Synergy Effect Of Nano-Organic Palygorskite On The Properties Of Star-Shaped SBS-Modified Asphalt Part 1
Jul 24, 2023
Abstract: With the rapid development of economic construction, styrene-butadiene-styrene (SBS)- modified asphalt is being more and more widely used in highway engineering, but there are still many deficiencies in the process of its use. To further improve its performance for use, nano organic palygorskite (A-Pal) and star-shaped SBS were compounded to obtain modified asphalt in this study. The high-temperature stability of SBS-modified asphalt was enhanced after incorporation with A-Pal for the high-temperature stability test by a dynamic shear rheometer. The A-Pal should improve the surface free energy and adhesion of SBS-modified asphalt by the water stability test analysis. The aging test shows that A-Pal can reduce the thermal oxygen decomposition of SBS and improve the anti-aging performance and fatigue resistance of SBS-modified asphalt. A-Pal has a certain improvement effect on the low-temperature performance of SBS-modified asphalt as shown by a low-temperature crack resistance test. A-Pal-compounded SBS-modified asphalt features good storage stability in normal temperatures with the lowest critical compatibility temperature.
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Keywords: palygorskite; SBS; modified asphalt; rheological properties
1. Introduction
In recent years, nano-materials and nano-technology have been applied more frequently in the field of pavement traffic materials, and nano-modified asphalt has become one of the hot topics of research [1–4]. At present, nano-layered silicate materials are generally applied to asphalt materials because of their large output and good performance [5–7]. The nano-layered silicate with a special crystal structure, which makes asphalt molecules enter the layered structure, can increase the layer spacing, improving the form peel structure, which prevents oxygen from penetrating the asphalt and delaying its aging. Therefore, nano-modified asphalt has good anti-rutting and anti-aging properties [8–10]. At the same time, organically modified nanomaterials can also improve the dispersion degree of polymer in asphalt, providing broad development prospects for modified asphalt in the future.
Palygorskite (Pal), also known as attapulgite, is a layered chain of water-rich magnesium–aluminum silicate clay minerals. It has a reputation as “the king of earth” for its wide range of applications. The crystal structure of Pal is characterized by the double-layer Si-O tetrahedral sheets that are connected with the single-layer (Mg, Al)-O octahedron sheets, and the unit layers are connected by oxygen to form a pore-like crystal structure [11]. The pores are filled with zeolite water and crystal water to form a fibrous single crystal. The single fiber has a length of about 0.5 to 1.0 µm, some even up to 1 cm, and a diameter of about 20 to 30 µm [12]. Pal has been widely used in the fields of coating materials [13], cement [14], asphalt, and other building materials due to its good rheology, adsorbability, and lower cost [15–17].
Several studies show that the presence of nano-organic Pal can effectively improve the aging resistance of asphalt and compatibility between polymer and asphalt. Zhang et al. [10] synthesized organic-Pal under microwave irradiation and applied it to Styrene butadiene rubber (SBR)-modified asphalt. They found that organic-Pal improved the compatibility and storage stability of SBR-modified asphalt. Then, they studied the rheological and morphological properties of SBR-modified asphalt with organic-Pal and found that organic-Pal has a positive effect on improving the viscoelasticity and anti-rutting properties of SBR-modified asphalt [18]. Sun et al. [15] applied Pal to epoxy asphalt and found that it has a good effect on tensile and adhesive properties. Jin et al. [19] applied organic-Pal to asphalt and found that the aging resistance of asphalt was greatly improved. At present, the effect of organic-Pal on styrene-butadiene-styrene (SBS)-modified asphalt is rarely studied. To further understand its effect and improve the performance of SBS-modified asphalt, this study used the star-shaped SBS modifier YH-801 and the nano-organic palygorskite (A-Pal) to prepare compounded SBS-modified asphalt. The light part of the asphalt can be adsorbed by the Pal with strong adsorption, such that the colloidal structure of the asphalt can be changed and the temperature stability of the modified asphalt can be improved [20].
2. Materials Preparation and Test Method
2.1. Materials
The 70# asphalt (AH-70) was produced by Maoming Petrochemical Co., Ltd. (Guangzhou, China) with the basic performance test results shown in Table 1. The palygorskite originated in Jiangsu, China. The basic performance parameters are shown in Table 2. The star-shaped styrene-butadiene-styrene block copolymer YH-801 (SBS4303) was produced by Yueyang Baling Petrochemical (Hunan, China) with a block ratio of 30/70.
2.2. Preparation of A-Pal-Compounded SBS-Modified Asphalt
Based on our previous research [19,21], Pal was treated with 1 mol/L HCl solution at 60 ◦C for 1 h to remove some large particles and cationic outside the raw material, then washed to neutral and dried. The treated Pal and γ-aminopropyltriethoxysilane (APTES) were dispersed in a xylene solution, and the condensation reflux method was used for magnetic stirring for 10 h, then washed several times with the filtrate, dried and crushed to obtain A-Pal to enhance compatibility with the asphalt matrix. The amounts of 0 wt%, 1 wt%, 3 wt%, and 5 wt% A-Pal, which composited 5 wt% of the SBS-modified asphalt, were prepared by the melt blending method (named AH-70+5Y, AH-70+5Y+1A, AH-70+5Y+3A, and AH-70+5Y+5A, respectively).
2.3. Characterization
A fluorescence microscope (FM) was used to describe the phase morphology of modified asphalt with short-wave blue-purple light (λ = 420 nm) excitation (DM3000, Leica). The phase morphology of the fluorescent component in the asphalt was observed by optical microscopy to further study the correlation between microstructure and macroscopic properties [22].
2.4. High-Temperature Rheological Evaluation
The high-temperature performance of asphalt refers to the ability of asphalt to resist permanent deformation under load, which was evaluated by a dynamic shear rheometer (DSR, MCR 301, Anton Paar, Austria) for temperature scanning and frequency scanning tests. The temperature scanning test was carried out by the AASHTO T315-05 [23] to study the effect of temperature change on the complex shear modulus G* and phase angle δ of A-Pal-compounded SBS-modified asphalt, with a heating rate of 2 ◦C/min and a temperature of 40~90 ◦C. Most asphalt under the working temperature of pavement belongs to the pseudo-plastic non-Newtonian fluid, and the viscosity of asphalt decreases with increasing shear rate. When the shear rate was extremely high or very small, the viscosity of the pseudo-plastic non-Newtonian fluid approached a constant, and the region where the viscosity of asphalt did not change with the shear rate was called the first Newtonian flow region and the second Newtonian flow region. The viscosity of the pseudo-plastic non-Newtonian fluid was in the first region and reached a maximum when it was constant, which is called zero shear viscosity (ZSV) [24]. The viscosity of the pseudo-plastic non-Newtonian fluid was in the second region and reached a minimum when it was constant, which is called the interfacial shear viscosity (ISV). The test results were fitted by the Carreau model and the calculation of ZSV [25]. The test at a temperature of 60 ◦C according to the AASHTO T315-05, 25 mm of the oscillating plate and a film thickness of 1 mm was used for the frequency scanning test in the range of 0.01–100 Hz, and the curve was scanned by exponential growth.
2.5. Water Stability Evaluation
The contact angles of the A-Pal-compounded SBS-modified asphalt samples were measured by the contact angle measuring instrument (DSA100, Kruss, Germany). The sessile drop method was carried out with pure water, formamide, and ethylene glycol. The surface free energy was calculated by the Owens–Wendt–Rabel–Kaelble (OWRK) method [26], and the relationship between the three was expressed by the OWRK method as follows.
where γsl is the surface free energy of the solid–liquid phase, γl is the surface free energy of the liquid, γs is the surface free energy of the solid, γl d is the dispersion component of the liquid, γs d is the dispersion component of the solid, γl p is the polar component of the liquid and γs p is the polar component of the solid.
Based on the surface free energy data analysis of three common mineral materials, the work of adhesion (Was) for the asphalt on the surface of the mineral material was calculated as shown in Equation (3) [27,28].
Bring Equation (1) into Equation (2) to get:
where γa d is the dispersion component of the asphalt, γs d is the dispersion component of the mineral material, γ p a is the polar component of the asphalt, and γ p s is the polar component of the mineral material.
The change of Gibbs free energy (∆Gaws) in each stage of spalling damage can be expressed by the work of exfoliation [29] and the calculation expression as follows:
Bring Equation (1) into the above Equation to get:
where γw is the surface free energy of the water, γ d w is the dispersion component of the water, and γ p w is the polar component of the water.
2.6. Aging Performance Evaluation
The aging performance of A-Pal-compounded SBS-modified asphalt was evaluated by the short-term aging, long-term aging, and fatigue factor. The mass loss rate (MLR), softening point increment index (∆S), rutting factor aging index (RAI), and zero shear viscosity aging index (ZSVAI) of asphalt samples were analyzed after aging treatment in the rolling thin film oven test (TFOT) and pressure aging vessel (PAV) to simulate the short-term and long-term aging of asphalt by AASHTO R28-09 [30]. The critical temperature (fatigue limit temperature) grade corresponding to the fatigue factor (G* × sinδ >5000 kPa) was tested from the temperature fatigue test, as an index for evaluating the fatigue resistance of asphalt.
2.7. Low-Temperature Rheological Evaluation
The low-temperature performance of asphalt refers to the ability of asphalt to resist cracking under load. The low-temperature crack resistance of the modified asphalt after TFOT+PAV aging was evaluated by a bending beam rheometer (BBR), by the specification AASHTO T313-12 [31]. According to the specification, 6 ◦C was the test range until the asphalt’s performance did not meet the requirements. The flexural creep stiffness and m value were tested under the temperatures 0, −6, −12, −18, and −24 ◦C with a load of 0.980 ± 0.05 N for 240 s.
3. Results and Discussion
3.1. Morphological Characteristics
FM was carried out to observe the distribution and structure of SBS and A-Pal in the modified asphalt [32]. To enhance the discrimination between asphalt and modifiers, the asphalt part of the image is displayed as black, and the polymer part is shown as green bright spots by adjusting the brightness shown in Figure 1. Asphalt is displayed as the continuous phase, and the dispersed-phase SBS was dispersed as the form of an island in the matrix pitch [33]. Figure 1b shows a large amount of small blocky SBS crosslinks in the asphalt, which accounts for a small proportion and the scattered distribution of the asphalt without A-Pal. SBS features a low ability to absorb soft asphaltenes from asphalt, resulting in low compatibility. After adding 1 wt% A-Pal (Figure 1c), the proportion of fluorescent substances was slightly increased, and the dispersion was still unevenly distributed in the asphalt. The ability of SBS polymer to absorb soft asphaltenes after A-Pal was added had a certain increase, which leads to the volume expansion of SBS polymer and the increase in the swelling degree [22]. With the addition of A-Pal (Figure 1d,e), the proportion of fluorescent substances continues to increase, and the degree of dispersion becomes more and more uniform. After adding A-Pal, the compatibility of SBS polymer with asphalt was improved to some extent; the low temperature and fatigue performance of modified asphalt should improve [19].
3.2. High-Temperature Performance of A-Pal-Compounded SBS-Modified Asphalt
The high-temperature stability is an important indicator of asphalt. The variation of the rutted factor obtained by the temperature scanning test is shown in Figure 2. It can be seen that the addition of SBS and A-Pal contributes to the improvement of the rutting factor and the rutting resistance. After SBS was added to asphalt, the rutting factor of asphalt showed a large increase and more improved resistance to rutting. The rutting factor continued to increase with the incorporation of A-Pal to further increase the rutting resistance. Compared with the previous study, it is consistent and has not changed due to the different types of SBS [18,19]. The sample with A-Pal content of 5 wt% had the highest rutting factor and the strongest anti-rutting ability, indicating that the incorporation of A-Pal can improve the temperature stability of SBS-modified asphalt. The value of the rutting factor decreases with the increasing temperature, and the rate was the same, indicating that all the modified asphalt samples have the same rheological properties.
The rutting factor critical temperature is the corresponding temperature factor of G*/sin δ = 1.0 kPa in the rutting factor test in the Strategic Highway Research Program (SHRP). The critical temperature of each sample is shown in Table 3. SBS could raise the critical temperature by 7.2 ◦C, compared with AH-70. After adding A-Pal, the rutting factor critical temperature continuously increased, and the maximum temperature increased to 75.7 ◦C, which was 20% higher than the matrix asphalt.
The ZSV of the modified asphalt increases with the increase of A-Pal content, which was similar to the test result of the rutting factor (Table 4). The ZSV of the asphalt matrix increased by 296% with the addition of SBS. After adding 1 wt% A-Pal, the ZSV of the modified asphalt increased to 949.4 Pa·s, which was higher than that of the modified asphalt with only SBS. With the increasing A-Pal content, the value of ZSV continues to increase, and the ZSV value of the 5 wt% compounded SBS-modified asphalt increases to 1291.8 Pa·s, which was 423% higher than that of the asphalt matrix. It showed that the compounding method was effective in improving the high-temperature stability of the asphalt binder.
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