Temperature Effects in Gamma-Ray Irradiation of Organic Insulators

Chapter 24

Temperature Effects in Gamma-Ray Irradiation of Organic Insulators for Superconducting Magnets for Fusion Reactors Downloaded by MONASH UNIV on October 22, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch024

H. Kudoh, N. Kasai, T. Sasuga, and T. Seguchi Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma 370-12 Japan

The gamma-radiation-induced degradation at 77 K for glass fiber reinforced plastic (GFRP) was examined by flexural tests and gas analysis, and compared with room temperature irradiation results. The decrease in flexural strength at break (measured at 77 K) was much less in the case of 77 K irradiation than for RT irradiation. The evolution of CO and C O was also depressed at 77 K. The temperature dependence of the degradation closely relates to the local molecular motion of the matrix resin during irradiation. Results for other polymers such as polymethylmethacrylate (PMMA) and polytetrafluoroethylene (PTFE) are also reported in terms of change in mechanical properties and molecular weight. 2

Polymer and organic composite materials are planned to be used as the insulator for super conducting magnets in fusion reactors. The radiation resistance of the materials must be evaluated in cryogenic environments. There is only limited research work on the temperature dependence of radiation effects on polymer and composite materials (1-4). In particular, little work on temperature effects in radiation-induced mechanical property changes is available (5-7). Wilski has pointed out that real understanding of radiation resistance in these materials will require further measurements of mechanical property changes (8). Therefore, low temperature gamma-ray irradiation effects on glass fiberreinforcedplastic (GFRP) were studied in terms of changes in mechanical properties and gas evolution, and the results were compared with those obtained by irradiation at room temperature (RT). The results on PMMA and PTFE are also reported in terms of changes in mechanical properties and molecular weight. 0097-6156/96/0620-0313$12.00/0 © 1996 American Chemical Society In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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IRRADIATION OF POLYMERS

Downloaded by MONASH UNIV on October 22, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0620.ch024

Experimental Materials used in this work are commercially available bis-phenol A type epoxy resin GFRP (G10CR; Glass fiber/ diglycidyl ether of bis-phenol A hardened with dicyanodiamide) of 2 mm thick specimens (6 mm width, 70 mm length), P M M A of 3 mm thick (10 mm width, 100 mm length) and PTFE of 0.1 mm thick sheet. They were exposed to Co-60 gamma rays at 77K in liquid nitrogen and RT in flowing nitrogen gas atmosphere at a dose rate of 30 kGy/h at 77 K and 10 kGy/h at RT. PTFE was irradiated also at 4 K and 30 kGy/h. The irradiation at 77 K and 4 K was carried out with low temperature irradiation test equipment at JAERI Takasaki {9JO). Samples of GFRP and P M M A for gas analysis were irradiated in evacuated glass ampules. Mechanical properties were measured by three-point flexural tests at 77 K and RT, with span length 50 mm and crosshead speed 2 mm/min for GFRP and P M M A , and by tensile test for PTFE at R T with dumbbell shaped specimens and crosshead speed 200 mm/min. Decomposed gas accumulated in the sample tube from GFRP and P M M A was analyzed by gas chromatography (GC) at RT. Total gas evolution was determined by the pressure rise. H2, C O and CO2 were measured by GC. The molecular weight of P M M A was measured by gel permeation chromatography (GPC). For PTFE, molecular weight was determined by differential scanning calorimetry (DSC) using the method developed by Suwa et al (11). The heat of recrystallization was obtained by cooling from the melt. Molecular weight was calculated as Mn=2.1xl0 (Hc)" , where He is the heat of recrystallization. DSC was also applied to measure the glass transition temperature (Tg) of GFRP from changes in the curvature in the thermogram. The DSC apparatus used was a Perkin Elmer type 7, and all DSC thermograms were taken at a heating and cooling rate of 20 K/min. The relaxation spectrum of GFRP was obtained by visco-elasticiry measurement. The instrument used was a R H E S C A RD-1100 torsion pendulum type visco-elastometer. The measurement was performed in the temperature range of 110 to 393 K with 1.25 K/min and the frequency range of 0.5 Hz to 2 Hz. 10

516

Results and Discussion Irradiation Effects on GFRP (Glass/bisphenol A epoxy/dicyanodiamide). Mechanical Properties. Figure 1 shows the change in flexural strength at break of GFRP as a function of dose. Flexural strength at break of GFRP decreases with dose, and reaches half of the original strength at 25 MGy for 77 K irradiation and at 1 MGy for R T irradiation (All mechanical measurements were performed at 77 K). The poor radiation resistance of this GFRP to R T radiation perhaps comes from the aliphatic hardener. For specimens irradiated

In Irradiation of Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

24. KUDOH ET AL.

Organic Insulators for Superconducting Magnets

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and measured at 77 K, the strength scarcely changed upon the annealing at room temperature. These facts indicate that die radiation-induced reactions are extremely depressed at 77 K , and almost complete during irradiation. The degradation of GFRP is mainly attributed to the matrix resin. Figure 2 shows the change in Tg of GFRP determined by DSC. The Tg of GFRP corresponds to that of epoxy resin. It decreases with dose, indicating the destruction of network structure of the epoxy resin through chain scission. The behavior of Tg to dose is the same with flexural strength; the degradation upon 77 K irradiation is much less than that upon R T irradiation, which implies a lower probability of chain scission at 77 K. Gas Evolution. Figure 3 shows gas evolution from GFRP, indicating total gas and component gases of H2, C O and CO2. The total gas evolved from GFRP by irradiation increases linearly at low dose, then seems to level off above 1 MGy. The total gas evolution is less at 77 K than at RT, but it does not show as large a temperature dependence as flexural strength shows. H2 evolution is almost the same at 77 K and at RT. However, C O and CO2 evolutions are much less at 77 K than at RT. The evolution of C O and CO2 well reflect the irradiation temperature dependence of flexural strength. Irradiation Temperature Dependence. Most of the studies on the temperature dependence of polymer radiation effects refer to glass transition temperature (1,2). Though glass transition would play a most significant role in temperature dependence, other factors must be taken into consideration. The irradiation temperature dependence found in this study can not be explained by the molecular mobility above/below Tg, because both irradiations are carried out below Tg. We measured the relaxation spectra of unirradiated GFRP by visco-elasticity measurement. The gamma transition was found around 200 K corresponding to local molecular mobility(/2). The local molecular motion is restricted at 77 K and allowed at RT. The formation of C O and CC>2 come from the cleavages of >O0 and - C O - bonds in bis-phenol A epoxy resin. The probability of cleavages depends on the mobility of >C=0 and - C O - groups, and is reduced at 77 K. On the other hand, the H2 evolution from the cleavage of the C - H bond is less dependent on the irradiation temperature because the molecular motion of C - H bond is allowed even at 77 K.

Irradiation Effects on PMMA. Mechanical Properties. Figure 4 shows the change in flexural strength of P M M A measured at 77 K as a function of dose. A large temperature effect is observed; the degradation at 77 K irradiation is much less than that at R T irradiation. Molecular Weight Figure 5 shows the number average molecular weight



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1.5

0

10

20

30

Dose (MGy) Figure 1 Flexural strength of GFRP measured at 77 K irradiated at 77 K without warming(A), irradiated at 77 K after warrningOJ), irradiated at RT(#).

*

390

(MGy) Figure 2 Glass transition temperature (Tg) of GFRP irradiated at 77 K(A), and at RT(0).


24.

KUDOHETAL.



a

b

Dose (MGy) Figure 3 Gas evolution from GFRP (a)t o t a l ( 0 , « ) and C O ( 0 » , (b)H (D,B)andC02(A,A). Open symbols denote R T irradiation, and solid 77 K irradiation, respectively. 2


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200

100

1

2

3

4

Dose (MGy) Figure 4 Flexural strength of P M M A measured at 77K

irradiated at 77 K(A), and at RT(0).

Dose (MGy) Figure 5 Number average molecular weight of P M M A irradiated at 77 K(A), and at RT(0).


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of P M M A measured by GPC. Molecular weight decreases with dose and the reciprocal of M n increased linearly with dose, indicating that chain scission occurs. The chain scission probability is less at 77K than at RT. The G value of chain scission obtained is 1.7 at R T and 0.24 at 77 K , from Figure 5. The probability at 77 K is one seventh of that at R T , in agreement with the observation that the dose to half strength is seven times larger at 77 K than at R T in the case of mechanical properties measured at R T (10). Gas Evolution. Figure 6 shows the gas evolution from PMMA. The same tendency with the case of GFRP is observed, that is, total gas evolution is less at 77 K but does not show so large a temperature dependence as does the strength. H2 evolution is almost the same at 77 K and R T , while C O and C 0 evolutions are much less at 77 K than at RT.


2

Irradiation Temperature Dependence. The irradiation temperature dependence relates to local molecular mobility, because both irradiations were performed below Tg. Since the transition temperature of the ester side group of P M M A is around 270 K , the molecular motion of the ester branch is restricted at 77 K and allowed at RT. This transition would relate to the large difference in flexural strength, molecular weight and gas evolution between 77 K and RT irradiations.

Irradiation Effects on PTFE. Mechanical Properties. Figure 7 shows the change in elongation at break of PTFE measured at R T as a function of dose. The degradation by 77 K irradiation is much less than that by RT irradiation, and it should be noted that the degradation by 4 K irradiation is the same as that by 77 K irradiation. The dose at equivalent change is 5 times larger at 4 K and 77 K than at RT. Molecular Weight. Figure 8 shows the number average molecular weight of PTFE measured by D S C and Suwa's method. Molecular weight decreased with dose, which means that chain scission takes place. The chain scission probability is the same at 4 K and 77 K , and less than at RT. The dose at equivalent decrease is 5 times larger at 4 K and 77K than at RT, which agrees with the elongation behavior. The irradiation temperature dependence is related to molecular mobility as described above. The difference between 77 K and 4 K irradiation is not observed for PTFE. Considering that there is no transition in molecular motion related to the chain scission between 4 K and 77 K , the degradation at 4 K would be die same as that at 77 K, and this should be true of other polymers. This indicates that though the radiation resistance of candidate materials for fusion reactors should be evaluated at 4 K , 77 K irradiation experiments may be substituted.



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0

0.5

1

1.5

Dose (MGy)

b

Dose (MGy) Figure 6 Gas evolution from P M M A

(a)total(0,«) and C0(0,4), (b)H (n,B)andC02(A A). 2

f

Open symbols denote R T irradiation, and solid 77 K irradiation, respectively.


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0

0

Dose at RT (kGy) 2

4

10

20

Dose at 4 K . 77K (kGy) Figure 7 Elongation at break of PTFE measured at R T irradiated at 4 K(D), 77 K ( A ) and RT(0).

Dose at RT (kGy)

Dose at 4 K , 77K (kGy) Figure 8 Molecular weight of PTFE measured with D S C irradiated at 4 K(D), 77 K ( A ) and RT(0).


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Conclusion


The irradiation temperature dependence on the degradation of GFRP was studied by irradiation at 77 K and R T by measuring the changes in flexural strength and in the evolved gas. The degradation in mechanical properties at 77 K was much less than at RT, and the evolution of C O and CO2 follows the same pattern. The degradation is related to the local molecular motion of the epoxy resin in GFRP. The probability of scission decreases with decreasing molecular motion at low temperature, but would be constant below 77 K. Thus, the radiation resistance at 4 K can be evaluated by 77 K irradiation.

Literature Cited 1. Hill D. J. T., O'Donnell J. H., Perera M. C. S. and Pomery P. J., Radiat. Phys. Chem., 1992, 40, 127 2. Wundlich K., J. Polym. Sci. Polymer Phys. Edn., 1974, 11, 1293 3. Kempner E. S. and Verkman A. S., Radiat., Phys. Chem., 1988, 32, 341 4. Garrett R. W., Hill D. J. T., Le T. T., Milne K. A., J. H., O'Donnell J. H., Perera M. C. S. and Pomery P. J., In Temperature Dependence of Radiation Chemistry of Polymers, Radiation Effects on Polymers; Clough R. L. and Shalaby S. W., Eds.; ACS Symposium Series 475; American Chemical Society: Washington, D. C., 1991; 146-155 5. Yamaoka H. and Miyata K., J.Nucl.Mater., 1985, 133/134, 788 6. Coltman R. R. and Klabunde C. E., J. Nucl. Mater., 1981, 103/104, 717 7. Takamura S. and Kato T., J.Nucl.Mater., 1981, 103/104, 729 8. Wilski H., Radiat. Phys. Chem., 1987, 29, 1 9. Kasai N. and Seguchi T., 1990, JAERI-M 90-155 10. Kudoh H., Kasai N., Sasuga T. and Seguchi T., Radiat. Phys. Chem., 1994, 43, 329 11. Suwa T., Takehisa M. and Machi S., J. of Appl. Polym. Sci., 1973, 17, 3253 12. Sasuga T. and Udagawa A., Polymer, 1991, 32, 402 RECEIVED September 8, 1995


Temperature Effects in Gamma-Ray Irradiation of Organic Insulators

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