Bio-Based Polyurethane Asphalt Binder with Continuous Polymer-Phase Structure: Critical Role of Isocyanate Index in Governing Thermomechanical Performance and Phase Morphology.

Polyurethane asphalt (PUA) has attracted considerable attention in the field of pavement engineering. However, traditional PUA systems typically exhibit low concentrations of polyurethane (PU), leading to a continuous bitumen-dominated phase that adversely affects mechanical properties. Furthermore, the non-renewable nature of raw materials raises environmental concerns. To address these limitations, this study developed an eco-friendly and cost-efficient bio-based PUA binder (PUAB) featuring a continuous high-biomass PU matrix (over 70% biomass) and a high bitumen content (60 wt%). The effects of the isocyanate index (NCO/OH ratio) on the cure kinetics, rheological behavior (rotational viscosity over time), viscoelasticity, damping capacity, phase morphology, thermal stability, and mechanical performance were systematically investigated using Fourier-transform infrared spectroscopy, dynamic mechanical analysis, laser-scanning confocal microscopy, and tensile testing. Key findings revealed that while the rotational viscosity of PUABs increased with a higher isocyanate index, all formulations maintained a longer allowable construction time. Specifically, the time to reach 1 Pa·s for all PUABs at 120 °C exceeded 60 min. During curing, higher isocyanate indices reduced final conversions but enhanced the storage modulus and glass transition temperatures, indicating improved rigidity and thermal resistance. Phase structure analysis demonstrated that increasing NCO/OH ratios reduced bitumen domain size while improving dispersion uniformity. Notably, the PUAB with the NCO/OH ratio of 1.3 achieved a tensile strength of 1.27 MPa and an elongation at break of 238%, representing a 49% improvement in toughness compared to the counterpart with an NCO/OH ratio = 1.1. These results demonstrate the viability of bio-based PUAB as a sustainable pavement material, offering a promising solution for environmentally friendly infrastructure development.