Polymeric materials in low earth orbit will be exposed to a harmful environment mainly due to atomic oxygen and ultraviolet radiation. In geosynchronous earth orbit, the major hazards to such materials are energetic charged particles and ultraviolet radiation. The progress of studies on the effects of these hazards on a polyetherimide, a polyimide, and an epoxy adhesive is presented

Kiefer, Richard L.

Orwoll, Robert A.

English

NASA Technical Reports Server

SEMI-ANNUAL STATUS REPORT
NASA Cooperative Agreement NCC-I-126
THE EFFECTS OF THE INTERACTION
OF POLYMERIC MATERIALS WITH THE SPACE ENVIRONMENT
January i, 1991 to June 15, 1991 <
Principal Investigators:
Grantee Institution:
RICHARD L. KIEFER
ROBERT A. ORWOLL
THE COLLEGEOF WILLIAM AND MARY
WILLIAMSBURG, VIRGINIA 23185
(_!ASA-C_-ILiS659) THE _-FffCTS OF THF
INT!£RACTI!-3N C_F Pi3LYMCRIC MATERIALS WITH TH_
SPACi! _',;!V!i<_]_!:'_::_T Semi_annu_]l Status i_eport,
I J_n. - i::_ aun. I'_91 (Col|eqe of Wi] | iam
and _-,!_.}:_ry) 2F3 i) CSCL lie G3/27
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https://ntrs.nasa.gov/search.jsp?R=19910018017 2020-03-19T16:37:47+00:00Z
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PRE'CEDIHG PAGE BLA_'_K NOT FILMED
THE EFFECTS OF THE INTERACTION
OF POLYMERIC MATERIAI_ WITH THE SPACE ENVIRONMENT
Introduction
Polymeric materials in low earth orbit (LEO, 300 to 500 km) will be
exposed to a harmful enviroriment mainly due to atomic oxygen and ultraviolet
radiation. In geosynchronous earth orbit (GEO, 36,000 km), the major
hazards to such materials are energetic charged particles and ultraviolet
radiation. We report here theprogress of our studies on the effects of
these hazards on three materials, a polyetherimide, a polyimide, and an
epoxy adhesive.
Atomic oxygen (AO) is formed from the dissociation of molecular oxygen,
02, and ozone, 03, by ultraviolet radiation of short wavelength (< 200 nm)
The A0 concentration varies with the latitude, solar cycle, season of the
year, and local solar time. It is the predominant chemical species at LEO
8 9 _3
altitudes, varying in concentration from I0 -I0 atoms cm at 300 km to
6 8 _3
I0 -i0 atoms cm at 500 km. The particle density is multiplied by the
speed of a satellite to give the particle flux incident on the forward
_I
facing surfaces. A satellite in LEO with a typical speed of 8 km s will
14 15 _2 _1
undergo I0 - i0 collisions cm s of A0 with its forward facing
surfaces. Atomic oxygen is known to cause surface erosion on polymeric
materials.
Ultraviolet radiation in both LEO and GEO has an energy flux of 0.14
_2 _i
joules cm s In GEO, an additional radiation environment is caused by
energetic charged particles trapped by the earth's magnetic field. The
radiation consists primarily of electrons and protons, each with a flux of
8 _2 _I
I0 particles cm s Polymeric materials subjected to ultraviolet
3radiation and/or energetic charged particles are known to suffer
degradation ofproperties due to both chain sission and crosslinking.
a
Polyetherimide Studies
Films of a polyetherimide containing up to 25 percent by weight of bis-
(triphenyltin) oxide (BTO) were prepared by evaporation of the solvent from
a solution containing both materials as solutes. The repeat unit of the
polyetherimide and the structural formula of BTO are shown in Figs. I and 2.
The polymer was obtained commercially and is sold under the General Electric
tradename of Ultem. The effects of atomic oxygen on these films were
simulated in the oxygen atmosphere of a radio-frequency glow-discharge
chamber. Typical results are shown in Fig. 3, where the degradation rates
of polyetherimide films with and without 20% BTO are plotted as a function
of exposure time. After about 90 minutes of exposure, a layer of tin oxide
has formed on the surface of the film and after 200 minutes of exposure, the
erosion rate drops to approximately two thirds that of the undoped polymer.
(Fifty minutes in the discharge chamber corresponds to approximately I month
in orbit at 500 km.)
Films of the polyetherimide containing 10% BTO were exposed to either
I00 keV electrons or ultraviolet radiation. The difference in the infrared
absorbance spectrum of a film taken before and after exposure is plotted in
Fig. 4 as a function of wavenumber. Exposure to ultraviolet radiation for
170 hours at 1.5 suns produces no change in the infrared spectrum of the
9
film. However, exposure to 5 x i0 rads of I00 keV electrons produces a
noticable change at 1750 wavenumbers corresponding to a decrease in the
concentration of carbonyl groups (C=O) after exposure. Similar differences
in the infrared spectra were obtained when films of the pure polyetherimide
were exposed to energetic electrons or ultraviolet radiation. The
uv/visible spectrum was also taken for films exposed to these radiations.
Figures 5, 6, 7, and 8 show the spectra before and after irradiation. It is
important to note that the transparency in the visible region (400 to 700
4nm) is the same for the pure polyetherimide and films of the material doped
with 10% BT0.
These results show that, when BT0 is added to the polyetherimide, the
rate of erosion with atomic oxygen is reduced significantly over a period of
I0 hours. However, the BT0 additive does not reduce the resistance of the
polyetherimide to ultraviolet radiation, nor does it change the effects due
to exposure to energetic electrons.
Since the polyetherimide is readily soluble in several common solvents,
we studied the possibility of spraying a solution of the BT0 doped material
onto surfaces as an atomic-oxygen resistant film. A chloroform solution of
the polyetherimide and BTO was placed in a small sprayer (Sure Shot Model C,
Milwaukee Sprayer Mfg. Co.). The sprayer was pressurized to about 90 psi
with compressed nitrogen gas, and a thin, uniform film was applied to a
glass plate. The film was dried in an oven and removed from the plate with
water. The film was very thin and fragile making it extremely difficult to
study, however, one sample was exposed to ultraviolet radiation. The
infrared and uv/visible spectra were taken before and after. The sprayed
film showed the same resistance to ultraviolet radiation as the thicker
films.
Polyimide Studies
Studies have been initiated on a polyimid e formed by the reaction of
pyromellitic dianhydride and di-(4-aminophenyl) ether and sold commercially
i
with the duPont trade name Kapton. Typical of most polyimides, Kapton is
infusable and insoluble. Our purpose here was to introduce A0-resistant
additives during the synthesis at a point where the additive is easily mixed
with the soluble polyimide precursor. The same compounds tested with the
polyetherimide were tested here. It was found that five compounds were
soluble: octaphenylcyclotetrasiloxane (0PCTS), octamethylcyclotetrasiloxane
(OMCTS),decamethyltetrasiloxane (DMTS), diphenylmethylsilane (DPMS), and
bis-(triphenyltin) oxide (BTO).
Films of the polyamic acid were formed on a clean glass plate with a
doctor blade according to established procedure. The film was then
2thermally imidized with a temperature ramp given in the literature. All
films produced in this fashion were clear and of good quality. Films doped
with BTOwere madewith concentrations of 5_, 3_, and I_ (w/w) BTO. It was
found that the films containing 5_ BTOwere brittle and could not be pulled
from the glass plate, however, films containing the lower concentrations
were more flexible and could be separated easily from the plate. Two
thicknesses of films were made, 0.5-1.0 mil and 2.5-3.0 mil. In addition,
thin films (0.5-1.0 mil) containing 3_ (w/w) OPCTSwere also made
Someinitial atomic oxygen exposures have been performed with the
synthesized films both pure and doped with BTO. Exposure periods of ninety
minutes were chosen so that comparisons with previous studies on the
polyetherimide described above could be made. A polyimide film doped with
i_ BTOcompletely disappeared during the first ninety minute period, while a
pure polyimide film did not disappear until the second ninety minute period.
In an exposure with the pure synthesized film and pure commercial film, it
was found that the synthesized film eroded muchfaster. This result is
similar to our findings in previous studies with polyetherimide and
polysulfone films. We beleive that the surface characteristics are
different between our films drawn on a glass plate and the commercial films
which are extruded.
Epoxy Adhesive Studies
Our work has focused on an epoxy adhesive manufactured by Dexter-Hysol
and sold as EA934NA. Previous studies had shown that this material had very
little resistance to ultraviolet radiation. Thus, it was a good material in
which to test various ultraviolet absorbers. Ironically, the material was
muchmore resistant to atomic oxygen than any other epoxy material tested.
The ultraviolet radiation studies were carried out in a small vacuum
chamber at the NASALangley Research Center. The chambercan be evacuated
_8
to a pressure of I0 torr. An Inficon IQ 200 mass spectrometer is
incorporated in the chamber so that volatile products can be monitored.
Ultraviolet radiation enters the chamber through a quartz window and is
supplied by an Oriel 6141 system with a i000 watt xenon-arc lamp. The
intensity of radiation at the surface of the sample is approximately 1.5
suns.
Samples of pure EA934NA immediately decomposed when exposed to
ultraviolet radiation. So many volatile products were emitted that within
30 seconds the pressure in the vacuum chamber rose by two orders of
magnitude. Large peaks appeared in the mass spectrum at mass numbers 18,
28, and 44. After about I0 minutes, the rate of emission of volatile
products decreased and the pressure in the vacuum chamber went down. Since
the composition of the material is proprietary information, no attempt was
made to suggest any reaction mechanism.
Samples of the epoxy material were made incorporating two different
ultraviolet absorbing compounds, benzophenone and nickel acetylacetonate.
In both cases, i0% (w/w) samples were made. When exposed to ultraviolet
radiation, these samples decomposed more rapidly than the pure material. In
J
both cases, additional peaks appeared in the mass spectrum which could be
correlated with decomposition products of the added compounds. Possibly,
the added materials accumulated at the surface during the cure of the epoxy
adhesive. These materials would probably degrade more quickly than the
epoxy adhesive thus causing the increase in volatile products.
References
I. C. E. Sroog, A. L. Endrey, S. V. Abramo, C. E. Berr, W. M. Edwards, and
K. L. Olivier, _. PolM. Sci., Part A, 3, 1373 (1965).
2. F. W. Harris in Polyimides, eds. D. Wilson, H. D. Stenzenberger, and P.
M. Hergenrother, pp. 26-29, Chapman and Hall, Glasgow (1990).
O
II •
• (_ CH3
REPEAT UNIT OF THE POLYETHERIMIDE
Figure I. The repeat unit of-the polyetherimide.
BTO
Figure 2. The structural formula of bis(triphenyl tin) oxide or BTO.
COMPARISON OF RATES OF EROSION IN
ATOMIC OXYGEN
Mass Loss Rate
(# grams/cm2/min)
6-
4-
2-
m
without additive
0 "_.
°
I I0 I0 2oo 8o0
with 25% BTO
I I I I
400 6O0
Exposure to Oxygen Plasma (min)
Figure 3. Erosion rates of films in atomic oxygen. A comparison of
polyetherimide films pure and with 20_ BTO.
i0
Change in Absorbance
Baseline- Exposed
EXPOSURES OF POLYETHERIMIDE / BTO FILMS
(10% BTO)
100 keY electrons.
5 x 109Rads
UV 170 hr at 1.5 suns
2000
I
1500
Wavenumbers (cm -1)
1000
Figure 4. The difference in the infrared absorbance spectra of
polyetherimide/lO_ BTO films taken before and after exposure
to i00 keV electrons and ultraviolet radiation.
11
100
BASELINE
PURE POLYETHERIMIDE
170 HOUR UV EXPOSURE AT 1.5 SUNS
I I I
UVIRRADIATED
_T 50
40-
:to
0
325 350 375 400 425 450 475 500
WAVELENGTH(NI4)
i
525 550 575
Figure 5. The uv/visible .spectra of a pure polyetherimide film before and
after exposure to ultraviolet radiation.
12
iO0
POLYETHERIMIDE WITH . I0%
t70 HOUR UV EXPOSURE AT t.5 SUNS
I t I I I I I I
BTO
UV IRRADIATED
BASELINE
%T 50
40
3O
20
0
325 350 375 . 400 500 525 550 575425 450 475
, WAVELENGTH(NM)
Figure 6. The uv/visible spectra of a polyetherimide film with 10% BTO
before and after exposure to ultraviolet radiation.
13
t
PURE POLYETHERINIDE
5000 MRAO EXPOSURE TO IOO KEV ELECTRONS
! ! 1 ) ! I I
80
7O
60
%T 5o
40
30
20
BASELINE
ELECTRONIRRADIATION
0
325 350 375 ., 400 425 450 475 500 525 550 575
WAVELENGTH(NM)
Figure 7. The uv/visible spectra of a pure polyetherimide film before and
after exposure to i00 keV electrons.
14
%T
100
90
80
7O
60
5O
40
30
20
i0
325
POLYETHERIMIDE WITH 10X
5000 MRAO EXPOSURE TO 100 KEV ELECTRONS
I I I I 1 ! I I
BTO
• BASELINE
350 375 . 400 425 450 475 500 525 550 575
WAVELENGTH(NM)
i
/
Figure 8. The uv/visible spectra of a polyetherimide film with I0_ BTO
before and after exposure to I00 keV electrons.


The effects of the interaction of polymeric materials with the space environment

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