A 13C NMR and DSC Study of the Amorphous and Crystalline Phases

The amorphous and crystalline phases in asphalt have been identified and studied using low- temperature solid-state carbon-13 CP/MAS NMR and DSC ...
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Energy & Fuels 1999, 13, 602-610

A

13C

NMR and DSC Study of the Amorphous and Crystalline 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 lowtemperature solid-state carbon-13 CP/MAS NMR and DSC techniques. The NMR mass percent of the crystalline methylene carbons was shown to correlate linearly with the mass percent of crystalline wax in asphalts measured using DSC. While the internal methylene carbon content of long-chain alkanes in the crystalline phase in the asphalts varied, the internal methylene carbon 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 cracking parameter, the fracture temperature of an asphalt. It was found that 1% or less of aliphatic carbons 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 in the 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 properties of asphalts are of interest because low-temperature cracking is one of the primary modes of failure for asphalt pavements. At the molecular level, this type of failure mode has been attributed, in part, to crystalline waxes.The first evidence of a crystalline phase in asphalts was reported in 1966 by Smith et al.1 using infrared spectroscopy. These authors found that the 720 cm-1 band for amorphous methylene carbons in longchain hydrocarbons split into a doublet in waxy asphalts. They attributed this band splitting to the formation of crystalline wax in the asphalt. Noel and Corbett2 studied the crystalline phase in a variety of asphalts and showed that asphalts are largely amorphous and that the crystallizable components are largely found in the saturate fraction (alkanes) of an asphalt with lesser amounts in the naphthene-aromatic fraction. They also reported that the crystallizable material measured directly by differential scanning calorimetry (DSC) and the wax obtained by precipitation were not identical. The precipitated wax was found to be a mixture of hydrocarbons in the amorphous and crystalline phases. Daly et al.3 have conducted an extensive study on the crystallization process in asphalts. They reported that the crystallization process is very time dependent, and several annealing steps are required in (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.

order to study the phase transition thermodynamically. In addition, they reported that the crystalline components in asphalt exhibit distinct endothermic patterns in a DSC thermogram, and these patterns depend on the chemical structure of the crystalline components and their interaction with the amorphous phase. The influence of crystalline and amorphous phases on the rheological properties of asphalts has recently been reported.4-10 A high crystalline wax content in asphalts can reduce ductility, increase brittleness at low temperature, and cause deterioration of adhesion to aggregates.5 Claudy et al.6 speculated that the presence of a crystalline phase in asphalts could be an important factor in determining the tendencies of asphalt pavements to crack in a cold environment. McKay et al.4 found that macrocrystalline waxes11 (n-alkanes having a carbon number range from C18 to C40) in asphalts cause viscosity increases, whereas the microcrystalline (4) McKay, J. F.; Branthaver, J. F.; Robertson, R. E. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1995, 40 (4), 794. (5) Gawel, I.; Czechowski, F.; Baginska, K. Proceeding of the Eurasphalt & 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.; Marcel Dekker: 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.

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

Amorphous and Crystalline Phases in Asphalts

Energy & Fuels, Vol. 13, No. 3, 1999 603

Table 1. Elemental Composition and Molecular Weight of Asphalts carbon (wt %)

hydrogen (wt %)

nitrogen (wt %)

oxygen (wt %)

sulfur (wt %)

molecular weight,a Mn

A (Venezuela) B (Middle East) C (Italy) D (Africa) E (North Africa)

85.1 84.1 84.9 86.0 86.8

Asphalt (Source) Group 1b 10.0 0.54 10.1 0.43 10.2 0.46 11.2 0.57 10.8 0.58

ND ND ND ND ND

4.90 4.95 ND 2.15 1.84

880 1000 910 890 860

AAA-1 (Lloydminster) AAB-1 (Wyoming Sour) AAC-1 (Redwater) AAD-1 (California Coastal) AAF-1 (West Texas) AAG-1 (California Valley) AAK-1 (Boscan) AAM-1 (West Texas Intermediate)

83.9 82.3 86.5 81.6 84.5 85.6 83.7 86.8

Asphalt (Source) Group 2c 10.0 0.50 10.6 0.54 11.3 0.66 10.8 0.77 10.4 0.55 10.5 1.10 10.2 0.70 11.2 0.55

0.6 0.8 0.9 0.9 1.1 1.1 0.8 0.5

5.50 4.70 1.90 6.90 3.40 1.30 6.40 1.20

790 840 870 700 840 710 860 1300

a

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

waxes11 (branched alkanes with a carbon number range from C25 to C65) cause decreases in viscosity. Bahia and Anderson10,12 reported an important phenomenon in asphalts that they defined as low-temperature physical hardening. They observed a gradual change in density and mechanical stiffness with time when asphalts are held at low temperatures. Even though these authors have shown that there is a definite relationship between low-temperature physical hardening and the wax content of asphalt, they attribute the hardening to factors other than the morphology of the wax. However, Claudy et al.6 showed that the amount of physical hardening that occurs with time can be related to the number of molecules in the asphalt which coalesce to form microscopic crystalline or amorphous domains. These authors also suggest that asphalt should no longer be thought of as homogeneous in density but instead as a complex, two-phase structure more akin to a gel with enhanced viscoelastic properties. Another phenomenon observed for asphalts but occurring at ambient temperature is isothermal steric hardening.13 That is, molecular restructuring of the asphalt over a long period of time. Netzel et al.8 reported that the phenomenon of steric hardening in asphalts may also be related to the change in the amount of the crystalline wax fraction with time. These authors have shown that the formation of the crystalline waxes in asphalt at room temperature as measured using NMR continues for many months. In another study, Netzel et al.9 reported that the amount of mobile aliphatic carbons, those in the amorphous phase, can be qualitatively related to many of the rheological and performance properties of an asphalt. Generally, standard DSC measurements are used to determine the crystalline phase in asphalts.2,3,6,7,14 Recently, the technique of modulated DSC has been used to aid the determination of the crystalline phase.15 Solid-state NMR techniques have been widely used to determine the amount of amorphous and crystalline phases in polymers. However, these techniques have (12) Bahia, H. U.; Anderson, D. A. Association of Asphalt Paving Technologists: 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.

been applied only recently to study the amorphous and crystalline phases in asphalts.9,16 Netzel et al.17 have reported that the 13C NMR spectrum of the internal methylene carbons of long-chain n-alkanes show two resonances. The resonance at ∼32 ppm was assigned to the crystalline (all-trans conformation) methylene carbons, and the observed resonance at ∼30 ppm was assigned to the internal methylene carbons in the amorphous phase (gauche conformation). Because the crystalline resonance peak is associated with n-alkanes, this peak represents the amount of macrocrystalline wax in asphalts. Microcrystalline waxes (branched alkanes) have resonances at different chemical shift positions and would not easily be observed in a broad solid-state 13C spectrum of an asphalt. Thus, the crystalline internal methylene carbon content measured by NMR should correspond to the crystalline wax fraction as measured by DSC if the crystalline wax fraction is mainly macrocrystalline. In this paper, the crystalline internal methylene carbon content of eight asphalts, as measured by solidstate 13C NMR techniques, is compared to the crystalline wax fraction, as measured using different DSC procedures. In addition, the crystalline methylene carbon contents of the asphalts are plotted versus their respective fracture temperatures (a parameter to assess low-temperature cracking) in an effort to determine the effect of crystalline phase on the thermal cracking of asphalts. Experimental Section Asphalt Samples. The five asphalt samples in group 1, listed in Table 1, were provided by Elf-Antar France. These asphalts were selected based on their large range of crystalline wax content. Eight Strategic Highway Research Program (SHRP) core asphalts, comprising group 2, also listed in Table 1, were obtained from the Material Research Library (MRL).18 The elemental composition and molecular weight for these asphalts are given in Table 1, and the chemical class composition (based on SARA and SAPA analyses) and the crystalline wax content (DSC) are given in Table 2. (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, Washington, DC.

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Table 2. SARA and SAPA Composition Analysis and Crystalline Wax Content of Asphalts saturates (wt %) A (Venezuela) B (Middle East) C (Italy) D (Africa) E (North Africa) AAA-1 (Lloydminster) AAB-1 (Wyoming Sour) AAC-1 (Redwater) AAD-1 (California Coastal) AAF-1 (West Texas Sour) AAG-1 (California Valley) AAK-1 (Boscan) AAM-1 (West Texas Intermediate) a

aromatics (wt %)

resins (wt %)

asphaltenes (wt %)

crystalline wax (mass %) DSC procedure 1 DSC procedure 2

12.1 9.7 12.2 20.9 24.2

Asphalt (Source) Group 1a 54.3 17.6 58.1 17.9 58.0 16.3 52.3 22.7 53.9 10.9

16.0 14.4 13.4 4.1 11.1

0.2 1.9 2.6 3.8 5.6

0.6 4.9 7.5 9.8 12.5

6.0 7.2 9.7 4.4 5.9 4.6 3.6 6.6

Asphalt (Source) Group 2b 69.1 13.4 64.0 15.1 67.6 10.6 61.9 18.7 70.8 14.0 70.5 21.6 61.5 22.9 67.4 23.3

11.5 13.7 12.1 15.0 9.3 3.3 12.0 2.7

0.4c 2.5c 3.0c 0.7c 1.9c 0.0c 0.4c 3.2c

0.4 4.6 4.9 1.6 3.7 0.2 1.2 5.3

Chemical class data from Elf-Antar France. b Claudy et al.6

Nuclear Magnetic Resonance (NMR). A Chemagnetics 100/200 solid-state NMR spectrometer operating at a 13C frequency of 25 MHz was used for cross polarization with magic-angle spinning (CP/MAS) and dipolar-dephasing (DD) measurements at -45 °C. Asphalt samples were heated to a temperature between 100 and 170 °C and poured directly into a 7.5 mm zirconia pencil rotor assembly. These samples remained at room temperature in the rotor for 2 months to 1 year before conducting the low-temperature CP/MAS and DD experiments. Low temperatures were obtained using an electric refrigeration FTS systems XR series Air-Jet sample cooler. The cooled, ultradry air from the cooler was transferred to the NMR probe via an insulated 2.4-m transfer line. Parameters for both CP/MAS and DD included a pulse width of 5 µs (90°), a pulse delay of 1 s, a contact time of 1 ms, a sweep width of 16 kHz, a free induction decay size of 1024 points, 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 dipolardephasing time was varied from 1 to 160 µs. Chemical shifts were referenced internally to the terminal methyl carbons at 14 ppm of the long n-alkane chains. The NMR time domain spectra were transformed using first a Lorentzian line broadening factor of -30 Hz followed by a Gaussian line broadening factor of +30 Hz. This combination of line broadening factors resulted in greater resolution of the carbon types in the frequency domain spectra without severely affecting the overall line width of the aliphatic carbon region or signal-tonoise ratio. The areas for the different carbon types in the aliphatic region of the CP/MAS spectra were determined using curvefitting software developed by Chemagnetics. The carbon types and their chemical shift positions were assigned based on liquid-state NMR data for asphalts and similar materials.19-23 The line widths at half-height for the different carbon types were determined from a critical analysis of 16 dipolardephasing asphalt spectra with dephasing times varying from 1 to 160 µs. As the dephasing time increases, various carbon types disappear, thus permitting an evaluation of the line widths for the remaining carbons.16,24 The line widths and chemical shift positions, once determined, were fixed and used (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 Chemistry, A Series of Monographs; Academic Press: New York, 1972; Vol. 24. (23) Johnson, L. F.; Jankowski, W. C. Carbon-13 NMR Spectra, A Collection of Assigned, Coded, and Indexed Spectra; Robert F. Krieger Publishing Company: Huntington, NY, 1978.

c

Reference 28 and assuming 180 J/g for the average heat of fusion.

to curve-fit all the dipolar-dephased spectra for all asphalts. Only the peak intensities of all carbon types in all spectra were varied until a match of the observed spectral data was obtained. A 50:50 mix of a Gaussian and Lorentzian line shapes was used to fit the carbon peaks. Table 3 lists the carbon-type assignments, 13C chemical shift values, and the line widths used in deconvoluting the CP/MAS and DD spectra. Differential Scanning Calorimetry (DSC). Two experimental procedures were used to determine the crystalline wax content in asphalt. These methods differ in the annealing time before cooling and the rate of heating of the samples. The time-temperature profiles for procedures 1 and 2 are shown in Figure 1. A TA Instruments model 2920 modulated DSC was used for determining the crystalline wax content by procedure 1. The manufacturer’s recommended procedures were followed for temperature and enthalpy calibrations. For temperature calibration, the melting point of indium (156.6 °C), water (0.01 °C), and n-octane (-56.76 °C) were used, and for enthalpy calibration, the enthalpy of fusion of indium (28.57 J/g) was used. A Mettler TA 2000B DSC was used to determine the crystalline wax content by procedure 2. The calibration of this instrument and quantitative determination of the crystalline wax content are described by Claudy et al.6 and the literature cited therein. Procedure 1. Approximately 15 mg, weighted accurately, of the asphalt sample was spread evenly across the bottom of a hermetic aluminum DSC pan. Heat was applied via a heat lamp to promote uniform spreading of the sample. The sample was then heated to an annealing temperature of 150 °C and held there for 15 min before commencing the experiment. After the sample was annealed, it was cooled at a rate of 10 °C/min to below -60 °C, held at this temperature for 15 min, and then heated at a rate of 10 °C/min to the annealing temperature. The endotherm peak observed on heating is due to melting and dissolution of the crystallites into the asphalt matrix. The average heat of fusion (enthalpy) used for asphalt waxes was 180 J/g. Procedure 2. The pan containing an asphalt sample (between 30 and 40 mg) was conditioned at room temperature for 24 h and then cooled at a rate of 10 °C/min to -100 °C. After reaching this temperature, the sample was heated at a rate of 5 °C/min to 100 °C. The amount of crystallized fraction was determined using a quantitative method reported by Claudy et al.25 These authors also used an average enthalpy (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.

Amorphous and Crystalline Phases in Asphalts Table 3.

13C

Energy & Fuels, Vol. 13, No. 3, 1999 605

Chemical Shifts and Typical Line Width Values Used in the Deconvolution of the Aliphatic Carbon Types in Asphalts

peak no.

chemical shift (ppm)

line width (Hz)a

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 14c

42.43 48.05

157.1 200.5

typical carbon typeb

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. Other carbon types within the chemical shift range of (1 ppm are possible. c Peaks 13 and 14 arbitrarily assigned to fill the region between 40-60 ppm.

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

Results and Discussion The 13C CP/MAS spectra at -45 °C for the aliphatic carbons in the group 1 asphalts are shown in Figure 2. The 13C dipolar-dephasing spectra at -45 °C with a contact time of 1 ms and a dephasing time (τ) of 1 µs for three of the group 2 (SHRP) asphalts (AAA-1, AAB1, and AAM-1) are shown in Figure 3. The spectra were obtained at -45 °C to increase the signal-to-noise ratio via an increase in the C-H cross-polarization efficiency.24 This improvement in efficiency is the result of the reduction in molecular segmental and rotational motion of the aliphatic carbons at the low temperature. The dipolar-dephasing (DD) spectra at a dipolardephasing time of 1 s were assumed to be essentially identical to their unrecorded CP spectra (τ ) 0) at -45 °C. Thus, a comparison can be made of NMR spectral properties measured for the group 1 asphalts using the CP/MAS technique with the spectral properties for the group 2 asphalts using the DD technique 3 years earlier. The 13C solid-state spectra of asphalts shown in Figures 2 and 3 exhibit reasonably well-defined resonance regions for the different aliphatic carbon types. The peak centered at 14 ppm is due mainly to the carbon (25) Claudy, P.; Letoffe, J. M.; King, G. N.; Planche, J. P.; Brule, B. Fuel Sci. Technol. Int. 1991, 9 (11), 71.

resonance for the terminal CH3 group in long-chain n-alkanes. The broad resonance between 15 and 27 ppm is due to branched alkane CH3 groups, CH3 groups attached to mono- and diaromatic rings, geminal methyl groups, and the methylene carbons in branched and normal alkanes. The carbon resonances between 27 and 30 ppm are associated mainly with methylene carbons of cycloalkanes and the methine carbon associated with the geminal methyl groups. The internal methylene carbons of n-alkanes are present in the region of 3032 ppm. The carbon resonances between 33 and 60 ppm are assigned to methine and methylene carbon types associated with the many different organic compounds in the asphalts. Methylene Carbon Content in the Crystalline and Amorphous Phases. The spectra of asphalts shown in Figures 2 and 3 show two well-defined internal methylene carbons at 31.8 and 30.1 ppm. The peaks are assigned to the internal methylene carbons of long-chain n-alkanes in the crystalline and amorphous phases, respectively.17 The CP/MAS and DD spectra were deconvoluted into peaks corresponding to the major aliphatic carbon types known 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 actual observed data, and the line through the data points is the summation of the area of the deconvoluted peaks at any given chemical shift value. For a compound of

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

known structure, the area under any given peak is proportional to the number of carbon types present in the molecule. The carbon-type assignments of the peaks shown in Figure 4 are given in Table 3. In most cases, more than one carbon type can be assigned to a given peak position in the NMR spectrum of an asphalt because of the many different molecules present having carbons with similar chemical shift values. The disparities in areas between peaks associated with carbons within a group are attributed to the compositional heterogeneity within the peak region. In addition, the area measurement of an individual peak within a broad poorly resolved region may not accurately represent the number of carbons associated with that peak. This is because line widths and chemical shift values for deconvoluting the several peaks within a region can be altered significantly for each individual peak area without affecting the total area for all the peaks within a region. Because line widths and chemical shift values were held nearly constant with only the peak height changing, in deconvoluting all the NMR spectra, it is assumed that the relative areas for the different carbon types is a measure of the relative amounts of carbon types in the various asphalts. As shown in Figure 4, the crystalline methylene carbon resonance at 31.8 ppm has a narrower line width relative to the line widths for other carbon types. The narrow width is the result of the crystalline methylene carbons having conformationally ordered n-alkyl chains,

Figure 2. -45 °C.

13C

CP/MAS NMR spectra of group 1 asphalts at

whereas the broad line width observed for the amorphous methylene carbon type at 30.1 ppm is mainly the result of conformational heterogeneity of the n-alkyl chains. The NMR structural parameters derived from the spectra for the asphalts are given in Table 4. The carbon aromaticity and fraction of aliphatic carbon values were obtained from the integration of the total CP/MAS and DD 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-amorphous methylene carbons relative to the total aliphatic carbon content at -45 °C were obtained from the peak areas of the deconvoluted spectrum for each asphalt. The mass percent of crystalline and rigid-amorphous methylene carbons were calculated from the percent elemental carbon (% C), fraction of aliphatic carbons (fali), and the fraction of crystalline (fC) and rigid-amorphous meth-

Amorphous and Crystalline Phases in Asphalts

Energy & Fuels, Vol. 13, No. 3, 1999 607

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

Figure 4. Deconvoluted asphalt E at -45 °C.

13C

CP/MAS NMR spectrum of

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

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

(1)

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

(2)

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

and the sum of the mass percent of the rigid-amorphous and crystalline methylene carbons. It is reasonable to expect that as the saturate content composed mostly of normal (paraffins) and branched (isoparaffins) alkanes in the asphalts increases, the amount of paraffinic hydrocarbons also increases. Thus, asphalts having high saturate contents have a greater tendency to crystallize. With the exclusion of asphalt AAM-1, the correlation coefficient and standard deviation for the relationship are 0.99 and 1.54, respectively. It is the n-alkanes that can easily crystallize. The crystalline internal methylene carbon content listed in Table 4 varies by a factor 10 for the different asphalts. Except for asphalt AAM-1, the rigid-amorphous internal methylene carbon content at -45 °C is nearly the same for all asphalts with an average value of 11.2 ( 1.4 mass percent. Crystalline Wax Content. The DSC thermograms for eight asphalts using procedure 1 are shown in Figure 5. The DSC thermograms for the group 2 asphalts using experimental procedure 2 have been reported by Claudy et al.6 The crystalline wax content determined for the asphalts using DSC procedures 1 and 2 are listed in Table 2. The crystalline-phase content can vary depending upon the annealing process, rates of heating and cooling, and average value used for the heat of fusion for n-alkanes. The discrepancy between the data for the two DSC procedures is, in part, due to the annealing procedures and heating rates. However, to a greater extent, it is due to the method used to determine the crystalline wax content from the thermograms. Figure 6 shows the thermograms obtained for asphalt AAM-1 using procedures 1 and 2. There are significant differences in the thermograms. The thermogram using procedure 1 (Figure 6a) shows a broad envelope for the melting endotherm, whereas, the thermogram using procedure 2 (Figure 6b) shows two distinct melting endotherms. This difference is essentially due to the different annealing times and partly due to the two different heating rates used in the two procedures. In addition, the glass-transition temperature (Tg) also varies. 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, to be an exotherm due to cold crystallization.15 That is, some n-alkane paraffinic material when frozen in the amorphous state during the cooling cycle and, subsequently, during the heating cycle crystallizes when the temperature exceeds the glass-transition temperature. When the heating temperature reaches the Tg region, the methylene carbon segments of the n-alkane molecules have sufficient mobility to rearrange to an alltrans conformation, which is a necessary condition for the onset of crystallization.16 The crystallites associated with the cold crystallization exotherm melt at a higher temperature and are included in the area of the meltingdissolution endotherm used to calculate the crystalline wax content. The endotherm area is directly proportional to the crystalline wax content. The area was calculated for procedure 1 (Figure 6a) using a base line based upon a nonlinear regression fit to a cumulative Gaussian shape of the data from -70 to -10 °C and from +70 to 100 °C. The endotherm area for the melting-dissolution of

608 Energy & Fuels, Vol. 13, No. 3, 1999 Table 4.

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Michon et al.

NMR Structural Parameters for Asphalts at -45 °C

asphalt

carbon aromaticity, fa

aliphatic carbon fraction, fali

crystallinea methylene carbon fraction, fC

rigid-amorphousb methylene carbon fraction, fA

crystalline methylene carbon content, mass %

rigid-amorphous methylene carbon content, mass %

Ac Bc Cc Dc Ec AAA-1d AAB-1d AAM-1d

0.294 0.279 0.244 0.222 0.267 0.256 0.295 0.252

0.706 0.721 0.756 0.778 0.733 0.744 0.705 0.748

0.0075 0.0330 0.0468 0.0597 0.0930 0.0139 0.0380 0.0563

0.180 0.208 0.192 0.178 0.170 0.166 0.171 0.257

0.44 2.00 3.02 4.00 5.89 0.86 2.19 3.66

10.9 12.6 12.4 11.9 10.7 10.3 9.8 16.9

a Ratio of the crystalline methylene carbon area to the total aliphatic carbon area. b Ratio of amorphous methylene carbon area to the total 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 2 asphalts using procedure 1.

the crystallites used in calculating the crystalline wax content 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 calculated from a base line drawn from +70 to -10 °C. In effect, procedure 2 increases the area of the melting endotherm and, thus, the wax content and minimizes the cold crystallization exotherm. Because the amorphous methylene carbon content is nearly constant among the various asphalts studied, the crystalline carbon content determined from NMR data may be directly related to the crystalline wax content in asphalts determined from DSC data. Figure 7 shows the plot of the mass percent of crystalline methylene carbons determined from NMR data versus the crystalline wax content determined from DSC using procedure 2 for the asphalts in group 1 and some asphalts in group 2. As shown, a good correlation exists but it is nonlinear. A second-order polynomial was used to fit the data and gave an r2 of 0.996 with a y-intercept of 0.589. The data

Figure 6. DSC thermogram for asphalt AAM-1 showing endoand exotherm areas and base line measurements using procedures 1 and 2.

for asphalt AAM-1 was not used in the least-squares regression analysis. Figure 7 also shows the relationship between the mass percent of crystalline methylene carbons from NMR data and the mass percent of crystalline wax from DSC procedure 1 data. The best fit of the data is a straight line with a slope of 1.000, an r2 of 0.995, and a y-intercept of 0.228. The 1 to 1 correlation suggests that DSC procedure 1 may be the preferred method to correlate with NMR data. However, the 1 to 1 correlation is, in this case, somewhat fortuitous because the crystalline wax content by DSC depends on several factors, one factor being the average heat of fusion used for the wax. A typical average value for waxes in asphalts is 180 J/g (used here and by Claudy et al.6), but other values have been used.14 The

Amorphous and Crystalline Phases in Asphalts

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

heat of fusion depends on the characteristics of the chain repeating units,26 that is, the disorder of the crystal structure due to methyl substitution and/or the degree of gauche conformation. The higher the extent of methyl substitution and/or gauche conformation of the methylene carbons in the carbon chain length, the more amorphous the wax. A totally amorphous wax has no heat of fusion. The residual NMR mass percents of crystalline methylene carbons (y-axis intercepts of 0.228 and 0.589 for plots of the two DSC procedures) indicate the presence of carbon types not associated with the crystalline methylene carbons but have nearly the same chemical shift. These carbons are the C-3 carbon (∼32.4 ppm) in n-alkanes and the β-CH2 carbon (∼32.3) in long-chain alkyl substituents on an aromatic ring, among others. Low-Temperature Cracking of Asphalts. Lowtemperature (thermal) cracking is one of the primary asphalt pavement failure modes observed in cold climates. The cracking is a result of increased tensile stresses due to a decrease in specific volume induced in asphalt as the temperature decreases. Low-temperature cracking (transverse fractures) of the pavement occurs when these stresses exceed the binder strength of the asphalt. As the temperature decreases, the molecular motions of the components in asphalt decreases. The asphalt properties change from a viscoelastic material to a brittle, rigid-amorphous/crystalline solid. The chemical components and their structure in asphalts that are primarily responsible for cracking are not known. The tendency of asphalt pavement to crack in a cold environment has been attributed in part, to its crystalline wax content.6 More recently, Netzel et al.9 reported a linear correlation exists (Figure 8) between the mass percent of all mobile-amorphous aliphatic carbons in asphalts and their fracture temperature.27 As shown in Figure 8, the greater the amount of mobile-amorphous carbons in an asphalt measured at 23 °C, the lower its fracture temperature. Asphalt AAK-1, being a relatively (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 Professional Reference Book; American Chemical Society: Washington, DC, 1993. (27) Jung, D.; Vinson, T. S. Transp. Res. Rec. 1993, 1417, 12.

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

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

hard asphalt as deduced from its penetration data,18 has an unrealistically high mobile phase. This asphalt contains inordinate amounts of paramagnetic vanadium which effected the dipolar-dephasing relaxation data from which the mass percent of mobile-amorphous aliphatic carbons was calculated.9 If crystallinity is a factor in low-temperature cracking of asphalts, then a correlation should also exist between the mass percent of crystalline methylene carbons and the fracture temperature. That is, the more carbons in the crystalline phase, the higher the fracture temperature. Figure 9 shows a plot of the mass percent of crystalline methylene carbons determined directly from NMR data versus DSC data for the SHRP core asphalts (group 2)28 calculated using the linear expression for the correlation shown in Figure 7. It is apparent in Figure 9 that a simple correlation between the crystalline carbon content and fracture temperature for all of the asphalts does not exist. Closer examination of the data shows that separate linear correlations exist for as(28) Western Research Institute Final Report to FHWA (Contract No. DTFH61-92C-00170) Fundamental Properties of Asphalts and Modified Asphalts, 1998; Vol. 1, p 195.

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phalts containing less than 1 mass percent of crystalline methylene carbons (AAA-1, AAD-1, AAG-1, and AAK1) and for those asphalts containing approximately 1 or more mass percent of crystalline methylene carbons (AAA-1, AAB-1, AAC-1, and AAM-1). Asphalts containing less than 1 mass percent of crystalline methylene carbons can have fracture temperatures ranging from -16 to -32 °C. That is, the fracture temperature is controlled by factors other than crystallinity of the paraffins. The controlling factor for three of the four asphalts is apparently the initial amount of mobile-amorphous aliphatic carbons at 23 °C (see Figure 8). Thus, the reason that asphalt AAG-1 has a higher fracture temperature (-15.8 °C) than asphalt AAA-1 (-30.3 °C), even though it contains less crystalline methylene carbons, is because it has a lower amount of mobile-amorphous aliphatic carbons. For asphalts containing ∼1% or more of crystalline methylene carbons (AAA-1, AAB-1, AAC-1, and AAM1), the fracture temperature of an asphalt depends not only on its initial mobile-amorphous carbons but also on its crystalline carbon content (Figure 9). Thus, as the crystalline content of an asphalt increases, so does its fracture temperature. Asphalt AAF-1 lies outside the range for either correlation; this may be because this asphalt has the most aromatic carbons of all asphalts studied.29 The amount of aromatic carbons and polar functional groups should also have some influence on the fracture temperature of asphalts. However, the influence of polararomatic compounds on low-temperature cracking has not yet been investigated. Conclusions The crystalline and amorphous phases in asphalts with wax contents extending over a large range were (29) Jennings, P. W. SHRP-A-335, Binder Characterization and Evaluation by Nuclear Magnetic Resonance Spectroscopy; Strategic Highway Research Program; National Research Council: Washington, DC, 1993.

Michon et al.

investigated using low-temperature solid-state 13C NMR and DSC techniques. Although the crystalline methylene carbon content varied for the different asphalts, the amount of methylene carbons in the amorphous phase remains nearly the same with the exception of SHRP asphalt AAM-1. For the asphalts studied, no single correlation could be found between the crystalline methylene carbon content and the fracture temperature of the asphalts. The fracture temperature for asphalts containing less than 1% crystalline methylene carbon depends mainly upon the amount of mobile-amorphous methylene carbons. Crystalline methylene carbons influence the fracture temperature if concentrations are greater than 1%. Disclaimer This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for its contents or use thereof. The contents of this report reflect the views of Western Research Institute and Elf-Antar France which are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views of the policy of the Department of Transportation. Mention of specific brand names or models of equipment is for information only and does not imply endorsement of any particular brand to the exclusion of others that may be suitable. Acknowledgment. The authors acknowledge Jerry Forney for performing the DSC experiments and the Federal Highway Administration under Contract No. DTFH61-92C-00170 for providing financial support related to the SHRP asphalt studies. Laurent C. Michon acknowledges Elf-Antar France for his financial support. EF980184R