Emulsion Polymerization - ACS Publications - American Chemical

6 Emulsion Polymerization G A R Y W. POEHLEIN

Downloaded by UNIV OF MISSOURI COLUMBIA on September 8, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch006

School of Chemical Engineering, Georgia Institute of Technology, Atlanta, G A 30332

Reaction Ingredients and Mechanisms Aqueous Phase Phenomena Monomer Droplets Polymer P a r t i c l e s Monomer-Swollen Emulsifier M i c e l l e s Summary Types of Reactor Processes Batch Reactors Semibatch Reactors Continuous Reactors Emulsion Polymerization K i n e t i c s Other Preparation Methods Inverse Emulsion Polymerization Direct Emulsification

Emulsion polymerization is the process of choice for the commercial p r o d u c t i o n of many polymers used f o r c o a t i n g and a d h e s i v e applications, especially f o r those products that can be used i n l a t e x form. Emulsion polymerization uses free-radical polymerization mechanisms w i t h unsaturated monomers. The heterogeneous nature of the r e a c t i o n m i x t u r e , however, has a significant influence on the chemical and p h y s i c a l reaction mechanisms and on the nature of the f i n a l product. A simple recipe for emulsion polymerization would be comprised of hydrophobic monomers (40 to 60 volume p e r c e n t ) , a continuous aqueous phase (40 to 60 volume percent), a water-soluble initiator, and an e m u l s i f i e r or stabilizer. Other minor ingredients such as c h a i n t r a n s f e r agents, inhibitors or r e t a r d e r s , and b u f f e r s may also be present. Emulsion p o l y m e r i z a t i o n is c h a r a c t e r i z e d by a large number of reaction s i t e s (the polymer p a r t i c l e s ) that contain a s m a l l number of free r a d i c a l s . These free r a d i c a l s are i s o l a t e d because of the water phase between the p a r t i c l e s . T y p i c a l polymer 0097 6156/ 85/0285 0131 $06.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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p a r t i c l e diameters would be i n the range of 50 to 500 nm, although new technology i n v o l v i n g h i g h - s w e l l i n g p a r t i c l e s can produce p a r t i c l e s as large as 50 ym (50,000 nm). A c o n s i d e r a b l e amount of work has been p u b l i s h e d d u r i n g the past 20 years on a wide v a r i e t y of e m u l s i o n p o l y m e r i z a t i o n and l a t e x problems. A l i s t of 11, m o s t l y r e c e n t , g e n e r a l r e f e r e n c e books i s i n c l u d e d at the end of t h i s chapter. Areas i n which s i g n i f i c a n t advances have been reported include reaction mechanisms and k i n e t i c s , l a t e x characterization and a n a l y s i s , copolymerization and p a r t i c l e morphology c o n t r o l , r e a c t o r mathematical m o d e l i n g , c o n t r o l of adsorbed and bound surface groups, p a r t i c l e s i z e c o n t r o l reactor parameters. Readers who are interested i n a more in-depth study of e m u l s i o n p o l y m e r i z a t i o n w i l l find extensive l i t e r a t u r e sources. Reaction Ingredients and Mechanisms The c o l l o i d a l nature of the r e a c t i o n media has a s i g n i f i c a n t influence on the course of an emulsion polymerization reaction. A number of d i s t i n c t phases e x i s t d u r i n g d i f f e r e n t i n t e r v a l s of a batch r e a c t i o n . Chemical and p h y s i c a l phenomena w i t h i n these phases and at the i n t e r f a c e s can be important i n d e t e r m i n i n g reaction k i n e t i c s and the properties of the l a t e x product. At the beginning of a batch r e a c t i o n the continuous aqueous phase c o n t a i n s the w a t e r - s o l u b l e i n i t i a t o r , e m u l s i f i e r s , and buffers. Common i o n i c e m u l s i f i e r s w i l l be present as m o l e c u l a r l y d i s s o l v e d e l e c t r o l y t e s , as s u r f a c e a c t i v e agents at the v a r i o u s interfaces, and as molecular c l u s t e r s c a l l e d m i c e l l e s . The monomer w i l l be i n three d i f f e r e n t l o c a t i o n s . A s m a l l amount w i l l be d i s s o l v e d i n the water phase. Some w i l l be s o l u b l i z e d within the e m u l s i f i e r m i c e l l e s . The bulk of the monomer, however, w i l l e x i s t i n the r e l a t i v e l y l a r g e (ca. 5 um) monomer d r o p l e t s . Any o i l s o l u b l e components s u c h as c h a i n t r a n s f e r a g e n t s w i l l be d i s t r i b u t e d with the monomer i f the water s o l u b i l i t y i s s u f f i c i e n t to permit transport from the droplets. The polymer p a r t i c l e s that are formed after the reaction begins represent another d i s t i n c t phase. These p a r t i c l e s w i l l be swollen by monomer and other o i l - s o l u b l e i n g r e d i e n t s . When r e l a t i v e l y water-insoluble monomers are used, the p a r t i c l e formation period, c a l l e d I n t e r v a l 1, extends from the beginning of the polymerization to the point at which the system i s not capable of s t a b i l i z i n g any new p a r t i c l e s . At t h i s p o i n t , the free e m u l s i f i e r i s c o m p l e t e l y adsorbed on the surface of the polymer p a r t i c l e s , and any m i c e l l e s i n i t i a l l y present w i l l have disappeared. The polymer p a r t i c l e s are s w o l l e n w i t h monomer, but i n most cases the b u l k of the monomer s t i l l remains i n the droplets. Homogeneous nucleation of polymer p a r t i c l e s can be s i g n i f i c a n t throughout the conversion range with monomers that are more water s o l u b l e such as v i n y l acetate. These new p a r t i c l e s may not be s t a b l e , and they can f l o c c u l a t e onto the larger p a r t i c l e s that were formed e a r l i e r . The polymer p a r t i c l e s continue to grow during I n t e r v a l 2 with monomer being supplied by diffusion from the droplets through the aqueous phase. I n t e r v a l 3 begins when the monomer d r o p l e t s disappear. The monomer i n the polymer p a r t i c l e s c o n t i n u e s to p o l y m e r i z e d u r i n g I n t e r v a l 3, and the p a r t i c l e i n t e r i o r becomes


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more v i s c o u s w i t h some of the r e a c t i o n s becoming d i f f u s i o n controlled. Most of the important c h e m i c a l and p h y s i c a l phenomena t h a t occur i n emulsion polymerization are l i s t e d and discussed b r i e f l y i n the remainder of t h i s section.


Aqueous Phase Phenomena. A l t h o u g h the continuous phase i s not normally a locus for s i g n i f i c a n t conversion of monomer to polymer, a number of important r e a c t i o n phenomena can take p l a c e i n t h i s phase. These reaction components are the f o l l o w i n g : 1. 2.

3.

4.

F r e e - r a d i c a l generation. Water-soluble i n i t i a t o r s such as the persulfates and various redox systems are used to generate free radicals. Radical propagation. The free r a d i c a l s formed i n the aqueous phase are h y d r o p h i l i c and u s u a l l y charged. Thus, they are very l i k e l y to react with monomer that i s d i s s o l v e d i n the aqueous phase before entering p a r t i c l e s , m i c e l l e s , or droplets. P a r t i c l e nucleation. Polymer p a r t i c l e s can be formed by s e v e r a l mechanisms. Homogeneous n u c l e a t i o n , a term used by F i t c h and coworkers (I), can occur i n the water phase by p r e c i p i t a t i o n of the growing oligomeric r a d i c a l s . R a d i c a l t e r m i n a t i o n . Normal t e r m i n a t i o n r e a c t i o n s would be expected to take p l a c e i n the continuous phase. These reactions w i l l not be the dominant method of r a d i c a l terminat i o n , but water-phase t e r m i n a t i o n can be q u i t e important i f water-soluble monomers are used. Water-soluble polymer can be formed with such monomers even though most of the monomer w i l l copolymerize with the hydrophobic monomer i n the p a r t i c l e s .

Monomer Droplets. The monomer droplets serve p r i m a r i l y as reserv o i r s t h a t s u p p l y monomer to the r e a c t i o n s i t e s i n the polymer p a r t i c l e s . These droplets can a l s o contain a v a r i e t y of other o i l s o l u b l e ingredients i n c l u d i n g d i s s o l v e d polymer, chain transfer agents, and i n unusual cases o i l - s o l u b l e i n i t i a t o r . The monomer and other ingredients, i f they have the r e q u i s i t e water s o l u b i l i t y , are transported to the primary polymerization locus i n the polymer particles. R e a c t i o n phenomena t h a t can occur i n the monomer droplets include the f o l l o w i n g : 1.

2.

P o l y m e r i z a t i o n ( P r o p a g a t i o n and T e r m i n a t i o n ) . The number of p a r t i c l e s formed from standard recipes i s considerably greater ( u s u a l l y by two to four orders of magnitude) than the number of monomer d r o p l e t s present at the beginning of the r e a c t i o n . Thus, p o l y m e r i z a t i o n w i t h i n monomer d r o p l e t s i s u s u a l l y not c o n s i d e r e d to be s i g n i f i c a n t . U g e l s t a d , E l A a s s e r , and Vanderhoff (2), however, demonstrated that polymerization i n monomer d r o p l e t s can be s i g n i f i c a n t , even dominant, i f the droplets can be made s m a l l . I n i t i a t i o n . W a t e r - s o l u b l e i n i t i a t o r s are n o r m a l l y used i n emulsion polymerization, and droplet i n i t i a t i o n can only take p l a c e when a waterborne o l i g o m e r d i f f u s e s i n t o the monomer d r o p l e t . A l t h o u g h such d i f f u s i o n does take p l a c e , i n most emulsion polymerization systems the bulk of the i n i t i a t i o n and propagation occurs i n the p a r t i c l e s . O i l - s o l u b l e i n i t i a t o r s


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3.


can be used i n systems t h a t generate a l a r g e number of s m a l l monomer drops (2). In such cases the dominant i n i t i a t i o n reaction can take place i n the d r o p l e t s . Transport s i z e r e d u c t i o n . As mentioned e a r l i e r , the monomer d r o p l e t s s e r v e as a r e s e r v o i r f o r s u p p l y i n g o i l - s o l u b l e components to the r e a c t i o n s i t e i n the p a r t i c l e s . Thermodynamic d r i v i n g forces w i l l cause d i f f u s i o n of such components from the droplets to the p a r t i c l e s . Such transport w i l l take p l a c e even i f the d r o p l e t s c o n t a i n polymer or other wateri n s o l u b l e components. In such cases, however, the d i f f u s i o n t r a n s p o r t w i l l stop before the d r o p l e t s d i s a p p e a r , and the droplets, g r e a t l y reduced i n s i z e , w i l l be a part of the f i n a l p a r t i c l e population.

Polymer P a r t i c l e s . The polymer p a r t i c l e s are almost always the dominant s i t e f o r p o l y m e r i z a t i o n . The f o l l o w i n g phenomena p l a y important r o l e s : 1.

2.

3.

4.

5.

F r e e - r a d i c a l and reagent absorption. Free r a d i c a l s , monomers, and other reagents are transported i n t o the polymer p a r t i c l e s . The free r a d i c a l s are l i k e l y to be o l i g o m e r s because a h y d r o p h i l i c i o n - r a d i c a l would remain i n the aqueous phase. Monomers and other reagents can d i f f u s e from the monomer d r o p l e t s to the p a r t i c l e s i f they possess adequate water solubility. D i s s o l v e d polymer and other w a t e r - i n s o l u b l e ingredients would remain i n the droplets. Radical propagation. Free r a d i c a l s within the polymer p a r t i c l e s w i l l react with monomer u n t i l the propagation reaction i s stopped by t r a n s f e r or t e r m i n a t i o n r e a c t i o n s or u n t i l the monomer s u p p l y i s exhausted. Free r a d i c a l s t h a t c o n t a i n a h y d r o p h i l i c end group may have a somewhat reduced m o b i l i t y because the end group w i l l tend to remain at the p a r t i c l e surface. Radical transfer. F r e e - r a d i c a l transfer reactions with monomer, polymer, and added transfer agents can take place i n the p a r t i c l e s . The polymer t r a n s f e r r e a c t i o n w i l l be more important i n e m u l s i o n p o l y m e r i z a t i o n because of the h i g h concentration of polymer i n the p a r t i c l e s . Radical desorption. Data for a number of experimental studies have been modeled by a k i n e t i c scheme that includes desorption of free r a d i c a l s . Presumably, r a d i c a l d e s o r p t i o n f o l l o w s a r a d i c a l t r a n s f e r r e a c t i o n . The m o b i l e free r a d i c a l c o u l d p o s s i b l y c r o s s the p a r t i c l e - w a t e r i n t e r f a c e i n t o the water phase. Nomura (_3) has p u b l i s h e d a recent r e v i e w paper on r a d i c a l desorption. R a d i c a l t e r m i n a t i o n . F r e e - r a d i c a l termination reactions are very fast reactions. The combination of reaction speed and the s m a l l r e a c t o r volume ( i . e . , the polymer p a r t i c l e ) a l t e r s the k i n e t i c model i n some cases. The simplest model (Smith-Ewart Case 2) i s based on the assumption t h a t i n s t a n t t e r m i n a t i o n occurs when a free r a d i c a l e n t e r s a p a r t i c l e t h a t a l r e a d y c o n t a i n s an a c t i v e r a d i c a l . As the p a r t i c l e s become l a r g e r and/or the r a d i c a l m o b i l i t y decreases because of the g e l effect, the termination rate becomes slower.



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Monomer-Swollen E m u l s i f i e r M i c e l l e s . The r o l e of m i c e l l e s i n e m u l s i o n p o l y m e r i z a t i o n has been e x t e n s i v e l y d i s c u s s e d . The o r i g i n a l work of H a r k i n s (4) and Smith and Ewart (5) t r e a t e d m i c e l l e s as s i t e s f o r p a r t i c l e n u c l e a t i o n . They proposed t h a t p a r t i c l e nucleation occurs when a free r a d i c a l enters a monomerswollen m i c e l l e and begins polymerization. As the p a r t i c l e s grow the m i c e l l e s disband to p r o v i d e e m u l s i f i e r f o r the new o r g a n i c surface. Other workers suggest t h a t the m i c e l l e s s i m p l y e x i s t as r e s e r v o i r s of e m u l s i f i e r to s t a b i l i z e p a r t i c l e s nucleated i n the aqueous phase. Roe (7) demonstrated that the Smith-Ewart equations can be derived without evoking the concept of a m i c e l l e . Perhaps each of these concepts i s v a l i d under the proper r e a c t i o n conditions. Summary. A l l of the phases and the p h y s i c a l and c h e m i c a l mechanisms d i s c u s s e d i n t h i s s e c t i o n are important d u r i n g the course of an emulsion polymerization reaction. They influence the r e a c t i o n k i n e t i c s and the p r o p e r t i e s of the l a t e x produced. Not a l l o f t h e phenomena t h a t can o c c u r a r e u n d e r s t o o d i n a q u a n t i t a t i v e manner. Nevertheless, considerable advances have been made i n t h e f u n d a m e n t a l u n d e r s t a n d i n g and t h e c o m m e r c i a l e x p l o i t a t i o n of emulsion polymerization processes. The remainder of t h i s chapter w i l l focus on reactor types and reaction k i n e t i c s . Types of Reactor Processes Batch Reactors. Polymer latexes are produced i n a wide v a r i e t y of reactors. The b o t t l e polymerizer was employed for e a r l y product development s t u d i e s , and, i n f a c t , such equipment i s s t i l l used w i d e l y today. B o t t l e s are p a r t i a l l y f i l l e d w i t h the r e c i p e i n g r e d i e n t s , a t t a c h e d to a r o t a t i n g shaft t h a t i s immersed i n a temperature-controlled bath, and allowed to react for a fixed time. The l a t e x i s then removed f o r e v a l u a t i o n . The advantage of a b o t t l e polymerizer i s that a large number of recipe v a r i a t i o n s can be run simultaneously. Some polymerizers w i l l hold more than 100 bottles. Some e a r l y batch polymerization reactors were b u i l t on r o t a t i n g shafts to copy the action of b o t t l e polymerizers. These reactors were e x p e n s i v e and d i f f i c u l t to m a i n t a i n . They were r e p l a c e d by standard s t i r r e d v e s s e l s which are commonly used today. T y p i c a l batch reactors contain an a g i t a t o r that i s mounted i n the center of the r e a c t o r top. The r e a c t o r s are o f t e n g l a s s l i n e d and c o n t a i n one or more b a f f l e s t o enhance m i x i n g . Heat r e m o v a l i s accomplished by c i r c u l a t i n g a coolant through the reactor jacket. Numerous r e a c t o r d e s i g n v a r i a t i o n s have been employed i n commercial processes. Polished s t a i n l e s s s t e e l reactors have been used i n p l a c e of g l a s s - l i n e tanks f o r some systems. The smooth s u r f a c e s ( g l a s s or p o l i s h e d m e t a l ) are d e s i r a b l e to minimize surface f o u l i n g . The use of s t a i n l e s s s t e e l i n c r e a s e s heat t r a n s f e r r a t e s and reduces maintenance c o s t s . The s t a i n l e s s surface may, however, be more prone to f o u l i n g . The heat of p o l y m e r i z a t i o n can be removed by a number of techniques. Cooled reactor jackets are most common, but i n t e r n a l c o o l i n g s u r f a c e s i n the form of c o i l s or pipe b a f f l e s are a l s o



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used. Shell-and-tube heat exchangers can be used i n a c i r c u l a t i o n loop e x t e r n a l to the r e a c t o r . R e f l u x condensers are another alternative. These forms of c o o l i n g , however, add heat transfer surface that may become fouled and require frequent cleaning. When stirred-tank reactors are operated i n the batch mode, a l l i n g r e d i e n t s are added at or near the beginning of the r e a c t i o n c y c l e , the reaction i s allowed to proceed to a desired end point, and the product l a t e x i s removed f o r f u r t h e r p r o c e s s i n g . S t r i c t batch o p e r a t i o n has a number of d i s a d v a n t a g e s . F i r s t , the heat load on the c o o l i n g system can be very nonuniform. The production r a t e s from such r e a c t o r s can be l i m i t e d by the c a p a b i l i t y of the heat removal system d u r i n g the peak i n the exotherm. The use of mixed i n i t i a t o r systems (fast and slow) and the continuous addition of a f a s t i n i t i a t o r are two ways of t r y i n g to d e a l w i t h t h i s problem. A second p o s s i b l e problem w i t h batch reactors i s composition d r i f t of copolymer systems. As with bulk, s o l u t i o n , and suspension systems, the more r e a c t i v e monomer polymerizes f i r s t , and the l e a s t r e a c t i v e polymerizes l a s t . Two a d d i t i o n a l f a c t o r s must be c o n s i d e r e d i n e m u l s i o n p o l y m e r i z a t i o n . F i r s t , the water s o l u b i l i t i e s o f t h e monomers can i n f l u e n c e t h e c o u r s e o f t h e p o l y m e r i z a t i o n because of r e a c t i o n i n the water phase to produce copolymer oligomers or even water-soluble polymer. These molecules can be r i c h i n the water-soluble monomer even i f i t s r e a c t i v i t y i s r e l a t i v e l y low. Second, the high degree of s u b d i v i s i o n achieved by producing s m a l l polymer p a r t i c l e s can lead to phase domains that are smaller than those i n copolymer produced by other processes. Latex produced from the more w a t e r - i n s o l u b l e monomers i n a batch reactor normally would have a r e l a t i v e l y narrow p a r t i c l e s i z e d i s t r i b u t i o n (PSD). I n t e r v a l 1, the p a r t i c l e n u c l e a t i o n p a r t of the r e a c t i o n , i s u s u a l l y completed e a r l y i n the p o l y m e r i z a t i o n c y c l e , and thus a l l p a r t i c l e s i n the f i n a l latex would have about the same age and the same s i z e . S e v e r a l f a c t o r s can counter t h i s normal trend, however. A low i n i t i a t i o n rate w i l l extend I n t e r v a l 1 and broaden the PSD. P a r t i c l e nucleation l a t e r i n the reaction can a l s o generate broader s i z e d i s t r i b u t i o n s . Several factors can l e a d to a second n u c l e a t i o n . W a t e r - s o l u b l e i n i t i a t o r s generate surface a c t i v e oligomers that add to the s t a b i l i z i n g c a p a b i l i t y of the system. Any l i m i t e d f l o c c u l a t i o n w i t h i n the system would reduce surface area and p o s s i b l y free e m u l s i f i e r for s t a b i l i z i n g new p a r t i c l e s . In a d d i t i o n , the t o t a l i n t e r f a c i a l area decreases d u r i n g I n t e r v a l 3 because the d e n s i t y of polymer i s g r e a t e r than monomer. A second-stage n u c l e a t i o n can be caused by any one or a combination of the above factors. Semibatch R e a c t o r s . P a r t of the r e c i p e ingredients are withheld from the i n i t i a l charge i n semibatch (sometimes c a l l e d semicontinuous) operation. These i n g r e d i e n t s are added l a t e r i n a programmed manner to c o n t r o l the course of the r e a c t i o n and to produce a d e s i r e d product. One reason f o r u s i n g semibatch operation i s to c o n t r o l heat release and/or the rate of polymerization. This c o n t r o l i s most commonly accomplished by withholding p a r t of the monomer and f e e d i n g i t at a c o n t r o l l e d r a t e l a t e r i n the r e a c t i o n c y c l e . Such r e a c t i o n s operate i n a monomer-starved condition, and branching (polymer transfer) mechanisms can be more



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significant. L i k e w i s e , the h i g h polymer c o n c e n t r a t i o n i n the p a r t i c l e s can cause the t e r m i n a t i o n r e a c t i o n to be d i f f u s i o n c o n t r o l l e d . I f heat removal i s a problem the monomer feed rate can c o n t r o l t h e r e a c t i o n so t h a t d e s i r e d t e m p e r a t u r e s can be maintained. I f heat removal i s not a problem one may s t i l l choose to operate the monomer-starved regime because the d i f f u s i o n - l i m i t e d t e r m i n a t i o n step can cause h i g h e r r e a c t i o n r a t e s and hence increased reactor p r o d u c t i v i t y . A second, but l e s s common, method of reaction rate c o n t r o l i s by the programmed addition of a h i g h l y r e a c t i v e i n i t i a t i o n system. A second motivation for using a monomer-feed semibatch procedure i s to c o n t r o l copolymer c o m p o s i t i o n and/or p a r t i c l e morphology. Delayed feed of part of the more r e a c t i v e monomer can be used to e l i m i n a t e or reduce the c o m p o s i t i o n d r i f t of the copolymer. The d e l a y e d feed of a comonoraer mixture when the reactor i s operated i n the monomer-starved regime can a l s o be used to prevent copolymer c o m p o s i t i o n d r i f t . Such o p e r a t i o n s w i l l produce polymer p a r t i c l e s with more uniform morphology. Different monomer a d d i t i o n schemes can be employed to c o n t r o l nonuniform p a r t i c l e morphology (see papers by B a s s e t t et a l . i n G e n e r a l References 7 and 9). A t h i r d motivation for delayed monomer addition i s to produce h i g h - s o l i d s latexes. The use of semibatch operation permits one to pass through the f l o c c u l a t i o n - s e n s i t i v e parts of the reaction c y c l e and t h e n t o b u i l d the s o l i d s l e v e l n e a r t h e end o f t h e polymerization. Sometimes the addition of more e m u l s i f i e r i s used i n the production of h i g h - s o l i d s latexes. The nature of the PSD can a l s o be c o n t r o l l e d by semibatch operation. I f a narrow PSD i s desired the e m u l s i f i e r and i n i t i a t o r components are charged to y i e l d a very short p a r t i c l e - n u c l e a t i o n p e r i o d . Narrowly d i s t r i b u t e d seed l a t e x e s can a l s o be used f o r t h i s purpose. In such systems the age d i s t r i b u t i o n i s narrow and the s i z e d i s t r i b u t i o n f o l l o w s . I f the e m u l s i f i e r feed and i n i t i a t i o n system are formulated to y i e l d a long nucleation period, a r e l a t i v e l y broad PSD l a t e x can be produced. In extreme cases of delayed-emulsifier feed, m u l t i p l e n u c l e a t i o n p e r i o d s w i l l r e s u l t , and even broader PSDs can be produced. Broad d i s t r i b u t i o n s can be an a s s e t when h i g h - s o l i d s , l o w - v i s i b i l i t y products are desired. A semibatch system w i l l be i n f l u e n c e d d i f f e r e n t l y by the presence of i n h i b i t o r . I f i n h i b i t o r i s present i n the r e c i p e i n g r e d i e n t s of a batch r e a c t o r i t w i l l d e l a y the s t a r t of p o l y m e r i z a t i o n , a f t e r which the r e a c t i o n w i l l proceed i n a normal manner. I n h i b i t o r i n the d e l a y e d feed stream(s) to a semibatch system w i l l reduce the e f f e c t i v e r a t e of i n i t i a t i o n . This r e d u c t i o n may r e q u i r e the use of more i n i t i a t o r , and because the i n h i b i t o r r e a c t s r a p i d l y , the p o l y m e r i z a t i o n r a t e may i n c r e a s e d r a m a t i c a l l y when the delayed-feed part of the c y c l e i s f i n i s h e d . Continuous Reactors. A v a r i e t y of continuous reactor systems are used commercially, but the most common are comprised of a number of s t i r r e d - t a n k r e a c t o r s (CSTR) connected i n s e r i e s . Operation normally i n v o l v e s pumping a l l ingredients i n t o the f i r s t CSTR and removing the p a r t i a l l y c o n v e r t e d l a t e x from the f i n a l r e a c t o r . Intermediate feed streams can a l s o be employed. D e t a i l e d reviews



138


of e m u l s i o n p o l y m e r i z a t i o n i n continuous systems are g i v e n by P o e h l e i n et a l . (8, G e n e r a l References _2, 8, and 9). The most common d i f f e r e n c e s between continuous and batch systems w i l l be discussed i n the remainder of t h i s section. The differences between a s i n g l e CSTR and a batch reactor are s i m i l a r to those between semibatch and batch reactors, except that they are u s u a l l y more pronounced. The a d d i t i o n of more reactors to a s e r i e s system tends to reduce some of the observed performance differences. A t y p i c a l example of different behavior i s the heat release p r o f i l e . An advantage often c i t e d for continuous reactor systems i s a constant heat l o a d w i t h f u l l y used r e a c t o r volume. Batch reactors are not u s u a l l y operated f u l l , and the heat load i s nonuniform. In a d d i t i o n , portions of the batch reaction c y c l e are devoted to c h a r g i n g and emptying the r e a c t o r and sometimes f o r h e a t i n g the reagents to p o l y m e r i z a t i o n temperature. Thus, the p r o d u c t i o n r a t e per u n i t volume can be h i g h e r i n a continuous system. Uniform product q u a l i t y i s a l s o an advantage c l a i m e d f o r continuous reactors. I f a continuous reactor can be c o n t r o l l e d at a d e s i r e d steady s t a t e , product q u a l i t y t r a n s i e n t s s h o u l d be considerably l e s s than t y p i c a l batch-to-batch v a r i a t i o n s . Start-up t r a n s i e n t s and shut-down procedures can produce off-spec product; hence, frequent start-ups and/or product changes represent a r e a l problem for continuous systems. A s i n g l e CSTR i s a v a l u a b l e t o o l for the study of polymerizat i o n k i n e t i c s but not f o r commercial p r o d u c t i o n . As mentioned e a r l i e r , systems comprised of a s e r i e s of CSTRs are most common. Some of the e a r l y synthetic rubber processes contain as many as 15 CSTRs. More r e c e n t systems are comprised of t h r e e to f i v e reactors. Not a l l r e a c t o r s need to be the same s i z e . In f a c t t h e r e are s u b s t a n t i a l reasons f o r u s i n g r e a c t o r s of d i f f e r e n t sizes. N e a r l y a l l p a r t i c l e s are l i k e l y to be produced i n the f i r s t r e a c t o r . The s i z e of t h i s r e a c t o r w i l l i n f l u e n c e the number of p a r t i c l e s formed. I f a maximum number of p a r t i c l e s i s desired t h i s f i r s t reactor w i l l be operated at a r e l a t i v e l y s m a l l mean residence time and thus w i l l be s m a l l e r than the other r e a c t o r s i n the system. Another reason for using different reactor sizes along the CSTR t r a i n i s the v a r i a t i o n of p o l y m e r i z a t i o n r a t e w i t h monomer conversion. This factor i s not a major consideration i f the f i n a l c o n v e r s i o n i s modest as i n the case of s t y r e n e - b u t a d i e n e rubber (SBR) processes. Normal e x i t c o n v e r s i o n s are 55 to 65% i n such systems, and the r e s i d u a l monomer i s recovered and r e c y c l e d . I f a very high conversion i s desired one must deal with the problem that the p o l y m e r i z a t i o n r a t e i s low at h i g h c o n v e r s i o n s . The f i n a l r e a c t o r i n the s e r i e s needs to be v e r y l a r g e i f the d e s i r e d conversion approaches 100%. Likewise, batch reaction c y c l e times become large i f high conversions are desired. H i g h - c o n v e r s i o n continuous processes w i l l r e q u i r e l a r g e r e a c t o r s near the end of the CSTR s e r i e s . In f a c t i t may be advantageous to permit the r e a c t i o n to c o n t i n u e i n the product s t o r a g e tanks i n h i g h - c o n v e r s i o n processes. This condition w i l l reduce the l e v e l of r e s i d u a l monomer i n the end products or i n the downstream processing steps.


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Continuous processes i n v o l v i n g t u b u l a r r e a c t o r s have been r e p o r t e d i n the l i t e r a t u r e ( G e n e r a l References _1, 2, 2» 9.). Continuous tubular reactors have been used i n three ways. Gonzalez (9.) used a t u b u l a r p r e r e a c t o r to feed a CSTR system. The t u b u l a r prereactor served as a p a r t i c l e nucleation system and thus s o l v e d the problem of c o n v e r s i o n o s c i l l a t i o n s often observed i n CSTR systems. A t u b u l a r p r e r e a c t o r a l s o can be used to generate a higher p a r t i c l e concentration than would be produced with the same recipe i n a CSTR system. Emulsion p o l y m e r i z a t i o n r e a c t i o n s have a l s o been s t u d i e d i n r e a c t o r s c o n s i s t i n g o n l y of tubes. Such r e a c t o r s o f f e r the p o t e n t i a l advantage of a l a r g e area f o r heat t r a n s f e r per u n i t volume and hence a high polymerization rate. One p o t e n t i a l problem with tubular reactors, namely plugging, has discouraged commercial use. A number of s t u d i e s have been r e p o r t e d on once-through continuous t u b u l a r r e a c t o r s but commercial reactors of t h i s type have not been p u b l i c i z e d . A continuous t u b u l a r - l o o p process has been patented (10) and used f o r r e l a t i v e l y s m a l l - s c a l e p r o d u c t i o n . The l o o p process c o n s i s t s of a tube-pump system i n which the r a t e of l a t e x c i r c u l a t i o n around the tube l o o p i s c o n s i d e r a b l y g r e a t e r than the throughput rate. Thus, the d i s t r i b u t i o n of residence times should be nearly the same as that of a s i n g l e CSTR. The PSD of a l a t e x i s s t r o n g l y r e l a t e d to the p a r t i c l e age d i s t r i b u t i o n . Thus, one would expect l a t e x e s produced i n c o n tinuous systems to have s i z e c h a r a c t e r i s t i c s different from batch products. The most extreme differences are seen for a s i n g l e CSTR. The p a r t i c l e age d i s t r i b u t i o n i n the l a t e x product from a CSTR i s given by Equation 1.


a

f (

t)

= ie -t/e

n

d

(1)

where t i s p a r t i c l e age and 0 i s the r e a c t o r mean r e s i d e n c e time. T h i s d i s t r i b u t i o n i s very broad i n comparison to the narrow age d i s t r i b u t i o n of p a r t i c l e s produced i n a batch r e a c t o r . The d i s t r i b u t i o n of p a r t i c l e ages and the c o r r e s p o n d i n g PSDs become narrower as more CSTRs are connected i n s e r i e s . In f a c t r a t h e r n a r r o w - d i s t r i b u t i o n SBR l a t e x can be produced i n the commercial systems containing 12-15 reactors i n s e r i e s . The response of a CSTR system to i n h i b i t o r s i n the feed streams i s , i n some r e s p e c t s , s i m i l a r to a semibatch system. Because i n h i b i t o r e n t e r s w i t h the feed stream, the r a t e of i n i t i a t i o n i s reduced i n proportion to the i n h i b i t o r flow. In extreme cases, the f l o w of i n h i b i t o r may be s u f f i c i e n t to prevent any i n i t i a t i o n i n the f i r s t r e a c t o r . When t h i s happens the p a r t i c l e n u c l e a t i o n phenomena i s s h i f t e d to the second tank, and s e r i o u s c o n t r o l problems can be e x p e r i e n c e d . Increased i n i t i a t o r concentrations can be used to overcome high i n h i b i t o r concentrations, but such a course of action can produce i n i t i a t i o n rates that are considerably higher i n the downstream reactors. Continuous r e a c t o r s comprised of a CSTR t r a i n are o f t e n operated with a s i n g l e feed l o c a t i o n i n the f i r s t reactor. The use of i n t e r m e d i a t e feed l o c a t i o n s can be advantageous f o r s e v e r a l reasons. F i r s t , i f the c o n v e r s i o n i n the f i r s t r e a c t o r i s low or



140

A P P L I E D POLYMER SCIENCE

modest, as i s o f t e n the case, f e e d i n g the e n t i r e monomer charge through the f i r s t reactor i s an i n e f f i c i e n t use of reactor volume. Feeding part of the monomer downstream can i n c r e a s e process p r o d u c t i v i t y and perhaps even reduce w a l l f o u l i n g . A second reason for intermediate monomer feed l o c a t i o n s i s to c o n t r o l copolymer composition and p a r t i c l e morphology. Unlike the batch reactor i n which the more r e a c t i v e monomer reacts f i r s t , the copolymer produced i n a CSTR should have uniform composition. I f a l l monomers are fed to the f i r s t r e a c t o r , however, the polymer formed i n the downstream r e a c t o r s w i l l c o n t a i n l e s s of the most r e a c t i v e monomer. Thus, the use of intermediate feed l o c a t i o n s i n a CSTR system i s analogous to the programmed addition of r e a c t i v e monomers i n semibatch r e a c t o r s . The l o c a t i o n and r a t e s of the various monomer streams w i l l influence copolymer composition and p a r t i c l e morphology. A t h i r d reason f o r f e e d i n g some of the monomer to downstream r e a c t o r s i s to b u i l d the s o l i d s l e v e l of the product. This increase i n s o l i d s can occur without the f l o c c u l a t i o n and rheology problems that might e x i s t i f the organic phase concentration were high i n the e a r l y reactors. Other recipe ingredients such as e m u l s i f i e r s and chain transfer agents could be subdivided and introduced to the reactor system i n s e v e r a l l o c a t i o n s . E m u l s i f i e r s and/or s t a b i l i z e r s may be necessary to produce a s t a b l e e f f l u e n t . Chain transfer agents that are used to l i m i t branching reactions may be of p a r t i c u l a r importance i n the high-conversion end of the r e a c t i o n system. L i k e w i s e , i n i t i a t o r systems that do not have a long h a l f - l i f e w i l l tend to decompose i n the f i r s t few reactors, and downstream additions may be necessary to achieve reasonable polymerization rates. The methods used for introducing feed streams i n t o continuous reactors can be quite important. A l l ingredients are charged and mixed before the l a t e x i s formed i n most batch reactor processes. The major purposes of mixing a f t e r the r e a c t i o n begins are to f a c i l i t a t e heat removal through the c o o l i n g surface and to maintain mass t r a n s f e r from the monomer phase to the polymer p a r t i c l e s . With a CSTR reaction system, however, the feed streams are added to p a r t i a l l y c o n v e r t e d l a t e x e s , and other f a c t o r s need to be considered. I n i t i a t o r streams are normally e l e c t r o l y t e s o l u t i o n s that can cause f l o c c u l a t i o n . These streams should be as d i l u t e as f e a s i b l e and added at a l o c a t i o n where rapid mixing takes place. E m u l s i f i e r feed streams may cause l o c a l nucleation i n the reactor i f they are not mixed properly. Monomer additions can a l s o be a problem i f the d i s p e r s i o n i s not adequate to p r o v i d e s u f f i c i e n t mass t r a n s f e r r a t e s . T h i s s i t u a t i o n can be e s p e c i a l l y important i n systems containing gaseous monomers. In summary, continuous r e a c t i o n s w i l l be used w i t h i n c r e a s e d frequency as production requirements grow and as design procedures improve. I f continuous systems are to be employed i n a commercial p r o c e s s , product development and p i l o t p l a n t s t u d i e s s h o u l d use s m a l l - s c a l e continuous systems e a r l y i n the development process. Such s t u d i e s w i l l s u b s t a n t i a l l y i n c r e a s e the p r o b a b i l i t y of a successful commercial process.


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Emulsion Polymerization K i n e t i c s The k i n e t i c s of e m u l s i o n p o l y m e r i z a t i o n r e a c t i o n s are complex because of the numerous chemical and physical phenomena that can occur i n the multicomponent, multiphase mixture. A large amount of l i t e r a t u r e e x i s t s on k i n e t i c s problems. The g e n e r a l r e f e r e n c e s l i s t e d at the end of t h i s chapter c o n t a i n many important papers. The r e v i e w paper by U g e l s t a d and Hansen (11) i s a comprehensive treatment of batch k i n e t i c s . The purpose of the remainder of t h i s chapter i s to present the general k i n e t i c s problems and some of the p u b l i s h e d r e s u l t s . The reader s h o u l d use the r e f e r e n c e s c i t e d e a r l i e r for a more d e t a i l e d study. K i n e t i c s models are u s e f u l f o r designing commercial reactors and f o r s t u d y i n g the fundamental mechanisms of the important r e a c t i o n s . The 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 t h a t takes p l a c e i n e m u l s i o n systems i s c h a r a c t e r i z e d by three main r e a c t i o n s : i n i t i a t i o n , propagation, and termination. Various r a d i c a l transfer r e a c t i o n s can a l s o be important. The r a t e of p o l y m e r i z a t i o n f o r bulk, s o l u t i o n , and suspension processes can be expressed as shown by Equation 2: Rp = k

p

[M][R-]

(2)

where k i s the propagation r a t e c o n s t a n t , [M] i s the monomer concentration, and [R ] i s the f r e e - r a d i c a l concentration. In bulk, s o l u t i o n , and suspension polymerization, the problem of determining [R«] i s handled by assuming a steady-state ( a c t u a l l y s l o w l y changing) r a d i c a l concentration and thus equating the rates of i n i t i a t i o n and termination. -

R = kjR-]

2

(3)

±

where R i s the rate of i n i t i a t i o n , and k i s the termination rate constant. By combining Equations 2 and 3 one obtains i

t

R P

- —i k

V

w

(*)

t

The major problem associated with the use of Equation^ 4 for reactor design c a l c u l a t i o n s stems from the fact that (k / k ) v a r i e s with monomer conversion, sometimes by s e v e r a l orders of magnitude. The free r a d i c a l s i n emulsion polymerization are i s o l a t e d i n the polymer p a r t i c l e s , and [ R ] i s expressed by Equation 5: 2

t

#

[R*] = N/N

A

(5)

where i s the average number of free r a d i c a l s per p a r t i c l e , N i s the p a r t i c l e c o n c e n t r a t i o n ( u s u a l l y expressed as the number of p a r t i c l e s per l i t e r of aqueous phase), and N^ i s the Avogadro number. Note t h a t n i s the average number of free r a d i c a l s per p a r t i c l e for a monodisperse system. represents the average of n over the latex PSD.


142



Combining Equations 2 and 5 gives the f o l l o w i n g for the rate of emulsion polymerization:

relationship

where the subscript "p" on the monomer concentration indicates the c o n c e n t r a t i o n of monomer i n the polymer p a r t i c l e s t h a t are considered the dominant s i t e for conversion of monomer to polymer. Note t h a t R i n E q u a t i o n 6 w i l l a l s o be based on a u n i t volume of aqueous phase. Values for the propagation rate constant can be determined from bulk or s o l u t i o n experiments. Values of kp have been published for a wide v a r i e t y of monomers as a f u n c t i o n of temperature. With standard e m u l s i o n p o l y m e r i z a t i o n r e c i p e s the v a l u e of [M]p i s determined from e q u i l i b r i u m s w e l l i n g measurements i f a free monomer phase i s present and by a mass balance i f a l l the monomer i s i n the polymer p a r t i c l e s . One normally assumes that [M] i s not dependent on p a r t i c l e s i z e i n latexes comprised of d i f f e r e n t - s i z e d p a r t i c l e s . This assumption w i l l be questionable i n some systems, e s p e c i a l l y those i n v o l v i n g h i g h - s w e l l i n g p a r t i c l e s . By assuming the [ M ] can be determined by p u b l i s h e d methods, the problem of computing R i s reduced to the p r e d i c t i o n of and N. The v a l u e of N can be known i f a l a t e x seed i s used i n the r e a c t o r . Seeds are used i n many s c i e n t i f i c s t u d i e s because the problem of predicting N i s eliminated. I f a seed i s not employed N must be predicted by published c o r r e l a t i o n s or measured. Smith and Ewart (6) p u b l i s h e d an e a r l y theory on p a r t i c l e n u c l e a t i o n t h a t r e s u l t e d i n the r e l a t i o n s h i p shown i n Equation 7: p

N = k R 0.4 [S] 0.6

(7)

i

where [S] i s the e m u l s i f i e r concentration, and k i s a constant that depends on the assumption used i n the model. E q u a t i o n 7 g i v e s a r e a s o n a b l e p r e d i c t i o n of N f o r s t y r e n e p o l y m e r i z a t i o n s w i t h standard e m u l s i f i e r s , but i s not adequate for many systems. More d e t a i l e d and complete theories on p a r t i c l e nucleation have been published by F i t c h et a l . (General References 3, and 4) and by Hansen and U g e l s t a d (12). These p u b l i c a t i o n s c o n s i d e r s e v e r a l mechanisms for p a r t i c l e nucleation, and they present mathematical models t h a t account f o r these v a r i o u s mechanisms. The present state of the a r t , however, w i l l not permit one to compute N from a knowledge of the recipe ingredients and reaction conditions, except for s p e c i a l cases. Thus, most product and process development work should probably include the measurement of N as a function of the important v a r i a b l e s . One i n t e r e s t i n g comparison between a CSTR and a batch reactor i s , however, i n the p r e d i c t i o n of N. I f the Smith-Ewart concepts are a p p l i e d to a s i n g l e CSTR, the p a r t i c l e concentration p r e d i c t i o n i s given by Equation 8: N= k

f

1

[s] -

0

e-°-

6 7


(8)

6.


POEHLEIN

143


f

where k i s a c o n s t a n t , and 0 i s the mean r e s i d e n c e time of the reactor. The equation i s quite different from Equation 7. Hence, one s h o u l d expect p a r t i c l e n u c l e a t i o n models f o r batch and CSTR systems to be q u i t e d i f f e r e n t , even f o r i d e n t i c a l r e c i p e s and r e a c t i o n mechanisms. Such d i f f e r e n c e s are one reason f o r u s i n g continuous r e s e a r c h r e a c t o r s e a r l y i n the p r o d u c t - p r o c e s s development program. The use of batch data to design a continuous process i s a h i g h - r i s k venture. The second part of the R problem, the determination of i s the subject of numerous papers (5,10,12,13). Most of these papers are concerned with obtaining s o l u t i o n s to the Smith-Ewart recursion r e l a t i o n s h i p given by Equation 9: dN, ^7 V /

(

N

n

-

r

N

n

)

+

k d

{

( n + 1 ) N

n r +

n N

n} (9)

4^—J{(n+2)

(n+l)N

n+2

-n(n-l)N j>

=0

n

where p* i s the t o t a l rate of r a d i c a l absorption by the p a r t i c l e s , N i s tne number of p a r t i c l e s that contain n free r a d i c a l s , is a rate constant for desorption of r a d i c a l s from the p a r t i c l e s , and v i s the volume of the monomer-swollen p a r t i c l e s . Smith and Ewart (5) obtained s o l u t i o n s to Equation 9 for three s p e c i a l cases: n « 1.0, n = 0.5, and n » 1.0. Case 2 (n = 0.5) has been the most w i d e l y p u b l i c i z e d s o l u t i o n . Data f o r a broad range of styrene emulsion polymerization experiments are consistent w i t h t h i s model. More g e n e r a l s o l u t i o n s f o r E q u a t i o n 9 were p u b l i s h e d by Stockmayer (13), O'Toole (14), and U g e l s t a d et a l . (15). A r e l a t i o n s h i p for the average number of r a d i c a l s per p a r t i c l e i n a monodisperse system i s given by Equation 10: ff

ff

n

4

Vi

where the I's are modified Bessel functions of the f i r s t kind, m = k v / k , a = / 8 a , and a = p v/Nkj_. The parameter a cannot be d i r e c t l y e v a l u a t e d because P , the r a d i c a l absorption rate, i s dependent on i n i t i a t i o n rate, r a d i c a l d e s o r p t i o n from the p a r t i c l e s , and water-phase t e r m i n a t i o n . Ugelstad et a l . (15) derived the f o l l o w i n g equation r e l a t i n g these phenomena: d

t

A

A

1

a = a + mn - Y a

2

(11)

1

2

where a = p v / N k i p i s the rate of i n i t i a t i o n , Y = 2 N k k / k v , k i s the water-phase t e r m i n a t i o n c o n s t a n t , and k i s a r a d i c a l absorption c o e f f i c i e n t . Numerical computation schemes were used to s o l v e Equations 10 and 11 to y i e l d r e s u l t s such as those shown i n F i g u r e s 1 and 2. i

t w

t

±

t

a


t w

a

144


The v a r i o u s Smith-Ewart cases can be i d e n t i f i e d on these graphs. The h o r i z o n t a l l i n e at n = 0.5 with m = 0 would be Case 2. The lower l e f t area where n i s s m a l l would correspond to Case 1, and the c u r v e s i n the upper r i g h t s i d e (n > 0.5) would approach Case 3 or bulk k i n e t i c s . I f a l l the r e l e v a n t parameters are known (a , m, Y) one can use graphs such as F i g u r e s 1 and 2 to determine n . However, two l i m i t a t i o n s must be considered. F i r s t , as mentioned e a r l i e r , the s o l u t i o n i s for a monodisperse l a t e x . I f a broad s i z e d i s t r i b u t i o n e x i s t s , one must account for differences i n f r e e - r a d i c a l transport among t h e particles. If these differences are accounted f o r , one o b t a i n s t h a t i s an average of n over the distribution. The second l i m i t a t i o n stems from the f a c t t h a t n can change w i t h time d u r i n g the course of a p o l y m e r i z a t i o n r e a c t i o n because a , m, or Y can change. Thus, the course of a r e a c t i o n w i l l t r a c e a c u r v e f o r n i n the space [a , m, Y ] , Napper, G i l b e r t et a l . (16-19) have obtained s o l u t i o n s for the system of equations represented by Equation 9 without assuming that dN / d t = 0.0. They have, however, n e g l e c t e d r e - e n t r y of free r a d i c a l s that have desorbed from the p a r t i c l e s . Among other t h i n g s , they c o n c l u d e t h a t w a t e r - p h a s e t e r m i n a t i o n can be i m p o r t a n t , t h a t i s , Y > 0.0. Dubner, P o e h l e i n , and Lee (20) developed a s t e a d y - s t a t e k i n e t i c s model f o r a s i n g l e , seed-fed CSTR on the b a s i s of the 0 T o o l e - U g e l s t a d concepts. Dimensionless groups analogous to a , m, and Y are a ' , y, and Y . These groups are defined i n terms of v , the s i z e of the seed l a t e x p a r t i c l e s , i n s t e a d of the s i z e of the p o l y m e r i z i n g p a r t i c l e s . An a d d i t i o n a l d i m e n s i o n l e s s group, 8 = v / 0 K i [ M ] , i s needed to include the new parameter 0, the CSTR mean residence time. Kj i s a p a r t i c l e growth parameter. F i g u r e 3 i s a t y p i c a l r e s u l t of Dubner's work. The p l o t of versus a includes consideration for the v a r i a t i o n of n with p a r t i c l e s i z e i n the r e a c t i n g m i x t u r e . F i g u r e 3 i s analogous to the s i n g l e c u r v e f o r m = 0 i n F i g u r e 1. The parametric c u r v e s r e s u l t because of v a r i a t i o n s i n 8. As 8 decreases (higher 0 v a l u e s ) d e v i a t i o n s from S-E Case 2 b e h a v i o r occurs at s m a l l e r values of a . F i g u r e 4 shows the i n f l u e n c e of r a d i c a l d e s o r p t i o n from p a r t i c l e s (the parameter Y) on . The shape of the c u r v e s a r e , as expected, quite s i m i l a r to those reported by Ugelstad et a l . In t h i s case two parameters, 8 and Y , are f i x e d . Dubner et a l . a l s o r e p o r t that the r a d i c a l d e s o r p t i o n r a t e (as accounted f o r by Y) c o u l d have a very s u b s t a n t i a l i n f l u e n c e on the PSD of the CSTR effluent latex. Figure 5 i l l u s t r a t e s t h i s effect. The d i s t r i b u t i o n s are p l o t t e d i n terms of a d i m e n s i o n l e s s diameter, t h a t i s , p a r t i c l e diameter divided by s e e d - p a r t i c l e diameter. The c u r v e f o r Y = 0.0 i s s i n g l e peaked, s i m i l a r to p u b l i s h e d r e s u l t s for styrene. As Y increases the PSD has two peaks; one at the seed s i z e and one larger. I f Y i s increased even further, the d i s t r i b u t i o n becomes s i n g l e peaked again, but skewed to the s i z e of the seed p a r t i c l e s . Hence, the measurement of PSDs from a seed-fed CSTR would be one way to study r a d i c a l d e s o r p t i o n mechanisms and rates.


1

1

f

f

1

g

g

f

c

1

C



POEHLE1N


Figure 1

Figure 2



146


o

Figure 4



POEHLEIN


00

DIMENSIONLESS

DIAMETER

Figure 5

American Chemical Society

Library

1155 16th St., N.W. In Applied Polymer Science; Tess, R., et al.; D.C.Society: 20036Washington, DC, 1985. ACS Symposium Series;Washington, American Chemical

148


Other Preparation Methods


Synthetic polymeric latexes can be produced by processes that are d i f f e r e n t from the standard e m u l s i o n p o l y m e r i z a t i o n methods d e s c r i b e d i n t h i s chapter. Two such processes, i n v e r s e e m u l s i o n polymerization and d i r e c t e m u l s i f i c a t i o n , are described b r i e f l y i n order to make t h i s paper more complete. The l i t e r a t u r e on these processes i s l e s s e x t e n s i v e , but i n t e r e s t i n such processes has recently increased. I n v e r s e Emulsion P o l y m e r i z a t i o n . Water-soluble monomers can be polymerized by emulsifying water s o l u t i o n s of these monomers i n an o r g a n i c continuous phase. T h i s process, c a l l e d inverse emulsion p o l y m e r i z a t i o n , y i e l d s a product comprised of a c o l l o i d a l suspension of droplets of aqueous polymer s o l u t i o n . The o r i g i n a l study of an inverse system by Vanderhoff et a l . (20) i n v o l v e d the monomer sodium j>-viny lbenzene s u l f o n a t e , an o r g a n i c phase of o_xylene, Span 60 as the e m u l s i f i e r , and either benzoyl peroxide or potassium persulfate i n i t i a t o r . Later work by Kurenkov et a l . (21) i n v o l v e d a c r y l a m i d e i n a t o l u e n e continuous phase, potassium p e r s u l f a t e , and Sentamid-5 ( e m u l s i f i e r ) . DiStefano (22) examined t h r e e monomers: acrylamide, dimethylaminoethyacrylate hydrochloride, and methacrylamide. The analogies between standard and inverse emulsion polymerization are obvious, but not complete. High polymerization rates coupled with high molecular weights are s i m i l a r , but the d e t a i l e d mechanisms and k i n e t i c s appear to be q u i t e complex. E v i d e n c e i s presented f o r m u l t i p l e e m u l s i o n d r o p l e t s ( o i l - i n - w a t e r - i n - o i 1 ) , a s t r o n g g e l e f f e c t c a u s i n g an i n v e r s e r e l a t i o n s h i p between m o l e c u l a r weight and e m u l s i f i e r concentration, and s e n s i t i v i t y to i n i t i a t o r and s a l t concentrations (22). High molecular weights are important for many a p p l i c a t i o n s of water-soluble polymers, and thus inverse emulsion polymerization processes are becoming more important. Work presently i n progress should help to generate a better understanding of the chemical and physical mechanisms i n v o l v e d . Direct Emulsification. Polymer c o l l o i d s c a l l e d " a r t i f i c i a l latexes" can be prepared by dispersion of bulk polymers or polymer s o l u t i o n s into an aqueous medium. Direct e m u l s i f i c a t i o n processes are reviewed by ElAasser (23). The preparation procedures i n v o l v e mechanical dispersion that may be followed by removal of s o l v e n t . According to ElAasser "the e f f i c i e n c y of e m u l s i f i c a t i o n , " and hence the p a r t i c l e s i z e c h a r a c t e r i s t i c s of the l a t e x , " i s determined by the e f f i c i e n c y of formation of fine droplets and the e f f i c i e n c y of s t a b i l i z a t i o n of the formed droplets." Important parameters i n the process include the source of energy or a g i t a t i o n , i t s i n t e n s i t y , and d u r a t i o n ; type and c o n c e n t r a t i o n of e m u l s i f i e r s ; mode of addition of e m u l s i f i e r and the two phases; density r a t i o of the two phases; temperature; and the rheology of the two phases. D i r e c t e m u l s i f i c a t i o n can be used to produce l a t e x e s from polymers that cannot be polymerized by f r e e - r a d i c a l mechanisms and from n a t u r a l polymers or t h e i r d e r i v a t i v e s . Three methods or a combination of methods can be a p p l i e d i n d i r e c t e m u l s i f i c a t i o n


6.

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149


p r o c e s s e s : s o l u t i o n e m u l s i f i c a t i o n , phase i n v e r s i o n , and s e l f emulsification. The f i r s t method, as the name i m p l i e s , i n v o l v e s e m u l s i f i c a t i o n of a polymer s o l u t i o n followed by solvent s t r i p p i n g . Phase i n v e r s i o n occurs when a d i l u t e aqueous a l k a l i i s added to a polymer t h a t has been compounded w i t h a l o n g - c h a i n f a t t y a c i d . S e l f - e m u l s i f i c a t i o n can be used w i t h polymers t h a t are m o d i f i e d w i t h f u n c t i o n a l groups, such as amino or quaternary ammonium groups, so t h a t they can be d i s p e r s e d i n water or a c i d s w i t h o u t emulsifiers. Acknowledgment T h i s m a t e r i a l i s based, i n p a r t , upon work supported National Science Foundation under Grant No. CPE-801445.

by

the

L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

F i t c h , R. M . ; Tsai, C. H. In "Polymer Colloids"; Fitch, R. M., Ed.; Plenum: New York, 1971; Chap. 5 and 6. U g e l s t a d , J.; E l A a s s e r , M . ; Vanderhoff, J. P o l y . L e t t e r I I 1973, 503. Nomura, M. In "Emulsion P o l y m e r i z a t i o n " ; P i i r m a , I . , E d . , ; Academic P r e s s : New York, 1982; Chap. 5, p. 191-219. H a r k i n s , W. D. J. Am. Chem. Soc. 1947, 69, 1429. Smith, W. V . ; Ewart, R. H. J. Chem. Phys. 1948, 16, 592. Roe, C. P. Ind. Eng. Chem. 1968, 60, 20. P o e h l e i n , G. W.; Dougherty, D. J . Rubber Chem. T e c h n o l . 1977, 50(3), 601. G o n z a l e z , R. A. M.S. T h e s i s , Lehigh University, Bethlehem, Pa., 1974. G u l f Oil Canada Ltd., U.S. Patent 3 551 396, 1970. Ugelstad, J . ; Hansen, F. K. Rubber Chem. Technol. 1976, 49(3), 536. Hansen, F. K.; U g e l s t a d , J. J. Polym. Sci., Polym. Chem. Ed. 1978, 16, 1953-79; 1979, 17, 3033-45; 1979, 17, 3047-67. Stockmayer, W. H. J. Polym. Sci. 1957, 24, 314. O'Toole, J. T. J. A p p l . Polym. Sci. 1965, 9, 1291. U g e l s t a d , J.; Mørk, P. C.; Aasen, J. O. J. Polym. Sci. A.1 1967, 5, 2281. Gilbert, R. G.; Napper, D. H. J. Chem. Soc. Faraday Trans. 1 1974, 70, 391. Hawkett, B. S.; Napper, D. H . ; Gilbert, R. G. J . Chem. Soc. Faraday Trans. 1 1975, 71, 2288; 1977, 73, 690; 1980, 76, 1323. Lansdowne, S. W.; Gilbert, R. G.; Napper, D. H . ; Sangster, D. F. J. Chem. Soc. Faraday Trans. 1 1980, 76, 1344. Lichti, G.; Gilbert, R. G.; Napper, D. H. J. Polym. Sci., A-1 1980, 18, 1297. P o e h l e i n , G. W.; Dubner, W.; Lee, H. C. Brit. Polym. J. 1982, 14. Vanderhoff, J. W.; B r a d f o r d , E. B . ; T a r k o w s k i , H. L ; S h a f f e r , J. B.; W i l e y , R. M. In " P o l y m e r i z a t i o n and P o l y c o n d e n s a t i o n Processes"; ADVANCES IN CHEMISTRY SERIES No. 34, American Chemical Society: Washington, D.C., 1962; p. 32.


150


21. 22. 23.

K u r e n k o v , V. K . ; O s i p o v a , T. M . ; K u z n e t s o v , E . V . ; Myagchenkov, V. A. Vysokonol. S o l d i n . , Ser. B20 1978, 647. DiStefano, F. M.S. Thesis, Lehigh U n i v e r s i t y , Bethlehem, Pa., 1981. E l A a s s e r , M.S. Paper 15, s h o r t course notes, "Advances i n Emulsion P o l y m e r i z a t i o n and Latex Technology"; Lehigh U n i v e r s i t y , Bethlehem, Pa., June 1982.


General References 1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11.

"Emulsion P o l y m e r i z a t i o n " ; P i i r m a , I . ; Gardon, J L.; Eds.; ACS SYMPOSIUM SERIES No. 24, American Chemical S o c i e t y : Washington, D.C., 1976. " P o l y m e r i z a t i o n Reactors and Processes"; Henderson, J . N . ; Bouton, T. C.; ACS SYMPOSIUM SERIES No. 104, American Chemical Society: Washington, D.C., 1979. "Polymer Colloids"; Fitch, R. M . , Ed.; Plenum: New York, 1971. "Polymer Colloids II"; Fitch, R. M . , Ed.; Plenum: New York, 1980. "Emulsion P o l y m e r i z a t i o n and I t s A p p l i c a t i o n s in Industry"; Eliseeva, V. I.; I v a n c h e v , S. S.; Kuchanov, S. I . ; Lebedev, A. V.; translated from Russian by Teaque, S. J . ; Plenum: New York, 1981. "Emulsion Polymerization of V i n y l Acetate"; ElAasser, M. S.; Vanderhoff, J . W., Eds.; A p p l i e d Science P u b l : Englewood, N . J . , 1981. "Emulsion Polymers and Emulsion Polymerization"; Bassett, D. R.; Hamielec, A. E . , Eds.; ACS SYMPOSIUM SERIES No. 165, American Chemical Society: Washington, D.C., 1981. "Emulsion P o l y m e r i z a t i o n " ; P i i r m a , I., Ed.; Academic P r e s s : New York, 1982. "Science and Technology of Polymer Colloids"; P o e h l e i n , G. W.; Goodwin, J. W.; Ottewill, R. H . , Eds.; M a r t i n u s N i j h o f f Publ.: The Hague, Netherlands, 1983. "New Concepts i n Emulsion Polymers"; Hwa, J . ; Vanderhoff, J . W., Eds.; J. Polym. Sci. 1969, C27. B l a c k l e y , D. C. "Emulsion P o l y m e r i z a t i o n : Theory and Practice"; Applied Science P u b l . : London, 1975.


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