Development on the Rheological Properties and Micromorphology of Active Reagent-Rejuvenated SBS-Modified Asphalt

This study aimed to improve the recycling efficiency of waste styrene–butadiene–styrene (SBS)-modified asphalt pavements and achieve sustainable road resource development. The SBS-modified asphalt aged in a pressure aging vessel (SBSPAV) was rejuvenated utilizing active reagents [i.e., methylene-bis(4-cyclohexylisocyanate) (HMDI) and 1,6-hexanediol diglycidyl ether (HDE)] and epoxy soybean oil (ESO). The HMDI type (HMDI combined with ESO) and HDE type (HDE combined with ESO) were used as rejuvenators. The rheological behavior of rejuvenated asphalt binders at medium and high temperatures was evaluated utilizing multiple stress creep recovery tests, linear amplitude sweep tests, and rotational viscosity tests. The high-temperature rutting resistance, medium-temperature fatigue resistance, and fluidity of the rejuvenated asphalt were evaluated. The micromorphology, roughness, and adhesion of the rejuvenated asphalt were evaluated by atomic force microscopy. The thermal stability of the rejuvenated asphalt was analyzed by thermogravimetric analysis. The results reveal that the HMDI-rejuvenated asphalt combines with the active hydrogen in SBSPAV to generate a stable chemical structure that benefits the rutting resistance and traffic load grade of the rejuvenated asphalt. Due to the increase in stiffness, the fatigue resistance of the HMDI-rejuvenated asphalt could be damaged to some extent. The residual epoxy functional groups in the HDE-rejuvenated asphalt are dispersed in the asphalt in an independent phase state, which reduces the elastic recovery ability. The stiffness of the rejuvenated asphalt is lowered because it has a more flexible structure. The anti-fatigue effectiveness of the HDE-rejuvenated asphalt is improved. HMDI can boost the intermolecular force of the rejuvenated asphalt while decreasing its temperature sensitivity. The viscosity of the rejuvenated asphalt gradually decreases as the HDE content increases. This occurs because HDE combines with SBSPAV to generate flexible segments, thereby increasing the flexibility of the asphalt colloids. HMDI and SBSPAV undergo a chemical reaction, which partially repairs the structure of SBS and inhibits the free diffusion of the asphalt molecules. The number of the HMDI-rejuvenated asphalt bee-like structures increases, while the volume drops and adhesion increases. The number and volume of the bee-like structures decreased and adhesion increased as the HDE content increased. This occurs because HDE contains a high concentration of light components, which minimizes the amount of asphaltene in SBSPAV. Thermogravimetric analysis tests show that the HDE-rejuvenated asphalt and HMDI-rejuvenated asphalt have good thermal stability.