Bitumen is usually regarded as a colloidal suspension of asphaltene particles sound by resin in an oily continuous matrix. It can be divided into four generic groups (SARAs): saturates(s), aromatics (A), resin (R), and asphaltenes (As). Bitumen has been used since ancient time, its easy availability and low cost makes it a widely used material in the modern and high technology world, especially in road pavement as the binder of aggregate [1, 2].
However, bitumen is brittle and hard in cold environments and soft in hot environments. And there are many types of failures, such as the low temperature cracking, fatigue cracking, and the rutting (or permanent deformation) at high temperature, that can reduce the quality and performance of road pavement during “in life” service [3,4]. The modification of bitumen is an attempt to extend the service life and improve the performance of asphalt pavements. Various elastomer and plastomer modifiers have been sought to improve rut resistance, fatigue resistance, cracking resistance, and stripping resistance resulting from increases in bitumen elasticity and stiffness [5-8]. But there are relatively a few polymer modifiers suitable for bitumen modification. When the polymer was used as bitumen modifiers, the polymers should be compatible with bitumen in blended process with conventional mixing equipments and can maintain their main properties during prolonged storage a high temperature [9-11].
Among the polymer modifiers of bitumen, Styrene-butadiene-styrene copolymer (SBS) triblock copolymer presented the best results in improving the bitumen properties. The styrene is usually the dispersed phase and provides the strength of the material, while the butadiene is the continuous phase and contributes to the elasticity of SBS. But SBS and bitumen is immiscible, and SBS tends to separate from bitumen at high storage temperatures due to the lack of compatibility between SBS and bitumen . Compatibility is most often defined by the extent of segregation of the bitumen and polymer during hot storage. Pavement construction techniques require the modified bitumen to remain their premium properties during mixing, storage and application on the road [13, 14]. In practice, the bitumen is stored at high temperatures previous to its application on the road. The instability problems of the polymer modified bitumen due to the modifier-rich phase separation may affect the final properties of the pavement . Hence, a suitable degree of storage stability is required for modified bitumen. There are several researches to improve the high temperature storage stability of polymer modified bitumen [16, 17]. A common way of stably modified bitumen is adding a stabilizer to reduce the interfacial tension, increase the adhesion, and obtain required properties.
The grafting polymerization is a well known method to modify the chemical and physical structure of polymers and to tailor properties for a specifically application [18-21]. Graft polymerization can be initiated by various methods, such as [gamma] rays, plasma treatment, ultraviolet and chemical initiators (including grafting polymerization in solution and in melt). Maleic anhydride (MAH), acrylic acid, benzyl acrylate, glycidyl methacrylate, acryl amide, and long chain unsaturated monomers are most commonly grafted onto polymers containing butadiene [22-24]. Many studies have appeared dealing with grafting vinyl monomers onto polymers. Solvothermal method, as one of the most effective methods, has been widely used to prepare the grafting copolymers [25, 26]. Previous literatures have indicated that the SBS-g-MAH can improve the storage stability and compatibility of SBS modified bitumen. However, little information has been done on the effects of grafting degree (GD) on the properties of modified asphalt. Before the SBS-g-MAH modified asphalt was used, more studies should be provided. Thus, further studies are still necessary to establish.
In this study, the solvothermal method was used to prepare the SBS grafted with maleic anhydride (SBS-g-MAH) copolymers, and the effect of the concentration of MAH was test to obtain the SBS-g-MAH copolymers with different GD. After preparing the SBS-g-MAH modified bitumen, the related tests of bitumen including the softening point, penetration, ductility, elastic recovery, penetration index, viscosity, storage stability and dynamic shear properties were carried out to evaluate the effects of GD of grafted SBS on properties of bitumen.
The 60/80 pen grade bitumen, with penetration of 69 dmm (deci-millimetre, 25[degrees]C, ASTM D5), softening point of 46.8[degrees]C (ASTM D36), ductility of 179cm (25[degrees]C, ASTM D113) and viscosity of 0.35 Pa s at 135[degrees]C and 275 Pa s at 60[degrees]C (ASTM D4402), was used to prepared modified bitumen. Chemical compositions of the bitumen are listed in Table 1.
TABLE 1. Chemical composition of base binder. Composition Measured values (wt%) Saturates 23.24 Aromatics 32.17 Resins 33.86 Asphaltenes 10.73
The SBS used, Grade 1301, was produced by the Yueyang Petrochemical, China. This was a linear-like SBS, containing 30 wt% styrenes, and the average molecular weight of SBS is 120,000 g/mol.
Benzoyl peroxide (BPO) was purified by dissolution in chloroform at room temperature and precipitation in cool methanol.
MAH was produced by Shanghai Chemical Solvent Factory and used without further purification.
A flask equipped, with stirrer, thermometer and condenser, was employed to prepare SBS-g-MAH copolymer. First, amount of SBS and MAH was dissolved in toluene at 80[degrees]C [+ or -] 1[degrees]C Second, BPO was added into mixtures in nitrogen protected atmosphere and the reaction was terminated after 3 h. Third, the volatile component was removed by reduced pressure distillation. The nonvolatile sample was dried at room temperature. The residual MAH was extracted by ethanol (Soxhlet 24 h). The all purified polymer was collected and dried to constant weight in a vacuum oven at 80[degrees]C.
The samples were cast into films onto a potassium bromide (KBr) thin plate with chloroform as a solvent. IR spectroscopy information on SBS and grafted SBS was obtained on a Fourier transform infrared (FTTR) spectrophotometer (NEXUS, Thermo Nicolet). The spectra were obtained by 4 [cm.sup.-1] resolution. To compare the effect of the purification on the spectrum of the grafted SBS, we prepared several samples after further purification. No significant changes were observed in the FTIR spectrum of the grafted SBS after further purification. The results indicated that the experimental procedure was effective.
The GD of MAH was determined by a back titration procedure. One gram purified sample was dissolved in chloroform (about 100 mL), then 50 mL ethanol solution of NaOH (0.1 mol/L) was added. The mixed solution was refluxed for 1 h with stirring. And then back titration was performed by HCl (0.1 mol/L) with bromothymol blue as indicators. The GD% was calculated according to the following formula:
GD% = ((([m.sub.0]-[m.sub.1]) x C x M x [10.sup.-3])/2W) x 100 (1)
where [m.sub.0] and [m.sub.1] is the amount of HCl (ml) consumed by pure SBS and grafted sample, C the molar concentration of HCl (mol/L), M the molecular weight of MAH, and W is the weight of sample (g).
Bitumen was heated to 170[degrees]C [+ or -] 5[degrees]C in an oil-bath heating container until it flowed fully. The 4 wt% SBS or SBS-g-MAH was mixed into the bitumen under 5000 rpm rotation speed about 40 min to ensure the blend became essentially homogenous.
The conventional physical properties of bitumens (empirical theological tests), including penetration at 25[degrees]C, softening point, ductility at 15[degrees]C and elastic recovery at 25[degrees]C, were tested in accordance with ASTM D5, ASTM D36, ASTM D113, and ASTM D6084, respectively. The penetration is an empirical test used to measure the consistency of bitumen. The test as follow showed. A container of bitumen is brought to the standard test temperature in a thermostatically-controlled water bath. The sample is placed under a needle of prescribed dimensions. The needle is loaded with a 100 g weight and is allowed to penetrate the bitumen sample for 5 seconds. The depth of penetration is measured in units of 0.1 mm (dmm) and is reported as penetration units. Softening point is measured by ring and ball (R&B) method. The test consists of taking a brass ring filled with bitumen and suspending it in a breaker filled with water. A steel ball of specified dimensions and weight is placed in the center of the sample. The bath is heated at a controlled rate of 5[degrees]C/min. When the bitumen softens, the ball and bitumen sink toward the bottom of the breaker. The temperature is recorded at the instant when the softened bitumen sinks the prescribed distance and touches the bottom plate. The ductility measures the distance in centimeter that a standard briquette of bitumen will stretch before breaking. The cross section of the briquette at its smaller dimension is one square centimeter. The test sample is brought to temperature in a water bath which is maintained at the standard temperature. The two ends of the sample are separated at the rate of 5 cm/min until rupture. Elastic recovery is useful in confirming that a material has been added to the bitumen to provide a significant elastomeric characteristic. The elastic recovery test determines these characteristics by using the ductility apparatus to stretch a sample. After the sample has reached a specified elongation, it is cut and allowed to recover. The specimens are pulled to a specified distance at a specified speed and at a specified temperature. Unless otherwise specified, the test shall be made at a temperature of 25[degrees]C and with a speed of 5 cm/min. The viscosity of modified bitumen was measured at 60 and 135[degrees]C according to ASTM D4402 using Brook-field viscometer (model DV-II+, Brookfieid Engineering).
Dynamic shear properties were measured with dynamic shear rheometer (DSR, MCR300, Anton Paar) in a parallel plate configuration with a gap width of 1 mm. Rheological tests were performed under controlled strain condition. The principal rheological parameters obtained from the DSR were complex modulus (G’), storage modulus (G’), loss modulus (G”) and the phase angle ([DELTA]). G* is defined as the ratio of maximum shear stress to maximum strain and provides a measure of the total resistance to deformation. The [DELTA] is the phase shift between the applied stress and strain responses during a test and is a measure of the viscoelastic balance of the material behavior. Temperature sweeps (from 30 to 90[degrees]C) with 2[degrees]C increments were applied at a fixed frequency of 10 rad/s.
Hot storage test is used to evaluate the high temperature storage stability of modified bitumens. The storage stability of modified bitumens was tested as following procedure: the sample was poured into an aluminum tube (25 mm in diameter and 140 mm in height). The tube was sealed and stored vertically in an oven at 163[degrees]C [+ or -] 5[degrees]C for 48 h. Then the aluminum tube containing the modified bitumen was took out of the oven and cooled in a refrigerator at -7[degrees]C for 4 h [+ or -] 5 min. Finally, the tubes were cut into three equal sections. The sections from the top and bottom were placed in separate dishes in an oven at 163[degrees]C until bitumen had well fluid to pour into softening point rings. If the difference between softening point of the top and the bottom sections of the tube was less than 2.2[degrees]C, the sample could be regarded as storage stable blend.
RESULTS AND DISCUSSION
The grafting reaction was carried out at 80[degrees]C, which can increase solubility and reactivity of MAH and SBS. The sealed experimental system can also avoid the evaporation of MAH and solvent. And it is favorable to the grafting reaction. Figure 1 shows the FTIR spectra of SBS and grafted SBS. Different from the FTIR spectrum of pure SBS, the spectrum of grafted SBS showed two new absorbance peak at 1780 [cm.sup.-1] (C=0 stretching from anhydride) and 1735 [cm.sup.-1] (C=0 stretching from anhydride hydrolysis). And the C-O-C stretching vibrations peak at 1300-1100 [cm.sup.-1] region are observed in SBS-g-MAH copolymer besides the inherent vibrations of SBS. Thus, FTIR results indicated that MAH was successfully grafted onto SBS.
Determination of the GD%
Generally, the grafting monomer concentration has important effect on grafting reaction. The different GD of the grafted SBS could be obtained by control the MAH concentration. Figure 2 shows that the GD initially increased with the increasing of MAH concentration. This may be because the number of monomer molecules diffusing throughout the reaction medium and reaching the SBS backbone governed the extent of grafting. But the GD reached a maximum at 2.1 g/100 mL, and then decreased with the further increasing of MAH concentration. It is maybe attribute the homopolymerization of MAH when the MAH concentration increased. The results also mean that the prepared procedure was an effective method to the grafting copolymerization of MAH onto SBS. The different GD of the SBS-g-MAH was prepared by control the MAH concentration to investigate the effect of GD on the properties of bitumen.
Conventional Properties of Bitumen
The effects of GD on conventional properties of the grafted SBS modified bitumen are showed in Table 2. The results showed that the softening point, ductility and elastic recovery of modified bitumen increase and penetration decrease when the GD of grafted SBS increases. It is means that the high temperature performance of SBS modified bitumen is improved when MAH grafted onto SBS. And the increase in softening point and decrease in penetration is result of the stiffening effect of grafting. Increase in ductility and elastic recovery is indicated that low temperature cracking resistance of SBS modified bitumen is improved by grafting MAH onto SBS.
TABLE 2. Conventional properties of modified bitumen with different GD of the grafted SBS. Different GD Properties 0 1.41 3.15 4.81 5.23 Softening point ( C) 59.6 61.7 63.5 65.8 68.2 Penetration (drum) 58 56 53 50 49 Ductility (cm) 68 70 73 77 79 Elastic recovery (%) 83.2 84.8 86.1 87.2 87.6 penetration index (PI) 1.33 1.66 1.85 2.12 2.48
Temperature susceptibility is defined as the change in the consistency parameter as a function of temperature. The penetration index (PI) can be used to determine the temperature susceptibility of the bitumen. It was calculated mathematically from penetration and softening point as the Shell Bitumen Handbook introduced. The penetration index is described in Eq. 2.
PI = (1952-500 X log([Pen.sub.25])-20 x SP)/(50 x log([Pen.sub.25])-SP-120) (2)
where: Pen25 is the penetration at 25[degrees]C in tenth of millimeters and SP is the softening point of bitumen.
Higher PI values indicate lower thermal susceptibility. Asphalt mixtures containing bitumen with higher PI are more resistance to low temperature cracking as well as permanent deformation or rutting. As shown in Table 2, the modified bitumen with SBS-g-MAH showed high PI value compared to no grafting SBS modified bitumen. It is indicated that the temperature susceptibility of SBS modified bitumen was improved when MAH was grafted onto SBS. Thus, the SBS-g-MAH modified bitumen can be used in widely service temperature.
From above results, It can be concluded that the high temperature performance, low temperature resistance and Temperature susceptibility of SBS modified bitumen is modified by grafting MAH onto SBS.
Most modified bitumens are non-Newtonian fluids at mixing and compacting temperature range in situ currently. The effect of viscosity on bitumen’s workability is very important in selecting proper mixing and compacting temperatures. A rotational viscometer is usually used for measuring the viscosity of Newtonian or non-Newtonian fluids. The rotational viscosity was determined by measuring the torque acting the cylindrical spindle surface when the cylindrical spindle immersed in bitumen is rotated at given a rotational speed (often 20 rpm) at a constant temperature. The effect of GD of grafted SBS on the viscosity of SBS-g-MAH modified bitumen at 135[degrees]C is showed in Fig. 3. The results indicated that the viscosity of modified bitumen increase with the GD of grafted SBS. The viscosity of SBS modified bitumen is 1.34 and 3.23 Pa s when the GD of grafted SBS is zero and 5.23%. According to SHRP specifications, viscosity should be less than 3 Pa s at 135[degrees]C to ensure pump ability at the hot mix asphalt plant. As shown in Fig. 3, viscosity of SBS-g-MAH modified bitumen becomes higher with GD of grafted SBS, especially GD is 5.23%. From the aspect of viscosity, the appropriate GD should be selected before modified bitumen with MAH grafted SBS.
To further investigate the effects of GD of grafted SBS on the viscosity of SBS-g-MAH modified bitumen, the viscosity of bitumen at 60[degrees]C and modification index (Modified/nOrigina/) was tested and analyzed. The results showed in Table 3. The modification index increase when the GD of grafted SBS in test range. Compared with the modification index at 135[degrees]C, it is high modification index at 60[degrees]C for SBS-g-MAH modified bitumen with different GD. It is mean that the SBS-g-MAH have better rut resistance (antipermanent deformation) at service temperature and easier to mixing, compacting at high temperature. It is benefit to prepare asphalt mixture and form an eligible pavement surface.
TABLE 3. Effect of the GD of grafted SBS on the viscosity of SBS-g-MAH modified bitumen. GD [eta] [eta] [eta] modified/ [eta]modified/ (%) (Pa s) (Pa s) [eta]origina/ [eta]origina/ (135[degrees] (60[degrees] (135[degrees] (60[degrees] C) C) C) C) 0 1.34 1075 3.83 3.91 1.41 1.52 1335 4.34 4.85 3.15 1.87 1765 5.34 6.42 4.81 2.78 2455 7.94 8.93 5.23 3.23 2635 9.23 9.58
Effect of SBS-g-MAH on Rheological Properties
The most important effect of modifiers on bitumen is changing of viscoelasticity because there is strong correlation between rutting resistance at high temperature and elastic modulus. Dynamic shear test can be used to determine the complex modulus (G*) and the phase angle ([DELTA]). The former can be related to the material strength and the latter provides information about the ratio between elastic and viscous response during the shearing process. Figure 4 shows the curves of G* and 3 versus temperature for the SBS-g-MAH modified bitumen with different GD. It can be seen that MAH grafted onto SBS results in enhancement of complex modules and reduction of phase angle. The results also showed that the minor increase in G* at low temperatures but major increase was observed at high temperatures. It is indicated that the SBS-g-MAH modified bitumen with high GD have higher viscoelastic behaviors and rutting resistance at high temperature, which may be caused by the fine network structure in SBS-g-MAH modified bitumen. The phase angle is more sensitive to chemical and physical structure of modified bitumen. Phase angle is defined as the phase difference between stress and strain in an oscillatory test and it is zero and 90[degrees] for elastic and viscous materials. The phase angle of SBS-g-MAH modified bitumens is less than that of SBS modified bitumen at the same temperature. The decreasing extent of phase angle becomes greater with increasing the GD of grafted SBS in SBS-g-MAH modified bitumens at test temperature. This trend reveals that the elastic response of bitumen was improved compared to the SBS modified bitumen.
The Strategic Highway Research Program (SHRP) tests take advantage of the rheological measurements to analyze the properties of bitumens used in road pavements, which adopted the temperature of the bitumens when G*/ sin [delta] is equal to 1 kPa as a criterion for the bitumen at high temperature. The rheological parameter G*/sin [delta] is defined as rutting parameter to demonstrate resistance of bitumen to the permanent deformation under repeated loads. Plots of rutting parameter versus temperature are displayed in Fig. 5 for the SBS-g-MAH modified bitumen with different GD. The rutting parameter of modified bitumen is increased when MAH grafted onto SBS. As shown in Table 4, when G*/sin [delta] was equal to 1 kPa the temperature of bitumens was 76[degrees]C for pure SBS modified bitumen, 78.5[degrees]C, 82.1, 87.2, and 89.5 for SBS-g-MAH modified bitumen with GD of 1.41, 3.15, 4.81, and 5.23%, respectively. It is indicated that the modified bitumen added with grafted SBS have higher rut resistance than that added with pure SBS.
TABLE 4. Effect of the GD of grafted SBS on high performance grade of modified bitumen. GD [T.sub.SHRP] ([degrees]C, (%) G*/sin [delta] = 1 kPa) 0 76.0 1.41 78.5 3.15 82.1 4.81 87.2 5.23 89.5
The SBS modified bitumen are multiphase systems, the difference in the solubility parameter and density between SBS and bitumen, phase separation would take place during storage at higher temperatures. The storage stability problem of polymer modified bitumen is a key technical problem in the use of many bitumen/polymer blends as an actual alternative to neat bitumen. The polymer droplets in bitumen are usually flocculation and float on the top of the modified bitumen at a high temperature under static situation. On the other hand, polymer modified bitumen occurs phase separation at a high temperature due to Brownian motion followed by gravitational flocculation and later creaming. Effects of the GD of the grafted SBS on storage stability of the SBS-g-MAH modified bitumen are shown in Fig. 6. Obviously, for bitumen modified by SBS without grafted by MAH, the difference in the softening points was large, which imply that the phase separation of the SBS/bitumen blend was serious. When the MAH was grafted onto SBS to prepare modified bitumen, the storage stability of the SBS modified bitumen was improved significantly. Figure 6 showed the difference of softening point in the top and bottom section of SBS-g-MAH modified bitumen with different GD. The difference change from 3.1[degrees]C for no grafted SBS modified bitumen to 0.9[degrees]C for 5.23% of the GD of modified bitumen. It is maybe attribute to the grafted SBS restricting SBS and bitumen molecular motion. Thus, the storage stable SBS modified bitumen can be obtained by grafting MAH onto SBS. According to the rule of the difference of softening point measurement were not more than 2.2[degrees]C, the no grafted SBS modified bitumen can not be used in pavement after long time storage at high temperature. But modified bitumen by SBS with 3.16% GD meet the modified bitumen requirement about storage stability and it can be used in pavement after storing at high temperature for long time, which is very convenient for user.
To verify the storage stable SBS modified bitumen can be obtained by grafting MAH onto SBS, the polymer content was increased to 7 wt%. The results showed in Table 5. The results indicated that the storage stability of SBS modified bitumen decrease remarkably when SBS content increasing. The difference change from 12.6 [degrees]C for no grafted SBS modified bitumen to 4.8 [degrees]C for 5.23% of the GD of modified bitumen. Therefore, it is an effective system to prepare modified bitumen with better compatibility using SBS-g-MAH.
TABLE 5. Effects of the GD on storage stability of (he modified bitumen (Containing 7 wt% polymer). GD The delta between top and (%) bottom softening point ([DELTA]S) 0 12.6 1.41 9.2 3.15 7.5 4.81 6.1 5.23 4.8
The SBS-g-MAH copolymer was successfully synthesized by solvothermal method in the presence of BPO as initiator was proposed, and different GD of the SBS-g-MAH was obtained by control the MAH concentration. Compared with pure SBS modified bitumen, the SBS-g-MAH copolymer increase the softening point, ductility, elastic recovery, complex modulus and rutting parameter, and decrease penetration, phase angle and temperature susceptibility of modified bitumen. The compatibility and storage stability of SBS modified bitumen is enhanced when MAH is grafted onto SBS. The high temperature performance and low temperature cracking resistance of modified bitumen is improved. And the more GD of grafted SBS, the more improvement to modified bitumen.