Influence of Temperature and Composition on the Linear Viscoelastic

Energy & Fuels 2000, 14, 131-137

131

Influence of Temperature and Composition on the Linear Viscoelastic Properties of Synthetic Binders Francisco Martıńez-Boza,* Pedro Partal, Benjamıń Conde, and Crı´spulo Gallegos Departamento de Ingenierıá Quı´mica. Universidad de Huelva, Escuela Polite´ cnica Superior, Ctra. de Palos de la Frontera s/n, 21819, Huelva, Spain Received April 26, 1999. Revised Manuscript Received September 8, 1999

This paper deals with the influences that mineral oil, resin, and SBS triblock copolymer concentrations exert on the linear viscoelastic properties of these materials in a wide range of temperatures, with the overall objective of formulating pigmentable synthetic binders with adequate mechanical properties. The relationship between microstructure and linear viscoelastic properties is also studied. Three different regions, transition to glassy, plateau and beginning of the terminal region may be observed in the dynamic mechanical spectrum of these systems, depending on SBS concentration and temperature. Dynamic mechanical analysis tests confirm that the synthetic binders studied are not thermorheologically simple materials as a consequence of the formation of a multiphase system. A polymer-rich phase with high elastic properties and a resin-rich phase with predominant viscous characteristics were found. At high polymer concentration a continuous polymer-rich phase appears, while the continuous resin-rich phase is obtained at low polymer concentration or high resin content.

Introduction The main use of bitumen is as a binder that, blended with mineral aggregates, makes asphalt,1 a mix that is extensively used in road pavement.2,3 The performance of a road pavement is controlled by the mechanical properties of the bitumen, due to the fact that it forms a continuous matrix and is the only deformable component.4-6 Bitumen is the highly viscous residue of crude oil, obtained by removing most of its volatile components.7 The chemical composition of bitumen is very complex, although its components can be broadly categorized as maltenes and asphaltenes.8,9 The asphaltenes are the most polar fraction and have the highest molecular weight, giving its dark color to the bitumen. The maltene fraction consists of polar aromatics, naphthene aromatics, and saturates. The ratio of the asphaltenes to the maltenes has a significant effect on the viscoelastic properties of bitumen and, consequently, on its performance as road paving binders.10 Thus, road pavements may show different distresses depending on * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Vinogradov, G. V.; Isayev, A. I.; Zolotarev, V.; Verbskaya, E. A. Rheol. Acta 1977, 16, 266-281. (2) Whiteoak, D. Shell Bitumen Handbook; Shell Bitumen UK, Riversdell Hause: Surrey, 1990, 14-16. (3) Adedeji, A.; Grunfelder, T.; Bates, F. S.; Macosko, C. W.; StroupGardiner, M.; Newcomb, D. E. Polym. Eng. Sci. 1996, 36, 1707-1723. (4) Lewandowski, L. H. Rubber Chem. Technol. 1994, 67, 447-480. (5) Dongre´, R.; Youtcheff, J.; Anderson, D. A. Appl. Rheol. 1996, 6, 75-82. (6) Isacsson, U.; Lu, X. Mater. Struct. 1995, 28, 139-159. (7) Lesueur, D.; Gerard, J.; Claudy, P.; Letoffe, J. J. Rheol. 1996, 40, 813-836. (8) Whiteoak, D. Shell Bitumen Handbook; Shell Bitumen UK, Riversdell Hause: Surrey, 1990, 88-91. (9) Ho, R.; Adedeji, A.; Giles, D. W.; Hajduk, D. A.; Macosko, C. W.; Bates, S. F. J. Polym. Sci. B: Polym. Phys. 1997, 35, 2857-2877.

temperaturesfor example, rutting (or permanent deformation at high temperatures) related to the viscosity of the bitumen matrix, and low-temperature cracking, as a result of brittle fracture of the glassy bitumen matrix.3,4,6,9 The increasing traffic loading on road pavements has resulted in tightening of binder specifications in order to obtain a higher mechanical stability of asphalt roads. This has forced the use of natural or synthetic polymers to enhance the service properties of bitumen.11-13 Although, in many cases, polymer modification of bitumen may result in a multiphase material with a clear tendency to phase separation, steric stabilization by copolymers, such as styrene-butadiene-styrene (SBS) block copolymers, has been proved very efficient to improve road performance.14 This fact is related to the linear viscoelastic properties of SBS-modified bitumen. Thus, they are much more elastic than neat bitumen at high temperatures; on the contrary, in the low temperature region, the phase angle (relationship between the viscous and elastic components) of modified binders decreases more slowly than the corresponding base bitumen as the temperature decreases. In summary, SBS-modified bitumen shows less temperature susceptibility than neat bitumen.14,15 The research of a binder with characteristics similar to those of a bitumen but with a greater easiness for its (10) Ait-Kadi, A.; Brahimi, H.; Bousmina, M. Polym. Eng. Sci. 1996, 36, 1724-1733. (11) Collins, J. H.; Boulding, M. G.; Gelles, R.; Berker, A. Proc. Assoc. Asphalt Paving Technol. 1991, 60, 43-79. (12) Kraus, G. Rubber Chem. Technol. 1982, 55, 1389-1402. (13) King, G. N.; King H. W.; Harders, O.; Chaverot P.; Planche J. P. Proc. Assoc. Asphalt Paving Technol. 1992, 61, 29-66. (14) Vonk, W. C.; van Gooswilligen, G. Shell Laboratorium: Amsterdam, 1991; Report 8.18, 1-14. (15) Lu, X.; Isacsson, U. JTEVA 1997, 25, 383-390.

10.1021/ef990072f CCC: $19.00 © 2000 American Chemical Society Published on Web 12/29/1999

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Energy & Fuels, Vol. 14, No. 1, 2000

Martıńez-Boza et al.

Table 1. Properties of the Products Used in the Formulation of the Synthetic Binder Samples Studied SBS styrene content (% wt) solution viscosity SMS2406 (Pa‚s) density ISO 2701 (kg/m3)

Table 3. Technological Characterization of the Synthetic Binder Samples Studied P5S

31 4.0 940

P6S

P7S P8S P9S P10S

penetration (1/10 mm) 154 163 113 87 40 31 softening point (°C) 53.6 50.8 50.8 50.4 52.8 53.2 Fraass breaking point (°C) -18.0 -12.0 -8.0 -5.0 0.0 0.0 PI 3.4 2.9 1.4 0.4 -1.0 -1.4

Colophony resin resinic acids content (% wt) softennig point (Pa‚s) (°C) density (kg/m3)

90 84 1100 Process oil

viscosity at 25 °C (Pa‚s) viscosity at 50 °C (Pa‚s) density (kg/m3) at 25 °C

2.2 73.4 980

Table 2. Compositions of the Synthetic Binder Samples Studied P5S

P6S

P7S

P8S

P9S P10S P740 P765

oil (% wt) 50.0 45.8 41.6 37.5 33.3 resin (% wt) 40.0 45.0 50.0 55.0 60.0 SBS (% wt) 10.0 9.1 8.3 7.5 6.6

29.1 65.0 5.8

51.6 40.0 8.3

26.6 65.0 8.3

pigmentation has forced the oil industry to develop a new material. This binder should not be considered as a bitumen from the point of view of its chemical composition, although their physical characteristics have been considered as a goal and in several occasions improved. This new product called synthetic binder, or “clear binder” is basically a mixture of oil, resin, and polymer. Of course, the dark colored asphaltenes should be omitted in the formulation of a synthetic binder. Pigmentable synthetic binders are used for colored pavement applications, such as to alert the traffic to special situations, to improve the effect of illumination, etc.16 The overall objective of this work was to formulate pigmentable synthetic binders with adequate mechanical properties. With this aim, this paper deals with the influences that mineral oil, resin, and SBS triblock copolymer concentrations exert on the linear viscoelastic properties of these materials in a wide range of temperature. Experimental Section A nonmodified colophony resin (90% resinic acids) provided by Valcan S. A. (Spain), a process aromatic oil donated by Ertoil S. A. (Spain), and a styrene-butadiene-styrene triblock copolymer, Kraton D-1101CS, provided by Shell Chemical Company (U.K.) were used to formulate the synthetic binders. Some properties of these products are shown in Table 1. Melt model binders were prepared blending the three components in a low shear batch mixer at 150 °C and a rotating speed of 60 rpm. The compositions of the samples studied are shown in Table 2. Frequency sweeps, between 0.01 and 100 rad/s, in the linear viscoelasticity range were performed on a controlled-stress Haake RS100 rheometer, using a profiled plate-and-plate geometry (20 mm diameter and 1 mm gap). Previously, stress sweep tests, at the frequency of 1 Hz, were carried out on each sample to determine (16) Brule, B.; Le bourlot, F.; Potti, J. J. First Congress on Emulsion; EDS: Paris, 1993; 4-11-193/01-05.

the linear viscoelasticity region. Time sweep tests, at constant amplitude and frequency, were also carried out to confirm that no structural modifications occurred during the time required for each test. Measurements were done in a temperature range between 278 and 323 K. Dynamic mechanical analysis (DMA), in a temperature range between 278 and 350 K, were performed on a controlled-stress Haake RS150 rheometer, at a frequency of 0.1 Hz, using the same plate-and-plate geometry and a heating rate of 1 K/min. The morphologies of the binders were observed with a JEOL JSM-5410 scanning electron microscope (20 kV and 7500X), using samples coated with gold to avoid charging. The technical characterization of the samples was carried out according to the ASTM specifications. The results obtained are shown in Table 3. Results and Discussions Linear Viscoelastic Properties. Figure 1 shows the frequency dependence of the storage and loss moduli for different synthetic binder samples (P5S, P7S, and P10S), having the same oil/polymer ratio, as a function of temperature. Three different regions may be observed in the dynamic mechanical spectrum of these systems, depending on SBS concentration and temperature. Thus, the transition to the glassy region is mainly observed at low temperatures, although an increase in SBS concentration shifts the characteristic frequency for the beginning of this region up to higher values. This region is characterized by an increase in the slope of the log-log plots of the linear viscoelasticity functions, storage, G′, and loss, G′′, moduli versus frequency and tends to disappear as temperature increases. A plateau region develops at a high SBS concentration, as temperature increases, showing values of G′ higher than G′′. This region is characterized by a flattening of the slope of the storage modulus versus frequency.17 On the contrary, a shoulder in G′ is found when the SBS concentration is not sufficiently high, being the values of G′′ higher than G′. Finally, at high temperature (i.e., 323 K), the beginning of the terminal or viscous region is clearly noticed in the low-frequency range, although the slope of the log-log plot of the storage modulus versus frequency is not proportional to 2. Figure 2 shows the frequency dependence of the storage and loss moduli for synthetic binder samples having the same polymer concentration (i.e., samples P740 and P765), as a function of temperature. Once again, the previously mentioned regions of the dynamic mechanical spectrum clearly appear. As can be observed, a plateau region develops as temperature increases for the sample with the lowest resin concentration (P740). On the contrary, the beginning of the transition region, at a given temperature, is displaced to lower frequencies as resin concentration increases.

Linear Viscoelastic Properties of Synthetic Binders

Energy & Fuels, Vol. 14, No. 1, 2000 133

Figure 1. Frequency dependence of the storage and loss moduli for different synthetic binders (P5S, P7S, and P10S) and temperatures.

The evolution of the linear viscoelasticity functions with frequency, at 278 K, for all the binders studied, is observed in Figure 3. At this temperature, all the samples exhibit an apparent transition region in the frequency range tested. The highest values of the storage and loss moduli correspond to samples with the largest resin content (see Table 2). For samples with the same resin concentration but different polymer content, i.e., P10S and P765, the largest values of the viscoelastic moduli correspond to those samples with the highest polymer concentration. The behavior at 298 K is quite different, as can be observed in Figure 4. Thus, in the low-frequency region, a shoulder in G′ appears, dampening the decrease of G′ with frequency in a magnitude that depends on polymer concentration. Consequently, although in the transition region the values of G′ always increase with resin concentration, in the above-mentioned region the maximum values correspond to the sample with the highest polymer concentration, for samples having the same oil/ polymer ratio, when a plateau region is clearly observed. Finally, at 323 K, all the synthetic binder samples exhibit the beginning of the flow region, although in this case the values of the zero-shear-rate-limiting viscosity pass through a minimum at an intermediate polymer concentration, as has been confirmed by steady-state flow measurements.18 The experimental values of the linear viscoelasticity functions may be empirically superposed using a shift

factor, aT, that follows an Arrhenius-like dependence on temperature:

[ (

aT ) exp

Ea 1 1 R T T

)]

(1)

where Ea is the activation energy and To is the temperature of reference. Figure 5 shows the resulting “master” curves for two samples with very different polymer concentrations, P5S and P10S. The temperature of reference is 298 K. The different regions of the mechanical spectrum previously mentioned are apparent. Furthermore, the influence of polymer concentration is now clearly noticed. While the system with the lowest polymer concentration always shows higher values of the loss modulus and just a shoulder in G′, the system having the largest content in polymer (P5S) shows a well-developed plateau region, with higher values of the storage modulus in a wide range of frequencies. Nevertheless, these synthetic binders are not thermorheologically simple materials. Thus, the values of the shift factor obtained from dynamic linear viscoelasticity measurements cannot be used to superpose steadystate flow curves.18 Moreover, the dynamic mechanical analysis tests carried out on the above-mentioned (17) Ferry, J. D. Viscoelastic Properties of Polymers; John Wiley & Sons: New York, 1980. (18) Martıńez-Boza, F. J. Rheological characterisation of synthetic binders; Ph D. Thesis, University of Seville, 1996.

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Figure 2. Frequency dependence of the storage and loss moduli for different synthetic binders (P740 and P765) and temperatures.

Figure 3. Frequency dependence of the storage and loss moduli for different synthetic binders at 278 K.

Figure 4. Frequency dependence of the storage and loss moduli for different synthetic binders at 298 K.

samples confirm that the time-temperature superposition principle does not hold for these binders (Figure 6, parts a and b). This failure can be deduced from the comparison of the temperature dependence of the linear viscoelasticity functions obtained from frequency sweep tests (FS) and from temperature sweep tests (TS) carried out at a constant frequency. The values of the frequency sweep master curve have been converted using the Arrhenius-like relationship between the shift factor and temperature as follows:

where Ea is the activation energy obtained from frequency sweep tests, wexp is the reduced frequency calculated from the superposition of experimental frequency sweep curves at different temperatures, and wR is the frequency at which the temperature sweep tests are carried out. As may be seen, the differences between both types of curves are much more important as polymer concentration increases (sample P5S), a fact that corroborates significant microstructural changes in the binder depending on polymer concentration. Nevertheless, the previously mentioned regions of the mechanical spectrum are still observed, as well as a fully developed plateau region for the binder with the largest polymer concentration.

T)

EaTo RTo ln(ωexp/ωR) + Ea

(2)


Figure 5. Empirical master curves for samples P5S and P10S (temperature of reference: 298 K).

Figure 6. Evolution of the linear viscoelasticity functions, obtained from DMA tests and frequency sweeps tests (using an empirical time-temperature superposition method), for a commercial synthetic binder and two synthetic binders (P5S and P10S) as a function of temperature.

On the contrary, studies carried out on commercial synthetic binders containing a much lower polymer concentration (Figure 6c) demonstrate that the abovementioned principle is held in a wide range of temperatures, as other authors have previously stated for neat bitumen5 and polymer-modified bitumen.10 As may be observed in the above-mentioned figure, no plateau region is shown, and just a slight decrease in the slope of the storage modulus is noticed, indicating much less important hydrodynamic interactions between the phases in the commercial synthetic binder. The evolution with temperature of the loss tangent, obtained from DMA tests, also confirms the significant influence of polymer concentration on the behavior of the binder in the low-temperature region. Thus, for binders having the same oil/polymer ratio, the maxi-


mum in tan δ, which denotes the beginning of the transition to the glassy region, moves to lower temperatures as polymer concentration increases, a fact that has been related to an improvement of its performance. On the contrary, for samples containing an identical polymer concentration, an increase in resin content shifts the beginning of the transition to the glassy region to higher temperatures. Relationship between Microstructure and Linear Viscoelastic Properties. The experimental results obtained in this study may be satisfactorily explained taking into account the microstructure of these synthetic binders. As is well-known, SBS elastomers are essentially polybutadiene blocks tipped at all ends with polystyrene blocks. Polystyrene end-blocks separate into polystyrene domains, which cross-link the polybutadiene chains into a three-dimensional network.14 If an SBS elastomer is added to a neat bitumen, it absorbs part of the maltene fraction with which the polymer is extended. Consequently, SBS-modified bitumen is a multiphase system formed by a polymer-rich phase and an asphaltene-rich phase. The SBS builds a threedimensional network in the polymer-rich phase, which, depending on polymer concentration, may become the continuous one, forming a three-dimensional network throughout the whole bitumen.9 In the case of synthetic binders, the polymer-rich phase would result from the absorption of some of the components of the oil fraction, with which the polymer would extend. The other phase would consist of the remaining oil fraction and the resin. At low SBS concentration, the polymer-rich phase is dispersed in the resin-rich phase matrix and its rheological response is similar to the one predicted by the emulsion model. Thus, the whole mixture would be a two-phase system for which the shoulder in G′ is due to the deformationrelaxation process of the dispersed phase.19,20 If the SBS concentration is sufficiently high, the polymer-rich phase would be the continuous one. From a rheological point of view, the appearance of the plateau region in the dynamic mechanical spectrum of these binders, as polymer concentration or oil /polymer ratio increases, should indicate the development of the above-mentioned three-dimensional network.21,22 This response is clearly seen in Figures 1 and 2. Thus, an increase in polymer concentration (Figure 1), or a decrease in resin content at a constant polymer concentration yielding an increase in oil/polymer ratio (Figure 2), favors the development of a plateau region as the polymer-rich phase becomes more important. Both factors also seem to produce a decrease in the temperature at which the transition to the glassy region is noticed. In other words, an increase in polymer concentration (binders P5S, P7S, and P10S) or an increase in the oil/resin ratio at a constant polymer concentration (binders P740, P7S, and P765) decreases the elasticity of the binder in the low temperature region, a fact that could be related to a decrease in the glassy temperature of the resin. As can be seen, the temperature at which the maximum in the loss tangent appears is closely (19) Bousmina, M. Rheol. Acta 1999, 38, 73-83 (20) Bousmina, M.; Bataille, P.; Sapieha, S.; Schreiber, H. P. J. Rheol. 1995, 39, 499-516. (21) Bousmina, M.; Muller, R. Rheol. Acta 1996, 35, 369-381. (22) Bousmina, M.; Muller, R. J. Rheol. 1993, 37, 663-679.

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Figure 7. SEM micrographs for the synthetic binders studied.

related to the oil/resin ratio (i.e., 1.2 approximately for P5S and P740; 0.4 for P10S and P765). The morphology of the synthetic binders studied has been observed by SEM microscopy. Thus, the develop-

ment of the above-mentioned three-dimensional network can be clearly seen in Figure 7, where SEM microphotographs of binders containing a constant oil/ polymer ratio, at room temperature, are shown. Two


different phases are noticed in sample P10S, a disperse phase, with a high density of gray, which corresponds to a polymer-rich phase, and a continuous phase, with a low density of gray, corresponding to a resin-rich phase. As may be observed, a higher polymer concentration (P10S to P5S) increases the content in polymerrich phase. At an intermediate polymer content (sample P8S), this phase seems to be the continuous one forming a three-dimensional network throughout the binder. This may explain some of the results previously reported. For example, thermo-rheological simplicity fails for binders with high polymer concentration (i.e., P5S) in the temperature range studied (Figure 6a), due to a dramatic change in the rheological properties of the binder at a temperature of about 50 °C; while the linear viscoelasticity functions of binder P10S obtained in a frequency sweep test are not significantly different from that obtained in a DMA test, after the application of an empirical time-temperature superposition method (Figure 6b). This change in microstructure may also explain the singular behavior of these binders in the flow region of the dynamic mechanical spectrum of binders having the same oil/polymer ratio. Thus, as was previously mentioned, the zero-shear-rate-limiting viscosity passes through a minimum at an intermediate polymer concentration (sample P8S). A similar tendency is observed in the softening point values (see Table 3). Conclusions The mechanical spectrum of the binders studied shows different regions: transition to glassy, plateau


and beginning of the terminal region. The transition to the glassy region is mainly observed at low temperatures. An intermediate plateau region develops, as temperature increases, at high SBS concentration. The beginning of the terminal, or viscous region, is clearly noticed in the low-frequency range at high temperature. The experimental values of the linear viscoelasticity functions may be empirically superposed using a shift factor. Nevertheless, these synthetic binders are not thermorheologically simple materials. Thus, the values of the shift factor obtained from dynamic linear viscoelastic measurements cannot be used to superpose steady-state flow curves. Furthermore, dynamic mechanical analysis tests carried out confirm that the time-temperature superposition principle does not hold for these binders, above all for those containing a high polymer concentration. For binders having the same oil/polymer ratio, the beginning of the transition to the glassy region moves to lower temperatures as polymer concentration increases, a fact that has been related to an improvement of its performance. These results may be explained considering the development of a polymer-rich phase. If the SBS concentration is sufficiently high, this phase may become the continuous one. On the other hand, a continuous resin-rich phase may be observed at low polymer concentration, shifting the beginning of the transition to the glassy region to higher temperatures. EF990072F

Influence of Temperature and Composition on the Linear Viscoelastic

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