Electrodeposition of Carboxyl Containing Copolymers - Advances in

9 Electrodeposition of Carboxyl Containing Copolymers Basic Studies, Phase Growth, Counterion Fixation

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch009

A.

E.

RHEINECK

a

and A. M. U S M A N I

Polymers and Coatings Department, N o r t h Dakota State University, Fargo, N. D.

Two copolymers containing the potassium salts of acrylic or methacrylic acid were used in vehicles for electrodeposi tion at a constant voltage (125 volts). Zinc, steel, and tin were used as anodes. Up to 88.8% of the total charge trans ferred was used for anodic dissolution with the vehicle while 72.8%

acrylic

was used in anodic dissolution with

the methacrylic vehicle.

Electrodeposits contained only 0.48

to 0.77 wt % potassium; thus, counterion fixation was slight. NMR

results indicate that electrodeposition has only a minor

effect on tacticity and microstructure

of the

Infrared results confirm that decarboxylation

copolymers. occurs espe

cially at the higher voltages, but molecular weight deter minations indicate that macroradical recombination

is not

extensive.

"Decently

the e l e c t r o d e p o s i t i o n

of o r g a n i c

coating

c o m p o s i t i o n s has

s h o w n g o o d p o t e n t i a l , a n d the c o m m e r c i a l e x p l o i t a t i o n as a m e t h o d of c o a t i n g a p p l i c a t i o n is n o w a r e a l i t y . It has a n d w i l l c o n t i n u e to m a k e a firm i m p a c t i n c o a t i n g a p p l i c a t i o n operations s u c h as the p r i m i n g of a u t o m o b i l e b o d i e s , t h e c o a t i n g of a l u m i n u m extrusions, a n d h o m e a p p l i a n c e parts a n d r e l a t e d objects. H i s t o r i c a l l y , C r o s s e a n d B l a c k w e l l , L t d . (1,2), d e v e l o p e d a n d s h o w e d the possible p o t e n t i a l of t h e a p p l i c a t i o n s of "oleoresinous l a c q u e r s " b y a n e l e c t r i c current.

T o d a y , c o m m e r c i a l systems use v a r i o u s types of w a t e r

s o l u b l e or w a t e r d i s p e r s i b l e p o l y m e r i c c o m p o s i t i o n s . T h e s e are g e n e r a l l y a

Deceased, August 10, 1971. 130

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

9.

RHEiNECK

AND

131

Carboxyl Containing Copolymers

usMANi

carboxyl bearing polymeric compositions inherently insoluble i n water b u t c a p a b l e of c o m b i n i n g w i t h o r g a n i c or i n o r g a n i c bases. T h e c h e m i c a l c o m p o s i t i o n s a n d properties of s u c h systems are d e s c r i b e d b y G i l c h r i s t ( 3 ) , H a g e n (4),

a n d S u l l i v a n ( 5 ) . W h e n a n e l e c t r i c a l p o t e n t i a l is i m

pressed across their aqueous systems, a d e p o s i t i o n o n t h e anode w i l l o c c u r . I n a d d i t i o n to a s o l u b i l i z e d o r d i s p e r s e d r e s i n system, a n electro d e p o s i t i o n b a t h m a y c o n t a i n d i s p e r s e d p i g m e n t s , i o n i z a b l e a n d other c o m p o u n d s as d e s c r i b e d b y G l o y e r et al. (6).

T h u s , ions a n d surface-

c h a r g e d particles i n t h e broadest sense are i n v o l v e d . tinuous electrodeposition

o p e r a t i o n requires


m a i n t a i n a m a t e r i a l balance. c o m p o s i t i o n tends critical.

that

A successful c o n

a l l the

components

H o w e v e r , d u r i n g electrodeposition the bath

to change

u n e v e n l y , a n d u n e v e n changes m a y b e

If the s h i e l d e d areas o n objects to b e electrocoated

p r o p e r l y coated, t h e coatings l a c k t h r o w i n g p o w e r .

are n o t

M e t h o d s to deter

m i n e a n d o v e r c o m e these p r o b l e m s h a v e b e e n discussed b y B r e w e r a n d c o - w o r k e r s (7—10) i n their d e s c r i p t i o n s of t h e F o r d M o t o r C o . ' s electroc o a t i n g process.

I n a s i m i l a r v e i n , a n I t a l i a n process is d e s c r i b e d b y

Bono a n d Pagani ( I I ) . T h e a n o d i c electrode reactions h a v e b e e n s t u d i e d b y L e B r a s H a g e n et al. ( 1 3 ) , a n d M a y a n d S m i t h (14).

(12),

I n a previous report ( 1 5 ) ,

f r o m o u r l a b o r a t o r y , electrode reactions o c c u r r i n g d u r i n g electrodeposi t i o n i n t h e case of a m a l e i c a d d u c t of a p o l y m e r i c p o l y o l - o l e i c a c i d ester neutralized w i t h potassium hydroxide were described.

T h e important

r o l e o f m e t a l l i c cations generated f r o m t h e a n o d e w a s e l u c i d a t e d a n d the extent of cation—polyion i n t e r a c t i o n w a s discussed. F r e s h electrodeposited coatings are c u r e d b y b a k i n g o r m a y b e c u r e d b y i n d u c t i o n h e a t i n g . R e c e n t l y , t w o patents (16, 17) h a v e b e e n g r a n t e d to the F o r d M o t o r C o . T h e s e cover the c u r i n g b y r a d i o c h e m i c a l m e t h o d s a n d electrodeposited coating methods.

T h i s c u r i n g process merges elec

t r o d e p o s i t i o n a n d h i g h energy c u r i n g . T h e r e is n o d o u b t that this merger of processes represents a t e c h n o l o g i c a l a d v a n c e . T h e objective of this p a p e r is to e l u c i d a t e t h e m e c h a n i s m of electro d e p o s i t i o n at constant v o l t a g e w h e n c a r b o x y l c o n t a i n i n g a c r y l i c c o p o l y mers a r e u s e d as e l e c t r o d e p o s i t i o n resins. M e t h o d s i n c l u d e d e t e r m i n a t i o n of t h e mass of t h e electrodeposit as a f u n c t i o n of t i m e a n d charge trans f e r r e d , d e t e r m i n a t i o n of c u r r e n t v a r i a t i o n s w i t h t i m e , analysis of m e t a l content of electrodeposits, a n d i n v e s t i g a t i o n of film structure b y m o l e c u l a r w e i g h t d e t e r m i n a t i o n a n d i n f r a r e d a n d N M R spectroscopy.

T h e metals

a n a l y z e d i n c l u d e cations f o r m e d b y a n o d i c d i s s o l u t i o n a n d p o t a s s i u m ions w h i c h are present as counterions i n t h e e l e c t r o d e p o s i t i o n b a t h .

Since

the r e t e n t i o n o f counterions i n t h e electrodeposit m a y i n f l u e n c e

water

a n d salt spray resistance of t h e film, p o t a s s i u m content is of p a r t i c u l a r interest.


132

ELECTRODEPOSITION

O F

COATINGS

Experimental Electrodeposition. T w o a c r y l i c c o p o l y m e r s w e r e u s e d f o r t h e s t u d y ; the c o m p o s i t i o n s are i n d i c a t e d i n T a b l e I. T h e c o p o l y m e r i z a t i o n w a s c o n d u c t e d at 125 =b 2 ° C w i t h 2 - b u t o x y e t h a n o l as t h e solvent. T h e cata lyst w a s a 4 % d i - i e r i - b u t y l p e r o x i d e s o l u t i o n b a s e d o n t o t a l m o n o m e r i n b o t h reactions. T h e final n o n - v o l a t i l e content o f b o t h r e s i n solutions w a s 7 0 . 0 % . T h e a c i d values b a s e d o n solids w e r e 76.0 f o r c o p o l y m e r A a n d 75.0 f o r c o p o l y m e r M (as m g K O H / g r a m s o l i d s ) .


Table I.

Monomer Mole Composition of Electrodeposition Resins

Monomers

Copolymer A, Moles

Copolymer M, Moles

M e t h y l methacrylate n - R u t y l acrylate A c r y l i c acid Methacrylic acid

1.5 4.0 1.0 —

1.5 4.0 — 1.0

B o t h resins w e r e n e u t r a l i z e d to t h e extent of 9 3 % w i t h p o t a s s i u m h y d r o x i d e a n d t h e i r solids adjusted to 7 . 0 % w i t h d e i o n i z e d w a t e r . T h e electrodeposition vehicle obtained f r o m copolymer A w i l l be henceforth r e f e r r e d to as E D V - A a n d that f r o m c o p o l y m e r M as E D V - M . T h e p H of b o t h solutions w a s 8.3. A l l depositions w e r e r u n b y the constant voltage t e c h n i q u e . T h e voltage w a s a p p l i e d p r i o r to i m m e r s i o n of the anode into t h e electro d e p o s i t i o n c e l l . T h e anodes s t u d i e d w e r e t i n , z i n c , a n d c o l d r o l l e d steel. A l l electrodes w e r e 7.5 c m w i d e a n d i m m e r s e d 7.5 l i n e a r c m . T h e total area of the i m m e r s e d surface was 112.5 c m p e r electrode. T h e d e p o s i t i o n v o l t a g e w a s m a i n t a i n e d at 125 volts, unless o t h e r w i s e i n d i c a t e d , a n d the electrode residence t i m e i n the cells w a s v a r i e d f r o m 15 to 90 seconds. It w a s felt that the y i e l d s of the deposit p e r electrode o n 112.5 c m w e r e too s m a l l f o r t h e a n a l y t i c a l w o r k . T h e r e f o r e i n e a c h r u n , a d u p l i c a t e w a s also p r e p a r e d , a n d the d e p o s i t e d films c o m b i n e d . I n other w o r d s , the t o t a l d e p o s i t i o n surface w a s 2 X 112.5 = 225 c m f o r t h e results to be described. A l l panels after e l e c t r o d e p o s i t i o n w e r e w a s h e d a n d d r i e d i n a v a c u u m o v e n at 110 ± 2 ° C . T h e q u a n t i t y of e l e c t r i c i t y i n m i c r o f a r a d s , /xF, w a s d e t e r m i n e d f r o m t h e c o u l o m b i c curves. F o r t w o s i m i l a r runs, t h e y w e r e a d d e d to get the t o t a l μ¥. L i k e w i s e , the w e i g h t s of t w o s i m i l a r deposits w e r e d e t e r m i n e d a n d c o m b i n e d to get t h e t o t a l w e i g h t . Recovery and Analysis of Electrodeposits. T h e electrodes w i t h t h e d e p o s i t e d films after a v a c u u m d r y i n g w e r e exhaustively extracted w i t h m e t h y l e t h y l ketone i n a Soxhlet extractor. T h e extraction w a s c o n t i n u e d u n t i l n o o r g a n i c m a t e r i a l r e m a i n e d o n t h e electrodes. T h i s w a s c h e c k e d b y r e c o r d i n g a surface s p e c t r u m of the electrode i n the i n f r a r e d r e g i o n (18). M e t h y l e t h y l ketone w a s e v a p o r a t e d f r o m the s o l u b i l i z e d p o l y m e r films. T h e solids w e r e t h e n a s h e d at a b o u t 6 0 0 ° C i n a muffle f u r n a c e f o r six hours. A s m a l l p o r t i o n of p e r c h l o r i c a c i d was u s e d to assist the r e m o v a l of t h e o r g a n i c matter. A s controls, p o l y m e r s A a n d M per se, d i d n o t leave a n y r e s i d u e w h e n s i m i l a r l y ashed. T h e i n o r g a n i c r e s i d u e f r o m a s h e d e l e c t r o d e p o s i t e d films w a s d i s s o l v e d i n d i l u t e n i t r i c a c i d . 2

2

2



9.

RHEiNECK AND

usMANi


133

M e t a l contents of the ashed solutions, thus p r e p a r e d , w e r e deter m i n e d b y a t o m i c a b s o r p t i o n spectroscopy (19). F o r atomic absorption analysis, the l i q u i d s o l u t i o n s a m p l e w a s a t o m i z e d i n a flame. M e t a l l i c ions present w e r e r e d u c e d to n e u t r a l atoms. A n O s r a m l a m p w a s u s e d to o b t a i n p o t a s s i u m s p e c t r u m w h i l e h o l l o w cathode l a m p s of i r o n a n d z i n c w e r e u s e d to o b t a i n the spectra of i r o n a n d z i n c , r e s p e c t i v e l y (19). C a l i b r a t i o n curves w e r e p r e p a r e d for a l l the metals w i t h a n a l y t i c a l grade i n o r g a n i c salts. T h e w a v e l e n g t h s u s e d for p o t a s s i u m , z i n c , a n d i r o n w e r e 4665, 2139, a n d 2483 A , respectively. A P e r k i n - E l m e r m o d e l 421 spectrophotometer was u s e d to r e c o r d the i n f r a r e d spectra of electrodeposits. T h e s e spectra w e r e a n a l y z e d to f o l l o w the change i n c a r b o n y l content w i t h d e p o s i t i o n voltage. T h e s t a n d a r d base l i n e t e c h n i q u e c o u p l e d w i t h r a t i o n i n g the peak-areas was used. T h e m e t h o d is d e s c r i b e d b y R h e i n e c k et al. (15, 18). T h e c a r b o n y l content Yco is expressed as f o l l o w s : _

Y c

o

A r e a of c a r b o n y l peak A r e a of h y d r o c a r b o n peak

N M R spectra w e r e o b t a i n e d o n a V a r i a n A 6 0 A h i g h r e s o l u t i o n spec trometer. T h e p o l y m e r c o n c e n t r a t i o n was 2 0 % i n d e u t e r a t e d c h l o r o f o r m . T e t r a m e t h y l s i l a n e w a s u s e d as a n i n t e r n a l s t a n d a r d , a n d spectra w e r e r e c o r d e d at a m b i e n t temperature. N u m b e r average m o l e c u l a r w e i g h t s , M , w e r e d e t e r m i n e d w i t h a M e c h r o l a b v a p o r phase osmometer. B e n z i l , r e c r y s t a l l i z e d three times f r o m m e t h a n o l w a s u s e d to o b t a i n the c a l i b r a t i o n c u r v e . n

Results and Discussion Mass of Electrodeposit. E l e c t r o d e p o s i t i o n i n v o l v e s phase ( i n c l u d i n g phase d i s s o l u t i o n ) . C o h n (20)

extension

s t u d i e d the t a r n i s h i n g of silver

m e t a l b y halogens a n d b y a p p l y i n g the d i s o r d e r t h e o r y o b t a i n e d t w o t i m e dependencies.

M a t h e m a t i c a l l y , this m a y b e represented as f o l l o w s : am + m

2

(1)

= p*t

w h e r e : a is constant T h e a b o v e c o n c e p t c a n be u s e d i n e l e c t r o d e p o s i t i o n w h i c h is essentially a phase g r o w t h process.

T w o l i m i t i n g cases are p o s s i b l e : m is mass of phase g r o w t h ρ is p r o p o r t i o n a l i t y constant t is t i m e of r e a c t i o n

C a s e 1: w h e n t is s m a l l ; m

2

am. 2

m =

(ρ·0

T h u s , E q u a t i o n 1 reduces t o : (2) T h e n , E q u a t i o n 1 becomes:

1 / 2


(3)

134

ELECTRODEPOSITION

O F COATINGS

I n F i g u r e s 1 a n d 2, mass d e p o s i t e d vs. t i m e a n d square root of t i m e are plotted f o r zinc anode a n d E D V - A a n d È D V - M respectively.

Both the

a b o v e l i m i t i n g cases ( E q u a t i o n s 2 a n d 3 ) a r e satisfied. O l s o n (21) has suggested a n u n i m p e d e d p h a s e g r o w t h d u r i n g early stages of d e p o s i t i o n . D u r i n g a d v a n c e d d e p o s i t i o n stages d i f f u s i o n t h r o u g h t h e g r o w i n g phase becomes important.

It is also t o b e n o t e d f r o m these graphs that t h e

square root t i m e d e p e n d e n c y is v a l i d u p to 45 sec o f d e p o s i t i o n ; t h e i n t e r c e p t changes, a n d t h e n t h e r e l a t i o n s h i p h o l d s a g a i n . T h i s i n v a l i d a t e s the c o n c e p t u a l c o n s t a n c y o f p. H o w e v e r , o u r results suggest that ρ m i g h t h a v e t w o values, X a n d Y , w h i c h a r e t i m e a n d process d e p e n d e n t .

I n the


case of a c o l d r o l l e d steel a n o d e , t h e a b o v e d e s c r i b e d d e p e n d e n c i e s w e r e n o t o b e y e d f o r b o t h e l e c t r o d e p o s i t i o n vehicles. DEPOSITION 15

ι

Figure 1.

30

1

TIME, 45

1

Sec. 60

1

75

1

90

r

Deposit mass vs. time and square root of time for zinc anode and EDV-A

F i n n a n d H a n s i p (22) h a v e s h o w n m a t h e m a t i c a l l y that t h e r e c i p r o c a l of square of c u r r e n t s h o u l d g i v e a l i n e a r c u r v e w h e n p l o t t e d

against

d e p o s i t i o n t i m e . T h i s r e l a t i o n s h i p is o n l y v a l i d i f t h e specific resistance of t h e deposit r e m a i n s constant d u r i n g t h e entire d e p o s i t i o n p e r i o d . F i g ures 3 a n d 4 are plots o f r e c i p r o c a l of t h e square o f c u r r e n t , 1/c vs. d e p o 2

s i t i o n t i m e , t, at a d e p o s i t i o n voltage o f 125 volts f o r z i n c a n d c o l d r o l l e d steel electrodes r e s p e c t i v e l y .

T h e a b o v e r e l a t i o n s h i p is f o l l o w e d reason-


9.

RHEiNECK

AND usMANi

DEPOSITION 15

135


TIME,

30

sec.

60

45

B 0.6 a

'

K E Y

/


2 H 0 +

0

2

2

P r o t o n a t i o n results i n p r e c i p i t a t i o n , a n d t h e p r o t o n c a p t u r e p r o d u c t c o n tributes s u b s t a n t i a l l y to the t o t a l deposit. ( 2 ) T h e i n t e r a c t i o n of p o l y a n i o n s w i t h m u l t i v a l e n t cations b y a n o d i c d i s s o l u t i o n is represented,

generated

f o r example, i n the case of z i n c

anode as f o l l o w s : - I 2+ OH 2

H 0 L 2

I

pry

H 0

+

ι

OH

Ο D

^0

OH

/

,

2

OH

ΛΟΗ

2

2

2

2

/ 0 H

2

OH,


144

ELECTRODEPOSITION

(3)

OF

COATINGS

A n o d i c o x i d a t i o n of p o l y a n i o n s m a y y i e l d c a r b o x y l r a d i c a l s .

T h e s e m a y u n d e r g o d e c a r b o x y l a t i o n to m a c r o r a d i c a l s . T h e m a c r o r a d i c a l s m a y c o m b i n e or d i s p r o p o r t i o n a t e to g i v e the deposit.


-C0

2

\O H

OH

Radical Formation

\ OH

—Φ

HO

O^

Coupling Reaction

c

t S

O H

/

N

Φ O H

Ο

OH

(combination)

Electrodeposition of Carboxyl Containing Copolymers - Advances in

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