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Teflon FEP & Hubble Space Telescope (HST) Abstracts


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Solar Effects on Tensile and Optical Properties of Hubble Space Telescope Silver-Teflon Insulation

A section of the retrieved Hubble Space Telescope (HST) solar array drive arm (SADA) multilayer insulation (MLI), which experienced 8.25 years of space exposure, was analyzed for environmental durability of the top layer of silver-Teflon fluorinated ethylene propylene (Ag-FEP).  Because the SADA MLI had solar and anti-solar facing surfaces and was exposed to the space environment for a long duration, it provided a unique opportunity to study solar effects on the environmental degradation of Ag-FEP, a commonly used spacecraft thermal control material.  Data obtained included tensile properties, solar absorptance, surface morphology and chemistry.  The solar facing surface was found to be extremely embrittled and contained numerous through-thickness cracks.  Tensile testing indicated that the solar facing surface lost 60% of its mechanical strength and 90% of its elasticity while the anti-solar facing surface had ductility similar to pristine FEP.  The solar absorptance of both the solar facing surface (0.155  0.032) and the anti-solar facing surface (0.208  0.012) were found to be greater than pristine Ag-FEP (0.074).  Solar facing and anti-solar facing surfaces were microscopically textured, and locations of isolated contamination were present on the anti-solar surface resulting in increased localized texturing.  Yet, the overall texture was significantly more pronounced on the solar facing surface indicating a synergistic effect of combined solar exposure and increased heating with atomic oxygen erosion.  The results indicate a very strong dependence of degradation, particularly embrittlement, upon solar exposure with orbital thermal cycling having a significant effect.

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Effects of Vacuum Ultraviolet Radiation of Various Wavelength Ranges on Teflon FEP Film

This paper describes testing to investigate the effects of vacuum ultraviolet (VUV) radiation on Teflon® fluorinated ethylene propylene (FEP) film, examining differences in mechanical properties degradation for samples of 50.8 m thickness exposed to VUV of various lower cut-off wavelengths.   Samples were illuminated in a high vacuum facility by deuterium lamps, which provide radiation in the 115-400 nm wavelength range, but with the highest intensity being below 200 nm.   Windows of fused silica, crystalline quartz, and magnesium fluoride provided lower cut-off wavelengths of 155, 140, and 115 nm, respectively.  Lamp intensity was measured in the 115-200 nm wavelength range throughout the sample exposures. The determined intensities were used to estimate intensity and incident energy of various wavelength ranges, between 115 and 400 nm.  Samples were analyzed for tensile strength and elongation at failure.  The effects of radiation exposures of different wavelength ranges were compared and discussed in terms of the expected depth to which radiation of various wavelengths is deposited into FEP.

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The Effect of Heating on the Degradation of Ground Laboratory and Space Irradiated Teflon FEP

The outer most layer of the multilayer insulation (MLI) blankets on the Hubble Space Telescope (HST) is back surface aluminized Teflon¨ FEP (fluorinated ethylene propylene). As seen by data collected after each of the three servicing missions and as observed during the second servicing mission (SM2), the FEP has become embrittled in the space environment, leading to degradation of the mechanical properties and severe on-orbit cracking of the FEP. During SM2, a sample of aluminized-FEP was retrieved from HST that had cracked and curled, exposing its aluminum backside to space. Because of the difference in optical properties between FEP and aluminum, this insulation piece reached 200 ˇC on-orbit, which is significantly higher than the nominal MLI temperature extreme of 50 ˇC. This piece was more brittle than other retrieved material from the first and third servicing missions (SM1 and SM3A, respectively). Due to this observation and the fact that Teflon thermal shields on the solar array bi-stems were heated on-orbit to 130 ˇC, experiments have been conducted to determine the effect of heating on the degradation of FEP that has been irradiated in a ground laboratory facility or in space on HST. Teflon FEP samples were x-ray irradiated in a high vacuum facility in order to simulate the damage caused by radiation in the space environment. Samples of pristine FEP, x-ray irradiated FEP and FEP retrieved from the HST during SM3A were heat treated from 50 to 200 ˇC at 25ˇ intervals in a high vacuum facility and then tensile tested. In addition, samples were tested in a density gradient column to determine the effect of the radiation and heating on the density of FEP. Results indicate that although heating does not degrade the tensile properties of non-irradiated Teflon, there is a significant dependence of the percent elongation at failure of irradiated Teflon as a function of heating temperature. Irradiated Teflon was found to undergo increasing degradation in the elongation at failure as temperature was increased from room temperature to 200 ˇC. Rate of degradation changes, which were consistent with the glass I transition temperatures for FEP, appeared to be present in both tensile and density data. The results indicate the significance of the on-orbit temperature of Teflon FEP with respect to its degradation in the low Earth orbital space environment.

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Mechanical Properties of Teflon® FEP Retrieved From the Hubble Space Telescope

Teflon® FEP (fluorinated ethylene propylene) surfaces on the Hubble Space Telescope (HST) have experienced significant degradation in mechanical properties during nearly ten years of exposure in the low Earth orbit (LEO) environment. This paper describes results of mechanical properties testing of Teflon® FEP materials exposed on HST for 9.7 years between launch and the third servicing mission (SM3A) and for 2.8 years between the second servicing mission (SM2) and SM3A. Results of tensile testing, bend testing and microscopic examination of crack morphology are described. Effects of post-retrieval heating and air vs. vacuum storage on the mechanical properties of FEP surfaces are described as they significantly affect interpretation of results regarding the durability of FEP on HST. This paper provides comparisons of the properties of FEP surfaces retrieved during SM3A to previously reported results for FEP materials retrieved during the first servicing mission (SM1) and SM2. Environmental exposure conditions for the HST exposed materials are also described.

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Hubble Space Telescope Third Servicing Mission Retrieved Metallized Teflon FEP Analysis

Following the third servicing mission (SM3A in December ’99) to the Hubble Space Telescope, analysis was performed on the two returned panels of multilayer insulation (MLI) as well as two patches. The MLI panels had been in space since the telescope was launched in April ’90 (9.7 years), while the patches were installed during the second servicing mission in February ’97 (2.8 years). This paper provides an overview of the tests performed on the returned metallized Teflon FEP along with a summary of results. Testing including determination of mechanical and optical properties, crystallinity and fractography. Because of the amount of material retrieved and the nominal environmental exposures of the retrieved materials, these analyses resulted in a fairly complete understanding of the degradation process affecting the materials on the telescope. Test results from SM3A materials showed significantly better mechanical strength than second servicing mission (SM2) samples.

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Effect of Air and Vacuum Storage on the Tensile Properties of X-ray Exposed Aluminized-FEP

Metallized Teflon® FEP (fluorinated ethylene propylene), a common spacecraft thermal control material, from the exterior layer of the Hubble Space Telescope (HST) has become embrittled and suffers from extensive cracking. Teflon samples retrieved during Hubble servicing missions and from the Long Duration Exposure Facility (LDEF) indicate that there may be continued degradation in tensile properties over time. An investigation has been conducted to evaluate the effect of air and vacuum storage on the mechanical properties of x-ray exposed FEP. Aluminized-FEP (Al-FEP) tensile samples were irradiated with 15.3 kV Cu x-rays and stored in air or under vacuum for various time periods. Tensile data indicate that samples stored in air display larger decreases in tensile properties than for samples stored under vacuum. Air-stored samples developed a hazy appearance, which corresponded to a roughening of the aluminized surface. Optical property changes were also characterized. These findings indicate that air exposure plays a role in the degradation of irradiated FEP, therefore proper sample handling and storage is necessary with materials retrieved from space.

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Thermal Contributions to the Degradation of Teflon® FEP on the Hubble Space Telescope

Metallized Teflon® fluorinated ethylene propylene (FEP) thermal control material on the Hubble Space Telescope (HST) is degrading in the space environment. Teflon® FEP insulation was retrieved during servicing missions, which occurred in 1993, 1997 and 1999. During the second servicing mission (SM2), the 5 mil aluminized-FEP (Al-FEP) outer layer of multilayer insulation (MLI) covering the telescope was found to be cracked in many locations around the telescope. Teflon® FEP retrieved during SM2 was more embrittled than FEP retrieved 2.8 years later from a different location, during the third servicing mission (SM3A). Studies have been conducted to understand the degradation of FEP on HST, and the difference in the degree of degradation of FEP from each of the servicing missions. The retrieved SM2 material experienced a higher temperature extreme during thermal cycling (200°C) than first servicing mission (SM1) and SM3A materials (upper temperature of 50°C), therefore an investigation on the effects of heating FEP was also conducted. Samples of pristine FEP and SM1, SM2, and SM3A retrieved FEP were heated to 200°C and evaluated for changes in properties. Heating at 130°C was also conducted because FEP bi-stem thermal shields are expected to cycle to a maximum temperature of 130°C on-orbit. Tensile, density, x-ray diffraction (XRD) crystallinity and differential scanning calorimetry (DSC) data were evaluated. It was found that heating pristine FEP caused an increase in the density and practically no change in tensile properties. However, when as-retrieved space samples were heated, the density increased and tensile properties decreased. Upon heating, all samples experienced an increase in crystallinity, with larger increases in the space exposed FEP. These results indicate that irradiation of FEP in space causes chain scission, resulting in embrittlement, and that excessive heating allows increased mobility of space-environment-induced scissioned chains. Thermal exposure was therefore found to have a major impact on the extent of embrittlement of FEP on HST.

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Quantitative Study of Degradation of PTFE and FEP Films by Soft X-Rays

The deterioration of mechanical properties experienced by the thermal control blankets on the Hubble Space Telescope may be due, at least in part, to the soft x-ray component of the space environment. We have performed a quantitative study of the degradation of the mechanical properties of both Teflon PTFE and FEP films exposed to soft x-rays in high vacuum. A quantitative x-ray intensity spectrum of the radiation at the specimen surface (in photons/cm2-sec-eV) was calculated with the NIST Desk Top Spectrum Analyzer and experimentally verified in-situ with a silicon photodiode radiation detector. Most of the exposures were performed with a tube potential of 15.3 kV and with the radiation filtered through a Be plate to eliminate the softer x-rays that would be absorbed by the front surface of the specimen. Knowledge of the intensity spectrum allowed a calculation of the distribution of energy deposition within the specimen film. This distribution was reasonably uniform throughout the film for these conditions. The quantitative measure of radiation dose most closely correlated with the degradation of mechanical properties was found to be the energy absorbed per unit area – in kJ/m2 – throughout the film’s thickness (areal dose). The strength of the films was assessed by a hydrostatic burst test devised in our laboratory .It was found that the strength of the 75 µm thick PTFE films fell by 25% at an areal dose of 10 kJ/m2. This corresponded to an interior dose 58.3 kGy. The strength continued to decrease with exposure until it reached about 10% of its initial value at areal doses of 350 kJ/m2 In contrast, the 50 µm FEP films were much more robust than the PTFE films, retaining much of their original strength up to areal doses of 50 kJ/m2.

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Environmental Exposure Conditions for Teflon® FEP on the Hubble Space Telescope

The outer layer of Teflon® fluorinated ethylene propylene (FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2), 6.8 years after HST was launched into low Earth orbit (LEO). Comparatively minor embrittlement and cracking were also observed in FEP materials retrieved from solar-facing surfaces on HST at the time of the First Servicing Mission (3.6 years exposure). After SM2, a Failure Review Board was convened to address the problem of degradation of MLI on HST. In order for this board to determine possible degradation mechanisms, it was necessary to consider all environmental constituents to which the FEP MLI surfaces were exposed. Based on measurements and various models, environmental exposure conditions for FEP surfaces on HST were estimated including; number and temperature ranges of thermal cycles; equivalent sun hours; fluence and absorbed radiation dose of x-rays, trapped protons, and plasma electrons and protons; and atomic oxygen (AO) fluence. This paper presents the environmental exposure conditions for FEP on the Hubble Space Telescope, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on HST.

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Insights into the Damage Mechanism of Teflon® FEP from the Hubble Space Telescope

Metallized Teflon® FEP (fluorinated ethylene propylene) thermal control material on the Hubble Space Telescope (HST) has been found to be degrading in the space environment. Teflon® FEP thermal control blankets (space-facing FEP) retrieved during the first servicing mission (SM1) were found to be embrittled on solar facing surfaces and contained microscopic cracks. During the second servicing mission (SM2) astronauts noticed that the FEP outer layer of the multi-layer insulation (MLI) covering the telescope was cracked in many locations around the telescope. Large cracks were observed on the light shield, forward shell and equipment bays. A tightly curled piece of cracked FEP from the light shield was retrieved during SM2 and was severely embrittled, as witnessed by ground testing. A Failure Review Board (FRB) was organized to determine the mechanism causing the MLI degradation. Density, x-ray crystallinity and solid state nuclear magnetic resonance (NMR) analyses of FEP retrieved during SM1 were inconsistent with results of FEP retrieved during SM2. Because the retrieved SM2 material curled while in space, it experienced a higher temperature extreme during thermal cycling, estimated at 200°C, than the SM1 material, estimated at 50°C. An investigation on the effects of heating pristine FEP and FEP retrieved from the HST was therefore conducted. Samples of pristine, SM1, and SM2 FEP were heated to 200°C and evaluated for changes in density and morphology. Elevated temperature exposure was found to have a major impact on the density of the retrieved materials. Characterization of polymer morphology of as-received and heated FEP by NMR provided results that were consistent with the density results. Differential scanning calorimetry (DSC) was conducted on pristine, SM1 and SM2 FEP. DSC results provided evidence of chain scission and increased crystallinity in the space exposed FEP, which supported the density and NMR results. Samples exposed to simulated solar flare x-rays, thermal cycling and long-term thermal exposure provided information on environmental contributions to degradation. These findings have provided insight into the damage mechanisms of FEP in the space environment.

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Environmental Exposure Conditions for Teflon® FEP on the Hubble Space Telescope

The outer layer of Teflon® fluorinated ethylene propylene (FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2), 6.8 years after HST was launched into low Earth orbit (LEO). Comparatively minor embrittlement and cracking were also observed in FEP materials retrieved from solar-facing surfaces on HST at the time of the First Servicing Mission (3.6 years exposure). After SM2, a Failure Review Board was convened to address the problem of degradation of MLI on HST. In order for this board to determine possible degradation mechanisms, it was necessary to consider all environmental constituents to which the FEP MLI surfaces were exposed. Based on measurements and various models, environmental exposure conditions for FEP surfaces on HST were estimated including; number and temperature ranges of thermal cycles; equivalent sun hours; fluence and absorbed radiation dose of x-rays, trapped protons, and plasma electrons and protons; and atomic oxygen (AO) fluence. This paper presents the environmental exposure conditions for FEP on the Hubble Space Telescope, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on HST.

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Simulated Solar Flare X-Ray and Thermal Cycling Durability Evaluation of Hubble Space Telescope Thermal Control Candidate Replacement Materials

During the Hubble Space Telescope (HST) second servicing mission (SM2), astronauts noticed that the multi-layer insulation (MLI) covering the telescope was damaged. Large pieces of the outer layer of MLI (aluminized Teflon® fluorinated ethylene propylene (Al-FEP)) were cracked in several locations around the telescope. A piece of curled up Al-FEP was retrieved by the astronauts and was found to be severely embrittled, as witnessed by ground testing. The national Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) organized a HST MLI Failure Review Board (FRB) to determine the damage mechanism of the Al-FEP in the HST environment, and to recommend a replacement thermal control outer layer to be installed on HST during the subsequent servicing missions. Candidate thermal control replacement materials were chosen by the FRB and tested for environmental durability under various exposures and durations by GSFC and NASA Glenn Research Center (GRC). This paper describes durability testing at GRC of candidate materials which were exposed to charged particle radiation, simulated solar flare x-ray radiation, and thermal cycling under load. Samples were evaluated for changes in solar absorptance and tear resistance. Descriptions of environmental exposures and durability evaluations of these materials are presented.

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Synchrotron VUV and Soft X-Ray Radiation Effects on Aluminized Teflon® FEP

Surfaces of the aluminized Teflon® FEP multi-layer thermal insulation on the Hubble Space Telescope were found to be cracked and curling in some areas at the time of the second servicing mission in February 1997, 6.8 years after HST was deployed in low Earth orbit (LEO). As a part of a test program to assess environmental conditions which would produce embrittlement sufficient to cause cracking of Teflon® on HST, samples of Teflon® FEP with a backside layer of vapor deposited aluminum were exposed to vacuum ultraviolet (VUV) and soft x-ray radiation of various energies using facilities at the National Synchrotron Light Source, Brookhaven National Laboratory. Samples were analyzed for ultimate tensile strength and elongation. Results will be compared to those of aluminized Teflon® FEP retrieved from HST after 3.6 years and 6.8 years on orbit and will be referenced to estimated HST mission doses of VUV and soft x-ray radiation.

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A Comparison of Space and Ground Based Facility Environmental Effects on FEP Teflon®

Fluorinated Ethylene Propylene (FEP) Teflon® is widely used as a thermal control material for spacecraft, however, it is susceptible to erosion, cracking, and subsequent mechanical failure in low Earth orbit. One of the difficulties in determining whether FEP Teflon® will survive during a mission is the wide disparity of erosion rates observed for this material in space and in ground based facilities. Each environment contains different levels of atomic oxygen, ions, and vacuum ultraviolet (VUV) radiation in addition to parameters such as the energy of the arriving species and temperature. These variations make it difficult to determine what is causing the observed differences in erosion rates. This paper attempts to narrow down which factors affect the erosion rate of FEP Teflon® through attempting to change only one environmental constituent at a time. This was attempted through the use of a single simulation facility (plasma asher) environment with a variety of Faraday cages and VUV transparent windows. Isolating one factor inside of a radio frequency (RF) plasma proved to be very difficult. Two observations could be made. First, it appears that the erosion yield of FEP Teflon® with respect to that of polyimide Kapton is not greatly affected by the presence or lack of VUV radiation present in the RF plasma and the relative erosion yield for the FEP Teflon® may decrease with increasing fluence. Second, shielding from charged particles appears to lower the relative erosion yield of the FEP to approximately that observed in space, however, it is difficult to determine for sure whether ions, electrons, or some other components are causing the enhanced erosion.

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Analysis of Retrieved Hubble Space Telescope Thermal Control Materials

The mechanical and optical properties of the thermal control materials on the Hubble Space Telescope (HST) have degraded over the nearly 7 years the telescope has been in orbit. Astronaut observations and photographs from the second servicing mission (SM2) revealed large cracks in the metallized Teflon® FEP, the outer layer of the mulit-layer insulation (MLI), in many locations around the telescope. Also, the emissivity of the bonded metallized Teflon® FEP radiator surfaces of the telescope has increased over time. Samples of the top layer of the MLI and radiator material were retrieved during SM2, and a thorough investigation into the degradation following in order to determine the primary cause of damage. Mapping of the cracks on HST and the ground testing showed that thermal cycling with deep-layer damage from electron and proton radiation are necessary to cause the observes embrittlement. Further, strong evidence was found indicating that chain scission (reduced molecule weight) is the dominant form of damage to the metallized Teflon® FEP.

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Evaluation and Selection of Replacement Thermal Control Materials for the Hubble Space Telescope

The mechanical and optical properties of the metallized Teflon® FEP thermal control materials on the Hubble Space Telescope (HST) have degraded over the nearly 7 years the telescope has been in orbit. Given the damage to the outer layer of the multi-layer insulation (MLI) that was apparent during the second servicing mission (SM2), the decision was made to replace the outer layer during subsequent servicing missions. A Failure Review Board (FRB) was established to investigate the damage to the MLI and identify a replacement material. The replacement material had to meet the stringent thermal requirements of the spacecraft and maintain structural integrity for at least 10 years. Ten candidate materials were selected and exposed to ten-year HST-equivalent doses of simulated orbital environments. Samples of the candidates were exposed sequentially to low and high-energy electrons and protons, atomic oxygen, x-ray radiation, ultraviolet radiation, and thermal cycling. Following the exposures, the mechanical integrity and optical properties of the candidates were investigated using Optical Microscopy, Scanning Electron Microscopy (SEM), and a Laboratory Portable Spectroreflectometer (LPSR). Based on the results of these simulations and analyses, the FBR selected a replacement material and two alternates that showed the highest likelihood of providing the requisite thermal properties and surviving for 10 years in orbit.

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Mechanical Properties Degradation of Teflon® FEP Returned form the Hubble Space Telescope

After 6.8 years in orbit, degradation has been observed in the mechanical properties of second-surface metallized Teflon® FEP (fluorinated ethylene propylene) used on the Hubble Space telescope (HST) on the outer surface of the multi-layer insulation (MLI) blankets and on radiator surfaces. Cracking of FEP surfaces on HST was first observed upon close examination of samples with high solar exposure retrieved during the first servicing mission (SM1) conducted 3.6 years after HST was put into orbit. Astronaut observations and photographs from the second servicing mission (SM2), conducted after 6.8 years on orbit, revealed severe cracks in the FEP surfaces of the MLI on many locations around the telescope. This paper describes results of mechanical properties testing of FEP surfaces exposed for 3.6 and 6.8 years to the space environment on HST. These tests include bend testing, tensile testing, and surface micro-hardness testing.

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Ground Based Testing of Replacement Thermal Control Materials for the Hubble Space Telescope

The mechanical and optical properties of the metallized Teflon FEP thermal control materials on the Hubble Space Telescope (HST) have degraded over the nearly seven years the telescope has been in orbit. Given the damage to the outer layer of the multi-layer insulation (MLI) blanket that was apparent during the second servicing mission (SM2), the decision was made to replace the outer layer during subsequent servicing missions. A Failure Review Board was established to investigate the damage to the MLI and identify a replacement material. The replacement material had to meet the stringent thermal requirements of the spacecraft and maintain mechanical integrity for at least ten years. Ten candidate materials were selected and exposed to ten-year HST-equivalent doses of simulated orbital environments. Samples of the candidates were exposed sequentially to low- and high-energy electrons and protons, atomic oxygen, x-ray radiation, ultraviolet radiation, and thermal cycling. Following the exposures, the mechanical integrity and optical properties of the candidates were investigated using optical microscopy, scanning electron microscopy (SEM), a laboratory portable spectroreflectometer (LPSR) and a Lambda 9 spectroreflectometer. Based on the results of these simulations and analyses, the Failure Review Board selected a replacement material and two alternatives that showed the highest likelihood of providing the requisite thermal properties and surviving for ten years in orbit.

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Investigation of Teflon FEP Embrittlement on Spacecraft in Low Earth Orbit

Teflon FEP (fluorinated ethylene-propylene) is commonly used on exterior spacecraft surfaces in the low Earth orbit (LEO) environment for thermal control. Silverized or aluminized FEP is used for the outer layer of thermal control blankets because of its low solar absorptance and high thermal emittance. FEP is also preferred over other spacecraft polymers because of its relatively high resistance to atomic oxygen erosion. Because of its low atomic oxygen erosion yield, FEP has not been protected in the space environment. Recent, long term space exposures such as on the Long Duration Exposure Facility (LDEF, 5.8 years in space), and the Hubble Space Telescope (HST, after 3.6 years in space) have provided evidence of LEO environmental degradation because of long durations and the different conditions (such as differences in altitude) of the exposures. Samples of FEP from LDEF and from HST (retrieved during its first servicing mission) have been evaluated for solar induced embrittlement and for synergistic effects of solar degradation and atomic oxygen. Micro-indenter results indicate that the surface hardness increased as the ratio of atomic oxygen fluence to solar fluence decreased for the LDEF samples, but the solar exposures were higher. Cracks induced during bend testing were significantly deeper for the HST samples with the higher solar exposure than for LDEF samples with similar oxygen fluence to solar fluence ratios. If solar fluences are compared, the LDEF samples appear as damaged as the HST samples, except that HST had deeper induced cracks. The results illustrate difficulties in comparing LEO exposed materials from different missions. Because the HST FEP appears more damaged than the LDEF FEP based on the depth of embrittlement, other causes for FEP embrittlement in addition to the atomic oxygen and ultraviolet (UV) radiation, such as thermal effects and the possible role of soft x-ray radiation, need to be considered. FEP that was exposed to soft x-rays in a ground test facility, showed embrittlement similar to that witnessed in LEO, which indicates that the observed differences between LDEF and HST FEP might be attributed to the different soft x-ray fluences during these two missions.

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Degradation of FEP Thermal Control Materials Returned from the Hubble Space Telescope

After an initial 3.6 years of space flight, the Hubble Space Telescope (HST) was serviced through a joint effort with the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). Multi-layer insulation (MLI) was retrieved from the electronics boxes of the two magnetic sensing systems (MSS), also called the magnetometers, and from the returned solar array (SA-I) drive arm assembly. The top layer of each MLI assembly is fluorinated ethylene propylene (FEP, a type of Teflon). Dramatic changes in material properties were observed when comparing areas of high solar fluence to areas of low solar fluence. Cross sectional analysis shows atomic oxygen (AO) erosion values of up to 25.4m m (1 mil). Greater occurrences of through-thickness cracking and surface microscopy were observed in areas of high solar exposure. Atomic force microscopy (AFM) showed increases in surface microhardness measurements with increasing solar exposure. Decreases in FEP tensile strength and elongation were measured when compared to non-flight material. Erosion yield and tensile results are compared with FEP data from the Long Duration Exposure Facility (LDEF). AO erosion yield data, solar fluence values, contamination, micrometeoroid or debris (MMD) impact sites, and optical properties are presented.

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Effects of Heating on Teflon® FEP Thermal Control Material from the Hubble Space Telescope

Metallized Teflon® FEP (fluorinated ethylene propylene) thermal control material on the Hubble Space Telescope (HST) is degrading in the space environment. Teflon® FEP thermal control blankets (space-facing FEP) retrieved during the first service mission (SM1) were found to be embrittled on solar facing surfaces and contained microscopic cracks. During the second servicing mission (SM2) astronauts noticed that the FEP outer layer of the multi-layer insulation (MLI) covering the telescope was cracked in many locations around the telescope. Large cracks were observed on the light shield, forward shell, and equipment bays. A tightly curled piece of cracked FEP from the light shield was retrieved during SM2 and was severely embrittled, as witnessed by ground testing. A Failure Review Board (FRB) was organized to determine the mechanism causing the MLI degradation. Density, x-ray crystallinity, and solid state nuclear magnetic resonance (NMR) analyses of FEP retrieved during SM1 were inconsistent with results of FEP retrieved during SM2. Because the retrieved SM2 material curled while in space, it experienced a higher temperature extreme during thermal cycling, estimated at 200° C, than the SM1 material, estimated at 50° C. An investigation on the effects of heating pristine and FEP exposed on HST was therefore conducted. Samples of pristine, SM1, and SM2 FEP were heated to 200° C and evaluated for changes in density and morphology. Elevated temperature exposure was found to have a major impact on the density of the retrieved materials. Characterization of polymer morphology of as-received and heated FEP samples by NMR provided results that were consistent with the density results. These findings have provided insight to the damage mechanisms of FEP in the space environment.

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Hubble Space Telescope Metalized Teflon® FEP Thermal Control Materials: On-Orbit Degradation and Post-Retrieval Analysis

During the Hubble Space Telescope (HST) second servicing mission (SM2), degradation of unsupported Teflon® FEP (fluorinated ethylene propylene), used as the outer layer of the multi-layer insulation (MLI) blankets, was evident as large cracks on the telescope light shield. A sample of the degraded outer layer was retrieved during the mission and returned to Earth for ground testing and evaluation. The results of the Teflon® FEP sample evaluation and additional testing of pristine Teflon® FEP led the investigative team to theorize that the HST damage was caused by thermal cycling with deep-layer damage from electron and proton radiation which allowed the propagation of cracks along stress concentrations, and that the damage increased with the combined total dose of electrons, protons, ultraviolet and x-ray radiation along with thermal cycling. This paper discusses the testing and evaluation of the retrieved Teflon® FEP.

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Effects of Radiation and Thermal Cycling on Teflon® FEP

Surfaces of the aluminized Teflon® FEP (fluorinated ethylene propylene) multilayer thermal insulation on the Hubble Space Telescope (HST) were found to be cracked and curled in some areas at the time of the second servicing mission (SM2) in February 1997, 6.8 years after HST was deployed into low Earth orbit (LEO). In an effort to understand what elements of the space environment might cause such damage, pristine second-surface aluminized Teflon® FEP was tested for durability to various types of radiation, to thermal cycling and to radiation followed by thermal cycling. Types of radiation included synchrotron vacuum ultraviolet and soft x-ray radiation, electrons and protons. Thermal cycling was conducted in various temperature ranges to simulate HST orbital conditions for Teflon® FEP. Results of tensile testing of the exposed specimens showed that exposure to high fluences of radiation caused degradation in tensile properties of FEP. However, exposure to radiation alone in exposures comparable to those experienced by HST did not produce reduction in ultimate tensile strength and elongation of Teflon® similar to that observed for HST-retrieved aluminized Teflon®. Synergism of radiation exposure and thermal cycling was evident in the results of three experiments: thermal cycling following electron and proton irradiation, thermal cycling following x-ray exposure, and additional thermal cycling of a sample retrieved from HST. However, irradiation and thermal cycling with comparable HST SM2 exposure conditions did not produce the degradation observed in the FEP material retrieved during HST SM2.

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Thermal Cycling-Caused Degradation of Hubble Space Telescope Aluminized FEP Thermal Insulation

The Hubble Space Telescope (HST) was launched in April of 1990 and was visited during service missions in December of 1993 and February of 1997. During the latter servicing mission, astronauts observed that the top layer of multi-layer insulation, which consisted of second surface aluminized FEP Teflon®, has occasional tears in its 0.127 mm thick outer layer. A sample was retrieved which had torn and rolled up under its own stress such that the aluminized layer was on the exposed surface. The sample was found to have an increase in solar absorptance and has multiple cracks in the aluminization in a mud-tile configuration. Tests conducted in a ground laboratory high-rate thermal cycling system indicate that a signification portion of the observed increase in solar absorptance may have been caused by cracks in the fatigued aluminum as a result of approximately 40.000 thermal cycles it received in space.

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Effect of X-Rays on the Mechanical Properties of Aluminized FEP Teflon

Pieces of the multilayer insulation (MLI) that is integral to the thermal control of the Hubble Space Telescope (HST) have been returned by two servicing missions after 3.6 and 6.8 years in orbit. They reveal that the outer layer, which is made from 5 mil (0.13 mm) thick aluminized fluorinated ethylenepropylene (FEP) Teflon®, has become severely embrittled. Although possible agents of embrittlement include electromagnetic radiation across the entire solar spectrum, trapped particle radiation, atomic oxygen, and thermal cycling, intensive investigations have not yielded unambiguous causes. Previous studies utilizing monoenergenic photons in the 69-1900 eV range did not cause significant embrittlement, even at much higher doses than were experienced by the HST MLI. Neither did x-rays in the 3 to 10 keV range generated in a modified electron beam evaporator. An antidotal aluminized FEP sample that was exposed to an intensive dose from unfiltered Mo x-ray radiation from a rotating anode generator, however, did show the requisite brittlement. Thus, a study was undertaken to determine the effects of x-ray exposure on the embrittlement of aluminized FEP in hopes that it might elucidate the HST MLI degradation mechanism. Tensile specimens of aluminized 5 mil thick FEP were exposed to a constant fluence of unfiltered x-ray radiation from a Mo target whose maximum energy ranged from 20-60 kV. Other samples were annealed, thermally cycled (100x) between 77-333 K, or cycled and irradiated. Tensile tests and density measurements were then performed on the samples which had been irradiated had the drastically reduced elongation-to-break, characteristic of the HST samples. Thermal cycling may accelerate the embrittlement, but the effect was near the scatter in the measurements. Annealing and thermal cycling had no apparent effect. Only the samples which had been irradiated and annealed showed significant density increases, likely implicating polymer chain scission and annealing.

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Ground Laboratory Soft X-Ray Durability Evaluation of Aluminized Teflon® FEP Thermal Control Insulation

Metallized Teflon® fluorinated ethylene propylene (FEP) thermal control insulation is mechanically degraded if exposed to a sufficient fluence of soft x-ray radiation. Soft x-ray photons (4 to 8 Ĺ in wavelength or 1.55 to 3.2 keV) emitted during solar flares have been proposed as a cause of mechanical properties degradation of aluminized Teflon® FEP thermal control insulation on the Hubble Space Telescope (HST). Such degradation can be characterized by a reduction in elongation-to-failure of the Teflon® FEP. Ground laboratory soft x-ray exposure tests of aluminized Teflon® FEP were conducted to assess the degree of elongation degradation, which would occur as a result of exposure to soft x-rays in the range of 3 to 10 keV. Test results indicate that soft x-ray exposure in the 3 to 10 keV range, at mission fluence levels, does not alone cause the observed reduction in elongation of flight retrieved samples. The soft x-ray exposure facility design, mechanical properties degradation results, and implications will be presented.
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Curator:  Sandra.A.Zolo@nasa.gov  and NASA Official Responsible For Content:  Sharon.K.Miller@nasa.gov
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Last Updated: 02/04/2014