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NASA/TM— 2004-213098 




Atomic Oxygen Durability Evaluation 

of a UV Curable Ceramer Protective Coating 



Bruce A. Banks 

Glenn Research Center, Cleveland, Ohio 

Christina A. Karniotis 

QSS Group, Inc., Cleveland, Ohio 

David Dworak and Mark Soucek 
University of Akron, Akron, Ohio 



April 2004 



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NASA/TM— 2004-213098 




Atomic Oxygen Durability Evaluation 

of a UV Curable Ceramer Protective Coating 



Bruce A. Banks 

Glenn Research Center, Cleveland, Ohio 

Christina A. Karniotis 

QSS Group, Inc., Cleveland, Ohio 

David Dworak and Mark Soucek 
University of Akron, Akron, Ohio 



Prepared for the 

Seventh International Conference on Protection of Materials 

and Structures from Space Environment 

cosponsored by Materials and Manufacturing Ontario and 

The Centre for Research in Earth and Space Technology (CRESTech) 

Toronto, Canada, May 10-13, 2004 



National Aeronautics and 
Space Administration 

Glenn Research Center 



April 2004 



Acknowledgments 



The authors gratefully acknowledge Justin Tokash and Dr. Rex Ramsier, University of Akron, Physics Department, 

for their XPS contributions. 



Available from 



NASA Center for Aerospace Information 
7121 Standard Drive 
Hanover, MD 21076 



National Technical Information Service 
5285 Port Royal Road 
Springfield, VA 22100 



Available electronically at http: / /gltrs. grc.nasa.gov 



Atomic Oxygen Durability Evaluation of a UV Curable Ceramer 

Protective Coating 

Bruce A. Banks 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Christina A. Karniotis 

QSS Group, Inc. 
Cleveland, Ohio 44135 

David Dworak and Mark Soucek 
University of Akron 
Akron, Ohio 44325 



Abstract: The exposure of most silicones to atomic oxygen in low Earth orbit (LEO) results in the 
oxidative loss of methyl groups with a gradual conversion to oxides of silicon. Typically 
there is surface shrinkage of oxidized silicone protective coatings which leads to cracking of 
the partially oxidized brittle surface. Such cracks widen and branch crack with continued 
atomic oxygen exposure ultimately allowing atomic oxygen to reach any hydrocarbon 
polymers under the silicone coating. A need exists for a paintable silicone coating that is free 
from such surface cracking and can be effectively used for protection of polymers and 
composites in LEO. A new type of silicone based protective coating holding such potential 
was evaluated for atomic oxygen durability in an RF atomic oxygen plasma exposure 
facility. The coating consisted of a UV curable inorganic/organic hybrid coating, known as 
a ceramer, which was fabricated using a methyl substituted polysiloxane binder and 
nanophase silicon-oxo-clusters derived from sol-gel precursors. The polysiloxane was 
functionalized with a cycloaliphatic epoxide in order to be cured at ambient temperature via 
a cationic UV induced curing mechanism. Alkoxy silane groups were also grafted onto the 
polysiloxane chain, through hydrosilation, in order to form a network with the incorporated 
silicon-oxo-clusters. The prepared polymer was characterized by 'H and ^'Si NMR, FT-IR, 
and electrospray ionization mass spectroscopy. The paper will present the results of atomic 
oxygen protection ability of thin ceramer coatings on Kapton H as evaluated over a range of 
atomic oxygen fluence levels. 



Key words: Atomic oxygen, silicones 

1.0 INTRODUCTION 

Silicones are one of the few polymers that can be applied by painting or spraying over 
composite or other organic spacecraft materials which have afforded reasonable 
protection from low Earth orbital (LEO) atomic oxygen attack. The gradual oxidation of 
the silicones in LEO results in an oxidized silicone surface which becomes a silicate or 
silica (Refs. 1-3). This surface conversion from silicone to silica also tends to produce 
tensile stresses in the surface of the oxidized silicone with an increase in the surface 
microhardness as a result of atomic oxygen reaction with the silicone (Ref. 4). A variety 



NASA/TM— 2004-213098 



of approaches have been or are now being explored to identify silicones, silicone 
copolymers or silicone-hydrocarbon blends that provide flexibility as well as atomic 
oxygen protection (Refs. 5-8). Results to date indicate that hybrid polymers composed of 
inorganic and organic polymers hold potential to survive LEO atomic oxygen attack. The 
silicones which are dominated by a oxygen-to- silicon ratio of 1.5 have shown greater 
resistance to atomic oxygen attack than the silicones with a ratio of 1.0. Polyhedral 
oligomeric silsesquioxane (POSS) contains covalently bonded reactive functionalities 
appropriate for polymerization or grafting. It can be blended or copolymerized with 
many aerospace polymers and is being considered for atomic oxygen durability (Ref. 8). 
The resistance to atomic oxygen attack of silicone blended or copolymerized polymers 
has been dependent not only on the oxygen-to-silicone ratio but the fractional fill of the 
silicone. The challenge to make functional use of such blends has been to find an 
adequately silicone-filled polymer which contains the appropriate protective silicone such 
that it has acceptable atomic oxygen durability, volatility, optical, thermal, mechanical 
and ease of application properties. 

Because of their ability to provide atomic oxygen protection, thermal stability, flexibility, 
and stability; polysiloxanes are an attractive candidate solution to achieving ideal 
protection from the elements of space. However, this is just part of the solution. The 
vacuum ultraviolet (VUV) radiation and high energy particles can still damage and 
degrade the composite material. Therefore, to incorporate protection from those 
components as well, ceramer coatings; which are inorganic/organic hybrid materials, can 
be utilized. Ceramers are part ceramic (inorganic) and part polymer (organic) and can 
offer protection from atomic oxygen as well as UV radiation and high energy particles via 
the in situ fabrication of nanophase silicon-oxo-clusters (Refs. 9 and 10). The silicon- 
oxo-clusters are formed through a series of hydrolysis and condensation reactions 
between sol-gel precursors. 

The intention of a ceramer approach is to acquire a synergistic effect between the 
inorganic and organic phases on a nanoscale through the use of phase coupling agents, 
which for this system are alkoxy silanes pendant from the polysiloxane chain. There is 
confirmation of a synergy between the phases and this approach affords a uniformly 
distributed nanophase within a continuous organic phase (Ref. 11). Once the coating is 
exposed to atomic oxygen, a protective layer of silicon oxide is formed and, with the 
incorporation of silicon-oxo-clusters, the coating should protect the composite material 
against atomic oxygen erosion, high energy particles, and VUV radiation. Figure 1 is an 
overall diagram of the coating's function (Ref. 12). 



NASA/TM— 2004-213098 



High Energy' F'article 




> Substrate 



Figure 1 . - Depiction of the formation and function of protective silicon oxide layer and 

silicon-oxo-clusters . 



This paper summarizes an investigation of the high fluence atomic oxygen durability of 
ultraviolet radiation curable ceramer protective coating consisting of methyl substituted 
polysiloxane and compares the results with the commonly used silicone coating DC93- 
500. 

2.0 MATERIALS: METHYL SUBSTITUTED POLYSILOXANE 

2.1 Synthesis 

Experimental 

Materials 

Octamethylcyclotetrasiloxane, 1 ,3,5,7-tetramethylcyclotetrasiloxane, 1 , 1 ,33 
tetramethyldisiloxane, and vinyl triethoxysilane were purchased from Gelest, Inc. and 
used as supplied. Wilkinson's catalyst (chlorotris (triphenylphosphine) rhodium(I), 
99.99%), Amberlyst 15 ion-exchange resin, and 4-vinyl-l-cyclohexene 1,2-epoxide were 
purchased from Aldrich and used as supplied. Toluene, supplied by Aldrich Chemical 
Co., was distilled in order to eliminate any impurities. Irgacure 250 was supplied by Ciba 
Specialty Chemicals and used as received. Air sensitive materials were transferred and 
weighed in an inert atmosphere dry box under argon. 



NASA/TM— 2004-213098 



HjC CHj H]C H 

H£ Y ^0 CH3 H f ^0 CH3 HX CHj 

\/ \/ \/ \/ \ / 

gi S + Si Si 4 ffii-C^EiH 

/\ /< 

H3C CH3 H CHj 



IdiEsiiHtigeRfisin 



NjBlaiiket 



CHa Cris CHn CHg 

H — ao— fsioHsioHa — h 
I II I 

CHg H CH] CHg 




HjC=CHCH3Si{OEt)3 



RiCitaljfit 
NjBlantet 




CH3 CH5 CHt CH3 

■ 1 I 1 

I III 

CH3 (CHJ CHj CH3 




Figure 2. - Synthesis of poly (dimethylsiloxane-co-methylhydrosiloxane), hydride 
terminated. 



Synthesis ofpoly(dimethylsiloxane-co-methylhydrosiloxane), hydride terminated 

To a three neck round bottom flask equipped with a reflux condenser and nitrogen 
inlet/outlet was added octamethylcyclotetrasiloxane (90.00 g), 1,3,5,7- 
tetramethylcyclotetrasiloxane (5.33 g), 1,1,3,3-tetramethyldisiloxane (0.67 g), and 
Amberlyst 15 (20 wt%) and stirred at 90 °C, under nitrogen, for 15 h. The viscous solution 
was then filtered to obtain poly(dimethylsiloxane-co-methylhydrosiloxane), hydride 
terminated of various molecular weight ranges. Vacuum filtration was performed 
(< 1 mm Hg) in order to remove low molecular weight oligomers and unreacted starting 
materials. Weight average molecular weight was obtained from gel permeation 



NASA/TM— 2004-213098 



chromatography (GPC) analysis, M^ = 42,000, PDI = 1.66. Polymer characterization and 
Si-H functionality was confirmed/analyzed through 'HNMR, FT-IR, and titration. 
'HNMR ( (ppm), CDCI3): 4.675 (s, CHj-Si-H). FT-IR (cm ' , KBr Plate): 2150 (s, Si-H. 

Cycloaliphatic epoxide andalkoxy silane functionalization of prepared 
poly(dimethylsiloxane-co-methylhydrosiloxane), hydride terminated. 

To a three neck round bottom flask, equipped with a reflux condenser and nitrogen 
inlet/outlet was added was added poly(dimethylsiloxane-co-methylhydrosiloxane), hydride 
terminated (30 g), 4-vinyl-l-cyclohexene diepoxide (20 g), vinyl triethoxy silane (2 g), and 
Wilkinson's catalyst (0.004 g). Via a canula, distilled toluene (30 g) was added. The 
reaction was held at 75 °C by means of an oil bath and mechanically stirred under a 
nitrogen blanket. The disappearance of the Si-H functionality was monitored through FT- 
IR and the disappearance of the peak at -2160 cm ' indicated that the reaction was 
complete. Any solvent and unreacted starting materials were removed under vacuum 
(3-5 mm Hg). Cycloaliphatic epoxide and alkoxy silane functionalization was 
confirmed/analyzed through 'HNMR, FT-IR analysis, and titration. 

2.2 Structure 

Once cured the coating should form a strong interlocking network consisting of a cross 
linked polysiloxane phase with interconnected silicon-oxo-clusters (Figure 3). The 
silicon-oxo-clusters will be connected to the polysiloxane backbone though hydrolysis 
and condensation reactions with the tethered alkoxy silane. 



Coated Composite 




Figure 3. - Structure of cross linked polysiloxane phase with interconnected silicon-oxo- 
clusters. 



NASA/TM— 2004-213098 



3.0 APPARATUS AND PROCEDURE 

3.1 Coating Application 

The polysiloxane was diluted with dry toluene (25% w/w) in order to reduce the 
viscosity. Sol-gel precursor (5% w/w) and photo initiator (3% w/w) was also added to 
the solution and thoroughly mixed. A piece of Kapton H (~4 inches in diameter) was 
mounted onto a spinning stage and spun at a very high speed. The polysiloxane solution 
was dropped onto the center of the spinning Kapton sample. The sample was removed 
from the stage and passed through a UV curing chamber with a belt speed of 25 ft/min 
and an average intensity of 150 mW/cm^ The coating thickness was measured with a 
coating thickness gauge and AFM. Both methods confirmed an average of a two 
micrometer thickness. 

Instruments 

Viscosity measurements were taken on an AR 500 Rheometer (Thermal Analysis) equipped 
with a cup and bob sample holder and operated at 21.1°C. Pencil harness tests were 
conducted according to ASTM method D3363-00. Taber scratch test was performed using 
a Taber Shear/Scratch Tester model 502 and conforming to ASTM method G171-03. 
Taber Abrasion tests were conducted on a Taber Industries 5130 Abraser using a CS-10 test 
wheel. Taber abrasion studies corresponded to ASTM method D552-93a. 
Thermogravimetric analysis was performed on a TGA Q 500 (Thermal Analysis). 

X-ray photoelectron spectroscopy was completed on a Kratos Model ES3000 with a non- 
monochromatic 120 Watt Al K- Alpha radiation source under 10 '^torr. Scanning electron 
microscopy was performed on a Hitachi S-2150 operating at 15 kV. Atomic force 
microscopy was performed on a multimode scanning probe microscope (Digital 
Instruments) using the tapping mode. 

3.2 Atomic Oxygen Exposure 

Samples of the ceramer silicone-coated Kapton H polyimide (with silicone coat on both 
sides) were compared with samples of DC 93-500 silicone-coated Kapton H (with 
silicone coat on one side) for atomic oxygen durability. The same coatings were also 
applied to fused silica substrates for the purposes of obtaining changes in optical 
properties as well as noting evidence of tensile cracking. Optical properties changes 
(reflectance, transmittance and absorptance) and mass loss were documented at atomic 
oxygen effective fluence levels of 2.22x10^' and 1.38xl0^^atoms/cml Kapton H witness 
samples were used to determine the effective atomic oxygen fluence as described in 
ASTM E 2089-00, "Standard Practices for Ground Laboratory Atomic Oxygen 
Interaction Evaluation of Materials for Space Applications". (Ref. 13) All Kapton H 
substrates used for coating evaluation and fluence witnesses were made of 2.54 cm 
diameter by 0. 127 mm thick Kapton H polyimide. An additional set of ceramer and 
DC93-500 silicone coated samples were made that were scratched prior to exposing to 
atomic oxygen using finger wiping with laboratory dust. This was done to see if minor 



NASA/TM— 2004-213098 



abrasion of the silicone surface would cause preferential cracking of the silicone coatings 
with atomic oxygen exposure. Samples of silicone-coated Kapton H were punched out 
and vacuum dehydrated for 48 hours prior to weighing to minimize mass uncertainty due 
to weight loss as recommended by ASTM E 2089-00. (Ref. 13). Atomic oxygen testing 
was performed on samples that were placed in an SPI Plasma Prep II 13.56 MHz radio 
frequency plasma asher. The ashers are typically operated on air at a pressure of 12.7 to 
16 Pa (95-120 mTorr). The samples were each held down by fine wires attached to a 
metal frame (as shown in Figure 4) laying on a glass plate to minimize curling of the 
samples with atomic oxygen exposure from only one side. 

Curhng typically occurs for silicone coated samples that are coated on one side and could 
allow atomic oxygen to attack the uncoated back of the samples which would 
compromise the sample weight loss data. The plasma asher was operated at a Kapton 
effective flux of 4.69 x 10'^ atoms/(cm^sec) (Ref. 13). 

Because many silicones used on LEO spacecraft have a history of causing contamination 
on spacecraft as a result of evolution of volatile silicones and with subsequent oxidation 
and conversion to silica on neighboring spacecraft surfaces, cross contamination witness 
samples were placed in the plasma ashers next to the silicone coated samples to assess the 
degree of silicone transport and resulting contamination. Tests were performed prior to 
sample exposures to validate that any contamination deposited would be as a result of the 
samples contained within the plasma asher. Thicknesses of deposited contaminants were 
measured using a Dektak 6M stylus profilometer which scanned the contamination coated 




Figure 4. - Sample holder to prevent curling of 2.54 cm diameter samples when they 
were exposed to atomic oxygen. 



NASA/TM— 2004-213098 



fused silica slide from the deposited area to an area that was protected from 
contamination deposition by means of a tightly fitted aluminum foil mask. Optical 
properties changes from prior to and after atomic oxygen exposure were made using a 
Perkin Elmer Lambda- 19 spectrophotometer. 

4.0 RESULTS AND DISCUSSION 

4.1 Methyl Substituted Polysiloxane Characterization 

The abrasion and scratch resistance of the cured coating were studied to determine how 
susceptible it is to physical damage. The Taber abrasion and scratch tests yielded a value 
of 183 wear cycles per mil and a scratch value of 50 grams. Both of these values are low 
and show that the coating has poor abrasion resistance. These values were checked using 
the pencil hardness test, which gave a value of 2B/B. This value is also low and this 
trend could be a result of the very low glass transition temperature of the coating, which 
is approximately -130 °C. Such a low glass transition temperature makes the coating soft 
and vulnerable to damage. Varying the pendant group to raise the glass transition of the 
coating could be a potential answer in improving the abrasion resistance. 

Thermal gravimetric analysis was performed in order to observe the thermal stability of 
the crosslinked polysiloxanes. Irreversible changes to the crosslinked structure of 
silicone polymers unavoidably occur at high temperatures due to chain scission or 
oxidative cross-linking (Ref. 14). In an inert atmosphere, depolymerization occurs with 
the loss of volatile products, mostly low molecular weight cyclic oligomers; but is often 
catalyzed by traces of acids, bases, water, or residual catalyst used in the polymers 
original production (Ref. 15). Typically, depolymerization occurs near 400 °C for 
reasonably pure polydimethylsiloxane (Ref. 16). 

Thermal gravimetric analysis of the cured coatings illustrates the loss of small molecular 
weight oligomers in the early stages of the analysis (Figure 5). As expected, the 
depolymerization occurs near 400 °C for the sample tested. The range of molecular 
weights give way to the multiple slopes the curve exhibits. It is also important to note that 
the sample generated a small amount of residue (~ 13%), which can be attributed to the 
silicon-oxo-clusters formed during the polymerization process and high molecular weight 
chains that may not have completely volatized/degraded. 

One of the most important aspects of the coating is the presence of the silicon-oxo-clusters. 
By utilizing the AFM's tapping mode, it will be possible to detect "hard" (silicon-oxo- 
clusters) and "soft" (polymer) regions within the crosslinked polymer network. These 
clusters provide additional protection against high-energy particles and deep UV light. 
Figure 6 is an AFM image of a sample with 5% (w/w) sol-gel precursor added prior to 
casting. The silicon-oxo-clusters are clearly visible in the subjected sample. The average 
size for the methyl substituted polysiloxane nano phase is 125 nm. Figure 6 shows a more 
disperse and uniformly sized nanophase, which could be attributed to the small size of the 
pendant methyl group allowing more freedom to the growing nano clusters. 



NASA/TM— 2004-213098 




400 SOO 

Teiipei'atiii'e (°C) 



Figure 5. - Thermal Gravimetric Analysis of Cured Coating with 5% Sol-gel precursor 



•"•ssimOmi 





Figure 6. - AFM images of crosslinked methyl polysiloxane substituted with 5% sol-gel 
precursor. 



NASA/TM— 2004-213098 



Conductivity tests were performed on the crosslinked coatings. The coatings showed no 
signs of electrical conductivity, which is expected due to their insulating nature (Ref. 14). 

4.2 Atomic Oxygen Exposure Results 

Photographs of the silicone coated Kapton H samples and silicone coated fused silica 
samples after two different levels of atomic oxygen exposure are shown in Figure 7. 





0.5 mm 

2.54 cm diameter Kapton substrate Fused silica substrate 

a. Ceramer coated samples at a Kapton effective fluence of 2.22 x 10^' atoms/cm^ 





2.54 cm diameter Kapton substrate 



Fused silica substrate 



b. DC93-500 coated samples at a Kapton effective fluence of 2.22 x 10^' atoms/cm^ 

Figure 7. - Photographs of silicone coated Kapton H and silicone coated fused silica after 
atomic oxygen exposure to moderate and high fluence levels. 



NASA/TM— 2004-213098 



10 




.5^-^^^''^^ 





0.5 mm 



2.54 cm diameter Kapton substrate Fused silica substrate 

c. Ceramer coated samples at a Kapton effective fluence of 1.38 x 10^^ atoms/cm^ 




2.54 cm diameter Kapton substrate Fused silica substrate 

d. DC93-500 coated samples at a Kapton effective fluence of 1.38 x 10"^ atoms/cm". 

Figure 7. - (Concluded). Photographs of silicone coated Kapton H and silicone coated 
fused silica after atomic oxygen exposure to moderate and high fluence levels. 

As can be seen in Figure 7, atomic oxygen exposure of the ceramer and DC93-500 
provide excellent protection for moderate (2.22 x 10^' atoms/cm') fluence levels. The 
ceramer coating appears to be a significant improvement at moderate fluence levels in 
that there is no sign of microcracking as occurs for DC93-500. However, at high fluences 
(1.38 X 10^^ atoms/cm^) both the ceramer and DC93-500 develop microcracks. Unlike the 



NASA/TM— 2004-213098 



11 



DC93-500, the ceramer tends to detach from its substrate causing greater coating 
shrinkage due to atomic oxygen attack on both surfaces of the coating. This causes 
greater shrinkage and opening of the cracks thus allowing atomic oxygen to attack 
underlying Kapton. Thus, the ceramer coating is better for moderate fluences and could 
potentially be used for coating optical polymers such as Fresnel lenses for concentrators 
over solar cells. In such applications protective coatings that form microcracks would not 
be suitable due to loss in specular transmittance. 

The mass loss of coated Kapton as a function of atomic oxygen fluence for both the 
ceramer and DC93-500 coatings is shown in Figure 8. 





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-♦ Ceramer - scratched 

-■ Ceramer - unscratched 

-A — DC - scratched 
■ A- ■ - DC - unscratched 
-3K — untreated kapton 



Figure 8. - Mass loss of coated Kapton as a function of atomic oxygen fluence for both 
the ceramer and DC93-500 coatings. 



As can be seen from Figure 8, the ceramer coating as well as the DC93-500 coating does 
provide significant atomic oxygen protection for most fluences. However, at fluences 
above 1 x 10^'atoms/cm% the ceramer coating develops apertures in it due to 
microcracking and the rate of oxidation of the underlying Kapton greatly increases. There 
did not seem to be significant differences in atomic oxygen protection resulting from the 
laboratory dust abrasion of the coatings. This is thought to be due to the very shallow 
scratches resulting without any going all the way through the coatings. 



NASA/TM— 2004-213098 



12 



Atomic oxygen exposure of the ceramer coatings cause an increase in absorptance and 
therefore reduction in transmittance for wavelengths <800 nm with little change in 
reflectance as shown in Figure 9. 



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Figure 9. - Effects of atomic oxygen on total optical properties for creamer coated fused 
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NASA/TM— 2004-213098 



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Figure 9. - (Concluded). Effects of atomic oxygen on total optical properties for creamer 
coated fused silica. 



Similar results were observed for DC93-500 and the ceramer as shown in Figure 10. 
However, if one considers applications such as protective coatings for Fresnel 
concentrator solar cell arrays there is an important distinction between the two coatings 
for moderate fluences. The specular transmittance degradation caused by atomic oxygen 
exposure is in part due to increased absorption for wavelengths < 800 nm as well as 
microcracking as shown in Figure 11. 



NASA/TM— 2004-213098 



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Figure 10. - Effects of atomic oxygen on total optical properties for DC93-500 coated 
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NASA/TM— 2004-213098 



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Figure 10. - (Concluded). Effects of atomic oxygen on total optical properties for DC93- 
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Figure 1 1. - Effects of atomic oxygen on specular transmittance of ceramer and DC93- 
500 coated fused silica. 



NASA/TM— 2004-213098 



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w 




500 1000 1500 

Wavelength (nm) 



2000 



2500 



original 

2.22 E+21 atoms/cm2 

1.38 E+22 atoms/cm2 



b. DC93-500 

Figure 11.- (Concluded). Effects of atomic oxygen on specular transmittance of ceramer 
and DC93-500 coated fused silica. 



As can be seen by examining Figures 1 1 (a) and 7 (a and b), the absence or presence of 
microcracking at a fluence level of 2.22 x 10^' atoms/cm^ makes a significant impact on 
specularly transmitted light. The ceramer coating does allow much greater specular 
transmittance than the DC93-500 for fluences up to 2.22 x 10^' atoms/cml 

X-ray photoelectron spectroscopy was performed in order to confirm the presence of a 
protective oxide layer (Figure 12). It is important to note that sputtering was not performed 
during the analysis. This ensures that only the surface of the samples was analyzed. The 
initial XPS spectrum shows high amounts of both silicon and oxygen, which is expected 
since these elements are present in the polymer backbone. After atomic oxygen exposure 
the oxygen peak increased while the silicon peaks decreased. This is anticipated due to the 
protective oxide layer possessing a high amount of oxygen compared to silicon. The oxide 
layer should be composed of silicon atoms whose valences are filled by oxygen atoms, 
yielding a Si-O^ network. The presence of carbon after exposure to atomic oxygen is due to 
impurities on the surface of the film such as dust, dirt, etc. and is, therefore, always present 
(Ref. 12). 



NASA/TM— 2004-213098 



17 




300 400 

Energy (cV) 



- Before AO Exposure After AO Exposure 



Figure 12. - XPS Spectrum of the Cross-linked Methyl Substituted Polysiloxane Before 

and After Atomic Oxygen Exposure 

Cross contamination tests performed separately on the ceramer and DC93-500 coating 
using fused silica witness slides adjoining the silicone samples indicated that there was 
transport of short chain silicones to the fused silica that resulted in a silica deposit 857 
Angstroms for a Kapton effective fluence of 1 x 10^^ atoms/cm^ for the ceramer, but no 
measurable contamination from the DC93-500. This is probably due to the lack of 
vacuum stripping of the ceramer whereas the DC93-500 is vacuum stripped. A "no 
sample" test of the facility did, in fact, result in no deposit of oxidized silicone. 

5.0 CONCLUSIONS 

Atomic oxygen exposure of ceramer and DC93-500 silicone coated Kapton H and fused 
silica slides indicates the ceramer coating has superiority over DC93-500 coatings for 
moderate (up to 2.22 x 10^' atoms/cm^) Kapton effective atomic oxygen fluences. The 
ceramer coatings at this fluence resulted in low mass loss of coated Kapton samples and 
did not show evidence of the extended microcracking that occurred for DC93-500. This 
results in superior specular light transmittance for the ceramer coatings. This may allow 
its use as atomic oxygen protective coatings over silicone Fresnel concentrators for solar 
arrays. At high fluence levels (1.3 8x10^^ atoms/cm^) the ceramer coating develops 
microcracks that result in detachment of the coating causing exposure of the underlying 
Kapton which does not occur for DC93-500. Thus at high fluences DC93-500 would be a 
better choice of protective coating. 



NASA/TM— 2004-213098 



18 



The ceramer coating produced cross contamination of silica on witness slides which 
suggests that the ceramer should be vacuum stripped to prevent the transport of short 
chain silicones that can result in contamination. 

7.0 REFERENCES 

1. Banks, B.A., Dever, J. A., Gebauer, L., and Hill, CM., "Atomic Oxygen Interactions 
with FEP Teflon and Silicones on LDEF," presented at the 1st LDEF Post- Retrieval 
Symposium, Kissimmee, Florida, June 2-8, 1991. 

2. Banks, B.A., Rutledge, S.K., de Groh, K.K., Mirtich, M.J., Gebauer, L., OUe, R., and 
Hill, CM., "The Implications of the LDEF Results on Space Station Freedom Power 
System Materials," presented at the 5th International Symposium on Materials in a 
Space Environment, Cannes-Mandelieu, France, September 16-20, 1991. 

3. Banks, B., Rutledge, S., Sechkar, E., Stueber, T., Snyder, A., Hatas, C, and 
Brinker, D., "Issues and Effects of Atomic Oxygen Interactions With Silicone 
Contamination on Spacecraft in Low Earth Orbit," NASA/TM— 2000-210056, 
proceedings of the S"" International Symposium on Materials in a Space Environment 
and the 5* International Conference on Protection of Materials and Structures from 
the LEO Space Environment cosponsored by the CNES, Integrity Testing Laboratory, 
ESA, ONERA and the Canadian Space Agency, Arcachon, France, June 4-9, 2000. 

4. de Groh, K., Banks, B., Ma, D., "Ground-Laboratory To In-Space Effective Atomic 
Oxygen Fluence Determination for DC 93-500 Silicone," 7* International Conference 
on ""Protection of Materials and Structures form Space Environment," Toronto, 
Canada, May 10-13,2004. 

5. Rutledge, S., Cooper, J. and OUe, R., "The Effect of Atomic Oxygen on Polysiloxane- 
Polyimide for Spacecraft Applications in Low Earth Orbit," Proceedings of the Space 
Operations Applications and Research Symposium, NASA CP-3103, Albuquerque, 
NM, June 26-28, 1990. 

6. Hung, C, "Reaction and Protection of Wire Insulators in Atomic Oxygen 
Environment," NASA TM 106767, 1994. 

7. Zhang, C, Babonneau, F., Bonhomme, C, Laine, R.M. Soles, C.L., Hristov, H.A., 
Yee, A.F., "Highly Porous Polyhedral Silsesquioxane Polymers. Synthesis and 
Characterization," J. Am. Chem. Soc. 120, 8380-8391. 1998. 

8. Brunsvold, A.L., Minton, T.K., Gouzman, I., Grossman, E. and Gonzalez, R.I. "An 
investigation of the Resistance of POSS Polyimide to Atomic Oxygen 

Attack," submitted to Journal of High Performance Polymers, 2003. 

9. Soucek, M.D. and Tuman, S.J., J. Coat. TechnoL, 68 (854), 73 (1996). 



NASA/TM— 2004-213098 19 



10. Tuman, SJ., Chamberlain, D., Scholsky, K.M., and Soucek, M.D., Prog. Org. Coat, 

28,251(1996). 

11. Wold, C.R. and Soucek, M.D., J. Coat. Tech., 70, 43 (1997). 

12. Dworak, D.P. and Soucek, M.D., Prog. Org. Coat., 47, 448 (2003). 

13. ASTM E 2089-00, "Standard Practices for Ground Laboratory Atomic Oxygen 
Interaction Evaluation of Materials for Space Applications," June 2000. 

14. Smith, A.L. "The Analytical Chemistry of Silicones', Wiley, New York, (1991). 

15. Kang, D.W., Rajendran, G.P., and Zeldin, M., /. Polym. Sci. Part A. Polym. Chem., 
24, 1085 (1986). 

16. N. Grassie and I.G. Macfarlane, Eur. Polym. J., 14, 875 (1978). 



NASA/TM— 2004-213098 20 



REPORT DOCUMENTATION PAGE 


Form Approved 
0MB No. 0704-0188 


Public reporting burden for this coiiection of information is estimated to average 1 fiour per response, inciuding tfie time for reviewing instructions, searcfiing existing data sources, 
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1. AGENCY USE ONLY (Leai/e Wan/t) 


2. REPORT DATE 

April 2004 


3. REPORT TYPE AND DATES COVERED 

Technical Memorandum 


4. TITLE AND SUBTITLE 

Atomic Oxygen Durability Evaluation of a UV Curable Ceramer 
Protective Coating 


5. FUNDING NUMBERS 

WBS-22-319-20-E1 


6. AUTHOR(S) 

Bruce A. Banks, Christina A. Karniotis, David Dworak, and Mark Soucek 


7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

National Aeronautics and Space Administration 
John H. Glenn Research Center at Lewis Field 
Cleveland, Ohio 44135-3191 


8. PERFORMING ORGANIZATION 
REPORT NUMBER 

E-14573 


9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 

National Aeronautics and Space Administration 
Washington, DC 20546-0001 


10. SPONSORING/MONITORING 
AGENCY REPORT NUMBER 

NASA TM— 2004-213098 


11. SUPPLEMENTARY NOTES 

Prepared for the Seventh International Conference on Protection of Materials and Structures from Space Environment cosponsored 
by Materials Manufacturing Ontario; and The Centre for Research in Earth and Space Technology (CRESTech), Toronto, Canada, 
May 10-13, 2004. Bruce A. Banks, NASA Glenn Research Center; Christina A. Karniotis, QSS Group, Inc., 21000 Brookpark 
Road, Cleveland, Ohio 44135; and David Dworak and Mark Soucek, University of Akron, 302 Buchtel Mall, Akron, Ohio 44325. 
Responsible person, Bruce A. Banks, organization code 5480, 216-433-2308. 


12a. DISTRIBUTION/AVAILABILITY STATEMENT 

Unclassified - Unlimited 

Subject Category: 75 Distribution: Nonstandard 

Available electronicallv at http://nltrs.grc.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390. 


12b. DISTRIBUTION CODE 


13. ABSTRACT (Maximum 200 words) 

The exposure of most silicones to atomic oxygen in low Earth orbit (LEO) results in the oxidative loss of methyl groups 
with a gradual conversion to oxides of silicon. Typically there is surface shrinkage of oxidized silicone protective coatings 
which leads to cracking of the partially oxidized brittle surface. Such cracks widen and branch crack with continued 
atomic oxygen exposure ultimately allowing atomic oxygen to reach any hydrocarbon polymers under the silicone 
coating. A need exists for a paintable silicone coating that is free from such surface cracking and can be effectively used 
for protection of polymers and composites in LEO. A new type of silicone based protective coating holding such potential 
was evaluated for atomic oxygen durability in an RF atomic oxygen plasma exposure facility. The coating consisted of a 
UV curable inorganic/organic hybrid coating, known as a ceramer, which was fabricated using a methyl substituted 
polysiloxane binder and nanophase silicon-oxo-clusters derived from sol-gel precursors. The polysiloxane was 
functionalized with a cycloaliphatic epoxide in order to be cured at ambient temperature via a cationic UV induced curing 
mechanism. Alkoxy silane groups were also grafted onto the polysiloxane chain, through hydrosilation, in order to form 
a network with the incorporated silicon-oxo-clusters. The prepared polymer was characterized by ^H and ^^Si NMR, 
FT-IR, and electrospray ionization mass spectroscopy. The paper will present the results of atomic oxygen protection 
ability of thin ceramer coatings on Kapton H as evaluated over a range of atomic oxygen fluence levels. 


14. SUBJECT TERMS 

Atomic oxygen; Silicones 


15. NUMBER OF PAGES 

26 


16. PRICE CODE 


17. SECURITY CLASSIFICATION 
OF REPORT 

Unclassified 


18. SECURITY CLASSIFICATION 
OF THIS PAGE 

Unclassified 


19. SECURITY CLASSIFICATION 
OF ABSTRACT 

Unclassified 


20. LIMITATION OF ABSTRACT 



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