Perfluorinated Ion Exchange Membranes - ACS Publications

16

Perfluorinated Ion Exchange Membranes

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 13, 2015 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch016

TOSHIKATSU SATA and YASUHARU ONOUE Research and Development Division, Tokuyama Soda Co., Limited, Mikage-cho 1-1, Tokuyama City, 745 Yamaguchi Prefecture, Japan

The requirements f o r ion-exchange membranes i n membrane cell c a u s t i c - c h l o r i n e process are : high p e r m s e l e c t i v i t y , low e l e c t r i c r e s i s t a n c e , e x c e l l e n t chemical r e s i s t a n c e to oxidants and alkali, good heat r e s i s t a n c e , low d i f f u s i o n of s a l t and low p e r m e a b i l i t y to water and good mechanical p r o p e r t i e s . Chemical r e s i s t a n c e to oxidants i n p a r t i c u l a r could not be achieved in conventional hydrocarbon type ion-exchange membranes, although v a r i o u s e l e c t r o l y t i c methods were t r i e d to prevent ion-exchange membranes of that type from d e t e r i o r a t i n g by oxidants. In 1972, the difficulty i n chemical resistivity was overcome by perfluorocarbon ion-exchange membrane made by du Pont de Nemours and Co. (1). but t h i s membrane was still too low i n p e r m s e l e c t i v i t y while its electric r e s i s t a n c e was sufficiently low. The low e l e c t r i c r e s i s t a n c e and high permselectivity are f a c t o r s g e n e r a l l y forced to be mutually i n c o n s i s t e n t . However, the use of an a n i s o t r o p i c s t r u c t u r e f o r ion-exchange membranes enables both of these requirements to be achieved together (2,3). A n i s o t r o p i c ion-exchange membranes like reverse osmosis membrane are w e l l known, i . e . , monovalent c a t i o n and anion perms e l e c t i v e ion-exchange membranes in e l e c t r o d i a l y t i c c o n c e n t r a t i o n of sea water to make e d i b l e s a l t (4,5). In the case of ion-exchange membranes f o r the c a u s t i c - c h l o r i n e process, concentration of f i x e d ions i n the membrane should be kept high to prevent permeat i o n of hydroxide ions through the membrane. In general, however, a membrane which has a high c o n c e n t r a t i o n of f i x e d ions throughout a l s o shows high electric r e s i s t a n c e . Therefore, it is d e s i r a b l e that the ion-exchange membrane is composed of a t h i n l a y e r having high concentration of f i x e d ions and a t h i c k l a y e r of low e l e c t r i c resistance. Various methods can be used to achieve t h i s purpose, such as to d i f f e r e n t i a t e i o n exchange c a p a c i t y of s t r o n g l y a c i d i c ion-exchange groups along the c r o s s - s e c t i o n of the membrane, or to stratify weakly a c i d i c ion-exchange groups over the surface of an ion-exchange membrane which has s t r o n g l y a c i d i c i o n exchange groups. Various attempts were made by us to reduce the i o n exchange capacity of perfluorocarbon s u l f o n i c a c i d type membrane, i.e., decomposition or i n a c t i v a t i o n of i o n exchange groups by chemical

0097-6156/82/0180-0411$05.00/0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


412

PERFLUORINATED IONOMER

M E M B R A N E S

r e a c t i o n impregnation of hydrophobic m a t e r i a l s on the membrane surface and so on (6). However i t was found that these methods could not achieve adequate performance i n terms of e l e c t r i c a l r e s i s t a n c e and sodium i o n p e r m s e l e c t i v i t y . Therefore f u r t h e r e f f o r t s were d i r e c t e d to prepare an a n i s o t r o p i c membrane using some weakly a c i d i c c a t i o n exchange groups. We s e l e c t e d c a r b o x y l i c a c i d groups as the main component of weakly a c i d i c c a t i o n exchange groups, r a t h e r than s u l f o n i c a c i d amide w i t h a d i s s o c i a b l e hydrogen, phosphonic a c i d , phenolic hydroxide or p e r f l u o r o - t e r t - a l c o h o l exchange s i t e s , from the viewpoint o f ease of p r e p a r a t i o n , s t a b i l i t y and good performance i n e l e c t r o lysis , NEOSEPTA-F and i t s P r e p a r a t i o n Methods The chemical s t r u c t u r e o f NEOSEPTA-F made by us i s b a s i c a l l y as f o l l o w s : —(r C F - C F 2 2 1

CF -CF 2 CF CF CF

—f-CF -CF-3 ό CF CF CF 9

m

ό

( CF ) I 2 ρ S0 Na 3

2

n

( CF ) ι 2 q COONa

l/( m + η ) = 6 - 8 m/n = 5 - 2 0 p,q = 1 - 2 Although perfluorocarbon s u l f o n i c a c i d groups are very s t a b l e chemically as w e l l as t h e r m a l l y , perfluorocarbon s u l f o n y l h a l i d e , e s p e c i a l l y s u l f o n y l c h l o r i d e groups, are q u i t e r e a c t i v e . For example, s u l f o n y l c h l o r i d e groups react w i t h o x i d a n t s , reductants, various amines, phenol compounds, i o d i n e compounds, e t c . and give c a r b o x y l i c a c i d , s u l f i n i c a c i d , s u l f o n i c a c i d amide, -CF2I and so f o r t h . Some examples of how t h i s feature can be used to generate various kinds of membranes w i l l next be described 1) (7) A membranous m a t e r i a l having a thickness of 0.2 mm and composed of a copolymer of t e t r a f l u o r o e t h y l e n e ( monomer A ) and p e r f l u o r o ( 3,6-dioxa-4-methyl-7-octene-sulfonyl f l u o r i d e ) Γ monomer Β ) i n a mole r a t i o of about 7 : 1 , which had an i o n exchange c a p a c i t y upon h y d r o l y s i s of 0.91 m i l l i e q u i v a l e n t / g r a m of dry membrane ( meq./g.dry membrane of H-form ) was hydrolyzed an aqueous s o l u t i o n o f dimethyl s u l f o x i d e and potassium hydroxide to a f f o r d an ion-exchange membrane having sodium s u l f o n a t e groups. Sulfonate groups of the membrane was converted t o s u l f o n i c a c i d form completely by n i t r i c a c i d . The membrane was then d r i e d , and reacted a t 130 C f o r 75 hours i n a bath c o n s i s t i n g o f phosph orous p e n t a c h l o r i d e and phosphorous o x y c h l o r i d e . A f t e r the reac t i o n , the product was washed w i t h carbon t e t r a c h l o r i d e and d r i e d .

In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


16.

SATA A N D ONOUE

Perfluorinated Ion

Exchange

413

To examine the r e s u l t i n g membrane, the r e f l e c t i v e i n f r a r e d spectrum of t h i s membrane^was measured. I t was found that the ab s o r p t i o n band a t 1060 cm observed i n the s u l f o n i c acid-type mem brane disappeared, and a strong absorption band corresponding to the s u l f o n y l c h l o r i d e group was observed a t 580 and 1420 cm The membrane having s u l f o n y l c h l o r i d e groups and the membrane having s u l f o n i c a c i d groups were dipped i n η-butyl a l c o h o l . A i r was introduced i n the medium, and a p p l i e d uniformly t o the surface of the membrane f o r o x i d a t i o n r e a c t i o n ( at 110 C f o r 3 hours ) . The membrane were then washed w i t h methanol and water, and d r i e d . To examine t h e i r surface s t r u c t u r e s , the r e f l e c t i v e i n f r a r e d spec trum of the t r e a t e d membranes were measured. No a p p r e c i a b l e d i f f erence was seen i n the s u l f o n i c acid-type membrane before and a f t e r the treatment. I n the s u l f o n y l c h l o r i d e - t y p e membrane, the a b s o r p t i o n band a t 580 and 1420 cm" a s c r i b a b l e to s u l f o n y l c h l o r i d e t o t a l l y disappeared, and a new a b s o r p t i o n band a t 1790 cm was observed. This a b s o r p t i o n band i s assigned t o c a r b o x y l i c a c i d group. These membranes were t r e a t e d f o r h y d r o l y s i s w i t h methanol s o l u t i o n c o n t a i n i n g 10 % of sodium hydroxide ( f o r 16 hours a t 60 C ) , washed w i t h water, and d r i e d . The r e f l e c t i v e i n f r a r e d spec trum of the t r e a t e d membranes were measured. The a b s o r p t i o n band at 1790 cm disappeared which had been observed on s u l f o n y l c h l o r i d e - t y p e membrane. Instead, a new absorption band was observed at 1680 cm" . These membranes were each dipped i n a dye s o l u t i o n of 1 % c r y s t a l v i o l e t and 10 % ethanol i n a 0.5 N HC1 aqueous s o l u t i o n . Then the membranes were washed w i t h water and cut by a microtome, A microscopic examination i n d i c a t e d that the membrane d e r i v e d from s u l f o n i c a c i d type membrane was uniformly dyed deep green through out, whereas the membrane derived from s u l f o n y l c h l o r i d e - t y p e membrane was dyed deep green only i n i t s inner p a r t l e a v i n g the outer l a y e r s of 20 JUL each from i t s both surfaces f r e e from dyeing. This dyeing t e s t shows that i n the s u l f o n y l c h l o r i d e type membrane, the o x i d a t i o n treatment has converted the s u l f o n y l c h l o r i d e groups at the outer l a y e r s i n t o carboxyl groups to the extent of 20 jX from r e s p e c t i v e s u r f a c e s . The p r o p e r t i e s of the membrane hydrolyzed w i t h 10 % sodium hyroxide were measured, and a saturated sodium c h l o r i d e s o l u t i o n was e l e c t r o l y z e d using t h i s membrane. The r e s u l t s are given i n Table I . For comparison, the s u l f o n y l c h l o r i d e - t y p e membrane was t r e a t e d i n η-butyl a l c o h o l at 110 °C f o r 3 hours without i n t r o d u c i n g a i r . The t r e a t e d membrane was subjected t o h y d r o l y s i s t r e a t ment i n a methanol s o l u t i o n c o n t a i n i n g 10 % of sodium hydroxide. E l e c t r i c r e s i s t a n c e of the membrane was 450 Q —cm , and the c u r r ent e f f i c i e n c y could not be measured. E l e c t r i c r e s i s t a n c e was measured on the membrane which was placed p a r t i t i o n i n g 3.5 Ν NaCl s o l u t i o n to i t s one s i d e and 6.0 Ν NaOH s o l u t i o n to the other s i d e at 85 °C and the s o l u t i o n s were e l e c t r o l y z e d . The e l e c t r o l y s i s was c a r r i e d out by using a s a t u rated s o l u t i o n of sodium c h l o r i d e as an a n o l y t e , a t i t a n i u m l a t h 1

1

1

2



414

M E M B R A N E S

Table I

Membrane having sulfonic acid groups (blank)

Properties

Electric

Resistance - cm ) Ion Exchange Capacity ( Meq./g.dry membrane of H-form )


( Ω

2

Water Content (%) Catholyte Concn. ( Ν ) Current E f f i c i e n c y (%) NaCl i n Catholyte (ppm) ( as 48 % NaOH )

Membrane w i t h oxidation reaction

1.95

2.25

0.91

0.85

17 7.50 49

12 7.50 93 18

238

m a t e r i a l coated w i t h ruthenium oxide and t i t a n i u m oxide as an anode, a mesh-like m i l d i r o n as a cathode. Water was added t o the cathode compartment, and aqueous s o l u t i o n of sodium hydroxide was obtained i n a c e r t a i n c o n c e n t r a t i o n . The current d e n s i t y was 30 A/dm and the temperature o f the e l e c t r o l y t i c s o l u t i o n was 80 to 90°C. 2) (8) The same membranous m a t e r i a l as mentioned i n 1) having a thickness of 100 JJL was s e t i n a r e a c t o r of the design which would a l l o w only one surface o f the membrane to contact w i t h r e a c t i o n reagents. T h e r e a f t e r , the r e a c t o r compartment was f i l l e d w i t h vapour of phosphorous p e n t a c h l o r i d e ( at 170°C f o r an hour ) to have one surface o f the membrane reacted. The r e f l e c t i v e i n f r a r e d spectrum and dyeing t e s t r e s p e c t i v e l y showed that the mem brane had s u l f o n y l c h l o r i d e groups and that approximately 5 Jul o f non-dyed l a y e r was s t r a t i f i e d at the membrane surface where phosphorous p e n t a c h l o r i d e had contacted. The e l e c t r i c r e s i s t a n c e of t h i s membrane was about 1500 β - cm2 i n a 1.0 Ν h y d r o c h l o r i c a c i d s o l u t i o n at 25°C when measured by 1000 c y c l e A.C. The e l e c t r i c r e s i s t a n c e o f the same membrane before the r e a c t i o n w i t h phosphorous p e n t a c h l o r i d e was only 0.38 Λ - cm under the same conditions. The membrane which had a t h i n l a y e r of s u l f o n y l c h l o r i d e groups was t r e a t e d by t r i e t h y l a m i n e a t room temperature f o r 16 hours, washed w i t h water and then heated a t 170°C. T h e r e a f t e r , the membrane was a l s o immersed i n the same mixed s o l u t i o n composed of water, dimethyl s u l f o x i d e and potassium hydroxide as mentioned before. E l e c t r i c r e s i s t a n c e o f the membrane was 1.5S2- cm when measured i n the environment of 3.5 Ν sodium c h l o r i d e s o l u t i o n t o 2

2

2


16.

Perfluorinated Ion

SATA A N D ONOUE

415

Exchange

i t s one s i d e and 6,0 Ν sodium hydroxide s o l u t i o n t o the P C I 5 reacted s i d e of the membrane a t 85 °C, The measurement i n 3,5 Ν sodium c h l o r i d e s o l u t i o n of pH 0,5 ( adjusted by h y d r o c h l o r i c a c i d ) showed the e l e c t r i c r e s i s t a n c e ο f 432Q- cm a t 25.0 C, On the other hand, the e l e c t r i c r e s i s t a n c e of the membrane before the r e a c t i o n w i t h phosphorous p e n t a c h l o r i d e and t r i e t h y l a m i n e was 1.1Q- cm and 1,0Î2- cm r e s p e c t i v e l y when measured i n the environment of 3.5 Ν sodium c h l o r i d e s o l u t i o n to i t s one s i d e and 6.0 Ν sodium hydroxide s o l u t i o n to the other s i d e , and i n the environment o f 3.5 Ν sodium c h l o r i d e s o l u t i o n of pH 0,5, Accord ing to r e f l e c t i v e i n f r a r e d spectrum, the a b s o r p t i o n bands observed were d i f f e r e n t between the surfaces reacted w i t h phosphorous p e n t a c h l o r i d e and non-reacted. Namely the a b s o r p t i o n band at 1680cm"" corresponding to c a r b o x y l groups was observed, and the a b s o r p t i o n band a t lOoOcnT observed i n the s u l f o n i c a c i d type membrane disappeared on the s u r f a c e which had been reacted w i t h phosphorous p e n t a c h l o r i d e . Using the t r e a t e d membrane e l e c t r o l y s i s of sodium c h l o r i d e s o l u t i o n was c a r r i e d out under the same e l e c t r o l y s i s c o n d i t i o n s as 1). The t r e a t e d surface of the membrane was faced to the cathode s i d e i n the e l e c t r o l y z e r , When 6.5 Ν sodium hydroxide s o l u t i o n was obtained as c a t h o l y t e , the current e f f i c i e n c y was 93% and the c e l l v o l t a g e was 3,85v. On the other hand, the i o n exchange membrane not t r e a t e d by phosphorous p e n t a c h l o r i d e and t r i e t h y lamine showed the current e f f i c i e n c y of 52% and the c e l l v o l t a g e of 3.68v when 6.5 Ν sodium hydroxide was obtained as c a t h o l y t e . 3) (9) S i m i l a r membranous copolymer as mentioned i n 1) hav ing a thickness of 150 ju ( The i o n exchange c a p a c i t y of t h i s mem brane was 0,83 meq,/q.dry membrane of H-form. ) was s e t i n a hor i z o n t a l r e a c t o r o f the design which would a l l o w only one s u r f a c e of the membrane to contact w i t h r e a c t i o n reagents. Then f i n e c r y s t a l powder of phosphorous p e n t a c h l o r i d e was uniformly placed to cover one s u r f a c e of the membrane and heated ( at 155°C f o r 40 min. ) The membrane having the s u l f o n y l c h l o r i d e groups was t r e a t e d by an aqueous η-butyl amine s o l u t i o n f o r 2 hours, washed w i t h water, heated i n a i r f o r 24 hours at 90 C and then dipped i n 10% methanol s o l u t i o n of sodium hydroxide. The r e f l e c t i v e i n f r a r e d spectrum showed that a b s o r p t i o n band a s c r i b a b l e to the s u l f o n y l c h l o r i d e disappeared and new absorption bands appeared a t 1620, 1680 and 3400 cm"" , The e l e c t r i c r e s i s t a n c e and the e l e c t r o l y s i s r e s u l t s of both o f the t r e a t e d and untreated membranes are shown i n Table I I r e s p e c t i v e l y . 4) (10) The membranous copolymer as mentioned before r e i n forced by a p l a i n woven c l o t h of p o l y t e t r a f l u o r o e t h y l e n e was reacted w i t h vapour of phosphorous p e n t a c h l o r i d e to form a memb rane having s u l f o n y l c h l o r i d e groups on i t s only one s i d e . The membrane having s u l f o n y l c h l o r i d e groups on i t s only one s u r f a c e was t r e a t e d by 29 % aqueous ammonia s o l u t i o n f o r 30 min. at 25 C. A f t e r the ammonia treatment, the a b s o r p t i o n bands a s c r i b a b l e to 2


2

2

1

1

e

e

1


e


416

M E M B R A N E S

Table I I

Properties

Untreated Membrane

E l e c t r i c Resistance i n 6.0 Ν NaOH (£2-cm ) E l e c t r i c Resistance i n 1 N HC1 -cm ) Catholyte Concn. (N) Current E f f i c i e n c y (%) NaCl i n Catholyte (ppm) ( as 50 % NaOH ) C e l l Voltage (v) 2


2

3,, 6 0..43 6..5 58 143 3,.55

Treated Membrane 3..8 125 6,.5 94 12 3,.65

s u l f o n y l c h l o r i d e disappeared and new absorption bands were obser ved at 940, 1010, 1410, 1680 and 3400 cm~l. The absorption bands of 940 and 1010 ciir"! disappeared a f t e r the membrane was heated i n oxygen atmosphere. T h e r e a f t e r , the membrane was t r e a t e d by n i t r i c a c i d . The absorption band o f 1680 cm""l disappeared and a new ab s o r p t i o n band was observed. I t was deduced that s u l f o n i c a c i d amide groups and c a r b o x y l i c a c i d groups had been introduced by these treatments. In e l e c t r o l y s i s o f sodium c h l o r i d e s o l u t i o n under the same c o n d i t i o n s as mentioned before, 8,0 Ν sodium hydroxide was obtained as c a t h o l y t e at the current e f f i c i e n c y o f 95 % and the c e l l v o l t a g e of 4.1 V. 5) (11) S u l f o n i c a c i d groups of c a t i o n exchange membrane, Nafion 110 ( trademark f o r products o f Ε. I . du Pont de Nemours & Co. ) were converted to s u l f o n y l c h l o r i d e by a mixture of phos phorus p e n t a c h l o r i d e and phosphorus o x y c h l o r i d e . The r e s u l t i n g membrane having s u l f o n y l c h l o r i d e groups was s e t i n an o x i d a t i o n device which allows a uniform a i r c i r c u l a t i o n . A i r saturated with n-butanol vapor was introduced i n t o the o x i d a t i o n device at 110 °C to allow r e a c t i o n on one s i d e only. I t was found that i n the spectrum of the t r e a t e d s u r f a c e , the absorption band a s c r i b a b l e to the s u l f o n y l c h l o r i d e groups disappeared? and a new absorption band a s c r i b a b l e to the c a r b o x y l i c a c i d groups appeared at 1790 cnr-l. In the spectrum o f the other s u r f a c e , the absorption band of s u l f o n y l c h l o r i d e groups remained l i k e w i s e as before the t r e a t ment, and no absorption of the carboxyl a c i d groups was observed. As shown i n the above, s u l f o n i c a c i d groups of p e r f l u o r o c a r bon polymer can be e a s i l y changed to weakly a c i d i c c a t i o n exchange groups. There are various other methods to change s u l f o n i c a c i d groups to weakly a c i d i c c a t i o n exchange groups, i . e , , c o n t a c t i n g the membrane having s u l f o n y l c h l o r i d e groups with aromatic comp ounds with phenolic hyroxide groups, various amines, ammonium ions and so on (12),


16.

SATA A N D O N O U E

Perfluorinated Ion

Exchange

417

When these b i l a y e r or m u l t i l a y e r ion-exchange membranes were used i n the e l e c t r o l y s i s of a l k a l i metal s a l t s o l u t i o n , perform ance of the e l e c t r o l y s i s was e x c e l l e n t . S u l f o n i c a c i d groups and c a r b o x y l i c a c i d groups were s e l e c t e d f o r NEOSEPTA-F as the main i o n exchange groups.


General P r o p e r t i e s The NEOSEPTA-F membrane p r o p e r t i e s examined are mainly of those r e l a t i n g to the e l e c t r o l y s i s of sodium c h l o r i d e s o l u t i o n . Table I I I shows c h a r a c t e r i s t i c s of t y p i c a l grades of NEOSEPTA-F. These membranes are c h e m i c a l l y s t a b l e , i . e . , against a c i d , base, oxidants and reductants because the membranes have p e r f l u o r o c a r b o n backbone. And a l s o the membranes have strong mechanical s t r e n g t h because of reinforcement w i t h the f a b r i c of p o l y t e t r a f l u o r o e t h y lene. Used i n the e l e c t r o l y s i s of sodium c h l o r i d e s o l u t i o n , no d e t e r i o r a t i o n of performance or mechanical s t r e n g t h was observed i n continuous s e r v i c e f o r 2 years under appropriate e l e c t r o l y s i s c o n d i t i o n s . NEOSEPTA-F membranes are always improved to get b e t t e r performance i n the e l e c t r o l y s i s and v a r i o u s grades which show b e t t e r performance are developed. E l e c t r i c Resistance of the Membranes. F i g u r e 1 shows the r e l a t i o n s h i p between the e l e c t r i c r e s i s t a n c e of NEOSEPTA-F C-1000 and pH value of 3.5 Ν sodium c h l o r i d e s o l u t i o n ( pH was adjusted by adding h y d r o c h l o r i c a c i d ). The measurements were c a r r i e d out at 25.0 C using 1000 c y c l e A.C. NEOSEPTA-F C-2000 a l s o shows the s i m i l a r r e l a t i o n s h i p between the e l e c t r i c r e s i s t a n c e and pH value of sodium c h l o r i d e s o l u t i o n . I t i s recognized that these NEOSE PTA-F ion-exchange membranes have weakly a c i d i c c a t i o n exchange groups which are d i s s o c i a b l e i n the range between pH 2 and 3. G e n e r a l l y , when an ion-exchange membrane contacts w i t h h i g h l y concentrated s o l u t i o n , i t s h r i n k s and then the e l e c t r i c r e s i s t a n c e increases remarkably. These phenomena were observed i n the case of NEOSEPTA-F a l s o . Figure 2 shows the e l e c t r i c r e s i s t a n c e of the membrane measured w i t h 1000 c y c l e A.C. at 80 C i n sodium hydroxide s o l u t i o n of v a r i o u s c o n c e n t r a t i o n s . F i g u r e 3 shows the e l e c t r i c r e s i s t a n c e of the membrane measured w i t h d i r e c t current under the same c o n d i t i o n s as the e l e c t r o l y s i s was to be c a r r i e d out. ( The membrane was placed p a r t i t i o n i n g 3.5 Ν sodium c h l o r i d e s o l u t i o n to one s i d e and sodium hydroxide s o l u t i o n of v a r i e d c o n c e n t r a t i o n to the other s i d e and the d i r e c t current was passed at the current d e n s i t y of 30 A/cm . ) The e l e c t r i c r e s i s t a n c e measured w i t h d i r e c t current was c o n s i d e r a b l y lower than that measured w i t h a l t e r n a t i n g c u r r e n t , w h i l e the d i f f e r e n c e should normally be very minute i f measured under e x a c t l y the same c o n d i t i o n s . The above s i g n i f i cant d i f f e r e n c e i s seemingly a t t r i b u t a b l e to that i n case of d i r e c t current measurement anolyte i s sodium c h l o r i d e s o l u t i o n kept at constant c o n c e n t r a t i o n of 3.5 N, which would increase water content of the membrane due to h i g h l y hydrated sodium ions passing through, thus lowering the e l e c t r i c r e s i s t a n c e .



418

M E M B R A N E S


Table I I I C h a r a c t e r i s t i c s o f NEOSEPTA-F C-1000 and C-2000

NEOSEPTA-F C-1000

Name

NEOSEPTA-F C-2000

Backing

Polytetrafluoro ethylene f a b r i c

Polytetrafluoro ethylene f a b r i c

Resin

Perfluorocarbon

Perfluorocarbon

Main Ion Exchange Groups Ion Exchange Capacity E l e c t r i c Resistance** (Λ-cm Water Content T e n s i l e Strength**** ( Kg/cm ) 2

-SO Na -COONa

-SO Na -COONa

0.83

0.91

2.0

1.7

10.3

11.7

10.6

10.6

*. Meq./g. dry r e s i n of Η -form. **. Measured by e l e c t r o l y s i s o f 3.5 Ν NaCl | 6,0 Ν NaOH ( C-1000 ) and 3.5 Ν NaCl I 9.0 Ν NaOH ( C-2000 ) a t the current d e n s i t y of 30 A/dm a t 80 °C. ***. g.H20/ g. d r y r e s i n of Η -form. Measured i n atmosphere a f t e r the membrane was e q u i l i b r a t e d w i t h 6.0 Ν NaOH ( C-1000 ) and 9.0 Ν NaOH ( C-2000 ) a t room temperature. ****. Both wet and d r y . 2



16.


Perfluorinaîed Ion Exchange

419

™ 10

Q L .

,

,

,

,

,

,

,

,

,

2 U 6 8 10 12 Concentration of NaOH ( Ν ) Figure 2. Relationship between electric resistance of NEOSEPTA-F and concen tration of NaOH. Key: O , NEOSEPTA-F C-1000; φ , NEOSEPTA-F C-2000. Measured by 1000 cycle alternating current at 80° C.




420

M E M B R A N E S

2.5h

GJ

6 7 8 9 10 Concentration of NaOH in Catholyte ( Ν ) Figure 3. Relationship between electric resistance of NEOSEPTA-F measured by direct current and concentration of catholyte. Key: Q, NEOSEPTA-F C-1000; (J), NEOSEPTA-F C-2000. Measurements were made with direct current under the same conditions as the electrolysis of NaCl solution was to be carried out.



16.

Perfluorinated Ion Exchange


0

2

4 6 8 10 12 Concentration of NaOH ( N )

All

14

F/gare 4. Change of H 0 content of NEOSEPTA-F C-1000 with concentration of NaOH solution. The membrane was immersed in NaOH solution of various concentration after boiling for 1 h in pure H 0. Measurement was made at 20° C after the membrane had been immersed in NaOH solution of various concentration for 4 days at room temperature. 2

2



422


M E M B R A N E S

Figure 5. Relationship of current efficiency and cell voltage to NaOH concentration of catholyte. Key: Q, NEOSEPTA-F C-1000;fl),NEOSEPTA-F C-2000. Current density is 20 A/dm . 2




Per fluorinated Ion Exchange

423

Figure 6. Relationship between NaCl in caustic product and concentration of NaOH in catholyte. Key: Q, NEOSEPTA-F C-1000; φ , NEOSEPTA-F C-2000.


424


M E M B R A N E S


F i g u r e 4 shows the water content o f the membrane correspond ing to v a r i o u s concentrations o f sodium hydroxide. The extent o f decrease o f the water content w i t h i n c r e a s e o f c o n c e n t r a t i o n o f the e x t e r n a l s o l u t i o n i s remarkable i n comparison w i t h the case o f c r o s s l i n k e d hydrocarbon type c a t i o n exchange membranes, Performance o f NEOSEPTA-F i n Sodium C h l o r i d e S o l u t i o n E l e c t rolysis. F i g u r e 5 shows the r e l a t i o n s h i p of the c e l l v o l t a g e and the current e f f i c i e n c y r e s p e c t i v e l y w i t h the c o n c e n t r a t i o n o f sodium hydroxide i n c a t h o l y t e when e l e c t r o l y s i s o f sodium c h l o r i d e s o l u t i o n was c a r r i e d out a t the current d e n s i t y o f 30 A/cm . From the economical viewpoint, i . e . , the e l e c t r o l y s i s power c o s t , de p r e c i a t i o n o f equipment c o s t , membrane cost and so on, the optimum c o n c e n t r a t i o n o f sodium hydroxide f o r NEOSEPTA-F C-1000 i s about 20 % and that f o r NEOSEPTA-F C-2000 i s about 27 %. In the case o f NEOSEPTA-F C-2000, the current e f f i c i e n c y i n creases w i t h i n c r e a s e o f sodium hydroxide c o n c e n t r a t i o n i n catho lyte. I t i s thought that the water i n the membrane surface por t i o n o f cathode s i d e i s dehydrated and the c o n c e n t r a t i o n o f f i x e d i o n i n the membrane i n c r e a s e s . The presumption that the cathode s i d e o f the membrane surface would s h r i n k w i t h the i n c r e a s e o f sodium hydroxide c o n c e n t r a t i o n i s o b v i o u s l y proved i n the r e l a t i o n ship between the sodium hydroxide c o n c e n t r a t i o n i n c a t h o l y t e and sodium c h l o r i d e c o n c e n t r a t i o n i n the product ( F i g u r e 6 ) , The d i f f u s e d amount o f sodium c h l o r i d e decreased remarkably w i t h i n crease o f sodium hydroxide c o n c e n t r a t i o n . Conclusion NEOSEPTA-F i s one o f the perfluorocarbon i o n exchange memb ranes f o r C h l o r - A l k a l i e l e c t r o l y t i c process. I t i s considered t o be o f an i d e a l membrane s t r u c t u r e which i s a n i s o t r o p i c a l l y com posed o f s u l f o n i c a c i d groups and weakly a c i d i c groups as i o n exchange groups. S u l f o n i c a c i d groups give high conductance t o the membrane because o f the high water content. And a t h i n l a y e r of c a r b o x y l i c a c i d groups i s a b a r r i e r f o r leakage o f hydroxide ions. Literature Cited 1. Grot, W. G. ; Munn, G. E. ; Walmsley, P. N. " P e r f l u o r i n a t e d Ion Exchange Membranes", presented at the 141 s t N a t i o n a l Meet ing , the E l e c t r o c h e m i c a l S o c i e t y , Houston, Texas, May ( 1972 ), 2. Sata, T. ; Murakami, S. ; Murata, Y. Japan. Pat. A p p l i c a t i o n P u b l i c a t i o n No. 14595/1979, U.S.Pat. 4166014. 3. Sata, Τ. , Murakami, S. , Murata, Y. Japan. Pat. A p p l i c a t i o n P u b l i c a t i o n No. 14596/1979, U.S.Pat. 4169023, B r i t . P a t . 1493164, Ger. Pat. 2504622. 4. Sata, T. J . C o l l o i d I n t e r f a c e S c i . 1973, 44 393 ; Sata, T.;



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Izuo, R. C o l l o i d and I n t e r f a c e S c i . 1978, 256, 757. ; Sata, T. ; Yamane, R. ; M i z u t a n i , Y. J . Polymer S c i . P o l y mer Chem. 1979, 17 2071, e t c . 5. M i z u t a n i , Y. ; Yamane, R. ; Sata, T. Japan. Pat. A p p l i c a t i o n P u b l i c a t i o n No. 23607/1971, U.S.Pat. 3510417, 3510418, B r i t . Pat. 1238656, ; M i z u t a n i , Y. ; Yamane, R. ; Sata, T. ; Izuo, R., Japan. Pat. A p p l i c a t i o n P u b l i c a t i o n No. 3801/1972, 3802/1972, U.S.Pat. 3647086, B r i t . Pat. 1251550, e t c . 6. Sata, T. ; Nakahara, A. ; Murata, Y. ; I t o , J. Japan. Pat. Open P u b l i c a t i o n No. 26284/1978, 26285/1978, 26286/1978, 138489/1977, ; Sata, T. ; Nakahara, A. ; Murata, Y. ; I t o , J . ; Shirouzu, M. Japan. Pat. Open P u b l i c a t i o n No. 55383/ 1978, 58493/1978. 7. Onoue, Y. ; Sata, T. ; Nakahara, A. ; I t o , J. Japan. Pat. Open P u b l i c a t i o n No. 132069/1978, U.S.Pat. 4200711 ( 1980 ). 8. Sata, T. ; Nakahara, A. ; I t o , J . ; Shirouzu, M. Japan. Pat. Open P u b l i c a t i o n No. 64090/1979. 9. Sata, T. ; Nakahara, A. ; I t o , J. ; Shirouzu, M. Japan. Pat. Open p u b l i c a t i o n No. 41287/1979. 10. Sata, T. ; Nakahara, A. ; I t o , J. ; Shirouzu, M. Japan. Pat. Open P u b l i c a t i o n No. 21478/1979. 11. Onoue, Y. ; Sata, T. ; Nakahara, A. ; I t o , J . Japan. Pat. Open P u b l i c a t i o n No. 83982/1979. 12. Sata, T. ; Nakahara, A. ; I t o , J . Japan. Pat. Open P u b l i c a t i o n No. 125974/1978, 137888/1978, ; Sata, T. ; Nakahara, A. ; I t o , J . ; Shirouzu, M. Japan. Pat. Open P u b l i c a t i o n No. 20981/1979. R E C E I V E D August 26, 1981.


Perfluorinated Ion Exchange Membranes - ACS Publications

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