Responsible Use of Polyphosphoric Acid (PPA) Modification of

Responsible Use of Polyphosphoric Acid (PPA)

Modification of Asphalt Binders

PUBLICATION NO. FHWA-HIF-23-005 February 2023

Figure Source: PTSi

FOREWORD

Polyphosphoric acid (PPA) has been used to chemically modify asphalt binders to improve high

temperature rheological properties, without adversely affecting low temperature rheological

properties, since the early 1970s.

(1)

Since the introduction of Superpave performance-grade (PG)

binders, PPA has been used as an additive for adjusting rheological properties to meet PG

specification parameters. PPA has also been used to modify asphalt binders that need an extended

range between the high and low temperature performance requirements to meet PG specification

limits. Since the early 1990s, PPA has also been used in combination with polymer modifiers in

polymer modified asphalt binders to enhance the quality of paving grade asphalt binders. This

report discusses use of PPA as an asphalt binder modifier and presents information on detection

and quantification of PPA in asphalt binders.

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation

(USDOT) in the interest of information exchange. The U.S. Government assumes no liability for

the use of the information contained in this document.

The U.S. Government does not endorse products, manufacturers, or outside entities. Trademarks,

names, or logos appear in this report only because they are considered essential to the objective of

the document. They are included for informational purposes only and are not intended to reflect a

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Non-Binding Contents

Except for the statutes and regulations cited, the contents of this document do not have the force

and effect of law and are not meant to bind the public in any way. This document is intended only

to provide clarity to the public regarding existing requirements under the law or agency policies.

While this document contains nonbinding technical information, you must comply with the

applicable statutes and regulations.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve

Government, industry, and the public in a manner that promotes public understanding. Standards

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its

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ensure continuous quality improvement.

TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HIF-23-005

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Responsible Use of Polyphosphoric Acid (PPA).

5. Report Date

February 2023

6. Performing Organization Code

7. Author(s)

Gaylon Baumgardner (ORCID: 0000-0002-4791-1093),

Adam J. T. Hand (ORCID: 0000-0002-5041-7491), Elie Y.

Hajj (ORCID: 0000-0001-8568-6360) and Timothy B.

Aschenbrener (ORCID: 0000-0001-7253-5504)

8. Performing Organization Report

No.

9. Performing Organization Name and Address

Department of Civil and Environmental Engineering

University of Nevada

1664 North Virginia Street

Reno, NV 89557

10. Work Unit No.

11. Contract or Grant No.

693JJ31850010

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration

Office of Preconstruction, Construction and Pavements

1200 New Jersey Avenue, SE

Washington, DC 20590

13. Type of Report and Period

Covered

Final Report

September 2018–September 2019

14. Sponsoring Agency Code

FHWA-HICP-40

15. Supplementary Notes

FHWA Agreement Officer’s Representative: Timothy B. Aschenbrener, PE.

16. Abstract

Polyphosphoric acid (PPA) is a chemical modifier employed to improve high temperature

rheological properties without adversely affecting low temperature rheological properties that has

been used since the early 1970s.

(1)

Implementation of Superpave performance-grade (PG) asphalt

binder acceptance specifications lead to use of PPA to aid in meeting rheological parameters of some

polymer modified asphalt binders. PPA has also been used as a binder modifier to extend the asphalt

binder range between the high and low temperature performance limits of the specification.

This report provides information to supplement existing publications communicating responsible

use of Polyphosphoric Acid (PPA) in asphalt binder formulations. Information is provided on current

uses of PPA, available qualitative and quantitative methods to detect presence of phosphorus in

asphalt binders, and suggestions as to how phosphorus might indicate the presence and amount of

PPA in asphalt binders.

17. Key Words

Asphalt polyphosphoric acid, PPA, chemically

modified asphalt, asphalt binder

18. Distribution Statement

No restrictions. This document is available to the

public through the National Technical

Information Service, Springfield, VA 22161.

http://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

22. Price

N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.

SI* (MODERN METRIC) CONVERSION FACTORS

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol

LENGTH

in inches 25.4 millimeters mm

ft feet 0.305 meters m

yd yards 0.914 meters m

mi miles 1.61 kilometers km

AREA

square inches 645.2 square millimeters mm

square feet 0.093 square meters m

square yard 0.836 square meters m

ac acres 0.405 hectares ha

square miles 2.59 square kilometers km

VOLUME

fl oz fluid ounces 29.57 milliliters mL

gal gallons 3.785 liters L

cubic feet 0.028 cubic meters m

cubic yards 0.765 cubic meters m

NOTE: volumes greater than 1000 L shall be shown in m

MASS

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t")

TEMPERATURE (exact degrees)

F Fahrenheit 5 (F-32)/9 Celsius

or (F-32)/1.8

ILLUMINATION

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m

cd/m

FORCE and PRESSURE or STRESS

lbf poundforce 4.45 newtons N

lbf/in

poundforce per square inch 6.89 kilopascals kPa

APPROXIMATE CONVERSIONS FROM SI UNITS

Symbol When You Know Multiply By To Find Symbol

LENGTH

mm millimeters 0.039 inches in

m meters 3.28 feet ft

m meters 1.09 yards yd

km kilometers 0.621 miles mi

AREA

square millimeters 0.0016 square inches in

square meters 10.764 square feet ft

square meters 1.195 square yards yd

ha hectares 2.47 acres ac

square kilometers 0.386 square miles mi

VOLUME

mL milliliters 0.034 fluid ounces fl oz

L liters 0.264 gallons gal

cubic meters 35.314 cubic feet ft

cubic meters 1.307 cubic yards yd

MASS

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T

TEMPERATURE (exact degrees)

C Celsius 1.8C+32 Fahrenheit

ILLUMINATION

lx lux 0.0929 foot-candles fc

cd/m

candela/m

0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS

N newtons 0.225 poundforce lbf

kPa kilopascals 0.145 poundforce per square inch lbf/in

iii

TABLE OF CONTENTS

INTRODUCTION..........................................................................................................................1

BACKGROUND ............................................................................................................................1

COMMON CONCERNS...............................................................................................................4

Effects of PPA

.....................................................................................................................4

PPA Quantification

............................................................................................................6

SUMMARY ....................................................................................................................................9

ADDITIONAL INFORMATION .................................................................................................9

REFERENCES .............................................................................................................................10

LIST OF FIGURES

Figure 1. Phosphorus detection test results (a) negative, no phosphorus, (b) and (c) both positive

detections of phosphorus.

(Figure source (19))

...........................................................................................6

Figure 2. EDXRF calibration curve for PG64-22 asphalt binder containing PPA loadings from

0.0 to 1.0 percent.

(Figure source PTSi)

......................................................................................................8

LIST OF TABLES

Table 1. Composition of mixed polyphosphoric acids.

(Table Source PTSi, Data Source (1,12))

........................2

Table 2. Example of improved PG binder properties with PPA modification.

(Table Source PTSi, Data

Source (8,9))

...........................................................................................................................................3

Table 3. Example of improved asphalt binder aging properties with PPA modification.

(Table Source

PTSi)

...................................................................................................................................................4

Table 4. Points of calibration curve for each base asphalt.

(Table Source PTSi)

........................................8

LIST OF ABBREVIATIONS AND SYMBOLS

Abbreviations

AA atomic absorption

AASHTO American Association of State Highway and Transportation Officials

ASTM ASTM International

cps counts per second

DOT Department of Transportation

DSR dynamic shear rheology

EDXRF energy dispersive X-ray fluorescence

FHWA Federal Highway Administration

G* complex modulus

phosphoric acid

ICP inductively coupled plasma spectroscopy

LADOTD Louisiana Department of Transportation and Development

P2O

phosphorus pentoxide

phosphorus pentasulfide

)

red phosphorus

PAV pressure aging vessel

PG performance grade

PPA polyphosphoric acid

ppm parts per million

PTSi Paragon Technical Services, Inc.

RTFO rolling thin-film oven

SBS styrene-butadiene-styrene

SHRP Strategic Highway Research Program

TFHRC Turner Fairbank Highway Research Center

UNR University of Nevada, Reno

U.S. United States

USPTO United States Patent and Trademark Office

UTI useful temperature interval

WDXRF wavelength dispersive X-ray fluorescence

WYDOT Wyoming Department of Transportation

XRF X-ray fluorescence

ΔT

delta T

δ phase angle

INTRODUCTION

Asphalt binder is used in more than 200 applications, including asphalt pavements.

(2)

Non-

asphaltic additives are sometimes used in asphalt binder formulations to modify characteristics to

enhance properties of end use performance. Examples of such additives include adhesion

promoters, chemical additives, warm-mix additives, waxes, refined bio-oils, and polymers.

Phosphorus pentoxide (P

) and polyphosphoric acid (PPA), a reactive oligomer or short chain

polymer, are specific examples of non-asphaltic additives successfully used in asphalt binder

formulations for more than seventy years.

(1,3,4,5,6)

First use of PPA as a paving asphalt binder modifier was reported in 1973.

(1)

In this application,

PPA modified asphalt was formulated to meet acceptance specifications set forth by the Louisiana

Department of Transportation and Development (LADOTD) for a specific grade of asphalt. This

specification required an asphalt having a penetration of 60–70 dmm at 25 degrees Celsius and a

minimum viscosity of 3,600 poises at 60 degrees Celsius. Use of PPA modification allowed for

maintaining the minimum penetration while increasing the desired viscosity.

Since the end of the Strategic Highway Research Program (SHRP)

[1]

in 1993 and implementation

of the Superpave system performance grade (PG) binder specification in the mid 1990s, use of

PPA to improve the overall properties of asphalt binders has increased.

(8,9,10,11)

Industry experience

has shown that use of small amounts of PPA, as a tool for chemical modification of asphalt binders

used alone or in conjunction with polymers, can improve high-temperature PG without adversely

affecting the low-temperature PG. PPA modified asphalt binders can improve asphalt pavements

performance.

Though use of PPA to enhance asphalt binder performance has a long track record, its use is often

debated, possibly due to a lack of understanding of PPA benefits in improving asphalt binder

performance. Some State Departments of Transportation (State DOTs) do not allow use of PPA as

an asphalt binder modifier.

(10)

Other State DOTs have restrictions allowing use of PPA but limiting

use through implementation of use levels, typically ranging for 0.25 percent to 1.5 percent. A 2022

review of published State DOT specifications showed fifteen States allowing usage or limited

usage of PPA. A common maximum limit was 0.5 percent; however, some States allowed up to

0.75 and 1.0 percent maximums. Three States allowed usage with prior approval, twenty-nine

States had no stated restrictions, and five States did not allow use of PPA.

BACKGROUND

Chemical modification of asphalt with phosphorus compounds like phosphorus pentoxide (P

stable phosphorus sulfides, e.g., phosphorus pentasulfide (P

), and red phosphorus ((P

)

), was

performed as early as 1948.

(3)

In 1973, the United States Patent and Trademark Office (USPTO)

issued a patent to Stephen H. Alexander, US Patent number 3,751,278, described as, “Method of

Treating Asphalt.”

(1)

The object of the invention was to provide a treatment method to increase the

viscosity of a vacuum distilled asphalt. More specifically, the object was to substantially increase

viscosity of asphalt without significantly decreasing the asphalt penetration. Another object was

The Strategic Highway Research Program (SHRP) was a 5-year, $150 million applied research program authorized

by the Surface Transportation and Uniform Relocation Act of 1987.

to provide an asphalt composition with unique temperature susceptibility characteristics to meet

the desired specification of the Louisiana State Highway Department.

(1)

The method describes

mixtures of condensed derivatives of phosphoric acid with H

equivalents of greater than 100

percent used to modify asphalt binders.

Since its first reported use in the early 1970s, PPA modification has increasingly been used as a

means of producing modified binders in North America. PPA provides the benefit of phosphoric

acid and P

modification without the risks associated with combining hot asphalt with water

containing orthophosphoric acid, 85 percent phosphoric acid, or the handling risks associated with

solid P

(6)

Similar to polymer modification, modification with PPA stiffens the asphalt at high

temperature with improved resistance to permanent deformation and has no detrimental effects on

low temperature properties.

(6,8)

Phosphoric acid, also known as orthophosphoric acid, is typically a colorless liquid of 85 percent

in water. PPA offered commercially is a mixture of orthophosphoric acid and oligomers,

short chain polymers, of phosphoric acid, for example, pyrophosphoric acid, triphosphoric and

higher acids and is sold on the basis of its calculated equivalent content of H

. PPA is obtained

by condensation of mono-phosphoric acid by hydration of P

. It is a viscous liquid at 25 degrees

Celsius, from 840–60,000 centipoises depending on calculated equivalent content of H

. PPA

is a non-oxidant compound highly soluble in organic compounds. Asphalt applications typically

use PPA with H

concentrations greater than 100 percent. The more common PPA used for

asphalt applications, superphosphoric acid, is 105 percent. Orthophosphoric acid and PPA of

concentrations less than 105 percent may be used but are generally avoided. PPA of lower

than 100 percent contain water and present risk of foaming and corrosion in refinery or

terminal. Table 1 presents the varied composition of mixed polyphosphoric acids which comprise

mixtures having an H

equivalent of greater than 100 percent concentration.

Table 1. Composition of mixed polyphosphoric acids.

(Table Source PTSi, Data Source (1,12))

Weight Percent Percentage of the Strong Phosphoric Acids

Ortho Pyro

Tri-

poly

Tetra-

poly

Penta-

poly

Hexa-

poly

Hepta

-poly

Octa-

poly

Nona-

poly

High-

poly

101 73.0 81.5 18.5 – – – – – – – –

104 75.0 60.2 35.4 4.4 – – – – – – –

105.5 76.2 49.2 42.0 8.4 0.4 – – – – – –

107 77.1 38.5 46.8 12.2 2.5 – – – – – –

108.7 78.3 28.0 49.1 16.5 5.2 1.2 – – – – –

110 79.2 20.5 46.2 20.6 8.8 3.4 0.5 – – – –

111.3 80.1 15.4 39.1 24.4 12.7 5.7 2.3 0.5 – – –

114.7 83.0 5.6 18.7 17.8 14.7 12.0 8.6 7.2 5.1 2.5 7.8

116.5 83.4 3.2 9.1 10.8 11.3 10.4 8.8 8.3 8.5 6.8 22.0

– not present.

Since the advent of the American Association of State Highway and Transportation Officials

(AASHTO) M 320, Standard Specification for Performance-Graded Asphalt Binder, commonly

referred to as the PG binder specification, PPA has been used in PG asphalt binders. AASHTO M

320, which is not a Federal requirement, assumes that the PG for large loads, or slow traffic, can

be met by an increase in the higher temperature of the binder grade. For example, standard grade

PG64-22 for normal traffic is shifted to PG70-22 for slower heavy traffic and to PG76-22 for heavy

standing or interstate conditions. With these different asphalt binder grades, the temperature range

would respectively span 86, 92, and 98 degrees Celsius between the upper and lower PG limits,

commonly referred to as the “Useful Temperature Interval” (UTI). Typically, PGs spanning more

than 90 degrees Celsius are difficult to produce from conventional refining methods requiring

asphalt binder modification. This is somewhat similar to the LADOTD AC-40 discussed in the

introduction.

Polymer modification is the more common method of asphalt binder modification. However,

asphalt binders that need minimal improvement of upper temperature performance limits may be

modified using polyphosphoric acid (PPA) alone. The advantage of using PPA is that it improves

the high temperature rheological properties without affecting the low temperature grade.

Since the implementation of the PG binder specification, PPA has also been used in combination

with polymer modifiers in polymer modified asphalt binders to enhance the quality of paving grade

asphalt binders.

(8,9)

When used in conjunction with polymer modification, PPA allows

achievement of the specified acceptance limits (Dynamic Shear Rheology [DSR] parameter

G*/sinδ, Elastic Recovery, etc.) while limiting the increase in asphalt binder rotational viscosity

measured at 135 degrees Celsius. Results have shown an overall improvement of high temperature

stiffness using a combination of PPA and SBS exceeding the additive stiffening improvement of

each of the two materials.

(11)

Table 2 presents PG binder properties of a neat PG64-22 compared

to the same base binder modified with PPA, styrene-butadiene-styrene block copolymer (SBS),

and SBS plus PPA. These data show improvement in properties of the neat PG64-22 with

improvements in the high, intermediate, as well as low temperature performance. The asphalt

binder modified with a combination of SBS and PPA show the synergistic effect discussed as well

as improvement of rotational viscosity results.

Table 2. Example of improved PG binder properties with PPA modification.

(Table Source PTSi,

Data Source (8,9))

Asphalt Binder

High

Temperature

Limit

(Passing RTFO

DSR G*/sinδ)

(°C)

Intermediate

Temperature

Limit

(Passing

PAV DSR

G*sinδ)

(°C)

Low

Temperature

Limit

(Passing

m-value

@ 60 sec)

(°C)

Rotational

Viscosity

@ 135°֩C

(Pa•s)

Elastic

Recovery

@ 60°֩C

(Percent)

Useful

Temperature

Interval

(UTI)

PG64-22

(Neat Base Binder)

66.1 26.1 –23.6 450 NT 89.7

PG67-22

(0.25 Percent PPA)

68.6 22.0 –24.1 477 NT 92.7

PG76-22

(4.75 Percent SBS)

76.8 22.0 –26.5 4,575 87.5 103.2

PG76-22

(3.4 SBS/0.25 PPA)

77.4 21.8 –27.1 2,235 85.0 104.5

Notes: DSR=dynamic shear rheometer; RTFO=rolling thin-film oven; PAV=pressure aging vessel; G*=complex

modulus; δ=phase angle; NT=not tested.

COMMON CONCERNS

Some concerns about PPA are listed below and addressed in the following sections.

• What are potential adverse effects of PPA?

• How does one know if an asphalt binder contains PPA?

• How can one determine how much PPA is used in an asphalt binder?

Effects of PPA

While PPA modification has been compared to oxidation or “air blowing” of asphalt, PPA

modification differs from air blowing. There is no asphalt oxidation, and PPA modified asphalt

has good low temperature properties compared to air blown asphalt. Opposed to oxidation, PPA

may exhibit anti-oxidative characteristics in asphalt binders.

(3,4)

PPA modification is a functional

economic tool that can be used by binder suppliers to produce PG asphalt binders either with or

without polymers depending on the specification and performance requirements. A common

parameter used to identify additive effects on low temperature performance of asphalt binders is

delta T

critical

or ΔT

(13,14)

Research indicates that more negative values of ΔT

appear to be

correlated to non-load related cracking and other destresses related to poor relaxation properties.

(15)

Suggested considerations for potential ΔT

specification criteria limit warning values of –2.5

degrees Celsius at 20-hour PAV aging and a failure limit value of –5 degrees Celsius at 40-hour

PAV aging.

(15)

As previously stated, an advantage of PPA is improvement high temperature

rheological properties without affecting low temperature theological properties. Therefore,

addition of PPA to asphalt binders is not typically expected to have a detrimental effect on low

temperature properties. Table 3 presents data from a PG64-22 asphalt binder modified with

increasing levels of PPA. Table 3 indicates PPA modification can improve aging and enhance ΔT

Table 3. Example of improved asphalt binder aging properties with PPA modification.

(Table

Source PTSi)

Asphalt

Binder

(PPA

Dosage)

Original

DSR

G*/sinδ

at 64°C

[at 70°C]

(kPa)

RTFO

DSR

G*/sinδ

at 64°C

[at 70°C]

(kPa)

PAV

DSR

G*sinδ

at 25°C

(kPa)

BBR

Stiffness

20 hour

P/F

Temp

(°C)

BBR

m-value

20 hour

P/F

Temp

(°C)

ΔT

hour

BBR

Stiffness

40 hour

P/F

Temp

(°C)

BBR

m-value

40 hour

P/F

Temp

(°C)

ΔT

hour

Aging

Index

PG64-22

(0.00)

1.85

[0.889]

4.12

[–]

4,470 –13.3 –14.1 0.8 –11.6 –10.7 –0.9 2.23

PG64-22

(0.25)

2.38

[1.13]

–

[2.42]

4,670 –13.0 –14.7 1.7 –11.6 –10.8 –0.8 2.14

PG64-22

(0.50)

2.79

[1.33]

–

[2.76]

4,500 –13.7 –15.2 1.5 –11.8 –10.8 –1.0 2.08

PG64-22

(1.00)

3.75

[1.80]

–

[3.71]

4,180 –14.4 –16.0 1.6 –13.1 –12.5 –0.6 2.06

Notes: DSR=dynamic shear rheometer; BBR=bending beam rheometer RTFO=rolling thin-film oven; PAV=pressure aging

vessel; G*=complex modulus; δ=phase angle; P/F=Pass/Fail; –not measured.

Data presented in Table 2 and Table 3 are in line with findings from previous laboratory studies.

The stiffening effect and improved aging of PPA is crude source dependent, with anywhere from

0.5 to 3.0 percent of PPA needed to increase the binder grade.

(9,10,11,16)

Increased moisture damage protentional can be of concern when PPA modified asphalt binders are

used. Several laboratory studies have evaluated the moisture damage potential of mixtures

produced with PPA modified asphalt binders.

(9,10)

Asphalt mixture testing in these studies indicated

that moisture damage potential of mixtures with PPA modified asphalt binder was not noted with

modification rates of 1.0 to 1.5 percent or greater for certain asphalt-aggregate combinations.

Nonetheless, an increase in moisture damage potential was not noted with use levels below 1.0

percent.

(16)

Reported findings suggest that PPA modified asphalt binders can mitigate moisture

damage potential with amine, hydrated lime, and phosphate ester anti-strips, results of which are

asphalt and aggregate dependent. Field testing supports these findings with no negative

performance related to PPA asphalt binder modification.

(16)

Typical asphalt mixture design and

verification testing, including moisture damage testing, is sufficient for the PPA asphalt binder

modification levels discussed. PPA asphalt binder modification levels greater than about 1.5

percent may require additional evaluation to determine adverse or deleterious effects due to

interactions between PPA modified asphalt binder, aggregates, or other asphalt additives in the

mixture.

PPA Detection

It is not possible to determine the presence of or measure the content of H

or PPA in asphalt

binder.

(17)

However, since asphalt binder does not naturally contain phosphorus, the assumption

could be made that asphalt binder containing phosphorus may contain PPA. In this case it would

then be possible to calculate the theoretical content of PPA used to modify the asphalt binder.

Wet chemistry qualitative methodologies are available to determine the presence of phosphorus in

organic compounds. However, these methodologies are primarily used for non-oil or non-

petroleum compounds. A common method uses ammonium molybdate. In this method the organic

compound is heated with an oxidizing agent, typically nitric acid, to convert the phosphate to

phosphoric acid. The resulting phosphoric acid is boiled with ammonium molybdate yielding a

precipitate of ammonium phosphomolybdate and a yellow coloration indicating the presence of

phosphorus. It may be apparent why this method may not be particularly applicable to asphalt

binder as any yellow coloration would be masked by the asphalt binder.

Asphalt researchers at the FHWA Turner-Fairbank Highway Research Center (TFHRC) developed

a wet chemistry methodology to detect the presence of phosphorus in asphalt binder. This

methodology was later adopted as AASHTO T 377, Standard Method of Test for Detecting the

Presence of Phosphorus in Asphalt Binder.

(18 )

Use of AASHTO methods and specifications are

not a Federal requirement.

In this method, phosphorus present in the asphalt binder is extracted using butyl alcohol, and the

extracted phosphorus is transferred to an aqueous phase. If phosphorus is present, treating this

aqueous phase with ammonium molybdate, antimony potassium tartrate, and ascorbic acid will

yield an antimony-phospho-molybdate complex and a blue color.

Figure 1 shows the results obtained from the AASHTO T 377 acid detection test; the control

sample (a) (PG64-22) does not contain PPA. Sample (b) (PG64-22 plus 0.5 percent PPA) and

sample (c) (PG64-22 plus 0.5 percent PPA and 0.5 percent liquid antistrip additive) both contain

PPA, which is indicated by the blue color as described in the test method. It is of importance to

note that this is not a quantitative method but a qualitative method which only determines the

presence of phosphorus, it is not specific to PPA. A positive result only indicates the presence of

phosphorus that may be from other sources than PPA.

Figure 1. Phosphorus detection test results (a) negative, no phosphorus, (b) and (c) both

positive detections of phosphorus.

(Figure source (19))

PPA Quantification

The qualitative methods discussed have limited to no function as a quantitative tool in determining

the amount of phosphorus in asphalt; however, instrumental quantitative methods for

determination of phosphorus in asphalt do exist. Several instrumental analytical approaches are

available for determination and quantification of phosphorous in asphalt binders. Such analytical

approaches include graphite furnace atomic adsorption spectroscopy (Furnace AA),

(20)

inductively

coupled plasma spectroscopy (ICP),

(,21,22)

energy dispersive x-ray fluorescence spectroscopy

(EDXRF),

(23)

and wavelength dispersive x-ray fluorescence spectrometry (WDXRF).

(24)

This

discussion will focus on x-ray fluorescence. EDXRF and WDXRF will be collectively referred to

simply as x-ray fluorescence (XRF), both approaches can be used for asphalt binder analysis. In

comparison, WDXRF instruments offer better signal intensity and resolution, however, this comes

at a considerable comparative instrument investment, and longer data acquisition time that creates

a risk of overheating samples. EDXRF instruments may be more suitable for asphalt binder

analysis at considerable reduced investment, while providing decreased data acquisition time and

reduced sample heating. EDXRF instruments are available in both benchtop and handheld

versions. Handheld instruments, while unique, may not be the more practical approach for asphalt

binder analysis.

All instrumental methods discussed are capable of identifying the presence of and determining

phosphorous content in asphalt binders. Some may be more suitable than others. For example,

sample preparation for AA and ICP requires reducing the asphalt to sufficient viscosity to be

sprayed through a nebulizer into a flame. Compared to the XRF approach, this is a disadvantage

as the XRF approach requires no sample preparation allowing phosphorus to be directly

determined in the asphalt matrix as received. Standardized XRF methods are available for

determining the amount of phosphorus in oil matrices,

(23,24)

however, there are no available

standardized XRF methods for determination of phosphorus in an asphalt matrix. In fact,

standardized methods for determining phosphorus in asphalt matrices with AA or ICP are non-

existent as well. Some suppliers and State DOTs have developed XRF methods for determination

of phosphorus in asphalt matrices.

(17,25,26)

These methods typically use existing methods for

(a) CLEAR – Negative (b) BLUE COLOR – Positive (c) BLUE COLOR – Positive

determination of phosphorus in other matrices as the starting point for preparing samples.

Considering the stated disadvantages of AA and ICP and that some State DOTs and suppliers have

developed analytical methods for asphalt binders, the remainder of this discussion will focus on

EDXRF.

EDXRF can be used to analyze many types of matrices including solid, liquid, powder, etc. The

elemental range typically includes sodium to uranium on the periodic table. The concentration

range is typically from (sub) ppm levels to 100 percent. The elements with high atomic numbers

have better detection limits than the lighter elements with low atomic numbers. In an EDXRF X-

ray produced by the source, an X-ray tube, irradiates the sample. The elements present in the

sample will emit fluorescent X-ray radiation with discrete energies that are characteristic for these

elements. By measuring the sample’s radiation energies, it is possible to determine which elements

are present (qualitative analysis); therefore, determination of phosphorus presence in asphalt

binder is a straightforward procedure. By measuring the intensities of the emitted energies, it is

possible to determine how much of each element is present in the sample (quantitative analysis);

nonetheless this method is somewhat more involved.

XRF instruments do not normally require frequent calibration; however, calibration for specific

analysis method elements is important upon initial implementation of specific analysis methods.

Instruments should be calibrated in accordance with manufacturer’s recommendations. Custom

prepared calibration standards for elements critical to asphalt analysis (calcium, copper,

molybdenum, phosphorous, sulfur, and zinc) are readily available from commercial sources.

Calibration is also important when analysis-critical components, such as X-ray source or detector,

of the instrument are maintained or replaced.

In analytical chemistry, a calibration curve, also known as a standard curve, is a general method

for determining the concentration of a substance in an unknown sample by comparing the unknown

to a set of standard samples of known concentration. The calibration or standard curve is a plot of

how the instrumental response changes with the concentration of the substance to be measured.

This is the method proposed to be employed for determination of the estimated concentration of

PPA in asphalt binder samples.

After ensuring instrument operation and calibration, the first step in asphalt binder analysis is

preparation of an asphalt binder calibration curve specific to the element of interest. In the current

case the element of interest is phosphorous or more specifically, PPA. In the asphalt binder

calibration curve step PPA modified asphalt binder blends are prepared by addition of PPA to non

PPA containing asphalt binder. PPA modified asphalt binders are prepared with increasing

quantities of PPA ranging from zero to a desired maximum depending on asphalt binder

specification limits or expected use levels. Limiting the total number of calibration samples

reduces the number of XRF runs. An eleven-point calibration curve consisting of a sample of neat

asphalt binder, or no PPA, and ten samples of this binder modified with equally spaced increasing

loading of PPA to a target maximum is suggested. Since some State DOTs limit use levels to less

than 1.0 percent, and not many asphalt binders require more than about 1.2 percent to change one

full PG, an acceptable range might be 0.0 to 1.0 percent in 0.1 percent increments. Figure 2 presents

a calibration curve PG64-22 modified with PPA (105 percent) with eleven-point loadings from 0.0

to 1.0 percent in 0.1 percent increments as described.

Figure 2. EDXRF calibration curve for PG64-22 asphalt binder containing PPA loadings

from 0.0 to 1.0 percent.

(Figure source PTSi)

To cover greater ranges, increments could be varied maintaining eleven-points of calibration. For

example, 0.0 to 1.5 percent or 0.0 to 2.0 percent. Table 4 provides an example of points on an

eleven-point calibration curve for a range up to 2.0 percent.

Table 4. Points of calibration curve for each base asphalt.

(Table Source PTSi)

Percent PPA Weight of Asphalt

Binder (g)

Weight of PPA (g) Total Weight (g)

0.0 100.0 0.0 100

0.1 99.9 0.1 100

0.2 99.8 0.2 100

0.3 99.7 0.3 100

0.4 99.6 0.4 100

0.6 99.4 0.6 100

0.8 99.2 0.8 100

1.0 99.0 1.0 100

1.4 98.7 1.3 100

1.6 98.4 1.6 100

2.0 98.0 2.0 100

Instrumental quantification methods are limited by the accuracy and effectiveness of the

calibration generated. With any instrumental method used to quantify phosphorus and estimate

PPA content in asphalt binders, it is important to consider the impact of significant changes in

binder source or chemistry on the effectiveness of the calibration curve. For example, overlapping

of fluorescing energies of phosphorus and sulfur caused by asphalt binders with higher

concentrations of sulfur may confound or interfere with accurate determination of the amount of

y = 0.0282x - 0.0278

R² = 0.9996

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Phosphorus Concentration (ppm)

Phosphorus Concentration (cps)

PPA Concentration in PG64-22 Asphalt (percent)

phosphorus in a sample.

(17)

Spectral overlap of sulfur on phosphorus can be lessened through use

of beam filters. To address possible confounding of phosphorus concentration determination, at

least four base asphalts with sulfur content ranging from 2.0 to 6.0 percent should be considered

in development of calibration curves. A calibration curve using linear regression is obtained after

the analysis of the prepared calibration standards in the XRF spectrometer and plotting intensity

readings in terms of counts per second (cps) versus concentration in percent phosphorus as PPA.

This is done for each of the base asphalt binders used in preparation of calibration samples.

Accuracy is improved with a correlation coefficient of 0.9950 or better.

Evaluation of unknown samples is straightforward; the samples are analyzed in the same manner

used for analyzing the samples prepared for development of calibration curves and the percent

phosphorus as PPA is estimated using the prepared calibration curves.

SUMMARY

Polyphoshoric Acid (PPA) is a non-asphaltic reactive oligomer or short chain polymer that has

been used in asphalt binder formulations for more than seventy years. First reported use of PPA as

a paving asphalt binder modifier was in 1973. Since the end of the SHRP in 1993 and

implementation of the SuperPave PG binder specification in the mid 1990s, use of PPA to improve

the overall properties of asphalt binders has increased. Industry experience has shown that use of

small amounts of PPA, as a tool for chemical modification of asphalt binders used alone or in

conjunction with polymers, can improve high-temperature PG without adversely affecting the low-

temperature PG.

Use of PPA to enhance performance of asphalt binders has a long track record; however, its use is

often debated. Some State DOTs do not allow use of PPA as an asphalt binder modifier; others

allow restricted use. A common maximum limit is 0.5 percent with some States allowing up to

0.75 and 1.0 percent maximums.

This report has discussed effects of PPA on asphalt binder properties and performance as well as

methods to detect the presence and amount of PPA. Indications are that PPA improves high

temperature performance of asphalt binder without adversely effecting asphalt low temperature

properties or aging performance. In fact, use of PPA is shown to possibly provide improved binder

aging performance. Presence of PPA can be indicated through chemical tests or via instrumental

analysis. Instrumental methods are available to efficiently quantify dosage levels of PPA in asphalt

binders.

ADDITIONAL INFORMATION

Available literature may help address issues such as use level limits in PPA modification of asphalt

binders, performance characteristics of PPA modified asphalt binders with respect to multiple

stress creep and recovery (MSCR) testing, and moisture sensitivity. The following list provides

additional information about PPA modified asphalt binders:

• Asphalt Institute Publication IS-220: “Polyphosphoric Acid Modification of Asphalt”

• Association of Asphalt Paving Technologists Symposium, “Polyphosphoric Acid

Modification,” March 2010

• Federal Highway Administration, TechBrief: “The Use of Performance Asphalt Binder

Modified with Polyphosphoric Acid (PPA),” March 2012

• Applied Research Associates, Inc., Report No. 0001946-1: “Performance of Asphalt

Mixtures Containing Polyphosphoric Acid,” September 2014

• Transportation Research Board, National Cooperative Highway Research Program

(NCHRP) Synthesis 511: “Relationship Between Chemical Makeup of Binders and

Engineering Performance,” (2017)

REFERENCES

1. Alexander, S. H., (1973), “Method of Treating Asphalt,” United States Patent 3,751,278,

United States Patent and Trademark Office (USPTO), Alexandria, VA.

2. Asphalt Institute (2007), “MS-4: The Asphalt Handbook,” Manual Series number 4, 7

Edition, Lexington, KY.

3. Hoiberg, A. J., (1948), “Air-Blown Asphalt and Catalytic Preparation Thereof,” United

States Patent and Trademark Office (USPTO), Alexandria, VA.

4. Hoiberg, A. J., (1962), “Asphalt Composition,” United States Patent and Trademark Office

(USPTO), Alexandria, VA.

5. Shearon, W.H. and A.J. Hoiberg, (1953), “Catalytic Asphalt,” Journal of Industrial and

Engineering Chemistry, 45(10), 2122-2312.

6. Fee, D. Maldonado, R., Reinke, G., and H. Romagosa, (2010), “Phosphoric Acid

Modification of Asphalt,” Transportation Research Record: Journal of the Transportation

Research Board, N0 21790, pp49-57, Transportation Research Board of the Notional

Academies, Washington, D.C.

7. American Association of State Highway and Transportation Officials (2004). AASHTO M

266-04 Standard Specification for Viscosity-Graded Asphalt Binder, AASHTO, Washington,

D.C.

8. Baumgardner, G.L., Martin, J-V, Powell, R., and P. Turner (2007), “Binder Modified with a

Combination of Polyphosphoric Acid and Styrene-Butadiene-Styrene Block Co-polymer:

The NCAT Test Track Experience over the First Two Research Cycles,” Fifty-Second

Annual Conference of the Canadian Technical Asphalt Association (CTAA), Niagra Falls,

Ontario.

9. Transportation Research Board, (2009), “Polyphosphoric Acid Modification of Asphalt

Binders, A Workshop,” Transportation Research Circular E-C160, Washington D.C.

10. Arnold, T.S., (2014), “The Use of Phosphoric Acid to Stiffen Hot Mix Asphalt Binders,”

U.S. Department of Transportation, Federal Highway Administration, McClean, VA.

11. Bukowski, J., Youtcheff, J. and M. Harman, (2012), “The Use of Performance of Asphalt

Binder Modified with Polyphosphoric Acid (PPA),” U.S. Department of Transportation,

Federal Highway Administration, Washington D.C.

12. Huhti, A.L. and P.A. Gartaganis, (1956), “The Composition of the Strong Phosphoric

Acid,” Canadian Journal of Chemistry, 34(6), pp.785-797.

13. Asphalt Institute, “State-Of-The-Knowledge: Use of the Delta Tc Parameter to Characterize

Asphalt Binder Behavior,” Asphalt Institute Informational Series, IS-240, 2019.

14. Baumgardner, G. L., (2021), “Delta Tc Binder Specification Parameter,” [tech brief]. No.

FHWA-HIF-21-042. United States. Federal Highway Administration. Office of

Preconstruction, Construction, and Pavements.

15. Anderson, R. M., G. N. King, D. I. Hanson, and P. B. Blankenship, (2011), “Evaluation of

the Relationship Between Asphalt Binder Properties and Non-Load Related Cracking,”

Journal, Association of Asphalt Paving Technologists, Volume 80.

16. Youtcheff, J., (2012), TechBrief: “The Use of Performance Asphalt Binder Modified with

Polyphosphoric Acid (PPA),” U,S, Department of Transportation, Federal Highway

Administration, McClean, VA.

17. Reinke, G. and S. Glidden, (2010), “Analytical Procedures for Determining Phosphorus

Content in Asphalt Binder and Impact of Aggregate on Quantitative Recovery of Phosphorus

from Asphalt Binders,” Journal of the Association of Asphalt Paving Technologists, Vol 79,

pp. 695-718, Association of Asphalt Paving Technologists, Lin

18. American Association of State Highway and Transportation Officials (2021). AASHTO T

377-17 Standard Method of Test for Detecting the Presence of Phosphorus in Asphalt Binder,

AASHTO, Washington, D.C.

19. Mahmud, I., Hossain, Z., and G. Baumgardner, (2016). “Effects of Polyphosphoric Acid

(PPA) Modification of Asphalts,” Proceedings of the International Society for Asphalt

Pavements, ISAP 2016, Jackson Hole, WY.

20. ASTM International, (2010), E1184-21 Standard Practice for Determination of Elements by

Graphite Furnace Atomic Absorption Spectrometry,” ASTM International, West

Conshohocken, PA.

21. American Oil Chemists Society, (2017), Official Method Ca 20-99 for Phosphorus in Oil by

Inductively Coupled Plasma Optical Emission Spectroscopy, AOCS, Urbana, IL.

22. ASTM International, (2017), D8110-17 Standard Test Method for Elemental Analysis of

Distillate Products by Inductive Coupled Plasma Mass Spectrometry (ICP-MS), ASTM

International, West Conshohocken, PA.

23. ASTM International, (2019), D6481-14 Standard Test Method for Determination of

Phosphorus, Sulfur, Calcium, and Zinc in Lubrication Oils by Energy Dispersive X-ray

Fluorescence Spectroscopy, ASTM International, West Conshohocken, PA.

24. ASTM International, (2019), D6443-14 Standard Test Method for Determination of Calcium,

Chlorine, Copper, Magnesium, Phosphorus, Sulfur, and Zinc in Unused Lubricating Oils and

Additives by Wavelength Dispersive X-ray Fluorescence Spectrometry (Mathematical

Correction Procedure), ASTM International, West Conshohocken, PA.

25. Wyoming Department of Transportation (2015), “Percent Phosphoric Acid and Phosphorus

Content in Asphalt by X-Ray Fluorescence Spectrometry,” WYDOT Materials Testing

Manual Method 838.0, Wyoming Department of Transportation, Cheyenne, WY.

26. Barborak, R.C., Coward, C.E. Jr., R.E. Lee, (2016) “Detection and Estimation of Re-Refined

Engine Oil Bottoms in Asphalt Binders – Texas Department of Transportation,”

Transportation Research Record: Journal of the Transportation Research Board, No. 2574,

pp. 48-56, Transportation Research Board, Washington, DC.