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Article

Study on the Performance Restoration of Aged Asphalt Binder with Vegetable Oil Rejuvenators: Colloidal Stability, Rheological Properties, and Solubility Parameter Analysis

by
Heng Yan
1,
Xinxin Cao
2,
Wei Wei
2,
Yongjie Ding
2,* and
Jukun Guo
3
1
School of Civil Engineering, Chongqing Jiaotong University, Chongqing 400074, China
2
College of Metropolitan Transportation, Beijing University of Technology, Beijing 100124, China
3
Shandong Transport Vocational College, Weifang 261206, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(8), 917; https://doi.org/10.3390/coatings15080917 (registering DOI)
Submission received: 3 June 2025 / Revised: 22 June 2025 / Accepted: 23 June 2025 / Published: 6 August 2025
(This article belongs to the Section Environmental Aspects in Colloid and Interface Science)
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Review Reports Versions Notes

Abstract

This study evaluates the effectiveness of various rejuvenating oils, including soybean oil (N-oil), waste frying oil (F-oil), byproduct oil (W-oil), and aromatic hydrocarbon oil (A-oil), in restoring aged asphalt coatings by reducing asphaltene flocculation and improving colloidal stability. The rejuvenators were incorporated into aged asphalt binder via direct mixing at controlled dosages. Their effects were assessed using microscopy, droplet diffusion analysis, rheological testing (DSR and BBR), and molecular dynamics simulations. The aim is to compare the compatibility, solubility behavior, and rejuvenation potential of plant-based and mineral-based oils. The results indicate that N-oil and F-oil promote asphaltene aggregation, which supports structural rebuilding. In contrast, A-oil and W-oil act as solvents that disperse asphaltenes. Among the tested oils, N-oil exhibited the best overall performance in enhancing flowability, low-temperature flexibility, and chemical compatibility. This study presents a novel method to evaluate rejuvenator effectiveness by quantifying colloidal stability through grayscale analysis of droplet diffusion patterns. This integrated approach offers both mechanistic insights and practical guidance for selecting bio-based rejuvenators in asphalt recycling.

1. Introduction

The aging of asphalt concrete, particularly bitumen, has become a major concern in the road construction industry due to its significant impact on pavement performance and service life [1]. Over the years, various methods have been explored to address the aging of asphalt, including the use of rejuvenators, modifiers, and additives [2,3]. These substances help restore the properties of bitumen that are lost due to oxidation, thermal effects, and mechanical stress. Common rejuvenators include plant oils, synthetic polymers, and even waste products derived from fossil fuels. Studies have shown that plant oils, in particular, exhibit promising potential as rejuvenators, as they can reverse the aging effects and improve the flexibility and durability of asphalt mixtures [4].
In asphalt pavement construction, the use of reclaimed asphalt pavement (RAP) technology is a widely adopted approach to enhance sustainability. The economic and environmental benefits of using RAP are well recognized, as it not only reduces construction costs by 14% to 34% when the RAP content ranges from 20% to 50% [5] but also reduces the need for new raw materials, thereby saving energy and lowering pollution emissions associated with material extraction and transportation [6,7]. However, RAP presents challenges in terms of fatigue resistance and low-temperature performance. The aged binder in RAP often exhibits brittle behavior, leading to premature cracking and failure of the pavement. To address these issues, the use of asphalt rejuvenators has been proposed to restore the binder’s properties and improve the overall performance of RAP mixtures [8]. Recent studies have focused on the application of plant oils as effective rejuvenators. These natural oils, due to their chemical composition, are capable of reducing the viscosity of oxidized bitumen and improving its low-temperature crack resistance and high-temperature rutting resistance. Several studies have highlighted the potential of plant oils in enhancing the physicochemical properties of aged bitumen [3,9]. Therefore, this study focuses on exploring the role of vegetable oil rejuvenators in mitigating asphalt aging, contributing to both the economic and environmental benefits of asphalt pavement construction.
The rejuvenator consists of base oil and additives, with base oil making up approximately 70%. Currently, aromatic hydrocarbons are mainly used as the base oil in rejuvenators, but they are known for their high carcinogenicity [10]. Aromatic hydrocarbons (PAHs), when used as rejuvenators, release significant amounts of polycyclic aromatic compounds into the air during the high-temperature mixing of asphalt. For instance, a study analyzed the PAH emissions from asphalt mixing plants, revealing that the concentration of PAHs in fine particulate matter (PM2.5) ranged from 0.51 to 60.73 ng/m3, with an average concentration of 11.54 ng/m3. The highest concentrations were observed during the winter months. These PAHs may be transported through the air, potentially affecting the health of nearby residents [11]. They can also infiltrate groundwater through road runoff, causing environmental pollution [12]. Additionally, using aromatic hydrocarbons as the base oil in rejuvenators is costly and offers poor anti-aging properties [13]. Therefore, there is an urgent need to explore rejuvenator base oils with lower levels of polycyclic aromatic compounds, better affordability, and reliable performance for use in asphalt reclaimer mixtures.
In recent years, bio-rejuvenators have emerged as a promising new technology for asphalt pavements, especially in recycled asphalt. These bio-rejuvenators are sourced from various origins [14], each with distinct mechanisms and effects. Existing research mainly explored the performance and mechanisms of recycled asphalt and mixtures from vegetable-based materials. However, studies on the mechanisms and efficacy of rejuvenators from animals and algae are still ongoing [15,16,17].
Vegetable oils, known for their availability and renewability, have become a major research focus worldwide as a replacement for traditional petroleum in chemical production [18]. Compared to aromatic hydrocarbons, vegetable oils have higher molecular weights and boiling points, which improve construction safety and aging resistance. Additionally, vegetable oils lack aromatic components, providing superior environmental benefits. Consequently, there is increasing interest in alternative rejuvenators, especially those derived from vegetable oils, known for their renewability and low toxicity [19].
Several studies have explored the use of rejuvenators and various oils in regenerating aged asphalt, providing valuable insights into their efficacy. Hallizza et al. [20] examined the physical properties of asphalt rejuvenated from different aging stages using deep-fried waste oil. Analyzing penetration, softening point, and Brookfield viscosity through one-way ANOVA, the study found no significant difference between virgin asphalt and recycled asphalt. Sun et al. [21] tested the chemical compositions of vegetable-based rejuvenators from various sources to aid in rejuvenator selection. The study found that these rejuvenators’ primary components closely resemble the light components in asphalt, which can resist molecular aggregation during aging, replenish missing light components, and restore the rheological properties of aged asphalt. Martins et al. [22] investigated six rejuvenators: waste vegetable oil (WV oil), waste vegetable oil esters (WV grease), organic oil, tall oil, aromatic fraction extracts, and waste engine oil (WEO). All rejuvenators successfully reduced PG94-12 to PG64-22. Compared to virgin asphalt, these products provided excellent resistance to rutting, prolonged fatigue life, and reduced critical cracking temperature. Du et al. [23] explored the impact of two rejuvenators, peanut oil and soybean oil, on the rheological and microscopic properties of differently aged SBS-modified asphalt binders. The results indicated that both rejuvenators exhibited a restorative effect on aged asphalt binders, with peanut oil being the most effective. Pratik et al. [24] used watermelon seed oil and composite castor oil as rejuvenators to achieve 100% recycling of RAP. The study revealed that when added at a 5% concentration, the recycled asphalt binder showed superior resistance to rutting and fatigue, surpassing those of virgin asphalt. Zhang et al. [25] studied asphalt modification using nine types of bio-oils and mineral oils to investigate the relationship between the glass transition temperature of modified asphalt binders and their low-temperature performance. The glass transition temperature serves as a rapid and accurate indicator of asphalt’s low-temperature fracture properties. The study revealed that the glass transition temperatures of oil-modified asphalts varied and were consistently lower than that of virgin asphalt, with the lowest temperature observed for bio-waste oil. Cao et al. [26] used vegetable oil to rejuvenate aged asphalt, noting its enhancement of low-temperature and fatigue performance. The study also observed a physical dilution of polar large molecules in the aged asphalt. Behnood et al. [27] systematically reviewed the effects of rejuvenators on the performance of aged asphalt, discussing the varying impacts of different rejuvenator types. Lv et al. [28] found that adding bio-oil significantly influenced the high- and low-temperature performance of aged asphalt, identifying the interaction as a physical reaction. Ding et al. [29] used wood cellulose as a raw material and employed hydrothermal liquefaction to produce bio-oil. The study indicated that bio-asphalt with 10% bio-oil content exhibited performance comparable to 70# asphalt. Furthermore, bio-oil-recycled asphalt showed minimal performance differences compared to virgin asphalt and even outperformed it in some aspects, according to the studies. Consequently, further research on bio-oil as an asphalt rejuvenator is warranted. Recycled asphalt using vegetable oil exhibited favorable mechanical properties, but blending vegetable oil with asphalt posed risks of colloidal instability. Phenomena such as flocculation and precipitation of asphalt components could occur due to mixing and blending crude oils with significant property differences [30].
The substantial compositional differences between bio-asphalt and mineral asphalt made the colloidal stability after their mixture a critical area that required immediate research attention [19]. Hassan et al. [31] separated vegetable-oil-recycled asphalt into four components and evaluated asphalt colloidal stability using the colloidal instability coefficient. However, results varied significantly due to thin-layer chromatography separation with differing eluting agents and solvents, rendering the assessment inappropriate [32,33]. Gong et al. [34] tested the thermal storage stability of various bio-oils mixed with asphalt at different temperatures, finding reduced stability with higher bio-oil content and storage temperature. Jun et al. [35] used molecular simulation studies and found that the composite rejuvenator made of plant oil (pine needle oil, PNO) and mineral oil (dioctyl phthalate, DOP) significantly improved the rheological properties and low-temperature performance of aged asphalt, with the compatibility between plant oil and mineral oil being a key factor for its effectiveness. This suggests the potential compromise of asphalt colloidal stability by vegetable oil, but current research in this area remains insufficient.
Although various bio-based rejuvenators have emerged, their effectiveness in asphalt recycling has not been comprehensively and systematically evaluated. The existing literature mainly focuses on individual types of rejuvenators, often based on small-scale laboratory experiments, and lacks long-term validation under real-world conditions. Moreover, comparative studies of different types of rejuvenating oils are relatively scarce, especially those analyzing the effects of vegetable oils versus mineral oils on the restoration of asphalt properties. Therefore, this study aims to systematically evaluate the effectiveness of various vegetable and mineral oils as rejuvenators in restoring the properties of aged asphalt and to explore their mechanisms, particularly by using droplet diffusion methods and molecular simulations to further analyze their impact on asphalt colloidal stability. The primary goal of this research is to fill the gaps in current studies by providing a comprehensive comparative analysis that assesses the role of different rejuvenators in restoring asphalt performance and enhancing its durability. This will provide theoretical foundations and practical guidance for optimizing asphalt recycling technologies and promoting the widespread application of bio-based rejuvenators.

2. Experimental Section

2.1. Materials

The native soybean oil (N-oil) is sourced from supermarkets, as shown in Figure 1a, and it is obtained through pressing. The waste soybean oil (F-oil) comes from the frying process in restaurants and is then filtered to remove impurities, as shown in Figure 1b. The W-oil is a byproduct of the purification of soybean oil at a processing temperature of 400 °C, as shown in Figure 1c; the oil contains linoleic acid, oleic acid, and triglycerides. The aromatic hydrocarbon oil (A-oil) is a product obtained from residue oil through a series of catalytic and cracking reactions, sourced from a company in Shandong, primarily containing benzene, toluene, and xylene. Figure 1d shows this oil, obtained through a series of catalytic and cracking reactions from residue oil. N-oil, F-oil, and W-oil are classified as vegetable oils, while A-oil is classified as a mineral oil. Table 1 presents the physical and chemical properties of these oils, and Table 2 presents their elemental compositions. According to ASTM D6373 standards [36], the key performance indicators of both base asphalt and aged asphalt binder meet the aging resistance requirements. The base asphalt has a 25 °C penetration of 64.8 (0.1 mm), which falls within the standard range of 60–80 (0.1 mm). After aging, the penetration decreases to 50.7 (0.1 mm), with a residue penetration ratio of 78.2% (≥50%). The ductility at 10 °C decreases from >100 cm for the base asphalt to 6.5 cm for the aged asphalt, still meeting the equivalent requirement of ASTM D6373 for a 15 °C ductility of ≥10 cm (the temperature difference has been adjusted according to climate zone standards). The softening point increases from 49.4 °C to 64.3 °C, indicating improved high-temperature stability of the aged asphalt, which complies with the allowable range for softening point changes in the RTFOT test. These results demonstrate that the aging resistance of the asphalt meets the requirements of road engineering specifications.
In the subsequent experiments, N-asphalt refers to the rejuvenation of aged asphalt using N-oil (native soybean oil); F-asphalt is the result of rejuvenating aged asphalt by adding F-oil (waste soybean oil); W-asphalt is asphalt rejuvenated with W-oil (a byproduct of soybean oil refining); and A-asphalt is asphalt rejuvenated using A-oil (aromatic hydrocarbon oil).
The virgin asphalt produced in China was AH-70, and the aged asphalt underwent Rolling Thin Film Oven Test (RTFOT) using the RP-0610 rotary thin film oven (U-Therm International (H.K.) Limited, Hong Kong, China) and Pressure Aging Vessel (PAV) test using the SYD-0630 asphalt pressure aging vessel (Shanghai Changji Geological Instrument Co., Ltd., Shanghai, China). The Rolling Thin Film Oven Test simulated the short-term aging of asphalt during high-temperature mixing, while the Pressure Aging Vessel test simulated the long-term aging of asphalt during its usage (6–7 years). Following ASTM D2872, short-term aging was conducted at 163 °C for 85 min with an airflow rate of 4000 mL/min. Subsequently, the asphalt aged in the short term was introduced into the Pressure Aging Vessel (PAV) for long-term aging, following the experimental procedure outlined in ASTM D6521. The aging conditions included a temperature of 90 °C, a pressure of 2.1 MPa, and an aging duration of 20 h. Table 3 presents the physical properties of the virgin asphalt and aged asphalt.
The research roadmap is illustrated in Figure 2. Three types of vegetable oils (N-oil, F-oil, W-oil) and one mineral oil (A-oil) are selected as rejuvenators. Microscopic observation and a modified spot test are utilized to analyze the colloidal stability of the rejuvenators with asphalt. Additionally, this study investigates the rheological properties of different recycled asphalts through DSR and BBR tests.

2.2. Test Program

2.2.1. Microscopic Observation

Microscope observation was utilized to monitor the flocculation of asphaltene during the addition of rejuvenators, offering the advantages of room temperature, atmospheric pressure, and direct observation. Jesper et al. [37] introduced n-heptane into crude oil and observed the flocculation of asphaltene using a microscope. Zhao et al. [38] observed the initiation point of asphaltene flocculation under a microscope by adding normal alkanes to crude oil, with the appearance of flocculated bodies with a particle size of 0.5 μm marking the initiation point. Due to the higher viscosity of asphalt binder compared to crude oil, this study initially dissolved the asphalt in toluene and then added n-heptane to observe the flocculation state of asphaltene.
In this study, 10 g of asphalt binder was taken and added to a beaker, followed by the addition of 10 g of toluene to dissolve the asphalt binder, and then 70 g of n-heptane. For N-oil, F-oil, W-oil, and A-oil, 2 g, 3 g, 4 g, 5 g of n-heptane was added to the toluene-asphalt solution. After standing for 15 min, the mixture was stirred with a glass rod to prepare slides for microscope observation. Optical observation was conducted using a Leica fully automatic optical microscope DM6B (Leica Microsystems, Wetzlar, Germany).

2.2.2. Droplet Diffusion Method

Fourest et al. [39] evaluated the colloidal stability of recycled asphalt binder using the drop diffusion method according to the ASTM D4740-04 (2014) standard [40], which has the advantage of being quick and easy to use. n-heptane was added to crude oil, and the mixture of crude oil and n-heptane was dropped onto filter paper. Different colors of circles and rings appeared on the filter paper, with lighter colors indicating poorer colloidal stability. Fabricio et al. [41] added toluene solvent to crude oil and utilized the drop diffusion method to observe the colloidal stability of different crude oils under the influence of n-heptane.
In this study, 10 g of asphalt binder was placed in a 150 mL conical flask. Initially, a small amount of n-heptane was added to dilute the oil sample, followed by a gradual increase in the amount of n-heptane, preparing mixtures of 58 mL, 66 mL, 74 mL, 82 mL, and 90 mL of n-heptane with asphalt binder. Subsequently, filter paper was dried in a constant-temperature oven at 100 °C for 10 min, then cooled in a desiccator for later use. Finally, before taking each sample drop, the mixture was stirred evenly with a glass rod, and a droplet was released onto the center of the filter paper, allowing the droplet to freely diffuse and form a spot pattern.

2.2.3. Molecular Simulation

Direct simulation of the complex mixture structure of real asphalt binder is impractical; therefore, this study employs a four-component (asphaltene, resins, saturates, and aromatics) twelve-molecule asphalt binder model to construct an aged asphalt binder model [42]. The four components consist of 12 molecules, with their specific parameters detailed in Table 4. The molecular models were constructed using the Amorphous Cell module in Materials Studio software, Materials Studio software, Version 2020, created by Dassault Systèmes, sourced from Waltham, MA, USA. forming a four-component monomer model, as shown in Figure 3.
Following geometric and dynamic structural optimizations of the twelve individual molecules, the models were constructed using the Amorphous module. Subsequently, further structural and dynamic optimizations were performed. The optimized structure is presented in Figure 4. Previous studies report that the density of aged asphalt binder ranges from 1.01 to 1.05 g·cm–3, with an error margin of 3% to 8% compared to real aged asphalt binder [43,44,45]. The density of the optimized aged asphalt binder model in this study is 1.02 g·cm–3, indicating a relatively accurate and reliable model, as shown in Figure 5.
Vegetable oils (N, F, W oils) predominantly consist of linoleic acid (C18H32O2), oleic acid (C18H34O2), and triglycerides (C39H74O6), whereas mineral oils (A oil) mainly comprise aromatic hydrocarbons [22]. Two rejuvenator oil models were assembled and optimized structurally, as shown in Figure 6 and Figure 7.

3. Results and Discussion

3.1. Effect of Rejuvenator on Asphaltene Flocculation

3.1.1. Asphaltene Flocculation of Aged Asphalt Binder

This study investigated the changes in asphaltene flocculation status after asphalt binder aging, observing asphaltene flocculation images of both virgin asphalt binder and aged asphalt binder under a microscope, as shown in Figure 8.
Figure 8a,b show that the size of asphaltene flocculation particles in aged asphalt binder is significantly larger than that in virgin asphalt binder, indicating that asphaltene becomes more prone to flocculation after aging, resulting in a reduction in the colloidal stability of aged asphalt binder. This increase in particle size can be attributed to the aging process, during which the loss of lighter components, such as volatile oils and aromatic hydrocarbons, causes a relative increase in the asphaltene content. This leads to stronger intermolecular interactions, promoting the aggregation of asphaltene molecules into larger flocs. Additionally, the color of asphaltene flocculation particles in aged asphalt binder appears darker compared to those in virgin asphalt binder, which is likely due to the increased presence of aromatic compounds and higher-molecular-weight aggregates formed during the oxidation and polymerization reactions in the aging process. Since the asphaltene flocculation process is three-dimensional, measuring the thickness of the particles is challenging. Therefore, this study primarily focused on the changes in the planar dimensions of the flocculation particles, as they provide a more feasible means to analyze the extent of aggregation and the reduction in colloidal stability.

3.1.2. The Planar Dimensions of Flocculation Particles Under Different Rejuvenators

This study analyzed the average size of asphaltene flocculation particles at rejuvenator/aged asphalt binder mass ratios of 20%, 30%, 40%, and 50%, as depicted in Figure 9.
Figure 9 demonstrates that as the mass of N-oil increased, the average length of asphaltene flocculation particles progressively increased, reaching 11, 18, 20, and 22. This indicates that N-oil effectively promotes asphaltene flocculation. A similar trend was observed with F-oil, where the average length of the flocculation particles increased to 7, 8, 14, and 20 as the mass of F-oil increased, suggesting that F-oil also promotes asphaltene flocculation. Both N-oil and F-oil likely promote this process through interactions with asphaltene molecules, possibly by altering the surface properties of asphaltenes and facilitating the aggregation of smaller particles into larger flocs. In contrast, the effect of A-oil differed significantly. As the mass of A-oil increased, the average length of the asphaltene flocculation particles decreased progressively to 10, 5, 1, and 0. This suggests that A-oil dissolves asphaltene, disrupting particle aggregation and promoting the solubilization of asphaltenes. This behavior is likely due to the solvent properties of A-oil, which enable it to interact with the polar components of asphaltene, leading to the breakdown of flocculated structures.
Similarly, W-oil exhibited a solvent effect on asphaltene. As the mass of W-oil increased, the average length of asphaltene flocculation particles decreased to 26, 17, 13, and 10, suggesting that W-oil dissolves asphaltene particles over time. The solvent effect of W-oil can be attributed to its chemical structure and polarity, which facilitate the solubilization of asphaltenes by disrupting their intermolecular interactions. In summary, vegetable oils, N-oil and F-oil, promoted asphaltene flocculation by enhancing the aggregation of asphaltene particles, likely due to their ability to interact with the polar components of asphaltene. On the other hand, mineral oil A-oil and vegetable oil W-oil exhibited solvent effects, dissolving asphaltene and preventing flocculation. These contrasting behaviors highlight the complex mechanisms of asphaltene interaction with different oils, where oils with suitable polarity can either promote flocculation or dissolve asphaltene depending on their chemical properties.

3.2. Colloidal Stability of Recycled Asphalt Binder

3.2.1. Optimal Dosage of Rejuvenators

The penetration index is a highly effective indicator for determining the optimal content of rejuvenators [46]. The penetration index measured at various concentrations of asphalt binder rejuvenators allows for the assessment of the flowability and viscosity changes of asphalt binder after rejuvenator addition. The study introduced N-oil (2%, 4%, 6%), F-oil (2%, 4%, 6%, 8%), A-oil (2%, 4%, 6%, 8%), and W-oil (3%, 6%, 9%, 12%) to aged asphalt binder. With the penetration index of aged asphalt binder at 25.1, the standard for restoring the penetration index of recycled asphalt binder to that of virgin asphalt binder (64.8) was utilized to determine the optimal content of the rejuvenators.
Figure 10 illustrates that as the content of rejuvenators increased, the penetration index of recycled asphalt binder gradually rose, indicating a linear relationship between the penetration index of recycled asphalt binder and the content of rejuvenators. The slopes of the penetration index versus rejuvenator content for N-asphalt, F-asphalt, A-asphalt, and W-asphalt were 12.96, 8.65, 6.14, and 4.07, respectively, suggesting that under the same rejuvenator production, N-oil had the most significant impact on the penetration index of aged asphalt binder, followed by F-oil, A-oil, and W-oil with decreasing effects. The optimal content of rejuvenators, calculated through interpolation, was 3.38% for N-oil, 5.13% for F-oil, 6.76% for A-oil, and 9.76% for W-oil. At these optimal content levels, the rejuvenators could restore the penetration index of aged asphalt binder to the level of virgin asphalt binder.
The penetration index is used to measure the hardness or softness of asphalt binder, with an increase indicating a reduction in viscosity and an improvement in flowability, both of which are crucial for asphalt binder performance. As asphalt binder ages, the asphaltene and resin components become harder and more brittle. N-oil has the most significant effect on increasing the penetration index, suggesting that the fatty acids and other polar groups in N-oil may reduce the cohesive forces between asphalt binder particles, making them more deformable under pressure. This, in turn, lowers viscosity while enhancing flowability and flexibility. The higher slope (12.96) further indicates that N-oil is the most effective in restoring asphalt binder workability.
The softening point and penetration before and after aging were compared at the optimal rejuvenator dosage, as shown in Table 5. The addition of vegetable oils significantly improved the aging resistance of asphalt binder, particularly N-oil. The penetration value of the base asphalt binder was 64.8 before aging and decreased to 25.1 after aging, a reduction of approximately 61.2%, indicating significant aging-induced hardening. N-asphalt exhibited the smallest change in penetration, with a decrease of only 9.2%, demonstrating that vegetable oils can significantly enhance aging resistance. F-asphalt and W-asphalt showed penetration changes of 13.3% and 19%, respectively, though their effects were slightly less pronounced than N-asphalt. In contrast, A-asphalt (containing mineral oil) experienced the greatest penetration decrease, reaching 25.0%, indicating that mineral oil does not effectively slow down aging.
Regarding the softening point, the base asphalt binder’s softening point increased from 49.4 to 53.5, with a change of about 8.3%. N-asphalt exhibited the smallest increase in softening point, rising by 5%, indicating the best performance in aging resistance. F-asphalt and W-asphalt showed changes of 7% and 7.9%, respectively, also improving aging resistance. In contrast, A-asphalt showed the largest increase in softening point, reaching 9.1%, demonstrating that mineral oil accelerates the aging process.
Plant oil-based asphalts (N-asphalt, F-asphalt, and W-asphalt) demonstrate superior aging resistance compared to mineral oil-based A-asphalt. Among them, N-asphalt exhibits the best performance, with a ductility reduction of only 16.5%. In contrast, F-asphalt and W-asphalt show reductions of 21.2% and 30.5%, respectively. In comparison, A-asphalt experiences a significant 50% decrease in ductility, indicating its poorer aging resistance. Overall, plant oil-based asphalts, particularly N-asphalt, offer enhanced aging resistance.

3.2.2. Droplet Diffusion Images of Recycled Asphalt Binder

The optimal amounts of N-oil, F-oil, A-oil, and W-oil were added to aged asphalt binder to prepare rejuvenated asphalt binder, named N-asphalt, F-asphalt, A-asphalt, and W-asphalt. Following that, 58 mL, 66 mL, 74 mL, 82 mL, and 90 mL of n-heptane were individually dropped onto the four types of rejuvenated asphalt binder, and the mixed solutions were applied to filter paper to generate diffusion spot patterns, as depicted in Figure 11.
In Figure 11a–e, representing different n-heptane amounts (58 mL, 66 mL, 74 mL, 82 mL, and 90 mL) in the drop diffusion images, inner circles and outer rings were observed. The inner circles had a darker color, and the inner circles in a, b, c, d, e exhibited similar colors. As the n-heptane titration volume increased, the outer rings underwent significant changes, gradually lightening in color. While the color change in the outer ring could distinguish alterations in asphalt binder colloid stability, quantitative analysis was essential.
This study employed image processing to calculate the average grayscale difference between the inner circle and outer ring for quantitative analysis of the diffusion images. Initially, the grayscale values of each point in the image were extracted, and a three-dimensional grayscale value image was established, as depicted in Figure 12.
Based on the characteristics of relatively constant grayscale values in the inner circle and significant grayscale changes in the outer ring, this study calculated the grayscale difference ratio between different outer rings and inner circles, defined as the grayscale ratio.
grayness   ratio   = the   gray   value   of   the   outer   ring   −   the   gray   value   of   the   inner   circle the   gray   value   of   the   inner   circle
Figure 13a–e show three-dimensional grayscale images that accurately represent the color changes observed in Figure 11a–e. As the n-heptane titration volume increased, the grayscale difference between the outer ring and inner circle gradually increased. Therefore, the grayscale difference in drop diffusion images provided an accurate quantification of the color difference between the outer ring and inner circle.

3.2.3. Grayness Ratio of Different Recycled Asphalt Binder

The grayscale ratio of different recycled asphalts (N-asphalt, F-asphalt, W-asphalt, A-asphalt) was calculated to analyze colloidal stability, with virgin asphalt binder selected as the reference sample. Figure 14 illustrates the grayscale ratios for the five asphalt binders at n-heptane titration volumes of 58 mL, 66 mL, 74 mL, 82 mL, and 90 mL.
The grayscale ratio linearly increased with the n-heptane titration volume, as shown in Figure 14. The slopes for N-asphalt, F-asphalt, W-asphalt, A-asphalt, and virgin asphalt binder were 0.077, 0.058, 0.046, 0.044, and 0.035, respectively, indicating that the sensitivity of the grayscale ratio to n-heptane decreased sequentially for recycled asphalt binder. Grayscale ratios for N-asphalt, F-asphalt, W-asphalt, A-asphalt, and virgin asphalt binder decreased sequentially at the same n-heptane titration volume. This indicated a sequential increase in colloidal stability for N-asphalt, F-asphalt, W-asphalt, A-asphalt, and virgin asphalt binder. Consequently, a sequential increase in stability of aged asphalt binder by N-oil, F-oil, W-oil, and A-oil was observed.

3.3. The Rheological Properties of Recycled Asphalt Binder

It is evident from the above studies that differences exist in the colloidal stability of recycled asphalt binder with different rejuvenators. Further analysis focused on variations in pavement performance. Well correlated with pavement performance, the rheological properties of asphalt binders were such that the rutting factor reflected high-temperature performance, and the stiffness modulus and m-value reflected low-temperature performance [22]. Consequently, we conducted DSR and BBR tests to evaluate the rheological properties of various recycled asphalt binders.

3.3.1. The High-Temperature Performance of Recycled Asphalt Binder

This study employed the AR2000 DSR to test the rutting factor of N-asphalt, F-asphalt, W-asphalt, A-asphalt, virgin asphalt binder, and aged asphalt binder. The temperature scan range was 40–80 °C, and the frequency was 10 rad/s, following the testing method outlined in AASHTO T315-02. The rutting factors of the six types of asphalt binder at different temperatures are illustrated in Figure 15.
It can be observed from Figure 15 that the rutting factor increased after the aging of the virgin asphalt binder, indicating that aging benefited the high-temperature performance of asphalt binder. The rutting factor of recycled asphalt binder was lower than that of aged asphalt binder but higher than that of the virgin asphalt binder. This suggests that the rejuvenator reduced the high-temperature performance of aged asphalt binder, although it remained superior to the virgin asphalt binder.
The rutting factor continuously decreased with the increase in temperature, indicating that higher temperatures were detrimental to the high-temperature performance of asphalt binder. At the same temperature, the rutting factors of N-asphalt, F-asphalt, W-asphalt, and A-asphalt were close. This indicates that at the optimal dosage, the impact of N-oil, F-oil, W-oil, and A-oil on the high-temperature performance of aged asphalt binder was generally consistent. The influence of colloidal stability on the high-temperature performance of recycled asphalt binder was not significant. These results further emphasize the potential of vegetable oils, such as N-oil and F-oil, as effective rejuvenators for asphalt binder. While the rejuvenator slightly reduced the high-temperature performance of recycled asphalt binder compared to the aged version, it still provided better high-temperature stability than virgin asphalt binder. This demonstrates that vegetable oils, particularly N-oil and F-oil, can play an essential role in improving the high-temperature performance of recycled asphalt binder, offering a viable alternative to traditional rejuvenators and contributing to more sustainable and durable asphalt binder recycling practices. The fact that the rutting factors of the different oils were similar at optimal dosages further supports that vegetable oils are promising and versatile rejuvenators, ensuring consistent high-temperature performance improvements across various formulations.

3.3.2. The Low-Temperature Performance of Recycled Asphalt Binder

This study employed SYD-0627 BBR testing to determine the creep stiffness and m-values of N-asphalt, F-asphalt, W-asphalt, A-asphalt, virgin asphalt binder, and aged asphalt binder, with a test temperature of −12 °C and following the ASTM D 6648-01 standard. The creep stiffness and m-values of the six asphalt binder types are shown in Figure 16.
As shown in Figure 16, aging caused an increase in the bending creep stiffness of asphalt binder, accompanied by a decrease in the m-value. When rejuvenators (N-oil, F-oil, W-oil, and A-oil) were added to the aged asphalt binder, the bending creep stiffness of N-asphalt, F-asphalt, and W-asphalt was similar, while A-asphalt exhibited slightly higher values. This indicates that N-oil, F-oil, and W-oil were more effective in improving the properties of aged asphalt binder than A-oil. The bending creep stiffness of the four rejuvenated asphalt binders was lower than that of virgin asphalt binder, suggesting that the low-temperature performance of aged asphalt binder improved with N-oil, F-oil, W-oil, and A-oil was superior to that of virgin asphalt binder. The m-values of N-asphalt, F-asphalt, W-asphalt, and A-asphalt increased in sequence, indicating that the low-temperature performance of aged asphalt binder was enhanced by N-oil, F-oil, W-oil, and A-oil, with the effect strengthening progressively. However, the m-values of the four rejuvenated asphalt binders remained lower than that of virgin asphalt binder, indicating that while N-oil, F-oil, W-oil, and A-oil improved the low-temperature performance of aged asphalt binder, it was not fully restored to the level of virgin asphalt binder. The trend in the colloidal stability of the rejuvenated asphalt binders closely mirrored the trend in the m-values. These results suggest that vegetable oils, such as N-oil, F-oil, and W-oil, are more effective than mineral oils in improving the low-temperature performance of aged asphalt binder, particularly in enhancing low-temperature toughness.
Although these rejuvenators did not fully restore the low-temperature performance to the level of virgin asphalt binder, they still show significant potential for enhancing the low-temperature performance and reducing the brittleness of recycled asphalt binder. This makes them particularly suitable for rejuvenating aged asphalt binder and extending its service life. Therefore, vegetable oil-based rejuvenators hold great promise in asphalt binder recycling, particularly for improving low-temperature performance.

3.4. Model Calculation

Solubility Parameter

The solubility parameter is a key measure of compatibility between two systems [47] and an important indicator of intermolecular forces within a material system. In this study, the solubility parameter is employed to assess the compatibility between vegetable oil, mineral oil, and aged asphalt binder. First, the aged asphalt binder and rejuvenator oil models were annealed and optimized under the NVT ensemble. Subsequently, the solubility parameters for the aged asphalt binder model and the two rejuvenator oil models were calculated. The results are presented in Table 6.
As shown in Table 6, the solubility parameter of the aged asphalt binder is 17.538 (J/cm3)0.5, while that of soybean oil is 19.86 (J/cm3)0.5, and aromatic oil has a solubility parameter of 20.277 (J/cm3)0.5. The differences in solubility parameters reflect variations in the compatibility between the rejuvenator oils and the aged asphalt binder, primarily influenced by molecular structure, polarity, and solvent compatibility.
The primary component of soybean oil is triglycerides, which contain long-chain fatty acids such as linoleic acid (C18H32O2) and oleic acid (C18H34O2). These fatty acids exhibit non-polar characteristics; however, triglycerides also contain polar groups, enabling interaction with the polar components of asphaltene molecules in the asphalt binder [48,49]. The oxidation of the aged asphalt binder increases the content of polar functional groups, such as carboxyl, ester, and phenol. As a result, the aged asphalt binder exhibits relatively good compatibility with the fatty acids and polar components in soybean oil. The solubility parameter difference of 2.322 (J/cm3)0.5 between soybean oil and the aged asphalt binder allows soybean oil to effectively dissolve both non-polar and some polar components in the asphalt binder.
In contrast, aromatic oils (such as benzene, toluene, and xylene) primarily consist of aromatic hydrocarbons, which exhibit strong non-polar characteristics and relatively rigid molecular structures. The solubility parameter of aromatic oil is 20.277 (J/cm3)0.5, significantly higher than that of the aged asphalt binder, indicating a substantial difference in solubility between the two. Although some non-polar molecules in aromatic oil can dissolve certain non-polar components of the asphalt binder, the lack of sufficient polar interactions and the smaller molecular size (typically associated with lower molecular weights) hinder effective intermolecular interactions with the complex macromolecular system in the asphalt binder. Therefore, aromatic oil has much lower solubility compared to soybean oil. Additionally, the polar functional groups in the aged asphalt binder further decrease the solubility of aromatic oil, leading to poor compatibility between the two.
From the perspective of solubility theory, the solubility parameter is a key indicator of the compatibility between a solvent and a solute. When the solubility parameters of the solvent and solute are similar, their interactions are stronger, resulting in better dissolution. The solubility parameter of soybean oil is relatively similar to that of the aged asphalt binder, leading to stronger interactions and better dissolution of certain components in the asphalt binder. In contrast, the solubility parameter of aromatic oil differs significantly from that of the aged asphalt binder, reducing its ability to dissolve the asphalt binder compared to soybean oil. Therefore, vegetable oils (such as soybean oil) have solubility parameters that are relatively similar to aged asphalt binder, resulting in good compatibility and high solubility. In contrast, aromatic oils, due to their significant differences in solubility parameters, molecular structure mismatches, and differing polarity characteristics, are less effective at dissolving aged asphalt binder compared to vegetable oils.

4. Conclusions

This investigation into the influence of rejuvenators on aged asphalt binder yielded significant findings, providing valuable insights into asphalt binder aging and asphaltene flocculation. This study highlighted the potential of rejuvenators to restore key asphalt binder properties. Based on the experimental results, the following conclusions can be drawn:
(1) N-oil and F-oil promote asphaltene flocculation and enhance aggregation, while A-oil and W-oil prevent flocculation by dissolving asphaltene. N-oil and F-oil facilitate aggregation through interactions with the polar components of asphaltene, whereas A-oil and W-oil exhibit solvent effects that disrupt asphaltene aggregation. The results highlight the key role of oil polarity in asphaltene flocculation.
(2) Vegetable oils, especially N-oil and F-oil, show significant potential for use in recycled asphalt binder. These oils not only restore the penetration index and improve flowability but also enhance colloidal stability. The optimal rejuvenator dosages are 3.38% for N-oil, 5.13% for F-oil, 6.76% for A-oil, and 9.76% for W-oil, effectively restoring the performance of aged asphalt binder. N-oil, with its high concentration of polar triglycerides and unsaturated fatty acids, exhibits the strongest rejuvenating effect, while F-oil is somewhat less effective. A-oil and W-oil show weaker effects. This study demonstrates that N-oil has the greatest potential for improving the flowability and stability of recycled asphalt binder.
(3) The droplet diffusion method, based on grayscale analysis, effectively reflects changes in the colloidal stability of recycled asphalt binder. It fills a gap in existing studies by providing a quantitative evaluation. The results indicate a linear increase in the grayness ratio with higher n-heptane droplets. The slopes for N-asphalt, F-asphalt, W-asphalt, A-asphalt, and virgin asphalt binder were 0.077, 0.058, 0.046, 0.044, and 0.035, respectively.
(4) In terms of high-temperature performance, the rutting factor of recycled asphalt binder was lower than that of aged asphalt binder but higher than that of virgin asphalt binder, indicating a slight reduction in high-temperature performance due to rejuvenation. However, the high-temperature performance of the rejuvenated asphalt binder remained better than that of virgin asphalt binder, and at optimal dosages, the rutting factors for N-oil, F-oil, W-oil, and A-oil were similar, suggesting that vegetable oils (such as N-oil and F-oil) are effective in improving the high-temperature performance of recycled asphalt binder.
(5) In terms of low-temperature performance, BBR test results showed that although aging increased the bending creep stiffness and decreased the m-value, rejuvenators (N-oil, F-oil, W-oil, A-oil) improved the low-temperature properties. N-oil, F-oil, and W-oil were more effective than A-oil in enhancing low-temperature performance, with their m-values gradually increasing. However, the low-temperature performance of the rejuvenated asphalt binder was not fully restored to the level of virgin asphalt binder. Nevertheless, vegetable oils significantly improved the low-temperature toughness of recycled asphalt binder and reduced its brittleness. Therefore, vegetable oils, particularly N-oil and F-oil, show great potential as rejuvenators, improving both high-temperature and low-temperature performance and extending the service life of recycled asphalt binder.
(6) The solubility parameter of soybean oil is 19.86 (J/cm3)0.5, which differs by 2.322 (J/cm3)0.5 from aged asphalt binder 17.538 (J/cm3)0.5, indicating good compatibility and dissolution capacity. In contrast, aromatic oil has a solubility parameter of 20.277(J/cm3)0.5, which differs more significantly from aged asphalt binder, leading to weaker dissolution capacity. Overall, vegetable oils exhibit better compatibility and dissolution effects because their solubility parameters are closer to that of aged asphalt binder, making them more suitable for asphalt binder rejuvenation.
This study delves into the effects of plant oils (N-oil, F-oil, and W-oil) and mineral oil (A-oil) as rejuvenators for recycled asphalt binder, offering new insights and innovative contributions. The research demonstrates that plant oils effectively promote asphaltene flocculation, improving both the high-temperature and low-temperature performance of recycled asphalt binder. Notably, N-oil and F-oil show significant potential in enhancing the flowability, colloidal stability, and recovery of asphalt binder. Additionally, the use of the droplet diffusion method combined with grayscale analysis introduces a novel, quantitative approach to assess colloidal stability, providing valuable guidance for optimizing the performance of asphalt binder rejuvenators. These findings not only fill gaps in existing research but also offer a new direction for the development of more sustainable and environmentally friendly asphalt binder rejuvenators.

Author Contributions

Conceptualization, X.C. and H.Y.; investigation, X.C., W.W. and J.G.; methodology, Y.D. and X.C.; writing—original draft, X.C. and H.Y.; writing—review and editing, H.Y. and X.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Four kinds of asphalt rejuvenators: (a) soybean oil; (b) waste soybean oi; (c) byproduct oil; (d) mineral oil.
Figure 1. Four kinds of asphalt rejuvenators: (a) soybean oil; (b) waste soybean oi; (c) byproduct oil; (d) mineral oil.
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Figure 2. The roadmap of the research.
Figure 2. The roadmap of the research.
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Figure 3. Twelve aged asphalt binder molecules.
Figure 3. Twelve aged asphalt binder molecules.
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Figure 4. Optimization and comparison of aged asphalt binder models.
Figure 4. Optimization and comparison of aged asphalt binder models.
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Figure 5. Optimization of structural density of aged asphalt binder model.
Figure 5. Optimization of structural density of aged asphalt binder model.
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Figure 6. Molecular model of vegetable oil.
Figure 6. Molecular model of vegetable oil.
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Figure 7. Aromatic hydrocarbon oil molecular model.
Figure 7. Aromatic hydrocarbon oil molecular model.
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Figure 8. The asphaltene flocs of virgin asphalt binder (a) and aged asphalt binder (b).
Figure 8. The asphaltene flocs of virgin asphalt binder (a) and aged asphalt binder (b).
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Figure 9. Size of asphaltene flocs with different dosages of rejuvenator.
Figure 9. Size of asphaltene flocs with different dosages of rejuvenator.
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Figure 10. Penetration of recycled asphalt binder with different dosages of rejuvenator.
Figure 10. Penetration of recycled asphalt binder with different dosages of rejuvenator.
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Figure 11. Droplet diffusion of N-asphalt with n-heptane contents of (a) 58 mL; (b) 66 mL; (c) 74 mL; (d) 82 mL; and (e) 90 mL.
Figure 11. Droplet diffusion of N-asphalt with n-heptane contents of (a) 58 mL; (b) 66 mL; (c) 74 mL; (d) 82 mL; and (e) 90 mL.
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Figure 12. Principle of droplet diffusion image analysis.
Figure 12. Principle of droplet diffusion image analysis.
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Figure 13. The greyscale of droplet diffusion of N-asphalt with n-heptane content of (a) 58 mL; (b) 66 mL; (c) 74 mL; (d) 82 mL; and (e) 90 mL.
Figure 13. The greyscale of droplet diffusion of N-asphalt with n-heptane content of (a) 58 mL; (b) 66 mL; (c) 74 mL; (d) 82 mL; and (e) 90 mL.
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Figure 14. Different grayness ratios of recycled asphalt binder with different dosages of n-heptane.
Figure 14. Different grayness ratios of recycled asphalt binder with different dosages of n-heptane.
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Figure 15. Rutting factors of recycled asphalt binder at different temperatures.
Figure 15. Rutting factors of recycled asphalt binder at different temperatures.
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Figure 16. Stiffness modulus and m-value of asphalt binder.
Figure 16. Stiffness modulus and m-value of asphalt binder.
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Table 1. Physical and chemical indicators of N-oil, F-oil, W-oil, A-oil.
Table 1. Physical and chemical indicators of N-oil, F-oil, W-oil, A-oil.
RejuvenatorSpecific Gravity
(g/cm3)		ColourMolecular WeightViscosity at 60 °C (mPa·S)Viscosity at 90 °C (mPa·S)
N-oil0.917pale yellow14312010
F-oil0.951brown19652411
W-oil0.966dark brown232028480
A-oil0.987dark green63919246
Table 2. Elementary composition of N-oil, F-oil, W-oil, A-oil.
Table 2. Elementary composition of N-oil, F-oil, W-oil, A-oil.
RejuvenatorC/%H/%O/%C/H
N-oil80.4711.566.580.58
F-oil79.7410.997.260.60
W-oil77.7710.3110.260.63
A-oil85.519.464.650.75
Table 3. Physical and chemical indicators of virgin and aged asphalt.
Table 3. Physical and chemical indicators of virgin and aged asphalt.
PropertyVirgin AsphaltAged Asphalt Binder
Penetration (25 °C, 5 s)/0.1 mm64.850.7
Ductility (10 °C)/cm>1006.5
Softening point/°C49.464.3
Table 4. Molecular composition of aged asphalt binder model.
Table 4. Molecular composition of aged asphalt binder model.
MoleculesMolecular
Formula		Molecular
Weight (Da)		Number of
Molecules
asphaltene-AC42H46O5630.85
asphaltene-BC66H67NO7988.24
asphaltene-CC51H54SO57795
saturate-AC30H62422.85
saturate-BC35H62482.96
aromatic-AC35H36O4520.79
aromatic-BC30H42O2434.710
resin-AC40H55NO2581.96
resin-BC40H56O3S616.96
resin-CC18H10O2S2322.418
resin-DC36H53NO2531.86
resin-EC29H48O2428.77
Table 5. Penetration, softening point, and ductility before and after aging.
Table 5. Penetration, softening point, and ductility before and after aging.
ParametersN-AsphaltF-AsphaltW-AsphaltA-AsphaltVirgin Asphalt
Penetration (0.1 mm)64.864.864.864.864.8
Softening point (°C)54.153.25351.649.4
Ductility (cm)7.98.510.510.2100.8
ParametersAged
N-asphalt		Aged
F-asphalt		Aged
W-asphalt		Aged
A-asphalt		Aged
asphalt
Penetration (0.1 mm)58.856.252.548.650.7
Softening point (%)56.856.957.256.353.5
Ductility (cm)6.66.77.35.16.4
Table 6. Solubility parameter.
Table 6. Solubility parameter.
TypeSolubility Parameter (J/cm3)0.5Diversity (J/cm3)0.5
Aromatic oil20.2772.739
vegetable oil19.862.322
aged asphalt binder17.5380
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MDPI and ACS Style

Yan, H.; Cao, X.; Wei, W.; Ding, Y.; Guo, J. Study on the Performance Restoration of Aged Asphalt Binder with Vegetable Oil Rejuvenators: Colloidal Stability, Rheological Properties, and Solubility Parameter Analysis. Coatings 2025, 15, 917. https://doi.org/10.3390/coatings15080917

AMA Style

Yan H, Cao X, Wei W, Ding Y, Guo J. Study on the Performance Restoration of Aged Asphalt Binder with Vegetable Oil Rejuvenators: Colloidal Stability, Rheological Properties, and Solubility Parameter Analysis. Coatings. 2025; 15(8):917. https://doi.org/10.3390/coatings15080917

Chicago/Turabian Style

Yan, Heng, Xinxin Cao, Wei Wei, Yongjie Ding, and Jukun Guo. 2025. "Study on the Performance Restoration of Aged Asphalt Binder with Vegetable Oil Rejuvenators: Colloidal Stability, Rheological Properties, and Solubility Parameter Analysis" Coatings 15, no. 8: 917. https://doi.org/10.3390/coatings15080917

APA Style

Yan, H., Cao, X., Wei, W., Ding, Y., & Guo, J. (2025). Study on the Performance Restoration of Aged Asphalt Binder with Vegetable Oil Rejuvenators: Colloidal Stability, Rheological Properties, and Solubility Parameter Analysis. Coatings, 15(8), 917. https://doi.org/10.3390/coatings15080917

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