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 DOI: 10.1177/0954008308089707

2008 20: 429High Performance PolymersM. Moser, C.O.A. Semprimoschnig, M.R.J. Van Eesbeek and R. Pippan

Telescope Solar ArraysSurface and Bulk Degradation of Teflon® FEP Retrieved from the Hubble Space

  

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Surface and Bulk Degradation of Teflon R� FEPRetrieved from the Hubble Space Telescope SolarArrays

M. MOSER1

Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben,Franz-Josef-Strasse 18, A – 8700 Leoben, Austria

C. O. A. SEMPRIMOSCHNIGM. R. J. VAN EESBEEKEuropean Space Research and Technology Centre ESTEC, Noordwijk, The Netherlands

R. PIPPANErich Schmid Institute of Materials Science, Austrian Academy of Sciences and DepartmentMaterial Physics, Montanuniversität Leoben, Leoben, Austria

(Received 8 May 2007� accepted 4 January 2008)

Abstract: Teflon R� fluorinated ethylene propylene (FEP) is used on the Hubble Space Telescope (HST) asthe outer layer of multilayer insulation (MLI) blankets. During space shuttle servicing missions (SM) toHST through thickness cracks were observed in the FEP layers. Material brought to Earth for investigationshowed signs of severe degradation. During servicing mission 3B, the 4th servicing mission to HST inMarch 2002, the second pair of European Space Agency ESA built solar arrays was retrieved and flownback to Earth after 8.25 years in space. Samples of the MLI thermal control material were taken from thesolar array drive arm (SADA) and investigated for surface and bulk degradation. The MLI was envelopedaround the SADA and thus allowed the examination of FEP degradation in dependence of the orientationof space exposed areas with respect to the Sun. Therefore micrographs of FEP surfaces and fractographsof through thickness cracks were taken and the solar absorptance �S and the normal emittance �N wasmeasured on the entire MLI. Differential scanning calorimetrical analyses as well as electron spectroscopyfor chemical analyses were conducted to analyze chemical changes of the bulk polymer and the surfacelayer, respectively. Tensile properties and mass losses of the space exposed material were evaluated aswell. Results of the investigated thermal control material are compared with samples exposed on theground to thermal cycling, soft X-rays and vacuum ultra-violet radiation as well as to previously reportedresults on space-exposed FEP. The paper thus gives further insight into the mechanism and processescontributing to the on orbit degradation of Teflon R� FEP and correlates the presented data with exposurelevels and exposure conditions.

Key Words: Hubble Space Telescope, Teflon R� FEP, low Earth orbit degradation, spacecraft material durability

High Performance Polymers, 20: 429–446, 2008 DOI:10.1177/0954008308089707

��2008 SAGE Publications

Figures 1, 3, 4 appear in color online: http://hip.sagepub.com

430 M. MOSER ET AL.

Figure 1. Right: Hubble Space Telescope with axis system. The position of the investigated multilayerinsulation (MLI) on the solar array drive arm (SADA) is indicated� Left: SADA MLI with grid system.

1. INTRODUCTION

Tetrafluoroethylene-hexafluoropropylene copolymer (trade name Teflon R� FEP) is com-monly used for space applications as a top layer in multilayer insulation or as a secondsurface mirror on radiator panels because of its good thermo-optical properties and re-sistivity to most environmental conditions. During the first servicing mission (SM1) tothe Hubble Space Telescope (HST) in December 1993, 3.6 years after the telescope waslaunched into low Earth orbit, a first set of European Space Agency (ESA) built solar ar-rays (HST/SA1) was replaced by a new set (HST/SA2), which remained on HST for 8.25years until March 2002, when the fourth servicing mission (SM3B) took place. Metal-lized FEP retrieved from various surfaces of HST during SM1 and SM2 in February 1997,as well as material investigated from the solar arrays was found with cracks and severedegradation [1–3].

HST/SA2 was deinstalled and flown back to Earth during SM3B and the –V2 solarpanel wing (figure 1) was shipped to ESA for post-flight investigation. Visual inspectionshowed extensive degradation, discoloration and through thickness cracks on virtuallyall FEP heat-shields. The samples to be discussed herein originate from the solar arraydrive arm (SADA) of the –V2 solar array. The arm was enwrapped with four identicalmultilayer insulation (MLI) blankets. The top and space-exposed layer of the MLI was a127 �m (5 mil) FEP foil backside-coated with a 160 nm Ag/Inconel film and embossedevery 1.5 in. (3.8 cm) with 0.5-cm-diameter vent-holes. This layer was fixed with acrylic

DEGRADATION OF TEFLON R� FEP RETRIEVED 431

adhesive to a polytetrafluoroethylene (PTFE)-impregnated glass fiber cloth. The packagewas followed by 16 layers of double-sided aluminized Kapton R� with Dacron net andanother layer of glass fiber cloth [1]. Of the four MLIs the inner-most layer– next to thesolar array drive and HST body — showed the strongest degradation (figure 1) and wastherefore used for extensive analyses. As shown in figure 1 a grid system was applied tounmistakably trace back samples to their position on the MLI.

On orbit the solar arrays and thus the SADA and the MLI had stable position with re-gard to the Sun. The orientations are given in figure 1 with the solar-facing +V3 directionbeing between row C and row D and the anti-solar-facing –V3 direction point from rowH. It was therefore possible to study the influences of dominating space environmentaleffects on the degradation of FEP by investigating the material along a grid-column (i.e.circumferential around SADA – from 1 to 4 in the –V2 direction). Comparing the analysesof the HST/SA2 retrieved material with results from ground-based exposures of FEP tovacuum ultra-violet (VUV), soft X-rays and thermal cycling and analyzing these data toprevious investigations of retrieved HST material, provided further insight into the degra-dation processes of space-exposed FEP for various exposure levels and different exposureconditions.

2. ENVIRONMENTAL EXPOSURE CONDITIONS

The space environmental conditions that the FEP on the MLI was exposed to varied greatlywith its location around the SADA. As the solar arrays had a stable orientation trackingthe Sun, the +V3 direction always pointed to the Sun and the –V3 was anti-solar-facing.Therefore it is possible to derive environmental exposure conditions from data calculatedby ESA and NASA for material retrieved at previous servicing missions to the HST asthey are given in Drolshagen [4] and Dever et al. [5]. Extrapolated data for equivalent Sunhour (ESH) exposure as well as the number of thermal cycles and temperature range aregiven in table 1 for the 8.25 year HST/SA2 mission duration. The fluences of X-ray andparticle environments – i.e electron and proton radiation – however, are presented for thetimeframe between launch of HST and SM3B, the actual particle fluences HST/SA2 wasexposed to between SM1 and SM3B are therefore lower than those given herein (table 1).The space environmental data presented in table 1 differs from the data provided by NASAin de Groh et al. [7] for the SADA timeframe, but the values given herein are of the sameorder of magnitude.

In order to study important contributors to the on-orbit degradation of the polymer,pristine 127 �m Teflon R� FEP material, coated on one side with vapor-deposited alu-minum (VDA), was exposed on the ground to thermal cycling, VUV and soft X-ray radia-tion. Accelerated thermal cycling was performed between –100 �C and +100 �C for 6350cycles with a cycle duration of 7 min. VUV and soft X-ray exposures were performed in asolar radiation simulator facility [8] at a base pressure below 10�7 mbar. The VUV sourceswere two Hamamatsu type 7293 Deuterium lamps, capable of producing VUV radiation

432 M. MOSER ET AL.

Table 1. Exposure conditions for Hubble Space Telescope solar array drive arm sur-faces as they are derived from Drolshagen [4] and Dever et al. [5].

Exposure SM3BESH (h)Solar facing 46 028Anti-solar facing 14 330Thermal cycling / 45 000 cycles /Temperatures range � 100 �CX-raya 1–8 Å b 0.5–4Å b

Solar facing (J m�2) � 355.8 � 22.6Several strong solar eventsbetween 1999 and 2001 c

Electron fluencea (e� cm�2) � 40 keV � 3.89 � 1013

Proton fluencea (e� cm�2) � 40 keV � 3.84 � 1010

AO fluence (atom cm�2)Solar facing 2.7 � 1020d

Anti-solar facing 3.4 � 1020d

aData given for the period between launch and SM3B.bDever et al. [5].cNational Oceanic and Atmospheric Administration [6].De Groh et al. [7].

in the wavelength range between 115 and 400 nm with an acceleration factor of approxi-mately 13 [Moser, M., internal research, ESA ESTEC, 2005]. Samples were exposed fora duration of 7385 ESH. Soft X-ray radiation was simulated by a 40-h (215 000 ESH)exposure to an Al K� source type XR3E2 from Fisons [Moser, M., internal research, ESAESTEC, 2005].

3. EXPERIMENTAL PROCEDURES

Digital micrographs of MLI surfaces and cross-sections of through-thickness crackedHST/SA2 FEP were acquired with a CETI optical microscope. Measurements of thethermo-optical properties were performed on the MLI blanket at each grid position. Thesolar absorptance �S was determined with a UV/VIS/NIR spectrometer, type Cary 500(Varian), with integrating sphere. This spectrometer measures the transmittance andreflectance in the ultraviolet, visible and near infrared spectrum of the light over a wave-length range of 250 to 2500 nm. The sphere integrates both the specular and the diffuseparts of the beam. The instrument is calibrated with an internal reference standard by mea-suring its spectrum and comparing it to the certified spectrum of an aluminum specularreflectance standard. From the reflectance spectra the solar absorptance �S is calculated.The normal thermal emittance �N is measured using an infrared reflectometer, type DB

DEGRADATION OF TEFLON R� FEP RETRIEVED 433

100 (Gier-Dunkle). The instrument is calibrated with an internal working standard. Boththermo-optical analyses were performed according to ECSS-Q-70-09 [9].

Root-mean-squared (RMS) surface roughness of FEP on various positions on the MLIwas acquired with a LEICA TCS NT confocal microscopy stage of a LEICA optical mi-croscope. The x/y resolution is 0.18 �m (FWHM) and a corresponding z-resolution ofbetter than 0.35 �m (FWHM) can be achieved.

Mass measurements were done with a Sartorius BP211D micro-balance on sampleswith a diameter of 4 mm that were punched out of the MLI. Three samples were taken fromevery investigated area and the averaged mass measurement with its standard deviation isreported. Sample thicknesses were calculated from the mass with the known diameter anddensity of the material and correlated to the beginning of life (BOL) value with its knownthickness of 5 mil. It has however to be mentioned that this method of thickness determi-nation does not account for possible density changes in the space-exposed material.

Electron spectroscopy for chemical analyses (ESCA) measurements were performedwith a Surface Science Instruments (SSI) M-Probe Spectrometer operating at a base pres-sure of 3 � 10�9 mbar. The samples were irradiated with monochromatic Al K� X-rays(1486.6 eV) using an X-ray spot size of 1000 �m� 400 �m and� 180 W power. Surveyspectra were recorded with pass energy of 150 eV, from which the surface chemical com-positions were derived. In addition, selected high resolution spectra were recorded withpass energy of 25 eV, from which the chemical states of those elements were determined.

Differential scanning calorimetry (DSC) experiments were done on 4 mm disks pun-ched from row 2 of the MLI (figure 1). These samples were heated in an inert atmosphere(N2 purge rate of 70 mL min�1) in a Mettler Toledo 822 DSC from 35 to 300 �C at aheating rate of 10 K min�1. The recorded data was analysed using the STARe EvaluationSoftware V8.01 and compared with pristine material.

Tensile testing was conducted using a Lloyd LR5K test frame fitted with Ondio con-trol software and a 500 N load cell. Specimens were held in manually tightened grips andtested using a crosshead separation rate of 10.0 mm min�1. The load–displacement tracewas recorded and the ultimate tensile stress (UTS), maximum strain and linear modulusdetermined. The sample specimens used consisted of 3 mm wide FEP stripes, whose endswere bonded on the flat side of abrasive paper to counteract slippage in the clamps of thetensile test machine. The stripes were punched directly from the MLI. A gauge length of20 mm was utilized. Sample holders of the ground exposure facilities required a differenttensile test sample design. These specimens were dumbbell shaped with a gauge lengthof 6 mm and a test length of 5 mm. After irradiation the samples were bonded to abrasivepaper as described above. Bonding the ends of the tensile test samples has been elabo-rated in extensive test runs to provide reliable results on exposed and un-exposed material[Garcia-Martin, G. and Semprimoschnig, C., internal research, ESA ESTEC, 2004]. Notehowever that these sample configurations do not conform to ASTM D882 [10] also due tothe large sample size and high test speed.

In both, tensile tests and mass measurements, the metallic coating, the adhesive layerand the glass fiber cloth were removed from the backside of the FEP using a 1 mol/lsodium hydroxide solution.

434 M. MOSER ET AL.

Figure 2. Micrographs of the cracked vapor-deposited aluminum coating on the A2 (a) and the H2 (b)areas. Eroded and discolored FEP surface on the solar-facing D2 grid position (c).

4. RESULTS

4.1. Optical investigation

Mud-tile-like cracking of the reflective Ag/Inconel layer was observed on solar and anti-solar-facing surfaces of the HST/SA2 MLI. The cracks were uniformly distributed alongthe �V2 axis, the longitudinal SADA direction. Examples of these crack patterns aregiven in figure 2(a) and (b) for material from the A2 and H2 positions, respectively. Crack-ing of the metallic coating originates from a mismatch between the thermal expansion ofFEP and the Ag/Inconel film� the coefficients of thermal expansion at room temperature ofFEP and silver are roughly 140and 19.7 �m m�1 �C�1, respectively [11]. From the over-all geometry it is reasonable to assume that the maximum tensile stresses from thermalexpansion appear tangential to the V2 SADA axis, leading to the observed �V2-directedlongitudinal cracking. On the anti-solar-facing surfaces very fine fractures of FEP sur-faces were observed, similar to pattern described in Dever et al. [12]. The solar-facingFEP surfaces between row C and D were strongly discoloured (figures 1 and 2(c)) and fea-tured an ochre, rough (RMS roughness of 13.6 � 0.6 �m, as compared to around 3.7 �mon the other HST/SA2 surfaces and below 3 �m for pristine material) surface, which mayoriginate from erosion and contamination. The polymer on the solar-facing areas was in-tensively cracked and a through-thickness crack through the FEP/acrylic-adhesive/glassfiber combination along the �V2 axis has split the entire MLI perpendicular to the +V3axis (figure 3). The strong degradation of FEP, especially on the solar-facing surfaces,clearly documents the important influence of solar radiation on the resistance of the poly-mer and the thus resulting on-orbit cracking.

Due to the reduced transparency (see below, figure 5(b)) on the solar-facing surfacesit was not possible to investigate cracking of the metallic coating.

In grid C2–D2 the through-thickness crack branches as shown in figure 3. Theirfracture surfaces were investigated, yielding three distinct crack surface appearances asthey are representatively shown in figure 3(a) to (c). The fractograph of figure 3(a) wastaken close to the branching point. The image shows a crack profile with striations, which

DEGRADATION OF TEFLON R� FEP RETRIEVED 435

Figure 3. Left: Overview of a branched through-thickness crack located between grid positions C-D1-2. Fractographs (right) are taken as indicated at the positions (a), (b) and (c).

resembles the crack growth pattern observed in fatigue fracture of metals and structuralpolymers. In contrast to metals, however, fatigue cracks in polymers may not propagatesteadily but may grow in bursts or spurts. These spurts are associated with a large numberof stress cycles. They can produce microscopic markings that appear very similar to stri-ations but which do not correspond to single load cycles [13]. A similar cracking pattern,with possible lines of rest, was observed by Van Eesbeek et al. [1] at fracture surfaces in-vestigated from a SADA MLI retrieved from the first HST solar arrays (HST/SA1). Theseauthors state that theoretical calculations yield stresses created during thermal cyclingin the FEP/acrylic–adhesive/glass fiber combination which are above the tensile strengthlimit of the polymer. The fracture surface in figure 3(b) and (c), further away from thebranch point, appears smoother, and only traces of rest lines may be found. A smoothfracture appearance was reported by Wang et al. [14] for aluminum-coated 5 mil FEP re-trieved at SM2. There this pattern is described to stem from very slow crack propagationunder the influence of (space) environmental effects and relatively low stresses. Furtherin these images the embrittled and roughened top layer of the polymer is visible as a shinyfringe on the top edge.

4.2. Thermo-optical properties

Literature values of the solar absorptance �S and normal emittance �N are � 0.09 and� 0.75, respectively [15]. Measurements on pristine Ag/Inconel-coated FEP yielded �S of

436 M. MOSER ET AL.

Figure 4. Plot of the solar absorptance distribution on the space-facing multilayer insulation surfaces.The grid positions are given on the x and y axis, �S is indicated on the z-axis and by a colour code.

Table 2. Solar absorptance �S, normal emittance �N and ��� ratio data of the investi-gated solar array drive arm multi-layer insulation surfaces.

Grid Row 1 Row 2 Row 3�S �N ��� �S �N ��� �S �N ���

ratio ratio ratioB 0.150 0.765 0.196 0.171 0.775 0.221 0.143 0.780 0.183C 0.260 0.760 0.342 0.333 0.770 0.432 0.314 0.785 0.400D 0.312 0.770 0.405 0.352 0.775 0.454 0.323 0.770 0.419E 0.242 0.765 0.316 0.159 0.780 0.204 0.203 0.775 0.262F 0.129 0.770 0.168 0.104 0.775 0.134 0.117 0.780 0.150G 0.131 0.760 0.172 0.102 0.775 0.132 0.111 0.775 0.143H 0.127 0.765 0.166 0.144 0.775 0.186 0.140 0.780 0.179I 0.129 0.765 0.169 0.176 0.785 0.224 0.192 0.785 0.245

0.081 � 0.009 and �N of 0.792 � 0.003. Comparing these data with the results of theabsorptance measurements of the HST/SA2 blanket (figure 4 and table 2) major increasesin �S as compared to the BOL martial were found on the entire MLI. As expected fromthe strong discoloration and blurring of the solar-facing B–D grids an approximately four-fold increase to a maximum value of 0.352 at the D2 surface was measured there. Thescans on the anti-solar-facing H and I grids revealed a more than doubling of �S with a

DEGRADATION OF TEFLON R� FEP RETRIEVED 437

maximum absorptance of 0.192 at the I3 surface. Only small increases were found on theintermediate areas with absorptance values around 0.120. The surfaces near A and F werecovered by wiring and thus shielded from solar radiation and the particle environmentresulted in near-to-pristine values of absorptance.

No clear correlation was found between solar exposure and the normal emittance ofFEP MLI surfaces. The strongest decrease in �N to 0.760 was found on the C1 and theG1 surfaces. The highest measured emittance of 0.785, however, was found on the C3surface� a grid with very similar exposure conditions to the C1 grid. Furthermore, high �N

values were measured at the anti-solar-facing I2 and I3 surfaces.Summarizing the results obtained by the thermal emittance measurements we can

observe that no obvious effect was noted due to the different exposure conditions or loca-tions. This was a surprise – as we will show later – a significant mass loss was noticedon samples facing the Sun (C/D grid). The observed mass loss should have reduced thethermal emittance to about 0.65. One explanation for this discrepancy is that the thermalemittance instrument determines an integrated value over a sample area of about 16 mm indiameter whereas for the mass loss measurements about three times smaller samples wereused. The mass loss measurement can therefore pick up smaller variations. In addition,the sun-facing areas did also show a significant increase in surface roughness. As in-creased surface roughness increases the thermal emittance this corroborates the suggestedexplanation of the observed discrepancy.

On the HST/SA1 samples, a solar absorptance �S of 0.101 and 0.102 and a thermalemittance �N of 0.792 and 0.803 was found on solar-facing and anti-solar-facing surfaces,respectively [3]. Moreover these MLIs are described to have whitish, yellow and browndiscoloration.

Absorptance and transmittance spectra of HST/SA2 FEP samples compared to groundexposed material, as presented in figure 5(a) and (b), respectively, give further insight intothe thermo-optical degradation. Strong decreases in the reflectance and the transmittancein comparison with the reference material within the entire wavelength range were de-tected for FEP positioned on the solar-facing D1 surface of the SA2 MLI, clearly indicat-ing a degradation of the bulk polymer as the reason for the losses of the thermo-opticalproperties.

An on-ground thermally cycled sample (indicated as TC in figure 5(a) and (b)) wasfound to have decreased reflectance in the investigated wavelength region – please notethat the ground exposed samples were aluminum coated and thus have different spectraand cut off wavelengths to the Ag/Inconel-coated HST/SA2 specimen. In contrast, thetransmittance spectrum of this sample showed no change in comparison with the referencematerial. Microscopic analyses of the thermally cycled material revealed cracking of theAl coating, comparable to those found above in the Ag/Inconel coating of HST/SA2 FEP.These cracks caused an apparent “increase” in absorptance and the investigation revealsthat thermal cycling causes no traceable changes in the thermo-optical properties of FEP.

Irradiation with VUV or X-rays led to absorption losses from � 500 nm to smallerwavelengths in both the reflectance and transmittance spectra and shows thus that high en-ergetic electromagnetic radiation can play a role in the thermo-optical degradation proces-

438 M. MOSER ET AL.

Figure 5. Reflectance (a) and transmittance (b) spectra of Hubble Space Telescope exposed materialcompared with ground exposed and pristine FEP.

DEGRADATION OF TEFLON R� FEP RETRIEVED 439

Figure 6. Mass and thickness distribution of FEP from column 2 of the investigated solar array drivearm multilayer insulation.

ses of FEP. Losses similar to those of the ground irradiated samples were found at theHST/SA2 G1 surface, with decreases in transmittance and reflectance at wavelengths be-low � 750 nm.

4.3. Mass and thickness distribution

Results of the mass measurements on samples from column 2 of the HST/SA2 MLI arepresented in figure 6. The highest mass losses of close to 1 mg occur on the solar-facing Cand D surfaces. Theses losses correspond to a decrease in thickness of more than 38 �m.Small mass losses were found on the anti-solar-facing H and I surfaces, with thicknessreductions of less than 1 �m. No changes and even increased sample masses were deter-mined on the intermediate F–G and J–A areas which were covered on HST by cabling,and thus protected from AO and radiation environment.

Higher thickness losses on solar-facing MLI areas as compared to anti-solar-facingsurfaces were reported for samples investigated from HST/SA1 [3]. There a maxi-mum mass loss of 23 �m is reported which corresponds to a ratio of 2.615 �10�3 �g mm�2 ESH�1. As expected from the comparable exposure conditions this value isclose to the 2.023� 10�3 �g mm�2 ESH�1 found in this investigation for the solar-facingHST/SA2 surface.

On-ground VUV exposure resulted in a mass loss ratio of 0.794 � 10�3 �g mm�2

ESH�1, whereas the ratio of 9.55 � 10�8 �g mm�2 ESH�1 found for X-ray-exposedmaterial was five orders of magnitude smaller.

440 M. MOSER ET AL.

Table 3: ESCA data of the investigated multilayer insulation surfaces.

Position B1 B2 BC1 C1 D2 DE1 F2 H1 H2ElementCarbon 32.9 32.4 33.6 35.0 34.8 35.5 24.1 25.8 26.4C–H/CH–CF 2.0 1.7 2.5 3.4 4.1 3.9 13.1 7.9 4.8C–O/CH—CF2 1.5 1.2 1.6 1.8 1.6 1.7 2.8 1.4 1.2C=O/CF 2.6 2.4 2.9 2.8 2.9 2.9 2.1 1.5 1.6CF2 23.4 23.8 24.1 24.2 23.7 24.1 5.6 13.8 17.6CF3 3.3 3.3 2.9 2.8 2.5 2.9 0.5 1.3 1.3Fluorine 65.3 66.4 64.2 62.2 62.2 61.6 18.9 37.8 45.8C–C/(CF2+CF3) 0.075 0.063 0.092 0.125 0.154 0.142Oxygen 1.9 1.2 2.2 2.9 3.1 2.9 38.7 24.0 19.2C–O–C 1.3 0.8 1.6 2.3 2.5 2.4O–CF2 0.6 0.4 0.6 0.5 0.6 0.5SiO2/O–C 38.7 24.0 19.2Silicon 17.4 12.4 8.6

Thus the ground-based tests show that VUV radiation has a dominant contributionto the thinning of the polymer. The small mass losses after X-ray exposure result fromdifferences of attenuation length, the depth within the majority of radiation is absorbedin the material, between VUV and X-ray radiation. VUV radiation is energetic enoughto break the bonds of the polymer but cannot penetrate deep into the material� it thuscauses photoetching [16, 17]. Soft X-rays however have attenuation lengths of tens tohundreds of micrometers depending on their energy and cause thus chain scissoning inthe bulk polymer. Dever et al. [5] report a thickness loss due to AO � 2.0 �m of +V3exposed HST surfaces retrieved during SM3. Thus one can conclude that the mass losson HST results from the influence of AO erosion and energetic electromagnetic radiation,probably also accelerated by the thermal environment.

4.4. Electron spectroscopy for chemical analyses

Selected samples were punched from the retrieved material for ESCA analysis. The re-sults are presented in table 3 and the sample location on the MLI is given according to thegrid system given in figure 1. The measurements showed a silica layer on the anti-solarF–H areas. This layer could be due to condensation of silicone and subsequent oxidationto silica, which forms a protective layer. On the solar-facing side an increased carbonlevel (C–C/C–H) was detected which correlated well with the increase in solar absorp-tance. In addition, a decrease from the ideal F/C ratio of 2 was noted in combinationwith increases in the C–C/CF2+CF3) ratio from anti-solar-facing surface to the directlysun-exposed DE1 surface. This indicates that abundant carbon is formed due to degrada-tion of the polymer as consistently observed in Milintchouk et al. [18] for HST and long

DEGRADATION OF TEFLON R� FEP RETRIEVED 441

duration exposure facility (LDEF) samples. There the decreases in F/C ratio could be di-rectly linked to increases in solar exposure, chain scissioning of FEP into macromoleculesand subsequent defluorination near the surface with the creation of C=C bonds. Further, aresidual amount of oxygen was also noted on the B1 to DE1 surfaces.

4.5. Differential scanning calorimetry

The heat of fusion and the heat of crystallization of samples taken from the various surfaceareas of the SADA MLI are given in figure 7(a) and (b), respectively. The heat of fusionconsumed for melting the exposed FEP varied dramatically, depending upon the surfacelocation that a sample was taken from. The energy was higher at the solar- and anti-solar-facing side of the MLI compared to the BOL material and has increased from –18.5 J g�1 measured for the pristine material by roughly 2.75 J g�1 to –21.25 J g�1 formaterial taken from the solar-facing D2 grid and to close to –19.5 J g�1 for samplesfrom the anti-solar-facing I2 area. A similar result is given for the heat of crystallization.Increased energies of crystallization are found at the solar-facing and anti-solar-facingsurfaces, the error margin in this data is, however, higher than determined for the heat offusion data. Increased heats of fusion and crystallization indicate increased crystallinityof the polymer. The decreased heat of fusion and large variation in heat of crystallinityfound for the G2 samples could be linked to a partial on-orbit covering of this area bycabling. The DSC scans furthermore revealed a decreased onset temperature of meltingin all investigated samples. A maximum decrease of 5.3 �C was found on the solar-facingD grid. Decreases in the onset temperature are considered to be a sign of bond scissoningand degradation of the polymer.

Heating of FEP may increase the crystallinity in the polymer. This was shown by deGroh et al. [19] with the increases in the heat of fusion of pristine FEP aged at 200 �C. Inde Groh et al. [19] the space-exposed samples did not display increases in heat of fusion,but the investigated material was thermally cycled to only +50 �C on-orbit as comparedto the SADA FEP that experienced cycling between �100 �C. It can thus be assumed thatthe higher temperatures support movement of X-ray scissioned necessary for an increasedcrystallinity. This is in accordance with de Groh et al. [19], which states that increasedcrystallinity of FEP aged at 130 �C for 332 h. Furthermore, in de Groh et al. [20] it isreported that X-ray exposure combined with increased thermal exposure caused greaterincreases in crystallinity than heating alone.

Milintchouk et al. [18] found decreased chain lengths and increased crystallinity ofFEP exposed for 69 months on the LDEF as well as on material retrieved from HST/SA1.The increases were only found on solar-facing surfaces. Moreover, it has to be mentionedthat the DSC data presented herein fit well with recent microthermal analyses of the SADAMLI surface layers as reported by Fischer and Semprimoschnig [21], who also foundindications for chain scissioning on the solar-facing surfaces of the polymer.

442 M. MOSER ET AL.

Figure 7. Heat of fusion (a) and heat of crystallization (b) of FEP samples taken from column 2 of themultilayer insulation.

DEGRADATION OF TEFLON R� FEP RETRIEVED 443

Table 4. Tensile test results of HST/SA2 solar array drive arm material (top), HST/SA1solar array drive arm samples (center) [22], and ground exposed FEP (bottom).

HST/SA2 samplesa UTS Modulus Elongation to failure(MPa) (MPa) (%)

BOL 24.47 � 1.28 432.4 � 81.02 446.9 � 50.64C3 13.17 � 0.57 631.9 � 23.03 5.5 � 0.68F3 14.21 � 0.05 558.3 � 69.39 130.5 � 14.82H1 16.05 � 0.39 510.2 � 73.69 237.0 � 29.38HST/SA1 samplesb UTS (MPa) Elongation to failure (%) Thickness (�m)BOL 19.3 183 127

18.8 190 50Solar facing – – 50Intermediate 1 12.2 17 127Intermediate 2 14.3 75 127Anti-solar facing 19.4 190 50Ground exposurec UTS (MPa) Elongation to failure (%)BOL 19.28 � 0.97 469.5 � 22.70X-ray 14.70 � 1.27 109.01 � 79.26VUV 14.74 � 1.22 237.74 � 57.32Thermal cycling 19.68 � 0.63 435.42 � 29.59

aStripes, test area 3 � 20 mm, 10 mm min�1 test speed.bDumbbell shape, test area 5 � 15 mm, 510 mm min�1 test speed [18].cDumbbell shape, test area 5 � 7 mm, 5 mm min�1 test speed.

4.6. Tensile testing

Tensile test specimens were investigated from three surfaces of the SADA MLI, the solar-facing C3, the anti-solar-facing H1 and the intermediate F2 grid position. The averagedresults of the tensile tests done on three samples from the investigated MLI surface areasare presented in table 4 and compared with data from the pristine material (average of 15tests).

The data show that a major decrease in mechanical properties was found on the space-exposed samples. Decreases of elongation to break and UTS combined with the increasesin the Young’s modulus are a clear indicator for bulk embrittlement of the FEP layer. Asexpected the most drastic decline in tensile properties was observed at samples taken fromthe +V3 solar-facing C3 grid with relative decreases in elongation and UTS of approxi-mately 99 and 46% and increases in Young’s modulus of 46%, respectively.

Results from the anti-solar-facing H1 grid also indicate severe bulk degradation ofFEP with decreases in elongation and UTS to 237% and 16.05 MPa, respectively. Thedegradation found on this surface was smaller in comparison with the C3 grid materialand samples from the F3 area. The data presented herein is not consistent with tensile testdata presented by de Groh et al. [7] from SADA MLI. In their study strong decreases in

444 M. MOSER ET AL.

elongation to break and UTS were only found for solar-facing surfaces, whereas on anti-solar-facing surfaces no degradation was observable. In de Groh et al. [7] tensile testswere, however, made on FEP with intact adhesive and scrim layers.

Due to differences in the sample configurations a one-to-one comparison betweenthe tensile test results of HST/SA1 and ground exposed material cannot be done. Thusthe bulk degradation of FEP can only be qualitatively compared� however, the effectsleading to the degradation can give an additional insight. Tensile test data from HST/SA1[18] and ground exposed samples along with data for pristine material tested with thesame configuration are given in table 4. No significant degradation was observed onsamples of the shadowed rear-side of HST/SA1, and a pristine sample exposed to rapidthermal cycling. Significant degradation in both, UTS and maximum strain is observedfor solar-exposed, internally heated power harness 27 �m HST/SA1 samples and materialirradiated on the ground with VUV and X-rays. In the case of HST/SA1 the authorsreport a full loss of the mechanical integrity of samples from areas receiving direct solarillumination (+V3) [18]� the material was strongly embrittled and could be broken bygentle touching.

The HST samples had a stable position relative to the Sun. Thus the +V3-orientatedsurfaces not only were constantly irradiated by the Sun – when the telescope was notin the Earth’s shadow – but were also exposed to a number of solar flares during theirmission duration. Seven main events were reported during the HST/SA1 flight, all ofwhich occurred in 1991, a year of maximum solar activity. The HST/SA2 were installedon the telescope close to a solar minimum but retrieved during the maximum [6]. Severalvery strong solar flares were reported between 1999 and 2002, showing that these sampleswere also exposed to high doses of X-rays.

Samples irradiated on the ground with high doses of X-rays were found with rela-tive losses of nearly 80% in elongation and 25% in UTS, respectively. Nevertheless, thesesamples were exposed to higher ESH than the space-irradiated material, but only to mono-chromatic radiation in contrast to the polychromatic electromagnetic radiation in space.

VUV radiation led to a decay of the tensile properties of FEP. The maximum strainwas found to be half and the UTS had lost a quarter of the BOL value after exposureto just one-sixth of the HST/SA2 ESH amount. Literature reports that VUV irradiation,especially in the regions between 130–140 and 165–175 nm, was able to degrade the me-chanical strength of FEP [22–24]. A model describing the degradation caused by ultravio-let irradiation of FEP is proposed by Skurat et al. [23]. It indicates three layers consistingof an eroded and roughened surface, an embrittled surface layer and small damage to thebulk material.

No significant changes in comparison with the pristine material were found after ten-sile testing of thermally cycled FEP. Thermal aging of both, pristine, previously irradiatedand space-exposed FEP, however, was reported to cause embrittlement of the polymer asa result of the temperature-induced higher mobility of the polymer chains, which led to anincreased crystallinity [19, 20].

DEGRADATION OF TEFLON R� FEP RETRIEVED 445

5. SUMMARY and CONCLUSIONS

Teflon R� FEP sample specimens retrieved after 8.25 years in space from solar arrays of theHubble Space Telescope were found with severe mass losses and unprecedented increasesin solar absorptance. The polymer was shown to have decreased tensile properties andsigns of increased crystallinity. The degradation effects were especially strong on solar-facing surfaces where more than 30% of mass was lost, where the solar absorptance wasfound to have a more than four-fold increase, and where tensile tests revealed a total lossof mechanical properties. DSC analyses suggest increased crystallinity on solar-facingand anti-solar-facing surfaces. Furthermore material from solar-facing surfaces was ochrecolored and severely cracked, with a through-thickness crack splitting the entire space-exposed layer along the V2 axis. Investigation of the fracture surfaces revealed threedistinct fracture surface appearances which originate from cyclic stresses or slow crackgrowth under the influence of space environmental effects. The ESCA of the retrievedFEP confirmed the severe degradation on the solar-facing areas whereas on the anti-solar-facing surfaces a protective silica layer was found, which may have been formed due to thecondensation of silicone contamination with subsequent interaction with atomic oxygen.

Laboratory tests show that VUV radiation causes erosion of the FEP surface. It isthus, together with AO, an important contributor to the mass and thickness losses of thepolymer. VUV radiation was furthermore identified to cause bulk embrittlement of FEP,whereas thermal cycling seems not to directly affect the properties of the polymer. Ther-mal ageing at temperatures allowing increased polymer chain mobility contributes to thedegradation of FEP by increasing its crystallinity leading to a decreased toughness of thepolymer. Soft X-rays from solar flares are a very important factor in the degradationprocesses of the material as they can cause polymer chain scissioning in the bulk furtherfacilitating chain movement and weakening of FEP. The degradation of space-exposedmaterial was found to be higher than that observed for ground exposed material, thusshowing that synergetic effects take place.

NOTE

1. Author to whom correspondence should be addressed: e-mail: Martin.Moser@mu-leoben.at

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446 M. MOSER ET AL.

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