Fillers and Reinforcements for Plastics

14 The Effects of Moisture on the Properties of High Performance Structural Resins and

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Composites C. E .

BROWNING

and

J.

M.

WHITNEY

A i r Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio 45433

Graphite- and boron-fiber reinforced composites, as well as castings of current resin systems, were evaluated to deter mine the effects of moisture and/or high humidity on their physical properties and their room and elevated temperature mechanical properties. All of the neat resin castings ab sorbed moisture and swelled and showed a loss in elevated temperature tensile strength. All composite systems showed weight and thickness increases when subjected to high humidity. However, the effect of absorbed moisture on the elevated temperature mechanical properties is determined principally by the lay-up of the laminate and/or the test being applied. Thus, fiber controlled composite properties are relatively unaffected by absorbed moisture whereas matrix controlled properties are adversely affected. For both castings and composites the effects of moisture were reversible.

TTigh A

A

performance

s t r u c t u r a l composites

h a v e gone t h r o u g h several

stages o f d e v e l o p m e n t t o w h e r e t h e y a r e b e i n g r e a d i e d f o r u s e i n

actual A i r Force hardware.

A t this stage t h e i r p e r f o r m a n c e d u r i n g ex-

posures to s i m u l a t e d aircraft enviroments m u s t b e e v a l u a t e d .

Important

e n v i r o n m e n t a l factors i n c l u d e m o i s t u r e ( h i g h h u m i d i t y ) a n d extremes i n t e m p e r a t u r e . A m a j o r c o n c e r n is r e t e n t i o n of h i g h t e m p e r a t u r e c o m p o s i t e properties after exposure to h i g h h u m i d i t y . T h i s p r o g r a m w a s u n d e r t a k e n to d e t e r m i n e t h e effects of h i g h h u m i d i t y o n t h e m e c h a n i c a l a n d p h y s i c a l properties of h i g h composites.

G r a p h i t e - a n d boron-fiber r e i n f o r c e d composites

performance as w e l l as

137 Deanin and Schott; Fillers and Reinforcements for Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

138

FILLERS

AND

REINFORCEMENTS FOR

PLASTICS

t h e i r m o r e i m p o r t a n t associated cast r e s i n systems w e r e e v a l u a t e d . F l e x u r a l a n d tensile properties w e r e m e a s u r e d as a f u n c t i o n of m o i s t u r e , t e m p e r a t u r e , t i m e of exposure

to m o i s t u r e , a n d n u m b e r of

exposure

cycles. Q u a s i - i s o t r o p i c l a m i n a t e s w e r e s t u d i e d because the c r i t i c a l d e s i g n p r o p e r t i e s are the i n - p l a n e p r o p e r t i e s of

m u l t i d i r e c t i o n a l composites.

U n i d i r e c t i o n a l p r o p e r t i e s w e r e also o b t a i n e d because c e r t a i n ones flex)

Table I. Cast Downloaded by EAST CAROLINA UNIV on November 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0134.ch014

(e.g.,

are v e r y sensitive to m a t r i x properties. Materials Systems Evaluated Resins

N a r m c o 2387 Erl-2256 Erla-4617 E p o n 828 X-2546

Table II. Fiber AS HT-S HM-S Boron

Modulus (10* psi) 32.0 38.0 50.0 60.0

Composites B o r o n 5505 ( b o r o n / 2387) HT-S/Erla-4617 HT-S/Adx-516 HT-S/P13N A-S/X-2546 HT-S/X-2546 HM-S/X-2546 HT-S/X-911 HT-S/HT-epoxide A-S/Erla-4617 Fiber Properties Strength (10 psi) s

400 380 320 500

Density (grams/cc) 1.79 1.75 1.90 2.63

Experimental T h e cast resins a n d r e i n f o r c e d composites e v a l u a t e d are s h o w n i n T a b l e I. B o r o n - r e i n f o r c e d composites w e r e f a b r i c a t e d f r o m A v c o 5505 p r e p e g tape. G r a p h i t e fiber-reinforced composites u t i l i z e d H e r c u l e s t y p e A - S , H T - S , or H M - S g r a p h i t e fibers. T h e properties of these r e i n f o r c e ments are g i v e n i n T a b l e I I . E r l a - 4 6 1 7 , E r l - 2 2 5 6 , a n d E p o n 828 w e r e e v a l u a t e d because of t h e i r extensive use as l a m i n a t i n g e p o x y r e s i n systems, p a r t i c u l a r l y w i t h g r a p h i t e fibers. X - 2 5 4 6 is a n e w l y d e v e l o p e d , m o d i f i e d e p o x y r e s i n f r o m U n i o n C a r b i d e h a v i n g a h i g h heat d i s t o r t i o n t e m p e r a t u r e ( 4 8 5 ° F ) a n d c o n c u r r e n t usefulness i n composites at 3 5 0 ° F . A v c o 2387 w a s e v a l u a t e d because i t is the r e s i n system u s e d i n A v c o 5505 b o r o n / e p o x y p r e p r e g t a p e ( t h e m o s t w i d e l y u s e d m a t e r i a l s system i n advanced composite developmental programs). C a s t r e s i n a n d c o m p o s i t e test specimens w e r e s u b j e c t e d to t h e stepw i s e e n v i r o n m e n t a l exposure c y c l e s h o w n i n T a b l e I I I . T h e c y c l i c e x p o sure is representative of t h e severe e n v i r o n m e n t s a n a i r c r a f t c a n see d u r i n g service. D a t a w e r e o b t a i n e d at r o o m t e m p e r a t u r e , 350° F after a 1-hr soak,

Deanin and Schott; Fillers and Reinforcements for Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

14.

Moisture

BROWNING A N D WHITNEY

Table III.

139

Effects

Stepwise Environmental Exposure Cycle

Step 1. R e l a t i v e h u m i d i t y = = 2 2 J ^ hrs.

95-100%.

Temperature

=

120°F.

Time

Step 2. Specimens are r e m o v e d f r o m c h a m b e r a n d p l a c e d u n d e r n o r m a l r o o m c o n d i t i o n s for 15 m i n . Step 3. T e m p e r a t u r e = - 6 5 ° F . T i m e = 1 h r . S t e p 4. S t e p 2 repeated. S t e p 5. T e m p e r a t u r e = 250°F. T i m e = Y h r . Downloaded by EAST CAROLINA UNIV on November 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0134.ch014

2

S t e p 6. S t e p 2 repeated.

a n d at 3 5 0 ° F ( 1 - h r soak) after 2, 10, 15, a n d 3 0 exposure cycles. W h e n o n l y h u m i d i t y a g i n g w a s of c o n c e r n , t h e h u m i d i t y a n d t e m p e r a t u r e c o n d i t i o n s s h o w n i n step 1 w e r e u s e d f o r 24 h r s . A g i n g times g e n e r a l l y consisted of 15 or 30 days. 350° F is t h e m a x i m u m use t e m p e r a t u r e that most of these h i g h p e r f o r m a n c e composites w i l l see i n a c t u a l use a n d , therefore, is t h e test t e m p e r a t u r e of p r i m e concern. B o r o n a n d g r a p h i t e composite test specimens w e r e also subjected to w a t e r b o i l exposures of v a r y i n g d u r a t i o n . Specimens w e r e either b o i l e d u n t i l t h e i r w e i g h t p i c k - u p s w e r e e q u i v a l e n t to t h e i r w e i g h t gains r e c o r d e d d u r i n g cycling—"equivalent water boir—or u n t i l their weight pick-ups r e a c h e d a constant v a l u e — " e q u i l i b r i u m w a t e r b o i l / ' A secondary f a c t o r of interest w a s w h e t h e r o r n o t w a t e r b o i l exposures c o u l d b e u s e d as q u i c k a n d effective screening tests p r i o r to c y c l i c exposures. S p e c i m e n s w e r e tested to d e t e r m i n e i f t h e same m e c h a n i c a l properties w e r e f o u n d for t w o groups of specimens h a v i n g t h e same w e i g h t gains b u t w i t h one set h a v i n g b e e n c y c l e d to t h e p a r t i c u l a r w e i g h t g a i n a n d t h e other set h a v i n g b e e n w a t e r b o i l e d to t h e same w e i g h t g a i n . T a b l e IV shows t h e s h o r t - b e a m shear strengths at r o o m t e m p e r a t u r e a n d 350° F f o r b o t h c y c l e d a n d w a t e r b o i l e d b o r o n / e p o x y specimens. A t 350° F after 30 cycles ( w e i g h t g a i n = 0 . 8 2 % ) a n d at 3 5 0 ° F after 11 hrs of w a t e r b o i l

Table I V . Comparison of the Shear Strengths of Water-Boiled and Cycled Specimens of A v c o 5505 (Boron/Epoxy) Test Conditions

Shear Strength (10 psi) s

Cyclic Exposures RT 350°F (dry) 350°F (after 30 cycles) Water Boil Exposures RT 350°F ( d r y ) 350°F (after 11 h r s w a t e r boil)

Weight Gain (%)

% Retention of RT Strength

13.3 6.0 5.6

— — 0.82

— 45.1 42.1

14.4 6.9

— —

— 48.0

6.7

0.81

46.5


140

FILLERS

AND

REINFORCEMENTS FOR

PLASTICS

( w e i g h t g a i n = 0 . 8 1 % ) the p e r c e n t r e t e n t i o n of r o o m t e m p e r a t u r e shear strength is a p p r o x i m a t e l y t h e same. F l e x u r a l testing was d o n e o n 4.0 i n c h l o n g X 0.5 i n c h w i d e s p e c i mens u s i n g t h e t h r e e - p o i n t l o a d i n g m e t h o d w i t h a s p a n - t o - d e p t h r a t i o of 32 to 1. S h o r t - b e a m shear strengths w e r e m e a s u r e d b y the t h r e e - p o i n t l o a d i n g m e t h o d , u s i n g a s p a n - t o - d e p t h r a t i o of 4 to 1. T e s t specimens w e r e 1.00 i n c h l o n g X 0.25 i n c h w i d e . T a b b e d t e n s i l e test c o u p o n s w e r e 6.0 inches l o n g X 0.5 i n c h w i d e .


3 0

ω UJ25 Ο >ο

S

ί ^ =;π; I

1-2256

/

^Α-2387

' '

Κ ΙΟΙ-

//

/

'

/ /

i l

'

r

^Χ-2546

7 /

20

(Λ Ο CL

3 Ζ

Γ7/

4617-/

'

y ' '

5

te-T 0.5

I 1.0

I I I I I I I 1.5 2.0 2.5 3.0 3.5 4.0 45

I I L 5.0 5.5 6.0 65

7.0

PERCENT WEIGHT GAIN

Figure

Results and

1.

Effect of cyclic exposures on the gains of cast resin systems

weight

Discussion

C a s t Resins. W e i g h t gains f r o m a b s o r b e d m o i s t u r e f o r the cast resins are p l o t t e d as a f u n c t i o n of the n u m b e r of exposure cycles i n F i g u r e 1. T h e lowest w e i g h t g a i n after 30 cycles w a s a b o u t 1 % b y E p o n 828 w h i l e t h e h i g h e s t w a s a b o u t 6 . 5 % b y X - 2 5 4 6 . T h e shapes of the curves i n d i c a t e that n o n e of the systems has r e a c h e d e q u i l i b r i u m e v e n after 30 cycles. E a c h system also s h o w e d c o n c u r r e n t thickness increases, v a r y i n g f r o m 1.3% for 828 to 2 . 8 % for X - 2 5 4 6 . T h e effect of c y c l i c exposures o n t h e tensile strengths of the cast epoxies is i l l u s t r a t e d i n F i g u r e 2. T h e b a r g r a p h shows p e r c e n t r e t e n t i o n of the u n e x p o s e d r o o m t e m p e r a t u r e ( R T ) tensile strength as a f u n c t i o n of the n u m b e r of exposure cycles.

E a c h system was tested near its p r o

p o s e d use t e m p e r a t u r e — i . e . , 2256 was tested at 2 5 0 ° F w h i l e the others w e r e tested at 350 ° F .

A l l systems s h o w e d

large strength r e d u c t i o n s


14.

Moisture

BROWNING A N D W H I T N E Y

141

Effects

c a u s e d b y t e m p e r a t u r e ( 0 exposure c y c l e s ) .

A f t e r 30 exposure

cycles

n o n e of the 350° F systems s h o w e d strength retentions greater t h a n 1 5 % . E r l a - 4 6 1 7 f a r e d so p o o r l y t h a t testing w a s d i c o n t i n u e d after 15 cycles. I n a separate e x p e r i m e n t , specimens of the 4617 system w e r e

ambient

a g e d [ R T a n d 5 0 % r e l a t i v e h u m i d i t y ( R H ) ] f o r six m o n t h s , after w h i c h t h e i r 350° F s t r e n g t h d r o p p e d f r o m a n u n a g e d v a l u e of 5700 p s i to a n a g e d v a l u e of 1330 p s i ( 7 7 %

reduction).

T h e s e specimens w e r e t h e n

d r i e d at 250° F in vacuo to constant w e i g h t , r e s u l t i n g i n the r e c o v e r y of t h e i r o r i g i n a l , u n a g e d 350° F strength. Downloaded by EAST CAROLINA UNIV on November 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0134.ch014

T h i s r e v e r s i b i l i t y aspect of the m o i s t u r e d e g r a d a t i o n process is f u r t h e r i l l u s t r a t e d i n F i g u r e 3 w h e r e t h e heat d i s t o r t i o n temperatures ( H D T ) of 4617 ° C are p l o t t e d as a f u n c t i o n of h u m i d i t y a g i n g . T h e H D T is the t e m p e r a t u r e at w h i c h a deflection of 0.02 i n c h occurs u n d e r a constant l o a d . T h e s o l i d l i n e is the u n a g e d c o n t r o l . T h e l i n e furthest to the left

ERL-2256 (250°F)

100

80

Li_ Ο

UJ

2 Ο

Lu CC

I-

II I-

Lu

gΟ

X-2546 (350°F)

NARMCO 2387 (350°F)

ERLA-4617 (350°F)

-

80

40

20

ce

LU CL

0

15 30

0

LL

15 30

0 15 30

NUMBER OF EXPOSURE

Figure 2.

0

LL 15

CYCLES

Effect of cyclic exposures on the tensile strengths of cast epoxy resins

w a s r e c o r d e d after 20 days h u m i d a g i n g (step 1 c o n d i t i o n s for 24 h r s ) , s h o w i n g the H D T b e i n g l o w e r e d a p p r o x i m a t e l y 3 5 ° C ; m o r e i m p o r t a n t l y , the t e m p e r a t u r e at w h i c h t h e i n i t i a l deflection o c c u r s — a n i n d i c a t i o n of the softening p o i n t of the r e s i n a n d , i n t u r n , the softening p o i n t of the composite—was

l o w e r e d a p p r o x i m a t e l y 90 ° C .

The corresponding mois

t u r e p i c k - u p after 20 days a g i n g w a s 3 1 / 2 % .

T o illustrate the reversi

b i l i t y of the m o i s t u r e a b s o r p t i o n or p l a s t i c i z i n g process, specimens w h i c h


142

FILLERS

A N D REINFORCEMENTS F O R PLASTICS

h a d b e e n a g e d f o r 20 days w e r e d r i e d to constant w e i g h t , g i v i n g t h e d o t t e d c u r v e w h i c h is almost i d e n t i c a l t o t h e c u r v e of the o r i g i n a l , u n -


exposed specimens.

20

50

100 TEMPERATURE

Figure

3.

150

180

°C

Effect of humid aging on the heat distortion curves of Erla-4617 cast epoxy resin

temperature

Results and Discussion Composites.

T h e effect of c y c l i c exposures o n t h e tensile strengths

of q u a s i - i s o t r o p i c H T - S g r a p h i t e l a m i n a t e s is s h o w n i n F i g u r e 4.

Test

temperatures a n d r e s i n systems are i n d i c a t e d at t h e t o p of t h e

figure.

T h e s e are e i g h t - p l y l a m i n a t e s h a v i n g p l y orientations of 0 ° , + 4 5 ° ,

—45°,

9 0 ° , 9 0 ° , - 4 5 ° , + 4 5 ° , 0 ° , w i t h t h e test d i r e c t i o n p a r a l l e l to t h e outer 0 ° p l y . 4617 a n d A D X - 5 1 6 ( a n e p o x y - p o l y a r y l s u l f o n e )

composites are

so a d v e r s e l y affected b y t h e t e m p e r a t u r e t h a t p l a s t i c i z a t i o n b y m o i s t u r e c o u l d cause n o f u r t h e r s t r e n g t h r e d u c t i o n s . H T - S composites w i t h P 1 3 N (a polyimide from C i b a - G e i g y ) , X-2546, a n d X-911 ( a n epoxy-phenolic f r o m F i b e r i t e ) s h o w n e g l i g i b l e 350° F s t r e n g t h losses as a f u n c t i o n of cycling.

P 1 3 N composites

reductions.

w e r e also tested at 5 0 0 ° F w i t h n o strength

B e c a u s e o f t h e p l y orientations (fiber d i r e c t i o n )

a n d test

d i r e c t i o n , this p a r t i c u l a r p r o p e r t y ( q u a s i - i s o t r o p i c t e n s i o n ) is fiber d o m i n a t e d ( o r c o n t r o l l e d ) to s u c h a n extent t h a t s u b s t a n t i a l m o i s t u r e a b s o r p t i o n b y t h e m a t r i x does n o t cause significant strength r e d u c t i o n s .

Even

t h e 4617 system w h i c h r e t a i n e d o n l y 5 % o f its p u r e r e s i n c a s t i n g s t r e n g t h w h e n tested at 350° F composite

after 15 cycles, g a v e a q u a s i - i s o t r o p i c g r a p h i t e

t h a t r e t a i n e d 5 0 % of its tensile strength at 3 5 0 ° F after 30

cycles.


14.

BROWNING A N D W H I T N E Y

Moisture

143

Effects

T h e effects of m o i s t u r e o n t h e flexural o r b e n d i n g strengths of b o t h u n i d i r e c t i o n a l a n d q u a s i - i s o t r o p i c g r a p h i t e composites F i g u r e 5. A l l tests w e r e d o n e at 350 ° F .

are i l l u s t r a t e d i n

Here, however, a n equivalent

w a t e r b o i l (see e x p e r i m e n t a l ) w a s u s e d as a n a c c e l e r a t e d s c r e e n i n g test. F l e x o r b e n d i n g d a t a s h o w h o w t h e test m e t h o d , fiber orientations, a n d r e s i n H D T a l l influence t h e effects of a b s o r b e d m o i s t u r e . Several interesting comparisons c a n be made w i t h the data i n F i g u r e 5.

T h e quasi-isotropic tension i n F i g u r e 4 c a n b e compared w i t h the

q u a s i - i s o t r o p i c flex i n F i g u r e 5. T h e X - 2 5 4 6 a n d X - 9 1 1 composites

after


being given equivalent water boils showed substantial reductions i n their R T a n d 3 5 0 ° F b e n d i n g s t r e n g t h because of a b s o r b e d

moisture. T h e

X - 9 1 1 system w h i c h p r e v i o u s l y s h o w e d n o tensile losses, lost over 4 0 %

HT-S/4617 (350°F)

HT-S/ADX-516 HT-S/PI3N HT-S/PI3N (350°F) (350°F) (500°F)

HT-S/X-2546 HT-S/X-9II (350°F) (350°F)

100

80

>=!

60-

40

-

20

-

0 15 30

0 15 30

0 15 30

0 15 30

0

15 30

0 15 30

NUMBER OF EXPOSURE CYCLES

Figure

4.

Effect

of cyclic exposures on the tensile strengths of quasiisotropic graphite composites

of its R T flex s t r e n g t h a n d 3 0 % o f its 3 5 0 ° F strength. U n i d i r e c t i o n a l X - 2 5 4 6 d a t a s h o w h o w m o i s t u r e affects a m a t r i x c o n t r o l l e d p r o p e r t y s u c h as flex. T h e r e is a c o n t i n u a l , stepwise decrease i n t h e 350° F u n i d i r e c t i o n a l flex strength w i t h i n c r e a s i n g w a t e r b o i l exposure t i m e u n t i l , after 26 h r s , t h e strength r e t e n t i o n w a s o n l y a b o u t 5 0 % . A c o m p a r i s o n c a n also b e m a d e of fiber d o m i n a t e d vs. r e s i n d o m i n a t e d l a y - u p s . U n i d i r e c t i o n a l 4617 composites w i t h a l l o f t h e i r fibers i n t h e 0 ° d i r e c t i o n a r e n o t n e a r l y as a d v e r s e l y affected b y t h e t e m p e r a t u r e as are t h e q u a s i - i s o t r o p i c composites w h i c h h a v e o n l y t w o plies o f e i g h t o r i e n t e d at 0 ° . R e s i n H D T


144

FILLERS

AND

REINFORCEMENTS FOR

PLASTICS

c a n also be c o m p a r e d . T h e 4617 system w h i c h has a H D T of a b o u t 350° F gave a q u a s i - i s o t r o p i c c o m p o s i t e h a v i n g a 350° F less t h a n 2 0 %

s t r e n g t h r e t e n t i o n of

w h i l e X - 2 5 4 6 w h i c h has a + 4 5 0 ° F H D T g a v e q u a s i -

i s t o t r o p i c composites h a v i n g 350° F s t r e n g t h retentions greater t h a n 8 0 % . HT-S/4617 QUASI-ISOTROPIC (350°F)

HT-S/4617 UNIDIRECTIONAL (350°F)


l i l O O p

!fc

80

HT-S/X-9II QUASI-ISOTROPIC (350°F)

HT-S/X-2546 QUASI-ISOTROPIC (350°F) HT-S/2546 UNIDIRECTIONAL (350°F)

-

60

40

20

0.5 0 0.5 0 2 16 0 2 16 26 WATER BOIL EXPOSURE TIME (HOURS)

Figure

5.

Effect of moisture (water-boil strengths of graphite/epoxy

0

22

exposure) on the composites

ftexural

S t r e n g t h r e t e n t i o n i n b o t h t e n s i o n a n d flex for b o r o n / e p o x y

com-

posites is s h o w n i n F i g u r e 6. T h e t e n s i o n d a t a w e r e o b t a i n e d o n q u a s i i s o t r o p i c coupons,

a n d the flex d a t a o n u n i d i r e c t i o n a l specimens.

s h o w n , strength r e d u c t i o n s are n e g l i g i b l e at 350° F after 30

As

exposure

cycles for the q u a s i - i s o t r o p i c tension, e v e n t h o u g h the system h a d a w e i g h t p i c k - u p of a b o u t 1 % .

In

flexure,

however, a drastic strength

r e d u c t i o n of ca. 5 5 % o c c u r r e d because of a b s o r b e d m o i s t u r e ( e q u i v a l e n t water b o i l ) .

F i g u r e 7 shows t h e l o a d - d e f l e c t i o n curves f r o m this same

flex testing of the b o r o n / e p o x y c o m p a r e d w i t h t h e 350° F

specimens.

If the 350° F " d r y " c u r v e is

" w e t " c u r v e , there is a c o n s i d e r a b l e loss i n

l o a d - c a r r y i n g a b i l i t y w i t h a s i m u l t a n e o u s large increase i n deflection caused b y absorbed

moisture.

T h e test w a s a c t u a l l y s t o p p e d

deflection s h o w n w i t h o u t h a v i n g b r o k e n a n y of the b o r o n

at the filaments.

W h e n tested at 350° F d r y there is a l w a y s filament b r e a k a g e i n the t e n s i o n ( b o t t o m ) face of the c o u p o n . I n other w o r d s , m o i s t u r e a b s o r p t i o n c a u s e d



Moisture

QUASI-ISOTROPIC TENSION (350°F)

Effects

UNIDIRECTIONAL FLEX (350°F)

100 Γ -

80


60

z

ce

LU LU H CL LU 5 * LU

-

40

20

0 15 30 0 II EXPOSURE CYCLES( Nr.) WATER BOIL(Hrs.)

Figure 6. Effect of moisture on the tensile (quasi-isotropic) and flexural (unidirectional) strengths of boron/epoxy composites

250

r

-RT CONTROL 350°F

CONTROL

3 5 0 ° F AFTER II Hr. WATER BOIL (EQUIVALENT WATER BOIL )

J0.05

0.10

0.15

0.20

DEFLECTION (in.)-^

Figure

7.

Load-deflection curves for unidirectional epoxy composite test specimens

boron/


146

FILLERS

AND

REINFORCEMENTS FOR

PLASTICS

a different f a i l u r e m o d e to o c c u r ; t h e " d r y " 350° F c o n t r o l gave a s h a r p tensile f a i l u r e w i t h fiber b r e a k a g e i n the tensile f a c e of the c o u p o n ; t h e w e t ( e q u i v a l e n t w a t e r b o i l ) 3 5 0 ° F s p e c i m e n s h o w e d o n l y p l a s t i c defor m a t i o n w i t h n o filament b r e a k a g e (i.e., t h e m o i s t u r e has p l a s t i c i z e d the e p o x y m a t r i x to s u c h a n extent t h a t at 3 5 0 ° F i t cannot efficiently transfer l o a d f r o m fiber to

fiber).

QUASI-ISOTROPIC

σ

±45°

90° Ε


lOOr-

80

Q_ 60 ' CL

40-

20

-

0 30

0 30

0 30

0 30

0 30

0

30

0 30

0

30

DAYS AT l 2 0 ° F / 9 5 - I O O % R.H.

Figure 8. Effects of humid aging on the strengths (σ) and moduli (E) of A-S/Erh-4617 composites as a function of composite stacking sequence

T h e effects of m o i s t u r e as a f u n c t i o n of c o m p o s i t e s t a c k i n g sequence, l a y - u p or p l y fiber orientations are i l l u s t r a t e d i n F i g u r e 8. S h o w n is the p e r c e n t r e t e n t i o n of o r i g i n a l , u n e x p o s e d r o o m t e m p e r a t u r e tensile strength (σ)

a n d m o d u l u s (E)

b e f o r e a n d after 30 days h u m i d a g i n g

(120°F/

9 5 - 1 0 0 % R H ) . T h e c o m p o s i t e system is t y p e A - S g r a p h i t e a n d E r l a - 4 6 1 7 e p o x y r e s i n w i t h a l l t e s t i n g d o n e i n t e n s i o n at 3 0 0 ° F .

A s the g r a p h is

v i e w e d f r o m l e f t - t o - r i g h t , there are t w o sets of c o l u m n s f o r the strengths a n d m o d u l i f o r e a c h s t a c k i n g sequence shown—i.e., the test goes f r o m fiber

c o n t r o l l e d to m a t r i x c o n t r o l l e d .

U n i d i r e c t i o n a l a n d quasi-isotropic

specimens, w i t h a l a r g e p e r c e n t a g e of t h e i r fibers p a r a l l e l to t h e d i r e c t i o n of t h e test, s h o w less i l l effects f r o m t h e h u m i d i t y ( e v e n t h o u g h t h e y h a d s u b s t a n t i a l w e i g h t g a i n s ) t h a n d o the ± 4 5 ° a n d 9 0 ° composites

which

h a v e n o 0 ° p l i e s . T h e strength a n d m o d u l u s r e t e n t i o n values for b o t h t h e 0 ° a n d q u a s i - i s o t r o p i c specimens w e r e over 6 0 % after 3 0 days h u m i d


14.

Moisture


147

Effects

a g i n g w h i l e the ± 4 5 ° a n d 90° specimens s h o w e d less t h a n 1 0 % r e t e n t i o n after e q u i v a l e n t a g i n g . O n e d i s t u r b i n g r e s u l t of this s t u d y w a s that q u a s i i s o t r o p i c specimens t h a t h a d b e e n h u m i d a g e d for 30 days l a r g e cracks after setting i n a m b i e n t c o n d i t i o n s .

developed

F i g u r e 9 is a

photo-

m i c r o g r a p h ( s i d e v i e w ) of one of these c r a c k e d specimens, s h o w i n g the c r a c k r u n n i n g i n the 90° p l i e s . Conclusions


T h e effect of a b s o r b e d m o i s t u r e o n t h e e l e v a t e d t e m p e r a t u r e

me-

c h a n i c a l properties of composites is d e t e r m i n e d p r i n c i p a l l y b y the l a y - u p of the l a m i n a t e a n d / o r the test b e i n g a p p l i e d — i . e . , the m e t h o d b y w h i c h l o a d is i n t r o d u c e d i n t o the l a m i n a t e . T h i s means t h a t a g i v e n t y p e of l a m i n a t e u n d e r g o i n g a specific m e t h o d of e l e v a t e d t e m p e r a t u r e m e c h a n i c a l t e s t i n g m a y s h o w n o loss i n the p a r t i c u l a r m e c h a n i c a l p r o p e r t y b e i n g measured amount

of

(at temperature) moisture.

e v e n t h o u g h i t has a b s o r b e d a significant

O n the other

h a n d , this same

system, h a v i n g

a n e w l a y - u p , u n d e r g o i n g a different h i g h t e m p e r a t u r e m e c h a n i c a l test (different m e t h o d of l o a d i n t r o d u c t i o n ) a n d h a v i n g a b s o r b e d a n e q u i v a l e n t a m o u n t of m o i s t u r e m a y s h o w a s u b s t a n t i a l loss i n t h e p a r t i c u l a r

Figure 9. Photomicrograph of humid-aged isotropic tension specimen (side view, 112.5 mechanical property value being measured. d e m o n s t r a t e d b y the b o r o n / e p o x y

composite

quasiX)

T h i s b e h a v i o r is most a p t l y system.

A

quasi-isotropic

l a m i n a t e tested i n tension at 350° F after 30 exposure cycles has essentially the same tensile p r o p e r t i e s as i t d i d at 3 5 0 ° F b e f o r e a n y exposure e v e n t h o u g h i t has a b s o r b e d

a significant a m o u n t of m o i s t u r e .

This


same

148

FILLERS A N D R E I N F O R C E M E N T S FOR PLASTICS

b o r o n / e p o x y system, h a v i n g a u n i d i r e c t i o n a l l a y - u p a n d b e i n g tested i n flexure,

shows almost a 5 0 % loss i n its 350° F flexural s t r e n g t h after i t has

absorbed

a n a m o u n t of m o i s t u r e e q u i v a l e n t to t h a t p i c k e d u p b y

the

quasi-isotropic/tension laminate. W a t e r behaves as a p l a s t i c i z i n g agent, a p p a r e n t l y d i s r u p t i n g the strong hydrogen

bonding

present i n the h i g h l y p o l a r e p o x y

systems.

E v i d e n c e for this are t h e r e v e r s i b i l i t y of the w a t e r a b s o r p t i o n effect a n d t h e c h a n g e of f a i l u r e m o d e f r o m d r y to w e t specimens.

T h e test results

after w a t e r b o i l exposures c a n be c o r r e l a t e d w i t h those of h i g h h u m i d i t y Downloaded by EAST CAROLINA UNIV on November 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0134.ch014

exposures b a s e d o n e q u i v a l e n t w a t e r w e i g h t gains. T h e use temperatures of several r e s i n systems are too close to t h e i r heat d i s t o r t i o n temperatures. The

h i g h heat

distortion temperature

resins are not

as significantly

affected b y m o i s t u r e at 350° F as are t h e l o w e r heat d i s t o r t i o n t e m p e r a t u r e resins.

T h e m e c h a n i c a l properties of b o t h " w e t " a n d " d r y " composites

( a l l systems) are essentially unaffected u p to 2 5 0 ° F . T h e effect of m o i s t u r e is r e v e r s i b l e .

D r y i n g of w e t test

results i n t h e r e c o v e r y of the o r i g i n a l d r y strengths.

specimens

T h e cast r e s i n sys-

tems are not h y d r o l y t i c a l l y u n s t a b l e as e v i d e n c e d b y the r e v e r s i b i l i t y of moisture absorption. RECEIVED October 11, 1973.


Fillers and Reinforcements for Plastics

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