TEOS Tetraethyl Orthosilicate

Overview

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). Supplied 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. 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: "Pure TEOS" — the monomeric Si(OEt)₄ at ≥98.5% purity, used as a tetra-functional crosslinker; "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. 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 cure-time variability.

TEOS chemistry centres on the hydrolysis-condensation mechanism: hydrolysis — water attacks the Si–O–Et bond under acid or base catalysis, releasing ethanol and forming Si–OH (silanol) groups; condensation — two silanols on adjacent TEOS-derived molecules react, releasing water and forming a Si–O–Si bond; propagation — the condensation continues across the four available reactive sites of each Si centre, building a three-dimensional crosslinked silicate network. Acid catalysis (HCl, formic acid, p-TSA at 0.1–1.0%) favours linear and lightly-branched chain growth; base catalysis (ammonia, NaOH at 0.1–1.0%) favours dense particle-like condensation giving silica gel particles.

Technical Specifications

Applications

RTV-2 Silicone Mould Rubber Crosslinker

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. 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.

Sol-Gel Optical and Protective Coatings

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

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.

Fumed Silica Precursor

TEOS combusted with H₂/O₂ flame at 1200°C to produce nano-silica particles — the alternative pyrogenic pathway to fumed silica synthesis from chlorosilanes. Approximately 5–10% of global fumed silica production.

Anti-Corrosive Primer Crosslinker

Solvent-borne and 2K coating systems use 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.

Selection Guide: Pure TEOS vs ES-40

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: maximum tetra-functional crosslink density is required (rare in RTV-2; more common in sol-gel coatings); precise control of starting silicate molecular weight is needed (laboratory-scale R&D); cost optimisation drives the choice (pure TEOS is typically 5–10% cheaper per kg than ES-40).

Equivalent Grades

• Wacker Silbond / Geniosil — primary European TEOS supply
• Dow Z-6403 — alternative North American supply
• Shin-Etsu KBE-04 — Japanese equivalent
• Generic 98.5% TEOS — Chinese commodity grade