VOLUME 17, NUMBER 3
MAY/JUNE 2003
© Copyright 2003 American Chemical Society
Articles Rheological Effects of Waxes in Bitumen Y. Edwards* and P. Redelius Division of Highway Engineering, Royal Institute of Technology (KTH), S-100 44, Stockholm Sweden, Nynas, S-149 82 Nyna¨ shamn, Sweden Received September 9, 2002. Revised Manuscript Received December 30, 2002
Rheological effects of adding two bitumen waxes (isolated from SEC-II fraction) to three bitumens were studied using dynamic mechanical analysis (DMA). Also, a commercially available slack wax was used in the study. The results show that the magnitude and type of effect on bitumen rheology depend on the bitumen and type of crystallizing fraction in the bitumen. Effects due to wax content shown in DMA temperature sweeps are well related to the corresponding effects shown in DSC thermograms. The slope of the logarithm of the complex modulus between 25 °C and 60 °C is introduced as a possible proper factor for predicting rutting sensitivity due to wax content.
Introduction Bitumen is the black adhesive that binds flexible pavements on roads and airfields together. Bitumen is also used in other areas of application, such as waterproofing, flooring and joint materials. Almost all bitumen originates from crude oil and is the residue of a refining process. It is well-known that bitumen is a very complex and temperature dependent material consisting of hydrocarbon molecules. Naphtenic-base crude oils often give a large yield of bitumen that may be of good quality, while paraffinic crude oils may give bitumen of good quality or yield bitumen not suitable for road construction. Wax is a term used for all kinds of waxlike solids and liquids in nature, as well as for synthetic compounds with waxy physical characteristics.1 Some crude oils contain substantial amounts of petroleum wax, which crystallize at falling temperature and may cause severe problems in pipelines and process equipment by pre* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Warth, A. B. The Chemistry and Technology of Waxes; Reinhold: New York, 1956.
cipitation.2,3 However, petroleum waxes are also useful industrial products (for candles, polish, etc.). Wax in bitumen is petroleum wax coming from refining of waxy paraffinic crude oils. However, different users have not yet come to any agreement concerning a most suitable definition for wax in bitumen.4 The two fundamental petroleum wax classes generally recognized in crude oils and distillates are paraffin waxes and microcrystalline waxes. It has been suggested that wax in bitumen could be classified in the same two groups. Bitumen may contain also other types of molecules identified as wax.5 Macrocrystalline paraffin waxes mainly are composed of n-paraffins (n-alkanes) with minor amounts of isoand cycloparaffins. They crystallize as plates or needles. (2) Srivastava, S. P.; Handoo, J.; Agrawal, K. M.; Joshi, G. C. PhaseTransition Studies in n-Alkanes and Petroleum-Related Waxes-A Review. J. Phys. Chem. Solids 1993, 54, 639-670. (3) Lira-Galeana, C.; Hammami A. Wax Precipitation from Petroleum Fluids. Dev. Pet. Sci. 2000, 40B. (4) Carbognani, L.; Duarte, D.; Rosales, J.; Villalobos, J. Isolation and Characterization of Paraffinic Components from Venezuelan Asphalts. Effects of Paraffin Dopants on Rheological Properties of Some Asphalts; Venezuela, 1998. Petroleum Science and Technology 16 (9&10), 1085-1111 (1998). (5) Redelius, P.; Lu, X.; Isacsson, U. Non-Classical Wax in Bitumen. Road Materials and Pavement Design 2002, 1.
10.1021/ef020202b CCC: $25.00 © 2003 American Chemical Society Published on Web 03/21/2003
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The melting point of isolated macrocrystalline paraffin waxes lies around 50-70 °C, in a distinct melting area, but in bitumen the melting point of the paraffins decreases with about 20-30 °C.6 Microcrystalline waxes are aliphatic hydrocarbon compounds containing a considerable amount of iso- and cycloparaffins and minor amounts of n-paraffins. They crystallize as small needles. A microcrystalline petroleum wax is characterized also by a less distinct melting area and its high average molecular weight giving higher viscosity compared to macrocrystalline paraffin wax.7 Wax in bitumen has normally been considered as a negative indication of the quality of the bitumen. Consequently, many bitumen specifications include requirements concerning wax content. Factors influencing the effect of waxes are chemical composition (source of bitumen) and rheological behavior of the bitumen as well as content, composition and crystallinity of the wax. According to the literature, negative effects on bitumen due to high wax content may show in different ways. The viscosity may suddenly decrease in the melting temperature range of the crystallized wax (typically around 60-80 °C). Other feared effects are brittleness, physical hardening, poor ductility and poor adhesion. As a rule of thumb, it has been proposed that the wax content in bitumen should not exceed 3 wt % (% by weight).8 Depending on definition of wax in bitumen, this may or may not be a relevant figure. Results and experience concerning negative effect of wax in bitumen are based mainly on laboratory studies and some are listed below.9-12 Wax in bitumen may: show negative effects during compaction of an asphalt mix as a result of a suddenly increasing viscosity. The asphalt concrete may also get sensible to rutting as a result of decreasing viscosity caused by wax at higher temperatures. make the bitumen more brittle and the asphalt concrete prone to cracking at low temperatures. Physical hardening at low temperature is considered to be, at least partly, caused by wax. show a negative effect on the adhesion between bitumen and aggregate. The ability to wet the aggregate may decrease because of the hydrophobic character of the wax. reduce the cohesion of the bitumen, which is critical especially for thin asphalt layers. Inhomogenities in the bitumen, caused by wax crystals, may reduce ductility at lower temperatures.
Somewhat contradictory, also positive effects of wax in bitumen have been reported, particularly improved low-temperature properties and lower handling temperatures (for better compaction).5,6 Several methods are used for determination of wax content. They all give different results for the same bitumen, which has caused problems for many years.13,14 This obstacle was obvious also in the European harmonizing work.15,16 Most test methods for determining wax content in bitumen contain two steps, separation and wax precipitation, respectively. The separation has been done by adsorption of polar compounds combined with solvent extraction of neutral oils and waxes or even by distillation (destructive). Later techniques used are nonaqueous ion exchange chromatography (IEC) and size exclusion chromatography (SEC) to separate the neutral oils from the polar compounds. Wax precipitation means isolating the waxes from the rest of the neutral fraction by using a solvent, that will not dissolve the wax at low temperature (de-waxing step). The wax will crystallize and can then be filtrated. The result both quantitatively and qualitatively depends on the solvent and precipitation temperature used. Another method for studying waxes is differential scanning calorimetry (DSC), which has been used since the 1970s for studying the precipitation of wax and determination of wax content.14,17 The wax content or crystallizing fraction (CF) determined designates the amount of material taking part in the phase transition over a given temperature range. In a DSC temperature sweep, the enthalpy changes are measured in a cooling cycle for determination of crystallizing material and in a heating cycle for calculation of amount of melting material. Wax influence on the rheological behavior of bitumen has been studied and documented by several authors. Some of these studies using a rheometer are mentioned below. McKay et al.18 isolated bitumen wax using the IEC neutral fractions and wax precipitation in methyl ethyl ketone (MEK) at -25 °C. From the rheological data presented (dynamic viscosity and tan δ at 25 °C and 1.0 rad/s), it was concluded, in general and speculatively, that paraffin waxes in bitumen appear to impart higher stiffness and may result in poor low-temperature properties, and that microcrystalline waxes appear to reduce stiffness and may improve low-temperature properties of bitumens.
(6) Butz, T.; Rahimian, I.; Hildebrand, G. Modifikation von Strassenbitumen mit Fischer-Tropsch Paraffin. Bitumen 2000, 3. (7) Musser, B. J.; Kilpatrik, P. K. Molecular Characterization of Wax Isolated from a Variety of Crude Oils. Energy Fuels 1998, 12, 715725. (8) Gawel, I.; Czechowski, F.; Baginska, K. Study of Wax Isolated from Bitumen. Eurasphalt & Eurobitume Congress 1996. (9) Fritsche, H. Zur Bestimmung des Paraffingehaltes in Bitumen. Bitumen 1995, 1, 29-33. (10) King, G. N. Using Rapid Analytical Methods To Define Bitumen Chemical Structure: A Refiners Approach. In Proceedings of the International Symposium on Chemistry of Bitumens, Rome, 1991; Vol II, pp 742-769. (Copyright by the University of Wyoming Research Corporation.) (11) Que, G.; Liang, W.; Chen, Y.; Liu, C.; Zhang, Y. Relationship between Chemical Composition and Performance of Paving Asphalt. In Proceedings of the International Symposium Chemistry of Bitumens, Rome, 1991; Vol. II, pp 517-527. (12) Planche, J. P.; Claudy, P. M.; Le´toffe´, J. M.; Martin D. Using Thermal Analysis Methods To Better Understand Rheology. Thermochim. Acta 1998, 223-227.
(13) Krom, C. J. Determination of the Wax Content of Bitumens. J. Inst. Pet. 1968, 54, 232-240. (14) Noel, F.; Corbett, L. W. A Study of the Crystalline Phases in Asphalts. J. Inst. Pet. 1970, 56, 261-268. (15) Bitumen and Bituminous Binders-Determination of the Paraffin Wax Content-Part 1: Method by Distillation. European Standard EN 12606-1; 1999 CEN Central Secretarial rue de Stassart 36 B-1050 Brussels. (16) , Bitumen and Bituminous Binders-Determination of the Paraffin Wax Content-Part 2: Method by Extraction. European Standard EN 12606-2; 1999 CEN Central Secretarial rue de Stassart 36 B-1050 Brussels. (17) Claudy, P.; Letoffe, J. M.; Planche, J. P. Using Thermoanalytical Methods To Characterize Bitumen Structure. In Proceedings of the Eurobitume Congress, Stockholm, 1993; Report No 1.08. (18) McKay, J. F.; Branthaver, J. F.; Robertson, R. E. Isolation of Waxes from Asphalts and the Influence of Waxes on Asphalt Rheological Properties. In Proceedings of the ACS 210th National Meeting, Chicago, August 20-25, 1995; Division of Petroleum Chemistry, American Chemical Society: Washington, DC, 1995.
Energy & Fuels, Vol. 17, No. 3, 2003 513 Table 1. Bitumens Used in This Study wax (wt %) penetration softening obtained (dmm) point (°C) using DSC
bitumen nonwaxy Venezuelan, NV Middle East, ME waxy Venezuelan, WV
176 205 214
39 39 39
0 4 2
Bitumen wax was isolated by Carbognani et al.,4 using clay absorption of the polar compounds followed by Soxhlet extraction of neutral oils and wax precipitation in MEK at -10 °C. Frequency sweeps were performed at temperatures from 10 to 70 °C. Significant differences were registered only for the high wax level at the highest and lowest temperatures. At high temperature, the paraffinic waxes, and especially the nalkanes in these waxes, were found to slightly impair the rheological behavior of bitumen (decrease in complex modulus). At low temperature, waxes from the paraffinic bitumen increased the dynamic shear modulus, while wax from the naphteno-aromatic bitumen decreased the modulus. Carbognani et al. concluded that this improvement (lower modulus at 10 °C) was due to cocrystallized aromatic and resin types. Bitumens with Tafelparaffin (TP) and Gatschparaffin (GP), respectively, were studied by Obertu¨r et al.19 TP is a macroparaffin wax (90 wt % n-paraffins) and GP is a half-solid microparaffin wax (about 70 wt % iso- and cycloparaffins). Temperature sweeps from -10 to 70 °C at low frequency were performed. At low temperatures, the complex modulus increased with increasing TP content. At higher temperatures and depending on paraffin content, the paraffin was dissolved in the bitumen. At even higher temperatures, the TP showed a thinning effect. Adding GP showed a thinning effect up to 10 °C. Butz et al.6 also studied the effect of adding FischerTropsch (FT) paraffin to bitumens. According to the authors, FT paraffin is similar to bitumen wax, but contains much longer molecules (from C40 to C115). The wider wax molecule distribution means larger melting area. The effects of FT-paraffins on the rheological behavior of the bitumens were measured using temperature sweeps from 10 to 60 °C at low and high frequencies. The conclusion drawn was that stiffness and elasticity increased at temperatures from 10 to 60 °C. The purpose of the work described in this paper was to study the effects of two bitumen waxes (isolated from the SEC-II fraction of the bitumens) on the rheological behavior of three bitumens by dynamic mechanical analysis (DMA). Also a commercially available slack wax was used in this study. Experimental Section Materials. Bitumen. Three different bitumens were used for the study, one nonwaxy and the other two containing approximately 2 and 4 wt % wax, respectively, according to DSC measurements. Characteristics of the bitumens are presented in Table 1. The bitumens are the same as those used in an earlier published study by Redelius et al.5 Crystallizing wax from two of the bitumens in Table 1 (Middle East, ME, and waxy Venezuelan, WV) and slack wax were used for doping the original bitumens. (19) Obertu¨r, U.; Rahimian, I. Einfluss derzugesetzten Paraffine und deren Struktur auf die Eigenschaften von Bitumen. Bitumen 1997, 4.
Bitumen Wax. Crystallizing bitumen wax was isolated from the SEC-II fractions of the two bitumens ME and WV used in this study. The isolated material is referred to as SEC-II wax and is considered to be the crystallizing fraction of bitumen. The procedure used is as follows.5 First preparative size exclusion chromatograhy (PSEC) was used to separate bitumen into two fractions, SEC-I and SECII. In the separation, 16 g of the bitumen sample was used and about 1600 mL of SEC-II eluate was obtained. The cutpoint between SEC-I and SEC-II was initially (first few samples) defined as the point where the eluate changes from nonfluorescence (black) to fluorescence (bright blue). The consecutive samples were run a specific time to cut-point decided by the initial runs. Most of the solvent (toluene) was then removed using a rotary evaporator. The SEC-II was diluted in toluene by a ratio of 5 g to 14 mL, and further by 2-butanone 11.25 mL for each gram of SEC-II. The solution was cooled to -20 °C and held at that temperature for 1 h, after which it was poured into a chilled funnel (vacuum) having a frit of 10-16 µm porosity. The filter cake was finally rinsed with 25 mL cold 2-butanone, and the bitumen wax fraction was obtained. The waxes are called wax 36 and wax 40 in this paper. Characteristics of the waxes5 are presented in Table 2. The composition of the two waxes are chemically different. Note that both the isolated waxes are not of the type macrocrystalline paraffin waxes, because there are no (wax 36) or very little (wax 40) n-alkanes according to GC-MS (rough estimate by comparing GC peak areas of internal standard and bitumen wax fractions). Both bitumen waxes show rather high amounts of asphaltenes and low amounts of saturates according to Iatroscan analysis. Wax 40 shows much higher amounts of aromatics than wax 36 according to Iatroscan. Furthermore, IR analysis shows high content of saturated hydrocarbons and no or only traces of aromatic and polar groups indicating that the two waxes mainly contain noninteracting molecules. Wax 36 appears medium hard and wax 40 hard and friable. The contradictions in results from chemical analysis are discussed by Redelius et al.5 Slack Wax. The slack wax was a commercially available product (Terhell Paraffin Type JA 201 from Hans-Otto Schumann GmbH& Co) with a congealing point (ASTM D 938) of 41/42 °C according to the product data sheet. Identification using GC-MS is shown in Figure 1. The eluted fraction of the slack wax (53%) was mainly composed of n-alkanes (about 90%) with carbon numbers from 19 to 36. The other 10% was mainly branched alkanes. The not eluted fraction may have larger molecular weights with carbon number higher than 36 and higher boiling point (which makes it difficult to analyze under normal GC-MS conditions). The slack wax was made into a solution at 1.0 mg/g (1000 ppm by weight) in hexane (>99%, by GC, Merck, Germany). The standard used for this analysis was docosane (Merck, Germany), and it was made into a standard solution at 70 ppmw in hexane. The instrument used was a Varian 3400 gas chromatograph with a Finnigan SSQ 7000 mass spectrometry detector. GC injector was kept at 275 °C to facilitate the vaporization of analytes with larger molecular in the GC injector. The transfer line was kept at 250 °C throughout the test. Splitless injection time was 30 s. The GC column used was a DB5-MS nonpolar capillary column (J&W Scientific, Folsom, CA), 30m, i.d. 0.25 mm, film thickness 0.10 µm. The initial GC column temperature was 120 °C for 1 min, and then ramped to 250 °C at 10 °C/min, and held at 250 °C for 60 min. The GC injection volume was 1 µL throughout the test. The mass detector was operated at 70 ev using EI mode. The source vacuum was 18.21 millitor and its temperature 150 °C. Manifold vacuum was 3.5E-07 Torr and its temperature 70 °C. The solvent delay time of MS filament was 3 min. MS full-scan was applied ranging from 45 to 550 m/z at 2 scans per second. The computer-based MS
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Table 2. Properties of Isolated Bitumen Waxes Used in This Study
a
crystallizing wax in bitumen
iatroscan analysisa
IR analysis
GC-MS (% n-alkanes)
crystallization temp (°C)/ enthalpy (J/g)
wax36 (Middle East)
21/37/23/19
0
73/74.5
wax40 (waxy Venezuelan)
15/16/52/17
high content of saturates, lack of aromatic and polar groups high content of saturates, lack of aromatic and polar groups
3
72/59.5
Saturates/aromatics/resins/asphaltenes.
Figure 1. Gas chromatogram of slack wax. spectrum library used was the NIST mass spectral search program, version 1.7. Under current experimental conditions, n-alkanes with carbon numbers from 19 to 36 were identified by GC-MS as shown in Figure 1. The small chromatographic peaks between n-alkanes peaks were identified as branched alkanes. Semiquantitative results were also obtained by using docosane as an external standard. The concentration of n-alkanes was estimated by the equation, Cn ) (An/Astd)Cstd, where Cn and Cstd are the concentrations of n-alkane and docosane, and An and Astd are the peak areas of n-alkane and docosane, respectively. All the calculation was based on the assumption that all the compounds show the same or similar response factor as that of the standard docosane in GC-MS. Such an assumption is practical and feasible for a rough estimation of the mass balance of the composition. Preparation of Samples. Prior to mixing, the bitumen wax was cooled to -20 °C, grinded to fine particles and homogenized. Samples were prepared by adding calculated amount of bitumen wax to approximately 10 g of bitumen, after which the sample was heated 30 min at 140 °C. The samples were then placed in preheated blocks and homogenized by shaking 2 × 90 s. Levels of 1, 3, and 6 wt % bitumen wax or slack wax were used. Bitumen and slack wax were mixed in larger quantities. Methods of Analysis. Dynamic Mechanical Analysis, DMA. Rheological measurements were performed with temperature and frequency sweeps using a rheometer (Rheometrics, RDA II). Experiments were carried out in the total temperature range of -30 °C to 100 °C. For the temperature range -30 to +80 °C, parallel plates with 8 mm diameter and gap 1.5 mm were used, while for the range 10 to 100 °C, the plate diameter was 25 mm and the gap 1 mm. The temperature sweeps started at the lower temperature and the temperature was increased by 2 °C/min. A sinusoidal strain was applied and the actual strain and torque were measured.
Dynamic shear modulus |G*| and phase angle δ were calculated. Frequency sweeps were performed from 0.1 to 100 rad/s. Differential Scanning Calorimetry, DSC. DSC analysis was performed using a Mettler TA3000 system. Approximately 15 mg of bitumen sample was weighed in an open pan and placed in the DSC cell under nitrogen blanket. The sample was heated to 120 °C and then cooled at 10 °C/min to -70 °C, followed by heating to 120 °C at the same rate. The method was used for determining the DSC wax content of the prepared (doped) bitumen/wax samples. Both cooling and heating cycles were used for calculation of the amount of crystallizing material (crystallizing material in the cooling cycle plus recrystallizing material in the heating cycle). For the calculation of amount of wax a value of ∆H ) 121.3 J/g is used. This figure is an estimated average value for wax in bitumen. However, different types of wax have different melting enthalpies why the amounts of crystallized fractions measured with DSC should be interpreted with care. Low-Temperature Creep Test, BBR. Creep tests were carried out using the bending beam rheometer (TE-BBR, Cannon Instrument Company). Test temperatures used were -15, -25, and -35 °C. A sample beam (125 mm long 12.5 mm wide and 6.25 mm thick) was submerged in a constant-temperature bath and kept at test temperature for 30 min. A constant load of 100 g was then applied to the beam of the binder, which was supported at both ends, and the deflection of center point was measured continuously. Creep stiffness S and creep rate m of the binders were determined at a loading time of 60 s.
Results and Discussions DMA temperature sweeps were performed for bitumen/wax mixtures over a wide range of temperatures (-30 to 80 °C) at a frequency of 10 rad/s. The three bitumens, NV, ME and WV, with 0, 1, 3, and 6 wt % wax 36, wax 40 or slack wax were used. Rheological behavior at medium and high temperatures was further studied using temperature sweeps from 10 up to 100 °C at a lower frequency (1 rad/s) as well as frequency sweeps at two different temperatures (50 and 20 °C). Within the wide temperature sweep from -30 to 80 °C three different processes occur; glass transition, wax crystallization and melting of the wax, respectively. In Figures 2-4 the results using the three bitumens with 0 and 6 wt % of each wax are illustrated. To further illustrate the relation between melting and crystallization of waxes in the bitumens and their rheological properties, recordings of thermal flow from DSC experiments were combined with the corresponding results from DMA experiments. To make the diagrams comparable, identical temperature scales were used. However, it should be pointed out that the heating rate is 10 °C/min for the DSC scan and 2 °C/min for the
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Figure 2. Complex modulus and phase angle (frequency 10 rad/s) of waxy Venezuelan bitumen compared to heat flow by DSC at identical temperatures. Figure 4. Complex modulus and phase angle (frequency 10 rad/s) of nonwaxy Venezuelan bitumen compared to heat flow by DSC at identical temperatures.
Figure 3. Complex modulus and phase angle (frequency 10 rad/s) of ME bitumen compared to heat flow by DSC at identical temperatures.
DMA experiment. To make the DSC curves more clear, only samples with the highest content of wax (6%) are included.
The interpretation of the DSC curve from bitumen is rather complicated since there are several overlapping phenomena to consider. The glass transition of bitumen occurs over a large temperature range due to the complex mixture of different molecules. Starting at the lowest temperature (-30 °C) in the heat flow diagrams (cf. Figures 2-4, bottom diagrams), the glass transition shows as a shift in specific heat versus temperature (a second-order transition). The glass transition is followed by a weak exothermic effect at around -10 °C caused by the cold crystallization of wax which could not crystallize through the cooling cycle due to limited mobility. However, on heating through the glass transition region, molecular mobility increases and the wax may crystallize. A broad endothermic effect is then observed in the heat flow diagrams. Most often this broad effect contains several overlapping thermal effects due to the melting of crystallizing fractions or other phenomena. Above +10 °C, mainly melting of the wax occurs. At around +60 °C to +70 °C the wax is completely melted. Figures 2-4 show the DMA diagrams with temperature sweeps from -30 to 80 °C at higher frequency (10 rad/s), while Figure 7 shows the same samples at sweeps from 10 to 100 °C at lower frequency (1 rad/s). Rheological Effects at Medium and Higher Temperatures. Adding wax 36 to waxy Venezuelan bitumen (WV) only showed a marginal effect at temperatures over about 50 °C but a stiffening effect at lower temperatures (cf. Figures 2 and 7). Adding wax 40 to WV showed greater effect than adding wax 36. The
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Figure 5. Relative slope variation of complex modulus for WV, ME, and NW with 6 wt % wax 36, wax 40, or slack wax.
DMA curves for WV +3 wt % wax 40 and WV +6 wt % wax 36 are practically identical. The great difference between samples containing slack wax and samples containing extra bitumen wax is due to the fact that the slack wax lowers the complex modulus at temperatures over about 40 °C to a markedly lower level compared to original bitumen. There is, as just mentioned, none or just a marginal such effect for samples containing wax 36 or wax 40 over about 40 °C but a clear stiffening effect at temperatures lower than about 40 °C. WV with 6 wt % wax 40 is stiffer than WV with 6 wt % slack wax throughout the whole temperature sweep.
Edwards and Redelius
Figure 6. Slope of logarithm of complex modulus between 25 and 60 °C for the three bitumens at different wax levels.
In practice, the binder modulus at high service temperatures is an important factor influencing plastic deformation of asphalt mixtures. The maximum in-situ temperature taken in Sweden is +60 °C. Too low binder stiffness and elasticity (storage modulus) at such pavement surface temperatures makes the asphalt pavement sensitive to rutting. The DMA results in this study show that WV bitumen containing wax 36 or wax 40 has the same complex modulus at +60 °C while bitumen containing slack wax has a lower modulus, which means that the latter may be more sensitive to rutting. Since the selection of a proper binder is based on the penetration value at +25 °C also the stiffening effect at +25 °C has to be taken into consideration. Consequently, a
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Figure 7. The complex modulus and phase angle (frequency 1rad/s) for NV, ME, and WV with 6 wt % wax 36, wax 40 or slack wax.
proper factor for rutting sensitivity could be the slope of the logarithmic function of the complex modulus between +25 and +60 °C. Table 3 shows such slope values for the complex modulus between these two temperatures based on the DMA temperature sweeps in Figures 2-4 and corresponding data from lower wax levels. The slopeT1-T2 is given as (log(G*(T2) - log(G*(T1))/ (T2 - T1), where G*(T2) and G*(T1) is the complex modulus at temperature T2 and T1, respectively. However, both the positive stiffening effect in one part of the temperature range and the sudden negative
Figure 8. Complex modulus and phase angle (frequency 10 rad/s) at temperatures below +5 °C.
decreasing viscosity effect in the other part of the temperature range will contribute to the decrease of such a value which may be misleading. (In Table 3 the complex modulus is given for both temperatures.) Figure 5 shows the variation of the slope over the total temperature range for WV, ME and NV. The appearances of these curves are very similar to the corresponding DSC curves (in Figures 2, 3 and 4). The negative slope25-60 value is typically higher for the samples with slack wax than for samples with wax 36 or wax 40. Figure 6 shows the slope25-60 versus wax content for all samples in the study.
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Table 3. Slope of Logarithm of Complex Modulus between +25 and +60 °C (S25-60) and Complex Modulus (CM) Values in Pa at These Temperatures WV added wax
wt %
S25-60 ×10-2
CM (60 °C) ×102
1 3 6 0 1 3 6 0 1 3 6
-6.20 -6.51 -6.74 -7.09 -6.31 -6.51 -7.11 -8.00 -6.06 -6.46 -7.26 -8.97
1.41 1.52 1.79 2.35 1.23 1.42 2.47 5.67 1.21 1.32 1.69 3.69
9.65 8.04 7.77 7.75 7.58 7.43 8.06 9.09 9.27 7.26 4.86 2.65
36
40
slack
ME
CM (25 °C) ×105
NV
S25-60 ×10-2
CM (25 °C) ×105
CM (60°C) ×102
-7.17 -7.29 -7.57 -7.91 -7.17 -7.34 -7.71 -8.26 -7.00 -7.31 -8.03 -9.17
2.26 2.19 2.73 3.50 1.90 2.16 2.89 4.74 1.89 1.61 2.37 3.21
6.99 6.12 6.17 5.98 5.89 5.77 5.72 6.06 6.77 4.47 3.65 1.96
The decrease in phase angel for all bitumens containing wax indicates a higher elasticity of these samples. This effect starts already at low temperatures where there is still little effect on the complex modulus. This shows that the wax crystals form a kind of gel or network structure in the binder. When comparing DMA analysis at the lower frequency of 1 rad/s (Figure 7), this becomes even more evident. The effect is noticeable until all wax is completely melted and the binder approaches a Newtonian fluid. Adding wax 36 or wax 40 to Middle East bitumen (ME) (Figure 3) showed mainly the same pattern as for WV bitumen, but a weaker effect on the complex modulus and phase angle at higher temperatures. ME is less effected than WV by the addition of bitumen wax 36, although ME originally contains 4 wt % (by DSC) of wax 36. For ME the effect of adding 6 wt % of wax 36 is slightly greater than adding 3 wt % of wax 40. The difference between samples containing slack wax and samples containing added bitumen wax is the same as for WV as discussed above. The smaller effect of adding wax to ME bitumen may be due to the fact that ME originally contains quite a large amount of wax (4% by DSC). The natural wax might already contribute to a higher stiffness at medium and higher temperatures and the extra addition of wax does not give the same stiffening effect. Adding wax 36 or wax 40 to nonwaxy Venezuelan bitumen (NV) gave small effects for all levels (Figure 4). Only for levels of 3 and 6 wt %, the phase angle decreased somewhat in the medium temperature range. At quite low temperatures, wax 36 makes the bitumen somewhat less stiff (lower complex modulus and higher phase angle compared to original NV). Wax 40 shows a slightly larger stiffening effect than wax 36 as a whole. NV seems, like ME, to be slightly more affected by addition of 6 wt % wax 36 than by addition of 3 wt % wax 40. At low temperatures (below about -10 °C), adding slack wax makes NV significantely less stiff. The small effect of added wax to the nonwaxy bitumen is due to some solubility of the wax in the bitumen. This is evident from the experiment with addition of small amount of wax, where no effect at all was seen at levels of 1%. Diagrams showing effects in the temperature range from 10 to 100 °C at lower frequency (1 rad/s) are presented in Figure 7. The diagrams show that the decrease in phase angle (increased elasticity) for waxy
S25-60 ×10-2
CM (25°C) ×105
CM (60 °C) ×102
-6.63 -6.71 -6.83 -7.23 -6.54 -6.74 -6.91 -7.54 -6.86 -6.89 -6.89 -8.51
1.53 1.42 1.48 1.66 1.14 1.59 1.77 3.08 1.70 1.45 0.93 1.79
7.27 6.29 6.02 4.93 5.90 6.99 6.67 7.03 6.76 5.64 3.58 1.87
bitumen is more evident at lower frequency. In bitumens ME and WV containing 6% slack wax, there is a remarkable decrease of the phase angle at the melting of the wax. A pronounced deviation from the smooth phase angle profile between 40 and 55 °C appears. The corresponding strong effect is not seen at higher freqency. A corresponding plateau effect appears for the dynamic modulus at about 102 Pa, which is shown very clearly in the temperature sweep at lower frequency (1rad/s), but is hardly noticeable at higher frequency (10 rad/s). Such frequency dependent behavior indicates weak interactions, which readily loose at higher frequencies, and/or may reflect an intermediate (orthorhombic) phase transition for the paraffinic wax.2 In summary, bitumen wax 36 from ME affected all three bitumens less than did bitumen wax 40 from WV and much less than the slack wax. WV is most affected by addition of wax, ME less affected and NV the least affected. It should be noted that NV originally is very low in wax content, WV contains about 2 wt % of bitumen wax 40 and ME contains about 4 wt % of bitumen wax 36 according to DSC measurements. The great difference in the high temperature range (above approximately 40 °C) between samples containing slack wax and samples containing added bitumen wax 36 and 40, is also clearly illustrated in Figures 2-4, showing no pronounced lower complex modulus for the samples containing added bitumen wax (compared to original bitumen and sample with slack wax). These melting effects show even more clearly in the low frequency diagrams in Figure 7. For the samples containing added bitumen wax, no complex modulus plateau appears and no deviation from the smooth phase angle curve. The slope25-60 is higher for all samples containing slack wax. Comparing the bitumens, Middle East shows the highest slope25-60 due to the fact that the total wax content is higher for this bitumen originally having a high natural wax content. The relation between wax content and slope25-60 is illustrated for all bitumens in Figure 7. Results from the frequency sweeps at 50 and 20 °C (not shown) support the results and conclusions drawn from the temperature sweeps discussed above. Rheological Effects at Low Temperatures. Also in the low-temperature area (lower than +5 °C), adding wax had some stiffening effect, but not for all samples in the study. This is more closely illustrated for WV and NV in Figure 8.
Energy & Fuels, Vol. 17, No. 3, 2003 519 Table 4. Crystallizing Fractions, Temperatures, and Enthalpies for Prepared Samples with 6 wt % Wax 36, Wax 40, or Slack Wax, Obtained Using DSC bitumen samples
original wax content according to DSC %
+wax 36 %;°C; J/g
+wax 40 %; °C; J/g
+slack wax %; °C; J/g
nonwaxy Venezuelan, NV Middle East, ME waxy Venezuelan, WV
0 4 2
2.5/37/2.8 6/51/7.8 5/46/5.9
2.5/35/3.1 6/49/7.3 5/45/6.2
4/30/5.0 8.5/41/10.4 6.5/38/7.2
Table 5. Comparison of Wax Content by DSC to Rheological Effects by DMA for Samples with 6 wt % Added Wax 36, Wax 40, and Slack Wax bitumen sample
CF (%) DSC
NV + wax 36 ME + wax 36 WV + wax 36 NV + wax 40 ME + wax 40 WV + wax 40 NV + slackwax ME + slackwax WV + slackwax
2.5
wax effect on stiffness compared to original bitumen +40 °C no decrease of G* no decrease of G* no decrease of G* no decrease of G* no decrease of G* no decrease of G* markedly lower G* markedly lower G* markedly lower G*
slope25-60 ×10-2 -7.23 -7.91 -7.09 -7.54 -8.26 -8.00 -8.51 -9.17 -8.97
Figure 9. BBR stiffness at -25 °C for NV, ME, and WV doped with slack wax (0, 1, 3, and 6 wt %).
sured by DSC for prepared samples at the 6 wt % level are presented in Table 4. By carefully comparing the DSC thermograms with the DMA analysis in Figures 2-4 (adding information also from Figures 7-9), a number of observations can be made. These are summarized in Table 5. The shape of the endothermic peak, representing the melting of the crystallized fraction in different samples (6 wt % level), differs mostly depending on type of wax. For all samples with 6 wt % slackwax, the peak is very pronounced, while for samples with 6 wt % wax 36, the peak profile is broader and shows no pronounced peak (most likely due to lack of n-alkanes). For the samples with 6 wt % wax 40, the peak profile is a combination of the two profiles just mentioned (for samples with slack wax and wax 36, respectively). This is most clearly illustrated for bitumen NW (in Figure 4) as this bitumen originally contains no wax.
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Energy & Fuels, Vol. 17, No. 3, 2003
Conclusions The rheological behavior and effects of adding wax (crystallizing material as determined by DSC) to bitumen were studied using DMA. The following conclusions are drawn based on the results obtained in the study: Magnitude and type of effect on bitumen rheology depend on the bitumen and type of crystallizing fraction in the bitumen. N-alkane rich crystallizing material (slack wax) in bitumen gives markedly negative effects by lowering the complex modulus at temperatures over about 40 °C. At higher wax levels, a pronounced deviation from the smooth phase angle curve and a corresponding plateau effect for the dynamic modulus appear (at low frequency). Marked stiffening effects at lower temperatures than about 40 °C occur as well, but not for nonwaxy Venezuelan at lower wax levels (up to 3 wt %). Crystallizing material with no or very low n-alkane content (wax 36 and wax 40) gives no negative complex modulus lowering effects at higher temperatures. Stiffening effects occur at temperatures below about 50 °C, and may go down to very low temperatures (-5 to -30 °C). Wax content in bitumen obtained by DSC gives a relative amount of material taking part in the total phase transition over the entire test temperature range. The exothermic and endothermic peak profiles give some indications on type of wax and rheological effects due to crystallization and melting of these fractions in
Edwards and Redelius
the bitumen. Unfortunately DSC does not distinguish between good and bad effects on bitumen rheological behavior, as shown in this study. Rheological measurements by DMA temperature sweeps over a wide temperature range give useful and necessary information concerning type of effects due to crystallizing material (low complex modulus/high phase angle at higher temperatures, stiffening effects at medium and lower temperatures and/or softening effects at low temperatures). Effects due to wax content shown in DMA temperature sweeps are well related to the corresponding effects shown in DSC thermograms. Specifying a wax criteria on DMA measurements should be more relevant than specifying certain maximum values for wax content by DSC or other test method for determining wax content in bitumen known from the literature, such as the methods according to EN 12602. A factor for rutting sensitivity could be the slope25-60 for the logarithm of the complex modulus in a DMA temperature sweep at 10 rad/s as described in this study. Acknowledgment. This research was financially supported by Nyna¨s Petroleum. The skillful assistance by Jonas Ekblad and Britt Wideman at performing the rheological measurements is greatly acknowledged. EF020202B
