Degradation Behaviors of Natural, Guayule, and Synthetic Isoprene Rubbers

Abstract

Abstract The degradation behavior of three isoprene based rubbers, NR, GR, and IR, under accelerated thermal and uv radiative environments have been investigated, and the extents of the degradation are compared. The oxidative degradation introduces oxygen atoms into the elastomer chains to form carbon—oxygen linkages via hydroxyl, ether, carbonyl, and carboxyl functional groups. Some polynuclear aromatic compounds are also found on the severely thermal-degraded surfaces. The secondary atomic constituents of the vulcanizates, such as nitrogen and sulfur, also oxidized during the aging. The formations of sulfate, sulfite, and zinc oxide are observed, but no direct oxidation of nitrogen to nitro or nitroso groups is detected. The atomic surface concentration of carbon decreases significantly (1/2 to 2/3), while that of oxygen increases substantially (2 to 3 times) during the oxidative aging test. The concentrations of minor elements, S and Zn, increase to a few percent more than two fold, but that of N increases only slightly. The surface enrichment of these elements is probably derived from segregation, migration, and oxidation. The element F is present with varying concentrations (less than 10%). The element is derived from remnants of the mold releasing agent containing fluorine. The flex cracking is observed on some severely thermally degraded specimens, but none is observed under the uv radiation. The formation of aromatic cyclic compounds from these linear chain elastomers is positively identified from the π−π* excitation peak in the XPS spectra. The thick, hardened laminated layers are the visual evidences of the formation. In short, NR is found to be slightly more stable than GR and IR in the present aging test. However, the thermal and uv radiation stabilities are strongly influenced by the rubber formulations. The presence of carbon black tends to nullify the differences existing between the degradation behavior of these vulcanizates. The antioxidant in the formulation is believed to be more effective than the characteristics and the origins of the gums in controlling the degradation behavior.