Structure, Properties and Applications of 1,1,1,3,5,5,5-Heptamethyltrisiloxane（HC-YM9360）

  1,1,1,3,5,5,5-Heptamethyltrisiloxane is a chemical compound with specific structural and physicochemical properties that exhibit a wide range of uses and potentials in synthesis and applications. Heptamethyltrisiloxane (Heptamethyltrisiloxane), with the chemical formula C₇H₂₂O₂Si₃, is a linear organosiloxane compound. As a low molecular weight siloxane, its properties include:

Low viscosity: 1.5-2.5 cSt at 25°C, close to water (1 cst), giving it excellent flowability;

High volatility: boiling point 152-155℃, volatile at room temperature, very low residue;

Chemically inert: acid and alkali resistant (pH 2-12), oxidation resistant, and compatible with most organics;

Low surface tension: approx. 18-20 mN/m, significantly lower than conventional mineral oils (30 mN/m).

Chemical structural formula:

Full chemical name: 1,1,1,3,5,5,5-heptamethyltrisiloxane (code: HMTS)

Alias: Bis(trimethylsiloxy)methylsilane

Molecular Formula: C7H22O2Si3

Molecular weight: 222.51

CAS: 1873-88-7

I. Synthesis method, production process and its properties

Synthesis method, production process：

Heptamethyltrisiloxane is mainly prepared by silane hydrolysis and condensation, and the typical process is as follows:

Raw material: Methyl trichlorosilane (MeSiCl₃) mixed with dimethyl dichlorosilane (Me₂SiCl₂);

Hydrolysis: Hydrolyzed in acidic aqueous solution (HCl) to produce the intermediate silanol;

Condensation: Control pH (3-5) and temperature (60-80°C) to promote linear condensation:

2Me₃SiOH+Me₂Si(OH)₂→Me₃Si-O-SiMe₂-O-SiMe₃+ 2H₂O

Purification: distillation to remove low-boiling substances (e.g., hexamethyldisiloxane), product purity up to 99.5%.

Properties：

1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) is a symmetrical trisiloxane with a molecular structure that contains three silicon atoms (Si), with neighboring silicon atoms alternately connected by oxygen bridges (Si-O-Si) with seven methyl (-CH3) substituents.

The spatial configuration of HMTS is based on the bond angles and substituent arrangement of the Si-O-Si bonds, and its core (trisiloxane backbone) has a nonlinear folded structure.

Si-O-Si bond angles: typically between 140°-160°, not completely linear.

Methyl Substituent Distribution: The methyl group (-CH3) is arranged in a tetrahedral arrangement. Due to the sp³ hybridization of the silicon atoms, this arrangement reduces intermolecular interactions and improves the volatility and hydrophobicity of the compounds.

Overall molecular morphology: Due to the high rotational freedom of the Si-O bond, HMTS may exhibit a curved or umbrella-like conformation, and is more likely to form oligomers or weakly interacting monomers in nonpolar solvents.

Physical Properties: HMTS has a low density, low surface tension and high volatility, making it widely used in hydrophobic coatings, lubricants and modification of silicone materials.

II. Chemical properties and solubility analysis

Chemical stability: HMTS has high chemical stability and is not easy to decompose at room temperature and pressure. However, its Si-O-Si bond may be hydrolyzed or rearranged under the catalytic reaction of acid or alkali.

Heat Resistance: When heated above 200°C, cracking may occur to form oligosiloxanes. The thermal stability of HMTS molecules can be improved by the introduction of high temperature resistant groups (e.g., aromatic or fluorinated substituents), which can be attached to the molecule using reactions such as silica-hydrogen addition and polycondensation, allowing them to be used in higher-temperature environments.

Oxidation resistance: Stable in air, but may degrade in high temperature oxidizing environments.

Hydrolyzability: insoluble in water, but under the action of strong acid or alkali, the Si-O-Si bond may be broken, generating small molecule silanol.

Solubility studies: The solubility of HMTS depends on the polarity of the solvent.

Insoluble in water: It is insoluble in water because its hydrophobic methyl group covers the Si-O-Si skeleton, making it difficult to form hydrogen bonds with water.

Soluble in organic solvents: Soluble in low-polar or non-polar solvents, such as: alkanes (e.g., n-hexane, n-heptane) aromatic hydrocarbons (e.g., toluene, xylene) chlorinated hydrocarbons (e.g., dichloromethane, carbon tetrachloride) ethers (e.g., tetrahydrofuran THF, diethyl ether).

Good solubility allows for easy dispersion and mixing in coatings, lubricants and silicone oil systems.

III. Areas of application

Due to its special structure and physicochemical properties, HMTS has a wide range of applications in many industries.

Modulation of siloxane polymerization as a capping agent: HMTS can be used as a capping agent for oligosiloxanes (e.g., PDMS) to terminate the growth of Si-O-Si chains and effectively improve the hydrophobicity and stability of siloxane materials.

As a hydrophobic additive for silicone coatings: Applied to the waterproof and self-cleaning coating , it can significantly improve the waterproof and pollution resistance of the material.

As lubricant and defoamer: used for precision instrument lubrication and industrial defoaming system.

Application in silicone-modified polymers: It can improve the water resistance, weatherability and flexibility of polyurethane and silicone-propylene emulsions.

Applications in Aerospace: HMTS and its modified derivatives have potential applications in aerospace .

High Temperature Lubricants : Due to the high temperature resistance of the Si-O-Si bond, HMTS can be used in ultra-high temperature environments (such as engine bearings and spacecraft components), where it maintains lower viscosity change than conventional lubricants, providing more consistent low friction loss and effectively extending component life.

Heat-resistant protective coating : Can be used as a component of high and low temperature resistant coatings to improve the stability of materials at extreme temperatures.

Antioxidant Silicone Fluids: Used as a low volatility antioxidant silicone fluid base in aerospace electronics and specialty environment lubricants .

Surfactant: As an effective surfactant, heptamethyltrisiloxane reduces surface tension, improves wettability, and is used as an additive in coatings, inks, textiles, and pesticides. It improves the leveling of coatings, improves the printing quality of inks, enhances the water repellency of textiles, and improves the spraying effect of pesticides.

IV. Security

While focusing on the performance and application of HMTS, its safety should not be neglected. HMTS flash point is 18°C (closed cup), which is a flammable liquid, and should be kept away from fire and heat sources during storage and use, and should strictly comply with the relevant fire prevention regulations. At the same time, avoid direct contact between HMTS and skin and eyes, if contacted inadvertently, immediately flush with plenty of water and seek medical attention.  

V. Market and development trends

At present, the global HMTS market size shows a steady growth trend, the main production regions are concentrated in Asia, Europe and North America. With the increasing demand for high-performance silicone materials in electronics, automotive, aerospace and other industries, the market demand for HMTS is expected to continue to expand. In the future, with the continuous innovation of silicone material technology, HMTS may show more potential application value in new energy, biomedicine and other emerging fields, further expanding its application scope.

Conclusion：

1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) is a low-viscosity, highly hydrophobic, and highly volatile siloxane compound with a unique Si-O-Si structure that gives it a wide range of application value. In addition to conventional applications in coating, lubrication and sealants, HMTS and its modified products have shown potential value in high-temperature lubrication and protective coatings in the aerospace industry. With the development of new silicone materials, the application prospect of HMTS will be broader.

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