Quantitative Analysis of Resin Matrix Aging by Gel Permeation

33 Quantitative Analysis of Resin Matrix Aging by Gel Permeation Chromatography and Differential Scanning Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 23, 2018 | https://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch033

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

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 23, 2018 | https://pubs.acs.org Publication Date: August 28, 1980 | doi: 10.1021/bk-1980-0132.ch033

460

Figure 2.

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

0

v

10

15

2 0

ELUTION TIME - MINUTES


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 .


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


462


o/6


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




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



464


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


5

10



15


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)


Lot

039



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



33.

CRABTBEE

Resin

Matrix

467

Aging

ο*ρ

η *

5

10


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


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.


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.


Quantitative Analysis of Resin Matrix Aging by Gel Permeation

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