Sequential and synergistic effects during accelerated aging of polymers and the utility of oxidation-induction-time measurements in life-assessment methodology

The study reported here had two objectives. The first was to identify possible sequential and synergistic aging effects arising in common nuclear-qualified electric cable insulation materials, in response to concerns raised by the United States Nuclear Regulatory Commission about current Class 1E qualification procedures. The second was to develop a methodology for and an understanding of the utility of oxidation-induction-time (OIT) measurements as part of material life-assessment technology. Both of these objectives were accomplished. Accelerated aging experiments involving sequential and simultaneous applications of thermal and gamma-radiation stress components were performed with two common ethylene propylene rubber products and two cross-linked polyethylene products. One of each group was in plaque form, the other in stripped-cable-insulation form. These experiments involved temperatures of 110, 120, 130, and 140 degrees Centigrade and radiation doses of nominally 0.12 and 0.25 MGy. Relative elongation-to-rupture and relative OIT results of these experiments were used to identify sequential and synergistic effects observed for the four materials. The utility of OIT measurements was shown in part through favorable point-by-point and plot-by-plot comparisons of OIT data with appropriate relative-elongation data. Experimental correlations of OIT data from these experiments and others closely related to them were developed which defined the behavior of OIT for the above four and other materials as a function of radiation dose, antioxidant concentration, thermal aging time and temperature, and the temperature of the OIT-test temperature.