Polyphosphoric acid modified asphalt: Proposed mechanisms

Asphalt binders have been chemically modified with polyphosphoric acid (PPA) to improve high temperature rheological properties without adversely affecting low temperature rheological properties since the early 1970's. More recently, PPA has been used in Superpave performance-grade (PG) binders that need an extended range between the high and low temperature performance requirements. The mechanism of chemical modification of asphalt with PPA remains in great part unknown. This paper presents results that will help to better understand the mechanisms of chemical modification with PPA. PPA modified and unmodified asphalts from different crude sources were analyzed for chemical composition by asphaltene precipitation, thin-layer chromatography (TLC) and nuclear magnetic resonance (NMR), by gel-permeation chromatography (GPC) and atornic force microscopy (AFM). The results indicate that the mechanism of PPA action depends on the base asphalt. In one case PPA affected a phase dispersed in asphalt, in the other case it affected the asphalt matrix. In both cases, PPA caused stiffening of the modified phase. Several stiffening mechanisms are proposed.