Tires Back into Tires: Devulcanization of Tire Rubbers

The optimal process conditions for a high ratio of de-vulcanization to polymer degradation have been elaborated for tire rubbers: SBR, BR, NR and CIIR. These polymers all show their own particular breakdown characteristics, and the temperature dependence of the breakdown mechanism was investigated by measuring sol fractions and crosslink densities. For SBR and BR, the highest reduction in crosslink density was found at a temperature of 220°C, together with a moderate increase in sol content. According to the Horikx theory, which correlates sol fraction and decrease in crosslink density, this is the result of a high degree of crosslink scission. Higher process temperatures result in a lower decrease in crosslink density due to recombination of active chain fragments. NR and CIIR show different behaviour. Breakdown of NR in this temperature range results in an almost complete destruction of the polymer network; crosslink density is reduced to almost zero and the sol fraction is close to 100%. The same result is found for CIIR at higher temperatures. The different rubbers within this study react all according to their own specific de-vulcanization mechanisms. The best de-vulcanization conditions for whole passenger car tire material are determined as a compromise of the individual parameters.