What Is Silicone Oil (PDMS)? Viscosity Grades, Properties, and Industrial Uses

The same name, silicone oil, sits on a drum of thin, fast-spreading liquid and on a drum of fluid as thick as cold honey. One lubricates a valve. Another damps a gauge needle, another releases a molded rubber part, and another quiets the foam in a reactor. They are all the same chemistry, and the viscosity grade is what decides which job a given drum can do.

Silicone oil is polydimethylsiloxane (PDMS), also called dimethicone (CAS 63148-62-9 / 9006-65-9): a clear, colorless, odorless, chemically inert and thermally stable silicone fluid that is graded by viscosity in centistokes (cSt). The same base chemistry runs from thin 5 cSt carrier fluids to 100,000 cSt damping gums; chain length sets the viscosity, and viscosity is what you actually specify when you buy. Those properties make it a workhorse for lubrication, mold release, damping, dielectric insulation, polishes, personal care, and foam control, with the grade matched to the application.

What silicone oil is

Silicone oil is a polymer of repeating silicon-oxygen (siloxane) units carrying methyl groups, polydimethylsiloxane (PDMS, CAS 63148-62-9, PubChem). The same material named in personal-care and pharmaceutical-excipient contexts is dimethicone, and the CAS 9006-65-9 you see on some spec sheets is the polymer family name for the same fluid. The longer the polymer chain, the higher the viscosity, which is why one chemistry can span a thin pourable liquid and a fluid you can stand a spoon in. It is clear, odorless, and largely inert; it does not readily oxidize, char, or evaporate under normal use, and its viscosity changes far less with temperature than a mineral oil’s does.

That last point matters at the purchase order. A 30-weight motor oil thins dramatically when it gets hot; a 350 cSt silicone fluid holds much of its viscosity across the same swing, which is why it shows up in damping and heat-transfer service where a hydrocarbon would thin out and fail. If you are choosing between a silicone fluid and an organic oil, that flat viscosity-temperature curve is usually the deciding spec.

The viscosity range: how buyers actually shop

Viscosity, measured in centistokes (cSt) at 25 degrees C, is the defining variable, and choosing the grade is most of the buying decision. The table below maps the common grades to where they land. RawSource stocks 100, 350, 1000, 10000, and 12500 cSt as standard inventory grades, shown in bold, and can source the rest of the range to order.

The pattern reads cleanly: low cSt is volatile, carrier, and release; mid cSt is the general lubricant, antifoam, and personal-care band; high cSt is for damping, dielectric, and mold release. If you are not sure where your job lands, start with the function and the temperature, then pick the grade. The grade-by-grade selection logic is covered in the silicone oil viscosity guide.

Key properties

The reason one fluid does so many jobs is a stack of properties that organic oils do not deliver together. The figures below are typical reference values for general-purpose PDMS; validate the exact numbers against the Certificate of Analysis for the grade you buy.

One honest caveat lives in that volatility row. Low-viscosity grades are useful precisely because they evaporate, which also means a 5 or 10 cSt fluid is the wrong choice where you need the lubricant to stay put. If a film has to survive heat and time, move up the viscosity ladder.

Types of silicone oil

Most of what ships as silicone oil is straight PDMS, but the family has a few important variants. The right type is a function of what you need beyond viscosity: a higher refractive index, more thermal headroom, surface reactivity, or fast evaporation.

• Dimethicone / PDMS (standard): the default, methyl-terminated fluid covered above. Most lubrication, release, damping, polish, and personal-care use runs on this. Buy it as dimethicone (PDMS).
• Phenyl silicone oils: some methyl groups replaced with phenyl, raising the refractive index and extending high-temperature and radiation stability. Used in optical coupling, specialty heat-transfer, and high-temperature lubrication where straight PDMS oxidizes too soon.
• Amino- and reactive-modified silicones: functional groups (amino, hydroxyl, epoxy, vinyl) grafted on so the fluid can bond to a substrate or cure. These are the workhorses of textile softening, paper release, and water-repellent finishing where you want the silicone to stay anchored, not migrate.
• Cyclomethicone (volatile): cyclic siloxanes that flash off fast and leave a dry, non-greasy feel. Primarily a personal-care carrier and a delivery vehicle that vanishes after application; not a persistent lubricant.

Applications by industry

The same drum can do very different work depending on the grade and the modifier. Here is where silicone oil earns its keep, with the grade band that usually fits.

• Lubricants and release agents: low-friction, wide-temperature lubrication for plastic-on-plastic, rubber, and plastic-on-metal contacts where a hydrocarbon would attack the elastomer or wash off. 100 to 1000 cSt covers most of it; mold and process release usually runs 100 to 350 cSt.
• Antifoam and defoamers: silicone’s low surface tension collapses foam in coatings, fuels, fermentation, and chemical processing. Higher-viscosity fluids (1000 cSt and up, often compounded with silica) are the active base for defoamer concentrates.
• Personal care and cosmetics: dimethicone delivers slip, a non-greasy skin feel, and a smoothing film in lotions, hair products, and skin care; cyclomethicone carries actives and flashes off dry. Personal-care use is governed by its own regulations and labeling.
• Dielectric, transformer, and electronics: high dielectric strength plus thermal stability make qualified silicone fluids an insulating and cooling medium for transformers, capacitors, and electronics potting; this is a low-moisture, purpose-qualified grade, not general-purpose oil.
• Heat-transfer and damping fluids: the flat viscosity-temperature curve and high flash point suit closed-loop heat transfer and viscous damping in dashpots, instruments, and dampers, where 1000 to 12500 cSt grades set the damping rate.
• Mold release: a thin, clean-releasing film for rubber, urethane, and plastic molding that does not transfer or stain; 100 to 1000 cSt depending on the surface and cure.
• Polishes and surface treatments: gloss, water repellency, and protection for automotive, furniture, and industrial finishes; mid grades give durable shine, and the hydrophobic film sheds water.
• Textile and leather softeners: reactive amino-functional silicones bond to fiber for a durable soft hand and water repellency that survives washing; 350 cSt and modified grades are typical.
• Medical-device lubricity (informational): medical-grade silicone fluids are used to lubricate device surfaces for low-friction movement. This is informational only; device-grade material, qualification, and any medical use are governed by separate regulation and are not addressed here.
• Paint and coating additive: small additions improve flow, leveling, and anti-cratering and reduce surface defects; dosing is low and grade selection matters because the wrong silicone can cause the very fisheyes you are trying to prevent.

A note on grades: not all 50 cSt is the same

One distinction shows up at the purchase order and trips up first-time buyers. A standard silicone oil and a purpose-purified grade can share a viscosity number yet not be the same product. For electrical and heat-transfer service, a low-moisture, qualified grade is used, which is a different specification from a general-purpose silicone oil of the same viscosity. That grade is covered separately in silicone heat transfer oil. When you specify silicone oil, specify the grade as well as the viscosity, and put the application on the RFQ so we quote the right material rather than the cheapest fluid that matches the number.

Buying silicone oil in bulk

RawSource supplies silicone oil as dimethicone (PDMS) across the viscosity range, in drums, IBC totes, and bulk, with a Certificate of Analysis on every lot. The five stocked grades cover most industrial work:

• Silicone oil 100 cSt — general lubricant, mold release, polish base, personal care
• Silicone oil 350 cSt — lubricant, release agent, antifoam base, softener
• Silicone oil 1000 cSt — damping, lubrication, dielectric and polish service
• Silicone oil 10000 cSt — heavy damping, dashpots, dielectric and mold release
• Silicone oil 12500 cSt — heavy damping, instrument fluids, high-viscosity additive

Grades outside this set, and the phenyl, amino-functional, and volatile types, are sourced to order from the silicone fluids and oils line. To get a clean quote the first time, tell us three things: the viscosity grade in cSt, the volume and packaging (drums, totes, or bulk), and the application. We return pricing, lead time, and a CoA, and a sample to validate performance on your own system before you commit to a volume order. For larger sourcing programs, the chemical procurement guide walks through specs, documentation, and supply continuity.

Frequently Asked Questions

What is silicone oil?

Silicone oil is polydimethylsiloxane (PDMS), also called dimethicone, a clear, colorless, odorless, chemically inert, and thermally stable synthetic fluid. It is sold across a wide viscosity range, from about 5 cSt to over 100,000 cSt, and used for lubrication, mold release, damping, dielectric insulation, polishes, personal care, and foam control, with the grade chosen to fit the job.

Is silicone oil the same as dimethicone and PDMS?

Yes. Silicone oil, dimethicone, and polydimethylsiloxane (PDMS) all name the same base chemistry. PDMS is the chemical name, dimethicone is the name used in personal-care and pharmaceutical-excipient contexts, and silicone oil is the common industrial term. The viscosity grade and any purification or modification define fitness for a given use, not the name on the label.

What does cSt or viscosity mean for silicone oil?

Centistokes (cSt) is the unit of kinematic viscosity, measured at 25 degrees C, and it is how silicone oil is graded and sold. A 5 cSt fluid is thin and pourable; a 100,000 cSt fluid is near-gum thick. Because viscosity is set by polymer chain length, it is the single number that most determines whether a grade fits your application.

What is silicone oil used for?

Silicone oil is used as a lubricant and release agent, an antifoam and defoamer base, a dielectric and heat-transfer fluid, a viscous damping fluid, a mold-release agent, a polish and water-repellent treatment, a textile and leather softener, a paint and coating additive, and a personal-care ingredient. Low grades carry and release; mid grades lubricate and defoam; high grades damp and insulate.

How do I choose a silicone oil viscosity grade?

Start from the function and the temperature, then pick the grade. Low viscosity (5 to 50 cSt) carries, spreads, and releases. Mid viscosity (100 to 1000 cSt) lubricates, defoams, and serves personal care. High viscosity (5000 cSt and up) damps and acts as an additive base. When in doubt, request a sample of the stocked grade nearest your target and validate it on your own system.

Is silicone oil food-grade or safe?

Silicone oil is chemically inert and low in toxicity, and specific qualified grades exist for food-contact, personal-care, and pharmaceutical use under their own regulations. A general-purpose industrial grade is not automatically food-grade or device-grade. Confirm the exact grade, its qualification, and the Certificate of Analysis against your requirement, and consult the SDS before use.

Is silicone oil flammable?

Silicone oil is not classified as flammable in the way light solvents are; higher-viscosity grades carry high flash points, often above 300 degrees C, which is part of why they suit heat-transfer and electrical service. It is still a combustible organic fluid that will burn at sufficient temperature. Always confirm the flash point and handling guidance on the specific grade’s SDS.

Is silicone oil biodegradable?

No. PDMS is low in toxicity and chemically inert, but it is environmentally persistent and not readily biodegradable, so it is handled, stored, and disposed of as an industrial fluid. It is not a biodegradable or “green” product, and it should be managed per the SDS and local regulations rather than treated as an environmentally benign material.

Editorial note. This article is general technical guidance for industrial and professional buyers. Silicone oil here is an industrial PDMS fluid; nothing here is a cosmetic, medical, or efficacy claim, and personal-care (dimethicone) and device uses are governed by their own regulations. Property and viscosity figures are typical reference values to validate for your grade and application; the Certificate of Analysis governs the material you buy. The fluid is low in toxicity and chemically inert but is environmentally persistent, not a biodegradable or “green” product. Always consult the current Safety Data Sheet (SDS) before handling. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.