Abstract
Due to the diversity of plant asphalt sources and refining processes, the composition and properties of the various plant asphalts are also different. The plant asphalts are used to preserve the green environment. The natural material is eco-friendly; hence, the usage of plant asphalts is getting promoted these days. There are some problems in the road performance of some source plant asphalt, so the change characteristics of the microstructure of the modified plant asphalt are analyzed in this paper. Firstly, the raw materials are prepared, then the experimental plan is made, and finally the microstructure change characteristics of asphalt at different dosages of 0%, 0.25%, 0.5%, 0.75%, and 1% are studied using the soft computing techniques. It is found that with the increase of the content of polyphosphate, the structure of modified asphalt has changed. The most obvious change is that the volume of asphaltenes becomes smaller, the number of asphaltenes increases, the area of asphalt micelle becomes smaller, and the area of dispersion medium becomes larger. The experimental study is performed to guide the right material for road construction which is eco-friendly for maintaining the green environment.
1. Introduction
The traffic flow and driving frequency on the highways have increased sharply. Similarly, the axle load of freight cars has increased continuously. It is generally carried out in one-way driving by lanes, which requires three important aspects to be taken care of such as(1)Improvement of the road surface’s anti-fluidity, that is, the ability to resist rutting at high temperature(2)The ability to improve flexibility and elasticity of the road so as to resist cracking of the road at low temperature(3)The ability to improve wear resistance and prolong service life
These requirements lead to many changes that have taken place in modern highways and roads. Large span prestressed roof panel is widely being used in modern buildings, which requires roof waterproof materials to adapt to large displacement, more resistant to severe high and low temperature climatic conditions, better durability, self-adhesive, convenient construction, and reducing maintenance workload. These changes in the use environment pose a severe challenge to the performance of petroleum asphalt. The present asphalt industry faces two major issues: (a) need for eco-friendly asphalt mixtures and (b) reducing the cost of raw materials [1]. In order to provide eco-friendly solution to the problem which is also cost-effective, researchers have experimented with different possible solutions. In [2], authors proposed the use of reclaimed asphalt to reduce the costs of aggregates. At the same time, they overcome the quality issue by adding waste engine oil (WEO) and waste vegetable oil (WVO). Authors study the effects of these two oily based rejuvenators on utilization of RAP in bituminous mixtures. In [3], authors prepared asphalt modified by mixing asphalt 60/70 with epoxidized natural rubber (ENR) in different percentages. The results were evaluated using Marshall test to assess the asphalt pavement performance depending on the curing time and hardener concentration. In [4], authors investigate the possibility of WCO as a rejuvenator for aged bitumen. They also demonstrate the optimum percentage of waste cooking oil for the rejuvenated bitumen using a penetration blending chart. In [5], researchers have focused on environment-friendly-less additives to be mixed with the asphalt. They analyzed the effect of adding cashew nut liquid shell (LCC) and organically modified clay vermiculite (OVMT). They also have assessed its mechanical and rheological properties. In [6], authors have modified the asphalt binder using natural rubber (NR) based on cup lump. The results show improved performance and service. The DSR analysis was carried out which revealed that the presence of elastic NR within the bitumen network enhances the viscosity and stiffness and reduces temperature susceptibility which in turn increases the rutting property.
Although researchers have paid attention to address the abovementioned problems, the most of them have paid more attention to the modification of petroleum asphalt to meet the requirements. Most of the materials commonly used as asphalt modifiers are polymer, such as SBS and SBR [7]. The most of the domestic road asphalt is ordinary asphalt, with high wax content that leads to poor adhesion. The high price of these asphalt modifiers limits the development of modified asphalt technology. Another problem is that it does not address the environmental issues that have become more and more serious. For instance, the haze pollution caused by crop burning has brought many problems to people’s health and life. Our studies of existing literature show that the existing solutions are still not environ-friendly and at the same time are not cost-effective.
We also studied the existing literature on improved methods of asphalt performance and the development of new asphalt-modified materials [8]. Modified asphalt is a kind of asphalt binder made by adding rubber, resin, high polymer, ground rubber powder, or other fillers, or taking measures such as slight oxidation of asphalt to improve the performance of asphalt or asphalt mixture. There are two mechanisms of modified asphalt: one is to change the chemical composition of asphalt, and the other is to make the modifier evenly distributed in asphalt to form a certain spatial network structure. Plant asphalt is the waste residue in the process of plant oil processing, which is mainly burned as waste. At present, the preliminary research shows that it contains about 10% sterol, 5% vitamin E, and a large number of high-grade fatty acids, which are mainly used in the production of casting binder, rubber softener, cement-preformed separator, black printing ink, asphalt coating, coating, surface active carbon, leather assistant, heavy fuel, etc. Different raw materials and processing methods have different properties [9, 10]. Some plant bitumen can be mixed into matrix asphalt, resulting in poor temperature sensitivity and aging resistance of mixed plant asphalt, and poor water stability of asphalt mixture. This makes it difficult for some bio-asphalt with performance defects to be directly used in road engineering [11]. But scholars at home and abroad have basically the same research methods for these plant asphalts, that is, to firstly analyze the chemical composition of plant asphalt and then evaluate the performance of plant asphalt. This shows that the research of plant asphalt is still at the level of asphalt performance, and there is little research on the blending process and mechanism of plant asphalt and petroleum asphalt, and the performance of plant asphalt mixture. In this way, many problems that can only be shown in the layer of remix are ignored, and the engineering application of plant asphalt cannot be guided. Therefore, for the project of plant asphalt, we need to analyze the specific problems and develop a comprehensive and systematic research program according to the characteristics of plant asphalt. In view of the above problems, this paper takes polyphosphate as the raw material for the modification of plant asphalt and analyzes the microstructure changes of asphalt under different polyphosphate contents. This study determines a set of research methods of plant asphalt and conducts a comprehensive and systematic study on the characteristics of the microstructure changes of asphalt after the modification of plant asphalt.
The highlights of the studies are as follows:(1)To study the existing methods for asphalt preparation which are eco-friendly(2)To study different modifiers, their composition, and effects on asphalt preparation(3)To study the application of set of research methods for plant asphalt(4)To perform comprehensive and systematic study on the characteristics of the microstructure changes of asphalt after the modification of plant asphalt
The paper has been organized as follows:(1)Introduction(2)Proposed methodology(3)Process description(4)Illustrating the experimental method(5)Result analysis(6)Conclusion
2. Proposed Methodology
The proposed methodology divides the research into the following phases.(1)Study and identification of raw material that includes asphalt preparation, polyphosphate, modifier(2)Describing the processing phase(3)Describing the experimental method(4)Analysis of the change characteristics of asphalt microstructure after the modification of plant asphalt(5)Result analysis(6)Conclusion
2.1. Raw Materials
2.1.1. Asphalt Preparation
The properties of plant asphalt are very important to the performance of polyphosphate-modified asphalt. Because of the different chemical components of different oil sources and different grades of base asphalt, its compatibility with polyphosphate modifier is quite different. Not every base asphalt is suitable for modification. Some studies have shown that the matrix asphalt with a larger resin content will show better performance after modification by polyphosphate. Therefore, after comparative analysis, this paper adopts SPC90. The modification of No. 2 base asphalt with polyphosphate is studied.
The technical indexes of SPC90 base asphalt are shown in Table 1.
2.1.2. Polyphosphate
Polyphosphate is used as the modified material of plant asphalt in this study, which is also known as tetra phosphoric acid or polyphosphoric acid, and it is a colorless, transparent, and viscous liquid, which is corrosive. It is a second-class inorganic acid corrosive material [2–5], easy to deliquesce, and not crystallize, and can react with water to produce orthophosphoric acid. Its melting point is 48∼50°C, boiling point is 856°C, density is 2100 kg/m3, relative molecular weight is 337.93, and representative formula is H6P4O13. Polyphosphate is widely used in chemical, pharmaceutical, and leather industries. According to the percentage of phosphoric acid, polyphosphate can be divided into different grades, as shown in Table 2. Polyphosphate with the content of H3PO4 of 105%, 110%, and 115% is often used to make modified asphalt.
Classification of the polyphosphate has been shown in Table 2.
For the same base asphalt, different grades of polyphosphate have different effects on its modification. This is because the chain length of polyphosphate product with H3PO4 content is different, so the performance of modified asphalt is different. The higher the grade of polyphosphate is, the better the viscosity of modified asphalt is. This is because 105% phosphoric acid contains more dimers, short units, pyrophosphoric acid, and orthophosphate. However, although the content of 110% phosphoric acid dimer is the same, the number of units is relatively small, and the number of compound chains after reaction is n > 3. For 115% phosphoric acid, the number of units is almost zero, and the number of chemical chains after reaction is from n to 14 units. The increase of chain length will make the interaction between the chains and then make the viscosity of modified asphalt increase. By analyzing the performance of different grades of polyphosphate-modified asphalt, it is pointed out that 110% polyphosphate-modified asphalt has the best effect and high cost performance. Therefore, 110% of polyphosphate produced by a chemical reagent factory in Chengdu is used in the study of modified asphalt.
2.1.3. Modifier
In this paper, SBS4303 particles of Yanshan Petrochemical Company and SBR1502 powder with an effective content of 80% produced by a Chemical Technology Co., Ltd., of Shandong Province are selected as polymer modifiers for indoor preparation of modified asphalt.
The technical indexes of polymer modifier are shown in Tables 3 and 4.
2.2. Process Description
In order to avoid the influence of repeated heating and different heating times on the performance of asphalt, the prepared modified asphalt should be put into different stainless steel containers. Each test should ensure that the asphalt sample has the same heating process and heating times. In the preparation process of modified asphalt, heating temperature, shear time, and shear rate are the key technologies that affect the performance of modified asphalt. The reaction temperature is too low, the modified asphalt is not easy to mix and disperse, and the mixing and swelling reaction of polymer modifier and asphalt is not sufficient; the reaction temperature is too high, which will lead to the aging of matrix asphalt and modifier. The reaction time is an important factor affecting the swelling of the modifier. Too short time will lead to uneven mixing of the modifier and asphalt, and too long time will lead to aging of asphalt. For the shear rate [6–8], high-speed shear or colloid mill is usually used to realize the dispersion and mixing of modifier and asphalt in the large-scale processing of modified asphalt in the factory; high-speed shear is generally used to prepare modified asphalt in the laboratory, and the shear rate is mostly set to more than 4000 r/min. Therefore, based on the analysis on the advantages and disadvantages of the preparation process of polyphosphate-modified asphalt in the domestic and foreign literature, this paper modified the plant asphalt.
In the test, the amount of modifier is 5.0%, and the processing of modified asphalt is as follows:
Firstly, heat the asphalt to 160–180°C, and weigh 800 g for use.
Secondly, put the modifier into the oven, dry it at 50°C for 5 hours, weigh the modifier of the required weight, add it to the base asphalt in batches, and stir it evenly with a glass rod; thirdly, start the emulsifying machine, set the initial speed to about 500 r/min, gradually accelerate to 4000 r/min, and stop the machine after mixing for 20 min; fourthly, put the modified asphalt sample stirred by the emulsifier into the oven at 160°C for 5 hours, so that the modifier can fully swell in the asphalt; fifthly, start the emulsifier again, adjust the rotation speed to 4000 r/min, mix the modified asphalt sample for 10 min, and then gradually reduce the rotation speed of the emulsifier to stop the machine, so that the bubbles in the asphalt are discharged, to complete the processing of the sample, and inject the mold to be tested.
2.3. Experimental Method
Four component methods are used to separate asphalt into four components with similar chemical properties, namely, asphaltene, saturated component, aromatic component, and gum, by using the selective dissolution of different organic solvents and the selective adsorption of different adsorbents.
According to JTG e20-2011 test code for asphalt and asphalt mixture of highway engineering, the test flow of four components in this paper is as shown in Figure 1.
3. Analysis of the Change Characteristics of Asphalt Microstructure after the Modification of Plant Asphalt
3.1. Image Feature Acquisition of Asphalt Microstructure
Because there will be some chemical reaction after the modification of plant asphalt, which will change the colloidal structure of asphalt, so it is impossible to obtain the change results of asphalt microstructure from the microscopic point of view by common observation means. Therefore, the change characteristics of asphalt microstructure are analyzed by the combination of FTIR and AFM.
Fourier transform infrared (FTIR) spectroscopy, thermal analysis, gel permeation chromatography (GPC), and chemometrics theory are used to analyze the chemical composition, molecular structure of polyphosphoric acid-modified asphalt, and the reaction process between polyphosphoric acid [10–12] and asphalt, so as to explain the chemical modification mechanism of polyphosphoric acid-modified asphalt from the microscopic level.
The test flowchart is shown in Figure 1.
It is suitable for quantitative analysis of gases, liquids, and solids. According to the Lambert–Beer law, when light with intensity of L passes through a certain distance in a uniform material and after a certain distance, the light absorbed by the material is uniform. Its principle of spectral analysis is shown in Figure 2.
Its mathematical expression is given in
In equation (1), is the incident radiation intensity, and is the transmission radiation intensity and the light attenuation coefficient of material.
Asphalt is the product of crude oil after treatment. It is a mixture of a variety of extremely complex hydrocarbons and derivatives of these compounds. Its chemical components are mainly carbon and hydrogen (more than 97%), followed by a small amount of oxygen, sulfur, nitrogen, and trace amounts of vanadium, iron, manganese, nickel, and other metal elements.
Due to the extremely complex chemical composition structure of asphalt and the limitation of analytical and testing methods, it is difficult to extract a single substance with certain chemical composition from asphalt. Therefore, on the basis of thermal analysis, the microscopic change characteristics of asphalt are obtained by atomic force microscope [12–14].
Atomic force microscopy (AFM) [15] scans the surface of the sample with a nanoscale probe to obtain the micro-information of the sample. As a new type of surface structure analysis instrument, AFM is beyond the limitation of light and electron wavelength on the resolution of the microscope. It can observe the shape of the material from the perspective of three-dimensional. Compared with the traditional methods, AFM has the advantages of high precision, no damage to the sample, strong localization, and simple sample preparation. It can be applied to the study of the surface structure of conductor, semiconductor, and insulator materials, so it is widely used in the fields of surface science, polymer materials, life science, and biology. The program NOVA of AFM is used to collect data, and the data are generated into mdt computer file. The image analysis tool is used to open the data result file, which contains two graphs: a phase graph and a height graph. For each graph, there are two-dimensional and three-dimensional imaging methods. The imaging illustration is given in Figure 3.
(a)
(b)
3.2. Selection of Image Parameters of Asphalt Microstructure
The microstructure image of polyphosphate-modified asphalt obtained by AFM is processed and analyzed by computer, and the structural parameters of characterization significance are selected, such as asphaltene area, micelle area, dispersed medium area composed of saturated phenol and aromatic phenol, asphaltene number, micelle number, asphaltene length, asphaltene height, and maximum micelle radius. By comparing the change of asphaltene area, micelle area, and the area of dispersion medium composed of saturated phenol and aromatic phenol, the change degree of asphaltene micelle structure after adding polyphosphate is explored; special attention is paid to the morphological description of asphaltene, the core of asphaltene colloidal structure. By comparing the change of asphaltene area, number, length, height, and other morphological indexes with a different dosage, the modification mechanism of polyphosphate-modified asphalt can be explored. In addition, the asphalt material is a kind of colloidal dispersion system, and the size of its colloidal composition “micelle” will directly affect its performance. Therefore, this paper also studies the characteristic parameter of “maximum micelle radius.”
3.2.1. Determination of Area of Three Components
The general idea of image preprocessing is firstly collecting the data of van der Waals force from the image of AFM, which means to generate a new data image, then processing the grayscale image, modifying the image data according to the result of histogram, and finally recognizing the “peak form” structure of asphaltene, the dispersing medium composed of white micelles and saturated phenol and aromatic phenol, so as to generate black, white, and gray tricolor graphs.
The process of area algorithm for three substances is traverse all pixels[16–19] and count the black and white gray points as their respective areas. It is not that the area is only the relative value of the pixel area at this time. The real area is calculated as
In equation (2), represents the area of each substance and represents the number of pixels.
Because the transformed image data are a floating-point two-dimensional array, the pixel coordinate position corresponds to the data one by one in the form of matrix. The coordinate range of Y-axis is 0–255, 256 pixels in total. The area of sampling is , so the formula for calculating the area of each substance is the above formula. However, for the sake of comparison, the area ratio of each substance is used as the characterization parameter. The specific process of algorithm is firstly traverse all the pixel points in the image, record the location of the black point, take each black point as the center, and traverse the pixels around it. If they are all black, it is considered that the point and the surrounding points belong to the same “bee” structure. The number of asphaltenes can be determined by the above cycle operation.
3.2.2. Determination of Asphaltene Length and Height
The volume characterization parameters of asphaltene are length and height. The significance of the study is that the three-dimensional imaging characteristics of AFM can be used to observe the change of asphaltene with the amount of polyphosphate modifier and further explore the chemical modification mechanism of polyphosphate-modified asphalt. Firstly, the coordinates of the edge points of the “peak form” structure are determined. Through the coordinates of the top point, the bottom point, the left point, and the right point, the angle between the “peak form” structure and the horizontal, as well as the length of the horizontal and vertical directions can be determined. Finally, the length of the “bee” structure can be calculated according to the Pythagorean law.
3.2.3. Determination of the Maximum Micelle Radius
The effect of adding polyphosphate modifier on the change of asphalt micelle size is also one of the important aspects reflecting the modification effect of polyphosphate-modified asphalt. The algorithm of maximum micelle radius is shown in Figure 4.
In the figure, the black part is asphaltene, the gray part is the dispersion medium composed of saturated phenol and aromatic phenol, the white part is asphalt micelle, and the circle in the shadow part is half of the maximum micelle radius.
The calculation formula is given as
In the above formula, represents the pixels in the overall image, represents the distance parameter between pixels, and represents the micelle size.
The calculation formula is as above. Firstly, all pixels of the whole image are traversed, then the white pixels are filtered out, each white pixel is taken as the center, and the distance from the nearest black pixel or gray pixel is calculated; after the traverse, the maximum distance is multiplied by 2, which is the radius of the largest micelle.
On this basis, for image data enhancement, the linear dynamic method is used to enhance the image, and the schematic diagram is shown in Figure 5.
The program processing formula of linear dynamic range adjustment is given as
In equation (4), represents the largest point in the image, represents the smallest point in the image, and represents the adjustment range of the image.
The flowchart of specific program implementation is shown in Figure 6.
3.3. Result Analysis
We have analyzed the relationship between the amount of polyphosphate and the number, length, and height of asphaltenes. We studied the microstructure of the asphalt modified by polyphosphate [20] with different contents (0%, 0.25%, 0.5%, 0.75%, and 1%) under the atomic force microscope. The image processing program of atomic force microscope and spectral analysis method is used to obtain the useful information from the image, and the structural characteristic parameters (three-component area ratio, three-component area size, asphaltene length, asphaltene number, asphaltene height and maximum micelle radius, etc.).
The results obtained by spectral analysis and AFM is shown in Table 5.
It can be seen from Table 5 that the biggest factor affecting the performance of modified asphalt is the content of polyphosphate. In this paper, we will focus on the influence of different contents of modifiers on the microstructure characteristic parameters of polyphosphate-modified asphalt. Compared with the phase diagram of the microstructure of polyphosphate-modified asphalt with different contents, it can be seen intuitively that with the increase of the content of polyphosphate, each structure of polyphosphate-modified asphalt has changed, the most obvious change is that the volume of asphaltene is smaller, the number of asphaltene is increased, the area of asphalt micelle is smaller, and the area of dispersion medium is larger. At the same time, the boundary between asphalt micelle and dispersion medium is clearer.
According to the results of spectral analysis, the reason for this effect is the chemical reaction between polyphosphate and asphalt, resulting in new functional groups. These functional groups change the molecular structure of asphalt. The following is the quantitative analysis of the microstructure parameters of the asphalt modified by polyphosphate with different contents and the relationship between these structural parameters and the content of polyphosphate modifier. The influence of modified plant asphalt on asphalt area is shown in Figures 7–9.
The relationship between the content of polyphosphate and the area ratio can be seen directly in the comparison diagram: with the increase of polyphosphate, the asphaltene area increases, the area of the agglomerate decreases, and the area of the dispersing medium increases. On the AFM phase diagram of polyphosphate-modified asphalt, the honeycomb structure decreases, but the number increases, the area of the agglomerate decreases, and the dispersing medium increases obviously. This shows that when polyphosphate is added, the asphaltene clusters are broken, which shows that the dispersion of asphaltene in the soft components of asphalt is increased, thus forming a stable spatial network.
The relationship between the dosage of polyphosphate modifier and the morphological parameters of asphaltene is shown in Figures 10 and 11.
Figures 10 to 11 show the relationship between the amount of polyphosphate and the number, length, and height of Asphaltenes. With the increase of the amount of polyphosphate, the number of asphaltenes increases and the length decreases. In the AFM phase diagram of polyphosphate-modified asphalt, asphaltenes become dispersed with the increase of the amount. However, asphalt height does not show obvious regularity change, which may be related to the big error of 3D data acquisition.
4. Conclusions
To sum up, this paper focuses on the study of the microstructure of the asphalt modified by polyphosphate with different contents (0%, 0.25%, 0.5%, 0.75%, and 1%) under the atomic force microscope. Using the image processing program of atomic force microscope and spectral analysis method, a large amount of effective information is obtained from the image, the structural characteristic parameters (three-component area ratio, three-component area size, asphaltene length, asphaltene number, asphaltene height, maximum micelle radius, etc.) which are of significance for characterization are selected for quantitative calculation and analysis, and the following conclusions are obtained:(1)With the increase of polyphosphate dosage, the volume of asphaltene decreases, the number of asphaltenes increases, the area of asphalt micelle decreases, the maximum micelle radius decreases, and the area of dispersion medium increases. At the same time, the boundary between asphalt micelle and dispersion medium is clearer. This indicates that after adding polyphosphoric acid, the colloid type of asphalt has the tendency to change from soluble gel-type asphalt to “gel-type” asphalt.(2)The statistical analysis shows that at the 5% significance level, the change of polyphosphate dose has a significant effect on the maximum micelle radius. This shows that the biggest change to the modified asphalt is to break up its micelles, make them change from the original large-scale micelles to dispersed and smaller micelles, and form network structure in space, so that the performance of asphalt can be improved.(3)There is a good correlation between the microstructure of polyphosphate and the evaluation index of penetration classification and anti-rut factor. The results show that the number of asphaltenes, the area of asphaltene, the maximum radius of micelle, and the length of asphaltene have obvious influence on the penetration, softening point, ductility, and anti-rutting factor, and the penetration, ductility, and phase angle decrease with the increase of the number of asphaltenes; with the increase of the number of asphaltenes, the maximum radius of micelles and the length of asphaltenes, the penetration, ductility, and phase angle decrease, and the softening point and anti-rutting factor increase. This shows that the addition of polyphosphate will make the asphalt thicken, break the asphaltene clusters, enhance the dispersion of asphaltene in the soft components of asphalt, form a stable space network between the dispersed asphaltenes, improve the high-temperature performance of asphalt, and thus improve the temperature sensitivity of asphalt.
Data Availability
All the data pertaining to this article are included in the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
This study is self-funded.