Rheological characterization of storage-stable SBS-modified asphalts

Polymer Testing 21 (2002) 295–302 www.elsevier.com/locate/polytest

Material Characterisation

Rheological characterization of storage-stable SBS-modified asphalts Guian Wen b

a,*

, Yong Zhang a, Yinxi Zhang a, Kang Sun b, Yongzhong Fan

b

a Polymeric Materials Research Institute, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, People’s Republic of China

Received 13 June 2001; accepted 7 August 2001

Abstract The high-temperature storage stability of styrene–butadiene–styrene triblock copolymer (SBS) modified asphalt can be improved significantly with the addition of elemental sulfur. The dynamic mechanical properties of SBS-modified asphalts before and after adding sulfur were characterized by using dynamic shear rheometry. The addition of sulfur to SBS modified asphalt resulted in the formation of a chemically vulcanized SBS network structure in the modified binders, and the high temperature performance of the binders was improved and their temperature susceptibility was reduced to a great extent. The SBS content has a great effect on the rheological properties of the asphalts. The rheological properties of SBS modified asphalts depended strongly on the sulfur level. Increasing sulfur levels led to increasing crosslinking density in the modified binders, and consequently the rheological properties of SBS-modified asphalt was improved. A comparison was made among the properties of the asphalts modified by three different SBS structures. The SBS structure affected the compatibility and storage stability of SBS-modified asphalts, which were improved by the addition of sulfur. As determined by a rotational viscometer, the increase in asphalt viscosity is not directly proportional to the SBS content before and after adding sulfur. The morphology of SBS-modified asphalts, which was characterized by optical microscopy, showed that the compatibility and storage stability of SBS modified asphalt were improved by the addition of sulfur.  2002 Elsevier Science Ltd. All rights reserved. Keywords: SBS triblock copolymer; Polymer modified asphalt; Rheological properties; Storage stability

1. Introduction The use of Styrene–Butadiene–Styrene triblock copolymer (SBS) as an asphalt modifier was developed by the Shell Chemical Company [1,2]. It has been recognized that the physical and mechanical properties and rheological behavior of conventional asphalt compositions can be improved by the addition of SBS [3]. SBS exhibits a two-phase morphology consisting of glassy polystyrene (PS) domains connected together by the rubber polybutadiene (PB) segments at the temperatures

* Corresponding author. Tel.: +86-21-5474-2671; fax: +8621-5474-1297. E-mail address: [email protected] (G. Wen).

between glass transition temperatures of the PB and PS, so SBS exhibits crosslinked elastomer network behavior. Above the glass transition temperature of PS, the PS domains soften and SBS becomes melt processable. This behavior of a thermoplastic elastomer has allowed SBS to become one of the promising candidates in asphalt modification [4]. Because of the poor compatibility between SBS and asphalt, however, the storage stability of SBS-modified asphalt is usually poor at elevated temperatures. It was reported that the stability of polymer-modified asphalt (PMA) can be improved by the addition of sulfur, frequently in the form of elemental sulfur. In 1958, Welborn and Babashak [5] reported that storage stability of natural and/or synthetic rubbers or their latex modified asphalts can be improved significantly by the addition

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of sulfur. Maldonado et al. [6] disclosed a process for preparing storage-stable SBS-modified asphalt by adding sulfur. However, due to the excessive viscosity, the product could not be used in practice. In 1990, storagestable PMA with addition of sulfur has been prepared and applied into practical road paving [7]. After that, some methods for preparation of practical storage-stable SBS-modified asphalt have been developed [8–10]. It is commonly believed that sulfur chemically crosslinks the polymer molecules and chemically couples polymer and asphalt through sulfide and/or polysulfide bonds. Few publications to date concern studies on rheological characteristics of PMA in the presence of sulfur. The present work seeks to identify the favorable rheological characteristics and morphology of asphalts modified by SBS with the addition of sulfur, to compare the asphalts with the modified binder without sulfur, and to study the effect of the SBS structures and the sulfur level on the rheological characteristics of the asphalts.

closing the tube without air enclosure, it was stored vertically at 163°C in an oven for 48 h, then the tube containing the modified asphalt was cooled to ambient temperature and cut horizontally into three equal sections. The difference in softening points between the top and the bottom sections of the tube was measured. When the difference is less than 2.5°C, the sample can be regarded as having good storage stability. 2.4. Rheological characterization A strain-controlled rheometry (Advanced Rheology Expanded System, ARES of Rheometric Scientific Co., USA) with parallel plate geometry (25 mm in diameter) was used to determine the rheological characteristics of

2. Materials and measurements 2.1. Materials AH-90 paving asphalt was obtained from Zhongyou Xingneng Asphalt Factory of Jiangyin, Jiangsu, China. The physical properties of this asphalt are as follows: penetration: 90 dmm (25°C, ASTM standard D5); softening point: 45.0°C (ASTM standard D36); viscosity: 332 Pa s (135°C, ASTM standard D4402). SBS 4303, 1301 and 1401 were produced by Yanshan Petrochemical Co., Ltd., China. SBS 4303 is a star-like polymer, containing 30 wt% styrene, Mw=350,000. SBS 1301 and 1401 are linear polymers containing 30 and 40 wt% styrene, respectively, with about the same Mw, 110,000. Sulfur is a commercial product (industrial grade) of Jinghai Chemical Co., Ltd., China 2.2. Preparation of samples Asphalt was heated to 160°C in a small container until it flowed fully. A given part of SBS (based on 100 parts asphalt) was mixed into the asphalt under high-speed stirring for 1 h and the blend became essentially homogenous. The storage-stable SBS-modified asphalt was obtained when a given level of sulfur was added into the blend and the stirring continued for 60 min under the same stirring condition. 2.3. Storage stability test The storage stability of asphalt binders was tested as follows. The sample was poured into an aluminum foil tube, 32 mm in diameter and 160 mm in height. After

Fig. 1. Effect of sulfur on storage stability of SBS-modified asphalt. (Asphalt 100, SBS 1301, 3.5 wt%, sulfur 5.0 wt% based on SBS). (a) Before adding sulfur; (b) After adding sulfur.

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Table 1 The effect of SBS content on the performance grade of SBS modified asphalt (sample: SBS1301; sulfur level: 5.0 wt%) SBS content

The temperature when G*/sin d=1 kPa (°C) Before adding sulfur

0.0 2.0 3.5 5.0 6.0

After adding sulfur

68.3 74.3 76.4 84.1 87.0

76.4 79.8 87.8 90.4

Fig. 2. Complex modulus and tan d versus temperature for original and SBS1301 modified asphalt before and after adding sulfur.

PMA. A temperature sweep was applied over the range 30–100°C at a fixed frequency of 10 rad/s and variable strain. About 1.0 g sample was put onto the lower plate. After the sample was heated to flow, the upper parallel plate was lowered to contact tightly with the sample and the sample trimmed. The final gap was adjusted to 1.2 mm. All the samples were held at a defined, constant temperature for 10 min and then the temperature was varied in 2°C increments. Various viscoelastic parameters, such as G*, G⬘, G⬙, and tan d were collected automatically by the RSI Orchestrator software. 2.5. Viscosity measurement The flow properties of asphalt samples were determined by a rotational viscometer (Model DV-II+, Brookfield Engineering Inc., USA) at given temperatures, 110, 120, 135, 160 and 180°C and a rotating speed of 60 rpm. 2.6. Morphology observation The sample morphology was observed using an optical microscope with hot plate, made by Leica Co., Germany. Squashed slides of modified binders were prepared using very small amounts of the heated sample and viewed under the microscope at a magnification of 400 at 160°C.

3. Results and discussion 3.1. Storage stability of SBS-modified asphalts with and without sulfur Fig. 3. Effect of polymer content on rheological properties of SBS1301 modified asphalts. (a) Before adding sulfur; (b) After adding sulfur.

Due to the difference in the solubility parameter and density between SBS and asphalt, phase separation would take place in SBS-modified asphalts during storage at elevated temperatures. Droplets of the SBS melt dispersed in asphalt are usually accumulated and float on

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the top of the asphalt at a high temperature and static state. When the storage stability of SBS-modified asphalts were tested as mentioned earlier, the softening point of the top section of modified asphalts after a period of storage became much higher than that of the rest of the material because of the higher concentration of SBS. Storage-stable SBS-modified asphalts can be prepared by reaction with sulfur at high temperature under high shear mixing. Fig. 1 shows the storage stability of the SBS-modified asphalts with and without sulfur. The sulfur level is 5.0 wt% based on the rubber for all the samples shown in Fig. 1. It can be seen that the modified asphalts with varying SBS content show good storage stability after reaction with sulfur. The same results of improved storage stability for the modified asphalts containing other grades of SBS and sufficient sulfur were obtained also. 3.2. Viscoelasticity of SBS modified asphalts with and without sulfur Fig. 2 shows the comparison of rheological properties of SBS-modified asphalt before and after adding sulfur and original asphalt. With increasing the temperature, tan d increased and G* decreased for the original asphalt. The varying trend of tan d and G* was slowed down when 3.5 phr SBS1301 was added to the asphalt. A similar phenomenon has been reported in an early publication by Lu and Iascsson [3]. Nevertheless, the addition of sulfur led to the increase in G* more significantly at elevated temperatures, and the tan d curve became flatter over a wide range of tested temperatures. It indicates that the elasticity of the modified binder had been improved effectively with the addition of sulfur due to the formation of a chemically crosslinked network in the modified binders. In accordance with the Strategic Highway Research

Fig. 4.

Fig. 5. Effect of SBS structure on the rheological properties of modified asphalts before and after adding sulfur. (a) Before adding sulfur; (b) After adding sulfur.

Effect of sulfur level on rheological properties of SBS modified asphalts (SBS content of 3.5 wt%).

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Table 2 The performance grade of SBS modified asphalt (SBS: 3.5 wt%; sulfur: 5.0 wt%) SBS grade

SBS4303 SBS1301 SBS1401

The temperature when G*/sin d=1 kPa (°C) Before adding sulfur

After adding sulfur

76.7 74.3 73.4

86.4 78.4 79.8

Fig. 6. Effect of SBS content on asphalt viscosity (test conditions: shear rate: 20.4 s⫺1; temperature: 135°C).

Program (SHRP) method, the temperature of asphalt binders when G*/sin d is equal to 1000 kPa are 68.3°C for original AH-90 asphalt, 74.3°C for the modified asphalt containing 3.5 wt% SBS1301, 78.4°C for the modified asphalt containing 3.5 wt% SBS1301 and 5.0 wt% sulfur, respectively. It means that the storagestable SBS-modified asphalt containing sulfur has high service temperature. 3.3. Effect of SBS content Fig. 3 shows the dynamic mechanical properties of SBS-modified asphalts with various SBS contents before and after adding sulfur. Before adding sulfur, as SBS content increases, G* of the modified binders increases at high temperature, which becomes apparent when the SBS content is over 5.0 wt%. There is virtually no increase at SBS content of 2.0 wt%. At this content, the SBS acts as a dispersed polymer. From 2.0 to 5.0 wt%, the SBS begins to form a localized network structure. At SBS content over 5.0 wt%, a continuous network is formed throughout the binder [11]. After adding sulfur, the

Fig. 7. Effect of temperature on asphalt viscosity: (a) original AH-90; (b) 3.5 wt% SBS modified asphalt; (c) 6.0 wt% SBS modified asphalt.

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Table 3 Effect of polymer content on activation energy of SBS-modified asphalts SBS content, %wt

Eh (kJ/mol) Before adding sulfur

0 3.5 6.0

64.9 70.5 69.1

After adding sulfur

3.4. Effect of sulfur level

70.0 68.4

changes in tan d with increasing temperatures are smaller than those of modified binders before adding sulfur. The change in tan d of modified asphalt containing 3.5 wt% SBS is not more than 10, at the test temperature. At high SBS content, tan d of the modified asphalts is very small and changes very slightly at a given temperature. Moreover, the G* of the modified binders increases significantly with increasing SBS content. It can be related to the formation of network structure in the modified binders by chemical crosslinking.

Fig. 8.

We can calculate the temperature of the modified binders when G*/sin d is equal to 1 kPa, and relate it to different SBS content, as shown in Table 1. The temperature of the modified binders with the addition of sulfur is higher than that of the binders without sulfur.

The effect of sulfur level on dynamic mechanical properties of SBS modified asphalts is showed in Fig. 4. It can be seen that G* of the modified binder containing 10.0 wt% sulfur is higher than that of the modified binder containing 5.0 wt% sulfur at high temperatures and the change in tan d is reduced to a little extent. It is obvious that the increasing sulfur level enhances the degree of chemical vulcanization of SBS, by which dynamic mechanical properties of the modified binders containing SBS and sulfur are influenced. 3.5. Effect of SBS structure In this work, three different grades of SBS with different structure were used as asphalt modifiers. As shown

Morphology development with the mixing time of SBS-modified asphalt with 5.0 wt% sulfur.

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in Fig. 5, the properties of the asphalts modified by SBS 1301 and 1401 are very similar. The asphalt modified by SBS 4303 has somewhat higher G* and lower tan d at high temperature. This may be accounted for by the differences in the structure of SBS rubbers. SBS 4303 is a star-like tri-block copolymer, with a high Mw of 350,000. SBS1301 and SBS1401 are linear tri-block copolymers with their Mw being about 1/3 of the Mw of SBS4303. Because of the apparent physical network by chain entanglement in SBS4303, it is reasonable that before adding sulfur in such SBS content the change in the tan d of the SBS4303 modified binder is lower than that of the SBS1301 and SBS1401 modified binders. Table 2 lists the temperatures when G*/sin d is equal to 1 kPa for the asphalts modified by three structures of SBS with and without sulfur. It can be seen that SBS4303 modified asphalt can be used at the highest service temperature whenever adding sulfur. For the asphalts modified by other SBS, the addition of sulfur can significantly improve the high temperature properties.

Fig. 9.

301

3.6. Viscosity behavior The effect of polymer content on the viscosity of SBSmodified asphalts is shown in Fig. 6. The increase in asphalt viscosity is not directly proportional to the SBS content. Before adding sulfur, a somewhat marked change is observed when the SBS content exceeds 3.5 wt%. This suggests that certain molecular interactions may exist among SBS molecules and between SBS and asphalt components, such as swelling and/or solvation [12]. After adding sulfur, the asphalt viscosity is similar to that of the blend without sulfur when the SBS content is below 2.0 wt%. There are chemical vulcanization and other reactions between sulfur and asphalt components [13], which is reflected as significantly increasing viscosity in this study. The viscosity of the modified asphalt at SBS content of 6.0% is over 10 times higher than that of the original asphalt and about two times higher than that of SBS-modified asphalts without adding sulfur.

Morphology development with the storage time of SBS4303-modified asphalt without sulfur at 160°C.

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The effect of temperature on asphalt viscosity can be described by the Arrhenius equation, h(T)=KeEh/RT (where h is the viscosity, K the material coefficient, R the gas constant, and Eh the activation energy). By plotting log h(T) versus 1/T, the activation energy of asphalts is calculated from the slope of plots, as shown in Fig. 7, and listed in Table 3. The activation energy of asphalts is influenced slightly by the addition of SBS before and after adding sulfur. 3.7. Morphology The compatibility between polymer and asphalt is critical to the properties of PMAs [14]. The morphology of PMAs was investigated using optical microscopy by characterizing the distribution and the fineness of polymer in the asphalt matrix. As shown in Fig. 8, the addition of sulfur has a significant effect on the compatibility of SBS-modified asphalt. It can been seen that the fine white SBS particles are dispersed in the asphalt matrix (appearing black) after one hour high-speed shearing, Fig. 8(a). The particle size of SBS become smaller and smaller during the preparation process of the modified binders containing 3.5 wt% SBS and 5.0 wt% sulfur. Finally, SBS modified asphalt has a very fine network structure at a magnification of 400, as shown in Fig. 8(d). However, SBS-modified asphalt without sulfur was unstable and had phase separation at elevated temperature [15], as shown in Fig. 9. Brownian flocculation is the main mechanism of gross phase separation. The morphology of the modified binder containing SBS and sulfur at elevated temperature is also shown in Fig. 9. The morphology of the modified binder containing SBS4303 and sulfur stays unchanged during the storage time of one hour, as shown in Fig. 8(d). It means that the compatibility of SBS modified asphalts have been improved significantly with the addition of sulfur, and the blend with sulfur can be stored at high temperature.

Acknowledgements The present work is supported by the National Natural Science Foundation of China. The registered number is 59973011.

References

[1] G. Holden, E.T. Bisshop, N.R. Legge, Thermoplastic elastomers, J. Polym. Sci. Part C 26 (1969) 37. [2] R.H. Bowering, Modified bitumens, AAPA Conference Papers, Members Conference Wirest Point Casino, Hobart, Tasmania, vol. 27, 1984. [3] X. Lu, U. Isacsson, Rheological characterization of styrene–butadiene–styrene copolymer modified bitumens, Constr. Building Mater. 11 (1997) 23. [4] T.S. Shuler, T.H. Collins, J.P. Kirkpatrick, Polymer-modified asphalt properties related to asphalt concrete performance, in: O.E. Briscoe (Ed.), Asphalt: Relationship to Mixture. ASTM STP 941 American Society for Testing and Materials, Philadelphia, 1987, p. 179. [5] J.T. Welborn, J.F. Babashak, New rubberized asphalt for roads, Journal of Highway Division, Proceedings of American Society of Civil Engineering 1651 (1958). [6] P. Maldonado, J. Mas, T.K. Phung, Process for preparing polymer–bitumen compositions, US Patent 4145322, 1979. [7] J.-P. Planche, Method for the preparation of bitumen– polymer compositions, PCT WO 9002776, 1990. [8] R.C. Bellomy, E.L. McGinnis, Bitumen composition containing bitumen, polymer and sulfur, US Patent 5371121, 1994. [9] G.E. William, Polymer enhanced asphalt, WO 9745488, 1997. [10] A.G. William, L.R. Baton, Compatible asphalt–polymer blends, US Patent 5672642, 1997. [11] Y.J. Lee, L.M. France, M.C. Hawley, The effect of network formation on the rheological properties of SBR modified asphalt binders, Rubber Chem. Technol. 70 (1997) 256. [12] X. Lu, U. Isacsson, Influence of styrene–butadiene–styrene polymer modification on bitumen viscosity, Fuel 76 (1997) 1353. [13] P. De Filippis, C. Giavarini, M.I. Santarelli, Reaction of visbreaker bitumens with sulfur, Pet. Sci. Technol. 15 (1997) 743. [14] M.H. Lewandowsky, Polymer modification of paving asphalt binders, Rubber Chem. Technol. 67 (1994) 447. [15] S.A. Hesp, R.T. Woodhams, Stabilization mechanisms in polyolefin–asphalt emulsions, in: In K.R. Wardlaw, S. Scott (Eds.), Polymer Modified Asphalt Binders. ASTM STP 1108, American Society for Testing and Materials, Philadelphia, 1992, p. 1.