Study on the Grafting Reaction and Road Performance of MAH‑g‑SBS Modified Asphalt Sealants

Abstract

With the increase in service life of transportation infrastructure, highway crack repair materials have become a critical factor restricting road maintenance quality. Therefore, in order to cope with the technical bottlenecks of insufficient high-temperature stability and poor low-temperature ductility of traditional sealants, a new chemical grafting modified asphalt sealant, maleic anhydride grafted styrene–butadiene block copolymer (MAH-g-SBS) modified asphalt sealant, was developed. Benzoyl peroxide (BPO) was used as the initiator to generate MAH-g-SBS by maleic anhydride (MAH) grafting, modifying styrene–butadiene block copolymer (SBS). Through the coupling of completely randomized design and orthogonal experimental design, a MAH-g-SBS grafting rate prediction model was obtained by multiple nonlinear regression analysis, which considers the effects of reaction time, initiator dosage, reaction monomer dosage, and reaction temperature. Meanwhile, the significance of factors on grafting rate was clarified by the analysis of variance (ANOVA) and the prediction model validity was verified by error analysis. The prediction model and test data were both considered, and the maximum grafting rate was 20%. Simultaneously, by combining Fourier transform infrared spectroscopy (FTIR), the modification mechanism was revealed through the relationship between the carbonyl characteristic peak intensity and the road performance of the material. Subsequently, the road performances of MAH-g-SBS modified asphalt sealants with different grafting rates were studied by a series of laboratory tests and it was found that with the increase of grafting rate, the ductility, viscosity, and resilience recovery significantly increased and the softening point first decreased and then increased, but the cone penetration decreased. Meanwhile, based on the gray relational analysis, the grafting rate had the most significant impact on ductility and the least impact on cone penetration.

1. Introduction

Pavement cracks are one of the prevalent diseases in road engineering, which not only accelerates the water damage of the pavement structural layer but also significantly reduces the comfort and safety of driving. In order to slow down the expansion of cracks and restore pavement functionality, asphalt crack filling materials have become core materials for crack repair due to their excellent adhesion, flexibility, and construction convenience. However, the conventional asphalt sealants can easily soften and flow at high temperature, are brittle at low temperature, and can easily have problems such as peeling failure with the old pavement in long-term service, which seriously restricts its engineering application. The reason is that the asphalt material has high temperature sensitivity and insufficient compatibility with polymer modifiers, so there is an urgent need to improve its wide temperature range performance balance through molecular design. −

In recent years, polymer-modified asphalt, as an innovative pavement material, has received wide attention and application in road engineering. − The repair of pavement cracks produced by this material under long-term loading and an external environment is a problem that needs to be solved at present. Through grafting and block copolymerization technologies, two chain segments which are hydrophilic and lipophilic, acidic and alkaline, plastic and highly elastic, and otherwise incompatible can be bonded, to give the material a series of unique properties. Among them, styrene–butadiene block copolymer (SBS), as an important thermoplastic elastomer, not only perfectly combines the excellent elasticity of rubber with the thermoplastic characteristics of resin but also shows excellent crack repair effect in the field of modified asphalt crack filling materials due to its excellent elastic recovery ability and high strength. −

Through research, a highly stable asphalt sealant consisting of synergistically modifying asphalt with SBS and carbon nanotubes (CNTs), the synergistic effect of SBS, lightweight calcium carbonate, and soybean oil, was developed. The optimal process parameters were determined by combining a low-temperature BBR test and LAS test, and an orthogonal test and hierarchical analysis method clarified the influence of different factors on the performance of asphalt sealants.

In terms of physical modification and structural characterization, Al₂O₃ could significantly enhance the high temperature performance and storage stability of polymer modified asphalt. The incorporation of dithiobimorpholine (DTDM) could regulate the storage stability and rheological response of SBS-modified asphalt. The uniform dispersion characteristics of nanoparticles were revealed by using fluorescence microscopy. The advantages of aging resistance and rheological properties of nano OMMT/SBS modified asphalt was confirmed by an Atomic Force Microscope (AFM) test. The microscopic phase evolution was clarified by Fourier transform infrared spectroscopy (FTIR) and fluorescence microanalysis. On the introduction of cationic waterborne polyurethane (PU+), a combination of rheology and microanalysis verified the effect of adhesion enhancement over a wide range of temperature.

In terms of chemical modification, the SBS-modified asphalt performed better than most thermoplastic materials in terms of rheological properties and mixture adaptability. The maleic anhydride (MAH) grafting with SBS could significantly improve the mechanical properties of clay nanocomposites. The synergistic improvement of waste polyethylene (PE) and PE-g-MAH modified asphalt properties and storage stability by twin-screw extrusion technology was achieved. The high-temperature compacting could produce chemical reactions, which can produce cross-linking reactions with matrix asphalt and build cross-linking networks to modify the properties of rubber asphalt. The MAH grafted polypropylene (PP-g-MAH) was prepared through a twin-screw extrusion test by SBS and styrene isoprene styrene block copolymers (SIS). According to the FTIR of gutta-percha grafted products, the grafted modification mechanism was characterized.

In summary, physical modification can improve the material properties in short-term service, while chemical grafting provides a new way to improve the long-term durability by constructing a stable interface through covalent bonding. But there has been little research on the quantitative relationship between the reaction mechanism and the process parameters of chemical grafting modification. Meanwhile, the characterization of the modification mechanism of chemical grafting is still insufficient. All of the above restrict the application of chemical graft-modified materials. Therefore, in this paper, benzoyl peroxide (BPO) was used as an initiator to graft-modify SBS with MAH to produce MAH-g-SBS, and then, MAH-g-SBS was added into matrix asphalt to prepare MAH-g-SBS modified asphalt sealants. Through the coupling of completely randomized design and orthogonal experimental design, a MAH-g-SBS grating rate prediction model, which considered the coupling effect of the reaction time, initiator dosage, reaction monomer dosage, and reaction temperature, was established. Meanwhile, the analysis of variance (ANOVA) was used to study the influence of factors on the grafting rate, and the error analysis was used to verify the validity and reliability of the prediction model. Furthermore, the modification mechanism of the grafting reaction was investigated based on an FTIR test. In addition, a series of laboratory tests (cone penetration test, ductility test, softening point test, rotational viscosity test, and resilience recovery test) were conducted to investigate the effects of different grafting rates on the road performance of MAH-g-SBS modified asphalt sealants. Finally, based on gray correlation analysis, the mechanism of grafting rate on road performance was revealed. The above research aims to provide guidance for the development and application of high-performance sealants with theoretical depth and engineering feasibility.

2. Reaction Mechanism

The core modification mechanism of MAH is to improve the properties of SBS by linking the main chain of the copolymer with other types of molecular chains. The anhydride modification technology leads to significant changes in the colloidal structure and chemical composition of asphalt, which have a direct effect on the macroscopic road performance. Compared to traditional physical modification methods, MAH modification exhibits different characteristics in terms of modification effect and approach. The modification process of MAH on asphalt is the combined effects of the Diels–Alder addition reaction, alternating copolymerization reaction, and electron transfer. The potential reaction pathways is as follows, − as Figure shows.

• (1)Decomposition of the initiator, generating free radicals:

$$R-R\to 2R^{*}$$

• (2) R ^* attack of the active hydrogen (a-H) in the chain of the SBS molecule, forming a chain radical.
• (3)Chain radicals grafted to MAH to form MAH-g-SBS.

1.

Schematic diagram of grafting mechanism.

3. Materials and Sample Preparation

3.1. Materials

The molecular numbers of SBS range from tens of thousands to hundreds of thousands. The SBS polymer chain consists of different types of segments in tandem, including rigid plastic segments (hard segments) and flexible rubber segments (soft segments), which are arranged to form a “microstructure” similar to that found in alloys, − as Figure illustrates.

2.

Schematic diagram of SBS structure.

The YH-792 SBS produced by China Sinopec was used in the test; the basic technical indexes are shown in Table .

1. Technical Indexes of YH-792 SBS.

Item	Value
S/B ratio	40/60
Oil filling rate (%)	0
Oil filling rate (≤%)	0.70
Ash (≤%)	0.2
300% constant tensile stress (≥MPa)	2.8
Tensile strength (≥MPa)	20
Tearing elongation (≥%)	700
Tear (≤%)	55
(A)	90 ± 5
Tear strength (≥KN/m)	40
Melt flow rate (g/min)	0.10–5.00

The 90# matrix asphalt produced by Changchun Pengwei Road Construction Co. Ltd., MAH taken from Shanghai McLean Biochemical Technology Co. Ltd., BPO produced by Xilong Science Co. Ltd., cyclohexane produced by Xilong Science Co. Ltd., ethanol absolute produced by Jiangsu Huahang Biotechnology Co. Ltd., and acetone produced by Shanghai Hengli Chemical Co. Ltd. were used for the preparation of MAH-g-SBS modified asphalt sealants.

According to the Chinese specification of Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011), the technical indexes of 90# matrix asphalt were tested, and the results are detailed in Table , which shows that the asphalt meets the specification requirements.

2. Technical Indexes of 90# Matrix Asphalt.

Test item	Specification	Test result	Test method
Penetration (25 °C)/0.1 mm	60–80	65	T0604
Softening point/°C	≥43	48.3	T0606
Ductility (15 °C)/cm	≥100	>100	T0605
After RTFOT (163 °C, 85 min)
Quality change/%	±0.8	–0.11	T0609
Residual penetration ration (25 °C)/%	≥58	63	T0604
Residual ductility/cm	≥15	19	T0605

3.2. Sample Preparation

3.2.1. Preparation of MAH Grafted SBS

A three-necked round-bottomed flask (500 mL) was used as a reaction vessel. First analytically pure toluene solvent (100 mL) was injected into it, followed by the addition of SBS (10.0 g), and the mixture was stirred at water bath temperature until the polymer was completely dissolved. Under a constant temperature, the grafted monomer and the appropriate amount of BPO initiator were injected sequentially according to the preset mass ratio. After the reaction was terminated, the system was cooled to room temperature, a sufficient amount of absolute ethanol was injected until the precipitate was completely precipitated, and the solid phase products were separated by Brinell funnel vacuum filtration. Then the resulting filter cake was placed in a vacuum drying oven at 60 °C for 12 h to remove residual solvent. To further purify the product, the dried sample was placed in a Soxhlet extraction device and continuously refluxed with acetone as solvent for 24 h. The final product was dried by vacuum twice, to obtain the high-purity grafted modified SBS materials, − as shown by the process in Figure .

3.

Schematic diagram of the process.

3.2.2. Preparation of MAH-g-SBS Modified Asphalt Sealants

The matrix asphalt was placed in a constant temperature oven at 170 ± 2 °C, and when the asphalt was in the molten state, MAH-g-SBS grafted modifier was added. The process is divided into three stages (Figure ): in the first stage, a high-speed shear homogenizer was used to implement high-intensity dispersion for 60 min to ensure that the modifier and the asphalt to form a uniform dispersion system, in the second stage, the mixture was transferred to a conventional mechanical stirring device for continuous mixing for 120 min, to promote the enhancement of interfacial compatibility, and in the third stage, functional additives such as stabilizer, antiaging agents, and others were added based on the mix ratio and mixing was continued for 30 min to complete system optimization, finally giving the MAH-g-SBS modified asphalt sealants. −

4.

MAH-g-SBS modified asphalt sealant preparation process.

3.3. Determination of Grafting Rate

The grafting rate of the grafted products was determined by gravimetric analysis. First, the grafted SBS polymers were dried in a vacuum oven at 60 °C. Subsequently, acetone was used to extract the graft monomer for 24 h to remove the incomplete reacted graft monomers. After completion of the extraction, vacuum drying was performed again, and the weight after drying was measured. Based on the measured weight, the grafting rate was calculated by eq .

$$GD=\frac{W_1-W_0}{W_0}\times 100\%$$ 1

where GD is the grafting rate in %, W ₁ is the yield of purified grafts in g, and W ₀ is the mass of SBS in g.

4. Results and Discussion

4.1. Grafting Rate Prediction Model of MAH-g-SBS

The grafting rate of MAH grafted SBS directly affects its polarity, interfacial compatibility, and application properties. The effects of reaction time, initiator dosage, reaction monomer dosage, and reaction temperature on the grafting rate of MAH-g-SBS are investigated by completely randomized design, where the initiator dosage is the mass ratio of the initiator to SBS, and the reaction monomer dosage is the mass ratio of MAH to SBS. Then a four-factor, four-level orthogonal experimental design is established, and based on the completely randomized design fitting equations, the grafting rate prediction model of MAH-g-SBS under multifactor coupling is determined.

4.1.1. Effect of Single Factor on Grafting Rate

The effects of single factors (reaction time, initiator dosage, reaction monomer dosage, and reaction temperature) on the grafting rate of MAH-g-SBS are studied by completely randomized design. When studying a certain factor, the rest of the factors are fixed values; for example, in order to investigate the effect of reaction time on grafting rate of MAH-g-SBS, the reaction time is set to 4, 6, 8, and 10 h, respectively, and the initiator dosage, reaction monomer dosage, and reaction temperature are set to 1.1%, 30%, and 80 °C, respectively. The completely randomized design scheme is listed in Table and test results are shown in Figure .

3. Completely Randomized Design Scheme.

Sample	Reaction time (h)	Initiator dosage (%)	Reaction monomer dosage (%)	Reaction temperature (°C)
C1	4	1.1	30	80
C2	6
C3	8
C4	10
C5	8	0.7	30	80
C6		0.9
C7		1.1
C8		1.3
C9	8	1.1	10	80
C10			20
C11			30
C12			40
C13	8	1.1	30	40
C14				60
C15				80
C16				100

5.

Relationship curves and fitting curves between grafting rate and single factor: (a) reaction time, (b) initiator dosage, (c) reaction monomer dosage, and (d) reaction temperature.

As Figure (a) shows, the grafting rate increased significantly from 12.2% to 17.6% with the increase of reaction time from 4 to 8 h, which is attributed to the efficiently free radical grafting reaction of the double bond between MAH and SBS butadiene chain segments. However, when the reaction time further increased to 10 h, the grafting rate decreased to 15.1%, which might be due to the long reaction time, resulting in side reactions such as MAH self-polymerization, SBS main chain degradation, or ester bond reversal. From Figure (b), the grafting rate gradually increased from 15.3% to 17.6% as the initiator dosage increased from 0.7% to 1.1%, indicating that the appropriate amount of initiator can effectively promote the generation of free radicals and accelerate the grafting reaction. However, when the initiator dosage increased to 1.3%, the grafting rate decreased to 14.4%, which might be due to the chain termination reaction caused by the excessive initiator and the side reactions such as MAH self-polymerization or oxidative degradation of the SBS main chain being aggravated, resulting in a decrease in the grafting rate. Figure (c) indicated that the grafting rate increased significantly from 8.1% to 17.6% with the increase of the reaction monomer dosage from 10% to 30%, showing that the increase of MAH dosage could increase the effective collision probability between MAH and SBS double bond and promote the positive direction of free radical. When the reaction monomer dosage increased to 40%, the grafting rate decreased slightly to 17.1%, from which is was presumed that the self-polymerization reaction was triggered by the excess monomer or the mass transfer restriction was caused by the increased viscosity of the system, which inhibited the grafting reaction between MAH and SBS. Figure (d) shows that the grafting rate increased significantly from 10.5% to 17.6% as the temperature increased from 40 to 80 °C, which is attributed to the accelerated decomposition of the initiator and the increase in the concentration of free radicals at higher temperature, which enhance the reactivity of the MAH and SBS double bond. However, when the temperature was increased to 100 °C, the grafting rate decreased to 15.4% because of high temperature triggered MAH self-polymerization, thermal degradation of the SBS main chain, or the intensification of radical chain termination reaction, resulting in the decrease in grafting rate.

As Figure expresses, the relationship between the grafting rate and reaction time, initiator dosage, reaction monomer dosage, and reaction temperature can be calculated by eqs –, respectively

$$Y=-0.4063{X_1}^2+6.1925X_1-6.135{\mathit{R}}^2=0.9946$$ 2

$$Y=-28.125{X_2}^2+55.4X_2-9.8938{\mathit{R}}^2=0.8726$$ 3

$$Y=-0.0092{X_3}^2+0.7955X_3+0.575{\mathit{R}}^2=0.9232$$ 4

$$Y=-0.0041{X_4}^2+0.6645X_4-9.64{\mathit{R}}^2=0.9807$$ 5

where Y is the grafting rate in %, X ₁ is the reaction time in h, X ₂ is the initiator dosage in %, X ₃ is the reaction monomer dosage in %, and X ₄ is the reaction temperature in °C.

The coefficients of determination R ² in the fitting equations of completely randomized design are close to 1, which characterizes the effectiveness of the model.

4.1.2. Determination of the Grafting Rate Prediction Model

Section 4.1.1 shows that the reaction time, initiator dosage, reaction monomer dosage, and reaction temperature under the influence of a single factor have a quadratic relationship with the grafting rate; however, there is an interaction between factors. Therefore, in order to study the comprehensive influence of multiple factors on the grafting rate, based on the completely randomized design fitting equations, the grafting rate prediction model under multifactor coupling was obtained by orthogonal experimental design (Table ), which was established to determine the MAH-g-SBS grafting rate under different influencing factors.

4. Parameters in Orthogonal Experimental Design.

	Factors
Levels	Reaction time (h)	Initiator dosage (%)	Reaction reaction monomer dosage (%)	Reaction temperature (°C)
1	4	0.7	10	40
2	6	0.9	20	60
3	8	1.1	30	80
4	10	1.3	40	100

A multiple nonlinear regression model for the MAH-g-SBS grafting rate is established by combining the fitting equations of the single factors with the grafting rate, as eq shows

$$GP=a_0+\sum _{i=1}^k{a}_i{X}_i+\sum _{i=1}^k{a}_{ii}{X}_i^2$$ 6

where GP is the grafting rate in %, X is the influencing factor, k stands for the number of items, and a ₀, a _i , a _ii are fitting parameters.

Based on the orthogonal experimental data, multiple nonlinear regression analysis is carried out, and the various regression coefficients of the equation are determined, as shown in Table . Then the grafting rate prediction model of MAH-g-SBS is obtained, and R ² is 0.9559, indicating that the equation has a good fitting effect

$$GP=-11.12+1.698X_1-7.775X_2+0.548X_3+0.351X_4-0.119X_1^2+1.25X_2^2-0.005X_3^2-0.002X_4^2{\mathit{R}}^2=0.9559$$ 7

where GP is the grafting rate in %, X ₁ is the reaction time in h, X ₂ is the initiator dosage in %, X ₃ is the reaction monomer dosage in %, and X ₄ is the reaction temperature in °C.

5. Fitting Parameters.

		95% confidence interval for difference
Parameters	Value	Lower bound	Upper bound
a 0	–11.12	–30.633	8.393
a 1	1.698	–0.564	3.959
a 2	–7.775	–39.949	24.399
a 3	0.548	0.184	0.912
a 4	0.351	0.098	0.604
a 11	–0.119	–0.279	0.041
a 22	1.25	–14.773	17.273
a 33	–0.005	–0.012	0.002
a 44	–0.002	–0.004	–0.001

From the value of the regression coefficients in Table , the confidence intervals for difference of a ₃ (0.548, CI: 0.184–0.912) and a ₄ (0.351, CI: 0.098–0.604) do not cross the zero value, indicating that the two parameters are statistically significant at the α = 0.05 level. Meanwhile, the ANOVA method was used to clarify the influence of the reaction time, initiator dosage, reaction monomer dosage, and reaction temperature on the grafting rate based on orthogonal experimental design data. The results of the ANOVA between the grafting rate and the above four factors are listed in Table .

6. ANOVA Results.

Factor	SS	d	MS	F value	F critical value	Significance evaluation
X 1	4.813	3	1.604	1.214	F 0.01(3,15) = 5.42	least significant
X 2	24.223	3	8.074	6.112	F 0.05(3,15) = 3.29	most significant
X 3	146.532	3	48.844	36.972		most significant
X 4	13.957	3	4.652	3.521		significant
SS	3.963	15	1.321

Table revealed that the significance of influence on grafting rate could be ranked from high to low as follows: reaction monomer dosage, initiator dosage, reaction temperature, and reaction time. Reaction monomer dosage and initiator dosage exhibited the most significant effect, while reaction time showed the least significance. Notably, although the reaction temperature is statistically nonsignificant, the moderate F value suggests potential practical relevance under specific conditions. The stabilized effects of the reaction monomer dosage are remarkable, as its optimal level could substantially enhance the grating rate stability. In contrast, the weak influence of the reaction time implies that chemical reaction efficiency may be prioritized over precise timing in practical operations. The reason is that the chain links between molecules due to chemical reactions can significantly enhance the stability of the grafting rate.

Furthermore, in order to verify the accuracy and application effect of the prediction model, the results of a set of completely randomized design are randomly selected and brought into the prediction model for error analysis, and the selected test data are shown in Table .

7. Parameter Values for Error Analysis.

Reaction time (h)	Initiator dosage (%)	Reaction monomer dosage (%)	Reaction temperature (°C)
8	1.1	30	40

The predicted values of MAH-g-SBS grafting rate are calculated by the prediction model and the error is quantified by eqs and to obtain Table

$$AE=|Y_{\mathrm{T}\mathrm{e}\mathrm{s}\mathrm{t}}-Y_{\mathrm{P}\mathrm{r}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{t}}|$$ 8

$$RE=\frac{AE}{Y_{\mathrm{T}\mathrm{e}\mathrm{s}\mathrm{t}}}$$ 9

where AE is absolute error in % and RE is relative error in %.

8. Error Analysis Results.

Parameter	Test value (%)	Predicted value (%)	AE (%)	RE (%)
Grafting rate	10.5	10.588	0.088	0.838

As Table shows, the predicted grafting rate of the model for the predicted value is 10.588%, which is highly consistent with the test value of 10.5%, with AE of 0.088% and RE of 0.838%, manifesting that the prediction model for MAH-g-SBS grafting rate considering the combined effect of various factors is reliable.

4.2. FTIR Characterization

As Figure shows, FTIR analysis of the MAH-g-SBS modifier indicates that MAH is successfully grafted to the SBS. No characteristic absorption peak is detected at 1710 cm^–1 for ungrafted SBS, whereas significant carbonyl (CO) stretching vibration peaks are observed in both MAH-g-SBS-10% and MAH-g-SBS-20% samples. The peak area increases by approximately 1.8-fold with the rise in grafting rate from 10% to 20%, indicating a positive correlation between the MAH content in the grafted product and grafting rate. In addition, no anhydride CO antisymmetric stretching vibration peak is observed within the range 1770–1790 cm^–1 for all grafted samples, illustrating that the MAH is completely ring-opened and chemically bonded to SBS via esterification. This conclusion is further confirmed by the emerging ester bond C–O–C stretching vibration peak at 1180 cm^–1, while pure SBS do not show any response in this region, confirming that the grafting reaction constructs a stable molecular chain connection through ester bonds.

6.

FTIR of MAH-g-SBS modifier.

The chemoselectivity of the grafting reaction on the SBS chain segments can be revealed by changes in the characteristic double bond peaks. Typical double bond peaks at 964 cm^–1 and 910 cm^–1 are present for ungrafted SBS. After grafting, the 964 cm^–1 peak intensity of MAH-g-SBS-10% decreased by 30%, while that of MAH-g-SBS-20% further decreased by 55%, indicating that MAH preferentially attacked the trans-double-bonding sites of the butadiene chain segments: the higher the grafting rate, the more significant the double-bond depletion. It is worth noting that the peak shape and intensity of all samples remained stable at 696.2 cm^–1 and 1601 cm^–1, showing that the grafting reaction does not destroy the aromatic structure of the styrene chain segment, which is consistent with the characteristic of the butadiene segment as the active reaction site in SBS graft modification.

4.3. Effect of Grafting Rate on MAH-g-SBS Modified Asphalt Sealants

Considering the prediction model and test data, the maximum grafting rate was determined to be 20%. Accordingly, MAH-g-SBS samples were categorized into five gradients at 5% intervals (i.e., 5%, 10%, 15%, 20%) for subsequent performance evaluations. MAH-g-SBS powder with 0%, 5%, 10%, 15% and 20% grafting rates is added into 90# matrix asphalt by an internal blending method according to the mass ratio of 6% to prepare MAH-g-SBS modified asphalt sealants. According to the Chinese specification of JTG E20-2011, and Hot poured sealants for pavement JT/T 740-2024, a cone penetration test, ductility test, softening point test, rotational viscosity test, and compression resilience recovery test on MAH-g-SBS modified asphalt sealants were conducted to investigate the effects of grafting rate on the road performance of the modified asphalt sealants.

4.3.1. Cone Penetration

As shown in Figure , the cone penetration decreases with an increasing grafting rate. For the ungrafted sample, the cone penetration is 43.2; when the grafting rate increases to 20%, this value drops significantly to 18.6. Notably, a transient recovery in cone penetration is observed at 10% grafting rate, which can be attributed to the plasticizing effect of the MAH short chains. Specifically, at low grafting rates, MAH segments are sparsely distributed along the SBS molecular chains and fail to form effective cross-linking networks. Instead, these short, flexible MAH chains act as molecular lubricants, enhancing the mobility of SBS chains and temporarily softening the materialthus leading to the slight increase in cone penetration. However, when the grafting rate exceeds 15%, the mechanical behavior shifts dramatically. With more MAH groups participating in the grafting reaction, the number of chemical cross-linking points between MAH and SBS molecular chains increases significantly, forming a dense three-dimensional network structure. This enhanced cross-linking restricts the sliding and rearrangement of molecular chains under external force, strengthens intermolecular interactions, and ultimately improves the material’s stiffness and resistance to deformation, as manifested by the sharp decrease in cone penetration.

7.

Cone penetration at different grafting rates.

4.3.2. Ductility

As Figure shows, ductility improves continuously as grafting rate increased, from 26.9 cm at 0% to 41.7 cm at 20%. The polar groups of MAH strengthen intermolecular secondary bonds (e.g., hydrogen bonds, dipole–dipole interactions), which act as “molecular bridges”, promoting uniform stress distribution under external forces and avoiding localized concentration, that causes premature fracture, thus facilitating better ductility. The branched structures enhance chain entanglement and enable more extensive segmental sliding and orientation during stretching. This process dissipates more energy, further improving ductility and toughness through synergistic effects of enhanced intermolecular cohesion and optimized chain mobility.

8.

Ductility at different grafting rates.

4.3.3. Softening Point

Figure shows that the softening point shows first a decreasing and then an increasing trend. The softening point of the ungrafted sample is 81 °C and decreases to 67.5 °C at 5%, which is attributed to the incorporation of MAH destroying the original physical cross-linking network of SBS, leading to the reduction of thermal stability. With the further increase of grafting rate, the softening point gradually rises to 73 °C because the polar groups of MAH enhance the intermolecular hydrogen bonding, partially compensating for the loss of cross-linked structure. At 20% grafting rate, the softening point increases significantly to 81.7 °C, even exceeding that of the ungrafted sample. This substantial improvement indicates that at high grafting rates chemical cross-linking begins to dominate the thermal behavior of the material. The formation of a more stable three-dimensional network structure through chemical cross-linking effectively enhances the high-temperature stability, leading to a significant elevation of the softening point. This transition from physical to chemical cross-linking dominance marks a critical shift in the material’s thermal performance mechanism.

9.

Softening point at different grafting rates.

4.3.4. Viscosity

From Figure , the viscosity increases monotonically with the grafting rate, rising from 1325 mPa s at 0% to 1563 mPa s at 20%. This consistent upward trend arises from the synergistic effect of structural changes induced by MAH grafting. The primary driver is the introduction of polar groups onto SBS molecular chains. These polar groups enhance intermolecular frictional resistance by strengthening dipole–dipole interactions, directly impeding the relative movement of chains during flow. Simultaneously, they form temporary physical cross-links through hydrogen bonding-dynamic structures that act as reversible “chain anchors”, further restricting molecular mobility and elevating flow resistance. A secondary contributing factor is the slight increase in molecular weight caused by the grafting reaction. Higher molecular weight amplifies chain entanglement, as longer chains exhibit greater interpenetration. This enhanced entanglement creates an additional network-like barrier to flow, synergistically reinforcing the viscosity increase initiated by polar group interactions.

10.

Viscosity at different grafting rates.

4.3.5. Resilience Recovery

As Figure indicates, the resilience recovery increases steadily with the grafting rate, rising from 95% at 0% to 98.4% at 20%. The introduction of polar cross-linking points onto the SBS molecular chains through MAH grafting increases the density of the dynamic cross-linking network within the material. These polar cross-links act as elastic “molecular hinges”, when subjected to external forces, which can effectively store elastic potential energy and facilitate rapid retraction of chain segments upon unloading, directly contributing to the improvement in resilience recovery. Additionally, the MAH grafting enhances the interfacial compatibility between the modifier and the matrix, which reduces the occurrence of microstructural defects (such as voids or weak interfaces), otherwise leading to permanent deformation under cyclic loading. Consequently, the material exhibits enhanced resistance to fatigue failure under dynamic loading conditions, further highlighting the beneficial effect of graft modification on the material’s dynamic mechanical performance.

11.

Resilience recovery rate at different grafting rates.

4.4. Gray Relational Analysis

In order to quantify the effect of grafting rate on the road performance of MAH-g-SBS modified asphalt sealants, gray relational analysis is used to construct a model with grafting rate as the comparison sequence and the cone penetration, ductility, softening point, viscosity, and resilience recovery as the reference sequence, as Table shows.

9. Determination of the Analysis Sequence.

Comparison sequence	Reference sequence
Grafting rate (%)	Cone penetration (0.1 mm)	Ductility (cm)	Softening point (°C)	Viscosity (mPa s)	Resilience recovery (%)
5	40.6	29.5	67.5	1465	97.1
10	42.7	33.1	72.8	1490	97.5
15	26.4	38.6	73.0	1510	97.7
20	18.6	41.7	81.7	1563	98.4

After eliminating the difference in magnitude by the initialization, the absolute difference and correlation between the grafting rate and each performance index are calculated by eq . −

$$\gamma (X_0,X_{\mathrm{i}})=\frac{1}{n}\sum _{k=1}^n\frac{\min {\mathrm{\Delta }}_{0i}(k)+\rho \max {\mathrm{\Delta }}_{0i}(k)}{{\mathrm{\Delta }}_{0i}(k)+\rho \max {\mathrm{\Delta }}_{0i}(k)}$$ 10

Generally that when ρ = 0.5, γ _i ≥ 0.6, indicating that the comparison sequence is well correlated with the reference sequence, otherwise, poorly correlated. The correlation between the grafting rate and each performance index is shown in Table .

10. Gray Relational Analysis Results.

Cone penetration	Ductility	Softening point	Viscosity	Resilience recovery
0.6034	0.6466	0.6315	0.6233	0.6207

Gray relational analysis results show that the correlation between the grafting rate and each performance index is as follows: ductility > softening point > viscosity > resilience recovery > cone penetration. This manifests that the grafting rate has the most significant effect on the ductility with a correlation of 0.6466, which is consistent with the test data that the increase in grafting rate up to 20% is 41.3%; the introduction of MAH polar groups effectively improves the ductility of the material by enhancing the hydrogen bonding between the molecular chains. The softening point is the second highest its value increases in a stepwise manner with the increase of grafting rate, which may be related to the increase of cross-linking network density. It is worth noting that the cone penetration decreased by 54.2%, but it has the lowest correlation, suggesting that this parameter may experience more interference by external conditions such as temperature.

5. Conclusion

In this paper, a series of laboratory studies were conducted on the MAH-g-SBS grafting rate under multifactor coupling conditions and its effect on the road performance of MAH-g-SBS modified asphalt sealants; the findings are as follows.

• (1)Based on the coupling method of completely randomized design and orthogonal experimental design, the MAH-g-SBS grafting rate prediction model was established through multiple nonlinear regression analysis, whose validity and reliability was verified by error analysis. Considering the MAH-g-SBS grafting rate prediction model and test data, the maximum grafting rate was 20%.
• (2)The influences of reaction monomer dosage and initiator dosage on the MAH-g-SBS grafting rate were the most significant and the impact of reaction temperature was significant, while the effect of reaction time was the least significant through the ANOVA method. The chemical reaction had a greater impact on the stability of the grafting rate than the construction process.
• (3)From FTIR characterization, a significant CO characteristic peak appeared at 1720 cm^–1 in the SBS molecular chain after grafting, and the peak intensity was positively correlated with the grafting rate.
• (4)The ductility, viscosity, and resilience recovery of MAH-g-SBS modified asphalt sealants increased as grafting rate increased and the softening point first decreased and then increased, but the cone penetration gradually decreased.
• (5)Gray relational analysis showed that the grafting rate had the greatest influence on the ductility, followed by the softening point, and had the least effect on the cone penetration.

The authors declare no competing financial interest.