Influence of Extraction Solvents on the Properties of Binders Recovered From Bituminous Mixtures Produced with Polyphosphoric Acid Containing Asphalt
By Gerald Reinke
The use of polyphosphoric acid (PPA) to modify asphalt, either as a sole a or as an adjunct to polymer modifiers such as SBS or ethylene terpolymer, has had a stormy history within the bituminous supply and hot mix paving industries. While the use of PPA as an asphalt modifier has been known for many years, its popularity increased significantly with the implementation of the PG grading system for asphalt binders. Relatively low levels of PPA, on the order of 0.2 percent when using polymer and on the order of 1 percent when used alone, could result in a one, or in some cases, a two PG grade increase in the high temperature grade of a given binder.
Some concerns were raised and data developed that demonstrated the possibility of neutralization of the improved properties resulting from the use of PPA. Additionally, forensic data from certain construction projects appeared to show that binders recovered from bituminous mixtures produced with binders containing PPA exhibited a decrease in binder stiffness relative to the PG grade shipped to the project.
Mathy Technology and Engineering Services, Inc. (MTE) was called upon to investigate the recovered binder properties of projects produced using PPA modified binder wherein it was felt by the user agency that the recovered binder failed to meet the proper grade.
After some investigation and several puzzling results, an understanding was finally arrived at of why one laboratory could extract a PPA-containing mix and obtain no binder stiffness decrease and another laboratory could extract binder from mix on the same project and observe a decrease in stiffness. Commercial suppliers of the two most commonly used halogenated extraction solvents, trichloroethylene and n-propyl bromide add an acid scavenger to stabilize their solvents. Without the acid scavenger these solvents will generate hydrochloric or hydrobromic acids which can have a corrosive effect on equipment (such as centrifugal extractors) and which can artificially stiffen the binder during the recovery process.
The acid scavenger typically used is 1,2 epoxy butane which irreversibly reacts with the acid protons, thus removing them from the solvent. Any acid that might be present in the binder will also preferentially react with the epoxy butane thus destroying the reaction of the acid with asphalt binder. A few examples in the table below will suffice to show the extent to which the acid scavenger can impact the recovered binder properties.
In the table, the first two samples were simply solubilized in the indicated solvent and then recovered using ASTM D-5404 (the rotary evaporator method). The result shows that n-propyl bromide with the stabilizer results in the loss of approximately 0.8 kPa of stiffness. The use of toluene resulted in the loss of 0.2 kPa, some of which could be attributed to incomplete removal of the higher boiling point solvent.
For the New York cores, the binder contained approximately 0.8 percent PPA based on independent binder tests for phosphorus content. The binder extracted and recovered from the two cores, which were taken transversely at the same pavement location, shows substantially different stiffness values for the binders obtained from the two solvents.
The situation for the cores taken in Ontario, Canada, is similar to the New York results. In this case the PPA level is unknown, but the PG grade of the binder was a PG 64-28. With n-propyl bromide the recovered binder fails to meet a PG 64 whereas the binder recovered from using toluene does meet the required PG grade.
The above data is a portion of the information that has been accumulated relative to the impact of solvent type on the recovered properties of mixes produced with PPA containing binders. If you are responsible for the extraction and recovery of binder from mixes where PPA may be present, it is imperative that a solvent which does not contain an acid scavenger be used. This solvent can be a research grade of trichloroethylene or n-propyl bromide, toluene, or tetrahydrofuran (THF).
Care must obviously be taken when using all solvents, but especially with toluene which is flammable and THF which is flammable and quite reactive with oxygen. With an appropriate solvent choice and a careful recovery procedure, an accurate determination of the in-place properties of any binder can be obtained.
Note: Take precautions to read and follow all manufacturers’ warnings and precautions when handling these materials.
|Gerald Reinke is President of MTE Services, Inc. in Onalaska, Wisconsin.