The Formation of Coagulum in Emulsion Polymerization - ACS

11 The Formation of Coagulum in Emulsion Polymerization J. W. VANDERHOFF

Downloaded by PURDUE UNIVERSITY on June 13, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch011

Emulsion Polymers Institute and Department of Chemistry, Lehigh University, Bethlehem, PA 18015 In emulsion polymerization, a water-immiscible monomer is emulsified in water using an oil-in-water emulsifier and polymerized using a water-soluble or oil-soluble initiator. The product is a latex, i.e., a colloidal sol comprised of submicroscopic polymer spheres suspended in water. The emulsion polymerization may be carried out by: (i) batch polymerization in which all ingredients are added to the reactor and the mixture is heated under agitation to the polymerization temperature: (ii) semi-continuous polymerization in which the monomer as well as other ingredients are added continuously or in increments over the course of the polymerization to remove the heat of reaction which would otherwise exceed the cooling capacity of the reactor; (iii) continuous polymerization in which all ingredients are added continuously to a tubular reactor or a continuous stirredtank reactor, either singly or in series, and partially or completely polymerized latex is removed continuously. All three types of emulsion polymerization can be carried out using seeded emulsion polymerization, i.e., by adding monomer, initiator, and emulsifier to a previously-prepared small-particle-size latex, the particles of which grow in size without initiation of a new crop of particles. The purpose of seeded emulsion polymerization is to avoid the uncertainties of the particle initiation stage, obtain better batch-to-batch reproducibility, and give a stable latex of the desired particle size. The usual description of these different polymerization processes suggests that all produce stable latexes and various hypotheses have been advanced to explain the stability of these latexes to such factors as added electrolyte, mechanical shear and freezing and thawing. In the literature, there is little mention of the fact that many of these polymerizations produce varying amounts of coagulum, i.e., polymer recovered in a form other than that of a stable latex. This coagulum is produced in all sizes of polymerization reactors, ranging from the smallest laboratory

0097-6156/81/0165-0199$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.


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r e a c t o r t o t h e l a r g e s t p r o d u c t i o n r e a c t o r . I t may be a mere nuisance i n small l a b o r a t o r y p o l y m e r i z a t i o n r e a c t o r s , but i n l a r g e - s c a l e p o l y m e r i z a t i o n s i t may p r e v e n t t h e s c a l e - u p o f a comm e r c i a l l y - a c c e p t a b l e l a t e x o r exact a heavy economic p e n a l t y i n l o n g e r c y c l e t i m e s and r e d u c e d y i e l d s . The f o r m a t i o n o f c o a g u l u m i s o b s e r v e d i n a l l t y p e s o f e m u l s i o n polymers: ( i ) s y n t h e t i c rubber l a t e x e s such as butadienes t y r e n e , a c r y l o n i t r i l e - b u t a d i e n e , and b u t a d i e n e - s t y r e n e - v i n y l p y r i d i n e c o p o l y m e r s a s w e l l a s p o l y b u t a d i e n e , p o l y c h l o r o p r e n e , and p o l y i s o p r e n e ; ( i i ) c o a t i n g s l a t e x e s s u c h as s t y r e n e - b u t a d i e n e , a c r y l a t e e s t e r , v i n y l a c e t a t e , v i n y l c h l o r i d e , and e t h y l e n e c o polymers; ( i i i ) p l a s t i s o l r e s i n s such as p o l y v i n y l c h l o r i d e ; ( i v ) s p e c i a l t y l a t e x e s s u c h as p o l y e t h y l e n e , p o l y t e t r a f l u o r o e t h y l e n e , and o t h e r f l u o r i n a t e d p o l y m e r s ; ( v ) i n v e r s e l a t e x e s o f p o l y a c r y l a m i d e and o t h e r w a t e r - s o l u b l e p o l y m e r s p r e p a r e d b y i n v e r s e e m u l s i o n polymerization. T h e r e a r e no m a j o r l a t e x c l a s s e s p r o d u c e d b y e m u l s i o n p o l y m e r i z a t i o n t h a t a r e c o m p l e t e l y f r e e o f coagulum f o r mation during o r a f t e r polymerization. The c o a g u l u m f o r m e d d u r i n g p o l y m e r i z a t i o n may t a k e many f o r m s and i s commonly r e f e r r e d t o b y many names, o f t e n c o l l o q u i a l , e . g . , r e a c t o r f o u l i n g , f i l t e r a b l e s o l i d s , button, sediment, s i l t , g r i t , seeds, sand, waste, s c r a p , o r worse. I n this discussion,the t e r m " c o a g u l u m w i l l be u s e d t o d e n o t e a n y p o l y m e r r e c o v e r e d i n a form o t h e r than s t a b l e l a t e x . 11

F o r m a t i o n o f Coagulum The c o a g u l u m formed i n a l a t e x c a n be d i v i d e d i n t o t h r e e main types: ( i ) c o a g u l u m f o r m e d d u r i n g p o l y m e r i z a t i o n and r e c o v ered from the l a t e x afterwards by f i l t r a t i o n o r s e d i m e n t a t i o n ; ( i i ) coagulum d e p o s i t e d on t h e r e a c t o r s u r f a c e s d u r i n g p o l y m e r i z a t i o n , on the w a l l s , r o o f , bottom, a g i t a t o r , b a f f l e , thermowell, o r c o o l i n g c o i l ; ( i i i ) coagulum formed i n t h e l a t e x a f t e r p o l y m e r i z a t i o n , during storage o r t r a n s p o r t a t i o n . The c o a g u l u m formed d u r i n g p o l y m e r i z a t i o n may r a n g e f r o m a s i n g l e lump o f p o l y m e r w i t h l i t t l e f l u i d l a t e x t o t i n y s a n d - l i k e g r a i n s i n an o t h e r w i s e s t a b l e l a t e x . I ti s unusual f o r a l a t e x to coagulate completely d u r i n g p o l y m e r i z a t i o n , t o s o l i d i f y i n the r e a c t o r and c a u s e t h e a g i t a t o r t o s e i z e ; n e v e r t h e l e s s , i t o c c u r s o f t e n enough t o w a r r a n t d e v e l o p m e n t o f means t o remove t h e s o l i d i f i e d polymer. U s u a l l y , t h e l a t e x coagulates o n l y p a r t i a l l y , t o f o r m lumps o f c o a g u l u m w h i c h c a n b e removed b y f i l t r a t i o n o r s e d i mentation o r which remain i n the r e a c t o r a f t e r t h e l a t e x i s d r a i n ed; t h e l a t e x i s u s u a l l y r e l a t i v e l y s t a b l e a f t e r r e m o v a l o f t h e c o a g u l u m . The lumps o f c o a g u l u m f i l t e r e d f r o m t h e l a t e x may be s o l i d o r p o r o u s . The s o l i d lumps may b e s o f t and s t i c k y o r h a r d , f r i a b l e , and g r a n u l a r a c c o r d i n g t o t h e p o l y m e r and i t s d e g r e e o f p l a s t i c i z a t i o n w i t h monomer. The p o r o u s lumps a r e u s u a l l y r e l a t i v e l y h a r d and f r i a b l e . O c c a s i o n a l l y , t h e coagulum i s t o o f i n e i n p a r t i c l e s i z e t o be removed e a s i l y b y f i l t r a t i o n o r s e d i m e n t a -



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t i o n ; i n t h i s case, i t remains d i s p e r s e d i n the l a t e x , but manif e s t s i t s e l f a s s u r f a c e r o u g h n e s s o r h a z i n e s s when a f i l m i s c a s t from t h e l a t e x . The c o a g u l u m d e p o s i t e d o n t h e r e a c t o r s u r f a c e s may b e lumps and a c c r e t i o n s o n t h e r e a c t o r w a l l s o r r o o f a n d o n t h e a g i t a t o r s h a f t a n d b l a d e s , b a f f l e s , t h e r m o w e l l , o r c o o l i n g c o i l ; i t may b e deposited on the r e a c t o r as a surface l a y e r o r " s k i n " . The lumps and a c c r e t i o n s o n t h e v a r i o u s r e a c t o r s u r f a c e s may b e s t i c k y o r hard and g r a n u l a r a c c o r d i n g t o t h e polymer c o m p o s i t i o n and i t s deg r e e o f p l a s t i c i z a t i o n w i t h monomer. S i m i l a r l y , t h e s u r f a c e l a y e r on t h e r e a c t o r s u r f a c e may d i s p l a y t h e p r o p e r t i e s o f t h e p o l y m e r i f i t i s r e l a t i v e l y t h i n and u n p l a s t i c i z e d ; a t h i c k e r f i l m p l a s t i c i z e d w i t h monomer may b e s o f t a n d s t i c k y . O f t e n , c o a g u l u m may a l s o b e f o u n d o n t h e b o t t o m o f t h e r e a c t o r where i t i s d e p o s i t e d when t h e l a t e x i s d r a i n e d f r o m t h e r e a c t o r . G e n e r a l l y , t h e c o a g u lum l e f t i n t h e r e a c t o r r e s e m b l e s t h e lumps o f c o a g u l u m f i l t e r e d f r o m t h e l a t e x ; h o w e v e r , i n some c a s e s , i t i s d i f f e r e n t , s u g g e s t i n g t h a t i t was f o r m e d b y a d i f f e r e n t mechanism. The c o a g u l u m f o r m e d i n t h e l a t e x a f t e r p o l y m e r i z a t i o n d u r i n g s t o r a g e o r t r a n s f e r may r e s u l t f r o m f l o c c u l a t i o n o f t h e l a t e x b y m e c h a n i c a l s h e a r , e . g . , b y pumping t h e l a t e x f r o m t h e r e a c t o r t o t h e s t o r a g e t a n k and f r o m t h e s t o r a g e t a n k t o t h e t a n k c a r o r t r u c k , t r a n s p o r t a t i o n t o t h e c u s t o m e r s p l a n t , a n d pumping f r o m the tank c a r o r t r u c k i n t o the customer s storage tank. Coagulum may a l s o b e f o r m e d b y t e m p e r a t u r e v a r i a t i o n s w i t h i n t h e l a t e x d u r i n g s t o r a g e o r t r a n s p o r t a t i o n . Of c o u r s e , f r e e z i n g o f t e n causes f l o c c u l a t i o n o f l a t e x e s ; however, t h e s t a b i l i t y o f l a t e x e s v a r i e s w i t h t e m p e r a t u r e above t h e f r e e z i n g p o i n t , and a l a t e x t h a t i s s t a b l e a t room t e m p e r a t u r e may s l o w l y f l o c c u l a t e when t h e temperature i s r a i s e d o r lowered s u f f i c i e n t l y . This slow f l o c c u l a t i o n may n o t b e n o t i c e a b l e o v e r a s h o r t t i m e , b u t may r e s u l t i n the f o r m a t i o n o f s i g n i f i c a n t coagulum over a l o n g e r t i m e . Indeed a c c e l e r a t e d s t o r a g e s t a b i l i t y t e s t s , e.g., h e a t i n g t h e l a t e x a t 50°, a r e o f t e n u s e d i n i n d u s t r y t o p r e d i c t s t o r a g e s t a b i l i t y a t a m b i e n t c o n d i t i o n s . A l s o , some l a t e x e s may c o n t a i n m i c r o s c o p i c p o l y m e r p a r t i c l e s t h a t s l o w l y s e t t l e o r cream d u r i n g s t o r a g e . The c r i t i c a l p a r t i c l e s i z e f o r s e t t l i n g o r c r e a m i n g c a n b e e s t i m a ted using Overbeek s c r i t e r i o n ( 1 ) , i . e . , a p a r t i c l e that s e t t l e s o r creams a t a r a t e o f o n l y 1 mm i n 24 h o u r s a c c o r d i n g t o S t o k e s law w i l l n e v e r s e t t l e o r cream i n p r a c t i c e because o f t h e Brownian m o t i o n o f t h e p a r t i c l e s and t h e chance thermal convect i o n w i t h i n t h e sample^ F o r example, f o r p o l y s t y r e n e p a r t i c l e s ( d e n s i t y = 1.050 gm/cm ) t h e c r i t i c a l s i z e f o r s e t t l i n g i n w a t e r i s 0.65 ym, w h i c h h a s b e e n c o n f i r m e d b y many o b s e r v a t i o n s o f t h e s t o r a g e s t a b i l i t y o f m o n o d i s p e r s e p o l y s t y r e n e l a t e x e s . The s e t t l i n g o r c r e a m i n g o f t h e s e m i c r o s c o p i c p a r t i c l e s may f o r m a d e n s e l a y e r which w i l l n o t pass through a f i l t e r and thus appear a s c o agulum . The f o r m a t i o n o f c o a g u l u m d u r i n g o r a f t e r p o l y m e r i z a t i o n presents s e r i o u s problems i n i n d u s t r i a l l a t e x p r o d u c t i o n f o r s e v 1

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e r a l reasons: ( i ) i t d e c r e a s e s t h e y i e l d o f t h e polymer by t h e amount o f c o a g u l u m f o r m e d ; ( i i ) i t i n c r e a s e s t h e "down t i m e " o f the r e a c t o r , thus l e n g t h e n i n g t h e c y c l e time u n p r e d i c t a b l y ; ( i i i ) i t n e c e s s i t a t e s c l e a n i n g t h e r e a c t o r between s u c c e s s i v e p o l y m e r i zation batches; ( i v ) i t increases the batch-to-batch v a r i a t i o n i n l a t e x q u a l i t y a n d p r o p e r t i e s ; ( v ) t h e c o a g u l u m must b e d i s p o s e d o f u s u a l l y by i n c i n e r a t i o n o r b u r i a l i n s a n i t a r y l a n d f i l l ; ( v i ) t h e s w e l l i n g o f c o a g u l u m w i t h monomer may p o s e a h e a l t h h a z a r d , p a r t i c u l a r l y f o r monomers t h a t h a v e b e e n shown, o r a r e s u s p e c t e d , t o be t o x i c , e . g . , v i n y l c h l o r i d e a n d a c r y l o n i t r i l e . These d i s a d v a n t a g e s e m p h a s i z e t h e i m p o r t a n c e o f d e t e r m i n i n g t h e mechanism o f c o a g u l u m f o r m a t i o n a n d d e v e l o p i n g methods t o r e d u c e o r e l i m i n a t e i t . M e c h a n i s m o f F o r m a t i o n o f Coagulum Two mechanisms p r o p o s e d f o r t h e f o r m a t i o n o f c o a g u l u m a r e : (i) a f a i l u r e o f thec o l l o i d a l s t a b i l i t y o f the l a t e x during o r a f t e r t h e p o l y m e r i z a t i o n , t o cause f l o c c u l a t i o n o f the p a r t i c l e s and e v e n t u a l l y f o r m m i c r o s c o p i c a n d m a c r o s c o p i c c o a g u l u m ; ( i i ) p o l y m e r i z a t i o n o f t h e monomer b y a mechanism o t h e r t h a n t h a t o f emulsion p o l y m e r i z a t i o n , t o g i v e polymer o f d i f f e r e n t form than latex particles. Latex

Stability Failure

The f a i l u r e o f l a t e x s t a b i l i t y , a n d t h e r e s u l t a n t f l o c c u l a t i o n o f t h e l a t e x p a r t i c l e s , m a y cause t h e f o r m a t i o n o f coagulum t h a t i s r e c o v e r e d from t h e l a t e x a f t e r p o l y m e r i z a t i o n as w e l l a s a b u i l d u p on t h e r e a c t o r s u r f a c e s . M o r e o v e r , t h e i n h e r e n t i n s t a b i l i t y o f t h e l a t e x may a l s o c a u s e f l o c c u l a t i o n d u r i n g s t o r a g e o r t r a n s p o r tation. The e x p e r i m e n t a l f a c t o r s t h a t a f f e c t t h e s t a b i l i t y o f t h e l a tex d u r i n g and a f t e r p o l y m e r i z a t i o n a r e t h e r e c i p e used f o r t h e p o l y m e r i z a t i o n , t h e type and i n t e n s i t y o f a g i t a t i o n d u r i n g and a f t e r t h e p o l y m e r i z a t i o n , t h e temperature o f p o l y m e r i z a t i o n and s t o r a g e , a n d t h e a g e a n d s t o r a g e c o n d i t i o n s o f t h e l a t e x . The r e c i p e u s e d i n t h e p o l y m e r i z a t i o n i n c l u d e d t h e mode o f p o l y m e r i z a t i o n , t h e monomer-water r a t i o , t h e s o l u b i l i t y o f t h e monomer i n water, t h e e m u l s i f i e r type and c o n c e n t r a t i o n , i n i t i a t o r type and c o n c e n t r a t i o n , t o t a l e l e c t r o l y t e c o n c e n t r a t i o n , and i m p u r i t i e s present i n the system. The s t a b i l i t y o f l a t e x e s d u r i n g a n d a f t e r p o l y m e r i z a t i o n may be a s s e s s e d a t l e a s t q u a l i t a t i v e l y b y t h e t h e o r e t i c a l r e l a t i o n ships describing the s t a b i l i t y o f lyophobic c o l l o i d s . The V e r w e y O v e r b e e k t h e o r y (2) c o m b i n e s t h e e l e c t r o s t a t i c f o r c e s o f r e p u l s i o n between c o l l o i d a l p a r t i c l e s w i t h t h e London-van d e r Waals f o r c e s of a t t r a c t i o n . The e l e c t r o s t a t i c f o r c e s o f r e p u l s i o n a r i s e f r o m the s u r f a c e charge, e.g., from adsorbed e m u l s i f i e r i o n s , s u r f a c e s u l f a t e endgroups i n t r o d u c e d by p e r s u l f a t e i n i t i a t o r , o r i o n i c g r o u p s i n t r o d u c e d b y u s i n g f u n c t i o n a l monomers. T h e s e e l e c t r o -



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s t a t i c r e p u l s i o n forces are greatest c l o s e s t to the p a r t i c l e surf a c e and decrease e x p o n e n t i a l l y w i t h i n c r e a s i n g d i s t a n c e from the p a r t i c l e surface. The r a t e o f e x p o n e n t i a l d e c r e a s e i s d e p e n d e n t upon t h e e l e c t r o l y t e c o n c e n t r a t i o n : the higher the concentration and t h e h i g h e r t h e v a l e n c e o f t h e c o u n t e r i o n s , t h e more r a p i d t h e d e c r e a s e . The L o n d o n - v a n d e r W a a l s f o r c e s o f a t t r a c t i o n a r i s e from the d i f f e r e n c e i n d i e l e c t r i c c o n s t a n t between the p a r t i c l e s and t h e medium. These a t t r a c t i v e f o r c e s a r e g r e a t e s t c l o s e s t t o the p a r t i c l e s u r f a c e and decrease e x p o n e n t i a l l y w i t h i n c r e a s i n g d i s t a n c e from t h e p a r t i c l e s u r f a c e . U n l i k e the e l e c t r o s t a t i c r e p u l s i o n f o r c e s , t h e London-van der Waals f o r c e s a r e o n l y s l i g h t l y a f f e c t e d by e l e c t r o l y t e c o n c e n t r a t i o n and o t h e r parameters o f t h e s y s t e m . The n e t o v e r a l l i n t e r a c t i o n b e t w e e n c o l l o i d a l p a r t i c l e s i s d e s c r i b e d b y t h e sum o f t h e p o t e n t i a l e n e r g i e s o f r e p u l s i o n and a t t r a c t i o n . The u s u a l r e s u l t f o r s t a b l e c o l l o i d a l s o l s i s a deep p o t e n t i a l e n e r g y w e l l o f a t t r a c t i o n a t v e r y s m a l l i n t e r p a r t i c l e d i s t a n c e s , w i t h the p o t e n t i a l energy o f i n t e r a c t i o n r i s i n g s h a r p l y t o a r e p u l s i o n peak a t s l i g h t l y g r e a t e r d i s t a n c e s , f o l lowed by a slow decrease t o zero w i t h i n c r e a s i n g i n t e r p a r t i c l e distance. F o r r e l a t i v e l y l a r g e p a r t i c l e s , the slow decrease from t h e r e p u l s i o n peak may g i v e a s h a l l o w s e c o n d a r y minimum o f a t t r a c tion a t relatively large interparticle distances. Thus t h e o v e r a l l i n t e r a c t i o n w i l l r e s u l t i n f l o c c u l a t i o n i f t h e energy o f c o l l i s i o n i s g r e a t enough t o surmount t h e r e p u l s i o n peak and t h e i n t e r p a r t i c l e d i s t a n c e d e c r e a s e s t o t h a t c o r r e s p o n d i n g t o t h e deep primary a t t r a c t i o n p o t e n t i a l energy w e l l . Once f l o c c u l a t e d i n the primary p o t e n t i a l energy w e l l , the p a r t i c l e s a r e d i f f i c u l t t o s e p a r a t e and w i l l r e m a i n i n t h e f l o c c u l a t e d s t a t e . An i n c r e a s e i n e l e c t r o l y t e concentration o r valence o f the counterion lowers t h e r e p u l s i o n peak a n d t h u s f a v o r s f l o c c u l a t i o n . S i m i l a r l y , part i c l e s f l o c c u l a t e d i n t h e s e c o n d a r y a t t r a c t i o n minimum may b e r e dispersed without d i f f i c u l t y . I n a d d i t i o n t o e l e c t r o s t a t i c r e p u l s i o n and London^van der W a a l s a t t r a c t i o n , s t e r i c s t a b i l i z a t i o n (3) must b e t a k e n i n t o a c count. S t e r i c s t a b i l i z a t i o n r e s u l t s from the a d s o r p t i o n o f an uncharged hydrated species on the p a r t i c l e s u r f a c e , which h i n d e r s the approach o f o t h e r p a r t i c l e s and thus a c t s a s a m e c h a n i c a l b a r r i e r to flocculation. S t e r i c s t a b i l i z e r s i n c l u d e such uncharged w a t e r - s o l u b l e polymers a s m e t h y l c e l l u l o s e o r n o n i o n i c e m u l s i f i e r s such as nonylphenol-ethylene o x i d e a d d u c t s . The p o t e n t i a l e n e r g y of i n t e r a c t i o n between s t e r i c a l l y - s t a b i l i z e d c o l l o i d a l p a r t i c l e s i s r e p u l s i o n a t v e r y s m a l l i n t e r p a r t i c l e d i s t a n c e s , f o l l o w e d by a r a p i d d e c r e a s e t o a s h a l l o w a t t r a c t i o n w e l l a t somewhat l a r g e r d i s t a n c e s , a n i n c r e a s e t o a r e l a t i v e l y l o w r e p u l s i o n peak a t s l i g h t l y g r e a t e r d i s t a n c e s , and a s l o w d e c r e a s e t o z e r o w i t h i n creasing distance. Thus s t e r i c a l l y - s t a b i l i z e d p a r t i c l e s t e n d t o f l o c c u l a t e i n the shallow p o t e n t i a l energy w e l l and a r e r e l a t i v e l y easy t o r e d i s p e r s e . I t s h o u l d b e n o t e d , h o w e v e r , t h a t t h e same u n c h a r g e d p o l y m e r m o l e c u l e s t h a t g i v e r i s e t o s t e r i c s t a b i l i z a t i o n a l s o may c a u s e


204

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AND



1 1

f l o c c u l a t i o n o f c o l l o i d a l p a r t i c l e s by " b r i d g i n g (4). Steric s t a b i l i z a t i o n r e s u l t s from the a d s o r p t i o n o f an uncharged h y d r a t e d polymer molecule on a s i n g l e p a r t i c l e , perhaps w i t h l o o p s o f t h e p o l y m e r c h a i n e x t e n d i n g i n t o t h e a q u e o u s p h a s e . F l o c c u l a t i o n may r e s u l t f r o m t h e a d s o r p t i o n o f t h e same m o l e c u l e o n more t h a n o n e p a r t i c l e t o f o r m a " b r i d g e " b e t w e e n them; a s u f f i c i e n t number o f s u c h b r i d g e s w o u l d c a u s e c o m p l e t e f l o c c u l a t i o n o f t h e l a t e x . The r e s u l t o f a d d i t i o n o f such polymers steric stabilization or flocculation i s determined by t h e s p e c i f i c system, t h e r e l a t i v e c o n c e n t r a t i o n o f p o l y m e r a n d p a r t i c l e s , a n d t h e method o f m i x i n g t h e polymer w i t h t h e l a t e x . Such f l o c c u l a t i o n i s a l s o o b served w i t h polymer m o l e c l u e s c o n t a i n i n g i o n i c groups. I t should be e m p h a s i z e d t h a t t h e u s e o f f u n c t i o n a l monomers s u c h a s a c r y l i c a c i d o r 2 - s u l f o e t h y l m e t h a c r y l a t e may f o r m s u c h p o l y m e r s i n t h e aqueous serum o f t h e l a t e x . The f l o c c u l a t i o n o f c o l l o i d a l p a r t i c l e s c a n be d i v i d e d into two m a i n t y p e s : (i) diffusion-controlled flocculation; ( i i ) agitation-induced flocculation. The d i f f u s i o n - c o n t r o l l e d flocculat i o n has been a n a l y z e d t h e o r e t i c a l l y by v o n Smoluchowski ( 5 ) . Each p a r t i c l e i s c o n s i d e r e d a s a c e n t e r t o w h i c h o t h e r p a r t i c l e s d i f f u s e b y B r o w i a n m o t i o n . Thus t h e r a t e o f f l o c c u l a t i o n i s p r o p o r t i o n a l t o t h e s q u a r e o f t h e number o f p a r t i c l e s . The o r i g i n a l t r e a t m e n t assumed t h a t a l l i n t e r p a r t i c l e c o l l i s i o n s w e r e e f f e c t i v e i ncausing f l o c c u l a t i o n . L a t e r m o d i f i c a t i o n s ( 6 ) assumed t h a t t h e p o t e n t i a l energy b a r r i e r between p a r t i c l e s r e s i s t s f l o c c u l a t i o n and t h a t o n l y those c o l l i s i o n s w i t h s u f f i c i e n t energy t o overcome t h i s b a r r i e r w i l l c a u s e f l o c c u l a t i o n . The a g i t a t i o n induced f l o c c u l a t i o n has a l s o been a n a l y z e d t h e o r e t i c a l l y ( 7 ) . I n t h i s c a s e , t h e r a t e o f f l o c c u l a t i o n depends upon t h e c o l l i s i o n r a d i i o f t h e p a r t i c l e s and t h e r a t e o f shear. A c o m b i n a t i o n o f t h e two t h e o r i e s ( 8 ) shows t h a t d i f f u s i o n - c o n t r o l l e d flocculation i s p r e d o m i n a n t a t p a r t i c l e s i z e s o f c a . O.lym, b u t a g i t a t i o n - i n d u c e d f l o c c u l a t i o n becomes e q u i v a l e n t a t s i z e s o f c a . l y m a n d b e comes p r e d o m i n a n t a t l a r g e r p a r t i c l e s i z e s . This finding applied to l a t e x e s s u g g e s t s t h a t t h e i n i t i a l s t a g e s o f f l o c c u l a t i o n a r e d i f f u s i o n - c o n t r o l l e d , b u t a s t h e a g g r e g a t e s grow i n s i z e , t h e y e v e n t u a l l y r e a c h t h e s i z e where a g i t a t i o n - i n d u c e d f l o c c u l a t i o n becomes c o m p e t i t i v e , a n d grow much more r a p i d l y when t h e a g i t a t i o n - i n d u c e d f l o c c u l a t i o n becomes p r e d o m i n a n t . Thus a n i n h e r e n t i n s t a b i l i t y o f a l a t e x d u r i n g p o l y m e r i z a t i o n may r e s u l t i n a n autoaccelerating f l o c c u l a t i o n o f thep a r t i c l e s as the polymerizat i o n proceeds. The f o r m a t i o n o f c o a g u l u m c a n o f t e n b e c o r r e l a t e d w i t h : ( i ) c o n v e r s i o n ; ( i i ) a g i t a t i o n r a t e ; ( i i i ) mode o f a d d i t i o n o f i n g r e dients. G e n e r a l l y , t h e amount o f c o a g u l u m f o r m e d i n c r e a s e s w i t h i n c r e a s i n g c o n v e r s i o n , b u t t h i s o b s e r v a t i o n may b e t h e r e s u l t o f an i n i t i a l s t a b i l i t y f a i l u r e w h i c h becomes more o b v i o u s a s t h e p o l y m e r i z a t i o n proceeds. S i n c e l a t e x e s c a n be f l o c c u l a t e d by m e c h a n i c a l s h e a r , t h e a g i t a t i o n r a t e u s e d must b e s u f f i c i e n t t o g i v e good m i x i n g a n d h e a t t r a n s f e r , y e t n o t so g r e a t a s t o c a u s e


11.

VANDERHOFF

Coagulum

in Emulsion

Polymerization

205

flocculation. Generally, f o r batch polymerizations, the a g i t a t i o n r a t e s h o u l d b e v i g o r o u s t o f o r m t h e monomer e m u l s i o n , moderate from the i n i t i a t i o n o f p o l y m e r i z a t i o n t o t h e disappearance o f t h e monomer d r o p l e t s , and m i l d ( o n l y s u f f i c i e n t t o m a i n t a i n good h e a t t r a n s f e r a n d m i x i n g ) t h e r e a f t e r . The mode o f a d d i t i o n o f i n g r e d i e n t s a l s o a f f e c t s t h e f o r m a t i o n o f coagulum, e.g., c o n t i n uous a d d i t i o n o f monomer o f t e n r e d u c e s t h e amount o f c o a g u l u m compared t o t h a t formed i n b a t c h p o l y m e r i z a t i o n .


D i f f e r e n t Mechanism o f P o l y m e r i z a t i o n P o l y m e r i z a t i o n o f monomer b y a d i f f e r e n t mechanism a l s o c a u s e s t h e f o r m a t i o n o f c o a g u l u m . Coagulum f o u n d i n t h e l a t e x may r e s u l t f r o m p o l y m e r i z a t i o n i n l a r g e monomer d r o p l e t s o r a s e p a r a t e l a y e r o f monomer. I n most p o l y m e r i z a t i o n s , monomer e m u l s i o n d r o p l e t s a r e l-10ym i n d i a m e t e r . The c h a n c e e n t r y o f a r a d i c a l i n t o these d r o p l e t s causes t h e i r p o l y m e r i z a t i o n t o form m i c r o s c o p i c (as opposed t o s u b m i c r o s c o p i c ) p a r t i c l e s ; however, although these microscopic p a r t i c l e s a r e l e s s s t a b l e than the submicroscopic p a r t i c l e s , t h e presence o f o n l y a few such p a r t i c l e s i s u s u a l l y not s u f f i c i e n t t o r e s u l t i n the formation o f coagu l u m . I n l a r g e r e a c t o r s , h o w e v e r , t h e r e may e x i s t l a r g e monomer d r o p s a s w e l l a s a s e p a r a t e l a y e r o f monomer, p a r t i c u l a r l y i f t h e a g i t a t i o n i s n o t s u f f i c i e n t t o mix a l l p a r t s o f t h e system equally well. I n t h i s case, e n t r y o f r a d i c a l s i n t o these l a r g e drops and t h e s e p a r a t e l a y e r w i l l c a u s e a b u l k p o l y m e r i z a t i o n t h a t w i l l c e r t a i n l y r e s u l t i n t h e f o r m a t i o n o f coagulum. The c o a g u l u m d e p o s i t e d o n t h e r e a c t o r s u r f a c e s may b e t h e r e s u l t o f p o l y m e r i z a t i o n i n l a r g e monomer d r o p s o r a s e p a r a t e monomer l a y e r , o r i t may b e t h e r e s u l t o f p o l y m e r i z a t i o n o f t h e monomer i n t h e v a p o r s p a c e a b o v e t h e l a t e x o r a s u r f a c e p o l y m e r i z a t i o n on the w a l l s and roof o f the r e a c t o r . P o l y m e r i z a t i o n i n the vapor space o f the r e a c t o r w i l l form s o l i d polymer i n t h e f o r m o f p a r t i c l e s w h i c h may s t i c k t o t h e r e a c t o r s u r f a c e s o r f a l l i n t o the l a t e x ; i n the l a t e r case, these p a r t i c l e s serve as nuc l e i f o r t h e f o r m a t i o n o f c o a g u l u m . P o l y m e r i z a t i o n o f monomer o n t h e r e a c t o r s u r f a c e s w i l l f o r m s o l i d p a r t i c l e s t h a t become s w o l l e n w i t h monomer a n d grow by f l o c c u l a t i o n o f t h e l a t e x p a r t i c l e s . The s u r f a c e p o l y m e r i z a t i o n c a n b e r e l a t e d t o t h e s m o o t h n e s s o f the r e a c t o r s u r f a c e ; t h e smoother t h e s u r f a c e , t h e l e s s e r t h e tendency f o r s u r f a c e p o l y m e r i z a t i o n and f o r m a t i o n o f coagulum. In t h i s respect, g l a s s - l i n e d reactors are superior t o s t a i n l e s s s t e e l r e a c t o r s , although h i g h l y polished s t a i n l e s s s t e e l surfaces a r e o f t e n s a t i s f a c t o r y . Spots where t h e g l a s s l i n i n g has been f r a c t u r e d and i s m i s s i n g , o r scratches i n the s t a i n l e s s s t e e l w a l l s , s e r v e a s n u c l e i f o r t h e f o r m a t i o n o f c o a g u l u m . The i m p o r t a n t p a r a m e t e r i s t h e w e t t i n g o f t h e r e a c t o r s u r f a c e b y t h e monomer-polymer phase; such w e t t i n g i s f a c i l i t a t e d b y s u r f a c e roughn e s s o r d i s c o n t i n u i t i e s a n d i n h i b i t e d b y a p e r f e c t l y smooth s u r face.


206

EMULSION POLYMERS

AND



E l i m i n a t i o n o f Coagulum T h e r e a r e no g e n e r a l methods o r a p p r o a c h e s t o e l i m i n a t e t h e f o r m a t i o n o f coagulum i n e m u l s i o n p o l y m e r i z a t i o n s . Each polymeri z a t i o n s y s t e m must b e c o n s i d e r e d s e p a r a t e l y , a n d a l t h o u g h remed i e s w o r k e d o u t i n one s y s t e m may be u s e f u l i n a n o t h e r s y s t e m , t h e s e d i f f e r e n c e s a r e s u c h t h a t t h e r e a r e o n l y a few g e n e r a l i z a t i o n s t h a t c a n be a p p l i e d . G e n e r a l l y , t h e c o a g u l u m c a n be r e duced o r e l i m i n a t e d b y m o d i f i c a t i o n o f t h e p o l y m e r i z a t i o n r e c i p e and t e c h n i q u e o r by m o d i f i c a t i o n o f t h e r e a c t o r . There a r e seve r a l approaches t o reduce o r e l i m i n a t e coagulum by m o d i f i c a t i o n of t h e p o l y m e r i z a t i o n r e c i p e and technique: ( i ) the substitution of a seeded system f o r a unseeded system o f t e n e l i m i n a t e s t h e f o r m a t i o n o f a coagulum and d e c r e a s e s t h e b a t c h - t o - b a t c h v a r i a t i o n s i nproduct p r o p e r t i e s ; ( i i ) a d d i t i o n o f s t a b i l i z i n g emulsif i e r a t t h e a p p r o p r i a t e c o n v e r s i o n may s t a b i l i z e p a r t i c l e s t h a t would o t h e r w i s e f l o c c u l a t e ; ( i i i ) r i g o r o u s temperature c o n t r o l o f t e n d e c r e a s e s t h e amount o f c o a g u l u m f o r m e d w h e r e a s a l l o w i n g the temperature t o r i s e s l i g h t l y d u r i n g t h e r a p i d exothermic p o l y m e r i z a t i o n g i v e s r i s e t o f o r m a t i o n o f coagulum; ( i v ) v a r y i n g t h e mode o f monomer a d d i t i o n may e l i m i n a t e t h e f o r m a t i o n o f c o a g u l u m , e . g . , a d d i n g t h e monomer c o n t i n u o u s l y i n s t e a d o f a t t h e b e ginning o f t h e r e a c t i o n ; (v) v a r i a t i o n o f the a g i t a t i o n r a t e , u s u a l l y a d e c r e a s e a t each stage o f t h e r e a c t i o n t o t h e l e v e l j u s t s u f f i c i e n t t o m a i n t a i n good t r a n s f e r a n d m i x i n g ; ( v i ) a b e t t e r u n d e r s t a n d i n g o f t h e p o l y m e r i z a t i o n s y s t e m may s u g g e s t a l t e r a t i o n s o f t h e r e c i p e t h a t w i l l be e f f e c t i v e . There a r e s e v e r a l approaches t o reduce o r e l i m i n a t e t h e f o r m a t i o n o f coagulum by m o d i f i c a t i o n o f t h e r e a c t o r d e s i g n : ( i ) s u b s t i t u t i o n o f a semi-continuous process f o r a b a t c h process and a continuous process for a semi-continuous process; ( i i ) s u b s t i t u t i o n o f a d i f f e r e n t r e a c t o r c o n f i g u r a t i o n , e.g., a tumbler type may g i v e l e s s c o a g u l u m t h a n a s t i r r e d t a n k r e a c t o r a n d a t u b e r e a c t o r u s e d f o r c o n t i n u o u s p o l y m e r i z a t i o n may g i v e l e s s c o a g u l u m than a b a t c h r e a c t o r ; ( i i i ) m o d i f i c a t i o n o f t h e a g i t a t o r and b a f f l e system t o ensure u n i f o r m a g i t a t i o n throughout t h e r e a c t o r and complete b u t m i l d m i x i n g o f t h e i n g r e d i e n t s ; ( i v ) b e t t e r temperat u r e c o n t r o l ; ( v ) d i f f e r e n t mode o f a d d i t i o n o f i n g r e d i e n t s , e.g., a d d i t i o n o f monomer b e l o w t h e s u r f a c e o f t h e l a t e x r a t h e r than by dropping i t through t h e vapor space t o t h e upper s u r f a c e of t h e l a t e x . Despite these g e n e r a l i z a t i o n s , the reduction o r e l i m i n a t i o n o f coagulum i s u s u a l l y b e s t a c c o m p l i s h e d by a "systems a p p r o a c h " , i . e . , a c o n s i d e r a t i o n o f l a t e x p r o p e r t i e s t o be a c h i e v e d i n t h e emulsion p o l y m e r i z a t i o n , t h e economics o f t h e p o l y m e r i z a t i o n process, and t h e d e l i b e r a t e design o f t h e r e a c t o r system f o r that p a r t i c u l a r p o l y m e r i z a t i o n system. Each p o l y m e r i z a t i o n system must be c o n s i d e r e d a s a s e p a r a t e s y s t e m a n d t r e a t e d a s s u c h . The most e f f e c t i v e a p p r o a c h t o r e d u c e o r e l i m i n a t e t h e f o r m a t i o n o f c o a g u l u m i s t o d e t e r m i n e t h e mechanism b y w h i c h i t i s f o r m e d a n d


11. VANDERHOFF

Coagulum in Emulsion Polymerization 207

the approximate stage in the conversion at which it is formed. Once this is known, the remedies usually suggest themselves. Without this knowledge, the efforts are limited to cut-and-try methods without basis in mechanism or logic.


Abstract Coagulum is formed in many emulsion polymerizations, from the smallest laboratory size to the largest production reactors. It is observed in many forms, from a single lump of polymer with lit tle or no fluid latex to tiny sand-like grains suspended in an otherwise stable latex. Usually, it is found as lumps in the la tex or deposited on the reactor surfaces. The type and amount of coagulum formed depends upon the polymer system and the polymeri zation recipe and technique. Two mechanisms are proposed for the formation of coagulum: (i) a failure of the stability of the la tex, giving rise to flocculation and growth of the aggregates to macroscopic size; (ii) a different mechanism of polymerization, e.g., polymerization in large monomer drops or a separate monomer layer in the vapor space above the latex and on the reactor sur faces. The factors affecting latex stability during polymeriza tion (electrostatic repulsion, London-van der Waals attraction, steric stabilization, flocculation by "bridging") as well as the possibility of polymerization by mechanisms other than emulsion polymerizations are discussed in terms of the parameters of poly merization. Generalizations on the formation of coagulum as a function of the parameters of the polymerization are reviewed, and recommendations are made to reduce or eliminate coagulum in specific cases. Generally, however, the reduction or elimination of coagulum depends upon the examination of the particular system and the determination of the mechanism and the time of coagulum formation, which determines the appropriate remedy. Literature Cited 1.

Overbeek, J. Th. G., "Colloid Science", Vol. 1, H. R. Kruyt, editor, Elsevier, New York, 1952, p. 80. 2. Verwey, E. J. W., and Overbeek, J. Th. G., "Theory of Stabil ity of Lypohobic Colloids", Elsevier, New York, 1948. 3. Heller, W., and Pugh, T. L . , J. Chem. Phys. 1954, 22, 1778; Ottewill, R. Η., "Nonionic Surfactants", M. Schick, editor, Marcel Dekker, New York, 1967. 4. Linke, W. F., and Booth, R. Β., Trans. Am. Inst. (Metall.) Engrs. 1959, 217, 364; Michaels, A. S., and Morelos, O., Ind. Eng. Chem. 1955, 47, 1801; Healy, T. W., and La Mer, V. Κ., J. Phys. Chem., 1962, 66, 1835; see also Audsley, Α., Mineral Processing Information Note No. 5 "Flocculation of Suspen sions of Solids with Organic Polymers --- A Literature Sur vey", Warren Spring Laboratory, Herts, England, 1965. 5. von Smoluchowski, W., Z. Physik, 1916, 17, 557, 585; ibid. Z. Physik, 1917, Chem. 92, 129.


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6. 7.

Fuchs, Ν., Z. Physik, 1934, 89, 736. von Smoluchowski, Μ., Z. Physik, 1917, Chem. 92, 155; Tuorila, P., Kolloidchem. Beihefte, 1927, 24, 1; Mueller, H., Kolloidchem. Beihefte, 1928, 27, 223 Overbeek, J. Th. G., "Colloid Science", Vol. 1, H. R. Kruyt, editor, Elsevier, New York, 1952, P. 290.

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April 6, 1981.


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The Formation of Coagulum in Emulsion Polymerization - ACS

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