Molecular dynamics simulation on vegetable oil modified model asphalt

Abstract

Vegetable oil is a common waste in the food and hospitality industries. It has been proposed as a modifier for asphalt to improve its performance on road pavement. In order to understand the functional mechanism of vegetable oil modifying the properties of model asphalt mixture, investigating the chemical composition, microstructure and property relationship of vegetable oil modified asphalt mixture in a wide temperature range is important. To understand the mechanism with molecular details, molecular dynamics (MD) simulations have been conducted on major fatty acid components in waste vegetable oil (WVO) (including linoleic acid, oleic acid, palmitic acid) and their mixture with model asphalt at different temperatures. One model WVO ternary mixture was built based on the literature data of the chemical composition of three kinds of fatty acids. Major properties such as density, diffusion coefficient, radial distribution function, thermal conductivity, molecular correlation functions, and viscosity were calculated. It was found that oleic acid, linoleic acid and palmitic acid have similar densities, diffusion activation energies, diffusion coefficient prefactors, and thermal conductivities from the room temperature to the hot-mix asphalt temperature. The bulk viscosities of fatty acids predicted using Green-Kubo method are similar as well, and are close to available literature data. All three kinds of fatty acids and their mixture can mix well with components in model asphalt mixture at different temperatures, and can help components in asphalt to mix more homogeneously. The zero-shear-rate viscosities of fatty acids modified asphalts at 443 K, 400 K, and 358 K were calculated running non-equilibrium MD simulations, while the viscosity at the room temperature was predicted using the Debye-Stoke-Einstein equation based on the correlation function and relaxation time calculation. Based on result in this project, implementing vegetable oil improves the density of model asphalt mixture and the mixing of components in asphalt. It does not change the viscosity of asphalt significantly at high temperature but decreases the viscosity at low temperature. The thermal conductivities of pure fatty acids, model vegetable oil ternary mixture, and model vegetable oil modified asphalt mixtures are very similar and show negligible dependence on temperature. The findings in this project suggest the functional mechanism of vegetable oil on asphalt that implementing vegetable oil can improve the performance of asphalt by facilitating the mixing of components in asphalt mixture and reorientation relaxation of asphalt at low temperatures but not at high temperatures, thus reducing the viscosity of mixture at low temperatures while having negligible effect at high temperatures.