To elucidate the modification mechanism of waterborne polyurethane (WPU) on emulsified asphalt, anionic and cationic WPUs are utilized as modifiers. As well, their effects on physical properties, microstructure, and compatibility are characterized using basic performance tests, Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). The results show that WPU-modified emulsified asphalt exhibited a higher softening point, reduced penetration, and decreased ductility, suggesting enhanced high-temperature stability but diminished low-temperature flexibility. Among all samples, the combination of cationic WPU with cationic emulsified asphalt shows the highest softening point (54.1 °C), whereas cationic emulsified asphalt alone exhibits the lowest one (52.9 °C). Anionic emulsified asphalt demonstrates the highest penetration (79 mm), while non-ionic WPU combined with cationic emulsified asphalt shows the lowest one (59.3 mm). The ductility decreases from 90.3 cm to 28.7 cm. The storage stability varies with WPU ion type. Cationic WPU-modified samples showed the poorest storage stability (0.7% residue), while anionic-modified samples exhibit the best one (0.4% residue). FTIR analysis confirms the presence of characteristic WPU absorption peaks, indicating that physical blending occurs, and chemical interaction is limited. AFM observations reveal that anionic WPUs provide superior compatibility, forming fine, uniformly distributed particles with the lowest surface roughness (5.655 nm). In contrast, cationic WPUs form chain-like structures that cure effectively but exhibit poor dispersion. This study provides a basis for the development of high-performance WPU-modified emulsified asphalt.

To elucidate the modification mechanism of waterborne polyurethane (WPU) on emulsified asphalt, anionic and cationic WPUs are employed as modifiers, and their effects on physical properties, microstructure, and compatibility are characterized through basic performance tests, Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). The results showed that WPU-modified emulsified asphalt exhibited a higher softening point and reduced penetration and decreased ductility, indicating to enhance high-temperature stability but decrease low-temperature flexibility. Among all samples, cationic WPU + cationic emulsified asphalt (CWpu+Ce) presented the highest softening point (54.1 °C), while cationic emulsified asphalt (Ce) showed the lowest one (52.9 °C). Anionic emulsified asphalt (Ae) exhibited the highest penetration (79 mm), whereas nonionic WPU + cationic emulsified asphalt (Wpu+Ce) showed the lowest one (59.3 mm). The ductility ranged from 90.3 cm (Ae) to 28.7 cm (Wpu+Ae). The storage stability varied with WPU ion type. Cationic WPU-modified samples showed the poorest storage stability (0.7% residue), while anionic-modified samples exhibited the best one (0.4% residue). FTIR analysis confirmed the presence of characteristic WPU absorption peaks, indicating that physical blending occurred, and chemical interaction was limited. AFM observations revealed that anionic WPUs provided superior compatibility, forming fine, uniformly distributed particles with the lowest surface roughness (5.655 nm). In contrast, cationic WPUs formed chain-like structures that cured effectively but exhibited poor dispersion. This study provides a basis for the development of high-performance WPU-modified emulsified asphalt.