- Email: [email protected]
The methodology study of time accelerated irradiation of elastomers
Nuclear Instruments and Methods in Physics Research B 236 (2005) 229–234 www.elsevier.com/locate/nimb
The methodology study of time accelerated irradiation of elastomers Masayuki Ito
*
Advanced Research Institute for Science and Engineering, Waseda University 3-4-1 Ookubo, Shinjuku-Ku, Tokyo 169-8555, Japan Available online 31 May 2005
Abstract The article studied the methods how to shorten the irradiation time by increasing dose rate without changing the relationship between dose versus properties of degraded samples. The samples used were nine kinds of EPDM which have different compounding formula. The different dose of Co-c ray was exposed to the samples. The maximum dose was 2 MGy. The reference condition to be compared with two short time test conditions is irradiation of 0.33 kGy/h at room temperature. Two methods shown below were studied as the time-accelerate irradiation conditions. (1) Irradiation of 4.2 kGy/h in 0.5 MPa oxygen at room temperature. (2) Irradiation of 5.0 kGy/h in air at 70 °C. After irradiation the mechanical properties of samples were measured at room temperature. The changes in 100% modulus suggest that irradiation in 0.5 MPa oxygen increases slightly scission reaction and irradiation at 70 °C increases slightly crosslinking, compared with the results obtained under low dose rate irradiation (the reference condition). The deviation was mostly in ±0.25 for 100% modulus and was ±0.5 for ultimate elongation throughout the all doses, where the value obtained at the reference condition referred to as 1.0. Thus, it was found out that two methods mentioned above are available as time accelerated irradiation conditions. Ó 2005 Elsevier B.V. All rights reserved. PACS: 82.35.Lr Keywords: Degradation by irradiation; Ethylene-propylene elastomer; Time accelerated test; Electric cable for nuclear power plant
1. Introduction *
Tel./fax: +81 42 346 3467. E-mail address: [email protected]
It is necessary to estimate the lifetime of elastomers in the various environments where they are
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.039
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used. The estimating the lifetime requires the application of accelerated aging techniques. Electric wires and cables used in nuclear power plant are exposed by low dose rate irradiation during the lifetime of nuclear power plant beyond their expected 40-year time frame. The accelerated aging method for electric wires and cables required a high dose rate irradiation. It was pointed out that the exposure of polymers in air results only the oxidation of surface when the dose rate was higher [1,2]. Dissolved oxygen in the polymer is consumed by reaction with free radicals generated by irradiation. Therefore, when the rate of the consumption of oxygen is faster than the rate of diffusion from the surrounding atmosphere through the polymer, oxidation occurs only in regions near the sample surface. Clough et al. studied to identify the occurrence of heterogeneous degradation, and have summarized those in an article [3]. They have developed the modulus profiling technique [4,5]. By using this techniques they studied ‘‘time–temperature–dose rate superposition’’ [6]. Seguchi et al. studied ‘‘thickness of oxidized layer–oxygen concentration–dose rate relationship’’ [7,8]. The thickness of oxidized layer Lc is expresses by Eqs. (1) and (2). Lc ¼ ðKP ox =IÞ0.5 ;
ð1Þ
K ¼ 2DS=U.
ð2Þ
In Eq. (1) Pox is pressure of oxygen and I is dose rate. D is diffusion coefficient, S is solubility coefficient and U is amount of oxygen consumption by unit dose in Eq. (2). There are two methods to increase the thickness of oxidized layer. One is irradiation under pressurized oxygen and the other is the irradiation at elevated temperature at which polymer dose not affect by heat alone. The later method is based on the data that the increase of diffusion coefficient of oxygen in the polymer increases with raising the temperature. This article compare two method of time accelerated aging technique as shown below.
(1) Irradiation of 4.2 kGy/h in 0.5 MPa oxygen at room temperature. (2) Irradiation of 5.0 kGy/h in air at 70 °C. Dose rate and temperature of the reference condition was 0.33 kGy/h and ambient temperature, respectively.
2. Experimental 2.1. Materials The properties of ethylene-propylene-diene rubber (EPEM) depend on their compounding formula. There are many studies on the relationship between the compound formula and physical properties and durability for heat aging. On the other hand, radiation resistant properties of EPDM have not been studied systematically. The use of a few kinds of samples that have different compounding formula might be necessary to establish the short time test and to evaluate the lifetime of EPDM. The article studied the radiation-induced degradation of nine kinds of EPDM samples at three conditions. The compounding formula [9] was reported previously. The thickness of the samples was about 0.5 mm. 2.2. Irradiation Dose rate and temperature used as the reference condition was 0.33 kGy/h and ambient temperature in air, respectively. The samples are oxidized homogeneously at this condition [10]. The article studied two accelerated radiation-aging methods as shown below. (1) Irradiation of 4.2 kGy/h in 0.5 MPa oxygen at room temperature.The irradiation under this condition oxidizes the samples of 1.0 mm homogenously [11]. (2) Irradiation of 5.0 kGy/h in air at 70 °C.The thickness of the oxidized layer was measured by the method of color change of DPPD mixed in EP07P (JSR) pure vulcanized. The color of DPPD increases the darkness by
M. Ito / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 229–234
oxidation. The dark layer from the surface of the sample that irradiated under this condition was 1.8 mm.
2.3. Tensile testing After irradiation sample sheets were cut into a dumbbell shape (Japan Industrial standard, K6301, No. 4). Tensile test were performed using Shimazu Model SC-500 Test Machine equipped with pneumatic grips. Samples were strained at ambient temperature at 500 mm/min using an initial jaw distance of 50 mm. At each aging condition four samples were tested and calculated to obtain the results according to JIS K6301.
3. Result and discussion 3.1. Changes in mechanical properties Fig. 1 shows the changes of 100% modulus, ultimate elongation and tensile strength of EPDM-1 by irradiation at the reference condition and two time accelerated environment. Figs. 2 and 3 show the results of EPDM-2 and EPDM-3 respectively as the similar manner as in Fig. 1. Tensile strength
231
and ultimate elongation decreases with increasing dose in all samples and in all of the irradiation conditions applied. On the other hand, there are three types on the changes of 100% modulus by irradiation. EPDM-1 increases 100% modulus with increasing dose. 100% modulus increases with up to a certain dose and decreases from the dose in EPDM-2. The significant change can not been observed on 100% modulus by irradiation on sample 3. I have studied the effect of quantitative addition of scission and crosslinking on modulus and ultimate elongation of an elastomer [12]. The crosslinking predominate condition increases modulus of the elastomer (case 1). When scission is predominating, modulus decreases with increasing dose (case 2). Addition of the same amount of scission and crosslinking keeps the modulus the constant value (case 3). But ultimate elongation decreases with increasing dose for all three cases. This tendency of decreasing ultimate elongation was found to be the smallest in case 3. Fig. 4 shows the relationship between 100% modulus and ultimate elongation of case 1 (C S), case 2 (S > C) and case 3 (S = C). In case 1, the amount of crosslinking added is five times of scission [12]. Therefore, case 1 considered to be the typical type of crosslinking predominant
Fig. 1. Changes of mechanical properties of EPDM-1 by irradiation. s 0.33 kGy/h in air at room temperature; d 4.2 kGy/h in 0.5 Mpa O2 at room temperature; h 5.0 kGy/h in air at 70 °C.
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Fig. 2. Changes of mechanical properties of EPDM-2 by irradiation. Caption is the same as Fig. 1.
Fig. 3. Changes of mechanical properties of EPDM-3 by irradiation. Caption is the same as Fig. 1.
condition. The slope of the locus is 1.0 in the case as shown Fig. 4. Figs. 5–7 show the relationship between 100% modulus and ultimate elongation of EPDM-1, 2 and 3 respectively. There is not significant difference between the reference condition and time accelerated irradiation on the relationship in Fig. 5. This means almost the same ratio of crosslinking and scission was added to EPDM-1 along with aging for three irradiation conditions. Fig. 6 shows that crosslinking is predominant in the earlier stage of the aging of EPDM-2,
but chain scission predominates from a certain dose. Irradiation of EPDM-3 in 0.5 MPa oxygen brought about higher ratio of scission to crosslinking. On the other hand, crosslinking predominate during irradiation at 70 °C. But, the differences of changes of mechanical properties between three irradiation conditions are not significant on the view point of short time test. The procedure described below examine whether the time-accelerated irradiation conditions are appropriate or not for the reference condition [9].
M. Ito / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 229–234
Fig. 4. Creation of modulus–ultimate elongation plotting. The modulus and the ultimate elongation of sample in circle which have a certain network concentration changes the relationship between the modulus and the ultimate elongation along with addition of scission (S) and crosslinking (C).
233
Fig. 6. Modulus–ultimate elongation plotting for EPDM-2. Non irradiated sample has the highest ultimate elongation.
Fig. 5. Modulus–ultimate elongation plotting for EPDM-1. Non irradiated sample has the highest ultimate elongation.
Fig. 7. Modulus–ultimate elongation plotting for EPDM-3. Non irradiated sample has the highest ultimate elongation.
(1) The data obtained by reference condition is plotted on a liner section paper. The vertical axis is dose and the horizontal axis is 100% modulus or ultimate elongation. (2) The second procedure draws the line between point and point to obtain locus. This procedure refers to the mathematical interpolation. (3) The value of 100% modulus or ultimate elongation obtained by time-accelerated condition is divided by the extrapolated reference value on the same dose.
(4) The values obtained in procedure (3) are plotted on re-scaled graph, where the reference value is defined as 1.0. Fig. 8 shows a result obtained by the procedure described above with focussing 100% modulus. The symbols in the figure which are put by the line are data obtained irradiation at 70 °C. The other symbols are that obtained under 0.5 MPa oxygen. The line 1.0 refers to the 100% modus of the samples irradiated at 0.33 kGy/h at room temperature. The deviations of time accelerated results from the
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Fig. 8. Comparison of the different irradiation condition on 100% modulus along with irradiation. Ratio = (100% modulus of EPDM aged in time accelerated irradiation at a certain dose)/(100% modulus of EPDM aged in reference condition at the corresponding dose). The symbols which are put by the line are data obtained by irradiation at 70 °C. The other symbols are that obtained under 0.5 MPa oxygen. s EPDM-1; h EPDM-2; n EPDM-3; , EPDM-4; } EPDM-5; þ EPDM-6; EPDM-7; EPDM-8; EPDM-9.
Fig. 9. Comparison of the different irradiation condition on ultimate elongation along with irradiation. Ratio = (ultimate elongation of EPDM aged in time accelerated irradiation at a certain dose)/(ultimate elongation of EPDM aged in reference condition at the corresponding dose) The caption is the same as in Fig. 8.
reference condition are in ±25% even in higher dose. The values of ratio irradiated at 70 °C are higher than that of irradiated pressurized oxygen at higher dose. This shows that irradiation at 70 °C tends to bring about crosslinking, and irradiation in pressurized oxygen gives a higher scission compared to the reference condition. Fig. 9 shows the result for ultimate elongation. As the same as Fig. 8 the symbols in the figure which are put by the line are data obtained irradiation at 70 °C. The ranges of deviations are in ±50% for most of the samples. Thus, it was found out that two methods mentioned above are available as time accelerated irradiation conditions.
References [1] M. Dole, Report of Symposium IV Chemistry and Physics of Radiation Dosimetry, Army Chemical Center, Maryland, 1952, p. 120. [2] A. Charlesby, Proc. Roy. Soc. (London) A 215 (1952) 187. [3] R.L. Clough, K.T. Gillen, C.A. Quintana, J. Polym. Sci., Polym. Chem. Ed. 23 (1985) 359. [4] R.L. Clough, K.T. Gillen, C.A. Quintana, Polym. Degrad. Stab. 17 (1987) 31. [5] R.L. Clough, K.T. Gillen, Polym. Eng. Sci. 29 (1989) 29. [6] R.L. Clough, K.T. Gillen, Polym. Degrad. Stab. 24 (1989) 137. [7] T. Seguchi, S. Hasimoto, W. Kawaqkami, E. Kuriyama, Report of Japan Atomic Energy Research Institute JAERI-M 7315, 1997. [8] T. Seguchi, Y. Yamamoto, Repot of Japan Atomic Energy Research Institute JAERI-1299 (1986). [9] M. Ito, Y. Kusama, T. Yagi, S. Okada, K. Yosida, Mater. Life 5 (1993) 18. [10] M. Ito, S. Okada, I. Kuriyama, Radiat. Phys. Chem. 16 (1961) 481. [11] K. Arakawa, T. Seguchi, Kobunshi ronbunshyu 41 (1984) 733. [12] M. Ito, J. Soc. Rubber Ind. Jpn. 59 (1985) 169.
The methodology study of time accelerated irradiation of elastomers Masayuki Ito
*
Advanced Research Institute for Science and Engineering, Waseda University 3-4-1 Ookubo, Shinjuku-Ku, Tokyo 169-8555, Japan Available online 31 May 2005
Abstract The article studied the methods how to shorten the irradiation time by increasing dose rate without changing the relationship between dose versus properties of degraded samples. The samples used were nine kinds of EPDM which have different compounding formula. The different dose of Co-c ray was exposed to the samples. The maximum dose was 2 MGy. The reference condition to be compared with two short time test conditions is irradiation of 0.33 kGy/h at room temperature. Two methods shown below were studied as the time-accelerate irradiation conditions. (1) Irradiation of 4.2 kGy/h in 0.5 MPa oxygen at room temperature. (2) Irradiation of 5.0 kGy/h in air at 70 °C. After irradiation the mechanical properties of samples were measured at room temperature. The changes in 100% modulus suggest that irradiation in 0.5 MPa oxygen increases slightly scission reaction and irradiation at 70 °C increases slightly crosslinking, compared with the results obtained under low dose rate irradiation (the reference condition). The deviation was mostly in ±0.25 for 100% modulus and was ±0.5 for ultimate elongation throughout the all doses, where the value obtained at the reference condition referred to as 1.0. Thus, it was found out that two methods mentioned above are available as time accelerated irradiation conditions. Ó 2005 Elsevier B.V. All rights reserved. PACS: 82.35.Lr Keywords: Degradation by irradiation; Ethylene-propylene elastomer; Time accelerated test; Electric cable for nuclear power plant
1. Introduction *
Tel./fax: +81 42 346 3467. E-mail address: [email protected]
It is necessary to estimate the lifetime of elastomers in the various environments where they are
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.04.039
230
M. Ito / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 229–234
used. The estimating the lifetime requires the application of accelerated aging techniques. Electric wires and cables used in nuclear power plant are exposed by low dose rate irradiation during the lifetime of nuclear power plant beyond their expected 40-year time frame. The accelerated aging method for electric wires and cables required a high dose rate irradiation. It was pointed out that the exposure of polymers in air results only the oxidation of surface when the dose rate was higher [1,2]. Dissolved oxygen in the polymer is consumed by reaction with free radicals generated by irradiation. Therefore, when the rate of the consumption of oxygen is faster than the rate of diffusion from the surrounding atmosphere through the polymer, oxidation occurs only in regions near the sample surface. Clough et al. studied to identify the occurrence of heterogeneous degradation, and have summarized those in an article [3]. They have developed the modulus profiling technique [4,5]. By using this techniques they studied ‘‘time–temperature–dose rate superposition’’ [6]. Seguchi et al. studied ‘‘thickness of oxidized layer–oxygen concentration–dose rate relationship’’ [7,8]. The thickness of oxidized layer Lc is expresses by Eqs. (1) and (2). Lc ¼ ðKP ox =IÞ0.5 ;
ð1Þ
K ¼ 2DS=U.
ð2Þ
In Eq. (1) Pox is pressure of oxygen and I is dose rate. D is diffusion coefficient, S is solubility coefficient and U is amount of oxygen consumption by unit dose in Eq. (2). There are two methods to increase the thickness of oxidized layer. One is irradiation under pressurized oxygen and the other is the irradiation at elevated temperature at which polymer dose not affect by heat alone. The later method is based on the data that the increase of diffusion coefficient of oxygen in the polymer increases with raising the temperature. This article compare two method of time accelerated aging technique as shown below.
(1) Irradiation of 4.2 kGy/h in 0.5 MPa oxygen at room temperature. (2) Irradiation of 5.0 kGy/h in air at 70 °C. Dose rate and temperature of the reference condition was 0.33 kGy/h and ambient temperature, respectively.
2. Experimental 2.1. Materials The properties of ethylene-propylene-diene rubber (EPEM) depend on their compounding formula. There are many studies on the relationship between the compound formula and physical properties and durability for heat aging. On the other hand, radiation resistant properties of EPDM have not been studied systematically. The use of a few kinds of samples that have different compounding formula might be necessary to establish the short time test and to evaluate the lifetime of EPDM. The article studied the radiation-induced degradation of nine kinds of EPDM samples at three conditions. The compounding formula [9] was reported previously. The thickness of the samples was about 0.5 mm. 2.2. Irradiation Dose rate and temperature used as the reference condition was 0.33 kGy/h and ambient temperature in air, respectively. The samples are oxidized homogeneously at this condition [10]. The article studied two accelerated radiation-aging methods as shown below. (1) Irradiation of 4.2 kGy/h in 0.5 MPa oxygen at room temperature.The irradiation under this condition oxidizes the samples of 1.0 mm homogenously [11]. (2) Irradiation of 5.0 kGy/h in air at 70 °C.The thickness of the oxidized layer was measured by the method of color change of DPPD mixed in EP07P (JSR) pure vulcanized. The color of DPPD increases the darkness by
M. Ito / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 229–234
oxidation. The dark layer from the surface of the sample that irradiated under this condition was 1.8 mm.
2.3. Tensile testing After irradiation sample sheets were cut into a dumbbell shape (Japan Industrial standard, K6301, No. 4). Tensile test were performed using Shimazu Model SC-500 Test Machine equipped with pneumatic grips. Samples were strained at ambient temperature at 500 mm/min using an initial jaw distance of 50 mm. At each aging condition four samples were tested and calculated to obtain the results according to JIS K6301.
3. Result and discussion 3.1. Changes in mechanical properties Fig. 1 shows the changes of 100% modulus, ultimate elongation and tensile strength of EPDM-1 by irradiation at the reference condition and two time accelerated environment. Figs. 2 and 3 show the results of EPDM-2 and EPDM-3 respectively as the similar manner as in Fig. 1. Tensile strength
231
and ultimate elongation decreases with increasing dose in all samples and in all of the irradiation conditions applied. On the other hand, there are three types on the changes of 100% modulus by irradiation. EPDM-1 increases 100% modulus with increasing dose. 100% modulus increases with up to a certain dose and decreases from the dose in EPDM-2. The significant change can not been observed on 100% modulus by irradiation on sample 3. I have studied the effect of quantitative addition of scission and crosslinking on modulus and ultimate elongation of an elastomer [12]. The crosslinking predominate condition increases modulus of the elastomer (case 1). When scission is predominating, modulus decreases with increasing dose (case 2). Addition of the same amount of scission and crosslinking keeps the modulus the constant value (case 3). But ultimate elongation decreases with increasing dose for all three cases. This tendency of decreasing ultimate elongation was found to be the smallest in case 3. Fig. 4 shows the relationship between 100% modulus and ultimate elongation of case 1 (C S), case 2 (S > C) and case 3 (S = C). In case 1, the amount of crosslinking added is five times of scission [12]. Therefore, case 1 considered to be the typical type of crosslinking predominant
Fig. 1. Changes of mechanical properties of EPDM-1 by irradiation. s 0.33 kGy/h in air at room temperature; d 4.2 kGy/h in 0.5 Mpa O2 at room temperature; h 5.0 kGy/h in air at 70 °C.
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Fig. 2. Changes of mechanical properties of EPDM-2 by irradiation. Caption is the same as Fig. 1.
Fig. 3. Changes of mechanical properties of EPDM-3 by irradiation. Caption is the same as Fig. 1.
condition. The slope of the locus is 1.0 in the case as shown Fig. 4. Figs. 5–7 show the relationship between 100% modulus and ultimate elongation of EPDM-1, 2 and 3 respectively. There is not significant difference between the reference condition and time accelerated irradiation on the relationship in Fig. 5. This means almost the same ratio of crosslinking and scission was added to EPDM-1 along with aging for three irradiation conditions. Fig. 6 shows that crosslinking is predominant in the earlier stage of the aging of EPDM-2,
but chain scission predominates from a certain dose. Irradiation of EPDM-3 in 0.5 MPa oxygen brought about higher ratio of scission to crosslinking. On the other hand, crosslinking predominate during irradiation at 70 °C. But, the differences of changes of mechanical properties between three irradiation conditions are not significant on the view point of short time test. The procedure described below examine whether the time-accelerated irradiation conditions are appropriate or not for the reference condition [9].
M. Ito / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 229–234
Fig. 4. Creation of modulus–ultimate elongation plotting. The modulus and the ultimate elongation of sample in circle which have a certain network concentration changes the relationship between the modulus and the ultimate elongation along with addition of scission (S) and crosslinking (C).
233
Fig. 6. Modulus–ultimate elongation plotting for EPDM-2. Non irradiated sample has the highest ultimate elongation.
Fig. 5. Modulus–ultimate elongation plotting for EPDM-1. Non irradiated sample has the highest ultimate elongation.
Fig. 7. Modulus–ultimate elongation plotting for EPDM-3. Non irradiated sample has the highest ultimate elongation.
(1) The data obtained by reference condition is plotted on a liner section paper. The vertical axis is dose and the horizontal axis is 100% modulus or ultimate elongation. (2) The second procedure draws the line between point and point to obtain locus. This procedure refers to the mathematical interpolation. (3) The value of 100% modulus or ultimate elongation obtained by time-accelerated condition is divided by the extrapolated reference value on the same dose.
(4) The values obtained in procedure (3) are plotted on re-scaled graph, where the reference value is defined as 1.0. Fig. 8 shows a result obtained by the procedure described above with focussing 100% modulus. The symbols in the figure which are put by the line are data obtained irradiation at 70 °C. The other symbols are that obtained under 0.5 MPa oxygen. The line 1.0 refers to the 100% modus of the samples irradiated at 0.33 kGy/h at room temperature. The deviations of time accelerated results from the
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M. Ito / Nucl. Instr. and Meth. in Phys. Res. B 236 (2005) 229–234
Fig. 8. Comparison of the different irradiation condition on 100% modulus along with irradiation. Ratio = (100% modulus of EPDM aged in time accelerated irradiation at a certain dose)/(100% modulus of EPDM aged in reference condition at the corresponding dose). The symbols which are put by the line are data obtained by irradiation at 70 °C. The other symbols are that obtained under 0.5 MPa oxygen. s EPDM-1; h EPDM-2; n EPDM-3; , EPDM-4; } EPDM-5; þ EPDM-6; EPDM-7; EPDM-8; EPDM-9.
Fig. 9. Comparison of the different irradiation condition on ultimate elongation along with irradiation. Ratio = (ultimate elongation of EPDM aged in time accelerated irradiation at a certain dose)/(ultimate elongation of EPDM aged in reference condition at the corresponding dose) The caption is the same as in Fig. 8.
reference condition are in ±25% even in higher dose. The values of ratio irradiated at 70 °C are higher than that of irradiated pressurized oxygen at higher dose. This shows that irradiation at 70 °C tends to bring about crosslinking, and irradiation in pressurized oxygen gives a higher scission compared to the reference condition. Fig. 9 shows the result for ultimate elongation. As the same as Fig. 8 the symbols in the figure which are put by the line are data obtained irradiation at 70 °C. The ranges of deviations are in ±50% for most of the samples. Thus, it was found out that two methods mentioned above are available as time accelerated irradiation conditions.
References [1] M. Dole, Report of Symposium IV Chemistry and Physics of Radiation Dosimetry, Army Chemical Center, Maryland, 1952, p. 120. [2] A. Charlesby, Proc. Roy. Soc. (London) A 215 (1952) 187. [3] R.L. Clough, K.T. Gillen, C.A. Quintana, J. Polym. Sci., Polym. Chem. Ed. 23 (1985) 359. [4] R.L. Clough, K.T. Gillen, C.A. Quintana, Polym. Degrad. Stab. 17 (1987) 31. [5] R.L. Clough, K.T. Gillen, Polym. Eng. Sci. 29 (1989) 29. [6] R.L. Clough, K.T. Gillen, Polym. Degrad. Stab. 24 (1989) 137. [7] T. Seguchi, S. Hasimoto, W. Kawaqkami, E. Kuriyama, Report of Japan Atomic Energy Research Institute JAERI-M 7315, 1997. [8] T. Seguchi, Y. Yamamoto, Repot of Japan Atomic Energy Research Institute JAERI-1299 (1986). [9] M. Ito, Y. Kusama, T. Yagi, S. Okada, K. Yosida, Mater. Life 5 (1993) 18. [10] M. Ito, S. Okada, I. Kuriyama, Radiat. Phys. Chem. 16 (1961) 481. [11] K. Arakawa, T. Seguchi, Kobunshi ronbunshyu 41 (1984) 733. [12] M. Ito, J. Soc. Rubber Ind. Jpn. 59 (1985) 169.