12 A Review of Mechanistic Considerations and Process
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 29, 2015 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch012
Design Parameters for Precipitation Polymerization M . R. J U B A Research Laboratories, Eastman Kodak Company, Rochester, N Y 14650
P r e c i p i t a t i o n p o l y m e r i z a t i o n s g e n e r a l l y c o n s i s t o f two phases: the d i l u e n t phase and the s o l i d polymer p a r t i c l e s . The d i l u e n t i s a s o l v e n t f o r the monomer and i n i t i a t o r and a nonsolvent f o r the polymer. The polymer p a r t i c l e s are not s t a b i l i z e d and tend t o agglomerate t o form a polymer paste o r s l u r r y . In a d d i t i o n , the p o l y m e r i z a t i o n r a t e i s independent o f the number o f p a r t i c l e s (_1) . Some t y p i c a l examples o f p r e c i p i t a t i o n p o l y m e r i z a t i o n s a r e :
Monomer Diluent methyl methacrylate cyclohexane (2) styrene methanol (3) acrylonitrile bulk (4) vinylidene chloride bulk (_5) vinyl chloride bulk (6) Since these p r e c i p i t a t i o n p o l y m e r i z a t i o n s produce polymeric s o l i d s a t very high p o l y m e r i z a t i o n r a t e s and very high p u r i t y ( i . e . , f r e e from e m u l s i f i e r s , suspending agents, e t c . ) , t h e i r p o p u l a r i t y as manufacturing processes i s i n c r e a s i n g . T h i s creates some i n t e r e s t i n g challenges f o r the process design engineer who i s searching f o r a r e l a t i o n s h i p among the r e a c t i o n parameters and the p h y s i c a l v a r i a b l e s o f the r e a c t i o n . These r e l a t i o n s h i p s are g e n e r a l l y determined e m p i r i c a l l y , because o f the complex k i n e t i c s o f the p r e c i p i t a t i o n polymerizat i o n process and the l a r g e v a r i a t i o n s from one r e a c t i o n system to another. Nevertheless, a review o f the l i t e r a t u r e presents u s e f u l g u i d e l i n e s f o r process design experiments.
0-8412-0506-x/79/47-104-267$05.00/0 © 1979 American Chemical Society In Polymerization Reactors and Processes; Henderson, J. Neil, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 29, 2015 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch012
268
POLYMERIZATION REACTORS AND PROCESSES
P a r t i c l e Formation. E l e c t r o n microscopy and o p t i c a l microscopy are the d i a g n o s t i c t o o l s most o f t e n used to study p a r t i c l e formation and growth i n p r e c i p i t a t i o n p o l y m e r i z a t i o n s (7,8). However, i n t y p i c a l p o l y m e r i z a t i o n s of t h i s type, the p a r t i c l e formation i s normally completed i n a few seconds or tens o f seconds a f t e r the s t a r t o f the r e a c t i o n (9), and the p h y s i c a l processes which are i n v o l v e d are d i f f i c u l t to measure i n a r e a l time manner. As a r e s u l t , the a c t u a l p a r t i c l e formation mechanism i s open to a v a r i e t y of i n t e r p r e t a t i o n s and the r e s u l t s could f i t more than one t h e o r e t i c a l model. B a r r e t t and Thomas (10) have presented an e x c e l l e n t review o f the four p h y s i c a l processes i n v o l v e d i n the p a r t i c l e formation: oligomer growth i n the d i l u e n t oligomer p r e c i p i t a t i o n t o form p a r t i c l e n u c l e i capture o f oligomers by p a r t i c l e n u c l e i , and coalescence or agglomeration o f primary p a r t i c l e s . The f i r s t process begins as i n i t i a t o r decomposes i n the d i l u e n t phase and polymerizes the monomer to form oligomers which p r e c i p i t a t e from s o l u t i o n upon reaching a c r i t i c a l molecu l a r weight. This c r i t i c a l molecular weight i n c r e a s e s with the s o l u b i l i t y of the polymer and i s low enough so t h a t a l l the oligomers are captured or nucleate p a r t i c l e s before t h e i r r a d i c a l s are terminated. As a r e s u l t , n e a r l y a l l p o l y m e r i z a t i o n takes p l a c e i n the p a r t i c l e s and the polymer c o n c e n t r a t i o n i n the d i l u e n t phase i s low. The polymer s o l u b i l i t y can be estimated using s o l u b i l i t y parameters (11) and the value o f the c r i t i c a l oligomer molecular weight can be estimated from the Flory-Huggins theory of polymer s o l u t i o n s (12), but the optimum d i l u e n t i s s t i l l u s u a l l y chosen empirically. B a r r e t t and Thomas (10) l i s t e d the f o l l o w i n g e f f e c t s o f i n c r e a s i n g the d i l u e n t ' s solvency: r e t a r d s the onset of p a r t i c l e formation, i n c r e a s e s the d u r a t i o n o f p a r t i c l e formation, and produces fewer, l a r g e r p a r t i c l e s with a broader p a r t i c l e size distribution. Once the oligomers have formed, two mechanisms, s e l f n u c l e a t i o n and aggregate n u c l e a t i o n , are used to d e s c r i b e p a r t i c l e nucleation. In s e l f - n u c l e a t i o n , the extended oligomer chain c o l l a p s e s upon i t s e l f to nucleate a p a r t i c l e . In aggregate n u c l e a t i o n the oligomers r e v e r s i b l y a s s o c i a t e with each other u n t i l the aggregate reaches a c r i t i c a l s i z e above which i t i s thermodynamically s t a b l e and continues to grow. The aggregation of oligomers r e q u i r e s a lower average degree of p o l y m e r i z a t i o n f o r n u c l e i formation than the s e l f n u c l e a t i o n model where a l a r g e r i n d i v i d u a l chain i s r e q u i r e d .
In Polymerization Reactors and Processes; Henderson, J. Neil, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 29, 2015 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch012
12.
JUBA
Precipitation
Polymerization
269
Therefore, aggregation i s considered the primary mode o f p a r t i c l e n u c l e a t i o n i n most systems. A f t e r the p a r t i c l e n u c l e i form, they capture the oligomers growing i n the d i l u e n t phase and e s s e n t i a l l y no new p a r t i c l e n u c l e i are formed. Two models have been employed t o e x p l a i n t h i s capture. The f i r s t i s d i f f u s i o n capture. T h i s theory was o r i g i n a l l y proposed by F i t c h and T s a i (13) f o r the aqueous p o l y m e r i z a t i o n of methyl methacrylate. According t o t h i s theory, any oligomer which d i f f u s e s t o an e x i s t i n g p a r t i c l e before i t has a t t a i n e d the c r i t i c a l s i z e f o r n u c l e a t i o n i s i r r e v e r s i b l y captured. The rate o f n u c l e a t i o n i s equal t o the r a t e o f i n i t i a t i o n minus the rate o f capture. The r a t e o f capture i s p r o p o r t i o n a l t o both the s u r f a c e area and the number o f p a r t i c l e s . In the e q u i l i b i r u m capture model, on the other hand, there i s a dynamic e q u i l i b r i u m between the growing oligomers and the surface o f the p a r t i c l e s as w e l l as the p o s s i b i l i t y o f some interchange with the i n t e r i o r o f the p a r t i c l e s . Although both o f these models provide a reasonable d e s c r i p t i o n o f the p r e c i p i t a t i o n p o l y m e r i z a t i o n process, they do not i l l u s t r a t e the r e l a t i o n s h i p between the r e a c t o r v a r i a b l e s and the polymer p a r t i c l e p r o p e r t i e s . Perhaps the only process where such c o r r e l a t i o n s have been p u b l i s h e d i s the bulk p o l y m e r i z a t i o n o f v i n y l c h l o r i d e as reported by Ray, J a i n and Salovey (14). The p o l y m e r i z a t i o n occurs i n four stages. Bulk PVC Process Nucleation o f the primary p a r t i c l e p o p u l a t i o n F l o c c u l a t i o n o f p a r t i c l e s and capture o f oligomers t o a p o i n t o f constant p a r t i c l e p o p u l a t i o n Polymerization u n t i l the separate monomer phase i s consumed P o l y m e r i z a t i o n o f absorbed monomer i n the polymer p a r t i c l e s The commercial process as d e s c r i b e d by J . C h a t e l a i n (15) c o n s i s t s o f two stages as shown i n F i g u r e 1. In the prepolymerizer, the polymer p a r t i c l e s are formed and polymerized t o 7-8 wt. % conversion before being t r a n s f e r r e d t o the autoclave where the p a r t i c l e s are polymerized t o a s o l i d powder a t about 88% conversion. The f i n a l polymer p a r t i c l e s have a 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 , F i g u r e 2 (15), and the mean p a r t i c l e s i z e i s a strong f u n c t i o n o f the a g i t a t i o n i n the prepolymerizer, Figures 3 and 4 (16). In a d d i t i o n , s e v e r a l patents, d i s c u s s the e f f e c t s o f various a d d i t i v e s on the p a r t i c l e s i z e o f the f i n a l product. Ray, J a i n and Salovey (14) modeled these phenomena using the k i n e t i c constant o f coalescence as t h e i r major parameter. This constant was a f u n c t i o n o f p a r t i c l e s i z e , a g i t a t i o n r a t e , and the s u r f a c e p r o p e r t i e s o f the p a r t i c l e s , and i t s f u n c t i o n a l form suggested t h a t the p r o b a b i l i t y o f coalescence was p r o p o r t i o n a l t o the s u r f a c e area p e r u n i t volume o f the
In Polymerization Reactors and Processes; Henderson, J. Neil, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
POLYMERIZATION REACTORS AND PROCESSES
DEGASING V.C. FEEDING
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 29, 2015 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch012
P R E P O
DEGASING
V.C. FEEDING
PVC UNLOADING British Polymer Journal Figure
1.
Reactor system for bulk PVC (15)
100
100
120
140
160
Particle size (/im) British Polymer Journal Figure
2.
Particle
size distribution
for bulk PVC (15)
In Polymerization Reactors and Processes; Henderson, J. Neil, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
12.
JUBA
Precipitation
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 29, 2015 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch012
|
