Effect of Zn/Al Layered Double Hydroxide Containing 2-Hydroxy-4-n-octoxy-benzophenone on UV Aging Resistance of Asphalt
UV radiation is a main factor to reduce the service life of asphalt pavement due to the UV aging of asphalt binder. To obtain enhanced UV aging resistance, an organic UV absorber called 2-hydroxy-4-n-octoxy-benzophenone (HNOB) had been intercalated into an inorganic UV absorber called Zn/Al layered double hydroxide (LDH) to play a combined anti-UV role in asphalt binder. Fourier transform infrared spectroscopy revealed that HNOB anions have been intercalated into the interlayer galleries of Zn/Al-LDH containing HNOB anions (Zn/Al-HNOB−-LDH). X-ray diffraction results of Zn/Al-LDH containing anions () and /styrene-butadiene-styrene (SBS) modified asphalt disclosed that asphalt molecules entered into LDH interlayer galleries to form an expanded phase structure. UV-Vis absorbance patterns showed that Zn/Al-HNOB−-LDH has a better capacity of blocking UV light due to the synergetic effect of HNOB and Zn/Al-LDH. The chemical fractions analysis, conventional physical tests, and rheological tests of SBS modified asphalt, modified asphalt, and Zn/Al-HNOB−-LDH/SBS modified asphalt before and after UV aging testified that Zn/Al-HNOB−-LDH can improve the UV aging resistance of SBS modified asphalt more significantly.
Asphalt is an important byproduct from crude oil and is widely used in most of highways due to its excellent viscoelastic property. However, the unsaturated aromatic composition in asphalt can be easily oxidized due to the complex effect of UV radiation and oxygen. Consequently, high-temperature rutting and low-temperature cracking may occur in the short lifetime of pavement. The maintenance procedure is costly and the damaged pavement may threaten the safety of traffic. Therefore, it is very essential to mitigate the UV aging process and thereby prolong the service time of asphalt pavement.
In the last few years, layered clays have attracted attention to retard aging process of asphalt binder. These cationic layered clays include kaolinite , montmorillonite , rectorite , and vermiculite . Compared with these cationic layered silicates, an anionic clay called layered double hydroxide (LDH) can be incorporated with organic anions more easily [5–7]. The chemical formula of LDH is . While M2+ is a divalent metal cation, M3+ is a trivalent metal cation and is an interlayer anion . LDH has been used as a new type of UV absorber in asphalt due to the special layered structure. At present, there are some published articles concerning the application of LDH in asphalt binder. The effect of magnesium (Mg)/aluminum (Al) LDH containing anions (denoted as ) on the aging properties of pristine asphalt was investigated by Huang et al. ; their study showed that LDH improved the physical properties of UV aged asphalt. The rheological properties of UV aged asphalt containing were studied by Wu et al. . They found that is beneficial for enhancing the aging resistance of base asphalt. The effect of on the aging properties of crumbed rubber modified asphalt (CRMA) was studied by Pang et al. . The results displayed that the addition of slowed down the degradation of CRMA when subjected to UV aging. The UV blocking properties of zinc and as the additives for asphalts were compared by Wang et al. . They found that increased particle size and decreased band gap resulted in a better UV aging resistance for . However, the above articles did not consider the difference of hydrophilic clay surface and hydrophobic asphalt. Therefore, it has the potential to modify the LDH with organic anions to make it more compatible with asphalt binder.
In this research, Zn/Al LDH intercalated with benzophenone acid anions (Zn/Al-HNOB−-LDH) was prepared using calcination recovery method from calcined , which can be synthesized by coprecipitation method. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and transmission electron microscope (TEM) were used to characterize the structural properties of Zn/Al-HNOB−-LDH and . UV-Vis spectrometer was used to evaluate the UV adsorbing capacity of Zn/Al-HNOB−-LDH, , and HNOB. The UV aging resistance of asphalt/LDH composites was evaluated by chemical fraction analysis, conventional physical tests, and rheological tests before and after UV aging process.
Zn(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, 2-hydroxy-4-n-octoxy-benzophenone (HNOB), and Na2CO3 were provided from Shanghai Chemical Industrial Company (China). All chemicals and reagents were of analytical grade and deionized water was carbonate-free throughout the preparation process. Styrene-butadiene-styrene (SBS) modified asphalt was produced by China Best Modified Asphalt Co., Ltd., Hubei, China, and had a penetration of 48 dmm at 25°C, a ductility of 30 cm at 5°C, a softening point of 81°C, and a dynamic viscosity of 2.4 Pa·s at 135°C. The chemical structure of HNOB is shown in Figure 1.
2.2. Preparation of and Zn/Al-HNOB−-LDH
200 mL Zn(NO3)2·6H2O (1 mol·L−1) solution and 100 mL Al(NO3)3·9H2O (1 mol·L−1) solution were mixed and then the mixture was slowly added to 100 mL Na2CO3 (1 mol·L−1) solution under N2 atmosphere at 70°C. The pH of the reaction mixture was adjusted 9.0 by dropwise addition of NaOH solution (1 mol·L−1). After 24 h stirring, the white precipitate was washed and filtered several times using hot deionized water. The residual solid was dried in an oven at 105°C for 12 h and was ground to obtain shown in Figure 2(a). The Zn/Al-HNOB−-LDH shown in Figure 2(b) was prepared from by calcination recovery. First, 200 g of was calcined at 500°C for 2 h in a muffle furnace to remove anions. Second, a cooling process was performed in a closed storage pan, from which the air had to be continuously evacuated using a vacuum pump to prevent the accumulation of carbon dioxide. Third, the calcined product and 500 mL of HNOB solution (at a concentration of 20 wt%) were mixed in a glass flask by stirring for 30 min. Finally, the mixture was then filtered, and the residue was dried at 105°C for 24 h and ground to obtain Zn/Al-HNOB−-LDH.
2.3. Preparation of LDH Modified Asphalt
Asphalt was heated at around 150°C in a mixer until it transferred to a well fluid. Then, 5 wt% Zn/Al-HNOB−-LDH or was added to asphalt and the mixture was blended in a high shear mixer at 4000 rpm for approximately 90 min to ensure the uniform dispersion of LDHs in the asphalt. The pristine asphalt without LDHs was also prepared following the same procedures due to oxidation in hot mixing process .
2.4. UV Aging Simulation
The UV aging procedure was performed in an oven equipped with an UV lamp to simulate the UV aging of asphalt in real condition. The melted asphalt was placed on a steel pan ( mm) for 12 days. The testing temperature was chosen as 60°C. The intensity and wavelength of UV radiation were 5000 μW/cm2 and 300 nm.
The FTIR spectra of the HNOB, Zn/Al-HNOB−-LDH, and were obtained using a Nexus FTIR spectrometer from Thermo Nicolet Corp. (New York, USA). The scan range was between 4000 cm−1 and 400 cm−1 with a resolution of 4 cm−1. XRD graphs of the Zn/Al-HNOB−-LDH and were obtained using an X-ray diffractometer (D/MX-IIIA, Rigaku, Japan) with Cu Kα radiation ( nm; 40 kV, 50 mA). The data were collected in step-scan mode at a scanning rate of 0.02°/s. The diffractograms were scanned from 5° to 20°. TEM analyses of the Zn/Al-HNOB−-LDH and powders were conducted using a JEM-2100F electron microscope (JEOL, Japan) at a 200 kV accelerating voltage. The UV-Vis absorbance spectra of the HNOB, Zn/Al-HNOB−-LDH, and were recorded by a UV spectrometer (Shimadzu-3600, Japan) equipped with an integrating sphere. BaSO4 was used as a background and the wavelength range was selected within the range of 250–700 nm. AFM (Model DI Nanoscope IV, American Veeco Company) was used to observe the structure change of asphalt before and after aging at the nanometer-scale resolution. Asphalt was carefully placed on a 10 mm × 10 mm × 1 mm glass plate and the scan region area was 10 μm × 10 μm.
2.6. Chemical Fraction Analysis
The chemical compositions of asphalt before and after UV aging can be measured by thin-layer chromatography with flame ionization detection (TLC-FID), which is purchased from Iatron Laboratories Ltd. in Japan. The analysis process was as the following steps. First, 100 mg of asphalt sample was dissolved in the 5 mL of dichloromethane to form a solution. Second, the chromarods were cleaned and activated by FID-flame and 1 μL of the prepared solution for each sample was spotted on a chromarod. Third, 70 mL of -heptane, toluene/-heptane (4/1 by volume), and toluene/ethanol (11/9 by volume) were, respectively, spotted on the chromarods. When they expanded to 100 mm, 50 mm, and 25 mm, then they were placed in an oven at 80°C to evaporate the solvent. The four chemical compositions of the samples can be detected by a thin-layer chromatography with flame ionization detection (TLC-FID). Each sample was measured four times repeatedly to obtain the average number.
2.7. Conventional Physical Tests of Asphalt
According to the standard JTJ052-2000 (Standard Test Methods of Asphalt and Bituminous Mixtures for Highway Engineering, China), the conventional physical properties of asphalt are including penetration, ductility, softening point, and dynamic viscosity. The difference of these test results before and after UV aging can be considered as the important aging indexes to evaluate the UV aging resistance of different asphalts .
2.8. Rheological Properties
Rheological properties of asphalt were evaluated by dynamic shear rheometer (DSR, Gemini 150, Bohlin, USA). Temperature sweep was conducted at a constant frequency of 10 rad/s and the temperature range was from 0°C to 80°C. Frequency sweep was performed at various temperatures of 20°C, 40°C, 60°C, and 80°C and frequency range was from 0.1 rad/s to 400 rad/s. The tested asphalt was placed between two parallel plates with a diameter of 8 mm and a gap of 2 mm.
3. Results and Discussion
3.1. FTIR Analysis
The FTIR spectra of , HNOB, and Zn/Al-HNOB−-LDH were given in Figure 3. Figure 3(a) showed that the peak appearing at 3480 cm−1 was due to the bending vibrations of hydroxyl group and interlayer water molecules. The obvious absorption peak appearing at 1376 cm−1 was associated with the anions . The peaks observed within the range of 400–600 cm−1 were attributed to the translation vibration caused by Zn/OH and Al-OH and deformation vibration caused by HO-Zn/Al-OH . The FTIR spectrum of HNOB was shown in Figure 3(b). It can be seen that the peak at 1641 cm−1 was corresponding to the stretching vibration of C=O bond in carboxylic acid. The peaks at 1570 cm−1, 1505 cm−1, 1256 cm−1, and 1155 cm−1 were ascribed to the stretching vibration of C=C bond in the aromatic ring. The FTIR spectrum of Zn/Al-HNOB−-LDH was shown in Figure 3(c). Five obvious peaks at 1639 cm−1, 1571 cm−1, 1502 cm−1, 1259 cm−1, and 1159 cm−1 can be observed at nearly the same positions with HNOB. Moreover, compared with , Zn/Al-HNOB−-LDH showed no peak at around 1376 cm−1, indicating that HNOB anions have completely displaced anions.
The XRD patterns of , Zn/Al-HNOB−-LDH, modified asphalt, and Zn/Al-HNOB−-LDH/SBS modified asphalt are displayed in Figure 4. Figure 4(a) exhibited the characteristic peaks at 11.67°, which is due to the (003) diffraction peak of . Meanwhile, a characteristic peak at 6.43° can be observed in the XRD pattern of Zn/Al-HNOB−-LDH shown in Figure 4(b). Furthermore, a shift of the diffraction peak of SBS modified asphalt containing Zn/Al-HNOB−-LDH to a lower angle can be observed in Figure 4(b). All XRD data calculated by Bragg formula () for , Zn/Al-HNOB−-LDH, and their corresponding SBS modified asphalt are listed in Table 1.
It is clear that the interlayer distance of Zn/Al-HNOB−-LDH is wider than that of . This is attributed to the intercalation of HNOB into LDH by replacing anions. Figure 4(c) showed that the diffraction peak of the SBS modified asphalt containing appeared at 2θ angle of 11.67°, which was similar to that of . It implied that the interlayer distance of modified asphalt does not change. That means that asphalt molecules do not enter into interlayer galleries of and a separated phase structure is formed by asphalt and . Figure 4(d) showed that the diffraction peak in XRD pattern of Zn/Al-HNOB−-LDH/SBS modified asphalt appeared at 2θ angle of 4.59° and the calculated interlayer distance was 1.92 nm, which was apparently larger than that of Zn/Al-HNOB−-LDH. It reveals that asphalt molecules intercalated into Zn/Al-HNOB−-LDH to form an expanded phase structure. The main reason is that HNOB contains benzene ring, which has a similar structure to the aromatic component in asphalt. According to the principle of “better compatibility with similar polarities,” asphalt molecular chains can intercalate into Zn/Al-HNOB−-LDH to improve the compatibility between Zn/Al-HNOB−-LDH and asphalt. The schematic models of separated phase and expanded phase structures are presented in Figure 5.
3.3. Morphology of and Zn/Al-HNOB−-LDH
The TEM photomicrographs for and Zn/Al-HNOB−-LDH were shown in Figures 6(a) and 6(b). Figure 6(a) showed that the crystallites consisted of homogeneous platelets with dimensions ranging from 400 nm to 500 nm. Figure 6(b) showed that the distribution of Zn/Al-HNOB−-LDH platelets is less uniform than that of the platelets. This difference is attributed to the decarbonation of during calcinations . The release of anions caused the collapse of some metal cation layers, resulting in a rearrangement of the platelets. The memory effect of LDH led to the recovery of the Zn/Al-HNOB−-LDH platelets via the intercalation of HNOB, resulting in defects and interconnected edges. Figures 6(c) and 6(d) showed that the interlayer distances of and Zn/Al-HNOB−-LDH were 0.78 nm (×10−9 m) and 1.37 nm, which were consistent with the interlayer distance of the (003) plane obtained from the X-ray analysis. Therefore, the TEM photomicrographs further confirm that the interlayer distance of Zn/Al-HNOB−-LDH is wider than that of .
3.4. UV-Vis Absorbance
The UV-Vis absorption curves of HNOB, Zn/Al-HNOB−-LDH, and were shown in Figure 7. had an apparent UV absorption band within the range of 240 and 320 nm. Meanwhile, Zn/Al-HNOB−-LDH showed a stronger UV absorption band in the same range. The UV absorption was above 70% within the range of 260 and 330 nm and even reached 93% at 300 nm, indicating that Zn/Al-HNOB−-LDH has an excellent UV absorption capacity. Moreover, Figure 7 showed that an additional adsorption occurred within the range of 400 and 500 nm, while HNOB and had a complete UV light transmission in this range. This is because multilayers structure formed by host layers and guest anions had a synergetic effect on absorbing UV light, which enhanced the UV absorption intensity of Zn/Al-HNOB−-LDH. The UV adsorption and reflection mechanism of Zn/Al-HNOB−-LDH was illustrated in Figure 8. When the wavelength of UV light is similar to the interlayer distance of Zn/Al-LDH, the maximum UV reflection rates can be achieved . As a result, Zn/Al-HNOB−-LDH laminates with different interlayer distances lead to a wider range of UV absorption band. In summary, the expanded phase structures in Zn/Al-HNOB−-LDH contribute to a better UV barrier capacity.
3.5. Chemical Fraction Analysis
The chemical components of asphalt are divided into four fractions, which are including saturates, aromatics, resins, and asphaltenes. The schematic molecular formulas were shown in Figure 9. Saturate fraction consists of hydrocarbons with straight or branched chains and saturated rings. The molecular weight of saturates is usually within the range of 500 and 800 g/mol. Aromatic fraction is made of naphthenic aromatic compounds and its molecular weight is from 800 to 1000 g/mol. Resin fraction is composed of condensed aromatic compounds and its molecular weight is from 800 to 1000 g/mol. Asphaltene fraction consisted of polycyclic aromatic compounds with a chain length larger than 40 carbons and its molecular weight is between 1000 and 10000 g/mol. Loeber et al.  reported that the different fractions in the asphalt binder follow a colloidal law and the colloidal index (Ic) as is defined as follows:
The smaller Ic value is, the more “gel-like” asphalt becomes. UV aging can prompt the transition from sol to gel, which results in lowering Ic values of asphalts. Therefore, the aging degree of asphalts before and after UV aging can be determined by the decrement of Ic values.
The four fractions of SBS modified asphalt, modified asphalt, and Zn/Al-HNOB−-LDH/SBS modified asphalt before and after UV aging were shown in Table 2. Before UV aging, it can be seen that the Ic values of SBS modified asphalt, modified asphalt, and Zn/Al-HNOB−-LDH/SBS modified asphalt are 3.96, 3.98, and 3.97, respectively. Compared with , Zn/Al-HNOB−-LDH can slightly reduce Ic value of asphalt binder. It may be caused by the incorporation of organic HNOB− anions. After UV aging, the Ic values of asphalts decreased to different extents. The decrements of SBS modified asphalt, modified asphalt, and Zn/Al-HNOB−-LDH/SBS modified asphalt were 1.56, 1.12, and 0.57, respectively. It indicated that Zn/Al-HNOB−-LDH has a better UV aging resistance than .
3.6. Physical Properties of SBS Modified Asphalt
To evaluate the aging index, the differences of physical properties of asphalt can be considered as important aging indexes. The penetration, ductility, softening point, and viscosity of asphalt before and after UV aging can be measured to calculate the corresponding aging indexes [21, 22].
The aging indexes of SBS modified asphalt, modified asphalt, and Zn/Al-HNOB−-LDH/SBS modified asphalt were shown in Table 3. Compared with SBS modified asphalt, modified asphalt had higher RP and RD values but lower VAI and SPI values. For instance, RP and RD increased from 32% and 36% to 42% and 47%, while SPI and VAI decreased from 8.1°C and 152% to 6.9°C and 138%, respectively. It implied that UV resistance of SBS modified asphalt after addition of had been enhanced. Compared with , Zn/Al-HNOB−-LDH exhibited the increased RP (49%) and RD (54%) as well as the decreased SPI (5.3°C) and VAI (118%). It indicated that Zn/Al-HNOB−-LDH has a better UV resistance than . This is because the organic modification of LDH by HNOB− can significantly improve the compatibility and asphalt molecule can be well shielded in interlayer galleries. The oxidation, which is the main reason of asphalt aging, will be effectively prevented due to this shielding effect. Therefore, Zn/Al-HNOB−-LDH/SBS modified asphalt improves the UV aging resistance of SBS modified asphalt more evidently.
3.7. Rheological Tests
3.7.1. Aging Index of Complex Modulus
Due to the close correlation between complex modulus () and asphalt strength, the ratio between the value of aged asphalt and the value of the pristine asphalt can be used to evaluate the UV aging degree . This ratio can be defined as the UV aging index (AI), which can be calculated by (2). The lower AI value corresponds with a better UV aging resistance of this sample:
The AI values of pristine asphalt, , and Zn/Al-HNOB−-LDH were shown in Figure 10. The temperature range was from 0°C to 30°C and the scan rate is 10 rad/s. After the UV aging process, the AI values of and Zn/Al-HNOB−-LDH were lower than the control asphalt, indicating that both and Zn/Al-HNOB−-LDH can improve the UV aging resistance of asphalt. In addition, the AI values of Zn/Al-HNOB−-LDH were lower than , implying that Zn/Al-HNOB−-LDH can further improve the UV aging resistance of asphalt.
3.7.2. Frequency Sweep
The main distress factor of asphalt pavement is traffic load, which occurs at dynamic frequencies and temperatures. Generally, rheological performances of asphalt under this dynamic condition can be fitted by the time temperature equivalence principles. Any relaxation process of asphalt can be completed in a short time at a high temperature or in a long time at a low temperature. Therefore, raising the temperature or lowering the temperature has the same effect with prolonging the observing time or shortening the observing time [24, 25]. The frequency sweep curves of complex modulus at different temperatures () can be shifted along the frequency coordination to form a master curve. The shift factor can be defined as
In this equation, represents the frequency at the reference temperature, Hz; represents the shifted frequency at a certain temperature. represents the shift factor. The complex modulus curve at 20°C can be fitted by serving as a baseline according to time temperature equivalence principle. The master curves of complex modulus for each asphalt sample before and after aging were shown in Figures 11 and 12. Fitting shift factors were shown in Table 4.
The interaction extent between internal components of asphalt can be determined by activation energy, which can be calculated by
The plots of against (which were shown in Figure 13) can be drawn and can be calculated as the slope of the linear fitting line of the plots. It is known that UV aging can lead to the physical hardening, which corresponds to the increased flow resistance of asphalt. Obviously, the hardening asphalt needs more activation energy () to make it flow. Therefore, values of asphalt before and after UV aging can be used to determine the aging degrees.
Table 5 displayed the values of asphalt binders before and after UV aging. Before UV aging, the active energy of asphalts containing both and Zn/Al-HNOB−-LDH is higher than that of the pristine asphalt. It indicated that the interaction between asphalt fractions is stronger with the introduction of and Zn/Al-HNOB−-LDH, thus leading to the increased hardening components. After UV aging, both the values of the three samples increased, which can be explained by the increased Ic values as previously discussed. It was noteworthy that the difference of values of asphalt containing Zn/Al-HNOB−-LDH before and after UV aging was lowest among these three samples. That means Zn/Al-HNOB−-LDH can slow down the formation of hardening components in asphalt binder, resulting in a better UV aging resistance.
Zn/Al-HNOB−-LDH was prepared by calcination method from . FTIR spectra exhibited that HNOB− anions intercalated into the calcined . XRD patterns indicated that the interlayer distance increases from 0.78 nm to 1.37 nm when the HNOB− anions had intercalated into Zn/Al-LDH. Separated phase structure and expanded phase structure were formed in modified asphalt and Zn/Al-HNOB−-LDH/SBS modified asphalt, respectively. TEM morphologies further testified that the interlayer distance of Zn/Al-HNOB−-LDH is wider than that of . UV-Vis adsorption curves displayed that Zn/Al-HNOB−-LDH has a high UV absorption rate within the wavelength range of 260–330 nm. The chemical analysis, physical property tests, and rheological tests of asphalt before and after UV aging indicated that Zn/Al-HNOB−-LDH can significantly improve the UV aging resistance of SBS modified asphalt.
Conflict of Interests
The authors declare that there is no conflict of interests.
This work is financially supported by the Ministry of Transport of the People’s Republic of China (Grant no. 2013 318 800 020), the Independent Innovation Foundation of Wuhan University of Technology (Grant no. 145201016), the National Basic Research Program of China (973 Program) (2014CB932104), and the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2011BAE28B04). The authors gratefully acknowledge them.