29 Experimental Analysis of Hydrothermal Aging in Fiber-
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Reinforced Composites D.
H.
KAELBLE
R o c k w e l l International Science C e n t e r , T h o u s a n d O a k s , CA 91360
There r e c e n t l y has been an i n c r e a s i n g i n t e r e s t and research e f f o r t i n c h a r a c t e r i z i n g the i n f l u e n c e of moisture absorption on the physical response of polymer matrix composite m a t e r i a l s . Three recent workshops have been e x c l u s i v e l y devoted to t h i s s u b j e c t ( 1 - 3 ) . E a r l i e r s t u d i e s i n d i c a t e d t h a t moisture degrades both the interfacial bond strength and modifies the matrix bulk response by i n t e r n a l plasticization ( 4 , 5 ) . Recent s t u d i e s show that the a p p l i c a t i o n of polyurethane p r o t e c t i v e coatings on graphite fiber i n epoxy matrix (Thornell 300/NARMCO 5208) composite delayed but d i d not prevent the moisture degradation o f composite strength (6). Q u a n t i t a t i v e chemical a n a l y s i s programs f o r characterizing the composite prepreg for q u a l i t y assurance have now been developed ( 7 - 9 ) . These systematic approaches t o chemical separat i o n and a n a l y s i s of polymer composites and adhesives provide an important new t o o l f o r a n a l y z i n g the physiochemical mechanisms o f enviromental degradation. The o b j e c t i v e o f the present study is t o implement an i n t e grated chemical, physical and mechanical t e s t program to analyze the environmental d u r a b i l i t y o f a standard 177°C (350°F) s e r v i c e c e i l i n g graphite/epoxy composite based on T300 g r a p h i t e f i b e r (Union Carbide Corp.) and 5208 epoxy matrix (NARMCO D i v i s i o n , Celanese Corp). In order to more c l e a r l y h i g h l i g h t the i n f l u e n c e of polymer matrix chemistry, comparison data f o r two other c l o s e l y r e l a t e d graphite/epoxy composites are included as reference s y s tems. A u n i a x i a l r e i n f o r c e d panel o f g r a p h i t e T300 i n 5208 epoxy prepreg (NARMCO Lot 823 R o l l 3) was layed up by using 48 p l i e s o f 0.0132 cm t h i c k and 30 cm wide prepreg to produce a f l a t panel o f dimensions by 100 cm length by 0.63 cm t h i c k n e s s . D e t a i l s of the standard f a b r i c a t i o n and cure c y c l e are summarized i n Table I. The cure c y c l e described i n Table I i s standard f o r t h i s composite. The o u t l i n e o f experimental methods a p p l i e d f o r physiochemical c h a r a c t e r i z a t i o n and e v a l u a t i o n of environmental 0-8412-0567-l/80/47-132-395$05.75/0 © 1980 American Chemical Society May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Table I F a b r i c a t i o n and Curing Cycles f o r T300 Graphite F i b e r i n 5208 Epoxy Matrix Composite (Vol une F r a c t i o n Fiber V = 0.60) f
Step No*
Procedure
1
Lay up 48 l i n e a r l y a l i g n e d p i i e s graphite epoxy prepreg between 6 bleeder p i i e s of 120 glass c l o t h and 6 p i i e s o f 181 glass and vacuum bag.
2
Vacuun on part with heating r a t e of 1.2 t o 3.3°C/min from 22 t o 135°C.
3
Hoid part under vacuum f o r 60 mi η at 135°C a t ambient external pressure.
4
Raise external pressure t o 6.0 Kg/cm and r e l e a s e part vacuum at 2.0 Kg/cm external pressure. Raise temperature to 180°C a t heating r a t e o f 1. 2 t o 3.3°C per min. and hold 120 mi η at 180°C.
2
2
5
2
6
Cool from 180°C t o 65°C under 6.0 Kg/cm pressure with thermal gradient i n part o f l e s s than 25°C.
7
Remove vacuum bag and r e t u r n unconstrained oven post cure at 180°C f o r 6 h r .
part f o r
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Hydrothermal Aging
d u r a b i l i t y are summarized i n Table I I . D e t a i l s o f the t e s t methods are o u t l i n e d i n e a r l i e r r e p o r t s (9-11). Results and Discussion In order t o focus t h e d i s c u s s i o n , the r e s u l t s o f t h i s study are subgrouped under s p e c i f i c c a t e g o r i e s . Chemical Network Defects Continued development and a p p l i c a t i o n o f the physiochemical a n a l y s i s system o u t l i n e d i n the upper p o r t i o n o f Table I I provide f u r t h e r i n s i g h t i n t o the chemical network defects which 1 i m i t t h e environmental d u r a b i l i t y o f present 177°C (350°F) s e r v i c e c e i l i n g graphite-epoxy composites. This system o f a n a l y s i s i d e n t i f i e s the chemical network s t r u c t u r e s and c u r i n g r e a c t i o n s o f F i g . 1 as being common t o the graphite-epoxy composites studied i n t h i s program (9-11). The o r i g i n a l c h a r a c t e r i z a t i o n program o f Table I I has p r e v i ously been applied to u n i a x i a l Type AS graphite f i b e r i n 3501-5 epoxy matrix and t o Union Carbide T300 graphite f i b e r i n f i b e r i t e 934 epoxy matrix. The T300 i n f i b e r i t e 934 epoxy matrix composite forms the primary s k i n s t r u c t u r e o f the cargo bay doors o f the Space Shuttle O r b i t e r and thus i s exposed t o the environmental regions o f ambient t e r r e s t i a l exposure, space f l i g h t , and r e - e n t r y to earth atmosphere (12). By comparing these two reference comp o s i t e material s with the T300 i n 5208 epoxy o f t h i s study, a much c l e a r e r p i c t u r e o f the r o l e o f chemical s t r u c t u r e defects on e n v i ronmental d u r a b i l i t y i s e s t a b l i s h e d . The lower p o r t i o n of F i g . 1 shows two c r o s s l i n k i n g r e a c t i o n s and chemical c r o s s l i n k s t r u c t u r e s . Current a n a l y s i s shows t h a t the epoxy can homopolymerize ( c r o s s l i n k r e a c t i o n 1) to form t h e c r o s s l i n k shown i n the lower l e f t o f F i g . 1 ( 1 3 , 14). Conversely, the c o - r e a c t i o n o f epoxide with amine ( c r o s s l i n k r e a c t i o n 2) produces the c r o s s l i n k shown i n the lower r i g h t view o f F i g . 1 ( 1 3 , 14). The data of Table I I I show that the matrix glass t r a n s i t i o n temperature Tg i s lowered and e q u i l i b r i u m moisture uptake i s i n creased by chemical compositions that favor r e a c t i o n 1 over r e a c t i o n 2 i n F i g . 1. The use o f the B F c a t a l y s t i s known t o favor r e a c t i o n 1 and to lower the temperature range f o r c u r i n g f o r easy p r o c e s s a b i l i t y , as shown i n the DSC ( d i f f e r e n t i a l scanning c a l o r i m e t e r ) thermograms of F i g . 2. In the curves o f F i g . 2, the r a t e o f chemical c u r i n g correl a t e s d i r e c t l y with the amplitude dH/dt o f heat r e l e a s e r a t e . Thermal p o l y m e r i z a t i o n o f pure DGMDA epoxy r e q u i r e s very high temperature. A d d i t i o n o f 2% by weight c a t a l y s t i s shown to i n i t i a t e c u r i n g at r e l a t i v e l y low temperature. The c o - r e a c t i o n o f DGMDA epoxy with DDS c i r a t i v e i s shown to i n i t i a t e and complete during at intermediate temperatures. The curves o f F i g . 2 and data o f Table I I I show that the network dominantly formed from 3
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Table I I Outline f o r Composite D u r a b i l i t y C h a r a c t e r i z a t i o n Part 1:
A n a l y s i s o f Separated F i b e r and M a t r i x l a . Obtain and separate uncured prepreg components l b . Analyze f i b e r and matrix s u r f a c e energies lc. Analyze r e s i n chemistry and c u r i n g mechani sm Id. Define c u r i n g k i n e t i c s and network s t r u c t u r e l e . Analyze hydrothermal aging e f f e c t s on network structure
Part 2:
A n a l y s i s o f Composite Laminate Aging 2a. Obtain composite laminates f o r aging s t u d i e s 2b. Measure k i n e t i c s o f water d i f f u s i o n i n t o composite 2c. Determine i n t e r l a m i n a r shear s t r e n g t h versus moisture content. Outline f o r Composite D u r a b i l i t y C h a r a c t e r i z a t i o n 2d. 2e.
Part 3:
Determine f r a c t u r e energy versus moisture content Measure dynamic mechanical (NDT) response versus moisture content
Data A n a l y s i s and NDT Methodology 3a. Determine r e l a t i o n between s t r e n g t h degradation mechani sms and NDT methodology 3b. Design NDT experiments and s t a t i s t i c a l a n a l y s i s f o r t r a c k i n g strength degradation 3c. Define improved matrix and i n t e r f a c e c h e m i s t r i e s
Table I I I E f f e c t s o f Chemical Composition on Tg(dry) and Moisture Uptake of Cured Resin (cured 5 hr at 188°C)
Composition (wt%) 71 29
Epoxy (TGMDA) C u r a t i v e (DDS)
70 28 2 98 2 100
Epoxy (TGMDA) C u r a t i v e (DDS) BF Catalyst Epoxy (TGMDA) BF Catalyst Epoxy
Tg(°C)
Equil ibrium Moisture Uptake at 100°C Immersion (wt%)
249
5.5 - 6.0
228
5.5 - 6.0
3
202
3
198
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Figure 1.
C H 2 ~ CH ~ CHp
1
Ν
0 " C
H
H
2 N
S
Ό " Ό
0 N H
2
= 251.5 GM/MOLE
0
V
- CH2
0
2 - © N \ CHo - CH - CHo
C H 2 - CH
(350°F)
L
J
j 0
0
N - Q - S - Q - N
4
X
CHo - CH - CHo I OH
C H ? - CH - CHo -
OH
H2N-©4-S
service temperature epoxy matrix
OH
I
OH I - CH - C H 2
\/
- CHo - CH - CH?
- CH2
1
CHo - CH - C H ;
C R O S S L I N K REACTION 2:(62.7% BY WEIGHT Ε + 37.3% BY WEIGHT C)
Suggested curing mechanisms for 177°C resins
CH.
CLL
- 0 - CH - C H 2 - Ο
I
2
Ι
• CH - C H
+ C H 2 - CH - C H 2
C R O S S L I N K REACTION 1: ( 100/, BY WEIGHT Ε )
0
\/
/ CHo - CH - CHo
C H 2 - CH - C H 2
0
TETRAGLYC IDYL METHYLENE DI A N I LI NE (TGMDA); M.W. - 422 gm/MOLE
CURATIVE (C): DI AMI NOD IPHENYLSULFONE (DDS); M . W .
EPOXY (Ε):
RESINS F O R A E R O S P A C E
Figur 2. DSC thermograms for reactions of known compositions: (ΦΧ pure TGMDA epoxy, (O) 71 wt % TGMDA epoxy + 219 wt % DDS curative, (A) epoxy + 2wt% BF3 catalyst
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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cure r e a c t i o n 1 (see F i g . 1) can be expected t o d i s p l a y lower environmental d u r a b i l i t y than that formed from r e a c t i o n 2. Chemical Defects and Environmental Response The DSC thermograms f o r the commercial epoxy matrix material s are shown i n F i g . 3. The low temperature i n i t i a t i o n o f c u r i n g i n F i b e r i t e 934 and Hercules 3501-5 r e s i n s c o r r e l a t e s w i t h the BFo c a t a l y s t - a s s i s t e d r e a c t i o n 1, shown i n Figs. 1 and 2. NARMCO 5208 shows a c u r i n g k i n e t i c s i n F i g . 3 which c l o s e l y c o r r e l a t e s w i t h r e a c t i o n 2 of Figs. 1 and 2. The thermograms of F i g . 3 thus show that F i b e r i t e 934 and Hercules 3501-5 r e s i n s are more r e a d i l y cured at low temperatures than i s NARMCO 5208. The c a t a l y z e d r e a c t i o n o f epoxy groups i n F i b e r i t e 934 and Hercules 3501-5 i s shown by the curves of F i g . 4 to produce onset o f the g l a s s t r a n s i t i o n damping peak a t lower temperature than i n amine c r o s s l i n k e d NARMCO 5208 composite. The e f f e c t o f exposure o f these cured comp o s i t e s t o moisture aging, f o l l o w e d by thermal scanning o f damping p r o p e r t i e s , i s shown i n F i g . 5. F i b e r i t e 934 and Hercules 3501-5 r e s i n s soften and d i s p l a y onset o f high damping at 150°C due t o higher moisture s e n s i t i v i t y o f t h e i r c r o s s l i n k networks as compared t o NARMCO 5208 which r e t a i n s low damping sol i d s t a t e response t o 200°C. The curves o f F i g s . 3-5 f o r commercial epoxy r e s i n s r e f l e c t the r e l a t i o n between chemical network composition, p r o c e s s a b i l i t y , and environmental d u r a b i l i t y , which are c o n s i s t e n t with the two network r e a c t i o n s of F i g . 1 and the thermograms o f F i g . 2 f o r known r e a c t i o n mixtures. Methods of Chemical
Characterization
A knowledge of prepreg chemical p r o p e r t i e s i s now e s t a b l i s h e d as important i n f o r m a t i o n f o r q u a l i t y assurance and manufacturing process o p t i m i z a t i o n . The summary f o r chemical c h a r a c t e r i z a t i o n o f graphite-epoxy pregreg material s i s shown i n Table IV. The methodologies used are based on instrumental techniques and y i e l d q u a n t i t a t i v e i n f o r m a t i o n concerning the physiochemical p r o p e r t i e s of the pregreg material (7_-9). For example, the higher l e v e l o f f r e e DDS c u r a t i v e i n NARMCO 5208 c o r r e l a t e s with the lower degree of cure and higher heat o f p o l y m e r i z a t i o n , as shown i n the data summary o f Table IV. The data o f Table IV shows the importance o f d i f f e r e n t i a t i o n between t o t a l DDS c u r a t i v e , as measured by IR spectroscopy, and f r e e amine, as measured by q u a n t i t a t i v e molec u l a r separation using l i q u i d chromatography. The numeric i n f o r mation r e l a t i n g the chemical composition o f the prepreg systems summarized i n Table IV forms part of the m a t e r i a l s and processes approach t o chemical d e f e c t s d e f i n i t i o n o u t l i n e d i n e a r l i e r
reports
{J^9).
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Figure 3. DSC thermograms for curing reactions of commercial epoxy matrix materials extracted from prepreg (DSC scan rate φ = 20° C/min): ( Q ) Fiberite 934, (O) Hercules 3501-5, (A) Narmco 5208
0
50
100
150
200
250
300
350
Temperature ( ° C ) Figure 4. Rheovibion thermal scans for flexural damping in cured uniaxial rein forced graphite—epoxy composite in the dry unaged condition: (O) Fiberite 934/T300, (Π) Hercules 3501-5/AS, (A) Narmco 5208/T300
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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403
Hydrothermal Aging
50
TOO
150
200
250
300
350
Temperature ( ° C ) Figure 5. Rheovibion thermal scans of flexural damping in cured uniaxial rein forced graphite-epoxy composite in the wet-aged condition: (O)Fiberite 934/ T300, (Π) Hercules 3501-5/AS, (A) Narmco 5208/T300
Table IV Chemical C h a r a c t e r i z a t i o n o f Graphite-Epoxy Prepreg M a t e r i a l s Reference Systems
This Study
Epoxy M a t r i x
Hercules 3501-5
Fiberite 934
NARMCO 5208
Graphite Fiber
Hercules Type AS
U. Carbide T300
U. Carbide T300
% Total DDS C u r a t i v e by IR Spectroscopy
29.2
27.8
22.1
% Free DDS C u r a t i v e by L i q u i d Chromatography
18.1
14.5
17.8
Epoxide Equivalent
205
227
173
Wt. % B F Type Boron
0.047
0.022
.0005
R e l a t i v e Degree o f Cure by L i q u i d Chromatography
22
27
6.9
107
107
140
3
Heat o f P o l y m e r i z a t i o n by DSC ( C a l / g polymer)
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Table V Environmental D u r a b i l i t y C h a r a c t e r i z a t i o n of Major 177°C (350°F) S e r v i c e Temperature Graphite Epoxy Composites (Standard Cure, U n i a x i a l ) T h i s Study
Reference System 1.
General
Epoxy M a t r i x
Hercul es 3501-5
Fiberite 934
NARMCO 5208
Graphite Fiber
Hercules Type AS
U. Carbide T300
U. Carbide T300
Fiber Finish
None
1% by F i b e r wt bisphenol-A epoxy
1% by F i b e r wt bisphenol-A epoxy
Post Cure Cycle
3 hr at
2 hr at
6 hr a t
188°C
177°C
180°C
0.60
0.60
0.60
Fiber Vol ume Fraction
Surface Energies (dyn/cm) Uncured M a t r i x 30.9 ± 2.1 Yd
Cured matrix
d
Yd
28.8 ± 2 . 0 12.4 ± 2 . 1
28.0 14.5
28.5 13.2
± 2.1 ± 2.3
26.7 ± 3 . 0 29.9 ± 5 . 8
24.4 ± 2.6 23.4 ± 3.8
± 2.2
75°C M a t r i x Moisture Absorption Max. Ho0 Uptake fwt%) 6.50 Diffusion Coefficient D (10 cm /sec) R
4.
± 2.3
Y
YP
3.
29.2 13.4
10.1
γρ
Fi ber
29.9 ± 2.4 14.4 ± 2 . 9
γΡ
_ a
z
2.08
± 2.2 ± 2.4
+ 2.8
24.9 ± 2.6 25.2 ± 3.8
6.39
5.84
.89
1.45
Dynamic Mechanical Damping (110 Hz, F l e x u r e , Unaged) Alpha T r a n s i t i o n (°C) 252.0 258TÔ 282.0 tan 6 , 110 Hz m a x
Range (V2tan
