Pore size control technique in the spinning of polysulfone hollow fiber ultrafiltration membranes

Desalination, 80 (1991) 1 67-180

167

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

Pore Size Control Technique in the Spinning of Polysulfone Hollow Fiber Ultrafiltration Membranes SIIOICHI DOT and KATSUHIKO HAMANAKA Separation Membrane Plant Asahi ChemicallndusttyCo ., Ltd., 2-1, Samejirna,

ji-shi, Shizuoka-

ken, 416 (Japan)

(Received December 10, 1990)

SUMMARY

Conditions for finger-like void formation in polysulfone (PS) ultrafiltration membranes were studied empiri cally. It was found that both the "mixability parameter" which was obtained in separate titration experiments and the viscosity of coagulants could be used to predict membrane morphology. It was also found that finger-like void layers appeared under the conditions which dope had concentrations quite apart from the phase separation area and which coagulant had both a low mixability parameter and low viscosity. These factors proved to play a very important role in controlling the pore size of membranes . The results of this study were applied to obtaining pore size variations of PS hollow fiber membranes .

INTRODUCTION

In 1976 a PS hollow fiber ultrafiltration membrane was formed from a dope of PS in dimethylacetoamide (DMAC) . The hollow fiber had a "5-layer structure," as shown in Fig. 1, consisting of an inner skin layer, an inner void layer, a central sponge layer, an outer void layer and an outer skin layer. The inner and outer skin layers are the most dense in the cross-section . They are composed of very small polymer particle aggregations . The result is a "double skin structure ." Twice ultrafiltration is performed during a single filtration operation .

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16 8

Fig. 1 . 5-layer structure of a polysulfone hollow fiber.

As shown in scanning micrographs of the inner and outer surfaces, no pores are observed, even under x 10,000 (Fig . 1). The second and fourth layers are called the "inner void" and "outer void" layers, respectively . They have a finger-like void structure . These layers show low hydrodynamic resistance to liquid flow and therefore contribute to high membrane permeability . The central sponge layer contributes to the strength of the membrane with the skin layers . The PS hollow fiber membrane showed excellent characteristics and performance particularly regarding reliability and permeability . Subsequent studies proved some of these excellent characteristics were the result of the 5-layer structure . One advantage is that even if one surface is damaged, the other will continue to reject solutes and prevent contaminant leakage, and no CMW change is observed . Generally, the inner and outer skin layers should have

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the same pore size to fully utilize the advantages of the "double skin ." The highly reliable permselectivity of the double skin is valuable in any application, especially in bioseparation and pharmaceutical production where leakage must never occur . Another major advantage is higher flux . Table I shows that the 5-layer hollow fiber membrane can provide a much higher pure water permeability than a usual sponge hollow fiber membrane under the same rejection conditions . It is considered that void layers give much lower hydrodynamic resistance to water flow than a sponge structure . TABLE I

Effects of morphology on performance 5-layer

Sponge

0.90

0.49

Cross-section

PWP M3/M2

• hr • kgf/cm2

Solute rejection, % Burst pressure, kgflcm2

10 20

10 19

The first aim of this study was to clearly define the conditions for obtaining the above-mentioned excellent 5-layer structure hollow fiber membranes . Such a membrane can be considered as a combination of two flat membranes, each with a finger-like void layer . Therefore, the problem of how to get a 5-layer hollow fiber membrane can be restated as how to get a flat membrane with a finger-like void layer . Another purpose of the study was to establish a pore size control technique in the spinning of the 5-layer structure hollow fiber membrane .

REVIEW

Many researchers have reported on the relationship between preparation conditions and membrane morphology. Kesting [1j, Frommer [2], and Strathmann [3] are very well-known for their pioneer work in this area .

1 70

Cabasso and Klein [4] also described the morphology of a PS hollow fiber at almost the same time as our work . They showed that high viscosity of spinning dope and relatively moderate quenching conditions in the bore fluid should be used to get a macrovoidless membrane . Uragami [5] showed that it was very important to substitute solvents with coagulants at a high velocity to make a finger-like void structure . Munari and Bottino [6] discussed the relationship between void formation and the ratio of solvent flow from dope vs . nonsolvent flow from coagulants. Fane and Fell [7] showed that a "coagulation value," that is, a parameter concerning the coagulation power of nonsolvents, was effective in predicting the morphology of polyamide membranes . The theoretical approaches of Smolders and Mulder [8], McHugh [9], and Kamide and Manabe [10] are of particular interest with their rigorous application of thermodynamics to resolution of the membrane formation mechanism .

OUR OWN APPROACH

We empirically approached determining the conditions forming a fingerlike void structure utilizing an original parameter. We considered that the substituting rate of the solvent and the coagulant is important in determining morphology. One of our key parameters was the "mixability parameter" defined in eq . (a) for titration by PS solution to the cloud point with the nonsolvent . In addition to the mixability parameter, from a kinetic view, viscosity of the coagulant was taken into consideration . We considered that the substituting rate of the solvent and the coagulant was also affected by the coagulant viscosity . A concentration dependent mutual diffusivity should be investigated, but viscosity was used at the first stage of our approach .

EXPERIMENTS

1. Determination of mixability parameter

About 49 ml of N-methyl-2-pyrrolidone (NMP) was used as a solvent to obtain 2 wt% PS solution. NMP and all nonsolvents used here were of analytical grade. The titration was performed in a water bath kept at 25°C (Fig . 2) .

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(Nonsolvent) (ml) (a) Misabilityparameter = (Nonsolvent + Solvent) (ml)

Dope casted

Doctor blade Water bath Glass plate 2 wt% Polymer solution Nonsolvent stirrer chip I1

Magnetic stirrer

f

Coagulation bath

Fig. 2 . Determination of mixabifity parameter by titration .

Fig. 3. Preparation of flat membranes.

2. Preparation of flat membranes As illustrated in Fig. 3, each of the nonsolvents was used as the coagulant in a bath for preparation of a PS flat membrane . The wt% of PS in the dope was 18. The dope was casted on the glass plate to form a flat membrane of 200 u thickness by a doctor blade . Then it was put immediately in the bath . The temperature was kept at 25°C. 3. Preparation

of

hollow fiber membranes

The apparatus shown in Fig. 4 was used to obtain hollow fiber membranes with water as the internal coagulant (IC) and one of several nonsolvents as the external coagulant, that is, coagulation bath media (CB) . The wt% of PS in the dope was also 18 . The temperature was kept at 25°C, and the air gap height was about 5 cm . 4. Measurement of solute rejection of membranes We used the apparatus shown in Fig. 5 in a series of experiments to measure pure water permeability (PWP) and solute rejection . In the measurement of solute rejection ultrafiltration was performed for a 500 ppm



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dextran solution. Nominal molecular weight of the dextran (Pharmacia, Inc .) was usually 10,000. Pressure indicator

Internal coagulant (IC)

9

Dope

Hollow fibers i

Control valve

Roll

a

l Refractive index detector

PMP 500 ppm dextran solution

Coagulation bath (CB) Fig. 4. Hollow fiber spinning apparatus .

Fig. 5 . Schematic diagram of solute rejection measurement system .

RESULTS AND DISCUSSION

Conditions of finger-like void formation Meaning of mixability parameter One of our key parameters, the "mixability parameter," is defined in eq. (a), and it shows the mixability of nonsolvent in the PS/NMP solution . This method is almost the same as Fane's [7], where the coagulation value was defined as the volume of nonsolvent titrated into 25 ml of polymer solution . It is useful, but some other researchers defined the volume of polymer solution as 50 ml or other value ; then the coagulation value would change in accordance with the initial polymer solution volume . The mixability parameter is considered as dimensionless with a normalized coagulation value . A lower value to zero means high coagulation power, and a higher value to 1 means a low coagulation power . Characteristics of the coagulant Fig. 6 shows that the mixability parameter of various nonsolvents which were obtained by the above-mentioned titration was mapped against the viscosity of the nonsolvents . Then the results were compared with the membrane structure obtained experimentally . The formed flat membranes are shown in Fig . 7 in a cross-sectional micrograph, positioned on Fig . 6.

0

0 .5

1 .0

methanol

•

acetone

1

• water

ethanol

• 0

ormamide

iso-propanol

10

ethylene-glycol-mono-butyl-ether

diMtethyl-solfoxide

Fig. 6 . Nonsolvents mapped by mixability parameter vs . viscosity.

li 41

1



•

4

1000

glycerin

Viscosity (Pa •s )

100

propylene-glycol

• tetra-ehylene-glycol .

x14s

Fig. 7. Effects of mbcability parameter and viscosity of coagulant media on morphology of membranes .

1 75

The map shows that coagulants exhibiting a low viscosity as well as a low mixability are most effective for obtaining a finger-like void layer in the membrane . The substituting rate of solvent and coagulant is considered to be high under these conditions . The formed hollow fiber membranes are shown in Fig. 8 in a cross-section and surface micrograph, positioned on Fig. 6.

Fig. 8. Effects of mixability parameter and viscosity of coagulation bath media on morphology of hollow fiber membranes.

The 5-layer structure was obtained only with water as the external coagulant. The results show that the importance of the balance in the coagulating conditions of the internal and external coagulants . They also show the effects of the external coagulant viscosity on the finger-like void length and on the outer surface structure . Three systems in which the coagulant had almost the same mixability parameter (a, b and c in Fig . 8) shows void length decreased in accordance with increasing viscosity . Also the outer surface changes from porous and flat to an open network . These show that not only the mixability parameter but also viscosity are effective for morphology change. Conditions of dope

There are also important conditions in the initial concentrations of the dope. Fig . 9a-c shows the effect of the dope of three PS/NMP/additive systems. Both the internal and external coagulant were water (IC= CB =water) .

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Fig. 9 . Morphology and phase diagram of PS/NMP/additive . (IC s CB - water)

Even if using water of low mixability and low viscosity as a coagulant, a 5-layer structure can only be obtained in the area where concentrations are apart from the phase separation area . Therefore, not only coagulant characteristics but also initial dope conditions are important to determine the morphology of resulting membranes.

Pore size control technique in 5-layer hollow fiber membranes The next major question is how to obtain the desired pore size, or CMW, while keeping a 5-layer structure. Many spinning conditions can affect both the pore size and the morphology . The influence of two major conditions the spinning dope conditions and coagulant characteristics - are investigated here. Conditions of dope The relative concentration ratio of the dope constituents strongly influences morphology and solute rejection . The phase diagram in Fig. 9b shows the morphology of membranes spun from dopes consisting of PS, NMP and tetra-ethylene-glycol (TEG) with water as the internal and the external coagulant . As discussed above, a 5-layer structure can be obtained

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only in the shadowed area . The solute rejection contour lines are put on the same map . As shown in Fig. 10, an especially high TEG content results in low solute rejection. It is believed that a high TEG content may cause increasing polymer particle growth by microphase separation which then results in larger interparticle openings.

Fig. 10 . Solute rejection and morphology on the phase diagram of PS/NMP/TEG . (IC = CB = water)

Characteristics of the coagulant

The coagulant content has a similar strong effect . Fig. 11 shows the influence of increasing NMP or TEG content in an aqueous-based internal coagulant with water as the external coagulant . Solute rejection decreases with increasing NMP or TEG content . This may be an effect of the increased coagulant mixability, in which microphase separation in the dope slowed at the interface, thereby increasing polymer particle growth and ultimately resulting in larger interparticle openings . TEG shows a higher effect than NMP . Viscosity of 100% TEG is about 50 (KPa • s) while of 100% NMP is about 1 (KPa • s). Therefore, TEG solution is considered to be more viscous than NMP one at the same wt% value. It can be considered that coagulant viscosity is also effective as well as the mixability parameter .



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W4P a

tent 0

Fig. 11 . Effect of NMP or TEG in IC on morphology and solute rejection .

Performance of obtained hollow, fiber membranes

Fig . 12 shows the morphology and solute rejection curves of PS hollow fiber membranes having the 5-layer structure in four variations (A-D) obtained by control of the dope composition, coagulant content and other spinning conditions . The rejection was measured with several nominal molecular weight dextrans . CMW of A is 30,000 and of D is more than 1,000,000 on these coordinates under these measurement conditions .

179

Fig. 12 . Pore size variations line-up .

CONCLUSIONS

1 . Conditions for a finger-like void formation were investigated to confirm that (a) the coagulant should have both a lower mixability parameter and a lower viscosity, and (b) dope should have concentrations apart from phase separation. In other words, we can predict what kind of morphology we will get before preparing a membrane under certain conditions in the discussed system. 2. Pore size variations of 5-layer structure hollow fiber membranes were obtained by mainly controlling (a) the coagulant characteristics-mixability and viscosity, and (b) dope conditions-tendency to phase separation .

REFERENCES 1 2 3 4

R .E . Kesting, Synthetic Polymeric Membranes, 2nd Ed ., Wiley, New York, 1985 . MA . Frommer et al ., Polymer Reprints, 12 (1971) 245. H . Strathmann et al ., J. of Appl . Poly. Sci., 15 (1971) 811 . I. Cabasso et al., J. of Appl. Poly. Sci., 21 (1977) 165.

180 T. Uragami et al., Desalination, 37 (1981) 293 . A . Bottino, S . Munari et aL, Chimicaoggi aprile (1987) 11 . A .G . Fane, CID. Fell, et al., J . of Membrane Sd., 38 (1988) 113 . CA . Smolders. et al., e.g ., J . of Appl. Poly. Sci., 21 (1977) 199; Desalination, 38 (1981) 349 ; Notes prepared for ICOM'90 short course, Chicago, 1990 . 9 AJ. McHugh, Notes prepared for ICOM'90 short course, Chicago, 1990 . 10 K. Kamide and S. Manabe, ACS Symp. Series, 269 (1985) 197 . 5 6 7 8