Thermo-oxidative degradation behavior of natural rubber vulcanized by different curing systems

In this work, the effects of three types of curing systems: conventional vulcanization (CV), semi-efficient vulcanization (SEV) and efficient vulcanization (EV) on the degradation efficiency of sulfur cured natural rubber (NR) were investigated. NR vulcanizates were prepared and degraded at 210 °C in air for different periods of time. The sol fraction, molecular weight, element contents and degradation kinetics were studied in detail. The degradation efficiency of the NR vulcanizates was decreased in the order of CV, SEV and EV cured NR, which was caused by the lower crosslink density, apparent activation energy and bond dissociation energy of polysulfide bonds formed in the NR vulcanizates with CV, as compared to others. The results of Horikx theory and contents of carbonyl and sulfone group in degraded rubber indicated both sulfide scission and main chain scission occurred in the NR vulcanizates prepared with CV and SEV. Both the type of sulfidic crosslinks and crosslink density affect the degradative efficiency and products. The type of sulfidic crosslinks was the main factor affecting the degradation degree of NR vulcanizates, while the crosslink density mainly influenced the sol molecular weight of the degraded products. Moreover, the degradation mechanism of the three types of NR vulcanizates was proposed. This work demonstrated that the thermo-oxidative degradation efficiency of vulcanized rubber is affected by curing systems, which may provide guidance for upcycling waste rubber effectively.