D4 Octamethylcyclotetrasiloxane

Overview

D4 — octamethylcyclotetrasiloxane, CAS 556-67-2 — is the eight-membered Si–O ring cyclic siloxane with two methyl groups on each silicon atom. Molecular formula C₈H₂₄O₄Si₄, molecular weight 296.6 g/mol, density 0.956 g/cm³ at 25°C, refractive index 1.396. Supplied as a clear colourless mobile liquid at ≥99.5% purity by GC, with D3 impurity ≤0.2%, D5+ impurity ≤0.3%, water content ≤30 ppm, acidity (as HCl) ≤2 ppm, and APHA colour ≤10. Boiling point 175.7°C at atmospheric pressure (115°C at 50 mmHg for vacuum distillation), flash point 57°C closed cup.

D4 is produced by fractional distillation of the DMC mixture under reduced pressure — typically a 50-plate distillation column with 5–10:1 reflux ratio at 50–100 mmHg pressure separates D4 from the surrounding D3 and D5 cyclics with a typical fractional purity of 99.5–99.9%. The cost premium over DMC is 25–35% on a per-kg basis, reflecting the distillation energy and capital cost of the fractionation column. Choose D4 over DMC when application-specific purity requirements justify the cost premium: cosmetic-grade silicone fluid manufacture (EU REACH 2027 residual D4 limits in personal-care products); pharmaceutical USP-NF dimethicone; optical-clarity silicone for LED encapsulation; laboratory hydrosilylation R&D.

EU REACH SVHC designation: D4 listed in 2018 under the vPvB criterion. Industrial use of D4 as silicone polymer feedstock remains permitted under REACH industrial-use registrations. Personal-care leave-on products are subject to declining D4 residual limits — ≤0.1% from 2027, with further restrictions under ECHA review. SEMITECH provides full REACH compliance documentation packet on every shipment: registrant ECHA number reference, SVHC notification template, quantitative D4 content declaration, SDS in EU/GHS format with full SVHC classification disclosure, toxicology dossier reference.

Technical Specifications

Applications

Cosmetic-Grade Silicone Fluid

The largest single application for fractionated D4. Hair-care silicone fluid (smoothening serum, anti-frizz spray), skin-care emollient (volatile-vehicle silicone for foundation, BB cream), and personal-care emulsifier all use D4-derived dimethicone with documented cyclic-content control. EU REACH 2027 timeline restricts D4 residual content in leave-on personal-care products to ≤0.1% by weight; achieving this in finished cosmetic silicone fluid requires starting from controlled-purity D4 feedstock and implementing rigorous post-polymerisation vacuum stripping.

Pharmaceutical USP-NF Dimethicone

Medical-device coatings, drug-delivery silicone, pharmaceutical-process anti-foam (specifically MeFoam-FG) use D4 as the controlled-purity feedstock. The USP-NF dimethicone monograph requires documented control of cyclic siloxane content, heavy-metal residue, and microbial limits.

Optical-Clarity LED Encapsulation

Trace D3 and D5 impurities can phase-separate under the high-temperature operating conditions (120–150°C continuous LED junction temperature) and produce photonic clarity defects. Use D4 at 99.9%+ specification for optical-grade silicone encapsulation.

Laboratory Hydrosilylation R&D

D4 as a model cyclic substrate for kinetic and mechanism studies because of its single-cyclic chemistry and well-characterised thermodynamics.

Selection Guide vs DMC

Order DMC for commodity silicone polymer manufacture (95% of the volume use case). Order fractionated D4 only when: cosmetic leave-on regulatory limits apply; pharmaceutical USP-NF compliance pathway is required; optical-clarity application demands trace-impurity control; laboratory R&D requires single-cyclic substrate.

Equivalent Grades

• Dow Cyclomethicone — primary cosmetic-grade D4
• Wacker AK Cyclo — alternative European cosmetic-grade
• Shin-Etsu KF-994 — Japanese cosmetic-grade
• Generic 99.5% D4 — Chinese commodity grade with REACH packet