Reverse Osmosis and Ultrafiltration - American Chemical Society

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Downloaded by OHIO STATE UNIV LIBRARIES on July 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch002

Development of Polyether Sulfone Ultrafiltration Membranes M. KAI, K. ISHII, H. TSUGAYA, and T. MIYANO Daicel Chemical Industries, Ltd., 1 Teppo-cho, Sakai-shi 590, Japan A parameter was introduced successfully to collectively describe the effect of casting conditions on the performance of polyethersulfone ultrafiltration membranes. This parameter is related to the amount of water to be absorbed by as cast solution during evaporation period. Another parameter, Coagulation Value which corresponds to the amount of water necessary to start the phase separation of casting solution clearly elucidated the role of the additives. Between these independent parameters was found a relationship which extends the applicable range of solvents, additives and casting conditions. On the basis of this study, commercial ultrafiltration membranes thermally more stable than conventional polysulfone membranes have been developed. Increasing demands for mechanically strong and stable UF membranes which should stand strong chemicals and high temperature resulted in the development and commercial production of Udel polysulfone (PSF) membranes by several manufactures. In contrast to many studies on PSF membranes (1^2) which are not stable enough for heat sterilization at a temperature higher than 100°C, little work (3) has been reported on UF membranes made from Victrex polyethersulfone (PES) which should be more promising than PSF due to its higher thermal stability. Their chemical structures are as follows. Polyethersulfone (PES)

Polysulfone (PSF)

0097-6156/85/0281-0021$06.00/0 © 1985 American Chemical Society

In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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REVERSE OSMOSIS A N D ULTRAFILTRATION

In this paper, the effect of parameters on membrane performance in membrane fabrication by phase inversion method, such as casting solution composition and casting conditions, is discussed with emphasis on the characteristic importance of water. Experimental


Polymers. Commercially available polymers, Victrex 300P, one of PES polymers supplied by ICI and Udel P-1700, PSF by UCC were used without further purification after the purchase. Glass Transition Temperature. The glass transition temperatures (Tg) of the polymers were measured by a torsional braid analyzer, Model 100-B1, Chemical Instruments Corp., under following conditions; pulse interval, 5 minutes and heating rate, l°C/min. The effect of glycerine absorbed by the polymers on Tg was measured with the samples soaked overnight in 50 % glycerine aqueous solution and then dried. Preparation of Membranes. Membranes were prepared by the following process unless otherwise mentioned. The polymer was completely dissolved with a solvent or a mixture of solvent and non-solvent to obtain a homogeneous solution (dope). The dope was cast 180 microns thick on a glass plate. The cast dope was allowed to stand for 30 seconds. The casting atmosphere was controlled at 25 °C and 55 % RH, and the velocity of air stream over the cast dope, below 20 cm/min. The cast dope was immersed into a water bath kept at 17 °C. The membrane obtained was left in the water bath over 30 minutes. The residual solvent was leached out of the membrane completely in a hot water bath at 90°C for 15 minutes. Evaluation of Membranes. Membranes were evaluated by use of thin channel flat cells with 200 microns channel height and 25 cm effective membrane area and at the mean flow velocity of 1.3 X 10 cm/sec. Pure water and 0.2 % ovalbumin aqueous solution were fed under 0.5 Kg/cm and at 25 °C. The concentration of the solute was determined by high performance GPC. Coagulation Value. The coagulation value (C.V.) of a dope, a parameter which represents the resistance to yield polymer precipitate from the dope by the addition of water, was measured as follows. The test solution was prepared in the same way to make the corresponding dope, except only 2g of polymer was dissolved in 98g of the same liquid composition. Water was added dropwise like titration into the test solution kept stirred at 30 °C. The amount of water necessary to yield the first precipitate refers to the C.V. of the corresponding dope. Results and Discussion Thermal Stability of Polymers. It is well known that the thermal stability of amorphous polymer can be estimated by Tg in greneral. This criterion alone, however, is not enough for membrane, due to its highly fine structure and strong interaction with water. For instance, hydrophobic PSF UF membranes are thermally more stable


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Polyether Sulfone UF Membranes

than relatively hydrophilic cellulose acetate UF membranes, in spite of their almost the same Tgs observed of solid polymers. The experimental results given in Table I show that the Tgs of cellulose acetates were lowered 40-50 °C by the glycerine treatment, while that of PSF decreased only 15°C. This observation gives a likely account to the above mentioned difference. Judging from its high Tg and insensitivity to the glycerine treatment, PES should be more promising than PSF to obtain thermally stable UF membranes.


Table I.

Thermal Stability of Some Polymers (4-6) Observed Tg

Polymer

non--treatment

(%) Hydrophilic Polymers

Hydrophobic Polymers

*1 ASTM, D570

(°C)

Water *1 Absorption

CDA *2

glycerine treatment

195

145-155

195-205

150-160

1.7-6.5 CTA *3

PSF

0.3

190

175

PES

0.43

225

225

*2 Cellulose diacetate

*3 Cellulose triacetate

Effect of Dope Composition. In order to determine an optimal solvent for PES and PSF, several high boiling and water miscible solvents were examined in binary dopes. As shown in Table II, no water flux was measured of the membranes prepared with other solvents than 2-pyrrolidone (2-PN). Observation by scanning electron microscopy (SEM) revealed that these unproductive membranes had porous structure but with dense skin on the bottom surface in addition to top skin. Table II. The Performance of the Membranes Prepared from Binary Dopes (polymer concentration, 20 wt%)

PES Solvents

„, Flux 3

2

(m /m day) N-methyl-2 ... ,

PSF

„ .

„_

Rejection (%)

0

„ .

Flux 3

2

(m /m day)

Rejection (%)

0

-

-pyrrolidone 2-pyrrolidone N,N-dimethyl -formamide N,N-dimethyl -acetamide

0.92

100

1.15

79

~

~

~

~



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In order to see whether the bottom skin prevented the water permeation, this layer was scraped off the membrane prepared with N-methyl-2-pyrrolidone (NMP). This pealed membrane, the crosssection of which is shown in Figure 1-A, exhibited water flux and solute rejection. On the other hand, the removal of the top skin showed no effect at all. The mechanism of the formation of these non-permeable bottom skin is not clear, but specific interaction between the dope and the glass plate (7), as well as the gelation mechanism, should be taken into consideration. According to well-known procedure to improve the membrane performance, ternary systems with a solvent and an additive were examined. As a solvent, NMP and 2-PN were used; as an additive, all the compounds listed in Table III were tried and the concentration ranged from 5 to 20 wt% according to the nature of the additive. Table III.

Acids

Water Miscible Organic Additives

Acetic Acid, Propionic Acid, Lactic Acid

Esters Amide Amine Ketone

Ethyl Lactate, Triethyl Phosphate N-Methylformamide Pyridine Cyclohexanone

An important aspect of the additives' role is the effect to increase the sensitivity of the dope to imbibed water, that is, to decrease C.V.. Among many parameters proposed to elucidate the effect of additives, C.V. was chosen, because this parameter well represents the sensitivity of each dope to intruded water. The effect of the additives to change the unproductive NMP binary dopes into productive ones is clearly shown in Figure 2, wherein pure water flux under 1 kg/cm (PWF/P) is plotted against C.V.. PES and PSF membranes were obtained from NMP ternary dopes of which polymer concentration was 17 wt%. Common to both polymers, PWF/P takes off from zero at critical C.V. somewhat below that of the original binary dope and increases with decrease in C.V. regardless of the kind of additives. Both curves show that the more sensitive to the intruded water the dope is, the higher the PWF/P of the membrane is. In Figure 3 similar relationship can be seen for the PES membranes obtained from the ternary dopes composed of PES (20 wt%), an additive and 2-PN, which gave productive membrane from the binary dope among the solvents examined (Table II). Effect of Casting Conditions. In order to improve the membrane performance, the effect of casting conditions were studied with PES/2-PN binary dope. As 2-PN is not volatile, the standing time (evaporation time) was not expected to have significant influence on the membrane performance. The outcome, however, was more than a simple effect as seen in Figure 4, where the performance was shown as the function of the standing time.


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Top Skin (Air Side)

Bottom Skin (Glass Side)

A) 50 microns

Top Skin (Air Side)

Bottom Skin (Glass Side)

B) 50 microns Figure 1. The SEM pictures of the cross-sections after the scraping treatment. A) The bottom skin was removed. B) The top skin was removed.



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REVERSE OSMOSIS AND ULTRAFILTRATION

Figure 2. The performance of PES and PSF membranes obtained from NMP ternary dopes (polymer concentration, 17 wt%): 0 » PES; A , PSF. Dark points represent binary dopes.

Figure 3. The performance of PES membranes obtained from 2-PN ternary dopes (polymer concentration, 20 wt%). Dark point represents binary dope.



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Figure 4. The effect of casting conditions. The binary dope composed of 20 wt% PES and 2-PN was cast on PET paper. The velocities of air stream over cast dopes were, Q=20, A =4.1X10 , V=1.26X10 and [3=1.76X10 cm/min. The humidity was 75 % RH.


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In order to elucidate these complicate results, the effect of the water absorbed from air by the cast dope should be taken into consideration, because the dopes were very sensitive to water (8). The amount of the water to be imbibed during the standing time was calculated as follows. The absorption rate of water from the air to the unit area of the cast dope (dW/dt, in g/cm min) is proportional to the concentration of water in the air stream (h, in g/cm )


dW/dt = k-h where k (cm/min) is the overall Equation 1 is integrated to obtain

(1) coefficient

of

mass

transfer.

W = k-h.t + Wo

(2)

2 where Wo (g/cm ) is the amount of water present in the dope before casting. The effect of the velocity of the air stream over the cast dope (U, in cm/min) can be written as follows. k = F-U a

(3)

where exponent a is 0.5 for laminar flow, and 0.8 for turbulent flow and F is a constant which mainly depends on the apparatus. Replacing h by the relative humidity (hr) and the saturated vapor pressure of water (Ps, in mmHg) and combining Equations 2 and 3, we obtain W = F.Ps.hr-U* t + Wo = F.G + Wo where G (Gelation Factor) = Ps.hr-Ua. t

(4) (5)

Equation 4 represents the amount of the water contained in the unit surface layer of the cast dope at the moment of the immersion as the sum of the water contained before casting and that imbibed during the standing time. The latter is given by the product of two parameters F and G. Although their physical meaning can not be clearly defined, G is the proper product of all the principal measurable variables. So, if the dope and the apparatus are given, the effect of different casting conditions should be able to be estimated by G. As shown in Figure 5, four curves in Figure 4 converged into one curve by use of G. This curve covered the data taken under the casting conditions varied over wide range of 40-80 % RH for hr, 0.17-5.0 minutes for t, and 20-1.26 x 10 cm/min for U, as well as the data shown in Figure 4. The curve in Figure 5 may be divided in three regions. In region A membrane performances do not change with G; in region B water flux increases but rejection decreases; in region C water flux decreases steeply to zero with increase in G. This nature of the curve might be explained as follows. In region A, the amount of the absorbed water is too little to influence the membrane performance. Further imbibition of



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Figure 5. The relationship between G and membrane performance. Dark points represent the data shown in Figure 4.


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water beyond region A advances the phase separation in top surface and grows the pores greater. This results in the increase of water flux and the decrease of solute rejection. In region C, further advancement in the phase separation breaks the finelypored structure and forms impermeable layer leaving much less but large-sized pores. The introduction of G, through which the effects of independent variables of casting conditions can be converted each other, and the curve, which represents the relationship between membrane performance and G for a given dope, made it easier to set up optimal casting conditions. Relationship between C.V. and G. A ternary dope was prepared from PES, 2-PN and lactic acid. From this dope, C.V. of which was smaller than that of the binary dope, membranes were made under various casting conditions, that is, various G values. The plotting of water flux vs. G for these membranes formed a curve which had similar shape to that for the binary dope membranes and was shifted in the direction to decrease G as shown in Figure 6. This suggests that the effect of the decrease in C.V. on membrane performance can be obtained by the increase in G controlled independently of C.V.. In order to see the relationship between the effects of C.V. and G, G values at specific points of the curves, the border of region B and region C for instance, were plotted against C.V.s of the corresponding curves. As shown in Figure 7, a linear relationship was found between C.V. and log G. If one wants to obtain the membranes of the same performance, that at the border of region B and C for instance, from different dopes, the effect of the difference in C.V. can be compensated by the corresponding change in G given by the line in this figure. The utilization of the relationship described above extends the range of choice of the additives and the casting conditions to achieve superior performance. Thermal Stability of PES UF membrane On the basis of the study described above, commercial PES UF membranes, Molsep DUS tubular and flat membranes have been developed successfully. The separation performance of DUS-40 is characterized by the sharp cutoff at 4 x 10 Dalton as shown in Figure 8. The thermal stability of DUS-40 is shown in Figure 9, in comparison with that of PSF membrane which has nearly the same cutoff point as DUS-40. DUS-40 kept its performance uninfluenced up to 130 °C, in contrast with a PSF membrane of which performance changed remarkably after thermal treatment over 100°C. Conclusion Commercial polyethersulfone UF membranes thermally more stable than conventional polysulfone membranes were developed through: 1. The application of 2-PN as the solvent. 2. The control of the sensitivity of the casting dope to water by the proper choise of additives. 3. The control of the casting conditions by use of a newly



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Figure 6. The relationship between G and the performance of membranes made from a ternary dope (20 wt% PES, 6 wt% lactic acid and 74 wt% 2-PN) in comparision with that for the binary dope membranes. Points and broken line indicate the ternary dope membranes. Solid line indicates the binary dope shown in Figure 5.

Figure 7.

The relationship between C.V. and G.


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REVERSE OSMOSIS AND ULTRAFILTRATION

Figure 8.

The separation performance of DUS-40.

Figure 9. The thermal stability of DUS-40: O . DUS-40; A , a UF membrane made from PSF. Membranes were immersed into a hot water bath in an autoclave for 30 min.


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introduced parameter which corresponds to the amount of the water absorbed by the cast membrane during the evaporation time. Literature Cited 1. 2.


3. 4. 5. 6. 7. 8.

Nishimura, M.; Muro, T.; Tsujisaka, Y. Kobunshi Ronbunshu 1977, 34, 713-8. (an example for flat membranes) Cabasso, I.; Klein, E.; Smith, J. K. J. Appl. Polym. Sci. 1977, 21, 165-180. (an example for hollow fibers) Tweddle, T. A.; Kutowy, O.; Thayer, W. L.; Sourirajan, S. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 320-6. Union Carbide Corp., Product Data, 1981. Sumitomo Chemical Industries, Ltd., Technical Service Information Note, 1979. Daicel Chemical Industries, Ltd., Technical Bulletin. Tsugaya, H.; Miyano, T. Japanese Patent Kokai 57-207505. Kamizawa, C.; Matsuda, M.; Masuda, H.; Tanaka, H. Koubunshi Ronbunshu 1983, 40, 401-4.

RECEIVED February 22, 1985


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Reverse Osmosis and Ultrafiltration - American Chemical Society

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