About thermo-oxidative ageing at moderate temperature of conventionally vulcanized natural rubber

This article is dedicated to the understanding of the mechanisms involved in the evolution of the structure of a “conventionally” vulcanized rubber during thermo-oxidative (TO) aging. Based on swelling measurements, WAXS and DSC, a scenario of these mechanisms is proposed. Our results show that the crosslinking reactions are far from being complete at the end of the vulcanization process. During TO aging carried out at a moderate temperature 77 °C (350 K), the creation of long bridges first takes place via the consumption of residual sulfur; this mechanism is mainly responsible of the increase in the density of elastically active chains. The presence of residual antioxidants inhibits in the early stages of aging the chemical reactions involving oxygen. For longer aging time (here 7 days), these reactions can then occur and create "unidentified" bridges whose formation may also involves sulfur grafted onto the polymer chains. Finally, all these cross-linking reactions seem to enhance the heterogeneity of the spatial distribution of the crosslinks - already existing in the initial material - and to create highly crosslinked domains. •In conventionally crosslinked system, vulcanization process is not finished at t98.•TH ageing at 350 K involves chemical reactions different from those of the process.•At 350 K, TH ageing creates weak long polysulfidic bridges from the residual free S.•TO at 350 K lead to supplementary crosslinks in “conventional” NR.•TO ageing enhances the heterogeneity of the crosslinks spatial distribution.