The Interaction of Polyphosphoric Acid with Bituminous Binders

Abstract

The purpose of this study is to evaluate how different binders react with polyphosphoric acid (PPA), and to contribute to a better understanding of this interaction. A number of bituminous binders were characterized before and after adding PPA, using rheological and chemical test methods, including FT-IR, uv-vis spectroscopy, NMR and gel permeation chromatography. The interaction with PPA is quantified as the change in softening point after adding a specific percentage of PPA. Most bitumen show an increase in R&B. Also in rheology, the stiffness increases, while the phase angle decreases when adding PPA. These observations are well-known and have been reported frequently by other authors. However, for one of the binders, small amounts of PPA lead to a decrease in R&B and to an increase in penetration. Rheological tests on this binder confirmed that the complex modulus decreases after adding PPA. These effects could be attributed to a precipitation, induced by PPA. This is very clear when investigating a drop of the bitumen-PPA blend in an optical microscope, and is also confirmed by storage stability tests. In conclusion, the findings suggest that PPA interacts with conjugated aromatic compounds, and this can in some cases lead to a precipitation.