Si-75 (Bis-TESPD)

What is Si-75?

Si-75, designated chemically as Bis[3-(triethoxysilyl)propyl] disulfide (TESPD), is a bifunctional polysulfide silane coupling agent with CAS number 211519-85-6 and molecular formula C₁₈H₄₂O₆S₂Si₂. Si-75 is the disulfide analog of Si-69 (TESPT): it has the same overall structure — two triethoxysilylpropyl groups flanking a central sulfur chain — but the central chain contains only two sulfur atoms (disulfide, –S₂–) instead of four (tetrasulfide, –S₄– in Si-69).

This single structural difference — two sulfur atoms instead of four — has significant practical consequences for rubber compound processing. The disulfide bridge in Si-75 is more thermally stable than the tetrasulfide in Si-69. It releases sulfur more slowly and at higher temperature. This means that during high-temperature mixing cycles in internal rubber mixers (dump temperatures of 155–170 °C), Si-75 produces fewer premature crosslinks in the compound than Si-69, reducing scorch incidents and processing rejects.

The tradeoff is that Si-75 provides less available sulfur per molecule for vulcanization crosslinking. To achieve equivalent crosslink density compared with a Si-69-based compound, Si-75 formulations require either a higher Si-75 loading or supplemental sulfur from the curative system. In practice, tire compound formulators add a calculated sulfur supplement (typically 0.3–0.5 phr extra sulfur) to a Si-75 compound compared with the Si-69 reference formulation.

Si-75 is marketed internationally by Evonik (as TESPD-grade products), Momentive, Struktol, and various Chinese suppliers under the Si-75 or TESPD designation. CAS 211519-85-6 is the definitive identifier.

Key Properties and Performance

Si-75 is a yellow to amber liquid, visually similar to Si-69 but typically slightly less viscous due to the shorter polysulfide chain. Sulfur content is 13–14 wt% (compared with 22–24 wt% for Si-69, reflecting the two versus four sulfur atoms). Flash point exceeds 150 °C. Density at 25 °C is approximately 1.050 g/cm³.

Commercial Si-75 is a polysulfide mixture (predominantly disulfide with some monosulfide and trisulfide) similar to how Si-69 is predominantly tetrasulfide with other chain lengths present. The "≥90% polysulfide mixture" purity specification applies to both.

The critical performance comparison between Si-75 and Si-69 is their behavior during high-temperature mixing:

The silanization efficiency — how well the silane bonds to the silica surface — is similar for Si-75 and Si-69 at equivalent temperatures and mixing conditions. The key difference is exclusively in the polysulfide center's behavior toward premature crosslinking.

Applications in Industry

Passenger Car Tire Treads at Elevated Dump Temperature

Si-75 is increasingly specified in passenger car tire tread compounds where the tire manufacturer's internal mixer operates at high productivity settings that result in dump temperatures above 155 °C. Modern high-speed mixing with large internal mixers (270–550 liter capacity) and high fill factors generates significant frictional heat in the compound, and dump temperatures of 160–170 °C are not unusual. At these temperatures, Si-69's tetrasulfide chain releases sulfur and initiates premature crosslinking (scorch), producing unusable compound and requiring mixer clean-out.

Si-75's disulfide bridge is stable to approximately 165 °C, providing a buffer of 10–15 °C over Si-69 at the same mixer conditions. Tire plants that experience scorch issues with Si-69 during summer months (when ambient temperatures elevate compound temperatures) or with high-performance tire compounds that use high-Mooney SBR grades (which generate more mixing heat) have successfully solved the scorch problem by switching to Si-75.

High-Performance Silica/Rubber Masterbatch Production

Silica/rubber masterbatches — pre-compounded silica-rubber blends used to improve dispersion consistency at the final mixing stage — are produced in dedicated compounders that may run at high throughput with elevated dump temperatures. Si-75 is the preferred silane for masterbatch production when Si-69 causes scorch during the masterbatch mixing stage.

Tire Compounds Requiring Tight Cure Time Windows

In compounds where the difference between fully cured and over-cured (reversion onset) is narrow — typical of high-performance tire treads with high accelerator loadings — reducing the premature sulfur input from the silane's polysulfide groups gives the formulator more control over the cure plateau. Si-75's lower sulfur release helps maintain a wider process window between the curing onset and the reversion plateau.

Handling, Dosage, and Storage

Dosage in tire compounds:

• Passenger car tire tread: 6–9 wt% on silica weight (typically 7–8 wt% for 80 phr silica)
• Additional sulfur supplement vs Si-69 reference: typically 0.3–0.5 phr extra elemental sulfur

Si-75 is added to the mixer at the same point as Si-69 — in the non-productive first mixing stage with silica, at a dump temperature of 145–165 °C. The silanization reaction (ethoxysilane + silica silanol → Si–O–Si + ethanol) proceeds at the same rate for Si-75 and Si-69. The ethanol evolution during mixing is similar.

Safe handling precautions are identical to Si-69: ventilated mixing area, standard rubber industry PPE, eye protection.

Storage: cool dry location below 30 °C, sealed drums. Shelf life: 12 months. The amber color may deepen slightly on storage without affecting performance.

Frequently Asked Questions

Can Si-75 replace Si-69 in all applications? Yes, with formulation adjustment (sulfur supplement of 0.3–0.5 phr to compensate for lower sulfur content). The silanization efficiency and silica coupling performance are equivalent. The scorch safety improvement is the key reason to switch, and the supplemental sulfur cost is negligible compared with the reduction in compound waste from scorch incidents.

Is Si-75 more expensive than Si-69? Typically yes, by 5–15%, reflecting the more complex synthesis of the disulfide-predominant polysulfide distribution compared with the tetrasulfide distribution in Si-69. For high-throughput tire plants where scorch rejects represent significant material waste, the Si-75 premium is easily justified.

Does Si-75 provide the same rolling resistance improvement as Si-69? Yes, when formulated with the sulfur supplement at equivalent effective sulfur crosslink density. The rolling resistance benefit comes from the silica-silane coupling (reduction of Payne effect) and from the crosslink density, both of which are fully achievable with Si-75 + supplemental sulfur at the same level as Si-69. Comparative rolling resistance data from major tire producers confirm equivalent tan δ at 60 °C performance between optimized Si-69 and Si-75 formulations.

Why does the dump temperature matter for scorch with polysulfide silanes? The polysulfide S–S bond in both Si-69 and Si-75 is thermally labile. At elevated temperatures, the sulfur chain fragments, releasing active sulfur species that begin accelerating vulcanization crosslinking reactions between adjacent rubber polymer chains. The tetrasulfide in Si-69 has lower thermal stability than the disulfide in Si-75, so sulfur release and incipient crosslinking begins at lower temperature with Si-69. If the compound is processed above the critical temperature for more than 2–3 minutes (as happens at 155–165 °C with Si-69), visible scorch can develop.