siloxane

Siloxane is a chemical compound containing silicon–oxygen–silicon (Si–O–Si) linkages with organic substituents attached to silicon. The term spans a broad family from small cyclic oligomers to high‑molecular‑weight polymers commonly known as silicones.

The core motif is Si–O–Si, with silicon atoms bearing organic groups such as methyl, phenyl, or vinyl. In organosiloxanes the backbone is typically [-SiR2-O-]n. Cyclic siloxanes such as octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) are common feedstocks; linear polymers include polydimethylsiloxane (PDMS).

Properties of siloxanes include thermal stability, chemical resistance, and low surface energy, which confer flexibility and hydrophobicity. They span a wide range of viscosities and mechanical properties, from fluids to elastomers and resins, and they often exhibit low glass transition temperatures.

Synthesis and processing usually involve hydrolysis or condensation of silanes or silanols, followed by polymerization or cyclization, often with catalysts. The resulting materials can be tailored for specific applications through choice of substituents and molecular weight.

Applications are broad: lubricants and fluids, sealants and coatings, adhesives, cosmetics, medical devices, and electronics. PDMS is widely used as a silicone oil and elastomer; cyclic siloxanes appear in personal care products; siloxane resins provide weather and chemical resistance.

Safety and regulation vary by compound and use. Siloxanes generally have low acute toxicity, but certain volatile cyclic siloxanes raise environmental and regulatory concerns in some jurisdictions, particularly in cosmetics, necessitating appropriate handling and disposal.