1 Elastomer-Modified Epoxy Resins in Coatings Applications R. S. DRAKE and D. R. EGAN Β F Goodrich Chemical Group, Cleveland, OH 44131
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W. T. MURPHY Β F Goodrich Company, Research and Development Center, Brecksville, OH 44141 Elastomer-modified epoxy resins have grown in usage in the past 15 years i n several application areas connected with structural adhesives, composites, civil engineering/construction, e l e c t r i c a l laminates/ encapsulants and corrosion resistance. Some of this increased attention has come about through utility of telechelic butadiene/acrylonitrile l i q u i d polymers. Both carboxyl and amine reactive l i q u i d polymers (CTBN and ATBN) have provided chemistries amenable to this modification with the polybutadiene/acrylonitrile co polymer providing s o l u b i l i t y parameters close to if not equaling those of base epoxy resins. I t has only been within the last few years that similar elasto mer-modified epoxy resins have been examined i n epoxy coatings and primers. This chapter reviews a portion of that r e l a t i v e l y new journal and patent l i t e r a t u r e involving epoxy coal-tar, powder, photo-curable and solventless heavy duty coatings as well as metal primers. These examples are employed to i l l u s t r a t e such benefits of elastomer inclusion as reverse im pact, bending/crimping, corrosion resistance, ther mal shock resistance and coating adhesion. The chapter concludes with an initial reporting on in d u s t r i a l maintenance and marine coatings based on elastomer-modified epoxy resin models. Similar benefits are noted with these t r a d i t i o n a l solvent -based coatings under ambient cure. Numerous b l e n d s o r a l l o y s o f t h e r m o s e t t i n g r e s i n s w i t h e l a s t o m e r s have been d e v e l o p e d o v e r t h e p a s t f o u r d e c a d e s a n d f o u n d t h e i r way i n t o c o n t i n u i n g c o m m e r c i a l u s e . A g e n e r o u s amount o f t h i s h a s been r e l a t e d t o t o u g h e n i n g needs : O v e r c o m i n g t h e b r i t t l e n e s s o f h i g h l y c r o s s l i n k e d g l a s s y p o l y m e r s . W i t h i t came t h e r e c o g n i t i o n t h a t n o t o n l y was d e g r e e o f e l a s t o m e r / r e s i n c o m p a t i b i l i t y i m p o r t a n t b u t a l s o t h e need f o r a c h i e v i n g a d i s p e r s e d p h a s e 0097-6156/8 3/0221-0001$06.00/0 © 1983 American Chemical Society
In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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EPOXY RESIN CHEMISTRY
with particle size integrity. Thus, b o t h b l e n d e d o r a l l o y e d p r o d u c t s , a s w e l l as t h e p r o c e s s i n g t e c h n i q u e s f o r a c h i e v i n g them, were o p t i m i z e d and l a t e r i m p r o v e d upon. P r o d u c t s f o r b o t h s t r u c t u r a l and n o n - s t r u c t u r a l a p p l i c a t i o n s p r o l i f e r a t e d . Among t h e e a r l i e r d e v e l o p m e n t s o f e l a s t o m e r / r e s i n b l e n d s , t h e p o l y b u t a d i e n e / a c r y l o n i t r i l e or n i t r i l e * elastomer m o d i f i c a t i o n of p h e n o l - f o r m a l d e h y d e c o n d e n s a t i o n r e s i n s s e r v e s a s a t y p i c a l exam p l e and a p i v o t f o r t h i s p a p e r ' s t h e s i s a s w e l l . The n i t r i l e p h e n o l i c c a t e g o r y o f p r o d u c t s a t t e s t s t o a l a r g e and c o n t i n u i n g l i t e r a t u r e s h o w i n g f i r s t t h e u s a g e o f n i t r i l e e l a s t o m e r s them s e l v e s and l a t e r t h e i r c a r b o x y l i c a n a l o g s . Systematic evaluations o f e l a s t i c p h e n o l - f o r m a l d e h y d e m o l d i n g c o m p o s i t i o n s , e.g., w i t h b o t h c a r b o x y l i c and a n a l o g o u s n o n c a r b o x y l i c e l a s t o m e r s a r e u s e f u l i n demonstrating the s t r o n g e r reinforcement p r o p e r t i e s of the c a r boxy 1 - r e a c t i v e s p e c i e s . I t i s these s o l i d c a r b o x y l i c n i t r i l e e l a s t o m e r s which began t o show u t i l i t y i n t h e m o d i f i c a t i o n o f e p o x y r e s i n s . P r o c e s s i n g needs f o r s o l i d e l a s t o m e r i n c l u s i o n , p a r t i c u l a r l y i n l i q u i d epoxy r e s i n s , have n o t always been advantageous. A s s o c i a t e d problems i n c l u d e g e l , v i s c o s i t y t h r e s h o l d l i m i t a t i o n s and a c h i e v i n g d e s i r e d r u b b e r l e v e l s i n e x c e s s o f 5-6 p h r . Sometimes p r o c e s s i n g must be c a r r i e d out i n s e l e c t e d s o l v e n t s , not always a d e s i r a b l e or tolerable step. In the mid-60's c a r b o x y l - t e r m i n a t e d p o l y b u t a d i e n e / a c r y l o n i t r i l e (CTBN) l i q u i d p o l y m e r s w e r e i n t r o d u c e d f o r t h e p u r p o s e o f epoxy r e s i n m o d i f i c a t i o n . These t e l e c h e l i c polymers a r e e s s e n t i a l l y macromolecular d i a c i d s . They o f f e r p r o c e s s i n g e a s e (and t h e r e f o r e advantage) over the s o l i d c a r b o x y l i c n i t r i l e e l a s t o m e r s . I t i s no s u r p r i s e t h a t t h e epoxy p r e p r e g i n d u s t r y ( a d h e s i v e and n o n - a d h e s i v e v a r i e t i e s ) f o u n d t h e l i q u i d and s o l i d c a r b o x y l i c n i t r i l e e l a s t o m e r s p e c i e s u s e f u l t o g e t h e r i n p r o c e s s i n g l i q u i d and l o w e r m o l e c u l a r w e i g h t s o l i d epoxy r e s i n s w h e r e e l a s t o m e r m o d i f i c a t i o n was n e e d e d . L a t e r , i n 1974, amine r e a c t i v e v e r s i o n s o f t h e l i q u i d n i t r i l e p o l y m e r s (ATBN) w e r e i s s u e d , t h e r e b y o f f e r i n g a n o t h e r way t o i n t r o d u c e r u b b e r y segments i n t o a c u r e d e p o x y r e s i n n e t w o r k . Refer ences a r e c i t e d which p r o v i d e d e t a i l e d d i s c u s s i o n s of n i t r i l e r u b ber, c a r b o x y l i c n i t r i l e r u b b e r and b o t h c a r b o x y l - and a m i n e - t e r minated n i t r i l e l i q u i d polymers (1-4). T a b l e I i l l u s t r a t e s CTBN and ATBN p r o d u c t s s t r u c t u r a l l y . Table I I provides p r o p e r t i e s f o r t y p i c a l s o l i d carboxylic n i t r i l e elastomers. I t i s the purpose of t h i s r e v i e w paper t o examine the r e l a t i v e l y newer u s e s o f t h e s e e l a s t o m e r t y p e s a s r e s i n m o d i f i e r s w i t h i n the broad range of epoxy-based c o a t i n g systems.
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E l a s t o m e r - M o d i f i e d Epoxy R e s i n P r e p a r a t i o n Wide-ranging documentation e x i s t s which covers m o d i f i c a t i o n o f epoxy r e s i n s u s i n g c a r b o x y l - t e r m i n a t e d p o l y b u t a d i e n e / a c r y l o n i t r i l e l i q u i d polymers i n which a d d i t i o n e s t e r i f i c a t i o n ( a l k y -
In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
DRAKE ET A L .
Elastomer-Modified Epoxy Resins
TABLE I CARBOXYL-TERMINATED (CT) LIQUID POLYMERS.... 0
Ç
HO-C-R-f (CH -CH=CH-CH ) -(CH -CH) 1-R-C-0H L I J 2
2
2
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y
M
Carboxyl Equivalent Weight (Typical) =1850
AMINE-TERMINATED (AT) LIQUID POLYMERS 0
0
H-N^N-(CH ) -N-8^^ 2
2
Amine Equivalent Weight (Typical) = 900
TABLE I I SOLID CARBOXYLIC BUTADIENE/ACRYLONITRILE ELASTOMERS
Physical Form
Percent Acrylonitrile
EPHR Carboxyl
Crumb*
26-28
0.07-0.08
Mooney Viscosity (ML-4 @ 100°C) 1
35-45
* H y c a r 1 4 7 2 , a p r o d u c t o f BF G o o d r i c h C h e m i c a l Group
In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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EPOXY RESIN CHEMISTRY
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d r o x y l e s t e r i f i c a t i o n ) r e a c t i o n s a r e employed t o p r e p a r e the epoxy base ( 5 - 8 ) . While such r e a c t i o n s proceed uncatalyzed at excess epoxy e q u i v a l e n t s n o r m a l l y g r e a t e r t h a n 3 / 1 - e p o x y / c a r b o x y l , i n s t r u c t i v e examples a r e g i v e n here f o r t-phosphine o r q u a t e r n a r y phosphonium s a l t c a t a l y z e d systems. Table I I I i l l u s t r a t e s t h i s f o r a common l i q u i d 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 (DGEBA) a t t h r e e s e l e c t e d temperatures w i t h t i m e - t o - z e r o - a c i d measured. T a b l e I V shows a v a r i a t i o n on t h i s w i t h m i x e d d i a c i d s o f v a r y i n g pKa v a l u e e m p l o y e d . T a b l e s V and V I g i v e r e c i p e s f o r t h e p r e p a r a t i o n o f e l a s t o m e r - m o d i f i e d s o l i d DGEBA e p o x y r e s i n s e i t h e r u s i n g a o n e - s t e p method ( d i r e c t m o d i f i c a t i o n ) o r a t w o - s t e p method ( a d vancement t o h i g h e r m o l e c u l a r w e i g h t w i t h b i s p h e n o l A a f t e r com p l e t i n g the carboxyl/epoxy r e a c t i o n ) . E l a s t o m e r - m o d i f i e d epoxy r e s i n s y s t e m s w i t h more c o m p l e x i t y t o t h e i r p r e p a r a t i o n scheme h a v e b e e n d e m o n s t r a t e d . Two e x a m p l e s suffice. S h e l l e y and C l a r k e (9) i n s t r u c t t h a t a ' v u l c a n i z a t i o n p r o c e d u r e c a n be s u c c e s s f u l l y e m p l o y e d t o i m p r o v e e l e v a t e d t e m p e r a t u r e p r o p e r t i e s i n t h e c u r e d r e s i n mass. T h i s s t e p o c c u r s s u b sequent to the e s t e r i f i c a t i o n regime. I t c a n be p r a c t i c e d w i t h i m p u n i t y a t low r u b b e r c o n t e n t s (7.5-10%) w i t h o u t g e l a t i o n o r i n d e e d v e r y much v i s c o s i t y i n c r e a s e . P e r o x i d e s a p p e a r t o be p r e f e r r e d over s u l f u r / s u l f u r donor systems. T a b l e V I I d i s p l a y s an e x a m p l e o f t h i s p r o c e d u r e w i t h a s o l i d DGEBA r e s i n . A n o t h e r chosen example stems from t h e p e r c e i v e d b e n e f i t s o f c o m b i n i n g s o l i d and l i q u i d c a r b o x y l i c n i t r i l e e l a s t o m e r s i n t h e same m o d i f i c a t i o n scheme. T h i s i s p a r t i c u l a r l y u s e f u l when a degree o f ' t a c k i s r e q u i r e d i n the system. Table V I I I e x h i b i t s one a p p r o a c h f o r p r e p a r i n g s u c h an e l a s t o m e r - m o d i f i e d s o l i d DGEBA resin. In t h i s i n s t a n c e a r e s i n (epoxide e q u i v a l e n t weight of 650) was p r e p a r e d by a d v a n c i n g two a v a i l a b l e l i q u i d epoxy r e s i n adducts — one u t i l i z i n g a l i q u i d n i t r i l e , t h e o t h e r a s o l i d n i t r i l e elastomer. To a l e s s e r e x t e n t , a m i n e - t e r m i n a t e d p o l y b u t a d i e n e / a c r y l o n i t r i l e (ATBN) l i q u i d s a r e a l s o u s e d by e p o x y f o r m u l a t o r s . T h i s p o l y m e r i c d i s e c o n d a r y amine i s e m p l o y e d i n a d m i x t u r e w i t h a m i n e s , a m i d o a m i n e s o r f a t t y p o l y a m i d e s o f c h o i c e ( 1 0 , 1 1 ) . T h u s , one a d m i x e s c h e m i c a l and p o l y m e r i c a m i n e s t o c r e a t e a n o v e l t o u g h e n i n g o r f l e x i b i l i z i n g hardener package. I n each i n s t a n c e of n i t r i l e e l a s t o m e r m o d i f i c a t i o n - whether r u b b e r i s added t o t h e epoxy p o r t i o n o r t o t h e h a r d e n e r p o r t i o n the l e v e l of rubber l a r g e l y determines whether a toughened o r a f l e x i b i l i z e d epoxy r e s u l t s . The f o r m e r i s c h a r a c t e r i z e d by l i t t l e loss i n thermal/mechanical p r o p e r t i e s . The l a t t e r shows a d o m i n a n t i n f l u e n c e o f t h e added r u b b e r . C o n s e q u e n t t o d o c u m e n t a t i o n s u r r o u n d i n g methods o f e m p l o y i n g r e a c t i v e n i t r i l e e l a s t o m e r s t o modify epoxy r e s i n s i s a growing body o f l i t e r a t u r e w h i c h s e r v e s t o c h a r a c t e r i z e and e l u c i d a t e these systems. Such t o p i c s as m o r p h o l o g y i n t h e c u r e d and uncured s t a t e , t r a n s i t i o n s from toughening to f l e x i b i l i z a t i o n , v i s c o e l a s t i c e f f e c t s , e q u i l i b r i u m p h y s i c a l p r o p e r t i e s and p h a s e s t r u c t u r e are a v a i l a b l e to the i n v e s t i g a t o r (12-17). 1
1
In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
DRAKE ET AL.
Elastomer-Modified Epoxy Resins
TABLE I I I ADDUCTS OF LIQUID EPOXY AND CTBN Parts DGEBA L i q u i d Epoxy (EEW, 185) CTBN (1300X8) Triphenylphosphine
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T e m p e r a t u r e (°C)
(Weight)
62.50 37.50 0.25
Time t o Z e r o A c i d ( H r s )
60 80 120
6.0 1.3
