Rheological Behavior of Bitumen Modified by Synthesized Polyurethane Based on MDI–PPG Reactive Prepolymers

A polyurethane prepolymer (PU) based on polypropylene glycol (PPG) and diphenylmethane diisocyanate (MDI) as the soft and hard segments and 3,3'-dichloro-4,4'-diaminodiphenylmethane (MOCA) as a chain extender was synthesized and used to modify the viscoelastic behavior of bitumen. The interaction between the PU-prepolymer and unreacted MDI and, the polar groups present in the base bitumen provide a route to enhance the miscibility between the blend components. The rheological behavior of the bitumen, with varying PU content, was investigated from strain sweep and time sweep measurements, with particular attention given to the analysis of Lissajous-Bowditch (LB) curves and the normalized third relative intensities (I3/I1). Three different methods based on strain sweep tests were employed to determine the critical strain at which the viscoelastic behavior transitions from the linear to nonlinear regime. The distinct rheological behavior observed for bitumen with different PU content was supported from observations of the blend morphology and confirmation of interactions between blend components from Fourier transform infrared spectroscopy (FTIR). This study reveals that the rheological behavior of the PU modified bitumen is closely related to the combined effects of interfacial interaction, phase morphology and phase distribution of the blend components.