Thermo-oxidative degradation behavior of natural rubber induced by sulfur crosslinking structures

With the increasing volume of end-of-life tires, the efficient recycling and reclaimation of natural rubber (NR) has become an urgent challenge in the rubber industry. In this work, the thermo-oxidative degradation behavior of sulfur-vulcanized NR was systematically investigated, with an emphasis on the role of sulfur crosslinking bonds in inducing main-chain scission. A series of NR vulcanizates with varying sulfur crosslink densities and types were subjected to thermo-oxidative degradation treatment, and their structural evolution was analyzed using FTIR, XPS, ¹³CNMR , and DSC. The experimental results showed that higher sulfur crosslink density led to lower molecular weight, shortened oxidative induction time, and enhanced formation of oxygen-containing functional groups, indicating that sulfur bonds facilitate polymer degradation. Building on previous studies that report β-scission occurring in NR main chains, as well as structural analyses conducted before and after chain cleavage, a mechanistic hypothesis is proposed: sulfur crosslinks undergo oxidative cleavage, generating reactive sulfur-containing intermediates that likely lower the bond dissociation energy of adjacent C–C bonds and promote preferential main-chain scission near the crosslinking sites. These findings provide structural insight into the degradation behavior of sulfur-crosslinked NR and offer a theoretical basis for designing efficient strategies for controlled degradation and advanced recycling of crosslinked rubber materials.