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

Polyphosphoric acid (PPA) has commonly been added to bitumen to increase bitumen's stiffness and elasticity. However, the effect of PPA on bitumen properties is highly affected by the environment as well as interaction between PPA and other bitumen modifiers. Among the external factors affecting PPA's efficacy are environmental factors such as moisture and ultraviolet exposure. This paper presents a multiscale approach to examine the effect of moisture conditioning on PPA-modified bitumen. To understand how water molecules moderate the role of PPA in bitumen, we used computational modeling with density functional theory (DFT), along with laboratory experiments. Based on the DFT results, shortening the chain length of PPA, due to water exposure, reduces the networking effect of PPA oligomers in the bitumen matrix, negatively affecting the elasticity of the PPA-modified bitumen. Our DFT results show that the interaction energy between PPA and a quinoline-resin representative molecule of bitumen decreased from-70.8 to-33.6 kcal/mol; this was attributed to a reduction in PPA's chain length from seven to two repeating units. The latter was reflected in PPA-modified bitumen's elasticity (G/sinÎ) decreasing from 6.1 to 3.0 kPa after exposure to water for 100 h. Water conditioning also affected the elasticity of unmodified bitumen (without PPA), but the trend was in the opposite direction; after 100 h water exposure, unmodified bitumen's elasticity (G/sinÎ) increased from 2.0 to 2.7 kPa. This increase was attributed to the formation of molecular clusters in the bitumen matrix. This was further supported by our DFT results demonstrating that intermolecular attraction is greater for water-grafted asphaltene planes than for pristine asphaltene planes, as shown by the spacing of a water-grafted asphaltene dimer at 5.8 Å being significantly lower than that of a pristine asphaltene dimer at 7.12 Å. The study outcome provides insights into precautions to be taken with PPA-modified bituminous composites used in humid and high-precipitation regions.