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Article

Modified Asphalt Prepared by Coating Rubber Powder with Waste Cooking Oil: Performance Evaluation and Mechanism Analysis

by
Jianwei Zhang
1,
Meizhu Chen
1,2,*,
Yuan Yan
2,
Muyan Han
1 and
Yuechao Zhao
1
1
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China
2
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(7), 844; https://doi.org/10.3390/coatings15070844
Submission received: 30 June 2025 / Revised: 12 July 2025 / Accepted: 17 July 2025 / Published: 18 July 2025
(This article belongs to the Section Environmental Aspects in Colloid and Interface Science)
Browse Figures
Versions Notes

Abstract

Waste cooking oil (WCO) plays different roles in modified asphalt and significantly affects the performance of the binder. However, a systematic comparative study is still lacking in the existing research. This study investigates the effects of WCO used as a swelling agent for rubber powder (RP) and as a compatibilizer in rubber powder-modified asphalt (RPMA) on the performance of modified asphalt. Specifically, the microstructure and functional groups of WCO-coated RP were first characterized. Then, RPMAs with different RP dosages were prepared, and the storage stability and rheological properties of RPMAs were thoroughly investigated. Finally, the flue gas emission characteristics of different RPMAs at 30% RP dosing were further analyzed, and the corresponding inhibition mechanisms were proposed. The results showed that the RP coated by WCO was fully solubilized internally, and the WCO formed a uniform and continuous coating film on the RP surface. Comparative analysis revealed that when WCO was used as a swelling agent, the prepared S-RPMA exhibited superior storage stability. At a 30% RP content, the softening point difference value of S-RPMA was only 1.8 °C, and the reduction rate of the segregation index reached 40.91%. Surprisingly, after WCO was used to coat the RP, the average concentrations of VOCs and H2S in S-RPMA30 were reduced to 146.7 mg/m3 and 10.6 ppm, respectively, representing decreases of 20.8% and 22.1% compared with the original RPMA30. These findings demonstrate that using WCO as a swelling agent enhances both the physical stability and environmental performance of RPMA, offering valuable insights for the rational application and optimization of WCO incorporation methods in asphalt modification. It also makes meaningful contributions to the fields of coating science and sustainable materials engineering.

1. Introduction

Asphalt pavements are widely used in the construction of high-grade highways around the world due to their advantages, such as short construction periods, skid resistance, wear resistance and driving comfort [1,2,3,4]. However, asphalt is prone to aging, leading to a decline in pavement performance. Modern traffic, with high traffic volume, heavy loads and vehicle channelization, has further exacerbated the occurrence of pavement diseases [5,6]. Conventional petroleum asphalt is increasingly unable to meet the growing demands of transportation. Currently, polymer modifiers, such as styrene-butadiene-styrene (SBS), are commonly used to enhance the performance of petroleum asphalt [7,8,9]. However, the high cost and complex preparation processes of most polymer-modified asphalts have hindered their large-scale application in road construction. Therefore, it is essential to identify a cost-effective and easily processable modifier to improve the performance of asphalt pavements.
On the other hand, as the automotive industry continues to grow, the demand and replacement rate of automotive tires have increased significantly, which in turn has led to more waste tires [10,11]. According to statistics, the world produces more than 1 billion used tires every year, and more than 50% of them are landfilled or incinerated, with this number showing a rising trend year by year [12]. The main component of tires is rubber; there are crosslinking agents, stabilizers, additives and other complex components of rubber, which, in the natural environment, are not easy to degrade. Many of the chemical components in the combustion of large amounts of hydrogen sulfide and phenol, both of which are highly toxic compounds, cause environmental pollution while also harming the human body [13,14]. Fortunately, waste tire rubber has been widely used in civil engineering applications due to its elasticity, toughness and environmental benefits. According to recent reviews, its applications span asphalt modification, geotechnical structures, concrete enhancement and noise reduction barriers [15,16]. These diversified uses not only contribute to waste management but also offer performance benefits in various engineering systems. In the context of asphalt modification, rubber powder derived from end-of-life tires remains one of the most effective reuse pathways [17,18]. Existing studies have shown that crushing waste tires and further processing them into rubber powder (RP) for incorporation into asphalt can significantly improve the high- and low-temperature performance of asphalt, as well as enhancing its aging resistance [19,20,21]. Moreover, rubber powder modified asphalt (RPMA) enables the recycling of waste tires, offering substantial economic and environmental benefits.
However, RPMA suffers from poor thermal storage stability, high viscosity and a tendency to produce more volatile organic compounds (VOCs) during application due to the structure and composition of RP [22,23]. Due to significant differences between RP and asphalt in terms of density, solubility parameters, molecular polarity and surface functional groups, the interfacial compatibility between the two materials is inherently poor. Also, the surface of the RP particles often lacks active polar functional groups that could interact effectively with the polar components in asphalt. As a result, phase separation and sedimentation tend to occur during the mixing and storage processes, adversely affecting the homogeneity and long-term stability of rubberized asphalt binders [24,25,26]. Moreover, to ensure sufficient strength during tire manufacturing, additives, such as vulcanizing agents and plasticizers, are introduced. While these components enhance the mechanical properties of the tires, they also generate more toxic and hazardous gases at high temperatures, leading to more severe flue gas emissions from the resulting RPMA [27,28].
To address the compatibility issues between RP and asphalt, researchers have mainly undertaken the following approaches. First, by optimizing the preparation process parameters of RPMA, such as increasing the shear temperature and extending the mixing time, they aim to enhance the contact and interaction between RP and asphalt, thereby promoting better swelling of RP within the asphalt matrix [29,30]. Second, targeted pretreatment of RP is conducted in advance, such as microwave activation, chemical modification or solution oxidation, to alter the surface structure of RP and enhance its interfacial bonding with asphalt [31,32,33]. Third, various compatibilizers are added to the asphalt and RP, most of which are materials rich in lightweight components, which mainly act as solubilizers of the RP [34,35].
Regarding the serious issue of flue gas emissions from RPMA, an increasing number of researchers have devoted efforts to developing effective emission-reducing materials. Among these, biochar has received significant attention due to its high specific surface area, porous structure, and rich surface functional groups, which enable it to adsorb VOCs and other harmful gaseous byproducts during the heating process of asphalt. In addition, various nanomaterials, including nanosilica, nanoclays and graphene oxide, have also been investigated for their adsorption capabilities and catalytic degradation functions. These nanomaterials are capable of trapping and decomposing small molecular weight VOCs and sulfur-containing compounds due to their high surface reactivity and catalytic properties [36,37,38].
Waste cooking oil (WCO), another common waste product, was initially used more as a rejuvenating agent for aged asphalt to improve the performance of asphalt, while in recent years it has also been widely used in research on the storage stability of RPMA [39,40]. The application of WCO in RPMA is mainly used as a compatibilizer between RP and asphalt to supplement the lighter components of the asphalt, thus improving the compatibility between RP and asphalt [41,42]. A few studies have also explored the use of WCO as a swelling agent for RP prior to incorporation, where RP is first swollen with WCO and then the WCO-coated RP is added to asphalt to improve its performance.
Despite increasing attention to the reuse of WCO and RP in asphalt modification, existing studies often treat them in isolation and lack a systematic comparison of their combined or differentiated effects. Moreover, the roles of WCO as a swelling agent for RP and as a compatibilizer in RPMA have not been clearly distinguished, especially in terms of their influence on material performance and emission behavior. In fact, due to the different sequences of WCO addition, its role in RPMA varies, which can significantly impact the performance of the final RPMA product. Unfortunately, there is currently a lack of in-depth comparative studies addressing this aspect.
Therefore, this study aims to fill these research gaps by comparing the effects of WCO in two different functional roles. Based on this, the authors’ team used WCO as the swelling agent of RP and the compatibilizer of RPMA, respectively. Firstly, RPMA samples with varying dosages were prepared in the laboratory. Then, the storage stability of different types of RPMA was systematically compared and analyzed, and the effects of the addition sequence of WCO on the rheological properties and flue gas emission of RPMA were further investigated. Finally, the mechanisms by which WCO functions as a swelling agent and as a compatibilizer in RPMA were explored, and the specific flowchart is shown in Figure 1. The related studies are of great scientific value for the efficient and rational application of WCO in RPMA.

2. Materials and Methods

2.1. Raw Materials

In this research, the RP was sourced from shredded waste truck tires and sieved to a particle size of 60 mesh. It was selected as the modifier for the preparation of modified asphalt, and its basic performance indexes are shown in Table 1.
The WCO was collected from a local restaurant and filtered to remove food residues. It was mainly used in the pre-swelling treatment of RP and in the component compatibility of the RPMA prepared at a later stage, and its key physical–chemical properties were measured and listed in Table 2.
In this study, the commonly used 70 petroleum asphalt, as the base asphalt, was measured in accordance with the requirements of the Chinese standard (JTG E20-2011) [43]. Its main technical properties are shown in Table 3.

2.2. The Process of WCO Pre-Treatment RP

In this study, WCO was first used as a swelling agent for RP, and its pretreatment process for RP is shown in Figure 2. The appropriate amount of WCO was first added to the weighed dry RP at room temperature and stirred uniformly using a glass rod. After all the WCO was coated on the surface of the RP, the mixture was put into the oven at 100 °C for 5 h for development, allowing for the RP to be fully solubilized. Finally, the developed pretreated RP was loaded and stored for subsequent preparation of RPMA.
The mass ratio of WCO to RP in this study was 1:2. It was based on preliminary experiments and literature reviews, which indicated that this proportion provided the optimal balance between improving the compatibility of materials and enhancing the performance of modified asphalt [44]. As shown in Figure 2, when the WCO–RP ratio was 1:1, a large amount of oil exuded from the surface of the RP and agglomerates. This indicates that the residual WCO molecules were excessive, which would seriously affect the high-temperature performance of modified asphalt in the later stages. When the WCO–RP ratio was 1:3, the surface of the RP was relatively dry. At this ratio, WCO was unable to fully coat all of the RP, thus failing to achieve the intended pretreatment effect on RP. Only when the WCO–RP ratio was 1:2 was the WCO enough to fully dissolve the RP, and there was no significant oil flooding.

2.3. The Preparation of Different RPMAs

The preparation process of modified asphalt was different when WCO was used as a compatibilizer and solubilizer, respectively, as shown in Figure 3. Figure 3a illustrates the preparation process when WCO was used as a compatibilizer. Firstly, the base asphalt was heated to a fluid state, then a predetermined amount of RP was added to the hot asphalt and stirred at a constant speed for 15 min to ensure uniform mixing. At this point, the pre-weighed WCO was added. The mixture of the three components was then subjected to high-speed shearing at 175 °C and 5000 rpm for 30 min. After shearing, the prepared RPMA was placed in an oven for curing before subsequent testing. When WCO was used as a swelling agent for RP, the preparation of RPMA simply involves adding the WCO-coated RP into the hot asphalt, while keeping all other preparation parameters unchanged, as shown in Figure 3b.
To facilitate analysis, the controlled variable method was adopted in this study, in which the mass ratio of WCO to RP was maintained at 1:2 when WCO was used as either a compatibilizer or a swelling agent. To further investigate the effect of WCO on the compatibility of RPMA, RPMA samples with varying RP contents were prepared. The specific experimental sample codes are listed in Table 4.

2.4. Characterization Method of RP

2.4.1. Scanning Electron Microscope (SEM) Test

The microstructure of RP changes before and after being coated with WCO. Therefore, SEM was used to characterize the morphological differences. During the experiment, a small amount of sample was directly mounted on conductive adhesive, and gold-coated for 45 s using a Quorum SC7620 sputter coater. The imaging was performed at an accelerating voltage of 3 kV.

2.4.2. Fourier Transform Infrared Spectroscopy (FT-IR) Test

To determine whether RP was fully coated by WCO, the FT-IR test was used to characterize the chemical functional group of RP before and after coating. Additionally, FT-IR analysis was employed to further investigate the modification mechanism of WCO on RPMA. During the test, the sample surface was pressed tightly against the crystal surface of the ATR accessory to collect the infrared spectrum. The resolution was set to 4 cm−1, with 32 scans per sample, and the spectral range was 500–4000 cm−1.

2.4.3. Thermogravimetric (TG) Test

To evaluate the thermal stability of WCO-coated RP at high temperatures, TG analysis was conducted on RP before and after WCO coating. The temperature range for the test was from room temperature to 800 °C, with a heating rate of 10 °C/min. The entire test was carried out under a nitrogen atmosphere.

2.5. Characterization Method of RPMA

2.5.1. Storage Stability Test

The storage stability of different samples was evaluated using the thermo-tube method. First, 50 g of the heated sample was poured into an aluminum tube and left standing at 163 °C for 48 h. The tube was then cooled at −20 °C for 4 h. Finally, the aluminum tube was cut into three equal segments: upper, middle and lower. The softening points of the upper and lower segments were measured, and the softening point difference (SPD) was calculated. In addition, the rutting factor (G*/sin δ) of the upper and lower segments of the aluminum tube was further tested. The segregation index (SI) was then calculated using Equation (1). The detailed experimental procedure followed the Chinese standard JTG-E20 2011.
SI = ( G * / sin   δ ) max −   ( G * / sin   δ ) avg ( G * / sin   δ ) avg   ×   100 %  
where (G*/sin δ)max was the G*/sin δ of either the upper or lower part, and (G*/sin δ)avg was the average value of both parts.

2.5.2. Rheological Performance Test

A dynamic shear rheometer (DSR) was used to further investigate the rheological properties of the different samples, specifically measuring the complex modulus and phase angle of each sample. The test was conducted over a temperature range of 30–80 °C at a frequency of 10 rad/s. The detailed experimental procedure followed the Chinese standard JTG-E20 T0628.

2.5.3. Flue Gas Collection and Testing Methods

RPMA releases a large amount of toxic and hazardous fumes at high temperatures. Based on this, the study further employed a portable gas detector to analyze the flue gas emissions of RPMA prepared before and after RP was coated with WCO. The detector collects gas samples using a mercury vacuum suction injection technique and employs multiple built-in sensors to monitor the concentrations of VOCs and H2S released from the samples.
As shown in Figure 4, a quantified sample was first placed into a sealed chamber and preheated at 160 °C for 30 min using electric heating equipment. Then, the temperature was increased to 180 °C, and the stirring speed was set to 50 r/min using an electric stirrer. Subsequently, the gas monitoring device was connected. As the sample begins to release fumes under high-temperature conditions, the device continuously records the real-time emission concentrations of VOCs and H2S released from the asphalt.

3. Results and Discussion

The purpose of modifying asphalt with RP and WCO in this study is threefold. The first is to enhance the high-temperature performance of asphalt by increasing its softening point and improving its resistance to permanent deformation. The second objective is to improve the storage stability of RPMA and mitigate the phase separation issues commonly observed in RPMA. The third objective is to reduce harmful flue gas emissions during asphalt production and paving, in response to growing environmental concerns. The following sections present and analyze the experimental results with respect to these performance goals.

3.1. Performance Characterization of WCO-Coated RP

3.1.1. Morphology

To investigate the effect of WCO coating on the surface structure of RP, the SEM was used to characterize the micro-morphology of untreated RP and WCO-treated RP, and the results are shown in Figure 5. From Figure 5a,b, it can be observed that the surface of the untreated RP shows an obvious rough structure, the edge of the particles is relatively sharp, and the morphology is loose and has irregular protrusions, which may not be conducive to the interfacial bonding between the RP and the asphalt, thus affecting its modification effect. In contrast, the surface of the RP is significantly changed after WCO treatment. At 600× magnification (Figure 5c), the surface of the RP particles is transformed from the original rough edges to a smoother and more rounded morphology, and the particles show obvious aggregation and adhesion between them, indicating that WCO forms a uniform and continuous coating film on the surface. At higher magnification (Figure 5d), the presence of this coating film is more obvious, and the microstructure of RP particles is covered; the surface shows strong densification and flatness, which suggests that the WCO forms a good coating effect on the RP.
In summary, WCO coating not only changes the surface roughness and pore structure of the RP, but also enhances the affinity between the RP and asphalt through the formation of an oil film coating layer, which contributes to the improvement of its dispersion and compatibility. In addition, the cladding layer may play a certain barrier and retardation role for the VOCs released from the RP during heat treatment, thus having a positive significance for the development of environmentally friendly RPMA.

3.1.2. Functional Groups

To verify the coating of WCO on the surface of the RP, FT-IR spectroscopy is conducted to characterize the chemical functional groups of RP, WCO and WCO-coated RP, and the results are shown in Figure 6. It can be seen that the overall spectrum of RP is relatively flat, and no obvious characteristic absorption peaks are observed, indicating that it is a highly cross-linked structure with low IR activity. The WCO exhibits characteristic absorption peaks at 2920 cm−1 and 2850 cm−1, corresponding to the C-H stretching vibrations of -CH2 and -CH3 groups, respectively. A prominent peak at 1745 cm−1 is attributed to the C=O stretching vibration of ester groups. Additionally, absorption peaks observed at 1460 cm−1 and 1165 cm−1 correspond to C-H bending vibrations and C-O-C stretching, respectively; these are all typical characteristic peaks of fatty acid esters.
Notably, the WCO-coated RP spectra retained all the major characteristic peaks in WCO as described above, indicating that the fatty acid ester components in WCO were successfully coated and immobilized on the RP surface to form an organic thin film layer with a certain thickness. This conclusion is consistent with the SEM results, further confirming that WCO exhibits good adhesion and coverage on the surface of the RP. In summary, the ester compounds in WCO not only effectively coat the RP surface but also provide abundant polar functional groups, which are expected to enhance the interfacial compatibility between RP and asphalt.

3.1.3. Thermal Stability

To further verify the thermal stability of WCO coated on the RP surface, thermogravimetric analysis of RP and WCO-coated RP was carried out, and the results are shown in Figure 7. As seen from the TG curves, the total mass loss of the untreated RP is 70.93%, while the WCO-coated RP is 56.12%, and its residual carbon amount also decreases from about 29% to 17%, indicating that the WCO coating affected the thermal decomposition behavior of the RP. Compared to RP, the pyrolysis onset temperature of WCO-coated RP is slightly lower, probably due to the volatilization or initial decomposition of the lighter components of WCO at lower temperatures. The DTG curves show that the RP exhibits a single major weight loss peak at 381.41 °C, whereas the WCO-coated RP displays a primary decomposition peak at 389.65 °C along with a distinct secondary weight loss peak at 445.80 °C. This can be attributed to the decomposition of pyrolysis products in WCO, such as fatty acids or esters, further supporting the successful coating of WCO on the RP surface.

3.2. Storage Stability Analysis of RPMA

3.2.1. Softening Point Difference (SPD)

To evaluate the effect of WCO, when used as a compatibilizer and as a swelling agent, on the storage stability of RPMA, the softening point differences (SPD) of RPMA with different RP dosages were measured, and the results are shown in Figure 8. With the increase in the RP dosage, the SPD of RPMA gradually increased, especially at 30% dosing, where the SPD reaches 2.8 °C, which exceeds the critical value of 2.5 °C, indicating poor storage stability. When WCO is used as a compatibilizer (C-RPMA), there is a decrease in the SPD values at all dosing levels, indicating that WCO helps to enhance the interfacial compatibility between RP and base asphalt. This is due to the fact that both WCO and asphalt are byproducts of petroleum and they have similar compatibility properties. Notably, when WCO as a swelling agent was pre-mixed with RP and then blended into asphalt, the SPD value further decreases, and the decrease is the most significant at 30% blending, from 2.7 °C to 1.8 °C, indicating that this method can effectively inhibit RP settlement and make RP sufficiently swollen in advance, which significantly improves the storage stability of RPMA.

3.2.2. Segregation Index (SI)

Figure 9 shows the SI values of RPMA, C-RPMA and S-RPMA at different RP dosages. It can be seen that the SI value increases with the increase in RP dosing for either RPMA, which confirms that the increase in RP leads to a decrease in the storage stability performance of the modified asphalt. Under the same dosage of RP, the addition of WCO can all reduce the SI value of the modified asphalt. Compared with RPMA, when the RP dosage is 10%, the SI values of C-RPMA10 and S-RPMA10 decrease by 27.59% and 44.83%, respectively. At an RP dosage of 20%, the SI values of C-RPMA20 and S-RPMA20 decrease by 25.00% and 38.89%, respectively. When the RP content reaches 30%, the SI values of C-RPMA30 and S-RPMA30 are reduced by 20.45% and 40.91%, respectively. This is consistent with the findings in Section 3.2.1, which indicate that when WCO is used as a swelling agent for RP, the modified asphalt exhibits better storage stability across all RP content levels. Overall, compared to its role as a compatibilizer, WCO demonstrates greater potential in enhancing the storage stability of RPMA when used as a swelling agent to pre-coat RP, indicating broader application prospects.

3.3. Rheological Properties Analysis of RPMA

3.3.1. Complex Modulus (G*) and Phase Angle (δ)

Figure 10 presents the variation trends of G* and δ for different samples within 30–80 °C, which are used to characterize their high-temperature performance and structural stability [45]. As shown in Figure 10a, the G* of modified asphalt gradually increases with the increase in RP dosing at the same temperature, indicating that the addition of RP effectively enhances the structural stiffness and rutting resistance of asphalt. With the introduction of WCO, the G* values of both S-RPMA and C-RPMA samples show varying degrees of reduction at the same RP content, indicating that their high-temperature performance is somewhat compromised. Among them, the G* value of S-RPMA is slightly lower than that of RPMA but significantly higher than that of C-RPMA. This suggests that when WCO is used to pre-swell RP, most of its components are absorbed or swollen into the RP, resulting in fewer free light components in the system. In contrast, when WCO is used as a compatibilizer, it is mixed with asphalt in advance, limiting the swelling effect on RP. Consequently, more free light components remain in the modified asphalt, leading to the most pronounced decrease in system stiffness.
As further shown in Figure 10b, the δ of all samples increases gradually with rising temperature, indicating a trend of reduced elasticity. Overall, at the same RP content, RPMA exhibits the lowest δ values, demonstrating a stronger ability to retain elasticity under high-temperature conditions. The δ values of S-RPMA fall at an intermediate level, while those of C-RPMA are the highest, indicating poorer elastic recovery at high temperatures and a system dominated by viscous behavior. In summary, although the introduction of WCO inevitably leads to a reduction in the high-temperature performance of the RPMA system, using WCO as a swelling agent for RP offers a better balance between structural stability and interfacial improvement. This approach not only enhances storage stability but also helps maintain favorable high-temperature viscoelastic properties of modified asphalt.

3.3.2. Rutting Factor (G*/sin δ)

The G*/sin δ is often employed to characterize the rutting resistance of modified asphalt at high temperatures [46,47]. The results are shown in Figure 11. As the temperature increases, G*/sin δ decreases for all samples, which indicates that the asphalt begins to change from elastic to viscous. Compared with untreated RPMA, there is a corresponding decrease in G*/sin δ for C-RPMA and S-RPMA at the same RP dosing level, and the decrease is greater for C-RPMA. This indicates that the addition of WCO softens RPMA to some extent, making it more easily deformed at high temperatures. Compared to direct addition to asphalt, the high-temperature rutting resistance of the prepared RPMA can be further improved if WCO is used to solubilize RP in advance, which is also consistent with the conclusion in Section 3.3.1.

3.4. Flue Gas Emission Analysis of RPMA

To investigate the flue gas emission behavior of RPMA under different treatment methods, this study further tested the dynamic release of VOCs and H2S during the heating process for each sample at an RP content of 30%, and the results are presented in Figure 12. Figure 12a,b show the variations in VOCs and H2S emission concentrations over 900 s for the three samples. It is evident that the RPMA30 sample rapidly releases a large amount of fume during the initial heating stage, followed by a plateau phase. Ultimately, the VOCs concentration reaches 201 mg/m3, and the H2S concentration reaches 16 ppm, both representing the highest levels observed. In contrast, the WCO-treated samples exhibit significant emission suppression, with both C-RPMA30 and S-RPMA30 showing markedly reduced release levels.
The average concentrations are shown in Figure 12c,d, further quantifying the emission-reduction effectiveness of WCO treatment. The results indicate that the average VOCs and H2S concentrations for S-RPMA30 are 146.7 mg/m3 and 10.6 ppm, respectively, representing reductions of 20.8% and 22.1% compared to the original RPMA30. Although C-RPMA30 also shows decreased VOCs and H2S concentrations, its suppression effect is less pronounced than that of S-RPMA30. The above results indicate that when WCO is used as a swelling agent for RP, it can effectively seal the surface micropores and boundary cracks of RP, thereby significantly reducing the emission of harmful gases during the heating process while enhancing the storage stability of modified asphalt. In contrast, when WCO is used as a compatibilizer in the asphalt matrix, it primarily disperses within the asphalt phase and fails to provide sufficient encapsulation for RP, resulting in a less pronounced emission reduction effect.

3.5. Modification Mechanism of WCO to RPMA

Figure 13 shows the IR spectra of different samples. It can be seen that RPMA30 exhibits only typical peaks characteristic of RP and asphalt, including the -CH2 and -CH3 telescopic vibration peaks of 2850 cm−1 and 2920 cm−1, the bending vibration peaks at 1454 cm−1 and 1375 cm−1. These features indicate that the material is primarily composed of aliphatic and aromatic carbon skeletons. With the introduction of WCO, the characteristic peaks of WCO appear in both C-RPMA30 and S-RPMA30 samples. Notably, the C=O stretching vibration peak at 1745 cm−1 is significantly intensified in S-RPMA30, indicating that the ester compounds in WCO have effectively penetrated and adsorbed onto the RP surface. Similarly, the presence of the C=O stretching peak at 1745 cm−1 in C-RPMA30 also confirms the incorporation of WCO in the system. It is worth noting that the addition of WCO does not introduce any new characteristic peaks in the modified asphalt. Therefore, whether used as a compatibilizer or a swelling agent, the modification of RPMA by WCO is essentially a physical blending process.

3.6. Flue Gas Suppression Mechanism of WCO to RPMA

To further investigate the environmental role of WCO in RPMA, a fume-suppression model is proposed based on the infrared spectroscopy and fume emission test results. As shown in Figure 14, a large number of microcracks and pores existed on the surface of the untreated RP, which provided abundant escape channels for light components (e.g., aromatic hydrocarbons, hydrogen sulfide, etc.) during the pyrolysis process. After WCO coating, its fatty acid ester molecules formed a layer of polymer-like coating structure through good wettability and adhesion, covering the surface of the RP and penetrating into its microscopic pores. Under high temperatures, the coating layer plays an effective role of physical closure and isolation, blocking the escape path of the internal components and reducing the migration rate of the light volatile components, thus significantly weakening the intensity of VOCs and H2S release from the RPMA at the later stage. Combined with the results of SEM, FT-IR and TG analyses, the mechanism was fully verified and the average concentrations of VOCs and H2S were reduced by more than 20%, respectively, indicating that the RPMAs prepared by WCO-coated RP had less flue gas emissions.

4. Conclusions

This study investigated the dual role of waste cooking oil (WCO) in rubber powder modified asphalt (RPMA) by evaluating its effects when used, respectively, as a swelling agent for rubber powder (RP) and as a compatibilizer in the asphalt matrix. The influence of WCO on the storage stability, rheological properties and fume emissions of RPMA was systematically analyzed, and a fume suppression mechanism for WCO-coated RP was proposed. The main conclusions are as follows:
(1)
After being coated with WCO, the surface of the RP becomes smoother, and noticeable aggregation and adhesion between particles are observed, indicating the formation of a uniform and continuous coating layer on the RP surface. The presence of the WCO coating film on the surface of the RP is confirmed by FT-IR and TG analyses. Moreover, the peak temperature of thermal weight loss for the coating film is observed at 389.65 °C, indicating that no significant thermal degradation occurs during the application of RPMA.
(2)
Compared with a compatibilizer, when WCO is used as a swelling agent for RP, the storage stability of the prepared RPMA is better, and the SPD value of RPMA with 30% RP dosing is only 1.8 °C, and the reduction of SI value reaches 40.91%. Meanwhile, comparing different RPMAs, it is found that the high-temperature performance of modified asphalt with the addition of WCO is all reduced, but S-RPMA shows better high-temperature rutting resistance compared with C-RPMA.
(3)
The addition of WCO can effectively suppress the fume emissions of RPMA to a certain extent. Notably, when WCO coated the RP, the average concentrations of VOCs and H2S in the prepared S-RPMA30 are 146.7 mg/m3 and 10.6 ppm, respectively, which are reduced by 20.8% and 22.1% compared to the original RPMA30. The WCO-coated RP forms a polymer-like encapsulation structure, which serves as an effective physical barrier at elevated temperatures, significantly mitigating the release of harmful emissions from RPMA.
Compared to previous studies focusing solely on the use of RP or WCO in asphalt modification, this research was novel in systematically comparing the two functional roles of WCO—as a swelling agent and as a compatibilizer—in modifying rubberized asphalt. However, the current study was conducted under controlled laboratory conditions, and only one type of base asphalt and WCO source was used. Future studies should explore the long-term field performance, multi-source material variability and lifecycle cost–benefit analysis to validate and extend the findings of this research.

Author Contributions

Conceptualization, J.Z. and M.H.; methodology, J.Z.; formal analysis, Y.Z. and Y.Y.; investigation, J.Z.; writing-original draft preparation, J.Z.; writing-review and editing, M.C.; project administration, M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (No. 52478466, No. 52178437 and No. 52408490). The authors would like to acknowledge their financial support.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. The research flowchart of this study.
Figure 1. The research flowchart of this study.
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Figure 2. WCO’s coating process for RP. (a) Specific coating process; (b) The ratio of WCO to RP.
Figure 2. WCO’s coating process for RP. (a) Specific coating process; (b) The ratio of WCO to RP.
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Figure 3. Preparation of different RPMAs. (a) C-RPMA; (b) S-RPMA.
Figure 3. Preparation of different RPMAs. (a) C-RPMA; (b) S-RPMA.
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Figure 4. Flue gas collection and testing.
Figure 4. Flue gas collection and testing.
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Figure 5. Morphology of RP before and after being coated with WCO: (a) uncoated ×600; (b) uncoated ×3000; (c) coated ×600; (d) coated ×3000.
Figure 5. Morphology of RP before and after being coated with WCO: (a) uncoated ×600; (b) uncoated ×3000; (c) coated ×600; (d) coated ×3000.
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Figure 6. FT-IR spectra of RP, WCO-coated RP and WCO.
Figure 6. FT-IR spectra of RP, WCO-coated RP and WCO.
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Figure 7. TG and DTG curves of RP and WCO-coated RP: (a) TG; (b) DTG.
Figure 7. TG and DTG curves of RP and WCO-coated RP: (a) TG; (b) DTG.
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Figure 8. SPD values of different samples.
Figure 8. SPD values of different samples.
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Figure 9. SI values of different samples.
Figure 9. SI values of different samples.
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Figure 10. G* and δ of all samples: (a) G*; (b) δ.
Figure 10. G* and δ of all samples: (a) G*; (b) δ.
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Figure 11. G*/sin δ of all samples.
Figure 11. G*/sin δ of all samples.
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Figure 12. Flue gas emission of different samples: (a,b) concentration variation of VOCs and H2S; (c,d) average concentration of VOCs and H2S.
Figure 12. Flue gas emission of different samples: (a,b) concentration variation of VOCs and H2S; (c,d) average concentration of VOCs and H2S.
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Figure 13. FT-IR spectra of different samples.
Figure 13. FT-IR spectra of different samples.
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Figure 14. Flue gas suppression mechanism of RPMA by WCO.
Figure 14. Flue gas suppression mechanism of RPMA by WCO.
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Table 1. Basic performance indexes of RP.
Table 1. Basic performance indexes of RP.
Physical propertiesPerformance indicatorsRelative density (%)Water content (%)Metal content (%)Fiber content (%)
Requirements1.10–1.30≤1≤0.05≤0.1
Values1.140.430.0070.07
Chemical propertiesPerformance indicatorsAsh content (%)Acetone extract (%)Carbon Black (%)
Requirements≤8≤2228–38
Values51334
Table 2. Performance indexes of WCO.
Table 2. Performance indexes of WCO.
Performance IndicatorsValues
Density (25 °C, g/cm3)0.91
Viscosity (60 °C, mm2/s)72
Flash point (°C)221
Table 3. Basic performance indicators of base asphalt.
Table 3. Basic performance indicators of base asphalt.
Performance IndicatorsRequirementsValuesTest Methods
Penetration (25 °C, 0.1 mm)60–8061JTG E20-2011 T0604
Softening point (°C)44–5452JTG E20-2011 T0605
Ductility (15 °C, 5 cm/min)>100>100JTG E20-2011 T0606
Dynamic viscosity (60 °C, Pa·s)≥180201JTG E20-2011 T0625
Density (g/cm3)1.017/JTG E20-2011 T0603
Wax content (%)≤2.21.7JTG E20-2011 T0615
Aging test (163 °C, 5 h)Mass change (%)≤±0.8−0.14JTG E20-2011 T0609
Ratio of penetration (25 °C, %)≥5864
Residual ductility (10 °C, cm)≥45.7
Table 4. Composition and label of the sample.
Table 4. Composition and label of the sample.
Sample IDComposition of Sample
RPMA1070# base asphalt + 10%RP
RPMA2070# base asphalt + 20%RP
RPMA3070# base asphalt + 30%RP
C-RPMA10Compatibilizer + 70# base asphalt + 10%RP
C-RPMA20Compatibilizer + 70# base asphalt + 20%RP
C-RPMA30Compatibilizer + 70# base asphalt + 30%RP
S-RPMA10Swelling agent + 70# base asphalt + 10%RP
S-RPMA20Swelling agent + 70# base asphalt + 20%RP
S-RPMA30Swelling agent + 70# base asphalt + 30%RP
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MDPI and ACS Style

Zhang, J.; Chen, M.; Yan, Y.; Han, M.; Zhao, Y. Modified Asphalt Prepared by Coating Rubber Powder with Waste Cooking Oil: Performance Evaluation and Mechanism Analysis. Coatings 2025, 15, 844. https://doi.org/10.3390/coatings15070844

AMA Style

Zhang J, Chen M, Yan Y, Han M, Zhao Y. Modified Asphalt Prepared by Coating Rubber Powder with Waste Cooking Oil: Performance Evaluation and Mechanism Analysis. Coatings. 2025; 15(7):844. https://doi.org/10.3390/coatings15070844

Chicago/Turabian Style

Zhang, Jianwei, Meizhu Chen, Yuan Yan, Muyan Han, and Yuechao Zhao. 2025. "Modified Asphalt Prepared by Coating Rubber Powder with Waste Cooking Oil: Performance Evaluation and Mechanism Analysis" Coatings 15, no. 7: 844. https://doi.org/10.3390/coatings15070844

APA Style

Zhang, J., Chen, M., Yan, Y., Han, M., & Zhao, Y. (2025). Modified Asphalt Prepared by Coating Rubber Powder with Waste Cooking Oil: Performance Evaluation and Mechanism Analysis. Coatings, 15(7), 844. https://doi.org/10.3390/coatings15070844

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