Bis(trimethylsilyl)amine, (Me₃Si)₂NH; clear colourless liquid; bp 125°C; ammonia byproduct; flammable.

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. SEMITECH supplies HMDS 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. The compound has a 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.

Fumed silica hydrophobisation, MQ resin synthesis, analytical GC derivatisation, semiconductor photolithography adhesion.

Fumed silica hydrophobic surface modification is 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. SEMITECH supplies approximately 8–12% of global HMDS demand for this application.

MQ silicone resin synthesis uses 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 uses 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 uses 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 uses 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.

TMS-N-TMS attacks substrate-OH; releases NH₃; full silylation requires venting/scrubbing.

The silylation reaction proceeds via nucleophilic attack of substrate-OH on the Si-N bond of HMDS, displacing one TMS group as TMS-substrate ester and producing TMS-NH₂ as intermediate. The TMS-NH₂ further reacts with a second substrate-OH, displacing the second TMS group and releasing NH₃ as the final byproduct. Reaction kinetics are favoured by: (1) elevated temperature (80–150°C drops reaction time from hours to minutes); (2) acid catalysis (trace TFA, p-TSA at 0.1–1.0% accelerates the kinetics 5–10×); (3) anhydrous conditions (water competes with substrate-OH and consumes HMDS without productive silylation).

• Fumed silica silylation — 5–15 wt% HMDS on silica, 80–150°C, 1–4 hr in fluidised bed or stirred tank with N₂ purge for NH₃ removal
• MQ resin synthesis — HMDS as M-unit, TEOS as Q-unit, cohydrolysis at 60–120°C with acid catalyst
• GC derivatisation — HMDS at 5–10 mol equivalents on analyte, 60–80°C, 30–60 min in pyridine solvent
• Semiconductor photolithography — HMDS vapour at 100–150°C, 1–2 minute wafer treatment in dedicated HMDS vapour-prime tool

Ammonia management is the dominant industrial-handling consideration — an ammonia scrubber (sulphuric acid solution, citric acid solution) is required for any HMDS use generating >1 kg NH₃ per day. The TMS-OH (trimethylsilanol) byproduct of partial reactions condenses to hexamethyldisiloxane (HMDSO, CAS 107-46-0) which is a useful low-boiling silicone fluid that can be recovered and recycled. Industrial fumed silica silylation plants typically achieve 95%+ HMDS utilisation through closed-loop NH₃ scrubbing and HMDSO recovery.

CoA per shipment; 200 kg drum; 12-month shelf life; flash point 19°C — Class IB flammable, requires segregation.

SEMITECH issues a CoA on every batch with: HMDS purity (GC, target ≥99%), water content (Karl Fischer, target ≤500 ppm), free trimethylsilanol content (GC, target ≤0.5%), 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 fumed silica producers and MQ resin manufacturers; 25 kg amber glass bottles for laboratory-scale GC derivatisation customers. MOQ 200 kg per order for industrial use, 1 kg for laboratory samples. Lead time 2–4 weeks ex-Zhejiang to Asia ports, 4–6 weeks to Europe and North America after sea freight (Class IB flammable shipment requires reefer container or expedited DG processing).

Storage: HMDS has flash point 19°C — Class IB flammable liquid (low flash point, 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; HMDS-specific spill kit with neutraliser (5% citric acid solution) on site. Shelf life 12 months sealed under nitrogen blanket below 25°C; opened drums should be re-blanketed and consumed within 90 days. Health and reactivity: HMDS is a flammable, mild skin-and-eye irritant; ammonia byproduct on contact with moisture is acutely irritant to respiratory system; full PPE — chemical-splash goggles, nitrile gloves, lab coat, respirator (P95 minimum); only handle in a well-ventilated fume hood. Spill response: cool with water to prevent ignition (do not contact directly with water at high concentration — exothermic ammonia release possible), absorb on inert media, neutralise residue with 5% citric acid solution, dispose as flammable + ammonia hazardous waste. REACH and regulatory: REACH-registered for industrial use; SDS in EU/GHS format with full flammability and ammonia-byproduct classification.

HMDS is the workhorse silylation reagent — installs trimethylsilyl groups on Si-OH surfaces. Largest application is fumed silica hydrophobisation (5–15 wt% on silica). Class IB flammable (flash point 19°C) requires dedicated DG storage. Ammonia scrubber required for industrial-scale silylation. ≥99% purity, 200 kg drum, 12-month shelf life under N₂.

SEMITECH stocked grade; CoA per batch.