Bis[3-(triethoxysilyl)propyl] disulfide (Si-75, TESPD, CAS 56706-10-6) is the two-sulfur counterpart of Si-69 and the answer to its main processing problem. Where the tetrasulfide delivers a high crosslink density but scorches easily, the disulfide trades a little coupling efficiency for a much wider processing window. PolyBlueChem supplies the disulfide as the neat liquid and as a 50% carrier-bound powder, with per-batch COA and SDS.
Specifications
| Property | Specification |
|---|---|
| CAS number | 56706-10-6 |
| Chemical name | Bis[3-(triethoxysilyl)propyl] disulfide; TESPD; bis(triethoxysilylpropyl) disulfane |
| Formula / MW | C18H42O6S2Si2 / 474.8 g/mol |
| Appearance | Pale yellow clear liquid (standard form) |
| Sulfur content | About 13.5% total sulfur (two sulfur per molecule) |
| Assay / purity | Confirmed per batch on the COA |
| Chloride content | Confirmed per batch on the COA |
| Density and refractive index | Confirmed per batch on the COA |
| Storage | Sealed, dry, cool; protect from moisture |
Tetrasulfide or disulfide: the real trade-off
| Criterion | Si-69 (tetrasulfide, 40372-72-3) | Si-75 (disulfide, 56706-10-6) |
|---|---|---|
| Sulfur per bridge | 4 (average rank 3.7-4.0) | 2 |
| Total sulfur content | About 23.5% | About 13.5% |
| Coupling efficiency | Higher crosslink density at equal loading | Slightly lower, because there is less sulfur to open the bridge |
| Scorch resistance | Lower – the tetrasulfide releases sulfur readily | Higher – a wider, safer processing window |
| Mixing approach | Usually staged, with a temperature ceiling in the silane stage | More forgiving; can be run in fewer stages in some compounds |
| Ethanol release per mole | Same (three ethoxy groups either way) | Same |
| Typical reason to switch | – | Scorch problems, mixer temperature limits, or a plant that cannot stage the mix |
The decision is almost never “one is better”. It is: can your mixer keep the tetrasulfide below its reaction threshold? If yes, Si-69 gives more coupling per kilogram. If not, Si-75 lets you run a compound you could not otherwise run – and the lower sulfur load also frees up the cure package.
Where Si-75 wins
- High-temperature mixing lines. Compounds that must be mixed hot, or in a single stage, exceed the safe window for a tetrasulfide.
- Thick sections and long cure cycles. Less free sulfur means less risk of premature crosslinking during shaping.
- Compounds where the cure system is already sulfur-rich. The disulfide adds less to the total sulfur budget, leaving more headroom.
- Retread and low-hysteresis formulations where the coupling requirement is moderate and process robustness matters more.
How it works
The mechanism is identical to the tetrasulfide, with a shorter bridge: the ethoxy groups hydrolyse to silanols (releasing ethanol) and condense onto the silica surface; the disulfide bridge opens during vulcanisation to bond the filler to the rubber. Because there are two sulfur instead of four, the bridge is shorter and stiffer, and it releases its sulfur less aggressively – which is precisely the source of both the improved scorch safety and the slightly lower coupling efficiency.
Forms supplied
| Form | Active content | Typical use |
|---|---|---|
| Neat liquid | 100% | Plants with liquid dosing into the internal mixer |
| Carrier-bound powder | About 50% on a carrier | Weighing by bag, no liquid handling; compare price per active kilogram |
Buying notes
- Do not compare Si-69 and Si-75 by price per kilogram. Their molecular weights differ (539.0 vs 474.8) and their sulfur contents differ by a factor of nearly two, so the useful comparison is cost per mole of coupling silane, adjusted for the sulfur you are buying.
- Ask for sulfur rank distribution, not only the average – a broad distribution behaves differently from a narrow one.
- Ask for volatile content: partially hydrolysed silane is the most common hidden cause of “the silane stopped working”.
- Specify the form (neat or 50% carrier) on the purchase order, because the same CAS is traded in both.
Packaging and supply
The neat liquid ships in steel drums or IBCs with moisture protection; the carrier-bound form ships in 20-25 kg bags. Both are supplied with per-batch COA and SDS, and FOB or CIF terms are quoted against destination. Store sealed and dry – hydrolysis is the failure mode, and a drum that has been opened repeatedly in a humid plant is a slow decline in coupling efficiency rather than a visible defect.
Related products
- Si-69 / TESPT – bis[3-(triethoxysilyl)propyl] tetrasulfide (CAS 40372-72-3)
- 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
- 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] disulfide (Si-75 / TESPD, CAS 56706-10-6) as neat liquid and 50% carrier-bound powder, with per-batch COA and SDS and FOB/CIF delivery. Tell us your mixing constraints and we will help you decide between the disulfide and the tetrasulfide on a cost-per-mole basis.
