Influence of Water-Soluble Polymers on Rheology of Pigmented Latex

21 Influence of Water-Soluble Polymers on Rheology of Pigmented Latex Coatings

Downloaded by UNIV OF PITTSBURGH on December 19, 2014 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch021

J. E. Glass Department of Polymers and Coatings, North Dakota State University, Fargo, N D 58105

The components of a coating are examined with respect to the mechanism each exhibits in determining the rheological response of the total formulation. In particular, the sensitivities of formulations containing hydrophobically modified, water-soluble polymers are addressed. The sensitivities are examined systematically with variation in the following formulation components: pigment volume concentration, percent nonvolatiles, formulation stabilizers (i.e., dispersant and surfactant levels), latex type and size distribution, and thickener type and molecular weight. A quantitative interpretation of the sensitivities observed across this broad-spectrum investigation cannot be offered because there are too many variables with matrix interactions that are difficult to quantify statistically, but general trends in this new area of technology are discussed.

THE INFLUENCE OF VARIATION IN FORMULATION COMPONENTS on coating rheology and on the sensitivity of formulations containing hydrophobically modified, water-soluble polymers (known in the coatings area as associative thickeners) will be discussed in this chapter. Hydrophobically modified, acid-swellable latex thickeners will not be discussed. A multiplicity of interactions can influence the associations of a water-soluble polymer (note the general discussion in Chapter 5) with the dispersed components of a coatings formulation. In pigmented waterborne latex coatings, this matrix of interactions can potentially interrupt the association between the hydrophobes of the hydrophobically modified, water-soluble polymer and the latex. If higher surface energy pigments, particularly T i 0 , are not properly stabilized, adsorption will occur on these higher energy surfaces. L o w molecular weight copolymers containing carboxylate groups are added to the formulation to ensure pigment dispersion. A formulation surfactant also is added to 2

0065-2393/86/0213-0391$07.50/0 © 1986 American Chemical Society

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

392

WATER-SOLUBLE POLYMERS

ensure d i s p e r s e d c o m p o n e n t s t a b i l i t y . T h e f o r m u l a t i o n s u r f a c t a n t is g e n e r a l l y a n o n i o n i c e t h o x y l a t e t y p e , w h e r e a s the s u r f a c t a n t u s e d i n the synthesis o f the l a t e x is a n i o n i c o r a n a n i o n i c - n o n i o n i c c o m b i n a t i o n . A s d i s c u s s e d i n C h a p t e r 5, n o n i o n i c s u r f a c t a n t s w i l l g e n e r a l l y d i s p l a c e a n i o n i c s f r o m t h e s u r f a c e o f l a t e x p a r t i c l e s . I f the h i g h - e n e r g y p i g m e n t s u r faces are s u i t a b l y s t a b i l i z e d , a s s o c i a t i o n b e t w e e n the h y d r o p h o b e s o f the


h y d r o p h o b i c a l l y m o d i f i e d , w a t e r - s o l u b l e p o l y m e r a n d the l a t e x is f a v o r e d o n a statistical basis. A c o h e s i v e i n t e r a c t i o n b e t w e e n the s t a b i l i z i n g moieties a n d the h y d r o p h o b e s of the thickener appears to b e i m p o r tant i f the d e s i r e d p r o p e r t i e s o f the c o a t i n g are to b e o b t a i n e d . W i t h the m u l t i t u d e of c o m p o n e n t s i n a p i g m e n t e d w a t e r b o r n e latex c o a t i n g , d i r e c t e v i d e n c e o f a s s o c i a t i o n is d i f f i c u l t t o o b t a i n . I n d i r e c t e v i d e n c e is f o u n d i n the c o m p a r i s o n o f r h e o l o g i c a l p r o f i l e s ( F i g u r e 1) a n d t h i c k e n i n g e f f i c i e n c y d a t a ( T a b l e I) i n the f o r m u l a t i o n s c o n t a i n i n g a s m a l l (117-nm)-particle latex. H i g h e r high-shear-rate viscosities ( H S V s ) a n d l o w e r l o w - s h e a r - r a t e viscosities ( L S V s ) are o b s e r v e d w i t h w a t e r s o l u b l e p o l y m e r s c o n t a i n i n g h y d r o p h o b e s at t h i c k e n e r c o n c e n t r a t i o n s c o m p a r a b l e t o t h o s e o f c e l l u l o s e ethers r e q u i r e d t o a c h i e v e a g i v e n K r e b u n i t ( K U ) v i s c o s i t y (an i n d u s t r i a l m e a s u r e m e n t p r a c t i c e i n f o r m u l a t i n g waterborne coatings). W i t h o u t h y d r o p h o b e m o d i f i c a t i o n , l o w molecular

SHEAR

RATE (s* ) 1

Figure 1. Viscosity (Pas) dependence on the shear rate (s' ) of an interior coating formulation: 57% PVC, 32% NVV, 90-KU-containing small-particle (100-nm), all-acrylic latex. Water-soluble thickeners: (B) (Hudroxypropyl)methykellulose; (O) SMAT; (O) HEUR 708; () M 10 poly(oxyethylene). Reproduced with permission from reference 11. Copynght 1984 Oil and Colour Chemists' Association. 1

r

r

s


21.

Influence on Rheology of Pigmented Latex Coatings

GLASS

393

Table I. Thickener Studies for Mechanistic Interpretation Thickener

Concn (wt%)

24-h Viscosity (KU)

M 1.8 Χ 1 0 poly(oxyethylene) M 1.0 Χ 10 poly(oxyethylene) H E U R 708 SMAT HPMC

7.51 3.10 0.40 0.46 0.36

92 89 91 93 93

4

r

5

r

a


e

H P M C is (hydroxypropyl)methylcellulose.

w e i g h t (18,000) p o l y ( o x y e t h y l e n e ) ( P O E ) p r o v i d e s the l o w e r L S V a n d h i g h e r H S V d e s i r e d , b u t the r h e o l o g y is r e l a t e d to the c o n c e n t r a t i o n (7.5 w t %) a n d m o l e c u l a r w e i g h t o f the t h i c k e n e r e m p l o y e d , n o t to a n a s s o c i a t i o n . T h e c o a t i n g t h i c k e n e d w i t h this m o l e c u l a r w e i g h t P O E i m p a r t s r h e o l o g y a p p r o x i m a t e l y e q u a l to those c o n t a i n i n g h y d r o p h o b i c a l l y m o d i f i e d , w a t e r - s o l u b l e p o l y m e r s , b u t the latter are e f f e c t i v e at a 1 0 - f o l d l o w e r c o n c e n t r a t i o n . A n i n c r e a s e i n P O E m o l e c u l a r w e i g h t (100,000) decreases the a m o u n t r e q u i r e d to e f f e c t a g i v e n K r e b u n i t v i s c o s i t y (a v i s c o s i t y a s s o c i a t e d w i t h a m o d e r a t e 2 5 - 7 5 - s " shear r a t e ) , b u t the L S V s i n c r e a s e a n d H S V s decrease, a general p h e n o m e n o n noted i n small-particle latex formulations thickened with nonmodified water-soluble polymers. T h e g e n e r i c structures o f t w o h y d r o p h o b i c a l l y m o d i f i e d t h i c k e n e r s [an e t h o x y l a t e d u r e t h a n e ( H E U R 708) a n d a m o d i f i e d s t y r e n e - m a l e i c a c i d t e r p o l y m e r ( S M A T ) ] a r e i l l u s t r a t e d i n structures I a n d II. B o t h h y d r o p h o b i c a l l y m o d i f i e d , w a t e r - s o l u b l e p o l y m e r s are e f f e c t i v e i n p r o m o t i n g the r h e o l o g i c a l r e s p o n s e ( F i g u r e 1) o f the l o w e r m o l e c u l a r w e i g h t P O E w i t h o u t large amounts of either h y d r o p h o b i c a l l y m o d i f i e d , waters o l u b l e p o l y m e r r e q u i r e d to a c h i e v e the s t a n d a r d f o r m u l a t i o n K r e b u n i t viscosity. 1

T h e s e c h a n g e s a n d the sensitivities that arise i n coatings w i t h h y d r o p h o b e m o d i f i c a t i o n o f w a t e r - s o l u b l e p o l y m e r s are r e v i e w e d to

Ο Ο U Ί Η R-hhWO-CHt-CHt^O-C-N-R- Ν- C*OCH KMt*ir|- 0-C-N-R' Η Η Η Η t

R - C . - C

κ.·ο-««β

R'=C-C. R

8

CT - C

n

M

s

'"

4

: Structura* or* al encompassing I



394


II

u n d e r s t a n d the i n d i v i d u a l c o n t r i b u t i o n s a n d the c o m b i n e d i n t e r a c t i o n s o f f o r m u l a t i o n c o m p o n e n t s o n a coating's r h e o l o g y .

Influence of Thickener-Dispersed Component Interactions on Low-Shear-Rate Viscosities G e n e r a l . A W e i s s e n b e r g r h e o m e t e r has b e e n e m p l o y e d ( J , 2) to s i m u l a t e c o n d i t i o n s d u r i n g a p p l i c a t i o n o f a c o a t i n g . T h e c o a t i n g is s h e a r e d at a c o m p a r a t i v e l y h i g h rate (ca. 3000 s" ); after a constant v i s c o s i t y response is r e a c h e d , the d e f o r m a t i o n is i n s t a n t a n e o u s l y c h a n g e d to a l o w d e f o r m a t i o n rate ( v i a a n o s c i l l a t o r y m o d e ) a n d the v i s c o u s a n d elastic responses are o b s e r v e d w i t h t i m e . 1

H S V s a n d L S V s are a f f e c t e d b y s e v e r a l f o r m u l a t i o n v a r i a b l e s . T h e i n f l u e n c e o f the t w o k e y c o m p o n e n t s , the t h i c k e n e r a n d the l a t e x , a n d t h e i r s y n e r g i e s , o n the l o w d e f o r m a t i o n rate r e c o v e r y (3) o f the a p p l i e d c o a t i n g is i l l u s t r a t e d i n F i g u r e 2. T h e three t h i c k e n e r s are a n a c r y l i c a c i d - e t h y l a c r y l a t e c o p o l y m e r , H E C , a n d a n a l k a l i - s w e l l a b l e latex that has e x p a n d a b l e s u r f a c e a c i d segments a n d is n o t h y d r o p h o b i c a l l y m o d i f i e d . T h e i r i n f l u e n c e o n the coating's r e c o v e r y response i n the p r o t e c t i v e c o l l o i d s t a b i l i z e d , v i n y l - a c r y l i c l a t e x f o r m u l a t i o n is u n d e r s t a n d a b l e i n t e r m s o f the t h i c k e n e r s d e g r e e o f s o l v a t i o n b y w a t e r a n d d e c r e a s e d selfa s s o c i a t i o n . A s u r p r i s i n g l y s t r o n g r e l a t i o n s h i p (3) b e t w e e n the coating's f l o w o u t a n d the p r o x i m i t y o f the t h i c k e n e r ' s s o l u b i l i t y p a r a m e t e r to that o f w a t e r [ 1 7 - 2 3 ( k c a l / c m ) ] w a s n o t e d . W i t h the h i g h elastic r e s p o n s e o f the l a t e x - a c i d s w e l l a b l e t h i c k e n e r ( w h i c h is n o t h y d r o p h o b i c a l l y m o d i f i e d ) , the i n f l u e n c e o f l a t e x p a r t i c l e s i z e o n the f o r m u l a t i o n ' s r h e o l o g i c a l r e s p o n s e is n e g l i g i b l e . 3 M

T h e o t h e r l a t e x i n v e s t i g a t e d i n this s t u d y (response i l l u s t r a t e d i n F i g u r e 2) is that o f a s m a l l (117-nm) a l l - a c r y l i c r e s i n . S m a l l - p a r t i c l e l a t i c e s w e r e p r e f e r r e d i n the e a r l y 1960s b e c a u s e o f t h e i r b e t t e r p i g m e n t b i n d i n g e f f i c i e n c y . T h e h i g h elastic r e s p o n s e of the s m a l l - p a r t i c l e latex


21.


GLASS

395

1000

900

ε ο c >» •σ &

8oo

< 3 0 0


200

100

°0

20

40

60

80

100

120

140

TIME, sec

Figure 2. Time dependence of G * recovery: 11212 paints; 90 KU. Strain amplitude is 28 μm. Open symbols, 347-nm vinyl acetate-butyl acrylate (BMD) latex with ΗEC-stabilizing segments chemically attached to the surface; closed symbols, 117-nm all-acrylic (SMD) latex. Thickener: (Ο, Φ) acrylic acid-ethyl acrylate copolymer (PAAC), (A, A) HEC; and (Π, M) alkali-swellable latex thickener (MAP). Reproduced with permission from reference 14. Copynght 1978 Federation of Societies for Coatings Technology.

even with the most hydrophilie thickener is the consequence of floeculation of the small-particle latex, which results in poorer flowout, and a film integrity dependent on the thinnest sections of the film. Parameters influencing the flowout time of a coating that could be controlled b y formulation components are shown in equation 1 (4): h = η/σΧ

3

(1)

where t% is the half-life (s) for an amplitude decrease in surface irregular ity; η is the non-Newtonian viscosity (Pa s) (the influence of the elastic component on the response was not recognized); σ is the surface tension at the coating-air interface; and X is the film thickness of the applied coating. T h e film thickness is primarily influenced by the coating's H S V , which can be affected by the median particle size (5) of the latex. Thus, the better pigment binding efficiency of small-particle latices sought by coatings chemists of the 1960s is offset by their poor flowout produced b y unfavorable viscosities at both high and low shear rates (5) (Figure 3).



396


SHEAR STRESS—dynes/cm

2

SHEAR STRESS—dynes/cm

2

Figure 3. Viscosity dependence on particle-size distnbution: (a) high shear rate and (b) low shear rate. Curve A is paint with 0.63-μτη latex. Curve Β is paint with 70% 0.63-μτη latex and 30% 0.103-μπι latex.

A s w a t e r b o r n e latex coatings i m p r o v e d to g r a d u a l l y d o m i n a t e a l l s e g m e n t s o f t h e trade-sales m a r k e t , t h e i n a b i l i t y to a c h i e v e g o o d flowout w i t h s m a l l p a r t i c l e s i z e l a t i c e s w a s m e c h a n i s t i c a l l y i n t e r p r e t e d as a n i n t e r b r i d g i n g p h e n o m e n o n ( 6 - 9 ) . I n t h e e a r l y 1970s, o n e o f t h e m a n u f a c turers o f c e l l u l o s e ethers i n t r o d u c e d (10) S M A T . T h e t e r m o n o m e r was a h y d r o p h o b e - m o d i f i e d styrene m o i e t y obtained f r o m the reaction o f a n o n y l p h e n o l e t h o x y l a t e (40 m o l a v e r a g e ) o x y a n i o n w i t h v i n y l b e n z y l chloride. D u r i n g the same approximate time p e r i o d , H E U R , another h y d r o p h o b e - m o d i f i e d water-soluble p o l y m e r , w a s i n t r o d u c e d to the E u r o p e a n coatings industry. T h e influence of both types of h y d r o p h o b e - m o d i f i e d water-soluble p o l y m e r s o n t h e r h e o l o g y o f a s m a l l p a r t i c l e s i z e l a t e x is i l l u s t r a t e d i n


21.

GLASS


397


F i g u r e 4; t h e v i s c o s i t i e s at l o w shear rates a r e l o w e r a n d v i s c o s i t i e s at h i g h shear rates a r e h i g h e r t h a n o b s e r v e d i n f o r m u l a t i o n s p r e p a r e d f r o m the s a m e g r i n d b u t t h i c k e n e d w i t h a c e l l u l o s e ether. T h e m o l e c u l a r weight, m o l e c u l a r w e i g h t distribution, a n d surface tension of aqueous h y d r o p h o b e - m o d i f i e d , w a t e r - s o l u b l e p o l y m e r s are g i v e n (11) i n T a b l e I I . M e c h a n i s t i c Interpretation of L S V Response. T h e l o w e r L S V s p r o d u c e d b y the h y d r o p h o b i c a l l y m o d i f i e d , w a t e r - s o l u b l e p o l y m e r s c a n b e i n t e r p r e t e d i n t e r m s o f a n elastic r e s p o n s e (as r e f l e c t e d i n first n o r m a l stress d i f f e r e n c e s , N\) o f the c o a t i n g f o r m u l a t i o n . I n d i s p e r s i o n s c o n t a i n i n g o n l y a w a t e r - s o l u b l e p o l y m e r a n d a n H E C - s t a b i l i z e d l a t e x , the s l o p e a p p r o a c h e s a v a l u e o f 2 at l o w shear rates ( F i g u r e 5). T h i s r e l a t i o n s h i p is d e f i n e d (12,13) i n e q u a t i o n 2 N

l

(2)

=

w h e r e Ni is the first n o r m a l stress d i f f e r e n c e ( P a ) , Ψ is its c o e f f i c i e n t , a n d 7 2 i is the shear rate ( s ) . I n a series o f c o m m e r c i a l p a i n t s (14), s u c h a r e l a t i o n s h i p w a s n o t o b s e r v e d ( F i g u r e 6) a n d the Ni b e h a v i o r w a s i n t e r p r e t e d i n t e r m s o f y i e l d stress c h a r a c t e r i s t i c s . I n studies c o m p a r i n g h y d r o p h o b i c a l l y m o d i f i e d , w a t e r - s o l u b l e p o l y m e r a n d c e l l u l o s e ether _ 1

H

SHEAR RATE (s )

Figure 4. Viscosity (Fas) dependence on the shear rate (s~ ) of an interior formulation; 57% PVC, 32% NVV, 120-KU-containing small-particle (117-nm), all-acrylic latex. Water-soluble thickeners: (CD) HEC-MHR; (O) SMAT; (Q) HEUR 708; ( 0 ) HEUR 200; (o ) HEUR 100; (A) HEUR L 75; and ( ν ) HEUR LR 8500. Reproduced with permission from refer ence 11. Copyright 1984 Oil and Colour Chemists' Association. 1


398


Table II. Molecular Weight and Surface Tension Data of Aqueous Solutions (0.1 wt %) of Synthetic Thickeners Thickener


H E U R 100 H E U R 200 H E U R 708 SMAT

M

w

10,848 93,992 37,051 32,331

M„

Polydispersity (M /M )

Surface Tension (mN/m)

8,679 46,638 23,216 6,395

1.3 2.0 1.6 5.1

49 56 54 58

w

n

thickened formulations, the predicted slope is approached in the hydro phobically modified, water-soluble polymer thickened coatings (Fig ure 7 ) ; thus, these thickeners are effective in inhibiting flocculation of the dispersed components and thereby yield stress behavior. T h e elasticity reflected in the complex reflected in Ni-y

modulus recovery data (Figure 4 ) is also

(Figure 8 ) analysis of the same coatings (i.e., the slope

approaches 2 in the protective colloid stabilized, vinyl acetate-acrylate latex formulation when the thickener is the well-solvated acrylic a c i d ethyl acrylate copolymer) paralleling the low G * recovery.

1 0

1 0

1

1

10°

·—

ι — - — . . . . . ..i 10 10 1

S H E A R R A T E , sec

2

.—

I 10?

1

Figure 5. First normal stress difference dependence on the shear rate of aqueous water-soluble polymer solutions thickened to —127 KU. The aqueous solutions also contained a vinyl-acrylic latex in amounts equiva lent to that used in formulation 11212. For latex and thickener identification, see the legend to Figure 2. Reproduced with permission from reference 14. Copyright 1978 Federation of Societies for Coatings Technology.


21.

GLASS

Influence on Bheology of Pigmented Latex Coatings

399

LU Ο Ζ LU OC LU


CO CO LU OC

< OC

O CO DC

1

10P

1

ΙΟ"

10

3

10

1

SHEAR RATE, sec"

Figure 6. Shear-rate dependence of the first normal stress difference (commercial trade-sale paints). Key: •, E-80; Ο, A-95; Δ , F-97;

Influence of Water-Soluble Polymers on Rheology of Pigmented Latex

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