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

Abstract

Bitumen without modification is prone to suffer from damage under extreme temperature and traffic conditions, leading to distress such as rutting, fatigue cracking, and thermal cracking. Polyurethane-modified bitumen can effectively be prepared at lower temperatures and combine low-carbon and environmental-protection concepts. The resulted polyurethane can also improve the in-service performance of bitumen, such as rutting resistance. In this study, 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 on the base bitumen provides a route to enhancing 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 (FTIR) spectroscopy. This study reveals that the rheological behavior of a PU-modified bitumen is closely related to the combined effects of interfacial interaction, phase morphology, and phase distribution of the blend components. Illustrating this relationship can enable the design of tailored bituminous materials with excellent in-service performance.