A Reactive Wanol-based polyurethane for asphalt modification: Mechanism, performance, and synergy with SBS

Reactive polyurethane (PU)-based modifiers have attracted increasing attention in asphalt modification because they can form chemically bonded networks within the binder, improving mechanical performance and reducing phase separation compared with conventional polymers. However, existing PU-modified systems still face challenges related to reaction control, storage stability, and the limited understanding of how synthesis parameters affect binder-scale performance. Meanwhile, SBS, although widely used, interacts mainly through physical blending, which may limit its effectiveness under severe thermal, loading conditions. Therefore, hybrid systems combining chemical and physical modification mechanisms are needed. In this study, a reactive PU modifier was synthesized using Wanol R2305 polyol and methylene diphenyl diisocyanate (MDI), and its performance was evaluated both individually and in combination with SBS. The novelty of the study is the systematic linkage between the NCO/OH ratio, polyurethane network formation, and asphalt binder performance, together with the development of a PU–SBS hybrid modification strategy. FTIR analysis confirmed urethane linkage formation and the development of a crosslinked polymer structure. Modified binders were evaluated through conventional tests, rotational viscosity, DSR, MSCR, BBR, and storage stability analysis. The results showed that PU modification improved stiffness, elasticity, temperature stability, rutting resistance. More importantly, the PU–SBS hybrid system exhibited a pronounced synergistic effect, providing superior elastic recovery, viscoelastic behavior, and high-temperature deformation resistance compared with single-modified binders. The hybrid binder also maintained acceptable workability limits. The hybrid binder containing 2% SBS and 6% PU exhibited the best overall performance, showing G*/sinδ values up to 4.1 times higher than the 4SBS binder and reducing the storage stability difference from 18.2 °C to 4.6 °C. Moreover, the hybrid system maintained acceptable workability despite the significant increase in stiffness and elastic response. Overall, the proposed PU system offers a controlled, mechanism-based modification approach, while its combination with SBS provides an effective pathway for producing high-performance asphalt binders.