The optimal process conditions for a high ratio of de-vulcanization to polymer degradation have been investigated for tire rubbers: SBR, BR, NR and CIIR. These polymers all show their own particular breakdown characteristics. 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 2200C, 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. Although different rubbers react Via other de-vulcanization mechanisms, the best de-vulcanization conditions for whole passenger car tire material are optimized.