Dimethylcyclosiloxanes
[CAS# 69430-24-6]

Identification

• Classification: Chemical reagent >> Organic reagent >> Siloxane
• Name: Dimethylcyclosiloxanes
• Synonyms: DMC;Cyclomethicone;Cyclodimethylsiloxanes;Silicone TSA 405A;Silwet FZ 2166; UP 1004; X 2-1401
• Molecular Formula: [(CH₃)_2SiO]_{n, n=3~6}
• CAS Registry Number: 69430-24-6
• EC Number: 614-966-1

Properties

• Density: 0.956-0.958 g/cm³ (25 °C)
• Refractive index: 1.3960-1.397 (25 °C)
• Flash point: 55 °C

Safety Data

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Flammable liquids	Flam. Liq.	3	H226
Chronic hazardous to the aquatic environment	Aquatic Chronic	3	H412
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413
Reproductive toxicity	Repr.	2	H361
Reproductive toxicity	Repr.	2	H361f
Chronic hazardous to the aquatic environment	Aquatic Chronic	1	H410

Discovery and Applications

Dimethylcyclosiloxanes are a group of cyclic organosilicon compounds consisting of repeating units of siloxane (Si–O–Si) bonds with two methyl groups attached to each silicon atom. These compounds form a family of cyclic siloxanes, commonly denoted as D₃, D₄, D₅, and D₆, where the subscript indicates the number of repeating siloxane units in the ring. Among these, octamethylcyclotetrasiloxane (D₄) and decamethylcyclopentasiloxane (D₅) are the most studied and widely used.

The discovery of dimethylcyclosiloxanes traces back to the early developments in silicone chemistry during the 20th century. The fundamental siloxane chemistry was pioneered in the 1940s and 1950s, enabling the synthesis of cyclic and linear siloxane polymers. These cyclic siloxanes were identified as important intermediates and raw materials in the production of silicone polymers and elastomers.

Dimethylcyclosiloxanes are characterized by their volatility, low surface tension, chemical inertness, and thermal stability. These properties arise from the strong Si–O bonds and the hydrophobic methyl substituents. Due to their unique attributes, dimethylcyclosiloxanes have been extensively utilized in various industrial and consumer products.

A primary application of dimethylcyclosiloxanes is as intermediates in the manufacture of silicone polymers, including silicone oils, resins, and elastomers. The cyclic siloxanes are ring-opened and polymerized to form long-chain polydimethylsiloxanes (PDMS), which are widely used as lubricants, antifoaming agents, and release agents.

In personal care and cosmetic formulations, volatile dimethylcyclosiloxanes such as D₄ and D₅ serve as carriers, emollients, and spreading agents. Their rapid evaporation and skin-friendly profile make them valuable in deodorants, skin creams, hair conditioners, and antiperspirants, where they provide a smooth, non-greasy feel.

Dimethylcyclosiloxanes also find use in industrial applications such as in the formulation of silicone-based coatings, adhesives, and sealants. Their chemical inertness and stability under various environmental conditions enhance product durability and performance.

Environmental and safety assessments have noted that certain cyclic dimethylsiloxanes, especially D₄ and D₅, exhibit persistence in the environment and potential bioaccumulation. This has led to regulatory scrutiny and efforts to manage their use and release. However, these compounds are generally considered to have low acute toxicity to humans.

Manufacturing of dimethylcyclosiloxanes typically involves the hydrolysis and condensation of dimethyldichlorosilane, resulting in a mixture of cyclic and linear siloxanes. The cyclic compounds are then separated by distillation due to their differing boiling points.

In summary, dimethylcyclosiloxanes are cyclic organosilicon compounds discovered in the mid-20th century as part of silicone chemistry advancements. They serve as crucial intermediates in silicone polymer production and are widely applied in personal care, industrial coatings, and adhesives due to their volatility, stability, and low surface tension. Their environmental persistence has prompted regulatory attention, but their overall chemical versatility continues to sustain their broad use.

References

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