HMDS Hexamethyldisilazane

Overview

HMDS — hexamethyldisilazane, also written as bis(trimethylsilyl)amine or 1,1,1,3,3,3-hexamethyldisilazane, CAS 999-97-3 — is a silylation reagent with the structure (CH₃)₃Si–NH–Si(CH₃)₃. Molecular formula C₆H₁₉NSi₂, molecular weight 161.4 g/mol, density 0.774 g/cm³ at 20°C, refractive index 1.408, boiling point 125°C at atmospheric pressure, flash point 19°C closed cup. Supplied as a clear colourless mobile liquid at ≥99% purity by GC, with water content ≤500 ppm (HMDS is moisture-sensitive but more tolerant than alkoxysilanes), free trimethylsilanol ≤0.5%, and APHA colour ≤30. Strong characteristic ammonia odour from trace decomposition; full odour will be released on contact with moisture and acidic surfaces during silylation reactions.

HMDS functions as a silylation reagent — its purpose is to install trimethylsilyl (TMS, –Si(CH₃)₃) groups onto reactive hydroxyl, amine, or carboxyl groups on a target substrate, replacing the H of those functional groups with the TMS group. The mechanism: HMDS reacts with substrate-OH (for example, on the surface of fumed silica or at the chain-end of α,ω-dihydroxy-PDMS) to form substrate-O-Si(CH₃)₃ + (CH₃)₃Si-NH₂; the trimethylsilylamine intermediate further reacts with another substrate-OH to form a second substrate-O-Si(CH₃)₃ + ammonia. Net stoichiometry: one mole of HMDS silylates two moles of substrate-OH, releasing one mole of ammonia. The ammonia byproduct must be vented or scrubbed during industrial-scale silylation operations.

Class IB flammable liquid (flash point 19°C — vapour ignites at room temperature with ignition source). Storage requirements: dedicated flammable-storage cabinet or warehouse area at 5–25°C with positive ventilation and explosion-proof electrical fittings; ground all transfer equipment; no open flames within 10 metres of transfer operations; spill containment for full drum capacity. Shelf life 12 months sealed under nitrogen blanket below 25°C; opened drums should be re-blanketed and consumed within 90 days.

Technical Specifications

Applications

Fumed Silica Hydrophobic Surface Modification

The largest single application — converting hydrophilic fumed silica (surface Si-OH) to hydrophobic fumed silica (surface Si-O-Si(CH₃)₃) for use as a thickening, anti-sag, and rheology-modifying additive in non-aqueous formulations like silicone sealants, defoamer compounds, polyester moulding compound, and inks. Fumed silica producers dose HMDS at 5–15 wt% on silica weight in a fluidised-bed reactor or stirred-tank silylation reactor, typically at 80–150°C for 1–4 hours, achieving 90–99% conversion of surface silanol to TMS.

MQ Silicone Resin Synthesis

HMDS as the M-unit (mono-functional trimethylsilyl) source paired with TEOS as the Q-unit (tetra-functional silicate) source — in the cohydrolysis-condensation reaction, HMDS provides the chain-terminating TMS groups that control molecular weight and processing characteristics of the finished MQ resin. Typical M:Q ratio in MQ resins is 0.6–1.0 by mole.

Analytical GC Derivatisation

HMDS as a silylation reagent to convert polar, non-volatile analyte hydroxyl groups (sugars, alcohols, organic acids, amines) into volatile TMS-derivatised analogues for gas chromatography analysis.

Semiconductor Photolithography

HMDS as an adhesion promoter — the silicon wafer surface is exposed to HMDS vapour before photoresist coating to convert surface Si-OH (oxide) to surface Si-O-Si(CH₃)₃, providing the hydrophobic surface needed for uniform photoresist adhesion.

PDMS Chain-End Capping

HMDS to install TMS groups on α,ω-dihydroxy-PDMS chain ends, converting the OH-terminated polymer into a chain-end-capped methyl-terminated PDMS for applications where moisture-cure reactivity is undesirable.

Industrial Handling Considerations

NH₃ scrubbing is mandatory for any HMDS use generating >1 kg NH₃ per day. Standard configuration: vent the silylation reactor headspace through a packed-bed scrubber containing 5–10% sulphuric acid solution (or citric acid for less corrosive operation) to absorb NH₃ as (NH₄)₂SO₄ or ammonium citrate. Industrial fumed silica silylation plants achieve 95%+ NH₃ capture efficiency and recover the (NH₄)₂SO₄ as a sellable agricultural fertiliser byproduct. Scale your scrubber capacity for the maximum HMDS feed rate (each kg HMDS releases 0.105 kg NH₃ at full conversion) plus a safety factor of 1.5–2× for upset conditions.

Equivalent Grades

• Evonik Dynasylan HMDS — primary European supply
• Wacker SILRES HMDS — alternative European supply
• Shin-Etsu KBM-303 — Japanese equivalent
• Generic 99% HMDS — Chinese commodity grade