Electrodeposition of Coatings - ACS Publications

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10 Electrochemistry of Polymer Deposition ZLATA KOVAC-KALKO

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PPG Industries, Inc., Coatings & Resins Division, Springdale, Pa. 15144

The electrodeposition of oil-modified polyesters and epoxy esters was investigated at constant current density, j, and at constant preset applied voltage, E . app

electrodeposition

At constant j, the

starts after an induction time, τ.

Film

thickness and electrode potential increase linearly with time, t, for t > τ. Coulombic efficiency and film resistivity are independent

of t. Coulombic efficiency increases and film

resistivity decreases with increased j. At constant

E , app

at

the beginning of electrodeposition the electrical field is high, and the growth of the film follows the logarithmic time law. The film resistance is non-ohmic.

With increased thickness

the electrical field decreases, the growth follows the

√t

law, and resistance becomes ohmic. Coulombic efficiency is independent

of t but increases with increased

E p. ap

Q i n c e t h e classical w o r k o f F i n k a n d F e i n l e i b ( I ) m a n y p u b l i c a t i o n s ^

h a v e a p p e a r e d o n t h e e l e c t r o c h e m i c a l aspects of this process

(2-8).

O u r interest was to find o u t h o w t h e e l e c t r o d e p o s i t i o n of p o l y m e r s begins, w h a t l a w or l a w s d e t e r m i n e t h e g r o w t h of films, a n d h o w a n o d e p o t e n t i a l , c o u l o m b i c efficiency, a n d film resistance d e p e n d u p o n p l a t i n g t i m e a n d voltage u s e d i n c o m m e r c i a l p r a c t i c e .

I n a d d i t i o n t o constant

experiments characteristic o f i n d u s t r i a l use, constant

current

voltage measure­

ments w e r e also m a d e to o b t a i n a d d i t i o n a l i n f o r m a t i o n .

Experimental T h e e l e c t r o d e p o s i t i o n o f o i l - m o d i f i e d polyesters a n d e p o x y esters has b e e n i n v e s t i g a t e d at constant a p p l i e d voltages, E , a n d at constant c u r r e n t densities, /. E x p e r i m e n t s w e r e c a r r i e d o u t i n a s t i r r e d e m u l s i o n at constant t e m ­ perature. A n o d e potentials, E , w e r e m e a s u r e d w i t h respect to a refer­ ence electrode ( saturated c a l o m e l or P t electrode ) via a L u g g i n c a p i l l a r y ( t o a v o i d t h e i R - v o l t a g e d r o p t h r o u g h p a i n t ) u s i n g a K e i t h l e y 660 a p P

a

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

150

ELECTRODEPOSITION

O F COATINGS

electrometer. C u r r e n t s w e r e m e a s u r e d w i t h a K e i t h l e y 6 0 0 B electrometer. B o t h a n o d e p o t e n t i a l a n d current w e r e r e c o r d e d o n a d u a l c h a n n e l B r u s h r e c o r d e d ( M a r k 2 8 0 ) as a f u n c t i o n o f e l e c t r o d e p o s i t i o n t i m e . A t y p i c a l j—t a n d E —t g r a p h is s h o w n f o r t h e b e g i n n i n g o f e l e c t r o d e p o s i t i o n i n F i g u r e 1. T h e a m o u n t o f charge flow w a s r e c o r d e d w i t h a c o u l o m e t e r ( Vari-Tech model VT-1176B). a

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80 Ε < Ε

60 40 20

Figure 1. Current-time and anode potential-time curves for oil-modified polyester systems at E^ = 450 volts

Substrates w e r e p r e b a k e d a n d w e i g h e d p r i o r to electrodeposition. P l a t i n g areas w e r e either 4 o r 115 c m . A stainless steel c a t h o d e w a s p l a c e d p a r a l l e l t o t h e anode. T h e thickness o f d e p o s i t e d films after bak­ i n g w e r e d e t e r m i n e d u s i n g a P e r m a s c o p e ( T w i n C i t y T e s t i n g C o . ). A l s o the w e i g h t o f b a k e d films w a s m e a s u r e d . 2

Results and Discussion Beginning of Electrodeposition. E l e c t r o d e p o s i t i o n c a n b e c a r r i e d o u t at a constant c u r r e n t d e n s i t y (/) o r at a constant voltage.

T h e current

densities g e n e r a l l y u s e d i n e l e c t r o d e p o s i t i o n are o n the o r d e r of a f e w

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

10.

Polymer Deposition

KOVAC-KALKO

ma/cm .

W h e n the

2

151

current is a p p l i e d , the

electrochemical

reaction

starts. W e w i l l assume that the m a i n reactions are: Anodic : o x i d a t i o n of water

2 H 0 = 4 H+ + 0 2

and d i s s o l u t i o n of substrate

Μ = Μ

η

+

+ 4e~

2

(1)

+ ne~

(2)

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discharge of water

2 H 0 + &r 2

= 20H~ + H

(3)

2

T h e c o n c e n t r a t i o n of these ions at the interface is g i v e n b y S a n d s equation:

C - . - C .

||/5

+

(4)

i

w h e r e C =-.o is the c o n c e n t r a t i o n at the interface, C x

is the b u l k c o n c e n ­

h

t r a t i o n , / is the c u r r e n t density, t is time, F is F a r a d a y ' s constant, a n d D is the d i f f u s i o n coefficient. T h e c o n c e n t r a t i o n of H

+

a n d m e t a l l i c ions at

the anode a n d O H " ions at the c a t h o d e w i l l increase w i t h t i m e a n d c u r r e n t density.

T h e p r o d u c t i o n of these ions w i l l g i v e rise to a c o n c e n t r a t i o n

g r a d i e n t across a b o u n d a r y l a y e r adjacent processes

to the electrode.

Diffusion

set i n to d i m i n i s h this increase i n c o n c e n t r a t i o n — s o m e

diffuse a w a y .

ions

F o r a n i o n i c d e p o s i t i o n , after a c e r t a i n t i m e , τ, k n o w n as

the i n d u c t i o n t i m e , c o n c e n t r a t i o n of H ions w i l l be h i g h e n o u g h to r e a c h +

the s o l u b i l i t y p r o d u c t , for a g i v e n system—i.e., [H+] ( R C O O - ] = Ka

(5)

A f t e r this c r i t i c a l c o n c e n t r a t i o n is r e a c h e d , the film w i l l f o r m at the anode. T h i s is i n d i c a t e d b y an increase i n E

a

with t (Figure 2).

F r o m E q u a t i o n 4 it f o l l o w s that / y/ Τ is a constant for a g i v e n system since n, F, π, a n d D are constants.

So / y/ τ is a characteristic for

a g i v e n substrate a n d p o l y m e r e m u l s i o n (5, 6).

F r o m F i g u r e 3 it c a n

b e seen that / y/ τ is smaller ( 3.0 Χ 10" amps s e e 3

172

c m " ) on untreated

steel t h a n o n Z n - p h o s p h a t e d steel (4.9 Χ 10" a m p s e e 3

2

172

cm" ). 2

Dissolu­

t i o n of the substrate c a n account for these differences because of the h i g h e r charge ( F e

2 +

or F e

3 +

vs. H ) w h i c h is m o r e effective i n c o a g u l a t i o n . +

F r o m Sand's e q u a t i o n it is possible to calculate the i n t e r f a c i a l c o n ­ centration of H

+

ions a n d the p H at the electrode surface w i t h Z n - p h o s ­

p h a t e d steel. T h e p H at the surface was c a l c u l a t e d to be 2.2 ( p H b a t h =

8.9). I n contrast to / =

constant, experiments w h e r e it takes a f e w seconds

for the f o r m a t i o n of p o l y m e r film to start at a constant voltage, E

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

a p p

, an

152

ELECTRODEPOSITION

O F

COATINGS

electrode is c o m p l e t e l y c o v e r e d w i t h a film w i t h i n a f r a c t i o n of a s e c o n d . T h i s is c a u s e d b y the large currents deposition.

flowing

at the b e g i n n i n g of electro­

( T h e p e a k currents are o n the o r d e r of 1 0 - 1 0 0 m a / c m d e ­ 2

p e n d i n g u p o n voltage a p p l i e d a n d the c o n d u c t i v i t y of the p a i n t e m u l s i o n ,

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w h i c h are b e t w e e n 3 0 0 - 3 0 0 0 m h o s . )

SECONDS Figure 2. Anode potential v s . deposition time in oilmodified polyesters at constant current density. Zincphosphated steel substrate. G r o w t h o f F i l m . If the film is a n e l e c t r o n i c insulator, i t cannot trans­ p o r t the electrons w h i c h are r e q u i r e d f o r Reactions 1 a n d 2.

Therefore,

the c h a r g e transfer c a n take p l a c e o n l y at the m e t a l / f i l m interface. T h e ions f o r m e d i n Reactions 1 a n d 2 t h e n c a r r y c u r r e n t t h r o u g h t h e

film.

T h e y react c h e m i c a l l y w i t h t h e c a r b o x y l i c ions a r r i v i n g f r o m t h e b a t h , g i v i n g rise to the f o r m a t i o n of n e w layers of film. H e n c e , film thickness increases. T h e transport of t h e ions t h r o u g h t h e film is c a u s e d b y the presence of a h i g h e l e c t r i c a l field. O n e m a y ask, w h a t is the most general r e l a t i o n b e t w e e n i o n i c flux o r c u r r e n t d e n s i t y a n d t h e e l e c t r i c a l field i n a n y i o n i c

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

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10.

KOVAC-KALKO

153

Polymer Deposition

SECONDS Figure 3. Anode potential vs. deposition time in oilmodified polyesters. Untreated steel substrate. conductor?

T h e a n s w e r a c c o r d i n g to the textbooks of e l e c t r o c h e m i s t r y

is ( 9 ) :

j = A sinh ^

(6)

w h e r e / is c u r r e n t density, A is a constant g i v e n b y t h e c o n d u c t i v i t y of a system, q is the charge o n a n i o n , a is t h e distance t r a v e l e d b y a n i o n b e t w e e n successful j u m p s , F is the e l e c t r i c a l constant, a n d Τ is absolute t e m p e r a t u r e .

field,

k is t h e B o l t z m a n n

T h e p r o d u c t kT is t h e measure

of t h e r m a l energy. E q u a t i o n 6 c a n b e r e d u c e d to the s i m p l e r f o r m s i n t h e f o l l o w i n g s p e c i a l cases ( 10,11 ) : ( 1 ) L o w field a p p r o x i m a t i o n , w h e n

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

154

ELECTRODEPOSITION

OF

COATINGS

i.e., w h e n the w o r k d o n e b y e l e c t r i c a l field o n a n i o n is m u c h smaller t h a n t h e r m a l energy.

I n this case

. , qaF sinn —

_ -

qaF k

T

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and

3 = Ρ

F

(7)

i.e., c u r r e n t d e n s i t y is a l i n e a r f u n c t i o n of the

field.

w h e r e ρ is the specific resistivity of the w e t d e f i n e d as E / 8 ( I I ) a

wet

where E

T h i s is O h m s l a w ;

film.

Electrical

field

is

is anode p o t e n t i a l a n d δ is thickness of

a

film. T h e rate of increase of film thickness is :

di w h e r e M/nF

' m

=

( 8 )

is the e l e c t r o c h e m i c a l e q u i v a l e n t w e i g h t a n d d is density

of a film. At / =

constant, M

dl/dt — a

constant or δ =

j

(t

— τ)

(9)

F i l m thickness after i n d u c t i o n t i m e r, increases l i n e a r l y w i t h t i m e , as does E At E

a

( cf. F i g u r e s 2, 3 ).

a p p

=

constant,

dJ

_ ~

constant δ

or after i n t e g r a t i o n δ =

constant

\T~t

i.e., thickness increases l i n e a r l y w i t h square root of t i m e . (2)

H i g h field a p p r o x i m a t i o n

(10): qaF

w »

1

i.e., e l e c t r i c a l field is m u c h greater t h a n t h e r m a l energy; t h e n

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

(10)

10.

κο ν A c -κ A L K O

155

Polymer Deposition

A 2

qaF Θ

Χ

Ρ

i.e., c u r r e n t is a n e x p o n e n t i a l f u n c t i o n o f t h e I n this case at £

= constant

a p p

(11)

kT field—non-ohmic

behavior.

(10): (12)

δ = constant In t

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T h i c k n e s s is a l o g a r i t h m i c f u n c t i o n of t i m e .

I • Δ •

.96 1.93 2.89

p

2.78-10 v/em 3.27 3.5 5

500

400

Ο > 300

200

100

1-5

1-0 THICKNESS Figure 4.

20

10 cm

Anode potential vs. thickness of baked film at constant j in oil-modified polyester.

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

156

ELECTRODEPOSITION

O F COATINGS

W h a t values of a n e l e c t r i c a l field exist d u r i n g t h e e l e c t r o d e p o s i t i o n of p o l y m e r s , a n d h o w are t h e y d e t e r m i n e d ? by plotting E

(11);

a

F i e l d is d e f i n e d as

E /8 a

vs. δ ( F i g u r e 4 ) , o n e obtains a d i f f e r e n t i a l field. I n

F i g u r e 4 these lines intersect at t h e same point, i n d i c a t i n g that voltage d r o p caused b y the interfaces is i n d e p e n d e n t of c u r r e n t a n d is s m a l l i n c o m p a r i s o n w i t h a v o l t a g e d r o p across t h e film thickness.

I n these plots,

the thickness of t h e b a k e d film is u s e d instead of t h e w e t films, a s s u m i n g that t h e error i n t r o d u c e d is s u c h that a constant c a n b e i n t r o d u c e d . T h e fields

o b t a i n e d are s u c h that w h e n a g i v e n / w a s p l u g g e d i n t o t h e c o m ­

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p u t e r , i t c a l c u l a t e d the h y p e r b o l i c sines r e l a t i o n a c c o r d i n g to E q u a t i o n 6.

T h e plots are g i v e n i n F i g u r e 5 f o r a l o w m o l e c u l a r w e i g h t system

a n d i n F i g u r e 6 f o r a h i g h m o l e c u l a r w e i g h t system.

F o r l o w molecular

1-0 •9

•8 •7

*X

life 0 σ* X

c

•6 •5

•4 •3 •2 •1

V.

ι

1

ι

2

1

1

3

4

} m A / cm Figure 5.

w e i g h t systems,

Hyperbolic sine function vs. ionic current density in an epoxy ester

sinh varied between

0.37 a n d 0.9 since t h e

v a r i e d b e t w e e n 0.3 a n d 0.8 o r F b e t w e e n

1.2 to 2.8 Χ

10

5

qaF/kT

volts/cm.

H o w e v e r f o r h i g h m o l e c u l a r w e i g h t systems, s i n h c h a n g e d f r o m 5 to 25

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

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10.

K O V A C: - K A L K O

Figure 6.

since qaF/kT

157

Polymer Deposition

Hyperbolic sine junction vs. ionic current density in an oil-modified polyester

changes f r o m 2 - 5 or F f r o m 3 to 5 Χ 1 0 v o l t s / c m . F r o m 5

the c o m p u t e r d a t a one c a n calculate qa. If q is a s s u m e d to b e -{-1, t h e n i n the l o w m o l e c u l a r w e i g h t system, α is 8 A a n d i n the h i g h system is 35 A . T h e distances b e t w e e n the c a r b o x y l i c groups of the p o l y m e r i n these t w o systems ( 12 ) are 10 a n d 20 A f o r l o w a n d h i g h M, r e s p e c t i v e l y . T h i s w o u l d i n d i c a t e that H ions w i l l h a v e e n o u g h energy to m o v e f r o m +

one c a r b o x y l i c g r o u p to the other. F r o m the a b o v e equations, one expects that thickness w o u l d v a r y l i n e a r l y w i t h t i m e at / =

constant; this is s h o w n i n F i g u r e 7. A t

constant, f o r l o w to m o d e r a t e fields, δ =

y/1

E

a

p

p

=

at 4 seconds a n d longer,

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

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158

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

10

20

30

40

50

60

SECONDS Figure 7. Thickness of baked films vs. time in an oilmodified polyester at constant current density

2.0[

Figure 8.

Thickness of bakedfilmsvs. λ / time in an epoxy ester at constant applied voltage

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

10.

KOVAC-KALKO

159

Polymer Deposition

w h i c h is s h o w n i n F i g u r e 8. A t t h e b e g i n n i n g of e l e c t r o d e p o s i t i o n a n d at high E

a p p

w h e r e qaF/kT

> 1 there is d e v i a t i o n f r o m t h e y/~t l a w .

T a b l e I shows the field after successive times at E a h i g h m o l e c u l a r w e i g h t system. be e x p e c t e d .

a p p

=

350 volts i n

T h i s shows w h a t t y p e of g r o w t h c a n

F o r short times qaF i s — 6 kT, w h i c h means that thickness

w o u l d v a r y l i n e a r l y w i t h l o g t, a n d at l o n g e r times w h e r e qaF kT, the y/1

=

T h i s is r e v e a l e d i n F i g u r e s 9 a n d 10. Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch010

1-2

d e p e n d e n c e w o u l d b e a p p r o a c h e d after 9 to 16 seconds.

Table I. Time, sec

3 50-volt Oil-Modified Polyester Coating j,

0.27 0.826 1.14 2.25 4.0 16.0 36.0 81.0

ma/cm

2

44.8 15.2 9.97 4.40 1.74 0.76 0.67 0.49

Field, volts/cm X 10

qaF/kT

6.37 5.25 4.81 3.96 3.00 2.16 2.02 1.72

6.13 5.05 4.63 3.81 2.89 2.08 1.94 1.66

b

•app. 25 0

VOLTS

Figure 9. Thickness of bakedfilmvs. log time in an oilmodified polyester at constant applied voltage. Short electrodeposition times.

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

160

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

2.5 h

Ε

2.0h

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1.5 Ζ S

λ.Ο\

too

1

2

3

4

5

6

7

8

V

9

Figure 10. Thickness of baked film vs y/ time in an oil-modified polyester at constant applied voltage A t constant t i m e , thickness increases l i n e a r l y w i t h E

, as s h o w n

&OV

f o r a n o i l - m o d i f i e d polyester system i n F i g u r e 11. T h e slopes a n d i n t e r ­ cepts o f these lines differ f o r short ( 4 seconds) a n d l o n g ( 8 1 seconds) times because o f t h e differences i n t h e t w o g r o w t h processes. T h e c a l c u ­ l a t i o n is b a s e d o n the differences b e t w e e n e l e c t r i c a l a n d t h e r m a l energies. 2.5 20 h

Ο Δ

1.5 (Λ

ζ ν

.5

1 οο

200

300

400

500

VOLTS Figure 11.

Thickness of bakedfilmvs. applied voltage in an oil-modified polyester at constant electrodeposition time

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

10.

KOVAC-KALKO

Polymer Deposition

161

H o w is i t k n o w n that kT is n o t m u c h larger t h a n assumed?

To

c l a r i f y this, the t e m p e r a t u r e of the substrate w a s m e a s u r e d d u r i n g elec­ trodeposition w i t h

a Thermistor

(DynaSense

electronic

thermometer

m o d e l 8390-3) w h i c h w a s i n t h e r m a l b u t n o t e l e c t r i c a l contact w i t h the substrate because one does n o t w a n t to measure E M F c a u s e d b y passage of c u r r e n t t h r o u g h i t b u t the E M F caused b y t h e r m a l changes.

T h e data

for the highest possible AT c h a n g e are s h o w n i n F i g u r e 12. T h i s

figure

shows that i n the s t i r r e d system AT changes o n l y b y a f e w degrees C ,

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w h i c h w o u l d m a k e a n e g l i g i b l e c o n t r i b u t i o n to the c a l c u l a t i o n s .

Figure 12.

Temperature of substrate vs. electrodeposition time in an oil-modified polyester

C o u l o m b i c E f f i c i e n c y . C o u l o m b i c efficiency, CL, is d e f i n e d as m g of b a k e d film p e r c o u l o m b passed. A t / =

constant, w h e r e F is constant,

one has CE constant, o r m g vs. c o u l o m b s a r e l i n e a r , as s h o w n i n F i g u r e 13.

T h e slope of these lines is the CE, f r o m w h i c h one c a n c a l c u l a t e the e l e c t r o c h e m i c a l e q u i v a l e n t w e i g h t .

M/n—i.e., At E

a p P

=

constant, the c o u l o m b i c efficiency is also i n d e p e n d e n t of

the e l e c t r o d e p o s i t i o n t i m e b u t increases w i t h a n i n c r e a s e d a p p l i e d v o l t a g e as s h o w n i n F i g u r e 14. If the p o l y m e r system h a d a n a r r o w m o l e c u l a r w e i g h t d i s t r i b u t i o n a n d w o u l d deposit as a s t o i c h i o m e t r i c c o m p o u n d , s u c h variations i n the e q u i v a l e n t w e i g h t w o u l d n o t b e possible. H o w e v e r , w e are d e a l i n g w i t h systems w h i c h have n e i t h e r a n a r r o w m o l e c u l a r w e i g h t d i s t r i b u t i o n n o r are they d e p o s i t e d as " p u r e " c o m p o u n d s . ( C h e m i -

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

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162

ELECTRODEPOSITION

O F COATINGS

COULOMBS Figure 13. Weight of baked film vs. coulombs passed in an oil-modified polyester at constant current density

Figure 14.

cal

Weight of baked film vs. coulombs passed in an oil-modified poly­ ester at constant applied voltage

analysis

of w e t

films

showed

that

they

also

contain

trapped

counter ions. ) Resistance. A n o d e p o t e n t i a l ( £ ) represents t h e s u m of v o l t a g e d r o p s a

across t h e m e t a l - f i l m interface, across t h e film thickness, a n d across t h e

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

10.

KOVAC-KALKO

film-paint

Polymer Deposition

163

interface. E p l u s the IR d r o p t h r o u g h the p a i n t e m u l s i o n a n d a

the c a t h o d e - p a i n t interface equals E

a p p

.

Since most o f t h e v o l t a g e d r o p

occurs across t h e film itself, t h e ratio of E

a

a n d current density passing

to d e p o s i t a film at a g i v e n t i m e represents resistance, R, of w e t , g r o w i n g film

a n d is g i v e n i n Ω / c m . 2

A s p o i n t e d o u t earlier at / =

constant c u r r e n t R film w o u l d b e a

l i n e a r f u n c t i o n o f t i m e since ρ =

constant.

T h i s is s h o w n i n F i g u r e 15.

F r o m k n o w n R a n d a thickness of b a k e d film, t h e specific resistivities w e r e c a l c u l a t e d a n d w e r e f o u n d to b e 8.9 Χ Ι Ο Ω c m a n d 8.8 Χ Ι Ο Ω

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7

8

c m f o r the e p o x y ester a n d t h e o i l m o d i f i e d polyesters, r e s p e c t i v e l y .

ΙΟ

20

30

40

50

60

70

80

SECONDS Figure 15. Film resistance vs. time in an oil-modified polyester at constant current density H o w e v e r , at E

a p p

=

constant, n o n - o h m i c b e h a v i o r s h o u l d b e ex­

p e c t e d at h i g h fields. N o n - o h m i c c o n d u c t i o n i n p o l y m e r films w a s p r e v i ­ ously m e n t i o n e d b y B e c k (13)

a n d C o o k e (14).

B e c k a s s u m e d that this

b e h a v i o r is c a u s e d b y the presence of a space charge r e g i o n , a n d C o o k e a s s u m e d i t w a s c a u s e d b y t h e second W i e n effect—i.e., d i s s o c i a t i o n of w e a k electrolytes.

field-induced

T h e fields d u r i n g t h e e l e c t r o d e p o s i t i o n

of p o l y m e r s are c e r t a i n l y h i g h e n o u g h to cause the d i s s o c i a t i o n of d e ­ p o s i t e d free acids o r m e t a l l i c soaps. Since i n t h e present w o r k the W i e n effect a n d its c o n t r i b u t i o n to t h e n o n - o h m i c c o n d u c t i o n i n t h e films w e r e not m e a s u r e d d i r e c t l y , R w a s c a l c u l a t e d o n l y f o r the e p o x y ester system, w h e r e l o w to m o d e r a t e fields w e r e found—i.e., w h e r e O h m ' s l a w is v a l i d . Specific resistivities of these films are g i v e n i n T a b l e I I .

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

164

ELECTRODEPOSITION

Table II. Voltage, volts

Specific Resistivity" for Epoxy Ester Time, sec.

200 volts

275 volts

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α

O F COATINGS

Ω cm X 10

4

3.2

9 49

7.6 9.6

4

6.0

9 49

Resistivity increases with time and E

7

&pp

9.0 10.8

in this system.

Summary D u r i n g t h e e l e c t r o d e p o s i t i o n o f p o l y m e r s at constant voltage, t h e c u r r e n t d e n s i t y is a f u n c t i o n o f the h y p e r b o l i c sine o f the e l e c t r i c a l

field.

F i l m g r o w t h at the b e g i n n i n g o f e l e c t r o d e p o s i t i o n f o l l o w s t h e l o g a r i t h m i c t i m e l a w . I n this r e g i o n t h e film resistance is n o n - o h m i c i n b e h a v i o r . T h i s is t h e r e g i o n o f h i g h e l e c t r i c a l field strength. A s t h e film thickness increases, t h e field strength across i t decreases, a n d t h e g r o w t h of t h e film f o l l o w s t h e y/1 l a w . I n this r e g i o n t h e film resistance f o l l o w s O h m ' s l a w . T h i s is also a r e g i o n o f m o d e r a t e t o l o w field

strength. A t constant current, t h e thickness a n d film resistance v a r y l i n e a r l y

w i t h t i m e . C o u l o m b i c efficiency a n d specific resistivity are constant since the e l e c t r i c a l field is m a i n t a i n e d constant

during the deposition.

l o m b i c efficiency increases, a n d film resistance decreases w i t h

Cou­

increased

c u r r e n t density. Acknowledgment T h e author thanks C . H i g g i n b o t h a m f o r h i s s k i l l f u l

experimental

assistance. G r a t e f u l a c k n o w l e d g m e n t is g i v e n to P . P i e r c e , N . F r i c k , a n d M . W i s m e r f o r their m a n y h e l p f u l a n d c h a l l e n g i n g discussions. Literature

Cited

1. 2. 3. 4. 5. 6.

Fink, C. G., Feinleib, M . , Trans. Electrochem. Soc. (1948) 94, 309. Tawn, A. H . R., Beery, I. R., J. Oil Colour Chem. Assoc. (1965) 48, 790. Beck, F., Farbe Lack (1966) 72, 218. Finn, S. R., Mell, C. G., J. Oil Colour Chem. Assoc. (1964) 47, 219. Netillard, J. P., Double Liaison (1970) 17, 225, 233. Saatweber, D . , Vollmert, B., Angew. Makromol. Chem. (1969) 8, 80; (1970) 10, 143. 7. Finn, S. R., Hasnip, J. Α., J. Oil Colour Chem. Assoc. (1965) 48, 1121. 8. Beery, I. R., Paint Technol. (1963) 27, 12.

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

10.

KOVAC-KALKO

Polymer Deposition

165

9. Bockris, J . O'M., Reddy, A . K., " M o d e r n Electrochemistry," V o l . I, p . 391, Plenum, N e w York, 1970. 10. Cabrera, H., Mott, N. F., Rep. Progr. Phys. (1948-49) 12, 163. 11. Young, L., " A n o d i c F i l m s , " Academic, N e w York, 1961. 12. Pierce, P., private communication ( 1 9 7 0 ) . 13. Beck, F., Ber. Bunseng., Physik. Chem. (1968) 72, 445. 14. Cooke, Β. Α., Paint Technol. (1970) 34, 12. May

28,

1971

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RECEIVED

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