Development of Epoxy Resin-Based Binders for Electrodeposition

4 Development of Epoxy Resin-Based Binders for Electrodeposition Coatings with High Corrosion Resistance

Downloaded by LOUISIANA STATE UNIV on November 15, 2014 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch004

W. RAUDENBUSCH Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), Badhuisweg 3, 1031 CM Amsterdam-Noord, The Netherlands Electrodeposition (ΕD) has proved to be a most attractive process for industrial coating, in particular for the priming of automobile bodies. Epoxy resins play a leading role in this field, nearly 6 % of a l l epoxy resins sold for coatings being used in ED systems. Until the early 1970s, a l l ED binders used industrially were based on maleinised natural oils, acidic alkyd resins or epoxy resin esters. Later, binders based on maleinised polybutadiene oils (LMPBs = low-molecular-weight polybutadienes) also came into industrial use, particularly in Western Europe. A l l these systems were of the anionic type, i.e. their water solubility and their deposition at the anode were due to the presence of carboxylate groups in the binder molecules. More recently, cathodic ED, with binders carrying cationic groups (e.g. -N+R H), has proved to be an interesting alternative. This paper describes development work directed towards improving the corrosion resistance of anionic, and more briefly, cationic ED binders. 2

The p r o p e r t i e s , described i n broad terms, o f the d i f f e r e n t c l a s s e s o f a n i o n i c binders are shown i n Table I. The systems i n i t i a l l y used (epoxy e s t e r s , m a l e i n i s e d o i l s and a c i d i c alkyds) general l y performed q u i t e w e l l and the r a p i d r i s e o f t h e LMPBs, notwith standing t h e i r bath s t a b i l i t y problems, was t h e r e f o r e r a t h e r surprising. In f a c t , the advance o f LMPB-based binders was due t o one key advantage: t h e i r superior c o r r o s i o n r e s i s t a n c e when a p p l i e d t o nonphosphated s t e e l substrates. This property i s important i n modern automotive a p p l i c a t i o n s s i n c e most cars now used i n Western Europe are o f the i n t e g r a l body type on which spray-phosphating always leaves some p o o r l y phosphated or even bare areas. Primers o f f e r i n g good c o r r o s i o n p r o t e c t i o n on p o o r l y phosphated s t e e l are thus quite e s s e n t i a l f o r ensuring a long l i f e t i m e o f modern c a r s . The s u p e r i o r performance o f LMPBs i n t h i s respect was a s c r i b e d t o the low water p e r m e a b i l i t y o f polybutadiene f i l m s Our more recent work, however, has thrown a d i f f e r e n t l i g h t on t h i s theory.

0-8412-0525-6/79/47-114-057$05.00/0 © 1979 American Chemical Society

In Epoxy Resin Chemistry; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

In Epoxy Resin Chemistry; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. very good poor

very good very good

adequate t o very good

very good

good t o very good

Mechanical c o a t i n g properties

good poor

good t o very good

very good

good t o very good

ED p r o p e r t i e s

Corrosion r e s i s t a n c e - on phosphated s t e e l - on hare s t e e l

occasional difficulties

very good

stability

adequate t o very good

Bath

M a l e i n i s e d LMPBs

Epoxy r e s i n esters

Maleinised o i l s , a c i d i c alkyds

PERFORMANCE OF ANIONIC ED BINDERS

TABLE I


4.

RAUDENBUSCH

Fundamental Studies


59

Epoxy Resin-Based Binders on the Factors Governing Corrosion

Resistance

When comparing two t y p i c a l ED b i n d e r s , a commercial LMPB system and a l o n g - o i l epoxy e s t e r , i n the standard ASTM s a l t - s p r a y t e s t (25 yrn c l e a r coatings on bare s t e e l , stoved at 180 C f o r 30 min), we f i r s t observed that the r a t i n g s (good with LMPB, poor with the epoxy e s t e r ) d i d not depend on the method o f a p p l i c a t i o n used (ED or spraying from solvent s o l u t i o n ) . Therefore, we subsequently a p p l i e d a l l binders simply from organic solvent s o l u t i o n s , which g r e a t l y speeded up our f u r t h e r work. Our second observation was that the epoxy e s t e r coatings f a i l e d by l o s s o f adhesion r a t h e r than by f i l m degradation. We assumed that o f the d i f f e r e n t p o l a r groups present (carboxyl, ester, ether) the carboxyls play the major r o l e i n adhesion promotion, and l o s e t h i s f u n c t i o n upon s a l t formation with the aqueous a l k a l i formed during s a l t - s p r a y t e s t i n g . Consequently, we i n q u i r e d which other, more a l k a l i - r e s i s t a n t , adhesion-promoting groups might be present i n LMPB b i n d e r s . By i n f r a r e d spectroscopy we discovered that during c u r i n g o f LMPB f i l m s at 180 °C f o r 30 min about h0% o f the carbon double bonds i n i t i a l l y present disappear and s u b s t a n t i a l amounts o f hydroxyl and (ketonic) carbonyl groups are formed. The assumption that these groups are p r i m a r i l y r e s p o n s i b l e f o r the high c o r r o s i o n r e s i s t a n c e on bare s t e e l was supported by the f a c t that LMPB coatings cured only by a i r - d r y i n g (20 °C/T d) contained much l e s s OH and l e s s C=0 and showed poor s a l t - s p r a y r a t i n g s . E l a b o r a t i n g on the hypothesis that hydroxyl groups may be important a l k a l i - r e s i s t a n t adhesion promotors, we i n v e s t i g a t e d the s a l t - s p r a y performance o f c e r t a i n epoxy-resin-based coatings which a f t e r c u r i n g contain reasonably w e l l - d e f i n e d l e v e l s o f unreacted hydroxyl groups. One such system c o n s i s t e d o f t h e s o l i d epoxy r e s i n EPIKOTE 1007 (E-1007) cured with various amounts o f hexamethoxymethylmelamine (HMMM). When the hydroxyl content o f the r e s i n i s known, and the equivalent weight o f HMMM i s taken as 130 (assuming for s t e r i c reasons that only three o f t h e s i x a v a i l a b l e methoxymethyl groups are r e a c t i v e ) , i t i s p o s s i b l e t o c a l c u l a t e the amount of r e s i d u a l OH groups i n cured coatings quite a c c u r a t e l y . These experiments (see Table I I ) showed that good s a l t - s p r a y r e s i s t a n c e on bare s t e e l may be expected i f t h e coating contains between 200 and kOO meq hydroxyl groups per 100 g. I t i s i n t e r e s t i n g t o note that good s a l t - s p r a y r e s i s t a n c e appeared t o be u n r e l a t e d t o t h e degree o f c r o s s l i n k i n g achieved i n t h i s system. The strong b e n e f i c i a l i n f l u e n c e o f high hydroxyl l e v e l s on c o r r o s i o n r e s i s t a n c e was a l s o seen i n other model experiments, at l e a s t where epoxy r e s i n s formed the major coating i n g r e d i e n t s . Some systems, however, most o f which were based on alkyds r a t h e r than on epoxies, f a i l e d t o give s i m i l a r l y good s a l t - s p r a y r a t i n g s even though t h e i r r e s i d u a l OH l e v e l was high (300toU00 meq/100g). Thus other f a c t o r s c e r t a i n l y a l s o p l a y a r o l e . From our subsequent work, some o f these f a c t o r s appear t o be:



95 111 115 133 15^

Buchholζ hardness

HMMM weight ratio

99/1 95/5 90/10 85/15 67/33

Mechanical

E-1007/

< 5 >100 >100 < 5 < 5

BS impact, direct, in. lb

properties

poor good good very good very good

Methyl ethyl ketone resistance

2 0

9i

10 10

9 9 7 0 0

1 day 8 days

ASTM B117-6U salt-spray resistance^*

350 305 250 190 ^0

Residual OH content of cured films, meq/100 g

a

a. Clear 25 pm coatings a p p l i e d by spraying from organic solvent s o l u t i o n and stoved at 180 °C f o r 30 min. Systems contained 0.5 %vr o f p - t o l u o l s u l f o n i c a c i d as c a t a l y s t . b. Ratings: 10 = unaffected, 9 = 5 mm l o s s o f adhesion from s c r a t c h , 8= 10 mm l o s s o f adhesion, 7= 15 mm l o s s o f adhesion, e t c . , 0 = t o t a l l o s s of adhesion.

5

h

1 2 3

Exp. no.

PROPERTIES OF "EPIKOTE" 1007/HMMM COATINGS ON BARE STEEL

TABLE I I


4.

Epoxy Resin-Based Binders

RAUDENBUSCH

61

- S u s c e p t i b i l i t y o f cured coatings t o a l k a l i n e h y d r o l y s i s . - H y d r o p h i l i c i t y o f cured f i l m s . Poor s a l t - s p r a y r a t i n g s were found t o r e s u l t from the presence of too many p o l a r groups i n the coatings. - S t e r i c (?) f a c t o r s . Generally we observed that the use o f even modest amounts o f f a t t y acids or t h e i r e s t e r s i n OH-rich binders s u b s t a n t i a l l y decreases the c o r r o s i o n r e s i s t a n c e . A p o s s i b l e explanation would be s t e r i c a l s h i e l d i n g of the adhesion-promoting groups by the long f a t t y a c i d t a i l s .


Development o f Anionic

ED Binders with High Corrosion

Resistance

The "high-hydroxyl" p r i n c i p l e was discovered i n experiments i n which the coatings were c o n v e n t i o n a l l y a p p l i e d from organic solvent s o l u t i o n s . We then set out t o develop ED binders conforming t o t h i s p r i n c i p l e . Apart from o f f e r i n g good c o r r o s i o n r e s i s t a n c e , such binders must o f course meet a number of f u r t h e r requirements, such as -

s o l u b i l i t y or d i s p e r s i b i l i t y i n water, good bath s t a b i l i t y , good ED behaviour, c r o s s l i n k a b i l i t y on s t o v i n g , good mechanical and chemical f i l m p r o p e r t i e s , acceptable p r e p a r a t i o n methods and c o s t s .

Note: The formulations described i n t h i s s e c t i o n are covered by the German Patent A p p l i c a t i o n OLS 2700537 and other a p p l i c a t i o n s i n a number o f c o u n t r i e s . 5

Much of our work was based on the f o l l o w i n g approach: Into a s o l i d epoxy r e s i n (EPIKOTE type 1001 or 100U) t e r m i n a l hydroxyl groups were introduced by r e a c t i n g the epoxy groups with s t o i c h i o metric q u a n t i t i e s o f hydroxy acids such as l a c t i c o r , p r e f e r a b l y , d i m e t h y l o l p r o p i o n i c a c i d (DMPA). The r e s u l t i n g h y d r o x y l - r i c h "backbones" (8-12 OH groups/molecule) were then reacted with a c y c l i c anhydride such as s u c c i n i c anhydride (SA) t o introduce the carboxyl groups necessary f o r s o l u b i l i s a t i o n i n water: CH -0H t CH^-0C0-C-CH 2 CH -0H I 3 — OH CH -0H 2 HOOC-C-CH3 — OH CH2OH ^ — OH — OH —OH (DMPA) —OH CH„-0H

CH -0C0-CH -CH -C00H ,2 2 2 CH-0C0-C-CH2 , 3 -OH CH -0C0-CH -CH -C00H CH -C0 -OH 2 -OH CH2-CO ^ 0H OH (SA) OH CH -0C0-CH -CH -C00H

CH -0C0 - C-CH 2 , 3 CH -0H

CH -0C0-C-CH 2• , 3 CH -0C0-CH -CH -C00H

OH-RICH BACKBONE

WATER-SOLUBLE RESIN

o

2

o

^0

t

o

2

OH OH OH OH

o

EPIKOTE 100U

0

2

o

o

o

o

I

>

Ë

2

2

2

o

2


2

2


62

EPOXY RESIN CHEMISTRY

I t soon became c l e a r that these o i l - f r e e binders were t o o "hard", i . e . when a p p l i e d by ED they flowed p o o r l y during the a c t u a l d e p o s i t i o n step and consequently formed rough coatings (nos. 1 and 2 i n Table I I I ) . B e t t e r - f l o w i n g binders were obtained when the s u c c i n i c anhydride was r e p l a c e d by a s u b s t i t u t e d m a t e r i a l , v i z . a premaleinised f a t t y a c i d (no. 3, Table I I I ) . U n f o r t u n a t e l y , how ever, t h i s approach always l e d t o a d e t e r i o r a t i o n i n s a l t - s p r a y r e s i s t a n c e , an e f f e c t t y p i c a l f o r m o d i f i c a t i o n s with n a t u r a l f a t t y acids (see above). We then discovered that we could improve the softness and the flow o f the binders by i n c o r p o r a t i n g a d e r i v a t i v e o f a s y n t h e t i c f a t t y a c i d . T h i s d e r i v a t i v e i s the g l y c i d y l e s t e r o f an α-branched monocarboxylic a c i d with 10 carbon atoms and i s commercially a v a i l able under the trade-name CARDURA. E10 (CE 10). Being a monoepoxy compound, CE 10 could be e a s i l y i n c o r p o r a t e d by r e a c t i o n with part o f the carboxyl groups present i n the b i n d e r s :

CH -0C0-CH -CH -C00H I 2 2 2 CH -0-C0-C-CH 2 I 3 -OH CH -0C0-CH -CH -C00H o

o

o

o

o

d

I—OH -OH -OH -0H -OH

d

d

^

Q

2

CH -CH-CH -0C0-C H ±-Σ±(CE 10)

CH -0C0-CH -CH -C00H I 2 2 2 CH -0-C0-C-CH o

o

o

o

o

I

ά

3

CH -0C0-CH -CH -C00H 2

2

2

WATER-SOLUBLE RESIN OH

I

CH -0C0-CH -CH -C00-CH -CH-CH -C00-C H J

o

o

d

d

CH -0-C0-C-CH OH OH -OH 0 H

' }-0H .OH

o

o

o

n

d

d

d

y

Λ n

I

y

o

CH -0C0-CH -CH -C00H ά

OH ? CH -0C0-CH -CH -C00-CH -CH-CH -0C0-C H ^ H

2

2

2

2

2

9

1

CH -0-C0-C-CH 2 , 3 CH -0C0-CH -CH -C00H o

2

2

2

OH-RICH ED BINDER (No. h i n Table I I I )



4.

RAUDENBUSCH


63

TABLE I I I HYDROXYL-RICH ANIONIC ED BINDERS

No.

Binder composition^ Molar r a t i o s

ED behaviour^

Hydroxyl content , 0

a

Bath stability

d

Mechanical coating properties

( T d A o °C)

meq/100 g

Saltspray rating (10 d) e

1

E-100U/LA/SA 1/2/1.5

hok

poor

fair

good

9

2

E-IOOU/DMPA/SA 1/ 2 / 1 . 5

h!9

poor

fair

good

9i

3

E-1001/DMPA/MAFA 1/ 2 /2

358

good

very good

good

5 - 6e

h

E-1OOU/DMPA/SA/CE 10 1/ 2 /I* /2

3U1

good

fair

good

9

f

5

E-1001/AA/DMPA/TMA/CE 10 2/1/2 /2.3/2.U

good

very good

good

9

f

6

Commercial LMPB binder (comparison)

good

good

fair

9

f

-

e

a. A l l binders (except no. 6) combined with HMMM (weight r a t i o 9 5 / 5 ) ; 25 ym t h i c k c l e a r coatings a p p l i e d t o bare s t e e l by spraying or by ED, followed by s t o v i n g at 180 °C f o r 30 min. b. E-1001, E-100U: s o l i d ΕΡΙΚ0ΤΕ epoxy r e s i n s ; LA: l a c t i c a c i d ; DMPA: d i m e t h y l o l p r o p i o n i c a c i d ; SA: s u c c i n i c anhydride; TMA: t r i m e l l i t i c anhydride; MAFA: p r e maleinised f a t t y a c i d ; CE 10: CARDURA E10; AA: a d i p i c a c i d . c. C a l c u l a t e d . d. 10 % aqueous s o l u t i o n s c o n t a i n i n g ethylene g l y c o l mono-n-butyl ether (30 % on b i n d e r s ) , and t r i e t h y l a m i n e f o r n e u t r a l i s a t i o n . e. Spray-applied c o a t i n g s ; i f. Both ED- and spray-applied c o a t i n g s ; f

f

°

rs a l t

s

- P

r a v

r a t i n e s see Table I I r a t i n g s see Table I I .


64

EPOXY RESIN CHEMISTRY


A binder of t h i s type (no. k i n Table I I I ) , i n combination with small amounts o f melamine or phenolic r e s i n s as c r o s s l i n k e r s , showed good ED behaviour and good mechanical f i l m p r o p e r t i e s . I t s s a l t - s p r a y r e s i s t a n c e was only s l i g h t l y lower than that of a CARDURA-free analogue (no. 2 i n Table I I I ) . CE 10 seems t o have a l e s s e r detrimental e f f e c t on the s a l t - s p r a y r e s i s t a n c e than n a t u r a l f a t t y a c i d s , p o s s i b l y because i t s hydrocarbon chain i s much s h o r t e r . The h y d r o l y t i c a l s t a b i l i t y o f binder no. k was not very good and our next task was t o improve t h i s aspect. Binders rendered water-soluble by the a c i d i c h a l f e s t e r p r i n c i p l e contain a h y d r o l y t i c a l l y weak s i t e , v i z . the h a l f e s t e r l i n k a g e .

OH-RICH J— OH + 0* BACKBONE

OH-RICH BACKBONE

C= 0

CYCLIC ANHYDRIDE

C00E

• 0-CO

ACIDIC HALFESTER

The reason i s probably that the s a p o n i f i c a t i o n o f the h a l f e s t e r i s a s s i s t e d by the neighbouring carboxyl f u n c t i o n . We screened the h y d r o l y t i c s t a b i l i t y o f binders c o n t a i n i n g d i f f e r e n t types of h a l f e s t e r s by s t o r i n g aqueous s o l u t i o n s (10 % s o l i d s content) at ko C and p e r i o d i c a l l y checking the pH, the c o n d u c t i v i t y and the ED behaviour. In t h i s way we found that binders c o n t a i n i n g h a l f e s t e r s of maleic o r p h t h a l i c a c i d are quite unstable. The s t a b i l i t y of binders with s u c c i n i c h a l f e s t e r s i s b e t t e r but probably s t i l l i n s u f f i c i e n t f o r most i n d u s t r i a l a p p l i c a t i o n s . H a l f e s t e r s o f t r i m e l l i t i c anhydride (TMA) are considerably more s t a b l e than those of s u c c i n i c a c i d . To our s u r p r i s e we found that m o d i f i c a t i o n of the t r i m e l l i t i c h a l f e s t e r s with the above-mentioned monoepoxide CE 10 enhanced the s t a b i l i t y even f u r t h e r . I f about equimolar amounts o f TMA and CE 10 are used there are s u f f i c i e n t c a r b o x y l i c functions l e f t t o guarantee a good s o l u b i l i t y o f the binders i n water:

OH-RICH IBACKBONE

/C^/^V^COOH OH + 0 I (\ Τ

0 / \ + CH^-CH-CH -0-C0-C H o

n

Η

0

TMA

CE 10

OH

I

C00-CH -CH-CH - •0-CO-C H

OH-RICH BACKBONE

9

C00H' CARDURA E10-TMA MODIFIED ED BINDER


15


4.

RAUDENBUSCH


65

From our l a b o r a t o r y experience we b e l i e v e that such systems are about as s t a b l e as the i n d u s t r i a l l y well-known binders which are rendered water-soluble with premaleinised f a t t y a c i d s . A t y p i c a l h y d r o x y l - r i c h binder which i s modified with TMA and and CE 10 i s no. 5 i n Table I I I . In t h i s binder the backbone i s s l i g h t l y d i f f e r e n t from that o f the preceding b i n d e r s , i . e . , EPIKOTE 100U i s r e p l a c e d by a combination o f EPIKOTE 1001 (two moles) and a d i p i c a c i d (one mole). T h i s was done t o f u r t h e r improve the mechanical p r o p e r t i e s o f the c o a t i n g s . With melamine or phenolic r e s i n s as c r o s s l i n k e r s binder no. 5 shows an a t t r a c t i v e combination o f good ED behaviour, good mechan i c a l f i l m p r o p e r t i e s and high bath s t a b i l i t y . I t s s a l t - s p r a y r e s i s t a n c e i s comparable t o that o f a commercial LMPB system (no. 6 i n Table I I I ) . Binder no. 5 a l s o shows s e l f - c r o s s l i n k i n g behaviour, i . e . i t gives coatings with an i n t e r e s t i n g l e v e l o f performance when stoved i n the absence o f e x t e r n a l c r o s s l i n k e r s . In view o f these encouraging r e s u l t s , we decided t o submit binder no. 5 t o customers under the code LR-2052. Customer r e a c t i o n s so f a r have been quite p o s i t i v e and our views regarding the high s a l t - s p r a y r e s i s t a n c e o f t h i s binder on non-phosphated s t e e l were f u l l y confirmed. With a view t o lowering raw m a t e r i a l costs (the DMPA used i n binder LR-2052 i s r a t h e r expensive), we l a t e r developed another OH-rich b i n d e r , designated LR-2053. This system i s based on a l i n e a r backbone c o n t a i n i n g a l i q u i d epoxy r e s i n , EPIKOTE 828, d i p h e n y l o l propane, a d i p i c a c i d and CE 10 (molar r a t i o 3 / 2 / 2 / 2 ) and again i s rendered water-soluble with a TMA/CE 10 combination (2.5 moles TMA and 2.6 moles CE 10 per mole o f backbone). This binder performs as w e l l as LR-2052, with an only m a r g i n a l l y lower s a l t - s p r a y r e s i s t a n c e . With both b i n d e r s , LR-2052 and LR-2053, we have prepared pigmented ED p a i n t s . We used a simple T1O2 based pigmentation without a c t i v e i n g r e d i e n t s . T y p i c a l data f o r these p a i n t s are shown i n Table IV. The general performance o f both pigmented systems was very s a t i s f a c t o r y and the s a l t - s p r a y r a t i n g s were comparable t o those o f unpigmented coatings. In c o n c l u s i o n , we can say that i t i s p o s s i b l e t o design anionic ED binders based on epoxy r e s i n s which give coatings with a much b e t t e r s a l t - s p r a y r e s i s t a n c e on bare s t e e l than that o f coatings based on conventional anionic b i n d e r s . The main design features o f the epoxy r e s i n based binders are (1) the presence of a s u f f i c i e n t l y high l e v e l o f hydroxyl groups i n the c r o s s l i n k e d coatings, and (2) the omission o f n a t u r a l f a t t y a c i d s . In l a t e r work we a p p l i e d the same p r i n c i p l e s t o c a t i o n i c ED b i n d e r s . C a t i o n i c ED Binders A c h i e f advantage claimed f o r coatings a p p l i e d by the cathodic ED i s t h e i r s u p e r i o r c o r r o s i o n r e s i s t a n c e , and t h i s has been the main i n c e n t i v e f o r our work i n t h i s area. I n i t i a l l y , we t r i e d t o e s t a b l i s h whether the cathodic d e p o s i t i o n process as such would l e a d t o improved c o r r o s i o n r e s i s t a n c e o f the c o a t i n g s . To t h i s end



composition

properties

0

d

6

f

mm

s

um

Q.cm C min V V cm/V weeks

#w

steel.

100

9-9-2

passed

>3

>3

passed 8^

Gt

0

Gt

0

good

22-28 170

good

>k

16/200

195

very

22-28

>k

250

2 150-200

2

30

150-175 220 15/175

1280

7.5

1U.8

0.80

100/U0.5

0.28

85/15

Setaliet

30

a

LR-2053

1260

7.1

1U.8

100/1+0.5 0.83

0.28

85/15

100

LR-2052

Setaliet

Thermoreactive p h e n o l i c r e s i n (Synthèse B.V., H o l l a n d ) . Titanium dioxide/carbon b l a c k / c l a y (weight r a t i o 3 6 / 2 / 2 ) . Diethylene g l y c o l monobutyl ether/isophorone (weight r a t i o 2 / 1 ) . C i r c u l a t i o n i n open c o n t a i n e r s , no u l t r a f i l t r a t i o n .

e. A l l c o a t i n g s a p p l i e d t o bare f . For r a t i n g s see Table I I .

a. b. c. d.

10 d)

1 8 0 °C/30 minutes)

Film thickness, F i l m appearance Pendulum hardness (Konig), Adhesion ( c r o s s h a t c h , DIN 5 3 1 5 1 ) E r i c h s e n impact, r e v e r s e , C o n i c a l mandrel bend (ASTM D 5 2 2 - 6 0 ) S a l t - s p r a y r e s i s t a n c e (ASTM B 1 1 7 - 6 U ;

Cured f i l m p r o p e r t i e s ( S t o v i n g :

P

H ( 2 5 °C) Specific resistance ( 2 5 C), Deposition temperature, Deposition time, Deposition v o l t a g e , Rupture v o l t a g e , Throwing power (Ford t e s t ) , S t a b i l i t y o f p a i n t at 2 8 °C,

Paint

13

Crosslinkin'g r e s i n B i n d e r / c r o s s l i n k e r weight r a t i o Pigment /binder weight r a t i o B i n d e r / s o l v e n t s weight r a t i o Degree o f n e u t r a l i s a t i o n ( t r i e t h y l a m i n e ) Paint s o l i d s content,

Paint

Binder

PIGMENTED ED PAINTS

TABLE IV


a

4.

RAUDENBUSCH

67


we modified a given a n i o n i c ED b i n d e r , LR-38 ( a m a l e i n i s e d l o n g - o i l epoxy r e s i n e s t e r modified with a styrene/methacrylic a c i d copolymer), so that i t could be e l e c t r o d e p o s i t e d a n o d i c a l l y or cathod i c a l l y depending on the pH. Such "amphoteric" behaviour was achieved by converting t h e ( s u b s t i t u t e d s u c c i n i c ) anhydride r i n g s present i n LR-38 with 2-dimethylaminoethanol i n t o amino a c i d f u n c t i o n s , e.g. |LR-38 -I— CH-COOH CH2-COO-CH2-CH2-N(CH3)


2

S a l t - s p r a y data ( f o r nos. 1 and 2 i n Table V) i n d i c a t e d t h a t , on zinc-phosphated s t e e l , cathodic ED indeed l e d t o c o n s i d e r a b l y b e t t e r c o r r o s i o n r e s i s t a n c e . On bare s t e e l , however, both cathodic and anodic coatings f a i l e d completely. F u r t h e r , we prepared a number o f d i f f e r e n t , amine-modified epoxy r e s i n e s t e r s c o n t a i n i n g between 30 and 50 %w of d r y i n g f a t t y acids (e.g. no. 3 i n Table V). These could a l l be deposited c a t h o d i c a l l y and again a t t a i n e d e x c e l l e n t s a l t - s p r a y r a t i n g s on phosphated s t e e l but performed p o o r l y on bare s t e e l . We concluded that cathodic ED prevents phosphate l a y e r degradation (a well-known phenomenon with anodic ED) and thus leads t o s u p e r i o r c o r r o s i o n r e s i s t a n c e on p r e t r e a t e d s t e e l . On bare s t e e l the binders performed too p o o r l y t o allow comparison between cathodic and anodic ED. In the l i g h t o f our more recent d i s c o v e r y o f the "highh y d r o x y l " p r i n c i p l e , the poor performance o f our e a r l y c a t i o n i c systems on bare s t e e l can now be explained by t h e i r l a c k o f s u f f i c i e n t hydroxyl groups (

Development of Epoxy Resin-Based Binders for Electrodeposition

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