WO2014004794A1

WO2014004794A1 - Seals having been pretreated with atomic oxygen for protection in space environments

Info

Publication number: WO2014004794A1
Application number: PCT/US2013/048133
Authority: WO
Inventors: Christopher Daniels; Janice WASOWSKI
Original assignee: University of Akron
Current assignee: University of Akron
Priority date: 2012-06-27
Filing date: 2013-06-27
Publication date: 2014-01-03
Anticipated expiration: 2014-12-27

Description

SEALS PRETREATED WITH ATOMIC OXYGEN

[0001] This invention was made with United States government support under contract NNC08CA35C awarded by the National Aeronautics and Space Administration (NASA) . The U.S. government has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] The present application claims the benefit of U.S. Provisional Application No. 61/665,044, filed June 27, 2012.

FIELD OF THE INVENTION

[0003] The present invention generally relates to seals, and more particularly to seals and sealing constructs useful in space environments. In some embodiments, this invention relates to seals and sealing constructs which have been pretreated with atomic oxygen before being placed in a space environment.

BACKGROUND OF THE INVENTION

[0004] Space docking systems are utilized to join two or more manned spacecraft together. They are designed to operate in low-Earth-orbit (LEO), in high-Earth-orbit (HEO), on extraterrestrial surfaces, and in deep space locations. Space docking systems and other components of objects exposed to space environments include seals in various locations, and these seals are sometimes exposed to the space environment. Some examples include seals at the interface between two vehicles, at windows, and at fluid and electrical connections. Precious resources necessary for manned and unmanned spaceflight, such as potable water, coolant, and breathable air and refrigerant are retained and confined by seals. Any damage to or other compromising of the seal increases the leak rate and loss of the pressurized fluid or gas that the seal is intended to retain.

[0005] These seals must withstand multiple uses and hold up against the operational temperate extremes to which they are exposed, and are therefore manufactured from polymers. The polymers are negatively affected by the space environment, including solar radiation in the form of ultraviolet light and reactive elements such as atomic oxygen. The radiation and reactive elements (such as atomic oxygen) to which these seals are exposed compromises the sealing surfaces of the seal in short time spans.

[0006] The exposure to radiation rays and to reactive elements in the space environment compromises the sealing surface and the seal in general. These seals tend to be formed of elastomers and the radiation and reactive elements cause the elastomer to become brittle and to erode at the molecular level and shrink and crack. This compromises the functioning of the seal because cracked surfaces do not form good seals. Mission profiles and durations are limited by the ability of a seal to resist the space environment. Therefore, there is a need in the art to improve upon the ability of a seal to resist the detrimental effects of the space environment.

SUMMARY OF THE INVENTION

[0007] A first embodiment of this invention provides a method of improving the seal performance of a seal exposed to solar radiation and/or reactive elements in a space environment, the method comprising: pre-treating the seal terrestrially with atomic oxygen before use in a space environment.

[0008] A second embodiment of this invention provides a method as in the first embodiment, wherein the seal is manufactured from the group consisting of butyl rubber, silicone, fluorocarbon, polyurethane, nitrile, neoprene, ethylene propylene, fluorosilicone, natural rubber, butadiene, polytetrafluoroethylene, styrene- butadiene and fluorinated ethylene propylene.

[0009] A third embodiment provides a method as in either the first or second embodiment, wherein the seal is a silicone elastomer.

[0010] A fourth embodiment provides a method as in any of the first through third embodiments, wherein the silicone elastomer has a collectible volatile condensable material of about less than 0.1%.

[0011] A fifth embodiment provides a method as in any of the first through fourth embodiments, wherein the seal is pretreated by exposure of between 6.5 x 10¹⁹ atoms/in.² and 6.5 x 10²¹ atoms/in.² of atomic oxygen. [0012] A sixth embodiment provides a method as in any of the first through fifth embodiments, wherein the seal is pretreated by exposure of between 3.2 x 10²⁰ atoms/in.² and 3.2 x 10²¹ atoms/in.² of atomic oxygen.

[0013] A seventh embodiment provides a method as in any of the first through sixth embodiments, wherein the seal is pretreated by exposure of between 6.5 x 10²⁰ atoms/in.² and 1.3 x 10²¹ atoms/in.² of atomic oxygen.

[0014] An eighth embodiment provides a sealing construct in a space environment comprising: a seal-bearing object; a seal on said seal-bearing object, said seal including a seal body having a sealing surface wherein said seal body is pretreated by terrestrial exposure to atomic oxygen; and a seal-engaging object selectively engaged with said seal-bearing object through said seal, said seal- engaging object having a sealing surface, wherein, when said seal-engaging object is selectively engaged with said seal-bearing object, said sealing surface of said seal-engaging object engages said sealing surface of said seal, and said seal is compressed between the seal-bearing object and the seal-engaging object.

[0015] A ninth embodiment provides a sealing construct as in the eighth embodiment, wherein the seal is manufactured from the group consisting of butyl rubber, silicone, fluorocarbon, polyurethane, nitrile, neoprene, ethylene propylene, fluorosilicone, natural rubber, butadiene, polytetrafluoroethylene, styrene- butadiene and fluorinated ethylene propylene.

[0016] A tenth embodiment provides a sealing construct as in the eighth or ninth embodiments, wherein the seal is a silicone elastomer.

[0017] An eleventh embodiment provides a sealing construct as in any of the eighth through tenth embodiments, wherein the silicone elastomer has a collectible volatile condensable material of about less than 0.1%.

[0018] A twelfth embodiment provides a sealing construct as in any of the eighth through eleventh embodiments, wherein the seal body is pretreated by exposure of between 6.5 x 10¹⁹ atoms/in.² and 6.5 x 10²¹ atoms/in.² of atomic oxygen.

[0019] A thirteenth embodiment provides a sealing construct as in any of the eighth through twelfth embodiments, wherein the seal body is pretreated by exposure of between 3.2 x 10²⁰ atoms/in.² and 3.2 x 10²¹ atoms/in.² of atomic oxygen. [0020] A fourteenth embodiment provides a sealing construct as in any of the eighth through thirteenth embodiments, wherein the seal body is pretreated by exposure of between 6.5 x 10²⁰ atoms/in.² and 1.3 x 10²¹ atoms/in.² of atomic oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Fig. 1 is a schematic representation of an embodiment of the sealing construct in a front elevation view, but with a portion thereof, namely a seal- engaging member, not shown;

[0022] Fig. 2 is a schematic representation of a cross section of the sealing construct of Fig. 1, taken along the line 2— 2 of Fig. 1 and showing a seal-engaging member before engagement with the seal;

[0023] Fig. 3 is a schematic representation of a cross section of the sealing construct as in Fig. 2, but shown with the seal-engaging member engaging and compressing the seal;

[0024] Fig. 4 is a schematic representation of the mass point leak rate test system employed in the experimental proof of the present invention, the test system serving to quantify the mass of dry air that passed the seal location;

[0025] Fig. 5 is a schematic representation of the test fixture employed in the system of Fig. 4;

[0026] Fig. 6 is a graph of the leak rate performance of S0383-70 flight specimens post on-orbit exposure; and

[0027] Fig. 7 is a graph of the leak rate performance of S0899-50 flight specimens post on-orbit exposure.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0028] To better understand the interaction between materials and the environment of space, a series of experiments was undertaken called the Materials International Space Station Experiment (MISSE) . During the MISSE experiment, select materials were placed on the outside of the International Space Station (ISS) where they were exposed to a low-Earth orbit environment. Materials studied within the body of work were exposed for 537 days. It was unexpectedly found that the pretreatment of a seal surface with exposure to atomic oxygen improved the ability of the seal to better resist the detrimental effects of a space environment.

[0029] In particular embodiments, silicone elastomer seals are of interest. It is posited that during pretreatment with atomic oxygen, the atomic oxygen reacts with the surface of the silicone elastomer making up the surface of the seal to create a higher concentration of silicon oxides (SiOx) at the surface of the seal. The SiOx, in turn, protects the bulk elastomer from further degradation, and reduces permeation through the elastomer.

[0030] One embodiment of the invention is directed to a method of improving the seal performance of a seal exposed to solar radiation and/or solar elements in a space environment. This method includes pre-treating the seal terrestrially with atomic oxygen before use in a space environment. The pre-treatment of the seal with atomic oxygen prior to space flight has been found to form a means of protection from the effects of space environment.

[0031] The atomic oxygen may be produced by any source, such as, but not limited to, radio frequency (RF) plasma, as long as other degrading environments, such, but not limited to, ultraviolet radiation, are not present.

[0032] The type of seal for which this method can be use is selected from the group consisting of docking seals , window seals, human transfer hatch seals, equipment transfer hatch seals, fluid transfer connection seals, electrical transfer connection seals, cargo bay door seals, control surface seals, inter panel seals, joint seals and landing gear door seals.

[0033] The seal of the method is formed of any suitably compressible material for use in space environments. In some embodiments, the material of the seal is an elastomer. In some embodiments, the elastomer is selected from the group consisting of butyl rubber, silicone, fluorocarbon, polyurethane, nitrile, neoprene, ethylene propylene, fluorosilicone, natural rubber, butadiene, polytetrafluoroethylene, styrene-butadiene and fluorinated ethylene propylene. In some embodiments, the elastomer has a collectible volatile condensable material of less than 1%. In other embodiments, the elastomer has a collectible volatile condensable material of less than 0.5%, in other embodiments, less than 0.25%, in other embodiments, less than 0.15% and, in other embodiments, less than 0.1%. [0034] The seals are pretreated with atomic oxygen simply by exposing one or more surfaces thereof to atomic oxygen terrestrially. By terrestrially it is meant that the exposure is performed in a controlled environment with limited exposure to other damaging contaminants, including ultraviolet radiation and orbital debris. The exposure can be to all surfaces at once or to one or more surfaces. For example, a seal laying on a flat surface could not have the surface on which it is laying treated by atomic oxygen, so it would need to be reoriented after treatment of exposed sides in order to treat the side on which it is laying, if such treatment is desired.

[0035] In one embodiment, the seal is pretreated by the exposure of between 6.5 x 10¹⁹ atoms/in.² and 6.5 x 10²¹ atoms/in.² of atomic oxygen. In yet another embodiment, the seal is pretreated by exposure of between 3.2 x 10²⁰ atoms/in.² and 3.2 x 10²¹ atoms/ in.² of atomic oxygen. And, in yet another embodiment, the seal is pretreated by exposure of between 6.5 x 10²⁰ atoms/in.² and 1.3 x 10²¹ atoms/ in.² of atomic oxygen.

[0036] Portions of an embodiment of a sealing construct in accordance with this invention are shown with reference to Figs. 1-3. Fig. 1 provides a front elevation view of a seal bearing object 112 bearing a seal 114, and the entire sealing construct is shown in cross section in Figs. 2 (seal-engaging object 120 disengaged) and 3 (seal-engaging object 120 engaged), the cross section being taken along the line 2— 2 in Fig. 1. As seen in Figs. 2 and 3, the sealing construct is designated by the numeral 110.

[0037] The sealing construct 110 includes a seal-bearing object 112, bearing a seal 114 with a sealing surface 116 that is exposed when not engaged by a seal engaging object 120. The sealing construct 110 also includes a seal-engaging object 120 that selectively engages with the seal-bearing object 112 through the seal 114. The seal-engaging object 120 has a sealing surface 122 that engages the sealing surface 116 of the seal 114, when the seal-engaging object 120 is selectively engaged with the seal-bearing object 112, as generally depicted by the movement of the seal-engaging object 120 in the direction of the arrows in Fig. 2. Upon selective engaging movement, the seal 114 is compressed between the seal- bearing object 112 and the seal-engaging object 120 as shown in Fig. 3. At least a portion of the surface 116 of the seal 114 is pretreated with atomic oxygen as per the teachings herein. Thus the sealing construct 110 is improved over those of the prior art.

[0038] In light of the foregoing, it should be appreciated that the present invention significantly advances the art by providing a seal having been pretreated for protection in a space environment that is structurally and functionally improved in a number of ways. While particular embodiments of the invention have been disclosed in detail herein, it should be appreciated that the invention is not limited thereto or thereby inasmuch as variations on the invention herein will be readily appreciated by those of ordinary skill in the art. The scope of the invention shall be appreciated from the claims that follow.

EXPERIMENTAL

Test Specimens

[0039] The test specimens were standard AS568A size 2-106 O-rings manufactured from two silicone elastomer compounds, Parker Hannifin S0383-70 and, Parker Hannifin S0899-50. The measurements of specimen durometer hardness, prior to any exposures, were 68.1 and 36.8, respectively. The nominal dimensions are shown in Table 1. Table 1. Nominal dimensions of the test specimens.

[0040] A characteristic of the chosen silicone elastomer compounds was that they contained minimal amounts of low weight molecules that are released when exposed to a vacuum environment. This desirable characteristic minimizes the amount of material that would collect on spacecraft optics, solar panels, and instruments and is achieved during the post-cure cycle of silicone manufacture. Verifying this characteristic leads credibility that the specimens were properly manufactured. Therefore, random samples from each batch of O-rings were verified to be low outgassing per ASTM E595-07, as defined by having a total mass loss (TML), less than 1% and a collectible volatile condensable material (CVCM) less than 0.1%, as shown in Table 2. Table 2. Outgassing Values.

Surface Pretreatment

[0041] Select flight specimens were pretreated with atomic oxygen (AO) to nominal /In fluence of 1.5xl0²¹ atoms/in.². The quantities, compound designation, and description of the test specimens are shown in Table 3.

Table 3. Quantities of flight test specimens and surface pretreatments.

Control Specimens

[0042] The control specimens were surface pretreated (select samples) and mounted into their assembly at the same time and utilizing identical procedures as those used for the on-orbit flight samples. The assembly was placed in a secure location while the flight samples were exposed on-orbit. The secure location was in a laboratory environment, without strict temperature or humidity control. The control specimens were disassembled and leak rate tested at the same time period using the identical test apparatus utilized for the flight exposed samples. On-Orbit Exposure

[0043] The flight specimens ascended into orbit attached to the MISSE 6B Passive Experiment Container (PEC) abroad Space Shuttle flight STS-123. The PEC was placed external to ISS on March 13, 2008, retrieved September 1, 2009, for total on-orbit duration of 537 days, and returned to Earth aboard STS-128. To determine the AO fluence that impinged upon the ram-facing and wake-facing surfaces, a series of Kapton® H polyimide samples were placed in an adjacent location to the test specimens. The AO fluence was determined individually from ram-facing and wake-facing specimens by quantifying the erosion of the polyimide as outlined in ASTM E 2089-00, which is herein incorporated by reference.

Description of the testing apparatus and procedures

[0044] The leak rate of each test specimen was measured using a mass point leak rate system. The test apparatus quantified the mass of dry air that passed the seal location, including the amount of gas that was transported through any leakage points (e.g., through microcracks and at the sealing interfaces) and that which permeated through the elastomer compound. The system consisted of a gas reservoir immersed in a water bath to moderate any temperature fluctuations of the gas within the closed system, as shown in Fig. 4. The temperature of the water was monitored by a resistance temperature detector with an accuracy of ±0.35°F (0.20°C). The gas reservoir was connected to the test fixture containing the test specimen.

[0045] The test fixture was manufactured from stainless steel an assembled such that each seal specimen was compressed to 25% of its nominal height. The specimen was compressed between two flat surfaces, as shown in Fig. 5, each with roughness better than 16 μίη. (0.4 μπι). Dry air was used as the test gas and was supplied o the volume at the test specimen's interior. The exterior of the seal was subjected to vacuum pressure such that the targeted pressure differential across the seal was 14.7 psid (101 kPa). The gas pressure of the volume was monitored using two pressure transducers. The pressure transducers provided 0.05% full-scale accuracy over a range of 0-20 psig (0-140 kPa). To ensure that the system of supply lines from the gas reservoir to the test section was hermetic, the system was checked with a helium leak detection system and was found to leak no greater than lxlO^"9 atm-cc/s. Hence, any decrease in the mass of gas within the closed system was attributed to the test specimen leakage.

[0046] The leak rate of each test specimen was quantified using the mass point leak rate technique with comprehensive error analysis. This technique returns the leak rate of test specimen, using the specific gas of interest (air), as well as the uncertainty of the individual measurement.

Results of On-Orbit AO and {/^exposure

[0047] Using accepted techniques, the effective AO fluence was determined using Kapton polyimide. Polyimide samples were weighed before and after flight. The difference in their weight indicates the amount of AO the sample received.

[0048] The diameter of the polyimide specimens was 0.39 in. The weights of the ram- and wake-facing polyimide specimens were reduced from 7.1364x10^" ^s±2.9xl0^"8 and 7.1798x10 ^s ±3.1xl0^"8 lbm before flight to 5.6831x10 ^s and 7.1055x10 ^s lbm after flight, respectively. As shown in Table 4, the weight loss of the Kapton specimens indicated that the ram-facing exposure assembly was subjected to 1.2xl0²² atoms/in.² while on-orbit, while the wake-facing test specimens received approximately 5% of that AO exposure, 6.0 xlO²² atoms/in.². A difference in AO fluence between the ram- and wake-facing sides of the PEC was expected as previous research has shown the predominance of AO flux occurs on the ram-facing side during orbit. The control specimens were reported to have an AO fluence of -6.2xl0^"19±4.4xl0¹⁹ atoms/in.², indicating the experimental scatter was on the order of 10% of the wake-facing fluence measurement. The erosion of the polyimide specimens was attributed to the impingement and reaction of AO only, per the ASTM standard; however, it is acknowledged that simultaneous exposure to UV may have altered the erosion rate.

Table 4. On-Orbit exposure levels of the polyimide specimens.

[0049] The UV exposure of the ram- and wake-facing exposure assemblies was quantified through numerical analyses. The Boeing Integrated ISS TRASYS model used ISS telemetry data to compute the number of orbits, beta angle, and solar exposure including albedo. The model provided estimates of 2620 and 1950 equivalent sun hours (ES ) for the ram- and wake-facing directions, respectively. Additionally, the temperature and vacuum conditions of the PECs were assumed to be identical. Any variation between the test specimens exposed in ram- and wake- facing orientations was assumed to be attributable to the resilience of the elastomer compound and the different AO fluences.

Leak Rate Results of the On-Orbit specimens

[0050] The leak rate of the ram-facing, wake-facing, and control specimens of the two elastomer compounds were quantified and in general, the leak rate performance of the S0383-70 silicone elastomer compound was superior to the other compound investigated.

[0051] The leak rates of the S0383-70 test specimens, after having been exposed to approximately 18 months of on-orbit space environment, were found to have increased relative to the control test specimens.

[0052] The leak rate of the test specimens manufactured from S0383-70 increased with increased exposure to AO, as shown in Fig. 6. Of particular interest, specimens with AO pretreatment exhibited lower leak rate after similar levels of on-orbit exposure than those specimens without pretreatment. For the wake-facing specimens on the left of Fig. 6, the average leak rate of those without pretreatment was 2.2 times greater than those with AO pretreatment.

[0053] When similar comparisons were made for the six ram-facing specimens on the right of Fig. 6, for those having received AO pretreatment, the average leak rate was 3.0 times greater than for those specimens without A O pretreatment than those with pretreatment.

[0054] The leak rates of the S0899-50 test specimens, after having been exposed to approximately 18 months of on-orbit space environment, were found to have increased relative to the control test specimens.

[0055] Similar to observations of the S0783-70 compound, the AO pretreated S0899-50 specimens had a lower leak rate than those without pretreatment. The average leak rate of those specimens without pretreatment was 2.1 times greater than those specimens with pretreatment, as shown on the left portion of Fig. 7. Similarly, analysis of the ram-facing specimens showed that the S0899-50 specimens without pretreatment exhibited a 1.4 times greater leak rate than the specimens with pretreatment, indicating that the AO pretreatment process has a beneficial effect on leak rate performance.

Claims

CLAIMS is claimed is:

A method of improving the seal performance of a seal exposed to solar radiation and/or reactive elements in a space environment, the method comprising: pre-treating the seal terrestrially with atomic oxygen before use in a space environment.

The method of claim 1 wherein the seal is manufactured from the group consisting of butyl rubber, silicone, fluorocarbon, polyurethane, nitrile, neoprene, ethylene propylene, fluorosilicone, natural rubber, butadiene, polytetrafluoroethylene, styrene-butadiene and fluorinated ethylene propylene.

The method of claim 2 wherein the seal is a silicone elastomer.

The method of claim 2 wherein the silicone elastomer has a collectible volatile condensable material of about less than 0.1%.

The method of claim 1 wherein the seal is pretreated by exposure of between 6.5 x 10¹⁹ atoms/in.² and 6.5 x 10²¹ atoms/in.² of atomic oxygen.

The method of claim 1 wherein the seal is pretreated by exposure of between 3.2 x 10²⁰ atoms/in.² and 3.2 x 10²¹ atoms/in.² of atomic oxygen.

The method of claim 1 wherein the seal is pretreated by exposure of between 6.5 x 10²⁰ atoms/in.² and 1.3 x 10²¹ atoms/in.² of atomic oxygen.

8. A sealing construct in a space environment comprising:

a seal-bearing object; a seal on said seal-bearing object, said seal including a seal body having a sealing surface wherein said seal body is pretreated by terrestrial exposure to atomic oxygen; and

a seal-engaging object selectively engaged with said seal-bearing object through said seal, said seal-engaging object having a sealing surface, wherein, when said seal-engaging object is selectively engaged with said seal-bearing object, said sealing surface of said seal-engaging object engages said sealing surface of said seal, and said seal is compressed between the seal-bearing object and the seal-engaging object.

9. The sealing construct of claim 8 wherein the seal is manufactured from the group consisting of butyl rubber, silicone, fluorocarbon, polyurethane, nitrile, neoprene, ethylene propylene, fluorosilicone, natural rubber, butadiene, polytetrafluoroethylene, styrene-butadiene and fluorinated ethylene propylene.

10. The sealing construct of claim 9 wherein the seal is a silicone elastomer.

11. The sealing construct of claim 10 wherein the silicone elastomer has a collectible volatile condensable material of about less than 0.1%.

12. The sealing construct of claim 8 wherein the seal body is pretreated by exposure of between 6.5 x 10¹⁹ atoms/in.² and 6.5 x 10²¹ atoms/in.² of atomic oxygen.

13. The sealing construct of claim 8 wherein the seal body is pretreated by exposure of between 3.2 x 10²⁰ atoms/in.² and 3.2 x 10²¹ atoms/in.² of atomic oxygen.

14. The sealing construct of claim 8 wherein the seal body is pretreated by exposure of between 6.5 x 10²⁰ atoms/in.² and 1.3 x 10²¹ atoms/in.² of atomic oxygen.