Solar ultraviolet testing was not developed which will provide highly accelerated (20 to 50X) exposures that correlate to flight test data. Additional studies are required to develop an exposure methodology which will assure that accelerated testing can be used for qualification of materials and coatings for long duration space flight. Some conclusions are listed: Solar UV radiation is present in all orbital environments; Solar UV does not change in flux with orbital altitude; UV radiation can degrade most coatings and polymeric films; Laboratory UV simulation methodology is needed for accelerated testing to 20 UV solar constants; Simulation of extreme UV (below 200 nm) is needed to evaluate requirements for EUV in solar simulation

Slemp, Wayne S.

English

NASA Technical Reports Server

ULTRAVIOLET RADIATION EFFECTS
Wayne S. Slemp
Materials Division
NASA Langley Research Center
Hampton, Virginia 23665-5225
N89- 2355 1
425
https://ntrs.nasa.gov/search.jsp?R=19890014180 2020-03-20T03:13:46+00:00Z
SOLAR SPECTRUM
The irradiance of the solar spectrum for air mass 0 is presented in the figure for the
wavelength range of 0.2 micrometers to 2.6 micrometers.
Irradiance,
mw/cm2um
240I
160
8O
0J
.2 .6 1.0 1.4 1.8 2.2
Wavelength, um
I
2.6
426
THE ELECTROMAGNETIC SPECTRUM
The electromagnetic spectrum from gamma rays to radio waves is represented in
the figure. The ultraviolet, visible, and near-infrared radiation found in the space
solar spectrum is only a small part of this electromagnetic spectrum. The UV
spectrum is divided into three parts--the vacuum or extreme UV below 200 nm, the
far UV from 200 nm to 300 nm, and the near UV from 300 nm to 400 nm. Nine
percent of the solar energy is found in the UV.
"7rays IX ra s I ultra infra
Y ;J violet red
visible
vac:um/1°02iOfa_iOnea4iO
uviOVlOVl
11 miCrOs ;aadvi°
700
waves
3000 nm
I- 90/0 _1
iTM 49% -, 98% r
427
ULTRAVIOLET ABSORPTION AND PHOTOCHEMICAL EFFECTS
The chemical changes resulting from exposure of a polymer to ultraviolet light are
illustrated in the figure. The first law states that only those radiations that are absorbed by a
material can produce a chemical change. If a material does not absorb the particular
wavelength of UV incident upon the material, then UV cannot cause a chemical change in
the material. The second law states that each molecule taking part in a reaction absorbs one
quantum (h_) of energy. If moles are substituted in the Stark-Einstein law, the Bohr law is
obtained. Since this equation is divided by wavelength, the smaller the UV wavelength,
the greater the UV energy.
1) Only those radiations that are absorbed by a material can produce
a chemical change (Grotthuss-Draper)
2) Energy absorbed by a reacting molecule is given by hX (Plank's
const, x freq. of absorbed light) (Stark and Einstein)
3) Change molecules to moles (Bohr Law)
Equation: E=Nh =Nhc/2
E = 2.86 x 10 5/_. k cal/mole
Examples:
N - Avagadro's No.
C - Velocity of light
_. - Wavelength, angstroms
-- 4000A the Einstein is 71 k cal/mole
-- 2537A the Einstein is 113 k cal/mole
428
TYPICAL VALUES OF BOND ENERGIES
The typical values of bond energies are shown in this table. This illustrates that several
chemical bonds can be broken by the 113 k cal/mole energy of the 2537A wavelength of
UV radiation. Since the solar radiation in space extends to wavelengths as low as 1000A,
most polymer bonds can be broken with UV radiation.
CHEMICAL BOND ENERGY
_K cal./mole, 25*)Bond Bond Energy Term E
C-C 82.6
C-C 145.8
C_C 199.6
C-N 72.8
C=N 147
CmN 212.6
C-O 85.5
C.,O aldehydes 176
C-O ketones 179
C-S 65
N-N 39
N=N I00
SI-O silicones i06? b
SAil values are deduced from aliphatic compounds and are
taken from T.L. Cottrell, "The Strengths of Chemical Bonds,"
Butterworths Scientific Publications, London, 1958, pp. 270-
275.
b? , doubtful value
429
ULTRAVIOLET RADIATION EFFECTS
Most organic molecules lie in a singlet ground state (RH). Absorption of a photon
raises the molecule to an excited singlet or triplet state (RH*). If the molecule has sufficient
energy in the excited stated, bond dissociation may take place (R. + H.). This
decomposition process must compete with other deexcitation processes. The excited
molecule may revert to the ground state by emission of heat or energy (hv) in the form of
fluorescence or phosphorescence. The later processes allow the excited molecule to return
to the ground state without producing a chemical change. Revision to the ground state may
also occur by the transfer of electronic energy from one group to another group in the
vicinity of the excited molecule. An example of this occurs in polymethylphenylsiloxane
where the phenyl group absorbs the UV energy then transfers this to the methyl group
where degradation occurs.
h_
RH v RH*
._R. + H.
RH* --
OTHER POSSIBLE REACTIONS
RH* -RH + HEAT
RH* _ RH + h_
M * + M ---_ M + H *
{FLUORESCENCE
PHOSPHORESCENCE}
{ELECTRONIC ENERGY
TRANSFER }
430
EFFECT OF ULTRAVIOLET RADIATION ON THE
TENSILE STRENGTH OF MYLAR
q'his figure illustrates that UV radiation can degrade the mechanical properties of
polymeric materials although the major research emphasis has been on changes in optical
properties of polymer films.
PERCENT
TENSILE
I00
80
60
40
2O
0
ORIGINAL
STRENGTH
1"-'"""-'-';_-,-_ /-ALUMINIZED
- _k MYLAR
_,TO0 BRITTLE
× TO TEST1 I l I I i
25 50 75 I00 125 150 175
HOURS IRRADIATION (BH-6 U.V. LAMP)
431
LEO SPACECRAFT THERMAL CONTROL ENVIRONMENT
The thermal control environment for a low Earth orbital satellite consists of the direct
solar radiation, the Earth albedo (sunlight reflected from clouds, terain, and water), and the
emitted radiation from the Earth. The absorptance of the spacecraft with its view factor to
each of these heat sources is balanced against the emission of heat from the spacecraft.
Direct
solar
Spacecraft
emission
Earth
emitted
Earth
Albedo
Earth
432
STRUCTURAL REFLECTANCE OF ZINC OXIDE-SILICONE
•This figure illustrates the change in spectral reflectance due to UV exposure in vacuum
for a zinc-oxide, pigmented silicone paint S-13. The figure also illustrates that upon
introduction of air (oxygen) into the vacuum system, bleaching occurs which eliminates the
UV degradation to this coating. This bleaching of white paints has led to the need for in situ
testing of spacecraft coatings.
IO0
90 S
z 80
k--
C) 70
w
d
u_
w 60
cr
w 50
>
_- 40
..J
w
a: 30
2O
0.2 04 0.6 0.8 I0 12 1.4 16 1.8 20 2.2 2.4 26
WAVELENGTH, MICRONS
433
LUNAR ORBITER IV
The radiator on the Lunar Orbiter spacecraft was coated with the S-13 paint before
in situ testing was found to be required. Lunar Orbiters I and II experienced very dramatic
temperature increases due to UV and solar wind plasma degradation of the coating. To
offset the increase in solar absorptance of the white paint, about 20 percent of the radiator
area on Lunar Orbiters IV and V were coated with quartz optical solar reflectors.
I
/
(3RIGINAL PAGE
BI_A,(_K.I ._IND WHITE P; ,r_-FO_RAP!-!
434
FLEXIBLE SECOND-SURFACE MIRROR (SSM) THERMAL CONTROL COATING
This figure is a schematic of a flexible second-surface mirror coating. The coating
consists of a polymeric film that is transparent in the solar wavelength region and is coated
on the back side with a reflecting, opaque metal like aluminum or silver. An adhesive is
placed behind the metal to hold the coating to the spacecraft. Typically, the larger the
thickness of the polymeric film, the stronger the absorption bands are in the infrared, out of
the solar wavelength region, and therefore, the higher the emittance of the coating. An
example of an SSM coating is silvered- perfluorinated ethylene propylene
copolymer (FEP) Teflon.
Sunlight
Thk_kness
(mils)
0.5 - 5.0
0.008
2.0
\
Heat
Transparent
Polymeric
Finn
Reflecting Metal
Adhesive
435
CHARACTERISTICS OF THERMAL CONTROL COATINGS
The figure helps to explain the radiation characteristics of the SSM coating in the
previous figure. The reflectance of this coating and the transparency of the polymeric
film must occur from 0.2 to 3.0 micrometers, the region of maximum solar energy. But
the coating is radiating heat away from a spacecraft which has a maximum temperature of
about 100°C; therefore, this energy is found in the infrared from 10 to 50 micrometers.
The characteristic absorption bands of polymers provide this infrared emittance.
Reflectance,
%
I,o #
6O
4O
20
Reflectance
/
/
/
I
0.2
L [ 1 1 _4- I 1 1 I
0 0.5 1.0 2.0 5 10 20 50
Wavelength, p m
- 1.0
- 0.8
- 0.6
- 0.4
- 0.2
0
Em_tance
436
NON-SPECULAR SILVERED TEFLON
The non-specular reflecting silvered Teflon SSM coating used on the Orbiter's radiators
was developed at NASA Langley. The FEP Teflon is embossed with a special roller to
provide light scattering on the metal coating side leaving the outside smooth to prevent
trapping contamination in the Teflon surface. The process reduces the specular reflectance
to about 15 percent but maintains the 0.09 solar absorptance of the smooth silvered Teflon
SSM.
1O0
80
60NONSPECULAR
REFLECTANCE,
PERCENT 40
20
EMBOSSED
• SMOOTH
2 6 10 14 18 22 26 30
WAVFLENGTH.MICROMETERS
ORIGINAL PAGE
BLACK AND WHITE PHO_
437
EFFECT OF UV RADIATION ON TRANSMISSION OF
TRANSPARENT POLYIMIDE FILMS
This figure illustrates some current studies being conducted at NASA Langley on
highly transparent polyimide films. These films are stable to about 300°C and are soluble
in the imide form, which means they are sprayable. Some of these experimental films have
exhibited high stability to simulated solar UV and high energy electron radiation.
Transmission,
percent
.;.oo
8o
6o
SONTROL F[LNS
(BEFORE
IRRADIATION)
\
200 300
100
6f-SbSO 2 \ __
oF-APB_,,';_ODPA-BDAF j
_DSDA APB' £1Y I "/ f 80
/U
4Q
/,,;,_ TON 20
400 500 6_0
WAVELENGTH,
200 300
UV IR;_JOIATION
30oo_n 6FBDAF
/ U,,: J
/"6F-DDSO)f
I
Soiur /"_XlmL£" (500 nlr)
400 5OO 600
WAVELENGTH, nm
438
ALUMINUM FOIL COATING FOR COMPOSITE TUBES
A concept for thermal control and atomic oxygen protection for the composite tubes
used as structural elements in the Space Station Freedom has been developed at NASA
Langley and demonstrated under contract with Boeing Aerospace Company. Aluminum
foil 0.008 cm thick is anodized or sputter coated with SiOx to achieve the desired radiation
properties of 0.3 solar absorptance and 0.65 emittance. This aluminum foil is then
adhesively bonded to the exterior of the graphite/epoxy tube. Process specifications have
been developed for achieving the radiation properties with chromic acid anodizing.
O
O
0
0
0
Can be anodized or sputter coated to achieve desired
optical properties
AppJication techniques can provide a non-specular
efiecting coating
Provides atomic oxygen protection for composite tubes
Re_>;[ant to abrasion and UV degradation
f_emonstrated on 2 inch dia, X 8 feet tong P75/934
>_raphite epoxy composite tubes
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
439
SPECULAR REFLECTANCE OF CHEMICALLY TREATED ALUMINUM
It is highly desirable to have a low solar absorptance coating on the composite structural
members which would not be a specular reflector. With the large number of structural
members in the Space Station Freedom, sunlight reflected from these members can interfere
with optical experiments on the Freedom. This figure shows that the chromic acid
anodizing process provides less than 5 percent specular reflectance at 0.5 micrometers, the
peak solar wavelength, where the sulfuric acid process has nearly 40 percent specular
reflectance.
Specular
reflectance
%
8O
6O
4O
2O
J
J
J
Bright finished AI (unanodized)
J
Sulfuric acid anodized AI
Chromic acid anodized AI
--t -_..... t....... T....... i....... i
0.4 0.5 0.6 0.7 0.8
Wavelength, micrometers
440
EFFECTS OF UV ON SOLAR ABSORPTANCE OF
SEALED CHROMIC-ACID ANODIZED 1145 AL 3 MIL FOIL
The chromic acid anodized aluminum foil used to coat the composite tubes for the
Space Station Freedom has been exposed to simulated solar ultraviolet radiation at NASA
Langley. These results show that the coating is very stable to UV radiation having only a
0.02 increase in solar absorptance in 4,000 equivalent solar hours (2,000 hours x 2 solar
constants).
2x ESH, air mass zero
.4-
Solar
absorptance
.3 _
,2 -
.1 --
0
) 0 0 0 0 0
I 1 I
1000 2000 3000
Exposure time, ESH
I
4000
441
SOLAR IRRADIANCE SIMULATION WITH XENON
This figure compares the spectral irradiance of a xenon short-arc lamp with a quartz
envelope to the solar irradiance at air mass zero. The figure clearly shows that xenon has a
good UV solar match from approximately 0.2 to 0.7 micrometers but is much more intense
in the infrared region. This IR radiation leads to over heating of test specimens when
accelerated exposure is attempted. Experimental results indicate that acceleration factors of
only 3X are possible without substantially overheating the test specimens.
Relative
spectral
irradiance
.25
.2O
.15
.10
.O5
'l't].
- li \ '-,
_ _ _.., "-,_,Xenon...
- )_ ,b°lar-/ _
II I ..... -.-,I
0 .5 1 1.5 2.0
Wavelength (micrometers)
442
PROBLEMS ASSOCIATED WITH SOLAR SIMULATION
When most investigators expose materials to simulated solar UV, the irradiance of the
UV beam is measured and referred to as a percentage of the solar irradiance at air mass 0.
Since many spacecraft rotate, then this must be taken into account in calculating one solar
constant. Also the shape factor for the spacecraft surface and the orbital parameters must
be considered. Many spacecraft will have only 25 percent of their time in orbit in the sun.
This would be a 4 to 1 acceleration factor even if the laboratory exposures were conducted
at one solar constant.
• One solar constant assumes nonrotating spacecraft in constant
sunlight, but most rotate
• Shape factor of 1 where, in reality, shape factor is near 0.5
• Near Earth orbit is approximately 90 min. with about 30 min. in
solar occult
• Reality is 25% to 40% of time in sunlight for spacecraft surfaces
443
TRUSS TUBE TEMPERATURES
This is a steady state calculation for a composite tube with an cue of .25/.25 in the
proposed Space Station Freedom orbit. This projects the thermal cycle range and shows
the typical occult for these conditions.
(R/E:",25"/.2_ )
0.6 O.B L
IME - (HOURS
444
COMPARISON OF FLIGHT AND LABORATORY DATA ON
ZINC OXIDE-POTASSIUM SILICATE COATING Z-93
This figure was prepared a number of years ago from flight and laboratory data
conducted using short arc xenon UV source and a 3 kev solar wind proton source with
thermal electrons for charge neutralization. The combined UV and solar wind plasma
experienced on Lunar Orbiter V was under simulated in the laboratory. The UV
degradation experience by OSD-III and Pegasus was over simulated in the laboratory test.
No changes in procedures or equipment which have been made since these tests were
conducted would alter these results.
CHANGE
IN SOLAR
ABSORPTANCE
Ao S
•12
.08
.04
0
_ .u._.OR.,.._v ' P.+_-,
-_
_.i.____- -.._._---- _EcAsus
I I I I I 1
0 400 800 1200 1600 2060
EQUIVALENT SOLAR-HOURS
445
SUMMARY
Solar ultraviolet testing has not been developed which will provide highly accelerated
(20 to 50X) exposures that correlate to flight test data. Additional studies are required to
develop an exposure methodology which will assure that accelerated testing can be used for
qualification of materials and coatings for long-duration space flight.
• Solar UV radiation is present in all orbital environments
• Solar UV does not change in flux with orbital altitude
• UV radiation can degrade most coatings and polymeric
films
• Laboratory UV simulation methodology is needed for
accelerated testing to 20 UV solar constants
• Simulation of extreme UV (below 200 nm) is needed to
evaluate requirements for EUV in solar simulation
446


Ultraviolet radiation effects

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