Physico-chemical analysis of five hard bitumens: Identification of chemical species and molecular organization before and after artificial aging

Abstract

The present paper focuses on a physico-chemical analysis of five different types of bitumens, both before and after aging. These bitumen samples were chosen based on: the original crude oil (straight-run bitumens with different asphaltene and crystallized fraction contents), mode of refining (straight-run vs. half-blown bitumen), and modifier characteristics (straight-run vs. polyphosphoric acid (PPA)-modified bitumen). The aim of this study is to determine both the aging effect on chemical species and the chemical organization as a function of the type of bitumen sample. In order to obtain information on bitumen chemistry, n-heptane precipitation (Standard NF EN 12591), IATROSCAN chromatography (coupling between a thin-layer liquid chromatography on a silica gel and a flame ionization detector), FTIR spectroscopy and differential scanning calorimetry (DSC) have all been used. Size exclusion chromatography under “high-speed” conditions (HS-SEC), which yields information relative to asphaltene associations, was also introduced. Several years of road aging were simulated through 25 h of a pressure aging vessel (PAV) test. The coupling of IATROSCAN chromatography and n-heptane precipitation made it possible to identify and quantify polar resins. In accordance with previous studies, the use of HS-SEC combined with IATROSCAN chromatography indicates that a modification by PPA leads to an increase in asphaltene content and a more dispersed asphaltene structure than that found in pure bitumen. This same conclusion can be drawn from observations of the half-blown bitumen sample. The half-blown bitumen actually contains less asphaltene than one of the straight-run bitumens in the study; furthermore, its asphaltenes are more highly agglomerated. These results demonstrate that asphaltene association does not systematically depend on quantity alone, as its chemical type also enters into play. Moreover, during aging, even though asphaltene content is increasing for all bitumen samples, its agglomeration is still highly dependent on the type of bitumen. It would therefore appear that the presence of crystallized fractions exerts a major influence on this process.