Turnover Studies on Amino Crosslinked Electrocoating Paints

15 Turnover Studies on Amino Crosslinked

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Electrocoating Paints WERNER J. BLANK American C y a n a m i d C o . , Industrial Chemicals and Plastics Division, 1937 West M a i n St., Stamford, C o n n . 06904

Radioactive tracer work was found to be a useful tool in determining the amine balance in an electrocoating tank. The amine which codeposited with the polymer, the amine which was removed from the bath with the drag out liquid, and the amine content of the feed composition were found to determine

the steady state of an electrocoating tank.

Using the data obtained in the radioactive tracer work it was possible to predict accurately the rise in amine content from a 35%

neutralized bath to a steady-state level of 65%

neutralization.

The ability to predict the amine equilibrium

will simplify future turnover studies and will decrease the amount of work required to select an amine for a certain resin and paint

formulation.

i n c r e a s e d d u r i n g t h e last f e w years.

T h e i r p e r f o r m a n c e makes

them

e s p e c i a l l y suitable f o r w h i t e a n d p a s t e l colors a n d f o r a p p l i c a t i o n s r e q u i r i n g i n c r e a s e d o u t d o o r d u r a b i l i t y a n d h i g h detergent resistance. advantage

Their

o v e r self-crosslinked p o l y m e r systems lies i n t h e ease w i t h

w h i c h t h e i r c r o s s l i n k i n g d e n s i t y c a n b e adjusted a c c o r d i n g t o t h e r e q u i r e ments. T h e p a i n t s t u d i e d i n the present w o r k w a s a h i g h reflectivity w h i t e f o r m u l a t i o n b a s e d o n a n a c r y l i c resin, c r o s s l i n k e d w i t h a n a m i n o crossl i n k i n g agent. T h i s f o r m u l a t i o n gives the m i n i m u m r e q u i r e d r e f l e c t i v i t y of 8 5 % f o r l i g h t i n g

fixtures.

I n our previous w o r k ( I ) w e demonstrated

the p e r f o r m a n c e a n d static s t a b i l i t y of s u c h a system.

I n this p a p e r w e

show the consideration for achieving turnover stability. 216

Brewer; Electrodeposition of Coatings Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

15.

BLANK

217

Amino Crosslinked Paints


Experimental Formulation. T h e f o r m u l a t i o n u s e d i n this s t u d y w a s d e v e l o p e d f o r w h i t e a p p l i c a t i o n s r e q u i r i n g h i g h r e f l e c t i v i t y at a b o u t 1 m i l film thickness. This demanded h i g h pigment-to-binder ratio i n t h e electrocoating bath. T h e r a t i o u s e d here w a s 60/100. A s b a c k b o n e r e s i n w e selected a n a c r y l i c p o l y m e r w i t h a n a c i d n u m b e r of a p p r o x i m a t e l y 100. T h i s m a t e r i a l is a v a i l a b l e u n d e r t h e c o m m e r c i a l n a m e o f X C - 4 0 1 0 ( A m e r i c a n C y a n a m i d C o . I n d u s t r i a l C h e m i c a l s a n d Plastics D i v . , W a y n e , N . J. ). T h e c r o s s l i n k i n g agent selected w a s a h e x a ( a l k o x y m e t h y l ) m e l a m i n e w h i c h is a v a i l a b l e u n d e r the c o m m e r c i a l n a m e of C r o s s - L i n k i n g A g e n t 1116 ( C L A 1116) ( A m e r i c a n C y a n a m i d C o . , W a y n e , N . J . ) . T h e r e s i n - t o - c r o s s l i n k i n g agent r a t i o u s e d w a s 72/28. T h e p i g m e n t was a n a l u m i n u m oxide-treated t i t a n i u m d i o x i d e r u t i l e pigment (Unitane OR-600 titanium dioxide) (American C y a n a m i d C o . Pigments D i v . , B o u n d Brook, N . J . ) . T h e n e u t r a l i z i n g a m i n e u s e d w a s d i i s o p r o p a n o l a m i n e . It w a s selected because i t a l l o w s the p r e p a r a t i o n o f stable f e e d materials at l o w n e u t r a l i zations. T h e i n i t i a l n e u t r a l i z a t i o n of the b a t h w a s 35%—i.e., 3 5 % of a l l a v a i l a b l e c a r b o x y l g r o u p s o f the r e s i n w e r e n e u t r a l i z e d w i t h a m i n e . B a t h a n d f e e d f o r m u l a t i o n are s h o w n i n T a b l e I. Table I.

Bath and Feed Formulation

X C - 4 0 1 0 resin C r o s s l i n k i n g agent 1116 Diisopropanolamine Unitane O R - 6 0 0 t i t a n i u m dioxide D e i o n i z e d water B a t h liquid

Bath

Feed

66.7 12.5 4.3 37.5 79.0 —

61.5 9.0 2.3 46.0 81.2

T h e electrocoating p a i n t w a s p r e p a r e d b y g r i n d i n g t h e X C - 4 0 1 0 resin, c r o s s l i n k i n g agent 1116, d i i s o p r o p a n o l a m i n e , a n d t i t a n i u m d i o x i d e b l e n d o n a t h r e e - r o l l m i l l a n d d i s p e r s i n g the r e s u l t i n g viscous paste o n a h i g h s p e e d dissolver. T h e w a t e r m u s t b e a d d e d to t h e paste v e r y s l o w l y to ensure u n i f o r m d i s p e r s i o n . T h e c o m p o s i t i o n o f the f e e d m a t e r i a l w a s b a s e d o n the c o n s i d e r a t i o n that t h e r e d u c t i o n i n b a t h solids takes p l a c e b y d e p o s i t i o n a n d b y d r a g out ( m e c h a n i c a l e n t r a i n m e n t ) of t h e b a t h l i q u i d . T o o b t a i n a steadystate c o n d i t i o n , t h e f e e d m a t e r i a l m u s t h a v e t h e w e i g h t e d average c o m p o s i t i o n of the d e p o s i t e d film a n d t h e d r a g o u t l i q u i d . B y a n a l y z i n g t h e d e p o s i t e d film a n d the d r a g o u t l i q u i d as d e s c r i b e d b e l o w , w e a r r i v e d at a f e e d c o m p o s i t i o n w i t h a p i g m e n t - t o - b i n d e r ratio o f 85/100 a n d a resint o - c r o s s l i n k i n g agent r a t i o o f 76.5/23.5. T h e n e u t r a l i z a t i o n of 2 0 % w a s the lowest possible since at l o w e r levels t h e e m u l s i f i e d p a i n t h a d a v e r y large p a r t i c l e size a n d w a s unstable. T h e c o a t i n g b a t h r e f e r r e d to as b a t h l i q u i d w a s u s e d t o e m u l s i f y the f e e d m a t e r i a l . T h e a m i n e content i n t h e c o a t i n g b a t h e n h a n c e d the e m u l s i f i c a t i o n o f h i g h solids f e e d . Analysis of the Deposited Film. A t t e n u a t e d t o t a l reflectance spec troscopy (2) ( A T R ) w a s u s e d t o a n a l y z e the ester-to-triazine ratio i n t h e


218

E L E C T R O D E P O S I T I O N O F COATINGS


d e p o s i t e d film. T h e p i g m e n t content o f the d e p o s i t e d film w a s d e t e r m i n e d b y solvent w a s h i n g o f the d e p o s i t e d u n c u r e d p a i n t i n t o a c r u c i b l e . A f t e r e v a p o r a t i o n o f the solvent at 105 ° C f o r 2 hours the r e s i n a n d c r o s s l i n k i n g a g e n t w e r e b u r n e d off, a n d t h e i n o r g a n i c r e s i d u e w a s w e i g h e d . Determination of the Amine Content by a Radioactive Tracer Method. PREPARATION OF THE RADIOACTIVE AMINE. T r i t i a t e d d i i s o p r o p a n o l a m i n e ( D I P A ) w a s p r e p a r e d b y e x p o s i n g 0.73 g r a m o f finely g r o u n d i n a c t i v e D I P A to 4.5 curies of p u r e t r i t i u m gas f o r 2 7 d a y s . A f t e r exposure, the r a d i o a c t i v e D I P A w a s degassed several times t o r e m o v e a n y excess t r i t i u m gas. T o r e m o v e a n y w a t e r - e x c h a n g e a b l e t r i t i u m , t h e a m i n e w a s d i s s o l v e d i n 15 m l o f w a t e r , filtered, a n d e v a p o r a t e d t o dryness. T h i s process w a s r e p e a t e d u n t i l t h e specific a c t i v i t y o f t h e D I P A - H ( 3 ) n o l o n g e r c h a n g e d . O n l y t h e t r i t i u m atoms a t t a c h e d t o t h e Ν a t o m a n d Ο of D I P A c a n b e e x c h a n g e d w i t h the h y d r o g e n s o f t h e w a t e r . B y e v a p o r a t i o n o f the w a t e r the e x c h a n g e a b l e t r i t i u m is r e m o v e d as a c o m p o n e n t of the w a t e r , l e a v i n g the stable t r i t i u m - c a r b o n b o n d s u n a f f e c t e d . A f t e r this r e p e a t e d e v a p o r a t i o n process, 0.5751 g r a m o f D I P A w a s r e c o v e r e d a n d d i s s o l v e d i n 10 m l o f d e i o n i z e d w a t e r , t h e n .05 m l o f this s o l u t i o n w a s d i l u t e d w i t h d e i o n i z e d w a t e r to 100 m l . A n a l i q u o t c o n t a i n i n g 2.875 /Agrams D I P A w a s d i s s o l v e d i n a m o d i f i e d B r a y ( 3 ) s o l u t i o n a n d c o u n t e d , g i v i n g a n o b s e r v e d a c t i v i t y o f 38,950 c o u n t s / m i n . W i t h a c o u n t i n g efficiency o f 2 9 % , this a c t i v i t y c o r r e s p o n d e d t o 21 m i l l i c u r i e s / g r a m o r 46,700 a b s o l u t e d i s i n t e g r a t i o n s p e r m i n u t e p e r /*gram o f D I P A . A P a c k a r d M 3375 l i q u i d s c i n t i l l a t i o n spectrometer w a s u s e d f o r t h e c o u n t i n g . F o r a c o n v e n i e n t a n d precise h a n d l i n g , t h e a c t i v e a m i n e w a s b l e n d e d w i t h 1.9250 grams o f i n a c t i v e a m i n e a n d d i l u t e d t o 2 5 m l w i t h deionized water. ADDITION OF ACTIVE AMINE TO THE COATING BATH. F i v e m l o f t h e above-described amine solution was used i n combination w i t h inactive amine to prepare 1 liter of a 1 0 % electrocoating bath. T h e formulation of this b a t h w a s t h e same as d e s c r i b e d f o r t h e f o r m u l a t i o n o f t h e tank, except that t h e n e u t r a l i z a t i o n w a s 2 0 % . T h e n e u t r a l i z a t i o n w a s r a i s e d d u r i n g the e x p e r i m e n t b y a d d i n g i n a c t i v e a m i n e to 40, 60, 80, a n d 1 0 0 % neutralization. SPECIFIC ACTIVITY IN THE DEPOSITED ENAMEL. T h e u n c u r e d electrod e p o s i t e d e n a m e l o n t h e p a n e l w a s a n a l y z e d f o r a m i n e content b y dis s o l v i n g a k n o w n a m o u n t of e l e c t r o d e p o s i t e d p a i n t i n 20 m l of B r a y s o l u t i o n a n d a n a l y z i n g a l i q u o t s o f 0.4-4 m l as d e s c r i b e d a b o v e . Bath Analysis during Turnover. SOLIDS DETERMINATION. A 1 0 - m l sample ( accurately measured ) was taken f r o m the well-stirred bath a n d t r a n s f e r r e d i n t o a w e i g h e d a l u m i n u m d i s h o f 9.5 c m d i a m e t e r , t h e n d r i e d i n a n o v e n f o r 2 0 m i n u t e s a t 175 ° C . P H MEASUREMENTS. T h e p H w a s m e a s u r e d w i t h a n e l e c t r o n i c p H m e t e r w i t h glass electrodes w i t h a n a c c u r a c y o f ± 0 . 0 5 - p H units. CONDUCTIVITY MEASUREMENTS. A W h e a t s t o n e b r i d g e w i t h a fre q u e n c y o f 1000 H z w a s u s e d . T h e p l a t i n u m b l a c k - c o a t e d electrodes of the c o n d u c t i v i t y c e l l gave a m e a s u r i n g r a n g e f r o m 0.1 t o 2000 m i c r o m h o s / c m . T h e measurements w e r e m a d e at 2 5 ° C ± 1 ° C . RESIN-TO-CROSSLINKING AGENT RATIO. s c r i b e d i n the film analysis w a s u s e d .

T h e same m e t h o d


(2) de


15.

BLANK

219


PIGMENT-TO-BINDER RATIO. T e n m l o f b a t h l i q u i d w e r e p i p e t t e d i n t o a t a r e d c r u c i b l e a n d d r i e d f o r 2 hours at 1 2 5 ° C . T h e c r u c i b l e w a s w e i g h e d , the b i n d e r b u r n e d off i n a f u r n a c e , a n d the c r u c i b l e w a s r e w e i g h e d . AMINE TITRATION. T e n m l b a t h l i q u i d w e r e d i l u t e d i n a 2 5 0 - m l b e a k e r w i t h 5 0 m l o f d e i o n i z e d w a t e r ; 2 grams o f T r i t o n X - 1 0 0 surfactant ( R o h m & Haas C o . ) were added, a n d the b l e n d was agitated w i t h a magnetic stirrer f o r 5 m i n u t e s to o b t a i n c o m p l e t e d i s p e r s i o n . T h e b l e n d w a s t i t r a t e d w i t h 1 - m l increments o f 0.1 Ν H C 1 i n t h e c o n v e n t i o n a l m a n n e r to a final p H o f a b o u t 3. F r o m a p l o t o f p H vs. m l H C 1 , the v o l u m e o f H C 1 u s e d to r e a c h p H o f 4.6 w a s d e t e r m i n e d . COULOMBIC YIELD. T h e c o u l o m b i c y i e l d o f the p a i n t w a s d e t e r m i n e d b y d e p o s i t i o n at a constant v o l t a g e o n a n u n t r e a t e d steel p a n e l . D u r i n g the d e p o s i t i o n process the c u r v e o f c u r r e n t vs. t i m e w a s i n t e g r a t e d m a n u a l l y o r w i t h a L e c t r o c o u n t I I integrator ( R o y s o n E n g i n e e r i n g C o . , H a t b o r o , P a . ) . T h e panels w e r e r e w e i g h e d after b a k i n g . T h e c o u l o m b i c y i e l d is expressed i n m g / c o u l o m b . Method of T u r n o v e r Determination. CONTINUOUS COATING APPA RATUS. F o r this s t u d y w e u s e d a n A b r e x ( A b r e x S p e c i a l t y C o a t i n g s L t d . , 280 W y e c r o f t R d . , O a k v i l l e , O n t a r i o , C a n a d a ) d e p l e t o r w h i c h u t i l i z e s a r o t a t i n g steel w h e e l as t h e anode. T h e u n c u r e d p a i n t is c o n t i n u o u s l y s c r a p e d off t h e w h e e l . T h i s m a c h i n e p e r m i t s a h i g h t u r n o v e r rate a n d the d e t e r m i n a t i o n o f the f e e d c o m p o s i t i o n . It does not s i m u l a t e the i n t r o d u c t i o n o f a n y i m p u r i t i e s (e.g., f r o m m e t a l p r e t r e a t m e n t ) u n d e r p r o d u c t i o n c o n d i t i o n s . T h e i n i t i a l t u r n o v e r rate w a s h a l f a t u r n o v e r p e r d a y , a n d i t w a s later i n c r e a s e d to one t u r n o v e r p e r d a y . A t the e n d o f e a c h t u r n o v e r , checks o f a l l possible parameters w e r e m a d e u s i n g t h e t e c h n i q u e s d e s c r i b e d above. T h e w h e e l w a s r u n at a b o u t 1 r p m , w h i c h gives a d r a g out o f a p p r o x i m a t e l y 1 5 % . T h e d e p o s i t i o n voltage w a s 1 3 0 - 1 4 0 volts w i t h a n i m m e r s i o n t i m e o f 2 0 - 2 5 seconds. FEED ADDITION. D u r i n g t h e t u r n o v e r t h e solids fluctuated b e t w e e n 10 a n d 8 % . W h e n the solids l e v e l f e l l t o 8 % , i t w a s i n c r e a s e d to 1 0 % b y a d d i n g a h i g h l y c o n c e n t r a t e d f e e d m a t e r i a l . T h e a m i n e deficient f e e d w a s e m u l s i f i e d o n a h i g h s p e e d dissolver b y t h e s l o w a d d i t i o n o f b a t h l i q u i d . T h e c o m p o s i t i o n o f the f e e d m a t e r i a l r e m a i n e d u n c h a n g e d d u r i n g the entire series o f t u r n o v e r s .

Results Film Composition.

RESIN-TO-CROSSLINKING AGENT RATIO. B a s e d o n

the i n f r a r e d ( I R ) analysis o f the d e p o s i t e d film, a f e e d m a t e r i a l c o n t a i n i n g a r e s i n - c r o s s l i n k i n g agent r a t i o o f 76.5/23.5 w a s u s e d . sponds t o a p p r o x i m a t e l y 8 5 %

This

corre

m i g r a t i o n efficiency a n d c o d e p o s i t i o n o f

the a m i n o c r o s s l i n k i n g agent—i.e., t h e p r o p o r t i o n o f c r o s s l i n k i n g agent i n the d e p o s i t e d b i n d e r w a s 8 5 % o f its p r o p o r t i o n i n the b i n d e r c o n t e n t of the b a t h . PIGMENT-TO-BINDER RATIO. T h e p i g m e n t - t o - b i n d e r r a t i o o f t h e d e p o s i t e d film, b y analysis, w a s 90/100. A s s u m i n g a d r a g o u t o f a b o u t ( f o u n d to be substantially the value i n actual operation)

15%

and the drag

out t o h a v e the same c o m p o s i t i o n as the b a t h , w e a r r i v e d a t a p i g m e n t t o - b i n d e r ratio o f 85/100 f o r the f e e d .


220

E L E C T R O D E P O S I T I O N O F COATINGS

AMINE CODEPOSITION. T h e a m o u n t o f a m i n e c o d e p o s i t e d w i t h t h e p a i n t at t h e a n o d e has a significant i n f l u e n c e o n t h e a m i n e b a l a n c e d u r i n g a t u r n o v e r . T o o b t a i n steady state, t h e a m o u n t o f a m i n e a d d e d d u r i n g the t u r n o v e r m u s t b e t h e same as t h e a m o u n t lost b y e v a p o r a t i o n , d r a g out, a n d c o d e p o s i t i o n . A m i n e losses t h r o u g h e v a p o r a t i o n w e r e f o u n d t o b e n e g l i g i b l e f o r this system. T h e necessary a m i n e content o f t h e f e e d m a t e r i a l d e p e n d s o n t h e h y d r o p h o b i c - h y d r o p h i l i c b a l a n c e o f t h e b i n d e r system a n d t h e s t a b i l i t y


requirements of the feed.

F i g u r e 1 shows that t h e percentage

a m i n e i n t h e d e p o s i t e d film rises w i t h t h e d e p o s i t i o n voltage.

of the T h e or-

% NEUTRALIZATION O F RESIN IN BATH

< 10h "

50

200

100 DEPOSITION V O L T A G E

Figure 1. Radioactive tracer studies on high re flectivity paint. Resin, XC-4010; crosslinking agent, CLA-1116; pigment, Unitane OR-600 titanium di oxide; amine, tritium-labeled diisopropanolamine. d i n a t e i n these graphs gives t h e percentage o f a m i n e i n the film r e l a t i v e to t h e b a t h content as 1 0 0 % . T h u s i f t h e b a t h r e s i n w a s 4 0 % n e u t r a l i z e d , a 1 7 % a m i n e content i n t h e film corresponds t o 6 . 8 % n e u t r a l i z a t i o n i n the

film.

F i g u r e 1 also demonstrates

that t h e percentage

of amine co-

d e p o s i t e d decreases w i t h h i g h e r n e u t r a l i z a t i o n a n d reaches a m i n i m u m at a b o u t 6 0 % n e u t r a l i z a t i o n . I n the p r e v i o u s w o r k ( I ) w e h a v e s h o w n that this l e v e l o f n e u t r a l i z a t i o n p r o v i d e s t h e o p t i m u m i n t h r o w i n g p o w e r ( 4 ). T h e percentage o f a m i n e c o d e p o s i t e d a t n e u t r a l i z a t i o n h i g h e r t h a n 6 0 % a n d h i g h e r voltages increases, thus r e d u c i n g t h e t h r o w i n g p o w e r . I n F i g u r e 2 t h e percentage of a m i n e c o d e p o s i t e d is s h o w n as a f u n c tion of the deposition time.

T h e a m i n e c o n c e n t r a t i o n does n o t change


15.

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Turnover Studies on Amino Crosslinked Electrocoating Paints

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