DMC (Dimethyl Cyclic Siloxane Mixture, D3–D7 Cyclic Blend, CAS 69430-24-6): The Dominant Feedstock for Silicone Polymer Manufacture
SEMITECH DMC is the dimethyl cyclic siloxane mixture — a D3–D7 blend dominated by D4 at 50–70% — the primary feedstock for ring-opening polymerisation of all standard silicone fluids, gums, and rubbers. ~25% lower cost than fractionated D4. 200 kg HDPE drum or 25 t ISO-tank.
Contents
| 69430-24-6 | 50–70% D4 | 25 t |
|---|---|---|
| CAS number | Cyclic distribution | ISO-tank quantity |
Chemistry & Specifications
D3–D7 cyclic siloxane mixture; clear colourless mobile liquid; ~25% lower cost than fractionated D4.
DMC (CAS 69430-24-6) — unfractionated hydrolysis product of dimethyldichlorosilane. Typical distribution: D3 1–5%, D4 50–70%, D5 20–35%, D6 3–8%, D7+ 1–3%. Total cyclic ≥98%, water ≤50 ppm, acidity ≤5 ppm, APHA ≤30. Viscosity 2.5–3.5 cP, density 0.95 g/cm³. Downstream polymerisation reaches thermodynamic equilibrium independent of starting cyclic distribution, making fractionated D4 unnecessary for most applications.
Polymerisation Pathways: ROP and Equilibration to PDMS Polymer
KOH or H₂SO₄ catalyst opens cyclic ring; equilibrates chain length to thermodynamic distribution; produces PDMS fluid/gum.
Ring-opening polymerisation under acid or base catalysis at 80–150°C. The equilibrium is independent of starting cyclic distribution.
- 50–500 cP fluid — cosmetic, lubricant, mould-release; 30–200 Si-O units
- 1,000–60,000 cP fluid — hydraulic, dielectric, defoamer base; 200–1000 units
- >100,000 cP gum — HCR/HTV rubber base polymer; 1000–10,000 units
- OH-terminated PDMS — RTV sealant base; produced by water-quenched ROP
After polymerisation, residual cyclics are vacuum-stripped and recycled — DMC manufacturing operates as a closed-loop process.
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Applications & Procurement Context
Silicone fluid manufacture (largest), HCR/HTV gum, RTV base polymer, silicone resin, defoamer base.
Silicone fluid (largest) — methyl silicone fluid, modified silicone fluids, emulsifiable cosmetic fluids. HCR/HTV gum — 95–98% on total monomer. RTV base polymer — OH-terminated PDMS for sealants. Silicone resin — D-unit source for MQ and DT resins. Defoamer base — medium-viscosity PDMS for MeFoam compounds. DMC is a commodity with 600,000+ MT/year global capacity (China 70%+).
Procurement, Storage and Quality Control
CoA per shipment; 200 kg drum or 25 t ISO-tank; 24-month shelf life; non-flammable Class III combustible.
CoA per batch: total cyclic (≥98%), individual D3/D4/D5/D6/D7 by GC, water (≤50 ppm), acidity (≤5 ppm), APHA (≤30). MOQ 200 kg; 25 t ISO-tank for bulk. Lead time 1–2 weeks Asia, 4–6 weeks Europe/North America. Flash point 49°C for D4-rich blends. Protect from moisture. D4/D5 content triggers REACH SVHC classification — industrial feedstock use permitted; downstream products must comply with residual-cyclic limits.
DMC Specification Sheet
SEMITECH stocked grade; CoA per batch with full cyclic distribution.
| Property | Specification | Test Method |
|---|---|---|
| Chemical name | Dimethyl cyclic siloxane mixture (D3-D7) | — |
| CAS number | 69430-24-6 | — |
| Total cyclic content | ≥98% | Gas chromatography |
| D3 content | 1–5% | GC |
| D4 content | 50–70% | GC |
| D5 content | 20–35% | GC |
| D6 content | 3–8% | GC |
| D7+ content | 1–3% | GC |
| Linear oligomer impurity | ≤2% | GC |
| Water content | ≤50 ppm | Karl Fischer |
| Acidity (as HCl) | ≤5 ppm | Potentiometric titration |
| APHA colour | ≤30 | ASTM D1209 |
| Density (20°C) | 0.95 g/cm³ | ASTM D1475 |
| Viscosity (25°C) | 2.5–3.5 cP | Brookfield |
| Flash point | 49°C (closed cup, D4-rich blend) | ASTM D93 |
| Boiling range | 134–250°C (atmospheric) | — |
| EU REACH classification | SVHC (D4/D5 vPvB) — industrial-feedstock use permitted | — |
| Packaging | 200 kg HDPE-lined steel drum / 25 t ISO-tank | — |
| Shelf life | 24 months sealed below 30°C | — |
FAQ
+DMC or fractionated D4 for silicone fluid manufacturing?
DMC for commodity grades (5–60,000 cP), HCR/HTV gum, RTV base polymer — polymerisation equilibrates regardless of starting distribution. D4 only when regulatory residual-cyclic control is required (cosmetic, pharmaceutical, optical-clarity).
+How does the D4/D5 SVHC designation affect downstream products?
Personal-care leave-on products must report D4 >0.1% (2027 limit); wash-off D5 limits phased 2030+. Industrial silicone is exempt from cosmetic limits but still requires REACH registration and SDS disclosure.
+Typical DMC-to-finished-fluid conversion efficiency?
Commercial plants achieve 92–96% mass conversion. Modern closed-loop cyclic-recovery systems reach 96%+; older plants without recovery operate at 88–92%.