Specimen heating radiation dose

Specimens used in tests were sections of cables with PVC outer coating. PVC was plasticized with DOF softener. The materials considered were exposed to the radiation and thermal aging. The samples have been irradiated at room temperature by hard gamma rays with 10 rad/sec dose power. A number of samples had been heated for long different times at 90°C. Besides a special specimens were cut out from outer coating for test on tensile machine like "Instron". The total doses of irradiation, times of heating and elongations at break obtained with "Instron" are listed in Table 1. 

Figures 1(a) and (b) show relations between the currents measured and the voltages applied for the specimens with and without H, respectively, at several ionizing dose rates for the first cycle. The currents at the applied voltage increased as the ionizing dose rate increased. Moreover, the irradiation temperature was also gone up to 473 K by gamma-heating. The increment of the irradiation temperature has a little influence on the increment of the current. However, it is, for the temperature below 473 K, much lower than the radiation effects.

The first conventional UHMWPE material was a control, virgin, unirradiated material. The second conventional UHMWPE material was gamma-radiation sterilized in nitrogen with a dose of 30 kGy (referred to as 30 kGy, Y-N2). The two highly crosslinked UHMWPEs were both gamma irradiated with an absorbed dose of 100 kGy. One of the crosslinked materials was heat treated until the specimen center reached 110°C for 2 hours (referred to as 100 kGy, 110°G). The second was heat treated at 150°G for 2 hours (referred to as 100 kGy, 150°G). The degree of crystallinity of the four material types was determined by differential scanning calorimetry (DSC), see Table 14.2. 

This energy deposition rate, 4 GGy s corresponds to a temperature increase of ten million degrees per second for a thermally isolated material of specific heat 0.4 J g K Irradiation of a large piece of material at a dose rate of 1 GGy s" would result in its vaporization in a fraction of a second. Apart from transmission electron microscopes, only nearby nuclear explosions and devices that give short pulses of irradiation for the study of fast radiation processes [96] produce such high dose rates. It is important to realize that continuous irradiation in the electron microscope, even at such dose rates, does not have to cause a large rise in the temperature of the specimen. This is because a very small object like the illuminated area in a TEM has a large surface area per unit volume, and so it is efficiently cooled by thermal conduction into the rest of the specimen. 

PA 11 and PA 12 are easily sterilized with ionizing radiation. For instance, exposing 30 to 100 mm thick polyamide 12 films to irradiation at a dose rate of 50 kGy did not result in changes in the film. Only above a dose rate of 100 kGy signs of slight crosslinking were observed. Thicker test specimens of heat stabilized PA 12 were treated with 400 kGy electron radiation without any measurable changes. 

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