Resins for Aerospace - American Chemical Society

1 Status of High-Temperature Laminating Resins and Adhesives P. M . HERGENROTHER and N. J. JOHNSTON

Downloaded by 46.161.62.102 on October 18, 2016 | http://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch001

National Aeronautics and Space Administration, Langley Research Center, Hampton, VA 23665

The q u e s t for high t e m p e r a t u r e p o l y m e r s for u s e as functional resins (e.g., coatings and sealants) a n d structural resins (e.g., a d h e s i v e s , c o m p o s i t e matrices a n d foams) b e g a n in the m i d - 1 9 5 0 ' s . Early work on high t e m p e r a t u r e p o l y m e r s c o n c e n t r a t e d primarily on thermal stability a n d paid little or n o attention to cost, processability, p e r f o r m a n c e o r actual u s e t e m p e r a t u r e . As w o r k progressed, it was quickly recognized that a successful p o l y m e r must exhibit a favorable combination of price, processability and performance and also that t h e r m a l stability differed from dimensional stability or heat resistance. Work on most of the more exotic a n d costly p o l y m e r s was s o o n discontinued as a result of this realism a n dthemore c u r r e n t emphasis h a s generally b e e n restricted to the d e v e l o p m e n t of materials for specific applications. In fact, this review will be confined to those high t e m p e r a t u r e p o l y m e r s currently b e i n g developed as a d h e s i v e s a n d c o m p o s i t e matrices. The t e r m " h i g h t e m p e r a t u r e " in this c o n t e x t refers to the ability of a structural material to perform properly in air for thousands of hours at 232°C, h u n d r e d s o f h o u r s a t 3l6°C a n d / o r m i n u t e s a t 538°C. The i d e a l h i g h t e m p e r a t u r e p o l y m e r i c s y s t e m f o r u s e as a p r o d u c t i o n - l i n e adhesive o r laminating r e s i n would e x h i b i t a v a r i e t y o f d e s i r a b l e f e a t u r e s such as t h e f o l l o w i n g : easy t a p e and p r e p r e g p r e p a r a t i o n , good s h e l f - l i f e , a c c e p t a b l e t a c k and d r a p e , acceptable q u a l i t y c o n t r o l procedures, low p r o c e s s i n g temperature and p r e s s u r e , n o v o l a t i l e e v o l u t i o n , dense g l u e l i n e o r m a t r i x ( n o v o i d s ) , good m e c h a n i c a l performance o v e r t h e d e s i r e d t e m p e r a t u r e range, t i m e , and e n v i r o n m e n t a l c o n d i t i o n s , a c c e p t a b l e r e p a i r a b i l i t y and c o s t - e f f e c t i v e n e s s . I r o n i c a l l y , no c o m m e r c i a l l y a v a i l a b l e s y s t e m e x i s t s w h i c h a c t u a l l y o f f e r s any r e a s o n a b l e combinat i o n o f these features. I n s t e a d , t h e few c o m m e r c i a l l y a v a i l a b l e m a t e r i a l s r e p r e s e n t a compromise o f t h e s e p r o p e r t i e s .

0-8412-0567-l/80/47-132-003$05.00/0 © 1980 American Chemical Society May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

RESINS

4

FOR AEROSPACE


H i s t o r i c a l Development o f H i g h Temperature S t r u c t u r a l R e s i n s To p l a c e t h e c u r r e n t s t a t u s o f h i g h t e m p e r a t u r e p o l y m e r s i n p r o p e r p e r s p e c t i v e , a b r i e f c h r o n o l o g i c a l review on each o f t h e more p o p u l a r h i g h t e m p e r a t u r e p o l y m e r s i s a p p r o p r i a t e . I n '1955 * Ε. I . DuPont de Nemours a n d Company, I n c . , was a w a r d e d t h e f i r s t o f s e v e r a l p a t e n t s (2) o n a r o m a t i c p o l y i m i d e s ( P I ) c o n s i d e r e d t o be t h e f i r s t f a m i l y o f h i g h t e m p e r a t u r e p o l y m e r s . Several companies s u c h as W e s t i n g h o u s e , M o n s a n t o , G e n e r a l E l e c t r i c , Amoco and DuPont d e v o t e d a s i g n i f i c a n t e f f o r t t o P I d e v e l o p m e n t . I n f a c t , more e f f o r t h a s gone i n t o P I d e v e l o p m e n t t h a n a l l o f t h e o t h e r h i g h t e m p e r a t u r e p o l y m e r s c o m b i n e d a n d , now t h a t t h e DuPont P I p a t e n t s a r e e x p i r i n g , i t w i l l b e i n t e r e s t i n g t o s e e i f com p a n i e s who a r e b a s i c i n c e r t a i n P I monomers ( e . g . , G u l f O i l C h e m i c a l s Company who s e l l s 3*3 , ^ , V - b e n z o p h e n o n e t e t r a c a r b o x y l i c a c i d dianhydride) w i l l enter t h e P I marketplace. The m a j o r p o r t i o n o f t h e e a r l i e r P I work i n v o l v e d t h e p r e p a r a t i o n o f s o l u b l e p r e c u r s o r p o l y ami c a c i d s f r o m t h e r e a c t i o n o f a r o m a t i c t e t r a c a r b o x y l i c a c i d d i a n h y d r i d e s and a r o m a t i c d i a m i n e s and t h e i r subsequent c y c l o d e h y d r a t i o n t o P I (Eq. l ) . f

P r e c u r s o r polyamic a c i d s e x h i b i t severe problems i n s h e l f l i f e and p r o c e s s i n g due t o h y d r o l y t i c i n s t a b i l i t y , w a t e r r e l e a s e d d u r i n g t h e c y c l o d e h y d r a t i o n and g e n e r a l l a c k o f m e l t f l o w . I n s p i t e o f t h e s e p r o b l e m s , s e v e r a l P I made b y t h i s g e n e r a l s y n t h e t i c r o u t e became c o m m e r c i a l l y a v a i l a b l e about i960, s u c h as P y r e ML lacquers, Vespel m o l d i n g m a t e r i a l s and m-3h adhesive. Concurrent w i t h t h e development o f P I , t h e p o l y b e n z i m i d a z o l e s ( P B I ) a s i n i t i a l l y r e p o r t e d b y M a r v e l (3) w e r e b e i n g evaluated. Polybenz imidazoles are g e n e r a l l y prepared by t h e p o l y c on dens at i on o f a r o m a t i c b i s (o^-di amines ) w i t h d i p h e n y l e s t e r s o f aromatic d i c a r b o x y l i c a c i d s (Eq. 2). C0 2

-2n H 0 2

May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

1.

HERGENROTHER

A N D JOHNSTON

High-Temperature

5

Laminating

C o n s i d e r a b l e amounts o f p h e n o l and w a t e r a r e e v o l v e d w h i c h caused pronounced f a b r i c a t i o n problems d u r i n g the p r o c e s s i n g o f c o m p o s i t e s and a d h e s i v e s . P B I was c o m m e r c i a l l y a v a i l a b l e i n t h e m i d I 9 6 0 s ( e . g . , I m i d i t e 850 f r o m Narmco M a t e r i a l s ) a n d i s c u r r e n t l y b e i n g e v a l u a t e d f o r u s e as h o l l o w f i b e r s i n w a t e r p u r i f i c a t i o n , f l a m e r e s i s t a n t f i b e r s and l o w d e n s i t y foams. D u r i n g t h e I 9 6 0 ' s a v a r i e t y o f new p o l y m e r s w e r e r e p o r t e d as h i g h t e m p e r a t u r e m a t e r i a l s p r i m a r i l y o n t h e b a s i s o f t h e i r w e i g h t l o s s p e r f o r m a n c e as m e a s u r e d b y t h e r m o g r a v i m e t r i c a n a l y s i s (^,5)« Few o f t h e s e new m a t e r i a l s were i n v e s t i g a t e d a s s t r u c t u r a l resins. One o f s e v e r a l s y s t e m s w h i c h d i d r e c e i v e c o n s i d e r a b l e a t t e n t i o n , h o w e v e r , was t h e p o l y p h e n y l q u i n o x a l i n e s ( P P Q ) . These m a t e r i a l s , as f i r s t r e p o r t e d i n 1967 (0 > a r e r e a d i l y p r e p a r e d from t h e p o l y c y c l o c o n d e n s a t i o n o f aromatic b i s ( o - d i a m i n e s ) and a r o m a t i c d i b e n z i l s ( E q . 3 ) as s o l u b l e , h i g h m o l e c u l a r w e i g h t thermoplastics. A l t h o u g h t h e y d i s p l a y e x c e l l e n t p o t e n t i a l as h i g h t e m p e r a t u r e s t r u c t u r a l r e s i n s , p a r t i c u l a r l y as a d h e s i v e s ,


1

COCO φ

-4nH 0 2

m t h e PPQ a r e r e l a t i v e l y e x p e n s i v e , r e q u i r e h i g h t e m p e r a t u r e p r o c e s s i n g and a t p r e s e n t a r e n o t c o m m e r c i a l l y a v a i l a b l e . As more p o t e n t i a l a p p l i c a t i o n s d e v e l o p e d f o r h i g h t e m p e r a t u r e s t r u c t u r a l r e s i n s , a l t e r n a t e s y n t h e t i c r o u t e s t o P I were i n v e s t i gated t o improve p r o c e s s a b i l i t y . These r o u t e s c o n c e n t r a t e d on a l l e v i a t i n g the e v o l u t i o n of the v o l a t i l e s responsible f o r f a b r i c a t i o n problems. I n t h e l a t e I 9 6 0 ' s TRW (j) d e v e l o p e d a r o u t e t o n a d i c end-capped imide o l i g o m e r s w h i c h c o u l d be t h e r m a l l y p o l y m e r i z e d t h r o u g h t h e u n s a t u r a t i o n ( E q . k). T h i s t e c h n o l o g y became known a s P13N w i t h t h e Ρ s t a n d i n g f o r p o l y i m i d e , t h e 1 3 r e p r e s e n t i n g an a v e r a g e o l i g o m e r i c m o l e c u l a r w e i g h t o f 1 3 0 0 , and t h e Έ Ο Ο o

AMIC ACID

A~200°C

-5.34H-0

THERMOSET


RESINS F O R


6

AEROSPACE

r e f e r r i n g t o n a d i c end-caps. The t h e r m a l c h a i n e x t e n s i o n , o f t e n r e f e r r e d t o as p y r o l y t i c p o l y m e r i z a t i o n , a p p a r e n t l y o c c u r s through a combination of addition reactions i n v o l v i n g the v i n y l g r o u p s o f t h e n a d i m i d e m o i e t y and m a l e i m i d e / c y c l o p e n t a d i e n e p r o d u c t s formed from a r e v e r s e D i e l s - A l d e r r e a c t i o n o f the nadimide. T h i s p r o c e s s i s a c c o m p l i s h e d -with t h e e v o l u t i o n o f a s m a l l amount o f c y c l o p e n t a d i e n e . A s i g n i f i c a n t i m p r o v e m e n t i n p r o c e s s i n g o v e r t h e p o l y a m i c a c i d s was r e a l i z e d b u t t h e r e s u l t a n t r e s i n was b r i t t l e . The P13N p a t e n t s w e r e p u r c h a s e d b y t h e C i b a G e i g y C o r p o r a t i o n who was u n s u c c e s s f u l i n d e v e l o p i n g a P13N product l i n e . Work a t R h o n e - P o u l e n c on p o l y i m i d e s r e s u l t e d i n t h e d e v e l o p ment o f m a l e i m i de e n d - c a p p e d o l i g o m e r s w h i c h c o u l d be c h a i n e x t e n d e d t h e r m a l l y a n d / o r b y r e a c t i o n w i t h an a r o m a t i c d i a m i n e (Eq. 5) (8*)· The K e r i m i d and K i n e l p r o d u c t s as m a r k e t e d b y R h o d i a i n t h e U. S. e v o l v e d f r o m t h i s t e c h n o l o g y . They e x h i b i t e x c e l l e n t m e l t f l o w p r o p e r t i e s and e a s i l y f o r m v o i d - f r e e p a r t s . Although these m a t e r i a l s are used i n c e r t a i n a p p l i c a t i o n s such

(5)

RESIN F R O M ADDITION T H R O U G H D O U B L E BOND A N D T H R O U G H M I C H A E L R E A C T I O N OF A M I N E WITH D O U B L E B O N D

as c i r c u i t b o a r d s , t h e y h a v e n o t r e c e i v e d a c c e p t a n c e as h i g h temperature s t r u c t u r a l r e s i n s because o f t h e i r l i m i t e d t h e r m a l stability. Current High Temperature S t r u c t u r a l Resins The P13N c h e m i s t r y was e x t e n d e d a t NASA-Lewis R e s e a r c h C e n t e r t o t h e d e v e l o p m e n t o f PMR-15, PMR s t a n d i n g f o r P o l y m e r i z a t i o n o f M o n o m e r i c Re a c t a n t s (£). The c h e m i s t r y i s t h e same as i n P13N e x c e p t t h e a v e r a g e o l i g o m e r i c m o l e c u l a r w e i g h t was i n c r e a s e d t o 1500 a n d t h e m a t e r i a l i n i t i a l l y u s e d f o r p r e p r e g f o r m a t i o n i s a m i x t u r e o f monomers. The d i a n h y d r i d e o f 3,3**+, V b e n z o p h e n o n e t e t r a c a r b o x y l i c a c i d and n a d i c a n h y d r i d e a r e s e p a r a t e l y r e a c t e d w i t h an e x c e s s o f m e t h a n o l t o f o r m t h e h a l f methyl esters. h k*-Methylene d i a n i l i n e i s added t o a m e t h a n o l i c s o l u t i o n o f t h e two h a l f - e s t e r s . The r e s u l t a n t v a r n i s h i s u s e d t o impregnate the r e i n f o r c e m e n t . Tack and drape are o b t a i n e d i n 9



1.

HERGENROTHER

A N D JOHNSTON

High-Temperature

7

Laminating

the prepreg through r e s i d u a l methanol which f r e q u e n t l y evaporates and l e a v e s t h e p r e p r e g b o a r d y . E s t e r i f i c a t i o n beyond t h e h a l f e s t e r stage has been o b s e r v e d and has been t h e source o f p r o c e s s i n g problems. R e s i d u a l v o l a t i l e s a r e removed d u r i n g a B - s t a g e c y c l e w i t h a d d i t i o n a l cure o c c u r r i n g through t h e nadimide endc a p s . PMR-15 h a s shown c o n s i d e r a b l e p o t e n t i a l f o r u s e i n h i g h temperature s t r u c t u r a l composites and i s c u r r e n t l y under e v a l u a t i o n i n many l a b o r a t o r i e s . PMR-15 p r e p r e g i s c o m m e r c i a l l y a v a i l able. I n an attempt t o improve p r o c e s s a b i l i t y and i n t r o d u c e t a c k and d r a p e c h a r a c t e r i n t o a P I p r e p r e g w i t h o u t u s i n g s o l v e n t , w o r k was i n i t i a t e d a t N A S A - L a n g l e y R e s e a r c h C e n t e r i n t h e m i d 1970 s w h i c h c l i m a x e d i n t h e d e v e l o p m e n t o f LARC-l60 ( E q . 6)(l0). A m i x t u r e o f n a d i c anhydride and 3»3 h,V-benzophenone t e t r a carboxylic a c i d dianhydride i s converted t o t h e h a l f - e t h y l 1

f

X

/

NH

2

-H 0, 2

" „ .

V

;

-EtOH

(6) THERMOSET^JL

B - S T A G E D IMIDE F O R M

esters using essentially a stoichiometric quantity o f ethanol. A l i q u i d o l i g o m e r i c a m i n e , J e f f a m i n e AP-22 o r Ancamine-DL, i s then blended w i t h t h e two h a l f - e t h y l e s t e r s w i t h t h e s t o i c h i o m e t r y a d j u s t e d t o p r o v i d e an o l i g o m e r w i t h an a v e r a g e m o l e c u l a r w e i g h t o f l600. T h e r e s u l t a n t m i x t u r e o f monomers i s a v i s c o u s l i q u i d a t room t e m p e r a t u r e a n d c o n t a i n s n o r e s i d u a l s o l v e n t . The monomeric m i x t u r e i s amenable t o m e l t i m p r e g n a t i o n t o p r o v i d e p r e p r e g w i t h t a c k and drape w i t h o u t t h e use o f r e s i d u a l s o l v e n t . I n c o m p o s i t e f a b r i c a t i o n , t h e c o n d e n s a t i o n v o l a t i l e s a r e removed d u r i n g a B-stage c y c l e t o form imide oligomers which e x h i b i t e x c e l l e n t f l o w . These a r e t h e n t h e r m a l l y c h a i n - e x t e n d e d a n d c r o s s - l i n k e d t h r o u g h t h e nadimide end-groups. Composites o f LARC-I60 e x h i b i t g o o d p e r f o r m a n c e a t h i g h t e m p e r a t u r e s . Both neat r e s i n and p r e p r e g c a n be o b t a i n e d c o m m e r c i a l l y . Because o f i t s enhanced p r o c e s s a b i l i t y , LARC-I60 i s c u r r e n t l y b e i n g e v a l u a t e d as a m a t r i x m a t e r i a l i n many l a b o r a t o r i e s . Fabrication d e v e l o p m e n t a c t i v i t i e s o n b o t h PMR-15 a n d LARC-160 a r e b e i n g s p o n s o r e d b y N A S A - L a n g l e y a t B o e i n g A e r o s p a c e Company a n d Rockwell International, respectively.


8

RESINS F O R A E R O S P A C E

I n 1 9 6 9 , Hughes A i r c r a f t Company b e g a n work u n d e r A i r F o r c e f u n d i n g o n a c e t y l e n e - t e r m i n a t e d i m i d e o l i g o m e r s (Eq. 7) w h i c h c o u l d be t h e r m a l l y p o l y m e r i z e d t h r o u g h a c e t y l e n e end-groups


Ο

THERMOSET

(ll). T h i s work c u l m i n a t e d i n t h e development o f a m a t e r i a l c a l l e d HR-600. I t was i n i t i a l l y p r o p o s e d t h a t c h a i n e x t e n s i o n o f HR-600 o c c u r r e d t h r o u g h a r o m a t i z a t i o n o r a c e t y l e n i c t r i m e r i z a t i o n (12). R e c e n t w o r k has shown, h o w e v e r , t h a t t h e t h e r m a l r e a c t i o n o f an e t h y n y l group p r o v i d e s a v e r y complex m i x t u r e o f p r o d u c t s ( 1 3 ) . T h i s t e c h n o l o g y was l i c e n s e d b y G u l f O i l Chemi c a l s Company and t h e HR-600 r e s i n i s now m a r k e t e d a s T h e r m i d 600. G r a p h i t e r e i n f o r c e d c o m p o s i t e s f r o m T h e r m i d 600 h a v e shown g o o d p e r f o r m a n c e a t 3l6°C a f t e r a g i n g . F u r t h e r development o f Thermid 600 as a l a m i n a t i n g r e s i n u n d e r AFML/NASA s p o n s o r s h i p was c u r t a i l e d due t o p r o b l e m s a s s o c i a t e d w i t h i n a d e q u a t e f l o w and s h o r t gel time. DuPont c o n t i n u e d t h e i r w o r k i n t h e P I f i e l d a n d i n 1 9 7 2 a n n o u n c e d t h e N R - 1 5 0 s e r i e s o f P I (lj+). One o f t h e s e m a t e r i a l s , NR-150B2, i s prepared by mixing 2,2-bis(3 ,V-dicarboxyphenyl) h e x a f l u o r o p r o p a n e w i t h 9 5 m o l e % p a r a - and 5 m o l e % m e t a p h e n y l e n e d i a m i n e i n a s u i t a b l e s o l v e n t s u c h as N - m e t h y l p y r r o l i d o n e (Eq. 8). The m o n o m e r i c v a r n i s h i s u s e d f o r t h e p r e p a r a t i o n o f p r e p r e g w h i c h r e t a i n s t a c k a n d d r a p e due t o r e s i d u a l solvent. The f a b r i c a t i o n o f l a m i n a t e s [ e . g . , 30.5cm χ 30.5cm χ 0.32cm ( 1 2 " χ 1 2 " χ 0 . 1 2 5 " ) ] i s d i f f i c u l t b e c a u s e o f t h e r e m o v a l o f h i g h b o i l i n g s o l v e n t and c o n d e n s a t i o n v o l a t i l e s ; c o n s e q u e n t l y , l o n g c u r e and p o s t c u r e c y c l e s must b e u s e d . As w i t h most c o m p o s i t e m a t e r i a l s , s m a l l amounts (PMR-15~LARC-l60. E a s e o f p r o c e s s i n g a n d


10

RESINS F O R A E R O S P A C E


TABLE I - E f f e c t Flexural

o fElevated

Strength

Flexural Matrix

Strength,GPa[Kpsi],

260°C a f t e r

RT

Temperature

o fUnidirectional

500 h r j

Exposure on HT-S L a m i n a t e s

{% r e t e n t i o n

288°C a f t e r

500 h r .

260°C i n a i r

@288°C i n a i r

o f RT p r o p e r t i e s ) 3l6°C a f t e r

500 h r .

@316°C i n a i r

NR-150B2

l.*»3[2073

1.30[1893 ( 9 1 )

1.21E1751

(85)

1 . 1 4 [ 1 6 51

(80)

PMR-15

1.43C208]

1.84[267] (128)

1.24[l80] (87)

0.97[1^0]

(67)

Thermid-600

1.28C185]

1.17C169]

(91)

0.99C1443

(78)

1.04[151] (82)

•LARC-160

2.13C309]

1.52[220]

(71)

14.4C2093

(68)

TABLE I I - E f f e c t on

Interlaminar

Interlaminar Matrix

RT

o fElevated Shear

Shear

Temperature

Strength

available

Exposure

o fUnidirectional

Strength,MPa[Kpsi](?

Not

HT-S L a m i n a t e s

r e t e n t i o n o f RT p r o p e r t i e s )

260°C a f t e r 5 0 0 h r .

288°C a f t e r

§260°C i n a i r

@288°C i n a i r

500 h r .

3l6°C a f t e r 5 0 0 h r §316°C i n a i r

NR-150B2

70.3C10.2]

40.7C5-91 (58)

39.3C5.7] ( 5 6 )

34.5C5.03 ( 4 9 )

PMR-15

lll.OCl6.l3

53.8C7.8] ( 4 8 )

47.6C6.93 ( 4 3 )

17.9C2.63 ( 1 6 )

Thermid-600

83.^[12.1]

60.OC8.73 ( 7 2 )

51.0C7.4] ( 6 1 )

4l.4C6.03 (50)

95.8[13.9]

53.8C7.8] ( 5 6 )

46.9C6.83 ( 4 9 )

Not

LARC-160

available


1.

HERGENROTHER

A N D JOHNSTON

High-Temperature

11

Laminating

q u a l i t y o f p r e p r e g f o l l o w e d t h e t r e n d : LARC-l60>PMR-15> 600>NR-150B2. W i t h r e s p e c t t o c o s t t h e f o l l o w i n g t r e n d LARC-160

Resins for Aerospace - American Chemical Society

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