Thermo-rheological behavior and compatibility of modified asphalt with various styrene–butadiene structures in SBS copolymers

Materials and Design 88 (2015) 177–185

Contents lists available at ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/jmad

Thermo-rheological behavior and compatibility of modified asphalt with various styrene–butadiene structures in SBS copolymers Ming Liang a, Peng Liang a, Weiyu Fan a,⁎, Chengduo Qian a, Xue Xin a, Jingtao Shi b, Guozhi Nan a a b

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, Shandong Province 266580, PR China PetroChina Fuel Oil Co., Ltd. Research Center, Qingdao 266580, PR China

a r t i c l e

i n f o

Article history: Received 7 August 2015 Received in revised form 31 August 2015 Accepted 1 September 2015 Available online 5 September 2015 Keywords: Modified asphalt Viscoelastic characteristics SBS Compatibility Structure parameters

a b s t r a c t The knowledge of the morphological structure or effect of polymer structure on the performance of polymermodified asphalt (PMA) is not systematic and completed yet. In this paper, SBS-modified asphalts were prepared by asphalts with different compositions and styrene–butadiene–styrene (SBS) copolymers with various styrene– butadiene structures, which in turn were subjected to frequency sweep tests, viscous measurements and fluorescence microscopy. The results revealed that the SBS-modified asphalt containing 30 wt.% styrene had the optimal viscoelastic functions and the highest viscosity, indicating enhanced viscoelastic characteristics and less sensitivity to temperature changes. Furthermore, it is less susceptible to shear forces for asphalts as the increase of styrene content because larger and stronger aggregated polystyrene domains can render deformation and movement more difficult. For system studied, the compatibility becomes poorer as the increase of styrene contents and polymer phase sizes decrease with the enhancement of styrene contents as well as their volume proportions. The scope of distribution curves becomes narrower and the swelling degree is lower as the increase of styrene contents by image analysis. As a conclusion, there is a moderate styrene content for SBS to acquire equilibrium between the compatibility and viscoelastic characteristics. © 2015 Published by Elsevier Ltd.

1. Introduction Asphalt is primarily applied in the paving industry as an adhesive for mineral aggregates owing to its cohesiveness, viscoelastic properties, strength and so on [1]. However, vehicle with heavier load and increased traffic volume as well as extreme climate lead to pavement distress, such as permanent deformation [2–5], fatigue and low temperature cracking [6–7]. While unexpected distress causes the increase of maintenance cost and shortens the life of road. It is noteworthy that most of the above-mentioned distress can be closely related to the properties of asphalt binders such as viscoelastic behavior, strength, and rate of plastic deformation, in which the first factor is placed special emphasis especially [8–10]. Polymer-modified asphalt (PMA) is the incorporation of polymers in asphalt by mechanical mixing or chemical reaction, which was invented to enhance mechanical properties, thermal susceptibility and aging resistance of asphalt [11–12]. A great variety of polymers have been used as modifiers which can be subdivided into three main categories: thermoplastic elastomers, plastomers, and reactive polymers [13]. It is well accepted that the most frequently used category of polymers for asphalt modification are thermoplastic block copolymers, among which styrene–butadiene– ⁎ Corresponding author. E-mail addresses: [email protected] (M. Liang), [email protected] (W. Fan), [email protected] (X. Xin).

http://dx.doi.org/10.1016/j.matdes.2015.09.002 0264-1275/© 2015 Published by Elsevier Ltd.

styrene (SBS) copolymer is the preferred one. SBS copolymers are composed of glassy polystyrene domains connected by polybutadiene segments, which present a two-phase morphology [11,13]. Glass transition temperature (Tg) of the PS blocks is around 95 °C while that of the PB blocks is around − 80 °C [14]. When ambient temperature lies between the glass transition temperature of PS blocks and PB blocks, the PS blocks are glassy and apt to enhance the stiffness of SBS, while the PB blocks are rubbery and can offer the elasticity [11,15]. With the special properties, SBS has been well accepted as the asphalt modifier. After SBS copolymers are mixed with asphalt, the system gradually becomes a biphasic microstructure where a polymer-rich phase formed by maltenes–swollen-polymer is dispersed in an asphaltene-rich phase (Fig. 1) [16–18]. In general, asphalt can be broadly divided into saturates, aromatics, resins and asphaltenes by solvent, which are commonly referred as SARA fractions [19–21]. However, each component has a different solubility parameter and presents different degrees of compatibility with PS and PB blocks in the SBS. The compatibility between asphalt and SBS has been identified as a critical factor which generates a profound effect on the thermomechanical properties, rheological properties and morphology. Some efforts [1,11,13,22–24] have been devoted to reveal the compatibility between asphaltic component and SBS copolymers. As a rule of thumb, asphalt with high aromatic content or adding aromatic oil to asphalt with low aromatic content is easier to obtain a compatible and stable SBS-modified asphalt [25–26]. More importantly, SBS molecular

178

M. Liang et al. / Materials and Design 88 (2015) 177–185

Fig. 1. Schematic illustration of the influence of SBS on asphalt from different dimensions.

architectures, such as styrene–butadiene diblock content, the S/B ratio, and molecular weight, have a strong impact on the mechanical, rheological, and morphological properties of SBS-modified asphalt. Although some authors [27–30] have studied the effects of the polymer molecular parameters on the basic properties for asphalt, information about the detailed descriptions to morphology, swelling and the interactions of PB midblock as well as PS endblock with the asphalt components are still unclear. Furthermore, the microstructure is not analyzed quantitatively and how various styrene–butadiene structures in SBS affect the thermo-rheological properties remains to be investigated. The objective of the present study is to determine the influence of various styrene–butadiene structures in SBS on the thermorheological properties of SBS-modified asphalt and the compatibility between SBS and asphalt component. To achieve this goal, SBS-modified asphalts were prepared by asphalts with different chemical compositions and SBS with various styrene–butadiene structures by means of high-shear mixer, which in turn were subjected to frequency sweep tests in the linear viscoelastic region, viscous measurements and fluorescence microscopy. The compatibility between SBS and asphalt was evaluated by the rheological compatibility criteria. In the meantime, the microstructures of modified asphalts were analyzed quantitatively with the aid of fluorescent microscope, image collecting system and professional software in order to reveal details about swelling and interaction of SBS with asphaltic component. 2. Experiment and methods 2.1. Materials Two neat asphalts with 80/100 pen grade, coded as A and B, were selected in this research. The asphalts with same penetration grade and different chemical compositions are chosen in order to evaluate the interaction of SBS with the asphalt component. Physical properties and

Table 1 Physical properties and chemical compositions of the neat asphalts. Measured values Specifications Physical properties Penetration (25 °C, 0.1 mm) Penetration index PI Softening point (R&B, °C) Ductility (15 °C, cm) Kinematic viscosity (135 °C, Pa·s) Density (15 °C, g·cm−3) Chemical compositions (wt.%) Saturates Aromatics Resins Asphaltenes

A

B

ASTM D5 [31] ASTM D5 ASTM D36 [32] ASTM D113 [33] ASTM D4402 [34] ASTM D70 [35]

89 −0.69 44.4 186 0.375 1.020

90 −0.67 41.7 171 0.453 1.029

ASTM D4124 [36]

19.7 43.0 25.7 11.6

18.9 39.4 28.3 13.4

chemical compositions of the neat asphalts are presented in Table 1. SBS copolymers with the form of porous pellets were provided by Yueyang Baling Petrochemical, China. The selected SBS in this study is a linear structure polymer with various styrene–butadiene structures and average molecular weight (Mw) of 100,000 g/mol. The main properties of these polymers are displayed in Table 2. Schematic representation of linear SBS molecular structure is described in Fig. 1. Polystyrene (PS) polymer was also selected to compare with SBS. 2.2. Preparation of SBS-modified asphalt samples SBS-modified asphalt samples were prepared by melt blending method. The process conditions were optimized on the basis of previous study in our group in order to maximize the rheological properties and minimize asphalt degradation. Great concern was taken to insure the repeatable data because the rheological behavior of asphalt is strongly dependent on the heating history. The neat asphalts were first heated in the cylindrical vessel to 175 °C and the SBS copolymers (3 wt.% of the blend) were added to asphalt and sheared for 30 min on a high shear mixer at 4000 rpm. Afterwards, asphalt–SBS blends were stirred for 3 h to ensure homogeneous mixtures. Analyses described in Section 2.3. were performed on the samples. Being compared with SBS-modified asphalt, polystyrene modified asphalt sample was also prepared under the identical condition. For comparing various samples easily, SBS with 20%, 30%, and 40% styrene modified asphalt A are coded as SBSA-1, SBSA-2, and SBSA-3 respectively and the rest can be analogized in the same way. 2.3. Tests and analysis The conventional tests were carried out to evaluate the properties and compositions of base asphalts, including penetration (ASTM D5), penetration index PI (ASTM D5), softening point (ASTM D36), ductility (ASTM D113), kinematic viscosity (ASTM D4402), density (ASTM D70) and SARA fractions (ASTM D3279). Table 2 Properties of SBS copolymers. Items

Molecular structure Styrene–butadiene ratio Average molecular weight Volatility Ash content Tensile strength Breaking elongation Shore hardness Melt flow rate

Units

Copolymers SBS-1

SBS-2

SBS-3

104 g·mol−1 % % MPa % A g·min−1

Linear 20/80 10.0 0.7 0.2 8 800 65 ± 7 0.1–5

Linear 30/70 10.0 0.7 0.2 15 750 75 ± 7 0.1–5

Linear 40/60 9.8 0.7 0.2 20 700 90 ± 5 0.5–5

M. Liang et al. / Materials and Design 88 (2015) 177–185

Rheological characterizations were performed on a strain-controlled rheometer (Advances Rheology Expanded System, ARES of Rheometric Scientific Co., USA) with parallel disk geometry (8 mm and 25 mm in diameter). Strain and stress sweep tests were previously employed on each sample to obtain the linear viscoelastic domain and then small oscillatory shear tests in the form of isothermal (5, 25, 50 and 75 °C) frequency sweep (0.1–50 rad/s) were performed in the linear viscoelastic domain. A sample of asphalt, about 0.1 g (8 mm) or 1 g (25 mm), was placed on the bottom disk and the disk was then mounted in the rheometer. The sample was subsequently heated in order to soften it, the upper disk was lowered to contact tightly with the sample and then the squeezed sample was trimmed. The final gap was adjusted to 1 mm (25 mm) or 1.5 mm (8 mm). It is worth noting that 25 mm diameter disks were used for measurements at room temperature or above while 8 mm disks were applied for tests below room temperature. Two replicates of specimen needed to be done to obtain the reliable results. Steady state flow tests were also employed to measure viscous flow behavior on the rheometer with a plate-and-plate geometry (25 mm diameter and 1 mm gap). The measurements were conducted in a wide shear rate range (10−3–102 s−1) at 60 °C. The time–temperature superposition method was applied in this study in order to obtain the linear viscoelasticity functions covering many orders of the reduced independent variable. The time–temperature superposition states that the effect of increasing the loading time (or decreasing frequency) on mechanical properties of a material is equivalent to that of raising temperature [37]. Various viscoelastic parameters from the experimental, such as storage modulus (G′), loss modulus (G″) and complex modulus (G⁎), could be empirically superposed by a shifting factor. Morphology of PMA samples were observed using a fluorescence microscope of Olympus BX51 made by Japan. Samples were irradiated using a blue light for excitation, and then the fluorescent yellow light re-emitted by swollen polymer phase was observed under optical microscopy of Olympus BX51 equipped with DP72 digital camera. A small amount of molten samples were first placed and squashed carefully between two glass slides. Subsequently, thin glass slide with samples was viewed under the microscope with a magnification of 100 × and the aid of Cellsens Standard software at room temperature. The image processing and analysis techniques were applied in this paper to quantify the dimensions and distributions of polymer-rich phase in the asphalt. This method is usually implemented by a professional image analyzer, among which Image-Pro-Plus analysis program is widely used. Image-Pro-Plus analysis program is the versatile software capable of providing significant information from the captured images to determine the particle size distribution of polymer, catalysts and other civil engineering materials. In order to obtain the reliable results, fifty images were captured equally from one sample and analyzed. 3. Results and discussion 3.1. Viscoelasticity behavior The performance of asphalt pavement is mainly dependent on the mechanical and viscoelastic behavior of asphalt which is impacted pronouncedly by the polymer properties. What's more, the lineal viscoelasticity function (storage modulus G′ and loss modulus G″) is highly sensitive to changes of component and internal structure of asphalt. Therefore, it is an excellent method using the dynamic rheological measurements to study the influence of SBS copolymer with various styrene–butadiene structures on the performance of modified asphalt. The most important aspect of the rheological properties of asphalt is the dependence on time or frequency, and frequency dependence of G ′ and G″ of SBS-modified asphalt are displayed in Fig. 2. As can be seen, G′ as well as G″ enhances drastically with the increase of frequency and attains the increment for four orders of magnitude when

179

frequency changes from 0.1 to 100 rad/s. A discrepancy between G′ and G″ becomes narrow as frequency increases. It seems that G′ would be nearly equal to or even higher than G″ when frequency approaches a certain value. Loss modulus G″ is always greater than storage modulus G′ in the tested frequency region at 50 °C, which indicates that the bulk rheological behavior is dominated by viscous properties. Asphalt modified by SBS with different S/B ratios shows different mechanical characteristics although the difference is quite small. The viscoelastic functions of SBS-modified asphalt containing 30% styrene are always greater than that of remaining SBS-modified asphalt. Moduli of SBSA-1 and SBSA-3 are almost equivalent while the values of modulus of SBSB-1 exceed SBSA-3. This is due to the fact that the latter sample has higher asphaltene content and a stronger entanglement as well as interaction between polymer and asphaltene is formed. In addition, PS modified asphalt shows the inferior viscoelastic behavior comparing with SBS. It is quite controversial as to whether asphalt is a thermosimplicity material, i.e. whether the time–temperature superposition (TTS) principle holds for asphalt. Notwithstanding, it is believed that polymer modified asphalt can be described reasonably well by the master curves in the form of their linear viscoelastic material functions [38–40]. The time–temperature superposition method was applied to study the different viscoelastic properties of modified asphalt with various styrene–butadiene ratios in this research. The master curves, using 25 °C as the reference temperature, of storage and loss modulus of various samples were created by a horizontal shift factor, αT, as can be observed in Fig. 3. It can be seen that TTS is well applicable for all samples in this study. In a high frequency zone (low temperature), an overlap of master curves occurs for various samples, the onset of which is closed to the glassy modulus. The mechanical properties of asphalt in the glassy region are poorly affected by polymer modifiers [41]. However, different mechanical characteristics are displayed for modified asphalt containing various SBS in a low frequency zone (high temperature). This is due to that asphaltenes are fully peptized and dispersed in the maltenes at the high temperature. Hence, polymer relaxation processes are the main contribution to the bulk rheological behavior. Asphalt modified by SBS with 30 wt.% styrene content showed a maximum value for storage modulus, followed by samples containing 20 wt.% and 40 wt.% styrene. PS modified asphalt shows the inferior viscoelastic behavior comparing with SBS-modified asphalt. The higher of styrene content, the studied system is more apt to present the rigid blend and more difficult to obtain a homogeneous dispersion. G′ of SBS-modified asphalt containing 20 wt.% remains lower than that of SBS with 30 wt.% styrene because of a more flexible blend. It can be concluded that a moderate styrene content in SBS (20–40 wt.%) can acquire an equilibrium point between rigid and flexible properties, indicating better compatibility between SBS and asphalt as well as enhanced viscoelastic characteristics. It is important to underline that all samples showed a terminal region in the low-frequency range, where the slope of storage modulus G′ versus logω approaches a value of 2 and loss modulus G″ becomes proportional to ω, suggesting a pure viscous behavior at this temperature region. The temperature dependence of the horizontal shifting factor can be represented by the Arrhenius or Williams–Landel–Ferry relations [42,43]. In this study, as seen in Fig. 4, the variation of αT with temperature was described by an Arrhenius-like equation in the studied temperature region as follows: αT ¼ exp

   Ea 1 1 − R T T0

where Ea presents the activation energy, which is associated with material temperature susceptibility, R denotes the universal gas constant, T is the current temperature, and T0 represents the reference temperature, which was chosen to be room temperature. Although the horizontal shifting factors of various SBS modified samples are similar,

180

M. Liang et al. / Materials and Design 88 (2015) 177–185

Fig. 2. Frequency dependence of G′ and G″ of SBS-modified asphalt with various styrene–butadiene structures at 50 °C.

the differences can be distinguished and quantified by the slope of Arrhenius equations, i.e. Ea. Some researchers argued that the activation energy was related to molecular characteristics such as polymer molecular weight, molecular configuration, degree of branching and so on [44,45]. Activation energy values calculated from shift factors (αT) for various samples in this study are listed in Table 3. SBSmodified asphalt with 30 wt.% styrene content exhibited the minimum value of the activation energy compared to the other two SBS-modified asphalts, which implies that the blend is less sensitive to temperature changes for the studied samples. As a result, it should be noted that moderate styrene content in SBS is appropriate for modification of the selected asphalt, which prone to obtain modified asphalt with enhanced viscoelastic behavior and less thermo-susceptibility. The activation energy corresponding to asphalt B with the higher asphaltene was lower than those obtained from asphalt A. In addition, the results regarding activation energy is related to oil source, since properties of asphalt can dependent significantly upon the source. Moreover, the activation energy of PS modified asphalt was lower than that of SBS-modified asphalt because of the more rigid system brought by PS.

3.2. Viscous flow behavior Various asphalts have been studied in the linear viscoelastic region of small deformations in Section 3.1. In order to clearly reveal the distinctions of SBS with various styrene–butadiene structures and the correlations of structure with mechanical properties, steady state shear

measurements were applied in the larger deformation region, where the polymer chains undergo markedly conformational changes. Viscous flow curves of various samples, in a way of shear viscosity verse shear rates at 60 °C, are shown in Fig. 5. Evolutions of viscosity of SBSmodified asphalt show a dependence on shear rates. At the low shear rates region, viscosity exhibits a slight decline trends as the increase of shear rates. This is due to the “movement” of polystyrene (PS) domains together with diffusion of the individual polymer chains [13]. On the other hand, a drastically decrease for viscosity occurs at the high shear rates zone especially when shear rates exceed 1 s−1, corresponding to a shear-thinning behavior. This phenomenon is caused by sample squeezing out of the plate and associated with the asphaltenic micelles structure. SBS-modified asphalt with 30 wt.% styrene shows the highest viscosity in the low shear rates zone, followed by samples containing 20 wt.% and 40 wt.% styrene. PS modified asphalt presents a lowest value in viscosity comparing with SBS, which is related to the swelling degree of PS. The difference in viscosity among various SBSmodified asphalt B are greater than that of asphalt A owing to the different component constitute as well as the source of base asphalt. The distinctions of viscous flow curves among various modified asphalt caused by polymer molecular architecture can be described and quantified by the Carreau model, which fit the flow curves fairly well: η 1 ¼  b 2 s η0 1 þ γγb c

M. Liang et al. / Materials and Design 88 (2015) 177–185

181

Fig. 3. Master curves of various samples for asphalts A and B at the reference temperature of 25 °C.

where s is related to the slope of the shear-thinning region, γbc is a critical shear rate corresponding to the turning-point of shearthinning region, and η0 is the asymptotic viscosity where the shear

rates approach zero. The fitting results of parameters from Carreau model are shown in Table 4. As can be observed that η0 shows a trend from rise to decline as the increase of styrene content, implying that

Fig. 4. Variations of empirical shift factor as a function of temperature.

182

M. Liang et al. / Materials and Design 88 (2015) 177–185

Table 3 Activation energy values calculated from shift factors (αT) for various samples. Block ratio S/B 20/80 30/70 40/60 PS

Table 4 The fitting results of parameters from Carreau model.

Ea/KJ·mol−1

The S/B ratio

Asphalt A

Asphalt B

222.263 214.174 218.294 171.268

218.294 210.356 213.635 170.666

moderate styrene content in SBS is crucial to attain ideal properties. SBS containing 30 wt.% styrene is highly recommended for asphalt modification. Meanwhile, the critical shear rate for the onset of the shearthinning region shifts to higher shear rate as the increase of styrene content, which is less susceptible to shear forces. Polymer architecture accounts for this phenomenon. Apparently, with the increase of styrene, polystyrene (PS) domains become larger and the aggregation effects are stronger. Subsequently, the movement and shifting of PS domains become difficult, which need more force and energy. 3.3. Compatibility and the correlation with morphology In order to achieve the desired performance of pavement, separation between polymer and asphalt is necessary to avoid during the period of storing, pumping, applying polymer modified asphalt. Undoubtedly, a number of factors cause the incompatibility and instability, among which the discrepancy in the solubility parameter, polarity, density and molecular architecture between polymers and asphalt contribute to incompatibility pronouncedly [46]. However, the SBS molecular structures, such as styrene–butadiene diblock content, and the S/B ratio, have a great influence on the morphology and viscoelastic properties of modified asphalt. However, it is generally recognized that swelling as well as interactions of the PB midblock and PS endblocks with the asphalt components is different, but the details are still unclear. Therefore, it is important to study the compatibility between asphalt and SBS to reveal how SBS structures influence the rheological properties and morphology. A lot of attempts have been made to evaluate and quantify the compatibility between SBS and asphalt. Softening point variation between the top and the bottom of the samples after the stability test is commonly applied to identify whether the substantial phase separation occur. Specifications [47] stipulated that the difference of softening point between the top and bottom sections should not be more than 2.5 °C. Actually, rheological compatibility method is more sensitive than

Asphalt A η0 × 10−3 (Pa·s) γbc (s−1) s Asphalt B η0 × 10−3 (Pa·s) γbc (s−1) s

PS

20/80

30/70

40/60

PS

0.781 4.28 0.339

0.855 5.60 0.523

0.754 9.58 0.919

0.452 12.8 0.566

0.721 7.65 0.705

0.795 7.51 0.675

0.604 8.75 0.723

0.323 24.0 1.14

softening point variation to monitor differentiation in compatibility for SBS-modified asphalt with different structures. Yvonne Becker M [48] studied various rheological compatibility methods including master curves, Cole–Cole diagrams, Han diagrams and black curves to predict compatibility between SBS and polymers. They found that Cole–Cole plots were the most effective methods to identify compatibility and these plots have been used widely to study compatibility in polyblends. As a consequence, Cole–Cole diagrams are employed to evaluate the compatibility of SBS-modified asphalt with various styrenes. Cole–Cole diagrams consist of representations of the complex viscosity components (η⁎ = η′ − iη″) in the complex plane (η′, η″). Evolution of η″ with η′ in conformity with symmetrical parabolas is considered as the proof of compatibility, while deviation from this symmetry is related to incompatibility for SBS-modified asphalt. Figs. 6 and 7 show Cole–Cole diagrams of SBS-modified asphalts A and B at 50 °C. As can be observed, η″ shows its descending following its ascending as the increase of η′, where occurs peak value in the curves. What's more, the form of Cole–Cole plots differ from each other and SBS-modified asphalt with 20 wt.% styrene together with 30 wt.% styrene shows the best symmetrical parabolas, followed by 40 wt.% styrene (Fig. 6). Data concentrated on the left side of the Cole–Cole curves mean the essential elastic properties at the tested temperature, whereas those shifted to the right show a transition from an elastic behavior at low temperature to a viscous behavior as the temperature increases. For all samples studied at 50 °C in this research, the curves occurred in the right side, predicting the dominant viscous behavior. Furthermore, for asphalt B at 50 °C, data appears an overlap in the high frequency region and presents a great difference in the low frequency region. The peak of the curves shifted to the right side for samples containing lower styrene because of a more flexible blend. The curves of goodness of fit for parabolas show that the compatibility of

Fig. 5. Viscous flow curves of SBS-modified asphalt with various styrene–butadiene structures and PS modified asphalt at 60 °C.

M. Liang et al. / Materials and Design 88 (2015) 177–185

183

Fig. 6. Cole–Cole diagrams of asphalt A at 50 °C. Fig. 8. Cole–Cole diagrams of asphalt A at 75 °C.

modified asphalt with 20 wt.% styrene contents is the best, followed by 30 wt.% and 40 wt.% styrene contents. It is noteworthy that the form of Cole–Cole plots drastically changed and deviated from symmetrical parabolas at the higher temperatures (Figs. 8 and 9). SBS copolymers show a two-phase morphology where spherical domains formed by the polystyrene blocks disperse in a matrix of polybutadiene, thus giving rise to different relaxation mechanisms with different levels of thermal as well as time (frequency) dependence. Yvonne Becker M [48] pointed out that either the presence of various relaxation process or the superposition of different relaxation functions as a consequence of complex relaxation processes results in deviations from the parabola in the Cole–Cole plots. The hard glassy domain (PS) exhibited a broad distribution of relaxation times. Because the segment of PB block shows similar relaxation process with light component due to similarity of molecular structure. As a consequence, deviations from the parabola become more remarkable as the increase of styrene contents. However, swelling and interactions among PS domain, PB segments and asphalt component can have a great effect on the compatibility and the form of Cole–Cole plots. Morphology of modified asphalt may shed some light on the issue about the compatibility. Fluorescence microscopy of SBS-modified asphalt with various styrene contents were displayed in Fig. 10. As can be seen, SBS copolymers were homogeneously dispersed in asphalt in the form of round or spherical particle where light area denotes dispersed polymer particle phase formed by SBS particles swelling light fractions (saturates and aromatics) and

dark zone represents asphalt phase. Apparently, polymer phase sizes decrease as the increase of styrene contents for SBS copolymers and for their volume proportions in the field of microscope. As previously mentioned, Image-Pro-Plus analysis program was taken to quantify the dimensions and distributions of polymer-rich phase in the asphalt and the results are shown in Fig. 11. As the styrene contents increase, the scope of distribution curves become narrower and the proportions of smaller size increase, indicating polymer-rich phase distributions are more uniform. For example, polymer-rich phase size distributes from 5 to 30 μm for 20 wt.% styrene contents while the range is 5 to 20 μm for 40 wt.% styrene contents. The results mean that polymerrich phase tend to smaller size as the increase of styrene contents. However, it is generally believed that aggregation energy of the PS domains becomes more pronounced and the SBS is more difficult to be broken into smaller particle as the increase of styrene contents. It can be deduced that swelling degree influences the dispersions state of polymer particles. The solubility parameter is helpful to illustrate the swelling among PS domain, PB segments and asphalt component. Literature [49] pointed out that the solubility parameter of the PB blocks were similar with that of saturates and the PS blocks were similar with aromatics. Therefore, the PB blocks are swollen by saturates while the PS blocks are swollen by aromatics. When the styrene contents are low (lower S/B ratio), the PS domains are small and the PB blocks from the soft segment tend to form more rubberized structures. Furthermore, the rubbery

Fig. 7. Cole–Cole diagrams of asphalt B at 50 °C.

Fig. 9. Cole–Cole diagrams of asphalt B at 75 °C.

184

M. Liang et al. / Materials and Design 88 (2015) 177–185

Fig. 10. Fluorescence microscopy of SBS-modified asphalt A with 20% styrene contents (a), 30% styrene contents (b), 40% styrene contents (c) and PS modified asphalt (d).

structure are easily swollen by saturates and their volumes become significantly larger than its initial volume. On the other hand, in the case of high styrene contents, the PS domains become larger and the degree of crystalline for SBS increases, yielding larger and superior perfect crystalline PS domains. Thus, the swelling degree is lower for the case of high styrene contents. Moreover, the swelling degree can be verified from proportion of polymer-rich phase area (Table 5). For PS modified asphalt, the PS particles dispersed in asphalt were very large. In other words, it is more difficult to disperse PS because of highly rigidity and crystalline. 4. Conclusions Influence of various styrene–butadiene structures in SBS on thermorheological properties and the compatibility of modified asphalt were evaluated by charactering the linear viscoelasticity, viscous flow behavior, morphology and Cole–Cole diagrams. The SBS-modified asphalt

containing 30 wt.% styrene shows the highest viscoelastic functions, indicating enhanced viscoelastic characteristics. TTS is well applicable in a wide frequency region for modified asphalt in this study and the activation energy calculated from Arrhenius equation implies that the SBSmodified asphalt with 30% styrene content is less sensitive to temperature changes. Furthermore, in terms of flow behavior, the SBS-modified asphalt with 30 wt.% styrene content shows the highest viscosity in the low shear rates zone, followed by samples containing 20 wt.% and 40 wt.% styrene. However, it is less susceptible to shear forces for asphalt as the increase of styrene because larger and stronger aggregated polystyrene domains make the movement more difficult and more force is needed. For system studied, the compatibility of the modified asphalt with 20 wt.% styrene contents is the best, followed by 30 wt.% and 40 wt.% styrene contents, from the curves of goodness of fit for parabolas in Cole–Cole diagrams. Nevertheless, the form of Cole–Cole plots drastically changed and deviated from symmetrical parabolas at the high temperatures, implying the poorer compatibility and more phase separation tendency of the blends. In microscopy, SBS copolymers are homogeneously dispersed in asphalt in the form of round or spherical particle and polymer phase sizes decrease as the increase of styrene contents and for their volume proportions. Furthermore, by means of image analysis program, the scope of distribution curves become narrower as the styrene contents increase, suggesting polymer-rich phase tend to smaller size. Thus, the swelling degree becomes lower as the increase of styrene contents, which can be verified from proportion of polymer-rich phase area. As a conclusion, SBS with higher styrene has poorer compatibility for SBS-modified asphalt owing to lower swelling degree. There is an optimal viscoelastic performance as the increase of styrene contents in SBS.

Table 5 Proportion of polymer-rich phase area. The S/B ratio

Fig. 11. The mean particle size distributions for polymer-rich phase by statistics.

Polymer-rich phase percentage/vol.%

PS

20/80

30/70

40/60

PS

39.4

14.0

6.41

12.4

M. Liang et al. / Materials and Design 88 (2015) 177–185

Consequently, there is a moderate styrene content in SBS in order to acquire an equilibrium point between the compatibility and viscoelastic characteristics. Acknowledgments This work is part of a research project supported by the Fundamental Research Funds for the Central Universities (Research Project No. 24720156046A) and China University of Petroleum Postgraduate Innovation Project (Research Project No. 15CX06046A). The authors gratefully acknowledge their financial support. References [1] D. Lesueur, The colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen modification, Adv. Colloid Interf. 145 (2009) 42–82. [2] H. Fu, L. Xie, D. Dou, L. Li, M. Yu, S. Yao, Storage stability and compatibility of asphalt binder modified by SBS graft copolymer, Constr. Build. Mater. 21 (2007) 1528–1533. [3] X. Lu, U. Isacsson, Modification of road bitumens with thermoplastic polymers, Polym. Test. 20 (2000) 77–86. [4] U. Isacsson, X. Lu, Testing and appraisal of polymer modified road bitumens—state of the art, Mater. Struct. 28 (1995) 139–159. [5] M. Liang, X. Xin, W. Fan, H. Sun, Y. Yao, B. Xing, Viscous properties, storage stability and their relationships with microstructure of tire scrap rubber modified asphalt, Constr. Build. Mater. 74 (2015) 124–131. [6] V. Carrera, P. Partal, M. García-Morales, C. Gallegos, A. Paéz, Influence of bitumen colloidal nature on the design of isocyanate-based bituminous products with enhanced rheological properties, Ind. Eng. Chem. Res. 48 (2009) 8464–8470. [7] L. Filippelli, L. Gentile, C.O. Rossi, G.A. Ranieri, F.E. Antunes, Structural change of bitumen in the recycling process by using rheology and NMR, Ind. Eng. Chem. Res. 51 (2012) 16346–16353. [8] R. Blanco, R. Rodríguez, M. García-Garduño, V.M. Castaño, Rheological properties of styrene–butadiene copolymer-reinforced asphalt, J. Appl. Polym. Sci. 61 (1996) 1493–1501. [9] J.K.J. Newman, Dynamic shear rheological properties of polymer-modified asphalt binders, J. Elastomers Plast. 30 (1998) 245–263. [10] A.A. Yousefi, Polyethylene dispersions in bitumen: the effects of the polymer structural parameters, J. Appl. Polym. Sci. 90 (2003) 3183–3190. [11] J. Zhu, B. Birgisson, N. Kringos, Polymer modification of bitumen: advances and challenges, Eur. Polym. J. 54 (2014) 18–38. [12] J.C. Munera, E.A. Ossa, Polymer modified bitumen: optimization and selection, Mater. Des. 62 (2014) 91–97. [13] G. Polacco, J. Stastna, D. Biondi, L. Zanzotto, Relation between polymer architecture and nonlinear viscoelastic behavior of modified asphalts, Curr. Opin. Colloid Interface 11 (2006) 230–245. [14] L. Zanzotto, J. Stastna, O. Vacin, Thermomechanical properties of several polymer modified asphalts, Appl. Rheol. 10 (2000) 185–191. [15] M.C.C. Lucena, S.A. Soares, J.B. Soares, Characterization and thermal behavior of polymer-modified asphalt, Mater. Res. 7 (2004) 529–534. [16] M. Liang, X. Xin, W. Fan, H. Luo, X. Wang, B. Xing, Investigation of the rheological properties and storage stability of CR/SBS modified asphalt, Constr. Build. Mater. 74 (2015) 235–240. [17] J.F. Masson, P. Collins, G. Robertson, J.R. Woods, J. Margeson, Thermodynamics, phase diagrams, and stability of bitumen–polymer blends, Energy Fuel 17 (2003) 714–724. [18] G. Hernández, E.M. Medina, R. Sánchez, A.M. Mendoza, Thermomechanical and rheological asphalt modification using styrene–butadiene triblock copolymers with different microstructure, Energy Fuel 20 (2006) 2623–2626. [19] F.J. Navarro, P. Partal, F. Martınez-Boza, C. Valencia, C. Gallegos, Rheological characteristics of ground tire rubber-modified bitumens, Chem. Eng. J. 89 (2002) 53–61. [20] V. Carrera, M.G. Morales, F.J. Navarro, P. Partal, C. Gallegos, Bitumen chemical foaming for asphalt paving applications, Ind. Eng. Chem. Res. 49 (2010) 8538–8543.

185

[21] R.B. Boysen, J.F. Schabron, The automated asphaltene determinator coupled with saturates, aromatics, and resins separation for petroleum residua characterization, Energy Fuel 27 (2013) 4654–4661. [22] R. He, H. Dou, D. Li, Experimental research on compatibility between SBS modifier and base asphalt, Appl. Mech. Mater. 633 (2014) 1090–1094. [23] P. Wloczysiak, A. Vidal, E. Papirer, P. Gauvin, Relationships between rheological properties, morphological characteristics, and composition of bitumen–styrene butadiene styrene copolymers mixes. I. A three-phase system, J. Appl. Polym. Sci. 65 (1997) 1595–1607. [24] X. Lu, U. Isacsson, Compatibility and storage stability of styrene–butadiene–styrene copolymer modified bitumens, Mater. Struct. 30 (1997) 618–626. [25] G. Kraus, Modification of asphalt by block polymers of butadiene and styrene, Rubber Chem. Technol. 55 (1982) 1389–1402. [26] E.J. Van Beem, P. Brasser, Bituminous binders of improved quality containing Cariflex thermoplastic rubber, J. Inst. Pet. 59 (1973). [27] E. Martínez, C. Chávez, A. Herrera, N. Herrera, Comparative study of the effect of sulfur on the morphology and rheological properties of SB- and SBS-modified asphalt, J. Appl. Polym. Sci. 115 (2010) 3409–3422. [28] Q. Zhang, T. Wang, W. Fan, W. Ying, Y. Wu, Evaluation of the properties of bitumen modified by SBS copolymers with different styrene–butadiene structure, J. Appl. Polym. Sci. 131 (2014) 40398. [29] P.H. Yeh, Y.H. Nien, W.C. Chen, W.T. Liu, Evaluation of thermal and viscoelastic properties of asphalt binders by compounding with polymer modifiers, Polym. Compos. 31 (2010) 1738–1744. [30] F. Dong, W. Zhao, Y. Zhang, J. Wei, W. Fan, Y. Yu, Z. Wang, Influence of SBS and asphalt on SBS dispersion and the performance of modified asphalt, Constr. Build. Mater. 62 (2014) 1–7. [31] ASTM D5-06. Standard test method for penetration of bituminous materials. [32] ASTM D36-06. Standard test method for softening point of bitumen (ring and ball apparatus). [33] ASTM D113-99. Standard test method for ductility of bituminous materials. [34] ASTM D4402-06. Standard test method for viscosity determination of asphalt at elevated temperatures using a rotational viscometer. [35] ASTM D70-03. Standard test method for density of semi-solid bituminous materials (pycnometer method). [36] ASTM D4124-01. Standard test methods for separation of asphalt into four fractions. [37] R.M. Christensen, Theory of Viscoelasticity, 2nd ed. Dover, Mineola (N.Y.), 2003. [38] G. Polacco, J. Stastna, D. Biondi, F. Antonelli, Z. Vlachovicova, L. Zanzotto, Rheology of asphalts modified with glycidylmethacrylate functionalized polymers, J. Colloid Interface Sci. 280 (2004) 366–373. [39] P. Partal, F.J. Martínez-Boza, B. Conde, C. Gallegos, Rheological characterization of synthetic binders and unmodified bitumens, Fuel 78 (1999) 1–10. [40] G. Polacco, O.J. Vacin, D. Biondi, J. Stastna, L. Zanzotto, Dynamic master curves of polymer modified asphalt from three different geometries, Appl. Rheol. 13 (2003) 118–124. [41] J. Stastna, L. Zanzotto, O. Vacin, Viscosity function in polymer-modified asphalts, J. Colloid Interface Sci. 259 (2003) 200–207. [42] J.M. Dealy, K.F. Wissbrun, Melt Rheology and Its Role in Plastics Processing, Kluwer Academic Publishers, Dordrecht, Netherland, 1999. [43] J.D. Ferry, Viscoelastic Properties of Polymers, Wiley, New York, 1980. [44] M. García-Morales, P. Partal, F.J. Navarro, F. Martínez-Boza, C. Gallegos, Linear viscoelasticity of recycled EVA-modified btumens, Energy Fuel 18 (2004) 357–364. [45] A. Ait-Kadi, B. Brahimi, M. Bousmina, Polymer blends for enhanced asphalt binders, Polym. Eng. Sci. 36 (1996) 1724–1733. [46] M. García-Morales, P. Partal, F.J. Navarro, F.J. Martínez-Boza, C. Gallegos, Processing, rheology, and storage stability of recycled EVA/LDPE modified bitumen, Polym. Eng. Sci. 47 (2007) 181–191. [47] AASHTO-PP5, The Laboratory Evaluation of Modified Asphalt Systems, American Association of State Highway and Transportation Officials, United States, 1993. [48] M.Y. Becker, J. Alejandro, Müller, Y. Rodriguez, Use of rheological compatibility criteria to study SBS modified asphalts, J. Appl. Polym. Sci. 90 (2003) 1772–1782. [49] R.M. Ho, A. Adedeji, D.W. Giles, D.A. Hajduk, C.W. Macosko, F.S. Bates, Microstructure of triblock copolymers in asphalt oligomers, J. Polym. Sci. B Polym. Phys. 35 (1997) 2857–2877.