Current epoxy resins utilized in high performance structural composites absorb moisture from high humidity environments. Such moisture absorption causes plasticization of the resin to occur with concurrent swelling and lowering of the glass transition temperature. Similar effects are observed in composites. Data are presented showing the effects of absorbed moisture on Hercules AS/3501-5 graphite/epoxy composites. Prediction of moisture content and distribution in composites, along with reduction in mechanical properties, are discussed

Husman, G. E.

Whitney, J. M.

English

NASA Technical Reports Server

ENVIRONMENTAL EFFECTS ON POLYMERIC 
MATRIX COMPOSITES 
J. M. Whitney and G. E. Husman 
Air F o r c e  Mater ia ls  Laboratory 
SUMMARY 
Current  epoxy r e s ins  utilized in high performance s t ruc tura l  composites 
absorb  mois ture  f rom high humidity environments. 
tion causes  plasticization of the r e s in  to occur with concurrent swelling and 
lowering of the glass transit ion temperature .  
composites.  
Hercules  AS/3501- 5 graphite/epoxy composites.  Predict ion of mois ture  
content and distribution in composites, along with reduction in mechanical 
proper  t i e s, a r e  dis  cus s ed . 
Such mois ture  absorp-  
Similar  effects are observed in 
Data a re  presented showing the effects of absorbed mois ture  on 
INTRODUCTION 
The  g l a s s  transit ion temperature,  T g ,  of a polymer i s  defined as the 
tempera ture  above which the polymer is soft and below which it i s  hard .  
epoxy r e s ins  the T 
glassy solid to a rubbery solid. 
p r ia te  to d iscuss  a g lass  transit ion tempera ture  region r a the r  than a single 
glass  transit ion temperature ,  as the change f rom a ha rd  polymeric ma te r i a l  
to a soft ma te r i a l  takes  place over a tempera ture  range. The concept of a T g  
is for convenience and refers to the tempera ture  at which there  i s  a ve ry  
g' rapid change in physical p roper t ies .  
F o r  
i s  the tempera ture  a t  which the polymer goes f rom a g 
F r o m  a prac t ica l  standpoint i t  i s  m o r e  appro- 
A s  a resul t ,  there  i s  no p rec i se  T 
It is well recognized (ref. 1) that the T of a polymer can be lowered by 
tempera ture  than the polymer.  This process  i s  r e fe r r ed  to  as plasticization. 
Thus, mois ture  ac t s  as a diluent in cu r ren t  r e s ins  being utilized in high per- 
formance s t ruc tura l  composites,  result ing in a lowering of the T 
a r e  indications ( r e f .  2) that s imi l a r  effects occur in epoxy ma t r ix  composites. 
Data  (ref. 2) a l so  indicates that the lowering of the Tg in both neat r e s ins  and 
mixing with it a miscible  liquid (diluent) that a as a lower g lass  transit ion 
There  
g' 
687 
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derived composites can be estimated f r o m  the Kelley- Bueche plasticization 
theory (ref.  3 ) .  Thus, absorbed moisture  reduces the temperature  range 
over which ma t r ix  dominated composite propert ies  remain  stable.  F r o m  a 
pract ical  standpoint, change of failure mode due to plasticization i s  of pr i -  
m a r y  concern. 
In the present  paper ,  the prediction of mois ture  content in conjunction 
In addition, data i s  with laboratory characterization i s  discussed in detail.  
presented which shows the effect of absorbed mois ture  on the flexure 
strength of unidirectional Hercules  AS/3501- 5 graphite/epoxy composites. 
The flexure tes t  i s  an excellent example of absorbed moisture  inducing a 
change in failure mode. 
PREDICTION O F  MOISTURE DIFFUSION 
Fick 's  Law 
It h a s  been shown ( r e f .  4) that mois ture  diffusion in laminated compo- 
s i tes  can be predicted by Fick's second law. 
nes s  of an  infinite plate, the diffusion equation i s  given by 
F o r  diffusion through the thick- 
where m i s  the percent  mois ture  gain per  unit thickness, D, is the diffusivity 
through the thickness, t denotes t ime, and z i s  the thickness coordinate. 
Consider the following boundary and initial conditions for a plate of thickness 
h 
m(0, t) = m ( h , t )  = m = constant ( 3) 1 
where m i  i s  the initial moisture  distribution in the materia1,and m l  i s  the 
surface moisture  concentration, which i s  a function of the relative humidity. 
A solution to equation (1) which satisfies the conditions of equations (2)  and 
(3) can be obtained by classical  separation of variables with the resul t  
688 
where 
00 
4 1 C (2n- 11 n= 1 m ( z ,  t) = ml - n (ml -  mi) 
x [sin (2n- 1 ) n z  exp [-(2n- 1) x tg: I]
8 
The total weight gain of mois ture  in the plate i s  given by 
M = ( m d z  
0 
(4 )  
Integration of equation (4)  yields 
Appli cat  ion to Char  ac te r i za  t i  on 
Consider an experiment  where an  initially d r y  specimen is  exposed to 
a constant environment ( t empera tu re  and humidity) f o r  a given period of t ime 
t l .  It i s  then put in a d r y  environment and the tempera ture  ramped a t  a con- 
stant r a t e  to a given level  a t  t ime tz .  A t es t  i s  then per formed on the speci- 
men  over some period of t ime. 
character izat ion of mois ture  effects on the mechanical behavior of laminates .  
It i s  often desirable  to control both the mois ture  content and distribution 
during such a characterization. 
a purpose. 
Such a procedure i s  used during laboratory 
Equations (41 and ( 7 )  can be modified for  such 
F o r  the interval 0 & t L t l ,  equations (4)  and (7)  can be  used direct ly  
with m. = M. = 0 and D, = D,(T1), where  T 
this time interval.  
pera ture .  
and as a resul t ,  Dz will  va ry  with t ime. 
ing t:k in the following manner  ( r e f .  5\ 
denotes the tempera ture  during 
1 1 1 
It should be noted that the diffusivity i s  a function of tem- 
In the interval tl4kGt2 the tempera ture  will be varying with t ime 
This can  be  accounted for  by defin- 
689 
Note that in the derivation of equation (8) it is assumed that the temperature 
gradient has negligible effect on diffusivity, as the heat diffusivity is several 
orders of magnitude greater than moisture diffusivity. For this interval the 
initial distribution can be obtained from equation (4). with the result 
where 
In addition, 
D, = D,(T) = D,(t) (12) 
If equation (9) is expressed a s  a Fourier series,  then the moisture profile 
for this time interval becomes 
4 , 1  
n= 1 
m(z, t) = n 
(2n- 1 ) n z  
- exp [-(2n-1) h 
690 
I 
2 (2n- 1) 
For the interval t L t  
2- 
4 m  
x mi(a) = - n= 1 
(2n- 1 ) ~  z 
h x exp [-[zn-l)  2 2  tz 1 sin 
where 
D (s )  ds 1 - Z 
5 
The boundary conditions a r e  those of equation { 11). 
the form of a Fourier ser ies ,  the moisture proEile for this time interval 
becomes 
Since equation (14) is in 
(2n- 1) n z  
- exp [-(zn-ll tf  h 
I 
2 2  
and the total. moisture gain is given 
8M1 M(t) = -
n= 1 2 R 
69 1 
where 
CHANGE IN FAILURE MODES 
Filament Dominated Laminates 
In most engineering usage of fiber reinforced composites, laminate 
stacking geometry is  chosen such that stiffness and strength a re  controlled 
by fiber modulus and strength, respectively. Thus, sorne matrix softening 
can be accommodated in such applications without serious consequences. If 
considerable matrix softening occurs, however, the ability of the resin to 
support the fiber is severely reduced, along with the ability to transfer load 
through the matrix to the fibers. The result is a change in failure mode from 
filament dominated to matrix dominated. 
unidirectional compression, where a significant loss in matrix stiffness leads 
to local instabilities and a reduction in compression strength. Thus, any loss 
in resin Tg due to moisture absorption can lead to a reduction in the useful 
temperature range of the composite laminate. 
The classical example is  that of 
Flexure Strength 
Unidirectional flexure tests a r e  commonly used for quality control, and 
0 degree flex strength is considered to be a filament dominated property. F o r  
state-of-the-art high-performance epoxy resins, 0 degree dry flex strength is  
relatively insensitive to temperatures below 300°F. With increasing moisture 
content, however, measurable strength degradation can occur a t  temperatures 
considerably below 300OF. This is  illustrated in Table I, where 0 degree flex 
strengths a r e  shown for Hercules AS/3501- 5 graphite/epoxy composites. 
These results were obtained on eight-ply composites subjected to a standard 
four-point bend test with a 32:l span-to-depth ratio. A cursory examination 
of these results reveals that a severe loss in 0 degree flex strength occurs at 
a tempkrature as low as  Z O O O F  after exposure to equilibrium moisture content 
in a 95% relative humidity environment. The Tg of this material under these 
conditions has been shown to be approximately 2100F (ref.  2) .  
69 2 
Under dry conditions the shear strength of the matrix, for temperatures 
less than 300°F, is high enough to prevent interlaminar shear failure and 
assure that the flex strength is governed by fiber breakage. A s  moisture 
induces matrix softening below 300oF, the high temperature flex strength 
becomes dominated by interlaminar shear yielding. This conclusion can be 
supported by examining failed specimens and noting that the 3000F wet com- 
posites did not display fiber breakage as  the mode of failure, but were per- 
manently deformed near the load noses where the shear stress was largest. 
Furthermore, the load deflection curves for these cases produced a classic 
example of an elastic-plastic material. 
failure was induced, fiber breakage occured between the loading pins where 
the interlaminar shear stress vanishes. 
another classic example of a change in failure mode induced by matrix 
softening . 
For conditions under which brittle 
Thus, the 0 degree flex strength is 
Interlaminar shear stress- strain behavior relative to  the 0 degree flex 
test is illustrated in figure 1 .  
from a f 45 degree tensile test as described by Rosen (ref. 6). 
stress-strain curve is not shown, but is terminated at  the stress level where 
the maximum interlaminar shear s t ress  occurs in the 0 degree flex test, 
This value can be calculated from classical beam theory with the following 
result for quarter-point loading. 
These 0 degree shear results were obtained 
The entire 
U f 
- s  - -  t max 
where t 
the flex E%, up is the flex strength, and S is the span-to-depth ratio of the 
test specimen. 
stress- strain behavior is observed. 
ior occurs at very low s t ress  levels. 
is  the maximum value of the interlaminar stress obtained during 
For the high temperature tests considerable non-linear shear 
For the wet tests, the non-linear behav- 
To further illustrate the change in failure mode, 0 degree flex strength 
This plot resembles typical log modulus versus temperature 
for composites o r  f o r  assessing the useful 
is plotted on a log scale in figure 2 as  a function of temperature for wet and 
Iry conditions. 
x r v e s  found in classical viscoelastic polymeric materials. Thus, the flex 
:est may be useful in assessing T 
:emperature range of the materia f for various moisture contents. 
693 
CONCLUDING REMARKS 
It has been shown that a solution to Fick's law can be obtained which is 
relavent to laboratory characterization of composite materials containing 
moisture. 
determining total weight gain due to moisture absorption. Data presented 
also indicates that the widely utilized unidirectional flexure test can be a 
valuable tool in assessing the useful temperature range of composite laminate: 
for various moisture contents. 
This solution provides a detailed moisture profile in addition to 
REFERENCES 
1. 
2. 
3.  
4. 
5. 
6. 
Bueche, F. : Physical Properties of Polymers. Interscience Publishers, 
New York, 1962. 
Browning, C. E.  ; Husman, G. E. ; and Whitney, J. M. : Moisture Effects 
in Epoxy Matrix Composites. Composite Materials: Testing and 
Design (Fourth Conference), American Society f o r  Testing and Mater- 
ials, 1976. 
Bueche, F.: and Kelley, F.N.:  Viscosity and Glass Temperature Rela- 
tions for Polymer-Diluent Systems. Journal of Polymer Science, 
Vol. 45, 1961, pp. 549-556. 
Shen, Chi-Hung; and Springer, G. S.: Moisture Absorption and Desorption 
of Composite Materials. Journal of Composite Materials, vol. 10, 
January, 1976, pp. 2-20. 
Weitzman, Y. : Diffusion With Time-Varying Diffusivity, With Applica- 
tion to Moisture-Sorpticn in Composites. 
Materials, vol. 10, July, 1976. 
Journal of Composite 
Rosen, B. W. : A Simple Procedure for Experimental Determination of 
the Longitudinal Shear Modulus of Unidirectional Composites. 
of Composite Materials, vol. 6, October, 1972, pp. 552-554. 
Journal 
694 
T A B L E  I. - UNIDIRECTIONAL F L E X  STRENGTH,  
AS13501- 5 GRAPHITE/EPOXY 
TEMPERATURE 
R T  
Z O O 0  F 
25c0 F 
300° F 
D (WET - 1 e 7 %) ** f D (WET-l. lYo)* Df(DRP) f 
259 KSI 265 KSI 252 KSI 
259 KSI 210 KSI 180 KSI 
242 KSI 166 KSI 135  KSI 
233  KSI 125 KSI 90 KSI 
:k EXPOSED T O  EQUILIBRIUM AT 757'0 RELATIVE HUMIDITY AND 
160°F, % WT. GAIN = 1.1%. 
*:: EXPOSED T O  EQUILIBRIUM A T  9 5 %  RELATIVE HUMIDITY AND 
160°F, % WT. GAIN = 1.7%. 
695 
I 
r 250°F -DRY = 63 "10 
I . .  , 1 , . , I .  I A 
0 I 2 3 4 5 6 7 8 
r, ( O/O 1 
Figure 1,- Shear stress-strain curves for unidirectional 
composites, 
75% relative humidity and 1600F. 
Wet = 1.1% equilibrium weight gain at 
( O )*T A S / 3 5 0 1 - 5  
V+ 63 '10 
DRY 
\ EQUILIBRIUM AT 75% RH, 160°F A EQUILIBRIUM AT 95% RH, 160°F 5.0 I 
I ,  I I I I I 1 
50 I 00 150 200 250 300 350 
4.91 
0 
T("F) 
Figure 2.- Unidirectional flex strength as a 
function of temperature. 
696 


Environmental effects on polymeric matrix composites

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