Grafting Reactions of (Hydroxyethyl)-cellulose During Emulsion

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch018

Grafting Reactions of (Hydroxyethyl)cellulose During Emulsion Polymerization of Vinyl Monomers D . H . Craig Research Center, Hercules, Inc., Wilmington, DE 19894

The aqueous phases of vinyl-acrylic and all-acrylic latexes manufactured in the presence of (hydroxyethyl)cellulose (HEC) were analyzed for residual HEC content and HEC molecular weight distribution by using colorimetry and high-performance size-exclusionchromatography, respectively. Vinyl acetate monomer, contrary to traditional thought, grafted very little to HEC, although acrylic monomers grafted extensively under the same emulsion polymerization conditions. In both cases extensive degradation of the HEC molecular weight occurred. A scheme is proposed whereby radicals generated along the HEC backbone, a result of chain transfer from initiator to protective colloid, mediate both the HEC grafting reaction and HEC molecular weight reduction to an extent that depends on the relative reactivities of the monomers used.

THE COMPOUND H ( YDROXYETHYLC )ELLULOSE (HEC) IS WIDELY EMPLOYED as a p r o t e c t i v e c o l l o i d i n the c o m m e r c i a l m a n u f a c t u r e of v i n y l acetate c o n t a i n i n g h o m o p o l y m e r a n d c o p o l y m e r latices d e s t i n e d f o r use as b i n d e r s i n latex p a i n t s ( I ) . T h e m a i n reasons f o r this usage are that l o w levels of H E C i m p a r t g o o d mechanical stability a n d processing c h a r a c t e r i s t i c s to the latex, p a r t i c u l a r l y u n d e r l e a n surfactant c o n d i t i o n s . I n c o m m e r c i a l p a i n t latices, l o w levels of surfactants are r e q u i r e d to m i n i m i z e the w a t e r s e n s i t i v i t y o f the d r i e d f i l m . T h e s m a l l a m o u n t s o f H E C that are u s e d to r e p l a c e the c o r r e s p o n d i n g l y h i g h e r surfactant l e v e l s a c t u a l l y i m p r o v e the f i l m f o r m a t i o n a n d d u r a b i l i t y o f p a i n t latices as w e l l b y p r o v i d i n g a latex s u r f a c e that is m o r e c o m p a t i b l e w i t h p i g m e n t s a n d b y c o n t r o l l i n g the rate o f w a t e r e v a p o r a t i o n ; thus, p r o p e r c o a l e s c e n c e o f latex p a r t i c l e s is a l l o w e d .

T h e e n h a n c e d m e c h a n i c a l s t a b i l i t y i m p a r t e d to v i n y l acetate b a s e d latexes b y i n c l u s i o n o f H E C i n the p o l y m e r i z a t i o n r e c i p e has t r a d i t i o n 0065-2393/86/0213-0351$06.00/0 ® 1986 American Chemical Society

In Water-Soluble Polymers; Glass, J. E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1986.


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a l l y b e e n e x p l a i n e d as a r i s i n g f r o m the extensive g r a f t i n g o f v i n y l acetate o n t o the H E C b a c k b o n e ; this g r a f t i n g p r o v i d e s H E C - e n c a p s u l a t e d l a t e x p a r t i c l e s t h a t a r e s t a b i l i z e d s t e r i c a l l y . T h i s f e a t u r e has b e e n s u r m i s e d f r o m the g e n e r a l l y h i g h c h a i n - t r a n s f e r constants o f the p o l y ( v i n y l a c e tate) r a d i c a l d e t e r m i n e d f r o m b u l k , s o l u t i o n , o r s u s p e n s i o n p o l y m e r i z a t i o n m e a s u r e m e n t s , w h e n c o m p a r e d to the c h a i n - t r a n s f e r constants o f other radicals d e r i v e d f r o m monomers often used i n emulsion p o l y m e r i z a t i o n , s u c h as m e t h y l m e t h a c r y l a t e (2,3). T h e k n o w n t e n d e n c y f o r p o l y ( v i n y l acetate) t o c h a i n t r a n s f e r t o itself (4) s e e m e d to c o n f i r m this v i e w p o i n t . H o w e v e r , the traditional theory does not p r o v i d e an explanat i o n as to h o w the g r o w i n g p o l y ( v i n y l acetate) r a d i c a l , w h i c h w o u l d b e l o c a t e d i n the g r o w i n g p o l y m e r p a r t i c l e s , c h a i n transfers to H E C , w h i c h is p r e s e n t i n t h e a q u e o u s p h a s e . U n t i l r e c e n t l y , n o d e f i n i t i v e studies a d d r e s s e d this issue. R e c e n d y , studies h a v e b e g u n t o a p p e a r d e t a i l i n g the c h e m i s t r y o f H E C i n t h e p r e s e n c e o f p e r s u l f a t e i n i t i a t o r s a l o n e (5-8) o r u n d e r the c o n d i t i o n s o f v i n y l acetate e m u l s i o n p o l y m e r i z a t i o n (9-14). D o n e s c u et a l . (5) s u g g e s t e d that H E C a n d p e r s u l f a t e f o r m a r e d o x p a i r at the h i g h temperatures encountered i n thermally catalyzed emulsion p o l y m e r i z a t i o n a n d s h o w e d t h a t t h e a c t i v a t i o n e n e r g y f o r the d e c o m p o s i t i o n o f p e r s u l f a t e w a s l o w e r i n the p r e s e n c e o f H E C . T h i s result w a s c o r r o b o r a t e d b y K i s l e n k o et a l . (7), w h o d e t e r m i n e d the a c t i v a t i o n e n e r g y f o r p e r s u l f a t e d e c o m p o s i t i o n t o b e 20.8 a n d 34.0 k c a l / m o l i n the p r e s e n c e a n d a b s e n c e o f H E C , r e s p e c t i v e l y . S t e p i n et a l . (11) a n a l y z e d the p o l y m e r s o l i d s o f a d i b u t y l m a l e a t e - v i n y l acetate c o p o l y m e r s y n t h e s i z e d i n the p r e s e n c e o f H E C a n d f o u n d H E C - g r a f t c o p o l y m e r s as w e l l as the e x p e c t e d d i b u t y l m a l e a t e - v i n y l acetate c o p o l y m e r s . T h e authors s t a t e d that the H E C - g r a f t c o p o l y m e r s are c o n c e n t r a t e d at the p a r t i c l e i n t e r f a c e ; this f e a t u r e ensures c o l l o i d a l s t a b i l i t y . J a k u b e c a n d C h o c h o l a t y (14) d i s c o v e r e d a s y s t e m to suppress the o x i d a t i v e d e g r a d a t i o n of H E C d u r i n g p e r s u l f a t e - i n i t i a t e d e m u l s i o n p o l y m e r i z a t i o n o f v i n y l acetate b y e m p l o y i n g f o r m a t e salts; the p r o d u c t i o n o f h i g h - v i s c o s i t y v i n y l acetate h o m o p o l y m e r d i s p e r s i o n s results. I n c o n t r a s t t o v i n y l acetate, p e r s u l f a t e - i n i t i a t e d e m u l s i o n p o l y m e r i zation of acrylic monomers or v i n y l - a c r y l i c m o n o m e r blends of high a c r y l i c c o n t e n t , i n the p r e s e n c e o f H E C , r e s u l t e d i n e x t r e m e t h i c k e n i n g a n d e v e n t u a l c o a g u l a t i o n , e s p e c i a l l y at r e l a t i v e l y h i g h p o l y m e r solids (i.e., g r e a t e r t h a n 30%). T h u s , the b e n e f i t s o f H E C s t a b i l i z a t i o n , s u c h as m e c h a n i c a l s t a b i l i t y at l o w s u r f a c t a n t l e v e l s , w e r e n o t r e a l i z e d i n a l l a c r y l i c l a t i c e s ( J ) . T h i s c o n t r a s t i n b e h a v i o r r e l a t i v e to that o f v i n y l a c e tate h a d g e n e r a l l y b e e n e x p l a i n e d as b e i n g d u e to the l a c k o f g r a f t i n g o f a c r y l i c m o n o m e r s o n t o H E C ; this l a c k o f g r a f t i n g p r e v e n t s the f o r m a t i o n o f t h e p r o t e c t i v e l a y e r o f H E C at the p a r t i c l e i n t e r f a c e , a d i r e c t


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e x t r a p o l a t i o n o f the c h a i n - t r a n s f e r constant studies m e n t i o n e d f o r v i n y l acetate (2,3). N e v e r t h e l e s s , the p o t e n t i a l i m p r o v e m e n t s i n a c r y l i c l a t e x s t a b i l i t y r e s u l t i n g f r o m the successful i n c o r p o r a t i o n o f H E C f o s t e r e d r e s e a r c h a i m e d at t h e d e v e l o p m e n t o f p r o c e s s e s p e r m i t t i n g the i n c l u s i o n o f H E C (or r e l a t e d c o m p o u n d s ) d u r i n g the e m u l s i o n p o l y m e r i z a t i o n o f a c r y l i c m o n o m e r s (15-22). F o r i n s t a n c e , p r o c e s s e s h a v e b e e n p a t e n t e d (15, 16) w h e r e b y t h e p r o t e c t i v e c o l l o i d is g r a d u a l l y a d d e d d u r i n g the c o u r s e o f p o l y m e r i z a t i o n , w h i c h p r o v i d e latices o f c o a r s e p a r t i c l e s i z e a n d h i g h v i s c o s i t y . H i g h - s o l i d s H E C - s t a b i l i z e d a c r y l i c latices w e r e p r o d u c e d t h r o u g h t h e a d d i t i o n o f b r o m o t r i c h l o r o m e t h a n e 2 - 5 m i n after the f o r m a t i o n o f " p o l y m e r m i c e l l e s " (17). A l t e r n a t i v e l y , use o f H E C p r e t r e a t e d w i t h p e r o x i d i c c o m p o u n d s ( 18,19) o r use o f h i g h l y d e g r a d e d s t a r c h (20) as a p r o t e c t i v e c o l l o i d e n a b l e d the p r o d u c t i o n o f l o w - v i s c o s i t y a c r y l i c l a t i c e s o f h i g h s t a b i l i t y (18,19) o r latices o f i n c r e a s e d b l o c k i n g resistance (20). T w o p a r t i c u l a r l y c o n v e n i e n t m e t h o d s p r o d u c e d l o w - v i s c o s i t y , s h e a r - s t a b l e H E C - c o n t a i n i n g a c r y l i c p o l y m e r d i s p e r s i o n s t h r o u g h the s i m p l e i n c l u s i o n o f w a t e r - s o l u b l e " r e g u l a t o r s " , s u c h as m e r c a p t o a c e t i c a c i d o r c y c l o h e x y l a m i n e (21), o r " e m u l s i o n s t a b i l i z e r s " , s u c h as a l l y l a l c o h o l (22), a l o n g w i t h the H E C at the b e g i n n i n g o f the p o l y m e r i z a t i o n reaction. O n l y i n o n e i n s t a n c e w a s a n y e x p l a n a t i o n g i v e n as to w h y a n y o f the a f o r e m e n t i o n e d a c r y l i c systems a c t u a l l y w o r k e d . O g a t a a n d W a k a b a y a s h i (22) s u g g e s t e d that the g r a f t p o l y m e r i z a t i o n r e a c t i o n b e t w e e n a c r y l i c o r m e t h a c r y l i c m o n o m e r a n d the p r o t e c t i v e c o l l o i d w a s m o d e r a t e d b y the p r e s e n c e o f a l l y l a l c o h o l a n d r e l a t e d substances; the c o a g u l a t i o n that w o u l d o r d i n a r i l y r e s u l t f r o m the b r i d g i n g o f the latex p a r t i c l e s w a s p r e v e n t e d . T h i s e x p l a n a t i o n w a s c o n t r a r y to t r a d i t i o n a l t h o u g h t ( v i d e supra) b u t n e v e r t h e l e s s m o r e r e a d i l y e x p l a i n e d the t h i c k e n i n g o f H E C - c o n t a i n i n g a c r y l i c systems b e f o r e c o a g u l a t i o n o c c u r s . W i t h these a r g u m e n t s i n m i n d , w e e s t a b l i s h e d a p r o g r a m to h e l p d e f i n e the c h e m i s t r y o f H E C u n d e r the c o n d i t i o n s e n c o u n t e r e d d u r i n g the p r o d u c t i o n o f H E C - s t a b i l i z e d a c r y l i c a n d v i n y l - a c r y l i c c o p o l y m e r latices. R e s u l t s f r o m that s t u d y are p r e s e n t e d h e r e .

Experimental Section A series of polymerization reactions were carried out according to the procedures outlined in Tables I-III by varying the levels of the components as listed. A number of variations of the recipe in Table I were run including (1) the presence and absence of the water-soluble regulator (i.e., chain-transfer agent) triethanolamine (TEA) (2), an extension of the work detailed in reference 21, which showed that stable, low-viscosity polymer dispersions were produced only in the presence of water-soluble regulators; (2) the substitution of an 85:15



Table I. Acrylic Emulsion Polymerization Recipe

Mixture 1

Parts by Weight (%)

Component water Triton X-102

û

sodium dodecyl benzenesulfonate potassium persulfate (5%) HEC regulator or stabilizer ( T E A ) monomer potassium persulfate (5%) monomer b


C

2 3

4

total

45.00-53.00 0.00-2.00 0.00-0.50 2.00-4.00 0.00-1.00 0.00-1.00 10.00 0.00-2.00 34.00 100.00

N O T E : The procedure used was as follows: (1) add mixture 1 to kettle under an N 2 blanket and heat to 85 °C; (2) begin addition of mixture 3 via the metering pump; (3) add mixture 2 after 50% of monomer has been added; and (4) maintain at 85 ° C for 2 h beyond addition of monomer. Triton X-102 is a-[4-(l,l,3,3-tetramethylbutyl)phenyl]-u>-hydroxypoly (oxy-l,2-ethanediyl). H E C is (hydroxyethyl)cellulose [Natrosol 250 LR H E C (Hercules, Inc.), 5% aqueous Brookfield viscosity = 93 cP and weight-average molecular weight = 80,000]. T E A is triethanolamine. The monomer is composed of butyl acrylate (42.7%), methyl methacrylate (56.8%), and methacryhc acid (0.5%). û

b

C

d

T a b l e II. V i n y l A c e t a t e - A c r y l i c E m u l s i o n Polymerization Recipe

Mixture 1

Component water sodium bicarbonate (5%) Aerosol A-102" Triton X-102 HEC monomer* potassium persulfate (5%) monomer Aerosol A-102 total b

2 3 4 5

c

Parts by Weight (%) 43.60 1.44 1.40 0.70 0.50 10.00 2.48 38.00 0.88 100.00

N O T E : The procedure used was as follows: (1) add mixture 1 to kettle under an Ν2 blanket and heat to 85 °C; (2) add mixture 2 and 3 and when refluxing subsides, begin addition of step 4 via the metering pump; (3) add mixture 5 after 50% of monomer addition; and (4) maintain at 85 ° C for 2 h after completion of monomer addition. Aerosol A-102 is the disodium salt of the sulfosuccinate half-ester of ethyoxylated fatty alcohol. H E C is Natrosol 250 MR H E C (Hercules, Inc.) (2% aqueous Brookfield viscosity = 5000 cP and weight-average molecular weight = 650,000) or Natrosol 250 LR H E C (see Table I). T h e monomer is composed of vinyl acetate (85%) and butyl acrylate (15%). e

b

c


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Table III. Vinyl Acetate Homopolymer Emulsion Polymerization Recipe Mixture


1

2 3 4

Component water HEC° potassium persulfate vinyl acetate sodium bicarbonate (5%) vinyl acetate total

Parts by Weight (%) 43.45 1.96 0.15 10.00 1.44 43.00 100.00

N O T E : The procedure used was as follows: (1) add mixture 1 to kettle under N2 blanket and heat to 85 °C; (2) add mixtures 2 and 3 and when refluxing subsides, begin adding mixture 4 via metering pump; and (3) maintain at 85 °C for 2 h beyond final addition of vinyl acetate. See Table II for a description of H E C .

fl

mixture of vinyl acetate and butyl acrylate for the acrylic monomer mixture listed; (3) the portionwise versus batch addition of the persulfate initiator; and (4) the slow addition of all the monomer feed versus the semibatch method listed. The resulting latices were characterized for p H , Brookfield viscosity, average particle diameter (Coulter nanosizer), grit content (visual assessment), and mechanical stability (10 min in a Waring blender at the highest setting). The samples subjected to high shear were analyzed for both particle size and viscosity. For determination of the amount of grafted H E C , the latices were centrifuged at 17,000 rpm to remove the polymer solids, and the supernatant liquid was analyzed by high-performance size-exclusion chromatography (23) for the concentration of residual H E C (estimated by the area under the chromatogram tracing) and H E C molecular weight distribution. The estimates of H E C concentration determined from the chromatograms were refined by colorimetric determination of H E C (24). Controls were run whereby the latex was produced in the absence of H E C and postthickened with H E C after cooling to room temperature. Analysis of these latices proceeded as described. In addition, solvent extraction and fractionation of the centrifuged polymer solids were performed on a number of samples to distinguish between grafted and physically absorbed H E C .

Results and Discussion T o distinguish between grafted H E C and absorbed H E C , we ran a number of experiments, including the synthesis of control latices in the absence of H E C (according to Table I) with subsequent postaddition of H E C . Centrifugation of these latices and analysis of the supernate revealed that 100% of the postadded H E C remained in the aqueous phase with no reduction in molecular weight (see Table I V , which pro-



356


v i d e s s o m e t y p i c a l a n a l y t i c a l d a t a o n latices p r e p a r e d a c c o r d i n g to T a b l e I). In a d d i t i o n , extensive solvent extractions of selected p o l y m e r s o l i d s s a m p l e s o b t a i n e d f r o m c e n t r i f u g i n g d e m o n s t r a t e d that o n l y l o w l e v e l s (

—Ι

oc

10

/S

*0

30

TIME(m1n.)

Figure 7. Relative weight percent grafted HEC vs. time of HEC addition. All-acrylic latices were produced according to Table I in the presence of 1.0% Triton X-102,0.15% SDBS, 0.5% TEA, and 0.5% HEC. Concentrations are based on the total recipe.


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YOID VOLUME

-th

12

14

INJECT SAMPLE

16

ELUTION

18

20

DISTANCE(cm)

Figure 8. Size-exclusion chromatography of residual HEC in the aqueous phase of the latices of Figure 7. Key: A, time = 0 min; B, time = 5-30 min; and C, HEC standard. VOID VOLUME

14 INJECT SAMPLE

16 ELUTION

18

20

22

DISTANCE(cm)

Figure 9. Size-exclusion chromatography of residual HEC in the aqueous phase of the acrylic latices produced according to Table I in the presence of 1.0% Triton X, 0.15% SDBS, 0.5% TEA, and 0.5% HEC: A, all the initiator was present at the beginning: B, half of the initiator was present at the beginning and half was added after 50% monomer addition; and C, HEC standard.


362



VOID VOLUME

11 12 INJECT SAMPLE

14

•' 16 ELUTION

18

20

22

24

26

DISTANCE(cm)

Figure 10. Size-exclusion chromatography of residual HEC in the aqueous phase of the acrylicfoticesproduced according to Table I in the presence and absence of surfactants. Key: A, no surfactant, no TEA, and 0.5% HEC; B, 1.0% Triton X, 0.15% SDBS, 0.5% HEC, and no TEA; C, no surfactant, 0.5% TEA, and 0.5% HEC; D, 1.0% Triton X-102,0.15% SDBS, 0.5% TEA, and 0.5% HEC; and E, HEC standard.

cases. T h e s e d a t a i m p l y that surfactants p a r t i c i p a t e i n g r a f t i n g r e a c t i o n s as w e l l . A l t h o u g h g r a f t i n g r e a c t i o n s o f surfactants a r e k n o w n (25), it a p p e a r s that s u r f a c t a n t s d o n o t c o m p e t e e f f e c t i v e l y w i t h c o m p o u n d s s u c h as H E C f o r the free r a d i c a l s g e n e r a t e d d u r i n g i n i t i a t i o n . P e r h a p s m o r e i n t e r e s t i n g t h a n m o s t o f the d a t a p r e s e n t e d so f a r are the s i z e - e x c l u s i o n c h r o m a t o g r a m s o f F i g u r e 11. C u r v e A represents the H E C s t a n d a r d , c u r v e Β the r e s i d u a l H E C i n the l a t e x s u p e r n a t e of a n a l l - a c r y l i c l a t e x p r e p a r e d i n t h e p r e s e n c e o f H E C a n d surfactants a n d i n the a b s e n c e o f T E A ( a c c o r d i n g to T a b l e I ) , a n d c u r v e C the r e s i d u a l H E C p r o f i l e o f t h e l a t e x s u p e r n a t e g e n e r a t e d w h e n the a c r y l i c m o n o m e r s o f c u r v e Β h a v e b e e n r e p l a c e d b y a n 85:15 m i x t u r e o f v i n y l acetate a n d b u t y l a c r y l a t e , a l l o t h e r things b e i n g e q u a l . O n e c a n i m m e d i a t e l y s p o t the e x t e n s i v e H E C d e g r a d a t i o n b u t d i s t i n c t l a c k o f H E C g r a f t i n g that o c c u r s i n t h e v i n y l - a c r y l i c s y s t e m . C o n t r a r y to t r a d i t i o n a l t h o u g h t , v i n y l acetate has m u c h less t e n d e n c y to g r a f t o n t o H E C t h a n d o a c r y l i c m o n o m e r s u n d e r the conditions encountered i n emulsion p o l y m e r i z a t i o n . T h e s e o b s e r v a t i o n s l e d t o a n e x t e n s i v e i n v e s t i g a t i o n o f the synthesis o f H E C - s t a b i l i z e d v i n y l acetate h o m o p o l y m e r a n d c o p o l y m e r latices (see r e c i p e s i n T a b l e s I I a n d I I I ) . T y p i c a l d a t a are p r o v i d e d i n T a b l e V a n d F i g u r e 12, a l t h o u g h , u n l i k e a c r y l i c s y s t e m s , a m o d e r a t e d e g r e e o f v a r i a b i l i t y w a s o b s e r v e d i n the m a n u f a c t u r e o f the l a t i c e s . A s e x p e c t e d ,


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VOID VOLUME

/

°\~-"~ INJECT SAMPLE

12

14

16

18

20

22

ELUTION DISTANCE(cm)

Figure 11. Size-exclusion chromatography of residual HEC in the aqueous phase of the latices produced according to Table I in the presence of 1.0% Triton X-102, 0.15% SDBS, 0.5% HEC, and no TEA. Key: A, HEC standard; B, all-acrylic monomer mixture of Table I; and C, vinyl-acrylic monomer mixture of Table II, in the recipe of Table I.

Figure 12. Size-exclusion chromatography of residual HEC in the aqueous phase of the latices produced according to Table II (curve C) and Table III (curve B). Curve A is HEC standard.


In Water-Soluble Polymers; Glass, J. E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1986. 1.15 1.15:0.50 1.15:0.50 1.15:0.50 1.00 1.00:0.50 1.15:0.50

Concn (wt %) 40 — 1480 670 >10,000 3200 125

Viscosity (cP)

T

d

c

b

a

NOTE: VA is vinyl acetate and Β A is butyl acrylate. The recipes are listed in Table III. Natrosol 250 L R H E C . The recipes are listed in Table II. Natrosol 250 M R H E C .

r

b

b

r

c4

c

1.96 1.40:0.50 1.96 1.40:0.50

a

H E C (low M ) > surfactant-HEC (low M ) > H E C (high M ) surfactant-HEC (high M )

VA V A - B A (85:15) VA V A - B A (85:15) T

Concn (wt%)

Stabilizer

750 390 1600 250

Viscosity (cps)

Table V . V i n y l Acetate Latices

Monomer

h

NOTE: BA is butyl ί acrylate, M M A is methyl methacrylate, and M A A is methacrylic acid. "The recipes are listed in Table I. HEC was added after surfactant addition.

fo

surfactant surfactant-HEC surfactant-HEC post a d d surfactant-HEC-TEA HEC HEC-TEA surfactant-HEC

BA-MMA-MAA BA-MMA-MAA BA-MMA-MAA BA-MMA-MAA BA-MMA-MAA BA-MMA-MAA V A - B A (85:15)

0

Stabilizer

Monomer

Table I V . A c r y l i c Latices

1.20 0.39 1.31 0.63

Average Particle Diameter (μηι)

0.1 >4.0 0.1 3.6 2.0 >4.0 0.3

Average Particle Diameter (μτη)

yes yes yes yes

Shear Stability

no yes yes yes yes yes yes

Shear Stability


0 4 6 5

%HEC Grafted

83 0 45 93 61 5

%HEC Grafted

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v e r y l i t t l e g r a f t i n g o f H E C o c c u r r e d d e s p i t e the h i g h r e a c t i v i t y o f H E C t o w a r d d e g r a d a t i o n i n these s y s t e m s . T h e c h r o m a t o g r a m s p r o v i d e d i n F i g u r e 12 s h o w that l i t t l e d i f f e r e n c e exists i n the g r a f t i n g o r d e g r a d a t i o n r e a c t i o n s o f H E C i n t h e p r e s e n c e o f v i n y l acetate m o n o m e r a l o n e v e r s u s a n 85:15 m i x t u r e o f v i n y l a c e t a t e a n d b u t y l a c r y l a t e o r i n the p r e s e n c e o r a b s e n c e o f surfactants. T h e s e e x p e r i m e n t s c o n f i r m e d that v i n y l acetate has l i t t l e t e n d e n c y to g r a f t o n t o H E C , d e s p i t e the h i g h r e a c t i v i t y o f the p o l y ( v i n y l acetate) r a d i c a l .

Scheme I. Proposed scheme outlining the chemistry of HEC in the presence of persulfate and acrylic monomers under standard emulsion polymerization conditions.



366


A l t h o u g h the data presented are contrary to established explana t i o n s , t h e y a r e c o m p l e t e l y consistent w i t h S c h e m e I . T h u s , f r e e r a d i c a l s generated a l o n g the H E C b a c k b o n e b y chain transfer f r o m persulfate a n i o n r a d i c a l s c a n a c t as c o m m o n i n t e r m e d i a t e s f o r i n i t i a t i n g either grafting or oxidative degradation. In the presence of reactive acrylic m o n o m e r s , e x t e n s i v e g r a f t i n g w i l l o c c u r unless a r t i f i c i a l l y c o n t r o l l e d v i a r e g u l a t o r s s u c h as T E A , w h i c h a p p e a r to f u n c t i o n b y p r e v e n t i n g t h e f o r m a t i o n o f H E C p o l y m e r r a d i c a l s i n t h e first p l a c e . P r e c e d e n c e f o r this r e s u l t a l r e a d y exists i n t h e l i t e r a t u r e (26). I n t h e p r e s e n c e o f less r e a c t i v e m o n o m e r s s u c h as v i n y l acetate, s i g n i f i c a n t l y less H E C g r a f t i n g occurs while persulfate-mediated oxidative degradation predominates. T h i s l a c k o f H E C g r a f t i n g s h o u l d n o t i n f l u e n c e t h e w e l l - k n o w n selfb r a n c h i n g o f p o l y ( v i n y l acetate) (4) a r i s i n g f r o m c h a i n transfer t o p o l y m e r resin, w h i c h occurs i n the p o l y m e r particles a n d not i n the aqueous phase as d o e s t h e p r o p o s e d s c h e m e .

Conclusions U n d e r s i m i l a r e m u l s i o n p o l y m e r i z a t i o n c o n d i t i o n s , v i n y l acetate m o n o m e r h a s m u c h less o f a t e n d e n c y t o g r a f t t o H E C r e l a t i v e t o a c r y l i c m o n o m e r s t h a n w a s p r e v i o u s l y t h o u g h t . T h e extensive g r a f t i n g o f acrylic m o n o m e r s to H E C produces the characteristic viscosity b u i l d u p that o c c u r s p r i o r t o c o a g u l a t i o n i n t r a d i t i o n a l a c r y l i c - H E C latex systems.

Acknowledgments T h e h e l p o f H . G . B a r t h , Κ. V . H a n n o n , H . J . G o l d y , E . W . S c h w a r z , F . S. S z c z e p k o w s k i , a n d H . C . T i l l s o n is g r e a t l y a p p r e c i a t e d .

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