Best practice for the devulcanization of sulfur-cured SBR rubber

In the present paper, special attention will be devoted to thermo-chemical devulcanization of sulfur-cured styrene butadiene rubber (SBR) using diphenyldisulfide (DPDS) as devulcanization aid. SBR is the main component in whole passenger car tire rubber and, at the same time, the most critical one in terms of devulcanization. This study is the first step to realize an effective devulcanization process for whole passenger car tire rubber. Diphenyldisulfide was found to be an effective agent for reclaiming of mainly natural rubber based sulfur-cured materials: DPDS acts as a radical scavenger and helps in preventing broken rubber chains to recombine. In this study, the effect of DPDS on the ratio of crosslink to polymer scission is investigated. The most important process parameter is the temperature, at which the polymer network is broken down. A reduction of the crosslink density of the devulcanizate is observed with increasing devulcanization temperature from 180C to 220C. However, above a temperature threshold of 220C, the crosslink density rises again. This is due to intramolecular rearrangements of chain fragments of butadiene moieties from uncontrolled degradation and oxidation effects. Oxidation stabilizers are added during the devulcanization process in order to reduce the degradation and interrupt the oxidation cycles. Above a temperature threshold of 220C, a further decrease in crosslink density without creating more sol fraction can be achieved this way. The combination of DPDS and oxidation stabilizers significantly enhances the devulcanization efficiency of SBR versus the one obtained when DPDS is used alone. The results are interpreted in terms of mechanisms of main chain and sulfur bridge scissions and the degradative cycles triggered by the presence of oxygen.