Emulsion Polymerization

1 Emulsification and Emulsion Polymerization of Styrene Using Mixtures of Cationic Surfactant and Long Chain Fatty Alcohols or Alkanes as Emulsifiers A. R. M. AZAD, J. UGELSTAD, R. M. FITCH, and F. K. HANSEN

Downloaded by NEW YORK UNIV on May 20, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch001

Laboratory of Industrial Chemistry, The University of Trondheim, The Norwegian Institute of Technology, N-7034 Trondheim-NTH, Norway

In some recent papers Ugelstad et al.(1,2,3) have reported results on the emulsion polymerization of styrene with a mixed emulsifier system consisting of Na hexadecyl sulphate and hexadecanol. Under given condi tions for emulsification and polymerization it was found that the monomer droplets became the main loci for initiation of polymerization. The first part of the present work describes emulsification experiments using a mixture of a cationic emulsifier, octadecyl pyridinium bromide (OPB) with n-fatty alcohols of different chain lengths, applying ordinary stirring equipment. In order to get a comparison of the monomer droplet size with that of the particles in the final latex, it was necessary to develop a method for obtaining electron microscope pictures of the monomer emulsions (4). Polymerizations of the monomer emulsions were carried out with o i l - s o l u b l e initiators. Oil-soluble i n i t i a t o r s have often been employed in emulsion poly merization recipes and are generally used in suspension polymerization. Whereas in the latter case the initi ation naturally takes place in the monomer droplets, the locus of initiation and growth of particles in emulsion polymerization with o i l - s o l u b l e i n i t i a t o r s has been open to some doubt. However, the fact that the particle size and size distribution is not very d i f f e rent from the results with water-soluble i n i t i a t o r s and that the particles are generally much smaller than the droplets in the monomer emulsions indicates that with T h i s w o r k i s p a r t o f t h e t h e s i s b y A.R.M.A. a t t h e Norwegian I n s t i t u t e o f T e c h n o l o g y , Trondheim, Norway 1

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

2

EMULSION

POLYMERIZATION

ordinary emulsifier recipes initiation o f particles takes p l a c e mainly i nt h e aqueous phase. An a d d i t i o n a l p a r t d e s c r i b e s some r e s u l t s i n w h i c h t h e e m u l s i f i c a t i o n of t h e monomer i s c a r r i e d o u t w i t h a M a n t o n G a u l i n homogenizer. S o m e e x p e r i m e n t s we're a l s o c a r r i e d o u t where t h e hexadecanol was r e p l a c e d by hexadecane.


Experimental

part

M a t e r i a l s : T h e s t y r e n e monomer was d i s t i l l e d t w i c e , the second time immediately p r i o r t o e m u l s i f i cation. A s m a l l amount o f i n h i b i t o r (p-benzoquinone, 500 mg/kg monomer) w a s added t o a v o i d p o l y m e r i z a t i o n i n the e m u l s i f i c a t i o n experiments. OPB w a s s y n t h e s i z e d from o c t a d e c y l bromide ( p u r e , Koch L i g h t L a b . L t d . , England) and p y r i d i n e (pure, Merck, Germany); t h e p r o d u c t was washed f o u r times w i t h d r y e t h e r a n d r e c r y s t a l 1 i z e d t w i c e f r o m a c e t o n e , m.p. 7 5 ° C . Hexadecanol (HD)( H y f a t o l 1 6 , Aarhus O l i e f a b r i k ) and octadecanol (0D) ( p u r e , F l u k a , S w i t z e r l a n d ) were d i s t i l l e d t w i c e i nvacuum. 2,2'-azobisisobutyronitrile ( A I B N ) ( p u r e , F l u k a ) , w a s c r y s t a l l i z e d t w i c e f r o m 96% ethanol. E i c o s a n o l , ES ( A r a c h i d i c a l c o h o l , p u r e Koch L i g h t L a b . L t d . , E n g l a n d ) , t e t r a d e c a n o l , TD ( p u r e , S c h u c h a r d t , M u n i c h ) , h e x a d e c a n e ( p u r i s , K o c h L i g h t Lab.) b e n z o y l p e r o x i d e , B P ('97% M e r c k ) , c u m e n e h y d r o p e r o x i d e , CHP ( 7 0 % i n c u m e n e , M e r c k - S c h u c h a r d t ) , o s m i u m t e t r a oxide (Merck) and d i o c t y l sodium s u l f o s u c c i n a t e (pure, Merck) were used w i t h o u t f u r t h e r p u r i f i c a t i o n . R e d i s t i l l e d water was used. Osmium t e t r o x i d e s t a i n i n g o f monomer e m u l s i o n . 0.01 c m o f t h e e m u l s i o n w a s d i l u t e d w i t h 0 . 5 c m w a t e r s a t u r a t e d w i t h s t y r e n e . To t h i s was added g r a d u a l l y w i t h m i x i n g a s a t u r a t e d s o l u t i o n o f OSO4 i n w a t e r p r e p a r e d i m m e d i a t e l y b e f o r e u s e . T h e a m o u n t o f OSO4 s o l u t i o n added was 0.13 c m , c o r r e s p o n d i n g t o a s t y r e n e :0sÛ4 m o l a r r a t i o o f 1 : 1 . 5 . A n i m m e d i a t e b l a c k e n i n g o f the emulsion took p l a c e . Samples (2-3 μΐ) were w i t h drawn a f t e r 5 a n d 10 m i n u t e s p l a c e d o n a f o r m v a r a n d carbon-coated g r i d and a l l o w e d t o d r y . The g r i d s were examined i na Siemens e l e c t r o n microscope. Some experiments were c a r r i e d o u t i n which t h e 0s04:styrene molar r a t i o and time o f r e a c t i o n were v a r i e d . Inferior e l e c t r o n m i c r o s c o p e p i c t u r e s w e r e o b t a i n e d when t h e m o l a r r a t i o o f 0 s Û 4 : s t y r e n e w a s l e s s t h a n 1:1 o r h i g h e r than 2:1. P r o l o n g e d r e a c t i o n t i m e s g r e a t e r t h a n 30 minutes a l s o i n v a r i a b l y gave i n f e r i o r r e s u l t s . After d r y i n g on t h e g r i d s t h e p a r t i c l e s were s t a b l e and 3

3

3


1.

AZAD

ET AL.

Emuhion

Polymerization

of

3

Styrene

p r e p a r a t i o n s c o u l d be l e f t f o r a week w i t h o u t a n y n o t i c e a b l e change i n t h ee l e c t r o n m i c r o s c o p e p i c t u r e s . In some e x p e r i m e n t s t h e s t a i n e d p a r t i c l e s w e r e s h a d o w e d w i t h a 80/20 Pt/Pd a l l o y a t an a n g l e o f 30° t o a s c e r t a i n that t h ep a r t i c l e s were s p h e r i c a l . Apparatus

and procedure

The e m u l s i f i c a t i o n e x p e r i m e n t s w e r e c a r r i e d o u t i n a 500 cm glass vessel w i t h a paddle s t i r r e r f i t t e d w i t h thermometer, manometer and equipment f o r c h a r g i n g and s a m p l i n g . H o t w a t e r was p a s s e d t h r o u g h t h e o u t e r jacket o f t h ereactor. 0 Ρ Β , f a t t y a l c o h o l a n d H2O w e r e f i r s t mixed w i t h s t i r r i n g a t 70-80 °C, t h e t e m p e r a t u r e depending upon t h ec h a i n l e n g t h o f t h ea l c o h o l . After c o o l i n g t o 60 ° C , t h e s t y r e n e was added a n d t h e s t i r r i n g continued a t 600 rpm. Samples were withdrawn a t i n t e r v a l s through a bottom stopcock and analysed f o r e m u l s i f i e r i n t h e aqueous phase a f t e r c e n t r i f u g a t i o n as d e s c r i b e d i na p r e v i o u s p a p e r (2) w i t h t h e e x c e p t i o n t h a t 0 . 0 0 2 M d i o c t y l sodium s u l f o s u c c i n a t e was used f o r t h e t i t r a tion. From some o f t h e s a m p l e s o f t h e monomer emulsions e l e c t r o n m i c r o g r a p h s were o b t a i n e d i n the manner d e s c r i b e d above. E m u l s i o n s f r o m t h e M a n t o n G a u l i n homog e n i z e r w e r e s u b s e q u e n t l y s t i r r e d a t 6 0 °C a n d a n a l y s e d i n t h e *ame m a n n e r . The p o l y m e r i z a t i o n e x p e r i m e n t s w e r e c a r r i e d o u t a s d e s c r i b e d p r e v i o u s l y {2). The i n i t i a t o r was d i s s o l v e d i n t h e monomer p r i o r t o e m u l s i f i c a t i o n . Samples were w i t h d r a w n t h r o u g h t h e bottom v a l v e i n t o 25 c m o f a short stop s o l u t i o n o f p-benzoquinone i nmethanol. The c o n v e r s i o n was d e t e r m i n e d by e v a p o r a t i o n o f t h e v o l a t i l e s a t 60 °C. E l e c t r o n micrographs o f t h el a t e x p a r t i c l e s were obtained i n t h eusual way.


3

3

Results

and discussion

E f f e c t o f η-fatty a l c o h o l a s a d d i t i v e : As found p r e v i o u s l y i n t h ea n i o n i c mixed e m u l s i f i e r systems w i t h f a t t y a l c o h o l s , t h e method o f p r e p a r i n g t h e emul s i o n s i s c r u c i a l when moderate s t i r r i n g i s employed. T h u s , i f t h ef a t t y a l c o h o l was added t o t h e monomer p r i o r t o m i x i n g w i t h an aqueous s o l u t i o n o f t h e e m u l s i f i e r , only very coarse emulsions r e s u l t e d , which sepa rated within a few minutes. With t h ep r e s e n t method there took place a rapid e m u l s i f i c a t i o n , with t h e r e s u l t that a t moderate i n i t i a l e m u l s i f i e r c o n c e n t r a t i o n s more than 9 7 . 5 % o f t h ee m u l s i f i e r was a d s o r b e d


4

EMULSION

POLYMERIZATION

w i t h i n 15-20 m i n u t e s . T a b l e I g i v e s some r e s u l t s o f t h e amount o f e m u l s i f i e r a d s o r b e d o n t h e d r o p l e t s a f t e r 15 m i n u t e s s t i r r i n g w i t h 2 g / d m H^O o f O P B a n d w i t h v a r i o u s m o l a r r a t i o s o f h e x a d e c a n o l (HD)t o OPB. I t a p p e a r s t h a t a HD:0PB r a t i o o f 2:1 i s s u f f i c i e n t t o bring about p r a c t i c a l l y t o t a l adsorption o f e m u l s i f i e r on t h e d r o p l e t s . F i g . 1 g i v e s t h e r e s u l t s o f some e x p e r i m e n t s w i t h OPB a n d h e x a d e c a n o l i n w h i c h t h e a m o u n t o f O P B a d s o r b e d on t h e d r o p l e t s w a s f o l l o w e d a s a f u n c t i o n o f t i m e a t 60 °C a n d w i t h s t i r r i n g a t 6 0 0 r p m . I t a p p e a r s t h a t i n a l l c a s e s t h e a m o u n t o f OPB a d s o r b e d o n t h e d r o p l e t s r a p i d l y reaches an optimal value. As t h e s t i r r i n g i s c o n t i n u e d t h e OPB i s g r a d u a l l y t r a n s p o r t e d b a c k t o t h e aqueous phase, i n d i c a t i n g a gradual degradation o f t h e emulsion. This d e g r a d a t i o n c o u l d be f o l l o w e d by e l e c tron micrographs o f the emulsion a f t e r OSO4-Staining as d e s c r i b e d a b o v e . Fig. 2 gives an e l e c t r o n micrograph o f one o f t h e e m u l s i o n s a f t e r 15 m i n u t e s s t i r r i n g . The droplet s i z e s a r e i n t h e r a n g e o f 0.4 t o 1.5 ym. F i g . 3 s h o w s a n e l e c t r o n m i c r o g r a p h o f t h e same e m u l s i o n a f t e r 21 h s t i r r i n g a t 6 0 ° C . T h e d r a s t i c i n c r e a s e i nd r o p l e t s i z e i s c l e a r l y apparent. F i g . 4 g i v e s some r e s u l t s w i t h f a t t y a l c o h o l s o f different chain length. I t a p p e a r s t h a t w i t h t h e C-j^ f a t t y a l c o h o l t h e e m u l s i f i c a t i o n i spoor a n d t h e emulsion i s r e l a t i v e l y unstable. As t h e chain l e n g t h of t h e f a t t y a l c o h o l i si n c r e a s e d the s t a b i l i t y o f t h e emulsion gradually increases. With t h e Cpo a l c o h o l t h e amount o f e m u l s i f i e r a d s o r b e d on t h e d r o p l e t s a f t e r 20 h s t i r r i n g a t 6 0 ° C i s o n l y r e d u c e d f r o m 9 5 t o 9 0 %. F o r c o m p a r i s o n some e x p e r i m e n t s w e r e c a r r i e d o u t i n w h i c h t h e f a t t y a l c o h o l was d i s s o l v e d i n t h e monomer phase. The r e s u l t s o f these experiments are given i n T a b l e I I . I t appears t h a t i n t h i s case t h e amount o f OPB a d s o r b e d o n t h e d r o p l e t s a f t e r 1 5 m i n u t e s i s v e r y low. A f t e r 20 h s t i r r i n g t h e amount a d s o r b e d i s s t i l l o n l y c a . 50 % w h i c h , when compared t o t h e r e s u l t s i n F i g . 1, seems t o c o r r e s p o n d t o an " e q u i l i b r i u m " v a l u e at the given temperature and s t i r r i n g r a t e . The p r o n o u n c e d d e p e n d e n c e o f t h e d e g r e e o f e m u l s i f i c a t i o n upon t h e order o f mixing w i t h f a t t y a l c o h o l s is not y e t s a t i s f a c t o r i l y explained. Several p o s s i b i l i t i e s may be a d v a n c e d . a ) D u r i n g t h e s t i r r i n g , i n r e g i o n s of high shearrate i nthe neighbourhood o f the s t i r r e r , t h e r e w i l l be formed c o n t i n u o u s l y small d r o p l e t s w h i c h , without s t a b i l i z a t i o n , will continuously coalesce with the l a r g e r d r o p l e t s inthe bulk o f the mixture. In the p r e s e n c e o f t h e m i x e d e m u l s i f i e r t h e s e d r o p l e t s may b e


3


1.

AZAD E T AL.

Emulsion

Polymerization

of Styrene

Total OPB added

ο CM

X

1.5

CO

ε •opietsi

en

1.0 A

Ό

3:1

1:1

>rbed

0.5

(Λ Ο

OPB


s

0

200

A00

600 800 Time, minutes

1000

1200

U00

1800

1600

Figure 1. OPB adsorbed on the monomer droplets as a function of time at different molar ratios HD:OPB. Styrene — 83.3 g, H 0 — 250 g, OPB — 2.0 g/dm H 0. Temp. = 60° C. Stirring — 600 rpm. 2

Figure 2. Electron micrograph of monomer emulsion after 15 min stirring for HD:OPB. Molar ratio 6:1 (Figure I, curve E).

3

2

Figure 3. Electron micrograph of mono mer emubion of Figure 2, after 21 hr stirring at 600 rpm and 60°C


EMULSION POLYMERIZATION

total OPB added


Eicosanc il _^Octa decanol

Hexac leca nol

Tetradi canol

Time, minutes Figure 4. OPB adsorbed on the monomer droplets as a function of time for different long chain fatty alcohols. Styrene = 83.3 g, H O = 250 g, OPB = 2.0 g/dm H O. Temp. = 60°C. Stirrings600 rpm. Molar ratio fatty alcohoWPB = 3:1. s

3

B


1.

AZAD

Table

ET AL.

Emulsion

Polymerization

of

7

Styrene

I . OPB a d s o r b e d o n t h e monomer d r o p l e t s a f t e r 1 5 minutes o f s t i r r i n g . HD d i s s o l v e d i n t h e water phase p r i o r t o e m u l s i f i c a t i o n . Temp. = 6 0 ° C . S t i r r i n g = 6 0 0 r p m . OPB = 2 . 0 g / d m H 0 , H 0 = 2 5 0 g . Styrene = 83.3 g. 3


2

2

M o l a r r a t i o Amount o f OPB i n t h e aqueous p h a s e , g/dm^ w a t e r HD:0PB 1.88 0.21 0.06 0.05 0.06

0:1 1:1 2:1 3:1 4:1

Amount o f OPB a d s o r b e d on monomer d r o p l e t s . q/dm3 w a t e r 0.12 1.79 1.94 1.95 1.94

T a b l e I I . OPB a d s o r b e d o n s t y r e n e d r o p l e t s a f t e r d i f f e r e n t s t i r r i n g t i m e s w h e r e HD i s d i s s o l v e d i n t h e monomer Temp. = 6 0 ° C , S t i r r i n g = 600 rpm. OPB = 2.0 g/dnv H 0 . H 0 = 250 g . S t y r e n e = 8 3 . 3 g . 2

Molar r a t i o HD:0PD

2

Time o f Amount o f OPB i n s t i r r i n g t h e aqueous phase (min) g/dm w a t e r 15 1.82 1230 1.06 15 1.76 1330 0.95 3

4:1 10:1

Amount o f OPB a d s o r b e d o n monomer d r o p l e t s g/dm w a t e r 0.18 0.94 0.24 1.05 3



8

EMULSION

POLYMERIZATION

r a p i d l y covered with a complex l a y e r o f e m u l s i f i e r and f a t t y a l c o h o l , which w i l l prevent the coalescence f o r the time n e c e s s a r y f o r d i s p e r s i o n o f a l l t h e monomer to take p l a c e . b)During s t i r r i n g , f r e s h o i l - w a t e r i n t e r faces a r e created by t h e paddle s t i r r e r . The alcohol and e m u l s i f i e r p r e s e n t i n t h e w a t e r p h a s e d i f f u s e r a p i d l y t o t h i s f r e s h l y formed i n t e r f a c e , r e s u l t i n gi n a momentary high c o n c e n t r a t i o n o f a l c o h o l a t t h e interphase. T h i s may c a u s e a local lowering o f t h e i n t e r f a c i a l t e n s i o n w h i c h may d r a s t i c a l l y f a c i l i t a t e t h e e m u l s i f i c a t i o n . As shown b y D a v i e s a n d Haydon ( 6 ) , a high c o n c e n t r a t i o n o f f a t t y a l c o h o l i n a d d i t i o n t o e m u l s i f i e r a t the i n t e r f a c e w i l l lower γ t o a p p r o x i m a t e l y z e r o v a l u e w h i c h may l e a d t o s p o n taneous e m u l s i f i c a t i o n . D a v i e s a n d Haydon have added the f a t t y a l c o h o l t o t h e o i l phase p r i o r t o mixing with the water solution o f the e m u l s i f i e r . In this case, t h e r e f o r e , they had t o apply a r e l a t i v e l y very high concentration o f f a t t y a l c o h o l . c ) I t i s also possible that the development o f t r a n s i e n t i n t e r f a c i a l t e n s i o n g r a d i e n t s o n t h e monomer d r o p l e t s f o r m e d may f a c i l i t a t e emulsification with f a t t y alcohol present in t h e aqueous phase. Immediately a f t e r a droplet i s separated i n t o two d r o p l e t s , the i n t e r f a c i a l tension, γ, w i l l t e n d t o be h i g h e r a t t h e p o i n t s o f c l o s e s t a p p r o a c h than a t t h e more d i s t a n t p a r t s o f t h e i n t e r faces. The ensuing gradient i n γ tends t o suck aqueous s o l u t i o n between t h e newly formed d r o p l e t s f o r c i n g them a p a r t a n d hence p r o v i d i n g them w i t h time to s t a b i l i z e themselves against coalescence a f t e rt h e i n t e r f a c i a l tension gradient has vanished ( 5 J . Fig. 5 gives electron micrographs o f latexes prepared w i t h AIBN a s i n i t i a t o r from e m u l s i o n s w i t h a c o n s t a n t styrene:HoO w e i g h t r a t i o = 1:3, a c o n s t a n t a m o u n t o f OPB a n d w i t h HD:0PB m o l a r r a t i o s o f 0, 1 : 1 , 3:1 a n d 4:1 r e s p e c t i v e l y . I n t h e f i r s t c a s e t h e l a t e x contains only small p a r t i c l e s with a r e l a t i v e l y broad p a r t i c l e s i z e d i s t r i b u t i o n . W i t h a HD:0PB r a t i o o f 1:1 t h e p a r t i c l e s a r e s o m e w h a t l a r g e r a n d m o r e m o n o disperse. I n both cases t h e p a r t i c l e n u c l e a t i o n apparently takes place completely i nt h e aqueous phase. The l a r g e r p a r t i c l e s i n t h e s e c o n d c a s e p r o b a b l y s t e m from t h e f a c t t h a t t h e amount o f e m u l s i f i e r i n t h e water phase i slower, due t o t h e f a c t that a r e l a t i v e l y l a r g e p a r t i s a d s o r b e d o n t h e monomer d r o p l e t s . However there i s s t i l l enough e m u l s i f i e r present i nt h e aqueous phase t o b r i n g about p r a c t i c a l l y complete p a r t i c l e n u c l e a t i o n i n t h a t p h a s e . I n t h e c a s e s o f H D : 0 P B = 3:1 a n d 4 : 1 t h e s i t u a t i o n i s c o m p l e t e l y d i f f e r e n t . We h a v e in both cases a bimodal d i s t r i b u t i o n . The major part


Emulsion

Polymerization

of

Styrene


AZAD E T AL.

Figure 5. Electron micrographs of final latexes with varying amounts of hexa decanol (HD). Styrene = 166.7 g, H 0 = 500 g, OPB = 2.0 g/dm H 0. Temp. = 60°C. A1BN — 1.0 g in 166.7 g styrene. Molar ratios ΕΌ.ΟΡΒ = (A): 0, (B): 1:1, (C): 3:1, and (D): 4:1. OPB remaining in the water phase after emulsification of monomer (A) —1.78, (B) = 0.35, (C) = 0.08 g/dm H 0. 3

2

3

2

2



10

EMULSION

POLYMERIZATION

by w e i g h t c o n s i s t s o f p a r t i c l e s o f d i a m e t e r 0.4 t o 1.5 urn. By c o m p a r i n g w i t h t h e e l e c t r o n m i c r o g r a p h s o f t h e c o r r e s p o n d i n g m o n o m e r e m u l s i o n s i n F i g . 6, i t i s apparent t h a t these p a r t i c l e s stem from i n i t i a t i o n i n monomer d r o p l e t s . In addition, there are a consider a b l e n u m b e r o f p a r t i c l e s o f a b o u t 0 . 2 ym n o t p r e s e n t i n the monomer e m u l s i o n . These p a r t i c l e s t h e r e f o r e most probably stem from n u c l e a t i o n i n t h e aqueous phase. In F i g . 7 a r e g i v e n t h e r e s u l t s o f k i n e t i c m e a s u r e m e n t s o f t h e l a t e x e s g i v e n i n F i g . 5. I t a p p e a r s that i nthe case o f pure e m u l s i f i e r the k i n e t i c r e s u l t s a r e i n a g r e e m e n t w i t h common e x p e r i e n c e i n e m u l s i o n polymerization o f styrene with a water soluble i n i t i ator. The r a t e i s a p p r o x i m a t e l y c o n s t a n t up t o high conversion. I n c a s e C a n d D, h o w e v e r , t h e s i t u a t i o n i s completely d i f f e r e n t . The i n i t i a l rate i s r e l a t i v e l y high a n d d e c r e a s e s s i g n i f i c a n t l y up t o about 30 % c o n v e r s i o n when i t s t a r t s i n c r e a s i n g s l o w l y . W i t h t h e r e a c t i o n t a k i n g p l a c e i n t h e monomer d r o p l e t s t h e r a t e should be g i v e n by r = k [M] p

M

(D

(

w h e r e LM]M i s t h e c o n c e n t r a t i o n o f m o n o m e r i n t h e d r o p l e t s , \X\\\ t h e c o n c e n t r a t i o n o f i n i t i a t o r i n t h e d r o p l e t s , Vjv| t h e t o t a l v o l u m e o f m o n o m e r d r o p l e t s p e r 1 H2O a n d r i s g i v e n i n m o l s e c " p e r 1 H 0 . W i t h l i t e r a t u r e values o f k = 300 1/mole-sec, k t = 1 0 1 / m o l e - s e c , k j = 1 . 0 x 1 0 " s e c " ' a n d [M]M = 8 . 4 m o l e / 1 , V = 0 . 3 8 3 1/1 H 0 , t h e c a l c u l a t e d i n i t i a l r a t e i s approximately 20 g PS/1 H 0 / h . T h e e x p e r i m e n t a l i n i t i a l r a t e i s f o u n d t o be 20 g PS/1 H 0 / h , i n good agreement with the c a l c u l a t e d value. In t h e b e g i n n i n g t h e r a t e d e c r e a s e s w i t h ^ i n c r e a s i n g conversion i naccordancewith e q u a t i o n ( 1 ) ([MJm d e creases). Beyond 30 % c o n v e r s i o n t h e r a t e i n c r e a s e s s l i g h t l y , probably due t o a continuous decrease i n k . 1

2

8

D

5

M

2

2

2

t

peroxide gave r e s u l t s comparable t o those with AIBN: i n i t i a t i o n both i n monomer d r o p l e t s a n d i n t h e aqueous phase, a l t h o u g h t o v a r y i n g d e g r e e s , was o b s e r v e d , a s s h o w n i n F i g s . 8 a n d 9. U n d e r t h e s a m e c o n d i t i o n s o f e m u l s i f i e r and f a t t y a l c o h o l , benzoyl peroxide gave t h e h i g h e s t d e g r e e o f monomer d r o p l e t i n i t i a t i o n , w h i l e cumene h y d r o p e r o x i d e l e d t o more n u c l e a t i o n i n t h e aqueous phase. A more f i n e l y d i s p e r s e d monomer d r o p l e t emulsion could be achieved by i n c r e a s i n g t h e c o n c e n t r a t i o n o f


AZAD E T AL.

Emuhion

Polymerization

of

Styrene


1.

Figure 7. Polymer formed as a function of time with varying amounts of HD for the experiments given in Figure 5. (Letters (A), (B), (C), and (D) refer to Figure 5.)


11


EMULSION

POLYMERIZATION

Figure 8. Electron micrographs of monomer emulsion (A) and final latex (B) when benzoyl peroxide was used as the oil soluble initiator. Styrene = 166.7 g, H 0 = 500 g, HD:OPB — 4:1, OPB = 2.0 g/dm H 0. Temp. = 60°C. BP — 2.0 g in 166.7 g styrene. 2

3

2

Figure 9. Electron micrographs of monomer emulsion (A) and final latex (B) when cumene hydroperoxide was used as the oil soluble initiator. Styrene = 166.7 g, H 0 = 500 g, HD:OPB = 4:1, OPB = 2.0 g/dm H 0. Temp. = 60°C. CHP = 2.5 g in 166.7 g styrene. 2

3

2



1.

AZAD E T A L .

Emulsion

Polymerization

of

Styrene

13

the mixed e m u l s i f i e r system ( F i g . 1 0 ) . However, t h e monomer d r o p l e t s u r f a c e does n o t i n c r e a s e p r o p o r t i o n a t e l y t o t h eamount o f e m u l s i f i e r added s o t h a t t h e con c e n t r a t i o n o f e m u l s i f i e r l e f t i n t h e aqueous phase a f t e r e m u l s i f i c a t i o n i s i n t h i s case i n c r e a s e d . I t turns o u tthat this f a c t leads t o that one gets a pre dominant n u c l e a t i o n i n t h e aqueous phase ( F i g . 1 1 ) . The r e s u l t s a r e i n a c c o r d a n c e w i t h p r e v i o u s r e s u l t s w i t h m i x e d e m u l s i f i e r s y s t e m s o f Na h e x a d e c y l sulphate a n d h e x a d e c a n o l w i t h I ^ S p O g a s i n i t i a t o r (2). Also i n the present case with o i l s o l u b l e i n i t i a t o r and with OPB i t i s t h e r e f o r e a n e c e s s a r y c o n d i t i o n f o r o b t a i n i n g a h i g h d e g r e e o f monomer d r o p l e t i n i t i a t i o n t h a t n o t o n l y a f i n e d i s p e r s i o n o f monomer i s a c h i e v e d b u t t h a t a t t h e same t i m e t h e c o n c e n t r a t i o n o f e m u l s i f i e r l e f t in t h e aqueous phase i svery low. E f f e c t o f hexadecane as a d d i t i v e : In a s e r i e s o f papers H a l l w o r t h and C a r l e s s (7,8,9,10) have i n v e s t i gated t h e e f f e c t o f t h enature oT t h e i n t e r n a l phase on t h e s t a b i l i t y o f o i l i n w a t e r e m u l s i o n s a s w e l l a s the e f f e c t o f a d d i t i o n o f long chain f a t t y a l c o h o l s with sodium dodecyl sulphate o r sodium hexadecyl s u l p h a t e as t h ei o n i c e m u l s i f i e r . They found t h a t l i g h t petroleum and chlorobenzene emulsions prepared o n l y w i t h s o d i u m h e x a d e c y l s u l p h a t e w e r e much l e s s s t a b l e than those produced using t h elonger chain paraffins, white s p i r i t and light l i q u i d p a r a f f i n s . Most i n t e r e s t i n g l y ,however, they found t h a t a d d i t i o n of smal1 amounts o f l i g h t l i q u i d p a r a f f i n s t o t h e 1 i g h t petroleum o r chlorobenzene l e d t o an increase i n s t a b i 1 i t y which even surpassed t h a t which was o b t a i n e d w i t h 1 o n gc h a i n f a t t y a l c o h o l s . I t s h o u l d be noted , h o w e v e r , t h a t Hal 1 w o r t h e t a l . i n p r e p a r i ng t h e i r emulsions a p p l i e d t h eusual method o f a d d i t i o n o f t h e a d d i t i v e s t o t h e main component o f t h ei n t e r n a l phase before mixing with the water s o l u t i o n o f t h e anionic emulsifier. As shown above a n d d i s c u s s e d i n p r e v i o u s papers, t h i s procedure does n o t lead t o r a p i d e m u l s i f i c a t i o n w i t h f a t t y a l c o h o l s when o r d i n a r y , m o d e r a t e s t i r r i n g i sapplied f o r preparing t h e emulsions. In order t o i n v e s t i g a t e t h ep o s s i b l e a p p l i c a t i o n s o f t h e r e s u l t s o f Hal 1 w o r t h a n d C a r l ess on e m u l s i o n polymeriz a t i o n we h a v e c a r r i e d o u t s o m e e x p e r i m e n t s w i t h h e x a decane as a d d i t i v e . I tturned o u tthat with o r d i nary s t i r r i ng e q u i p m e n t , a d d i t i o n o f h e x a d e c a n e d i d n o t g i v e the r a p i d e m u l s i f i c a t i o n which c o u l d be o b t a i ned w i t h the 1ongchai η f a t t y a l c o h o l s . Therefore a s e r i e s o f e x p e r i m e n t s were c a r r i ed o u t i nwhich t h e e m u l s i o n s a f t e r p r e m i x i ng w e r e h o m o g e n i z e d w i t h a M a n t o n G a u l i η


14

POLYMERIZATION


EMULSION

Figure 10. Electron micrographs of monomer emulsions prepared with varying concentrations of OPB. Styrene = 166.7 g, H 0 = 500 g. Temp. = 60°C. Molar ratio ΗΌ.ΟΡΒ = 3:1. OPB, = (A) 2.0, (B) 4.0, (C) 6.0, and (D) 8.0 g/dm H 0. OPB remaining in the water phase after emulsification of monomer, (A) = 0.10, (B) = 0.53, (C) = 1.12, and (D) — 3.14 g/dm H 0. 2

3

3

2


2

Emulsion

Polymerization

of Styrene


AZAD E T A L .

Figure 11. Electron micrographs of final latexes from the experiments given in Figure 10 when polymerized using AIBN as initiator. A1BN = 4.0 g in 166.7 g styrene. (Letters (A),(B),(C), and (D) refer to Figure 10.)



16

EMULSION

POLYMERIZATION

laboratory homogenizer. As expected* the order o f m i x i n g h a d no d e t e c t a b l e i n f l u e n c e o n the r e s u l t i n g e m u l s i o n s even w i t h l o n g c h a i n f a t t y a l c o h o l s , when such an e f f e c t i v e homogenizing d e v i c e was a p p l i e d . In F i g . 12 a r e g i v e n some r e s u l t s w h e r e t h e e f f e c t of hexadecane i s compared with that o f hexadecanol on the s t a b i l i t y o f the emulsions u s i n g the Manton Gaul i n h o m o g e n i z e r a n d w i t h t h e c a t i o n i c OPB e m u l s i f i e r . T h e f i g u r e a l s o i n c l u d e s a r e s u l t o f a n e x p e r i m e n t w i t h OPB without anya d d i t i v e , which as expected led t o a very unstable emulsion. As s h o w n , t h e a p p l i c a t i o n o f t h e h o m o g e n i z e r f o r p r e p a r i n g the emulsions d i d not lead t o a n y i n c r e a s e i n the s t a b i l i t y o f the emulsion w i t h hexadecanol as a d d i t i v e as determined by m e a s u r i n g the amount o f adsorbed OPB a s a f u n c t i o n o f t i m e w i t h s t i r r i n g a t 6 0 ° C . A d d i t i o n o f hexadecane leads t o an extremely s t a b l e emulsion even a t the r e l a t i v e l y severe c o n d i t i o n s o f s t i r r i n g a t 60 °C. I n f a c t , the emulsion w i t h hexa decane a s a d d i t i v e i s even more s t a b l e than the o n e obtained with n-eicosanol. In F i g s . 1 3 , 1 4 , 15 a n d 16 a r e g i v e n e l e c t r o n micrographs o f the emulsions with hexadecanol and hexa decane i m m e d i a t e l y a f t e r p r e p a r a t i o n a n d a f t e r a b o u t 20 h r s s t i r r i n g a t 6 0 ° C . When c o m p a r i n g F i g . 1 3 a n d F i g . 2, b o t h w i t h h e x a d e c a n o l , i t appears that the a p p l i c a t i o n o f the homogenizer has r e s u l t e d i nthe formation o f a l a r g e r number o f s m a l l d r o p l e t s i n the range o f 0 . 2 - 0 . 3 urn. T h i s d o e s n o t , h o w e v e r , l e a d t o a m o r e stable emulsion. A f t e r 20 h s t i r r i n g a t 6 0 °C a l l t h e small d r o p l e t s have disappeared and the e l e c t r o n micro graph o f the emulsion, F i g . 14, i s r a t h e r s i m i l a r t o t h e o n e shown i n F i g . 3. T h i s i s i n a g r e e m e n t w i t h t h e r e s u l t s o f t h e m e a s u r e m e n t s o f a d s o r p t i o n o f OPB ( F i g . 12). With hexadecane the e l e c t r o n micrographs of the e m u l s i o n i m m e d i a t e l y a f t e r p r e p a r a t i o n ( F i g . 1 5 ) show a p p r o x i m a t e l y t h e same s i z e a n d s i z e d i s t r i b u t i o n a s obtained with hexadecanol. With hexadecane, however, the e l e c t r o n micrograph taken a f t e r 23 h s t i r r i n g a t 60 °C ( F i g . 1 6 ) , r e v e a l s t h a t t h e s m a l l d r o p l e t s t o a large extent are s t i l l present and that only a r e l a t i v e l y smal1 number o f 1 a r g e r d r o p l e t s i n the range o f 1 ym h a v e b e e n f o r m e d . F i g . 17 g i v e s r e s u l t s o f a p o l y m e r i z a t i o n e x p e r i ment w h e r e A I B N w a s a d d e d t o t h e monomer b e f o r e homo g e n i z i n g the mixture. The e l e c t r o n micrograph o f the l a t e x s h o u l d be compared w i t h t h a t o f t h e monomer ( F i g . 15). I t a p p e a r s t h a t we a g a i n h a v e i n i t i a t i o n b o t h i n the monomer d r o p l e t s a n d i n t h e aqueous p h a s e .


1.

Emulsion

AZAD E T A L .

Polymerization

17

of Styrene


Total OPB added

800 Time,

1600 minutes

3200

2400

Figure 12. OPB adsorbed on the monomer droplets as a function of time for pure OPB, OPB + hexadecanol (HD), and OPB + hexadecane as emulsifier, homogenized with the Manton Gaulin homogenizer and afterwards stirred at 60°C using the paddle stirrer (600 rpm). Styrene — 333.8 g/dm H 0, OPB = 2.0 g/dm H 0. Temp. = 60°C. Molar ratio hexadecanohOPB — 4:1, hexa decane :OP Β = 4:1. 3

3

2

2

Figure 13. Electron micrograph of monomer emuhion of Figure 12 with OPB + hexadecanol (HD) immediately after homogenization

Figure 14. Electron micrograph of mono mer emulsion of Figure 13 after 21 hr stirring at 60°C and 600 rpm



18

EMULSION

Figure 15. Electron micrograph of monomer emulsion of Figure 12 with OPB + hexadecane immediately after homogenization

POLYMERIZATION

Figure 16. Electron micrograph of monomer emulsion of Figure 15 after 23 hr of stirring at 60°C and 600 rpm

Figure 17. Electron micrograph of final latex from an emuhion prepared as the one in Figure 15 with A1BN added to the styrene before homogenization, homogenized, and polymerized. Styrene = 250 g, H 0 = 750 g, OPB — 2.0 g/dm H O. A1BN = 6.0 g in 250 g styrene. Molar ratio hexadecane:OPB = 4:1. Temp. = 60°C. 2


3

s


1.

AZAD E T AL.

Emulsion

Polymerization

of

Styrene

19

H a l l w o r t h and C a r l e s s (H)) d i s c u s s s e v e r a l p o s s i b i l i t i e s f o r t h ee f f e c t o f l i g h t l i q u i d p a r a f f i n on t h e s t a b i l i t y o f e m u l s i o n s w i t h l i g h t p e t r o l e u m o r chlorobenzene as the main components. They seem t o p r e f e r an e x p l a n a t i o n p r e v i o u s l y advanced by them a n d several other authors f o r the e f f e c t o f f a t t y a l c o h o l , namely t h a t t h ei n c r e a s e d s t a b i l i t y i sdue t o t h e formation o f an i n t e r f a c i a l complex between t h e a d d i t i v e and sodium hexadecyl sulphate. The condenced mixed f i l m w i l l r e s i s t coalescence p r i m a r i l y by v i r t u e of i t s r h e o l o g i c a l p r o p e r t i e s . With mixed f i l m s o f t h e present t y p e , t h eimportance o f t h ef i l m v i s c o e la s t i c i ty l i e s i n i t s a b i l i t y t o maintain e l e c t r i c a l repulsion between approaching d r o p l e t s by preventing l a t e r a l d i s placement o f t h eadsorbed i o n s . The e f f e c t i v e paraff i n i e o i l has chains a t l e a s t as long as those o f t h e a l k y ! s u l p h a t e a n d w i l l b e a s s o c i a t e d b y v a n d e r Waal s' f o r c e s with t h ehydrocarbon chain o f the alkyl sulphate at t h e i n t e r f a c e . Davies a n d Smith (11) measured t h es t a b i l i t y o f a s e r i e s o i l i nwater emuTsions i n c l u d i n g benzene and hexane as the main component o f t h ei n t e r n a l phase and w i t h a d d i t i o n o f small amounts o f hexadecane and long c h a i n f a t t y a l c o h o l s . They measured t h echange i n d r o p l e t s i z e with time and found a remarkable increase of t h es t a b i l i t y by a d d i t i o n o f hexadecane which was f a r more e f f e c t i v e than h e x a d e c a n o l . Davies and Smith reject t h e explanation o f Hallworth and Carless o f t h e e f f e c t o f hexadecane on t h es t a b i l i t y o f hexane emul sions on thermodynamic grounds. The a c t i v i t y c o e f f i c i e n t o f an a l k a n o l i n a l k a n e i s i n t h er e g i o n o f 15 a t 25 ° C , a n d i s a p p r o x i m a t e l y i n d e p e n d e n t o f c h a i n length. The n o n - i d e a l i t y o f alcohol as solutes i n alkanes c a n t h e r e f o r e be a t t r i b u t e d e n t i r e l y t o t h e hydroxy! group. S i m i l a r l y , t h ea d s o r p t i o n o f a l k a n o l s f r o m t h e a l k a n e p h a s e t o t h e o/w i n t e r f a c e i s d o m i n a t e d by t h e c h a n g e i n t h e e n v i r o n m e n t o f t h e h y d r o x y ! g r o u p and n o t t o t h e m e t h y l e n e g r o u p s . On t h e o t h e r h a n d , the long c h a i n alkane (Ci5) w i l l behave almost ideally i n a s h o r t e r c h a i n o i l (C5) a n d t h e r e s h o u l d be no tendency for t h elonger chain f r a c t i o n t o concentrate at t h e i n t e r f a c e . Davies and Smith suggest t h a t the e f f e c t o f a d d i t i o n o f small amounts o f hexadecane on s t a b i l i t y may b e d u e t o a p r e v e n t i o n o f e m u l s i o n d e g r a d a t i o n b y molecular diffusion. This approach t o emulsion i n s t a b i l i t y was f i r s t presented by H i g u c h i and M i s r a (12), and was b a s e d o n t h e f a c t t h a t s m a l l d r o p l e t s w i l l demonstrate deviations i nphysical properties as compared t o l a r g e r d r o p l e t s o r plane s u r f a c e s .


EMULSION

20

for

For the case o f an o i l d r o p l e t low o i l solubilities:


C

- C

r

i n water one has

exp (2 M)/(r RT)

œ

Y

POLYMERIZATION

(2)

P

where C i s the s o l u b i 1 i t y o f d r o p l e t s o f radius r , C the s o l u b i 1 ity o f an i n f ini t e l y 1arge d r o p l e t , M is the molecular weight o f the o i l , γ i s the i n t e r f a c i a l t e n s i o n , ρ the d e n s i t y o f the o i l , R the g a s constant and Τ t h e a b s o l u t e t e m p e r a t u r e . The increase i n solu b i l i t y w i t h d e c r e a s i n g r wi11 make t h e s m a l 1 d r o p l e t s t h e r m o d y n a m i c a l l y more u n s t a b l e wi t h r e s p e c t t o t h e 1arger ones. The r a t e o f d i s s o l u t i o n o r g r o w t h o f a d r o p l e t can be e x p r e s s e d a s (3)

G = 47rDr(C -C ) s

0

w h e r e Ce i s t h e c o n c e n t r a t i o n o f o i 1 i n t h e w a t e r p h a s e i n e q u i l i b r i urn w i t h t h e d r o p l e t , C i s t h e concentra t i o n a t some d i s t a n c e f r o m t h e d r o p l e t , 1 a r g e a s com pared t o r ,and D i s the d i f f u s i o n c o e f f i c i e n t o f t h e oi1 in water. Because the r a t e o f growth o r d i s s o l u t i o n o f a d r o p l e t must be equal t o i t s r a t e o f change o f m a s s , o n e a l s o may w r i t e 0

6

= 4wr p 2

(4)

dr/dt

Higuchi andMisra consider a bimodal emulsion w i t h n d r o p l e t s o f r a d i us r a n d n ] d r o p l e t s o f r a d i u s r]. From the above e q u a t i o n s a n d mass b a l a n c e conside r a t i o n s the r a t e o f change o f the smal1 s i z e d d r o p l e t s ( r-| ) w a s f o u n d t o b e : 2

2

(5) where Κ = (2yM)/(pRT). The r a t e o f c h a n g e w i 1 1 b e d i r e c t l y p r o p o r t i o n a l to the s o l u b i 1 i t y o f the o i l . M o r e o v e r , i t s h o u l d be noted t h a t i f the r a d i u s o f the d r o p l e t i s decreased ten f o l d the r a t e o f change w i l l occur a thousand times f a s t e r . H i g u c h i a n d M i s r a have a l s o t r e a t e d the more complex s i t u a t i o n o f a polydi sperse emulsion. Davi s a n d Smith f i n d that the equation developed f o r degradation by t h e m o l e c u l a r d i f f u s i o n r o u t e a s e x p r e s s e d b y E q . (5) may e x p l a i n t h e r a t e o f d e g r a d a t i o n o f d i f f e r e n t emulsions and the e f f e c t o f the s o l u b i 1 i t y o f t h e


1.

AZAD E T A L .

Emulsion

Polymerization

of

21

Styrene

i n t e r n a i phase on t h es t a b i l i t y o f the emulsion. Higuchi and Misra a l s o c o n s i d e r the e f f e c t o f a d d i t i o n o f a s m a l 1 q u a n t i t y o f a n a d d i t i ve w h i c h i s c o n s i d e r a b l y 1 ess water s o l u b l e than the main c o n s t i t u e n t o f the i n t e r n a l phase. F o r a t w o c o m p o n e n t c a s e o n e may w r i t e : C

xW

" x xL

k

C

e

x

P ( V >

C

zW

» z zL

k

C

e

x

P ( V

r

6

7

>

where C i s t h e e q u i 1 i b r i urn c o n c e n t r a t i o n o f t h e m a i η component! χ i nw a t e r wi t h a d r o p l e t s i z e r , C l i s the c o n c e n t r a t i o n i n the d r o p l e t , C w and C l a r e t h e c o r r e s p o n d i ng v a l u e s f o r t h e mi n o r c o m p o n e n t ζ,


w

x

z

z

Κ

χ

= 2 V /RT, Y

X

K

z

= 2 V /RT Y

Z

(8)

k and k a r e t h er e s p e c t i v e di s t r i b u t i o n c o e f f i c i e n t s , and V a n d V a r e t h e r e s p e c t i v e p a r t i a l m o l a l v o l u m e s of t h ecomponents χ and ζ i n the d r o p l e t . I f k z

Emulsion Polymerization

Recommend Documents