(3-Mercaptopropyl)trimethoxysilane (CAS 4420-74-0) – KH-590, A-189, MPTMS – is the original mercaptosilane: the fastest-reacting coupling agent in the sulfur silane family, and the one with the widest literature base. Its trimethoxy head hydrolyses quickly, which makes it effective at low temperature and difficult to store, and releases methanol in the process. PolyBlueChem supplies it as a clear liquid with per-batch COA and SDS.
Specifications
| Property | Specification |
|---|---|
| CAS number | 4420-74-0 |
| Chemical name | (3-Mercaptopropyl)trimethoxysilane; 3-(trimethoxysilyl)-1-propanethiol; MPTMS |
| Trade designations | KH-590; Silquest A-189; Si-189 class |
| Formula / MW | C6H16O3SSi / 196.34 g/mol |
| Appearance | Clear to slightly yellowish liquid |
| Sulfur content | About 16.3% (one thiol 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 light and oxygen |
Trimethoxy or triethoxy: 4420-74-0 vs 14814-09-6
Both are mercaptosilanes and both are sold as “the mercapto silane”, which is why they are confused. The difference is the alkoxy leaving group, and it decides how the product behaves in your plant.
| Property | Trimethoxy (this product) | Triethoxy (CAS 14814-09-6) |
|---|---|---|
| Formula / MW | C6H16O3SSi / 196.34 | C9H22O3SSi / 238.42 |
| Moles per kilogram | 5.09 | 4.19 |
| Sulfur per kilogram of product | About 163 g | About 134 g |
| Hydrolysis by-product | Methanol | Ethanol |
| Hydrolysis rate | Faster – couples at lower temperature in less time | Slower, more controllable |
| Regulatory pressure | Higher – methanol is the more restricted alcohol in most markets and at most sites | Lower |
| Cost per mole of silane | Usually lower | Usually higher |
The practical rule: if methanol is acceptable at your site and in the final article, the trimethoxy form gives you more silane per kilogram for less money. If methanol is a problem, pay for the ethoxy form. This is a genuine engineering decision and it is decided by regulation and ventilation, not by datasheet preference.
Why the thiol is the reactive one
A mercaptosilane bonds to rubber by free-radical addition of the thiol to an unsaturated bond – no sulfide bridge has to open first. Consequences:
- Coupling starts earlier and at lower temperature than with a tetra- or disulfide silane.
- It will also pre-couple or scorch if the mix is not controlled, which is why mercaptosilanes are used in formulated systems rather than as a drop-in replacement for Si-69.
- The thiol binds strongly to metals, which is why this molecule appears in adhesion promoters for steel and aluminium as well as in rubber compounds.
- Oxidative ageing is the storage problem: two thiols slowly combine into a disulfide, and the product loses its fast-reacting character while still passing a “total sulfur” specification. Free mercaptan assay is the test that matters.
Where it is used
- Silica-filled rubber – tread and technical compounds needing low-temperature coupling.
- Coatings, sealants and adhesive primers – the thiol anchors to metal; the silane anchors to the binder or to a mineral filler.
- Filler and pigment surface treatment – mercaptofunctionalisation of silica, silicates and oxide pigments.
- Polymer grafting and thiol-ene chemistry – the SH group is a clean radical handle for surface and polymer modification.
Buying notes
- State the CAS: 4420-74-0 and 14814-09-6 are routinely interchanged in enquiries and cannot be substituted without reformulating.
- Ask for free mercaptan, not just total sulfur – oxidised material still contains sulfur.
- Ask for moisture and chloride: both drive premature hydrolysis and both shorten shelf life.
- Plan the methanol question early: it affects your VOC abatement design and, in some markets, whether the final article is acceptable at all.
- Pack against consumption: this is the least storage-stable of the sulfur silanes.
Packaging and supply
Supplied as a liquid in steel drums or IBCs, sealed under moisture and light protection. Each lot ships with a COA and SDS and FOB or CIF terms are quoted against destination. Storage should be cool, dry and dark; open drums oxidise at the surface, so prefer the smallest package that matches your consumption rate rather than the largest that matches your freight budget.
Related products
- 3-Mercaptopropyltriethoxysilane, KH-580 / A-1891 (CAS 14814-09-6)
- Si-69 / TESPT – bis[3-(triethoxysilyl)propyl] tetrasulfide (CAS 40372-72-3)
- Si-75 / TESPD – bis[3-(triethoxysilyl)propyl] disulfide (CAS 56706-10-6)
- 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 (3-mercaptopropyl)trimethoxysilane (CAS 4420-74-0, KH-590 / A-189 class) as a clear liquid with per-batch COA and SDS and FOB/CIF delivery. Tell us whether methanol is acceptable in your process and we will confirm whether the trimethoxy or the triethoxy mercaptosilane is the right choice.
