Comparative Investigation of the De-vulcanization Parameters of Tire Rubbers

The optimal process conditions for a high ratio of devulcanization to polymer degradation have been investigated for tire rubbers: styrene–butadiene rubber (SBR), butadiene rubber (BR), natural rubber (NR), and chloro-isobutylene–isoprene rubber (CIIR). These polymers all show their own particular breakdown characteristics. The temperature dependence of the breakdown mechanism was investigated by measuring sol fractions and cross-link densities. For SBR and BR, the highest reduction in cross-link 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 a decrease in cross-link density, this is the result of a high degree of cross-link scission. Higher process temperatures result in a lower decrease in cross-link density due to recombination of active chain fragments. NR and CIIR show different behavior. Breakdown of NR in this temperature range results in an almost complete destruction of the polymer network; cross-link 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 devulcanization mechanisms, the best devulcanization conditions for whole passenger car tire material are optimized.