DMC Cyclic Mixture

Overview

DMC — dimethyl cyclic siloxane mixture, CAS 69430-24-6 — is the unfractionated cyclic siloxane blend obtained from the hydrolysis of dimethyldichlorosilane in the Müller-Rochow direct synthesis pathway. The mixture composition typically runs: D3 (hexamethylcyclotrisiloxane) at 1–5%, D4 (octamethylcyclotetrasiloxane) at 50–70%, D5 (decamethylcyclopentasiloxane) at 20–35%, D6 at 3–8%, D7+ at 1–3%. Supplied at ≥98% total cyclic siloxane content (linear oligomer impurity ≤2%), water content ≤50 ppm, acidity (as HCl) ≤5 ppm, and APHA colour ≤30. The compound is a clear colourless mobile liquid (viscosity 2.5–3.5 cP at 25°C) with a faint silicone odour, density 0.95 g/cm³ at 20°C, boiling range 134–250°C at atmospheric pressure.

DMC is the dominant feedstock for silicone polymer manufacture in China and globally — the cost differential to fractionated D4 is typically 25–35% on a per-kg basis, which is significant in commodity silicone fluid and gum production where raw-material cost dominates. Downstream polymerisation processes — ring-opening polymerisation (ROP) for silicone fluid and gum, anionic equilibration for high-MW gum, and acid- or base-catalysed equilibration for blended fluids — all give equivalent chain-length distributions whether starting from DMC or fractionated D4 because the polymerisation reaches thermodynamic equilibrium that is determined by the temperature and catalyst, not the cyclic feedstock distribution.

DMC undergoes ring-opening polymerisation (ROP) under acid (sulphuric acid, triflic acid) or base (KOH, CsOH, tetramethylammonium hydroxide) catalysis at 80–150°C reactor temperature. The catalyst initiates ring-opening at a cyclic siloxane Si–O bond, then propagates chain growth through reaction with additional cyclic monomer. The polymerisation is reversible and reaches a thermodynamic equilibrium that is essentially independent of the starting cyclic distribution — running the reaction long enough at constant temperature gives the same chain-length distribution starting from pure D4, pure D5, or DMC mixture. Use fractionated D4 only when the application demands tight feedstock-purity control (cosmetic grade, pharmaceutical grade, optical-clarity silicone).

EU REACH SVHC designation: D4 (CAS 556-67-2) and D5 (CAS 541-02-6) are on the SVHC candidate list as vPvB (very persistent, very bioaccumulative); the DMC mixture inherits these designations on the basis of D4/D5 content. Industrial use of DMC as feedstock for silicone polymer manufacture is permitted, but downstream silicone products must comply with EU REACH residual-cyclic-siloxane limits (D4 ≤0.1% in personal-care leave-on products from 2027; further restrictions under review). Vacuum-stripping post-polymerisation reduces residual cyclic siloxane below applicable limits in the finished silicone fluid.

Technical Specifications

Applications

Silicone Fluid Manufacture

The largest application — methyl silicone fluid (5–60,000 cP grades), modified silicone fluids (amino-, hydroxy-, vinyl-modified), and emulsifiable silicone fluids for cosmetic and personal-care formulations. DMC is metered into a continuously-stirred reactor, polymerised under acid or base catalysis, neutralised, and stripped to give the target viscosity grade.

HCR/HTV Silicone Rubber Gum

DMC at 95–98% on total monomer, polymerised to >100,000 cP gum and supplied to compounders for downstream filler-mixing and peroxide-cure compounding.

RTV Silicone Base Polymer

α,ω-Dihydroxy-PDMS (50–500 cP) for one-component and two-component RTV silicone sealant uses DMC as feedstock with water co-monomer to introduce the OH-terminated chain ends required for condensation cure.

Silicone Resin Manufacture

High-temperature coatings and electronic encapsulation use DMC as the difunctional D-unit source in polymerisation with monofunctional M-units (HMDS) and tetrafunctional Q-units (TEOS) to give MQ resins and DT silicone resins.

Defoamer Base Fluid

DMC-derived medium-viscosity PDMS as the active component in MeFoam-S, MeFoam-MO, and MeFoam-FG defoamer compounds.

Selection Guide vs Fractionated D4

Order DMC for: commodity silicone fluid grades (5–60,000 cP) where the polymerisation reaches thermodynamic equilibrium that is independent of starting cyclic distribution; HCR/HTV silicone rubber gum where high-MW polymerisation also equilibrates; RTV base polymer for sealant compounds; cost-optimised manufacturing where 25–35% feedstock cost reduction matters more than polymer batch-to-batch consistency at parts-per-million purity. Order fractionated D4 for: cosmetic-grade silicone fluid (EU REACH 2027 residual limits); pharmaceutical USP-NF dimethicone; optical-clarity silicone for LED encapsulation; laboratory hydrosilylation R&D.

Equivalent Grades

• Wacker / Dow / Shin-Etsu DMC — globally-traded commodity grade
• Tangshan Sanyou DMC — primary Chinese supplier
• Shandong Dongyue DMC — alternative Chinese supplier
• Generic Chinese DMC — commodity grade ≥98% total cyclic