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Effects of Radiation and Thermal Cycling
on Teflon® FEP
Surfaces of the aluminized Teflon® FEP (fluorinated
ethylene propylene) multilayer thermal insulation on the Hubble Space
Telescope (HST) were found to be cracked and curled in some areas at
the time of the second servicing mission (SM2) in February 1997, 6.8
years after HST was deployed into low Earth orbit (LEO). In an effort
to understand what elements of
the space environment might cause such damage, pristine second-surface
aluminized Teflon® FEP was tested for durability to various
types of radiation, to thermal cycling and to radiation followed by
thermal cycling. Types
of radiation included synchrotron vacuum ultraviolet and soft x-ray
radiation, electrons and protons. Thermal cycling was conducted in
various temperature ranges to simulate HST orbital conditions for Teflon®
FEP. Results of tensile testing of the exposed specimens showed that
exposure to high fluences of radiation caused degradation in tensile
properties of FEP. However, exposure to radiation alone in exposures
comparable to those experienced
by HST did not produce reduction in ultimate tensile strength and
elongation of Teflon® similar to that observed for
HST-retrieved aluminized Teflon®. Synergism of radiation
exposure and thermal cycling
was evident in the results of three experiments: thermal cycling
following
electron and proton irradiation, thermal cycling following x-ray
exposure,
and additional thermal cycling of a sample retrieved from HST. However,
irradiation and thermal cycling with comparable HST SM2 exposure
conditions
did not produce the degradation observed in the FEP material retrieved
during
HST SM2.
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Evaluation of Low Earth Orbit
Environmental Effects on International Space Station Thermal Control
Materials
Samples of International Space Station (ISS) thermal control coatings
were exposed to simulate low Earth orbit (LEO) environmental conditions
to determine effects on optical properties. In one test, samples of the
white
paint coating Z-93P were coated with outgassed products from Tefzel®
(ethylene tetrafluoroethylene copolymer) power cable insulation as may
occur on ISS. These samples were then exposed, along with an
uncontaminated Z-93P witness sample, to vacuum ultraviolet (VUV)
radiation to determine solar absorptance degradation. The Z-93P samples
coated with Tefzel® outgassing products experienced greater
increases in solar absorptance than witness samples not coated with
Tefzel® outgassing products. In another test, samples of
second surface silvered Teflon® FEP (fluorinated ethylene
propylene), SiOx (where x(2)-coated silvered Teflon®
FEP, and Z-93P witness samples were exposed to the combined
environments
of atomic oxygen and VUV radiation to determine optical properties
changes
due to these simulated ISS environmental effects. This test verified
the
durability of these materials in the absence of contaminants.
Thermal Cycling-Caused Degradation of
Hubble Space Telescope Aluminized FEP Thermal Insulation
The Hubble Space Telescope (HST) was launched in April of 1990 and
was visited during service missions in December of 1993 and February of
1997. During the latter servicing mission, astronauts observed that the
top layer of multi-layer insulation, which consisted of second surface
aluminized FEP Teflon®, has occasional tears in its 0.127 mm
thick outer layer. A sample was retrieved which had torn and rolled up
under its own stress such that the aluminized layer was on the exposed
surface. The sample was found to have an increase in solar absorptance
and has multiple cracks in the aluminization in a mud-tile
configuration.
Tests conducted in a ground laboratory high-rate thermal cycling system
indicate that a signification portion of the observed increase in solar
absorptance may have been caused by cracks in the fatigued aluminum as
a result of approximately 40.000 thermal cycles it received in space. |
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