The retrieved LDEF had varying amounts of visible contamination films (brown stains) at many locations. FTIR spectra of heavy film deposits at vents and of optical windows from tray E5 indicated methyl silicone and silica in the contaminant films. Two possible sources of the methyl silicone are DC-710 phenyl methyl silicone in the shuttle-bay-liner beta cloth, and the shuttle tile waterproofing silane. It is concluded that much of the silicon and silica contamination came from ground operations and the orbiter

Harvey, Gale A.

English

NASA Technical Reports Server

N93 • 1 2777
SOURCES AND TRANSPORT OF SILICONE NVR
Gale A. Harvey
NASA Langley Research Center
Hampton, VA 23665-5225
Phone: 804-864-6742, FAX: 804-864-7790
_Z_
SUMMARY
The retrieved LDEFhad varying amounts of visible contamination films (brown
stains) at many locations. FTIR spectra of heavy film deposits at vents and of optical
windows from tray E5 indicated methyl silicone and silica in the contaminant films. Two
possible sources of the methyl silicone are DC-710 phenyl methyl silicone in the
shuttle-bay-liner beta cloth, and the shuttle tile waterproofing silane. It is concluded that
much of the silicone and silica contamination came from ground operations and the
orbiter.
INTRODUCTION
A brown stain of varying thickness was present on most of the retrieved LDEF
(ref. 1). Several analyses have indicated significant silicone and silica in these stains
(ref. 2). The source of the silicon, silica, or precursor silicones is not understood since
most of the exposed surfaces of LDEF were anodized aluminum of urethane based paints.
Organic silicones have strong absorptions in the 10 micrometer region of the spectrum.
FTIR spectroscopy has been applied to LDEF samples, suspected silicones, and spacecraft
facility witness plates at KSC in order to better understand the silicone contamination of
LDEF.
MEASUREMENT TECHNIQUE
Fourier transform infrared spectroscopy (FTIR) was used for the identification of
silicones and silica. The spectra are 4 cm "1 resolution and the spectrometer was
optimized for the 5 to 10 micrometer region. The sample spectra are ratioed to a
background spectrum to give transmission spectra. Sample residues are placed on IR
transmitting windows (i. e., CaF2, MgF2, NaC1) and centered in the IR beam at the beam
focus in the sample compartment. Additional information regarding FTIR spectroscopy
for contamination analysis is in reference 3.
Solvent-wash plates are used in aerospace cleanrooms to measure accumulation
of organic films. These witness plates are typically syringed with an aggressive solvent
such as freon, chloroform, or methylene chloride. This solvent is allowed to evaporate in
a fume hood and the residue is transferred to a weighing pan for mass measurement.
Organic films can also be removed from hardware or facility surfaces by wetting
the surfaces with a solvent and then wiping the surface with an extracted cleanroom
wipe (refs. 4, 5). The wipes are then extracted again by soaking for 30 minutes in
spectroscopic grade isopropyl alcohol (IPA), the alcohol is evaporated, and the organic
residue transferred to an IR window for measurement and analysis.
175
https://ntrs.nasa.gov/search.jsp?R=19930003589 2020-03-17T10:35:36+00:00Z
LDEF DATA
Some of the heaviest organic film deposits were on the ram direction edge of the
1/4 inch thick aluminum end plates (Figure 1). These end plates were at vent openings to
the interior of LDEF and hence the organic films result primarily from outgassing from
the interior. FTIR spectra of a scraping of film from an end plate is presented in figure 2.
The absorption at 1260 cm "1 is identified as resulting from methyl silicone (SiCH3). This
absorption is normally spectral sharp and stable in frequency, and hence is a convenient
and reliable indicator of a methy silicone group in a molecule.
Calcium fluoride and magnesium fluoride windows were flown in tray E5 of
LDEF. An IR spectrum of the calcium fluoride window is presented as figure 3. The
1260 cm "1 methyl silicone absorption is more pronounced here than in the vent scraping.
The optical windows in tray E5 were mounted with Chorlastic RS00 silicon rubber gaskets
on the back side. Microscope examination revealed the windows had thin, brittle
contamination films on both sides. However, the film on the center of the back surface of
the magnesium fluoride window (figure 4) adhered to the gasket. The IR spectrum of
only the front surface film is presented in figure 5. Again the 1260 cm "1 methyl silicone
absorption is present. The broad absorption at 1050 cm "1 suggests silica. The 1260 cm "1
absorption in the front surface film suggests an external source rather than an interior
source for much of the silicone.
The IR spectra of contamination on LDEF can be compared to spectra of typical
spacecraft and cleanroom organic films. Figure 6 is the IR spectra of residue from
Kapton multi-layer insulation which was used as a witness plate. Figure 7 is the IR
spectra of residue from the cleanroom wipes used during integration and processing of
the Upper Atmosphere Research Satellite (UARS). These spectra are dominated by
strong carbonyl absorption at 1750 cm "1 and are not similar to spectra of contamination
on LDEF.
Brown stains, similar to those on LDEF, have been reported in orbiter bays and on
flight hardware. Such a stain in the Columbia bay after the LDEF retrieval is reported in
reference 6. Photographs ofi_aint brown stains in the Discovery bay after STS-48 were
taken October 9, 1991, and are presented as figures 8 and 9. These stains suggest a
possible nonpayload source of contamination.
?
F
E
|
BETA CLOTH OIL
176
Beta cloth is a woven fabric used for thermal control of spacecraft and is used to
line much of the orbiters' bays. The fibers are about 10 micron diameter glass fibers
coated with about a 10 micron layer of Teflon. Silicone oil is usually added to the fabric
during manufacturing to enhance the mechanical properties (increase flexibility and
reduce particle shedding) of the fabric. The DC-704 and DC-710 have vapor pressures of
about 10 -7 Torr at room temperature (ref. 7 and 8). Although the vapor pressure is low,
all of this material is expected t0 outgas during long-term vacuum exposure. The
expected mode of contamination at standard temperature and pressure is by contact
transfer. The molecular structure for DC-704 (ref. 9) is presented as figure i0_ Some beta
clothshave as much as 100 mg/ft 2 of DC'710 extractable bysoaking the cloth for
30 minutes in IPA. The beta cloth used in the Discovery bay for STS-48 had 10 mg/ft 2 of
extractable silicone residue. The IR spectra of the residue from the Discovery bay liner
beta cloth is presented in figure 11. The SiCH3 absorption as well as the SiO absorptions
are characteristically sharp and well defined in figure 11.
z
RTV-142 _UE
RTV-142 is a silicone potting compound that was used on the UARS spacecraft and
is believed to be representative of silicone potting compounds used sparingly on LDEF.
Four measurements of NVR from RTV-142 were made. The four independent
measurements are mass loss during vacuum bake, residue from 30 minute soak in IPA,
mass spectroscopy via residual gas analyser, and baking in a vacuum gas cell.
The mass loss during a 24 hour bake at 70°C was --0.08 percent. The mass loss
during a 24 hour bake at 160°C was =0.23 percent. The recovered NVR from a 30 minute
soak in IPA was =0.5 percent. The IR spectra from the IPA soak is presented as
figure 12. These spectra and 3.4 micrometer spectra indicate the residue is a phenyl
methyl silicone. The RGA gives principal mass fragments of 15 (CH3), 29 (CHO & C2H5),
31, and 43. Spectra of residue in the heated gas cell were similar to those from the IPA
soak.
All of these tests indicated small mass loss of RTV-142 under vacuum. The
higher mass recovered from IPA soaking compared to a 24 hour bake indicates slow
outgassing.
PAYLOAD CHANGEOUT ROOM WASH PLATES
One foot square aluminum wash plates were exposed in the Payload Changeout
Room (launch complex 39 PCR) during processing of the UARS spacecraft. Two wash
plates were exposed during the period July 23 to August 13, 1991, near station 900 of the
orbiter in the PCR. The plates were syringed with CH2C12 and the residues weighed.
The residues were analyzed by the KSC Microchemical Analysis Branch (ref. 10). IR
spectra of a transfer of residue with hexane is presented as figures 13 and 14. The
spectra clearly show silicones, primarily dlmethyl silicones. IR spectra of transfers of
residue with CH2C12, a more aggressive solvent, show primarily carbonyl and C-O
absorption. The hexane transferred residues indicate a light or volatile silicone.
STS-48 RESIDUES
A 14-inch by 14-inch square of 5 mil Kapton multi-layer insulation was attached to
the UARS airborne support equipment (UASE) module during the STS-48 mission. The
IR spectra of residue from an IPA syringe of this witness plate is presented as figure 15.
Although only 0.01 mg of residue was recovered from this witness plate, SiCH3 and SiO
absorptions are strongly indicated. This witness plate was covered until August 13, 1991,
and was retrieved from the shuttle bay October 9, 1991. The delay in retrieving this
witness plate was due to the orbiter landing at Dryden rather than at KSC as planned,
and conflicts in orbiter operations in the orbiter processing facilities.
Four dry wipes of the UARS Airborne Support Equipment (UASE) toolbox,using
extractedpolyesterwipes (ref.4),were taken. IR spectraofa UASE dry wipe ispresented
as figure16. Siliconeabsorption isindicatedin the residue. However, some siliconeis
alsoindicatedin residualresidue ofthe controlwipe (fig.17) so caution should be
exercisedin interpretationof the UASE wipe data.
ORBITER TILEWATERPROOFING
The heat protective tile and upper surface external blankets of the orbiter are
waterproofed to avoid unnecessary water absorption prior to launch. The waterproofing
compound used for the STS-48 mission was dimethylethoxy silane (DMES). A diagram of
the molecular structure of this material is presented as figure 18, and an IR spectrum is
presented as figure 19. A strong Sill absorption is present at 2200 cm "1 and methyl and
methylene absorptions are also present at 2900 cm -1.
177
This silane compound is used because it chemically reacts with the silica in the
tiles to bond methyl silicones to the silica (ref. 11). About 200 pounds of DMES were used
to waterproof the orbiter for STS-48'. DMES is extremely volatile; i.e., its vapor
pressure is about 230 mm Hg at 20°C (ref. 12). The PCR wash plates suggest transfer of
waterproofing compound in orbiter processing facilities.
A few drops of DMES were transferred to CaF2 and NaC1 windows placed on a
deuterium lamp. A thin SiO2 film (figure 20) was left on both of these windows.
Therefore, DMES is a precursor to both methyl silicones (ref. 11) and silica.
DMES is not the silane used to rewaterproof the Challenger for the LDEF
deployment. Hexamethyldisilizane (HMDS) was used to rewaterproof the Challenger for
mission 41C.
* Palou, J., private comm., December 1991:
CONCLUSIONS
Several potential sources Of silicone contamination from-orbiter and spacecraft
processing at KSC have been identified. The most suspect source is the orbiter tile
waterproofing compound, a volatile silane. Further work is needed in order to better
understand the role of the waterproofing compound in the production of the silicones and
silica detected on LDEF.
REFERENCES
1. Harvey, G. A., "Organic Contamination of LDEF, " Proc of First LDEF Post-Retrieval
Data Conference, June 1991, NASA CP 3134, pp. 179-197, January 1992.
f
.
.
.
.
,
7. Hall, D. F., T. B. Stewart, and R. R. Hayes, "Photo-Enhanced Spacecraft
Contamination Deposition," AIAA 20th Thermophysics Conference, June 1985,
Williamsburg, VA, p. 1-9, AIAA-85-0953.
..... c ........ -- .......
8. Barrington, A. E., High Vacuum En_neerin_, Prentice-Hall, Inc., 1963.
Crutcher, E. R., and K. T. Warner, "Molecular Films Associated with LDEF," Proc.
of First LDEF Post-Retrieval Symposium, June 1991, NASA CP 3134,
January 1992.
Harvey, G. A., and J. L. Raper, "Halogen Occulation Experiment (HALOE) Optical
Witness-Plate Program," NASA TM 4081, February 1989.
Harvey, G. A', J. LI Raper, and D. C. Zellers, "Measuring Low-Level Nonvolatile
Residue Contamination on Wipes, Swabs, and Gloves," Microcontamination, 8 (11):
pp. 43-46, 69, 1990.
Raper, J. L., and G. A. Harvey, "Halogen Occulation Experiment (HALOE)
Contamination Control Program," Cleanroom Technology Forum Proceeding,
pp. 38-51 San Jose, CA, October 1991.
Maag, C. R., J. Houseman, and W. K. Linder, "Results of the Interim Operational
Contamination Monitor (IOCM) as Flown on the STS-32 Mission," JPL Report D-
8170, p. 150, March 1991.
178
9. Anon., Information about Silicone Fluids, Dow Corning, 1976.
10. Carman, W. R., "Analysis of NVR Samples STS-48 PCR Exposure," Laboratory
Request MCB-0721-91, KSC Microchemical Analysis, September 1991.
11. Johnson, R. E., and D. Ford, "Studies of Silane Stability to Humidity and
Temperature," LeTourneau University Research and Advanced Development
Institute, April, 1990.
12. Anon., "Material Safety Data Sheet for Dimethylethoxysilane," HULS America Inc.,
1992.
ACKNOWLEDGMENTS
Phil Klich (LaRC) and Sharon Straka (GSFC) assisted in obtaining HALOE and
UARS witness plates at KSC. Barbara Lambert (GSFC) provided photographs of the
Discovery bay. Jaime Palou (KSC) implemented the IR analysis of witness from the PCR.
Bill Carman (KSC) performed the IR analysis of the PCR residues and reviewed the
analysis in detail with the author. Tommy Leiffel (LaRC) provided samples of tile
waterproofing compound (DMES).
7
179
\J
u01sstwsueJi
o
"10
c
o
c c
g
_ E
t_
ID
i
iT
o
=
i
|
|
_i
180
t-
eD
q_
r-
E
c::
o
t"
e-
0
0
t-
O
e=
,e-
ID
_r
/
/-
.J
o
u0!ss!wsueJl
r
•- 0
O
_ E
_ 2
g
Q_
i
_r
o_
ORIGINAL PAGE
BLACK AND WH1TE PHOTOGRAP__
=
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
\
..... •=,...... _ .... ___ ; ...... _ _....
u0!ss LUSUejJ.
o=
o
*o
_6
1]
__o°
3
C_
Lo E
.O
CO
03
O3
E-
=_.
03
E
.S
(/}
r-
o
.O
E
"6
t-
.90
O
t-
Q.
LL
t
u0!sS!wSUeJl
°
o
¢m
E
O
01
¢_.
o
t_
= u01ss!tusueJ.L
03
J
O
O
u
E
i ,_o
t-
O
3
o° ¢z
- (/1
:3
E
==
181
u0jssjtusu_Jl
oo
le
o
oo
C
o >
oo
o _ o
uo!ss!Lusue_J.
_J
_r
I--
i?-
==
oi
"8
E
2
Q)
Q.
:D
182
Z
i
i-
e-
c
o \
-J I
u ',
r_
uJ
"0
_r
..J _j
0 Z
I uJ
O:r.O
Z ,- _I:)
o T
._ x_ _ "-7-
0 T,
_- W /"
o _ !2 _0
f" _ ...],l _
0
o
I
I
",.)
i
"r
I
_ --"2
I
"I"
_ >_
I,- J_ C
Q;_ O
e- ._ IIJ
_ _J _
_- O CXO
0
0
r-.
6
r_
0
2
_¢
0
LL

\-
$
\
u_
Thermal Control Coatings, Protective Coatings,
and Surface Treatments
Co-Chairmen: Ann Whitaker and Wayne Slemp
Recorder: Johnny Golden
185
!i:


Sources and transport of silicone NVR

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server