(477h) Molecular Dynamics Simulations of Engine Oil Additives in Asphalt

Asphalt materials that are used in roadways are designed to meet engineering specifications that address factors such as creep at hot temperatures and stiffness at cold temperatures; extremes of local climate conditions serve to define "hot" and "cold". Some of these temperature-dependent specifications are quantified by rheological measures that relate to complex or dynamic modulus. Asphalt suppliers seek low-cost additives that alter the complex modulus at one or both extreme conditions. A problematic example from recent decades is "recycled engine oil bottoms" (REOB), a byproduct of unvaporized residue that is generated by distilling used motor oil to extract lubricants for fresh uses. REOB includes metals from engine wear debris and nonvolatile engine oil additives that survive long-time, high mileage use in a vehicle. Problems arose when REOB appeared to be a promising additive on a laboratory scale yet was considered (by some) to cause pavement deterioration and premature cracking when it was used in road construction. As part of a project toward understanding the mechanisms of REOB in asphalt, all-atom molecular dynamics (MD) simulations have been conducted of oligomeric poly(isobutylene) (PIB, 20 repeat units) and of calcium phenate as examples of engine oil additives that may be introduced into asphalt through this use of REOB. The model asphalt is a 12-component system that was previously shown to exhibit mechanical relaxations in MD simulations that resemble those of real asphalt. Here, 10 mass% systems of viscosity index improver (PIB) or over-based detergent (Ca phenate) in asphalt have been simulated. Computation of complex modulus by sampling stress fluctuations at equilibrium provides estimates of frequency-dependent storage modulus and loss modulus (G', G'') across multiple temperatures. Comparisons with complex modulus results from MD for the unmodified asphalt indicate the effects of these idealized REOB compounds at higher temperatures, i.e. the extents that storage and loss modulus change in the presence of REOB. Time-temperature superposition enables bringing these comparisons to some frequency and temperature ranges of practical interest. The rates of rotational relaxations for individual molecules within the simulations illustrate REOB effects over local molecular length and time scales.