D5 Decamethylcyclopentasiloxane

Overview

D5 — decamethylcyclopentasiloxane, CAS 541-02-6 — is the ten-membered Si–O ring cyclic siloxane with two methyl groups on each silicon atom. Molecular formula C₁₀H₃₀O₅Si₅, molecular weight 370.8 g/mol, density 0.957 g/cm³ at 25°C, refractive index 1.396. Supplied as a clear colourless mobile liquid at ≥99.5% purity by GC, with D4 impurity ≤0.2%, D6+ impurity ≤0.3%, water content ≤30 ppm, acidity (as HCl) ≤2 ppm, and APHA colour ≤10. Boiling point 210°C at atmospheric pressure (130°C at 50 mmHg), flash point 78°C closed cup.

D5 is produced by the same fractional distillation process used for D4, but separated as the heavier-cyclic fraction. Distillation typically gives 30–35% D5 yield from the DMC mixture, vs 50–60% D4 yield. The higher boiling point of D5 (210°C vs 175°C for D4) is the key functional differentiator in cosmetic applications: D5 evaporates more slowly from skin and hair, providing the smooth dry-feel typical of antiperspirant aerosol carriers and dry-shampoo formulations, vs D4's faster evaporation that gives the immediate weightless feel of foundation and BB cream vehicles. Cost premium over D4 is typically 5–15% on a per-kg basis at Chinese-origin spot pricing — D5 is the slightly higher-value cyclic in the value chain due to lower yield from fractionation.

EU REACH SVHC designation: D5 listed in 2018 alongside D4 under the vPvB criterion. The regulatory timeline for D5 differs from D4: wash-off cosmetic products face a 0.1% D5 limit on a phased timeline ending 2030+; leave-on cosmetic products are under continued ECHA review with no specific limit announced yet but expected to follow the wash-off timeline plus 2–3 years. Industrial silicone products (sealants, lubricants, electronic encapsulation) are exempt from the cosmetic limits but still require REACH downstream-user registration.

Technical Specifications

Applications

Cosmetic and Personal-Care Direct Ingredient

The largest application — D5 is added directly to antiperspirant aerosol formulations (5–25%), hair-care leave-on products (smoothening serum, anti-frizz spray, dry shampoo at 10–40%), foundation and BB cream (volatile-vehicle carrier at 15–30%), sun-care lotion (water-resistant SPF formulations at 5–20%), and shave gel and aftershave (emollient and dry-feel agent at 3–10%). D5 deposits the active formulation onto skin or hair, then evaporates over 5–30 minutes leaving a non-greasy dry-feel finish.

D5-Derived Silicone Fluid

Medium-volatility silicone fluid (5–10 cP grades) where the residual D5 monomer in the finished fluid is the desired functional component. Unlike D4-feedstock production where the goal is to minimise residual cyclics, D5-derived fluid manufacture deliberately retains a controlled fraction (10–30%) of unpolymerised D5 in the final product to provide volatile-carrier behaviour.

Specialty Silicone Gum

D5 in conjunction with D4 to control chain-length distribution and polymer flexibility for high-modulus elastomer applications.

Optical-Clarity LED Encapsulation

D5 over D4 in some formulations because the higher boiling point reduces volatile loss during high-temperature curing of the encapsulant.

Selection Guide vs D4 in Cosmetic Formulation

D4 (boiling point 175°C, faster evaporation) for: foundation and BB cream (immediate dry-down to powder feel within 2–5 minutes); fast-evaporating mascara and eyeliner; lightweight skincare emulsions where weightless dry-feel matters more than residence time. D5 (boiling point 210°C, slower evaporation) for: antiperspirant aerosol (carrier must persist 30+ seconds during application); hair-care leave-on (smoothening serum needs 5–30 min residence to deliver smoothing effect); shave preparations where the moist phase must hold during shaving. Many cosmetic formulations blend D4 and D5 — typical antiperspirant uses 60% D5 + 40% D4; typical foundation 30% D5 + 70% D4.

Equivalent Grades

• Dow Cyclopentasiloxane — primary cosmetic-grade D5
• Wacker AK Cyclo D5 — alternative European cosmetic-grade
• Shin-Etsu KF-995 — Japanese cosmetic-grade
• Generic 99.5% D5 — Chinese commodity grade with REACH packet