17 Epoxy Resins Containing a Specific Vulnerability
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JAMES R. GRIFFITH Chemistry Division, Naval Research Laboratory, Washington, DC 20375
The concept of a coating with a built in vulnerability to destruction by specific reagents has been used successfully in some applications. For example, floor finishes which can be removed readily when renewal is required by the application of dilute ammonia, or another mild reagent, are available and convenient. The extension of this concept to rugged, durable coatings, such as epoxies, entails the requirement that a means be found to degrade an "infinite network" molecular structure which is inherently resistant to attack. A need exists for such materials, however, because the factors of toughness and durability which are desirable when epoxy coatings are applied become expensive, troublesome obstacles to removal when the time arrives for renewal. The present work concerns some efforts to synthesize epoxy resins which contain a cleavable weak link that allows the cured network to be chemically degraded into smaller fragments. The consequence of such degradation should be that such coatings would peel from surfaces or become soluble in common solvents after treatment with a "triggering" reagent. For convenience, such a selectively destructible polymer is called a command-destruct resin. P r o p e r t i e s o f An I d e a l Command-Destruct
Resin
An i d e a l command-destruct r e s i n should r e t a i n the d e s i r a b l e p r o p e r t i e s o f conventional epoxies; i n c l u d i n g , reasonable c o s t , cure by a v a i l a b l e c u r i n g agents, a b i l i t y t o be compounded with s o l v e n t s , pigments, etc. t o form h i g h - q u a l i t y p a i n t s , and provide long s e r v i c e l i f e . In a d d i t i o n , the cured coatings should r e t a i n the a b i l i t y t o be destroyed q u i c k l y and e f f e c t i v e l y by s e l e c t e d reagents not l i k e l y t o be encountered i n the s e r v i c e environment. A high degree o f destruct agent s p e c i f i c i t y insures against a c c i d e n t a l l o s s o f the coating. Also, the destruct
This chapter not subject to U.S. copyright. Published 1979 American Chemical Society
Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
EPOXY RESIN CHEMISTRY
260
agents should be m i l d chemicals without harmful environmental effects. A l l of the i d e a l p r o p e r t i e s may never be found i n r e a l systems because of a m u l t i p l i c i t y of problems concerning economics, s y n t h e s i s p o s s i b i l i t i e s , d e s t r u c t e f f e c t i v e n e s s l i m i t a t i o n s and environmental s t a b i l i t y . However, some o f the necessary a t t r i b u t e s of e f f e c t i v e m a t e r i a l s are being incorporated i n t o the f o l l o w i n g r e s i n types.
Downloaded by CORNELL UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch017
Cleavage o f O l e f i n i c Bonds One approach t o an e f f e c t i v e command-destruct epoxy i s that of i n c o r p o r a t i n g one or more o l e f i n i c bonds i n t o each molecule such that the f i n a l network can be degraded by the well-known r e a c t i o n of o l e f i n s with powerful o x i d i z i n g agents, such as a c i d i c permanganate:
!
R-CH=CH-R
f
> RC00H + H00CR
One o f the simpler compounds o f t h i s type i s the d i g l y c i d y l ether of h ,if-dihgdroxystilbene, although the expense and high-melting point {2Qk C) o f t h i s diphenol make the substance i m p r a c t i c a l : C H ^ C H C ^ O ^ ^ C H = C H ^ ^ 0 C H C H CH, 2
0 The product obtained i n a standard d i g l y c i d y l ether r e a c t i o n with e p i c h l o r o h y d r i n and a l k a l i was an o f f - w h i t e c r y s t a l l i n e powder melting at 191°-193°C. A lower m e l t i n g diphenol which contains s e v e r a l o x i d i z a b l e s i t e s i s curcumin. This i s an i n t e n s e l y c o l o r e d pH i n d i c a t o r which undergoes v a r i o u s v i v i d c o l o r changes during the s y n t h e s i s of the d i g l y c i d y l ether. The f o l l o w i n g formula shows the simplest form, although the a c t u a l products obtained t o date have apparently been polymeric: H C0 3
CHgCHCHgO 0
0
0
CH=CHCCH CCH=CH 2
0CH
3
OC^CH
CH
2
0
A t h i r d compound o f the o l e f i n i c type, and the most promising obtained t o date, i s the d i g l y c i d y l ether o f 2,6d i v a n i l l y l i d e n e cyclohexanone, which i s produced by the conventional r e a c t i o n o f the diphenol with e p i c h l o r o h y d r i n :
Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
17.
Epoxy Resins with a Specific Vulnerability
GRIFFITH
CH 0
ο
3
CH CHCH 0
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^OCH
3
=CH ^ )
CH= ^
2
2
OC^CH
\y
o
261
CH
2
0
This b r i g h t yellow, c r y s t a l l i n e compound has a m e l t i n g range of 1^3 -ihQ C, and i t i s v i r t u a l l y i n s o l u b l e i n common epoxy s o l v e n t s . However, i t -will react with polyamide c u r i n g agents, and f i l m s of good q u a l i t y have been produced by use o f dimethyl s u l f o x i d e as solvent. The f i l m s so produced are r e a d i l y destroyed by a s o l u t i o n of potassium permanganate i n 50% a c e t i c a c i d - water. Permanganate i n water alone i s i n e f f e c t i v e , and the solvency of the a c e t i c a c i d i s necessary i n order t o s w e l l and s o f t e n the cured f i l m s enough f o r the permanganate t o penetrate to the o l e f i n i c s i t e s . After degradation, the f i l m residue may be e a s i l y brushed away from the surface t o which the f i l m was formerly t i g h t l y bonded. Another approach t o a d i g l y c i d y l ether of t h i s type which r e s u l t s i n a product of improved s o l u b i l i t y c h a r a c t e r i s t i c s i s the r e a c t i o n of two moles of a pure d i g l y c i d y l ether of Bisphenol-A w i t h one mole of 2 , 6 - d i v a n i l l y l i d e n e cyclohexanone. This r e s u l t s i n a r e s i n with g l y c i d y l ether t e r m i n a l s which i s p a r t i a l l y s o l u b l e i n ketone s o l v e n t s , p a r t i c u l a r l y methyl e t h y l ketone. Further work w i l l be necessary t o produce p r a c t i c a l p a i n t s from the 2 , 6 - d i v a n i l l y l i d e n e cyclohexanone system, but as an example of the p o s s i b i l i t i e s of the o l e f i n i c approach t o c l e a v a b l e epoxy r e s i n s , i t has been reasonably s u c c e s s f u l . Cleavable T e r t i a r y Carbamate Epoxy Resins T e r t i a r y carbamates, or urethanes, are not produced as r e a d i l y as the primary and secondary compounds, but an e f f e c t i v e s y n t h e s i s has been d e s c r i b e d ( l ) . This molecular u n i t can be r e a d i l y cleaved by warm s o l u t i o n s o f m i n e r a l a c i d s ( 2 ) , and the ease of cleavage may be c o n t r o l l e d by proper s e l e c t i o n o f the components o f the t e r t i a r y r a d i c a l :
0 f
R R"R"*
H+
COCNHR
>R'R"R"' COH + C0
2
+ H NR
Δ Epoxy r e s i n s may be produced which c o n t a i n t e r t i a r y carbamates by the use of g l y c i d o l as the c h a i n - t e r m i n a t i n g alcohol i n a diisocyanate - d i o l reaction. For example, a stepwise r e a c t i o n has been used i n which one e q u i v a l e n t of g l y c i d o l has been reacted w i t h 1,6-hexamethylene d i i s o c y a n a t e t o produce a mixture represented i d e a l l y by the r e a c t i o n :
Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
EPOXY RESIN CHEMISTRY
262 CH CHCH 0H + 0=C=N(CH ).N=C=0 o
o
>
o
0 0 CH CHCH OCNH(CH )^N= C= 0 0
0
2
0
2
\ / 0
2
6
Downloaded by CORNELL UNIV on October 8, 2016 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch017
The a v a i l a b l e remaining isocyanate may then react with a t e r t i a r y a l c o h o l when d i b u t y l t i n d i a c e t a t e i s used as catalyst. An i n t e r e s t i n g d i o l c o n t a i n i n g both primary and t e r t i a r y terminals i s h y d r o x y c i t r o n e l l o l , a derivative o f n a t u r a l products : CH„
CH
HOC CH
0
(CH ) C 2
CH CH 0H
3
2
2
3
The complex epoxy r e s i n obtained from t h i s system i s a lowmelting wax which i s nominally represented by the formula:
0 II,
0 CHCH ΝI I (CH ) C I 3 3
x
3
x
CH CHCH 0CNH(CH ).NHC0C \ ds d do O
o
o
0
0
d
CH
0
0 H ,
0
x
U
0
CH CH 0CNH(CH )^NHC0CH CH C H d d do d \ / d O
o
o
O
3
Q
0
This r e s i n may be melted and blended with amine c u r i n g agents t o produce tough polymers which are v u l n e r a b l e t o cleavage at the t e r t i a r y linkage. As with the o l e f i n - c o n t a i n i n g epoxies, much i s y e t t o be done with t e r t i a r y carbamate epoxies i n order t o produce p r a c t i c a l systems, but the s y n t h e s i s p o s s i b i l i t y has been established. A v a r i e t y o f d i i s o c y a n a t e s and t e r t i a r y a l c o h o l s are a v a i l a b l e . A general s y n t h e s i s e f f o r t i s needed t o screen the most e f f e c t i v e compositions with regard t o solvency, compounding, cure, environmental s t a b i l i t y , and the e f f e c t i v e n e s s of v a r i o u s d e s t r u c t agents and c o n d i t i o n s .
Literature Cited 1.
Bailey, W. J. and Griffith, J. R. Polymer Preprints, 1964, (1), 266-278.
2.
Bailey, W. J. and Griffith, J. R. Polymer Preprints, 1964, 5 (1), 279-290.
5
RECEIVED May 21, 1979.
Bauer; Epoxy Resin Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
