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Investigation of influence factors on low temperature properties of SBS modified asphalt
Construction and Building Materials 154 (2017) 609–622
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation of influence factors on low temperature properties of SBS modified asphalt Peng Lin, Weidong Huang ⇑, Yi Li, Naipeng Tang, Feipeng Xiao The Key Laboratory of Road and Traffic Engineering, Ministry of Education, Tongji University, Shanghai 201804, PR China
h i g h l i g h t s Show the key factors influencing low temperature properties of binder. Optimum modification scheme for SBS modified asphalt is investigated. Component distribution, thermal behavior and FTIR spectrum are presented. Rubber processing oil can supplement the loss of maltene fraction. Evaluate SBS modified asphalt mixture with TSRST test.
a r t i c l e
i n f o
Article history: Received 12 March 2017 Received in revised form 16 June 2017 Accepted 19 June 2017
Keywords: SBS Polymer type Sulphur Rubber processing oil DSC TSRST
a b s t r a c t Styrene-butadiene-styrene (SBS) is widely used in asphalt modification and the investigations about SBS modified asphalt are focused on high temperature property, storage stability and the compatibility between SBS and asphalt. While the study on influencing factors on low temperature properties are not sufficient yet. In this paper, a systematical research was conducted to evaluate the influencing level of important factors such as SBS polymer type, SBS content, sulfur content and addition of rubber processing oil. Firstly, bending beam rheometer (BBR) test associated with Burger’s model was used to evaluate the low temperature property and relaxation capacity. Furthermore, FTIR, GPC and DSC tests were conducted to investigate the chemical changes in SBS modified asphalt. At last, the low temperature performance of SBS modified asphalt mixture was investigated with thermal stress restrained specimen test (TSRST). In this paper, the optimum modification scheme of SBS modified asphalt binder and mixture was investigated and summarized. SBS polymer is swollen by saturates and aromatics which made the base binder lack of maltene fraction. While the further addition of rubber processing oil supplemented the maltenes which improved the low temperature property. Ó 2017 Published by Elsevier Ltd.
1. Introduction The history of asphalt modification to achieve better performance in road engineering is long and styrene-butadienestyrene (SBS) is one of most outstanding modifier widely used all around the world. When SBS copolymers are mixed in asphalt, SBS is swollen by the maltenes in base asphalt. The system gradually becomes a biphasic microstructure where polymer-rich phase formed by maltenes-swollen-polymer is dispersed in an asphaltene-rich phase [1–4]. As literature indicates, base binder can be divided into four fractions, saturates, aromatics, resins and asphaltenes, which is referred as SARA fractions [5–9]. As the maltenes, mainly ⇑ Corresponding author. E-mail address: [email protected] (W. Huang). http://dx.doi.org/10.1016/j.conbuildmat.2017.06.118 0950-0618/Ó 2017 Published by Elsevier Ltd.
composed with saturates and aromatics, is absorbed by SBS copolymer, the base binder will become lack of maltenes. The compatibility between asphalt and SBS has been identified as a critical factor which has a significant influence on the rheological properties and morphology of SBS modified asphalt [10,11]. Literatures indicates that, asphalt with high aromatic content compared to asphalt with low aromatic content is easier to obtain a compatible state and a better low temperature performance [12–14]. In 1982, Kraus showed that SBS is compatible with asphalt with high aromatic content and incompatible with a highly paraffinic content asphalt [12]. Wloczysiak et al. showed that asphalt with 70–80 wt% aromatic not only caused PS to swell but also caused antiplasiticization of PS, an increase in its glass transition temperature [15]. Rubber processing oil, rich in aromatic and saturates fractions, is used as a component in manufacturing of rubber product [16–20]. Addition of rubber processing oil has a profound effect in
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improving the compatibility and low temperature performance of SBS modified asphalt while the investigation is not sufficient. In the system of SBS modified asphalt, sulfur is usually used as a cross-linking agent during the preparation process to increase the storage stability. The sulfur reacts with the alkene moieties presents in SBS and creates chemical crosslinks which help to stabilize the system [21–25]. In 2002, Wen studied rheological characterization of storage-stable SBS modified asphalt with phase angle and complex modulus. He also studied the morphological changes of storage-stable SBS/sulfur modified asphalt [26,27]. Zhang et al. investigated some other important properties of SBS/sulfur modified asphalt including dynamic viscosity, thermal stability [28,29]. However, the influence of sulfur on the low temperature properties of SBS modified asphalt is not clear yet. In this paper, low temperature performance of SBS modified asphalt is studied systematically by evaluating the influence of important factors such as SBS type, SBS content, sulfur content and rubber processing oil content.
the modified asphalt composition is present in Table 1. It should be noted that, for more conveniently contrast in figures, 4.5L and 4.5L_0.15S are two names of the same kind of SBS modified asphalt binder. Similarly, 4.5R and 4.5R_0.15S are the same kind of asphalt binder too. SBS modified asphalt samples were prepared as follows. Firstly, the SBS was added to base binder and sheared for 30 min at 180 °C with a shear speed of 4000 r/min. Secondly, the blend was stirred for 60 min using mechanical stirrer at 180 °C. Thirdly, sulfur was added to the blend and stirred for 90 min at 180 °C. At last, the rubber processing oil was added to the blend and stirred for another 60 min at 180 °C. The prepared binders satisfied the storage stability test. All samples were performed RTFO aging procedure at the end of samples blending to make sure there was no phase separation in the samples. This is especially important for samples without cross-linking agent. After the RTFO aging procedure, samples were performed PAV aging procedure [24]. 3.2. Bending beam rheometer (BBR) test Creep test was carried out at different low temperatures (12, 18 and 24 °C) using a bending beam rheometer (BBR) according to ASTM D6648 [30]. The stiffness (S) and the creep rate (m-value) of the sample beam were determined. All combinations were replicated three times for reliability reasons. 3.3. Burgers modelling
2. Objectives 1. To evaluate the influencing level of SBS type, SBS content, sulfur content and rubber processing content on the low temperature property of SBS modified asphalt binder. 2. To find out the optimum modification scheme of SBS modified asphalt for low temperature property. 3. To track the chemical changes in SBS modified asphalt through GPC test, FTIR test and DSC test. 4. To evaluate the low temperature performance of SBS modified mixture with TSRST test.
Compared with Kelvin model and Maxwell model, Burger’s model is better in characterizing viscoelastic property of asphalt binder. Thus Burger’s model was used for further investigation of low temperature of the samples. It is made of two components, springs are associated with storage and dashpots are associated with dissipation of deformation energy. Therefore, the parameters of Burger’s model is able to indicate the material’s capacity for energy dissipation. So, the burger’s model was used to calculate the dissipated and stored energy for each BBR test. According to the original data obtained from BBR software, the flexural creep compliance D(t) can be calculated with the following equation. 3
DðtÞ ¼
4bh dðtÞ
ð1Þ
PL3
where D(t) is the flexural creep compliance at time t; P is the applied constant load; L is the distance between supports; b is the width of specimen; h is the depth of specimen; d(t) is the deflection in middle-span of specimen at time t. The creep compliance can be expressed by Burgers model [31] using the following equation:
3. Materials and methods 3.1. Materials and preparation One base binder provided by a company (E70, PG 64-16) was used in this study. The linear styrene-butadienestyrene (SBS) polymer has average molecule weight of w = 110,000), containing 30%wt styrene. The radial styrene–buta 110,000 g/mol (M dienestyrene (SBS) has average molecule weight of 230,000 g/mol, containing 30wt% of styrene. The amount of SBS modifier was ranged from 3.0% to 6.0% by the weight of base binder. Sulfur is commonly used as a cross-linking agent to enhance the storage stability of polymer modified asphalt. The reaction between SBS and sulfur is similar to that of vulcanization in rubber industry [23,26]. In this study, the sulfur was used as a cross-linking agent and the amount of sulfur was ranged from 0% to 0.25%. Rubber processing oil, commonly utilized in rubber industry, has similar chemical components with saturates and aromatics fraction in base binder. The content of rubber processing oil was ranged from 0% to 8%. The detail of
DðtÞ ¼
E 1 t 1 2t þ þ 1 e g2 E1 g1 E2
ð2Þ
where E1 is the instantaneous elastic modulus; g1 is the viscous coefficient; E2 and g2 are viscoelastic indicators. To estimate the nonlinear fitting parameters of the model, Excel SOLVER tool was used with method of least squares. The four Burgers parameters were obtained according to the modeling results [31]. The parameters obtained from Burgers model could be calculate as follows:
k¼
g1
ð3Þ
E1
Table 1 Description of asphalt composition. Bitumen Category
Linear SBS (%)
Radial SBS (%)
Rubber processing Oil(%)
Sulfur(%)
E70 (Base Binder) 3L 4.5L 6L 3R 4.5R 6R 4.5L_0S 4.5L_0.15S 4.5L_0.25S 4.5R_0S 4.5R_0.15S 4.5R_0.25S 4.5L_4R 4.5L_8R 4R 8R
0 3 4.5 6 0 0 0 4.5 4.5 4.5 0 0 0 4.5 4.5 0 0
0 0 0 0 3 4.5 6 0 0 0 4.5 4.5 4.5 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 4 8 4 8
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0 0.15 0.25 0 0.15 0.15 0.15 0.15 0 0
Note: (1) 4.5L and 4.5L_0.15S are the same kind of modified asphalt binder. (2) 4.5R and 4.5R_0.15S are the same kind of modified asphalt binder. (3) E70 stands for Esso70 #, is the base binder.
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Aromatic index : IAR ¼ AR1600 =
X
ð9Þ
ARv~ P
The sum of the area represents: ARv~ ¼ AR1700 þ AR1600 þ AR1460 þ AR1310 þ AR1030 þ AR1030 þ AR965 þ AR864 þ AR814 þ AR743 þ AR725 þ AR700 3.5. Solubility test and gel permeation chromatography (GPC) test About 20 mg virgin binder sample was dissolved with tetrahydrofuran (THF) in a 10 mL volumetric flask for 24 h before solubility test and GPC test. The solution was filtered through a 0.45 lm PTFE filter and collected in a 0.5 mL centrifugal tube for GPC test. The remaining solution in the volumetric flask was accordingly filtrated through the weighted filter. After that, about 5 mL clean THF was filtrated through the filter until the filtrated solution was colourless. The filter was kept in a vacuum oven at 35 °C for about 12 h until the retained THF was fully volatilized. Then the filter was weighted again and the increase weight of the filer can be used to describe the weight of insoluble component in the asphalt. The solubility was calculated using as follows:
Solubility % ¼
Fig. 1. Fractions of the chromatogram based on molecular weight.
W s ðtÞ ¼ r20
E 2E 1 1 2t 2t þ 1 2e g2 þ e g2 E1 2E2
W d ðtÞ ¼ r20
t
g1
þ
ð4Þ
E 1 2t 1 e g2 2E2
ð5Þ
W d ðtÞ W s ðtÞ
ð6Þ
Dissipation energy ratio ¼
where kðsÞ is relaxation time, W s ðtÞðMPaÞ is stored energy per volume, W d ðtÞðMPaÞ is dissipated energy per volume, t(s) is the loading time of the test, and r0 (MPa) is the stress at the mid-span. It can be seen from Eqs. (4) and (5) that stored energy and dissipated energy were functions of time t. Considering stiffness and m-value calculated at 60 s, dissipation energy ratio was calculated at t = 60 s. 3.4. Attenuated Total Reflection (ATR) Fourier Transform Infrared (FT-IR) Spectroscopy The infrared spectra values were collected using a Bruker TENSOR FT-IR spectrometer equipped with a reflection diamond ATR accessory. To quantify the change in IR absorption, band areas rather than peak absorbance value were used. Three replicates for each binder sample was tested to ensure that the variability was P small. The ARv~ values were normalized to the total sum of all band areas ( ARv~ ), and some of the indices were computed as follows [32,33]:
Polystyrene index : IPS ¼ AR700 =
X
Polybutadiene index : IPB ¼ AR965 =
ð7Þ
ARv~
X
ð8Þ
ARv~
W binder W filter after filtration W filter before filtration 100% W binder
Waters 1515 High-Pressure Liquid Chromatography (HPLC) Pump and Waters 2414 Refractive Index (RI) detector were used to perform GPC test. THF (HPLC grade) was selected as mobile phase solvents. A combination of three columns was used for separating constituents of asphalt binder by molecular size. The calibration curve was built with ShodexÒ Polystyrene Standards in order to convert the retention time to molecular weight. The chromatogram was divided into three slices based on the molecular weight of eluting species [34]. The three fractions were dissolved polymers (molecular weight greater than 19,000), apparent asphaltenes (molecular weight from 19,000 to 3000) and maltenes (molecular weight less than 3000) [34], as shown in Fig. 1. The contents of insoluble components, dissolved polymers, apparent asphaltenes and maltenes were calculated accordingly based on Eqs. (5)–(8). Two replicates were used for both of solubility test and GPC test and the average values were reported.
Insoluble Components % ¼ 1 Solubility% Dissolv ed Polymers % ¼
Areadissolv ed polymers Solubility% Total area under chromatogram
Apparent Asphaltenes % ¼
Maltenes % ¼
Areaapparent asphaltenes Solubility % Total area under chromatogram
Areamaltenes Solubility % Total area under chromatogram
ð12Þ
ð13Þ
ð14Þ
A Netzsch DSC 204 F1 Differential Scanning Calorimeter was used to determine the glass transition temperature (Tg). The instrument was calibrated with n-octane, indium and zinc and the measurements were conducted under nitrogen atmosphere. In all the cases, pans of aluminium of 40 ll were used and the weight of
SBS Type and Sulfur Content
500
Stiffness at -18 C (MPa)
ð11Þ
3.6. Differential Scanning Calorimetry test (DSC)
550 SBS Type and SBS Content
ð10Þ
450 400 350 300 250 200
Fig. 2. Stiffness of asphalt binders at 18 °C.
Rubber Processing Oil
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the samples were kept in the range of 5–10 mg. The heating scans were recorded from 70 °C to 100 °C at a heating rate of 10 °C min1. Tg was obtained from the inflection point of the curves corresponding to the first scan.
3.7. Thermal Stress Restrained Specimen Test (TSRST) TSRST was carried out according to the procedure of AASHTO TP 10-93 [35]. The tests were performed in the MTS apparatus. The 50 50 250 mm rectangular prisms cut from compacted slabs were used for testing. The specimens were placed in the temperature chamber and conditioned for two hours at +5 °C. During the test the temperature was lowered at a rate of 10 °C/h. Three replicates were conducted for each asphalt mixture.
4. Result and discussion 4.1. Low temperature properties analysis The low temperature rheological properties of SBS modified asphalt binders were evaluated with BBR test results. The Stiffness and m-value of PAV aged samples at 18 °C were illustrated in Figs. 2 and 3. It was be found in Fig. 2 that stiffness values of both linear and radial SBS modified asphalt declined with an increase of SBS content. When the SBS content is the same, the stiffness values of linear SBS modified asphalt and radial SBS modified asphalt were nearly the same. The stiffness values of SBS modified asphalt are lower than that of base binder, indicating SBS modifier had a positive effect on the stiffness of asphalt binder at low temperature, while the type of SBS had little influence. For the samples with different content of sulfur, the samples with 0.15% content of sulfur had lower stiffness than those of samples with 0% or 0.25% content of sulfur. Sulfur could trigger the cross-linking reaction and the polymer network reduced the possibility of segregation. However, an excessive amount of crosslinking agent could also decrease the flow ability which led to an increase of stiffness. The results implied that, the 0.15% was the optimum content of sulfur for SBS modified asphalt when SBS content was 4.5%. When rubber processing oil was added to base binder and 4.5% linear SBS modified asphalt, the stiffness values decreased sharply, especially when rubber processing oil content was 8%. The results indicated that rubber processing oil had a positive effect on the stiffness. It can be explained as the rubber processing oil is mainly composed of aromatic and aliphatic components, which can improve the flow ability of asphalt binder at low temperature.
The creep rate (m-value) is an important parameter to evaluate the creep ability of asphalt binder at low temperature. As shown in Fig. 3, the linear and radial SBS modified asphalt had higher mvalues than those of E70 (base binder), and the m-values increased along with the increase of SBS content. However, there was no significant difference between linear SBS modified asphalt and radial SBS modified asphalt with the same content of SBS. It indicated that both linear and radial SBS had positive influence on the creep ability of asphalt binder. In terms of m-value, the optimum content of sulfur for 4.5% linear SBS modified asphalt was 0.25%. While for radial SBS modified asphalt, optimum sulfur content was 0.15%. The reason may be the shapes of linear SBS and radial SBS are different, and the lumpshaped macromolecular compound of radial SBS needs less content of sulfur to form the cross-linking structure. As shown in Fig. 3, m-value of 4.5L_8R was 0.06 higher than that of 4.5L and m-value of 8R was only 0.04 higher than that of base binder. It indicated that adding rubber processing oil to linear modified asphalt was more efficient than adding rubber processing oil to base binder in improving m-value. Refferred to ASTM D6648 [30], PG low temperatures were calculated according to stiffness (S = 300 MPa) and m-value (m = 0.3). As illustrated in Fig. 4, when SBS content is 3.0% and 4.5%, the PG low temperature of radial SBS modified aspahlt was lower than that of linear SBS modified apshalt. However the PG low temperaute of 6% linear SBS modified asphalt was nearly the same with that of 6% radial SBS modified aspahlt. Considering the SBS content used in practical engineering is usually from 3% to 5%, it indicated the low temperautre performance of radial SBS modified asphalt was better than that of linear SBS modfied aspaht. Sulfur content had an significant influence on the PG low temerpature of SBS modfied asphalt. The optimum content of sulfur for linear SBS modified asphalt was 0.25%, while the optimum content of sulfur for radial SBS modfied aspahlt was 0.15%. In summary, the low temperature properties of SBS modified asphalt were mainly controlled by two key factors, polymer and maltenes. Both the addition of maltenes (rubber processing oil) and SBS polymer can improve the low temperautre properties. If the two factors are independent, the addition order of polymer and rubber processing oil will not influence the PG low temperautre. Compared with base asphalt, if 8% content of rubber processing oil was added first and 4.5% linear SBS polymer was added later, PG low temperature decrease was 2.9 °C and 2.2 °C. However, when linear SBS polymer was added first and
0.36 SBS Type and SBS Content
SBS Type and Sulfur Content
0.34
m-value at -18 C
0.32 0.3 0.28 0.26 0.24 0.22 0.2
Fig. 3. M-value of asphalt binders at 18 °C.
Rubber Processing Oil
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-20.0
PG Low Temperature ( C )
-21.0 -22.0 -23.0 -24.0 -25.0 -26.0 -27.0 -28.0 -29.0 -30.0
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
Fig. 4. PG low temperature of SBS modified asphalt.
PG Low Temperature Difference ( C )
2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
Fig. 5. PG low temperature difference of SBS modified asphalt.
rubber processing oil was added later, the decrease of PG low temperature was 1.3 °C and 3.8 °C. The difference was caused by the negative effect of lack of maltenes when SBS polymer was added first. Furthermore, the increase of PG low temperature can be roughly calculated as 0.9 °C. The computing process are as follows.
T maltenes first ¼ T 8R T E70 ¼ 26:4 ð23:5Þ ¼ 2:9 ¼ T maltnes T polymer later ¼ T 4:5L
8R
ð15Þ
T 8R ¼ 28:6 ð26:4Þ ¼ 2:2 ¼ T polymer ð16Þ
T polymer first ¼ T 4:5L T E70 ¼ 24:8 ð23:5Þ ¼ 1:3 ¼ T polymer T maltnes lack T maltnes later ¼ T 4:5L
8R
ð17Þ
T 4:5L ¼ 28:6 ð24:8Þ ¼ 3:8
¼ T maltnes lack þ T polymer
ð18Þ
T maltenes lack ¼ ðT polymer þ T polymer lack Þ ðT polymer Þ ¼ 1:3 ð2:2Þ ¼ 0:9 C
ð19Þ
As we know, PG low temperature calculated according to mvalues (Tm) and PG low temperature calculated according to stiffness (Ts) are different. To find out the key factor of low temperature properties of SBS modified asphalt, temperature difference ðDT c ¼ T s T m Þ was calculated, T s stands for the PG low temperature calculated according to stiffness and T m stands for the PG low temperature calculated according to m-value. If the DT c > 0; the binder is more likely to be damaged due to high stiffness value (stiffness controlled asphalt) and if DT c < 0, the binder is more likely to be damaged as the low m-value (m-value controlled asphalt). As shown in Fig. 5, DT c values of SBS modified asphalt were below 0 °C indicating the SBS modified asphalt were m-value controlled and the main problem of SBS modified asphalt were lack of creep capacity. The DT c values of radial SBS modified asphalt were
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280.0 260.0
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
Relaxation time (s)
240.0 220.0 200.0 180.0 160.0 140.0 120.0 100.0
Fig. 6. Relaxation time of SBS modified asphalt binders.
Dissipation Energy Ratio (%)
80.0 75.0
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
70.0 65.0 60.0 55.0 50.0 45.0 40.0
Fig. 7. Dissipation energy ratio of SBS modified asphalt binders.
relatively closer to 0 °C than those of linear SBS modified asphalt, indicating the stiffness and the m-value of radial SBS modified asphalt were more balanced. Content of sulfur also had a significant influence on DT c of SBS modified asphalt. When content of sulfur increased, DT c of linear and radial SBS modified asphalt binders tended to be zero, revealing that the addition of sulfur promoted the balance between stiffness and m-value. It should be noted that if radial SBS modified was produced without sulfur, it would be seriously lack of m-value. As we can see in Figs. 2 and 3, rubber processing oil can not only improve the stiffness of SBS modified asphalt binder and base binder, but also improve its m-value. It was seen in Fig. 5, the improvement in m-value was more significant which made it became stiffness controlled binders. The addition of rubber processing oil can solve the problem of lack m-value for SBS modified asphalt. Relaxation time was calculated with the Burger’s parameters according to Eq. (3) (k ¼ g1 =E1 ), which is an important parameter reflecting the stress relaxation capacity of material. The shorter relaxation time is, the more rapid the stress dissipation is [36]. Both m-value and relaxation time are parameters describing the relaxation capacity, while relaxation time has clearer physical meaning.
As shown in Fig. 6, relaxation time decreased along with the addition of SBS, implying adding SBS was able to promote the relaxation capacity. When SBS content is the same, relaxation time of linear SBS modified asphalt is lower than those of radial SBS modified asphalt. It demonstrated that linear SBS modified asphalt had better relaxation capacity and it might be due to the linear structure of linear SBS polymer. Sulfur and rubber processing oil also had significant influence on the relaxation time. Both for linear and radial SBS modified asphalt, if sulfur was not added during producing, the relaxation time would be very high. It indicated that sulfur was important for the relaxation capacity of SBS modified asphalt binders. Addition of Rubber processing oil also could significantly improve the relaxation capacity, especially for SBS modified asphalt binders. This phenomenon can be explained as the rubber processing oil supplements the light components in the base binder which was absorbed by the SBS polymer. Dissipation Energy Ratio (DER) was calculated with Burger’s model parameters according to Eq. (5), which can reflect the material’s capacity for energy dissipation. For viscoelastic material, the external work can be transformed into the energy of the following forms: elastic strain energy stored in materials consumed energy,
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100
80 70 60 50 40 30 20
100
E70 3R 4.5R 6R
(b)
90
Normalized Refractive Index
(a)
90
Normalized Refractive Index
E70 3L 4.5L 6L
80 70 60 50 40 30 20 10
10
0 12
0 12
14
16
18
20
22
24
26
14
16
Normalized Refractive Index
E70 4.5L_0w 4.5L_0.15w 4.5L_0.25w
(c)
90 80 70 60 50 40 30 20 10 0
20
22
100
Normalized Refractive Index
100
18
24
26
Retention Time (min)
Retention Time (min)
E70
(d)
90
4.5R_0w
80
4.5R_0.15w
70
4.5R_0.25w
60 50 40 30 20 10
12
14
16
18
20
22
24
26
0
12
14
16
Retention Time (min)
18
20
22
24
26
Retention Time (min)
Fig. 8. Chromatogram of SBS modified asphalt binders.
Insoluble Component E70 3L 4.5L 6L 3R 4.5R 6R 4.5L_0S 4.5L_0.15S 4.5L_0.25S 4.5R_0S 4.5R_0.15S 4.5R_0.25S 4.5L_4R 4.5L_8R
0.5
Dissolved Polymers
15.6 18.3
2.7 4.1 6.8 2.5
8.4
76.1
18.1
75.3
14.3
76.4
15.2
78.6
14.2
78.6
4.7 2.5 1.5 4.3
2.0
15.8
73.8
15.4
2.7 4.1
79.3
18.1
75.3
3.6 2.8
14.5
77.5
3.7 2.8
14.5
77.9
4.7 2.5
14.2
5.3 1.3 1.2 4.2 1.4 4.0
0%
Maltenes
83.2
1.7 3.9
2.4 3.8
Asphaltenes
78.6
13.8
78.7
17.1
77.5
15.5
10%
79.1
20%
30%
40%
50%
60%
70%
80%
90%
100%
Fig. 9. Component distribution of SBS modified asphalt binders.
because of the material flow, and surface energy because of crack occurrence and development. Therefore, the larger dissipation energy ratio implies the material has a good internal flow, that is, material has a good capacity to resist the cracking in low temperature. As shown in this Fig. 7, the DER values of linear SBS modified asphalt were much better than those of radial SBS modified asphalt. It demonstrates that linear SBS can improve the
anti-cracking capacity, while radial SBS can not. Furthermore, adding 0.15% content of sulfur to SBS modified asphalt could improve the DER significantly, while further addition of sulfur had little effect. It indicates that sulfur is important for anti-cracking capacity for SBS modified asphalt binders at low temperature. Finally, addition of rubber processing oil can significantly improve the DER of SBS modified asphalt. But the increase of DER for base binder was not remarkable. It was because SBS polymer absorbed the
P. Lin et al. / Construction and Building Materials 154 (2017) 609–622
Insoluble Component and and Dissolved Polymers (%)
616
12.00 10.00 8.00 6.00 4.00 2.00 0.00
Fig. 10. Insoluble component and dissolved Polymers in Sbs modified asphalt.
Fig. 11. FTIR spectrum of SBS modified asphalt with different SBS content.
Fig. 12. FTIR spectrum of SBS modified asphalt with different sulphur content.
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1.8
SBS Type and SBS Content
SBS Type and Sulfur Content
Polystyrene Index (%)
1.6
Rubber Processing Oil
1.4 1.2 1 0.8 0.6 0.4 0.2 0
Fig. 13. Polystyrene index of SBS modified asphalt binders.
Polybutadine Index (%)
3
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
2.5 2 1.5 1 0.5 0
Fig. 14. Polybutadiene index of SBS modified asphalt binders.
Aromaticity Index (%)
4.8
SBS Type and SBS Content
SBS Type and Sulfur Content
4.6 4.4 4.2 4 3.8 3.6
Fig. 15. Aromaticity index of SBS modified asphalt binders.
Rubber Processing Oil
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maltenes fraction in base binder and rubber processing oil can supplement the maltenes fraction.
4.2. Analysis of GPC test results GPC test can be used for quantitative analysis of the molecular components distribution of asphalt binders. In this paper, GPC test was conducted to find out the molecular distribution of SBS modified asphalt and the change due to the addition of sulfur and rubber processing oil. To explain, the insoluble component is not defined as solid, but polymer whose molecular size above 0.45 lm. It was caused by the high content of SBS polymer and addition of cross-linking agent (sulfur). The insoluble component was gel-like and mainly consisted of cross-linked polymer and the maltenes absorbed by the insoluble polymer. In Fig. 8, the peak at about 14.5 min retention time can be used to evaluate the content of dissolved polymer in the asphalt binder. It demonstrates in Fig. 8(a) that, the peak of 4.5L (4.5% linear SBS modified asphalt) was higher than that of 3L and 6L. Associate with the fraction distribution results in Fig. 9, the insoluble component of 6L was much higher than that of 4.5L. It was because the polymer peak is higher along with the increase of SBS content. However, when the content of SBS was more than 4.5%, the linear SBS polymer could not be dissolved in THF sufficiently and the SBS polymer was filtered out, which lead to a decrease in the peak in the chromatogram. Similar phenomenon was illustrated in Figs. 8 (b) and 9 that the polymer peak of 3R was higher than that of 4.5R and 6R due to the insoluble of radial SBS polymer. As illustrated in Fig. 8(c) and (d), the addition of sulfur had a significant influence on the component distribution. The increase of sulfur content led to a decrease of polymer peak in chromatogram and an increase of insoluble component both for linear and radial SBS modified asphalt. Compared with linear SBS modified asphalt, the insoluble component of radial SBS modified asphalt was higher. It implies that sulfur, used as a cross-linking agent, was able to promote the cross-linking reaction between SBS polymer and asphaltenes which increase the insoluble content in SBS modified asphalt. The insoluble content of radial SBS modified asphalt is higher than that of linear SBS modified asphalt when sulfur content is the same. Furthermore, the addition of rubber processing oil lead to a decrease of insoluble component indicating the aromatic components might be important for dissolve of SBS polymer.
To observe the change of polymer content, total polymer (total polymer = insoluble component + dissolved polymers) was calculated and results were shown in Fig. 10. As we know, SBS polymer absorbs saturates and aromatics fraction in base asphalt and volume of SBS polymer increase by 4–5 times. In the GPC test, when the insoluble component was filtered and kept in vacuum oven for 24 h, THF was fully volatilized, while absorbed saturates and maltenes stayed in the insoluble components. Total polymer was actually consisted of insoluble polymer, dissolved polymer and part of maltenes component absorbed in insoluble polymer. That was the reason why total polymer was more than the SBS content. As shown in Fig. 10, the total of linear SBS modified asphalt and radial SBS modified asphalt increased along with the increase of SBS content. It was because the total polymer was mainly consisted of SBS polymer. Meanwhile, the total polymer content of samples with 0.15% sulfur were a little higher than those of samples with 0% or 0.25% content of sulfur. It was because the sulfur, used as cross-linking agent, could influence the content of maltenes absorbed by SBS polymer. When content of sulfur was too little, the SBS polymer could not be dispersed in asphalt uniformly and stably, which cause the content of maltenes absorption decreased. When content of sulfur was too much, network structure of SBS polymer lead to a decrease of maltenes absorption content. Thus, 0.15% sulfur was optimum content both for radial and linear SBS modified asphalt. However, when rubber process oil was added, total polymer decreased significantly. It was because rubber process oil enhanced the dissolving capacity of base binder which led to a sharp decrease of insolubility component. As mentioned above, the total polymer consisted of dissolved polymer, insoluble polymer and the maltenes absorbed by the insoluble polymer. The decrease of content of insoluble component led to a decrease of maltenes in insoluble polymer as well. In this way, the total polymer decreased when rubber processing oil was added and it implied that addition of rubber processing oil improved the compatibility of SBS polymer. 4.3. Analysis of FTIR test results Fourier Transform Infrared (FTIR) is an important test for quantitative analysis of the chemical composition change in SBS modified asphalt. In this paper, the polystyrene index (IPS) and polybutadiene index (IPB) were used to for quantitative analysis of polymer in SBS modified asphalt binder and aromaticity index
Fig. 16. Heat flow of linear SBS modified asphalt binders.
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Fig. 17. First derivative curves of linear SBS modified asphalt binders.
Fig. 18. Heat flow curves of radial SBS modified asphalt binders.
(IAR) was used for quantitative analysis of aromatic content. The calculation of the indexes were described in Eqs. (7)–(9). The FITR spectrums of SBS modified asphalt are illustrated in Figs. 11 and 12 and the indexes were illustrated in Figs. 13–15. As shown in Fig. 11, the peak at 700 cm1 reflects polystyrene and peak at 965 cm1 reflects polybutadiene. The polystyrene peak (700 cm1 peak) of 6L was much higher than those of 4.5L and 3L and polystyrene peak of 6R was also higher than those of 4.5R and 3R. It indicates that the polystyrene detected in FTIR test increased along with the increase of SBS content. Similarly, the polybutadiene peak (965 cm1 peak) increased along with the increase of SBS content both for linear and radial SBS modified asphalt binders. As shown in Fig. 12, the polystyrene peak (700 cm1) of samples with 0.15% content of sulfur were higher than those of samples with 0% or 0.25% content of sulfur. It indicates that 0.15% is the optimum content of SBS content for polystyrene detected in FTIR test. Furthermore, the polybutadiene peak (965 cm1) of sample with 0.15% content of sulfur were also higher than those of
samples with 0% or 0.25% content sulfur. It is coherent with the GPC test that the total polymer of samples with 0.15% content of sulfur were higher. The results of polystyrene index (IPS) were illustrated in Fig. 13. The IPS of base binder was almost zero indicating the polystyrene mainly came from SBS polymer. As illustrated in Fig. 13, the IPS values were mainly controlled by the content of SBS content and the influence of SBS polymer type and the addition rubber processing oil were very slight. Meanwhile, when the content of sulfur was 0.15%, IPS values of samples were a little higher. It can be seen from Fig. 14, the variation of IPB was similar with that of IPS. IPB values were mainly controlled by the content of SBS content and IPB was slightly influenced by polymer type and addition of rubber processing oil. When sulfur content was 0.15%, the IPB reached the peak. Aromaticity index can reflect the content of aromatic component in binder and the results of aromaticity index (IAR) were illustrated in Fig. 15. The IAR decreased along with the increase of SBS
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3.0 E70 3R 4.5R
First Derivative (mw/°C)
2.0
4.5R_0.25w
1.0
6R
0.0 -1.0
-2.0 -3.0
-50
-40
-30
-20
-10
0
10
20
30
40
50
40
50
Temperature (°C) Fig. 19. First derivative curves of radial SBS modified asphalt binders.
3.0
2.0
4.5L_8R
First Derivative (mw/°C)
4.5L_4R
1.0
E70 4.5L
0.0 -1.0 -2.0 -3.0
-50
-40
-30
-20
-10
0
10
20
30
Temperature (°C) Fig. 20. First derivative curves of SBS modified asphalt binders with different content of rubber processing oil.
content. It imply the SBS polymer swelled more aromatic component from the base binder when the content of polymer increased. For linear SBS modified asphalt, the influence of adding sulfur was very small. However, for radial SBS modified asphalt, SBS polymer swelled more aromatic component along with the increase of sulfur content. Furthermore, the addition rubber processing oil led to an increase of IAR for SBS modified asphalt and base binder. It is because the rubber processing is rich in aromatic component. 4.4. Glass transition temperature analysis Thermal behavior of asphalt shows clues of changes of chemical components as different chemical structure in asphalt composition played different roles in determining the overall thermal behavior of asphalt binders. In this paper, thermal behavior of SBS modified asphalt binders was tested with DSC test. To determine the transition temperature (Tg), two tangents were fitted linear parts of
signal below and above the transition. The Tg was identified as the temperature at half height of the change in heat capacity and the results. The heat flow of linear SBS modified asphalt was shown in Fig. 16 and its derivative curve was shown in Fig. 17. The derivative heat flow allows for improved analysis of the Tg over heat flow alone. The Tg between 30 °C and 20 °C arose from the maltene phase and it is important for the low temperature properties of asphalt binders. The intensity at Tg reflects the content of amorphous material in phase responsible for the Tg. As we can see in Figs. 16 and 17, Tg decreased along with the increase of SBS content. Especially in Fig. 17, the decrease of Tg was highlighted by the dotted line. Meanwhile, the Tg of maltene lose intensity along with the increment of SBS content. It implied that SBS was swollen by maltene in base binder which was made of saturate and aromatic. The absorption of maltene led to a loss of amorphous material in
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Fracture Temperature ( )
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-40 -38 -36 -34 -32 -30 -28 -26 -24 -22 -20
Fig. 21. Fracture temperature in TSRST test of SBS modified asphalt mixture.
Table 2 Summary of optimum modification scheme of SBS modified asphalt for different parameters. Test Methods
BBR
Burger’s Model GPC FTIR DSC TSRST
Parameters
SBS Content (%)
Stiffness M-value PG Low Temperature Relaxation Time Dissipation Energy Ratio Total Polymer Polystyrene Index Polybutadiene Index Decrease of Tg Fracture Temperature
Sulfur Content (%)
Linear SBS
Radial SBS
Linear SBS
Radial SBS
6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 4.5
6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 4.5
0.15 0.25 0.25 0.25 0.15 0.15 0.15 0.15 – –
0.15 0.15 0.15 0.25 0.15 0.15 0.15 0.15 – 0.15
Rubber Processing Oil Content (%) 8.0 8.0 8.0 8.0 8.0 – 8.0 8.0 8.0 8.0
Note: The slash in the cell means the content variation had no effect or the tests were not done.
maltene phase which caused the loss in intensity at Tg. According (T New g
A
T gA
B
T Bg ),
to the rule of mixture ¼W þW the proportion in maltene between aromatic and saturate changed and the content of the saturate increased. The saturate fraction is likely to be rich in alkanes and cyclo-alkanes. Furthermore, the addition of rubber processing oil lead to a slight decrease of Tg. It is because the rubber processing oil is rich in aromatic and saturate which has lower Tg. As shown in Figs. 18 and 19, the Tg of radial SBS modified asphalt decreased along with the increment of SBS content and the degree of decrease was similar to those of linear SBS modified asphalt. It was also because that the SBS was swollen by the saturate and aromatic in the base binder which changed the proportion of the maltene. Furthermore, the increment of sulfur had no influence on Tg of radial modified asphalt. It indicated that addition of sulfur did not influence the absorption of saturate and aromatic Fig. 19. As illustrated in Fig. 20, the peak of 4.5L (4.5% linear SBS modified asphalt) was lower than that of base binder. As mentioned before, it was because SBS was swollen by saturate and aromatic fraction in base binder which caused a loss in intensity at Tg. However, when rubber processing oil was added, the intensity of Tg increased significantly. When the content of rubber processing oil was 8%, the peak at Tg was as high as that of base binder. It implied when content of rubber processing oil was 8%, it supplemented the lost amorphous fraction in base binder. Furthermore, the Tg of 4.5% SBS modified asphalt with 8% rubber processing oil decreased for about 3 °C compared with base binder. It was because the rubber processing oil was rich in saturate and aromatic.
4.5. Thermal stress restrained specimen test result For further investigation on the low temperature performance of SBS modified asphalt mixture, TSRST test was conducted according to AASHTO TP 10-93 [35]. During specimen preparation, sup12.5 was chosen as the gradation. The designed asphalt content is 5% (by weight) and air void is controlled at about 8%. The fracture temperature results of the SBS modified asphalt mixtures were illustrated in Fig. 21. As illustrated in Fig. 21, the failure temperature of SBS modified asphalt mixture was lower than that of base asphalt mixture. However, when the content of SBS was more than 4.5%, the further addition of SBS led to an increase of fracture temperature. The optimum SBS content for low temperature performance of mixture seem to be 4.5% both for linear and radial SBS modified asphalt mixture and it was not coherent with the results in BBR tests. It was observed that all the samples failed in a catastrophic mode. The failure temperature results in TSRST test might be not able to reflect the true low temperature performance of the SBS modified asphalt mixture with high SBS content. If the samples were notched before TSRST test, the results might be more accurate in reflecting the low temperature performance of SBS modified asphalt mixture. For radial SBS modified asphalt mixture, the optimum sulfur content seem to be 0.15%. While, the difference between 4.5R_0.15S and 4.5R_0.25S was very small. Furthermore, the addition of rubber processing oil improved the low temperature performance of SBS modified asphalt mixture significantly. It seems that, the modification by adding maltene fraction to asphalt is more effective compared with adding SBS polymer.
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5. Summary and conclusions 5.1. Summary In this paper, rheological and chemical tests were used to evaluate the effect of SBS type, SBS content, sulfur content and rubber processing oil on the low temperature properties of SBS modified asphalt binder. To find out the optimum modification scheme of the SBS modified asphalt for low temperature properties, the results of tests are summarized in Table 2. As illustrated in Table 2, optimum content of SBS for binder was 6% and optimum SBS content for mixture was 4.5%. For linear SBS modified asphalt, the optimum content of sulfur was 0.15% for stiffness, DER, total polymer and FTIR parameters. While, the optimum content of sulfur was 0.25% considering relaxation capacity. As for radial SBS modified asphalt, 0.15% content of sulfur is best choice for most parameters. Rubber processing oil can promote the low temperature properties of SBS modified asphalt and base asphalt. The better content of rubber processing oil in this study was 8%. 5.2. Conclusions (1) According to the low temperature rheological test and thermal stress restrained specimen test results, the optimum modification scheme of SBS modified asphalt binder and mixture was summarized. (2) Low temperature properties of SBS modified asphalt are mainly limited due to lack of m-value. Addition of sulfur and rubber processing oil can improve its m-value and promote SBS modified asphalt to be balanced in stiffness and mvalue. (3) According to the results of GPC, FTIR and DSC test, SBS polymer is swollen by saturate and aromatic fraction in base asphalt which led to a decrease of maltenes content, while further addition of rubber processing oil can supplement the decrease of maltenes. It is speculated that addition of polymer led to a lack of maltenes which had bad effect on low temperature properties. (4) The fracture temperature results in TSRST test indicated that 4.5% of SBS was optimum content, addition of sulfur and rubber processing can improve the low temperature performance.
Acknowledgements The authors gratefully acknowledge the financial supports by the Transportation Science and Technology Project of Fujian under Grant number 201449 and Technology Project of Anhui traffic Holding Group Co., Ltd. References [1] M. Liang, P. Liang, W. Fan, C. Qian, X. Xin, J. Shi, et al., Thermo-rheological behavior and compatibility of modified asphalt with various styrene– butadiene structures in SBS copolymers, Mater. Des. 88 (2015) 177–185. [2] D.O. Larsen, J.L. Alessandrini, A. Bosch, M.S. Cortizo, Micro-structural and rheological characteristics of SBS-asphalt blends during their manufacturing, Constr. Build. Mater. 23 (8) (2009) 2769–2774. [3] K. Stangl, A. Jäger, R. Lackner, The effect of styrene-butadiene-styrene modification on the characteristics and performance of bitumen, Monatshefte Fuer Chemie/chemical Monthly 138 (4) (2007) 301–307. [4] J.F. Masson, S. Bundalo Perc, A. Delgado, Glass transitions and mixed phases in block SBS, J. Polym. Sci. Part B Polym. Phys. 43 (3) (2005) 276–279.
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[12] G. Kraus, Modification of asphalt by block polymer of butadiene and styrene, Rubber Chem. Technol. 55 (5) (1982) 1389–1402. [13] P. Maldonado, J. Mas, T.K. Phung, Process for preparing bitumen-polymer compositions, US, 1979. [14] P. Vanelstorfe, Brasser, Improvement in bitumen by the addition of thermoplastic rubber. Revue Generale Des Routes Et Des Aerodromes, 1975. [15] P. Wloczysiak, A. Vidal, E.N. 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 (8) (2015) 1595–1607. [16] E. Ardrizzi, R. Vivirito. Rubber processing oil and rubber products containing it, CA, 1996. [17] C.O. Okieimen, F.E. Okieimen, Effect of natural rubber processing sludge on the degradation of crude oil hydrocarbons in soil, Bioresour. Technol. 82 (1) (2002) 95–97. [18] F. Wang, Study on Environmental Friendly Rubber Processing Oil, Chemical Production & Technology, 2004. [19] C.O. Okieimen, F.E. Okieimen, Bioremediation of crude oil-polluted soil–effect of poultry droppings and natural rubber processing sludge application on biodegradation of petroleum hydrocarbons, Environ. Sci. 12 (1) (2005) 1–8. [20] L. Ya, F. Qiu, J. Yin, Status and exploration for environment-friendly rubber processing oil, China Synthetic Rubber Industry, 2010. [21] J.S. Chen, C.C. Huang, Fundamental characterization of SBS-modified asphalt mixed with sulfur, J. Appl. Polym. Sci. 103 (5) (2007) 2817–2825. [22] I. Cho, B.K. Min, W.J. Sang, Y. Sohn, One-dimensional single crystalline antimony sulfur iodide, SbSI, Mater. Lett. 86 (11) (2012) 132–135. [23] Í.A. de Carcer, R.M. Masegosa, M.T. Viñas, M. Sanchez-Cabezudo, C. Salom, M.G. Prolongo, et al., Storage stability of SBS/sulfur modified bitumens at high temperature: influence of bitumen composition and structure, Constr. Build. 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Han, Effects of thermal oxidative ageing on dynamic viscosity, TG/DTG, DTA and FTIR of SBS- and SBS/sulfur-modified asphalts, Constr. Build. Mater. 25 (1) (2011) 129–137. [30] American Society of Testing Materials, Standard test method for determining the flexural creep stiffness of asphalt binder using the bending beam rheometer (BBR), In: American Society of Testing Materials, ASTM D6648, West Conshohocken, PA, 2001. [31] S. Liu, W. Cao, S. Shang, H. Qi, J. Fang, Analysis and application of relationships between low-temperature rheological performance parameters of asphalt binders, Constr. Build. Mater. 24 (4) (2010) 471–478. [32] I. Yut, A. Zofka, Attenuated total reflection (ATR) fourier transform infrared (FT-IR) spectroscopy of oxidized polymer-modified bitumens, Appl. Spectrosc. 65 (7) (2011) 765–770. [33] J. Lamontagne, P. Dumas, V. Mouillet, J. 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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation of influence factors on low temperature properties of SBS modified asphalt Peng Lin, Weidong Huang ⇑, Yi Li, Naipeng Tang, Feipeng Xiao The Key Laboratory of Road and Traffic Engineering, Ministry of Education, Tongji University, Shanghai 201804, PR China
h i g h l i g h t s Show the key factors influencing low temperature properties of binder. Optimum modification scheme for SBS modified asphalt is investigated. Component distribution, thermal behavior and FTIR spectrum are presented. Rubber processing oil can supplement the loss of maltene fraction. Evaluate SBS modified asphalt mixture with TSRST test.
a r t i c l e
i n f o
Article history: Received 12 March 2017 Received in revised form 16 June 2017 Accepted 19 June 2017
Keywords: SBS Polymer type Sulphur Rubber processing oil DSC TSRST
a b s t r a c t Styrene-butadiene-styrene (SBS) is widely used in asphalt modification and the investigations about SBS modified asphalt are focused on high temperature property, storage stability and the compatibility between SBS and asphalt. While the study on influencing factors on low temperature properties are not sufficient yet. In this paper, a systematical research was conducted to evaluate the influencing level of important factors such as SBS polymer type, SBS content, sulfur content and addition of rubber processing oil. Firstly, bending beam rheometer (BBR) test associated with Burger’s model was used to evaluate the low temperature property and relaxation capacity. Furthermore, FTIR, GPC and DSC tests were conducted to investigate the chemical changes in SBS modified asphalt. At last, the low temperature performance of SBS modified asphalt mixture was investigated with thermal stress restrained specimen test (TSRST). In this paper, the optimum modification scheme of SBS modified asphalt binder and mixture was investigated and summarized. SBS polymer is swollen by saturates and aromatics which made the base binder lack of maltene fraction. While the further addition of rubber processing oil supplemented the maltenes which improved the low temperature property. Ó 2017 Published by Elsevier Ltd.
1. Introduction The history of asphalt modification to achieve better performance in road engineering is long and styrene-butadienestyrene (SBS) is one of most outstanding modifier widely used all around the world. When SBS copolymers are mixed in asphalt, SBS is swollen by the maltenes in base asphalt. The system gradually becomes a biphasic microstructure where polymer-rich phase formed by maltenes-swollen-polymer is dispersed in an asphaltene-rich phase [1–4]. As literature indicates, base binder can be divided into four fractions, saturates, aromatics, resins and asphaltenes, which is referred as SARA fractions [5–9]. As the maltenes, mainly ⇑ Corresponding author. E-mail address: [email protected] (W. Huang). http://dx.doi.org/10.1016/j.conbuildmat.2017.06.118 0950-0618/Ó 2017 Published by Elsevier Ltd.
composed with saturates and aromatics, is absorbed by SBS copolymer, the base binder will become lack of maltenes. The compatibility between asphalt and SBS has been identified as a critical factor which has a significant influence on the rheological properties and morphology of SBS modified asphalt [10,11]. Literatures indicates that, asphalt with high aromatic content compared to asphalt with low aromatic content is easier to obtain a compatible state and a better low temperature performance [12–14]. In 1982, Kraus showed that SBS is compatible with asphalt with high aromatic content and incompatible with a highly paraffinic content asphalt [12]. Wloczysiak et al. showed that asphalt with 70–80 wt% aromatic not only caused PS to swell but also caused antiplasiticization of PS, an increase in its glass transition temperature [15]. Rubber processing oil, rich in aromatic and saturates fractions, is used as a component in manufacturing of rubber product [16–20]. Addition of rubber processing oil has a profound effect in
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improving the compatibility and low temperature performance of SBS modified asphalt while the investigation is not sufficient. In the system of SBS modified asphalt, sulfur is usually used as a cross-linking agent during the preparation process to increase the storage stability. The sulfur reacts with the alkene moieties presents in SBS and creates chemical crosslinks which help to stabilize the system [21–25]. In 2002, Wen studied rheological characterization of storage-stable SBS modified asphalt with phase angle and complex modulus. He also studied the morphological changes of storage-stable SBS/sulfur modified asphalt [26,27]. Zhang et al. investigated some other important properties of SBS/sulfur modified asphalt including dynamic viscosity, thermal stability [28,29]. However, the influence of sulfur on the low temperature properties of SBS modified asphalt is not clear yet. In this paper, low temperature performance of SBS modified asphalt is studied systematically by evaluating the influence of important factors such as SBS type, SBS content, sulfur content and rubber processing oil content.
the modified asphalt composition is present in Table 1. It should be noted that, for more conveniently contrast in figures, 4.5L and 4.5L_0.15S are two names of the same kind of SBS modified asphalt binder. Similarly, 4.5R and 4.5R_0.15S are the same kind of asphalt binder too. SBS modified asphalt samples were prepared as follows. Firstly, the SBS was added to base binder and sheared for 30 min at 180 °C with a shear speed of 4000 r/min. Secondly, the blend was stirred for 60 min using mechanical stirrer at 180 °C. Thirdly, sulfur was added to the blend and stirred for 90 min at 180 °C. At last, the rubber processing oil was added to the blend and stirred for another 60 min at 180 °C. The prepared binders satisfied the storage stability test. All samples were performed RTFO aging procedure at the end of samples blending to make sure there was no phase separation in the samples. This is especially important for samples without cross-linking agent. After the RTFO aging procedure, samples were performed PAV aging procedure [24]. 3.2. Bending beam rheometer (BBR) test Creep test was carried out at different low temperatures (12, 18 and 24 °C) using a bending beam rheometer (BBR) according to ASTM D6648 [30]. The stiffness (S) and the creep rate (m-value) of the sample beam were determined. All combinations were replicated three times for reliability reasons. 3.3. Burgers modelling
2. Objectives 1. To evaluate the influencing level of SBS type, SBS content, sulfur content and rubber processing content on the low temperature property of SBS modified asphalt binder. 2. To find out the optimum modification scheme of SBS modified asphalt for low temperature property. 3. To track the chemical changes in SBS modified asphalt through GPC test, FTIR test and DSC test. 4. To evaluate the low temperature performance of SBS modified mixture with TSRST test.
Compared with Kelvin model and Maxwell model, Burger’s model is better in characterizing viscoelastic property of asphalt binder. Thus Burger’s model was used for further investigation of low temperature of the samples. It is made of two components, springs are associated with storage and dashpots are associated with dissipation of deformation energy. Therefore, the parameters of Burger’s model is able to indicate the material’s capacity for energy dissipation. So, the burger’s model was used to calculate the dissipated and stored energy for each BBR test. According to the original data obtained from BBR software, the flexural creep compliance D(t) can be calculated with the following equation. 3
DðtÞ ¼
4bh dðtÞ
ð1Þ
PL3
where D(t) is the flexural creep compliance at time t; P is the applied constant load; L is the distance between supports; b is the width of specimen; h is the depth of specimen; d(t) is the deflection in middle-span of specimen at time t. The creep compliance can be expressed by Burgers model [31] using the following equation:
3. Materials and methods 3.1. Materials and preparation One base binder provided by a company (E70, PG 64-16) was used in this study. The linear styrene-butadienestyrene (SBS) polymer has average molecule weight of w = 110,000), containing 30%wt styrene. The radial styrene–buta 110,000 g/mol (M dienestyrene (SBS) has average molecule weight of 230,000 g/mol, containing 30wt% of styrene. The amount of SBS modifier was ranged from 3.0% to 6.0% by the weight of base binder. Sulfur is commonly used as a cross-linking agent to enhance the storage stability of polymer modified asphalt. The reaction between SBS and sulfur is similar to that of vulcanization in rubber industry [23,26]. In this study, the sulfur was used as a cross-linking agent and the amount of sulfur was ranged from 0% to 0.25%. Rubber processing oil, commonly utilized in rubber industry, has similar chemical components with saturates and aromatics fraction in base binder. The content of rubber processing oil was ranged from 0% to 8%. The detail of
DðtÞ ¼
E 1 t 1 2t þ þ 1 e g2 E1 g1 E2
ð2Þ
where E1 is the instantaneous elastic modulus; g1 is the viscous coefficient; E2 and g2 are viscoelastic indicators. To estimate the nonlinear fitting parameters of the model, Excel SOLVER tool was used with method of least squares. The four Burgers parameters were obtained according to the modeling results [31]. The parameters obtained from Burgers model could be calculate as follows:
k¼
g1
ð3Þ
E1
Table 1 Description of asphalt composition. Bitumen Category
Linear SBS (%)
Radial SBS (%)
Rubber processing Oil(%)
Sulfur(%)
E70 (Base Binder) 3L 4.5L 6L 3R 4.5R 6R 4.5L_0S 4.5L_0.15S 4.5L_0.25S 4.5R_0S 4.5R_0.15S 4.5R_0.25S 4.5L_4R 4.5L_8R 4R 8R
0 3 4.5 6 0 0 0 4.5 4.5 4.5 0 0 0 4.5 4.5 0 0
0 0 0 0 3 4.5 6 0 0 0 4.5 4.5 4.5 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 4 8 4 8
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0 0.15 0.25 0 0.15 0.15 0.15 0.15 0 0
Note: (1) 4.5L and 4.5L_0.15S are the same kind of modified asphalt binder. (2) 4.5R and 4.5R_0.15S are the same kind of modified asphalt binder. (3) E70 stands for Esso70 #, is the base binder.
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Aromatic index : IAR ¼ AR1600 =
X
ð9Þ
ARv~ P
The sum of the area represents: ARv~ ¼ AR1700 þ AR1600 þ AR1460 þ AR1310 þ AR1030 þ AR1030 þ AR965 þ AR864 þ AR814 þ AR743 þ AR725 þ AR700 3.5. Solubility test and gel permeation chromatography (GPC) test About 20 mg virgin binder sample was dissolved with tetrahydrofuran (THF) in a 10 mL volumetric flask for 24 h before solubility test and GPC test. The solution was filtered through a 0.45 lm PTFE filter and collected in a 0.5 mL centrifugal tube for GPC test. The remaining solution in the volumetric flask was accordingly filtrated through the weighted filter. After that, about 5 mL clean THF was filtrated through the filter until the filtrated solution was colourless. The filter was kept in a vacuum oven at 35 °C for about 12 h until the retained THF was fully volatilized. Then the filter was weighted again and the increase weight of the filer can be used to describe the weight of insoluble component in the asphalt. The solubility was calculated using as follows:
Solubility % ¼
Fig. 1. Fractions of the chromatogram based on molecular weight.
W s ðtÞ ¼ r20
E 2E 1 1 2t 2t þ 1 2e g2 þ e g2 E1 2E2
W d ðtÞ ¼ r20
t
g1
þ
ð4Þ
E 1 2t 1 e g2 2E2
ð5Þ
W d ðtÞ W s ðtÞ
ð6Þ
Dissipation energy ratio ¼
where kðsÞ is relaxation time, W s ðtÞðMPaÞ is stored energy per volume, W d ðtÞðMPaÞ is dissipated energy per volume, t(s) is the loading time of the test, and r0 (MPa) is the stress at the mid-span. It can be seen from Eqs. (4) and (5) that stored energy and dissipated energy were functions of time t. Considering stiffness and m-value calculated at 60 s, dissipation energy ratio was calculated at t = 60 s. 3.4. Attenuated Total Reflection (ATR) Fourier Transform Infrared (FT-IR) Spectroscopy The infrared spectra values were collected using a Bruker TENSOR FT-IR spectrometer equipped with a reflection diamond ATR accessory. To quantify the change in IR absorption, band areas rather than peak absorbance value were used. Three replicates for each binder sample was tested to ensure that the variability was P small. The ARv~ values were normalized to the total sum of all band areas ( ARv~ ), and some of the indices were computed as follows [32,33]:
Polystyrene index : IPS ¼ AR700 =
X
Polybutadiene index : IPB ¼ AR965 =
ð7Þ
ARv~
X
ð8Þ
ARv~
W binder W filter after filtration W filter before filtration 100% W binder
Waters 1515 High-Pressure Liquid Chromatography (HPLC) Pump and Waters 2414 Refractive Index (RI) detector were used to perform GPC test. THF (HPLC grade) was selected as mobile phase solvents. A combination of three columns was used for separating constituents of asphalt binder by molecular size. The calibration curve was built with ShodexÒ Polystyrene Standards in order to convert the retention time to molecular weight. The chromatogram was divided into three slices based on the molecular weight of eluting species [34]. The three fractions were dissolved polymers (molecular weight greater than 19,000), apparent asphaltenes (molecular weight from 19,000 to 3000) and maltenes (molecular weight less than 3000) [34], as shown in Fig. 1. The contents of insoluble components, dissolved polymers, apparent asphaltenes and maltenes were calculated accordingly based on Eqs. (5)–(8). Two replicates were used for both of solubility test and GPC test and the average values were reported.
Insoluble Components % ¼ 1 Solubility% Dissolv ed Polymers % ¼
Areadissolv ed polymers Solubility% Total area under chromatogram
Apparent Asphaltenes % ¼
Maltenes % ¼
Areaapparent asphaltenes Solubility % Total area under chromatogram
Areamaltenes Solubility % Total area under chromatogram
ð12Þ
ð13Þ
ð14Þ
A Netzsch DSC 204 F1 Differential Scanning Calorimeter was used to determine the glass transition temperature (Tg). The instrument was calibrated with n-octane, indium and zinc and the measurements were conducted under nitrogen atmosphere. In all the cases, pans of aluminium of 40 ll were used and the weight of
SBS Type and Sulfur Content
500
Stiffness at -18 C (MPa)
ð11Þ
3.6. Differential Scanning Calorimetry test (DSC)
550 SBS Type and SBS Content
ð10Þ
450 400 350 300 250 200
Fig. 2. Stiffness of asphalt binders at 18 °C.
Rubber Processing Oil
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the samples were kept in the range of 5–10 mg. The heating scans were recorded from 70 °C to 100 °C at a heating rate of 10 °C min1. Tg was obtained from the inflection point of the curves corresponding to the first scan.
3.7. Thermal Stress Restrained Specimen Test (TSRST) TSRST was carried out according to the procedure of AASHTO TP 10-93 [35]. The tests were performed in the MTS apparatus. The 50 50 250 mm rectangular prisms cut from compacted slabs were used for testing. The specimens were placed in the temperature chamber and conditioned for two hours at +5 °C. During the test the temperature was lowered at a rate of 10 °C/h. Three replicates were conducted for each asphalt mixture.
4. Result and discussion 4.1. Low temperature properties analysis The low temperature rheological properties of SBS modified asphalt binders were evaluated with BBR test results. The Stiffness and m-value of PAV aged samples at 18 °C were illustrated in Figs. 2 and 3. It was be found in Fig. 2 that stiffness values of both linear and radial SBS modified asphalt declined with an increase of SBS content. When the SBS content is the same, the stiffness values of linear SBS modified asphalt and radial SBS modified asphalt were nearly the same. The stiffness values of SBS modified asphalt are lower than that of base binder, indicating SBS modifier had a positive effect on the stiffness of asphalt binder at low temperature, while the type of SBS had little influence. For the samples with different content of sulfur, the samples with 0.15% content of sulfur had lower stiffness than those of samples with 0% or 0.25% content of sulfur. Sulfur could trigger the cross-linking reaction and the polymer network reduced the possibility of segregation. However, an excessive amount of crosslinking agent could also decrease the flow ability which led to an increase of stiffness. The results implied that, the 0.15% was the optimum content of sulfur for SBS modified asphalt when SBS content was 4.5%. When rubber processing oil was added to base binder and 4.5% linear SBS modified asphalt, the stiffness values decreased sharply, especially when rubber processing oil content was 8%. The results indicated that rubber processing oil had a positive effect on the stiffness. It can be explained as the rubber processing oil is mainly composed of aromatic and aliphatic components, which can improve the flow ability of asphalt binder at low temperature.
The creep rate (m-value) is an important parameter to evaluate the creep ability of asphalt binder at low temperature. As shown in Fig. 3, the linear and radial SBS modified asphalt had higher mvalues than those of E70 (base binder), and the m-values increased along with the increase of SBS content. However, there was no significant difference between linear SBS modified asphalt and radial SBS modified asphalt with the same content of SBS. It indicated that both linear and radial SBS had positive influence on the creep ability of asphalt binder. In terms of m-value, the optimum content of sulfur for 4.5% linear SBS modified asphalt was 0.25%. While for radial SBS modified asphalt, optimum sulfur content was 0.15%. The reason may be the shapes of linear SBS and radial SBS are different, and the lumpshaped macromolecular compound of radial SBS needs less content of sulfur to form the cross-linking structure. As shown in Fig. 3, m-value of 4.5L_8R was 0.06 higher than that of 4.5L and m-value of 8R was only 0.04 higher than that of base binder. It indicated that adding rubber processing oil to linear modified asphalt was more efficient than adding rubber processing oil to base binder in improving m-value. Refferred to ASTM D6648 [30], PG low temperatures were calculated according to stiffness (S = 300 MPa) and m-value (m = 0.3). As illustrated in Fig. 4, when SBS content is 3.0% and 4.5%, the PG low temperature of radial SBS modified aspahlt was lower than that of linear SBS modified apshalt. However the PG low temperaute of 6% linear SBS modified asphalt was nearly the same with that of 6% radial SBS modified aspahlt. Considering the SBS content used in practical engineering is usually from 3% to 5%, it indicated the low temperautre performance of radial SBS modified asphalt was better than that of linear SBS modfied aspaht. Sulfur content had an significant influence on the PG low temerpature of SBS modfied asphalt. The optimum content of sulfur for linear SBS modified asphalt was 0.25%, while the optimum content of sulfur for radial SBS modfied aspahlt was 0.15%. In summary, the low temperature properties of SBS modified asphalt were mainly controlled by two key factors, polymer and maltenes. Both the addition of maltenes (rubber processing oil) and SBS polymer can improve the low temperautre properties. If the two factors are independent, the addition order of polymer and rubber processing oil will not influence the PG low temperautre. Compared with base asphalt, if 8% content of rubber processing oil was added first and 4.5% linear SBS polymer was added later, PG low temperature decrease was 2.9 °C and 2.2 °C. However, when linear SBS polymer was added first and
0.36 SBS Type and SBS Content
SBS Type and Sulfur Content
0.34
m-value at -18 C
0.32 0.3 0.28 0.26 0.24 0.22 0.2
Fig. 3. M-value of asphalt binders at 18 °C.
Rubber Processing Oil
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-20.0
PG Low Temperature ( C )
-21.0 -22.0 -23.0 -24.0 -25.0 -26.0 -27.0 -28.0 -29.0 -30.0
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
Fig. 4. PG low temperature of SBS modified asphalt.
PG Low Temperature Difference ( C )
2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
Fig. 5. PG low temperature difference of SBS modified asphalt.
rubber processing oil was added later, the decrease of PG low temperature was 1.3 °C and 3.8 °C. The difference was caused by the negative effect of lack of maltenes when SBS polymer was added first. Furthermore, the increase of PG low temperature can be roughly calculated as 0.9 °C. The computing process are as follows.
T maltenes first ¼ T 8R T E70 ¼ 26:4 ð23:5Þ ¼ 2:9 ¼ T maltnes T polymer later ¼ T 4:5L
8R
ð15Þ
T 8R ¼ 28:6 ð26:4Þ ¼ 2:2 ¼ T polymer ð16Þ
T polymer first ¼ T 4:5L T E70 ¼ 24:8 ð23:5Þ ¼ 1:3 ¼ T polymer T maltnes lack T maltnes later ¼ T 4:5L
8R
ð17Þ
T 4:5L ¼ 28:6 ð24:8Þ ¼ 3:8
¼ T maltnes lack þ T polymer
ð18Þ
T maltenes lack ¼ ðT polymer þ T polymer lack Þ ðT polymer Þ ¼ 1:3 ð2:2Þ ¼ 0:9 C
ð19Þ
As we know, PG low temperature calculated according to mvalues (Tm) and PG low temperature calculated according to stiffness (Ts) are different. To find out the key factor of low temperature properties of SBS modified asphalt, temperature difference ðDT c ¼ T s T m Þ was calculated, T s stands for the PG low temperature calculated according to stiffness and T m stands for the PG low temperature calculated according to m-value. If the DT c > 0; the binder is more likely to be damaged due to high stiffness value (stiffness controlled asphalt) and if DT c < 0, the binder is more likely to be damaged as the low m-value (m-value controlled asphalt). As shown in Fig. 5, DT c values of SBS modified asphalt were below 0 °C indicating the SBS modified asphalt were m-value controlled and the main problem of SBS modified asphalt were lack of creep capacity. The DT c values of radial SBS modified asphalt were
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280.0 260.0
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
Relaxation time (s)
240.0 220.0 200.0 180.0 160.0 140.0 120.0 100.0
Fig. 6. Relaxation time of SBS modified asphalt binders.
Dissipation Energy Ratio (%)
80.0 75.0
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
70.0 65.0 60.0 55.0 50.0 45.0 40.0
Fig. 7. Dissipation energy ratio of SBS modified asphalt binders.
relatively closer to 0 °C than those of linear SBS modified asphalt, indicating the stiffness and the m-value of radial SBS modified asphalt were more balanced. Content of sulfur also had a significant influence on DT c of SBS modified asphalt. When content of sulfur increased, DT c of linear and radial SBS modified asphalt binders tended to be zero, revealing that the addition of sulfur promoted the balance between stiffness and m-value. It should be noted that if radial SBS modified was produced without sulfur, it would be seriously lack of m-value. As we can see in Figs. 2 and 3, rubber processing oil can not only improve the stiffness of SBS modified asphalt binder and base binder, but also improve its m-value. It was seen in Fig. 5, the improvement in m-value was more significant which made it became stiffness controlled binders. The addition of rubber processing oil can solve the problem of lack m-value for SBS modified asphalt. Relaxation time was calculated with the Burger’s parameters according to Eq. (3) (k ¼ g1 =E1 ), which is an important parameter reflecting the stress relaxation capacity of material. The shorter relaxation time is, the more rapid the stress dissipation is [36]. Both m-value and relaxation time are parameters describing the relaxation capacity, while relaxation time has clearer physical meaning.
As shown in Fig. 6, relaxation time decreased along with the addition of SBS, implying adding SBS was able to promote the relaxation capacity. When SBS content is the same, relaxation time of linear SBS modified asphalt is lower than those of radial SBS modified asphalt. It demonstrated that linear SBS modified asphalt had better relaxation capacity and it might be due to the linear structure of linear SBS polymer. Sulfur and rubber processing oil also had significant influence on the relaxation time. Both for linear and radial SBS modified asphalt, if sulfur was not added during producing, the relaxation time would be very high. It indicated that sulfur was important for the relaxation capacity of SBS modified asphalt binders. Addition of Rubber processing oil also could significantly improve the relaxation capacity, especially for SBS modified asphalt binders. This phenomenon can be explained as the rubber processing oil supplements the light components in the base binder which was absorbed by the SBS polymer. Dissipation Energy Ratio (DER) was calculated with Burger’s model parameters according to Eq. (5), which can reflect the material’s capacity for energy dissipation. For viscoelastic material, the external work can be transformed into the energy of the following forms: elastic strain energy stored in materials consumed energy,
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100
80 70 60 50 40 30 20
100
E70 3R 4.5R 6R
(b)
90
Normalized Refractive Index
(a)
90
Normalized Refractive Index
E70 3L 4.5L 6L
80 70 60 50 40 30 20 10
10
0 12
0 12
14
16
18
20
22
24
26
14
16
Normalized Refractive Index
E70 4.5L_0w 4.5L_0.15w 4.5L_0.25w
(c)
90 80 70 60 50 40 30 20 10 0
20
22
100
Normalized Refractive Index
100
18
24
26
Retention Time (min)
Retention Time (min)
E70
(d)
90
4.5R_0w
80
4.5R_0.15w
70
4.5R_0.25w
60 50 40 30 20 10
12
14
16
18
20
22
24
26
0
12
14
16
Retention Time (min)
18
20
22
24
26
Retention Time (min)
Fig. 8. Chromatogram of SBS modified asphalt binders.
Insoluble Component E70 3L 4.5L 6L 3R 4.5R 6R 4.5L_0S 4.5L_0.15S 4.5L_0.25S 4.5R_0S 4.5R_0.15S 4.5R_0.25S 4.5L_4R 4.5L_8R
0.5
Dissolved Polymers
15.6 18.3
2.7 4.1 6.8 2.5
8.4
76.1
18.1
75.3
14.3
76.4
15.2
78.6
14.2
78.6
4.7 2.5 1.5 4.3
2.0
15.8
73.8
15.4
2.7 4.1
79.3
18.1
75.3
3.6 2.8
14.5
77.5
3.7 2.8
14.5
77.9
4.7 2.5
14.2
5.3 1.3 1.2 4.2 1.4 4.0
0%
Maltenes
83.2
1.7 3.9
2.4 3.8
Asphaltenes
78.6
13.8
78.7
17.1
77.5
15.5
10%
79.1
20%
30%
40%
50%
60%
70%
80%
90%
100%
Fig. 9. Component distribution of SBS modified asphalt binders.
because of the material flow, and surface energy because of crack occurrence and development. Therefore, the larger dissipation energy ratio implies the material has a good internal flow, that is, material has a good capacity to resist the cracking in low temperature. As shown in this Fig. 7, the DER values of linear SBS modified asphalt were much better than those of radial SBS modified asphalt. It demonstrates that linear SBS can improve the
anti-cracking capacity, while radial SBS can not. Furthermore, adding 0.15% content of sulfur to SBS modified asphalt could improve the DER significantly, while further addition of sulfur had little effect. It indicates that sulfur is important for anti-cracking capacity for SBS modified asphalt binders at low temperature. Finally, addition of rubber processing oil can significantly improve the DER of SBS modified asphalt. But the increase of DER for base binder was not remarkable. It was because SBS polymer absorbed the
P. Lin et al. / Construction and Building Materials 154 (2017) 609–622
Insoluble Component and and Dissolved Polymers (%)
616
12.00 10.00 8.00 6.00 4.00 2.00 0.00
Fig. 10. Insoluble component and dissolved Polymers in Sbs modified asphalt.
Fig. 11. FTIR spectrum of SBS modified asphalt with different SBS content.
Fig. 12. FTIR spectrum of SBS modified asphalt with different sulphur content.
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1.8
SBS Type and SBS Content
SBS Type and Sulfur Content
Polystyrene Index (%)
1.6
Rubber Processing Oil
1.4 1.2 1 0.8 0.6 0.4 0.2 0
Fig. 13. Polystyrene index of SBS modified asphalt binders.
Polybutadine Index (%)
3
SBS Type and SBS Content
SBS Type and Sulfur Content
Rubber Processing Oil
2.5 2 1.5 1 0.5 0
Fig. 14. Polybutadiene index of SBS modified asphalt binders.
Aromaticity Index (%)
4.8
SBS Type and SBS Content
SBS Type and Sulfur Content
4.6 4.4 4.2 4 3.8 3.6
Fig. 15. Aromaticity index of SBS modified asphalt binders.
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maltenes fraction in base binder and rubber processing oil can supplement the maltenes fraction.
4.2. Analysis of GPC test results GPC test can be used for quantitative analysis of the molecular components distribution of asphalt binders. In this paper, GPC test was conducted to find out the molecular distribution of SBS modified asphalt and the change due to the addition of sulfur and rubber processing oil. To explain, the insoluble component is not defined as solid, but polymer whose molecular size above 0.45 lm. It was caused by the high content of SBS polymer and addition of cross-linking agent (sulfur). The insoluble component was gel-like and mainly consisted of cross-linked polymer and the maltenes absorbed by the insoluble polymer. In Fig. 8, the peak at about 14.5 min retention time can be used to evaluate the content of dissolved polymer in the asphalt binder. It demonstrates in Fig. 8(a) that, the peak of 4.5L (4.5% linear SBS modified asphalt) was higher than that of 3L and 6L. Associate with the fraction distribution results in Fig. 9, the insoluble component of 6L was much higher than that of 4.5L. It was because the polymer peak is higher along with the increase of SBS content. However, when the content of SBS was more than 4.5%, the linear SBS polymer could not be dissolved in THF sufficiently and the SBS polymer was filtered out, which lead to a decrease in the peak in the chromatogram. Similar phenomenon was illustrated in Figs. 8 (b) and 9 that the polymer peak of 3R was higher than that of 4.5R and 6R due to the insoluble of radial SBS polymer. As illustrated in Fig. 8(c) and (d), the addition of sulfur had a significant influence on the component distribution. The increase of sulfur content led to a decrease of polymer peak in chromatogram and an increase of insoluble component both for linear and radial SBS modified asphalt. Compared with linear SBS modified asphalt, the insoluble component of radial SBS modified asphalt was higher. It implies that sulfur, used as a cross-linking agent, was able to promote the cross-linking reaction between SBS polymer and asphaltenes which increase the insoluble content in SBS modified asphalt. The insoluble content of radial SBS modified asphalt is higher than that of linear SBS modified asphalt when sulfur content is the same. Furthermore, the addition of rubber processing oil lead to a decrease of insoluble component indicating the aromatic components might be important for dissolve of SBS polymer.
To observe the change of polymer content, total polymer (total polymer = insoluble component + dissolved polymers) was calculated and results were shown in Fig. 10. As we know, SBS polymer absorbs saturates and aromatics fraction in base asphalt and volume of SBS polymer increase by 4–5 times. In the GPC test, when the insoluble component was filtered and kept in vacuum oven for 24 h, THF was fully volatilized, while absorbed saturates and maltenes stayed in the insoluble components. Total polymer was actually consisted of insoluble polymer, dissolved polymer and part of maltenes component absorbed in insoluble polymer. That was the reason why total polymer was more than the SBS content. As shown in Fig. 10, the total of linear SBS modified asphalt and radial SBS modified asphalt increased along with the increase of SBS content. It was because the total polymer was mainly consisted of SBS polymer. Meanwhile, the total polymer content of samples with 0.15% sulfur were a little higher than those of samples with 0% or 0.25% content of sulfur. It was because the sulfur, used as cross-linking agent, could influence the content of maltenes absorbed by SBS polymer. When content of sulfur was too little, the SBS polymer could not be dispersed in asphalt uniformly and stably, which cause the content of maltenes absorption decreased. When content of sulfur was too much, network structure of SBS polymer lead to a decrease of maltenes absorption content. Thus, 0.15% sulfur was optimum content both for radial and linear SBS modified asphalt. However, when rubber process oil was added, total polymer decreased significantly. It was because rubber process oil enhanced the dissolving capacity of base binder which led to a sharp decrease of insolubility component. As mentioned above, the total polymer consisted of dissolved polymer, insoluble polymer and the maltenes absorbed by the insoluble polymer. The decrease of content of insoluble component led to a decrease of maltenes in insoluble polymer as well. In this way, the total polymer decreased when rubber processing oil was added and it implied that addition of rubber processing oil improved the compatibility of SBS polymer. 4.3. Analysis of FTIR test results Fourier Transform Infrared (FTIR) is an important test for quantitative analysis of the chemical composition change in SBS modified asphalt. In this paper, the polystyrene index (IPS) and polybutadiene index (IPB) were used to for quantitative analysis of polymer in SBS modified asphalt binder and aromaticity index
Fig. 16. Heat flow of linear SBS modified asphalt binders.
P. Lin et al. / Construction and Building Materials 154 (2017) 609–622
619
Fig. 17. First derivative curves of linear SBS modified asphalt binders.
Fig. 18. Heat flow curves of radial SBS modified asphalt binders.
(IAR) was used for quantitative analysis of aromatic content. The calculation of the indexes were described in Eqs. (7)–(9). The FITR spectrums of SBS modified asphalt are illustrated in Figs. 11 and 12 and the indexes were illustrated in Figs. 13–15. As shown in Fig. 11, the peak at 700 cm1 reflects polystyrene and peak at 965 cm1 reflects polybutadiene. The polystyrene peak (700 cm1 peak) of 6L was much higher than those of 4.5L and 3L and polystyrene peak of 6R was also higher than those of 4.5R and 3R. It indicates that the polystyrene detected in FTIR test increased along with the increase of SBS content. Similarly, the polybutadiene peak (965 cm1 peak) increased along with the increase of SBS content both for linear and radial SBS modified asphalt binders. As shown in Fig. 12, the polystyrene peak (700 cm1) of samples with 0.15% content of sulfur were higher than those of samples with 0% or 0.25% content of sulfur. It indicates that 0.15% is the optimum content of SBS content for polystyrene detected in FTIR test. Furthermore, the polybutadiene peak (965 cm1) of sample with 0.15% content of sulfur were also higher than those of
samples with 0% or 0.25% content sulfur. It is coherent with the GPC test that the total polymer of samples with 0.15% content of sulfur were higher. The results of polystyrene index (IPS) were illustrated in Fig. 13. The IPS of base binder was almost zero indicating the polystyrene mainly came from SBS polymer. As illustrated in Fig. 13, the IPS values were mainly controlled by the content of SBS content and the influence of SBS polymer type and the addition rubber processing oil were very slight. Meanwhile, when the content of sulfur was 0.15%, IPS values of samples were a little higher. It can be seen from Fig. 14, the variation of IPB was similar with that of IPS. IPB values were mainly controlled by the content of SBS content and IPB was slightly influenced by polymer type and addition of rubber processing oil. When sulfur content was 0.15%, the IPB reached the peak. Aromaticity index can reflect the content of aromatic component in binder and the results of aromaticity index (IAR) were illustrated in Fig. 15. The IAR decreased along with the increase of SBS
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3.0 E70 3R 4.5R
First Derivative (mw/°C)
2.0
4.5R_0.25w
1.0
6R
0.0 -1.0
-2.0 -3.0
-50
-40
-30
-20
-10
0
10
20
30
40
50
40
50
Temperature (°C) Fig. 19. First derivative curves of radial SBS modified asphalt binders.
3.0
2.0
4.5L_8R
First Derivative (mw/°C)
4.5L_4R
1.0
E70 4.5L
0.0 -1.0 -2.0 -3.0
-50
-40
-30
-20
-10
0
10
20
30
Temperature (°C) Fig. 20. First derivative curves of SBS modified asphalt binders with different content of rubber processing oil.
content. It imply the SBS polymer swelled more aromatic component from the base binder when the content of polymer increased. For linear SBS modified asphalt, the influence of adding sulfur was very small. However, for radial SBS modified asphalt, SBS polymer swelled more aromatic component along with the increase of sulfur content. Furthermore, the addition rubber processing oil led to an increase of IAR for SBS modified asphalt and base binder. It is because the rubber processing is rich in aromatic component. 4.4. Glass transition temperature analysis Thermal behavior of asphalt shows clues of changes of chemical components as different chemical structure in asphalt composition played different roles in determining the overall thermal behavior of asphalt binders. In this paper, thermal behavior of SBS modified asphalt binders was tested with DSC test. To determine the transition temperature (Tg), two tangents were fitted linear parts of
signal below and above the transition. The Tg was identified as the temperature at half height of the change in heat capacity and the results. The heat flow of linear SBS modified asphalt was shown in Fig. 16 and its derivative curve was shown in Fig. 17. The derivative heat flow allows for improved analysis of the Tg over heat flow alone. The Tg between 30 °C and 20 °C arose from the maltene phase and it is important for the low temperature properties of asphalt binders. The intensity at Tg reflects the content of amorphous material in phase responsible for the Tg. As we can see in Figs. 16 and 17, Tg decreased along with the increase of SBS content. Especially in Fig. 17, the decrease of Tg was highlighted by the dotted line. Meanwhile, the Tg of maltene lose intensity along with the increment of SBS content. It implied that SBS was swollen by maltene in base binder which was made of saturate and aromatic. The absorption of maltene led to a loss of amorphous material in
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Fracture Temperature ( )
P. Lin et al. / Construction and Building Materials 154 (2017) 609–622
-40 -38 -36 -34 -32 -30 -28 -26 -24 -22 -20
Fig. 21. Fracture temperature in TSRST test of SBS modified asphalt mixture.
Table 2 Summary of optimum modification scheme of SBS modified asphalt for different parameters. Test Methods
BBR
Burger’s Model GPC FTIR DSC TSRST
Parameters
SBS Content (%)
Stiffness M-value PG Low Temperature Relaxation Time Dissipation Energy Ratio Total Polymer Polystyrene Index Polybutadiene Index Decrease of Tg Fracture Temperature
Sulfur Content (%)
Linear SBS
Radial SBS
Linear SBS
Radial SBS
6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 4.5
6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 4.5
0.15 0.25 0.25 0.25 0.15 0.15 0.15 0.15 – –
0.15 0.15 0.15 0.25 0.15 0.15 0.15 0.15 – 0.15
Rubber Processing Oil Content (%) 8.0 8.0 8.0 8.0 8.0 – 8.0 8.0 8.0 8.0
Note: The slash in the cell means the content variation had no effect or the tests were not done.
maltene phase which caused the loss in intensity at Tg. According (T New g
A
T gA
B
T Bg ),
to the rule of mixture ¼W þW the proportion in maltene between aromatic and saturate changed and the content of the saturate increased. The saturate fraction is likely to be rich in alkanes and cyclo-alkanes. Furthermore, the addition of rubber processing oil lead to a slight decrease of Tg. It is because the rubber processing oil is rich in aromatic and saturate which has lower Tg. As shown in Figs. 18 and 19, the Tg of radial SBS modified asphalt decreased along with the increment of SBS content and the degree of decrease was similar to those of linear SBS modified asphalt. It was also because that the SBS was swollen by the saturate and aromatic in the base binder which changed the proportion of the maltene. Furthermore, the increment of sulfur had no influence on Tg of radial modified asphalt. It indicated that addition of sulfur did not influence the absorption of saturate and aromatic Fig. 19. As illustrated in Fig. 20, the peak of 4.5L (4.5% linear SBS modified asphalt) was lower than that of base binder. As mentioned before, it was because SBS was swollen by saturate and aromatic fraction in base binder which caused a loss in intensity at Tg. However, when rubber processing oil was added, the intensity of Tg increased significantly. When the content of rubber processing oil was 8%, the peak at Tg was as high as that of base binder. It implied when content of rubber processing oil was 8%, it supplemented the lost amorphous fraction in base binder. Furthermore, the Tg of 4.5% SBS modified asphalt with 8% rubber processing oil decreased for about 3 °C compared with base binder. It was because the rubber processing oil was rich in saturate and aromatic.
4.5. Thermal stress restrained specimen test result For further investigation on the low temperature performance of SBS modified asphalt mixture, TSRST test was conducted according to AASHTO TP 10-93 [35]. During specimen preparation, sup12.5 was chosen as the gradation. The designed asphalt content is 5% (by weight) and air void is controlled at about 8%. The fracture temperature results of the SBS modified asphalt mixtures were illustrated in Fig. 21. As illustrated in Fig. 21, the failure temperature of SBS modified asphalt mixture was lower than that of base asphalt mixture. However, when the content of SBS was more than 4.5%, the further addition of SBS led to an increase of fracture temperature. The optimum SBS content for low temperature performance of mixture seem to be 4.5% both for linear and radial SBS modified asphalt mixture and it was not coherent with the results in BBR tests. It was observed that all the samples failed in a catastrophic mode. The failure temperature results in TSRST test might be not able to reflect the true low temperature performance of the SBS modified asphalt mixture with high SBS content. If the samples were notched before TSRST test, the results might be more accurate in reflecting the low temperature performance of SBS modified asphalt mixture. For radial SBS modified asphalt mixture, the optimum sulfur content seem to be 0.15%. While, the difference between 4.5R_0.15S and 4.5R_0.25S was very small. Furthermore, the addition of rubber processing oil improved the low temperature performance of SBS modified asphalt mixture significantly. It seems that, the modification by adding maltene fraction to asphalt is more effective compared with adding SBS polymer.
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5. Summary and conclusions 5.1. Summary In this paper, rheological and chemical tests were used to evaluate the effect of SBS type, SBS content, sulfur content and rubber processing oil on the low temperature properties of SBS modified asphalt binder. To find out the optimum modification scheme of the SBS modified asphalt for low temperature properties, the results of tests are summarized in Table 2. As illustrated in Table 2, optimum content of SBS for binder was 6% and optimum SBS content for mixture was 4.5%. For linear SBS modified asphalt, the optimum content of sulfur was 0.15% for stiffness, DER, total polymer and FTIR parameters. While, the optimum content of sulfur was 0.25% considering relaxation capacity. As for radial SBS modified asphalt, 0.15% content of sulfur is best choice for most parameters. Rubber processing oil can promote the low temperature properties of SBS modified asphalt and base asphalt. The better content of rubber processing oil in this study was 8%. 5.2. Conclusions (1) According to the low temperature rheological test and thermal stress restrained specimen test results, the optimum modification scheme of SBS modified asphalt binder and mixture was summarized. (2) Low temperature properties of SBS modified asphalt are mainly limited due to lack of m-value. Addition of sulfur and rubber processing oil can improve its m-value and promote SBS modified asphalt to be balanced in stiffness and mvalue. (3) According to the results of GPC, FTIR and DSC test, SBS polymer is swollen by saturate and aromatic fraction in base asphalt which led to a decrease of maltenes content, while further addition of rubber processing oil can supplement the decrease of maltenes. It is speculated that addition of polymer led to a lack of maltenes which had bad effect on low temperature properties. (4) The fracture temperature results in TSRST test indicated that 4.5% of SBS was optimum content, addition of sulfur and rubber processing can improve the low temperature performance.
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