Interplay between wax and polyphosphoric acid and its effect on bitumen thermomechanical properties

Most bitumen contains some level of wax, and the widespread use of wax-based additives in asphalt industry further increases bitumen's wax content. Wax-based additives reduce viscosity at high temperatures, facilitating the mixing and compaction of asphalt mixtures. However, the added wax leads to increased stiffness and physical hardening at low temperatures. In addition, wax may have an interplay with other bitumen additives and modifiers such as polyphosphoric acid (PPA), which is typically used by refineries and asphalt blending terminals. PPA has been shown to enhance high-temperature properties without negatively impacting the low-temperature performance of asphalt pavement. However, due to the interplay between wax and PPA, it is hypothesized that the presence of wax may moderate PPA-bitumen interactions. This paper uses laboratory experiments and computational modeling to investigate mechanisms of interaction between PPA and bitumen in the absence and presence of wax. The results of a molecular dynamics simulation showed that PPA has a strong interaction with fused aromatics such as those classified as resins in bitumen, such interactions lead to reduced solubility of resins in heptane, causing them to precipitate out in heptane and fall into the asphaltene category. The presence of wax appears to reduce the efficacy of PPA on increasing bitumen's elasticity, as evidenced by lower complex modulus and higher creep compliance compared to those of no-wax bitumen doped with PPA. This was further observed in our computational analysis, which showed a good interaction between resin molecules and PPA molecules. However, the interaction between resins and PPA was reduced when wax was added, causing resins to become more scattered, as evidenced by a significant reduction in the radial distribution function of resin molecules. The study results help inform formulators and manufacturers about interplay between bitumen additives such as wax and PPA which may lead to unwanted consequences on pavement performance. (C) 2020 Elsevier Ltd. All rights reserved.