Study on aging mechanism of SBS/SBR compound-modified asphalt based on molecular dynamics

Hu Shao; Jianya Tang; Wenzheng He; Shuang Huang; Tengjiang Yu

doi:10.1515/rams-2023-0106

Article Open Access

Study on aging mechanism of SBS/SBR compound-modified asphalt based on molecular dynamics

Hu Shao , Jianya Tang , Wenzheng He , Shuang Huang and Tengjiang Yu

Published/Copyright: August 14, 2023

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From the journal REVIEWS ON ADVANCED MATERIALS SCIENCE Volume 62 Issue 1

Abstract

Component ratio change is considered to be the main reason leading to the deterioration of asphalt properties, but there are few studies on the aging mechanism from the perspective of modifier molecules. To reveal the aging mechanism of styrene–butadiene–styrene block copolymer (SBS)/styrene butadiene rubber (SBR) compound-modified asphalt, the micro mechanism in the aging process was studied by combining molecular dynamics (MD) and Fourier transform infrared spectroscopy (FTIR). First, MD was used to establish the micro models of SBS/SBR compound-modified asphalt at different aging stages (non-aging, short-term aging, and long-term aging) and to verify its rationality. Second, the micro characteristics of the SBS/SBR compound-modified asphalt micro model, such as solubility parameters, diffusion coefficient, interface interaction energy, and radial distribution function, were analyzed by calculation. Finally, the FTIR results proved the rationality of the simulation and explained the aging mechanism of SBS/SBR compound-modified asphalt. The results show that the cohesiveness density and solubility parameters of SBS/SBR compound-modified asphalt increase, the diffusion coefficient decreases, and the molecular interface stability increases during the aging process. And, the carbonyl index, sulfoxide index, and aromatic ring index increased in different degrees after aging. The study explains the aging mechanism of SBS/SBR compound-modified asphalt from the perspective of modifier molecules and provides a theoretical basis for the research of asphalt anti-aging.

Keywords: asphalt; SBS/SBR; molecular dynamics; aging mechanism; FTIR

1 Introduction

SBS/SBR compound-modified asphalt is a modified asphalt with a dynamic viscosity greater than 50 kPa·s at 60℃, which is formed by adding styrene–butadiene–styrene block copolymer (SBS) and styrene butadiene rubber (SBR) into the virgin asphalt [1,2]. The higher dynamic viscosity is beneficial to improve the adhesion relationship between asphalt and mineral aggregate and ensure the mixture pavement performance. Although SBS/SBR compound-modified asphalt has better fatigue resistance, high temperature performance, and aging resistance, in the actual service process, SBS/SBR compound-modified asphalt will still face the impact of aging [3]. At the same time, the lack of relevant micro explanation on the aging mechanism of SBS/SBR composite-modified asphalt restricts the further improvement of its durability [4]. Therefore, the aging mechanism of SBS/SBR compound-modified asphalt needs to be explored and deeply understood.

With the increasing demand for asphalt mixture pavement performance, the modification of asphalt material properties has become a focus of attention for researchers [5]. Among them, after SBS modifier is added to virgin asphalt, the asphalt performance is improved significantly [6]. For example, Chen et al. studied the relationship between modifier and asphalt in SBS-modified asphalt, summarized the factors affecting the performance of SBS-modified asphalt, and improved the preparation method [7]; Wang et al. used a dynamic shear rheometer to evaluate the low temperature performance of SBS-modified asphalt and established the correlation between asphalt critical temperature and T _g [8]; Geng et al. studied the properties of SBS-modified asphalt under wet and dry cycles, revealing the physical properties and chemical composition of asphalt during bonding [9]. Therefore, SBS-modified asphalt can better adapt to the area with large temperature difference and significantly improve the strain capacity of asphalt pavement. This is because SBS modifier molecules have a chain-like spatial structure, which greatly improves the mechanical properties of asphalt under the action of asphalt components, just like adding reinforcement to concrete [10]. However, although the addition of SBS modifier can improve the anti-aging property of asphalt mixture relatively, the performance improvement is not obvious in the face of low temperature environment.

The experimental results show that the SBS modifier and SBR modifier added at the same time have better fatigue resistance, high temperature resistance, and aging resistance than the asphalt material added alone [11]. For example, Zhu et al. studied the ratio interval of SBS modifier and SBR modifier, thereby obtaining SBS/SBR compound-modified asphalt with better performance and improving its anti-aging performance [12]; Chen et al. explored the aging behavior of SBS/SBR compound-modified asphalt under extreme environment and the influence of changes in mechanical properties and rheological properties on the asphalt and mixture performance under long-term hot oxygen conditions [13]; Liu conducted a comparative study of aging under thin film oven test and ultraviolet aging conditions to explore the anti-aging properties of modified asphalt [14]. The modification mechanism of SBR modifier is that SBR molecules combine with light components in the matrix asphalt and embed into the network structure formed by SBS molecules to form a more stable spatial structure [15]. The essence of performance improvement is that the interaction between modifier molecules and asphalt component molecules in SBS/SBR compound-modified asphalt is enhanced [16]. But under the influence of complex environment, SBS/SBR compound-modified asphalt is still facing the threat of aging, which leads to the deterioration of pavement performance for asphalt mixture.

In conclusion, SBS/SBR compound-modified asphalt can obviously improve the asphalt material properties, especially in durability. A large number of scholars have also conducted a lot of exploration and experiments on its aging characteristics and obtained a wealth of research results. However, there are few studies on the aging micro mechanism of SBS/SBR compound-modified asphalt from the micro perspective. Therefore, in the study, molecular dynamics (MD) and Fourier transform infrared spectroscopy (FTIR) were used to analyze SBS/SBR compound-modified asphalt with different aging so as to reveal its micro characteristics in the aging process. The aging micro mechanism of SBS/SBR compound-modified asphalt performance degradation was revealed from the perspective of molecular changes of modifier.

2 Materials and models

2.1 SBS/SBR compound-modified asphalt

The asphalt used in the study is AH-90# virgin asphalt, and the test results of basic technical indexes of virgin asphalt are shown in Table 1.

Table 1

Basic technical indexes of virgin asphalt

Technical indexes	Technical requirements	Test result
Penetration (25℃, 5 s, 100 g) (0.1 mm)	80–100	94
Softening point (℃)	≥43	44.6
60℃ dynamic viscosity (135℃) (Pa·s)	≥160	197
Penetration index	−1.5 to +1.0	−0.76
10℃ ductility (cm)	≥45	91.2
15℃ ductility (cm)	≥100	>100
Density (25℃) (g·cm⁻³)	0.9–1.1	−

The SBS modifier’s professional name is styrene–butadiene–styrene block copolymer, which belongs to styrene thermoplastic elastomer, and is a common asphalt modifier. The SBR modifier is obtained by polymerization of butadiene and propylene, and the incorporation of SBR into asphalt can improve the low temperature performance of asphalt [17]. The modifiers are shown in Figure 1, and the basic technical indexes of modifiers are shown in Table 2.

Figure 1

Modifiers and SBS/SBR compound-modified asphalt: (a) SBS modifier; (b) SBR modifier; and (c) SBS/SBR compound-modified asphalt.

Table 2

Technical indexes of SBS modifier and SBR modifier

Modifier	Tensile strength (MPa)	Elongation (%)	Hardness JIS	S/B	Specific gravity
SBS	32	800	75	31/69	0.93
SBR	32	320	65	−	0.94

Based on the preliminary research results of the project, the compound-modified asphalt of 4% SBS + 3% SBR has a good compound effect, so the study adopted 4% SBS + 3% SBR as the optimal mixing amount. The specific preparation of SBS/SBR compound-modified asphalt refers to relevant research results [18]. SBS modifier was added to virgin asphalt under the condition of 170℃, SBR modifier was added after high-speed stirring, and the compound-modified asphalt was formed after high-speed shear. SBS/SBR compound-modified asphalt is shown in Figure 1.

In order to study the aging micro mechanism, the asphalt was aged to obtain SBS/SBR composite-modified asphalt with different aging degrees. In the aging test, asphalt thin-film oven test method (GB/T 5304) was used [19]. Firstly, SBS/SBR compound-modified asphalt was placed in the tray spread evenly. Secondly, it is placed in the electric control oven 163℃ for heating aging, aging process to open the automatic air blast option. Finally, after 5 h, part of the asphalt was taken out and stored, and the remaining asphalt was heated for another 15 h. After the aging test, the SBS/SBR compound-modified asphalt was aged, and the short-term aged SBS/SBR compound-modified asphalt (aged for 5 h) and long-term aged SBS/SBR compound-modified asphalt (aged for 20 h) were obtained.

2.2 Micro models of SBS/SBR compound-modified asphalt

Asphalt is a kind of mixed material mainly composed of hydrocarbons, and it is obviously impractical to establish exact chemical formulas for each compound. From the perspective of chemical components, asphalt components can be mainly divided into four types, namely asphaltene, resin, saturate, and aromatic [20]. Traditional aging mechanism believes that asphalt aging is also realized through the mutual transformation of components, that is, the macro properties of asphalt (penetration, ductility, viscosity, etc.) will change with the change of component proportion. After summarizing the research data of asphalt component molecules, the four-component chemical formula used in the research is shown in Figure 2 [21,22].

Figure 2

The four components of asphalt: (a) asphaltene C₅₄H₅₉NOS; (b) resin C₁₀₀ H₁₀₆; (c) saturate C₂₂ H₄₆; and (d) aromatic C₁₂H₁₂.

According to the chemical composition of the modifier, both SBS molecule and SBR molecule are polymers composed of styrene and 1,3-butadiene, among which SBS is block copolymer and SBR is random copolymer. Using the research results of Zhu et al. as reference, the SBS molecular model was formed by polymerization of 2 styrene molecules, 18 butadiene molecules, and 2 styrene molecules, while the SBR molecular model was formed by polymerization of 9 trans-1,4-polybutadiene, 3 cis-1,4-polybutadiene, 3 1,2-polybutadiene, and 2 polystyrene polymer molecules, as shown in Figure 3 [23]. Meanwhile, referring to the research results of Xiang et al., the middle of SBS and SBR molecules was broken, and aldehyde and carboxyl groups were added at the fracture, respectively, to form short-term aged SBS and SBR molecules and long-term aged SBS and SBR molecules, as shown in Figure 3 [24].

Figure 3

SBS modifier molecule and SBR modifier molecule: (a) SBS molecule C₁₀₄H₁₄₂; (b) SBR molecule C₇₆H₁₀₈; (c) short-term aged SBS molecule C₅₂H₇₀ O; (d) long-term aged SBS molecule C₅₂H₇₀O₂; (e) short-term aged SBR molecule C₃₈H₅₄O; and (f) long-term aged SBR molecule C₃₈H₅₄O_2.

It was necessary to determine the number of relevant molecules before establishing the micro models of SBS/SBR compound-modified asphalt. The number of asphalt four component molecules was calculated by the method of mass percentage and molar mass conversion. Among them, the mass percentage of asphalt four components was obtained through four components test, and its main process was referred to Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering [25]. The mass percentage of four components for SBS/SBR compound-modified asphalt unaged, short-term aged and long-term aged, and the molecular number of components after conversion are shown in Table 3. Since the component molecules can only be integers, the number of molecules is treated as integers in the calculation process.

Table 3

The number of component molecules in asphalt microscopic model

Components	Asphalt	Asphaltene	Resin	Saturate	Aromatic	SBS	SBR
Mass percent	Unaged (%)	5.13	16.21	28.29	43.37	4	3
	Short-term aged (%)	14.14	24.06	14.71	40.09	4	3
	Long-term aged (%)	23.57	26.51	10.65	32.27	4	3
Number of molecules	Unaged	1	2	18	55	1	1
	Short-term aged	3	3	9	51	1	1
	Long-term aged	6	4	6	41	1	1

After the molecular formula and dosage of the asphalt four components and modifiers were determined, the SBS/SBR compound-modified asphalt micro models were established. The specific steps are as follows:

Asphalt four-component molecular models, SBS molecular model, and SBR molecular model were established according to the chemical formula, and preliminary geometric optimization of the molecular model was carried out;
Micro models were assembled according to the number of molecules in Table 3, and condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field was used to obtain the preliminary SBS/SBR compound-modified asphalt micro models;
Annealing operation was carried out on the asphalt micro models, and the annealing was cyclic for 10 times within the range of 300–500 K, and geometric optimization was required for each completion, and more stable asphalt micro models were obtained;
Dynamic simulation was carried out on the asphalt micro models, and the temperature was 298.0 K, the pressure was 1.0 × 10^–4 GPa, the cutoff distance was 12.5 Å, the ensemble was NPT (constant-pressure, constant-temperature), the force field was COMPASS, and the running time was 300 ps.

After the simulation, the density of the asphalt micro models was stable and tends to be stable in a certain range. The microscopic model of asphalt is shown in Figure 4. The average density of the latter 250–300 ps was taken as the model density; the densities of the unaged, short-term aged, and long-term aged SBS/SBR compound-modified asphalt micro models are 0.940, 0.995, and 1.026 g·cm⁻³, respectively. The density of asphalt micro models is very close to that of real asphalt, which verifies its rationality [26,27].

Figure 4

Establishment of SBS/SBR compound-modified asphalt micro models.

3 Micro mechanism of aging for SBS/SBR compound-modified asphalt

3.1 Characteristic analysis of micro models

3.1.1 Compatibility analysis

Solubility parameter is a physical constant that measures the compatibility degree of liquid materials: the smaller the solubility, the better the compatibility. According to Hildebrand’s studies, the material solubility parameter has a good relationship with compatibility [28]. Therefore, the solubility parameter can be used as the compatibility evaluation index of SBS/SBR compound-modified asphalt. The calculation formula of solubility parameter is shown in the following formula:

(1) δ = E coh V ,

where δ is the solubility parameter; E _coh is the material cohesive energy; and V is the true molecular volume.

A good solubility parameter can explain the compatibility degree between different materials, thus providing a guarantee for its stable performance. The solubility parameters of SBS/SBR compound-modified asphalt micro models were calculated, and the calculation results are shown in Table 4.

Table 4

Solubility parameters of SBS/SBR compound-modified asphalt micro models

SBS/SBR compound-modified asphalt	Cohesive energy density (J·m⁻³)	Solubility parameter (J·cm⁻³)^0.5	Density (g·cm⁻³)
Unaged	3.429 × 10⁸	18.517	0.940
Short-term aged	3.659 × 10⁸	19.128	0.995
Long-term aged	3.729 × 10⁸	19.310	1.026

It can be seen from Table 4 that the aging has a certain impact on the compatibility of SBS/SBR compound-modified asphalt micro models. Microscopically, the solubility parameters, cohesion energy density, and density of SBS/SBR compound-modified asphalt increase gradually with the aging process. The increase of solubility parameters indicates that aging reduces the compatibility of asphalt micro models, and the increase of cohesion energy density indicates that the interaction between asphalt components has been enhanced, and stronger energy is needed to separate the components, which has also been proved by the increase of density. Combined with the micro models, it can be seen that the SBS/SBR chain structure fracture leads to the decrease of surface tension in the aging process of SBS/SBR compound-modified asphalt, and the interfacial relationship between modifier molecules and components molecules tends to be stable.

3.1.2 Diffusion property analysis

The essence of material diffusion property is the constant migration of its constituent particles in space. Due to the large number of particles in the micro model, it is impossible to accurately analyze the diffusion path of each particle. Therefore, the parameters of statistical particles were usually used for overall analysis to calculate the continuous diffusion behavior of particles in the system [29]. The diffusion property can be characterized by the diffusion coefficient, which can be calculated by using the slope of the mean square displacement curve, as shown in the following formula:

(2) D = 1 6 lim t → ∞ d d t ∑ i = 1 N 〈 ∣ r i ( t ) − r i ( 0 ) ∣ 2 〉 ,

where r _i(t) is the displacement of molecule i at time t; < > is the average of all molecules in the system; r _i(0) is the displacement of molecules at the initial time i; N is the number of molecules; D is the diffusion coefficient of molecules; and dt is the derivative of the mean square displacement curve with respect to time t.

After calculating mean square displacement of the SBS/SBR compound-modified asphalt micro models, the relationship between mean square displacement and time is shown in Figure 5, and the fitting results of mean square displacement slope are shown in Table 5.

Figure 5

Relationship between mean square displacement and time.

Table 5

Fitting results of mean square displacement slope

SBS/SBR compound-modified asphalt	Fitting results	R²	Slope of curve	Diffusion coefficient (Å²/ps)
Unaged	y = 0.1136x + 8.2105	R² = 0.9957	0.1136	0.0189
Short-term aged	y = 0.0893x + 6.3072	R² = 0.9866	0.0893	0.0148
Long-term aged	y = 0.0629x + 4.5723	R² = 0.9984	0.0629	0.0104

It can be seen from Table 5 that the diffusion coefficient of SBS/SBR-modified asphalt micro models decreases with aging time. This indicates that aging effect reduces the diffusion performance of asphalt materials, which is not conducive to the adhesion behavior of asphalt on the aggregate surface. Based on the fracture and oxidation behavior of SBS/SBR molecular chains, it can be inferred that the relationship between SBS/SBR molecules and asphalt component molecules becomes stable gradually. In addition, the decrease of diffusion property leads to the weakening of asphalt ductility, which is not conducive to the durability of the mixture properties.

3.2 Analysis of interaction between component molecules and modifier molecules

3.2.1 Analysis of interaction performance

In order to reasonably describe the relationship between SBS/SBR molecules and asphalt component molecules, the interfacial interaction energy (interaction energy) was used as the index. The interaction energy is composed of van der Waals force and Coulomb force, and its negative value indicates that the two components attract each other [30]. The greater the absolute value of the interaction energy between different molecules, the greater the force between them.

The SBS/SBR molecules and asphalt component molecules in the SBS/SBR compound-modified asphalt micro models were calculated, and the calculation formula is shown in formula (3). The calculation results of the interaction energy between SBS/SBR molecules and asphalt component molecules are shown in Table 6.

(3) γ interaction = E SBS/SBR + E asphalt − E total 2 A ,

where E _SBS/SBR is the energy removal of asphalt component molecules; E _asphalt is energy removal of SBS/SBR molecules; E _total is the total energy of SBS/SBR compound-modified asphalt micro models; and A is the interface area.

Table 6

Interaction energy between the asphalt component molecules and the modifier molecules

SBS/SBR compound-modified asphalt	E _total (kcal·mol⁻¹)	E _asphalt (kcal·mol⁻¹)	E _SBS/SBR (kcal·mol⁻¹)	A (Å²)	Interfacial energy (kcal·mol⁻¹)·Å
Unaged	−1523.78	−1401.55	338.5092	1076.3439	0.2140
Short-term aged	−1553.20	−1331.25	196.3730	1021.8962	0.2046
Long-term aged	−1654.58	−1343.06	68.94136	1036.2600	0.1835

As can be seen from Table 6, the total energy value of SBS/SBR compound-modified asphalt micro models decreases with the aging process, which indicates that the micro models tend to be stable on the whole. The energy value of modifier molecules decreased with the aging process, which was caused by the fracture of modifier molecular chains under the influence of aging and the formation of aldehyde or carboxyl groups at the fracture site. The oxidation at the fracture site changed the relationship between modifier molecules and asphalt component molecules. According to the absolute value of interaction energy between modifier molecules and asphalt component molecules, it can be found that the interaction energy increases with the deepening of the aging degree. This indicates that the interaction between modifier molecules and asphalt component molecules was enhanced, the asphalt material becomes more dense, and the overall performance becomes brittle and hard.

3.2.2 Relationship between modifier molecules and asphalt component molecules

The radial distribution function (RDF) (also known as pair correlation function) is the ratio of the number density of an atomic group to its average density at each spherical position, which describes how the density varies with the distance from the reference particle [31]. Therefore, the RDF reflects the relative distribution relationship between different particles, and the larger the peak of the function, the more likely the molecules are to surround each other. The RDF can be calculated according to the following formula:

(4) g a b ( r ) = V N a N b ∑ i = 1 N a ∑ j = i + 1 N b 〈 δ ( r − ∣ r i → ( t ) − r j → ( t ) ∣ ) 〉 t ,

where a is the reference set of particles; b is the calculation set of particles; and V is the volume of the box. At time t, the mean density of particles b in the shell with radius r and thickness d _r from the reference particle a _i is obtained by traversing the delta (d _r) function of all b particles and then averaging.

The RDF of modifier molecules (SBS molecules and SBR molecules) and asphalt component molecules (asphaltene, resin, saturate, and aromatic) in the SBS/SBR composite-modified asphalt models were calculated, and the calculated results are shown in Figure 6.

Figure 6

RDF of modifier molecules and asphalt component molecules: (a) unaged SBS molecule and asphalt component molecules; (b) unaged SBR molecule and asphalt component molecules; (c) short-term aged SBS molecule and asphalt component molecules; (d) short-term aged SBR molecule and asphalt component molecules; (e) long-term aged SBS molecule and asphalt component molecules; and (f) long-term aged SBR molecule and asphalt component molecules.

According to the calculation results of RDF between modifier molecules and asphalt component molecules (as shown in Figure 6), different modifier molecules have different relationships with different asphalt component molecules. For the unaged SBS/SBR compound-modified asphalt, the peaks of SBS/SBR molecules and asphaltene molecules are relatively obvious, reaching 1.71 and 1.83, respectively, and reaching equilibrium at 16.5 Å for the first time, and then at 34.5 Å for the second time, but the interaction decreases and eventually tends to be stable (around 1.0). After short-term aged, the peak of SBS molecules and asphaltene molecules decreased and the first equilibrium position decreased after SBS molecules were broken and oxidized. The interaction between SBR molecules and bituminous component molecules is similar, but the first equilibrium position is near 18 Å and the second equilibrium position is near 30 Å. After long-term aged, the peak of modifier molecules and asphalt component molecules decreases further, and the equilibrium distance also decreases, which indicates that the properties of asphalt materials are further reduced. In general, during the aging process of SBS/SBR compound-modified asphalt, the interaction between SBS/SBR molecular chain and asphalt component molecules gradually becomes stable due to the fracture and oxidation of SBS/SBR molecular chain, and the equilibrium distance decreases at the same time, so it is easier to reach the equilibrium and stable state under the same conditions.

4 Infrared spectrum characteristics of SBS/SBR compound-modified asphalt

4.1 Infrared spectroscopy test

FTIR is one of the common methods used to test chemical structures of asphalt material. Its main principle is caused by the energy-induced internal tensile vibration of covalent bonds in the process of molecules absorbing infrared light [32]. Therefore, the number, size, and intensity of absorption peaks for different functional groups can be measured by infrared spectroscopy in different wavelength regions.

After FTIR test of SBS/SBR compound-modified asphalt (unaged, short-term aged, and long-term aged), FTIR results were obtained, as shown in Figure 7.

Figure 7

FTIR test results of SBS/SBR compound-modified asphalt: (a) unaged; (b) short-term aged; and (c) long-term aged.

As can be seen from Figure 7, the chemical composition structure of SBS/SBR compound-modified asphalt is mainly composed of aliphatic compounds, aromatic compounds, and heteroatomic derivatives. After aged (short-term aged and long-term aged), the absorption zone mainly appears in the C═O segment, which means that the oxidation reaction occurs in the SBS/SBR compound-modified asphalt.

4.2 Quantitative analysis of FTIR

In order to directly illustrate the specific structural characteristics of SBS/SBR compound-modified asphalt in the aging process, it is necessary to quantitatively analyze the peak area of specific functional groups in the FTIR [33,34]. The defining formulas of peak area ratio for characteristic functional groups are shown as follows:

(5) I B = A 1 , 371 A 1 , 462 + A 1 , 371 ,

(6) I Ar = A 1 , 620 ∑ A ,

(7) I C = O = I 1 , 700 ∑ A ,

(8) I S = O = I 1 , 032 ∑ A ,

(9) I C = C = I 966 ∑ A ,

where A is the absorption peak area; ΣA is the sum of the areas of different peaks in a wavelength range; I _B is the aliphatic functional group index; I _Ar is the aromatic ring index; I _C═O is the index of carbonyl functional groups; I _S═O is the index of sulfoxyl functional groups; and I _C═C is the index of olefin functional groups.

In asphalt research, different researchers choose different ΣA when studying asphalt FTIR, so the corresponding relative content change law will be different eventually. In the study, the common 4,000 to 700 cm⁻¹ full spectrum was used to calculate the value of ΣA, and the formula is ΣA = A _2,921 + A _2,857 + A _2,166 + A _1,602 + A _1,521 + A _1,455 + A _1,371 + A _1,034 + A ₉₆₅ + A ₈₇₃ + A ₈₃₁. After calculation, the functional group parameters of SBS/SBR compound-modified asphalt were obtained, as shown in Table 7.

Table 7

Change characteristics of the functional group parameters

Asphalt	I _B	I _Ar	I _C═O	I _S═O	I _C═C
Unaged	0.51	0.0053	0.0042	0.0274	0.0671
Short-term aged	0.46	0.0064	0.0074	0.0359	0.0557
Long-term aged	0.38	0.0083	0.0096	0.0463	0.0433

As can be seen from Table 7, functional group characteristics of SBS/SBR compound-modified asphalt with different aging degrees are different. After short-term aged, the aliphatic index (I _B) and olefin index (I _C═C) of SBS/SBR compound-modified asphalt decreased after aging, while the aromatic ring index (I _Ar), the carbonyl functional group index (I _C═O), and the sulfoxide functional group index (I _S═O) increased regularly. The results showed that sulfoxide functional groups and carbonyl functional groups increased during aging, while the aliphatic functional groups and olefin functional group decreased. After long-term aged, the regularity of functional group parameter changes was further enhanced, because the increase of aging time intensifies the oxidation reaction. Therefore, it can be seen from the FTIR test that a series of complex reactions occur in the aging process, including chemical bond fracture, oxidation, and polycondensation. This corresponds to the fracture and oxidation (increase of aldehyde and carboxyl groups) setting of SBS/SBR molecular chain in MD simulation, which indicates the rationality and accuracy of MD simulation results.

In summary, combining MD simulation and FTIR test, it can be seen that the aging process of SBS/SBR compound-modified asphalt not only changes in macro properties, but also has a molecular scale aging mechanism. In addition to the change of asphalt four-component ratio, the SBS/SBR molecular chain will also have aging behavior during the aging process, affecting its relationship with asphalt component molecules. The fracture and oxidation of modifier molecule chain (aldehyde groups or carboxyl groups) behavior alter the efficacy of SBS/SBR molecules. It can be found from the MD simulation that the SBS/SBR compound-modified asphalt micro models show the characteristics of increasing solubility parameters and cohesive energy density during the aging process, and the diffusion coefficient decreases with aging, indicating that the stability of the interface structure formed by molecule was enhanced. Meanwhile, aging results in the change of interaction between SBS/SBR molecules and asphalt components (attraction is enhanced, repulsion is weakened), and the first equilibrium distance between molecules was reduced, resulting in increased density, and asphalt materials become brittle and hard. In the FTIR test, it can be found that the aging of SBS/SBR compound-modified asphalt leads to different peaks in the infrared spectrum results. After short-term and long-term aged, the I _Ar, I _C═O, and I _S═O of SBS/SBR compound-modified asphalt were increased to varying degrees, while the I _B and I _C═C were decreased after aging, which indicates that the olefin functional group fracture and oxidation effect occur in the aging process of SBS/SBR compound-modified asphalt. This proves the rationality of the MD simulation and explains the micro aging mechanism of SBS/SBR compound-modified asphalt.

5 Conclusions

The aging mechanism of SBS/SBR compound-modified asphalt was explored from the perspective of MD simulation and FTIR test, and the main conclusions were as follows:

Besides the change of asphalt four-component ratio, SBS/SBR molecular chain fracture and oxidation (increase of aldehyde and carboxyl groups) are also important reasons for the deterioration of aging SBS/SBR compound-modified asphalt properties;
The fracture and oxidation of modifier molecules lead to the decrease of the solubility parameters and diffusion coefficient of SBS/SBR compound-modified asphalt micro models, which is not conducive to the adhesion behavior of asphalt on aggregate surface;
Aging results in the change of interaction between SBS/SBR molecules and asphalt components (attraction is enhanced, repulsion is weakened), and the first equilibrium distance between molecules was reduced, resulting in increased density, and asphalt materials become brittle and hard;
The contents of I _Ar, I _C═O, and I _S═O in infrared spectrum increase with the aging time, while the contents of I _B and I _C═C decrease with the aging time, which indicated that aging resulted in the fracture and oxidation of the olefin functional groups of SBS/SBR molecules.

Acknowledgments

The authors gratefully appreciate supports from the project of Research and Engineering Demonstration of Intelligent Construction Site Technology for Expressway Facing Industrialization (No. JKKJ-2019-12), Hezong High-speed Transportation Science and Technology Demonstration Project (No. YJZX2371006), Development of Intelligent Site Platform Information System for Hezong Expressway (No. XXRK2031001), Development of Smart Internet of Things Data Acquisition System for Hezong Expressway (No. XXJC2032006), and the state key laboratory of road in Northeast Forestry University.

Funding information: This work was supported by the Anhui Traffic Control Technology Projects (No. JKKJ-2019-12, No. YJZX2371006, No. XXRK2031001, No. XXJC2032006).
Author contributions: Hu Shao: discussion, supervision, and project administration; Jianya Tang: methodology, resources, material preparation, and performing the experiment; Wenzheng He: conceptualization, data analysis, discussion, writing, and revision; Shuang Huang: language modification, syntax check; Tengjiang Yu: model building, software running, and result collation. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: The raw/processed data required to reproduce these findings cannot be shared at this time, as the data form part of an ongoing study.

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Received: 2023-02-08

Revised: 2023-06-08

Accepted: 2023-07-24

Published Online: 2023-08-14

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/rams-2023-0106

Keywords for this article

asphalt; SBS/SBR; molecular dynamics; aging mechanism; FTIR

Creative Commons

BY 4.0