Role of methylene diphenyl diisocyanate (MDI) additives on SBS-modified asphalt with improved thermal stability and mechanical performance

Poly(styrene-butadiene-styrene), or SBS, is commonly added to asphalt mixtures to improve their thermal stability and mechanical performance under working conditions in pavements and roads. However, the resulting SBS-modified blend suffers from separation between the SBS polymer and asphalt molecular components, thereby decreasing the expected mechanical performance of the material in the long term. Here, we present how adding methylene diphenyl diisocyanate (MDI)-based additives may improve the phase stability of SBS-modified asphalt as measured by AFM and separation testing. We then discuss the fundamental mechanisms that involve SBS, MDI, and asphalt molecules to achieve such an improvement. To this end, we utilize molecular modeling methods of semiempirical tight binding, density functional theory, and reaction rate calculations to simulate and characterize the intermolecular interactions of SBS and MDI with asphaltene molecules, a key component of asphalt bitumen. We find that, while noncovalent π-π stacking does not significantly explain the macroscopic properties of asphalt blends, reactions between asphaltene and MDI likely occur. As such, we propose that MDI acts as a compatibilizing agent between asphaltenes and SBS, which enhances the phase stability of MDI-SBS-modified asphalt. We also demonstrate that MDI additives with asphalt have lower chemical softness and polarizability, indicating a lower tendency toward degradation by oxidative agents.