Teflon fluorinated ethylene propylene (FEP) (DuPont) is
commonly used on exterior spacecraft surfaces for thermal control
in the low-Earth orbit environment. Silverized or aluminized Teflon
FEP is used for the outer layers of the thermal control blanket
because of its high reflectance, low solar absorptance, and high
thermal emittance. Teflon FEP is also desirable because, compared
with other spacecraft polymers (such as Kapton), it has relatively
high resistance to atomic oxygen erosion. Because of its comparably
low atomic oxygen erosion yield, Teflon FEP has been used unprotected
in the space environment.
Recent, long-term space exposures, such as on the Long Duration
Exposure Facility (LDEF, 5.8 years in space) and the Hubble Space
Telescope (after 3.6 years in space), have provided evidence of
low-Earth orbit environmental degradation of Teflon FEP. These
exposures provide unique opportunities for studying environmental
degradation because of their long durations and different conditions
(such as differences in altitude). Samples of Teflon FEP from
LDEF and the Hubble Space Telescope (retrieved during its first
servicing mission) were evaluated for solar-induced embrittlement
and for synergistic effects of solar degradation and atomic oxygen.
Surface hardness measurements were obtained with unique Nano Indenter
(Nano Instruments, Inc., Oak Ridge, Tennessee) techniques for
polymers, which can measure hardness versus depth. Samples were
bend tested to induce surface cracking, and then the bend-tested
samples were cross-sectioned to determine crack depth. Tensile
testing was conducted on Hubble samples and compared with LDEF
data. Surface morphologies of Hubble and LDEF Teflon FEP samples
were compared by using scanning electron microscopy and atomic
force microscopy.
Nano Indenter results indicate that the surface hardness increased
as the ratio of atomic oxygen fluence to solar exposure fluence
(in equivalent Sun hours) decreased for the LDEF samples. This
occurred because the atomic oxygen eroded away part, or all, of
the solar-embrittled layer. Teflon FEP multilayer insulation retrieved
from Hubble provided evidence of severe embrittlement on solar-facing
surfaces. Some areas were cracked through the thickness of the
5-mil film. Nano Indenter measurements indicated higher surface
hardness values for these samples. Cracks induced during bend
testing were significantly deeper for the Hubble samples with
highest solar exposure than for LDEF samples with similar atomic
oxygen/solar exposure.
These results underscore the need to conduct further studies and the necessity to consider causes for Teflon FEP embrittlement in addition to direct atomic oxygen/solar exposure, such as the possible role of soft x-ray radiation, which is dependent on solar flares. Teflon FEP that was exposed to soft x-rays in a ground test facility showed similar embrittlement, which indicates that the observed differences between LDEF and Hubble Teflon might be due to varying soft x-ray fluences during these two missions.
Previous articleLast updated April 30, 1997
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