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)
Cathodic : 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
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.