A 13C NMR and DSC Study of the Amorphous andCrystalline Phases in Asphalts

Laurent C. Michon, Daniel A. Netzel,* and Thomas F. Turner

Western Research Institute, 365 North 9th Street, Laramie, Wyoming 82072-3380

Didier Martin and Jean-Pascal Planche

Elf-Antar France, Centre de Recherche d’Elf Solaize, B.P. 22, 69360, Solaize, France

Received September 9, 1998. Revised Manuscript Received January 20, 1999

The amorphous and crystalline phases in asphalt have been identified and studied using low-temperature solid-state carbon-13 CP/MAS NMR and DSC techniques. The NMR mass percentof the crystalline methylene carbons was shown to correlate linearly with the mass percent ofcrystalline wax in asphalts measured using DSC. While the internal methylene carbon contentof long-chain alkanes in the crystalline phase in the asphalts varied, the internal methylenecarbon content of the long-chain alkanes in the amorphous phase remained relatively constant.The NMR crystalline methylene carbon content was plotted against a low-temperature crackingparameter, the fracture temperature of an asphalt. It was found that 1% or less of aliphaticcarbons in the crystalline phase has little effect on the fracture temperature. For these asphalts,the fracture temperature depends mainly on the initial amount of mobile aliphatic carbons inthe amorphous phase at 23 °C. For asphalts containing 1% or more of crystalline aliphatic carbons,the fracture temperature increases with increasing crystalline methylene carbon content.

Introduction

The low-temperature physical and rheological proper-ties of asphalts are of interest because low-temperaturecracking is one of the primary modes of failure forasphalt pavements. At the molecular level, this type offailure mode has been attributed, in part, to crystallinewaxes.The first evidence of a crystalline phase inasphalts was reported in 1966 by Smith et al.1 usinginfrared spectroscopy. These authors found that the 720cm-1 band for amorphous methylene carbons in long-chain hydrocarbons split into a doublet in waxy as-phalts. They attributed this band splitting to theformation of crystalline wax in the asphalt. Noel andCorbett2 studied the crystalline phase in a variety ofasphalts and showed that asphalts are largely amor-phous and that the crystallizable components are largelyfound in the saturate fraction (alkanes) of an asphaltwith lesser amounts in the naphthene-aromatic fraction.They also reported that the crystallizable materialmeasured directly by differential scanning calorimetry(DSC) and the wax obtained by precipitation were notidentical. The precipitated wax was found to be amixture of hydrocarbons in the amorphous and crystal-line phases. Daly et al.3 have conducted an extensivestudy on the crystallization process in asphalts. Theyreported that the crystallization process is very timedependent, and several annealing steps are required in

order to study the phase transition thermodynamically.In addition, they reported that the crystalline compo-nents in asphalt exhibit distinct endothermic patternsin a DSC thermogram, and these patterns depend onthe chemical structure of the crystalline components andtheir interaction with the amorphous phase.

The influence of crystalline and amorphous phaseson the rheological properties of asphalts has recentlybeen reported.4-10 A high crystalline wax content inasphalts can reduce ductility, increase brittleness at lowtemperature, and cause deterioration of adhesion toaggregates.5 Claudy et al.6 speculated that the presenceof a crystalline phase in asphalts could be an importantfactor in determining the tendencies of asphalt pave-ments to crack in a cold environment. McKay et al.4found that macrocrystalline waxes11 (n-alkanes havinga carbon number range from C18 to C40) in asphaltscause viscosity increases, whereas the microcrystalline

(1) Smith, C. D.; Scheutz, R. S.; Hodgson, R. S. Ind. Eng. Chem.,Prod. Res. Dev. 1966, 5, 153.

(2) Noel, F.; Corbett, L. W. J. Inst. Pet. 1970, 56, 261.(3) Daly, W. H.; Qiu, Z.; Negulescu, I. Transp. Res. Rec. 1996, 1535,

54.

(4) McKay, J. F.; Branthaver, J. F.; Robertson, R. E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40 (4), 794.

(5) Gawel, I.; Czechowski, F.; Baginska, K. Proceeding of theEurasphalt & Eurobitume Congress, Strasburg, France, May 7-10,1996, E&E.5.139, 1.

(6) Claudy, P.; Letoffe, J. M.; Rondelez, F.; Germanaud, L.; King,G.; Planche, J.-P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992,37, 1408.

(7) Planche, J.-P.; Martin, D.; Rey, C.; Champion, L.; Gerard, J. F.Proceedings of the 5th International RILEM Symposium 1997, 167.

(8) Netzel, D. A.; Turner, T. F.; Forney, G. E.; Serres, M. Am. Chem.Soc. Div. Polym. Chem., Prepr. 1997, 38, 829.

(9) Netzel, D. A.; Miknis, F. P.; Wallace, J. C.; Butcher, C. H.;Thomas, K. P. Asphalt Science and Technology; Usmani, A., Ed.; MarcelDekker: New York, 1997; Chapter 2.

(10) Bahia, H. U.; Anderson, D. A. Prepr. Pap.sAm. Chem. Soc.,Div. Fuel Chem. 1992, 37, 1397.

(11) Giavarini, C.; Pochetti, F. J. Therm. Anal. 1973, 5, 83.

602 Energy & Fuels 1999, 13, 602-610

10.1021/ef980184r CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 03/09/1999

waxes11 (branched alkanes with a carbon number rangefrom C25 to C65) cause decreases in viscosity.

Bahia and Anderson10,12 reported an important phe-nomenon in asphalts that they defined as low-temper-ature physical hardening. They observed a gradualchange in density and mechanical stiffness with timewhen asphalts are held at low temperatures. Eventhough these authors have shown that there is a definiterelationship between low-temperature physical harden-ing and the wax content of asphalt, they attribute thehardening to factors other than the morphology of thewax. However, Claudy et al.6 showed that the amountof physical hardening that occurs with time can berelated to the number of molecules in the asphalt whichcoalesce to form microscopic crystalline or amorphousdomains. These authors also suggest that asphaltshould no longer be thought of as homogeneous indensity but instead as a complex, two-phase structuremore akin to a gel with enhanced viscoelastic properties.

Another phenomenon observed for asphalts but oc-curring at ambient temperature is isothermal sterichardening.13 That is, molecular restructuring of theasphalt over a long period of time. Netzel et al.8 reportedthat the phenomenon of steric hardening in asphaltsmay also be related to the change in the amount of thecrystalline wax fraction with time. These authors haveshown that the formation of the crystalline waxes inasphalt at room temperature as measured using NMRcontinues for many months. In another study, Netzelet al.9 reported that the amount of mobile aliphaticcarbons, those in the amorphous phase, can be quali-tatively related to many of the rheological and perfor-mance properties of an asphalt.

Generally, standard DSC measurements are used todetermine the crystalline phase in asphalts.2,3,6,7,14

Recently, the technique of modulated DSC has beenused to aid the determination of the crystalline phase.15

Solid-state NMR techniques have been widely used todetermine the amount of amorphous and crystallinephases in polymers. However, these techniques have

been applied only recently to study the amorphous andcrystalline phases in asphalts.9,16 Netzel et al.17 havereported that the 13C NMR spectrum of the internalmethylene carbons of long-chain n-alkanes show tworesonances. The resonance at ∼32 ppm was assignedto the crystalline (all-trans conformation) methylenecarbons, and the observed resonance at ∼30 ppm wasassigned to the internal methylene carbons in theamorphous phase (gauche conformation). Because thecrystalline resonance peak is associated with n-alkanes,this peak represents the amount of macrocrystallinewax in asphalts. Microcrystalline waxes (branchedalkanes) have resonances at different chemical shiftpositions and would not easily be observed in a broadsolid-state 13C spectrum of an asphalt. Thus, the crys-talline internal methylene carbon content measured byNMR should correspond to the crystalline wax fractionas measured by DSC if the crystalline wax fraction ismainly macrocrystalline.

In this paper, the crystalline internal methylenecarbon content of eight asphalts, as measured by solid-state 13C NMR techniques, is compared to the crystal-line wax fraction, as measured using different DSCprocedures. In addition, the crystalline methylene car-bon contents of the asphalts are plotted versus theirrespective fracture temperatures (a parameter to assesslow-temperature cracking) in an effort to determine theeffect of crystalline phase on the thermal cracking ofasphalts.

Experimental Section

Asphalt Samples. The five asphalt samples in group 1,listed in Table 1, were provided by Elf-Antar France. Theseasphalts were selected based on their large range of crystallinewax content. Eight Strategic Highway Research Program(SHRP) core asphalts, comprising group 2, also listed in Table1, were obtained from the Material Research Library (MRL).18

The elemental composition and molecular weight for theseasphalts are given in Table 1, and the chemical class composi-tion (based on SARA and SAPA analyses) and the crystallinewax content (DSC) are given in Table 2.

(12) Bahia, H. U.; Anderson, D. A. Association of Asphalt PavingTechnologists: Austin, TX, 1993; p 1.

(13) Petersen, J. C. Transp. Res. Rec. 1984, 999, 13.(14) Brule, B.; Planche, J. P.; King, G.; Claudy, P.; Letoffe, J. M.

Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1990, 35 (3), 330.(15) Turner, T. F.; Branthaver, J. F. Asphalt Science and Technology;

Usmani, A., Ed.; Marcel Dekker: New York, 1997; Chapter 3.

(16) Netzel, D. A. Transp. Res. Rec. 1998, 1638, 23.(17) Netzel, D. A.; Michon, L. C.; Serres, M. L.; Wieseler, K. M.

Proceedings of the 25th North American Thermal Analysis Society,McLean, VA, 1997, 741.

(18) Material Reference Library, National Research Council, Wash-ington, DC.

Table 1. Elemental Composition and Molecular Weight of Asphalts

carbon(wt %)

hydrogen(wt %)

nitrogen(wt %)

oxygen(wt %)

sulfur(wt %)

molecularweight,a Mn

Asphalt (Source) Group 1b

A (Venezuela) 85.1 10.0 0.54 ND 4.90 880B (Middle East) 84.1 10.1 0.43 ND 4.95 1000C (Italy) 84.9 10.2 0.46 ND ND 910D (Africa) 86.0 11.2 0.57 ND 2.15 890E (North Africa) 86.8 10.8 0.58 ND 1.84 860

Asphalt (Source) Group 2c

AAA-1 (Lloydminster) 83.9 10.0 0.50 0.6 5.50 790AAB-1 (Wyoming Sour) 82.3 10.6 0.54 0.8 4.70 840AAC-1 (Redwater) 86.5 11.3 0.66 0.9 1.90 870AAD-1 (California Coastal) 81.6 10.8 0.77 0.9 6.90 700AAF-1 (West Texas) 84.5 10.4 0.55 1.1 3.40 840AAG-1 (California Valley) 85.6 10.5 1.10 1.1 1.30 710AAK-1 (Boscan) 83.7 10.2 0.70 0.8 6.40 860AAM-1 (West Texas Intermediate) 86.8 11.2 0.55 0.5 1.20 1300

a Data obtained at WRI, VPO at 60 °C. b Data from Elf-Antar France. c Data from MRL.18

Amorphous and Crystalline Phases in Asphalts Energy & Fuels, Vol. 13, No. 3, 1999 603

Nuclear Magnetic Resonance (NMR). A Chemagnetics100/200 solid-state NMR spectrometer operating at a 13Cfrequency of 25 MHz was used for cross polarization withmagic-angle spinning (CP/MAS) and dipolar-dephasing (DD)measurements at -45 °C. Asphalt samples were heated to atemperature between 100 and 170 °C and poured directly intoa 7.5 mm zirconia pencil rotor assembly. These samplesremained at room temperature in the rotor for 2 months to 1year before conducting the low-temperature CP/MAS and DDexperiments. Low temperatures were obtained using anelectric refrigeration FTS systems XR series Air-Jet samplecooler. The cooled, ultradry air from the cooler was transferredto the NMR probe via an insulated 2.4-m transfer line.Parameters for both CP/MAS and DD included a pulse widthof 5 µs (90°), a pulse delay of 1 s, a contact time of 1 ms, asweep width of 16 kHz, a free induction decay size of 1024points, a rotor spinning rate of 4.5 kHz, and 3600 acquisitions.In addition, a dipolar-dephasing pulse sequence with a 180°refocusing pulse was used to obtain the DD data. The dipolar-dephasing time was varied from 1 to 160 µs. Chemical shiftswere referenced internally to the terminal methyl carbons at14 ppm of the long n-alkane chains. The NMR time domainspectra were transformed using first a Lorentzian line broad-ening factor of -30 Hz followed by a Gaussian line broadeningfactor of +30 Hz. This combination of line broadening factorsresulted in greater resolution of the carbon types in thefrequency domain spectra without severely affecting theoverall line width of the aliphatic carbon region or signal-to-noise ratio.

The areas for the different carbon types in the aliphaticregion of the CP/MAS spectra were determined using curve-fitting software developed by Chemagnetics. The carbon typesand their chemical shift positions were assigned based onliquid-state NMR data for asphalts and similar materials.19-23

The line widths at half-height for the different carbon typeswere determined from a critical analysis of 16 dipolar-dephasing asphalt spectra with dephasing times varying from1 to 160 µs. As the dephasing time increases, various carbontypes disappear, thus permitting an evaluation of the linewidths for the remaining carbons.16,24 The line widths andchemical shift positions, once determined, were fixed and used

to curve-fit all the dipolar-dephased spectra for all asphalts.Only the peak intensities of all carbon types in all spectra werevaried until a match of the observed spectral data wasobtained. A 50:50 mix of a Gaussian and Lorentzian lineshapes was used to fit the carbon peaks. Table 3 lists thecarbon-type assignments, 13C chemical shift values, and theline widths used in deconvoluting the CP/MAS and DD spectra.

Differential Scanning Calorimetry (DSC). Two experi-mental procedures were used to determine the crystalline waxcontent in asphalt. These methods differ in the annealing timebefore cooling and the rate of heating of the samples. Thetime-temperature profiles for procedures 1 and 2 are shownin Figure 1. A TA Instruments model 2920 modulated DSCwas used for determining the crystalline wax content byprocedure 1. The manufacturer’s recommended procedureswere followed for temperature and enthalpy calibrations. Fortemperature calibration, the melting point of indium (156.6°C), water (0.01 °C), and n-octane (-56.76 °C) were used, andfor enthalpy calibration, the enthalpy of fusion of indium (28.57J/g) was used. A Mettler TA 2000B DSC was used to determinethe crystalline wax content by procedure 2. The calibration ofthis instrument and quantitative determination of the crystal-line wax content are described by Claudy et al.6 and theliterature cited therein.

Procedure 1. Approximately 15 mg, weighted accurately, ofthe asphalt sample was spread evenly across the bottom of ahermetic aluminum DSC pan. Heat was applied via a heatlamp to promote uniform spreading of the sample. The samplewas then heated to an annealing temperature of 150 °C andheld there for 15 min before commencing the experiment. Afterthe sample was annealed, it was cooled at a rate of 10 °C/minto below -60 °C, held at this temperature for 15 min, and thenheated at a rate of 10 °C/min to the annealing temperature.The endotherm peak observed on heating is due to meltingand dissolution of the crystallites into the asphalt matrix. Theaverage heat of fusion (enthalpy) used for asphalt waxes was180 J/g.

Procedure 2. The pan containing an asphalt sample (be-tween 30 and 40 mg) was conditioned at room temperaturefor 24 h and then cooled at a rate of 10 °C/min to -100 °C.After reaching this temperature, the sample was heated at arate of 5 °C/min to 100 °C. The amount of crystallized fractionwas determined using a quantitative method reported byClaudy et al.25 These authors also used an average enthalpy

(19) Hagen, A. P.; Johnson, M. P.; Randolph, B. B. Fuel Sci. Technol.Int. 1989, 7 (9), 1289.

(20) Netzel, D. A.; McKay, D. R.; Heppner, R. A.; Guffey, F. D.;Cooke, S. D.; Varie, D. L.; Linn, D. E. Fuel 1981, 60, 307.

(21) Alemany, L. B. Magn. Reson. Chem. 1989, 27, 1065.(22) Strothers, J. B. Carbon-13 NMR Spectroscopy. Organic Chem-

istry, A Series of Monographs; Academic Press: New York, 1972; Vol.24.

(23) Johnson, L. F.; Jankowski, W. C. Carbon-13 NMR Spectra, ACollection of Assigned, Coded, and Indexed Spectra; Robert F. KriegerPublishing Company: Huntington, NY, 1978.

(24) Netzel, D. A.; Miknis, F. P.; Soule, J. L.; Taylor, A. E.; Serres,M. L. Handbook of Asphalt Binder Technology; Youtcheff, J., Ed.;Marcel Dekker: New York, in press.

Table 2. SARA and SAPA Composition Analysis and Crystalline Wax Content of Asphalts

crystalline wax (mass %)saturates(wt %)

aromatics(wt %)

resins(wt %)

asphaltenes(wt %) DSC procedure 1 DSC procedure 2

Asphalt (Source) Group 1a

A (Venezuela) 12.1 54.3 17.6 16.0 0.2 0.6B (Middle East) 9.7 58.1 17.9 14.4 1.9 4.9C (Italy) 12.2 58.0 16.3 13.4 2.6 7.5D (Africa) 20.9 52.3 22.7 4.1 3.8 9.8E (North Africa) 24.2 53.9 10.9 11.1 5.6 12.5

Asphalt (Source) Group 2b

AAA-1 (Lloydminster) 6.0 69.1 13.4 11.5 0.4c 0.4AAB-1 (Wyoming Sour) 7.2 64.0 15.1 13.7 2.5c 4.6AAC-1 (Redwater) 9.7 67.6 10.6 12.1 3.0c 4.9AAD-1 (California Coastal) 4.4 61.9 18.7 15.0 0.7c 1.6AAF-1 (West Texas Sour) 5.9 70.8 14.0 9.3 1.9c 3.7AAG-1 (California Valley) 4.6 70.5 21.6 3.3 0.0c 0.2AAK-1 (Boscan) 3.6 61.5 22.9 12.0 0.4c 1.2AAM-1 (West Texas Intermediate) 6.6 67.4 23.3 2.7 3.2c 5.3

a Chemical class data from Elf-Antar France. b Claudy et al.6 c Reference 28 and assuming 180 J/g for the average heat of fusion.

604 Energy & Fuels, Vol. 13, No. 3, 1999 Michon et al.

value of 180 J/g for the melting and/or dissolution of thecrystalline wax.

Results and Discussion

The 13C CP/MAS spectra at -45 °C for the aliphaticcarbons in the group 1 asphalts are shown in Figure 2.The 13C dipolar-dephasing spectra at -45 °C with acontact time of 1 ms and a dephasing time (τ) of 1 µsfor three of the group 2 (SHRP) asphalts (AAA-1, AAB-1, and AAM-1) are shown in Figure 3. The spectra wereobtained at -45 °C to increase the signal-to-noise ratiovia an increase in the C-H cross-polarization ef-ficiency.24 This improvement in efficiency is the resultof the reduction in molecular segmental and rotationalmotion of the aliphatic carbons at the low temperature.The dipolar-dephasing (DD) spectra at a dipolar-dephasing time of 1 s were assumed to be essentiallyidentical to their unrecorded CP spectra (τ ) 0) at -45°C. Thus, a comparison can be made of NMR spectralproperties measured for the group 1 asphalts using theCP/MAS technique with the spectral properties for thegroup 2 asphalts using the DD technique 3 years earlier.

The 13C solid-state spectra of asphalts shown inFigures 2 and 3 exhibit reasonably well-defined reso-nance regions for the different aliphatic carbon types.The peak centered at 14 ppm is due mainly to the carbon

resonance for the terminal CH3 group in long-chainn-alkanes. The broad resonance between 15 and 27 ppmis due to branched alkane CH3 groups, CH3 groupsattached to mono- and diaromatic rings, geminal methylgroups, and the methylene carbons in branched andnormal alkanes. The carbon resonances between 27 and30 ppm are associated mainly with methylene carbonsof cycloalkanes and the methine carbon associated withthe geminal methyl groups. The internal methylenecarbons of n-alkanes are present in the region of 30-32 ppm. The carbon resonances between 33 and 60 ppmare assigned to methine and methylene carbon typesassociated with the many different organic compoundsin the asphalts.

Methylene Carbon Content in the Crystallineand Amorphous Phases. The spectra of asphaltsshown in Figures 2 and 3 show two well-defined internalmethylene carbons at 31.8 and 30.1 ppm. The peaks areassigned to the internal methylene carbons of long-chainn-alkanes in the crystalline and amorphous phases,respectively.17

The CP/MAS and DD spectra were deconvoluted intopeaks corresponding to the major aliphatic carbon typesknown to be present in all asphalts.24 As an example,the deconvoluted CP/MAS spectrum of asphalt E at -45°C is shown in Figure 4. The solid points are the actualobserved data, and the line through the data points isthe summation of the area of the deconvoluted peaksat any given chemical shift value. For a compound of

(25) Claudy, P.; Letoffe, J. M.; King, G. N.; Planche, J. P.; Brule, B.Fuel Sci. Technol. Int. 1991, 9 (11), 71.

Table 3. 13C Chemical Shifts and Typical Line Width Values Used in the Deconvolution of the Aliphatic Carbon Typesin Asphalts

peak no. chemical shift (ppm) line width (Hz)a typical carbon typeb

1 11.56 39.3

2 13.99 53.2

3 18.40 106.8

4 20.06 33.1

5 22.30 113.7

6 25.75 39.3

7 27.81 84.7

8 30.16 82.7

9 31.76 24.8

10 32.84 70.3

11 35.21 66.1

12 38.02 117.8

13c 42.43 157.114c 48.05 200.5

a Line width at half-height based on the analyses of dipolar dephasing data from 1-160 µs dephasing time. 50% Gaussian and 50%Lorentzian line shape factors were applied in fitting the resonance peaks. b Carbon types listed are for illustrative purposes only. Othercarbon types within the chemical shift range of (1 ppm are possible. c Peaks 13 and 14 arbitrarily assigned to fill the region between40-60 ppm.

Amorphous and Crystalline Phases in Asphalts Energy & Fuels, Vol. 13, No. 3, 1999 605

known structure, the area under any given peak isproportional to the number of carbon types present inthe molecule. The carbon-type assignments of the peaksshown in Figure 4 are given in Table 3. In most cases,more than one carbon type can be assigned to a givenpeak position in the NMR spectrum of an asphaltbecause of the many different molecules present havingcarbons with similar chemical shift values. The dispari-ties in areas between peaks associated with carbonswithin a group are attributed to the compositionalheterogeneity within the peak region. In addition, thearea measurement of an individual peak within a broadpoorly resolved region may not accurately represent thenumber of carbons associated with that peak. This isbecause line widths and chemical shift values fordeconvoluting the several peaks within a region can bealtered significantly for each individual peak areawithout affecting the total area for all the peaks withina region. Because line widths and chemical shift valueswere held nearly constant with only the peak heightchanging, in deconvoluting all the NMR spectra, it isassumed that the relative areas for the different carbontypes is a measure of the relative amounts of carbontypes in the various asphalts.

As shown in Figure 4, the crystalline methylenecarbon resonance at 31.8 ppm has a narrower line widthrelative to the line widths for other carbon types. Thenarrow width is the result of the crystalline methylenecarbons having conformationally ordered n-alkyl chains,

whereas the broad line width observed for the amor-phous methylene carbon type at 30.1 ppm is mainly theresult of conformational heterogeneity of the n-alkylchains.

The NMR structural parameters derived from thespectra for the asphalts are given in Table 4. The carbonaromaticity and fraction of aliphatic carbon values wereobtained from the integration of the total CP/MAS andDD spectra at -45 °C. At -45 °C, approximately 85%of amorphous methylene carbons are in the rigid state.9

The fractions of the crystalline and rigid-amorphousmethylene carbons relative to the total aliphatic carboncontent at -45 °C were obtained from the peak areasof the deconvoluted spectrum for each asphalt. The masspercent of crystalline and rigid-amorphous methylenecarbons were calculated from the percent elementalcarbon (% C), fraction of aliphatic carbons (fali), and thefraction of crystalline (fC) and rigid-amorphous meth-

Figure 1. DSC time-temperature profile: (a) procedure 1,(b) procedure 2.

Figure 2. 13C CP/MAS NMR spectra of group 1 asphalts at-45 °C.

606 Energy & Fuels, Vol. 13, No. 3, 1999 Michon et al.

ylene carbons (fA) using eqs 1 and 2, respectively.

The mass percents of internal methylene carbons inthe crystalline and rigid-amorphous phases are givenin Table 4. There is a direct relationship between theweight percent of saturates in the asphalts (Table 2)

and the sum of the mass percent of the rigid-amor-phous and crystalline methylene carbons. It is reason-able to expect that as the saturate content composedmostly of normal (paraffins) and branched (isoparaffins)alkanes in the asphalts increases, the amount of para-ffinic hydrocarbons also increases. Thus, asphalts hav-ing high saturate contents have a greater tendency tocrystallize. With the exclusion of asphalt AAM-1, thecorrelation coefficient and standard deviation for therelationship are 0.99 and 1.54, respectively. It is then-alkanes that can easily crystallize. The crystallineinternal methylene carbon content listed in Table 4varies by a factor 10 for the different asphalts. Exceptfor asphalt AAM-1, the rigid-amorphous internal me-thylene carbon content at -45 °C is nearly the samefor all asphalts with an average value of 11.2 ( 1.4 masspercent.

Crystalline Wax Content. The DSC thermogramsfor eight asphalts using procedure 1 are shown in Figure5. The DSC thermograms for the group 2 asphalts usingexperimental procedure 2 have been reported by Claudyet al.6 The crystalline wax content determined for theasphalts using DSC procedures 1 and 2 are listed inTable 2. The crystalline-phase content can vary depend-ing upon the annealing process, rates of heating andcooling, and average value used for the heat of fusionfor n-alkanes. The discrepancy between the data for thetwo DSC procedures is, in part, due to the annealingprocedures and heating rates. However, to a greaterextent, it is due to the method used to determine thecrystalline wax content from the thermograms. Figure6 shows the thermograms obtained for asphalt AAM-1using procedures 1 and 2. There are significant differ-ences in the thermograms. The thermogram usingprocedure 1 (Figure 6a) shows a broad envelope for themelting endotherm, whereas, the thermogram usingprocedure 2 (Figure 6b) shows two distinct meltingendotherms. This difference is essentially due to thedifferent annealing times and partly due to the twodifferent heating rates used in the two procedures. Inaddition, the glass-transition temperature (Tg) alsovaries. Procedures 1 and 2 give Tg values of -22.0 and-29.2 °C, respectively. The shaded area centered at ∼0°C in Figure 6a has been shown, by modulated DSC, tobe an exotherm due to cold crystallization.15 That is,some n-alkane paraffinic material when frozen in theamorphous state during the cooling cycle and, subse-quently, during the heating cycle crystallizes when thetemperature exceeds the glass-transition temperature.When the heating temperature reaches the Tg region,the methylene carbon segments of the n-alkane mol-ecules have sufficient mobility to rearrange to an all-trans conformation, which is a necessary condition forthe onset of crystallization.16 The crystallites associatedwith the cold crystallization exotherm melt at a highertemperature and are included in the area of the melting-dissolution endotherm used to calculate the crystallinewax content.

The endotherm area is directly proportional to thecrystalline wax content. The area was calculated forprocedure 1 (Figure 6a) using a base line based upon anonlinear regression fit to a cumulative Gaussian shapeof the data from -70 to -10 °C and from +70 to 100°C. The endotherm area for the melting-dissolution of

Figure 3. 13C dipolar dephasing NMR spectra of group 2asphalts at -45 °C. Dephasing time 1 µs.

Figure 4. Deconvoluted 13C CP/MAS NMR spectrum ofasphalt E at -45 °C.

mass % CH2-crystalline ) % C × fali × fC (1)

mass % CH2-amorphous ) % C × fali × fA (2)

Amorphous and Crystalline Phases in Asphalts Energy & Fuels, Vol. 13, No. 3, 1999 607

the crystallites used in calculating the crystalline waxcontent of asphalts via DSC experimental procedure 2,as reported in the literature,6 is shown in Figure 6b.For procedure 2, the endotherm area was calculatedfrom a base line drawn from +70 to -10 °C. In effect,procedure 2 increases the area of the melting endothermand, thus, the wax content and minimizes the coldcrystallization exotherm.

Because the amorphous methylene carbon content isnearly constant among the various asphalts studied, thecrystalline carbon content determined from NMR datamay be directly related to the crystalline wax contentin asphalts determined from DSC data. Figure 7 showsthe plot of the mass percent of crystalline methylenecarbons determined from NMR data versus the crystal-line wax content determined from DSC using procedure2 for the asphalts in group 1 and some asphalts in group2. As shown, a good correlation exists but it is nonlinear.A second-order polynomial was used to fit the data andgave an r2 of 0.996 with a y-intercept of 0.589. The data

for asphalt AAM-1 was not used in the least-squaresregression analysis. Figure 7 also shows the relationshipbetween the mass percent of crystalline methylenecarbons from NMR data and the mass percent ofcrystalline wax from DSC procedure 1 data. The bestfit of the data is a straight line with a slope of 1.000, anr2 of 0.995, and a y-intercept of 0.228. The 1 to 1correlation suggests that DSC procedure 1 may be thepreferred method to correlate with NMR data. However,the 1 to 1 correlation is, in this case, somewhatfortuitous because the crystalline wax content by DSCdepends on several factors, one factor being the averageheat of fusion used for the wax. A typical average valuefor waxes in asphalts is 180 J/g (used here and byClaudy et al.6), but other values have been used.14 The

Table 4. 13C NMR Structural Parameters for Asphalts at -45 °C

asphaltcarbon

aromaticity, fa

aliphatic carbonfraction, fali

crystallinea

methylene carbonfraction, fC

rigid-amorphousb

methylene carbonfraction, fA

crystallinemethylene carboncontent, mass %

rigid-amorphousmethylene carboncontent, mass %

Ac 0.294 0.706 0.0075 0.180 0.44 10.9Bc 0.279 0.721 0.0330 0.208 2.00 12.6Cc 0.244 0.756 0.0468 0.192 3.02 12.4Dc 0.222 0.778 0.0597 0.178 4.00 11.9Ec 0.267 0.733 0.0930 0.170 5.89 10.7AAA-1d 0.256 0.744 0.0139 0.166 0.86 10.3AAB-1d 0.295 0.705 0.0380 0.171 2.19 9.8AAM-1d 0.252 0.748 0.0563 0.257 3.66 16.9

a Ratio of the crystalline methylene carbon area to the total aliphatic carbon area. b Ratio of amorphous methylene carbon area to thetotal aliphatic carbon area. c NMR data from 13C CP/MAS spectrum at -45 °C, contact time 1 ms. d NMR data from 13C DD spectrum at-45 °C, contact time 1 ms, dephasing time 1 µs.

Figure 5. DSC thermograms for group 1 and three group 2asphalts using procedure 1.

Figure 6. DSC thermogram for asphalt AAM-1 showing endo-and exotherm areas and base line measurements usingprocedures 1 and 2.

608 Energy & Fuels, Vol. 13, No. 3, 1999 Michon et al.

heat of fusion depends on the characteristics of the chainrepeating units,26 that is, the disorder of the crystalstructure due to methyl substitution and/or the degreeof gauche conformation. The higher the extent of methylsubstitution and/or gauche conformation of the meth-ylene carbons in the carbon chain length, the moreamorphous the wax. A totally amorphous wax has noheat of fusion.

The residual NMR mass percents of crystalline me-thylene carbons (y-axis intercepts of 0.228 and 0.589 forplots of the two DSC procedures) indicate the presenceof carbon types not associated with the crystallinemethylene carbons but have nearly the same chemicalshift. These carbons are the C-3 carbon (∼32.4 ppm) inn-alkanes and the â-CH2 carbon (∼32.3) in long-chainalkyl substituents on an aromatic ring, among others.

Low-Temperature Cracking of Asphalts. Low-temperature (thermal) cracking is one of the primaryasphalt pavement failure modes observed in cold cli-mates. The cracking is a result of increased tensilestresses due to a decrease in specific volume induced inasphalt as the temperature decreases. Low-temperaturecracking (transverse fractures) of the pavement occurswhen these stresses exceed the binder strength of theasphalt. As the temperature decreases, the molecularmotions of the components in asphalt decreases. Theasphalt properties change from a viscoelastic materialto a brittle, rigid-amorphous/crystalline solid. Thechemical components and their structure in asphaltsthat are primarily responsible for cracking are notknown.

The tendency of asphalt pavement to crack in a coldenvironment has been attributed in part, to its crystal-line wax content.6 More recently, Netzel et al.9 reporteda linear correlation exists (Figure 8) between the masspercent of all mobile-amorphous aliphatic carbons inasphalts and their fracture temperature.27 As shown inFigure 8, the greater the amount of mobile-amorphouscarbons in an asphalt measured at 23 °C, the lower itsfracture temperature. Asphalt AAK-1, being a relatively

hard asphalt as deduced from its penetration data,18 hasan unrealistically high mobile phase. This asphaltcontains inordinate amounts of paramagnetic vanadiumwhich effected the dipolar-dephasing relaxation datafrom which the mass percent of mobile-amorphousaliphatic carbons was calculated.9

If crystallinity is a factor in low-temperature crackingof asphalts, then a correlation should also exist betweenthe mass percent of crystalline methylene carbons andthe fracture temperature. That is, the more carbons inthe crystalline phase, the higher the fracture temper-ature. Figure 9 shows a plot of the mass percent ofcrystalline methylene carbons determined directly fromNMR data versus DSC data for the SHRP core asphalts(group 2)28 calculated using the linear expression for thecorrelation shown in Figure 7. It is apparent in Figure9 that a simple correlation between the crystallinecarbon content and fracture temperature for all of theasphalts does not exist. Closer examination of the datashows that separate linear correlations exist for as-

(26) Mandelkern, L. In Physical Properties of Polymers, 2nd ed.;Mark, J. E., Eisenberg, A., Graessley, W. W., Mandelkern, L. M.,Samulski, E. T., Koenig, J. L., Wignall, G. D., Eds.; ACS ProfessionalReference Book; American Chemical Society: Washington, DC, 1993.

(27) Jung, D.; Vinson, T. S. Transp. Res. Rec. 1993, 1417, 12.

(28) Western Research Institute Final Report to FHWA (ContractNo. DTFH61-92C-00170) Fundamental Properties of Asphalts andModified Asphalts, 1998; Vol. 1, p 195.

Figure 7. Plots of the NMR crystalline internal methylenecarbons versus the crystalline wax content measured by DSCprocedure 1 (9) and procedure 2 (b). (O) asphalt AAM-1 byDSC procedure 2 not used in nonlinear regression fit.

Figure 8. Plot of the mobile-amorphous aliphatic carbons(ref 9) versus the fracture temperature (ref 27) for group 2asphalts. (Asphalt AAK-1 not used in the linear least-squarefitting of the data.)

Figure 9. Plot of the NMR crystalline methylene carbonsversus the fracture temperature (ref 27) for group 2 asphalts.(b) Measured directly from NMR data; (9) conversion of DSCdata (ref 28) using Figure 7.

Amorphous and Crystalline Phases in Asphalts Energy & Fuels, Vol. 13, No. 3, 1999 609

phalts containing less than 1 mass percent of crystallinemethylene carbons (AAA-1, AAD-1, AAG-1, and AAK-1) and for those asphalts containing approximately 1or more mass percent of crystalline methylene carbons(AAA-1, AAB-1, AAC-1, and AAM-1).

Asphalts containing less than 1 mass percent ofcrystalline methylene carbons can have fracture tem-peratures ranging from -16 to -32 °C. That is, thefracture temperature is controlled by factors other thancrystallinity of the paraffins. The controlling factor forthree of the four asphalts is apparently the initialamount of mobile-amorphous aliphatic carbons at 23°C (see Figure 8). Thus, the reason that asphalt AAG-1has a higher fracture temperature (-15.8 °C) thanasphalt AAA-1 (-30.3 °C), even though it contains lesscrystalline methylene carbons, is because it has a loweramount of mobile-amorphous aliphatic carbons.

For asphalts containing ∼1% or more of crystallinemethylene carbons (AAA-1, AAB-1, AAC-1, and AAM-1), the fracture temperature of an asphalt depends notonly on its initial mobile-amorphous carbons but alsoon its crystalline carbon content (Figure 9). Thus, asthe crystalline content of an asphalt increases, so doesits fracture temperature.

Asphalt AAF-1 lies outside the range for eithercorrelation; this may be because this asphalt has themost aromatic carbons of all asphalts studied.29 Theamount of aromatic carbons and polar functional groupsshould also have some influence on the fracture tem-perature of asphalts. However, the influence of polar-aromatic compounds on low-temperature cracking hasnot yet been investigated.

Conclusions

The crystalline and amorphous phases in asphaltswith wax contents extending over a large range were

investigated using low-temperature solid-state 13C NMRand DSC techniques. Although the crystalline methyl-ene carbon content varied for the different asphalts, theamount of methylene carbons in the amorphous phaseremains nearly the same with the exception of SHRPasphalt AAM-1.

For the asphalts studied, no single correlation couldbe found between the crystalline methylene carboncontent and the fracture temperature of the asphalts.The fracture temperature for asphalts containing lessthan 1% crystalline methylene carbon depends mainlyupon the amount of mobile-amorphous methylenecarbons. Crystalline methylene carbons influence thefracture temperature if concentrations are greater than1%.

Disclaimer

This document is disseminated under the sponsorshipof the Department of Transportation in the interest ofinformation exchange. The U.S. Government assumesno liability for its contents or use thereof.

The contents of this report reflect the views ofWestern Research Institute and Elf-Antar France whichare responsible for the facts and the accuracy of the datapresented herein. The contents do not necessarily reflectthe official views of the policy of the Department ofTransportation.

Mention of specific brand names or models of equip-ment is for information only and does not imply en-dorsement of any particular brand to the exclusion ofothers that may be suitable.

Acknowledgment. The authors acknowledge JerryForney for performing the DSC experiments and theFederal Highway Administration under Contract No.DTFH61-92C-00170 for providing financial supportrelated to the SHRP asphalt studies. Laurent C. Michonacknowledges Elf-Antar France for his financial support.

EF980184R

(29) Jennings, P. W. SHRP-A-335, Binder Characterization andEvaluation by Nuclear Magnetic Resonance Spectroscopy; StrategicHighway Research Program; National Research Council: Washington,DC, 1993.

610 Energy & Fuels, Vol. 13, No. 3, 1999 Michon et al.