| HS Code | 703381 |
| Product Name | Heptamethyl Trisiloxane |
| Cas Registry Number | 1873-88-7 |
| Einecs Number | 217-502-8 |
| Molecular Formula | C7H22O2Si3 |
| Molecular Weight | 222.50 g/mol |
| Synonyms | 1,1,1,3,3,5,5-Heptamethyltrisiloxane; Bis(trimethylsiloxy)methylsilane |
| Appearance | Colorless clear liquid |
| Physical State | Liquid |
| Odor | Mild silicone-like odor |
| Density | 0.819 g/mL at 25 °C |
| Boiling Point | 142 °C |
| Flash Point | 35 °C |
| Refractive Index | 1.382 at 20 °C |
| Water Solubility | Insoluble in water |
| Viscosity | Low viscosity, approximately 1 cSt |
| Purity | Typically ≥95% |
| Storage Conditions | Store in a cool, dry, well-ventilated place; protect from moisture |
As an accredited Heptamethyl Trisiloxane factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Packed in a 1 L amber glass bottle with a chemical-resistant screw cap, lot-numbered GHS-compliant label, and secure secondary containment. |
| Container Loading (20′ FCL) | Heptamethyl Trisiloxane is packed in drums and loaded into a 20-foot FCL container, securely stowed and sealed for safe transport. |
| Shipping | Ship Heptamethyl Trisiloxane as a flammable liquid in UN-approved, tightly sealed containers, protected from moisture and ignition sources. Store in a cool, dry, ventilated area. Follow the SDS and applicable DOT/IATA/IMDG requirements for UN number, packing group, labeling, and documentation. |
| Storage | Store Heptamethyl Trisiloxane in a tightly closed, labeled container in a cool, dry, well-ventilated area away from heat, sparks, open flames, and strong oxidizers. Keep away from moisture and incompatible materials. Use grounded containers and equipment, protect from physical damage, and avoid vapor accumulation. Ensure adequate ventilation and avoid inhalation. Follow local regulations and the SDS; keep spill containment available. |
| Shelf Life | Typically 24 months when stored in a sealed container at ambient temperature, away from moisture, heat, and direct sunlight. |
A significant downstream use of heptamethyl trisiloxane (HMTS), a monohydridosiloxane with the structure (CH3)3Si–O–SiH(CH3)–O–Si(CH3)3, is as the hydrophobic precursor in the synthesis of agricultural trisiloxane alkoxylates. In a batch hydrosilylation, an allyl-terminated poly(ethylene oxide-co-propylene oxide) monomethyl ether is dried to a moisture content below 100 ppm before charging to a glass-lined reactor equipped with a turbine agitator, reflux condenser, and nitrogen sparge. A Karstedt platinum(0) complex in vinyl-terminated polydimethylsiloxane is added at 5–10 ppm Pt, and the reactor is heated to 80–100 °C under positive nitrogen pressure. The Si–H:allyl molar ratio is maintained between 1.00:1.05 and 1.00:1.15, with the excess allyl polyether used to drive residual hydride consumption and to prevent silanol formation. Reaction progress is monitored by FTIR; the Si–H stretching absorption at 2160 cm⁻¹ is reduced below the detection limit before cooling and filtration. The resulting polyalkyleneoxide-modified trisiloxane is a nonionic silicone surfactant that reduces the equilibrium surface tension of water to 20–23 mN/m at 0.1 wt% when tested according to ASTM D1331. In agricultural spray systems, this surface activity contributes to wetting and spreading on waxy leaf cuticles, although actual field deposition depends on spray volume, nozzle type, water hardness, and tank-mix compatibilizers. The main process bottleneck is water control: residual moisture in the polyether above 100 ppm hydrolyzes Si–H to silanol, which condenses to produce turbidity and elevated viscosity. Amine and sulfur-containing impurities must be excluded because they poison the platinum catalyst and leave unreacted Si–H, which can create odor and instability in the final adjuvant. Published data for specific commercial adjuvant formulations is limited, but the synthetic window is representative of low-molecular-weight silicone surfactant production records.
The process control windows in Table 1 apply to hydrosilylation of HMTS with different olefin substrates. These ranges are derived from batch process records and are not product specifications.
| Functional substrate | Si–H:olefin molar ratio | Pt catalyst loading | Temperature | Endpoint criterion |
|---|---|---|---|---|
| Allyl-terminated polyether | 1.00:1.05–1.15 | 5–10 ppm | 80–100 °C | Si–H peak at 2160 cm⁻¹ below FTIR detection limit |
| 1-Octene | 1.00:1.10–1.20 | 5–8 ppm | 90–110 °C | Residual Si–H ≤ 0.02 mmol/g |
| 1-Dodecene | 1.00:1.10–1.20 | 8–12 ppm | 100–120 °C | Residual Si–H ≤ 0.02 mmol/g |
In solventless addition-cure silicone release coatings, the base polymer is a vinyl-functional polydimethylsiloxane with a vinyl content of 0.15–0.35 mmol/g, and the crosslinker is a poly(methylhydrosiloxane-co-dimethylsiloxane) copolymer with a Si–H content of 2–15 mmol/g. Because heptamethyl trisiloxane contains only one Si–H group per molecule, it cannot function as a crosslinker; instead it acts as a monofunctional chain stopper when added at low dosage. Addition levels from 0.5 wt% to 5 wt% based on total formulation cap vinyl-terminated chains, reduce the effective crosslink density after platinum-catalyzed addition, and shift the cured film from low to higher release force. Release force is measured by FINAT FTM 3 after lamination and conditioning; low-release commercial coatings typically fall between 8 cN/25 mm and 35 cN/25 mm, but the precise response to HMTS concentration is formulation-specific. Curing is performed in a forced-air oven at 120–150 °C for 20–40 s, with platinum catalyst at 80–120 ppm. At HMTS levels above 6 wt%, the network becomes under-crosslinked and the extractable content rises, leading to blocking in the release liner stack and poor anchorage on the substrate. Below 0.2 wt%, the effect on release force is usually within the repeatability limit of the test method. Residual Si–H in the cured film can hydrolyze at relative humidity above 60%, generating silanol groups that alter surface polarity and release aging; therefore, high-humidity storage must be validated by release-force retention data. HMTS cannot replace the multifunctional Si–H crosslinker because network formation requires multiple reactive sites per molecule. Published data for specific commercial coating platforms is limited, but the chain-stopper mechanism is established from addition-cure silicone chemistry.
The Si–H functionality of heptamethyl trisiloxane reacts with terminal α-olefins such as 1-octene, 1-dodecene, and 1-hexadecene in the presence of a platinum catalyst to yield alkyl-modified trisiloxanes. The reaction is carried out at 90–120 °C with an olefin:Si–H molar ratio between 1.05:1 and 1.20:1 and a Pt loading of 5–15 ppm. The product class consists of short siloxane backbones carrying one long alkyl chain; viscosity at 25 °C typically remains below 50 mPa·s for C6–C12 alkyl derivatives, while longer alkyl chains raise the pour point and reduce compatibility with polar organic solvents. Reaction completion is monitored by iodine value for residual olefin and by FTIR for disappearance of the Si–H band at 2160 cm⁻¹. Terminal α-olefins are required; internal or vinylidene olefins react slowly and may leave unreacted hydride, which causes odor, moisture sensitivity, and formulation instability. Alkyl-modified trisiloxanes prepared from HMTS are used as low-surface-energy emollients, lubricants, and intermediates for further functionalization. In personal care applications, the final alkyl-silicone ingredient is assessed separately under cosmetic regulations; HMTS is not the cosmetic active. Process equipment for α-olefin hydrosilylation is usually a stainless-steel or glass-lined vessel with nitrogen blanketing because the olefin and platinum catalyst are sensitive to oxygen and moisture. Published data for specific alkyl chain lengths is limited in public literature; product control parameters include viscosity, refractive index, residual Si–H content, and iodine value rather than molecular weight alone.
Surface treatment of fumed silica and precipitated silica with HMTS proceeds through the reaction of Si–H with surface silanol groups, yielding a siloxane-linked hydrophobic surface and releasing hydrogen gas. In a fluidized-bed reactor or ploughshare mixer, silica with a BET surface area of 150–300 m²/g is pre-dried at 120–150 °C to remove physisorbed water, then contacted with 1–5 wt% HMTS under nitrogen. The treatment temperature is held at 100–160 °C for 30–120 min, depending on bulk density, agitator speed, and silica type. Hydrogen release requires explosion protection: the lower explosive limit of hydrogen in air is 4 vol%, so the headspace is purged with nitrogen and the off-gas is vented through a dilution or thermal oxidation system. Completion is assessed by FTIR for residual silanol intensity and by a methanol wetting test; untreated silica is hydrophilic, whereas treated material floats on water and resists moisture uptake. Water contact angle values above 110° are commonly recorded for fully treated high-surface-area silica, but published data for HMTS-treated grades is limited compared with established organosilane treatments. The critical operational boundary is adsorbed water: moisture converts Si–H to silanol, reduces hydrophobization efficiency, and increases hydrogen evolution unpredictably. Amine-containing substrates must be avoided because amines catalyze siloxane redistribution and may generate low-molecular-weight cyclic siloxanes. Treated silica from this route is evaluated in silicone sealants, defoamer concentrates, and hydrophobic coatings, where it modifies rheology, sedimentation, and moisture sensitivity.
Condensation-cure silanol-terminated polydimethylsiloxane compounds can be modified with low levels of heptamethyl trisiloxane as a reactive chain stopper. In a planetary mixer under vacuum, 0.5–3 wt% HMTS is added to the silanol-terminated polymer before or during the compounding of filler and condensation catalyst. The Si–H group reacts with terminal silanol groups, releasing hydrogen and forming an Si–O–Si linkage that caps the polymer chain. This capping reduces molecular weight build-up, lowers extrusion viscosity, and controls green strength before cure. A tin catalyst such as dibutyltin dilaurate at 0.1–0.5 wt% or a titanate catalyst is used for the condensation reaction; the mixer is operated under vacuum to remove hydrogen and prevent void formation. The hydrogen released by Si–H/silanol reaction is flammable, and the vacuum pump exhaust must be vented outside the process area; the lower explosive limit of hydrogen in air is 4 vol%. Excess HMTS above 5 wt% may introduce excessive monofunctional ends that reduce the modulus of the cured elastomer below specification. The final mechanical properties are measured by ISO 37 for tensile strength and elongation at break, and by ISO 7619 for hardness. Published data for HMTS as a chain stopper in condensation-cure systems is limited because trimethylsiloxy-terminated polydimethylsiloxane is the conventional capping agent; HMTS is considered where a Si–H functional intermediate is already available and where hydrogen handling is acceptable.
Foam-control formulations based on HMTS derivatives are built from polyether-modified trisiloxanes or alkyl-modified trisiloxanes rather than from HMTS itself. The derivatives are compounded with hydrophobic silica, silicone oil, and organic carriers in a high-shear rotor-stator mixer operating at 3000–6000 rpm. The low molecular weight of the trisiloxane backbone produces a compact hydrophobic segment that can orient at the air/liquid interface; the polyether or alkyl chain controls the cloud point and the solubility limit in the target foam system. In aqueous surfactant media, polyether-modified trisiloxanes exhibit cloud points between 30 °C and 60 °C, and the defoaming action changes when the process temperature approaches or exceeds the cloud point because the surfactant collapses into a dispersed hydrophobic phase. This phase inversion is formulation-specific and is influenced by electrolyte concentration, pH, and the presence of anionic surfactants. The defoamer is tested by air-release time and foam height reduction according to ASTM D892 for high-temperature foaming tendency, although that method is designed for lubricating oils and must be adapted for aqueous process fluids. Published data for specific commercial defoamer concentrates is limited, and the relationship between HMTS derivative structure and foam-control performance is established through comparative screening rather than a linear property model. The main processing constraint is the high shear required to disperse hydrophobic silica without destroying the silicone polyether structure; prolonged mixing above 6000 rpm can generate heat that exceeds the cloud point and causes the batch to invert prematurely.
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Heptamethyl trisiloxane (CAS 1873-88-7; molecular formula C7H22O2Si3; relative molecular mass 222.5 g/mol) is a linear trisiloxane in which the middle silicon atom carries one hydrogen substituent and one methyl substituent, while the terminal silicon atoms are trimethylsiloxy groups. The systematic structure is 1,1,1,3,5,5,5-heptamethyltrisiloxane; it contains a single silicon-hydride moiety and is used as a monofunctional hydride donor rather than as a high-polymer chain builder. The boiling point at 101.3 kPa is generally listed as 142 °C. Density at 20 °C is approximately 0.819 g/mL by ASTM D4052; refractive index at 20 °C is approximately 1.381 by ASTM D1218. The closed-cup flash point is approximately 33 °C by ASTM D56. The Si-H functionality distinguishes it from octamethyltrisiloxane, which is the fully methylated analogue with no hydrosilylation-reactive site. Product codes are supplier-defined; the material is commonly requested by CAS number and GC purity rather than by a universal model designation.
The theoretical active-hydrogen content of heptamethyl trisiloxane is 0.45 wt% hydrogen or 4.49 mmol Si-H per gram. Incoming quality-control documents therefore report active hydrogen more often than simple moisture. The Si-H value is determined by alkali decomposition manometry, iodometric titration, or 1H NMR integration of the Si-H resonance near 4.7 ppm; Fourier-transform infrared spectroscopy observes the Si-H stretch near 2160 cm⁻¹. Gas-chromatographic purity is measured against an internal standard, and supplier methods may reference ASTM E260 for general GC practice. A typical commercial certificate may list GC-FID purity at 97–99%, with the balance consisting of lower methylsiloxanes, branched trisiloxane isomers, and trace silanols. If the Si-H content is depleted by hydrolysis or premature redox reactions, the product no longer behaves as a true monofunctional silicon hydride. In an addition-cure network, a 5 mol% reduction in Si-H versus vinyl can leave unreacted vinyl groups, lower the crosslink density, and shift the Shore A hardness below the formulation target. Because the product is volatile, off-spec material cannot simply be over-dosed by large margins without altering volatile organic content and cure stoichiometry.
Table 1 lists representative physical-property windows used for incoming lot acceptance when the product is intended for hydrosilylation-grade service.
| Parameter | Typical Value or Range | Test Method |
|---|---|---|
| Molecular formula | C7H22O2Si3 | Structural assignment by 29Si NMR |
| Relative molecular mass | 222.5 g/mol | Calculated from atomic weights |
| Normal boiling point | 142 °C at 101.3 kPa | Distillation curve |
| Density at 20 °C | 0.819 g/mL | ASTM D4052 |
| Refractive index at 20 °C | 1.381 | ASTM D1218 |
| Flash point, closed cup | 33 °C | ASTM D56 |
| Active hydrogen | 4.49 mmol/g | Calculated / iodometric titration |
In a typical hydrosilylation capping reaction, the heptamethyltrisiloxane Si-H is reacted with an allyl-terminated polyether or an alpha-olefin in the presence of a Karstedt platinum catalyst at 80–90 °C. The reagent is fed below the liquid surface through a dip tube into a glass-lined reactor blanketed with dry nitrogen; the internal pressure is maintained at 0.2–0.5 bar gauge to exclude atmospheric oxygen, which can deactivate platinum and promote silanol formation. Reaction progress is followed by FTIR disappearance of the 2160 cm⁻¹ band. In a 2000 L production vessel, the dosing rate is controlled so that the batch temperature does not exceed 100 °C; the heat of reaction per mole of Si-H is lower than that of multifunctional polymethylhydrosiloxane because only one addition occurs per molecule. Published data for this specific reactor configuration is limited. Operators monitor the induction period, which is strongly dependent on residual sulfur and phosphorus content in the feed; phosphine and sulfide impurities at parts-per-million levels can extend the induction time and require additional catalyst. If the reaction temperature is raised above 120 °C to compensate, isomerization of the allyl ether to the unreactive propenyl ether can become competitive, reducing the final conversion. This processing window, typically 80–120 °C, is therefore treated as a critical control range in silicone polyether production.
In anionic ring-opening polymerization of octamethylcyclotetrasiloxane, addition of heptamethyltrisiloxane at the late stage stops chain growth by introducing a trimethylsilyl terminus while leaving a Si-H site for subsequent functionalization. This process is conducted under vacuum to remove cyclics; the reactor is often a wiped-film evaporator operated at 120–140 °C and 5–20 mbar. The volatile unreacted trisiloxane is stripped with the residual cyclics. Published data for this specific configuration is limited. The resulting Si-H-capped polydimethylsiloxane can be converted to telechelic polymers by hydrosilylation with allyl-functional monomers. This route is used when a single hydride terminus is preferred over a hydroxyl or vinyl terminus.
Silicone-polyether surfactants for polyurethane foam require control of the siloxane-to-polyether architecture. Heptamethyltrisiloxane supplies one terminal or pendant Si-H site and therefore produces a short siloxane block with one attached polyether segment. The molar ratio of Si-H to allyl-terminated polyether is typically set at 1.00:1.00 to 1.05:1.00, with the slight excess used to compensate for evaporative loss of the trisiloxane during the reaction. At 85 °C and 5–10 ppm platinum, conversion is normally complete within 4–8 h depending on the polyether molecular weight and moisture content. The resulting product is a clear, low-viscosity liquid; turbidity indicates unreacted polyether, water, or silanol condensation. Surface-tension reduction is measured by ASTM D1331; formulators use the value to compare the hydride-derived surfactant to conventional dimethyl silicone fluids. Because the hydride site is consumed during addition, the final silicone-polyether no longer releases hydrogen and can be handled as a non-reactive copolymer.
Silicon-hydride hydrolysis becomes significant when the material is exposed to water in the presence of acids, bases, or certain transition-metal residues. The reaction generates hydrogen and converts the Si-H to a silanol; in a closed container, the pressure rise can be hazardous. Plant handling therefore specifies dry nitrogen blankets with a dew point below -40 °C, and drums are opened only under a nitrogen sweep when ambient relative humidity exceeds 60%. Stainless steel and glass-lined equipment are preferred. Copper-containing alloys are avoided in high-temperature service because copper residues can catalyze dehydrogenative coupling. Storage is conducted at 5–30 °C in sealed, electrically grounded containers. The product is incompatible with strong bases, concentrated acids, and some amines; addition of an amine-containing corrosion inhibitor or stabilizer should not be made without compatibility testing. Vent lines on storage tanks must be fitted with flame arresters and pressure/vacuum relief because the flash point is 33 °C.
Under GHS classification used on supplier safety data sheets, the material is typically classified as flammable liquid and may be assigned aspiration toxicity and skin irritation categories. Transport requires closed containers and electrical bonding because the flash point is 33 °C. In the European Union, CAS 1873-88-7 falls within the general silicone intermediate regulatory framework; downstream users must verify that the supplier’s REACH registration covers the intended use. Classification and labelling should be checked against CLP Regulation (EC) No 1272/2008. The product is not intended for direct food-contact use unless a specific food-contact compliance evaluation is completed against the relevant national or regional legislation. Residual moisture, peroxide content, and acidity are not specified in every commercial grade, but when the material is used as an electronic-grade capping agent, additional lot tests for trace chloride and total metals may be required. No single product model designation is recognized across all global suppliers; ordering is normally by CAS number, minimum GC purity, and moisture limit.
In a silicone distillation unit, heptamethyltrisiloxane is isolated from mixed siloxane streams by vacuum fractionation. The atmospheric boiling point is 142 °C, but column operation at 50–100 mbar lowers the reboiler temperature and reduces thermal redistribution. Batch-to-batch variation is normally observed in the 0.5–1.5% range for residual low-boiling siloxanes and in trace silanol content. High silanol levels are problematic because they participate in acid- or base-catalyzed redistribution and can alter the molecular weight distribution of downstream siloxane polymers. For hydrosilylation-grade product, buyers may set an incoming silanol limit by Karl Fischer or 29Si NMR, along with density and refractive index acceptance windows. In a continuous addition-cure compounding line, feed lines are jacketed at 20 °C to avoid vapor lock because the vapor pressure is sufficient to cause pump cavitation at higher feed temperatures. When drums from different batches are blended, a simple density check is not sufficient to detect isomer variation; GC-FID or 29Si NMR is required. Published data for this specific configuration is limited.
Octamethyltrisiloxane functions as an inert volatile silicone fluid, whereas heptamethyltrisiloxane is a reactive hydride-functional analogue. Direct substitution is not recommended where an unreactive volatile carrier is needed, because the Si-H group can react with nucleophiles, generate hydrogen under basic or acidic aqueous conditions, and alter surface and electrical properties. In hydrosilylation capping, however, heptamethyltrisiloxane is used specifically because it provides one addition site. The differences relevant to process selection are shown in Table 2.
| Parameter | Heptamethyltrisiloxane | Octamethyltrisiloxane | 1,1,3,3-Tetramethyldisiloxane |
|---|---|---|---|
| CAS registry number | 1873-88-7 | 107-51-7 | 3277-26-6 |
| Si-H sites per molecule | 1 | 0 | 2 |
| Relative molecular mass | 222.5 g/mol | 236.5 g/mol | 134.3 g/mol |
| Normal boiling point | 142 °C | 153 °C | 70–71 °C |
| Primary role | Monofunctional hydride donor, chain stopper | Inert volatile carrier | Difunctional hydride crosslinker or chain extender |
In solventless silicone release coatings, the choice between heptamethyltrisiloxane and tetramethyldisiloxane governs the network architecture: heptamethyltrisiloxane terminates, tetramethyldisiloxane extends, and octamethyltrisiloxane acts as a non-reactive diluent. This functional distinction determines the molecular weight between crosslinks and the peel force of the cured film.
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