Emulsion Polymerization

17 Mechanical Stability of Vinyl Chloride Homopolymer and Copolymer Latices O. PALMGREN

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch017

Research Centre, Norsk Hydro a.s., 3900 Porsgrunn, Norway

The object of this study was to c l a r i f y some aspects of the mechanism of shear-induced floccula tion i n c o l l o i d a l dispersions. Vinyl chloride homopolymer and copolymer l a t i c e s were prepared by emulsion polymerization using sodium dodecyl sulphate as emulsifier. Agglomeration behavior i n these l a t i c e s was studied by measuring the mechanical stability using a high speed s t i r r i n g test. The latex p a r t i c l e size was measured by an analytical c e n t r i fuge. Molecular areas of emulsifier i n the saturated adsorption layer at the surface of homopolymer and copolymer latex particles were estimated from ad sorption titration data. Experimental Materials. A l l monomers were of commercial quality: v i n y l chloride (VCM), v i n y l acetate and VeoVa 10. The l a t t e r i s a v i n y l ester of a saturated monocarboxylic acid with a highly branched structure containing 10 carbon atoms. This monomer i s produced by Shell Chemicals. The emulsifier, sodium dodecyl sulphate (SDS), was the commercial material Berol 474 from Berol Kemi, Sweden. Na S O supplied by Noury & Van der Lande, was used as initiator. The NaHCO3 used as buffer and for adjustment of the electrolyte concentration, was Merck p.a. grade. 2

2

8

Polymerization. The polymerizations were carried out i n a 4 l i t e r reactor. Water, emulsifier, i n i t i a t o r and buffer were charged, de-aeration followed, and monomer was added. Then the reaction was started by r a i s i n g the temperature of the batch C

258

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

17.

PALMGREN

Vinyl Chloride

Homopolymer

and Copolymer

Latices

259


t o 50 C T h e r e m a i n i n g e m u l s i f i e r s o l u t i o n w a s a d d e d continuously during the polymerization, following a procedure that took i n t o account p a r t i c l e t o t a l sur face. The l a t i c e s were i n t h i s way prepared a t al o w l e v e l o f s t a b i l i t y , w i t h good s h e l f s t a b i l i t y , b u t practically zero s t i r r i n g stability. P a r t i c l e s i z e measurement. The p a r t i c l e s i z e was m e a s u r e d b y a n a n a l y t i c a l c e n t r i f u g e ( J j . I n t h i s technique the sedimentation a n d separation o f particles i s followed through an optical/electronic system based on the s c a t t e r i n g and l i g h t - a b s o r b i n g properties o f p a r t i c l e s . Themonodisperse l a t i c e s obtained are characterized b y the p a r t i c l e s i z e c o r r e s p o n d i n g t o the maximum l i g h t a b s o r p t i o n , approximately equal t o the weight average p a r t i c l e size. I n some c a s e s p a r t i c l e s i z e m e a s u r e m e n t s w e r e also oerfcrmed by electron microscopy and nitrogen adsorption. D e t e r m i n a t i o n o f r e s i d u a l VCM. t i o n was done b y g a s chromatograph.

This

determina

Surface tension measurement. Adsorption t i t r a t i o n , a l s o c a l l e d s o a p t i t r a t i o n , (2,3) w a s c a r r i e d out b y the drop volume method a t d i f f e r e n t polymer concentrations. The equivalent concentration of salt was h e l d c o n s t a n t . T h e a m o u n t o f e m u l s i f i e r n e c e s s a r y to reach the c r i t i c a l m i c e l l e concentration (CMC) i n the l a t e x was determined b y each t i t r a t i o n . The t o t a l weight o f e m u l s i f i e r present i n the l a t e x i s the weight o f e m u l s i f i e r i n the water plus the weight of e m u l s i f i e r adsorbed. The l i n e a r p l o t o f e m u l s i f i e r concentration ( t o t a l amount o f e m u l s i f i e r c o r r e s p o n d i n g t o the end-point o f each t i t r a t i o n ) versus poly mer c o n c e n t r a t i o n g i v e s t h e CMC a s t h e intercept and t h e s l o p e d e t e r m i n e s t h e amount o f e m u l s i f i e r adsorbed on the polymer surface i ne q u i l i b r i u m with e m u l s i f i e r i n s o l u t i o n a t t h e CMC ( E ) . E i s r e l a t e d t o the p a r t i c l e dïameter b y m

= E

m

=

9-96-M ç-D_-

A.

m Here M i s the molecular weight o f the e m u l s i f i e r , ç i s the polymer d e n s i t y , D i s t h e volume-to-surface average p a r t i c l e diameter^in Angstrom u n i t s , a n dA i s the area per e m u l s i f i e r molecule on the polymer surface i nÂ . o


260

EMULSION


I n some c a s e s t h e s u r f a c e t e n s i o n a l s o m e a s u r e d b y t h e r i n g m e t h o d u s i n g a d u Noîiy meter.

POLYMERIZATION

was tensio-

Latex s t a b i l i t y measurement. Latex s t a b i l i t y was m e a s u r e d by a h i g h speed s t i r r i n g t e s t (4-9)· The s t a b i l i t y o f a l a t e x i s g i v e n b y t h e l e n g t h of time r e q u i r e d t o produce complete f l o c c u l a t i o n by h i g h s p e e d s t i r r i n g o f 150 grams of the l a t e x i n a 600 m l g l a s s b e a k e r . T h e s t i r r i n g s p e e d w a s 14000 r p m w i t h a c i r c u l a r d i s k 21 mm i n d i a m e t e r . Although the absolute significance of the s t a b i l i t y test i s not clear, the test i s suitable for comparison of s t a b i l i t y l e v e l s under d i f f e r e n t test conditions. U s u a l l y t h e r e was no t r o u b l e w i t h f o a m i n g . In some c a s e s a n t i f o a m i n g a g e n t w a s a d d e d t o t h e latex p r i o r to s t a b i l i t y measurements. This e s s e n t i a l l y eliminated the foaming without a f f e c t i n g the numeri cal value of the s t a b i l i t y of these latices. B o t h ASTM and B S I h a v e d e a l t w i t h t h e s t a n d a r d i s a t i o n of the h i g h speed s t i r r i n g t e s t (10,11). V a r i o u s methods for t e s t i n g the m e c h a n i c a l s t a b i l i t y of l a t i c e s have been d e s c r i b e d by o t h e r a u t h o r s (12-15)· Results

and

discussion

Polymerization. F i g u r e 1. shows how t h e forma t i o n of p a r t i c l e s d u r i n g the polymerization depends o n t h e i n i t i a l c o n c e n t r a t i o n o f SDS i n t h e aqueous phase. In order to achieve a monodisperse latex, the p a r t i c l e n u c l e a t i o n m u s t be c o n f i n e d t o t h e i n i t i a l s t a g e o f t h e p o l y m e r i z a t i o n . No new p a r t i c l e s c a n b e p e r m i t t e d to form d u r i n g the r e a c t i o n , and agglome r a t i o n o f l a t e x p a r t i c l e s must be p r e v e n t e d . Thus b o t h a t o o h i g h and a t o o low e m u l s i f i e r concentra t i o n must be a v o i d e d (16). T h i s was a c c o m p l i s h e d by careful postaddition oTemulsifier. The l i n e a r r e l a t i o n s h i p b e t w e e n t h e logarithm o f t h e r e s u l t i n g number o f p a r t i c l e s and t h e loga rithm of the i n i t i a l emulsifier concentration, either i n the presence or i n the absence of m i c e l l e s , has a l s o been r e p o r t e d by U g e l s t a d , Mork, D a h l and Rangnes (17). Nucleation of l a t e x p a r t i c l e s has r e c e n t l y b e e n d i s c u s s e d b y G o o d w i n , H e a r n , Ho a n d O t t e w i l l (18) and by F i t c h (1£).



17.

PALMGREN

Vinyl

Chloride

Homopolymer

and Copolymer

Latices

261

Latex s t a b i l i t y * Effect of p a r t i c l e size and emulsifier level* Latex s t a b i l i t y data for three latices with different particle size, are plotted i n F i g u r e 2. A t a g i v e n e m u l s i f i e r l e v e l , e x p r e s s e d a s weight per cent of polymer, the s t a b i l i t y increases w i t h i n c r e a s i n g p a r t i c l e s i z e . The l o g a r i t h m o f t h e stability i s a linear function of the emulsifier concentration (20). I n F i g u r e 3· t h e same s t a b i l i t y d a t a a r e p l o t t e d v e r s u s t h e s u r f a c 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 . The surface concentration i s given as per cent of t o t a l coverage of the p a r t i c l e surface, as determined by adsorption t i t r a t i o n . The a d s o r p t i o n i s o t e r m s o f SDS on p o l y v i n y l c h l o r i d e (PVC) l a t e x p a r t i c l e s i n t h e p r e s e n c e of v a r i o u s concentrations of sodium ions i n t h e bulk s o l u t i o n , have been determined by B i b e a u and Matijevic (21_). I n t h e l a t i c e s w h i c h w e r e e x a m i n e d by u s , the surface c o n c e n t r a t i o n of e m u l s i f i e r was confined to the region w e l l below the s a t u r a t i o n l e v e l . Under these circumstances only a negligible f r a c t i o n o f t h e t o t a l amount o f e m u l s i f i e r w i l l be d i s s o l v e d i n t h e aqueous phase. At a given surface concentration of e m u l s i f i e r the s t a b i l i t y decreases with increasing p a r t i c l e size. The l o g a r i t h m o f t h e s t a b i l i t y i s a l i n e a r f u n c t i o n of the surface concentration of emulsifier with a s l o p e w h i c h i s i n d e p e n d e n t o f t h e p a r t i c l e s i z e . The s t a b i l i t y l e v e l c o u l d be r a i s e d by r e d u c i n g t h e polymer c o n c e n t r a t i o n or i t c o u l d be lowered by i n c r e a s i n g t h e s a l t c o n c e n t r a t i o n . The s l o p e remains unaltered. I n F i g u r e 4· t h e s a m e s t a b i l i t y d a t a a r e p l o t t e d versus the surface concentration of emulsifier. But t h i s time the surface concentration i s expressed as number o f m o l e c u l e s p e r s q u a r e cm. Van den H u l and Vanderhoff (22) have shown t h a t polystyrene l a t i c e s polymerized with a persulphate i n i t i a t o r have permanently charged groups present on the p a r t i c l e s u r f a c e , r e s u l t i n g from t h e i o n i c free r a d i c a l i n i t i a t o r . S u r f a c e charges on p o l y s t y r e n e l a t e x p a r t i c l e s have a l s o been s t u d i e d by Hearn, O t t e w i l l a n d S h a w (23). A permanent p a r t i c l e surface c h a r g e was a l s o f o u n d f o r PVC l a t e x p a r t i c l e s b y B i b e a u ^ a n d M a t i j e v i c . T h e p a r t i c l e s i z e w a s 48ΟΟ Â . 1.5*10 J e n d g r o u p s p e r s q u a r e c m w e r e f o u n d o n t h e s e p a r t i c l e s . T h i s c o r r e s p o n d s t o a p p r o x i m a t e l y one end group a t t h e p a r t i c l e surface per each of seven p o l y m e r m o l e c u l e s . F o r a p a r t i c l e s i z e o f 2000 A t h i s β


262

EMULSION

POLYMERIZATION


Emulsifier concentration g SDS / 100 ml H2O

Figure 1. Polymerization—formation of partides. Effect of initial emulsifier concentration. Na*: 0.01 mol/l.

-~ 10 0.01

Total amount of emulsifier

Polymer: 45% by weight. Na+: 0.01 mol/l.

0-5

0.6

0.7

08

0.9

10


11

1.2

Vinyl

PALMGREN

17.

Chloride

Homopolymer

and Copolymer

Latices

263

Surface concentration of emulsifier •/.of E m

3


10

16

20

24

28

32

36

40

Figure 3. Mechanical stability of PVC latices. Effect of particle size and emulsifier level. Polymer: 45% by weight. Na*: 0.01 mol/l.



264

EMULSION

POLYMERIZATION

w i l l r e p r e s e n t 6-10 c h a r g e s p e r s q u a r e cm. The sur face concentration of emulsifier recorded i n Figure 4· i s a p p r o x i m a t e l y 10 t i m e s t h i s v a l u e . S i m i l a r r e s u l t s to those obtained here by the s t a b i l i t y measurements have been r e p o r t e d by Roe and B r a s s (7,8). T h e y s t u d i e d p o l y s t y r e n e l a t e x s t a b i l i z e d b y p o t a s s i u m p a l m i t a t e . The a n a l y s i s s u p p l i e d by t h e s e a u t h o r s shows t h a t t h e o r d e r o f magnitude o f t h e s l o p e o f t h e s t a b i l i t y c u r v e s c a n be a c c o u n t e d for as an entropie e f f e c t of crowding of adsorbed m o l e c u l e s d u r i n g an e n c o u n t e r between two p a r t i c l e s . They p o i n t e d t h i s out as a p o s s i b l e e x p l a n a t i o n as t h e amount o f e m u l s i f i e r a d s o r b e d s t r o n g l y affects the s t a b i l i t y without a l t e r i n g the electrophoretically derived double-layer potential. Bibeau and M a t i j e v i c s t u d i e d the s t a b i l i t y of a PVC l a t e x b y a d d i t i o n o f e l e c t r o l y t e s . They a l s o found surface i o n c o n c e n t r a t i o n s derived by electro p h o r e s i s t o be p o o r i n d i c a t o r s o f l a t e x s t a b i l i t y . T h e i r s t a b i l i t y r e s u l t s were found t o compare favor a b l y w i t h DLVO t h e o r y p r e d i c t i o n s , u s i n g t h e a c t u a l surface concentration of potential-determining s p e c i e s a s t h e b a s i s f o r i n t e r p r e t a t i o n . That means t a k i n g i n t o account both f i x e d charges and adsorbed emulsifier. L a t e x s t a b i l i t y w i l l be d e t e r m i n e d by t h e com b i n e d e f f e c t o f two f a c t o r s : the p r o b a b i l i t y of c o l l i s i o n between p a r t i c l e s and the f r a c t i o n of the encounters between p a r t i c l e s which lead to permanent c o n t a c t . Tha f i r s t f a c t o r , t h e c o l l i s i o n frequency, w i l l i n c r e a s e w i t h i n c r e a s i n g p a r t i c l e s i z e and p a r t i c l e number. It w i l l a l s o i n c r e a s e w i t h i n c r e a s i n g s h e a r r a t e . The i n f l u e n c e o f v a r i o u s t e s t conditions on t h e s e c o n d f a c t o r o u g h t t o be d i s c u s s e d on t h e b a s i s o f t h e DLVO t h e o r y o f c o l l o i d s t a b i l i t y . I n the t h e o r y developed by D e r j a g u i n and Landau (24) and Verwey and Overbeek (2^.) the s t a b i l i t y of c o l l o i d a l d i s p e r s i o n s i s treated i n terms of the e n e r g y changes w h i c h t a k e p l a c e when p a r t i c l e s a p p r o a c h one a n o t h e r . The t h e o r y i n v o l v e s estimations of the energy of a t t r a c t i o n (London-van der Walls f o r c e s ) and the energy of r e p u l s i o n ( o v e r l a p p i n g of e l e c t r i c double layers) i n terms of inter-particle d i s t a n c e . But i n a d d i t i o n to e l e c t r o s t a t i c inter a c t i o n , s t e r i c r e p u l s i o n h a s a l s o t o be c o n s i d e r e d . The s u b j e c t o f f l o c c u l a t i o n k i n e t i c s a n d t h e s t a b i l i z a t i o n o f d i s p e r s i o n s h a s been d e a l t w i t h i n many r e c e n t l y p u b l i s h e d p a p e r s . Some o f t h e m a r e cited h e r e (26-34)*



17.

PALMGREN

Vinyl

Chloride

Homopolymer

and Copolymer

Latices

265

The a d s o r b e d l a y e r o f e m u l s i f i e r on t h e p a r t i c l e surface can a f f e c t the s t a b i l i t y of l a t i c e s i n three main ways: 1. B y i n c r e a s i n g t h e c h a r g e o n t h e p a r t i c l e s , w h i c h w i l l increase the r e p u l s i v e forces. 2. B y a l t e r i n g t h e v a l u e o f t h e e f f e c t i v e H a m a k e r c o n s t a n t , w h i c h means m o d i f y i n g t h e i n h e r e n t attractive forces. 3· B y s t e r i c a l l y h i n d e r i n g c o n v e r g e n c e o f the particles. Bibeau and M a t i j e v i c used a f i x e d v a l u e f o r the Hamaker c o n s t a n t and i n t e r p r e t e d t h e i n c r e a s e d s t a b i l i t y by a d d i t i o n of e m u l s i f i e r as b e i n g e x c l u s i v e l y an e f f e c t of i n c r e a s e d s u r f a c e charge. But the v a r i o u s s t a b i l i z i n g mechanisms are not m u t u a l l y e x c l u s i v e a n d may f u n c t i o n c o - o p e r a t i v e l y . The e f f e c t o f a d s o r b e d l a y e r s on t h e e n e r g y o f a t t r a c t i o n between two p a r t i c l e s h a s b e e n c o n s i d e r e d b y V o i d (35.) * by Vincent (36). E x c e l l e n t r e v i e w s on c o l l o i d s t a b i l i t y a r e given b y N a p p e r a n d H u n t e r (32) and by O t t e w i l l (38). a n c

L a t e x s t a b i l i t y . E f f e c t o f r e s i d u a l monomer. S t a b i l i t y depends on t h e age o f t h e l a t e x , a s shown i n f i g u r e 5· P r e s u m a b l y t h e r a i s i n g o f t h e s t a b i l i t y l e v e l by s t o r a g e c a n be a t t r i b u t e d t o t h e d e c r e a s i n g c o n t e n t o f r e s i d u a l monomer. The d a t a i n t h i s figure w e r e o b t a i n e d some y e a r s a g o b e f o r e s p e c i a l a t t e n t i o n was f o c u s e d on t h e r e s i d u a l monomer b e c a u s e o f the h e a l t h hazard a r i s i n g from exposure to i t . I n i t i a l l y t h e monomer c o n t e n t i n t h e p a r t i c l e s o f t h i s l a t e x w a s 6-8 p e r c e n t . O n l y a s m a l l f r a c t i o n o f t h e monomer i s d i s s o l v e d i n the aqueous phase. E x p e r i m e n t a l p o l y m e r i z a t i o n s a r e now r u n t o h i g h e r c o n v e r s i o n t h a n p r e v i o u s l y , a n d t h e monomer is s t r i p p e d from the l a t e x p a r t i c l e s by t h e end of poly m e r i z a t i o n t o a c o n t e n t o f Ο.25 per cent by weight of polymer. This r a i s e s the s t a b i l i t y l e v e l by a f a c t o r o f m o r e t h a n 100 as compared t o a l a t e x w i t h approximately 3 P c e n t V C M , a s shown i n F i g u r e 6. A short evacuation of t h i s 3 P c e n t VCM l a t e x h a s t h e same e f f e c t a s s e v e r a l d a y s o f s t o r a g e . The p r o c e s s c a n a l s o be r e v e r s e d b y a d d i t i o n o f V C M , m a k i n g t h e l a t e x more u n s t a b l e . S t o r a g e h a s no e f f e c t on t h e l o w monomer l a t e x , a n d f u r t h e r l o w e r i n g o f the monomer c o n t e n t b y a d d i t i o n a l s t r i p p i n g h a s no e f f e c t on t h e s t a b i l i t y l e v e l . The r e s i d u a l monomer c o n t e n t may c h a n g e t h e i n herent a t t r a c t i v e f o r c e s between the p a r t i c l e s . These f o r c e s depend on t h e n a t u r e o f t h e p o l y m e r m a t e r i a l e

r

e

r


266

EMULSION

POLYMERIZATION

Total amount of emulsifier g SDS/ 100g polymer 1θ3


Age of latex, days:

Figure 5. Mechanical stability of PVC latices. Effect of age of latex. Polymer: 45% by weight. Na+: 0.01 mol/l.

OA

OS

0.6

07

28 14 7

08

0.9

10

Total amount of emulsifier g SDS/ KOg polymer

Figure 6. Mechanical stability of PVC latices. Effect of residual VCM in the latex particles. Polymer: 45% by weight. Na+: 0.05 moles/l.


1.1

17.

PALMGREN

Vinyl

Chloride

Homopolymer

and Copolymer

267

of which the p a r t i c l e s c o n s i s t , p a r t i c u l a r l y the number o f atoms p e r u n i t volume and t h e p o l a r i s a b i l i t y . A r e s i d u a l monomer c o n t e n t o f 3 P cent w i l l presum a b l y change these p r o p e r t i e s v e r y l i t t l e . The r e s i d u a l monomer c o n t e n t may c h a n g e t h e r e p u l s i v e f o r c e s b e t w e e n t h e p a r t i c l e s . To some extent m i g r a t i o n o f monomer t h r o u g h t h e p o l y m e r - w a t e r inter face w i l l take place during the s t a b i l i t y test. This migration could d i s t u r b the double-layer potential. The m i g r a t i o n w i l l be more p r o n o u n c e d t h e m o r e m o n o m e r - s w o l l e n t h e p a r t i c l e s a r e . Monomer may be found i n the l o c a l i t y of the charged surface, ad j o i n i n g the adsorbed emulsifier molecules, g i v i n g a s h i e l d i n g effect of surface charge. The r e s i d u a l monomer c o n t e n t w i l l b y e x t e r n a l p l a s t i c i z a t i o n cause a considerable lowering of the polymer glass t r a n s i t i o n temperature. A c o r r e l a t i o n between s t a b i l i t y and s o f t n e s s of the polymer p a r t i c l e s may e x i s t . The h y d r o p h o b i c p a r t o f the e m u l s i f i e r m o l e c u l e s may p a r t l y p e n e t r a t e t h e p a r t i c l e s u r f a c e and t h u s be a n c h o r e d t o t h e s u r f a c e t o some extent. The r e s i s t a n c e t o d e f o r m a t i o n o f s u c h a s t a b i l i z i n g l a y e r , when s u b j e c t e d t o m e c h a n i c a l s h e a r , i s assumed t o be d e p e n d e n t on t h e p o l y m e r particle softness. With soft p a r t i c l e s polymer chain entangle ment may a l s o o c c u r o n p a r t i c l e - t o - p a r t i c l e contact, making redispersion of agglomerates more u n l i k e l y . e


Latices

r

Latex s t a b i l i t y . Effect of copolymerization. When c o p o l y m e r i z i n g VCM w i t h v i n y l e s t e r s i t appears t o be t h e c o m b i n a t i o n o f two c o m p e t i n g e f f e c t s w h i c h determines the latex s t a b i l i t y . A s t a b i l i t y i n c r e a s i n g e f f e c t seems t o a r i s e from i n c r e a s i n g t h e p o l a r i t y of the polymer p a r t i c l e s u r f a c e , and a s t a b i l i t y de c r e a s i n g e f f e c t from i n c r e a s i n g the softness of the polymer p a r t i c l e s by i n t e r n a l p l a s t i c i z a t i o n . Copolymerization w i t h v i n y l acetate has a strong e f f e c t on t h e n a t u r e o f t h e s u r f a c e o f t h e p o l y m e r p a r t i c l e s , but the p l a s t i c i z a t i o n e f f e c t i s compara t i v e l y weak. W i t h i n c r e a s i n g c o n t e n t o f v i n y l acetate i n the copolymer the l a t e x s t a b i l i t y w i l l pass through a d i s t i n c t maximum b e f o r e d e c r e a s i n g b e l o w the s t a b i l i t y l e v e l of the homopolymer. C o p o l y m e r i z a t i o n w i t h VeoVa h a s l i t t l e e f f e c t on the nature of the p a r t i c l e surface, but the p l a s t i c i z a t i o n e f f e c t i s somewhat more p r o n o u n c e d t h a n w i t h v i n y l a c e t a t e . W i t h i n c r e a s i n g c o n t e n t of VeoVa i n the copolymer the l a t e x s t a b i l i t y w i l l pass through a s m a l l maximum b e f o r e d e c r e a s i n g f a r b e l o w t h e s t a b i l i t y l e v e l of the homopolymer.



268

EMULSION

POLYMERIZATION

The s t a b i l i t y o f t h e c o p o l y m e r l a t i c e s shown i n F i g u r e 7· a r e n e a r t h e maximum l e v e l b o t h f o r the v i n y l a c e t a t e and the VeoVa copolymer. V i n y l a c e t a t e u n i t s i n the copolymer a r e more h y d r o p h i l i c t h a n the v i n y l c h l o r i d e u n i t s , and t h e y w i l l t o some e x t e n t h y d r o l y s e , i n t r o d u c i n g h y d r o x y l g r o u p s on t h e p a r t i c l e s u r f a c e . The b r a n c h e d , b u l k y s t r u c t u r e of VeoVa makes i t s e s t e r group d i f f i c u l t to hydrolyse. T h e s p e c i f i c d e n s i t y w e r e 1·39, 1-36 and 1-33 g r a m p e r c u b i c cm f o r h o m o p o l y m e r , copolymer w i t h 1C n e r c e n t v i n y l a c e t a t e , a n d c o p o l y m e r w i t h 10 p e r cent VeoVa r e s p e c t i v e l y . The l a t i c e s w e r e o f low viscosity. The o u t l i n e d r e l a t i o n s h i p b e t w e e n s t a b i l i t y a n d p o l y m e r c h a r a c t e r i s t i c s h a s t o be c o n f i r m e d by f u r t h e r investigations. L a t e x s t a b i l i t y . E f f e c t of pH. The pH o f the l a t i c e s were a d j u s t e d to about 8 b e f o r e d o i n g any of the p r e v i o u s l y described s t a b i l i t y measurements. The e f f e c t o f pH on t h e s t a b i l i t y i s d e s c r i b e d i n F i g u r e 8. A s h a r p d r o p i n s t a b i l i t y o c c u r s below pH 2. T h i s i s p o s s i b l y due t o d e i o n i z a t i o n o f t h e s u l p h a t e group of the e m u l s i f i e r , i n d i c a t i n g an e l e c t r o s t a t i c re p u l s i o n to have been operative. Latex s t a b i l i t y . Effect of temperature. Usually no t e m p e r a t u r e c o n t r o l was i m p o s e d . The r i s e i n t e m p e r a t u r e d u r i n g t h e t e s t was 1-2 C For purposes of temperature control the bottle containing the sample was p r o v i d e d w i t h a w a t e r j a c k e t t h r o u g h w h i c h w a t e r a t s p e c i f i e d t e m p e r a t u r e was c i r c u l a t e d . The effect of t e m p e r a t u r e on t h e s t a b i l i t y i s d e s c r i b e d i n F i g u r e 9· To a v o i d c o n f u s i o n t h e e x p e r i m e n t a l p o i n t s a r e not i n d i c a t e d on t h e f i g u r e , except those obtained at room temperature. The t e m p e r a t u r e d e p e n d e n c e was f o u n d to obey the A r r h e n i u s e q u a t i o n . At any g i v e n e m u l s i f i e r l e v e l a l i n e a r p l o t of the logarithm of the s t a b i l i t y v e r s u s 1/T c o u l d b e o b t a i n e d . L a t e x s t a b i l i t y . E f f e c t of s t i r r i n g speed and spindle disk diameter. F i g u r e s 1 0 . and 1 1 . show how sensitive the s t a b i l i t y test i s w i t h regard to the speed of s t i r r i n g and the diameter of s p i n d l e d i s k . Characterization a comparison i s given t i o n by the a n a l y t i c a scope and by n i t r o g e n

of p a r t i c l e s i z e . In Figure 12. between p a r t i c l e s i z e determina l centrifuge, by e l e c t r o n m i c r o a d s o r p t i o n on t h e carefully


17.

Vinyl

PALMGREN

Chloride

Homopolymer

and Copolymer

Latices

269

Total amount of emulsifier g SDS/ 100g polymer


7

Figure 7. Mechanical stability of PVC copolymer latices. Effect of copolymerization. Polymer: 45% by weight. Na*: 0.05 moles/I

pH 10° Copolymer with 10% VeoVa I J2

g SDS/BOg polymer

2050Â, VCM 0.25%

Figure 8. Mechanical stability of PVC copolymer latices. Effect of pH. Polymer: 45% by weight. Na*: 0.05 mol/l.


EMULSION

270

POLYMERIZATION


Total amount of emulsifier g SDS/K30g polymer

45% by weight. Na+: 0.05 mol/l.

02

03

0Â

05

0-6

07

08

09

Total amount of emulsifier g SDS/100g polymer

Figure 10. Mechanical stability of PVC latices. Effect of stirring speed. Polymer: 45% by weight. Na : 0.05 mol/l. +

02

03

0Â

0.5

0.6


07

0.8

17.

Vinyl Chloride

PALMGREN

Homopolymer

and Copolymer

Latices

271


Total amount of emulsifier g SDS/100g polymer

03

04

0.5

0.6

07

0.8

0.9

10

Figure 11. Mechanical stability of PVC latices. Effect of spindle disk diameter. Polymer: 45% by weight. Νa*: 0.05 mol/l.

diameter, Â Analytical centrifuge max. light absorption [ISO! [^6Ô| [2ÎÂÔ Electron microscopy number average

1535 1460 1560

surface average

1690 1650 1740

Nitrogen adsorption surface average

A Β C

2000 2200 2250

PVC Copolymer, 10% VeoVa Copolymer, 10% vinyl acetate

Figure 12. Particle size measurements by different methods


272

EMULSION

POLYMERIZATION


Particle diameter, Λ

Figure 13. Histogram: particle size distribu tion obtained by electron microscopy. Curve: the normal distribution with mean 1530 and standard deviation 340. Analytical centrifuge: 1960 A.

2000

2800

Â / molecule Residual VCM Ό % PVC

364-39.9

Residual VCM 0-25% Figure 14. D measured by analytical centrifuge. E determined by soap titration. Estimation of A from these data. p

m

m

PVC

37.2-42.0

Copolymer, 10% VeoVa

39.1-43.5

Copolymer, 10% vinyl acetate

664-68-3



17.

PALMGREN

Vinyl

Chloride

Homopolymer

and Copolymer

Latices

273

d r i e d powder obtained by f r e e z i n g l a t e x . The r e s u l t s obtained by e l e c t r o n microscope are corrected f o r a s h r i n k i n g of 20 per cent. Some s i n t e r i n g of the powder during d r y i n g i s a p o s s i b l e explanation of the some what high values obtained by n i t r o g e n adsorption. V i n y l c h l o r i d e i s b u i l t i n t o the copolymer some what f a s t e r than the v i n y l e s t e r s . The l a t i c e s de scribed i n Figure 12. were prepared w i t h the same i n i t i a l concentration of e m u l s i f i e r . The d i f f e r e n c e i n p a r t i c l e s i z e between the homopolymer and the co polymer l a t i c e s are considered t o be w i t h i n e x p e r i mental e r r o r . In Figure 13- the histogram shows a p a r t i c l e s i z e d i s t r i b u t i o n obtained by e l e c t r o n microscopy. This d i s t r i b u t i o n may be described by a normal d i s t r i b u t i o n , i l l u s t r a t e d by the dotted curve. C h a r a c t e r i z a t i o n of adsorption of e m u l s i f i e r . The area occupied by each adsorbed e m u l s i f i e r molecule at the polymer-water i n t e r f a c e ( A ) was estimated from the mean p a r t i c l e s i z e of the l a t i c e s , determined by the a n a l y t i c a l c e n t r i f u g e , and the amount of emul s i f i e r adsorbed a t the i n t e r f a c e corresponding t o f u l l coverage, determined by adsorption t i t r a t i o n . The r e s u l t s obtained a f t e r having examined several l a t i c e s are summarized i n Figure 14· The values found f o r A do not d i f f e r s i g n i f i c a n t l y f o r the homopolymer an8 the VeoVa copolymer. For the v i n y l acetate copolymer the area i s d i s t i n c t l y increased as a r e s u l t of increased i n t e r f a c e p o l a r i t y (39)* The molecular areas recorded i n Figure 14· has to be adjusted upwards by approximately 15 per cent for not using the surface average diameter. Acknowledgment The author i s g r a t e f u l t o Mr. K . G i l l , Mr. A.Hansen, Mrs. K.MUrer and Mr. A.Talmoen f o r c a r r y i n g out the p o l y m e r i z a t i o n , s t a b i l i t y and adsorption experiments, and t o Norsk Hydro f o r permission t o p u b l i s h t h i s work.

Literature cited 1. Rangnes P. and Palmgren O., J.Polymer S c i . C 33 (1971), 181 2. Maron S . H . , Elder M.E. and Ulevitch I . N . J.Colloid S c i . 9 (1954), 89 3. Paxton T.R., J.Col.Interface S c i . 31 (1969), 19


274 4. 5. 6.


7. 8. 9. 10. 11. 12. 13.

EMULSION POLYMERIZATION Jordan H . F . , Brass P . D . and Roe C.P., Ind.Eng.Chem.Anal.Ed. 9 (1937), 1 8 2 Dawson H . G . , Anal.Chem. 21 (1949), 1066 Madge E.W., Collier H.M. and Duckworth I.H., Trans.Inst.Rubber Ind. 28 (1952), 15 Roe C.P. and B r a s s P.D., J.Colloid Sci. 10 ( 1 9 5 5 ) , 194 Roe C.P., I n d . E n g . C h e m . 60 ( 1 9 6 8 ) , 20 Roe C.P., J.Col.Interface Sci. 37 (1971), 93 ASTM specification D 1076-71 BSI specification 1672-72 and 3397-70 Maron S . H . and Ulevitch I.N., A n a l . C h e m . 25 (1953), 1087 Stamberger P., J.Colloid Sci. 17 ( 1 9 6 2 ) , 146

14.

Greene B.W. and Sheetz D.P., J.Col.Interface Sci. 32 ( 1 9 7 0 ) , 9 6 15. Utracki L.A., J.Col.Interface Sci. 4 2 ( 1 9 7 3 ) , 185 16. Vanderhoff J.W., Vitkuske J.F., Bradford E.B. and Alfrey T., J.Polymer Sci. 20 ( 1 9 5 6 ) , 2 2 5 17. Ugelstad J., M ö r k P.C., D a h l P . a n d R a n g n e s P., J.Polymer Sci. C 27 ( 1 9 6 9 ) , 4 9

1 8 . Goodwin J.W., Hearn J., Ho C. and Ottewill R.H., Br.Polym.J. 5 ( 1 9 7 3 ) , 347 1 9 . Fitch R.M., B r . P o l y m . J . 5 ( 1 9 7 3 ) , 467 20. Palmgren O., IUPAC Symp. on M a c r o m o l e c u l e s , Helsinki 1972, Priprint Vol.4, III-49 2 1 . B i b e a u A.A. and Matijevic E., J.Col.Interface Sci. 43 ( 1 9 7 3 ) , 330 2 2 . V a n den H u l H.J. and V a n d e r h o f f J.W., B r . P o l y m . J . 2 ( 1 9 7 0 ) , 121 2 3 . Hearn J., Ottewill R . H . a n d Shaw J.N., B r . P o l y m . J . 2 (1970), 116 24. D e r j a g u i n B.V. and Landau L., A c t a p h y s . - c h i m . URSS 14 ( 1 9 4 1 ) , 633 2 5 . Verwey B.V. and Overbeek J.T.G., "Theory o f t h e Stability of Lyophobic Colloids", Elsevier, 1948 26. Watillon A . and J o s e p h - P e t i t A.M., D i s c . F a r a d a y Soc. 42 ( 1 9 6 6 ) , 143 27. Ottewill R . H . and Shaw J.N., D i s c . F a r a d a y S o c . 42 ( 1 9 6 5 ) , 154 2 8 . Napper D.H., I n d . E n g . C h e m . P r o d . R e s . D e v e l o p . 9 ( 1 9 7 0 ) , No4, 467 29. Dunn A.S. and Chong L.C.H., Br.Polym.J. 2 ( 1 9 7 0 ) , 49 3 0 . Napper D . H . and N e t s c h e y Α . , J.Col.Interface Sci. 37 ( 1 9 7 1 ) , 528 3 1 . H e s s e l i n k F.T., Vrij A . and Overbeek J.T.G., J . P h y s . C h e m . 75 ( 1 9 7 1 ) , No14, 2094



17.

PALMGREN

Vinyl Chloride

Homopolymer

and Copolymer

Latices

275

32. Hatton W. and McFadyen P . , J.Chem.Soc., Faraday Trans. 70 (1970), No4, 655 33. Ottewill R.H. and Walker T., J.Chem.Soc., Faraday Trans. 70 (1970), No5, 917 34. Khanna R . K . , J.Oil.Col.Chem.Assoc. 57 (1974), 161 35. Vold M.J., J . C o l l o i d S c i . 16 (1961), 1 36. Vincent Β . , J.Col.Interface S c i . 42 (1973), 270 37. Napper D.H. and Hunter R.J., MTP I n t . R e v . S c i . , Series I 7 (1972), Chap.8 38. Ottewill R . H . , Specialist Periodical Reports, C o l . S c i . 1, Chap.5, The Chemical Soc., London 1973 39. Yeliseyeva V.I. and Zuikov A . V . , Polymer Preprints 16 (1975), No1, 143


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