Fig. 6-13 Radiation resistance of cross-linked PTFE The tensile elongation and strength of the pristine PTFE decrease abruptly at irradiation of 5 kGy in air, and the PTFE cannot be used. In contrast, those of the cross-linked PTFE kept large values even after irradiation of 400 to 600 kGy and this PTFE is usable, showing that cross-linking modifies the radiation resistance of PTFE by over 100 times.

Fig. 6-14 Modification in properties of composite made of using cross-linked PTFE The mechanical properties of the composite made using cross-linked PTFE are modified more than those of one prepared using pristine PTFE. Radiation resistance of the composite using cross-linked PTFE increases as shown in Fig. 6-13. PTFE/glass fiber composite is filled with 20% of glass fiber by weight and PTFE/carbon fiber composite is filled with 10 wt % carbon fiber.

Polytetrafluoroethylene (PTFE) is a very important engineering plastic as it has a high performance having high resistance against heat and chemicals. However, its applications are limited in the fields of sterilization for medical goods, and of nuclear and space environments, because PTFE is degraded owing to main chain scission induced by a small amount of radiation. On the other hand, PTFE had been considered not to be cross-linking by energetic radiation, but it has been found in JAERI that it can be cross-linked between molecular chains by irradiation just above its melting temperature (327 degrees cent.) in the absence of oxygen. This cross-linked PTFE shows excellent stiffness, and resistance for creep and high radiation level (Fig. 6-13). Polymer composite, in which polymer material is reinforced by glass or carbon fibers, is used as a structural material for airplanes, etc. Development of fiber-reinforced PTFE has been tried, but adhesion between PTFE and glass or carbon fibers is not good and the strength of the fiber-reinforced PTFE becomes half of the pristine PTFE. By use of the cross-linked PTFE for composite, the flexural strength of the composite is increased, the flexural modulus is elevated to 4 times higher than that of the pristine PTFE (Fig. 6-14) and realizes high radiation stability. It is considered that this modification is caused by an increase in the tightness between the fiber and the cross-linked PTFE as the result of increased homogeneity due to the disappearance of crystallites by cross-linking. This technology in the cross-linked PTFE/fiber composite is expected to lead to a path for using PTFE in radiation environments, especially in space for artificial satellites, in which lightweight and high mechanical properties are required.

Reference A. Oshima et al., Radiation-Induced Crosslinking of Short Fiber-Filled Polytetrafluoroethylene (PTFE), JAERI-Tech 99-012 (1999).

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