Efficient de-vulcanization of sulfur-vulcanized SBR

Enabling recycling loops for used passenger car tires is a challenge and an opportunity: The challenge lies in the presence of SBR as the main elastomer in this type of tires, which makes this material difficult to reclaim due to the tendency of the elastomer chain fragments to re-combine. The opportunity lies in the wide availability of the material and in the fact that passenger car tires form a huge potential market for recycled rubber. The general problem of reclaiming rubber is the fact that besides crosslinks also polymer chains are broken in this process, and this influences the properties and reduces the quality of the recycled material. An efficient devulcanization is needed in order to achieve a high-quality recycled rubber. Within this study, recycling of sulfur-cured SBR in a thermal de-vulcanization process is investigated in order to elaborate the correlation between the process conditions and the ratio of polymer chain to crosslink scission. The temperature range for the de-vulcanization is varied from 180 °C to 300 °C, and the treatments are done in air and in a nitrogen atmosphere. Depending on the parameters used, the sol fraction increases as expected, and crosslink density first decreases, but increases again above a temperature threshold of 220 °C. The reason for this increase in crosslink density is a complicated intra-molecular rearrangement of chain fragments due to uncontrolled degradation and oxidation effects. Preventing oxidation during thermal treatment reduces the degree of rearrangement and results in significantly improved SBR devulcanizate properties.