Perfluorocarboxylate Polymer Membranes - American Chemical Society

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Perfluorocarboxylate Polymer Membranes

HIROSHI UKIHASHI and MASAAKI Y A M A B E Research Laboratory, Asahi Glass Company, Limited, Yokohama, Japan

A new process using i o n exchange membranes has r e c e n t l y g a i n ed wide acceptance in the c h l o r - a l k a l i i n d u s t r y from the viewpoint of energy saving and environmental c o n t r o l . One of the important breakthroughs f o r t h i s process was to develop a membrane of high performance. A novel p e r f l u o r o c a r b o x y l a t e membrane named Flemion was de veloped by Asahi Glass Co., L t d . in 1975, r e s u l t i n g from its long experience both with manufacture of v a r i o u s fluorochemicals and with e l e c t r o d i a l y s i s using membranes. Flemion is q u i t e d i f f e r e n t from p r i o r membranes in that it i s based on s p e c i f i c p e r f l u o r i n a t e d copolymers with pendant c a r b o x y l i c a c i d as a f u n c t i o n a l group. The i n t r o d u c t i o n of c a r b o x y l i c functions i n the polymer has r e a l i z e d high p e r m s e l e c t i v i t y in c a t i o n transport with high c o n d u c t i v i t y , which is indispensable to e l e c t r o c h e m i c a l a p p l i c a t i o n of i o n exchange membranes. This chapter summarizes the p r e p a r a t i o n and the f a b r i c a t i o n of p e r f l u o r o c a r b o x y l a t e polymers and t h e i r fundamental p r o p e r t i e s i n c l u d i n g those of the i o n i z e d s a l t - t y p e membranes. The a p p l i c a t i o n of Flemion in c h l o r - a l k a l i e l e c t r o l y s i s is a l s o d e s c r i b e d . P r e p a r a t i o n of P e r f l u o r o c a r b o x y l a t e Polymer P e r f l u o r o c a r b o x y l a t e polymers were prepared by copolymeriza t i o n of t e t r a f l u o r o e t h y l e n e and c a r b o x y l a t e d p e r f l u o r o v i n y l e t h e r . The g e n e r a l formula of copolymers a r e shown as f o l l o w s . -(CF CF ^ I(CF2CF2>2l

CF2=CF2 + 12 oleum

-> OCF CF CF2Ç=0 2

2

CH3OH

-> FOCCF2CF2CO2CH3 CF2-CF-CF3 > FOC(CFOCF2ÎH^-CF2CF2C02CH3 CF3 CF =CFO(CF2CFO^-(CF2)3C02CH3 2

C F 3

Mi (m = 0) , M2 (m = 1) Copolymerization of t e t r a f l u o r o e t h y l e n e and carboxylated p e r f l u o r o v i n y l ether i s c a r r i e d out e i t h e r i n s o l u t i o n , b u l k o r emuls i o n system w i t h a r a d i c a l i n i t i a t o r . A t y p i c a l copolymer composit i o n curve i s given i n F i g u r e 1, where M l or M2 was copolymerized w i t h t e t r a f l u o r o e t h y l e n e i n b u l k system a t 70°C. The monomer r e a c t i v i t y r a t i o s of t e t r a f l u o r o e t h y l e n e and each v i n y l ether a r e c a l c u l a t e d as 7.0 and 0.14, r e s p e c t i v e l y . Copolymers c o n t a i n i n g up to 35 mole% of carboxylated v i n y l ether were synthesized by r e g u l a t i n g the r e a c t i o n pressure of t e t r a f l u o r o e t h y l e n e i n the c o p o l y m e r i z a t i o n procedure. The s t u d i e s of the copolymer w i t h X-ray d i f f r a c t i o n and d i f f e r e n t i a l scanning c a l o r i m e t r y revealed that the c r y s t a l l i n i t y of the copolymer decreased w i t h i n c r e a s i n g v i n y l ether content as shown i n F i g u r e 2. The copolymer became amorphous a t the v i n y l ether content of 20 mole%. The g l a s s t r a n s i t i o n temperature of the amorphous copolymer l i e s around 10°C. Copolymers having high molecular weight of more than 3 χ 105 can be obtained by adopting b u l k or emulsion system w i t h extremely p u r i f i e d f u n c t i o n a l comonomers. Fabrication The e s t e r type copolymer as polymerized has a n o n - c r o s s l i n k e d l i n e a r s t r u c t u r e and i s melt processable.

Eisenberg and Yeager; Perfluorinated Ionomer Membranes ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

17.

Perfluorocarboxylate

UKIHASHI A N D Y A M A B Ε

Ml

Membranes

or M 2 Content in feed (mole%)

Figure 1. Copolymer composition curve.


429

430

PERFLUORINATED IONOMER

M E M B R A N E S


17.



Membranes

431

Figure 3 shows the dependence of apparent v i s c o s i t y of the copolymer upon shear r a t e . Since apparent v i s c o s i t y decreases w i t h i n c r e a s i n g shear r a t e , the copolymer i s regarded as a k i n d of p s e u d o p l a s t i c s . Apparent v i s c o s i t y of c a . 1 0 p o i s e i s obtained around the shear r a t e of 10^ s e c " l i n the temperature range of 230 ~ 250°C. F i g u r e 4 p l o t s l o g a r i t h m of apparent v i s c o s i t y of the c o p o l y mer a g a i n s t r e c i p r o c a l temperature. Apparent v i s c o s i t y decreases w i t h i n c r e a s i n g temperature w i t h the apparent a c t i v a t i o n energy of ca. 12 kcal/mole. In a d d i t i o n , the copolymer i s t h e r m a l l y s t a b l e as i s r e p r e sented by the thermal decomposition temperature of 320°C measured by thermogravimetry as shown i n F i g u r e 5. A c c o r d i n g l y , the copolymer can be molded by press o r e x t r u s i o n method i n t o f i l m s o f a r b i t r a r y t h i c k n e s s under proper c o n d i t i o n s . A f t e r f a b r i c a t i o n , the e s t e r type f i l m s are hydrolyzed i n a c a u s t i c s o l u t i o n to be converted to c a r b o x y l i c a c i d type membranes. F i g u r e 6 shows the change of i n f r a r e d spectrum by h y d r o l y s i s of the e s t e r type f i l m i n 25 wt% c a u s t i c s o l u t i o n a t 90°C f o r 16 h r s . Complete h y d r o l y s i s i s i n d i c a t e d by the f a c t that the a b s o r p t i o n at 1780 cm" due to V =0 ( - C O O C H 3 ) i s w h o l l y s h i f t e d to 1680 cm" of v =0 (-COONa). 4

1

1

c

c

Fundamental

Properties

E s s e n t i a l p r o p e r t i e s are described mainly f o r the sodium type membrane made of copolymers of t e t r a f l u o r o e t h y l e n e and carboxylated v i n y l ether ( M l ) , soaked i n c a u s t i c s o l u t i o n s . The content of c a r boxylated v i n y l ether i n the copolymer determines i o n exchange c a p a c i t y of the r e s u l t i n g membrane, which i s expressed as m i l l i e q u i v a l e n t of c a r b o x y l i c a c i d group per gram weight of dry sodium type membrane. Physical properties tensile properties Sodium type membranes behave q u i t e d i f f e r e n t l y from e s t e r type ones under t e n s i l e s t r e s s . I n F i g u r e 7, s t r e s s - s t r a i n curves are compared between both types of mem branes. By i n c o r p o r a t i o n of ions i n t o the membrane, s i g n i f i c a n t i n c r e a s e of t e n s i l e modulus and decrease of e l o n g a t i o n , which are o f t e n acknowledged i n h i g h c r o s s l i n k i n g of polymers, a r e brought about. Such a change i n nature i s due to the s t r u c t u r a l change i n the membrane, which i s suggested by the advant o f a new peak i n the s m a l l angle X-ray s c a t t e r i n g p a t t e r n of the sodium type mem brane as shown i n F i g u r e 8. I t appears that ions form some k i n d of aggregations, which presumably a c t as t r a n s i e n t c r o s s l i n k s . The presence of ions imparts s p e c i f i c f e a t u r e s on the mechan i c a l p r o p e r t i e s of the membrane; being r e l a t i v e l y l i t t l e dependent upon temperature and being a f f e c t e d by the c o n c e n t r a t i o n of c a u s t i c s o l u t i o n i n which i t i s soaked. The sodium type membrane e x h i b i t s higher t e n s i l e s t r e n g t h than the e s t e r type one. I t s s t r e n g t h i s


PERFLUORINATED IONOMER

M E M B R A N E S

230-2501

I

10

I

io

I 2

io

3

1

S h e a r rate (sec ) Figure 3. Melt flow behavior of polymer.


17.



Membranes

433

c (0 k_

C0 QQ_

Perfluorocarboxylate Polymer Membranes - American Chemical Society

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