Investigating on polymerization process and interaction mechanism of thermosetting polyurethane modified asphalt

Abstract

Thermosetting polyurethane (PU) can significantly improve the high-temperature stability, mechanical strength, and low-temperature flexibility of asphalt. To broaden its application, a systematic understanding of how the PU interacts with the asphalt is needed. Currently, limited effort has been focused on the polymerization and modification mechanism of thermosetting polyurethane modified asphalt (TPUA). The interaction mechanism of thermosetting PU with asphalt was revealed from three aspects: molecular structure, microstructure, and surface free energy (SFE). First, the polymerization process of thermosetting PU in asphalt was analyzed based on curing reaction kinetics, that is, reaction order and activation energy. Then, the physical and chemical interactions between modifier and asphalt were investigated quantitatively and qualitatively by infrared spectroscopy and nuclear magnetic resonance spectrometer (1H NMR). Besides, the two-phase structure of PU and asphalt was observed by fluorescence microscope (FM) and scanning electron microscope (SEM). Finally, the effect of thermosetting PU on the SFE of asphalt was revealed by the sessile drop method. The results showed that asphalt did not affect the secondary reaction characteristics of the PU modifier. The relationship between the curing time and the reaction degree and temperature was identified, which well predicted the curing time required to finish the reaction under different temperatures. Moreover, the isocyanate component in PU reacted with the active hydrogen in asphalt to form carbamate (–NHCOO–), and the exact degree of chemical modification was asphalt-source dependent. With the increase of modifier content, the continuous phase in TPUA gradually changed from asphalt to PU, and the phase transition point was near 30 wt%. When the critical content was exceeded, asphalt was dispersed in the PU skeleton as microspheres. In addition, adding PU increased asphalt polarity, and the SFE of TPUA increased gradually as the modifier content increased.