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 as the primary crosslinker in RTV-2 condensation-cure silicone mould rubber, as feedstock for sol-gel ceramics and optical fibre cladding, in foundry investment-casting binders, and as a precursor for fumed silica synthesis. ≥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 — tetraethyl orthosilicate, also called ethyl silicate or silicic acid tetraethyl ester, CAS 78-10-4 — is a tetra-functional silicate ester with formula Si(OC₂H₅)₄. Molecular weight 208.3 g/mol, density 0.934 g/cm³ at 20°C, refractive index 1.382, viscosity 0.7 cP at 25°C (mobile clear liquid), boiling point 168°C at atmospheric pressure, flash point 49°C (closed cup, Class IB flammable). SEMITECH supplies TEOS at ≥98.5% purity by GC, with water content ≤0.1% by Karl Fischer, free silicic acid ≤0.05%, acidity (as HCl) ≤5 ppm, and APHA colour ≤30. The compound is fully soluble in nearly all anhydrous organic solvents (toluene, hexane, ethanol, acetone) and undergoes rapid hydrolysis on contact with water to form silicic acid (Si(OH)₄) plus four moles of ethanol per mole of TEOS — this hydrolysis-condensation chemistry is the basis of every TEOS application.
Two grades dominate commercial TEOS supply: (1) “Pure TEOS” — the monomeric Si(OEt)₄ at ≥98.5% purity, used as a tetra-functional crosslinker; (2) “Ethyl silicate-40” or “ES-40” — partially-hydrolysed TEOS oligomer with 40% silica content (vs 28.8% for pure TEOS), supplied as a thicker viscous solution that has already undergone partial condensation to form linear and branched silicate oligomers. ES-40 is the practical workhorse for RTV-2 silicone mould rubber and sol-gel coatings — the partial pre-hydrolysis improves miscibility with silicone polymer base and reduces the cure-time variability of fully-monomeric TEOS. SEMITECH stocks both grades; the page below focuses on monomeric TEOS as the reference grade.
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.
TEOS chemistry centres on the hydrolysis-condensation mechanism: (1) hydrolysis — water attacks the Si-O-Et bond under acid or base catalysis, releasing ethanol and forming Si-OH (silanol) groups; (2) condensation — two silanols on adjacent TEOS-derived molecules react, releasing water and forming a Si-O-Si bond; (3) propagation — the condensation continues across the four available reactive sites of each Si centre, building a three-dimensional crosslinked silicate network. The kinetics depend strongly on pH: acid catalysis (HCl, formic acid, p-TSA at 0.1–1.0%) favours linear and lightly-branched chain growth giving thicker fluid; base catalysis (ammonia, NaOH at 0.1–1.0%) favours dense particle-like condensation giving silica gel particles.
- 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
For RTV-2 silicone mould rubber, TEOS or ES-40 is mixed with α,ω-dihydroxy-PDMS at 5–10% loading; the tin catalyst (DBTDL or DOTL) catalyses both the silanol-silanol condensation and the silanol-ethoxy condensation, building crosslinks between PDMS chains via tetra-functional silicate junctions. The cure releases approximately 1 mole of ethanol per mole of TEOS — a typical RTV-2 mould rubber formulation produces 1–2% ethanol byproduct that volatilises during cure. For sol-gel optical coatings (anti-reflective films, scratch-resistant hard coats, optical fibre cladding), TEOS is hydrolysed in ethanol/water solvent under acid catalysis, deposited as a thin film by spin-coating or dip-coating, and heat-cured at 100–500°C to form a dense silica film.
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 condensation-cure silicone mould rubber is the largest single application — prototype tooling, dental impression, decorative resin casting moulds, candle moulds, food-contact silicone moulds (with food-grade certification). TEOS or ES-40 at 5–10% on PDMS base + 0.1–0.5% tin catalyst (DBTDL or DOTL) gives a 2-component RTV system with 30–60 minute pot-life and 12–24 hour cure to Shore A 25–60 elastomer. Sol-gel optical and protective coatings use TEOS at 30–60% in IPA/water solvent with HCl or formic acid catalyst at 0.1–0.5% to produce anti-reflective coatings, scratch-resistant hard coats, and optical fibre cladding. Application techniques include spin-coating (semiconductor industry), dip-coating (architectural glass), and spray-coating (large-area substrates).
Foundry investment-casting binders use ES-40 (more than monomeric TEOS) at 25–40% in ethanol/water solvent with ammonia gel catalyst — the binder coats a wax pattern, gels around it, and fires to a ceramic mould shell that captures fine detail of the wax pattern. Fumed silica precursor uses TEOS combusted with H₂/O₂ flame at 1200°C to produce nano-silica particles (the alternative pyrogenic pathway to fumed silica synthesis from chlorosilanes); SEMITECH-supplied TEOS feeds approximately 5–10% of global fumed silica production. Anti-corrosive primer crosslinker in solvent-borne and 2K coating systems uses TEOS at 1–5% to produce additional Si-O-Si crosslink density alongside titanate or zirconate-driven Ti-O-C / Zr-O-C ester crosslinks.
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.
SEMITECH issues a CoA on every batch with: TEOS purity (GC, target ≥98.5%), water content (Karl Fischer, target ≤0.1%), free silicic acid (target ≤0.05%), acidity as HCl (potentiometric, target ≤5 ppm), APHA colour (target ≤30), density at 20°C, refractive index, and flash point. Standard packing 200 kg HDPE-lined steel drums; 1 t IBC totes for high-volume RTV-2 silicone manufacturers; 25 t ISO-tank for bulk fumed silica precursor and sol-gel coating customers. MOQ 200 kg per order. Lead time 1–2 weeks ex-Zhejiang to Asia ports for stocked grade, 4–6 weeks to Europe and North America after sea freight.
Storage: TEOS has flash point 49°C — Class IB flammable liquid under OSHA 29 CFR 1910.106 and EU CLP Flammable Liquid Category 3. Storage requirements: dedicated flammable-storage cabinet or warehouse area with adequate ventilation; ground all transfer equipment for static dissipation; no open flames within 5 metres; spill containment for full drum capacity. Moisture protection is critical — TEOS hydrolyses on contact with atmospheric moisture, releasing ethanol vapour and forming silicic acid hydrate that drops the active TEOS assay; recommended storage under nitrogen blanket below 25°C; opened drums should be re-blanketed and consumed within 90 days. Shelf life 18 months sealed for nitrogen-blanketed storage. Health and regulatory: REACH-registered for industrial silicone polymer feedstock and crosslinker use; mild skin and eye irritant; vapour inhalation hazard at flash point and above; standard PPE — nitrile gloves, splash goggles, fume hood ventilation. Ethanol vapour released during downstream cure should be vented or scrubbed for compliance with local volatile organic compound regulations. SDS in EU/GHS format issued with every shipment.
TEOS is the tetra-functional silicate crosslinker — Si(OEt)₄ at ≥98.5% purity for RTV-2 silicone mould rubber, sol-gel optical coatings, foundry investment-casting binders, and fumed silica precursor synthesis. Class IB flammable (flash point 49°C) requires dedicated storage. Ethyl silicate-40 (ES-40) partial-hydrolysate is the practical workhorse for most RTV-2 and sol-gel applications.
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?
For RTV-2 mould rubber, ES-40 is almost always the better choice — the partial pre-hydrolysis (40% SiO₂ content vs 28.8% for pure TEOS) provides better miscibility with PDMS base, more consistent cure kinetics, and lower ethanol-byproduct vapour during cure. Use pure TEOS only when: (1) you need maximum tetra-functional crosslink density (rare in RTV-2; more common in sol-gel coatings); (2) the formulation requires precise control of starting silicate molecular weight (laboratory-scale R&D); (3) cost optimisation drives the choice — pure TEOS is typically 5–10% cheaper per kg than ES-40 on Chinese-origin spot pricing. Most production-scale RTV-2 manufacturers specify ES-40 in their formulations and SEMITECH supplies both grades.
+How do I prevent premature gelling of TEOS in storage?
TEOS gels through hydrolysis-condensation when exposed to moisture. Prevention: (1) store under dry nitrogen blanket below 25°C — moisture-free storage gives 18+ month stability; (2) avoid storage above 30°C — elevated temperature accelerates any moisture-driven condensation that has begun; (3) re-blanket the drum headspace with dry nitrogen after each draw; (4) decant only the volume needed for immediate use, and consume opened drums within 90 days. Diagnostic for partial hydrolysis: visible cloudiness, viscosity rise above 1 cP at 25°C, and dropped GC purity below 95%. If viscosity has risen but purity is still above 95%, the material is usable for ES-40 applications (RTV-2, sol-gel) at adjusted dosing; if purity below 95%, dispose for the application.
+Can TEOS be used as the only crosslinker for sol-gel anti-reflective coatings, or does it need a co-crosslinker?
TEOS alone produces a brittle, dense silica film with refractive index ~1.45 — useful for some optical applications (single-layer anti-reflective at specific wavelength) but lacking the flexibility and adhesion needed for most sol-gel coatings. Practical formulations blend TEOS with: (1) MTES (methyltriethoxysilane) at 10–30% to introduce methyl groups for flexibility and reduced thermal expansion; (2) GLYMO or other functional silane at 5–15% to introduce reactive functional groups for adhesion to substrate or for downstream functionalisation; (3) longer-chain alkoxysilane (octyl-, hexadecyl-trimethoxysilane) at 1–10% to introduce hydrophobic character and crack resistance. Pure TEOS sol-gel is used in research and development; commercial coatings are nearly always multi-component formulations.