Effect of Polyphosphoric Acid on the Coupled Aging Behavior of SBR-Modified Asphalt Under Intense UV Radiation and Large Temperature Differences

Asphalt pavements in high-altitude cold regions are subjected to the coupled effects of intense ultraviolet (UV) radiation and large temperature differences, which accelerate the oxidative aging and cracking of styrene-butadiene rubber-modified asphalt (SBR-MA), severely compromising their service durability. To improve aging resistance, polyphosphoric acid (PPA) was incorporated into SBR-MA to produce a PPA/SBR-modified binder (PPA/SBR-MA). This study systematically investigated the multiscale evolution of rheological properties, chemical structure, molecular weight distribution, nanoscale morphology, and nanomechanical properties of both SBR-MA and PPA/SBR-MA under coupled aging conditions of intense UV radiation and large temperature differences. The results indicated that macroscopic surface cracking in PPA/SBR-MA was significantly less severe than that in SBR-MA after aging. In terms of high-temperature rutting resistance and fatigue life, the PPA/SBR-MA exhibited a three-stage evolution pattern of “initial enhancement—subsequent deterioration—subsequent recovery,” while the SBR-MA showed only a two-stage pattern of “initial enhancement—subsequent deterioration.” Notably, PPA/SBR-MA consistently outperformed SBR-MA both before and throughout aging. Mechanistically, PPA reacts with polar asphaltene components to form phosphate ester linkages, thereby enhancing crosslinking between SBR and asphalt, suppressing polymer chain scission and stabilizing the colloidal structure. This retards internal asphaltene decomposition and small-molecule migration while suppressing asphaltene aggregation near the surface layer, thereby alleviating surface embrittlement. Compared with SBR-MA, the PPA/SBR-MA exhibits a 68% reduction in the surface-to-bulk modulus ratio, indicating a significantly mitigated gradient aging effect along the depth direction, which in turn suppresses macroscopic surface cracking. These findings reveal a phosphorylation-driven stabilization mechanism, offering a rational materials design paradigm for pavement applications in extreme environments.