OXYGEN DIFFUSION EFFECTS IN THERMALLY AGED ELASTOMERS

We have obtained detailed, time-dependent modulus profiles on 2.2-mm thick samples of Neoprene and styrene-butadiene rubber (SBR) that were subjected to aging at temperatures of 80-150-degrees-C in air. The materials undergo strongly heterogeneous oxidation throughout this temperature range, with degradation much higher near surface regions than in the sample interior. The samples exhibit a latent onset of heterogeneous effects, particularly at the lower temperatures. As aging proceeds, the modulus in the exterior regions rises rapidly, whereas modulus changes in the interior region may slow down or stop. Numerous experiments with Neoprene formulations employing hindered phenol stabilizers of very different molecular weights demonstrate that heterogeneous oxidation is not the result of selective depletion of antioxidant from the surface regions of the sample. The primary cause of the incipient heterogeneous oxidation effects is a dramatic decrease in oxygen permeability coefficient as aging proceeds.