Influence of the formation of an oxidized layer on the dynamic-mechanical properties of natural rubber

Natural rubber cylinders vulcanized with two different systems, TMTD and S/CBS, were aged for more than 2000 h in an air oven at 100 °C. During the ageing an oxidized layer formed on the surfaces of the specimens. To study the differences between the interior of the materials and their surfaces, and to find out how the interior of the materials was affected by the ageing, measurements were performed both on whole specimens and on specimens from which the surface layer had been removed. Dynamic mechanical (complex) modulus and loss-angle were recorded intermittently. The surface layer was hard and brittle because of thermo-oxidative crosslinking and probably had a low permeability to oxygen. The modulus of the interior material decreased slightly during the ageing. After ageing, the specimens cracked easily during deformation. The difference between surface and interior was greater, and the formation of the surface layer started earlier, when the material was vulcanized with S/CBS.