Preparation and Microscopic Characterization of Asphalt Modified by Ethylene Metal-free Phthalocyanine-dispersed Graphene

HUANG Gang, HUANG Hao,2, ZHANG Xia, HE Junxi, ZHOU Chao, TAN Yan,FENG Manman, LÜ Chuncheng

(1. National and Local Joint Engineering Laboratory of Traffic Civil Engineering Materials, Chongqing Jiaotong University, Chongqing 400074, China; 2. Xinjiang Transportation Planning, Survey, Design and Research Institute Co., Ltd, Wulumuqi 830006, China; 3. China Merchants Chongqing Testing Center for Highway Engineering Co., Ltd, Chongqing 400074, China)

Abstract: Graphene-modified asphalt (GMA) for road application was prepared via using metal-free phthalocyanine-dispersed to modify an SK-70# base asphalt with graphene. The preparation parameters are as follows: the content of graphene is 0.26% based on the mass percentage of absolute ethanol, the content of nonmetal phthalocyanine is 190% based on the mass percentage of graphene, and then the GMA is prepared via unique high-speed shearing with continuing to ventilate nitrogen, which can prevent the aging of modified asphalt in the high-speed shearing process, and effectively evaluate the modifier. The penetration, softening point, force ductility, and fracture energy of GMA were significantly improved based on the base asphalt.Thus, the incorporation of graphene could enhance the base asphalt’s high- and low- temperature stability.The modification mechanism was researched via metallographic microscopy, computed tomography (CT),Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). Adsorption and physical dispersion of the asphaltenes and resins in the phthalocyanine-graphene system were confirmed.

Key words: ethylene metal-free phthalocyanine; GMA; performance; dispersion and adsorption;asphaltene; resin

1 Introduction

Graphene is a honeycomb two-dimensional inorganic nanomaterial formed by hybridized carbon(C)-atoms and has various excellent properties. For example, graphene has a large specific surface area and π-bonds, as in benzene rings. Additionally, the C-atoms can resist damage by bending deformation[1-6]. Honget al[7]used sulfuric acid intercalated graphite oxide(SIGO) to prepare graphene via rapid reduction and expansion stripping at just above 100 ℃, and pointed out that SIGO was easy to use the direct oxidation offspring of graphite in sulfuric acid. Graphene is exceptionally stable but aggregates easily. Therefore,improving the applicability and dispersion of graphene is an active research area. As a common road construction material, asphalt is widely used in the pavement.Due to the structural similarity between graphene and asphalt components, it is speculated that there is mutual adsorption between them in an ideal state[8].Chenet al[9]found that graphene/tourmaline can be used as the original mineral powder substitute to add to the asphalt mixture. The added composite powder can reduce smoke emission in the asphalt. Liuet al[10]found the stability of asphalt mixture can be effectively improved by adding graphene oxide. The research on asphalt binder shows that graphene oxide’s addition can effectively enhance the surface free energy (SFE)potential of asphalt binder and mixture[11]. Wanget al[12-14]showed that expanded graphite nanoflakes into asphalt could effectively improve the recovery of the asphalt mixture’s fracture energy. Yaoet al[15]added graphite nanoflakes into asphalt to improve the corresponding high- and low- temperature properties. And the complex shear modulus and the anti rutting factor,and the water damage of the final prepared asphalt mixture are significantly improved. Huanget al[16]provedthat the expanded graphene and asphalt exist in intercalation and peel off, and part of them are dispersed in asphalt. Liuet al[17]found that the graphene oxide (GO)can significantly improve the viscosity, consistency,stiffness, and other properties of asphalt, but has no significant impact on the low-temperature performance during characterizing the modified asphalt, which was prepared via adding GO and SBS at the same time.Chenet al[18]designed a composite modified asphalt designed via adding graphene nanoplatelets (GNPs) to rubber modified asphalt and evaluated the viscoelasticity under high temperature, different stress, and other temperature. The result showed that the high- and lowtemperature performance and viscoelasticity of rubber modified asphalt could be effectively improved via adding GNPs. The addition of graphene in asphalt can effectively improve the high-temperature performance and mechanical properties of asphalt, proving that the addition of graphene can improve the performance of asphalt[9,19,20]. To effectively disperse graphene in asphalt, phthalocyanine is selected to disperse graphene in asphalt to enhance asphalt performance.

Table 1 Process specifications for graphene prepared by ball milling

Table 2 Performance parameters of SK-70# base asphalt

Asphalt is the most important pavement material,which is used in road engineering. The objective of this study was to incorporate the excellent properties of graphene to comprehensively improve the performance of viscoelastic asphalt by reducing or eliminating rutting, cracking, surface abrasion in asphalt pavement,and the pavement cost in the service life cycle. This study provides reference for the development of durable long-life asphalt pavement with a high performance.

2 Experimental

2.1 Materials

The graphene used in this study was prepared by ball milling. The process are shown in Table 1. Absolute ethanol (analytical purity >99.9%) was used as the base solvent for the graphene mother liquor, trichloroethylene was used as the base solvent for asphalt, the dispersant was non-metallic phthalocyanine produced by Shanghai Aladdin Biochemical Technology Co., Ltd,and the base asphalt was SK-70# asphalt. The asphalt materials were tested and evaluated based on theStandard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering(JTG E20-2011) and theTechnical Specifications for Construction of Highway Asphalt Pavements(JTG F40-2004), as shown in Table 2. The preferred dispersant is metal-free phthalocyanine. Relevant parameters are shown in Table 3.

Table 3 Parameters of metal-free phthalocyanine dispersant

2.2 Equipment

Preparation of modified asphalt: the materials were weighed using an AUW320 electronic analytical balance (with 0.000 1-g divisions) from Shimadzu Corporation, Japan; a heating test was performed using a DL-1 universal electric furnace from Beijing Zhongxing Weiye Instrument Co., Ltd., China; an ultrasonic test was conducted using a JP-040 ultrasonic cleaner from Shenzhen Jiemeng Cleaning Equipment Co., Ltd.,China; centrifugation was performed using a H1850 desktop high-speed centrifuge from Hunan Xiangyi Centrifuge Co., Ltd., China; rotary evaporation was performed using an R-215 rotavapor system from Buchi Laboratory Equipment, Switzerland; and a shear test was carried out using a BME200L high-shear mixing emulsifier (motor power: 0.4 kW; speed: 0-10 000 rpm) from Shanghai Weiguang Machinery Manufacturing Co., Ltd., China.

Performance testing of modified asphalt: the penetration, softening point, and force ductility were measured using a SYD-2801D penetration index tester, a SYD-2806E softening point tester, and an SYD-45DBF ductility/tension tester with temperature and speed regulation, respectively, from Shanghai Changji Geological Instrument Co., Ltd., China.

Microscopic analysis of modified asphalt: the morphology of the modified asphalt was characterized using a DM2700M metallographic microscope from Leica Instrument Co., Ltd., Germany; the dispersion of the modified asphalt was measured using a CD-5BX/nCT A-type nano-3D CT analyzer (voltage: 50 kV; current: 130 μA; pixel width: 1.724 μm) from Chongqing True Test Technology Co., Ltd., China; a Solver PRO SPM AFM (non-contact scanning) from NT-MDT Russia was used to characterize the component morphology and thickness of the modified asphalt and to investigate the modification mechanism; and the physical and chemical properties of the modified asphalt were characterized using a Nicolet iS50 FTIR spectrometer from Thermo Fisher Scientific (China) Co., Ltd.

2.3 Preparation of graphene mother liquor

The graphene dispersion mother liquor was prepared based on previous research results. The designed graphene content was 0.26 wt% of absolute ethanol mass, and the metal-free phthalocyanine content was 190 wt% of graphene mass. The graphene and metal-free phthalocyanine were weighed using an electronic analytical balance (0.000 1-g divisions) and placed in a 1 000-mL beaker. A total of 350 g of absolute ethanol was added and gently stirred with a glass rod. The solution mixture was heated in water bath at 80 ℃ for 10 min, with slight stirring. Next, the solution mixture was cooled to room temperature, covered with plastic film, and sonicated for 30 min. The sonicated solution mixture was centrifuged at 4 000 rpm for 1.0 h. After centrifugation, the supernatant was collected as the graphene mother liquor. The main process for preparing the graphene mother liquor is shown in Fig.1.

2.4 Preparation of GMA

The following method was used to prepare GMA.A total of 350 g of the base asphalt was completely dissolved in 250 mL trichloroethylene, and the resulting solution behaved almost like a Newtonian fluid. The prepared graphene dispersion mother liquor was added to the asphalt-trichloroethylene solution. The obtained mixture was stirred with a glass rod, followed by sonication for 30 min. The purpose of the whole process is to make the dispersed graphene fully diffuse in the asphalt. The solution was placed in an oven (6 h, 60 ℃)to remove the absolute ethanol. The trichloroethylene was subsequently completely removed by rotary evaporation in an oil bath (110 ℃) at 85-90 rpm for 60 min.Next, the trichloroethylene-free asphalt was poured into a container, followed by special high-speed shearing to obtain the GMA, which can prevent aging of modified asphalt during the high-speed shearing and evaluate the modifier effectively[21]. The shearing parameters were as follows: a shearing temperature of 140 ℃, a shearing speed of 6 500 rpm, and a shearing time of 3 h. The preparation process of the GMA is shown in Fig.2.

3 Characterization results and analysis

3.1 Performance test

Penetration reflects the rheological properties of asphalt[22,23], which represents the corresponding consistency of asphalt at 25 ℃. The standard needle is 100 g and the injection time is 5 s. The softening point is used to evaluate the temperature sensitivity of asphalt and the critical temperature of reaction asphalt from solid to liquid[24]. Ductility reflects the deformation ability of asphalt in the whole process of stretching to fracture in a 5 ℃ water bath at a speed of 1 cm/min[25,26]. The high-temperature stability and low-temperature deformation resistance of the prepared GMA were evaluated by measuring the penetration, softening point, and ductility. The SK-70# base asphalt and phthalocyanine asphalt (PA) were also tested for comparison. The results are shown in Table 4.

According to Table 4, the penetration of PA and GMA (based on matrix asphalt) decreased significantly, 8.5 and 12.3 mm, respectively, which means that the penetration of PA and GMA decreased by 15.7 and 24.5%, respectively. And the softening points of PA and GMA (based on matrix asphalt ) increase by 0.6 and 1.4 ℃, respectively, and the softening points of PA and GMA increase by 1.2 and 2.8%, respectively. According to the results of the force ductility test, the maximum force value, ductility value, and fracture energy of PA and GMA (based on matrix asphalt) increased significantly,i e, 65.5 and 70.0 N, respectively, 72.0 and 99.5 mm, respectively, and 2 455.5 and 2 901.4 N·mm, respectively.

Table 4 reveals that adding the phthalocyanine dispersant does not degrade the asphalt. The penetration of GMA was significantly reduced from that of the base asphalt, which indicated that the base asphalt was hardened. Graphene addition increased the softening point and viscosity of the base asphalt, which stated that the base asphalt viscoelasticity had been modified.Thus, the high-temperature performance of asphalt was improved. The 5 ℃ force ductility test results showed that the maximum force, ductility, and fracture energy of the base asphalt all increased significantly, indicating that graphene incorporation can dramatically improve the low-temperature fracture toughness and tensile strength of asphalt. The main reason for this result could be attributed to the partial intercalation of dispersed graphene sheets into the asphaltenes, causing the asphaltenes to adsorb on the graphene surface[27,28]and enhancing the ability of the asphalt structure to resist damage and maintain stability.

To sum up, phthalocyanine as a dispersant can effectively improve the performance of asphalt. Simultaneously, through the change of penetration, softening point, ductility value, and fracture energy of GMA dispersed by phthalocyanine, all indicators are positive changes. The performance of asphalt is significantly improved.

3.2 Metallographic microscopy

A metallographic microscope can quickly observe the three-dimensional micromorphology of materials[29].Fig.3 shows the metallographic microscope at room temperature. The base asphalt, PA, and GMA morphology were characterized by metallographic microscopy at 100 × and 500 × magnification. Figs.3 ((a)-(b)) were the base asphalt image, Figs.3 ((c)-(d)) were PA image and Figs.3 ((e)-(f)) were GMA image.

Fig.3 (a) shows that the base asphalt exhibited a dendritic-cluster-like network structure. Fig.3(b) shows the dendritic structure at 500× magnification. The black substance that formed the dendritic structure was presumed to be aggregates,i e, micelles, of asphaltenes and resins. The other glossy morphologies were supposed to be saturated and aromatics.

The morphology of the PA (Fig.3(c)) was compared with that of the base asphalt (Fig.3(a)) at 100×magnification. The dendritic-cluster-like network structure shown in Fig.3(a) was transformed into the disordered stripes shown in Fig.3(c). The morphologies of the PA and base asphalt were also characterized under 500× magnification. A blue substance was adsorbed around the stripes in the PA, and the stripe morphology was disordered. The blue importance was not found around the dendritic structure in the base asphalt. Therefore, it was inferred that phthalocyanine was adsorbed around the asphaltene- resin micelles and dispersed the micelles.

Table 4 Performance indicators of asphalt

The morphologies of the GMA at 100× and 500×magnification are shown in Figs.3 ((e)-(f)) for comparison with those of the base asphalt and the PA. The base asphalt exhibited some linear stripes, the PA exhibited disordered stripes, and the GMA had a large number of uniform stripes that transformed into multichains.The chains were regularly bent at the rear and straight in the front. The chain formation was significant and fairly organized. Both phthalocyanine and graphene are nanomaterials, and nano-scale morphology cannot be observed under a metallographic microscope. However,layered structures were observed around the asphaltene- resin micelles in the GMA (Fig.3(f)). The layered structures were presumed to be micro-scale graphene aggregates or graphene-phthalocyanine aggregates.

In summary, there is a stable network structure in the base asphalt before any substance is added. When the asphalt’s network structure is opened after adding phthalocyanine, the modified asphalt will form a stable chain structure after adding graphene. Because of the change of the system, the performance of asphalt will be affected. The shift in micromorphology is consistent with the change of asphalt performance in Table 4.

3.3 CT scanning results

In CT imaging, X-rays orγ-rays are passed through the object to be detected without damaging the item. The condition of the original object is best reflected in the CT images. The CT results of the three asphalts are shown in Fig.4.

In Fig.4(c), 4(f), and 4(i), thex-axis represents the distance of the mark, and the higher part of they-axis is the height of the bright white spot. The CT images of the base asphalt, the PA, and the GMA are shown in Figs.4 ((a)-(i)), for comparison. White bright spots and black dark spots were observed in the base asphalt.The white bright spots were 14.50 μm in diameter and 127 μm in height. Figs.4 ((d)-(f)) show that the white bright spots and black dark spots persisted following the addition of the phthalocyanine dispersant, and the white bright spots were 28.45 μm in diameter and 255 μm in height. Figs.4 ((g)-(i)) show that upon adding both phthalocyanine and graphene dispersants, the small white bright spots remained but not the black dark spots. The white bright spots were 17.18 μm in diameter and 255 μm in height. Graphene has high surface tension and interlayer forces and is therefore difficult to disperse uniformly. In CT images, the larger the CT value is, the greater the density of the material is and the higher the brightness is. Similarly, the smaller the CT value is, the lower the brightness is[30]. The changes in the asphalt before and after the addition of phthalocyanine and graphene can be effectively distinguished by comparing the densities and sizes of the white spots.

The height of the white spots is 127 μm in Figs.4((a)-(c)) and 255 μm in Figs.4 ((d)-(i)). Therefore, the white bright spots in the base asphalt are the impurities mixed during transportation and processing, and the bright spots in Figs.4 ((d)-(f)) are high-density phthalocyanine aggregate. Figs.4 ((g)-(i)) white highlights in asphalt are composed of phthalocyanine and graphene.The diameter of the white bright spot changed from 28.45 μm (in Fig.4(f)) to 17.18 μm (in Fig.4(i)), which means that the addition of metal-free phthalocyanine can disperse graphene aggregates, in order to make more uniform dispersion of graphene in asphalt.

3.4 AFM

The AFM working principle is described here.The sample exerts a force on the microscope tip, which is fed back to a computer to simulate the sample topography. The simulated data are automatically processed to generate an image[31,32]. The base asphalt, PA, and GMA were characterized by AFM. The base asphalt,PA, and GMA were dissolved and diluted at the same ratio. At normal temperature, the test results are shown in Fig.5.

Bright spots were observed in the AFM images of the base asphalt (Fig.5 ((a)-(c))) with an approximate height of 30-50 nm. “Snowflake” structures were distributed uniformly around the bright spots. The main components of asphalt are asphaltenes, resins, aromatics, saturates, and waxes. The SK-70# base asphalt used in this study is a low-wax asphalt. In the SK-70#base asphalt, the asphaltene molecules have the highest polarity and molecular weight, the long-chain hydrocarbon resin has the second highest polarity, and the wax and oil have the lowest polarity[33]. The asphalt nuclei are primarily asphaltenes, to which resins adsorb to form micelles dispersed in the oils. It has been shown that asphaltene molecules tend to form irregular aggregates[34]. The average diameter of asphaltene molecules is approximately 5 nm. Micelles of asphaltene molecules have average sizes of approximately 20 nm or even higher. Therefore, the bright spots were asphaltenes, the “snowflakes” were resins, and the remaining structures were waxes and phenols, which confirmed the metallographic microscopy results. This phenomenon has not been effectively explained in the existing literature, and no convincing evidence has been found to prove it. The whole process is based on the composition analysis of asphalt four components(SARA). The specific breakdown is as follows: In the SARA system of asphalt, the polarity of asphaltene and resin is stronger, and the polarity of saturated and aromatic components is weaker than that of resin and asphaltene. In the process of sample preparation, the asphalt is dissolved in the solvent, and the resin exists around the asphaltene after the solvent volatilization.Therefore, it shows that large particles’ bright spot is asphaltene, and the “snowflakes” around asphaltene are resins. In the solvent system, the asphaltene particles are surrounded by resin to form micelles during solvent evaporation through further analysis. Asphaltene is a stable particle, and resin is a semi-solid material; the two’s polarities are more extensive, and the adsorption capacity are more robust. With the solution’s evaporation, the asphaltene forms brights white spots, and the resins forms the “snowflakes”.The AFM images of PA shown in Figs.5 ((d)-(f)) were compared with those of the base asphalt shown in Figs.5 ((a)-(c)).The maximum height of the bright spots in the PA was greater than 50 nm, the medium-intensity spots were approximately 45 nm in height, and most of the small bright spots were approximately 20 nm in height. Compared to the base asphalt, the height and quantity of the spots in the PA were both higher, and the position and appearance of the “snowflakes” was significantly different. The difference between the morphology and thickness of the PA and the base asphalt resulted from the dispersion of the asphaltenes and resins by the phthalocyanine molecules, which was consistent with the metallographic microscopy results for the PA(Figs.3 ((d)-(f))). Phthalocyanine molecules have been shown to adsorb asphaltenes via π-π interactions[35].In the petroleum industry, phthalocyanine is used to associate asphaltenes into large micelles that can be extracted. This mechanism resulted in the thickening of the asphaltenes in the experiment. At the same time, the resins and other components were uniformly dispersed and became thinner, forming a new micellar structure.The results also showed that phthalocyanine changed the asphalt structure, which was consistent with the changes in asphalt performance. Among the material characteristics, the low-temperature fracture energy of PA exhibited the largest change from the base asphalt,which suggested that the low-temperature performance of the asphalt was improved. Compared to the base asphalt, the penetration of the PA was reduced by 8.5 mm (0.1 mm) with very little change in the softening point. The penetration index reflects the viscosity and temperature susceptibility of an asphalt. The softening point reflects the asphalt thermal stability. The results indicated that phthalocyanine addition changed the morphology of the asphaltenes and resins, dispersed these particles more uniformly, and increased the viscosity. Saturates and aromatics adsorbed on the surface of the phthalocyanine molecules, thereby enhancing the low-temperature asphalt performance. Asphaltenes and resins contain large aromatic ring systems and a large number of heteroatoms. Aggregation of asphaltenes and resins occurs via the π-π stacking of aromatic ring systems and the formation of hydrogen bonds with heteroatoms, primarily by inter-ring π-π interactions[36]. Phthalocyanine has a special two-dimensional conjugated π-π electronic structure, so that phthalocyanine molecules and asphalt molecules are likely to form supramolecular aggregates or supramolecular dispersions[35].

Figs.5 ((g)-(i)) showed that the asphaltene of the GMA was thinner than those of the base asphalt and phthalocyanine, with maximum and minimum thicknesses of approximately 40 and 15 nm, respectively.The original single large micelle became a large micelle surrounded by small micelles. The overall distribution was more uniform than that of the base asphalt,and the resins “snowflakes” disappeared. A circle of dark stripes with a thickness of 2.5-5 nm appeared around the asphaltene micelles. Graphene cannot be directly observed in asphalt by AFM. However, two bright spots were observed in the GMA. Based on the metallographic microscopy results, these bright spots were considered to be graphene aggregates. Asphalt is a colloidal dispersion of solid asphaltenes. Asphaltene has a polycyclic aromatic hydrocarbon flaky structure.The π-π interaction of aromatic rings between the flaky molecules results in partial lamellar stacking in some asphaltenes due to the π-π bond adsorption between graphene and phthalocyanine. Phthalocyanine and the graphene sheets dispersed by phthalocyanine both adsorbed asphaltenes and loosely separated the asphaltene stacking structure, making the asphaltene dispersion more uniform. Macroscopically, the deformation resistance, the high-temperature stability, and the low-temperature cracking resistance of the GMA were all enhanced.

3.5 FTIR spectroscopy

IR spectroscopy is a type of absorption spectroscopy, wherein molecular vibrations in compounds results in the absorption of IR light with specific wavelengths[37,38]. The wavelength of adsorbed IR light was associated with the chemical bond vibration depending on the structural characteristics of the materials.Therefore, IR spectroscopy can be used to elucidate the molecular structure and chemical bond changes in materials and determine the composition of compounds[39-41]. The working principle of IR is described here. The vibrations of functional groups are associated with absorption peaks of characteristic intensities and shapes[42-44], which can be analyzed to determine the structural composition of substances and changes in chemical bonds. Fig.6 shows the comparative analysis of base asphalt, PA, and GMA by IR spectroscopy in the range of 4 000-400 cm-1. Fig.6(a) is an overall view, and Fig.6(b) is a partially enlarged view.

Asphalt has four components: asphaltenes, resins,saturates, and aromatics. Asphalt contains two main elements, C and H, and small amounts of S, N, O, and metal elements, which do not significantly affect the material characteristics. In Fig.6 (a), the two strong and sharp absorption peaks at 2 919 and 2 849 cm-1appeared for the base asphalt, PA, and GMA. These peaks were attributed to the antisymmetric stretching vibration of -CH2- and the symmetrical stretching vibration of C-H. A peak corresponding to the C-H deformation vibration appeared in the 1 500-1 300 cm-1range. Two sharp peaks appeared at 1 456 and 1 375 cm-1, which were attributed to the vibrations of -CH2and -CH3, respectively. A strong peak was observed at 1599 cm-1for the base asphalt, PA, and GMA, which was attributed to the C=C bond vibration of the benzene ring. Fig.6(b) is a partial enlargement of the spectra to facilitate analysis. An abnormal peak appeared at 1 539 cm-1in the PA spectrum shown in Fig.6(b). The molecular formula of phthalocyanine is C32H18N8. The C=C skeleton vibration may have produced the 2-4 peaks in this region. These peaks were not observed in the base asphalt. However, a peak at 1 539 cm-1was observed in the PA, which was attributed to the C=N stretching vibration. The characteristic peak at 1 539 cm-1was not observed in the base asphalt but appeared in the PA and disappeared in the GMA. This result can be explained in terms of the strong physical adsorption of graphene to cyclic phthalocyanine molecules via π-π[45]interactions, which resulted in the disappearance of the characteristic peak. Due to the physical effect of π-π adsorption, the vibration of C=N bond is limited by graphene added into PA, resulting in the disappearance of the characteristic peak in GMA. Zhanget al[46]proposed the structure of asphaltene through molecular simulation, observed the structure of asphaltene, and analyzed the π-π adsorption in asphalt. Due to the π-π interaction of asphaltene itself, graphene has a huge specific surface area. When graphene is added into asphalt, graphene accumulates on asphaltene, and the two adsorb each other through π-π interaction. The metallurgical microscopic results confirmed that the addition of graphene and phthalocyanine dispersants significantly changed the asphalt microstructure. Therefore,phthalocyanine was an effective graphene dispersant for GMA.

4 Conclusions

a) A GMA was prepared with the following properties: a penetration of 5.03 mm, a softening point of 49.8 ℃, a maximum force in the force ductility test at 5 ℃ of 70.0 N, a ductility of 99.5 mm, and a fracture energy of 2 901.4 N·mm. The performance of this modified asphalt was significantly higher than that of the SK-70# base asphalt. Among the material properties,the largest difference between the modified and base asphalts was observed for the fracture energy, with an increase of 45.1%.

b) Metallurgical microscopy showed that under the influence of a phthalocyanine dispersant, the addition of graphene transformed the dendritic aggregates of asphaltenes and resins in the base asphalt into a large number of uniform stripes, which subsequently transformed into multichains. The chains were regularly bent at the rear and straight in the front. The chain formation was significant and organized. The phthalocyanine dispersant significantly changed the morphology of the GMA.

c) CT images showed that the action of the phthalocyanine dispersant transformed large graphene aggregates in the GMA into small graphene aggregates with a more uniform distribution. The CT value of the GMA and the number of dense white bright spots also increased. The results indicated that the phthalocyanine and graphene changed the structure of the asphalteneresinmicelles in the base asphalt.

d) AFM results showed that phthalocyanine-dispersed graphene was primarily via π-π interactions.Phthalocyanine and the graphene sheets were dispersed by phthalocyanine. Macroscopically, the deformation resistance, the high-temperature stability, and the low-temperature cracking resistance of the GMA were all improved over that of the base asphalt, which was attributed to physical adsorption.

e) In the FTIR spectra, a 1 539 cm-1characteristic peak appeared in the PA that was not observed either in the base asphalt or the GMA. This result was attributed to the separation of the asphaltenes and resins in the base asphalt upon phthalocyanine addition, which were then re-linked via graphene-phthalocyanine adsorption into a supramolecular structure. The strong physical adsorption of graphene to the cyclic phthalocyanine molecules via π-π interactions was confirmed.

f)The study used phthalocyanine to disperse graphene, but failed to completely disperse graphene.Further research about how can graphene be completely inserted into the asphalt, so that graphene and asphalt can be ideally linked to improve asphalt performance is needed.