Investigating the modifying mechanism of polyphosphoric acid on asphalt: From the colloidal component perspective

Abstract

This study separated four colloidal components of polyphosphoric acid (PPA) modified asphalts, which were made with three base asphalts from different oil sources and two grades of PPA. Frequency sweep tests were conducted on 27 of these oily components to obtain viscoelastic parameters according to the Williams-Landel-Ferry (WLF), Arrhenius, and standard logistic models. Infrared spectroscopy was used to gather molecular structural information of all components, and a semi-quantitative analysis method was applied to assess the molecular structural changes of each component. The results show that the rheological properties of the same components from different base asphalts vary to some extent, indicating differences in molecular structure. This leads to differential effects of the two grades of PPA on each oily component. Specifically, PPA significantly increases the complex modulus (G*) of the resins. Moreover, PPA with longer molecular chains shows a greater increase in the G* for the resins with lower aromaticity. Meanwhile, the G* of the aromatics and saturates exhibits distinct trends after PPA modification. First, this result indicates that PPA can indeed react with each asphalt oily component to some extent, causing changes in the rheological properties of the lighter components. Second, the significance of this reaction is determined by the aromaticity of the asphalt molecular structure and the polarity of the PPA molecules. The infrared spectroscopy results show that PPA significantly increases the polarizability of asphaltenes and resins. Furthermore, the functional groups of PPA mainly precipitate in the asphaltenes. This indicates that there is a substance transfer occurring within the asphalt during PPA modification. The study results point out that after PPA modification, the mechanical properties of the lighter components (serve as the dispersed phase) will change to varying degrees, which is also one of the important factors leading to the changes in asphalt performance. The oil-source-induced variability of PPA-modified asphalt is affected by the molecular weight and aromaticity of the asphalt molecules.