Bitumen Structural Stability Characterization Using Turbidimetric

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Energy & Fuels 2003, 17, 1407-1415

1407

Bitumen Structural Stability Characterization Using Turbidimetric Titration R. Karlsson* and U. Isacsson Division of Highway Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received September 17, 2002. Revised Manuscript Received June 2, 2003

The structural stability of mixtures of old and new bituminous binders was studied using threedimensional turbidimetric titration, where the three dimensions are a visualization of the Hansen solubility parameters showing dispersive, polar, and hydrogen-bonding components. The study was initiated to investigate the possible consequences of mixing old and new binders during asphalt recycling. However, the titration method used is new, and an assessment was needed before evaluation of the structural stability of mixtures of old and new binders could begin. The results showed that three-dimensional titration gives more-detailed information regarding structural stability, compared to one-dimensional Heithaus titration. It was also found that the technique used for automatic detection of precipitation must be further developed. The results regarding mixtures of old and new binders indicated that structural stability is not a limiting factor in asphalt recycling, provided that the old and new binders are compatible.

Introduction During the last two decades, asphalt recycling has become increasingly common, and, today, a diversity of methods is in use. Asphalt recycling has normally been proven to be viable and economically sound, but legal provisions and an awareness of the importance of recycling have also promoted the use of recycled asphalt. In Sweden, all reclaimed asphalt currently is recycled in some form, and the current legislation imposes that at least 90% is to be recycled for the same purpose as it had before. However, asphalt recycling is only beneficial if it is done properly. If the recycled asphalts do not perform satisfactorily, recycling may not be acceptable from neither an economical nor environmental point of view. The performance of recycled pavements has mostly met the expectations, but more fundamental knowledge is needed to facilitate further development. Properties of recycled asphalt are, of course, dependent on properties of the recycled aggregate and binder. The recycled binder could be less reliable, compared to a new binder, because the properties of the old binder are neither fully known nor controllable during the recycling process. Questions arise in regard to what possible consequences this could have in practice, and if properties of recycled asphalt can be assessed and altered, if necessary, to obtain as-new material. A research project, of which the result described in this paper is one part, has studied the extent to which old and new binders mix. It was assumed that the mixing of old and new binders is dependent on mechanical mixing and diffusion, as well as chemical compatibility and stability. The process of diffusion in which old and new binders merge together has been addressed in previous papers.1,2 It was concluded, among other * Author to whom correspondence should be addressed. E-mail: [email protected].

things, that diffusion is greatly influenced by temperature but relatively unaffected by aging. This paper addresses the compatibility between binders used in recycling and the stability of the recycled asphalt binder. By studying the structural stability of mixtures of old and new binders, the intent of the work described in this paper is to acquire new knowledge of the interaction between mixed binders and the expected quality of recycled binders. Structural stability of bituminous binders, in this context, is regarded as the ability to avoid phase separation and, consequently, is related to the distribution of chemical functionalities of binders. The binder stability studies were performed using turbidimetric titration with three different titrants: isooctane, methyl ethyl ketone, and iso-octanol. These titrants represent the three major types of intermolecular interactions: dispersive, polar, and hydrogen bonding. Initially, work was performed to evaluate the suitability of turbidimetric titration as a tool for characterizing the structural stability of bitumen. Mixtures of old and new binders were then studied. An important issue, when considering old binders in recycled asphalt, is the degree of aging. Oxidative aging leads to an irreversible change in the chemical composition of a binder.3 The oxidation products of the old binder will inevitably be added to the recycled binder. A question arises, in regard to whether excessively aged binder components influence the performance of recycled asphalt, besides its stiffness, such as a decrease in structural stability. Another question that arises is (1) Karlsson, R.; Isacsson, U. J. Mater. Civ. Eng. 2003, 15, 157165. (2) Karlsson, R.; Isacsson, U. Int. J. Road Mater. Pavement Des. 2002, 3, 167-182. (3) Yen, T. F., Chilingarian, G. V., Eds. Asphaltenes and Asphalts; Developments in Petroleum Science 40B; Elsevier Science: New York, 2000; Vol. 2.

10.1021/ef0202034 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/27/2003

1408 Energy & Fuels, Vol. 17, No. 6, 2003

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whether there is a limit regarding the number of times asphalt can be recycled, to avoid the negative influence of aging. Theory Molecular interactions are of utmost importance in bituminous binders. These interactions are related to chemical functionalities and solubility properties of the molecules that comprise bitumen. Hansen parameters describe solubility by contributions from dispersive (δd), polar (δp), and hydrogen-bonding (δh) interactions.4 These parameters can be illustrated graphically by representing each type of interaction with an axis in a three-dimensional space. The ability to dissolve or be dissolved is then represented by a sphere determined by the coordinates δd, δp, δh and radius R. Substances within the distance R from each other are likely to mix homogeneously. According to Hansen,5 the dispersive axis should be scaled by one half, to achieve a spherical shape of the solubility space. The R value of a system of a solvent (1) and solute (2) is consequently written as

R ) x[2(1δd - 2δd)]2 + (1δp - 2δp)2 + (1δh - 2δh)2

(1)

Redelius6 used data from Rahimian and Zenke7 of bitumen solubility in different solvents and plotted δh against δp. Redelius found an area of solubility for bitumen that was characterized by low polar and hydrogen-bonding interactions and drew the conclusion that the Hansen parameters can describe the solubility of bitumen better than previously used one-parameter models. It is stated that the solubility parameters of a mixture of two miscible solvents i and j are additive, i.e.,

φ δ + jφjδ δ h) i φ + jφ

(2)

where φ is the volume fraction of each of the liquids. This is the key to the method of automated turbidimetric titration used in this paper, in which the solubility parameters of a solvent can be continuously changed from a good solvent toward a nonsolvent. Heithaus8 developed a method for the determination of “asphaltene peptization” in bitumen, which is a concept used earlier in other disciplines of petroleum chemistry. He used toluene as a solvent and n-heptane as a titrant to determine when flocculation occurs. Using flocculation ratio/dilution ratio analysis, several aspects of asphaltene peptization and solubility could be studied. The flocculation ratio (FR) and dilution ratio (C) were defined as

(3)

and

C)

WA VS + VT

WA the weight of bitumen in the sample. When plotting values of FR and C for tests using different concentrations of bitumen in toluene, the values fell on a straight line (cf. Figure 1). Extrapolation of the regression line to the interceptions with the FR- and C-axes rendered FRmax and Cmin values. FRmax was considered to be closely related to the peptizability of asphaltenes (the least-soluble components), and, in this case, there is no influence of bitumen on the solubility power (WA ) 0 when C ) 0). Cmin was regarded to be related to the state of peptization or stability of the bitumen and its sensitivity toward the addition of a certain miscible substance, because, in this case, there is no influence of the solvent used (VS ) 0 when FR ) 0). The flocculation ratio/dilution ratio analysis used by Heithaus also involved parameters that have been derived to describe the level of peptizability or solubility of the asphaltenes (pa) and the peptizing power or solvency of the maltenes (po), as well as the state of peptization in the bitumen (P).

pa ) 1 - FRmax

(4)

where VS is the volume of solvent (toluene), VT the volume of titrant (isooctane) needed for flocculation, and

(

(5)

)

(6)

po 1 ) +1 1 - pa Cmin

(7)

po ) FRmax P)

i i

VS FR ) VS + VT

Figure 1. Schematic of flocculation ratio/dilution ratio analysis, after Heithaus.8

1 +1 Cmin

These three parameters were used in the analysis of the test results that have been presented. However, the analysis did not contribute any valuable information, and, therefore, the parameters are not discussed further in this paper. Newcomb et al.9 used Heithaus titration to investigate the compositional effects of rejuvenators and concluded that the method could be used to characterize the effects of rejuvenators on aged bitumen. Strong indications of correlation were found between parameters that were derived from Heithaus tests and properties of bitumen, such as low aging index (viscosity at 140 °F after RTFO (Rolling Thin Film Oven)/initial viscosity at 140 °F) and ductility. Pauli et al.10 found correlations between the structural stability and the (4) Hansen, C. M. The Three-Dimensional Solubility Parameter and Solvent Diffusion Coefficient; Danish Technical Press: Copenhagen, 1967. (5) Hansen, C. M. Hansen Solubility ParameterssA User’s Handbook; CRC Press: Boca Raton, FL, 2000. (6) Redelius, P. G. Fuel 2000, 79, 27-35. (7) Rahimian, I.; Zenke, G. Bitumen 1986, 1, 2-8. (8) Heithaus, J. J. J. Inst. Pet. 1962, 48, 458, 45. (9) Newcomb, D. E.; Nusser, B. J.; Kiggundu, B. M.; Zallen, D. M. Transp. Res. Rec. 1984, 968, 66-77. (10) Pauli, A. T.; Branthaver, J. F. Rheological and Compositional Definitions of Compatability as They Relate to the Colloidal Model of Asphalt and Residua. Presented before the Division of Petroleum Chemistry at the 217th National Meeting of the American Chemical Society, Anaheim, CA, 1999.

Bitumen Structural Stability Characterization

Energy & Fuels, Vol. 17, No. 6, 2003 1409

Table 1. Solvent and Titrants Used in Titration and Their Solubility Parametersa Hansen solubility parameters [(MJ/m3)0.5]

a

substance

abbreviation

δd

δp

δh

toluene (solvent) isooctane (2,2,4-trimethylpentane) methyl ethyl ketone (2-butanone) iso-octanol (2-ethyl-1-hexanol)

Tol IC8 MEK IC8ol

18.0 14.3 16.0 16.0

1.4 0 9.0 3.3

2.0 0 5.1 11.9

From Barton.12 Table 2. Binders Used in Three-Dimensional Turbidimetric Titration notation

type and source

A-85 B-14 C-12 D-60 E-180 F G H-180 I-180 J

extracted from road sample extracted from road sample extracted from recovered asphalt Laguna, Venezuela Laguna, Venezuela origin unknown 3.8/96 tar-containing binder and H-180 Mexico Laguna, Venezuela Visbreaker (Laguna, Venezuela)

ratio between the viscosity of bitumen and the viscosity of maltenes (SEC II) (maltenes from Size Exclusion Chrom. fraction II). In terms of Hansen parameters, the use of a single titrant, such as n-heptane, to investigate a solubility space is far from sufficient. Redelius suggested the use of three titrants, each of which showing one predominant type of molecular interaction. These titrants were isooctane, methyl ethyl ketone (MEK), and iso-octanol (cf. Table 1), which are all miscible with toluene. Earlier attempts have been made to use alcohols as titrants but with poor results, because the alcohols separated from the mixture rather than the components of the bitumens tested.11 The use of three titrants permitted a more comprehensive picture of the stability of bitumen to be obtained.6 Experimental Section Three-dimensional turbidimetric titration was selected to investigate the conditions for the mixing of old and new binders. For this purpose, mixtures of one new binder and binders extracted from roads were prepared. The mixtures were intended to correspond to recycled binders. The tests were also intended to give information on the interaction between the old and new binders. The titration method was first assessed during a series of tentative tests using several binders, as described below, after which point the stability toward the addition of isooctane, MEK, and iso-octanol was tested. Materials. The binders used in the studies are given in Table 2. Three asphalt cores were taken from roads and the binders were recovered. The binders obtained were referenced as A-83, B-14, and C-12, where 83, 14, and 12 are the penetration, in dmm, at 25 °C. Binder A-83 was extracted from a 25-year-old dense-grade wearing course, binder B-14 was extracted from milled asphalt granules, and binder C-12 was from a core that was extracted from a wearing course. The solvent used for extraction was dichloromethane. It is likely that the initial penetration grades of all three binders were >85 dmm, and that all these binders originated from Laguna, Venezuela, because this bitumen source dominated the Swedish market during the period when the mixtures were originally manufactured. The extracted binders were mixed with new binder (Laguna), referenced as R-180 (where R denotes rejuvenator and 180 denotes the penetration grade), in proportions of 20/80 and

comments

straight run; may have been slightly oxidized straight run; may have been slightly oxidized used in practice but to a limited extent binders mixed in laboratory also TFO-aged to 65, 38 and 25 dmm straight run

80/20 (by weight percent). The reason for choosing these ratios were a compromise that was based on the following: (i) interest in studying stabilities of mixtures upon the addition of small amounts of old material, (ii) expectations of finding the lowest stability at a proportion of ∼50/50%,12 (iii) possible problems in the detection of small changes in stability, (iv) the need to limit the number of tests, and (v) recycling ratios in practice may be similar to 20% new material in hot in-place asphalt recycling and 80% new material in in-plant asphalt recycling. Binders D-60, E-180, G, H-180, I-180, and J was used to evaluate the method of turbidimetric titration. Binder G contained 3.8 wt % of a coal tar-containing binder. To investigate the effects of aging, binder H-180 was aged in a thin film oven (TFO). Toluene was used as the solvent, and isooctane, MEK, and iso-octanol were used as titrants. The solubility parameters and abbreviations of solvent and titrants are given in Table 2. Three-Dimensional Turbidimetric Titration. The main parts of the method have been described by Redelius.6 The principle of the method used is to make solutions of bitumen in toluene and then add titrant until precipitation occurs. After the amount of titrant required for precipitation at different concentrations of bitumen in toluene has been determined, the parameters that describe the state of peptization are calculated using eqs 3-7. A major problem arises when trying to detect the precipitation, because bitumen absorbs light very efficiently over a wide range of wavelengths. A new method for detection of the precipitation point has been developed by Nynas AB,13 where an ultraviolet/visual spectroscopy-attenuated total reflectance (UV/Vis-ATR) probe is used. As the precipitation occurs, the absorbance of light on the ATR probe increases sharply, making the precipitation point normally easy to detect. Bitumen was dissolved in toluene in concentrations of 10, 20, 30, 40, and 50 wt % the day before titration for the binders to have remained in solution for approximately the same duration, because the titration procedure requires ∼5 h. The equipment used for automated titration was a titrator (Schott (11) Davison, R. R.; Bullin, J. A.; Glover, C. J.; Chaffin, J. M.; Peterson, G. D.; Lunsford, K. M.; Cipione, C. A. Development of Gel Permeation Chromatography, Infrared and Other Tests to Characterize Asphalt Cements with Field Performance; Research Report 458-1F; Chemical Engineering Department and Texas Transportation Institute, Texas A&M University: College Station, TX, 1989. (12) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; 2nd ed.; CRC Press: Boca Raton, FL, 1991. (13) Redelius, P. G., personal communication, 2000.

1410 Energy & Fuels, Vol. 17, No. 6, 2003

Figure 2. Absorbance at 480 nm detected during titration with MEK in a 20% solution of bitumen H-180 in toluene, and the regression procedure used for determination of the precipitation point. Titronic Universal) and a UV/Vis spectrophotometer (Varian Cary model 50 Bio) that had been fitted with an ATR probe (Equitech fiber-optic with a three-bounce sapphire ATR lens). The temperature was kept constant at 25 °C, using circulating water. The spectrophotometer and the titrator were set up in such a way that recording and dosing could start simultaneously. The samples were continuously stirred. Absorbance at both 480 and 740 nm was recorded concurrently. In the method described by Redelius, the absorbance at 740 nm was used. In this study, the absorbance at 480 nm was also monitored, because absorbance at this wavelength is stronger (cf. Figure 4, presented later in this work) and, thereby, is less noisy. At wavelengths 50% is difficult, because the viscosity of the solutions becomes too high and certain bitumens are more difficult to dissolve.

Results Because the method of turbidimetric titration of bitumens using three different titrants, as described by Redelius,6,13 is a new method, it was decided to evaluate the method and assess its reliability before investigating the properties of “recycled” binders, i.e., mixtures of old and new binders. Evaluation of Three-Dimensional Turbidimetric Titration. Initially, some tests were related to studies of the precipitation process. Tests on several bitumens were performed to get an opinion about the variation in FRmax and Cmin values. A testing program was set up, in collaboration with Nynas AB, to compare the results that were obtained in the two laboratories using three binders. The diagram shown in Figure 2 is typical for changes in absorbance during titration for all three titrants used. The evolution of the UV/Vis absorbance recorded using an ATR probe can be divided into three stages: (i) the initial stage of decreasing absorbance during the addition of the first few drops of titrant, (ii) the intermediate plateau stage of low absorbance, and (iii) the final stage of increasing absorbance. Some work was performed to investigate the process of precipitation and the UV/Vis spectra obtained using the ATR probe, to better understand the mechanisms behind the measurements. During a limited number of titrations, spectra were scanned over a range of 400800 nm at additions of 0.8 mL of titrant per scan. The result of such a test is illustrated in Figure 4 for five volumes of titrant added (0, 2, 8.4, 10, and 14 mL). The spectra are selected to show an initial decrease and final increase in absorbance after the precipitation point (at 8.5 mL of titrant). Note that the titrants and toluene show negligible absorbance in the range of wavelengths that has been studied. The spectra obtained from all titrations of bitumen were very similar, using different titrants at >480 nm, but also were similar to the spectrum of the bitumen itself, whereas minor differences were observed at 100%. Note that the large fraction of precipitate found in isooctane by centrifugation may be unexpected to someone with experience from using more-common methods to obtain asphaltenes, i.e., dissolving at higher temperatures and using techniques for filtering and

Figure 6. Determination of FRmax and Cmin for I-180 titrated with IC8, showing a severe shift of the precipitation line.

cleaning the asphaltenes. Only the fraction that is truly dissolved in isooctane will remain after centrifugation. On a few occasions, a tendency was observed, that precipitation occurred slightly earlier each time the titration was repeated using IC8 as the titrant. The results of the, by far, most serious case of such a drift is illustrated in Figure 6, where the results are shifted upward each time a new measurement is performed (I180). In the study illustrated in Figure 6, all five solutions that contained 20% bitumen were titrated first, and then all five solutions that contained 50% were titrated. For the 20% solutions (the group of data points second from the left), no shift was observed, whereas at 50% bitumen (the far right group of data points), the upward shift became obvious. At a different occasion, the 10%, 30%, and 40% solutions were titrated in five series, in the order of 10%, 30%, and 40%, showing a clear upward shift for each series. This indicates that the observed shift was amplified at higher concentrations. Even though the shift was considered unacceptable for one single case, shifts well within confidence intervals were observed at titrations with IC8 in three other cases. Note that titrations performed by Nynas AB showed no shift. This example of systematic error shows the importance of understanding the mechanisms behind automated detection using an UV/Vis-ATR probe, because the FRmax and Cmin values obtained for I-180 using the first set of tests (20% and 50%) and the second set (10%, 30%, and 40%) would have been different. This issue is considered further in the discussion section. To evaluate the method, and to get an opinion about the range of FRmax and Cmin values of various bitumens

1412 Energy & Fuels, Vol. 17, No. 6, 2003

Karlsson and Isacsson

Table 3. Mean Values of FRmax and Cmin for Five Binders of Which One (H-180) Was Aged to Different Grades of Penetrationa Isooctane

MEK

Iso-octanol

FRmax

binder

FRmax

Cmin

FRmax

Cmin

FRmax

Cmin

sum rank

D-60 E-180 F G H-180 H-65 (TFO) H-38 (TFO) H-25 (TFO)

0.29 (0.28-0.29) 0.26 (0.26-0.27) 0.58 (0.56-0.60) 0.30 (0.27-0.33) 0.36 (0.35-0.37) 0.42 (0.41-0.44) 0.42c (0.40-0.43) 0.42c (0.39-0.45)

0.38 (0.36-0.42) 0.34 (0.33-0.35) 1.07 (0.93-1.28) 0.39 (0.31-0.56) 0.64 (0.57-0.73) 0.59 (0.55-0.65) 0.71c (0.61-0.86) 0.83c (0.63-1.26)

0.43 (0.42-0.46) 0.39 (0.35-0.42) 0.45 (0.43-0.47) 0.37b (0.32-0.42) 0.49 (0.46-0.52) 0.52 (0.46-0.57) 0.51 (0.50-0.53) 0.49 (0.49-0.49)

0.53 (0.49-0.59) 0.54 (0.42-0.79) 0.61 (0.52-0.73) 0.56b (0.40-1.13) 0.61 (0.52-0.75) 0.59 (0.43-1.04) 0.63 (0.59-0.68) 0.63 (0.62-0.65)

0.38 (0.35-0.40) 0.37 (0.36-0.39) 0.58 (0.58-0.59) 0.39 (0.38-0.40) 0.44 (0.43-0.45) 0.44 (0.44-0.44) 0.45 (0.44-0.46) 0.44 (0.44-0.45)

0.91 (0.71-1.32) 0.62 (0.53-0.74) 1.28 (1.21-1.36) 0.75 (0.68-0.84) 1.16 (1.05-1.28) 0.91 (0.89-0.93) 0.87 (0.79-0.96) 0.99 (0.91-1.09)

1.1 1.02 1.61 1.06b 1.29 1.38 1.38 1.35

3 1 8 2b 4 6 6 5

a The mean values of FR max and Cmin are given in bold type. The range for each given to the right of the mean value in parentheses is the 95% confidence interval. b Precipitation was detected visually before it was detected by UV/Vis-ATR. c No precipitation was detected for the 40% or 50% solutions.

prior to the studies on “recycled” binders presented in the next section, five binders were tested and the FRmax and Cmin values were determined. The results are given in Table 3. The 95% confidence interval is also given beside each mean value. The width of the confidence intervals given may be underestimated, because there might be other errors that are related to reproducibility, as shown previously. The confidence intervals indicate the low precision by which Cmin is determined, compared to FRmax. The confidence intervals for Cmin are too broad, from the tests presented using MEK as the titrant, for any conclusions to be drawn. Binders D-60 and E-180 are regarded as high performing, whereas binder F is known to show poor structural stability. Binder G is known to contain coal tar and was visually observed to be almost ready to undergo phase separation (further addition of tarcontaining binder caused separation). When binder G was titrated with MEK, precipitation was sometimes visually observed long before it was detected by UV/ Vis-ATR. In this case, MEK revealed the low stability, but the method used for detection failed to show low stability (high values of FRmax and Cmin). Binder H-180 was aged using the TFO to penetration values of 65, 38, and 25 dmm. Particulates were observed while solutions of H-25 were being prepared. However, no major changes in stability due to TFO aging could be found by titration. In addition to the results given in Table 3, the difficulties experienced in detecting precipitation at higher concentrations of H-25 and H-38 when titrating with isooctane should be mentioned. In the right columns of Table 3, the structural stability of the binders are ranked based on the sum of FRmax values from all three titrants. The idea behind this is that the structural stability is dependent on sufficient stability toward all three titrants. Comprehensive testing was performed on three different binders in the laboratories of Nynas AB and Royal Institute of Technology. Five repetitions were performed for each concentration of binder in toluene, which required a total of 225 titrations in each laboratory. The results are summarized as mean values in Table 4. The results agree fairly well. The only major disagreement between the laboratories involved the titration of H-180 with MEK. The H-180 was evaluated earlier in the tentative tests that have been previously described and presented in Table 3. These results agree better with the Nynas results. The reason for this discrepancy is unknown. Generally, the FRmax values agree better than the Cmin values.

Table 4. Mean Values of FRMax and Cmin Obtained at Two Different Laboratoriesa and Using Three Binders FRmax binder

KTH

I-180 H-180 J

0.28 0.36 0.62

I-180 H-180 J I-180 H-180 J

Cmin Nyn

KTH

Nyn

Isooctane 0.29 0.39 0.61

0.34 0.67 1.03

0.36 0.63 1.30

0.44 0.58b 0.57

MEK 0.44 0.48 0.56

0.52 0.50b 0.69

0.56 0.66 0.77

0.37 0.47 0.66

Iso-octanol 0.39 0.48 0.64

0.72 1.00 1.35

0.64 0.95 2.00

a The two laboratories were the Royal Institute of Technology (denoted as KTH) and Nynas AB (denoted as Nyn). b The tests were repeated, giving a result similar to those obtained at Nynas (cf. Table 3).

Binders F and J were regarded as low-stability binders. Poor stability seems to be indicated by values of FRmax > 0.5 and Cmin > 1.0 for IC8 and IC8ol. For all the binders studied, the results obtained using MEK could not be found to clearly distinguish between the binders of low and high stability. FRmax and Cmin of Mixtures of Old and New Binders. Binders A-83, B-14, C-12, and F were mixed with R-180, and their stability toward the addition of IC8, MEK, and IC8ol was tested for each mix, as well as that of the pure binders. The purpose of this activity was to simulate the mixing of old and new binders that normally is performed during asphalt recycling, to get an opinion about the stability of recycled binders. The results are presented in Table 5, showing mean values of FRmax and Cmin, together with corresponding confidence intervals. Binder F was included as an example of a binder that is considered to be less suitable for mixing with most of the other binders, because of its relatively low stability. Titrating with isooctane changes the solvent properties of the bitumen/toluene mixture, mainly toward low dispersion, but also, low polar and hydrogen-bonding interactions. The FRmax and Cmin values obtained from titrations of the three old binders, as well as mixes with R-180, using isooctane, seem toexhibit acceptable stability, because none have Cmin values that are >1 (cf. Table 5). The values of FRmax and Cmin for the old binders were larger than those for R-180, as expected. For the mixes of A-83 and B-14 with R-180, the values of FRmax are

Bitumen Structural Stability Characterization

Energy & Fuels, Vol. 17, No. 6, 2003 1413

Table 5. Mean Values and Confidence Intervals for FRmax and Cmin at Titration of Four Different Binders Mixed with R-180a binder

0%

20%

80%

100%

0.34 (0.30-0.37) 0.32 (0.30-0.33) 0.32 (0.30-0.34) 0.49 (0.47-0.51)

0.29 (0.28-0.31) 0.28 (0.27-0.29) 0.34 (0.31-0.36) 0.58 (0.56-0.60)

A-83 B-14 C-12 F

0.26 (0.25-0.27) 0.26 (0.25-0.27) 0.26 (0.25-0.27) 0.26 (0.25-0.27)

FRmax Values in Isooctane 0.31 (0.30-0.32) 0.33 (0.33-0.34) 0.33 (0.30-0.35) 0.31 (0.28-0.35)

A-83 B-14 C-12 F

0.38 (0.35-0.42) 0.38 (0.35-0.42) 0.38 (0.35-0.42) 0.38 (0.35-0.42)

FRmax Values in MEK 0.43 (0.42-0.44) 0.43 (0.42-0.43) 0.42 (0.41-0.43) 0.42 (0.41-0.44)

0.36 (0.35-0.37) 0.39 (0.38-0.40) 0.34 (0.32-0.35) 0.48 (0.47-0.48)

0.43 (0.36-0.42) 0.38 (0.37-0.38) 0.32 (0.31-0.34) 0.45 (0.43-0.47)

A-83 B-14 C-12 F

0.38 (0.38-0.39) 0.38 (0.38-0.39) 0.38 (0.38-0.39) 0.38 (0.38-0.39)

FRmax Values in Iso-octanol 0.40 (0.37-0.43) 0.37 (0.36-0.38) 0.37 (0.36-0.37) 0.43 (0.42-0.43)

0.37 (0.35-0.39) 0.31 (0.30-0.32) 0.31 (0.30-0.32) 0.55 (0.55-0.55)

0.36 (0.35-0.38) 0.29 (0.27-0.30) 0.29 (0.29-0.29) 0.58 (0.58-0.59)

A-83 B-14 C-12 F

0.35 (0.31-0.40) 0.35 (0.31-0.40) 0.35 (0.31-0.40) 0.35 (0.31-0.40)

Cmin Values in Isooctane 0.37 (0.35-0.41) 0.41 (0.39-0.44) 0.36 (0.31-0.45) 0.44 (0.33-0.78)

0.35 (0.28-0.49) 0.58 (0.50-0.68) 0.73 (0.58-1.01) 0.91 (0.79-1.09)

0.44 (0.38-0.53) 0.55 (0.46-0.70) 0.40 (0.32-0.58) 1.07 (0.93-1.28)

A-83 B-14 C-12 F

0.67 (0.51-1.06) 0.67 (0.51-1.06) 0.67 (0.51-1.06) 0.67 (0.51-1.06)

Cmin Values in MEK 0.53 (0.50-0.56) 0.49 (0.47-0.52) 0.50 (0.47-0.54) 0.49 (0.45-0.55)

0.60 (0.53-0.69) 0.44 (0.42-0.46) 0.59 (0.49-0.78) 0.55 (0.53-0.58)

0.55 (0.46-0.70) 0.43 (0.42-0.45) 0.54 (0.47-0.64) 0.61 (0.52-0.73)

A-83 B-14 C-12 F

0.64 (0.59-0.70) 0.64 (0.59-0.70) 0.64 (0.59-0.70) 0.64 (0.59-0.70)

Cmin Values in Iso-octanol 0.57 (0.47-0.75) 0.57 (0.51-0.65) 0.59 (0.58-0.61) 0.71 (0.68-0.75)

0.62 (0.53-0.77) 0.37 (0.34-0.39) 0.37 (0.34-0.42) 1.16 (1.14-1.18)

0.72 (0.61-0.88) 0.31 (0.28-0.35) 0.34 (0.32-0.35) 1.28 (1.21-1.36)

a The titrants were isooctane, methyl ethyl ketone (MEK), and iso-octanol. The binder amounts were 0%, 20%, 80%, and 100%. The mean values are given in bold type, and the range given in parentheses to the right of the mean value is the 95% confidence interval.

Figure 7. Mean values and confidence intervals for FRmax and Cmin at titration, with iso-octanol, of binders C-12 and F mixed with R-180. Values have been taken from Table 5.

larger, compared to the pure binders, but the difference is small, and only the test with B-14 shows a significant difference. Binder F shows a very low stability, and the FRmax and Cmin values seem to be almost proportional to the fraction of binder F in the mixture. The addition of MEK during titration mainly contributes to increasing the polar interactions of the solvent. Both pure binders and mixes seem to be stable to the addition of MEK. The effect of aging also seems to be small. Binder F seems slightly less stable than the other binders. By titrating with iso-octanol, the stability toward the addition of hydrogen-bonding interactions is manifested. Opposite to titration with isooctane, the content of old binders gives a more stable binder, with regard to titration with iso-octanol. The most aged binders, such

as B-14 and C-12, show a linear decrease in both the FRmax and Cmin values, relative to the amount of binder added (cf. Figure 7; confidence intervals are indicated with dotted lines). In contrast, the addition of binder F shows a linear increase in both the FRmax and Cmin values, relative to the amount of binder added. Discussion The discussion starts with the key concepts and models of stability and solubility that have been applied to interpretation of data from turbidimetric titration of bitumen. The method for turbidimetric titration then is evaluated, and, finally, the significance of the results for asphalt recycling is discussed. Modeling of Solubility and Stability of Bitumen. Structural instability (phase separation) is indisputably

1414 Energy & Fuels, Vol. 17, No. 6, 2003

detrimental to binder performance. The degree of structural stability is also of interest. A few hypotheses can be formulated, regarding the importance of binder structural stability: (1) Because phase separation means a drastic change in binder properties, it is favorable to have a buffer against phase separation, i.e., high stability. The stability of a binder will probably be affected in operation when mixed with stone material, filler, and additives. (2) Oxidative aging produces components that increase the viscosity of a binder; however, the hardening susceptibility is dependent on how well peptized these viscosity-building components (e.g. asphaltenes) are. As earlier described, structural stability is related to the degree of peptization. (3) The beneficial properties of bitumen is a consequence of the complex constitution of bitumen in which different types of molecules, such as viscosity builder, aggregate adhesive, and solvent, show different functions for these types of molecules. Excessively high stability may be a sign of bitumen with a comparatively uniform constitution. Application of a theory of solubility, such as that proposed by Hansen,4 which was originally developed for systems with a few components, may cause problems when applied to bitumens, which exhibit a very complex constitution and, most likely, a range of amphophilic components (i.e., both hydrophilic and hydrophobic). The theory developed by Hansen predicts that a solute with solubility parameters outside the solvent solubility sphere will precipitate. It is common opinion that there are components in bitumen (often called resins) that show an ability to keep nonpolar or oily fractions together with the asphaltenes in a stable solution.3 Without these “bridging” components, the oily and asphaltene fractions are not miscible, which means that bitumen stability is related to the mutual solubility of all bitumen components and their distribution of solubility parameters. Consequently, the relatively simple model that was proposed by Hansen may show limitations for studies of structural stability in such a complex system as bitumen. When interpreting results from titration tests, it is important to distinguish between the effects of Hansen’s solubility parameters and structural stability. The parameters describe the center of solubility, in terms of the strength of intermolecular interactions, whereas structural stability is related to the “fourth” solubility parameter: the radius R of the solubility sphere (see eq 1). In other words, R, being the maximum difference between the solvent and (mean) bitumen solubility parameters, could be interpreted as a measure of structural stability. Solubility parameters and structural stability of bitumen cannot be studied separately using a single titrant, as earlier indicated. The previous discussion may contribute in the interpretation of results presented in this paper regarding aged bitumen. The severely aged B-14 and C-12 showed higher values of FRmax and Cmin (indicating lower stability) for IC8 and lower values of FRmax and Cmin (indicating higher stability) for IC8ol, compared to other binders that are assumed to be of the same origin (cf. Table 5). These results could illustrate the common pattern for longterm aged bitumen, whereas the pattern might be

Karlsson and Isacsson

different for high-temperature, short-term aging, as indicated in Table 3 for TFO-aged bitumen H-180. Generally, oxidative aging of bitumen leads to a certain increase in polar components; however, most of the molecules remain unaffected. Consequently, the distribution of the bitumen solubility parameters broadens, which, at a first glance, would suggest a decrease in structural stability (broad and discontinuous distribution of solubility parameters leads to phase separation). However, the results presented in Table 5 illustrate that bitumen aging increases stability toward the addition of hydrogen-bonding substances, such as IC8ol. Examples of products of bitumen aging are ketones, anhydrides, and carboxylic acids,3 which can be assumed to form hydrogen bonds with alcohols such as IC8ol. This observation could be interpreted as a shifting of the (mean) solubility parameters of bitumen toward greater hydrogen-bonding attractions (increasing δh), whereas the structural stability is less affected (small change in R). It is also possible to interpret this observation as the aged binder components “bridging” between IC8ol and the remaining binder, hence creating a more stable mixture. Note that a one-dimensional Heithaus titration using a nonpolar titrant (such as IC8), in this case, would have given limited information regarding changes in the solubility and stability. As previously indicated, the mechanisms behind the structural stability of bituminous binders are very complex, and comprehensive research is needed to draw more-definite conclusions. Assessment of the Three-Dimensional Turbidimetric Titration Method. When performing turbidimetric titration, as described in this paper, errors occur in each step, from sample preparation to analysis. These errors are put into focus one by one and are discussed in this section. The main steps involved in the method are (i) preparation of bitumen solutions in toluene, (ii) dosing of the titrant, and (iii) detection by UV/Vis-ATR and determination of the precipitation point. To achieve equilibrium, the bitumen solutions were prepared one day in advance. The fact that toluene is used as the solvent may be a restriction, because toluene may not be a perfect solvent for all types of bituminous binders to be tested. Theoretically, Cmin is supposed to consider this problem by extrapolation of the test data to values that are independent of the solvent. Regarding dosing of the titrant and the simultaneous collection of data, no problems were identified, because of the high accuracy of the dosimeter used. For example, the time required for dosing 16 mL over a period of 10 min deviated by