Eight-membered Si-O ring with 8 methyl groups; clear colourless liquid; bp 175°C; SVHC vPvB classification.

D4 — octamethylcyclotetrasiloxane, CAS 556-67-2 — is a cyclic siloxane consisting of an eight-membered Si-O ring 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. SEMITECH supplies D4 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 require controlled-purity feedstock); pharmaceutical USP-NF dimethicone (regulatory pathway requires documented cyclic-content control); optical-clarity silicone for LED encapsulation (trace cyclic impurity affects photonic clarity); laboratory hydrosilylation R&D (single-cyclic substrate for kinetic studies).

Cosmetic silicone fluid, USP-NF dimethicone, LED optical encapsulation, hydrosilylation chemistry research.

Cosmetic-grade silicone fluid manufacture is the largest single application for fractionated D4. 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. 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 through the manufacturing chain.

Pharmaceutical USP-NF dimethicone for medical-device coatings, drug-delivery silicone, and pharmaceutical-process anti-foam (specifically MeFoam-FG) uses D4 as the controlled-purity feedstock. The USP-NF dimethicone monograph requires documented control of cyclic siloxane content, heavy-metal residue, and microbial limits — fractionated D4 is the regulatory-compliant feedstock for this pathway. Optical-clarity silicone for LED and photonic encapsulation uses D4 at 99.9%+ specification because 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. Laboratory hydrosilylation chemistry uses D4 as a model cyclic substrate for kinetic and mechanism studies because of its single-cyclic chemistry and well-characterised thermodynamics.

D4 listed on REACH SVHC candidate list (vPvB); industrial-feedstock use permitted with documented compliance pathway.

D4 is on the European Chemicals Agency (ECHA) REACH SVHC candidate list — listed in 2018 under the vPvB (very persistent, very bioaccumulative) criterion based on environmental persistence in soil and aquatic systems. Implications for industrial supply chain: (1) D4 supply for use as silicone polymer feedstock is permitted under REACH industrial-use registrations; (2) downstream silicone products containing >0.1% D4 are subject to SVHC notification and Annex XIV authorisation requirements; (3) personal-care leave-on products are subject to declining D4 residual limits — ≤0.1% from 2027, with further restrictions under ECHA review.

SEMITECH provides a full REACH compliance documentation packet on every D4 shipment: (1) REACH industrial-use registration reference (registrant ECHA number); (2) SVHC notification template for downstream-user submission; (3) quantitative declaration of D4 content (always 99.5–99.9% by GC); (4) SDS in EU/GHS format with full SVHC classification disclosure; (5) toxicology dossier reference for downstream regulatory pathway. Customer regulatory affairs teams should review their finished-product compliance pathway against REACH downstream-user requirements before specifying D4 feedstock for personal-care or medical-device manufacturing.

CoA per shipment with cyclic GC breakdown; 200 kg drum; 24-month shelf life; flash point 57°C requires Class IB storage.

SEMITECH issues a CoA on every batch with: D4 purity (GC, target ≥99.5%), D3 impurity (GC, target ≤0.2%), D5+ impurity (GC, target ≤0.3%), water content (Karl Fischer, target ≤30 ppm), acidity as HCl (potentiometric, target ≤2 ppm), APHA colour (target ≤10), density at 25°C, refractive index, and flash point. Standard packing 200 kg HDPE-lined steel drums; 1 t IBC totes for high-volume cosmetic and pharmaceutical customers; 25 t ISO-tank for bulk silicone polymer manufacturers (rare for D4 vs DMC ISO-tank). MOQ 200 kg per order. Lead time 2–4 weeks ex-Zhejiang to Asia ports, 4–6 weeks to Europe and North America after sea freight (REACH compliance documentation customs review can add 1–3 days).

Storage: D4 has flash point 57°C — Class IB flammable liquid under OSHA 29 CFR 1910.106 and EU CLP Flammable Liquid Category 3. Storage requirements: dedicated flammable-storage cabinet or warehouse area with adequate ventilation; ground all transfer equipment for static dissipation; no open flames within 5 metres of transfer operations; spill containment for full drum capacity. Shelf life 24 months sealed below 30°C in original packaging; opened drums subject to slow ring-opening to higher cyclics on prolonged ambient exposure — re-test cyclic distribution if storage exceeds 12 months opened. Health and regulatory: REACH-registered for industrial silicone polymer feedstock use; FDA 21 CFR 175.300 listed for indirect food-contact in adhesive components; mild skin and eye irritant; standard PPE — nitrile gloves, splash goggles, fume hood ventilation. Full REACH SVHC documentation packet issued with every shipment.

D4 is fractionated cyclic siloxane — ≥99.5% pure octamethylcyclotetrasiloxane for cosmetic-grade, pharmaceutical-grade, and optical-clarity silicone where DMC mixture purity is insufficient. EU REACH SVHC vPvB classification — industrial feedstock use permitted with full compliance documentation. ~25% cost premium over DMC.

SEMITECH stocked grade; CoA per batch with cyclic GC distribution.