Spectroscopic Analysis to Determine the Chemical Nature of Asphalt
Asphalt is composed of many chemical structures. WRI uses spectroscopy to identify subtle differences in the chemistry of asphalts and to relate how those variations influence performance properties in highway and industrial applications. Techniques WRI uses to study the chemical composition of asphalt include Fourier transform infrared (FTIR) spectroscopy, non-aqueous potentiometric titration (NAPT), nuclear magnetic resonance (NMR) spectroscopy, and magnetic resonance imaging (MRI) spectroscopy.
FTIR is used to identify and quantify the different functional groups present in asphalt. In the simplest characterization, the functional groups are classified as carbonyls and sulfoxides. However, a more elaborate infrared method is also available. This method relies on chemical reactions and differential spectra to isolate seven different functional groups: pyrroles, phenols, ketones, carboxylic acids, 2-quinolones, anhydrides and sulfoxides. Typically, the presence of ketones and sulfoxides increases with aging.
NAPT is not a spectroscopic technique but is used to quantify the acids and bases in asphalt. Typically, acids and bases are classified as strong, moderate, and weak. Recent work at WRI shows that with aging, the strong and weak acids and the weak bases increase in concentration. However, the data seem to indicate that the concentration of strong and moderate bases is not affected by aging.
NMR spectroscopy provides information about the chemical environment in which various nuclei exist. In general, the relative aliphatic and aromatic distributions of the hydrogen and carbon moieties are determined. In addition, other hydrogen and carbon types can be identified. For example, the olefinic hydrogens in styrene-butadiene co-polymers can be detected in polymer-modified asphalts. More recently, the different chemical environments in which phosphorous nuclei exist have been determined for polyphosphoric acid modified asphalts.
MRI spectroscopy is being used to measure the contact angle of water in asphalt. From the data, asphalt-in-water and asphalt-in-air surface tensions, spreading coefficients, and works of adhesion can be calculated. The surface tension data compare reasonably well with published data for the different asphalts that have been studied.