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Microstructures and thermal aging mechanism of expanded vermiculite modified bitumen
Construction and Building Materials 47 (2013) 919–926
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Microstructures and thermal aging mechanism of expanded vermiculite modified bitumen Henglong Zhang a,⇑, Hongbin Xu a, Xiaoliang Wang a, Jianying Yu b a b
College of Civil Engineering, Hunan University, Changsha 410082, PR China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China
h i g h l i g h t s This work evaluates a novel bitumen modification through the use of expanded vermiculite. A new approach to determine the microstructures of OEVMt modified bitumen is reported. Thermal aging properties of OEVMt modified bitumen are evaluated. Aging mechanism of the binders is revealed by using atomic force microscopy.
a r t i c l e
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Article history: Received 14 May 2013 Received in revised form 21 May 2013 Accepted 24 May 2013 Available online 17 June 2013 Keywords: Bitumen Expanded vermiculite Composite materials Microstructure Atomic force microscopy Aging
a b s t r a c t This work evaluates a novel bitumen modification through the use of expanded vermiculite (EVMt) which has been traditionally used as thermal insulation material. In order to improve the compatibility between EVMt and bitumen, EVMt was modified by octadecyl dimethyl benzyl ammonium chloride and marked as organo-expanded vermiculite (OEVMt). A new approach to determine the microstructures of OEVMt modified bitumen was reported by combining the results from X-ray diffraction analysis of OEVMt modified bitumen and the dissolving–filtrating procedure. Aging mechanism of the binders was revealed by using atomic force microscopy (AFM). The results show that OEVMt modified bitumen forms a semi-exfoliated nanostructure. As a result of thin film oven test (TFOT) and situ thermal aging, mass change rate and viscosity aging index are increased, while retained penetration and ductility are decreased of binders. However, these physical changes can be effectively prevented by OEVMt. Aging influences bitumen morphology significantly. TFOT leads to the association of the dispersed domains and the single phase trend of bitumen. These changes are accelerated by situ thermal aging. However, these morphology changes of bitumen are inhibited obviously with the introduction of OEVMt, indicating the good thermal aging resistance of OEVMt modified bitumen with semi-exfoliated nanostructure. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Bitumen, residue from crude oil distillation, is a complex mixture of different organic molecules. Its chemical composition primarily depends on both crude oil source and processing procedure involved [1,2]. Bitumen has been widely used in road construction as a binder due to its good viscoelastic properties [3]. The increasing demands of traffic on road building materials in recent years have resulted in a search for binders with improved performance relative to normal penetration grade bitumens [4–6]. Furthermore, as other organic substances, bitumen is subjected to aging, which influences its chemical structures and physical properties. Bitumen aging is one of the principal factors causing the deterioration of asphalt pavements. Important aging related modes ⇑ Corresponding author. Tel./fax: +86 731 88823937. E-mail address: [email protected] (H.L. Zhang). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.05.099
of failure are traffic and thermally induced cracking, and ravelling [7]. Consequently, more and more modified bitumens are used for paving applications through the addition of virgin polymers (SBS, SBR, EVA, etc.) and waste polymers (plastics from agriculture, crumb tyre rubber, etc.) [2,8,9]. Recently, layered silicates have been used to modify bitumen. The main layered silicates include montmorillonite (Mt), rectorite, vermiculite (VMt) and kaolinite clay. More attention of the researchers has been paid to Mt modified bitumen. To increase the compatibility between the Mt and bitumen, Mt is commonly exchanged with organic cations, particularly alkylammonium ions, making the Mt become lipophilic, and the interlayer spacing is enlarged. It has been found that physical properties, rheological behaviors of bitumen and polymer modified bitumen could be obviously improved due to the introduction of organo-montmorillonite (OMt) [10–13]. Even more exciting is that OMt can significantly improve the thermo-oxidative and photo-oxidative aging
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resistance of bitumen [14,15], which is usually attributed to barrier properties of Mt by formation of intercalated or exfoliated nanostructure. However, the microstructures of Mt and OMt modified bitumens are only characterized by X-ray diffraction (XRD). The true microstructures may be hidden by test error during XRD analysis for modified bitumen. Like Mt, VMt is a mica-type silicate, belongs to the general family of 2:1 layered silicates. Each layer consists of octahedrally coordinated cations (typically Mg, Al and Fe) sandwiched by tetrahedrally coordinated cations (typically Si and Al). The isomorphous substitution of Si4+ by Al3+ leads to a net negative surface charge that is compensated by an interlayer of exchangeable hydrated cations (Ca2+, Mg2+, Cu2+, Na+) [16]. When pristine VMt flakes are strongly heated at high temperature (about 900 °C) during a short period of time, the water situated between layers is quickly converted into steams, exerting a disruptive effect upon the structure. As a consequence, a highly porous material named expanded vermiculite (EVMt) is formed and it is an efficient thermal insulator [17]. On these grounds, this work deals with the use of EVMt as a potential modifier in bitumen modification, which may somewhat become a new approach in the asphalt industry. In contrast to Mt, EVMt shows a better improvement on aging resistance of bitumen [18]. However, the aging mechanism of EVMt modified bitumen as well as the pristine bitumen is still not well understood. As mentioned before, most of published articles were limited in the physical or rheological properties of the layered silicates modified bitumens, few researchers paid attention to the microstructures of the binders except the using of XRD. New analytical methods should be taken to better understand the interaction of EVMt with bitumen. The atomic force microscopy (AFM) is a very high-resolution type of scanning probe microscopy. The AFM images a structure by scanning with a tiny tip over the sample surface. It is capable of measuring topographic features at the nanometer-scale or even at atomic-scale resolution. In recent decades, AFM has been used to investigate the morphology of bitumen by more and more researchers [19–21]. It allows to visualize precise details of nanoscale structures of bitumen without special techniques of sample preparation. In summary, the main objective of this research was to study bitumen modification degrees achieved by adding organo-expanded vermiculite (OEVMt). The microstructures of OEVMt modified bitumen were characterized by combining XRD and dissolving–filtrating method. Thermal aging properties and mechanism of OEVMt modified bitumen and the pristine bitumen were investigated by AFM and the physical properties changes of the binders before and after thin film oven test and situ thermal aging.
Table 1 Physical properties of the pristine bitumen. Physical properties
Measured values
Penetration (25 °C, 0.1 mm) Softening point (°C) Ductility (15 °C/10 °C, cm) Viscosity (60 °C, Pa s) Viscosity (135 °C, Pa s)
75 48.6 150.0/15.6 258 0.54
vermiculite (OEVMt) with a particle size of 300 mesh. The ammonium content of OEVMt determined by thermogravimetric analysis under helium atmosphere was 18.6 wt%. 2.3. Preparation of OEVMt modified bitumen Modified bitumen was prepared using a high shear mixer. Bitumen was heated to 150 ± 5 °C in an oil-bath heating container until it flowed fully. Then 3 wt% OEVMt was added into bitumen, and the mixture was blended at 5000 r/min for 60 min. The same process was also performed on the pristine bitumen in order to compare with the OEVMt modified bitumen. 2.4. Thermal aging procedures Thermal aging of the binders can be performed using standard aging procedures, the thin film oven test (TFOT, ASTM D1754) [22], and pressurized aging vessel (PAV, ASTM D6521) [23]. However, OEVMt platelets automatically accumulated to reduce their surface area under tough testing conditions in the PAV. This accumulation weakened the barrier properties of the OEVMt. So the PAV was not suitable for the simulation of long-term thermal aging of OEVMt modified bitumen. In this paper, short-term thermal aging of the binders was performed using the thin film oven test (TFOT), while the situ thermal aging was used to perform the longterm thermal aging. TFOT was conducted in an oven with a rotating platform on an axis, and the rotation of platform was carried out by the axis. The binders in pans were put on the platform and heated for 5 h at 163 °C. The situ thermal aging was performed in oven. The residue from TFOT was aged at 70 °C for 30 days, the thickness of bitumen film was about 3.2 mm. The aged binders were blended at 150 °C for 10 min to insure a homogeneous system for measuring physical properties, microstructure and morphology. Aging susceptibility of the binders may be evaluated by means of an aging index, which is defined as the ratio of a chemical or physical parameter of the aged binder to that of the original binder [7]. In this paper, aging indices obtained using mass and physical measurements were used to evaluate the aging properties of the binders. The mass change rate (MCR) is computed in accordance with the following formula:
MCR ¼ ðM 1 M 0 Þ=M 0 100
where M0 and M1 are mass of the binders before and after aging, respectively. The retained penetration (RP), viscosity aging index (VAI) and retained ductility (RD) of the binders are computed as formulas (2)–(4), respectively.
RP ¼
Aged penetration value 100 Unaged penetration value
VAI ¼ 2. Experimental 2.1. Materials The 60/80 pen grade bitumen was used as base binder in this study. The physical properties of the pristine bitumen are listed in Table 1. EVMt, 300 mesh, was used as bitumen modifying agent. Octadecyl dimethyl benzyl ammonium chloride, chemically pure, was used to modify the EVMt.
2.2. Preparation of organo-expanded vermiculite A 500-mL round-bottom, three-necked flask with a mechanical stirrer, thermometer, and condenser with a drying tube was used as a reactor. EVMt (10 g) was gradually dissolved in 200 mL deionised water and stirred for 30 min. Then octadecyl dimethyl benzyl ammonium chloride was added into this solution. The resultant suspension was vigorously stirred for 10 h. The treated EVMt was repeatedly washed with deionised water. The filtrate was titrated with 0.1 N AgNO3 until no precipitate of AgCl was formed. The filter cake was then placed in a vacuum oven to dry at 80 °C for 24 h. The dried cake was then ground to obtain organo-expanded
ð1Þ
RD ¼
Aged viscosity value Unaged viscosity value 100 Unaged viscosity value
Aged ductility value 100 Unaged ductility value
ð2Þ
ð3Þ
ð4Þ
2.5. Physical properties test The physical properties of the binders, including softening point, penetration (25 °C), and ductility (5 °C), were tested according to ASTM D36, ASTM D5, and ASTM D113, respectively [24–26]. Brookfield viscometer was employed to measure the rotational viscosity of the binders at 135 °C and 60 °C according to ASTM D4402 [27]. 2.6. Microstructure characterization X-ray diffraction graphs of EVMt, OEVMt and modified bitumens were obtained using a Rigaku D/max D/MAX-IA diffractometer with Cu Ka radiation (k = 0.15406 nm; 40 kV, 50 mA) at room temperature. The scanning rate was 2°/ min. The diffractograms were scanned from 0.5° to 10°.
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H.L. Zhang et al. / Construction and Building Materials 47 (2013) 919–926 2.7. Micro-morphology analysis AFM was applied to investigate the micro-morphology of the binders. A hot liquid drop of bitumen at 140 °C was carefully placed on a 10 mm 10 mm 1 mm steel disk, which was heated for 1–2 min on a hot plate at about 140 °C, a temperature high enough to melt bitumen, but not so high that it would oxidize rapidly. The bitumen was spread out with a blade to form a round film of about 5 mm in diameter. This hot film was left on the hot plate undisturbed for an additional 1 min to allow the surface to flow to a smooth and glossy finish [20]. For AFM analysis, the film was then cooled to ambient temperature(about 5 °C), covered by a glass cap to prevent dust pick-up and annealed for a minimum of 24 h before imaging [28]. Topographic and phase images were scanned using an etched silicon probe. Cantilever was 125 lm long with curvature radius at 5–10 nm and height at 15–20 lm. The drive frequency was 260 kHz and the drive amplitude was 56 mw. Test was operated in tapping mode. All the microphotographs showed a 15 lm 15 lm region unless otherwise indicated.
3. Results and discussion 3.1. Microstructures of OEVMt and modified bitumen The XRD patterns of EVMt and OEVMt are shown in Fig. 1. The basal spacings (d001) are calculated according to the Bragg equation (k = 2d sinh). The calculated d001 values are given in Table 2. As shown in Fig. 1, the first diffraction peak of the XRD curve for EVMt is at d001 = 1.42 nm. After organic modification by octadecyl dimethyl benzyl ammonium chloride, the peak is shifted to a lower angle which means an increase in d001 value. As indicated, d001 value increases from 1.42 nm to 5.33 nm for OEVMt. The increased d001 value will contribute to the intercalation of bitumen molecules into the galleries of OEVMt easily. The OEVMt modified bitumen was also characterized by XRD in order to evaluate whether intercalation occurred. Interestingly enough, upon melt blending with bitumen, the OEVMt shows no diffraction peak in the XRD pattern. The disappearance of the diffraction peak of OEVMt after compounding with bitumen implies either the formation of an intercalated nanocomposite with a basal spacing larger than 17.60 nm, the formation of an exfoliated nanostructure or a very disorganized structure of the silicate platelets. Further characterizations are necessary to determine exactly microstructure of the OEVMt modified bitumen. In filled polymers areas, transmission electron microscope (TEM) has been proved to an extremely useful technique to characterize the nanocomposite microstructures by combining with XRD analysis. In this paper, TEM method was tried but failed because some small bitumen molecules easily evaporated due to the vac-
Fig. 1. XRD patterns of EVMt, OEVMt, OEVMt modified bitumen and separated OEVMt from modified bitumen.
Table 2 Interlayer spacing of EVMt, OEVMt and OEVMt in modified bitumen. Samples
2h (°)
d001 (nm)
d001 variation (nm)
EVMt OEVMt OEVMt in modified bitumen Separated OEVMt
6.23 1.66 – 1.83
1.42 5.33 >17.60 4.82
– 3.91 – 3.40
uum environment at ambient temperature. In an effort to determine the exact microstructure of OEVMt modified bitumen, dissolving–filtrating procedure was performed as follows: first, OEVMt modified bitumen was dissolved in trichloroethylene, then the OEVMt was filtered from this solution. After that the separated OEVMt was characterized by XRD. The XRD patterns of separated OEVMt were shown in Fig. 1d. It can be seen that separated OEVMt shows an obvious diffraction peak in XRD curve, the corresponding d001 value is 4.82 nm. It implies that not all OEVMt are peeled off, there are still some regular layered structures of OEVMt in modified bitumen. Even more exciting is that this d001 value is lower than that of OEVMt. The reasons can be attributed to the existed organic compounds in EVMt interlayers. The bitumen molecules intercalated into the interlayers of OEVMt are dissolved by trichloroethylene, which contributes to the decreased d001 value of separated OEVMt. All above discussion indicates that OEVMt modified bitumen forms a semi-exfoliated nanostructure.
3.2. Morphology of bitumen and effect of OEVMt modification A typical topographic AFM image of unmodified bitumen is shown in Fig. 2a. The dispersed domains (the dark-looking region) display a succession of pale and dark lines, which are often called ‘bee-like’ structures. The appearance of ‘bee-like’ structures is caused by the existence of asphaltenes in the bitumen according to our previous research [29]. The dimension of ‘bee-like’ structures is decreased obviously in OEVMt modified bitumen, as shown in Fig. 2b. However, the ‘bee-like’ structures with decreased dimension disperse in the OEVMt modified bitumen more homogeneously in comparison with unmodified bitumen. The appearance of the ‘bee-like’ structures can be attributed to the crystallization of the microcrystalline waxes contained in the asphaltenes, as shown in Fig. 3. According to XRD analysis, bitumen molecules intercalate into the layers of OEVMt during the melt blending process, and OEVMt modified bitumen forms a semiexfoliated nanostructure. The silicate layers of OEVMt with high aspect ratio greatly obstruct the movement of bitumen molecule chains. Therefore, the crystallization of microcrystalline waxes and waxy molecules in bitumen can be prevented during cooling from high to testing temperature (about 5 °C), which contributes to the decreasing of ‘bee-like’ structures dimensions. AFM Phase images of unmodified bitumen and OEVMt modified bitumen are shown in Fig. 4. In AFM analysis, both the topographic and phase images are viewed side-by-side in real time. Phase images can be used to map variations of surface properties such as viscoelasticity and elasticity. As shown in Fig. 4a, there are obviously two phases in unmodified bitumen, which are corresponding to the dispersed domains and the matrix in topographic image. However, the contrast between the dispersed phase and the matrix is decreased with the introduction of OEVMt as seen in a comparison of Fig. 4a and b. The boundary between the dispersed phase and the matrix is clear in unmodified bitumen, while the boundary is ambiguous in OEVMt modified bitumen. This indicates a change in the tip–sample interactions during the AFM analysis, which is affected by the sample modulus [30]. The decreased constrast is related to differences in the rise of bitumen stiffness with the
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Fig. 2. Topographic images of (a) unmodified bitumen, and (b) OEVMt modified bitumen on a scale of 15 lm 15 lm. The color contrast covers a height variation of 120 nm.
stiffness of the two phases is decreased after modification. It implies that OEVMt shows a better interaction with the matrix, which contributes to the increasing of the relative stiffness of the matrix.
3.3. Effect of TFOT on binder physical properties and morphology
Fig. 3. Three-dimensional topographic images of unmodified bitumen on a scale of 15 lm 15 lm.
introduction of OEVMt. The relative stiffness of the dispersed phase is higher than that of the matrix [9]. However, the difference in
The changes of physical properties of the binders before and after TFOT are given in Table 3. As shown in Table 3, both mass change rate and viscosity aging index of OEVMt modified bitumen decrease significantly after TFOT. In addition, retained ductility and retained penetration of the OEVMt modified bitumen are higher than that of the unmodified bitumen, indicating the good aging resistance of OEVMt modified bitumen. The improvement on the aging resistance performance is resulted from the barrier properties of the OEVMt particles which hinder the penetration of the oxygen molecules and increase their average path length by forming the semi-exfoliated nanostructure. Another reason can be the decrease of the volatility of the oil components of bitumen, which prevents the hardening of the binders. The topographic images of unmodified bitumen and OEVMt modified bitumen after TFOT are shown in Fig. 5. The phase images of unmodified bitumen and OEVMt modified bitumen after TFOT are shown in Fig. 6. Compared with unaged samples, the contrast between the matrix and the dispersed domains in the aged samples decreases remarkably, especially for unmodified bitumen,
Fig. 4. Phase images of (a) unmodified bitumen, and (b) OEVMt modified bitumen on a scale of 15 lm 15 lm.
H.L. Zhang et al. / Construction and Building Materials 47 (2013) 919–926 Table 3 Influence of OEVMt on physical properties of bitumen before and after TFOT. Aging indices
Unmodified bitumen
OEVMt modified bitumen
Mass change rate (%) Retained penetration (%) Viscosity aging index (%, 60 °C) Retained ductility (%, 15 °C)
0.09 80 88 20
0.02 90 43 46
the boundary between the matrix and the dispersed domains disappears (Fig. 5a). It indicates that an obvious association of the dispersed domains in bitumen appears during TFOT. It can be explained that oxidation leads to the decrease of the difference in chemical properties between the dispersed domains and the matrix during the TFOT. Bitumen is a complex system made of different constituents, namely saturates, aromatics, resins and asphaltenes. TFOT decreases aromatics and at the same time increases the content of resins and asphaltenes, which contributes to the following transformation of different fractions, aromatics ? resins ? asphaltenes. As a result, the content of high molecular weight fraction is increased and the whole stiffness of the bitumen is obviously increased, which lead to the reduced difference in stiffness between the two phases in bitumen. According to Fig. 6a, it can be seen clearly that the contrast between the dispersed phase and the matrix phase is decreased obviously and their boundary is also disappearing in unmodified bitumen after TFOT. It indicates that there is a single-phase trend of the bitumen during the TFOT. As discussed before, the relative stiffness of the dispersed phase is higher than that of the matrix. It is reasonable to assume that the stiffness of the matrix in bitumen is increased more obviously than that of the dispersed phase during TFOT. In addition, the topographic images in Fig. 5a show that the dimension and the amount of ‘bee-like’ structures are reduced after TFOT. As mentioned before, the appearance of the ‘bee-like’ structures is attributed to the crystallization of the microcrystalline waxes or waxy molecules contained in the asphaltenes. Due to bitumen aging, some chemical reactions occurred and molecular structures of bitumen are changed simultaneously, which introduces an increase in the overall polarity of the bitumen. Similarly, the structure and chemical properties of the microcrystalline waxes and waxy molecules also may be changed, which reinforces their solubility in bitumen. Their crystallization after aging is prevented. Consequently, dimension and the amount of ‘bee-like’ structures are decreased.
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The surface of aged OEVMt modified bitumen is relatively smooth, which is similar to that of the unaged samples. The slight changes of the morphology after TFOT indicate the better aging resistance of OEVMt modified bitumen in comparison with unmodified bitumen, which is in accordance with changes in physical properties of unmodified and OEVMt modified bitumens after TFOT. Oxidation is the most important aging mechanism of bitumen, which leads to the hardening of bitumen as a result of the chemical changes, e.g. the transformation from aromatics and resins to asphaltenes [7]. However, the oxidation of bitumen is reduced remarkably by OEVMt which hinders the permeability of oxygen in the bitumen during TFOT. As a result, the hardening of the bitumen can be effectively prevented. Therefore, the aging resistance of OEVMt modified bitumen is improved remarkably. Interestingly enough, there are two clear phases for OEVMt modified bitumen after TFOT. The contrast between the matrix and the dispersed domains is increased compared to that of unaged samples (Fig. 6b). As mentioned earlier, OEVMt shows a better interaction with the matrix compared to the dispersed domains, which contributes to the decreased contrast between the matrix and the dispersed domains after modification. However, stiffness of the dispersed domains increases obviously by the transformation from aromatics and resins to asphaltenes during the TFOT. As a consequence, the influence of OEVMt on stiffness of the two phases is decreased, which leads to the increased difference in stiffness between the matrix and the dispersed domains. Compare with unmodified bitumen, the association of the dispersed domains is prevented obviously in OEVMt modified bitumen, indicating its good short-term aging resistance. 3.4. Effect of situ thermal aging on binder physical properties and morphology The changes of physical properties of the binders before and after situ thermal aging (30 d) are given in Table 4. As can be seen from Table 4, situ thermal aging make the physical properties of the binders reduced more remarkably in comparison with that after TFOT. According to our previous research, it has been found that PAV simulation corresponds to 30 days of situ thermal aging based on the changes of the physical properties of the binders before and after aging. For plain bitumen, it has been established that this simulation corresponds to a service life of the order of 4– 5 years [9]. The results show that, after situ thermal aging, mass change rate and viscosity aging index of OEVMt modified bitumen
Fig. 5. Topographic images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after TFOT on a scale of 15 lm 15 lm. The color contrast covers a height variation of 80 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Phase images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after TFOT on a scale of 15 lm 15 lm.
Table 4 Influence of OEVMt on physical properties of bitumen before and after situ thermal aging. Aging indices
Unmodified bitumen
OEVMt modified bitumen
Mass change rate (%) Retained penetration (%) Viscosity aging index (%, 60 °C) Retained ductility (%, 15 °C)
0.66 43 982 5
0.49 49 609 12
are lower than that of unmodified bitumen, while retained ductility and retained penetration are higher than that of the unmodified bitumen, which is in accord with the TFOT results. It indicates that OEVMt can contribute to the improved both short-term and longterm thermal aging resistance of the bitumen. The topographic and phase images of unmodified bitumen and OEVMt modified bitumen after situ thermal aging are shown in Figs. 7 and 8, respectively. The surface of unmodified bitumen becomes very rough after situ thermal aging in comparison with that after TFOT. In addition, the contrast between the matrix and the dispersed domains further decreases. There is only one phase for unmodified bitumen after situ thermal aging in Fig. 8a. As described in aging procedure, situ thermal aging was performed after
TFOT. It indicates that the single-phase trend of the bitumen is accelerated during situ thermal aging compared with TFOT. The accelerated single-phase trend of the unmodified bitumen can be attributed to the enhanced compatibility between the dispersed phase and the matrix phase. The aging conditions of situ thermal aging are more rigorous as compared to the TFOT. Consequently, the morphology and chemical structures of the bitumen molecules are more obviously affected. As a result, the overall polarity of both the matrix and the dispersed domains further increases, which leads to the increased solubility with bitumen molecules and the single-phase trend. As compared with phase image of OEVMt modified bitumen after TFOT aging in Fig. 6b, the areas of the dispersed domains are increased. On the contrary, the areas of the matrix are decreased. Moreover, the boundary between the dispersed domains and the matrix becomes blurred, which implies that the association of the dispersed domains also occurs in OEVMt modified bitumen during situ thermal aging. However, there are still two obvious phases in OEVMt modified bitumen after situ thermal aging. The above discussion shows that although the aging mechanism of the bitumen is not changed by OEVMt, the aging tendency of the bitumen is prevented remarkably by the introduction of OEVMt, indicating the good long-term thermal aging resistance of OEVMt modified bitumen.
Fig. 7. Topographic images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after situ thermal aging on a scale of 15 lm 15 lm. The color contrast covers a height variation of 80 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. Phase images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after situ thermal aging on a scale of 15 lm 15 lm.
Interestingly, the dimension and the amount of ‘bee-like’ structures increase obviously after situ thermal aging compared to TFOT. As indicated in Tables 2 and 3, mass change rate of unmodified bitumen increases from 0.09% to 0.66%, which shows an obvious oxidation of bitumen molecules during situ thermal aging. As can be seen, during situ thermal aging, the asphaltenes content is increased. On the other hand, the colloidal structure of bitumen is transformed from sol–gel type to gel type, which causes the solid-like effect of bitumen. Thus, these high molecular weight fractions cannot be dissolved effectively by other bitumen molecules, which contributes to the formation of the ‘bee-like’ structures. 4. Conclusions The microstructures, thermal aging properties and mechanism of OEVMt modified bitumen were evaluated. According to the results of XRD analysis of OEVMt modified bitumen and the dissolving–filtrating procedure (first, OEVMt modified bitumen was dissolved in trichloroethylene, then the OEVMt was filtered from this solution. After that the separated OEVMt was characterized by XRD), OEVMt modified bitumen is proved to form the semiexfoliated nanostructure. As a result of TFOT and situ thermal aging, the physical properties of aged bitumens become more solid-like, as indicated by increased mass change rate and viscosity aging index as well as the decreased retained penetration and ductility. Aging also influences bitumen morphology significantly. The association of the dispersed domains and the single phase trend of bitumen occur during TFOT, which is accelerated by situ thermal aging. However, these physical and morphology changes of bitumen after TFOT and situ thermal aging are prevented obviously with the introduction of OEVMt, indicating the good aging resistance of OEVMt modified bitumen with semi-exfoliated nanostructure. Such a result suggests that OEVMt can be effectively used as a modifier to improve the aging resistance of bitumen. Acknowledgments This work was supported by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No. 2011BAE28B04), and was financially supported by the Project of Young Teacher Growth of Hunan University (No. 2012-161). The work was also supported by Open Fund of Key Laboratory of Special Environment Road Engineering
of Hunan Province (Changsha University of Science & Technology, No. kfj120401). The authors gratefully acknowledge their financial support. References [1] Becker Y, Muller AJ, Rodriguez Y. Use of rheological compatibility criteria to study SBS modified asphalts. J Appl Polym Sci 2003;90(7):1772–82. [2] Cuadri AA, Navarro FJ, Garcia-Morales M, Gallegos C. Bitumen chemical modification by thiourea dioxide. Fuel 2011;90(6):2294–300. [3] Airey GD. Rheological properties of styrene butadiene styrene polymer modified road bitumens. Fuel 2003;82(14):1709–19. [4] Isacsson U, Lu X. Laboratory investigation of polymer modified bitumens. J Assoc Asphalt Paving Technol 1999;68:35–63. [5] Martinez A, Paez A, Martin N. Rheological modification of bitumens with new poly-functionalized furfural analogs. Fuel 2008;87(7):1148–54. [6] Lu XH, Isacsson U. Rheological characterization of styrene–butadiene–styrene copolymer modified bitumens. Constr Build Mater 1997;11(1):23–32. [7] Lu XH, Isacsson U. Effect of ageing on bitumen chemistry and rheology. Constr Build Mater 2002;16(1):15–22. [8] Garcia-Morales M, Partal P, Navarro FJ, Gallegos C. Effect of waste polymer addition on the rheology of modified bitumen. Fuel 2006;85(7, 8):936–43. [9] Durrieu F, Farcas F, Mouillet V. The influence of UV aging of a Styrene/ Butadiene/Styrene modified bitumen: comparison between laboratory and on site aging. Fuel 2007;86:1446–51. [10] Yu JY, Zeng X, Wu SP, Wang L, Liu G. Preparation and properties of montmorillonite modified asphalts. Mater Sci Eng A 2007;447:233–8. [11] Polacco G, Kriz P, Filippi S, Stastna J, Biondi D, Zanzotto L. Rheological properties of asphalt/SBS/clay blends. Eur Polym J 2008;44:3512–21. [12] Yu JY, Wang L, Zeng X, Wu SP, Li B. Effect of montmorillonite on properties of styrene–butadiene–styrene copolymer modified asphalt. Polym Eng Sci 2007;47(9):1289–95. [13] Jahromi SG, Khodaii A. Effects of nanoclay on rheological properties of bitumen binder. Constr Build Mater 2009;23(8):2894–904. [14] Yu JY, Feng PC, Zhang HL, Wu SP. Effect of organo-montmorillonite on aging properties of asphalt. Constr Build Mater 2009;23(7):2636–40. [15] Zhang HL, Yu JY, Wang HC, Xue LH. Investigation of microstructures and ultraviolet aging properties of organo-montmorillonite/SBS modified bitumen. Mater Chem Phys 2011;129(3):769–76. [16] Du XS, Xiao M, Meng YZ, Hay AS. Synthesis of poly(arylene disulfide)vermiculite nanocomposites via in situ ring-opening polymerization of macrocyclic oligomers. Polym Int 2004;53(6):789–93. [17] Yu JY, He J, Ya CQ. Preparation of phenolic resin/organized expanded vermiculite nanocomposite and its application in brake pad. J Appl Polym Sci 2011;119(1):275–81. [18] Zhang HL, Shi CJ, Han J, Yu JY. Effect of organic layered silicates on flame retardancy and aging properties of bitumen. Constr Build Mater 2013;40(1):1151–5. [19] Loeber L, Sutton O, Morel J, Valleton JM, Muller G. New direct observations of asphalts and asphalt binder by scanning electron microscopy and atomic force microscopy. J Microsc 1996;182:32–9. [20] Masson J-F, Leblond V, Margeson J. Bitumen morphologies by phase-detection atomic force microscopy. J Microsc 2006;221:17–29. [21] Baumgardner GL, Masson J-F, Hardee JR, Menapace AM, Williams AG. Polyphosphoric acid modified asphalt: proposed mechanisms. Proc Assoc Asphalt Paving Technol 2006;74:283–305. [22] ASTM D1754. Standard test method for effects of heat and air on asphaltic materials (Thin-Film Oven Test); 2009.
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[23] ASTM D6521. Standard practice for accelerated aging of asphalt binder using a pressurized aging vessel (PAV); 2008. [24] ASTM D36. Standard test method for softening point of bitumen (Ring-andBall Apparatus); 2012. [25] ASTM D5. Standard test method for penetration of bituminous materials; 2013. [26] ASTM D113. Standard test method for ductility of bituminous materials; 2007. [27] ASTM D4402. Standard test method for viscosity determination of asphalt at elevated temperatures using a rotational viscometer; 2012.
[28] Zhang HL, Wang HC, Yu JY. Effect of aging on morphology of organomontmorillonite modified bitumen by atomic force microscopy. J Microsc 2011;242(1):37–45. [29] Magonov SN, Elings V, Whangbo MH. Phase imaging and stiffness in tappingmode atomic force microscopy. Surf Sci 1997;375:385–91. [30] Jager A, Lackner R, Eisenmenger-Sittner C, Blab R. Identification of microstructural components of bitumen by means of atomic force microscopy (AFM). Proc Appl Math Mech 2004;4:400–1.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Microstructures and thermal aging mechanism of expanded vermiculite modified bitumen Henglong Zhang a,⇑, Hongbin Xu a, Xiaoliang Wang a, Jianying Yu b a b
College of Civil Engineering, Hunan University, Changsha 410082, PR China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China
h i g h l i g h t s This work evaluates a novel bitumen modification through the use of expanded vermiculite. A new approach to determine the microstructures of OEVMt modified bitumen is reported. Thermal aging properties of OEVMt modified bitumen are evaluated. Aging mechanism of the binders is revealed by using atomic force microscopy.
a r t i c l e
i n f o
Article history: Received 14 May 2013 Received in revised form 21 May 2013 Accepted 24 May 2013 Available online 17 June 2013 Keywords: Bitumen Expanded vermiculite Composite materials Microstructure Atomic force microscopy Aging
a b s t r a c t This work evaluates a novel bitumen modification through the use of expanded vermiculite (EVMt) which has been traditionally used as thermal insulation material. In order to improve the compatibility between EVMt and bitumen, EVMt was modified by octadecyl dimethyl benzyl ammonium chloride and marked as organo-expanded vermiculite (OEVMt). A new approach to determine the microstructures of OEVMt modified bitumen was reported by combining the results from X-ray diffraction analysis of OEVMt modified bitumen and the dissolving–filtrating procedure. Aging mechanism of the binders was revealed by using atomic force microscopy (AFM). The results show that OEVMt modified bitumen forms a semi-exfoliated nanostructure. As a result of thin film oven test (TFOT) and situ thermal aging, mass change rate and viscosity aging index are increased, while retained penetration and ductility are decreased of binders. However, these physical changes can be effectively prevented by OEVMt. Aging influences bitumen morphology significantly. TFOT leads to the association of the dispersed domains and the single phase trend of bitumen. These changes are accelerated by situ thermal aging. However, these morphology changes of bitumen are inhibited obviously with the introduction of OEVMt, indicating the good thermal aging resistance of OEVMt modified bitumen with semi-exfoliated nanostructure. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Bitumen, residue from crude oil distillation, is a complex mixture of different organic molecules. Its chemical composition primarily depends on both crude oil source and processing procedure involved [1,2]. Bitumen has been widely used in road construction as a binder due to its good viscoelastic properties [3]. The increasing demands of traffic on road building materials in recent years have resulted in a search for binders with improved performance relative to normal penetration grade bitumens [4–6]. Furthermore, as other organic substances, bitumen is subjected to aging, which influences its chemical structures and physical properties. Bitumen aging is one of the principal factors causing the deterioration of asphalt pavements. Important aging related modes ⇑ Corresponding author. Tel./fax: +86 731 88823937. E-mail address: [email protected] (H.L. Zhang). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.05.099
of failure are traffic and thermally induced cracking, and ravelling [7]. Consequently, more and more modified bitumens are used for paving applications through the addition of virgin polymers (SBS, SBR, EVA, etc.) and waste polymers (plastics from agriculture, crumb tyre rubber, etc.) [2,8,9]. Recently, layered silicates have been used to modify bitumen. The main layered silicates include montmorillonite (Mt), rectorite, vermiculite (VMt) and kaolinite clay. More attention of the researchers has been paid to Mt modified bitumen. To increase the compatibility between the Mt and bitumen, Mt is commonly exchanged with organic cations, particularly alkylammonium ions, making the Mt become lipophilic, and the interlayer spacing is enlarged. It has been found that physical properties, rheological behaviors of bitumen and polymer modified bitumen could be obviously improved due to the introduction of organo-montmorillonite (OMt) [10–13]. Even more exciting is that OMt can significantly improve the thermo-oxidative and photo-oxidative aging
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resistance of bitumen [14,15], which is usually attributed to barrier properties of Mt by formation of intercalated or exfoliated nanostructure. However, the microstructures of Mt and OMt modified bitumens are only characterized by X-ray diffraction (XRD). The true microstructures may be hidden by test error during XRD analysis for modified bitumen. Like Mt, VMt is a mica-type silicate, belongs to the general family of 2:1 layered silicates. Each layer consists of octahedrally coordinated cations (typically Mg, Al and Fe) sandwiched by tetrahedrally coordinated cations (typically Si and Al). The isomorphous substitution of Si4+ by Al3+ leads to a net negative surface charge that is compensated by an interlayer of exchangeable hydrated cations (Ca2+, Mg2+, Cu2+, Na+) [16]. When pristine VMt flakes are strongly heated at high temperature (about 900 °C) during a short period of time, the water situated between layers is quickly converted into steams, exerting a disruptive effect upon the structure. As a consequence, a highly porous material named expanded vermiculite (EVMt) is formed and it is an efficient thermal insulator [17]. On these grounds, this work deals with the use of EVMt as a potential modifier in bitumen modification, which may somewhat become a new approach in the asphalt industry. In contrast to Mt, EVMt shows a better improvement on aging resistance of bitumen [18]. However, the aging mechanism of EVMt modified bitumen as well as the pristine bitumen is still not well understood. As mentioned before, most of published articles were limited in the physical or rheological properties of the layered silicates modified bitumens, few researchers paid attention to the microstructures of the binders except the using of XRD. New analytical methods should be taken to better understand the interaction of EVMt with bitumen. The atomic force microscopy (AFM) is a very high-resolution type of scanning probe microscopy. The AFM images a structure by scanning with a tiny tip over the sample surface. It is capable of measuring topographic features at the nanometer-scale or even at atomic-scale resolution. In recent decades, AFM has been used to investigate the morphology of bitumen by more and more researchers [19–21]. It allows to visualize precise details of nanoscale structures of bitumen without special techniques of sample preparation. In summary, the main objective of this research was to study bitumen modification degrees achieved by adding organo-expanded vermiculite (OEVMt). The microstructures of OEVMt modified bitumen were characterized by combining XRD and dissolving–filtrating method. Thermal aging properties and mechanism of OEVMt modified bitumen and the pristine bitumen were investigated by AFM and the physical properties changes of the binders before and after thin film oven test and situ thermal aging.
Table 1 Physical properties of the pristine bitumen. Physical properties
Measured values
Penetration (25 °C, 0.1 mm) Softening point (°C) Ductility (15 °C/10 °C, cm) Viscosity (60 °C, Pa s) Viscosity (135 °C, Pa s)
75 48.6 150.0/15.6 258 0.54
vermiculite (OEVMt) with a particle size of 300 mesh. The ammonium content of OEVMt determined by thermogravimetric analysis under helium atmosphere was 18.6 wt%. 2.3. Preparation of OEVMt modified bitumen Modified bitumen was prepared using a high shear mixer. Bitumen was heated to 150 ± 5 °C in an oil-bath heating container until it flowed fully. Then 3 wt% OEVMt was added into bitumen, and the mixture was blended at 5000 r/min for 60 min. The same process was also performed on the pristine bitumen in order to compare with the OEVMt modified bitumen. 2.4. Thermal aging procedures Thermal aging of the binders can be performed using standard aging procedures, the thin film oven test (TFOT, ASTM D1754) [22], and pressurized aging vessel (PAV, ASTM D6521) [23]. However, OEVMt platelets automatically accumulated to reduce their surface area under tough testing conditions in the PAV. This accumulation weakened the barrier properties of the OEVMt. So the PAV was not suitable for the simulation of long-term thermal aging of OEVMt modified bitumen. In this paper, short-term thermal aging of the binders was performed using the thin film oven test (TFOT), while the situ thermal aging was used to perform the longterm thermal aging. TFOT was conducted in an oven with a rotating platform on an axis, and the rotation of platform was carried out by the axis. The binders in pans were put on the platform and heated for 5 h at 163 °C. The situ thermal aging was performed in oven. The residue from TFOT was aged at 70 °C for 30 days, the thickness of bitumen film was about 3.2 mm. The aged binders were blended at 150 °C for 10 min to insure a homogeneous system for measuring physical properties, microstructure and morphology. Aging susceptibility of the binders may be evaluated by means of an aging index, which is defined as the ratio of a chemical or physical parameter of the aged binder to that of the original binder [7]. In this paper, aging indices obtained using mass and physical measurements were used to evaluate the aging properties of the binders. The mass change rate (MCR) is computed in accordance with the following formula:
MCR ¼ ðM 1 M 0 Þ=M 0 100
where M0 and M1 are mass of the binders before and after aging, respectively. The retained penetration (RP), viscosity aging index (VAI) and retained ductility (RD) of the binders are computed as formulas (2)–(4), respectively.
RP ¼
Aged penetration value 100 Unaged penetration value
VAI ¼ 2. Experimental 2.1. Materials The 60/80 pen grade bitumen was used as base binder in this study. The physical properties of the pristine bitumen are listed in Table 1. EVMt, 300 mesh, was used as bitumen modifying agent. Octadecyl dimethyl benzyl ammonium chloride, chemically pure, was used to modify the EVMt.
2.2. Preparation of organo-expanded vermiculite A 500-mL round-bottom, three-necked flask with a mechanical stirrer, thermometer, and condenser with a drying tube was used as a reactor. EVMt (10 g) was gradually dissolved in 200 mL deionised water and stirred for 30 min. Then octadecyl dimethyl benzyl ammonium chloride was added into this solution. The resultant suspension was vigorously stirred for 10 h. The treated EVMt was repeatedly washed with deionised water. The filtrate was titrated with 0.1 N AgNO3 until no precipitate of AgCl was formed. The filter cake was then placed in a vacuum oven to dry at 80 °C for 24 h. The dried cake was then ground to obtain organo-expanded
ð1Þ
RD ¼
Aged viscosity value Unaged viscosity value 100 Unaged viscosity value
Aged ductility value 100 Unaged ductility value
ð2Þ
ð3Þ
ð4Þ
2.5. Physical properties test The physical properties of the binders, including softening point, penetration (25 °C), and ductility (5 °C), were tested according to ASTM D36, ASTM D5, and ASTM D113, respectively [24–26]. Brookfield viscometer was employed to measure the rotational viscosity of the binders at 135 °C and 60 °C according to ASTM D4402 [27]. 2.6. Microstructure characterization X-ray diffraction graphs of EVMt, OEVMt and modified bitumens were obtained using a Rigaku D/max D/MAX-IA diffractometer with Cu Ka radiation (k = 0.15406 nm; 40 kV, 50 mA) at room temperature. The scanning rate was 2°/ min. The diffractograms were scanned from 0.5° to 10°.
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H.L. Zhang et al. / Construction and Building Materials 47 (2013) 919–926 2.7. Micro-morphology analysis AFM was applied to investigate the micro-morphology of the binders. A hot liquid drop of bitumen at 140 °C was carefully placed on a 10 mm 10 mm 1 mm steel disk, which was heated for 1–2 min on a hot plate at about 140 °C, a temperature high enough to melt bitumen, but not so high that it would oxidize rapidly. The bitumen was spread out with a blade to form a round film of about 5 mm in diameter. This hot film was left on the hot plate undisturbed for an additional 1 min to allow the surface to flow to a smooth and glossy finish [20]. For AFM analysis, the film was then cooled to ambient temperature(about 5 °C), covered by a glass cap to prevent dust pick-up and annealed for a minimum of 24 h before imaging [28]. Topographic and phase images were scanned using an etched silicon probe. Cantilever was 125 lm long with curvature radius at 5–10 nm and height at 15–20 lm. The drive frequency was 260 kHz and the drive amplitude was 56 mw. Test was operated in tapping mode. All the microphotographs showed a 15 lm 15 lm region unless otherwise indicated.
3. Results and discussion 3.1. Microstructures of OEVMt and modified bitumen The XRD patterns of EVMt and OEVMt are shown in Fig. 1. The basal spacings (d001) are calculated according to the Bragg equation (k = 2d sinh). The calculated d001 values are given in Table 2. As shown in Fig. 1, the first diffraction peak of the XRD curve for EVMt is at d001 = 1.42 nm. After organic modification by octadecyl dimethyl benzyl ammonium chloride, the peak is shifted to a lower angle which means an increase in d001 value. As indicated, d001 value increases from 1.42 nm to 5.33 nm for OEVMt. The increased d001 value will contribute to the intercalation of bitumen molecules into the galleries of OEVMt easily. The OEVMt modified bitumen was also characterized by XRD in order to evaluate whether intercalation occurred. Interestingly enough, upon melt blending with bitumen, the OEVMt shows no diffraction peak in the XRD pattern. The disappearance of the diffraction peak of OEVMt after compounding with bitumen implies either the formation of an intercalated nanocomposite with a basal spacing larger than 17.60 nm, the formation of an exfoliated nanostructure or a very disorganized structure of the silicate platelets. Further characterizations are necessary to determine exactly microstructure of the OEVMt modified bitumen. In filled polymers areas, transmission electron microscope (TEM) has been proved to an extremely useful technique to characterize the nanocomposite microstructures by combining with XRD analysis. In this paper, TEM method was tried but failed because some small bitumen molecules easily evaporated due to the vac-
Fig. 1. XRD patterns of EVMt, OEVMt, OEVMt modified bitumen and separated OEVMt from modified bitumen.
Table 2 Interlayer spacing of EVMt, OEVMt and OEVMt in modified bitumen. Samples
2h (°)
d001 (nm)
d001 variation (nm)
EVMt OEVMt OEVMt in modified bitumen Separated OEVMt
6.23 1.66 – 1.83
1.42 5.33 >17.60 4.82
– 3.91 – 3.40
uum environment at ambient temperature. In an effort to determine the exact microstructure of OEVMt modified bitumen, dissolving–filtrating procedure was performed as follows: first, OEVMt modified bitumen was dissolved in trichloroethylene, then the OEVMt was filtered from this solution. After that the separated OEVMt was characterized by XRD. The XRD patterns of separated OEVMt were shown in Fig. 1d. It can be seen that separated OEVMt shows an obvious diffraction peak in XRD curve, the corresponding d001 value is 4.82 nm. It implies that not all OEVMt are peeled off, there are still some regular layered structures of OEVMt in modified bitumen. Even more exciting is that this d001 value is lower than that of OEVMt. The reasons can be attributed to the existed organic compounds in EVMt interlayers. The bitumen molecules intercalated into the interlayers of OEVMt are dissolved by trichloroethylene, which contributes to the decreased d001 value of separated OEVMt. All above discussion indicates that OEVMt modified bitumen forms a semi-exfoliated nanostructure.
3.2. Morphology of bitumen and effect of OEVMt modification A typical topographic AFM image of unmodified bitumen is shown in Fig. 2a. The dispersed domains (the dark-looking region) display a succession of pale and dark lines, which are often called ‘bee-like’ structures. The appearance of ‘bee-like’ structures is caused by the existence of asphaltenes in the bitumen according to our previous research [29]. The dimension of ‘bee-like’ structures is decreased obviously in OEVMt modified bitumen, as shown in Fig. 2b. However, the ‘bee-like’ structures with decreased dimension disperse in the OEVMt modified bitumen more homogeneously in comparison with unmodified bitumen. The appearance of the ‘bee-like’ structures can be attributed to the crystallization of the microcrystalline waxes contained in the asphaltenes, as shown in Fig. 3. According to XRD analysis, bitumen molecules intercalate into the layers of OEVMt during the melt blending process, and OEVMt modified bitumen forms a semiexfoliated nanostructure. The silicate layers of OEVMt with high aspect ratio greatly obstruct the movement of bitumen molecule chains. Therefore, the crystallization of microcrystalline waxes and waxy molecules in bitumen can be prevented during cooling from high to testing temperature (about 5 °C), which contributes to the decreasing of ‘bee-like’ structures dimensions. AFM Phase images of unmodified bitumen and OEVMt modified bitumen are shown in Fig. 4. In AFM analysis, both the topographic and phase images are viewed side-by-side in real time. Phase images can be used to map variations of surface properties such as viscoelasticity and elasticity. As shown in Fig. 4a, there are obviously two phases in unmodified bitumen, which are corresponding to the dispersed domains and the matrix in topographic image. However, the contrast between the dispersed phase and the matrix is decreased with the introduction of OEVMt as seen in a comparison of Fig. 4a and b. The boundary between the dispersed phase and the matrix is clear in unmodified bitumen, while the boundary is ambiguous in OEVMt modified bitumen. This indicates a change in the tip–sample interactions during the AFM analysis, which is affected by the sample modulus [30]. The decreased constrast is related to differences in the rise of bitumen stiffness with the
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Fig. 2. Topographic images of (a) unmodified bitumen, and (b) OEVMt modified bitumen on a scale of 15 lm 15 lm. The color contrast covers a height variation of 120 nm.
stiffness of the two phases is decreased after modification. It implies that OEVMt shows a better interaction with the matrix, which contributes to the increasing of the relative stiffness of the matrix.
3.3. Effect of TFOT on binder physical properties and morphology
Fig. 3. Three-dimensional topographic images of unmodified bitumen on a scale of 15 lm 15 lm.
introduction of OEVMt. The relative stiffness of the dispersed phase is higher than that of the matrix [9]. However, the difference in
The changes of physical properties of the binders before and after TFOT are given in Table 3. As shown in Table 3, both mass change rate and viscosity aging index of OEVMt modified bitumen decrease significantly after TFOT. In addition, retained ductility and retained penetration of the OEVMt modified bitumen are higher than that of the unmodified bitumen, indicating the good aging resistance of OEVMt modified bitumen. The improvement on the aging resistance performance is resulted from the barrier properties of the OEVMt particles which hinder the penetration of the oxygen molecules and increase their average path length by forming the semi-exfoliated nanostructure. Another reason can be the decrease of the volatility of the oil components of bitumen, which prevents the hardening of the binders. The topographic images of unmodified bitumen and OEVMt modified bitumen after TFOT are shown in Fig. 5. The phase images of unmodified bitumen and OEVMt modified bitumen after TFOT are shown in Fig. 6. Compared with unaged samples, the contrast between the matrix and the dispersed domains in the aged samples decreases remarkably, especially for unmodified bitumen,
Fig. 4. Phase images of (a) unmodified bitumen, and (b) OEVMt modified bitumen on a scale of 15 lm 15 lm.
H.L. Zhang et al. / Construction and Building Materials 47 (2013) 919–926 Table 3 Influence of OEVMt on physical properties of bitumen before and after TFOT. Aging indices
Unmodified bitumen
OEVMt modified bitumen
Mass change rate (%) Retained penetration (%) Viscosity aging index (%, 60 °C) Retained ductility (%, 15 °C)
0.09 80 88 20
0.02 90 43 46
the boundary between the matrix and the dispersed domains disappears (Fig. 5a). It indicates that an obvious association of the dispersed domains in bitumen appears during TFOT. It can be explained that oxidation leads to the decrease of the difference in chemical properties between the dispersed domains and the matrix during the TFOT. Bitumen is a complex system made of different constituents, namely saturates, aromatics, resins and asphaltenes. TFOT decreases aromatics and at the same time increases the content of resins and asphaltenes, which contributes to the following transformation of different fractions, aromatics ? resins ? asphaltenes. As a result, the content of high molecular weight fraction is increased and the whole stiffness of the bitumen is obviously increased, which lead to the reduced difference in stiffness between the two phases in bitumen. According to Fig. 6a, it can be seen clearly that the contrast between the dispersed phase and the matrix phase is decreased obviously and their boundary is also disappearing in unmodified bitumen after TFOT. It indicates that there is a single-phase trend of the bitumen during the TFOT. As discussed before, the relative stiffness of the dispersed phase is higher than that of the matrix. It is reasonable to assume that the stiffness of the matrix in bitumen is increased more obviously than that of the dispersed phase during TFOT. In addition, the topographic images in Fig. 5a show that the dimension and the amount of ‘bee-like’ structures are reduced after TFOT. As mentioned before, the appearance of the ‘bee-like’ structures is attributed to the crystallization of the microcrystalline waxes or waxy molecules contained in the asphaltenes. Due to bitumen aging, some chemical reactions occurred and molecular structures of bitumen are changed simultaneously, which introduces an increase in the overall polarity of the bitumen. Similarly, the structure and chemical properties of the microcrystalline waxes and waxy molecules also may be changed, which reinforces their solubility in bitumen. Their crystallization after aging is prevented. Consequently, dimension and the amount of ‘bee-like’ structures are decreased.
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The surface of aged OEVMt modified bitumen is relatively smooth, which is similar to that of the unaged samples. The slight changes of the morphology after TFOT indicate the better aging resistance of OEVMt modified bitumen in comparison with unmodified bitumen, which is in accordance with changes in physical properties of unmodified and OEVMt modified bitumens after TFOT. Oxidation is the most important aging mechanism of bitumen, which leads to the hardening of bitumen as a result of the chemical changes, e.g. the transformation from aromatics and resins to asphaltenes [7]. However, the oxidation of bitumen is reduced remarkably by OEVMt which hinders the permeability of oxygen in the bitumen during TFOT. As a result, the hardening of the bitumen can be effectively prevented. Therefore, the aging resistance of OEVMt modified bitumen is improved remarkably. Interestingly enough, there are two clear phases for OEVMt modified bitumen after TFOT. The contrast between the matrix and the dispersed domains is increased compared to that of unaged samples (Fig. 6b). As mentioned earlier, OEVMt shows a better interaction with the matrix compared to the dispersed domains, which contributes to the decreased contrast between the matrix and the dispersed domains after modification. However, stiffness of the dispersed domains increases obviously by the transformation from aromatics and resins to asphaltenes during the TFOT. As a consequence, the influence of OEVMt on stiffness of the two phases is decreased, which leads to the increased difference in stiffness between the matrix and the dispersed domains. Compare with unmodified bitumen, the association of the dispersed domains is prevented obviously in OEVMt modified bitumen, indicating its good short-term aging resistance. 3.4. Effect of situ thermal aging on binder physical properties and morphology The changes of physical properties of the binders before and after situ thermal aging (30 d) are given in Table 4. As can be seen from Table 4, situ thermal aging make the physical properties of the binders reduced more remarkably in comparison with that after TFOT. According to our previous research, it has been found that PAV simulation corresponds to 30 days of situ thermal aging based on the changes of the physical properties of the binders before and after aging. For plain bitumen, it has been established that this simulation corresponds to a service life of the order of 4– 5 years [9]. The results show that, after situ thermal aging, mass change rate and viscosity aging index of OEVMt modified bitumen
Fig. 5. Topographic images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after TFOT on a scale of 15 lm 15 lm. The color contrast covers a height variation of 80 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Phase images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after TFOT on a scale of 15 lm 15 lm.
Table 4 Influence of OEVMt on physical properties of bitumen before and after situ thermal aging. Aging indices
Unmodified bitumen
OEVMt modified bitumen
Mass change rate (%) Retained penetration (%) Viscosity aging index (%, 60 °C) Retained ductility (%, 15 °C)
0.66 43 982 5
0.49 49 609 12
are lower than that of unmodified bitumen, while retained ductility and retained penetration are higher than that of the unmodified bitumen, which is in accord with the TFOT results. It indicates that OEVMt can contribute to the improved both short-term and longterm thermal aging resistance of the bitumen. The topographic and phase images of unmodified bitumen and OEVMt modified bitumen after situ thermal aging are shown in Figs. 7 and 8, respectively. The surface of unmodified bitumen becomes very rough after situ thermal aging in comparison with that after TFOT. In addition, the contrast between the matrix and the dispersed domains further decreases. There is only one phase for unmodified bitumen after situ thermal aging in Fig. 8a. As described in aging procedure, situ thermal aging was performed after
TFOT. It indicates that the single-phase trend of the bitumen is accelerated during situ thermal aging compared with TFOT. The accelerated single-phase trend of the unmodified bitumen can be attributed to the enhanced compatibility between the dispersed phase and the matrix phase. The aging conditions of situ thermal aging are more rigorous as compared to the TFOT. Consequently, the morphology and chemical structures of the bitumen molecules are more obviously affected. As a result, the overall polarity of both the matrix and the dispersed domains further increases, which leads to the increased solubility with bitumen molecules and the single-phase trend. As compared with phase image of OEVMt modified bitumen after TFOT aging in Fig. 6b, the areas of the dispersed domains are increased. On the contrary, the areas of the matrix are decreased. Moreover, the boundary between the dispersed domains and the matrix becomes blurred, which implies that the association of the dispersed domains also occurs in OEVMt modified bitumen during situ thermal aging. However, there are still two obvious phases in OEVMt modified bitumen after situ thermal aging. The above discussion shows that although the aging mechanism of the bitumen is not changed by OEVMt, the aging tendency of the bitumen is prevented remarkably by the introduction of OEVMt, indicating the good long-term thermal aging resistance of OEVMt modified bitumen.
Fig. 7. Topographic images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after situ thermal aging on a scale of 15 lm 15 lm. The color contrast covers a height variation of 80 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. Phase images of (a) unmodified bitumen, and (b) OEVMt modified bitumen after situ thermal aging on a scale of 15 lm 15 lm.
Interestingly, the dimension and the amount of ‘bee-like’ structures increase obviously after situ thermal aging compared to TFOT. As indicated in Tables 2 and 3, mass change rate of unmodified bitumen increases from 0.09% to 0.66%, which shows an obvious oxidation of bitumen molecules during situ thermal aging. As can be seen, during situ thermal aging, the asphaltenes content is increased. On the other hand, the colloidal structure of bitumen is transformed from sol–gel type to gel type, which causes the solid-like effect of bitumen. Thus, these high molecular weight fractions cannot be dissolved effectively by other bitumen molecules, which contributes to the formation of the ‘bee-like’ structures. 4. Conclusions The microstructures, thermal aging properties and mechanism of OEVMt modified bitumen were evaluated. According to the results of XRD analysis of OEVMt modified bitumen and the dissolving–filtrating procedure (first, OEVMt modified bitumen was dissolved in trichloroethylene, then the OEVMt was filtered from this solution. After that the separated OEVMt was characterized by XRD), OEVMt modified bitumen is proved to form the semiexfoliated nanostructure. As a result of TFOT and situ thermal aging, the physical properties of aged bitumens become more solid-like, as indicated by increased mass change rate and viscosity aging index as well as the decreased retained penetration and ductility. Aging also influences bitumen morphology significantly. The association of the dispersed domains and the single phase trend of bitumen occur during TFOT, which is accelerated by situ thermal aging. However, these physical and morphology changes of bitumen after TFOT and situ thermal aging are prevented obviously with the introduction of OEVMt, indicating the good aging resistance of OEVMt modified bitumen with semi-exfoliated nanostructure. Such a result suggests that OEVMt can be effectively used as a modifier to improve the aging resistance of bitumen. Acknowledgments This work was supported by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No. 2011BAE28B04), and was financially supported by the Project of Young Teacher Growth of Hunan University (No. 2012-161). The work was also supported by Open Fund of Key Laboratory of Special Environment Road Engineering
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