Evolution of Composite Reverse Osmosis Membranes - ACS

12 Downloaded by UNIV OF ROCHESTER on August 25, 2013 | http://pubs.acs.org Publication Date: December 12, 1985 | doi: 10.1021/bk-1985-0269.ch012

Evolution of Composite Reverse Osmosis Membranes JOHN E. CADOTTE FilmTec Corporation, Minneapolis, MN 55435

The development of composite reverse osmosis membranes is reviewed with emphasis on those types that have sur vived the selection for commercial development. A composite r e v e r s e osmosis membrane i s composed o f a t h i n , dense polymer s k i n ( b a r r i e r l a y e r ) formed over a m i c r o p o r o u s s u p p o r t f i l m . Such membranes a r e d i r e c t descendants o f the L o e b - S o u r i r a j a n membrane developed i n 1960 ( 1 ) . I n the e a r l y s i x t i e s i t was shown t h a t the L o e b - S o u r i r a j a n c e l l u l o s e a c e t a t e membrane was asymmetric, h a v i n g a t h i n dense l a y e r about 200 nm (2000 Angstroms) t h i c k over a m i c r o porous f i l m (20. Thus, knowing t h i s , i t was l o g i c a l t o attempt t o form the dense s u r f a c e l a y e r s e p a r a t e l y from the porous body as a way t o form e q u i v a l e n t o r improved composite membranes. The d i s t i n c t i o n o f composite membranes v e r s u s asymmetric membranes r e s t s w i t h the mode o f f o r m a t i o n . Asymmetric membranes r e s u l t from c a s t i n g a s i n g l e polymer s o l u t i o n i n one s t e p t h a t produces a m i c r o p o r o u s f i l m w i t h a t h i n , dense l a y e r over one s u r f a c e . Composite membranes, on the o t h e r hand, a r e formed i n two s t e p s — c a s t i n g o f the m i c r o p o r o u s support f i r s t , f o l l o w e d by d e p o s i t i o n o f the b a r r i e r l a y e r on the surface of t h i s microporous support l a y e r . The advantage g a i n e d by u s i n g the " c o m p o s i t e " approach i s t h a t each m a t e r i a l used f o r the m i c r o p o r o u s s u p p o r t f i l m and f o r the b a r r i e r l a y e r can be o p t i m i z e d s e p a r a t e l y t o p r o v i d e improved membrane p e r f o r m a n c e . A s c h e m a t i c diagram o f a t y p i c a l commercial composite membrane i s p r e s e n t e d i n F i g u r e 1. The m i c r o p o r o u s p o l y s u l f o n e s u p p o r t f i l m i s c a s t on a woven o r nonwoven b a c k i n g m a t e r i a l , u s u a l l y made from polyester fibers. The p o l y s u l f o n e s u p p o r t i s a p p r o x i m a t e l y 50 μπι (two m i l s ) i n t h i c k n e s s . About h a l f o f t h i s t h i c k n e s s s i t s above the p o l y e s t e r b a c k i n g m a t e r i a l , and about h a l f o f i t i s embedded i n the f i b r o u s c a r r i e r web. The t h i c k n e s s o f t h e a p p l i e d b a r r i e r l a y e r may range from 20 t o over 500 nm, depending upon the c o m p o s i t i o n o f the b a r r i e r l a y e r . The methods t h a t have been used t o form composite membranes may be grouped i n t o the f o u r g e n e r a l t y p e s l i s t e d i n T a b l e I .

0097-6156/ 85/0269-O273S06.50/0 © 1985 American Chemical Society

In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Downloaded by UNIV OF ROCHESTER on August 25, 2013 | http://pubs.acs.org Publication Date: December 12, 1985 | doi: 10.1021/bk-1985-0269.ch012

Figure 1.

Schematic representation of the structure of FT-30 t h i n - f i l m composite membrane.


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CADOTTE Table

A. B.

C.


D.

I.

Composite Reverse Osmosis Membranes General

Methods f o r Forming Composite Membranes

275 (3)

C a s t i n g o f the b a r r i e r l a y e r s e p a r a t e l y f o l l o w e d by l a m i n a t i o n to the support f i l m . D i p - c o a t i n g of the support f i l m i n a polymer s o l u t i o n and d r y i n g , or i n a r e a c t i v e monomer or prepolymer s o l u t i o n f o l l o w e d by c u r i n g w i t h heat or r a d i a t i o n . Gas-phase d e p o s i t i o n o f the b a r r i e r l a y e r from a glow d i s c h a r g e plasma. I n t e r f a c i a l p o l y m e r i z a t i o n of r e a c t i v e monomers on the s u r f a c e of the support f i l m .

B a r r i e r l a y e r s have been a p p l i e d by a l l of the above methods on m i c r o p o r o u s support f i l m w i t h a f i b r o u s b a c k i n g (as i l l u s t r a t e d i n F i g u r e 1 ) , or on m i c r o p o r o u s h o l l o w f i b e r s . N o r m a l l y , except f o r Method C i n T a b l e I (gas-phase d e p o s i t i o n ) , the support f i l m i s used i n the wet "as c a s t " c o n d i t i o n f o r a p p l y i n g the r e a g e n t s to form the b a r r i e r l a y e r . The f i n a l s t e p of p r e p a r a t i o n , d r y i n g of the membrane, f r e q u e n t l y a i d s i n bonding the b a r r i e r l a y e r to the support film. Methods B, C and D i n T a b l e I use i n s i t u f o r m a t i o n methods i n c o n t r a s t to Method A where the b a r r i e r l a y e r i s p r e f o r m e d . The i n s i t u methods are g e n e r a l l y more p r a c t i c a l f o r machine p r o d u c t i o n o f composite membrane. S e p a r a t e C a s t i n g of the B a r r i e r Layer The f i r s t composite r e v e r s e osmosis membrane r e p o r t e d i n the t e c h n i c a l l i t e r a t u r e was d e v e l o p e d by P e t e r F r a n c i s of N o r t h S t a r Research I n s t i t u t e i n 1964 ( 4 ) . T h i s membrane was formed by f l o a t - c a s t i n g an u l t r a t h i n f i l m of c e l l u l o s e a c e t a t e (CA) upon a water s u r f a c e , r e moving the membrane from the water s u r f a c e by l a m i n a t i o n onto a pre-formed m i c r o p o r o u s support f i l m and d r y i n g t o bond the membrane to the s u p p o r t . T h i s f l o a t - c a s t i n g p r o c e d u r e has s i n c e been des c r i b e d i n the t e c h n i c a l l i t e r a t u r e f o r both f l a t sheet and t u b u l a r membranes (5, 6, 7 ) . The i n i t i a l m i c r o p o r o u s support f i l m s used i n the work were made from c e l l u l o s e a c e t a t e by a m o d i f i c a t i o n o f the L o e b - S o u r i r a j a n procedure. L a t e r work showed t h a t s e v e r a l types of the membrane f i l t e r s m a n u f a c t u r e d by M i l l i p o r e C o r p o r a t i o n and Gelman S c i e n c e s , I n c . , p e r f o r m e d as w e l l and a l l o w e d h i g h e r f l u x . A continued search f o r a more c o m p r e s s i o n - r e s i s t a n t support f i l m l e d to the development of p o l y c a r b o n a t e , p o l y p h e n y l e n e oxide and p o l y s u l f o n e m i c r o p o r o u s f i l m s i n 1966 to 1967 ( 8 ) . Of t h e s e , m i c r o p o r o u s p o l y s u l f o n e f i l m p r o v e d to have the b e s t p r o p e r t i e s . The p o l y s u l f o n e support was made by c a s t i n g a l i q u i d l a y e r of a 12.5 to 15 p e r c e n t s o l u t i o n o f Union C a r b i d e Udel P3500 p o l y s u l f o n e i n dimethylformamide onto a g l a s s p l a t e a t 4 to 7 m i l s (100-175 ym) t h i c k n e s s , then c o a g u l a t i n g the f i l m i n water. The c e l l u l o s e a c e t a t e , n o r m a l l y a medium v i s c o s i t y type h a v i n g 39 to 40 p e r c e n t a c e t y l c o n t e n t , was d i s s o l v e d at 5 to 10 p e r c e n t s o l i d s i n a s o l v e n t w i t h s l i g h t water s o l u b i l i t y such as c y c l o h e x -


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anone. F l o w i n g of t h i s s o l u t i o n onto a water s u r f a c e w i t h m e c h a n i c a l drawing produced t h i n membranes t h a t f l o a t e d on the water s u r f a c e . The t h i c k n e s s of the membranes c o u l d be c o n t r o l l e d i n a range of 20 to 500 nm. O p t i c a l i n t e r f e r e n c e c o l o r s of the u l t r a t h i n membrane s e r v e d as a g u i d e f o r e s t i m a t i n g t h i c k n e s s ; a c t u a l measurements were made by i n t e r f e r o m e t r i c or g r a v i m e t r i c methods. U l t r a t h i n f i l m s c o u l d be c a s t on a water s u r f a c e from many o t h e r commercial polymers f o r p r e p a r a t i o n of composite membranes. However, o n l y those formed from c e l l u l o s i c or r e l a t e d p o l y s a c c h a r i d e e s t e r s e x h i b i t e d adequate f l u x f o r use i n r e v e r s e o s m o s i s . S i m i l a r composite membranes were d e v e l o p e d by the team of Merten, R i l e y and L o n s d a l e at about the same time (9, 10, 1_1). T h e i r u l t r a t h i n c e l l u l o s e a c e t a t e f i l m s were formed by c a r e f u l l y c o n t r o l l e d w i t h d r a w a l of a g l a s s p l a t e from a d i l u t e acetone s o l u t i o n of c e l l u lose acetate. F i l m t h i c k n e s s was a c c u r a t e l y c o n t r o l l e d by the r a t e of w i t h d r a w a l of the g l a s s p l a t e . Immersion of the d r i e d p l a t e i n water r e l e a s e d the t h i n f i l m which was then mounted on a pre-formed microporous support f i l m . These t y p e s of c e l l u l o s e a c e t a t e composite membranes are of h i s t o r i c a l i n t e r e s t only. D u r i n g the p e r i o d when t h i s r e s e a r c h was done the composite membranes made u s i n g v e r y t h i n c e l l u l o s e a c e t a t e b a r r i e r l a y e r s (under 100 nm) looked a t t r a c t i v e f o r t h e i r h i g h f l u x properties. However, l a t e r o p t i m i z a t i o n of the asymmetric c e l l u l o s e a c e t a t e membrane p r o c e s s improved f l u x and, i n g e n e r a l , outdistanced composite CA t y p e s f o r p r a c t i c a l , low c o s t membrane m a n u f a c t u r e . Dip

Coating

Methods

I t might appear t h a t the most l o g i c a l approach f o r p r e p a r a t i o n of composite membranes would be a d i p - c o a t i n g method (Example Β i n T a b l e I ) . A pre-formed m i c r o p o r o u s support would be d i p p e d or o t h e r wise c o a t e d i n some manner w i t h a polymer s o l u t i o n and d r i e d . Addi t i o n a l v a r i a t i o n s of the method might i n c l u d e i n c o r p o r a t i o n of c r o s s l i n k i n g agents i n the polymer s o l u t i o n or the use of r e a c t i v e mono mers i n p l a c e of the polymer. The monomer c o m p o s i t i o n s a p p l i e d on the s u p p o r t f i l m c o u l d be c u r e d w i t h heat or u l t r a v i o l e t r a d i a t i o n to form the composite membrane. In s p i t e of t h i s apparent v e r s a t i l i t y f o r the d i p - c o a t i n g p r o c e s s , few membranes of commercial i n t e r e s t have been d e v e l o p e d . The main o b s t a c l e i n t h i s method appears to be the f i l l i n g of p o r e s i n the s u r f a c e of the s u p p o r t to produce low flux. T h i s p o r e - s e a l i n g problem has u s u a l l y been overcome i n some manner i n the more e f f e c t i v e d i p - c o a t e d membranes. One of the e a r l i e s t membranes formed by d i p - c o a t i n g was reported by L o n s d a l e and R i l e y ( j U , 12)· The s u r f a c e of a c e l l u l o s e a c e t a t e c e l l u l o s e n i t r a t e (CA/CN) m i c r o p o r o u s f i l m was f i r s t c o a t e d w i t h an aqueous p o l y a c r y l i c a c i d s o l u t i o n . T h i s formed a p r o t e c t i v e l a y e r on the CA/CN f i l m t o c o n f e r r e s i s t a n c e to the subsequent s o l v e n t coating operation. A c o a t i n g of c e l l u l o s e t r i a c e t a t e i n c h l o r o f o r m was then a p p l i e d and d r i e d . The membranes e x h i b i t e d s a l t r e j e c t i o n s as h i g h as 99 p e r c e n t . Two t y p e s of composite r e v e r s e osmosis membranes, formed by the d i p - c o a t i n g approach, are known to be of commercial i n t e r e s t at the p r e s e n t time and are d i s c u s s e d below. One type c o n s i s t s of membranes made on a m i c r o p o r o u s support f i l m by a c i d - c a t a l y z e d c o n d e n s a t i o n of


12.

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Composite Reverse Osmosis Membranes

211


f u r f u r y l a l c o h o l or i t s mixtures with 1,3,5-tris(hydroxyethyl)cyanuric acid. The o t h e r type c o n s i s t s of membranes formed on a m i c r o porous support f i l m by h e a t - c u r i n g o f a s u l f o n a t e d p o l y s u l f o n e r e s i n l a y e r d e p o s i t e d from aqueous o r a l c o h o l i c s o l u t i o n s . F u r f u r y l A l c o h o l Condensation Reactions. A membrane d e s i g n a t e d "NS-200" was f i r s t d i s c o v e r e d by the author a t N o r t h S t a r Research I n s t i t u t e d u r i n g 1972. T h i s membrane was made by d i p - c o a t i n g a p o l y s u l f o n e support f i l m i n a 2:2:1 s o l u t i o n o f f u r f u r y l a l c o h o l : s u l f u r i c acid:Carbowax 20M i n 80:20 water : i s o p r o p a n o l (13-15). The excess c o a t i n g s o l u t i o n was a l l o w e d t o d r a i n o f f . Oven d r y i n g a t 125 t o 140°C produced a b l a c k composite membrane h a v i n g a s u l f o n a t e d polyfurane b a r r i e r layer. The most o u t s t a n d i n g p r o p e r t y o f the NS-200 membrane was i t s h i g h s a l t r e j e c t i o n . Tests of laboratoryformed membranes i n s y n t h e t i c seawater a t 1000 p s i ( 6 8 9 5 k P a s c a l s ) f r e q u e n t l y produced s a l t r e j e c t i o n s g r e a t e r than 99.9 p e r c e n t a t 20 g f d (33 L / s q m/hr) f l u x . Attempts t o decrease the t h i c k n e s s o f the b a r r i e r l a y e r o f NS-200 membranes by a p p l y i n g a t h i n n e r c o a t i n g r e s u l t e d i n a l a r g e l o s s i n membrane f l u x . T h i s s u g g e s t s t h a t the membrane was somewhat asymmetric w i t h a dense s u r f a c e s k i n and a h i g h e r f l u x type o f s t r u c t u r e underneath i n the pores o f the support film. The s u r f a c e o f the h i g h l y c a t a l y z e d f u r f u r y l a l c o h o l s o l u t i o n c o a t i n g p r o b a b l y undergoes s l i g h t e v a p o r a t i o n b e f o r e the onset o f the r a p i d p o l y m e r i z a t i o n t o produce a s u r f a c e s k i n . From the b e g i n n i n g o f i t s development, problems w i t h the long term s t a b i l i t y o f the NS-200 membrane were o b s e r v e d . A gradual i n c r e a s e i n water f l u x o c c u r r e d , accompanied by a c o r r e s p o n d i n g decrease i n s a l t r e j e c t i o n . Three f a c t o r s may have been i n v o l v e d : (a) a tendency toward i r r e v e r s i b l e s w e l l i n g i n monovalent s a l t s o l u t i o n s due t o h y d r a t i o n o f the s u l f o n i c a c i d groups, (b) c h e m i c a l l i n k a g e s t h a t were u n s t a b l e t o h y d r o l y s i s ( s u l f a t e e s t e r l i n k a g e s have been suggested as a p o t e n t i a l weak l i n k ) and ( c ) a s e n s i t i v i t y to o x i d a t i o n . The c h e m i c a l h e t e r o g e n e i t y o f the s u l f o n a t e d p o l y furane r e s i n made the i n s t a b i l i t y problems d i f f i c u l t t o r e s o l v e . I t s s w e l l i n g b e h a v i o r was d e c r e a s e d by i n c r e a s i n g the oven temperatures d u r i n g the h e a t - c u r e s t e p t o g i v e a t i g h t e r , a p p a r e n t l y more c r o s s l i n k e d , membrane. A l s o , the s w e l l i n g b e h a v i o r c o u l d be l i m i t e d by i o n i c c r o s s - l i n k i n g v i a p e r i o d i c treatment w i t h p o l y v a l e n t metal i o n s such as barium. F u r t h e r s t u d i e s by o t h e r groups (16>> _17) d i d not r e s o l v e the l o n g term i n s t a b i l i t y problems o f NS-200 membranes. A r e c e n t p a t e n t g r a n t e d t o K u r i h a r a and co-workers o f Toray I n d u s t r i e s d e s c r i b e s membranes formed by a c i d - c a t a l y z e d c o n d e n s a t i o n r e a c t i o n s o f a v a r i e t y o f r e a c t i v e monomers on p o l y s u l f o n e support f i l m s under c o n d i t i o n s s i m i l a r t o those used f o r NS-200 membranes (18). The data (Example 2 i n the p a t e n t ) show t h a t 1 , 3 , 5 - t r i s ( h y d r o x y e t h y l ) i s o c y a n u r i c a c i d (THEIC), when p o l y m e r i z e d a t 150°C on a p o l y s u l f o n e support f i l m , produced a membrane e x h i b i t i n g 96.7 p e r c e n t s a l t r e j e c t i o n w i t h a f l u x o f 0.49 g f d (0.82 L / s q m/hr) when t e s t e d toward 0.25 p e r c e n t NaCl a t 568 p s i (3916 k P a s c a l s ) a n d 25°C.

1,3,5-tris(hydroxyethyl)isocyanuric (THEIC)


acid


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M A T E R I A L S S C I E N C E O FS Y N T H E T I C

M E M B R A N E S

A number o f c o - r e a c t a n t s , i n c l u d i n g p o l y a l c o h o l s and p o l y c a r b o x y l i c a c i d s , d i d not i n c r e a s e the membrane f l u x s i g n i f i c a n t l y . However, when the r e a c t i o n was c a r r i e d out u s i n g one p a r t THEIC t o two p a r t s f u r f u r y l a l c o h o l and t e s t e d under the same c o n d i t i o n s the f l u x i n c r e a s e d t o 12.3 g f d (20.5 L / s q m/hr) w i t h 99.9 p e r c e n t s a l t r e j e c t i o n (Example 40 i n the p a t e n t ) . An i n t e r e s t i n g p r o p e r t y o f t h i s type o f membrane made from the 1:2 T H E I C : f u r f u r y l a l c o h o l c o m p o s i t i o n was i t s high r e j e c t i o n of organic s o l u t e s . F o r example, r e j e c t i o n s o f e t h a n o l and p h e n o l by NS-200 membrane r e p o r t e d by C a d o t t e e t a l were 65 and 83 p e r c e n t r e s p e c t i v e l y (19). The c o r r e s p o n d i n g r e j e c t i o n s o f e t h a n o l and p h e n o l r e p o r t e d by Harumiya e t a l f o r the 1:2 T H E I C : f u r f u r y l a l c o h o l membrane was 97 and 99 p e r c e n t (20)· In t h i s c o m b i n a t i o n of r e a c t a n t s t o form the composite membrane, the THEIC component c o n t r i b u t e s toward h i g h r e j e c t i o n o f o r g a n i c s o l u t e s and the f u r f u r y l a l c o h o l component c o n t r i b u t e s h i g h f l u x and s a l t r e j e c t i o n p r o p e r t i e s . The PEC-1000 membrane o f Toray I n d u s t r i e s , I n c . , has been desc r i b e d by K u r i h a r a e t a l (21^). T h i s membrane was c h a r a c t e r i z e d as a t h i n - f i l m composite type made by an a c i d c a t a l y z e d p o l y m e r i z a t i o n on the s u r f a c e . Membrane performance r e p o r t e d f o r seawater t e s t s was 99.9 p e r c e n t TDS r e j e c t i o n a t f l u x e s o f 5.0 t o 7.4 g f d (8.3 t o 12.3 L/sq m/hr) when t e s t e d w i t h 3.5 p e r c e n t s y n t h e t i c seawater a t 800 p s i (5516 k P a s c a l s ) . The membrane was s t a b l e i n 1500-hour t e s t s i n s p i r a l wrap elements and e x h i b i t e d s t a b i l i t y i n a temperature range o f 25 t o 55°C and i n a pH range from 1 t o 13. High o r g a n i c r e j e c t i o n s were r e p o r t e d f o r the PEC-1000 membrane; r e j e c t i o n o f dimethylformamide from a 10 p e r c e n t s o l u t i o n was 95 p e r c e n t and s i m i l a r t e s t s w i t h d i m e t h y l s u l f o x i d e showed 96 p e r c e n t r e j e c t i o n . The c o m p o s i t i o n and c o n d i t i o n s f o r p r e p a r a t i o n o f PEC-1000 membrane i s n o t d i s c l o s e d i n R e f e r e n c e 21. A p p a r e n t l y i t i s a d i p - c a s t membrane r e l a t e d t o comp o s i t i o n s d e s c r i b e d by K u r i h a r a , Watanaba and Inoue i n R e f e r e n c e 18. S u l f o n a t e d P o l y s u l f o n e Membranes. A membrane o f t h i s composition, made by the d i p - c o a t i n g p r o c e s s , i s o f i n t e r e s t because o f i t s h i g h degree o f c h l o r i n e r e s i s t a n c e . The membrane was made by d i p - c o a t i n g of p o l y s u l f o n e s u p p o r t s ( f l a t sheets o r h o l l o w f i b e r s ) i n a water o r w a t e r - a l c o h o l s o l u t i o n o f s u l f o n a t e d p o l y s u l f o n e , d r y i n g and h e a t curing. The use o f s u l f o n a t e d a r o m a t i c polymers f o r r e v e r s e osmosis membranes began i n the l a t e I960's w i t h the work o f Plummer, Kimura and LaConte o f G e n e r a l E l e c t r i c ( 2 2 ) . P o l y p h e n y l e n e o x i d e was s u l f o n a t e d u s i n g c h l o r o s u l f o n i c a c i d i n c h l o r o f o r m , and asymmetric membranes were c a s t from a c h l o r o f o r m / m e t h a n o l s o l v e n t . Later a Rhone-Poulenc p a t e n t d e s c r i b e d the p r e p a r a t i o n o f asymmetric s u l f o n a t e d p o l y s u l f o n e membranes (23^)· The degree o f s u l f o n a t i o n was l i m i t e d so t h a t the p r o d u c t would be w a t e r - i n s o l u b l e . The asymm e t r i c membrane was c a s t from a dimethylformamide s o l u t i o n . C a d o t t e , Steuck and P e t e r s e n a t Midwest Research I n s t i t u t e i n 1976 d e v e l o p e d a composite s u l f o n a t e d p o l y s u l f o n e membrane ( 2 4 ) . P o l y s u l f o n e was s u l f o n a t e d t o a degree t o produce water s o l u b i l i t y . This material i n water o r w a t e r - a l c o h o l s o l u t i o n s was used f o r d i p - c o a t i n g o f p o l y s u l f o n e support f i l m s . Oven d r y i n g o f the c o a t e d support f i l m a t 100 t o 140°C caused the f o r m a t i o n o f s u l f o n e c r o s s - l i n k s , t h a t s e r v e d t o i m m o b i l i z e the s u l f o n a t e d p o l y s u l f o n e b a r r i e r l a y e r . A l t e r n a t i v e l y the a d d i t i o n o f p o l y a l c o h o l s o r p o l y p h e n o l s t o the s o l u t i o n c o u l d be used t o produce s u l f o n a t e e s t e r c r o s s - l i n k s .


12.

CADOTTE


279

G e n e r a l l y , the s a l t r e j e c t i o n s o b s e r v e d f o r these membranes i n s e a water r e v e r s e osmosis t e s t s d i d not exceed 80 p e r c e n t . This process was a p p l i e d at A l b a n y I n t e r n a t i o n a l to form composite membranes on h o l l o w p o l y s u l f o n e f i b e r s (25). S a l t r e j e c t i o n s on the h o l l o w f i b e r membranes were above 98 p e r c e n t at an average f l u x of about 1.5 g f d (2.5 L/sq m/hr) i n a 12 000-hour t e s t u s i n g 30 000 ppm seawater at 1000 p s i . In o t h e r 5000-hour t e s t s u s i n g 3500 ppm b r a c k i s h water at 400 p s i w i t h a d d i t i o n of 100 ppm c h l o r i n e at pH 8 f l u x and s a l t r e j e c t i o n remained c o n s t a n t at 1 g f d and 98 p e r c e n t r e s p e c t i v e l y ( F i g u r e s 8 and 12 i n R e f e r e n c e 25 a ) .


Gas

Phase D e p o s i t i o n of the B a r r i e r L a y e r

Plasma p o l y m e r i z a t i o n to form a b a r r i e r l a y e r over a dry m i c r o p o r o u s support f i l m was r e p o r t e d i n 1970 by Buck and Davan (26) and i n 1973 by Yasuda and Lamaze (2^7) and by H o l l a h a n and Wydeven (28)· The plasma r e a c t i o n s are q u i t e h e t e r o g e n i o u s , not o n l y i n v o l v i n g p o l y m e r i z a t i o n , but d e p o l y m e r i z a t i o n and m o d i f i c a t i o n of f u n c t i o n a l g r o u p s . Yasuda and Marsh showed t h a t i n t e r a c t i o n o f the plasma w i t h the polymer of the support f i l m a l s o p l a y e d a p a r t i n f o r m a t i o n of the b a r r i e r l a y e r ( 2 9 ) . In a d d i t i o n to the u s u a l v i n y l monomers, most o r g a n i c compounds h a v i n g adequate vapor p r e s s u r e c o u l d be used to d e p o s i t a b a r r i e r l a y e r on porous s u p p o r t s . A d d i t i o n a l copolymers c o u l d be formed by i n c l u s i o n of n i t r o g e n i n the r e a c t a n t g a s e s . The s u p p o r t s used i n c l u d e d M i l l i p o r e f i l t e r s , porous p o l y s u l f o n e f i l m s and porous g l a s s tubes. Examples were p r e s e n t e d of plasma formed membranes w i t h 99 p e r c e n t s a l t r e j e c t i o n , 38 g f d f l u x (63.3 L/sq m/hr) and low f l u x d e c l i n e i n seawater t e s t s . A r e c e n t r e p o r t by H e f f e r n a n et a l d e s c r i b e s gas phase d e p o s i t i o n of membranes on h o l l o w f i b e r s ( 3 0 ) . A membrane d e s i g n a t e d " S o l r o x " made by Sumitomo Chemical Company i s c l o s e l y r e l a t e d to the above plasma p o l y m e r i z e d composite membranes. A 1980 r e p o r t by T. Sano d e s c r i b e d the Sumitomo p r o c e s s ( 3 1 ) . A support f i l m was c a s t from a p o l y a c r y l o n i t r i l e copolymer c o n t a i n ing at l e a s t 40 mole p e r c e n t a c r y l o n i t r i l e . The support f i l m was d r i e d and exposed t o a h e l i u m or hydrogen plasma t o form a t i g h t c r o s s - l i n k e d s u r f a c e s k i n on the porous p o l y a c r y l o n i t r i l e support film. Data i n a U.S. Patent i s s u e d i n 1979 to Sano e t a l showed t h a t the u n m o d i f i e d support f i l m had a water f l u x o f 87 g f d (145 L/ sq m/hr) at 142 p s i (10 kg/sq cm). A f t e r the plasma treatment a r e v e r s e osmosis t e s t u s i n g 0.55 p e r c e n t NaCl at 710 p s i (4895 kPa) showed 10.5 g f d (17.5 L/sq m/hr) f l u x at 98.3 p e r c e n t s a l t r e j e c t i o n (32). T h i s membrane appears to f a l l between a c o n v e n t i o n a l asymm e t r i c membrane and a composite membrane. I f the s u r f a c e s k i n i s o n l y c r o s s - l i n k e d , one might c a l l i t a m o d i f i e d asymmetric membrane. However, i f the s u r f a c e s k i n i s s u b s t a n t i a l l y m o d i f i e d c h e m i c a l l y to make i t d i s t i n c t from the b u l k of the membrane i t c o u l d be cons i d e r e d as a composite t y p e . I n t e r f a c i a l Polymerization

of the B a r r i e r Layer

I n t e r f a c i a l r e a c t i o n s to produce t h i n polymer f i l m s date back to the e a r l y work of W a l l a c e C a r o t h e r s s t a r t i n g i n 1929. Synthetic polyamides f o r p r o d u c i n g n y l o n f i b e r s were formed by l o n g , c a r e f u l l y



280

M A T E R I A L S SCIENCE O F SYNTHETIC M E M B R A N E S

c o n t r o l l e d high-temperature condensation r e a c t i o n s . By the e a r l y 1940's, r e s e a r c h was b e i n g done t o produce the same type of l i n e a r h i g h polymers i n low-temperature c o n d e n s a t i o n r e a c t i o n s . The lowtemperature r e a c t i o n s were d i v i d e d i n t o two t y p e s : (a) a s o l u t i o n p o l y c o n d e n s a t i o n i n which e q u i m o l a r amounts of a diamine and d i a c i d c h l o r i d e p l u s t e r t i a r y amine were d i s s o l v e d i n an i n e r t s o l v e n t , and (b) an i n t e r f a c i a l p o l y c o n d e n s a t i o n i n which the diamine was d i s s o l v e d i n water and the d i a c i d c h l o r i d e i n a non-water m i s c i b l e solvent. Upon c o n t a c t of the two s o l u t i o n s , a polyamide membrane was formed a t the i n t e r f a c e . The i n t e r f a c i a l method has been of wide g e n e r a l i n t e r e s t a l t h o u g h i t s commercial use has not been extensive. The i n t e r f a c i a l methods are a p p l i c a b l e t o p r o d u c t i o n of a wide v a r i e t y of p o l y a m i d e s , p o l y s u l f o n a m i d e s , p o l y u r e t h a n e s and polyesters. R e a c t i o n s i n v o l v e d i n both the s o l u t i o n p o l y c o n d e n s a t i o n methods and i n t e r f a c i a l p o l y c o n d e n s a t i o n s were reviewed i n a book by P.W. Morgan p u b l i s h e d i n 1965 (33). The use o f i n t e r f a c i a l l y formed t h i n f i l m s f o r membrane s e p a r a t i o n s was, t h e r e f o r e , o b v i o u s . Reference t o such membranes i n Morgan's book c h a r a c t e r i z e s them as r e t e n t i v e to dyes but permeable to s a l t s . E a r l y attempts t o use i n t e r f a c i a l l y formed membranes at the N o r t h S t a r Research L a b o r a t o r i e s i n the l a t e i 9 6 0 ' s tended to c o n f i r m the low s a l t r e t e n t i o n d e s c r i b e d by Morgan. In these e a r l y attempts t o p r e p a r e composite membranes p o l y s u l f o n e support f i l m s were s a t u r a t e d i n s o l u t i o n s of v a r i o u s d i a m i n e s , a c i d a c c e p t o r s and surfactants. A f t e r d r a i n i n g , the f i l m s were c o n t a c t e d w i t h hexane s o l u t i o n s of v a r i o u s d i a c y l c h l o r i d e s . In t h i s i n i t i a l work, s a l t r e j e c t i o n s of these membranes were g e n e r a l l y too low f o r p r a c t i c a l use i n seawater or b r a c k i s h water a p p l i c a t i o n s . S i m i l a r l y poor r e s u l t s were d e s c r i b e d i n a p a t e n t by S c a l a and co-workers, d a t i n g to t h a t e r a , wherein i n t e r f a c i a l c o n d e n s a t i o n membranes were produced (34). P o l y m e r i c Amine R e a c t a n t s : NS-100. In 1970 a d i f f e r e n t approach was t r i e d t h a t produced an immediate and s i g n i f i c a n t improvement i n membrane p r o p e r t i e s . A p o l y s u l f o n e support f i l m was s a t u r a t e d i n a water s o l u t i o n of a p o l y m e r i c amine (0.5 t o 1.0 p e r c e n t s o l u t i o n of p o l y e t h y l e n i m i n e ) which was r e a c t e d w i t h t o l y l e n e d i i s o c y a n a t e or i s o p h t h a l o y l c h l o r i d e (0.1 t o 1.0 p e r c e n t i n hexane). The membrane was d r i e d i n a 110°C oven (3_5). T h i s p r o c e s s gave s a l t r e j e c t i o n s i n seawater t e s t s i n excess of 99 p e r c e n t . P o l y e t h y l e n i m i n e , a conv e n i e n t , c o m m e r c i a l l y a v a i l a b l e w a t e r - s o l u b l e polyamine, c o n t a i n s a h i g h c o n c e n t r a t i o n of amine groups f o r h i g h r e a c t i v i t y w i t h the i s o cyanate and a c y l c h l o r i d e r e a c t a n t s . The amine groups (one per each two c a r b o n atoms) are i n a p e r c e n t r a t i o of 30:40:30 p r i m a r y : secondary: t e r t i a r y (36). The m o l e c u l a r weight of the more e f f e c t i v e grades of p o l y e t h y l e n i m i n e was i n a range of 10 000 to over 60 000 . Models of t h i s h i g h l y branched polymer i n d i c a t e t h a t i t i s s p h e r o i d a l i n nature (36). Two types of r e a c t i o n s i n v o l v e d i n the p r e p a r a t i o n of NS-100 membranes are i l l u s t r a t e d i n F i g u r e 2. The s t r u c t u r a l r e p r e s e n t a t i o n of p o l y e t h y l e n i m i n e (PEI) i s s i m p l i f i e d to show o n l y the r e a c t i v e p r i m a r y and secondary amine groups. In the f i r s t s t e p the amine groups r e a c t r a p i d l y w i t h i s o p h t h a l o y l c h l o r i d e a t the i n t e r f a c e t o produce a polyamide s u r f a c e s k i n , w h i l e amine groups below



CADOTTE


NH2

Isophthaloyl Chloride i n Hexane Phase Interface

NH rNH

H N-f

Polyethylenimine i n Water Phase

2

*^-'

·» _ ' F i r s t Step Interfacial

v

^^cJCJ Lev Nti

^ N ^

^ÎNH

H th2

/

—

V

Reaction Polyamide S u r f a c e Skin Interface Unreacted P o l e t h y l e n e i n Water

Heat Cure Step

Î

H

NS-100 Polyamide B a r r i e r Layer over C r o s s - l i n k e d Polyethylenimine

Ν ;

Figure 2.

Reactions

+

N H

3

involved i n NS-100 membrane formation.



282


the i n t e r f a c e remain u n r e a c t e d . In the second s t e p ( d r y i n g and h e a t c u r i n g a t 110°C) i n t e r n a l c r o s s - l i n k i n g o f p o l y e t h y l e n i m i n e t a k e s p l a c e w i t h the e l i m i n a t i o n o f ammonia between a d j a c e n t amine groups. These r e a c t i o n s produce a membrane t h a t has t h r e e d i s t i n c t zones o f d e c r e a s i n g p o r o s i t y : (1) the p o l y s u l f o n e support f i l m , (b) a t h i n , c r o s s - l i n k e d p o l y e t h y l e n i m i n e zone e x t e n d i n g i n t o the pores o f the support f i l m and ( c ) the dense polyamide ( o r p o l y u r e a ) s u r f a c e s k i n which a c t s as the h i g h r e t e n t i o n b a r r i e r . I t i s noteworthy t h a t s i m p l e h e a t - c u r i n g o f a p o l y e t h y l e n i m i n e c o a t i n g on the m i c r o p o r o u s p o l y s u l f o n e , o m i t t i n g the i n t e r f a c i a l r e a c t i o n s t e p , produced a c r o s s - l i n k e d p o l y e t h y l e n i m i n e membrane t h a t gave 70 p e r c e n t s a l t r e j e c t i o n a t 55 g f d (91.7 L / s q m/hr) f l u x i n the seawater t e s t . A l s o , i f a f u l l y formed NS-100 membrane was d r i e d a t 75°C r a t h e r than h e a t - c u r e d a t 110°C, s a l t r e j e c t i o n f e l l to 96 p e r c e n t . The e v i d e n c e suggests t h a t the polyamide s u r f a c e s k i n i s v e r y t h i n and t h a t the c r o s s - l i n k e d p o l y e t h y l e n i m i n e underl a y e r i s needed f o r a d d i t i o n a l s u p p o r t . Morgan p r e s e n t e d e v i d e n c e p r o v i n g t h a t i n t e r f a c i a l polyamide r e a c t i o n s o c c u r r e d p r i m a r i l y on the s o l v e n t s i d e o f the i n t e r f a c e ; t h a t i s , the amine r e a c t a n t m i g r a t e d a c r o s s the i n t e r f a c e i n t o the o r g a n i c phase i n o r d e r f o r r e a c t i o n to occur. S i n c e the b u l k y , h y d r o p h i l i c p o l y e t h y l e n i m i n e macromolecule has e s s e n t i a l l y no s o l u b i l i t y i n the hexane phase i n the NS-100 r e a c t i o n , the r e s u l t i n g dense s u r f a c e s k i n i s e x t r e m e l y thin. The c r o s s - l i n k e d p o l y e t h y l e n i m i n e g e l zone under t h i s v e r y t h i n b a r r i e r l a y e r i s b e l i e v e d t o be n e c e s s a r y f o r h i g h s a l t r e j e c tion. Other P o l y m e r i c Amine R e a c t a n t s . A l t h o u g h the NS-100 membrane showed h i g h s a l t r e j e c t i o n c a p a b i l i t y , i t had some l i m i t a t i o n s : (a) the f l u x , 18 g f d i n seawater t e s t s a t 1500 p s i , might be i n c r e a s e d , (b) the s e n s i t i v i t y o f NS-100 t o c h l o r i n e l e d t o attempts t o modify the amine s t r u c t u r e t o i n c r e a s e i t s o x i d a t i o n r e s i s t a n c e and ( c ) s u r f a c e b r i t t l e n e s s o f NS-100 due t o i t s h i g h l y c r o s s - l i n k e d s t r u c t u r e gave problems i n m e c h a n i c a l p r o c e s s i n g o f the membrane. T h i s l a t t e r problem c o u l d be overcome t o a degree by o v e r - c o a t i n g t h e membrane with a p o l y v i n y l a l c o h o l p r o t e c t i v e layer. These l i m i t a t i o n s , which appeared i n e f f o r t s t o s c a l e up NS-100 i n t o commercial p r o d u c t i o n , l e d t o e x p e r i m e n t a t i o n on a wide v a r i e t y o f polyamines and a c y l c h l o r i d e or isocyanate r e a c t a n t s . Riley and co-workers d e v e l o p e d a s e r i e s o f membranes i n t o commerc i a l production using a polyetheramine i n place of polyethylenimine (.37). The p o l y e t h e r a m i n e , a l s o c a l l e d p o l y e p i a m i n e , i s an adduct of p o l y e p i c h l o r o h y d r i n with ethylenediamine. It functioned s i m i l a r l y to p o l y e t h y l e n i m i n e i n NS-100, except i t e x h i b i t e d an i n c r e a s e d f l u x and an i n c r e m e n t a l i n c r e a s e i n c h l o r i n e r e s i s t a n c e . The f i r s t o f t h i s s e r i e s o f membranes, d e s i g n a t e d PA-300, was s u b s e q u e n t l y used i n the 3.2 mgd d e s a l i n a t i o n p l a n t a t Jeddah, Saudi A r a b i a (38»). The PA-300 membrane i s formed from p o l y e t h e r a m i n e and i s o p h t h a l o y l chloride. A c l o s e l y - r e l a t e d membrane, d e s i g n a t e d RC-100, i s bel i e v e d t o be the t o l y l e n e d i i s o c y a n a t e a n a l o g o f PA-300. A r e c e n t p a t e n t g r a n t e d t o Toray I n d u s t r i e s , I n c . , d e s c r i b e d the use o f a s i m i l a r amine polymer i n composite membranes ( 3 9 ) . P o l y e p i i o d o h y d r i n was r e a c t e d w i t h 4 - ( a m i n o m e t h y l ) p i p e r a z i n e t o form t h e r e a c t i v e amine polymer. T h i s polymer, 3 p a r t s , p l u s 1 p a r t



12.

CADOTTE

283


4 - ( a m i n o m e t h y l ) p i p e r a z i n e o r r e l a t e d monomers was c o a t e d on a p o l y s u l f o n e support f i l m and r e a c t e d i n t e r f a c i a l l y w i t h i s o p h t h a l o y l chloride. The a d d i t i o n o f the monomer r e a c t a n t was b e n e f i c i a l i n a t t a i n i n g a minimum s a l t r e j e c t i o n o f 99.5 p e r c e n t i n seawater r e v e r s e osmosis t e s t s . Many a d d i t i o n a l polyamines have been proposed t o r e p l a c e p o l y e t h y l e n i m i n e i n the NS-100 membrane t y p e . Some o f t h e s e polymers have o n l y secondary amine groups i n the c h e m i c a l s t r u c t u r e , and would presumably n o t undergo i n t e r n a l c r o s s - l i n k i n g by e l i m i n a t i o n of ammonia, which o c c u r s i n p o l y e t h y l e n i m i n e . N o t a b l e among t h e s e l a t t e r r e p o r t s a r e two U.S. p a t e n t s i s s u e d t o Kawaguchi e t a l o f T e i j i n i n 1981 and 1982. The f i r s t o f these p a t e n t s (40) d e s c r i b e s a v a r i e t y o f r e a c t i v e l i n e a r polymers c o n t a i n i n g a n h y d r i d e , a c y l c h l o r i d e o r s u l f o n y l c h l o r i d e groups, t h a t were r e a c t e d w i t h an exc e s s o f p o l y f u n c t i o n a l amines t o produce w a t e r - s o l u b l e polymers c o n t a i n i n g r e a c t i v e p r i m a r y o r secondary amine g r o u p s . These polymers were r e a c t e d i n t e r f a c i a l l y on p o l y s u l f o n e support f i l m s w i t h a r o m a t i c d i - o r t r i - a c y l c h l o r i d e s o r s u l f o n y l c h l o r i d e s t o form composite membranes. F o r example, a polyamine produced from the r e a c t i o n o f t r i - e t h y l e n e t e t r a m i n e with a maleic anhydride-methyl a c r y l a t e copolymer was c o a t e d onto a p o l y s u l f o n e s u p p o r t . After interfacial r e a c t i o n u s i n g a m i x t u r e o f i s o p h t h a l o y l c h l o r i d e and t r i m e s o y l c h l o r i d e (1,3,5-benzene t r i c a r b o n y l c h l o r i d e ) , the membrane was d r i e d a t 115 t o 120°C (Example 1 i n the p a t e n t ) . T e s t e d w i t h 0.5 p e r c e n t sodium c h l o r i d e s o l u t i o n a t 25°C and 600 p s i (4137 kPa) the membranes e x h i b i t e d a f l u x a f t e r 200 hours o f 59 g f d (98 L / s q m/ h r ) and s a l t r e j e c t i o n o f 96.8 p e r c e n t . The second p a t e n t o f Kawaguchi e t a l (41) d e s c r i b e s a number o f composite, i n t e r f a c i a l l y - f o r m e d membranes t h a t a r e a m p h o t e r i c ; t h a t i s , they have both c a t i o n i c and a n i o n i c f u n c t i o n a l i t y . T h i s c o n t r i butes a degree o f s p e c i f i c i t y t o the membrane. A l a r g e p r o p o r t i o n of an i o n i c s o l u t e such as sodium c h l o r i d e w i l l pass through the membrane w h i l e noncharged s o l u t e s such as s u c r o s e a r e r e t a i n e d t o a h i g h degree. S e v e r a l s y n t h e t i c r o u t e s were g i v e n f o r the p r e p a r a t i o n o f v a r i o u s membranes. The approach u t i l i z e d p o l y m e r i c amines h a v i n g b o t h r e a c t i v e and n o n - r e a c t i v e amine g r o u p s . The r e a c t i v e amine groups p r o v i d e d c a t i o n i c s i t e s i n the membrane. T r i - o r t e t r a f u n c t i o n a l a c y l c h l o r i d e s were used i n the s o l v e n t phase f o r the i n t e r f a c i a l r e a c t i o n . H y d r o l y s i s o f a p o r t i o n o f the a c y l groups of t h e s e r e a g e n t s p r o v i d e d the a n i o n i c s i t e s i n the membrane. The membranes formed i n these r e a c t i o n s were t e s t e d u s i n g feed s o l u t i o n s c o n t a i n i n g one p e r c e n t sodium c h l o r i d e and t h r e e p e r c e n t s u c r o s e . S e v e r a l o f the examples c i t e d i n the p a t e n t a l l o w e d more than 80 p e r c e n t o f the sodium c h l o r i d e t o pass through the membrane w h i l e r e j e c t i n g more than 97 p e r c e n t o f the s u c r o s e . One approach used by Kawaguchi e t a l f o r p r e p a r a t i o n o f the amphoteric membrane was the use o f p o l y e t h y l e n i m i n e h a v i n g p a r t o f the amine groups n e u t r a l i z e d w i t h h y d r o c h l o r i c a c i d . The i n t e r f a c i a l r e a c t a n t s were t r i m e s o y l c h l o r i d e (TMC), 3 - c h l o r o s u l f o n y l i s o p h t h a l o y l c h l o r i d e (SPC) and p y r o m e l l i t i c a c i d c h l o r i d e (PMC).

TMC

SPC

PMC



284

M A T E R I A L S SCIENCE OF SYNTHETIC

MEMBRANES

The n e u t r a l i z e d amine groups d i d not r e a c t w i t h the a c y l c h l o r i d e r e a g e n t s i n the i n t e r f a c i a l r e a c t i o n step and d i d not undergo con d e n s a t i o n r e a c t i o n s d u r i n g the heat c u r e step of membrane f o r m a t i o n . A l o o s e l y c r o s s - l i n k e d membrane was formed t h a t was v e r y h y d r o p h i l i c due to the h i g h c o n c e n t r a t i o n of r e s i d u a l amine and c a r b o x y l g r o u p s . A n o t h e r approach used f o r d e a c t i v a t i n g p a r t of the amine groups of p o l y e t h y l e n i m i n e was to use a p a r t i a l q u a t e r n i z a t i o n of p o l y e t h y l enimine w i t h e t h y l i o d i d e . The membranes formed were s i m i l a r i n p r o p e r t i e s to those made by the p a r t i a l p o l y e t h y l e n i m i n e n e u t r a l i z a tion. S t i l l a n o t h e r type of amine polymer was p r e p a r e d by f r e e r a d i c a l p o l y m e r i z a t i o n of a m i x t u r e of d i a l l y l a m i n e h y d r o c h l o r i d e , d i m e t h y l d i a l l y l ammonium c h l o r i d e and s u l f u r d i o x i d e . T h i s polymer i n the f r e e base form f o r i n t e r f a c i a l r e a c t i o n had r e a c t i v e secondary amine groups and n o n - r e a c t i v e q u a t e r n a r y amine g r o u p s :

The membranes formed were s i m i l a r i n p r o p e r t i e s polyethylenimine types.

to the

modified

Monomeric Amine R e a c t a n t s : NS-300. For s e v e r a l y e a r s i t was thought t h a t a p o l y m e r i c amine was needed to span the p o r e s i n the s u p p o r t f i l m s u r f a c e to o b t a i n h i g h s a l t r e j e c t i o n i n the i n t e r f a c i a l t h i n f i l m composite membrane. However, a 1975 r e s e a r c h r e p o r t by the N o r t h S t a r D i v i s i o n of Midwest Research I n s t i t u t e (42) showed t h a t a monomeric amine, p i p e r a z i n e , i n i n t e r f a c i a l r e a c t i o n s w i t h i s o p h t h a l o y l c h l o r i d e gave s a l t r e j e c t i o n s of 90 t o 96 p e r c e n t i n seawater tests. The improved membrane performance had been a c h i e v e d through o p t i m i z a t i o n of the i n t e r f a c i a l r e a c t i o n , c o n c e n t r a t i o n of r e a c t a n t s , a c i d a c c e p t o r s and s u r f a c t a n t s . C r e d a l i et a l had p r e v i o u s l y p r e p a r e d asymmetric membranes from pre-formed p o l y p i p e r a z i n e a m i d e s and r e p o r t e d them to be c h l o r i n e r e s i s t a n t ( 4 3 ) . Therefore, poly(pipera z i n e i s o p h t h a l a m i d e ) i n t e r f a c i a l membranes had been made w i t h the o b j e c t i v e of f o r m i n g a c h l o r i n e r e s i s t a n t membrane. Because p i p e r a z i n e i s a d i - s e c o n d a r y amine, i t was thought at t h a t time t h a t the p i p e r a z i n e p o l y a m i d e s , h a v i n g no a m i d i c hydrogen atoms, would r e s i s t chlorine oxidation. A c t u a l l y , these membranes were found to be more c h l o r i n e - r e s i s t a n t than NS-100, but not t o the degree t h a t they c o u l d be used w i t h c o n t i n u o u s c h l o r i n a t i o n of the feedwater. Because of t h e i r lower f l u x and s a l t r e j e c t i o n p r o p e r t i e s as compared w i t h NS-100, the p i p e r a z i n e - i s o p h t h a l a m i d e membranes were never d e v e l o p e d f u r t h e r , but m o d i f i c a t i o n of t h i s r e a c t i o n p r o v e d to be of c o n s i d e r a b l e i n t e r e s t . P a r t or a l l of the i s o p h t h a l o y l c h l o r i d e was replaced w i t h the t r i f u n c t i o n a l a c y l c h l o r i d e , t r i m e s o y l c h l o r i d e (44)· The i n t e r f a c i a l r e a c t i o n of t r i m e s o y l c h l o r i d e w i t h p i p e r a z i n e i s be l i e v e d to produce p r i m a r i l y the l i n e a r polyamide, w i t h o n l y moderate cross-linking: ρ 1

C0C1

C00H


η


12.

CADOTTE

285


Perhaps through the i n t r o d u c t i o n o f f r e e c a r b o x y l i c a c i d groups, a much h i g h e r f l u x membrane was formed. T h i s membrane, which was named NS-300, e x h i b i t e d p a r t i c u l a r l y h i g h r e t e n t i o n o f s a l t s h a v i n g polyvalent anions. S a l t r e j e c t i o n s by the NS-300 membrane toward s y n t h e t i c s e a water improved as the i s o p h t h a l a m i d e c o n t e n t o f the b a r r i e r l a y e r increased. S u r p r i s i n g l y , membrane f l u x peaked r a t h e r than simply d e c l i n i n g as a f u n c t i o n o f i n c r e a s i n g i s o p h t h a l a m i d e c o n t e n t . This i s i l l u s t r a t e d by the data i n T a b l e I I . Maximum water p e r m e a b i l i t y c h a r a c t e r i s t i c s were found a t an approximate copolymer r a t i o o f 67 p e r c e n t i s o p h t h a l i c and 33 p e r c e n t t r i m e s i c g r o u p s . The d i f f e r e n c e s i n the magnesium s u l f a t e v e r s u s sodium c h l o r i d e r e j e c t i o n appear t o be due t o the a n i o n i c a l l y c h a r g e d n a t u r e o f the membrane b a r r i e r l a y e r , which i s r i c h i n c a r b o x y l a t e g r o u p s . Table

I I . E f f e c t o f the I s o p h t h a l o y l : T r i m e s o y l C h l o r i d e R a t i o on the Performance o f NS-300 Membranes i n Reverse Osmosis T e s t s

Acid Chloride

Trimesoyl

Ratio»

Isophthaloyl

Reverse Osmosis T e s t R e s u l t s 3.5% S y n t h e t i c 0.5% Magnesium SulSeawater-1500 p s i g f a t e - 200 p s i g Flux Flux (gfd(gfdL/sq Salt Rej. L/sq Salt Rej. m/hr) (percent) m/hr) (percent)

100

0

26 (43)

99.3

80 (133)

68

75

25

31 (52)

99.3

96 (160)

64 65

33

67

77 (128)

99.9

94 (157)

25

75

58 (97)

99.6

73 (121)

78

10

90

18 (30)

99.9

33 (55)

96

99.0

20 (33)

98

0

100

4 (6.7)

* Aqueous phase c o n t a i n e d 1% p i p e r a z i n e , 1% t r i s o d i u m phosphate, 0.5% d o d e c y l sodium s u l f a t e ; hexane phase c o n t a i n e d 1% (w/v) of a c y l c h l o r i d e s . T a b l e I I I i l l u s t r a t e d t h i s phenomenon, w h e r e i n a s i n g l e t e s t specimen (made w i t h the p i p e r a z i n e trimesamide homopolymer) was s e q u e n t i a l l y exposed t o f e e d s o l u t i o n s o f sodium c h l o r i d e , magnesium c h l o r i d e , sodium s u l f a t e and magnesium s u l f a t e . The c h l o r i d e s a l t s were both p o o r l y r e t a i n e d w h i l e r e t e n t i o n o f the s u l f a t e s a l t s was excellent. Thus, s a l t r e t e n t i o n i n the c a r b o x y l a t e - r i c h NS-300 membrane was c o n t r o l l e d by the a n i o n s i z e and c h a r g e . T h i s membrane c o u l d not d i s t i n g u i s h between the u n i v a l e n t sodium i o n and the d i v a l e n t magnesium i o n , which i s the o p p o s i t e o f the b e h a v i o r o b s e r v e d f o r asymmetric c e l l u l o s e a c e t a t e membranes. S a l t passage through the NS-300 membrane may be d e s c r i b e d as a n i o n - c o n t r o l l e d .


286

M A T E R I A L S SCIENCE OF SYNTHETIC M E M B R A N E S

Table

III.

Solute

NaCl MgCl

2

Na S0. 2 4 MgS0 o


4

E f f e c t of C a t i o n and A n i o n V a l e n c e on S a l t R e j e c t i o n P r o p e r t i e s of NS--300 Membranes Flux (gfd) (L/sq

m/hr)

Salt Rejection (percent)

42

(70)

50

32

(53)

46

41

(68)

97.8

32

(53)

97.9

Reverse osmosis t e s t c o n d i t i o n s : 0.5% s a l t c o n c e n t r a t i o n , 200 p s i (1380 k P a ) , 25°C, p o l y ( p i p e r a z i n e t r i m e s a m i d e ) membrane.

Attempts t o s c a l e up the NS-300 membrane p r e p a r a t i o n from the l a b o r a t o r y s c a l e to c o n t i n u o u s machine p r o d u c t i o n l e d to a h i g h degree of v a r i a b i l i t y i n membrane p r o p e r t i e s (45)· The d i f f e r e n c e was a t t r i b u t e d i n p a r t to the v a r i a b i l i t y of machine-made p o l y s u l fone support f i l m . P r o p e r t i e s of the machine-made p o l y s u l f o n e s u p p o r t s d i f f e r e d from the l a b o r a t o r y c a s t support f i l m s . One of the major f a c t o r s a f f e c t i n g t h i s d i f f e r e n c e was t h a t the machinemade support f i l m was c a s t on a nonwoven p o l y e s t e r b a c k i n g m a t e r i a l which can v a r y i n p r o p e r t i e s . Thus, the machine support f i l m , which was q u i t e adequate f o r NS-100 type membranes u s i n g p o l y m e r i c amine r e a c t a n t s , s t i l l remained a l i m i t a t i o n f o r the monomeric amine r e a c t i o n of NS-300. An approach t o overcome t h i s problem was replacement of p i p e r a z i n e i n the NS-300 r e a c t i o n w i t h p i p e r a z i n e - t e r m i n a t e d oligomers. P i p e r a z i n e o l i g o m e r s had been i n v e s t i g a t e d i n an e a r l i e r program ( 4 4 ) . They were made by the r e a c t i o n of excess p i p e r a z i n e w i t h d i - or t r i a c y l c h l o r i d e s i n an i n e r t s o l v e n t such as 1,2-dichloroethane, d i c h l o r o m e t h a n e or t - b u t a n o l . The r e s u l t i n g a m i n e - t e r m i n a t e d p o l y a mide o l i g o m e r had low s o l u b i l i t y i n the s o l v e n t system and p r e c i p i tated. T h i s s e r v e d t o l i m i t the degree of p o l y m e r i z a t i o n of the oligomer. Even so, a p o r t i o n of the p r o d u c t s was i n s o l u b l e i n water and was f i l t e r e d out d u r i n g p r e p a r a t i o n of the aqueous o l i g o m e r i c amine s o l u t i o n f o r the i n t e r f a c i a l r e a c t i o n s t e p . T a b l e IV l i s t s the b e s t performance d a t a o b t a i n e d f o r p i p e r a z i n e o l i g o m e r membranes i n t e r f a c i a l l y r e a c t e d w i t h i s o p h t h a l o y l c h l o r i d e . The o b j e c t i v e of t h e s e t e s t s was to a c h i e v e s i n g l e - p a s s seawater d e s a l i n a t i o n membranes. As such, the p r e s e n c e of f r e e c a r b o x y l a t e groups was a v o i d e d ; use was made of the t r i m e s o y l c h l o r i d e or a l t e r nate t r i a c y l h a l i d e s i n the o l i g o m e r f o r m a t i o n s t e p , and d i a c y l c h l o r i d e s i n the i n t e r f a c i a l r e a c t i o n s t e p . A few samples of s e a water d e s a l i n a t i o n membranes were o b t a i n e d . Best r e s u l t s were seen f o r p i p e r a z i n e - c y a n u r a t e p r e - p o l y m e r s i n t e r f a c i a l l y c r o s s - l i n k e d by i s o p h t h a l o y l c h l o r i d e , but f l u x e s were low i n view of the o p e r a t i n g t e s t p r e s s u r e of 1500 p s i (10 342 k P a s c a l ) . A l s o , i n d i v i d u a l membrane r e s u l t s w i t h p i p e r a z i n e o l i g o m e r s were e q u a l l y as e r r a t i c as those experienced f o r piperazine d i r e c t l y . The o n l y n o t a b l e advantage of the p i p e r a z i n e o l i g o m e r approach was the a b i l i t y t o i n c o r p o r a t e


12.

CADOTTE

287


c y a n u r a t e r i n g s i n t o the membrane s t r u c t u r e . C y a n u r i c c h l o r i d e was too prone to h y d r o l y s i s to p r o v i d e good i n t e r f a c i a l membranes w i t h piperazine otherwise. T a b l e IV.

Membranes Formed U s i n g P i p e r a z i n e thaloyl Chloride

Reactant used to p r e p a r e Acid P i p e r a z i n e Oligomer

trimesoyl

Isoph-

Reverse Osmosis T e s t Data (a) Salt Rej. Flux (L/sq*m /hr) (7o) (gfd)

chloride

NaOH

13

(22)

99.0

chloride

triethylamine

58

(97)

93.8

cyanuric

chloride

NaOH

14

(23)

99.2

cyanuric

chloride

triethylamine

24

(40)

98.0

Ν,Ν'-dimethylpiperazine

45

(75)

93.9

triethylamine

34

(57)

92.4

9

(15)

99.0

trimesoyl


Acceptor

Oligomers and

phosphorus

oxychloride

1:1 t r i m e s o y l : isophthaloyl chloride cyanuric

chloride

(b)

NaOH

(a) Twenty to 24 hour t e s t s , 1500 p s i (10 342 k P a s c a l ) 25°C, s y n t h e t i c seawater (b) Reacted w t i h a 6:1 molar r a t i o of p i p e r a z i n e : m o r p h o l i n e

3.5%

In c o n c l u s i o n , some of the h i g h s a l t r e j e c t i o n p r o p e r t i e s found w i t h i n t e r f a c i a l p o l y p i p e r a z i n e a m i d e membranes i n the l a b o r a t o r y c o u l d not be a t t a i n e d by a machine-made membrane. However, the machine-formed membrane may s t i l l f i n d a p p l i c a t i o n s where h i g h r e j e c t i o n of monovalent s a l t s i s not r e q u i r e d but where h i g h f l u x and r e j e c t i o n of l a r g e r s o l u t e s are u s e f u l . A l i m i t e d e f f o r t to r e p l a c e p i p e r a z i n e i n the r e a c t i o n w i t h p i p e r a z i n e - t e r m i n a t e d oligomers d i d not appear to r e s o l v e the membrane r e p r o d u c a b i l i t y problem. Recent s t u d i e s made on the use of p i p e r a z i n e t e r m i n a t e d o l i g o m e r s i n compos i t e membrane p r e p a r a t i o n were r e p o r t e d by R. Sudak et a l at Membrane Systems, Inc. (46) and by J.F. Wolfe et a l at S t a n f o r d Research I n s t i t u t e (47). Monomeric Amine R e a c t a n t s : FT-30. In 1978 the FT-30 membrane was d e v e l o p e d at F i l m T e c C o r p o r a t i o n ( 4 8 ) . The FT-30 membrane was p r o duced by the i n t e r f a c i a l r e a c t i o n of a r o m a t i c diamines i n the aqueous phase w i t h t r i a c y l c h l o r i d e s i n the s o l v e n t phase (49)· As w i t h the p i p e r a z i n e r e a c t i o n s d e s c r i b e d i n the p r e c e d i n g p a r a g r a p h s , r e a c t i o n s of diamines w i t h a t r i a c y l c h l o r i d e such as t r i m e s o y l c h l o r i d e can l e a d to two t y p e s of s i d e r e a c t i o n s i n a d d i t i o n to the l i n e a r c h a i n formation. These r e a c t i o n s are e i t h e r h y d r o l y s i s of the t h i r d a c y l c h l o r i d e group of t r i m e s o y l c h l o r i d e to a c a r b o x y l i c a c i d or to r e a c t i o n w i t h a n o t h e r diamine m o l e c u l e to produce c h a i n b r a n c h i n g or cross-linking. A g a i n , i t i s l i k e l y t h a t both of these r e a c t i o n s o c c u r , and f o r FT-30 i t i s b e l i e v e d on the b a s i s of ESCA s t u d i e s t h a t the r a t i o of b r a n c h i n g to c a r b o x y l group f o r m a t i o n i s a p p r o x i m a t e l y 1:1 ( 5 0 ) . For example, the f o l l o w i n g e q u a t i o n shows the r e a c t i o n between m-phenylene diamine and t r i m e s o y l c h l o r i d e to produce a


288


polyamide h a v i n g b r a n c h i n g s i t e s and f r e e c a r b o x y l

groups:

2

tor"".

C10I


NH

0

0C1

C=0

Evolution of Composite Reverse Osmosis Membranes - ACS

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