Abstract
Thermal oxidation behaviors of unfilled and filled nitrile rubber (NBR) vulcanizates were studied by the measurement of the proton spin-spin relaxation time T2. The T2 signal for a degraded specimen was resolved into two components (short T2 (T2S) and long T2 (T2L)), while the T2 signal for an undegraded specimen was fitted by a single component. The T2S component may correspond to motionally constrained regions in the network chains, and the T2L component (comparable to the T2 component for the undegraded specimen) to relatively free regions. The fraction of T2S increases with the increase of the heating time. The increased fraction of T2S at a given period depends on both the surface activity and the amount of the filler particles, associated with the cumulative oxygen uptake. In the thermal oxidation process of NBR systems, the crosslinking reactions predominantly occur, and the results from the FT-IR depth profile analysis reflect the appearence of the crosslink density gradients from the outer to the inner in the degraded specimen. It is considered that the T2S component is assigned to the locally constrained regions with high crosslink density, and that the constrained regions in the network chains is inhomogeneously distributed from the outer to the inner. It has becomes clear that the increased fraction of T2S is closely connected with the lowering of the breaking stretch ratio in tension important for the practical performance, and that the failure of the overall system occurs sensitively under the influence of the constrained regions. The pulsed NMR method is expected to be one of the most suitable tools for studying degradation behaviors of rubber parts in engineering applications.
