Microscopic Mechanism Underlying SCA’s Influence on Water Stability of Polyurethane

Abstract Polyurethane mixtures (PUMs) are promising replacements for asphalt mixtures owing to their low construction energy consumption, minimal emissions, and excellent road performance; however, their applications are limited by poor water stability. Most studies have focused on improving the hydrolysis resistance of the polyurethane bulk material, and studies of enhancing the polyurethane–aggregate interfacial adhesion to improve the water stability of PUMs remain limited. In this study, silane coupling agents (SCAs) were employed to enhance the interfacial adhesion between polyurethane and aggregates under moisture exposure, thus establishing correlations between the macroscopic properties and microscopic structure and elucidating the mechanisms of interfacial reinforcement by SCAs. The macroscopic effects of SCA type and dosage (0%–4%) on polyurethane hydrophobicity and interfacial performance were quantified by contact-angle measurements and surface energy analysis. The evolution of the polyurethane molecular structure, functional groups, micromorphology after SCA incorporation, and the adhesion enhancement mechanisms at the polyurethane–aggregate interface under moist conditions were examined using Fourier-transform infrared spectroscopy, X-ray diffraction analysis, and atomic force microscopy. The polyurethane contact angle increased significantly with increasing SCA dosage. The adhesion work initially increased and then decreased, and the stripping work exhibited the opposite trend, indicating an optimal SCA dosage of 2%. SCAs reacted with polyurethane to form extensive cross-linked Si─O─Si structures with low surface energy. The resulting increase in RMS roughness increased the contact angle of the modified polyurethane. The Si─O─Si networks improved molecular-chain rigidity and overall stability, which limited soft-segment diffusion and chain dissociation, thereby reducing the hydrolysis rate. These cross-linked Si─O─Si structures exhibited stronger bonding with Si─O groups on aggregate surfaces, increasing the adhesion work at the polyurethane–aggregate interface. Excessive SCA dosage induced excessive cross-linking, which disrupted the soft–hard segment balance and promoted microphase separation.