Hexamethyldisiloxane (HMDS) Storage & Tank Compatibility

Storing Hexamethyldisiloxane (HMDS)? Start Here

Hexamethyldisiloxane (HMDS), C₆H₁₈OSi₂, is a colorless, water-thin organosilicon liquid built from two trimethylsilyl groups bridged by an oxygen atom. With a viscosity near 0.65 cSt, a boiling point around 100 C, and a flash point near -1 C, it behaves as a volatile, nonpolar, lipophilic solvent rather than a heavy silicone oil. It is practically insoluble in water but freely miscible with hydrocarbons and most organic solvents, which makes it valued as a fast-drying carrier and cleaning fluid in coatings, electronics, cosmetics, and silicone manufacturing. Because its vapor is highly flammable and its small, nonpolar molecule readily permeates polyolefins, HMDS demands flammable-solvent handling discipline and metal construction. It is also very toxic to aquatic life, so spill containment and vapor control are essential wherever it is stored or transferred.

Is HMDS Compatible With Polyethylene (HDPE / XLPE) Tanks?

No. Hexamethyldisiloxane is not suitable for storage in polyethylene (HDPE or crosslinked XLPE) or polypropylene tanks. Although heavy, high-molecular-weight silicone oils are often tolerated by polyolefins, HMDS is a very different material: it is a low-viscosity (about 0.65 cSt), volatile, nonpolar solvent that behaves much like a light hydrocarbon. Nonpolar organic solvents of this type are absorbed into the polyethylene matrix, causing the wall to swell, soften, lose tensile strength, and eventually craze or weep. The molecule is also small and volatile enough to permeate the tank wall, which both loses product and creates a flammable-vapor hazard around the vessel. For these reasons the standard plastic-tank resistance charts rate nonpolar hydrocarbon-type solvents as not recommended for polyethylene. Store HMDS in stainless steel (316) or properly bonded and grounded carbon steel vessels with PTFE or FKM seals, rated for a Class IB flammable liquid. Reserve polyethylene only for short-term secondary containment, never for bulk or long-term storage.

Material compatibility at a glance

HMDS is a volatile, highly flammable, nonpolar silicone solvent. Store and handle in stainless steel (316) or properly bonded and grounded carbon steel with PTFE or FKM seals. Polyethylene and polypropylene tanks are not suitable: this low-viscosity lipophilic solvent permeates and swells polyolefins. Treat HMDS as a flammable-solvent service with explosion-proof equipment, vapor control, and aquatic-spill protection.

Ratings: S suitable · C conditional / limited · U unsuitable. Verify against the cited resistance charts and your concentration/temperature before specifying.

The safety that actually matters

• Highly flammable liquid and vapor (flash point about -1 C). Keep away from heat, sparks, open flame, and hot surfaces; use only non-sparking, explosion-proof equipment and eliminate all ignition sources.
• Bond and ground all containers and transfer lines to prevent static-spark ignition; this low-conductivity solvent can accumulate a static charge during pumping.
• Provide adequate ventilation or local exhaust; vapor is harmful if inhaled and can form an explosive mixture with air (about 0.5 to 21.8 percent by volume).
• Wear chemical-resistant gloves and eye protection; HMDS causes skin irritation and prolonged or repeated exposure can cause organ damage.
• Very toxic to aquatic life with long-lasting effects; prevent any release to drains, soil, or waterways and provide spill containment.
• Store in tightly closed metal containers in a cool, well-ventilated, fire-rated area away from strong oxidizers.

Common questions

Can I store hexamethyldisiloxane in a poly (HDPE or XLPE) tank?

No. HMDS is a volatile, nonpolar solvent that permeates and swells polyethylene, so HDPE and XLPE are rated unsuitable for bulk storage. Use stainless steel (316) or properly bonded and grounded carbon steel with PTFE or FKM seals instead.

Why is HMDS rated incompatible with PE when silicone oils usually are not?

Heavy silicone oils are large, high-viscosity molecules that polyethylene tolerates. HMDS is the opposite: a small, water-thin (about 0.65 cSt), volatile solvent that acts like a light hydrocarbon. Nonpolar solvents of this kind are absorbed into the PE wall, causing swelling and loss of strength.

How flammable is hexamethyldisiloxane?

Very. Its flash point is near -1 C (30 F), giving it an NFPA flammability rating of 4. It must be handled as a Class IB flammable liquid with explosion-proof equipment, bonding and grounding, and rigorous ignition-source control.

What are the best materials of construction for HMDS?

Stainless steel 316 is preferred, with properly bonded and grounded carbon steel acceptable for the dry solvent. Use PTFE or FKM (Viton) for seals and gaskets, and avoid EPDM and polyolefins, which swell in nonpolar solvents.