bk-1983-0221.ch006

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 9, 2016 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch006

6 Polyfunctional Chelating Agents for Improved Durability of Epoxy Adhesion to Steel 1

ANTHONY J. D E N I C O L A , JR. and JAMES P. BELL University of Connecticut, Institute of Materials Science, Storrs, CT 06268 The serious drop i n strength of epoxy bonded carbon steel structural joints exposed to humid environments for extended periods of time i s greatly reduced by surface treating the bond surfaces with polyfunctional mercaptoester coupling agents. Adhesion performance was evaluated by accelerated aging of torsion joints i n water at 57°C. Surface treatment involves preactivation of the steel surface with aqueous ammonium c i t r a t e solution and treatment with the mercaptoester i n organic solution. The d u r a b l e a d h e s i o n o f epoxy r e s i n s t o s t e e l i n t h e p r e s e n c e o f m o i s t u r e h a s b e e n shown t o b e d i f f i c u l t t o a c h i e v e ( 1 ) . The s t r e n g t h o f t h e s e systems i s t y p i c a l l y a d e q u a t e f o r most s t r u c t u r a l a p p l i c a t i o n s . However, t h e d e l e t e r i o u s e f f e c t m o i s t u r e h a s o n t h e b o n d i n g b e t w e e n t h e m e t a l a n d a d h e s i v e must be t a k e n i n t o a c c o u n t i f epoxy r e s i n s a r e t o b e u s e d f o r b o n d i n g s t e e l i n m o i s t o r humid e n v i r o n m e n t s . T h i s p l a c e s a s e v e r e l i m i t a t i o n o n t h e number o f s t r u c t u r a l b o n d i n g a p p l i c a t i o n s where e p o x i d e a d h e s i v e s c o u l d be u s e d . S e v e r a l r e s e a r c h groups have s t u d i e d the e f f e c t s o f water on the d u r a b i l i t y o f s t r u c t u r a l j o i n t s . K e r r , M a c D o n a l d , a n d Orman (2) i n t h e i r s t u d y o f epoxy a d h e s i v e s w h i c h were e x p o s e d t o s a t u r a t e d atmospheres o f water and e t h a n o l found t h a t hydrogen b o n d i n g i n t e r a c t i o n s between t h e a d h e s i v e and m e t a l were d i s r u p t e d by the a c c u m u l a t i o n o f water a t h y d r o p h i l i c s i t e s on t h e o x i d e s u r f a c e . The w a t e r d i s p l a c e s t h e r e s i n where

1

Current address: Hercules Incorporated, Hercules Research Center, Wilmington, DE 19899 0097-6156/83/0221-0119$06.00/0 © 1983 American Chemical Society

Bauer; Epoxy Resin Chemistry II ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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secondary v a l e n c e f o r c e s e x i s t , thereby g r e a t l y weakening the bond. I n c o n t r a s t , i t was f o u n d t h a t e t h a n o l , w h i l e a f f e c t i n g t h e s t r e n g t h o f t h e j o i n t by a b s o r p t i o n i n t o t h e m a t r i x , had l e s s t e n d e n c y t o a d s o r b a t t h e i n t e r f a c e and d i s p l a c e t h e r e s i n from the s u r f a c e . The a u t h o r s s u g g e s t t h e t e n d e n c y t o a g g r e g a t e at the i n t e r f a c e i s r e l a t e d t o the c a p a c i t y of the penetrant t o h y d r o g e n bond, w a t e r h a v i n g a much h i g h e r c a p a c i t y t h a n ethanol. S i m i l a r s t u d i e s c o n f i r m t h i s concept o f adhesive f a i l u r e by p r e f e r e n t i a l a d s o r p t i o n and a d h e s i v e d i s p l a c e m e n t a t the adherend s u r f a c e ( 3 ) . Thermodynamic e v i d e n c e f o r p r e f e r e n t i a l d i s p l a c e m e n t o f t h e a d h e s i v e f r o m t h e i n t e r f a c e by w a t e r i s p r o v i d e d by t h e w o r k o f G l e d h i l l and K i n l o c h ( 1 ) . By a p p r o x i m a t i n g t h e v a l u e o f t h e w o r k o f a d h e s i o n b e t w e e n an epoxy a d h e s i v e and i r o n o x i d e s u r f a c e , t h e y were a b l e t o show t h a t i n t h e a b s e n c e o f w a t e r t h i s v a l u e was p o s i t i v e and s t a b l e . I f , however, water i s p r e s e n t a t t h e i n t e r f a c e t h e work o f a d h e s i o n s h i f t s t o a n e g a t i v e , and h e n c e u n s t a b l e v a l u e . I t f o l l o w s t h a t u n d e r m o i s t c o n d i t i o n s , t h e l o c u s o f f a i l u r e w i l l l i k e l y be a t t h e i n t e r f a c e , r e s u l t i n g i n a l o w e r s t r e n g t h . T h i s i s what one observes e x p e r i m e n t a l l y . F r o m t h e above d i s c u s s i o n , i t f o l l o w s t h a t i t s h o u l d be p o s s i b l e t o i m p r o v e t h e d u r a b i l i t y o f bonded j o i n t s by t h e i n t r o d u c t i o n of s u i t a b l e c o u p l i n g agents a t the i n t e r f a c e . If the c o u p l i n g agent i s c a p a b l e of c h e m i c a l l y i n t e r a c t i n g w i t h the m e t a l or i t s o x i d e , d i s p l a c e m e n t o f the adhesive a t the i n t e r f a c e w i l l be p r e v e n t e d . Our e f f o r t s h a v e f o c u s e d on t h e p o s s i b i l i t y o f a t t a c h i n g o r g a n i c compounds o n t o t h e s u r f a c e o f 1018 low c a r b o n s t e e l through a complexation r e a c t i o n w i t h i r o n . We h a v e e x a m i n e d m e r c a p t o e s t e r c h e l a t i n g a g e n t s as p o t e n t i a l c o u p l i n g a g e n t s . T h e s e a r e compounds w h i c h a r e c a p a b l e o f c o m p l e x i n g w i t h m e t a l i o n s by f o r m i n g r i n g o r g a n o m e t a l l i c c h e l a t e s t r u c t u r e s . E x p e r i m e n t a l . The a d h e s i v e employed was a d i g l y c i d y l e t h e r o f B i s p h e n o l A (Epon R e s i n 828*) epoxide r e s i n . The c u r i n g a g e n t s u s e d t o c u r e t h e e p o x i d e r e s i n and c u r e c o n d i t i o n s a r e g i v e n i n T a b l e 1.

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Table I .

Amine C u r i n g A g e n t s and C o n d i t i o n s

AMINE ** V e r s a m i d 140 Polyamide *** Diethylenetriamine ****DMP-30


***

Polyfunctional Chelating Agents

Methylenedianiline

PHR, AMINE 50 10 10 25.6

CURE CYCLE 72 66 1 2 1 2

hrs., hrs., nr., nr., nr., nr.,

25°C 25°C 120°C 150°C 120°C 150°C

o f Cure

ACCELERATOR (0.2phr)Resorcinol

A t u b u l a r b u t t j o i n t g e o m e t r y d e v e l o p e d b y L i n and B e l l ( 4 , 5 ) was u s e d t o t e s t t h e s h e a r s t r e n g t h o f t h e e p o x y / s t e e l a d h e s i v e system. T h i s geometry o f f e r s s e v e r a l advantages over some o f t h e more c o n v e n t i o n a l t e s t i n g s y s t e m s . The a p p l i e d s t r e s s i s very n e a r l y simple shear. This eliminates geometric dependence o f t h e d a t a due t o c o m p l e x a p p l i e d s t r e s s e s . A second advantage i s t h a t t h e c o n t a c t a r e a between t h e a d h e s i v e and s u b s t r a t e i s r e l a t i v e l y s m a l l . The j o i n t i s d e s i g n e d s o a s t o a l l o w water p e n e t r a t i o n from b o t h s i d e s o f a r a i s e d annular r i n g w h i c h i s 0.16 cm i n w i d t h . The maximum r a d i a l d i f f u s i o n d i s t a n c e i s 0.08 cm. T h i s i s t h e d i s t a n c e w a t e r w o u l d h a v e t o d i f f u s e i n o r d e r t o c o m p l e t e l y cover t h e i n t e r f a c e . Epoxy r e s i n c o n t r o l systems (600 g r i t p o l i s h e d o n l y ) l o s e about 9 5 % o f t h e i r i n i t i a l s t r e n g t h w i t h i n a week when s o a k e d i n d i s t i l l e d water a t 57°C T h i s a l l o w s a c c e l e r a t e d wet s t r e n g t h t e s t i n g w i t h o u t h a v i n g t o use extreme temperatures. A l l j o i n t s u r f a c e s were c a r e f u l l y p o l i s h e d w i t h 600 g r i t s a n d p a p e r . J o i n t s were t r e a t e d p r i o r t o a p p l y i n g t h e a d h e s i v e by t o t a l l y i m m e r s i n g them i n a r e f l u x i n g o r g a n i c s o l u t i o n c o n t a i n i n g t h e c o u p l i n g a g e n t f o r 15 m i n u t e s . A f t e r s o a k i n g i n p u r e s o l v e n t f o r 5 m i n u t e s a t room t e m p e r a t u r e , t h e j o i n t s were t h e n a i r d r i e d . The r e s i n m i x t u r e (44 mg) was a p p l i e d t o t h e r a i s e d a n n u l a r r i n g o f t h e j o i n t a p p r o x i m a t e l y one h o u r a f t e r treatment. The r e s i n w e i g h t was m e a s u r e d a c c u r a t e l y ( i O . l mg) by w e i g h i n g t h e h a l f o f t h e j o i n t c o n t a i n i n g t h e r a i s e d r i n g p r i o r t o and a f t e r c o a t i n g . Both t h e adhesive j o i n t assembly and s t e e l s l e e v e p l a c e d a r o u n d t h e j o i n t t o i n s u r e p r o p e r a l i g n m e n t a r e composed o f 1018 c a r b o n s t e e l . Shear t e s t i n g o f t h e a d h e s i v e j o i n t s was c o n d u c t e d o n a n I n s t r o n U n i v e r s a l T e s t i n g M a c h i n e , M o d e l TM-S. T o r q u e was a p p l i e d t o t h e j o i n t w i t h an I n s t r a l a b t o r s i o n a l d e v i c e . A 1000-lb r e v e r s i b l e l o a d - c e l l was u s e d t o measure t h e f o r c e a t b r e a k . The ** Reg. trademark o f Henkel C o r p o r a t i o n *** A l d r i c h C h e m i c a l Co. **** Reg. t r a d e m a r k o f Rohm a n d Haas Company


121

122


c r o s s - h e a d speed was 0.2 i n / m i n f o r a l l t e s t s . Applied torque, Mt, i s r e l a t e d t o t h e maximum s h e a r s t r e s s , tmax, by t h e following:

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C o u p l i n g Agents H e x a n e t r i o l t r i t h i o g l y c o l a t e (HTTHG). 1 , 2 , 6 - H e x a n e t r i o l (18 gm) and m e r c a p t o a c e t i c a c i d (37.5 gm) were added t o 50 ml o f x y l e n e and h e a t e d t o 150°C f o r 4.5 h o u r s . Water formed d u r i n g t h e r e a c t i o n was a z e o t r o p e d w i t h x y l e n e . Vacuum was a p p l i e d a t 160°C and 10 t o r r p r e s s u r e f o r 1 h o u r t o d i s t i l l o f f e x c e s s r e a g e n t s . The p r o d u c t was r e p e a t e d l y washed w i t h d i s t i l l e d water. T h i r t y - f i v e ml o f t h e p r o d u c t was i s o l a t e d . Ethylmercaptoacetate. C h l o r o a c e t y l c h l o r i d e (15 m l ) was added t o e t h a n o l (100 m l ) . P o t a s s i u m c a r b o n a t e was added and t h e s o l u t i o n was s t i r r e d a t room t e m p e r a t u r e f o r 1 h o u r . The p o t a s s i u m c a r b o n a t e was f i l t e r e d o f f . E x c e s s e t h a n o l was s t r i p p e d on a r o t a r y e v a p o r a t o r . The r e s i d u e was added t o 40 ml o f t o l u e n e and t h e s o l u t i o n was washed w i t h w a t e r . The t o l u e n e was s t r i p p e d on an e v a p o r a t o r , y i e l d i n g 15 ml o f e t h y l c h l o r o acetate. N a s h (20 gm) was d i s s o l v e d i n 200 m l o f r e f l u x i n g a c e t o n e . E t h y l c h l o r o a c e t a t e (10 m l ) was added i n two p a r t s . After 3 h o u r s o f r e a c t i o n t o l u e n e (400 ml) and w a t e r (200 m l ) were a d d e d . The t o l u e n e l a y e r was i s o l a t e d and r e p e a t e d l y washed w i t h water. The s o l v e n t was s t r i p p e d on an e v a p o r a t o r , y i e l d i n g 8 ml o f e t h y l m e r c a p t o a c e t a t e . H e x a n e t r i o l t r i m e r c a p t o b u t y r a t e (HTTMB). 4 - c h l o r o b u t y r y l c h l o r i d e was added t o 1 , 2 , 6 - h e x a n e t r i o l and t h e p r o c e d u r e f o r s y n t h e s i z i n g e t h y l m e r c a p t o a c e t a t e was f o l l o w e d . P e n t a e r i t h r i t o l t e t r a t h i o g l y c o l a t e (PETG) and p e n t a e r i t h r i t o l t e t r a m e r c a p t o p r o p r i o n a t e (PETMP) were p r o v i d e d by E v a n s C h e m e t i c s Company. R e s u l t s and D i s c u s s i o n . The c u r i n g a g e n t u s e d t o c r o s s l i n k an epoxy r e s i n u s u a l l y r e p r e s e n t s a s i g n i f i c a n t f r a c t i o n o f the resin. I n most s i t u a t i o n s i t i s t h e most i m p o r t a n t d e t e r m i n i n g f a c t o r i n f l u e n c i n g the p r o p e r t i e s of the r e s i n , s i n c e the v a s t m a j o r i t y o f epoxy r e s i n s o f c o m m e r c i a l i n t e r e s t a r e b a s e d on b i s p h e n o l A. I t i s then not s u r p r i s i n g t o f i n d t h a t the c u r i n g


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a g e n t c a n have a s i z a b l e e f f e c t on t h e d u r a b i l i t y o f t h e s y s t e m . F i g u r e s 1 and 2 i l l u s t r a t e t h e i n f l u e n c e o f c u r i n g a g e n t on a d h e s i v e j o i n t d u r a b i l i t y . A l l of the c o n t r o l systems s t u d i e d r a p i d l y l o s t t h e i r i n i t i a l d r y s t a t e p r o p e r t i e s when e x p o s e d t o w a t e r . However, t h o s e s y s t e m s w h i c h a r e e i t h e r c a p a b l e o f s h i e l d i n g formed h y d r o x y l groups o r w h i c h c o n t a i n a low c o n c e n t r a t i o n o f h y d r o x y l o r o t h e r p o l a r g r o u p s a r e much more r e s i s t a n t t o t h e e f f e c t o f w a t e r . T h i s i s t h e c a s e f o r t h e V e r s a m i d 140 and DMP-30 c u r e d e p o x y s y s t e m s . F i g u r e 1 i s a c o m p a r i s o n o f DETA and V-140 room t e m p e r a t u r e c u r e d e p o x i e s . The V e r s a m i d c o m p o s i t i o n i s much l e s s s u s c e p t i b l e t o t h e e f f e c t o f water, owing t o the s h i e l d i n g a b i l i t y of the f a t t y p o r t i o n s o f the m o l e c u l e ( 6 ) . F i g u r e 2 i l l u s t r a t e s t h a t t h e same e f f e c t i s p r e s e n t f o r h i g h t e m p e r a t u r e c u r e d e p o x y s y s t e m s . The DMP-30 c o m p o s i t i o n i s more d u r a b l e t h a n t h e m e t h y l e n e d i a n i l i n e c u r e d m a t e r i a l b e c a u s e i t i n v o l v e s p o l y m e r i z a t i o n o f the epoxy group, f o r m i n g a p o l y e t h e r n e t w o r k . H e n c e , t h e number o f p o l a r g r o u p s i s r e d u c e d and t h e w a t e r d u r a b i l i t y o f t h e a d h e s i v e j o i n t i s i m p r o v e d . F i g u r e 2 a l s o p o i n t s out another important p o i n t . Despite the v e r y h i g h shear s t r e n g t h achieved w i t h m e t h y l e n e d i a n i l i n e i n the dry s t a t e , t h i s system i s a c t u a l l y the l e a s t d u r a b l e o f the f o u r c o m p o s i t i o n s e x a m i n e d . D r y s t r e n g t h a d h e s i o n i s n o t a good measure o f w a t e r d u r a b i l i t y . The e f f e c t o f h e x a n e t r i o l t r i t h i o g l y c o l a t e on t h e d u r a b i l i t y o f t h e V e r s a m i d 140 c u r e d s y s t e m i s p r e s e n t e d i n F i g u r e 3. Treatment of the s t e e l s u r f a c e w i t h t h i s c o u p l i n g agent r e s u l t s i n a considerable increase i n water d u r a b i l i t y . A previous study of B-diketone c o u p l i n g agents performed i n our l a b o r a t o r y ( 7 ) showed t h a t a p r e - a c t i v a t i o n o f t h e s u r f a c e was necessary f o r bond improvement. T h i s p r e - a c t i v a t i o n i n v o l v e s e x p o s i n g t h e p o l i s h e d s t e e l j o i n t s t o a 3% aqueous ammonium c i t r a t e s o l u t i o n (pH 7.0) a t 70°C p r i o r t o i m m e r s i o n i n a m e t h a n o l s o l u t i o n (2.5 χ 10~^M) o f t h e c o u p l i n g a g e n t a t r e f l u x . A prior citrate p r e t r e a t m e n t was u s e d f o r a l l s y s t e m s p r e s e n t e d i n t h i s p a p e r unless otherwise stated. The e f f e c t o f ammonium c i t r a t e t r e a t m e n t a l o n e was e x a m i n e d and i s p r e s e n t e d i n F i g u r e 4. The u s e o f c i t r i c a c i d p r o v i d e s s h o r t t e r m improvement i n d u r a b i l i t y f o r t h e V e r s a m i d c u r e d system. There i s a decay i n s t r e n g t h t o the l e v e l o f the c o n t r o l s a f t e r prolonged soaking at 57°C C i t r i c a c i d i s known t o be a s t r o n g c h e l a t i n g a g e n t f o r m e t a l i o n s ( 8 ) . The i r o n c o m p l e x o f c i t r i c a c i d i s w a t e r soluble. T h i s drop i n d u r a b i l i t y a f t e r prolonged soaking i s t h o u g h t t o be due t o t h e f a c t t h a t c i t r i c a c i d f o r m s a w a t e r soluble chelate with iron. Such a c o m p l e x w o u l d be e x p e c t e d t o be s e n s i t i v e t o t h e d i f f u s i o n o f w a t e r i n t o t h e j o i n t . It s h o u l d be n o t e d t h a t a l l o f t h e c o u p l i n g a g e n t s i n v e s t i g a t e d form water i n s o l u b l e c h e l a t e s .


124



ω

6000

2

4 6 θ IMMERSION TIMECDAYS)

10

Figure 1. The e f f e c t of water immersion (57°C) on the t o r s i o n a l shear strength of Versamid 140 (O) and DETA ( Φ ) room temperature cured adhesive j o i n t s .

~ 100001-

2

4 6 8 IMMERSION TIMECDAYS)

10

Figure 2. The e f f e c t of water immersion (57°C) on the t o r s i o n a l shear strength of methylene d i a n i l i n e (0) and DMP-30 (·) high temperature cured adhesive j o i n t s .


6.

DE NICOLA AND BELL

Ρolyfunctional Chelating Agents

125


£10000

2

4 IMMERSION

6 8 TIMECDAYS)

10

Figure 3. E f f e c t of HTTHG treatment on bond d u r a b i l i t y ( 5 7 ° C ) of Versamid 140 cured j o i n t s : (0) HTTHG, (9) c o n t r o l .

£ 10000"

g

750α

2


10

Figure 4. E f f e c t of ammonium c i t r a t e treatment on bond d u r a b i l i t y ( 5 7 ° C ) of Versamid 140 cured j o i n t s : (O) ammonium c i t r a t e , ( Φ ) control.


126



The w a t e r a b s o r p t i o n p r o p e r t i e s and s h e a r s t r e n g t h o f t h e V e r s a m i d 140 c u r e d c o m p o s i t i o n i s p r e s e n t e d i n F i g u r e s 5 and 6, respectively. A 16 m i l p l a t e o f r e s i n was c u r e d b e t w e e n g l a s s plates. A f t e r c u r i n g i t was c u t i n t o s t r i p s . Weight g a i n s t u d i e s were p e r f o r m e d by s o a k i n g t h e s a m p l e s i n d i s t i l l e d w a t e r at 57°C The s h e a r s t r e n g t h o f t h e r e s i n as a f u n c t i o n o f i m m e r s i o n t i m e i n w a t e r a t 57°C was d e t e r m i n e d by a punch t y p e s h e a r d e v i c e s p e c i f i e d i n ASTM D732-46. E q u i l i b r i u m water uptake i s achieved i n a v e r y s h o r t time w i t h t h i n samples. An e q u i l i b r i u m w e i g h t g a i n o f 3.8% was a c h i e v e d a f t e r a p p r o x i m a t e l y 48 h o u r s a t 57°C. A 3 3 % l o s s o f shear s t r e n g t h occurs a f t e r soaking i n d i s t i l l e d water a t 57°C The s h a p e o f t h e c u r v e ( F i g u r e 6) seems t o i n d i c a t e t h a t t h i s l o s s i n s t r e n g t h i s due t o p l a s t i c i z a t i o n o f t h e r e s i n . F i g u r e 7 p o i n t s o u t t h e tremendous improvement i n w a t e r d u r a b i l i t y p o s s i b l e v i a the use o f c o u p l i n g agents. The methylenedianiline cured c o n t r o l f a l l s to a strength l e v e l of 300 p s i a f t e r 3 d a y s i n w a t e r a t 5 7 ° C The t r e a t e d j o i n t s , on t h e o t h e r h a n d , m a i n t a i n a s t r e n g t h o f 3900 p s i a f t e r 32 d a y s i n water. S e v e r a l f a c t o r s a r e t h o u g h t t o be i n v o l v e d i n t h e m e c h a n i s m o f d u r a b i l i t y i m p r o v e m e n t . The o b j e c t i v e o f t h i s s t u d y was t o e x a m i n e compounds w h i c h a r e c a p a b l e o f i n t e r a c t i n g c h e m i c a l l y w i t h the s t e e l s u r f a c e . I n a d d i t i o n , r e a c t i o n w i t h the epoxide resin i s also desirable. The p o l y f u n c t i o n a l m e r c a p t o e s t e r compounds meet b o t h o f t h e s e r e q u i r e m e n t s i n t h a t t h e m e r c a p t o e s t e r m o i e t y p r o v i d e s a means o f c h e l a t i n g i r o n c a t i o n s and i n a d d i t i o n i s v e r y r e a c t i v e t o w a r d s t h e e p o x i d e g r o u p i n t h e p r e s e n c e o f an a m i n e . The p o l y f u n c t i o n a l n a t u r e o f t h i s compound e n a b l e s i t t o a t t a c h t o t h e s t e e l s u r f a c e a t more t h a n one s i t e and p r o v i d e s a g r o u p w h i c h c a n c h e m i c a l l y a t t a c h t o t h e resin. The d r a m a t i c improvement i n bond d u r a b i l i t y s u p p o r t s such a h y p o t h e s i s . ESCA a n a l y s i s o f ammonium c i t r a t e and HTTHG t r e a t e d s t e e l i s c u r r e n t l y b e i n g u n d e r t a k e n i n an a t t e m p t t o b e t t e r u n d e r s t a n d the chemical processes which are o c c u r i n g a t the s u r f a c e . The f o l l o w i n g p r o p o s a l i s p r e s e n t e d i n an a t t e m p t t o e x p l a i n t h e bond improvement mechanism. As s t a t e d e a r l i e r , t h e ammonium c i t r a t e p r e t r e a t m e n t was f o u n d t o be a n e c e s s a r y component o f t h e t r e a t m e n t . C i t r i c a c i d i s known t o be an e f f e c t i v e treatment f o r the removal of i r o n o x i d e from s t a i n l e s s s t e e l ( 9 ) , d i s s o l v i n g t h e o x i d e by f o r m i n g a w a t e r s o l u b l e c h e l a t e with iron. C a r b o n s t e e l ( 1 0 1 8 ) was u s e d i n t h e p r e s e n t s t u d y . C i t r i c a c i d d i s s o l v e s t h e o x i d e f i l m and r e p l a c e s i t w i t h a t h i n f i l m o f c i t r a t e complex. Subsequent treatment w i t h a m e r c a p t o e s t e r c o u p l i n g agent r e s u l t s i n a replacement of the c i t r a t e by t h e m e r c a p t o e s t e r , t h e r e b y f o r m i n g a t i g h t l y bound i n s o l u b l e c o m p l e x l a y e r on t h e s u r f a c e . The c i t r i c a c i d , i n


6.

DE NICOLA AND BELL


111


4.5

2


10

Figure 5. % weight gain as a function of water immersion (57°C) of Versamid 140 cured epoxy: (0) 0.4 mm p l a t e , (©) 1.6 mm plate.

UJ 5000 CO

52500UJ

X CO

2


10

Figure 6. E f f e c t of water immersion on the shear strength of Versamid 140 room temperature cured epoxy (0.4 mm thick sample).




128

θ

16

24

IMMERSION TIMECDAYS)

32

Figure 7. E f f e c t of HTTHG treatment on bond d u r a b i l i t y (57°C) of methylene d i a n i l i n e cured adhesive j o i n t s : (0) HTTHG, (·) c o n t r o l .


6.

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129

e f f e c t , provides a source of i r o n cations. The c o u p l i n g a g e n t cannot e f f e c t i v e l y complex w i t h i r o n i n the o x i d e form. T h i s i s i l l u s t r a t e d i n T a b l e I I . When c i t r a t e i s n o t u s e d p r i o r t o t r e a t m e n t w i t h t h e c o u p l i n g a g e n t , a f t e r 4 days i m m e r s i o n t h e s t r e n g t h o f t h e MDA c u r e d r e s i n h a s d r o p p e d t o 3800 p s i . T h i s compares w i t h 7800 p s i when c i t r a t e i s u s e d w i t h t h e c o u p l i n g agent.


T a b l e I I E f f e c t o f HTTHG T r e a t m e n t on J o i n t Pretreatment:

J o i n t No.

1. 2.

Durability

T e s t A - T r e a t e d w i t h HTTHG i n a r e f l u x i n g s o l u t i o n ( m e t h a n o l ) f o r 1/2 h o u r . T e s t Β - T r e a t e d w i t h a 3% aqueous ammonium c i t r a t e (pH 7.0) s o l u t i o n . S u b s e q u e n t t r e a t m e n t w i t h HTTHG i n m e t h a n o l f o r 15 m i n u t e s . Shear S t r e n g t h Test A

(psi) Test

Β

1 2 3

4300 4600 2600

8600 7100 7700

AVG:

3800

7800

J o i n t s immersed i n d i s t i l l e d w a t e r a t 57°C f o r 96 h o u r s . Cured w i t h m e t h y l e n e d i a n i l i n e .

A d d i t i o n a l evidence t o support the concept t h a t c i t r i c a c i d p r o v i d e s i r o n c a t i o n s i s p r e s e n t e d i n T a b l e I I I . I f ammonium c i t r a t e was a c t i n g o n l y a s a c h e m i c a l o x i d e d i s s o l v e r , t h e n m e c h a n i c a l p o l i s h i n g u n d e r n i t r o g e n w o u l d be e x p e c t e d t o h a v e t h e same e f f e c t . M e c h a n i c a l l y p o l i s h i n g t h e s t e e l j o i n t s a n d a p p l y i n g HTTHG u n d e r n i t r o g e n r e s u l t e d i n a bond s t r e n g t h o f 3500 p s i a f t e r 240 h o u r s i m m e r s i o n i n w a t e r a t 57°C. Polishing j o i n t s i n a i r a n d t h e n t r e a t i n g w i t h ammonium c i t r a t e a n d HTTHG i n n i t r o g e n r e s u l t e d i n a bond s t r e n g t h o f 7000 p s i f o r t h e same immersion time. A c t i v a t i o n o f t h e s u r f a c e b y ammonium c i t r a t e i s apparently involved.


130


T a b l e I I I C o m p a r i s o n o f Ammonium C i t r a t e and M e c h a n i c a l P o l i s h i n g i n N i t r o g e n Pretreatment:


Joint 1 2 3 4 5 AVG: 1. 2. 3. 4.

No.

T e s t A - P o l i s h e d w i t h 600 g r i t i n a n i t r o g e n a t m o s p h e r e ( 0 . 3 % 0£ max.) T e s t Β - P o l i s h e d w i t h 600 g r i t i n a i r , t h e n t r e a t e d w i t h a 3% ammonium c i t r a t e s o l u t i o n under n i t r o g e n . Shear Strength ( p s i ) Test Β Test A 3900 4200 3000 3100 3400

8300 6400 6400

3500

7000

A l l j o i n t s were t r e a t e d w i t h HTTHG i n n i t r o g e n . O2 c o n c e n t r a t i o n was d e t e r m i n e d w i t h a mass s p e c t r u m . J o i n t s were immersed i n d i s t i l l e d w a t e r a t 57°C f o r 240 hours. Cured w i t h m e t h y l e n e d i a n i l i n e .

Ammonium c i t r a t e s o l u t i o n s w e r e f o u n d t o l o s e t h e i r e f f e c t i v e n e s s when aged a t room t e m p e r a t u r e f o r e x t e n d e d p e r i o d s of time. Table IV i l l u s t r a t e s the e f f e c t o f a g i n g o f the ammonium c i t r a t e . T e s t s p e c i m e n s A were t r e a t e d w i t h ammonium c i t r a t e p r e p a r e d one week p r i o r t o u s e . T e s t s p e c i m e n Β were treated with s o l u t i o n prepared j u s t p r i o r to use. A l l j o i n t s w e r e s u b s e q u e n t l y t r e a t e d w i t h PETG i n e t h y l a c e t a t e , t h e n immersed f o r 214 h o u r s i n w a t e r a t 5 7 ° C The bond s t r e n g t h o f t h e f r e s h c i t r a t e t r e a t e d j o i n t s i s more t h a n f o u r t i m e s g r e a t e r t h a n t h e j o i n t s t r e a t e d w i t h aged c i t r a t e . The r e a s o n f o r t h i s e f f e c t i s n o t p r e s e n t l y known, b u t p e r h a p s i t i n v o l v e s t h e f o r m a t i o n o f c a r b o n a t e s a l t s o f t h e ammonium g r o u p v i a r e a c t i o n w i t h carbon d i o x i d e i n a i r . I t i s t h e r e f o r e important t o use fresh solutions to obtain meaningful r e s u l t s .


6.

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131


T a b l e I V E f f e c t o f Ammonium C i t r a t e A g i n g on J o i n t D u r a b i l i t y Pretreatment:


J o i n t No.

1. 2. 3.

Test A - Treated citrate solution Test Β - Treated citrate solution

with aged with made

3% aqueous ammonium f o r one week i n a i r . 3% aqueous ammonium just prior to use.

Shear S t r e n g t h Test A

1 2

450 1000

3 4 5 6

900 750 750 2500

AVG:

1070

(psi) Test

Β

5140 3950 5300

4800

A l l j o i n t s w e r e t r e a t e d w i t h PETG i n e t h y l a c e t a t e . Immersed f o r 214 h o u r s i n d i s t i l l e d w a t e r a t 5 7 ° C Cured w i t h methylene d i a n i l i n e .

P e n t a e r i t h r i t o l t e t r a t h i o g l y c o l a t e (PETG) a n d p e n t a e r i t h r i t o l tetramercaptopropionate (PETMP) were e x a m i n e d as c o u p l i n g a g e n t s t o d e t e r m i n e t h e e f f e c t o f f u n c t i o n a l i t y and c h e l a t e r i n g s i z e o n bond d u r a b i l i t y . B o t h c o u p l i n g a g e n t s p r o v i d e d a tremendous improvement i n bond d u r a b i l i t y r e l a t i v e t o the c o n t r o l . F i g u r e s 8 a n d 9 i l l u s t r a t e t h e e f f e c t o f PETG and PETMP o n bond d u r a b i l i t y f o r a m e t h y l e n e d i a n i l i n e c u r e d r e s i n . The s t r e n g t h l o s s c u r v e s f o r t h e s e two m a t e r i a l s r e s e m b l e t h e HTTHG c u r v e . A p p a r e n t l y , t h e s t r e n g t h l o s s p r o c e s s i s common t o a l l three c o u p l i n g agents. A comparison o f the v a r i o u s surface treatments i s presented i n T a b l e V. The r e s i n u s e d was m e t h y l e n e d i a n i l i n e c u r e d a t h i g h t e m p e r a t u r e . A l l o f t h e j o i n t s were p o l i s h e d w i t h 600 g r i t emery c l o t h and t r e a t e d w i t h ammonium c i t r a t e , w i t h t h e e x c e p t i o n o f t h e c o n t r o l and s a n d b l a s t e d j o i n t s . Sandblasting t h e s u r f a c e i n c r e a s e s t h e s u r f a c e a r e a a n d p r o v i d e s a means o f mechanical i n t e r l o c k i n g o f the adhesive with the substrate surface. T h i s does i m p r o v e t h e w a t e r d u r a b i l i t y t o some e x t e n t , b u t t h e e f f e c t i s s m a l l when compared t o t h e r e s u l t s a c h i e v e d w i t h the c o u p l i n g agents. Ammonium c i t r a t e , i n a d d i t i o n t o a c t i v a t i n g t h e s u r f a c e , has a b e n e f i c i a l e f f e c t on d u r a b i l i t y . I t i s c o n c e i v a b l e t h a t c h e m i c a l r e a c t i o n s b e t w e e n t h e ammonium c a r b o x y l a t e g r o u p s o f t h e c i t r a t e c o m p l e x and t h e epoxy g r o u p s of the r e s i n i s o c c u r i n g .



132



16

Figure 8 . E f f e c t of PETG treatment on bond d u r a b i l i t y methylene d i a n i l i n e cured adhesive j o i n t s : (·) c o n t r o l .


(57°C) of (0) PETG,

16

Figure 9 . E f f e c t of PETMP treatment on bond d u r a b i l i t y (57°C) of methylene d i a n i l i n e cured adhesive j o i n t s : (0) PETMP, (·) c o n t r o l .


DE NICOLA AND

6.


BELL

133

Table V Comparison of the Various Surface Treatments


Pretreatment Control Sandblasting Ammonium c i t r a t e Ethylmercaptoacetate HTTMB HTTHG PETG PETMP 1. 2. 3. 4.

Shear S t r e n g t h

(psi)

0 1560 3900 4580 7000 6525 6450 6525

J o i n t s were immersed i n d i s t i l l e d w a t e r a t 57°C f o r 240 hours. J o i n t s were p o l i s h e d w i t h 600 g r i t emery c l o t h . J o i n t s were t r e a t e d w i t h t h e c o u p l i n g a g e n t s a f t e r a p r e a c t i v a t i o n w i t h ammonium c i t r a t e . Methylenedianiline cured.

Ethylmercaptoacetate, a monofunctional " c o u p l i n g agent", p r o v i d e s a m i n o r improvement r e l a t i v e t o t h e c i t r i c a c i d , p e r h a p s by p r o v i d i n g a more h y d r o p h o b i c s u r f a c e . A d e c r e a s e i n f u n c t i o n a l i t y has a d e t r i m e n t a l e f f e c t on bond d u r a b i l i t y r e l a t i v e t o the p o l y f u n c t i o n a l c o u p l i n g agents. Adequate c o u p l i n g t o b o t h t h e s u b s t r a t e and r e s i n a r e n e c e s s a r y f o r l o n g t e r m bond d u r a b i l i t y . H e x a n e t r i o l t r i m e r c a p t o b u t y r a t e was e x a m i n e d t o d e t e r m i n e t h e e f f e c t o f a s e v e n memebered r i n g c h e l a t e on bond d u r a b i l i t y . The s t a b i l i t y o f a s e v e n membered r i n g c h e l a t e w o u l d be e x p e c t e d t o be l o w e r t h a n a f i v e o r s i x membered r i n g . T h i s d o e s n o t a p p e a r t o be t h e c a s e . Relative c h e l a t e d i s s o c i a t i o n does n o t c o r r e l a t e w i t h bond s t r e n g t h . The same i s t r u e o f f u n c t i o n a l i t y , p r o v i d e d a minimum f u n c t i o n a l i t y i s present. Conclusions. A t u b u l a r b u t t a d h e s i v e j o i n t was f o u n d t o be o f p a r t i c u l a r a d v a n t a g e f o r s t u d y i n g t h e e f f e c t s o f w a t e r on t h e bond d u r a b i l i t y o f e p o x y a d h e s i o n t o 1018 low c a r b o n s t e e l . T h i s geometry l e n d s i t s e l f t o a c c e l e r a t e d immersion t e s t i n g w i t h o u t having t o r e s u l t t o extreme temperatures. P o l y f u n c t i o n a l m e r c a p t o e s t e r c h e l a t o r s have been s y n t h e s i z e d and e v a l u a t e d as p o t e n t i a l c o u p l i n g a g e n t s f o r t h e a d h e s i o n o f epoxy r e s i n s t o s t e e l . The a d h e s i v e s y s t e m s were e v a l u a t e d by s u b j e c t i n g t o r s i o n a l j o i n t s to shear f a i l u r e a f t e r a c c e l e r a t e d immersion t e s t i n g i n d i s t i l l e d water. The f o l l o w i n g r e s u l t s a r e o f p a r t i c u l a r s i g n i f i c a n c e : (1) The c u r i n g a g e n t has an o b s e r v a b l e e f f e c t on bond d u r a b i l i t y - those c u r i n g agents capable of s h i e l d i n g p o l a r


134



groups i n the r e s i n or forming c r o s s l i n k s t r u c t u r e s not c o n t a i n i n g h i g h l y p o l a r groups p r o v i d e a h i g h e r degree o f water durability. (2) P r e - a c t i v a t i o n o f t h e s t e e l s u r f a c e by ammonium c i t r a t e was n e c e s s a r y f o r maximum bond p r o t e c t i o n . (4) I n c r e a s i n g the f u n c t i o n a l i t y o f the c o u p l i n g agent from three t o f o u r d i d not s i g n i f i c a n t l y a l t e r the magnitude o f the bond s t r e n g t h . However, r e d u c i n g t h e f u n c t i o n a l i t y t o one d e c r e a s e d t h e bond s t r e n g t h d r a m a t i c a l l y . (5) Bond s t r e n g t h was n o t d e p e n d e n t upon c h e l a t e r i n g s i z e .

Literature Cited 1. G l e d h i l l , R. Α.; Kinloch, A. J . J . Adhesion, 1974, 6, 315. 2. Kerr, C.; MacDonald, N. C.; Orman, J . J . Appl. Chem., 1967, 17, 62. 3. DeLollis, N. J . ; Montaya, O. J . Appl. Poly. S c i . , 1967, 11, 983. 4. L i n , C. J . ; Bell, J . P. J . Appl. Poly. S c i . , 1972, 16, 1721. 5. McCarvill, W. T.; B e l l , J . P. J . Adhesion, 1974, 6(3), 185. 6. Witcoff, H. J . Oil and Colour Chem. Ass., 1964, 47(4), 273. 7. DeNicola, A. J . ; Bell, J . P. Org. Coat. Plast. Chem. Preprints, Aug. 1981, 45, 120. 8. Blume, W. J . ASTM STP 538, 1973, 43. RECEIVED

December 2,

1982


bk-1983-0221.ch006

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