In the rapidly evolving landscape of automotive technology, the shift toward electric vehicle car platforms has ushered in a new era of performance demands for tire components. As a researcher deeply involved in polymer science and tire engineering, I have focused on addressing the critical challenges posed by electric vehicle car designs: the need for reduced rolling resistance to extend battery range, enhanced wear resistance to handle increased vehicle mass and torque, and superior wet-grip performance to ensure safety under diverse driving conditions. Among the various elastomers explored, solution-polymerized styrene-butadiene rubber (SSBR) stands out due to its tunable microstructure and favorable balance of properties. However, conventional SSBR often falls short in meeting the stringent “magic triangle” requirements—low rolling resistance, high wet-skid resistance, and excellent wear resistance—simultaneously. This has driven the development of advanced modification techniques, with end-chain functionalization emerging as a pivotal strategy. In this comprehensive study, I delve into the application of double-end modified star-shaped SSBR in tread compounds specifically formulated for electric vehicle car tires, comparing its performance against single-end modified counterparts through a detailed experimental investigation.
The proliferation of electric vehicle car models has fundamentally altered tire design paradigms. Unlike internal combustion engine vehicles, electric vehicle car systems are characterized by higher instantaneous torque, greater overall mass due to battery packs, and a paramount emphasis on energy efficiency. Consequently, tires for electric vehicle car applications must exhibit exceptionally low hysteresis losses to minimize rolling resistance and maximize driving range, while also providing robust mechanical integrity to withstand accelerated wear from rapid starts and stops. Additionally, the safety imperative necessitates outstanding anti-skid performance on both dry and wet surfaces, as electric vehicle car platforms often require shorter braking distances. My research is motivated by the need to develop next-generation tread compounds that can satisfy these multifarious demands, leveraging advanced polymer architectures to achieve synergistic enhancements.
SSBR, synthesized via anionic polymerization, offers a versatile backbone that can be tailored through variations in styrene content, vinyl content, and molecular architecture. The incorporation of functional groups at the polymer chain ends has proven particularly effective in enhancing filler-polymer interactions, especially with silica, which is essential for modern high-performance tire formulations. Single-end modification, typically involving the attachment of a silane or amine group, improves silica dispersion and reduces the Payne effect. However, double-end modification, where both termini of the polymer chain are functionalized, coupled with a star-shaped topology, promises even greater benefits. The star-shaped architecture reduces the number of free chain ends, which are known to contribute to hysteresis, while dual functionalization creates multiple anchor points for chemical bonding with silica surfaces. This study aims to systematically evaluate how this advanced double-end modified star-shaped SSBR influences the processing, mechanical, and dynamic viscoelastic properties of tread compounds designed for electric vehicle car tires, providing a foundation for optimized formulation strategies.
To contextualize the material’s behavior, it is essential to consider the fundamental viscoelastic principles governing tire performance. The key dynamic mechanical properties are often represented by the loss factor, tan δ, which is the ratio of the loss modulus (G”) to the storage modulus (G’):
$$ \tan \delta = \frac{G”}{G’} $$
For electric vehicle car tire treads, a high tan δ at around 0°C correlates with improved wet-skid resistance, as it indicates greater energy dissipation on cold, wet surfaces. Conversely, a low tan δ at elevated temperatures (e.g., 60°C) signifies lower hysteresis and thus reduced rolling resistance, a critical factor for the energy efficiency of an electric vehicle car. The glass transition temperature (Tg) of the polymer blend also plays a crucial role, as it influences the temperature dependence of these properties. The Payne effect, a measure of filler-filler network breakdown under strain, is quantified by the difference in storage modulus at low and high strain amplitudes:
$$ \Delta G’ = G’_{1\%} – G’_{100\%} $$
A lower ΔG’ indicates better filler dispersion and weaker filler networks, which typically translates to lower hysteresis and better overall performance. The double-end modified star-shaped SSBR is hypothesized to significantly reduce ΔG’ by promoting stronger polymer-filler bonds and limiting free chain end mobility.
| Component | Compound A (Single-End Modified SSBR) | Compound B (Double-End Modified Star-Shaped SSBR) |
|---|---|---|
| SSBR Type A (Single-end modified, 37.5 phr oil extended) | 68.8 | 0 |
| SSBR Type B (Double-end modified star-shaped, 20 phr oil extended) | 0 | 60.0 |
| SSBR Type C (High vinyl, 5 phr oil extended) | 26.25 | 26.25 |
| SSBR Type D (Low Tg, non-oil extended) | 25.0 | 25.0 |
| Environmental-Friendly Process Oil | 1.3 | 10.0 |
| Highly Dispersible Silica | 80.0 | 80.0 |
| Silane Coupling Agent (Si75) | 6.4 | 6.4 |
| Carbon Black N234 | 5.0 | 5.0 |
| Zinc Oxide | 3.0 | 3.0 |
| Stearic Acid | 1.5 | 1.5 |
| Antioxidant System | 2.5 | 2.5 |
| Paraffinic Wax | 1.5 | 1.5 |
| Sulfur and Accelerators | 7.0 | 7.0 |
The materials employed in this investigation were selected to reflect state-of-the-art formulations for electric vehicle car tire treads. Two primary SSBR variants were compared: a commercial single-end modified SSBR (denoted as Type A) and a novel double-end modified star-shaped SSBR (Type B). Their fundamental characteristics, as determined through Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and gel permeation chromatography (GPC), are summarized below. The other polymer components, Type C and Type D SSBR, were included to adjust the overall glass transition temperature and balance processability with performance, a common practice in designing compounds for electric vehicle car applications. The filler system was dominated by highly dispersible silica, crucial for achieving low rolling resistance, complemented by a small amount of carbon black for reinforcement and conductivity. The silane coupling agent was essential for forming covalent bonds between silica and the polymer matrix.
| Property | SSBR Type A (Single-End Modified) | SSBR Type B (Double-End Modified Star-Shaped) |
|---|---|---|
| Mooney Viscosity [ML(1+4) @ 100°C] | 85 | 91 |
| Bound Styrene Content (%) | 36.0 | 36.9 |
| Vinyl Content (1,2-structure) (%) | 26.0 | 25.3 |
| Glass Transition Temperature, Tg (°C) | -32 | -29 |
| Weight-Average Molecular Weight, Mw (×104 g/mol) | 139.4 | 78.0 |
| Number-Average Molecular Weight, Mn (×104 g/mol) | 66.9 | 43.4 |
| Polydispersity Index (PDI = Mw/Mn) | 2.08 | 1.80 |
| Volatile Matter (%) | 0.05 | 0.07 |
| Ash Content (%) | 0.04 | 0.04 |
The experimental work was conducted using standard laboratory equipment for rubber compounding and testing. Mixing was performed in an internal mixer following a three-stage process to ensure optimal silica silanization and dispersion, a critical step for achieving the desired performance in electric vehicle car tire treads. The first stage involved mastication of the rubbers and incorporation of two-thirds of the silica and coupling agent. The second stage added the remaining silica, coupling agent, and other ingredients except curatives. The final stage introduced the sulfur and accelerators at a lower temperature to prevent premature scorch. Vulcanization was carried out at 161°C for 20 minutes to achieve optimum cure. Characterization included Mooney viscosity, oscillating disk rheometry, rubber process analyzer (RPA) strain sweeps, dynamic mechanical analysis (DMA), tensile testing, hardness, rebound resilience, DIN abrasion, and dry/wet friction coefficient measurements on asphalt simulants. This comprehensive test matrix allows for a holistic evaluation of the compounds’ suitability for the demanding service conditions of an electric vehicle car.
The physicochemical analysis reveals distinct differences between the two primary SSBRs. While both polymers have similar styrene and vinyl contents—key determinants of Tg and traction properties—the double-end modified star-shaped SSBR (Type B) exhibits a slightly higher Mooney viscosity and a notably narrower molecular weight distribution (PDI of 1.80 vs. 2.08). The higher Mooney viscosity of Type B, despite its lower molecular weight (Mw of 780,000 vs. 1,394,000 for Type A), suggests significant chain branching or star-shaped architecture leading to greater chain entanglements. The narrower PDI indicates a more uniform polymer structure, which can contribute to more consistent processing and property profiles. The Tg of Type B is approximately 3°C higher than that of Type A, which can be attributed to the restricted chain mobility imposed by the dual functional groups and star topology. This fundamental difference in polymer architecture sets the stage for the observed variations in compound behavior, particularly relevant for the dynamic performance of an electric vehicle car tire.
The processing characteristics of the uncured compounds are critical for manufacturing efficiency. The Mooney viscosity and scorch safety data are presented below. The compound containing the double-end modified star-shaped SSBR (Compound B) showed a lower Mooney viscosity compared to the single-end modified SSBR compound (Compound A), despite the higher raw polymer viscosity of Type B. This counterintuitive result can be explained by the enhanced interaction between the functionalized chain ends of Type B and the silica surface. The strong polymer-filler bonding effectively reduces the hydrodynamic volume of the polymer chains in the compound, leading to lower overall viscosity and improved processability—a valuable asset in tire production lines. The scorch time (t5) was slightly longer for Compound B, indicating better processing safety, which is advantageous for complex tread extrusion operations in electric vehicle car tire manufacturing.
| Parameter | Compound A (Single-End Modified SSBR) | Compound B (Double-End Modified Star-Shaped SSBR) |
|---|---|---|
| Mooney Viscosity [ML(1+4) @ 100°C] | 120 | 116 |
| Mooney Scorch Time, t5 @ 127°C (min) | 19.2 | 21.2 |
| Rheometer Data @ 161°C | ||
| Minimum Torque, FL (dN·m) | 3.29 | 2.76 |
| Maximum Torque, Fmax (dN·m) | 18.47 | 17.41 |
| Torque Rise (Fmax – FL) (dN·m) | 15.18 | 14.65 |
| t30 (min) | 4.04 | 4.12 |
| t60 (min) | 5.20 | 5.34 |
| t95 (min) | 14.04 | 14.34 |
The curing kinetics, as monitored by the moving die rheometer, show comparable cure rates for both compounds, with only minor differences in cure times. The slightly lower maximum torque (Fmax) and torque rise (Fmax – FL) for Compound B suggest a marginally lower crosslink density. This could be related to the lower molecular weight of the Type B SSBR, which might result in a slightly lower number of effective network chains per unit volume. However, as will be shown, this does not detrimentally affect the mechanical properties; rather, it may contribute to a favorable balance between stiffness and hysteresis. The Payne effect, a direct indicator of filler network strength and dispersion, was evaluated via RPA strain sweeps at 60°C. The results, summarized in Table 4, demonstrate a significant reduction in ΔG’ for the compound containing the double-end modified star-shaped SSBR. This reduction, approximately 9%, unequivocally confirms superior silica dispersion and weaker filler-filler interactions. The enhanced polymer-filler coupling via the dual functional ends effectively restricts the formation of a percolating silica network, which is a primary source of energy loss in rolling tires. For an electric vehicle car, where minimizing every source of energy dissipation is paramount, this improvement directly contributes to lower rolling resistance.
| Parameter | Compound A (Single-End Modified SSBR) | Compound B (Double-End Modified Star-Shaped SSBR) |
|---|---|---|
| Storage Modulus at 1% Strain, G’1% (kPa) | 1211.4 | 1131.0 |
| Storage Modulus at 100% Strain, G’100% (kPa) | 209.7 | 219.5 |
| ΔG’ = G’1% – G’100% (kPa) | 1001.7 | 911.5 |
The extrusion behavior, assessed using a Garvey die, revealed that Compound B exhibited slightly smoother edges and less tearing compared to Compound A. This improved extrudate quality is consistent with the better silica dispersion and lower Payne effect, as a well-dispersed filler system promotes more homogeneous flow and reduces elastic turbulence during extrusion. This processing advantage is beneficial for manufacturing precision tread patterns, which are essential for optimizing noise, wear, and hydroplaning resistance in electric vehicle car tires.
The static mechanical properties of the vulcanizates provide insight into their structural integrity and potential durability. The data, encompassing hardness, modulus, tensile strength, elongation, resilience, and abrasion resistance, are compiled in Table 5. Compound B, based on the double-end modified star-shaped SSBR, displayed lower hardness and 300% modulus compared to Compound A. This is consistent with the rheometer data indicating a slightly lower crosslink density. However, the tensile strength and elongation at break remained essentially equivalent, demonstrating that the modified polymer architecture effectively compensates for the crosslink density difference through enhanced filler reinforcement. The most notable improvement is in abrasion resistance: the DIN abrasion volume loss decreased from 102 mm³ to 98 mm³, corresponding to an increase in the abrasion index from 146% to 152%. This enhancement in wear resistance is critically important for electric vehicle car tires, which are subjected to higher shear forces due to the instant torque delivery of electric motors and the greater vehicle mass. The improved filler dispersion and stronger polymer-filler bonds in Compound B distribute stress more uniformly and reduce localized wear mechanisms. Furthermore, after thermal aging at 100°C for 72 hours, both compounds maintained good property retention, with Compound B showing slightly better retention of elongation, indicating robust network stability under severe conditions that might be encountered in an electric vehicle car.
| Property | Compound A (Single-End Modified SSBR) | Compound B (Double-End Modified Star-Shaped SSBR) |
|---|---|---|
| Shore A Hardness (points) | 72 | 68 |
| 300% Modulus (MPa) | 17.2 | 16.3 |
| Tensile Strength (MPa) | 22.0 | 21.4 |
| Elongation at Break (%) | 366 | 371 |
| Rebound Resilience @ 60°C (%) | 57 | 58 |
| DIN Abrasion Volume Loss (mm³) | 102 | 98 |
| Abrasion Index (%) | 146 | 152 |
| Tear Strength (kN/m) | 40 | 40 |
| After Aging (100°C × 72 hours) | ||
| Shore A Hardness (points) | 76 | 73 |
| 100% Modulus (MPa) | 5.2 | 4.6 |
| Tensile Strength (MPa) | 21.5 | 21.4 |
| Elongation at Break (%) | 288 | 297 |
| Rebound Resilience @ 60°C (%) | 59 | 61 |
| Tear Strength (kN/m) | 41 | 40 |
The dynamic mechanical analysis (DMA) provides the most direct correlation with key tire performance indicators: rolling resistance, wet grip, and handling. The temperature sweep data from -60°C to 80°C at 10 Hz are summarized in Table 6 and depicted graphically. The tan δ values at specific temperatures are of paramount importance. For the electric vehicle car tire application, the tan δ at 0°C is a proxy for wet-skid resistance, while tan δ at 60°C correlates with rolling resistance. Compound B exhibited a substantial 22% increase in tan δ at 0°C (0.409 vs. 0.334), indicating a significant enhancement in wet traction potential. This improvement can be attributed to the higher overall Tg of the compound (approximately -14.7°C vs. -18.7°C for Compound A) and the more effective energy dissipation mechanism provided by the constrained chain ends interacting with silica. The restricted molecular motion at the chain termini allows for greater viscoelastic damping at lower temperatures, which is beneficial for grip on wet surfaces—a critical safety feature for any electric vehicle car.
Conversely, at the service temperature of 60°C, Compound B showed a 14% reduction in tan δ (0.091 vs. 0.106). This decrease signifies lower hysteresis and, consequently, lower rolling resistance. The dual functionalization and star-shaped structure effectively “anchor” the polymer chains to the filler, reducing the amount of free volume and the mobility of chain segments that contribute to energy loss under cyclic deformation. This leads to less heat build-up during rolling, directly translating to improved energy efficiency and extended driving range for the electric vehicle car. The peak tan δ value (tan δmax) was also higher for Compound B, which is often associated with a more homogeneous material and better filler dispersion, corroborating the RPA findings. The overall DMA profile demonstrates that the double-end modified star-shaped SSBR successfully decouples the traditionally conflicting requirements of high wet grip and low rolling resistance—a breakthrough for electric vehicle car tire technology.
| Property | Compound A (Single-End Modified SSBR) | Compound B (Double-End Modified Star-Shaped SSBR) |
|---|---|---|
| Glass Transition Temperature, Tg (°C)* | -18.7 | -14.7 |
| tan δ @ 0°C | 0.334 | 0.409 |
| tan δ @ 25°C | 0.158 | 0.155 |
| tan δ @ 60°C | 0.106 | 0.091 |
| Maximum tan δ (tan δmax) | 0.686 | 0.747 |
*Tg determined from the peak of the tan δ curve.
The practical implications of these dynamic properties were further validated through dry and wet friction coefficient measurements on an asphalt simulator. The results, presented in Table 7, show that Compound B achieved marginally higher friction coefficients on both dry and wet surfaces compared to Compound A. This empirical confirmation aligns perfectly with the DMA predictions, underscoring the compound’s superior braking performance. For an electric vehicle car, which may have regenerative braking systems but still relies on mechanical brakes for ultimate safety, a high-friction tread compound is indispensable for achieving short stopping distances under all conditions.
| Test Condition | Compound A (Single-End Modified SSBR) | Compound B (Double-End Modified Star-Shaped SSBR) |
|---|---|---|
| Dry Friction Coefficient (slip speed 1 m/s) | 0.858 | 0.875 |
| Wet Friction Coefficient (slip speed 1 m/s) | 0.906 | 0.919 |
The superior performance of the double-end modified star-shaped SSBR can be rationalized through a molecular and microstructural lens. The star-shaped architecture, typically with three or more arms radiating from a central core, inherently reduces the number of free chain ends per molecule. In linear polymers, these free ends act as dangling chains that contribute disproportionately to hysteresis losses. By tying these ends into a branched structure, their mobility is restricted. Furthermore, the chemical modification of both termini of each arm with functional groups (e.g., silane or tin-based moieties) creates multiple active sites for covalent bonding with the silanol groups on the silica surface. This results in a strongly bound, crosslinked polymer-filler interface. The effectiveness of this interfacial bonding can be conceptually described by an enhancement factor, ξ, which modifies the classical Guth-Gold equation for the modulus of filled rubber:
$$ G’_{\text{compound}} = G’_{\text{matrix}} (1 + 2.5 \phi_f + 14.1 \phi_f^2) \cdot \xi $$
where φf is the filler volume fraction, and ξ ≥ 1 represents the additional reinforcement due to strong polymer-filler interactions. For the double-end modified system, ξ is significantly higher than for the single-end modified system, leading to better stress transfer, higher effective crosslink density from filler bonds, and reduced filler network formation. This microstructural control is the key to unlocking the performance triad needed for electric vehicle car tires. Additionally, the narrower molecular weight distribution of the modified polymer promotes more uniform stress distribution within the elastomer matrix, further enhancing durability—a vital consideration given the higher loads and torques typical of an electric vehicle car.
From a formulation perspective, the use of this advanced SSBR also allows for optimization of other compound ingredients. For instance, the oil extension level can be adjusted, as seen in the formulations where Compound B required a higher dosage of process oil (10 phr vs. 1.3 phr) to achieve comparable processing viscosity. This oil acts as a plasticizer but is effectively immobilized by the polymer-filler network, minimizing its negative impact on hysteresis. The balance of other SSBR types (C and D) in the blend is crucial to fine-tune the overall Tg window, ensuring that the high tan δ at 0°C does not come at the expense of winter performance or ride comfort. The synergy between the double-end modified star-shaped SSBR and highly dispersible silica, mediated by an efficient silane, creates a nanocomposite-like structure that delivers exceptional performance. This formulation strategy is particularly suitable for the next generation of electric vehicle car tires, which may also incorporate sustainable materials; the enhanced efficiency could potentially allow for reduced material usage, contributing to the lightweighting goals of electric vehicle car design.
In conclusion, this in-depth investigation demonstrates that double-end modified star-shaped SSBR represents a significant advancement over single-end modified SSBR for use in tread compounds of electric vehicle car tires. The unique polymer architecture—characterized by a star-shaped topology with functional groups at both termini of each arm—confers a suite of benefits that directly address the core challenges of electric vehicle car tire development. These benefits include improved silica dispersion (evidenced by a 9% reduction in the Payne effect ΔG’), enhanced processability with lower compound viscosity and better extrusion characteristics, superior abrasion resistance (approximately 4% improvement in DIN abrasion index), and a breakthrough dynamic mechanical profile. Specifically, the compound exhibits a 22% increase in tan δ at 0°C for better wet-skid safety and a 14% decrease in tan δ at 60°C for lower rolling resistance. These improvements are empirically validated by higher dry and wet friction coefficients. The molecular design effectively mitigates the classic trade-off between traction and rolling resistance, while simultaneously boosting wear resistance to handle the demands of electric vehicle car platforms. Therefore, the adoption of double-end modified star-shaped SSBR is a highly promising strategy for formulating high-performance, energy-efficient, and durable tread compounds that can meet and exceed the expectations of the evolving electric vehicle car market. Future work could explore the long-term durability under extreme load cycles, the compatibility with other sustainable fillers, and the performance in full-scale tire testing to fully validate these laboratory findings for real-world electric vehicle car applications.