine.

6. The membrane of claim 4 wherein the second layer polymer also contains styrene.

7. A process for preparing a membrane of claim 1 comprising forming a thin, fluid impermeable sheet of a terpolymer of styrene, divinylbenzene and at least one member of the group consisting of 2-vinylpyridine and 4-vinylpyridine, treating the said sheet with a halogenated solvent for a time sufficient to swell the sheet and sulfonating said sheet with a solution of sulfur trioxide in sulfur dioxide at 40° to 25°C
for a time sufficient to sulfonate the said sheet.

8. A process according to claim 7 wherein only one side of the swollen, fluid impermeable sheet is sulfonated at the stated temperature for a time sufficient to sulfonate said sheet in a graduated manner.

9. A process according to claim 7 wherein both sides of the swollen, fluid impermeable sheet are sulfonated and including the steps of contacting the sulfonated sheet with a solution of an alkali metal hydroxide for a time sufficient to convert substantially all -SO3H groups to -SO3Me where Me is an alkali metal, drying the sulfonated sheet, applying a monomer mixture of divinylbenzene and at least one member of the group consisting of 2 vinylpyridine and 4-vinylbenzene to one side of the sulfonated sheet, removing any solvent and heating the resulting sheet to polymerize the monomer mixture.