Novel ion exchange membranes based on an aromatic polyethersulfone

Journal of Membrane Science, 22 (1985) 3 2 5 -332

325

Elsevier Science Publishers B . V ., Amsterdam - Printed in The Netherlands

NOVEL ION EXCHANGE MEMBRANES BASED ON AN AROMATIC POLYETHERSULFONE*

P, ZSCHOCKE and D . QUELLMALZ Forschungsinstitut Berghof GmbH, Postfach 1523, 7400 Tubingen 1 (F.R.G.) (Received April 1, 1984 ; accepted in revised form September 12, 1984)

Summary From the aromatic polyethersulfones 1700 and 3500, possessing the repeating unit shown below, cation as well as anion exchange membranes were obtained The synthesis of these ion exchange membranes is described .

C H3

Their properties, to the extent they have been determined, are comparable to those of other commercially available membranes . The alkaline stability of the polysulfone anion exchange membrane, however, was found to be significantly better .

Introduction Electrodialysis (ED) becomes more and more important for separation processes, for example, for the desalination of dilute solutions, or for the general separation of ions from neutral molecules . A prerequisite for the efficiency of an ED process is the use of highly efficient ion exchange membranes. These membranes should fulfill the following requirements : - high permselectivity, in the range of 90%, in 0 .5 : 1 N KCl, - low electrical resistance, if possible lower than 5 Il-cm 2 in 1 N KCl, - high mechanical stability, - high chemical stability, especially in the presence of strong oxidizing agents like chlorine or oxygen, - a fairly low swelling or shrinking behaviour with change of the surrounding electrolyte concentration, - a high thermal stability up to 80 ° C, and - low production costs . *Paper presented at the 4th Symposium on Synthetic Membranes in Science and Industry, Tubingen, F .R.G ., September 6-9, 1983 .

0376-7388/85/$03 .30

© 1985 Elsevier Science Publishers B . V .



326

F F X

+ y

F\C - C` F \F

F/

E\C-C / F F/

--~

\S02F

F F

I I CI I ~

~ C - ~ ~X F

F

O f

-i

F

I I I F

50 2 E

I

i ~ y( ~ C -i F

Y

50 3H

Fig . 1 . Principle of synthesis of a cation exchange material by copolymerization .

The ion exchange materials, that is to say, the ion exchange membranes, can be prepared in one of two alternative routes . The first is by copolymeri zation of functionalized monomers, as is done to obtain perfluor cation exchange membranes (Nafion) (Fig. 1) ; the second is by functionalizing a suitable base polymer with the desired cationic or anionic groups, as shown in Fig. 2 . H

H

___-C-C--

H

-

+ SO2 + C(2 -~~ ---C-C--__ I I

H H Polyethylene

H

A C13 ~-- P02

F

S02C1

H

H

I

I

H

SO3 H

___-

jCL 2 C/C-

'I

I

Polystyrene

PCt a

I ~/ PO(OH) 2

Fig . 2 . Principle of synthesis of ion exchange materials starting from base polymers .

We decided for reasons of cost to functionalize suitable uncrosslinked base polymers containing aromatic groups in the main chain . This concept has the following advantages : In aromatic groups the functional groups can be introduced according to ionic mechanisms, which in general take place under milder conditions than radical mechanisms, and chemical attack of the base polymer can be better avoided . If uncrosslinked and soluble base polymers are used, it should be possible to obtain soluble ion exchange materials, which allow preparation of phase

327 inversion ion exchange membranes . These membranes possess a sufficiently thin dense skin layer, a requirement for low electrical resistance, and since these membranes are supported by a porous substructure, a sufficient mechanical stability can be obtained . Since the electrical resistance of an ion exchange membrane is directly proportional to its thickness and inversely proportional to its fixed ion concentration, it can be expected that for an extremely thin ion exchange membrane, a lower ion exchange capacity would be required than for thicker completely homogenous membranes . With lower fixed ion exchange capacity, undesired swelling or shrinking behaviour of a membrane is decreased . Description of the work performed To determine which base polymers were suitable, samples consisting of aromatic polymers like aromatic polyethersulfone, poly(2,6-dimethyl phenyl ether) (PPO) and polystyrene, as well as aliphatic polymers like sulfonated polyethylene, polyvinylidene fluoride and polyacrylic acid, were exposed to 40% NaOH at 70-80° C for 300 hours. The results of these stability tests are shown in Table 1 . TABLE 1

Alkaline stability of commercially available polymers Polymer

Influence of 300 hour exposure to 40% NaOH at 70-80 ° C

Polysulfone 1700 (3500) Poly(2,6-dimethyl phenyl ether-1,4) Polystyrene Sulfonated polyethylene Polyvinylidene fluoride Polyacrylic acid

none cracks none film destroyed embrittlement film destroyed

As can be seen, the best polymers were polystyrene and polysulfone . Since a number of studies have already been carried out on ion exchange membranes based on polystyrene, we decided to investigate polysulfone . The synthetic pathways for obtaining cationic and anionic ion exchange materials by acidic and basic functionalization of polysulfone are shown in Fig. 3 . The cationic exchange material is easily obtained by sulfonation of polysulfone, either by means of chlorosulfonic acid at -10 ° C or by means of sulfur trioxide weakly bound to phosphoric acid triethylester at room temperature . The anionic exchange material was obtained by a two-step synthesis, consisting of chloromethylation of polysulfone in the first step with subsequent quaternary amination by means of trimethylamine in the second step.



328

By variation of the molar ratio of the reactants, ion exchange materials having different ion exchange capacities in the range of 0 .5-1 .8 meq/g were obtained . Both the cationic and the anionic ion exchange materials were soluble in classical polymer solvents, like N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc) or N-methylpyrrolidone (NMP) . It was therefore possible to obtain asymmetric ion exchange membranes by forming a thin casting film from which the solvent, DMF, was partly evaporated, followed by subsequent precipitation in water . Polysulfone

CISO3 H

C

exchange

cation

I

I

material

(C 2 H 5) 3 P = 0 x SO3

3

Polysulfone

1700 (3500) CICH 2 OCH 3 /znO

CH3

CH 3 CH 2 CI i

(C H 3 )3N

0--+ CH3 CH 2 N(CH3 ) 3

CI

Polysulfone anion exchange matericl

Fig . 3 . Synthesis of ion exchange materials based on aromatic polyethersulfone .

The asymmetric structure of these membranes is demonstrated in the scanning electron micrograph of the membrane's cross-section shown in Fig . 4 . The 12-µm thick, homogenous membrane is supported by a 40 jtm thick porous substructure .



329

Fig. 4 . Scanning electron micrograph of the cross-section of a sulfonated polysulfone cation exchange membrane (dried) . i

100 -

-10

90

70 N

s .J +n 50

o_

0.8

10

12 14 I EC (meq/g) Fig. 5 . Polysulfone cation exchange membrane : electrical resistance, R, and permselectivity, S, as a function of the ion exchange capacity, IEC.



330 The cation as well as the anion exchange membranes possessed permselectivities in the range of 80-90% in 0 .5 : 1 N KCl and electrical resistances in the range of 0 .5-5 cl-cm 2 in 1 N KCI . The relationship between their ion exchange capacity, IEC, electrical resistance, R, and permselectivity, S, is shown in Figs. 5 and 6 .

20

90 I 50 15 70

N E

a 0

25 5 4 1 I

•

3

•A

10

A

k

I

A

2

•

1

i I I ( 1 I 07

09

11

13 15 IEC (meq/g)

17

Fig . 6 . Polysulfone anion exchange membrane : electrical resistance, R, and permselectivity, S, as a function of the ion exchange capacity, IEC.

It can be seen, that the optimal regions for the best permselectivityelectrical resistance combination occur at relatively low ion exchange capacities; in the case of cation exchange membranes in the range of 1 .0-1 .2 meq/g, and for anion exchange membranes in the range of 1 .25-1 .35 meq/g . The membranes possess a mechanical tensile strength of 2-3 kp/mm 2 and a fairly good chemical stability .

331

Exposed to 1 N HO at 70-80 ° C the cation exchange membrane as well as the anion exchange membrane had a lifetime of 4000 hours . The same was found for the cation exchange membrane, when exposed to 1 N NaOH at 70-80° C, whereas the anion exchange membrane had a lifetime of only 500 hours. It should be noted, however, that this lifetime is more than twice that of the best commercially available anion exchange membrane . Finally we like to report the results of the first electrodialysis experiment performed with the polysulfone cation and anion exchange membrane pairs, in comparison with the CMV/AMV membrane pairs from the Asahi Glass Company . We determined the electrical work and the desalting time required to desalt different electrolyte solutions from 3000 ppm to 300 ppm . The electrodialysis test apparatus used was the BEL-2, made by the Forschungsinstitut Berghof GmbH, in the form of a 10 cell stack, as shown in Fig . 7 . In Table 2, the results obtained from both membrane pairs are presented ; it can be seen that the values are very similar . Lf //

Lf

I, S.

P

0

Fig . 7 . Schematic diagram of the laboratory electrodialysis apparatus BEL-2 of Forschungsinstitut Berghof GmbH . E = electrode rinsing, D = diluate circulation, K = concentrate circulation, P = pump, + = anode connection, - = cathode connection, Lf = conductivity measurement, 0 = drain valve .

332 TABLE 2 Comparison of the membrane pairs CMV (cation) and AMV (anion) with polysulfone cation and anion membranes with respect to electrical work and desalination times for 90% desalination of several electrolytes with salt content 3000 ppm Salt

MgCl ' CaCI, NaCl Na2 SO, NaHCO 3 Na 2 HPO,

Asahi - CMV/AMV

Berghof - MK-3/MA-11

Electrical work (W-hr/1)

Desalination time (hr)

Electrical work (W-hr/1)

Desalination time (hr)

4.11 3.2 3.34 2 .92 2 .49 3 .07

1 .51 1 .31 1 .25 1 .41 1 .53 2 .20

1 .94 3 .1 3 .21 2 .97 1 .96 2 .81

1 .3 1 .3 1 .3 1 .75 2 .05 2 .5

Acknowledgement We would like to thank both, Dr . Strathmann, who initiated this project, and the Bundesministerium fur Forschung and Technologie, which sponsored this work .