Effect of Surfactant Structure on the Electrodeposition of Cationic

18 Effect of Surfactant Structure o n the Electrodeposition of Cationic Latexes

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch018

E. H. WAGENER, S. R. KUROWSKY, D. S. GIBBS, and R. A. WESSLING Physical Research Laboratory, The Dow Chemical Co., Midland, Mich. 48640

The effect of emulsifier structure on the electrodeposition of cationic latexes has been investigated. The electrocoating performance of these systems can be correlated with the r e d u c i b i l i t y of the emulsifier. This phenomenon has no counterpart i n conventional electrodeposition processes. The electrophoretic deposition of aqueous colloids has been known for many years. An extensive technology was developed to fabricate a r t i c l e s from natural rubber latex. Noble (1) traces its origins back to 1908. The use of electrodeposition to apply paint i s a comparatively recent innovation. The history of this development has been detailed by Brewer (2,3). The electrodeposition of paint, or electrocoating as it i s commonly labeled, i s derived from the old rubber latex technology and has many features i n common; but it d i f fers i n one important respect-- it provides rapid and complete current cutoff. The modern process of electrodeposition can thus be described as a combination of three basic elements: (a) Electrophoresis - migration of charged polymer par t i c l e s to metal surface; (b) Deposition - c o l l o i d a l des t a b i l i z a t i o n of particles at the metal-bath interface; and (c) Insulation - formation of an adherent, nonconductive layer of resin on the metal surface. The last named element i s responsible for the high throwing power which can be achieved with the electrocoating process. Commercial electrocoating formulations are made up with low molecular weight resins containing ionizable groups. Typically, they are converted to aqueous d i s persions by f i r s t dissolving the resin i n a watermiscible coupling solvent, then adding the appropriate 276

Piirma and Gardon; Emulsion Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


18.

Electrodeposition

WAGENER E T AL.

of Cationic

Latexes

277

s o l u b i l i z i n g agent and f i n a l l y adding water to form the dispersion. Both anodic and cathodic e l e c t r o c o a t i n g systems are i n use. The a n o d i c systems c o n t a i n c a r b o x y l a t e d polymers where the a c i d groups are n e u t r a l i z e d w i t h amines o r KOH. The c a t h o d i c systems c o n t a i n a m i n o f u n c t i o n a l r e s i n s where the amine groups are n e u t r a l i z e d w i t h an a c i d . The c o l l o i d a l s t a t e i n t h e s e systems i s p o o r l y de fined. I t depends on the l e v e l of o r g a n i c s o l v e n t , the number o f i o n i z a b l e groups i n the r e s i n and the degree of n e u t r a l i z a t i o n . In an anodic system, f o r example, t h e p o l y m e r may b e c o m p l e t e l y s o l u b l e a t h i g h p H a n d change g r a d u a l l y t o a h y d r o p h o b i c c o l l o i d as t h e pH i s d e c r e a s e d t o 7. At lower pH, the system f l o c c u l a t e s . The u n c e r t a i n t y o f t h e c o l l o i d a l s t a t e makes i t very d i f f i c u l t to study the process of electrodeposi t i o n i n these systems q u a n t i t a t i v e l y . However, the consensus i s that d e p o s i t i o n takes place p r i m a r i l y by a charge n e u t r a l i z a t i o n mechanism ( 3 ) : R(COO") (COOH) x

y

+

xH

+

x O H "

+

^

R(COOH)

x

+

y

+

(1)

Anodic R[R NH"*"] [R N] 2

x

2

y

^

R(R N) 2

x

+

y

*

(2)

Cathodic The H and OH" ions are s u p p l i e d by the s i m u l t a n e o u s e l e c t r o l y s i s of water during electrodeposition. Elec trode reactions i n v o l v i n g the r e s i n s are of l i t t l e s i g nificance. However, o x i d a t i o n of the metal substrate does p l a y a r o l e i n a n o d i c electrocoating. +

M° 1-11

M

+

—>

R(COO~)

M n

+ n

—>

ne"

+ 11

M*

(3)

R(COO") >r n

(4)

Latexes can a l s o be e l e c t r o d e p o s i t e d but i f they are s t a b i l i z e d by i o n i z e d groups such as c a r b o x y l i o n s , the same p r o b l e m w i t h p H d e p e n d e n t p a r t i c l e c h a r g e a n d c o l loidal stability is encountered. Latexes s t a b i l i z e d w i t h f u l l y i o n i z e d groups such as s u l f o n a t e i o n s a r e w e l l known i n t h e c o a t i n g s field. But, they have not been u t i l i z e d i n e l e c t r o d e p o s i t i o n b e c a u s e t h e y do n o t c u t o f f c u r r e n t . This i s not the case for c e r t a i n c a t i o n i c latexes. An e l e c t r o d e p o s i t i o n process based on s u l f o n i u m s t a b i l i z e d c o l l o i d s has been r e p o r t e d ( 4 ) . The s u l f o n i u m i o n , l i k e the a n a l o -



278

EMULSION

POLYMERIZATION

gous q u a t e r n a r y ammonium i o n , i s h i g h l y i o n i z e d o v e r a w i d e pH range so t h a t p a r t i c l e c h a r g e i s independent of pH. M o r e o v e r , i t has been shown t h a t s u l f o n i u m s y s tems c a n e l e c t r o d e p o s i t w i t h c u r r e n t c u t o f f comparable to t h a t of an a m i n e - s t a b i l i z e d l a t e x , whereas quater n a r y ammonium l a t e x d e p o s i t e d c o n d u c t i v e f i l m s . This c o n d i t i o n holds over a wide range of coating c o n d i t i o n s (5). A comparison of coating weight deposited a t v a r i o u s p H ' s i s s h o w n i n F i g u r e 1. The q u a t e r n a r y ammonium s t a b i l i z e d l a t e x d e p o s i t e d as a c o n d u c t i v e gel o v e r a pH range o f 2-10. The s u l f o n i u m l a t e x deposited u n d e r t h e same c o n d i t i o n s y i e l d e d a n i n s u l a t i n g film. Not evident from the f i g u r e i s the f a c t t h a t d e p o s i t i o n had e s s e n t i a l l y stopped i n the sulfonium case after a b o u t 15 s e c o n d s ; i t was s t i l l i n c r e a s i n g i n t h e q u a t e r n a r y ammonium s y s t e m when t h e e x p e r i m e n t was t e r m i n a t e d (2 m i n u t e s ) . S i n c e p a r t i c l e charge i n these l a t e x e s i s n o t pH dependent, the mechanism o u t l i n e d above f o r the conven t i o n a l e l e c t r o c o a t i n g systems cannot apply. I t was c l e a r from the s t a r t that the difference i n the behav i o r o f s u l f o n i u m a n d q u a t e r n a r y ammonium s t a b i l i z e d latexes i s r e l a t e d to the greater r e a c t i v i t y of the sulfonium ion. Though s t a b l e i n d i l u t e aqueous solu t i o n s , s u l f o n i u m i o n s m i g h t be e x p e c t e d t o u n d e r g o r a p i d decomposition under the conditions obtained at the cathode surface w h i l e current i s flowing. However, the s p e c i f i c r e a c t i o n s i n v o l v e d were not known. The present: s t u d y was u n d e r t a k e n t o determine whether the e l e c t r o c h e m i c a l r e d u c i b i l i t y of the stabil i z i n g c a t i o n s was a n i m p o r t a n t f a c t o r . I n o r d e r t o s i m p l i f y the problem experimentally, a l a t e x s t a b i l i z e d w i t h a d s o r b e d e m u l s i f i e r was s e l e c t e d as t h e m o d e l s y s tem. This permits study of the c a t i o n independent of the polymer p a r t i c l e . A s t u d y o f r e d u c t i o n p r o c e s s e s was a n a t u r a l choice s i n c e we w e r e d e a l i n g w i t h c a t h o d i c electrodeposition; b u t t h e m o t i v a t i o n t o l o o k a t t h i s a s p e c t was generated by e a r l i e r work w i t h b e n z y l i c s u l f o n i u m s a l t s on mer cury cathodes (6,7). I t was shown t h a t t h e s e s a l t s c o u l d be r e d u c t T v e l y c o u p l e d i n water whereas the cor r e s p o n d i n g q u a t e r n a r y ammonium s a l t s w e r e u n r e a c t i v e . Reductive coupling of sulfonium salts i n water turns out to be a v e r y g e n e r a l r e a c t i o n . It can take place not o n l y on mercury, but on h a r d metals l i k e s t e e l as well. T h i s t e c h n i q u e was u s e d t o d e v e l o p u n i q u e e l e c trocoating processes (8,9). The c a t h o d i c p r o c e s s e s are i l l u s t r a t e d schematically below:


18.

WAGENER E T

AL.

Electrodeposition

e


2 Ri-

>

Ri ·

— >:

of Cationic

-f-

R

2

Latexes

*~ S —R 3

(5)

(6)

Ri-Ri RH

279

+

OH

(7)

The r e d u c i b i l i t y o f t h e c a t i o n s c a n be i n f l u e n c e d b y changing the s u b s t i t u e n t s on the h e t e r o atom. We u s e d t h i s approach to synthesize surfactants w i t h different reduction potentials. The e l e c t r o d e p o s i t i o n of latexes s t a b i l i z e d b y t h e s e s u r f a c t a n t s was t h e n s t u d i e d a n d the r e s u l t s correlated w i t h reduction p o t e n t i a l . Results The d a t a i n T a b l e I show t h a t a l l o f t h e s u l f o n i u m emulsifiers are e a s i l y reduced r e l a t i v e to water (^-2.0 V i n this experiment). The f i r s t wave u s u a l l y a p p e a r s at ^-1.0 V, but an e l e c t r o n t r a n s f e r process at very p o s i t i v e p o t e n t i a l s i s seen i n the n i t r o b e n z y l substi t u t e d s u l f o n i u m compound. It is believed that the n i t r o group i t s e l f i s i n v o l v e d i n t h i s process though the s u l f o n i u m group i s t h e one t h a t u l t i m a t e l y g e t s re duced. An a r y l s u b s t i t u e n t on the s u l f u r leads to a s l i g h t l o s s i n r e d u c i b i l i t y (more n e g a t i v e E ^ ) , b u t the b i g g e s t change comes when no a c t i v a t i n g g r o u p s are p r e s e n t as i n the a l k y l s u l f o n i u m s . No significant s t e r i c e f f e c t s are evident i n any of the salts. The e f f e c t o f s u b s t i t u e n t s t r u c t u r e i s more p r o nounced i n the n i t r o g e n compounds. The "aromatic oniums r e d u c e as e a s i l y as s u l f o n i u m s , b u t the quat e r n a r y ammonium s a l t s a r e r e d u c i b l e o n l y when t h e n i t r o g e n has a c t i v a t i n g substituents. E l e c t r o n w i t h d r a w i n g g r o u p s l i k e F , CF3 and NO2 on the b e n z y l s u b s t i t u e n t favor r e d u c i b i l i t y . The f l u o r o s u b s t i t u t e d c o m p o u n d #12 i s r e d u c i b l e o n l y c o m p e t i t i v e l y w i t h w a t e r s i n c e i t s f i r s t wave appears as a shoulder on the s o l v e n t wave. These data p e r t a i n to reductions on mercury only. On o t h e r m e t a l s , t h e r e d u c t i o n p o t e n t i a l may b e shifted to more n e g a t i v e v a l u e s , and s i n c e the hydrogen overv o l t a g e w o u l d be l o w e r , the onium ions might not reduce preferentially. We s p e c u l a t e , h o w e v e r , t h a t t h e order of r e d u c i b i l i t y w o u l d n o t be changed i n comparisons on a given substrate. T h i s does seem t o be a reasonable approximation for sulfonium salts at least (11). , ,



+

10

C

9

2

2 5

2

2

Ci2H 5-^-CH -

2

2

2

H 5-^-CH -

2

Ci2H 5-^-CH -

8

1 2

Ci2H -^^-CH -

2

7

2

HO-CH2CH2-

HO-CH2CH2-

C3H7-

C2H5-

CH 3

Ci2H 5-^-CH -

2

6

2

Ci2H 5-^-CH -

2

5

2

0 N-^-CH -

C12H25-

4

R 3

HO-CH2CH2-

3

CH -

3

C Hy—

C2H5-

3

CH -

3

CH -

3

CH -

3

3

CH -

O-CH2-

2

C12H25-

3

R

CH -

C2H5-

k

A"

0-

C12H25-

2

Ri

;

3

P r o p e r t i e s of E m u l s i f i e r s

Sulfonium S a l t s , R i - S - R

Ci2H25"

-

1

No.

Table I-A

Cl"

Cl"

Cl"

Cl"

Cl"

Cl"

Cl"

CI*"

Cl"

Cl"

A

-0.99

-0.88, -1.03, -1.36

-1.09

-0.97, -1.11, -1.45

-1.00, -1.14, -1.44

-1.02, -1.13, -1.42

-0.64, -1.08, -1.38

-1.11

-1.35

-1.63

V (SCE)

\



Ci2H 5-^-CH -

17

20

19

18

No.

2

Cli»H2 7""

C12H25-Q-CH2-

C12H25-

R

C2H5-

3

Effect of Surfactant Structure on the Electrodeposition of Cationic

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