Throwing Power as Related to Material Properties with Analysis by

13 Throwing Power as Related to Material Properties with Analysis by Digital Computer Simulation

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. 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 .


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 -


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-


13.


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


(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



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

(250)

150

£ σ

Ο \

I00h

CO

£

jo 3 Ο

Ο

— · — —· *—— Χ — — Χ ο

· χ.

Λ

— ο — — ο

-Ο

50

0

50

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.


13.


197

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


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


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


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


13.


199

Τ hr ο wing Power


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.


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


13.


201

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


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.


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


13.


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


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.

Davey, W . P., U . S. Patent 1,294,627 (Feb. 18, 1919). Sumner, G. C., Trans. Faraday Soc. (1940) 36, 272. Phillips, G . , "Electropainting for the Seventies," London, 1969. Gilchrist, A. E., U . S. Patent 3,230,162 (Jan. 18, 1966). Gilchrist, A. E., U . S. Patent 3,304,250 (Feb. 14, 1967). Gilchrist, A. E., U . S. Patent 3,351,575 (Nov. 7, 1967). Gilchrist, A. E., U . S. Patent 3,351,675 (Nov. 7, 1967). Gilchrist, A. E., U . S. Patent 3,362,899 (Jan. 9, 1968). Finn, S. R . , Hasnip, J. Α., J. Oil Colour Chemists Assoc. (1965) 48, 1121. Tawn, A. R . H . , Berry, J. R., J. Oil Colour Chemists Assoc. (1965) 48, 790. Olsen, D. Α., Boardman, P. J., J. Electrochem. Soc. (1967) 114, 445. Brewer, G . E . F., Hamilton, C . C., Horsh, M . E . , J. Paint Technol. (1969) 41, 114. Hays, D. R . , White, C . S., J. Paint Technol. (1969) 41, 461. LeBras, L. R . , J. Paint Technol. (1966) 38, 85. Cooke, Β. Α., J. Paint Technol. (1970) 34, 12. Finn, S. R . , Mell, C . C., J. Oil Colour Chemists Assoc. (1964) 47, 219. Giboz, J. P., Lahaye, J., J. Paint Technol. (1970) 42, 371. Gilchrist, A. E . , U . S. Patent 3,290,235 (Dec. 6, 1966). Brewer, G . E . F., Horsch, M . E., Madarasz, M . F., J. Paint Technol. (1966) 38, 453. Brewer, G . E . F., Strosberg, G . G., Horsch, M . E., J. Paint Technol. (1967) 39, 551. Maron, S. H . , Prutton, C. F., "Principles of Physical Chemistry," pp. 573574, MacMillan, New York, 1958. Groening, Α. Α., Cady, H . P., J. Phys. Chem. (1926) 30, 1597.

RECEIVED May 28, 1971.


Throwing Power as Related to Material Properties with Analysis by

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