High-temperature rutting and low temperature cracking of bitumen because of severe temperature susceptibility limits its further application. Therefore, it is necessary to modify bitumen . The addition of polymers to bitumen has been proved to be effective to improve the performance significantly. The pavement with polymer modified bitumens (PMB) exhibits greater resistance to rutting and thermal cracking, decreased fatigue damage, stripping, and temperature susceptibility [2-4]. Among the polymer modifiers of bitumen, styrene-butadiene-styrene (SBS) block copolymer became the best modifiers of bitumen because the physical and mechanical properties and rheological behavior of conventional bitumen can be improved significantly with the addition of SBS. SBS exhibits a two-phase morphology consisting of soft block and hard block [5, 6]. The styrene, referred as the hard block, is usually the dispersed phase, and provides the strength of the material; while the butadiene, the soft block, contributes to the elasticity of SBS . Unfortunately, SBS is destined to separate from the bitumen when stored at high temperature because of the poor compatibility between SBS and bitumen . Furthermore, SBS tends to degrade by exposure to heat, oxygen, and UV light since SBS contains an unsaturated bond, which may make a contribution to the deterioration of bitumen pavement [9-11].
Layered silicate is a type of mineral, which consists of layers of tetrahedral silicate sheets and octahedral hydroxide sheets . It mainly includes montmorillonite (MMT), vermiculite (VMT), and kaolinite clay (KC). Recently, layered silicates have been widely used for the modification of polymers [13-15]. Polymer chains can be intercalated into the interlayer of clay and make the clay disperse into the polymer matrix at nanometer-scale, which leads to significant improvements in thermal, mechanical, and barrier properties of polymers [16, 17]. SBS/KC composites have been successfully used to improve the high-temperature storage stability of SBS modified bitumen [7, 9]. The improvement in high-temperature storage stability could be attributed to the KC in the SBS/KC composites for it decreased the difference of densities between SBS and bitumen. However, properties except for high-temperature storage stability of SBS modified bitumen cannot be improved obviously by KC, which may be attributed that the layer of KC is not benefit to be intercalated, not forming the nanocomposite. Compared with KC, MMT has been proved to form nanocomposite easily.
In this article, clay/SBS modified bitumen composites were prepared by the melt blending with Na-MMT and OMMT, respectively. OMMT is the organic cationic surfactants modified MMT. In such case, the inorganic actions in the galleries of the pristine Na-MMT are exchanged for tallow composites/surfactants, which make Na-MMT become compatible with the asphalt. The effects of Na-MMT and OMMT on physical properties, dynamic theological behaviors, and aging properties of SBS modified bitumen were investigated.
Bitumen, AH-90 paving bitumen was obtained from SK Chemicals, Korea. The physical properties of the bitumen were listed in Table 1. SBS, Grade 791H, was produced by the Yueyang Petrochemical, China. It was a linear-like SBS containing 30% styrene, and the weight-average molecular weight was 120,000 g [mol.sup.-1]. The sodium montmorillonite (Na-MMT), with interlayer cation of [Na.sup.+] and having a cation exchange capacity (CEC) of 90 meq/100 g, particle size of 200 mesh, and the aspect ratio being 200-500, was supplied by Fenghong Clay Chemical Factory, Zhejiang, China. The organophilic montmorillonite (OMMT), which is MMT exchanged with octadecylammonium ions and having a CEC of 130 meq/100 g and the same particle size with Na-MMT was purchased from the same factory.
Preparation of MMT/SBS Modified Bitumen Composites
SBS modified bitumens were prepared using a high shear mixer at 170[degrees]C and a shearing speed of 4000 rpm. First, bitumen was heated to become a fluid in an iron container, then upon reaching about 170[degrees]C, SBS was added to the bitumen and sheared for 40 min to produce SBS modified bitumen. After that, Na-MMT or OMMT was added into SBS modified bitumen, and the mixtures were blended at a fixed rotate speed about 60 min to produce MMT/SBS modified bitumen composites. When compared with MMT/SBS modified bitumen, SBS modified bitumen in the absence of MMT was prepared under the same conditions.
X-ray diffraction (XRD) graphs were obtained using a Rigaku D/max 2400 diffractometer with Cu K[alpha] radiation ([lambda] = 0.154 nm, 40 kV, 120 mA) at room temperature, the diffract to grams were scanned from 1.5[degrees] to 15[degrees] in the 2[theta] range in 0.02[degrees] steps, scanning rate was 2[degrees]/min.
Physical Properties Test
Classical Tests. The physical properties of bitumen, including softening point, penetration (25[degrees]C), and ductility (5[degrees]C), were tested according to ASTM D36, ASTM D5, and ASTM D113-86, respectively.
Brookfield viscometer (Model DV-II+, Brookfield Engineering, USA) was employed to measure the viscosity of modified bitumens at 135[degrees]C according to ASTM D4402.
High-Temperature Storage Stability Test. Static storage tests were used to estimate high-temperature storage stability of modified bitumens. The experimental system consisted of a tube (32 mm in diameter and 160 mm in height), vertically placed in an oven, at 163[degrees]C for 48 h and, then it was taken out, followed by cooling down to room temperature and being cut into three equal sections. If the difference between the softening points of the top and the bottom sections was less than 1[degrees]C, the sample was considered to have good high-temperature storage stability. Otherwise, it was designated to be unstable.
Dynamic Rheological Characterization
Dynamic rheological measurements for all the samples (with MMT and without MMT) were performed in plate-plate mode (diameter 2.5 cm), in the Dynamic shear rheometer (Model AR2000, TA). Temperature sweeps (from 50 to 80[degrees]C) with 1[degrees]C increments were applied at a fixed frequency of 10 rad/s and variable strain. The rheological parameters were measured for calculating viscoelastic parameters such as complex modulus (G*), phase angle ([delta]), and rut factor (G*/sin[delta]).
Thin film oven test (TFOT) and the rolling thin film oven test (RTFOT) were used to simulate the change in the properties of bitumen during the plant hot mixing and the lay down process, and the pressure aging vessel (PAV) that utilizes the residue from RTFOT was used to simulate long-term aging after 5-10 years of service. Short-term and long-term laboratory aging of SBS modified bitumen and MMT/SBS modified bitumen composites were performed using RTFOT (ASTM D 2872) and PAV (AASHTO PP1), respectively. The standard aging procedures of 163[degrees]C and 75 min for the RTFOT and 100[degrees]C, 2.1 MPa, and 20 h for the PAV were used.
RESULTS AND DISCUSSION
Structure of MMT/SBS Modified Bitumen Composites
Similar to polymer/layered silicate nanocomposites, layered silicate modified bitumen has two structure types, namely intercalated structure and exfoliated structure, as shown in Fig. 1. The intercalated structures correspond to well-ordered multilayered structures where the bitumen chains are inserted into the gallery space between the silicate layers. The exfoliated structures correspond to delaminating structures where the individual silicate layers are no longer close enough to interact with the gallery cations .
The degree of exfoliation of silicate layers of Na-MMT and OMMT in the bitumen was investigated by using XRD techniques from the position, shape, and the intensity of the basal reflections in the XRD patterns. Several XRD curves for Na-MMT, OMMT, Na-MMT/SBS modified bitumen composite, and OMMT/SBS modified bitumen composite were shown in Fig. 2. As reported earlier, when [d.sub.001] peak shift to a lower angle, the interlayer of the Na-MMT or OMMT will be widened . The interlayer spacing can be calculated according to the Bragg equation (2d sin [theta] = [lambda]), which were given in Table 2. It can be found that the crystalline peak of the pristine Na-MMT was at 2[theta] = 5.88 ([d.sub.001] = 1.50 nm) and it was at 2[theta] = 3.02 ([d.sub.001] = 2.92 nm) in the Na-MMT/SBS modified bitumen composite. Therefore, we can conclude that the bitumen and SBS were intercalated into the MMT gallery and Na-MMT/SBS modified bitumen composite may form an intercalated structure. According to Fig. 2, we could not observe any crystalline peak in XRD for the OMMT/SBS modified bitumen composite, which implied that the interlayer d-spacing of OMMT in the OMMT/SBS modified bitumen composite was more than 4.4 nm. It may suggest that the layer of OMMT had already been peeled off and OMMT/SBS modified bitumen composite may form an exfoliated structure.
The above-mentioned results can be explained by different microstructure between Na-MMT and OMMT. Na-MMT layers were hydrophilic and the spaces between them were small, which made the intercalation and peeling of layers harder; while OMMT interlayer were already enlarged by organic molecules, which made the micro-structures of OMMT layers change and OMMT become lipophilic . This kind of structure of OMMT provided the benefits for the insertion of bitumen molecules.
Effects of MMT on the Physical Properties of SBS Modified Bitumen
Effects on Softening Point, Ductility, and Penetration. The effects of the Na-MMT and OMMT content on the physical properties of SBS modified bitumens can be seen in Table 3 as an increase in softening point and a decrease in penetration with increasing MMT content. The results showed that MMT can improve the high temperature properties of SBS modified bitumen. When compared with Na-MMT, OMMT showed better effect in improving properties of SBS modified bitumen. This may be the reason that OMMT/SBS modified bitumen composites formed an exfoliated structure, which made OMMT disperse equably in bitumen. So, the properties of OMMT/SBS modified bitumens composites were better than Na-MMT/SBS modified bitumens composites.
Effects on Viscosity. Figure 3 showed that the viscosity of MMT/SBS modified bitumen composites at 135[degrees]C tends to increase with the increase in the content of MMT. The increase in the viscosity may be due to the formation of intercalated and exfoliated structure in MMT/SBS modified bitumen composites, because the movement of bitumen molecule chains was obstructed by the layer of MMT at high temperature. In addition, the viscosity of OMMT/SBS modified bitumen composites was higher than Na-MMT/SBS modified bitumen composites at the same content of MMT. Such different behaviors can be explained by Na-MMT and OMMT respective dispersing situations in bitumen.
Effects on High-Temperature Storage Properties. Because SBS is not totally compatible with bitumen, if a mixture of SBS and bitumen is kept at high temperature, the SBS will often separate out from the bitumen. When the mixture is kept under quiescent conditions at high temperatures, SBS will aggregate to form coarse particles, which will result in the difference in properties between top and bottom sections . The high-temperature storage stabilities of MMT/SBS modified bitumen composites were shown in Fig. 4. The difference in softening point between top and bottom was 2.2[degrees]C for SBS modified bitumen in the absence of MMT. When SBS modified bitumen including 1% OMMT or 2% Na-MMT, the difference in softening point between top and bottom was only 0.3[degrees]C and 0.7[degrees]C, respectively, which showed that the storage stability of SBS modified bitumen was improved. However, when the content of OMMT or Na-MMT exceeded 1 or 2%, respectively, the difference in softening point increased with increasing MMT content. This may be a consequence of the precipitation of excessive MMT particles, which were not intercalated or exfoliated. In addition, the storage stability of the OMMT/SBS modified bitumen composites was better than that of the Na-MMT/SBS modified bitumen composites, which indicated that OMMT had better compatibility and dispersing ability with SBS modified bitumen than Na-MMT.
Effects of MMT on Dynamic Rheological Properties of SBS Modified Bitumen
Dynamic shear tests are advantageous because the data can be acquired within the linear range of the bitumen blend in a loading mode that is similar to that of traffic loading . Figure 5 showed the curves of complex modulus (G*) versus temperature for the MMT/SBS modified bitumen composites. G* is defined as the ratio of maximum shear stress to maximum strain and provides a measure of the total resistance to deformation when the bitumen is subjected to shear loading. According to Fig. 5, increase in the G* value exhibited a more viscoelastic behavior of MMT/SBS modified bitumen composites than that of SBS modified bitumen at high temperature. Moreover, it can be seen that with increasing MMT contents, the G* value of the modified bitumens increased. When compared with Na-MMT/SBS modified bitumen composites, OMMT/SBS modified bitumen composites exhibited higher complex modulus at the same content of MMT, which may be caused by the exfoliation of OMMT layers in SBS modified bitumen. These results suggested that both Na-MMT and OMMT can improve the viscoelastic behaviors of SBS modified bitumen.
Figure 6 showed the result of phase angles ([delta]) against temperature. The phase angle, defined as the phase difference between stress and strain in an oscillatory test, is a measure of the viscoelastic balance of the material behavior. Measurement of phase angle is generally considered to be more sensitive to the chemical and physical structure than complex modulus for the modification of bitumens . According to Fig. 6, we can see that obvious decrease in phase angle with the addition of MMT at high temperature. The deduction in [delta] value exhibits a more elastic behavior of bitumen. The decreasing extent of phase angle became greater when the content of MMT increased. Additionally, OMMT/SBS modified bitumen composites exhibited lower phase angle than Na-MMT/SBS modified bitumen composites, which may be caused by their respective dispersing structures in SBS modified bitumen.
In Strategic Highway Research Program (SHRP) specifications, the rheological parameter, G*/sin[delta], was selected to express the contribution of the bitumen binder to permanent deformation. This value reflects the total resistance of a binder to deform under repeated loading (G*) and the relative amount of energy dissipated into nonrecoverable deformation (sin[delta]) during a loading cycle . The G*/sin[delta] value should be larger than 1 kPa at 10 rad/s (1.6 Hz) for the binder at a maximum pavement design temperature. With a higher value of the parameter rate, there is higher resistance to permanent deformation. Figure 7 indicated that, when temperature ranges from 50 to 80[degrees]C, there was a increase in rut factors of MMT/SBS modified bitumen composites compared with SBS modified bitumen, which can be attributed to the increase in G* and decrease in phase angle when MMT content increases. Figure 7 also showed the isochronal plots of G*/sin[delta], revealing distinct differences because of the ability of MMT modifiers to interact with bitumen. The effect of Na-MMT and OMMT contents on the performance grade of the modified bitumen were listed in Table 4. It can be seen that the performance grade of MMT/SBS modified bitumen composites were improved with the addition of MMT the MMT/SBS modified bitumen composites had a higher performance grade than SBS modified bitumen. When G*/sin[delta] = 1 kPa, the temperatures of MMT/SBS modified bitumen composites were higher than that of SBS modified bitumen composites, which indicated that MMT is helpful for the improvement of rutting resistance.
Effects of MMT on Aging Properties of SBS Modified Bitumen
Aging is a very complex process in bitumen, and the degree of complexity increases when PMB are involved. Changes in the properties of aged PMB were dependent on a combined effect of bitumen oxidation and polymer degradation . Figures 8-10 showed the changes in the physical properties of SBS modified bitumen and SBS modified bitumen with 3% MMT after aging.
Viscosity data often function as a window through which other characteristics of bitumen may be observed . Accordingly, the change of viscosity before and after aging makes sense to the study on the aging process of bitumen. The viscosity aging index (VAI) was used to characterize the aging extent, and calculated by measuring the viscosity of the samples before and after the test as shown in Eq. 1 . The higher the VAI value, the more aged the sample is .
VAI(%) = [[Aged viscosity value – Unaged viscosity value]/Unaged viscosity value] x 100 (1)
Figure 8 showed the VAI of modified bitumens after RTFOT and PAV aging. An increase of the viscosities was observed in all the samples after RTFOT and PAV; however, the VAI of SBS modified bitumen decreased obviously after the addition of MMT, which indicated that MMT/SBS modified bitumen composites have better aging resistance than SBS modified bitumen. It can be concluded that resistance to aging of SBS modified bitumen can be improved by the addition of MMT, which was ascribed to barrier of the intercalated or exfoliated structure to oxygen, reducing efficiently the oxidation of bitumen, and the degradation of SBS. When compared with Na-MMT/SBS modified bitumen composite, OMMT/SBS modified bitumen composite exhibited lower VAI values. It indicated that exfoliated structure was more effective than intercalated structure for enhancing the aging resistance of SBS modified bitumen.
Similar to the VAI, the change of softening point is an indication of the sensitivity to aging of bitumen. It can be calculated as Eq. 2. A smaller change of softening point reflects lower influence of aging.
Change of softening point ([degrees]C) = aged softening point – unaged softening point (2)
Figure 9 revealed the changes of softening point of the modified bitumens after RTFOT and PAV. At the end of the RTFOT, the softening point of the samples increased. However, the softening point of all the samples decreased after PAV. Two possible reasons for the changes are (1) the molecular weight of the base bitumen is increased due to oxidation, which increased the softening point; (2) polymer molecules are degraded in size, and as a result, the bitumen-polymer interactions may be reduced dramatically, which induce the reduction of the softening point . The increase of softening point after RTFOT means that effect of the oxidation of bitumen exceeded influence of the degradation of SBS, while the result was contrary after PAV.
We can also see from Fig. 9, the change values of softening point increased in the order: OMMT/SBS modified bitumen composites, Na-MMT/SBS modified bitumen composites, and SBS modified bitumen in the absence of MMT. These results indicated that aging resistance of SBS modified bitumen had been improved by the addition of MMT.
The retained ductility can also be used to evaluate the resistant ability to aging. It can be calculated as Eq. 3. A higher value of retained ductility shows better resistance to aging.
Retained ductility (%) = [aged ductility/unaged ductility] x 100. (3)
The retained ductility of modified bitumens after aging can be seen in Fig. 10. The difference of retained ductility of all the samples was rather small after RTFOT aging. However, MMT/SBS modified bitumens, especially OMMT/SBS modified bitumen, showed larger retained ductility compared to SBS modified bitumen after PAV aging, which indicated that the resistance to long-term aging of modified bitumens was improved with the addition of MMT. OMMT/SBS modified bitumen exhibited the highest retained ductility, owing to the formation of exfoliated structure.
Clay/SBS modified bitumen composites were prepared by melt blending with different amounts of Na-MMT and OMMT. The XRD results showed that the Na-MMT/SBS modified bitumen composite may form an intercalated structure, whereas the OMMT/SBS modified bitumen composite may form an exfoliated structure.
The addition of MMT to SBS modified bitumen increased both the softening point and viscosity. However, a decrease was shown on the values of ductility due to the addition of MMT. The high-temperature storage stability can be improved by MMT with a proper amount added. Both Na-MMT and OMMT can improve the dynamic rheological properties of SBS modified bitumen. MMT/SBS modified bitumen composites exhibited higher complex modulus, lower phase angle, and higher rutting resistance. MMT/SBS modified bitumen composites showed better resistance to aging than SBS modified bitumen, which was ascribed to barrier of the intercalated or exfoliated structure to oxygen, reducing efficiently the oxidation of bitumen and the degradation of SBS. When compared with Na-MMT, OMMT had greater effects in improving properties of SBS modified bitumen, which indicated that exfoliated structure was more effective than intercalated structure in enhancing properties of SBS modified bitumen.
We are grateful to Dr. Shanjun Gao and Dr. Lili Wu for helping with this study.
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Jianying Yu, Lin Wang, Xuan Zeng, Shaopeng Wu, Bin Li
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of China
Correspondence to: Jianying Yu; e-mail: email@example.com
Contract grant sponsor: Ministry of Communications of the People’s Republic of China and the Science and Technology Department of Hubei Province, People’s Republic of China.