High Temperature Laminating Resins Based on Melt Fusible

38 High Temperature Laminating Resins Based on Melt Fusible Polyimides 1

HUGH H. GIBBS and C. V. BREDER

Downloaded by CORNELL UNIV on June 2, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0142.ch038

Plastics Department, Ε. I. duPont de Nemours & Co., Wilmington, Del. 19898

Incorporation of theperfluoroisopropylidenegroup as the flexibilizing linkage in the aromatic dianhydride 2,2-bis(3',4'-dicarboxyphenyl)hexafluoropropane dianhydride (6F) has resulted in a new class of melt fusible polyimide binders. Polyimides based on 6F and aromatic diamines are ideally suited for preparing lami nates having very low void levels. The low molecular weight prepolymer undergoes thermally induced condensation polymeri zation to produce a high molecular weight polyimide which is both linear and amorphous. Voids formed as a result of the elimi nation of volatiles can then be readily removed by applying pres sure above the glass transition temperature (T ). Varying the structure of the aromatic diamine varies the T over the range 229°-385°C. Such polymers are tough and have unusually good thermal-oxidative stability. g

g

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or s o m e t i m e n o w , t h e r e h a s b e e n a n e e d f o r a p o l y i m i d e b i n d e r w h i c h c o u l d b e u s e d t o m a k e l o w v o i d (less t h a n 3 % ) l a m i n a t e s , t h a t w o u l d h a v e a h i g h d e g r e e of p r o p e r t y r e t e n t i o n a n d t h e r m a l - o x i d a t i v e s t a b i l i t y i n t h e 2 6 0 ° 3 7 1 ° C t e m p e r a t u r e r a n g e , a n d t h a t w o u l d n o t b e c r o s s l i n k e d . T h e last f e a t u r e w o u l d n o t o n l y s e r v e t o p r o m o t e m e l t f u s i b i l i t y b u t w o u l d also c o n t r i b u t e t o toughness. I n addition, the b i n d e r s h o u l d be based o n a h i g h solids, l o w v i s c o s i t y s o l u t i o n o f t h e k i n d n o r m a l l y u s e d i n p r e p r e g g i n g o p e r a t i o n s . It s h o u l d also b e c a p a b l e o f b e i n g u s e d i n r a p i d , h i g h p r e s s u r e , m a t c h e d - d i e m o l d i n g as w e l l as i n l o w e r p r e s s u r e ( 1 0 0 - 2 0 0 p s i ) v a c u u m b a g - a u t o c l a v e m o l d i n g . N o k n o w n p o l y i m i d e b i n d e r m e t these criteria c o m p l e t e l y . Conventional aromatic polyimide binders polymerize b y thermally i n d u c e d c o n d e n s a t i o n r e a c t i o n s . I n t h e m o l d i n g o f a t y p i c a l p r e p r e g , as m u c h as 2 0 % w e i g h t loss c a n o c c u r . T h i s loss is a t t r i b u t a b l e t o a c o m b i n a t i o n o f r e s i d u a l 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 ( N M P ) o r d i m e t h y l f o r m a m i d e ( D M F ) and the water a n d alcohol evolved i n the polymerization a n d imidization r e a c t i o n s (see R e a c t i o n 1 ) .

Present address: Food and Drug Administration, Department of Health, Educa tion, and Welfare, Washington, D.C. 1

442

Platzer; Copolymers, Polyblends, and Composites Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

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If t h e d i a n h y d r i d e is a l l o w e d to p r e r e a c t w i t h a l c o h o l t o f o r m t h e d i e s t e r d i a c i d d e r i v a t i v e , n o p o l y m e r i z a t i o n o c c u r s w h e n t h e d i a m i n e is a d d e d . If t h i s ester-forming reaction were not a l l o w e d , the p o l y a m i d e a c i d f o r m a t i o n w o u l d o c c u r i m m e d i a t e l y . I n t h i s case t h e m o l e c u l a r w e i g h t w o u l d d e p e n d o n l y o n the m o l a r i m b a l a n c e of the m o n o m e r s ( R e a c t i o n 2 ). It is g e n e r a l l y n o t d e s i r a b l e to h a v e a h i g h m o l e c u l a r w e i g h t p o l y m e r s o l u t i o n as b i n d e r b e c a u s e at h i g h s o l i d s c o n t e n t t h e s o l u t i o n is t o o v i s c o u s t o h a n d l e , a n d at l o w s o l i d s c o n c e n t r a t i o n t h e r e is u s u a l l y i n s u f f i c i e n t p i c k u p i n o n e pass i n t h e p r e p r e g g i n g o p e r a t i o n . I n the e a r l y stages of c u r e , w h i c h n o r m a l l y t a k e p l a c e at 150°-200°C, t h e m o l e c u l a r w e i g h t r a p i d l y a d v a n c e s to t h e p o i n t at w h i c h t h e h i g h l y f l u i d b i n d e r s o l i d i f i e s ( g e l s ) . S i n c e t h i s o c c u r s b e f o r e a l l of t h e v o l a t i l e s h a v e b e e n e l i m i n a t e d , t h e n e t r e s u l t is a l a m i n a t e t h a t g e n e r a l l y h a s a h i g h v o i d c o n t e n t ( 1 0 - 2 0 % ). W h e n 3,3',4,4'-benzophenone tetracarboxylic d i a n h y d r i d e ( B T D A ) is u s e d as t h e d i a n h y d r i d e , it is v e r y d i f f i c u l t to e l i m i n a t e these v o i d s , e v e n at ele v a t e d temperatures a n d pressures. Side reactions a p p a r e n t l y occur d u r i n g p o l y m e r i z a t i o n w h i c h c o u l d l e a d to l o n g c h a i n b r a n c h i n g a n d c r o s s l i n k i n g , p o s s i b l y via t h e i n t e r a c t i o n of t h e c e n t r a l c a r b o n y l g r o u p i n B T D A a n d a n a m i n e g r o u p l e a d i n g to a n a z o m e t h i n e ( > C = N — ) l i n k a g e . S u c h a r e a c t i o n w o u l d greatly r e d u c e t h e m e l t f u s b i l i t y of t h e c u r e d p o l y i m i d e . T h e a d v e r s e effects o f t h e v o i d s so p r o d u c e d i n s u c h l a m i n a t e s h a v e b e e n d i s c u s s e d p r e v i o u s l y ( 1 , 2, 3, 4, 5).


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A n o v e l a p p r o a c h to e l i m i n a t i n g v o i d s has b e e n t o d e v e l o p b i n d e r s b a s e d o n i m i d e oligomers c o n t a i n i n g unsaturated aliphatic e n d groups (6, 7 ) . A t e l e v a t e d temperatures these oligomers m e l t a n d u n d e r g o a free r a d i c a l p o l y m e r i z a t i o n r e a c t i o n w h i c h d o e s n o t i n v o l v e t h e loss o f a n y v o l a t i l e b y p r o d u c t s . F o r t h i s r e a s o n v e r y l o w v o i d l a m i n a t e s c a n b e p r o d u c e d . H o w e v e r , t h i s class o f p o l y i m i d e s is i n h e r e n t l y v e r y b r i t t l e b e c a u s e o f e x t e n s i v e c r o s s l i n k i n g w h i c h occurs d u r i n g p o l y m e r i z a t i o n . Also, such materials cannot be used for long m u c h a b o v e 2 2 5 ° C b e c a u s e of t h e o x i d a t i v e i n s t a b i l i t y r e s u l t i n g f r o m t h e p r e s e n c e of a l i p h a t i c h y d r o g e n a t o m s i n t h e p o l y m e r c h a i n s . T h e o b j e c t i v e o f t h i s s t u d y w a s to d e v e l o p a p o l y i m i d e b i n d e r s y s t e m w h i c h w o u l d not u n d e r g o side reactions d u r i n g p o l y m e r i z a t i o n b u t w o u l d result i n a linear h i g h m o l e c u l a r w e i g h t a m o r p h o u s p o l y m e r . S u c h a c u r e d p o l y i m i d e w o u l d h a v e to h a v e s u f f i c i e n t m e l t f u s i b i l i t y t o u n d e r g o c o a l e s c e n c e r e a d i l y i n order to eliminate t r a p p e d voids w h e n heated u n d e r pressure above t h e glass t r a n s i t i o n t e m p e r a t u r e (T ). T h e a p p r o a c h has b e e n to p r e p a r e a n e w f a m i l y of p o l y i m i d e s , a l l b a s e d o n 2 , 2 - b i s ( 3 ' , 4 ' - d i c a r b o x y p h e n y l ) h e x a f l u o r o p r o pane dianhydride ( 6 F ) and various aromatic diamines. g

Some work involving polyimides prepared from p e r f l u o r o c a r b o n g r o u p s has a l r e a d y b e e n d o n e . Rogers

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of p o l y i m i d e s i n w h i c h t h e c o m m o n d e n o m i n a t o r w a s t h e h e x a f l u o r o i s o p r o p y l i dene b r i d g e d 6 F D A . A l t h o u g h the 6 F / 6 F D A p o l y i m i d e w a s prepared, there w a s n o r e p o r t of p o l y i m i d e s p r e p a r e d w i t h o u t t h e f l u o r o c a r b o n b r i d g e d d i a m i n e . C r i t c h l e y et al. ( 9 , J O ) h a v e d e s c r i b e d p o l y i m i d e s c o n t a i n i n g l i n e a r p e r fluoroalkylene bridges i n either one or b o t h of the monomers. T h i s paper, therefore, describes the p r e p a r a t i o n a n d properties of p o l y i m i d e s based o n 6 F and aromatic diamines w h i c h have unexpected utility i n preparing low-void l a m i n a t e s t h a t a r e t o u g h , r e t a i n t h e i r p r o p e r t i e s at e l e v a t e d t e m p e r a t u r e s , a n d possess a n u n u s u a l d e g r e e of t h e r m a l - o x i d a t i v e s t a b i l i t y .

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6FDA Experimental Preparation of U n r e i n f o r c e d P o l y i m i d e s . T h e polyimides characterized i n this s t u d y w e r e p r e p a r e d i n t w o w a y s . W h e n l a m i n a t i n g operations w e r e n o t g o i n g t o b e c a r r i e d o u t , t h e p o l y m e r s w e r e m o s t r e a d i l y p r e p a r e d via t h e a m i d e a c i d r o u t e i n p y r i d i n e . W h e n t h e l a m i n a t e - f o r m i n g c h a r a c t e r i s t i c s w e r e to b e s t u d i e d u l t i m a t e l y , t h e n e a t resins w e r e p r e p a r e d b y s i m p l e t h e r m a l p o l y m e r i zation of b i n d e r solutions. A m i d e A c i d R o u t e . T o a 3 - l i t e r , 3 - n e c k e d r o u n d - b o t t o m e d flask e q u i p p e d w i t h a thermometer, m e c h a n i c a l stirrer, condenser, a n d n i t r o g e n p u r g e w a s a d d e d 100 g ( 0 . 5 0 0 m o l e , 6 % m o l a r excess) 4 , 4 ' - o x y d i a n i l i n e ( O D A ) . T h e diamine was washed i n with 700 m l distilled pyridine w h i c h h a d been dried



446

COPOLYMERS,

POLYBLENDS,

AND

COMPOSITES

o v e r m o l c e u l a r sieves ( 5 A ) just p r i o r t o u s e . T h e m i x t u r e w a s s t i r r e d a n d h e a t e d t o 5 0 ° - 6 0 ° C t o e f f e c t s o l u t i o n . T h e n 6 F ( 2 1 3 . 6 g , 0.48 m o l e ) w a s a d d e d as a d r y s o l i d i n s m a l l p o r t i o n s d u r i n g a b o u t 5 m i n w i t h s t i r r i n g . L a s t traces w e r e w a s h e d i n w i t h 100 m l p y r i d i n e . T h e s l i g h t l y e x o t h e r m i c r e a c t i o n w a s e a s i l y m a i n t a i n e d at 5 0 ° - 6 0 ° C b y i n t e r m i t t e n t c o o l i n g w i t h a n i c e b a t h . A t the e n d of 45 m i n , a l l the d i a n h y d r i d e w a s i n s o l u t i o n a n d the p o l y a m i d e acid containing amine e n d groups was produced. Phthalic anhydride c a p p i n g a g e n t ( 5 . 6 9 g, 0 . 0 3 8 5 m o l e ) w a s a d d e d a l l at o n c e a n d w a s h e d i n w i t h 1 0 0 m l p y r i d i n e , stirring was c o n t i n u e d for an a d d i t i o n a l 45 m i n , a n d t h e n acetic anhydride (330 m l ) was a d d e d . I m i d i z a t i o n occurred almost instantly. T h e t e m p e r a t u r e rose f r o m a b o u t 3 0 ° to 4 5 ° C . T h e 6 F / O D A p o l y i m i d e s o l u t i o n w a s stirred for 30 m i n , t h e n c o o l e d to r o o m t e m p e r a t u r e a n d p o u r e d s l o w l y , w i t h v i g o r o u s s t i r r i n g , i n t o a l a r g e excess o f m e t h a n o l i n a W a r i n g b l e n d e r . T h i s p a r t of t h e o p e r a t i o n w a s u s u a l l y c a r r i e d o u t i n t h r e e stages u s i n g a 1 - g a l l o n b l e n d e r . T h e l i g h t c r e a m - c o l o r e d g r a n u l a r p o l y m e r w h i c h c a m e o u t of s o l u t i o n w a s s e p a r a t e d b y filtration a n d w a s h e d o n t h e filter w i t h t h r e e v o l u m e s o f m e t h a n o l . It w a s d r i e d o v e r n i g h t i n a v a c u u m o v e n at 1 5 0 ° C a n d t h e n h e a t e d to 2 6 0 ° C f o r 2 h r s to r e m o v e last t r a c e s of v o l a t i l e s . T h e i n h e r e n t viscosity was 0.42. T h e molecular weight was normally controlled b y using a 4 - 6 % m o l a r excess of d i a m i n e . N o t e t h a t t h e use of p h t h a l i c a n h y d r i d e as a c a p p i n g agent was not essential. W i t h o u t it, acetamide e n d groups w o u l d h a v e been f o r m e d . T h e more inert phenylene e n d groups were preferred i n order to m i n i m i z e m o l e c u l a r w e i g h t c h a n g e s d u r i n g m o l d i n g of t h e p o l y m e r . P r e p a r a t i o n o f B i n d e r S o l u t i o n s . I n a t y p i c a l r u n , 6 F ( 4 4 4 g , 1.00 m o l e ) w a s c h a r g e d i n t o a 2 - l i t e r 3 - n e c k e d r o u n d b o t t o m e d flask fitted w i t h a t h e r m o m e t e r , stirrer, a n d condenser. T h e n , 92.0 g (2.00 m o l e ) absolute ethanol a n d 3 5 7 m l D M F w e r e a d d e d ( N M P w a s also u s e d as a s o l v e n t i n t h i s s t u d y ) . T h e s l u r r y w a s h e a t e d w i t h s t i r r i n g t o 1 1 0 ° - 1 2 0 ° C at w h i c h p o i n t a l l o f t h e d i a n h y d r i d e was i n solution, a n d the diester was f o r m e d . T h e dark amber s o l u t i o n w a s s t i r r e d at t h i s t e m p e r a t u r e f o r 3 0 m i n , t h e n c o o l e d t o 8 0 ° C ; O D A ( 2 0 0 g , 1.00 m o l e ) w a s a d d e d s l o w l y w i t h v i g o r o u s s t i r r i n g o v e r a 5 - m i n p e r i o d . T h e O D A was w a s h e d i n w i t h an a d d i t i o n a l 100 m l D M F . T h e temperature w a s m a i n t a i n e d at 7 5 ° - 8 5 ° C f o r 3 0 m i n . A t t h e e n d o f t h i s t i m e , a l l o f t h e d i a m i n e was i n solution, a n d the b i n d e r was r e a d y for use. S o l u t i o n viscosity w a s 4 - 6 poises at 2 5 ° C as d e t e r m i n e d w i t h t h e B r o o k f i e l d v i s c o m e t e r . The c a l c u l a t e d c o n c e n t r a t i o n w a s 5 2 w t % o n a c u r e d r e s i n b a s i s . I n a l l cases, stoichiometric concentrations of the m o n o m e r s w e r e u s e d . P o l y m e r i z a t i o n of B i n d e r S o l u t i o n s . T o assess t h e p r o p e r t i e s of t h e u n r e i n f o r c e d resins d e r i v e d f r o m b i n d e r s o l u t i o n , t h e s o l u t i o n s w e r e p l a c e d i n a n a l u m i n u m p a n a n d t h e n c u r e d i n a v a c u u m o v e n . T h e f o a m e d m a s s of h i g h m o l e c u l a r w e i g h t p o l y m e r ( i n h e r e n t v i s c o s i t y g r e a t e r t h a n 0.5) was then m e c h a n i c a l l y c u t i n t o a finely d i v i d e d g r a n u l a r p r o d u c t . T h e c u r e c y c l e f o r p o l y i m i d e s w i t h a T,, less t h a n 3 0 0 ° C w a s 2 h r s at 2 0 0 ° C f o l l o w e d b y 3 h r s at 3 0 0 ° C . F o r t h o s e w i t h a T of 3 0 0 ° - 3 5 0 ° C , a n a d d i t i o n a l 3 h r s at 3 6 0 ° C w a s u s e d . F o r m e m b e r s of the f a m i l y w i t h the highest softening temperatures ( 3 5 0 ° - 3 8 5 ° C ) , t h e c u r e w a s e x t e n d e d e v e n f u r t h e r t o i n c l u d e a 3 - h r p e r i o d at 371 ° C . A l l p o l y m e r i z a t i o n s of this t y p e w e r e r u n u n d e r v a c u u m . g

C o m p r e s s i o n M o l d i n g o f U n r e i n f o r c e d R e s i n s . T h e finely d i v i d e d p o l y m e r w a s c h a r g e d i n t o a p o s i t i v e p r e s s u r e m o l d b e t w e e n t w o sheets o f g r a p h i t e c o a t e d a l u m i n u m f o i l , h e a t e d t o t h e m o l d i n g t e m p e r a t u r e at 4 0 0 0 p s i , a n d h e l d t h e r e f o r 10 m i n . A f t e r b e i n g c o o l e d u n d e r p r e s s u r e t o r o o m t e m p e r a t u r e , t h e c o m p r e s s i o n - m o l d e d s p e c i m e n was r e m o v e d . Test bars c o u l d then be c u t f r o m the p l a q u e w h i c h was n o r m a l l y about % - i n c h thick. T h e m o l d i n g temperature w a s u s u a l l y a b o u t 5 0 ° C h i g h e r t h a n t h e T of t h e p a r t i c u l a r p o l y i m i d e b e i n g used. g


38.

GIBBS

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A N D B R E D E R

Table I .

447

Resins

T y p i c a l F i n a l M o l d i n g Conditions for L a m i n a t e s

Rapid Matched-Die Cycle, min/°C/psi

Matrix Polymer 6F/0DA 6F/PPD 6F/1, 5-ND

Vacuum BagAutoclave Cycle, hrs/°C/psi

10/325/2500 10/371/2500 30/440/2500

4/316/200 3/371/200 1/400/200

C h a r a c t e r i z a t i o n of U n r e i n f o r c e d R e s i n s . T h e i n h e r e n t v i s c o s i t i e s w e r e d e t e r m i n e d i n c o n c e n t r a t e d ( 9 5 - 9 7 % ) s u l f u r i c a c i d at 2 5 ° C at a p o l y m e r concentration of 0 . 5 % . F o r t h e r m o g r a v i m e t r i c analyses ( T G A ) , t h e D u p o n t m o d e l 9 5 0 t h e r m o g r a v i m e t r i c a n a l y z e r w a s o p e r a t e d at a 1 5 ° C / m i n h e a t i n g rate. T h e T v a l u e s o f u n r e i n f o r c e d resins w e r e d e t e r m i n e d w i t h t h e D u p o n t m o d e l 9 0 0 differential s c a n n i n g calorimeter c e l l ( D S C ) operated at a h e a t i n g rate of 1 0 ° C / m i n .


n

P r e p a r a t i o n of L a m i n a t e s . T h e s t a c k e d p l i e s o f p r e p r e g w e r e l a i d u p i n a v a c u u m b a g a c c o r d i n g t o t h e p r o c e d u r e p r e v i o u s l y o u t l i n e d (11, 12, 13). Slight v a c u u m ( 1 - 2 p s i ) was applied, a n d the temperature w a s brought to 2 0 0 ° C at t h e rate of about 5 ° - 8 ° C / m i n . T h e s e conditions w e r e m a i n t a i n e d f o r one h r , a n d then t h e v a c u u m w a s increased to 1 2 - 1 5 p s i f o r another 4 5 m i n . W h e n 6 F / O D A binder was used, the partly cured laminate w a s discharged f r o m t h e v a c u u m b a g a n d c u t i n t o bars V 2 - i n . w i d e a n d 5 - 6 i n . l o n g . L a m i n a t e s m a d e w i t h p o l y m e r s h a v i n g h i g h e r TJs w e r e u s u a l l y h e a t e d f o r a n a d d i t i o n a l 2 h r s at 2 5 0 ° C b e f o r e b e i n g d i s c h a r g e d f r o m t h e v a c u u m b a g . A t t h i s p o i n t , t h e TJs w e r e g e n e r a l l y l o w ( 1 6 0 ° - 1 8 0 ° C ) a n d s e v e r a l w t % o f v o l a t i l e s w e r e n o t y e t d r i v e n off. N o r m a l l y , i f a n a u t o c l a v e w e r e t o b e u s e d , the l a m i n a t i n g cycle w o u l d b e c o n t i n u e d i n t h e o r i g i n a l v a c u u m b a g . I n this study, h o w e v e r , a l l laminations w e r e r u n i n a press. T o k e e p laminates (espe cially those w h i c h w e r e u n i d i r e c t i o n a l ) f r o m m o v i n g o u t sideways w i t h t h e a p p l i c a t i o n of pressure, it w a s necessary to place the m a c h i n e d bars i n a positive pressure m o l d , r e b a g t h e m o l d , a n d t h e n c o n t i n u e t h e cycle. I n this w a y t h e sides o f t h e m o l d c o n s t r a i n e d t h e l a m i n a t e . T h e r e m a i n d e r o f t h e m o l d i n g c y c l e v a r i e d a c c o r d i n g t o t h e p a r t i c u l a r b i n d e r a n d t h e p r e s s u r e u s e d (see T a b l e I for a s u m m a r y ) . P r e l i m i n a r y C h a r a c t e r i z a t i o n of L a m i n a t e s . A l l l a m i n a t e s w e r e r o u t i n e l y c h e c k e d f o r v o i d content b y c o m p a r i n g the actual density w i t h that calculated f r o m t h e concentrations of t h e components a n d their densities. T h e resin content c o u l d b e d e t e r m i n e d b y K j e l d a h l nitrogen analysis. T h e T w a s most conveniently determined with the D u p o n t model 941 thermomechanical analyzer operated at 5 ° C / m i n i n the expansion m o d e ( T M A - e x p a n s i o n ) . a

Discussion Properties of P o l y i m i d e s . E F F E C T O F S T R U C T U R E O N T . A s u m m a r y of t h e p o l y i m i d e s p r e p a r e d i n t h i s s t u d y b y t h e a m i d e a c i d r o u t e is g i v e n i n T a b l e I I . I t i s a p p a r e n t t h a t t h e T i n c r e a s e s w i t h d e c r e a s i n g flexibility o f t h e d i a m i n e . 6 F / R B A , t h e m o s t flexible c o m b i n a t i o n , h a d t h e l o w e s t T ( 2 2 9 ° C ) . A t t h e other extreme, 6 F / 1 , 5 - N D , w h i c h contains t h e b u l k y , r i g i d naphthalene ring, h a d t h e h i g h e s t T ( 3 6 5 ° C ) . I t is o f i n t e r e s t t o n o t e t h a t t h e d i a m i n e s t h a t contain t h e similarly sized — Ο — , — S — , a n d — C H — linkages a l l gave polyimides h a v i n g similar T values ( 2 8 0 ° - 2 9 1 ° C ) . Change i n the position of t h e a m i n o g r o u p i n p h e n y l e n e d i a m i n e f r o m t h e m e t a t o t h e p a r a p o s i t i o n caused t h e T to increase significantly ( 2 9 7 ° - 3 2 6 ° C ) . g

g

g

g

2

g

g

American Chemical Society Library 1155 16th st Η. ΨΙ.

Platzer; Copolymers, Polyblends, and Composites Washington, 0. C. 20036 Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

448

COPOLYMERS,

T a b l e II.

POLYBLENDS,

A N D COMPOSITES

Glass Transition Temperatures of 6 F Polyimides M a d e via A m i d e A c i d Route

Inherent T , Viscosity °C g

Code


Diamine

XH

2

A l l p o l y i m i d e s p r e p a r e d u s i n g 6 F as t h e d i a n h y d r i d e w e r e f o u n d t o b e a m o r p h o u s . A l l w e r e r e a d i l y s o l u b l e i n s o l v e n t s s u c h as p y r i d i n e , D M F , a n d N M P , as w e l l as i n c o n c e n t r a t e d s u l f u r i c a c i d . A l t h o u g h D S C c u r v e s s o m e t i m e s showed weak melting endotherms o n freshly precipitated polymer, the endotherm was permanently destroyed b y melting a n d could not be detected i n a


38.

GIBBS

A N D

T a b l e III.

Laminating

B R E D E R

449

Resins

R o o m Temperature M e c h a n i c a l Properties of 6 F C o p o l y i m i d e s M a d e via A m i d e A c i d R o u t e

Molar Ratio

Diamines'

1

ODA/RBA

90/10 80/20 60/40 65/35 62/38 51/49 41/59 25/75

ODA/1, 5-ND ODA/PPD

Tensile Strength, " psi

T , °C

Inherent Viscosity 0.39 0.40 0.39 0.35 0.38 0.34 0.36 0.36

Elongation, %

1

g

277 269 254 311 298 304 307 315

b

15,000 14,700 14,600 15,700 14,500 — — —

11 13 12 6 5 — — —

« See Table II. A S T M D-1708.


b

rerun

of the sample.

6F/1,5-ND)

were

Furthermore,

tested

t w o of the polyimides

for crystallinity

b y x-ray

(6F/ODA and

diffraction;

none w a s

detected. T h e ability to vary the T

o v e r a b r o a d r a n g e is o f c o n s i d e r a b l e

g

significance.

Laminates based o n polyimides having l o w T

g

practical

values w o u l d be

r e l a t i v e l y e a s y t o p r e p a r e b u t w o u l d suffer i n t e r m s o f h i g h t e m p e r a t u r e m e chanical property retention.

T h i s is b e c a u s e s u c h p r o p e r t i e s g e n e r a l l y f a l l off

quite r a p i d l y i n the v i c i n i t y of the T .

O n the other h a n d , if a p o l y i m i d e w i t h a

g

high Τ

g

were used, properties c o u l d be maintained through m u c h higher tem

peratures, b u t then the fabrication temperature w o u l d have to b e correspond ingly increased. final

T h i s is b e c a u s e , f o r o p t i m u m r e s u l t s i n v o l v i n g l a m i n a t e s , t h e

m o l d i n g temperature

is u s u a l l y h i g h e r

than the T .

I n any particular

g

a p p l i c a t i o n i t is n e c e s s a r y t o b a l a n c e off ease o f p r o c e s s i n g vs. e n d - u s e p e r f o r m ance.

F r o m t h e d a t a c i t e d i n T a b l e I I , i t is a p p a r e n t t h a t a w i d e r a n g e o f T

g

values w a s attainable

i n the 6 F f a m i l y of p o l y i m i d e s s i m p l y b y choosing t h e

appropriate diamine. Yet another w a y of v a r y i n g the T

g

imides using mixed diamines 1

g

80

I

70

1

(Table 1

was t h r o u g h the preparation of copoly

I I I ) . T h e d a t a i n d i c a t e t h a t t h e r e is a n

1

1

1

1

I

1

50 40

-

30 20 10 0

6F/0DA

IN

6F/0DA

IN

AIR

\ \

NITROGEN

\\

PMDA/ODA

IN

AIR

PMDA/ODA

IN

NITROGEN

\

N

\ \ *·. \

-

•*·\ 1

0

100

1

200

1

300

—

I

1

1

1

1

1

400

500

600

700

800

900

TEMPERATURE, °C Figure 1.

Programmed temperature thermogravimetric analysis of polyimides


450

C O P O L Y M E R S ,

P O L Y B L E N D S ,

A N D

C O M P O S I T E S

approximately linear relationship b e t w e e n the T values of the i n d i v i d u a l poly imides a n d the concentration of the diamines i n the c o p o l y i m i d e . T H E R M A L - O X I D A T I V E S T A B I L I T Y B Y T G A . T y p i c a l programmed tempera t u r e T G A c u r v e s r u n i n a i r a n d n i t r o g e n f o r 6 F / O D A are p r e s e n t e d i n F i g u r e 1. F o r c o m p a r i s o n , c u r v e s f o r P M D A / O D A are also g i v e n . I t is a p p a r e n t t h a t g

Ο

Ο

I

II


ο

ο

Ο

Pyromellitic dianhydride (PMDA) w h i l e b o t h p o l y m e r s w e r e quite stable, the latter p o l y i m i d e p e r f o r m e d some w h a t b e t t e r i n t h i s k i n d o f test. I n i s o t h e r m a l w e i g h t loss s t u d i e s c a r r i e d o u t at 3 9 0 ° C , b o t h p o l y m e r s d e g r a d e d a t t h e r a t e o f 0 . 2 - 0 . 3 % / h r i n n i t r o g e n a n d 0 . 4 - 0 . 6 % / h r i n air. M E C H A N I C A L P R O P E R T I E S O F U N R E I N F O R C E D P O L Y I M I D E S . Typical room temperature properties of the h o m o p o l y i m i d e s a n d the c o p o l y i m i d e s are pre s e n t e d i n T a b l e I V a n d T a b l e I I I r e s p e c t i v e l y . I t c a n b e s e e n t h a t as t h e d i a m i n e m o i e t y b e c o m e s b u l k i e r a n d less f l e x i b l e , t h e r e i s a p r o g r e s s i v e i n c r e a s e i n t h e T v a l u e s , a n d t h e p o l y m e r s also b e c o m e stiffer a n d s o m e w h a t m o r e brittle. Àt 2 5 0 ° C , the 6 F / O D A p o l y i m i d e still h a d a tensile strength of 4 8 0 0 psi, a tensile m o d u l u s of 148,000 p s i , a n d 7 8 % elongation. L O N G T E R M A I R A G I N G O F U N R E I N F O R C E D P O L Y I M I D E S . A summary of the data o n a i r o v e n aging of 6 F / O D A , 6 F / P P D , a n d 6 F / 1 , 5 - N D r u n at 2 6 0 ° , 3 1 6 ° , a n d 3 7 1 ° C respectively is g i v e n i n T a b l e V . N o t e that 6 F / O D A r e t a i n e d 7 7 % o f its o r i g i n a l tensile strength after 10,000 h r s at 2 6 0 ° C . g

Table IV. R o o m T e m p e r a t u r e M e c h a n i c a l Properties of P o l y i m i d e s M a d e b y T h e r m a l l y C u r i n g D M F - b a s e d B i n d e r Solutions Polyimide Property T , °C Tensile strength/psi Tensile modulus/ psi Flexural strength/psi Flexural modulus/psi Compressive strength/psi Elongation/ % Notched Izod impact / ft-lb/in. Density, g/cm g

3

6F/RBA 229 13,400 — — — — 11 1.0 —

6F/ODA 285« 17,600 496,000 22,000 500,000 62,400 15 1.0 1.42

6F/PPD 326 17,500 570,000 — — — 8 0.8 1.42

T values as high as 3 0 0 ° C were obtained when the solvent was Ν M P . To values as high as 3 8 5 ° C were obtained when the solvent was Ν M P . « A S T M D-1708.

High Temperature Laminating Resins Based on Melt Fusible

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