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.