Asymmetric reverse osmosis and ultrafiltration membranes prepared from sulfonated polysulfone

Asymmetric reverse osmosis and ultrafiltration membranes prepared from sulfonated polysulfone

Desalination, 36 (1981) 39-62 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands ASYMMETRIC REVERSE OSMOSIS AND ULTRAF...

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Desalination, 36 (1981) 39-62

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

ASYMMETRIC REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES PREPARED FROM SULFONATED POLYSULFONE* C. FRIEDRICH, A. DRIANCOURT, C . NOEL AND L. MONNERIE Laboratoire de Physicochimie Structurale et Macromoleculaire, 10, rue Vauquelin 75231 Paris, Cedex 05 (France)

(Received December 6, 1979)

SUMMARY

The concept of solubility parameter, developed by Hansen, has been applied to the determination of solubility diagram of polysulfone and sulfonated polysulfone. Data showing the effect of various parameters such as thermodynamic quality of the solvent, casting solution composition and viscosity, solvent evaporation period and precipitation bath composition, on membrane structure and performance have been presented . By adjusting these parameters a wide variety of sulfonated polysulfone membranes useful for both ultrafiltration and reverse osmosis applications could be obtained . INTRODUCTION

Ultrafiltration and reverse osmosis are fast emerging as new and versatile unit operations in separation technology. Since the time of Bechhold [1] who coined the term "Ultrafiltration" in 1906, until the middle of the century, all the research work reported was mainly associated with collodion membranes and cellophane films . Only the development of asymmetric membranes [2] in the beginning of the second half of this century, has made industrial scale membrane filtration feasible . The original preparation technique for asymmetric membranes as well as subsequent modifications were developed rather empirically . Only recently an attempt has been made (3-5) to rationalize the various preparation process parameters determining the structure of the membranes within the framework of a considerable theory . These studies have been essentially limited to membranes made from cellulosic polymers. * Presented at the Symposium on New Aspects of Ultrafiltration and Reverse Osmosis, May 21-22, 1979, Technical University, Vienna . 0011-9164/81/0000-0000/$02 .50 © 1981 Elsevier Scientific Publishing Company



40

C. FRIEDRICH et al -

However, recent progress in polymer chemistry, has considerably extended the scope of membrane filtration with the synthesis of new polymers which show rather promising properties for use as reverse osmosis or ultrafiltration membranes. We wish to report on the preparation and characterization of sulfonated polysulfone membranes. Indeed, polysulfone is a high-performance, tough, high-temperature resistant thermoplastic resin . The objective of this work was to apply the same general technique developed by Loeb and Sourirajan for the preparation of asymmetric reverse osmosis membranes to another polymer and to determine the parameters which are essential for the formation and the filtration properties of the membranes obtained . EXPERIMENTAL

Polysulfone (I) [P 1700, Union Carbide] was sulfonated in organic solution using chlorosulfonic acid (C1SO 3 H) as the sulfonating agent [6, 71

O_>



X0

CH 3 O

C CH3

Sulfonated polysulfone was studied either in the free acid tPSSH) or in the sodium salt (PSSNa) form . In both cases, the degree of sulfonation was 0.476 . All the solvents used were of C.P. grade. To obtain the solubility diagram, 3 g of polymer were mixed with 6 ml of each of the solvents tested . After mixing for 2 days at room temperature, the solubility of polymer was examined . The viscosity of the various concentrated solutions was determined at 25° C by employing either a Hoeppler Viscometer (Haake type B) or Weissenberg Rheogoniometer. The casting solutions were prepared by dissolving the requisite quantity of the polymer and different additives . The mixtures were stirred until completely clear. The casting solutions were cast in a ca 0 .20 mm thick film on a glass plate and either directly or after a predetermined evaporation time immersed into a water bath. The other details are given in Tables and Figures . The reverse osmosis (RO) membranes were tested at 25 ° C in thin-channel flow-through high-pressure cells . The operating pressure was 30 Bars and the test solution consisted of 0.5 W % NaCl . Salt rejection was determined by conductivity measurements. An aqueous solution containing 4 .0 % wt/volume dextran [Sephadex G 10 Pharmacial was selected as the candidate material for characterization of the



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

41

ulfrafiltration (UF) membranes . The membrane testing was carried out in a laboratory test unit supplied by Amicon . The operating pressure was 2 bars . Dextran was determined by light scattering. For the scanning electron micrographs of membrane cross-sections, samples were prepared by fracturating the membrane at liquid nitrogen temperature and coating with gold. Extensive use was made of the transmission electron microscope in this work . Samples were prepared in an atmosphere of given humidity (88%) by surface drying the wet membrane and trimming the film to the proper size to fit in a sample holder which was immediately immersed in liquid Freon 22 at -160 ° C. The tiny ice crystals produced were sublimed from the specimen at 100°C into a vacuum of 1()r 4 Torr. Cross-sections of the membrane were then obtained (-170 ° C) and shadowed with evaporated platinum-carbon after which they were coated with a layer of evaporated carbon . Sulfonated polysulfone was dissolved and the replica were observed with a Philips M 300 electron microscope .

RESULTS AND DISCUSSION

Solubility diagram The polymer solubility diagram, which was introduced by Klein and Smith [8], is a valuable tool in the prediction of casting compositions for any polymer. In Fig. 1-3, the solubilities of a large number of liquids were plotted on a three-dimensional diagram with 8d, Sp and Sh as coordinates. These terms are derived from the expression relating the total solubility parameter 8 to the contributions arising from hydrogen bonding (8h), permanent dipoles (Sp ) and dispersion forces (Sd) : 82 = 8 h + Sp + 82 d

The values used for 0h, Sp and Sd were taken from the reference [9] . As shown in Fig. 1-3, an ellipsoid corresponding to the region of polymer solubility is defined by those solvents found to dissolve the particular polymer (Table I) . From the ellipsoid centers, the following total solubility parameters S and components 6d, Sh and Sp can be estimated :

PS PSSNa PSSH

S 10.85 11 .8 11.2

Sd 8.7 5 8.6 8 .45

Sp 5.4 5.95 5.5

Sh 3.4 5.4 5 4.9

The components Sd and Sp thus obtained for polysylfone are in agreement with the values of 8 .7 and 5 .9, respectively, determined by Shaw [10] using the method developed by Gordon [111 .



C . FRIEDRICH et al .

• Swells or partially n Insoluble ~k P S

a

Fig. 1 . Solubility diagram for polysulfone .

o Soluble

soluble

REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

a

Swells or partially

43

soluble

0 Insoluble Sh

*

PSSH

0

Soluble

19 18

10

2

3

4

5

6

7

8 6

10

11 12

Sp

Fig . 2. Solubility diagram for sulfonated polysuifone in the free acid form .

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C . FRIEDRICH et al .

. Swells or partially soluble o Insoluble * PSSNa o Soluble

Fig. 3 . Solubility diagram for sulfonated polysulfone in the sodium salt form.

Name

Diacetone alcohol 2 •butoxyethanol 2(2 •b utoxyethoxy)ethanol 2-ethoxyethanol 2(methoxyethoxy)ethanol 2 •rnethoxyethanol Isophorone Tetramethylurea Dimethylacetamide Triethylphosphate Trimethylphoaphate Tetrahydrofuran Benzonitrile Dimethylformamide Acetophenone Cyclohexanone N-methyl •2 pyrrolidone Dimethylaulfoxide Morpholine Pyridine 'y-Butyrolactone Dioxane Epichlorhydrine

Code

I 2 3 4 6 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 10.12 10,22 9,97 11,43 10.73 12,11 9.72 10.6 11 .11 10 .9 12,37 9 .51 10.98 12.15 9.69 9 .89 11,17 13.03 10.51 10.6 12 .81 1.0 .01 10.7

7 .65 7 .76 7 .80 7.85 7 .90 7.90 8.10 8.2 8.2 8.2 8.2 8 .22 8.60 8.52 8.55 8.65 8.75 9.00 9.20 9.25 9.26 9.30 9.3

6d

5 0 cal" ,

4 .00 3.10 3.4 4.5 3.80 4.60 4.00 4 .00 5 .60 5 .60 7.80 2 .80 6 .50 6.70 4.20 4 .10 6.00 8.00 2.40 4.3 8.10 0.9 6 .0

cm 3/2 Sp

6.30 5.90 5,20 7 .00 6,20 8.00 3 .60 5.4 6.00 4.60 6,00 3,90 2.50 6.60 1,80 2 .60 3,5 5.00 4 .50 2.9 3 .6 3.6 1.8

Sh

SOLUBILITY OF POLYSULFONE AND SULFONATED POLYSULFONE IN THE FREE ACID AND THE SODIUM SALT FORMS

TABLE I

PSSH, PSSNa PSSH, PSSNa PSSH PSSH, PSSNa, PS PSSH, PSSNa, PS PSSH, PSSNa PSSH,PSSNa,PS PSSH PSSH, PS PSSH, PSSNa PSSH PSSH PSSH,PSSNa,PS PSSH, PSSNa PSSH PSSH,PSSNa,PS PSSH, PSSNa PSSH PSSH, PS

PSSH

Solubility

C. FRIEDRICH et al .

46

From comparison (Fig . 4) of the solubility regions of PS, PSSH and PSSNa it appears at once that the substitution by free acid groups -SO 3 H improves the polymer solubility . Basic solvents such as dioxane and tetrahydrofuran and hydrogen bonding solvents dissolve PSSH . On the other hand, the sulfonated polysulfone in the salt form is found to be soluble principally in polar solvents.

a4 i

2

18 ` 14

8

g,n

l P

to •, 2o , 0

~ t2 i

, 11

a16

1

~~

17

~•

X13

"~

11

Fig. 4 . Influence of sulfonation on the solubility of polysulfone . PSSH A ; PSSH and PSSNa • ; PSSH and PS * ; PSSH, PSSNa and PS it ,



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

47

Having defined the region of solubility one can proceed to use it in solving practical problems . Solvent selection can be systematically worked out according to the following requirements : - The thermodynamic quality of the solvent or its ability to solvate large quantities of polymer. It is related to the difference between the solubility parameters of the solvent and the polymer . As the solubility parameter of the solvent approaches that of the polymer, that is to say as the distance d decreases, the solvent thermodynamic quality increases . Note that one can choose a mixed, good solvent composed exclusively of non solvents, by choosing non solvents located, respectively, on opposite sides of the region of solubility . - The solvent or the solvent mixture position must be on the diagram between the polymer position, i.e . the solubility maximum, and the quench fluid position* . Under these conditions, during the immersion step, the ingress of the quench fluid continuously shifts the surface of the membrane, destined to contain the dense, asymmetric layer, to less soluble compositions and phase separation . - The vapor pressure which governs the evaporation rate of the solvent and the hygroscopy which determines the rate of water ingress in the film during the immersion step . - The amount of quench fluid required for initiating phase separation . The addition of a quench fluid shifts the solvent position along the line joining the coordinates of the original solution and the point representing the quench fluid . As the composition reaches the solubility boundary, incipient precipitation occurs . The concentration of quench fluid required for polymer precipitation is thus related to the distance D . However it also depends on the thermodynamic quality of the solvent, i .e. the distance d . The smaller the solubility parameter disparity of solvent and polymer, the better is the thermodynamic affinity of the solvent and the higher is the precipitant concentration at the point of precipitation .

The viscosity of the casting solution The viscosity of the casting solution is a function of the polymer concentration and the solvent system at constant temperature . Figs . 5 and 6 show representative plots of log t versus polymer concentration . The viscosity of equiconcentrated solutions of sulfonated polysulfone may differ by a factor of 10-102 . However, the difference between the viscosity in good and poor solvents is not uniform over the whole concentration range . At a critical concentration, which is all the higher as the thermodynamic affinity of the

* In most cases water is used as quenched medium .



48

C. FRIEDRICH et al .

solvent is poor, there occurs a change in slope due to overlapping of macromolecules and entangling of coils . The difference in the behaviour of dilute and concentrated solutions should not cause surprise. There is a quite widely held point of view that the inl rl cp

PSSH

1C?9 8 1

1

7-

6 , 5432-

x 14 • 9 • 17

• 18 A benzyl A

alcohol

6

e 20 • 21 10 • carbitol • 5 4 8 • 11

1 5

10

Fig . 5. Viscosities of PSSH solutions .

15

20

25

30

Cu/dl

REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

e-

DMF DMAC NMP DMSO BENZYL ALCOHOL METHOXYETHANOL PYRIDINE X BUTYROLACTONE TEP CARBITOL METHYLCARBITOL TETRAMETHYLUREA TRIMETHYLPHOSPHATE

Fig. 6 . Viscosities of PSSNa solutions .

49



50

C . FRIEDRICH et al

c trinsic viscosity [n] increases as the quality of the solvent becomes better . The polymeric coil is quite likely to be expanded in thermodynamically better solvents . In solutions of low concentration where isolated macromolecules are present, the solvent exerts an effect only on their conformation and effective dimensions . Apart from macromolecular conformation, an important role is played in concentrated solutions by the interactions between macromolecules, which leads to structure-formation and to the formation of a fluctuating network made up of macromolecular aggregates . In this case, the solvent affects both the molecular conformation and the interaction between molecules . In order to eliminate the effect of the solvent viscosity (rlo) we have calculated the dependence of reduced viscosity (rlr) upon concentration (C) in grams of sulfonated polysylfone per dl of solution :

17T = 77-170 n0 C In Figs. 7 and 8, the reduced viscosity is plotted versus the reciprocal distance "d" as a measure of the thermodynamic quality of the solvent . At low concentration (1 .5 g/dl), for both PSSH and PSSNa, one observes a monotonic increase in the reduced viscosity with increasing solvent affinity. The too high values observed for solutions in dimethylformamide are due to the polyelectrolyte behaviour of sulfonated polysulfone . When the macromolecules do not overlap, only expansion of coils plays it part owing to the increase in the solvent thermodynamic quality . At high concentration (20 g/dl), the viscosity falls, reaches a minimum and, then, gradually increases as the solvent thermodynamic affinity increases . By analogy with the expressions [12, 131 derived for the melt of an amorphous polymer, the viscosity of concentrated polymer solution can be regarded as proportional to the radius of gyration of the expanded coil, and the strength and the number of entanglements on the polymer chain . In solvents under conditions, the strength of the polymer-polymer contacts is strong which results in high viscosities of concentrated solutions . As the solvent quality is improved, there is a sharp decrease in the strength of the polymer entanglements . Simultaneously, macromolecules expand . However, the effect of expansion is small in comparison with the sharp decrease of the entanglement strength and this leads to a fall in the viscosity . When the strength of entanglements becomes low, the increase in the radius of gyration prevails and results in increased viscosity of concentrated solutions .

Some ultrafiltration and reverse osmosis experiments Two types of structure, each associated with characteristic membrane properties, can be distinguished for asymmetric membranes . A foam-like structure with a dense, homogeneous layer at the surface, is suitable for re-



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

51

verse osmosis . On the contrary, a "sponge-like" structure, which has no ability to retain salts owing to the pore collapse which occurs at pressures above 10 bars, is well adapted to ultrailtration .

2

1$

1

OS

Y Fig. 7 . Dependence of reduced viscosity of FSSH concentrated solutions upon the thermodynamic quality of the solvent .



C. FRIEDRICH et al .

52

Ultra filtration

From the literature [3, 4, 151 which describes the formation of "spongelike" structures and from our own experience [5], the solvent selection can be systematically worked out according to the following requirements : The polymer concentration in the casting solution has to be adjusted in such a way that, in the leaching bath, the system passes the border of the red . 9

C = 2s/ican P55 Na

Z(

• DMA C x DMF

Triethyl phosphate , NMP

010 red- q C =1 .5g/400rnI

o DM50 O Trimethyl phosphate

1°_

• Tetramsthy l urea • Y( . Butyrolactone

@11

∎ Z .(Methoxyethozy)ethanol + Pyridine o Mathoxy ethanol

1(

5

05

0

0.5

1

1.S

% Fig. 8 . Dependence of reduced viscosity of PSSNa concentrated solutions upon the thermodynamic quality of the solvent .



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

53

two-phase region of the polymersolvent-precipitant phase diagram at the diluted side of the critical point (Fig . 9). Under these conditions, few small nuclei of the polymer-rich phase will be formed with a large difference in composition from the original solution and dispersed in the polymer-poor phase . When the precipitation process will come to an end, the polymer will be solidified and, through coalescence or sticking of the polymer spheres, an open-structure will be obtained . However, a minimum polymer concentration is essential to prevent the formation of a latex and ensure a certain mechanical strength . With increasing polymer concentration pores become smaller (Fig . 19) and water permeability decreases . The viscosity of the casting solution must fall between two critical values which depend on the solvent . A very low viscosity clearly favors "finger-like" °olymer

Solvent

Precipitant

Fig. 9 . Schematic phase diagram for a ternary system polymer-solvent-precipitant showing the precipitation paths at various polymer concentrations and relative rates of precipitant diffusing in and solvent diffusing out of the casting solution .

54

C. FRIEDRICH et al .

a

C

d

e Fig. 10. Effects of polymer concentration in casting solution on membrane structure (X 5.600). PSSH (a) 33 .3 g (b) 20 g (c) 18 g (d) 17 g (e) 16 g ; Water (a), (b), (c) : 4 g ; (d), (e) : 5 g ; Dimethylformamide 50 g ; Mg(CI04)2 : 0 .75 g ; Solvent evaporation period : 2s; Quench fluiwater d: (22 °C). Temperature of casting atmosphere : 22°C; Water permea bility (1/m2 -day) (a) 115 ;(b) 1475 ;(c) 3130 ;(d) 6855 ;(e) 10 .920.



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

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structures while a high viscosity yields dense membranes . For equiconcentrated solutions but different solvents, the higher the viscosity of casting solution, the smaller the pore size (Fig . 11). As a consequence, poor solvents such as trimethylphosphate, methoxyethanol, 'r-butyrolactone, 2-methoxyethoxyethanol are not suitable for ultrafiltration membranes . The rate of precipitation must be high in order that only small amounts of solvent diffuse out the immersed film before gelation sets in . The effect of additives to the casting solution or precipitant is to change the activity coefficient of the polymer, the solvent and/or the precipitant . The action of casting solution additives, such as inorganic salts, is to decrease the difference in the chemical potential of the water between the casting solution and the wash bath (14). This causes a decrease in the rate at which acetone leaves and an increase in the rate at which water enters the cast film . As the addition of salts to the casting solution also decreases the amount of water required to cause polymer precipitation, inorganic salts in the casting solution increase the speed of precipitation and, as a consequence, the size of pores of resulting membranes (Fig. 12). On the other hand, inorganic salts added to the precipitant reduce the rate of precipitation and favor a more dense structure (Fig . 13) . Some typical results summarized in Tables II and III confirm the criteria established for creating sulfonated polysulfone ultrafiltration membranes for practical purposes . Reverse osmosis Referring to previous data [3-5] and experimental results reported in this paper, casting solution compositions and film-casting conditions have to be selected according to the following requirements : - The polymer concentration in the casting solution has to be adjusted in such a way that, in the leaching bath, the system undergoes asol-gel transition before passing the border of the two-phase region of the polymer-solventprecipitant phase diagram. (Fig. 9). Thus, it will not be able to separate into two phases and an homogeneous structure will be formed . - The disparity of the solubility parameters of the polymer and the solvent or the solvent mixture must be small which tends to decrease the size of supermolecular polymer aggregates in the casting solution and results in the formation of smaller size pores on the membrane surface . - The membrane making procedure must involve an evaporation period large enough to cause sol-gel transition at the film surface (Fig . 14) but short enough to avoid the complete gelation of the medium prior to contact with the quench fluid . The effect of droplet growth in the interdispersed phase during film formation is predominent up to a certain critical evaporation time after which the simultaneous effect of pore depletion by droplet coalescence and partial surface shrinkage begin to control membrane performance.

C. FRIEDRICH et al .

56

a

d

1= 58 p

G = 600 tl=221p

G = 535

q=32 .7p G=540

e

rj=200p G=510

rl = 29 .4 p

G = 260

Fig. 11 . Effects of casting solution viscosity on membrane structure . Casting solution : water : 7 .7 g ; Mg(CI0 4 )2 : 2 .3 g ; PSSNa (a) : 40 g : (b) (c) (d) (e) : 30 g ; Solvent : 60 g (a) THF (b) Dioxane (c) pyridine (d) NW (e) DMAc. Solvent evaporation period : 2 s. Quench fluid : water (2 °C). Temperature of casting atmosphere : 2 ° C.



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

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a

b

Fig. 12 . Effect of various salts contained in the casting solution on the membrane structure. Casting solution composition : PSSH : 20 g, water : 4 g, n-methylpyrrolidone 50 g, salt ; 1 .5 g (a); Mg(Cl04 )2 ; ( b) NaNO 3 ; (c) KSCN . The order of the volume of water required for precipitating PSSH from the solution is : KSCN < NaNO 3 < Mg(C104 )2 .

58

a

C. FRIEDRICH et al .

b

Fig. 13 . Dependence of the membrane porosity on the concentration of NaCI in the leaching bath . Casting solution : PSSH : 17 g, Water 5 g, Mg(C104)2 0.75 g, Dimethylformamide : 50 g. Leaching bath (22 ° C) : (a) Water (b) 0 .1% (W/V) NaCI aqueous solution (c) 0.2% (W/V) NaCI aqueous solution . Evaporation period : 2 s at 22 ° C.

The rate of coagulation must be low. The effect of all successful additives is to adjust both the volume of precipitant required for membrane coagulation and the relative rates of solvent loss and precipitant gain in the leaching bath so as to produce membranes which fall in an optimum porosity range of 0 .5 to 0.75 . When, during the coagulation step, the amount of quench liquid required for membrane coagulation is high and/or the solvent leaves the immersed film faster than water enters, the result is less porous membranes . On this basis for experimentation we have chosen, from the diagram shown in Fig . 3, an initial solvent composition containing dioxane : 60 g as volatile solvent and water : 8 g. Membranes were prepared from casting solutions containing PSSNa : 40 g and certain organic acids, the effect of which is to increase the casting solution viscosity and to decrease the solvent evaporation rate to different degrees, depending on their chemical nature . In some cases membranes were subjected to a heat treatment procedure, i .e. membranes were treated for 10 minutes at 75 ° C in 33% NaNO 3 aqueous solution . In all -



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

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TABLE II DATA ON SOLUTE SEPARATION AT 2 BARS GIVEN BY ULTRAFILTRATION MEMBRANES. Film casting conditions : temperature of casting solution and casting atmosphere 22 ° C solvent evaporation period : 2 s. coagulation in water (22 ° C) Solvent g

Salt g

PSSH Water Viscosi- Pure water g g ty of permeacasting tion solution 1/m 2 .day P

Aqueous solution of dextran flux 1/m 2 . day

Solute separation %

DMF 50 g

KSCN

0 .75 0 .75 1 .5 NaNO3 0.75 Mg(CI04 )2 0 .75

25 20 25 20 20

4 4 4 4 4

66.5 27 .2 76 .1 27.2 30.5

823 3920 2183 1070 1475

475 2050 922 901 1455

46 13.2 32.7 25.7 19.2

NMP 50 g

KSCN

0 .75 1 .5 NaNO 3 0 .75 1 .5 Mg(Ct04)2 0 .75 1 .5

20 20 20 20 20 20

4 4 4 4 4 4

44.4 54.5 48.8 47 45 .2 55 .3

650 2476 380 823 310 645

462 1225 307 584 273 521

50 79.7 59.2 85.8 57 .7 79.3

DMA C 50 g KSCN 0.75 NaNO3 0.75 Mg(C104 )2 0 .75

20 20 20

4 4 4

33 .1 36 .75 34 .6

1371 288 335

922 367 333

59 66_5 31 .2

Carbitol50g KSCN

20

4

314.5

490

684

15

0.75

TABLE III DEPENDENCE OF THE ULTRAFILTRATION PROPERTIES OF SULFONATED POLYSULFONE ON THE CONCENTRATION OF NaCl IN THE LEACHING BATH Casting solution

PSSH

NaCl

Pure water permeation I/m' .day

Aqueous solution of dextran Flux l/m' .da} Solute separation

D\IF H_0 mg(C104)2

17 g 0 50g 0 .1 5 g 0 .75g 0 .2

6854 5770

53 2 7 4646

2 20

4014

3719

30 .7

PSSH Pyridine

40 g 60 g

0 0

Evaporation period 60 s

14500 380

75

2 2

Evaporation period 60 s

460

71

135 Temperature of casting solution and casting atmosphere 22 ° C Coagulation in water (22 ° C) Evaporation period, generally 2 s



C . FRIEDRICH et al .

60

I•A BLE IV DEPENDENCE OF THE MEMBRANE PERFORMANCE ON THE CASTING SOLtI r[ON COMPOSITION AND EVAPORA 1'ION PERIOD Acid concentration, q

Evaporation period, s .

Flux I/m2 .day

Salt Rejection . %

Water content,

Thickness p

rl(25'(') P

Initial evaporation rate, : s gent

None

0 30 60 120

246 380 306 235

? t q 3, 55 .5 75 .2

78 .7 76 .5 73 .2 71 .5

255 220 212 201

1 13

3 .3 10 L'

Act-tie acid 2 .47

0 30 60 120

435 322 235 294

35.3 609 57 68.2

79 .8 76 .3 76 .2 72.2

230 221 210 212

137

29 .10 6

0 30

532 154

28.9 34 .9

76 .2 758

223 215

192

1 .4 .3 10 6

259 334 208 207

53 .2 46 .5 51 .3 83

77 .9

13 6 10 6

71 .5 70.3

266 227 222 193

168

CLI,CIIOH000H

0 30 60 120

~lalunti. acid 4 .28 HOOt :CH .000H

0 30 60

80-1 862 261

28.2 32 .9 69

75 .3 70.4 68 .5

212 219 194

210

21 1 106

0 30 60 120

775 975 462 150

38 .7 35 .5 57 .7 93 .1

73

192 189

200

18 .7 106

67 .5 65 .3

186

0

156

27 .1

74 .2

212

203

8 106

120

111

88 .2

66 .5

188

0 30 60 120

564 403 339 358

36 36 54 .2 58 .2

74 .7

261 230 222 192

348

29 .4

CH,000H ( ;Ivtulic acid 3 13 HO011 . •C OOH Lactic at,d .3 .7

Malen• at,d 4 .78 IIOO( •( 'II ('ti('OOH but can ic it ad 4 h6 1-1000CHO1i1 . COOH Tartaric Acid 6 .18 flOU("1('HUH1, ( ()()If

68.3

TABLE V EFFECT OF THE HEAT TREATMENT PROCEDURE ON MEMBRANE PERFORMANCE

Acid concentration g None Acetic acid 2 .47 Glycolic acid 3 .13 Lactic acid 3 .7 Malonic acid 4 .28 Maleic acid 4 .78

Evaporation period : 45 seconds

Flux 1/m2 . day

Salt rejection `Yo

265 404 300 296 420 596

80 .1 79 .4 92 .6 86 .2 73 .8 83 .2



REVERSE OSMOSIS AND ULTRAFILTRATION MEMBRANES

2 sec

61

120sec

Fig. 14 . Effect of evaporation time on the membrane structure . Casting solution : PSSNa : 40 g, Water : 8 g, Dioxane : 60 g, Maleic acid : 4_78 g. Evaporation and coagulation at 22°C.

cases asymmetric skin type membrane structure have been obtained . A'though the membranes were not obtained as the result of a thorough optimization of preparation conditions typical results, summarized in Tables IV and `d confirm the criteria established for preparing sulfonated polysulfone R.O . membranes. CONCLUSION

This work confirms the conclusions stated earlier for cellulose acetate as to the dependence of the porous structure of the membrane surface on the casting solution composition and viscosity, solvent evaporation period, total gelation environment and membrane post-treatment procedure . By changing one of these parameters, a wide variety of sulfoneted polysulfone useful for both utirafiltration and reverse osmosis can be obtained

REFERENCES 1. H. S. Bechhold, Phys. Chem., 60 (1507) 257 . 2. S. Loeb and S. Sourirajan, Advan. Chem, Ser., 38 (1962) 117 .



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C . FRIEDRICH et al .

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