Silicone oil’s CAS number does not fix its viscosity, and the viscosity decides the job

Your plant reorders silicone oil for a line of soft-close lids, quoting the same name and CAS number as last year, and the new lot passes incoming identity checks. On the line, the lids now close too fast. Nothing on the certificate of analysis (CoA) is false. Dimethicone, CAS 9006-65-9, names a family of fluids running from thin and free-flowing to thick and slow to pour. The damping torque of every hinge was set by a viscosity nobody wrote on the purchase order.

Silicone oil earns its place in industry through a short list of jobs: viscous damping and torque transfer, release, lubrication, foam control, polishes and dielectric duty. Each job draws on the same chemistry, a flexible siloxane backbone carrying methyl groups, and each needs a different viscosity. The sections below cover what the identity documents for dimethicone do and do not fix, how each job maps to viscosity, and the plant decision that comes first.

What does CAS 9006-65-9 tell you about the fluid in the drum?

Less than the paperwork suggests. In its 2026 record, FDA’s Global Substance Registration System (GSRS) codes CAS 9006-65-9 as “GENERIC (FAMILY)” rather than as a single substance. It defines dimethicone as a mixture whose component may be any one of a list of viscosity-named grades, from dimethicone 20 to dimethicone 12500. The same record carries a viscosity property of 50 cSt to 1,000 cSt, flagged as non-defining and unreferenced, which is narrower than the record’s own grade list.

PubChem shows the same gap from the other side. More than half of its depositor records for 9006-65-9 standardize to one of two small, discrete molecules: hexamethyldisiloxane (CID 24764) and octamethyltrisiloxane (CID 24705). That is how a catalog entry for dimethicone ends up printing the formula C8H24O2Si3 and a molecular weight of 236.53 g/mol. C8H24O2Si3 is the formula PubChem gives for octamethyltrisiloxane, a three-silicon molecule, so both figures describe that oligomer rather than a polymer chain.

The honest formula for the fluid is its repeat unit, (C2H6OSi)n, and the chain length hiding behind that n is what viscosity tracks. In practice, two certificates can carry the same CAS number for fluids that will not do the same job. The identity block confirms that you bought a dimethicone. Only the viscosity line, with its temperature and test method, confirms that you bought the right one.

Which viscosity does each industrial job need?

The direction holds even where the exact number belongs to the equipment designer. Thin fluids spread and wet. Mid-range fluids lubricate. Thick fluids resist shear and form the base of antifoam compounds. Damper grades are whatever the damper designer specified. The table sets out what the fluid does in each job and what a wrong grade looks like.

Damping is where the viscosity line becomes a part number. The application notes in the dimethicone catalog record list crankshaft and driveline torsional vibration dampers, viscous couplings, rotary dampers, control valves, dashpots, aviation gyroscopes, and soft-close hinges, lids and knobs. For the rotary and soft-close cases the notes state that viscosity sets the damping torque, and in a viscous damper of any kind the fluid is part of the tuning. A viscosity change on a reorder is an engineering change, not a purchasing substitution.

Plastics processors meet a different boundary. The fluid works as a slip and release aid on surfaces, but slip additives melt-blended into a masterbatch use ultra-high-molecular-weight silicone gum, not this fluid. If the request is for an additive that goes into the polymer melt, the fluid is the wrong form whatever its viscosity.

As a lubricant, plain silicone oil carries less load than an additized hydrocarbon oil, so it is the wrong choice for heavily loaded metal-on-metal gears and bearings without an additive package. Where it wins is clean, non-staining duty on plastic and organic-rubber parts that petroleum oils can swell or attack.

Release agents and polishes raise one more identity question: is the silicone fluid on the quote linear dimethicone at all?

Decamethylcyclopentasiloxane (CAS 541-02-6), a cyclic siloxane that PubChem lists with a boiling point of 210 °C, is used as a volatile carrier and solvent. It evaporates after application, while a linear dimethicone fluid of ordinary viscosity stays on the surface; the very thinnest linear grades are volatile too. Ask which kind the quote describes, and check the EU restriction status of any cyclopentasiloxane carrier, because the two do different jobs on the part.

Why does the paint shop get a veto before any grade is chosen?

The property that makes silicone oil useful, its low surface tension, is the same property that makes it a contaminant. A trace of stray silicone on a substrate causes craters and fisheyes in a paint or coating film, because the coating cannot wet the contaminated spot. A release agent that transfers to a molded part, a lubricant mist that settles across a shop, or a polishing cloth carried between bays can each put silicone where a coater does not want it.

That makes the first question on any silicone oil purchase a plant question, not a grade question. Does anything this fluid touches get painted, coated, printed or bonded later, in your plant or at your buyer’s? If yes, keep silicone out of that flow entirely with a non-silicone release agent or lubricant. Otherwise, write the segregation down: separate storage, dedicated dispensing equipment and a cleaning step validated on the parts that go to coating. Settling this on the requisition costs less than settling it after a fisheye complaint.

Is a cP quote the same as a cSt specification?

No, and the mismatch is easy to miss. Kinematic viscosity, in centistokes (cSt), is what a gravity-driven capillary viscometer measures. Dynamic viscosity, in centipoise (cP) or pascal-seconds, is what a rotational viscometer reports. The Cosmetic Ingredient Review’s 2022 amended safety assessment notes that viscosity for this ingredient class is reported in both forms. Converting kinematic to dynamic means multiplying by the fluid’s relative density and dividing by 1000 to reach Pa·s.

Registries trip on this too. FDA’s GSRS records a blend named “Dimethicone and Dimethiconol (6500 cP)” with an approximate viscosity of 6500 cP at 25 °C. The value sits under a property labeled VISCOSITY:KINEMATIC, although centipoise is a dynamic unit, and the sibling blend at 3200 cP is filed the same way. If a federal substance registry can file a dynamic value under a kinematic label, a supplier’s data sheet can too.

Because dimethicone fluids are a little less dense than water, a fluid’s cP figure comes out slightly below its cSt figure. That density gap is small; the larger traps are an unstated test temperature and, for high-viscosity fluids, a rotational reading taken at a shear rate where the fluid thins. Ask for the unit, the temperature and, for any figure in cP, the density at that temperature.

What belongs on the purchase specification?

1. Identity: dimethicone (polydimethylsiloxane), with the CAS number the supplier’s documents use, read as confirming the family and not the grade.

2. Kinematic viscosity in cSt at a stated temperature, an acceptance range agreed in writing, and the test method named on every CoA, for example ASTM D445.

3. For any figure reported in cP, the density at the same temperature, so you can check the conversion yourself.

4. For dampers and viscous couplings, heat and shear stability, and a viscosity result on every lot, not only the first.

5. For foam control, whether you need straight fluid, a silica-filled compound or an emulsion, because the three are not interchangeable in a foaming system.

6. For plastics, whether the silicone goes on a surface (fluid) or into the melt (gum masterbatch).

7. For the plant, written confirmation that the receiving area and the end use sit outside any silicone-free zone.

Methodology: identity and registry data are from PubChem, FDA’s GSRS, the EU CosIng inventory and ECHA CHEM; the viscosity conversion is from the Cosmetic Ingredient Review’s 2022 amended safety assessment, and the application notes are from RawSource product records. Viscosity directions by job are qualitative, so confirm them against your equipment maker’s requirement, and read the hazard statements in the current Safety Data Sheet (SDS) before handling.