The Space Exposure Experiment of PEEK Sheets Under Tensile Stress∗

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The Space Exposure Experiment of PEEK Sheets under Tensile Stress∗

Takashi NAKAMURA∗∗, Hiroshi NAKAMURA∗∗, Osamu FUJITA∗∗, Toru NOGUCHI∗∗ and Kichiro IMAGAWA∗∗∗

To find out the degradation behavior of polymer in the real space, space exposure experiments utilizing the International Space Station (ISS) were scheduled. PEEK sheets under tensile stresses were exposed to the environment around the ISS orbit, and were irradiated by atomic oxygen (AO), ultraviolet ray, and electron beam (EB) in the ground test facility. This study introduces the outline of these experiments, and shows the results of AO and EB pilot irradiation tests as follows: (1) Test piece surfaces after AO exposure exhibited significant morphological damages characterized by micron-sized conical pits. (2) Thickness reductions of the test pieces by AO exposure increased with increasing tensile stress. (3) Residual strength after AO exposure could be estimated by taking account of thickness reduction. (4) No significant change was observed on surface morph, mass, chemical structure, and tensile properties of the test pieces after EB exposure regardless of tensile stress.

Key Words: High Polymer Materials, Tensile Properties, Material Testing, PEEK, Interna- tional Space Station, Low Earth Orbit, Atomic Oxygen, Ultraviolet Ray, Elec- tron Beam

vironment. 1. Introduction To investigate these phenomena, we organized two In recent years, polymeric materials are getting im- kinds of experiments: one was a space exposure experi- portant for fabricating space structures. Especially, poly- ment and the other was a ground control experiment. Fig- meric sheets are essentially needed for inflatable struc- ure 1 shows an outline of these experiments. The space tures(1), the state of the art technology in construction of exposure experiments expose Poly-Ether-Ether-Ketone space facilities such as moon bases, large space antennas. (PEEK) sheets with tensile loads to the real space envi- These space-based structural members have to hold a cer- ronment. This research is a part of a space exposure ex- tain amount of load in the real space. However, space periment program named as Micro-Particles Capturer and can be extremely harsh to polymers due to the presence of Space Environment Exposure Device (MPAC&SEED) ex- (4) several types of radiation and atomic oxygen (AO)(2).In periment implemented by the Japan Aerospace Explo- particular, it is known that many polymeric materials are ration Agency (JAXA). This exposure experiment started damaged by AO in low earth orbit, altitudes from 200 to in October 2001, utilizing the ISS Russian Service Mod- 700 km where the International Space Station (ISS) goes ule, and is still ongoing now. In contrast, the ground con- around(3). At this stage, we have insufficient data on inter- trol experiments irradiate AO, ultraviolet ray (UV) and relation between tensile stress and degradation behavior electron beam (EB) artificially to the PEEK sheets of the of polymeric materials suffered from the “real” space en- same kind using ground facilities of JAXA. This report introduces the outline of the space and ∗ Received 26th January, 2004 (No. 04-4036) the ground experiments, and shows the first available re- ∗∗ Division of Mechanical Science, Graduate School of Engi- sults of pilot irradiation tests carried out before the regular neering, Hokkaido University, North 13, West 8, Kita-ku, ground experiments. After exposure in space, data will be Sapporo, Hokkaido 060–8628, Japan. analyzed together with these ground results to investigate E-mail: [email protected] the degradation process of tension-loaded polymers in the ∗∗∗ Institute of Space Technology and Aeronautics, Japan space environment. Aerospace Exploration Agency (JAXA), 2–1–1 Sengen, Tsukuba, Ibaraki 305–8505, Japan

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Fig. 1 Outline of the space exposure experiment program using PEEK sheets under tensile stresses

Fig. 2 Chemical structure of PEEK

2. Space Exposure Experiment Program 2. 1 Material Materials to be used in space must have a high heat resistance. More specifically, we have to select materials capable of withstanding temperatures higher than +100◦C∼+150◦C according to estimated temperatures of exposure devices. After narrowing the candidate materials to several varieties in terms of heat resistance, PEEK was finally selected by the AO and UV pre-irradiation test. Fig. 3 Test pieces for the space exposure experiments PEEK is one of crystalline thermoplastic polymers. Its molecular structure is shown in Fig. 2. We use FS- 1100C (produced by Sumitomo Bakelite Co., Ltd.) with a 0.4 mm thickness as our PEEK sheet. The high heat resistance (Continuous service temperature: 260◦C) of this material is achieved by the aromatic structures. PEEK has an excellent heat resistance and a radiation proof property similar to polyimide, which is a well-known space material at present. Compared with the polyimide, PEEK has a better workability to fabricate thin sheet type products resulting from its thermo-plastic properties. 2. 2 Space exposure experiments Figure 3 shows the test pieces for the space exposure experiments. Three types with different widths were fab- ricated from sheets with a 0.4 mm thickness. The axial direction of each test piece was same as the drawing di- Fig. 4 Test piece attachment for the space exposure experi- (5) rection of the sheets. A test piece attachment is shown in ments Fig. 4(5). A tensile load was applied to the specimen by a tension spring. The initial stresses were determined to 0, mounted were four test piece attachments: two were with 1.57, and 4.70 MPa by the different specimen widths. The no stress but one each was with low stress and high stress. ratios of initial stresses to yield stress (≈ 85 MPa) were 0, Onboard also were many other samples proposed by other 2, and 5%, respectively. researchers. Three sets of sample holders were fixed to A sample holder is shown in Fig. 5. On this holder the outside of the service module. The surfaces of the test

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Table 1 Performance of the Combined Space Eﬀects Test Fa- cility of JAXA

Fig. 5 Sample holder for the space exposure experiments pieces were set perpendicular to the forward direction of the ISS. The ISS goes around its orbit at an altitude of about 400 km at a velocity of about 8 km/s. The individual sample holders were scheduled to be exposed for 1, 2, and 3 years with an annual recovery by a Soyuz spacecraft as Fig. 6 Test piece attachment for the ground control experi- shown in Fig. 1. One sample holder exposed for one year ments in space was returned to the earth in November 2002, and the analyses and evaluations of the test pieces are ongoing similar to those in the space exposure experiments were at Hokkaido University. scheduled for the irradiated samples. The major contents of the analyses are as follows: Figure 6 shows the test piece attachment for the • Physical factors: Change in mass loss, thickness, ground control tests. Four test pieces were mounted on surface morph, and reaction efficiency the attachment, and two of them were loaded by different • Chemical factors: Change in chemical structure, coil springs. Although the test piece shapes were slightly molecular weight, and crystallinity different from those shown in Fig. 3, as the widths were • Mechanical factors: Change in tensile properties 25 mm, 7.8 mm, 11 mm for no stress, low stress, and high (elastic modulus, yield strength, tensile strength, necking stress, respectively, the initial stresses were same as those stress, elongation, and energy at break) and fracture proof space exposure experiments. Two AO monitors (Kap- cesses ton) are also shown in Fig. 6. The AO fluence was mea- By measuring these factors, we hope to clarify the sured by using mass losses of these monitors. time dependant degradation process affected by different 3. Pilot Irradiation Tests before the Ground Control exposure periods. We understand, however, that it will Experiments be difficult to distinguish environmental effects clearly be- cause AO and other various types of radiation will affect Before the regular ground control experiments men- test pieces at a time. To address this problem, we planned tioned in section 2.3, we conducted pilot irradiation tests ground control experiments. using AO and EB beams. The pilot irradiation tests were 2. 3 Ground control experiments programmed to blush up experimental methods and analy- Ground control experiments use AO, UV, and EB as sis points of the ground control experiments. This chapter representatives of the residual atmosphere, direct sunlight, introduces main results of the pilot irradiation tests. and particulate radiation, respectively. Irradiation tests are 3. 1 AO exposure ongoing using the Combined Space Effects Test Facility 3. 1. 1 Experimental procedure The experimen- of JAXA. As shown in Table 1, this facility can irradiate tal conditions of pilot AO exposure tests are shown in Ta- single source of AO, UV, and EB to test pieces in the vac- ble 2. The AO fluence was 2.79 × 1020 atoms/cm2.Thisis uum environment. The fluence of each irradiation was se- an average value measured by the two AO monitors, and is lected to be equivalent to that during six months, one year, equivalent to about 1 – 3 weeks on the ISS orbit. The AO and three years on the ISS orbit. Analyses and evaluations velocity, 8.1 km/s, was same as the ISS speed. The trans-

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Table 2 Conditions of AO exposure

(a) Before exposure

Fig. 7 Test piece surface after AO exposure lational energy of the AO beam with this velocity is 5 eV. The applied stresses during exposure were same as those of the regular space exposure and ground control experiments. Before and after AO exposure, mass loss, exposed area, surface morph, and tensile properties were exam- ined. Mass loss was measured by using an electron bal- ance (Sartorius, ME215P). The surface appearance was observed by a digital microscope (Scalar, HDM2100 V), and the surface morph was measured by an Atomic Force Microscope (AFM: Digital Instruments, NanoScope III). Tensile strength tests were carried out according to the (b) After exposure ASTM D882-95a, with a strain rate of 0.1 mm/mm/min, Fig. 8 AFM images of the test piece surfaces before and after and at a temperature of 23 ± 2◦C and a relative humidity AO exposure of 50 ± 5%. Before the tensile tests, each specimen had been cured for 48 hours under the same conditions. Each Table 3 Thickness reductions by AO exposure test piece for tensile tests was cut from the ground control specimens with a width of 1 mm. 3. 1. 2 Experimental results Figure 7 shows a specimen surface after AO exposure. The central circular area corresponded to the irradiated region, and was clearly distinguished from unexposed area. This phenomenon was observed on all exposed samples regardless of ap- ered to be major factors on mass loss of polymers by AO plied stress. Figure 8 shows AFM images of specimen attack. According to an experiment using polyimide(2), surfaces before and after exposure. The specimen surface AO reacted with the polymer preferentially at a particu- after exposure exhibited significant damage characterized lar site in the chemical structures under a relatively low by conical pits of 1 µm sizes with a few µm depths. This energy AO beam with 5 eV although a sputtering effect cone-like structure is similar to the surface of polyimide was significant under a high energy AO beam with 50 eV. and some other polymers after AO exposure(3), (6).There This result indicates that chemical reactions may be dom- was no significant difference on the surface feature among inant in this study since the translational energy of AO applied stresses. was 5 eV. In such a case, the increase of the thickness re- Table 3 shows the average thickness reductions of the duction with increasing stress may be explained by using test pieces calculated by using mass losses and exposed thermal activation process(7) in which tensile stress low- areas. The thickness reduction by AO attack was about ers the activation energy of chemical reaction between AO 8–10 µm, and increased with increasing stress. Generally, and material. The validity of this idea must be checked by both chemical reactions and sputtering effects are consid- further experiments including higher stress and larger flu-

Series A, Vol. 47, No. 3, 2004 JSME International Journal 369 ence; however, the results shown in Table 3 are important 3. 2 EB exposure to use polymers to space structures since structural mem- 3. 2. 1 Experimental procedure The experimen- bers usually sustain certain amount of stress in the real tal conditions of pilot EB exposure tests are shown in Ta- space environment. ble 4. The EB fluence, 1.77 × 1012 e/cm2, was equiva- The strength of the material after AO exposure may lent to about 0.5 year of the ISS orbit, and the EB dose be affected by several factors such as the reduction of spec- was 0.886 kGy. Some reports investigating a durability of imen thickness, the stress concentration resulting from the PEEK against EB have used high dose from 100 kGy to surface morph as shown in Fig. 8, and the chemical reac- 100 MGy(8), (9). Compared with the values, we used small tion layer with AO, etc. To know the main factor that influ- dose focusing on a practical use on the ISS orbit. The ap- ences strength properties, we carried out tensile strength plied stresses during exposure were 0, 1.02, and 3.66 MPa, tests before and after AO exposures. and were about the same or slightly lower compared with Tensile strengths and elongations are shown in Figs. 9 those of the regular space exposure and ground control ex- and 10 respectively. Here, tensile strengths after AO expo- periments. sure were calculated by using reduced thickness (reduced Before and after EB exposure, surface appearance, section area) of the test piece. Tensile strengths after AO mass, tensile strength properties, chemical structures, Tg exposures were almost same as those of pristine samples (Glass transition temperature), and Tm (Melting temper- regardless of the applied stress. Elongations also did not ature) were measured. Experimental procedures of sur- show significant changes after exposure. Other tensile face observations, mass measurement, and tensile strength properties, for example, yield stress, and tensile energy tests were same as those of AO exposure tests. Chemi- to break, were almost same between exposed and pris- cal structures were analyzed by using Fourier transform tine samples. These results suggest that chemical reaction infrared spectroscopy (FT-IR, (ATR), Spectrac, Iµ-II). Tg layer of exposed sample is too thin to affect mechanical and Tm at surface layers with 0.1 mm depth of the spec- behavior of PEEK sheets with a 0.4 mm thickness, and imens were measured by using a DSC (SEICO Instru- that residual strength of this material after AO exposure ments, DSC6200). can be estimated by taking account of thickness decrease. 3. 2. 2 Experimental results Surface observations clarified that no significant change in surface morphs of the specimens before and after exposures. Mass reduction of 0.01% was measured after EB exposure; however, the change is too small for us to recognize a significant difference. Tensile strengths and elongations are shown in Figs. 11 and 12 respectively. Tensile strengths after EB exposures were almost same as those of pristine samples

Table 4 Conditions of EB exposure

Fig. 9 Tensile strength before and after AO exposure

Fig. 10 Elongation before and after AO exposure Fig. 11 Tensile strength before and after EB exposure

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creased with increasing stress. ( 3 ) The residual strength after AO exposure can be estimated by taking account of thickness decrease regardless of tensile stress. ( 4 ) No significant change was observed on surface morph, mass, chemical structure, and tensile properties of test piece after EB exposure regardless of tensile stress. Acknowledgement This study is carried out as a part of “Ground-based Research Announcement for Space Utilization” promoted Fig. 12 Elongation before and after EB exposure by Japan Space Forum. The authors wish to acknowl- edge Minoru Fujita in the Division of Mechanical Science, Table 5 Heat properties of test pieces before and after EB ex- Hokkaido University, for his technical cooperation to this posure experiment. References ( 1 ) Nowak, P.S., Sadeh, W.Z. and Janakus, J., Feasibil- ity Study of Inflatable Structures for a Lunar Base, J. Spacecraft Rockets, Vol.31, No.3 (1994), pp.453–457. ( 2 ) Tagawa, M., Suetomi, T., Kinoshita, H., Umeno, M. and Ohmae, N., Surface Reaction of a Low-Flux Atomic Oxygen Beam with a Spin-Coated Polyimide regardless of applied stress. Elongations also did not show Film: Synergetic Effect of Atomic Oxygen and Ultra- significant changes after exposure. Other tensile proper- violet Exposures, Trans. Japan Soc. Aero. Space Sci., Vol.42, No.135 (1999), pp.40–45. ties, for example, yield stress, and tensile energy to break, ( 3 ) Tennyson, R.C., Atomic Oxygen Effects on Polymer- were almost same between exposed and pristine samples. Based Materials, Can. J. Phys., Vol.69 (1991), FT-IR analyses showed no significant change in spec- pp.1190–1208. tra before and after exposures. Table 5 showed thermal ( 4 ) JAXA, Homepage, http://matdb1n.tksc.jaxa.jp/mpac properties measured by DSC. The Tg, Tm, and other ther- seed/index.html mal parameters after exposure were almost same with (5) IHI.,SM/MPAC&SEED, Input Data Package for De- those of pristine samples. tail Design Review Board, (in Japanese), JRE2AR- According to the data mentioned above, we clarified 003A, Chapter 6.5-7, (1999). ( 6 ) Kleiman, J.I., Gudimenko, Y.I., Iskanderova, Z.A., that radiation resistance of PEEK is so high that EB en- Tennyson, R.C. and Morison, W.D., Surface Struc- ff vironment for 0.5 year on the ISS orbit gave no e ect on ture and Properties of Polymers Irradiated with Hyper- material properties. thermal Atomic Oxygen, Surf. Interface Anal., Vol.23, 4. Conclusions No.5 (1995), pp.335–341. ( 7 ) Zhurkov, S.N. and Tomashevsky, E.E., An Investiga- To clarify the effect of tensile stress on degradation tion of Fracture Process of Polymers by the Electron behavior of polymer in space, space exposure experiments Spin Resonance Method, Physical Basis of Yield and of PEEK sheets under tensile stresses were scheduled to- Fracture, Institute of Physics, (1966), pp.200–208. ( 8 ) Funk, J.G. and Sykes, G.F., Jr., Space Radiation Effects gether with the ground control experiments. The outline of on Poly[Aryl-Ether-Ketone] Thin Films and Compos- both experiments was introduced. The main results of pi- ites, SAMPE Quarterly, Vol.19, No.3 (1988), pp.19– lot irradiation tests using AO and EB beams for the ground 26. control experiments were shown as follows: ( 9 ) Oyabu, M., Kobayashi,Y., Seguchi, T., Sasuga, T. and ( 1 ) Test piece surfaces after AO exposure exhibited Kudoh, H., Analysis of Electron-Irradiated Poly-Ether significant damages characterized by micron-sized conical Ether Ketone by X-Ray Photoeletron Spectroscopy, pits. Bunseki Kagaku, (in Japanese), Vol.44, No.3 (1995), ( 2 ) The thickness reduction by AO exposure in- pp.195–201.

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