Polyphosphoric acid (PPA) is used as a modifier in asphalt binder to improve performance properties and is also the topic of much discussion among asphalt technologists.
In 2012, a workshop sponsored by The Federal Highway Administration, the Transportation Research Board and the Minnesota Department of Transportation spawned a technical brief. That brief provides an overview of PPA use and compiles facts from many sources about performance.
Here is a sampling of some of the information:
One of the first patents for binder modification was in 1973.
Increase in binder stiffness from the addition of PPA was found to be crude source dependent.
There is no PPA applications rate that will work for all asphalts.
SBS (styrene-butadiene-styrene) + PPA modified asphalt binders can provide fatigue and durability resistance similar to asphalt binders solely modified with SBS.
A binder with one percent or less PPA does not indicate any increased absorption of water or loss of strength in testing.
Hydrated lime treatment of the aggregate was found to work very well as an anti-stripping agent.
PPA works as a stiffener and reactant when used with polymers such as SBS and ethylene terpolymers.
Limestone aggregate could not reverse or reduce the stiffening effect of PPA on the binder.
When over 27 test sections were placed on the NCAT (National Center for Asphalt Technology) test track with SBS + PPA combinations, all performed well with little rutting or cracking and no indication of moisture damage.
Several states that have used PPA extensively indicated that there have been no instances of negative performance that can be attributed to PPA.