Within a silica-filled passenger tread formulation, the substitution of high cis-1,4 polybutadiene produced by a neodymium carboxylate/alkylaluminium catalyst system modifies the viscoelastic loss spectrum at 0°C far more than conventional cobalt or nickel polybutadienes. The neodymium-catalysed grade is specified with a cis-1,4 content of at least 96 mol%, a vinyl content below 0.8 mol%, a glass transition temperature between -105°C and -110°C when measured by differential scanning calorimetry in accordance with ISO 22768:2017, and a Mooney viscosity ML(1+4) at 100°C of 40–50 MU when tested according to ISO 289-1:2018. The main loss factor peak of this polymer lies near -100°C, so at a wet grip reference temperature of 0°C the polybutadiene phase contributes only low hysteresis. In a tread compound where precipitated silica with a CTAB surface area of 160–180 m²/g is added at 80–95 phr and coupled with bis[3-(triethoxysilyl)propyl] tetrasulfide at 8–10 phr, the polymer-filler network and the silane grafting density determine whether the 0°C loss factor remains above the threshold associated with wet grip class B or C under UNECE Regulation 117.02. Since high cis neodymium polybutadiene is nonpolar and linear, its incorporation reduces silica flocculation only when silanization is completed before the dump temperature reaches 155°C; otherwise the unreacted silanol population promotes filler-filler contacts that raise storage modulus at low strain and obscure the viscoelastic contribution of the polymer. Wet grip development in a silica tread is therefore not controlled by polymer Tg alone but by the competition between polymer phase damping and filler network stiffness under dynamic strain at 0°C and 10 Hz.

How Does High Cis-1,4 Content Modify the Viscoelastic Loss Spectrum at 0°C?

The loss factor at 0°C is commonly extracted from a forced non-resonance dynamic mechanical analysis temperature sweep conducted from -80°C to 80°C at a heating rate of 2°C/min, a frequency of 10 Hz, and a strain amplitude of 0.1% in a shear sandwich or tensile geometry according to ISO 4664-1:2011. For a solution SBR with 25–40% bound styrene and 25–50% vinyl, the main chain relaxation peak is located between -30°C and -10°C, producing a tan δ at 0°C of 0.25–0.45 after silica reinforcement. High cis neodymium polybutadiene exhibits a tan δ maximum at approximately -100°C and a tan δ at 0°C below 0.05, so its addition to a tread compound reduces the composite loss factor through a blend rule that is monotonic at low strains. In a 75/25 SSBR/Nd-BR blend, the 0°C loss factor typically falls by 0.03–0.08 for each 10 phr of neodymium polybutadiene replacement when the SSBR vinyl content is 35–50%. The reduction is sharper for high-vinyl SSBR because the 0°C reference point lies on the shoulder of the SSBR loss peak, where small Tg shifts and peak broadening affect tan δ strongly. The same blend can retain acceptable wet grip if the silica microdispersion improves simultaneously, because the resulting drop in low-strain Payne-effect stiffness allows the deformation to be distributed more uniformly through the polymer matrix. Dynamic mechanical testing on an RPA 2000 at 100°C and 1 Hz, with a strain sweep from 0.1% to 100%, is used to separate filler network breakdown from polymer loss; a compound with optimized high cis Nd-BR and silane coupling typically shows a Payne effect ΔG′ of 250–600 kPa at 80 phr silica, whereas an under-silanized compound can exceed 1000 kPa and exhibit reduced loss factor sensitivity to polymer phase changes. Frequency-temperature equivalence is handled with WLF constants C1 = 17.4 and C2 = 51.6 K relative to the polymer Tg, allowing DMA data at 10 Hz to be compared with high-frequency wet braking deformation.

On a production-scale 270 L Banbury-type internal mixer with four-wing tangential rotors and a fill factor of 0.75, the masterbatch stage for a silica tread containing high cis neodymium polybutadiene is run with an initial rotor speed of 40–50 rpm and a ram pressure of 0.5–0.6 MPa. Silica is split into two or three additions to prevent the batch temperature from exceeding 150°C before the silanization reaction is complete. The second addition of silica is made at 120°C, and the TESPT silane is dosed together with the first silica fraction to ensure sufficient contact with silanol groups during the reactive mixing window of 135–155°C. If the dump temperature exceeds 160°C, the polysulfane sulfur in TESPT can undergo premature cleavage and generate scorch in the presence of zinc oxide and stearic acid, producing a rise in minimum torque of more than 0.5 dN·m in the subsequent MDR cure trace at 160°C. Neodymium polybutadiene tends to generate less shear heating than high-vinyl SSBR because its glass transition is lower, but its narrow molecular weight distribution and high linearity contribute more cold flow after bale packaging, so the polymer is commonly oil-extended or stored at temperatures below 25°C. Batch-to-batch variation in neodymium catalyst residues, particularly aluminum alkyl co-catalysts, can shift the silanization rate by altering the effective moisture content; therefore the silica moisture content is controlled to 4–7% by Karl Fischer titration and the silane is not added after the batch has dropped below 130°C.

When Silane Grafting and Filler Flocculation Compete in the Mixing Chamber

When the silane grafting reaction is conducted between 140°C and 155°C, the ethoxy groups of TESPT hydrolyze with adsorbed water and silanol groups, releasing ethanol and forming a covalent siloxane bridge to the silica surface. Below 135°C, the reaction rate is too slow to suppress filler flocculation, and the resulting silica network produces a high Payne effect. Above 155°C, the tetrasulfane group begins to participate in polymer-filler coupling and premature crosslinking, which reduces scorch safety in the final compound. The high cis-1,4 polybutadiene phase contains less than 1 mol% vinyl unsaturation and therefore does not undergo the same silane-assisted branching that high-vinyl SSBR can experience at elevated dump temperatures. This means the Nd-BR-rich phase remains less polar and less reactive toward the silane sulfur bridge, so the silane is preferentially grafted to the silica surface rather than to the polybutadiene backbone. The resulting morphology is a dispersed silica network with reduced filler-filler contacts, but the wet grip contribution of the polybutadiene phase remains low because the molecular relaxation is far below the wet braking frequency. In compounds where the Nd-BR content is raised above 30 phr of the polymer blend, the 0°C loss factor can fall below 0.20, and wet grip grading under UNECE Regulation 117.02 may shift by one or more classes depending on the other compounding variables. For high wet grip targets, the neodymium polybutadiene content is therefore limited to 15–25 phr unless the SSBR is modified with higher vinyl or higher styrene to compensate.

The cure system interacts with the high cis Nd-BR phase through sulfur solubility and accelerator partitioning. Because sulfur solubility in polybutadiene is typically higher than in SBR at 100°C, the Nd-BR phase can deplete the surrounding matrix of curative, reducing crosslink density near the silica interface. This effect is mitigated by adding insoluble sulfur at 1.5–2.5 phr and by using a higher DPG level to maintain silanol shielding in the silica-filled system. The resulting equilibrium swelling ratio in toluene, measured on vulcanized specimens according to ISO 1817:2015, should remain within the range of 3.0–4.5 for a wet grip tread; higher swelling indicates lower crosslink density and greater tread block deformation under wet braking, while lower swelling indicates excessive stiffness that can reduce the loss factor at 0°C. Vulcanization kinetics are monitored with an MDR at 160°C for 30 min according to ASTM D5289-19a; a typical silica tread containing 20 phr high cis Nd-BR reaches 90% cure in 6–10 min, but the cure curve is no longer monotonic if TESPT was pre-scorched during mixing above 160°C.

Wet Grip Roadway Protocols and Standardised Braking Friction Measurements

Outdoor wet grip evaluation under ISO 23671:2015 and the related UNECE Regulation 117.02 grading procedure quantifies the relative braking performance of a candidate tire against a reference tire on a standardized wet surface. The test vehicle is instrumented with ABS and deceleration data are normalized for surface friction variation by repeated reference runs. The resulting wet grip index is reported as a dimensionless value, with the reference tyre assigned 100. A compound with a tan δ at 0°C of 0.25 may correspond to a wet grip index of 1.25 or higher when the tread pattern and vehicle test conditions are favorable, whereas a compound with a tan δ at 0°C below 0.15 is generally associated with a wet grip index below 1.10. These correlations are sensitive to macrotexture, water depth, and ambient temperature; at 5°C, the polymer viscoelastic contribution increases, and the high cis neodymium polybutadiene phase moves closer to its loss peak, so the wet grip penalty of Nd-BR can be smaller than at 25°C. In winter tyre tread formulations, where low-temperature wet grip is critical, high cis Nd-BR is often limited to 10–20 phr and combined with a high-vinyl SSBR having a Tg of -25°C to -15°C. The use of ASTM E303-93(2018) pendulum slip resistance on laboratory specimens cannot replace whole-tire wet braking data because the pendulum test does not capture tread block deformation, water drainage, or the strain history of the contact patch.

Because high cis neodymium polybutadiene is nonpolar, the selection of process oils and antidegradants in a silica tread formulation affects both the silanization efficiency and the wet grip indicator. Naphthenic or TDAE oils partition preferentially into the polybutadiene phase, lowering its loss peak further and reducing the 0°C tan δ contribution; this effect is more pronounced above 10 phr oil. Aromatic oils, where allowed, would increase the 0°C loss factor but are restricted under REACH Annex XVII for PAH content below 3% by mass. Amine-containing antiozonants such as IPPD can adsorb on the silica surface through acid-base interactions and compete with silane coupling, so their concentration is controlled to less than 2 phr and they are added in the final mixing stage at temperatures below 110°C. Silica storage at relative humidity above 60% increases bound water beyond the 4–7% optimal range, accelerating silane hydrolysis before the silica is wetted by polymer and causing ethanol evolution that cools the batch and leaves uncondensed silanol groups. The resulting batch-to-batch variation in wet grip indicator can reach ±0.02 tan δ units at 0°C, which is sufficient to shift a wet grip class under UNECE Regulation 117.02. To maintain process capability, silica is pre-dried or moisture-adjusted to a Karl Fischer water content of 5–6%, and the silane addition is interlocked with the batch temperature rather than a fixed time.

Incompatibility between high cis neodymium polybutadiene and highly polar processing aids is observed when the total ester softening agent exceeds 5 phr, because phase separation at the silica-polymer interface reduces the effectiveness of the silane coupling layer and lowers low-strain storage modulus. This operational boundary is particularly relevant for compounds that must meet both wet grip class C or higher and rolling resistance class B or higher under Regulation (EC) No 1222/2009 and UNECE Regulation 117.02; the use of high cis Nd-BR above 30 phr may require a reduction in silica loading from 95 phr to 75 phr to avoid excessive compound stiffness at low temperatures. Pre-drying is required at relative humidity above 60%, and the final blend should not be stored for more than 72 hours before extrusion because moisture uptake can shift the tan δ at 0°C by 0.01–0.02. The combination of high cis Nd-BR with amine-based accelerators or aldehyde-amine antioxidants should be avoided above the stated limits due to premature crosslinking and silica adsorption. Published data for this specific configuration is limited, particularly for outdoor wet grip index measurements on tread compounds with high cis Nd-BR contents above 35 phr, so formulation decisions should be confirmed by whole-tire tests under ISO 23671:2015 rather than by DMA indicators alone.