Cyclopentasiloxane: Properties, Formulation Function & Sourcing

A blender rejects your tote of “cyclomethicone.” The certificate of analysis (CoA) lists a closed-cup flash point of 55 °C and a boiling point near 175 °C. The antiperspirant on that line was qualified on D5, which boils at 210 °C and flashes at 73 °C. What arrived was D4-heavy. The fill line is down, QC has opened a deviation, and a “silicone is silicone” assumption just cost a production day. The fix starts with treating cyclopentasiloxane as a specific molecule with specific numbers, not a category.

The short version. Cyclopentasiloxane (INCI) is decamethylcyclopentasiloxane, or D5: CAS 541-02-6, formula C10H30O5Si5, molecular weight 370.77 (PubChem CID 10913). It is a volatile silicone fluid — boiling point 210 °C, closed-cup flash point 73 °C, vapor pressure 0.3 mm Hg at 25 °C, viscosity 3.9 cSt, surface tension 18.5 dyne/cm, and effectively water-insoluble (about 0.017 mg/L at 25 °C, log Kow 8.06). In a formulation it functions as a volatile carrier and solvent for higher-molecular-weight silicones and oil-soluble actives, as a spreading agent that lowers surface tension, and as a low-residue diluent that flashes off cleanly. Above the 60 °C flammable cutoff it is a combustible liquid, not a flammable one. The EU restricts D4, D5, and D6 in cosmetics under REACH Annex XVII, so residual D4 is the number that decides whether a lot clears for your target market.

What cyclopentasiloxane is

Cyclopentasiloxane is the INCI name for decamethylcyclopentasiloxane, the cyclic silicone the trade calls D5. It carries CAS 541-02-6, molecular formula C10H30O5Si5, and a molecular weight of 370.77 (PubChem CID 10913). It is a colorless, near-odorless volatile liquid. The “5” counts five silicon-oxygen units in the ring, and that ring size sets nearly every property a buyer writes onto a spec sheet.

The structure explains the behavior. D5 is a closed ring of alternating silicon and oxygen atoms, each silicon capped with two methyl groups that shield the backbone, so the molecule has almost no polarity and almost no reactivity at use temperatures. The result is a fluid that wets a surface, then evaporates intact instead of reacting or polymerizing. Its functional value is physical, not chemical. PubChem notes that on heating to decomposition it emits acrid smoke and irritating vapors — a handling concern at process temperatures, not a shelf-life one.

That physical value shows up as a set of formulation functions, defined by volatility and low surface tension rather than by any performance claim: D5 acts as a volatile carrier and solvent for higher-molecular-weight silicones and oil-soluble actives, as a spreading agent, and as a low-viscosity diluent that flashes off rather than remaining as a film. For the non-volatile silicones it is most often paired with, see our guide to polydimethylsiloxane (PDMS / dimethicone).

D5 sits in the middle of a family. Drop one ring unit and you get D4 (octamethylcyclotetrasiloxane, CAS 556-67-2). Add one and you get D6 (dodecamethylcyclohexasiloxane, CAS 540-97-6). The three are chemical neighbors with sharply different volatility, and the older blanket term “cyclomethicone” once covered all of them. Modern personal-care cyclomethicone is predominantly D5, but the label alone does not tell you the isomer split. The CoA does.

Grade and trade-name landscape

D5 reaches you under an INCI label, a cyclomethicone designation, or a supplier trade code. The molecule is the same; the grade is the contract. The table below maps the names you will see on quotes and CoAs against what they imply.

Trade-code equivalence is the trap. Two drums labeled as D5 equivalents can carry different residual D4 and different nonvolatile (high-boiling silicone) carryover, and both move the sensory profile and the regulatory exposure. Residual D4 is the single number that turns a routine purchase into a compliance problem, because D4 is the most heavily restricted member of the family. Treat the trade name as a starting point and the CoA as the spec.

D5 also ships inside finished blends, not only as a neat fluid. A typical hair- or skin-care concentrate pairs it with dimethicone for slip, phenyl trimethicone for refractive index and gloss, and a non-silicone emollient to tune the film. RawSource lists cyclopentasiloxane within its Beauty & Personal Care range alongside the related cyclic and linear fluids. When you buy a blend, the same discipline applies: ask which isomer the cyclomethicone fraction is, and at what residual D4.

How do D4, D5, and D6 differ in volatility?

Volatility is the property buyers most often get wrong, because it drives dry-down time, process loss during heating, and the flash point your safety data sheet (SDS) reports. The numbers below are experimental values from PubChem.

Read across the row and the trend is plain: each ring unit you add raises the boiling point by roughly 35 °C, drops the vapor pressure by about an order of magnitude, and pushes the flash point up. D5 evaporates fast enough to give a controlled dry-down, while sitting above the regulatory and handling thresholds that follow D4. D6 lingers longer and behaves more like a light fixed fluid than a true volatile carrier. If your formula was built on a D5/D6 ratio, a shift in that ratio changes the dry-down rate before any other ingredient does. The lower flash point of D4 is exactly why a D4 contaminant pulls a D5 lot toward the flammable line.

The dry-down rate is measurable, not a marketing word. One PubChem-cited study put the evaporation of neat D5 from an open surface at a mean of 0.029 mg/cm2/min at 23 °C, rising to 0.060 mg/cm2/min at 32 °C, with the rate roughly halved when D5 was held inside a finished emulsion matrix. That sensitivity is why dry-down behavior shifts with both ambient conditions and the rest of your formula, so a lab panel run in winter can read differently from a summer production batch. D5 is also effectively water-insoluble (about 0.017 mg/L at 25 °C, log Kow 8.06), so it stays in the oil phase. Where a silicone-free system is the target, the closest functional analogs are volatile hydrocarbons and lighter emollient esters — different molecules with different SDSs, qualified as reformulations rather than drop-in swaps.

Spec red flags on a cyclopentasiloxane CoA

A CoA that lists only “cyclomethicone, passes” is not a spec. Five things to question before you accept a lot:

1. No isomer breakdown. Demand the gas-chromatography split for D4, D5, and D6 as a percentage. A lot sold as D5 can carry meaningful D4, and that single number drives both the sensory profile and compliance. 2. Residual D4 above your market’s limit. For EU-bound product the rinse-off ceiling is 0.1 percent by weight. Ask for the measured residual D4, not a “complies” checkbox, and ask for the method behind the figure. 3. Flash point reported below about 70 °C. D5 should land near 73 °C closed cup. A lower figure points to lighter cyclics in the blend and can change how the lot is classified for transport and storage. 4. Missing acidity or chloride figure. Trace acid or chloride can drive slow hydrolysis and odor on aging. Ask for acid value or chloride in ppm, plus a confirmed appearance and clarity. 5. High nonvolatile residue. Carryover of high-boiling silicone leaves a film D5 is specified to avoid. A nonvolatile-content limit on the spec protects the low-residue behavior buyers select D5 for in the first place.

None of these checks slow a qualified supplier down. A principal who hesitates to put a residual-D4 number in writing is telling you something. Build the isomer split and residual D4 into the purchase specification, not just the incoming QC test, so the requirement travels with every reorder.

Is cyclopentasiloxane flammable or combustible?

The distinction is worth money, because it changes how a lot is stored and shipped. D5’s closed-cup flash point is 73 °C (163 °F) per PubChem. Both the GHS and the OSHA Hazard Communication Standard (29 CFR 1910.1200) draw the flammable-versus-combustible line at a 60 °C flash point. At 73 °C, D5 is a combustible liquid, not a flammable one, which keeps it out of the most restrictive transport and warehousing tiers.

That margin is thin enough to protect. D4 flashes at 55 °C, below the cutoff, so a D5 lot diluted with D4 can slide from combustible toward flammable and trigger a different transport classification. Aggregated supplier data on PubChem shows most registrants report D5 as not meeting GHS hazard criteria, with a minority flagging combustible-liquid (H227) and chronic aquatic (H413) endpoints — the latter the thread that runs through the EU restriction below.

Two physical numbers shape handling. D5 is water-insoluble and lighter than water at 0.9593 g/cm3, so a spill floats on a water surface instead of dissolving into it, and its 18.5 dyne/cm surface tension lets it creep into threads and seams, so seal integrity on totes and drums is worth checking at receipt. Bulk D5 typically moves in 200 kg steel drums, IBC totes, or ISO tanks, often with nitrogen blanketing to limit moisture pickup and slow evaporative loss in storage.

Regulatory status by region

Region drives the spec. In the EU, D4, D5, and D6 are restricted in cosmetics under REACH Annex XVII. The 0.1 percent by-weight limit in rinse-off products has applied since 2020, and a 2024 amendment broadened the restriction toward leave-on products and to D6. D4 separately carries the heavier flag: it holds the EU’s SVHC designation and is treated as PBT and vPvB. The practical takeaway is that residual D4 in your D5 is the number a European customer’s regulatory team will check first.

The environmental angle is physical. D5’s high Henry’s Law constant (33.0 atm-m3/mol at 25 °C) means it partitions out of water into air rather than persisting dissolved, which is part of why the regulatory focus has centered on aquatic and bioaccumulation endpoints for the cyclic siloxanes.

In the US there is no FDA pre-market approval for cosmetic ingredients under the Federal Food, Drug, and Cosmetic Act; responsibility for ingredient and product safety sits with the manufacturer of the finished good. The industry Cosmetic Ingredient Review panel has assessed the cyclomethicones, and D5 is listed as active on the TSCA inventory. None of this substitutes for a market-by-market check: a formula cleared for US sale can still need a residual-D4 reformulation for an EU launch, so confirm current regulatory status and limits for your application and jurisdiction, and build that into the sourcing spec early.

Grades, specs, and bulk sourcing

Properties tell you what to buy; sourcing discipline tells you how to keep buying it to the same number. RawSource supplies cyclopentasiloxane (D5) to cosmetic and industrial formulators from domestic US stock, in drums, IBC totes, and ISO-tank quantities, with a CoA per lot. Three checks pay for themselves on a bulk program.

First, write the purchase spec to a residual-D4 ceiling and an isomer split, not just to “cyclopentasiloxane.” The EU restriction concentrates demand on low-residual-D4 grades, so a single-source plan is also a single point of compliance failure; qualify a second source to the same numbers. Second, sample-test the isomer split on incoming lots, not only at first qualification, because trade-code equivalence does not guarantee lot-to-lot equivalence. A standard GC method and a disciplined incoming spec catch the failures that otherwise surface on the fill line. Third, hold the SDS and the CoA against each other: the flash point on the SDS should match the lot, and any drift toward 60 °C is an early warning of D4 creep.

Volatility also belongs in the total-cost math. The 0.3 mm Hg vapor pressure at 25 °C and the roughly 60 kJ/mol enthalpy of vaporization (PubChem) mean D5 carries off measurable mass during any open or heated step, so yield loss is a real line item on a hot-process formula. That is the trade-off: the same volatility you pay a premium for over a fixed silicone is what you lose to evaporation, so judge the buy on cost per validated kilogram on the line, not price per drum at the dock.

Formulation function, by application

D5 earns its place wherever a formula needs spreading and slip without a residual film. Each role below is a physical function of its volatility and 18.5 dyne/cm surface tension, not a performance promise:

• Antiperspirants and deodorants. A volatile carrier that distributes higher-MW silicones and oil-soluble actives across the application surface, then evaporates. Its 210 °C boiling point and 0.3 mm Hg vapor pressure set a controlled dry-down rate.
• Hair-care formulations. A volatile carrier and spreading agent for silicones such as dimethicone and phenyl trimethicone; it sheets across the substrate and flashes off, leaving the deposited silicone behind without the carrier.
• Color cosmetics and foundations. A spreading agent and volatile vehicle for long-wear, film-forming systems; pairs with silicone elastomers and waxes.
• Skin-care formulations. A low-viscosity volatile diluent and emollient-class fluid that flashes off rather than forming an occlusive film, used to reduce the residual oily film a fixed-oil system would otherwise leave in the formula’s sensory profile.
• Household and industrial care. Beyond personal care, D5 appears in automotive detailing fluids and polishes as a fast-flashing carrier that leaves gloss without residue.

Cyclopentasiloxane is a precise, well-characterized fluid once you stop treating it as a generic “silicone.” Pin down the isomer split and the residual D4, confirm the flash point, and the rest of the spec follows.

Request a quote or specification sheet

Tell us your target market (US, EU, or both), your residual-D4 ceiling, the isomer split you formulate to, and the packaging your line receives — drums, IBC totes, or ISO tank. We will return a current specification sheet, the measured residual-D4 figure, and a quote against domestic US stock, with a sample available to qualify the lot on your own system before you commit a formula. Send the CAS (541-02-6) and your annual volume to open an RFQ.

Frequently asked questions

Is cyclopentasiloxane the same as cyclomethicone?

Not exactly. Cyclopentasiloxane (D5, CAS 541-02-6) is one specific cyclic silicone; “cyclomethicone” is an older blanket term that can mean D4, D5, D6, or a blend. Modern personal-care cyclomethicone is usually D5-dominant, but the label alone does not confirm the isomer split or the residual D4 — the CoA does. Specify D5 by CAS and require the GC breakdown.

What residual D4 level should I specify in cyclopentasiloxane?

Tie it to your market. For EU-bound rinse-off products, REACH Annex XVII sets a 0.1 percent by-weight ceiling on D4, D5, and D6, and a 2024 amendment broadened the scope. Write a measured residual-D4 limit and the test method into the purchase spec, ask for the figure on every lot’s CoA, and confirm the current limit for your specific product type and jurisdiction.

What CoA data should I require before accepting a D5 lot?

At minimum: the GC isomer split (D4/D5/D6 as a percentage), the measured residual D4 with method, the closed-cup flash point, an acid value or chloride figure in ppm, appearance and clarity, and a nonvolatile-residue limit. A CoA that says only “cyclomethicone, passes” is not a specification and should not be accepted into a bulk program.

Can a D5 grade be swapped supplier-to-supplier without requalification?

Treat it as a requalification, not a drop-in. Two materials sold as D5 equivalents can differ in residual D4 and in nonvolatile carryover, both of which move sensory behavior and regulatory exposure. Qualify any second source to the same residual-D4 and isomer spec, and sample-test incoming lots rather than relying on trade-code equivalence.

Editorial note. This article is general technical guidance for cosmetic and industrial formulation and procurement professionals. Physical properties, volatility, regulatory status, and sensory behavior depend on your specific grade, isomer split, residual D4, formulation, and process, and must be validated on your own system; the Certificate of Analysis governs the lot you buy. Figures are compiled from authoritative public sources (PubChem, ECHA, OSHA) and are typical reference values, not a guaranteed specification. Regulatory limits change and vary by jurisdiction — confirm current REACH, TSCA, and market requirements for your product before you commit a formula. Products are sold for industrial and professional use only. Nothing here is a medical, health, cosmetic-efficacy, or safety claim. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.