Electrodeposition of Coatings

8 Studies in Cathodic Electrodeposition R. A. WESSLING, D. S. GIBBS, W. J. S E T T I N E R I , and E. H. WAGENER

Downloaded by UNIV ILLINOIS URBANA on May 2, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch008

Physical Research Laboratory and Designed Products Department, T h e D o w Chemical C o . , M i d l a n d , M i c h . 48640

Coatings have been electrodeposited

on a steel cathode

from an alkaline bath containing a cationic sulfonium latex. As with quaternary ammonium stabilized latexes, the charge on the polymer particles is independent operation on the alkaline side.

of pH, permitting

In a comparative

study,

however, only the sulfonium latex deposited a smooth, thin (< 1 mil) coating with rapid current cut off and low residual current.

The mechanism of anodic electrodeposition was

reviewed and compared with the results of this study.

The

protonated amine systems appear to destabilize at the elec trode by a neutralization mechanism, in analogy with the neutralization of carboxyl groups in the anodic process, but this mechanism is not available to the sulfonium system. Therefore, a unique mechanism of deposition must be oper ating in the latter case.

/

T p h e process of e l e c t r o d e p o s i t i n g o r g a n i c coatings o n m e t a l A

has b e e n i n v e s t i g a t e d .

cathodes

C a t h o d i c e l e c t r o d e p o s i t i o n has a n u m b e r of

i n h e r e n t advantages over t h e c o m m o n l y u s e d a n o d i c processes, b u t these are best r e a l i z e d i f t h e process c a n b e c a r r i e d o u t i n a n e u t r a l or a l k a l i n e m e d i u m . A r e v i e w of the literature suggests that t h e major p r o b l e m i n d e v e l o p i n g c a t h o d i c e l e c t r o d e p o s i t i o n has b e e n t h e l a c k of c a t i o n i c e m u l sions that are stable a b o v e p H 7 a n d s t i l l deposit e l e c t r i c a l l y i n s u l a t i n g films o n t h e c a t h o d e

surface.

A n i n v e s t i g a t i o n o f t h e c h e m i s t r y of s u l f o n i u m salts i n o u r l a b o r a tories suggested that e m u l s i o n s s t a b i l i z e d b y these groups m i g h t h a v e the d e s i r e d characteristics.

I n recent years, t h e c a p a b i l i t y of p r e p a r i n g

s u c h emulsions has b e e n a c h i e v e d ( I ) . T h i s p a p e r gives a p r e l i m i n a r y a c c o u n t of t h e i r p e r f o r m a n c e i n e l e c t r o d e p o s i t i o n a n d p r o v i d e s f u r t h e r i n s i g h t i n t o t h e m e c h a n i s m of d e p o s i t i o n at t h e cathode.

T h e results

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

8.

WESSLiNG

111

Cathodic Electrodeposition

ET AL.

suggest that significant differences exist b e t w e e n a n o d i c a n d c a t h o d i c processes. E l e c t r o d e p o s i t i o n of o r g a n i c coatings onto m e t a l surfaces has b e e n k n o w n f o r a b o u t 40 years, b u t n o t u n t i l t h e e a r l y 1960s has i t c o m e i n t o i n d u s t r i a l p r o m i n e n c e , l a r g e l y because of the efforts of B r e w e r , B u r n s i d e , a n d c o - w o r k e r s at F o r d M o t o r C o . ( 2 ) . A t present, u s e of this process is g r o w i n g r a p i d l y as e v i d e n c e d b y the n u m b e r of installations a n d patents r e l a t i n g to the process ( 3 , 4).


Review of Anodic Systems T o a p p r e c i a t e the p o t e n t i a l advantages of a c a t h o d i c system, p a r t i c u l a r l y a n a l k a l i n e c a t h o d i c system, a b r i e f r e v i e w of t h e m e c h a n i s m of a n o d i c e l e c t r o d e p o s i t i o n is h e l p f u l .

T h e a n o d i c systems

developed

to date h a v e b e e n g e n e r a l l y b a s e d o n the i n c o r p o r a t i o n of t h e c a r b o x y l a t e a n i o n ( R - C O O " ) either as a p a r t of t h e p o l y m e r m o l e c u l e itself or the surfactant of a n e m u l s i o n . M a n y studies h a v e b e e n m a d e o n the m e c h a n i s m of e l e c t r o d e p o s i t i o n i n v o l v i n g c a r b o x y l a t e s t a b i l i z e d systems (5-11 ), a n d i t is g e n e r a l l y agreed that t h e f o l l o w i n g reactions c o n t r i b u t e to t h e f o r m a t i o n o f a n e l e c t r i c a l l y i n s u l a t i n g film o n t h e a n o d e : (1)

E l e c t r o l y s i s of w a t e r : (a) H 0 -> O H · + H + e~ (b) 2 O H · - » H 0 + Ο · (c) 2 0 · - > 0 2

2

2

(2)

O x i d a t i o n of t h e anode : M ° —> M " + +

(3)

ner

R e a c t i o n of the c a r b o x y l a t e i o n w i t h H + a n d M Ο

Ο

Il

II

R—Ο—Ο- + H+ Ο

Il

* R — C — Ο - + M » + ->

n

+

R—C—OH

ο II

(RC—0)„—M

I n a d d i t i o n , B e r r y ( I I ) has suggested that c o n c e n t r a t i o n c o a g u l a t i o n c o u l d o c c u r s i m u l t a n e o u s l y w i t h the c a r b o x y l a t e reactions s h o w n a b o v e . T h i s m e c h a n i s m w o u l d a l l o w some of the c a r b o x y l a t e salt t o b e d e p o s i t e d i n the film intact. T h i s is a s i m p l i f i e d v i e w of t h e a n o d i c d e p o s i t i o n m e c h a n i s m w h i c h leaves o u t m a n y p a r a m e t e r effects s t u d i e d i n d e t a i l b y w o r k e r s i n t h e field,

b u t i t b a s i c a l l y describes most a n o d i c systems i n that t h e d e p o s i t e d

films a p p e a r t o c o n t a i n the p r o d u c t s p r e d i c t e d b y these reactions. A c -

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

112

ELECTRODEPOSITION OF COATINGS

c e p t i n g this m e c h a n i s m as t h e basic m o d e l f o r a n o d i c e l e c t r o d e p o s i t i o n , several disadvantages b e c o m e i m m e d i a t e l y e v i d e n t . ( 1 ) T h e system is h i g h l y p H d e p e n d e n t . A n y l o w e r i n g o f the p H s i g n i f i c a n t l y b e l o w 7 w o u l d coagulate the system since s t a b i l i z a t i o n de p e n d s o n t h e c a r b o x y l a t e ion's b e i n g present. H i g h p H ' s h a v e b e e n s h o w n t o p r o d u c e u n d e s i r a b l e side effects s u c h as gassing. M o s t systems operate i n a p H r a n g e o f 7 t o 8.7. I n a d d i t i o n , t h e e l e c t r o d e p o s i t i o n p e r f o r m a n c e is sensitive t o changes i n p H since the c a r b o x y l a t e i o n c o n c e n t r a t i o n is p H d e p e n d e n t . ( 2 ) T h e free c a r b o x y l g r o u p p r o d u c e d b y r e a c t i o n w i t h H r e m a i n s i n the c o a t i n g a n d is u s u a l l y not d e c o m p o s e d d u r i n g the b a k i n g step. T h e i o n i z a b l e g r o u p presents a p o i n t o f v u l n e r a b i l i t y to m o i s t u r e t r a n s m i s s i o n t h r o u g h the c o a t i n g .


+

( 3 ) M e t a l ions are i n c l u d e d i n the film. I n the case o f a c o p p e r substrate, this has l e d t o g r e e n coatings (12). C o a t i n g s o n steel m a y b e s i m i l a r l y d i s c o l o r e d (13). T h e m e t a l carboxylates o r m e t a l h y d r o x i d e s (10) f o r m e d i n the c o a t i n g are points of v u l n e r a b i l i t y for m o i s t u r e trans m i s s i o n . T h e m e t a l carboxylates w o u l d b e susceptible to h y d r o l y s i s , w h i c h some w o r k e r s b e l i e v e c o u l d result i n c o r r o s i o n at t h e c o a t i n g - m e t a l i n t e r f a c e (14). ( 4 ) T h e electrolysis o f w a t e r at the a n o d e p r o d u c e s o x y g e n . T h e g e n e r a l l y a c c e p t e d m e c h a n i s m of its f o r m a t i o n i n v o l v e s b o t h the h y d r o x y l r a d i c a l a n d the o x y g e n r a d i c a l . S o m e w o r k e r s (15) r e p o r t that reactions of these species w i t h the c o a t i n g h a v e a n adverse effect o n the c o a t i n g p e r f o r m a n c e p r o p e r t i e s a n d e v e n c h a n g e the r e s i n character i n s o l u t i o n . T h e oleoresinous systems seem t o b e p a r t i c u l a r l y affected a n d r e q u i r e antioxidants. ( 5 ) I n g e n e r a l , r e l a t i v e l y l a r g e a m o u n t s o f base a r e r e q u i r e d t o create the c a r b o x y l a t e a n i o n . I f amines are u s e d , t h e y g e n e r a l l y r e q u i r e f r o m a b o u t 0.3 t o 0.5 m i l l i e q u i v a l e n t s / g r a m o f p o l y m e r solids t o p r o d u c e acceptable performance. T h e s e d i s a d v a n t a g e s h a v e o b v i o u s l y n o t s t o p p e d the g r o w t h of a n o d i c e l e c t r o d e p o s i t i o n . H o w e v e r , i t is i n t e r e s t i n g that the t h e o r e t i c a l solutions to some of these p r o b l e m s h a v e b e e n k n o w n to be o b t a i n a b l e b y c a t h o d i c d e p o s i t i o n , yet n o c o m m e r c i a l c a t h o d i c system has a p p e a r e d .

Review of Cathodic Systems S e v e r a l advantages o f c a t h o d i c e l e c t r o d e p o s i t i o n m a k e i t a w o r t h w h i l e area o f i n v e s t i g a t i o n . R e d u c t i o n takes p l a c e a t t h e c a t h o d e ; there fore, t h e m e t a l surface does n o t i o n i z e a n d r e m a i n s passive d u r i n g t h e d e p o s i t i o n process.

N o ions enter the c o a t i n g e v e n w i t h easily o x i d i z a b l e

substrates. T h e electrolysis of w a t e r at the c a t h o d e p r o d u c e s the h y d r o g e n radical, hydrogen, and hydroxide ion. 2 H 0 + 2 e~ -* 2 O H ~ + 2 H · 2

2 H · —> H

2


8.

WESSLING ET AL.

113


R e a c t i o n o f these species w i t h t h e c o a t i n g m a t e r i a l w i l l p r o b a b l y h a v e a less h a r m f u l effect o n c o a t i n g p r o p e r t i e s t h a n t h e c o r r e s p o n d i n g oxidation reaction mentioned above. M o s t of t h e c a t h o d i c systems d e s c r i b e d i n the l i t e r a t u r e a r e s i m p l e analogs of the a n o d i c resins w h e r e the c a r b o x y l a t e g r o u p ( - C O O " ) has been replaced b y a n amino group ( — N — H ) . +

T h e s e resins a r e p r e p a r e d

i n a n o n a q u e o u s r e a c t i o n a n d post e m u l s i f i e d i n w a t e r to f o r m a n electroc o a t i n g system.

T h e y g e n e r a l l y h a v e a charge/mass

ratio h i g h enough


to m a k e the p o l y m e r w a t e r s o l u b l e o r at least w a t e r d i s p e r s a b l e . C o n v e n - t i o n a l latexes m a d e b y e m u l s i o n p o l y m e r i z a t i o n are n o t c o m m o n l y u s e d . A recent b r o a d r e v i e w (16)

o n t h e c h e m i s t r y a n d t e c h n o l o g y of c a t i o n i c

polyelectrolytes focused o n quaternary a m m o n i u m polyelectrolytes b u t also d e s c r i b e d other types of p o l y e l e c t r o l y t e s s u c h as s u l f o n i u m a n d phosp h o n i u m . T h e l o n g list of i n d u s t r i a l a p p l i c a t i o n s i d e n t i f i e d i n this p a p e r d i d n o t i n c l u d e electrocoating, i n d i c a t i n g that o n i u m i o n p o l y e l e c t r o l y t e s s u i t a b l e f o r this use h a d n o t b e e n d e v e l o p e d . T w o i m p o r t a n t a p p l i c a tions f o r q u a t e r n a r y p o l y e l e c t r o l y t e s , e l e c t r o c o n d u c t i v e coatings a n d a n t i static agents, r e q u i r e t h e p o l y m e r to r e t a i n c h a r g e

for performance.

E l e c t r o c o a t i n g u s u a l l y requires a r a p i d d e s t r u c t i o n of c h a r g e i n o r d e r t o o p t i m i z e t h r o w i n g p o w e r , c u r r e n t efficiency, r e d u c t i o n of gassing, etc. T h i s suggests that the q u a t e r n a r y a m m o n i u m g r o u p a n d e l e c t r o d e p o s i t i o n are n o t v e r y c o m p a t i b l e . I n c o m p a r i s o n w i t h t h e v o l u m i n o u s literature o n q u a t e r n a r y a m m o n i u m p o l y e l e c t r o l y t e s a n d surfactants, the s u l f o n i u m analogs h a v e r e c e i v e d little s t u d y . T h e r e v i e w c i t e d a b o v e i n t r o d u c e d s u l f o n i u m p o l y e l e c t r o l y t e s w i t h t h e statement "the s u l f o n i u m q u a t e r n a r y u n i t does n o t seem to h a v e a n y s p e c i a l efficacy over t h e m o r e r e a d i l y a v a i l a b l e a m m o n i u m q u a t e r n a r y g r o u p / ' It also m e n t i o n s t h e i r c h e m i c a l i n s t a b i l i t y ( c o m p a r e d w i t h q u a t e r n a r y systems) as a d i s a d v a n t a g e . c a t i o n i c surfactants c o m m e n t s :

J u n g e r m a n n (17),

i n his book o n

"It is d i f f i c u l t to i m a g i n e w h a t a d v a n

tages these o n i u m ( non-nitrogeneous-surfactants ) c o m p o u n d s w o u l d pos sess over t h e c o r r e s p o n d i n g a m m o n i u m c o m p o u n d except p e r h a p s i n t h e area of b i o l o g i c a l a c t i v i t y . " T h e s e c o m m e n t s i n d i c a t e a c o m m o n a t t i t u d e t o w a r d the use o f sulfo n i u m salts i n p o l y m e r t e c h n o l o g y . (18),

Nonetheless, as H a t c h p o i n t e d o u t

t h e i n s t a b i l i t y a n d h i g h r e a c t i v i t y of s u l f o n i u m salts are a n a d v a n

tage i n c e r t a i n cases. A s c o n v e n t i o n a l coatings f o r e x a m p l e , s u l f o n i u m b a s e d systems i n c l u d i n g b o t h p o l y e l e c t r o l y t e s (19, 20) 22)

a n d latexes

(21,

c a n b e d e p o s i t e d f r o m aqueous m e d i a a n d easily c u r e d to h y d r o

phobic products. T h e results o f o u r s t u d y i n d i c a t e that the h i g h e r c h e m i c a l r e a c t i v i t y of s u l f o n i u m c o m p o u n d s is also a n a d v a n t a g e i n e l e c t r o d e p o s i t i o n of


114

ELECTRODEPOSITION

o r g a n i c coatings, b u t heretofore

OF

COATINGS

s u c h systems h a v e not b e e n

reported.

T h e c a t i o n i c e l e c t r o d e p o s i t i o n systems w h i c h h a v e b e e n

described

i n the l i t e r a t u r e i n c l u d e b o t h s o l u b l e systems a n d emulsions, b u t a l l h a v e b e e n b a s e d o n n i t r o g e n cations. (1)

T w o types h a v e b e e n r e p o r t e d :

P r o t o n a t e d a m i n e based systems.


—+N

(2)

—

H

Q u a t e r n a r y a m m o n i u m systems. (RV-N

W o r k o n the first t y p e of system is r e p o r t e d i n a series of patents b y Spoor, P o h l e m a n n , a n d c o - w o r k e r s ( 2 3 - 2 5 ) i n w h i c h t h e y describe elec t r o d e p o s i t i o n of b o t h solutions a n d dispersions h a v i n g a as the a t t r a c t i v e v e h i c l e .

—N —H +

cation

I n general, t h e y e m p l o y a m i n o a l k y l esters of

a c r y l i c a c i d s u c h as m o n o ( Ν , Ν - d i m e t h y l a m i n o ) e t h y l

methacrylate(I)

or N - v i n y l i m i d a z o l e ( I I ) as c o m o n o m e r s i n their p o l y m e r i c c o m p o s i t i o n s . T h e cations are f o r m e d b y a d j u s t i n g the p H w i t h either acetic or h y d r o c h l o r i c a c i d to g i v e solutions ( o r opalescent s o l u t i o n s ) w h i c h are s u b s e q u e n t l y electrocoated

o n the cathode.

Their general conditions for

e l e c t r o d e p o s i t i o n are: pH A p p l i e d voltage Solids Electrodeposition time

3.5-6.0 2 0 - 1 0 0 volts 8-10% 2 minutes

T h e y r e p o r t coatings r a n g i n g f r o m 0.46-1.32 m i l s t h i c k after b a k i n g w h i c h h a v e resistance of v a r y i n g degrees to salt w a t e r a n d a l k a l i n e w a t e r . Slater a n d T h o w (26)

d e s c r i b e e l e c t r o c o a t i n g of e p o x y ester d i s p e r

sions u s i n g salted a m i n e d i s p e r s i n g agents, a l l of w h i c h are the

—N —H +

type.

I n t h e i r examples, t h e y describe dispersions m a d e w i t h d i m e t h y l

soya

amine,

1-hydroxyethyl-s-heptadecenyl

hydrogenated tallow amine.

l e c u l a r w e i g h t s to f o r m t h e a m i n e cations. reported

imidazoline and dimethyl

T h e y c l a i m o r g a n i c acids of v a r y i n g m o F o r m i c a c i d is u s e d i n t h e i r

examples.

G e n e r a l c o n d i t i o n s f o r e l e c t r o d e p o s i t i o n are: pH A p p l i e d voltage

3.4-4.2 2 0 0 - 2 5 0 volts


8.

WESSLiNG

ET

AL.

115


Solids Electrodeposition time

10% Continuous coil coating

T h e resultant coatings are 1.0-1.1 m i l s t h i c k o n steel cathodes. T a w n (27)

r e p o r t e d a n i t r o g e n - b a s e d c a t i o n i c system w h i c h c o m

prises a d i s p e r s i o n of a n e p o x y r e s i n w i t h a " p a r t i a l l y n e u t r a l i z e d a m i n o a m i d e or i m i d a z o l i n e ( w h i c h m a y o r m a y not b e p o l y m e r i c ) or w i t h a n e p o x y - a d d u c t thereof." H e c l a i m s several n o v e l advantages, s u c h as r a p i d d e p o s i t i o n at l o w voltages ( 2 0 - 5 0 0 ) , h i g h c u r r e n t efficiencies, v e r y d i l u t e solutions ( 1 % or l e s s ) , a n d coatings w h i c h c u r e at r o o m t e m p e r a t u r e or Downloaded by UNIV ILLINOIS URBANA on May 2, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch008

s l i g h t l y above. N o d a t a are g i v e n , b u t the system p r o b a b l y operates at a p H < 7 b a s e d o n the — N — H structures m e n t i o n e d . +

F o u r a d d i t i o n a l patents o n c a t h o d i c e l e c t r o d e p o s i t i o n h a v e b e e n re c e n t l y issued (28-30),

b u t the t e c h n i q u e s

u s e d i n these cases to

c a t h o d i c d e p o s i t i o n are essentially the same as that d e s c r i b e d

get

above.

T h e s e patents d i f f e r p r i m a r i l y i n r e s i n c o m p o s i t i o n a n d p o s t - c u r i n g mechanisms. A l l of the a b o v e m e n t i o n e d systems are b a s e d o n a m i n e salts. (31)

McCoy

has r e p o r t e d the o n l y case of e l e c t r o d e p o s i t i o n of q u a t e r n a r y a m m o

nium-based emulsions.

Three

specific d e r i v a t i v e s h e m e n t i o n e d

are:

N-dodecylbenzyl-N,N-diethyl-l-ethanol ammonium chloride; l-(2-aminoethyl)-2-l-alkenyl-2-imidazole; and octadecenylmethyl di-2-hydroxyethylammonium chloride. A s p h a l t emulsions s t a b i l i z e d w i t h these emulsifiers w e r e

electrode-

p o s i t e d u n d e r the f o l l o w i n g c o n d i t i o n s : pH A p p l i e d voltage Solids Electrodeposition time

3.0-6.0 3 0 - 1 0 0 volts 10-30% Varied

T h e coatings d e p o s i t e d w e r e c h a r a c t e r i z e d b y w e i g h t / s q i n c h w i t h no thickness g i v e n . T h e d a t a w e r e presented to s h o w that c a t i o n i c e m u l sions deposit coatings at a h i g h e r rate a n d a h i g h e r efficiency t h a n a n i o n i c emulsions. T h e w e i g h t s of asphalt d e p o s i t e d w e r e g e n e r a l l y i n the r a n g e of 200 m g / s q i n c h , w h i c h is r e l a t i v e l y h i g h c o m p a r e d w i t h the results o b t a i n e d f r o m the s u l f o n i u m emulsions u s e d i n this s t u d y . F u r t h e r treat m e n t of M c C o y ' s d a t a is p r e s e n t e d i n the d i s c u s s i o n section. T o the authors' k n o w l e d g e , n o n e of these c a t i o n i c systems is of sig nificant c o m m e r c i a l i m p o r t a n c e t o d a y .

The

r e m a i n d e r of this

paper

demonstrates some of the p r o b l e m s that m i g h t be e n c o u n t e r e d w i t h the systems a b o v e a n d shows that s u l f o n i u m - b a s e d systems c a n

overcome

these difficulties.


116

ELECTRODEPOSITION

O F COATINGS


Experimental T h e m a j o r i t y o f t h e experiments w e r e c o n d u c t e d i n a p o l y e t h y l e n e c e l l 10.7 c m X 7.9 c m X 2.54 c m . T w o , e q u a l size g r a p h i t e anodes w e r e p l a c e d at t h e ends of t h e l o n g axis o f t h e c e l l . A 10.2 c m X 1.27 c m m e t a l s a m p l e w a s p l a c e d e q u i d i s t a n t b e t w e e n t h e g r a p h i t e anodes ( u s u a l l y 4.6 c m ) so that t h e flat p l a n e o f t h e s a m p l e w a s n o r m a l to a l i n e j o i n i n g t h e t w o anodes. T h e p o w e r source w a s c a p a b l e o f p r o d u c i n g three-phase d i r e c t c u r r e n t at 500 volts a n d 40 a m p s . A B a u s c h a n d L o m b V O M - 5 r e c o r d e r w a s u s e d to p l o t c u r r e n t as a f u n c t i o n of t i m e . T h e 20 i n c h e s / m i n u t e s p e e d w a s u s e d f o r a l l experiments to e x p a n d t h e c u r r e n t - t i m e profile best. O c c a s i o n a l l y , a s e c o n d V O M - 5 r e c o r d e r w a s u s e d i n c o n j u n c t i o n w i t h a s t a n d a r d c a l o m e l e l e c t r o d e to p l o t t h e voltage d r o p b e t w e e n t h e c a t h o d e a n d t h e s o l u t i o n as a f u n c t i o n of t i m e . T h e area u n d e r the c u r r e n t - t i m e c u r v e w a s i n t e g r a t e d u s i n g a n I n f o t r o n i c s C R S - 1 1 0 integrator p r o v i d e d w i t h a V i c t o r D i g i t - m a t i c data printer. A Microflex t i m e r w a s p l a c e d i n t h e c i r c u i t so that t i m e i n t e r v a l s as short as 1/10 of a s e c o n d u p to 2 m i n u t e s c o u l d b e a c c u r a t e l y r e p r o d u c e d . T h e system w a s a u t o m a t e d so that a b u t t o n a c t i v a t e d t h e t i m e r w h i c h a l l o w e d a preset v o l t a g e to b e a p p l i e d across t h e c e l l f o r a p r e c i s e i n t e r v a l , at t h e e n d o f w h i c h t h e i n t e g r a t e d area ( c o u l o m b s ) w a s p r i n t e d out. E l a b o r a t e safety controls w e r e p r o v i d e d f o r t h e system. D a t a o n c o u l o m b s , c o a t i n g w e i g h t , a n d c u r r e n t efficiencies represent t h e average of f o u r experiments. T h e m e t a l u s e d f o r this s t u d y w a s steel w i t h B o n d e r i t e 37 c o a t i n g . E a c h s a m p l e w a s a c c u r a t e l y c u t so that t h e area of t h e s a m p l e c o a t e d v a r i e d no m o r e t h a n 1 % . F o r s m a l l c o u p o n s this area w a s 7.3 s q c m . I n a t y p i c a l e x p e r i m e n t , 70 grams of m a t e r i a l at 1 0 % solids w e r e p l a c e d i n t h e c e l l c o n t a i n i n g t h e m e t a l sample. T h e p o w e r source w a s preset f o r 200 volts, a n d t h e e x p e r i m e n t w a s a c t i v a t e d as d e s c r i b e d above. A t the e n d of 2 minutes, the sample was removed f r o m the solution a n d rinsed thoroughly w i t h deionized water. T h e sample was air d r i e d a n d w e i g h e d . T h e c u r r e n t efficiency w a s d e t e r m i n e d f o r e a c h s a m p l e f r o m the w e i g h t of p o l y m e r d e p o s i t e d p e r c o u l o m b ( c ) passed b y t h e f o l l o w i n g equation: C u r r e n t efficiency = Weigjrt (mg) coulombs P e r c e n t solids ( t o t a l w e i g h t b a s i s ) , p H , c o n d u c t i v i t y , s t a b i l i t y of the latexes, a n d emulsifier coverage w e r e d e t e r m i n e d b y s t a n d a r d techniques. P a r t i c l e size w a s d e t e r m i n e d b y l i g h t scattering ( B r i c e - P h o e n i x ) a n d w a s occasionally checked b y electron microscopy.

Results T o supplement the l i m i t e d experimental data published o n the nitro gen-based systems, several latex a n d w a t e r - s o l u b l e systems w e r e synthe sized a n d characterized.

The

— N—H +

literature was somewhat


vague

8.

ET AL.

WESSLiNG

117


about the colloidal nature of their systems, referring to them as "disper sions" or "opalescent solutions." In this work, a latex svstem with well defined colloidal properties was synthesized using a — N — H emulsifier +


(Figure 1). T h e latex characteristics are also given.

CP Figure 1.

I +

Schematic of an —N —H +

stabilized latex

Latex characteristics: Composition 60 butyl acryhte/40 styrene Emulsifier concentration 0.10 meq/gram polymer Particle size 1100 A Solids 10% pH 2.5

1.0 .9 .8

200 Volts - 2 Minutes Bonderite 37 Room Temperoture

.7 cn-6

ΦΙ

.4

- Ν - Η LATEX I

.3 .2 .1 5

10

15

SECONDS Figure 2.

I

Current vs. time for — N—Η +

stabilized htex


118

ELECTRODEPOSITION

O F COATINGS

T h e c u r r e n t - t i m e c u r v e f o r this system is s h o w n i n F i g u r e 2. T h e c u r v e shows a r a p i d cutoff of current a n d l o w r e s i d u a l c u r r e n t w h i c h i n d i c a t e s that the system w o u l d h a v e g o o d t h r o w i n g p o w e r (32). T h e e l e c t r o d e p o s i t i o n p e r f o r m a n c e d a t a are s h o w n i n T a b l e I.


Table I.

— N — H Latex Electrodeposition Performance +

C o a t i n g weight C o a t i n g thickness C o a t i n g appearance C o a t i n g efficiency R e s i d u a l current

0.97 mg/sq c m 0.6 m i l smooth, uniform 8.3 mg/c 0.14 ma/sq c m

A s n o t e d earlier, these experiments w e r e r u n at a p H of 2.5. T h e i r per f o r m a n c e c o u l d n ' t b e e v a l u a t e d above a p H of 7 o w i n g to c o a g u l a t i o n of t h e latex. T w o q u a t e r n a r y a m m o n i u m systems w e r e s y n t h e s i z e d : a w a t e r s o l u b l e p o l y e l e c t r o l y t e a n d a latex ( F i g u r e s 3 a n d 4 ) . T h e p o l y m e r c o m p o s i t i o n a n d o n i u m i o n structure w e r e m a d e as n e a r l y a l i k e as pos sible to a v o i d effects f r o m s t r u c t u r a l changes. T h e t w o systems d i f f e r e d significantly i n t h e n u m b e r of charges p e r u n i t mass of p o l y m e r as w o u l d b e expected i n c o m p a r i n g a s o l u b l e p o l y e l e c t r o l y t e a n d a latex. T h e w a t e r - s o l u b l e system was electrodeposited at 5 % solids a n d w a s s t a b i l i z e d w i t h 2.1 m e q of the q u a t e r n a r y m o i e t y p e r g r a m of p o l y m e r . T h e resultant c u r r e n t - t i m e c u r v e is s h o w n i n F i g u r e 5. A s t h e c u r v e indicates, the w a t e r - s o l u b l e system r e m a i n s c o n d u c t i v e a n d deposits o n l y a g e l - l i k e c o a t i n g , r o u g h a n d b u b b l y i n a p p e a r a n c e a n d generally u n a c c e p t a b l e as

Figure 3.

Schematic of an ( R ^ N polymer

+

stabilized water-soluble


8.

WESSLING ET AL.

119


+

3

Figure 4. Schematic of an ( R ^ N stabilized htex Latex characteristics: Composition 60 butyl acrylatel40 styrène Emuhifier concentration 0.04 meq/gram polymer Particle size 1100 A Solids 10% pH 7.7


+

a m e t a l c o a t i n g . T h e b a t h t e m p e r a t u r e rose r a p i d l y as t h e c u r r e n t c o n t i n u e d to flow. V a r y i n g solids a n d voltage d i d v e r y l i t t l e to give a better c o a t i n g f r o m this system. T h e q u a t e r n a r y a m m o n i u m latex, o n t h e other h a n d , p r o d u c e d a significantly better c u r r e n t - t i m e c u r v e as s h o w n i n F i g u r e 5. T h e c u r r e n t d i d d r o p , b u t t h e e l e c t r o d e p o s i t i o n p e r f o r m a n c e of t h e q u a t e r n a r y a m m o n i u m latex w a s n o t as g o o d as t h e e l e c t r o d e p o s i t i o n p e r f o r m a n c e of the — N — H latex as s h o w n i n T a b l e I I . +

0 LI 0 Figure 5.

I

5

I

10 SEC0N0S

I

15

L

20

Current-time curves of water-soluble and emulsion quaternary ammonium systems


120

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

Table II.

Electrodeposition Performance for — N ( R ) 3 +

and — N — H Latexes +

+

-N(R),


C o a t i n g weight C o a t i n g thickness C o a t i n g appearance C u r r e n t efficiency Residual current

7.3 m g / s q c m 4.0 m i l s rough, bubbly 42 mg/c 0.82 m a / s q c m

0.97 m g / s q c m 0.6 smooth, uniform 8.3 mg/c 0.14 m a / s q c m

S i n c e t h e q u a t e r n a r y a m m o n i u m systems are f u l l y i o n i z e d , the sta b i l i t y o f these latexes is m o r e i n d e p e n d e n t o f p H . T h e d a t a c o m p a r i n g the e l e c t r o d e p o s i t i o n p e r f o r m a n c e i n T a b l e I I s h o u l d b e q u a l i f i e d o n t w o p o i n t s : the p H of t h e q u a t e r n a r y system w a s 7.7 vs. 2.5 f o r t h e — N — H +

latex, a n d t h e e m u l s i f i e r c o n c e n t r a t i o n w a s h i g h e r (0.1 m e q / g r a m

vs.

0.04 m e q / g r a m ) f o r the — N — H system. H o w e v e r , the p e r f o r m a n c e of +

the q u a t e r n a r y a m m o n i u m latex system d i d n o t i m p r o v e at l o w e r p H or h i g h e r emulsifier c o n c e n t r a t i o n .

The — N — H +

system w o u l d p r o b a b l y

p e r f o r m e v e n better w i t h respect t o c u r r e n t efficiency i f the emulsifier c o n c e n t r a t i o n w e r e l o w e r e d to 0.04 m e q / g r a m . s i o n that the — N — H +

T h e r e f o r e , the c o n c l u

system p e r f o r m s better w i t h respect t o d e s i r a b l e

c o a t i n g thickness, r a p i d c u r r e n t cutoff, l o w r e s i d u a l a m p e r a g e , film a p pearance, a n d coating w e i g h t remains v a l i d . In

g e n e r a l , t h e d a t a o n t h e e l e c t r o d e p o s i t i o n p e r f o r m a n c e of t h e

n i t r o g e n b a s e d systems c a n b e s u m m e d u p as f o l l o w s :

System

Electrodeposition Performance

— N + — H Solution

Good

— N + — H Latex

Good

(R) N

Solution

Unacceptable

(R) N

Latex

Fair

4

4


8.

WESSLiNG

ET

121


AL.

A s d i s c u s s e d later, the differences b e t w e e n the — N — H s o l u t i o n a n d +

+

N(R)

s o l u t i o n p e r f o r m a n c e is p r o b a b l y c a u s e d b y the l a c k of a s i m p l e

4

charge d e s t r u c t i o n m e c h a n i s m :

ι +

ι

— N H + O H - -• — Ν

I f o r the N ( R ) Downloaded by UNIV ILLINOIS URBANA on May 2, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch008

+

the N ( R ) +

dergo

4

4

+

H 0 2

I

s o l u t i o n . B e t t e r p e r f o r m a n c e of the N ( R ) +

4

latex over

s o l u t i o n c a n b e a t t r i b u t e d to the f a c t that the latex c a n u n

destabilization

(concentration

coagulation)

to

deposit

a

film

whereas a w a t e r - s o l u b l e p o l y m e r w o u l d not r e a d i l y coagulate. A s u l f o n i u m p o l y e l e c t r o l y t e (essentially the same as the p r e v i o u s l y d e s c r i b e d q u a t e r n a r y a m m o n i u m p o l y e l e c t r o l y t e except f o r t h e o n i u m i o n s t r u c t u r e ) s t a b i l i z e d w i t h 2.3 m e q of the s u l f o n i u m m o i e t y p e r g r a m of p o l y m e r w a s s y n t h e s i z e d a n d subjected to the same e l e c t r o d e p o s i t i o n c o n d i t i o n s d e s c r i b e d earlier f o r the q u a t e r n a r y a m m o n i u m p o l y e l e c t r o lyte. T h e p e r f o r m a n c e u n d e r these c o n d i t i o n s w a s not s i g n i f i c a n t l y better t h a n that of the q u a t e r n a r y a m m o n i u m p o l y e l e c t r o l y t e . T h e r e f o r e ,

atten

t i o n w a s c e n t e r e d o n the s u l f o n i u m latex systems w h i c h gave s u p e r i o r coatings. T h e s u l f o n i u m latex w a s c o m p a r a b l e w i t h the q u a t e r n a r y a m m o n i u m latex a b o v e w i t h respect to p o l y m e r c o m p o s i t i o n , p a r t i c l e size, emulsifier structure, emulsifier c o n c e n t r a t i o n , a n d p o l y m e r c o n c e n t r a t i o n . ference a g a i n w a s i n the o n i u m i o n structure.

T h e dif

T h e latex

characteristics,

c u r r e n t - t i m e curves, a n d e l e c t r o d e p o s i t i o n p e r f o r m a n c e

are s h o w n i n

F i g u r e 6.

N o t e that the s u l f o n i u m system was r u n u n d e r a l k a l i n e c o n

d i t i o n s at a p H of 7.6. U n d e r the i d e n t i c a l e l e c t r o d e p o s i t i o n c o n d i t i o n s u s e d f o r the q u a ternary a m m o n i u m s t a b i l i z e d latex, the s u l f o n i u m s t a b i l i z e d latex

+

^^S-(R)

2

Figure 6. Schematic of an (R) S stabilized latex Latex characteristics: Composition 60 butyl acrylatel40 styrene Emulsifier concentration 0.04 meq I gram polymer ? article size 1100 A Solids 10% pH 7.6 3

+


de-

122

ELECTRODEPOSITION OF COATINGS

p o s i t e d a film w i t h better t h r o w i n g p o w e r (as i n d i c a t e d b y t h e slope of the c u r v e s )

( 3 2 ) , l o w e r r e s i d u a l c u r r e n t , a n d better

appearance.

In

a d d i t i o n , less heat w a s g e n e r a t e d i n t h e b a t h t h a n w i t h t h e q u a t e r n a r y a m m o n i u m latex. T h e film w a s s m o o t h a n d u n i f o r m w i t h a thickness i n the 1.0 m i l range.

T h e c o m p a r a t i v e results are s h o w n i n F i g u r e 7 a n d


Table III.

SECONDS Figure 7.

Current-time curves for sulfonium and quaternary ammonium latexes

Table III.

Electrodeposition Performance for — S ( R ) 2 and — N ( R ) Latexes +

+

3

-S(R) C o a t i n g weight C o a t i n g thickness C o a t i n g appearance C u r r e n t efficiency Residual current

2

1.6 mg/sq c m 0.8 m i l smooth, uniform 44 m g / c 0.09 m a / s q c m

—N(R), 7.3 m g / s q c m 4.0 m i l s rough, b u b b l y 42 mg/c 0.82 ma/sq c m

Discussion B e f o r e d i s c u s s i n g t h e q u a t e r n a r y a m m o n i u m vs. s u l f o n i u m some c o m m e n t s s h o u l d b e m a d e a b o u t t h e u t i l i t y o f t h e — N — H +


latexes, systems.


8.

WESSLING

123


ET AL.

A t first glance t h e y are attractive systems since b o t h solutions a n d latex deposit g o o d films w i t h g o o d e l e c t r o d e p o s i t i o n p e r f o r m a n c e . Some of these systems are b e i n g field e v a l u a t e d a l t h o u g h n o c o m m e r c i a l use has b e e n a d o p t e d . T h e reason p r o b a b l y lies i n the l a c k of a clear advantage over t h e a n o d i c systems. T h e d a t a of W i s m e r a n d Bosso ( 3 3 ) i n d i c a t e that t h e p o s t u l a t e d inherent c a t h o d i c advantages of n o o x i d a t i o n of the substrate ( n o m e t a l ions i n the film) a n d n o o x i d a t i o n of the film are v a l i d . H o w e v e r , these advantages m a y b e o u t w e i g h e d b y t h e fact that these systems must operate at a n a c i d p H w h e r e c o r r o s i o n of the e q u i p m e n t a n d p o s s i b l y the m e t a l b e i n g coated w o u l d b e a p r o b l e m . I n a d d i t i o n , t h e p e r f o r m a n c e of these systems, l i k e t h e p e r f o r m a n c e of a n o d i c c a r b o x y l a t e ( C O O " ) systems, w o u l d b e v e r y p H sensitive. T h e b i g a d v a n t a g e of the q u a t e r n a r y a m m o n i u m a n d s u l f o n i u m sys tems over the — N — H systems is less sensitivity to p H . T h e q u a t e r n a r y +

salts, b e i n g strong electrolytes, c a n b e u s e d above p H 7. T h e i r h i g h degree of i o n i z a t i o n results i n t h e c a t i o n c o n c e n t r a t i o n b e i n g m o r e c o n stant d u r i n g p H changes i n t h e b a t h . T h i s s h o u l d b e reflected i n m o r e stable e l e c t r o d e p o s i t i o n p e r f o r m a n c e . T h e s e advantages, w h e n a d d e d t o the i n h e r e n t c a t h o d i c process advantages, m a k e the q u a t e r n a r y systems w o r t h i n v e s t i g a t i n g . Y e t , of the q u a t e r n a r y a m m o n i u m a n d s u l f o n i u m systems i n v e s t i g a t e d , o n l y the s u l f o n i u m latex p e r f o r m e d w e l l e n o u g h to c o m p e t e w i t h the c o m m e r c i a l a n o d i c systems. A s m e n t i o n e d , one possible reason f o r the p o o r b e h a v i o r of t h e q u a ternary a m m o n i u m system is that t h e s i m p l e a c i d - b a s e charge d e s t r u c t i o n m e c h a n i s m of the — N — H a n d a n o d i c systems is n o longer a v a i l a b l e . I n +

the a n o d i c systems, protons are p r o d u c e d b y the electrolysis of w a t e r , w h i c h reacts w i t h the i n c o m i n g c a r b o x y l a t e s t a b i l i z e d resin to f o r m a water i n s o l u b l e p r o d u c t . Ο

II

II

C—OH

C—Ο- + H+ - * Water soluble

Water insoluble

S i n c e h y d r o x y l ions are generated at t h e cathode, a s i m i l a r m e c h a n i s m c o u l d b e e n v i s i o n e d f o r the — N — H +

systems, w h e r e

N+—Η + - O H - *

Ν + H 0 2


124

ELECTRODEPOSITION

OF

COATINGS

the c h a r g e is d e s t r o y e d b y the a c i d - b a s e r e a c t i o n a b o v e . T h i s m e c h a n i s m is a v a i l a b l e to b o t h a m i n e p o l y e l e c t r o l y t e s a n d a m i n e - s t a b i l i z e d emulsions. T h e q u a t e r n a r y a m m o n i u m systems, o n the other h a n d , are not desta b i l i z e d b y p H changes. T h e r e f o r e , a different m e c h a n i s m is r e q u i r e d to get d e p o s i t i o n of a n o n - c o n d u c t i v e p o l y m e r out of a q u a t e r n a r y a m m o n i u m s t a b i l i z e d system. O u r results s h o w that q u a t e r n a r y a m m o n i u m p o l y e l e c t r o l y t e s deposit c o n d u c t i v e , w a t e r s w o l l e n gels. T h i s correlates w i t h t h e i r p e r f o r m a n c e as e l e c t r o c o n d u c t i v e resins (16), w h i c h is b a s e d o n the r e t e n t i o n of the R N g r o u p i n t h e d r y p o l y m e r . I n o u r studies, the fact that r e s i d u a l q u a t e r n a r y a m m o n i u m g r o u p s r e m a i n i n the c o a t i n g w a s c o n f i r m e d b y mass s p e c t r a l analysis.


4

+

S i n c e the w a t e r - s o l u b l e q u a t e r n a r y a m m o n i u m p o l y e l e c t r o l y t e s are lyophilic colloids, they can be irreversibly coagulated only b y destroying the c h a r g e d groups. A p p a r e n t l y this does not o c c u r i n the electrodeposi t i o n process of q u a t e r n a r y a m m o n i u m , so the p o l y e l e c t r o l y t e is i n effect o n l y c o n c e n t r a t e d at t h e electrode u n d e r the influence of the a p p l i e d electric field. It is u n l i k e l y that a n y t h r o w i n g p o w e r c o u l d b e a c h i e v e d w i t h s u c h a system. T h e q u a t e r n a r y a m m o n i u m s t a b i l i z e d latex, h o w e v e r , is a l y o p h o b i c c o l l o i d . T h e r e f o r e , the m e c h a n i s m s of c o n c e n t r a t i o n c o a g u l a t i o n a n d elec t r o l y t e c o a g u l a t i o n are a v a i l a b l e to d e s t a b i l i z e the system. U n d e r the c o n d i t i o n s u s e d i n o u r experiments, electrolyte c o a g u l a t i o n seems the less l i k e l y of the t w o . H y d r o x i d e ions are generated at the electrode surface b u t p r o b a b l y not i n sufficient q u a n t i t y to flocculate the latex particles at the interface. C o n c e n t r a t i o n c o a g u l a t i o n is the better p o s s i b i l i t y . B o t h the sulfo n i u m and quaternary a m m o n i u m can be irreversibly coagulated simply b y d r y i n g . A cast layer of latex q u i c k l y coalesces i n t o a c o n t i n u o u s , w a t e r i n s o l u b l e film as the w a t e r is r e m o v e d . T h e forces b r i n g i n g a b o u t coales c e n c e at the electrode surface are of course q u i t e different, b u t the same g e n e r a l p r i n c i p l e a p p l i e s . If t h e latex particles are b r o u g h t close e n o u g h together, the d i s p e r s i o n forces cause i r r e v e r s i b l e coalescence, b u t to d o this, the electrostatic r e p u l s i o n b e t w e e n t h e c h a r g e d particles m u s t b e o v e r c o m e . I n e l e c t r o d e p o s i t i o n , this is d o n e b y the process of electroendo-osmosis. T h e a b o v e q u a t e r n a r y a m m o n i u m latex system, e v e n t h o u g h it does h a v e some t h r o w i n g p o w e r because of d e s t a b i l i z a t i o n , deposits a r e l a t i v e l y t h i c k c o a t i n g w h i c h is r o u g h a n d b u b b l y i n appearance. I n the c u r r e n t s t u d y , 7.3 m g / s q c m of p o l y m e r w e r e d e p o s i t e d . T h i s c o a t i n g w a s 4.0 m i l s t h i c k . A d d i t i o n a l d a t a s h o w i n g that q u a t e r n a r y a m m o n i u m systems deposit t h i c k coatings comes f r o m the s t u d y of M c C o y (31) m e n t i o n e d earlier. U s i n g N - d o d e c y l b e n z y l - ^ I V - d i e t h y l - i V - e t h a n o l a m m o n i u m c h l o r i d e as the emulsifier, h e reports the f o l l o w i n g :


8.

WESSLING

ET AL.

Electrodeposition Time, minutes

pH 5.4 5.0 4.5 3.0

125

Cathodic Electrodeposition Asphalt mg/sq

1.0 2.0 5.0 3.0

Deposited, in./min

Asphalt mg/sq in.

82 70 48 67

Deposited, {mg/sq cm)

82 140 240 201

(11.2) (19.2) (32.8) (27.5)

T h e s e w e i g h t s are c o n s i d e r a b l y h i g h e r p e r square i n c h t h a n those f o u n d i n this s t u d y . I n s t i l l another e x a m p l e , h e reports that a n e m u l s i o n sta bilized w i t h octadecenyl

methyl di-2-hydroxyethylammonium chloride


d e p o s i t e d 2,660 m g / s q i n c h after 30 m i n u t e s . A p p a r e n t l y the q u a t e r n a r y a m m o n i u m latex systems r e t a i n e n o u g h charge i n the d e p o s i t i n g l o n g e n o u g h to a l l o w the film to b u i l d u p to r e l a t i v e l y t h i c k

film

coatings

w h i c h w o u l d b e u n a c c e p t a b l e f o r most e l e c t r o d e p o s i t i o n a p p l i c a t i o n s . E v e n i f these t h i c k coatings w e r e acceptable, i t is d o u b t f u l that these films w o u l d h a v e the d e s i r e d w a t e r a n d c o r r o s i o n resistance.

T h e residual

q u a t e r n a r y a m m o n i u m groups w o u l d a l l o w w a t e r to permeate the c o a t i n g even faster t h a n r e s i d u a l c a r b o x y l or a m i n e groups o w i n g to their h i g h i o n i c character.

Since they are r e l a t i v e l y stable, t h e r m a l d e c o m p o s i t i o n

of the r e s i d u a l q u a t e r n a r y a m m o n i u m groups w o u l d b e difficult. A s s h o w n i n F i g u r e 3 a n d T a b l e I I I , the s u l f o n i u m latex gave a n excellent c o a t i n g w i t h g o o d e l e c t r o d e p o s i t i o n p e r f o r m a n c e .

T h e differ

ence i n p e r f o r m a n c e b e t w e e n the s u l f o n i u m a n d q u a t e r n a r y a m m o n i u m latexes c o u l d b e a t t r i b u t e d to the h i g h e r c h e m i c a l r e a c t i v i t y of the sulfo n i u m g r o u p w h i c h causes m o r e r a p i d emulsifier g r o u p d e s t a b i l i z a t i o n r e s u l t i n g i n m o r e r a p i d c o a g u l a t i o n of the latex. S e v e r a l m e c h a n i s m s f o r charge d e s t r u c t i o n f o r q u a t e r n a r y t y p e sys tems c o u l d b e i n v o l v e d d u r i n g e l e c t r o d e p o s i t i o n . M o s t f a v o r the s u l f o n i u m g r o u p s r e a c t i n g at a h i g h e r rate.

Investigations into these m e c h a n i s m s

are c o n t i n u i n g a n d w i l l b e r e p o r t e d i n subsequent papers.

I n i t i a l results

i n d i c a t e that several m e c h a n i s m s m a y b e o p e r a t i n g either

concurrently

or i n d e p e n d e n t l y i n sequence d u r i n g t h e v e r y short t i m e i t takes to deposit a coating.

Conclusions A n e w a p p r o a c h to e l e c t r o c o a t i n g has b e e n f o u n d w h i c h i n v o l v e s c a t i o n i c e l e c t r o d e p o s i t i o n of a n o r g a n i c c o a t i n g over a w i d e p H range. T h i s a p p r o a c h i n v o l v e s the use of a s u l f o n i u m system w h i c h , u n d e r e q u a l c o n d i t i o n s , o u t p e r f o r m s the q u a t e r n a r y a m m o n i u m systems r e p o r t e d p r e viously.

A different e l e c t r o d e p o s i t i o n m e c h a n i s m is p r o b a b l y i n v o l v e d

f r o m that suggested f o r the a n o d i c systems a n d the — N — H +

systems.


cathodic

126

ELECTRODEPOSITION

O F COATINGS

T h e c a t h o d i c s u l f o n i u m system has m a n y advantages over t h e c o m m e r c i a l a n o d i c systems: ( a ) I t is a latex system w h i c h a l l o w s w e l l - c h a r a c t e r i z e d c o l l o i d a l properties a n d p o l y m e r c o m p o s i t i o n . F i n n a n d M e l l (34) s h o w e d that the e l e c t r i c a l c u r r e n t efficiency f o r latex systems is p o t e n t i a l l y s i g n i f i c a n t l y h i g h e r t h a n w a t e r - s o l u b l e systems.


( b ) I t has w i d e p H range o p e r a b i l i t y — g o o d e l e c t r o d e p o s i t i o n p e r f o r m a n c e has b e e n o b t a i n e d f r o m p H 2 - 1 0 . I n general, e l e c t r i c a l effi c i e n c y increases w i t h i n c r e a s i n g p H . ( c ) S u l f o n i u m groups are easily t h e r m a l l y d e c o m p o s e d ( 9 0 ° - 1 2 5 ° C ) to g i v e h y d r o p h o b i c p r o d u c t s , l e a v i n g n o i o n i c g r o u p s i n t h e film ( 1 9 , 21, 22). (à) I n t h e s u l f o n i u m latex system, f e w e r s u l f o n i u m g r o u p s p e r u n i t p o l y m e r mass are n e e d e d f o r e l e c t r o d e p o s i t i o n t h a n t h e n u m b e r o f a n i o n i c g r o u p s u s e d i n t h e a n o d i c system. ( e ) N o m e t a l ions a r e f o r m e d at t h e cathode t o d i s c o l o r t h e film o r increase t h e c o r r o s i o n rate o f t h e m e t a l substrate. T h e latter effect has r e c e n t l y b e e n d e m o n s t r a t e d b y M a y (14) i n a n o d i c electrodepositions. the

( f ) N o o x i d a t i o n of t h e film occurs since h y d r o g e n is p r o d u c e d at cathode.

( h ) E a r l y i n d i c a t i o n s are that t h e s u l f o n i u m systems r u n cooler t h a n the a n o d i c system. ( i ) T h i s c a t i o n i c e l e c t r o d e p o s i t i o n process is a p p l i c a b l e to a v a r i e t y of p o l y m e r compositions.

Acknowledgments T h e authors a r e e s p e c i a l l y i n d e b t e d to H . D . C l a r e y a n d L . D . Yats f o r t h e i r excellent s u p p o r t i n g w o r k i n these studies. gratefully acknowledge

T h e authors

also

t h e t e c h n i c a l assistance o f B . W . M i l l e r a n d

J . L . T o w n s e n d a n d t h e s t i m u l a t i n g conversations w i t h T . A l f r e y , J r .

Literature Cited 1. Gibbs, D . S., Wessling, R. Q., Wagener, Ε . H . , Belgian Patent 741,085 (1970). 2. Brewer, G . E. F., Burnside, G . L . , U.S. Patent 3,355,373 (1967). 3. Phillips, G . , Electroplating for the 70's Conference, Paper No. 15, London, England, Oct. 28-29 (1969). 4. Kiefer, D. M . , Chem. Eng. News (1969) 47, 32. 5. Hays, D . R., White, C . S., J. Paint Technol. (1969) 41, 461. 6. Nakamura, Y., Oki, F., Nozaki, Α., Bull. Chem. Soc. Japan (1969) 42, 1534. 7. Rheineck, A. E., Usmani, A. M . , J. Paint Technol. (1969) 41, 597. 8. Brushwell, W., Australian Paint J. (1969) 18. 9. May, C. Α., Smith, G . , J. Paint Technol. (1968) 40, 493. 10. Mercouris, S., Graydon, W . F., J. Electrochem. Soc. (1970) 1117, 717. 11. Berry, J. R., Paint Tech. (1963 ) 27, 13; (1964) 28, 24. 12. Hutchinson, C . O., Plating (1967) 54, 1246. 13. Deibert, R. J., J. Paint Technol. (1966) 38, 421. 14. May, C. Α., J. Paint Technol. (1971) 43, 43.



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ET AL.


127

15. Sullivan, M. R., J. Paint Technol. (1966) 38, 424. 16. Hoover, M. F., J. Macromol. Sci. (Chem.) (1970) 1327. 17. Jungermann, E., "Cationic Surfactants," p . 192, M a r c e l Dekker, N e w York, 1970. 18. H a t c h , M. J., Chem. Eng. News (1960) 38, 104. 19. H a t c h , M. J., Meyer, F . J., L l o y d , W . D., J. Appl. Polymer Sci. (1969) 3, 721. 20. F a n g , J . C . , U . S . Patent 3,310,540 (1967). 21. Kangas, D. Α., U . S . Patent 3,322,737 ( 1 9 6 7 ) . 22. L l o y d , W. G., U . S . Patent 3,409,660 (1968). 23. Spoor, H., Florus, G., Pohlemann, H., Schauder, F., U . S . Patent 3,455,806 (1969). 24. Spoor, H., Florus, G., Pohlemann, H., Schauder, F . , U . S . Patent 3,454,482 (1969). 25. Spoor, H., Pohlemann, H., U . S . Patent 3,458,420 ( 1 9 6 9 ) . 26. Slater, W . W., T h o w , L. E., U . S . Patent 3,468,779 ( 1 9 6 9 ) . 27. T a w n , A . R . W., Paint, Oil, Colour J. (1969) 821. 28. Brockmann, F. J., Canadian Patent 863,924 ( 1 9 7 1 ) . 29. M u n n , R . Η. E., Holder, M. V., M c N e e n e y , P . , British Patent 1,235,975 (1971). 30. Bosso, J. F . , Wismer, M., W . German Patent 2,003,123 ( 1 9 7 1 ) ; 2,033,123 (1971). 31. M c C o y , P. E., U. S. Patent 3,159,558 ( 1 9 6 4 ) . 32. Olson, D. Α., J. Paint Technol. (1966) 38, N o . 499, 429. 33. Wismer, M., Bosso, J. F . , Chem. Eng. (1971) 78, 114. 34. F i n n , S. R., M e l l , C . C., J. Oil Colour Chem. Assn. (1964) 219. RECEIVED M a y 28, 1971.


Electrodeposition of Coatings

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