Radiation and Thermal Effects on the Dielectric Relaxation Properties of PEEK

J. Ind. Eng. Chem., Vol. 13, No. 2, (2007) 250-256 Radiation and Thermal Effects on the Dielectric Relaxation Properties of PEEK Phil Hyun Kang, Chung Lee, and Ki Yup Kim Radiation Application Research Division, Korea Atomic Energy Research Institute, Jeonbuk 580-195, Korea Received September 15, 2006; Accepted December 26, 2006 Abstract: The physical properties of γ-ray-irradiated and thermally aged Poly(ether ether ketone) (PEEK) have been investigated. To evaluate the degradation level of PEEK, analyses of the mechanical, electrical, and dielectric relaxation properties were carried out for accelerated-aged PEEK. Studies of the temperature dependency of the dielectric properties indicated that the glass transition temperature of the aged PEEK increased after the radiation and thermal aging. The frequency dependency of the dielectric properties implied that the magnitude of the induced dipoles and ions increased upon increasing the radiation doses and thermal aging. The values of the relaxation intensity calculated using Cole-Cole s circular arc could be useful for evaluation of the degradation level of PEEK. Keywords: radiation, thermal aging, dielectric relaxation, PEEK Introduction 1) Polymeric insulating materials are widely used in a broad range of applications in the power supply industry. However, the electrical performance of these materials could be compromised by their working environments; one of the most deleterious thing is nuclear radiation exposure, such as that encountered in nuclear reactors and radiation facilities [1]. PEEK, a low-density, crystallizable thermoplastic exhibiting excellent heat and radiation resistance, is attracting attention as a next-generation electrical insulation material. Even though radiation hardening materials such as PEEK are used in nuclear power plants, where they must have radiation and thermal resistance at the same time, only a few studies on both the radiation and thermal degradation of these materials have been carried out. In the present paper, the elongation at break was measured for virgin and γ-ray-irradiated PEEK, and the volume resistivity was measured for γ-ray-irradiated and thermally aged PEEK. In addition, molecular relaxation in the irradiated, thermally aged, and thermal-radiation accelerated-aged PEEK samples were investigated using dielectric analyses. To whom all correspondence should be addressed. (e-mail: phkang@kaeri.re.kr) Experimental Sample Preparation The sample used was a PEEK film (300-µm thick, medium viscosity grade 450 G; Victrex plc Inc.) cut into 250-by-250 mm squares for electrical and dielectric measurements. The samples used for the mechanical testing were of dumbbell type. Figure 1 shows the repeating unit of the PEEK used in this research. Thermal Accelerated Aging In this work, the thermal decomposing activation energy of PEEK was calculated, for a prediction of the equivalent lifetime, using accelerated thermal aging. A thermogravimetric analysis (TGA) run and the Flynn- Wall-Ozawa equation [Eq. (1)] were used to obtain the activation energy of the thermal decomposition reaction [2]. The calculated thermal decomposing activation energy was 241.87 kj/mol. log F(α) = log - logβ - 2.315-0.4567 (1) where A is the pre-exponential factor, β is the heating rate [K/min], R is the gas constant (8.314 J mol -1 K -1 ),

Radiation and Thermal Effects on the Dielectric Relaxation Properties of PEEK 251 Figure 1. Repeating unit of PEEK. Table 1. Equivalent Lifetime and Accelerated Thermal Aging Period of PEEK at T 1 = 130 C and T 2 =90 C Sample No. Accelerated thermal aging period, k 1 (h) Equivalent lifetime, k 2 (year) PEEK 0 0 Virgin PEEK 10 193 10 PEEK 20 386 20 PEEK 30 579 30 PEEK 40 772 40 PEEK 50 965 50 and E a is the thermal decomposing activation energy [kj/mol]. The thermal degradation was accelerated at 130 o C by the Arrhenius exploit method, using the activation energy calculated by TGA [Eqs.(2), (3)]. (2) ln (3) where k 1 is the length of time of thermal aging [hr], k 2 is the equivalent lifetime [hr], T 1 is the temperature for thermal aging [K], and T 2 is the operating temperature [K]. The equivalent lifetime and accelerated thermal aging period of PEEK are shown in Table 1. Radiation and Thermal-Radiation Aging For the purpose of the dielectric relaxation measurements with radiation degradation, the sample was irradiated with gamma rays in the presence of air at room temperature in a 60 Co facility at the Korea Atomic Energy Research Institute. The overall doses were 3000 kgy at a dose rate of 5 kgy/hr. For investigating the physical properties of the multi-aged PEEK, i.e., both thermal and radiation aging, accelerated thermally aged PEEK samples, named PEEK 10, PEEK 30, and PEEK 50, were irradiated with gamma rays of 400 and 1000 kgy doses. A nomination of the aged samples used subscripts for thermal aging and superscripts for radiation aging. Measurement The virgin and irradiated PEEK samples were subjected to a tensile mechanical testing following the ASTM standard D 638. The tensile properties of the sample at room temperature were evaluated using a Universal testing machine (Zwick Roell Z010, Zwick Inc.). A crosshead speed of 5 mm/min and a gauge length of 50 mm were used. The specimen load was sensed by a 1,000 kg capacity load cell. This cell was calibrated mechanically by precision standard weights prior to testing of each set of samples. From these experiments, the elongation at break of each sample was obtained. An average of 10 samples was tested. All the tensile tests were run in the time mode. Elongations at break (tensile strain, ε break ) were computed from the elongation (ΔL) of the sample divided by its initial length (L), ε break = ΔL / L The volume resistivity was measured at 1 kv using a guard ring electrode and a High resistance meter (Model 6517 A, Keithley) and a resistivity test fixture (Model 8009, Keithley). The surface morphology of aged PEEK was inspected using a scanning electron microscope (SEM) (XL Series 30S Philips Co.). For dielectric analysis, the PEEK sample was placed in direct contact with the sensor. The electrodes transmited an applied oscillating voltage to the sample and sensed the response of the sample from the applied voltage. The optimum sensor agreement for monitoring the bulk properties of PEEK was the ceramic parallel plate sensor. The parallel plate sensor consisted of lower and upper electrods. The lower electrode was the excitation electrode; it contained a resistance temperature detector (RTD) to accurately monitor the sample temperature. The upper electrode was the response electrode; it contained a guard ring to prevent fringing effects. A sample of a PEEK sheet was placed on the ceramic parallel plate sensor, after purging for 3 min with dry nitrogen gas, the upper ram was lowered to exert 300 newtons of force on the sample. Data were acquired while heating at a rate of 2 o C/min from 50 to 250 o C and applying a multiplexing frequency (1, 3, 10, 30, 100, or 300 Hz and 1, 3, 10, 30, or 100 khz). Results and Discussion Elongation at Break, Volume Resistivity, and Morphology Figures 3, 4, and 5 show the elongation at break and volume resistivity behavior of γ-ray-irradiated and accelerated-aged PEEK samples. Elongation at break decreased linearly below 1500 kgy of irradiation. Irradiation reduced its ductility. The elongation at break for virgin PEEK was ε break < 115 %. It has been reported that the ductility does not change much below the glass tran-

252 Phil Hyun Kang, Chung Lee, and Ki Yup Kim Figure 2. Arrhenius plot of PEEK (conversion level: 5 %). Figure 4. Volume resistivity of γ-ray-irradiated PEEK. Figure 3. Elongation at break of virgin and γ-ray-irradiated PEEK samples. sition temperature of PEEK [3]. These abrupt decreases of the elongation at break are considered to be due to a loss of the ductility because the predominant radiation crosslinking reaction made structural changes within the amorphous regions. The volume resistivity of the irradiated and thermally aged PEEK samples decreased with increasing radiation dose and thermal aging. The volume resistivity of irradiated PEEK decreased abruptly above 1500 kgy of irradiation. Figure 6 shows scanning electron micrographs of the surfaces of the aged PEEK samples. No change was observed between the neat and irradiated PEEK samples. PEEK is a ductile polymer. However, Harsha and coworkers [4] observed that the degradation mechanism loes not reflect any ductility; instead a brittle failure appearance is reflected in the micrographs. It has been reported that at low doses (up to 66 MGy) radiation-induced changes manifest themselves most clearly in reduced rates of crystallization and reorganization, in response to the formation of cross-links within the amorphous regions [5]. Figure 5. Volume resistivity of thermally accelerated-aged PEEK. Temperature Dependency of ε r ' and ε r '' Figures 7 and 8 show the temperature dependency of ε r ' and ε r '' of γ-ray-irradiated and thermally accelerated- aged PEEK samples. The results of the temperature dependency of ε r ' are described first. At f = 1 khz, the value of ε r ' of the non-irradiated sample (PEEK 0 ) began increasing rapidly at 145 o C, but it slowed above 150 o C. Because this temperature is above the glass transition temperature (T g = 150 o C), it is a principal dispersion (α dispersion) occurring by a dipole orientation involving main chain segment movement [6]. For a practical polymer, two kinds of dielectric absorptions and dispersions of the temperature are above T g. In general, absorption at a low frequency (high temperature) has a higher activation energy than absorption at a high frequency (low temperature). In accordance with previous research [7-9], PEEK has two kinds of absorptions and dispersions at 150 and 300 o C. The temperature at whichε r ' begins to increase is shifted rapidly at higher temperatures with thermal aging and a radiation dose. The variation in magnitude of the free volume caused by

Radiation and Thermal Effects on the Dielectric Relaxation Properties of PEEK 253 Figure 6. Scanning electron micrographs of the surfaces of aged PEEK samples. Figure 7. Temperature dependency of γ-ray-irradiated PEEK. degradation, such as that of electron beam irradiation, was studied previously using positron annihilation [10,11]. Fujita and coworkers found that the magnitude of the mean free volume increases rapidly from T g and that this increase is suppressed by electron beam irradiation. It was found that 60 MGy of gamma ray irradiation caused cross-linking among the molecules in PEEK, especially at temperatures above T g [12]. It was also found that the C-C strength decreases with the radiation dose, whereas that of C-O and C=O increases for oxidation using XPS [13]. For these reasons, the radiation and thermal aging disintegrates the molecules in PEEK and promotes cross-linking reactions at the same time, increasing the number of dipoles so that the aged samples showed larger values of ε r '. At a certain temperature, ε r ' increased with an increasing dose and thermal aging time. This result is interpreted as a consequence of the orientation polarization increasing due to the increasing number of dipoles with increasing radiation dose. Above 200 o C, ε r ' rapidly increased again. This phenomenon was caused by the ionic conductivity and space charge polarization due to movement of the impurities. The values of ε r '' of the non-irradiated PEEK sample Figure 8. Temperature dependency of thermally acceleratedaged PEEK. peaked near 163 o C. Because this absorption appears above T g, it may be anαabsorption. The temperature at which this absorption appears shifted to higher temperatures upon increasing the radiation dose and thermal aging time. The degradation mechanism of aged PEEK occurs through chain scissions between benzene rings and carbon, and oxygen atoms during γ-ray irradiation; therefore, the molecular weight decreased. The broken parts of PEEK combined with oxygen in the atmosphere and finally became oxidized. Frequency Dependency of ε r ' and ε r '' Figures 9 and 10 show the frequency dependency of ε r ' and ε r '' at 160 o C. In both cases (non-irradiated and aged PEEK), the value of ε r ' decreased with an increasing frequency. The value of ε r '' peaked from 150 to 190 o C, so called, an absorption. These phenomena are explained by considering dipole orientations with segment movements of the main chains, as described previously. The value of ε r ' decreased with both an increasing radiation dose and thermal aging time, as in Figures 6 and 7; this

254 Phil Hyun Kang, Chung Lee, and Ki Yup Kim Figure 9. Frequency dependency of γ-ray-irradiated PEEK. Figure 11. Cole-Cole s circular arc plots of γ-ray-irradiated PEEK. Figure 10. Frequency dependency of thermally accelerated aged PEEK. situation is due to the structural changes in PEEK during aging, as described for the temperature dependency of ε r ' and ε r '' [13]. The decreases of ε r ' and ε r '' with increasing frequency were due to extermination of the atomic and electronic polarization at low frequency. It is believed that the values of ε r ' and ε r '' decrease at a certain frequency because of the orientation polarization and space charge polarization, which agrees with the Debye equation [14,15]. Cole-Cole s Circular Arc For many dielectric materials, the frequency properties of the complex relative permittivity (ε r *) agree well with the Cole-Cole s circular arc law. Equation (4) corresponds to this arc [16]. ε r * = ε r + (0 < β 1) (4) where ε rs is the equilibrium permittivity, ε r is the instantaneous permittivity, ω is the angular frequency, τ 0 is the mean relaxation time, and β is a parameter in- Figure 12. Cole-Cole s circular arc plots of thermally accelerated aged PEEK. dicating the relaxation time distribution. The relaxation intensity Δε r is given by equation (5): Δε r = ε r0 - ε r (5) Cole-Cole s circular arcs of the aged PEEK samples are plotted for the frequency dependency of ε r ' and ε r '' in Figures 11, 12, and 13. The circular arcs increased upon increasing the radiation dose and the thermal aging time. The values of the relaxation intensity (Δε r ) as a function of the aging are derived in Table 2. The values of Δε r increased with aging. Jeffery and Damon interpreted these dielectric relaxation characteristics to be due to ionic conduction with ethylene propylene rubber [17]; it seems to be a similar case for PEEK as well. The Δε r value of PEEK 1000 (1000 kgy-irradiated PEEK) increased rapidly when compared with that of the non-irradiated PEEK; those of PEEK 2000 and PEEK 3000 increased slightly. This result indicates that radiation-induced dipoles and impurities were produced abruptly at a 1000 kgy dose; thereafter, recombination and disintegration of the induced electric charges dominated at doses

Radiation and Thermal Effects on the Dielectric Relaxation Properties of PEEK 255 Conclusions Figure 13. Cole-Cole s circular arc plots of γ-ray-irradiated PEEK after thermal aging. Table 2. Dielectric Relaxation Intensity of Aged PEEK at 160 C Sample Aging No. Δε r Virgin PEEK 0 0.784 1000 kgy PEEK 1000 0.9353 2000 kgy PEEK 2000 0.9543 3000 kgy PEEK 3000 1.0398 10 yr PEEK 10 0.7983 30 yr PEEK 30 0.9009 50 yr PEEK 50 0.9828 10 yr + 400 kgy 400 PEEK 10 1.0713 10 yr + 1000 kgy 1000 PEEK 10 1.1466 30 yr + 400 kgy 400 PEEK 30 1.0578 30 yr + 1000 kgy 1000 PEEK 30 1.1853 50 yr + 400 kgy 400 PEEK 50 1.1035 50 yr + 1000 kgy 1000 PEEK 50 1.2017 above 1000 kgy. As for the thermally aged PEEK, the Δε r value of PEEK 10 increased gradually when compared with that of PEEK 0 ; those of PEEK 30 and PEEK 50 increased abruptly. It is believed that the generation and disintegration of the induced dipoles occurs competitively, and the space charge polarization increases by rapidly increasing the impurity ions more than PEEK 10. With regard to the thermal-radiation aged PEEK, the magnitude of Δε r decreased as follows: PEEK 10 400 > PEEK 30 400 > PEEK 50 400 > PEEK 10 1000 > PEEK 30 1000 > PEEK 50 1000. These results indicate that the degradation of PEEK depends mainly on the radiation degradation, and that it is influenced by thermal aging as well. The mechanical, electrical, and dielectric properties of γ-ray-irradiated PEEK, thermally aged PEEK, and PEEK irradiated after thermal aging were studied and the following results were obtained. For γ-ray-irradiated PEEK, the mechanical properties degraded abruptly above 1500 kgy and the electrical properties degraded at 2000 kgy. Radiation-induced electric charges were produced abruptly in the irradiated PEEK at 1000 kgy. Thereafter, recombination and disintegration of the induced electric charges occurred simultaneously above 1000 kgy. For thermal aging, the generation of thermally induced electric charges was maintained in the PEEK 10 sample, which was accelerated-aged for 193 h at 130 o C. The degradation level of the thermal-radiation aged PEEK depended upon the radiation degradation. The relaxation intensity values obtained from Cole-Cole s circular arcs can be useful for evaluating the degradation level of PEEK. Acknowledgment This project was performed under the Nuclear R&D Program of the MOST. References 1. S. S. Bamji, IEEE Trans. EI., 27, 402-404 (1992). 2. Annual book of ASTM standards, E1641, Standard test method for decomposition kinetics by thermogravimetry, pp. 1041-1045 (1994). 3. C. W. Extrand, Electrical Overstress/Electrostatic Discharge Symposium Proceeding 2000, 26-28 Sept. 2000, pp. 161-165 (2000). 4. A. P. Harsha, U. S. Tewari, and B. Venkatraman, Wear, 254, 693-712 (2003). 5. A. S. Vaughan and G. C. Stevens, Polymer, 36, 1531-1540 (1995). 6. C. J. Pratt and M. J. A. Smith, 7 th Int l Conf. Dielectr. Materi. Measure. Applic., pp. 64-67 (1996). 7. K. Y. Kim, C. Lee, H. K. Kang, B. H. Ryu, and K. J. Lim, Proc. of the KIEEME Annual Summer Conf. 2003, Vol. 4, No. 1, pp. 485-488 (2004). 8. A. A. Goodwin and G. P. Simon, Polymer, 37, 991-995 (1996). 9. E. Boinard, R. A. Pethrick, and C. J. MacFarlane, Polymer, 41, 1063-1076 (2000). 10. K. Shinyama, M. Baba, and S. Fujita, Proc. of 1998 Int l Symp. on Electr. Insul., pp. 387-391 (1998). 11. K. Shinyama and S. Fujita, IEEE Trans. on Dielect.

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