High Temperature Polymers from Thermally Curable Oligomers - ACS

1 High Temperature Polymers from Thermally Curable Oligomers PAUL M. HERGENROTHER

Downloaded by LAURENTIAN UNIV on December 5, 2014 | http://pubs.acs.org Publication Date: July 9, 1985 | doi: 10.1021/bk-1985-0282.ch001

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

The synthesis, physical and mechanical properties of thermally curable oligomers primarily for use as high temperature composite matrices are reviewed. High temperature in this context is defined as having usable mechanical properties at 177°C and higher. In the l a t e 1950's r e s e a r c h was i n i t i a t e d on high t e m p e r a t u r e p o l y m e r s , p r i m a r i l y t o meet the demand f o r f u n c t i o n a l and s t r u c t u r a l r e s i n s f o r advanced a i r c r a f t and weapon systems and the e l e c t r o n i c s industry. Many d i f f e r e n t polymers w i t h remarkable thermal s t a b i l i t y e v o l v e d from t h i s e f f o r t . However, most of t h e s e high temperature polymers f o r s t r u c t u r a l a p p l i c a t i o n s e x h i b i t e d p r o c e s s i n g problems due t o the e v o l u t i o n of v o l a t i l e s and/or poor f l o w and w e t t i n g even a t temperatures of ~371°C and p r e s s u r e s - 1 . 3 8 MPa (200 p s i ) . In the l a t e 1960's, r e s e a r c h was d i r e c t e d towards improving the processability of high t e m p e r a t u r e p o l y m e r s . The approaches p r i m a r i l y i n v o l v e d i n c o r p o r a t i n g more f l e x i b l e m o i e t i e s w i t h i n the polymer backbone t o permit p r o c e s s i n g as t h e r m o p l a s t i c s and p l a c i n g r e a c t i v e groups on the ends of o l i g o m e r s which c o u l d be t h e r m a l l y reacted to chain extend. The r e a c t i v e o l i g o m e r approach has s e r v e d as an a t t r a c t i v e r o u t e t o high performance/high temperature m a t e r i a l s f o r a v a r i e t y o f a p p l i c a t i o n s . To p l a c e r e a c t i v e o l i g o m e r s i n proper p r o s p e c t i v e , the advantages and d i s a d v a n t a g e s are b r i e f l y r e v i e w e d . The r e a c t i v e o l i g o m e r route a l l e v i a t e s many shortcomings of the high m o l e c u l a r weight l i n e a r polymer route by p r o v i d i n g p r o c e s s a b l e m a t e r i a l s w i t h b e t t e r s o l u b i l i t y and w e t t a b i l i t y , lower melt or s o f t e n i n g t e m p e r a t u r e and minimal v o l a t i l e e v o l u t i o n d u r i n g c u r e . It a l s o p r o v i d e s c u r e d m a t e r i a l s w i t h b e t t e r s o l v e n t and m o i s t u r e r e s i s t a n c e and g e n e r a l l y h i g h e r i n i t i a l e l e v a t e d temperature mechanical p r o p e r ties. The d i s a d v a n t a g e s are a p r o c e s s a b l e form w i t h s h o r t e r s h e l f l i f e and a cured m a t e r i a l w i t h l e s s toughness and lower l o n g term thermooxidative s t a b i l i t y . T h i s paper reviews t h e r m a l l y c u r a b l e o l i g o m e r s t h a t a f f o r d high performance/high temperature polymers primarily as laminating resins and i s not intended t o be

This chapter not subject to U.S. copyright. Published 1985, American Chemical Society

In Reactive Oligomers; Harris, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

REACTIVE OLIGOMERS

comprehensive. High temperature i n t h i s c o n t e x t i s h a v i n g u s a b l e mechanical p r o p e r t i e s at 177°C and h i g h e r .

defined

as

R e s u l t s and D i s c u s s i o n Epoxies - The term " r e a c t i v e o l i g o m e r " i s r e l a t i v e l y new but t h e concept i s f a i r l y o l d . T h i s concept has been used f o r many y e a r s w i t h systems such as e p o x i e s , p h e n o l i c s , u n s a t u r a t e d e s t e r s , cyanates, i s o c y a n a t e s and many o t h e r c r o s s l i n k e d s y s t e m s . An example of a 177°C c u r i n g epo*y system (Narmco's 5208) which was i n t r o d u c e d i n t o t h e m a r k e t p l a c e about 1971 i s shown i n E q . U


A

CH -CH-CH 2

2

N-/CH 2 -CH-ChO \ /2 0

V

0 I

MY720(100 PARTS)

CH„ DDS (28 PARTS)

0

0

CH -CH-CH -0-^|_^-0-CH -CH-CH 2

2

2

CH

SOLUBLE TACKY OLIGOMER

2

3

SU-8(8.2 PARTS) 177-204°C THERMOSET

The tetraglycidyl derivative of 4,4 -methylenedianiline (CibaG e i g y ' s MY-720) and an epoxy n o v o l a c ( C e l a n e s e ' s SU-8) a r e p a r t i a l l y r e a c t e d w i t h 4 , 4 ' - d i a m i nodi p h e n y l s u l f o n e (DDS) t o y i e l d a s o l u b l e tacky oligomer ( 1 ) . T h i s o l i g o m e r i s used t o melt impregnate a r e i n f o r c e m e n t sucfi as g l a s s o r c a r b o n / g r a p h i t e . The r e s u l t i n g p r e p r e g i s t a c k y , d r a p a b l e and has nominal s h e l f l i f e a t ambient t e m p e r a t u r e of two weeks. Composites f a b r i c a t e d w i t h a f i n a l c u r e of 2 hr a t 204°C under 0 . 3 4 MPa (50 p s i ) e x h i b i t good s t r e n g t h p r o p e r t i e s from -54°C t o 177°C but poor ambient temperature damaqe t o l e r a n c e ( a b i l i t y t o c a r r y a l o a d a f t e r s u s t a i n i n g impact damage). There a r e many o t h e r 177°C epo*y systems available such as F i b e r i t e ' s 934 and 9 7 6 , H e x c e l ' s 2 6 3 , and H e r c u l e s 3501 and 3502. The c o m p o s i t i o n of t h e s e systems v a r y . Some employ a c a t a l y s t such as boron t r i f l u o r i d e monoethylamine w h i l e o t h e r s use v a r i o u s o t h e r components ( e . g . g l y c i d y l e s t e r s ) t o modify t h e f o r m u l a t i o n . Cyanates - Cyanate c h e m i s t r y s e r v e s as another example where a r e a c t i v e o l i g o m e r i s i n i t i a l l y formed and s u b s e q u e n t l y cured t o a highly crosslinked resin. The i n i t i a l report on cyanates as l a m i n a t i n g r e s i n s i n 1968 (2) i n v o l v e d t h e d i c y a n a t e of 2 , 2 ' - b i s ( 4 hydroxyphenyl)propane ( b i s p h e n o l - A ) . More r e c e n t work has i n v o l v e d other dicyanates i n c l u d i n g 1,3-dicyanatobenzene ( r e s o r c i n o l d i c y a n a t e ) , a w h i t e c r y s t a l l i n e s o l i d m e l t i n g a t 80°C ( 3 ) . which i s t r a n s formed by h e a t i n g t o a t a c k y o l i g o m e r . T h i s form has been used f o r melt i m p r e g n a t i o n of g l a s s or c a r b o n / g r a p h i t e r e i n f o r c e m e n t t o y i e l d a p r e p r e g w i t h room temperature t a c k and d r a p e . Carbon/graphite r e i n f o r c e d composites e x h i b i t e d e x c e l l e n t mechanical p r o p e r t i e s a t ,


1.

HERGENROTHER

Thermally Curable Oligomers

26 t o 177°C (4·, 5 · ) . Cured neat d i c y a n a t e are given i n T a b l e I.

T a b l e I.


T e s t Temp.,

resin

properties

1 of

resorcinol

R e s o r c i n o l D i c y a n a t e Cured R e s i n P r o p e r t i e s °C

26

177

Tensile Strength, MPa ( K s i )

124 (18)

88.2 (12.8)

43.4 (6.3)

T e n s i l e Modulus, GPa (Ksi)

4 . 7 6 (690)

3 . 9 3 (570)

3 . 0 3 (440)

3.2

4.2

9.2

Flexural Strength, MPa (Ksi)

224 ( 3 2 . 5 )

158 ( 2 2 . 9 )

6 8 . 9 (10)

F l e x u r a l Modulus, GPa (Ksi)

5 . 0 3 (730)

3 . 7 2 (540)

2 . 6 9 (390)

227 (33)

111.7 ( 1 6 . 2 )

7 5 . 8 (11)

Compressive M o d u l u s , GPa (Ksi)

2 . 9 0 (420)

1.93 (280)

Izod I m p a c t - N o t c h e d , Ft. lb./in

0.35

Elongation, %

Compressive Y i e l d S t r e n g t h , MPa ( K s i )

232

1.52 (220)

Source: Reference 4 . Cyanamides - Cyanamides a l s o r e p r e s e n t a c l a s s of m a t e r i a l s where r e a c t i v e o l i g o m e r s have been p r e p a r e d . A representative example of t h e t y p e of m o d i f i c a t i o n done t o cyanamides t o moderate the i n i t i a l reaction to obtain l i n e a r soluble melt-processable o l i g o m e r s i s shown i n E q . 2 . A b i s ( a r y l s u l f o n y l cyanamide) was i n i t i a l l y r e a c t e d w i t h two moles of a b i s (cyanamide) t o y i e l d an oligomeric mixture (ideally r e p r e s e n t e d i n E q . 2 as a s i m p l e compound). These f i r e - r e s i s t a n t m a t e r i a l s have shown p r o m i s i n g p r o p e r t i e s as composite r e s i n m a t r i c e s ( 6 ) . B i s m a l e i m i d e s - B i s m a l e i m i d e s r e s i n s were f i r s t i n t r o d u c e d i n t o t h e market i n t h e e a r l y 1970*s. As w i t h o t h e r r e s i n s y s t e m s , t h e r e are many v a r i a t i o n s of b i s m a l e i m i d e s . The Kermid and K i n e l b i s m a l e i m i d e p r o d u c t s as marketed i n t h e U. S . by Rhodia are r e p r e s e n t a t i v e examples. Bismaleimide chemistry i s represented i n Eq. 3 where c u r i n g can be accomplished t h e r m a l l y through the u n s a t u r a t i o n i n t h e mal e i mi de o r by way of t h e M i c h a e l R e a c t i o n where an a p p r o p r i a t e c u r i n g agent such as a r o m a t i c diamine adds a c r o s s t h e a c t i v a t e d double bond. In most i n s t a n c e s , a combination of c u r i n g t h e r m a l l y through the double bond and v i a an a r o m a t i c diamine i s used i n a c t u a l p r a c t i c e . B i s m a l e i m i d e s are f r e q u e n t l y f o r m u l a t e d


4

REACTIVE OLIGOMERS

NC-N

„-®CH -t^- 7 - ®cH ior S0 C N

2

9

I

Φ

'

I 2 Φ

N C

N H

1

f

NH

NH-CN

2

NH

NC-HN

NH-CN


\ φ

ù

I φ

ί

SOLUBLE OLIGOMER IC MIXTURE

Y

'ni W

|

(2)

THERMOSET CONTAINING S-TRIAZINE MOIETIES

a

w i t h o t h e r m a t e r i a l s t o o b t a i n t h e proper combination of p r o p e r ties. As an example, m a t e r i a l s known as t h e BT r e s i n s (B stands f o r b i s m a l e i m i d e and Τ means t r i a z i n e ) a r e blends o f v a r i o u s amounts of a b i s m a l e i m i d e and a d i c y a n a t e (bisphenol A d i c y a n a t e ) (2)· One form of t h e BT r e s i n i s thought t o be t h e p r i n c i p a l component i n a l a m i n a t i n g r e s i n system known as 5245-C (&) which has d i s p l a y e d excellent t r a n s l a t i o n of f i b e r properties i n composites. In g e n e r a l , b i s m a l e i m i d e f o r m u l a t i o n s o f f e r a h i g h e r use temperature than epo*y s y s t e m s . SuIfones - Although thermosets as e x e m p l i f i e d by t h e cured e p o x i e s , cyanates and b i s m a l e i m i d e s a r e high modulus m a t e r i a l s , they are b r i t t l e when v o i d of toughening agents ( e . g . rubbers or t h e r m o -



1.

HERGENROTHER

5


plastics). In a d d i t i o n , t h e s e m a t e r i a l s absorb m o i s t u r e which lowers t h e i r use temperature and s t i f f n e s s . This i s e s p e c i a l l y apparent i n the compression s t r e n g t h at e l e v a t e d temperature of composites which have been exposed t o hot/humid e n v i r o n m e n t s . In an attempt t o develop a composite m a t r i x r e s i n w i t h improved m o i s t u r e r e s i s t a n c e , a s i g n i f i c a n t e f f o r t has been devoted t o the s y n t h e s i s and p o l y m e r i z a t i o n of b i s ( 3 - e t h y n y l p h e n o x y - 4 - p h e n y l ) s u l f o n e , commonl y r e f e r r e d t o as a c e t y l e n e - t e r m i n a t e d s u l f o n e ( A T S ) . Several d i f f e r e n t forms of ATS, c o n t a i n i n g v a r i o u s amounts of o l i g o m e r s , have been e v a l u a t e d ( 9 - 1 2 ) . The presence of o l i g o m e r s i s a d v a n t a geous s i n c e they depress the m e l t i n g p o i n t t o p r o v i d e a t a c k y m a t e r i a l at room temperature ( d e s i r a b l e f o r melt i m p r e g n a t i o n and t a c k and drape i n p r e p r e g ) . The most recent s y n t h e t i c r o u t e (12) t o ATS i s shown i n E q . 4 . Neat r e s i n moldings cured f o r 1 hr e a c f T a t Br

177 and 204°C and 18 hr at 218°C gave t h e p r o p e r t i e s i n Table II. Room temperature p r o p e r t i e s of u n i d i r e c t i o n a l A S - 4 g r a p h i t e f i b e r l a m i n a t e s (cured 2 hr each at 177 and 250°C) i n c l u d e d f l e x u r a l s t r e n g t h of 1.56 GPa (226,000 p s i ) , f l e x u r a l modulus of 117.2 GPa ( 1 7 , 0 0 0 , 0 0 0 p s i ) and s h o r t beam shear s t r e n g t h of 7 9 . 3 MPa (11,500 psi) (13). Laminate p r o p e r t i e s were a l s o r e p o r t e d t o be good at 177°C a f t e r exposure t o hot/humid environment ( 9 ) . Unfortunately, the b r i t t l e n e s s of cured ATS as i n d i c a t e d by f r a c t u r e energy of 35 J/M l i m i t s i t s use i n composite a p p l i c a t i o n s . M a t e r i a l s w i t h low toughness and o n l y moderate s t i f f n e s s e x h i b i t low l e v e l s of impact 2

Table I I .

Neat R e s i n P r o p e r t i e s of ATS

T e n s i l e S t r e n g t h , MPa ( K s i ) T e n s i l e M o d u l u s , MPa ( K s i ) Elongation, % G l a s s T r a n s i t i o n Temperature F r a c t u r e Energy ( G ) , J / M I c

1. 2.

2

(Tg),

°C

48.9 (7.1) 3619.4 (525) 1.4 300 35 2

Source: Reference 1 3 . Source: Reference 1 4 .


REACTIVE OLIGOMERS

6

resistance. In a d d i t i o n , m i c r o c r a c k i n g of t h e r e s i n i n a composite i s frequently encountered. Other a c e t y l e n e - t e r m i n a t e d resins (ATR) s i m i l a r t o ATS as shown i n E q . 5 have a l s o been prepared ( 1 3 , 1 5 ) . The cured Η 0 - » Γ - „ „ . Β φ Β Γ Γ

_

^

Β ^ - , , - O ^ B r

J

I HCEC-C(ChL)OH Pd cat. \

CH


HCEC^O/O-Ar-O-T^CECH

^

V

CH

3

3

H0-C-C = C ^ ^ 0 - A r - 0 ^ V c - C - C - 0 H

HERE A r =

^Toi

s o

i^,^ ioT s

io)iigr. tgusx

(

AND CH

'

(5)

0

a c e t y l e n e - t e r m i n a t e d r e s i n based upon b i s p h e n o l A was r e p o r t e d t o absorb s i g n i f i c a n t l y l e s s m o i s t u r e under hot/humid environment t h a n ATS (13). The cured hexafluoropropylidene containing resin e x h i b i t e d t h e lowest weight l o s s upon aging at 316°C f o r 200 h r i n air. More work w i l l undoubtedly be f o r t h c o m i n g on t h e s e ATR which will i n c l u d e toughening and more comprehensive e v a l u a t i o n as composite r e s i n m a t r i c e s . P r i o r t o t h e ATS work, mal e i mi d e - t e r m i n a t e d s u l f o n e s (16) were p r e p a r e d as d e p i c t e d i n E q . 6 . A f t e r c u r i n g t h e m a l e i m i d e - t e r m i n a t ed s u l f o n e s t r u c t u r e where η = 1 at 275°C f o r 0 . 5 h r , t h e s h e a r modulus was 1.2 χ 1 0 dyne/cm (173,880 p s i ) and t h e Tg = 239°C. When η = 2 , t h e shear modulus and Tg dropped t o 1.0 χ 1 0 dyne/cm (144,900 p s i ) and 185°C r e s p e c t i v e l y . A r e s i n t o be used as a composite m a t r i x s h o u l d have a high shear modulus t o prevent m i c r o b u c k l i n g o f t h e f i b e r s under a compression l o a d . The shear modulus i s r e l a t e d t o t h e Young's modulus by t h e e q u a t i o n G = E/2 (1 + v) where G = shear modulus, Ε = Young's modulus and ν - P o i s s o n ' s ratio. A cured r e s i n w i t h a Young's modulus of - 3 . 4 4 GPa (500,000 p s i ) and a f r a c t u r e energy of - 5 0 0 0 J / M i s d e s i r e d f o r use as a composite r e s i n matrix ( 1 7 ) . U n i d i r e c t i o n a l carbon/graphite f i b e r ( e . g . H e r c u l e s A S - 4 ) r e f n T o r c e d composites s h o u l d have a minimum c o m p r e s s i v e s t r e n g t h at room temperature of - 1 . 3 8 GPa (200,000 p s i ) and a t 82°C a f t e r m o i s t u r e s a t u r a t i o n of - 1 . 0 3 GPa (150,000 p s i ) f o r use i n commercial a i r p l a n e s . 1 0

2

1 T Î

2

2

Nadimi d e - t e r m i n a t e d p o l y s u l f o n e s (norbornene-terminated), as shown i n E q . 7 where t h e l i n e a r s u l f o n e segment had a number average m o l e c u l a r weight (M ) o f - 2 0 , 0 0 0 g / m o l e were f i r s t reported i n 1980 ( 1 8 ) . T h i s work was performed i n an attempt t o develop a tough s o l v e n t r e s i s t a n t r e s i n f o r use i n c o m p o s i t e s . The t h e r m a l l y induced reaction o f t h e nadimi de t e r m i n a l groups involves a n


HERGENROTHER


1.


0

M ~ 20,000 g/mole

7

0

c o m b i n a t i o n of r e a c t i o n s of the nadimide and m a l e i m i d e / c y c l o p e n t a diene formed from a r e t r o D i e l s - A d l e r r e a c t i o n . Graphite f a b r i c ( T - 3 0 0 , 8-Harness s a t i n ) r e i n f o r c e d composites gave RT f l e x u r a l s t r e n g t h of 0 . 6 3 GPa (91,900 p s i ) and RT f l e x u r a l s t r e n g t h a f t e r 28 days i n methylene c h l o r i d e of 0.12 GPa (17,000 p s i ) ( 1 9 ) . It was not s u r p r i s i n g t h a t a c r o s s l i n k e d polymer w i t h a l i n e a r s u l f o n e segment of M of - 2 0 , 0 0 0 g / m o l e was found t o be s e n s i t i v e to methylene c h l o r i d e , a p r i n c i p a l component i n p a i n t s t r i p p e r s , and an extremely a g g r e s s i v e s o l v e n t f o r p o l y s u l f o n e s . Ethynyl-terminated sulfone oligomers (ETS) were prepared by end-capping hydroxy-terminated sulfone oligomers with 4-ethynylbenzoyl chloride as i n d i c a t e d i n Eq. 8 (20). The n



8

REACTIVE OLIGOMERS

(8) CHAIN EXTENSION, BRANCHING, CROSSLINKING n=~7 TO-59 (M -3,000 TO -26.000 g/mole) t h e r m a l l y cured ETS e x h i b i t e d h i g h e r T ' s , b e t t e r s o l v e n t r e s i s t a n c e ; and s l i g h t l y b e t t e r a d h e s i v e and composite p r o p e r t i e s at 177°C than a comparable l i n e a r p o l y s u l f o n e (Union C a r b i d e ' s UDEL-P1700, from t h e r e a c t i o n of b i s p h e n o l - A and 4 , 4 ' - d i c h l o r o d i p h e n y l s u l f o n e ) (21)· The p r o p e r t i e s of ETS where t h e s u l f o n e segment had M of 4TJÏT0, 8000 and 12000g/mole are p r e s e n t e d i n Table III. g

n

Table I I I .

P r o p e r t i e s of Cured E t h y n y l - T e r m i n a t e d

M , g/mole of s u l f o n e segment T g , °C (250°C c u r e ) Film Swelling in chloroform, % G i c , J/M RT Thin F i l m T e n s i l e S t r e n g t h , MPa ( K s i ) RT Thin F i l m T e n s i l e M o d u l u s , GPa ( K s i ) RT Thin F i l m E l o n g a t i o n , % n

2

4000 202 < 10 790

— — —

8000 200 - 20 1300

— — —

Sulfones

12000 196 - 55 2100 83.4 (12.1) 2.45

(355)

4.6

Source: Reference 2 2 . I mi des - P o l y i m i d e s (PI) have been c o n v e n t i o n a l l y prepared by t h e chemical o r thermal c y c l o d e h y d r a t i o n of p o l y a m i c a c i d s formed from the s o l u t i o n r e a c t i o n of a r o m a t i c t e t r a c a r b o x y l i c 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 . The e a r l y PI were i n s o l u b l e and r e l a t i v e l y intractable. The polyamic a c i d was the p r o c e s s a b l e i n t e r m e d i a t e . However, t h e p o l y a m i c a c i d p r e c u r s o r has two major s h o r t c o m i n g s , h y d r o l y t i c i n s t a b i l i t y and the e v o l u t i o n of v o l a t i l e s d u r i n g t h e thermal c o n v e r s i o n t o P I . In a d d i t i o n , r e s i d u a l s o l v e n t was l e f t i n a d h e s i v e tapes and prepregs t o o b t a i n t a c k , drape and f l o w . During t h e f a b r i c a t i o n of components, the e v o l u t i o n of v o l a t i l e s caused p r o c e s s i n g problems and l e d t o p o r o s i t y i n t h e p a r t . As work p r o g r e s s e d on P I , o t h e r s y n t h e t i c r o u t e s were i n v e s t i g a t e d (e.g. r e a c t i o n of e s t e r s of 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 s w i t h diamines



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9

and r e a c t i o n of a r o m a t i c d i i s o c y a n a t e s w i t h a r o m a t i c t e t r a c a r b o x y l i c d i a n h y d r i d e s ). Information on s t r u c t u r e / p r o p e r t y relationships p e r m i t t e d t h e d e s i g n of t h e r m o p l a s t i c P I . However, u n t i l r e c e n t l y , l i t t l e work was done t o o p t i m i z e the m o l e c u l a r w e i g h t / p r o c e s s a b i l i t y / m e c h a n i c a l p r o p e r t i e s of t h e r m o p l a s t i c P I . G e n e r a l l y , tempera t u r e s 80 t o 120°C h i g h e r than t h e g l a s s t r a n s i t i o n temperature (Tg) and p r e s s u r e > 1.38 MPa were r e q u i r e d i n compression m o l d i n g . In most i n s t a n c e s , p r o c e s s a b l e PI were o b t a i n e d by compromising t h e use temperature. To improve t h e p r o c e s s a b i l i t y of PI and t o a l l e v i a t e or e l i m i n a t e v o l a t i l e s , work began i n t h e l a t e 1960's under NASA Lewis s p o n s o r s h i p ( 2 3 ) , t o end-cap o l i g o m e r s w i t h r e a c t i v e g r o u p s . The f i r s t r e p o r t i n v o l v e d t h e use of 3 , 6 - e n d o m e t h y l e n e - l , 2 , 3 , 6 - t e t r a h y drophthali c anhydri de (5-norbornene-2,3-di carboxy1i c anhydri d e , n a d i c a n h y d r i d e ) and a l k y l d e r i v a t i v e s t h e r e o f (e.g. citraconic a n h y d r i d e ) and 1 , 2 , 3 , 4 - t e t r a h y d r o p h t h a l i c a n h y d r i d e ( 4 - c y c l o h e x e n e 1 , 2 - d i c a r b o x y l i c a n h y d r i d e ) as t h e r e a c t i v e end group on i m i d e oligomers ( 2 3 - 2 5 ) . T h i s i n i t i a l e f f o r t l e d t o t h e development of P13N [ w i t h Ύ f o r p o l y i m i d e , 13 f o r M of ~1300g/mole and Ν f o r n a d i c end cap] whose s t r u c t u r e i s shown below. n

The amic a c i d o l i g o m e r c o u l d be t h e r m a l l y c y c l o d e h y d r a t e d t o y i e l d the nadimi d e - t e r m i n a t e d o l i g o m e r which e x h i b i t e d poor f l o w at temperature < 275°C. The end-groups begin t o r e a c t at a moderate r a t e at ~275°C w i t h t h e e v o l u t i o n of a s m a l l amount (1-2%) of cyclopentadiene. The thermal c h a i n e x t e n s i o 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 , o c c u r s through a c o m b i n a t i o n of a d d i t i o n r e a c t i o n s i n v o l v i n g t h e u n s a t u r a t i o n of t h e nadimide moiety 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 formed from a r e v e r s e D i e l s - A l d e r r e a c tion. T h i s route p r o v i d e d an improvement i n p r o c e s s i n g over t h e l i n e a r p o l y a m i c a c i d s and ester/ami ne d e r i v e d p o l y i m i d e s but t h e cured P13N r e s i n was b r i t t l e . E a r l y work r e p o r t e d good high temper a t u r e p r o p e r t i e s f o r s m a l l dense g l a s s r e i n f o r c e d composites ( 2 5 ) . Problems ( e . g . p o r o s i t y and m i c r o c r a c k i n g ) were encountered in~~the f a b r i c a t i o n of l a r g e composites and f i l l e d moldings a p p a r e n t l y due t o t h e e v o l u t i o n of water (from i n c o m p l e t e i m i d i z a t i o n ) , r e s i d u a l s o l v e n t ( Ν , Ν - d i m e t h y l f o r m a m i d e , DMF) and c y c l o p e n t a d i e n e , poor r e s i n f l o w and r e s i n s h r i n k a g e . In a d d i t i o n , v a r i a b i l i t y i n t h e s t a b i l i t y of t h e amic a c i d s o l u t i o n was a p r o b l e m . To r e s o l v e t h e s e s h o r t c o m i n g s , work was i n i t i a t e d at NASA Lewis Research C e n t e r which l e d t o t h e development of PMR-15 ( w i t h PMR meaning t h e i n - s i t u p o l y m e r i z a t i o n of monomeric r e a c t a n t s and 15 f o r an average M l i n e a r segment of - 1 5 0 0 g/mole) (26, 2 7 ) . The e a r l y work i n v o l v e d t h e dimethyl e s t e r of S ^ ' ^ V - b e n z o p h e n o n e t e t r a c a r b o * y l i c a c i d , 4 , 4 ' - m e t h y l e n e d i a n i l i n e and the monomethyl e s t e r n


10

REACTIVE OLIGOMERS

of 5 - n o r b o r n e n e - 2 , 3 - d i c a r b o ) ^ y l i c a c i d where t h e s t o i c h i o m e t r y was 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 M of - 1 5 0 0 g / m o l e . A methanol s o l u t i o n of t h e s e monomers was used t o impregnate the reinforcement. R e s i d u a l s o l v e n t (-11%) was r e q u i r e d t o p r o v i d e t a c k and drape t o the p r e p r e g . In the f a b r i c a t i o n of c o m p o s i t e s , the cure c y c l e was programmed t o f i r s t remove the r e s i d u a l s o l v e n t and then most of the v o l a t i l e s (water and methanol) from thermal c y c l i z a t i o n t o the i m i d e . This c y c l e i s commonly c a l l e d B - s t a g i n g . As t h e temperature was i n c r e a s e d , the B - s t a g e d r e s i n s t i l l had s u f f i c i e n t f l o w under p r e s s u r e t o compact f u r t h e r and t h e r e b y p r o v i d e dense (low v o i d c o n t e n t ) c o m p o s i t e s . In a c t u a l p r a c t i c e , complete c o n v e r s i o n t o imide p r i o r t o p o l y m e r i z a t i o n through t h e norbornenyl end-group may not o c c u r . T h i s i s only of academic concern s i n c e dense composites w i t h e x c e l l e n t mechanical p r o p e r t i e s have been f a b r i c a t e d (28, 2 9 ) . PMR-15 composites have a s i g n i f i c a n t data base and are b e i n g used i n a v a r i e t y of h i g h p e r f o r m a n c e , h i g h temperature a p p l i c a t i o n 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 a b l e from s e v e r a l major p r e p r e g s u p p l i e r s . To o b t a i n tack and drape i n the p r e p r e g w i t h o u t the use of r e s i d u a l s o l v e n t , the P13N and PMR-15 t e c h n o l o g y was f u r t h e r m o d i f i e d at NASA Langley Research C e n t e r . The h a l f - e t h y l e s t e r s of 3,3' ,4,4'-benzophenonetetracarboxylic a c i d and n a d i c a c i d were blended w i t h a l i q u i d e u t e c t i c amine m i x t u r e , J e f f a m i ne A P - 2 2 o r Ancamine-DL (homologs and isomers of m e t h y l e n e d i a n i l i n e ) , 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 M of ~1600g/ mole ( 3 0 ) . T h i s m a t e r i a l was c a l l e d LARC-160. The v i s c o u s monomeric m i x t u r e i s amenable t o melt 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 tack and drape w i t h o u t e x c e s s i v e r e s i d u a l s o l v e n t . As w i t h PMR-15, the p r e p r e g i s B - s t a g e d t o remove c o n d e n s a t i o n v o l a t i l e s and subsequently cured t o y i e l d dense composites w i t h good high temperature p r o p e r t i e s . LARC-160 i s a l s o a v a i l a b l e from s e v e r a l prepreggers. Nadic a n h y d r i d e was a l s o used t o t e r m i n a t e an amic a c i d o l i g o m e r prepared from the r e a c t i o n of 3 , 3 * , 4 , 4 ' - b e n z o p h e n o n e t e t r a carboxylic dianhydride and 3,3'-methylenedianiline where the s t o i c h i o m e t r y was a d j u s t e d t o y i e l d a nadimi d e - t e r m i n a t e d imide o l i g o m e r w i t h M ~1300g/mole. T h i s m a t e r i a l has been r e f e r r e d t o as LARC-13 and has been e v a l u a t e d as a high temperature a d h e s i v e ( 3 1 ) . A high t e n p e r a t u r e a d h e s i v e (BXR10214-151C) s i m i l a r t o PMR-15 i s commercially a v a i l a b l e (32). About 1970, r e s e a r c h was i n i t i a t e d under A i r Force f u n d i n g on a c e t y l e n e - t e r m i n a t e d imide o l i g o m e r s (ATI) which c o u l d be t h e r m a l l y c h a i n extended through the a c e t y l e n i c end-groups ( 3 3 , 3 4 ) . This e f f o r t r e s u l t e d i n the development of HR-600 ( E q . 9) and subsequent c o m m e r c i a l i z a t i o n by G u l f O i l Chemicals Company i n the form of T h e r mid - 6 0 0 . Neat r e s i n p r o p e r t i e s of HR-600 are p r e s e n t e d i n Table IV w h i l e p r e l i m i n a r y composite p r o p e r t i e s are g i v e n i n Table V. Although HR-600/Thermid-600 p r o v i d e d p r o m i s i n g neat r e s i n and composite p r o p e r t i e s , major p r o c e s s i n g problems have plagued t h e s e as w e l l as o t h e r a c e t y l e n e - t e r m i n a t e d o l i g o m e r s . R e s i n f l o w and w e t t i n g i s i n h i b i t e d due t o the r e a c t i o n of the t e r m i n a l e t h y n y l groups p r i o r t o the f o r m a t i o n of a complete melt or s o f t s t a t e . This becomes even more severe due t o heat t r a n s f e r problems as


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High Temperature Polymers from Thermally Curable Oligomers - ACS

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