Asphalt is composed of hundreds of thousands of molecules with various properties. By identifying and separating discrete asphalt components (also known as fractionation), WRI is able to study their relationship to asphalt performance.
WRI routinely uses ASTM D 4124 (the Corbett method) to determine the components of asphalt. This method separates asphalt into fractions defined as saturates, naphthene aromatics, polar aromatics, and asphaltenes.
The combined saturates, naphthene aromatics, and polar aromatics make up the maltenes (petrolenes), while the asphaltenes are defined by the solvent used to precipitate them, usually n-heptane. The asphaltenes are a solubility class and exist in asphalt as the dispersed phase, while the maltenes are the continuous phase. The polar aromatics, also referred to as the resins, serve as the peptizing agent for the asphaltenes. The chemical composition and volume of asphaltenes contribute to the viscous nature of asphalt.
Characterization techniques WRI routinely uses in its highway materials research include adsorption chromatography, gel permeation (size exclusion) chromatography, and ion-exchange chromatography.
Gel permeation chromatography separates the components of asphalt by molecular volume. In this procedure, the large components elute from the column first, while the smaller components elute later. The asphaltenes make up the bulk of the material that elutes from the column first. WRI has demonstrated that the presence of polymer in polymer-modified asphalt can also be identified using this technique.
A more detailed investigation of the chemical nature of asphalt can be made by separating asphalt using the ion-exchange chromatographic (IEC) procedure. This procedure separates asphalt into fractions defined as neutrals, strong and weak acids, and strong and weak bases. The procedure can also be modified to isolate a fraction defined as amphoterics. WRI is currently evaluating the effects of these various IEC fractions on the moisture susceptibility of asphalt.