Thermomechanical Analysis (TMA), Differential Scanning Calorimetry (DSC), and Thermogravimetric Analysis (TGA) were performed on samples of Halar exposed on the LDEF Mission for 6 years in orbit and unexposed Halar control samples. Sections 10-100 microns thick were removed from the exposed surface down to a depth of 1,000 microns through the 3 mm thick samples. The TMA and DSC results, which arise from the entire slice and not just its surface, showed no differences between the LDEF and the control samples. TMA scans were run from ambient to 300 C; results were compared by a tabulation of the glass transition temperatures. DSC scans were run from ambient to 700 C; the enthalpy of melting was compared for the samples as a function of section depth with the sample. The TGA results, which arise from the surface of the sample initially, showed a sharp increase in the topmost 50 micron section (the exposed, discolored side) in the weight loss of 170 C in oxygen. This weight loss dropped to bulk values in the range of depth of 50-200 microns. The control sample showed only a slight increase in weight loss as the top surface was approached. The LDEF Halar sample appears to be mechanically undamaged, with a surface layer which oxidizes faster as a result of orbital exposure

Bauer, Robert A.

Brower, William E., Jr.

Holla, Harish

English

NASA Technical Reports Server

N93 • 1 £788
EFFECTS OF ORBITAL EXPOSURE ON HALAR
DURING THE LDEF MISSION
William E. Brower, Jr., Harish Holla, and Robert A. Bauer
Department of Mechanical and Industrial Engineering
Marquette University
1515 W. Wisconsin Avenue
Milwaukee, Wisconsin 53233
ABSTRACT
Thermomechanical Analysis (TMA), Differential Scanning Calorimetry (DSC), and
Thermogravimetric Analysis (TGA) were performed on samples of Halar exposed on the
LDEF Mission for 6 years in orbit and unexposed Halar control samples. Sections 10-100
microns thick were removed from the exposed surface down to a depth of 1,000 microns
through the 3 mm thick samples. The TMA and DSC results, which arise flora the entire
slice and not just its surface, showed no differences between the LDEF and the control
samples. TMA scans were run from ambient to 300 C; results were compared by a tabulation
of the glass transition temperatures. DSC scans were run from ambient to 700 C; the
enthalpy of melting was compared for the samples as a function of section depth within the
sample. The TGA results, which arise from the surface of the sample initially, showed a sharp
increase in the topmost 50 micron section (the exposed, discolored side) in the weight loss
of 170 C in oxygen. This weight loss dropped to bulk values in the range of depth of 50-200
microns. The control sample showed only a slight increase in weight loss as the top surface
was approached. The LDEF Halar sample appears to be mechanically undamaged, with a
surface layer which oxidizes faster as a result of orbital exposure.
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391
https://ntrs.nasa.gov/search.jsp?R=19930003600 2020-03-17T10:34:33+00:00Z
INTRODUCTION
The first reports of the effects of prolonged orbital exposure by Whitaker (1) showed
some weight loss data for a range of solar array materials. Tennyson et al (2) reported
dimensional changes and changes in thermal expansion coefficients for a range of composite
samples. B.J. Dunbar (3) reported on the general effects encountered by the LDEF samples
- atomic oxygen, particle strikes, and UV exposure. Some of the Mylar 5 rail coatings were
completely gone; this result gives added interest to the Halar and RTV studies of this
investigation. Steckel and Le (4) were the first to report degradation as a function of depth
in the sample, although their results were calculated from bulk weight loss data. The thrust
of this investigation was to determine the depth profile of the damage to the Halar and RTV
LDEF samples. Results for the Halar samples are reported here. Thermomechanical
Analysis (TMA), Thermogravimetric Analysis (TGA), and Differential Scanning Calorimetry
(DSC) were employed to assess the effects of orbital exposure during the LDEF Mission.
EXPERIMENTAL PROCEDURE
The procedure for preparing samples from the piece of LDEF exposed Halar and the
Halar control is shown schematically in Figure 1. First, 1 cm x 1/4 cm pieces were cut from
the full Halar pieces. These pieces were best suited for sectioning in the Edmund Model
DK-10 microtome. Although the nominal minimum section thickness was 10 microns for the
micrometer, the typical section was 50 microns thick. Wide variations in section thicknesses
between sections and within a section occurred as shown inTables 3-8,* due to bending of the
microtome blade, play in the micrometer drive, and the inherent toughness of the Halar.
Table 1 shows the dimensions of the samples that were cut from the fully exposed and control
samples of Halar. The density, calculated from the measured volume and the measured
weight of the cut samples, did not appear to vary between the exposed and the control. Piece
392
*Tables 1 through 8 are cited in text.
7 (exposed) did have a significantly lower density than the rest of the exposed sample and the
control samples. It is hard to imagine such a sharp variation of density within the exposed
sample of Halar. The test conditions during the various thermal analyses are given in Table
2. The heating rates were all the same, whereas the temperature range varied with the
technique. TGA and DSC could be performed well above the glass transition temperature,
but TMA could not. The TGA atmosphere was oxygen to assess oxidation rates.
RESULTS AND DISCUSSION
The results are presented for each technique by showing some thermograms, the
output of the thermal analysis run. Tables of peak temperatures, peak integrals, or baseline
shift amounts (weight changes, penetrations) have been compiled from all the thermograms.
All the thermograms used to obtain the data in Tables 3-8 are given in Appendices A, B, and
C. These temperatures, integrals, or shifts are then plotted versus section depth for the three
techniques employed; TMA, TGA, and DSC. Since the section thicknesses varied within the
section itself, each section was weighed, and its depth is given in the tables as the calculated
average depth from the weight of the section and the density of the Halar from Table 1.
The penetration versus temperature TM,_ thermogram is shown in Figure 2 for the
top section of the LDEF Halar sample. Although visible discoloration was present in this top
section, the glass transition temperature, 253 C in Figure 2, was essentially the same as the
control, 254 C, as shown in Figure 3. The glass transition temperatures for all the sections
analyzed in the TMA are given in Table 3 for the LDEF exposed Halar sample, and in Table
4 for the control Halar sample. The temperatures were determined by the inflection points
of the plots within the transition. Figure 4 is a plot of the transition temperatures as a
function of section depth in the sample. All the temperatures are within +/- 2 C. There is no
trend with depth, and the control is essentially the same as the LDEF exposed sample.
393
Figure 5 is the TGA thermogram for weight gain or loss while heating in oxygen for
the topmost LDEF exposed sample. Significant weight losses occurred at 170 C and in the
range 300-500 C. As can be seen in Figure 6, the weight loss at 170C is far less for the top-
most control sample than for the exposed Halar sample, while the weight loss at the higher
temperature range is similar for both samples. TGA weight losses at 170 C, 290 C, and 420
C are given in Table 5 for all the LDEF exposed Halar sections and in Table 6 for all the
Halar control sections. The plot of weight loss at 170 C versus section depth is shown in
Figure 7. The LDEF exposed Halar shows a dramatic increase in weight loss as compared
to the control samples for the first two sections from the top. Discoloration was evident in
both of the top two TGA sections of the exposed sample. Apparently the oxidation rate
differs from the control for the LDEF exposed Halar only to a depth of about 50 microns.
The DSC thermogram is shown in Figure 8 for the topmost LDEF exposed sample.
A noisy melting endotherm is evident at 235C, and a strong exotherm at 446 C. The top-
most control sample, Figure 9, showed a weak melting endotherm at 234 C. The second
section of the control sample, Figure 10, showed an endotherm at 235 C very similar to
the LDEF sample. Plots of melting temperature versus section depth and melting enthalpy
(the integral of the melting endotherm) versus depth are shown in Figures 11 and 12. In both
cases, there appears to be no difference between the LDEF and the control samples. No
significant variation with section depth is evident for either melting temperature or for
enthalpy of melting.
The TMA and DSC techniques measure the response of the whoie sample section
which is placed in the analyzer. Near surface effects that are truncated in several atom layers
would not be resolvable in the roughly 50 micron thick sections. The TGA, however,
measures the oxidation rate at the surface of the section placed in the analyzer. The top-
most section had as its top surface the actual top surface given the orbital exposure. The
394
J
other side of the section was produced by the microtome. Thus, the TGA is the most surface
sensitive of the three techniques employed, and it is the only technique to sense damage from
orbital exposure. This 50 micron damage depth is in rough agreement with the observation
of severe damage to 125 micron thick Mylar (3).
CONCLUSIONS
The orbital exposure during the LDEF Mission did not appear to mechanically
damage the Halar sample. To a surface section resolution of about 50 microns, no
thermodynamic damage was detectible via differential thermal analysis. The top 50 microns
of the LDEF exposed sample did exhibit a higher oxidation rate than the control samples,
which correlates to the depth of the discoloration.
3_
References
(1) A. F. Whitaker, "Coatings Could Protect Composites From Hostile Space
Environments," Adv. Materials & Processes, 4 (1991), 30-32.
(2) R. C. Tennyson, G. E. Mabson, W. D. Morison, and J. Kleinman, "LDEF Mission
Update: Composites in Space," Adv. Materials & Processes, 5 (1991), 33-36.
(3) B. J. Dunbar, "A Materials Scientist in Space," MRS Bulletin, May (1991), 36-41.
(4) G. L. Steckel and T. D. Le, "LDEF Mission Update: Composites Survive Space
Exposure," Adv. Materials & Processes, 8 (1991), 35-38.
306
Table 1 Thickness and Density Measurements for Cut Halar LDEF and Control
Samples Before Sectioning
Dimensional Characteristics of Halar Samples
Measured Thickness
Pc#l Pc#2 Pc#3 Pc#4 Pcfl5 Pc#6 Pcfi7
Control Control Control Exposed Exposed Control Exposed
(inches) (inches) (inches) (inches) (inches) (inches) (inches)
0.1209 0.1220 0.1257 0.1215 0.1224 0.1256 0.1180
0.1218 0.1256 0.1202 0.1218 0.1241 0.1202
0.1218 0.1254 0.1196 0.1198 0.1252 0.1202
Calculated Density
Pc#l Pc#2
Control Control
( gr/cc ) ( gr/cc )
1.651 1.549
Pc#3 Pc#4 Pc#5 Pc#6 Pc#7
Control Exposed Exposed Control Exposed
( gr/cc ) ( gr/cc ) ( gr/cc ) ( gr/cc ) ( gr/cc )
1.558 -1.515 1.568 1.573 1.205
Table 2 Test Conditions for LDEF Samples for Thermal Analysis
Technique
TMA
TGA
DSC
Test Atmosphere Heating Rate, C/min Temp Range,C
flowing Ar i0 25-300
flowing 02 i0 25-700
flowing Ar i0 25-600
397
Table 3 Glass Transition Temperatures as Determined by Thermomechanical Analysis
for Halar LDEF Samples
Halar in
Exposed
Piece #4
Area=
Density =
Sample ID
H4Cl
H4CIA
H4C2
H4C3
H4C4
H4C5
H4C6
H4C7
H4C8
H4C9
H4CI0
H4Cll
H4C!2
H4C13
H4C14
H4C15
H4C16
H4C17
cutoff
total
original
TMA Transition temp.
o.248=^2
1.515 gr/cm^ 3
Wt Thick
(gr)
0.0027 71.9
0.0027 71.9
0.0047 125.1
0.0013 34.6
0.0030 79.8
0.0038 i01.i
0.0045 119 8
0.0044 117.1
0.0014 37 3
0.0080 212.9
0.0028 74 5
0.0051 135.7
0.0019 50 6
0.0053 141.1
0.0039 103 8
0.0050 133.1
0.0019 50 6
0.0078 207.6
0.0486 1293 5
0.1161 3161.9
0.1148 3053.0
Depth
<_n)
36.0
36.0
134.4
214.3
271.5
362.0
472.4
590.9
668.1
793.1
936.9
1042.0
1135.2
1231.0
1353.4
1471.8
1563.7
1692.7
determined by inflection pt
Temp C
253.3
251.0
254.1
252.2
251.1
252.0
252.0
398
Table 4 Glass Transition Temperatures as Determined by Thermomechanical Analysis
for Halar Control Samples
Halar in
Control
Piece #3
TMA Transition temp. determined by inflection pt
Area = 0.3035 cm^2
Density= 1.558 gr/cm^3
Sample ID Wt Thick Depth
(gr) (tma) (tlm)
H3Cl 0.0016 33.8 16.9
H3C2 0.0010 21.1 44.4
H3C3 0.0050 105.7 107.8
H3C4 0.0007 14.8 168.0
H3C5 0.0182 384.9 367.8
H3C6 0.0011 23.3 572.0
H3C7 0.0001 2.1 584.7
H3C8 0.0095 200.9 686.2
H3C9 0.0007 14.8 794.0
H3CI0 0.0090 190.3 896.6
H3Cll 0.0052 110.0 1046.7
H3C12 0.0006 12.7 1108.0
H3C13 0.0084 177.6 1203.2
H3C14 0.0003 6.3 1295.2
H3C15 0.0100 211.5 1404.0
H3C16 0.0016 33.8 1526.7
cutoff 0.0734 1552.3
Temp C
253.7
254.1
253.4
252.7
253.2
252.7
252.7
total 0.1464 3096.1
original 0.1508 3190.0
Table 5 Weight Losses at Various Temperature Ranges as Determined by
Thermogravimetric Analysis for Halar LDEF Samples
Halar in TGA Weight changes occur after onset temperatures
Exposed
Piece #7
Area:
Density=
0.270 cra^2
1.205 gr/cm^3
Sample ID Wt(gr)
H7CI 0.0004
H7C2 0.0029
H7C3 0.0003
H7C4 0.0048
H7C5 0.0006
H7C6 0.0040
H7C7 0.0006
H7C8 0.0066
H7C9 0.0029
H7CI0 0.0047
H7Cll 0.0011
H7C12 0.0044
H7C13 0.0037
H7C14 0.0039
H7C15 0.0007
H7C16 0.0057
H7C17 0.0008
thick mean 170 C 290 C 420 C
(t_n) depth d %wt d %wt d %wt
12.3 6.2 23.0 61.6 14.3
89.1 56.9 3.9 68.7 27.3
9.2 106.0 0.0 78.0 20.6
147.5 184.4 I.I 74.7 24.8
18.4 267.4 0.0 82.6 24.2
122.9 338.1 0.5 72.8 26.8
18.4 408.8 0.0 84.9 22.0
202.9 519.4 0.4 64.1 35.7
89.1 665.4 0.0 67.7 31.8
144.5 782.2 0.3 65.3 34.7
33.8 871.4 0.0 69.5 29.8
135.2 955.9 nd 68.5 31.3
113.7 1080.4
119.9 1197.2
21.5 1267.9
175.2 1366.2
24.6 1466.1
cutoff 0.0509 1564.5
total 0.0990 3042.9
original 0.0994 3053
400
E
Table 6 Weight Losses at Various Temperature Ranges as Determined by
Thermogravimetric Analysis for Halar Control Samples
Halar in TGA Weight changes occur after onset temperatures
Control
Piece #6
Area:
Density=
0.2639 cm^2
1.573 gr/cm^ 3
Sample ID
H6CI
H6C2
H6C3
H6C4
H6C5
H6C6
H6C7
H6C8
H6C9
H6CI0
H6CII
H6C12
H6C13
H6C14
H6C15
H6C16
H6C17
H6C18
cutoff
wt(gr)
0.0046
0. 0040
0.0049
0 0030
0 0008
0 0047
0 0006
00040
00033
0.0030
0.0007
0.0039
0.0005
0.0041
0.0007
0.0043
0.0022
0.0059
0.0803
thick mean 170 C 290 C 420 C
(um) depth d %wt d %wt d %wt
110.8 55.4 1.3 66.2 32.5
96.4 159.0 0.8 72.5 27.1
118.0 266.2 0.2 68.0 31.8
72.3 361.3 0.0 66.7 32.8
19.3 407.1 0.0 74.9 25.1
113.2 473.4 0.0 67.4 32.2
14.5 537.2
96.4 592.6 0.0 65.9 34.0
79.5 680.5
72.3 756.4 0.0 68.6 31.4
16.9 801.0
93.9 856.4 0.0 69.3 30.7
12.0 909.4
98.8 964.8 0.i 66.5 33.5
16.9 I022.6
103.6 1082.8 0.0 67.4 33.7
53.0 1161.1
142.1 1258.7 0.3 65.0 34.6
1934.4
total 0.1355 3264.2
original 0.1320 3180
401
Table 7 Transition Temperatures and Enthalphy of Melting as
Differential Scanning Calorimetry for Halar LDEF Samples
Halar in DSC
Exposed
Piece #5
Temperatures determined by peaks
Area= 0.1281 crn^2
Density= 1.568 gr/c_n^3
San_le ID Wt Thick
(gr)
H5CI 0.0042 209.1
H5C2 0.0018 89.6
H5C3 0.0039 194.2
H5C4 0.0021 104.6
H5C5 0.0048 239.0
H5C6 0.0019 94.6
H5C7 0.0034 169.3
H5C8 0.0032 159.3
H5C9 0.0022 109.5
H5CI0 0.0070 348.5
HSCll 0.0016 79.7
H5C12 0.0034 169.3
H5C13 0.0038 189.2
H5C14 0.0013 64.7
H5C15 0.0076 378.4
H5C16 0.0010 49.8
H5C17 0.0027 134.4
Depth
104.6
253.9
395.8
545.2
716.9
883.7
1015.6
1179.9
1314.3
1543.4
1757.4
1881.9
2061.1
2188.1
2409.6
2623.7
2715.8
te_p C temp C
Determined
cutoff 0.0388 1931.7
total 0.0947 4714.7
original 0.0883 3094.0
H
445.6 235.1
449.7 237.5 6.26
445.4 235.9 4.35
435.6 237.7 3.24
441.0 235.6 5.80
449.3 236.2 2.49
by
4O2
Table 8 byTransition Temperatures and Enthalphy of Melting as Determined
Differential Scanning Calorimetry for Halar Control Samples
Ha!ar in DSC
Control
Piece #2
TenTperatures determined by peaks
Area= 0.265 cm^2
Density= 1.549 gr/cm^3
Sample ID Wt Thick Depth
(gr) (_rta) (_'n)
H2CI 0.0012 29.2 14.6
H2C2 0.0058 141.3 99.8
H2C3 0.0006 14.6 177.8
H2C4 0.0053 129.1 249.6
H2C5 0.0003 7.3 317.8
H2C6 0.0053 129.1 386.0
H2C7 0.0067 163.2 532.2
H2C8 0.0048 116.9 672.2
H2C9 0.0023 56.0 758.7
H2CI0 0.0042 102.3 837.8
H2CII 0.0066 160.8 969.4
H2C12 0.0005 12.2 1055.9
H2C13 0.0059 143.7 1133.8
H2C14 0.0024 58.5 1235.0
H2C15 0.0025 60.9 1294.7
H2C16 0.0020 48.7 1349.4
H2C17 0.0023 56.0 1401.8
H2C18 0.0027 65.8 1462.7
cutoff 0.0463 1127.9
total 0.1077
original 0.1207
2623.7
temp C temp C H
448.8 234.7 3.02
452.0 235.1 6.78
448.0 238.0
447.7 236.0 3.60
450.2 235.5 3.32
449.5 236.7 2.23
3094.0
LDEF or CONTROL Sample
//
/
/" /
/
Top, Exposed Surface
\
____-->
I
I
J /
1,/4 x 1 om out Pieoe
Microtomed Section
DENSITY FROM
WEIGHT AND DIMENSIONS
Average Thickness
fro_ Length, Width,
and Density
Thermal Analyses" TMA, TGA, and DSC
Figure 1 Schematic Diagram of LDEF Sample Sectioning Procedure p
404
.5 _- H4Ct Force- 100 mb
253 C
DELTA SERZES TH,_7
Figure 2 TMA Plot for the Top Section of the Halar, LDEF Sample Which Showed
Visible Discoloration
405
•ii_
13pm
_0.0 C/mtn
DELTA SERZES _'I,_, 7
Figure 3 TMA Plot for the Top Section of the Halar Control Sample
406
m
A
o
v
J
Figure 4
Melting Temperature by TMA
Ha_, _n _ Almol_hlm
+
+
o
÷
4- 4-
o
o
-- I I 1 I I I I I
0 200 400 _ !100
c_c_ _ _ um_c. Cum)
+ control Poll13 o exposed Pc#4
TMA Glass Transition Temperature as a Function of Section Depth for Halar
LDEF and Control Samples
l 407
HALAR IN OP._/AR. H7Cl
_ST DER]3,'AT;VE OF F:ZLE:P_TC'.
45
Io
Date: dun t4. 199t O: 12pro Temperature (C]
Scanning Rmte: t0.0 C/mtn
Smmple Wt: 0.407 mg Dlsk: RB05
,=_1='_ct _u_R DEL T,_ SERIES TGA7
Figure 5 TGA Plot for the Top Section of the Halar LDEF Sample Showing a Large
Weight Loss at 170 ° C
b
408
i00
._ 75
I-!
rid
Z
0
Date: Hay 30. t99t 7: tGpm
Scanning Rate: tO.O C/rain
Saaple Wt: 4.725 mg Disk: _905
Ft2_'/_5_'d _L45q
HALAR IN O2/_q° H6Ct i
\,X / i
\ \ ,-., / i\\ / ,.., ,
m.--'_---__ _k._ _._ _._ _-_-------_._----_._---_.® _._
Temperature (C)
L7E4TA SERZES TGA7
Figure 6 TGA Plot for the Top Section of the Halar Control Sample
409
|
o
"3
i
t
Figure 7
24
22
20
18
16
14
12
10
B
6
4
2
0
Percent Weight Loss in TGA
I-_l(sr In 02/M At_rno_Dher_.Onset 170C
0 200 400 SO0 BOO
¢alcul_.cl _th from m_'f_:e (urn)
+ control Pc#6 ¢. exposed Pc#7
TGA Weight Loss at 170" C as a Function of Sectioning Depth for Halar,
LDEF and Control Samples
410
o
t_
v
HSCi
WT: 4.20 mg
ATMOSPHERE: ARGON
SCAN RATE:
40 cc/min
iO.O0 deg/mln
446 C
I,f
03
.d
¢.2
BAUER
DATE:
Figure 8
235 c
213 C
iiO.O0 160.00 2i0.00 _._
FILE."/_Ci. Dr
9i/05/23 TIME: 16: 20
!)0 410.00 480.O0
TEMPERATURE (C)
!
5t0.00
DTA
DSC Plot for the Top Section of the Halar LDEF Sample Showing an
Endotherm at 235* C and an Exotherm at 446* C
411
o
ca
'v
r.j
LLJ
0_
._I
rJ
's-
J HT: t.20 mg SCAN RATE:
ATMOSPHER_ ARGON 40 cc/min
80.00 i30.OO i80.00
235 C
t0.00 deg/mtn
449 C
 o.oo  .oo
BAUER FZL_" /.t2Cl. D T
DATE: 9t/05/22 TTHE: 14:37
TEMPERATURE (C) DTA
Figure 9 DSC Plot for the Top Section of the Halar Control Sample Showing a Weak
Endotherm at 234 ° C and a Weak Exotherm at 449 ° C
412
U
UJ
U3
.-I
U
Z
235 C
&4_ F.FL_ _. DF
DATE: 9t/04/t5 TZNE: t4: i2
TEMPERATURE (C)
Figure 10 DSC Plot for the Second Section of the Halar Control Sample Showing a
Strong Endotherm at 235* C and a Strong Exotherm at 452 ° C
DTA
413
23g
O
-., 237
!
E
Figure 11
Melting Temperature by DSC
I-ialar In Am"
+
+
4-
<>
¢,
+
0
i i I i I I s a
200 400 600 BOO
calculo_l depth from Burfact (urn)
control Pc#2 _ exposed Pc#5
DSC Melting Endotherm Temperature as a Function of Section Depth for
Halar LDEF and Control Samples
414
10
Melting Enthalpy by DSC
Haiar in Ar Abno_ohem
u
o
tJ
Figure 12
9 m
8-
7-
6-
5-
4-
3-+
2-
1-
0
0
+
<>
+
I I I ! I I i I
200 400 600 800
calcutatld dopb_ from surface Cure)
control Pc#2 _> exposed Pc#5
DSC Melting Enthalpy as a Function of Section Depth for Halar LDEF and
Control Samples
415



Effects of orbital exposure on Halar during the LDEF mission

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