Influence of Bitumen Colloidal Nature on the Design of Isocyanate

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Ind. Eng. Chem. Res. 2009, 48, 8464–8470

Influence of Bitumen Colloidal Nature on the Design of Isocyanate-Based Bituminous Products with Enhanced Rheological Properties Virginia Carrera,† Pedro Partal,*,† Moise´s Garcıá-Morales,† Crı´spulo Gallegos,† and Antonio Paéz‡ Departamento Ingenierıá Quı´mica, Facultad de Ciencias Experimentales, Campus de ‘El Carmen’, UniVersidad de HuelVa, 21071 HuelVa, Spain, and REPSOL YPF, Technology Centre, N-V Road, km 18, 28931 Mo´stoles, Spain

This work deals with the influence that bitumen colloidal nature exerts on the rheological properties of bitumen samples modified by isocyanate-based reactive polymers. Shear rheology tests, modulated differential scanning calorimetry (MDSC), chemical characterization by TLC-FID, and AFM microstructural analysis were carried out on four different 150/200 penetration neat bitumen samples and the corresponding MDI-PPG (a low molecular weight polypropylene glycol functionalized with a polymeric 4,4′-diphenylmethane diisocyanate) modified binders. The results obtained demonstrate that the bitumen modification degree depends on bitumen reactivity and microstructure. Thus, the highest modification capability is obtained with neat bitumen samples that exhibit both a well-developed three-dimensional network and a high chemical reactivity with the isocyanate groups. The results obtained may be used to improve the performance of these materials, according to their final application. 1. Introduction Bitumen is a colloidal dispersion of asphaltenes into an oily matrix constituted by saturates, aromatics, and resins (which make up the maltene fraction). This composition is commonly known as SARA fractions.1 The chemical composition of bitumen depends primarily on its crude source and processing. Bitumen physicochemical behavior depends on the relative concentration of its different fractions. Thus, a variation in its composition strongly affects its mechanical properties and chemical reactivity.2,3 Bitumen has some properties, such as impermeability, adhesiveness, elasticity, and ductility, which make it the most suitable material to be used as a binder of mineral aggregates for paving applications.4 However, the performance of bitumen for road applications has been questioned, due to the fact that is brittle and hard in cold environments and soft in hot environments,5 yielding different road distresses. Bitumen modification with polymers may help to overcome such road distresses, i.e., rutting at high temperature, fatigue cracking, and thermal cracking.6,7 Three main categories of polymers are generally considered for bitumen modification: thermoplastic elastomers, plastomers, and reactive polymers.8,9 The first two classes of polymers usually present a very low compatibility with bitumen. The addition of reactive polymers, containing functional groups supposedly able to chemically interact with certain bitumen compounds, may yield some advantages in the resulting binder.2,3,8,10 In this sense, an MDI-PPG (polypropylene glycol functionalized with 4,4′-diphenylmethane diisocyanate) prepolymer has been used in this research. Bitumen modification by this prepolymer is expected to take place by reaction of the isocyanate groups of the polymer with polar groups (-OH; >NH) in asphaltenes and resin molecules. Previous studies pointed out that bitumen modification with MDI-functionalized prepolymers is a complex process.11 Thus, bitumen modification takes place by reaction of isocyanate groups with some bitumen * To whom correspondence should be addressed. Tel.: +34 959 21 99 89. Fax: +34 959 21 99 83. E-mail: [email protected]. † Universidad de Huelva. ‡ REPSOL YPF.

compounds, followed by a long-term modification process due to chemical reactions with air moisture. Thus, water reacts with the remaining isocyanate groups, yielding an unstable carbamic acid which decomposes into an amine and carbon dioxide. The highly reactive amine is expected to react with isocyanate groups left in MDI-PPG chains previously linked to bitumen compounds. Consequently, low-molecular-weight-isocyanate-based reactive prepolymer could be used for the design of polymermodified bituminous products with improved properties.11,12 However, the final characteristics of the modified binders seem to be strongly dependent on the crude oil source and the refining process (bitumen chemical composition). This research is focused on the role that bitumen colloidal nature plays in the thermomechanical properties of bitumen modified by isocyanate-based reactive prepolymers. In this sense, the rheological response, thermal behavior, chemical composition, and microstructure of different 150/200 penetration bitumen samples, and their resulting MDI-PPG modified binders, were characterized. The results obtained may be used to improve the performance of these materials, according to their final applications (asphalt, building materials, etc.). 2. Experimental Section Four bitumen samples (150/200 penetration grade), referred to with letters A-D, were used as base materials for polymer modification. The results of penetration grade trials, according to ASTM D5,13 and ring and ball softening temperature (R&B) tests, determined according to ASTM D36,14 are presented in Table 1. The polymer used was a polypropylene glycol (PPG) functionalized by polymeric MDI (4,4′-diphenylmethane diisocyanate), henceforth MDI-PPG. This polymer was synthesized by reaction of PPG (Alcupol D-0411 donated by Repsol YPF, Table 1. Penetration and Ring and Ball Softening Temperature Values for the Different Neat Bitumen Samples Studied

penetration (dmm) R&B softening point (°C)

bitumen A

bitumen B

bitumen C

bitumen D

145 41.5

160 39.5

111 45.5

165 38

10.1021/ie9004404 CCC: $40.75  2009 American Chemical Society Published on Web 08/11/2009

Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009

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Table 2. SARA Fractions for Neat Bitumen, Noncured Modified Binders, and Cured Modified Binders bitumen A

bitumen B

4% MDI-PPG compounds a

saturates (%) aromatics (%)a resins (%)a asphaltenes + NCO/polars (%)a a

bitumen C

4% MDI-PPG

bitumen D

4% MDI-PPG

4% MDI-PPG

neat

0 days

30 days

neat

0 days

30 days

neat

0 days

30 days

neat

0 days

30 days

8.24 52.46 26.65 12.65

7.53 51.22 24.45 16.80

5.99 50.40 24.66 18.95

8.42 45.78 21.98 23.82

8.72 42.63 17.86 30.79

9.96 32.35 27.93 29.02

9.42 45.13 23.05 22.4

8.07 42.85 15.29 33.79

7.79 39.34 15.99 36.88

7.4 52.71 22.19 17.17

7.84 42.75 22.04 27.37

6.99 37.42 25.76 29.80

Chromatographic peak percentage area.

Spain) and polymeric MDI (supplied by Dow Chemical, Spain), selecting a PPG/MDI molar ratio of 1/3, in N2 atmosphere, at 40 °C, for 48 h and under agitation. The polymer shows an average Mw of 2800 g · mol-1, polydispersity (Mw/Mn) of 1.33, and an average functionality of 2.8. Blends of bitumen and MDI-PPG (4 wt %) were processed, for 1 h, in a cylindrical vessel (60 mm diameter, 140 mm height), at 90 °C and an agitation speed of 1200 rpm, using an IKA RW-20 stirring device (Germany) equipped with a four-blade turbine. After processing, this hot modified bitumen was poured onto aluminum foil, forming a thin layer, and was cured for 30 days at room conditions. In addition, a polymer modified bitumen with 3 wt % commercial styrene-butadiene-styrene (SBS) was prepared in the same device (180 °C, 2 h) for the sake of comparison. Viscous flow measurements, at 60 and 135 °C, were carried out in a controlled-stress rheometer (RS150, Haake, Germany), using a plate-and-plate geometry (35 mm diameter, 1 mm gap). Temperature sweep tests in oscillatory shear, according to AASHTO TP515 (1 °C/min heating rate, 10 rad/s, and 1% strain), were conducted in a Gemini rheometer (Bohlin, U.K.) between 10 and 110 °C. At least two replicates of each test were done. Modulated differential scanning calorimetry (MDSC) tests were performed with a Q-100 calorimeter (TA Instruments, USA). Samples of 5-10 mg were subjected to the same testing procedure: temperature range between -80 and 110 °C; heating rate of 5 °C/min; amplitude of modulation of (0.5 °C, a period of 60 s; and nitrogen as purge gas, with a flow rate of 50 mL/ min. In order to provide the same recent thermal history to the samples, neat and modified bitumen samples were placed in hermetic aluminum pans for 24 h before measurement. Bitumen SARA fractions (Table 2) were determined by thin layer chromatography coupled with a flame ionization detector (TLC/FID), using an Iatroscan MK-6 analyzer (Iatron Corporation Inc., Japan). Elution was performed in hexane, toluene, and dichloromethane/methanol (95/5), following the procedure outlined elsewhere.16 The microstructural characterization of the samples was carried out by means of atomic force microscopy (AFM), with a MultiMode AFM connected to a Nanoscope IV scanning probe microscope controller (Digital Instruments, Veeco Metrology Group Inc., Santa Barbara, CA). All the images were acquired in tapping mode at 30 and 50 °C. The samples were prepared by heat-casting, a method that causes a negligible effect on the material morphology compared to solvent-casting.17 3. Results and Discussion 3.1. Viscous Flow Behavior. Viscous flow curves, at 60 °C (common pavement temperature reached in warm climates), for the different neat bitumen samples studied and for their corresponding noncured (0 days) and cured (30 days) MDIPPG modified binders, are presented in Figures 1 and 2. As

Figure 1. Viscous flow curves, at 60 °C, for neat, 3% SBS, and 4% MDI-PPG modified (noncured and 30-days-cured) binders from bitumens A and D.

Figure 2. Viscous flow curves, at 60 °C, for neat, 3% SBS, and 4% MDI-PPG modified (noncured and 30-days-cured) binders from bitumens B and C.

can be observed, neat bitumen samples A and D show an almost constant viscosity in the whole shear rate range studied. However, a shear-thinning behavior is apparent when these samples are modified with MDI-PPG (see Figure 1). Moreover, bitumen modification (bitumens A and D) yields a slight increase in viscosity just after processing, which results from polymerbitumen chemical reactions. Thus, isocyanate (-NCO) groups in polymeric molecules are known to react with some bitumen compounds containing active hydrogen atoms, namely hydroxyl and amine groups, leading to the formation of urethane and urea

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Table 3. Carreau’s Model Parameters for Neat Bitumen, and Noncured and 30-days-Cured MDI-PPG Modified Binders Carreau’s model parameters binders

η0 (Pa · s)

γ˙ c (1/s)

s

bitumen A neat 4% MDI-PPG; 0 days 4% MDI-PPG; 30 days bitumen B neat 4% MDI-PPG; 0 days 4% MDI-PPG; 30 days bitumen C neat 4% MDI-PPG; 0 days 4% MDI-PPG; 30 days bitumen D neat 4% MDI-PPG; 0 days 4% MDI-PPG; 30 days

6.4 × 10 9.2 × 101 2.8 × 102 8.5 × 101 3.5 × 103 1.6 × 105 6.7 × 101 5.8 × 102 3.1 × 105 6.7 × 101 9.3 × 101 2.2 × 102

8.3 × 10-1 2.6 × 10-2 1.2 × 10-1 2.9 × 10-3 1.1 × 10-1 7.0 × 10-3 3.5 × 10-1 2.1 × 10-1

4.2 × 10-2 4.6 × 10-2 5.0 × 10-1 5.0 × 10-2 2.3 × 10-1 3.7 × 10-2 5.3 × 10-2 3.4 × 10-2

1

linkages, respectively.5,18 This fact could explain short-term bitumen modification, i.e., that occurring during processing.11 In addition, viscosity curves present a significant increase in viscosity after a curing period of 30 days, being quite close to the values obtained for 3 wt % SBS modified bitumen, a concentration commonly used in asphalt paving. According to Martıń-Alfonso et al.,11 long-term bitumen modification would be related to the slow diffusion of air moisture into the bitumen and its reaction with free -NCO groups, giving rise to an increase in the molecular weight of the polymer-asphaltene molecules, and reducing the amount of reactive groups in the mixture. As a result, 4 wt % MDI-PPG seems to be a suitable reactive polymer concentration to obtain modified binders with viscosity similar to that of 3 wt % SBS modified bitumen, at high in-service temperatures (i.e., 60 °C). On the contrary, bitumen B and C reactive modification results in a remarkable increase in viscosity (see Figure 2). Thus, for instance, the addition of 4 wt % MDI-PPG to bitumen B yields, just after its processing, an increase in viscosity of more than 1 order of magnitude in relation to that of the binder containing 3 wt % SBS. Moreover, after 30 days curing, binder viscosity increases further, yielding values of around 3 orders of magnitude higher than neat bitumen B. On the other hand, 4 wt % MDI-PPG modified binders from bitumens B and C show an apparent non-Newtonian viscous behavior, suggesting the development of highly structured systems. Carreau’s model fits the flow behavior of the different neat and modified samples studied fairly well: η ) η0

1

[ ( )] 1+

γ˙ γ˙ c

2 s

(1)

where η0 is the zero-shear-rate-limiting viscosity, γ˙ c, is the critical shear rate for the onset of the shear-thinning region, and s is a parameter related to the slope of this last region. The values of Carreau’s model parameters for all the samples studied are shown in Table 3. The zero-shear-rate-limiting viscosity is a useful parameter to quantify the degree of bitumen modification at high temperature (60 °C). In this sense, a modification index (MI) has been defined as follows: MI )

η0,mod - η0,neat η0,neat

(2)

where η0,mod is the zero-shear-rate-limiting viscosity of the modified bitumen, and η0,neat is the zero-shear-rate-limiting viscosity of the neat bitumen, at 60 °C. Hence, viscosity

Figure 3. Modification index values for noncured (A) and 30-days-cured (B) 4% MDI-PPG modified binders, as a function of bitumen composition.

enhancement due to polymer addition, in the low-shear-rate range, is quantified as the difference between the limiting viscosity of the modified binder and that corresponding to the neat bitumen, at the same temperature. Figure 3 shows the values of the modification index for the different samples studied, as a function of curing. Although the binder from bitumen B modification presents a significantly higher MI value than the other binders just after processing (Figure 3A), the binder from bitumen C shows the highest MI value after 30 days curing (Figure 3B). This fact would suggest that bitumen B is more reactive during the mixing process, leading to an improved shortterm bitumen modification, while long-term modification is larger for bitumen C. In any case, both modified binders, from bitumens B and C, respectively, exhibit much higher MI values after long-term modification compared with modified binders from bitumens A and D, which display a quite poor viscosity enhancement. Moreover, despite this large viscosity increase found for modified binders from bitumens B and C, the viscous flow curves, at 135 °C, for cured binders demonstrate that these samples have lower viscosity than the limiting value stated by AASHTO MP1.19 Thus, viscosities are always lower than 3 Pa · s in the whole shear rate range tested (Figure 4). 3.2. Linear Viscoelastic Behavior. Temperature sweep tests in oscillatory shear at 10 rad/s, from 10 to 110 °C, were carried out on neat bitumen and modified binders (see Figures 5 and 6). As can be observed, the storage, G′, and loss, G′′, moduli always decrease as temperature increases. Neat bitumen always shows G′′ values higher than G′ in the whole temperature range studied, indicating a predominantly viscous behavior. This viscous behavior is also found for the modified binders A and D, even after 30 days curing (Figure 5). On the contrary, bitumen B and C modification results in different linear viscoelastic characteristics for noncured and cured binders. Thus, G′ versus temperature curves for noncured samples show a shoulder at temperatures of around 70 °C, dampening the differences between the loss and storage moduli in the region of high temperature (Figure 6). On the contrary, 30-days-cured modified binders B and C show quite similar values of G′′ and G′ in a wide range of temperature, and a crossover between both linear viscoelastic functions at an intermediate temperature. The above-mentioned shoulder in G′, for the noncured samples, was previously reported for synthetic binders containing SBS, resin, and process oil in their formulations.20 SBS


Figure 4. Viscous flow behavior, at 135 °C, for 30-days-cured MDI-PPG modified binders, as a function of bitumen composition (bitumens A-D).

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Figure 7. Nonreversing heat flow curves for neat bitumen (A) and cured modified binders (B).

Figure 8. Colloidal index values for neat bitumen, and noncured and cured modified binders. Figure 5. Temperature sweep tests in oscillatory shear for neat bitumen and modified binders A and D.

Figure 6. Temperature sweep tests in oscillatory shear for neat bitumen and modified binders B and C.

molecules form a polymer-rich phase, which may become the continuous one as polymer concentration increases by forming

a three-dimensional network throughout the whole bitumen. As a result, predominantly elastic characteristics are found at high in-service temperatures (G′ > G′′) above a critical SBS concentration. On the contrary, at low SBS concentration, the polymer-rich phase remains dispersed in a continuous oily matrix and the rheological response of the binder 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 deformation-relaxation process of the dispersed phase.21,22 In the same way, for noncured MDI-PPG modified bitumens, the dispersed polymer-rich phase would arise from the local reactions between -NCO groups and bitumen compounds. However, during the long-term bitumen modification, this dispersed phase seems to evolve toward an almost extended three-dimensional network, as may be deduced from the viscoelastic behavior found for both bitumens after 30 days curing (Figure 6). 3.3. Bitumen Modification and Microstructure. The thermal behavior of neat and MDI-PPG modified bitumen samples has been evaluated by means of modulated differential scanning calorimetry (MDSC), a technique which allows for the characterization of reversing and nonreversing thermal events.23,17 Figure 7A displays nonreversing heat flow thermograms for all the neat bitumen samples studied, while Figure 7B shows those ones corresponding to 30-days-cured 4% MDI-PPG modified binders. Neat bitumen usually displays four overlapping thermal

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Figure 9. AFM micrographs, at 30 °C, for neat bitumen samples A (A), B (B), C (C), and D (D). Window 50 × 50 µm.

events in MDSC.17 Upon heating, the most relevant thermal event is an endothermic background, which goes from -60 to 70-90 °C, depending on the bitumen nature. However, this event is not always clearly noticed (e.g., bitumen D). Furthermore, three other overlapping events may be noticed (Figure 7A): an exothermic peak, at around -20 °C, for bitumen samples B and C, or close to -5 °C for bitumen samples A and D; a second exothermic peak, located at ca. 40 °C; and an endothermic event, at about 50 °C, for bitumen samples A, B, and C. As can be seen in Figure 7B, 4 wt % MDI-PPG bitumen modification leads to a remarkable increase in the energy associated with the endothermic background (and, therefore, with the endothermic peak located close to 50 °C). This result would suggest the reaction between polymer and bitumen compounds, giving rise to more complex structures that absorb more energy during melting (the area of the endothermic background is larger). In this sense, it is worth mentioning that bitumen D develops a noticeable endothermic background after polymer addition. The development of those more complex structures has been reported to relate to the reaction between functional groups of the reactive polymer and the most polar bitumen compounds, i.e., mainly the asphaltene and resin fractions.11 Such a set of reactions seems to be a much more complex process, being strongly affected by the bitumen nature, though. Thus, Table 2 gathers the evolution of the SARA fractions, obtained by TLCFID, for both neat bitumen and 4 wt % MDI-PPG modified binders. As can be observed, the bitumen asphaltene fraction increases during binder processing (noncured sample). Primarily,

this increase should be attributed to the polymer reaction with the bitumen asphaltene fraction, and the remaining nonreacted polymer, which is not eluted by any of the solvents used in the chromatographic method. This interpretation seems to explain the results found for bitumen A, which experiences an increase close to 4 wt % (that is, the actual polymer concentration added) in the asphaltene fraction, after processing. On the contrary, modified bitumen samples B, C, and D showed asphaltene concentration increments comprised between 7 and 11% as compared with neat bitumen, much higher than that corresponding to the actual MDI-PPG added. This increase would result from the reaction between some of the bitumen compounds being less polar than asphaltenes (e.g., aromatics and/or resins) and NCO groups in polymer molecules during processing, a fact that would explain the changes observed in its corresponding fractions (see Table 2). In addition, long-termcuring reactions seem to increase the concentration of the most polar groups (resins and asphaltenes), with slight changes in the saturate fraction. It is worth mentioning that, in some cases (i.e., bitumen B), long-term curing mainly favors an increase in the resin fraction. Aiming for further insight into this issue, a modified Gaestel colloidal index, IcR, which accounts for chemical composition changes due to the reactive modification by MDI-PPG, may be defined as follows: IcR )

saturates + asphaltenes + NCO/polars aromatics + resins

(3)

where NCO/polars refers to the resulting species formed by chemical reactions between reactive polymer and polar bitumen


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Figure 10. AFM micrographs, at 50 °C, for neat bitumen and modified binders: (A1) neat bitumen A, (A2) 30-days-cured modified bitumen A, (B1) neat bitumen B, and (B2) 30-days-cured modified bitumen B. Window 30 × 30 µm.

compounds, which cannot be eluted by any of the solvents employed in the chromatographic method. Figure 8 displays the evolution of the modified colloidal index, for the different bitumen samples studied, prior to and after reactive modification. According to the above-defined IcR, a higher modified colloidal index would mean larger asphaltene clusters, leading to materials with more significant gel-like behaviors (i.e., higher elasticity). As may be seen, neat bitumen samples B and C have the highest colloidal index values and, consequently, show higher elastic modulus values at high in-service temperatures (60 °C) than neat bitumens A and D. In addition, modification (Figure 3B) and colloidal indexes (Figure 8) are in a good agreement after 30 days curing. In that way, those neat bitumens with the highest colloidal index undergo the most remarkable enhancement in viscosity during long-term modification. However, just after the processing, binder C presents a higher IcR, but much lower MI values, than bitumen B. This fact would suggest a high chemical reactivity in bitumen C during processing, as may be deduced from its larger increase in IcR during short-term bitumen modification. Nevertheless, MDIPPG molecules seem to react with bitumen compounds of lower molecular weight than in the case of bitumen B, which leads to a lower increase in viscosity of binder C after mixing (if compared with bitumen B; see Figure 3A). Some of the thermo-rheological results so far reported, and their interpretation, may be complemented with atomic force microscopy (AFM) observations in tapping mode. Phase imaging provides images by monitoring the difference between the oscillation signal sent to the instrument cantilever and its actual oscillation as affected by tip-sample interactions. Thus, this

technique provides for the mapping of domains with different rheological or mechanical properties.24 Figure 9 shows AFM micrographs corresponding to the four samples of neat bitumen studied, at 30 °C. Different dark and light regions can be distinguished in bitumens B and C. Thus, solid particles of asphaltenes (black and white streaks) appear covered by a solid shell of resins (light gray areas) surrounded by to the molten maltenic matrix (darkest areas), in which asphaltene micelles (i.e., asphaltenes peptized by resins) are dispersed. These micrographs support the well-known bitumen colloidal model.25-27 However, the above-mentioned picture cannot be clearly observed in bitumens A and D. Thus, the asphaltene phase is almost vanished, showing a rather different microstructure. Figure 10 shows AFM images, at 50 °C, for neat bitumen and 30-days-cured MDI-PPG modified binders A (Figure 10A1 and 10A2, respectively), and B (Figure 10B1 and 10B2, respectively). Larger dark areas for neat bitumen samples are observed at this temperature (see Figures 9 and 10), because, as stated by the colloidal model, the width of the resin layer around the asphaltene micelles will decrease at 50 °C. On the contrary, 4 wt % MDI-PPG bitumen modification yields larger light region areas in both binders. However, quite different microstructures are noticed in these modified binders: a welldeveloped three-dimensional network for binder B and a highly dispersed micelle microstructure for binder A. In any case, polymer addition gives rise to a modified binder microstructure with lower thermal susceptibility. As a conclusion, neat bitumen microstructure, linked to its chemical composition, plays a relevant role in its modification by low-molecular-weight

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reactive polymers. Thus, neat bitumen with a well-developed colloidal microstructure should be selected to achieve larger modification index values after MDI-PPG addition (Figure 3). Moreover, the highest MI values after long-term modification are obtained with bitumens that exhibit both a well-developed microstructure and a high chemical reactivity with the isocyanate groups (e.g., bitumen C). 4. Conclusions The effect of bitumen colloidal nature on the short-term (during processing) and long-term (during curing) modification of bitumen with an isocyanate-based reactive polymer has been evaluated. The results obtained demonstrate very different modification levels depending on bitumen characteristics. The final bitumen modification degree, represented by a modification index, seems to be the combination of two different factors: bitumen reactivity with isocyanate-based polymers and bitumen microstructural capability for the formation of a threedimensional polymer-bitumen network, which develops further during binder curing. Accordingly, larger bitumen modification is obtained for neat bitumen samples with high concentrations in polar compounds (polar aromatics, resins, and asphaltenes) able to react with prepolymer -NCO groups, and with a welldeveloped asphaltene-rich colloidal microstructure (i.e., neat bitumen samples B and C). As a result, neat bitumen chemical composition and its microstructure play a relevant role in the bitumen modification by low-molecular-weight reactive polymers. The highest modification capability is obtained for neat bitumen that exhibits both a well-developed complex microstructure and a high chemical reactivity with the isocyanate groups. Acknowledgment This work is part of a research project sponsored by a MECFEDER program (Research Project No. MAT2007-61460). The authors gratefully acknowledge its financial support. We also appreciate fruitful discussions with Mr. Fermıń Monge Flores from REPSOL YPF. Literature Cited (1) Lesueur, D. The colloidal structure of bitumen: Consequences on the rheology and on the mechanisms of bitumen modification. AdV. Colloid Interface Sci. 2009, 145, 42. (2) Becker, Y.; Mu¨ller, A. J.; Rodrı´guez, Y. Use of rheological compatibility criteria to study SBS modified asphalts. J. Appl. Polym. Sci. 2003, 90, 1772. (3) Iqbal, M. H.; Hussein, I. A.; Al-Abdul Wahhab, H. I.; Amin, H. B. Rheological Investigation of the influence of acrylate polymers on the modification of asphalt. J. Appl. Polym. Sci. 2006, 102, 3446. (4) Whiteoak, D. The Shell Bitumen Handbook; Shell Bitumen U.K.: Surrey, 1990. (5) Singh, B.; Tarannum, H.; Gupta, M. Use of isocyanate production waste in the preparation of improved waterproofing bitumen. J. Appl. Polym. Sci. 2003, 90, 1365.

(6) Newman, J. K. Dynamic shear rheological properties of polymer modified asphalts binder. J. Elastomers Plast. 1998, 30, 245. (7) Yousefi, A. A. Polyethylene dispersions in bitumen: the effects of the polymer structural parameters. J. Appl. Polym. Sci. 2003, 90, 3183. (8) Polacco, G.; Stastna, J.; Biondi, D.; Antonelli, F.; Vlachovicova, Z.; Zanzotto, L. J. Rheology of asphalts modified with glycidylmethacrylate functionalized polymers. J. Colloid Interface Sci. 2004, 280, 366. (9) Navarro, F. J.; Partal, P.; Garcıá-Morales, M.; Martıńez-Boza, F. J.; Gallegos, C. Bitumen modification with a low-molecular-weight reactive isocyanate-terminated polymer. Fuel 2007, 86, 2291. (10) Polacco, G.; Stastna, J.; Vlachovicova, Z.; Biondi, D.; Zanzotto, L. Temporary networks in polymer-modified asphalts. J. Polym. Eng. Sci. 2004, 44, 2185. (11) Martıń-Alfonso, M. J.; Partal, P.; Navarro, F. J.; Garcıá-Morales, M.; Gallegos, C. Role of water in the Development of New IsocyanateBased Bituminous Products. Ind. Eng. Chem. Res. 2008, 47, 6933. (12) Martıń-Alfonso, M. J.; Partal, P.; Navarro, F. J.; Garcıá-Morales, M.; Gallegos, C. Use of a MDI-functionalized reactive polymer for the manufacture of modified bitumen with enhanced properties for roofing applications. J. Eur. Polym. 2008, 44, 1451. (13) American Society for Testing and Materials. Standard test method for penetration of bituminous materials. ASTM D5. (14) American Society for Testing and Materials. Standard test method for softening point of bitumen (ring and ball apparatus). ASTM D36. (15) American Association of Stage Highway and Transportation Officials. Standard specification for performance graded asphalt binder using a dynamic shear rheometer (DSR). AASHTO designation TP5. Gaithersburg; 1993. (16) Eckert, A. The application of Iatroscan-technique for analysis of bitumen. Pet. Coal 2001, 43, 51. (17) Masson, J. F.; Polomark, G. M.; Collins, P. Time-dependent microstructure of bitumen and its fractions by modulated differential scanning calorimetry. Energy Fuels 2002, 16, 470. (18) Singh, B.; Gupta, M.; Kumar, L. J. Bituminous polyurethane network: Preparation, properties, and end use. Appl. Polym. Sci. 2006, 101, 217. (19) American Association of Stage Highway and Transportation Officials. Standard specification for performance graded asphalt binder. AASHTO designation MP1. Gaithersburg; 1993. (20) Martıńez-Boza, F. J.; Partal, P.; Conde, B.; Gallegos, C. Influence of temperature and composition on the linear viscoelastic properties of synthetic binders. Energy Fuels 2000, 14, 131. (21) Bousmina, M.; Bataille, P.; Sapieha, S.; Schreiber, H. P. Comparing the effect of corona treatment and block copolymer addition on rheological properties of polystyrene/polyethylene blends. J. Rheol. 1995, 39, 499. (22) Bousmina, M. Rheology of polymer blends: linear model for viscoelastic emulsions. Rheol. Acta 1999, 38, 73–83. (23) Masson, J. F.; Leblond, V.; Margeson, J. Bitumen morphologies by phase-detection atomic force microscopy. J. Microsc. 2006, 221, 17. (24) Masson, J. F.; Polomark, G. M. Bitumen microstructure by modulated differential scanning calorimetry. Thermochim. Acta 2001, 374, 105. (25) Redelius, P. G. Solubility parameters and bitumen. Fuel 2000, 79, 27. (26) Palade, L. I.; Attane´, P.; Camaro, S. Linear viscoelastic behavior of asphalt and asphalt based mastic. Rheol. Acta 2000, 39, 180. (27) Lesueur, D.; Gerard, J. F.; Claudy, P.; Letoffe, J. M.; Planche, J. P.; Martin, D. A structure-related model to describe asphalt linear viscoelasticity. J. Rheol. 1996, 40, 813.

ReceiVed for reView March 17, 2009 ReVised manuscript receiVed July 17, 2009 Accepted July 22, 2009 IE9004404

Influence of Bitumen Colloidal Nature on the Design of Isocyanate

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