2 Elastomeric Polysiloxane Modifiers for Epoxy Networks
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Synthesis of Functional Oligomers and Network Formation Studies J. S. RIFFLE—Union Carbide Corporation, S. Charleston, WV 25303 I. YILGOR, C. TRAN, G. L. WILKES, and J. E. McGRATH1Virginia Polytechnic Institute and State University, Department of Chemistry, Chemical Engineering, and Polymer Materials and Interfaces Laboratory, Blacksburg, VA 24061 Α. Κ. BANTHIA—Indian Institute of Technology, Materials Science Centre, Kharagpur 721302, India Novel elastomeric polysiloxane modifiers for epoxy networks have been synthesized and characterized. In addition, curing studies of conventional epoxy resins incorporating these oligo mers have been conducted. Structures were prepared having either epoxide, primary amine and/or secondary amine endgroups. The polymerization step for the siloxanes consisted of a base cata lyzed equilibration of the appropriately functionalized disiloxane and octamethylcyclotetrasiloxane. In the case of hydroxy piperazine terminated modifiers, polymers were f i r s t synthesized with epoxy endgroups. These endgroups were then subsequently capped with an excess of piperazine prior to curing. The oligomers were characterized by FTIR, 'H and 13C NMR, GPC, endgroup analysis and vapor pressure osmometry. The secondary amine terminated oligomers basically act as linear modifiers. By contrast, the aminopropyl functional siloxanes produce a crosslinked network. Piperazine terminated oligomers show much better compatibility with the epoxy resins compared to the aminopropyl terminated oligomers. This is especially true if the modifier contains an amide group. The curing reactions of the epoxy resins (EPON resin-828) using these oligomers and a cycloaliphatic diamine (PACM-20) were followed under a variety of conditions by DSC. Percent conversion versus time plots showed that r e a c t i v i t i e s of the functional siloxanes are higher than the cycloaliphatic amine at comparable temperatures. X-ray photoelectron spectroscopy shows the surfaces of the polysiloxane modified materials to be predominantly siloxane even at quite low bulk levels of the reactive oligomers. Preliminary mechanical property studies are encouraging and detailed morphology-property studies are continuing. Epoxy resins are one of the most important classes of thermosetting polymers. They are widely used for many important 1
To whom correspondence should be directed. 0097-6156/83/0221-0021$09.50/0 © 1983 American Chemical Society
In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
EPOXY RESIN CHEMISTRY
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a p p l i c a t i o n s such as c o a t i n g s , a d h e s i v e s , r e i n f o r c e d p l a s t i c s and m a t r i x r e s i n s f o r a d v a n c e d c o m p o s i t e m a t e r i a l s (J_~4). I f the n e t w o r k s t r u c t u r e i s p r o p e r l y g e n e r a t e d , epoxy m a t e r i a l s can e x h i b i t e x c e l l e n t c o r r o s i o n and s o l v e n t r e s i s t a n c e , good a d h e s i o n t o many s u b s t r a t e s and a d e q u a t e e l e c t r i c a l p r o p e r t i e s . However, i n c o n t r a s t to such d e s i r a b l e c h a r a c t e r i s t i c s , epoxy n e t w o r k s a r e t y p i c a l l y r a t h e r b r i t t l e and d i s p l a y r a t h e r low f r a c t u r e t o u g h ne'ss. M o r e o v e r , the n a t u r e o f t h e c r o s s l i n k i n g agent and i t s c o n c e n t r a t i o n may a l s o i n d u c e r e l a t i v e l y h i g h w a t e r a b s o r p t i o n characteristics. F o r e x a m p l e , a s t r u c t u r a l epoxy s y s t e m may be c u r e d w i t h a r o m a t i c a m i n e s . The r e s u l t i n g n i t r o g e n c o n t a i n i n g c r o s s l i n k p r o v i d e s s i t e s f o r hydrogen bonding to water. Absorp t i o n o f w a t e r has t h e e f f e c t o f p l a s t i c i z i n g t h e s e m a t e r i a l s and t h u s d e t e r i o r a t i n g one o f t h e i r more d e s i r a b l e c h a r a c t e r i s t i c s . M o r e o v e r , the l a c k o f t o u g h n e s s has l i m i t e d t h e a p p l i c a t i o n c h a r a c t e r i s t i c s o f these systems. Many s t u d i e s have been c o n d u c t e d w h i c h have a t t e m p t e d t o improve t h e f r a c t u r e and/or i m p a c t e n e r g i e s o f t h e n e t w o r k s , w h i l e at t h e same t i m e r e t a i n i n g t h e other properties. I n o r d e r t o do t h i s , one must c l o s e l y d e f i n e t h e p a r a m e t e r s such as t h e m o d i f i e r s t r u c t u r e , m o l e c u l a r w e i g h t , s o l u b i l i t y p a r a m e t e r , and c o n c e n t r a t i o n o f t h e e l a s t o m e r . In a d d i t i o n , t h e s t r u c t u r e and r e a c t i v i t y o f t h e h a r d e n e r and t h e c u r i n g agent must be c a r e f u l l y c o n t r o l l e d C5-8). I t i s important i n i t i a l l y t o d e f i n e e l a s t o m e r systems t h a t a r e s u f f i c i e n t l y "miscible w i t h t h e u n c u r e d epoxy f o r m u l a t i o n t h a t t h e y can be a d e q u a t e l y d i s p e r s e d . D u r i n g the f o r m a t i o n o f the network (e.g. the c u r i n g s t e p ) , i f the s o l u b i l i t y parameter o f the e l a s t o m e r and epoxy m a t r i x s t r u c t u r e d i f f e r s u f f i c i e n t l y , i t has been p o s s i b l e to d e v e l o p phase s e p a r a t e d e l a s t o m e r epoxy m a t e r i a l s . T h u s , s m a l l domains o f t h e r u b b e r phase w h i c h would be t h e m i n o r i t y component may be d i s p e r s e d i n t h e c o n t i n u o u s m a t r i x com p o s e d o f the epoxy r e s i n and t h e h a r d e n e r . T h i s dynamic f o r m a t i o n o f the m u l t i p h a s e s t r u c t u r e i s v e r y s e n s i t i v e to r e a c t i o n c o n d i t i o n s and the c h a r a c t e r i s t i c s o f t h e r u b b e r , such as i t s molecular weight. T h e r e f o r e , the f u n c t i o n a l i t y and r e a c t i v i t y o f t h e e l a s t o m e r as compared w i t h t h e h a r d e n e r a r e a l s o i m p o r t a n t parameters. P r e v i o u s l y , most o f t h e r e s e a r c h on t h e t o u g h e n i n g o f epoxy n e t w o r k s has u t i l i z e d t h e c o m m e r c i a l l y a v a i l a b l e c a r b o x y t e r m i n a t e d b u t a d i e n e - a c r y l o n i t r i l e (NBR) r u b b e r s (6>, 9-11). The c a r b o x y m a t e r i a l s a r e c a p a b l e o f i n t e r a c t i n g w i t h t h e epoxy g r o u p s to g e n e r a t e a h y d r o x y e s t e r as shown i n Scheme 1. Thus, t h i s r o u t e has p r o v i d e d a mechanism f o r c h e m i c a l l y b o n d i n g t h e e l a s t o m e r i c component t o t h e r i g i d m a t r i x . The c h e m i c a l l y bonded system then presumably should p r o v i d e a r e l a t i v e l y s t a b l e l i n k a g e b e t w e e n t h e m o d i f i e r and t h e m a t r i x . I f t h e r u b b e r p a r t i c l e s a r e o f a p p r o p r i a t e s i z e , t h e y may m o d i f y t h e p r o p e r t i e s o f t h e epoxy s y s t e m i n a somewhat a n a l o g o u s way t o t h e t o u g h e n i n g o f t h e r m o p l a s t i c s ( 2 3 ) . I t i s t h o u g h t , f o r e x a m p l e , t h a t t h e mechanism o f c r a z e y i e l d i n g may be i n f l u e n c e d by t h e p r e s e n c e o f t h e e l a s t o m e r 1 1
In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch002
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