Throwing Power as Related to Material Properties with Analysis by

sons show that electrodeposition throwing power is a char acteristic of the ... role of metallic ions, actually present much data which support the ac...
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13 Throwing Power as Related to Material Properties with Analysis by Digital Computer Simulation

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A. E . GILCHRIST and D . O. SHUSTER Glidden-Durkee, Division S C M Corp., Dwight P. Joyce Research Center, Strongsville, Ohio 44136

Electrodeposition

is a process that can be defined by known

electrochemical laws and inherent properties of the material deposited.

Throwing power is that characteristic of the ma­

terial being deposited that promotes uniform deposited film thickness, at varying cathode-to-anode distances on the same deposition electrode.

Process variables such as deposition

voltage, time, electrode areas and spacing, and tank size can­ not be used as material properties. Electrical equivalent cir­ cuits and mathematical relations were developed to represent circuit actions derived.

Such relations were combined as a

digital computer program. Results of such computer calcula­ tions were tested by actual laboratory

electrodeposition

operations, and the results were compared.

These compari­

sons show that electrodeposition throwing power is a char­ acteristic of the deposition material and is not governed by application process variables.

" e l e c t r o d e p o s i t i o n as a m e t h o d of a p p l y i n g p a i n t to a c o n d u c t i v e object ^

w a s p a t e n t e d b y D a v e y ( 1 ) i n 1919. Successful l a b o r a t o r y scale a p ­

p l i c a t i o n of coatings o n the i n s i d e of t i n p l a t e cans, after f a b r i c a t i o n , w a s c a r r i e d o u t i n the 1930's ( 2 ) . C o m m e r c i a l e x p l o i t a t i o n of this c o a t i n g t e c h n i q u e has o n l y a p p e a r e d , h o w e v e r , i n the past 10 years ( 3 ) . A c o m ­ prehensive d e s c r i p t i o n of the process, p a i n t c o m p o s i t i o n s , a n d q u a n t i t a t i v e aspects of t h e e l e c t r i c a l efficiency of t h e e l e c t r o d e p o s i t i o n m e t h o d of a p p l y i n g p a i n t a p p e a r e d i n 1966 (4).

S i n c e then, a d d i t i o n a l details, s u c h

as the use of electrodialysis to c o n t r o l b a t h c o m p o s i t i o n ( 5 ) , resinous c o a t i n g materials (6, 7 ) , a n d the requirements f o r m a i n t a i n i n g the b a t h 191 In Electrodeposition of Coatings; Brewer, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

192

ELECTRODEPOSITION

c o m p o s i t i o n i n a c o n t i n u o u s e l e c t r o d e p o s i t i o n process

OF

(8)

COATINGS

have

been

issued. T h e r e is almost u n a n i m o u s agreement

that the d e p o s i t i o n process

obeys F a r a d a y ' s l a w (4, 9, 10, 11)—i.e., the w e i g h t of deposit f o r m e d is p r o p o r t i o n a l to the n u m b e r of c o u l o m b s of e l e c t r i c i t y passed. (12),

Voltage

resin c o n c e n t r a t i o n , b a t h t e m p e r a t u r e , c u r r e n t density, a n d several

substrates (10)

d o not influence the c o u l o m b i c y i e l d significantly. H o w ­

ever, the c o u l o m b i c y i e l d does v a r y i n v e r s e l y w i t h the degree of n e u t r a l ­ i z a t i o n of the r e s i n (10, 11,

13).

Since e l e c t r o d e p o s i t i o n obeys F a r a d a y ' s l a w , e l e c t r o l y s i s — n o t

elec­

t r o p h o r e s i s — i s the process g o v e r n i n g the d e p o s i t i o n . O h m ' s l a w c a n be Downloaded by UNIV LAVAL on July 15, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch013

a p p l i e d to the system to d e t e r m i n e the a m o u n t of current f l o w i n g a n d voltage losses i n the resistive elements c o m p r i s i n g the system (9, 10, 13).

12,

I n contrast to e l e c t r o p l a t i n g , the specific c o n d u c t i v i t y of t y p i c a l

electrocoatings at o p e r a t i n g solids a n d temperatures, 5 0 0 - 4 0 0 0 m i c r o m h o / c m , is so large that the b a t h resistance m u s t be i n c l u d e d i n a n analysis of t h e e l e c t r i c a l c i r c u i t i n v o l v e d (9, 13).

A m o r e d r a m a t i c a n d significant

difference f r o m e l e c t r o p l a t i n g , h o w e v e r , is the e l e c t r i c a l i n s u l a t i n g p r o p ­ erty of the d e p o s i t e d

film.

It is the h i g h specific resistance of the elec-

t r o d e p o s i t e d film w h i c h makes possible the u n i f o r m coverage of i r r e g ­ u l a r l y s h a p e d objects a n d t h e c o a t i n g of the i n s i d e of s u c h restrictive areas as r o c k e r panels o n a u t o m o b i l e bodies.

T h i s p r o p e r t y of electro-

c o a t i n g is c a l l e d t h r o w i n g p o w e r a l t h o u g h u n i f o r m thickness c o v e r i n g p o w e r w o u l d be a m o r e accurate t e r m f o r a p i g m e n t e d p a i n t

film.

M u c h w o r k has b e e n d o n e o n the influence of the various reactions o c c u r r i n g at the

anode a n d at the

cathode

o n the

electrodeposition

process. I n a n o d i c d e p o s i t i o n it has b e e n d e m o n s t r a t e d that 1 0 0 % of the current

flowing

at the c a t h o d e c a n be a c c o u n t e d for b y the electrolysis

of w a t e r to generate h y d r o g e n gas (14).

T w o c o m p e t i n g reactions at the

a n o d e are p r o p o s e d . T h e electrolysis of w a t e r at the anode w i l l generate o x y g e n gas a n d h y d r o g e n ions. R e s i n w i l l p r e c i p i t a t e o n the anode as a result of the l o w e r p H because of the increased c o n c e n t r a t i o n of

the

h y d r o g e n ions. A l t e r n a t e l y , it is p r o p o s e d that m e t a l ions generated b y o x i d a t i o n of the anode substrate or m e t a l pretreatment react to p r e c i p i t a t e the resin. T a w n a n d B e r r y (10),

o f t e n falsely a c c u s e d of s u p p o r t i n g the

role of m e t a l l i c ions, a c t u a l l y present m u c h d a t a w h i c h s u p p o r t the a c i d c o a g u l a t i o n theory.

T h e y d e m o n s t r a t e d s u c h e v i d e n c e as the fact that

the i r o n content of films o n m i l d steel e l e c t r o d e p o s i t e d panels w a s n o greater t h a n o n panels d i p p e d into the same resins. T h e a m o u n t of i r o n f o u n d i n the e l e c t r o d e p o s i t e d film w a s o n l y a s m a l l f r a c t i o n of that w h i c h w o u l d be r e q u i r e d b y the s t o i c h i o m e t r y of the m e t a l i o n - p r e c i p i t a t i o n m e c h a n i s m or that r e q u i r e d to account for the o x i d a t i o n reactions

oc­

c u r r i n g at the anode b a s e d o n F a r a d a y ' s l a w . P r e c i p i t a t e d resin b y a d d i -

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

13.

GILCHRIST AND SHUSTER

Throwing

193

Power

t i o n of ferrous sulfate w a s i n s o l u b l e i n o r g a n i c solvents s u c h as m e t h a n o l a n d acetone whereas the e l e c t r o d e p o s i t e d r e s i n , w h i c h w i l l also redissolve i n t h e b a t h w h e n d e p o s i t i o n is t e r m i n a t e d , w a s s o l u b l e i n t h e o r g a n i c solvents.

I n d e p e n d e n t l y , i t has b e e n p o i n t e d o u t that t h e clear film o b ­

t a i n e d w h e n a n u n p i g m e n t e d resin is d e p o s i t e d o n a n i r o n a n o d e is f u r t h e r e v i d e n c e that t h e r o l e o f i r o n ions is i n c o n s e q u e n t i a l ( 1 5 ) . It has also b e e n o b s e r v e d that e l e c t r o d e p o s i t i o n c a n b e o b t a i n e d o n s u c h inert sur­ faces as p l a t i n u m a n d c a r b o n

(16).

Recent

studies of d e p o s i t i o n over

i r o n a n d p l a t i n u m (17) demonstrate the d o m i n a n c e of the a c i d p r e c i p i t a ­ t i o n m e c h a n i s m d u r i n g t h e first t w o m i n u t e s o r so o f d e p o s i t i o n , a t i m e s p a n most often f o u n d i n c o m m e r c i a l p r a c t i c e .

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W e t o n w e t d e p o s i t i o n o f a w h i t e p a i n t after a b l a c k , o r v i c e - v e r s a , was f o u n d to y i e l d g r a y films (10).

T h i s e x a m p l e of t h e fact that t h e

d e p o s i t i o n l a y e r b u i l d s u p at t h e d e p o s i t - s u b s t r a t e interface rather t h a n at t h e d e p o s i t - b a t h interface has b e e n successfully d e m o n s t r a t e d b y a p a t e n t e d process

(18).

I n this process

t h e c o r r o s i o n resistance

of a n

electrocoated object is i m p r o v e d b y i m m e r s i n g t h e w e t c o a t e d object i n a s o l u t i o n c o n t a i n i n g m e t a l - t r e a t i n g oxyanions a n d e l e c t r o d e p o s i t i n g t h e m u n d e r the p a i n t film. W e h a v e d e p o s i t e d r e d p a i n t after b l a c k p a i n t a n d vice-versa.

Since t h e r e d p i g m e n t particles a r e m u c h larger t h a n t h e

b l a c k , there is m o r e surface d i s c o l o r a t i o n o f a n i n i t i a l b l a c k c o a t i n g w h e n r e d is f o r c e d t h r o u g h i t o n subsequent e l e c t r o c o a t i n g t h a n i n t h e reverse situation.

I n either case, h o w e v e r , m i c r o s c o p i c e x a m i n a t i o n of a cross-

section t h r o u g h t h e layers o f c o a t i n g c l e a r l y shows that t h e p i g m e n t e d system w h i c h w a s d e p o s i t e d second is f o u n d b e t w e e n

the pigmented

m a t e r i a l w h i c h is d e p o s i t e d first a n d t h e c o n d u c t i n g substrate. Experimental L a b o r a t o r y measurements o f t h r o w i n g p o w e r h a v e b e e n most n o t e d for t h e i r v a r i e t y o f test apparatus (10,13,19).

T h e r e is one e x a m p l e o f a

p r a c t i c a l a p p l i c a t i o n of t h r o w i n g p o w e r testing (20), a. t h e o r e t i c a l analysis (11), a s u m m a r y of t h e r e l a t i o n s h i p s b e t w e e n m a t e r i a l properties, d e p o ­ sition parameters, a n d t h r o w (13), a n d a p r o p o s e d r e l a t i o n s h i p b e t w e e n t h r o w i n g p o w e r a n d voltage, b a t h specific resistance a n d c o u l o m b i c y i e l d (11).

A l m o s t u n i v e r s a l l y t h r o w i n g p o w e r is r e c o r d e d as inches o f some­

thing.

T h i s n u m b e r suffers because the basis f o r t h e m e a s u r e m e n t has

not b e e n d e f i n e d , just as a specific resistance o r specific c o n d u c t i v i t y as a n u m b e r is meaningless unless t h e t e m p e r a t u r e at w h i c h t h e measure­ m e n t w a s t a k e n is also r e c o r d e d . I n order f o r a t h r o w i n g p o w e r measure­ m e n t to have a n y r e a l significance, t h e film thickness d e v e l o p e d o n a reference anode m u s t also b e r e c o r d e d

(19).

O u r p u r p o s e i n this i n v e s t i g a t i o n w a s t w o f o l d .

First, w e w a n t e d to

i d e n t i f y t h e m a t e r i a l properties o f electrocoatings w h i c h i n f l u e n c e d t h r o w -

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

194

ELECTRODEPOSITION

OF

COATINGS

i n g p o w e r . T h r o w i n g p o w e r is a f u n c t i o n or a n i n d e x of d e p o s i t e d

film

thickness u n i f o r m i t y . It is n o t a f u n c t i o n of total a m o u n t of film d e p o s i t e d n o r the d e p o s i t i o n c o n d i t i o n s s u c h as voltage a n d t i m e . E l e c t r o d e p o s i t i o n f o l l o w s F a r a d a y ' s l a w : one g r a m e q u i v a l e n t w e i g h t of matter is c h e m i c a l l y a l t e r e d at e a c h electrode f o r 96,501 i n t e r n a t i o n a l c o u l o m b s of electricity passed t h r o u g h the electrolyte. T h e current passed c a n be d e r i v e d f r o m O h m ' s l a w : voltage equals the p r o d u c t of flowing

a n d c i r c u i t resistance i n ohms.

amperes

W h e n c i r c u i t resistance is c o n ­

stant, changes i n voltage c a n o n l y p r o d u c e changes i n amperes.

When

c o m b i n e d w i t h t i m e variations this p r o d u c e s o n l y changes i n t o t a l q u a n ­ t i t y of m a t e r i a l d e p o s i t e d — n o t its d i s t r i b u t i o n or l o c a t i o n . T h e o n l y factor Downloaded by UNIV LAVAL on July 15, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch013

i n f l u e n c e d b y the e l e c t r o d e p o s i t i o n m a t e r i a l is the c i r c u i t resistance, w h i c h is a c o m b i n a t i o n of resistances of b a t h s o l u t i o n a n d d e p o s i t e d film m a ­ terial.

If t h r o w i n g p o w e r is a n inherent p r o p e r t y of a p a r t i c u l a r elec­

t r o c o a t i n g f o r m u l a t i o n , s u c h o p e r a t i n g variables as voltage a n d d e p o s i t i o n t i m e are not parameters w h i c h d e t e r m i n e t h r o w i n g p o w e r . S e c o n d l y , w e w a n t e d a m a t h e m a t i c a l d e s c r i p t i o n of the f o r m a t i o n of a c o a t i n g layer i n a restrictive c h a n n e l of definable geometry.

If w e

c o u l d d e v e l o p this m a t h e m a t i c a l d e s c r i p t i o n , w e c o u l d use a c o m p u t e r to p e r f o r m the t h r o w i n g p o w e r experiment, b a s e d u p o n a n i n p u t set of relevant m a t e r i a l properties, p h y s i c a l d i m e n s i o n s of the test set u p , a n d o p e r a t i n g c o n d i t i o n s . Success i n step t w o of o u r objectives w o u l d enable us to d e v e l o p a q u a n t i t a t i v e r e l a t i o n s h i p b e t w e e n the relevant m a t e r i a l properties a n d t h r o w i n g p o w e r . T h e most i m p o r t a n t relevant properties are c u r r e n t r e q u i r e d f o r d e p o s i t i o n , ( c o u l o m b s / g r a m ), specific resistance of b a t h s o l u t i o n , a n d w e t d e p o s i t e d film specific reistance.

Differences

b e t w e e n l a b o r a t o r y e x p e r i m e n t a n d c o m p u t e r s i m u l a t i o n c o u l d b e ana­ l y z e d to p r o v i d e a n even better u n d e r s t a n d i n g of the t h r o w i n g p o w e r p r o b l e m a n d thus l e a d to p r a c t i c a l a p p l i c a t i o n s .

Apparatus Electrocoating

s o l u t i o n specific resistance measurements

f o r m e d i n the m a n n e r established for electrolytes.

are

per­

T h e Y e l l o w Springs

Instrument C o . m o d e l 31 c o n d u c t i v i t y b r i d g e , w i t h an a u x i l i a r y d e c a d e c a p a c i t o r v a r i a b l e f r o m 0.01 to 1.0 m f d has p r o v e d to be u s e f u l f o r this measurement.

F o r r o u t i n e use i n the l a b o r a t o r y a c o n d u c t i v i t y c e l l was

f a b r i c a t e d w i t h p o l i s h e d stainless steel electrodes h e l d r i g i d l y i n p o s i t i o n i n a m a c h i n e d b l o c k of P l e x i g l a s . T h i s c e l l is c a l i b r a t e d w i t h K C 1 s o l u ­ tions i n the s t a n d a r d m a n n e r .

It is r u g g e d , easily c l e a n e d , a n d m o r e

efficient to use t h a n p l a t i n i z e d p l a t i n u m electrodes w h i c h r a p i d l y b e c o m e surface c o n t a m i n a t e d w i t h p i g m e n t , thus r e q u i r i n g f r e q u e n t c l e a n i n g a n d r e p l a t i n i z i n g i n o r d e r to m a i n t a i n a c c u r a c y of measurement.

W i t h pol-

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

13.

GILCHRIST AND SHUSTER

195

Throwing Power

i s h e d stainless steel electrodes 1000 H z is m a n d a t o r y t o p r e v e n t p o l a r i z a ­ t i o n rather t h a n 60 H z c o m m o n l y u s e d w i t h p l a t i n i z e d electrodes. L a b o r a t o r y evaluations o f t h e d e p o s i t i o n p r o p e r t i e s , d e p o s i t e d specific resistance a n d e l e c t r i c a l efficiency c i p r o c a l ) , are c a r r i e d o u t as d e s c r i b e d

film

( coulombs/gram or the re­

p r e v i o u s l y (4).

Current-time

curves a r e r e c o r d e d b y a Sargent r e c o r d e r m o d e l S R t y p e o r a H e a t h k i t model E U W - 2 0 .

C o u l o m b s o f e l e c t r i c i t y passed are m e a s u r e d either b y

a d i s c integrator m o u n t e d o n t h e Sargent r e c o r d e r o r b y a S e l f - O r g a n i z ­ i n g System, I n c . m o d e l S I 100 e l e c t r o n i c integrator.

N e t coating weight

d e p o s i t e d is d e t e r m i n e d b y w e i g h i n g t h e 4 - s q i n c h panels, before a n d after c o a t i n g , o n a l a b o r a t o r y a n a l y t i c a l b a l a n c e s u c h as a M e t t l e r t y p e H 5

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(160 grams c a p a c i t y ) . 36 cm

c

; / / / / / / /

**|

>///////////////////////{

/ /

A /

/////////////At////////.

/ / / / / /

/

// /

-•13.0 k ! cm I Figure 1. Plexiglas throwing power tank (top view; 0.4 scale). C, cathode; A , reference anode; TP, throwing power section. Base, 17 X 39 cm. Laboratory

t h r o w i n g p o w e r experiments

are performed w i t h the

test apparatus s h o w n i n F i g u r e 1. T h e t a n k is 10 c m d e e p a n d is filled w i t h 9 c m of bath.

F o u r - s q i n c h ( 10 X 10 c m ) panels are u s e d f o r t h e

c a t h o d e a n d t h e reference anode.

I n t h e t h r o w i n g p o w e r section a n y

surface u p to a 4 X 12 i n c h ( 1 0 X 3 0 c m ) p a n e l m a y b e u s e d .

When

m u l t i p l e sectional panels are p l a c e d i n the t h r o w i n g p o w e r slot, a sequence shunt s w i t c h is u s e d to g i v e c u r r e n t readings at discrete t i m e intervals so that a c u r r e n t - t i m e c u r v e m a y b e t r a c e d f o r each segment f r o m t h e points r e c o r d e d .

Results T h r o w i n g p o w e r measurements

are d i s p l a y e d b y t h e m e t h o d s h o w n

i n F i g u r e 2. F i l m thickness at several points a l o n g t h e t h r o w i n g p o w e r

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

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196

ELECTRODEPOSITION

10

20

O F COATINGS

30

Distance from Cathode End Figure 2.

Method of displaying throwing power

p a n e l is m e a s u r e d w i t h a P e r m a s c o p e h a v i n g t h e electronic c i r c u i t a n d m e a s u r i n g h e a d a p p r o p r i a t e f o r t h e t y p e of substrate i n v o l v e d . Q u a l i t a ­ t i v e effects of d e p o s i t e d film specific resistance a n d b a t h specific resistance o n t h r o w i n g p o w e r are i l l u s t r a t e d i n F i g u r e 2. T h e c o m m o n d e n o m i n a t o r for t h e three examples s h o w n is t h e same film thickness o n the reference a n o d e i n each experiment. F i l m resistance c a n b e v a r i e d essentially i n d e ­ p e n d e n t l y of other e l e c t r o c o a t i n g properties b y a d d i n g a n o r g a n i c solvent

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100

150

Applied Voltage Figure 3. Coulombs/gram v s . voltage effect of level of pig­ ment loading (composition by weight). ·, 100% vehicle; X , 3 vehicle/l pigment; O , 2 vehicle/l pigment.

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

13.

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Throwing Power

to t h e v e h i c l e before s o l u b i l i z a t i o n , thus a l t e r i n g the v i s c o s i t y o f the d e ­ posited

film.

S u c h v i s c o s i t y changes o f d e p o s i t e d film, w h e t h e r b y t e m ­

p e r a t u r e o r b y solvent d i l u t i o n , alter t h e specific resistance o f t h e d e ­ p o s i t e d m a t e r i a l . B a t h resistance c a n b e v a r i e d i n d e p e n d e n t l y o f other p r o p e r t i e s b y r e d u c i n g t h e m a t e r i a l to different solids levels. T h e r e l a t i o n s h i p b e t w e e n e l e c t r i c a l efficiency a n d v o l t a g e is s h o w n i n F i g u r e 3. A b o v e ca. 5 0 volts, t h e c o u l o m b s / g r a m is essentially c o n ­ stant.

B e l o w 50 volts t h e curves c l i m b t o w a r d u n d e f i n e d values as t h e

voltage is r e d u c e d to t h e 5 - 1 0 v o l t range. A r e p l o t of t h e d a t a i n F i g u r e 3 is s h o w n i n F i g u r e 4. I t is, h o w e v e r , s t i l l i m p o s s i b l e t o extrapolate t h e

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curves t o a zero v a l u e o f v o l t a g e a n d / o r m g / c o u l o m b .

0l 0

1

1

50

100

1—

150

Applied Voltage Figure 4.

Milligrams/coulomb vs. voltage (replot of Figure 3)

T o define t h e F a r a d a y b e h a v i o r o f electrocoatings at l o w voltages, p o l a r o g r a p h y experiments w e r e p e r f o r m e d . T h e c u r r e n t - v o l t a g e r e l a t i o n ­ s h i p i n a t y p i c a l p o l a r o g r a p h y e x p e r i m e n t is s h o w n i n F i g u r e 5 . T h e p o r t i o n of this c u r v e i n the r e g i o n f r o m 0 - 5 volts is f o u n d i n almost a l l p h y s i c a l c h e m i s t r y texts i n t h e first c h a p t e r o n e l e c t r o c h e m i s t r y .

This

section o f t h e c u r v e shows the classic shape of t h e c u r r e n t - v o l t a g e c u r v e f o r a n electrolyte b e t w e e n inert electrodes (21,22).

T h e c u r v e as s h o w n

i n F i g u r e 5 w a s t a k e n d i r e c t l y f r o m t h e s t r i p c h a r t r e c o r d i n g trace o f a p r o g r a m m e d voltage d c p o w e r s u p p l y ( selected l i n e a r increase o f v o l t a g e w i t h t i m e ) f o r a c e l l i n w h i c h t w o c l e a n e d c o l d r o l l e d steel

electrodes

w e r e i n s e r t e d i n a P l e x i g l a s t a n k at a n electrode s e p a r a t i o n o f 5 c m . T h e surface area o n e a c h electrode exposed t o t h e b a t h w a s 2.5 X 5 c m o r 12.5 s q c m . A s i n d i c a t e d , the slope of the straight l i n e p o r t i o n of t h e c u r v e

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

198

ELECTRODEPOSITION

O F COATINGS

is t h e system resistance, w h i c h , f o r this experiment w a s c a l c u l a t e d f r o m the slope to b e 62.5 ohms. D e c o m p o s i t i o n voltages i n t h e range o f 1.7-2.2 volts c a n b e f o u n d i n literature o n electrolysis f o r electrolytes i n w h i c h

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h y d r o g e n is e v o l v e d at the c a t h o d e a n d o x y g e n at the anode.

Voltage, volts Figure 5.

Decomposition voltage and minimum deposition voltage for an electrocoating material

T h e r e is a p p a r e n t l y o n l y o n e literature reference t o a t h r e s h o l d d e p o s i t i o n voltage o f a p a i n t (8) a l t h o u g h m i n i m u m c u r r e n t

densities

are m e n t i o n e d f r e q u e n t l y . T h e r e is a n e x a m p l e (16) i n w h i c h , f o r t w o materials, ( F i g u r e s 6 a n d 7 i n R e f . 8) t h e flow o f a finite a m o u n t of c u r r e n t ( a t 1.5 a n d 6 volts r e s p e c t i v e l y ) w i t h a y i e l d i n each case of 0 grams o f d e p o s i t e d d r y film w e i g h t occurs.

I

Re

Figure 6.

2

Re

3

Re Ν

Re

Electrical resistance circuit analyzed by a com­ puter

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

13.

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199

Τ hr ο wing Power

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Distance from Cathode End,cm Figure 7. Comparison of experimental results with computer simulation. X , ma­ terial A; O , material B; —, computer simulations. If t h e e x p e r i m e n t as i l l u s t r a t e d b y F i g u r e 5 is p e r f o r m e d at different electrode spacings, t h e m i n i m u m d e p o s i t i o n v o l t a g e ( M D V ) is a l i n e a r f u n c t i o n o f t h e distance b e t w e e n t h e electrodes, the b a t h resistance.

caused, o f course, b y

A p l o t o f m i n i m u m d e p o s i t i o n v o l t a g e vs. electrode

separation e x t r a p o l a t e d to 0 s e p a r a t i o n gives the t r u e m i n i m u m d e p o s i t i o n voltage. Since t h e r e l a t i o n s h i p is l i n e a r , o n l y t w o measurements, at 5 a n d 10 c m s e p a r a t i o n f o r e x a m p l e , are n e e d e d .

B y s u b t r a c t i n g t h e difference

b e t w e e n t h e M D V ' s at 5 a n d 10 c m f r o m t h e v a l u e at 5 c m , t h e M D V at zero separation is o b t a i n e d . D i g i t a l computer simulation of the experimental measurement of t h r o w i n g p o w e r w a s b a s e d o n the f o l l o w i n g assumptions. ( 1 ) A continuous, non-linear, time-dependent function c a n be treated as t h e s u m m a t i o n o f l i n e a r steps i f t h e size of e a c h d i f f e r e n t i a l t i m e i n ­ t e r v a l is s m a l l e n o u g h . ( 2 ) T h e e l e c t r i c a l resistance o f a n e l e c t r o c o a t i n g b a t h c o n f i n e d b y a g i v e n g e o m e t r y c a n b e c a l c u l a t e d f r o m t h e specific resistance of t h e b a t h at t h e t e m p e r a t u r e of t h e e x p e r i m e n t , t h e d i m e n s i o n s of t h e c h a m b e r , a n d t h e area o f t h e electrodes. ( 3 ) T h e e l e c t r i c a l resistance of the d e p o s i t e d film is a l i n e a r f u n c t i o n of the film thickness a n d c a n b e c a l c u l a t e d b y m e a s u r i n g t h e area o f t h e e l e c t r o d e surface b e i n g c o a t e d a n d u s i n g t h e specific resistance o f t h e d e p o s i t e d film i n t h e w a y u s e d t o c a l c u l a t e t h e b a t h resistance. ( 4 ) T h e c u r r e n t flowing to a g i v e n area of the electrode b e i n g c o a t e d at a n y t i m e i n t e r v a l c a n b e d i v i d e d i n t o t h a t p o r t i o n w h i c h causes d e p o s i ­ t i o n at t h e net c o u l o m b s / g r a m rate a p p r o a c h e d as a constant v a l u e (see F i g u r e 3 ) a n d t h e r e m a i n d e r o f t h e c u r r e n t w h i c h causes electrolysis b u t y i e l d s n o d e p o s i t i o n film. ( 5 ) T h e e x p e r i m e n t is p e r f o r m e d u n d e r i s o t h e r m a l c o n d i t i o n s o r u n d e r c o n d i t i o n s sufficiently close t o i s o t h e r m a l that t h e rate o f heat d i s ­ s i p a t i o n is m u c h greater t h a n t h e rate of heat g e n e r a t i o n a n d does n o t i n t r o d u c e a t e m p e r a t u r e effect o n the film a n d b a t h resistance.

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

200

ELECTRODEPOSITION

O F

COATINGS

T h e e q u i v a l e n t e l e c t r i c a l resistance c i r c u i t w h i c h the c o m p u t e r c a l ­ culates is s h o w n i n F i g u r e 6.

F o r a g i v e n geometry t h r o w i n g p o w e r

e x p e r i m e n t (see F i g u r e 1, f o r e x a m p l e ) the v a l u e f o r Re a n d Rs i n F i g ­ u r e 6 c a n b e c a l c u l a t e d d i r e c t l y f r o m the specific resistance of the electro­ c o a t i n g b a t h at the t e m p e r a t u r e a n d solids u s e d f o r t h e e x p e r i m e n t . F o r a cross-section of ( 1 c m X 9 c m ) = 9 s q c m , Re is 1/9 o f the specific resist­ ance p e r centimeter of slot l e n g t h .

Since the distance f r o m t h e center

l i n e of the slot to the a n o d e surface is 0.5 c m , Rs is 1/2 of Re f o r a section of a n o d e area 1 c m w i d e a n d 9 c m h i g h . Rf is a v a r i a b l e resistance w h i c h is 0 f o r n o d e p o s i t e d film a n d is a l i n e a r f u n c t i o n of the film thickness. T h e d i g i t a l c o m p u t e r p e r f o r m s the f o l l o w i n g s e q u e n c e of calculations Downloaded by UNIV LAVAL on July 15, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch013

for each i n t e r v a l of t i m e selected a n d a c c u m u l a t e s the results of successive b u i l d u p s of d e p o s i t e d film o n t h e a n o d e segments A i , A , etc. 2

( 1 ) T h e resistance is c a l c u l a t e d at the n o d e points starting at p o i n t η a n d e n d i n g at p o i n t 1 w h i c h is the t o t a l resistance f o r the entire c i r c u i t d u r i n g the t i m e i n t e r v a l . ( 2 ) B y d i v i d i n g this resistance into the a p p l i e d voltage, the total c u r r e n t flowing t h r o u g h the c i r c u i t d u r i n g the t i m e i n t e r v a l is c a l c u l a t e d . ( 3 ) V o l t a g e d r o p a n d c u r r e n t flow f o r each Re a n d (Rs + calculated.

Rf) are

( 4 ) T h e p o r t i o n of the c u r r e n t flowing t h r o u g h e a c h element ( Rs + Rf) w h i c h results i n d e p o s i t i o n d u r i n g the t i m e i n t e r v a l is c a l c u l a t e d b y (Vs - MOV)/(Rs + Rf). ( 5 ) B a s e d o n t h e l a b o r a t o r y measurements of the net c o u l o m b s / g r a m of deposit, specific resistance of the d e p o s i t e d film, area of t h e anode segment, a n d specific g r a v i t y of the d e p o s i t e d film, t h e a m o u n t of film thickness a d d e d d u r i n g the i n t e r v a l a n d a n e w v a l u e f o r Rf b a s e d o n t h e n e w film thickness are c a l c u l a t e d . ( 6 ) T h e c o m p u t e r repeats the c y c l e of c a l c u l a t i o n s i n steps 1 - 5 f o r a specific n u m b e r of t i m e intervals o r u n t i l a selected film thickness is o b t a i n e d o n the first a n o d e segment, A i . T h e a c t u a l p r i n t o u t of i n f o r m a ­ t i o n c a n b e m e r e l y a table of the anode segment n u m b e r a n d t h e film thickness d e v e l o p e d o n that segment after a g i v e n p e r i o d of t i m e . A l t e r ­ nately, p r i n t o u t s at selected t i m e intervals of a n y of the d e s i r e d voltages, currents, or other v a r i a b l e s m a y b e o b t a i n e d .

Discussion It w a s necessary to establish w h e t h e r o r n o t the m o d e l chosen f o r the d i g i t a l c o m p u t e r s i m u l a t i o n w a s a g o o d one. A t y p i c a l e x a m p l e of t h e k i n d of agreement b e t w e e n the l a b o r a t o r y experiments a n d the c o m p u t e r s i m u l a t i o n is s h o w n b y F i g u r e 7 f o r t w o materials h a v i n g w i d e l y different t h r o w i n g p o w e r . A s F i g u r e 7 shows, the agreement b e t w e e n theory a n d e x p e r i m e n t is excellent.

O b v i o u s l y w e h a v e b e e n able to i d e n t i f y t h e

r e l e v a n t m a t e r i a l properties a n d their i n t e r r e l a t i o n s h i p s as t h e y relate to t h r o w i n g p o w e r . T h o s e properties a r e : specific resistance of t h e d e p o s i t e d

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

13.

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Throwing Power

film a n d specific resistance o f t h e electrocoat b a t h at t h e t e m p e r a t u r e at the start o f t h e e x p e r i m e n t , d e c o m p o s i t i o n v o l t a g e o f t h e system f o r t h e electrodes e m p l o y e d , m i n i m u m d e p o s i t i o n voltage ( M D V ) f o r the system, net e l e c t r i c a l efficiency of d e p o s i t i o n ( c o u l o m b s / g r a m o r m g / c o u l o m b ) , a n d t h e specific g r a v i t y o f t h e d e p o s i t e d c o a t i n g .

V i s u a l evidence

that

voltage a n d t i m e are n o t parameters w h i c h i n f l u e n c e t h e t h r o w i n g p o w e r of e l e c t r o c o a t i n g is f o u n d i n F i g u r e 8. T h e r e is n o a r g u m e n t that i n ­ creased voltage o r t i m e does g i v e m o r e "inches of t h r o w " f o r a g i v e n e l e c t r o c o a t i n g ; h o w e v e r a p r o p o r t i o n a l increase i n film thickness occurs on t h e reference a n o d e as w e l l .

T h e r e f o r e t h e t h r o w i n g p o w e r was. n o t

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changed.

Distance from Cathode End Figure 8.

Effect of voltage or time on deposition

A n e x a m p l e o f the p r a c t i c a l use of t h e d i g i t a l c o m p u t e r s i m u l a t i o n to e v a l u a t e the c o n t r i b u t i o n of one o f t h e r e l e v a n t p r o p e r t i e s o f electrocoatings t o t h r o w i n g p o w e r is s h o w n i n F i g u r e 9. T h e c u r v a t u r e as i n d i -

Distance from Cathode End, cm Figure 9. Effect of minimum deposition voltage on throwing power. Digital com­ puter simulation for: MDV A — 4 volts, Β = 6 volts, C = 8 volts.

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

202

ELECTRODEPOSITION

cated

i n F i g u r e 9 is c o m p u t e r p r e d i c t e d .

O F COATINGS

W e h a v e f o u n d that the

m i n i m u m d e p o s i t i o n voltage o f a n e l e c t r o c o a t i n g d e p e n d s o n t h e a n o d e material.

Q u a l i t a t i v e l y t h e M D V increases

following manner: zinc phosphate

f o r steel substrates

i n the

(Bonderite 3 7 ) , cleaned cold rolled

steel, i r o n p h o s p h a t e ( B o n d e r i t e 1000), a n d u n t r e a t e d g a l v a n i z e d steel. C l e a n e d a l u m i n u m has a M D V h i g h e r t h a n a n y of t h e above

surfaces.

T h r o w i n g p o w e r f o r a g i v e n e l e c t r o c o a t i n g m a t e r i a l w a s s i m i l a r to that s h o w n i n F i g u r e 9 f o r z i n c phosphate, c l e a n g a l v a n i z e d a n d u n t r e a t e d a l u m i n u m substrates i n order o f i n c r e a s i n g m i n i m u m d e p o s i t i o n voltages. T h e most p r o n o u n c e d differences b e t w e e n l a b o r a t o r y e x p e r i m e n t a l d e t e r m i n a t i o n s of t h r o w i n g p o w e r a n d c o m p u t e r s i m u l a t i o n result f r o m Downloaded by UNIV LAVAL on July 15, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch013

heat d i s s i p a t i o n p r o b l e m s .

It w a s possible t o p r o v i d e either n o f o r c e d

c i r c u l a t i o n o r vigorous c i r c u l a t i o n i n t h e slot of t h e t h r o w i n g p o w e r tank ( F i g u r e 1 ) d u r i n g d e p o s i t i o n . I n F i g u r e 10 c u r v e A is o b t a i n e d w i t h v i g o r o u s a g i t a t i o n a n d c u r v e Β w i t h n o agitation. T o evaluate t h e heat d i s s i p a t i o n p r o b l e m further, a test s t r i p w a s c o a t e d i n s i d e a p i p e W h e n t h e cathode w a s p l a c e d at t h e b o t t o m e n d of t h e tube,

(20). results

Distance from Cathode End Figure 10. Effect of heat dissipation of film thickness distribution. A: cath­ ode at top of tube or vigorous circula­ tion in slot. B: cathode at bottom of tube or no circulation in slot. s i m i l a r to c u r v e Β i n F i g u r e 10 w e r e o b t a i n e d . W h e n t h e t h r o w i n g p o w e r t u b e w a s i n v e r t e d so that the cathode w a s at t h e t o p e n d , results s i m i l a r to c u r v e A , F i g u r e 10, w e r e o b t a i n e d .

W e have i n d e p e n d e n t l y deter­

m i n e d the film specific resistance a n d t h e b a t h specific resistance over a t e m p e r a t u r e range f r o m 7 0 ° to 1 1 0 ° F . specific resistance

T h e t e m p e r a t u r e effect o n b a t h

is slight over this t e m p e r a t u r e

range; h o w e v e r the

effect o n t h e specific resistance o f t h e d e p o s i t e d film is severe.

Since the

d e p o s i t i o n occurs at the s u b s t r a t e - f i l m interface a n d t h e heat is generated at this p o i n t a n d must b e d i s s i p a t e d t h r o u g h the substrate a n d t h r o u g h

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

13.

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Throwing Power

203

the d e p o s i t e d film l a y e r , t h e t e m p e r a t u r e effect o n t h e specific resistance of t h e d e p o s i t e d film m u s t b e t h e d o m i n a n t factor.

Summary E l e c t r o d e p o s i t i o n is a r e v e r s i b l e process, as are other s i m p l e elec­ trolysis reactions i n w a t e r s o l u t i o n . D e p o s i t i o n occurs at t h e e l e c t r o d e deposited

film

interface

i f t h e a p p l i e d v o l t a g e exceeds t h e m i n i m u m

d e p o s i t i o n v o l t a g e f o r t h e system. achieve

commercially acceptable

O p e r a t i n g c o n d i t i o n s necessary to

rates of c o a t i n g a p p l i c a t i o n are f a r

r e m o v e d f r o m e q u i l i b r i u m c o n d i t i o n s . It is possible t o s i m u l a t e the a c t u a l

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d e p o s i t i o n of a n e l e c t r o c o a t i n g i n a c a v i t y of d e f i n e d g e o m e t r y b y a n electrical circuit w h i c h may be analyzed b y a digital computer. T h e major d e v i a t i o n s b e t w e e n t h e m o d e l system a n d t h e a c t u a l e x p e r i m e n t arise u n d e r c o n d i t i o n s s u c h that i s o t h e r m a l b e h a v i o r is n o l o n g e r v a l i d . A l t h o u g h t h e energy i n p u t p e r u n i t t i m e i n t e r v a l to e a c h segment of t h e system is easily c a l c u l a t e d , t h e rate a n d n a t u r e of t h e e n e r g y d i s s i p a t i o n are m u c h m o r e c o m p l e x p r o b l e m s .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

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RECEIVED May 28, 1971.

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