A thermal-vacuum outgassing model and test protocol for predicting outgassing times and dimensional changes for polymer matrix composites is described. Experimental results derived from a 'control' sample are used to provide the basis for analytical predictions to compare with the outgassing response of Long Duration Exposure Facility (LDEF) flight samples

Matthews, R.

Tennyson, R. C.

English

NASA Technical Reports Server

N93-28262
OUTGASSING AND DIMENSIONAL CHANGES OF POLYMER MATRIX COMPOSITES IN SPACE
R. C. Tennyson and R. Matthews
University of Toronto Institute for Aerospace Studies
North York, Ontario, Canada, M3H 5T6
Phone: 416/667-7710, Fax: 416/667-7799
SUMMARY
This report describes a thermal-vacuum outgassing model and test protocol for predicting outgassing
times and dimensional changes for polymer matrix composites. Experimental results derived from a
"control" sample are used to provide the basis for analytical predictions to compare with the outgassing
response of LDEF flight samples.
THERMAL-VACUUM OUTGASSING AND DIMENSIONAL CHANGES OF LDEF POLYMER
MATRIX COMPOSITES (AO 180)
The UTIAS experiment consisted of a variety of graphite, aramid and boron fiber reinforced epoxy
matrix composites located at station D-12 on LDEF (i.e., -82 ° relative to velocity vector). Selected samples
were instrumented with strain and temperature gauges that were sampled every 16 hours over the first 370
days in orbit. Data were stored on a magnetic tape cassette using a space-qualified data acquisition system
designed and constructed at UTIAS. Details on this aspect of our experiment can be obtained from ref. 1. It
was found that the strain/thermal gauge measuring system worked flawlessly, as evidenced by the measured
response of a stainless steel calibration specimen which remained unchanged throughout the 5.75 years in
orbit. Typical time/temperature and strain/temperature data for one material (graphite/epoxy, 5208/T300) are
shown in Figs. 1 and 2, respectively. This data can be re-plotted as strain vs. temperature as given in Fig. 3
for the 90 ° laminate. It can be seen that a "total" dimensional strain change of -1600x 10 -6 occurred after
about 80 days in orbit. It should be noted that no microcracks were observed in this laminate and full
recovery of the dimensional change occurred once the sample was returned to Earth and exposed to the
ambient environment.
From this data, it is possible to estimate the coefficient of thermal expansion (CTE) from the final slope
once all outgassing is essentially finished. Using this CTE value, one can correct for the temperature
variations on-orbit, giving the strain change of the sample, over time, independent of temperature. The
formula used to do this is:
where
At
ct
Tt
TRef
(X
A t = E t - (T t - TRef).Ot
= strain change at time t
= measured strain at time t
= temperature at time t
= reference temperature = 75°F
= CTE of material.
(1)
At was then plotted against time and an adjustment factor (A adj) was added to every point. This had the
effect of shifting the graph so that the final strain was zero, allowing the total strain change to be read easily.
Figure 4 shows the adjusted A t versus time curve for graphite/epoxy (5208/T300). From this graph it is
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evident that outgassing was completed in about 80-100 days. It is clear that outgassing was very rapid over
the first 25 days, then slowed due to the low temperatures encountered (see Fig. 1). Outgassing then
increased after 50 days as the sample temperature increased, and eventually no further measured dimensional
change occurred after about 80-100 days exposure. Similar behaviour was exhibited by the other composite
materials (ref. 1). The outgassing time required to reach an equilibrium state in space depends on such
factors as the initial moisture concentration, the volatile content, laminate thickness, ambient temperature and
constituent material diffusion properties.
MOISTURE DESORPTION AND DIMENSIONAL CHANGES
As with many other published analyses, the moisture desorption can be estimated using Fick's law,
given by (for example, ref. 2):
c3c c32c
-_=D_ bX 2 (2)
where c = moisture concentration at x
t = time
x = thickness coordinate
h = thickness
D = diffusion coefficient in the x-direction
and the following boundary conditions apply:
c(x, o) = Co (initial concentration)
c(o, t) = c(h, t) = coo (ambient concentration)
c(x, oo)= coo
Noting that the total mass change over time is given by
M = fo h c(x, t)dx (3)
a solution was obtained to Eq. (2) in terms of the moisture content variation with time:
M(T, t) = Mo( 1 - G) (4)
where Mo = initial moisture content (wt %), assuming the final moisture content = 0 in space, and
878
k=_0( 1 exp -(2k + 1)2_2Dt]/
where
I fDq0751= 1-exp -7.3l_h2] ]
D = diffusion coefficient = Doexp(-Ed/RT) (from ref. 3)
R = gas constant
T = absolute temperature
Ecl= activation (diffusion) energy
(5)
(6)
For constant temperature, Shen and Springer (ref. 4) have shown that the diffusion coefficient (D) can
be calculated knowing the moisture content at different times, i.e.,
where
Mo
M1, M2
DT=c°nst- 16M 2 _11 ]
= initial moisture content at start of desorption test
= moisture contents at times tl and t2, respectively
(7)
Rather than measure moisture content during a test, one can employ strain data. Noting that
E = M[3 (8)
where 13= coefficient of moisture expansion (CME), then substituting into Eq. (4) yields:
e(T, t) = eo(1 - G) (9)
Substituting Eq. (8) into Eq. (7) also gives
/1;h2 [ E2_E 1 ]2
D'r=_ons,= -i-_o2[_- _,_-1 (10)
PREDICTING MOISTURE OUTGASSING FOR VARYING TEMPERATURES
To develop a model for predicting outgassing of materials in space, it is necessary to take temperature
into account. However, Fick's law as previously described applies to constant temperature, constant
879
humidity environments. In space, the humidity level (i.e., vacuum) is constant. It is possible to determine
the diffusion coefficient by performing outgassing tests at different temperatures (T1 and T2). Using the
Arrhenius relation given by Eq. (6), one obtains the following two equations:
E •
In(D1) = In(Do) -
Jl (11)
ln(D2) = in(Do) _ E*
T2 (12)
where E* = Ed/R. Solving for E* and Do yields:
E* = [In(D2)- ln(D1)][1 12] ,13)
D°:exPl(ln :)1nO )+ O)1W
T2
(14)
Substituting back into Eq. (6) gives:
D(T) = exp ln(D2) - ln(D1) + ln(D 1) 1.
1 T1 rT2
exp 1 T
(15)
This equation can be used to calculate the diffusion coefficient at any temperature, T, as long as the diffusion
coefficients D1 and D 2 at temperatures T1 and T2 are known. All of the above temperatures must be
absolute (K).
APPLICATION TO LDEF SAMPLES
Experimental Input
.
,
Control samples were subjected to vacuum outgassing at elevated temperature to obtain their "dry
weight" values.
For given temperature (T) and % RH, record moisture uptake (%) for given material from its "dry" state
as a function of time (t) to saturation.
88O
io
%
tlr2
. Measure sample strain (_) as a function of time (t) in vacuum for a given temperature (T1). Repeat
experiment at another temperature (T2). Both experiments employ samples having the same Mo.
E(T,t)
\ T 2 \\ \
\ \
\
Slope = D
Eo
Desorption Response
Analytical Procedure
1. Using the e(T, t) curves, calculate the initial slope from Eq. (10) to obtain DI(TI) and D2(T2).
2. Determine D(T) from Eq. (15) based on DI(T1), D2(T2), T1 and T2.
3. Using the LDEF temperature/time profile obtained in-orbit (see Fig. 1), calculate the dynamic strain
change E(t) for given time steps (At), using the above D(T) equation evaluated at the appropriate
temperature. The e(t) function is given by
Et=et-l-Et-,(1-exp[-7.3(D(_)'At)75 0 (16)
where Tt = average temperature over At, assuming % is known at t = 0 from the outgassing test. By using
this equation at every time step over the temperature history, it is possible to calculate the strain change of
the sample due to outgassing, taking into account temperature effects.
881
COMPARISON WITH LDEF DATA
Before starting the outgassing tests, the samples were completely dried out under vacuum at elevated
temperature. The dry weight and length of each sample were measured and recorded. The samples were
then placed in a hygroscopic chamber to absorb moisture. Each sample was left until it absorbed the same
amount of moisture as was outgassed from the equivalent LDEF sample. This value was determined by
measuring the CME (13) of each material and then dividing the strain change measured on-orbit by 13to give
the total change in moisture content. Typical moisture absorption curves for three polymer matrix
composites flown on LDEF are shown in Fig. 5. At this point in time, results will be presented only for one
graphite/epoxy system -- T300/5208, having the form of a 4 ply, 90" laminate [90]4. The ground-based
simulator tests were conducted on two tubes: a control sample (5T5) that had remained under ambient
laboratory conditions since the manufacture of the LDEF flight specimens and a flight sample (2T13). Both
22"C and 50"C outgassing tests were performed on the control and flight samples. From CME calculations,
it was determined that the flight data from sample 3T6 (Fig. 3) indicated a total moisture content change of
.50%. Therefore, for all the tests on the T300/5208 [90]4 samples, a moisture content as close as possible
to this value was used, as summarized in Table I. The initial straight-line portions of the tests are plotted in
Figs. 6 to 9 and a comparison of the 13and D results presented in Table I. The nonlinearity at the beginning
of these curves is due to temperature corrections. For about the first hour the samples had not reached
equilibrium. On average, the 22"C and 50"C tests took about 13 days and 6 days to complete, respectively.
Note that the diffusion coefficients of the control and flight samples agree quite well, indicating no
significant changes occurred after 69 months of space exposure.
Using the temperature history of the flight sample (3T6) shown in Fig. 1, it was possible to predict its
outgassing behaviour based on the results in Table I and Eqs. (15) and (16). A comparison with the flight
data is given in Fig. 10. It is obvious that the predicted values do not fit the actual results very closely.
However, if the predicted diffusion coefficient is reduced to 13.4% of its measured value, the predicted
response is extremely close to the actual data as shown in Fig. 11. This indicates that the model itself is
correct. The discrepancies may be due to differences between the test conditions and the space environment,
such as a higher pressure or the presence of surface contaminants. It is also important to note that there is a
large variability in the thickness and uniformity of the samples. Manufacturing variations may have caused
the different diffusion coefficients of the samples. Ideally, it would be best to measure D(T) from sample
3T6, and see how well this prediction fits the flight data. Unfortunately, it was not yet possible to perform
this test, although this will be done in the near future.
ACKNOWLEDGEMENTS
This research was sponsored by the Canadian Space Agency (CSA) under contract
025SR.9F009-1-1435, and two Ontario Centres of Excellence: the Institute for Space and Terrestrial
Science and the Ontario Centre for Materials Research. Special thanks are extended to Dr. D. G. Zimcik of
the CSA for his continued support of our work.
REFERENCES
, Tennyson, R. C.: Composite Materials in Space- Results from the LDEF Satellite. J. Canadian
Aeronautics and Space Institute, vol. 37, no. 3, Sept. 1991.
2. Crank, J.; and Park, G. S.: Diffusion in Polymers. Academic Press, London, 1968.
882
.,
Whitney, J. M.; and Browning, C. E.: Some Anomalies Associated with Moisture Diffusion in Epoxy
Matrix Composite Materials. Advanced Composite Materials - Environmental Effects, ASTM STP 658,
J. R. Vinson, ed.; American Society for Testing and Materials, 1978.
Shen, Chi-Hung.; and Springer, George S.: Moisture Absorption and Desorption of Composite
Materials. J. Composite Materials, vol. 10, Jan. 1976.
Table I. Summary of Simulator Outgassing Test Results for Graphite/Epoxy (T300/5208) Tube Laminates
[90]4
Sample No. Type Temp [°C] Mi [%] AE [I.tE] CME [I.tE/%] D [mm2/h]
5T5 Control 22 .49 - 1200 -2449 .0001
5T5 Control 50 .55 - 1939 -3525 .00047
2T13 Flight 22 .505 - 1212 -2400 .00013
2T13 Flight 50 .632 - 1517 -2400 .00078
200
150
ii
o
v 100
E
_ 50
-5o
Fig. 1 Thermal History of LDEF Specimen (3T6)
Graphite/Epoxy [90 °]
5208/-1-300
0 100 200 300 400
Elapsed Time from Initiate (days)
883
::L
v
¢-
O3
"OQ_
1200
8OO
400
0
-400
-800
-1200
-1600
-2000
-2400
0
Fig. 2 Strain-Time History for LDEF Specimen (3T6)
Graphite/Epoxy [90 °]
5208/-1"300
100 200 300
Elapsed Time from Initiate (days)
Fig. 3 Thermal/Strain Response of LDEF Specimen (31"6)
Graphite/Epoxy [90 °]
5208/T300
400
Q_
0
° 2-"
Post Flight
Arnt ient ,--_ '_ _ _,_ _ _J
".3 ,o ._ --"
80 days
-351_i
Temperature (deg F)
884
m1600
O
1200 _)
_b
1000
@
O800
o
600 m
400
200
-200
0
Fig. 4 Plot of A Strain vs Time in Orbit
for LDEF Specimen (3T6)
Graphite/Epoxy -
5208/T300
(8x90 °, Tube, (90°)) _
I
°
lq 21
Days
O
v
¢-
0
e-
0
o
-1
°_
0
0.7
0.6
O.S
0.4
0.3
0.2
0.1
Fig. 5 LDEF Control Moisture Absorption Test
4 Ply 90 deg -- 50 C -- 75% RH
+ +
+
+ []
13 SP. 328
+ T300/934
O T300/5208
12 16 20
Root Time (clays 1/2)
24 28
885
Fig. 60utgassing Test on Control Specimen (5T5) @ 22°C
Graphite/Epoxy [90 °]
5208/T300
.so.........................i .............._ ................._ ................_ ................i .............: .........._........ T ....................i ..............
@-15o .....................i ...............i ...........i................i .. ! i i i
_ -_o.......................4 .............4 .............i ............4 ............i ..........._ .........i ..............i .................! ...........
-400 i
0 0.5 1 1.t5 2 2.5 3 3.5 4.5 5
Root Time (hours 1/2)
Fig. 70utgassing Test on Control Specimen (5T5) @ 55°C
Graphite/Epoxy [90 °]
5208/-I-300
._E
o3
°1 i
" i i
-1100 _
0 0.5 1 1.5 2 _5 3 3.5 4 4.5 5
Root Time (hours 1/2)
886
¢--
£
r..)
C
-50-
-100-
-150-
-200-
Fig. 80utgassing Test on LDEF Flight Specimen (2T13) @ 22°C
Graphite/Epoxy[90°]
5208/T300
-250
0 0.5
r t
1.5 2 2.5 3 3.5 4 4.5
Root Time (hours 1/2)
¢--
Fig. 90utgassing Test on LDEF Flight Specimen (2T13) @ 50°C
Graphite/Epoxy[90°]
5208/1300
5OO
i li i i i¸  .
£
o
E
c
,m
L_
u)
200- •
100-
O-
0 0.5 1 1.5 2 2_5
Root Time (hours 1/2)
888
c_
r-
E
U3
b
m
Fig. 10 Comparison of Predicted Strain/Time Response in Space
with LDEF Flight Specimen
Graphite/Epoxy [90 ° ]
5208/T300
Time (days)
Fig. 11 Comparison of Corrected Prediction of Strain/Time Response in Space
with LDEF Flight Specimen (D* = 0.134D)
Graphite/Epoxy [90 °]
5208/T300
Time (days)


Outgassing and dimensional changes of polymer matrix composites in space

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