Influence of Polymer Concentration on the Microstructure and

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Influence of Polymer Concentration on the Microstructure and Rheological Properties of High-Density Polyethylene (HDPE)-Modified Bitumen A. Pe´rez-Lepe,* F. J. Martı´nez-Boza, and C. Gallegos Dpto. Ingenierı´a Quı´mica, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain Received October 1, 2004. Revised Manuscript Received January 24, 2005

The overall objective of this work was to study the influence of high-density polyethylene (HDPE) concentration on the rheological properties and microstructure of HDPE-modified binders prepared in a rotor-stator mixing device. With this aim, viscous and linear viscoelastic measurements, as well as morphological analysis and modulated differential scanning calorimetry tests, were performed on the modified binders. From the experimental results obtained, it can be concluded that the addition of HDPE to bitumen enhances the mechanical properties of the binders, especially in the high-temperature region, where pavements can be submitted to permanent deformation. The addition of 3% HDPE to bitumen is able to readily modify the structure of the binder, by forming a gel-type polymer structure dispersed within the bituminous matrix, providing it with enhanced elastic properties.

Introduction It is generally assumed that bitumen is a complex colloidal system in which the asphaltenes are dispersed in a matrix of the remaining components, the maltenes.1,2 The relationship between the complex colloidal structure of bitumen and its thermomechanical properties has been a subject of scientific and technical interest,3 because of the fact that bitumen has been widely used for road paving applications.4 Polymer additives are well-known to improve the mechanical properties of bitumen. The purpose of bitumen modification is to achieve the desired engineering properties to prevent asphalt from the main pavement defects, such as rutting at high temperatures, fatigue strength, and crack initiation and propagation in the low-temperature region.5 The addition of polymers to bitumen is known to impart enhanced service properties, such as improved thermomechanical resistance, elasticity, and adhesivity.6 Among the many polymers used to modify bitumen, high-density polyethylene * Author to whom correspondence should be addressed. Telephone: +34959219985. Fax: +34959219983. E-mail address: antonio.perez@ diq.uhu.es. (1) Lesueur, D.; Gerard, J.; Claudy, P.; Letoffe, J. A StructureRelated Model to Describe Asphalt Linear Viscoelasticity. J. Rheol. 1996, 40, 813-836. (2) Stastna, J.; Zanzotto, L.; Ho, K. Fractional Complex Modulus Manifested in Asphalts. Rheol. Acta 1994, 33, 344-354. (3) Gonza´lez, O.; Pen˜a, J. J.; Mun˜oz, M. E.; Santamarı´a, A.; Pe´rezLepe, A.; Martı´nez-Boza, F.; Gallegos, C. Rheological Techniques as a Tool to Analyze Polymer-Bitumen Interactions: Bitumen Modified with Polyethylene and Polyethylene-Based Blends. Energy and Fuels 2002, 16, 1256-1263. (4) Whiteoak, D. Shell Bitumen Handbook; Shell Bitumen UK: Riversdell Hause, Surrey, U.K., 1990. (5) Ait-Kadi, A.; Brahimi, H.; Bousmina, M. Polymer Blends for Enhanced Asphalt Binders. Polym. Eng. Sci. 1996, 36, 1724-1733. (6) Collins, J. H.; Bouldin, M. G.; Gelles, R.; Berker, A. Improved Performance of Paving Asphalt by Polymer Modification. Proc. Assoc. Asphalt Paving Technol. 1991, 60, 43-79.

(HDPE) is known to exert an important change in the mechanical behavior of the bitumen.5,7-11 In a previous paper,7 the authors studied the optimization of the manufacturing process of different polymer-modified bitumens. It was concluded that a high-energy mixing process is always necessary to well disperse a polymer in polymer-modified binders. This influence was even more remarkable for HDPE-modified bitumen, because this polymer was more difficult to disperse within the bitumen than others, such as styrene-butadienestyrene (SBS) or low-density polyethylene (LDPE). The overall objective of this work was to study the influence of HDPE concentration on the rheological properties and microstructure of HDPE-modified binders that have been prepared in a rotor-stator mixing device. With this aim, viscous and linear viscoelastic measurements, as well as morphological analysis and modulated differential scanning calorimetry tests, have been performed on the modified binders. Experimental Section A 60/70 penetration-grade bitumen provided by Cepsa (Spain), with an asphaltene content of 19.34% (ASTM D3279) and a specific gravity of 1.01 (25 °C/25 °C) (ASTM D70), has (7) Pe´rez-Lepe, A.; Martı´nez-Boza, F.; Gallegos, C.; Gonza´lez, O.; Pen˜a, J. J.; Mun˜oz, M. E.; Santamarı´a, A. Influence of the Processing Conditions on the Rheological Behaviour of Polymer-Modified Bitumen. Fuel 2003, 82, 1339-1348. (8) Fawcett, A. H.; McNally, T.; McNally, G. M.; Andrews, F.; Clarke, J. Blends of Bitumen with Polyethylenes. Polymer 1999, 40, 63376349. (9) Newman, J. K. J. Dynamic Shear Rheological Properties of Polymer-Modified Asphalt Binders. Elastom. Plast. 1998, 30, 245263. (10) Lu, X.; Isacsson, U. Rheological Characterisation of StyreneButadiene-Styrene Copolymer Modified Bitumens. Constr. Build. Mater. 1997, 11, 23-32. (11) Hınıslıogˇlu, S.; Agˇar, E. Use of High-Density Polyethylene as Bitumen Modifier in Asphalt Concrete Mix. Mater. Lett. 2004, 58, 267271.

10.1021/ef0497513 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005

Influence of Polymers on HDPE-Modified Bitumen been used as base binder. A pelletized high-density polyethylene (HDPE 6006-L), from Repsol-YPF (Spain), with a density of 0.957 g/cm3 (ASTM D-792) and an average molecular weight of 171500 g/mol, has been used as a binder modifying agent. Polymer concentration in the modified bitumen was in the range of 0-5 wt %. Blends of bitumen and polyethylene were prepared in a pilot-plant-scale rotor-stator device (a rotor-stator SD41 Super-dispax SD 41 mixer, from IKA (Germany)). Blends were manufactured at 180 °C and a rotation speed of 8200 rpm. These blends were laid over an aluminum foil sheet and were stored at -18 °C, immediately after their preparation by the mixing process, to avoid polymer-bitumen phase separation. The rheological study was performed using a CS-rheometer (RheoStress RS150, Haake Gbr. (Germany)). Frequency and temperature sweep tests in oscillatory shear and steady-state flow measurements were performed. Frequency sweep runs were applied over a range of 0.01100 rad/s under isothermal conditions, using a serrated plateand-plate geometry, to avoid possible slip phenomena (diameters of 10, 20, and 35 mm; gaps of 1 and 2 mm). Temperature sweep tests were performed at 0.628 rad/s and a heating rate of 1 °C/min. Both temperature and frequency sweep tests were performed within the linear viscoelastic range of the materials. For the flow tests, the same geometries were used in a shear rate range, which was dependent on material viscosity. A data acquisition time of 300 s was established, at each shear stress, to ensure a steady-state viscosity measurement. Modulated differential scanning calorimetry (MDSC) tests were conducted (model DSC Q-100, TA Instruments Waters (USA)), using 10-20 mg samples. The samples were pressed into aluminum pans and subjected to the same measurement procedure. Thus, a cool-heat-cool-heat scan was programmed, to provide the same thermal history for all the samples and avoid the influence of the polymerization process on the thermal transitions of the polymer and the possible effects of crystallization and/or rearrangements during sample storage. A heating rate of 10 °C/min was selected. Temperature ranged from - 80 °C to 200 °C. Optical microscopy was used to study the morphology of polymer-modified bitumens. A drop of a heated sample was placed between the microscope slides. Samples were observed using an optical microscope (Model IX70, Olympus (Japan)).

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Figure 1. Microscopic observations of HDPE-modified bitumen: (A) 1% HDPE, (B) 2% HDPE, (C) 3% HDPE, and (D) 5% HDPE, using optical microscopy.

Results and Discussion Morphological Characterization. An optical characterization was performed to understand the morphology of the bituminous binders modified with HDPE. Microscopic observations of the bituminous binders modified with 1%, 2%, 3%, and 5% HDPE are displayed in panels A, B, C, and D, respectively, in Figure 1. In the case of the addition of 1% HDPE, independent polymer inclusions, with an average radius of 40 µm and a volume fraction of 0.10 (as obtained by image analysis), are dispersed within the bituminous matrix. The addition of 1% HDPE does not yield an important change in the structure of the binder. For the 2%-HDPEmodified binder, nonspherical independent polymer inclusions are observed, although small dispersed polymer associations, yielding some type of weak polymer structure within the bitumen matrix, are also obtained. In the case of modification with 3% HDPE, interconnected polymer inclusions, including the bitumen phase within the compound, are observed. This morphology would give, as a result, a somewhat structured material, because of the interconnection of the dispersed phase. In the case of the 5%-HDPE-modified binder, the polymer phase forms an interconnected three-dimen-

Figure 2. Differential scanning caloirmetry (DSC) thermograms of different HDPE-modified bitumens.

sional spongelike structure, yielding a gel-type morphology, which would remain stable with temperature, until the melting temperature of the polymer-rich phase is attained. Thermal Behavior. To study the effect of HDPE concentration on the thermal behavior of HDPE-modified binders, differential scanning calorimetry (DSC) tests were conducted. The thermograms obtained for all the binders studied are presented in Figure 2. The different curves are vertically shifted to improve clarity. The endothermic fusion peaks of the bituminous blends appear at approximately the same temperature (∼122.5 °C), except for the binder that contains 5% HDPE (123.58 °C). This increase in melting temperature as the percentage of polymer concentration increases in polymer/ bitumen blends has been previously reported in the literature.8,12 The fusion peaks of the binders are shifted (12) Fawcett, A. M.; McNally, T. Blends of Bitumen with Various Polyolefins. Polymer 2000, 41, 5315.

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Table 1. Melting Temperature (Tm), Melting Enthalpy of the Polymer/Bitumen Blend (∆Hblend), Melting Enthalpy Corresponding to the Polymer Concentration Percentage in the Blend (∆Hpol), (∆Hpol ) (∆Hblend/(% HDPE) × 100)), and Crystalline Fraction of the Polymer in the Blends (Fcblend) for Bitumen 60/70 Modified with 1%, 2%, 3%, and 5% HDPE Value parameter

1% HDPE

2% HDPE

3% HDPE

5% HDPE

Tm (°C) ∆Hblend (J/g) ∆Hpol (J/g) Fcblend

122.61 0.848 84.80 28.94

122.27 2.615 130.75 44.62

122.53 4.800 160.00 54.61

123.58 8.598 171.96 58.69

∼8-10 °C toward lower temperatures, in relation to that of the HDPE. The presence of bitumen causes polyethylene crystals to break, yielding smaller and lessperfect crystals (because of the migration of some bitumen light components, such as the paraffinic and aromatic compounds, to the polymer-rich phase, as can be deduced from optical microscopy observations), which melt at lower temperatures.13 The fusion enthalpies vary from 0.84 J/g, in the case of 1%-HDPE-modified bitumen, to 8.60 J/g, for 5%HDPE-modified bitumen. The value of the fusion enthalpy divided by the polymer fraction in the blend would represent the fraction of polymer that melts as the sample is heated, or the fraction that is crystalline.8 These values are displayed in Table 1. In the case of pure crystalline HDPE, the fusion temperature, as obtained by means of X-ray scattering, is 293 J/g.8 Thus, by dividing the value of enthalpy of each blend by the polymer percentage and 293, the crystalline fraction (Fc) is obtained. The Fc value increases with polymer content in the blend, as reported in the literature in the case of LDPE.8 The Fc value is dependent, in any case, on the thermal history of the sample (fusion and/or crystallization events during blend preparation and storage).14 Rheological Behavior and Its Relationship with Binder Microstructure. As is well-known, neat bitumen is a viscoelastic material. Nevertheless, its behavior is dramatically influenced by temperature, because it exhibits predominantly elastic behavior at low temperature and viscous-like behavior at moderate and high temperatures. The degree of modification exerted by the addition of polyethylene (HDPE) has been quantified by characterizing the rheological properties (viscous flow and linear viscoelasticity) of bituminous binders modified with HDPE in different proportions. Thus, 1%-, 2%-, 3%-, and 5%-HDPE-modified bitumens were manufactured using the mixing device and mixing conditions described previously. At low temperature (-10 °C), a quite similar behavior is observed for all the binders modified with HDPE, as can be observed in Figure 3, where the evolution of the storage and loss moduli (G′ and G′′, respectively) with frequency (ω) is represented. Notwithstanding, different mechanical characteristics are obtained for the binders containing different proportions of HDPE at high temperature (75 °C), as can be observed in Figure 4. This (13) Hoffman, J. D.; Miller, R. L. Kinetic of Crystallization from the Melt and Chain Folding in Polyethylene Fractions Revisited: Theory and Experiment. Polymer 1997, 38, 3151-3212. (14) Le´toffe´, J. M.; Chamion-Lapalu, L.; Martin, D.; Planche, J. P.; Ge´rard, J. F.; Claudy, P. Bull. Lab. Ponts Chaussees 2000, 229, 1320.

Figure 3. Frequency (ω) dependence of the storage modulus (G′) and loss modulus (G′′), at -10 °C, for different HDPEmodified bitumens.

Figure 4. Frequency dependence (ω) of the storage modulus (G′) and loss modulus (G′′), at 75 °C, for different HDPEmodified bitumens.

is due to the fact that, at 75 °C, binders soften sufficiently, and, hence, polymer relaxation processes are the main contribution to the bulk rheological behavior of the complex mixture.10 Thus, the results obtained indicate that HDPE-modified asphalts show enhanced viscoelastic characteristics at high temperature. However, the influence of HDPE on the rheological characteristics of bitumen is dependent on its concentration in the binder. As can be observed in Figure 4, the binder containing 1% HDPE shows a terminal region in the low-frequency range and at high temperature. The terminal zone is approached when the material presents a pure viscous behavior. At the end of the terminal zone, the storage modulus of the viscoelastic material (G′) becomes proportional to ω2, while G′′ becomes proportional to ω.15 The binder with 2% HDPE shows a slight elastic enhancement in the low-frequency region. In contrast, the 3%-HDPE-modified bitumen shows a tendency toward a plateau region at low frequencies. The binder containing 5% HDPE shows a well-developed plateau region, with values of the storage modulus higher than the loss modulus, over the entire frequency range studied. In Figure 5, the evolution of the loss tangent (tan δ) with frequency ω is represented for all the binders (15) Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1980.

Influence of Polymers on HDPE-Modified Bitumen

Figure 5. Frequency dependence (ω) of the loss tangent (tan δ) at (A) 5 °C and (B) 75 °C for different HDPE-modified bitumens.

studied. At low temperature (5 °C), the tan δ values are of a quite similar order for all the binders. In contrast, the differences are remarkable at high temperature (75 °C). Thus, the binder containing 5% HDPE shows tan δ values that are lower than that over the entire frequency domain. This observation indicates a remarkable enhancement of the mechanical properties of the 5%-HDPE-modified bitumens, in relation to those shown by bitumen modified with 1% and 2%, at this temperature. These results may be explained taking into account that the effect of polymer modification on the linear viscoelastic properties of bitumen is related to the presence of a polymer phase. This phase may become a continuous phase, depending on polymer concentration and its ability to swell with maltene molecules.2 The modification of the bitumen with 1% or 2% HDPE yields a dispersed polymer phase, as has been previously discussed. On the other hand, the addition of 3% or 5% HDPE to bitumen yields a structured polymer phase (see Figure 1), which contributes to an enhancement of the binder elasticity. As has been previously reported,1,16,17 bitumen thermorheological simplicity has become quite controversial, particularly for polymer-modified bitumen.18 Mavridis19 has proposed a very simple method to evaluate material thermorheological simplicity. Thus, if the time-temperature superposition principle holds for a given material, the tan δ values (or the phase angle δ), plotted versus the corresponding values of the complex modulus, obtained at different temperatures, should match on a single curve (Black diagram). In contrast, if the curves obtained at different temperatures do not superpose and, furthermore, they are not parallel, the material shows a nonuniform dependence on temperature and can be considered as a thermorheologically complex material.20 The effect of polymer concentration (16) Stastna, J.; Zanzotto, L. Linear Response of Regular Asphalt to External Harmonic Fields. J. Rheol. 1999, 43, 719-734. (17) Stastna, J.; Zanzotto, L. Response to “Letter to Editor”. J. Rheol. 1996, 40, 813-836. (18) Garcı´a-Morales, M.; Partal, P.; Navarro, F. J.; Martı´nez-Boza, F.; Gallegos, C. Energy Fuels 2004, 18, 357-364. (19) Mavridis, H.; Shroff, R. N. Temperature Dependence of Polyolefin Melt Rheology. Polym. Eng. Sci. 1992, 32, 1778-1791. (20) Lesueur, D.; Ge´rard, J. F.; Claudy, P.; Le´toffe´, J. M.; Martin, D.; Planche, J. P. Polymer Modified Asphalts as Viscoelastic Emulsions. J. Rheol. 1998, 42, 1059-1074.

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Figure 6. Evolution of the phase angle (δ) with the complex modulus, at different temperatures, for the HDPE-modified bitumens studied: (A) 1% HDPE, (B) 2% HDPE, (C) 3% HDPE, and (D) 5% HDPE.

on the Black diagrams of the binders studied can be observed in Figure 6. For the binder containing 1% HDPE (Figure 6A), a tendency toward viscous behavior appears at high temperatures, with δ values close to 90°. At low temperatures, a tendency toward the glassy region, with values of the complex modulus close to 109, can be observed for all the binders. For the binder containing 2% HDPE (Figure 6B), δ values close to 85° are observed at high temperatures, rendering a slight enhancement of the elastic properties of the binder. For the binders containing 1% and 2% HDPE, a continuity of the curves at different temperatures is observed, and, consequently, they could be empirically superposed to obtain a master frequency-sweep curve, which is typical of thermorheologically simple materials. In contrast, binders containing 3% and 5% HDPE do not show a tendency toward viscous behavior at high temperatures in their Black diagrams and display an apparent thermorheologically complex behavior. Furthermore, an important change in the mechanical behavior of the binders is observed at high temperatures. At 75 °C, δ values as low as 20° are observed for the binder containing 5% HDPE. Thus, the addition of 3% and 5% HDPE to bitumen enhances the elastic properties of the binders, yielding polymer-modified bitumens with improved resistance to permanent deformation at high temperatures. The steady-state flow properties of the polymermodified binders studied are shown in Figure 7. This figure also includes the values of the complex viscosity, to determine if the Cox-Merz rule21 is followed. It may be observed that, at low temperatures (5 °C; see Figure 7A), all the binders modified with HDPE show a zeroshear-rate-limiting viscosity, with higher values of viscosity as the HDPE concentration increases, although higher viscosities are apparent for the binder containing 5% HDPE. On the other hand, at high temperatures (75 °C; see Figure 7B), although a zero-shear-rate-limiting viscosity is apparent for the bitumens modified with 1% and 2% HDPE, only a tendency toward a limiting viscosity at low shear rates is observed for the binders modified with 3% and 5% HDPE. This behavior may be (21) Cox, W. P.; Merz, E. H. Correlations of Dynamic and Steady Viscosities. J. Polym. Sci. 1958, 28, 619-622.

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Figure 7. Shear rate (γ˘ ) and frequency (ω) dependence of the steady-state and complex viscosities, respectively, at (A) 5 °C and (B) 75 °C for different HDPE-modified bitumens.

Figure 8. Temperature (T) dependence of the storage modulus G′ for different HDPE-modified bitumens.

related to the presence of strong interactions among the polymeric microphases.10 These interactions would be developed as a consequence of swelling processes of the polymer by the maltenic oils, and dispersion processes, favored by the high energy supplied by the mixer device and processing temperature. Nevertheless, the Cox-Merz rule is not followed at frequencies and shear rates outside the zero-shear-ratelimiting viscosity region for any of the binders and temperatures studied. Temperature sweep tests, in the linear viscoelasticity range, were also performed on these binders, to study more deeply the viscoelastic behavior in the hightemperature region. The evolution of the storage modulus with temperature is represented in Figure 8, as a function of HDPE concentration. For the binder containing 1% HDPE, the storage modulus (G′) decreases gradually with temperature, within the temperature range studied. The binder with 2% HDPE shows a more

Pe´ rez-Lepe et al.

remarkable decrease in the G′ values at ∼122 °C. The binders modified with 3% and 5% HDPE show a dramatic decrease in the storage modulus values at approximately the same temperature, respectively. This temperature almost coincides with the melting temperatures of the binders, which were obtained via DSC. This fact notes that, as the fusion temperature of the polyethylene “structure” is attained, the G′ value suddenly decreases as the crystals melt, causing the structural cross-linking sites to break.8 In the case of 1%-HDPE-modified bitumen, no polyethylene structure is formed. Thus, G′ starts to decrease before the melting of the polyethylene crystals occurs, because the bitumen phase governs the rheological behavior of the binder. The bitumen phase is, at the same time, reinforced by the noncrystalline fraction of the polyethylene. Conclusions The addition of high-density polyethylene (HDPE) to bitumen enhances the mechanical properties of the modified binder prepared in a rotor-stator device; however, the extent of the effect is dependent on the polymer content. The effect of the polymer is more apparent at high temperatures (75 °C). Thus, the modification exerted by the HDPE enhances the elastic properties of the binder at high temperatures, in the region where permanent deformation can affect the pavement service. The addition of 1% and 2% HDPE to bitumen is not able to modify the mechanical behavior of the bitumen effectively. In these cases, morphologies of independent polymer inclusions within the bitumen matrix are obtained. With the addition of 3% HDPE, an important change in the mechanical spectrum of the binders is attained. Thus, interconnected polymer inclusions are observed, yielding a structured material. For the 5%HDPE-modified binder, this effect is even more evident. A highly structured interconnected polymer phase is dispersed within the bitumen matrix, forming a spongelike structure. Thus, the interactions become sufficiently strong to yield a plateau region in the mechanical spectrum of the binder. As the polymer-rich phase (polymer swollen by the light components of the bitumen) reaches its melting temperature, the polymer structure collapses, as the elastic modulus of the binder abruptly decreases to lower values. The swelling of the polymer by the light components of the bitumen shifts the melting temperature of the polymer phase toward lower values (a plasticizing effect). Acknowledgment. This work is part of a research project sponsored by a MCYT-FEDER program (Research Project No. MAT2001-0066-C02-02). The authors gratefully acknowledge its financial support. EF0497513