Development of Solid Properties and Thermochemistry of Asphalt

Energy & Fuels 1996, 10, 855-864

855

Development of Solid Properties and Thermochemistry of Asphalt Binders in the 25-65 °C Temperature Range David A. Storm,* Ronald J. Barresi, and Eric Y. Sheu Texaco Research and Development, P.O. Box 509, Beacon, New York 12508 Received December 20, 1995X

The effects of temperature and composition on the rheology of asphalt binders is of practical importance. The asphalt binder should be solid enough at higher temperatures to prevent the asphalt from flowing, and it should be fluid enough at lower temperatures to prevent the asphalt from cracking. In this work it is shown that there is a transition from a higher temperature Newtonian fluid phase to a lower temperature viscoelastic fluid phase at 45 °C in the vacuum residue of Arabian medium/heavy crude oil that is associated with the heptane insoluble asphaltenes. The asphaltene content must be greater than 15 wt % for this transition to occur. Solid properties increase by a factor of 3 beyond the phase boundary. The low-frequency dynamic viscosity increases by a factor of 5. The transition is first order with an enthalpy change on heating of 0.5 cal/g. The position of the phase boundary in a temperature-composition diagram can be predicted by calculating points at which the free volume of the asphaltenic colloidal particles in the Newtonian fluid state vanishes. According to small angle X-ray scattering the microstructure of the viscoelastic phase consists of irregularly shaped clusters with a characteristic length of 500 Å. There is a polydispersity of cluster sizes. The fractal dimension of the average cluster is 1.5. The clusters are themselves composed of smaller scattering centers with a characteristic length of 40 Å. The time-temperature superposition principle applies in each microphase, but the shift factors are discontinuous at the phase boundary. However, only the relaxation times and density of Maxwell elements must be rescaled by constant factors at the phase boundary.

Introduction Asphalt binders are usually the vacuum bottoms of asphaltic crude oils. They are mixed with aggregate (stones) to make road asphalt. The two vacuum residues studied in this and previous work, Arabian medium/ heavy and Ratawi, are typical asphalt binders, on the basis of NMR and HP-GPC analyses. 1 The rheological properties of asphalt binders are of great practical importance. The asphalt may flow at higher temperatures under the stress of heavy traffic, if the binder is not viscous enough; and it may crack at lower temperatures, if it is too brittle or if it shrinks too much when the temperature falls rapidly. The thermorheology of Strategic Highway Research Program (SHRP) asphalt binders has been studied by Bhatia and Anderson.2 The thermochemistry of processes occurring in asphalt binders has been studied by Claudy et al.3 One process involves waxes. McKay et al. have recently reported on rheological changes in SHRP asphalt binders when waxes from certain binders were added back into the binders.4 It is generally believed that wax processes affect asphalt performance.2-4 It was previously reported that the heptane insoluble asphaltenes in several petroleum vacuum residues are Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Jennings, P. W. Private communication. (2) Bahia, H. V.; Anderson, D. A. Asphalt Paving Technol. 1993, 62, 93-129. (3) Claudy, P.; Letoffe, J. M.; King, G. N.; Planche, J. P.; Brule, B. Fuel Sci. Technol. Int. 1991, 1, 71-92. (4) McKay, J. F.; Branthaver, J. F.; Roberson, R. E. Symposium on Petroleum Chemistry and Processing, 210th National Meeting of the ACS, Chicago, IL, Aug 20-23, 1995. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40, 794-798.

colloidal particles at 93 °C.5 Many researchers have in fact observed scattering centers in the colloidal size range in heavy crude oils, and crude oil residues, in small angle X-ray scattering experiments over the years, and it has been suspected for some time that these scattering centers are associated with the asphaltenes. The colloidal particles at 93 °C are approximately 100 Å in diameter,5 which is large compared to normal molecular dimensions. There is a question at present as to whether the scattering centers observed at room temperature are large molecules, micelles, or some other microstructure formed by the self-association of smaller asphaltenic molecules.6,7 In at least one case, the residue of Arabian medium/heavy, there is also thermochemical evidence suggesting that the colloidal particles at higher temperatures are micelles that form when the residue contains more than 4-5 wt % heptane insoluble asphaltenes.8 Although the physiochemical state of asphaltenic molecules appears to be a somewhat esoteric question with regard to road asphalts, we believe this subject is relevant to understanding how asphalt properties depend on temperature and chemical composition. If indeed the colloidal-sized scattering centers are large molecules, one could reasonably expect the same types of molecular motions, or relaxation modes, in binders

X

0887-0624/96/2510-0855$12.00/0

(5) Storm, D. A.; Sheu, E. Y.; DeTar, M. M. Fuel 1993, 72, 977981. (6) Klm, Hyo-gun; Long, R. B. Ind. Eng. Chem. Fundam. 1979, 18, 60-63. (7) Herzog, P.; Tchoubar, D.; Espinat, D. Fuel 1988, 67, 245-250. (8) Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Symposium on Petroleum Chemistry and Processing, 210th National Meeting of the ACS, Chicago, IL, Aug 20-25, 1995. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40, 776-779.

© 1996 American Chemical Society

856

Energy & Fuels, Vol. 10, No. 3, 1996

as for molecules of polymer in solutions, or melts, and therefore use concepts developed in polymer rheology to predict how asphalt properties would change with temperature and chemical composition. Indeed, much progress has been made by applying the time-temperature superposition principle and the WLF equation to asphalt rheology.2 On the other hand, if the colloidal particles at 93 °C are micelles, one might suspect that different microphases could form as the temperature and chemical composition are changed. The relaxation modes might be different in these different microphases, and hence each microphase could have different rheological properties. The time-temperature superposition principle might not apply uniformly in all of the microphases. In this case the phase diagram for the asphalt binder would have to be understood to understand how properties depend on temperature and chemical composition. It was also previously reported that asphaltenic colloidal particles in several vacuum residues behave under shear at 93 °C as a suspension of dispersed particles.9,10 These vacuum residues are Newtonian fluids at this temperature. In a more detailed study at this temperature with Ratawi vacuum residue, it was found that the shear rheology is that for a suspension of soft spheres.10 However, for temperatures less than approximately 65 °C, non-Newtonian shear thickening was observed, and the residue appeared to develop a yield stress.11 It was suggested in that work that a phase transition occurs at this temperature from the higher temperature dispersed particle phase to a lower temperature phase that has solid properties. It was surmised that this transition is initially analogous to the order-disorder transition for hard sphere suspensions, in that there is a decrease in free volume as the phase boundary is approached; however, as the transition proceeds, there is a molecular rearrangement, and nonasphaltenic molecules, previously confined to the region around the asphaltenic particles, gain longer range mobility. The thermochemistry of this transition and the changes in rheology are subjects of this work. The importance of a glass transition in asphalt binders was recognized by many workers in the 1960s.12 Effects of asphalt binder composition on glass transition temperature have been studied by Connor and Spiro,13 Noel and Corbett,14 Huynh et al.,15 and Giavarini et al.16 Typically, glass transition temperatures are between -10 and -25 °C, depending on the asphalt binder. Bahia and Anderson have made a study of the thermorheological properties of SHRP asphalt binders and, in particular, the influence of the glass transition on an effect called physical hardening.2 As discovered by Bahia and Anderson, physical hardening is due to a slow shrinkage that occurs in a metastable state that is formed when the asphalt binder is cooled below its glass transition temperature too rapidly. It is generally believed that the glass transition has some influence on cracking in road asphalts at low temperatures. (9) Storm, D. A.; Barresi, R. J.; DeCanio, S. J. Fuel 1991, 70, 779782. (10) Storm, D. A.; Sheu, E. Y. Fuel 1993, 72, 233-237. (11) Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Energy Fuels 1995, 9, 168-176. (12) Schmidt, R. J.; Barrall, E. M. J. Inst. Pet. 1965, 51, 161-168. (13) Connor, H. J.; Spiro, J. G. J. Inst. Pet. 1968, 54, 137-139. (14) Noel, F.; Corbett, L. W. J. Inst. Pet. 1970, 56, 261-268. (15) Huynh, H. K.; Khong, T. D.; Malhotra, S. L.; Blanchard, L. P. Anal. Chem. 1978, 50, 976-979. (16) Giavarini, C.; Pochetti, F. J. Therm. Anal. 1973, 5, 83-94.

Storm et al.

Figure 1. Dynamic viscosity at frequency of 0.628 s-1 versus weight fraction asphaltenes for natural and synthetic AMH residues.

Figure 2. Dynamic viscosity at frequency of 0.628 s-1 versus temperature for AMH vacuum residue.

The thermochemical aspects of the glass transition have been studied by Claudy et al.3 These authors find that the glass transition is associated with a smooth change in heat capacity, occurring typically around -25 °C. These authors also elucidate a largely endothermic process on heating in the temperature range of 0-90 °C due to the dissolution of crystallized waxes. McKay et al. have recently investigated the effect of added waxes on the dynamic viscosity and tan δ for SHRP asphalt binders.4 It is generally believed that wax processes affect asphalt performance.2-4 In this work we suggest that another phase transition occurs in the 25-65 °C range for asphalt binders containing more than approximately 15 wt % heptane insoluble asphaltenes. This transition depends on both asphaltene concentration and temperature. It is responsible for the increase in viscosity shown in Figure 1 for asphaltene concentrations greater than 15 wt %, and we believe it is largely responsible for the increase in viscosity shown in Figure 2 for temperatures less than 45 °C. A thermochemical analysis indicates that the asphaltene transition is first order. The enthalpy change is small, however, approximately 0.5 cal/g (endothermic on heating). Solid properties develop as the phase transition occurs. The line of transitions in the temperature-concentration phase diagram can be predicted by using rheological parameters determined in the higher temperature phase in a free volume calculation. Although not investigated, it seems possible that there could be a change in specific volume associated with this transition. If so, the phase transition could also be relevant to the cracking of asphalt as the temperature swings between 0 and 50 °C during ambient temperature cycles.

Thermochemistry of Asphalt Binders

Energy & Fuels, Vol. 10, No. 3, 1996 857

Table 1. Properties of Ratawi and Arabian Medium/ Heavy (AMH) Vacuum Residues C/H S (%) N (%) metals (ppm) Ratawi AMH

1.4 1.4

6.3 4.8

0.5 0.5

174 176

C7I

CAR CAR/HAR

21.3 32.7 25.0 32.2

4.9 4.6

Experimental Section The vacuum residue was made from Arabian medium/heavy (AMH) crude oil from Saudi Arabia by vacuum distillation. It has an apparent boiling point greater than 1050 °F, and it is quite similar to the Ratawi residue previously studied. In particular, both contain more than 20 wt % heptane insoluble asphaltenes (C7I). Some properties of these two residues are shown in Table 1. These vacuum residues are similar to SHRP asphalt binders B and F on the basis of their aromatic carbon content.1 The ratio of aromatic carbon to aromatic hydrogen is slightly outside the SHRP asphalt binder range, however, indicating that the aromatics are more highly substituted than those in SHRP asphalts B and F.1 As in previous work, samples of synthetic residue were prepared by dispersing some of the residue in the fraction that is soluble in heptane, the heptane solubles. The heptane solubles were prepared by precipitating the asphaltenes at room temperature with 40 parts of heptane for 1 part of residue. The slurry was stirred overnight, and the asphaltenes were removed by filtration. After the residue was dispersed in the heptane solubles, the samples were allowed to heal for 2 weeks in closed sample jars placed in a vacuum oven with a small nitrogen purge at 110 °C before use. This procedure appears to result in an imperceptible perturbation to the normal state of the heptane insoluble asphaltenes in the residue. Waxes were removed from the heptane soluble fraction by first adding 3.75 mL of toluene/g of heptane solubles and then adding 11.25 mL of 2-butanone/g of heptane solubles. Following Branthaver et al.,17 the mixture was stirred at 0 °C for 2 h and then filtered to recover the waxes. The heptane solubles contained 2.8 wt % wax according to this procedure. The solvents were removed from the dewaxed heptane solubles by vacuum distillation; samples with various amounts of wax were made by dispersing amounts of the heptane solubles into this dewaxed heptane soluble fraction. As in the previous study with the Ratawi vacuum residue, the shear viscosity of the AMH residue was measured in a Couette-type viscometer in which the cup and bob were enclosed in a high-temperature furnace designed by Bohlin Instruments. The characteristics of this viscometer were described previously.11 The loss and storage moduli were also measured at 25, 35, 45, 55, and 65 °C in oscillatory experiments. This instrument was also supplied by Bohlin and consisted of two metal plates, 25 mm in diameter, enclosed in a heated chamber. The instrument had an enhanced resolution capability for the angular deflection. With a gap of 0.5 mm, the detection limit for the strain is approximately 2.5 × 10-5. The samples were allowed to equilibrate at 25 °C overnight before the first measurement was made. After the measurement at 25 °C, the samples were then heated to the higher temperatures and allowed to equilibrate for 1 h after the set point had been reached before the next measurement was made. In the Bohlin instrument an oscillating stress is applied to the top plate, and its angular deflection is measured. Complementary experiments must be done to delineate the linear viscoelastic region as a function of applied stress and frequency. In general, the frequency range of 0.1-30 Hz was covered in the oscillatory experiments with an applied stress of 10 Pa. Some of the data at lower frequencies had to be discarded for the less viscous samples at the higher temper(17) Branthaver, J. F.; Thomas, K. P.; Dorrence, S. M.; Heppner, R. A.; Ryan, M. J. Liq. Fuels Technol. 1983, 1, 127-146.

atures because the strains were too large, and some of the data at higher frequencies had to be discarded for the most viscous materials at low temperatures because the strains were too small. Also for some samples, and some combinations of temperature and frequency, the strain wave had a wavelength approaching 20 times the gap dimension. These data were also discarded. Only data in which the ratio of the wavelength of the strain wave to gap dimension exceeds 40 are reported. According to Bohlin, the errors in phase are less than 1° and the errors in the relative velocity gradient are less than 1%. The heat capacities of the residues, natural or synthetic, were measured relative to that for the heptane soluble fraction using a calorimeter purchased from SETARAM, Model BT2.15D. There are two sample cells in this calorimeter: the reference cell, which contained the nonasphaltenic fraction, or heptane solubles; and the sample cell, which contained the samples of synthetic, or natural, residue. The difference in heat flow to the two cells was measured as the temperature was increased from 25 to 93 °C at a rate of 0.2 °C/min. Ten gram samples were used in the cells, and the samples were allowed to equilibrate at 25 °C overnight before the measurements were made. According to the manufacturer, one can expect a detection limit for power to be better than 20 µW, which, if multiplied by the duration of the experiment, could lead to an error in the difference of heat capacities of 0.2 mcal g-1 °C-1. The error in enthalpy could be 0.01 cal/g. These are very conservative estimates, since the manufacturer states the detection limit for a change in energy to be 1000 times smaller than that calculated by assuming the errors in power at each instance add in phase during the experiment. There is a bias in the electronics of this particular instrument so that it appears that less heat is needed to heat the sample cell than the reference cell. Therefore, in the final calculation for the difference in heat capacities, the amount of heat measured in a baseline experiment with heptane solubles in both cells was subtracted from the amount measured when the sample of residue was in the sample cell. Repeatability of the baseline experiment was excellent. Experiments were also done to study the heat change in the heptane soluble fraction that occurs in this temperature range due to the melting of the waxes. Samples with wax concentrations of 0.7, 1.4, 2.1, and 2.8 wt % were placed in the sample cell, while the dewaxed sample was placed in the reference cell. The samples were allowed to equilibrate at 25 °C overnight, and a baseline experiment with dewaxed heptane solubles in both cells was used to correct for the electronic bias in the instrument. Small angle X-ray scattering (SAXS) was performed with the 10 m SAXS instrument at Oak Ridge National Laboratory (ORNL). The X-rays were generated from a Rigaku-Denki rotating anode with a copper target, operating with 4 kW power. The KR photons with a wavelength of 1.54 Å are selected using a pyrolytic graphite monochromator. The selected photons pass through a set of collimators and a series of pinholes to produce a 1 mm diameter X-ray beam at the sample position. The sample to detector distance was set at 5.176 m, which gives a scattering vector range, Q, from ∼0.01 to ∼0.1 Å-1, where Q ) 4π/λ sin(θ/2) (λ is the wavelength and θ is the scattering angle). A beam stop made of lead titanium with a 1.5 cm diameter hole was installed in front of the detector on the beam axis to stop the transmitted X-ray beam. A two-dimensional continuous wire detector was used to detect the scattered photons. Samples were loaded into 1 mm aluminum cells with Kapton windows and held in a vacuum chamber to minimize the air scattering. Each sample was measured at room temperature for 3-4 h to ensure good counting statistics. The raw data were corrected for dark current and background and reduced to the absolute intensity according to a calibrated polyethylene standard of known scattering cross section at the peak position.

858


Figure 3. Creep compliance for AMH vacuum residue at 25 °C.

Results The dynamic viscosities at a frequency of 0.1 Hz for AMH residue and samples of synthetic residue at 25 °C are shown as a function of asphaltene concentration in Figure 1. There is a large increase in viscosity for asphaltene concentrations greater than 15 wt %. The arrow is the phase boundary, calculated as described below. The dynamic viscosity of AMH residue as a function of temperature is shown in Figure 2. There is also a large increase in viscosity starting at 45 °C. Again the arrow is the phase boundary. It should be noted that the wax concentration is constant for the data in Figure 1 because of the way in which the samples were prepared. However, one could attribute part of the increase in dynamic viscosity in Figure 2 to wax crystallization.4 It should be recognized, however, that effects due to waxes only involve 2.8 wt % of the sample, whereas the phase boundary shown in Figure 2 involves 25 wt % of the sample. As shown in Figure 3, the AMH vacuum residue is clearly a viscoelastic liquid at room temperature. The creep compliance shows that except for some questionable data at short times (less than 0.5 s), the material flows with a well-defined viscosity. The viscosity obtained from this experiment was 4.93 × 104 Pa s, which is somewhat higher than the dynamic viscosity measured at a frequency of 0.628 s-1, 4.51 × 104 Pa s. The stress was 10 times larger in the compliance experiment, and so the discrepancy is likely due to shear thickening.11 During the stress recovery period a steady state compliance Je° of 2.5 × 10 -4 was obtained, which is comparable to that measured for dilute solutions of un-cross-linked polymers. The stress in this creep experiment was 100 Pa, which might appear large for a material that is apparently Newtonian. A high stress is necessary, however, to capture some of the short time response. As shown in Figure 4, a fairly high stress is needed at an angular frequency of 94.2 s-1 to be in the linear viscoelastic range and to cause strains that are detectable with this instrument (approximately 2.5 × 10-5). It is interesting to note that an extrapolation of the linear part of the strain-stress relationship shown in Figure 4 to zero strain suggests there is a yield stress of approximately 1 Pa. The shear viscosities of the various samples of residue relative to that for the heptane soluble fraction are listed in Table 2 as a function of temperature in the 93-300 °C range. As discussed previously for Ratawi vacuum residue, the changes in relative viscosity with changes in as-

Storm et al.

Figure 4. Strain versus applied stress at frequency of 94.2 s-1 for AMH vacuum residue at 25 °C. Table 2. Relative Viscosity of Residue WA

93 °C

150 °C

200 °C

250 °C

300 °C

0.013 0.018 0.023 0.035 0.045 0.058 0.070 0.093 0.115 0.138 0.161 0.183 0.208 0.231 0.251

1.18 1.15 1.11 1.35 1.44 1.51 1.64 1.96 2.27 2.54 3.01 3.80 4.56 5.15 6.17

1.11 1.08 1.13 1.20 1.26 1.31 1.35 1.55 1.70 1.82 2.01 2.32 2.59 2.79 3.08

1.09 1.08 1.10 1.15

1.06 1.05 1.08 1.11 1.16 1.17 1.22 1.29 1.39 1.44 1.51 1.65 1.76 1.89 1.96

1.05 1.05 1.05 1.05 1.08 1.12 1.16 1.18 1.22 1.30 1.35 1.41 1.50 1.58 1.71

1.24 1.26 1.42 1.52 1.61 1.70 1.90 2.06 2.19 2.34

phaltene concentration are what one would measure for a suspension of soft spheres.11 A convenient way to model the structure in the hightemperature state is to use the equation

ηr ) (1 - KWA)-2.5

(1)

where ηr is the relative viscosity (ratio of viscosity of the residue to that of the heptane solubles), WA is the weight fraction of asphaltenes in the residue, and K is a solvation constant that accounts for the fact that the volume occupied by the asphaltenes in the residue (hydrodynamic volume) is different from that in the solid state. The solvation constant can be written as

K ) F/FA(sol)

(2)

where the unknown FA(sol) is the asphaltene density in the residue. Equation 1 is shown in Figure 5 for the data in Table 2 corresponding to 93 and 300 °C. As discussed previously, one obtains consistent solvation constants when applying many different theories for the viscosity of suspensions of spheres.11 In effect, KWA is the effective volume fraction of the asphaltenes in the residue, a concept that is well-established but usually written as [η]c for dilute solutions, where c is the concentration of the substance in weight per unit volume and [η] is the intrinsic viscosity measured as c approaches zero. Equation 1 allows this idea of an effective volume to be applied in more concentrated suspensions. For later reference one can calculate the size of the solvated asphaltenic particle to the unsolvated particle



Figure 7. Loss modulus versus frequency for natural and synthetic residues: asphaltene concentration 0.25 (AMH) (b), 0.21 (9), 0.18 ([), 0.16 (O), 0.14 (0), and 0.12 (]). Figure 5. Relative viscosity at 93 and 300 °C versus weight fraction asphaltenes in natural and synthetic AMH residues: 93 oC (4); 300 oC (O).

Figure 8. Loss modulus versus frequency for AMH vacuum residue for various temperatures: 25 °C (b); 35 °C (9); 45 °C (2); 55 °C (1); 65 °C ([).

Figure 6. Solvation constant versus 1/T: Ratawi (9); Ratawi corrected (0); AMH (b); AMH corrected (O).

from the solvation constant by rearranging eq 2

R/R0 ) [FA(dry)/FK]1/3

(3)

where R is the radius of the solvated particle and R0 is the radius of the asphaltenic particle. When the temperature dependence of the solvation constant was analyzed, it was found that it varies exponentially with 1/T. This is shown in Figure 6 for Ratawi and AMH. The solvation shells get larger at lower temperatures. The solvation constants for temperatures less than 65 °C deviate from the Arrhenius equation based on the higher temperature data, because of the transition to a viscoelastic state that we will discuss below. As with Ratawi,11 the unfilled data point corresponding to 25 °C is calculated by neglecting the relative viscosities for these samples having rheology that has been altered by this transition. The loss moduli are shown in Figure 7 for the AMH residue and several samples with lower asphaltene content at 25 °C. The loss modulus is a measure of the tendency of the strain energy to be dissipated as heat, as in a viscous liquid. There appear to be two groups. In one group, samples with an asphaltene concentration 18 wt % or greater, the loss modulus increases significantly with increasing asphaltene concentration. In the second group, samples with asphaltene concentration

less than 18 wt %, the loss modulus has only a slight dependence on asphaltene concentration. There is a noticeable change in the magnitude of the loss modulus for all frequencies as one moves from one group to another. The loss moduli for AMH residue as a function of frequency are plotted in Figure 8 for each of the temperatures. The loss modulus depends linearly on frequency for all samples. There may be a slight flattening of the curves in the high-frequency region. The flattening could be due to the instrument error since the strain in these experiments is approaching the limit of detection. If the flattening is indeed real, it would indicate some motions are too slow to dissipate energy on this time scale. This would be analogous to entanglement coupling in polymer rheology, although in this case it would be due to the solvated particles not having enough time to slip past one another in these high-frequency experiments. The storage modulus is shown in Figure 9 for these same samples. Again there appear to be two groups of samples. The storage modulus is a measure of the tendency of the strain energy to be stored, as with a solid. There is a significant increase in the magnitude of storage modulus for all frequencies for residues containing 18 wt % asphaltenes or more, and the moduli for this group depend more strongly on asphaltene concentration. The storage modulus also depends linearly on frequency. The ratio of the storage modulus to loss modulus is shown in Figure 10 for an angular frequency of 0.628 s-1. Again the arrows are the predicted positions of the phase boundaries. Solid properties develop at these phase boundaries, as indicated by an increase in G′ relative to G′′.

860


Storm et al.

A

B

C Figure 9. Storage modulus versus frequency for natural and synthetic residues: asphaltene concentration 0.25 (AMH) (b), 0.21 (9), 0.18 ([), 0.16 (O), 0.14 (0), and 0.12 (]).

D Figure 11. Heat required to heat sample (uncorrected) relative to heptane soluble reference from room temperature to 93 °C: (A) AMH residue; (B) asphaltene concentration 0.21; (C) asphaltene concentration 0.18; (D) heptane solubles.

Figure 10. Ratio of storage modulus to loss modulus at 25 °C versus weight fraction asphaltenes: 25 °C (b); 35 °C (9); 45 °C ([).

In the thermochemical experiments a 10 g sample of the heptane solubles was placed in the reference cell, and a sample of the natural or synthetic residue containing known amounts of asphaltenes was placed in the sample cell. After approximately 12 h for equilibration, the temperature was increased linearly at a rate of 0.2 °C/min. As discussed above, one measures the difference in heat flow to the sample and reference cell. The results for AMH, a sample with asphaltene concentration of 21 wt %, a sample with asphaltene concentration of 18 wt %, and the heptane solubles are shown in Figure 11. The curve for the heptane solubles in Figure 11 illustrates the bias in the electronics, since the heptane soluble fraction was in both cells. Therefore, to calculate the change in heat relative to that for the reference cell, the area under the curve for the heptane solubles in Figure 11 was subtracted from the areas of the corresponding curves for samples containing asphaltenes. The heat capacity of the residue relative to that for the heptane soluble fraction was calculated by dividing these areas by the sample weight (10g) and the temperature interval (68 °C). The heat capacity of the heptane solubles was found to be larger than that for the residue, and so the calculation based on the difference in areas yields the absolute value of the difference in heat capacities. These absolute values are shown in Figure 12, where it is seen that there is a discontinuity at an asphaltene concentration of 15 wt %. To determine the sign of the enthalpy change, one must know that a larger area under the curves in Figure 11 corresponds to less heat being required to heat the sample cell than the reference cell. The absolute value of the differences in the heat capacity shown in Figure

Figure 12. Absolute value of heat capacity of natural and synthetic residues relative to that for heptane soluble fraction versus weight fraction asphaltenes.

12 indicates that approximately 0.5 cal/g of heat is absorbed in the transition from the lower temperature viscoelastic fluid state to the higher temperature Newtonian fluid state. The heat change involved in the dissolution of waxes was investigated by having the dewaxed heptane soluble sample in the reference cell and samples containing various amounts of wax in the sample cell. The curves for the heptane solubles with various wax contents are shown in Figure 13 after correction for the bias in electronics. The dissolution process is clearly endothermic. The heat change is 20.03 ( 2.6 cal/g of wax, obtained by averaging the results for the four samples. This is less than half of the value reported by Claudy et al. for normal alkanes3 and may be the result of having highly branched molecules in the saturated hydrocarbon fraction. Since the waxes also undergo an endothermic phase change in this temperature range, and the enthalpy change is about the same as the one we have measured for the fluid-fluid transition, there is a danger that wax dissolution is confounding the thermochemistry of the fluid-fluid transition. Since the dissolution of the waxes is occurring in the reference cell, an error can only occur if the waxes are not melting in the sample cell, because they did not crystallize due to the presence of the asphaltenes. Since the dissolution is endothermic in the reference cell, it would appear as an exothermic



A

B

C

D Figure 13. Heat required to heat samples containing natural wax relative to dewaxed sample: wax concentrations (A) 0.7 wt %, (B) 1.4 wt %, (C) 2.1 wt %, and (D) 2.8 wt %.

process in the sample cell, thereby resulting in a larger area. That is, an error due to an unbalanced wax dissolution would make the absolute value of the difference in heat capacity larger. The data in Figure 12 clearly show this is not responsible for the observed transition at 15 wt % asphaltenes, since the samples with the least asphaltenes are the ones with the largest area. However, when viewing the curve for the dissolution of waxes in Figure 13, one might suspect that an unbalanced wax dissolution in the two cells is causing the wiggles in the curves shown in Figure 11. It could also account for the increase in the relative heat capacities of samples more concentrated than 15 wt % asphaltenes. On the basis of the measured heat for dissolution of the waxes in this work, one would estimate that approximately half of the waxes are prevented from crystallizing in the AMH residue. To learn more about the structure of the viscoelastic phase, SAXS measurements were made at room temperature with AMH residue. The scattering data were analyzed according to the method of Liu et al.18 In this analysis one assumes that basic units such as spherical particles have clustered to form a fractal object. The fit to the scattering intensity using this model, and the model of polydispersed spheres used for the Newtonian fluid state at 93 °C, are compared to the experimental points in Figure 14. The fractal model fits the experimental points quite well, while the model of polydispersed spheres deviates significantly. One obtains from the fractal model that the fractal dimension associated with the scattering centers, or clusters, is 1.5. The characteristic dimension of the basic spherical particles is 40 Å (diameter), and the number of these particles in the average cluster is 12. Therefore, the average cluster has a characteristic length of approximately 500 Å (diameter), or 0.05 µm.

Figure 14. SAXS intensity for AMH residue at room temperature: experimental data (O); fit according to polydispersed spheres (Schultz distribution) (‚ ‚ ‚); fit according to fractal model (s).

As discussed above, and previously for the case of Ratawi,11 all of the vacuum residues we have studied more concentrated in asphaltenes than approximately 2-5 wt % behave as Newtonian fluids in the temperature range of 65-150 °C. The shear rheology is that of a suspension of spheres. The asphaltenic particles,

however, must interact strongly with some of the molecules in the nonasphaltenes and, therefore, appear in the rheological analyses to be solvated by a deformable layer of these molecules. This is of course the role assigned to the resins many years ago in the model of Pfeiffer and Saal.19 One can calculate the amount of solvation by applying many different theories for the rheology of hard spheres. One very simple way is to apply eq 1; the solvation constant is the slope of the line shown in Figure 5. It might be mentioned that eq 1 has a defect, but this does not affect the solvation constants obtained from it in the Newtonian fluid state.11 The equation should break down when the sample is too dilute to be micellar or so concentrated that the transition that we are discussing has already occurred. In both microphases, a different rheology is observed, and eq 1 is not accurate. In the argument below, eq 1 will be applied to the Newtonian fluid state, and then the solvation constants obtained from it will be extrapolated to the lower temperature region to calculate the position of the phase boundary in a temperature-asphaltene concentration diagram. The position of the phase boundary is calculated by using a free volume argument based on the observation that the effective size of the asphaltenic particles in the residue is determined by the solvation constant, as shown in eq 3, and since the solvation constant is temperature dependent, the asphaltenic particles will have more or less of the volume to move about in as the temperature is changed. A condition can be reached in which the solvation shells have grown so large that they begin to overlap. That is, the free volume becomes zero. Alternatively, one could reach this condition by holding the temperature constant and increasing the concentration of asphaltenes. As the free volume in the Newtonian fluid state vanishes, one might expect a transition to a new structure in which the nonasphaltenic molecules that were in the solvation shells now spread out over several asphaltenic particles. One

(18) Liu, Y. C.; Sheu, E. Y.; Chen, S. H.; Storm, D. A. Fuel 1995, 74, 1352-1356.

(19) Pfeiffer, J. Ph.; Saal, R. V. J. J. Phys. Colloid Chem. 1940, 44, 139-148.

Discussion

862


Storm et al.

Figure 15. Phase diagram for AMH residue.

might imagine that in this fluid state the nonasphaltenes are a glue making the structure of the asphaltenic particles more rigid. In the vocabulary of asphalt chemistry, the Newtonian fluid state is a sol, while the viscoelastic fluid state is a gel. However, according to Figure 3 the residue at 25 °C is a viscoelastic liquid, except possibly for applied stresses less than 1 Pa. One can use this free volume concept to develop predictions for the phase boundary in a temperatureconcentration diagram by the following argument. If one assumes the asphaltenic spheres pack as hard spheres at closest approach (fcc), then the average interparticle spacing between centers in the fluid is given by

L*/R ) (0.74FA/FWA)1/3

(4)

where the asphaltene density is the dry density. Since the interparticle distance can be written as

L* ) 2R0 + 2L + 2δ

(5)

where 2δ is the “free distance” between centers, and since the thickness of the solvation layer L in units of R0 can be calculated from eq 3, we have by rearrangement of eq 5

δ/R0 ) (0.74FA/FWA)1/3 - (FA/FK)1/3

(6)

Taking the “free distance” to be zero at the phase boundary, we arrive at the simple condition that the concentration at the phase boundary for a particular temperature is given by

WAc ) 0.74/K(T)

(7)

One can therefore extrapolate the solvation constants to lower temperatures using the Arrhenius equation and thereby estimate the critical asphaltene concentration for the transition to the viscoelastic fluid state. The theoretical phase diagram is shown in Figure 15. The rheological observations agree closely with the phase diagram. The low-frequency dynamic viscosity, shown in Figure 1, increases at the boundary as indicated by the arrow. Figure 10 shows that the AMH residue has essentially no solid properties at 45 °C, which agrees with the phase diagram which predicts the AMH residue is on the boundary of the Newtonian fluid state. If the temperature is reduced somewhat, however, the transition to the viscoelastic state occurs, and there is an increase in dynamic viscosity, as shown in Figure 2. The phase boundary also shows up as the significant increase in loss and storage moduli with

Figure 16. Time-temperature superposition for natural and synthetic residues: asphaltene concentrations 0.25 (AMH) (O), 0.21 (]), 0.18 (0), 0.16 (4), and 0.14 (3).

asphaltene concentration, shown to occur in Figures 7, 9, and 10 at an asphaltene concentration of 18 wt %. It should be pointed out that the exact position of the phase boundary in the temperature-concentration diagram depends on the particular asphalt binder. For example, in Ratawi, the phase boundary at 25 °C is at 0.17, instead of 0.18, because, as shown in Figure 16, the solvation shells are larger for Ratawi asphaltenes than for AMH asphaltenes at the same temperature. The structure of the viscoelastic state is somewhat unresolved at present. The SAXS results suggest that it consists of large regions, or clusters, with a characteristic length scale of approximately 500 Å. There is a polydispersity of cluster sizes. The clusters are composed of smaller scattering centers that have a characteristic dimension of 40 Å. The large regions, or clusters, have irregular shapes; the fractal dimension is 1.5, not 3 as it would be for the asphaltenic particles in the Newtonian fluid state. Therefore, the structure suggested by SAXS is consistent with the “solidification” of the dispersed asphaltenic particles to form a viscoelastic state in which the nonasphaltenic molecules act as a glue holding the asphaltenic molecules or particles together in clusters. Considering the thermochemistry of the transition, it can be seen that the change in the specific heat of the residue relative to that for the heptane insolubles occurs at 15 wt % instead of the predicted 18 wt %. Although the difference is small when one considers the complexity of the material and the simplicity of the theory used to make the prediction of the phase boundary, it is interesting to speculate that the difference is real. One could surmise that there are really two transitions involved. The transition from the hightemperature Newtonian fluid state occurs because of a decrease in free volume; such transitions are usually second order, with no change in enthalpy. This is the transition predicted by the rheological analyses, because the rheology is so sensitive to the decrease in free volume. In the thermochemical measurements, however, the boundary is being approached from the viscoelastic state, and there must be a molecular rearrangement of at least the nonasphaltenic molecules to form the solvation shells. This would be expected to be first order, and it could occur at a slightly lower asphaltene concentration. The endothermic nature of the transition corresponds to a “melting” of the microstructure in the viscoelastic state to form the Newtonian fluid state. It would appear



Table 3. Shift Parameters for G′′ ∆T aT ∆WA aA

25/35 °C 0.1 0.25/0.21 0.60

35/45 °C 0.12 0.25/0.18 0.55

45/55 °C 0.20 0.16/0.14 1.60

55/65 °C 0.25 0.16/0.12 1.60

that entropic effects are complicated. Asphaltenic particles lose translational and rotational freedom in the viscoelastic state, but the nonasphaltenic molecules, which were confined to the solvation shells in the Newtonian fluid state, gain freedom since they can spread out over a larger structure of asphaltenic particles. The waxes only partly crystallize, and so some wax molecules have more entropy than those in the crystallized state. The change in heat between the AMH residue and the sample with 16 wt % asphaltenes at the phase boundary is 0.35 cal/g of residue, or 12 cal/g of wax. Thus, about half the waxes may be prevented from crystallizing in the viscoelastic state at an asphaltene concentration of 25 wt %. One principle that has been of great use in polymer rheology, and applied with success in asphalt rheology, is the time-temperature superposition principle. It is seen in Figures 7-9 that it cannot apply over the full range of asphaltene concentrations and temperatures. There is a significant increase in the moduli over a narrow temperature or concentration interval at the phase boundary. However, both G′ and G′′ depend linearly on the frequency, and since the curves are parallel over the whole range in temperature and concentration, one might suspect that superposition is possible. This is indeed the case as shown in Figure 16, if one breaks the G′′ data for the AMH residue into two groups: one group corresponds to temperatures below the phase boundary temperature, 45 °C, and the other group to temperatures greater than 45 °C. The data for 45 °C are included in both groups since it is on the phase boundary. The magnitudes of G′′ are renormalized in Figure 16 by the ratio of temperatures, as is common in polymer rheology.20 The shift factors are shown in Table 3, and it is seen that there are two groups of shift factors. For temperatures less than 45 °C, the shift factor is approximately 0.1/10 °C, whereas above 45 °C, the shift factor is approximately 0.2/10 °C. Thus, it is clear that the time-temperature superposition principle applies in each phase, but there is a discontinuity in shift factors at the phase boundary. The linearity of G′ and G′′ and the parallel nature of the curves for samples with various asphaltene concentrations imply that there is also a superposition principle for asphaltene concentration. This is shown in Figure 17. Again the data for G′′ fall into two groups: more concentrated than 18 wt % and less concentrated than 18 wt %. In this case G′′ is renormalized by the ratio of asphaltene concentrations.20 As seen in Table 4, the shift factors fall in two groups. Above the concentration at the phase boundary they are approximately 0.6 for a change in asphaltene concentration of 0.03 wt %, and below the phase boundary concentration they are 1.60 for a change in asphaltene concentration of 0.02 wt % asphaltenes. The WLF equation is another important concept from polymer rheology that has been applied in asphalt rheology. It is usually written as (20) Ferry, J. D. Viscoelastic Properties of Polymers, 2nd ed.; Wiley: New York, 1970.

Figure 17. 17. Time-asphaltene concentration superposition for natural and synthetic residues: asphaltene concentrations 0.25 (AMH) (O), 0.21 (]), 0.18 (0), 0.16 (4), 0.14 (3), 0.12 (+), and 0.09 (×).

log aT ) -c1(T - TR)/(c2 + T - TR)

(8)

where aT is the shift factor for the time-temperature superposition and TR is the reference temperatue (25 °C). It is seen from Table 3 that this equation holds with c2 large compared to (T - TR) and c1/c2 approximately equal to 0.23 for samples in the viscoelastic state, and with c2 large and c1/c2 equal to 0.16 for samples in the Newtonian fluid state. Thus, the WLF equation holds in each microphase. An equation of the form of eq 8 also hold for changes in asphaltene concentration in the viscoelastic state with aT replaced by aA, T by WA, and TR by 0.25, the asphaltene concentration in the reference state. In this case c2 is large compared to (W - WA)and c1/c2 is approximately -17. In the Newtonian fluid state, c2 is large and the ratio c1/c2 is 23.5. In fact, this factor holds well over a 2-fold change in asphaltene concentration. As shown in Figure 16, there is a slight deviation at the lower frequencies for the 9 wt % sample. However, one should expect this: G′′/ω would depend on asphaltene concentration in the Newtonian fluid state at low frequencies since it is the shear viscosity. The linearity of G′ and G′′ and the parallel nature of the curves on both sides of the transition imply that the types of relaxation modes have not changed in the transition; only their contribution to the “rigidity” has changed.20 That is, all relaxation times scale the same way across the transition, and the density of Maxwell elements of form exp(-t/τ) increases uniformly. Following Tanner G′′ can be written as21

G′′(ω) )

∫0∞ωτ/(1 + ω2τ2)H(τ) d ln τ

(9)

where H(τ) is the density of Maxwell element exp(-t/τ) in the interval d ln τ. Equation 9 is not only linear in ω on both sides of the boundary but also has the same slope. One way this can occur is if all of the relaxation times τ scale as aτ and the density scales as H(aτ) ) bH(τ) across the boundary; then eq 9 becomes

G′′VEP(Rω) )

∫0∞(Rω)τ/[1 + (Rω)2τ2][bH(τ)] d ln τ

(10)

which has the frequency shifted by a and the density of (21) Tanner, R. I. Engineering Rheology; Oxford University Press: New York, 1985.

864


Storm et al.

Maxwell elements increased by b. The density H(τ) d lnτ is called the contribution to rigidity by Ferry,20 and so each relaxation mode contributes more to the rigidity in the viscoelastic state. Equation 10 implies that it will be possible to shift G′′ across the boundary by rescaling the frequency and the magnitude of H(τ). Finally, it is of interest to speculate about a change in specific volume at this transition. Since

∆H ) ∆E + P∆V

(11)

one could calculate an upper bound on the change in specific volume by assuming that ∆E is zero. Since ∆H is approximately 0.5 cal/g, ∆V cannot be larger than 20.7 cm3/g. However, this is very large from a practical point of view, and so ∆V could be significant as far as asphalt cracking is concerned even if there is a reasonable ∆E associated with the transition. For example, if ∆E is 0.48 cal/g, then ∆V could be 0.83 cm3/g. Conclusions Asphalt binders obtained from the vacuum bottoms of asphaltic crude oils have a simple microstructure in the 65-150 °C temperature. The asphaltenic colloidal particles are well-dispersed in the fluid in this temperature range. The particles, however, attract nonasphaltenic molecules, forming a solvation shell. Their effective volume fraction is therefore larger than that based on the volume of the asphaltenic particles alone. Due to the steric repulsive forces between solvation shells, the fluid is Newtonian.11 As the temperature is reduced the solvation shells get larger; and in the temperature range of 65-45 °C, the solvation shells begin to overlap. This loss of free volume for the asphaltenic particles induces a transition to a new microstructure. Solid properties develop at the phase boundary, and the viscosity increases significantly. There is a phase transition from the Newtonian fluid state to a viscoelastic fluid state. The microstructure of the viscoelastic fluid appears to be large regions or clusters of irregular shape that are composed of smaller scattering centers. One could imagine that a molecular rearrangement occurs when the solvation shells overlap, and the nonasphaltenic molecules, previously confined to the solvation shells,

now spread out over several asphaltenic particles to form clusters that have a characteristic length of 500 Å and a fractal dimension of 1.5. One can predict the position of the phase boundary in a temperature-composition diagram using the solvation constants obtained from shear viscosity measurements in the higher temperature Newtonian fluid state. The predictions agree well with the increase in viscosity and the development of solid properties as the temperature is reduced or asphaltene concentration increased. The transition from the Newtonian fluid state to the viscoelastic state is first order. The change in enthalpy is approximately 0.5 cal/g. It occurs for an asphaltene concentration greater than 15 wt %. It appears that natural waxes are prevented from crystallizing in the viscoelastic state. It also seems quite likely that there is a change in specific volume associated with this transition. Thus, this transition should affect asphalt performance. Although the time-temperature superposition principle fails at the phase boundary, the principle does apply in each microphase. The shift factors have a simple discontinuity at the phase boundary. The relaxation modes remain the same, while the density of Maxwell elements is increased. Each mode therefore contributes more to the rigidity in the viscoelastic phase. There is also a superposition principle for asphaltene concentration in the viscoelastic phase, and the principle applies for a limited concentration range near the phase boundary in the Newtonian fluid state. The WLF equation for the time-temperature shift factors applies in each phase with different constants, and a WLF equation for asphaltene concentration holds in the viscoelastic fluid phase and in the Newtonian fluid phase over a limited concentration range near the phase boundary. Acknowledgment. We thank Professor P. W. Jennings for supplying NMR and HP-GPC data for the Ratawi and AMH vacuum residue. We also thank the Solid State Division of Oak Ridge National Laboratory for granting SAXS beam time. This research was supported in part by the Division of Material Sciences, U.S. Department of Energy, under Contract DEAC0584OR21400, with Martin Marietta Energy System, Inc. EF9502564

Development of Solid Properties and Thermochemistry of Asphalt

Recommend Documents