Cyclopentasiloxane for Skin: Safety, Pores, REACH Clock

A contract manufacturer ships a leave-on hair serum to a sourcing partner in Hamburg. The cyclopentasiloxane carrier is 30% of the formula, and the certificate of analysis (CoA) puts residual octamethylcyclotetrasiloxane (D4) at 0.14% by weight. Under REACH Annex XVII entry 70, that one number can hold the lot at the border. The silicone is performing on the hair. The trace impurity it carries is the compliance problem.

That gap, between a material that works on skin and a material that clears a regulator, is where most cyclopentasiloxane decisions now get made. If you formulate, source, or resell personal-care products into more than one market, you are buying two things at once: a sensory ingredient and a moving regulatory position. This guide separates the two, with the chemistry that explains the skin feel and the named deadlines a buyer has to put on a calendar.

What cyclopentasiloxane is, and why formulators reach for it

Cyclopentasiloxane is the INCI name for decamethylcyclopentasiloxane, the cyclic silicone known across the trade as D5. Its CAS number is 541-02-6, molecular formula C10H30O5Si5, molecular weight 370.77 (PubChem, CID 10913). It is a colorless, low-odor liquid, listed by the Merck Index as a volatile liquid, with viscosity of 3.9 cSt at 25 C and a surface tension of 18.5 dyne/cm.

Two of those constants explain the skin feel buyers pay for. The low surface tension lets a drop spread fast and thin across the stratum corneum, giving the instant slip a formulator wants in a primer or an antiperspirant. The high volatility, boiling point 210 C and vapor pressure near 0.2 to 0.3 mmHg at 25 C, means the fluid flashes off within minutes and leaves a dry, powdery finish instead of grease. That is why D5 shows up as the carrier in foundations, hair serums, sunscreens, and roll-on antiperspirants: it delivers actives, then disappears.

It also carries other ingredients without water. Cyclopentasiloxane is effectively water-insoluble, 1.7 x 10-2 mg/L at 25 C, with a log Kow of 8.06. For the solubility side of that behavior, see our companion note on whether cyclopentasiloxane is water-soluble; for the broader property profile, see understanding cyclopentasiloxane benefits, uses, and properties.

Does cyclopentasiloxane clog pores or cause acne?

This is the question that drives consumer search, and the answer sits in the volatility data. A comedogenic ingredient earns that label by forming a film that traps sebum and keratin inside the follicle. D5 does the opposite. Researchers measuring evaporation from cosmetic surfaces recorded mean release rates of 0.029 mg/cm2/min at 23 C and 0.060 mg/cm2/min at 32 C (PubChem). At skin temperature, the fluid is leaving, not settling in.

Because it does not persist as an occlusive layer, formulators treat cyclopentasiloxane as non-comedogenic, and it commonly scores 0 on legacy comedogenicity scales. The honest caveat: comedogenicity is a property of the finished formula, not a single raw material. A serum that breaks someone out can still contain D5; the cause is usually a heavier emollient, a wax, a film-forming polymer, or a fragrance riding in the same base. D5’s own risk file is environmental, not dermatological, which is the reason the regulatory pressure below targets persistence in water, not safety on skin.

One practical consequence of the volatility: D5 does not build up across a wearing day the way a non-volatile oil can. The same property that makes it read as non-comedogenic also means a finished product has a short window to perform before the carrier is gone, so any active or pigment it delivers has to deposit during application, not after.

What changed in the regulations, and when

The skin-safety story is settled. The supply story is not. The European Union has been tightening the cyclic siloxanes for years, and 2024 widened the net.

On 13 June 2018, the Member State Committee identified D4, D5, and D6 as Substances of Very High Concern (SVHC). D4 is classified persistent, bioaccumulative, and toxic (PBT), and as a vPvB substance. D5 and D6 inherit the PBT classification when they contain 0.1% or more by weight of D4, which is the reason D4 trace content governs the whole family. The number to manage is the impurity, not the headline silicone.

Commission Regulation (EU) 2018/35 created REACH Annex XVII entry 70, barring D4 and D5 from wash-off cosmetic products at 0.1% or more by weight after 31 January 2020. Then Commission Regulation (EU) 2024/1328, dated 16 May 2024, rewrote that entry. It adds D6, and it sets a general rule: the three siloxanes shall not be placed on the EU market as a substance, as a constituent of other substances, or in mixtures at 0.1% or more by weight after 6 June 2026, with a separate leave-on cosmetics deadline of 6 June 2027. ECHA keeps the consolidated picture on its siloxanes hot-topics page.

The United States runs on a different clock. D5 remains permitted in US cosmetics, and the Cosmetic Ingredient Review supports that use. EPA granted a manufacturer-requested risk evaluation for D4 under TSCA in October 2020 and released the draft risk evaluation for D4 in September 2025 for public comment and peer review. Nothing forces a US reformulation today, but the direction of travel is visible.

How D5 compares to its siloxane siblings

When you reformulate away from D5, the obvious neighbors carry the same restriction. The spec table makes the trade clear.

Constants from PubChem CIDs 11169, 10913, and 10911. Heavier D6 flashes off more slowly and feels less dry; lighter D4 evaporates faster but is the regulated impurity everyone is trying to keep out. None of the three is an EU exit on its own, which pushes formulators toward non-siloxane volatiles such as isododecane when a clean leave-on claim matters.

Who the restrictions affect

Map this to where D5 sits in your catalog. The exposure is widest in beauty and personal care, where leave-on products are the workhorse: foundations, primers, color cosmetics, sunscreens, hair serums, and roll-on or stick antiperspirants all lean on volatile silicones for slip and dry-down. Those are the SKUs that hit the 6 June 2027 EU leave-on deadline.

Beyond the skin, D5 and its silicone-fluid family appear in household and institutional cleaning, in textile finishing, and as a carrier in industrial care products. The 2024 regulation also bars use as a dry-cleaning solvent for textiles, leather, and fur after 6 June 2026. A buyer with both an EU cosmetics line and US distribution faces the hardest case: the same cyclopentasiloxane SKU is compliant in one market and on a countdown in the other.

Private-label and contract-manufacturing arrangements widen that exposure. If a third party formulates your brand, the D5 sits inside their base and the residual D4 rides on their incoming silicone lots, so the compliance liability lands on a spec you may never see. Brands that sell into the EU through a sourcing partner inherit the deadline regardless of who blended the product.

Compliance steps, each with a deadline

1. Inventory now. List every SKU that contains D5, D4, or D6, by INCI and by weight percent. A label that reads only “cyclomethicone” is a flag, because the term can cover a D4/D5/D6 blend.
2. Pull CoAs now. Confirm residual D4 content is below 0.1% by weight on each lot. This single spec governs whether D5 and D6 inherit the PBT classification.
3. Wash-off EU products: already in force. D4 and D5 above 0.1% have been barred since 31 January 2020. Verify, do not assume.
4. Other products and mixtures into the EU: before 6 June 2026 for the general 0.1% placing-on-market limit, and the same date for dry-cleaning solvent uses.
5. Leave-on EU cosmetics: before 6 June 2027. This is the long pole for most personal-care lines and the deadline to build a reformulation schedule around.
6. US SKUs: no action required, but track the EPA D4 draft risk evaluation opened in September 2025 and keep your dual-market formulas separable.

Typical documentation gaps buyers miss

The most common miss is a CoA that certifies D5 assay but says nothing about residual D4. Ask the supplier for D4 quantified by GC-MS against the 0.1% threshold, in writing, per lot. A purity figure for the main ingredient does not answer the question the regulator asks.

The second gap is the “cyclomethicone” label. The trade name is generic and can describe different ring sizes or blends, so confirm the exact CAS numbers present instead of accepting the family name. The third gap is treating US permissibility as EU permissibility; the two markets have diverged, and a single global spec sheet no longer covers both. The fourth is forgetting D6, which the 2024 regulation pulled in alongside D4 and D5.

Cost and timeline of staying compliant

The real cost here is time, not unit price. A leave-on reformulation is not a drop-in swap. Moving from D5 to a volatile alternative changes the evaporation curve and the dry-down feel. It can shift the refractive index and disturb SPF or pigment dispersion, so each affected SKU needs fresh sensory work plus a new stability and compatibility study before it can ship.

Count backward from 6 June 2027 for EU leave-on lines. A reformulation and a stability hold, then dossier and artwork updates, consume most of the runway that looks generous today. A typical accelerated stability study alone occupies a fixed block of calendar before a single lot can move, and that block does not compress because the deadline is near. The analytical burden is smaller but recurring: GC-MS confirmation of residual D4 below 0.1% on incoming lots is the test that keeps a compliant formula compliant. For US-only lines, the near-term cost is monitoring, not reformulation, with the EPA draft as the trigger to revisit. The trade-off a buyer weighs is real: hold D5 for its sensory edge and absorb a hard EU exit date, or move early to a non-siloxane volatile and accept a finish that testers can usually distinguish from the original.

What to ask your supplier

• What is the residual D4 content of this D5 lot, by GC-MS, expressed as percent by weight against the 0.1% threshold?
• Can you supply per-lot CoAs that quantify D4, D5, and D6 separately instead of a single cyclomethicone figure?
• What is your roadmap and lead time for an EU-compliant volatile alternative, and can you support sensory matching to our current D5 grade?
• Do you certify the same material for both US and EU markets, and where do the specifications diverge?

A buyer who can recite this material, the 210 C boiling point that gives the dry finish, the 0.1% D4 rule that governs classification, and the 6 June 2027 leave-on date, is the buyer who keeps lots moving while competitors discover the deadline at customs. RawSource stocks cyclopentasiloxane and its silicone-fluid neighbors with lot documentation built for exactly these questions, so a sourcing team can verify D4 trace and plan a reformulation window on the same call.

Sources and methodology: physical constants are drawn from PubChem (CIDs 10913, 11169, 10911); regulatory dates and thresholds are quoted from Commission Regulations (EU) 2018/35 and 2024/1328, ECHA, and the EPA TSCA D4 risk-evaluation page, each linked above. No proprietary RawSource figures are used in this article.

RawSource Editorial

Commercial & Sourcing Desk