Comparative Investigation of the Devulcanization Parameters of Tire Rubbers

Rubber is a very durable material, in particular tire rubber, and this poses a major challenge for recycling. For end-of-life tires, incineration is currently the most important outlet, impeding the re-use of this valuable raw material in new rubber products. A considerable share of material recycling can only be achieved if tire material can be used in real recycling loops: tires back into tires.

Passenger car tire material is a blend of different polymers, with the main component being SBR. These polymers all show their own particular breakdown characteristics. The balance of crosslink to polymer scission is crucial for the quality of the recycled material, and it can be shifted by using a devulcanization aid, which chemically enhances crosslink scission or suppresses main chain scission. Additionally, the optimal process conditions for a high ratio of devulcanization to polymer degradation have to be adjusted not only for SBR, but also for BR, NR and IIR.

Within this study, the above mentioned polymers were investigated concerning their tendency for crosslink versus main chain scission, with the lead polymer being SBR. 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 behavior. 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 mechanism behind the different breakdown processes will be discussed, and the best devulcanization conditions for whole passenger car tire material will be elaborated in this presentation.