Energy & Fuels 2006, 20, 2623-2626
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Thermomechanical and Rheological Asphalt Modification Using Styrene-Butadiene Triblock Copolymers with Different Microstructure Gabriel Herna´ndez,*,† Eva M. Medina,† Ricardo Sa´nchez,† and Ana Marı´a Mendoza‡ Technical Assistance and DeVelopment Department, Dynasol Elasto´ meros S.A. de C.V. Km. 28.5, Carr. Tampico-Mante, Altamira, Tamaulipas, Me´ xico 89600, and Instituto Tecnolo´ gico de Ciudad Madero, DiVisio´ n de Estudios de Posgrado e InVestigacio´ n, JuVentino Rosas y Jesu´ s Urueta, Col. Los Mangos, Ciudad Madero, Tamaulipas, Me´ xico 89440 ReceiVed NoVember 24, 2005. ReVised Manuscript ReceiVed August 19, 2006
This paper shows the state of the art of Mexican asphalt, AC-20 grade, modified by linear styrene-butadiene triblock copolymers (SBS), which include variations in their chemical structures, specifically, total styrene and 1,2-butadiene vinyl contents. The main objective of this work was studying the influence of variation in the 1,2-butadiene vinyl content between 9 to 41% (wt/wt), and also variation in the styrene content between 10 and 45% (wt/wt). Both variables were changed in the structure for the SBS copolymer in order to see the effect on thermal and rheological properties when they are used as asphalt modifiers. The research includes composition characterization and thermal properties of raw and modified asphalt. Analytical characterization of SBS polymers was carried out too by means of differential scanning calorimetry, fluorescence microscopy, and a parallel plate rheometer. Results obtained showed that SBSs with a molecular weight of 200 000 ( 10 000 Daltons get the maximum performance in the softening point and maximum rheological failure temperature when the styrene content is around 30% and vinyl groups move from 20 to 30% into the polymer microstructure with an appropriate viscosity level at 135 °C according to Strategic Highway Research Program standards.
Introduction Asphalt is one of the main components for paving binders. Raw asphalt1 is an interesting material which can undergo different physical states with variation in the temperature. At room temperature and below 0 °C, asphalt is bright, rigid, and brittle. When heated above 25 °C, asphalt begins to soften (between 60 and 80 °C). However, the softening point depends on the nature and composition of the asphalt. At 120 °C, asphalt behaves like a Newtonian liquid, and finally, at 200 °C, asphalt starts decomposition, generating residues. Styrene-butadiene thermoplastics (commonly named as SBSs) are materials with retractile properties and may be used as asphalt modifiers to enlarge the temperature range where properties are optimum for paving.2 For example, a typical asphalt pave formulation may work well in places at temperatures between 10 and 50 °C; however, a SBS modified asphalt may work well from -20 to +76 °C. Figure 1 shows a schematic representation for linear SBS. The structure involves a starting polystyrene block, followed by an intermediate polybutadiene block, finishing with another polystyrene block. When butadiene3 is introduced in the synthesis of SBS by anionic polymerization, it may be linked as 1,4-butadiene (forming cis and trans groups) or 1,2-butadiene (forming vinyl groups). * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Dynasol Elasto ´ meros S.A. de C.V. Km. 28.5. ‡ Instituto Tecnolo ´ gico de Ciudad Madero. (1) Dongre, R.; Sharma M. G.; Anderson D. A. Characterization of Failure Properties of Asphalt Binders; ASTM STP 1241; Hardin, J. C., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1995; p 117-136. (2) Lewandowski, L. H. Rubber Chem. Technol. 1994, 67, 447-451.
Figure 1. Schematic representation of SBS structure.
Polystyrene blocks undergo intermolecular interactions (Figure 2), creating anchoring points in asphalt to give elastic properties. Some authors4-6 have reported that, at around 6% (wt/wt) polymer in the asphalt, SBS polymers formed an elastic network, where the anchoring points get a high affinity with the aromatic compounds in the asphalt. Regarding asphalt’s nature, experimental studies7 (Hardin) had reported that styrene-butadiene polymers have better affinity in those asphalts with a high aromatic content (3040% from the whole composition) and a low asphaltene content. However, these asphaltenes are needed to reinforce the modified asphalt; without it, asphalt is a soft, highly ductile, and easy malleable material. (3) Quirk, R. P.; Hsieh, H. L. Anionic Polimerization; Marcel Dekker Inc.: New York, 1996. (4) Blanco, R.; Rodrı´guez, R.; Garcı´a-Gardun˜o, M.; Castan˜o, V. M. J. Appl. Polym. Sci. 1995, 56, 57-64. (5) Shingo, K.; Tafaka, S.; Zhang, X.; Dewen, D. Polym. J. 2000, 33, 33-37. (6) Xiaohu, L.; Isacsson U.; Ekbland J. J. Mater. CiV. Eng. 1998, 77, 961-972. (7) Hardin, John C. Physical Properties of Asphalt Cement Binders; ASTM STP 1241 (ASTM PCN 04-012410-08); American Society for Testing and Materials: Philadelphia, PA, 1995.
10.1021/ef050393t CCC: $33.50 © 2006 American Chemical Society Published on Web 09/19/2006
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Figure 5. Effect of vinyl content in softening-point temperature in modified asphalt using SBS with different styrene levels. Figure 2. Intermolecular interactions between polystyrene blocks in SBS polymer chains.
Table 1. General Characteristics for Synthesized SBS sample
styrene %
vinyl content %
description
SBS-1 SBS-2 SBS-3 SBS-4 SBS-5 SBS-6 SBS-7 SBS-8 SBS-9
10 12 12 30 29 30 45 45 43
11 22 40 10 21 40 9 23 41
low styrene content and different vinyl level copolymers medium styrene content and different vinyl level copolymers high styrene content and different vinyl level copolymers
Table 2. Raw Asphalt Properties property
method
General Properties penetration, 0.1 mm ASTM D5 softening ring and ball temperature, °C ASTM D36 Brookfield viscosity at 135 °C, cP AASHTO TP48 Brookfield viscosity at 160 °C, cP AASHTO TP48 Brookfield viscosity at 190 °C, cP AASHTO TP48
Figure 3. Chemical structure from SBS copolymers used as asphalt modifiers.
Figure 4. Relation between glass transition temperature (Tg) and vinyl content in SBS copolymers.
Several authors8-10 have reported the use of SBSs as asphalt modifiers, being of commercial interest those polymers with 30% (wt/wt) styrene and 70% (wt/wt) butadiene. Microstructure (8) Laaly, H. O. The Science and Technology of Traditional and Modern Roofing Systems; Laaly Scientific Publishing: Los Angeles, CA, 1992; Chapters 25 and 26. (9) Bruˆle, B.; Brion, Y.; Tanguy, A. J. Assoc. Asphalt PaVing Technol. 1998, 57, 41-64. (10) Carreau, P.; Bousmina, M.; Bonniot, F. Can. J. Chem. Eng. 2000, 78, 495-503.
Rheological Properties G*/sin δ at 58 °C, kPa AASHTO TP5 G*/ sin δ at 64 °C, kPa AASHTO TP5 G*/sin δ at 70 °C, kPa AASHTO TP5 G* at 58 °C, kPa AASHTO TP5 G* at 64 °C, kPa AASHTO TP5 G* at 70 °C, kPa AASHTO TP5 phase angle at 58 °C, degrees AASHTO TP5 phase angle at 64 °C, degrees AASHTO TP5 phase angle at 70 °C. degrees AASHTO TP5 maximum rheological failure AASHTO TP5 temperature, °C
value 86 52 582 195 71 3.46 1.64 0.83 3.41 1.63 0.83 80.0 82.5 84.6 68.4
parameters from SBS polymers which have a strong impact on modified asphalt are (a) molecular weight, (b) coupling level and styrene-butadiene diblock content, (c) molecular configuration (radial or linear), (d) styrene content, and (e) vinyl group content (1,2-butadiene unsaturation). Modified asphalt quality is regulated in the United States by the Strategic Highway Research Program (SHRP) and SUPERPAVE11 by setting the standard specifications for performancegrade asphalt binders. Thus, the aim of this work was to observe rheological behavior as well as other thermal and mechanical properties from modified asphalt when structural parameters vary, such as 1,2-butadiene vinyl unsaturations and styrene content from SBS. Experimental Section Nine SBS linear triblock copolymers were synthesized by anionic polymerization3,12 using sequential addition from monomers, controlling the mass molecular weight to be 200 000 ( 10,000 Daltons, (11) SUPERPAVE: Design of PaVing Manual; Asphalt Emulsion Manufacturers Association (AEMA) and Asphalt Institute: Annapolis, MD, 2002. (12) Morton, M. Rubber Technology; Chapman Hall: New York, 1995; p 223-225.
Asphalt Modification
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Figure 6. Fluorescence microscopy (scale 1000 µm) of modified asphalt using SBS copolymers with different styrene contents: (a) lower styrene content, (b) medium styrene content, and (c) high styrene content. Vinyl content was between 9 and 11% (wt/wt). Table 3. Composition from Raw Asphalt Determined by Iatroscan19 composition by Iatroscan: MK-6 in application for non-modified Bitumen method. Conditions:stationary phase Chromarod-SIII; mobile phase (first n-hexane 100% 10 cm, second toluene/n-heptane 80:20 5 cm, third methylene dichloride/methanol 95:5 2 cm); flow rate of gases, H2 160 mL/min, air 2L/min; scanning speed 30 s/scan; playback attenuation 32 mV
Figure 7. Relation of maximum failure temperature versus vinyl content in SBS copolymers for the modified asphalt.
Figure 8. Effect of vinyl content on Brookfield viscosity at 135 °C from modified asphalt.
and all styrene content was in block form. Table 1 shows the main characteristic from these polymers, and Figure 3 describes the chemical structure. The molecular weight characterization was done using a Waters GPC chromatographer model 2690. Related to the structure, styrene blocks were equivalent in length and weight, and they varied as function of composition. 1,2-Butadiene (vinyl group) in polybutadiene blocks was controlled in different proportions in (13) American Standard Test Method for Measuring the Softening Point Temperature (Ring and Ball Temperature); ASTM D36; American Society for Testing and Materials: Philadelphia, PA, 2005. (14) American Standard Test Method for Penetration of Bituminous Materials; ASTM D5; American Society for Testing and Materials: Philadelphia, PA, 2005. (15) Standard Test Methods for Determining the Rheological Properties of Asphalt Binders Using Dynamic Shear Rheometer (DSR); AASHTO TP5; American Association of State Highway and Transportation Officials: Washington, DC, 1993.
composition
method
value
saturates, % (wt/wt) aromatics, % (wt/wt) resins, % (wt/wt) asphaltenes, % (wt/wt)
IATROSCAN MK-6 IATROSCAN MK-6 IATROSCAN MK-6 IATROSCAN MK-6
4.3 31.1 29.1 35.5
order to study the effect in modified asphalt. Quantification of vinyl groups was done using a TA Instruments dynamic scanning calorimeter (DSC) DSCQ1000 model. Typical paving formulations using SBS polymers include between 2 and 5% (wt/wt) polymer in the asphalt; however, in this case, a 4% (wt/wt) SBS polymer in asphalt was used. Modified asphalt was prepared using a high-shear mixer brand ROSS model ME100LC at 2500 rpm; the blend was heated at 190 ( 5 °C using a Watlow heater 93 model. Each polymer was dispersed within Mexican AC-20 asphalt from the Madero City refinery, whose properties are described in Tables 2 and 3. The time dispersion was 1.5 h until complete polymer dispersion, which was verified by fluorescence microscopy using a Zeiss microscope Axiotecy 20X model. Polymer modified asphalt blends were characterized by means of their softening point13 (“ring and ball temperature”) according to ASTM D36 (ASTM is the American Society for Testing and Materials); penetration was determined using a Koheler penetrometer model K95500;14 dynamic viscosity was obtained with a Brookfield viscometer model RDVS-II+, and rheological properties were studied according to SHRP and American Association of State Highway and Transportation Officials (AASHTO) methods15-17 with a Paar Physica rheometer MCR-300-SP model.
Results and Discussion DSC characterization from different copolymers permitted the finding of a proportional relation between the glass transition temperature and vinyl content (1,2-butadiene). This trend is shown in Figure 4 and can be used as a quick tool to estimate the vinyl content in this kind of copolymer. An important variable for polymer modified asphalt (PMA) which is related (16) Bahia, H. U.; Anderson, D. The New Proposed Rheological Properties of Asphalt Binders, Physical Properties of Asphalt Cement Binders; ASTM STP 1241; Hardin, J. C., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1995; pp 1-27. (17) Bahia, H. U.; Anderson, D. The DeVelopment of the Bending Beam Rheometer: Basis and Critical EValuation of the Rheometer; ASTM STP 1241; Hardin, J. C., Ed.; American Society for Testing and Materials: Philadelphia, PA, 1995; pp 28-50.
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Figure 9. Fluorescence microphotographs (scale 1000 µm) for modified asphalt using SBS with different vinyl contents: (a) lower vinyl content (10%), (b) medium vinyl content (21%), and (c) high vinyl content (40%). Styrene content was between 29 and 30% (wt/wt).
to ductility and thermal stability is the softening-point temperature (ring and ball temperature). PMA using synthesized SBS copolymers showed a maximum value for the softening point with vinyl content values between 20 and 30%, and this effect was also seen for SBS copolymers with different styrene contents, as can be seen in Figure 5. Also, the softening point is lower in copolymers with a higher styrene content (between 40 and 50%) because those systems are more rigid and a homogeneous dispersion is difficult to achieve. On the other hand, when the styrene content is lower (5 to 15%), a slight improvement can be seen for the softening point; however, it remains lower compared to other styrene systems because of a more flexible blend. A medium content of styrene (between 25 and 35%) allowed an equilibrium point between rigid and flexible behavior, improving the compatibility between SBS and asphalt, permitting higher values for the softening point. Fluorescence microscopy showed that variation in the styrene content undergoing an inversion phase when asphalt is modified; in other words, when the styrene content is lower in SBS polymers (5-15%), the microscopy showed a good dispersion of the polymer into the asphalt matrix where polymer domains formed networks, and these results are consistent with those found in the literature18 (Masson et al.). When SBS polymers have a medium styrene content (between 25 and 35%), a regular distribution from the SBS network is observed; the morphology for this distribution exhibited small circles like “salami” dispersed over the asphalt matrix. Finally, when SBS polymers have a high styrene content (between 40 and 50%), the inversion phase is observed because SBS polymers formed small and agglomerated particles, and the high content of asphalt is the dominant phase (Figure 6). All of these asphalt blends include a vinyl content between 9 and 11% weight in the SBS polymer. Another important parameter for paving design using asphalt binders and following the SHRP standard is the maximum failure temperature, which is calculated by the complex modulus divided by the phase angle, and its value is equal to 1.0 kPa (G*/sin δ ) 1.0 kPa). This rheological parameter is affected by the temperature and mechanical deformation. Figure 7 shows the rheological behavior observed at the maximum rheological failure temperature. It can be seen that a medium styrene content in SBS exhibits the highest values for this parameter. The vinyl content improves the maximum rheological failure temperature as well. (18) Masson, J.-F.; Collins, P.; Robertson, G.; Woods, J. R.; Margeson, J. Energy Fuels 2003, 17, 714-724. (19) Ecker, A. Pet. Coal 2001, 43, 51-53.
The Brookfield viscosity for modified asphalt is reduced when the styrene content increases in the SBS structure because of polybutadiene domains, which generates more molecular entanglements which absorb asphalt and increase the viscosity, as can seen in Figure 8. On the contrary, the vinyl content reduces the viscosity value. All modified asphalt blends studied showed viscosity values below the maximum value permitted by SHRP for Brookfield viscosity measurements at 135 °C, which is 3000 cP. Figure 9 shows the fluorescence microscopy for asphalt blends with different vinyl contents in SBS molecules with a medium styrene content. It can be observed that a lower vinyl content for these copolymers formed particles which tend to agglomerate. Modified asphalt with a medium vinyl content in the SBS achieved a good dispersion. However, if the vinyl content continues to increase, high entanglement domains are obtained, which require more agitation time in order to get a good dispersion in asphalt. Conclusion Thermoplastic SBS polymers showed a variety of structural characteristics which permit an improvement of the properties of raw asphalt, and the resultant modified asphalt can be approved in paving design. This study shows state-of-the-art research for modified asphalt using styrene-butadiene thermoplastic defined as linear SBS copolymers with differences in their structure, particularly in the styrene block and vinyl content. It was observed that a styrene content between 25 and 35% helps to disperse these copolymers in asphalt, reaching optimal properties such as the softening point and maximum temperature failure. On the other hand, lower-styrene-content copolymers (5-15% styrene) make dispersion in the asphalt difficult because of the complicated molecular entanglement of the bonded butadiene; and finally, a higher styrene content (more than 35%) produces rigid systems because of a loss of viscoelastic properties. The microstructure from the polybutadiene block is also important, especially for 1,2-butadiene unsaturations (vinyl groups) containing samples. It was observed that a vinyl content between 20 and 30% allows a maximum performance related to the softening-point temperature, good viscosity levels, and optimal rheological temperature failure according to SHRP. A higher vinyl level (30%) produces morphologies in modified asphalt similar to fibers which can be observed clearly by fluorescence microscopy. These structures require more agitation time in order to be well-dispersed in the asphalt. EF050393T
