3-(Triethoxysilyl)propyl thiocyanate (CAS 34708-08-2) is the thermally robust member of the sulfur silane family: a thiocyanate-functional coupling agent that survives mixing temperatures which would prematurely react a tetrasulfide or a mercaptosilane. It is the practical answer for plants that need to mix hot, and it is one of the least-understood products in the sulphur silane group. PolyBlueChem supplies it as a liquid with per-batch COA and SDS.
Specifications
| Property | Specification |
|---|---|
| CAS number | 34708-08-2 |
| Chemical name | 3-(Triethoxysilyl)propyl thiocyanate; 1-propanethiocyanic acid, 3-(triethoxysilyl)- |
| Trade designations | Si-264 class; thiocyanatopropyl silane |
| Formula / MW | C10H21NO3SSi / 263.43 g/mol |
| Appearance | Clear to pale yellow liquid |
| Sulfur content | About 12.2% (one thiocyanate sulfur per molecule) |
| Assay / purity | Confirmed per batch on the COA |
| Chloride content | Confirmed per batch on the COA |
| Moisture | Confirmed per batch on the COA |
| Storage | Sealed, dry, cool; protect from moisture |
What a thiocyanate does that a sulfide cannot
In a tetrasulfide or a disulfide, the sulfur bridge is already formed and starts to open at mixing temperature. A mercaptosilane is more reactive still. A thiocyanate is different: the -SCN group is thermally stable enough to pass through a hot mixing stage intact, and only decomposes under vulcanisation conditions to generate the sulfur species that bond the filler to the rubber.
That changes the engineering, not just the chemistry:
- No temperature ceiling in the silane stage. The compound can be mixed in fewer stages, at higher dump temperatures, without the silane reacting before it reaches the filler.
- Better scorch safety than a mercaptosilane by a wide margin, and generally better than a tetrasulfide in a hot mix.
- The trade-off is coupling efficiency: the thiocyanate must decompose before it can couple, so the coupling step depends on cure conditions. Where the tetrasulfide couples during mixing and vulcanisation both, the thiocyanate couples only in the vulcanisation window.
- By-products. Thiocyanate decomposition releases nitrogen- and sulfur-containing species; the compound and the plant ventilation must be designed for them.
The sulfur silane family in one table
| Silane | CAS | S in the functional group | Thermal stability in mixing | Best fit |
|---|---|---|---|---|
| Mercapto (triethoxy) | 14814-09-6 | 1 (thiol, SH) | Lowest – reacts early | Fast, low-temperature coupling |
| Mercapto (trimethoxy) | 4420-74-0 | 1 (thiol, SH) | Very low | Same, where methanol is acceptable |
| Disulfide (Si-75) | 56706-10-6 | 2 | Moderate | Safe window, staged or single-stage mix |
| Tetrasulfide (Si-69) | 40372-72-3 | 4 | Moderate to low | Maximum coupling per kilogram |
| Thiocyanate (this product) | 34708-08-2 | 1 (SCN) | Highest | Hot mixing, few stages, high dump temperature |
Where it earns its place
- High-temperature mixing lines – where the plant cannot hold the silane stage cool and a polysulfide would pre-react.
- Single-stage or shortened mix cycles – fewer passes means less silane loss and lower energy, but it demands a thermally stable coupling agent.
- Compounds with demanding dump-temperature requirements – silica compounds that need thorough dispersion, which requires heat.
- Rubber-to-metal and rubber-to-textile bonding systems, where a thermally stable sulfur donor is useful in the primer or the skim compound.
Buying notes
- Do not substitute on CAS alone. A thiocyanate silane is not interchangeable with a mercapto or a tetrasulfide; the cure package and the mixing schedule both change.
- Ask for thiocyanate assay and free chloride: residual chloride and unreacted starting material are the two impurities that matter.
- Ask for moisture content: like every alkoxysilane, hydrolysis is the ageing route, and a hydrolysed lot couples poorly however good the assay looks.
- Confirm the alkoxy group – the ethoxy form is standard; a methoxy analogue exists and would release methanol on hydrolysis.
Packaging and supply
Supplied as a liquid in steel drums or IBCs with moisture protection. Each lot ships with a certificate of analysis and safety data sheet; FOB or CIF terms are quoted against destination. Store sealed, cool and dry – the thiocyanate group is thermally robust but the alkoxysilane end is not, so moisture remains the limiting factor for shelf life. Pack size is agreed against consumption so that the material in your warehouse is always within its useful life.
Related products
- Si-69 / TESPT – bis[3-(triethoxysilyl)propyl] tetrasulfide (CAS 40372-72-3)
- 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)
- 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 3-(triethoxysilyl)propyl thiocyanate (CAS 34708-08-2) as a liquid with per-batch COA and SDS and FOB/CIF delivery. Tell us your mixing temperature profile and we will confirm whether a thiocyanate, a disulfide or a tetrasulfide is the right coupling agent for the compound.
