Emulsion Polymers and Emulsion Polymerization - American

34 Experimental Study of the Seeded Polymerization of Vinyl Acetate in a Tube 1

1

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C. K. LEE and T. H. FORSYTH The University of Akron, Akron, OH 44325 While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3-6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6). Because of the solubility of vinyl acetate in water, Smith-Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7) and Friis (8) seem to be more appropriate. The objective of this study was to investigate the feasibility of using a tubular reactor for the seeded emulsion polymerization of vinyl acetate, and to study the effect of process variables on conversion rate and latex properties. EXPERIMENTAL Figure 1 shows a schematic view of the tubular reactor. Seamless tubing, of 1/4-inch OD, type 316 stainless steel, was used for the preheater and the reactor. The reactor itself was 187 feet long, wound into a helix of 1 ft. diameter. The reactor and preheater were immersed in separate drums, which were filled with water, and maintained at constant temperature. Two feed tanks were used, one to hold the vinyl acetate emulsion and the other to hold the initiator solution. Constant 1

Current address: BFGoodrich R&D Center, Brecksville, Ohio 44141 0097-6156/81 /0165-0567$05.00/0 © 1981 American Chemical Society

In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

EMULSION POLYMERS AND EMULSION


568

POLYMERIZATION

feed flow r a t e s w e r e p o s s i b l e by u s i n g g e a r p u m p s , w i t h the c a p a c i t y of the e m u l s i o n p u m p twice as g r e a t as the i n i t i a t o r s o l u t i o n p u m p . T h e p r e h e a t e r was u s e d to o b t a i n a r e a c t i o n t e m p e r a t u r e of 5 0 ° C , p r i o r to c o m b i n i n g the two s t r e a m s to s t a r t the r e a c t i o n . F l o w r a t e s v a r i e d b e t w e e n 0 . 5 a n d 1.2 c c / s e c , and these w e r e l a m i n a r c o n d i t i o n s . U p o n s t a r t - u p , and b e t w e e n r u n s , the r e a c t o r was f u l l of water. The i n i t i a l run r e q u i r e d operation f o r a time equivalent to 2. 1 r e s i d e n c e t i m e s to d i s p l a c e the w a t e r , w i t h 1.5 r e s i d e n c e t i m e s b e t w e e n e a c h r u n . T h e d i f f e r e n c e is b e c a u s e the p r e h e a t e r was not f i l l e d w i t h w a t e r b e t w e e n r u n s . S a m p l e s w e r e r e g u l a r l y and p e r i o d i c a l l y c o l l e c t e d a f t e r 1.5 o r 2. 1 r e s i d e n c e t i m e s f o r f u r t h e r c h a r a c t e r i z a t i o n . M o l e c u l a r w e i g h t s and d i s t r i o u t i o n s w e r e m e a s u r e d by G P C . I n i t i a l B a t c h R e a c t o r S t u d i e s . A n a g i t a t e d 2000 m l t h i c k w a l l e d g l a s s r e a c t o r was b l a n k e t e d w i t h n i t r o g e n and o p e r a t e d at 5 0 ° C . V i n y l a c e t a t e c o n t a i n i n g about 15 p p m h y d r o q u i n o n e was u s e d without p u r i f i c a t i o n . T h e i o n i c e m u l s i f i e r was S i p e x E S T - 3 0 , a d v e r t i s e d as a s o d i u m t r i d e c y l e t h e r s u l f a t e , a n d the n o n i o n i c s u r f a c t a n t was S i p o n i c L - 2 5 , a l a u r y l a l c o h o l e t h o x y late. T a b l e I shows the r e c i p e s and p r o p e r t i e s of the t h r e e s e e d l a t e x e s p r o d u c e d in the b a t c h r e a c t o r . E s s e n t i a l l y c o m p l e t e c o n v e r s i o n s w e r e o b t a i n e d i n 30 to 45 m i n u t e s , but w i t h a t e m p e r a t u r e r i s e of a l m o s t 5 0 ° C . TABLE I R e c i p e u s e d to P r o d u c e Seed ( P r e p a r e d i n a 1500 m l B a t c h )

Water Vinyl Acetate Sipex E S T - 3 0 Siponic L - 2 5 Sodium Carbonate P o t a s s i u m P e r s u l f a te

_i_ 100 52 0.57

0. 19 0.08 0.25 Anionic: non-ionic Shelf L i f e > 6 month M . " 8. 64 M / M 3.6 Particle Size, A 1690 5

w

W

N

U_ 100 52 0.93 0.31 0.08

III 100 52 1.89 0. 63 0. 08 0. 19

0. 19 3:1

8. 75 3. 1 1270

7.58 3.5 (smallesi



34.

LEE AND FORSYTH

Seeded Polymerization

of Vinyl

569

Acetate

Tubular Reactor Studies. The first run in the tubular reactor was with the same recipe as for Seed I in Table I, but the conversion was very low, and there were two distinct phases. The residence time in the tube was equal to the batch reaction time. Apparently the more nearly constant temperature of the tubular reactor prevented rapid polymerization. In the next run, initiator and emulsifier levels were doubled, but still conversion was low, although phase separation was not so severe. With seed latex and still more emulsifier, Run I shown in Table II, monomer conversions of about 60% were obtained at 50 minutes average residence time in the reactor. No phase separation was evident, but later tests indicated that some phase separation was occurring. T A B L E II Recipes for Tubular Reactor Studies

T04

Run

Total E m u l s i f i e r (%)

Initiator (%) Seed Polymer (%)

2.2

0.5

0.4

I T06 T08

2.2

1.0

0.4

Seed Latex Type II

III

T09

T10 T i l

T12

T13 T14

2.2

2.2

2.2

2.2

4.4

6.9

1.0

1.0

1.0

1.0

1.0

1.0

0.4

1.3

2.6

0.4

0.4

0.4

2.2

1.5

0.4

Percentages based on total water; 0. 08 gm N a C O / 1 0 0 gm water in each 2

3

Water: Vinyl Acetate 2:1 and ionic emulsifier: nonionic emulsifier 3:1 The monomer conversion in the tubular reactor was regul a r l y measured during the run, but it did not always reach a steady state value, even after reaction times equivalent to four residence times. At low conversions, steady state was n o r mally obtained. At exit conversions between approximately 30 and 60%, conversion increased to a maximum at two residence


570

POLYMERIZATION


EMULSION POLYMERS AND EMULSION

Figure 2.

Effect of initiator concentration on conversion rate in the tubular reactor ((I): (O) 0.49%; ( Ο ) 0.98%; (A) 1.47%)


34.

LEE AND FORSYTH

Seeded Polymerization of Vinyl Acetate

571


t i m e s , then o s c i l l a t e d w i t h a constant f r e q u e n c y a n d a m p l i t u d e . T h i s o s c i l l a t i o n i s due to c o m p e t i n g f u n c t i o n s of the s u r f a c t a n t . A s c o n v e r s i o n i n c r e a s e s , the p a r t i c l e s i z e i n c r e a s e s , and m o r e s u r f a c t a n t i s r e q u i r e d to c o v e r the s u r f a c e . A s the s u r f a c t a n t is a d s o r b e d on the s u r f a c e , t h e r e a r e f e w e r t o t a l p a r t i c l e s , a n d the r e a c t i o n rate d e c r e a s e s . A t l o n g r e s i d e n c e t i m e s , steady states w e r e o b s e r v e d i n s e v e r a l r u n s , w i t h o s c i l l a t i o n s i n o t h e r r u n s . T h e u s e of l a t e x seeds and a g i t a t i o n i n the e m u l s i o n tank r e d u c e d the o s c i l l a t i o n s . A t l o w c o n v e r s i o n s of v i n y l acetate, m u c h of the i o n i c a n d n o n i o n i c e m u l s i f i e r w i l l be w i t h i n the p o o l s of v i n y l a c e t a t e . A s these p o o l s d i s a p p e a r at h i g h e r c o n v e r s i o n s , the e m u l s i f i e r b e c o m e s a v a i l a b l e to s t a b i l i z e the p o l y m e r p a r t i c l e s . C o n v e r s i o n Rate. F i g u r e s 2-5 show the c o n v e r s i o n - t i m e c u r v e s a s a f u n c t i o n of i n i t i a t o r c o n c e n t r a t i o n , s e e d c o n c e n t r a t i o n , m i x e d e m u l s i f i e r c o n c e n t r a t i o n and s i z e of s e e d l a t e x . T h e v a l u e s of c o n v e r s i o n , as p l o t t e d i n F i g u r e s 2 to 5, w e r e s e l e c t e d a t t i m e s e q u i v a l e n t to two r e s i d e n c e t i m e s , s i n c e this was e i t h e r a steady state v a l u e o r a c o n s i s t e n t m a x i m u m v a l u e . W h e r e d u p l i c a t e s a m p l e s w e r e c o l l e c t e d at two r e s i d e n c e t i m e s , the range of a c t u a l v a l u e s i s shown i n the p l o t . It c a n be s e e n i n F i g u r e s 2 a n d 3 that i n c r e a s e d i n i t i a t o r a n d e m u l s i f i e r l e v e l s i n c r e a s e d the rate of r e a c t i o n . W h i l e F i g u r e 2 i s c o n s i s t e n t w i t h t h e o r i e s of v i n y l acetate p o l y m e r i z a t i o n , the i n c r e a s e d a m o u n t s of n o n i o n i c e m u l s i f i e r s i n F i g u r e 3 m i g h t be e x p e c t e d to r e t a r d p o l y m e r i z a t i o n , as e x p l a i n e d b y N e t s c h e y (9K Since the n e g a t i v e l y c h a r g e d o l i g o m e r i c r a d i c a l i s u n a b l e to p e n e t r a t e the m i c e l l e b o r d e r b e c a u s e of the v i s c o u s n o n i o n i c s u r f a c t a n t and the a n i o n i c s u r f a c t a n t , the rate of p o l y m e r i z a t i o n is r e tarded. F o r the data p r e s e n t e d i n F i g u r e 3, h o w e v e r , t h e r e i s a d d i t i o n a l a n i o n i c s u r f a c t a n t (used to c o l l o i d a l l y s t a b i l i z e the e m u l s i o n ) w h i c h i s a l s o a v a i l a b l e to c r e a t e new m i c e l l e s a n d i n c r e a s e the r e a c t i o n r a t e . F i g u r e 4 shows that h i g h e r c o n c e n t r a t i o n s of s e e d l a t e x d e c r e a s e the r e a c t i o n r a t e , at constant m i x e d e m u l s i f i e r c o n c e n t r a t i o n s , w h i l e F i g u r e 5 shows that the s m a l l e s t l a t e x p a r t i c l e gave the l o w e s t r e a c t i o n r a t e . T h i s e f f e c t i s e x p l a i n e d b y the d e c r e a s e d a v a i l a b i l i t y of e m u l s i f i e r to c r e a t e m i c e l l e s , s i n c e the s m a l l s e e d l a t e x p a r t i c l e o r h i g h s e e d l a t e x c o n c e n t r a t i o n s a d s o r b m o r e of the s u r f a c t a n t , thus r e m o v i n g i t f r o m the w a t e r p h a s e . N a p p e r and P a r t s (10) h a v e s h o w n that the r e a c t i o n r a t e i s a c c e l e r a t e d w h e n the p o l y m e r s e p a r a t e s f r o m the w a t e r p h a s e


572

EMULSION POLYMERS AND EMULSION POLYMERIZATION


100 ι

0

Figure 3.

1

10

1

1

1

20 30 40 RESIDENCE TIME (min.)

Γ

50

60

Effect of mixed emulsifier concentration on conversion in tubular reactor ((E): (O) 2.2% ;(0) 4.4%; (A) 6.9%)

100 ι

ο ι 0

1

ι

1

1

10

20

1

1

30

1

ι

1

1

*

iO

50

60

RESIDENCE TIME (min.) Figure 4.

Effect of seed concentration on conversion in tubular reactor ((S): (O) 0.38%; (O) 1.29%; (A) 2.61%)



34.

LEE AND FORSYTH

Figure 5.

Seeded Polymerization

of Vinyl

Acetate

573

Effect of different seed latexes on conversion in the tubular reactor ((seed): (O) Latex I; (0) Latex II; (A) Latex III)


574

EMULSION POLYMERS AND EMULSION POLYMERIZATION


to f o r m a p o l y m e r p a r t i c l e . T h i s i n d i c a t e s that the p r i n c i p l e l o c u s of p o l y m e r i z a t i o n is the m o n o m e r - s w o l l e n p o l y m e r p a r t i c l e . F o r a l l of the data i n F i g u r e s 2-5, t h e r e a r e p o l y m e r p a r t i c l e s (seed) a v a i l a b l e , so t h e r e i s no s l o w r e t a r d e d p o l y m e r i z a t i o n r a t e , d u r i n g w h i c h the p o l y m e r p a r t i c l e s a r e b e i n g produced. P a r t i c l e S i z e . E l e c t r o n m i c r o s c o p y showed a wide, b i m o d a l range of p a r t i c l e s i z e s . M a n y p a r t i c l e s w e r e about 1000 A i n d i a m e t e r a n d w e r e p r o d u c e d b y s e c o n d a r y n u c l e a t i o n . T h e r e w e r e p a r t i c l e s of 10, 000 to 30, 000 A, p r o d u c e d b y g r o w t h of the s e e d l a t e x p a r t i c l e s . F u r t h e r s t u d i e s a r e p l a n n e d to e l i m i n a t e the s e c o n d a r y n u c l e a t i o n a n d p r o d u c e a n a r r o w e r r a n g e of p a r t i c l e s . M o l e c u l a r Weight. M o l e c u l a r weight values w e r e obtained w i t h a G P C unit. T h i s unit was s t a n d a r d i z e d f o r p o l y s t y r e n e , so the p o l y v i n y l acetate v a l u e s a r e not q u a n t i t a t i v e . T h e weight a v e r a g e m o l e c u l a r weights w e r e between 500, 000 a n d 900, 000. No t r e n d s of m o l e c u l a r weight a v e r a g e o r d i s t r i b u t i o n c o u l d be deduced. L a t e x S h e l f L i f e . T h e s h e l f l i f e of s e e d I was g r e a t e r than 6 m o n t h s , but none of the l a t e x e s p r o d u c e d i n the t u b u l a r r e a c t o r h a d a s h e l f l i f e of m o r e than a week. I n i t i a l l y , a t h i n l i q u i d l a y e r f o r m e d a t the top. A f t e r s e v e r a l w e e k s , a s o l i d l a y e r f o r m e d i n the b o t t o m of the c o n t a i n e r . D e s p i t e the h i g h l e v e l s of e m u l s i f i e r u s e d , the l a r g e p o l y v i n y l a c e t a t e p a r t i c l e s s e p a r a t e d q u i c k l y . No s t u d i e s w e r e m a d e to i n c r e a s e the s h e l f life. CONCLUSIONS T h i s study of the continuous, t u b u l a r , s e e d e d e m u l s i o n p o l y m e r i z a t i o n of v i n y l a c e t a t e has l e d to the f o l l o w i n g c o n c l u sions: 1.

Complete m o n o m e r conversion is possible with high c o n c e n t r a t i o n s of i n i t i a t o r a n d m i x e d e m u l s i f i e r .

2.

T h e u s e of s e e d i m p r o v e s the l a t e x s t a b i l i t y , but p h a s e s e p a r a t i o n s s t i l l o c c u r at l o w m o n o m e r c o n v e r s i o n s .


34. LEE AND FORSYTH

Seeded Polymerization of Vinyl Acetate

LITERATURE CITED 1.

2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Ghosh, M . ; Foster, D. W.; Lenczyk, J. P.; and Forsyth, T. H . ; "Continuous Polymerization Reactors", edited by T. C. Bouton and D. C. Chappelear, A.I.Ch. Ε. Sympo sium, 160, 102 (1976). Wallis, J. P. Α.; Ritter, R. Α . ; and Andre, Η.; A. I. Ch. E. J., 21, 686 (1975). Ghosh, M. and Forsyth, Τ. Η., in "Emulsion Polymeri zation", edited by I. Piirma and J. L . Gardon, ACS Sym posium 24, p. 367 (1976). Feldon, M . ; McCann, R. F. and Laundrie, R. W., India Rubber World, 128, 51 (1953). Rollin, A. L.; Patterson, I.; Huneault, R. and Rataille, P.; Can. J. Chem. Eng., 55, 565 (1977). I d e m , "Polymerization Reactors and Processes Symposium", edited by Henderson and Bouton, ACS Sympo sium Series, 104, 113 (1979). Dodge, J. S.; Woods, M. E . and Krieger, I. M . , J. Paint Technology, 42, 71 (1970). Dunn, A. S. and Taylor, P. Α . , Die Makromol. Chem., 83, 207 (1965). Ugelstad, J.; Mork, P. C . ; Dahl, P. and Rangnes, P . , J. Pol. Sci., C-27, 49 (1969. Friis, N. and Nyhagen, L., J. App. Pol. Sci., 19, 97 (1975). Netschey, Α.; Napper, D. H. and Alexander, Α. Ε . ; J. Pol. Sci. B-7, 829 (1969). Napper, D. H. and Parts, A. G . , J. Pol. Sci., 61, 113 (1962). Nomura, M. and Harada, Μ., in "Emulsion Polymeriza tion", edited by I. Piirma and J. L . Gardon, ACS Sympo sium 24, p. 102 (1976).

RECEIVED May 14,

1981.


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Emulsion Polymers and Emulsion Polymerization - American

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