Abstract

EU passenger vehicle CO₂ regulations are driving automotive R&D toward new materials and technologies that improve energy efficiency. A key avenue is reducing tire rolling resistance to improve fuel economy, while maintaining or even enhancing safety and service life. “Green tires” have already significantly cut rolling resistance by using synthetic elastomer blends with silica-silane filler systems. Functionalizing the elastomer can further improve silica-rubber interaction for additional performance gains. This study prepared model tread compounds using functionalized and non-functionalized elastomers with various silica grades and loadings, characterizing processability, cure behavior, and static/dynamic mechanical properties. Results show that elastomer functionalization affects nearly all tread properties: pre-cure filler-rubber interactions increase compound viscosity and hurt processing, but functionalized rubber in model tread formulations lowers rolling resistance (tanδ) without significant mechanical property loss.

Keywords: tire tread compound; functionalized SBR; silica; rolling resistance; wet grip; Payne effect

1. Introduction

1.1 Background

Road transport continues to grow: EU road transport greenhouse gas emissions rose over 30% from 1990–2019, with 70%+ from light commercial vehicles and passenger cars. EU regulations mandate: 55% and 50% CO₂ cuts for new passenger and van vehicles by 2030–2034 vs. 2021, and 100% reduction by 2035. Carmakers are reducing emissions through aerodynamics, engine efficiency, and new powertrains. Lowering tire rolling resistance is another major lever: less power is needed to maintain motion, cutting fuel consumption and heat buildup. For EVs, rolling resistance directly affects range.

Tread compound is the core factor determining rolling resistance. Early treads used emulsion SBR with carbon black — a trade-off among rolling resistance, wear, and grip where improving one worsens others. In the early 1990s, “green tires” appeared: SBR/BR blends with silica-silane filler systems, simultaneously improving rolling resistance and wet grip while maintaining wear — the so-called “magic triangle.”

Current research focuses on rubber functionalization; several commercial functionalized SBR grades exist. The goal is improving the interfacial interaction between rubber and fillers, especially silica in modern passenger tires.

1.2 Objectives

Primary: investigate how functionalized SBR affects tread compound processability, static mechanicals, and dynamic performance. Secondary: study elastomer blends, different silica surface areas, and varying silica loadings.

2. Literature Review

2.1 Brief History of Rubber

The earliest recorded rubber use dates to the late 15th century — Indigenous South Americans collected tree latex for ceremonial balls and waterproofing. In the 18th century, “rubber” and “caoutchouc” entered Western vocabulary. John Hancock’s masticator machine revolutionized processing. In 1839, Charles Goodyear discovered vulcanization — adding sulfur and accelerators under heat dramatically improved tensile strength, wear, and solvent resistance, enabling rubber’s entry into tire manufacturing.

Natural rubber (NR), from Hevea brasiliensis, boomed with bicycles and automobiles in the late 19th–early 20th century. Plantations spread to Sri Lanka and Singapore after 1876; global output grew from ~100,000 tons in 1909 to over 14 million tons by 2021. To address supply gaps, synthetic rubber development began as early as 1906. Butadiene rubber (BR) was among the first successful synthetics; SBR (styrene-butadiene copolymer) followed. Both remain widely used in tread compounds.

2.2 Tread Elastomers

Natural rubber (cis-1,4-polyisoprene) has high molecular weight, low Tg, and strain-induced crystallization — delivering high tensile strength, excellent wear and tear resistance. Off-road tires can contain up to 45% NR by weight. However, NR has poor oil/ozone resistance and weak wet grip, so high-speed passenger treads replace it with synthetics.

Butadiene rubber (BR) — high-cis BR (>90% cis) is preferred for treads: linear chains give strain-induced crystallization, low Tg, excellent wear, resilience, fatigue life, and tear strength. BR’s main weakness is poor wet grip, so it is blended with SBR.

Functionalization has four routes: (1) initiator end-group functionalization; (2) chain-end capping; (3) functional monomers during copolymerization; (4) post-modification of side-chain vinyl groups. Tread SBR typically incorporates polar functional groups to improve affinity with polar silica filler — better dispersion, fewer agglomerates, stronger filler-polymer interaction, lower heat buildup at high temperature (better rolling resistance), and improved wet grip. The trade-off: bulky functional groups restrict chain mobility, raising compound Mooney viscosity and worsening processability.

2.3 Reinforcing Systems

Reinforcing fillers improve both tensile strength and modulus while maintaining elongation. Carbon black and silica are the dominant choices. Silica surfaces have abundant silanol groups; silane coupling agents react with silanol on one end and rubber chains during vulcanization on the other, building a silica-silane-rubber bridge. After curing, covalent bonds create strong interface coupling. Silica-silane systems vs. comparable carbon black: higher modulus, better tear/wear, and improved dynamic hysteresis — enabling near-carbon-black wear with better wet grip, lower rolling resistance, and less heat buildup.

2.4 Viscoelasticity

Rubber is viscoelastic — both elastic and viscous. DMA applies sinusoidal strain and measures stress response. Storage modulus (G’/E’) is the elastic component; loss modulus (G”/E”) is the viscous component; tanδ = G”/G’ characterizes energy loss. Temperature sweeps from -80°C to 80°C produce the tanδ-temperature curve: tanδ at 0°C indicates wet grip; tanδ at 60°C indicates rolling resistance; the peak corresponds to Tg.

2.5 Tire Performance

EU tire labeling (mandatory since 2012, updated 2021) covers rolling resistance, wet grip, and noise, with new snow/extreme climate markings. Tire noise comes primarily from the “air pumping effect” — air squeezed out of tread grooves on contact and released on separation. Rolling resistance is the second largest energy loss in ICE vehicles after drivetrain; DMA tanδ at 60°C is the lab proxy. Wet grip depends on both compound and groove water evacuation; tanδ at 0°C evaluates intrinsic compound grip.

3. Experimental

3.1 Materials

Three high-dispersion silica grades with increasing surface area were used: Solvay Zeosil 1165MP (bead form), Evonik Ultrasil 7000GR and Ultrasil 9100GR (granulated form). Compounds followed classic green-tire formulations with silica-silane systems. Four series were prepared:

• S1: Two pure SBR grades compared (non-functionalized vs. end-functionalized)
• S2: SBR blended with two BR grades at 80/20
• S3: Varying SBR/BR blend ratios (100/0, 80/20, 60/40)
• S4: Different silica surface area grades

3.2 Mixing and Curing

A Brabender Plastograph 350E internal mixer was used, followed by a Polymix 80T two-roll mill. Three mixing stages: masterbatch (rubber + silica + additives), remill (accelerators), and final (sulfur). Fill factor 66%; initial chamber temperature 50°C; 50 rpm. Cure was on a Wickert WLP 1600 press at 100 bar, 160°C. Because silica compounds show marching modulus from flocculation, all cures were completed within 16–20 hours after mixing. Fixed cure time of 30 min (35 min for thick cylindrical specimens).

3.3 Characterization

• Mooney viscosity: Alpha Mooney 2000, ISO 289, ML(1+4) at 100°C
• Payne effect: TA-RPA Elite, 60°C, 1.667 Hz, strain sweep 0.075%–100%; ΔG’ between low and high strain evaluates filler network
• Cure characteristics: RPA Elite, ISO 6502, 0.5° strain, 1.67 Hz, 160°C, 30 min
• Hardness: Zwick Shore A, ASTM D2240
• Tensile: Zwick Z01, ISO 37, dumbbell specimens, 500 mm/min
• Rebound: Zwick 5109, ISO 4662
• DMA: Metravib Visco-analyser 2000, ISO 4664, -80°C to +80°C, 2°C/min, 10 Hz, tension mode

4. Results and Discussion

4.1 Series 1: Pure SBR — Functionalized vs. Non-functionalized

SBR 4601 = non-functionalized; SBR 4602 = end-functionalized. At equal silica loading, functionalized SBR 4602 showed higher mixing torque and discharge temperature due to pre-cure silica-rubber interaction. Mooney viscosity was significantly higher for 4602, and the gap widened with silica loading. Payne effect ΔG’ was also higher for 4602, indicating stronger filler-rubber interaction and network.

Minimum torque (S’min) was higher for 4602; scorch time (ts2) slightly shorter. Hardness increased nearly linearly with silica loading, with minimal difference between SBR grades. Tensile strength and elongation at break were comparable. Rebound decreased with silica loading, with little difference between SBR grades.

At 60 phr silica: Tg was similar; 60°C tanδ was lower for 4602 (better rolling resistance); 0°C tanδ was comparable (similar wet grip). The mechanism: end-group functionalization strengthens silica-rubber bonding, reducing free chain ends and hysteresis loss.

4.2 Series 2: SBR/BR 80/20 — BR Grade Comparison

The viscosity/torque difference between SBR 4601 and 4602 was partially mitigated by adding 20 phr BR. BR grade had minor effects; the dominant variable remained SBR functionalization. Mixing temperature was slightly higher with CB-24 vs. Cis-132 (likely day-to-day variation). Mooney and Payne effect followed the same SBR-functionalization trend.

SBR 4602 showed higher S’min; replacing with CB-24 extended scorch time except at 90 phr. Hardness was governed by silica loading. Tensile and rebound were similar between BR grades. Adding BR shifted Tg lower; 0°C tanδ dropped slightly (some wet grip loss); 60°C tanδ was slightly lower.

4.3 Series 3: SBR 4602/Cis-132 — Blend Ratio (100/0, 80/20, 60/40)

Higher BR content improved processing slightly. Mooney viscosity decreased with BR content. Payne effect ΔG’ decreased as BR increased — weaker filler network. Both S’min and S’max decreased with BR content.

Hardness was nearly independent of blend ratio, governed by silica loading. Higher SBR content raised M100 and M300 modulus; increasing BR lowered modulus and raised elongation. Rebound improved significantly with BR (Cis-132 has much higher resilience than SBR 4602). DMA: higher BR shifted Tg lower; 0°C tanδ dropped noticeably (worse wet grip); 60°C tanδ rose slightly (slightly worse rolling resistance).

Trade-off: More BR improves wear but sacrifices wet grip and slightly worsens rolling resistance.

4.4 Series 4: Silica Surface Area (80/20 blend)

Z1165MP = low surface area; U7000GR = medium; U9100GR = high. Higher surface area silica increased mixing torque, discharge temperature, and Mooney viscosity. U9100GR with functionalized SBR at ≥75 phr was very difficult to process. Z1165MP had the lowest processing load.

Payne effect ΔG’ increased with surface area — stronger filler network. U9100GR showed dramatically higher ΔG’ than the other two. Functionalized SBR 4602 consistently showed higher ΔG’ than 4601.

Higher surface area gave higher modulus at equal loading. Rebound was higher with lower surface area silica (less filler-filler interaction, less hysteresis). DMA: near Tg, U9100GR showed lower tanδ due to more bound rubber and constrained polymer layers. At 0°C, Z1165MP had the highest tanδ (best wet grip); at 60°C, Z1165MP had the lowest tanδ (best rolling resistance). Higher surface area worsened both wet grip and rolling resistance but offered wear potential.

5. Conclusions

5.1 Summary

1. Processing: Functionalized elastomers worsen processability — pre-cure silica-rubber interaction raises viscosity. SBR/BR blending can offset this. Do not pair high-surface-area silica with functionalized SBR — viscosity spikes make mixing extremely difficult. Low-surface-area silica allows higher loading with acceptable processing.
2. Cure: Functionalization has only minor effects on cure characteristics. Silica loading and surface area significantly affect cure — above the percolation threshold, torque is governed by filler network; surface area changes crosslink density.
3. Mechanicals: Functionalization has no significant negative impact on static properties. Higher BR content reduces some moduli. Changing silica surface area alters mechanics via crosslink density.
4. Dynamic performance: Functionalized SBR in green tread formulations modestly improves both rolling resistance and wet grip. More BR improves wear but hurts wet grip and slightly worsens rolling resistance. Higher surface area silica improves wear potential but hurts processing and rolling resistance. Low-surface-area silica gives better rolling resistance at some wear cost.

5.2 Recommendations

1. Processing: When using functionalized SBR, pair with low-to-medium surface area silica; at high loadings, must use SBR-BR blends to offset viscosity; avoid high-surface-area silica directly with end-functionalized SBR.
2. Curing: Strictly control the time between mixing and curing for silica compounds to suppress marching modulus from flocculation; re-optimize cure conditions for each silica grade.
3. Mechanicals: SBR/BR blend ratio is the key lever for balancing wear vs. wet grip; higher BR improves wear but lowers modulus and grip — optimize through small-scale trials.
4. Dynamics: To balance rolling resistance and wet grip, functionalized SBR with medium-surface-area silica is a promising combination; high-surface-area silica suits applications prioritizing wear over rolling resistance.