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kaolinite clay compound modified asphalts
EUROPEAN POLYMER JOURNAL
European Polymer Journal 42 (2006) 446–457
www.elsevier.com/locate/europolj
Thermo-rheological properties and storage stability of SEBS/kaolinite clay compound modified asphalts Chunfa Ouyang *, Shifeng Wang, Yong Zhang, Yinxi Zhang State Key Laboratory of Metal Matrix Composites, Research Institute of Polymer Materials, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China Received 9 November 2004; received in revised form 13 June 2005; accepted 5 July 2005 Available online 24 August 2005
Abstract Styrene–ethylene–butadiene–styrene block copolymer (SEBS) modified asphalts with improved high-temperature storage stability are prepared by incorporating kaolinite clay (KC) into the SEBS compounds. The effect of KC on the high-temperature storage properties, dynamic rheological and mechanical properties and morphologies of the modified asphalts are studied. It is found that the SEBS/KC ratio in the compound has a great effect on the high-temperature storage behavior. The modified asphalts are stable when the ratio of SEBS/KC is around 2. However, KC decreases the dynamic rheological and mechanical properties of the modified asphalts to some extent. The high-temperature storage property can be increased by improving the compatibility and decreasing the density difference between SEBS and asphalt. 2005 Elsevier Ltd. All rights reserved. Keywords: SEBS; Kaolinite clay; Asphalt; Storage stability; Rheological property
1. Introduction Asphalt as the binder of aggregate has been widely used in road pavement. Unfortunately, high-temperature rutting and low temperature cracking of asphalt cement or coating layer due to severe temperature susceptibility limits its further application [1]. Therefore, it is necessary to modify asphalt. Among the modifiers of asphalt, styrene–butadiene–styrene block (SBS) copolymer is most widely used in road pavement. Nevertheless, SBS tends to degrade by exposure to heat and UV light since SBS contains an unsaturated bond
* Corresponding author. Tel.: +86 21 54742671; fax: +86 21 54741297. E-mail address: [email protected] (C. Ouyang).
[2–4]. The development of the thermal unstable structure within SBS modified asphalt will lead to physical hardening of the asphalt [5]. Hydrogenated styrene-block copolymers are claimed to have improved resistance to degradation processes [6], a typical example being styrene–ethylene–butadiene–styrene block copolymer (SEBS). When used in asphalt modification, SEBS is much more resistant to thermal degradation as compared with SBS [7]. However, one of the most important aspects in road pavement is the storage stability of the modified asphalt. It is known that storage stability of polymer modified asphalts depends on many factors, particularly asphalt composition, polymer characteristics and content. By a proper selection of base asphalt, a stable polymer modified asphalt system can be obtained. However, in most cases, the polymer modified asphalt system is not stable for a given base asphalt.
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.07.004
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457
As mentioned before, the storage stability of polymer modified asphalts is very important. However, just like SBS, SEBS will easily separate from the asphalt when stored at high temperature because of the incompatibility between SEBS and asphalt, which is the major obstacle to the application of SEBS modified asphalts in paving [8]. Some measures have been taken to resolve the problem, such as chemical method (SEBS functionalized with maleic anhydride) [9]. In previous work, SBS modified asphalts with high-temperature storage stability were successfully prepared by equal-density method (premixing SBS and filler to make modified asphalt) [10,11]. In the present work, SEBS modified asphalts with high-temperature storage stability were prepared by incorporating kaolinite clay (KC) into SEBS and mixing this into asphalt. For comparison, asphalts modified by directly adding SEBS and KC were also prepared. The effects of KC on high-temperature properties, the hightemperature storage stability and the mechanical and rheological properties were analyzed. 2. Materials and experimental 2.1. Materials Two base asphalts, AH-90 and AH-70 paving asphalts, were obtained from the Zhenhai Petroleum Asphalt Factory, Zhejiang Province, China. The physical properties of the asphalt are shown in Table 1. The chemical characteristics of the asphalt are shown in Table 2. The generic fractions were determined using Thin Layer Chromatography-Flame Ionization Detector (TLC-FID). Three SEBSs are used. Grade 502 was produced by the Yueyang Petrochemical Co., Ltd., China. This is a linear-like SEBS, containing 30 wt.% styrene, and the average molecular weight was 65,000 g/mol. Grade 3150 and 3151 were obtained from TSRC Co., Ltd., China, these are linear-like SEBS. The average molecular weight of Grade 3150 was 60,000 g/mol and that of Grade 3151 was 150,000 g/mol. KC, with an average particle size of 325 mesh, was non-calcined type and produced by Fengyang Huagu-
447
Table 2 Chemical composition of the base asphalts Binder
Asphaltenes (%)
Resins (%)
Saturates (%)
Aromatics (%)
ZH-70 ZH-90
10.67 6.30
33.33 32.75
6.96 6.29
49.05 54.67
ang Powder-Material Factory, Anhui Province, China. The morphology is shown in Fig. 1. 2.2. Preparation of SEBS/KC compounds SEBS and KC were mixed to form SEBS/KC compounds in a HAAKE rheometer at 160 C for 5 min. Under these conditions, SEBS melted and mixed well with KC to form SEBS/KC compound, as shown in Fig. 2. 2.3. Preparation of modified asphalts All the modified asphalts were prepared using a high shear mixer (made by Weiyu Machine Co., Ltd., China) at 180 C and a shearing speed of 4000 rpm, and the shearing time was 40 min. First, 600 g asphalt was heated to become a fluid in an iron container, then upon reaching about 180 C, the modifier was added to the asphalts. 2.4. High-temperature storage stability test After mixing, some of the modified asphalt 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 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 SBS modified asphalts by
Table 1 Conventional physical properties of the base asphalts Binder
Penetration at 25 C (dm m)a
Softening point (C)b
Viscosity at 135 C (Pa s)c
AH-70 AH-90
68 90
49.5 46.5
0.38 0.35
a b c
ASTM D5. ASTM D36. ASTM D4402.
Fig. 1. Morphology of KC (SEM).
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at 163 C under an optical microscope (Leica Co., Germany) with a magnification of 400 times. KC was immersed in absolute alcohol filled in a beaker and dispersed by a low power ultrasonic instrument for half an hour. Then a drop of KC/alcohol mixture was taken out from the beaker for morphological observation. The observation was performed on scanning electron microscopy (SEM, S-2150, Japan) with a resolution of 4.5 nm.
35 30
TQ/(N.M)
25 20 15 10 5
3. Results and discussion
0 0
2
4
6
8
10
3.1. KC modified asphalts
Time, min
Fig. 2. Torque (TQ) variations with time for SEBS/KC compound at 160 C in HAAKE rheometer.
measuring their softening points and viscosities at 135 C. If the difference of the softening points between the top and the bottom sections was less than 2.5 C, and the viscosities for the top sections and the bottom ones were nearly the same, the samples was considered to have good high-temperature storage stability. If the softening points differed by more than 2.5 C or if the ratio of the two viscosities was above 1.1 or below 0.9, the SBS modified asphalt was labeled as unstable. 2.5. Rheological characterization A strain-controlled rheometry, advanced rheometer (Bohlin Gemini 200, UK) with parallel plate geometry (25 mm in diameter), was used to determine the rheological behavior of the asphalts before and after modification. Temperature sweeps (from 30 to 100 C) with 2 C increments were applied at a fixed frequency of 10 rad/s and variable strain. In each test, about 1.0 g of sample placed on the bottom plate, covering the entire surface, and the plate was then mounted in the rheometer. After the sample was heated to become a melt, the top plate was brought into contact with the sample, and the sample was trimmed. The final gap was adjusted to 1 mm. The actual strain and torque were measured to calculate various viscoelastic parameters such as complex modulus (G*) and phase angle (d). Strain sweeps between 1% and 100% at 30, 50, 70, 80, 90 and 95 C were carried out to establish the linear viscoelasticity range (LVR) of all samples. The maximum strain in LVR was selected to obtain sufficient stress response. 2.6. Morphological analysis A small drop of the asphalt was placed between two heated microscope glass slides and squashed to form a thin film. The morphology of the asphalt was observed
In order to understand the SEBS/KC compound modified asphalt, the KC modified asphalt was first studied. The effect of KC on the properties of asphalt is shown in Table 3. A small amount of KC had almost no effect on the softening point and high-temperature storage stability of asphalts. The morphology of asphalt changed little whether KC was added or not, as shown in Fig. 3. 3.2. SEBS/KC compound modified asphalts 3.2.1. High-temperature storage properties and morphology A minimum compatibility between the polymer and the asphalt is necessary to avoid separation during storage, pumping, and applying the asphalt and to achieve the expected properties in the pavement [12]. Stability tests can determine whether the interactions created between the polymers and the asphalts during mixing are strong enough to resist a separation of the polymer in the conditions in which it is stored. Two approaches have been accepted to find out if phase separation occurred during the high-temperature storage stability test: softening point variation and phase compatibility. Softening points between the top and the bottom of the samples after the high-temperature storage stability test should not be higher 2.5 C, indicating that there is no
Table 3 Effect of KC content on the properties of original asphalta KC content (%)
0 1 2.5 a
Softening point (C)
47.5 47.5 48.0
Viscosity at 135 C (Pa s)
0.30 0.30 0.31
Softening point (C)b Top
Bottom
47.5 48.0 49.0
47.5 48.2 48.8
ZH-90 base asphalt was used. Below is the same if not noted particularly. b Stored in an oven at 163 C for 48 h.
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Fig. 3. Micrographs of original asphalt and 2.5% KC modified asphalt (optical microscopy). (a) Original asphalt (·400), (b) 2.5% KC modified asphalt (·400).
substantial phase separation (storage stability). Samples are also compared by optical microscopy. For true stability the top section of the sample should have the same phase distribution as the bottom section. The high-temperature storage stabilities of SEBS/KC compound modified asphalts were presented in Table 4. Obviously, for asphalts modified by SEBS in the absence of KC, the differences in the softening points and viscosities were large, indicating that the phase separation was serious. The SEBS modified asphalt was unstable even at 3% SEBS content. With increasing SEBS content, the phase separation became more obvious. When the SEBS content increased to 6%, SEBS seriously separated from the asphalt.
The asphalts modified with 4% SEBS was still unstable when the SEBS/KC ratio was 100/10 or 100/30. When the ratio was 100/50 or 100/70, the storage stability of SEBS/KC compound modified asphalts was improved significantly. However, the asphalt modified by directly adding SEBS and KC, it was unstable when the ratio of SEBS/KC was 100/50. So it can be concluded that SEBS/KC compounding is critical to obtain the high-temperature storage stability. When the SEBS content was 3%, 5% and 6%, the modified asphalts with high-temperature storage stability were obtained by incorporating SEBS/KC (100/50) compounds into asphalts respectively, as shown in Table 4.
Table 4 Effect of KC content on the high-temperature storage stabilities of SEBS/KC compounds modified asphaltsa Viscosity at 135 C (Pa s)
Formulation SEBS, % (w/w)
SEBS/KC (w/w)
Top
Bottom
3
100/0 100/50
0.825 0.642
0.600 0.658
4
100/0 100/10 100/30 100/50 100/50b 100/70
1.058 1.008 0.975 0.790 1.552 0.875
5
100/0 100/50
6
100/0 100/50 a b c d
gt =gb c
Softening point (C)
S t S b (C)d
Top
Bottom
1.38 0.98
53.0 52.5
50.0 52.8
3.0 0.3
0.742 0.751 0.800 0.825 1.210 0.883
1.43 1.34 1.22 0.96 1.28 0.99
57.0 56.5 55.0 55.5 59.0 52
53.8 50.8 52.0 55.8 52.0 52.5
3.2 5.7 3.0 0.3 7.0 0.5
2.083 1.117
0.992 1.192
2.10 0.94
70.5 57.0
58.0 58.0
12.5 1.0
2.783 1.558
1.258 1.550
2.21 1.01
85.0 59
67.5 60.5
17.5 0.5
ZH-90 base asphalt and 502 SEBS were used. Below is the same if not noted particularly. SEBS and KC added directly. gt/gb = Top/bottom ratio of viscosity. St Sb = Bottom softening point subtracted from top one.
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A continuous and direct approach to study the hightemperature storage stability of SEBS modified asphalts
was obtained by observing the morphology at high temperature as a function of time. The morphology of
Fig. 4. Morphology of SEBS modified asphalt before and after adding KC (SEBS/KC = 100/50) at 163 C for some time (A) 3% SEBS modified asphalt for 0 min; (B) 3% SEBS modified asphalt for 5 min; (C) 3% SEBS/KC (100/50) modified asphalt for 0 min; (D) 3% SEBS/KC (100/50) modified asphalt for 1 h; (E) 4% SEBS modified asphalt for 0 min; (F) 4% SEBS modified asphalt for 5 min; (G) 5% SEBS modified asphalt for 0 min; (H) 5% SEBS modified asphalt for 5 min; (I) 5% SEBS/KC (100/50) modified asphalt for 0 min; (J) 5% SEBS/KC (100/50) modified asphalt for 1 h; (K) 6% SEBS modified asphalt for 0 min; (L) 6% SEBS modified asphalt for 5 min; (M) 6% SEBS/KC (100/50) modified asphalt for 0 min; (N) 6% SEBS/KC (100/50) modified asphalt for 1 h.
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Fig 4. (continued)
SEBS/KC compound modified asphalts are shown in Fig. 4. The morphology of the modified asphalt depends mainly on the concentration of SEBS and the presence of KC. The modified asphalts with a low polymer content and without KC, as shown in Fig. 4(A, C, E), show polymer domains dispersed in asphalt. While the SEBS content increased to 5%, as shown in Fig. 4(G), the polymer had a tendency to become a continuous phase. When the SEBS content was 6% (Fig. 4(M)), the continuous phase of the modified asphalt changed to the polymer. The morphology of SEBS modified asphalt without KC changed quickly with time, independent of the content. The SEBS aggregated quickly to form coarse particles. Five minutes later, as shown in Fig. 4, the coarser particles of SEBS formed, indicating that the SEBS modified asphalts without KC were not stable when stored at high temperature. However, as for the SEBS/
KC compound modified asphalts, the morphology had changed little even set at high temperature for an hour, indicating their stability at high temperature. 3.2.2. Softening point and TSHRP The softening point and the corresponding temperature (TSHRP) at G*/sin(d) = 1 kPa according to Strategic Highway Research Program (SHRP) are usually used to characterize the high-temperature properties of asphalt [10]. The higher the softening point (or corresponding temperature), the better the properties at high temperature. Generally, polymer can enhance the high-temperature properties of asphalt. In this study, the result is in accordance with the general viewpoint, as shown in Table 5 and Fig. 5. With increasing SEBS content, the softening point and TSHRP of the modified asphalt become higher,
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Table 5 Effect of SEBS/KC on the high-temperature properties of asphalts SEBS content (%)
0 3 4 5 6
Softening point (C)
SHRP temperature (C) (G*/ sin(d) = 1 kPa)
Original SEBS
SEBS/KC (100/50)
Original SEBS
SEBS/KC (100/50)
47.5 51.5 55.0 64.5 83.0
47.5 50.0 53.5 56.0 62.5
n.d. 68.5 72.7 83.4 92.6
n.d. 68.2 72.5 78.1 86.2
n.d. = not determined. Below is the same if not noted particularly.
indicating that the improvement of high-temperature property of the modified asphalt. Moreover, the softening point and TSHRP increases significantly when the SEBS content increased to 5% or 6%. The phenomenon might be explained by the morphology of the modified asphalts. As for the asphalts with low SEBS content, the SEBS domain dispersed in the asphalt (as shown in Fig. 4(A and C)). With increasing the content to 5% or 6%, the SEBS had the tendency to become a continuous phase (as shown in Fig. 4(G)) and further formed a continuous phase (as shown in Fig. 4(N)).
The effect of KC on the high-temperature properties of SEBS modified asphalts is shown in Tables 5 and 6 and Fig. 5. As for the asphalt modified by SEBS with a low content (3% and 4%), the introduction of KC slightly decreases the softening point and TSHRP, as shown in Table 5, which indicated that the KC had a little effect on the high-temperature properties of the SEBS modified asphalts. For further study the effect of KC on the high-temperature properties of low content SEBS modified asphalt, 4% SEBS modified asphalts with different KC content were prepared. The high-temperature properties of 4% SEBS modified asphalts with different ratio of SEBS/KC are shown in Table 6 and Fig. 5. As can be seen, KC had a slight influence on the high-temperature properties of 4% SEBS modified asphalt However, with increasing SEBS content to 5% and 6%, the high-temperature properties of the modified asphalts became more complicated. The introduction of KC worsened the high-temperature properties of the SEBS modified asphalts, as shown in Table 5. There would be a great difference of high-temperature properties between original SEBS modified asphalts and the SEBS/KC compound modified asphalt, which agree with the morphology of the modified asphalt, as shown in Fig. 4(G–N). There would be a possible explanation as follows. Because KC has many active functional groups on its surface [13], there will be rather strong
4% SEBS 4% SEBS/KC (100/30) 5
10
4% SEBS/KC (100/50) 4% SEBS/KC (100/70)
4
G*/sin(δ), Pa
10
G */sin(δ)=1000 Pa
3
10
2
10
30
40
50
60
70
80
90
100
110
o
Temperature, C
Fig. 5. Curve of G*/sin(d) versus temperature for 4% SEBS modified asphalt with different ratio of SEBS/KC.
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457 Table 6 Effect of KC content on the high-temperature properties of 4% SEBS/KC compound modified asphalt Property
Formulation SEBS/KC ratio (w/w)
Softening point (C) SHRP temperature (C)
0/0
100/10
100/30
100/50
100/70
55.0
53.0
54.0
53.5
53.5
72.7
n.d.
72.5
72.5
72.1
n.d. = not determined. Below is the same if not noted particularly.
interaction between SEBS and KC when blending them. After mixing asphalt and the SEBS/KC compound, the KC hindered the forming of the network of SEBS, while it would form in original SEBS modified asphalt with high content (5% and 6%), which contribute to the enhanced high-temperature properties. 3.2.3. Viscosity, needle penetration and low temperature ductility The effect of KC on the viscosity, needle penetration and low temperature ductility of the modified asphalt was shown in Table 7. As for the original SEBS modified asphalts, viscosity and ductility of the asphalt became higher, but the needle penetration became smaller with increasing SEBS content. The influence of KC on the viscosity, needle penetration and ductility of the SEBS modified asphalts was dependant on the SEBS content. When the SEBS content was low (3%, 4% and 5%), the KC had a little influence on them. When the SEBS content was 6%, the viscosity and ductility of the modified asphalt decreases significantly, and the needle penetration decreases to some extent. The phenomenon is in accordance with the change of morphology of the SEBS modified asphalts with introduction of the KC. Table 7 Effect of KC on the viscosity, needle penetration and ductility of asphalt SEBS, % (w/w)
SEBS/KC, (w/w)
Viscosity at 135 C (Pa s)
Needle penetration, 25 C (dm m)
Ductility, cm (5 C)
3
100/0 100/50 100/0 100/10 100/30 100/50 100/70 100/0 100/50 100/0 100/50
683 650 875 900 842 850 867 1292 1225 1742 1442
58 57 56 55 55 54 52 50 48 45 42
10 9 12 10 11 11 10 14 13 18 14
4
5 6
453
3.2.4. Dynamic viscoelastic parameters The most significant effect of polymers on asphalt is the improvement of elasticity. There is a strong correlation between rutting resistance at high temperature and complex modulus. Increasing complex modulus (elastic modulus) is to be expected because it reflects a promising rutting resistance at high temperature. Isochronal plots of complex modulus (G*) versus temperature at 10 rad/s for SEBS modified asphalts are shown in Fig. 6. For original SEBS modifies asphalt, the isochronal plots show a difference between the base asphalt and the SEBS modified asphalt, particularly at upper and lower ends of the temperature range. Although there is only minor increase in G* at low temperatures due to SEBS modification, there is considerable evidence of extreme polymeric modification at high temperatures. With increasing SEBS content, there is a significant increase in G* over entire temperature range especially for 5% or 6% SEBS modified asphalt, which is an evidence of dominant polymer network. Phase angle isochrones at 10 rad/s for the SEBS modified asphalts are presented in Fig. 7. Measurement of tan d is generally considered to be more sensitive to the chemical and physical structure than complex modulus for the modification of asphalts. The phase angle isochrones clearly illustrate the improved elastic response (reduced phase angles) of the modified asphalts compared to the base asphalt. Whereas the phase angles of the base asphalt approach 90 and therefore predominantly show viscous behavior with increasing temperature. SEBS significantly improve the elastic response at high temperature, which can be attributed to the viscosity of the base asphalt being low enough to allow the elastic network of the polymer to influence the mechanical properties of the modified asphalt. The plateau regions and decreased phase angles at intermediate and high temperatures is synonymies with the plateau region defined for polymers and demonstrates the ability of the polymer to form a continuous elastic network especially for SEBS (high content) modified asphalt. The presence and nature of the plateau and polymer network is a function of the chemical and physical properties of the SEBS polymer and base asphalt. The isochronal plots of G* and tan d indicate the degree of SEBS modification at high temperatures (above 50 C). Compared to original SEBS modified asphalts, the isochronal plots of SEBS/KC compound modified asphalts show a similar tendency with increasing the SEBS content, as shown in Figs. 6–9. What is noticeable is the considerable difference in the extent of the modification between the original SEBS and the SEBS/KC compound. For 3% and 4% SEBS/KC compound modified asphalts, the introduction of the KC had a little influence on the rheological properties of SEBS modified asphalt, except for a little viscous properties as compared
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10
4
G*, Pa
10
5
10
10
1
Base asphalt
2
3% SEBS
3
3% SEBS/KC (100/50)
4
4% SEBS
5
4% SEBS/KC (100/50)
6
5% SEBS
7
5% SEBS/KC (100/50)
8
6% SEBS
9
6% SEBS/KC (100/50)
8
3
9 6 4 7 5 2 3 1
2
20
30
40
50
60
70
80
90
100
110
o
Temperature, C
Fig. 6. Isochronal plots of G* versus temperature at 10 rad/s for asphalts modified by different content of SEBS/KC compound.
Base asphalt 3 % SEBS 3 % SEBS/KC (100/50) 4 % SEBS 4 % SEBS/KC (100/50) 5 % SEBS 5 % SEBS/KC (100/50) 6 % SEBS 6% SEBS/KC (100/50)
70
60
50
Tan(δ)
40
30
20
10
0
20
30
40
50
60
70
80
90
100
110
o
Temperature, C
Fig. 7. Isochronal plots of tan(d) versus temperature at 10 rad/s for asphalts modified by different content of SEBS/KC compound.
with original SEBS modified asphalt. WhatÕs more, for 4% SEBS modified asphalts, the isochronal plots of complex modulus nearly intersected as shown in Figs. 8 and 9, indicating that KC content nearly had little
influence on the rheological properties of SEBS/KC compound asphalts. However, increasing the SEBS content to 5% above, the KC had a significantly effect on the rheological properties of SEBS modified asphalts. The
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455
Asphalt ZH90# 4% SEBS 4% SEBS/KC (100/30) 4% SEBS/KC (100/50)
5
10
4% SEBS/KC (100/70)
4
G*, Pa.s
10
3
10
2
10
20
30
40
50
60
70
80
90
100
110
Temperature,oC
Fig. 8. Isochronal plots of G* versus temperature at 10 rad/s for asphalts modified by 4% SEBS/KC compound with different ratios.
70
Asphalt, ZH90# 4 % SEBS
60
4 % SEBS/KC (100/30) 4 % SEBS/KC (100/50) 50
4 % SEBS/KC (100/70)
Tan (δ)
40
30
20
10
0
20
30
40
50
60
70
80
90
100
110
Temperature,oC
Fig. 9. Isochronal plots of tan(d) versus temperature at 10 rad/s for asphalts modified by 4% SEBS/KC compound with different ratios.
isochronal plots of 5% or 6% SEBS/KC compound modified asphalt show a little sharp tendency as compared with corresponding SEBS (same content) modified asphalt, indicating that SEBS modified asphalt
had more obvious elastic response. The differences in rheological properties might be attributed to the morphologies of the modified asphalts. The introduction of the KC might hinder the forming of network of SEBS
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domain in asphalt matrix, leading to the elastic worsening of the asphalt.
Table 8 Hildebrand solubility parameters of asphalt components and of blocks in the SBS and SEBS copolymer
3.3. Storage-stable mechanism
Asphalt component
d (cal1/2/cm3/2)
Polymer
d (cal1/2/cm3/2)
Based on adsorption–desorption methods, asphalt can be separated into four components: asphaltenes, polar aromatics, naphthene aromatics and saturates. Each class has different characteristic solubility parameter values as listed in Table 2. Asphalt is traditionally considered as a dynamic colloid system consisting of a suspension of high molecular weight asphaltene micelles dispersed in a lower molecular weight oily medium (maltenes) [14]. Asphalts represent an extreme example of a complex block copolymer additive. Each of the numerous constituents will be distributed in varying concentrations between the microphase separated domains and macrophase separated regions. Asphaltene possesses the highest molecular weight fractions in asphalt and have very large solubility parameter values; maltene is composed of smaller molecular weight fractions and has solubility parameter values comparable to those of S and EB (see Table 8) [15–17]. Therefore, macrophase separation in the asphalt/SEBS mixture will occur even when the SEBS content is decreased to 3%, though the separation is determined by the ratio of asphaltene to SEBS [8]. Block copolymers can interact with additives, such as inorganic fillers or solvents or homopolymers, in several ways. If the mixing thermodynamics are governed by an unfavorable heat of mixing (DH > 0), macroscopic phase separation will occur. However, if one of the blocks is compatible with additive, favorable swelling may result, which can induce microstructural changes because the effective volume fraction and interfacial tension will be altered [18–20]. KC as a kind of inorganic filler might partly miscible with the blocks of the SEBS and asphalt and there can lead to microstructural change of the modified asphalt. In the study of the compatibility between asphalt oligomers and SEBS, Ho and the collaborator made a thorough study by means of
Asphaltene Saturates Naphthene aromatics Polar aromatics
12.2–16.1 8.5–9.8 9.3–11.0
PS PB P(EB)
9.1–9.7 8.1–8.6 7.8–8.1
10.7–13.0
Maltene consists of polar aromatics, naphthene aromatics, and polar aromatics.
transmission electron microscopy (TEM), small angle X-ray scattering (SAXA), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC) [8]. The experiments indicated that asphaltene is essentially immiscible with both blocks of SEBS, while maltene is miscible with SEBS. So the introduction of KC mainly influenced the compatibility between SEBS and asphaltene under the interaction of SEBS and KC by means of premixing technique, consequently improve the compatibility between SEBS and asphalt. Moreover, KC hindered the forming of network of SEBS in the modified asphalts at the interface and decreased the high-temperature properties. On the other hand, SEBS modified asphalt can be considered as a suspended system. For a suspended system, the particles in the liquid with the buoyancy force and gravitational force, and the falling velocity of the particles in the system follow StokeÕs law [7]: V ¼ 2ðq0 q1 Þgr2 =9g
ð1Þ
where q0 is the density of asphalt, q1 is the density of SEBS, g is the gravitational force constant, r is the average radius of the SEBS particles and g is the viscosity of the modified asphalt. To prevent the phase separation of SEBS from asphalt, a critical way is to reduce the falling velocity of the particles. As shown in Eq. (1), there are two methods
Table 9 High-temperature storage stability of 4% SEBS/KC modified asphalt with proper ratio Viscosity at 135 C (Pa s)
Formulation Asphalt/SEBS
SEBS/KC, (w/w)
Top
Bottom
ZH-70/502
100/0 100/60 100/0 100/60 100/0 100/40
1.125 0.830 1.058 0.810 2.783 1.538
0.730 0.845 0.742 0.808 1.258 1.550
ZH-90/3150 ZH-90/3151 a b
gt/gb = Top/bottom ratio of viscosity. St Sb = Bottom softening point subtracted from top one.
gt =gb a
1.54 0.98 1.43 0.99 2.21 0.99
Softening point (C) Top
Bottom
59.0 56.5 57.0 54.5 69.0 61.0
54.0 56.2 53.8 54.5 51.0 60.5
S t S b (C)b
5.0 0.3 3.2 0.0 18.0 0.5
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457
to do this, one is to reduce the particle size, the other is to decrease the density difference. The density of the SEBS was 0.93 g/cm3 and the density of asphalt here was 1.02 g/cm3 at room temperature. The density difference of the SEBS and asphalt became larger at high temperatures because the SEBS swelled in the oily fraction of the asphalt [16,17]. The density of KC is around 2.57 g/cm3 at room temperature. When the KC attached to SEBS, the density difference is decreased and the force for driving separation becomes zero at a certain content of KC, so the high-temperature storage stability is improved. Therefore, the improvement of high-temperature storage stability of the asphalt modified by SEBS maybe originates from two sources: one is KC mainly influenced the compatibility between SEBS and asphaltene, the other is KC decreased the density difference between asphalt and SEBS. To verify the effectiveness of the assumption above, ZH-70 base asphalt with higher content of asphaltenes and two other SEBSs named as 3150 and 3151 were selected to prepare the modified asphalts. By selecting the proper ratio of SEBS and KC, the stable system at high temperature was obtained, as shown in Table 9.
4. Conclusions The rheological and high-temperature properties of the road asphalts can be improved by means of SEBS polymer and SEBS/KC compound modification. With increasing SEBS content, the extent of polymer modification has differed depending on the polymer content and the presence of KC. When the SEBS content was 6%, the properties of SEBS modified asphalt were improved significantly. KC with a proper amount added can dramatically improve the compatibility between SEBS and base asphalt by means of premixing SEBS and KC, which might be explained that KC hindered the coalescence of SEBS from asphalt matrix at high temperature and decreased density deference between asphalt and SBS. However, the introduction of KC can influence the properties of SEBS modified asphalt. The extent influence of KC differed depending on the SEBS content. When the content was low (such as 3% or 4%), the properties including high-temperature, mechanical, rheological properties and morphology had a little change. However, increasing SEBS content to 5% or 6%, the KC had a dramatic influence on the properties asphalts. The morphology of SEBS/KC modified asphalt showed SEBS dispersed in asphalt while that of original SEBS modified asphalt showed that the SEBS network nearly formed, especially when SEBS
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content was 6%. The worsened elastic properties of SEBS modified asphalts were demonstrated using rheological parameters of complex modulus and phase angle. The worsening properties might be attributed to the introduction of the KC, which hindered the forming of SEBS network and decreased the high-temperature properties of SEBS modified asphalt.
Acknowledgements The authors acknowledge the financial support provided by the National High Technology Research and Development Program of China (863 Program), and the registered number is 2002AA335100.
References [1] Shingo K, Shigeru T, Zhan X, Dong D, Norihiro I. Polym J 2001;3(33):209–13. [2] Jellinek HHG. Degradation and stabilization of polymers, vol. 1. New York: Elsevier Science Publishers Inc; 1987. [3] Klemchuk PP. Polymer stabilization and degradation. ACS Symposium Series, vol. 280. Washington, D.C.: American Chemical Society; 1985. [4] Wang SM, Chang JR, Tsiang RCC. Polym Degrad Stab 1996;52:51–7. [5] Lu XH, Ulf I. Constr Build Mater 2000;14:79–88. [6] Gergen WP, Lutz RG, Davidson S. Hydrogenated copolymers in IPN. In: Golden G et al., editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser; 1996. [7] Lu XH, Ulf I. Polym Test 2001;20:77. [8] Ho RM, Adeyinka A, Giles WD, et al. J Polym Sci B 1997;35:2857–77. [9] Yvonne BM, Alejandro JM, Yajaira R. J Appl Polym Sci 2003;90:1772–82. [10] Wang SF, Zhang Y, Zhang YX. Polym Polym Compos 2003;11(6):477–85. [11] Ouyang CF, Wang SF, Zhang Y, Zhang YX. Symposium of international rubber conference 2004. p. 642–8. [12] Gorbaty ML, Peiffer DG, McHugh DJ. US Patent 5, 1994, 348, 994. [13] Luckham FP, Sylvia R. Adv Colloid Interf Sci 1999;82(1):43–92. [14] Loeber L, Muller G, Morel J. Fuel 1998;77(13):1443–50. [15] Hagen AP, Jones R, Hofener RM, Rabdolph BB, Johnson MP. Proc Assoc Asphalt Paving Technol 1984;53:119. [16] Brandrup J, Immergut EH, editors. Polymer handbook. 3rd ed. New York: Wiley; 1989. [17] Barton AFM, editor. CRC handbook of polymer–liquid interaction parameters and solubility parameters. Boca Raton, FL: CRC Press; 1990. [18] Hong KM, Noodlandi J. Macromolecules 1983;16:1083. [19] Cohen RE, Torradas JM. Macromolecules 1984;17:1101. [20] Tanaka H, Hasegawa H, Hashimoto T. Macromolecules 1991;24:240.
European Polymer Journal 42 (2006) 446–457
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Thermo-rheological properties and storage stability of SEBS/kaolinite clay compound modified asphalts Chunfa Ouyang *, Shifeng Wang, Yong Zhang, Yinxi Zhang State Key Laboratory of Metal Matrix Composites, Research Institute of Polymer Materials, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China Received 9 November 2004; received in revised form 13 June 2005; accepted 5 July 2005 Available online 24 August 2005
Abstract Styrene–ethylene–butadiene–styrene block copolymer (SEBS) modified asphalts with improved high-temperature storage stability are prepared by incorporating kaolinite clay (KC) into the SEBS compounds. The effect of KC on the high-temperature storage properties, dynamic rheological and mechanical properties and morphologies of the modified asphalts are studied. It is found that the SEBS/KC ratio in the compound has a great effect on the high-temperature storage behavior. The modified asphalts are stable when the ratio of SEBS/KC is around 2. However, KC decreases the dynamic rheological and mechanical properties of the modified asphalts to some extent. The high-temperature storage property can be increased by improving the compatibility and decreasing the density difference between SEBS and asphalt. 2005 Elsevier Ltd. All rights reserved. Keywords: SEBS; Kaolinite clay; Asphalt; Storage stability; Rheological property
1. Introduction Asphalt as the binder of aggregate has been widely used in road pavement. Unfortunately, high-temperature rutting and low temperature cracking of asphalt cement or coating layer due to severe temperature susceptibility limits its further application [1]. Therefore, it is necessary to modify asphalt. Among the modifiers of asphalt, styrene–butadiene–styrene block (SBS) copolymer is most widely used in road pavement. Nevertheless, SBS tends to degrade by exposure to heat and UV light since SBS contains an unsaturated bond
* Corresponding author. Tel.: +86 21 54742671; fax: +86 21 54741297. E-mail address: [email protected] (C. Ouyang).
[2–4]. The development of the thermal unstable structure within SBS modified asphalt will lead to physical hardening of the asphalt [5]. Hydrogenated styrene-block copolymers are claimed to have improved resistance to degradation processes [6], a typical example being styrene–ethylene–butadiene–styrene block copolymer (SEBS). When used in asphalt modification, SEBS is much more resistant to thermal degradation as compared with SBS [7]. However, one of the most important aspects in road pavement is the storage stability of the modified asphalt. It is known that storage stability of polymer modified asphalts depends on many factors, particularly asphalt composition, polymer characteristics and content. By a proper selection of base asphalt, a stable polymer modified asphalt system can be obtained. However, in most cases, the polymer modified asphalt system is not stable for a given base asphalt.
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2005.07.004
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457
As mentioned before, the storage stability of polymer modified asphalts is very important. However, just like SBS, SEBS will easily separate from the asphalt when stored at high temperature because of the incompatibility between SEBS and asphalt, which is the major obstacle to the application of SEBS modified asphalts in paving [8]. Some measures have been taken to resolve the problem, such as chemical method (SEBS functionalized with maleic anhydride) [9]. In previous work, SBS modified asphalts with high-temperature storage stability were successfully prepared by equal-density method (premixing SBS and filler to make modified asphalt) [10,11]. In the present work, SEBS modified asphalts with high-temperature storage stability were prepared by incorporating kaolinite clay (KC) into SEBS and mixing this into asphalt. For comparison, asphalts modified by directly adding SEBS and KC were also prepared. The effects of KC on high-temperature properties, the hightemperature storage stability and the mechanical and rheological properties were analyzed. 2. Materials and experimental 2.1. Materials Two base asphalts, AH-90 and AH-70 paving asphalts, were obtained from the Zhenhai Petroleum Asphalt Factory, Zhejiang Province, China. The physical properties of the asphalt are shown in Table 1. The chemical characteristics of the asphalt are shown in Table 2. The generic fractions were determined using Thin Layer Chromatography-Flame Ionization Detector (TLC-FID). Three SEBSs are used. Grade 502 was produced by the Yueyang Petrochemical Co., Ltd., China. This is a linear-like SEBS, containing 30 wt.% styrene, and the average molecular weight was 65,000 g/mol. Grade 3150 and 3151 were obtained from TSRC Co., Ltd., China, these are linear-like SEBS. The average molecular weight of Grade 3150 was 60,000 g/mol and that of Grade 3151 was 150,000 g/mol. KC, with an average particle size of 325 mesh, was non-calcined type and produced by Fengyang Huagu-
447
Table 2 Chemical composition of the base asphalts Binder
Asphaltenes (%)
Resins (%)
Saturates (%)
Aromatics (%)
ZH-70 ZH-90
10.67 6.30
33.33 32.75
6.96 6.29
49.05 54.67
ang Powder-Material Factory, Anhui Province, China. The morphology is shown in Fig. 1. 2.2. Preparation of SEBS/KC compounds SEBS and KC were mixed to form SEBS/KC compounds in a HAAKE rheometer at 160 C for 5 min. Under these conditions, SEBS melted and mixed well with KC to form SEBS/KC compound, as shown in Fig. 2. 2.3. Preparation of modified asphalts All the modified asphalts were prepared using a high shear mixer (made by Weiyu Machine Co., Ltd., China) at 180 C and a shearing speed of 4000 rpm, and the shearing time was 40 min. First, 600 g asphalt was heated to become a fluid in an iron container, then upon reaching about 180 C, the modifier was added to the asphalts. 2.4. High-temperature storage stability test After mixing, some of the modified asphalt 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 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 SBS modified asphalts by
Table 1 Conventional physical properties of the base asphalts Binder
Penetration at 25 C (dm m)a
Softening point (C)b
Viscosity at 135 C (Pa s)c
AH-70 AH-90
68 90
49.5 46.5
0.38 0.35
a b c
ASTM D5. ASTM D36. ASTM D4402.
Fig. 1. Morphology of KC (SEM).
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at 163 C under an optical microscope (Leica Co., Germany) with a magnification of 400 times. KC was immersed in absolute alcohol filled in a beaker and dispersed by a low power ultrasonic instrument for half an hour. Then a drop of KC/alcohol mixture was taken out from the beaker for morphological observation. The observation was performed on scanning electron microscopy (SEM, S-2150, Japan) with a resolution of 4.5 nm.
35 30
TQ/(N.M)
25 20 15 10 5
3. Results and discussion
0 0
2
4
6
8
10
3.1. KC modified asphalts
Time, min
Fig. 2. Torque (TQ) variations with time for SEBS/KC compound at 160 C in HAAKE rheometer.
measuring their softening points and viscosities at 135 C. If the difference of the softening points between the top and the bottom sections was less than 2.5 C, and the viscosities for the top sections and the bottom ones were nearly the same, the samples was considered to have good high-temperature storage stability. If the softening points differed by more than 2.5 C or if the ratio of the two viscosities was above 1.1 or below 0.9, the SBS modified asphalt was labeled as unstable. 2.5. Rheological characterization A strain-controlled rheometry, advanced rheometer (Bohlin Gemini 200, UK) with parallel plate geometry (25 mm in diameter), was used to determine the rheological behavior of the asphalts before and after modification. Temperature sweeps (from 30 to 100 C) with 2 C increments were applied at a fixed frequency of 10 rad/s and variable strain. In each test, about 1.0 g of sample placed on the bottom plate, covering the entire surface, and the plate was then mounted in the rheometer. After the sample was heated to become a melt, the top plate was brought into contact with the sample, and the sample was trimmed. The final gap was adjusted to 1 mm. The actual strain and torque were measured to calculate various viscoelastic parameters such as complex modulus (G*) and phase angle (d). Strain sweeps between 1% and 100% at 30, 50, 70, 80, 90 and 95 C were carried out to establish the linear viscoelasticity range (LVR) of all samples. The maximum strain in LVR was selected to obtain sufficient stress response. 2.6. Morphological analysis A small drop of the asphalt was placed between two heated microscope glass slides and squashed to form a thin film. The morphology of the asphalt was observed
In order to understand the SEBS/KC compound modified asphalt, the KC modified asphalt was first studied. The effect of KC on the properties of asphalt is shown in Table 3. A small amount of KC had almost no effect on the softening point and high-temperature storage stability of asphalts. The morphology of asphalt changed little whether KC was added or not, as shown in Fig. 3. 3.2. SEBS/KC compound modified asphalts 3.2.1. High-temperature storage properties and morphology A minimum compatibility between the polymer and the asphalt is necessary to avoid separation during storage, pumping, and applying the asphalt and to achieve the expected properties in the pavement [12]. Stability tests can determine whether the interactions created between the polymers and the asphalts during mixing are strong enough to resist a separation of the polymer in the conditions in which it is stored. Two approaches have been accepted to find out if phase separation occurred during the high-temperature storage stability test: softening point variation and phase compatibility. Softening points between the top and the bottom of the samples after the high-temperature storage stability test should not be higher 2.5 C, indicating that there is no
Table 3 Effect of KC content on the properties of original asphalta KC content (%)
0 1 2.5 a
Softening point (C)
47.5 47.5 48.0
Viscosity at 135 C (Pa s)
0.30 0.30 0.31
Softening point (C)b Top
Bottom
47.5 48.0 49.0
47.5 48.2 48.8
ZH-90 base asphalt was used. Below is the same if not noted particularly. b Stored in an oven at 163 C for 48 h.
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457
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Fig. 3. Micrographs of original asphalt and 2.5% KC modified asphalt (optical microscopy). (a) Original asphalt (·400), (b) 2.5% KC modified asphalt (·400).
substantial phase separation (storage stability). Samples are also compared by optical microscopy. For true stability the top section of the sample should have the same phase distribution as the bottom section. The high-temperature storage stabilities of SEBS/KC compound modified asphalts were presented in Table 4. Obviously, for asphalts modified by SEBS in the absence of KC, the differences in the softening points and viscosities were large, indicating that the phase separation was serious. The SEBS modified asphalt was unstable even at 3% SEBS content. With increasing SEBS content, the phase separation became more obvious. When the SEBS content increased to 6%, SEBS seriously separated from the asphalt.
The asphalts modified with 4% SEBS was still unstable when the SEBS/KC ratio was 100/10 or 100/30. When the ratio was 100/50 or 100/70, the storage stability of SEBS/KC compound modified asphalts was improved significantly. However, the asphalt modified by directly adding SEBS and KC, it was unstable when the ratio of SEBS/KC was 100/50. So it can be concluded that SEBS/KC compounding is critical to obtain the high-temperature storage stability. When the SEBS content was 3%, 5% and 6%, the modified asphalts with high-temperature storage stability were obtained by incorporating SEBS/KC (100/50) compounds into asphalts respectively, as shown in Table 4.
Table 4 Effect of KC content on the high-temperature storage stabilities of SEBS/KC compounds modified asphaltsa Viscosity at 135 C (Pa s)
Formulation SEBS, % (w/w)
SEBS/KC (w/w)
Top
Bottom
3
100/0 100/50
0.825 0.642
0.600 0.658
4
100/0 100/10 100/30 100/50 100/50b 100/70
1.058 1.008 0.975 0.790 1.552 0.875
5
100/0 100/50
6
100/0 100/50 a b c d
gt =gb c
Softening point (C)
S t S b (C)d
Top
Bottom
1.38 0.98
53.0 52.5
50.0 52.8
3.0 0.3
0.742 0.751 0.800 0.825 1.210 0.883
1.43 1.34 1.22 0.96 1.28 0.99
57.0 56.5 55.0 55.5 59.0 52
53.8 50.8 52.0 55.8 52.0 52.5
3.2 5.7 3.0 0.3 7.0 0.5
2.083 1.117
0.992 1.192
2.10 0.94
70.5 57.0
58.0 58.0
12.5 1.0
2.783 1.558
1.258 1.550
2.21 1.01
85.0 59
67.5 60.5
17.5 0.5
ZH-90 base asphalt and 502 SEBS were used. Below is the same if not noted particularly. SEBS and KC added directly. gt/gb = Top/bottom ratio of viscosity. St Sb = Bottom softening point subtracted from top one.
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A continuous and direct approach to study the hightemperature storage stability of SEBS modified asphalts
was obtained by observing the morphology at high temperature as a function of time. The morphology of
Fig. 4. Morphology of SEBS modified asphalt before and after adding KC (SEBS/KC = 100/50) at 163 C for some time (A) 3% SEBS modified asphalt for 0 min; (B) 3% SEBS modified asphalt for 5 min; (C) 3% SEBS/KC (100/50) modified asphalt for 0 min; (D) 3% SEBS/KC (100/50) modified asphalt for 1 h; (E) 4% SEBS modified asphalt for 0 min; (F) 4% SEBS modified asphalt for 5 min; (G) 5% SEBS modified asphalt for 0 min; (H) 5% SEBS modified asphalt for 5 min; (I) 5% SEBS/KC (100/50) modified asphalt for 0 min; (J) 5% SEBS/KC (100/50) modified asphalt for 1 h; (K) 6% SEBS modified asphalt for 0 min; (L) 6% SEBS modified asphalt for 5 min; (M) 6% SEBS/KC (100/50) modified asphalt for 0 min; (N) 6% SEBS/KC (100/50) modified asphalt for 1 h.
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457
451
Fig 4. (continued)
SEBS/KC compound modified asphalts are shown in Fig. 4. The morphology of the modified asphalt depends mainly on the concentration of SEBS and the presence of KC. The modified asphalts with a low polymer content and without KC, as shown in Fig. 4(A, C, E), show polymer domains dispersed in asphalt. While the SEBS content increased to 5%, as shown in Fig. 4(G), the polymer had a tendency to become a continuous phase. When the SEBS content was 6% (Fig. 4(M)), the continuous phase of the modified asphalt changed to the polymer. The morphology of SEBS modified asphalt without KC changed quickly with time, independent of the content. The SEBS aggregated quickly to form coarse particles. Five minutes later, as shown in Fig. 4, the coarser particles of SEBS formed, indicating that the SEBS modified asphalts without KC were not stable when stored at high temperature. However, as for the SEBS/
KC compound modified asphalts, the morphology had changed little even set at high temperature for an hour, indicating their stability at high temperature. 3.2.2. Softening point and TSHRP The softening point and the corresponding temperature (TSHRP) at G*/sin(d) = 1 kPa according to Strategic Highway Research Program (SHRP) are usually used to characterize the high-temperature properties of asphalt [10]. The higher the softening point (or corresponding temperature), the better the properties at high temperature. Generally, polymer can enhance the high-temperature properties of asphalt. In this study, the result is in accordance with the general viewpoint, as shown in Table 5 and Fig. 5. With increasing SEBS content, the softening point and TSHRP of the modified asphalt become higher,
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Table 5 Effect of SEBS/KC on the high-temperature properties of asphalts SEBS content (%)
0 3 4 5 6
Softening point (C)
SHRP temperature (C) (G*/ sin(d) = 1 kPa)
Original SEBS
SEBS/KC (100/50)
Original SEBS
SEBS/KC (100/50)
47.5 51.5 55.0 64.5 83.0
47.5 50.0 53.5 56.0 62.5
n.d. 68.5 72.7 83.4 92.6
n.d. 68.2 72.5 78.1 86.2
n.d. = not determined. Below is the same if not noted particularly.
indicating that the improvement of high-temperature property of the modified asphalt. Moreover, the softening point and TSHRP increases significantly when the SEBS content increased to 5% or 6%. The phenomenon might be explained by the morphology of the modified asphalts. As for the asphalts with low SEBS content, the SEBS domain dispersed in the asphalt (as shown in Fig. 4(A and C)). With increasing the content to 5% or 6%, the SEBS had the tendency to become a continuous phase (as shown in Fig. 4(G)) and further formed a continuous phase (as shown in Fig. 4(N)).
The effect of KC on the high-temperature properties of SEBS modified asphalts is shown in Tables 5 and 6 and Fig. 5. As for the asphalt modified by SEBS with a low content (3% and 4%), the introduction of KC slightly decreases the softening point and TSHRP, as shown in Table 5, which indicated that the KC had a little effect on the high-temperature properties of the SEBS modified asphalts. For further study the effect of KC on the high-temperature properties of low content SEBS modified asphalt, 4% SEBS modified asphalts with different KC content were prepared. The high-temperature properties of 4% SEBS modified asphalts with different ratio of SEBS/KC are shown in Table 6 and Fig. 5. As can be seen, KC had a slight influence on the high-temperature properties of 4% SEBS modified asphalt However, with increasing SEBS content to 5% and 6%, the high-temperature properties of the modified asphalts became more complicated. The introduction of KC worsened the high-temperature properties of the SEBS modified asphalts, as shown in Table 5. There would be a great difference of high-temperature properties between original SEBS modified asphalts and the SEBS/KC compound modified asphalt, which agree with the morphology of the modified asphalt, as shown in Fig. 4(G–N). There would be a possible explanation as follows. Because KC has many active functional groups on its surface [13], there will be rather strong
4% SEBS 4% SEBS/KC (100/30) 5
10
4% SEBS/KC (100/50) 4% SEBS/KC (100/70)
4
G*/sin(δ), Pa
10
G */sin(δ)=1000 Pa
3
10
2
10
30
40
50
60
70
80
90
100
110
o
Temperature, C
Fig. 5. Curve of G*/sin(d) versus temperature for 4% SEBS modified asphalt with different ratio of SEBS/KC.
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457 Table 6 Effect of KC content on the high-temperature properties of 4% SEBS/KC compound modified asphalt Property
Formulation SEBS/KC ratio (w/w)
Softening point (C) SHRP temperature (C)
0/0
100/10
100/30
100/50
100/70
55.0
53.0
54.0
53.5
53.5
72.7
n.d.
72.5
72.5
72.1
n.d. = not determined. Below is the same if not noted particularly.
interaction between SEBS and KC when blending them. After mixing asphalt and the SEBS/KC compound, the KC hindered the forming of the network of SEBS, while it would form in original SEBS modified asphalt with high content (5% and 6%), which contribute to the enhanced high-temperature properties. 3.2.3. Viscosity, needle penetration and low temperature ductility The effect of KC on the viscosity, needle penetration and low temperature ductility of the modified asphalt was shown in Table 7. As for the original SEBS modified asphalts, viscosity and ductility of the asphalt became higher, but the needle penetration became smaller with increasing SEBS content. The influence of KC on the viscosity, needle penetration and ductility of the SEBS modified asphalts was dependant on the SEBS content. When the SEBS content was low (3%, 4% and 5%), the KC had a little influence on them. When the SEBS content was 6%, the viscosity and ductility of the modified asphalt decreases significantly, and the needle penetration decreases to some extent. The phenomenon is in accordance with the change of morphology of the SEBS modified asphalts with introduction of the KC. Table 7 Effect of KC on the viscosity, needle penetration and ductility of asphalt SEBS, % (w/w)
SEBS/KC, (w/w)
Viscosity at 135 C (Pa s)
Needle penetration, 25 C (dm m)
Ductility, cm (5 C)
3
100/0 100/50 100/0 100/10 100/30 100/50 100/70 100/0 100/50 100/0 100/50
683 650 875 900 842 850 867 1292 1225 1742 1442
58 57 56 55 55 54 52 50 48 45 42
10 9 12 10 11 11 10 14 13 18 14
4
5 6
453
3.2.4. Dynamic viscoelastic parameters The most significant effect of polymers on asphalt is the improvement of elasticity. There is a strong correlation between rutting resistance at high temperature and complex modulus. Increasing complex modulus (elastic modulus) is to be expected because it reflects a promising rutting resistance at high temperature. Isochronal plots of complex modulus (G*) versus temperature at 10 rad/s for SEBS modified asphalts are shown in Fig. 6. For original SEBS modifies asphalt, the isochronal plots show a difference between the base asphalt and the SEBS modified asphalt, particularly at upper and lower ends of the temperature range. Although there is only minor increase in G* at low temperatures due to SEBS modification, there is considerable evidence of extreme polymeric modification at high temperatures. With increasing SEBS content, there is a significant increase in G* over entire temperature range especially for 5% or 6% SEBS modified asphalt, which is an evidence of dominant polymer network. Phase angle isochrones at 10 rad/s for the SEBS modified asphalts are presented in Fig. 7. Measurement of tan d is generally considered to be more sensitive to the chemical and physical structure than complex modulus for the modification of asphalts. The phase angle isochrones clearly illustrate the improved elastic response (reduced phase angles) of the modified asphalts compared to the base asphalt. Whereas the phase angles of the base asphalt approach 90 and therefore predominantly show viscous behavior with increasing temperature. SEBS significantly improve the elastic response at high temperature, which can be attributed to the viscosity of the base asphalt being low enough to allow the elastic network of the polymer to influence the mechanical properties of the modified asphalt. The plateau regions and decreased phase angles at intermediate and high temperatures is synonymies with the plateau region defined for polymers and demonstrates the ability of the polymer to form a continuous elastic network especially for SEBS (high content) modified asphalt. The presence and nature of the plateau and polymer network is a function of the chemical and physical properties of the SEBS polymer and base asphalt. The isochronal plots of G* and tan d indicate the degree of SEBS modification at high temperatures (above 50 C). Compared to original SEBS modified asphalts, the isochronal plots of SEBS/KC compound modified asphalts show a similar tendency with increasing the SEBS content, as shown in Figs. 6–9. What is noticeable is the considerable difference in the extent of the modification between the original SEBS and the SEBS/KC compound. For 3% and 4% SEBS/KC compound modified asphalts, the introduction of the KC had a little influence on the rheological properties of SEBS modified asphalt, except for a little viscous properties as compared
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10
4
G*, Pa
10
5
10
10
1
Base asphalt
2
3% SEBS
3
3% SEBS/KC (100/50)
4
4% SEBS
5
4% SEBS/KC (100/50)
6
5% SEBS
7
5% SEBS/KC (100/50)
8
6% SEBS
9
6% SEBS/KC (100/50)
8
3
9 6 4 7 5 2 3 1
2
20
30
40
50
60
70
80
90
100
110
o
Temperature, C
Fig. 6. Isochronal plots of G* versus temperature at 10 rad/s for asphalts modified by different content of SEBS/KC compound.
Base asphalt 3 % SEBS 3 % SEBS/KC (100/50) 4 % SEBS 4 % SEBS/KC (100/50) 5 % SEBS 5 % SEBS/KC (100/50) 6 % SEBS 6% SEBS/KC (100/50)
70
60
50
Tan(δ)
40
30
20
10
0
20
30
40
50
60
70
80
90
100
110
o
Temperature, C
Fig. 7. Isochronal plots of tan(d) versus temperature at 10 rad/s for asphalts modified by different content of SEBS/KC compound.
with original SEBS modified asphalt. WhatÕs more, for 4% SEBS modified asphalts, the isochronal plots of complex modulus nearly intersected as shown in Figs. 8 and 9, indicating that KC content nearly had little
influence on the rheological properties of SEBS/KC compound asphalts. However, increasing the SEBS content to 5% above, the KC had a significantly effect on the rheological properties of SEBS modified asphalts. The
C. Ouyang et al. / European Polymer Journal 42 (2006) 446–457
455
Asphalt ZH90# 4% SEBS 4% SEBS/KC (100/30) 4% SEBS/KC (100/50)
5
10
4% SEBS/KC (100/70)
4
G*, Pa.s
10
3
10
2
10
20
30
40
50
60
70
80
90
100
110
Temperature,oC
Fig. 8. Isochronal plots of G* versus temperature at 10 rad/s for asphalts modified by 4% SEBS/KC compound with different ratios.
70
Asphalt, ZH90# 4 % SEBS
60
4 % SEBS/KC (100/30) 4 % SEBS/KC (100/50) 50
4 % SEBS/KC (100/70)
Tan (δ)
40
30
20
10
0
20
30
40
50
60
70
80
90
100
110
Temperature,oC
Fig. 9. Isochronal plots of tan(d) versus temperature at 10 rad/s for asphalts modified by 4% SEBS/KC compound with different ratios.
isochronal plots of 5% or 6% SEBS/KC compound modified asphalt show a little sharp tendency as compared with corresponding SEBS (same content) modified asphalt, indicating that SEBS modified asphalt
had more obvious elastic response. The differences in rheological properties might be attributed to the morphologies of the modified asphalts. The introduction of the KC might hinder the forming of network of SEBS
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domain in asphalt matrix, leading to the elastic worsening of the asphalt.
Table 8 Hildebrand solubility parameters of asphalt components and of blocks in the SBS and SEBS copolymer
3.3. Storage-stable mechanism
Asphalt component
d (cal1/2/cm3/2)
Polymer
d (cal1/2/cm3/2)
Based on adsorption–desorption methods, asphalt can be separated into four components: asphaltenes, polar aromatics, naphthene aromatics and saturates. Each class has different characteristic solubility parameter values as listed in Table 2. Asphalt is traditionally considered as a dynamic colloid system consisting of a suspension of high molecular weight asphaltene micelles dispersed in a lower molecular weight oily medium (maltenes) [14]. Asphalts represent an extreme example of a complex block copolymer additive. Each of the numerous constituents will be distributed in varying concentrations between the microphase separated domains and macrophase separated regions. Asphaltene possesses the highest molecular weight fractions in asphalt and have very large solubility parameter values; maltene is composed of smaller molecular weight fractions and has solubility parameter values comparable to those of S and EB (see Table 8) [15–17]. Therefore, macrophase separation in the asphalt/SEBS mixture will occur even when the SEBS content is decreased to 3%, though the separation is determined by the ratio of asphaltene to SEBS [8]. Block copolymers can interact with additives, such as inorganic fillers or solvents or homopolymers, in several ways. If the mixing thermodynamics are governed by an unfavorable heat of mixing (DH > 0), macroscopic phase separation will occur. However, if one of the blocks is compatible with additive, favorable swelling may result, which can induce microstructural changes because the effective volume fraction and interfacial tension will be altered [18–20]. KC as a kind of inorganic filler might partly miscible with the blocks of the SEBS and asphalt and there can lead to microstructural change of the modified asphalt. In the study of the compatibility between asphalt oligomers and SEBS, Ho and the collaborator made a thorough study by means of
Asphaltene Saturates Naphthene aromatics Polar aromatics
12.2–16.1 8.5–9.8 9.3–11.0
PS PB P(EB)
9.1–9.7 8.1–8.6 7.8–8.1
10.7–13.0
Maltene consists of polar aromatics, naphthene aromatics, and polar aromatics.
transmission electron microscopy (TEM), small angle X-ray scattering (SAXA), dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC) [8]. The experiments indicated that asphaltene is essentially immiscible with both blocks of SEBS, while maltene is miscible with SEBS. So the introduction of KC mainly influenced the compatibility between SEBS and asphaltene under the interaction of SEBS and KC by means of premixing technique, consequently improve the compatibility between SEBS and asphalt. Moreover, KC hindered the forming of network of SEBS in the modified asphalts at the interface and decreased the high-temperature properties. On the other hand, SEBS modified asphalt can be considered as a suspended system. For a suspended system, the particles in the liquid with the buoyancy force and gravitational force, and the falling velocity of the particles in the system follow StokeÕs law [7]: V ¼ 2ðq0 q1 Þgr2 =9g
ð1Þ
where q0 is the density of asphalt, q1 is the density of SEBS, g is the gravitational force constant, r is the average radius of the SEBS particles and g is the viscosity of the modified asphalt. To prevent the phase separation of SEBS from asphalt, a critical way is to reduce the falling velocity of the particles. As shown in Eq. (1), there are two methods
Table 9 High-temperature storage stability of 4% SEBS/KC modified asphalt with proper ratio Viscosity at 135 C (Pa s)
Formulation Asphalt/SEBS
SEBS/KC, (w/w)
Top
Bottom
ZH-70/502
100/0 100/60 100/0 100/60 100/0 100/40
1.125 0.830 1.058 0.810 2.783 1.538
0.730 0.845 0.742 0.808 1.258 1.550
ZH-90/3150 ZH-90/3151 a b
gt/gb = Top/bottom ratio of viscosity. St Sb = Bottom softening point subtracted from top one.
gt =gb a
1.54 0.98 1.43 0.99 2.21 0.99
Softening point (C) Top
Bottom
59.0 56.5 57.0 54.5 69.0 61.0
54.0 56.2 53.8 54.5 51.0 60.5
S t S b (C)b
5.0 0.3 3.2 0.0 18.0 0.5
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to do this, one is to reduce the particle size, the other is to decrease the density difference. The density of the SEBS was 0.93 g/cm3 and the density of asphalt here was 1.02 g/cm3 at room temperature. The density difference of the SEBS and asphalt became larger at high temperatures because the SEBS swelled in the oily fraction of the asphalt [16,17]. The density of KC is around 2.57 g/cm3 at room temperature. When the KC attached to SEBS, the density difference is decreased and the force for driving separation becomes zero at a certain content of KC, so the high-temperature storage stability is improved. Therefore, the improvement of high-temperature storage stability of the asphalt modified by SEBS maybe originates from two sources: one is KC mainly influenced the compatibility between SEBS and asphaltene, the other is KC decreased the density difference between asphalt and SEBS. To verify the effectiveness of the assumption above, ZH-70 base asphalt with higher content of asphaltenes and two other SEBSs named as 3150 and 3151 were selected to prepare the modified asphalts. By selecting the proper ratio of SEBS and KC, the stable system at high temperature was obtained, as shown in Table 9.
4. Conclusions The rheological and high-temperature properties of the road asphalts can be improved by means of SEBS polymer and SEBS/KC compound modification. With increasing SEBS content, the extent of polymer modification has differed depending on the polymer content and the presence of KC. When the SEBS content was 6%, the properties of SEBS modified asphalt were improved significantly. KC with a proper amount added can dramatically improve the compatibility between SEBS and base asphalt by means of premixing SEBS and KC, which might be explained that KC hindered the coalescence of SEBS from asphalt matrix at high temperature and decreased density deference between asphalt and SBS. However, the introduction of KC can influence the properties of SEBS modified asphalt. The extent influence of KC differed depending on the SEBS content. When the content was low (such as 3% or 4%), the properties including high-temperature, mechanical, rheological properties and morphology had a little change. However, increasing SEBS content to 5% or 6%, the KC had a dramatic influence on the properties asphalts. The morphology of SEBS/KC modified asphalt showed SEBS dispersed in asphalt while that of original SEBS modified asphalt showed that the SEBS network nearly formed, especially when SEBS
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content was 6%. The worsened elastic properties of SEBS modified asphalts were demonstrated using rheological parameters of complex modulus and phase angle. The worsening properties might be attributed to the introduction of the KC, which hindered the forming of SEBS network and decreased the high-temperature properties of SEBS modified asphalt.
Acknowledgements The authors acknowledge the financial support provided by the National High Technology Research and Development Program of China (863 Program), and the registered number is 2002AA335100.
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