Effect of Surfactants and Cosolvents on the Behavior of Associative

20 Effect of Surfactants and Cosolvents on the Behavior of Associative Thickeners in Latex Systems J. C . Thibeault, P. R. Sperry, and E. J. Schaller

Downloaded by EMORY UNIV on February 21, 2016 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch020

Research Laboratories, Rohm and Haas Company, Spring House, PA 19477

Adsorption isotherms and viscosity-shear-rate profiles of model anionic and nonionic associative thickeners with acrylic latices are strongly dependent on the latex particle size and system surfactant and cosolvent content. The adsorption data indicate a level of latex-thickener interaction substantially greater than that with the conventional nonassociative thickener (hydroxyethyl)cellulose. Addition of sodium dodecyl sulfate surfactant or butyl Carbitol water-miscible cosolvent results in desorption of the associative thickeners from the latex surface and produces concomitant changes in rheology. These effects provide the coatings formulator with a unique means of independently adjusting the viscosity at both low shear rate (leveling, sagging, and settling) and high shear rate (brushing and film build).

FORMULATING A LATEX PAINT

r e q u i r e s c a r e f u l a d j u s t m e n t o f the f i n a l r h e o l o g y . V i s c o s i t y at h i g h shear rates m u s t b e c o n t r o l l e d i n a n a r r o w r a n g e to g i v e s u f f i c i e n t f i l m b u i l d a n d h i d i n g w i t h o u t e x t r e m e b r u s h o r r o l l e r d r a g , w h i l e l o w - s h e a r - r a t e v i s c o s i t y m u s t b e h i g h e n o u g h to p r e v e n t s e t t l i n g d u r i n g storage a n d excessive s a g g i n g after a p p l i c a t i o n b u t l o w e n o u g h t o a l l o w the f l o w a n d l e v e l i n g n e c e s s a r y f o r o p t i m u m a p p e a r a n c e ( J ) . H i s t o r i c a l l y , r h e o l o g i c a l c o n t r o l i n l a t e x c o a t i n g s has been achieved w i t h high molecular weight water-soluble polymers such as c e l l u l o s i c s a n d p o l y a c r y l a t e s , w h i c h t h i c k e n b o t h b y e n t r a p p i n g w a t e r molecules w i t h i n their h i g h l y s w o l l e n p o l y m e r coils a n d b y c h a i n e n t a n g l e m e n t (2). A l t h o u g h s u c h t h i c k e n e r s h a v e b e e n u s e d q u i t e s u c c e s s f u l l y o v e r the y e a r s , t h e y s u f f e r c e r t a i n l i m i t a t i o n s w i t h r e s p e c t t o t h e i r v i s c o s i t y - s h e a r rate p r o f i l e s , w h i c h are i n h e r e n t i n t h e i r t h i c k e n i n g mechanism (3-5). R e c e n t l y , a n u m b e r o f n e w t h i c k e n e r s h a v e b e e n d e v e l o p e d (6) that t h i c k e n b y a n e n t i r e l y d i f f e r e n t m e c h a n i s m a n d that o f f e r the coatings 0065-2393/86/0213-0375$06.00/0 © 1986 American C h e m i c a l Society

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


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formulator the potential for significant improvements i n rheology. These t h i c k e n e r s a r e r e l a t e d t o t h e t r a d i t i o n a l t h i c k e n e r s i n that t h e y a r e b a s e d on similar water-soluble polymers, although generally of somewhat l o w e r m o l e c u l a r w e i g h t . T h e k e y d i f f e r e n c e is that t h e y also c o n t a i n h y d r o p h o b i c g r o u p s , w h i c h a l l o w t h e m t o associate w i t h v a r i o u s o t h e r h y d r o p h o b i c components of the coating. A simple illustration of some of the p r a c t i c a l d i f f e r e n c e s b e t w e e n c o n v e n t i o n a l t h i c k e n e r s a n d a s s o c i a t i v e t h i c k e n e r s is s h o w n i n T a b l e I w h e r e s o m e k e y r h e o l o g i c a l p a r a m e ters a r e g i v e n f o r t h r e e l a t e x p a i n t s , w h i c h a r e i d e n t i c a l e x c e p t f o r t h e t h i c k e n e r . T h e first p a i n t l i s t e d c o n t a i n s a c o m m e r c i a l ( h y d r o x y e t h y l ) cellulose ( H E C ) representative o f conventional thickeners. T h e other t w o contain t w o recently c o m m e r c i a l i z e d associative thickeners based, respectively, o n nonionic and anionic water-soluble polymer backbones. S t o r m e r v i s c o s i t y is a l o w - s h e a r v i s c o s i t y o b t a i n e d f r o m a p a d d l e viscometer w i d e l y used i n the paint industry. Shear rate, although not w e l l - d e f i n e d , is e s t i m a t e d t o b e a b o u t 5 0 - 1 0 0 s ~ . T h i s is t h e shear-rate r e g i o n that d e t e r m i n e s p r o p e r t i e s s u c h as b r u s h p i c k u p a n d p o u r a b i l i t y a n d is r e l a t e d t o w h a t is g e n e r a l l y r e g a r d e d as t h e c o n s i s t e n c y o f t h e p a i n t . A l t h o u g h t h e S t o r m e r v i s c o s i t y is g i v e n i n a r b i t r a r y K r e b u n i t s , t h e i m p o r t a n t p o i n t w i t h r e g a r d t o o u r p r e s e n t p u r p o s e is that t h e p a i n t s i n T a b l e I have b e e n adjusted to similar low-shear viscosities b y v a r y i n g the t h i c k e n e r use l e v e l . T h u s , t h e l o w - s h e a r t h i c k e n i n g e f f i c i e n c i e s o f o u r t h r e e t h i c k e n e r s a r e i n v e r s e l y p r o p o r t i o n a l t o these use l e v e l s . C l e a r l y , associative thickeners c a n span a w i d e range of low-shear efficiencies. H o w e v e r , u n l i k e c o n v e n t i o n a l t h i c k e n e r s , this e f f i c i e n c y is n o t n e c e s sarily l i n k e d to molecular weight. T h u s , the l o w molecular weight n o n i o n i c a s s o c i a t i v e t h i c k e n e r is s i g n i f i c a n t l y m o r e e f f i c i e n t at a l o w shear r a t e t h a n t h e s u b s t a n t i a l l y h i g h e r m o l e c u l a r w e i g h t ( h y d r o x y e t h y l ) c e l l u l o s e , w h i l e t h e i n t e r m e d i a t e m o l e c u l a r w e i g h t a n i o n i c associative t h i c k e n e r is less e f f i c i e n t . W e w i l l s h o w l a t e r that this d i f f e r e n c e i n e f f i c i e n c y c o r r e l a t e s w i t h d i f f e r e n c e s i n t h e d e g r e e o f associative character built into the polymers. l

I C I v i s c o s i t y is a h i g h - s h e a r - r a t e v i s c o s i t y (10,000 s " ) that correlates w e l l w i t h p e r f o r m a n c e i n a p p l i c a t i o n s p r o c e s s e s s u c h as b r u s h i n g , r o l l i n g , a n d s p r a y i n g . I C I viscosities c a n b e o b t a i n e d b y u s i n g associative 1

Table I. Properties of Thickeners

Thickener Type (Hydroxyethyl)cellulose Nonionic associative Anionic associative

M

w

850,000 40,000 400,000

Amt of Thickener (lb/100 gal) 3.34 0.60 4.75

Stormer Viscosity (KU) 81 82 83

ICI Viscosity (P) 0.43 0.20 0.90


Leveling 2 8 7

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Behavior of Associative Thickeners in Latex Systems 377


t h i c k e n e r s that are b o t h h i g h e r a n d l o w e r t h a n those o b t a i n e d b y u s i n g c o n v e n t i o n a l t h i c k e n e r s ( T a b l e I ) . I n a sense, h o w e v e r , the l o w v a l u e o b t a i n e d w i t h the n o n i o n i c a s s o c i a t i v e t h i c k e n e r is m i s l e a d i n g . I f the I C I d a t a a r e n o r m a l i z e d to the s a m e t h i c k e n e r use l e v e l , the n o n i o n i c a s s o c i a t i v e t h i c k e n e r is s i g n i f i c a n t l y m o r e I C I e f f i c i e n t i n t e r m s of p o i s e p e r p o u n d t h a n either o f the other t h i c k e n e r s . A s a n a s i d e , this last f a c t p r o v i d e s the basis f o r a v e r y u s e f u l f o r m u l a t i o n t r i c k . T h e d e g r e e o f a s s o c i a t i o n , a n d t h e r e f o r e the l o w - s h e a r - r a t e e f f i c i e n c y , o f a s s o c i a t i v e t h i c k e n e r s c a n b e g r a d u a l l y d e c r e a s e d b y the c a r e f u l a d d i t i o n o f s u r f a c t a n t s o r c o s o l v e n t s . B e c a u s e this m e a n s the t h i c k e n e r use l e v e l c a n b e i n c r e a s e d , a n d w i t h it t h e I C I v i s c o s i t y , w h i l e the s a m e S t o r m e r v i s c o s i t y is m a i n t a i n e d , a m e t h o d f o r i n d e p e n d e n t l y a d j u s t i n g t h e h i g h - a n d l o w - s h e a r v i s c o s i t i e s that c a n b e o f s u b s t a n t i a l p r a c t i c a l v a l u e to the coatings f o r m u l a t o r is o b t a i n e d . L e v e l i n g , a l t h o u g h n o t a p r i m a r y r h e o l o g i c a l p a r a m e t e r , is n e v e r t h e less a n i m p o r t a n t o n e i n c h a r a c t e r i z i n g the p e r f o r m a n c e of a p a i n t . L e v e l i n g is a m e a s u r e of the a b i l i t y of a c o a t i n g to f l o w o u t after a p p l i c a t i o n so as to o b l i t e r a t e a n y s u r f a c e i r r e g u l a r i t i e s , s u c h as b r u s h m a r k s , p r o d u c e d b y the a p p l i c a t i o n p r o c e s s (7). I n the p r e s e n t case w e h a v e r a t e d l e v e l i n g s u b j e c t i v e l y o n a scale o f 0 to 10 b y c o m p a r i n g b r u s h o u t s o f e a c h p a i n t to a series o f s t a n d a r d s . B e c a u s e l e v e l i n g is a v e r y l o w shear rate p r o c e s s , r o u g h l y 1 0 s o r less, the h i g h l e v e l i n g v a l u e s o b t a i n e d f o r o u r t w o a s s o c i a t i v e t h i c k e n e r s r e l a t i v e to that of the H E C i m p l y that t h e y h a v e m u c h l o w e r v i s c o s i t i e s at v e r y l o w shear rates. T h i s r e s u l t is v e r y g e n e r a l a n d i l l u s t r a t e s o n e o f t h e p r i m a r y a d v a n t a g e s o f associative thickeners over conventional nonassociative thickeners. _ 1

- 1

T h e p r e c e d i n g illustrates s o m e o f the r h e o l o g i c a l p r o p e r t i e s c h a r a c teristic o f a s s o c i a t i v e t h i c k e n e r t h i c k e n e d systems. I n w h a t f o l l o w s , the results o f s o m e d i r e c t m e a s u r e m e n t s of the d e g r e e o f l a t e x - t h i c k e n e r association for t w o m o d e l associative thickeners under various c o n d i t i o n s a r e d i s c u s s e d , a n d a n a t t e m p t is m a d e to r e l a t e these results to r h e o l o g i c a l b e h a v i o r . A l t h o u g h l a t e x - t h i c k e n e r a s s o c i a t i o n is o n l y o n e o f the m a n y a s s o c i a t i v e i n t e r a c t i o n s that c a n o c c u r i n p r a c t i c a l associative t h i c k e n e r s y s t e m s , u n d e r m a n y c o n d i t i o n s this a s s o c i a t i o n is the d o m i n a n t i n t e r a c t i o n i n t e r m s o f the o v e r a l l r h e o l o g i c a l b e h a v i o r o f the system.

Experimental Section Two model associative thickeners were examined in this study, one a low molecular weight ethylene oxide based urethane copolymer containing a relatively high overall weight fraction of hydrophobe and the other the ammonium salt of an anionic acrylic copolymer of significantly higher molecular weight and lower hydrophobe content. Although not identical with the commercial associative thickeners of Table I, these associative thickeners are very similar and when compared directly show virtually identical behavior. Latices employed are



378


approximately monodisperse with average diameters of 50, 90,140, 300, 340, and 600 nm. All are copolymers of butyl acrylate and methyl methacrylate of approximately 0 °C glass temperature and contain about 1% of copolymerized carboxylic acid groups. These copolymers were prepared as described previously (3) with 0.4% ammonium persulfate initiator and 0.05-3.05% sodium dodecyl benzenesulfonate surfactant. All systems were adjusted to p H 9 with ammonia. Thickener adsorption measurements were carried out by centrifuging the equilibrated thickener-latex mixtures and analyzing the separated serums for free thickener. In the nonionic case, this analysis was conducted by using a modification of a colorimetric method originally developed for nonionic polyethylene oxide surfactants (8). For the anionic case, 0.1% 9-vinylanthracene was included during the thickener synthesis to serve as a fluorescent tag. Viscosity-shear-rate curves shown are a composite of data from several instruments: a Brookfield SynchroLectric viscometer with U L adapter, a Haake Rotovisco RV2 viscometer, and an ICI cone and plate viscometer.

Results and Discussion F i g u r e 1 shows adsorption isotherms for the t w o m o d e l associative t h i c k e n e r s o n a 3 0 0 - n m - d i a m e t e r a c r y l i c l a t e x at 25 v o l % s o l i d s . D a t a a r e

40

30

MEC 0

0

1

2

3

4

WEIGHT % THICKENER IN CONTINUOUS PHASE Figure I. Adsorption isotherms for HEC, nonionic associative, and anionic associative thickeners with a 300-nm latex. Key: A , anionic; nonionic; and HEC.


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also shown for a typical nonassociative H E C thickener (Natrosol 250 G R from Hercules, Inc.) (3). T h e strong adsorption observed for the associative thickeners relative to that for H E C even at very low thickener concentrations is direct evidence for the associative nature of these thickeners. T h e shapes of the associative thickener adsorption isotherms in F i g ure 1 resemble that of the classical Langmuir isotherm, with the strong adsorption observed at very low thickener concentrations gradually leveling off as the thickener level increases and the latex surface becomes saturated. Such behavior is consistent with a simple model in which localized hydrophobic sections of the predominantly hydrophilic thickener attach themselves to hydrophobic binder sites on the latex surface. Because the associative thickener molecules are multifunctional in the sense that each contains two or more hydrophobes, the result is a three-dimensional transient network in which the latex particles serve as the branch points and the thickener molecules act as the cross-links. Similar to a permanently cross-linked polymer network, these cross-links resist the stretching that must occur when the system begins to flow; thus, the cross-links contribute to the apparent viscosity of the system. O f course, the transient nature of the cross-links, that is, the fact they are continually forming, breaking, and re-forming, allows the system to flow rather than just de-form as would be the case in a permanently crosslinked system. As a final point with reference to Figure 1, the initial slope of the adsorption isotherm is significantly greater for the nonionic associative thickener than for the anionic. Because both thickeners appear to have fairly similar saturation values, this property implies that the former is more strongly associated with the latex surface than the latter. T h e greater association of the nonionic associative thickener can be attributed to its higher hydrophobe content making it a more highly associative thickener than the anionic associative thickener. Figure 2 shows viscosity-shear-rate profiles for a 300-nm latex at 25 vol % solids and containing 2% thickener by weight on the continuous phase. T h e extreme shear thinning behavior observed with H E C (Natrosol 2 5 0 M R ) is typical of thickeners of this type and illustrates their primary limitation with respect to coatings rheology. Clearly, for such a system to maintain both a high enough high-shear-rate viscosity for good film build and hiding and a low enough low-shear-rate viscosity for good leveling is impossible. T h e two associative thickeners, on the other hand, show a much more Newtonian profile, particularly in the lowshear-rate region, and therefore allow a more equitable balance of highand low-shear-rate viscosity. Put in slightly different terms, if the thickener levels are adjusted so as to give similar viscosities at some intermediate shear rate (e.g., the Stormer shear rate), the two associative thickeners, because of the flatness of their rheology profiles, must give sub-


380



1 0 0 0 0 0

0.1

1

10

100

SHEAR RATE

1000

10000

1/SEC

Figure 2. Viscosity-shear-rate profile for HEC, nonionic associative, and anionic associative thickeners with a 300-nm latex. Key: , HEC; —, nonionic; and , anionic.

s t a n t i a l l y l o w e r viscosities at v e r y l o w shear rates t h a n H E C . T h i s p r o p e r t y , as d i s c u s s e d e a r l i e r , translates i n t o m a r k e d l y b e t t e r l e v e l i n g behavior. T h e shapes o f the l a t e x - a s s o c i a t i v e t h i c k e n e r r h e o l o g y p r o f i l e s c a n b e u n d e r s t o o d i n t e r m s o f the t r a n s i e n t n e t w o r k p i c t u r e o f l a t e x - a s s o c i a t i v e t h i c k e n e r systems i f t w o f a i r l y r e a s o n a b l e a s s u m p t i o n s are m a d e : (1) that the v i s c o s i t y o f a p a r t i c u l a r l a t e x - a s s o c i a t i v e t h i c k e n e r s y s t e m is d i r e c t l y r e l a t e d to the n u m b e r of c r o s s - l i n k s p r e s e n t at a n y g i v e n t i m e (as is the m o d u l u s o f a p e r m a n e n t l y c r o s s - l i n k e d p o l y m e r n e t w o r k ) a n d (2) that the b r e a k i n g a n d r e - f o r m i n g of i n d i v i d u a l c r o s s - l i n k s o c c u r b y b o t h t h e r m a l a n d s h e a r - i n d u c e d p r o c e s s e s . A t v e r y l o w shear rates, the t h e r m a l p r o c e s s e s w i l l c l e a r l y d o m i n a t e a n d the e q u i l i b r i u m c o n c e n t r a t i o n o f c r o s s - l i n k s w i l l r e m a i n a p p r o x i m a t e l y constant at the t h e r m a l v a l u e i n d e p e n d e n t o f the shear r a t e ; the result is N e w t o n i a n - l i k e v i s c o s i t y . A s the shear rate a p p r o a c h e s a n d t h e n e x c e e d s s o m e c r i t i c a l v a l u e , h o w e v e r , s h e a r - i n d u c e d p r o c e s s e s w i l l start to p l a y a b i g g e r a n d b i g g e r r o l e a n d the e q u i l i b r i u m c o n c e n t r a t i o n of c r o s s - l i n k s w i l l d e c r e a s e r a p i d l y w i t h i n c r e a s i n g shear r a t e , h e n c e the s u d d e n d o w n w a r d t u r n i n


20.

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the a s s o c i a t i v e t h i c k e n e r v i s c o s i t y - s h e a r - r a t e c u r v e s o f F i g u r e 2 i n the 10-100-s" r e g i o n .


1

O n e f e a t u r e o b v i o u s i n F i g u r e 2 is the f a c t that the a n i o n i c a s s o c i a t i v e t h i c k e n e r s y s t e m is s i g n i f i c a n t l y less v i s c o u s t h a n the n o n i o n i c asso c i a t i v e t h i c k e n e r s y s t e m d e s p i t e the s u b s t a n t i a l l y h i g h e r m o l e c u l a r w e i g h t o f the f o r m e r . T h e n o n i o n i c associative t h i c k e n e r is a m u c h m o r e e f f i c i e n t a s s o c i a t i v e t h i c k e n e r (for latex) t h a n the a n i o n i c a s s o c i a t i v e t h i c k e n e r ; this f e a t u r e is t o t a l l y consistent w i t h the b e h a v i o r seen e a r l i e r i n the p a i n t d a t a . B o t h results, of c o u r s e , m u s t b e r e l a t e d to the c o n c l u s i o n r e a c h e d p r e v i o u s l y f r o m the i n i t i a l slopes o f the a s s o c i a t i v e t h i c k e n e r a d s o r p t i o n i s o t h e r m s , that is, that the n o n i o n i c a s s o c i a t i v e t h i c k e n e r associates m o r e s t r o n g l y w i t h the latex t h a n the a n i o n i c associative thickener. I n a d d i t i o n to s t r u c t u r a l v a r i a t i o n s i n the t h i c k e n e r , v a r i a t i o n s i n the l a t e x a r e also e x p e c t e d to a f f e c t the o v e r a l l d e g r e e o f a s s o c i a t i o n o f the latex-associative thickener system. O n e simple variation that should h a v e a r e a d i l y r a t i o n a l i z e d e f f e c t is latex p a r t i c l e size. F i g u r e 3 s h o w s

WEIGHT % THICKENER IN CONTINUOUS P H A S E Figure 3. Effect of latex particle size on the adsorption of a nonionic asso ciative thickener. Key: • , 50 nm, 90 nm; A , 140 nm; Φ, 300 nm; and • , 600 nm.




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a d s o r p t i o n i s o t h e r m s f o r o u r m o d e l n o n i o n i c associative t h i c k e n e r o n f i v e a c r y l i c latices o f p a r t i c l e size r a n g i n g f r o m 50 to 600 n m . N o t e that b o t h t h e s a t u r a t i o n v a l u e a n d t h e i n i t i a l s l o p e a p p e a r to i n c r e a s e as t h e p a r t i c l e s i z e d e c r e a s e s ; this r e s u l t i n d i c a t e s a n i n c r e a s e i n t h e d e g r e e o f l a t e x - t h i c k e n e r a s s o c i a t i o n . S u c h b e h a v i o r is c l e a r l y d u e t o t h e i n v e r s e r e l a t i o n s h i p b e t w e e n s u r f a c e a r e a a n d p a r t i c l e size. A s s u m i n g that t h e a v e r a g e n u m b e r o f h y d r o p h o b i c b i n d i n g sites a v a i l a b l e o n t h e latex s u r face p e r unit area depends o n l y o n the c o m p o s i t i o n of the latex, one w o u l d e x p e c t t h e s a t u r a t i o n v a l u e also to b e i n v e r s e l y r e l a t e d to p a r t i c l e s i z e . A s i m i l a r result is e x p e c t e d f o r the i n i t i a l s l o p e o f the i s o t h e r m i f o n e a d d s t h e a d d i t i o n a l a s s u m p t i o n that t h e l a t e x - a s s o c i a t i v e t h i c k e n e r b i n d i n g constant is also o n l y a f u n c t i o n o f t h e latex c o m p o s i t i o n . V e r y s i m i l a r results a r e o b t a i n e d w i t h the a n i o n i c associative t h i c k ener as is s h o w n i n F i g u r e 4.

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Effect of Surfactants and Cosolvents on the Behavior of Associative

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