Use of Polyphosphoric Acid in Asphalt Binders, Applications for Asphalt Paving — Introduction
Asphalt binders are often modified by the use of additives to improve properties and meet performance requirements. Polyphosphoric Acid (PPA) has been used for this purpose in asphalt paving in North America for over thirty years. It is currently estimated that up to 400 million tons of asphalt mix modified with PPA have been utilized on US highways in the past five years. This represents up to 14% per year of the asphalt pavement placed in the United States over that period.
Polyphosphoric Acid is currently used in the air-blowing oxidation process, as an additive in reactive polymer applications, and as a direct binder modifier. PPA is classified as a chemical modifier because it reacts with some of the components of asphalt. The other broad category of asphalt modification is referred to as polymer modification and involves the use of specific elastomeric polymers. Both approaches are increasingly used in North America to meet the high performance requirements for binders in Superpave™ applications. In neat asphalt paving, PPA increases the high temperature Performance Grade (PG) rating of the asphalt binder without affecting the low temperature properties. Significant improvement in the water-sensitivity of mixes is also obtained. In polymer-modified asphalt, PPA provides these same benefits and also allows for a significant reduction in the level of polymer required to meet elastic recovery requirements.
The use of Polyphosphoric Acid in asphalt modification was first reported in 1973. Earlier work dealt with the use of phosphorus pentoxide (P2O5) as a catalyst in the air-blown oxidation of asphalt. Significant research on the use of PPA with polymers to modify asphalts, conducted since the late 1990s, is illustrated in several patents. Considerable interest continues in understanding how PPA interacts with binders and developing technology to optimize its performance. There are a large number of recent papers and presentations.
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