Enhancing the Storage Stability and Rutting Resistance of Modified Asphalt through Surface Functionalization of Waste Tire Rubber Powder

Abstract

Waste tire rubber powder-modified asphalt (RMA) has been widely used in road construction, which was traditionally limited by the poor compatibility of RMA, affecting pavement performance. By synthesizing epoxy soybean oil with amide groups (ESO/TETA) and grafting it onto desulfurized rubber powder (DCR) through microwave irradiation, a surface-functionalized rubber powder (MDCR) was produced successfully. The effects of the physical properties, storage stability, thermal stability, and rheological behavior of the modified asphalt were studied. The results show that the MDCR with a polar surface improved the compatibility and adhesive interactions between the modified crumb rubber and the asphalt. The MDCR content could reach 50%, and the phase separation could meet the requirements of 2.2 °C, which has application conditions in engineering for stable storage. Additionally, the inclusion of MDCR in the asphalt formulations significantly mitigated the temperature sensitivity of the modified asphalt. Importantly, when the MDCR constituted from 20% to 50% of the asphalt, there was a noted reduction in the phase angle at temperatures above 70 °C, indicating a significant improvement in the elastic efficiency. The MDCR also led to substantial enhancements in the resistance of the asphalt to high-temperature and high-stress rutting, addressing the crucial limitations in the consumption ability of waste tire rubber powder and improving the overall performance of RMA in pavement applications.

1. Introduction

With the large-scale construction of asphalt pavement, the demand for petroleum-based asphalt is increasing [1,2]. Considering the heavy traffic load and changing temperatures, asphalt pavement is prone to various problems, such as rutting, fatigue, and thermal cracking [3,4,5]. To address these issues and enhance the engineering properties of asphalt binders, modifications are necessary.

In the realm of green technology, utilizing crumb rubber (CR) [6]-modified asphalt [7] offers a promising solution. This approach not only helps in consuming vast quantities of waste tires but also contributes to reducing carbon emissions during the construction process [8,9]. When integrated into the asphalt, the rubber particles absorb light components, causing them to expand significantly, which in turn improves the asphalt’s high-temperature performance. Furthermore, mixtures with rubber-modified adhesives are reported to reduce traffic noise [10], reduce maintenance costs [11], and enhance wear resistance, skid resistance, and rutting and cracking resistance [12]. However, integrating crumb rubber into asphalt faces significant challenges due to the inert characteristics of CR’s surface, which hinders its effective mixing with the asphalt binder [13]. This causes incompatibility and poor adhesion between the CR and the binder coupled with a compromised storage stability of the asphalt–rubber mixture [14,15].

Research efforts have led to the development of a pre-desulfurization process for CR, resulting in desulfurized rubber powder (DCR), which destroys the CR cross-link and reduces its molecular weight or controls its internal network structure to improve the compatibility of the CR and native asphalt [14,16]. The desulfurization process destroys the grid structure of the rubber powder, reduces the size and number of the rubber particles, and makes the desulfurized rubber powder swell more easily [16]. Yang et al. [17] found that DCR expanded more thoroughly than CR in matrix bitumen. Although the asphalt characteristics are enhanced with the increase in the DCR content, an excess amount of rubber cannot achieve a better performance improvement, and the adhesion between the DCR and binder will be reduced, leading to a series of problems such as an increased segregation degree, high viscosity, and difficult construction problems. Sheng et al. [18] found that the optimal amount of DCR with a satisfactory mixture performance was 20%. This greatly limits the consumption ability of waste tire rubber powder and the further control of the cost of rubber asphalt.

In parallel, the exploration of biomaterials as asphalt modifiers is gaining traction due to their affordability, renewability, and environmental benefits. Bio-oils from sources like waste cooking oil, castor oil, and soybean oil have shown potential as effective asphalt enhancers [19,20,21], which could potentially lead to a better dispersion of the crumb rubber within the asphalt. However, directly adding bio-oil to asphalt may reduce the viscosity of the asphalt mixture, potentially compromising its stability. In line with molecular design principles, crumb rubber, after being activated through grafting with acrylamide and utilized as an asphalt binder modifier, showed a reduction in the difference in softening points. However, the overall effect of this modification was found to be limited [22]. This study introduces an innovative approach involving the surface chemical grafting of an amidated epoxy soybean oil agent (ESO/TETA), aimed at improving asphalt’s performance across different temperatures and enhancing its storage stability. This development represents a step towards sustainable road construction, offering the dual benefits of reducing maintenance costs and extending the lifespan of road surfaces. It is expected to provide a new and environmentally friendly pavement material for the road engineering field, reduce maintenance costs, extend the service life of pavement, and promote the realization of sustainable road construction.

2. Materials and Methods

2.1. Materials

For the experiment, Donghai 90# asphalt was used as the base asphalt binder, which has a softening point of 45 °C, a ductility (5 °C) of 0.5 cm, and a penetration (25 °C) of 85 mm. Desulfurized rubber powder (DCR) was provided by Fenyang Ruifeng Co., Ltd. (Lvliang, China); commercial ESO (purity ≥ 98%) with an epoxy equivalent of 162.5 g/mol was kindly supplied by Adamas-bate (Shanghai, China); and trimethenetetramine (TETA) was purchased from Tianjin Damao Chemical Plant (Tianjin, China).

Table 1. Content of each component of the DCR.

Components	Content (%)
Operation oil	5.88
Rubber hydrocarbon	48.51
Carbon black	10.34
Ash	8.61

details the components of the DCR, as outlined by the GB/T 14837.1-2014 standard [23].

Table 2. Elemental analysis of the DCR.

C (%)	O (%)	Si (%)	S (%)
76.4	17.7	2.1	1.1

presents the results of the elemental analysis of the DCR, providing insights into its chemical composition tested by an element analyzer.

2.2. Preparation of Bio-Based Modifier (ESO/TETA)

A solution was prepared with a mole ratio of ESO to TETA of 1:1. The solution was stirred in a constant-temperature water bath at 50 °C. During this process, the amine group from the TETA reacted with the ester and epoxy groups in the ESO, resulting in amination and the opening of the rings. This reaction proceeded over a period of 4 h, culminating in the formation of the ESO/TETA compound.

2.3. Preparation of Surface-Modified Desulfurized Rubber Powder (MDCR)

After the DCR underwent purification and dehydration with the use of pure ethanol, 200 g of DCR was subsequently immersed in 200 g of the ESO/TETA solution and stirred for a period of 24 h. This step allowed the ESO/TETA compound to physically adhere to the rubber’s surface. Following this absorption process, the coated DCR was subjected to microwave radiation at a power of 1000 W (using a 2450 MHz microwave, manufactured by Panasonic, Osaka, Japan) for a duration of 4 min. To finalize the process, any excess ESO/TETA modifier was washed off with pure ethanol, resulting in the production of a bio-based, surface-functionalized waste rubber powder, referred to as modified devulcanized crumb rubber (MDCR). The detailed method of preparation is presented in Figure 1.

2.4. The Procedure of Asphalt Binder Modification

First, the dried MDCR was mixed with the matrix asphalt at 175 °C, and the stirring rate of the high-speed mixer was set to 1500 rpm for 60 min. After the initial mixing, the mixture was further processed in a high-speed shear emulsifier. Here, it was cut and emulsified at 7000 rpm at a temperature of 180 °C for 20 min. The resulting modified asphalt can be categorized based on the percentage of MDCR incorporated into the asphalt matrix. The designations MDCR20A, MDCR30A, MDCR40A, and MDCR50A correspond to modified asphalts with 20%, 30%, 40%, and 50% MDCR contents, respectively.

2.5. Testing Methods

Fourier transform infrared spectroscopy (FT-IR) can identify the differences between the functional groups and characteristic absorption peaks in ESO/TATA and ESO and TETA and analyze the synthesis mechanism. FT-IR (Invenio-S, Bruker, WI, USA) was used to evaluate the changes in the functional groups during the reaction. The scanning wavenumber range was 4000–400 cm⁻¹, and the cumulative number of scans and scanning rate were 32 and 4 cm⁻¹, respectively.

The nuclear magnetic resonance spectroscopy (NMR) analysis of the samples was conducted using a Bruker 600 MHz high-resolution NMR spectrophotometer (Bruker-electrospin 600 MHz Ultrashield, Bruker, WI, USA). ¹H-NMR spectra were used in the study of the ESO/TATA structure. The samples were prepared by dissolving 10 mg in deuterated CDCl₃, and the spectra were obtained at room temperature.

The primary element types and contents of the DCR and MDCR were examined using EDS (Gemini SEM 360, ZEISS, Jena, Germany) at a voltage with 5 kV.

The physical characteristics of each modified asphalt sample were tested following the JTG E20-2011 test specification for highway engineering asphalt and asphalt mixtures. The softening point was determined using the ball-and-ring method with a fully automatic asphalt softening point tester (SYD-2806G, Changji Geological Instrument Co., Ltd., Shanghai, China). The penetration was determined using an automatic penetration tester (SYD-2801H, Changji Geological Instrument Co., Ltd., Shanghai, China). The ductility and elastic recovery were determined using a ductility tester (SYD-4508G, Changji Geological Instrument Co., Ltd., Shanghai, China).

Segregation testing was performed to measure the degree of separation between the rubber and asphalt binder. At first, the samples were heated to 163 °C to allow them to pour easily into aluminum tubes. Then, they were placed in an upright position in a holder rack. Then, the tube tops were sealed and placed inside an oven at 163 °C for a continuous 48 h. After that, the rack was taken out and kept in a refrigerator for 4 h at −2 °C. After the asphalt solidified, one-third of the upper and lower parts were taken, put into a beaker, and melted in the oven. To evaluate the storage stability of the composite modified asphalt, the difference between the softening point of the top sample and the bottom sample was determined according to ASTM D5892 [24]; if the difference was less than 2.2 °C, the sample was considered stable in terms of storage.

A dynamic shear rheometer (DSR) was used to assess the high-temperature rheological performance of the asphalt binder using a dynamic rheometer (ARES-G2, TA Instruments Company, New Castle, DE, USA). Samples were prepared by placing them between two parallel plates, each with a diameter of 25 mm, and a sample thickness of 1 mm was maintained. DSR temperature sweep tests were performed. The asphalt binder was under the linear viscoelastic range when the tests were conducted. The temperature sweeps were operated with test temperatures changing from 52 °C to 82 °C at a certain frequency of 10 rad/s.

For the assessment of the recovery and rutting characteristics, the multiple stress creep recovery (MSCR) test was conducted using a rheometer (DHR, TA Instruments Company, New Castle, DE, USA). The oscillation rate was set to 10 rad/s, as this typically represents a traffic speed of 90 km/h. For the test, a set of 10 cycles of repetitive loading and unloading were measured by the instrument, with a loading time of 1 s and a relaxation time of 9 s. The measurements were taken for both stresses of 0.1 kPa and 3.2 kPa. The data were then utilized to calculate the percentage recoverable strain in the specimens. In addition, the rutting performance of the binder was assessed using the MSCR test, as outlined in the standards (ASTM-D7405, 2015).

3. Results

3.1. The Successful Synthesis of ESO/TETA

The FT-IR analysis of the ESO, TETA, and their reaction product, ESO/TETA, is presented in Figure 2. The characteristic bands of the ESO, TETA, and ESO/TETA at 2917–2856 cm⁻¹ were attributed to C-H elongation vibrations (CH₂, CH₃). The characteristic absorption peak at 1640 cm⁻¹ corresponded to the -C=N stretching vibration in the TETA, signifying the presence of imine groups. For the ESO, the absorption peaks at 1744 cm⁻¹ (characteristic of ESO’s triglyceride carbonyl group) and 824 cm⁻¹ were indicative of ester and epoxy groups, respectively, consistent with reports in the literature [25,26]. A notable decrease in the intensity of the epoxy characteristic peak at 824 cm⁻¹ in the ESO/TETA sample suggests a reaction between the epoxy group of the ESO and the amine group in the TETA. Additionally, the ESO/TETA spectrum exhibited peaks at 3291 cm⁻¹, 1560 cm⁻¹, and 1658 cm⁻¹, which were attributed to N-H stretching vibrations, N-H bending vibrations, and C=O stretching vibrations, respectively. These peaks are characteristic of secondary amide groups, indicating that the ester groups in the ESO underwent amination with the amine groups in the TETA.

To further verify whether the amine group and the epoxy group and ester group participated in the reaction, we tested the ¹H-NMR spectroscopy of the ESO, TETA, and ESO/TETA, as shown in Figure 3. The ¹H-NMR spectra facilitated the assignment of the synthetic ESO/TETA constituents based on chemical shifts, peak morphologies, and established coupling constants. By comparing the ESO, TETA, and ESO/TETA, the chemical shift range of 6.58–6.21 ppm was attributed to protons attached to nitrogen atoms, indicating the presence of amine functionalities. The chemical shift range of 2.69–2.61 ppm corresponded to protons on the hydroxyl groups formed via the ring-opening reaction of the epoxy groups, underscoring the occurrence of this reaction. Additionally, the absorption peaks within the 2.13–2.03 ppm range were associated with protons on the carbon atoms adjacent to the secondary amine group in the TETA, further confirming the modification of the amine group. These spectral findings substantiate the successful synthesis of the desired bio-oil-based modifier, demonstrating the reaction between the epoxy group of the ESO and the amine group of the TETA, as well as the involvement of ester groups in the process.

3.2. Surface Modification for DCR

The DCR was treated using the ESO/TETA bio-oil-based modifier, where the light molecular fractions of the modifier not only adhered to the rubber surface via physical adsorption but also penetrated the voids within the rubber clusters. Upon microwave irradiation, the cross-linked network structure within the DCR was altered, converting it into free polymer chains [27,28,29]. This process also facilitated the decomposition of chemical groups in the ESO/TETA, leading to the formation of amide free radicals that covalently bonded with free radicals on the rubber. As shown in Figure 4, the FTIR spectra of the DCR, MDCR, and ESO/TETA show that the infrared spectra of the DCR and MDCR had strong absorption peaks at wavelengths of 2850 cm⁻¹–3000 cm⁻¹, which indicates that the rubber powder contained “-CH₃, =CH₂ and ≡CH”. The spectrum for the DCR featured an absorption peak at 1537 cm⁻¹, attributed to the stretching vibration of the C=C skeleton, which indicates the presence of unsaturated bonds within the rubber powder. Notably, this peak was absent in the MDCR spectrum, suggesting the breakage of double bonds during the treatment. Conversely, new absorption peaks emerged at 1150 cm⁻¹ and 1095 cm⁻¹ in the MDCR spectrum, which were assigned to the formation of C-N bonds. This implies that the amide groups in the ESO/TETA reacted with the carbon free radicals of the DCR under the microwave irradiation to form C-N linkages, and at 1658 cm⁻¹, a C=O expansion and contraction vibration peak appeared, and the peak at 1738 cm⁻¹ was attributed to the ester characteristic peak in epoxy soybean oil, indicating a successful grafting of the ESO/TETA modifier onto the DCR surface via microwave irradiation. The introduction of these functional groups onto the rubber surface is presumed to increase its surface polarity. Such chemical modifications are expected to improve the compatibility and adhesive interactions between the modified crumb rubber and the asphalt, which is critical to the properties of modified asphalt.

The EDS analysis of the DCR and MDCR is illustrated in Figure 5. The relative contents of C, O, N, S, and Si on the DCR and MDCR surface are shown in Figure 6. It is clear that the new element N appeared in the MDCR compared to the EDS of the DCR, and the relative content of O decreased by 3.4%. The results showed that during the microwave activation process, the oxidation reaction did not occur on the DCR surface, but the amide group reacted with the carbon radicals of the DCR to form the C-N group, which was consistent with the above FT-IR results.

3.3. Road Performance of the MDCR-Modified Asphalt

Figure 7 presents the results for the penetration, softening point, low-temperature ductility, and elastic recovery of the modified asphalt. It can be observed that the incorporation of DCR significantly enhanced the ductility value, softening point, and elastic recovery of the composite modified asphalt, while the penetration value exhibited a notable decrease. This phenomenon can be attributed to the inherent characteristics of DCR as a cross-linked elastic material. Upon its addition to the asphalt, the DCR absorbed the lighter fractions, leading to their swelling and a consequent reduction in the proportion of light components within the asphalt. Simultaneously, there was an increase in the proportion of resin components, which resulted in decreased penetration values and improvements in the low-temperature flexibility, elastic recovery, and softening point [30]. The introduction of MDCR further enhanced the road performance of the modified asphalt. It can also be observed that an increase in the MDCR content enhanced the ductility, softening point, and elastic recovery of the asphalt composite while reducing its penetration. This trend is consistent with previous results reported by Ma [31]. Beyond the swelling effect, this improvement was largely due to the interaction between the amine and amide groups of the biological modifier and the carbon free radicals in the DCR, leading to the formation of polar groups. Consequently, the surface polarity of the rubber powder increased, promoting enhanced compatibility with the matrix asphalt. After modification, the MDCR content could comprise up to 50% of the mixture. This resulted in an asphalt with a penetration value of 39 mm at 25 °C, a softening point of 72.7 °C, a ductility of 10.8 cm at 5 °C, and an elastic recovery rate of 89%.

3.4. Storage Stability of the MDCR-Modified Asphalts

The stability of modified asphalt during storage is crucial for its practical application in engineering projects. To assess this, the compatibility of the modifiers with asphalt was evaluated using a segregation test, which measured the softening point difference (SPD) over time. The SPD of the modified asphalt decreased as storage time increased, indicating a change in compatibility. Figure 8 gives the SPDs of the modified asphalts with different DCR and MDCR contents. Specifically, with a 20% DCR content, the modified asphalt exhibited an SPD of 11.5 °C. This relatively high SPD suggests a lower compatibility of the DCR within the asphalt matrix, indicating potential issues with stability and homogeneity over time. When the same proportion of MDCR-modified asphalt was incorporated, the segregation of the MDCR within the asphalt was significantly reduced by 54.8%, with an SPD of 5.2 °C. This reduction is considerably greater than the 35% decrease in SPD observed in an asphalt binder containing acrylamide-grafted rubber, as determined through the cigar tube test sample, in comparison to a control asphalt with conventional rubber [22]. Furthermore, when the MDCR content reached more than 40%, the results of the phase separation test could meet the requirements of 2.2 °C outlined in ASTM D5892. Therefore, this achievement marks a significant improvement in the stability of the MDCR-modified asphalt, qualifying it as suitable for engineering applications.

3.5. Dynamic Rheological Properties of the MDCR-Modified Asphalts

In the investigation of the complex shear modulus (G*) behavior across the various asphalt formulations in response to temperature variations, as delineated in Figure 9a, a universal trend was observed, where G* diminished with an increase in temperature across all the examined asphalt types. This trend signifies a reduction in the materials’ resistance to deformation as temperatures escalate [32,33]. Compared with the DCR20A modified asphalt, the influence of temperature on the modulus of the MDCR20A modified asphalt notably diminished, indicating that MDCR can significantly reduce the temperature sensitivity of modified asphalt. This may be due to the addition of the MDCR, which enhanced the interfacial effect with the asphalt and increased the chain entanglement with light and colloidal components in the asphalt. Furthermore, with increasing concentrations of MDCR, this molecular entanglement intensified, successively bolstering the asphalt’s deformation resistance. At the molecular level, the entanglement involves cross-links and interactions that can significantly restrict the mobility of polymer chains, thereby increasing the material’s elasticity, effectively increasing its deformation resistance.

It was found that the d value of the matrix asphalt and DCR20A increased with the increase in temperature, indicating that the viscosity of the asphalt increased and the deformation recovery ability decreased. This phenomenon can be attributed to the breakdown of sulfur-carbon (S-C) and carbon-carbon (C-C) bonds within the crosslinked macromolecules, a process accelerated by the thermal energy accumulated during stirring and shearing. Such degradation reduced the mass of the rubber macromolecules, leading to diminished elasticity and recovery properties [34]. However, when the proportion of MDCR in the asphalt was between 20% and 50%, the parameter d showed a decrease above 70 °C, signifying a notable improvement in the elastic efficiency. It can be seen that the viscoelastic properties of the MDCR-modified asphalt have very complex influences, and more elastic components appeared in the MDCR-modified asphalt, which is further speculated to be related to a certain degree of chain entanglement in the asphalt colloid system. It is noted that the high-temperature performance of asphalt binders was improved after the addition of the MDCR.

Figure 10 gives the cumulative strain curve of the modified asphalt sample at 58 °C. The cumulative strain of the asphalt substrate increased obviously with the extension of the loading time. The matrix asphalt is mainly viscous, and high temperatures or overload lead to more serious permanent deformation of the matrix asphalt [35]. The cumulative strain of the DCR-modified asphalt was smaller than that of the matrix asphalt at each strain level, indicating that the DCR had a complementary effect on the elasticity of the asphalt and improved the elastic recovery ability. At a low strain level, the cumulative strain change rate was small, but at a high strain level, the cumulative strain increased significantly. This shows that the elastic recovery ability of the DCR-modified asphalt under overload conditions was insufficient. In contrast, the MDCR-modified asphalt had excellent elastic resilience. MDCR can form a more stable elastic network structure in asphalt, such that the asphalt is still in a viscoelastic state under high-temperature and high-strain conditions, which has a positive effect on improving the deformation recovery ability of modified asphalt.

The percentage recoverable strain (R) and non-recoverable creep compliance values (J_nr) at 58 °C and 76 °C are shown in Figure 11. At 58 °C, the matrix asphalt exhibited characteristics akin to a viscous flow state, with recovery rates nearing zero across the various stress levels. The incorporation of the DCR yielded an improvement in R at lower stress levels; however, this enhancement diminished with increasing temperature and stress. Specifically, at a stress of 3.2 kPa, the effect of the DCR on improving the recovery rate was very little, and at 76 °C, the R of the DCR-modified asphalt approached zero. This suggests that DCR’s capacity to mitigate permanent deformation due to viscous flow is limited to conditions representative of light traffic, and it does not significantly enhance the high-temperature deformation resistance in modified asphalt. Conversely, the use of MDCR in the asphalt modification led to a notable improvement in the recovery rates, indicating a substantial reduction in the permanent deformation caused by viscous flow and a decreased risk of rutting. This improvement was attributed to MDCR’s ability to form a more stable elastic network within the asphalt matrix, thereby significantly enhancing the rutting resistance.

The J_nr value serves as an indicator of the residual strain post recovery, with lower values signifying reduced residual strains and, consequently, smaller rut depths. The observed J_nr values at a lower stress level (J_nr 0.1) followed a similar trend to those at a higher stress level (J_nr 3.2) across the different temperatures, ranking from highest to lowest as follows: matrix asphalt, DCR20A, MDCR20A, MDCR30A, MDCR50A, and MDCR40A. The J_nr values of the rubberized asphalt were negatively correlated with the crumb rubber amount, suggesting that an appropriate amount would have a positive impact on the high-temperature performance. Liu et al. [36] also reported that the Shenoy non-recovered compliance (S_nr) values, as an alternative to J_nr, were dramatically decreased when the amount varied from 25% to 50%. It was also noted that at 58 °C, the J_nr 3.2 values for both the matrix and modified asphalt samples were greater than their J_nr 1.0 values. Compared to DCR, the MDCR modifications under identical dosages significantly reduced the likelihood of rutting at high temperatures, with increased dosages of MDCR correlating with more pronounced improvements in the high-temperature rutting resistance. These findings suggest that MDCR interacts with the asphalt matrix to form additional elastic components and a more stable elastic network. This is possibly due to a degree of molecular chain entanglement facilitated by covalent reactions between surface-modified colloids and the asphalt. Such interactions are speculated to substantially bolster the modified asphalt’s rutting resistance under conditions of high stress and at high temperature ranges.

4. Conclusions

The bio-oil-based modifier ESO/TETA was successful synthesized, and a surface-functionalized desulfurized crumb rubber (MDCR) was prepared with the ESO/TETA by microwave irradiation. Incorporating MDCR into asphalt led to notable enhancements in the asphalt’s physical and rheological properties as well as its stability, significantly affecting its performance on roads. The increased surface polarity of the MDCR improved its compatibility with the asphalt matrix, further contributing to the asphalt’s enhanced storage stability. Specifically, increasing the amount of MDCR in the asphalt improved its ductility, raised its softening point, and boosted its elastic recovery, while simultaneously decreasing its penetration. Moreover, the presence of the MDCR in the asphalt formulations markedly reduced the material’s sensitivity to temperature variations and significantly enhanced its viscoelastic properties, including better elastic recovery and increased resistance to rutting. These improvements can be attributed to the improved interfacial effects and molecular entanglement brought by the inclusion of the MDCR. Consequently, MDCR emerges as a promising modifier for asphalt, being particularly beneficial under conditions of simulating heavy traffic and high temperatures.