Effects of antioxidants on the D.C. conductivity of ethylene propylene rubber

The D.C. electrical conductivity of a number of samples of ethylene propylene rubber has been measured between 20° and 120° using a three terminal (guarded electrode) cell. Measurements were made with copper electrodes either pressed directly onto the polymer or placed on lead films evaporated on the samples A typical sample, 4×10⁻² cm thick, was prepared by milling the resin with dicumyl peroxide, clay filler and an antioxidant, then pressing and curing the material at 190°C. Before any measurements were made the samples were devolatilized at 90°C for a week. In a previous publication¹ the effects of varying the clay concentrates were described and discussed. Here the effects of varying the antioxidant concentrations are reported. The antioxidants used were Topanol and the Ethyl antioxidants numbers 330 and 702. Concentrations ranging from 0.5 to 4 wt% were studied. The current-electric field (I-E) characteristics were measured for fields as large as 8×10⁶ V/m over a range of temperatures from 20° to 120°C. With the exception of samples containing high clay concentrations the I-E characteristics were very nonlinear. At high clay concentrations (∼40wt%) the charge transport was more nearly ohmic. In general, the addition of the antioxidants in concentrations of 1wt% or more increases the conductivity at all temperatures. The effect is especially large at the higher temperatures (over 100°C) where the increase may be by more than a factor of ten. Between 1 and 4 wt% of antioxidant a monotonic increase of conductivity with antioxidant concentration was found. Below 1 wt% antioxidant the variation of the conductivity is irregular. All three antioxidants produced roughly the same increase. The increase was found to be approximately the same in samples containing 0, 20 at 40 wt% clay. An increase of the dicumyl peroxide concentration from 2 to 8 wt% did not affect conductivity.