Moderating Effects of Paraffin Wax on Interactions between Polyphosphoric Acid and Bitumen Constituents

Abstract

Despite the merits of using polyphosphoric acid (PPA) in modification of bitumen, its desirable stiffening effect on bitumen is totally composition-dependent. While asphaltenes and resins are correctly introduced as the immediate components affected by PPA, our laboratory rheological investigations show that bitumen's wax content could be a potential interference for PPA's modification function. As evidenced by the complex viscosity test, PPA is very effective in low-wax bitumen, but its effect on high-wax bitumen is quite negligible. In a high-wax medium, the trend of interaction energy improvement with the number of wax chains suggests that the crystalline networks of wax can interfere in interactions of PPA with either asphaltene or resin through concealing interaction sites of asphaltene and resin. This in turn leads to the reduction of their accessible active sites for interacting with PPA. To study the working mechanism of PPA on bitumen constituents, the density functional theory (DFT) approach and energy decomposition analysis (EDA) were performed on the pairs of PPA−asphaltene, PPA−resin, and PPA−wax molecules. In the case of the PPA−asphaltene interaction, EDA shows clear contributions of electrostatic, orbital, and dispersion forces. In the resin fraction, the strong interactions between basic sites of the resin molecules and PPA lead to the formation of ion-pair compounds stabilized through oppositely charged ions held together via Coulombic attraction. The formation of these new entities, which are often nonsoluble in heptane, can further explain the decrease of resin content after PPA modification. The reduction of resin is followed by an increase in the content of asphaltene with a smaller molecular weight in n-heptane precipitates, which is in line with our experiments and that of other researchers.

Understanding How Polyphosphoric Acid Changes Bitumen’s Response to Water Exposure

“…Therefore, nondirectional longrange electrostatic forces are the driving forces for the association of PPA-resin into pairs. Based on our energy decomposition analysis (EDA) results, 10 in addition to a significant electrostatic interaction (70.5%), an ion-pair of PPA-quinoline takes advantage of the orbital mixing interactions (22.4%) arising from charge donation−acceptation interactions between two fragments.…”

“…Another reason could be possible antagonism between PPA’s function and that of other bitumen modifiers. Considering the acidic nature of PPA, there has always been a concern about PPA’s probable reaction with basic antistripping agents such as lime and amines, which are commonly used in moisture-control additives. , The possible adverse reactions are not limited to PPA’s interaction with other bitumen additives; our recent studies on the rheological properties of PPA-modified bitumen show that the wax content of the bitumen could be a potential interference for PPA’s modification function. , Other laboratory evaluations conducted on warm-mix bitumen-containing PPA have also indicated that the low-temperature properties of bitumen are impaired. , Also contributing to the uncertainty in PPA application are some unknowns or perceived consequences associated with PPA in the long term; the most important of these is the hydrophilic nature of PPA, which causes moisture susceptibility of PPA-modified bitumen.…”

Future Directions for Applications of Bio-Oils in the Asphalt Industry: A Step to Sequester Carbon in Roadway Infrastructure

“…Bitumen is a colloidal mixture that consists of large molecules called asphaltenes in the discrete phase and saturates, aromatics, and resins in the continuous (liquid) phase of bitumen. 139 The resins-to-asphaltenes ratio in bitumen is a determinative factor in bitumen characteristics. 140 If the ratio of a mixture falls off toward the pure maltene, the mixture shows a solution behavior; if most of the mixture is asphaltenes, the mixture is in gelatinous form.…”

“…Aging affects the chemical composition of bitumen. Bitumen is a colloidal mixture that consists of large molecules called asphaltenes in the discrete phase and saturates, aromatics, and resins in the continuous (liquid) phase of bitumen . The resins-to-asphaltenes ratio in bitumen is a determinative factor in bitumen characteristics .…”

Resin–Paraffin Aggregation Processes in Waxy Oil Revealed by Vanadyl Porphyrin Spin Probe

“…24 In this case, the effect of modifiers of paraffin crystals depends on the oil composition, 25 and asphaltenes can interfere with the action of inhibitors. Apparently, the details of the interaction of paraffin with molecules of other types can be clarified only using the methods of molecular dynamics and quantum chemistry, 26,27 considering it at the molecular level, relying on the experimental data of adequate in situ methods.…”