Electrodeposition of Coatings

7 Pigmentation of Electrocoatings DAVID S. YOUNG and ARTHUR T. GRONET MPM Division, Pfizer Inc., 640 North 13th St., Easton, P a . 18042

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While it is generally agreed that the vehicle influences the performance of a pigmented electrocoat system more than the pigment, certain pigments do have a significant influence on many parameters of electrocoating.

Particle size of the

pigment, assuming good dispersion, has little effect on per formance.

A pigment with high specific resistance will usu

ally transfer more efficiently and give better bath stability. Increasing the pigment-to-binder (P/B) ratio will generally increase the specific resistance and throwing power of the bath but will have little effect on coulomb efficiency or rup ture voltage whereas the film becomes harder and the gloss decreases especially at P/B's above 1/1.

Pigments suitable

for electrocoat should have good alkali resistance, low solu bility in water, easy dispersibility,

minimum settling, con

trolled particle size, and a balance between their specific resistance and their specific conductivity in aqueous suspen sions that allows good mobility.

V l T T h i l e t h e o p e r a t i o n of e l e c t r o d e p o s i t i o n systems m a y b e r e l a t i v e l y s i m p l e , t h e m e c h a n i s m o f d e p o s i t i o n is c o m p l e x , i n v o l v i n g electroly sis, electrophoresis,

electroosmosis,

o x i d a t i o n o f m e t a l , coalescence of

c o l l o i d a l particles, a n d gas f o r m a t i o n . T h e b i n d e r of a n e l e c t r o d e p o s i t i o n c o a t i n g c a n b e a m o d i f i e d o i l , a l k y d , a c r y l i c , o r a n epoxyester resin. T h e p o l y m e r s u s e d f o r a n o d i c d e p o s i t i o n h a v e free a c i d o r a c i d a n h y d r i d e groups a t t a c h e d to t h e p o l y m e r c h a i n . A l t h o u g h these b i n d e r s m a y b e u s e d alone a n d clear films d e p o s i t e d , this p a p e r deals strictly w i t h p i g m e n t e d systems. A n e l e c t r o d e p o s i t i o n p r i m e r a n d / o r topcoat m a y b e v i s u a l i z e d as a suspension of p i g m e n t particles a n d p o l y m e r - c o a t e d p i g m e n t

particles

w i t h t h e i o n i z e d c a r b o x y l g r o u p s c o v e r i n g t h e surface so that t h e p a r t i c l e carries a negative e l e c t r i c a l charge.

S i n c e l i k e e l e c t r i c a l charges r e p e l

98

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

7.

99

Pigmentation of Electrocoatings

YOUNG AND GRONET

e a c h other, these negative charges are r e s p o n s i b l e f o r k e e p i n g the p a r ticles d i s p e r s e d i n the w a t e r . T h e s e charges also enable the particles to m i g r a t e t o w a r d the p o s i t i v e l y c h a r g e d anode d u r i n g the e l e c t r o d e p o s i t i o n process w h e n a d i r e c t c u r r e n t of 5 0 - 4 0 0 volts is a p p l i e d . T h e

film

is

f o r m e d b y c o a g u l a t i o n at the a n o d e of these n e g a t i v e l y c h a r g e d m a c r o m o l e c u l e s of the b i n d e r . A t the same t i m e this film is m a d e p r a c t i c a l l y a n h y d r o u s b y electroosmosis. T h e advantages of the e l e c t r o c o a t i n g process over c o n v e n t i o n a l m e t h ods of p a i n t a p p l i c a t i o n are:

(1)

u n i f o r m p i n h o l e - f r e e coatings,

(2)

d e p o s i t i o n of greater film thickness o n h a r d - t o - r e a c h recessed areas a n d areas of b r o k e n c o n t o u r , thus i n c r e a s i n g c o r r o s i o n resistance, Downloaded by UNIV LAVAL on April 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch007

mated operation, (4) lems,

(5)

(3)

auto

e l i m i n a t i o n of fire h a z a r d s a n d a i r p o l l u t i o n p r o b

l o w o p e r a t i n g costs,

and

(6)

fast t h r o u g h p u t .

The

main

disadvantages of the e l e c t r o c o a t i n g process are: ( 1 ) h i g h cost of i n s t a l l a tion, (2)

c o m p l e x q u a l i t y c o n t r o l measures

necessary

to ensure

good

s t a b i l i t y of the o p e r a t i n g tank, a n d ( 3 ) the fact that the system is pres e n t l y l i m i t e d to one-coat a p p l i c a t i o n s a n d d a r k or pastel colors.

Scope T h e e l e c t r o d e p o s i t i o n of p r i m e r s a n d one-coat p a i n t systems as i n d u s t r i a l p r o d u c t i o n finishes has b e e n p r o v e d a p r a c t i c a l a n d b e n e f i c i a l c o m m e r c i a l m e t h o d of c o a t i n g p r o d u c t i o n l i n e p r o d u c t s . present interest are ( 1 ) a u t o m o t i v e p r i m e r s , ( 2 ) (3)

g e n e r a l p u r p o s e one-coat

M a j o r areas

of

appliance primers, and

systems.

A s of S e p t e m b e r 1969 m o r e t h a n 400 electrocoat installations w e r e o p e r a t i n g i n v a r i o u s i n d u s t r i a l plants a r o u n d t h e w o r l d ( I ) .

These i n

stallations w e r e c o a t i n g a b o u t 18 m i l l i o n square feet of m e t a l surface p e r d a y . T h e s e 400 p r o d u c t i o n tanks w e r e split u p o n the five continents as follows : Number of Installations (1) Africa America Asia Australia Europe

Output, sqft/day (Estimated)

5 80 35 3 282

36,000 2,900,000 1,450,000 36,000 14,400,000

405

18,822,000

(1)

I n f o r m a t i o n a v a i l a b l e o n the 282 E u r o p e a n tanks shows that f o l l o w i n g types of articles w e r e e l e c t r o c o a t e d :


the

ELECTRODEPOSITION O F COATINGS

100

Number of Installations Automotive industry A u t o m o t i v e accessories Steel f u r n i t u r e Electro-industry Others

In.,

Output sq ft/day

%

In,

%

71

26

11,000,000

76.0

46

16

544,000

4.2

26

9

300,000

2.2

25

9

240,000

1.6

40

2,400,000

16.0

ÏÔÔ

14,484,000

100.0

114 282~

T h e n u m b e r o f electrocoat tanks i n s t a l l e d a n d t h e r e s u l t i n g p a i n t c a p a c i t y i n electrocoat operations i n N o r t h A m e r i c a i n c r e a s e d t r e m e n d o u s l y Downloaded by UNIV LAVAL on April 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch007

f r o m 1966 t o 1971.

T h e t o t a l c a p a c i t y o f electrocoat

tanks i n N o r t h

A m e r i c a w a s o n l y 81,000 gallons i n 1966 w h e r e a s i n 1971 i t w a s a p p r o x i m a t e l y 2 m i l l i o n gallons. A b r e a k d o w n of tanks, t a n k c a p a c i t y , a n d p a i n t usage i n 1971 f o r N o r t h A m e r i c a is s h o w n i n T a b l e I ( 2 ) . Table I.

Electrocoat Tanks and Usage in N o r t h America Capacity, Tanks

Autos and trucks A u t o parts Electrical Appliances A l u m i n u m extrusions Miscellaneous Total

Range

Average

32

8-76

16

1.5-24

6

35

0.2-18

38

1000 gal Total 1180

2

5.5

210

4 4

3-22

9

220

6

0.3-30

8

40

2.8-11

4.5

142

9

90

26 27

Painty million

250 1990

1 5 25

T h e latest figure f o r 1972 shows at least 1 6 0 tanks i n o p e r a t i o n i n N o r t h A m e r i c a , u s i n g a b o u t 2 . 2 m i l l i o n gallons w i t h a p a i n t v a l u e of a b o u t 2 8 m i l l i o n d o l l a r s ( 2 ) . I t is e s t i m a t e d that a p p r o x i m a t e l y 2 0 % of the passenger cars are p r i m e d b y electrocoat i n t h e U n i t e d States, 4 0 % i n E u r o p e , a n d o v e r 9 0 % i n J a p a n . T h e r e f o r e , at present, the a u t o m o t i v e i n d u s t r y c e r t a i n l y is the major user of e l e c t r o d e p o s i t e d paints.

Historical Background A great d e a l of i n f o r m a t i o n p e r t i n e n t to t h e p r o b l e m of e l e c t r o p h o r e t i c p a i n t d e p o s i t i o n c a n b e o b t a i n e d f r o m the studies r e p o r t e d a n d the literature o n the e l e c t r o d e p o s i t i o n of latex a n d v a r i o u s resins a n d r e l a t e d c o m p o s i t i o n s . O n e of t h e most significant o f the l i t e r a t u r e references ( 3 ) reports that t w o processes f o r d e p o s i t i n g r u b b e r e l e c t r i c a l l y f r o m a latex were developed i n the period 1925-1927. O n e was developed b y S h e p p a r d a n d E b e r l i n a n d t h e other b y P . K l e i n , a n d a n u m b e r o f patents


7.

YOUNG AND GRONET

101


w e r e issued. B o t h processes w e r e b a s e d o n t h e fact that t h e d i s p e r s e d particles o f r u b b e r i n t h e latex c a r r y a negative c h a r g e a n d i n a n electric field

migrate to a n d are deposited o n the anode where they

coagulate.

T h i s anaphoresis p r i n c i p l e w a s k n o w n to K n o x i n 1907 a n d C o c k e r i l l i n 1908,

a n d t h e y a t t e m p t e d t o coagulate r u b b e r f r o m t h e latex b y this

method. O n e o f the first i n d u s t r i a l a p p l i c a t i o n s o f electrophoreses w a s i n t h e dewatering of ceramic clay. This was developed b y C o u n t Schwerin i n G e r m a n y i n the e a r l y 1900's. W i t h a n electric i n p u t o f 8 0 k w h / t o n o f d r y c l a y is w a s p o s s i b l e t o d e w a t e r a s l u r r y f r o m 3 5 % solids suspension t o a p p r o x i m a t e l y 6 5 % solids p l a s t i c c l a y . T h e c e l l p o t e n t i a l w a s n o r m a l l y Downloaded by UNIV LAVAL on April 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch007

a b o u t 100 volts. A l t h o u g h a h i g h e r voltage gave a d r y e r p r o d u c t , m o r e energy w a s lost b y t h e electrolysis o f w a t e r a n d h e a t i n g o f t h e c e l l . B o t h electrophoretic m o v e m e n t o f t h e c l a y particles t o t h e a n o d e a n d t h e elect r o o s m o t i c m o v e m e n t o f t h e w a t e r t o t h e c a t h o d e take p l a c e i n t h e c e l l . T h e d e p o s i t i o n o f plastic, e l e c t r o p h o r e t i c a l l y , f r o m n o n a q u e o u s s o l u tions has b e e n s t u d i e d b y F e i n l i e b (4).

I n this s t u d y t h e r e s i n o r p l a s t i c

w a s t a k e n into s o l u t i o n i n a n a p p r o p r i a t e o r g a n i c solvent, a n d a n o n - s o l v e n t d i l u e n t w a s a d d e d to cause p r e c i p i t a t i o n as a disperse phase. U n d e r s u c h c o n d i t i o n s t h e d i s p e r s i o n w o u l d b e stable f o r a t least t h e t i m e r e q u i r e d to r u n t h e tests. F e i n l i e b s t u d i e d a v i n y l c h l o r i d e - v i n y l acetate copolymer w h i c h was electrophoretically deposited f r o m a solution of b u t y l acetate t o w h i c h 9 5 % e t h y l a l c o h o l w a s a d d e d as t h e p r e c i p i t a n t . A g o o d a d h e r e n t film w a s o b t a i n e d . I n a later a n d m o r e d e t a i l e d s t u d y , F i n k a n d F e i n l i e b investigated the electrodeposition of a number of syn thetic resin lattices ( a q u e o u s d i s p e r s i o n s ) o b t a i n e d as c o m m e r c i a l p r o d ucts f r o m t h e m a n u f a c t u r e r s .

T h e lattices tested i n c l u d e d :

(1) Copolymer of vinylidene chloride a n d acrylonitrile (2) A styrene-butadiene copolymer (3) A n unplasticized poly (vinyl chloride) (4) A plasticized poly (vinyl chloride) ( 5 ) P o l y ( v i n y l acetate) T h e " n e g a t i v e " lattices—i.e., a l l except t h e p o l y ( v i n y l

acetate)—gave

g o o d deposits o n t h e anode u n d e r satisfactory c o n d i t i o n s : voltages o n t h e o r d e r o f 1 v o l t a n d c u r r e n t densities o n t h e o r d e r o f 25 m a / s q i n c h . A c e l l w a s u s e d i n w h i c h the c a t h o d e w a s separated b y a d i a p h r a g m f r o m the b a t h to p r e v e n t h y d r o g e n gas b u b b l e s f r o m b e i n g c a r r i e d to t h e a n o d e a n d b e i n g d e p o s i t e d a l o n g w i t h the film. Deposits of cellulose were obtained b y M a n t e l l a n d C o z z a r e l l i f r o m 1 % s o l u t i o n o f cellulose i n t h e s o d i u m z i n c a t e .

T h e a n o d e deposit w a s

o b t a i n e d w i t h c e l l voltages f r o m 1.1 t o 1.28 volts a n d a c u r r e n t d e n s i t y of ca. 1 - 7 m a / s q i n c h .


102

ELECTRODEPOSITION

O F COATINGS

D e p o s i t s of m e t a l p o w d e r s a n d i n o r g a n i c r e f r a c t o r y carbides a n d oxides h a v e b e e n o b t a i n e d e l e c t r o p h o r e t i c a l l y f r o m n o n a q u e o u s suspen s i o n m e d i u m s . I n ane s t u d y b y M o s l e y a n d W a l l a c e ( 5 ) the l i q u i d s u s e d were isopropyl alcohol a n d nitromethane.

T h e a d h e s i o n of t h e a n o d e

deposit was i m p r o v e d b y a d d i n g 0 . 1 5 M a m m o n i u m hydroxide (using i s o p r o p y l a l c o h o l to d i l u t e the c o n c e n t r a t e d aqueous a m m o n i a s o l u t i o n ) . I n a s o m e w h a t different s t u d y b y P e a r l s t e i n , W i c k , a n d G a l l a c c i o ( 6 ), a n a t t e m p t w a s m a d e to deposit e l e c t r o p h o r e t i c a l l y finely d i v i d e d m e t a l p o w d e r s of s u c h metals as a l u m i n u m , t i t a n i u m , z i r c o n i u m , a n d t u n g s t e n to o b t a i n m e t a l coatings i n those cases w h e r e o r d i n a r y e l e c t r o p l a t i n g methods cannot be used.

It w a s f o u n d that a l u m i n u m e l e c t r o p h o r e t i c


deposits c o u l d b e o b t a i n e d at voltages r a n g i n g f r o m 20 to 320 w h e n u s i n g as s u s p e n d i n g m e d i u m a l i p h a t i c alcohols h a v i n g m o r e t h a n three c a r b o n atoms.

E t h e r s , esters, a n d ketones w e r e unsatisfactory.

I n t h e case of

a l u m i n u m the suspending m e d i u m used determined whether metal was d e p o s i t e d o n t h e a n o d e o r cathode, a n d t h e a d d i t i o n of a s m a l l a m o u n t of b u t y l a m i n e greatly i m p r o v e d t h e a d h e s i o n of t h e deposit. T h e d e p o s i t i o n of fine m i c a flakes w a s s t u d i e d b y H i r a y a m a a n d ( 7 ) B e r g . A silicone b i n d e r w a s u s e d , a n d b o t h silicone a n d m i c a p l a t e d o u t together at a p H of 8 to 9.5. T h i s system is sensitive to p H .

Technical Aspects of Pigmented Electrocoat in g Vehicles T h e m o v e m e n t of b o t h p i g m e n t a n d v e h i c l e particles i n a n electro coat system is associated w i t h t h e " e l e c t r o k i n e t i c p h e n o m e n o n . " T h i s p h e n o m e n o n i n c l u d e s a l l processes i n w h i c h e l e c t r i c a l l y c h a r g e d particles are a c t e d u p o n b y a n external e l e c t r i c a l field w h i c h results i n the c h a r g e d p a r t i c l e s m o v i n g t h r o u g h , or relative to, a fluid m e d i u m . If t h e p a r t i c l e is n o t free to m o v e , the system becomes m e c h a n i c a l l y strained. T h e d e v e l o p m e n t of k n o w l e d g e i n this area closely parallels t h e s t u d y of c o l l o i d s a n d is s a i d to have b e e n d i s c o v e r e d b y R e u s e i n 1808. F r o m a b o u t 1910 to 1935, m a n y t h e o r e t i c a l studies w e r e m a d e i n this area, a n d a n u m b e r w e r e t h e f u n d a m e n t a l basis f o r m u c h of the m o d e r n w o r k i n electro phoresis. E m u l s i o n s , c o l l o i d s , a n d sols consist of e l e c t r i c a l l y c h a r g e d particles of a range of sizes d i s p e r s e d i n a m e d i u m w h o s e d i e l e c t r i c properties p r e v e n t t h e charges f r o m b e i n g d i s s i p a t e d . T h e properties of t h e m e d i u m i n w h i c h the c h a r g e d particles are d i s p e r s e d greatly influence the p r o p erties of t h e system. If the m e d i u m has a h i g h d i e l e c t r i c constant, as does w a t e r , t h e m o v e m e n t of v e r y s m a l l charges ( ions f o r e x a m p l e ) takes p l a c e easily at potentials of less t h a n a c o u p l e of volts. H o w e v e r , i f the m e d i u m is a n o n p o l a r h y d r o c a r b o n , field potentials of several h u n d r e d to a t h o u s a n d volts m a y b e necessary to s h o w p a r t i c l e m o v e m e n t .

If the viscosity


7.

YOUNG AND GRONET

103


is l o w , as i n w a t e r , t h e rates o f m o t i o n o f c h a r g e d particles w i l l b e m u c h greater t h a n i n a viscous m e d i u m ( for the same e l e c t r i c a l field ) . T h e movement of a charged particle suspended i n a m e d i u m w h i c h is i n d u c e d b y a n external electric field is a c t u a l l y m o v e m e n t r e l a t i v e t o the m e d i u m . I f the c h a r g e d p a r t i c l e is fixed, as i n p o r o u s m e m b r a n e s , the l i q u i d w i l l m o v e t h r o u g h t h e m e m b r a n e .

C o n v e r s e l y , i f l i q u i d is

c a u s e d t o flow t h r o u g h a porous m e m b r a n e o r c a p i l l a r i e s , t h e m e m b r a n e or c a p i l l a r i e s a c q u i r e e l e c t r i c a l charges.

E v e n particles s e t t l i n g o u t o f a

suspension w i l l a c q u i r e a n e l e c t r i c a l charge f r o m m o v e m e n t t h r o u g h t h e liquid medium.


T h e particles of a c o l l o i d r e m a i n i n suspension b y reason o f t h e e l e c t r i c a l charges o n each p a r t i c l e .

Some c o l l o i d s h a v e particles w h o s e

charges are either p o s i t i v e o r negative.

A s a result, t h e particles of one

c o l l o i d m a y m i g r a t e to the anode, a n d o f another c o l l o i d t o t h e cathode. T h e m i x i n g o f o p p o s i t e l y c h a r g e d c o l l o i d s u s u a l l y results i n p r e c i p i t a t i o n a l t h o u g h different systems v a r y greatly i n sensitivity. T h e a d d i t i o n of a n electrolyte to a c o l l o i d w i l l o f t e n cause the c o l l o i d to p r e c i p i t a t e b y i n creasing the conductivity of t h e m e d i u m a n d permitting the colloid charges to b e n e u t r a l i z e d o r d i s s i p a t e d . M a n y o f t h e properties o f t h e c h a r g e d particles i n dispersions a r e r e l a t e d to t h e e l e c t r i c a l " d o u b l e l a y e r . " E a c h n e g a t i v e l y c h a r g e d p a r t i c l e , f o r e x a m p l e , w i l l b e s u r r o u n d e d b y a sheath o f p o s i t i v e charges, r e s u l t i n g f r o m t h e r e p u l s i o n o f electrons f r o m its n e i g h b o r h o o d i n t h e e n c l o s i n g m e d i u m . S u c h particles b e h a v e as a c h a r g e d c a p a c i t o r , a n d this aspect o f porous d i a p h r a g m s , electrodes, etc., has b e e n s t u d i e d extensively. T h e w e i g h t o f matter t r a n s p o r t e d f o r each c o u l o m b o f e l e c t r i c i t y passed t h r o u g h a c o l l o i d system f o l l o w s F a r a d a y ' s L a w a l t h o u g h i t is n o t as s i m p l e to d e t e r m i n e i n this case as i t is i n the case of ions i n electrolysis. W h e n i t is c o n s i d e r e d that i n a c o l l o i d t h e p a r t i c l e s are present i n a range o f sizes, w i t h t h e smallest m a n y times t h e size o f ions, a n d that e a c h p a r t i c l e m a y h a v e a range of e l e c t r i c a l charges o n t h e order o f h u n d r e d s of thousands of electrons, i t is to b e e x p e c t e d that average values of w e i g h t a n d c h a r g e w o u l d n e e d t o b e d e t e r m i n e d .

T h i s is d i f f i c u l t

since these factors w o u l d v a r y w i t h age, m e t h o d of p r e p a r a t i o n , a n d other factors. I n electrolysis, each i o n of the species c a n b e c o n s i d e r e d as h a v i n g a w e i g h t a n d a c h a r g e i d e n t i c a l to a l l t h e other ions o f that species, b u t w i t h t h e c o l l o i d s , statistical values m u s t b e u s e d . T h e basic

mathematical

r e l a t i o n f o r electrokinetic

p h e n o m e n a is

credited to H e l m h o l t z :


104

ELECTRODEPOSITION OF COATINGS u «

migration velocity of charged particle ( p i g m e n t or vehicle)

Ζ «= electrokinetic or zeta potential Ε — a p p l i e d voltage d i e l e c t r i c constant o f the l i q u i d phase η =

viscosity of the m e d i u m

4π — a f a c t o r r e l a t e d t o shape a n d size.


F o r ions o r v e r y s m a l l s p h e r i c a l p a r t i c l e s , t h e v a l u e a p p r o x i mates 6. F o r p a r t i c l e s o f l a r g e r size, t h e s m a l l e r f a c t o r is s a i d to b e c a u s e d b y the d i s t o r t i n g effect o f a n i n s u l a t i n g p a r t i c l e i n t h e electric field. T h e e q u a t i o n shows that t h e rate of p i g m e n t or v e h i c l e p a r t i c l e m o v e ment (electrophoresis)

o r flow o f l i q u i d phase (electroosmosis)

is p r o

p o r t i o n a l t o t h e a p p l i e d v o l t a g e , E. T h e zeta p o t e n t i a l m a y b e d e f i n e d as the v o l t a g e difference b e t w e e n the p a r t i c l e a n d t h e diffuse p o t e n t i a l value of the double layer surrounding it.

Influence of Pigment on Deposition Characteristics Particle Size.

Studies o f synthetic a n d n a t u r a l i r o n oxides a n d

extender p i g m e n t s s u c h as talc, barytes, a n d c a l c i u m carbonate

have

s h o w n that the p a r t i c l e size o f these p i g m e n t s ( w h i c h r a n g e d f r o m 0.5 to 8 m i c r o n s i n d i a m e t e r ) h a d l i t t l e o r n o effect o n t h e i r transfer p r o p e r t i e s i n electrocoat p r i m e r systems. H o w e v e r , p i g m e n t s w h i c h h a v e a t e n d e n c y to

flocculate

o r a g g l o m e r a t e w i l l h a v e l o w e r m o b i l i t y rates.

Generally,

the h i g h e r the resistance properties o f the electrocoat v e h i c l e , t h e better w i l l b e the transfer rates o f the p i g m e n t . Effect of p H . S i n c e a l l o f the electrocoat systems i n g e n e r a l p r o d u c t i o n use operate i n a p H r a n g e o f 7.5 t o 9.5, a s t u d y o f synthetic a n d n a t u r a l i r o n oxides i n w a t e r slurries a t p H levels o f 7.0 t o 10.0 s h o w e d that as p H increases, the p i g m e n t m o b i l i t y decreases s l i g h t l y , a n d at p H levels o v e r 10, some oxides a p p e a r to b e c o m e n e u t r a l i n charge o r e v e n reverse charge.

G e n e r a l l y , electrocoat p r i m e r s increase i n c o n d u c t i v i t y ,

h a v e l o w e r t h r o w i n g p o w e r a n d film thickness, i n c r e a s e d gassing, a n d d r a w greater c u r r e n t as the p H o f t h e b a t h is i n c r e a s e d . Settling Characteristics.

B e c a u s e o f t h e l o w viscosities a t w h i c h

electrocoat tanks o r baths are o p e r a t e d , settling o u t o f the p i g m e n t a n d / o r v e h i c l e is a serious p r o b l e m . A l l large c o m m e r c i a l tanks use either a g i tators o r elaborate p u m p i n g e q u i p m e n t t o m a i n t a i n a u n i f o r m tank. A s i n c o n v e n t i o n a l p a i n t systems, p i g m e n t s w h i c h h a v e a h i g h specific g r a v i t y a n d w h i c h are difficult t o g r i n d w i l l cause the most settling p r o b l e m s i n electrocoat tanks. H o w e v e r t h e m a n n e r i n w h i c h t h e d i s p e r s e d p i g m e n t e d


7.

YOUNG AND GRONET


105

paste g r i n d is l e t b a c k a n d t h e r e s u l t i n g s t a b i l i t y o f t h e r e d u c e d electro coat p a i n t system i n r e g a r d t o p r e v e n t i n g p a r t i n t h e s e t t l i n g characteristics

flocculation

o f a n electrocoat

play a n important system.

Generally

a b o u t 1 % o f a b e n e f i c i a t e d n a t u r a l hectorite, b e n t o n i t e , o r s y n t h e t i c c l a y m a y b e a d d e d t o a n electrocoat

p r i m e r system t o h e l p t h e s u s p e n s i o n

properties. Effect of Pigment/Binder Ratio on Electrical Resistance. U s u a l l y t h e specific resistance o f a n electrocoat p r i m e r system w i l l increase w h e n the p i g m e n t - t o - b i n d e r r a t i o is i n c r e a s e d . r e l a t e d t o t h e specific resistance

T h i s increase w i l l u s u a l l y b e

a n d conductivity of the prime and/or

extender p i g m e n t s u s e d t o increase t h e p i g m e n t l e v e l .


COULOMB EFFICIENCY. I n c r e a s i n g t h e P V C content o f a n i r o n o x i d e electrocoat p r i m e r w i l l g e n e r a l l y decrease t h e c o n d u c t i v i t y s l i g h t l y a n d m a y e i t h e r decrease o r increase s l i g h t l y t h e c o u l o m b efficiency o r m a y h a v e n o significant effect o n the c o u l o m b efficiency ( e s p e c i a l l y at p i g m e n t t o - b i n d e r ratios o f 0 . 1 / 1 t o 0 . 5 / 1 ) . THROWING POWER. T h e t h r o w i n g p o w e r o f a n electrocoat p r i m e r i s n o r m a l l y i n c r e a s e d as t h e P V C content increases. at P V C levels f r o m 1 t o 4 5 % .

T h i s is u s u a l l y t h e case

P i g m e n t s w i t h h i g h film resistance g i v e t h e

highest t h r o w i n g power. RUPTURE VOLTAGE. I n c r e a s i n g t h e P V C l e v e l f r o m 1 % t o 1 5 % i n a n electrocoat

p r i m e r system u s u a l l y h a s l i t t l e effect o n r u p t u r e v o l t a g e .

R u p t u r e v o l t a g e i s i n c r e a s e d m a r k e d l y w i t h i n c r e a s i n g solids a n d also b y v e h i c l e s w h i c h d e p o s i t a firm film o f h i g h viscosity. GLOSS OF FILM. S m o o t h films w i t h 6 0 ° gloss r e a d i n g s o f 7 0 ° t o 8 0 ° c a n b e o b t a i n e d at p i g m e n t - t o - b i n d e r ratios u p t o 0 . 5 / 1 .

Generally at

P / B o f 1 / 1 o r 1 . 5 / 1 r o u g h t e x t u r e d films result. T a s k e r a n d T a y l o r ( S ) s h o w e d i n i r o n o x i d e extender p r i m e r s t h a t t h e w e i g h t o f r e s i n d e p o s i t e d w a s constant, i r r e s p e c t i v e o f t h e P / B l e v e l a n d that t h e w e i g h t o f film d e p o s i t e d i n c r e a s e d w i t h i n c r e a s i n g P / B r a t i o ; therefore, m o r e p i g m e n t w a s present w i t h r e s u l t i n g l o w e r gloss. Effect of Bath Solids Level. S y n t h e t i c i r o n o x i d e m a l e i n i z e d o i l elec t r o d e p o s i t i o n p r i m e r s h a v e b e e n s u c c e s s f u l l y d e p o s i t e d at b a t h solids levels o f 7.5 t o 1 5 % .

N o significant differences c o u l d b e n o t e d i n d r y film

thickness o r film hardness.

N o n v o l a t i l e levels o f 7.5 t o 1 0 % a p p e a r t o b e

m o r e satisfactory f o r general a p p e a r a n c e o f t h e film b u t s l i g h t l y p o o r e r t h a n 12.5 t o 1 5 % levels f o r c o r r o s i o n resistance p r o p e r t i e s .

T h e normal

p r o d u c t i o n l e v e l is a b o u t 1 0 % solids i n t h e b a t h . Effect of Specific Resistance of Iron Oxide Pigments. T h e effect o f the s o l u b l e salt content a n d o f t h e specific resistance

of synthetic a n d

n a t u r a l i r o n oxides w a s e v a l u a t e d i n electrocoat p r i m e r systems.

A high

specific resistance a n d a l o w w a t e r - s o l u b l e salt content i s significant t o the efficiency o f the electrocoat process o n l y as i t is r e l a t e d t o a p a r t i c u l a r


106

ELECTRODEPOSITION

O F COATINGS

p i g m e n t . W a s h i n g t h e f o r e i g n electrolytes o u t of a p i g m e n t a n d increas i n g its specific resistance d o n o t necessarily m a k e this p i g m e n t p e r f o r m m o r e efficiently i n a n electrocoat system. T h e r e f o r e , i t is n o t t h e f o r e i g n electrolytes

b u t those

electrolytes

w h i c h c o r r e s p o n d to t h e s o l u b i l i t y

p r o d u c t s of t h e p i g m e n t , its c h e m i c a l c o m p o s i t i o n , a n d its specific c o n d u c t i v i t y i n aqueous suspensions that d e t e r m i n e t h e efficiency of a p a r t i c u l a r p i g m e n t i n a n electrocoat

system.

A l t h o u g h w a t e r - s o l u b l e salt content m a y n o t affect c o a t i n g efficiency a n d d e p o s i t e d film i n t e g r i t y , a l o w soluble-salt content is d e s i r a b l e since an

excessive a c c u m u l a t i o n of salts w i t h r e p e a t e d turnovers w i l l

cause

p o o r t a n k s t a b i l i t y a n d p r o d u c e gases, r e s u l t i n g i n films w i t h p o o r i n t e g r i t y . Downloaded by UNIV LAVAL on April 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch007

Effect on Resistance Characteristics of Deposited Film. T h e a d d i t i o n of p i g m e n t ( s y n t h e t i c r e d i r o n o x i d e ) at P V C l e v e l of 1 % to 1 5 % , to a n electrocoat p r i m e r system ( m a l e i n i z e d o i l t y p e ) , g e n e r a l l y w i l l p r o d u c e firmer,

t h i c k e r films w h i c h results i n i m p r o v e d c o r r o s i o n resistance of the

d e p o s i t e d electrocoat p r i m e r film. T h e a d d i t i o n of 2 % of the t o t a l p i g m e n t a t i o n as s t r o n t i u m c h r o m a t e a n d / o r 5 to 1 0 % of t h e p i g m e n t a t i o n as a basic l e a d p i g m e n t w i l l s i g n i f i c a n t l y i m p r o v e t h e salt f o g resistance of a n i r o n o x i d e electrocoat p r i m e r .

Selection of Pigments V a r i o u s selected

grades of synthetic r e d a n d y e l l o w i r o n oxides,

c h r o m i u m oxides, c a r b o n a n d l a m p b l a c k , t i t a n i u m d i o x i d e , p h t h a l o c y a n i n e b l u e a n d green, a n d some o r g a n i c y e l l o w s a n d reds h a v e b e e n successfully u s e d i n electrocoat

p a i n t systems, a l o n g w i t h t h e f o l l o w i n g

(see also

Table I I ) : ( 1 ) A n t i - c o r r o s i v e p i g m e n t s : v a r i o u s grades of l e a d p i g m e n t s ( basic w h i t e l e a d or s i l i c o c h r o m a t e ) , chromâtes w i t h l o w w a t e r s o l u b i l i t y ( s t r o n t i u m c h r o m a t e ) , z i n c sulfide a n d z i n c o x i d e . ( 2 ) E x t e n d e r p i g m e n t s : some grades of barytes, talc, c a l c i u m car bonate, k a o l i n , silicas, m i c a , a n d asbestos. ( 3 ) S u s p e n d i n g p i g m e n t s : b e n e f i c i a t e d b e n t o n i t e o r hectorite c l a y s , synthetic clays, a n d c o l l o i d a l silicas. A l t h o u g h r a w m a t e r i a l cost is s t i l l i m p o r t a n t i n electrocoat f o r m u l a tions, t h e p i g m e n t s selected m u s t h a v e g o o d transfer a n d s t a b i l i t y p r o p erties

with

the given vehicle

system

that

is u s e d .

Many

pigment

m a n u f a c t u r e r s h a v e d o n e extensive w o r k w i t h their l i n e of c o n v e n t i o n a l p i g m e n t s i n v a r i o u s types of electrocoat v e h i c l e systems; i n m a n y instances t h e y h a v e m a d e t a i l o r e d p r o d u c t s to meet t h e electrocoat

requirements.

T h e s e p i g m e n t s w i l l n e e d properties s u c h as excellent s t a b i l i t y i n a n a l k a l i n e m e d i a , l o w s o l u b i l i t y i n water, easy d i s p e r s i b i l i t y , m i n i m u m settling, c o n t r o l l e d p a r t i c l e size a n d shape ( m a x i m u m of 6 m i c r o n s ), a n d a b a l a n c e b e t w e e n specific resistance a n d specific c o n d u c t i v i t y i n aqueous suspensions that a l l o w s g o o d m o b i l i t y or transfer p r o p e r t i e s .


7.

YOUNG AND GRONET Table II.

T y p i c a l Formulation of Electrocoat Paints Ca. 100 gal Formula Parts by Weight

R e d Oxide Automotive Primer


107


S y n t h e t i c i r o n oxide B a s i c w h i t e lead B e n t o n i t e or hectorite clays A m i n e s o l u b i l i z e d m a l e i n i z e d o i l vehicle Triethylamine B u t y l Cellosolve Deionized water

20 2 0.2 13 1 3 17

Disperse i n p o r c e l a i n or sand m i l l Reduction A m i n e s o l u b i l i z e d m a l e i n i z e d o i l vehicle Diethanolamine 6 % Manganese drier Guaiacol D e i o n i z e d water

90 3 0.2 0.3 730 880.0 l b s

Pigment volume concentration N o n - v o l a t i l e content pH W e i g h t per g a l Specific resistance, 1300 ohms

5.0% 10.0% 7.8—8.0 8.8

R e d or B l a c k O n e - C o a t S y s t e m C a r b o n b l a c k or s y n t h e t i c I r o n O x i d e W a t e r reducible e p o x y - e s t e r vehicle B u t y l Cellosolve Triethylamine Deionized water D i s p e r s e i n p o r c e l a i n m i l l or sand m i l l Reduction W a t e r reducible e p o x y - e s t e r vehicle B u t y l Cellosolve Triethylamine Deionized water

3.5 13 0.4 0.4 17

87 2 3 720 846.3 l b s

Pigment volume concentration N o n - v o l a t i l e content pH W e i g h t per g a l Specific resistance, 1300 o h m s

1.0% 10.0% 8.5 8.5

Anti-Corrosion Pigments. M a n y o f t h e r e g u l a r a n t i - c o r r o s i o n p i g ments u s e d i n c o n v e n t i o n a l p a i n t systems are t o o s o l u b l e t o b e u s e d i n electrocoat systems.

T h i s is e s p e c i a l l y true o f most o f t h e c h r o m a t e


108

ELECTRODEPOSITION

OF

COATINGS

p i g m e n t s . T h e i r r e l a t i v e l y h i g h s o l u b i l i t y i n a n aqueous electrocoat t a n k w h e r e electrolysis is t a k i n g p l a c e m a y cause p r e c i p i t a t i o n of the m e t a l cations a n d release of f r e e c h r o m a t e ions into the b a t h ; these ions c a n react w i t h the electrocoat v e h i c l e a n d cause

flocculation

problems.

solubility lead and zinc pigments along w i t h strontium chromate to w o r k best i n most electrocoat v e h i c l e systems.

Low

appear

H o w e v e r each vehicle

system w i l l h a v e different properties, a n d each i n d i v i d u a l a n t i - c o r r o s i o n p i g m e n t m u s t be e v a l u a t e d f o r s t a b i l i t y i n each specific v e h i c l e system that is u s e d . Pigment Formulation Variables.

D e c r e a s i n g the p i g m e n t v o l u m e

c o n c e n t r a t i o n of a n electrocoat p a i n t or p r i m e r system w i l l u s u a l l y g i v e Downloaded by UNIV LAVAL on April 6, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch007

a softer film w i t h greater film smoothness b u t less h i d i n g p o w e r . Increas i n g the p i g m e n t v o l u m e c o n c e n t r a t i o n w i l l give h a r d e r films w h i c h are r o u g h e r i n appearance.

T h i s increase i n P V C w i l l u s u a l l y increase

t h r o w i n g p o w e r a n d also m a k e the d e p o s i t e d film m o r e susceptible

the to

pinholing. P r i m e , extender, a n d a n t i - c o r r o s i o n p i g m e n t s h a v e a definite effect o n the s t a b i l i t y of the electrocoat p a i n t system. P i g m e n t s w h i c h h a v e the d e s i r e d s t a b i l i t y i n a l k a l i n e médias, p r o p e r size, c h e m i c a l c o m p o s i t i o n , a n d l o w s o l u b i l i t y w i t h a c c o m p a n i e d h i g h specific resistance h a v e b e e n u s e d successfully at P V C levels of 1 to 2 0 % .

U s u a l l y the t y p e of v e h i c l e

has m o r e effect o n the s t a b i l i t y of the electrocoat system t h a n does the p i g m e n t , b u t it is s t i l l i m p o r t a n t that e a c h p i g m e n t or c o m b i n a t i o n of p i g m e n t s b e t h o r o u g h l y e v a l u a t e d ( at the d e s i r e d P V C l e v e l ) i n the l a b oratory b e f o r e large scale p r o d u c t i o n t a n k trials are m a d e .

Certainly

the i n c r e a s e d use of a p i g m e n t w h o s e electrolyte content is of the t y p e that contributes to the pigment's h i g h s o l u b i l i t y i n w a t e r w i l l u n d o u b t e d l y cause

flocculation

and tank stability problems.

Testing Methods for Performance E l e c t r o c o a t paints a n d / o r tanks are g e n e r a l l y tested f o r the f o l l o w i n g properties: ( 1 ) Throwing power: m e a s u r i n g the p e r c e n t of p a i n t d e p o s i t e d i n side a % - i n c h s t a n d a r d gas c o n d u i t ( w i t h or w i t h o u t a steel p a n e l i n s i d e ) or t h e percent p a i n t d e p o s i t e d o n the i n s i d e of either t w o or three phosp h a t e d steel panels that are separated b y % - i n c h shims. ( 2 ) pH and alkalinity: 50 m l of the p a i n t are u s u a l l y c h e c k e d w i t h a p H meter. T h e a l k a l i n i t y is u s u a l l y d e t e r m i n e d i n m i l l i e q u i v a l e n t s of a m i n e p e r 100 grams of p a i n t solids. ( 3 ) Conductivity: m a y b e m e a s u r e d w i t h a c i r c u l a t i o n c e l l c o n n e c t e d to a m e a s u r i n g b r i d g e of the W h e a t s t o n e t y p e , the p a i n t b e i n g the u n k n o w n resistance. (4)

Non-volatile content.


7.

YOUNG AND GRONET

109


( 5 ) Thickness of the cured film: m a y b e m e a s u r e d i n m i l s b y elec t r o n i c thickness tester o r i n m i c r o n s b y a P e r m a s c o p e . ( 6 ) Ash-binder ratio: 10 t o 15 m i l h e a t e d a t 2 2 0 ° F t o d e t e r m i n e n o n - v o l a t i l e content. T h e n o n - v o l a t i l e is t h e n ashed at 1 4 0 0 ° F f o r o n e h a l f h o u r . T h i s r a t i o is v e r y i m p o r t a n t because o n e c a n d e t e r m i n e t h e a s h - t o - b i n d e r r a t i o o f t h e t a n k a n d also that o f t h e d e p o s i t e d film. N o r m a l l y t h e p i g m e n t w i l l deposit m o r e efficiently t h a n t h e b i n d e r , a n d t h e p i g m e n t - t o - b i n d e r ratio w i l l decrease i n t h e tank as i t is w o r k e d . A d j u s t ments m u s t b e m a d e to k e e p this r a t i o i n b a l a n c e . ( 7 ) Coulombic yield: u s u a l l y expressed as t h e w e i g h t i n m i l l i g r a m s d e p o s i t e d o n t h e a n o d e f o r e a c h c o u l o m b c o n s u m e d a n d is c a l c u l a t e d f r o m a r e c o r d i n g amperemeter.


O t h e r l a b o r a t o r y tests m a y b e p e r f o r m e d s u c h as p u m p i n g s t a b i l i t y and

c o n t i n u o u s process testing o f t h e l i f e s p a n o f a n electrocoat b a t h .

A c o i l stock c o a t i n g d e v i c e w i t h v a r i a b l e speed d r i v e , heat a n d p a i n t f e e d e q u i p m e n t is u s e d .

exchangers,

F e e d , temperature, a n d throughput

are r u n t h e same as i n a p r o d u c t i o n tank.

T h e tank is r u n c o n t i n u o u s l y

u n t i l t h e d e p o s i t e d films d o n o t meet t h e specifications o r u s u a l l y u n t i l 20 times t h e w e i g h t o f solids u s e d as o r i g i n a l fill has b e e n c o n v e r t e d i n t o c o a t i n g . A 5 - g a l l o n l a b o r a t o r y tank w i l l c o m p l e t e l y t u r n over i n o n e d a y whereas i t takes a b o u t 2 0 d a y s f o r a p r o d u c t i o n tank t o t u r n over.

With

t h e present k n o w l e d g e o f electrocoat systems a n d b y u s i n g u l t r a f i l t r a t i o n the b a t h l i f e o f a n electrocoat a u t o m o t i v e p r i m e r t a n k c a n b e c o n t r o l l e d for i n d e t e r m i n a t e p e r i o d s o f t i m e .

Literature Cited 1. " T h e Influence of the Construction of Electrocoat Installation on Coating Results," D r . H. Frangen, Conference on Electro-Painting, L o n d o n 28 and 29, October, 1969, organized b y Business Conferences & Exhibitors, Ltd. 2. Levinson, S. B . , "Electrocoat, Powdercoat, Radiate," J. Paint Technol. (June 1972) 44, 569; Pt. I p. 49. 3. Hauser, Ernest Α., "Latex," translated b y W. J. Kelly, p p . 136-137, Chemical Catalog C o . , N e w York, 1930. 4. Feinlieb, M., "Electrodeposition of V i n y l Plastics," Trans. Electrochem. Soc. (1945) 88, 11. 5. Mosley, J. R., Wallace, T . C., "Electrophoretic Deposition, A Versatile Coat ing M e t h o d , " J. Electrochem. Soc. (1962) 109, 923. 6. Pearlstein, F . , W i c k , R., Gallaccio, Α., "Electrophoretic Deposition of Metals," J. Electrochem. Soc. (1963) 110, 843. 7. Berg, D., Hirayama, C., "Studies on the Electrophoretic Deposition of Mica," Electrochem. Technol. (1963) 1, 224. 8. Tasker, L., Taylor, J . R., J. Oil Colour Chemist's Assoc. (1965) 48, 121. RECEIVED M a y 28, 1971.


Electrodeposition of Coatings

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