Vinyltrimethoxysilane (VTMS) is the smallest commercially important vinyl-functional organosilane, and the one that turned polyethylene into a crosslinkable material without a peroxide curing line or an electron beam. If you have ever used a hot-water pipe or handled a low-voltage cable whose insulation was cured in a warm water bath, VTMS was probably part of the chemistry.
This guide covers the molecule (CAS 2768-02-7), how it is made, what the five specification numbers actually control, how the two-step and one-step crosslinking processes work, how it treats mineral fillers and promotes adhesion, and how to keep a moisture-sensitive silane from degrading in your store.
What Is Vinyltrimethoxysilane?
VTMS is a silane carrying one vinyl group and three methoxy groups on the same silicon atom. It is a small, mobile, low-viscosity liquid – which is exactly what a grafting monomer needs to be, since it has to disperse into a polymer melt within seconds of contacting a peroxide.
The molecule belongs to a family of organofunctional silanes built on the same principle: an organic group that likes polymers, a silicon centre that likes mineral surfaces, and hydrolysable groups that provide the chemical bridge between them. In this family the vinyl grade is the one chosen when the organic partner is a polyolefin and the reaction must be free-radical rather than condensation.
Chemical Identity: CAS 2768-02-7
| Item | Detail |
|---|---|
| Chemical name | Ethenyltrimethoxysilane (vinyltrimethoxysilane) |
| Abbreviations | VTMS, VTMO, A-171 type designation in some catalogues |
| CAS Registry Number | 2768-02-7 |
| Chemical class | Vinyl-functional trialkoxysilane; coupling agent and grafting monomer |
| Appearance | Colourless transparent liquid |
| Colour (Pt/Co) | Max 30 |
| Purity | Min 99.0% |
| Specific gravity (20 deg C) | 0.970 +/- 0.005 g/cm3 |
| Refractive index (n25D) | 1.3930 +/- 0.0050 |
| Reactive groups | One vinyl (free-radical), three methoxy (hydrolysis/condensation) |
The low specific gravity (below 1.0) and the refractive index around 1.393 are useful practical identifiers: both shift measurably if the silane has partially hydrolysed or been cut with methanol, which makes them fast incoming checks before a full GC assay.
How VTMS Is Produced
Industrial production is dominated by hydrosilylation:
- Hydrosilylation of acetylene. Trimethoxysilane is reacted with acetylene over a platinum or platinum-complex catalyst to give vinyltrimethoxysilane. This is the direct, most atom-efficient route and the basis of most capacity.
- From vinyl chloride or other vinyl sources. Alternative routes react a vinyl donor with trimethoxysilane or a chlorosilane followed by methoxylation; these exist mainly where feedstock economics differ.
Both routes end with distillation, and distillation is where the specification comes from: purity is set by the fraction cut, colour by the thermal history during distillation, and the residual methanol level by the top cut. A high-colour lot usually means the distillation ran hot or the product sat in a warm tank afterwards.
Specifications and What They Control
| Specification | What it protects |
|---|---|
| Appearance: colourless transparent liquid | Cloudiness means hydrolysis has started; a hazy drum should not go into a grafting line |
| Colour (Pt/Co) max 30 | Grafting efficiency and the colour of the finished compound; colour is also the earliest indicator of oxidation or metal contamination |
| Purity min 99.0% | The remaining percent is typically methanol, methyltrimethoxysilane or higher vinyl siloxanes – all of which change grafting kinetics and by-product load |
| Specific gravity 0.970 +/- 0.005 | Identity and consistency; a heavier drum usually means hydrolysis products or water contamination |
| Refractive index 1.3930 +/- 0.0050 | Second identity check, sensitive to methanol content and to partial condensation |
Two Reactive Ends, Two Mechanisms
The vinyl end: free-radical grafting
In the presence of a peroxide at extrusion temperature, the vinyl group takes part in radical chemistry and becomes covalently bound to the polymer backbone. This is a grafting reaction, not a polymerisation – a small number of silane units are attached along each polyethylene chain.
The methoxy end: hydrolysis then condensation
The three methoxy groups are hydrolysed by water to silanol groups, which then condense with each other (or with hydroxyls on a mineral surface) to form siloxane bonds. This step is what converts a grafted thermoplastic into a crosslinked network, or what bonds a silane to silica or glass.
| Step | Reaction | Controlled by |
|---|---|---|
| Grafting | Vinyl group attaches to the polymer radical | Peroxide type and dose, melt temperature, residence time |
| Hydrolysis | Methoxy to silanol, releasing methanol | Water availability and temperature |
| Condensation | Silanol to siloxane crosslink | Condensation catalyst, temperature, time |
Silane Crosslinking of Polyethylene
This is the application that consumes most VTMS worldwide. Two process routes are used, and both deliver the same product: a polyethylene whose chains are joined through siloxane bridges.
Two-step (Sioplas type)
- Graft masterbatch. PE is compounded with VTMS and a peroxide in a dedicated line, producing a grafted compound with a controlled silane content.
- Catalyst masterbatch. A separate masterbatch carries the silanol condensation catalyst, typically an organotin compound in a carrier resin.
- Conversion. The two masterbatches are let down together on the cable or pipe extrusion line, shaped, then cured in warm water or low-pressure steam.
One-step (Monosil type)
Silane, peroxide and catalyst are metered directly into a purpose-designed extruder, and the grafted melt goes straight to the die. Simpler logistics, but the extruder must handle the metering accuracy and the peroxide hazard, and the catalyst cannot contact the silane before the melt stage.
| Point | Two-step | One-step |
|---|---|---|
| Plant complexity | Needs a grafting line plus two masterbatches | Needs a specialised extruder |
| Logistics | Masterbatches can be shipped and stored | Silane handled on site |
| Typical use | Widely used for LV cable and pipe | Integrated cable plants |
| Graft consistency | Better, because grafting is done under controlled conditions | Depends on line control |
Curing
Crosslinking happens after shaping, in a water bath at roughly 80-95 deg C or in low-pressure steam, and continues for days at ambient conditions if the product is simply left in store. The practical consequence for the converter: cure is a function of time, temperature and wall thickness, and a thick section needs longer than a thin one.
Filler Treatment and Coupling
For mineral fillers, the silane is normally applied from solution or as a pre-treatment during filler manufacture. The methoxy groups hydrolyse and condense with surface silanols on silica, glass, talc, mica, aluminium hydroxide or magnesium hydroxide, leaving a vinyl-rich surface that then bonds to the polymer during compounding or peroxide crosslinking.
| Application | Typical silane dose | Notes |
|---|---|---|
| Silica / glass fibre | 0.5 – 1.5% on filler | Surface silanols are abundant; condensation is efficient |
| Talc, mica, wollastonite | 0.5 – 1.0% | Fewer surface hydroxyls; hydrolysis of the silane must be driven deliberately |
| ATH / MDH flame-retardant fillers | 0.5 – 1.5% | Improves mechanical properties at high loading and reduces viscosity |
| Mineral-filled PE compounds | 0.3 – 1.0% | Often added in a masterbatch or during compounding |
Two conditions decide whether silane treatment works: the filler must have surface hydroxyls (or an oxide layer that can be hydroxylated), and water must be present in small, controlled amounts to hydrolyse the methoxy groups. Too dry and the silane never activates; too wet and it condenses with itself before reaching the surface.
Adhesion Promotion and Copolymerisation
In coatings, adhesives and sealants, VTMS acts as an adhesion promoter on glass, metal oxides and mineral substrates, and as a crosslinker in moisture-curing systems. In copolymerisation it is used at low levels to introduce pendant trimethoxysilyl groups into EVA, acrylic and vinyl systems, giving moisture-curability and improved wet adhesion.
Handling, Hydrolysis Control and Safety
| Issue | Practice |
|---|---|
| Moisture | Keep containers sealed; use dry equipment; do not store partial drums open. Hydrolysis is the main degradation route |
| Methanol by-product | Hydrolysis releases methanol – ventilate the extruder feed, curing area and any open handling point |
| Catalyst separation | Keep the condensation catalyst out of contact with the silane until the melt stage; premature contact causes local gel |
| Flammability | Treat as a flammable liquid; consult the SDS for flash point and storage classification in your jurisdiction |
| Personal protection | Gloves, eye protection, and respiratory protection where mist or vapour can build up |
| Spill response | Absorb, keep away from drains and water courses – the silane hydrolyses and releases methanol |
Quality Control
- Appearance and colour (Pt/Co) – fastest screen; haze means hydrolysis, dark colour means thermal damage.
- Refractive index and specific gravity – two independent identity checks that catch dilution and partial hydrolysis.
- Assay by GC – confirms 99.0% min and identifies the main impurity (usually methanol or methyltrimethoxysilane).
- Water by Karl Fischer – low water is the point of the specification; a wet drum will gel in the grafting line.
The Wider Silane Family
VTMS is one member of a family in which the organic group is selected for the polymer and the hydrolysable group for the substrate. This matters when specifying: the same silicon chemistry with a different organic end behaves completely differently in a compound.
| Silane type | Organic end reacts with | Typical use |
|---|---|---|
| Vinyl (VTMS) | Polyolefins by free-radical grafting | Silane crosslinking, filler treatment in PE/PP |
| Amino | Epoxy, polyamide, phenolic, PVC | Glass-fibre composites, adhesives, foundry resins |
| Epoxy | Amine-cured systems, acrylics, polyurethanes | Coatings, sealants, filled systems |
| Methacryloxy | Acrylic and unsaturated polyester systems | Composites, filled acrylic, cable compounds |
| Alkyl (e.g. propyl, octyl) | Nothing reactive – hydrophobic only | Surface hydrophobation of fillers and pigments |
Packaging, Storage and Supply
| Item | Detail |
|---|---|
| Standard pack | 200 kg drum, dry sealed closure |
| Bulk pack | 1000 kg IBC on request |
| Storage | Dry, ambient, out of direct sunlight; use partial drums first |
| Documents | COA per batch (appearance, colour, assay, specific gravity, refractive index), SDS in English, shipping documents |
| Samples | Available for grafting and filler-treatment trials |
Frequently Asked Questions
Is VTMS the same as vinyltriethoxysilane? No. VTMS carries methoxy groups; the triethoxy analogue is a different CAS with slower hydrolysis and a different by-product alcohol.
Why is a low water content so important? Water hydrolyses the methoxy groups. A wet drum partially pre-condenses, which reduces grafting efficiency and can gel the compound.
Can VTMS be used without a peroxide? For grafting onto polyolefins, no – the vinyl group needs a radical source. For filler treatment the silane reacts by hydrolysis and condensation and no peroxide is required.
How long does silane crosslinking take? It depends on wall thickness and cure condition: hours in a hot water bath, days at ambient temperature.
Do you supply technical support for grafting trials? We can share typical starting parameters and connect you with documentation for the process route you are using.
Contact PolyBlueChem for specifications, samples, COA and quotations.