Modification of the Crumb Rubber Asphalt by Eucommia Ulmoides Gum under a High-Temperature Mixing Process

Abstract

The crumb rubber (CR) asphalt has some defects of high viscosity and poor storage stability, which brings great challenge to the high-quality construction of the CR asphalt pavement. To improve the interaction between the CR and base binder, the Eucommia ulmoides gum (EUG) with double-bond structure similar to trans-polyoctenamer rubber (TOR) was used to modify the CR asphalt. However, the original EUG double bond is basically inactive at room temperature and cannot form the effect of TOR. Open double bonds of EUG with asphalt and rubber powder form a network structure similar to TOR-modified rubber asphalt by high-temperature mixing with EUG in a torque rheometer. The effects of modified CR on rubber asphalt were analyzed by macro- and micro-experiments such as rotational viscosity tests, segregation tests, FTIR tests, and PG tests. It was found that the high-temperature mixing process works in both physical and chemical ways to mix the CR and EUG into an inseparable substance. The modified CR has higher chemical activity after desulfurization and degradation, which allows it to form a more effective chemical connection with asphalt. EUG can build a stable spatial crosslinking structure in CR asphalt due to the sulfurization reaction, which significantly improves the construction workability and system stability of the CR asphalt.

1. Introduction

Crumb rubber (CR), which is made of waste tires, can be used in the field of modified asphalt, and it brings some benefits of saving resources and protecting the environment. Rubber asphalt has unique advantages, such as the properties of anti-rutting deformation at high temperature and anti-cracking at low temperature [1,2]. However, the liquid–solid two-phase system and high viscosity of rubber asphalt result in the poor storage stability and construction workability. Therefore, it is necessary to further modify the performance of rubber asphalt [1,3,4]. In order to improve the compatibility between CR and asphalt and to establish an effective chemical connection between the CR and asphalt, the scholars have used trans-polyoctenamer rubber (TOR) to modify the rubber asphalt, which has achieved positive results [5]. TOR is a kind of polymer with a double-bond structure that can crosslink the sulfur in the asphalt with the sulfur on the surface of the CR to form a network structure, thus establishing a chemical connection between the CR and asphalt [6,7]. Although TOR has a good modification effect on the rubber asphalt, it is too expensive to be used widely in the construction of highways.

Through extensive investigations, one kind of polymer called Eucommia ulmoides gum (EUG) was found to be useful. EUG has double-bond structure, is a unique renewable resource in China, and can be used to modify the rubber asphalt with the similar effect of TOR. The EUG is a kind of natural high polymer material, and it has the same chemical composition as natural rubber, but the chemical constitution of them is different. EUG is the trans-polyisoprene, but the natural rubber is the cis-polyisoprene. The cis-polyisoprene is orderly and easy to gather and crystal. So it is a kind of flinty solid instead of an elastomer such as natural rubber. The EUG molecular chain has a lot of double bonds, and damaging its crystallization through the methods of cross-joining or grafting, which can damage its double bonds or orderly structure, can make its melting point drop, and even can make it become a kind of completely amorphous material such as natural rubber so that it can be used in some engineering occasion. To not depend on imports promoting efficient reactions between rubber crumb and asphalt and express the technical advantages of a dry-process rubber bitumen mixture with domestic natural rubber, the Eucommia ulmoides gum (EUG) was used to create chemical links between rubber crumb and asphalt. After rubber crumb was mixed with EUG, EUG’s own double bond structure can be vulcanized with the sulfur of rubber crumb, founded by the microscopic analysis and macroscopic representation of the mixture. EUG can also be grafted with maleic anhydride (MA) reacting with the asphalt amino-group, so chemical links were built between the EUG and asphalt. The EUG can also greatly improve the high- and low-temperature performance and system stability of rubber asphalt [8]. However, the original EUG double bond is basically inactive at room temperature and cannot form the effect of TOR. Through the preliminary test [9], it is found that when EUG is mixed with rubber powder at high temperature, the double bond can be opened by the sulfur atom of rubber powder, and open double bonds of EUG with asphalt and rubber powder form a network structure similar to TOR-modified rubber asphalt. However, existing studies are limited to the direct modification of EUG to rubber asphalt, which does not involve the modification of CR under high-temperature mechanical action. In this paper, a new process with high temperature was investigated. Firstly, quantitative CR and EUG were mixed in the torque rheometer at high temperature to form a new material (EUG-modified CR), then the EUG-modified CR was used to modify the asphalt binder. Finally, the effects of the high-temperature mixing process and EUG on the properties of CR are studied by laboratory tests.

2. Mechanism Analysis

2.1. Modification Mechanism of the High-Temperature Mixing Process on EUG and CR

The CR contains sulfur, while the double bonds on the molecular chains of different EUG can be crosslinked by sulfur during the sulfurization process. Gradually a crosslinking network with the increase of chemical crosslinking points was formed in the sulfurization process [10,11], as shown in Figure 1. Therefore, these properties can be used to combine the EUG with the CR organically.

Under the high-temperature condition and the intense mechanical action with the torque rheometer, the sulfurization rate and the sulfurization degree of EUG are both improved significantly. Through the actions of thermal energy and the shearing force, the CR will undergo the desulfurization reaction and the degradation reaction, which are conducive to improving the compatibility between the CR and the asphalt. With the existence of the “wrapping effect” of EUG, the CR cannot be excessively degraded, thus the EUG has a positive impact on the CR.

In the high-temperature mixing process, the sulfurization reaction can form a tight chemical crosslinking network between the EUG and the CR. And the intense extrusion of the torque rheometer’s rotor can integrate the EUG with the CR at the physical level. Finally, the two materials can be closely combined into a new composite material.

2.2. Modification Mechanism of EUG on Rubber Asphalt

Due to the poor compatibility between the CR and the asphalt, the rubber asphalt has some disadvantages, such as the high viscosity and the poor storage stability. The CR is prone to excessive swelling in the asphalt, so the light components of the asphalt can be excessively absorbed. However, the rubber particles tend to aggregate with each other, which leads to the high viscosity of the rubber asphalt and internal segregation phenomenon in the rubber asphalt.

The density of the CR is larger than that of asphalt, which leads to the deposition of CR in the asphalt. While the density of the EUG is smaller than that of asphalt, the composite material’s density is close to the density of the asphalt. According to the “equal density theory” [12,13], the composite of CR and EUG can be stably suspended in the asphalt.

After the desulfurization of CR, the broken sulfur bond on the surface of the rubber particle will react chemically with the asphalt, which can improve the compatibility between the CR and the asphalt. Moreover, the degraded rubber particles will be smaller, which can prevent them from aggregating into agglomerates and excessively swell in the asphalt. The “wrapping effect” of the EUG on the CR also has a similar effect as above. The double bond of the EUG can not only react with the sulfur in the CR but can also combine with the sulfur in the asphalt. Thus, the connection network among the EUG, the CR, and the asphalt can be formed through the above methods, which changes the interior of the rubber asphalt into a stable system. The modification mechanism of the EUG on the rubber asphalt is shown in Figure 2.

3. Materials and Experimental Plan

3.1. Materials

• EUG

EUG is a kind of natural high polymer material, and it has the same chemical composition as natural rubber, but the chemical constitution of them is different. EUG is the trans-polyisoprene, but the natural rubber is the cis-polyisoprene. The cis-polyisoprene is orderly and easy to gather and crystal. So it is a kind of flinty solid instead of an elastomer such as natural rubber. The main properties of the EUG used in this research are shown in

Table 1. Properties of EUG.

Specific Gravity	Hardness Shore A	Melting Point/°C	Softening Point/°C	Coefficient of Cubical Expansion/(/°C)	Thermal Conductivity/ (cal/ (cm·s·°C))
0.96~0.99	98	65	55	0.0008	3.1 × 104

.

2.

CR

The size of the CR used in this research is 30 mesh, and the main physical and chemical properties of CR are shown in

Table 2. Physical and chemical properties of CR.

Item	Relative Density	ω (Water)/ %	ω (Metal)/ %	ω (Fiber)/ %	ω (Ash Content)/%	ω (Acetone Extract)/%	ω (Carbon Black)/%	ω (Rubber Hydrocarbon)/%
Standard	1.10–1.30	<1.00	<0.05	<1.00	≤8.00	≤22.00	≥28.00	≥42.00
Test result	1.18	0.04	0.03	0.55	7.00	7.50	29.00	50.00

.

3.

Asphalt

The asphalt used in this research is one neat petroleum asphalt with penetration of 60/70, and the main technical properties of the neat asphalt are shown in

Table 3. Technical properties of asphalt.

Item	Penetration (25 °C, 100 g, 5 s)/(0.1 mm)	Ductility (5 cm/min, 15 °C)/cm	Softening Point/°C	RTFOT (163 °C, 5 h)
				Mass Loss/%	Penetration Ratio /%	10 °C Ductility/cm
Standard	60~80	≥100	≥46	−0.8~0.8	≥61	≥6
Test result	70	>150	47.0	0.15	67	7

.

3.2. Experimental Indicators (Test Method: Refer to the JTGE20-2011 Test Code for Asphalt and Asphalt Mixtures in Highway Engineering)

• The rotational viscosity test: The rotational viscosity shows the workability of the asphalt in construction. The instrument used in this test is an NDJ-1C rotational viscometer. The rotor type is 27#, and the rotational speed is 50 r/min. The rotational viscosity test was conducted at the temperature of 180 °C.
• Storage stability test of polymer-modified asphalt: The storage stability test was based on the results obtained from the softening point test. The smaller the difference in softening point is, the better the storage stability of the modified asphalt is.
• The Fourier-transform infrared spectroscopy (FTIR) test: Infrared spectroscopy can accurately detect the structure and structural changes of polymers in the asphalt, providing plenty of information to study the modification mechanism of the polymers on the asphalt. The instrument used in this test is the Nicolet 5700 Fourier infrared spectrometer.
• The scanning electron microscope (SEM) test: The morphological characteristics of polymers in the asphalt can be directly reflected by the microscopic images. The instrument used in this test is the SEM-3200M scanning electron microscope.
• The fluorescence microscope (FM) test: The distribution of the polymers in the asphalt can be observed by the fluorescence microscope. The instrument used in this test is the Eclipse 80i fluorescence microscope.

3.3. Experimental Scheme

Under the intense mechanical action in the high-temperature environment of the torque rheometer, CR will be desulfurized and degraded [9], but there are few references about the influence of the high-temperature mixing process on the CR in the presence of the EUG. Therefore, the relevant research was carried out according to the formula of the experimental materials in

Table 4. The formulae of experimental materials.

Formula	Different Rubber Asphalt Corresponding Modifier
Y-1	CR (unprocessed)
Y-2	Mixing of CR and raw EUG
Y-3	High temperature mixing of CR
Y-4	High temperature mixing of raw EUG and CR

. In this table, the ratio of EUG and rubber powder is 1:10.

According to the principle of the physical blending method, the EUG-modified CR was prepared by the XSS-300 torque rheometer (as shown in Figure 3). There are three process parameters in the use of the torque rheometer, which are temperature, time, and rotational speed. On the basis of previous studies, the following mixing process parameters of Y3 and Y4 were adopted in this research, which are 150 °C (mixing temperature), 15 min (mixing time), and 50 r/min (rotational speed). The speed and time of Y1 and Y2 are the same as Y4, and the temperature is 25 °C at room temperature.

4. Analysis of Experimental Results

4.1. The Tests of Rotational Viscosity and Softening Point Difference

The results of the rotational viscosity test and softening point difference test of the rubber asphalt with different formulas are shown in Figure 4 and Figure 5, respectively.

It can be seen from Figure 4 that the viscosity of the ordinary rubber asphalt (Y-1) is the highest among the four modified asphalt binders, indicating that the unmodified CR has undergone an excessive swelling reaction in the asphalt, which makes the viscosity of rubber asphalt increase sharply, thus affecting the binder’s workability in construction. Meanwhile, the viscosity of the mixed CR-modified asphalt (Y-3) is much lower than that of the ordinary rubber asphalt, which means that the mixed CR has undergone an intense desulfurization and degradation reaction. The rubber macromolecules are decomposed into a large number of small molecules, so the phenomenon of excessive swelling of the CR in the asphalt is changed. Furthermore, the viscosity of the raw EUG-modified rubber asphalt (Y-2) is almost unchanged compared with the ordinary rubber asphalt (Y-1), so it can be concluded that the raw EUG has a slight effect on the workability of the rubber asphalt in construction.

Figure 5 demonstrates that the subsidence phenomenon of the CR in the ordinary rubber asphalt (Y-1) is the most serious among the four modified asphalt binders. The system stability of the mixed CR-modified asphalt (Y-3) is superior to that of the ordinary rubber asphalt, indicating that the high-temperature mixing process could result in the depolymerization and the sulfur bond’s fracture of the CR, which not only decreases the binder’s molecular weight but also enhances the binder’s chemical activity [9]. At the same time, it is effective for the dispersion of the rubber particles in the asphalt and the formation of the crosslinking network between the CR and the asphalt; thereby, the storage stability of the modified asphalt is improved. In the same case of adding the EUG, the segregation degree of the modified asphalt without mixed CR (Y-2) is also larger than that of the modified asphalt with mixed CR (Y-4).

In Figure 4, the modification effect of the CR on the asphalt is further enhanced under the joint actions of the EUG and the high-temperature mixing process (Y-4). The EUG can crosslink the sulfur in the CR and the sulfur in the asphalt to form a network structure, and the network structure can separate the entangled rubber particles and reduce the molecular weight of the polymer. In addition, low melting point is a key characteristic of the EUG. The comprehensive effects of these factors have led to a significant decrease in the viscosity of the rubber asphalt.

As Figure 5 shows, the separation degree of the rubber asphalt under the joint actions of the EUG and the high-temperature mixing process (Y-4) is the smallest among the four modified asphalt binders, which shows that the EUG can promote the dispersion of the rubber particles in the asphalt and improve the compatibility between the CR and the asphalt. Therefore, the suspension stability of the CR in the asphalt is greatly improved.

4.2. Microscopic Examinations [14]

Figure 6a is a SEM image of the ordinary rubber asphalt (Y-1). It can be seen that the asphalt surface is uneven and the sizes of different rubber particles are very inconsistent. Moreover, the rubber particles are irregularly dispersed in the asphalt, and the aggregation of the rubber particles is very obvious. The above phenomena show that the swelling and diffusion reactions occur after the blending between the CR and the asphalt, but the rubber particles are still dispersed in the solid phase. Due to the poor compatibility between the CR and the asphalt, the ordinary rubber asphalt results in high viscosity and bad storage stability.

Figure 6c is a SEM image of the mixed CR-modified asphalt (Y-3). Compared with Figure 7a, it can be seen that the bulges on the asphalt surface are reduced and the agglomeration of rubber particles is improved. The above phenomena indicate that the molecular weight of the CR is reduced under the effect of the high-temperature mixing process. The fracture of the crosslinking bonds among rubber molecules leads to their inability to agglomerate, which improves the compatibility between the CR and the asphalt; therefore, the segregation degree of the modified asphalt during the storage process is decreased.

By comparing Figure 6d and Figure 6b, it can be found that the rubber particles in the mixed EUG and CR composite modified asphalt (Y-4) are smaller and more evenly distributed than those in the raw EUG-modified rubber asphalt (Y-2), which indirectly indicates that the high-temperature mixing process can improve the system stability of the rubber asphalt.

The intrinsic fluorescence properties of the CR and the asphalt are different, which show different colors under the blue light of FM. Therefore, the modifier shows an obvious yellow color, while the asphalt shows a dark one. Due to the viscoelasticity of the CR and the complex flow field in the blending process, the formation of the morphological structure of the rubber asphalt is a complex process. Moreover, the morphological structure is multi-layered, so there will be some granular, fibrous, or banded structures in the rubber asphalt.

Figure 7a is a FM image of the ordinary rubber asphalt (Y-1). It can be seen that the sizes of different rubber particles after excessive swelling are inconsistent, and the distribution of the rubber particles is uneven. It can be seen from Figure 7b that the distribution of the rubber particles in the mixed CR-modified asphalt (Y-3) is relatively regular, and the sizes of different rubber particles are relatively uniform, which indicates that CR can undergo a proper swelling reaction with the asphalt after the action of the high-temperature mixing process, thus the thermal storage stability of the mixed CR-modified asphalt (Y-3) is better than that of the ordinary rubber asphalt (Y-1).

Under the condition of adding the EUG, the microstructure of the mixed EUG and CR composite-modified asphalt (Y-4) is more orderly, and almost none of the big particles can be seen compared with the raw EUG-modified rubber asphalt (Y-2), which reflects the importance of the high-temperature mixing process.

4.2.1. The Effect of EUG on the Properties of Rubber Asphalt

Figure 6 shows that the surface of the mixed EUG and CR composite modified asphalt (Y-4) is very smooth and basically free of bulge, and the agglomeration phenomenon of the rubber particles disappears. It means that the compatibility between the CR and the asphalt is greatly improved by the joint actions of the EUG and the high-temperature mixing process, and the rubber particles are evenly dispersed in the asphalt. However, the separate action of the EUG and the high-temperature mixing process only has a limited modification effect on the CR. Therefore, it can be seen from Figure 6 that the microstate of the rubber particles in the raw EUG-modified rubber asphalt (Y-2) and the mixed CR-modified asphalt (Y-3) are both worse than that in the mixed EUG and CR composite modified asphalt (Y-4).

It can be seen from Figure 7 that the distribution characteristics of the rubber particles in the four modified asphalts under the observation of FM are basically consistent with the observation results of SEM. The distribution of the rubber particles in the raw EUG-modified rubber asphalt (Y-2) is more reasonable, and the sizes of the particles are smaller compared with the ordinary rubber asphalt (Y-1). Moreover, the microstructure of the mixed EUG and CR composite-modified asphalt (Y-4) is also superior to the mixed CR-modified asphalt (Y-3). These phenomena show that the EUG has a positive modification effect on the rubber asphalt.

4.2.2. A Summary of Microscopic Examinations

According to the three microscopic examinations, the fusion and dispersion of the rubber particles in the mixed EUG and CR composite modified asphalt (Y-4) are obviously better than the other three modified asphalt binders. It means that both the addition of the EUG and the high-temperature mixing process are helpful to improve the performance of the rubber asphalt. Therefore, the results of macroperformance tests are verified at the micro level.

4.3. Performance Grade Test (AASHTO M320: Standard Specification for Performance-Graded Asphalt Binder)

SBS-modified asphalt is a kind of high-performance modified asphalt binder, which is often used in road engineering. In order to fully prove the effectiveness of the EUG and the high-temperature mixing process, three kinds of asphalt binders were selected in comparison with the SBS-modified asphalt through the PG test. The results of the test are shown in

Table 5. Performance grade test results.

Test	Item		Temperature/°C	Y-1	Y-2	Y-4	SBS-Modified Asphalt	Requirement
DSR	Virgin test	(G*/sinδ)/kPa	64	7.729	8.235	13.318	14.752	≥1.0
			70	2.121	4.511	9.261	11.355
			76	1.653	2.375	4.524	5.349
			82	1.014	1.210	2.101	2.408
			88	0.360	0.312	0.862	0.992.
	RTFOT test		64	11.995	14.422	21.213	21.726	≥2.2
			70	5.804	7.517	14.670	15.440
			76	2.106	2.976	8.467	8.847
			82	—	1.724	4.596	5.020
			88	—	—	2.140	2.193
	PAV test	(G*·sinδ)/kPa	22	3345	2580	2029	2310	≤5000
			19	4921	4022	2840	3402
			16	5309	4910	3560	4023
			13	—	5340	4510	5122
			10	—	—	5063	—
BBR	PAV test	Creep stiffness/MPa	−12	182	165	124	130	≤300
			−18	361	324	225	244
			-24	—	—	343	371
		Creep rate	−12	0.320	0.373	0.422	0.392	≥0.3
			−18	0.282	0.291	0.375	0.304
			−24	—	—	0.293	0.280
Performance grade				PG70-22	PG76-22	PG82-28	PG82-28

.

According to the high-temperature indicator (G*/sinδ), it can be found that the raw EUG-modified rubber asphalt (Y-2) is one grade higher than the ordinary rubber asphalt (Y-1), and the mixed EUG and CR composite-modified asphalt (Y-4) is one grade higher than Y-2 and the same as SBS-modified asphalt. It means that the EUG and the high-temperature mixing process are both effective to improve the high-temperature performance of the rubber asphalt.

According to the intermediate-temperature indicator (G*·sinδ), it can be found that the raw EUG-modified rubber asphalt (Y-2) is one grade higher than the ordinary rubber asphalt (Y-1) and the same as SBS-modified asphalt. The mixed EUG and CR composite modified asphalt (Y-4) is one grade higher than SBS-modified asphalt, which shows that the EUG and the high-temperature mixing process are both effective to enhance the fatigue cracking performance of the rubber asphalt.

According to the low-temperature indicator (creep stiffness/creep rate), the raw EUG has little effect on the low-temperature performance of the rubber asphalt, and the joint action of the EUG and the high-temperature mixing process can improve the low-temperature performance of the rubber asphalt to the same level as the SBS-modified asphalt.

The results of the above test show that the performance of the rubber asphalt can be significantly enhanced under the joint action of the EUG and the high-temperature mixing process.

5. Conclusions

The following conclusions and findings can be drawn:

• The strong mechanical action in the high-temperature condition leads to the desulfurization and degradation of the rubber particles. The desulfurized and degraded rubber particles can react more intensely with the asphalt, which can improve the compatibility between the CR and the asphalt.
• The EUG can effectively prevent the occurrence of excessive swelling phenomena and the agglomeration phenomenon of the CR in asphalt. Moreover, a tight crosslinking network can be formed among the EUG, the CR, and the asphalt by the sulfurization reaction, and finally a stable spatial crosslinking structure can be established inside the rubber asphalt. Therefore, the workability of construction and the system stability of the mixed EUG and CR composite-modified asphalt are much better than that of the ordinary rubber asphalt.
• After the action of the high-temperature mixing process, a better chemical connection can be built between the CR and asphalt. The EUG can also improve the degree and the efficiency of the chemical reaction between the CR and the asphalt. Only under the joint action of these factors can the performance of the rubber asphalt be significantly enhanced.