TEOS (Tetraethyl Orthosilicate, Ethyl Silicate, CAS 78-10-4): Tetra-Functional Crosslinker for RTV-2 Silicone and Sol-Gel Chemistry
SEMITECH TEOS is the tetra-functional silicate crosslinker — Si(OEt)₄ — used in RTV-2 condensation-cure silicone mould rubber, sol-gel ceramics and optical coatings, foundry investment-casting binders, and as a fumed silica precursor. ≥98.5% purity, ≤0.1% water, 200 kg drum and ISO-tank packaging.
Contents
| 78-10-4 | ≥98.5% | 208.3 |
|---|---|---|
| CAS number | Purity | MW (g/mol) |
Chemistry & Specifications
Si(OEt)₄ tetra-alkoxide; clear colourless mobile liquid; bp 168°C; hydrolyses to silicic acid + 4 ethanol.
TEOS (CAS 78-10-4) has MW 208.3, density 0.934 g/cm³, viscosity 0.7 cP, bp 168°C, flash point 49°C. SEMITECH supplies at ≥98.5% purity, water ≤0.1%, free silicic acid ≤0.05%, acidity ≤5 ppm, APHA ≤30.
Two commercial grades: (1) "Pure TEOS" — monomeric Si(OEt)₄ at ≥98.5%; (2) "Ethyl silicate-40" (ES-40) — partially-hydrolysed oligomer with 40% silica content, the practical workhorse for RTV-2 and sol-gel coatings with better PDMS miscibility and more consistent cure.
Hydrolysis-Condensation Mechanism: Sol-Gel and RTV-2 Crosslinking
Hydrolysis to Si-OH; condensation to Si-O-Si network; releases ethanol; produces 3D crosslinked silicate.
- RTV-2 mould rubber crosslinker — TEOS or ES-40 at 5–10% on PDMS base; tin-catalysed condensation cure at 23°C overnight
- Sol-gel optical coatings — TEOS at 30–60% in IPA + water + acid catalyst; spin-coated and heat-cured
- Foundry investment binders — ES-40 at 25–40% in ethanol + water; gels around wax pattern, fired to ceramic shell
- Fumed silica precursor — TEOS combusted with H₂/O₂ flame at 1200°C to produce nano-silica
Kinetics depend on pH: acid catalysis favours linear chain growth; base catalysis favours dense particle-like condensation.
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Applications: RTV-2 Mould Rubber, Sol-Gel, Foundry, Fumed Silica Precursor
RTV-2 mould rubber crosslinker (largest), sol-gel optical coatings, foundry investment binders, fumed silica precursor.
RTV-2 mould rubber is the largest application — prototype tooling, dental impression, decorative and food-contact moulds. TEOS/ES-40 at 5–10% with tin catalyst gives 30–60 min pot-life and 12–24 hr cure to Shore A 25–60.
Sol-gel optical coatings for anti-reflective films, scratch-resistant hard coats, and optical fibre cladding. Foundry investment binders using ES-40 for ceramic shell moulds. Fumed silica precursor feeding ~5–10% of global production. Anti-corrosive primer crosslinker at 1–5% for additional Si-O-Si crosslink density.
Procurement, Storage and Quality Control
CoA per shipment; 200 kg drum or ISO-tank; 18-month shelf life; flash point 49°C — Class IB flammable storage.
Standard packing 200 kg drums; 1 t IBC for RTV-2 manufacturers; 25 t ISO-tank for bulk customers. MOQ 200 kg, lead time 1–2 weeks to Asia, 4–6 weeks to Europe/North America. Moisture protection critical — store under N₂ below 25°C; opened drums consumed within 90 days. REACH-registered.
TEOS Specification Sheet
SEMITECH stocked grade; CoA per batch.
| Property | Specification | Test Method |
|---|---|---|
| Chemical name | Tetraethyl orthosilicate / Ethyl silicate | — |
| CAS number | 78-10-4 | — |
| Molecular formula | Si(OC₂H₅)₄ / C₈H₂₀O₄Si | — |
| Molecular weight | 208.3 g/mol | — |
| Purity (TEOS) | ≥98.5% | Gas chromatography |
| Water content | ≤0.1% | Karl Fischer |
| Free silicic acid | ≤0.05% | Gravimetric |
| Acidity (as HCl) | ≤5 ppm | Potentiometric titration |
| APHA colour | ≤30 | ASTM D1209 |
| SiO₂ equivalent content | 28.4–28.8% | Gravimetric ignition |
| Density (20°C) | 0.934 g/cm³ | ASTM D1475 |
| Refractive index (20°C) | 1.382 | ASTM D1218 |
| Viscosity (25°C) | 0.7 cP | Brookfield |
| Boiling point | 168°C (atmospheric) | — |
| Flash point | 49°C (closed cup) | ASTM D93 |
| Flammability classification | Class IB / EU CLP Flammable Liquid Cat. 3 | — |
| Packaging | 200 kg HDPE-lined steel drum / 1 t IBC / 25 t ISO-tank | — |
| Shelf life | 18 months sealed below 25°C under N₂ | — |
FAQ
+Should I order pure TEOS or ethyl silicate-40 (ES-40) for my RTV-2 mould rubber formulation?
ES-40 is almost always better for RTV-2 — better PDMS miscibility, more consistent cure, and lower ethanol byproduct. Use pure TEOS only for maximum crosslink density (sol-gel), precise MW control (R&D), or when the 5–10% cost saving justifies it.
+How do I prevent premature gelling of TEOS in storage?
Store under dry N₂ below 25°C, re-blanket after each draw, consume opened drums within 90 days. Diagnostic for partial hydrolysis: cloudiness, viscosity above 1 cP, GC purity below 95%.
+Can TEOS be used as the only crosslinker for sol-gel anti-reflective coatings?
TEOS alone gives a brittle dense film. Practical formulations blend with MTES (10–30% for flexibility), GLYMO (5–15% for adhesion), and/or long-chain alkoxysilane for hydrophobicity and crack resistance.