Si-69 (Bis-TESPT)

What is Si-69?

Si-69, designated chemically as Bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT), is a bifunctional polysulfide silane coupling agent with CAS number 40372-72-3 and molecular formula C₁₈H₄₂O₆S₄Si₂. It is the most important silane coupling agent in the tire rubber industry and the key enabling chemical for green tire technology.

The molecule is symmetric: two triethoxysilylpropyl groups are connected through a central tetrasulfide chain (–S₄–). The silicon ends anchor to the silica filler surface through Si–O–Si bonds; the polysulfide center participates in sulfur vulcanization crosslinking of the rubber matrix. This bifunctional architecture is what makes Si-69 fundamentally different from other silane coupling agents — it does not merely bridge substrate to polymer at an interface; it is simultaneously a silica coupling agent and a sulfur vulcanization crosslinker built into a single molecule.

Si-69 is equivalent to Degussa/Evonik's TESPT product (the original developer), as well as Momentive's NXT silane series and Chinese grades marketed as TESPT or Si-69. CAS 40372-72-3 is the definitive identifier. Commercial Si-69 is actually a mixture of polysulfide chain lengths (S₂ through S₆) with tetrasulfide (S₄) predominating at approximately 70%, which is why the "purity" specification for Si-69 is typically ≥90% polysulfide mixture rather than a single-compound purity.

Key Properties and Performance

Si-69 is a yellow to amber viscous liquid with sulfur content of 22–24 wt%, reflecting the high proportion of sulfur in the tetrasulfide chain. Flash point is above 150 °C, making it a combustible liquid with better thermal stability than most silanes. Density at 25 °C is approximately 1.079 g/cm³. The characteristic odor is sulfurous, and the yellow-amber color is normal — it deepens slightly on storage but does not affect performance.

The key performance parameter in tire compounding is the Payne effect, which measures the strain-dependent stiffness of the filler network in uncured rubber. A high Payne effect indicates poor filler dispersion and strong silica-silica aggregation, which increases hysteresis (rolling resistance) in the cured tire. Si-69 reduces the Payne effect by covering the hydrophilic silica surface with organophilic silane, disrupting silica-silica hydrogen bonding and replacing it with silica-silane-rubber covalent connectivity.

In a well-optimized green tire formulation, Si-69 at 5–10 wt% on silica (typically 6.5–7.5 wt% for a 80 phr silica loading in SBR) achieves approximately 80–90% silica surface coverage, reduces Mooney viscosity by 15–25 MU versus untreated silica, and delivers rolling resistance measured by tan δ at 60 °C that is 25–40% lower than an equivalent carbon black formulation, while maintaining or improving wet traction (tan δ at 0 °C).

Applications in Industry

Green Tire Tread Compounding

Si-69 is the dominant silane in passenger car and light truck tire treads using silica reinforcement. The shift from carbon black to precipitated silica + Si-69 formulations in high-performance tire treads, which began commercially in the early 1990s with the Michelin Aquarelle concept, has expanded to the point where virtually all premium tire treads globally now use silica/Si-69 systems.

The standard process for silica/Si-69 tread compounding involves a two-stage mixing sequence. In the first stage (non-productive, no curatives), silica and Si-69 are added to the rubber at high temperature (140–160 °C dump temperature). The ethoxysilane groups on Si-69 react with silica surface silanols, releasing ethanol and bonding Si-69 to the silica. This silanization reaction requires time and temperature (2–4 minutes above 140 °C) and is incomplete in a single pass. A second or third re-mill at the same temperature improves silanization completeness. In the final mixing stage (productive), sulfur, accelerators, and other curatives are added at lower temperature (below 110 °C) to avoid premature crosslinking.

The silanization efficiency — the fraction of Si-69 that has reacted with silica rather than undergoing self-condensation or vaporizing — directly determines compound performance. At 150 °C dump temperature, silanization efficiency with Si-69 is approximately 70–80%. The remaining 20–30% contributes to compound crosslinking via the sulfur released from unreacted polysulfide groups.

High-Performance Truck and Commercial Vehicle Tires

In truck tire tread compounds designed for low rolling resistance (fuel economy improvement), Si-69 is used at slightly higher loadings (7–9 wt% on silica) because truck tire compounds typically use higher silica loadings (90–100 phr) than passenger car compounds. The fuel efficiency improvement from silica/Si-69 treads in long-haul truck tires (reduction in tire rolling resistance coefficient of 0.003–0.005) translates to 2–4% fuel saving over a 500,000 km tire life, with significant commercial value.

Silica-Reinforced Industrial Rubber Goods

Si-69 is used in industrial rubber goods that require white-filler reinforcement for technical or regulatory reasons. Conveyor belts for food contact applications cannot use carbon black but require high reinforcement — silica/Si-69 systems provide the mechanical performance. Hydraulic seals and O-rings in clean environments (medical, food, semiconductor) use silica-reinforced nitrile or EPDM compounds with Si-69 to achieve the required strength and compression set resistance.

Handling, Dosage, and Storage

Si-69 is added to rubber compounds during the non-productive mixing stage along with silica. Typical dosage:

• Passenger car tire tread: 5–8 wt% on silica weight (commonly 6.5–7.5 wt% for 80 phr silica loading)
• Truck tire tread: 7–9 wt% on silica weight
• Industrial rubber: 4–6 wt% on silica weight

Si-69 can be added directly as a liquid to the mixer through a gravimetric dosing system, or pre-blended with a portion of carbon black to improve handling (Si-69 is absorbed on carbon black to produce a free-flowing powder).

The silanization reaction in the mixer releases ethanol vapor. Mixer ventilation systems must handle ethanol concentrations from this source — a standard consideration in tire plant design.

Storage: cool, dry location below 30 °C, away from moisture and heat. Shelf life: 12 months in sealed drums. Once opened, reseal promptly.

Frequently Asked Questions

What is the difference between Si-69 and Si-75? Si-69 (TESPT) has a tetrasulfide bridge (–S₄–), while Si-75 (TESPD) has a disulfide bridge (–S₂–). Si-69 provides more sulfur for vulcanization crosslinking and is the historical standard for green tire compounding. Si-75 releases sulfur more slowly, giving better scorch safety at high dump temperatures (above 155 °C). Modern high-speed tire mixing often uses elevated dump temperatures where Si-75's slower sulfur release is preferred to reduce scorch rejects.

Can Si-69 be used with natural rubber? Yes, but natural rubber tire compounds historically use carbon black rather than silica, so Si-69/silica is less common in NR-based compounds than in SBR/BR passenger car tire treads. For specialty NR applications requiring white filler reinforcement, Si-69 works with the standard sulfur vulcanization chemistry used in NR.

Why does Si-69 turn dark on storage? The polysulfide mixture can undergo minor rearrangement reactions on prolonged storage, slightly deepening the amber color. This color change does not affect the sulfur content or coupling efficiency. An APHA color limit is specified in some procurement contracts, but performance-based acceptance (sulfur content, silane content) is more meaningful for rubber compounding applications.

What mixing temperature is required for silanization? Effective silanization of silica with Si-69 requires a dump temperature of 140–160 °C maintained for at least 2–3 minutes. Below 130 °C, silanization is negligible. Above 165 °C with excessive mixing time, precure of the compound can occur via premature polysulfide crosslinking. The optimal window is 145–160 °C dump.