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Polyphosphoric a c id m odifie d a spha lt :
propose d m e cha nism s
N R C C - 4 7 3 5 2
B a u m g a r d n e r , G . L . ; M a s s o n , J
-F . ; H a r d e e , J . R . ; M e n a p a c e ,
A . M . ; W i l l i a m s , A . G .
A version of this document is published in / Une version de ce document se trouve dans: Proceedings of the Association of
Asphalt Paving Technologists, Long Beach, CA., March 7, 2005, pp. 283-305
Gaylon L. Baumgardner1, J-F. Masson2, John R.Hardee3, Andy M.
Menapace4, Austin G. Williams5
Abstract
Asphalt binders have been chemically modified with polyphosphoric acid (PPA) to improve high temperature
rheological properties without adversely affecting low temperature rheological properties since the early 1970’s. More recently, PPA has been used in Superpave performance-grade (PG) binders that need an extended range between the high and low temperature performance requirements. The mechanism of chemical
modification of asphalt with PPA remains in great part unknown. This paper presents results that will help to better understand the mechanisms of chemical modification with PPA. PPA modified and unmodified asphalts from different crude sources were analyzed for chemical composition by asphaltenes precipitation and thin-layer chromatography (TLC), and by gel-permeation chromatography (GPC) and atomic force microscopy (AFM). The results indicate that the mechanism of PPA action depends on the base asphalt. In one case PPA affected a phase dispersed in asphalt, in the other case it affected the asphalt matrix. In both cases, PPA caused stiffening of the modified phase. Several stiffening mechanisms are proposed.
Key Words:
1
Executive Vice President, Paragon Technical Services, Inc. g.baumgardner@paratechlab.com
2
Senior Research Officer, National Research Council of Canada jean-francois.masson@nrc.gc.ca
3
Professor, Physical Chemistry, Henderson State University hardee@hsu.edu
4
Group Leader, Asphalt, Paragon Technical Services, Inc. a.menapace@paratechlab.com
5
Research Technician, Asphalt, Paragon Technical Services, Inc. a.williams@paratechlab.com
Introduction and Background
US Patent number 3,751,278 issued August 7, 1973 describes a, “Method of Treating Asphalt.” In summary the object of the invention was to provide a method to alter the viscosity-
penetration relationship of an asphalt. More specifically, the object was to substantially increase viscosity of asphalt without significantly decreasing the penetration of the asphalt. Another object was to provide an asphalt composition with unique temperature susceptibility characteristics (1). In the method,
mixtures of condensed derivatives of phosphoric acid (H3PO4)
with P2O5 equivalents of greater than 100% were used to modify
asphalt binders.
In the 1970’s viscosity was the method used to grade asphalt binders. Some states specified viscosity grades, in particular AC-40, in accordance with then current AASHTO guidelines, with additional requirements for minimum penetration values. These binders were specified in an attempt to obtain binders that would resist rutting while providing good performance against thermal cracking. While the concept was good, these binders were difficult to produce from conventional refining methods; therefore, super phosphoric acid was employed to increase the viscosity of a
standard AC-30 to that of an AC-40 minimally effecting the binder penetration.
More recently, with the advent of Superpave and the
application of performance grading (PG), it has been assumed that the performance requirements for large loads, or slow traffic could be met by an increase in the higher temperature of the performance grade. For example, standard grade PG64-22 for normal traffic, could be shifted to PG70-22 for slower heavy traffic and to PG76-22 for heavy standing or interstate conditions. Upon these
modifications, the grades would respectively span 86°C, 92°C and 98°C of performance. Typically, performance grades that span more than 90°C require asphalt modification. While polymer modification has been the more common, polyphosphoric acid (PPA) modification can also be used, the advantage being that it
improves the high temperature rheological properties without affecting the low temperature grade.
The use of PPA modified binders is often debated, to the point sometimes that agencies have banned the use of acid modified binders. Such actions result from a lack of understanding of the benefits of PPA as a tool to improve the performance of asphalt binders, combined with a lack of understanding of the mechanisms of the action of PPA. In an effort to better understand this
mechanism, modified and unmodified asphalts from two crude sources were analyzed for chemical composition by asphaltenes precipitation and thin-layer chromatography (TLC), and by gel-permeation chromatography (GPC) and atomic force microscopy (AFM).
Experimental Binder Formulations
Asphalt from Saudi and Venezuelan (Venz) crude sources with respective grades of PG64-22 and PG67-22 were modified with PPA to PG70-22 (Table 1). Just enough PPA was added to the original asphalt binder to achieve PG70-22. This was 1.2% and 0.62% by wt of binder for the respective Saudi and Venezuelan binders.
Table 1, Asphalt Identification and Characteristics Asphalt PG True PG Comment
Saudi 64-22 67.6-23.5 Control sample
Saudi Modified 70-22 72.25-25.08 PPA modified Saudi
Venz 67-22 68.5-24.02 Control sample
Asphalt Composition
Each binder was deasphaltened according to ASTM Method D-3279 “Standard Test Method for n-heptane Insolubles” to yield asphaltenes (As) and maltenes which is the n-heptane soluble portion.
The maltenes were further fractionated on an Iatroscan TH-10 Hydrocarbon Analyzer to yield the composition in saturates (S), cyclics (C) and resins (R). The method has been described in detail before (2,3). N-pentane was used to elute the saturates, and a 90/10 toluene/chloroform solution was used to elute the cyclics. The resins were not eluted and remained at the origin.
Gel-permeation Chromatography
Gel-permeation chromatography (GPC) was performed on the asphaltenes. GPC was performed on a Hewlett-Packard 1050
HPLC. A TSK-GELRG4000HHR column and a TSK-GELRG3000
column were used in series. An HXLL2 guard column was placed
in line before the two GELR columns. 0.05 g of asphaltenes were
dissolved in 5 ml of tetrahydrofuran (THF) and then stirred with a magnetic stirrer for 30 minutes. The resulting solution was passed through a 0.45 µ filter. Analysis was performed on a 25 µL injection, using THF as the solvent at a flow rate of 1 mL/min.
The columns were maintained at a temperature of 35oC and a
refractive index (RI) detector was used. Reported molecular weights are polystyrene (PS) equivalents obtained from the analysis of PS standards with molecular weights between 50,000 g/mol and 4130 g/mol.
Atomic Force Microscopy
Binders were prepared for atomic force microscopy by the application of a small bead to a steel stub. With a knife, the bead was scraped against the surface of the stub and the film heated to
Atomic force microscopy images were captured at room
temperature on a JEOL JSPM-5200 microscope. Both topographic and friction images were obtained after the asphalt films had been annealed 72 h to 96 h at room temperature. The silicon AFM probes (MikroMasch, Tallinn, Estonia) had a stiffness of 40 N/m. The basics of AFM and the origin of the topographic and friction signals were described before (4,5,6). The topographic images reveal vertical elevations and declinations associated with surface features, whereas the friction image allows for the differentiation of surface material based on changes in elastic or adhesive
properties. It thus reveals changes in surface composition, without revealing the nature of the change. All the microphotographs show
a 15 µm x 15 µm region unless otherwise indicated.
Results
Asphaltenes precipitation and TLC maltenes fractionation data are presented in Table 2. Modification of the Saudi asphalt binder with 1.2 wt% PPA increased asphaltenes content from 9.1 wt% to 14.7 wt% to produce the modified Saudi PG70-22. Similarly, modification of the Venezuelan asphalt with 0.62 wt% PPA increased asphaltenes from 10.5 wt% to 14.9 wt% to produce the modified Venezuelan PG70-22.
Table 2, Asphalt Composition
Asphalt A R C S
Saudi 64-22 9.1 11.2 75.5 4.4
Saudi Modified 70-22 14.7 10.5 74.4 0.4
Venz 67-22 10.5 21.5 65.2 2.8
Venz Modified 70-22 14.9 15.2 63.1 6.8
Further analysis of precipitated asphaltenes using P-31 Nuclear Magnetic Resonance (NMR) revealed no phosphorous compounds in the asphaltenes fraction of the non-modified asphalt binders, while phosphorous compounds were present in the precipitated asphaltenes from the PPA modified asphalt binders. P-31 NMR of the maltenes fractions from the non-modified and PPA modified asphalt binders revealed no phosphorous compounds. NMR
results support that PPA preferentially reacts with the asphaltenic phase of the asphalt as proposed by G. Orange et al. (7).
The AFM friction image for unmodified Saudi asphalt is shown in Figure 1. This asphalt shows two separate domains, a homogeneous matrix and flake-like domains dispersed in that matrix. The flakes are numerous and nearly form a co-continuous
phase. The average size of the flakes is about 1 µm. The
topographic image was monotonous and indicated that the surface was flat (not shown).
Figure 1, Phase Image of the Saudi Asphalt on a Scale of 15 µm x 15 µm.
The GPC trace for asphaltenes precipitated from the Saudi binder is shown in Figure 2 and Table 3. It reveals a maximum around 15 minutes of elution, which corresponds to a molecular
weight of 5200 g/mol. On the right of this maximum appears a broad shoulder for lower molecular weight material. The shoulder was centered near 1200 g/mol.
Figure 2, GPC Results for Precipitated Asphaltenes from Saudi and Saudi Modified Asphalts
-5 0 5 10 15 20 25 10 12 14 16 18 20 22 Time/minutes (Increasing Molecular Weight Right to Left)
RI R espo n se Saudi Saudi Modified
Table 3, Saudi and Saudi Modified Asphaltene Molecular Weight and Molecular Weight Distribution
Asphalt Molecular Weight Molecular Weight Range at Maximum at Half Height
Saudi 5200 g/mol 11,000 to 300 g/mol
Saudi Modified 1200 g/mol 7,300 to 170 g/mol
The microstructure of the Saudi asphalt was affected by PPA,
Figure 3. Ovoid domains about 2 µm on the long side were then
dispersed in a homogeneous matrix. These domains were larger, but less numerous than the flake-like domains in the original binder, and at the center of each domain was a small bee-like structure reminiscent of those observed by Pauli et al.(8). The topographic image (not shown) indicated that the dispersed phase
protruded very slightly from the matrix surface and that the bee-like centers were higher still and somewhat more visible.
Figure 3, Phase Image of PPA Modified Saudi Asphalt
PPA modification of the Saudi asphalt binder affected its molecular weight profile (Figure 2). The high molecular weight peak disappeared completely and only the original shoulder with a maximum at 1200 g/mol remained. The PPA modification also led to changes in the chemical composition of the binder as shown in Table 2, with the most important change being an apparent conversion of saturates into asphaltenes.
The friction image for the unmodified Venezuelan asphalt binder is shown in Figure 4. It showed small domains dispersed in
a homogeneous matrix. The average size of these domains was
about 0.6 µm. The topographic image was void of any features
and not shown. The GPC result showed a unimodal molecular weight distribution that indicated an average molecular weight of 2100 g/mol, Figure 7.
Figure 4, Phase Image of the Venezuelan Asphalt on a Scale of 15 µm x 15 µm.
Upon the modification of the Venezuelan asphalt binder with PPA, the dispersed phase remained unaffected, but its contrast with the matrix was reduced, Figure 5, and the matrix was no longer homogeneous, which was better seen at higher magnification, Figure 6. The asphalt matrix in the PPA modified Venezuelan
asphalt binder was peppered with very fine domains of about 60
nm (0.06 µm) in size and contained narrow strings of matter up to
about 1.5 µm long.
Figure 5, Phase Images of the PPA Modified Venezuelan Asphalt on a 15 µm x 15 µm
Figure 6, Phase Images of the PPA Modified Venezuelan Asphalt on a 5 µm x 5 µm Scale
The effect of PPA modification on the Venezuelan binder was to reduce the average molecular weight slightly from 2100 g/mol to 1700 g/mol, Figure 7 and Table 4, and to change the chemical composition. It raised both the asphaltenes and saturates contents, at the expense of the cyclics and resins (Table 2).
Figure 7, GPC Results for Precipitated Asphaltenes from Venezuelan and PPA Modified Venezuelan Asphalts
-5 0 5 10 15 20 25 10 12 14 16 18 20 22 Time/minutes (Increasing Molecular Weight Right to Left)
R I Re spon se Venz Venz Modified
Table 4, Venezuelan and PPA Modified Venezuelan Asphaltene Molecular Weight and Molecular Weight
Distribution
Asphalt Molecular Weight Molecular Weight Range at Maximum at Half Height
Venz 2100 11,000 to 160
Venz Modified 1700 7,300 to 140
Discussion
The AFM friction image arises from changes in composition and stiffness across a sample surface Figures 4-6. The greater the contrast between the domains in the friction image, the greater is the difference in stiffness and composition. Figure 1 shows that the Saudi binder had two phases of different composition, which is in agreement with the bimodal molecular weight distribution obtained by GPC. Upon PPA modification of the Saudi asphalt, the contrast between the matrix and the dispersed domains was
increased as seen in a comparison of Figures 1 and 3. Either the dispersed phase became stiffer or the matrix became softer, or both. This may be explained by a chemico-physical process where PPA first reacts with bitumen, followed by the segregation of polar material out of a non-polar matrix, the reordering leading to
domains with sharp boundaries. In accordance with the phase contrasts, the modified matrix would be softer and more adhesive than in the unmodified binder, whereas the dispersed phase would be stiffer and less adhesive, the stiffer phase leading to the rise in PG.
The proposed chemico-physical process does not address the nature of the reaction between PPA and the binder. The GPC results in Figure 2 do indicate that high molecular weight or associated material is involved in the change, which suggests that asphaltenes react with PPA. Based on GPC alone, it would be assumed that the PPA disperses associated asphaltenes into small domains, a scenario modeled in some detail by Orange et al. (7). With the binders under consideration here, the script must be incomplete, however, as Figures 1 and 3 do not indicate dispersion, but a greater association of polar, assumingly asphaltenes-rich, material. Moreover, a simple dispersion disregards the increase in asphaltenes and the fall in saturates (Table 2). Another mechanism of action must also be at play to explain the apparent conversion of saturates into asphaltenes.
To explain this apparent conversion, several scenarios have been considered as shown in Table 5. Scenarios A and B imply the co-precipication of the newly formed material with the asphaltenes in n-heptane. As indicated, however, no scenario is yet fully satisfactory, which reinforces the need for more comprehensive studies.
Table 5, Possible Mechanisms for the Conversion of Saturates into Polar Materials.
Action Pros Cons
A. Acid (cationic) polymerization of alkene containing saturates High molecular weight material formed Inconsistent with GPC. Alkenes often absent from saturates B. Copolymerization of
PPA and saturates
PPA chain may be inserted in alkyl chains through the displacement of a protonated alkyl sulfide groups C-O-P bonds have yet to be observed (7) Protonation→ Elimination→ Condensation of alkenes→Cyclisation C. →Aromatization Conversion of long chain saturates into alkyl-aromatics Highly orchestrated suite of reactions
The Venezuelan binder initially showed a fine dispersion in a continuous matrix. Interestingly, the GPC results did not provide a bimodal molecular weight distribution similar to that of the Saudi binder, but a unimodal molecular weight distribution, which indicates that the possible ordering of matter in solution (GPC) does not necessarily reflect the initial solid-state arrangement. For example, semi-crystalline matter shows as a separate phase in the solid-state but not in solution.
Upon PPA modification of the Venezuelan asphalt binder the contrast between the matrix and the dispersed phase decreased, Figures 5 and 6. Hence, PPA reduced the difference in stiffness between the phases. Given the modification of the matrix of this asphalt binder, Figure 6, and the likelihood of a PPA acidification
of the binder, it is reasonable to assume that the asphalt matrix increased in stiffness and that the dispersed phase was already relatively stiff. Given the low saturates content, the area of the dispersed phase in Figures 4 and 5, and the unimodal GPC distribution mentioned above, the dispersed phase is likely stiff semi-crystalline paraffinic and un-reactive material. The original matrix was more likely amorphous, more polar and more reactive with PPA. The result is a stiffer phase with a higher PG (Table 1).
At least two scenarios may concur to increase the stiffness of the continuous binder phase. First is the cross-linking of reactive segments to form a matrix of covalently linked matter., i.e., asphalt-PPA-asphalt, with unreacted or long chains of PPA being responsible for the observed strings (Figure 6); Second, is the PPA catalyzed cyclization of flexible carboxylic acid terminated alkyl chains bonded to aromatic rings (9), which would lead to stiffer naphthene aromatic ketones.
Further stiffening of the binder could be obtained from phosphate salts that aggregate and form ionic clusters as in ionic polymers. The ionic clusters are stiff and provide thermo-reversible cross-links (10). These clusters might explain the development of the very fine 60 nm dispersion.
These scenarios do not explain the increase in saturates in the Venezuelan binder after PPA modification. An acidolysis of the pendant alkyl chains on aromatic nuclei would be consistent with this increase. The breaking of alkyl aromatics in resins and asphaltenes into alkyl and stiff aromatic fragments would further explain the increase in n-heptane precipitates, and the segregation and formation of the additional domains
Conclusion
Two asphalts were modified with PPA and the resulting change in microstructure and composition was investigated by AFM, GPC and compositional analysis. The performance grade of both asphalts was raised by PPA, which was observed in AFM as a stiffening of one of the two main phases in asphalt. In one asphalt,
PPA affected the dispersed phase; in the other, it affected the matrix. The stiffening effect of PPA was thus asphalt dependent.
Several mechanisms were proposed to explain the stiffening from PPA modification of asphalt binders: co-polymerization of saturates; akyl aromatization of saturates; cross-linking of
neighboring asphalt segments; the formation of ionic clusters; and the cyclization of alkyl aromatics. Detailed physico-chemical analysis of PPA-modified asphalt binders will be required to determine which mechanism(s) prevail, but already the benefits of such a determination can be appreciated. They include a better control of asphalt stiffening and an improved forecast of the long-term and oxidative stability of PPA modified asphalts.
Acknowledgement
JFM wishes to thank Ms Valérie Leblond for the capture of the AFM images.
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