Many of the thermal control surfaces exposed to the space environment during the 5.8 year LDEF mission experienced changes in fluorescence. All of the thermal control coatings flown on LDEF experiments S0069 and A0114 were characterized for fluorescence under ambient conditions. Some of the black coatings, having protective overcoats, appear bright yellow under ultraviolet exposure. Urethane based coatings exhibited emission spectra shifts toward longer wavelengths in the visible range. Zinc oxide pigment based coatings experienced a quenching of fluorescence, while zinc orthotitanate pigment based and other ceramic type coatings had no measurable fluorescence

Gregory, John C.

Mell, Richard J.

Miller, Edgar R.

Peters, Palmer N.

Wilkes, Donald R.

Zwiener, James M.

English

NASA Technical Reports Server

N93 • 1 2781
FLUORESCENCE OF THERMAL CONTROL COATINGS ON S0069 AND A0114
James M. Zwiener, Richard J. Mell, Palmer N. Peters
NASA Marshall Space Flight Center
Huntsville, AL 35812
Donald R. Wilkes, Edgar R. Miller
AZ Technology
Huntsville, AL 35801
John C. Gregory
University of Alabama/Huntsville
Huntsville, AL 35899
INTRODUCTION
Many of the thermal control surfaces exposed to the space
environment during the 5.8 year LDEF mission experienced changes
in fluorescence. All of the thermal control coatings flown on
LDEF experiments $0069 and A0114 were characterized for fluores-
cence under ambient conditions. Some of the black coatings,
having protective overcoats, appear bright yellow under ultravio-
let exposure. Urethane based coatings exhibited emission spectra
shifts toward longer wavelengths in the visible range. Zinc
oxide pigment based coatings experienced a quenching of fluores-
cence, while zinc orthotitanate pigment based and other ceramic
type coatings had no measurable fluorescence.
233
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C.ATECOR_.-ES OR TYPES OF FLUORESCENCE EFFECTS OBSERVED
The specific fluorescence effects observed on the
experiments can be divided into three categories as outlined in
figure I. Urethane binder type coatings including the black
Z302 and the white A276 experienced similar shifts of fluores-
cence from the near ultraviolet toward the visible range, zinc
oxide pigmented coatings, using either the silicone or silicate
binders, demonstrated the same quenching of original (pre-
flight) fluorescence. Silver Teflon did not originally fluo-
resce, but now shows a weak, but measurable, fluorescence in the
visible.
234
z
o
Q
URETHANE BINDER TYPE COATINGS
"" BLACK COATINGS
','-WHITE COATINGS
ZINC OXIDE PIGMENTED COATINGS
•"SILICONE & SILICATE TYPE BINDERS
',-WHITE COATINGS
SILVER TEFLON COATINGS
"ACRYLIC ADHESIVE EFFECT
Figure 1. Three Categories of Fluorescence Effects Observed.
Note: Teflon is a trademark of Dupont.
PHOTOGRAPH OF THE VISUAL FLUORESCENCE OF S0069 SAMPLES
A pronounced visual demonstration of the post-flight fluo-
rescence glow of the urethane type paints with protective atomic
oxygen overcoats is provided in figure 2. Photographs were made
using either white or ultraviolet lighting. Black Z302/01650,
under ultraviolet lighting, shows a bright visible yellow fluo-
rescence. Even the white A276/O1650 shows a bright yellow
fluorescence under ultraviolet lighting. The other samples lack
sufficiently pronounced visible fluorescence for normal photo-
graphic observation.
ORIGINAL PAGE
BLACK AND WH1TE PHOTOGRAPH
White Light
r
Ultraviolet Light
Figure 2. Fluorescence of Thermal Control Coatings Comparison
of Samples Under White and Ultraviolet Light.
235
MF_SUR_m_T SETUP
Spectral measurements of the fluorescence of the samples
from both the S0069 and A0114 were made using the instrumentation
setup shown in figure 3. Monochromatic irradiation of the
samples was provided using a mercury/xenon high pressure dis-
charge source and attached prism monochr0ma£or, An excitation
wavelength centered at 280 nm was used for all measurements
described in this paper. All measurements utilized a test con-
trol sample of Mg0 to setup and verify:consistent system re-
sponse. In addition, sample controls were run for comparison.
:_ : i¸
DK.2A
SPECTROPHOTOMETER
4-
f 4_
FLUORESCENCE
SOURCEMONOCHROMATER
=28Onto
TESTSAMPLE
SPECULARREFLECTANCE
Figure 3. Schematic of Fluorescence Measurement.
236
FLUORESC_ SPl_-w/q_ OF Z302
A typical example of the measured spectral fluorescence of
the black Z302 samples is provided in figure 4. At about 280nm
the scattered signal of the incident excitation light is
recorded. An increase in the 280nm data indicates an increase in
scatter or a decrease in absorptance, whereas a decrease could be
attributed to an increase in the absorptance of the coating in
the wavelength region. Since the unprotected Z302 is eroded by
atomic oxygen, this increase over the ground control is most
likely caused by a surface roughening. Note, this sample was
exposed for only the I .6 years and was still black. The Z302
sample exposed for the full 5.8 years was eroded down to the base
primer, as can be seen in figure 2 (fourth sample from left, on
outer row). The ground control sample shows a weak but measura-
ble signal in the 400 to 500 nm wavelength range. In comparison
the flight sample shows a shift of fluorescence into the visible
region.
Thermal Control Surfaces Experiment $0069
Fluorescence Spectra of Z302
Relative Inton_i_y
0.5
350 400 450 500 550 600
WavetenQth in Nanometer8
650 700
Figure 4. Fluorescence Spectra of Z302.
237
FLUORESCENCE SPECTRA OF Z302 WITH OI650 OVERCOAT
The brightest visual fluorescence was observed for the OI650
overcoated urethane based paint samples, as was shown in Figure
2. An example of the measured emission spectra is provided by
the scan in Figure 5. It is interesting to note that the fluo-
rescenceof the Z302 and the 01650 are relatively distinct for the
stored control sample; whereas, after flight exposure this dis
tinction is not obvious. As compared to the Z302 sampie, the
Z302/OI650 emission shifts toward the visible region, But is
considerably stronger. Although the data is not corrected for
variations in instrumentation spectral response, the relative
response between different scans at the same wavelength are
comparable. A reference control was utilized to calibrate and
maintain consistent total system response.
Thermal Control Surfaces Experiment S0069
Fluorescence Spectra of Z302 with 01650 Overcoat
Relative Intensit_
0.5
0.4
0.3
0.2
0.1
Z302 with 01650 overcoat
Control
Flight P20 - 5.8 yrs exposure
l/_" " ContPol ..... _ Flight/5.8 y r,
0
250 300 350 400 450 500 550 600 650 700
WavelenQth in Nanometers
TRIAL20
i .
238
Figure 5. Fluorescence Spectra of Z302 with OI650 Overcoat.
Z93 WHITE PAINT VISIBLE FLUORESCENCE EFFECT
Photographs of Z93 samples from experiment A0114, under both
visible and ultraviolet light (Figure 6), clearly show the quench-
ing or reduction of the observed fluorescence emissions. These
samples had covers that exposed only half of the surface. In
white light, the exposed area is difficult to discern, whereas
under the ultraviolet light it becomes very clear which area was
exposed. Also note that the ram or leading edge sample and
trailing edge sample experienced the same quenching of fluores-
cence. Comparison of the exposed to covered sample areas
provides a good visible demonstration of the reflectance stabili-
ty of this material to the low earth orbit space environment for
extended periods.
COVERED EXPOSED COVERED EXPOSED
WHITE LIGHT ::_
LEADING EDGE TI qG EDGE
ULTRAVIOLET LIGHT
COVERED COVERED EXPOSED
White Light and Fluorescence of Z93
Samples from Experiment A0114
Figure 6. White Light and Ultraviolet Light Photographs of Z93.
239
FLUORESCENCE SPECTRA OF Z93 WHITE PAINT
f
!
Significant quenching of the fluorescence of the Z93 white
paint occurred within the first 1.6 years of 0n-orbit exposure.
Additional quenching occurred with continued exposure as shown in
the fluorescence spectra in Figure 7. S13G/LO also showed a
similar quenching of fluorescence. Both of these coatings are
based on a ZnO pigment, but have different binders. Z93 has a
silicate binder, whereas S13G/LO utilizes a silicone binder.
Previous work reported by Zerlaut and Harada at IITRI (ref. I)
observed a decrease of fluorescence in the zinc oxide material
after ultraviolet irradiation in vacuum. The original fluores-
cence was attributed to "interstitial zinc atoms or other crystal
imperfections," with the decrease attributed to a "s%abilization
or approach of stoichiometry" after ultraviolet irradiation expo-
sure.
Thermal Control Surfaces Experiment S0069
Fluorescence Spectra of Z93
Relative Intensity
0.8
0.6
0.4
0.2
,_5.8 yrs
Flight/1.6 yrs
Control
Flight P5 - 1.6 yrs exposure
Flight P6 - 5.8 yrs exposure
// \ Control
/ \\ /
. / . \ light/1.6 yrs
ht/5.8 yr=
0
250
TRIALIO
300 350 400 450 500 550
Wavelenati% in Nonometer_
600 650 700
Figure 7. Fluorescence Spectra of Z93.
=
240 Lt
IXi i
|
SILVER TEFLON SURFACE ON S0069 DURING ON-ORBIT _VERY
The first images returned of the front surface of experiment
S0069 were similar to the on-orbit photograph shown in Figure 8.
Originally the silver Teflon had the normal, specular mirror-like
surface, but, as seen in the photograph, it has turned a diffuse
whitish color with brown streaks. As reported previously, these
brown streaks are caused by cracks in the silver/inconel layer
which permits the adhesive (or components) to migrate between the
Teflon/silver interface. After exposure to the space environ-
ment, mainly solar ultraviolet irradiation, the adhesive degrades
to the observed brownish color. Note that the silver Teflon
covered by the side panels still has the original mirr0r-like
specular appearance.
Close Up View
Figure 8. Close Up View of S0069 During Recovery.
241
FLUORESCENCE OF SILVER TEFLOH SUP_ACE _ S0069
Several samples were cut from the front cover of S0069 in
Figure 8 to determine if fluorescence could be detected. As can
be seen in Figure 9, a weak but measurable fluorescence was
obtained. This fluorescence is considerably less than the scat-
tered light level, so that it cannot be detected during normal
visual inspection with an ultraviolet light.
Thermal Control Surfaces Experiment S0069
k _ , Fluorescence Spectra of Silver Teflon
Relative Intensity
1
o8 .......
0.6
0.4
02
l
t
Sample
[
Top Cover T13A
Sample
J
...... l ................................. i ................................. _ .................. _ .........
(
i
!
0 J
250 300 350 400 450 500 550 600 650 700
Wavelenath in Nanomgters
COVER2
L . . _
242
u
Figure 9. Fluorescence Spectra of Silver Teflon on S0069 After
Recovery.
!
_=_
B
i=
I|
L_
FLtK)RESCEN(_ OF SILVER TEFLONADHESIVE 3N(966)
Samples of the acrylic adhesive used to bond silver Teflon
to the S0069 front cover were exposed to simulated solar ultravi-
olet radiation for various times. These samples, including a
control, were measured to see if they fluoresced and to determine
the change, if any, from irradiation exposure. As can be seen
from the data in Figure 10, not only did the original adhesive
fluoresce, but after irradiation the emission shifts to the
visual region, similar to what was observed on the flight materi-
al. As can be seen by comparing the emission spectra of Figures
9 and 10, the fluorescence of the ground sample is considerably
stronger than the flight silver Teflon material. This can be
attributed to several factors: the adhesive on the ground samples
is totally exposed, while the flight samples have only very
little surface area of the adhesive exposed along the
silver/inconel cracks. In addition, the ground samples have only
been exposed in air (no long term vacuum exposure) and were not
covered with Teflon which could attenuate the signal. Further
testing is under way to more accurately simulate the flight
conditions.
Fluorescence Spectra of 3M Adhesive (966)
Used on LDEF S0069 Silver Teflon
Relative Intensity
0.5
Control
Control
Exposed: (1KW Mercury-Xenon Lamp)
60 MIn
MIn
15 MIn
5 MIn
0
250 300 350 400 450 500 550 600 650 700
.Wavelen,qlh in Nanometers
Figure 10. Fluorescence Spectra of 3M Adhesive (966) Used to
Attach Silver Teflon to S0069 Front Cover.
243
SUIWIRRY OF FINDINGS
Fluorescence was detected on all thermal control surfaces
flown and exposed to the space environment on S0069 and A0114
except the white Tedlar* and black ceramic paint D111 samples.
In some cases the fluorescence was extremely weak as for the YB71
ceramic white paint using a zinc orthotitanate pigment. In other
cases, the fluorescence was very striking, changing to the bright
yellow emission under ultraviolet lighting. This change is
similar to that shown by the black urethane, silicone overcoated
Z30_ samples.
The overall change in fluorescence emission characteristics
can be classified into three types. Urethane based pa!Dts showed
a shift in fluorescence from the near ultraviolet region toward
the visible, while the zinc oxide pigment based paints exhibit a
quenching of their fluorescence emission. In contrast, the
silver Teflon material which does not itself show any measurable
fluorescence, does exhibit a measurable fluorescence after recov-
ery from the LDEF mission. This appears to be caused by the
acrylic adhesive in the silver/inconel cracks.
Studies to fully document the fluorescence observed on
experiments S0069 and A0114 are continuing, and will be reported
in future papers.
* Tedlar is a trademark of Dupont.
REFERENCE
I. Zerlaut, Gene A. and Harada, Y: Stable White Coatings,
Subcontract under NAS7-100, contract 950111, Interim Report No.
IITRI-C207-27, January 9, 1964.
244
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Fluorescence of thermal control coatings on S0069 and A0114

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