Understanding H<sub>2</sub>O<sub>2</sub>-Induced Thermo-Oxidative Reclamation of Vulcanized Styrene Butadiene Rubber at Low Temperatures
Zhen Zhang, Jiayi Li, Chaoying Wan, Yuxin Zhang, Shifeng Wang
Abstract
Recombination reactions often occur in the process of chemical, thermal, or mechanical degradation of vulcanized styrene butadiene rubber (SBR), which hinders the recycling efficiency of waste tire rubbers. In this work, we developed an effective method to transform solid vulcanized SBR into reclaimed rubber with 100% sol fraction through a hydrogen peroxide (H2O2)-induced thermo-oxidative reclamation process at 100 °C with the assistance of soybean oil. The structural evolution of the vulcanized SBR and the role of H2O2 and soybean oil in the reclaiming process were investigated by sol–gel analysis, gel permeation chromatography (GPC), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR), and 13C nuclear magnetic resonance (13C NMR) spectroscopy. The results showed that vulcanized SBR underwent severe oxidative scission, the sol of the reclaimed samples increased from 20.4 wt % up to 100 wt % with a decreased molecular weight to 1.378 × 104 g/mol. Multiple functional groups such as carboxyl, hydroxyl, and ether groups were generated in the polymer chains as confirmed by 13C NMR and FTIR spectroscopy. Moreover, the reaction pathway of the oxidative reclamation was quantified by density functional theory (DFT) calculations, suggesting that hydroxyl radicals were generated from a homolytic cleavage reaction of H2O2 with SBR by hydrogen abstraction and accelerated the rate-determining step of the SBR oxidation. The apparent activation energy (Ea) of the initial stage of the oxidative reclamation decreased from 223 to 160 kJ/mol according to the model-free kinetics results.