Advanced Quantum Chemical Simulations to Achieve Sustainable Self-Repair of Oxidized SBS Polymers Using Reactive Repair Materials

The oxidative degradation of styrene-butadiene-styrene (SBS) polymers is the main reason for the substantial difficulty of the sustainable recycling of waste SBS asphalt pavements. Reactive repair materials containing reactive functional groups have the potential to reconnect and repair the broken chains of oxidized SBS polymers; however, conventional experimental methods struggle to reveal their repair mechanism. This paper addresses this issue by investigating the self-repair mechanism of isocyanate-based reactive repair materials for oxidized SBS polymers using quantum chemical simulations. Based on density functional theory, the molecular reactivity and local reactive sites of different isocyanates and oxidized SBS polymers at various oxidation levels were determined. According to transition state theory, the reaction energy pathways and reaction rates between isocyanates and oxidized SBS polymers were calculated, and the effects of temperature and pressure were considered. The results show that the isocyanate groups (─ N═C═O) in isocyanates and the hydroxyl (─ OH) and carboxyl groups (─ COOH) in oxidized SBS polymers are the potential reactive sites. During the reaction, the oxygen atom from the ─OH/─COOH groups in oxidized SBS donates electrons to form a new bond with the carbon atom in ─ N═C═O groups, thus reconnecting and repairing the oxidized SBS polymer. Methylene diphenyl diisocyanate (MDI) exhibits higher repair efficiency for oxidized SBS polymers than do hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI), and the increase in temperature and pressure can enhance the reaction rate between MDI and oxidized SBS polymers. Regarding the practical experimental condition (≥150°C, 1 atm), MDI can repair both lightly and heavily oxidized SBS polymers, whereas HDI and TDI can only repair lightly oxidized SBS polymers. The results of this study provide the theoretical foundation for the development of environmentally friendly recycling technologies for asphalt pavements.