Effects of Environment-Friendly Rejuvenator on the Rheological Properties and Microstructure of Aged Asphalt

Abstract

The purpose of this paper is to explore the feasibility of using waste soybean oil and the multi-epoxy compound trimethylolpropane triglycidyl ether (TMPGE) compound as a rejuvenator to recycle aged styrene-butadiene-styrene (SBS)-modified asphalt, and to develop an environment-friendly SBS-modified asphalt rejuvenator (ESMAR) to achieve dual waste reuse. The optimum ratio of each component of ESMAR and the production conditions of the formulation were firstly determined by the orthogonal test method. To evaluate the rejuvenation effect of ESMAR, the physical properties and rheological properties of SBS-modified asphalt before and after rejuvenation were studied. Physical properties test results showed that ESMAR can effectively soften the aged asphalt and decrease its softening point and viscosity, as well as increase its needle penetration. As far as rheological properties are concerned, ESMAR could improve the low-temperature cracking resistance and fatigue resistance of aged SBS-modified asphalt; however, there was a certain reduction in the rutting resistance. The microscopic characteristics of the rejuvenation of ESMAR were analyzed by fluorescence microscopy, infrared spectroscopy, and gel permeation chromatography tests for the aged SBS-modified asphalt before and after rejuvenation, which showed that ESMAR can not only reconstruct the degraded SBS molecules in the aged SBS-modified asphalt, but also has a “dilution” effect on the aged asphalt.

Keywords

infrastructure materials asphalt materials selection mix design asphalt production and use recycling

Asphalt pavements usually need to be maintained and repaired in their mid and late stages of service, which will generate much aged asphalt material when mitigating pavement distress and milling structurally deficient pavements ( 1 – 5 ). When these aged materials are discarded, they cause environmental pollution and take up a lot of land resources. At present, traditional mineral oil regenerant is mainly used to regenerate waste asphalt binder, but it is expensive and non-renewable. According to the world’s urgent requirements for resource reuse and environmental protection, it is urgent to develop and promote green, environment-friendly, and efficient rejuvenators for aged asphalt ( 6 , 7 ).

Bio-oil is a kind of viscous oil obtained by rapid pyrolysis, hydrothermal liquefaction, or filtration centrifugation of biomass such as straw, livestock manure, and oil. The sources of bio-oil are very wide, such as animal manure, corn stalks, rapeseed, soybeans, and so on. In numerous studies, bio-oil has been demonstrated to have the capacity to restore the physical and rheological characteristics of aged asphalt as well as enhance the road performance of recycled asphalt mixes ( 8 – 12 ). It is a green, environment-friendly, renewable resource. Soybean oil is the most commonly consumed bio-oil in the world, and China is the country with the largest total consumption of soybean oil in the world ( 13 ). According to statistics, from 2006 to the first quarter of 2022, the annual consumption of soybean oil in China surged from 7.6 million tons to nearly 40 million tons ( 14 ). China’s restaurant industry today produces about 20 million tons of waste soybean oil each year ( 15 ). The majority of waste soybean oil is discarded, which results in waste and also pollutes the environment ( 3 ).

Studies have shown that waste soybean oil has the characteristics of having many light components and large unsaturation, which has a significant dilution and softening effect on aged asphalt, and can improve the softness and fluidity of aged asphalt, and its low-temperature characteristics ( 16 – 18 ). Research on waste soybean oil asphalt regenerant is widely carried out worldwide.

Elkashef et al. showed that the incorporation of soybean oil in aged asphalt can markedly decrease the critical mid-temperature, thereby enhancing the fatigue resistance of recycled asphalt ( 19 ). Furthermore, the addition of soybean oil has been found to substantially ameliorate the low-temperature crack resistance of recycled asphalt mixes ( 20 ). Fang et al. revealed that the introduction of waste soybean oil in aged asphalt has the potential to modulate the distribution of constituents, thereby facilitating the rejuvenation of aged asphalt ( 21 ). Zhang et al. used molecular dynamics to simulate the diffusion behavior of soybean oil within aged asphalt binder and found that soybean oil has good rejuvenation performance and a high diffusion coefficient ( 22 ). At the same time, it could be uniformly distributed in the aged asphalt binder to form a cross-linked network structured to blend with good consistency and compatibility. However, the above studies only used a single soybean oil to improve the physical properties of aged asphalt without considering the chemical defects of the modified asphalt after aging ( 23 – 26 ). At the same time, pure soybean oil is competitive with the food streams, and the use of pure soybean oil for the regeneration of aged asphalt does not conform to the economic principle of the project.

The aging process of styrene-butadiene-styrene (SBS)-modified asphalt involves both the aging of the matrix asphalt and the degradation of the SBS modifier, in which the oxidative degradation of the SBS modifier is an essential factor leading to the deterioration of modified asphalt performance ( 27 – 31 ). While the physical properties of SBS-aged asphalt can be improved to some extent by waste soybean oil, degraded SBS modifiers cannot be restored. It has been reported that the tri-block structure of SBS can be degraded to styrene-butadiene (SB)-diblock structure under light or thermal oxidizing environment, and the degradation product—SB molecule—contains many oxygen-containing groups, including carbonyl, hydroxyl, and carboxyl groups ( 32 – 34 ). Because of the presence of these reactive groups, methods are proposed to restore the degraded SB-diblock structure. Compounds containing terminal groups can be used to reconfigure and react with the oxygen-containing terminal groups of aged SBS to reconnect the broken SBS blocks. There are a variety of compounds containing terminal groups, such as trimethylolpropane triglycidyl ether (TMPGE), 1, 4-butanediol diglycidyl ether (BUDGE), toluene diisocyanate (TDI), and isocyanate pre-polymer. In this study, TMPGE was used, which is expected to partially restore the tri-block structure and molecular weight of aged SBS through the reconstruction reaction, thus achieving the purpose of rejuvenating aged asphalt.

To fully and completely regenerate the aged SBS-modified asphalt, this paper proposes a rejuvenation method involving the combination of physical and chemical rejuvenation reactions, which uses the compounding of waste soybean oil and TMPGE to develop the environment-friendly SBS-modified asphalt rejuvenator (ESMAR), to adjust the chemical components of the matrix asphalt, as well as to reconstruct the structure of the fractured SBS modifier. The optimal ratio of each component in ESMAR and the formulated production conditions were determined by using the orthogonal test method. The physical properties, rheological properties, and microstructure of aged SBS-modified asphalt before and after rejuvenation were studied to evaluate the rejuvenation effect and applicability of ESMAR.

Objectives and Scope

Explore the feasibility of using waste soybean oil and TMPGE compound as a rejuvenator to recycle aged SBS-modified asphalt.

Determine the optimal content of each component of ESMAR and the optimal production process through orthogonal tests.

Evaluate the rheological properties of primary aged and secondary aged SBS-modified asphalt after rejuvenation.

Analyze the microscopic rejuvenation mechanism of ESMAR.

Materials and Experiments

Asphalt

The SBS-modified asphalt used in this study is “Donghai brand” SBS-modified asphalt, and the SBS modifier is YH-791H (1301) linear modifier produced by Baling Petrochemical, with S/B of 30/70. To obtain the aged asphalt required for the experiment, the original asphalt was initially put through a rolling thin-film oven test (RTFOT) to simulate the short-term aging of the asphalt mixture during the production process, as shown in Figure 1 ( 35 ). Then, the pressure aging vessel (PAV) test recommended by the Strategic Highway Research Program (SHRP) was used to simulate the long-term aging of asphalt during service, as shown in Figure 2 ( 36 ). The fundamental performance parameters of the two types of asphalt are presented in Table 1.

Figure 1.

Rolling thin-film oven test aging.

Figure 2.

Pressure aging vessel aging.

Table 1.

Physical Properties of Asphalt

Type	Penetration (25°C)/(0.1 mm)	Softening point/°C	Ductility (5°C)/cm	Rotational viscosity (135°C)/(Pa.s)
Original asphalt	58	71.5	36	1.85
Aged asphalt	19.6	78.2	1.2	2.75

Components and Configuration of ESMAR

Waste Soybean Oil

Waste soybean oil was prepared by heating in an oven at 200°C for 6 h to degrade it for use as a base oil fraction for ESMAR, as shown in Figure 3.

Figure 3.

Waste soybean oil deteriorated by oven heating for 6 h.

Multi-Epoxy Functional Group Compound TMPGE and Catalyst BDMA

In this study, TMPGE—a multi-epoxy functional compound—was selected as one of the raw materials for ESMAR to provide oxygen-containing functional groups for the rejuvenation process. To improve the reaction rate of the multiepoxy functional compounds and the diblock SB molecular chain segments generated by SBS-modified asphalt aged, N,N-dimethylbenzylamine (BDMA) was selected as the catalyst.

Preparation of Recycled Asphalt

Aged SBS-modified asphalt was heated to 170°C, waste soybean oil was added, and it was sheared at 2,000–3,000 rpm for 5 min to make a lightweight component; the lightweight component was heated to 175°C and TMPGE and BDMA were added, in turn, and it was sheared at 160°C–170°C at 2,000–3,000 rpm to the best test duration determined by orthogonal test.

Physical Properties Test

The physical properties of SBS-modified asphalt before and after rejuvenation were tested according to ASTM D5, ASTM D36, ASTM D4402, and ASTM D113, including penetration at 25°C, softening point, viscosity at 135°C, and ductility at 5°C.

Dynamic Shear Rheometer (DSR) Experiment

In this study, the rheological properties of original, aged, and recycled SBS-modified asphalt were studied by using SYD-0628 DSR produced by HTKYYQ^®. The rheological tests were performed under strain-controlled conditions at temperatures of −10°C to 80°C and a heating rate of 2°C/min. Below 30°C, a plate with a diameter of 8 mm and spacing of 2 mm was used, and in the rest of the conditions, a plate with a diameter of 25 mm and spacing of 1 mm was used. In this study, three samples were taken from the DSR test samples for repeated experiments.

Bending Beam Rheometer (BBR) Experiment

Based on AASHTO T313, the stiffness modulus and creep rate at −12°C, −18°C, and −24°C were measured by the RHE-102 BBR of the ATS company to evaluate the low-temperature performance of the SBS-modified asphalt. Three samples were taken for parallel experiments under each experiment condition.

Fluorescence Microscope Test

In this study, the fluorescence microscope was the FR-4A fluorescence microscope manufactured by BINGRU, and blue light was used for excitation observation. During the sample preparation, the asphalt was heated first, and the glass slide was preheated to prevent the rapid solidification of the asphalt. A clean and dry glass rod was used to drip the asphalt droplets onto the glass slides. The glass slides were placed flat in an oven at 130°C for 3 min and then removed. The cover glass was covered in a hot melt state and gently pressed flat. The fluorescence microscope used a 500 micron image measurement range and a multiple of 200 times.

Infrared Spectroscopy Test

A NICOLET-IS50 made by THERMO was used for this study to analyze the infrared spectra of SBS-modified asphalt before and after aged and after rejuvenation. The spectral acquisition area was 400–4,000 cm⁻¹, the scanning frequency was 32 times, and the resolution was 4 cm⁻¹. Infrared spectrum preparation used potassium bromide tablets.

Gel Permeation Chromatography Test

In this study, the Waters 2414 gel permeation chromatograph was used for the test, and the solvent was tetrahydrofuran (THF) solution. The test temperature was 40°C, the sample size was 20 μl, and the test time of each sample was about 15 min. The mobile phase flow rate of the test was maintained at 0.35 ml/min.

Preparation of ESMAR

Orthogonal Experiment Design

Orthogonal test design refers to a design method that analyzes multiple factors and levels through orthogonal tables. The principle is to select some representative horizontal combinations from many test schemes for testing. Through the analysis of this part of the test results, the optimal horizontal combination is found. In this study, waste soybean oil (factor A), TMPGE (factor B), and BDMA (factor C) accounting for the quality percentage of aging asphalt (%), and shear time (factor D) were selected as orthogonal test factors. Each factor selected three levels, that is, four-factor three-level orthogonal design—a total of 9 test groups. The crossover tests were conducted at different levels to reflect the influence of each factor on the three major indexes of asphalt and the viscosity at 135°C. The orthogonal test table is shown in Table 2, and the orthogonal test results are shown in Table 3.

Table 2.

Orthogonal Test Table

Numbering	A (%)	B (%)	C (%)	D (min)
1	(1) 4	(1) 2	(1) 0.05	(1) 15
2	(1) 4	(2) 3	(2) 0.1	(2) 30
3	(1) 4	(3) 4	(3) 0.2	(3) 60
4	(2) 5	(1) 2	(1) 0.05	(3) 60
5	(2) 5	(2) 3	(2) 0.1	(1) 15
6	(2) 5	(3) 4	(3) 0.2	(2) 30
7	(3) 6	(1) 2	(1) 0.05	(2) 30
8	(3) 6	(2) 3	(2) 0.1	(1) 15
9	(3) 6	(3) 4	(3) 0.2	(3) 60

Table 3.

Orthogonal Test Results of Recycled Asphalt

Numbering	25°C penetration (0.1 mm)	5°C ductility (cm)	Softening point (°C)	135°C viscosity (Pa·s)
1	57.4	20.4	53.3	2.55
2	64.1	25.6	53.3	0.93
3	69.1	29.3	50.4	0.83
4	59.4	33.7	52.2	1.19
5	63.0	37.0	51.0	1.29
6	70.3	38.9	49.9	0.47
7	60.6	34.9	51.9	0.90
8	62.0	40.0	47.0	0.88
9	70.3	39.4	49.1	0.52

Determination of the Best Ratio

To obtain the influence status of each orthogonal test factor on the three major indexes and 135°C viscosity performance of recycled asphalt, the test results in Table 3 were calculated using extreme difference analysis (R = X_max-X_min), as shown in Table 4. The results of each index analysis are summarized in Table 5.

Table 4.

Range Analysis Table of Various Factors on Each Index of Recycled Asphalt

Technology index	Factor A	Factor B	Factor C	Factor D
25°C penetration/0.1 mm	0.8	10.8	1.4	1.5
Softening point/°C	2.6	2.7	1.0	1.4
5°C ductility/cm	13.0	6.2	0.8	2.1
135°C viscosity/Pa·s	0.7	0.9	0.4	0.7

Table 5.

Summary of Analysis Results of Each Index

Technology index	Factor order of priority	Better conditions of orthogonal experiment
25°C penetration/0.1 mm	B>D>C>A	B₃D₂C₂A₃
Softening point/°C	B>A>D>C	B₁A₁D₂C₂
5°C ductility/cm	A>B>D>C	A₃B₃D₃C_3,2
135°C viscosity/Pa·s	B>A=D>C	B₃A₃D₂C₂/B₃D₂A₃C₂

Table 5 showed that factor A has the greatest effect on 5°C latency, and the better condition in the orthogonal test is level 3. Factor C has little effect on the four indicators. Considering the cost saving and ensuring the full and efficient response, factor C selects level 2. Factor D has a large effect on the penetration at 25°C, with the better condition of level 2 in the orthogonal test, thus factor D was selected as level 2. Factor B affects the penetration at 25°C, viscosity at 135°C, and softening point to the same extent, with better conditions in orthogonality at level 3 (4%) and level 1 (2%), which cannot determine the optimal amount of B for the time being. Keeping the level of factor B varying up and down in the range of 2% to slightly above 4%, the experiment is continued using one-way analysis. The test protocol is presented in Table 6 and the test results are presented in Table 7.

Table 6.

Further Verification of the Optimal Ratio Test Scheme

Numbering	Waste soybean oil content (%)	TMPGE content (%)	BDMA content (%)	Shearing time (min)
1	6	2	0.1	30
2	6	3	0.1	30
3	6	4	0.1	30
4	6	4.5	0.1	30

Note: BDMA = N,N-dimethylbenzylamine; TPMGE = trimethylolpropane triglycidyl ether.

Table 7.

Further Verification of the Optimum Ratio Test Results

Numbering	Softening point (°C )	5°C ductility (cm)	25°C penetration (0.1 mm)
1	49.9	32.0	66.8
2	49.9	33.1	67.1
3	50.2	37.2	68.2
4	50.0	37.6	69.2

From Table 7, it can be seen that the recycled asphalt performance of schemes 3 and 4 is better and the difference is small. Considering the influence of economic benefits, scheme 3 is selected as the best scheme; that is, factor B selects 4% level. Therefore, the final determination of ESMAR agent ratio and shear time is waste soybean oil dose of 6%, TMPGE dose of 4%, BDMA dose of 0.1%, and shear time of 30 min.

Rejuvenation Effect Verification of ESMAR

To evaluate the performance of ESMAR, a conventional physical properties and viscosity comparison analysis was selected with the widely used R regenerant produced by a company in Shaanxi at its recommended admixture (7%). The results of the tests are presented in Tables 8 and 9.

Table 8.

Results of Conventional Physical Properties of Asphalt after Incorporation of Two Regenerants

Type	25°C penetration (0.1 mm)	5°C ductility (cm)	Softening point (°C)	Elastic recovery at 25°C (%)
SBS-modified asphalt	58	36	71.5	92
Aged SBS-modified asphalt	19.6	1.2	78.2	14.2
Add ESMAR	68.2	37.2	50.2	91.1
Add R regenerant	57	24.3	49.0	69.9

Note: ESMAR = environment-friendly SBS-modified asphalt rejuvenator; SBS = styrene-butadiene-styrene.

Table 9.

Viscosity Test Results after Adding Regenerant

Type	Temperature (°C)
Type	90	105	120	135	150	165
SBS-modified asphalt	48.90	20.53	7.25	1.85	0.90	0.38
Aged SBS-modified asphalt	74.33	30.30	10.70	2.75	0.94	0.40
Add ESMAR	40.12	16.24	5.78	0.90	0.82	0.31
Add R regenerant	31.94	10.73	4.02	0.99	0.80	0.26

Note: ESMAR = environment-friendly SBS-modified asphalt rejuvenator; SBS = styrene-butadiene-styrene.

It can be seen in Tables 8 and 9 that both regenerants can significantly soften the aged asphalt, diminish its viscosity and softening point, and increase its penetration, yet the rejuvenation performance of ESMAR is better. Concerning 5°C ductility and elastic recovery, only the addition of ESMAR can restore the aged asphalt to the pre-aged level.

Rheological Properties Test

The fatigue factor (G*·sinδ) is usually applied to describe the fatigue resistance of asphalt, and the higher the value the worse the fatigue resistance. The stiffness modulus and the creep rate are used as indicators to evaluate the low-temperature cracking resistance. A lower stiffness modulus value and higher creep rate value represent asphalt with better low-temperature cracking resistance. To explore the applicability of ESMAR to secondary aged asphalt, after regeneration of primary aged asphalt, RTFOT and PAV experiments were conducted again to simulate the natural aging of asphalt to obtain secondary aged asphalt. Subsequently, the same rejuvenator and rejuvenation process as that of the primary aged asphalt were used to rejuvenate the secondary aged asphalt.

From Figure 4, a, c , d , and Table 10, compared with SBS-modified asphalt, after primary and secondary aging, the fatigue factor increases, stiffness modulus increases, and creep rate decreases, which reflects that asphalt has worse low-temperature cracking resistance and fatigue resistance after aging, and the aging process makes asphalt more prone to cracking. This is consistent with the conclusions of existing studies ( 37 – 40 ). It can be explained that the hardness of SBS-modified asphalt increases after aging because of the volatilization of light components in asphalt and oxidation of asphalt, which is also in accordance with the experiment results of physical indicators.

Figure 4.

Rheological test results: (a) effect of environment-friendly SBS-modified asphalt rejuvenator (ESMAR) on fatigue factor of asphalt, (b) effect of ESMAR on the rutting factor of asphalt, (c) stiffness modulus of asphalt at −12°C, −18°C, −24°C, and (d) creep rate of asphalt at −12°C, −18°C, −24°C.

Table 10.

Stiffness Modulus (m) and Creep Rate (S)

Type	−12°C		−18°C		−24°C
Type	S (MPa)	m	S (MPa)	m	S (MPa)	m
Original asphalt	65.4	0.462	171	0.352	344	0.271
Primary aged asphalt	80.7	0.385	199	0.302	408	0.244
Primary rejuvenation asphalt	65.6	0.458	169	0.359	344	0.279
Secondary aged asphalt	90.5	0.372	206	0.296	420	0.229
Secondary rejuvenation asphalt	67.3	0.45	175	0.346	350	0.269

After adding ESMAR, the fatigue factor and fatigue critical temperature of aged asphalt are significantly reduced. Meanwhile, in the temperature range of −10°C to 30°C, the fatigue factor of primary recycled asphalt is always lower than the original asphalt, and the fatigue factor of secondary recycled asphalt is also closer to the original asphalt. For the low-temperature cracking resistance index, stiffness modulus decreases and creep rate increases, which means that the low-temperature cracking resistance of recycled asphalt returns to a condition close to the original asphalt.

This indicates that ESMAR can restore the low-temperature cracking resistance and fatigue resistance of aged asphalt, as well as having good applicability to both primary and secondary aged asphalt. The reason for this is that the higher concentration of lighter compounds in the soybean oil contained in ESMAR may play a significant role in balancing the chemical composition of the aged asphalt. At the same time, the better flowability of waste soybean oil also helps to enhance the low-temperature cracking resistance and fatigue resistance of asphalt. On the other hand, the degraded SB-disconnected chains are structurally reorganized with TMPGE, which can also recover the low-temperature cracking resistance and fatigue resistance to some extent.

As shown in Figure 4b and Table 11, both primary aged and secondary aged resulted in a significant increase in the rutting factor of SBS-modified asphalt, with the performance grade both rising to PG82. This indicates that the aged behavior of SBS-modified asphalt contributes to the high-temperature deformation resistance of asphalt pavements, which is consistent with the findings of previous studies ( 41 – 43 ). After the addition of ESMAR, the rutting factor of recycled asphalt is reduced significantly, and the performance grade is reduced to PG76, which illustrates that ESMAR will reduce the high-temperature rutting resistance of SBS-modified asphalt. That is because the higher light and viscous components in the waste soybean oil can balance the chemical components of the aged asphalt and soften the aged asphalt significantly; meanwhile, the epoxy reaction can partially reconstruct the structure of SBS to improve the viscosity of aged asphalt.

Table 11.

Rutting Critical Temperature, High-Temperature Grade, and Fatigue Critical Temperature of Aged Asphalt and Recycled Asphalt

Type	Rutting critical temperature (°C)	Superpave^® performance grade	Fatigue critical temperature (°C)
Original asphalt	79.1	PG76	16.8
Primary aged asphalt	>80	PG82 (extrapolation)	22.1
Primary rejuvenation asphalt	76.1	PG76	15.7
Secondary aged asphalt	>80	PG82 (extrapolation)	25.3
Secondary rejuvenation asphalt	79.9	PG76	18.8

Study of the Microscopic Characteristics of Rejuvenation

Fluorescence Microscope (FM) Test

In the blue light region, there is little excited fluorescence in the asphalt material, and the SBS polymer modifier will be excited to produce obvious yellow-green fluorescence. Thus, the polymer phase is clearly distinguishable from the asphalt phase by FM imaging. It can be seen from Figure 5, a and b , that sparse weak spot fluorescence appears in the FM image of aged asphalt. Compared with the continuous yellow-green fluorescence in modified asphalt, the SBS content in aged SBS-modified asphalt has been reduced to a very low level, and the number of SBS modifiers after aging has been greatly reduced. It can be inferred that the SBS triblock copolymer underwent severe chain scission after asphalt aging, which is consistent with the findings of previous studies ( 27 – 31 ). As shown in Figure 5c, the SBS particulates in regenerated SBS-modified asphalt are significantly restored, and the distribution is closer to that of the original SBS-modified asphalt, where the SBS particulates are basically restored to the level when they were not aged. FM image analysis shows that ESMAR can effectively restore the molecular structure of SBS modifier. Figure 5d shows the results of FM tests of recycled asphalt without the TMPGE component, from which it can be seen that the SBS modifier molecules were not effectively recovered. This indicates that the reconstruction of the degraded SBS modifier molecules is a result of the presence of the TMPGE component.

Figure 5.

Fluorescence microscope photo of styrene-butadiene-styrene (SBS)-modified asphalt: (a) SBS-modified asphalt, (b) SBS-modified asphalt after pressure aging vessel aging, (c) SBS-modified asphalt after rejuvenation, and (d) SBS-modified asphalt after rejuvenation without trimethylolpropane triglycidyl ether.

Infrared Spectroscopy (FTIR)

The infrared spectrum analysis method was used for SBS-modified asphalt and recycled asphalt before and after aged, and the infrared spectrum is shown in Figure 6.

Figure 6.

Infrared spectra of original asphalt, aged asphalt, and recycled asphalt.

According to Lambert Beer’s law, the absorption peak size of the light energy group can characterize the content of the group ( 44 , 45 ). To quantitatively characterize the change of chemical structure during the aging process of SBS-modified asphalt, different functional group indexes were calculated by referring to the analysis method of Lamontagne et al. and the ratio of the peak area of the specific wavenumber segment of the infrared spectrum to the spectral area between 600cm⁻¹ and 2,000cm⁻¹ ( 46 ). The change of functional group index can reflect the change of chemical structure of asphalt before and after aging. The functional group absorption peak area A was obtained by ONMIC software, and sulfoxide index (SI), the carbonyl index (CI), and ether bond index (EI) were calculated by formulas 1–3 and shown in Table 12.

SI = \frac{A_{1031 c m^{- 1}}}{\sum A_{2000 - 600 c m^{- 1}}}

(1)

CI = \frac{A_{1695 c m^{- 1}}}{\sum A_{2000 - 600 c m^{- 1}}}

(2)

EI = \frac{A_{1010 c m^{- 1}}}{\sum A_{2000 - 600 c m^{- 1}}}

(3)

Table 12.

Sulfoxide Index (SI), Carbonyl Index (CI), Ether Bond Index (EI) Calculation Results

Type	SI	CI	EI
Original asphalt	0.002014	0	0
Aged asphalt	0.012704	0.003046	0
Recycled asphalt	0.008876	0.000677	0.032698

Compared with the original asphalt, the four absorption peaks representing aromatic molecules at 727–868 cm⁻¹ in the spectrum of aged asphalt gradually become smaller, indicating that the light components such as aromatics in aged asphalt are reduced, which is also the internal reason for the change of rheological properties of asphalt during aging. This is consistent with the findings of previous studies ( 47 , 48 ). This is mainly since the light component of asphalt is gradually transformed into heavy components such as asphaltene during the ageing process; at the same time, in the aging environment of higher temperature, the light components in asphalt have volatilization. From Table 12, it can be seen that, after aging of SBS-modified asphalt, the CI and SI indexes all showed an increasing trend, indicating that the unsaturated chain in the asphalt was oxidized during the aging process to form aging products such as carbonyl and sulfoxide groups. After aging of SBS-modified asphalt, the absorption peaks at 966 cm⁻¹ and 700 cm⁻¹, which can characterize the presence and content of SBS molecules, were significantly reduced, indicating that degradation of the SBS modifier in the original asphalt occurred during the aging process.

After adding ESMAR, new absorption peaks at 1,744 cm⁻¹ and 1,170 cm⁻¹ were generated, corresponding to the ester and epoxy groups in the polyepoxy compounds in ESMAR. The new absorption peak at 1,110 cm⁻¹ is generated by the C-O-C stretching vibration of the ether bond, which is the product of the polymerization reaction between the epoxy group and the hydroxyl group. At the same time, according to Table 12, the EI index increased from 0 to 0.032698, which indicated that the multi-epoxy compound in ESMAR opened the epoxy group and polymerized with the hydroxyl group in the SB-diblock segment structure. The absorption peaks at 966 cm⁻¹ and 700 cm⁻¹, which characterize the presence and content of SBS molecules, become larger and the results of the FM test strongly prove the occurrence of polymerization reaction. For the experimental group without the TMPGE component, the absorption peaks characterizing the presence and content of the SBS molecules did not show any significant change compared with the aged asphalt, and no new absorption peaks were generated at 1,110 cm⁻¹. This suggests that it is the TMPGE component in ESMAR that reconfigures the degraded SBS modifier molecules.

After adding ESMAR, the three indexes representing the aging degree of asphalt all showed a downward trend, and all decreased to the level before SBS-modified asphalt aging, indicating that adding ESMAR can reduce the aging degree of aging asphalt. On one hand, this is because the waste soybean oil component in ESMAR could adjust the ratio of components of aged asphalt as well as reduce the concentration of polar oxygen-containing functional groups of aged asphalt; that is, it has a “dilution” effect on the aged asphalt. On the other hand, the multi-epoxy group in ESMAR reacts with the hydroxyl group in the SB-diblock segment structure, so that the diblock SB-molecular chain is coupled and converted into a triblock SBS molecule again.

Gel Permeation Chromatography (GPC)

The GPC test can evaluate the molecular weight distribution of materials according to the characteristics of different elution time when different-sized molecules pass through the chromatographic column. Figure 7 shows the molecular weight distribution of asphalt after the GPC test. The number average molecular weight (M_n), weight average molecular weight (M_w), and dispersion coefficient (d) were calculated according to formulas 5–7 for quantitative analysis, as presented in Table 13.

M_{n} = \sum N_{i} \times M_{i} / \sum N_{i}

(5)

M_{w} = \sum W_{i} \times M_{i} / \sum W_{i}

(6)

d = M_{w} / M_{n}

(7)

where

M _i = the molecular weight,

W _i = the component weight of M_i, and

N _i = the molecular number of M_i.

Figure 7.

Molecular weight distribution of asphalt.

Table 13.

Molecular Weight and Distribution of Asphalt Samples

Type	M _W	M _n	d
Original asphalt
Characteristic peak A	2,604	1,106	2.354
Characteristic peak B	301,156	222,575	1.353
Whole sample	3,011	1,088	2.767
Aged asphalt
Characteristic peak A	98	43	2.279
Characteristic peak B	312,738	245,834	1.272
Whole sample	4,514	1,185	3.809
Recycled asphalt
Characteristic peak A	1,996	955	2.090
Characteristic peak B	267,532	224,678	1.191
Whole sample	3,612	1,113	3.245

Note: d = dispersion coefficient; M_n = number average molecular weight; and M_w = weight average molecular weight.

From Figure 7 and Table 13, it is clear that the intensity of peak A representing the molecular weight distribution of SBS polymer decreases, and M_w decreases from 2,604 to 98, indicating that SBS-modified asphalt will lead to the degradation of SBS polymer molecules after aging. The M_w of the whole sample increased from 3,011 to 4,514, an increase of 49.9%. The increase of M_w indicates that the molecular weight of the large mass in the asphalt molecule increases, indicating that the oxidation reaction occurs inside the aged asphalt to form macromolecules. The content of asphaltene macromolecules in aged asphalt increases, the intermolecular force increases, the relative movement becomes difficult, and the asphalt becomes hard and brittle, which is also the internal reason for the change of rheological properties of asphalt during aging.

From Table 13 it can be seen that the M_w of peak A of SBS-modified asphalt recovered from 98 to 1,996 after adding ESMAR, and the recovery amount reached 76.65% of the original asphalt. It indicates that SBS polymer molecular weight increases after asphalt rejuvenation, and ESMAR reconstructs the SBS modifier molecule to a certain extent, which is consistent with the results of FM and FTIR. At the same time, there was no significant change in peak A in the experimental group without the addition of TMPGE, reflecting that it is the TMPGE component in ESMAR that reconfigures the degraded SBS modifier molecule. At the same time, d decreases after rejuvenation, while d represents the width of the molecular weight distribution. The decrease in d indicates that the intermolecular forces of the regenerated asphalt are weakened and the internal stress distribution becomes more uniform. Macroscopically, the viscosity of the asphalt is reduced and the flexibility and ductility are increased, which is reflected by the physical property test results. The overall molecular weight of the regenerated asphalt showed a downward trend. As far as molecular weight is concerned, the number of small molecules (SMs) increased and the number of large molecules (LMs) decreased, indicating that the waste soybean oil component in ESMAR replenished the light component through “dilution” and increased the SM content in the asphalt, which is consistent with the findings of previous studies ( 49 ).

Through the GPC test, from the perspective of asphalt molecular weight distribution, it is shown that ESMAR can effectively repair partially degraded SBS polymer molecules, and has a “dilution” effect on aged asphalt, which can balance the chemical composition of aged asphalt.

Conclusion

Through orthogonal tests and three major asphalt performance indexes and viscosity tests, the best ratio (mass ratio) of ESNAR was determined as waste soybean oil: TMPGE:BDMA = 6:4:0.01. The best preparation process was shearing the components with 2,000–3,000 rpm for 30 min at 160°C–170°C.

Comparing the basic physical property indexes and viscosity test results of aged asphalt regenerated by adding ESNAR and commonly used asphalt regenerant (R regenerant), it showed that adding ESNAR can significantly improve the flexibility and low-temperature ductility of aged SBS-modified asphalt—all indexes were basically restored to the original SBS-modified asphalt level except for softening point and viscosity. From the rheological properties, ESMAR could restore the low-temperature cracking resistance and fatigue resistance of aged SBS-modified asphalt; moreover, it had good applicability to both primary and secondary aged asphalt. However, ESMAR has a degrading effect on the high-temperature rutting resistance of aged asphalt.

FM, FTIR, and GPC tests showed that the waste soybean oil component in ESMAR could adjust the ratio of components of aged asphalt as well as reduce the concentration of polar oxygen-containing functional groups of aged asphalt, which means it has a “dilution” effect on aged asphalt. In addition, ESMAR polyepoxide could open the epoxy group and polymerize with the hydroxyl group in the SB-diblock segment structure to reconstruct the SBS modifier molecule.

In addition, it is worth mentioning that this study is for recycled asphalt rather than recycled asphalt mixtures. Recycled asphalt mixtures are mainly composed of aged asphalt mixtures, rejuvenator, new asphalt, new minerals, and so forth, and their ratio design is a key factor affecting the performance of recycled asphalt mixtures. Because of the diversity of materials, the mixing processes and parameters are also different. With regard to the process of recycled asphalt mixture, some researchers believe that the rejuvenator will diffuse over time. For better diffusion and mixing, measures are often taken to mix the rejuvenator with the aged asphalt mixture first, then preheat it, and finally add new aggregate and new asphalt for mixing ( 50 – 53 ).

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: Q. Li, D. Liu, Y. Wen; data collection: Q. Li, D. Liu, Y. Wen; analysis and interpretation of results: Q. Li, D. Liu, Y. Wen; draft manuscript preparation: Q. Li, D. Liu, Y. Wen. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is partially sponsored by the Changsha University of Science and Technology Postgraduate Research Innovation Project (No. CXCLY2022008).

ORCID iD

Dongxu Liu

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