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Influence of ageing on rheology of SBR/sulfur-modified asphalts.

INTRODUCTION

Asphalt is an organic mixture that is widely used in road pavement due to its good viscoelastic properties (1). Unfortunately, asphalt becomes brittle at lower temperature and is a liquid at higher temperature, which can result in low-temperature cracking of pavement and high-temperature rutting. This temperature susceptibility limits its application. Therefore, in order to enhance the aggregate performance of asphalts, it is necessary to modify asphalts by adding polymer modifiers (2-6).

Among the polymer modifiers of asphalt, SBR has been found to be one of the most effective modifier for paving asphalt. An Engineering Brief from 1987 available at the US Federal Aviation Administration website (5) describes the benefits of SBR-modified asphalt in improving the properties of bituminous concrete pavement and seal coats. Low-temperature ductility is improved, viscosity is increased, elastic recovery is improved, and adhesive and cohesive properties of the pavement are improved. According to Becker et al., SBR latex polymers increase the ductility of asphalt pavement (7), which allows the pavement to be more flexible and crack resistant at low temperatures, as found by the Florida Department of Transportation (8). Unfortunately, some main drawbacks were also found in practice. The high-temperature property of SBR-modified asphalt is not very good. Though the low-temperature flexibility of asphalt increases evidently with the addition of SBR, the high-temperature rutting resistance cannot be improved effectively with increasing SBR content. Due to the much polybutadiene structures in SBR molecule, SBR is more susceptible to heat and oxygen, therefore the loss of low-temperature ductility of SBR-modified asphalt is very severe after oxidative ageing (9). For this reason, some countries have made the corresponding standards in evaluating the low-temperature ductility of SBR-modified asphalt. In our country, the low-temperature ductility of SBR-modified asphalt for road paving should not be less than 30 cm after short-term ageing (10). Besides the poor compatibility between SBR and asphalt is also another important problem to be considered. Phase separation would take place in SBR-modified asphalt at elevated temperatures. For these reasons, many measures have been taken, including adding various clays such as montmorillonite, organobentonite, organic palygorskite, weathered coal, carbon black (11-15). Compare with these measures, however, the most effective way to improve the high-temperature perfonnance and compatibility of SBR-modified asphalt is to add sulfur.

Some publications have concerned the studies on styrene-butadiene-styrene (SBS)/suIfur-modified asphalt. Wen et al. studied the rheological properties of SBS/sulfur-modified asphalt systematically and observed the morphology at elevated temperatures by optical microscopy (16-18). The similar studies also have been taken by other researchers (19), (20). It is commonly believed that sulfur chemically crosslinks polymer molecules and chemically couples polymer and asphalt through sulfide or polysulfide bonds, consequently the high-temperature performance and compatibility of the SBS/sulfur-modified asphalt are improved. Compare with SBS, SBR owns lower average molecule weight and is more reactive with sulfur due to lots of polybutadiene structures in the copolymer molecule, therefore the pronounced improvement on the high-temperature performance and compatibility of SBR-modified asphalt can be found by the addition of sulfur. In fact, sulfur has been used widely in the preparation of SBR-modified asphalt. Considering the loss of the low-temperature ductility of SBR-modified asphalt after short-term ageing, a suitable sulfur content should be determined in the preparation.

However, the performance of asphalt pavement mainly depends on the properties of aged asphalt, because oxidative ageing is an inevitable process in practical road paving. In the mixing of asphalt binder with aggregates, asphalt binder will endure short-term ageing, subsequently, asphalt binder will endure long-term ageing during the in-service life of pavement. It is reported that oxidative ageing has a great effect on the properties of polymer modified asphalts (PMAs) (21-23). Although the high-temperature performance of SBR-modified asphalt can be improved significantly with the addition of sulfur, if SBR/sulfur-modified asphalt still owns more better high-temperature performance after ageing is still unknown, moreover the influence of ageing on the rheological, physical properties, and the morphology of SBR/sulfur-modified asphalt is not reported in many publications.

The present work seeks to identify the changes on the rheological, physical properties of the SBR- and SBR/sulfur-modified asphalts by short-term and long-term ageing, to compare the SBR-modified asphalt with the SBR/sulfur-modified asphalt with the suitable sulfur content (0.03 wt%) by the morphological and thermogravimetric analysis, and finally to point out the main problems occurring in practical road paving of SBR/sulfur-modified asphalt.

MATERIALS AND EXPERIMENTAL

Materials

Asphalt, SK-90 paving asphalt, was obtained from the SK Petroleum Asphalt Factory, Southern Korea. The physical properties of the asphalt are as follows: softening point: 46.3 [degrees] C (ASTM D36); penetration: 87 dmm (25 [degrees] C, ASTM D5); viscosity: 0.32 Pa.s (135 [degrees] C, ASTM D 4402).

SBR was produced by the Lanzhou Petrochemical Co., Ltd., China. It was a star-like SBR, containing 27.3 wt% styrene, 0.64 wt% water soluble, 0.37 wt% volatile fraction and viscosity ([ML.sup.1+4 100 [degrees] C) 48-55.

Preparation of Samples

The modified asphalts were prepared using a high shear mixer (made by Weiyu Machine Co., Ltd., China). First, asphalt (400 g) was heated until it became a fluid in an iron container, then upon reaching about 170 [degrees] C, the SBR or sulfur powder (the amounts were based on 100 parts asphalt) were added to the asphalt, SBR content is always 4 wt%, sulfur content is 0.03 wt%, 0.065 wt%, 0.1 wt%, respectively, and then the blend was sheared for 40 min, the shearing temperature is 170CC, the shearing rate is 4000 rpm, subsequently the blend was stirred by a mechanical stirrer at 170 [degrees] C for 2 h to make sure the fully swelling of the modifiers in the asphalt. After that, the preparation has been finished.

Storage Stability Test

The storage stability of modified asphalts was measured as follows. The sample was transferred into an aluminum toothpaste tube (32 mm in diameter and 160 mm in height). The tube was sealed and stored vertically in an oven at 163 [degrees] C for 48 h, then taken out, cooled to room temperature, and cut horizontally into three equal sections. The samples taken from the top and bottom sections were used to evaluate the storage stability of the SBR-modified asphalts by measuring their softening points. If the difference of the softening points between the top and the bottom sections was less than 2.5 C, the sample was considered to have good high-temperature storage stability. If the softening points differed by more than 2.5 [degrees] C, the SBS-modified asphalt was considered to be unstable.

Standard Ageing Procedure

Oxidative ageing of modified asphalts was performed using the thin film oven test, TFOT (ASTM D1754), and pressurized ageing vessel, PAV (ASTM D6521). TFOT simulates short-term ageing that takes place during the hot mixing of asphalt binder with aggregates and during the pavement construction phase, while PAV simulates long-term ageing that occurs during the in-service life of asphalt pavement.

Physical Properties Test

The physical properties of asphalts, including softening point, penetration, viscosity (135 [degrees] C), ductility (5 [degrees] C), were measured in accordance with ASTM D36, D5, D4402, and Chinese specification GB/T 4508, respectively.

Thin-Layer Chromatography With Flame Ionization Detection

In the thin-layer chromatography with flame ionization detection (TLC-FID), 2% (w/v) solutions of the unaged and aged base asphalts were prepared in dichloromethane, and 1 /d sample solution spotted on chromarods using a spotter. The separation of bitumen into four generic fractions (saturates, aromatics, resins, and asphaltenes) was performed by a three-stage development using-heptane, toluene, and dichloromethane/methanol (95/5 by volume). respectively. The fractions were determined by means of latroscan MK-5 analyzer (latron Laboratories Inc., Tokyo, Japan).

Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) was performed using a strain-controlled rheometer (RDA II, Rheometrics). In DMA, frequency sweeps were applied over the range of 0.1 to 100 rad/s at a fixed strain amplitude at five temperatures: 25 [degrees] C, 30 [degrees] C, 40 [degrees] C, 50 [degrees] C, 60 [degrees] C, and temperature sweeps (from 30 to 138 [degrees] C) with 2 C increments were applied at a fixed frequency (10 rad/s) and at variable strains. High-temperature repeated creep tests were performed at 60 [degrees] C using a 1 s loading cycle followed by a recovery period of 9 s, and this was repeated 100 times. The repeated creep tests were made at a constant stress level of 30 Pa [24]. Parallel plates, gap 1.0 mm for [phi] 25 mm, were used. In each test, about 1.0 g of sample was applied to the bottom plate, covering the entire surface, and the plate was then mounted in the rheometer. After heating to the softening point of the binder, the top plate was brought into contact with the sample, and the sample was trimmed. The final gap was adjusted to 1 mm. A sinusoidal strain was then applied by an actuator. The actual strain and torque were measured and input to a computer for calculating various viscoelastic parameters such as complex modulus (G*) and phase angle ([delta]) and so on.

Morphology Observation

The sample morphology was observed using an optical microscope made by Nikkon Co., Japan. Squashed slides of modified binders were prepared using very small amounts of the heated sample and viewed under the microscope at a magnification of 400X.

RESULTS AND DISCUSSION

Storage Stability

Due to the difference in the solubility parameter and density between SBR and asphalt, phase separation would take place in SBR-modified asphalt during storage at elevated temperatures. Droplets of the SBR melt dispersed in asphalt are usually accumulated and float on the top of the asphalt at a high temperature and static state. The high-temperature storage stability of the SBR-modified asphalts with varying sulfur levels is tested and the results are shown in Table 1. Obviously, for the SBR-modified Asphalt, the difference in the softening points is large. As shown in Table 1, the difference in softening points is 6 [degrees] C for the SBR-modified asphalt. Storage-stable SBR-modified asphalt can be prepared by the addition of 0.03 wt% sulfur at high temperature under high shear mixing. With increasing the sulfur level, the difference in softening points declines slightly.

TABLE 1. Effect of sulfur on storage stability of SBR-modified
asphalts (SBR content: 4 wt%).

Sulfur content wt%   0  0.03 wt%  0.065 wt%  0.1 wt%

[DELTA]S/[degrees]C  6       1.8        0.8      0.6


Physical Properties

For the base asphalt, to confirm the influence of ageing on the chemical components and physical properties, the chemical components of the base asphalt before and after ageing were studied using TLC-FID, the four generic fractions, namely saturates, aromatics, resins, and asphaltenes, were determined and the physical properties of the aged binders were measured, as shown in Table 2. After TFOT ageing, the content of the hard asphalt components such as asphaltenes and resins increases, while the content of aromatics decreases. The variation becomes more apparent after PAV ageing, the content of saturates remains constant comparatively. The expected increase in softening point and viscosity and the decrease in penetration are observed after ageing, and this trend becomes very apparent with increasing the hard asphalt components. This reinforces the assumption that the variation in the physical properties of the base asphalt after ageing depends on the changes in the hard asphalt components.

TABLE 2. Chemical components and physical properties of base
asphalt before and after ageing.

                                Unaged  After TFOT    After PAV

Saturates (%)                     16.3        15.66        15.76
Aromatics (%)                    49.19        46.06        38.36
Resins (%)                       24.55        26.69         28.1
Asphaltenes (%)                   9.96        11.99        17.78
Softening point ([degrees]C)      46.3         50.2         54.7
Penetration (25'C, 0.1 mm)          87           54           46
Viscosity (135[degrees]C, Pa.s    0.32         0.62         0.93
Ductility (5 C, 0.1cm)             4.5  Unavailable  Unavailable


The physical properties of the SBR- and SBR/sulfur-modified asphalts are shown in Table 3. The high-temperature performance of the SBR-modified asphalt can be improved with the addition of 0.03 wt% suflur, as shown by the increase in the softening point and viscosity and the decline in the penetration in Table 3. With increasing the sulfur level, the varying trend in the physical properties becomes more pronounced and the high-temperature performance is improved further. As the result of dynamic vulcanization, a chemically crosslinking polymer network is formed in asphalt matrix and the increasing sulfur level enhances the crosslinking density further. However, the low-temperature ductility declines continually with further vulcanization. Because there are more polysulfide bonds formed among polymer molecules in this process, the movement of polymer molecules is restricted further as shown by the loss of the low-temperature flexibility. After TFOT ageing, as shown in Table 4, the softening point and viscosity of all binders decline to a little extent and the penetration increases accordingly. Usually the influence of oxidative ageing on the high-temperature performance of PMAs can be divided into the two major sides, on one hand, the increase of the hard asphalt components such as asphaltenes and resins by ageing improves the high-temperature performance of binders and declines the temperature susceptibility, on the other, the decomposition of the polymer dispersed in asphalt after ageing will lead to an opposite result. The changes in the physical properties after TFOT ageing show the polymer degradation to a little extent. For the SBR/sulfur-modified asphalts, the binder with 0.03 wt% sulfur owns a better low-temperature ductility after TFOT ageing, that can meet the professional standard as mentioned before (10). Considering the loss of the low-temperature ductility, the SBR/sulfur-modified asphalt with 0.03 wt% sulfur is suitable for practical road paving. After PAV ageing, as shown in Table 5, the obvious increase in the softening point, viscosity, and the decline in the penetration show the improvement on the high-temperature performance with further ageing, which can be attributed to the predominant influence of the increased hard asphalt components.

TABLE 3. Physical properties of SBR- and SBR/sulfur-modified asphalts
(SBR content: 4 wt%).

Sulfur
content   Softening     Penetration      Viscosity     Ductility
(wt%)       point       (25[degrees]C,      (135       (5[degrees]C,
           [degrees]C)      0.1 mm)       [degrees]C,     0.1 cm)
                                             Pa.s)

0             51.8            53.6         1.15           >200
0.03            53            51.4         1.28           >200
0.065         53.8            51.5         1.49          170.4
0.1           54.8            49.1         1.79          130.1

TABLE 4. Physical properties of SBR- and SBR/sulfur-modified
asphalts after TFOT ageing (SBR content: 4 wt%).

Sulfur
content    Softening     Penetration   Viscosity    Ductility
(wt%)        point           (25          (135           (5
         ([degrees]C)   [degrees]C,   [degrees]C,  [degrees]C,
                           0.1 mm)       Pa.s)       0.1 cm)

0                  51         54.2         0.83          150
0.03             52.5           52         0.97           65
0.065            51.8           53         1.05           27
0.1              52.5         49.8         1.26           12

TABLE 5. Physical properties of SBR- and SBR/sulfur-modified
asphalts after PAV ageing (SBR content: 4 wt%).

Sulfur
content    Softening     Penetration  Viscosity   Ductility
(wt%)        point           (25        (135         (5
         ([degrees]C)  [degrees]C,    [degrees]  [degrees]C,
                         0.1 mm)      C, Pa.s)     0.1 cm)

0                55.1         48.5       1.09           11
0.03             54.3         49.7       1.03          6.3
0.065            55.3           48        1.1          5.5
0.1              55.5         47.1       1.29          4.1


Dynamic Rheological Properties

Temperature Sweep. The rheological behavior of the SBR- and SBR/sulfur-modified asphalts before and after ageing is shown from Fig. 1a-c. Before ageing, the increasing complex modulus in the whole temperature range with the addition of sulfur shows the improved rheological performance of the SBR/sulfur-modified asphalts, correspondingly the isochronal plot of phase angle versus temperature becomes more flat with increasing the suflur content, which means acrosslinking polymer network is formed in asphalt (25) and the crosslinking density among polymer molecules increases further. When the sulfur level increases to 0.1 wt%, a continuous polymer network is formed and the susceptibility of the asphalt binder to temperature is alleviated to a great extent, as shown by the pronounced plateau of the phase angle plot. However, after TFOT ageing, it can be seen from Fig. 1b that the difference in complex modulus between the asphalt binders with and without sulfur is reduced to a great extent over the whole temperature range. For the vulcanized binders, the elevated isochronal plots of phase angle versus temperature at high temperature range show the crosslinked polymer network is destroyed and the susceptibility to temperature increases. The varying trend of the rheological behavior becomes more evident after PAV ageing, as shown in Fig. 1c, the difference in complex modulus among the binders is reduced further with further ageing, which indicates the improved rheological performance of the vulcanized binders is lost completely.

Frequency Sweep. The dependence of the rheological behavior of the PMAs on different loading frequencies (0.1-100 rad/s) at high temperature is shown from Fig. 2a-c, 60[degrees]C is chosen since it is usually used in evaluating the performance of asphalt pavement in hot climate. Before ageing, as shown in Fig. 2a, the lower isochronal plot of phase angle versus frequency with increasing the sulfur content shows the formation of a continuous cross-linking polymer network. In the isochronal plots of complex modulus versus frequency, the improved complex modulus of the SBR/sulfur-modified asphalts only can be observed at very low frequencies. From 0.1 rad/s to 2.46 rad/s, the improved complex modulus decreases quickly, after the transition frequency 2.46 rad/s, the SBR/sulfur-modified asphalts hold lower complex modulus compare with the SBR-modified asphalt with increasing the shear frequency, which becomes apparent for the binder with 0.1 wt% sulfur at 100 rad/s. Obviously, the vulcanized binder is more susceptible to dynamic shear due to the degradation of the crosslinked polymer network with increasing the shear frequency. After TFOT ageing, the susceptibility to shear frequency is reduced to some extent due to the partly degradation of the polymer network. As shown in Fig. 3b, the varying trend of complex modulus at high frequency range is reduced with the disappearance of the transition frequency and the difference in phase angle at low frequency range among the binders is reduced evidently. After PAV ageing, the improved complex modulus of the SBR/sulfur-modified asphalts is lost due to the further degradation of the crosslinked polymer network. In Fig. 2c, it can be seen the difference in complex modulus or phase angle among the binders is reduced further over the entire frequency range.

In the analysis for the isochronal plots of complex modulus versus phase angle, the drawn conclusion about the influence of ageing on the structural characteristics of the SBR- and SBR/sulfur-modified asphalts is similar to that by temperature sweep as mentioned before.

To understand the temperature dependence of the viscoelastic behavior in the normally operating temperature range of asphalt pavement, we conducted the frequency sweep test on each binder at other five temperatures: 25[degrees]C, 30[degrees]C, 40[degrees]C, 50[degrees]C, and calculated the shift factors by the master curves of complex modulus respectively according to the time-temperature superposition principle (TTSP), the reference temperature is 25[degrees]C. As has been previously reported by other researchers, the temperature dependence of the shift factor for bitumen may fit the Arrhenius equation (26), (27), Within the temperature range studied in this work, the temperature dependence of the shift factor for all the samples studied is described by an Arrhenius equation (Eq. 1) fairly well:

[alpha] [(T).sub.0] = exp[[E.sub.a]/R(1/T - 1/[T.sub.0])], (1)

where a[(T).sub.0] is the shift factor relative to the reference temperature; [E.sub.a] the activation energy; R = 8.314 J/mol K; T, the temperature (K); and [T.sub.0] is the reference temperature. The [r.sup.2] values ranged from 0.9977 to 0.9995. While the limited number of materials studied precludes making general conclusions, the differences that occur upon ageing these samples appear to be significant. The values of [E.sub.a] for each binder are shown in Table 6. As can be observed, the binder holds a higher activation energy after TFOT ageing, one reason is that oxidative ageing results in more interactions among asphalt components, this can be mainly attributed to the increased hard asphalt components as mentioned in Table 2. After PAV ageing, the interactions are enhanced further, as shown by the increased values of [E.sub.a]. Compared with the SBR-modified asphalt, the SBR/sulfur-modified asphalts always hold a lower [E.sub.a] before and after ageing due to the structural instability. Under the influence of dynamic shear or ageing, the substructure of the vulcanized polymer is easier to be destroyed, the interactions between asphalt components and polymer are alleviated obviously. With further ageing, the [E.sub.a] values of the vulcanized binders with different sulfur contents are similar, showing the further degradation of the crosslinked polymer network. Therefore sulfur has little effect on the activation energy and oxidation has the largest impact on [E.sub.a].

TABLE 6. Activation energy Ea of SBR- and SBR/sulfur-modified
asphalts before and after ageing (SBR content: 4 wt%).

Sulfur content          [E.sub.a] (kJ/mol)
(wt%)           Unaged      TFOT aged       PAV aged

0               150.56        163.89         177.49
0.03 wt%        147.97        162.08         175.36
0.065 wt%       148.34        162.28         175.38
0.1 wt%         148.65        162.98         175.40


Repeated Creep. Recently, the validity of the Superpave binder specification parameter G*/sin[delta] was questioned, Hu et al. (28) proposed the use of cyclic creep measurement for the study of high-temperature rutting resistance of binder. This issue was also examined in the National Cooperative Highway Research Program (NCHRP) Report 459 (24). Repeated creep testing was suggested as a better method for estimating the binder resistance to permanent strain accumulation. The viscous component of the creep stiffness was found to be a good indicator of the permanent strain accumulation and was proposed as a better specification parameter.

The repeated creep recovery test was conducted on all binders. The repeated creep recovery test simulates field conditions better as it applies a stress for a short duration of time and then leaves the material to recover for a longer duration of time, and repeats this many times. This in a way simulates vehicles passing on a pavement (29). The test consisted of 100 cycles of loading with a stress of 30 Pa for 1 s, and recovery for 9 s. These testing parameters are based on the suggestions from the NCHRP study (24).

Based on the recommendations, the time and strain data from the 50th and 51th cycles of the repeated creep recovery test were fitted to the four-element Burgers model as shown in Eq. 2

[gamma](t) = [[tau].sub.0]/[G.sub.0] + [[tau].sub.0] / [G.sub.1] (1 - exp (- t [G.sub.1] / [[eta].sub.1])) + [[tau].sub.0] t / [[eta].sub.0] (2)

where [gamma](t) is shear strain; [[tau].sub.0], constant shear strain; [G.sub.0], spring constant of Maxwell model; [G.sub.1], spring constant of Kelvin model; [[eta].sub.1], dashpot constant of Kelvin model; t, loading time; [[eta].sub.0], dashpot constant of Maxwell model.

Equation 3 represents the creep compliance, J(t), in terms of its elastic component ([J.sub.e]), its delayed-elastic component ([J.sub.de]), and its viscous component ([J.sub.v]):

J (t) = 1/[G.sub.0] + 1/[G.sub.1](1 - exp(- t[G.sub.1]/[[eta].sub.1])) + t/[[eta].sub.0] = [J.sub.e] + [J.sub.de] (t) + [J.sub.v] (t). (3)

The viscous component is inversely proportional to the viscosity, [[eta].sub.0], and directly proportional to stress and time of loading. Base on this separation of the creep response, the compliance could be used as an indicator of the contribution of binders to rutting resistance. Instead of using the compliance ([J.sub.v] (t)), which has a unit of 1/Pa, and to be compatible with the concept of stiffness introduced during SHRP, the inverse of the compliance, [G.sub.v], could be used. [G.sub.v] is defined as the viscous component of the creep stiffness (24), as shown in Eq. 4.

[G.sub.v] (t) = 1/[J.sub.v](t) = [[eta].sub.0]/t. (4)

The viscous component of the creep stiffness, [G.sub.v] (t), was found to be a good indicator of the permanent strain accumulation and is proposed as a better specification parameter. The rutting resistance of binder is evaluated by [G.sub.v] (t). A higher viscous stiffness is an indicator for higher resistance to permanent deformation of the binder.

The values of [G.sub.v] for each binder are shown in Table 7. Before ageing, the [G.sub.v] of the SBR/sulfur-modified asphalts is lower than that of the SBR-modified asphalt and the [G.sub.v] decreases continually with increasing the sulfur level, which means the SBR/sulfur-modified asphalts are very susceptible to repeated shear stress, especially for the binder with higher sulfur level. After ageing, the binders hold a higher [G.sub.v] and the values of [G.sub.v] increase with further ageing, which shows the improved rutting resistance of the aged binders as the result of the changed asphalt components. Compare with the SBR-modified asphalt, the lower [G.sub.v] of the SBR/sulfur-modified asphalts before and after ageing shows the poor rutting resistance. This still can be attributed to the structural instability of the SBR/sulfur-modified asphalts, the crosslinked polymer network in asphalt is easily destroyed under the influence of repeated shear force or ageing.

TABLE 7. [G.sub.v] of SBR- and SBR/sulfur-modified asphalts
before and after ageing (SBR content: 4 wt%).

                              [G.sub.v]/Pa (60[degrees]C)

Sulfur content (wt%)  Unaged           TFOT aged           PAV aged

0                     315.49                       361.14    499.92
0.03 wt%              216.01                       271.08    493.42
0.065 wt%             172.29                       209.34    446.76
0.1 wt%               162.31                       198.77    439.24


The structural instability of the SBR/sulfur-modified asphalts is mainly related to the relative weak polysulfide bonds -- [(S).sub.x] -- (x = 4-6) formed among SBR polymer molecules. There is a reactive [pi] bond in the carbon-carbon double bonds C=C of the butadiene structure. In the dynamic vulcanization of SBR without accelerants, the reactive [pi] bond is broken and converted into the carbon-sulfur bond C--S connected together by the polysulfide bond -- [(S).sub.x]-- (x = 4-6) among polymer molecules (30), so a crosslinked polymer network is formed in asphalt matrix. However, the bond length of the polysulfide bond is longer and the bond energy is lower. Under the influence of heat or dynamic shear, these weak polysulfide bonds exposed outside of the main molecule chains of SBR copolymers are broken, so the crosslinked polymer network is destroyed and the rutting resistance declines. For the SBR/sulfur-modified asphalt with higher sulfur level, there are lots of polysulfide bonds formed among polymer molecules. Under the influence of dynamic shear and ageing, more polysulfide bonds are broken so the susceptibility becomes more obvious.

In the opinion of conventional experiments, the high-temperature rutting resistance of asphalt binder can be evaluated by the values of softening point or viscosity, because there is a good correlation between them. The binder with higher softening point or viscosity often owns better rutting resistance. However, the drawn conclusions about rutting resistance by the creep tests do not conforms to the results of the conventional experiments, as shown in Tables 3 and 4, This is because the conventional experiments can not identify the differences in rheological behavior, it seems to be a problem when dealing with the more Theologically complicated PMAs.

Morphology Analysis

The compatibility between polymer and asphalt is critical to the properties of PMAs (31). The morphology of the PMAs before and after ageing was investigated using optical microscopy by characterizing the distribution and the fineness of polymer in asphalt matrix. The morphology of the SBR-modified asphalt and the SBR/sulfur-modified asphalt with the suitable sulfur content (0.03 wt%) before and after ageing is shown from Fig. 3a-f, respectively. For the SBR-modified asphalt, a continuous bitumen phase with lots of dispersed coarse SBR particles can be seen in Fig. 3a. The influence of ageing on the morphology is shown in Fig. 3b and c. Compared with Fig. 3a, the number of polymer particles dispersed in asphalt increases and their sizes decrease obviously after TFOT ageing, as shown in Fig. 3b. In Fig. 3c, it can be seen that the particle sizes decline further and the outlines become very dim after PAV ageing, which implies the further degradation of SBR (chain scission) and the compatibility between polymer and asphalt is improved further. The morphology of the SBR/sulfur-modified asphalt with 0.03 wt% sulfur before and after ageing is shown from Fig. 3d-f. Before ageing, as shown in Fig. 3d, an obvious morphological change can be observed by the addition of sulfur. Compared with the coarse polymer particles shown in Fig. 3a, the filamentous polymer network structure can be seen in Fig. 3d. The morphological change from the granular rubber particles to the filamentous polymer network shows the compatibility between SBR and asphalt is improved by dynamic vulcanization (32), (33). The influence of ageing on the morphology is shown in Fig. 3e and f. Compared with the SBR-modified asphalt, the SBR/sulfur-modified asphalt is more susceptible to ageing. As shown in Fig. 3e, the network is destroyed evidently and the sizes of the filamentous polymers decline due to the fusion of the vulcanizated SBR in TFOT ageing. As a consequence of the prolonged ageing time in PAV, the same conclusion can be confirmed further after ageing. In Fig. 3f, there are only a few unvulcanized polymer particles dispersed in asphalt matrix and the filamentous network disappears completely. Compared with the SBR-modified asphalt, the SBR/sulfur-modified asphalt is easier to be decomposed by ageing as the cause of the much better homogeneous system formed.

CONCLUSION

The DMA analysis illustrates the influence of oxidative ageing (short-term and long-term) on the Theological behavior of the SBR- and SBR/sulfur-modified asphalts to a great extent. On one hand, ageing prompts the degradation of the polymers in asphalt and leads to the increase in the viscous behavior, on the other, the increased hard asphalt components greatly contributes to the increase in the elastic behavior. Due to the structural instability of the SBR/sulfur-modified asphalts, the binders are more susceptible to oxidative ageing and dynamic shear, the crosslinked polymer network dispersed in asphalt degrades obviously under the influence of heat or dynamic shear and the binder with higher sulfur level shows the more obvious susceptibility.

The conventional softening point, viscosity tests are unable to identify the differences of the rheological behavior. This may be a limiting factor when dealing with the more rheologically complicated PMAs.

The major effect of ageing on the morphology of the modified binders is to homogenize the dispersion of polymer in asphalt matrix. The compatibility between polymer and asphalt can be improved evidently with further ageing due to the severe degradation of SBR (chain scission). For the SBR/sulfur-modified asphalt, the phase change from heterogeneous structure to homogenous one after ageing is more obvious due to the poor structural stability.

Because of the susceptibility of the SBR/sulfur-modified asphalt to ageing and dynamic shear, there are some severe problems existing in the practical road paving of the PMA. Therefore it is unreasonable to use sulfur solely in the preparation of SBR-modified asphalt.

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Feng Zhang, Jianying Yu, Shaopeng Wu

Key Laboratory of Silicate Materials Science and Engineering of Education Ministry, Wuhan, University of Technology, Wuhan 430070, People's Republic of China

Correspondence to: Jianying Yu; e-mail: wucifanying@163.com

DOI: 10.l002/pen.22047
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Author:Zhang, Feng; Yu, Jianying; Wu, Shaopeng
Publication:Polymer Engineering and Science
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Date:Jan 1, 2012
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