Advantages and Disadvantages of Associative Thickeners in Coatings

19 Advantages and Disadvantages of Associative Thickeners in Coatings Performance Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch019

F . G. Schwab Coatings Research Group, Cleveland, OH 44114

The introduction of new coatings raw materials is usually accompanied by enthusiastic descriptions of their apparent advantages. In critical evaluations, the stated claims may be confirmed or disproved or new advantages may be discovered. Usually some disadvantages are uncovered during the long and demanding process of formulating, manufacturing, and marketing the resulting products. This review examines the effects of associative thickeners on the performance of waterborne latex coatings, from the consumers viewpoint and from the perspective of the coatings manufacturer.

R E P L A C I N G A N Y RAW M A T E R I A L i n a c o a t i n g s f o r m u l a t i o n r e q u i r e s as the first step the d e t e r m i n a t i o n o f a l l o f the p r o p e r t i e s o n w h i c h the r a w m a t e r i a l has a n e f f e c t . T h i s statement is o b v i o u s b u t is also a p r i n c i p l e that is o f t e n i g n o r e d . S e v e r a l e x a m p l e s i l l u s t r a t e this statement: (1) B a s i c l e a d silicates w e r e f o r m e r l y u s e d i n e x t e r i o r l a t e x w o o d p r i m e r s to i n solubilize w o o d tannins a n d prevent t h e m f r o m staining latex topcoats. W h e n use o f the l e a d c o m p o u n d s i n c o n s u m e r p r o d u c t s w a s p r o h i b i t e d b y l a w , t h e y w e r e r e p l a c e d w i t h v a r i o u s l e a d - f r e e alternates. A l t h o u g h these substitutes b l o c k e d the t a n n i n s , s o m e t i m e s as e f f e c t i v e l y as the l e a d m a t e r i a l s , s o m e f o r m u l a t o r s o v e r l o o k e d the f a c t that the b a s i c l e a d silicates also c o n t r i b u t e d to c o r r o s i o n resistance. A s a r e s u l t , s o m e o f the l e a d - f r e e , s t a i n - b l o c k i n g p r i m e r s e x h i b i t e d severe n a i l h e a d r u s t i n g . (2) T h e r e p l a c e m e n t o f asbestos f i b e r s i n c o a t i n g s is a n o t h e r e x a m p l e . M a n y f o r m u l a t i o n s c o n t a i n i n g asbestos substitutes f a i l to d u p l i c a t e the p e r f o r m a n c e o f the a s b e s t o s - c o n t a i n i n g p r o d u c t s b e c a u s e the f o r m u l a t o r s o v e r l o o k e d e i t h e r the f i l m r e i n f o r c e m e n t o r the c o n t r o l o f c o n s i s t e n c y p r o p erties i m p a r t e d b y asbestos. (3) A f i n a l e x a m p l e i n v o l v e s the n o n i o n i c s u r f a c t a n t s o f the e t h o x y l a t e d p h e n o l t y p e . A f e w years a g o , w h e n 0065-2393/86/0213-0369$06.00/0 © 1986 American C h e m i c a l Society

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

Downloaded by UNIV OF PITTSBURGH on May 3, 2015 | http://pubs.acs.org Publication Date: May 5, 1986 | doi: 10.1021/ba-1986-0213.ch019

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these surfactants w e r e i n short s u p p l y a n d r e p l a c e m e n t s w e r e sought, o u r s t u d y ( u n p u b l i s h e d data) s h o w e d that these p r o d u c t s a f f e c t e d at least e i g h t p a i n t p r o p e r t i e s . L o c a t i o n o f substitutes that a p p e a r e d to b e g o o d e q u i v a l e n t s w a s r e l a t i v e l y easy o n l y t o f i n d o n c r i t i c a l e x a m i n a t i o n that t h e substitutes f a i l e d t o m a t c h the c o n t r o l s i n o n e o r m o r e o f e i g h t p r o p e r t i e s i m p o r t a n t i n a coating's p e r f o r m a n c e . C e l l u l o s e ethers w e r e t h e d o m i n a n t t h i c k e n e r s i n l a t e x p a i n t s f o r s e v e r a l d e c a d e s . T h e i r a t t r i b u t e s are d i s c u s s e d i n C h a p t e r 17. R e c e n t l y , h y d r o p h o b i c a l l y m o d i f i e d ( H M ) , w a t e r - s o l u b l e p o l y m e r s [a g e n e r i c s t r u c t u r e 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 , e t h o x y l a t e d u r e t h a n e ( H E U R ) t y p e is g i v e n i n C h a p t e r 21] w e r e d e v e l o p e d that o v e r c o m e s o m e m a j o r d e f i c i e n c i e s i n h e r e n t w i t h c e l l u l o s e ethers. T h e s e n e w t h i c k eners are r e f e r r e d to as a s s o c i a t i v e t h i c k e n e r s i n the c o a t i n g s i n d u s t r y . A l t h o u g h associative thickeners present a n o p p o r t u n i t y for i m p r o v i n g p a i n t p r o p e r t i e s , t h e y also h a v e s h o r t c o m i n g s i n c o m p a r i s o n w i t h c e l l u lose ethers. T h i s o v e r v i e w , b a s e d o n i n f o r m a t i o n g e n e r a t e d i n m y l a b o r a t o r y , e x a m i n e s the effects o f a s s o c i a t i v e t h i c k e n e r s o f the H E U R t y p e o n the a p p l i c a t i o n p e r f o r m a n c e o f l a t e x p a i n t s ; t h e i r a d v a n t a g e s are r e c o g n i z e d , a n d the c o m p r o m i s e s that m u s t b e c o n s i d e r e d i n r e p l a c i n g the t i m e - p r o v e n c e l l u l o s e ether t h i c k e n e r s are d i s c u s s e d .

Associative Thickeners: Advantages and Disadvantages W h e n the v a r i o u s a s s o c i a t i v e t h i c k e n e r s w e r e i n t r o d u c e d , the c l a i m e d advantages

were

fairly similar. T h e

advantages

and

disadvantages

o b s e r v e d are s u m m a r i z e d i n T a b l e I. Rheology. Paints containing associative thickeners have higher visc o s i t y at the shear rates d e v e l o p e d d u r i n g a p p l i c a t i o n . T h i s p r o p e r t y k e e p s the a p p l i c a t o r f r o m s p r e a d i n g t o o l i t t l e p a i n t o v e r t o o l a r g e a n

Table I. Associative Thickeners: H E U R Type Advantages

Controlled spreading Improved flow, leveling Cost savings Less roller spatter Improved compatibility Resistance to bacterial attack Convenient liquid form

Major

Disadvantages

phase separation Hiring or roll-up sensitivity to variation in formulation changes

Potential

Disadvantages

blocking and softer films freeze-thaw instability color problems water resistanee alkali resistance wet, chalk adhesion exterior durability open time a

3

These properties are real in hydrophobically modifed, alkali-swellable latex (LASHM) thickeners, which have gained acceptance in low-cost interior coatings. If the concern is the scrub resistance of and water-sensitivity of interior coatings, cellulose ethers would be the thickener of choice. In low-cost formulations, if higher viscosities at high shear rates are to be achieved, LASHM-type thickeners are the preferred types. a


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a r e a w i t h a b r u s h o r r o l l e r . C o n s i d e r i n g that the r e c o m m e n d e d f i l m t h i c k n e s s f o r o n e c o a t o f m o s t c o n s u m e r c o a t i n g s is a b o u t as t h i c k as a sheet o f b o n d p a p e r , the i m p o r t a n c e o f p r o p e r f i l m t h i c k n e s s b e c o m e s v e r y a p p a r e n t . A n o t h e r a d v a n t a g e o f a s s o c i a t i v e t h i c k e n e r s is that t h e y i m p a r t e x c e l l e n t f l o w a n d l e v e l i n g to l a t e x p a i n t s , p r o p e r t i e s n o t u s u a l l y attainable w i t h cellulosics. P r o d u c t i o n of latex paints w i t h a l k y d - l i k e f l o w a n d l e v e l i n g is r e l a t i v e l y easy; h o w e v e r , this o p e r a t i o n c a n also cause a p r o b l e m : the l a t e x p a i n t s h a v e a n a f t e r - f l o w t e n d e n c y . T h e user c a n a p p l y the p a i n t , o b s e r v e w i t h s a t i s f a c t i o n its e x c e l l e n t l e v e l i n g , a n d t h e n , s e v e r a l m i n u t e s l a t e r , f i n d n u m e r o u s sags that c a n n o t b e c o r r e c t e d b e c a u s e the p a i n t has a l r e a d y set u p . T h i s p r o b l e m c a n b e a v o i d e d , b u t usually w i t h some c o m p r o m i s e i n other properties. U s i n g associative t h i c k e n e r s p l u s o t h e r a d d i t i v e s ( c e l l u l o s i c s o r c l a y s ) , the f o r m u l a t o r c a n v a r y i n d e p e n d e n t l y the 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 o f a l a t e x p a i n t o v e r a f a i r l y w i d e r a n g e to o b t a i n the d e s i r e d r h e o l o g y o v e r a s p e c t r u m o f p o o r - (i.e., l o w - c o s t ) to e x c e l l e n t - q u a l i t y p a i n t s . C o s t S a v i n g s . A l t h o u g h associative t h i c k e n e r s are m o r e e x p e n s i v e t h a n c e l l u l o s e t h i c k e n e r s , l o w e r p a i n t costs are o f t e n o b t a i n e d w i t h the former thickeners because of their controlled spreading a n d i m p r o v e d f l o w p r o p e r t i e s . B e c a u s e t h i c k e r , s m o o t h e r f i l m s are o b t a i n e d , the v o l u m e s o l i d s o r the p r i m e p i g m e n t ( t i t a n i u m d i o x i d e ) c o n t e n t c a n b e l o w e r e d w i t h o u t a n unfavorable effect o n h i d i n g p o w e r . T h i s feature results i n r e d u c e d r a w m a t e r i a l s costs, i l l u s t r a t e d i n T a b l e I I f o r c o m m e r c i a l p a i n t s that are s o l d f o r the s a m e e n d uses. P a i n t C contains a n associative t h i c k e n e r . Improved

Compatibility. Improved

compatibility with

latices

y i e l d s h i g h e r gloss a n d less p o r o u s f i l m s . I m p r o v e d c o m p a t i b i l i t y w i t h c o l o r a n t s g i v e s b e t t e r a c c e p t a n c e o f t i n t i n g c o l o r s , r e d u c e s the q u a n t i t y o f c o l o r a n t r e q u i r e d , a n d i m p r o v e s the c o l o r s t a b i l i t y . O t h e r A d v a n t a g e s . A n y o n e w h o has a p p l i e d p a i n t w i t h a r o l l e r is a w a r e o f s p a t t e r ( d i s c u s s e d i n C h a p t e r 17), w h i c h is a n u i s a n c e that c a n b e r e d u c e d o r e l i m i n a t e d w i t h a s s o c i a t i v e t h i c k e n e r s . T h i s p r o p e r t y is a n i n h e r e n t a d v a n t a g e r e l a t e d to t h e i r l o w m o l e c u l a r w e i g h t . A n o t h e r a d v a n t a g e is t h e i r e n z y m a t i c s t a b i l i t y . U n l i k e the c e l l u l o s i c s , a s s o c i a tive thickeners d o not p r o v i d e f o o d for bacteria. S o m e associative t h i c k e n e r s are s u p p l i e d i n l i q u i d f o r m , w h i c h offers c o n v e n i e n t ease of i n c o r p o r a t i o n . M a j o r Disadvantages: Phase Separation and Picking. Oil-based p a i n t s u s u a l l y s h o w c l e a r l i q u i d s e p a r a t i o n a n d p i g m e n t s e t t l i n g after a g i n g i n the c a n , a n d the c o n s u m e r w i l l a c c e p t this. L a t e x p a i n t s are c h a r a c t e r i z e d as r e l a t i v e l y free o f s e p a r a t i o n a n d s e t t l i n g . M a n y c o m p a -


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Table I I . C o m m e r c i a l Paints Comparison Property Pounds of T i O / 1 0 0 gallons High-shear viscosity (P) Contrast ratio Drawdown Brushout Spreading rate of brushout (ftVgal.) 2

Paint A

Paint Β

Paint C

336 0.9

283 0.8

218 1.6

0.985 0.972 541

0.969 0.940 570

0.954 0.969 404

a

b

fo

value of 1 equates to complete hiding. Figures are averages from three operator results. Although all three paints had the same recommended spreading rate (400 f t /gal.), only paint C applied at that rate on brushing tests. The others were overspread by more than 35%. Paint C, containing 118 lb less of titanium dioxide/100 gal. than paint A, had approximately equal hiding to paint A because of its greater film thickness when applied by brush. b


2

nies w o u l d n o t a t t e m p t to m a r k e t a n e w l a t e x p a i n t that e x h i b i t s p h a s e s e p a r a t i o n to a n y s i g n i f i c a n t extent. P a i n t s c o n t a i n i n g associative t h i c k eners h a v e a m u c h greater t e n d e n c y to separate t h a n those t h i c k e n e d w i t h a cellulosic. T h e separation can take several forms. A clear l i q u i d l a y e r c a n b e o n t o p , s o m e t i m e s u p to a n i n c h i n d e p t h , a n d the c o l o r a n t c a n also f l o a t i n t o this l a y e r . P i g m e n t s e t t l i n g c a n o c c u r . T h e p h a s e s e p a r a t i o n p r o b l e m is a serious one. M a n y f o r m u l a t i n g v a r i a b l e s h a v e a n e f f e c t o n i t . R e m i x i n g is n o t a c c e p t a b l e to the c o n s u m e r . W h e n s o m e latex p a i n t s c o n t a i n i n g a s s o c i a t i v e t h i c k e n e r s are a p p l i e d b y b r u s h o r r o l l e r o v e r c e r t a i n u n d e r c o a t s o r a g e d p a i n t l a y e r s , the d e p o s i t e d f i l m is p i c k e d u p o n the s e c o n d pass o f the a p p l i c a t o r . T h e n e w l y a p p l i e d p a i n t is thus s t r i p p e d a w a y f r o m the b a s e c o a t . T h i s p r o p e r t y has b e e n t e r m e d " l i f t i n g " , " p i c k i n g " , o r " r o l l u p " . T h i s p r o b l e m r e s u l t e d i n r e m o v a l f r o m the m a r k e t o f a n a s s o c i a t i v e t h i c k e n e r o f the seventies (i.e., S M A T , d i s c u s s e d i n C h a p t e r 20). M o s t f o r m u l a t o r s n o w h a v e p r o p r i e t a r y t e c h n i q u e s f o r b y p a s s i n g this p r o b l e m . O t h e r p o t e n t i a l p r o b l e m s (listed i n T a b l e I) m u s t b e e v a l u a t e d . If the t w o p r o b l e m s d i s c u s s e d c a n b e o v e r c o m e a n d the r h e o l o g y is n o t n e g a t e d b y c e r t a i n f o r m u l a t i o n c o m p o n e n t s ( d i s c u s s e d i n C h a p t e r 20), p o s s i b l y w h a t e v e r other c o m p r o m i s e s necessary c a n b e e n d u r e d , u n t i l g r a d u a l i m p r o v e m e n t s i n the s e c o n d a r y p r o b l e m s c a n b e m a d e .

Problems from a Manufacturing Viewpoint C o a t i n g s f o r m u l a t o r s u n d e r t a k e a m a j o r p r o j e c t i n t r y i n g to r e p l a c e c e l l u l o s i c s w i t h a s s o c i a t i v e t h i c k e n e r s . U s e f u l p r a c t i c e s i n this p r o c e s s i n c l u d e a c a r e f u l s t u d y o f the effects of o r d e r o f a d d i t i o n o f i n g r e d i e n t s , f i l m p r o p e r t y d i f f e r e n c e r e l a t e d to t e s t i n g p r o c e d u r e s , a n d t i m e dependent changes, p a r t i c u l a r l y i n rheological properties. W i t h associa t i v e t h i c k e n e r s , a c o r r e l a t i o n a m o n g f i l m p r o p e r t i e s (i.e., t h i c k n e s s , f l o w a n d l e v e l i n g , gloss, a n d h i d i n g ) o b t a i n e d f r o m f i l m s p r e p a r e d b y s t a n d -


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a r d i n d u s t r i a l tests (i.e., d r a w d o w n b l a d e p r o c e d u r e s ) a n d f i l m s p r e p a r e d b y a c t u a l a p p l i c a t i o n w i t h b r u s h o r r o l l e r m a y n o t agree b e c a u s e the rate o f d e f o r m a t i o n d i f f e r e n c e s b e t w e e n the t w o p r e p a r a t i o n procedures.


W h e n m a n y tens o f t h o u s a n d s o f g a l l o n s o f l a t e x p a i n t h a v e b e e n m a d e i n a p l a n t o v e r the y e a r s u s i n g f a m i l i a r m a t e r i a l s a n d p r o c e d u r e s , c o n v i n c i n g a p r o d u c t i o n m a n a g e r that a n e w m e t h o d is n e c e s s a r y f o r e f f e c t i v e use o f a n e w r a w m a t e r i a l is s o m e t i m e s v e r y d i f f i c u l t . W i t h a s s o c i a t i v e t h i c k e n e r s , n e w m a n u f a c t u r i n g p r o c e d u r e s are f r e q u e n t l y r e q u i r e d . S o m e a s s o c i a t i v e t h i c k e n e r s are v i s c o u s m a t e r i a l s a n d are d i f f i c u l t to h a n d l e i n d r u m s . F r e q u e n t l y , p r e m i x e s o r p r e n e u t r a l i z a t i o n s are r e q u i r e d f o r b e s t results. T h i s step is a n e x t r a p r o d u c t i o n o p e r a t i o n . Variations of m o r e than 1 Ρ i n high-shear viscosity of a f o r m u l a c o n t a i n i n g e x a c t l y the s a m e t y p e s a n d a m o u n t s of i n g r e d i e n t s , b u t m a d e b y d i f f e r e n t p r o c e d u r e s , c a n o c c u r w h e n associative t h i c k e n e r s are e m p l o y e d . T h e o r d e r of a d d i t i o n is p a r t i c u l a r l y c r i t i c a l w i t h a s s o c i a t i v e t h i c k e n e r s . E n s u r i n g that the s p e c i f i e d a d d i t i o n o r d e r is c a r r i e d o u t i n the p l a n t m a y b e d i f f i c u l t . F o r e x a m p l e , p r o d u c t i o n p e r s o n n e l m a y n o t l i k e the o d o r of a s p e c i f i c p r o d u c t a n d t h e r e f o r e a d d the c o m p o n e n t last, regardless o f the s p e c i f i e d p r o c e d u r e s . W i t h a s s o c i a t i v e t h i c k e n e r s , the p a i n t v i s c o s i t y w i l l t a k e l o n g e r to s t a b i l i z e , r e l a t i v e to that f o r f o r m u l a t i o n s t h i c k e n e d w i t h c e l l u l o s e ethers. Q u a l i t y - c o n t r o l p r o b l e m s c o u l d result. A l s o , b o t h l o w - s h e a r a n d h i g h shear viscosities s h o u l d b e m o n i t o r e d f o r p r o p e r q u a l i t y c o n t r o l . B e c a u s e s m a l l v a r i a t i o n s i n s o m e i n g r e d i e n t s ( d i s c u s s e d i n s i m p l e systems o n a f u n d a m e n t a l basis i n C h a p t e r 20) c a n a f f e c t the h i g h - s h e a r - l o w - s h e a r v i s c o s i t y r e l a t i o n s h i p , p r o b a b l y n e w p r o c e d u r e s w i l l h a v e to b e estab l i s h e d to h a n d l e f a c t o r y b a t c h a d j u s t m e n t s . P r o c e d u r e s f o r d e t e r m i n i n g c o m p l e x i n t e r a c t i o n s as a f u n c t i o n o f t i m e s h o u l d b e e s t a b l i s h e d . T h e s i m p l e s t a n d m o s t sensitive tests are r h e o l o g i c a l ones. S o m e associative thickeners require difficult cleanup procedures (I). F o r example, one associative thickener requires a mixture of ethoxye t h a n o l - m e t h a n o l - w a t e r , a n d the l i t e r a t u r e * states that b e s t results are o b t a i n e d at 80 ° C (176 ° F ) , f o l l o w e d b y a h o t w a t e r r i n s e at 49 ° C (120 ° F ) . M a n y p a i n t p l a n t s w o u l d f i n d these r e q u i r e m e n t s e x t r e m e l y i n c o n v e n i e n t . F o r this s a m e t h i c k e n e r , the l i t e r a t u r e r e c o m m e n d s that t r a n s f e r l i n e s b e h e l d b e t w e e n 6 0 - 7 9 ° C (140-175 ° F ) , w h i c h is also n o t practical i n m a n y plants. T h e p o t e n t i a l a d v a n t a g e s to b e g a i n e d w i t h associative t h i c k e n e r s are v e r y i m p o r t a n t ones. T h e y j u s t i f y the v e r y e x t e n s i v e l a b o r a t o r y d e v e l o p m e n t w o r k that is r e q u i r e d to f o r m u l a t e t h e m i n t o w a t e r b o r n e latex coatings. R E C E I V E D for review February 19, 1985. A C C E P T E D December 9,

1985.

*Rohm and Haas Bulletin No. 81A40 "Experimental Rheology Modifier QR-708," pages 6 and 7.


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