Henry C. de Groh III, Bernadette J. Puleo, Deborah L. Waters, and Sharon K. Miller

Glenn Research Center, Cleveland, Ohio

Anti-adhesion Elastomer Seal Coatings for

Ultraviolet and Atomic Oxygen Protection

NASA/TM—2014-218334/REV1

February 2015

This Revised Copy, numbered as NASA/TM—2014-218334/REV1, February 2015, supersedes the

previous version, NASA/TM—2014-218334, September 2014, in its entirety. 
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Henry C. de Groh III, Bernadette J. Puleo, Deborah L. Waters, and Sharon K. Miller

Glenn Research Center, Cleveland, Ohio

Anti-adhesion Elastomer Seal Coatings for

Ultraviolet and Atomic Oxygen Protection

NASA/TM—2014-218334/REV1

February 2015

This Revised Copy, numbered as NASA/TM—2014-218334/REV1, February 2015, supersedes the

previous version, NASA/TM—2014-218334, September 2014, in its entirety. 

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

Chris C. Daniels, Nicholas G. Garafolo, Nicholas Penney, and William J. Anderer helped immensely with the accomplishment of 

this work: the conception of the test methods and hardware, maintenance of the custom software, and manufacture of specimens 

are due to them. Particular acknowledgement is due Heather A. Oravec for her help testing Medium-scale seals. This work was 

funded by the NASA Docking System project.

Available from

Level of Review: This material has been technically reviewed by technical management. 

Revised Copy

This Revised Copy, numbered as NASA/TM—2014-218334/REV1, February 2015, supersedes the previous version,

NASA/TM—2014-218334, September 2014, in its entirety. 

Text has been added throughout the entire document.

NASA STI Program

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This report is available in electronic form at http://www.sti.nasa.gov/ and http://ntrs.nasa.gov/
NASA/TM—2014-218334/REV1 1 

Anti-adhesion Elastomer Seal Coatings for 

Ultraviolet and Atomic Oxygen Protection 

Henry C. de Groh III, Bernadette J. Puleo, Deborah L. Waters, and Sharon K. Miller 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Radiation blocking sunscreen coatings have been developed for the protection of elastomer seals used 

in low-Earth-orbit (LEO). The coatings protect the seals from ultraviolet (UV) radiation and atomic 

oxygen (AO) damage. The coatings were developed for use on NASA docking seals. Docking seal 

damage from the UV and AO present in LEO can constrain mission time-line, flight mode options, and 

increases risk. A low level of adhesion is also required for docking seals so undocking push-off forces can 

be low. The coatings presented also mitigate this unwanted adhesion. Greases with low collected volatile 

condensable materials (CVCM) and low total mass loss (TML) were mixed with slippery and/or UV 

blocking powders to create the protective coatings. Coatings were applied at rates up to 2 mg/cm2

. 

Coated seals were exposed to AO and UV in the NUV (near-UV) and UV-C wavelength ranges 

(300 to 400 nm and 254 nm, respectively). Ground based ashers were used to simulate the AO of space. 

The Sun’s UV energy was mimicked assuming a nose forward flight mode, resulting in an exposure rate 

of 2.5 MJ/m2

-day. Exposures between 0 and 147 MJ/m2 (UV-C) and 245 MJ/m2 (NUV) were 

accomplished. The protective coatings were durable, providing protection from UV after a simulated 

docking and undocking cycle. The level of protection begins to decline at coverage rates less than 

0.9 mg/cm2

. The leakage of seals coated with Braycote+20%Z-cote ZnO sunscreen increased by a factor 

of 40 after moderate AO exposure; indicating that this coating might not be suitable due to AO 

intolerance. Seals coated with DC-7-16.4%Z-cote ZnO sunscreen were not significantly affected by 

combined doses of 2u1021 atoms/cm2

 AO with 73 MJ/m2 UV-C. Unprotected seals were significantly 

damaged at UV-C exposures of 0.3 MJ/m2

 and DC-7-16.4%Z-cote coated seals were undamaged at all 

exposures up to the limits tested thus far which were 147 MJ/m2

 UV-C and 245 MJ/m2 NUV. The 

coatings decreased adhesion sufficiently for docking seals at temperatures equal to or greater than –8 qC 

thus offer a simple and inexpensive way to mitigate adhesion. 

Introduction 

The National Aeronautics and Space Administration (NASA) is currently developing docking 

mechanisms and seals for their next manned spacecraft (Refs. 1 and 2). The docking mechanism and seal 

are expected to be located at the nose of a conical capsule similar in shape to the crew modules used during 

the Apollo era (Ref. 3). The docking system is expected to use a set of two silicone-rubber seals which are 

connected together by a web of the same material and held in place by a metal retaining ring. Silicone-based 

elastomers were chosen as the seal material primarily because silicone-rubbers meet the wide operating 

temperature range for space applications (Ref. 4) and do not lose mass due to atomic oxygen erosion. These 

seals are expected to be exposed to the space environment whenever the spacecraft is not docked. Several 

environmental hazards are present in space; these hazards depend on flight altitude and spacecraft 

orientation as well as several other factors such as seasonal variations (Refs. 5 and 6). Prior work has shown 

that the greatest environmental threats to the seal are ultraviolet (UV) radiation and atomic oxygen (AO) 

(Refs. 7 to 11). Protection from the effects of AO and UV are needed, especially for longer missions. 

Two UV protection methods were explored: 1) mixing a UV blocking compound into the elastomer 

prior to curing; 2) topical application of a UV blocking sunscreen coating. Sunscreen agents are generally 

classified as either organic (also referred to as chemical) or inorganic (also referred to as physical). The 
NASA/TM—2014-218334/REV1 2 

inorganic agents TiO2 and ZnO were examined because of their availability, common use, low reactivity 

and stability in vacuum (Refs. 12 to 14). Titanium dioxide was selected as the additive to the proprietary 

silicone elastomer compound, S0383-70. The sunscreen coating was made with ZnO because ZnO 

attenuates a wider swath of the UV spectrum. The objective of the research presented here was to 

determine the effectiveness of UV blocking inhibitors added to the elastomer compound and UV blocking 

coatings to prevent damage from UV and AO. 

A second problem that must also be addressed with the development of docking and hatch seals is 

related to adhesion. Silicone-based elastomers are naturally sticky. This stickiness helps to make a good 

seal, however can cause problems when spacecraft undock (Refs. 15 and 16). There are several options to 

mitigate this unwanted adhesion such as targeted use of AO (Refs. 10 and 15), radiation in the vacuumUV range 200 nm (Refs. 17 and 18), and application of a suitable grease (Ref. 19). The grease-based 

sunscreens developed to block UV were also designed to control adhesion; thus another objective of the 

research presented here was to sufficiently lower seal adhesive forces with the application of a UV 

blocking sunscreen. 

There are several other uses that the sunscreens presented here might be beneficially applied to; here 

are some examples: 

x An anti-adhesion coating could consist of boron nitride added to the grease- this would be applied to a 

hatch seal, or the seal between the heat shield and crew module to ensure low adhesion forces when 

the heat shield is released after re-entry, and before parachute deployment. 

x As a coating on seals used in motor vehicles, especially around the windshield and windows, or on 

tire side-walls. 

x As a coating on the rubber seals on solar panels. 

Space-qualified grease has been used in the past by NASA to lubricate gears and to ease the 

placement of elastomer seals (Refs. 20 and 21). The greases examined in this study were Braycote 

Micronic 601 EF, a space rated perfluoroether grease (Ref. 22); and Dow Corning 7 Release Compound 

(DC-7) which is a dimethyl silicone lubricant (Ref. 23). 

Vacuum leak rates of seals designed for spacecraft air locks are presented by Trout Jr. (Ref. 24). 

Prior work on the effects of the space environment on silicone rubber and elastomer seals can be found in 

(Refs. 7 to 9). Daniels et al. has presented the results of three different compounds of silicone rubber 

exposed in ground-based facilities and in orbit as part of NASA’s Materials International Space Station 

Experiment (MISSE-6) (Ref. 25); small #2-309 O-rings were exposed in space to an AO fluence up to 

1.2u1022 atoms/cm2

 and 2620 ESH (equivalent Sun hours, approximately 1045 MJ/m2) of UV. These 

compounds were designed by their manufacturers to be UV durable. Daniels et al. found that damage 

incurred during orbit was less when the seals were given an AO pretreatment prior to exposure in space. 

This decline in damage was perhaps due to a change in UV absorbance caused by changes in reflectivity 

brought about by the AO pretreatments which give silicone elastomers a glossy surface finish. The seals 

we are considering are expected to leak through bulk permeation as well as through the interface between 

the seal and mating counterface (Ref. 26). It is hoped that use of a protective sunscreen like those 

presented will both lower and stabilize leakage through the mating surface interface. Prior work by 

Gudimenko et al. presented a Si-rich coating (Photosil) for protection from AO and UV (Ref. 27) wherein 

Photosil was placed on various polymer films and thermal control paints for space applications. Mass loss 

was prevented, however, exposure to near-UV was not included. The lack of mass loss of silicone rubbers 

when exposed to AO is well known (Ref. 28) however, a solution to the deleterious combined effects of 

AO and UV in LEO was not found in the literature. 

Silicone elastomer seals were made and tested before and after exposure to AO and UV (with 

wavelengths near 254 nm (UV-C) and 350 nm (near-UV also known as NUV)). Seals were exposed and 

tested with and without sunscreen coatings. Seals of the base compound S0383-70 and with TiO2 added 

were included. Seal adhesion and leak rate were measured before and after exposure; leakage was 

determined using a pressure decay method previously presented (Ref. 29). 
NASA/TM—2014-218334/REV1 3 

Experimental Procedures 

Specimens 

Specimens were made from the silicone elastomer S0383-70, a rust colored silicone elastomer with a 

Shore Hardness A durometer rating of 70 supplied by the Parker Hannifin Corporation, CSS-Division. In 

an effort to increase the compound’s resistance to UV exposure, a second compound was created in which 

6 wt% TiO2 was added to the compound prior to molding and curing. 

The full-scale docking seal this research applies to is approximately 50 in. (1.27 m) in diameter. Two 

types of sub-scale seals were used in this study for exposure and leakage testing: 

x Medium-scale seals consisting of a double bulb and web between the bulbs, held in place with a 

retainer ring, similar in design to those presented in Reference 30. The centerline diameters of the two 

seal bulbs were about 10.2 in. (26 cm) and 11.5 in. (29.2 cm). 

x Small-scale seals with an inner diameter of 0.41 in. (10.4 mm) and an outer diameter of 0.83 in. 

(21.1 mm). Small-scale seals were made from 0.21 in. (5.3 mm) thick sheets of S0383-70; these 

washer style seals were cut from the sheets using a drill press and custom-made core drill lubricated 

with soap. Figure 1 shows examples of as-received small-scale seals made of S0383-70 and with 

6 wt% TiO2 added. 

Four different types of specimens were used for adhesion testing: flat topped buttons and 

#2-309 O-rings similar to those described previously (Ref. 15), sections cut from Medium-scale seals 

(Mss), and Medium-scale seals. Figure 2 shows examples of these adhesion test specimens after being 

tested. It was believed that some of the adhesion of sunscreen coated seals was caused by a vacuum effect 

between the greased flat surfaces. To test this, #2-309 O-rings were glued to a 1 in. square block which 

was vented to prevent any pressure effects from the inner volume; this vented block is shown in Figure 2. 

Figure 1.—Small-scale seals: baseline on left, with 6% TiO2 on right, 0.21 in. thick. 

 
NASA/TM—2014-218334/REV1 4 

Figure 2.—Adhesion specimens and 1 in. square vented block used 

with #2-309 O-rings. Top left: buttons and cut sections of Mediumscale seals (Mss) after adhesion testing. Center: Medium-scale 

seal after coating and adhesion testing. Bottom: #2-309 O-rings. 

Coatings 

The general scheme in the development of UV protective coatings was to combine a slippery release 

agent with a physical UV blocking additive. Several issues need to be considered when choosing the 

grease and blocking additive such as the outgassing of the grease, and the effect of the UV blocking 

additive on seal leakage. The following two greases and two blocking agents were down-selected and 

used in this study: 

x Braycote Micronic 601 EF, a low temperature, high vacuum perfluoroether grease made by Castrol. 

Braycote 601 is a flight qualified grease with low outgassing and an established history in NASA’s 

flight program. 

x Dow Corning 7 Release Compound (DC-7), is a very low outgassing silicone-based lubricant. 

x One UV blocking agent that was selected was ZnO in the form of uncoated Z-cote, a zinc oxide 

powder with a particle size less than 0.2 Pm; Z-cote is frequently used as an additive in sunscreens 

and cosmetics, and is made by BASF Corporation. 

x Boron nitride (BN) powder was examined because of its favorable friction and release characteristics. 

The BN powder used was grade SHP-325 (–325 mesh, 50 Pm diameter particles) supplied by 

Standard Oil Engineered Materials Company. 
NASA/TM—2014-218334/REV1 5 

The sunscreen mixtures were made by hand-mixing the selected powder into the grease. The 

following recipes were examined: 

x Braycote 601 

x Braycote-12 wt% Z-cote 

x Braycote-20 wt% Z-cote 

x DC-7 

x DC-7-16.4 wt% Z-cote 

x DC-7-17.6 wt% BN 

The mix of Braycote-20%Z-cote was used on small-scale seals and buttons, the Braycote-12%Z-cote 

was used on Medium-scale seals. Sunscreen coating rates of 0, 0.5, 0.9, 1.5, and 2 mg/cm2

 were 

examined. The rate of coverage was measured by weighing the specimen before and after coating 

application and the amount of coating adjusted by hand until the desired coverage was achieved. To put 

these application rates into perspective it may help the reader to know that the Sun Protection Factor 

(SPF) application rate standard for sunscreen on a person’s skin is 2 mg/cm2

. 

Atomic Oxygen Exposures 

The facilities used to expose specimens to AO are discussed in Reference 10 and located at the 

NASA Glenn Research Center (GRC). Atomic oxygen exposures were done using an isotropic air-based 

plasma created in SPI Plasma Prep II etchers; AO fluence was measured using Kapton H (DuPont) 

witness coupons prior to or during exposures. When considering AO in low-Earth-orbit (LEO) the “facing 

forward” (also known as ram, or windward) flight orientation receives AO at a rate of approximately 

4.4u1019 atoms/cm2

-day. The other flight modes of zenith, nadir, starboard and port are expected to 

receive about 1.76u1018 atom/cm2

-day. Groups of coated and uncoated small-scale seals were exposed to 

2.2u1020 atoms/cm2

 AO prior to near-UV (NUV) exposure. An exposure of 2.2u1020 atoms/cm2 is a level 

of AO expected during a period of about 5 days in LEO in the windward flight orientation, or about 

125 days in the zenith, nadir, starboard, or port flight orientation. Small-scale seals exposed to UV-C 

received AO exposures up to 2.2u1021 atoms/cm2

. 

UV Exposures 

The Sun delivers to spacecraft in LEO a UV irradiance (wavelengths 400 nm) of approximately 

110.9 W/m2

 (Ref. 6). A portion of this radiation will shine on a particular area of a spacecraft depending 

on the radiation view factor between the Sun and the surface being considered, as well as other factors 

such as the Sun being blocked by the Earth, or other parts of the spacecraft. The view factor of 0.26 used 

in this study is that expected for the forward facing windward, or “ram” flight mode. The UV dose for 

1 day of windward flight is then calculated as follows: 

Sun’s irradiance u 1 day u radiation view factor = 110.9 W/m2 u 1 day u 0.26 = 2,490,000 J/m2

. 

This value can then be used to determine a UV dose characteristic of a number of days of LEO flight in 

the windward orientation where 1 day would be a dose of 2.49 MJ/m2

, and 10 days would be 24.9 MJ/m2, 

and so on. 

The following three different facilities were used to expose specimens to UV radiation: 

x UV-C exposures at 254 nm were done at the NASA Glenn Research Center (GRC) in air at room 

temperature using an R-52G Mineralight UV lamp with the filter removed (manufactured by UltraViolet Products (UVP)). This lamp has an intensity peak at a wavelength of 254 nm. At the 4.5 cm 

exposure distance used, the intensity of the lamp was approximately 10 mW/cm2

. The intensity of the 

lamp was measured using a calibrated BLAK-RAY J-225 shortwave UV meter (manufactured by 
NASA/TM—2014-218334/REV1 6 

UVP). Coated and uncoated specimens were exposed to a variety of UV-C doses up to 147 MJ/m2. 

This maximum exposure is similar to the UV energy expected after about 60 days of windward LEO 

flight. However, these UV-C exposures delivered the energy in a narrow wavelength band (near 

254 nm), where the Sun would be delivering the radiation over a much wider wavelength range with 

the longer (300 to 400 nm) UV-A wavelengths having the greatest intensity. While these UV-C 

exposures do not mimic on-orbit UV exposure accurately, they do demonstrate the ability of the 

coatings and compound additives to prevent damage due to radiation in the UV-C wavelength range. 

x Near-UV exposures were done at GRC in a hard vacuum (~2u10–4 Pa) using a 500 W mercury 

(xenon) arc source (Oriel Model 66141) with intensity peaks in the 300 to 360 nm range (Ref. 10). 

The lamp’s intensity (9 mW/cm2

) inside the facility was measured in situ using a calibrated BLAKRAY J-221 longwave UV meter. Coated and uncoated specimens were exposed to NUV doses up to 

245 MJ/m2

. This maximum exposure is similar to the UV energy expected after about 100 days of 

windward LEO flight. These NUV exposures mimic the Sun’s radiation better (compared to UV-C) 

since NUV wavelengths are where the Sun’s UV radiation is most intense. A vacuum was imposed 

during these GRC NUV exposures, thereby better simulating LEO and any combined effects of UV 

and vacuum. 

x Near-UV exposures were also done at an outside vendor (Q-Lab Weathering Research Service). 

These Q-Lab NUV exposures were done near room temperature in air (not in a vacuum) using their 

Q-Sun Xenon test chamber which delivered an irradiance of approximately 87 W/m2

 (8.7 mW/cm2) 

and a peak intensity near 340 nm. Coated and uncoated specimens were exposed to NUV doses up to 

245 MJ/m2

. This group of exposures was done primarily to explore the effects of air and the absence 

of vacuum during UV exposure. 

Adhesion Testing 

Methods used to test adhesion have been presented in References 10, 15, 19, 30, and 31. 

All button style and #2-309 O-ring specimens were: mated against a clean, anodized 6061 T651 

aluminum alloy plate of average roughness of 13 Pin. (0.33 Pm); brought together at a rate of 0.01 in./s 

(0.254 mm/s), compressed 25 percent, and held for 20 hr at room temperature before unloading at a rate 

of 0.01 in./s. During unloading adhesion was measured using a calibrated force transducer. 

Adhesion test conditions and results for the Medium-scale tests are provided in Table 1. Mediumscale seals were adhesion tested between two plates using an Instron material test system, Model 5584, 

encased in a thermal environmental chamber. Tests at a variety of constant temperatures between 65 and 

–50 qC were accomplished. A laser extensometer was used to measure the displacement between plates. 

The dwell time for Medium-scale adhesion tests was 8 hr except for the case of cyclic tests which used a 

1 min. dwell time. Two Medium-scale seals (–0282 and –0283) were subjected to cyclic adhesion tests 

which consisted of 25 room temperature 1 min. dwell time adhesion tests. 

The rate of sunscreen coverage is listed in Table 1, however, frequently the same seal was adhesion 

tested more than once. In this way repeated docking/undocking cycles could be examined. When this 

occurred the coating was usually not replenished (and re-measured) between tests. The metal mating 

surface was thoroughly cleaned after each test which used an 8 hr dwell time. For example, seal –0306 

was first tested uncoated at room temperature (RT), then coated with Braycote and tested again at RT; 

then –0306 was tested at –8 qC without replenishing the coating. At this point, prior to this –8 qC test, the 

rate of Braycote coverage was unknown because some of the coating was likely left behind on the metal 

mating surface after the previous test. This has been conveyed in Table 1 by listing the rate of coverage 

for the –8 qC test of –0306 as “2nd cycle on 2.03”, with 2.03 mg/cm2

 being the original, 1st cycle, rate of 

coverage. If the coating was replenished between tests, the new rate of coverage appears in Table 1. The 

coating was not doctored in any way during short cyclic tests on seals –0282 and –0283, nor was the 

metal mating surface cleaned between cycles. The specific coating rate of coverage during these cyclic 

tests was not monitored; Table 1 lists the rate of coverage measured prior to the first cycle. 
NASA/TM—2014-218334/REV1 7 

TABLE 1.—TEST CONDITIONS AND ADHESION OF 12 in. DIAMETER MEDIUM-SCALE SEALS; 1 lbf = 4.448 N 

ID Compound Rate of coating coverage, 

mg/cm2

Adhesion test 

temperature, 

qC 

Adhesion, 

lbf

–0286 uncoated 0 RT 60.6 

–0287 uncoated 0 RT 58.4 

–0306 uncoated 0 RT 50.3 

–0306 Braycote only 1st cycle on 2.03 RT 1.1 

–0286 Braycote only 1st cycle on 1.94 RT 1 

–0306 Braycote only 2nd cycle on 2.03 –8.0 6.7 

–0286 Braycote only 2nd cycle on 1.94 –8.0 4.5 

–0306 Braycote only 3rd cycle on 2.03 65.0 0.8 

–0286 Braycote only 3rd cycle on 1.94 65.0 0.7 

–0287 Braycote-12%Z-cote 1st cycle on 1.69 RT 1.1 

–0305 Braycote-12%Z-cote 1st cycle on 2.00 RT 1 

–0305 Braycote-12%Z-cote 2nd cycle on 2.00 -8.0 4.2 

–0287 Braycote-12%Z-cote 2nd cycle on 1.69 -8.0 5.3 

–0305 Braycote-12%Z-cote 3rd cycle on 2.00 65.0 0.9 

–0287 Braycote-12%Z-cote 3rd cycle on 1.69 65.0 0.9 

–282 DC-7-16.4%Z-cote 2-26 cycles on 2.29 RT 1.9 to 12.8 increasing with 

each cycle 

–0283 DC-7-16.4%Z-cote 2-26 cycles on 1.89 RT 0.7 to 7.6 increasing with 

each cycle 

–0282 DC-7-16.4%Z-cote 1st cycle on 2.29 –8.0 6.5 

–0283 DC-7-16.4%Z-cote 1st cycle on 1.89 –8.0 13.5 

–0282 DC-7-16.4%Z-cote 1st cycle on 1.82 –50.0 322.3 

–0902 6% TiO2 uncoated 0 61.0 72.6 

–0903 6% TiO2 uncoated 0 –50.0 180.6 

Leakage Testing 

Leak rate test methods for small-scale (Refs. 9 and 18) and Medium-scale (Refs. 32 and 33) seals 

have been presented previously; however, a brief summary is provided here. 

Small-scale leakage tests were done between flat anodized aluminum alloy plates, at room 

temperature, with 15 percent compression of the seals’ height, and the volume inboard of the seal at a 

relative positive air pressure of 14.7 psig (0.1 MPa). The volume of the system was measured prior to 

testing. The pressure was measured using two pressure transducers which were averaged, and system 

temperature was measured using resistance temperature devices (RTD’s). 

The apparatus used to leak test the Medium-scale seals consisted of two stainless steel plates of 

16 Pin. or better surface finish, hermetic plumbing, and pressure and temperature sensors. The volume of 

the system was measured before testing. The top plate had a vent port located between the inner and outer 

bulbs which enabled isolation and testing of the inner seal. Dry air was supplied to the inner volume and a 

relative pressure of 14.7 psig (0.1 MPa) established and monitored by two pressure transducers. The 

apparatus was enclosed in a Tenney Benchmaster-BTRC environmental chamber with all leak tests done 

at 56 o

C. The leakage for small and Medium-scale seals was determined using a pressure decay method 

(Ref. 29) wherein the ideal gas law and definition of specific volume were used to determine air mass as a 

function of pressure, volume, and temperature. The mass of air in the system was thus measured over 

time; the slope of the mass versus time curve yielded leakage. 
NASA/TM—2014-218334/REV1 8 

Results 

Adhesion 

The average adhesion of the flat button, #2-309 O-ring, and cut Medium-scale seal specimens (Mss) 

are shown in Figure 3; the as-received condition is uncoated. The coating applied is noted with the 

specimen type. Each bar in Figure 3 is an average of two or more tests; the error bar is the average percent 

standard deviation of all tests. Figure 3 shows that the as-received adhesion of button, #2-309 O-ring, and 

specimens cut from a Medium-scale seal (Mss) is about the same as coated buttons; with the highest 

adhesion coming from the DC-7 coated buttons. Adhesion is due to molecular bonding between the 

surfaces (Ref. 34). Silicone elastomers are inorganic-organic polysiloxanes, with an inorganic Si-O-Si-O 

backbone and organic side groups attached, called radicals. The Si-O-Si bond angle and length are long 

compared to polymers with C-C backbones. These large bond lengths and angles allow for greater 

molecular movement and contribute to the flexibility and natural stickiness of silicone elastomers. 

Relatively mobile radicals at the surface are also available to contribute to adhesion (Ref. 35). Treatments 

that result in removal of radicals and/or limit backbone movement, can be expected to decrease adhesion 

(Ref. 36). Surface radicals can be removed by UV exposure in the presence of a vacuum; backbone 

movement can be limited by further cross-linking which can also be induced by UV exposure (Ref. 18). 

The high adhesion of coated buttons is caused by a surface tension driven vacuum effect created between 

the flat greased button and the flat metal mating surface (Ref. 34); when greased, the button acts like a 

suction cup. When the test article is crowned, or rounded, the edges release first thereby avoiding this 

vacuum effect. Figure 3 shows that the adhesion of non-flat test articles (i.e., O-rings and Mss) is lowered 

when coated. A crowned top prevents adhesion caused by vacuum when grease-based coatings are used. 

Figure 3 shows that the adhesion for coated buttons was lowered with the addition of boron nitride (BN); 

and that very small amounts (0.45 mg/cm2

) of the coating are sufficient to lower adhesion. The physical 

presence of the coatings prevent the molecular bonding between the surfaces that result in adhesion 

(Refs. 34 and 35). 

Figure 3.—Average adhesion of buttons and 2-309 O-rings coated with 0 or 2 mg/cm2, 

and cut sections from Medium-scale seals (Mss) coated with 0, 0.45, 1, and 2 mg/cm2; 

1 lb/in.2 = 6.9 kPa. 

0

2

4

6

8

10

12

14

16

Adhesion, lb/in2
NASA/TM—2014-218334/REV1 9 

Adhesion results for tests using full, uncut Medium-scale seals are shown in Table 1. The average 

1st cycle room temperature adhesion of Medium-scale seals was: 56.4 lbf (251 N) for uncoated seals and 

1.05 lbf (4.76 N) for seals coated with either Braycote or the Braycote-12%Z-cote. The average 2nd cycle 

room temperature adhesion for Medium-scale seals coated with DC-7-16%Z-cote was 1.3 lbf (5.78 N). 

These data show that the coatings lower room temperature adhesion by a factor of about 50. For this 

elastomer compound prior work has shown that adhesion tends to increase at lower temperatures, with 

adhesion for an uncoated Medium-scale seal of this type expected to be of the order of 300 lbf (1.33 kN) 

at –50 qC (Ref. 31). The adhesion of the DC-7 based sunscreen at –50 qC (Table 1) was 322 lbf (1.43 kN) 

which indicates the coating to be dysfunctional at this temperature; however, –50 qC is much lower than 

the current required use temperature specification for the docking seal. The minimum temperature 

specification for docking seal functionality is currently 4 qC (40 qF). There are future mission scenarios 

that are expected to require functionality at lower temperatures, thus improving the low temperature 

functionality of the sunscreens is an important facet of future work. Others have attributed the increase in 

adhesion at lower temperatures to increasing interfacial surface energy (Refs. 30 and 31). Another cause 

is believed to be the hardening of the DC-7 at –50 qC such that it acts more like a glue rather than a 

slippery lubricant. The service temperature range for DC-7 is –40 to 204 qC (Ref. 23). The average 

adhesion of Medium-scale seals coated with sunscreen (Braycote12%Z-cote or DC-7-Z-cote) at –8 qC 

was 7.4 lbf (33 N). This adhesion value can be scaled up to the full-scale docking seal by multiplying by 

5: 7.4 lbfu5 = 37 lbf (164 N) (this approximate scale-up factor is the ratio of the full-scale seal diameter 

50 in. to the Medium-scale’s diameter 10.85 in.) The maximum full-scale adhesion allowed is expected to 

be 200 lbf (890 N). The coatings thus appear to sufficiently lower adhesion to acceptable levels at 

currently specified use temperatures. 

Leakage 

Coated and uncoated specimens were exposed to UV-C, and NUV radiations, and to atomic oxygen 

in the form of an air plasma rich in AO. The effectiveness of the coating’s ability to prevent damage to 

the seal was quantified by measuring the seal’s leak rate. 

Response to AO 

A small group of small-scale seals were coated with 2 mg/cm2 of either Braycote-20%Z-cote or 

DC-7-16.4%Z-cote, exposed to AO only, and leak tested. The results of these tests are presented in 

Figure 4. Leakage increased by a factor of about 40 for the Braycote-Z-cote coated seals after small doses 

of AO compared to unexposed seals and to uncoated seals exposed to similar AO levels. It is clear this 

leakage increase is due to degradation of the coating and not the underlining elastomer because leakage 

for uncoated seals was unaffected by these low AO doses, as can be seen in Figure 4. The specimen 

coated with DC-7-Z-cote was unaffected by the AO. Dow Corning 7 is a silicone-based lubricant; as such, 

Si in the grease is expected to react with AO to form SiOx–rich compounds at the surface, as has been 

found for the silicone-based elastomer itself (Ref. 15). This prevents mass loss and may contribute to the 

durability of the DC-7-based sunscreens. Figure 5 shows images of three small-scale seals: one coated 

with Braycote-20%Z-cote and exposed to 1u1020 atoms/cm2

 AO; another coated with DC-7-16.4%Z-cote 

prior to AO exposure; and one coated with DC-7-16.4%Z-cote and exposed to 4.6u1020 atoms/cm2

 AO. 

All were coated at a rate near 2 mg/cm2

. The Braycote coated sample appears whiter because Braycote is 

thicker (more viscous) and less transparent than DC-7, and this particular recipe contained more Z-cote 

than the DC-7 coating. The whiter appearance of the Braycote coated sample might also be due to 

reaction with the AO. The important observation here is that after AO exposure the Braycote-20%Z-cote 

coating has cracked extensively in a mud-tile pattern; no such cracking was seen for the DC-7 sunscreen 

after AO exposure. 
NASA/TM—2014-218334/REV1 10 

Figure 4.—Leakage after AO exposure of small-scale seals as-received, coated with 

Braycote-20%Z-cote, or DC-7-16.4%Z-cote sunscreen; 1 lb/day = 0.454 kg/day. 

(a) (b) (c) 

Figure 5.—Optical images of small-scale seals: (a) coated with Braycote-20%Z-cote and exposed to 

1u1020 atoms/cm2 AO; (b) coated with DC-7-16.4%Z-cote prior to AO exposure; (c) coated with 

DC-7-16.4%Z-cote and exposed to 4.6u1020 atoms/cm2 AO. 

At first glance this susceptibility of Braycote to degrade when exposed to AO appears to be 

inconsistent with prior work presented by Christensen et al. (Ref. 8) who did not report a significant 

leakage increase for Braycote coated S0383 silicone seals exposed to AO. A small relative increase in 

leakage due to the presence of Braycote after AO exposure is however shown in their Figure 5 plot of 

leak rate versus equivalent hours of Sun exposure by comparing 11 to 12 percent compression nonlubricated and lubricated leakage results after AO exposure. We believe the reasons Christensen et al. did 

not see a large leakage increase for Braycote coated seals after AO exposure are because: 

x Most of the Braycote was removed prior to AO exposure. In Reference 8, seals were coated, then put 

through a set of six compression cycles with the grease being cleaned off the mating surface after 

each cycle- thereby lowering the coverage on the seal with each cycle. Only after this set of 

compression and release cycles were the seals exposed to AO. The amount of grease actually on the 

seal was not measured; and 

x Atomic oxygen exposure levels were low in Reference 8, about 8u1019 atoms/cm2

 for the coated 

seals. 

Response to UV-C and AO Exposure 

UV-C 

Figure 6 shows the response of small-scale seals made of the compound S0383-70 to UV-C radiation 

exposure. For the as-received, unprotected seals, significant damage becomes apparent at UV-C exposure 

levels of 0.4 MJ/m2

. For UV wavelengths 400 nm, the Sun is expected to deliver a total dose of about 

0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

5.E-05

6.E-05

0 1E+20 2E+20 3E+20 4E+20 5E+20

Leakage, lb/day

Atomic Oxygen, atoms/sq.cm

As-received

Braycote+Z-cote

DC-7-Z-cote
NASA/TM—2014-218334/REV1 11 

Figure 6.—Small-scale seal leakage after UV-C exposure, as-received, coated with 

2 mg/cm2 DC-7-16.4%Z-cote sunscreen, and with 6% TiO2 added to the compound; 

1 lb/day = 0.454 kg/day. 

2.5 MJ/m2

 UV per day to a surface oriented in the windward flight mode (view factor 0.26); only about 

0.11 MJ/m2

 of that dose would be from UV-C (280 nm) wavelengths (Ref. 6). Thus in the windward 

flight mode it would take 3 or 4 days for the Sun to deliver a UV-C dose of 0.4 MJ/m2

. This sensitivity 

indicates that ground-based simulations of LEO UV durability should include UV-C wavelengths. 

Addition of 6 percent TiO2 to the compound appears to provide a moderate level of protection from UV-C 

at doses less than 100 MJ/m2

 but addition of TiO2 appears to be ineffective at doses greater than 

100 MJ/m2

. Leakage for the seals coated with DC-7-16.4%Z-cote did not change as the level of UV-C 

exposure increased- indicating the seals were completely shielded from UV. The maximum exposure 

presented in Figure 6, 147 MJ/m2

, is the approximate total UV dose expected in space after 60 days of 

windward (ram) oriented LEO flight. The effectiveness of the coating to protect from UV is not 

unexpected since the transmittance of radiation through ZnO drops to near zero at wavelengths less than 

380 nm (Refs. 37 and 38) and ZnO is a well-known additive to commercial UV protective sunscreens and 

thermal control paints (Refs. 12 to 14, and 39). 

UV Attenuation 

Rutile and wurtzite are the most common and stable forms of TiO2 and ZnO, respectively. The 

attenuation of UV by these compounds is due to reflection, scattering, and absorption. The refractive 

index and reflectance of TiO2 and ZnO are high, similar to those for diamond (Ref. 40). The very high 

refractive index for TiO2 is partially responsible for its white appearance. Both compounds are 

semiconducting with electronic structures consisting of bands of orbitals separated by an energy band 

gap. Incoming UV radiation with energy equal to or greater than this energy band gap can excite an 

electron and move it from the valence band to the conduction band there-by absorbing the incoming 

radiation (Ref. 41). The UV absorbance of TiO2 and ZnO drop off at wavelengths greater than 380 nm 

because wavelengths greater than this are not energetic enough to overcome their band gap energies. Even 

though TiO2 and ZnO have similar band gap energies (about 3.1 eV) TiO2 absorbs more UV-B and -C 

radiation (340 nm) because it has more densely packed electron states that allow more absorption 

possibilities compared to ZnO (Refs. 13 and 14); ZnO however absorbs more UV-A (!350 nm). Thus a 

broader spectrum of UV protection should be able to be achieved through the inclusion of both TiO2 and 

ZnO particles. The inability of the addition of 6 percent TiO2 to the S0383-70 compound to protect it from 

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0.01 0.1 1 10 100

Leakage, lb/day

UV-C exposure, MJ/m2

As-received

DC-7-Z-cote

6%TiO2
NASA/TM—2014-218334/REV1 12 

UV is likely due to the low occurrence of TiO2 particles at the surface. Due to the proprietary nature of 

the S0383-70 compound the TiO2 particle size used is unknown; however, since absorbance is influenced 

by particle size it is believed improvements to the 6 percent TiO2 compound are possible through the use 

of TiO2 particles with diameters near 60 to 100 nm (Ref. 14). 

Another facet to consider in the development of protective coatings for silicone elastomers is the 

sensitivity of silicone to different UV wavelengths. The literature indicates that the absorption of UV in 

silicone elastomers drops off precipitously at wavelengths !300 nm (Refs. 42 and 43). Presumably 

because longer wavelengths are not energetic enough to break molecular bonds in the elastomer; and thus, 

are not energetic enough to do damage. The shorter wavelengths do break bonds and are thus absorbed in 

the process. However, attenuation increases with decreasing wavelength thus the depth of UV penetration 

decrease at the shorter wavelengths. This is why short VUV wavelengths are effective at lowering 

adhesion without doing significant damage to the elastomers: because the zone affected by the VUV is 

near the surface only. At the surface VUV breaks chemical bonds which contributes to additional crosslinking and a decline in adhesive characteristics (Ref. 18). Since silicone elastomers are more sensitive to 

wavelengths 300 nm, it is beneficial to use TiO2 in protective coatings since the absorbance of TiO2 is 

much higher at the lower wavelengths compared to ZnO. 

Atomic Oxygen 

Table 2 presents leakage results for small-scale seals coated with 2 mg/cm2

 DC-7-16.4%Z-cote and 

exposed to UV-C and AO. Assuming an AO exposure of 4.4u1019 atoms/cm2

-day, Table 2 AO exposures 

range from about 7 to 50 days of windward LEO exposure. Assuming 2.5 MJ/m2

-day UV exposure, the 

UV-C exposures varied in energy from 4 to 30 days of windward UV exposure. Table 2 data indicates the 

DC-7-Z-cote sunscreen effectively protects the seals from UV and AO at these exposures, since all 

leakage rates after exposure were very close to as-received, unexposed levels (Fig. 6, ~1u10-6 lb/day). The 

leakage of an uncoated seal at these exposure levels is of the order of 1u10–3 lb/day (Fig. 6). 

Response to NUV and AO 

Figure 7 shows the leakage results for small-scale seals exposed to both NUV and AO combined. 

Atomic oxygen exposures of 2.2u1020 atoms/cm2

 were done first. Near-UV exposures were then done at 

Q-Lab Weathering Research Service (Qlab) in air; or at the GRC in vacuum. Leakage differences 

between like exposures at Qlab and GRC were insignificant; thus the presence or absence of air had no 

noticeable effect on seal damage for these cases. The 113 and 240 MJ/m2

 NUV exposures caused leakage 

to increase markedly for the 6% TiO2 compound, rising 2 orders of magnitude above as-received leakage 

levels, indicating only moderate protection from UV due to the addition of TiO2 to the compound. 

Leakage levels were unchanged compared to as-received leakage (~1u10-6 lb/day, see Fig. 6) for all seals 

coated with DC-7-16.4%Z-cote (DC-7+Z in the legend of Fig. 7) indicating complete protection from 

NUV and no ill effects from AO at the exposure levels tested. 

TABLE 2.—LEAKAGE FOR SMALL-SCALE SEALS COATED 

WITH DC-7-Z-COTE AND EXPOSED TO UV-C AND AO; 

1 lb/DAY = 0.454 kg/DAY 

AO, atoms/cm2 UV-C, 

MJ/m2

Leakage, 

lb/day 

3.0E+20 10.14 1.01E-06 

4.6E+20 15.29 9.81E-07 

4.6E+20 15.29 6.04E-07 

4.6E+20 15.29 4.20E-07 

8.8E+20 29.41 1.00E-06 

2.2E+21 73.53 4.00E-06 
NASA/TM—2014-218334/REV1 13 

Figure 7.—Leakage, after NUV + AO exposure, of small-scale seals as-received, coated 

with 2 mg/cm2 DC-7-16.4%Z-cote sunscreen, and with 6% TiO2 added to the compound 

prior to curing; 1 lb/day = 0.454 kg/day. 

Figure 8.—Leakage, after 30 MJ/m2 UV-C exposure, of small-scale 

seals coated with various amounts of DC-7-16.4%Z-cote sunscreen; 

1 lb/day = 0.454 kg/day. 

Leakage Response to Rate of Coverage 

Figure 8 shows the leak rate of small-scale seals after being coated with various levels of 

DC-7-16.4%Z-cote and exposed to 30 MJ/m2

 UV-C. The characteristic of the lower coating levels 

(0.46 and 0.53 mg/cm2

) is that “all” of the coating is wiped off. The coating level could not be lowered 

further by wiping it off with clean nitrile gloves. Figure 8 shows that some protection from UV-C is 

present at very low coverage rates, and that the seal appears to be fully protected at coverage rates greater 

than 0.9 mg/cm2

. 

1.E-07

1.E-06

1.E-05

1.E-04

100 150 200 250

Leakage, lb/day

NUV, MJ/m2

Qlab DC-7+Z w/AO

Qlab As-Rec. w/AO

Qlab 6% TiO2 w/AO

GRC DC-7+Z w/AO

GRC As-Rec. w/AO

GRC 6% TiO2 w/AO

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0123

Leakage after 30 MJ/m2

UV-C, lb/day

Rate of Sunscreen coverage, mg/cm2
NASA/TM—2014-218334/REV1 14 

(a) (b) 

Figure 9.—(a) Coated 5.4 mm wide small-scale seal after UV exposure; 

(b) similar seal after UV exposure and leak testing. 

Coating Durability 

Figure 2 shows various seals and test specimens after being adhesion tested. In Figure 2 the 

Medium-scale seal, 1 button, 1 cut Medium-scale specimen, and 1 of the O-rings were coated prior to 

adhesion testing; the other specimens shown were not coated. The condition of the coating appears intact 

on the button, cut Medium-scale specimen, and Medium-scale seal, however, the coating appears to have 

moved off the crown of the O-ring. Such behavior is important since we would like to know if the seal is 

still protected after undocking, and if significant levels of the coating can be expected to be left behind on 

the mating surface. Studies of aspects affecting coating durability are planned and include time between 

application and use; and prior AO and UV exposure. Our observations to date are included here. 

The rate of coating coverage was measured before and after adhesion and leakage testing for a set of 

4 buttons and 4 small-scale seals coated with Braycote-20%Z-cote; the coverage decreased 10 percent for 

the buttons, and 27 percent for the small-scale seals. Coverage decreased an average of 4.5 percent for 

4 small-scale seals coated with DC-7-16.4%Z-cote. The optical micrographs of the DC-7-16.4%Z-cote 

coated small-scale seals before and after leak testing shown in Figure 9 indicate the coating to be durable 

to a leak test, which is similar to a docking/undocking operation. 

Figure 10(a) shows Medium-scale seal –0282 after coating with 1.82 mg/cm2

 DC-7-16.4%Z-cote but 

before any testing; Figure 10(b) shows –0282 after adhesion testing at –50 qC. Adhesion was very high 

for this test (see Table 1) and Figure 10(b) shows that much of the coating was removed during the 

adhesion test. However, the coating remained intact after adhesion tests at room temperature, –8 qC, and 

65 qC, as shown in Figure 10(c) and after multiple tests at these temperatures. Figure 11 shows the 

Medium-scale seal –0283 after being coated with 1.89 mg/cm2

 DC-7-16.4%Z-cote, adhesion tested at 

–8 qC with an 8 h dwell time, and then subjected to 25 room temperature adhesion tests using 1 min dwell 

times. Figure 11 shows that after 26 adhesion tests some of the sunscreen coating has been worn off. 

Future work includes UV and AO exposures after adhesion and/or leak testing so the level of sunscreen 

protection after a simulated docking operation can be determined. One related test has been accomplished 

to date: in this series of tests a small scale seal was coated with 2.04 mg/cm2

 DC-7-Z-cote and exposed to 

4.6u1020 atoms/cm2

 AO, then tested with leakage found to be 8u10–7 lb/day; the seal was then exposed to 

15.3 MJ/m2

 UV-C and leak tested again (resulting in 9.8u10–7 lb/day) which shows that the first leak test 

did not significantly disturb the sunscreen coating. Since the coating appears to be intact after a small 

number of compression cycles (1 to 3 adhesion and/or leak tests) and full protection is achieved at low 

mg/cm2

 levels of coverage, we currently believe the docking seal will remain protected from UV and AO 

after undocking in space. 

There is a use limitation in the DC-7 product literature against using DC-7 on silicone rubber O-rings 

due to possible degradation of the rubber; and Parker S0383-70 product literature recommends against 

using this compound with silicone oil (Ref. 44). Reactions between fluids and elastomer compounds are 

generally tested at elevated temperatures (302 qF) with complete immersion. Reactions between silicone 

elastomers and Dow Corning 4 Compound (a grease similar to DC-7) at room temperature are reported to 
NASA/TM—2014-218334/REV1 15 

 (a) (b) (c) 

Figure 10.—(a) Medium-scale seal –0282 after coating; (b) Coated –0282 after adhesion testing at –50 qC; 

(c) Coated Medium-scale seal –0283 after adhesion testing at –8 qC. 

Figure 11.—Coated Medium-scale seal –0283 

after adhesion testing at –8 qC (8 hr dwell) and 

then 25 more times at room temperature 

(1 min. dwell on each cycle). 

be moderate with a +5 percent volume change and –5 hardness point change after 1 day immersion 

(Ref. 45). We have been using sunscreens containing DC-7 on silicone O-rings for 2 years and have 

found no degradation. This absence of degradation is probably because our application does not require 

immersion, and our test temperatures are relatively low. A slight reaction with the silicone fluid may 

actually be beneficial in our applications wherein characteristic shrinking and hardening of S0383-70 

caused by the vacuum and UV of LEO are mitigated by DC-7. If for whatever reasons this combination 

(S0383-70 and DC-7) is found to be undesirable, a fluorosilicone elastomer compound could be used 

instead of S0383-70 since fluorosilicones are known to be more compatible with silicone fluids (Refs. 45 

and 46) and also have a very low use temperature (–73 qC). For reasons of AO durability, we believe it is 

important for both the elastomer compound and the sunscreen to be silicone-based. The presence of Si is 

key to establishing a stable sunscreen coating and avoiding AO erosion through the creation of durable 

SiOx-rich reaction products. 

Conclusions 

A class of grease-based coatings has been conceived and tested for use on elastomer docking seals 

used on spacecraft in low-Earth-orbit. The coatings use UV blocking additives to protect the coated article 

from UV radiation. The DC-7-16.4%Z-cote coating is silicone-based which may prevent its erosion by 

AO through the formation of non-volatile SiOx-rich compounds at the surface. Tests showed that the 

coatings successfully mitigate adhesion at temperatures greater than or equal to –8 qC. To realize low 

adhesion with a grease-based sunscreen the seal must be constructed using a curved crown to prevent 

suction-cup like adhesion caused by the vacuum formed between greased flat surfaces. Adhesion was 

very high for the DC-7-16.4%Z-cote coated seal tested at –50 qC indicating the coating’s inability to 

prevent high adhesion at this low temperature. The sunscreen made with Braycote performed badly when 
NASA/TM—2014-218334/REV1 16 

exposed to AO with leakage increasing by a factor of about 40 over base-line values. If further testing 

confirms Braycote’s intolerance to AO, use of Braycote on docking seals exposed to AO will not be 

recommended. Seals coated with DC-7-16.4%Z-cote were not significantly damaged after being exposed 

to a variety of AO and UV levels up to 2.2u1021 atoms/cm2

 and 245 MJ/m2. These exposure levels are 

characteristic to what might be expected after 50 to 100 days in LEO. Ultraviolet radiation exposures 

included UV-C and NUV wavelengths. The DC-7-16.4%Z-cote coating proved difficult to wipe 

completely off, with approximately 0.4 mg/cm2

 remaining after repeated attempts to wipe it off with clean 

nitrile gloves. Seals coated with DC-7-16.4%Z-cote appeared protected from UV at coverage rates as low 

as 0.9 mg/cm2

. The DC-7-16.4%Z-cote coating proved to be durable to small numbers (1 to 3) of 

compression cycles, and to provide UV protection after a compression cycle. The data presented indicate 

DC-7-16.4%Z-cote will protect docking seals from UV and AO and mitigate unwanted adhesion as long 

as use temperature is greater than or equal to –8 qC and the seal’s active surface is not flat. 

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