Bis[3-(triethoxysilyl)propyl] tetrasulfide (Si-69, TESPT, CAS 40372-72-3) is the workhorse sulfur silane of the tyre industry: the coupling agent that bonds silica to rubber and made low-rolling-resistance “green tyre” compounds possible. PolyBlueChem supplies the tetrasulfide in three commercial forms – the standard liquid, a 50% carrier-bound powder, and a polymer-bound pre-dispersed granule – with per-batch COA and SDS.
Specifications
| Property | Specification |
|---|---|
| CAS number | 40372-72-3 |
| Chemical name | Bis[3-(triethoxysilyl)propyl] tetrasulfide; TESPT; bis(triethoxysilylpropyl) tetrasulfane |
| Formula / MW | C18H42O6S4Si2 / 539.0 g/mol |
| Appearance | Pale yellow to amber clear liquid (standard form) |
| Sulfur content | About 23.5% total sulfur (four sulfur per molecule) |
| Assay / purity | Confirmed per batch on the COA |
| Chloride content | Confirmed per batch on the COA |
| Density (20 °C) | Confirmed per batch on the COA |
| Refractive index (nD25) | Confirmed per batch on the COA |
| Storage | Sealed, dry, cool; protect from moisture |
Batch-specific data for assay, chloride and physical constants is stated on the certificate of analysis. Sulfur rank (the average number of sulfur atoms in the bridge, typically quoted in the 3.7-4.0 range) is the specification item that most affects processing behaviour – ask for it explicitly if your compound is sensitive to scorch.
Three commercial forms, one molecule
| Form | What it is | When it is the right choice |
|---|---|---|
| Standard liquid (100%) | Neat TESPT, dosing by weight or by pump | Internal mixers with liquid injection; plants set up for neat silane |
| 50% carrier-bound powder | TESPT adsorbed on a carrier (carbon black or a mineral carrier) at about 50% active | Plants without liquid dosing; easier weighing, less spillage, better dispersion in a premix |
| Polymer-bound pre-dispersed granule | TESPT dispersed in a polymer binder as free-flowing granules | Continuous weighing lines; no liquid handling, no dust, consistent dosing in high-volume compounds |
All three deliver the same chemistry per active kilogram. The choice is about your plant, not about performance: compare price per kilogram of active silane, never per kilogram of product, because the 50% form carries half its weight in carrier.
What it does in a compound
Silica is hydrophilic and does not bond to hydrocarbon rubber on its own. TESPT solves that in two sequential reactions, and getting the sequence right is the whole art of silane coupling:
- Hydrolysis first. The three ethoxy groups react with traces of water to give silanol groups, releasing ethanol. This is why the compound needs a hydrolysis step and why ethanol is the dominant VOC in a silica compound.
- Then condensation. The silanols condense with the silanol groups on the silica surface, forming a covalent Si-O-Si bond to the filler.
- Finally, during vulcanisation. The tetrasulfide bridge opens and crosslinks with the rubber hydrocarbon, completing the filler-to-rubber bridge. The sulfur in TESPT is also available to the vulcanisation system, which is why switching silane type changes the cure package.
The payoff is that filler-filler networking drops, hysteresis drops, and rolling resistance drops – while wet grip is retained. That is the entire commercial case for green tyre compounds, and it is why TESPT became the largest-volume functional silane in the world.
The processing constraint that decides everything
| Parameter | Why TESPT is demanding | Practical control |
|---|---|---|
| Mixing temperature | The tetrasulfide bridge starts reacting at elevated temperature; too hot and the silane pre-reacts before the silane has coupled to the filler | Staged mixing with a temperature ceiling in the silane stage – the usual approach is to run the silane/silica stage cool and let the reaction complete in later stages |
| Scorch safety | Free sulfur from the tetrasulfide can interact with the cure system | Adjust the accelerator/sulfur package; ask for the sulfur rank figure of the lot |
| Moisture | Hydrolysis is required for coupling, but uncontrolled moisture causes premature condensation and gelling | Sealed storage, dry feed lines, controlled water addition in the mix |
| Ethanol release | Three moles of ethanol per mole of silane | Vent design and VOC abatement; not a product defect but a plant design issue |
Applications
- Silica-filled passenger and truck tyre treads – the dominant use; the reason green tyres exist.
- Other silica or silicate filled rubber articles – shoe soles, hose, belting, vibration mounts.
- Silica and mineral surface treatment – hydrophobic surface modification of fillers before compounding.
- Coupling in composite and coating systems – wherever a silica or silicate surface must bond to an organic matrix.
Buying notes
- Specify the form (neat, 50% carrier, pre-dispersed) as well as the CAS – they are priced and handled completely differently.
- Ask for sulfur rank, not just total sulfur: two lots with the same total sulfur can process differently if the rank distribution differs.
- Ask for chloride: residual chloride from the silane synthesis is a corrosion risk in the mixer and a vulcanisation interference.
- Ask for volatile content: a silane that has partially hydrolysed in storage delivers less coupling and more ethanol.
- Package against consumption – the neat liquid is moisture-sensitive, so a part-used drum is a liability.
Packaging and supply
Neat TESPT ships in steel drums, IBCs or ISO tanks with a dry-air blanket where required; carrier-bound and pre-dispersed forms ship in 20-25 kg bags or big bags. Every lot arrives with a COA and SDS. Because moisture is the main degradation route, packaging is agreed against your consumption rate rather than against the best freight economics, and nitrogen or dry-air padding is available on request. FOB and CIF terms are quoted against destination.
Related products
- Si-75 / TESPD – bis[3-(triethoxysilyl)propyl] disulfide (CAS 56706-10-6)
- 3-Mercaptopropyltriethoxysilane, KH-580 / A-1891 (CAS 14814-09-6)
- 3-Mercaptopropyltrimethoxysilane, KH-590 / A-189 (CAS 4420-74-0)
- Si-264 – 3-(triethoxysilyl)propyl thiocyanate (CAS 34708-08-2)
- VTMS – vinyltrimethoxysilane (CAS 2768-02-7)
Further reading
- Choosing a silane coupling agent: vinyl vs amino vs epoxy vs methacryloxy
- Vinyltrimethoxysilane: properties, production and applications
- Vinyltrimethoxysilane in silane-crosslinked PE for wire and cable
- Sulfur silanes in green tyre compounding: tetra-, di-, mono- and thiocyanate compared
- Si-69 vs Si-75: the mixing window that decides which silane you can run
- How silane coupling works: hydrolysis, condensation and the vulcanisation bridge
PolyBlueChem supplies bis[3-(triethoxysilyl)propyl] tetrasulfide (Si-69 / TESPT, CAS 40372-72-3) as neat liquid, 50% carrier-bound powder and polymer-bound pre-dispersed granule, with per-batch COA and SDS and FOB/CIF delivery. Tell us your mixing setup and the form you need, and we will quote against active silane content.
