Acetylene-Substituted Polyimides as Potential ... - ACS Publications

13 Acetylene-Substituted Polyimides as Potential HighTemperature Coatings N. BILOW

Downloaded by TUFTS UNIV on June 12, 2018 | https://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch013

Hughes Aircraft Company, Culver City, CA 90230

Early i n 1969, research was initiated, under U.S. A i r Force Materials Laboratory sponsorship1, aimed at the development of a new concept for chain extending and curing high temperature polymers, v i a an addition process. The need for such a process was apparent since u n t i l that time high temperature polymers such as polyimides, polyquinoxalines, polybenzimidazoles, e t c . , were all produced by condensation reactions which liberated large quantities of volatile byproducts during the polymerization, or cure. When prepolymers of these condensation polymers were used as molding compounds or laminating resins, they yielded porous structures with strengths and thermal oxidative stabilities w e l l below that which would be expected from theoretical considerations and well below that which would have been observed with nonporous structures. Furthermore, high molecular weight polyheterocyclics invariably were too intractable to fabricate into thick structures. After evaluating several potential cure concepts, it was found that uncatalyzed acetylene-terminated polyimide prepolymers could be chain extended and cured at temperatures of 200°C or above. I t was furthermore discovered that high strength polyimides could be produced having thermal s t a b i l i t i e s at least equivalent to those of conventional condensation type polyimides; thus, the polymerized acetylene groups had a high degree of thermal s t a b i l i t y . This suggested that aromatic moieties were being produced. Support for this conclusion was provided at Hughes i n a cursory study of one model compound which trimerized when heated i n the absence of catalysts, producing a benzenoid structure. However, an in-depth study of this homopolymerization was f i r s t conducted by P. Hergenrother

0-8412-0567-l/80/47-132-139$05.00/0 © 1980 American Chemical Society May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

RESINS F O R


140

AEROSPACE

H a v i n g made t h i s d i s c o v e r y , v a r i o u s t y p e s o f e t h y n y l s u b s t i t u t e d p o l y i m i d e prepolymers were s y n t h e s i z e d , molded i n t o v o i d - f r e e n e a t r e s i n s p e c i m e n s , and e v a l u a t e d as t o t h e i r m e c h a n i c a l p r o p e r t i e s . G l a s s and g r a p h i t e - f a b r i c r e i n f o r c e d c o m p o s i t e s w e r e a l s o f a b r i c a t e d and t e s t e d and shown t o r e t a i n h i g h s t r e n g t h s a t t e m p e r a t u r e s up t o 3 7 0 ° C I n g r a p h i t e f i b e r r e i n f o r c e d composi t e s , up t o 7 5 % o f t h e f l e x u r a l s t r e n g t h was r e t a i n e d a f t e r a g i n g i n a i r f o r 1000 h o u r s a t 3 2 0 ° C E a r l y d e s c r i p t i o n s o f t h i s work w e r e f i r s t p u b l i s h e d i n a s e r i e s o f l i m i t e d d i s t r i b u t i o n U.S. A i r F o r c e c o n t r a c t summary r e p o r t s , ! but the f i r s t p u b l i c d i s c l o s u r e d i d not occur u n t i l 1974.3. S u b s e q u e n t p a p e r s w e r e p r e s e n t e d a t m e e t i n g s o f t h e S o c i e t y f o r t h e Advancement o f M a t e r i a l s and P r o c e s s E n g i n e e r i n g (SAMPE) .-4*5>6 S u b s e q u e n t l y u p d a t e d p a p e r s w e r e p r e s e n t e d a t t h e American Chemical S o c i e t y Meeting i n Miami, FloridaZand Honolulu, Hawaii. These papers p r o v i d e d e x t e n s i v e m e c h a n i c a l p r o p e r t y d a t a . V a r i o u s U.S. and f o r e i g n p a t e n t s h a v e a l s o b e e n i s s u e d ^ - and s u b s e q u e n t l y h a v e b e e n l i c e n s e d t o t h e G u l f O i l C h e m i c a l Company, Hous t o n , Texas. Discussion Two s e r i e s o f p o l y i m i d e p r e p o l y m e r s w i t h t e r m i n a l a c e t y l e n e g r o u p s w e r e s y n t h e s i z e d f r o m monomers s u c h a s t h o s e i l l u s t r a t e d i n C h a r t s I and I I . B o t h i m i d i z e d p r e p o l y m e r s and t h e i r amic a c i d p r e c u r s o r s a r e s o l u b l e i n d i m e t h y 1 f o r m a m i de o r n - m e t h y l p y r r o l i d i n o n e and t h e s o l u t i o n s c a n be u s e d as v a r n i s h e s f o r t h e p r e p a r a t i o n o f c o a t i n g s , a d h e s i v e s , c o m p o s i t e s o l i d l u b r i c a n t s , and g l a s s o r g r a p h i t e f a b r i c r e i n f o r c e d l a m i n a t e d s t r u c t u r e s . S o l v e n t s such as t e t r a h y d r o f u r a n and a c e t o n e c a n o f t e n be u s e d when t h e p a r e n t d i a n h y d r i d e c o n t a i n s c e n t r a l g r o u p s s u c h a s 0, CH2, S, C(CF3>2 and c e r t a i n o t h e r m o i e t i e s ( s e e C h a r t s I and I I ) . P r e p o l y m e r o f t h e g e n e r a l type, i l l u s t r a t e d i n F i g u r e 1 (wherein n = l ) , melts a t 195°C -198°C, and when c u r e d a t 250°C, and s u b s e q u e n t l y p o s t c u r e d f o r 10 h o u r s a t 370°C, h a s a g l a s s t r a n s i t i o n t e m p e r a t u r e (Tg) as h i g h as 370°C. L e s s s e v e r e p o s t c u r e s g i v e l o w e r Tg r e s i n s as shown i n T a b l e I . M e c h a n i c a l p r o p e r t i e s o f t h e c u r e d p o l y m e r I ( w h e r e i n n = l ) a r e shown i n T a b l e I I . (compound I ( w h e r e i n n = l ) o r i g i n a l l y was d e s i g n a t e d HR600 b y t h e a u t h o r b u t t h e p o l y m e r i s c u r r e n t l y m a r k e t e d u n d e r t h e G u l f O i l C h e m i c a l Co. t r a d e name " T h e r m i d 600". When n=2, t h e o l i g o m e r i s d e s i g n a t e d HR602. H i g h e r t e l o m e r e h a v e b e e n d e s i g n a t e d i n an a n a l o g o u s mannerJ Thermogravimetric a n a l y s i s i n a i r i n d i c a t e s that the cured p o l y m e r i s s t a b l e t o o v e r 400°C a l t h o u g h l o n g t e r m (1000 h o u r ) a i r e x p o s u r e o f g r a p h i t e f i b e r r e i n f o r c e d l a m i n a t e s a t 316 C shows t h a t s t r e n g t h r e t e n t i o n i s a t b e s t 7 5 % , d e p e n d i n g upon t h e s p e c i f i c type o f g r a p h i t e f i b e r reinforcement used. This i s evidenced i n F i g u r e 3.

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

BILOW

Acetylene-Substituted

Polyimides

oc CM CM M ft

LU


et

HOl

DO

' ηU

flL

CM 41 CM

LU

Ζ

û. u.

i

ζ ο

Ο
=0

>

- 0 -

_o_

_o_

- 0 -

^C = 0 -CH2-

ζ

η °

- 0 -

;

^CiCF^

Χ

N

0

--

R

Ο

1

0

0

0

0

Ν MP, DMF THF THF (ACETONE)

0 0

200 - 210

Ν MP, DMF

175

134

THF (ACETONE)

150

CURE INCOMPLETE

212

253

370 263

296

182-185

ACETONE

0

195 - 200 160

324 294

Tg,°C OF C U R E D POLYMER

168-178

M.P., °C

Ο

ACETONE

SOLVENT

Ο

Type II oligomers

0

η

CHART IL


HR650

PARA ETHYNYL

HR600

NOTES

13.

BiLOw


TABLE I .

GLASS TRANSITION TEMPERATURES OF POLYIMIDE ( F I G 1» η =1) CURED UNDER VARIOUS CONDITIONS Postcure

Conditions m

Time, h r

Temp.,

ο C Λ

Tg,

°C

285 295 310 320 340

290 320 340 370 400

40 40 40 40 40


143

Polyimides

These v a l u e s , o b t a i n e d b y t h e r m o m e c h a n i c a l a n a l y s i s , w e r e somewhat l o w e r (20-25°C) t h a n t h o s e o b t a i n e d b y d y n a m i c m e t h o d s .

TABLE I I .

MECHANICAL PROPERTIES OF CURED POLYIMIDE I ( n = l ) ( 5 )

Property Tensile

Strength

14,000 p s i

T e n s i l e Modulus

550,000 p s i

Elongation

2.6%

Flexural

18,000-21,000 p s i

Strength

F l e x u r a l Modulus

650,000 p s i

Compressive

Up t o 66,000 p s i

Hardness

Strength

(Barcol)

85

SAMPE

D i f f e r e n t i a l t h e r m a l a n a l y s i s i n d i c a t e s a two s t e p p o l y m e r i z a t i o n , s i n c e a s m a l l e x o t h e r m i s o b s e r v e d a t a b o u t 200 C a n d a m a j o r e x o t h e r m a t 240-260°C. T h i s c o n c l u s i o n i s s u p p o r t e d b y p o l y m e r i z a t i o n mechanism s t u d i e s o f E.G. J o n e s , e t . a l . 2 - P r e p o l y mers h a v e g e n e r a l l y b e e n m o l d e d and c u r e d a t t h e l a t t e r t e m p e r a t u r e , a n d s u b s e q u e n t l y p o s t c u r e d i n a i r f o r 8-10 h o u r s a t 370 C. The l a t t e r t r e a t m e n t h a s b e e n f o u n d d e s i r a b l e when optimum l o n g term h i g h t e m p e r a t u r e s t a b i l i t y i s t o be r e a l i z e d . Higher degree o f p o l y m e r i z a t i o n prepolymers o f the type i l l u s t r a t e d i n F i g u r e 1 ( t o n=14) have T g s w h i c h a r e p r o g r e s s i v e l y lower as the degree o f p o l y m e r i z a t i o n i n c r e a s e s (see Table I I I ) ; t h u s , t h e i r maximum p o t e n t i a l h i g h t e m p e r a t u r e c a p a b i l i t y i s com p r o m i s e d d e p e n d i n g upon t h e a p p l i c a t i o n . Polymers w i t h the higher η v a l u e s ( e . g . , n=5) h a v e p r o v i d e d f i l m s i n s t u d i e s c o n d u c t e d to d a t e . f


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


144

(Polymide I) Figure 1.

Prepolymer of the general type

o

ô

Polyimide II

Figure 2.

Four acetylene-substituted polyimide oligomers


13.

BiLow


Polyimides f

TABLE I I I . T g s AS A FUNCTION OF (HR 600 TYPE PREPOLYMERS) η

145 η

Tg,

°c

1

320

2

268

3

254

5

240


P o s t c u r e s a t 340°C i n a i r

The a c e t y l e n e - s u b s t i t u t e d p o l y i m i d e s h a v i n g s t r u c t u r e s a n a l o gous t o t h a t o f F i g u r e 1 ( n = l ) w e r e made f r o m v a r i o u s c o m b i n a t i o n s of 4 diamines, 6 d i a n h y d r i d e s , and 2 a m i n o a r y l a c e t y l e n e s . S t r u c t u r e s o f t h e monomers u s e d a r e i l l u s t r a t e d i n C h a r t s I a n d I I . Several o f the acetylene-substituted polyimide oligomers (Chart I ) w e r e made w i t h o u t a d i a m i n e . F o u r o f t h e s e a r e i l l u s t r a t e d i n F i g u r e 2. P r e p o l y m e r s o f t h i s l a t t e r t y p e a r e s o l u b l e i n a c e t o n e and t e t r a h y d r o f u r a n , t h u s a l l o w i n g them t o b e u s e d i n a c e t o n e based v a r n i s h e s o r c o a t i n g s . D i e l e c t r i c p r o p e r t i e s o f t h e p o l y i m i d e o f F i g u r e l ( n = l ) were a l s o measured and found t o be v i r t u a l l y c o n s t a n t o v e r t h e tempera t u r e r a n g e o f 20-320°C a n d f r e q u e n c y r a n g e o f 9.0-12 GHZ. The d i e l e c t r i c c o n s t a n t was 3.13 +0.01 and t h e d i s s i p a t i o n f a c t o r was 0.5 +0.1. Prepolymers o f s t r u c t u r e I were a l s o found u s e f u l as a d h e s i v e s f o r t i t a n i u m . T y p i c a l T i - T i l a p s h e a r s t r e n g t h s depended upon t h e s p e c i f i c s t r u c t u r e , b u t t y p i c a l v a l u e s were as f o l l o w s : Temperature

[

n

=

1

n

=

2

*

Range, °C

20-260°C 20-260°C 20-260°C

Lap S h e a r Strengths, p s i 1200-2100 1600-3600 2700-4200

^ F i g u r e 1, n = l , w i t h t h e t e r m i n a l e t h y n y l p h e n y g r o u p s r e p l a c e d ! by 3 , 3 ' - e t h y n y l p h e n o x y p h e n y l . J A comprehensive r e v i e w o f t h e adhesive r e s e a r c h r e s u l t s has r e c e n t l y been s u b m i t t e d f o r p u b l i c a t i o n . l u The s t r e n g t h r a n g e i n d i c a t e d i n c l u d e s i n i t i a l a m b i e n t temper a t u r e v a l u e s a s w e l l a s v a l u e s o b t a i n e d a f t e r 1000 h r s . a g i n g a t 260 C. L o n g t e r m u s e a t t h e l a t t e r t e m p e r a t u r e t h u s i s h i g h l y practical. F l e x u r a l s t r e n g t h s o f g r a p h i t e f i b e r r e i n f o r c e d composites w e r e m e a s u r e d b e f o r e , d u r i n g , and a f t e r p r o l o n g e d t h e r m a l a g i n g i n a i r a t 316 C. R e s u l t s o f t h e s e t e s t s a r e shown i n F i g u r e s 3 and 4. The s t u d y d e m o n s t r a t e d t h a t u s e f u l p r o p e r t i e s a r e m a i n t a i n e d o v e r the l o n g term.




146

HRS IN AIR AT316°C

SAMPE Figure 3.

Flexural strength retention of graphite fiber-reinforced laminates made from prepolymer I (n = 1) (6). Refer to Figure 1.

200



13.

BiLOw


147

Polyimides

P o l y i m i d e I (n=l) n o t o n l y h a s been used t o p r e p a r e h i g h s t r e n g t h g l a s s and g r a p h i t e f i b e r r e i n f o r c e d l a m i n a t e s , b u t a l s o has p r o v i d e d v e r y h i g h c o m p r e s s i v e s t r e n g t h m o l d i n g s , a n d h i g h performance composite s o l i d l u b r i c a n t s . I n t e r l a m i n a r s h e a r s t r e n g t h s o f C e l i o n 3000 g r a p h i t e f i b e r r e i n f o r c e d l a m i n a t e s made f r o m P o l y i m i d e I (n=l) w e r e m e a s u r e d a s a f u n c t i o n o f b o t h t h e r m a l a g i n g a n d h u m i d i t y e x p o s u r e a t 70 C a n d 95% R.H. R e s u l t s o f t h e s e t e s t s a r e shown i n F i g u r e 5. I t i s e v i d e n t f r o m t h i s s t u d y t h a t e v e n a f t e r 1000 h o u r s o f c o n t i n u o u s e x p o sure, over 50% o f the i n i t i a l s t r e n g t h i s r e t a i n e d . Mechanical p r o p e r t i e s o f other cured polyimides o f the type i l l u s t r a t e d i n F i g u r e 1 vary w i t h the degree o f o l i g o m e r i z a t i o n as shown i n T a b l e s I V , V, a n d V I . T a b l e I V l i s t s m e c h a n i c a l p r o p e r t i e s , T a b l e V shows t h e v a r i o u s g l a s s t r a n s i t i o n t e m p e r a t u r e s , a n d T a b l e V I shows t y p i c a l g e l t i m e s . F o r a p p l i c a t i o n s such as i n c o a t i n g s , i t i s a n t i c i p a t e d that polymers w i t h the higher values o f η w i l l be most s a t i s f a c t o r y , s i n c e s u c h m a t e r i a l s h a v e a l o w e r c r o s s l i n k d e n s i t y a n d t e n d t o be t o u g h e r .

TABLE I V .

MECHANICAL PROPERTIES AS A FUNCTION OF η

HR600 TYPE PREPOLYMERS (STRUCTURE I ) Tensile Strength, KPSI

Modulus, MPSI

Elongation,

η

20°C

200°C

20°C

200°C

20°C

200°C

1 2 3 4

14 17 14 19

14.0 6.6 6.1 6.0

0.55 0.55 0.55 0.55

0.5 0.3 0.3 0.3

2.6 3.6 2.8 4.8

3.5 3.0 4.8

f

TABLE V.

T g s AS A FUNCTION OF η

η

Tg,

1 2 3 5

°c

320 268 254 240

P o s t c u r e s a t 340°C, Structure I

TABLE V I .

7

250°C

8.6 10.4 8.4

GEL TIME AS A FUNCTION OF η

η

Time, S e c

1 3 5 14

180 275 399 288

G e l t i m e a t 250°C, Structure I

American Chemical Society Library

May; Resins for Aerospace

1155 1Cth Chemical S t U. l¥. ACS Symposium Series; American Society: Washington, DC, 1980.



148

CELION 3000 FIBER REINFORCED LAMINATE, 36% RESIN

400 600 THERMAL AGING PERIOD, HRS.

Figure 5.

Interhminar shear strength as a function of air and humidity aging

Figure 6.

Structure I polymer adherent coatings on selected equipment


13.

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Polyimides

149


Research conducted t o date a l s o has i n c l u d e d an i n v e s t i g a t i o n of f l u i d i z e d bed c o a t i n g o f aluminum w i r e . T h i s i n v e s t i g a t i o n was p e r f o r m e d w i t h P o l y m e r I ( n = l ) by d r a w i n g h e a t e d w i r e t h r o u g h a T e f l o n TFE d i e w h i l e h e a t i n g t h e p r e p o l y m e r a t 188 C. The c o a t i n g p r o d u c e d was o f u n i f o r m t h i c k n e s s and a d h e r e d w e l l t o t h e a l u m i n u m w i r e . A n o t h e r s t u d y i n v o l v e d t h e c o a t i n g o f i r o n p i p e w i t h a 5% s o l u t i o n o f polymer I (n=l) i n dimethylformamide. C o a t i n g s were a i r d r i e d , t h e n c u r e d a t 250°C, and s u b s e q u e n t l y p o s t c u r e d up t o 320 C. A d h e r e n t c o a t i n g s p r o d u c e d f r o m t h e p o l y m e r o f s t r u c t u r e I ( n = l ) h a v e a l s o b e e n a p p l i e d t o b o t h c o n t r o l v a l v e s and pump i m p e l l e r s a s shown i n F i g u r e 6; h o w e v e r , t h e e v a l u a t i o n o f t h e s e c o a t i n g s has n o t y e t been completed. Synthesis

Procedure

I m i d i z e d prepolymers were p r e p a r e d by r e a c t i n g a 10% by w e i g h t s o l u t i o n o f d i a n h y d r i d e i n N - m e t h y l p y r r o l i d i n o n e (NMP) w i t h a 1 0 % s o l u t i o n o f t h e d i a m i n e i n t h e same s o l v e n t . A f t e r s t i r r i n g f o r 1/2-1 h o u r a t a m b i e n t t e m p e r a t u r e , t h e e t h y n y l a t e d amine i n NMP was a d d e d , and t h e r e a c t i o n was a l l o w e d t o c o n t i n u e f o r 1/2 h o u r . The m i x t u r e s w e r e h e a t e d a n d s u f f i c i e n t b e n z e n e was added t o a d j u s t the b o i l i n g p o i n t t o 150 C. When w a t e r e v o l u t i o n c e a s e d ( 4 - 6 h o u r s ) , t h e m i x t u r e s were c o o l e d , s o l v e n t was removed o n a r o t a r y e v a p o r a t o r , and t h e r e s i d u a l o l i g o m e r was s u b s e q u e n t l y t r i t u r a t e d i n e t h a n o l , then d r i e d . Summary Fourteen thermosetting acetylene-substituted polyimide prepolymers were s y n t h e s i z e d and e v a l u a t e d as t o t h e i r t h e r m a l p r o p e r t i e s , s o l u b i l i t y , and i n some c a s e s m e c h a n i c a l p r o p e r t i e s . Cured r e s i n T g s r a n g e d f r o m 212-410°C Low b o i l i n g s o l v e n t s s u c h a s a c e t o n e and t e t r a h y d r o f u r a n w e r e u s e a b l e i n some c a s e s , o p e n i n g up the p o s s i b i l i t y o f p r o d u c i n g s o l v e n t b a s e d c o a t i n g s f o r 300-370 C applications. C o a t i n g s p r e p a r e d t o d a t e h a v e shown c o n s i d e r a b l e promise a l t h o u g h t h e f i r s t comprehensive paper on t h e i r p r o p e r t i e s w i l l n o t be p u b l i s h e d u n t i l m i d 1980. f

Aknowledgements The a u t h o r w i s h e s t o e x p r e s s h i s a p p r e c i a t i o n t o Mr. A.A. C a s t i l l o , Mr. S. Goodman, D r . A.L. L a n d i s , Mr. W.H. F o s s e y a n d Mr. J . T e d e s c o f o r t h e i r v a l u a b l e c o n t r i b u t i o n s t o t h i s r e s e a r c h .

Literature 1. 2.

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