New polymeric materials for reverse osmosis membranes

New polymeric materials for reverse osmosis membranes

Desalination. 21 (1977) 35-M @ Elsevier Scientific Publishing NEW POLYMERIC Amsterdam MATERIALS R. ENDOH, T. TANAKA, Toray Id Company, - Printe...

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Desalination.

21 (1977) 35-M @ Elsevier Scientific Publishing

NEW POLYMERIC

Amsterdam

MATERIALS

R. ENDOH, T. TANAKA, Toray Id

Company,

- Printed in The Netherlands

FOR REVERSE

M. KURIHARA

OSMOSIS

MEMBRANES

AND K. IKEDA

Inc., Sonoyama, 01s~. Sh’ga (Japan)

(Received September 2, 1976)

SUMMARY

The results obtained from investigations on new aromatic polyamides containing carboxylic groups suitable for reverse osmosis are reported. The polymers are fabricated into asymmetric membranes by the Loeb-Sourirajan technique. The effects of fabrication conditions were also investigated to yield the optimum membranes for brackish water and sea water desalination, respectively.

Performance

characteristics

of the membranes

are 0.30 m3/m2

- day above

98 % rejection at 70 kg/cm’-35000 ppm NaCI, and 1.0-1.15 m3/m2 - day, 97-98x rejection at 40 kg/cm’-5000 ppm NaCI. They exhibited good resistance in the tests carried out in alkaline medium (pH 10) and acid medium (pH 4). lNfRODUCflON

Since the discovery Sourirajan

(I), the progress

of high flux cellulose made

in reverse

acetate

osmosis

membranes

membrane

by Loeb

technology

and

in the

past decade has been substantial. Research on new polymeric materials for reverse osmosis has progressed considerably (2). Among the new materials besides cellulose acetate which can be cited are aromatic polyamides (3, 4), polypiperazine-amides (5), sulfonated polyphenylene oxide (6), sulfonated polysuifone (7). and crosslinked polyethylenimine (8). However, the new membranes made from these new polymeric materials using the conventional fabrication methods such as asymm&-ic membrane techniques, hollow fibers and composite membranes are still inferior to cellulose acetate membranes in permselectivity and water permeability itself. The problem of finding new materials having properties analogous or superior to cellulose acetate and suitable for the fabrication of high flux membranes still remains to be solved. The interesting report (9) concerning the intrinsic permeability characteristics of the aromatic polyamide and cellulose acetate has stimulated us to investigate aromatic polyamides in more detail to search for new membranes. Our research work on new polymeric materials for reverse osmosis has been mainly directed to the study of new aromatic polyamides having carboxylic groups and

their

fabrication

into

asymmetric

membranes.

These

polymers

were

found

R. ENDOH,

?6

T. TANAKA,

M, KURMARA

AND K. fKEDA

to be extremely interesting because of their special chemical structure, pendant cat-boxy&z group in the main repeating unit, which seems to exhibit their consider-

ably good affinity with water. In this paper, the transport

properties of novel asymmetric membrane system based on a class of aromatic polyamides having a carboxylic group are describe3. These aromatic polyamides are characterized by their high degree of permsefectivity to dissotved saits, cunsiderabfy high fevef of wafer permeability, good mechanical strength and chemical stability. EXPERIMENTAL

Aromatic poiyamides were prepared by low temperature solution pofymerization (20, if) from the combinations of aromatic diamines and aromatic acid

derivatives. The aromatic diamine; used were 4, 4’-oxydianiline, p(m)-phenyIenediamine, 5,5’-methyfene bis(Z-aminophenol), 3,Sdiaminobenzoic acid, and the aromatic acid derivatives were iso(tere)-phthaloyl chloride, Pchtoroformylph’thalic anhydride. AI1 the materiafs used were purified either by distillation or recrystallization.

Al1 the membranes were prepared accurding tu &he usual technique; Wns of CG. 1% ,U thickness were cast from solution on a glass plate and after partial evaporation of the solvent, coagufated in water. The casting solution was generally composed of 10-20°/0 polymer, zS-S*/~additives such as LiCI, LiNO, and appropriate organic polar solvents such as ~-methyl-pyr~tidone {NIPS, dimethyfformamide (DMF) and d~methylace~mide (DMAC). The precipitated membranes were Beached out fur about f 5 hours under running water and then subjected to &he testing.

The Abcor Reverse Osmosis Test cell, a product of Abcor, Inc., Cambridge, Mass., was used. The membrane was supported by a sintered porous stainless steel disc covered with filter paper. Continuous agitation just above the membrane surFace was provided by a magnetic stirrer suspended from a Teflon bearing. The tests were run batchwise or continuously under 30-105 kg/cm’ of nitrogen pressure, employing 25~33sooO ppm NaCt sofution. For the calculation of membrane performance, the following eo~ve~tio~~l equations were used;

Rd(%)=

CF - CT x 100 r

_-Z.

0)

37

POt..YM&RSFOR REVERSE OSMOSB MEMBRANES

Flux Cm3,im2 - day) = Q/A

(2)

m = log~~~~*)/l~g

f3)

Wfo)

CF and C, are salt concentration in the feed and the permeate, respectively. Q is a A is an e82cttive membrane surface area. Ft and & are water flux of the membrane at the time t and tar respectively. The m value represents the iugafjrhmic flux deciine rate. water flux ~~r~u~~ tile membrane,

The structure of aramatic p~lyrtmide membmnes has been examined with the aid of the transmission electron microscope. The membranes, after having been carefully dried, were embedded with an acrylate resin, dyed with aqueous phosphoric tungsten acid (PTA) and were sectioned by using an ultramicrotome with a djam~~d knife. Cross sections of approximately So0 A thickness were examined by a Hitachi HU- 12 Transmission Electron Microscope, RESULT?2 AND

DiSCUSSK3N

Seven different kinds of aromatic polyamides were synthesized by low temperature solution ~~~yrn~r~~t~~~ as f&xi in Table 1. These polyamides are divided into four groups according to the combinations of aromatic amine and acid components used. Group

I;

(A-D)

(4)

IT: (AZ-D-AX-D)

(5)

IIf: {B-C)

(6)

IV: (A-C)

(7)

Ai an3 A2 represent

A: aromatic

diamine

component,

B: aromatic

diamine

with a carboxylic

Cr artlmatic

dicarboxylic

acid compcment.

D: aromatic

tr~~arb~xylic

acid component.

the different

ccrmponents.

group.

The polymers of Group ft have a different ~~nt~~~~ed amine ~~rnp~~~~t in the polymer chain as compared with those of Graup 2. The polymers of Group fif have carbaxylic groups in the amine component of polymer recurring unit. Those of group 1V represent the typical aromatic polyamide with no carboxylic gruup in the main chain (3)* AII the polymers listed in Tabie f have a sufRcientiy high molecular weight to prepare mechanic&y resistant membranes. .

R. ENDOH,

38 TABLE

T. TANAKA,

if. KURLZIARA AND

K. IKERA

I

POLYMERSrrtifcruRE

(NHRzNiKORzCO)m

(“NHR~INHCOR~IO)~

To fabricate asymmetric membranes with better and controllable membrane performance, it is quite important to control or optimize various parameters such as (I) the casting solution; pafymer concentration, sorts of additives, additive concentration, s&vent selection, and viscosity, (2) partiaf evaporation conditions, (3) coagulation, and (4) post treatment. The polymer concentration (PC) in the casting solution has a significant effect on the membrane flux and rejection characteristics as shown in Fig. i. The optimum polyamide concentration in casting solution necessary to obtain membranes with significant sait rejection and flux was in the range of IO to 16%. The po~ymer/add~tive/so~ven~ system in a casting s&&ion is also an important factor to prepare good membranes. One of the examples is shown in Table II. The solvent systems of casting solution are divided into two groups. The NMP/DMAc system is better than the DMAc/DMF system in the membrane p~rfo~rnaR~ by comparison of the water flux at the same salt rejection fevei. In both solvent systems, the use of LiNO, as additive in the casting solution generally produces membranes with high salt rejection and low water flux. LiCi alone can produce membranes with higher flux but with salt rejection below the desired level. Consequently, the combination of the two salts was investigated

POLYRtERSFQR REVERSEQSMUStS MEMBRANES

39

Polymer no. 3, PC: f6%

in order to arrive at a suitabie balance that would produce the desired high flux membmne with ca. 950/, salt rejection. ?-his effect of additives on the rn~rnbr~~e flux and rejection charxteristics is in goad ameement with the observation of the fine structure of the membrane cross section discussed below in section (ID). One of the most impo~nt steps in the membrane fabrication is the part&t evaporatkxx step trr control membrane structure and ~e~~~~e ~~~~~~~ in combination with the coagulation step. Other parameters are also optimized in these polyamides. For example, higher co~~~~t~on tern~~t~~ is apt to produce higher fiux membrane with lower salt rejection.

40

R. ENDOH,

T. TANAKA,

M. KWRIEIARA

AND

K. IKEDA

C. Membrane properries Table III shows the performances of the memb~~e obtained from the polymers listed in Table I by using the optimum membrane fabrication conditicms. The membrane performances are divided into two different groups, (HR) and high flux type (HF). It is clearly shown the aromatic polyamides having carboxyfic group

high rejectian type

that membranes prepared from (3, IF, iif)

are higher in water

than that of the pofyamide having no carboxyiic group (IV), in both the HR and the HF type. The higher water flux is probabfy connected with the presence of carboxylic groups in the polymer chain. A large difference in membrane performance is not observed among polymer group f, Ii, and XII, and in general it is better than that of cellulose acetate. Consequentty, the membrane performances flux

are not greatly affected by the structure af the amine component, and also the membrane performance itsetf is not affected by the position of the carboxylic group substitution in the polymer chain. Typical mechanical properties of the asymmetric membranes prepared

from aromatic

polyamides

having

carboxylic

groups show good mechanical strength and elasticity. The tenacity and the initial modulus of the membranes are in the range of 0.8 to 1.3 kg/mm’, and 15 to 20 kg/mm’ with an elongation of W-70 % at break, respectively. The membrane performances of polymer 3, 4 and 6 at different: pressures with various feed concentrations are listed in Table IV. The membrane obtained from polymer 6 keeps good rejection at 30-105 kg/cm’ and 2500-10000 ppm NaCl concentration, and the increase in fiux occurred as the pressure increased. Table V shows the effect of temperature on the membrane performance in

I 2

I

-

99.3

0.46

-

-

I::

97.0 -

1.33 -

IS -

3 4 5 6 7

I II Ii Ifl

99.2 99.4 99.3 99_ f

0.60 0.46 0.62 0.53 0.30

11 17 9 3

97.0 97.6 97.4 97.8 97.0

1.00 1.25 0.87 1.15 0.65

10 20 9 18 4

I

IV

99.i

A: Membrane cmxstant for water permeation (g/cd B: Membrane crlnstant for salt permeation
-s

- atm)

As/B: An index of measuring the Ieve af membrane perkrma~ce @I W,knP - 9 * atrn”)

R. ENDOH, T. TANAKA,

42 TABLE LIFE

3 6 7

TEST

hf. KURIHARA

AND K. IKEDA

VII OF MEMBRANES

I III IV

(30,

98.3 98.2 97.7

40

kg/cm2-25°C)

0.70 0.73 0.40

30 40 40

3500 5000 5000

800 120 120

-0.035 -0.015 -0.I20

-~ Active surface layer

Middle layer

Bottom layer

Fig. 2. Electron micrognphs of thin cross section of membranes. A: Membrane prepared from the 6.5% LiNOa as an additive in casting solution, polymer no. 3 (Group I). Membrane performance: 97.0% Rej.-1.0 mYm* day Flux. 3: Membrane prepared from the6.5% &Cl as an additive in casting solution, polymer no. 3 (Group I). Membrane performance: 87% Rej.-2.0 m3/m2 day Flux. C: Membrane prepared from the 6.5 % LiNOs as an additive in casting solution, polymer no. 6 (Group III). Membrane performance: 99.2% Rej.-0.53 m3/mz day Flux (40 kg/cm?5000 ppm NaCl, 25% pH 7).

POLYMERS

43

FOR REVERSE OSMOSIS MEMBRANES

the case of polymer no, 6. The water flux increases with the increase in temperature, as may he expected from the water viscosity change, whereas the salt rejection slightly decreases. of the pH values of feed solution on the membrane performance was studied in a permeation test as shown in Table VI. Although both the polymer 3 and 6 membranes show a little change in water flux and salt rejection at different pH values of feed solution, the excellent resistance of the membranes in the medium of the pH range 4 to IO has been confirmed. The endurance tests were carried out on membranes prepared from polymers 3, 6 and 7 for 120-800 hr. as shown in Table Vff. It has been revealed by such tests that the polyamides having carboxylic groups (polymers 3 and 6) are better in m value than the polyamide without carboxylic groups (polymer 7). D. Methrane strlictirre The membranes prepared from the polyamides described in this paper exhibit a typical asymmetric structure by transmission electron microscopy as shown in Figs. 2 A, B and C. Figs. 2 A and B illustrate the electron micrographs of the memb~nes prepared from polymer 3. The membranes prepared from LiNO, as an additive gives higher salt rejection and lower flux as compared with those prepared from LiCl as an additive. The membrane performances are in good agreement with the fme structure of the membrane cross sections in both the surface and the bottom layers. Fig. 2 C illustrates that the membrane prepared from polymer 6 gives the finest structure in ca. I .5 it of the surface layer which corresponds to the highest salt rejection among the three membranes shown in Figs. 2 A, B and C. CONCLUSlON

Aromatic polyamides containing carboxylic groups are a unique class of polymer exhibiting better properties than the aromatic polyamides with no carboxylic group, and cellulose acetate as the polymeric materials for reverse osmosis, The high water flux, which is one of the main properties of the membranes prepared from such polymers, is probably due to the nature of the carboxylic groups easily forming hydrogen bonds with water. The fabrication conditions are quite important to obtain controlled membrane performance for sea water and brackish water desalination. The membranes prepared from the polymers examined exhibit good durability at high pressure, high concentration and various pH values of the feed solution. ACKNOWLEDGEMENTS

The

authors

gratefully

acknowledge

the continuous

support

and interest

R. JZNDOH, T. TANAKA, hl. KURIHARA AND K. 1-A

44

of Dr. Y. Ito, the Managing Director, Mr. T. Kato, the Director of the Research and Development Div., Mr. M. Uchida, the General Manager of Engineering Research Laboratories, and Dr. S. Inoue, Assistant General Manager of Plastics Research

Laboratories,

of Toray

Ind. Inc. Special

thanks

are extended

to Dr. M.

out the electron microscopy and constructive discussions throughout this work. Finally, thanks are given to Messrs. S. Tokizane, T. Watanabe and coworkers for their constant assistance.

Tanimwa

for carrying

REFERENCE5 1. 2. 3. 4. 5. 6. 7. 8. 9. IO. 11.

S. LOEB AND S. SMJIUFUJAN, UCLA Report No. 60-62, 1961. H. K. LONSDALE, Desdinufiun. 13 (1973) 317. J. W. RICHTER AND H. H. HOEHN. U.S. Potent 3.567.632. R. MCKINNEY JR. AND 1. H. RHODES, Mucromdecuks, 4 (1971) 633. L. CREDALI. A. CHIOLLE AND P. PARRINI, Desdinution, 14 (1974) 137. A. B. LACONTI, P. J. CHLUDZINSKI AND A. P. FICKEIT, Reverse Osmosis Membrane Research, Edited by H. K. LONSDALE and H. E. PODAU, Plenum Press, New York N.Y., 1972, 263. R. CHAPURLAT, Proc. Fourth Intern. Symp. on Fresh Wafer from the Sea, 4 (1973) 83. 1. E. CADOTTEAND L. T. ROZELLE.Ofice of Saline Water, i&s. Devel. Progr. Rept. No. 927, 1972. M. A. FROMMER. J. S. MURDAY AND R. V. h4-, firopean Polymer J., 9 (1973) 367. N. YODA, K. IKEDA, M. KURIHARA, S. TOHYAMA AND R. NAKANISHI. J. Polymer Sci.. Pt. A-Z, 5 (1967) 2359. N. DOKOSHI, S. TOHYAMA. S. FUJITA, M. KIIRIHARA AND N. YODA, J. Polymer Sci., PI. A-l, 8 (1970) 2197.