Thermal aging of an insulation EPR-rubber compound

The stability of an EPR (ethylene-propylene rubber) compound, used as power cable insulation (138 kV) and for downhole pump (petroleum well) cables, was studied under various conditions of thermal stress. Samples were subjected to temperatures in the 50-300 degrees C range, for periods as long as 1000 h. After these treatments, electric and tensile strengths of the samples were determined, and the samples were also examined by spectroscopic (photoacoustic, infrared) methods. Gases evolved during rubber heating were analyzed by gas chromatography. It was found that: (1) electric strength is reduced: 17% of original value after 1 h at 300 degrees C and 40% after 1000 h at 200 degrees C, but no changes were found after 1000 h at 150 degrees C; (2) paraffin, used as a processing additive in the compound, exudes at temperatures as low as 50 degrees C; the exudate is more oxidized than the input material; (3) Pb/sub 3/O/sub 4/, which is also a constituent of the compound, is reduced to PbO at higher temperatures; and (4) methane is prevalent in the gaseous mixture released by the heated compound.<>