.A> I _ • " S AIAA-2001-1414 EFFECTS OF ELECTRONS, PROTONS, AND ULTRAVIOLET RADIATION ON THERMOPHYSICAL PROPERTIES OF POLYMERIC FILMS Dennis A. Russell', John W. Connell t, Lawrence B. Fogdalli, Werner W. Winkler ! ABSTRACT INTRODUCTION "l_he response of coated thin polymer films to Significant attention has recently been given to the ultraviolet (UV), electron and proton radiation simul- concept of using lightweight, compliant structures that taneously has been evaluated, with selected measure- can be folded into the compact volumes of conventional ments in situ. Exposure was intended to simulate the launch vehicles and subsequently deployed on orbit as a electron and proton radiation environment near the means to achieve very large structures in space, hence Earth-Sun Lagrangian points (L1 and L2) for five years the term Gossamer structures. Relative to on-orbit con- and ~1000 equivalent solar hours (ESH) UV. These struction this approach can offer significant reductions orbital environments are relevant to several potential in cost, complexity and risk to astronauts. These struc- missions such as the Next Generation Space Telescope tural concepts typically are comprised of a support and Geomagnetic Storm Warning, both of which may structure that can be deployed via mechanical, inflat- use thin film based structures for a sunshade and solar able or other means and subsequently rigidized; and a sail, respectively. The thin film candidates (12.5 p.m thin film that serves as an antenna, sail, reflector, thick) consisted of commercially available materials concentrator, sunshade, etc. Recent demonstrations of (Kapton® E, I-IN, Upilex® S, CP-1, CP-2, TOR-RC large deployable/inflatable film-based space structures and TOR-LMBP) that were metalized on one side with such as a 20 m solar sail (Znarnya-2, "New Light") and vapor deposited aluminum. All of the films are a 14 m lenticular inflatable antenna have provided aromatic polyimides, with the exception of TOR- glimpses into future possibilities for Gossamer space- LMBP, which is a copoly(arylene ether benzimidazole). craft. Consequently, a significant number of future The films were exposed as second surface mirrors and missions utilizing Gossamer spacecraft concepts have the effects of the exposure on solar absorptance (ct), been proposed by the National Aeronautics and Space thermal emittance (e) and tensile properties were Administration (NASA), Department of Defense determined. The in situ changes in solar absorptance (DOD), other United States Government and foreign from Kapton® and Upilex® were less than 0.1, space agencies. whereas the solar absorptance of TOR and CP films Materials are one of many technologies that must increased by more than 0.3 without saturating. The come together in an integrated fashion to successfully thermal emittance measurements also showed that the advance the Gossamer spacecraft concept to realization. Kapton® and Upilex® materials increased only 1-2%, Materials are enabling for both the support structure but the remaining materials increased 5-8%. Based on and the film portion of Gossamer spacecraft. They tensile property measurements made in air following must have specific combinations of properties, and the test, the failure stress of every type of polymer film maintain these properties over the life of the mission. decreased as a result of irradiation. The polymers most The property requirements can vary significantly, stable in reflectance, namely Upilex® and Kapton®, depending upon the mission and space environment. were also the strongest in tension before irradiation, and Earlier programs and space flights have laid a they retained the greatest percentage of tensile strength. good foundation for materials now being developed. The films less stable in reflectance were also weaker in Polyimides, polyesters, and perfluorinated ethylene- tension, and lost more tensile strength as a result of propylene copolymers have been used extensively on irradiation. The apparent failure strain (as a percent of spacecraft designed for a wide range of environmental original gage length) of every type of polymer film exposure levels. Some copolymers have been reason- except TOR-RC, decreased as aresult of irradiation. ably stable in reflectance when exposed to UV near one astronomical unit (AU). However, they lose significant Copyright © 2001 by The Boeing Co. *Boeing Phantom Works, Seattle WA 98124-3999 Published by the American Institute of *NASA-Langley Research Center, Hampton VA 23681-0001 Aeronautics and Astronautics, Inc., *Light Technologies, Inc., Seattle WA 98112-2906. Member AIAA with permission. IFormerly of the German Space Agency; Sankt Augustin, Germany 1 American Institute of Aeronautics and Astronautics amountosfreflectancaen,dchangreeflectivceharacter micrometers (0.02 mils) to deliver the very high dose if exposedtoelectronast fluxes near the peak rates indicated near the surface. Electrons of 40-keV energy measured at synchronous altitudes. Electrons typically with a much deeper dose depth profile were used to cause a bulk effect, with the polymer becoming a light- deliver the bulk dose. scattering medium. _ Polyimide films such as Kapton® have performed well in space, and have survived well Exposure Facilities in simulations of low-energy protons such as the 1-10 keV solar wind. However, they have degraded heavily Boeing's Combined Radiation Effects Test at high proton fluences, or when exposed to UV at Chamber (CRETC), diagramed in Figure 1, provided intensities representative of near-Sun missions, unless the appropriate experiment environment. The chamber protectively overcoated. 2 The U.S.-German HELIOS is equipped with sources for both a I0 to 50 kV rastered spacecraft survived thermal loading up to -I0 Suns for proton beam and a 30 to 60 kV electron beam. A several years with metalized polymer films and other water-cooled 6000 watt Xenon arc lamp provides advanced technologies including control of electrical continuum ultraviolet exposure from 200 to 400 lam at conductivity systemwide. 3 The HELIOS program an intensity of 1-3 UV suns. The clean, cryo-pumped studied gossamer films. Poly(paraxylylene) supported chamber is also fitted with a spectrophotometer that enabled in situ reflectance measurements to be made in a stainless steel mesh (a potential weight penalty) performed marginally, but bonded Al203/Al-overcoated periodically throughout exposures. It is not necessary quarter-mil Kapton® was stable even at accelerated to remove the samples from the sample holder or the vacuum to make the reflectance measurements. simulation intensities. The latest generation of materials includes self-supporting films having -13 micrometer (0.5-mil) thickness. Boeing's irradiation of samples of these materials has been documented for NASA, 4and is the subject of this paper. EXPERIMENTAL APPROACH Radiation Environment It was the goal of the program to provide a 5-year simulation of two regions of space, the environment at 0.98 astronomical units (AU) where the Geostorm CRETC satellite will orbit, and the environment at the second TOP VIEW Lagrangian point (L2) where the Next Generation Space Telescope (NGST) will be positioned. The Geostorm location between the Sun and the Earth is far beyond the influence of the Earth's magnetic field, making the environment of interest that of the solar wind and solar events. The L2 position, on the other hand, is located on the far side of the Earth away from the Sun. At this position, a spacecraft would pass through the Earth's geotail created by the interaction of the geomagnetic field with the solar wind. The best estimate available for the radiation environment was arrived upon by researching available information and by discussions with experts from NASA, academia and Figure 1 CRETC Chamber Layout industry. The levels present in these regions are continually being refined; however, it is understood that The UV intensity was measured across the overall by far the major contribution to both environments was beam-space that the specimen array occupied and was the solar wind. within ±10 percent when using approximately 1.5 UV The electron and proton fluence levels were suns. (One total sun is approximately 0.135 watt/cm2; determined by first generating a dose depth profile of a the sun's UV content is approximately 9.1% of its representative material (Kapton in this case) for the overall output, for a value of approximately 0.012 solar wind at L1. The goal then is to best approximate watt/cm2/UV-sun.) The areas of lowest UV intensity this profile with the beam energies available in the are small portions of the four corners of the array-space. chamber. This was accomplished by generating a test Characteristics of the proton and electron beams protocol that used 40-keV protons with a range of 0.52 were determined with Faraday cups that track the ,¢ American Institute of Aeronautics and Astronautics chambehrorizontaalndvertical centerlines (bisecting by the customer. Microscopy was used to determine the array of specimens). It was determined that the 40- machining direction (if applicable) as well as to assure keV electrons were quite uniform to ±5%. The 40-keV that the films would be exposed as second-surface proton beam, which is rastered with significant overlaps mirrors. The more fragile experimental polymer films to provide uniformity along with a larger beam size, were the most difficult to cut. Specimens that was uniform to ±15 percent over the sample array. developed ragged edges or tears were not used in the irradiations, but were set aside as extra controls. Test Specimen and Fixture The fabricated test fixture was wiped with isopropyl alcohol, then ultrasonically cleaned in a NASA Langley Research Center provided the test detergent wash and rinse, and finally given an ethanol specimens consisting of 7 types, commercially avail- solvent rinse and dry. able Kapton® E and HN, Upilex® S, CP-1 and CP-2 s Sample integration was performed using and 2 experimental films TOR-RC and TOR-LMBP 6. cleanroom gloves inside a clean laminar flow bench. Kapton® and Upilex® are mature film products that The first step of the integration was to attach the cut have been optimized for thermal and mechanical specimens to their holding bars (each bar is described properties through synthesis and processing. elsewhere as like a section of a very slightly curved Consequently, they exhibit significantly higher mandrel). Small pieces of Kapton tape were used as strengths, moduli and strain to failures than the batch needed to aid the initial securing of specimen ends cast experimental films. behind their hold-down metal strips. One at a time, The exposure area of the CRETC allowed for a each specimen was then mapped "down" and over the total of 15 specimens. The minimum area required for front surface of its mandrel, then looped over the top the emittance measurement and the maximum exposure and back of the mandrel, whereupon small weights area determined the number of specimens. In addition, were attached to each sample's bottom grip area, to the tensile strength measurements required a length of keep each specimen in mild tensile stress, but with unexposed material at each end. Therefore a custom freedom to shrink or elongate in response to radiation. test fixture sized for specimens approximately 75 mm The front cover shield was then attached, to define the (3 inches) long and 16 mm (0.65 inch) wide, with a overall exposure area of each specimen exactly. The central exposure and measurement section of 20 mm result was an array of 5 samples in each of 3 horizontal (0.8 inch) long was designed. rows on mandrel bars. Each row of specimens was mounted to a slightly curved shape mandrel-like bar that secures each test Property Measurement specimen in place during exposure. Figure 2 is an "exploded" view of the fixture. Each specimen was Reflectance. The Boeing CRETC has a double- secured from the end on the backside of the fixture. beam spectrophotometer that is optically coupled to the One feature of the fixture was a thin shield between the locations of test samples in the vacuum chamber rows of test specimens, to provide for a well-defined (Figure 1). With appropriate measuring light sources central irradiation section. (UV to near-IR), and with light detectors in situ, the value of a test surface's spectral reflectance, as modified by radiation or perhaps other stresses, is determined during measurements and retained for computer analysis. In Boeing's facility, an integrating sphere in the test chamber, between the detector and a sample being measured, produces a measurement of hemispherical reflectance. The spectral range is 250 nm to about 2500 nm. A sample is illuminated spectrally since the spectrophotometer optical path includes the monochromator after the light source(s). The spectral illumination begins with longest wavelength light (lowest eV value), and the measure- Figure 2 Exploded View of Specimen Fixture ment proceeds to shorter wavelengths. This is a non- destructive measurement. With opaque samples, solar Specimen Preparation and Mounting absorptance is derived by simple subtraction (using the appropriate solar wavelength weighting). The thin films (12.5 mm thick) were difficult to Emittance. A non-destructive measurement using handle. A cutting template tool was machined to aid in near-infrared radiation can be given to a film sample by cutting the specimens from the larger sheets provided laying it over an aperture provided in a Gier-Dunkle ,ig American Institute of Aeronautics and Astronautics EmittanceInspectionDevice(DB100). Boeing The average flux over the entire exposure period performeda seriesof theseroomtemperature was 4.8E8 p/cm2-s, 3.9E9 e/cm2-s and 1.5 equivalent measuremeanstpsartofthisprogramin,airfollowing UV suns. The chamber vacuum pressure level began at the in situ irradiation. A number of unirradiated 9E-7 at the start of the exposure and quickly leveled out samples cut from the same polymer sheets, were used at 2E-7 torr. The CRETC exposure systems are as unexposed comparison samples so all specimens designed to operate continuously except for brief were measured in the same run. The measuring device periods each day devoted to dosimetry and cleaning of illuminates each sample with polychromatic radiation, the UV source. The exposure was also interrupted and the apparatus circuitry computes a weighted during scheduled reflectance measurements. infrared reflectance value internally. With opaque specimens as in this program, the values of thermal Solar Absorptance emittance coefficients were derived by simple subtraction from the measured reflectance values. The solar absorptance values were calculated from Tensile. After completion of the emittance the spectral reflectance data measured by the spectro- measurements on all exposed samples as well as on photometer. From the 240 specific wavelengths avail- selected "comparison" or unexposed samples, able from each data set, 100 wavelengths representing mechanical property measurements were made. The the relative spectral weighting of the Sun's radiance test machine used for the property testing was a MII-50 curve were used in the calculation of solar absorptance. UD Satec universal test machine with a 440-kg (1000- As the exposure continued the TOR-LMBP sample type pound) load cell. The cell is calibrated down to 2 started to tear and by the third measurement level both pounds with a resolution down to 0.001 pounds. specimens had torn and were unmeasureable. Table 2 Instron hydraulic grips with rubber pads were used to lists all the individual values and Figure 3 plots the clamp each test film in turn. All measurements were average increase in the solar absorptance for each made at room temperature. specimen type as a function of the exposure. EXPERIMENTAL DATA Thermal Emittance The emittance results for both the unexposed and Exposure Summary exposed samples were calculated from the measured The simultaneous exposure of protons, reflectance values by subtraction for these opaque electrons and UV simulating a 5-year (60-month) materials. The bar graph in Figure 4 gives a quick mission at L1/L2 was divided into 5 exposure visual summary of the results. segments. Table 1lists the proton and electron fluences and the equivalent UV exposure hours for each Tensile Properties segment. While the total proton and electron fluences simulated the entire 60-month mission it was not Sixteen specimens failed in the gage section and possible to provide a UV exposure that simulated the nineteen failed in one or the other grip areas. In full mission within the scope of the contract. general, the values for failure strain in the case of grip failures were similar to the values obtained for the cases Therefore, the highest amount of UV exposure possible was accumulated as dictated by the exposure times of of gage failures. Figures 5, 6, and 7 summarize the the protons and electrons. results in bar graph format. The apparent modulus values were not affected Table 1Ex losure Summar,/ appreciably by the radiation exposure. However, the failure stress and strain were generally affected. Exposure Equivalent Proton Electron UV Segments Mission Duration Fluence Fluence Exposure Several exceptions include TOR-RC's failure strain Imonthsl (p/crn=) le/cm_) (hou,_) values, which were very low to begin with, remained 1 -3 3.6E+13 5.0E÷14 low; and stress at failure for Upilex®S showed a 2 12 2.0E+14 1.6E+15 decrease in only one of the two samples. These 3 24 3.9E+14 3.2E+15 remarks illustrate that the small number of exposed 4 42 7.1E+14 5.7E+t5 685 samples available combined with the difficulty of 5 6o 1.0E+I 5 8.0E+ 15 I0(0) making this type of measurement on very thin films reduces the usefulness of the results to primarily indicating trends. • 4 American Institute of Aeronautics and Astronautics Table 2 Individual Specimen In Situ Solar Absorptance Values Exposure 0 1 2 3 4 5 Segment Exposure 0 p/cm z 3.6E13 2.0E14 3.9E14 7.1E14 1.0EI5 Sample Levels Material ID 0 hr. UV 90 330 480 685 1000 2 0.300 0.326 0.328 0.335 0.352 0.373 Kapton® E 7 0.304 0.326 0.328 0.340 0.352 0.373 15 0.304 0.329 0.330 0.346 0.356 0.375 1 0.318 0.337 0.329 0.335 0.356 0.380 Kapton® HN 8 0.314 0.339 0.337 0.349 0.365 0.389 4 0.351 0.376 0.370 0.383 0.392 0.407 Upilex® S 11 0.355 0.381 0.383 0.398 0.399 0.413 9 0.213 0.246 0.339 0.409 0.473 0.546 CP-1 14 0.217 0.238 0.316 0.382 0.441 0.491 3 0.215 0.241 0.315 0.376 0.432 0.484 CP-2 6 0.211 0.233 0.289 0.360 0.406 0.458 10 0.194 0.258 0.374 0.440 0.496 0.560 TOR-RC 13 0.193 0.246 0.365 0.421 0.478 0.536 5 0.233 0.252 TOR-LMBP 12 0.227 0.280 0.4 0.35 TOR-RC CP-1 ,,_ CP-2_ Kapton_ 0.05 Kaptord9 HN_ 10 20 30 40 5O 6O Months In Orbit Figure 3 Increase in Solar Absorptance as a function of Exposure i 5 American Institute of Aeronautics and Astronautics KaptonOE KaptonOHN Upllex_ S CP-1 CP-2 TOR-RC Figure 4 Thermal Emittance 1 i-- Kapton_ E Kapton_ HN Uptlex_ S CP-1 CP-2 TOR-RC Figure 5 Apparent Modulus t 6 American Institute of Aeronautics and Astronautics 6O 40 2O 10 Kaptor_ E Kapto_lQHN Upl|exOS CP-1 CP-2 TOR-RC Figure 6 Apparent Failure Stress 110 lOO 90 80 70 _ 50 40 3O 2O 10 0 Kapton_ E Kaptor_ HN Upilex'_S CP-1 CP-2 TOR-RC Figure 7 Apparent Failure Strain g 7 American Institute of Aeronautics and Astronautics DISCUSSION and CONCLUSIONS recent irradiation of similar materials, 7 one sample- holding mandrel was machined to place a thermocouple Reflectance measured in situ was always found to near one corner specimen. The thermocouple was decrease after exposure to simulated space radiation. shielded from direct exposure to the simulated Sun Thus the computed values of each sample's solar source. As in this program, water cooling adjacent to, absorptance increased as exposure to radiation but thermally decoupled from, the mandrels provided a continued, to the end of the test without saturation. baseline set of conditions that included a temperature of Certain polymer films that were colorless prior to approximately 19 C for the samples and their mandrels irradiation became considerably more absorptive, and when the chamber interior was dark. Then, under an acquired a "bronze" color, during irradiation. The intensity of approximately 2 UV suns, the mandrel polymers that originally were colorless, more than temperature rose about five degrees Celsius (from -19 doubled their solar absorptance (from about 0.2 to C to 24 C or less) within minutes after beginning the nearly 0.5). On the other hand, Kapton® specimens exposure of samples to the solar beam. increased about 0.07 in solar absorptance, from base Further analysis has been done to estimate the values of about 0.3. Upilex®S was slightly more stable temperature of the polymer films themselves during for solar absorptance, increasing about 0.06 (from base irradiation. The film samples were not bonded to their values of about 0.35). TOR-RC nearly tripled in solar mandrels. Most samples had the majority of their area absorptance by the end of the test (from base values of in contact with their mandrel. However, some speci- approximately 0.2). TOR-LMBP distorted and then mens did not hang straight, though weighted. Despite disintegrated during the first quarter or so of the test being thin, they were not fully compliant with efforts to period. Figure 3 summarizes the increase in solar have them be draped uniformly around their mandrels. absorptance obtained on each of the irradiated polymer Therefore, portions of those samples did not have total films types. The experimental data divide into two thermal contact with their mandrel surfaces. Those principal groups, one of them having much more stable portions of films with no or poor thermal contact would reflectance than the other does. The changes in solar rise to higher values of equilibrium temperature during absorptance from Kapton® and Upilex® samples irradiation, depending on their absorptance and emit- remain less than 0.1, whereas the solar absorptance tance properties, the intensity of the simulated Sun, and changes in TOR and CP film samples increase by more the temperature of adjacent radiative surfaces such as than 0.3 without saturating. the mandrels. Calculations show that detached portions The thermal emittance values measured (in air) on of "low" absorptance films such as the CP series may the Kapton® specimens remained essentially equilibrate at about 80 C when illuminated by 1.5 suns, unchanged within experimental uncertainty. On the but could rise to ~170 C in their degraded states (Table other hand, the emittance of polymer CP-1 increased 2 and Figure 3). In contrast, portions of Kapton® and about eight percent in air (from about 0.47 to about 0.51 Upilex® samples that are irradiated while detached decimally) as a result of the combined UV/protorg- from cooled substrates would equilibrate and remain electron irradiation performed. The emittance of CP-2 under 150 C, since those materials are more stable and TOR-RC increased perhaps half as much. under UV, proton, and electron irradiation. Based on tensile property measurements made in More sophisticated instrumentation (designed to air following the test, the failure stress of every type of reveal stress patterns during tensile-property testing) polymer film decreased as a result of being irradiated. might indicate effects of film temperature excursions The polymers most stable in reflectance, namely during irradiation. Boeing did not employ such Upilex®S and Kapton®, were also the strongest in instrumentation during this work. We observed no tension before irradiation, and they retained the greatest visual differences in the responses or appearances of percentage tensile strength. The films less stable in various portions of samples, neither during irradiation reflectance were also weaker in tension, and lost more nor during post-irradiation testing. (The only exception tensile strength as a result of irradiation. This is being the physical tearing and distortion noted earlier illustrated in Figure 6. The apparent failure strain (as a for TOR-LMBP.) Therefore, the extent of sample percent of original gage length) of every type of contact (or lack of it) with its underlying polymer film except TOR-RC, decreased as a result of mandrel/substrate probably had no substantial effect, irradiation. The decrease was "dramatic" in Kapton®. and film temperature excursions during irradiation were Apparent modulus generally decreased so slightly due probably not great enough to cause fundamental to irradiation that the changes are not very significant. changes in polymer structure or strength. We conclude No direct measurement of sample temperature that the space radiation simulation that we conducted was made during the irradiation for this program. was effective in determining the most stable and least However, when preparing the same test fixture for a stable films for solar absorptance and tensile strength. 8 American Institute of Aeronautics and Astronautics RECOMMENDATIONS 7Russell, D. A., L. B. Fogdall, and G. Bohnhoff- The exposure of one or more, perhaps larger, film Hlavacek, "Effects of UV, Protons, and Electrons on samples to a test environment that has been optimized Polymer Films for Spacecraft Applications Including with respect to the UV/charge-particle exposure ratio NASA Next Generation Space Telescope." Final has been suggested. This environment would allow the Report to NASA-Goddard Space Flight Center for UV exposure to achieve a level that coincides with Contract S-40584-G, November 2000, electron and proton fluences reached in the same period. For example, a one-year exposure environment would include approximately 9000 UV hours and the NOTES equivalent one-year charge particle fluences. ACKNOWLEDGMENTS The authors wish to thank Dr. M. J. Meshishnek for his helpful discussions including the environment depth-dose calculations, and Mr. W. Blackwell for valuable discussions on his preliminary environment definition study. The authors appreciate the contributions of the following individuals during the course of this program. Loren D. Milliman for scientific data programming, James Beymer for fixture design and CAD support, Douglas Franich for test setup and monitoring, Jerry Hobson and preparation staff REFERENCES tFogdall, L. B. and S. S. Cannaday, "Effects of Space Radiation on Thin Polymers and Nonmetallics," Heat Transfer and Thermal Control Sst_Sy.._e__V,ol. 60, p290- 304, _ in Astronautics and Aeronautics, ed. L. S. Fletcher, 1978. 2Fogdall, L. B. and S. S. Cannaday, "Space Radiation Effects of aSimulated Venus/Mercury Flyby on Solar Absorptance and Transmittance Properties of Solar Cells, Cover Glasses, Adhesives and Kapton Film." AIAA Paper 71-452, April 1971. 3Winkler, W., "Material Performance Under Combined Stresses in the Hard Space Environment of the Sun- probe HELIOS-A," Acta Astronautica 10, 189-205 (1983). 4Russell, D.A,. L. B. Fogdall, and G. Bohnhoff- Hlavacek, "Simulated Space Environmental Testing on Thin Films," NASA/CR-2000-210101, Final Report for NASA Langley Research Center, April, 2000 5CP-1 and CP-2 manufactured by SRS Technologies. 6TOR-RC and TOR-LMBP manufactured by Triton Systems, Inc. 9 American Institute of Aeronautics and Astronautics