Degradation of Elastomers by Heat and/or Radiation

1. Is heat resistant elastomer durable for radiation? Fluorine containing elastomers have heat resistant property. AFLAS type showed higher radiation resistant than Viton type. Nevertheless, ultimate elongation of AFLAS was found out to decrease rapidly with increasing dose, compared with the other elastomers such as EPR, acrylic elastomer and hydrogenated nitril rubber. . 2. Dose the compounding for the thermal stability improve the radiation resistance? Nine kinds of compounding formula of ethylene-propylene elastomer (EPDM) were prepared to investigate if the compounding for heat resistant has durability for irradiation. The duration of thermal exposure was 384h at 140degree in air oven. The irradiation condition was 5kGy/h at 70degree, and total dose was 0.9MGy. It was found out that the compounding for improving the thermal stability did not bring radiation resistant on samples. 3. Synergistic relationship between heat and radiation on the degradation of elastomers. The measurement of decrease in ultimate elongation showed synergistic relationship between heat and radiation on the degradation of elastomer, but it was difficult to derive the magnitude of the synergism quantitatively from the result. Chemical stress relaxation measured the rate of scission of network chain. The rate constant of chemical stress relaxation was defined in equation. f (t) / f (0) = exp( -k't ) Where f (0) is stress at time 0 when sample is stretched at the certain length, f (t) is stress at time t and k' is the rate constant of the decay which refers to the rate of scission of network chain. The rate constant of continuous chemical stress relaxation under heat and radiation Kh+r was expressed as follows4). Kh+r Kh + Cc Iαexp (E' / RT ) Where Kh is the rate const of stress relaxation by heat alone, I is dose rate, E' is coefficient of synergism, R is gas constant, T is Temperature (K), and Cc is the constant which is refer to the sensitivity for radiation, a is dose rate dependence. The value of E' varied according to the kind of elastomer and the compounding formula