Quantitative Analysis of Resin Matrix Aging by Gel Permeation

Aug 28, 1980 - Quantitative Analysis of Resin Matrix Aging by Gel Permeation Chromatography and Differential Scanning Calorimetry. DAVID J. CRABTREE...
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Calorimetry DAVID J. CRABTREE N o r t h r o p C o r p o r a t i o n , H a w t h o r n e , CA 90250

The matrix resins widely u s e d with graphite fiber for t h e fabrication of composite aircraft structures a r e epoxy formulations which, while t h e y h a v e a t least six months s t o r a g e life a t - 18C, have appreciable reaction rates a t normal ambient t e m p e r a t u r e s . T h e s e p r e p r e g s c a n therefore b e ruined b y inadvertent e x p o s u r e t o a m b i e n t t e m p e r a t u r e . It is crucially i m p o r t a n t for t h e fabricator of c o m p o s i t e aircraft structures t o know how much a g i n g h a s o c c u r r e d in t h e s e p r e p r e g matrix resins prior t o fabrication. If significant a g i n g h a s occurred, t h e fabricator may h a v e t o change t h e cure cycle u s e d t o c u r e t h e structure in order to meet specifications for resin c o n t e n t and structural thickness. Because t h e amount o f resin f l o w t h a t o c c u r s during t h e n o r m a l autoclave cure process decreases with increasing age of the resin, structures c u r e d f r o m aged p r e p r e g will b e resin rich a n d overly thick u n l e s s a n a d j u s t m e n t is made. Ideally t h e prepreg will b e received f r o m t h e supplier f r o z e n i n d r y i c e and w i l l be kept r e f r i g e r a t e d i n c o l d s t o r a g e (-18C) p r i o r t o i t s u s e . I n t h e optimum f a b r i c a t i o n s e q u e n c e t h e p r e p r e g w i l l b e l a y e d up o n t h e m o l d and c u r e d w i t h i n two d a y s a f t e r r e m o v a l f r o m c o l d s t o r a g e . However, i n t e r r u p t i o n s i n t h e normal c y c l e o f event s can o c c u r which w i l l cause the p r e p r e g t o be e x p o s e d t o a m b i e n t t e m p e r a t u r e s f o r some p e r i o d o f t i m e . F o r example: (1) e r r o r s c a n o c c u r i n t h e shipment o f t h e p r e p r e g - f r o m t h e s u p p l i e r a n d t h e p r e p r e g may be e x p o s e d on a l o a d i n g d o c k f o r s e v e r a l d a y s , ( 2 ) power f a i l u r e s may o c c u r i n t h e r e f r i g e r a t i o n system a t t h e f a b r i c a t o r ' s p l a n t , and (3) t h e f a b r i c a t i o n sequence may r e q u i r e more t h a n 2-days l a p s e t i m e f o r c o m p l e t i o n . I t i s t h e r e f o r e n e c e s s a r y t o make a q u a n t i t a t i v e measurement o f t h e r e s i n m a t r i x s age p r i o r t o t h e time i t i s used i n t h e f a b r i c a t i o n of a s t r u c t u r e . N o r t h r o p u n d e r t o o k t h i s p r o g r a m t o d e v e l o p a n a l y t i c a l methods f o r q u a n t a t i v e l y d e t e r m i n i n g t h e amount o f a g i n g w h i c h h a s o c c u r r e d i n p r e p r e g m a t r i x r e s i n s . Our i n t e n t i o n was t o have a p r o c e d u r e w h e r e by t h e amount o f a g i n g w h i c h h a s o c c u r r e d i n e a c h b a t c h o f p r e p r e g can be t r a c k e d from the day o f i t s manufacture t o t h e day i t i s 1

0-8412-0567-l/80/47-132-459$05.00/0 ©

1980 A m e r i c a n C h e m i c a l Society

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

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

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460

Figure 2.

GPC analysis of 3501-6 resin (Lot 009) at 254 nm

0

v

10

15

2 0

ELUTION TIME - MINUTES

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

33.

Resin Matrix

CRABTREE

Aging

461

u s e d . G e l p e r m e a t i o n c h r o m a t o g r a p h y (GPC) and d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) w e r e s e l e c t e d a s a n a l y t i c a l methods f o r t h i s w o r k .

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Materials

Analyzed

M e t h o d s f o r q u a n t i t a t i v e l y m e a s u r i n g how much a g i n g o c c u r s d u r i n g a m b i e n t e x p o s u r e o f two e p o x y r e s i n f o r m u l a t i o n s i m p o r t a n t to the aerospace i n d u s t r y were developed. These m a t e r i a l s were: (1) 3501-6 ( m a n u f a c t u r e d by H e r c u l e s ) , and (2) 5208 ( m a n u f a c t u r e d by N a r m c o ) . B o t h o f t h e s e r e s i n s a r e w i d e l y u s e d f o r t h e manuf a c t u r e of g r a p h i t e f i b e r composites. R e s u l t s and

Discussion

GPC and DSC methods h a v e p r o v e d v e r y u s e f u l i n m e a s u r i n g changes t h a t o c c u r i n r e s i n m a t r i c e s due t o a m b i e n t e x p o s u r e . I n GPC a n a l y s i s t h e i n c r e a s e i n h i g h e r m o l e c u l a r w e i g h t comp o n e n t s as w e l l a s t h e d e c r e a s e i n t h e c u r i n g a g e n t c o n t e n t c a n be m o n i t o r e d . DSC a n a l y s i s c a n a l s o be u s e d f o r some r e s i n s . DSC q u a n t i t a t i v e l y m e a s u r e s t h e e x o t h e r m i c h e a t o f r e a c t i o n t h a t i s e v o l v e d by a r e s i n d u r i n g c u r e . Due t o a g i n g , c h a n g e s o c c u r i n t h e amount o f h e a t e v o l v e d . DSC was f o u n d u s e f u l i n m o n i t o r i n g a g i n g o f 3501-6 r e s i n . A. 3501-6 Epoxy R e s i n . The GPC a n a l y s e s o f unaged 3501-6 ( l o t 009) u s i n g t h e u l t r a v i o l e t d e t e c t o r a t 313 and 254 nanom e t e r s a r e shown i n F i g u r e s 1 and 2. The p e a k s a r e l a b e l e d and identified. T h e s e measurements can be made q u a n t i t a t i v e b y : (1) t h e p e r c e n t a g e s o f c u r i n g a g e n t a n d / o r h i g h m o l e c u l a r w e i g h t r e a c t i o n p r o d u c t s c a n be d e t e r m i n e d u s i n g peak h e i g h t s a n d / o r peak a r e a s ; and (2) t h e changes i n t h e r a t i o s o f s e l e c t e d p e a k s can be u s e d . T h e s e r a t i o s a r e u s e f u l b e c a u s e a s t h e r e s i n a g e s there i s e i t h e r a p r o g r e s s i v e i n c r e a s e or decrease i n the r a t i o s . R a t i o s t h a t a r e u s e f u l and t h e e f f e c t s a g i n g has upon them a r e summarized i n T a b l e I . TABLE I EFFECT OF AMBIENT AGING UPON GPC PEAK RATIOS FOR 3501-6 AND 5208 EPOXY RESINS

Peak R a t i o 2/3 2/4 4/5 3/4 1/3

3501-6 R e s i n Wavelength NM 313

254

Increase Increase

-

Decrease Increase

NM

Increase Increase Decrease Increase Increase

5208 R e s i n Wavelength 313 NM

254

Increase Increase

-

Decrease Increase

NM

Increase Increase

-

Decrease Increase

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

462

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

o/6

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c

g

0 6

Figure 3. GPC peak ratios at 313 nm of 3501-6 resin (Lot 009) after ambient ex­ posure: (O) 2/3 peak ratio, ( Q ) 2/4 peak ratio

DAYS AT ROOM TEMPERATURE

1 4

Figure 4. GPC peak ratios at 313 nm of 3501-6 resin (Lot 039) after ambient exposure: ( Q ) 2/3 peak ratio, (O) 2/4 peak ratio

• Φ L c ο D

DAYS AT ROOM TEMPERATURE

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

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33.

CRABTREE

Resin

Matrix

463

Aging

We h a v e f o u n d t h e u s e o f peak r a t i o s a n e f f e c t i v e method o f m o n i t o r i n g changes i n r e s i n advancement. I n F i g u r e 3 graphs o f the c h a n g e s t h a t o c c u r r e d i n t h e r a t i o o f p e a k s 2/3 a n d 2/4 a t 313 n a n o m e t e r s a r e shown. P r o g r e s s i v e i n c r e a s e s o c c u r r e d f o r b o t h t h e s e peak r a t i o s . I n F i g u r e 4 t h e peak r a t i o s f o r 3501-6 l o t 039 are shown. T h i s l o t o f r e s i n was i n a much more a d v a n c e d s t a t e when a n a l y z e d i n i t i a l l y . Note t h a t t h e i n i t i a l v a l u e s o f t h e peak r a t i o s a r e much h i g h e r a n d t h e y c o n t i n u e t o i n c r e a s e . A n e r r o r o c c u r r e d i n t h e s h i p m e n t o f b a t c h 039. I t was l o s t somewhere i n t r a n s i t and a r r i v e d a t the p l a n t i n an advanced s t a t e o f a g i n g . The peak r a t i o s f r o m t h e a n a l y s i s a t 254 n a n o m e t e r s f o r b o t h b a t c h e s 009 a n d 039 a r e shown i n F i g u r e s 5 a n d 6. H e r e a g a i n i t i s p o s s i b l e b o t h t o d e t e c t a n o v e r a g e c o n d i t i o n i n b a t c h 039 a n d t o d e t e c t changes t h a t o c c u r d u r i n g a m b i e n t t e m p e r a t u r e a g i n g . The DSC a n a l y s i s ( F i g u r e 7) shows t h r e e e x o t h e r m p e a k s w h i c h o c c u r a t a p p r o x i m a t e l y 135C, 170C, a n d 230C. P e a k s 1 a n d 2, w h i c h are n o t f u l l y r e s o l v e d , appear t o be g e n e r a t e d by t h e r e a c t i o n o f an a c c e l e r a t o r i n t h e r e s i n . The t h i r d peak i s g e n e r a t e d b y t h e r e a c t i o n o f the c u r i n g agent. During aging the h e i g h t s o f peaks 1 and 2 d e c r e a s e w i t h t i m e . Peak 3 i s n o t s e n s i t i v e t o a g i n g . The r a t i o o f p e a k s 2/3 d e c r e a s e s a s t h e a g e o f t h e r e s i n i n ­ c r e a s e s . F i g u r e 8 shows t h e c h a n g e s i n t h i s peak r a t i o a s l o t s 009 a n d 039 a r e a g e d a t a m b i e n t t e m p e r a t u r e . I t i s r e a d i l y a p p a r e n t t h a t l o t 039 i s more a d v a n c e d t h a n l o t 0 0 9 . B. 5208 Epoxy R e s i n . The GPC a n a l y s e s o f unaged 5208 made u s i n g t h e u l t r a v i o l e t d e t e c t o r a t 313 a n d 254 n a n o m e t e r s a r e shown i n F i g u r e s 9 a n d 10. The p e a k s a r e l a b e l e d a n d i d e n t i f i e d . Changes t h a t o c c u r r e d i n s e l e c t e d peak r a t i o s a t 313 a n d 254 n a n o ­ m e t e r s ( s e e T a b l e I ) a r e shown i n F i g u r e s 11 a n d 12. These p e a k r a t i o s d e l i n e a t e c h a n g e s i n t h e a g e o f t h e r e s i n m a t r i x . DSC measurements w e r e n o t u s e d f o r 5208 b e c a u s e t h e r e s i n o n l y shows a s i n g l e e x o t h e r m peak. T h e r e i s no a c c e l e r a t o r peak t o m o n i t o r for aging a n a l y s i s .

E x p e r i m e n t a l Equipment and C o n d i t i o n s A. GPC A n a l y s i s . GPC a n a l y s e s w e r e made u s i n g a W a t e r s A s s o c i a t e s L i q u i d C h r o m a t o g r a p h y M o d e l ALC/GPC-244. F i v e M i c r o s t r a g e l columns were used i n s e r i e s . The p o r e s i z e s o f t h e ο

ο

ο

columns w e r e 100A (2 c o l u m n s ) , 500A (2 c o l u m n s ) , a n d 1,000A (1 c o l u m n ) . U l t r a v i o l e t g r a d e t e t r a h y d r o f u r a n s o l v e n t was u s e d . The i n s t r u m e n t was o p e r a t e d a t room t e m p e r a t u r e w i t h a f l o w r a t e of 1 m l / m i n . C o n c e n t r a t i o n s o f t h e s a m p l e s u s e d f o r i n j e c t i o n s were t y p i c a l l y i n t h e r a n g e 2 t o 3 mg/ml. An u l t r a v i o l e t d e t e c t o r o p e r a t e d a t 254 a n d 313 n a n o m e t e r s was u s e d f o r a l l a n a l y s e s . B. DSC A n a l y s i s . DSC a n a l y s e s w e r e made o n a Du P o n t M o d e l 990 T h e r m a l A n a l y z e r u s i n g t h e M o d e l 910 DSC c e l l . A n a l y s e s were

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

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464

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

Figure 5. GPC peak ratios at 254 nm of 8501-6 resin (Lot 009) after ambient exposure: ( Q ) 3/4 peak ratio, (0) 2/3 peak ratio, (O) 1/3 peak ratio

Figure 6. GPC peak ratios at 254 nm of 3501-6 resin (Lot 039) after ambient exposure: ( Q ) 3/4 peak ratio, (0) 2/3 peak ratio, (O) 1/3 peak ratio

DAYS AT ROOM TEMPERATURE

5

10

DAYS AT ROOM TEMPERATURE

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

15

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CRABTREE

Resin Matrix

465

Aging

0.5

Figure 8. DSC peak ratios of 3501-6 resin (Lots 009 and 039) after ambient DAYS AT ROOM TEMPERATURE

exposure: (O)

Lot 009, (O)

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

Lot

039

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

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466

Figure 9.

GPC analysis of 5208 resin at 313 nm

ELUTION TIME - MINUTES

EPC XV ΈΑΚ 4 - ^

CURING AGENT PE/ Κ 3 χ

RE/ CTION PRODUCTS PEAK 2 ^

Figure 10.

GPC analysis of 5208 resin at 254 nm

1

Ar-f0 15 2 ELUTION TIME - MINUTES

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

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33.

CRABTBEE

Resin

Matrix

467

Aging

ο*ρ

η *

5

10

DAYS AT ROOM TEMPERATURE

15

Figure 12. GPC peak ratios at 254 nm of 5208 resin after ambient exposure: (O) 2/3 peak ratio, (O) 1/3 peak ratio

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

468

RESINS

FOR

AEROSPACE

made a t a h e a t up r a t e o f 10C/min. An a t m o s p h e r e o f f l o w i n g n i t r o g e n gas was m a i n t a i n e d i n t h e c e l l d u r i n g t h e a n a l y s e s . Sample s i z e s w e r e i n t h e r a n g e 1 0 t o 3 0 m i l l i g r a m . A n a l y s e s w e r e made w i t h i n s t r u m e n t s e n s i t i v i t i e s i n t h e r a n g e 0 . 5 t o 2 . 0 m e a l / second.

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Conclusions GPC and DSC h a v e b e e n u s e d t o m o n i t o r a g i n g t h a t o c c u r s i n matrix resins during storage. I t i s therefore possible to track w h a t e v e r changes o c c u r d u r i n g t h e l i f e t i m e o f a p r e p r e g . The f a b r i c a t o r c a n d e t e r m i n e how much a g i n g h a s o c c u r r e d i n a m a t e r i a l b e f o r e u s i n g i t , a n d i f s i g n i f i c a n t a g i n g h a s o c c u r r e d , h e may a l t e r t h e cure c y c l e as r e q u i r e d t o ensure cure o f a s t r u c t u r e w h i c h meets s p e c i f i c a t i o n s . RECEIVED

February

28,

1980.

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