Performance evaluation of gamma irradiated SiR-EPDM blends

Cable insulation materials (CIM) should perform their safety functions throughout their installed life in nuclear power plants (NPP). The CIM will be exposed to gamma irradiation at the installed locations. In order to forecast long-term performance of CIM, the short time accelerated testing was carried out. Due to its good mechanical strength, ethylene propylene diene monomer (EPDM) is widely used as CIM. Silicone rubber (SiR) is used in high temperature environments, due to its good di-electric properties/hydrophobicity. The blending of these two polymers may result in the improvement in their specific properties. This paper analyses the effects of gamma irradiation on the five different compositions (90-10; 70-30; 50-50; 30-70; 10-90) of SiR-EPDM blends. The blends were exposed to four different doses (25 Mrad, 100 Mrad, 200 Mrad and 250 Mrad) of gamma irradiation. The electrical and mechanical parameters like volume resistivity (VRY), surface resistivity (SRY), tensile strength (TS), elongation at break (EB), hardness (H) of the virgin and gamma irradiated blends were determined as per ASTM/IEC standards. The nature of degradation was investigated using Fourier transform infrared spectroscopy (FTIR). The simultaneous occurrence of cross-linking and chain scission is found to be the mechanism for ageing in SiR-EPDM blends. The electrical parameters such as volume resistivity and surface resistivity of all the blends are found to improve for all doses of gamma irradiation. To validate the influence of cross-linking reaction of the SiR-EPDM blends under a normal as well as in radiation environment, thermal analysis has been done using differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA). The morphological changes occurred on the surface of the virgin and gamma irradiated SiR-EPDM blends were investigated through the scanning electron microscopy (SEM) analysis. The blend C and the EPDM rich blends (D and E) are found to have the superior performance when compared with SiR rich blends (A and B). Hence among the three different compositions of EPDM rich blends, suitable blend can be selected for a particular NPP application.