Silicone Intermediates
Midstream chemistry connecting upstream chlorosilanes to downstream silicone end products — cyclic siloxanes (DMC / D4 / D5), TEOS tetra-functional crosslinker, and HMDS silylation reagent. Upstream feedstocks for silicone fluid, gum, RTV base polymer, and MQ resin manufacture.
Key Properties at a Glance
| Product Range | Cyclic siloxanes (D3-D7) / Quartz-derived alkoxides (TEOS) / Silylation reagents (HMDS) |
|---|---|
| Purity (typical) | ≥98% (DMC mixture) / ≥99.5% (D4, D5) / ≥98.5% (TEOS) / ≥99% (HMDS) |
| Water Content | ≤30 ppm (D4/D5) to ≤500 ppm (HMDS) — moisture-sensitive feedstocks |
| Flammability | Class IB (D4 49°C, TEOS 49°C, HMDS 19°C) / Class II (D5 78°C) |
| EU REACH Status | D4/D5 SVHC (vPvB) — industrial feedstock use permitted; cosmetic limits scheduled |
| Packaging | 200 kg HDPE-lined drum / 1 t IBC / 25 t ISO-tank for bulk |
| Lead Time | 1-2 weeks Asia / 4-6 weeks EU & North America |
Range shown is category-wide; refer to individual grade COA for precise specs.
Featured Grades
DMC Cyclic Mixture
CAS 69430-24-6
DMC (dimethyl cyclic siloxane mixture, CAS 69430-24-6) is the unfractionated D3-D7 cyclic blend obtained from dimethyldichlorosilane hydrolysis. It is the dominant feedstock for silicone fluid, HTV/RTV gum, and resin polymerisation — equivalent chain-length distributions to fractionated D4 at 25–35% lower cost per kg.
D4 Octamethylcyclotetrasiloxane
CAS 556-67-2
D4 (octamethylcyclotetrasiloxane, CAS 556-67-2) is the fractionated 8-membered cyclic siloxane at ≥99.5% purity — the controlled-purity feedstock for cosmetic-grade silicone fluid (EU REACH 2027 leave-on residual limits), pharmaceutical USP-NF dimethicone, and optical-clarity LED encapsulation.
D5 Decamethylcyclopentasiloxane
CAS 541-02-6
D5 (decamethylcyclopentasiloxane, CAS 541-02-6) is the fractionated 10-membered cyclic siloxane at ≥99.5% purity. Higher boiling point (210°C vs 175°C for D4) gives slower-evaporation dry-feel — the carrier of choice for antiperspirant aerosol, hair-care leave-on, and sun-care formulations.
All Grades/ 2
TEOS Tetraethyl Orthosilicate
CAS 78-10-4
TEOS (tetraethyl orthosilicate, ethyl silicate, CAS 78-10-4) is the tetra-functional silicate ester Si(OC₂H₅)₄ — the reference crosslinker for RTV-2 condensation-cure silicone mould rubber, sol-gel optical coatings, foundry investment-casting binders, and pyrogenic fumed silica precursor.
HMDS Hexamethyldisilazane
CAS 999-97-3
HMDS (hexamethyldisilazane, bis(trimethylsilyl)amine, CAS 999-97-3) is the workhorse silylation reagent — installs trimethylsilyl (TMS) groups on Si-OH surfaces. Largest application is fumed silica hydrophobic surface modification (5–15 wt% on silica). Also M-unit feedstock for MQ silicone resin synthesis.
Detailed Technical Guide
Overview
Silicone intermediates are the chemical building blocks that connect upstream silicon raw materials (metallurgical silicon, chlorosilanes) to downstream silicone end products (fluids, rubber, resins, silane coupling agents, fumed silica). The midstream of the organosilicon value chain — cyclic siloxanes (DMC, D4, D5), tetraethoxysilane (TEOS), hexamethyldisilazane (HMDS), and the functional polymer building blocks (methyl-hydrogen, vinyl, hydroxyl-terminated PDMS) — flows in multi-thousand-tonne global trade volumes because every silicone manufacturer needs continuous supply.
This pillar covers the eleven commercially significant intermediate classes, their downstream applications, and the supply-chain dynamics that make them the largest-volume traded category in the silicone industry. The matrix product site for SEMITECH-branded silicone intermediates is semitechnm.com.
Where Silicone Intermediates Sit in the Industry Chain
The organosilicon industry begins with metallurgical-grade silicon (Si) reduced from quartz, advances through chlorosilanes (HSiCl₃ / SiCl₄ / methylchlorosilanes from the Müller-Rochow direct synthesis) into hydrolysis intermediates, and terminates in the end-product silicone polymers. The midstream — between chlorosilanes and finished products — is where industrial trading is economically rational. Upstream chlorosilanes (TCS, STC, methylchlorosilanes) react violently with atmospheric moisture and are confined to integrated producer-to-consumer pipelines; polysilicon and metallurgical silicon are subject to US export controls (EAR, UFLPA forced-labour traceability) creating structural compliance risk for trading. Hydrolysed intermediates and alkoxide derivatives, by contrast, are stable, transportable, and serve a wide downstream customer base.
This site focuses on the commercially relevant midstream intermediates only. We deliberately do not cover trichlorosilane, silicon tetrachloride, methylchlorosilanes, polysilicon, or metallurgical silicon — these are out of scope for the silicon-materials authority hub for both safety and commercial-discipline reasons.
Tier 1: Cyclic Siloxanes (DMC / D4 / D5)
DMC (dimethylcyclosiloxane mixture, predominantly D3 / D4 / D5 / D6, CAS 69430-24-6) is the hydrolysis product of dimethyldichlorosilane and the single largest-volume silicone intermediate by tonnage. Every silicone fluid, every silicone rubber gum, every silicone resin starts from DMC via base- or acid-catalysed ring-opening polymerisation. Industrial purity is ≥99% with controlled D4:D5 ratio for downstream polymer molecular-weight control.
D4 (octamethylcyclotetrasiloxane, CAS 556-67-2) is the purified four-ring monomer separated from DMC by fractional distillation. D4 is the workhorse for high-quality PDMS fluid manufacture, HCR silicone gum, and textile softener finishes. EU REACH classifies D4 as a substance of very high concern (PBT) for cosmetic leave-on applications under specific exposure scenarios above 0.1% concentration; downstream cosmetic formulators must confirm regulatory compliance for their finished application.
D5 (decamethylcyclopentasiloxane, CAS 541-02-6) is the five-ring cyclic — the most important volatile silicone in cosmetics and personal care, used as carrier and skin-feel modifier in antiperspirants, sunscreens, hair conditioners, and colour cosmetics. EU REACH restrictions on D5 are evolving — currently restricted in wash-off cosmetic products with potential extension to leave-on applications.
Tier 1: TEOS and HMDS
TEOS (tetraethyl orthosilicate, Si(OC₂H₅)₄, CAS 78-10-4) is the most widely traded alkoxysilane on a tonnage basis. Three downstream applications dominate: precision-casting binder (lost-wax investment casting of turbine blades and complex metal parts using TEOS hydrolysed in ethanol with acid catalyst); sol-gel and anti-graffiti coatings (partial hydrolysis produces silica networks for protective coatings on natural-stone facades and glass anti-fogging); and ethyl silicate paint binder (TEOS-based zinc-rich primers for marine and offshore steel structures meeting ASTM D520).
HMDS (hexamethyldisilazane, (CH₃)₃SiNHSi(CH₃)₃, CAS 999-97-3) is a high-margin silylation reagent. Three signature applications: fumed silica surface treatment (HMDS reacts with surface silanols on hydrophilic fumed silica to produce hydrophobic grades — including SEMITECH's own Semisil R-series); semiconductor wafer photoresist primer (applied to silicon wafers before photoresist coating to improve adhesion of organic photoresist to silicon dioxide surface); analytical-chemistry silylation (HMDS converts polar OH and NH groups to TMS ethers/amines for GC-MS volatility, used in pharmaceutical impurity analysis and steroid metabolite quantitation).
Industrial-grade HMDS at ≥98% serves all fumed-silica processing and analytical silylation. Electronic-grade HMDS at ≥99.9% (with ultra-low chloride <1 ppm, ultra-low metal contamination <100 ppb total) is the semiconductor lithography requirement and carries a 4–6× price premium.
Tier 2: Functional Polymer Building Blocks
Three functional polymer classes complete the basic silicone-manufacturer feedstock list:
- Methyl hydrogen silicone oil (MD'ₓM) — supplied at three Si-H content levels: 0.18% (low-reactivity textile water-repellent finishes), 0.36% (release coatings), and 0.75% (highly reactive crosslinkers for addition-cure silicone rubber). The Si-H bond reacts with vinyl-PDMS in the presence of platinum catalyst to form the addition-cure network.
- Vinyl-terminated PDMS — supplied at three viscosities (1000 cSt for thin-section LSR, 10,000 cSt for medium RTV-2, 50,000 cSt for high-strength HCR addition cure). The matched A-side polymer for addition-cure silicone systems.
- Hydroxyl-terminated PDMS / 107 silicone — the foundation polymer for RTV-1 and RTV-2 condensation-cure silicone sealants. Supplied across the standard viscosity grades (25–20,000 cSt) covering one-component low-modulus sealants, high-modulus structural silicones, and tooling-grade RTV-2.
Tier 3: Specialty Intermediates
Three high-margin specialty classes serve smaller but growing segments: phenyl silicone oil (raises thermal stability to 250°C continuous and 300°C short-term — used in LED encapsulation, high-temperature electrical insulating fluids); fluorosilicone (3,3,3-trifluoropropyl-methyl PDMS, combines silicone temperature stability with fluoropolymer chemical resistance — fuel-system O-rings, premium cosmetics); MQ silicone resin (trimethylsilyl-functional silicone resin, the tackifier for silicone pressure-sensitive adhesives in medical tape and transdermal patches).
Procurement and Compliance
For SEMITECH-branded silicone intermediates at industrial trading volumes (200 kg drums to 25-tonne ISO-tank containers), the matrix product site is semitechnm.com which carries all eleven grade classes with CoA, REACH compliance documentation, and graduated MOQ pricing. The silicon-materials authority hub serves as the upstream technical reference and selection guide; transactional procurement flows to semitechnm.
For EU buyers: confirm REACH registration status of D4 / D5 cosmetic applications before specification. For US buyers: confirm TSCA inventory listing. For other regions: consult the relevant national chemical inventory authority before locking new formulations to production.